Science of Everyday Things: Real Life Chemistry

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SCIENCEOF

EVERYDAY THINGS

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SCIENCEOF

EVERYDAY THINGS volume 1: REAL-LIFE CHEMISTRY

edited by NEIL SCHLAGER written by JUDSON KNIGHT A SCHLAGER INFORMATION GROUP BOOK

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S C I E N C E O F E V E RY DAY T H I N G S VOLUME 1

Re a l - L i f e c h e m i s t ry

A Schlager Information Group Book Neil Schlager, Editor Written by Judson Knight Gale Group Staff Kimberley A. McGrath, Senior Editor Maria Franklin, Permissions Manager Margaret A. Chamberlain, Permissions Specialist Shalice Shah-Caldwell, Permissions Associate Mary Beth Trimper, Manager, Composition and Electronic Prepress Evi Seoud, Assistant Manager, Composition and Electronic Prepress Dorothy Maki, Manufacturing Manager Rita Wimberley, Buyer Michelle DiMercurio, Senior Art Director Barbara J. Yarrow, Manager, Imaging and Multimedia Content Robyn V. Young, Project Manager, Imaging and Multimedia Content Leitha Etheridge-Sims, Mary K. Grimes, and David G. Oblender, Image Catalogers Pam A. Reed, Imaging Coordinator Randy Bassett, Imaging Supervisor Robert Duncan, Senior Imaging Specialist Dan Newell, Imaging Specialist While every effort has been made to ensure the reliability of the information presented in this publication, Gale Group does not guarantee the accuracy of the data contained herein. Gale accepts no payment for listing, and inclusion in the publication of any organization, agency, institution, publication, service, or individual does not imply endorsement of the editors and publisher. Errors brought to the attention of the publisher and verified to the satisfaction of the publisher will be corrected in future editions. The paper used in the publication meets the minimum requirements of American National Standard for Information Sciences—Permanence Paper for Printed Library Materials, ANSI Z39.48-1984. This publication is a creative work fully protected by all applicable copyright laws, as well as by misappropriation, trade secret, unfair competition, and other applicable laws. The authors and editors of this work have added value to the underlying factual material herein through one or more of the following: unique and original selection, coordination, expression, arrangement, and classification of the information. All rights to this publication will be vigorously defended. Copyright © 2002 Gale Group, 27500 Drake Road, Farmington Hills, Michigan 48331-3535 No part of this book may be reproduced in any form without permission in writing from the publisher, except by a reviewer who wishes to quote brief passages or entries in connection with a review written for inclusion in a magazine or newspaper. ISBN

0-7876-5631-3 (set) 0-7876-5632-1 (vol. 1) 0-7876-5633-X (vol. 2)

0-7876-5634-8 (vol. 3) 0-7876-5635-6 (vol. 4)

Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

Library of Congress Cataloging-in-Publication Data Knight, Judson. Science of everyday things / written by Judson Knight, Neil Schlager, editor. p. cm. Includes bibliographical references and indexes. Contents: v. 1. Real-life chemistry – v. 2 Real-life physics. ISBN 0-7876-5631-3 (set : hardcover) – ISBN 0-7876-5632-1 (v. 1) – ISBN 0-7876-5633-X (v. 2) 1. Science–Popular works. I. Schlager, Neil, 1966-II. Title. Q162.K678 2001 500–dc21

2001050121

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CONTENTS

Introduction.............................................v Advisory Board ......................................vii

MEASUREMENT Measurement ......................................................3 Temperature and Heat......................................11 Mass, Density, and Volume ..............................23

NONMETALS AND METALLOIDS Nonmetals ......................................................213 Metalloids .......................................................222 Halogens .........................................................229 Noble Gases ....................................................237 Carbon............................................................243 Hydrogen ........................................................252

MATTER Properties of Matter .........................................32 Gases..................................................................48 ATOMS AND MOLECULES Atoms ................................................................63 Atomic Mass......................................................76 Electrons............................................................84 Isotopes .............................................................92 Ions and Ionization........................................101 Molecules........................................................109

BONDING AND REACTIONS Chemical Bonding .........................................263 Compounds....................................................273 Chemical Reactions........................................281 Oxidation-Reduction Reactions....................289 Chemical Equilibrium ...................................297 Catalysts..........................................................304 Acids and Bases ..............................................310 Acid-Base Reactions.......................................319

ELEMENTS SOLUTIONS AND MIXTURES Elements .........................................................119 Periodic Table of Elements ............................127 Families of Elements......................................140

Mixtures..........................................................329 Solutions.........................................................338 Osmosis ..........................................................347

METALS Metals..............................................................149 Alkali Metals...................................................162 Alkaline Earth Metals ....................................171 Transition Metals ...........................................181 Actinides .........................................................196 Lanthanides ....................................................205

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Distillation and Filtration..............................354 ORGANIC CHEMISTRY Organic Chemistry ........................................363 Polymers .........................................................372 General Subject Index ......................381

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INTRODUCTION

Overview of the Series Welcome to Science of Everyday Things. Our aim is to explain how scientific phenomena can be understood by observing common, real-world events. From luminescence to echolocation to buoyancy, the series will illustrate the chief principles that underlay these phenomena and explore their application in everyday life. To encourage cross-disciplinary study, the entries will draw on applications from a wide variety of fields and endeavors. Science of Everyday Things initially comprises four volumes: Volume 1: Real-Life Chemistry Volume 2: Real-Life Physics Volume 3: Real-Life Biology Volume 4: Real-Life Earth Science Future supplements to the series will expand coverage of these four areas and explore new areas, such as mathematics.

Arrangement of Real-Life Physics This volume contains 40 entries, each covering a different scientific phenomenon or principle. The entries are grouped together under common categories, with the categories arranged, in general, from the most basic to the most complex. Readers searching for a specific topic should consult the table of contents or the general subject index.

• How It Works Explains the principle or theory in straightforward, step-by-step language. • Real-Life Applications Describes how the phenomenon can be seen in everyday events. • Where to Learn More Includes books, articles, and Internet sites that contain further information about the topic. Each entry also includes a “Key Terms” section that defines important concepts discussed in the text. Finally, each volume includes numerous illustrations, graphs, tables, and photographs. In addition, readers will find the comprehensive general subject index valuable in accessing the data.

About the Editor, Author, and Advisory Board Neil Schlager and Judson Knight would like to thank the members of the advisory board for their assistance with this volume. The advisors were instrumental in defining the list of topics, and reviewed each entry in the volume for scientific accuracy and reading level. The advisors include university-level academics as well as high school teachers; their names and affiliations are listed elsewhere in the volume. N E I L S C H LAG E R is the president of

• Concept Defines the scientific principle or theory around which the entry is focused.

Schlager Information Group Inc., an editorial services company. Among his publications are When Technology Fails (Gale, 1994); How Products Are Made (Gale, 1994); the St. James Press Gay and Lesbian Almanac (St. James Press, 1998); Best Literature By and About Blacks (Gale,

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Within each entry, readers will find the following rubrics:

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2000); Contemporary Novelists, 7th ed. (St. James Press, 2000); and Science and Its Times (7 vols., Gale, 2000-2001). His publications have won numerous awards, including three RUSA awards from the American Library Association, two Reference Books Bulletin/Booklist Editors’ Choice awards, two New York Public Library Outstanding Reference awards, and a CHOICE award for best academic book. Judson Knight is a freelance writer, and author of numerous books on subjects ranging from science to history to music. His work on science titles includes Science, Technology, and Society, 2000 B.C.-A.D. 1799 (U*X*L, 2002), as well as extensive contributions to Gale’s seven-volume Science and Its Times (2000-2001). As a writer on history, Knight has published Middle Ages Reference Library (2000), Ancient

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Civilizations (1999), and a volume in U*X*L’s African American Biography series (1998). Knight’s publications in the realm of music include Parents Aren’t Supposed to Like It (2001), an overview of contemporary performers and genres, as well as Abbey Road to Zapple Records: A Beatles Encyclopedia (Taylor, 1999). His wife, Deidre Knight, is a literary agent and president of the Knight Agency. They live in Atlanta with their daughter Tyler, born in November 1998.

Comments and Suggestions Your comments on this series and suggestions for future editions are welcome. Please write: The Editor, Science of Everyday Things, Gale Group, 27500 Drake Road, Farmington Hills, MI 48331.

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ADVISORY BO T IATR LD E

William E. Acree, Jr. Professor of Chemistry, University of North Texas Russell J. Clark Research Physicist, Carnegie Mellon University Maura C. Flannery Professor of Biology, St. John’s University, New York John Goudie Science Instructor, Kalamazoo (MI) Area Mathematics and Science Center Cheryl Hach Science Instructor, Kalamazoo (MI) Area Mathematics and Science Center Michael Sinclair Physics instructor, Kalamazoo (MI) Area Mathematics and Science Center Rashmi Venkateswaran Senior Instructor and Lab Coordinator, University of Ottawa Ottawa, Ontario, Canada

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S C I E N C E O F E V E RY DAY T H I N G S real-life chemistry

MEASUREMENT MEASUREMENT T E M P E RAT U R E A N D H E AT M A S S , D E N S I T Y, A N D V O L U M E

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Measurement

MEASUREMENT

CONCEPT Measurement seems like a simple subject, on the surface at least; indeed, all measurements can be reduced to just two components: number and unit. Yet one might easily ask, “What numbers, and what units?”—a question that helps bring into focus the complexities involved in designating measurements. As it turns out, some forms of numbers are more useful for rendering values than others; hence the importance of significant figures and scientific notation in measurements. The same goes for units. First, one has to determine what is being measured: mass, length, or some other property (such as volume) that is ultimately derived from mass and length. Indeed, the process of learning how to measure reveals not only a fundamental component of chemistry, but an underlying—if arbitrary and manmade— order in the quantifiable world.

HOW IT WORKS Numbers

Obviously, there is an arbitrary quality underlying the modern numerical system, yet it works extremely well. In particular, the use of decimal fractions (for example, 0.01 or 0.235) is particularly helpful for rendering figures other than whole numbers. Yet decimal fractions are a relatively recent innovation in Western mathematics, dating only to the sixteenth century. In order to be workable, decimal fractions rely on an even more fundamental concept that was not always part of Western mathematics: place-value.

Place-Value and Notation Systems Place-value is the location of a number relative to others in a sequence, a location that makes it possible to determine the number’s value. For instance, in the number 347, the 3 is in the hundreds place, which immediately establishes a value for the number in units of 100. Similarly, a person can tell at a glance that there are 4 units of 10, and 7 units of 1. Of course, today this information appears to be self-evident—so much so that an explanation of it seems tedious and perfunctory—to almost anyone who has completed elementary-school arithmetic. In fact, however, as with almost everything about numbers and units, there is nothing obvious at all about place-value; otherwise, it would not have taken Western mathematicians thousands of years to adopt a placevalue numerical system. And though they did eventually make use of such a system, Westerners did not develop it themselves, as we shall see.

In modern life, people take for granted the existence of the base-10, of decimal numeration system—a name derived from the Latin word decem, meaning “ten.” Yet there is nothing obvious about this system, which has its roots in the ten fingers used for basic counting. At other times in history, societies have adopted the two hands or arms of a person as their numerical frame of reference, and from this developed a base-2 system. There have also been base-5 systems relating to the fingers on one hand, and base-20 systems that took as their reference point the combined number of fingers and toes.

R O M A N N U M E RA L S . Numeration systems of various kinds have existed since at least 3000 B.C., but the most important number

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instance, trying to multiply these two. With the number system in use today, it is not difficult to multiply 3,000 by 438 in one’s head. The problem can be reduced to a few simple steps: multiply 3 by 400, 3 by 30, and 3 by 8; add these products together; then multiply the total by 1,000—a step that requires the placement of three zeroes at the end of the number obtained in the earlier steps.

Measurement

But try doing this with Roman numerals: it is essentially impossible to perform this calculation without resorting to the much more practical place-value system to which we’re accustomed. No wonder, then, that Roman numerals have been relegated to the sidelines, used in modern life for very specific purposes: in outlines, for instance; in ordinal titles (for example, Henry VIII); or in designating the year of a motion picture’s release. H I N D U - A RA B I C

STANDARDIZATION IS CRUCIAL TO MAINTAINING STABILITY IN A SOCIETY. DURING THE GERMAN INFLATIONARY CRISIS OF THE 1920S, HYPERINFLATION LED TO AN ECONOMIC DEPRESSION AND THE RISE OF ADOLF HITLER. HERE, TWO CHILDREN GAZE UP AT A STACK OF 100,000 GERMAN MARKS—THE EQUIVALENT AT THE TIME TO ONE U.S. DOLLAR. (© Bettmann/Corbis.)

system in the history of Western civilization prior to the late Middle Ages was the one used by the Romans. Rome ruled much of the known world in the period from about 200 B.C. to about A.D. 200, and continued to have an influence on Europe long after the fall of the Western Roman Empire in A.D. 476—an influence felt even today. Though the Roman Empire is long gone and Latin a dead language, the impact of Rome continues: thus, for instance, Latin terms are used to designate species in biology. It is therefore easy to understand how Europeans continued to use the Roman numeral system up until the thirteenth century A.D.—despite the fact that Roman numerals were enormously cumbersome.

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N U M E RA L S .

The system of counting used throughout much of the world—1, 2, 3, and so on—is the HinduArabic notation system. Sometimes mistakenly referred to as “Arabic numerals,” these are most accurately designated as Hindu or Indian numerals. They came from India, but because Europeans discovered them in the Near East during the Crusades (1095-1291), they assumed the Arabs had invented the notation system, and hence began referring to them as Arabic numerals. Developed in India during the first millennium B.C., Hindu notation represented a vast improvement over any method in use up to or indeed since that time. Of particular importance was a number invented by Indian mathematicians: zero. Until then, no one had considered zero worth representing since it was, after all, nothing. But clearly the zeroes in a number such as 2,000,002 stand for something. They perform a place-holding function: otherwise, it would be impossible to differentiate between 2,000,002 and 22.

Uses of Numbers in Science

The Roman notation system has no means of representing place-value: thus a relatively large number such as 3,000 is shown as MMM, whereas a much smaller number might use many more “places”: 438, for instance, is rendered as CDXXXVIII. Performing any sort of calculations with these numbers is a nightmare. Imagine, for

S C I E N T I F I C N O TAT I O N . Chemists and other scientists often deal in very large or very small numbers, and if they had to write out these numbers every time they discussed them, their work would soon be encumbered by lengthy numerical expressions. For this purpose, they use scientific notation, a method for writing extremely large or small numbers by representing

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THE UNITED STATES NAVAL OBSERVATORY IN WASHINGTON, D.C., EXACT TIME OF DAY. (Richard T. Nowitz/Corbis. Reproduced by permission.)

them as a number between 1 and 10 multiplied by a power of 10. Instead of writing 75,120,000, for instance, the preferred scientific notation is 7.512 • 107. To interpret the value of large multiples of 10, it is helpful to remember that the value of 10 raised to any power n is the same as 1 followed by that number of zeroes. Hence 1025, for instance, is simply 1 followed by 25 zeroes. Scientific notation is just as useful—to chemists in particular—for rendering very small numbers. Suppose a sample of a chemical compound weighed 0.0007713 grams. The preferred scientific notation, then, is 7.713 • 10–4. Note that for numbers less than 1, the power of 10 is a negative number: 10–1 is 0.1, 10–2 is 0.01, and so on. Again, there is an easy rule of thumb for quickly assessing the number of decimal places where scientific notation is used for numbers less than 1. Where 10 is raised to any power –n, the decimal point is followed by n places. If 10 is raised to the power of –8, for instance, we know at a glance that the decimal is followed by 7 zeroes and a 1.

IS

AMERICA’S

PREEMINENT STANDARD FOR THE

calibration (discussed below) are very high, and the measuring instrument has been properly calibrated, the degree of uncertainty will be very small. Yet there is bound to be uncertainty to some degree, and for this reason, scientists use significant figures—numbers included in a measurement, using all certain numbers along with the first uncertain number. Suppose the mass of a chemical sample is measured on a scale known to be accurate to 10-5 kg. This is equal to 1/100,000 of a kilo, or 1/100 of a gram; or, to put it in terms of placevalue, the scale is accurate to the fifth place in a decimal fraction. Suppose, then, that an item is placed on the scale, and a reading of 2.13283697 kg is obtained. All the numbers prior to the 6 are significant figures, because they have been obtained with certainty. On the other hand, the 6 and the numbers that follow are not significant figures because the scale is not known to be accurate beyond 10-5 kg.

S I G N I F I CA N T F I G U R E S . In making measurements, there will always be a degree of uncertainty. Of course, when the standards of

Thus the measure above should be rendered with 7 significant figures: the whole number 2, and the first 6 decimal places. But if the value is given as 2.132836, this might lead to inaccuracies at some point when the measurement is factored into other equations. The 6, in fact, should be “rounded off ” to a 7. Simple rules apply to the

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rounding off of significant figures: if the digit following the first uncertain number is less than 5, there is no need to round off. Thus, if the measurement had been 2.13283627 kg (note that the 9 was changed to a 2), there is no need to round off, and in this case, the figure of 2.132836 is correct. But since the number following the 6 is in fact a 9, the correct significant figure is 7; thus the total would be 2.132837.

Fundamental Standards of Measure So much for numbers; now to the subject of units. But before addressing systems of measurement, what are the properties being measured? All forms of scientific measurement, in fact, can be reduced to expressions of four fundamental properties: length, mass, time, and electric current. Everything can be expressed in terms of these properties: even the speed of an electron spinning around the nucleus of an atom can be shown as “length” (though in this case, the measurement of space is in the form of a circle or even more complex shapes) divided by time. Of particular interest to the chemist are length and mass: length is a component of volume, and both length and mass are elements of density. For this reason, a separate essay in this book is devoted to the subject of Mass, Density, and Volume. Note that “length,” as used in this most basic sense, can refer to distance along any plane, or in any of the three dimensions—commonly known as length, width, and height—of the observable world. (Time is the fourth dimension.) In addition, as noted above, “length” measurements can be circular, in which case the formula for measuring space requires use of the coefficient π, roughly equal to 3.14.

This is simple enough. But what if the motorist did not know how much gas was in a gallon, or if the motorist had some idea of a gallon that differed from what the gas station management determined it to be? And what if the value of a dollar were not established, such that the motorist and the gas station attendant had to haggle over the cost of the gasoline just purchased? The result would be a horribly confused situation: the motorist might run out of gas, or money, or both, and if such confusion were multiplied by millions of motorists and millions of gas stations, society would be on the verge of breakdown. T H E VA L U E O F S TA N D A R D I Z AT I O N T O A S O C I E T Y. Actually,

there have been times when the value of currency was highly unstable, and the result was near anarchy. In Germany during the early 1920s, for instance, rampant inflation had so badly depleted the value of the mark, Germany’s currency, that employees demanded to be paid every day so that they could cash their paychecks before the value went down even further. People made jokes about the situation: it was said, for instance, that when a woman went into a store and left a basket containing several million marks out front, thieves ran by and stole the basket—but left the money. Yet there was nothing funny about this situation, and it paved the way for the nightmarish dictatorship of Adolf Hitler and the Nazi Party.

People use units of measure so frequently in daily life that they hardly think about what they are doing. A motorist goes to the gas station and pumps 13 gallons (a measure of volume) into an automobile. To pay for the gas, the motorist uses dollars—another unit of measure, economic

It is understandable, then, that standardization of weights and measures has always been an important function of government. When Ch’in Shih-huang-ti (259-210 B.C.) united China for the first time, becoming its first emperor, he set about standardizing units of measure as a means of providing greater unity to the country—thus making it easier to rule. On the other hand, the Russian Empire of the late nineteenth century failed to adopt standardized systems that would have tied it more closely to the industrialized nations of Western Europe. The width of railroad tracks in Russia was different than in Western Europe, and Russia used the old Julian calendar, as opposed to the Gregorian calendar adopted throughout much of Western Europe after 1582. These and other factors made economic exchanges between Russia and Western Europe

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REAL-LIFE A P P L I C AT I O N S Standardized Units of Measure: Who Needs Them?

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rather than scientific—in the form of paper money, a debit card, or a credit card.

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extremely difficult, and the Russian Empire remained cut off from the rapid progress of the West. Like Germany a few decades later, it became ripe for the establishment of a dictatorship—in this case under the Communists led by V. I. Lenin. Aware of the important role that standardization of weights and measures plays in the governing of a society, the U.S. Congress in 1901 established the Bureau of Standards. Today it is known as the National Institute of Standards and Technology (NIST), a nonregulatory agency within the Commerce Department. As will be discussed at the conclusion of this essay, the NIST maintains a wide variety of standard definitions regarding mass, length, temperature and so forth, against which other devices can be calibrated. T H E VA L U E O F S TA N D A R D I Z AT I O N T O S C I E N C E . What if a

nurse, rather than carefully measuring a quantity of medicine before administering it to a patient, simply gave the patient an amount that “looked right”? Or what if a pilot, instead of calculating fuel, distance, and other factors carefully before taking off from the runway, merely used a “best estimate”? Obviously, in either case, disastrous results would be likely to follow. Though neither nurses or pilots are considered scientists, both use science in their professions, and those disastrous results serve to highlight the crucial matter of using standardized measurements in science. Standardized measurements are necessary to a chemist or any scientist because, in order for an experiment to be useful, it must be possible to duplicate the experiment. If the chemist does not know exactly how much of a certain element he or she mixed with another to form a given compound, the results of the experiment are useless. In order to share information and communicate the results of experiments, then, scientists need a standardized “vocabulary” of measures. This “vocabulary” is the International System of Units, known as SI for its French name, Système International d’Unités. By international agreement, the worldwide scientific community adopted what came to be known as SI at the 9th General Conference on Weights and Measures in 1948. The system was refined at the 11th General Conference in 1960, and given its present name; but in fact most components of SI belong to a much older system of weights and measures

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developed in France during the late eighteenth century.

Measurement

SI vs. the English System The United States, as almost everyone knows, is the wealthiest and most powerful nation on Earth. On the other hand, Brunei—a tiny nationstate on the island of Java in the Indonesian archipelago—enjoys considerable oil wealth, but is hardly what anyone would describe as a superpower. Yemen, though it is located on the Arabian peninsula, does not even possess significant oil wealth, and is a poor, economically developing nation. Finally, Burma in Southeast Asia can hardly be described even as a “developing” nation: ruled by an extremely repressive military regime, it is one of the poorest nations in the world. So what do these four have in common? They are the only nations on the planet that have failed to adopt the metric system of weights and measures. The system used in the United States is called the English system, though it should more properly be called the American system, since England itself has joined the rest of the world in “going metric.” Meanwhile, Americans continue to think in terms of gallons, miles, and pounds; yet American scientists use the much more convenient metric units that are part of SI. HOW THE ENGLISH SYSTEM WORKS (OR DOES NOT WORK).

Like methods of counting described above, most systems of measurement in premodern times were modeled on parts of the human body. The foot is an obvious example of this, while the inch originated from the measure of a king’s first thumb joint. At one point, the yard was defined as the distance from the nose of England’s King Henry I to the tip of his outstretched middle finger. Obviously, these are capricious, downright absurd standards on which to base a system of measure. They involve things that change, depending for instance on whose foot is being used as a standard. Yet the English system developed in this willy-nilly fashion over the centuries; today, there are literally hundreds of units— including three types of miles, four kinds of ounces, and five kinds of tons, each with a different value. What makes the English system particularly cumbersome, however, is its lack of convenient

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conversion factors. For length, there are 12 inches in a foot, but 3 feet in a yard, and 1,760 yards in a mile. Where volume is concerned, there are 16 ounces in a pound (assuming one is talking about an avoirdupois ounce), but 2,000 pounds in a ton. And, to further complicate matters, there are all sorts of other units of measure developed to address a particular property: horsepower, for instance, or the British thermal unit (Btu). THE CONVENIENCE OF THE M E T R I C S Y S T E M . Great Britain, though

it has long since adopted the metric system, in 1824 established the British Imperial System, aspects of which are reflected in the system still used in America. This is ironic, given the desire of early Americans to distance themselves psychologically from the empire to which their nation had once belonged. In any case, England’s great worldwide influence during the nineteenth century brought about widespread adoption of the English or British system in colonies such as Australia and Canada. This acceptance had everything to do with British power and tradition, and nothing to do with convenience. A much more usable standard had actually been embraced 25 years before in a land that was then among England’s greatest enemies: France. During the period leading up to and following the French Revolution of 1789, French intellectuals believed that every aspect of existence could and should be treated in highly rational, scientific terms. Out of these ideas arose much folly, particularly during the Reign of Terror in 1793, but one of the more positive outcomes was the metric system. This system is decimal—that is, based entirely on the number 10 and powers of 10, making it easy to relate one figure to another. For instance, there are 100 centimeters in a meter and 1,000 meters in a kilometer. PREFIXES FOR SIZES IN THE M E T R I C S Y S T E M . For designating small-

er values of a given measure, the metric system uses principles much simpler than those of the English system, with its irregular divisions of (for instance) gallons, quarts, pints, and cups. In the metric system, one need only use a simple Greek or Latin prefix to designate that the value is multiplied by a given power of 10. In general, the prefixes for values greater than 1 are Greek, while Latin is used for those less than 1. These prefixes, along with their abbreviations and respective val-

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ues, are as follows. (The symbol µ for "micro" is the Greek letter mu.) The Most Commonly Used Prefixes in the Metric System giga (G) = 109 (1,000,000,000) mega (M) = 106 (1,000,000) kilo (k) == 103 (1,000) deci (d) = 10–1 (0.1) centi (c) = 10–2 (0.01) milli (m) = 10–3 (0.001) micro (µ) = 10–6 (0.000001) nano (n) = 10–9 (0.000000001) The use of these prefixes can be illustrated by reference to the basic metric unit of length, the meter. For long distances, a kilometer (1,000 m) is used; on the other hand, very short distances may require a centimeter (0.01 m) or a millimeter (0.001 m) and so on, down to a nanometer (0.000000001 m). Measurements of length also provide a good example of why SI includes units that are not part of the metric system, though they are convertible to metric units. Hard as it may be to believe, scientists often measure lengths even smaller than a nanometer—the width of an atom, for instance, or the wavelength of a light ray. For this purpose, they use the angstrom (Å or A), equal to 0.1 nanometers. • • • • • • • •

Calibration and SI Units T H E S E V E N B AS I C S I U N I T S .

The SI uses seven basic units, representing length, mass, time, temperature, amount of substance, electric current, and luminous intensity. The first four parameters are a part of everyday life, whereas the last three are of importance only to scientists. “Amount of substance” is the number of elementary particles in matter. This is measured by the mole, a unit discussed in the essay on Mass, Density, and Volume. Luminous intensity, or the brightness of a light source, is measured in candelas, while the SI unit of electric current is the ampere. The other four basic units are the meter for length, the kilogram for mass, the second for time, and the degree Celsius for temperature. The last of these is discussed in the essay on Temperature; as for meters, kilograms, and seconds, they will be examined below in terms of the means used to define each.

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C A L I B R AT I O N . Calibration is the process of checking and correcting the performance of a measuring instrument or device against the accepted standard. America’s preeminent standard for the exact time of day, for instance, is the United States Naval Observatory in Washington, D.C. Thanks to the Internet, people all over the country can easily check the exact time, and calibrate their clocks accordingly—though, of course, the resulting accuracy is subject to factors such as the speed of the Internet connection.

There are independent scientific laboratories responsible for the calibration of certain instruments ranging from clocks to torque wrenches, and from thermometers to laser-beam power analyzers. In the United States, instruments or devices with high-precision applications—that is, those used in scientific studies, or by high-tech industries—are calibrated according to standards established by the NIST. The NIST keeps on hand definitions, as opposed to using a meter stick or other physical model. This is in accordance with the methods of calibration accepted today by scientists: rather than use a standard that might vary—for instance, the meter stick could be bent imperceptibly—unvarying standards, based on specific behaviors in nature, are used.

Measurement

KEY TERMS The process of checking and correcting the performance of a measuring instrument or device against a commonly accepted standard. CALIBRATION:

A method used by scientists for writing extremely large or small numbers by representing them as a number between 1 and 10 multiplied by a power of 10. Instead of writing 0.0007713, the preferred scientific notation is 7.713 • 10–4. SCIENTIFIC NOTATION:

SI: An abbreviation of the French term Système International d’Unités, or International System of Units. Based on the metric system, SI is the system of measurement units in use by scientists worldwide.

Numbers included in a measurement, using all certain numbers along with the first uncertain number. SIGNIFICANT FIGURES:

M E T E R S A N D K I LO G RA M S . A

meter, equal to 3.281 feet, was at one time defined in terms of Earth’s size. Using an imaginary line drawn from the Equator to the North Pole through Paris, this distance was divided into 10 million meters. Later, however, scientists came to the realization that Earth is subject to geological changes, and hence any measurement calibrated to the planet’s size could not ultimately be reliable. Today the length of a meter is calibrated according to the amount of time it takes light to travel through that distance in a vacuum (an area of space devoid of air or other matter). The official definition of a meter, then, is the distance traveled by light in the interval of 1/299,792,458 of a second.

trary form of measure in comparison to the meter as it is defined today. Given the desire for an unvarying standard against which to calibrate measurements, it would be helpful to find some usable but unchanging standard of mass; unfortunately, scientists have yet to locate such a standard. Therefore, the value of a kilogram is calibrated much as it was two centuries ago. The standard is a bar of platinum-iridium alloy, known as the International Prototype Kilogram, housed near Sèvres in France. S E C O N D S . A second, of course, is a

One kilogram is, on Earth at least, equal to 2.21 pounds; but whereas the kilogram is a unit of mass, the pound is a unit of weight, so the correspondence between the units varies depending on the gravitational field in which a pound is measured. Yet the kilogram, though it represents a much more fundamental property of the physical world than a pound, is still a somewhat arbi-

unit of time as familiar to non-scientifically trained Americans as it is to scientists and people schooled in the metric system. In fact, it has nothing to do with either the metric system or SI. The means of measuring time on Earth are not “metric”: Earth revolves around the Sun approximately every 365.25 days, and there is no way to turn this into a multiple of 10 without creating a

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situation even more cumbersome than the English units of measure. The week and the month are units based on cycles of the Moon, though they are no longer related to lunar cycles because a lunar year would soon become out-of-phase with a year based on Earth’s rotation around the Sun. The continuing use of weeks and months as units of time is based on tradition—as well as the essential need of a society to divide up a year in some way. A day, of course, is based on Earth’s rotation, but the units into which the day is divided— hours, minutes, and seconds—are purely arbitrary, and likewise based on traditions of long standing. Yet scientists must have some unit of time to use as a standard, and, for this purpose, the second was chosen as the most practical. The SI definition of a second, however, is not simply one-sixtieth of a minute or anything else so strongly influenced by the variation of Earth’s movement.

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to vibrate 9,192,631,770 times. Expressed in scientific notation, with significant figures, this is 9.19263177 • 109. WHERE TO LEARN MORE Gardner, Robert. Science Projects About Methods of Measuring. Berkeley Heights, N.J.: Enslow Publishers, 2000. Long, Lynette. Measurement Mania: Games and Activities That Make Math Easy and Fun. New York: Wiley, 2001. “Measurement” (Web site). (May 7, 2001). “Measurement in Chemistry” (Web site). (May 7, 2001). MegaConverter 2 (Web site). (May 7, 2001). Patilla, Peter. Measuring. Des Plaines, IL: Heinemann Library, 2000.

Instead, the scientific community chose as its standard the atomic vibration of a particular isotope of the metal cesium, cesium-133. The vibration of this atom is presumed to be unvarying, because the properties of elements— unlike the size of Earth or its movement—do not change. Today, a second is defined as the amount of time it takes for a cesium-133 atom

Wilton High School Chemistry Coach (Web site). (May 7, 2001).

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Richards, Jon. Units and Measurements. Brookfield, CT: Copper Beech Books, 2000. Sammis, Fran. Measurements. New York: Benchmark Books, 1998. Units of Measurement (Web site). (May 7, 2001).

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Temperature and Heat

T E M P E R AT U R E A N D H E AT

CONCEPT Temperature, heat, and related concepts belong to the world of physics rather than chemistry; yet it would be impossible for the chemist to work without an understanding of these properties. Thermometers, of course, measure temperature according to one or both of two well-known scales based on the freezing and boiling points of water, though scientists prefer a scale based on the virtual freezing point of all matter. Also related to temperature are specific heat capacity, or the amount of energy required to change the temperature of a substance, and also calorimetry, the measurement of changes in heat as a result of physical or chemical changes. Although these concepts do not originate from chemistry but from physics, they are no less useful to the chemist.

HOW IT WORKS Energy The area of physics known as thermodynamics, discussed briefly below in terms of thermodynamics laws, is the study of the relationships between heat, work, and energy. Work is defined as the exertion of force over a given distance to displace or move an object, and energy is the ability to accomplish work. Energy appears in numerous manifestations, including thermal energy, or the energy associated with heat.

ing, and when those bonds are broken, the forces joining the atoms are released in the form of chemical energy. Another example of chemical energy release is combustion, whereby chemical bonds in fuel, as well as in oxygen molecules, are broken and new chemical bonds are formed. The total energy in the newly formed chemical bonds is less than the energy of the original bonds, but the energy that makes up the difference is not lost; it has simply been released. Energy, in fact, is never lost: a fundamental law of the universe is the conservation of energy, which states that in a system isolated from all other outside factors, the total amount of energy remains the same, though transformations of energy from one form to another take place. When a fire burns, then, some chemical energy is turned into thermal energy. Similar transformations occur between these and other manifestations of energy, including electrical and magnetic (sometimes these two are combined as electromagnetic energy), sound, and nuclear energy. If a chemical reaction makes a noise, for instance, some of the energy in the substances being mixed has been dissipated to make that sound. The overall energy that existed before the reaction will be the same as before; however, the energy will not necessarily be in the same place as before.

Another type of energy—one of particular interest to chemists—is chemical energy, related to the forces that attract atoms to one another in chemical bonds. Hydrogen and oxygen atoms in water, for instance, are joined by chemical bond-

Note that chemical and other forms of energy are described as “manifestations,” rather than “types,” of energy. In fact, all of these can be described in terms of two basic types of energy: kinetic energy, or the energy associated with movement, and potential energy, or the energy associated with position. The two are inversely related: thus, if a spring is pulled back to its maximum point of tension, its potential energy is

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coldness is a recognizable sensory experience in human life, in scientific terms, cold is simply the absence of heat.

Temperature and Heat

If you grasp a snowball in your hand, the hand of course gets cold. The mind perceives this as a transfer of cold from the snowball, but in fact exactly the opposite has happened: heat has moved from your hand to the snow, and if enough heat enters the snowball, it will melt. At the same time, the departure of heat from your hand results in a loss of internal energy near the surface of the hand, experienced as a sensation of coldness.

Understanding Temperature

DURING

HIS LIFETIME,

GALILEO

CONSTRUCTED A THER-

MOSCOPE, THE FIRST PRACTICAL TEMPERATURE-MEASURING DEVICE. (Archive Photos, Inc. Reproduced by permission.)

also at a maximum, while its kinetic energy is zero. Once it is released and begins springing through the air to return to the position it maintained before it was stretched, it begins gaining kinetic energy and losing potential energy.

Heat Thermal energy is actually a form of kinetic energy generated by the movement of particles at the atomic or molecular level: the greater the movement of these particles, the greater the thermal energy. When people use the word “heat” in ordinary language, what they are really referring to is “the quality of hotness”—that is, the thermal energy internal to a system. In scientific terms, however, heat is internal thermal energy that flows from one body of matter to another— or, more specifically, from a system at a higher temperature to one at a lower temperature. Two systems at the same temperature are said to be in a state of thermal equilibrium. When this state exists, there is no exchange of heat. Though in everyday terms people speak of “heat” as an expression of relative warmth or coldness, in scientific terms, heat exists only in transfer between two systems. Furthermore, there can never be a transfer of “cold”; although

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Just as heat does not mean the same thing in scientific terms as it does in ordinary language, so “temperature” requires a definition that sets it apart from its everyday meaning. Temperature may be defined as a measure of the average internal energy in a system. Two systems in a state of thermal equilibrium have the same temperature; on the other hand, differences in temperature determine the direction of internal energy flow between two systems where heat is being transferred. This can be illustrated through an experience familiar to everyone: having one’s temperature taken with a thermometer. If one has a fever, the mouth will be warmer than the thermometer, and therefore heat will be transferred to the thermometer from the mouth. The thermometer, discussed in more depth later in this essay, measures the temperature difference between itself and any object with which it is in contact.

Temperature and Thermodynamics One might pour a kettle of boiling water into a cold bathtub to heat it up; or one might put an ice cube in a hot cup of coffee “to cool it down.” In everyday experience, these seem like two very different events, but from the standpoint of thermodynamics, they are exactly the same. In both cases, a body of high temperature is placed in contact with a body of low temperature, and in both cases, heat passes from the high-temperature body to the low-temperature body. The boiling water warms the tub of cool water, and due to the high ratio of cool water to boiling water in the bathtub, the boiling water

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Temperature and Heat

BECAUSE

OF WATER’S HIGH SPECIFIC HEAT CAPACITY, CITIES LOCATED NEXT TO LARGE BODIES OF WATER TEND TO

STAY WARMER IN THE WINTER AND COOLER IN THE SUMMER.

CHICAGO’S

DURING

LAKEFRONT STAYS COOLER THAN AREAS FURTHER INLAND.

THE EARLY SUMMER MONTHS, FOR INSTANCE,

THIS

IS BECAUSE THE LAKE IS COOLED FROM

THE WINTER’S COLD TEMPERATURES AND SNOW RUNOFF. (Farrell Grehan/Corbis. Reproduced by permission.)

expends all its energy raising the temperature in the bathtub as a whole. The greater the ratio of very hot water to cool water, of course, the warmer the bathtub will be in the end. But even after the bath water is heated, it will continue to lose heat, assuming the air in the room is not warmer than the water in the tub—a safe assumption. If the water in the tub is warmer than the air, it will immediately begin transferring thermal energy to the lower-temperature air until their temperatures are equalized. As for the coffee and the ice cube, what happens is opposite to the explanation ordinarily given. The ice does not “cool down” the coffee: the coffee warms up, and presumably melts, the ice. However, it expends at least some of its thermal energy in doing so, and, as a result, the coffee becomes cooler than it was.

namics laws as a set of rules governing an impossible game. The first law of thermodynamics is essentially the same as the conservation of energy: because the amount of energy in a system remains constant, it is impossible to perform work that results in an energy output greater than the energy input. It could be said that the conservation of energy shows that “the glass is half full”: energy is never lost. By contrast, the first law of thermodynamics shows that “the glass is half empty”: no system can ever produce more energy than was put into it. Snow therefore summed up the first law as stating that the game is impossible to win.

ond of the three laws of thermodynamics. Not only do these laws help to clarify the relationship between heat, temperature, and energy, but they also set limits on what can be accomplished in the world. Hence British writer and scientist C. P. Snow (1905-1980) once described the thermody-

The second law of thermodynamics begins from the fact that the natural flow of heat is always from an area of higher temperature to an area of lower temperature—just as was shown in the bathtub and coffee cup examples above. Consequently, it is impossible for any system to take heat from a source and perform an equivalent amount of work: some of the heat will always be lost. In other words, no system can ever be perfectly efficient: there will always be a degree of

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THE LAWS OF THERMODYN A M I CS . These situations illustrate the sec-

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breakdown, evidence of a natural tendency called entropy. Snow summed up the second law of thermodynamics, sometimes called “the law of entropy,” thus: not only is it impossible to win, it is impossible to break even. In effect, the second law compounds the “bad news” delivered by the first with some even worse news. Though it is true that energy is never lost, the energy available for work output will never be as great as the energy put into a system. The third law of thermodynamics states that at the temperature of absolute zero—a phenomenon discussed later in this essay—entropy also approaches zero. This might seem to counteract the second law, but in fact the third states in effect that absolute zero is impossible to reach. The French physicist and engineer Sadi Carnot (1796-1832) had shown that a perfectly efficient engine is one whose lowest temperature was absolute zero; but the second law of thermodynamics shows that a perfectly efficient engine (or any other perfect system) cannot exist. Hence, as Snow observed, not only is it impossible to win or break even; it is impossible to get out of the game.

REAL-LIFE A P P L I C AT I O N S Evolution of the Thermometer A thermometer is a device that gauges temperature by measuring a temperature-dependent property, such as the expansion of a liquid in a sealed tube. The Greco-Roman physician Galen (c. 129-c. 199) was among the first thinkers to envision a scale for measuring temperature, but development of a practical temperature-measuring device—the thermoscope—did not occur until the sixteenth century. The great physicist Galileo Galilei (15641642) may have invented the thermoscope; certainly he constructed one. Galileo’s thermoscope consisted of a long glass tube planted in a container of liquid. Prior to inserting the tube into the liquid—which was usually colored water, though Galileo’s thermoscope used wine—as much air as possible was removed from the tube. This created a vacuum (an area devoid of matter, including air), and as a result of pressure differ-

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ences between the liquid and the interior of the thermoscope tube, some of the liquid went into the tube. But the liquid was not the thermometric medium—that is, the substance whose temperature-dependent property changes were measured by the thermoscope. (Mercury, for instance, is the thermometric medium in many thermometers today; however, due to the toxic quality of mercury, an effort is underway to remove mercury thermometers from U.S. schools.) Instead, the air was the medium whose changes the thermoscope measured: when it was warm, the air expanded, pushing down on the liquid; and when the air cooled, it contracted, allowing the liquid to rise. E A R LY T H E R M O M E T E R S : T H E S E A R C H F O R A T E M P E RAT U R E S CA L E . The first true thermometer, built by

Ferdinand II, Grand Duke of Tuscany (16101670) in 1641, used alcohol sealed in glass. The latter was marked with a temperature scale containing 50 units, but did not designate a value for zero. In 1664, English physicist Robert Hooke (1635-1703) created a thermometer with a scale divided into units equal to about 1/500 of the volume of the thermometric medium. For the zero point, Hooke chose the temperature at which water freezes, thus establishing a standard still used today in the Fahrenheit and Celsius scales. Olaus Roemer (1644-1710), a Danish astronomer, introduced another important standard. Roemer’s thermometer, built in 1702, was based not on one but two fixed points, which he designated as the temperature of snow or crushed ice on the one hand, and the boiling point of water on the other. As with Hooke’s use of the freezing point, Roemer’s idea of designating the freezing and boiling points of water as the two parameters for temperature measurements has remained in use ever since.

Temperature Scales T H E FA H R E N H E I T S CA L E . Not only did he develop the Fahrenheit scale, oldest of the temperature scales still used in Western nations today, but in 1714, German physicist Daniel Fahrenheit (1686-1736) built the first thermometer to contain mercury as a thermometric medium. Alcohol has a low boiling point, whereas mercury remains fluid at a wide range of temperatures. In addition, it expands and con-

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tracts at a very constant rate, and tends not to stick to glass. Furthermore, its silvery color makes a mercury thermometer easy to read. Fahrenheit also conceived the idea of using “degrees” to measure temperature. It is no mistake that the same word refers to portions of a circle, or that exactly 180 degrees—half the number of degrees in a circle—separate the freezing and boiling points for water on Fahrenheit’s thermometer. Ancient astronomers first divided a circle into 360 degrees, as a close approximation of the ratio between days and years, because 360 has a large quantity of divisors. So, too, does 180—a total of 16 whole-number divisors other than 1 and itself. Though today it might seem obvious that 0 should denote the freezing point of water, and 180 its boiling point, such an idea was far from obvious in the early eighteenth century. Fahrenheit considered a 0-to-180 scale, but also a 180to-360 one, yet in the end he chose neither—or rather, he chose not to equate the freezing point of water with zero on his scale. For zero, he chose the coldest possible temperature he could create in his laboratory, using what he described as “a mixture of sal ammoniac or sea salt, ice, and water.” Salt lowers the melting point of ice (which is why it is used in the northern United States to melt snow and ice from the streets on cold winter days), and thus the mixture of salt and ice produced an extremely cold liquid water whose temperature he equated to zero. On the Fahrenheit scale, the ordinary freezing point of water is 32°, and the boiling point exactly 180° above it, at 212°. Just a few years after Fahrenheit introduced his scale, in 1730, a French naturalist and physicist named Rene Antoine Ferchault de Reaumur (1683-1757) presented a scale for which 0° represented the freezing point of water and 80° the boiling point. Although the Reaumur scale never caught on to the same extent as Fahrenheit’s, it did include one valuable addition: the specification that temperature values be determined at standard sea-level atmospheric pressure.

Celsius scale. The latter was created in 1742 by Swedish astronomer Anders Celsius (1701-1744).

Temperature and Heat

Like Fahrenheit, Celsius chose the freezing and boiling points of water as his two reference points, but he determined to set them 100, rather than 180, degrees apart. The Celsius scale is sometimes called the centigrade scale, because it is divided into 100 degrees, cent being a Latin root meaning “hundred.” Interestingly, Celsius planned to equate 0° with the boiling point, and 100° with the freezing point; only in 1750 did fellow Swedish physicist Martin Strömer change the orientation of the Celsius scale. In accordance with the innovation offered by Reaumur, Celsius’s scale was based not simply on the boiling and freezing points of water, but specifically on those points at normal sea-level atmospheric pressure. In SI, a scientific system of measurement that incorporates units from the metric system along with additional standards used only by scientists, the Celsius scale has been redefined in terms of the triple point of water. (Triple point is the temperature and pressure at which a substance is at once a solid, liquid, and vapor.) According to the SI definition, the triple point of water—which occurs at a pressure considerably below normal atmospheric pressure—is exactly 0.01°C. T H E K E LV I N S CA L E . French physicist and chemist J. A. C. Charles (1746-1823), who is credited with the gas law that bears his name (see below), discovered that at 0°C, the volume of gas at constant pressure drops by 1/273 for every Celsius degree drop in temperature. This suggested that the gas would simply disappear if cooled to -273°C, which of course made no sense.

T H E C E L S I U S S CA L E . With its 32° freezing point and its 212° boiling point, the Fahrenheit system lacks the neat orderliness of a decimal or base-10 scale. Thus when France adopted the metric system in 1799, it chose as its temperature scale not the Fahrenheit but the

The man who solved the quandary raised by Charles’s discovery was William Thompson, Lord Kelvin (1824-1907), who, in 1848, put forward the suggestion that it was the motion of molecules, and not volume, that would become zero at –273°C. He went on to establish what came to be known as the Kelvin scale. Sometimes known as the absolute temperature scale, the Kelvin scale is based not on the freezing point of water, but on absolute zero—the temperature at which molecular motion comes to a virtual stop. This is –273.15°C (–459.67°F), which, in the Kelvin scale, is designated as 0K. (Kelvin measures do not use the term or symbol for “degree.”)

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Though scientists normally use metric units, they prefer the Kelvin scale to Celsius because the absolute temperature scale is directly related to average molecular translational energy, based on the relative motion of molecules. Thus if the Kelvin temperature of an object is doubled, this means its average molecular translational energy has doubled as well. The same cannot be said if the temperature were doubled from, say, 10°C to 20°C, or from 40°C to 80°F, since neither the Celsius nor the Fahrenheit scale is based on absolute zero. CONVERSIONS BETWEEN SCALES.

The Kelvin scale is closely related to the Celsius scale, in that a difference of one degree measures the same amount of temperature in both. Therefore, Celsius temperatures can be converted to Kelvins by adding 273.15. Conversion between Celsius and Fahrenheit figures, on the other hand, is a bit trickier. To convert a temperature from Celsius to Fahrenheit, multiply by 9/5 and add 32. It is important to perform the steps in that order, because reversing them will produce a wrong figure. Thus, 100°C multiplied by 9/5 or 1.8 equals 180, which, when added to 32 equals 212°F. Obviously, this is correct, since 100°C and 212°F each represent the boiling point of water. But if one adds 32 to 100°, then multiplies it by 9/5, the result is 237.6°F—an incorrect answer. For converting Fahrenheit temperatures to Celsius, there are also two steps involving multiplication and subtraction, but the order is reversed. Here, the subtraction step is performed before the multiplication step: thus 32 is subtracted from the Fahrenheit temperature, then the result is multiplied by 5/9. Beginning with 212°F, when 32 is subtracted, this equals 180. Multiplied by 5/9, the result is 100°C—the correct answer. One reason the conversion formulae use simple fractions instead of decimal fractions (what most people simply call “decimals”) is that 5/9 is a repeating decimal fraction (0.55555....) Furthermore, the symmetry of 5/9 and 9/5 makes memorization easy. One way to remember the formula is that Fahrenheit is multiplied by a fraction—since 5/9 is a real fraction, whereas 9/5 is actually a mixed number, or a whole number plus a fraction.

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Modern Thermometers MERCURY

THERMOMETERS.

For a thermometer, it is important that the glass tube be kept sealed; changes in atmospheric pressure contribute to inaccurate readings, because they influence the movement of the thermometric medium. It is also important to have a reliable thermometric medium, and, for this reason, water—so useful in many other contexts—was quickly discarded as an option. Water has a number of unusual properties: it does not expand uniformly with a rise in temperature, or contract uniformly with a lowered temperature. Rather, it reaches its maximum density at 39.2°F (4°C), and is less dense both above and below that temperature. Therefore alcohol, which responds in a much more uniform fashion to changes in temperature, soon took the place of water, and is still used in many thermometers today. But for the reasons mentioned earlier, mercury is generally considered preferable to alcohol as a thermometric medium. In a typical mercury thermometer, mercury is placed in a long, narrow sealed tube called a capillary. The capillary is inscribed with figures for a calibrated scale, usually in such a way as to allow easy conversions between Fahrenheit and Celsius. A thermometer is calibrated by measuring the difference in height between mercury at the freezing point of water, and mercury at the boiling point of water. The interval between these two points is then divided into equal increments—180, as we have seen, for the Fahrenheit scale, and 100 for the Celsius scale. VOLUME GAS THERMOMET E R S . Whereas most liquids and solids

expand at an irregular rate, gases tend to follow a fairly regular pattern of expansion in response to increases in temperature. The predictable behavior of gases in these situations has led to the development of the volume gas thermometer, a highly reliable instrument against which other thermometers—including those containing mercury—are often calibrated. In a volume gas thermometer, an empty container is attached to a glass tube containing mercury. As gas is released into the empty container; this causes the column of mercury to move upward. The difference between the earlier position of the mercury and its position after the introduction of the gas shows the difference between normal atmospheric pressure and the

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pressure of the gas in the container. It is then possible to use the changes in the volume of the gas as a measure of temperature.

the temperature of a object with which the thermometer itself is not in physical contact.

Temperature and Heat

Measuring Heat ELECTRIC

THERMOMETERS.

All matter displays a certain resistance to electric current, a resistance that changes with temperature; because of this, it is possible to obtain temperature measurements using an electric thermometer. A resistance thermometer is equipped with a fine wire wrapped around an insulator: when a change in temperature occurs, the resistance in the wire changes as well. This allows much quicker temperature readings than those offered by a thermometer containing a traditional thermometric medium. Resistance thermometers are highly reliable, but expensive, and primarily are used for very precise measurements. More practical for everyday use is a thermistor, which also uses the principle of electric resistance, but is much simpler and less expensive. Thermistors are used for providing measurements of the internal temperature of food, for instance, and for measuring human body temperature. Another electric temperature-measurement device is a thermocouple. When wires of two different materials are connected, this creates a small level of voltage that varies as a function of temperature. A typical thermocouple uses two junctions: a reference junction, kept at some constant temperature, and a measurement junction. The measurement junction is applied to the item whose temperature is to be measured, and any temperature difference between it and the reference junction registers as a voltage change, measured with a meter connected to the system. OTHER TYPES OF THERMOMET E R. A pyrometer also uses electromagnetic

properties, but of a very different kind. Rather than responding to changes in current or voltage, the pyrometer is gauged to respond to visible and infrared radiation. As with the thermocouple, a pyrometer has both a reference element and a measurement element, which compares light readings between the reference filament and the object whose temperature is being measured.

The measurement of temperature by degrees in the Fahrenheit or Celsius scales is a part of daily life, but measurements of heat are not as familiar to the average person. Because heat is a form of energy, and energy is the ability to perform work, heat is therefore measured by the same units as work. The principal SI unit of work or energy is the joule (J). A joule is equal to 1 newton-meter (N • m)—in other words, the amount of energy required to accelerate a mass of 1 kilogram at the rate of 1 meter per second squared across a distance of 1 meter. The joule’s equivalent in the English system is the foot-pound: 1 foot-pound is equal to 1.356 J, and 1 joule is equal to 0.7376 ft • lbs. In the British system, Btu, or British thermal unit, is another measure of energy, though it is primarily used for machines. Due to the cumbersome nature of the English system, contrasted with the convenience of the decimal units in the SI system, these English units of measure are not used by chemists or other scientists for heat measurement.

Specific Heat Capacity Specific heat capacity (sometimes called specific heat) is the amount of heat that must be added to, or removed from, a unit of mass for a given substance to change its temperature by 1°C. Typically, specific heat capacity is measured in units of J/g • °C (joules per gram-degree Celsius). The specific heat capacity of water is measured by the calorie, which, along with the joule, is an important SI measure of heat. Often another unit, the kilocalorie—which, as its name suggests—is 1,000 calories—is used. This is one of the few confusing aspects of SI, which is much simpler than the English system. The dietary Calorie (capital C) with which most people are familiar is not the same as a calorie (lowercase c)—rather, a dietary Calorie is the same as a kilocalorie.

Still other thermometers, such as those in an oven that register the oven’s internal temperature, are based on the expansion of metals with heat. In fact, there are a wide variety of thermometers, each suited to a specific purpose. A pyrometer, for instance, is good for measuring

capacity, the more resistant the substance is to changes in temperature. Many metals, in fact, have a low specific heat capacity, making them easy to heat up and cool down. This contributes

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to the tendency of metals to expand when heated, and thus affects their malleability. On the other hand, water has a high specific heat capacity, as discussed below; indeed, if it did not, life on Earth would hardly be possible. One of the many unique properties of water is its very high specific heat capacity, which is easily derived from the value of a kilocalorie: it is 4.184, the same number of joules required to equal a calorie. Few substances even come close to this figure. At the low end of the spectrum are lead, gold, and mercury, with specific heat capacities of 0.13, 0.13, and 0.14 respectively. Aluminum has a specific heat capacity of 0.89, and ethyl alcohol of 2.43. The value for concrete, one of the highest for any substance other than water, is 2.9. As high as the specific heat capacity of concrete is, that of water is more than 40% higher. On the other hand, water in its vapor state (steam) has a much lower specific heat capacity—2.01. The same is true for solid water, or ice, with a specific heat capacity of 2.03. Nonetheless, water in its most familiar form has an astoundingly high specific heat capacity, and this has several effects in the real world. E F F E C T S O F WAT E R ’ S H I G H S P E C I F I C H E AT C A PA C I T Y. For

instance, water is much slower to freeze in the winter than most substances. Furthermore, due to other unusual aspects of water—primarily the fact that it actually becomes less dense as a solid—the top of a lake or other body of water freezes first. Because ice is a poor medium for the conduction of heat (a consequence of its specific heat capacity), the ice at the top forms a layer that protects the still-liquid water below it from losing heat. As a result, the water below the ice layer does not freeze, and the animal and plant life in the lake is preserved. Conversely, when the weather is hot, water is slow to experience a rise in temperature. For this reason, a lake or swimming pool makes a good place to cool off on a sizzling summer day. Given the high specific heat capacity of water, combined with the fact that much of Earth’s surface is composed of water, the planet is far less susceptible than other bodies in the Solar System to variations in temperature.

18

(37°C), and, even in cases of extremely high fever, an adult’s temperature rarely climbs by more than 5°F (2.7°C). The specific heat capacity of the human body, though it is of course lower than that of water itself (since it is not entirely made of water), is nonetheless quite high: 3.47.

Calorimetry The measurement of heat gain or loss as a result of physical or chemical change is called calorimetry (pronounced kal-or-IM-uh-tree). Like the word “calorie,” the term is derived from a Latin root word meaning “heat.” The foundations of calorimetry go back to the mid-nineteenth century, but the field owes much to the work of scientists about 75 years prior to that time. In 1780, French chemist Antoine Lavoisier (1743-1794) and French astronomer and mathematician Pierre Simon Laplace (1749-1827) had used a rudimentary ice calorimeter for measuring heat in the formations of compounds. Around the same time, Scottish chemist Joseph Black (1728-1799) became the first scientist to make a clear distinction between heat and temperature. By the mid-1800s, a number of thinkers had come to the realization that—contrary to prevailing theories of the day—heat was a form of energy, not a type of material substance. (The belief that heat was a material substance, called “phlogiston,” and that phlogiston was the part of a substance that burned in combustion, had originated in the seventeenth century. Lavoisier was the first scientist to successfully challenge the phlogiston theory.) Among these were American-British physicist Benjamin Thompson, Count Rumford (1753-1814) and English chemist James Joule (1818-1889)—for whom, of course, the joule is named. Calorimetry as a scientific field of study actually had its beginnings with the work of French chemist Pierre-Eugene Marcelin Berthelot (1827-1907). During the mid-1860s, Berthelot became intrigued with the idea of measuring heat, and, by 1880, he had constructed the first real calorimeter.

The same is true of another significant natural feature, one made mostly of water: the human body. A healthy human temperature is 98.6°F

Essential to calorimetry is the calorimeter, which can be any device for accurately measuring the temperature of a substance before and after a change occurs. A calorimeter can be as simple as a styrofoam cup. Its quality as an insulator, which makes styro-

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foam ideal both for holding in the warmth of coffee and protecting the human hand from scalding, also makes styrofoam an excellent material for calorimetric testing. With a styrofoam calorimeter, the temperature of the substance inside the cup is measured, a reaction is allowed to take place, and afterward, the temperature is measured a second time.

sure of a gas: the greater the pressure, the less the volume, and vice versa. Even more relevant to the subject of thermal expansion is Charles’s law, which states that when pressure is kept constant, there is a direct relationship between volume and absolute temperature.

The most common type of calorimeter used is the bomb calorimeter, designed to measure the heat of combustion. Typically, a bomb calorimeter consists of a large container filled with water, into which is placed a smaller container, the combustion crucible. The crucible is made of metal, with thick walls into which is cut an opening to allow the introduction of oxygen. In addition, the combustion crucible is designed to be connected to a source of electricity.

as two systems that exchange no heat are said to be in a state of thermal equilibrium, chemical equilibrium describes a dynamic state in which the concentration of reactants and products remains constant. Though the concentrations of reactants and products do not change, note that chemical equilibrium is a dynamic state—in other words, there is still considerable molecular activity, but no net change.

In conducting a calorimetric test using a bomb calorimeter, the substance or object to be studied is placed inside the combustion crucible and ignited. The resulting reaction usually occurs so quickly that it resembles the explosion of a bomb—hence the name “bomb calorimeter.” Once the “bomb” goes off, the resulting transfer of heat creates a temperature change in the water, which can be readily gauged with a thermometer. To study heat changes at temperatures higher than the boiling point of water, physicists use substances with higher boiling points. For experiments involving extremely large temperature ranges, an aneroid (without liquid) calorimeter may be used. In this case, the lining of the combustion crucible must be of a metal, such as copper, with a high coefficient or factor of thermal conductivity—that is, the ability to conduct heat from molecule to molecule.

Temperature and Heat

C H E M I CA L E Q U I L I B R I U M A N D C H A N G E S I N T E M P E RAT U R E . Just

Calculations involving chemical equilibrium make use of a figure called the equilibrium constant (K). According to Le Châtelier’s principle, named after French chemist Henri Le Châtelier (1850-1936), whenever a stress or change is imposed on a chemical system in equilibrium, the system will adjust the amounts of the various substances in such a way as to reduce the impact of that stress. An example of a stress is a change in temperature, which changes the equilibrium equation by shifting K (itself dependant on temperature). Using Le Châtelier’s law, it is possible to determine whether K will change in the direction of the forward or reverse reaction. In an exothermic reaction (a reaction that produces heat), K will shift to the left, or in the direction of the forward reaction. On the other hand, in an endothermic reaction (a reaction that absorbs heat), K will shift to the right, or in the direction of the reverse reaction.

Temperature in Chemistry T H E GAS LAW S . A collection of statements regarding the behavior of gases, the gas laws are so important to chemistry that a separate essay is devoted to them elsewhere. Several of the gas laws relate temperature to pressure and volume for gases. Indeed, gases respond to changes in temperature with dramatic changes in volume; hence the term “volume,” when used in reference to a gas, is meaningless unless pressure and temperature are specified as well.

T E M P E R AT U R E AND REACT I O N RAT E S . Another important function

of temperature in chemical processes is its function of speeding up chemical reactions. An increase in the concentration of reacting molecules, naturally, leads to a sped-up reaction, because there are simply more molecules colliding with one another. But it is also possible to speed up the reaction without changing the concentration.

Among the gas laws, Boyle’s law holds that in conditions of constant temperature, an inverse relationship exists between the volume and pres-

By definition, wherever a temperature increase is involved, there is always an increase in average molecular translational energy. When temperatures are high, more molecules are col-

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Temperature and Heat

KEY TERMS ABSOLUTE ZERO:

The temperature,

The ability to accomplish

ENERGY:

defined as 0K on the Kelvin scale, at which

work—that is, the exertion of force over a

the motion of molecules in a solid virtual-

given distance to displace or move an

ly ceases. The third law of thermodynamics

object.

establishes the impossibility of actually

ENTROPY:

reaching absolute zero.

systems toward breakdown, and specifical-

CALORIE:

A measure of specific heat

capacity in the SI or metric system, equal to the heat that must be added to or

The tendency of natural

ly the tendency for the energy in a system to be dissipated. Entropy is closely related to the second law of thermodynamics. The oldest of

removed from 1 gram of water to change

FAHRENHEIT SCALE:

its temperature by 1°C. The dietary Calorie

the temperature scales still in use, created

(capital C), with which most people are

in 1714 by German physicist Daniel

familiar, is the same as the kilocalorie.

Fahrenheit (1686-1736). The Fahrenheit scale establishes the freezing and boiling

The measurement of

points of water at 32° and 212° respective-

heat gain or loss as a result of physical or

ly. To convert a temperature from the

chemical change.

Fahrenheit to the Celsius scale, subtract 32

CALORIMETRY:

CELSIUS SCALE:

The metric scale of

and multiply by 5/9.

temperature, sometimes known as the

FIRST LAW OF THERMODYNAMICS:

centigrade scale, created in 1742 by

A law which states the amount of energy in

Swedish astronomer Anders Celsius (1701-

a system remains constant, and therefore it

1744). The Celsius scale establishes the

is impossible to perform work that results

freezing and boiling points of water at 0°

in an energy output greater than the ener-

and 100° respectively. To convert a temper-

gy input. This is the same as the conserva-

ature from the Celsius to the Fahrenheit

tion of energy.

scale, multiply by 9/5 and add 32. Though

HEAT:

the worldwide scientific community uses

flows from one body of matter to another.

the metric or SI system for most measurements, scientists prefer the related Kelvin scale of absolute temperature. CONSERVATION OF ENERGY:

20

JOULE:

Internal thermal energy that The principal unit of energy—

and thus of heat—in the SI or metric system, corresponding to 1 newton-meter

A

(N • m). A joule (J) is equal to 0.7376 foot-

law of physics which holds that within a

pounds in the English system.

system isolated from all other outside fac-

KELVIN

tors, the total amount of energy remains

William Thompson, Lord Kelvin (1824-

the same, though transformations of ener-

1907), the Kelvin scale measures tempera-

gy from one form to another take place.

ture in relation to absolute zero, or 0K.

The first law of thermodynamics is the

(Units in the Kelvin system, known as

same as the conservation of energy.

Kelvins, do not include the word or symbol

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SCALE:

Established

by

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Temperature and Heat

KEY TERMS

CONTINUED

for degree.) The Kelvin scale, which is the

1°C. It is typically measured in J/g • °C

system usually favored by scientists, is

(joules per gram-degree Celsius). A calorie

directly related to the Celsius scale; hence

is the specific heat capacity of 1 gram of

Celsius temperatures can be converted to

water.

Kelvins by adding 273.15. SYSTEM:

In chemistry and other sci-

A measure of specific

ences, the term “system” usually refers to

heat capacity in the SI or metric system,

any set of interactions isolated from the

equal to the heat that must be added to or

rest of the universe. Anything outside of

KILOCALORIE:

removed from 1 kilogram of water to change its temperature by 1°C. As its name suggests, a kilocalorie is 1,000 calories. The dietary Calorie (capital C) with which

the system, including all factors and forces irrelevant to a discussion of that system, is known as the environment. THERMAL

kilocalorie.

resulting from internal kinetic energy.

KINETIC ENERGY:

The energy that

an object possesses by virtue of its motion. MOLECULAR TRANSLATIONAL EN-

The kinetic energy in a system

ERGY:

ENERGY:

Heat energy

most people are familiar is the same as the

THERMAL EQUILIBRIUM:

A situa-

tion in which two systems have the same temperature. As a result, there is no exchange of heat between them. The study of

produced by the movement of molecules

THERMODYNAMICS:

in relation to one another. Thermal energy

the relationships between heat, work, and

is a manifestation of molecular transla-

energy.

tional energy.

THERMOMETER:

A device that gauges

SECOND LAW OF THERMODYNAM-

temperature by measuring a temperature-

A law of thermodynamics which

dependent property, such as the expansion

states that no system can simply take heat

of a liquid in a sealed tube, or resistance to

from a source and perform an equivalent

electric current.

ICS:

amount of work. This is a result of the fact that the natural flow of heat is always from a high-temperature reservoir to a low-temperature reservoir. In the course of such a transfer, some of the heat will always be

THERMOMETRIC MEDIUM:

A sub-

stance whose physical properties change with temperature. A mercury or alcohol thermometer measures such changes.

lost—an example of entropy. The second

THIRD LAW OF THERMODYNAMICS:

law is sometimes referred to as “the law of

A law of thermodynamics stating that at

entropy.”

the temperature of absolute zero, entropy The

also approaches zero. Zero entropy contra-

amount of heat that must be added to, or

dicts the second law of thermodynamics,

removed from, a unit of mass of a given

meaning that absolute zero is therefore

substance to change its temperature by

impossible to reach.

SPECIFIC HEAT CAPACITY:

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liding, and the collisions that occur are more energetic. The likelihood is therefore increased that any particular collision will result in the energy necessary to break chemical bonds, and thus bring about the rearrangements in molecules needed for a reaction.

Ebbing, Darrell D.; R. A. D. Wentworth; and James P. Birk. Introductory Chemistry. Boston: Houghton Mifflin, 1995. Gardner, Robert. Science Projects About Methods of Measuring. Berkeley Heights, NJ: Enslow Publishers, 2000. MegaConverter 2 (Web site). (May 7, 2001).

WHERE TO LEARN MORE “About Temperature” (Web site). (April 18, 2001). “Basic Chemical Thermodynamics” (Web site). (May 8, 2001). “Chemical Thermodynamics” (Web site). (May 8, 2001).

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VOLUME 1: REAL-LIFE CHEMISTRY

Royston, Angela. Hot and Cold. Chicago: Heinemann Library, 2001. Santrey, Laurence. Heat. Illustrated by Lloyd Birmingham. Mahwah, N.J.: Troll Associates, 1985. Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996. Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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M A S S , D E N S I T Y, A N D VO LU M E

CONCEPT Among the physical properties studied by chemists and other scientists, mass is one of the most fundamental. All matter, by definition, has mass. Mass, in turn, plays a role in two properties important to the study of chemistry: density and volume. All of these—mass, density, and volume—are simple concepts, yet in order to work in chemistry or any of the other hard sciences, it is essential to understand these types of measurement. Measuring density, for instance, aids in determining the composition of a given substance, while volume is a necessary component to using the gas laws.

HOW IT WORKS

VOLUME AND DENSITY DEF I N E D . Volume, then, is measured in terms of

length, and can be defined as the amount of three-dimensional space an object occupies. Volume is usually expressed in cubic units of length—for example, the milliliter (mL), also known as the cubic centimeter (cc), is equal to 6.10237 • 10–2 in3. As its name implies, there are 1,000 milliliters in a liter. Density is the ratio of mass to volume—or, to put its definition in terms of fundamental properties, of mass divided by cubed length. Density can also be viewed as the amount of matter within a given area. In the SI system, density is typically expressed as grams per cubic centimeter (g/cm3), equivalent to 62.42197 pounds per cubic foot in the English system.

Mass Fundamental Properties in Relation to Volume and Density Most qualities of the world studied by scientists can be measured in terms of one or more of four properties: length, mass, time, and electric charge. The volume of a cube, for instance, is a unit of length cubed—that is, length multiplied by “width,” which is then multiplied by “height.”

M ASS D E F I N E D . Though length is easy enough to comprehend, mass is more involved. In his second law of motion, Sir Isaac Newton (1642-1727) defined mass as the ratio of force to gravity. This, of course, is a statement that belongs to the realm of physics; for a chemist, it is more useful—and also accurate—to define mass as the quantity of matter that an object contains.

Width and height are not, for the purposes of science, distinct from length: they are simply versions of it, distinguished by their orientation in space. Length provides one dimension, while width provides a second perpendicular to the third. Height, perpendicular both to length and width, makes the third spatial dimension—yet all of these are merely expressions of length differentiated according to direction.

Matter, in turn, can be defined as physical substance that occupies space; is composed of atoms (or in the case of subatomic particles, is part of an atom); is convertible into energy—and has mass. The form or state of matter itself is not important: on Earth it is primarily observed as a solid, liquid, or gas, but it can also be found (particularly in other parts of the universe) in a fourth state, plasma.

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Mass, Density, and Volume

ASTRONAUTS NEIL ARMSTRONG AND BUZZ ALDRIN, THE FIRST MEN TO WALK ON THE MOON, WEIGHED LESS ON MOON THAN ON EARTH. THE REASON IS BECAUSE WEIGHT DIFFERS AS A RESPONSE TO THE GRAVITATIONAL PULL OF THE PLANET, MOON, OR OTHER BODY ON WHICH IT IS MEASURED. THUS, A PERSON WEIGHS LESS ON THE MOON, BECAUSE THE MOON POSSESSES LESS MASS THAN EARTH AND EXERTS LESS GRAVITATIONAL FORCE. (NASA/Roger Ress-

THE

meyer/Corbis. Reproduced by permission.)

The more matter an object contains, the more mass. To refer again to the laws of motion, in his first law, Newton identified inertia: the tendency of objects in motion to remain in motion, or of objects at rest to remain at rest, in a constant velocity unless they are acted upon by some outside force. Mass is a measure of inertia, meaning that the more mass something contains, the more difficult it is to put it into motion, or to stop it from moving. who are not scientifically trained tend to think that mass and weight are the same thing, but this

It is understandable why people confuse mass with weight, since most weight scales provide measurements in both pounds and kilograms. However, the pound (lb) is a unit of weight in the English system, whereas a kilogram

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M ASS V S . W E I G H T. Most people

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is like saying that apples and apple pies are the same. Of course, an apple is an ingredient in an apple pie, but the pie contains something else— actually, a number of other things, such as flour and sugar. In this analogy, mass is equivalent to the apple, and weight the pie, while the acceleration due to gravity is the “something else” in weight.

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Mass, Density, and Volume

TO

DETERMINE WHETHER A PIECE OF GOLD IS GENUINE OR FAKE, ONE MUST MEASURE THE DENSITY OF THE SUB-

STANCE.

HERE,

A MAN EVALUATES GOLD PIECES IN

HONG KONG. (Christophe Loviny/Corbis. Reproduced by permission.)

(kg) is a unit of mass in the metric and SI systems. Though the two are relatively convertible on Earth (1 lb = 0.4536 kg; 1 kg = 2.21 lb), they are actually quite different.

REAL-LIFE A P P L I C AT I O N S

Weight is a measure of force, which Newton’s second law of motion defined as the product of mass multiplied by acceleration. The acceleration component of weight is a result of Earth’s gravitational pull, and is equal to 32 ft (9.8 m) per second squared. Thus a person’s weight varies according to gravity, and would be different if measured on the Moon; mass, on the other hand, is the same throughout the universe. Given its invariable value, scientists typically speak in terms of mass rather than weight.

Chemists do not always deal in large units of mass, such as the mass of a human body—which, of course, is measured in kilograms. Instead, the chemist’s work is often concerned with measurements of mass for the smallest types of matter: molecules, atoms, and other elementary particles. To measure these even in terms of grams (0.001 kg) is absurd: a single atom of carbon, for instance, has a mass of 1.99 • 10–23 g. In other words, a gram is about 50,000,000,000,000,000,000,000 times larger than a carbon atom—hardly a usable comparison.

Weight differs as a response to the gravitational pull of the planet, moon, or other body on which it is measured. Hence a person weighs less on the Moon, because the Moon possesses less mass than Earth, and, thus, exerts less gravitational force. Therefore, it would be easier on the Moon to lift a person from the ground, but it would be no easier to move that person from a resting position, or to stop him or her from moving. This is because the person’s mass, and hence his or her resistance to inertia, has not changed.

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Atomic Mass Units

Instead, chemists use an atom mass unit (abbreviated amu), which is equal to 1.66 • 10–24 g. Even so, is hard to imagine determining the mass of single atoms on a regular basis, so chemists make use of figures for the average atomic mass of a particular element. The average atomic mass of carbon, for instance, is 12.01 amu. As is the case with any average, this means that some atoms—different isotopes of carbon— may weigh more or less, but the figure of 12.01

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Mass, Density, and Volume

Earth (to same scale)

ALTHOUGH SATURN

IS MUCH LARGER THAN

EARTH,

IT IS MUCH LESS DENSE.

amu is still reliable. Some other average atomic mass figures for different elements are as follows: • • • • • • • • •

Hydrogen (H): 1.008 amu Helium (He): 4.003 amu Lithium (Li): 6.941 amu Nitrogen (N): 14.01 amu Oxygen (O): 16.00 Aluminum (Al): 26.98 Chlorine (Cl): 35.46 amu Gold (Au): 197.0 amu Hassium (Hs): [265 amu] The figure for hassium, with an atomic number of 108, is given in brackets because this number is the mass for the longest-lived isotope. The average value of mass for the molecules in a given compound can also be rendered in terms of atomic mass units: water (H2O) molecules, for instance, have an average mass of 18.0153 amu. Molecules of magnesium oxide (MgO), which can be extracted from sea water and used in making ceramics, have an average mass much higher than for water: 40.304 amu.

26

atomic mass of oxygen. In the case of magnesium oxide, the oxygen is bonded to just one other atom—but magnesium, with an average atomic mass of 24.304, weighs much more than hydrogen.

Molar Mass It is often important for a chemist to know exactly how many atoms are in a given sample, particularly in the case of a chemical reaction between two or more samples. Obviously, it is impossible to count atoms or other elementary particles, but there is a way to determine whether two items— regardless of the elements or compounds involved—have the same number of elementary particles. This method makes use of the figures for average atomic mass that have been established for each element.

These values are obtained simply by adding those of the atoms included in the molecule: since water has two hydrogen atoms and one oxygen, the average molecular mass is obtained by multiplying the average atomic mass of hydrogen by two, and adding it to the average

If the average atomic mass of the substance is 5 amu, then there should be a very large number of atoms (if it is an element) or molecules (if it is a compound) of that substance having a total mass of 5 grams (g). Similarly, if the average atomic mass of the substance is 7.5 amu, then there should be a very large number of atoms or molecules of that substance having a total mass of 7.5 g. What is needed, clearly, is a very large number by which elementary particles must be

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multiplied in order to yield a mass whose value in grams is equal to the value, in amu, of its average atomic mass. This is known as Avogadro’s number. AV O GA D R O ’ S N U M B E R. The first scientist to recognize a meaningful distinction between atoms and molecules was Italian physicist Amedeo Avogadro (1776-1856). Avogadro maintained that gases consisted of particles— which he called molecules—that in turn consisted of one or more smaller particles. He further reasoned that one liter of any gas must contain the same number of particles as a liter of another gas.

In order to discuss the behavior of molecules, it was necessary to set a large quantity as a basic unit, since molecules themselves are very small. This led to the establishment of what is known as Avogadro’s number, equal to 6.022137 ⫻ 1023 (more than 600 billion trillion.) The magnitude of Avogadro’s number is almost inconceivable. The same number of grains of sand would cover the entire surface of Earth at a depth of several feet. The same number of seconds, for instance, is about 800,000 times as long as the age of the universe (20 billion years). Avogadro’s number—named after the man who introduced the concept of the molecule, but only calculated years after his death— serves a very useful purpose in computations involving molecules. T H E M O L E . To compare two substances containing the same number of atoms or molecules, scientists use the mole, the SI fundamental unit for “amount of substance.” A mole (abbreviated mol) is, generally speaking, Avogadro’s number of atoms or molecules; however, in the more precise SI definition, a mole is equal to the number of carbon atoms in 12.01 g (0.03 lb) of carbon. Note that, as stated earlier, carbon has an average atomic mass of 12.01 amu. This is no coincidence, of course: multiplication of the average atomic mass by Avogadro’s number yields a figure in grams equal to the value of the average atomic mass in amu.

expressed in grams. A mole of helium, for instance, has a mass of 4.003 g (0.01 lb), whereas a mole of iron is 55.85 g (0.12 lb) These figures represent the molar mass for each: that is, the mass of 1 mol of a given substance.

Mass, Density, and Volume

Once again, the value of molar mass in grams is the same as that of the average atomic mass in amu. Also, it should be clear that, given the fact that helium weighs much less than air— the reason why helium-filled balloons float—a quantity of helium with a mass of 4.003 g must be a great deal of helium. And indeed, as indicated earlier, the quantity of atoms or molecules in a mole is sufficiently great to make a sample that is large, but still usable for the purposes of study or comparison.

Measuring Volume Mass, because of its fundamental nature, is sometimes hard to comprehend, and density requires an explanation in terms of mass and volume. Volume, on the other hand, appears to be quite straightforward—and it is, when one is describing a solid of regular shape. In other situations, however, volume measurement is more complicated. As noted earlier, the volume of a cube can be obtained simply by multiplying length by width by height. There are other means for measuring the volume of other straight-sided objects, such as a pyramid. Still other formulae, which make use of the constant π (roughly equal to 3.14) are necessary for measuring the volume of a cylinder, a sphere, or a cone. For an object that is irregular in shape, however, one may have to employ calculus—but the most basic method is simply to immerse the object in water. This procedure involves measuring the volume of the water before and after immersion, and calculating the difference. Of course, the object being measured cannot be water-soluble; if it is, its volume must be measured in a non-water-based liquid such as alcohol. LIQUID

AND

GAS

VOLUME.

The term “mole” can be used in the same way we use the word “dozen.” Just as “a dozen” can refer to twelve cakes or twelve chickens, so “mole” always describes the same number of molecules. Just as one liter of water, or one liter of mercury, has a certain mass, a mole of any given substance has its own particular mass,

Measuring liquid volumes is even easier than for solids, given the fact that liquids have no definite shape, and will simply take the shape of the container in which they are placed. Gases are similar to liquids in the sense that they expand to fit their container; however, measurement of gas volume is a more involved process than that used to measure either liquids or solids, because gases are

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highly responsive to changes in temperature and pressure. If the temperature of water is raised from its freezing point to its boiling point—from 32°F (0°C) to 212°F (100°C)—its volume will increase by only 2%. If its pressure is doubled from 1 atm (defined as normal air pressure at sea level) to 2 atm, volume will decrease by only 0.01%. Yet if air were heated from 32° to 212°F, its volume would increase by 37%; if its pressure were doubled from 1 atm to 2, its volume would decrease by 50%. Not only do gases respond dramatically to changes in temperature and pressure, but also, gas molecules tend to be non-attractive toward one another—that is, they tend not to stick together. Hence, the concept of “volume” in relation to a gas is essentially meaningless unless its temperature and pressure are known.

Comparing Densities In the discussion of molar mass above, helium and iron were compared, and we saw that the mass of a mole of iron was about 14 times as great as that of a mole of helium. This may seem like a fairly small factor of difference between them: after all, helium floats on air, whereas iron (unless it is arranged in just the right way, for instance, in a tanker) sinks to the bottom of the ocean. But be careful: the comparison of molar mass is only an expression of the mass of a helium atom as compared to the mass of an iron atom. It makes no reference to density, which is the ratio of mass to volume.

relates to the fact that it takes a much greater volume of feathers (measured in cubic feet, for instance) than of cannonballs to equal a ton. One of the interesting things about density, as distinguished from mass and volume, is that it has nothing to do with the amount of material. A kilogram of iron differs from 10 kg of iron both in mass and volume, but the density of both samples is the same. Indeed, as discussed below, the known densities of various materials make it possible to determine whether a sample of that material is genuine. C O M PA R I N G D E N S I T I E S . As noted several times, the densities of numerous materials are known quantities, and can be easily compared. Some examples of density, all expressed in terms of grams per cubic centimeter, are listed below. These figures are measured at a temperature of 68°F (20°C), and for hydrogen and oxygen, the value was obtained at normal atmospheric pressure (1 atm):

Comparisons of Densities for Various Substances: • • • • • • • • •

Oxygen: 0.00133 g/cm3 Hydrogen: 0.000084 g/cm3 Ethyl alcohol: 0.79 g/cm3 Ice: 0.920 g/cm3 Water: 1.00 g/cm3 Concrete: 2.3 g/cm3 Iron: 7.87 g/cm3 Lead: 11.34 g/cm3 Gold: 19.32 g/cm3

Specific Gravity Expressed in terms of the ratio of mass to volume, the difference between helium and iron becomes much more pronounced. Suppose, on the one hand, one had a gallon jug filled with iron. How many gallons of helium does it take to equal the mass of the iron? Fourteen? Try again: it takes more than 43,000 gallons of helium to equal the mass of the iron in one gallon jug! Clearly, what this shows is that the density of iron is much, much greater than that of helium. This, of course, is hardly a surprising revelation; still, it is sometimes easy to get confused by comparisons of mass as opposed to comparisons of density. One might even get tricked by the old elementary-school brain-teaser that goes something like this: “Which is heavier, a ton of feathers or a ton of cannonballs?” Of course neither is heavier, but the trick element in the question

28

VOLUME 1: REAL-LIFE CHEMISTRY

I S I T R E A L LY G O L D ? Note that pure water (as opposed to sea water, which is 3% more dense) has a density of 1.0 g per cubic centimeter. Water is thus a useful standard for measuring the specific gravity of other substances, or the ratio between the density of that substance and the density of water. Since the specific gravity of water is 1.00—also the density of water in g/cm3—the specific gravity of any substance (a number, rather than a number combined with a unit of measure) is the same as the value of its own density in g/cm3.

Comparison of densities make it possible to determine whether a piece of jewelry alleged to be solid gold is really genuine. To determine the answer, one must drop the sample in a beaker of water with graduated units of measure clearly

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Mass, Density, and Volume

KEY TERMS An SI unit

(cc), is equal to 6.10237 • 10-2 cubic inches

(abbreviated amu), equal to 1.66 • 10-24 g,

in the English system. As the name implies,

for measuring the mass of atoms.

there are 1,000 milliliters in a liter.

ATOMIC MASS UNIT:

AVERAGE ATOMIC MASS:

A figure

MOLAR MASS:

The mass, in grams, of

used by chemists to specify the mass—in

1 mole of a given substance. The value in

atomic mass units—of the average atom in

grams of molar mass is always equal to the

a large sample. The average atomic mass of

value, in atomic mass units, of the average

carbon, for instance, is 12.01 amu. If a sub-

atomic mass of that substance: thus, car-

stance is a compound, the average atomic

bon has a molar mass of 12.01 g, and an

mass of all atoms in a molecule of that sub-

average atomic mass of 12.01 amu.

stance must be added together to yield the average molecular mass of that substance.

The SI fundamental unit for

MOLE:

“amount of substance.” A mole is, general-

A figure,

ly speaking, Avogadro’s number of atoms,

named after Italian physicist Amedeo Avo-

molecules, or other elementary particles;

gadro (1776-1856), equal to 6.022137 ⫻

however, in the more precise SI definition,

⫹023.

a mole is equal to the number of carbon

AVOGADRO’S NUMBER:

Avogadro’s number indicates the

number of atoms, molecules, or other ele-

atoms in 12.01 g of carbon.

mentary particles in a mole. SPECIFIC GRAVITY:

The density of

The ratio of mass to vol-

an object or substance relative to the densi-

ume—in other words, the amount of mat-

ty of water; or more generally, the ratio

ter within a given area. In the SI system,

between the densities of two objects or

density is typically expressed as grams per

substances. Since the specific gravity of

cubic centimeter (g/cm3), equal to

water is 1.00—also the density of water in

62.42197 pounds per cubic foot in the Eng-

g/cm3—the specific gravity of any sub-

lish system.

stance is the same as the value of its own

DENSITY:

MASS:

The amount of matter an object

a number, without any unit of measure.

contains. MATTER:

density in g/cm3. Specific gravity is simply

Physical substance that occu-

VOLUME:

The

amount

of

three-

pies space, has mass, is composed of atoms

dimensional space an object occupies. Vol-

(or in the case of subatomic particles, is

ume is usually expressed in cubic units of

part of an atom), and is convertible to

length—for instance, the milliliter.

energy.

WEIGHT:

MILLILITER:

The product of mass multi-

One of the most com-

plied by the acceleration due to gravity (32

monly used units of volume in the SI sys-

ft or 9.8 m/sec2). A pound is a unit of

tem of measures. The milliliter (abbreviat-

weight, whereas a kilogram is a unit of

ed mL), also known as a cubic centimeter

mass.

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marked. Suppose the item has a mass of 10 g. The density of gold is 19 g/cm3, and because density is equal to mass divided by volume, the volume of water displaced should be equal to the mass divided by the density. The latter figure is equal to 10 g divided by 19 g/cm3, or 0.53 ml. Suppose that instead, the item displaced 0.88 ml of water. Clearly it is not gold, but what is it? Given the figures for mass and volume, its density is equal to 11.34 g/cm3—which happens to be the density of lead. If, on the other hand, the amount of water displaced were somewhere between the values for pure gold and pure lead, one could calculate what portion of the item was gold and which lead. It is possible, of course, that it could contain some other metal, but given the high specific gravity of lead, and the fact that its density is relatively close to that of gold, lead is a favorite gold substitute among jewelry counterfeiters. S P E C I F I C G RAV I T Y A N D T H E D E N S I T I E S O F P L A N E T S . Most

rocks near the surface of Earth have a specific gravity somewhere between 2 and 3, while the specific gravity of the planet itself is about 5. How do scientists know that the density of Earth is around 5 g/cm3? The computation is fairly simple, given the fact that the mass and volume of the planet are known. And given the fact that most of what lies close to Earth’s surface—sea water, soil, rocks—has a specific gravity well below 5, it is clear that Earth’s interior must contain high-density materials, such as nickel or iron. In the same way, calculations regarding the density of other objects in the Solar System provide a clue as to their interior composition.

30

planet, moon, or other body exerts is related to its mass, not its size. It so happens, too, that Jupiter is much larger than Earth, and that it exerts a gravitational pull much greater than that of Earth. This is because it has a mass many times as great as Earth’s. But what about Saturn, the secondlargest planet in the Solar System? In size it is only about 17% smaller than Jupiter, and both are much, much larger than Earth. Yet a person would weigh much less on Saturn than on Jupiter, because Saturn has a mass much smaller than Jupiter’s. Given the close relation in size between the two planets, it is clear that Saturn has a much lower density than Jupiter, or in fact even Earth: the great ringed planet has a specific gravity of less than 1. WHERE TO LEARN MORE Chahrour, Janet. Flash! Bang! Pop! Fizz!: Exciting Science for Curious Minds. Illustrated by Ann Humphrey Williams. Hauppauge, NY: Barron’s, 2000. “Density and Specific Gravity” (Web site). (March 27, 2001). “Density, Volume, and Cola” (Web site). (March 27, 2001). Ebbing, Darrell D.; R. A. D. Wentworth; and James P. Birk. Introductory Chemistry. Boston: Houghton Mifflin, 1995. “The Mass Volume Density Challenge” (Web site). (March 27, 2001). “MegaConverter 2” (Web site). (May 7, 2001). “Metric Density and Specific Gravity” (Web site). (March 27, 2001).

This brings the discussion back around to a topic raised much earlier in this essay, when comparing the weight of a person on Earth versus that person’s weight on the Moon. It so happens that the Moon is smaller than Earth, but that is not the reason it exerts less gravitational pull: as noted earlier, the gravitational force a

Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

VOLUME 1: REAL-LIFE CHEMISTRY

S C I E N C E O F E V E RY DAY T H I N G S

Robson, Pam. Clocks, Scales and Measurements. New York: Gloucester Press, 1993. “Volume, Mass, and Density” (Web site). (March 27, 2001).

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S C I E N C E O F E V E RY DAY T H I N G S real-life chemistry

M AT T E R P R O P E R T I E S O F M AT T E R GASES

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Properties of Matter

CONCEPT Matter is physical substance that occupies space, has mass, is composed of atoms—or, in the case of subatomic particles, is part of an atom—and is convertible to energy. On Earth, matter appears in three clearly defined forms—solid, liquid, and gas—whose varying structural characteristics are a function of the speeds at which its molecules move in relation to one another. A single substance may exist in any of the three phases: liquid water, for instance, can be heated to become steam, a vapor; or, when sufficient heat is removed from it, it becomes ice, a solid. These are merely physical changes, which do not affect the basic composition of the substance itself: it is still water. Matter, however, can and does undergo chemical changes, which (as with the various states or phases of matter) are an outcome of activity at the atomic and molecular level.

PROPERTIES O F M AT T E R

ties of energy available from even a tiny object traveling at that speed are enormous indeed. This is the basis for both nuclear power and nuclear weaponry, each of which uses some of the smallest particles in the known universe to produce results that are both amazing and terrifying. Even in everyday life, it is still possible to observe the conversion of mass to energy, if only on a very small scale. When a fire burns—that is, when wood experiences combustion in the presence of oxygen, and undergoes chemical changes—a tiny fraction of its mass is converted to energy. Likewise, when a stick of dynamite explodes, it too experiences chemical changes and the release of energy. The actual amount of energy released is, again, very small: for a stick of dynamite weighing 2.2 lb (1 kg), the portion of its mass that “disappears” is be equal to 6 parts out of 100 billion.

Einstein’s famous formula, E = mc2, means that every item possesses a quantity of energy equal to its mass multiplied by the squared speed of light. Given the fact that light travels at 186,000 mi (299,339 km) per second, the quanti-

Actually, none of the matter in the fire or the dynamite blast disappears: it simply changes forms. Most of it becomes other types of matter—perhaps new compounds, and certainly new mixtures of compounds. A very small part, as we have seen, becomes energy. One of the most fundamental principles of the universe is the conservation of energy, which holds that within a system isolated from all other outside factors, the total amount of energy remains the same, though transformations of energy from one form to another take place. In this situation, some of the energy remains latent, or “in reserve” as matter, while other components of the energy are released; yet the total amount of energy remains the same.

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HOW IT WORKS Matter and Energy One of the characteristics of matter noted in its definition above is that it is convertible to energy. We rarely witness this conversion, though as Albert Einstein (1879-1955) showed with his Theory of Relativity, it occurs in a massive way at speeds approaching that of light.

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Physical and Chemical Changes

what statement would contain the most information in the fewest words?”

In discussing matter—as, for instance, in the context of matter transforming into energy—one may speak in physical or chemical terms, or both. Generally speaking, physicists study physical properties and changes, while chemists are concerned with chemical processes and changes.

The answer he gave was this: “I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms—little articles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied.”

A physicist views matter in terms of its mass, temperature, mechanical properties (for example, elasticity); electrical conductivity; and other structural characteristics. The chemical makeup of matter, on the other hand, is of little concern to a physicist. For instance, in analyzing a fire or an explosion, the physicist is not concerned with the interactions of combustible or explosive materials and oxygen. The physicist’s interest, rather, is in questions such as the amount of heat in the fire, the properties of the sound waves emitted in the explosion of the dynamite, and so on. The changes between different states or phases of matter, as they are discussed below, are physical changes. If water boils and vaporizes as steam, it is still water; likewise if it freezes to become solid ice, nothing has changed with regard to the basic chemical structure of the H2O molecules that make up water. But if water reacts with another substance to form a new compound, it has undergone chemical change. Likewise, if water molecules experience electrolysis, a process in which electric current is used to decompose H2O into molecules of H2 and O2, this is also a chemical change. Similarly, a change from matter to energy, while it is also a physical change, typically involves some chemical or nuclear process to serve as “midwife” to that change. Yet physical and chemical changes have at least one thing in common: they can be explained in terms of behavior at the atomic or molecular level. This is true of many physical processes—and of all chemical ones.

S T R U C T U R E O F T H E AT O M . As Feynman went on to note, atoms are so tiny that if an apple were magnified to the size of Earth, the atoms in it would each be about the size of a regular apple. Clearly, atoms and other atomic particles are far too small to be glimpsed even by the most highly powered optical microscope. Yet physicists and other scientists are able to study the behavior of atoms, and by doing so, they are able to form a picture of what occurs at the atomic level.

In his highly readable Six Easy Pieces—a work that includes considerable discussion of chemistry as well as physics—the great American physicist Richard Feynman (1918-1988) asked, “If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures,

An atom is the fundamental particle in a chemical element. The atom is not, however, the smallest particle in the universe: atoms are composed of subatomic particles, including protons, neutrons, and electrons. These are distinguished from one another in terms of electric charge: as with the north and south poles of magnets, positive and negative charges attract one another, but like charges repel. (In fact, magnetism is simply a manifestation of a larger electromagnetic force that encompasses both electricity and magnetism.)

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Atoms

34

Indeed, what Feynman called the “atomic hypothesis” is one of the most important keys to understanding both physical and chemical changes. The behavior of particles at the atomic level has a defining role in the shape of the world studied by the sciences, and an awareness of this behavior makes it easier to understand physical processes, such as changes of state between solid, liquid, and gas; chemical processes, such as the formation of new compounds; and other processes, such as the conversion of matter to energy, which involve both physical and chemical changes. Only when one comprehends the atomic structure of matter is it possible to move on to the chemical elements that are the most basic materials of chemistry.

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Clustered at the center, or nucleus, of the atom are protons, which are positively charged, and neutrons, which exert no charge. Spinning around the nucleus are electrons, which exert a negative charge. The vast majority of the atom’s mass is made up by the protons and neutrons, which have approximately the same mass; that of the electron is much smaller. If an electron had a mass of 1—not a unit, but simply a figure used for comparison—the mass of the proton would be 1,836, and of the neutron 1,839.

Properties of Matter

Atoms of the same element always have the same number of protons, and since this figure is unique for a given element, each element is assigned an atomic number equal to the number of protons in its nucleus. Two atoms may have the same number of protons, and thus be of the same element, yet differ in their number of neutrons. Such atoms are called isotopes. The number of electrons is usually the same as the number of protons, and thus atoms have a neutral charge. In certain situations, however, the atom may lose or gain one or more electrons and acquire a net charge, becoming an ion. But electric charge, like energy, is conserved, and the electrons are not “lost” when an atom becomes an ion: they simply go elsewhere. It is useful, though far from precise, to compare the interior of an atom to a planet spinning very quickly around a sun. If the nucleus were our own Sun, then the electrons spinning at the edge of the atom would be on an orbit somewhere beyond Mars: in other words, the ratio between the size of the nucleus and the furthest edge of the atom is like that between the Sun’s diameter and an orbital path about 80 million miles beyond Mars. One of many differences between an atom and a solar system, however, is the fact that the electrons are spinning around the nucleus at a relative rate of motion much, much greater than any planet is revolving around the Sun. Furthermore, what holds the atom together is not gravitational force, as in the Solar System, but electromagnetic force. A final and critical difference is the fact that electrons move in much more complex orbital patterns than the elliptical paths that planets make in their movement around the Sun.

Molecules

A

COAL

GASIFICATION

PLANT.

MAKES IT POSSIBLE TO BURN

COAL GASIFICATION “CLEAN” COAL. (Roger Ress-

meyer/Corbis. Reproduced by permission.)

the world are not pure elements such as oxygen or iron. They are compounds in which atoms of more than one element join—usually in molecules. All molecules are composed of more than one atom, but not necessarily of more than one element: oxygen, for instance, generally appears in the form of molecules in which two oxygen atoms are bonded. Because of this, pure oxygen is represented by the chemical symbol O2, as opposed to the symbol for the element oxygen, which is simply O. One of the most well-known molecular forms in the world is water, or H2O, composed of two hydrogen atoms and one oxygen atom. The arrangement is extremely precise and never varies: scientists know, for instance, that the two hydrogen atoms join the oxygen atom (which is much larger than the hydrogen atoms) at an angle of 105°3’. Since the oxygen atom is much larger than the two hydrogens, its shape can be compared to a basketball with two softballs attached.

Though an atom is the fundamental unit of matter, most of the substances people encounter in

Other molecules are much more complex than those of water, and some are much, much more complex, a fact reflected in the sometimes

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lengthy names and complicated symbolic representations required to identify their chemical components. On the other hand, not all materials are made up of molecules: salt, for instance, is an ionic solid, as discussed below.

Quantifying Atoms and Molecules The nucleus of an atom is about 10–13 cm in diameter, and the diameter of the entire atom is about 10–8 cm—about 0.0000003937 in. Obviously, special units are required for describing the size of atoms, and usually measurements are provided in terms of the angstrom, equal to 10–10 m, or 10–8 cm. To put this on some sort of imaginable scale, there are 10 million angstroms in a millimeter. Measuring the spatial dimensions of an atom, however, is not as important as measuring its mass—and naturally, the mass of an atom is also almost inconceivably small. For instance, it takes about 5.0 • 1023 carbon atoms to equal just one gram of mass. Again, the numbers boggle the mind, but the following may put this into perspective. We have already established just how tiny an angstrom is; now consider the following. If 5.0 • 1023 angstrom lengths were laid end to end, they would stretch for a total of about 107,765 round trips from Earth to the Sun! It is obvious, then, that an entirely different unit should be used for measuring the mass of an atom, and for this purpose, chemists and other scientists use an atom mass unit (abbreviated amu). The latter is equal to 1.66 • 10–24 g. Even so, scientists can hardly be expected to be constantly measuring the mass of individual atoms; rather, they rely on figures determined for the average atomic mass of a particular element.

36

mass of individual atoms or molecules, scientists need a useful means for comparing atoms or molecules of different substances—and for doing so in such a way that they know they are analyzing equal numbers of particles. This cannot be done in terms of mass, because the number of atoms in each sample would vary: a gram of hydrogen, for instance, would contain about 12 times as many atoms as a gram of carbon, which has an average atomic mass of 12.01 amu. What is needed, instead, is a way to designate a certain number of atoms or molecules, such that accurate comparisons are possible. In order to do this, scientists make use of a figure known as Avogadro’s number. Named after Italian physicist Amedeo Avogadro (1776-1856), it is equal to 6.022137 ⫻ 1023 Earlier, we established the almost inconceivable scale represented by the figure 5.0 • 1023; here we are confronted with a number 20% larger. But Avogadro’s number, which is equal to 6,022,137 followed by 17 zeroes, is more than simply a mind-boggling series of digits. In general terms, Avogadro’s number designates the quantity of molecules (and sometimes atoms, if the substance in question is an element that, unlike oxygen, appears as single atoms) in a mole (abbreviated mol). A mole is the SI unit for “amount of substance,” and is defined precisely as the number of carbon atoms in 12.01 g of carbon. It is here that the value of Avogadro’s number becomes clear: as noted, carbon has an average atomic mass of 12.01 amu, and multiplication of the average atomic mass by Avogadro’s number yields a figure in grams equal to the value of the average atomic mass in atomic mass units. M O LA R M ASS A N D D E N S I T Y.

Average atomic mass figures range from 1.008 amu for hydrogen to over 250 amu for elements of very high atomic numbers. Figures for average atomic mass can be used to determine the average mass of a molecule as well, simply by combining the average atomic mass figures for each atom the molecule contains. A water molecule, for instance, has an average mass equal to the average atomic mass of hydrogen multiplied by two, and added to the average atomic mass of oxygen.

By comparison, a mole of helium has a molar mass of 4.003 g (0.01 lb) The molar mass of iron (that is, the mass of 1 mole of iron) is 55.85 g (0.12 lb) Note that there is not a huge ratio of difference between the molar mass of iron and that of helium: iron has a molar mass about 14 times greater. This, of course, seems very small in light of the observable differences between iron and helium: after all, who ever heard of a balloon filled with iron, or a skyscraper with helium girders?

AV O G A D R O ’ S N U M B E R A N D T H E M O L E . Just as using average atomic

mass is much more efficient than measuring the

The very striking differences between iron and helium, clearly, must come from something other than the molar mass differential between

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them. Of course, a mole of iron contains the same number of atoms as a mole of helium, but this says nothing about the relative density of the two substances. In terms of volume—that is, the amount of space that something occupies—the difference is much more striking: the volume of a mole of helium is about 43,000 times as large as that of a mole of iron. What this tells us is that the densities of iron and helium—the amount of mass per unit of volume—are very different. This difference in density is discussed in the essay on Mass, Density, and Volume; here the focus is on a larger judgment that can be formed by comparing the two densities. Helium, of course, is almost always in the form of a gas: to change it to a solid requires a temperature near absolute zero. And iron is a solid, meaning that it only turns into a liquid at extraordinarily high temperatures. These differences in overall structure can, in turn, be attributed to the relative motion, attraction, and energy of the molecules in each.

Molecular Attraction and Motion At the molecular level, every item of matter in the world is in motion, and the rate of that motion is a function of the attraction between molecules. Furthermore, the rate at which molecules move in relation to one another determines phase of matter—that is, whether a particular item can be described as solid, liquid, or gas. The movement of molecules generates kinetic energy, or the energy of movement, which is manifested as thermal energy—what people call “heat” in ordinary language. (The difference between thermal energy and heat is explained in the essay on Temperature and Heat.) In fact, thermal energy is the result of molecules’ motion relative to one another: the faster they move, the greater the kinetic energy, and the greater the “heat.” When the molecules in a material move slowly—merely vibrating in place—they exert a strong attraction toward one another, and the material is called a solid. Molecules of liquid, by contrast, move at moderate speeds and exert a moderate attraction. A material substance whose molecules move at high speeds, and therefore exert little or no attraction, is known as a gas. In short, the weaker the attraction, the greater the rate of relative motion—and the greater the amount of thermal energy the object contains.

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Properties of Matter

Types of Solids Particles of solids resist attempts to compress them, or push them together, and because of their close proximity, solid particles are fixed in an orderly and definite pattern. As a result, a solid usually has a definite volume and shape. A crystalline solid is a type of solid in which the constituent parts are arranged in a simple, definite geometric pattern that is repeated in all directions. But not all crystalline solids are the same. Table salt is an example of an ionic solid: a form of crystalline solid that contains ions. When mixed with a solvent such as water, ions from the salt move freely throughout the solution, making it possible to conduct an electric current. Regular table sugar (sucrose) is a molecular solid, or one in which the molecules have a neutral electric charge—that is, there are no ions present. Therefore, a solution of water and sugar would not conduct electricity. Finally, there are crystalline solids known as atomic solids, in which atoms of one element bond to one another. Examples include diamonds (made of pure carbon), silicon, and all metals. Other solids are said to be amorphous, meaning that they possess no definite shape. Amorphous solids—an example of which is clay—either possess very tiny crystals, or consist of several varieties of crystal mixed randomly. Still other solids, among them glass, do not contain crystals.

Freezing and Melting V I B RAT I O N S A N D F R E E Z I N G .

Because of their slow movement in relation to one another, solid particles exert strong attractions; yet as slowly as they move, solid particles do move—as is the case with all forms of matter at the atomic level. Whereas the particles in a liquid or gas move fast enough to be in relative motion with regard to one another, however, solid particles merely vibrate from a fixed position. As noted earlier, the motion and attraction of particles in matter has a direct effect on thermal energy, and thus on heat and temperature. The cooler the solid, the slower and weaker the vibrations, and the closer the particles are to one

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another. Thus, most types of matter contract when freezing, and their density increases. Absolute zero, or 0K on the Kelvin scale of temperature—equal to –459.67°F (–273°C)—is the point at which vibration virtually ceases. Note that the vibration virtually stops, but does not totally stop. In fact, as established in the third law of thermodynamics, absolute zero is impossible to achieve: thus, the relative motion of molecules never ceases. The lowest temperature actually achieved, at a Finnish nuclear laboratory in 1993, is 2.8 • 10–10 K, or 0.00000000028K—still above absolute zero. U N U S UA L C H A RAC T E R I S T I CS O F S O L I D A N D L I Q U I D WAT E R.

The behavior of water when frozen is interesting and exceptional. Above 39.2°F (4°C) water, like most substances, expands when heated. In other words, the molecules begin moving further apart as expected, because—in this temperature range, at least—water behaves like other substances, becoming “less solid” as the temperature increases. Between 32°F (0°C) and 39.2°F (4°C), however, water actually contracts. In this temperature range, it is very “cold” (that is, it has relatively little heat), but it is not frozen. The density of water reaches its maximum—in other words, water molecules are as closely packed as they can be— at 39.2°F; below that point, the density starts to decrease again. This is highly unusual: in most substances, the density continues to increase with lowered temperatures, whereas water is actually most dense slightly above the freezing point. Below the freezing point, then, water expands, and therefore when water in pipes freezes, it may increase in volume to the point where it bursts the pipe. This is also the reason why ice floats on water: its weight is less than that of the water it has displaced, and thus it is buoyant. Additionally, the buoyant qualities of ice atop very cold water helps explain the behavior of lake water in winter; although the top of a lake may freeze, the entire lake rarely freezes solid— even in the coldest of inhabited regions.

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lake does not freeze completely—only a layer at the top—and this helps preserve animal and plant life in the body of water. M E LT I N G . When heated, particles begin to vibrate more and more, and therefore move further apart. If a solid is heated enough, it loses its rigid structure and becomes a liquid. The temperature at which a solid turns into a liquid is called the melting point, and melting points are different for different substances. The melting point of a substance, incidentally, is the same as its freezing point: the difference is a matter of orientation—that is, whether the process is one of a solid melting to become a liquid, or of a liquid freezing to become a solid.

The energy required to melt 1 mole of a solid substance is called the molar heat of fusion. It can be calculated by the formula Q = smδT, where Q is energy, s is specific heat capacity, m is mass, and δT means change in temperature. (In the symbolic language often employed by scientists, the Greek letter δ, or delta, stands for “change in.”) Specific heat capacity is measured in units of J/g • °C (joules per gram-degree Celsius), and energy in joules or kilojoules (kJ)— that is, 1,000 joules. In melting, all the thermal energy in a solid is used in breaking up the arrangement of crystals, called a lattice. This is why water melted from ice does not feel any warmer than the ice did: the thermal energy has been expended, and there is none left over for heating the water. Once all the ice is melted, however, the absorbed energy from the particles—now moving at much greater speeds than when the ice was in a solid state—causes the temperature to rise.

Instead of freezing from the bottom up, as it would if ice were less buoyant than the water, the lake freezes from the top down—an important thing to remember when ice-fishing! Furthermore, water in general (and ice in particular) is a poor conductor of heat, and thus little of the heat from the water below it escapes. Therefore, the

For the most part, solids composed of particles with a higher average atomic mass require more energy—and hence higher temperatures— to induce the vibrations necessary for melting. Helium, with an average atomic mass of 4.003 amu, melts or freezes at an incredibly low temperature: –457.6°F (–272°C), or close to absolute zero. Water, for which, as noted earlier, the average atomic mass is the sum of the masses for its two hydrogen atoms and one oxygen atom, has an average molecular mass of 18.016 amu. Ice melts (or water freezes) at much higher temperatures than helium: 32°F (0°C). Copper, with an average atomic mass of 63.55 amu, melts at much, much higher temperatures than water: 1,985°F (1,085°C).

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Liquids The particles of a liquid, as compared to those of a solid, have more energy, more motion, and— generally speaking—less attraction to one another. The attraction, however, is still fairly strong: thus, liquid particles are in close enough proximity that the liquid resists attempts at compression. On the other hand, their arrangement is loose enough that the particles tend to move around one another rather than simply vibrate in place the way solid particles do. A liquid is therefore not definite in shape. Due to the fact that the particles in a liquid are farther apart than those of a solid, liquids tend to be less dense than solids. The liquid phase of a substance thus tends to be larger in volume than its equivalent in solid form. Again, however, water is exceptional in this regard: liquid water actually takes up less space than an equal mass of frozen water.

Boiling When a liquid experiences an increase in temperature, its particles take on energy and begin to move faster and faster. They collide with one another, and at some point the particles nearest the surface of the liquid acquire enough energy to break away from their neighbors. It is at this point that the liquid becomes a gas or vapor. As heating continues, particles throughout the liquid begin to gain energy and move faster, but they do not immediately transform into gas. The reason is that the pressure of the liquid, combined with the pressure of the atmosphere above the liquid, tends to keep particles in place. Those particles below the surface, therefore, remain where they are until they acquire enough energy to rise to the surface. The heated particle moves upward, leaving behind it a hollow space—a bubble. A bubble is not an empty space: it contains smaller trapped particles, but its small mass, relative to that of the liquid it disperses, makes it buoyant. Therefore, a bubble floats to the top, releasing its trapped particles as gas or vapor. At that point, the liquid is said to be boiling.

ing air. Normal atmospheric pressure (1 atm) is equal to 14 lb/in2 (1.013 x 105 Pa), and is measured at sea level. The greater the altitude, the less the air pressure, because molecules of air—since air is a gas, and therefore its particles are fastmoving and non-attractive—respond less to Earth’s gravitational pull. This is why airplanes require pressurized cabins to maintain an adequate oxygen supply; but even at altitudes much lower than the flight path of an airplane, differences in air pressure are noticeable. It is for this reason that cooking instructions often vary with altitude. Atop Mt. Everest, Earth’s highest peak at about 29,000 ft (8,839 m) above sea level, the pressure is approximately one-third of normal atmospheric pressure. Water boils at a much lower temperature on Everest than it does elsewhere: 158°F (70°C), as opposed to 212°F (100°C) at sea level. Of course, no one lives on the top of Mt. Everest—but people do live in Denver, Colorado, where the altitude is 5,577 ft (1,700 m) and the boiling point of water is 203°F (95°C). Given the lower boiling point, one might assume that food would cook faster in Denver than in New York, Los Angeles, or in any city close to sea level. In fact, the opposite is true: because heated particles escape the water so much faster at high altitudes, they do not have time to acquire the energy needed to raise the temperature of the water. It is for this reason that a recipe may include a statement such as “at altitudes above XX feet, add XX minutes to cooking time.” If lowered atmospheric pressure means a lowered boiling point, what happens in outer space, where there is no atmospheric pressure? Liquids boil at very, very low temperatures. This is one of the reasons why astronauts have to wear pressurized suits: if they did not, their blood would boil—even though space itself is incredibly cold. L I Q U I D T O GAS A N D B AC K AGA I N . Note that the process of changing a

overcome atmospheric pressure as they rise, which means that the boiling point for any liquid depends in part on the pressure of the surround-

liquid to a gas is similar to that which occurs when a solid changes to a liquid: particles gain heat and therefore energy, begin to move faster, break free from one another, and pass a certain threshold into a new phase of matter. And just as the freezing and melting point for a given substance are the same temperature—the only difference being one of orientation—the boiling

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point of a liquid transforming into a gas is the same as the condensation point for a gas turning into a liquid. The behavior of water in boiling and condensation makes possible distillation, one of the principal methods for purifying sea water in various parts of the world. First the water is boiled, then it is allowed to cool and condense, thus forming water again. In the process, the water separates from the salt, leaving it behind in the form of brine. A similar separation takes place when salt water freezes: because salt, like most crystalline solids, has a much lower freezing point than water, very little of it remains joined to the water in ice. Instead, the salt takes the form of a briny slush.

Gases A liquid that is vaporized, or any substance that exists normally as a gas, is quite different in physical terms from a solid or a liquid. This is illustrated by the much higher energy component in the molar heat of vaporization, or the amount of energy required to turn 1 mole of a liquid into a gas. Consider, for instance, what happens to water when it experiences phase changes. Assuming that heat is added at a uniform rate, when ice reaches its melting point, there is only a relatively small period of time when the H2O is composed of both ice and liquid. But when the liquid reaches its boiling point, the water is present both as a liquid and a vapor for a much longer period of time. In fact, it takes almost seven times as much energy to turn liquid water into pure steam than it does to turn ice into purely liquid water. Thus, the molar heat of fusion for water is 6.02 kJ/mol, while the molar heat of vaporization is 40.6 kJ/mol. Although liquid particles exert a moderate attraction toward one another, particles in a gas (particularly a substance that normally exists as a gas at ordinary temperatures on Earth) exert little to no attraction. They are thus free to move, and to move quickly. The overall shape and arrangement of gas is therefore random and indefinite—and, more importantly, the motion of gas particles provides much greater kinetic energy than is present in any other major form of matter on Earth.

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ing and thereby transferring kinetic energy back and forth without any net loss of energy. These collisions also have the overall effect of producing uniform pressure in a gas. At the same time, the characteristics and behavior of gas particles indicate that they tend not to remain in an open container. Therefore, in order to have any pressure on a gas—other than normal atmospheric pressure—it is necessary to keep it in a closed container.

The Phase Diagram The vaporization of water is an example of a change of phase—the transition from one phase of matter to another. The properties of any substance, and the points at which it changes phase, are plotted on what is known as a phase diagram. The phase diagram typically shows temperature along the x-axis, and pressure along the y-axis. For simple substances, such as water and carbon dioxide (CO2), the solid form of the substance appears at a relatively low temperature and at pressures anywhere from zero upward. The line between solids and liquids, indicating the temperature at which a solid becomes a liquid at any pressure above a certain level, is called the fusion curve. Though it appears to be a more or less vertical line, it is indeed curved, indicating that at high pressures, a solid well below the normal freezing point may be melted to create a liquid. Liquids occupy the area of the phase diagram corresponding to relatively high temperatures and high pressures. Gases or vapors, on the other hand, can exist at very low temperatures, but only if the pressure is also low. Above the melting point for the substance, gases exist at higher pressures and higher temperatures. Thus, the line between liquids and gases often looks almost like a 45° angle. But it is not a straight line, as its name, the vaporization curve, implies. The curve of vaporization demonstrates that at relatively high temperatures and high pressures, a substance is more likely to be a gas than a liquid.

The constant, fast, and random motion of gas particles means that they are regularly collid-

T H E C R I T I CA L P O I N T. There are several other interesting phenomena mapped on a phase diagram. One is the critical point, found at a place of very high temperature and pressure along the vaporization curve. At the critical point, high temperatures prevent a liquid from remaining a liquid, no matter how high the pressure.

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Properties of Matter

(y-axis) pressure

(liquid) (solid)

triple point

(x-axis)

A

(vapor)

temperature

PHASE DIAGRAM FOR WATER.

At the same time, the pressure causes gas beyond that point to become increasingly more dense, but due to the high temperatures, it does not condense into a liquid. Beyond the critical point, the substance cannot exist in anything other than the gaseous state. The temperature component of the critical point for water is 705.2°F (374°C)—at 218 atm, or 218 times ordinary atmospheric pressure. For helium, however, critical temperature is just a few degrees above absolute zero. This is, in part, why helium is rarely seen in forms other than a gas. THE

S U B L I M AT I O N

CURVE.

Another interesting phenomenon is the sublimation curve, or the line between solid and gas. At certain very low temperatures and pressures, a substance may experience sublimation, meaning that a gas turns into a solid, or a solid into a gas, without passing through a liquid stage. A well-known example of sublimation occurs when “dry ice,” made of carbon dioxide, vaporizes at temperatures above (–78.5°C). Carbon dioxide is exceptional, however, in that it experiences sublimation at relatively high pressures that occur in everyday life: for most substances, the sublimation point transpires at such a low pressure point that it is seldom witnessed outside of a laboratory.

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T H E T R I P L E P O I N T. The phenomenon known as the triple point shows how an ordinary substance such as water or carbon dioxide can actually be a liquid, solid, and vapor—all at once. Most people associate water as a gas or vapor (that is, steam) with very high temperatures. Yet, at a level far below normal atmospheric pressure, water can be a vapor at temperatures as low as –4°F (–20 °C). (All of the pressure values in the discussion of water at or near the triple point are far below atmospheric norms: the pressure at which water turns into a vapor at –4°F, for instance, is about 0.001 atm.)

Just as water can exist as a vapor at low temperatures and low pressures, it is also possible for water at temperatures below freezing to remain liquid. Under enough pressure, ice melts and is thereby transformed from a solid to a liquid, at temperatures below its normal freezing point. On the other hand, if the pressure of ice falls below a very low threshold, it will sublimate. The phase diagram of water shows a line between the solid and liquid states that is almost, but not quite, exactly perpendicular to the x-axis. But in fact, it is a true fusion curve: it slopes slightly upward to the left, indicating that solid ice turns into water with an increase of pressure. Below a certain level of pressure is the vaporiza-

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tion curve, and where the fusion curve intersects the vaporization curve, there is a place called the triple point. Just below freezing, in conditions equivalent to about 0.007 atm, water is a solid, liquid, and vapor all at once.

Other States of Matter P L A S M A . Principal among states of matter other than solid, liquid, and gas is plasma, which is similar to gas. (The term “plasma,” when referring to the state of matter, has nothing to do with the word as it is often used, in reference to blood plasma.) As with gas, plasma particles collide at high speeds—but in plasma the speeds are even greater, and the kinetic energy levels even higher.

The speed and energy of these collisions is directly related to the underlying property that distinguishes plasma from gas. So violent are the collisions between plasma particles that electrons are knocked away from their atoms. As a result, plasma does not have the atomic structure typical of a gas; rather, it is composed of positive ions and electrons. Plasma particles are thus electrically charged, and therefore greatly influenced by electric and magnetic fields. Formed at very high temperatures, plasma is found in stars. The reaction between plasma and atomic particles in the upper atmosphere is responsible for the aurora borealis, or “northern lights.” Though found on Earth only in very small quantities, plasma—ubiquitous in other parts of the universe—may be the most plentiful of all the states of matter. Q UAS I - S TAT E S . Among the quasistates of matter discussed by scientists are several terms describing the structure in which particles are joined, rather than the attraction and relative movement of those particles. Thus “crystalline,” “amorphous,” and “glassy” are all terms to describe what may be individual states of matter; so too is “colloidal.”

A colloid is a structure intermediate in size between a molecule and a visible particle, and it has a tendency to be dispersed in another medium—the way smoke, for instance, is dispersed in air. Brownian motion describes the behavior of most colloidal particles. When one sees dust floating in a ray of sunshine through a window, the light reflects off colloids in the dust, which are driven back and forth by motion in the air otherwise imperceptible to the human senses.

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DA R K M AT T E R. The number of states or phases of matter is clearly not fixed, and it is quite possible that more will be discovered in outer space, if not on Earth. One intriguing candidate is called dark matter, so described because it neither reflects nor emits light, and is therefore invisible. In fact, luminous or visible matter may very well make up only a small fraction of the mass in the universe, with the rest being taken up by dark matter.

If dark matter is invisible, how do astronomers and physicists know it exists? By analyzing the gravitational force exerted on visible objects in such cases where there appears to be no visible object to account for that force. An example is the center of our galaxy, the Milky Way. It appears to be nothing more than a dark “halo,” but in order to cause the entire galaxy to revolve around it—in the same way that planets revolve around the Sun, though on a vastly larger scale—it must contain a staggering quantity of invisible mass.

The Bose-Einstein Condensate Physicists at the Joint Institute of Laboratory Astrophysics in Boulder, Colorado, in 1995 revealed a highly interesting aspect of atomic behavior at temperatures approaching absolute zero. Some 70 years before, Einstein had predicted that, at extremely low temperatures, atoms would fuse to form one large “superatom.” This hypothesized structure was dubbed the BoseEinstein Condensate (BEC) after Einstein and Satyendranath Bose (1894-1974), an Indian physicist whose statistical methods contributed to the development of quantum theory. Cooling about 2,000 atoms of the element rubidium to a temperature just 170 billionths of a degree Celsius above absolute zero, the physicists succeeded in creating an atom 100 micrometers across—still incredibly small, but vast in comparison to an ordinary atom. The superatom, which lasted for about 15 seconds, cooled down all the way to just 20 billionths of a degree above absolute zero. The Colorado physicists won the Nobel Prize in physics in 1997 for their work. In 1999, researchers in a lab at Harvard University also created a superatom of BEC, and used it to slow light to just 38 MPH (61.2 km/h)—about 0.02% of its ordinary speed.

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Dubbed a “new” form of matter, the BEC may lead to a greater understanding of quantum mechanics, and may aid in the design of smaller, more powerful computer chips.

Some Unusual Phase Transitions At places throughout this essay, references have been made variously to “phases” and “states” of matter. This is not intended to confuse, but rather to emphasize a particular point. Solids, liquids, and gases are referred to as “phases” because many (though far from all) substances on Earth regularly move from one phase to another. There is absolutely nothing incorrect in referring to “states of matter.” But “phases of matter” is used in the present context as a means of emphasizing the fact that substances, at the appropriate temperature and pressure, can be solid, liquid, or gas. The phases of matter, in fact, can be likened to the phases of a person’s life: infancy, babyhood, childhood, adolescence, adulthood, old age. The transition between these stages is indefinite, yet it is easy enough to say when a person is at a certain stage. L I Q U I D C RY S TA L S . A liquid crystal is a substance that, over a specific range of temperature, displays properties both of a liquid and a solid. Below this temperature range, it is unquestionably a solid, and above this range it is just as certainly a liquid. In between, however, liquid crystals exhibit a strange solid-liquid behavior: like a liquid, their particles flow, but like a solid, their molecules maintain specific crystalline arrangements.

The cholesteric class of liquid crystals is so named because the spiral patterns of light through the crystal are similar to those which appear in cholesterols. Depending on the physical properties of a cholesteric liquid crystal, only certain colors may be reflected. The response of liquid crystals to light makes them useful in liquid crystal displays (LCDs) found on laptop computer screens, camcorder views, and in other applications.

lar energy levels. Liquefied natural gas (LNG) and liquefied petroleum gas (LPG), the latter a mixture of by-products obtained from petroleum and natural gas, are among the examples of liquefied gas in daily use. In both cases, the volume of the liquefied gas is far less than it would be if the gas were in a vaporized state, thus enabling ease and economy of transport. Liquefied gases are used as heating fuel for motor homes, boats, and homes or cabins in remote areas. Other applications of liquefied gases include liquefied oxygen and hydrogen in rocket engines; liquefied oxygen and petroleum used in welding; and a combination of liquefied oxygen and nitrogen used in aqualung devices. The properties of liquefied gases figure heavily in the science of producing and studying low-temperature environments. In addition, liquefied helium is used in studying the behavior of matter at temperatures close to absolute zero. C OA L GAS I F I CAT I O N . Coal gasification, as one might discern from the name, is the conversion of coal to gas. Developed before World War II, it fell out of favor after the war, due to the lower cost of oil and natural gas. However, increasingly stringent environmental regulations imposed by the federal government on industry during the 1970s, combined with a growing concern for the environment on the part of the populace as a whole, led to a resurgence of interest in coal gasification.

Though widely used as a fuel in power plants, coal, when burned by ordinary means, generates enormous air pollution. Coal gasification, on the other hand, makes it possible to burn “clean” coal. Gasification involves a number of chemical reactions, some exothermic or heatreleasing, and some endothermic or heat-absorbing. At one point, carbon monoxide is released in an exothermic reaction, then mixed with hydrogen released from the coal to create a second exothermic reaction. The energy discharged in these first two reactions is used to initiate a third, endothermic, reaction.

One interesting and useful application of phase change is the liquefaction of gases, or the change of gas into liquid by the reduction in its molecu-

The finished product of coal gasification is a mixture containing carbon monoxide, methane, hydrogen, and other substances, and this—rather than ordinary coal—is burned as a fuel. The composition of the gases varies according to the process used. Products range from coal synthesis gas and medium-Btu gas (both composed of car-

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Properties of Matter

KEY TERMS The temperature,

ABSOLUTE ZERO:

AVOGADRO’S NUMBER:

defined as 0K on the Kelvin scale, at which

named after Italian physicist Amedeo Avo-

the motion of molecules in a solid virtual-

gadro (1776-1856), equal to 6.022137 x

ly ceases. Absolute zero is equal to

1023. Avogadro’s number indicates the

–459.67°F (–273.15°C).

number of atoms, molecules, or other ele-

ATOM:

The smallest particle of an ele-

mentary particles in a mole.

ment that retains the chemical and physical

CHANGE OF PHASE:

properties of the element. An atom can

from one phase of matter to another.

exist either alone or in combination with other atoms in a molecule. Atoms are made up of protons, neutrons, and electrons. An atom that loses or gains one or more electrons, and thus has a net charge, is an ion. Atoms that have the same number of protons—that is, are of the same element— but differ in number of neutrons, are known as isotopes.

CRITICAL POINT:

An SI unit

(abbreviated amu), equal to 1.66 • 10–24 g,

A coordinate, plot-

stance cannot exist in anything other than the gaseous state. Located at a position of very high temperature and pressure, the critical point marks the termination of the vaporization curve. A substance made up of

atoms of more than one element. These atoms are usually, but not always, joined in molecules.

for measuring the mass of atoms. ATOMIC NUMBER:

The transition

ted on a phase diagram, above which a sub-

COMPOUND:

ATOMIC MASS UNIT:

The number of

CRYSTALLINE

SOLID:

A type of

protons in the nucleus of an atom. Since

solid in which the constituent parts have a

this number is different for each element,

simple and definite geometric arrange-

elements are listed on the periodic table in

ment that is repeated in all directions.

order of atomic number.

Types of crystalline solids include atomic

ATOMIC SOLID:

A form of crystalline

solids, ionic solids, and molecular solids. A substance made up of

solid in which atoms of one element bond

ELEMENT:

to one another. Examples include dia-

only one kind of atom.

monds (made of pure carbon), silicon, and ELECTRON:

all metals.

A negatively charged par-

ticle in an atom. Electrons, which spin A figure

around the protons and neutrons that

used by chemists to specify the mass—in

make up the atom’s nucleus, constitute a

atomic mass units—of the average atom in

very small portion of the atom’s mass. In

a large sample. If a substance is a com-

most atoms, the number of electrons and

pound, the average atomic mass of all

protons is the same, thus canceling out one

atoms in a molecule of that substance must

another. When an atom loses one or more

be added together to yield the average

electrons, however—thus becoming an

molecular mass of that substance.

ion—it acquires a net electric charge.

AVERAGE ATOMIC MASS:

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A figure,

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Properties of Matter

KEY TERMS The ability to accomplish

ENERGY:

CONTINUED

MOLAR HEAT OF FUSION:

The

work—that is, the exertion of force over a

amount of energy required to melt 1 mole

given distance to displace or move an

of a solid substance.

object.

MOLAR HEAT OF VAPORIZATION:

The boundary be-

The amount of energy required to turn 1

tween solid and liquid for any given sub-

mole of a liquid into a gas. Because gases

stance as plotted on a phase diagram.

possess much more energy than liquids or

FUSION CURVE:

solids, molar heat of vaporization is usualGAS:

A phase of matter in which mole-

ly much higher than molar heat of fusion.

cules move at high speeds, and therefore exert little or no attraction toward one

MOLAR MASS:

The mass, in grams, of

1 mole of a given substance. The value in

another.

grams of molar mass is always equal to the An atom or atoms that has lost or

value, in atomic mass units, of the average

gained one or more electrons, and thus has

atomic mass of that substance: thus car-

a net electric charge.

bon has a molar mass of 12.01 g, and an

ION:

IONIC SOLID:

A form of crystalline

average atomic mass of 12.01 amu. The SI fundamental unit for

solid that contains ions. When mixed with

MOLE:

a solvent such as water, ions from table

“amount of substance.” A mole is, general-

salt—an example of an ionic solid—move

ly speaking, Avogadro’s number of atoms,

freely throughout the solution, making it

molecules, or other elementary particles;

possible to conduct an electric current.

however, in the more precise SI definition,

ISOTOPES:

Atoms that have an equal

number of protons, and hence are of the

a mole is equal to the number of carbon atoms in 12.01 g of carbon. A form of crys-

same element, but differ in their number of

MOLECULAR SOLID:

neutrons.

talline solid in which the molecules have a neutral electric charge, meaning that there

KINETIC ENERGY:

The energy that

an object possesses by virtue of its movement. LIQUID:

are no ions present as there are in an ionic solid. Table sugar (sucrose) is an example of a molecular solid. When mixed with a

A phase of matter in which

solvent such as water, the resulting solution

molecules move at moderate speeds, and

does not conduct electricity.

therefore exert moderate attractions

MOLECULE:

toward one another.

ly of more than one element, joined in a

MATTER:

Physical substance that occu-

A group of atoms, usual-

structure. A subatomic particle that

pies space, has mass, is composed of atoms

NEUTRON:

(or in the case of subatomic particles, is

has no electric charge. Neutrons are found

part of an atom), and is convertible to

at the nucleus of an atom, alongside

energy.

protons.

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Properties of Matter

KEY TERMS

CONTINUED

The center of an atom, a

same mass—a mass that is many times

region where protons and neutrons are

greater than that of an electron. The num-

located, and around which electrons spin.

ber of protons in the nucleus of an atom is

NUCLEUS:

PHASE DIAGRAM:

A chart, plotted

the atomic number of an element.

for any particular substance, identifying

SOLID:

the particular phase of matter for that sub-

molecules move slowly relative to one

stance at a given temperature and pressure

another, and therefore exert strong attrac-

level. A phase diagram usually shows tem-

tions toward one another.

A phase of matter in which

perature along the x-axis, and pressure SUBLIMATION CURVE:

along the y-axis. PHASES OF MATTER:

The bound-

ary between solid and gas for any given The various

substance, as plotted on a phase diagram.

forms of material substance (matter), which are defined primarily in terms of the

In chemistry and other sci-

behavior exhibited by their atomic or

ences, the term “system” usually refers to

molecular structures. On Earth, three prin-

any set of interactions isolated from the

cipal phases of matter exist, namely solid,

rest of the universe. Anything outside of

liquid, and gas. Other forms of matter

the system, including all factors and forces

include plasma.

irrelevant to a discussion of that system, is

PLASMA:

One of the phases of matter,

known as the environment. “Heat” energy, a

closely related to gas. Plasma is found pri-

THERMAL ENERGY:

marily in stars. Containing neither atoms

form of kinetic energy produced by the rel-

nor molecules, plasma is made up of elec-

ative motion of atoms or molecules. The

trons and positive ions.

greater the movement of these particles,

PROTON:

A positively charged particle

the greater the thermal energy. The bound-

in an atom. Protons and neutrons, which

VAPORIZATION CURVE:

together form the nucleus around which

ary between liquid and gas for any given

electrons spin, have approximately the

substance as plotted on a phase diagram.

bon monoxide and hydrogen, though combined in different forms) to substitute natural gas, which consists primarily of methane. Not only does coal gasification produce a clean-burning product, but it does so without the high costs associated with flue-gas desulfurization systems. The latter, often called “scrubbers,” were originally recommended by the federal government to industry, but companies discovered that coal gasification could produce the same results for much less money. In addition, the waste products from coal gasification can be used

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SYSTEM:

VOLUME 1: REAL-LIFE CHEMISTRY

for other purposes. At the Cool Water Integrated Gasification Combined Cycle Plant, established in Barstow, California, in 1984, sulfur obtained from the reduction of sulfur dioxide is sold off for about $100 a ton.

The Chemical Dimension to Changes of Phase Throughout much of this essay, we have discussed changes of phase primarily in physical terms; yet clearly these changes play a significant role in chemistry. Furthermore, coal gasification

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serves to illustrate the impact chemical processes can have on changes of state. Much earlier, figures were given for the melting points of copper, water, and helium, and these were compared with the average atomic mass of each. Those figures, again, are: Average Atomic Mass and Melting Points of a Sample Gas, Liquid, and Solid • Helium: 4.003 amu; –457.6°F (–210°C) • Water: 40.304 amu 32°F (0°C) • Copper: 63.55 amu; 1,985°F (1,085°C). Something seems a bit strange about those comparisons: specifically, the differences in melting point appear to be much more dramatic than the differences in average atomic mass. Clearly, another factor is at work—a factor that relates to the difference in the attractions between molecules in each. Although the differences between solids, liquids, and gases are generally physical, the one described here—a difference between substances—is clearly chemical in nature. To discuss this in the detail it deserves would require a lengthy digression on the chemical dimensions of intermolecular attraction. Nonetheless, it is possible here to offer at least a cursory answer to the question raised by these striking differences in response to temperature.

Dipoles, Electron Seas, and London Dispersion Water molecules are polar, meaning that one area of a water molecule is positively charged, while another area has a negative charge. Thus the positive side of one molecule is drawn to the negative side of another, and vice versa, which gives water a much stronger intermolecular bond than, for instance, oil, in which the positive and negative charges are evenly distributed throughout the molecule. Yet the intermolecular attraction between the dipoles (as they are called) in water is not nearly as strong as the bond that holds together a metal. Particles in copper or other metals “float” in a tightly packed “sea” of highly mobile electrons, which provide a bond that is powerful, yet lacking in a firm directional orientation. Thus metals are both strong and highly malleable (that is, they can be hammered very flat without breaking.)

S C I E N C E O F E V E RY DAY T H I N G S

Water, of course, appears most often as a liquid, and copper as a solid, precisely because water has a very high boiling point (the point at which it becomes a vapor) and copper has a very high melting point. But consider helium, which has the lowest freezing point of any element: just above absolute zero. Even then, a pressure equal to 25 times that of normal atmospheric pressure is required to push it past the freezing point.

Properties of Matter

Helium and other Group 8 or Group 18 elements, as well as non-polar molecules such as oils, are bonded by what is called London dispersion forces. The latter, as its name suggests, tends to keep molecules dispersed, and induces instantaneous dipoles when most of the electrons happen to be on one side of an atom. Of course, this happens only for an infinitesimal fraction of time, but it serves to create a weak attraction. Only at very low temperatures do London dispersion forces become strong enough to result in the formation of a solid.

WHERE TO LEARN MORE Biel, Timothy L. Atom: Building Blocks of Matter. San Diego, CA: Lucent Books, 1990. Feynman, Richard. Six Easy Pieces: Essentials of Physics Explained by Its Most Brilliant Teacher. New introduction by Paul Davies. Cambridge, MA: Perseus Books, 1995. “High School Chemistry Table of Contents—Solids and Liquids” Homeworkhelp.com (Web site). (April 10, 2001). “Matter: Solids, Liquids, Gases.” Studyweb (Web site). (April 10, 2001). “The Molecular Circus” (Web site). (April 10, 2001). Paul, Richard. A Handbook to the Universe: Explorations of Matter, Energy, Space, and Time for Beginning Scientific Thinkers. Chicago: Chicago Review Press, 1993. “Phases of Matter” (Web site). (April 10, 2001). Royston, Angela. Solids, Liquids, and Gasses. Chicago: Heinemann Library, 2001. Wheeler, Jill C. The Stuff Life’s Made Of: A Book About Matter. Minneapolis, MN: Abdo & Daughters Publishing, 1996. Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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GASES Gases

CONCEPT The number of elements that appear ordinarily in the form of a gas is relatively small: oxygen, hydrogen, fluorine, and chlorine in the halogen “family”; and a handful of others, most notably the noble gases in Group 8 of the periodic table. Yet many substances can exist in the form of a gas, depending on the relative attraction and motion of molecules in that substance. A simple example, of course, is water, or H2O, which, though it appears as a liquid at room temperature, begins to vaporize and turn into steam at 212°F (100°C). In general, gases respond more dramatically to changes in pressure and temperature than do most other types of matter, and this allows scientists to predict gas behaviors under certain conditions. These predictions can explain mundane occurrences, such as the fact that an open can of soda will soon lose its fizz, but they also apply to more dramatic, life-anddeath situations.

HOW IT WORKS Molecular Motion and Phases of Matter On Earth, three principal phases or states of matter exist: solid, liquid, and gas. The differences between these three are, on the surface at least, easily perceivable. Clearly water is a liquid, just as ice is a solid and steam a vapor or gas. Yet the ways in which various substances convert between phases are often complex, as are the interrelations between these phases. Ultimately, understanding of the phases depends on an

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VOLUME 1: REAL-LIFE CHEMISTRY

awareness of what takes place at the molecular level. All molecules are in motion, and the rate of that motion determines the attraction between them. The movement of molecules generates kinetic energy, or the energy of movement, which is manifested as thermal energy. In everyday language, thermal energy is what people mean when they say “heat”; but in scientific terms, heat has a different definition. The force that attracts atoms to atoms, or molecules to molecules, is not the same as gravitational force, which holds the Moon in orbit around Earth, Earth in orbit around the Sun, and so on. By contrast, the force of interatomic and intermolecular attraction is electromagnetic. Just as the north pole of a magnet is attracted to the south pole of another magnet and repelled by that other magnet’s north pole, so positive electric charges are attracted to negative charges, and negatives to positives. (In fact, electricity and magnetism are both manifestations of an electromagnetic interaction.) The electromagnetic attractions between molecules are much more complex than this explanation makes it seem, and they play a highly significant role in chemical bonding. In simple terms, however, one can say that the greater the rate of motion for the molecules in relation to one another, the less the attraction between molecules. In addition, the kinetic energy, and hence the thermal energy, is greater in a substance whose molecules are relatively free to move. When the molecules in a material move slowly in relation to one another, they exert a strong attraction, and the material is called a solid. Molecules of liquid, by contrast, move at

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Gases

A

COMPARISON OF GAS, LIQUID, AND SOLID STATES.

moderate speeds and exert a moderate attraction. A material substance whose molecules move at high speeds, and therefore exert little or no attraction, is known as a gas.

Comparison of Gases to Other Phases of Matter WAT E R A N D A I R C O M PA R E D .

Gases respond to changes in pressure and temperature in a manner remarkably different from that of solids or liquids. Consider the behavior of liquid water as compared with air—a combination of oxygen (O2), nitrogen (N2), and other gases—in response to experiments involving changes in pressure and temperature. In the first experiment, both samples are subjected to an increase in pressure from 1 atm (that is, normal atmospheric pressure at sea level) to 2 atm. In the second, both experience an increase in temperature from 32°F (0°C) to 212°F (100°C). The differences in the responses of water and air are striking.

increase will decrease the volume by a whopping 50%, and an equivalent temperature increase results in a volume increase of 37%. Air and other gases, by definition, have a boiling point below room temperature. If they did not boil and thus become gas well below ordinary temperatures, they would not be described as substances that are in the gaseous state in most circumstances. The boiling point of water, of course, is higher than room temperature, and that of solids is much higher. T H E A R RA N G E M E N T O F PA R T I C L E S . Solids possess a definite volume and

a definite shape, and are relatively noncompressible: for instance, if one applies extreme pressure to a steel plate, it will bend, but not much. Liquids have a definite volume, but no definite shape, and tend to be noncompressible. Gases, on the other hand, possess no definite volume or shape, and are highly compressible.

A sample of water that experiences an increase in pressure from 1 to 2 atm will decrease in volume by less than 0.01%, while a temperature increase from the freezing point to the boiling point will result in only a 2% increase in volume. For air, however, an equivalent pressure

At the molecular level, particles of solids tend to be definite in their arrangement and close in proximity—indeed, part of what makes a solid “solid,” in the everyday meaning of that term, is the fact that its constituent parts are basically immovable. Liquid molecules, too, are close in proximity, though random in arrangement. Gas

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units of pressure represent some ratio of force to surface area.

Gases

U N I T S O F P R E SS U R E . The principal SI unit of pressure is called a pascal (Pa), or 1 N/m2. It is named for French mathematician and physicist Blaise Pascal (1623-1662), who is credited with Pascal’s principle. The latter holds that the external pressure applied on a fluid— which, in the physical sciences, can mean either a gas or a liquid—is transmitted uniformly throughout the entire body of that fluid.

A newton (N), the SI unit of force, is equal to the force required to accelerate 1 kg of mass at a rate of 1 m/sec2. Thus a Pascal is the pressure of 1 newton over a surface area of 1 m2. In the English or British system, pressure is measured in terms of pounds per square inch, abbreviated as lbs./in2. This is equal to 6.89 • 103 Pa, or 6,890 Pa.

THE

DIVERS PICTURED HERE HAVE ASCENDED FROM A

SUNKEN SHIP AND HAVE STOPPED AT THE METER)

DECOMPRESSION

LEVEL

TO

10-FT (3-

AVOID

GETTING

DECOMPRESSION SICKNESS, BETTER KNOWN AS THE

“BENDS.” (Jonathan Blair/Corbis. Reproduced by permission)

molecules are random in arrangement, but tend to be more widely spaced than liquid molecules.

Pressure There are a number of statements, collectively known as the “gas laws,” that describe and predict the behavior of gases in response to changes in temperature, pressure, and volume. Temperature and volume are discussed elsewhere in this book. However, the subject of pressure requires some attention before we can continue with a discussion of the gas laws. When a force is applied perpendicular to a surface area, it exerts pressure on that surface. Hence the formula for pressure is p = F/A, where p is pressure, F force, and A the area over which the force is applied. The greater the force, and the smaller the area of application, the greater the pressure; conversely, an increase in area—even without a reduction in force—reduces the overall pressure.

50

Another important measure of pressure is the atmosphere (atm), which is the average pressure exerted by air at sea level. In English units, this is equal to 14.7 lb/in2, and in SI units, to 1.013 • 105 Pa. There are two other specialized units of pressure measurement in the SI system: the bar, equal to 105 Pa, and the torr, equal to 133 Pa. Meteorologists, scientists who study weather patterns, use the millibar (mb), which, as its name implies, is equal to 0.001 bars. At sea level, atmospheric pressure is approximately 1,013 mb. The torr, also known as the millimeter of mercury (mm Hg), is the amount of pressure required to raise a column of mercury (chemical symbol Hg) by 1 mm. It is named for Italian physicist Evangelista Torricelli (1608-1647), who invented the barometer, an instrument for measuring atmospheric pressure. T H E B A R O M E T E R. The barometer constructed by Torricelli in 1643 consisted of a long glass tube filled with mercury. The tube was open at one end, and turned upside down into a dish containing more mercury: the open end was submerged in mercury, while the closed end at the top constituted a vacuum—that is, an area devoid of matter, including air.

Pressure is measured by a number of units in the English and SI systems. Because p = F/A, all

The pressure of the surrounding air pushed down on the surface of the mercury in the bowl, while the vacuum at the top of the tube provided an area of virtually no pressure into which the mercury could rise. Thus the height to which the

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Gases

Air cleaner Carburetor

Ignition coil

Intake manifold Push rod

Cylinder head

Rocker arm Valve spring Exhaust valve

Intake valve

Exhaust manifold

Spark plug Valve lifter Piston pin Piston

Camshaft

Crankshaft Exhaust manifold Starter solenoid Starter motor

Cylinder block Connecting rod Oil pan

Oil intake screen assembly

INTAKE STROKE Exhaust valve closed Exhaust valve

COMPRESSION STROKE

Intake valve open Rocker arm

Exhaust valve closed

Intake valve closed

POWER STROKE Exhaust valve closed

Spark plug

Valve spring

Intake valve

Top-deadcenter (T.D.C.)

Fuel/air mixture from the intake manifold

Intake valve closed

EXHAUST STROKE Exhaust valve open

Intake valve closed

Exhaust to exhaust manifold

Stroke

Piston Bottomdeadcenter (B.D.C.)

Push rod Lifter

THE

MAJOR COMPONENTS OF AN INTERNAL COMBUSTION ENGINE

TION SEQUENCE

(TOP),

AND THE FOUR STROKES OF ITS COMBUS-

(BOTTOM).

mercury rose in the glass tube represented normal air pressure (that is, 1 atm.) Torricelli discovered that at standard atmospheric pressure, the column of mercury rose to 760 mm (29.92 in). The value of 1 atm was thus established as equal to the pressure exerted on a column of mercury 760 mm high at a temperature of 0°C (32°F). In time, Torricelli’s invention became a fixture both of scientific laboratories and of households. Since changes in atmospheric pressure have an effect on weather patterns, many home indoor-outdoor thermometers today also include a barometer.

S C I E N C E O F E V E RY DAY T H I N G S

REAL-LIFE A P P L I C AT I O N S Introduction to the Gas Laws English chemist Robert Boyle (1627-1691), who made a number of important contributions to chemistry—including his definition and identification of elements—seems to have been influenced by Torricelli. If so, this is an interesting example of ideas passing from one great thinker to another: Torricelli, a student of Galileo Galilei (1564-1642), was no doubt influenced by Galileo’s thermoscope. Like Torricelli, Boyle conducted tests involving the introduction of mercury to a tube closed

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at the other end. The tube Boyle used was shaped like the letter J, and it was so long that he had to use the multi-story foyer of his house as a laboratory. At the tip of the curved bottom was an area of trapped gas, and into the top of the tube, Boyle introduced increasing quantities of mercury. He found that the greater the volume of mercury, the greater the pressure on the gas, and the less the volume of gas at the end of the tube. As a result, he formulated the gas law associated with his name. The gas laws are not a set of government regulations concerning use of heating fuel; rather, they are a series of statements concerning the behavior of gases in response to changes in temperature, pressure, and volume. These were derived, beginning with Boyle’s law, during the seventeenth, eighteenth, and nineteenth centuries by scientists whose work is commemorated through the association of their names with the laws they discovered. In addition to Boyle, these men include fellow English chemists John Dalton (1766-1844) and William Henry (17741836); French physicists and chemists J. A. C. Charles (1746-1823) and Joseph Gay-Lussac (1778-1850); and Italian physicist Amedeo Avogadro (1776-1856). There is a close relationship between Boyle’s, Charles’s, and Gay-Lussac’s laws. All of these treat one of three parameters—temperature, pressure, or volume—as fixed quantities in order to explain the relationship between the other two variables. Avogadro’s law treats two of the parameters as fixed, thereby establishing a relationship between volume and the number of molecules in a gas. The ideal gas law sums up these four laws, and the kinetic theory of gases constitutes an attempt to predict the behavior of gases based on these laws. Finally, Dalton’s and Henry’s laws both relate to partial pressure of gases.

Boyle’s, Charles’s, and GayLussac’s Laws BOYLE’S AND CHARLES’S LAW S . Boyle’s law holds that in isothermal

52

In this case, the greater the pressure, the less the volume and vice versa. Therefore, the product of the volume multiplied by the pressure remains constant in all circumstances. Charles’s law also yields a constant, but in this case the temperature and volume are allowed to vary under isobarometric conditions—that is, a situation in which the pressure remains the same. As gas heats up, its volume increases, and when it cools down, its volume reduces accordingly. Hence, Charles established that the ratio of temperature to volume is constant. A B S O LU T E T E M P E RAT U R E . In about 1787, Charles made an interesting discovery: that at 0°C (32°F), the volume of gas at constant pressure drops by 1/273 for every Celsius degree drop in temperature. This seemed to suggest that the gas would simply disappear if cooled to -273°C (-459.4°F), which, of course, made no sense. In any case, the gas would most likely become first a liquid, and then a solid, long before it reached that temperature.

The man who solved the quandary raised by Charles’s discovery was born a year after Charles died. He was William Thomson, Lord Kelvin (1824-1907); in 1848, he put forward the suggestion that it was molecular translational energy—the energy generated by molecules in motion—and not volume, that would become zero at -273°C. He went on to establish what came to be known as the Kelvin scale of absolute temperature. Sometimes known as the absolute temperature scale, the Kelvin scale is based not on the freezing point of water, but on absolute zero— the temperature at which molecular motion comes to a virtual stop. This is -273.15°C (-459.67°F). In the Kelvin scale, which uses neither the term nor the symbol for “degree,” absolute zero is designated as 0K. Scientists prefer the Kelvin scale to the Celsius, and certainly to the Fahrenheit, scales. If the Kelvin temperature of an object is doubled, its average molecular translational energy has doubled as well. The same cannot be said if the temperature were doubled from, say, 10°C to 20°C, or from 40°F to 80°F, since neither the Celsius nor the Fahrenheit scale is based on absolute zero.

conditions (that is, a situation in which temperature is kept constant), an inverse relationship exists between the volume and pressure of a gas. (An inverse relationship is a situation involving two variables, in which one of the two increases in direct proportion to the decrease in the other.)

GAY - LU SSAC ’ S LAW. From Boyle’s and Charles’s law, a pattern should be emerging: both treat one parameter (temperature in Boyle’s, pressure in Charles’s) as unvarying, while

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two other factors are treated as variables. Both, in turn, yield relationships between the two variables: in Boyle’s law, pressure and volume are inversely related, whereas in Charles’s law, temperature and volume are directly related. In Gay-Lussac’s law, a third parameter, volume, is treated as a constant, and the result is a constant ratio between the variables of pressure and temperature. According to Gay-Lussac’s law, the pressure of a gas is directly related to its absolute temperature.

of molecules. The ratio of mass between a mole of Element A and Element B, or Compound A and Compound B, is the same as the ratio between the mass of Atom A and Atom B, or Molecule A and Molecule B. Avogadro’s law describes the connection between gas volume and number of moles. According to Avogadro’s law, if the volume of gas is increased under isothermal and isobarometric conditions, the number of moles also increases. The ratio between volume and number of moles is therefore a constant.

Gases

Avogadro’s Law The Ideal Gas Law Gay-Lussac also discovered that the ratio in which gases combine to form compounds can be expressed in whole numbers: for instance, water is composed of one part oxygen and two parts hydrogen. In the language of modern chemistry, this is expressed as a relationship between molecules and atoms: one molecule of water contains one oxygen atom and two hydrogen atoms. In the early nineteenth century, however, scientists had yet to recognize a meaningful distinction between atoms and molecules, and Avogadro was the first to achieve an understanding of the difference. Intrigued by the whole-number relationship discovered by Gay-Lussac, Avogadro reasoned that one liter of any gas must contain the same number of particles as a liter of another gas. He further maintained that gas consists of particles—which he called molecules—that in turn consist of one or more smaller particles. In order to discuss the behavior of molecules, Avogadro suggested the use of a large quantity as a basic unit, since molecules themselves are very small. Avogadro himself did not calculate the number of molecules that should be used for these comparisons, but when that number was later calculated, it received the name “Avogadro’s number” in honor of the man who introduced the idea of the molecule. Equal to 6.022137 • 1023, Avogadro’s number designates the quantity of atoms or molecules (depending on whether the substance in question is an element or a compound) in a mole.

Once again, it is easy to see how Avogadro’s law can be related to the laws discussed earlier. Like the other three, this one involves the parameters of temperature, pressure, and volume, but it also introduces a fourth—quantity of molecules (that is, number of moles). In fact, all the laws so far described are brought together in what is known as the ideal gas law, sometimes called the combined gas law. The ideal gas law can be stated as a formula, pV = nRT, where p stands for pressure, V for volume, n for number of moles, and T for temperature. R is known as the universal gas constant, a figure equal to 0.0821 atm • liter/mole • K. (Like most figures in chemistry, this one is best expressed in metric rather than English units.) Given the equation pV = nRT and the fact that R is a constant, it is possible to find the value of any one variable—pressure, volume, number of moles, or temperature—as long as one knows the value of the other three. The ideal gas law also makes it possible to discern certain relationships: thus, if a gas is in a relatively cool state, the product of its pressure and volume is proportionately low; and if heated, its pressure and volume product increases correspondingly.

The Kinetic Theory of Gases

Today the mole (abbreviated mol), the SI unit for “amount of substance,” is defined precisely as the number of carbon atoms in 12.01 g of carbon. The term “mole” can be used in the same way we use the word “dozen.” Just as “a dozen” can refer to twelve cakes or twelve chickens, so “mole” always describes the same number

From the preceding gas laws, a set of propositions known collectively as the kinetic theory of gases has been derived. Collectively, these put forth the proposition that a gas consists of numerous molecules, relatively far apart in space, which interact by colliding. These collisions are responsible for the production of thermal energy, because when the velocity of the molecules increases—as it does after collision—the temperature increases as well.

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There are five basic postulates to the kinetic theory of gases: • 1. Gases consist of tiny molecular or atomic particles. • 2. The proportion between the size of these particles and the distances between them is so small that the individual particles can be assumed to have negligible volume. • 3. These particles experience continual random motion. When placed in a container, their collisions with the walls of the container constitute the pressure exerted by the gas. • 4. The particles neither attract nor repel one another. • 5. The average kinetic energy of the particles in a gas is directly related to absolute temperature. These observations may appear to resemble statements made earlier concerning the differences between gases, liquids, and solids in terms of molecular behavior. If so, that is no accident: the kinetic theory constitutes a generally accepted explanation for the reasons why gases behave as they do. Kinetic theories do not work as well for explaining the behaviors of solids and liquids; nonetheless, they do go a long way toward identifying the molecular properties inherent in the various phases of matter.

Laws of Partial Pressure In addition to all the gas laws so far discussed, two laws address the subject of partial pressure. When two or more gases are present in a container, partial pressure is the pressure that one of them exerts if it alone is in the container. Dalton’s law of partial pressure states that the total pressure of a gas is equal to the sum of its partial pressures. As noted earlier, air is composed mostly of nitrogen and oxygen. Along with these are small components, carbon dioxide, and gases collectively known as the rare or noble gases: argon, helium, krypton, neon, radon, and xenon. Hence, the total pressure of a given quantity of air is equal to the sum of the pressures exerted by each of these gases.

54

acid will ionize when introduced to water: one or more of its electrons will be removed, and its atoms will convert to ions, which are either positive or negative in charge.

Applications of Dalton’s and Henry’s Laws PA RT I A L P R E SS U R E : A M AT TER OF LIFE AND POSSIBLE D E AT H F O R S C U B A D I V E R S . The

gas laws are not just a series of abstract statements. Certainly, they do concern the behavior of ideal as opposed to real gases. Like all scientific models, they remove from the equation all outside factors, and treat specific properties in isolation. Yet, the behaviors of the ideal gases described in the gas laws provide a key to understanding the activities of real gases in the real world. For instance, the concept of partial pressure helps scuba divers avoid a possibly fatal sickness. Imagine what would happen if a substance were to bubble out of one’s blood like carbon dioxide bubbling out of a soda can, as described below. This is exactly what can happen to an undersea diver who returns to the surface too quickly: nitrogen rises up within the body, producing decompression sickness—known colloquially as “the bends.” This condition may manifest as itching and other skin problems, joint pain, choking, blindness, seizures, unconsciousness, permanent neurological defects such as paraplegia, and possibly even death. If a scuba diver descending to a depth of 150 ft (45.72 m) or more were to use ordinary air in his or her tanks, the results would be disastrous. The high pressure exerted by the water at such depths creates a high pressure on the air in the tank, meaning a high partial pressure on the nitrogen component in the air. The result would be a high concentration of nitrogen in the blood, and hence the bends. Instead, divers use a mixture of helium and oxygen. Helium gas does not dissolve well in blood, and thus it is safer for a diver to inhale this oxygen-helium mixture. At the same time, the oxygen exerts the same pressure that it would normally—in other words, it operates in accordance with Dalton’s observations concerning partial pressure.

Henry’s law states that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of the gas above the surface of the solution. This applies only to gases such as oxygen and hydrogen that do not react chemically to liquids. On the other hand, hydrochloric

O P E N I N G A S O DA CA N . Inside a can or bottle of carbonated soda is carbon diox-

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ide gas (CO2), most of which is dissolved in the drink itself. But some of it is in the space (sometimes referred to as “head space”) that makes up the difference between the volume of the soft drink and the volume of the container. At the bottling plant, the soda manufacturer adds high-pressure carbon dioxide (CO2) to the head space in order to ensure that more CO2 will be absorbed into the soda itself. This is in accordance with Henry’s law: the amount of gas (in this case CO2) dissolved in the liquid (soda) is directly proportional to the partial pressure of the gas above the surface of the solution—that is, the CO2 in the head space. The higher the pressure of the CO2 in the head space, the greater the amount of CO2 in the drink itself; and the greater the CO2 in the drink, the greater the “fizz” of the soda. Once the container is opened, the pressure in the head space drops dramatically. Once again, Henry’s law indicates that this drop in pressure will be reflected by a corresponding drop in the amount of CO2 dissolved in the soda. Over a period of time, the soda will release that gas, and eventually, it will go “flat.” A fire extinguisher consists of a long cylinder with an operating lever at the top. Inside the cylinder is a tube of carbon dioxide surrounded by a quantity of water, which creates pressure around the CO2 tube. A siphon tube runs vertically along the length of the extinguisher, with one opening in the water near the bottom. The other end opens in a chamber containing a spring mechanism attached to a release valve in the CO2 tube. FIRE

extinguisher, an aerosol can includes a nozzle that depresses a spring mechanism, which in turn allows fluid to escape through a tube. But instead of a gas cartridge surrounded by water, most of the can’s interior is made up of the product (for instance, deodorant), mixed with a liquid propellant.

Gases

The “head space” of the aerosol can is filled with highly pressurized propellant in gas form, and, in accordance with Henry’s law, a corresponding proportion of this propellant is dissolved in the product itself. When the nozzle is depressed, the pressure of the propellant forces the product out through the nozzle. A propellant, as its name implies, propels the product itself through the spray nozzle when the nozzle is depressed. In the past, chlorofluorocarbons (CFCs)—manufactured compounds containing carbon, chlorine, and fluorine atoms— were the most widely used form of propellant. Concerns over the harmful effects of CFCs on the environment, however, has led to the development of alternative propellants, most notably hydrochlorofluorocarbons (HCFCs), CFC-like compounds that also contain hydrogen atoms.

EXTINGUISHERS.

The water and the CO2 do not fill the entire cylinder: as with the soda can, there is “head space,” an area filled with air. When the operating lever is depressed, it activates the spring mechanism, which pierces the release valve at the top of the CO2 tube. When the valve opens, the CO2 spills out in the “head space,” exerting pressure on the water. This high-pressure mixture of water and carbon dioxide goes rushing out of the siphon tube, which was opened when the release valve was depressed. All of this happens, of course, in a fraction of a second—plenty of time to put out the fire.

Applications of Boyle’s, Charles’s, and Gay-Lussac’s Laws WHEN THE T E M P E R AT U R E C H A N G E S . A number of interesting results

occur when gases experience a change in temperature, some of them unfortunate and some potentially lethal. In these instances, it is possible to see the gas laws—particularly Boyle’s and Charles’s—at work. There are numerous examples of the disastrous effects that result from an increase in the temperature of combustible gases, including natural gas and petroleum-based products. In addition, the pressure on the gases in aerosol cans makes the cans highly explosive—so much so that discarded cans at a city dump may explode on a hot summer day. Yet, there are other instances when heating a gas can produce positive effects.

A E R O S O L CA N S . Aerosol cans are similar in structure to fire extinguishers, though with one important difference. As with the fire

A hot-air balloon, for instance, floats because the air inside it is not as dense than the air outside. According to Charles’s law, heating a gas will increase its volume, and since gas molecules exert little attraction toward one another,

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they tend to “spread out” even further with an increase of volume. This, in turn, creates a significant difference in density between the air in the balloon and the air outside, and as a result, the balloon floats. Although heating a gas can be beneficial, cooling a gas is not always a wise idea. If someone were to put a bag of potato chips into a freezer, thinking this would preserve their flavor, he would be in for a disappointment. Much of what maintains the flavor of the chips is the pressurization of the bag, which ensures a consistent internal environment so that preservative chemicals, added during the manufacture of the chips, can keep them fresh. Placing the bag in the freezer causes a reduction in pressure, as per Gay-Lussac’s law, and the bag ends up a limp version of its former self. Propane tanks and tires offer an example of the pitfalls that may occur by either allowing a gas to heat up or cool down by too much. Because most propane tanks are made according to strict regulations, they are generally safe, but it is not entirely inconceivable that the extreme heat of a summer day could cause a defective tank to burst. An increase in temperature leads to an increase in pressure, in accordance with GayLussac’s law, and could lead to an explosion. Because of the connection between heat and pressure, propane trucks on the highways during the summer are subjected to weight tests to ensure that they are not carrying too much gas. On the other hand, a drastic reduction in temperature could result in a loss in gas pressure. If a propane tank from Florida were transported by truck during the winter to northern Canada, the pressure is dramatically reduced by the time it reaches its destination. THE INTERNAL-COMBUSTION E N G I N E . In operating a car, we experience

two applications of the gas laws. One of these is what makes the car run: the combustion of gases in the engine, which illustrates the interrelation of volume, pressure, and temperature expressed in the laws attributed to Boyle, Charles, and GayLussac. The other is, fortunately, a less frequent phenomenon—but it can and does save lives. This is the operation of an airbag, which depends, in part, on the behaviors explained in Charles’s law.

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activates a throttle valve that sprays droplets of gasoline mixed with air into the engine. The mixture goes into the cylinder, where the piston moves up, compressing the gas and air. While the mixture is still at a high pressure, the electric spark plug produces a flash that ignites the gasoline-air mixture. The heat from this controlled explosion increases the volume of air, which forces the piston down into the cylinder. This opens an outlet valve, causing the piston to rise and release exhaust gases. As the piston moves back down again, an inlet valve opens, bringing another burst of gasoline-air mixture into the chamber. The piston, whose downward stroke closed the inlet valve, now shoots back up, compressing the gas and air to repeat the cycle. The reactions of the gasoline and air to changes in pressure, temperature, and volume are what move the piston, which turns a crankshaft that causes the wheels to rotate. T H E A I R B AG . So much for moving— what about stopping? Most modern cars are equipped with an airbag, which reacts to sudden impact by inflating. This protects the driver and front-seat passenger, who, even if they are wearing seatbelts, may otherwise be thrown against the steering wheel or dashboard.

In order to perform its function properly, the airbag must deploy within 40 milliseconds (0.04 seconds) of impact. Not only that, but it has to begin deflating before the body hits it. If a person’s body, moving forward at speeds typical in an automobile accident, were to smash against a fully inflated airbag, it would feel like hitting concrete—with all the expected results. The airbag’s sensor contains a steel ball attached to a permanent magnet or a stiff spring. The spring or magnet holds the ball in place through minor mishaps when an airbag is not warranted—for instance, if a car were simply to be “tapped” by another in a parking lot. But in a case of sudden deceleration, the magnet or spring releases the ball, sending it down a smooth bore. The ball flips a switch, turning on an electrical circuit. This in turn ignites a pellet of sodium azide, which fills the bag with nitrogen gas.

When the driver of a modern, fuel-injection automobile pushes down on the accelerator, this

At this point, the highly pressurized nitrogen gas molecules begin escaping through vents. Thus, as the driver’s or rider’s body hits the airbag, the deflation of the bag is moving it in the same direction that the body is moving—only

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Gases

KEY TERMS Tem-

derived by French physicist and chemist

perature in relation to absolute zero

J. A. C. Charles (1746-1823), which holds

(-273.15°C or -459.67°F), as measured on

that for gases in isobarometric conditions,

the Kelvin scale. The Kelvin and Celsius

the ratio between the volume and temper-

scales are directly related; hence, Celsius

ature of a gas is constant. This means that

temperatures can be converted to Kelvins

the greater the temperature, the greater the

(for which neither the word nor the sym-

volume, and vice versa.

bol for “degree” are used) by adding

DALTON’S LAW OF PARTIAL PRES-

273.15.

SURE:

ABSOLUTE TEMPERATURE:

ATMOSPHERE:

A measure of pres-

sure, abbreviated “atm” and equal to the average pressure exerted by air at sea level. In English units, this is equal to 14.7 lb/in2, and in SI units, to 101,300 pascals.

A statement, derived by the

English chemist John Dalton (1766-1844), which holds that the total pressure of a gas is equal to the sum of its partial pressures—that is, the pressure exerted by each component of the gas mixture. GAS:

A phase of matter in which mole-

A statement,

cules move at high speeds, and therefore

derived by the Italian physicist Amedeo

exert little or no attraction toward one

Avogadro (1776-1856), which holds that as

another.

AVOGADRO’S

LAW:

the volume of gas increases under isothermal and isobarometric conditions, the number of molecules (expressed in terms of mole number), increases as well. Thus, the ratio of volume to mole number is a constant. BAROMETER:

GAS LAWS:

A series of statements

concerning the behavior of gases in response to changes in temperature, pressure, and volume. The gas laws, developed by scientists during the seventeenth, eighteenth, and nineteenth centuries, include

An

instrument

for

measuring atmospheric pressure.

Avogadro’s law, Boyle’s law, Charles’s law, Dalton’s law of partial pressures, Gay-Lussac’s law, and Henry’s law. These are

A statement, derived

summed up in the ideal gas law. The kinet-

by English chemist Robert Boyle (1627-

ic theory of gases is based on observations

1691), which holds that for gases in

garnered from these laws.

BOYLE’S LAW:

isothermal conditions, an inverse relationship exists between the volume and pressure of a gas. This means that the greater the pressure, the less the volume and vice versa, and therefore the product of pressure multiplied by volume yields a constant figure. CHARLES’S

GAY-LUSSAC’S LAW:

A statement,

derived by French physicist and chemist Joseph Gay-Lussac (1778-1850), which holds that the pressure of a gas is directly related to its absolute temperature. Hence, the ratio of pressure to absolute temperature is a constant.

LAW:

A

statement,

S C I E N C E O F E V E RY DAY T H I N G S

HENRY’S LAW:

A statement, derived

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Gases

KEY TERMS

CONTINUED

by English chemist William Henry (1774-

MOLE:

1836), which holds that the amount of gas

“amount of substance.” The quantity of

dissolved in a liquid is directly proportion-

molecules or atoms in a mole is, generally

al to the partial pressure of the gas above

speaking, the same as Avogadro’s number:

the solution. This holds true only for gases,

6.022137 • 1023. However, in the more pre-

such as hydrogen and oxygen, which do

cise SI definition, a mole is equal to the

not react chemically to liquids.

number of carbon atoms in 12.01 g of car-

IDEAL GAS LAW:

A proposition, also

bon. When two or

known as the combined gas law, which

PARTIAL PRESSURE:

draws on all the gas laws. The ideal gas law

more gases are present in a container, par-

can be expressed as the formula pV = nRT,

tial pressure is the pressure that one of

where p stands for pressure, V for volume,

them exerts if it alone is in the container.

n for number of moles, and T for tempera-

Dalton’s law of partial pressure and

ture. R is known as the universal gas con-

Henry’s law relate to the partial pressure of

stant, a figure equal to 0.0821 atm •

gases.

liter/mole • K. ISOTHERMAL:

PASCAL:

Referring to a situa-

tion in which temperature is kept constant. ISOBAROMETRIC:

Referring to a sit-

uation in which pressure is kept constant. KINETIC ENERGY:

The principle SI or metric

unit of pressure, abbreviated “Pa” and equal to 1 N/m2. PRESSURE:

The ratio of force to sur-

face area, when force is applied in a direction perpendicular to that surface.

The energy that

an object possesses by virtue of its motion.

THERMAL ENERGY:

A form of kinet-

ic energy—commonly called “heat”—that

A set

is produced by the movement of atomic or

of propositions describing a gas as consist-

molecular particles. The greater the

ing of numerous molecules, relatively far

motion of these particles relative to one

apart in space, which interact by colliding.

another, the greater the thermal energy.

KINETIC THEORY OF GASES:

These collisions are responsible for the production of thermal energy, because when the velocity of the molecules increases—as it does after collision—the temperature increases as well. MILLIMETER OF MERCURY:

TORR:

An SI unit, also known as the

millimeter of mercury, that represents the pressure required to raise a column of mercury 1 mm. The torr, equal to 133 Pascals, is named for the Italian physicist Evange-

Anoth-

er name for the torr, abbreviated mm Hg.

58

The SI fundamental unit for

lista Torricelli (1608-1647), who invented the barometer.

much, much more slowly. Two seconds after impact, which is an eternity in terms of the processes involved, the pressure inside the bag has returned to 1 atm.

The chemistry of the airbag is particularly interesting. The bag releases inert, or non-reactive, nitrogen gas, which poses no hazard to human life; yet one of the chemical ingredients in

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the airbag is so lethal that some environmentalist groups have begun to raise concerns over its presence in airbags. This is sodium azide (NaN3), one of three compounds—along with potassium nitrate (KNO3) and silicon dioxide (SiO2)—present in an airbag prior to inflation. The sodium azide and potassium nitrate react to one another, producing a burst of hot nitrogen gas in two back-to-back reactions. In the fractions of a second during which this occurs, the airbag becomes like a solid-rocket booster, experiencing a relatively slow detonation known as “deflagration.” The first reaction releases nitrogen gas, which fills the bag, while the second reaction leaves behind the by-products potassium oxide (K2O) and sodium oxide (Na2O). These combine with the silicon dioxide to produce a safe, stable compound known as alkaline silicate. The latter, similar to the sand used for making glass, is all that remains in the airbag after the nitrogen gas has escaped. WHERE TO LEARN MORE “Atmospheric Pressure: The Force Exerted by the Weight of Air” (Web site). (April 7, 2001).

S C I E N C E O F E V E RY DAY T H I N G S

“Chemical Sciences Structure: Structure of Matter: Nature of Gases” University of Alberta Chemistry Department (Web site). (May 12, 2001).

Gases

“Chemistry Units: Gas Laws.” (February 21, 2001). “Homework: Science: Chemistry: Gases” Channelone.com (Web site). (May 12, 2001). “Kinetic Theory of Gases: A Brief Review” University of Virginia Department of Physics (Web site). (April 15, 2001). Laws of Gases. New York: Arno Press, 1981. Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998. Mebane, Robert C. and Thomas R. Rybolt. Air and Other Gases. Illustrations by Anni Matsick. New York: Twenty-First Century Books, 1995. “Tutorials—6.” Chemistrycoach.com (Web site). (February 21, 2001). Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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S C I E N C E O F E V E R Y D AY T H I N G S real-life chemistry

AT O M S AND MOLECULES AT O M S AT O M I C M A S S ELECTRONS ISOTOPES I O N S A N D I O N I Z AT I O N MOLECULES

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Atoms

CONCEPT Our world is made up of atoms, yet the atomic model of the universe is nonetheless considered a “theory.” When scientists know beyond all reasonable doubt that a particular principle is the case, then it is dubbed a law. Laws address the fact that certain things happen, as well as how they happen. A theory, on the other hand, attempts to explain why things happen. By definition, an idea that is dubbed a theory has yet to be fully proven, and such is the case with the atomic theory of matter. After all, the atom cannot be seen, even with electron microscopes—yet its behavior can be studied in terms of its effects. Atomic theory explains a great deal about the universe, including the relationship between chemical elements, and therefore (as with Darwin’s theory concerning biological evolution), it is generally accepted as fact. The particulars of this theory, including the means by which it evolved over the centuries, are as dramatic as any detective story. Nonetheless, much still remains to be explained about the atom—particularly with regard to the smallest items it contains.

HOW IT WORKS Why Study Atoms? Many accounts of the atom begin with a history of the growth in scientists’ understanding of its structure, but here we will take the opposite approach, first discussing the atom in terms of what physicists and chemists today understand. Only then will we examine the many challenges scientists faced in developing the current atomic model: false starts, wrong theories, right roads not taken, incomplete models. In addition, we

S C I E N C E O F E V E R Y D AY T H I N G S

AT O M S

will explore the many insights added along the way as, piece by piece, the evidence concerning atomic behavior began to accumulate. People who are not scientifically trained tend to associate studies of the atom with physics, not chemistry. While it is true that physicists study atomic structure, and that much of what scientists know today about atoms comes from the work of physicists, atomic studies are even more integral to chemistry than to physics. At heart, chemistry is about the interaction of different atomic and molecular structures: their properties, their reactions, and the ways in which they bond. WHAT THE ATOM MEANS TO CHEMISTRY. Just as a writer in English

works with the 26 letters of the alphabet, a chemist works with the 100-plus known elements, the fundamental and indivisible substances of all matter. And what differentiates the elements, ultimately, from one another is not their color or texture, or even the phase of matter—solid, gas, or liquid—in which they are normally found. Rather, the defining characteristic of an element is the atom that forms its basic structure. The number of protons in an atom is the critical factor in differentiating between elements, while the number of neutrons alongside the protons in the nucleus serves to distinguish one isotope from another. However, as important as elements and even isotopes are to the work of a chemist, the components of the atom’s nucleus have little direct bearing on the atomic activity that brings about chemical reactions and chemical bonding. All the chemical “work” of an atom is done by particles vastly smaller in mass than

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element. In fact, these two definitions do not form a closed loop, as they would if it were stated that an element is something made up of atoms. Every item of matter that exists, except for the subatomic particles discussed in this essay, is made up of atoms. An element, on the other hand, is—as stated in its definition—made up of only one kind of atom. “Kind of atom” in this context refers to the number of protons in its nucleus.

Atoms

JOHN DALTON.

either the protons or neutrons—fast-moving little bundles of energy called electrons. Moving rapidly through the space between the nucleus and the edge of the atom, electrons sometimes become dislodged, causing the atom to become a positively charged ion. Conversely, sometimes an atom takes on one or more electrons, thus acquiring a negative charge. Ions are critical to the formation of some kinds of chemical bonds, but the chemical role of the electron is not limited to ionic bonds. In fact, what defines an atom’s ability to bond with another atom, and therefore to form a molecule, is the specific configuration of its electrons. Furthermore, chemical reactions are the result of changes in the arrangement of electrons, not of any activity involving protons or neutrons. So important are electrons to the interactions studied in chemistry that a separate essay has been devoted to them.

What an Atom Is BASIC ATOMIC STRUCTURE. The definitions of atoms and elements seems, at first glance, almost circular: an element is a substance made up of only one kind of atom, and an atom is the smallest particle of an element that retains all the chemical and physical properties of the

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Protons are one of three basic subatomic particles, the other two being electrons and neutrons. As we shall see, there appear to be particles even smaller than these, but before approaching these “sub-subatomic” particles, it is necessary to address the three most significant components of an atom. These are distinguished from one another in terms of electric charge: protons are positively charged, electrons are negative in charge, and neutrons have no electrical charge. As with the north and south poles of magnets, positive and negative charges attract one another, whereas like charges repel. Atoms have no net charge, meaning that the protons and electrons cancel out one another. EVOLVING MODELS OF THE ATOM. Scientists originally thought of an atom

as a sort of closed sphere with a relatively hard shell, rather like a ball bearing. Nor did they initially understand that atoms themselves are divisible, consisting of the parts named above. Even as awareness of these three parts emerged in the last years of the nineteenth century and the first part of the twentieth, it was not at all clear how they fit together. At one point, scientists believed that electrons floated in a cloud of positive charges. This was before the discovery of the nucleus, where the protons and neutrons reside at the heart of the atom. It then became clear that electrons were moving around the nucleus, but how? For a time, a planetary model seemed appropriate: in other words, electrons revolved around the nucleus much as planets orbit the Sun. Eventually, however—as is often the case with scientific discovery—this model became unworkable, and had to be replaced by another. The model of electron behavior accepted today depicts the electrons as forming a cloud around the nucleus—almost exactly the opposite of what physicists believed a century ago. The use of the term “cloud” may perhaps be a bit mis-

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leading, implying as it does something that simply hovers. In fact, the electron, under normal circumstances, is constantly moving. The paths of its movement around the nucleus are nothing like that of a planet’s orbit, except inasmuch as both models describe a relatively small object moving around a relatively large one.

Atoms

The furthest edges of the electron’s movement define the outer perimeters of the atom. Rather than being a hard-shelled little nugget of matter, an atom—to restate the metaphor mentioned above—is a cloud of electrons surrounding a nucleus. Its perimeters are thus not sharply delineated, just as there is no distinct barrier between Earth’s atmosphere and space itself. Just as the air gets thinner the higher one goes, so it is with an atom: the further a point is from the nucleus, the less the likelihood that an electron will pass that point on a given orbital path.

Nucleons NIELS BOHR.

MASS NUMBER AND ATOMIC NUMBER. The term nucleon is used generi-

cally to describe the relatively heavy particles that make up an atomic nucleus. Just as “sport” can refer to football, basketball, or baseball, or any other item in a similar class, such as soccer or tennis, “nucleon” refers to protons and neutrons. The sum of protons and neutrons is sometimes called the nucleon number, although a more commonly used term is mass number. Though the electron is the agent of chemical reactions and bonding, it is the number of protons in the nucleus that defines an atom as to its element. Atoms of the same element always have the same number of protons, and since this figure is unique for a given element, each element is assigned an atomic number equal to the number of protons in its nucleus. The atoms are listed in order of atomic number on the periodic table of elements. ATOMIC MASS AND ISOTOPES.

A proton has a mass of 1.673 • 10–24 g, which is very close to the established figure for measuring atomic mass, the atomic mass unit. At one time, the basic unit of atomic mass was equal to the mass of one hydrogen atom, but hydrogen is so reactive—that is, it tends to combine readily with other atoms to form a molecule, and hence a compound—that it is difficult to isolate. Instead, the atomic mass unit is today defined as 1/12 of

S C I E N C E O F E V E R Y D AY T H I N G S

the mass of a carbon-12 atom. That figure is exactly 1.66053873 • 10–24 grams. The mention of carbon-12, a substance found in all living things, brings up the subject of isotopes. The “12” in carbon-12 refers to its mass number, or the sum of protons and neutrons. Two atoms may be of the same element, and thus have the same number of protons, yet differ in their number of neutrons—which means a difference both in mass number and atomic mass. Such differing atoms of the same element are called isotopes. Isotopes are often designated by symbols showing mass number to the upper left of the chemical symbol or element symbol—for instance, 12C for carbon-12. ELECTRIC CHARGE. Protons have a positive electric charge of 1, designated either as 1+ or +1. Neutrons, on the other hand, have no electric charge. It appears that the 1+ charge of a proton and the 0 charge of a neutron are the products of electric charges on the part of even smaller particles called quarks. A quark may either have a positive electric charge of less than 1+, in which case it is called an “up quark”; or a negative charge of less than 1–, in which case it is called a “down quark.”

Research indicates that a proton contains two up quarks, each with a charge of 2/3+, and

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Atoms

THE

EVOLUTION OF ATOMIC THEORY.

one down quark with a charge of 1/3–. This results in a net charge of 1+. On the other hand, a neutron is believed to hold one up quark with a charge of 2/3+, and two down quarks with charges of 1/3– each. Thus, in the neutron, the up and down quarks cancel out one another, and the net charge is zero. A neutron has about the same mass as a proton, but other than its role in forming isotopes, the neutron’s function is not exactly clear. Perhaps, it has been speculated, it binds protons— which, due to their positive charges, tend to repel one another—together at the nucleus.

Electrons An electron is much smaller than a proton or neutron, and has much less mass; in fact, its mass is equal to 1/1836 that of a proton, and 1/1839 that of a neutron. Yet the area occupied by electrons—the region through which they move— constitutes most of the atom’s volume. If the nucleus of an atom were the size of a BB (which, in fact, is billions of times larger than a nucleus), the furthest edge of the atom would be equivalent to the highest ring of seats around an indoor sports arena. Imagine the electrons as incredibly fast-moving insects buzzing constantly through the arena, passing by the BB but then flitting to

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the edges or points in between, and you have something approaching an image of the atom’s interior. How fast does an electron move? Speeds vary depending on a number of factors, but it can move nearly as fast as light: 186,000 mi (299,339 km) per second. On the other hand, for an item of matter near absolute zero in temperature, the velocity of the electron is much, much less. In any case, given the fact that an electron has enough negative charge to cancel out that of the proton, it must be highly energized. After all, this would be like an electric generator weighing 1 lb having as much power as a generator that weighed 1 ton. According to what modern scientists know or hypothesize concerning the inner structure of the atom, electrons are not made up of quarks; rather, they are part of a class of particles called leptons. It appears that leptons, along with quarks and what are called exchange particles, constitute the elementary particles of atoms— particles on a much more fundamental level than that of the proton and neutron. Electrons are perhaps the most intriguing parts of an atom. Their mass is tiny, even in atomic terms, yet they possess enough charge to counteract a “huge” proton. They are capable, in certain situations, of moving from one atom to

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another, thus creating ions, and depending on their highly complex configuration and ability to rearrange their configuration, they facilitate or prevent chemical reactions.

REAL-LIFE AP P LICATIONS Ancient Greek Theories of Matter The first of the Greek philosophers, and the first individual in Western history who deserves to be called a scientist, was Thales (c. 625-c. 547 B.C.) of Miletus. (Miletus is in Greek Asia Minor, now part of Turkey.) Among his many achievements were the correct prediction of a solar eclipse, and one of the first-ever observations of electricity, when he noted the electrification of amber by friction. But perhaps the greatest of Thales’s legacies was his statement that “Everything is water.” This represented the first attempt to characterize the nature of all physical reality. It set off a debate concerning the fundamental nature of matter that consumed Greek philosophers for two centuries. Later, philosophers attempted to characterize matter in terms of fire or air. In time, however, there emerged a school of thought concerned not with identifying matter as one particular thing or another, but with recognizing a structural consistency in all of matter. Among these were the philosophers Leucippus (c. 480-c. 420 B.C.) and his student Democritus (c. 460-370 B.C.) DEMOCRITUS’S “ATOMS”. Leucippus and Democritus proposed a new and highly advanced model for the tiniest point of physical space. Democritus, who actually articulated these ideas (far less is known about Leucippus) began with a “thought experiment,” imagining what would happen if an item of matter were subdivided down to its smallest piece. This tiniest fragment, representing an item of matter that could not be cut into smaller pieces, he called by a Greek term meaning “no cut”: atomos.

Democritus was not necessarily describing matter in a concrete, scientific way: his “atoms” were idealized philosophical constructs rather than purely physical units. Yet, he came amazingly close, and indeed much closer than any thinker for the next 22 centuries, to identifying the fun-

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damental structure of physical reality. Why did it take so long for scientists to come back around to the atomic model? The principal culprit, who advanced an erroneous theory of matter, also happened to be one of the greatest thinkers of all time: Aristotle (384-322 B.C..) ARISTOTLE’S

Atoms

“ELEMENTS”.

Aristotle made numerous contributions to science, including his studies in botany and zoology, as well as his explanation of the four causes, a significant attempt to explain events by means other than myth or superstition. In the area of the physical sciences, however, Aristotle’s impact was less than beneficial. Most notably, in explaining why objects fall when dropped, he claimed that the ground was their “natural” destination— a fallacy later overturned with the gravitational model developed by Galileo Galilei (1564-1642) and Sir Isaac Newton (1642-1727). The ideas Aristotle put forward concerning what he called “natural motion” were a product of his equally faulty theories with regard to what today’s scientists refer to as chemistry. In ancient times, chemistry, as such, did not exist. Long before Aristotle’s time, Egyptian embalmers and metallurgists used chemical processes, but they did so in a practical, applied manner, exerting little effort toward what could be described as scientific theory. Philosophers such as Aristotle, who were some of the first scientists, made little distinction between physical and chemical processes. Thus, whereas physics is understood today as an important background for chemistry, Aristotle’s “physics” was actually an outgrowth of his “chemistry.” Rejecting Democritus’s atomic model, Aristotle put forward his own view of matter. Like Democritus, he believed that matter was composed of very small components, but these he identified not as atoms, but as “elements”: earth, air, fire, and water. He maintained that all objects consisted, in varying degrees, of one or more of these, and based his explanation of gravity on the relative weights of each element. Water sits on top of the earth, he explained, because it is lighter, yet air floats above the water because it is lighter still—and fire, lightest of all, rises highest. Furthermore, he claimed that the planets beyond Earth were made up of a “fifth element,” or quintessence, of which little could be known. In fairness to Aristotle, it should be pointed out that it was not his fault that science all but

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died out in the Western world during the period from about A.D. 200 to about 1200. Furthermore, he did offer an accurate definition of an element, in a general sense, as “one of those simple bodies into which other bodies can be decomposed, and which itself is not capable of being divided into others.” As we shall see, the definition used today is not very different from Aristotle’s. However, to define an element scientifically, as modern chemists do, it is necessary to refer to something Aristotle rejected: the atom. So great was his opposition to Democritus’s atomic theory, and so enormous was Aristotle’s influence on learning for more than 1,500 years following his death, that scientists only began to reconsider atomic theory in the late eighteenth century.

noted that elements always react with one another in the same proportions.

A Maturing Concept of Elements

of Lavoisier and Proust influenced a critical figure in the development of the atomic model: English chemist John Dalton (1766-1844). In A New System of Chemical Philosophy (1808), Dalton put forward the idea that nature is composed of tiny particles, and in so doing he adopted Democritus’s word “atom” to describe these basic units. This idea, which Dalton had formulated five years earlier, marked the starting-point of modern atomic theory.

BOYLE’S IDEA OF ELEMENTS.

One of the first steps toward an understanding of the chemical elements came with the work of English physicist and chemist Robert Boyle (1627-1691). Building on the usable definition of an element provided by Aristotle, Boyle maintained no substance was an element if it could be broken down into other substances. Thus, air could be eliminated from the list of “elements,” because, clearly, it could be separated into more than one elemental substance. (In fact, none of the four “elements” identified by Aristotle even remotely qualifies as an element in modern chemistry.) Boyle, nonetheless, still clung to aspects of alchemy, a pseudo-science based on the transformation of “base metals,” for example, the metamorphosis of iron into gold. Though true chemistry grew out of alchemy, the fundamental proposition of alchemy was faulty: if one metal can be turned into another, then that means that metals are not elements, which, in fact, they are. Nonetheless, Boyle’s studies led to the identification of numerous elements—that is, items that really are elements—in the years that followed. LAVOISIER AND PROUST: CONSTANT COMPOSITION. A few years after

Boyle came two French chemists who extended scientific understanding of the elements. Antoine Lavoisier (1743-1794) affirmed the definition of an element as a simple substance that could not be broken down into a simpler substance, and

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Joseph-Louis Proust (1754-1826) put forward the law of constant composition, which holds that a given compound always contains the same proportions of mass between elements. Another chemist of the era had claimed that the composition of a compound varies in accordance with the reactants used to produce it. Proust’s law of constant composition made it clear that any particular compound will always have the same composition.

Early Modern Understanding of the Atom DALTON AND AVOGADRO: ATOMS AND MOLECULES. The work

Dalton recognized that the structure of atoms in a particular element or compound is uniform, but maintained that compounds are made up of compound atoms: in other words, water, for instance, is a compound of “water atoms.” However, water is not an element, and thus, it was necessary to think of its atomic composition in a different way—in terms of molecules rather than atoms. Dalton’s contemporary Amedeo Avogadro (1776-1856), an Italian physicist, became the first scientist to clarify the distinction between atoms and molecules. The later development of the mole, which provided a means whereby equal numbers of molecules could be compared, paid tribute to Avogadro by designating the number of molecules in a mole as “Avogadro’s number.” Another contemporary, Swedish chemist Jons Berzelius (1779-1848), maintained that equal volumes of gases at the same temperature and pressure contained equal numbers of atoms. Using this idea, he compared the mass of various reacting gases, and developed a system of comparing the mass of various atoms in relation to the lightest one, hydrogen. Berzelius also introduced the system

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of chemical symbols—H for hydrogen, O for oxygen, and so on—in use today. BROWNIAN MOTION AND KINETIC THEORY. Yet another figure

whose dates overlapped with those of Dalton, Avogadro, and Berzelius was Scottish botanist Robert Brown (1773-1858). In 1827, Brown noted a phenomenon that later had an enormous impact on the understanding of the atom. While studying pollen grains under a microscope, Brown noticed that the grains underwent a curious zigzagging motion in the water. The pollen assumed the shape of a colloid, a pattern that occurs when particles of one substance are dispersed—but not dissolved—in another substance. At first, Brown assumed that the motion had a biological explanation—that is, it resulted from life processes within the pollen—but later, he discovered that even pollen from long-dead plants behaved in the same way. Brown never understood what he was witnessing. Nor did a number of other scientists, who began noticing other examples of what came to be known as Brownian motion: the constant but irregular zigzagging of colloidal particles, which can be seen clearly through a microscope. Later, however, Scottish physicist James Clerk Maxwell (1831-1879) and others were able to explain this phenomenon by what came to be known as the kinetic theory of matter. Kinetic theory is based on the idea that molecules are constantly in motion: hence, the water molecules were moving the pollen grains Brown observed. Pollen grains are many thousands of times as large as water molecules, but since there are so many molecules in even a drop of water, and their motion is so constant but apparently random, they are bound to move a pollen grain once every few thousand collisions.

Mendeleev and the Periodic Table In 1869, Russian chemist Dmitri Mendeleev (1834-1907) introduced a highly useful system for organizing the elements, the periodic table. Mendeleev’s table is far more than just a handy chart listing elements: at once simple and highly complex, it shows elements in order of increasing atomic mass, and groups together those exhibiting similar forms of chemical behavior and structure.

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Reading from right to left and top to bottom, the periodic table, as it is configured today, lists atoms in order of atomic number, generally reflected by a corresponding increase in average atomic mass. As Mendeleev observed, every eighth element on the chart exhibits similar characteristics, and thus the chart is organized in columns representing specific groups of elements.

Atoms

The patterns Mendeleev observed were so regular that for any “hole” in his table, he predicted that an element would be discovered that would fill that space. For instance, at one point there was a gap between atomic numbers 71 and 73 (lutetium and tantalum, respectively). Mendeleev indicated that an atom would be found for the space, and 15 years after this prediction, the element germanium was isolated. However, much of what defines an element’s place on the chart today relates to subatomic particles—protons, which determine atomic number, and electrons, whose configurations explain certain chemical similarities. Mendeleev was unaware of these particles: from the time he created his table, it was another three decades before the discovery of the first of these particles, the electron. Instead, he listed the elements in an order reflecting outward characteristics now understood to be the result of the quantity and distribution of protons and electrons.

Electromagnetism and Radiation The contribution of Mendeleev’s contemporary, Maxwell, to the understanding of the atom was not limited to his kinetic theory. Building on the work of British physicist and chemist Michael Faraday (1791-1867) and others, in 1865 he published a paper outlining a theory of a fundamental interaction between electricity and magnetism. The electromagnetic interaction, as it later turned out, explained something that gravitation, the only other form of fundamental interaction known at the time, could not: the force that held together particles in an atom. The idea of subatomic particles was still a long time in coming, but the model of electromagnetism helped make it possible. In the long run, electromagnetism was understood to encompass a whole spectrum of energy radiation, including radio waves; infrared, visible, and ultraviolet light; x rays; and gamma rays. But this,

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too, was the product of work on the part of numerous individuals, among whom was English physicist William Crookes (1832-1919).

electrons were like raisins, floating in a positively charged “pudding”—that is, an undifferentiated cloud of positive charges.

In the 1870s, Crookes developed an apparatus later termed a Crookes tube, with which he sought to analyze the “rays”—that is, radiation— emitted by metals. The tube consisted of a glass bulb, from which most of the air had been removed, encased between two metal plates or electrodes, referred to as a cathode and an anode. A wire led outside the bulb to an electric source, and when electricity was applied to the electrodes, the cathodes emitted rays. Crookes concluded that the cathode rays were particles with a negative electric charge that came from the metal in the cathode plate.

Kelvin’s temperature scale contributed greatly to the understanding of molecular motion as encompassed in the kinetic theory of matter. However, his model for the distribution of charges in an atom—charming as it may have been—was incorrect. Nonetheless, for several decades, the “plum pudding model,” as it came to be known, remained the most widely accepted depiction of the way that electric charges were distributed in an atom. The overturning of the plum pudding model was the work of English physicist Ernest Rutherford (1871-1937), a student of J. J. Thomson.

RADIATION. In 1895, German physicist Wilhelm Röntgen (1845-1923) noticed that photographic plates held near a Crookes tube became fogged, and dubbed the rays that had caused the fogging “x rays.” A year after Röntgen’s discovery, French physicist Henri Becquerel (1852-1908) left some photographic plates in a drawer with a sample of uranium. Uranium had been discovered more than a century before; however, there were few uses for it until Becquerel discovered that the uranium likewise caused a fogging of the photographic plates.

RUTHERFORD IDENTIFIES THE NUCLEUS. Rutherford did not set out to dis-

Thus radioactivity, a type of radiation brought about by atoms that experience radioactive decay was discovered. The term was coined by Polish-French physicist and chemist Marie Curie (1867-1934), who with her husband Pierre (1859-1906), a French physicist, was responsible for the discovery of several radioactive elements.

The Rise and Fall of the Plum Pudding Model Working with a Crookes tube, English physicist J. J. Thomson (1856-1940) hypothesized that the negatively charged particles Crookes had observed were being emitted by atoms, and in 1897, he gave a name to these particles: electrons. The discovery of the electron raised a new question: if Thomson’s particles exerted a negative charge, from whence did the counterbalancing positive charge come? An answer, of sorts, came from William Thomson, not related to the other Thomson and, in any case, better known by his title as Lord Kelvin (1824-1907). Kelvin compared the structure of an atom to an English plum pudding: the

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prove the plum pudding model; rather, he was conducting tests to find materials that would block radiation from reaching a photographic plate. The two materials he identified, which were, respectively, positive and negative in electric charge, he dubbed alpha and beta particles. (An alpha particle is a helium nucleus stripped of its electrons, such that it has a positive charge of 2; beta particles are either electrons or positively charged subatomic particles called positrons. The beta particle Rutherford studied was an electron emitted during radioactive decay.) Using a piece of thin gold foil with photographic plates encircling it, Rutherford bombarded the foil with alpha particles. Most of the alpha particles went straight through the foil—as they should, according to the plum pudding model. However, a few particles were deflected from their course, and some even bounced back. Rutherford later said it was as though he had fired a gun at a piece of tissue paper, only to see the tissue deflect the bullets. Analyzing these results, Rutherford concluded that there was no “pudding” of positive charges: instead, the atom had a positively charged nucleus at its center.

The Nucleus Emerges PROTONS AND ISOTOPES. In addition to defining the nucleus, Rutherford also gave a name to the particles that imparted its positive charge: protons. But just as the identification of the electron had raised new questions that, in being answered, led to the discovery of the proton, Rutherford’s achievement only

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brought up new anomalies concerning the behavior of the nucleus. Together with English chemist Frederick Soddy (1877-1956), Rutherford discovered that when an atom emitted alpha or beta particles, its atomic mass changed. Soddy had a name for atoms that displayed this type of behavior: isotopes. Certain types of isotopes, Soddy and Rutherford went on to conclude, had a tendency to decay, moving toward stabilization, and this decay explained radioactivity. CLARIFYING THE PERIODIC TABLE. Soddy concluded that atomic mass, as

measured by Berzelius, was actually an average of the mass figures for all isotopes within that element. This explained a problem with Mendeleev’s periodic table, in which there seemed to be irregularities in the increase of atomic mass from element to element. The answer to these variations in mass, it turned out, related to the number of isotopes associated with a given element: the greater the number of isotopes, the more these affected the overall measure of the element’s mass. By this point, physicists and chemists had come to understand that various levels of energy in matter emitted specific electromagnetic wavelengths. Welsh physicist Henry Moseley (18871915) experimented with x rays, bombarding atoms of different elements with high levels of energy and observing the light they gave off as they cooled. In the course of these tests, he uncovered an astounding mathematical relationship: the amount of energy a given element emitted was related to its atomic number. Furthermore, the atomic number corresponded to the number of positive charges—this was in 1913, before Rutherford had named the proton—in the nucleus. Mendeleev had been able to predict the discovery of new elements, but such predictions had remained problematic. When scientists understood the idea of atomic number, however, it became possible to predict the existence of undiscovered elements with much greater accuracy. NEUTRONS. Yet again, discoveries—the nucleus, protons, and the relationship between these and atomic number—only created new questions. (This, indeed, is one of the hallmarks of an active scientific theory. Rather than settling questions, science is about raising new ones, and thus improving the quality of the questions that

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are asked.) Once Rutherford had identified the proton, and Moseley had established the number of protons, the mystery at the heart of the atom only grew deeper.

Atoms

Scientists had found that the measured mass of atoms could not be accounted for by the number of protons they contained. Certainly, the electrons had little to do with atomic mass: by then it had been shown that the electron weighed about 0.06% as much as a proton. Yet for all elements other than protium (the first of three hydrogen isotopes), there was a discrepancy between atomic mass and atomic number. Clearly, there had to be something else inside the nucleus. In 1932, English physicist James Chadwick (1891-1974) identified that “something else.” Working with radioactive material, he found that a certain type of subatomic particle could penetrate lead. All other known types of radiation were stopped by the lead, and therefore, Chadwick reasoned that this particle must be neutral in charge. In 1932, he won the Nobel Prize in Physics for his discovery of the neutron.

The Nuclear Explosion ISOTOPES AND RADIOACTIVITY. Chadwick’s discovery clarified another mys-

tery, that of the isotope, which had been raised by Rutherford and Soddy several decades earlier. Obviously, the number of protons in a nucleus did not change, but until the identification of the neutron, it had not been clear what it was that did change. At that point, it was understood that two atoms may have the same atomic number— and hence be of the same element—yet they may differ in number of neutrons, and thus be isotopes. As the image of what an isotope was became clearer, so too did scientists’ comprehension of radioactivity. Radioactivity, it was discovered, was most intense where an isotope was the most unstable—that is, in cases where an isotope had the greatest tendency to experience decay. Uranium had a number of radioactive isotopes, such as 235U, and these found application in the burgeoning realm of nuclear power—both the destructive power of atomic bombs, and later the constructive power of nuclear energy plants. FISSION VS. FUSION. In nuclear fission, or the splitting of atoms, uranium isotopes (or other radioactive isotopes) are bom-

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barded with neutrons, splitting the uranium nucleus in half and releasing huge amounts of energy. As the nucleus is halved, it emits several extra neutrons, which spin off and split more uranium nuclei, creating still more energy and setting off a chain reaction. This explains the destructive power in an atomic bomb, as well as the constructive power—providing energy to homes and businesses—in a nuclear power plant. Whereas the chain reaction in an atomic bomb becomes an uncontrolled explosion, in a nuclear plant the reaction is slowed and controlled. Yet nuclear fission is not the most powerful form of atomic reaction. As soon as scientists realized that it was possible to force particles out of a nucleus, they began to wonder if particles could be forced into the nucleus. This type of reaction, known as fusion, puts even nuclear fission, with its awesome capabilities, to shame: nuclear fusion is, after all, the power of the Sun. On the surface of that great star, hydrogen atoms reach incredible temperatures, and their nuclei fuse to create helium. In other words, one element actually transforms into another, releasing enormous amounts of energy in the process. NUCLEAR ENERGY IN WAR AND PEACE. The atomic bombs dropped by the

United States on Japan in 1945 were fission bombs. These were the creation of a group of scientists—legendary figures such as American physicist J. Robert Oppenheimer (1904-1967), American mathematician John von Neumann (1903-1957), American physicist Edward Teller (1908-), and Italian physicist Enrico Fermi (1901-1954)—involved in the Manhattan Project at Las Alamos, New Mexico. Some of these geniuses, particularly Oppenheimer, were ambivalent about the moral implications of the enormous destructive power they created. However, most military historians believe that far more lives—both Japanese and American—would have been lost if America had been forced to conduct a land invasion of Japan. As it was, the Japanese surrendered shortly after the cities of Hiroshima and Nagasaki suffered the devastating effects of fission-based explosions. By 1952, U.S. scientists had developed a “hydrogen,” or fusion bomb, thus raising the stakes greatly. This was a bomb that possessed far more destructive capability than the ones dropped over Japan. Fortunately, the Hiroshima and Nagasaki bombs were the only ones dropped

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in wartime, and a ban on atmospheric nuclear testing has greatly reduced the chances of human exposure to nuclear fallout of any kind. With the end of the arms race between the United States and the Soviet Union, the threat of nuclear destruction has receded somewhat—though it will perhaps always be a part of human life. Nonetheless, fear of nuclear power, spawned as a result of the arms race, continues to cloud the future of nuclear plants that generate electricity—even though these, in fact, emit less radioactive pollution than coal- or gas-burning power plants. At the same time, scientists continue to work on developing a process of power generation by means of nuclear fusion, which, if and when it is achieved, will be one of the great miracles of science. PARTICLE ACCELERATORS. One of the tools used by scientists researching nuclear fusion is the particle accelerator, which moves streams of charged particles—protons, for instance—faster and faster. These fast particles are then aimed at a thin plate composed of a light element, such as lithium. If the proton manages to be “captured” in the nucleus of a lithium atom, the resulting nucleus is unstable, and breaks into alpha particles.

This method of induced radioactivity is among the most oft- used means of studying nuclear structure and subatomic particles. In 1932, the same year that Chadwick discovered the neutron, English physicist John D. Cockcroft (1897-1967) and Irish physicist Ernest Walton (1903-1995) built the first particle accelerator. Some particle accelerators today race the particles in long straight lines or, to save space, in ringed paths several miles in diameter.

Quantum Theory and Beyond THE CONTRIBUTION OF RELATIVITY. It may seem strange that in this lengthy

(though, in fact, quite abbreviated!) overview of developments in understanding of the atom, no mention has been made of the figure most associated with the atom in the popular mind: German-American physicist Albert Einstein (18791955). The reasons for this are several. Einstein’s relativity theory addresses physical, rather than chemical, processes, and did not directly contribute to enhanced understanding of atomic structure or elements. The heart of relativity the-

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ory is the famous formula E = mc2, which means that every item of matter possesses energy proportional to its mass multiplied by the squared speed of light.

of the electron; but before this could be fully achieved, scientists had to develop a new understanding of the way electrons move around the nucleus.

The value of mc2, of course, is an enormous amount of energy, and in order to be released in significant quantities, an article of matter must experience the kinetic energy associated with very, very high speeds—speeds close to that of light. Obviously, the easiest thing to accelerate to such a speed is an atom, and hence, nuclear energy is a result of Einstein’s famous equation. Nonetheless, it should be stressed that although Einstein is associated with unlocking the power of the atom, he did little to explain what atoms are.

BOHR’S PLANETARY MODEL OF THE ATOM. As was often the case in the his-

However, in the course of developing his relativity theory in 1905, Einstein put to rest a question about atoms and molecules that still remained unsettled after more than a century. Einstein’s analysis of Brownian motion, combined with the confirmation of his results by French physicist Jean Baptiste Perrin (18701942), showed conclusively that yes, atoms and molecules do exist. It may seem amazing that as recently as 1905, this was still in doubt; however, this only serves to illustrate the arduous path scientists must tread in developing a theory that accurately explains the world. PLANCK’S QUANTUM THEORY.

A figure whose name deserves to be as much a household word as Einstein’s—though it is not— is German physicist Max Planck (1858-1947). It was Planck who initiated the quantum theory that Einstein developed further, a theory that prevails today in the physical sciences. At the atomic level, Planck showed, energy is emitted in tiny packets or “quanta.” Each of these energy packets is indivisible, and the behavior of quanta redefine the old rules of physics handed down from Newton and Maxwell. Thus, it is Planck’s quantum theory, rather than Einstein’s relativity, that truly marks the watershed, or “before and after,” between classical physics and modern physics. Quantum theory is important not only to physics, but to chemistry as well. It helps to explain the energy levels of electrons, which are not continuous, as in a spectrum, but jump between certain discrete points. The quantum model is now also applied to the overall behavior

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Atoms

tory of the atom, a man otherwise respected as a great scientist put forward a theory of atomic structure that at first seemed convincing, but ultimately turned out to be inaccurate. In this case, it was Danish physicist Niels Bohr (18851962), a seminal figure in the development of nuclear fission. Using the observation, derived from quantum theory, that electrons only occupied specific energy levels, Bohr hypothesized that electrons orbited around a nucleus in the same way that planets orbit the Sun. There is no reason to believe that Bohr formed this hypothesis for any sentimental reasons—though, of course, scientists are just as capable of prejudice as anyone. His work was based on his studies; nonetheless, it is easy to see how this model seemed appealing, showing as it did an order at the subatomic level reflecting an order in the heavens. ELECTRON CLOUDS. Many people

today who are not scientifically trained continue to think that an atom is structured much like the Solar System. This image is reinforced by symbolism, inherited from the 1950s, that represents “nuclear power” by showing a dot (the nucleus) surrounded by ovals at angles to one another, representing the orbital paths of electrons. However, by the 1950s, this model of the atom had already been overturned. In 1923, French physicist Louis de Broglie (1892-1987) introduced the particle-wave hypothesis, which indicated that electrons could sometimes have the properties of waves—an eventuality not encompassed in the Bohr model. It became clear that though Bohr was correct in maintaining that electrons occupy specific energy levels, his planetary model was inadequate for explaining the behavior of electrons. Two years later, in 1925, German physicist Werner Heisenberg (1901-1976) introduced what came to be known as the Heisenberg Uncertainty Principle, showing that the precise position and speed of an electron cannot be known at the same time. Austrian physicist Erwin Schrödinger (1887-1961) developed an

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Atoms

KEY TERMS The smallest particle of an ele-

ELECTRON: Negatively charged parti-

ment that retains all the chemical and

cles in an atom. Electrons, which spin

physical properties of the element. An

around the protons and neutrons that

atom can exist either alone or in combina-

make up the atom’s nucleus, constitute a

tion with other atoms in a molecule. Atoms

very small portion of the atom’s mass. The

are made up of protons, neutrons, and

number of electrons and protons is the

electrons.

same, thus canceling out one another; on

ATOM:

ATOMIC

MASS

UNIT:

An SI unit

(abbreviated amu), equal to 1.66 • 10-24 g,

electrons, it becomes an ion.

for measuring the mass of atoms.

ELEMENT SYMBOL: Another term for

ATOMIC

NUMBER:

The number of

chemical symbol. An atom or atoms that has lost or

protons in the nucleus of an atom. Since

ION:

this number is different for each element,

gained one or more electrons, and thus has

elements are listed on the periodic table of

a net electric charge.

elements in order of atomic number.

ISOTOPES: Atoms that have an equal

AVERAGE ATOMIC MASS: A figure

number of protons, and hence are of the

used by chemists to specify the mass—in

same element, but differ in their number of

atomic mass units—of the average atom in

neutrons.

a large sample.

MASS NUMBER: The sum of protons

CHEMICAL SYMBOL: A one- or two-

and neutrons in an atom’s nucleus.

letter abbreviation for the name of an

MOLECULE: A group of atoms, usually

element.

(but not always) representing more A substance made up of

than one element, joined in a structure.

atoms of more than one element. These

Compounds are typically made up of

atoms are usually joined in molecules.

molecules.

COMPOUND:

equation for calculating how an electron with a certain energy moves, identifying regions in an atom where an electron possessing a certain energy level is likely to be. Schrödinger’s equation cannot, however, identify the location exactly. Rather than being called orbits, which suggest the orderly pattern of Bohr’s model, Schrödinger’s regions of probability are called orbitals. Moving within these orbitals, electrons describe the shape of a cloud, as discussed much earlier in this essay; as a result, the “electron cloud” theory prevails today. This theory incorporates aspects of Bohr’s model, inasmuch as

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the other hand, if an atom loses or gains

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electrons move from one orbital to another by absorbing or emitting a quantum of energy.

WHERE TO LEARN MORE “The Atom.” Thinkquest (Web site). (May 18, 2001). “Elements” (Web site). (May 18, 2001). “Explore the Atom” CERN—European Organization for Nuclear Research (Web site). (May 18, 2001).

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Atoms

KEY TERMS

CONTINUED

A subatomic particle that

together form the nucleus around which

has no electric charge. Neutrons are found

electrons spin, have approximately the

at the nucleus of an atom, alongside

same mass—a mass that is many times

protons.

greater than that of an electron.

NEUTRON:

A generic term for the

QUARK: A particle believed to be a com-

heavy particles—protons and neutrons—

ponent of protons and neutrons. A quark

that make up the nucleus of an atom.

may either have a positive electric charge of

NUCLEON:

less than 1+, in which case it is called an NUCLEON NUMBER:

Another term

for mass number. NUCLEUS: The center of an atom, a

“up quark”; or a negative charge of less than 1-, in which case it is called a “down quark.”

region where protons and neutrons are

RADIATION:

located, and around which electrons spin.

tion can refer to anything that travels in a

PERIODIC TABLE OF ELEMENTS: A

chart that shows the elements arranged in order of atomic number, along with chem-

In a general sense, radia-

stream, whether that stream be composed of subatomic particles or electromagnetic waves. In a more specific sense, the term relates to the radiation from radioactive

ical symbol and the average atomic mass

materials, which can be harmful to human

(in atomic mass units) for that particular

beings.

element. Vertical columns within the periodic table indicate groups or “families” of elements with similar chemical character-

RADIOACTIVITY:

A term describing a

phenomenon whereby certain isotopes are subject to a form of decay brought about

istics.

by the emission of high-energy particles or PROTON:

A positively charged particle

in an atom. Protons and neutrons, which

Gallant, Roy A. The Ever-Changing Atom. New York: Benchmark Books, 1999. Goldstein, Natalie. The Nature of the Atom. New York: Rosen Publishing Group, 2001.

radiation, such as alpha particles, beta particles, or gamma rays.

“A Science Odyssey: You Try It: Atom Builder.” PBS—Public Broadcasting System (Web site). (May 18, 2001).

“A Look Inside the Atom” (Web site). (May 18, 2001).

Spangenburg, Ray and Diane K. Moser. The History of Science in the Nineteenth Century. New York: Facts on File, 1994.

“Portrait of the Atom” (Web site). (May 18, 2001).

Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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AT O M I C M A S S Atomic Mass

CONCEPT Every known item of matter in the universe has some amount of mass, even if it is very small. But what about something so insignificant in mass that comparing it to a gram is like comparing a millimeter to the distance between Earth and the nearest galaxy? Obviously, special units are needed for such measurements; then again, one might ask why it is necessary to weigh atoms at all. One answer is that everything is made of atoms. More specifically, the work of a chemist requires the use of accurate atomic proportions in forming the molecules that make up a compound. The measurement of atomic mass was thus a historic challenge that had to be overcome, and the story of the ways that scientists met this challenge is an intriguing one.

HOW IT WORKS Why Mass and Not Weight? Some textbooks and other sources use the term atomic weight instead of atomic mass. The first of these is not as accurate as the second, which explains why atomic mass was chosen as the subject of this essay. Indeed, the use of “atomic weight” today merely reflects the fact that scientists in the past used that expression and spoke of “weighing” atoms. Though “weigh” is used as a verb in this essay, this is only because it is less cumbersome than “measure the mass of.” (In addition, “atomic weight” may be mentioned when discussing studies by scientists of the nineteenth century, who applied that term rather than atomic mass.)

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One might ask why such pains have been taken to make the distinction. Mass is, after all, basically the same as weight, is it not? In fact it is not, though people are accustomed to thinking in those terms since most weight scales provide measurements in both pounds and kilograms. However, the pound is a unit of weight in the English system, whereas a kilogram is a unit of mass in the metric and SI systems. Though the two are relatively convertible on Earth, they are actually quite different. (Of course it would make no more sense to measure atoms in pounds or kilograms than to measure the width of a hair in light-years; but pounds and kilograms are the most familiar units of weight and mass respectively.) Weight is a measure of force affected by Earth’s gravitational pull. Therefore a person’s weight varies according to gravity, and would be different if measured on the Moon, whereas mass is the same throughout the universe. Its invariability makes mass preferable to weight as a parameter of scientific measure.

Putting an Atom’s Size and Mass in Context Mass does not necessarily relate to size, though there is enough of a loose correlation that more often than not, we can say that an item of very small size will have very small mass. And atoms are very, very small—so much so that, until the early twentieth century, chemists and physicists had no accurate means of isolating them to determine their mass. The diameter of an atom is about 10-8 cm. This is equal to about 0.000000003937 in—or to put it another way, an inch is about as long as 250

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million atoms lined up side by side. Obviously, special units are required for describing the size of atoms. Usually, measurements are provided in terms of the angstrom, equal to 10-10 m. (In other words, there are 10 million angstroms in a millimeter.)

Atomic Mass

Measuring the spatial dimensions of an atom, however, is not nearly as important for chemists’ laboratory work as measuring its mass. The mass of an atom is almost inconceivably small. It takes about 5.0 • 1023 carbon atoms to equal just one gram in mass. At first, 1023 does not seem like such a huge number, until one considers that 106 is already a million, meaning that 1023 is a million times a million times a million times 100,000. If 5.0 • 1023 angstrom lengths—angstroms, not meters or even millimeters—were laid end to end, they would stretch from Earth to the Sun and back 107,765 times!

Atomic Mass Units It is obvious, then, that an entirely different unit is needed for measuring the mass of an atom, and for this purpose, chemists and other scientists use an atom mass unit (abbreviated amu), which is equal to 1.66 • 10-24 g. Though the abbreviation amu is used in this book, atomic mass units are sometimes designated simply by a u. On the other hand, they may be presented as numbers without any unit of measure—as for instance on the periodic table of elements. Within the context of biochemistry and microbiology, often the term dalton (abbreviated Da or D) is used. This is useful for describing the mass of large organic molecules, typically rendered in kilodaltons (kDa). The Latin prefix kiloindicates 1,000 of something, and “kilodalton” is much less of a tongue-twister than “kilo-amu”. The term “dalton” honors English chemist John Dalton (1766-1844), who, as we shall see, introduced the concept of the atom to science. AVERAGE ATOMIC MASS. Since

1960, when its value was standardized, the atomic mass unit has been officially known as the “unified atomic mass unit.” The addition of the word “unified” reflects the fact that atoms are not weighed individually—a labor that would be problematic at the very least. In any case, to do so would be to reinvent the wheel, as it were,

S C I E N C E O F E V E R Y D AY T H I N G S

ERNEST RUTHERFORD.

because average atomic mass figures have been established for each element. Average atomic mass figures range from 1.008 amu for hydrogen, the first element listed on the periodic table of elements, to over 250 amu for elements of very high atomic number. Figures for average atomic mass can be used to determine the average mass of a molecule as well, since a molecule is just a group of atoms joined in a structure. The mass of a molecule can be determined simply by adding together average atomic mass figures for each atom the molecule contains. A water molecule, for instance, consists of two hydrogen atoms and one oxygen atom; therefore, its mass is equal to the average atomic mass of hydrogen multiplied by two, and added to the average atomic mass of oxygen.

Avogadro’s Number and the Mole Atomic mass units and average atomic mass are not the only components necessary for obtaining accurate mass figures where atoms are concerned. Obviously, as suggested several times already, it would be fruitless to determine the mass of individual atoms or molecules. Nor would it do to measure the mass of a few hundred, or even a few million, of these particles.

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becomes clear: as noted, carbon has an average atomic mass of 12.01 amu, and multiplication of the average atomic mass by Avogadro’s number yields a figure in grams equal to the value of the average atomic mass in atomic mass units.

Atomic Mass

REAL-LIFE A PPLICAT I O N S Early Ideas of Atomic Mass Dalton was not the first to put forth the idea of the atom: that concept, originated by the ancient Greeks, had been around for more than 2,000 years. However, atomic theory had never taken hold in the world of science—or, at least, what passed for science prior to the seventeenth century revolution in thinking brought about by Galileo Galilei (1564-1642) and others.

FREDERICK SODDY.

After all, as we have seen, it takes about 500,000 trillion million carbon atoms to equal just one gram—and a gram, after all, is still rather small in mass compared to most objects encountered in daily life. (There are 1,000 g in a kilogram, and a pound is equal to about 454 g.) In addition, scientists need some means for comparing atoms or molecules of different substances in such a way that they know they are analyzing equal numbers of particles. This cannot be done in terms of mass, because the number of atoms in each sample varies: a gram of hydrogen, for instance, contains about 12 times as many atoms as a gram of carbon, which has an average atomic mass of 12.01 amu. What is needed, instead, is a way to designate a certain number of atoms or molecules, such that accurate comparisons are possible. In order to do this, chemists make use of a figure known as Avogadro’s number. Named after Italian physicist Amedeo Avogadro (1776-1856), it is equal to 6.022137 x 1023. Avogadro’s number, which is 6,022,137 followed by 17 zeroes, designates the quantity of molecules in a mole (abbreviated mol). The mole, a fundamental SI unit for “amount of substance,” is defined precisely as the number of carbon atoms in 12.01 g of carbon. It is here that the value of Avogadro’s number

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Influenced by several distinguished predecessors, Dalton in 1803 formulated the theory that nature is formed of tiny particles, an idea he presented in A New System of Chemical Philosophy (1808). Dalton was the first to treat atoms as fully physical constructs; by contrast, ancient proponents of atomism conceived these fundamental particles in ideal or spiritual terms. Dalton described atoms as hard, solid, indivisible particles with no inner spaces—a definition that did not endure, as later scientific inquiry revealed the complexities of the atom. Yet he was correct in identifying atoms as having weight—or, as scientists say today, mass. THE FIRST TABLE OF ATOMIC WEIGHTS. The question was, how could any-

one determine the weight of something as small as an atom? A year after the publication of Dalton’s book, a discovery by French chemist and physicist Joseph Gay-Lussac (1778-1850) and German naturalist Alexander von Humboldt (1769-1859) offered a clue. Humboldt and GayLussac—famous for his gas law associating pressure and temperature—found that gases combine to form compounds in simple proportions by volume. For instance, as Humboldt and Gay-Lussac discovered, water is composed of only two elements: hydrogen and oxygen, and these two combine in a whole-number ratio of 8:1. By separating water into its components, they found that for every part of oxygen, there were eight parts of hydrogen. Today we know that water molecules

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are formed by two hydrogen atoms, with an average atomic mass of 1.008 amu each, and one oxygen atom. The ratio between the average atomic mass of oxygen (16.00 amu) and that of the two hydrogen atoms is indeed very nearly 8:1. In the early nineteenth century, however, chemists had no concept of molecular structure, or any knowledge of the atomic masses of elements. They could only go on guesswork: hence Dalton, in preparing the world’s first “Table of Atomic Weights,” had to make some assumptions based on Humboldt’s and Gay-Lussac’s findings. Presumably, Dalton reasoned, only one atom of hydrogen combines with one atom of oxygen to form a “water atom.” He assigned to hydrogen a weight of 1, and according to this, calculated the weight of oxygen as 8. AVOGADRO AND BERZELIUS IMPROVE ON DALTON’S WORK. The

implications of Gay-Lussac’s discovery that substances combined in whole-number ratios were astounding. (Gay-Lussac, who studied gases for much of his career, is usually given more credit than Humboldt, an explorer and botanist who had his hand in many things.) On the one hand, the more scientists learned about nature, the more complex it seemed; yet here was something amazingly simple. Instead of combining in proportions of, say, 8.3907 to 1.4723, oxygen and hydrogen molecules formed a nice, clean, ratio of 8 to 1. This served to illustrate the fact that, as Dalton had stated, the fundamental particles of matter must be incredibly tiny; otherwise, it would be impossible for every possible quantity of hydrogen and oxygen in water to have the same ratio. Intrigued by the work of Gay-Lussac, Avogadro in 1811 proposed that equal volumes of gases have the same number of particles if measured at the same temperature and pressure. He also went on to address a problem raised by Dalton’s work. If atoms were indivisible, as Dalton had indicated, how could oxygen exist both as its own atom and also as part of a water “atom”? Water, as Avogadro correctly hypothesized, is not composed of atoms but of molecules, which are themselves formed by the joining of two hydrogen atoms with one oxygen atom. Avogadro’s molecular theory opened the way to the clarification of atomic mass and the development of the mole, which, as we have seen, makes it possible to determine mass for large

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quantities of molecules. However, his ideas did not immediately gain acceptance. Only in 1860, four years after Avogadro’s death, did Italian chemist Stanislao Cannizzaro (1826-1910) resurrect the concept of the molecule as a way of addressing disagreements among scientists regarding the determination of atomic mass.

Atomic Mass

In the meantime, Swedish chemist Jons Berzelius (1779-1848) had adopted Dalton’s method of comparing all “atomic weights” to that of hydrogen. In 1828, Berzelius published a table of atomic weights, listing 54 elements along with their weight relative to that of hydrogen. Thus carbon, in Berzelius’s system, had a weight of 12. Unlike Dalton’s figures, Berzelius’s are very close to those used by scientists today. By the time Russian chemist Dmitri Ivanovitch Mendeleev (1834-1907) created his periodic table in 1869, there were 63 known elements. That first table retained the system of measuring atomic mass in comparison to hydrogen.

The Discovery of Subatomic Structures Until scientists began to discover the existence of subatomic structures, measurements of atomic mass could not really progress. Then in 1897, English physicist J. J. Thomson (1856-1940) identified the electron. A particle possessing negative charge, the electron contributes little to an atom’s mass, but it pointed the way to the existence of other particles within an atom. First of all, there had to be a positive charge to offset that of the electron, and secondly, the item or items providing this positive charge had to account for the majority of the atom’s mass. Early in the twentieth century, Thomson’s student Ernest Rutherford (1871-1937) discovered that the atom has a nucleus, a center around which electrons move, and that the nucleus contains positively charged particles called protons. Protons have a mass 1,836 times as great as that of an electron, and thus seemed to account for the total atomic mass. Later, however, Rutherford and English chemist Frederick Soddy (18771956) discovered that when an atom emitted certain types of particles, its atomic mass changed. ISOTOPES AND ATOMIC MASS.

Rutherford and Soddy named these atoms of differing mass isotopes, though at that point— because the neutron had yet to be discovered—

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they did not know exactly what had caused the change in mass. Certain types of isotopes, Soddy and Rutherford concluded, had a tendency to decay, moving (sometimes over a great period of time) toward stabilization. Such isotopes were radioactive. Soddy concluded that atomic mass, as measured by Berzelius, was actually an average of the mass figures for all isotopes within that element. This explained a problem with Mendeleev’s periodic table, in which there seemed to be irregularities in the increase of atomic mass from element to element. The answer to these variations in mass, it turned out, related to the number of isotopes associated with a given element: the greater the number of isotopes, the more these affected the overall measure of the element’s mass. A NEW DEFINITION OF ATOMIC NUMBER. Up to this point, the term “atomic

number” had a different, much less precise, meaning than it does today. As we have seen, the early twentieth century periodic table listed elements in order of their atomic mass in relation to hydrogen, and thus atomic number referred simply to an element’s position in this ordering. Then, just a few years after Rutherford and Soddy discovered isotopes, Welsh physicist Henry Moseley (1887-1915) determined that every element has a unique number of protons in its nucleus. Today, the number of protons in the nucleus, rather than the mass of the atom, determines the atomic number of an element. Carbon, for instance, has an atomic number of 6, not because there are five elements lighter—though this is also true—but because it has six protons in its nucleus. The ordering by atomic number happens to correspond to the ordering by atomic mass, but atomic number provides a much more precise means of distinguishing elements. For one thing, atomic number is always a whole integer—1 for hydrogen, for instance, or 17 for chlorine, or 92 for uranium. Figures for mass, on the other hand, are almost always rendered with decimal fractions (for example, 1.008 for hydrogen). NEUTRONS COMPLETE THE PICTURE. As with many other discoveries

along the way to uncovering the structure of the atom, Moseley’s identification of atomic number with the proton raised still more questions. In particular, if the unique number of protons identified an element, what was it that made isotopes

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of the same element different from one another? Hydrogen, as it turned out, indeed had a mass very nearly equal to that of one proton—thus justifying its designation as the basic unit of atomic mass. Were it not for the isotope known as deuterium, which has a mass nearly twice as great as that of hydrogen, the element would have an atomic mass of exactly 1 amu. A discovery by English physicist James Chadwick (1891-1974) in 1932 finally explained what made an isotope an isotope. It was Chadwick who identified the neutron, a particle with no electric charge, which resides in the nucleus alongside the protons. In deuterium, which has one proton, one neutron, and one electron, the electron accounts for only 0.0272% of the total mass—a negligible figure. The proton, on the other hand, makes up 49.9392% of the mass. Until the discovery of the neutron, there had been no explanation of the other 50.0336% of the mass in an atom with just one proton and one electron.

Average Atomic Mass Today Thanks to Chadwick’s discovery of the neutron, it became clear why deuterium weighs almost twice as much as ordinary hydrogen. This in turn is the reason why a large sample of hydrogen, containing as it does a few molecules of deuterium here and there, does not have the same average atomic mass as a proton. Today scientists know that there are literally thousands of isotopes—many of them stable, but many more of them unstable or radioactive—for the 100-plus elements on the periodic table. Each isotope, of course, has a slightly different atomic mass. This realization has led to clarification of atomic mass figures. One might ask how figures of atomic mass are determined. In the past, as we have seen, it was largely a matter of guesswork, but today chemists and physicist use a highly sophisticated instrument called a mass spectrometer. First, atoms are vaporized, then changed to positively charged ions, or cations, by “knocking off ” electrons. The cations are then passed through a magnetic field, and this causes them to be deflected by specific amounts, depending on the size of the charge and its atomic mass. The particles eventually wind up on a deflector plate, where the amount of deflection can be measured and compared with the charge. Since 1 amu has

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been calculated to equal approximately 931.494 MeV, or mega electron-volts, very accurate figures can be determined. CALIBRATION OF THE ATOMIC MASS UNIT. When 1 is divided by Avo-

gadro’s number, the result is 1.66 • 10-24—the value, in grams, of 1 amu. However, in accordance with a 1960 agreement among members of the international scientific community, measurements of atomic mass take as their reference point the mass of carbon-12. Not only is the carbon-12 isotope found in all living things, but hydrogen is a problematic standard because it bonds so readily with other elements. According to the 1960 agreement, 1 amu is officially 1/12 the mass of a carbon-12 atom, whose exact value (retested in 1998), is 1.6653873 • 10-24 g. 12

Carbon-12, sometimes represented as ᎏ6ᎏC, contains six protons and six neutrons. (As explained in the essay on Isotopes, where an isotope is indicated, the number to the upper left of the chemical symbol indicates the total number of protons and neutrons. Sometimes this is the only number shown; but if a number is included on the lower left, this indicates only the number of protons, which remains the same for each element.) The value of 1 amu thus obtained is, in effect, an average of the mass for a proton and neutron—a usable figure, given the fact that a neutron weighs only 0.163% more than a proton. Of all the carbon found in nature (as opposed to radioactive isotopes created in laboratories), 98.89% of it is carbon-12. The remainder is mostly carbon-13, with traces of carbon14, an unstable isotope produced in nature. By definition, carbon-12 has an atomic mass of exactly 12 amu; that of carbon-13 (about 1.11% of all carbon) is 13 amu. Thus the atomic mass of carbon, listed on the periodic table as 12.01 amu, is obtained by taking 98.89% of the mass of carbon-12, combined with 1.11% of the mass of carbon-13. ATOMIC MASS UNITS AND THE PERIODIC TABLE. The periodic table as it

is used today includes figures, in atomic mass units, for the average mass of each atom. As it turns out, Berzelius was not so far off in his use of hydrogen as a standard, since its mass is almost exactly 1 amu—but not quite, because (as noted above) deuterium increases the average mass somewhat. Figures increase from there along the

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periodic table, though not by a regular pattern. Sometimes the increase from one element to the next is by just over 1 amu, and in other cases, the increase is by more than 3 amu. This only serves to prove that atomic number, rather than atomic mass, is a more straightforward means of ordering the elements.

Atomic Mass

Mass figures for many elements that tend to appear in the form of radioactive isotopes are usually shown in parentheses. This is particularly true for elements with atomic numbers above 92, because samples of these elements do not stay around long enough to be measured. Some have a half-life—the period in which half the isotopes decay to a stable form—of just a few minutes, and for others, the half-life is but a fraction of a second. Therefore, atomic mass figures represent the mass of the longest-lived isotope.

Uses of Atomic Mass in Chemistry MOLAR MASS. Just as the value of atomic mass units has been calibrated to the mass of carbon-12, the mole is no longer officially defined in terms of Avogadro’s number, though in general its value has not changed. By international scientific agreement, the mole equals the number of carbon atoms in 12.01 g of carbon. Note that, as stated earlier, carbon has an average atomic mass of 12.01 amu.

This is no coincidence, of course: multiplication of the average atomic mass by Avogadro’s number yields a figure in grams equal to the value of the average atomic mass in amu. A mole of helium, with an average atomic mass of 4.003, is 4.003 g. Iron, on the other hand, has an average atomic mass of 55.85, so a mole of iron is 55.85 g. These figures represent the molar mass—the mass of 1 mole—for each of the elements mentioned. THE NEED FOR EXACT PROPORTIONS. When chemists discover new

substances in nature or create new ones in the laboratory, the first thing they need to determine is the chemical formula—in other words, the exact quantities and proportions of elements in each molecule. By chemical means, they separate the compound into its constituent elements, then determine how much of each element is present.

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Atomic Mass

KEY TERMS ATOM:

The smallest particle of an ele-

NUMBER:

A figure,

ment that retains all the chemical and

named after Italian physicist Amedeo Avo-

physical properties of that element.

gadro (1776-1856), equal to 6.022137 x

ATOMIC

MASS

UNIT:

An SI unit

(abbreviated amu), equal to 1.66 • 10-24 g, for measuring the mass of atoms. ATOMIC

NUMBER:

The number of

protons in the nucleus of an atom. Since this number is different for each element, elements are listed on the periodic table of elements in order of atomic number.

1023. Avogadro’s number indicates the number of atoms or molecules in a mole. COMPOUND:

A substance made up of

atoms of more than one element. These atoms are usually joined in molecules. DALTON:

An alternate term for atomic

mass units, used in biochemistry and microbiology for describing the mass of large organic molecules. The dalton

An old term for

(abbreviated Da or D) is named after

atomic mass. Since weight varies depend-

English chemist John Dalton (1766-1844),

ing on gravitational field, whereas mass is

who introduced the concept of the atom to

the same throughout the universe, scien-

science.

ATOMIC WEIGHT:

tists typically use the term “atomic mass”

ELEMENT:

A substance made up of only

instead.

one kind of atom, which cannot be chemi-

AVERAGE ATOMIC MASS: A figure

cally broken into other substances.

used by chemists to specify the mass—in

HALF-LIFE: The length of time it takes a

atomic mass units—of the average atom in

substance to diminish to one-half its initial

a large sample.

amount.

Since they are using samples in relatively large quantities, molar mass figures for each element make it possible to determine the chemical composition. To use a very simple example, suppose a quantity of water is separated, and the result is 2.016 g of hydrogen and 16 g of oxygen. The latter is the molar mass of oxygen, and the former is the molar mass of hydrogen multiplied by two. Thus we know that there are two moles of hydrogen and one mole of oxygen, which combine to make one mole of water. Of course the calculations used by chemists working in the research laboratories of universities, government institutions, and corporations are much, much more complex than the example we have given. In any case, it is critical that a chemist be exact in making these determinations,

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so as to know the amount of reactants needed to produce a given amount of product, or the amount of product that can be produced from a given amount of reactant. When a company produces millions or billions of a single item in a given year, a savings of very small quantities in materials—thanks to proper chemical measurement—can result in a savings of billions of dollars on the bottom line. Proper chemical measurement can also save lives. Again, to use a very simple example, if a mole of compounds weighs 44.01 g and is found to contain two moles of oxygen and one of carbon, then it is merely carbon dioxide—a compound essential to plant life. But if it weighs 28.01 g and has one mole of oxygen with one mole of carbon, it is poisonous carbon monoxide.

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Atomic Mass

KEY TERMS ION:

An atom or group of atoms that has

lost or gained one or more electrons, and thus has a net electric charge.

CONTINUED

SI definition, a mole is equal to the number of carbon atoms in 12.01 g of carbon. MOLECULE: A group of atoms, usually,

ISOTOPES: Atoms of the same element

but

(that is, they have the same number of pro-

than one element, joined in a structure.

tons) that differ in terms of mass. Isotopes

Compounds are typically made of up

may be either stable or unstable. The latter

molecules.

type, known as radioisotopes, are radioactive.

not

always, representing

more

PERIODIC TABLE OF ELEMENTS: A

chart that shows the elements arranged in

MASS: The amount of matter an object

order of atomic number, along with chem-

contains.

ical symbol and the average atomic mass

MOLAR MASS: The mass, in grams, of

(in atomic mass units) for that particular

1 mole of a given substance. The value in

element.

grams of molar mass is always equal to the

RADIOACTIVITY:

value, in atomic mass units, of the average

phenomenon whereby certain isotopes

atomic mass of that substance. Thus car-

known as radioisotopes are subject to a

bon has a molar mass of 12.01 g, and an

form of decay brought about by the emis-

average atomic mass of 12.01 amu.

sion of high-energy particles. “Decay” does

A term describing a

The SI fundamental unit for

not mean that the isotope “rots”; rather, it

“amount of substance.” A mole is, general-

decays to form another isotope, until even-

ly speaking, Avogadro’s number of atoms

tually (though this may take a long time), it

or molecules; however, in the more precise

becomes stable.

MOLE:

WHERE TO LEARN MORE “Atomic Weight” (Web site). (May 23, 2001). “An Experiment with ‘Atomic Mass’” (Web site). (May 23, 2001). Knapp, Brian J. and David Woodroffe. The Periodic Table. Danbury, CT: Grolier Educational, 1998. Oxlade, Chris. Elements and Compounds. Chicago: Heinemann Library, 2001.

S C I E N C E O F E V E R Y D AY T H I N G S

“Periodic Table: Atomic Mass.” ChemicalElements.com (Web site). (May 23, 2001). “Relative Atomic Mass” (Web site). (May 23, 2001). “What Are Atomic Number and Atomic Weight?” (Web site). (May 23, 2001).

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ELECTRONS Electrons

CONCEPT No one can see an electron. Even an electron microscope, used for imaging the activities of these subatomic particles, does not offer a glimpse of an electron as one can look at an amoeba; instead, the microscope detects the patterns of electron deflection. In any case, a singlecell organism is gargantuan in comparison to an electron. Even when compared to a proton or a neutron, particles at the center of an atom, electrons are minuscule, being slightly more than 1/2000 the size of either. Yet the electron is the key to understanding the chemical process of bonding, and electron configurations clarify a number of aspects of the periodic table that may, at first, seem confusing.

HOW IT WORKS The Electron’s Place in the Atom The atom is discussed in detail elsewhere in this book; here, the particulars of atomic structure will be presented in an abbreviated form, so that the discussion of electrons may proceed. At the center of an atom, the smallest particle of an element, there is a nucleus, which contains protons and neutrons. Protons are positive in charge, while neutrons exert no charge. Moving around the nucleus are electrons, negatively charged particles whose mass is very small in comparison to the proton or neutron: 1/1836 and 1/1839 the mass of the proton and neutron respectively. The mass of an electron is 9.109389 • 10–33 g. Compare the number 9 to the number

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1,000,000,000,000,000,000,000,000,000,000,000, and this gives some idea of the ratio between an electron’s mass and a gram. The large number— 1 followed by 33 zeroes—is many trillions of times longer than the age of the universe in seconds. IONS AND CHEMICAL BONDS.

Electrons, though very small, are exceedingly powerful, and they are critical to both physical and chemical processes. The negative charge of an electron, designated by the symbol 1- or -1, is enough to counteract the positive charge (1+ or +1) of a proton, even though the proton has much greater mass. As a result, atoms (because they possess equal numbers of protons and electrons) have an electric charge of zero. Sometimes an atom will release an electron, or a released electron will work its way into the structure of another atom. In either case, an atom that formerly had no electric charge acquires one, becoming an ion. In the first of the instances described, the atom that has lost an electron or electrons becomes a positively charged ion, or cation. In the second instance, an atom that gains an electron or electrons becomes a negatively charged ion, or anion. One of the first forms of chemical bonding discovered was ionic bonding, in which electrons clearly play a role. In fact, electrons are critical to virtually all forms of chemical bonding, which relates to the interactions between electrons of different atoms. MOVEMENT OF THE ELECTRON ABOUT THE NUCLEUS. If the size of the

nucleus were compared to a grape, the edge of the atom itself would form a radius of about a mile. Because it is much smaller than the nucle-

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Electrons

z

y

y

2py

y

x

x

TWO-LOBED

z

z

2px

x

2p2

ORBITALS.

us, an electron might be depicted in this scenario as a speck of dust—but a speck of dust with incredible energy, which crosses the mile radius of this enlarged atom at amazing speeds. In an item of matter that has been frozen to absolute zero, an electron moves relatively little. On the other hand, it can attain speeds comparable to that of the speed of light: 186,000 mi (299,339 km) per second. The electron does not move around the nucleus as a planet orbits the Sun—a model of electron behavior that was once accepted, but which has since been overturned. On the other hand, as we shall see, the electron does not simply “go where it pleases”: it acts in accordance with complex patterns described by the quantum theory of physics. Indeed, to some extent the behavior of the electron is so apparently erratic that the word “pattern” seems hardly to describe it. However, to understand the electron clearly, one has to set aside all ideas about how objects behave in the physical world.

Emerging Models of Electron Behavior The idea that matter is composed of atoms originated in ancient Greece, but did not take hold until early in the nineteenth century, with the

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atomic theory of English chemist John Dalton (1766-1844). In the years that followed, numerous figures—among them Russian chemist Dmitri Mendeleev (1834-1907), father of the periodic table of elements—contributed to the emerging understanding of the atomic model. Yet Mendeleev, despite his awareness that the mass of an atom differentiated one element from another, had no concept of subatomic particles. No one did: until very late in the nineteenth century, the atom might as well have been a hardshelled ball of matter, for all that scientists understood about its internal structure. The electron was the first subatomic particle discovered, in 1897—nearly a century after the scientific beginning of atomic theory. Yet the first hints regarding its existence had begun to appear some 60 years before. DISCOVERY OF THE ELECTRON. In 1838, British physicist and chemist

Michael Faraday (1791-1867) was working with a set of electrodes—metal plates used to emit or collect electric charge—which he had placed at either end of an evacuated glass tube. (In other words, most of the air and other matter had been removed from the tube.) He applied a charge of several thousand volts between the electrodes, and discovered that an electric current flowed between them.

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Electrons

A

COMPUTER-GENERATED MODEL OF A NEON ATOM.

THE

NUCLEUS, AT CENTER, IS TOO SMALL TO BE SEEN AT THIS SURROUNDING THE NUCLEUS ARE THE ATOM’S ELECTRON SPHERE), AND 2P (LOBED). (Photograph by Kenneth Eward/BioGrafx. National

SCALE AND IS REPRESENTED BY THE FLASH OF LIGHT. ORBITALS:

1S (SMALL

SPHERE),

2S (LARGE

Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.)

This seemed to suggest the existence of particles carrying an electric charge, and four decades later, English physicist William Crookes (1832-1919) expanded on these findings with his experiments using an apparatus that came to be known as a Crookes tube. As with Faraday’s device, the Crookes tube used an evacuated glass tube encased between two electrodes—a cathode at the negatively charged end, and an anode at the positively charged end. A wire led outside the bulb to an electric source, and when electricity was applied to the electrodes, the cathodes emitted rays. Crookes concluded that the cathode rays were particles with a negative electric charge that came from the metal in the cathode plate. English physicist J. J. Thomson (1856-1940) hypothesized that the negatively charged particles Crookes had observed were being emitted by atoms, and in 1897 he gave a name to these particles: electrons. The discovery of the electron raised questions concerning its place in the atom: obviously, there had to be a counterbalancing positive charge, and if so, from whence did it come? FROM PLUM PUDDINGS TO PLANETS. Around the beginning of the

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twentieth century, the prevailing explanation of atomic structure was the “plum pudding model,” which depicted electrons as floating like raisins in a “pudding” of positive charges. This was overturned by the discovery of the nucleus, and, subsequently, of the proton and neutron it contained. In 1911, the great Danish physicist Niels Bohr (1885-1962) studied hydrogen atoms, and concluded that electrons move around the nucleus in much the same way that planets move around the Sun. This worked well when describing the behavior of hydrogen, which, in its simplest form—the isotope protium—has only one electron and one proton, without any neutrons. The model did not work as well when applied to other elements, however, and within less than two decades Bohr’s planetary model was overturned. As it turns out, the paths of an electron’s movement around the nucleus are nothing like that of a planet’s orbit—except inasmuch as both models describe a relatively small object moving around a relatively large one. The reality is much more complex, and to comprehend the secret of the electron’s apparently random behavior is

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truly a mind-expanding experience. Yet Bohr is still considered among the greatest scientists of the twentieth century: it was he, after all, who first explained the quantum behavior of electrons, examined below.

on a continuum; rather, there are only certain energy levels possible for an atom of a given element. The energy levels are therefore said to be quantized.

Electrons

The Wave Mechanical Model

REAL-LIFE AP P LICATIONS Quantum Theory and the Atom Much of what scientists understand today about the atom in general, and the electron in particular, comes from the quantum theory introduced by German physicist Max Planck (1858-1947). Planck showed that, at the atomic level, energy is emitted in tiny packets, or “quanta.” Applying this idea to the electron, Bohr developed an idea of the levels at which an electron moves around the nucleus. Though his conclusions led him to the erroneous planetary model, Bohr’s explanation of energy levels still prevails. As has been suggested, the interaction between electrons and protons is electromagnetic, and electromagnetic energy is emitted in the form of radiation, or a stream of waves and particles. The Sun, for instance, emits electromagnetic radiation along a broad spectrum that includes radio waves, infrared light, visible light, ultraviolet light, x rays, and gamma rays. These are listed in ascending order of their energy levels, and the energy emitted can be analyzed in terms of wavelength and frequency: the shorter the wavelength, the greater the frequency and the greater the energy level. When an atom is at its ordinary energy level, it is said to be in a ground state, but when it acquires excess energy, it is referred to as being in an excited state. It may release some of that energy in the form of a photon, a particle of electromagnetic radiation. The amount of energy involved can be analyzed in terms of the wavelengths of light the atom emits in the form of photons, and such analysis reveals some surprising things about the energy levels of atoms. If one studies the photons emitted by an atom as it moves between a ground state and an excited state, one discovers that it emits only certain kinds of photons. From this, Bohr concluded that the energy levels of an atom do not exist

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In everyday terms, quantization can be compared to the way that a person moves up a set of stairs: by discrete steps. If one step directly follows another, there is no step in between, nor is there any gradual way of moving from step to step, as one would move up a ramp. The movement of electrons from one energy level to another is not a steady progression, like the movement of a person up a ramp; rather, it is a series of quantum steps, like those a person makes when climbing a set of stairs. The idea of quantization was ultimately applied to describing the paths that an electron makes around the nucleus, but this required some clarification along the way. It had been believed that an electron could move through any point between the nucleus and the edge of the atom (again, like a ramp), but it later became clear that the electrons could only move along specific energy levels. As we have seen, Bohr believed that these corresponded to the orbits of planets around the Sun; but this explanation would be discarded in light of new ideas that emerged in the 1920s. During the early part of that decade, French physicist Louis de Broglie (1892-1987) and Austrian physicist Erwin Schrödinger (1887-1961) introduced what came to be known as the wave mechanical model, also known as the particle-wave hypothesis. Because light appeared to have the properties of both particles and waves, they reasoned, electrons (possessing electromagnetic energy as they did) might behave in the same fashion. In other words, electrons were not just particles: in some sense, they were waves as well. UNDERSTANDING

ORBITALS.

The wave mechanical model depicted the movement of electrons, not as smooth orbits, but as orbitals—regions in which there is the highest probability that an electron will be found. An orbital is nothing like the shape of a solar system, but, perhaps ironically, it can be compared to the photographs astronomers have taken of galaxies. In most of these photographs, one sees an area of

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intense light emitted by the stars in the center. Further from this high-energy region, the distribution of stars (and hence of light) becomes increasingly less dense as one moves from the center of the galaxy to the edges. Replace the center of the galaxy with the nucleus of the atom, and the stars with electrons, and this is an approximation of an orbital. Just as a galaxy looks like a cloud of stars, scientists use the term electron cloud to describe the pattern formed by orbitals. The positions of electrons cannot be predicted; rather, it is only possible to assign probabilities as to where they will be. Naturally, they are most drawn to the positive charges in the nucleus, and hence an orbital depicts a high-density region of probabilities at the center—much like the very bright center of a galaxy. The further away from the nucleus, the less the probability that an electron will be in that position. Hence in models of an orbital, the dots are concentrated at the center, and become less dense the further away from the nucleus they are. As befits the comparison to a cloud, the edge of an orbital is fuzzy. Contrary to the earlier belief that an atom was a clearly defined little pellet of matter, there is no certainty regarding the exact edge of a given atom; rather, scientists define the sphere of the atom as the region encompassing 90% of the total electron probability. THE MYSTERIOUS ELECTRON.

As complex as this description of electron behavior may seem, one can rest assured that the reality is infinitely more complex than this simplified explanation suggests. Among the great mysteries of the universe is the question of why an electron moves as it does, or even exactly how it does so. Nor does a probability model give us any way of knowing when an electron will occupy a particular position. In fact, as German physicist Werner Heisenberg (1901-1976) showed with what came to be known as the Heisenberg Uncertainty Principle, it is impossible to know both the speed of an electron and its precise position at the same time. This, of course, goes against every law of physics that prevailed until about 1920, and in fact quantum theory offers an entirely different model of reality than the one accepted during the seventeenth, eighteenth, and nineteenth centuries.

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Orbitals Given the challenges involved in understanding electron behavior, it is amazing just how much scientists do know about electrons—particularly where energy levels are concerned. This, in turn, makes possible an understanding of the periodic table that would astound Mendeleev. Every element has a specific configuration of energy levels that becomes increasingly complex as one moves along the periodic table. In the present context, these configurations will be explained as simply as possible, but the reader is encouraged to consult a reliable chemistry textbook for a more detailed explanation. PRINCIPAL ENERGY LEVELS AND SUBLEVELS. The principal energy

level of an atom indicates a distance that an electron may move away from the nucleus. This is designated by a whole-number integer, beginning with 1 and moving upward: the higher the number, the further the electron is from the nucleus, and hence the greater the energy in the atom. Each principal energy level is divided into sublevels corresponding to the number n of the principal energy level: thus principal energy level 1 has one sublevel, principal energy level 2 has two, and so on. The simplest imaginable atom, a hydrogen atom in a ground state, has an orbital designated as 1s1. The s indicates that an electron at energy level 1 can be located in a region described by a sphere. As for the significance of the superscript 1, this will be explained shortly. Suppose the hydrogen atom is excited enough to be elevated to principal energy level 2. Now there are two sublevels, 2s (for now we will dispense with the superscript 1) and 2p. A p orbital is rather like the shape of a figure eight, with its center of gravity located on the nucleus, and thus unlike the s sublevel, p orbitals can have a specific directional orientation. Depending on whether it is oriented along an x-, y-, or zaxis, orbitals in sublevel p are designated as 2px, 2py, or 2pz. If the hydrogen atom is further excited, and therefore raised to principal energy level 3, it now has three possible sublevels, designated as s, p, and d. Some of the d orbitals can be imagined as two figure eights at right angles to one another, once again with their centers of gravity along the nucleus of the atom. Because of their more com-

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plex shape, there are five possible spatial orientations for orbitals at the d sublevel. Even more complex is the model of an atom at principal level 4, with four sublevels—s,p, d, and f, which has a total of seven spatial orientations. Obviously, things get very, very complex at increased energy levels. The greater the energy level, the further the electron can move from the nucleus, and hence the greater the possible number of orbitals and corresponding shapes. ELECTRON SPIN AND THE PAULI EXCLUSION PRINCIPLE.

Every electron spins in one of two directions, and these are indicated by the symbols ↑ and ↓. According to the Pauli exclusion principle, named after the Austrian-Swiss physicist Wolfgang Pauli (1900-1958), no more than two electrons can occupy the same orbital, and those two electrons must have spins opposite one another. This explains the use of the superscript 1, which indicates the number of electrons in a given orbital. This number is never greater than two: hence, the electron configuration of helium is written as 1s2. It is understood that these two electrons must be spinning in opposite directions, but sometimes this is indicated by an orbital diagram showing both an upward- and downward-pointing arrow in an orbital that has been filled, or only an upward-pointing arrow in an orbital possessing just one electron.

Electron Configuration and the Periodic Table THE FIRST 18 ELEMENTS. As one moves up the periodic table from atomic number 1 (hydrogen) to 18 (argon), a regular pattern emerges. The orbitals are filled in a neat progression: from helium (atomic number 2) onward, all of principal level 1 is filled; beginning with beryllium (atomic number 4), sublevel 2s is filled; from neon (atomic number 10), sublevel 2p— and hence principal level 2 as a whole—is filled, and so on. The electron configuration for neon, thus, is written as 1s22s22p6.

Note that if one adds together all the superscript numbers, one obtains the atomic number of neon. This is appropriate, of course, since atomic number is defined by the number of protons, and an atom in a non-ionized state has an equal number of protons and electrons. In noticing the electron configurations of an element, pay close attention to the last or highest principal

S C I E N C E O F E V E R Y D AY T H I N G S

energy level represented. These are the valence electrons, the ones involved in chemical bonding. By contrast, the core electrons, or the ones that are at lower energy levels, play no role in the bonding of atoms.

Electrons

SHIFTS IN ELECTRON CONFIGURATION PATTERNS. After argon, how-

ever, as one moves to the element occupying the nineteenth position on the periodic table— potassium—the rules change. Argon has an electron configuration of 1s22s22p63s23p6, and by the pattern established with the first 18 elements, potassium should begin filling principal level 3d. Instead, it “skips” 3d and moves on to 4s. The element following argon, calcium, adds a second electron to the 4s level. After calcium, the pattern again changes. Scandium (atomic number 21) is the first of the transition metals, a group of elements on the periodic table in which the 3d orbitals are filled. This explains why the transition metals are indicated by a shading separating them from the rest of the elements on the periodic table. But what about the two rows at the very bottom of the chart, representing groups of elements that are completely set apart from the periodic table? These are the lanthanide and actinide series, which are the only elements that involve f sublevels. In the lanthanide series, the seven 4f orbitals are filled, while the actinide series reflects the filling of the seven 5f orbitals. As noted earlier, the patterns involved in the f sublevel are ultra-complex. Thus it is not surprising than the members of the lanthanide series, with their intricately configured valence electrons, were very difficult to extract from one another, and from other elements: hence their old designation as the “rare earth metals.” However, there are a number of other factors—relating to electrons, if not necessarily electron configuration—that explain why one element bonds as it does to another. CHANGES IN ATOMIC SIZE. With the tools provided by the basic discussion of electrons presented in this essay, the reader is encouraged to consult the essays on Chemical Bonding, as well as The Periodic Table of the Elements, both of which explore the consequences of electron arrangement in chemistry. Not only are electrons the key to chemical bonding, understanding their configurations is critical to an understanding of the periodic table.

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Electrons

KEY TERMS ANION:

The negative ion that results

when an atom gains one or more electrons.

describe the pattern formed by orbitals.

ANODE: An electrode at the positively

EXCITED STATE:

charged end of a supply of electric current.

characteristics of an atom that has

ATOM:

The smallest particle of an

element. ATOMIC

acquired excess energy. GROUND STATE:

NUMBER:

The number of

A term describing the

A term describing

the state of an atom at its ordinary energy

protons in the nucleus of an atom. Since

level.

this number is different for each element,

ION:

elements are listed on the periodic table of

gained one or more electrons, and thus has

elements in order of atomic number.

a net electric charge.

CATHODE: An electrode at the negative-

ISOTOPES: Atoms that have an equal

ly charged end of a supply of electric cur-

number of protons, and hence are of the

rent.

same element, but differ in their number of

CATION:

The positive ion that results

An atom or atoms that has lost or

neutrons.

when an atom loses one or more electrons.

NEUTRON:

ELECTRODE: A structure, often a metal

has no electric charge. Neutrons are found

plate or grid, that conducts electricity, and

in the nucleus of an atom, alongside

which is used to emit or collect electric

protons.

charge.

NUCLEUS: The center of an atom, a

ELECTRON: A negatively charged parti-

region where protons and neutrons are

cle in an atom.

located, and around which electrons spin.

One of the curious things about the periodic table, for instance, is the fact that the sizes of atoms decrease as one moves from left to right across a row or period, even though the sizes increase as one moves from top to bottom along a column or group. The latter fact—the increase of atomic size in a group, as a function of increasing atomic number—is easy enough to explain: the higher the atomic number, the higher the principal energy level, and the greater the distance from the nucleus to the furthest probability range. On the other hand, the decrease in size across a period (row) is a bit more challenging to comprehend. However, all the elements in a period have their outermost electrons at a particular principal energy level corresponding to the num-

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A subatomic particle that

ber of the period. For instance, the elements on period 5 all have principal energy levels 1 through 5. Yet as one moves along a period from left to right, there is a corresponding increase in the number of protons within the nucleus. This means a stronger positive charge pulling the electrons inward; therefore, the “electron cloud” is drawn ever closer toward the increasingly powerful charge at the center of the atom.

WHERE TO LEARN MORE “Chemical Bond” (Web site). (May 18, 2001). “The Discovery of the Electron” (Web site). (May 18, 2001).

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Electrons

KEY TERMS

CONTINUED

A pattern of probabilities

ber integer, beginning with 1 and moving

regarding the regions that an electron can

upward. The higher the number, the fur-

occupy within an atom in a particular

ther the electron is from the nucleus, and

energy state. The orbital, complex and

hence the greater the energy in the atom.

ORBITAL:

imprecise as it may seem, is a much more accurate depiction of electron behavior

PROTON:

than the model once used, which depicted

in an atom.

A positively charged particle

electrons moving in precisely defined A term describing any

orbits around the nucleus, rather as planets

QUANTIZATION:

move around the Sun.

property that has only certain discrete values, as opposed to values distributed along

PERIODIC TABLE OF ELEMENTS: A

chart that shows the elements arranged in order of atomic number. Vertical columns within the periodic table indicate groups

a continuum. The quantization of an atom means that it does not have a continuous range of energy levels; rather, it can exist

or “families” of elements with similar

only at certain levels of energy from the

chemical characteristics.

ground state through various excited

PHOTON:

A particle of electromagnetic

radiation. PRINCIPAL ENERGY LEVEL: A value

states. RADIATION:

In a general sense, radia-

tion can refer to anything that travels in a

indicating the distance that an electron

stream, whether that stream be composed

may move away from the nucleus of an

of subatomic particles or electromagnetic

atom. This is designated by a whole-num-

waves.

Ebbing, Darrell D.; R. A. D. Wentworth; and James P. Birk. Introductory Chemistry. Boston: Houghton Mifflin, 1995. Gallant, Roy A. The Ever-Changing Atom. New York: Benchmark Books, 1999. Goldstein, Natalie. The Nature of the Atom. New York: Rosen Publishing Group, 2001. “Life, the Universe, and the Electron” (Web site).
S C I E N C E O F E V E R Y D AY T H I N G S

“A Look Inside the Atom” (Web site). (May 18, 2001). “Valence Shell Electron Pair Repulsion (VSEPR).” (May 18, 2001). “What Are Electron Microscopes?” (Web site). (May 18, 2001). Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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ISOTOPES Isotopes

CONCEPT Isotopes are atoms of the same element that have different masses due to differences in the number of neutrons they contain. Many isotopes are stable, meaning that they are not subject to radioactive decay, but many more are radioactive. The latter, also known as radioisotopes, play a significant role in modern life. Carbon-14, for instance, is used for estimating the age of objects within a relatively recent span of time—up to about 5,000 years—whereas geologists and other scientists use uranium-238 to date minerals of an age on a scale with that of the Earth. Concerns over nuclear power and nuclear weapons testing in the atmosphere have heightened awareness of the dangers posed by certain kinds of radioactive isotopes, which can indeed be hazardous to human life. However, the reality is that people are subjected to considerably more radiation from nonnuclear sources.

THE NUCLEUS AND ELECTRONS. Together with protons in the nucleus

Atoms and Elements

are neutrons, which exert no charge. The discovery of these particles, integral to the formation of isotopes, is discussed below. The nucleus, with a diameter about 1/10,000 that of the atom itself, makes up only a tiny portion of the atom’s volume, but the vast majority of its mass. Thus a change in the mass of a nucleus, as occurs when an isotope is formed, is reflected by a noticeable change in the mass of the atom itself.

The elements are substances that cannot be broken down into other matter by chemical means, and an atom is the fundamental particle in an element. As of 2001, there were 112 known elements, 88 of which occur in nature; the rest were created in laboratories. Due to their high levels of radioactivity, they exist only for extremely short periods of time. Whatever the number of elements—and obviously that number will increase over time, as new elements are synthesized—the same number of basic atomic structures exists in the universe.

Far from the nucleus (in relative terms, of course), at the perimeter of the atom, are the electrons, which have a negative electric charge. Whereas the protons and neutrons have about the same mass, the mass of an electron is less than 0.06% of either a proton or neutron. Nonetheless, electrons play a highly significant role in chemical reactions and chemical bonding. Just as isotopes are the result of changes in the number of neutrons, ions—atoms that are either positive or negative in electric charge—are the result of changes in the number of electrons.

HOW IT WORKS

92

What distinguishes one element from another is the number of protons, subatomic particles with a positive electric charge, in the nucleus, or center, of the atom. The number of protons, whatever it may be, is unique to an element. Thus if an atom has one proton, it is an atom of hydrogen, because hydrogen has an atomic number of 1, as shown on the periodic table of elements. If an atom has 109 protons, on the other hand, it is meitnerium. (Meitnerium, synthesized at a German laboratory in 1982, is the last element on the periodic table to have been assigned a name as of 2001.)

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Unless it loses or gains an electron, thus becoming an ion, an atom is neutral in charge, and it maintains this electric-charge neutrality by having an equal number of protons and electrons. There is, however, no law of the universe stating that an atom must have the same number of neutrons as it does protons and electrons: some do, but this is far from universal, as we shall see.

Isotopes

Neutrons The number of neutrons is variable within an element precisely because they exert no charge, and thus while their addition or removal changes the mass, it does not affect the electric charge of the atom. Therefore, whereas the importance of the proton and the electron is very clear to anyone who studies atomic behavior, neutrons, on the other hand, might seem at first glance as though they are only “along for the ride.” Yet they are all-important to the formation of isotopes. Not surprisingly, given their lack of electric charge, neutrons were the last of the three major subatomic particles to be discovered. English physicist J. J. Thomson (1856-1940) identified the electron in 1897, and another English physicist, Ernest Rutherford (1871-1937), discovered the proton in 1914. Rutherford’s discovery overturned the old “plum pudding” model, whereby atoms were depicted as consisting of electrons floating in a positively charged cloud, rather like raisins in an English plum pudding. As Rutherford showed, the atom must have a nucleus—yet protons alone could not account for the mass of the nucleus. There must be something else at the heart of the atom, and in 1932, yet another English physicist, James Chadwick (1891-1974), identified what it was. Working with radioactive material, he found that a certain type of subatomic particle could penetrate lead. All types of radiation known at the time were stopped by the lead, and therefore Chadwick reasoned that this particle must be neutral in charge. In 1932, he won the Nobel Prize in physics for his discovery of the neutron. NEUTRONS AND NUCLEAR FUSION. Neutrons played a critical role in the

development of the atomic bomb during the 1940s. In nuclear fission, atoms of uranium are bombarded with neutrons. The result is that the uranium nucleus splits in half, releasing huge

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A

MUSHROOM CLOUD RISES AFTER THE DETONATION OF

A HYDROGEN BOMB BY

FRANCE

IN A

1968

TEST.

DEU-

TERIUM IS A CRUCIAL PART OF THE DETONATING DEVICE FOR HYDROGEN BOMBS. (Bettmann/Corbis. Reproduced by per-

mission.)

amounts of energy. As it does so, it emits several extra neutrons, which split more uranium nuclei, creating still more energy and setting off a chain reaction. This explains the destructive power in an atomic bomb, as well as the constructive power—providing energy to homes and businesses—in a nuclear power plant. Whereas the chain reaction in an atomic bomb becomes an uncontrolled explosion, in a nuclear plant, the reaction is slowed and controlled. One of the means used to do this is by the application of “heavy water,” which, as we shall see, is water made with a hydrogen isotope.

Isotopes: The Basics Two atoms may have the same number of protons, and thus be of the same element, yet differ in their number of neutrons. Such atoms are called isotopes, atoms of the same element having different masses. The name comes from the Greek phrase isos topos, meaning “same place”: because they have the same atomic number,

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Isotopes

ENRICO FERMI.

isotopes of the same element occupy the same position on the periodic table. Also called nuclides, isotopes are represented m symbolically as follows: ᎏaᎏS, where S is the symbol of the element, a is the atomic number, and m is the mass number—the sum of protons and neutrons in the atom’s nucleus. For the stable sil93 ver isotope designated as ᎏ4ᎏ7 Ag, for instance, Ag is the element symbol; 47 its atomic number; and 93 the mass number. From this, it is easy to discern that this particular stable isotope has 46 neutrons in its nucleus. Because the atomic number of any element is established, sometimes isotopes are represented simply with the mass number, thus: 93Ag. They may also be designated with a subscript notation indicating the number of neutrons, so that this

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information can be obtained at a glance without having to do the arithmetic. For the silver isotope 93 shown here, this is written as ᎏ4ᎏ7 Ag46. Isotopes can also be indicated by simple nomenclature: for instance, carbon-12 or carbon-13.

Stable and Unstable Isotopes Radioactivity is a term describing a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of highenergy particles or radiation. Forms of particles or energy emitted in radiation include alpha particles (positively charged helium nuclei); beta particles (either electrons or subatomic particles called positrons); or gamma rays, which occupy the highest energy level in the electromagnetic

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radiation emitted by the Sun. Radioactivity will be discussed below, but for the present, the principal concern is with radioactive properties as a distinguishing factor between the two varieties of isotope. Isotopes are either stable or unstable. The unstable variety, known as radioisotopes, are subject to radioactive decay, but in this context, “decay” does not mean what it usually does. A radioisotope does not “rot”; it decays by turning into another isotope of the same element—or even into another element entirely. (For example, uranium-238 decays by emitting alpha particles, ultimately becoming lead-206.) A stable isotope, on the other hand, has already become what it is going to be, and will not experience further decay. Most elements have between two and six stable isotopes. On the other hand, a few elements—for example, technetium—have no stable isotopes. Twenty elements, among them gold, fluorine, sodium, aluminum, and phosphorus, have only one stable isotope each. The element with the most stable isotopes is easy to remember because its name is almost the same as its number of stable isotopes: tin, with 10. As for unstable isotopes, there are over 1,000, some of which exist in nature, but most of which have been created synthetically in laboratories. This number is not fixed; in any case, it is not necessarily important, because many of these highly radioactive isotopes last only for fractions of a second before decaying to form a stable isotope. Yet radioisotopes in general have so many uses, in comparison to stable isotopes, that they are often referred to simply as “isotopes.”

words, the lowest mass number is increasingly high in comparison to the atomic number. For example, uranium has an atomic number of 92, but the lowest mass number for a uranium isotope is not 184, or 92 multiplied by two; it is 218. The ratio of neutrons to protons necessary for a stable isotope creeps upward along the periodic table: tin, with an atomic number of 50, has a stable isotope with a mass number of 120, indicating a 1.4 to 1 ratio of neutrons to protons. For mercury-200, the ratio is 1.5 to 1. The higher the atomic number, by definition, the greater the number of protons in the nucleus. This means that more neutrons are required to “bind” the nucleus together. In fact, all nuclei with 84 protons or more (i.e., starting at polonium and moving along the periodic table) are radioactive, for the simple reason that it is increasingly difficult for the neutrons to withstand the strain of keeping so many protons in place. One can predict the mode of radioactive decay by noting whether the nucleus is neutronrich or neutron-poor. Neutron-rich nuclei undergo beta emission, which decreases the numbers of protons in the nucleus. Neutronpoor nuclei typically undergo positron emission or electron capture, the first of these being more prevalent among the lighter nuclei. Elements with atomic numbers of 84 or greater generally undergo alpha emission, which decreases the numbers of protons and neutrons by two each.

REAL-LIFE A PPLICAT ION S

Understanding Isotopes

Deuterium and Tritium

Before proceeding with a discussion of isotopes and their uses, it is necessary to address a point raised earlier, when it was stated that some atoms do have the same numbers of neutrons and protons, but that this is far from universal. In fact, nuclear stability is in part a function of neutronto-proton ratio.

Only three isotopes are considered significant enough to have names of their own, as opposed to being named after a parent atom (for example, carbon-12, uranium-238). These are protium, deuterium, and tritium, all three isotopes of hydrogen. Protium, or 1H, is hydrogen in its most basic form—one proton, no neutrons—and the name “protium” is only applied when necessary to distinguish it from the other two isotopes. Therefore we will focus primarily on the two others.

Stable nuclei with low atomic numbers (up to about 20) have approximately the same number of neutrons and protons. For example, the most stable and abundant form of carbon is carbon-12, with six protons and six neutrons. Beyond atomic number 20 or so, however, the number of neutrons begins to grow: in other

S C I E N C E O F E V E R Y D AY T H I N G S

Isotopes

Deuterium, designated as 2H, is a stable isotope, whereas tritium—3H—is radioactive. Both, in fact, have chemical symbols (D and T respec-

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tively), just as though they were elements on the periodic table. What makes these two so special? They are, as it were, “the products of a good home”—in other words, their parent atom is the most basic and plentiful element in the universe. Indeed, the vast majority of the universe is hydrogen, along with helium, which is formed by the fusion of hydrogen atoms. If all atoms were numbers, then hydrogen would be 1; but of course, this is more than a metaphor, since its atomic number is indeed 1. Ordinary hydrogen or protium, as noted, consists of a single proton and a single electron, the simplest possible atomic form possible. Its simplicity has made it a model for understanding the atom, and therefore when physicists discovered the existence of two hydrogen atoms that were just a bit more complex, they were intrigued. Just as hydrogen represented the standard against which atoms could be measured, scientists reasoned, deuterium and tritium could offer valuable information regarding stable and unstable isotopes respectively. Furthermore, the pronounced tendency of hydrogen to bond with other substances—it almost never appears by itself on Earth—presented endless opportunities for study regarding hydrogen isotopes in association with other elements. ISOLATION

OF

DEUTERIUM.

Deuterium is sometimes called “heavy hydrogen,” and its nucleus—with one proton and one neutron—is called a deuteron. It was first isolated in 1931 by American chemist Harold Clayton Urey (1893-1981), who was awarded the 1934 Nobel Prize in Chemistry for his discovery. Serving at that time as a professor of chemistry at Columbia University in New York City, Urey started with the assumption that any hydrogen isotopes other than protium must exist in very minute quantities. This assumption, in turn, followed from an awareness that hydrogen’s average atomic mass—measured in atomic mass units—was only slightly higher than 1. There must be, as Urey correctly reasoned, a very small quantity of “heavy hydrogen” on Earth. To separate deuterium, Urey collected a relatively large sample of liquid hydrogen: 4.2 quarts (4 l). Then he allowed the liquid to evaporate very slowly, predicting that the more abundant protium would evaporate more quickly than the

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isotope whose existence he had hypothesized. After all but 0.034 oz (1 ml) of the sample had evaporated, he submitted the remainder to a form of analysis called spectroscopy, adding a burst of energy to the atoms and then analyzing the light spectrum they emitted for evidence of differing varieties of atom. OF

CHARACTERISTICS AND USES DEUTERIUM. Deuterium, with an

atomic mass of 2.014102 amu, is almost exactly twice as heavy as protium, which has an atomic mass of 1.007825. Its melting point, or the temperature at which it changes from a solid to a liquid -426°F (-254°C), is much higher than for protium, which melts at -434°F (-259°C). The same relationship holds for its boiling point, or the temperature at which it changes from a liquid to its normal state on Earth, as a gas: -417°F (-249°C), as compared to -423°F (-253°C) for protium. Deuterium is also much, much less plentiful than protium: protium represents 99.985% of all the hydrogen that occurs naturally, meaning that deuterium accounts for just 0.015%. Often, deuterium is applied as a tracer, an atom or group of atoms whose participation in a chemical, physical, or biological reaction can be easily observed. Radioisotopes are most often used as tracers, precisely because of their radioactive emissions; deuterium, on the other hand, is effective due to its almost 2:1 mass ratio in comparison to protium. In addition, it bonds with other atoms in a fashion slightly different from that of protium, and this contrast makes its presence easier to trace. Its higher boiling and melting points mean that when deuterium is combined with oxygen to form “heavy water” (D2O), the water likewise has higher boiling and melting points than ordinary water. Heavy water is often used in nuclear fission reactors to slow down the fission process, or the splitting of atoms. DEUTERIUM IN NUCLEAR FUSION. Deuterium is also applied in a type of

nuclear reaction much more powerful that fission: fusion, or the joining of atomic nuclei. The Sun produces energy by fusion, a thermonuclear reaction that takes places at temperatures of many millions of degrees Celsius. In solar fusion, it appears that two protium nuclei join to form a single deuteron.

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During the period shortly after World War II, physicists developed a means of duplicating the thermonuclear fusion process. The result was the hydrogen bomb—more properly called a fusion bomb—whose detonating device was a compound of lithium and deuterium called lithium deuteride. Vastly more powerful than the “atomic” (that is, fission) bombs dropped by the United States over Japan in 1945, the hydrogen bomb greatly increased the threat of worldwide nuclear annihilation in the postwar years. Yet the power that could destroy the world also has the potential to provide safe, abundant fusion energy from power plants—a dream that as yet remains unrealized. Among the approaches being attempted by physicists studying nuclear fusion is a process in which two deuterons are fused. The result is a triton, the nucleus of tritium, along with a single proton. The triton and deuteron would then be fused to create a helium nucleus, with a resulting release of vast amounts of energy. TRITIUM. Whereas deuterium has a single neutron, tritium—as its mass number of 3 indicates—has two. And just as deuterium has approximately twice the mass of protium, tritium has about three times the mass, 3.016 amu. As is expected, the thermal properties of tritium are different from those of protium. Again, the melting and boiling points are higher: thus tritium heavy water (T2O) melts at 40°F (4.5°C), as compared with 32°F (0°C) for H2O.

Because it is radioactive, tritium is often described in terms of half-life, the length of time it takes for a substance to diminish to one-half its initial amount. The half-life of tritium is 12.26 years. As it decays, its nucleus emits a low-energy beta particle, and this results in the creation of the helium-3 isotope. Due to the low energy levels involved, the radioactive decay of tritium poses little danger to humans. Like deuterium, tritium is applied in nuclear fusion, though due to its scarcity, it is usually combined with deuterium. Furthermore, tritium decay requires that hydrogen bombs containing the radioisotope be recharged periodically. Also, like deuterium, tritium is an effective tracer. Sometimes it is released in small quantities into groundwater as a means of monitoring subterranean water flow. It is also used as a tracer in biochemical processes.

S C I E N C E O F E V E R Y D AY T H I N G S

Separating Isotopes Isotopes

As noted in the discussion of deuterium, tritium can only be separated from protium due to the differences in mass. The chemical properties of isotopes with the same parent element make them otherwise indistinguishable, and hence purely chemical means cannot be used to separate them. Physicists working on the Manhattan Project, the U.S. effort to develop atomic weaponry during World War II, were faced with the need to separate 235U from 238U. Uranium-238 is far more abundant, but what they wanted was the uranium-235, highly fissionable and thus useful in the processes they were attempting. Their solution was to allow a gaseous uranium compound to diffuse, or separate, the uranium through porous barriers. Because uranium238 was heavier, it tended to move more slowly through the barriers, much like grains of rice getting caught in a sifter. Another means of separating isotopes is by mass spectrometry.

Radioactivity One of the scientists working on the Manhattan Project was Italian physicist Enrico Fermi (19011954), who used radium and beryllium powder to construct a neutron source for making new radioactive materials. Fermi and his associates succeeded in producing radioisotopes of sodium, iron, copper, gold, and numerous other elements. As a result of Fermi’s work, for which he won the 1938 Nobel Prize in Physics, scientists have been able to develop radioactive versions of virtually all elements. Interestingly, the ideas of radioactivity, fission reactions, and fusion reactions collectively represent the realization of a goal sought by the medieval alchemists: the transformation of one element into another. The alchemists, forerunners of chemists, believed they could transform ordinary metals into gold by using various potions—an impossible dream. Yet as noted in the preceding paragraph, among the radioisotopes generated by Fermi’s neutron source was gold. The “catch,” of course, is that this gold was unstable; furthermore, the amount of energy and human mental effort required to generate it far outweighed the monetary value of the gold itself. Radioactivity is, in the modern imagination, typically associated with fallout from nuclear

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war, or with hazards resulting from nuclear power—hazards that, as it turns out, have been greatly exaggerated. Nor is radioactivity always harmful to humans. For instance, with its applications in medicine—as a means of diagnosing and treating thyroid problems, or as a treatment for cancer patients—it can actually save lives. HAZARDS ASSOCIATED WITH RADIOACTIVITY. It is a good thing that

radiation, even the harmful variety known as ionizing radiation, is not fatal in small doses, because every person on Earth is exposed to small quantities of radiation every year. About 82% of this comes from natural sources, and 18% from manmade sources. Of course, some people are at much greater risk of radiation exposure than others: coal miners are exposed to higher levels of the radon-222 isotope present underground, while cigarette smokers ingest much higher levels of radiation than ordinary people, due to the polonium-210, lead-210, and radon-222 isotopes present in the nitrogen fertilizers used to grow tobacco. Nuclear weapons, as most people know, produce a great deal of radioactive pollution. However, atmospheric testing of nuclear armaments has long been banned, and though the isotopes released in such tests are expected to remain in the atmosphere for about a century, they do not constitute a significant health hazard to most Americans. (It should be noted that nations not inclined to abide by international protocols might still conduct atmospheric tests in defiance of the test bans.) Nuclear power plants, despite the great deal of attention they have received from the media and environmentalist groups, do not pose the hazard that has often been claimed: in fact, coal- and oil-burning power plants are responsible for far more radioactive pollution in the United States. This is not to say that nuclear energy poses no dangers, as the disaster at Chernobyl in the former Soviet Union has shown. In April 1986, an accident at a nuclear reactor in what is now the Ukraine killed 31 workers immediately, and ultimately led to the deaths of some 10,000 people. The fact that the radiation was allowed to spread had much to do with the secretive tactics of the Communist government, which attempted to cover up the problem rather than evacuate the area.

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Another danger associated with nuclear power plants is radioactive waste. Spent fuel rods and other waste products from these plants have to be dumped somewhere, but it cannot simply be buried in the ground because it will create a continuing health hazard through the water supply. No fully fail-safe storage system has been developed, and the problem of radioactive waste poses a continuing threat due to the extremely long half-lives of some of the isotopes involved.

Dating Techniques In addition to their uses in applications related to nuclear energy, isotopes play a significant role in dating techniques. The latter may sound like a subject that has something to do with romance, but it does not: dating techniques involve the use of materials, including isotopes, to estimate the age of both organic and inorganic materials. Uranium-238, for instance, has a half-life of 4.47 • 109 years, which is nearly the age of Earth; in fact, uranium-dating techniques have been used to determine the planet’s age, which is estimated at about 4.7 billion years. As noted elsewhere in this volume, potassium-argon dating, which involves the isotopes potassium-40 and argon-40, has been used to date volcanic layers in east Africa. Because the half-life of potassium-40 is 1.3 billion years, this method is useful for dating activities that are distant in the human scale of time, but fairly recent in geological terms. Another dating technique is radiocarbon dating, used for estimating the age of things that were once alive. All living things contain carbon, both in the form of the stable isotope carbon-12 and the radioisotope carbon-14. While a plant or animal is living, there is a certain proportion between the amounts of these two isotopes in the organism’s body, with carbon-12 being far more abundant. When the organism dies, however, it ceases to acquire new carbon, and the carbon-14 present in the body begins to decay into nitrogen-14. The amount of nitrogen-14 that has been formed is thus an indication of the amount of time that has passed since the organism was alive. Because it has a half-life of 5,730 years, carbon-14 is useful for dating activities within the span of human history, though it is not without controversy. Some scientists contend, for instance, that samples may be contaminated by carbon from the surrounding soils, thus affecting ratios and leading to inaccurate dates.

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Isotopes

KEY TERMS ATOM:

The smallest particle of an ele-

ION:

An atom or atoms that has lost or

ment. Atoms are made up of protons, neu-

gained one or more electrons, and thus has

trons, and electrons. Atoms that have the

a net electric charge.

same number of protons—that is, are of the same element—but differ in number of neutrons are known as isotopes.

ISOTOPES: Atoms that have an equal

number of protons, and hence are of the same element, but differ in their number of

An SI unit

neutrons. This results in a difference of

(abbreviated amu), equal to 1.66 • 10-24 g,

mass. Isotopes may be either stable or

for measuring the mass of atoms.

unstable. The latter type is known as a

ATOMIC

MASS

UNIT:

radioisotope. ATOMIC

NUMBER:

The number of

protons in the nucleus of an atom. Since this number is different for each element, elements are listed on the periodic table of elements in order of atomic number. AVERAGE ATOMIC MASS: A figure

used by chemists to specify the mass—in atomic mass units—of the average atom in a large sample.

NEUTRON:

A subatomic particle that

has no electric charge. Neutrons are found at the nucleus of an atom, alongside protons. NUCLEUS: The center of an atom, a

region where protons and neutrons are located, and around which electrons spin. The plural of “nucleus” is nuclei. NUCLIDES: Another name for isotopes.

ELEMENT: A substance made up of only

one kind of atom. Hence an element can-

PERIODIC TABLE OF ELEMENTS: A

not be chemically broken into other sub-

chart that shows the elements arranged in

stances.

order of atomic number.

ELECTRON: Negatively charged parti-

cles in an atom, which spin around the protons and neutrons that make up the atom’s nucleus.

PROTON:

A positively charged particle

in an atom. Protons and neutrons, which together form the nucleus around which electrons spin, have approximately the same mass—a mass that is many times

HALF-LIFE: The length of time it takes a

greater than that of an electron. The num-

substance to diminish to one-half its initial

ber of protons in the nucleus of an atom is

amount.

the atomic number of an element.

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Isotopes

KEY TERMS In a general sense, radia-

about by the emission of high-energy par-

tion can refer to anything that travels in a

ticles or radiation, including alpha parti-

stream, whether that stream be composed

cles, beta particles, or gamma rays.

of subatomic particles or electromagnetic

RADIOISOTOPE:

waves. In a more specific sense, the term

the decay associated with radioactivity. A

relates to the radiation from radioactive

radioisotope is thus an unstable isotope.

RADIATION:

materials, which can be harmful to human beings.

An isotope subject to

TRACER: An atom or group of atoms

whose participation in a chemical, physiA term describing a

cal, or biological reaction can be easily

phenomenon whereby certain materials

observed. Radioisotopes are often used as

are subject to a form of decay brought

tracers.

RADIOACTIVITY:

WHERE TO LEARN MORE “Carbon 14 Dating Calculator” (Web site). (May 15, 2001). Ebbing, Darrell D.; R. A. D. Wentworth; and James P. Birk. Introductory Chemistry. Boston: Houghton Mifflin, 1995. “Exploring the Table of Isotopes” (Web site). (May 15, 2001).

100

CONTINUED

“Isotopes” University of Colorado Department of Physics (Web site). (May 15, 2001). Milne, Lorus Johnson and Margery Milne. Understanding Radioactivity. Illustrated by Bill Hiscock. New York: Atheneum, 1989. Smith, Norman F. Millions and Billions of Years Ago: Dating Our Earth and Its Life. New York: F. Watts, 1993.

Goldstein, Natalie. The Nature of the Atom. New York: Rosen Publishing Group, 2001.

“Stable Isotope Group.” Martek Biosciences (Web site). (May 15, 2001).

“The Isotopes” (Web site). (May 15, 2001).

“Tracking with Isotopes” (Web site). (May 15, 2001).

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I O N S A N D I O N I Z AT I O N

CONCEPT Atoms have no electric charge; if they acquire one, they are called ions. Ions are involved in a form of chemical bonding that produces extremely strong bonds between metals, or between a metal and a nonmetal. These substances, of which table salt is an example, are called ionic compounds. Ionization is the process whereby electrons are removed from an atom or molecule, as well as the process whereby an ionic substance, such as salt, is dissociated into its component ions in a solution such as water. There are several varieties of ionization, including field ionization, which almost everyone has experienced in the form of static electricity. Ion exchange, or the replacement of one ion by another, is used in applications such as water purification, while chemists and physicists use ions in mass spectrometry, to discover mass and structural information concerning atoms and molecules. Another example of ions at work (and a particularly frightening example at that) is ionizing radiation, associated with the radioactive decay following a nuclear explosion.

HOW IT WORKS Ions: Positive and Negative Atoms have no electric charge, because they maintain an equal number of protons (positively charged subatomic particles) and electrons, subatomic particles with a negative charge. In certain situations, however, the atom may lose or gain one or more electrons and acquire a net charge, becoming an ion.

S C I E N C E O F E V E R Y D AY T H I N G S

Aluminum, for instance, has an atomic number of 13, which tells us that an aluminum atom will have 13 protons. Given the fact that every proton has a positive charge, and that most atoms tend to be neutral in charge, this means that there are usually 13 electrons, with a negative charge, present in an atom of aluminum. Yet like all metals, aluminum is capable of forming an ion by losing electrons—in this case, three. CATIONS. Initially, the aluminum atom had a charge of +13 + (–13) = 0; in other words, its charge was neutral due to the equal numbers of protons and electrons. When it becomes an ion, it loses 3 electrons, leaving behind only 10. Now the charge is +13 + (–10) = +3. Thus the remaining aluminum ion is said to have a net positive charge of 3, represented as +3 or 3+. Chemists differ as to whether they represent the plus sign (or the minus sign, in the case of a negatively charged ion) before or after the number. Because both systems of notation are used, these will be applied interchangeably throughout the course of this essay.

When a neutral atom loses one or more electrons, the result is a positively charged ion, or cation (pronounced KAT-ie-un). Cations are usually represented by a superscript number and plus sign: Al+3 or Al3+, for instance, represents the aluminum cation described above. A cation is named after the element of which it is an ion: thus the ion we have described is either called the aluminum ion, or the aluminum cation. ANIONS. When a neutrally charged atom gains electrons, acquiring a negative charge as a result, this type of ion is known as an anion (ANie-un). Anions can be represented symbolically in much the same way as cations: Cl-, for

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Ions and Ionization

A

COMMON FORM OF FIELD IONIZATION IS STATIC ELECTRICITY.

HERE,

A GIRL PLACES HER HAND ON A STATIC ELEC-

TRICITY GENERATOR. (Paul A. Souders/Corbis. Reproduced by permission.)

instance, is an anion of chlorine that forms when it acquires an electron, thus assuming a net charge of –1. Note that the 1 is not represented in the superscript notation, much as people do not write 101. In both cases, the 1 is assumed, but any number higher than 1 is shown. The anion described here is never called a chlorine anion; rather, anions have a special nomenclature. If the anion represents, as was the case here, a single element, it is named by adding the suffix -ide to the name of the original element name: chloride. Such is the case, for instance, with a deadly mixture of carbon and nitrogen (CN-), better known as cyanide. Most often the -ide suffix is used, but in the case of most anions involving more than one element (polyatomic anions), as well as with oxyanions (anions containing oxygen), the rules can get fairly complicated. The general principles for naming anions are as follows: • -ide: A single element with a negative charge. Note, however, that both hydroxide (OH–) and cyanide (CN–) also receive the -ide suffix, even though they involve more than one element. • -ate: An oxyanion with the normal number of oxygen atoms, a number that depends on

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•

•

•

•

the nature of the compound. Examples include oxalate (C2O4- 2) or chlorate (ClO3-). -ite: An oxyanion containing 1 less oxygen than normal. Examples include chlorite (ClO2–). hypo____ite: An oxyanion with 2 less oxygens than normal, but with the normal charge. An example is hypochlorite, or ClO–. per____ate: An oxyanion with 1 more oxygen than normal, but with the normal charge. Perchlorate, or ClO4–, is an example. thio-: An anion in which sulfur has replaced an oxygen. Thus, SO4–2 is called sulfate, whereas S2O3–2 is called thiosulfate.

Elements and Ion Charges As one might expect, given the many differences among families of elements on the periodic table, different elements form ions in different ways. Yet precisely because many of these can be grouped into families, primarily according to the column or group they occupy on the periodic table, it is possible to predict the ways in which they will form ions. The table below provides a few rules of thumb. (All group numbers refer to the North American version of the periodic table;

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THE

DAMAGED REACTOR AT THE

CHERNOBYL

NUCLEAR PLANT IN THE FORMER

SOVIET UNION. THE 1986

ACCIDENT

AT THE PLANT RELEASED IONIZING RADIATION INTO THE ATMOSPHERE. (AP/Wide World Photos. Reproduced by permission.)

see Periodic Table of Elements essay for an explanation of the differences between this and the IUPAC version.) • Alkali metals (Group 1) form 1+ cations. For example, the ion of lithium (Li) is always Li+. • Alkaline earth metals (Group 2) form 2+ cations. Thus, beryllium (Be), for instance, forms a Be2+ ion. • Most Group 3 metals (aluminum, gallium, and indium) form 3+ cations. The cation of aluminum, thus, is designated as Al3+. • Group 6 nonmetals and metalloids (oxygen, sulfur, selenium, and tellurium) form 2– anions. Oxygen, in its normal ionized state, is shown as O2–. • Halogens (Group 7) form 1– anions. Fluorine’s anion would therefore be designated as Fl–. The metals always form positive ions, or cations; indeed, one of the defining characteristics of a metal is that it tends to lose electrons. However, the many elements of the transition metals family form cations with a variety of different charges; for this reason, there is no easy way to classify the ways in which these elements form cations.

S C I E N C E O F E V E R Y D AY T H I N G S

Likewise, it should be evident from the above table that nonmetals, such as oxygen or fluorine, gain electrons to form anions. This, too, is a defining characteristic of this broad grouping of elements. The reasons why these elements— both metals and nonmetals—behave as they do are complex, involving the numbers of valence electrons (the electrons involved in chemical bonding) for each group on the periodic table, as well as the octet rule of chemical bonding, whereby elements typically bond so that each atom has eight valence electrons.

REAL-LIFE A PPLICAT ION S Ionic Bonds, Compounds, and Solids German chemist Richard Abegg (1869-1910), whose work with noble gases led to the discovery of the octet rule, hypothesized that atoms combine with one another because they exchange electrons in such a way that both end up with eight valence electrons. This was an early model of ionic bonding, which results from attractions between ions with opposite electric charges:

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when they bond, these ions “complete” one another. As noted, ionic bonds occur when a metal bonds with a nonmetal, and these bonds are extremely strong. Salt, for instance, is formed by an ionic bond between the metal sodium (a +1 cation) and the nonmetal chlorine, a -1 anion. Thus Na+ joins with Cl- to form NaCl, or table salt. The strength of the bond in salt is reflected by its melting point of 1,472°F (800°C), which is much higher than that of water at 32°F (0°C). In ionic bonding, two ions start out with different charges and end up forming a bond in which both have eight valence electrons. In a covalent bond, as is formed between nonmetals, two atoms start out as most atoms do, with a net charge of zero. Each ends up possessing eight valence electrons, but neither atom “owns” them; rather, they share electrons. Today, chemists understand that most bonds are neither purely ionic nor purely covalent; rather, there is a wide range of hybrids between the two extremes. The degree to which elements attract one another is a function of electronegativity, or the relative ability of an atom to attract valence electrons. IONIC COMPOUNDS AND SOLIDS. Not only is salt formed by an ionic

bond, but it is an ionic compound—that is, a compound containing at least one metal and nonmetal, in which ions are present. Salt is also an example of an ionic solid, or a crystalline solid that contains ions. A crystalline solidis a type of solid in which the constituent parts are arranged in a simple, definite geometric pattern that is repeated in all directions. There are three types of crystalline solid: a molecular solid (for example, sucrose or table sugar), in which the molecules have a neutral electric charge; an atomic solid (a diamond, for instance, which is pure carbon); and an ionic solid. Salt is not formed of ordinary molecules, in the way that water or carbon dioxide are. (Both of these are molecular compounds, though neither is a solid at room temperature.) The internal structure of salt can be depicted as a repeating series of chloride anions and sodium cations packed closely together like oranges in a crate. Actually, it makes more sense to picture them as cantaloupes and oranges packed together, with the larger chloride anions as the cantaloupes and the smaller sodium cations as the oranges.

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If a grocer had to pack these two fruits together in a crate, he would probably put down a layer of cantaloupes, then follow this with a layer of oranges in the spaces between the cantaloupes. This pattern would be repeated as the crate was packed, with the smaller oranges filling the spaces. Much the same is true of the way that chlorine “cantaloupes” and sodium “oranges” are packed together in salt. This close packing of positive and negative charges helps to form a tight bond, and therefore salt must be heated to a high temperature before it will melt. Solid salt does not conduct electricity, but when melted, it makes for an extremely good conductor. When it is solid, the ions are tightly packed, and thus there is no freedom of movement to carry the electric charge along; but when the structure is disturbed by melting, movement of ions is therefore possible.

Ionization and Ionization Energy Water is not a good conductor, although it will certainly allow an electric current to flow through it, which is why it is dangerous to operate an electrical appliance near water. We have already seen that salt becomes a good conductor when melted, but this can also be achieved by dissolving it in water. This is one type of ionization, which can be defined as the process in which one or more electrons are removed from an atom or molecule to create an ion, or the process in which an ionic solid, such as salt, dissociates into its component ions upon being dissolved in a solution. The amount of energy required to achieve ionization is called ionization energy or ionization potential. When an atom is at its normal energy level, it is said to be in a ground state. At that point, electrons occupy their normal orbital patterns. There is always a high degree of attraction between the electron and the positively charged nucleus, where the protons reside. The energy required to move an electron to a higher orbital pattern increases the overall energy of the atom, which is then said to be in an excited state. The excited state of the atom is simply a step along the way toward ionizing it by removing the electron. “Step” is an appropriate metaphor, because electrons do not simply drift along a continuum from one energy level to the next, the way a person walks gently up a ramp. Rather,

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they make discrete steps, like a person climbing a flight of stairs or a ladder. This is one of the key principles of quantum mechanics, a cuttingedge area of physics that also has numerous applications to chemistry. Just as we speak of a sudden change as a “quantum leap,” electrons make quantum jumps from one energy level to another. Due to the high attraction between the electron and the nucleus, the first electron to be removed is on the outermost orbital—that is, one of the valence electrons. This amount of energy is called the first ionization energy. To remove a second electron will be considerably more difficult, because now the atom is a cation, and the positive charge of the protons in the nucleus is greater than the negative charge of the electrons. Hence the second ionization energy, required to remove a second electron, is much higher than the first ionization energy. IONIZATION ENERGIES OF ELEMENTS AND COMPOUNDS. First and

second ionization energy levels for given elements have been established, though it should be noted that hydrogen, because it has only one electron, has only a first ionization energy. In general, the figures for ionization energy increase from left to right along a period or row on the periodic table, and decrease from top to bottom along a column or group. The reason ionization energy increases along a period is that nonmetals, on the right side of the table, have higher ionization energies than metals, which are on the left side. Ionization energies decrease along a group because elements lower on the table have higher atomic numbers, which means more protons and thus more electrons. It is therefore easier for them to give up one of their electrons than it is for an element with a lower atomic number—just as it would be easier for a millionaire to lose a dollar than it would be for a person earning minimum wage. For molecules in compounds, the ionization energies are generally related to those of the elements whose atoms make up the molecule. Just as elements with fewer electrons are typically less inclined to give up one, so are molecules with only a few atoms. Thus the ionization energy of carbon dioxide (CO2), with just three atoms, is relatively high. Conversely, in larger molecules as with larger atoms, there are more electrons to

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give up, and therefore it is easier to separate one of these from the molecules.

Ions and Ionization

A number of methods are used to produce ions for mass spectrometry (discussed below) or other applications. The most common of these methods is electron impact, produced by bombarding a sample of gas with a stream of fast-moving electrons. Though easier than some other methods, this one is not particularly efficient, because it supplies more energy than is needed to remove the electron. An electron gun, usually a heated tungsten wire, produces huge amounts of electrons, which are then fired at the gas. Because electrons are so small, this is rather like using a rapid-fire machine gun to kill mosquitoes: it is almost inevitable that some of the mosquitoes will get hit, but plenty of the rounds will fire into the air without hitting a single insect. IONIZATION

PROCESSES.

Another ionization process is field ionization, in which ionization is produced by subjecting a molecule to a very intense electric field. Field ionization occurs in daily life, when static electricity builds up on a dry day: the small spark that jumps from the tip of your finger when you touch a doorknob is actually a stream of electrons. In field ionization as applied in laboratories, fine, sharpened wires are used. This process is much more efficient than electron impact ionization, employing far less energy in relation to that required to remove the electrons. Because it deposits less energy into the ion source, the parent ion, it is often used when it is necessary not to damage the ionized specimen. Chemical ionization employs a method similar to that of electron impact ionization, except that instead of electrons, a beam of positively charged molecular ions is used to bombard and ionize the sample. The ions used in this bombardment are typically small molecules, such as those in methane, propane, or ammonia. Nonetheless, the molecular ion is much larger than an electron, and these collisions are highly reactive, yet they tend to be much more efficient than electron impact ionization. Many mass spectrometers use a source capable of both electron impact and chemical ionization. Ionization can also be supplied by electromagnetic radiation, with wavelengths shorter than those of visible light—that is, ultraviolet light, x rays, or gamma rays. This process, called photoionization, can ionize small molecules,

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such as those of oxygen (O2). Photoionization occurs in the upper atmosphere, where ultraviolet radiation from the Sun causes ionization of oxygen and nitrogen (N2) in their molecular forms. In addition to these other ionization processes, ionization can be produced in a fairly simple way by subjecting atoms or molecules to the heat from a flame. The temperatures, however, must be several thousand degrees to be effective, and therefore specialized flames, such as an electrical arc, spark, or plasma, are typically used. MASS SPECTROMETRY. As not-ed, a number of the ionization methods described above are used in mass spectrometry, a means of obtaining structure and mass information concerning atoms or molecules. In mass spectrometry, ionized particles are accelerated in a curved path through an electromagnetic field. The field will tend to deflect lighter particles from the curve more easily than heavier ones. By the time the particles reach the detector, which measures the ratio between mass and charge, the ions will have been separated into groups according to their respective mass-to-charge ratios.

When molecules are subjected to mass spectrometry, fragmentation occurs. Each molecule breaks apart in a characteristic fashion, and this makes it possible for a skilled observer to interpret the mass spectrum of the particles generated. Mass spectrometry is used to establish values for ionization energy, as well as to ascertain the mass of substances when that mass is not known. It can also be used to determine the chemical makeup of a substance. Mass spectrometry is applied by chemists, not only in pure research, but in applications within the environmental, pharmaceutical, and forensic (crime-solving) fields. Chemists for petroleum companies use it to analyze hydrocarbons, as do scientists working in areas that require flavor and fragrance analysis.

Ion Exchange The process of replacing one ion with another of a similar charge is called ion exchange. When an ionic solution—for example, salt dissolved in water—is placed in contact with a solid containing ions that are only weakly bonded within its crystalline structure, ion exchange is possible. Throughout the exchange, electric neutrality is maintained: in other words, the total number of

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positive charges in the solid and solution equals the total number of negative charges. The only thing that changes is the type of ion in the solid and in the solution. There are natural ion exchangers, such as zeolites, a class of minerals that contain aluminum, silicon, oxygen, and a loosely held cation of an alkali metal or alkaline earth metal (Group 1 and Group 2 of the periodic table, respectively). When the zeolite is placed in an ionic solution, exchange occurs between the loosely held zeolite cation and the dissolved cation. In synthetic ion exchangers, used in laboratories, a charged group is attached to a rigid structural framework. One end of the charged group is permanently fixed to the frame, while a positively or negatively charged portion kept loose at the other end attracts other ions in the solution. RESINS AND SEMIPERMEABLE MEMBRANES. Anion resins and cation

resins are both solid materials, but in an anion resin, the positive ions are tightly bonded, while the negative ions are loosely bonded. The negative ions will exchange with negative ions in the solution. The reverse is true for a cation resin, in which the negative ions are tightly bonded, while the loosely bonded positive ions exchange with positive ions in the solution. These resins have applications in scientific studies, where they are used to isolate and collect various types of ionic substances. They are also used to purify water, by removing all ions. Semipermeable membranes are natural or synthetic materials with the ability to selectively permit or retard the passage of charged and uncharged molecules through their surfaces. In the cells of living things, semipermeable membranes regulate the balance between sodium and potassium cations. Kidney dialysis, which removes harmful wastes from the urine of patients with kidneys that do not function properly, is an example of the use of semipermeable membranes to purify large ionic molecules. When seawater is purified by removal of salt, or otherwise unsafe water is freed of its impurities, the water is passed through a semipermeable membrane in a process known as reverse osmosis.

Ionizing Radiation We have observed a number of ways in which ionization is useful, and indeed part of nature. But ionizing radiation, which causes ionization

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Ions and Ionization

KEY TERMS The negatively charged ion that results when an atom gains one or more electrons. An anion (pronounced “AN-ieun”) of a single element is named by adding the suffix -ide to the name of the original element—hence, “chloride.” Other rules apply for more complex anions. ANION:

The positively charged ion that results when an atom loses one or more electrons. A cation (pronounced KAT-ieun) is named after the element of which it is an ion and, thus, is called, for instance, the aluminum ion or the aluminum cation. CATION:

CRYSTALLINE SOLID: A type of solid

in which the constituent parts have a simple and definite geometric arrangement that is repeated in all directions. Types of crystalline solids include atomic solids, ionic solids, and molecular solids. ELECTRON: Negatively charged parti-

cles in an atom. The number of electrons and protons in an atom is the same, thus canceling out one another. When an atom loses or gains one or more electrons, however—thus becoming an ion—it acquires a net electric charge. A term describing the characteristics of an atom that has acquired excess energy. EXCITED STATE:

A term describing the characteristics of an atom that is at its ordinary energy level. GROUND STATE:

An atom or group of atoms that has lost or gained one or more electrons, and thus has a net electric charge. There are two types of ions: anions and cations. ION:

The process of replacing one ion with another of similar charge. ION

EXCHANGE:

IONIC BONDING:

A form of chemical

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bonding that results from attractions between ions with opposite electric charges. IONIC

COMPOUND:

A compound

(two or more elements chemically bonded to one another), in which ions are present. Ionic compounds contain at least one metal and nonmetal, joined by an ionic bond. IONIC SOLID:

A form of crystalline

solid that contains ions. When mixed with a solvent such as water, ions from table salt—an example of an ionic solid—move freely throughout the solution, making it possible to conduct an electric current. IONIZATION:

A term that refers to two

different processes. In one kind of ionization, one or more electrons are removed from an atom or molecule to create an ion. A second kind of ionization occurs when an ionic solid, such as salt dissociates into its component ions upon being dissolved in a solution. IONIZATION ENERGY: The amount of

energy required to achieve ionization in which one or more electrons are removed from an atom or molecule. Another term for this is ionization potential. NUCLEUS: The center of an atom, a

region where protons and neutrons are located, and around which electrons spin. ORBITAL:

A pattern of probabilities

regarding the position of an electron for an atom in a particular energy state. OXYANION:

An

anion

containing

oxygen. POLYATOMIC

ANION:

An

anion

involving more than one element. PROTON:

A positively charged particle

in an atom.

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in the substance through which it passes, is extremely harmful. It should be noted that not all radiation is unhealthy: after all, Earth receives heat and light from the Sun by means of radiation. Ionizing radiation, on the other hand, is the kind of radiation associated with radioactive fallout from nuclear warfare, and with nuclear disasters such as the one at Chernobyl in the former Soviet Union in 1986. Whereas thermal radiation from the Sun can be harmful if one is exposed to it for too long, ionizing radiation is much more so, and involves much greater quantities of energy deposited per area per second. In ionizing radiation, an ionizing particle knocks an electron off an atom or molecule in a living system (that is, human, animal, or plant), freeing an electron. The molecule left behind becomes a free radical, a highly reactive group of atoms with unpaired electrons. These can spawn other free radicals, inducing chemical changes that can cause cancer and genetic damage. WHERE TO LEARN MORE “The Atom.” Thinkquest (Web site). (May 18, 2001).

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“Explore the Atom” CERN—European Organization for Nuclear Research (Web site). (May 18, 2001). Gallant, Roy A. The Ever-Changing Atom. New York: Benchmark Books, 1999. “Ion Exchange Chromatography” (Web site). (June 1, 2001). “Just After Big Bang, Colliding Gold Ions Exploded.” UniSci: Daily University Science News (Web site). (June 1, 2001). “A Look Inside the Atom” (Web site). (May 18, 2001). “Portrait of the Atom” (Web site). (May 18, 2001). “A Science Odyssey: You Try It: Atom Builder.” PBS—Public Broadcasting System (Web site). (May 18, 2001). “Table of Inorganic Ions and Rules for Naming Inorganic Compounds.” Augustana College (Web site.) (June 1, 2001). “Virtual Chemistry Lab.” University of Oxford Department of Chemistry (Web site). (June 1, 2001).

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Molecules

CONCEPT Prior to the nineteenth century, chemists pursued science simply by taking measurements, before and after a chemical reaction, of the substances involved. This was an external approach, rather like a person reaching into a box and feeling of the contents without actually being able to see them. With the evolution of atomic theory, chemistry took on much greater definition: for the first time, chemists understood that the materials with which they worked were interacting on a level much too small to see. The effects, of course, could be witnessed, but the activities themselves involved the interactions of atoms in molecules. Just as an atom is the most basic particle of an element, a molecule is the basic particle of a compound. Whereas there are only about 90 elements that occur in nature, many millions of compounds are formed naturally or artificially. Hence the study of the molecule is at least as important to the pursuit of modern chemistry as the study of the atom. Among the most important subjects in chemistry are the ways in which atoms join to form molecules—not just the numbers and types of atoms involved, but the shape that they form together in the molecular structure.

HOW IT WORKS Introduction to the Molecule Sucrose or common table sugar, of course, is grainy and sweet, yet it is made of three elements that share none of those characteristics. The formula for sugar is C12H22O1 1, meaning that each

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MOLECULES

molecule is formed by the joining of 12 carbon atoms, 22 hydrogens, and 11 atoms of oxygen. Coal is nothing like sugar—for one thing, it is as black as sugar is white, yet it is almost pure carbon. Carbon, at least, is a solid at room temperature, like sugar. The other two components of sugar, on the other hand, are gases, and highly flammable ones at that. The question of how elements react to one another, producing compounds that are altogether unlike the constituent parts, is one of the most fascinating aspects of chemistry and, indeed, of science in general. Combined in other ways and in other proportions, the elements in sugar could become water (H2O), carbon dioxide (CO2), or even petroleum, which is formed by the joining of carbon and hydrogen. Two different compounds of hydrogen and oxygen serve to further illustrate the curiosities involved in the study of molecules. As noted, hydrogen and oxygen are both flammable, yet when they form a molecule of water, they can be used to extinguish most fires. On the other hand, when two hydrogens join with two oxygens to form a molecule of hydrogen peroxide (H2O2), the resulting compound is quite different from water. In relatively high concentrations, hydrogen peroxide can burn the skin, and in still higher concentrations, it is used as rocket fuel. And whereas water is essential to life, pure hydrogen peroxide is highly toxic. THE QUESTION OF MOLECULAR STRUCTURE. It is not enough, how-

ever, to know that a certain combination of atoms forms a certain molecule, because molecules may have identical formulas and yet be quite different substances. In English, for

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Molecules

MODELS

OF VARIOUS ELEMENTS.

instance, there is the word “rose.” Simply seeing the word, however, does not tell us whether it is a noun, referring to a flower, or a verb, as in “she rose through the ranks.” Similarly, the formula of a compound does not necessarily tell what it is, and this can be crucial. For instance, the formula C2H6O identifies two very different substances. One of these is ethyl alcohol, the type of alcohol found in beer and wine. Note that the elements involved are the same as those in sugar, though the proportions are different: in fact, some aspects of the body’s reaction to ethyl alcohol are not so different from its response to sugar, since both lead to unhealthy weight gain. In reasonable small quantities, of course, ethyl alcohol is not toxic, or at least only mildly so; yet methyl ether—which has an identical formula—is a toxin. But the distinction is not simply an external one, as simple as the difference between beer and a substance such as methyl ether, sometimes used as a refrigerant. To put it another way, the external difference reflects an internal disparity: though the formulas for ethyl alcohol and methyl ether are the same, the arrangements of the atoms within the molecules of each are not. The substances are therefore said to be isomers.

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In fact C2H6O is just one of three types of formula for a compound: an empirical formula, or one that shows the smallest possible wholenumber ratio of the atoms involved. By contrast, a molecular formula—a formula that indicates the types and numbers of atoms involved— shows the actual proportions of atoms. If the formula for glucose, a type of sugar (C6H12O6 ), were rendered in empirical form, it would be CH2O, which would reveal less about its actual structure. Most revealing of all, however, is a structural formula—a diagram that shows how the atoms are bonded together, complete with lines representing covalent bonds. (Structural formulas such as those that apply the Couper or Lewis systems are discussed in the Chemical Bonding essay, which also examines the subject of covalent bonds.) Chemists involved in the area of stereochemistry, discussed below, attempt to develop three-dimensional models to show how atoms are arranged in a molecule. Such models for ethyl alcohol and methyl ether, for instance, would reveal that they are quite different, much as the two definitions of rose mentioned above illustrate the two distinctly different meanings. Because stereochemistry is a highly involved and complex subject, it can only be touched upon very briefly in this essay; nonetheless, an under-

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Molecules

TWO

HIGH SCHOOL STUDENTS POSE WITH A MODEL OF A

standing of a molecule’s actual shape is critical to the work of a professional chemist. MOLECULES AND COMPOUNDS. A molecule can be most properly

defined as a group of atoms joined in a specific structure. A compound, on the other hand, is a substance made up of more than one type of atom—in other words, more than one type of element. Not all compounds are composed of discrete molecules, however. For instance, table salt (NaCl) is an ionic compound formed by endlessly repeating clusters of sodium and chlorine that are not, in the strictest sense of the word, molecules. Salt is an example of a crystalline solid, or a solid in which the constituent parts are arranged in a simple, definite geometric pattern repeated in all directions. There are three kinds of crystalline solids, only one of which has a truly molecular structure. In an ionic solid such as table salt, ions (atoms, or groups of atoms, with an electric charge) bond a metal to a nonmetal— in this case, the metal sodium and the nonmetal chlorine. Another type of crystalline solid, an atomic solid, is formed by atoms of one element bonding to one another. A diamond, made of pure carbon, is an example. Only the third type of crystalline solid is truly molecular in structure:

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DNA

MOLECULE. (James A. Sugar/Corbis. Reproduced by permission.)

a molecular solid—sugar, for example—is one in which the molecules have a neutral electric charge. Not all solids are crystalline; nor, of course, are all compounds solids: water, obviously, is a liquid at room temperature, while carbon dioxide is a gas. Nor is every molecule composed of more than one element. Oxygen, for instance, is ordinarily diatomic, meaning that even in its elemental form, it is composed of two atoms that join in an O2 molecule. It is obvious, then, that the defining of molecules is more complex than it seems. One can safely say, however, that the vast majority of compounds are made up of molecules in which atoms are arranged in a definite structure. In the essay that follows, we will discuss the ways atoms join to form molecules, a subject explored in more depth within the Chemical Bonding essay. (In addition, compounds themselves are examined in somewhat more detail within the Compounds essay.) We will also briefly examine how molecules bond to other molecules in the formation of solids and liquids. First, however, a little history is in order: as noted in the introduction to this essay, chemists did not always possess a clear understanding of the nature of a molecule.

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A Brief History of the Molecule In ancient and medieval times, early chemists— some of whom subscribed to an unscientific system known as alchemy—believed that one element could be transformed into another. Thus many an alchemist devoted an entire career to the vain pursuit of turning lead into gold. The alchemists were at least partially right, however: though one element cannot be transformed into another (except by nuclear fusion), it is possible to change the nature of a compound by altering the relations of the elements within it. Modern understanding of the elements began to emerge in the seventeenth century, but the true turning point came late in the eighteenth century. It was then that French chemist Antoine Lavoisier (1743-1794) defined an element as a simple substance that could not be separated into simpler substances by chemical means. Around the same time, another French chemist, JosephLouis Proust (1754-1826) stated that a given compound always contained the same proportions of mass between elements. The ideas of Lavoisier and Proust were revolutionary at the time, and these concepts pointed to a substructure, invisible to the naked eye, underlying all matter. In 1803, English chemist John Dalton (17661844) defined that substructure by introducing the idea that the material world is composed of tiny particles called atoms. Despite the enormous leap forward that his work afforded to chemists, Dalton failed to recognize that matter is not made simply of atoms. Water, for instance, is not just a collection of “water atoms”: clearly, there is some sort of intermediary structure in which atoms are combined. This is the molecule, a concept introduced by Italian physicist Amedeo Avogadro (1776-1856). AVOGADRO AND THE IDEA OF THE MOLECULE. French chemist and

physicist Joseph Gay-Lussac (1778-1850) had announced in 1809 that gases combine to form compounds in simple proportions by volume. As Gay-Lussac explained, the ratio, by weight, between hydrogen and oxygen in water is eight to one. The fact that this ratio was so “clean,” involving whole numbers rather than decimals, intrigued Avogadro, who in 1811 proposed that equal volumes of gases have the same number of particles if measured at the same temperature

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and pressure. This, in turn, led him to the hypothesis that water is not composed simply of atoms, but of molecules in which hydrogen and oxygen combine. For several decades, however, chemists largely ignored Avogadro’s idea of the molecule. Only in 1860, four years after his death, was the concept resurrected by Italian chemist Stanislao Cannizzaro (1826-1910). Of course, the understanding of the molecule has progressed enormously in the years since then, and much of this progress is an outcome of advances in the study of subatomic structure. Only in the early twentieth century did physicists finally identify the electron, the negatively charged subatomic particle critical to the bonding of atoms.

REAL-LIFE A PPLICAT I O N S Molecular Mass Just as the atoms of elements have a definite mass, so do molecules—a mass equal to that of the combined atoms in the molecule. The figures for the atomic mass of all elements are established, and can be found on the periodic table; therefore, when one knows the mass of a hydrogen atom and an oxygen atom, as well as the fact that there are two hydrogens and one oxygen in a molecule of water, it is easy to calculate the mass of a water molecule. Individual molecules cannot easily be studied; therefore, the mass of molecules is compared by use of a unit known as the mole. The mole contains 6.022137 ⫻ 1023 molecules, a figure known as Avogadro’s number, in honor of the man who introduced the concept of the molecule. When necessary, it is possible today to study individual molecules, or even atoms and subatomic particles, using techniques such as mass spectrometry.

Bonding Within Molecules Note that the mass of an atom in a molecule does not change; nor, indeed, do the identities of the individual atoms. An oxygen atom in water is the same oxygen atom in sugar, or in any number of other compounds. With regard to compounds, it should be noted that these are not the same thing as a mixture, or a solution. Sugar or salt can be dissolved in water at the appropriate tempera-

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tures, but the resulting solution is not a compound; the substances are joined physically, but they are not chemically bonded. Chemical bonding is the joining, through electromagnetic force, of atoms representing different elements. Each atom possesses a certain valency, which determines its ability to bond with atoms of other elements. Valency, in turn, is governed by the configuration of valence electrons at the highest energy level (the shell) of the atom. While studying noble gases, noted for their tendency not to bond, German chemist Richard Abegg (1869-1910) discovered that these gases always have eight valence electrons. This led to the formation of the octet rule: most elements (with the exception of hydrogen and a few others) are inclined to bond in such a way that they end up with eight valence electrons. When a metal bonds to a nonmetal, this is known as ionic bonding, which results from attractions between ions with opposite electric charges. In ionic bonding, two ions start out with different charges and form a bond in which both have eight valence electrons. Nonmetals, however, tend to form covalent bonds. In a covalent bond, two atoms start out as most atoms do, with a net charge of zero. Each ends up possessing eight valence electrons, but neither atom “owns” them; rather, they share electrons. ELECTRONEGATIVITY. Not all elements bond covalently in the same way. Each has a certain value of electronegativity—the relative ability of an atom to attract valence electrons. Elements capable of bonding are assigned an electronegativity value ranging from a minimum of 0.7 for cesium to a maximum of 4.0 for fluorine. The greater the electronegativity value, the greater the tendency of an element to attract valence electrons.

When substances of differing electronegativity values form a covalent bond, this is described as polar covalent bonding. Water is an example of a molecule with a polar covalent bond. Because oxygen has a much higher electronegativity (3.5) than hydrogen (2.1), the electrons tend to gravitate toward the oxygen atom. By contrast, molecules of petroleum, a combination of carbon and hydrogen, tend to be nonpolar, because carbon (2.5) and hydrogen have very similar electronegativity values.

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A knowledge of electronegativity values can be used to make predictions concerning bond polarities. Bonds that involve atoms whose electronegativities differ by more than 2 units are substantially ionic, whereas bonds between atoms whose electronegativities differ by less than 2 units are polar covalent. If the atoms have the same or similar electronegativity values, the bond is covalent.

Molecules

Attractions Between Molecules The energy required to pull apart a molecule is known as bond energy. Covalent bonds that involve hydrogen are among the weakest bonds between atoms, and hence it is relatively easy to separate water into its constituent parts, hydrogen and oxygen. (This is sometimes done by electrolysis, which involves the use of an electric current to disperse atoms.) Double and triple covalent bonds are stronger, but strongest of all is an ionic bond. The strength of the bond energy in salt, for instance, is reflected by its melting point of 1,472°F (800°C), much higher than that of water, at 32°F (0°C). Bond energy relates to the attraction between atoms in a molecule, but in considering various substances, it is also important to recognize the varieties of bonds between molecules— that is, intermolecular bonding. For example, the polar quality of a water molecule gives it a great attraction for ions, and thus ionic substances such as salt and any number of minerals dissolve easily in water. On the other hand, we have seen that petroleum is essentially nonpolar, and therefore, an oil molecule offers no electric charge to bond it with a water molecule. For this reason, oil and water do not mix. The bonding between water molecules is known as a dipole-dipole attraction. This type of intermolecular bond can be fairly strong in the liquid or solid state, though it is only about 1% as strong as a covalent bond within a molecule. When a substance containing molecules joined by dipole-dipole attraction is heated to become a gas, the molecules spread far apart, and these bonds become very weak. On the other hand, when hydrogen bonds to an atom with a high value of electronegativity (fluorine, for example), the dipole-dipole attraction between these molecules is particularly strong. This is known as hydrogen bonding.

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Molecules

KEY TERMS AVOGADRO’S

A figure,

NUMBER:

A chemical

named after Italian physicist Amedeo Avo-

formula that shows the smallest possible

gadro (1776-1856), equal to 6.022137 x

whole-number ratio of the atoms involved.

1023.

Compare with molecular formula and

Avogadro’s number indicates the

number of molecules in a mole.

structural formula.

BOND ENERGY: The energy required

HYDROGEN

to pull apart the atoms in a chemical bond.

dipole-dipole attraction between mole-

The joining,

cules formed of hydrogen along with an

CHEMICAL BONDING:

through electromagnetic force, of atoms

COMPOUND:

A substance made up of

atoms of more than one element. These atoms are usually joined in molecules. COVALENT

BONDING:

bonding that exists between molecules. This is not to be confused with chemical bonding, the bonding of atoms within a molecule.

share valence electrons.

ION:

An atom or group of atoms that has

lost or gained one or more electrons, and thus has a net electric charge.

A term describing an ele-

ment that exists as molecules composed of two atoms. DIPOLE-DIPOLE

A kind of

element having a high electronegativity.

A type of

chemical bonding in which two atoms

DIATOMIC:

BONDING:

INTERMOLECULAR BONDING: The

representing different elements.

IONIC BONDING:

bonding

A form of chemical

resulting

from

attractions

between ions with opposite electric ATTRACTION:

A

form of intermolecular bonding between molecules formed by a polar covalent bond.

charges. ISOMERS: Substances having the same

chemical formula, but which are chemically dissimilar due to differences in the

ELECTRON: A negatively charged parti-

arrangement of atoms.

cle in an atom.

LONDON DISPERSION FORCES: A

The relative

term describing the weak intermolecular

ability of an atom to attract valence

bond between molecules that are not

electrons.

formed by a polar covalent bond.

ELECTRONEGATIVITY:

Even a nonpolar molecule, however, must have some attraction to other nonpolar molecules. The same is true of helium and the other noble gases, which are highly nonattractive but can be turned into liquids or even solids at extremely low temperatures. The type of intermolecular attraction that exists in such a situation is described by the term London dispersion forces. The name has nothing to do with the capital of England: it is a reference to German-

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VOLUME 1: REAL-LIFE CHEMISTRY

American physicist Fritz Wolfgang London (1900-1954), who in the 1920s studied the molecule from the standpoint of quantum mechanics. Because electrons are not uniformly distributed around the nucleus of an atom at every possible moment, instantaneous dipoles are formed when most of the electrons happen to be on one side of an atom. Of course, this only happens for an infinitesimal fraction of time, but it serves to create a weak attraction. Only at very low tem-

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Molecules

KEY TERMS

CONTINUED

The SI fundamental unit for

POLAR COVALENT BONDING: The

“amount of substance.” A mole is, general-

type of chemical bonding between atoms

ly speaking, Avogadro’s number of mole-

that have differing values of electronegativ-

cules; however, in the more precise SI defi-

ity. Water molecules are an example of a

nition, a mole is equal to the number of

polar covalent bond.

MOLE:

carbon atoms in 12.01 g of carbon. MOLECULAR FORMULA: A chemical

formula that indicates the types and num-

SHELL:

The orbital pattern of the

valence electrons at the outside of an atom.

bers of atoms involved, showing the actual

STEREOCHEMISTRY:

proportions of atoms in a molecule. Com-

chemistry devoted to the three-dimension-

pare with empirical formula and structural

al arrangement of atoms in a molecule.

The area of

formula. MOLECULAR SOLID: A form of crys-

talline solid—a solid in which the constituent parts have a simple and definite geometric arrangement repeated in all directions—in which the molecules have a

STRUCTURAL FORMULA: A diagram

that shows how the atoms are bonded together, complete with lines representing covalent bonds. Compare with empirical formula and molecular formula.

neutral electric charge. Table sugar

VALENCE

(sucrose) is an example.

that occupy the highest energy levels in an

MOLECULE: A group of atoms, usually

atom. These are the only electrons involved

but not always representing more than one

in chemical bonding.

element,

joined

in

a

structure.

Compounds are typically made up of molecules.

ELECTRONS:

Electrons

VALENCY: The property of the atom of

one element that determines its ability to bond with atoms of other elements.

OCTET RULE: A term describing the

distribution of valence electrons that takes

VSEPR

place in chemical bonding for most ele-

TRON PAIR REPULSION) MODEL: A

ments, which end up with eight valence

means of representing the three-dimen-

electrons.

sional structure of atoms in a molecule.

peratures do London dispersion forces become strong enough to result in the formation of a solid. (Thus, for instance, oil and rubbing alcohol freeze only at low temperatures.)

Molecular Structure The Couper system and Lewis structures, discussed in the Chemical Bonding essay, provide a means of representing the atoms that make up a molecule. Though Lewis structures show the dis-

S C I E N C E O F E V E R Y D AY T H I N G S

(VALENCE

SHELL

ELEC-

tribution of valence electrons, they do not represent the three-dimensional structure of the molecule. As noted earlier in this essay, the structure is highly important, because two compounds may be isomers, meaning that they have the same proportions of the same elements, yet are different substances. Stereochemistry is the realm of chemistry devoted to the three-dimensional arrangement of atoms in a molecule. One of the most impor-

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tant methods used is known as the VSEPR model (valence shell electron pair repulsion). In bonding, elements always share at least one pair of electrons, and the VSEPR model begins with the assumption that the electron pairs must be as far apart as possible to minimize their repulsion, since like charges repel. VSEPR structures can be very complex, and the rules governing them will not be discussed here, but a few examples can be given. If there are just two electron pairs in a bond between three atoms, the structure of a VSEPR model is like that of a stick speared through a ball, with two other balls attached at each end. The “ball” is an atom, and the “stick” represents the electron pairs. In water, there are four electron pairs, but still only three atoms and two bonds. In order to keep the electron pairs as far apart as possible, the angle between the two hydrogen atoms attached to the oxygen is 109.5°.

116

Elements of Chemistry for Beginning Life Science Students. Merced, CA: Bioventure Associates, 1990. Burnie, David. Microlife. New York: DK Publishing, 1997. “Common Molecules” (Web site). (June 2, 2001). Cooper, Christopher. Matter. New York: DK Publishing, 2000. Mebane, Robert C. and Thomas R. Rybolt. Adventures with Atoms and Molecules, Book V: Chemistry Experiments for Young People. Springfield, NJ: Enslow Publishers, 1995. “The Molecules of Life” (Web site). (June 2, 2001). “Molecules of the Month.” University of Oxford (Web site). (June 2, 2001). “Molecules with Silly or Unusual Names” (Web site). (June 2, 2001).

WHERE TO LEARN MORE

“Theory of Atoms in Molecules” (Web site). (June 2, 2001).

Basmajian, Ronald; Thomas Rodella; and Allen E. Breed. Through the Molecular Maze: A Helpful Guide to the

Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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S C I E N C E O F E V E R Y D AY T H I N G S real-life chemistry

ELEMENTS ELEMENTS P E R I O D I C TA B L E O F E L E M E N T S FA M I L I E S O F E L E M E N T S

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Elements

CONCEPT The elements are at the heart of chemistry, and indeed they are at the heart of life as well. Every physical substance encountered in daily life is either an element, or more likely a compound containing more than one element. There are millions of compounds, but only about 100 elements, of which only 88 occur naturally on Earth. How can such vast complexity be created from such a small amount of elements? Consider the English alphabet, with just 26 letters, with which an almost infinite number of things can be said or written. Even more is possible with the elements, which greatly outnumber the letters of the alphabet. Despite the relatively great quantity of elements, however, just two make up almost the entire mass of the universe—and these two are far from abundant on Earth. A very small number of elements, in fact, are essential to life on this planet, and to the existence of human beings.

HOW IT WORKS The Focal Point of Chemistry Like physics, chemistry is concerned with basic, underlying processes that explain how the universe works. Indeed, these two sciences, along with astronomy and a few specialized fields, are the only ones that address phenomena both on the Earth and in the universe as a whole. By contrast, unless or until life on another planet is discovered, biology has little concern with existence beyond Earth’s atmosphere, except inasmuch as the processes and properties in outer space affect astronauts.

S C I E N C E O F E V E R Y D AY T H I N G S

ELEMENTS

While physics and chemistry address many of the same fundamental issues, they do so in very different ways. To make a gross generalization—subject to numerous exceptions, but nonetheless useful in clarifying the basic difference between the two sciences—physicists are concerned with external phenomena, and chemists with internal ones. For instance, when physicists and chemists study the interactions between atoms, unless physicists focus on some specialized area of atomic research, they tend to treat all atoms as more or less the same. Chemists, on the other hand, can never treat atoms as though they are just undifferentiated particles colliding in space. The difference in structure between one kind of atom and another, in fact, is the starting-point of chemical study.

Structure of the Atom An atom is the fundamental particle in a chemical element, or a substance that cannot be broken down into another substance by chemical means. Clustered at the center, or nucleus, of the atom are protons, which have a positive electric charge, and neutrons, which possess no charge. Spinning around the nucleus are electrons, which are negatively charged. The vast majority of the atom’s mass is made up by the protons and neutrons, which have approximately the same mass, whereas that of the electron is much smaller. The mass of an electron is about 1/1836 that of proton, and 1/1839 that of a neutron. It should be noted that the nucleus, though it constitutes most of the atom’s mass, is only a tiny portion of the atom’s volume. If the nucleus were the size of a grape, in fact, the electrons would, on average, be located about a mile away.

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Elements

THE

MOST COMMON AND/OR IMPORTANT CHEMICAL ELEMENTS.

Isotopes Atoms of the same element always have the same number of protons, and since this figure is unique for a given element, each element is

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assigned an atomic number equal to the number of protons in its nucleus. Two atoms may have the same number of protons, and thus be of the same element, yet differ in their number of neutrons. Such atoms are called isotopes.

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Isotopes are generally represented as follows: where S is the symbol of the element, a is the atomic number, and m is the mass number—the sum of protons and neutrons in the atom’s nucleus. For the stable silver isotope designated as 93 ᎏᎏAg, for instance, Ag is the element symbol (dis47 cussed below); 47 its atomic number; and 93 the mass number. From this, it is easy to discern that this particular stable isotope has 46 neutrons in its nucleus. m ᎏᎏS, a

Because the atomic number of any element is established, sometimes isotopes are represented simply with the mass number thus: 93Ag. They may also be designated with a subscript notation indicating the number of neutrons, so that one can obtain this information at a glance without having to do the arithmetic. For the silver isotope 93 shown here, this is written as ᎏ4ᎏ7 Ag46. Isotopes are sometimes indicated by simple nomenclature as well: for instance, carbon-12 or carbon-13.

Ions The number of electrons in an atom is usually the same as the number of protons, and thus most atoms have a neutral charge. In certain situations, however, the atom may lose or gain one or more electrons and acquire a net charge, becoming an ion. The electrons are not “lost” when an atom becomes an ion: they simply go elsewhere. Aluminum (Al), for instance, has an atomic number of 13, which tells us that an aluminum atom will have 13 protons. Given the fact that every proton has a positive charge, and that most atoms tend to be neutral in charge, this means that there are usually 13 electrons, with a negative charge, present in an atom of aluminum. Aluminum may, however, form an ion by losing three electrons.

represents the aluminum cation described above. (Some chemists represent this with the plus sign before the number—for example, Al+3.) A cation is named after the element of which it is an ion: thus the ion we have described is called either the aluminum ion, or the aluminum cation. ANIONS. When a neutrally charged atom gains electrons, and as a result acquires a negative charge, this type of ion is known as an anion (AN-ie-un). Anions can be represented symbolically in much the same way of cations: C1-, for instance, is an anion of chlorine that forms when it acquires an electron, thus assuming a net charge of –1. Note that the 1 is not represented in the superscript notation, much as people do not write 101. In both cases, the 1 is assumed, whereas any number higher than 1 is shown.

The anion described here is never called a chlorine anion; rather, anions have a special nomenclature. If the anion represents, as is the case here, a single element, it is named by adding the suffix -ide to the name of the original element name: hence it would be called chloride. Other anions involve more than one element, and in these cases other rules apply for designating names. A few two-element anions use the -ide ending; such is the case, for instance, with a deadly mixture of carbon and nitrogen (CN–), better known as cyanide. For anions involving oxygen, there may be different prefixes and suffixes, depending on the relative number of oxygen atoms in the anion.

REAL-LIFE A PPLICAT ION S The Periodic Table

CATIONS. After its three electrons have departed, the remaining aluminum ion has a net positive charge of 3, represented as +3. How do we know this? Initially the atom had a charge of +13 + (–13) = 0. With the exit of the 3 electrons, leaving behind only 10, the picture changes: now the charge is +13 + (–10) = +3.

Note that an ion is never formed by a change in the number of protons: that number, as noted earlier, is a defining characteristic of an element. If all we know about a particular atom is that it has one proton, we can be certain that it is an atom of hydrogen. Likewise, if an atom has 79 protons, it is gold.

When a neutral atom loses one or more electrons, the result is a positively charged ion, or cation (pronounced KAT-ie-un). Cations are represented by a superscript number and plus sign after the element symbol: Al3+, for instance,

Knowing these quantities is not a matter of memorization: rather, one can learn this and much more by consulting the periodic table of elements. The periodic table is a chart, present in virtually every chemistry classroom in the world,

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showing the elements arranged in order of atomic number. Elements are represented by boxes containing the atomic number, element symbol, and average atomic mass, in atomic mass units, for that particular element. Vertical columns within the periodic table indicate groups or “families” of elements with similar chemical characteristics. These groups include alkali metals, alkaline earth metals, halogens, and noble gases. In the middle of the periodic table is a wide range of vertical columns representing the transition metals, and at the bottom of the table, separated from it, are two other rows for the lanthanides and actinides.

An Overview of the Elements As of 2001, there 112 elements, of which 88 occur naturally on Earth. (Some sources show 92 naturally occurring elements; however, a few of the elements with atomic numbers below 92 have not actually been found in nature.) The others were created synthetically, usually in a laboratory, and because these are highly radioactive, they exist only for fractions of a second. The number of elements thus continues to grow, but these “new” elements have little to do with the daily lives of ordinary people. Indeed, this is true even for some of the naturally occurring elements: few people who are not chemically trained, for instance, are able to identify thulium, which has an atomic number of 69. Though an element can theoretically exist as a gas, liquid, or a solid, in fact the vast majority of elements are solids. Only 11 elements—the six noble gases, along with hydrogen, nitrogen, oxygen, fluorine, and chlorine—exist in the gaseous state at a normal temperature of about 77°F (25°C). Just two are liquids at normal temperature: mercury, a metal, and the non-metal bromine. (The metal gallium becomes liquid at just 85.6°F, or 29.76°C.) The rest are all solids. The noble gases are monatomic, meaning that they exist purely as single atoms. So too are the “noble metals,” such as gold, silver, and platinum. “Noble” in this context means “set apart”: noble gases and noble metals are known for their tendency not to react to, and hence not to bond with, other elements. On the other hand, a number of other elements are described as diatomic, meaning that two atoms join to form a molecule.

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Names of the Elements ELEMENT SYMBOL. As noted above, the periodic table includes the element symbol or chemical symbol—a one-or two-letter abbreviation for the name of the element. Many of these are simple one-letter designations: O for oxygen, or C for carbon. Others are two-letter abbreviations, such as Ne for neon or Si for silicon. Note that the first letter is always capitalized, and the second is always lowercase.

In many cases, the two-letter symbols indicate the first and second letters of the element’s name, but this is far from universal. Cadmium, for example, is abbreviated Cd, while platinum is Pt. In other cases, the symbol seems to have nothing to do with the element name as it is normally used: for instance, Au for gold or Fe for iron. Many of the one-letter symbols indicate elements discovered early in history. For instance, carbon is represented by C, and later “C” elements took two-letter designations: Ce for cerium, Cr for chromium, and so on. But many of those elements with apparently strange symbols were among the first discovered, and this is precisely why the symbols make little sense to a person who does not recognize the historical origins of the name. HISTORICAL BACKGROUND ON SOME ELEMENT NAMES. For many

years, Latin was the language of communication between scientists from different nations; hence the use of Latin names such as aurum (“shining dawn”) for gold, or ferrum, the Latin word for iron. Likewise, lead (Pb) and sodium (Na) are designated by their Latin names, plumbum and natrium, respectively. Some chemical elements are named for Greek or German words describing properties of the element—for example, bromine (Br), which comes from a Greek word meaning “stench.” The name of cobalt comes from a German term meaning “underground gnome,” because miners considered the metal a troublemaker. The names of several elements with high atomic numbers reflect the places where they were originally discovered or created: francium, germanium, americium, californium. Americium and californium, with atomic numbers of 95 and 98 respectively, are among those elements that do not occur naturally, but

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were created artificially. The same is true of several elements named after scientists—among them einsteinium, after Albert Einstein (18791955), and nobelium after Alfred Nobel (18331896), the Swedish inventor of dynamite who established the Nobel Prize.

Elements

Abundance of Elements IN THE UNIVERSE. The first two elements on the periodic table, hydrogen and helium, represent 99.9% of the matter in the entire universe. This may seem astounding, but Earth is a tiny dot within the vastness of space, and hydrogen and helium are the principal elements in stars.

All elements, except for those created artificially, exist both on Earth and throughout the universe. Yet the distribution of elements on Earth is very, very different from that in other places—as well it should be, given the fact that Earth is the only planet known to support life. Hydrogen, for instance, constitutes only about 0.87%, by mass, of the elements found in the planet’s crust, waters, and atmosphere. As for helium, it is not even among the 18 most abundant elements on Earth. ON EARTH. That great element essential to animal life, oxygen, is by far the most plentiful on Earth, representing nearly half—49.2%—of the total mass of atoms found on this planet. (Here the term “mass” refers to the known elemental mass of the planet’s atmosphere, waters, and crust; below the crust, scientists can only speculate, though it is likely that much of Earth’s interior consists of iron.) Together with silicon (25.7%), oxygen accounts for almost exactly three-quarters of the elemental mass of Earth. Add in aluminum (7.5%), iron (4.71%), calcium (3.39%), sodium (2.63%), potassium (2.4%), and magnesium (1.93%), and these eight elements make up about 97.46% of Earth’s material.

In addition to hydrogen, whose distribution is given above, nine other elements account for a total of 2% of Earth’s composition: titanium (0.58%), chlorine (0.19%), phosphorus (0.11%), manganese (0.09%), carbon (0.08%), sulfur (0.06%), barium (0.04%), nitrogen (0.03%), and fluorine (0.03%). The remaining 0.49% is made up of various elements.

S C I E N C E O F E V E R Y D AY T H I N G S

THIS

WOMAN’S GOITER IS PROBABLY THE RESULT OF A

LACK OF IODINE IN HER DIET. (Lester V. Bergman/Corbis. Repro-

duced by permission.)

Elements In the Human Body Fans of science-fiction are familiar with the phrase “carbon-based life form,” which is used, for instance, by aliens in sci-fi movies to describe humans. In fact, the term is a virtual redundancy on Earth, since all living things contain carbon. Essential though it is to life, carbon as a component of the human body takes second place to oxygen, which is an even larger proportion of the body’s mass—65.0%—than it is of the Earth. Carbon accounts for 18%, and hydrogen for 10%, meaning that these three elements make up 93% of the body’s mass. Most of the remainder is taken up by 10 other elements: nitrogen (3%), calcium (1.4%), phosphorus (1.0%), magnesium (0.50%), potassium (0.34%), sulfur (0.26%), sodium (0.14%), chlorine (0.14%), iron (0.004%), and zinc (0.003%). TRACE ELEMENTS. As small as the amount of zinc is in the human body, there are still other elements found in even smaller quantities. These are known as trace elements, because only traces of them are present in the body. Yet they are essential to human well-being: without enough iodine, for instance, a person can devel-

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Elements

KEY TERMS The negative ion that results when an atom gains one or more electrons. An anion (pronounced “AN-ie-un”) of an element is never called, for instance, the chlorine anion. Rather, for an anion involving a single element, it is named by adding the suffix -ide to the name of the original element—hence, “chloride.” Other rules apply for more complex anions.

ANION:

The smallest particle of an element. An atom can exist either alone or in combination with other atoms in a molecule. Atoms are made up of protons, neutrons, and electrons. An atom that loses or gains one or more electrons, and thus has a net charge, is an ion. Atoms that have the same number of protons—that is, are of the same element—but differ in number of neutrons are known as isotopes. ATOM:

An SI unit (abbreviated amu), equal to 1.66 • 10-24 g, for measuring the mass of atoms. ATOMIC

MASS

UNIT:

The number of protons in the nucleus of an atom. Since this number is different for each element, elements are listed on the periodic table in order of atomic number. ATOMIC

NUMBER:

AVERAGE ATOMIC MASS: A figure

used by chemists to specify the mass—in atomic mass units—of the average atom in

op a goiter, a large swelling in the neck area. Chromium helps the body metabolize sugars, which is why people concerned with losing weight and/or toning their bodies through exercise may take a chromium supplement. Even arsenic, lethal in large quantities, is a trace element in the human body, and medicines for treating illnesses such as the infection known as “sleeping sickness” contain tiny amounts of arsenic. Other trace elements include cobalt, cop-

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a large sample. If a substance is a compound, the average atomic mass of all atoms in a molecule of that substance must be added together to yield the average molecular mass of that substance. The positive ion that results when an atom loses one or more electrons. A cation (pronounced KAT-ie-un) is named after the element of which it is an ion and thus is called, for instance, the aluminum ion or the aluminum cation. CATION:

CHEMICAL SYMBOL:

Another term

for element symbol. A substance made up of atoms of more than one element. These atoms are usually joined in molecules.

COMPOUND:

A term describing an element that exists as molecules composed of two atoms. This is in contrast to monatomic elements.

DIATOMIC:

ELECTRON: Negatively charged parti-

cles in an atom. Electrons, which spin around the protons and neutrons that make up the atom’s nucleus, constitute a very small portion of the atom’s mass. The number of electrons and protons is the same, thus canceling out one another. But when an atom loses or gains one or more electrons, however—thus becoming an ion—it acquires a net electric charge.

per, fluorine, manganese, molybdenum, nickel, selenium, silicon, and vanadium. MAINTAINING AND IMPROVING HEALTH. Though these elements are present

in trace quantities within the human body, that does not mean that exposure to large amounts of them is healthy. Arsenic, of course, is a good example; so too is aluminum. Aluminum is present in unexpected places: in baked goods, for instance, where a compound containing alu-

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Elements

KEY TERMS

CONTINUED

ELEMENT: A substance made up of only

Compounds are typically made of up

one kind of atom. Unlike compounds, elements cannot be chemically broken into other substances.

molecules.

ELEMENT SYMBOL: A one- or twoletter abbreviation for the name of an element. These may be a single capitalized letter (O for oxygen), or a capitalized letter followed by a lowercase one (Ne for neon). Sometimes the second letter is not the second letter of the element name (for example, Pt for platinum). In addition, the symbol may refer to an original Greek or Latin name, rather than the name used now, is the case with Au (aurum) for gold.

MONATOMIC:

A term describing an ele-

ment that exists as single atoms. This in contrast to diatomic elements. NEUTRON:

A subatomic particle that

has no electric charge. Neutrons are found at the nucleus of an atom, alongside protons. NUCLEUS: The center of an atom, a

region where protons and neutrons are located, and around which electrons spin. PERIODIC TABLE OF ELEMENTS: A

chart that shows the elements arranged in

An atom that has lost or gained one or more electrons, and thus has a net electric charge.

order of atomic number, along with ele-

ISOTOPES: Atoms that have an equal

element. Vertical columns within the peri-

number of protons, and hence are of the same element, but differ in their number of neutrons.

odic table indicate groups or “families”

MASS NUMBER: The sum of protons

PROTON:

and neutrons in an atom’s nucleus. Where an isotope is represented, the mass number is placed above the atomic number to the left of the element symbol.

in an atom. Protons and neutrons, which

MOLECULE: A group of atoms, usu-ally

greater than that of an electron. The num-

but not always representing more than one element, joined in a structure.

ber of protons in the nucleus of an atom is

ION:

minum (baking powder) is sometimes used in the leavening process; or even in cheeses, as an aid to melting when heated. The relatively high concentrations of aluminum in these products, as well as fluorine in drinking water, has raised concerns among some scientists. Generally speaking, an element is healthy for the human body in proportion to its presence in the body. With trace elements and others that are found in smaller quantities, however, it is some-

S C I E N C E O F E V E R Y D AY T H I N G S

ment symbol and the average atomic mass (in atomic mass units) for that particular

of elements with similar chemical characteristics. A positively charged particle

together form the nucleus around which electrons spin, have approximately the same mass—a mass that is many times

the atomic number of an element.

times possible and even advisable to increase the presence of those elements by taking dietary supplements. Hence a typical multivitamin contains calcium, iron, iodine, magnesium, zinc, selenium, copper, chromium, manganese, molybdenum, boron, and vanadium. For most of these, recommended daily allowances (RDA) have been established by the federal government. Usually, people do not take

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sodium as a supplement, though—Americans already get more than their RDA of sodium through salt, which is overly abundant in the American diet. WHERE TO LEARN MORE Challoner, Jack. The Visual Dictionary of Chemistry. New York: DK Publishing, 1996. “Classification of Elements and Compounds” (Web site). (May 14, 2001).

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(May 14, 2001). “Elements, Compounds, and Mixtures” (Web site). (May 14, 2001). Knapp, Brian J. Elements. Illustrated by David Woodroffe and David Hardy. Danbury, CT: Grolier Educational, 1996. “Matter—Elements, Compounds, and Mixtures” (Web site). (May 14, 2001). Oxlade, Chris. Elements and Compounds. Chicago, IL: Heinemann Library, 2001.

“Elements and Compounds” (Web site). (May 14, 2001).

Stwertka, Albert. A Guide to the Elements. New York: Oxford University Press, 1998.

“Elements, Compounds, and Mixtures.” Purdue University Department of Chemistry. (Web site).

Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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Periodic Table of Elements

P E R I O D I C TA B L E OF ELEMENTS

CONCEPT In virtually every chemistry classroom on the planet, there is a chart known as the periodic table of elements. At first glance, it looks like a mere series of boxes, with letters and numbers in them, arranged according to some kind of code not immediately clear to the observer. The boxes would form a rectangle, 18 across and 7 deep, but there are gaps in the rectangle, particularly along the top. To further complicate matters, two rows of boxes are shown along the bottom, separated from one another and from the rest of the table. Even when one begins to appreciate all the information contained in these boxes, the periodic table might appear to be a mere chart, rather than what it really is: one of the most sophisticated and usable means ever designed for representing complex interactions between the building blocks of matter.

HOW IT WORKS Introduction to the Periodic Table As a testament to its durability, the periodic table—created in 1869—is still in use today. Along the way, it has incorporated modifications involving subatomic properties unknown to the man who designed it, Russian chemist Dmitri Ivanovitch Mendeleev (1834-1907). Yet Mendeleev’s original model, which we will discuss shortly, was essentially sound, inasmuch as it was based on the knowledge available to chemists at the time. In 1869, the electromagnetic force fundamental to chemical interactions had only recent-

S C I E N C E O F E V E R Y D AY T H I N G S

ly been identified; the modern idea of the atom was less than 70 years old; and another three decades were to elapse before scientists began uncovering the substructure of atoms that causes them to behave as they do. Despite these limitations in the knowledge available to Mendeleev, his original table was sound enough that it has never had to be discarded, but merely clarified and modified, in the years since he developed it. The rows of the periodic table of elements are called periods, and the columns are known as groups. Each box in the table represents an element by its chemical symbol, along with its atomic number and its average atomic mass in atomic mass units. Already a great deal has been said, and a number of terms need to be explained. These explanations will require the length of this essay, beginning with a little historical background, because chemists’ understanding of the periodic table—and of the elements and atoms it represents—has evolved considerably since 1869.

Elements and Atoms An element is a substance that cannot be broken down chemically into another substance. An atom is the smallest particle of an element that retains all the chemical and physical properties of the element, and elements contain only one kind of atom. The scientific concepts of both elements and atoms came to us from the ancient Greeks, who had a rather erroneous notion of the element and—for their time, at least—a highly advanced idea of the atom. Unfortunately, atomic theory died away in later centuries, while the mistaken notion of four “elements” (earth, air, fire, and water) survived

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In A New System of Chemical Philosophy (1808), Dalton put forward the idea that nature is composed of tiny particles, and in so doing he adopted the Greek word atomos to describe these basic units. Drawing on Proust’s law of constant composition, Dalton recognized that the structure of atoms in a particular element or compound is uniform, but maintained that compounds are made up of compound “atoms.” In fact, these compound atoms are really molecules, or groups of two or more atoms bonded to one another, a distinction clarified by Italian physicist Amedeo Avogadro (1776-1856).

Periodic Table of Elements

DMITRI MENDELEEV. (Bettmann/Corbis. Reproduced by permission.)

virtually until the seventeenth century, an era that witnessed the birth of modern science. Yet the ancients did know of substances later classified as elements, even if they did not understand them as such. Among these were gold, tin, copper, silver, lead, and mercury. These, in fact, are such an old part of human history that their discoverers are unknown. The first individual credited with discovering an element was German chemist Hennig Brand (c. 1630-c. 1692), who discovered phosphorus in 1674. MATURING CONCEPTS OF ATOMS, ELEMENTS, AND MOLECULES. The work of English physicist and

chemist Robert Boyle (1627-1691) greatly advanced scientific understanding of the elements. Boyle maintained that no substance was an element if it could be broken down into other substances: thus air, for instance, was not an element. Boyle’s studies led to the identification of numerous elements in the years that followed, and his work influenced French chemists Antoine Lavoisier (1743-1794) and Joseph-Louis Proust (1754-1826), both of whom helped define an element in the modern sense. These men in turn influenced English chemist John Dalton (1766-1844), who reintroduced atomic theory to the language of science.

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Dalton’s and Avogadro’s contemporary, Swedish chemist Jons Berzelius (1779-1848), developed a system of comparing the mass of various atoms in relation to the lightest one, hydrogen. Berzelius also introduced the system of chemical symbols—H for hydrogen, O for oxygen, and so on—in use today. Thus, by the middle of the nineteenth century, scientists understood vastly more about elements and atoms than they had just a few decades before, and the need for a system of organizing elements became increasingly clear. By mid-century, a number of chemists had attempted to create just such an organizational system, and though Mendeleev’s was not the first, it proved the most useful.

Mendeleev Constructs His Table By the time Mendeleev constructed his periodic table in 1869, there were 63 known elements. At that point, he was working as a chemistry professor at the University of St. Petersburg, where he had become acutely aware of the need for a way of classifying the elements to make their relationships more understandable to his students. He therefore assembled a set of 63 cards, one for each element, on which he wrote a number of identifying characteristics for each. Along with the element symbol, discussed below, he included the atomic mass for the atoms of each. In Mendeleev’s time, atomic mass was understood simply to be the collective mass of a unit of atoms—a unit developed by Avogadro, known as the mole—divided by Avogadro’s number, the number of atoms or molecules in a mole. With the later discovery of subatomic particles, which in turn made possible the discovery of isotopes, figures for atomic mass were clarified, as will also be discussed.

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Periodic Table of Elements

Main–Group Elements

Main–Group Elements

Atomic number Symbol N ame

1

IA 1 1

H

2

3 Li

1.00794

Period

5

6

2

radon

4 Be

13

11 22.989768 12 Na Mg

5

Transition Metals

magnesium

19

20

39.0983

3

4

5

6

7

III B

IV B

VB

VI B

VII B

21 44.955910 22

40.078

47.88

23

50.9415

24

51.9961

25

8

54.9305

26

10

VIII B

55.847

10.811

27 58.93320 28

58.69

11

12

IB

II B

29

63.546

30

65.39

15

IV A 6

12.011

14.00674

F

Ne

oxygen

fluorine

neon

15

16

17

18

28.0855

30.973762

Si

P

S

Cl

Ar

silicon

phosphorus

sulfur

chlorine

argon

31

69.723

32

72.61

33

74.92159

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Ga

Ge

As

vanadium

chromium

manganese

iron

cobalt

nickel

copper

zinc

gallium

germanium

arsenic

37

38

42

43

50

51

88.90585

40

91.224

41

92.90638

95.94

(98)

Y

Zr

Nb

Mo

zirconium

niobium

molybdenum technetium

71

72

73

55 132.90543 56 Cs Ba 87

137.327

67-70

*

barium (223)

88

Fr

Ra

francium

radium

(226)

89-102

†

174.967

178.49

180.9479

74

Tc 183.85

75

186.207

44

101.07

45 102.90550 46

106.42

47

107.8682

48

112.411

49

114.82

118.710

121.75

Rh

Pd

Ag

Cd

In

Sn

Sb

rhodium

palladium

silver

cadmium

indium

tin

antimony

77

78

79 196.96654 80

192.22

195.08

200.59

81

204.3833

82

207.2

78.96

35

79.904

36 Kr krypton

52

53 126.90447 54 Xe I

127.60

tellurium

83 208.98037 84

iodine (209)

85

(210)

86

Hf

Ta

W

Re

Os

Ir

Pt

Au

Hg

Tl

Pb

Bi

Po

At

Rn

hafnium

tantalum

tungsten

rhenium

osmium

iridium

platinum

gold

mercury

thallium

lead

bismuth

polonium

astatine

radon

(262)

104

(261)

105

Lr

Rf

rutherfordium dubnium

(262) 106

Db

(263)

107

(264)

108

(265)

109

(268)

110

(269)

111

(272)

112

114

(277)

Sg

Bh

Hs

Mt

Uun

Uuu

Uub

seaborgium

bohrium

hassium

meitnerium

ununnilium

unununium

ununbiium

116

(289)

Uuq ununquadium

131.29

xenon

Lu

lawrencium

83.80

Br bromine

lutetium

103

39.948

selenium

Te

Ru 190.2

34 Se

ruthenium

76

35.4527

Al

titanium

yttrium

32.066

20.1797

aluminum

Ti

39

helium

O

scandium 87.62

He 10

nitrogen

Sc

strontium

18.9984032

N

calcium

Sr

VII A 9

C

Ca 85.4678

15.9994

carbon

K

rubidium

17

VI A 8

B

potassium

Rb

16

VA 7

4.002602

boron

13 26.981539 14

9

24.3050

sodium

14

III A

9.012182

beryllium

cesium

7

18

VIII A

Rn

2 6.941

lithium

4

Atomic weight

(222)

II A

hydrogen

3

86

118

(289)

(222)

(293)

Uuh

Uuo

ununhexium

ununoctium

Inner–Transition Metals

*Lanthanides

57 La

138.9055

lanthanum

89

† Actinides

THE

PERIODIC TABLE

(227)

58

140.115

59 140.90765 60 Pr

cerium

praseodymium neodymium

90

232.0381

91

Nd (231)

Ac

Th

Pa

actinium

thorium

protactinium

(IUPAC

144.24

Ce

92

238.0289

U uranium

61

(145)

62

150.36

63

151.965

64

157.25

65 158.92534 66

162.50

67 164.93032 68

Pm

Sm

Eu

Gd

Tb

Dy

Ho

promethium

samarium

europium

gadolinium

terbium

dysprosium

holmium

93

(237)

94

(244)

95

(243)

96

(247)

97

(247)

98

(251)

Np

Pu

Am

Cm

Bk

Cf

neptunium

plutonium

americium

curium

berkelium

californium

99

167.26

Er (252)

Es einsteinium

erbium

100

(257)

69 168.93421 70

173.04

Tm

Yb

thulium

ytterbium

101

(258)

102

(259)

Fm

Md

fermium

mendelevium nobelium

No

SYSTEM)

In addition, Mendeleev also included figures for specific gravity—the ratio between the density of an element and the density of water—as well as other known chemical characteristics of an element. Today, these items are typically no longer included on the periodic table, partly for considerations of space, but partly because chemists’ much greater understanding of the properties of atoms makes it unnecessary to clutter the table with so much detail. Again, however, in Mendeleev’s time there was no way of knowing about these factors. As far as chemists knew in 1869, an atom was an indivisible little pellet of matter that could not be characterized by terms any more detailed than its mass and the ways it interacted with atoms of other elements. Mendeleev therefore ar-

S C I E N C E O F E V E R Y D AY T H I N G S

ranged his cards in order of atomic mass, then grouped elements that showed similar chemical properties. CONFIDENT PREDICTIONS. As Mendeleev observed, every eighth element on the chart exhibits similar characteristics, and thus, he established columns whereby element number x was placed above element number x + 8—for instance, helium (2) above neon (10). The patterns he observed were so regular that for any “hole” in his table, he predicted that an element to fill that space would be discovered.

Indeed, Mendeleev was so confident in the basic soundness of his organizational system that in some instances, he changed the figures for the atomic mass of certain elements because he was convinced they belonged elsewhere on the table.

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Later discoveries of isotopes, which in some cases affected the average atomic mass considerably, confirmed his suppositions. Likewise the undiscovered elements he named “eka-aluminum,” “eka-boron,” and “eka-silicon” were later identified as gallium, scandium, and germanium, respectively.

REAL-LIFE APPLICAT ION S Subatomic Structures Clarify the Periodic Table Over a period of 35 years, between the discovery of the electron in 1897 and the discovery of the neutron in 1932, chemists’ and physicists’ understanding of atomic structure changed completely. The man who identified the electron was English physicist J. J. Thomson (1856-1940). The electron is a negatively charged particle that contributes little to an atom’s mass; however, it has a great deal to do with the energy an atom possesses. Thomson’s discovery made it apparent that something else had to account for atomic mass, as well as the positive electric charge offsetting the negative charge of the electron. Thomson’s student Ernest Rutherford (1871-1937)—for whom, incidentally, rutherfordium (104 on the periodic table) is named— identified that “something else.” In a series of experiments, he discovered that the atom has a nucleus, a center around which electrons move, and that the nucleus contains positively charged particles called protons. Protons have a mass 1,836 times as great as that of an electron, and thus, this seemed to account for the total atomic mass. ISOTOPES AND ATOMIC MASS.

Later, working with English chemist Frederick Soddy (1877-1956), Rutherford discovered that when an atom emitted certain types of particles, its atomic mass changed. Rutherford and Soddy named these atoms of differing mass isotopes, though at that point—because the neutron had yet to be discovered—they did not know exactly what change had caused the change in mass. Certain types of isotopes, Soddy and Rutherford went on to conclude, had a tendency to decay by emitting particles or gamma rays, moving (sometimes over a great period of time) toward stabilization. In the process, these radioactive iso-

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topes changed into other isotopes of the same element—and sometimes even to isotopes of other elements. Soddy concluded that atomic mass, as measured by Berzelius, was actually an average of the mass figures for all isotopes within that element. This explained a problem with Mendeleev’s periodic table, in which there seemed to be irregularities in the increase of atomic mass from element to element. The answer to these variations in mass, it turned out, related to the number of isotopes associated with a given element: the greater the number of isotopes, the more these affected the overall measure of the element’s mass. A CLEARER DEFINITION OF ATOMIC NUMBER. Just a few years after

Rutherford and Soddy discovered isotopes, Welsh physicist Henry Moseley (1887-1915) uncovered a mathematical relationship between the amount of energy a given element emitted and its atomic number. Up to this point, the periodic table had assigned atomic number in order of mass, beginning with the lightest element, hydrogen. Using atomic mass and other characteristics as his guides, Mendeleev had been able to predict the discovery of new elements, but such predictions had remained problematic. Thanks to Moseley’s work, it became possible to predict the existence of undiscovered elements with much greater accuracy. As Moseley discovered, the atomic number corresponds to the number of positive charges in the nucleus. Thus carbon, for instance, has an atomic number of 6 not because there are five lighter elements—though this is also true—but because it has six protons in its nucleus. The ordering by atomic number happens to correspond to the ordering by atomic mass, but atomic number provides a much more precise means of distinguishing elements. For one thing, atomic number is always a whole integer—1 for hydrogen, for instance, or 17 for chlorine, or 92 for uranium. Figures for mass, on the other hand, are almost always rendered with whole numbers and decimal fractions (for example, 1.008 for hydrogen). If atoms have no electric charge, meaning that they have the same number of protons as electrons, then why do chemists not say that atomic number represents the number of protons or electrons? The reason is that electrons can easily be lost or gained by atoms to form ions,

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which have an electric charge. However, protons are very hard to remove.

mass take carbon-12, an isotope found in all living things, as their reference point.

NEUTRONS AND ATOMIC MASS.

It is inconvenient, to say the least, to measure the mass of a single carbon-12 atom, or indeed of any other atom. Instead, chemists use a large number of atoms, a value known as Avogadro’s number, which in general is the number of atoms in a mole (abbreviated mol). Avogadro’s number is defined as 6.02214199 • 1023, with an uncertainty of 4.7 • 1016. In other words, the number of particles in a mole could vary by as much as 47,000,000,000,000,000 on either side of the value for Avogadro’s number. This might seem like a lot, but in fact it is equal to only about 80 parts per billion.

By 1932, scientists had come a long way toward understanding the structure of the atom. Not only had the electron, nucleus, and proton been discovered, but the complex model of electron configuration (described later in this essay) had begun to evolve. Yet, one nagging question remained: the mass of the protons in the nucleus simply could not account for the entire mass of the atom. Neither did the electrons make a significant contribution to mass. Suppose a proton was “worth” $1,836, while an electron had a value of only $1. In the “bank account” for deuterium, an isotope of hydrogen, there is $3,676, which poses a serious discrepancy in accounting. Because deuterium is a form of hydrogen, it has one proton as well as one electron, but that only accounts for $1,837. Where does deuterium get the other $1,839? These numbers are not chosen at random, as we shall see. The answer to the problem of atomic mass came when English physicist James Chadwick (1891-1974) identified the neutron, a particle with no electric charge, residing in the nucleus alongside the protons. Whereas the proton has a mass 1,836 times as large as that of the electron, the neutron’s mass is slightly larger—1,839 times that of an electron. This made it possible to clarify the values of atomic mass, which up to that time had been problematic, because a mole of atoms representing one element is likely to contain numerous isotopes.

Periodic Table of Elements

When 1 is divided by Avogadro’s number, the result is 1.66 • 10–24—the value, in grams, of 1 amu. However, according to the 1960 agreement, 1 amu is officially 1/12 the mass of a carbon-12 atom, whose exact value (re-tested in 1998), is 1.6653873 ⫻ 10–24 g. Carbon-12, sometimes represented as (12/6)C, contains six protons and six neutrons, so the value of 1 amu thus obtained is, in effect, an average of the mass for a proton and neutron. Though atoms differ, subatomic particles do not. There is no such thing, for instance, as a “hydrogen proton”—otherwise, these subatomic particles, and not atoms, would constitute the basic units of an element. Given the unvarying mass of subatomic particles, combined with the fact that the neutron only weighs 0.16% more than a proton, the established value of 1 amu provides a convenient means of comparing mass. This is particularly useful in light of the large numbers of isotopes—and hence of varying figures for mass—that many elements have.

Average Atomic Mass Today, the periodic table lists, along with chemical symbol and atomic number, the average atomic mass of each element. As its name suggests, the average atomic mass provides the average value of mass—in atomic mass units (amu)—for a large sample of atoms. According to Berzelius’s system for measuring atomic mass, 1 amu should be equal to the mass of a hydrogen atom, even though that mass had yet to be measured, since hydrogen almost never appears alone in nature. Today, in accordance with a 1960 agreement among members of the international scientific community, measurements of atomic

S C I E N C E O F E V E R Y D AY T H I N G S

ATOMIC MASS UNITS AND THE PERIODIC TABLE. The periodic table as it

is used today includes figures in atomic mass units for the average mass of each atom. As it turns out, Berzelius was not so far off in his use of hydrogen as a standard, since its mass is almost exactly 1 amu—but not quite. The value is actually 1.008 amu, reflecting the presence of slightly heavier deuterium isotopes in the average sample of hydrogen Figures increase from hydrogen along the periodic table, though not by a regular pattern. Sometimes the increase from one element to the next is by just over 1 amu, and in other cases, the

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increase is by more than 3 amu. This only serves to prove that atomic number, rather than atomic mass, is a more straightforward means of ordering the elements. Mass figures for many elements that tend to appear in the form of radioactive isotopes are usually shown in parentheses. This is particularly true for elements with very, very high atomic numbers (above 92), because samples of these elements do not stay around long enough to be measured. Some have a half-life—the period in which half the isotopes decay to a stable form— of just a few minutes, and for others, the half-life is a fraction of a second. Therefore, atomic mass figures represent the mass of the longest-lived isotope.

are liquids at normal temperature: mercury, a metal, and the nonmetal halogen bromine. It should be noted that the metal gallium becomes liquid at just 85.6°F (29.76°C); below that temperature, however, it— like the elements other than those named in this paragraph—is a solid.

Chemical Names and Symbols For the sake of space and convenience, elements are listed on the periodic table by chemical symbol or element symbol—a one-or two-letter abbreviation for the name of the element according to the system first developed by Berzelius. These symbols, which are standardized and unvarying for any particular element, greatly aid the chemist in writing out chemical formulas, which could otherwise be quite cumbersome.

Elements As of 2001, there were 112 known elements, of which about 90 occur naturally on Earth. Uranium, with an atomic number of 92, was the last naturally occurring element discovered: hence some sources list 92 natural elements. Other sources, however, subtract those elements with a lower atomic number than uranium that were first created in laboratories rather than discovered in nature. In any case, all elements with atomic numbers higher than 92 are synthetic, meaning that they were created in laboratories. Of these 20 elements—all of which have appeared only in the form of radioactive isotopes with short half-lives—the last three have yet to receive permanent names. In addition, three other elements—designated by atomic numbers 114, 116, and 118, respectively—are still on the drawing board, as it were, and do not yet even have temporary names. The number of elements thus continues to grow, but these “new” elements have little to do with the daily lives of ordinary people. Indeed, this is true even for some of the naturally occurring elements: for example, few people who are not chemically trained would be able to identify yttrium, which has an atomic number of 39. Though an element can exist theoretically as a gas, liquid, or a solid, in fact, the vast majority of elements are solids. Only 11 elements exist in the gaseous state at a normal temperature of about 77°F (25°C). These are the six noble gases; fluorine and chlorine from the halogen family; as well as hydrogen, nitrogen, and oxygen. Just two

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Many of the chemical symbols are simple one-letter designations: H for hydrogen, O for oxygen, and F for fluorine. Others are two-letter abbreviations, such as He for helium, Ne for neon, and Si for silicon. Note that the first letter is always capitalized, and the second is always lowercase. In many cases, the two-letter symbols indicate the first and second letters of the element’s name, but this is not nearly always the case. Cadmium, for example, is abbreviated Cd, while platinum is Pt. Many of the one-letter symbols indicate elements discovered early in history. For instance, carbon is represented by C, and later “C” elements took two-letter designations: Ce for cerium, Cr for chromium, and so on. Likewise, krypton had to take the symbol Kr because potassium had already been assigned K. The association of potassium with K brings up one of the aspects of chemical symbols most confusing to students just beginning to learn about the periodic table: why K and not P? The latter had in fact already been taken by phosphorus, but then why not Po, assigned many years later instead to polonium? CHEMICAL SYMBOLS BASED IN OTHER LANGUAGES. In fact, potassi-

um’s symbol is one of the more unusual examples of a chemical symbol, taken from an ancient or non-European language. Soon after its discovery in the early nineteenth century, the element was named kalium, apparently after the Arabic qali or “alkali.” Hence, though it is known as potassium today, the old symbol still stands.

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The use of Arabic in naming potassium is unusual in the sense that “strange” chemical symbols usually refer to Latin and Greek names. Latin names include aurum, or “shining dawn” for gold, symbolized as Au; or ferrum, the Latin word for iron, designated Fe. Likewise, lead (Pb) and sodium (Na) are represented by letters from their Latin names, plumbum and natrium, respectively. Some chemical elements are named for Greek or German words describing properties of the element. Consider, for instance, the halogens, collectively named for a Greek term meaning “salt producing.” Chloros, in Greek, describes a sickly yellow color, and was assigned to chlorine; the name of bromine comes from a Greek word meaning “stink”; and that of iodine is a form of a Greek term meaning “violet-colored.” Astatine, last-discovered of the halogens and the rarest of all natural elements, is so radioactive that it was given a name meaning “unstable.” Another Greek-based example outside the halogen family is phosphorus, or “I bring light”—appropriate enough, in view of its phosphorescent properties. NAMES OF LATER ELEMENTS.

The names of several elements with high atomic numbers—specifically, the lanthanides, the transuranium elements of the actinide series, and some of the later transition metals—have a number of interesting characteristics. Several reflect the places where they were originally discovered or created: for example, germanium, americium, and californium. Other elements are named for famous or not-so-famous scientists. Most people could recognize einsteinium as being named after Albert Einstein (1879-1955), but the origin of the name gadolinium—Finnish chemist Johan Gadolin (1760-1852)—is harder for the average person to identify. Then of course there is element 101, named mendelevium in honor of the man who created the periodic table. Two elements are named after women: curium after French physicist and chemist Marie Curie (1867-1934), and meitnerium after Austrian physicist Lise Meitner (1878-1968). Curie, the first scientist to receive two Nobel Prizes—in both physics and chemistry—herself discovered two elements, radium and polonium. In keeping with the trend of naming transuranium elements after places, she commemorated the land of her birth, Poland, in the name of polonium. One of Curie’s students, French physicist Marguerite

S C I E N C E O F E V E R Y D AY T H I N G S

Perey (1909-1975), also discovered an element and named it after her own homeland: francium.

Periodic Table of Elements

Meitnerium, the last element to receive a name, was created in 1982 at the Gesellschaft für Schwerionenforschung, or GSI, in Darmstadt, Germany, one of the world’s three leading centers of research involving transuranium elements. The other two are the Joint Institute for Nuclear Research in Dubna, Russia, and the University of California at Berkeley, for which berkelium is named.

OF

THE IUPAC AND THE NAMING ELEMENTS. One of the researchers

involved with creating berkelium was American nuclear chemist Glenn T. Seaborg (1912-1999), who discovered plutonium and several other transuranium elements. In light of his many contributions, the scientists who created element 106 at Dubna in 1974 proposed that it be named seaborgium, and duly submitted the name to the International Union of Pure and Applied Chemistry (IUPAC). Founded in 1919, the IUPAC is, as its name suggests, an international body, and it oversees a number of matters relating to the periodic table: the naming of elements, the assignment of chemical symbols to new elements, and the certification of a particular research team as the discoverers of that element. For many years, the IUPAC refused to recognize the name seaborgium, maintaining that an element could not be named after a living person. The dispute over the element’s name was not resolved until the 1990s, but finally the IUPAC approved the name, and today seaborgium is included on the international body’s official list. Elements 110 through 112 had yet to be named in 2001, and hence were still designated by the three-letter symbols Uun, Uuu, and Uub respectively. These are not names, but alphabetic representations of numbers: un for 1, nil for 0, and bium for 2. Thus, the names are rendered as ununnilium, unununium, and ununbium; the undiscovered elements 114, 116, and 118 are respectively known as ununquadium, ununhexium, and ununoctium.

Layout of the Periodic Table TWO SYSTEMS FOR LABELING GROUPS. Having discussed the three items of

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information contained in the boxes of the periodic table—atomic number, chemical symbol/ name, and average atomic mass—it is now possible to step back from the chart and look at its overall layout. To reiterate what was stated in the introduction to the periodic table above, the table is arranged in rows called periods, and columns known as groups. The deeper meaning of the periods and groups, however—that is, the way that chemists now understand them in light of what they know about electron configurations—will require some explanation. All current versions of the periodic table show seven rows—in other words, seven periods—as well as 18 columns. However, the means by which columns are assigned group numbers varies somewhat. According to the system used in North America, only eight groups are numbered. These are the two “tall” columns on the left side of the “dip” in the chart, as well as the six “tall” columns to the right of it. The “dip,” which spans 10 columns in periods 4 through 7, is the region in which the transition metals are listed. The North American system assigns no group numbers to these, or to the two rows set aside at the bottom, representing the lanthanide and actinide series of transition metals. As for the columns that the North American system does number, this numbering may appear in one of four forms: either by Roman numerals; Roman numerals with the letter A (for example, IIIA); Hindu-Arabic numbers (for example, 3); or Hindu-Arabic numerals with the letter A. Throughout this book, the North American system of assigning Hindu-Arabic numerals without the letter A has been used. However, an attempt has been made in some places to include the group designation approved by the IUPAC, which is used by scientists in Europe and most parts of the world outside of North America. (Some scientists in North America are also adopting the IUPAC system.) The IUPAC numbers all columns on the chart, so that instead of eight groups, there are 18. The table below provides a means of comparing the North American and IUPAC systems. Columns are designated in terms of the element family or families, followed in parentheses by the atomic numbers of the elements that appear at the top and bottom of that column. The first number following the colon is the number in the North American system (as described above, a

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Hindu-Arabic numerical without an “A”), and the second is the number in the IUPAC system. Element

North

Family

American

IUPAC

1

1

2

2

Hydrogen and alkali metals (1, 87) Alkaline metals (4, 88) Transition metals (21,89)

3

Transition metals (22,104)

4

Transition metals (23,105)

5

Transition metals (24,106)

6

Transition metals (25,107)

7

Transition metals (26,108)

8

Transition metals (27,109)

9

Transition metals (28,110)

10

Transition metals (29,111)

11

Transition metals (30,112)

12

Nonmetals and metals (5,81) Nonmetals, metalloids, and metal (6,82)

3

13

4

14

5

15

Nonmetals, metalloids, and metal (7,83) Nonmetals, metalloids,

6

16

Halogens (9,85)

7

17

Noble gases (2,86)

8

18

(8,84)

Lanthanides (58,71)

No number group assigned in either system

Actinides (90,103)

No number group assigned in either system

Valence Electrons, Periods, and Groups The merits of the IUPAC system are easy enough to see: just as there are 18 columns, the IUPAC lists 18 groups. Yet the North American system is more useful than it might seem: the group number in the North American system indicates the number of valence electrons, the electrons that are involved in chemical bonding. Valence electrons also occupy the highest energy level in the atom—which might be thought of as the orbit farthest from the nucleus, though in fact the reality is more complex. A more detailed, though certainly far from comprehensive, discussion of electrons and energy levels, as well as the history behind these discoveries, appears in the Electrons essay. In what follows, the basics of electron configuration will be presented with the specific aim of making it clear exactly why elements appear in particular columns of the periodic table.

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PRINCIPAL ENERGY LEVELS AND PERIODS. At one time, scientists

thought that electrons moved around a nucleus in regular orbits, like planets around the Sun. In fact the paths of an electron are much more complicated, and can only be loosely defined in terms of orbitals, a set of probabilities regarding the positions that an electron is likely to occupy as it moves around the nucleus. The pattern of orbitals is determined by the principal energy level of the atom, which indicates a distance that an electron may move away from the nucleus. Principal energy level is designated by a whole-number integer, beginning with 1 and moving upward: the higher the number, the further the electron is from the nucleus, and hence the greater the energy in the atom. Each principal energy level is divided into sublevels corresponding to the number n of the principal energy level: thus, principal energy level 1 has one sublevel, principal energy level 2 has two, and so on. The relationship between principal energy level and period is relatively easy to demonstrate: the number n of a period on the periodic table is the same as the number of the highest principal energy level for the atoms on that row—that is, the principal energy level occupied by its valence electrons. Thus, elements on period 4 have a highest principal energy level of 4, whereas the valence electrons of elements on period 7 are at principal energy level 7. Note the conclusion that this allows us to draw: the further down the periodic table an element is positioned, the greater the energy in a single atom of that element. Not surprisingly, most of the elements used in nuclear power come from period 7, which includes the actinides. VALENCE ELECTRON CONFIGURATIONS AND GROUPS. Now to a

more involved subject, whereby group number is related to valence electron configuration. As mentioned earlier, the principal energy levels are divided into sublevels, which are equal in number to the principal energy level number: principal energy level 1 has one sublevel, level 2 has two sublevels, and so on. As one might expect, with an increase in principal energy levels and sublevels, there are increases in the complexity of the orbitals. The four types of orbital patterns are designated as s, p,d, and f. Two electrons can move in an s orbital pattern or shell, six in a p, 10 in a d,

S C I E N C E O F E V E R Y D AY T H I N G S

and 14 in an f orbital pattern or shell. This says nothing about the number of electrons that are actually in a particular atom; rather, the higher the principal energy level and the larger the number of sublevels, the greater the number of ways that the electrons can move. It does happen to be the case, however, that with higher atomic numbers—which means more electrons to offset the protons—the higher the energy level, the larger the number of orbitals for those electrons.

Periodic Table of Elements

Let us now consider a few examples of valence shell configurations. Hydrogen, with the simplest of all atomic structures, has just one electron on principal energy level 1, so in effect its valence electron is also a core electron. The valence configuration for hydrogen is thus written as 1s1. Moving straight down the periodic table to francium (atomic number 87), which is in the same column as hydrogen, one finds that it has a valence electron configuration of 7s1. Thus, although francium is vastly more complex and energy-filled than hydrogen, the two elements have the same valence-shell configuration; only the number of the principal energy level is different. Now look at two elements in Group 3 (Group 13 in the IUPAC system): boron and thallium, which respectively occupy the top and bottom of the column, with atomic numbers of 5 and 81. Boron has a valence-shell configuration of 2s22p1. This means its valence shell is at principal energy level 2, where there are two electrons in an s orbital pattern, and 2 in a p orbital pattern. Thallium, though it is on period 6, nonetheless has the same valence-shell configuration: 6s26p1. Notice something about the total of the superscript figures for any element in Group 3 of the North American system: it is three. The same is true in the other columns numbered on North American charts, in which the total number of electrons equals the group number. Thus in Group 7, the valence shell configuration is ns2np5, where n is the principal energy level. There is only one exception to this: helium, in Group 8 (the noble gases), has a valence shell configuration of 1s2. Were it not for the fact that it clearly fits with the noble gases due to shared properties, helium would be placed next to hydrogen at the top of Group 2, where all the atoms have a valence-shell configuration of ns2.

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Obviously the group numbers in the IUPAC system do not correspond to the number of valence electrons, because the IUPAC chart includes numbers for the columns of transition metals, which are not numbered in the North American system. In any case, in both systems the columns contain elements that all have the same number of electrons in their valence shells. Thus the term “group” can finally be defined in accordance with modern chemists’ understanding, which incorporates electron configurations of which Mendeleev was unaware. All the members of a group have the same number of valence electrons in the same orbital patterns, though at different energy levels. (Once again, helium is the lone exception.)

Some Challenges of the Periodic Table The groups that are numbered in the North American system are referred to as “representative” elements, because they follow a clearly established pattern of adding valence shell electrons. By contrast, the 40 elements listed in the “dip” at the middle of the chart—the transition elements— do not follow such a pattern. This is why the North American system does not list them by group number, and also why neither system lists two “branches” of the transition-metal family, the lanthanides and actinides. IRREGULAR

PATTERNS.

Even within the representative elements, there are some challenges as far as electron configuration. For the first 18 elements—1 (hydrogen) to 18 (argon)—there is a regular pattern of orbital filling. Beginning with helium (2) onward, all of principal level 1 is filled; then, beginning with beryllium (4), sublevel 2s begins to fill. Sublevel 2p—and hence principal level 2 as a whole—becomes filled at neon (10).

136

indeed, the transition elements are defined by the fact that they fill the d orbitals rather than the p orbitals, as was the pattern up to that point. After the first period of transition metals ends with zinc (30), the next representative element—gallium (31)—resumes the filling of the p orbital rather than the d. And so it goes, all along the four periods in which transition metals break up the steady order of electron configurations. As for the lanthanide and actinide series of transitions metals, they follow an even more unusual pattern, which is why they are set apart even from the transition metals. These are the only groups of elements that involve the highly complex f sublevels. In the lanthanide series, the seven 4f orbital shells are filled, while the actinide series reflects the filling of the seven 5f orbital shells. Why these irregularities? One reason is that as the principal energy level increases, the energy levels themselves become closer—i.e., there is less difference between the energy levels. The atom is thus like a bus that fills up: when there are just a few people on board, those few people (analogous to electrons) have plenty of room, but as more people get on, the bus becomes increasingly more crowded, and passengers jostle against one another. In the atom, due to differences in energy levels, the 4s orbital actually has a lower energy than the 3d, and therefore begins to fill first. This is also true for the 6s and 4f orbitals. CHANGES IN ATOMIC SIZE. The

subject of element families is a matter unto itself, and therefore a separate essay in this book has been devoted to it. The reader is encouraged to consult the Families of Elements essay, which discusses aspects of electron configuration as well as the properties of various element families.

After argon, as one moves to the element occupying the nineteenth position on the periodic table—potassium—the rules change. Argon, in Group 8 of the North American system, has a valence shell of 3s23p6, and by the pattern established with the first 18 elements, potassium should begin filling principal level 3d. Instead, it “skips” 3d and moves on to 4s. The element following argon, calcium, adds a second electron to the 4s sublevel.

One last thing should be mentioned about the periodic table: the curious fact that the sizes of atoms decreases as one moves from left to right across a row or period, even though the sizes increase as one moves from top to bottom along a group. The increase of atomic size in a group, as a function of increasing atomic number, is easy enough to explain. The higher the atomic number, the higher the principal energy level, and the greater the distance from the nucleus to the furthest probability range for the electron.

After calcium, as the transition metals begin with scandium (21), the pattern again changes:

On the other hand, the decrease in size across a period is a bit more challenging to com-

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Periodic Table of Elements

KEY TERMS ATOM:

The smallest particle of an ele-

GROUPS:

Columns on the periodic

ment that retains all the chemical and

table of elements. These are ordered

physical properties of the element.

according to the numbers of valence elec-

ATOMIC

MASS

UNIT:

An SI unit

(abbreviated amu), equal to 1.66 • 10-24 g, for measuring the mass of atoms. ATOMIC

NUMBER:

The number of

trons in the outer shells of the atoms for the elements represented. HALF-LIFE: The length of time it takes a

substance to diminish to one-half its initial

protons in the nucleus of an atom. Since

amount.

this number is different for each element,

ION:

elements are listed on the periodic table of

gained one or more electrons, thus acquir-

elements in order of atomic number.

ing a net electric charge.

AVERAGE ATOMIC MASS: A figure

ISOTOPES: Atoms that have an equal

used by chemists to specify the mass—in

number of protons, and hence are of the

atomic mass units—of the average atom in

same element, but differ in their number of

a large sample.

neutrons. This results in a difference of

An atom or atoms that has lost or

A figure,

mass. Isotopes may be either stable or

named after Italian physicist Amedeo Avo-

unstable. The latter type, known as

gadro (1776-1856), equal to 6.022137 x

radioisotopes, are radioactive.

AVOGADRO’S

NUMBER:

1023. Avogadro’s number indicates the number of atoms or molecules in a mole.

MOLE:

The SI fundamental unit for

“amount of substance.” A mole is, general-

CHEMICAL SYMBOL: A one- or two-

ly speaking, Avogadro’s number of atoms,

letter abbreviation for the name of an

molecules, or other elementary particles;

element.

however, in the more precise SI definition,

COMPOUND:

A substance made of two

a mole is equal to the number of carbon

or more elements that have bonded chem-

atoms in 12.01 g of carbon.

ically. These atoms are usually, but not

MOLECULE: A group of atoms, usually

always, joined in molecules.

but not always representing more than one

ELECTRON: A negatively charged parti-

element, joined by chemical bonds. Com-

cle in an atom. The configurations of

pounds are typically made of up mole-

valence electrons define specific groups on

cules.

the periodic table of elements, while the principal energy levels of those valence electrons define periods on the table.

NEUTRON:

A subatomic particle that

has no electric charge. Neutrons, together with protons, account for the majority of

ELEMENT: A substance made up of only

average atomic mass. When atoms have the

one kind of atom, which cannot be chemi-

same number of protons—and hence are

cally broken into other substances.

the same element—but differ in their

ELEMENT SYMBOL: Another term for

number of neutrons, they are called

chemical symbol.

isotopes.

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Periodic Table of Elements

KEY TERMS NUCLEUS: The center of an atom, a

ber integer, beginning with 1 and moving

region where protons and neutrons are

upward. The higher the principal energy

located. The nucleus accounts for the vast

level, the greater the energy in the atom,

majority of the average atomic mass.

and the more complex the pattern of

ORBITAL:

A pattern of probabilities

orbitals.

regarding the regions that an electron can

PROTON:

occupy within an atom in a particular

in an atom. The number of protons in the

energy state. The higher the principal ener-

nucleus of an atom is the atomic number

gy level, the more complex the pattern of

of an element.

orbitals.

A positively charged particle

RADIOACTIVITY:

A term describing a

A

phenomenon whereby certain isotopes

chart that shows the elements arranged in

known as radioisotopes are subject to a

order of atomic number, along with chem-

form of decay brought about by the emis-

ical symbol and the average atomic mass

sion of high-energy particles. “Decay” does

(in atomic mass units) for that particular

not mean that the isotope “rots”; rather, it

element.

decays to form another isotope— either of

PERIODS: Rows of the periodic table of

the same element or another—until even-

elements. These represent successive prin-

tually it becomes stable. This stabilizing

cipal energy levels for the valence electrons

process may take a few seconds, or many

in the atoms of the elements involved.

years.

PRINCIPAL ENERGY LEVEL: A value

VALENCE

indicating the distance that an electron

that occupy the highest energy levels in an

may move away from the nucleus of an

atom. These are the electrons involved in

atom. This is designated by a whole-num-

chemical bonding.

PERIODIC TABLE OF ELEMENTS:

prehend; however, it just takes a little explaining. As one moves along a period from left to right, there is a corresponding increase in the number of protons within the nucleus. This means a stronger positive charge pulling the electrons inward. Therefore, the “cloud” of electrons is drawn ever closer toward the increasingly powerful charge at the center of the atom, and the size of the atom decreases because the electrons cannot move as far away from the nucleus.

138

CONTINUED

ELECTRONS:

Electrons

“Elementistory” (Web site). (May 22, 2001). International Union of Pure and Applied Chemistry (Web site). (May 22, 2001). Knapp, Brian J. and David Woodroffe. The Periodic Table. Danbury, CT: Grolier Educational, 1998. Oxlade, Chris. Elements and Compounds. Chicago: Heinemann Library, 2001. “A Periodic Table of the Elements” Los Alamos National Laboratory (Web site). (May 22, 2001).

WHERE TO LEARN MORE

“The Pictorial Periodic Table” (Web site). (May 22, 2001).

Challoner, Jack. The Visual Dictionary of Chemistry. New York: DK Publishing, 1996.

Stwertka, Albert. A Guide to the Elements. New York: Oxford University Press, 1998.

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“Visual Elements” (Web site). (May 22, 2001).

S C I E N C E O F E V E R Y D AY T H I N G S

WebElements (Web site).
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FA M I L I E S O F E L E M E N T S Families of Elements

CONCEPT The term “family” is used to describe elements that share certain characteristics—not only in terms of observable behavior, but also with regard to atomic structure. All noble gases, for instance, tend to be highly nonreactive: only a few of them combine with other elements, and then only with fluorine, the most reactive of all substances. Fluorine is a member of another family, the halogens, which have so many shared characteristics that they are grouped together, despite the fact that two are gases, two are solids, and one—bromine—is one of only two elements that appears at room temperature as a solid. Despite these apparent differences, common electron configurations identify the halogens as a family. Families on the periodic table include, in addition to noble gases and halogens, the alkali metals, alkaline earth metals, transition metals, lanthanides, and actinides. The nonmetals form a loosely defined cross-family grouping, as do the metalloids.

HOW IT WORKS The Basics of the Periodic Table Created in 1869, and modified several times since then, the periodic table of the elements developed by Russian chemist Dmitri Ivanovitch Mendeleev (1834-1907) provides a highly useful means of organizing the elements. Certainly other organizational systems exist, but Mendeleev’s table is the most widely used—and with good reason. For one thing, it makes it possible to see at a glance families of elements, many

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of which either belong to the same group (column) or the same period (row) on the table. The periodic table is examined in depth within the essay devoted to that subject, and among the specifics discussed in that essay are the differing systems used for periodic-table charts in North America and the rest of the world. In particular, the North American system numbers only eight groups, leaving 10 columns unnumbered, whereas the other system— approved by the International Union of Pure and Applied Chemistry (IUPAC)—numbers all 18 columns. Both versions of the periodic table show seven periods. The groups numbered in the North American system are the two “tall” columns on the left side of the “dip” in the chart, as well as the six “tall” columns to the right of it. Group 1 in this system consists of hydrogen and the alkali metals; Group 2, the alkaline earth metals; groups 3 through 6, an assortment of metals, nonmetals, and metalloids; Group 7, halogens; and Group 8, noble gases. The “dip,” which spans 10 columns in periods 4 through 7, is the region in which the transition metals are listed. The North American system assigns no group numbers to these, or to the two rows set aside at the bottom, representing the lanthanide and actinide series of transition metals. The IUPAC system, on the other hand, offers the obvious convenience of providing a number for each column. (Note that, like its North American counterpart, the IUPAC chart provides no column numbers for the lanthanides or actinides.) Furthermore, the IUPAC has behind it the authority of an international body, founded in 1919, which oversees a number of matters

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Families of Elements

SIZE

REPRESENTATION OF THE ATOMIC RADII OF THE MAIN-GROUP ELEMENTS.

relating to the periodic table: the naming of elements, the assignment of chemical symbols to new elements, and the certification of a particular individual or research team as the discoverers of that element. For these reasons, the IUPAC system is coming into favor among North American chemists as well.

part, is that most American schools still use this system; furthermore, there is a reasoning behind the assignment of numbers to only eight groups, as will be discussed. Where necessary or appropriate, however, group numbers in the IUPAC system will be provided as well.

Despite the international acceptance of the IUPAC system, as well as its merits in terms of convenience, the North American system is generally the one used in this book. The reason, in

Principal Energy Levels

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Group numbers in the North American system indicate the number of valence electrons, or the electrons that are involved in chemical bonding.

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Valence electrons also occupy the highest energy level in the atom—which might be thought of as the orbit farthest from the nucleus, though in fact the term “orbit” is misleading when applied to the ways an electron moves. Electrons do not move around the nucleus of an atom in regular orbits, like planets around the Sun; rather, their paths can only be loosely defined in terms of orbitals, a pattern of probabilities regarding the areas through which an electron is likely to move. The pattern of orbitals is determined by the principal energy level of the atom, which indicates the distance an electron may move away from the nucleus. Principal energy level is designated by a whole-number integer, beginning with 1 and moving upward to 7: the higher the number, the further the electron is from the nucleus, and hence the greater the energy in the atom. The relationship between principal energy level and period is relatively easy to demonstrate. The number n of a period on the periodic table is the same as the number of the highest principal energy level for the atoms on that row—that is, the principal energy level occupied by its valence electrons. Thus, elements on period 1 have a highest principal energy level of 1, and so on.

Valence Electron Configurations When discussing families of elements, however, the periods or rows on the periodic table are not as important as the groups or columns. These are defined by the valence electron configurations, a subject more complicated than principal energy levels—though the latter requires a bit more explanation in order to explain electron configurations. Each principal energy level is divided into sublevels corresponding to the number n of the principal energy level: thus, principal energy level 1 has one sublevel, principal energy level 2 has two, and so on. As one might expect, with an increase in principal energy levels and sublevels, there are increases in the complexity of the orbitals. ORBITAL PATTERNS. The four basic

types of orbital patterns are designated as s, p,d, and f. The s shape might be described as spherical, though when talking about electrons, nothing is quite so neat: orbital patterns, remember, only identify regions of probability for the elec-

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tron. In other words, in an s orbital, the total electron cloud will probably end up being more or less like a sphere. The p shape is like a figure eight around the nucleus, and the d like two figure eights meeting at the nucleus. Again, these and other orbital patterns do not indicate that the electron will necessary follow that path. What it means is that, if you could take millions of photographs of the electron during a period of a few seconds, the resulting blur of images in a p orbital would somewhat describe the shape of a figure eight. The f orbital pattern is so complex that most basic chemistry textbooks do not even attempt to explain it, and beyond f are other, even more complicated, patterns designated in alphabetical order: g, h, and so on. In the discussion that follows, we will not be concerned with these, since even for the lanthanides and the actinides, an atom at the ground state does not fill orbital patterns beyond an f. SUBLEVELS AND ORBITAL FILLING. Principal energy level 1 has only an

s sublevel; 2 has an s and a p, the latter with three possible orientations in space; 3 has an s, p, and d (five possible spatial orientations); and 4 has an s, p, d, and f (seven possible spatial orientations.) According to the Pauli exclusion principle, only two electrons can occupy a single orbital pattern—that is, the s sublevel or any one of the spatial orientations in p, d, and f—and those two electrons must be spinning in opposite directions. Thus, two electrons can move in an s orbital pattern or shell, six in a p, 10 in a d, and 14 in an f orbital pattern or shell. Valence shell configurations are therefore presented with superscript figures indicating the number of electrons in that orbital pattern—for instance, s1 for one electron in the s orbital, or d10, indicating a d orbital that has been completely filled.

REAL-LIFE A PPLICAT I O N S Representative Elements Hydrogen (atomic number 1), with the simplest of all atomic structures, has just one electron on principal energy level 1, so, in effect, its valence electron is also a core electron. The valence configuration for hydrogen is thus written as 1s1. It should be noted, as described in the Electrons

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essay, that if a hydrogen atom (or any other atom) is in an excited state, it may reach energy levels beyond its normal, or ground, state.

From the Representative Elements to the Transition Elements

Moving straight down the periodic table to francium (atomic number 87), which is in the same column as hydrogen, one finds that it has a valence electron configuration of 7s1. Thus, although francium is vastly more complex and energy-filled than hydrogen, the two elements have the same valence shell configuration; only the number of the principal energy level is different. All the elements listed below hydrogen in Group 1 are therefore classified together as alkali metals. Obviously, hydrogen—a gas—is not part of the alkali metal family, nor does it clearly belong to any other family: it is the “lone wolf ” of the periodic table.

Groups 3 through 6, along with hydrogen and the four families so far identified, constitute the 44 representative or main-group elements. In 43 of these 44, the number of valence shell electrons is the same as the group number in the North American system. (Helium, which is in Group 8 but has two valence electrons, is the lone exception.) By contrast, the 40 elements listed in the “dip” at the middle of the chart—the transition metals—follow a less easily defined pattern. This is part of the reason why the North American system does not list them by group number, and also why neither system lists the two other families within the transition elements—the lanthanides and actinides.

Now look at two elements in Group 2, with beryllium (atomic number 4) and radium (88) at the top and bottom respectively. Beryllium has a valence shell configuration of 2s2. This means its valence shell is at principal energy level 2, where there are two electrons on an s orbital pattern. Radium, though it is on period 7, nonetheless has the same valence shell configuration: 7s2. This defines the alkaline earth metals family in terms of valence shell configuration. For now, let us ignore groups 3 through 6— not to mention the columns between groups 2 and 3, unnumbered in the North American system—and skip over to Group 7. All the elements in this column, known as halogens, have valence shell configurations of ns2np5. Beyond Group 7 is Group 8, the noble gases, all but one of whom have valence shell configurations of ns2np6. The exception is helium, which has an s2 valence shell. This seems to put it with the alkaline earth metals, but of course helium is not a metal. In terms of its actual behavior, it clearly belongs to the noble gases family. The configurations of these valence shells have implications with regard to the ways in which elements bond, a subject developed at some length in the Chemical Bonding essay. Here we will consider it only in passing, to clarify the fact that electron configuration produces observable results. This is most obvious with the noble gases, which tend to resist bonding with most other elements because they already have eight electrons in their valence shell—the same number of valence electrons that most other atoms achieve only after they have bonded.

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Families of Elements

Before addressing the transition metals, however, let us consider patterns of orbital filling, which also differentiate the representative elements from the transition elements. Each successive representative element fills all the orbitals of the elements that precede it (with some exceptions that will be explained), then goes on to add one more possible electron configuration. The total number of electrons—not just valence shell electrons—is the same as the atomic number. Thus fluorine, with an atomic number of 9, has a complete configuration of 1s22s22p5. Neon, directly following it with an atomic number of 10, has a total configuration of 1s22s22p6. (Again, this is not the same as the valence shell configuration, which is contained in the last two sublevels represented: for example, 2s22p6 for neon.) The chart that follows shows the pattern by which orbitals are filled. Note that in several places, the pattern of filling becomes “out of order,” something that will be explained below. Orbital Filling by Principal Energy Level • • • • • • • • • • •

1s (2) 2s (2) 2p (6) 3s (2) 3p (6) 4s (2) 3d (10) 4p (6) 5s (2) 4d (10) 5p (6)

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6s (2) 4f (14) 5d (10) 6p (6) 7s (2) 5f (14) 6d (10)

PATTERNS OF ORBITAL FILLING. Generally, the 44 representative elements

follow a regular pattern of orbital filling, and this is particularly so for the first 18 elements. Imagine a small amphitheater, shaped like a cone, with smaller rows of seats at the front. These rows are also designated by section, with the section number being the same as the number of rows in that section. The two seats in the front row comprise a section labeled 1 or 1s, and this is completely filled after helium (atomic number 2) enters the auditorium. Now the elements start filling section 2, which contains two rows. The first row of section 2, labeled 2s, also has two seats, and after beryllium (4), it too is filled. Row 2p has 6 seats, and it is finally filled with the entrance of neon (10). Now, all of section 2 has been filled; therefore, the eleventh element, sodium, starts filling section 3 at the first of its three rows. This row is 3s—which, like all s rows, has only two seats. Thus, when element 13, aluminum, enters the theatre, it takes a seat in row 3p, and eventually argon (18), completes that six-seat row. By the pattern so far established, element 19 (potassium) should begin filling row 3d by taking the first of its 10 seats. Instead, it moves on to section 4, which has four rows, and it takes the first seat in the first of those rows, 4s. Calcium (20) follows it, filling the 4s row. But when the next element, scandium (21), comes into the theatre, it goes to row 3d, where potassium “should have” gone, if it had continued filling sections in order. Scandium is followed by nine companions (the first row of transition elements) before another representative element, gallium (31), comes into the theatre. (For reasons that will not be discussed here, chromium and copper, elements 24 and 29, respectively, have valence electrons in 4s—which puts them slightly off the transition metal pattern.) According to the “proper” order of filling seats, now that 3d (and hence all of section 3) is filled, gallium should take a seat in 4s. But those seats have already been taken by the two preced-

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ing representative elements, so gallium takes the first of six seats in 4p. After that row fills up at krypton (36), it is again “proper” for the next representative element, rubidium (37), to take a seat in 4d. Instead, just as potassium skipped 3d, rubidium skips 4d and opens up section 5 by taking the first of two seats in 5s. Just as before, the next transition element— yttrium (39)—begins filling up section 4d, and is followed by nine more transition elements until cadmium (48) fills up that section. Then, the representative elements resume with indium (49), which, like gallium, skips ahead to section 5p. And so it goes through the remainder of the periodic table, which ends with two representative elements followed by the last 10 transition metals.

Transition Metals Given the fact that it is actually the representative elements that skip the d sublevels, and the transition metals that go back and fill them, one might wonder if the names “representative” and “transition” (implying an interruption) should be reversed. However, remember the correlation between the number of valence shell electrons and group number for the representative elements. Furthermore, the transition metals are the only elements that fill the d orbitals. This brings us to the reason why the lanthanides and actinides are set apart even from the transition metals. In most versions of the periodic table, lanthanum (57) is followed by hafnium (72) in the transition metals section of the chart. Similarly, actinium (89) is followed by rutherfordium (104). The “missing” metals—lanthanides and actinides, respectively—are listed at the bottom of the chart. There are reasons for this, as well as for the names of these groups. After the 6s orbital fills with the representative element barium (56), lanthanum does what a transition metal does—it begins filling the 5d orbital. But after lanthanum, something strange happens: cerium (58) quits filling 5d, and moves to fill the 4f orbital. The filling of that orbital continues throughout the entire lanthanide series, all the way to lutetium (71). Thus, lanthanides can be defined as those metals that fill the 4f orbital; however, because lanthanum exhibits similar properties, it is usually included with the lanthanides. Sometimes the term “lan-

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Families of Elements

KEY TERMS Those transition metals

LANTHANIDES: The transition metals

that fill the 5f orbital. Because actinium—

that fill the 4forbital. Because lan-

which does not fill the 5f orbital—exhibits

thanum—which does not fill the 4f

characteristics similar to those of the

orbital—exhibits characteristics similar to

actinides, it is usually considered part of

those of the lanthanides, it is usually con-

the actinides family.

sidered part of the lanthanide family.

ACTINIDES:

ALKALI METALS:

All members, except

MAIN-GROUP

The 44

ELEMENTS:

hydrogen, of Group 1 on the periodic table

elements in Groups 1 through 8 on the

of elements, with valence electron configu-

periodic table of elements, for which the

rations of ns1.

number of valence electrons equals the group number. (The only exception is heli-

ALKALINE EARTH METALS: Group 2

um.) The main-group elements, also called

on the periodic table of elements, with

representative elements, include the fami-

valence electron configurations of ns2.

lies of alkali metals, alkali earth metals,

ELECTRON CLOUD: A term used to

halogens, and noble gases, as well as other

describe the pattern formed by orbitals.

metals, nonmetals, and metalloids.

FAMILIES OF ELEMENTS:

Related

elements, including the noble gases, halogens, alkali metals, alkaline earth metals, transition metals,lanthanides, and actinides. In addition, metals, nonmetals, and metalloids form loosely defined families. Other family designations—such as carbon family—are sometimes used. GROUND STATE:

A term describing

METALLOIDS: Elements which exhibit

characteristics of both metals and nonmetals. Metalloids are all solids, but are not lustrous or shiny, and they conduct heat and electricity moderately well. The six metalloids occupy a diagonal region between the metals and nonmetals on the right side of the periodic table. Sometimes astatine is included with the metalloids, but in this book it is treated within the

the state of an atom at its ordinary energy

context of the halogens family.

level.

METALS: A collection of 87 elements

Columns on the periodic

that includes numerous families—the

table of elements. These are ordered

alkali metals, alkaline earth metals, transi-

according to the numbers of valence elec-

tion metals, lanthanides, and actinides, as

trons in the outer shells of the atoms for

well as seven elements in groups 3 through

the elements represented.

5. Metals, which occupy the left, center, and

GROUPS:

HALOGENS: Group 7 of the periodic

table of elements, with valence electron configurations of ns2np5.

part of the right-hand side of the periodic table, are lustrous or shiny in appearance, and malleable, meaning that they can be molded into different shapes without

An atom or atoms that has lost or

breaking. They are excellent conductors of

gained one or more electrons, and thus has

heat and electricity, and tend to form posi-

a net electric charge.

tive ions by losing electrons.

ION:

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Families of Elements

KEY TERMS NOBLE GASES: Group 8 of the peri-

(in atomic mass units) for that particular

odic table of elements, all of whom (with

element.

the exception of helium) have valence electron configurations of ns2np6.

PERIODS: Rows of the periodic table of

elements. These represent successive ener-

NONMETALS: Elements that have a dull

gy levels in the atoms of the elements

appearance; are not malleable; are poor

involved.

conductors of heat and electricity; and tend to gain electrons to form negative ions. They are thus the opposite of metals in most regards, as befits their name. Aside from hydrogen, the other 18 nonmetals occupy the upper right-hand side of the periodic table, and include the noble gases, halogens, and seven elements in groups 3 through 6. ORBITAL:

PRINCIPAL ENERGY LEVEL: A value

indicating the distance that an electron may move away from the nucleus of an atom. This is designated by a whole-number integer, beginning with 1 and moving upward. The higher the principal energy level, the greater the energy in the atom, and the more complex the pattern of orbitals.

A pattern of probabilities

regarding the position of an electron for an atom in a particular energy state. The high-

REPRESENTATIVE ELEMENTS:

See

main-group elements.

er the principal energy level, the more

TRANSITION METALS: A group of 40

complex the pattern of orbitals. The four

elements, which are not assigned a group

types of orbital patterns are designated as s,

number in the North American version of

p, d, and f—each of which is more complex

the periodic table. These are the only ele-

than the one before.

ments that fill the d orbitals.

PERIODIC TABLE OF ELEMENTS:

146

CONTINUED

A

VALENCE

ELECTRONS:

Electrons

chart that shows the elements arranged in

that occupy the highest energy levels in an

order of atomic number, along with chem-

atom. These are the electrons involved in

ical symbol and the average atomic mass

chemical bonding.

thanide series” is used to distinguish the other 14 lanthanides from lanthanum itself.

Metals, Nonmetals, and Metalloids

A similar pattern occurs for the actinides. The 7s orbital fills with radium (88), after which actinium (89) begins filling the 6d orbital. Next comes thorium, first of the actinides, which begins the filling of the 5f orbital. This is completed with element 103, lawrencium. Actinides can thus be defined as those metals that fill the 5f orbital; but again, because actinium exhibits similar properties, it is usually included with the actinides.

The reader will note that for the seven families so far identified, we have generally not discussed them in terms of properties that can more easily be discerned—such as color, phase of matter, bonding characteristics, and so on. Instead, they have been examined primarily from the standpoint of orbital filling, which provides a solid chemical foundation for identifying families. Macroscopic characteristics, as well as the ways that the various elements find application in

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daily life, are discussed within essays devoted to the various groups. Note, also, that the families so far identified account for only 92 elements out of a total of 112 listed on the periodic table: hydrogen; six alkali metals; six alkaline earth metals; five halogens; six noble gases; 40 transition metals; 14 lanthanides; and 14 actinides. What about the other 20? Some discussions of element families assign these elements, all of which are in groups 3 through 6, to families of their own, which will be mentioned briefly. However, because these “families” are not recognized by all chemists, in this book the 20 elements of groups 3 through 6 are described generally as metals, nonmetals, and metalloids. METALS AND NONMETALS. Metals are lustrous or shiny in appearance, and malleable, meaning that they can be molded into different shapes without breaking. They are excellent conductors of heat and electricity, and tend to form positive ions by losing electrons. On the periodic table, metals fill the left, center, and part of the right-hand side of the chart. Thus it should not come as a surprise that most elements (87, in fact) are metals. This list includes alkali metals, alkaline earth metals, transition metals, lanthanides, and actinides, as well as seven elements in groups 3 through 6—aluminum, gallium, indium, thallium, tin, lead, and bismuth.

Nonmetals have a dull appearance; are not malleable; are poor conductors of heat and electricity; and tend to gain electrons to form negative ions. They are thus the opposite of metals in most regards, as befits their name. Nonmetals, which occupy the upper right-hand side of the periodic table, include the noble gases, halogens, and seven elements in groups 3 through 5. These nonmetal “orphans” are boron, carbon, nitrogen, oxygen, phosphorus, sulfur, and selenium. To these seven orphans could be added an eighth, from Group 1: hydrogen. As with the metals, a separate essay—with a special focus on the “orphans”—is devoted to nonmetals.

S C I E N C E O F E V E R Y D AY T H I N G S

METALLOIDS AND OTHER “FAMILY” DESIGNATIONS. Occupying

Families of Elements

a diagonal region between the metals and nonmetals are metalloids, elements which exhibit characteristics of both metals and nonmetals. They are all solids, but are not lustrous, and conduct heat and electricity moderately well. The six metalloids are silicon, germanium, arsenic, antimony, tellurium, and polonium. Astatine is sometimes identified as a seventh metalloid; however, in this book, it is treated as a member of the halogen family. Some sources list “families” rather than collections of “orphan” metals, metalloids, and nonmetals, in groups 3 through 6. These designations are not used in this book; however, they should be mentioned briefly. Group 3 is sometimes called the boron family; Group 4, the carbon family; Group 5, the nitrogen family; and Group 6, the oxygen family. Sometimes Group 5 is designated as the pnictogens, and Group 6 as the chalcogens. WHERE TO LEARN MORE Bankston, Sandy. “Explore the Periodic Table and Families of Elements” The Rice School Science Department (Web site). (May 23, 2001). Challoner, Jack. The Visual Dictionary of Chemistry. New York: DK Publishing, 1996. “Elementistory” (Web site). (May 22, 2001). “Families of Elements” (Web site). (May 23, 2001). Knapp, Brian J. and David Woodroffe. The Periodic Table. Danbury, CT: Grolier Educational, 1998. Maton, Anthea. Exploring Physical Science. Upper Saddle River, N.J.: Prentice Hall, 1997. Oxlade, Chris. Elements and Compounds. Chicago: Heinemann Library, 2001. “The Pictorial Periodic Table” (Web site). (May 22, 2001). Stwertka, Albert. A Guide to the Elements. New York: Oxford University Press, 1998. “Visual Elements” (Web site). (May 22, 2001).

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S C I E N C E O F E V E R Y D AY T H I N G S r e a l - l i f e CHEMISTRY

M E TA L S M E TA L S A L KA L I M E TA L S A L KA L I N E E A R T H M E TA L S T RA N S I T I O N M E TA L S ACTINIDES LANTHANIDES

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M E TA L S

Metals

CONCEPT A number of characteristics distinguish metals, including their shiny appearance, as well as their ability to be bent into various shapes without breaking. In addition, metals tend to be highly efficient conductors of heat and electricity. The vast majority of elements on the periodic table are metals, and most of these fall into one of five families: alkali metals; alkaline earth metals; the very large transition metals family; and the inner transition metal families, known as the lanthanides and actinides. In addition, there are seven “orphans,” or metals that do not belong to a larger family of elements: aluminum, gallium, indium, thallium, tin, lead, and bismuth. Some of these may be unfamiliar to most readers; on the other hand, a person can hardly spend a day without coming into contact with aluminum. Tin is a well-known element as well, though much of what people call “tin” is not really tin. Lead, too, is widely known, and once was widely used as well; today, however, it is known primarily for the dangers it poses to human health.

HOW IT WORKS General Properties of Metals Metals are lustrous or shiny in appearance, and malleable or ductile, meaning that they can be molded into different shapes without breaking. Despite their ductility, metals are extremely durable and have high melting and boiling points. They are excellent conductors of heat and electricity, and tend to form positive ions by losing electrons.

S C I E N C E O F E V E R Y D AY T H I N G S

The bonds that metals form with each other, or with nonmetals, are known as ionic bonds, which are the strongest type of chemical bond. Even within a metal, however, there are extremely strong bonds. Therefore, though it is easy to shape metals (that is, to slide the atoms in a metal past one another), it is very difficult to separate metal atoms. Their internal bonding is thus very strong, but nondirectional. Internal bonding in metals is described by the electron sea model, which depicts metal atoms as floating in a “sea” of valence electrons, the electrons involved in bonding. These valence electrons are highly mobile within the crystalline structure of the metal, and this mobility helps to explain metals’ high conductivity. The ease with which metal crystals allow themselves to be rearranged explains not only metals’ ductility, but also their ability to form alloys, a mixture containing one or more metals.

Abundance of Metals Metals constitute a significant portion of the elements found in Earth’s crust, waters, and atmosphere. The following list shows the most abundant metals on Earth, with their ranking among all elements in terms of abundance. Many other metals are present in smaller, though still significant, quantities. • • • • • •

3. Aluminum (7.5%) 4. Iron (4.71%) 5. Calcium (3.39%) 6. Sodium (2.63%) 7. Potassium (2.4%) 8. Magnesium (1.93%)

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Metals

ALUMINUM IS THE MOST ABUNDANT METAL ON EARTH. THIS PHOTO SHOWS THE FIRST BLOBS OF ALUMINUM ISOLATED, A FEAT PERFORMED BY CHARLES HALL IN 1886. (James L. Amos/Corbis. Reproduced by permission.)

• 10. Titanium (0.58%) • 13. Manganese (0.09%) In addition, metals are also important components of the human body. The following list shows the ranking of the most abundant metals among the elements in the body, along with the percentage that each constitutes. As with the quantities of metals on Earth, other metals are present in much smaller, but still critical, amounts. • • • • • •

152

5. Calcium (1.4%) 7. Magnesium (0.50%) 8. Potassium (0.34%) 10. Sodium (0.14%) 12. Iron (0.004%) 13. Zinc (0.003%)

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EVER

Metals on the Periodic Table On the periodic table of elements, metals fill the left, center, and part of the right-hand side of the chart. The remainder of the right-hand section is taken by nonmetals, which have properties quite different from (and often opposite to) those of metals, as well as metalloids, which display characteristics both of metals and nonmetals. Nonmetallic elements are covered in essays on Nonmetals; Metalloids; Halogens; Noble Gases; and Carbon. In addition, hydrogen, the one nonmetal on the left side of the periodic table—first among the elements, in the upper left corner—is also discussed in its own essay. The total number of metals on the periodic table is 87—in other words, about 80% of all

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known elements. These include the six alkali metals, which occupy Group 1 of the periodic table, as well as the six alkaline earth metals, or Group 2. To the right of these are the 40 transition metals, whose groups are unnumbered in the North American version of the periodic table. Among the transition metals are two elements, lanthanum and actinium, often lumped in with the families of inner transition metals that exhibit similar properties. These are, respectively, the 14 lanthanides and 14 actinides. In addition, there are the seven “orphan” metals of periods 3 through 5, mentioned in the introduction to this essay, which will be discussed below.

Metals

The various families of elements are defined, not so much by external characteristics as by the configurations of valence electrons in their atoms’ shells. This subject will not be discussed in any depth here; instead, the reader is encouraged to consult essays on the specific metal families, which discuss the electron configurations of each. BEFORE THEY WERE DESTROYED BY TERRORIST ATTACKS SEPTEMBER 11, 2001, THE TWIN TOWERS OF THE WORLD TRADE CENTER IN NEW YORK CITY GLEAMED WITH A CORROSION-RESISTANT COVERING OF ALUMINUM.

ON

REAL-LIFE AP P LICATIONS Alkali Metals The members of the alkali metal family are distinguished by the fact that they have a valence electron configuration of s1 This means that they tend to bond easily with other substances. Alkali metals tend to form positive ions, or cations, with a charge of 1+. In contact with water, alkali metals form a negatively charged hydroxide ion (OH–). When alkali metals react with water, one hydrogen atom splits off from the water molecule to form hydrogen gas, while the other hydrogen atom remains bonded to the oxygen, forming hydroxide. Where the heavier members of the alkali metal family are concerned, reactions can often be so vigorous that the result is combustion or even explosion. Alkali metals also react with oxygen to produce either an oxide, peroxide, or superoxide, depending on the particular member of the alkali metal family involved. Shiny and soft enough to be cut with a knife, the alkali metals are usually white, though cesium is a yellowish white. When placed in a flame, most of these substances produce charac-

S C I E N C E O F E V E R Y D AY T H I N G S

(Bill Ross/Corbis. Reproduced by permission.)

teristic colors: lithium, for instance, glows bright red, and sodium an intense yellow. Heated potassium produces a violet color, rubidium a dark red, and cesium a light blue. This makes it possible to identify the metals by color when heated— a useful trait, since they so often tend to be bonded with other elements. The alkali metals, listed below by atomic number and chemical symbol, are: • • • • • •

3. Lithium (Li) 11. Sodium (Na) 19. Potassium (K) 37. Rubidium (Rb) 55. Cesium (Cs) 87. Francium (Fr)

Alkaline Earth Metals Occupying Group 2 of the periodic table are the alkaline earth metals, which have a valence electron configuration of s2. Like the alkali metals, the alkaline earth metals are known for their high reactivity—a tendency for bonds between atoms or molecules to be made or broken in such a way that materials are transformed. Thus they are sel-

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Metals

MOST “TIN”

ROOFS NOW CONSIST OF AN IRON OR STEEL ROOF COVERED WITH ZINC, NOT TIN. (John Hulme; Eye Ubiqui-

tous/Corbis. Reproduced by permission.)

dom found in pure form, but rather in compounds with other elements—for example, salts, a term that generally describes a compound consisting of a metal and a nonmetal. The alkaline earth metals are also like the alkali metals in that they have the properties of a base as opposed to an acid. An acid is a substance that, when dissolved in water, produces positive ions of hydrogen, designated symbolically as H+ ions. It is thus known as a proton donor. A base, on the other hand, produces negative hydroxide ions when dissolved in water. These are designated by the symbol OH-. Bases are therefore characterized as proton acceptors. The alkaline earth metals are shiny, and most are white or silvery in color. Like their “cousins” in the alkali metal family, they glow with characteristic colors when heated. Calcium glows orange, strontium a very bright red, and barium an apple green. Physically they are soft, though not as soft as the alkali metals; nor are their levels of reactivity as great as those of their neighbors in Group 1. The alkaline earth metals, listed below by atomic number and chemical symbol, are: • 4. Beryllium (Be) • 12. Magnesium (Mg)

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• • • •

20. Cadmium (Ca) 38. Strontium (Sr) 56. Barium (Ba) 88. Radium (Ra)

Transition Metals The transition metals are the only elements that fill the dorbitals. They are also the only elements that have valence electrons on two different principal energy levels. This sets them apart from the representative or main-group elements, of which the alkali metals and alkaline earth metals are a part. The transition metals also follow irregular patterns of orbital filling, which further distinguish them from the representative elements. Other than that, there is not much that differentiates the transition metals, a broad grouping that includes precious metals such as gold, silver, and platinum, as well as highly functional metals such as iron, manganese, and zinc. Many of the transition metals, particularly those on periods 4, 5, and 6, form useful alloys with one another, and with other elements. Because of their differences in valence electron configuration, however, they do not always combine in the same ways, even within an element. Iron, for

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instance, sometimes releases two electrons in chemical bonding, and at other times three. GROUPING THE TRANSITION METALS. There is no easy way to group the

transition metals, though some of these elements are traditionally categorized together. These do not constitute “families” as such, but they do provide useful ways to break down the otherwise rather daunting lineup of the transition metals. In two cases, there is at least a relationship between group number on the periodic table and the categories loosely assigned to a collection of transition metals. Thus the “coinage metals”— copper, silver, and gold—all occupy Group 9 on the IUPAC version of the periodic table. (These have traditionally been associated with one another because their resistance to oxidation, combined with their malleability and beauty, has made them useful materials for fashioning coins. Likewise the members of the “zinc group”— zinc, cadmium, and mercury—on Group 10 on the IUPAC periodic table have often been associated as a miniature unit due to common properties. Members of the “platinum group”—platinum, iridium, osmium, palladium, rhodium, and ruthenium—occupy a rectangle on the table, corresponding to periods 5 and 6, and groups 6 through 8. What actually makes them a “group,” however, is the fact that they tend to appear together in nature. Iron, nickel, and cobalt, found alongside one another on Period 4, may be grouped together because they are all magnetic to some degree. The transition metals, listed below by atomic number and chemical symbol, are: • • • • • • • • • • • • • • • •

21. Scandium (Sc) 22. Titanium (Ti) 23. Vanadium (V) 24. Chromium (Cr) 25. Manganese (Mn) 26. Iron (Fe) 27. Cobalt (Co) 28. Nickel (Ni) 29. Copper (Cu) 30. Zinc (Zn) 39. Yttrium (Y) 40. Zirconium (Zr) 41. Niobium (Nb) 42. Molybdenum (Mo) 43. Technetium (Tc) 44. Ruthenium (Ru)

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• • • • • • • • • • • • • • • • • • • • • • • •

45. Rhodium (Rh) 46. Palladium (Pd) 47. Silver (Ag) 48. Cadmium (Cd) 57. Lanthanum (La) 72. Hafnium (Hf) 73. Tantalum (Ta) 74. Tungsten (W) 75. Rhenium (Re) 76. Osmium (Os) 77. Iridium (Ir) 78. Platinum (Pt) 79. Gold (Au) 80. Mercury (Hg) 89. Actinium (Ac) 104. Rutherfordium (Rf) 105. Dubnium (Db) 106. Seaborgium (Sg) 107. Bohrium (Bh) 108. Hassium (Hs) 109. Meitnerium (Mt) 110. Ununnilium (Uun) 111. Unununium (Uuu) 112. Ununbium (Uub) The last nine, along with 11 of the actinides, are part of the list of metals known collectively as the transuranium elements—that is, elements with atomic numbers higher than 92. These have all been produced artificially, in most cases within a laboratory. Note that the last three had not been named as of 2001: the “names” by which they are known simply mean, respectively, 110, 111, and 112.

Metals

Lanthanides The lanthanides are the first of two inner transition metal groups. Note that in the list of transition metals above, lanthanum (57) is followed by hafnium (72). The “missing” group of 14 elements, normally shown at the bottom of the periodic table, is known as the lanthanides, a family which can be defined by the fact that all fill the 4f orbital. However, because lanthanum (which does not fill an f orbital) exhibits similar properties, it is usually included with the lanthanides. Bright and silvery in appearance, many of the lanthanides—though they are metals—are so soft they can be cut with a knife. They also tend to be highly reactive. Though they were once known as the “rare earth metals,” lanthanides

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only seemed rare because they were difficult to extract from compounds containing other substances—including other lanthanides. Because their properties are so similar, and because they tend to be found together in the same substances, the original isolation and identification of the lanthanides was an arduous task that took well over a century. The lanthanides (in addition to lanthanum, listed earlier with the transition metals), are: • • • • • • • • • • • • • •

58. Cerium (Ce) 59. Praseodymium (Pr) 60. Neodymium (Nd) 61. Promethium (Pm) 62. Samarium (Sm) 63. Europium (Eu) 64. Gadolinium (Gd) 65. Terbium (Tb) 66. Dysprosium (Dy) 67. Holmium (Ho) 68. Erbium (Er) 69. Thulium (Tm) 70. Ytterbium (Yb) 71. Lutetium (Lu)

Actinides Just as lanthanum is followed on the periodic table by an element with an atomic number 15 points higher, so actinium (89) is followed by rutherfordium (104). The elements in the “gap” are the other inner transition family, the actinides, which all fill the 5f orbital. These are placed below the lanthanides at the bottom of the chart, and just as lanthanum is included with the lanthanides, even though it does not fill an f orbital, so actinium is usually lumped in with the actinides due to similarities in properties. Most of the actinides tend to be unstable or radioactive, the later ones highly so. The first three members of the group (not counting actinium) occur in nature, while the other 11 are transuranium elements created artificially. In most cases, these were produced with a cyclotron or other machine for accelerating atoms, generally at the research center of the University of California at Berkeley during a period from 1940 to 1961. Einsteinium and fermium, however, were by-products of nuclear testing at Bikini Atoll in the south Pacific in 1952. The principal use of the actinides is in nuclear applications—weaponry

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or power plants—though some of them also have specialized uses as well. The actinides (in addition to actinium, listed earlier with the transition metals), are: • • • • • • • • • • • • • •

90. Thorium (Th) 91. Protactinium (Pa) 92. Uranium (U) 93. Neptunium (Np) 94. Plutonium (Pu) 95. Americium (Am) 96. Curium (Cm) 97. Berkelium (Bk) 98. Californium (Cf) 99. Einsteinium (Es) 100. Fermium (Fm) 101. Mendelevium (Md) 102. Nobelium (No) 103. Lawrencium (Lr)

“Orphan” Metals Seven other metals remain to be discussed. These are the “orphan” metals, which appear in groups 3, 4, and 5 of the North American periodic table (13, 14, and 15 of the IUPAC table), and they are called “orphans” simply because none belongs to a clearly defined family. Sometimes aluminum and the three elements below it in Group 3—gallium, indium, and thallium—are lumped together as the “aluminum family.” This is not a widely recognized distinction, but will be used here only for the purpose of giving some form of organization to the “orphan” elements. Though they do not belong to families, the “orphan” metals are nonetheless highly important. Aluminum, after all, is the most abundant of all metals on Earth, and has wide applications in daily life. Tin, too, is widely known, as is lead. Somewhat less recognizable is bismuth, but it appears in a product with which most Americans are familiar: Pepto-Bismol. The seven “orphans” are listed below, along with atomic number and chemical symbol: • • • • • • •

13. Aluminum (Al) 31. Gallium (Ga) 49. Indium (In) 50. Tin (Sn) 81. Thallium (Tl) 82. Lead (Pb) 83. Bismuth (Bi)

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Aluminum Named after alum, a salt known from ancient times and used by the Egyptians, Romans, and Greeks, aluminum is so reactive that it proved difficult to isolate. Only in 1825 did Danish physicist Hans Christian Ørsted (1777-1851) isolate an impure version of aluminum metal, using a complicated four-step process. Two years later, German chemist Friedrich Wöhler (1800-1882) obtained pure aluminum from a reaction of metallic potassium with aluminum chloride, or AlCl3. For years thereafter, aluminum continued to be so difficult to produce that it acquired the status of a precious metal. European kings displayed treasures of aluminum as though they were gold or silver, and by 1855, it was selling for about $100,000 a pound. Then in 1886, French metallurgist Paul-Louis-Toussaint Héroult (18631914) and American chemist Charles Martin Hall (1863-1914) independently developed a process that made aluminum relatively easy to extract by means of electrolysis. Thanks to what became known as the Hall-Héroult process, the price of aluminum dropped to around 30 cents a pound. America’s aluminum comes primarily from mines in Alabama, Arkansas, and Georgia, where it often appears in a clay called kaolin, used in making porcelain. Aluminum can also be found in deposits of feldspar, mica, granite, and other clays. The principal sources of aluminum outside the United States include France, Surinam, Jamaica, and parts of Africa. USES FOR ALUMINUM. Though aluminum is highly reactive, it does not corrode in moist air. Instead of forming rust the way iron does, it forms a thin, hard, invisible coating of aluminum oxide. This, along with its malleability, makes it highly useful for producing cans and other food containers. The World Trade Center Towers in New York City gleam with a corrosionresistant covering of aluminum. Likewise the element is used as a coating for mirrors: not only is it cheaper than silver, but unlike silver, it does not tarnish and turn black. Also, a thin coating of metallic aluminum gives Mylar balloons their silvery sheen.

Aluminum conducts electricity about 60% as well as copper, meaning that it is an extraordinarily good conductor, useful for transmitting electrical power. Because it is much more light-

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weight than copper and highly ductile, it is often used in high-voltage electric lines. Its conductivity also makes aluminum a favorite material for kitchen pots and pans. Unlike copper, it is not known to be toxic; however, because aluminum dissolves in strong acids, sometimes tomatoes and other acidic foods acquire a “metallic” taste when cooked in an aluminum pot.

Metals

Though it is soft in its pure form, when combined with copper, manganese, silicon, magnesium, and/or zinc, aluminum can produce strong, highly useful alloys. These find application in automobiles, airplanes, bridges, highway signs, buildings, and storage tanks. Aluminum’s high ductility also makes it the metal of choice for foil wrappings—that is, aluminum foil. Compounds containing aluminum are used in a wide range of applications, including antacids; antiperspirants (potassium aluminum sulfate tends to close up sweat-gland ducts); and water purifiers.

Other Elements in the Aluminum Group GALLIUM. When Russian chemist Dmitri Ivanovitch Mendeleev (1834-1907) created the periodic table in 1869, he predicted the existence of several missing elements on the basis of “holes” between elements on the table. Among these was what he called “eka-aluminum,” discovered in the 1870s by French chemist Paul Emile Lecoq de Boisbaudran (1838-1912). Boisbaudran named the new element gallium, after the ancient name of his homeland, Gaul.

Gallium will melt if held in the hand, and remains liquid over a large range of temperatures—from 85.6°F (29.76°C) to 3,999.2°F (2,204°C). This makes it highly useful for thermometers with a large temperature range. Used in a number of applications within the electronics industry, gallium is present in the compounds gallium arsenide and gallium phosphide, applied in lasers, solar cells, transistors, and other solidstate devices. INDIUM AND THALLIUM. As Boisbaudran would later do, German mineralogists Ferdinand Reich (1799-1882) and Hieronymus Theodor Richter (1824-1898) used a method called spectroscopy to discover one of the aluminum-group metals. Spectroscopy involves analyzing the frequencies of light (that is, the colors) emitted by an item of matter.

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However, Reich faced a problem when studying the zinc ores that he believed contained a new element: he was colorblind. Therefore he called on the help of Richter, his assistant. In 1863, they discovered an element they called indium because of the indigo blue color it emitted. Reich later tried to claim that he made the discovery alone, but Richter’s role is indisputable. Indium is used in making alloys, one of which is a combination with gallium that is liquid at room temperature. Its alloys are used in bearings, dental amalgams, and nuclear control roles. Various compounds are applied in solidstate electronics devices, and in electronics research. Two years before the discovery of indium, English physicist William Crookes (1832-1919) discovered another element using spectroscopic analysis. This was thallium, named after the Greek word thallos (“green twig”), a reference to a lime-green spectral line that told Crookes he had found a new element. Thallium is applied in electronic equipment, and the compound thallium sulfate is used for killing ants and rodents.

Three Other “Orphans” TIN. Known from ancient times, tin was combined with copper to make bronze—an alloy so significant it defines an entire stage of human technological development. Tin’s chemical symbol, Sn, refers to the name the Romans gave it: stannum. Just as tin added to copper gave bronze its strength, alloys of tin with copper and antimony created pewter, a highly useful, malleable material for pots and dinnerware popular from the thirteenth century through the early nineteenth century.

Tin is found primarily in Bolivia, Malaysia, Indonesia, Thailand, Zaire, and Nigeria—a wide geographical range. Like aluminum today, it has been widely used as a coating for other metals to retard corrosion. Hence the term “tin can,” referring to a steel can coated with tin. Also, tin has been used to coat iron or steel roofs, but because zinc is cheaper, it is now most often the coating of choice; nonetheless, these are still typically called “tin roofs.” LEAD. Due to knowledge of its toxic properties, lead is not frequently used today, but this was not always the case. The Romans, for instance, used plumbum as they called it (hence the chemical symbol Pb) as a material for making

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water pipes. This is the root of our own word plumber. (The same root explains the verb plumb or the noun plumb bob. A plumb bob is a lead weight, hung from a string, used to ensure that the walls of a structure are “plumb”—that is, perpendicular to the foundation.) Many historians believe that plumbum in the Romans’ water supply was one of the reasons behind the decline and fall of the Roman Empire. The human body can only excrete very small quantities of lead a day, and this is particularly true of children. Even in small concentrations, lead can cause elevation of blood pressure; reduction in the synthesis of hemoglobin, which carries oxygen from the lungs to the blood and organs; and decreased ability to utilize vitamin D and calcium for strengthening bones. Higher concentrations of lead can effect the central nervous system, resulting in decreased mental functioning and hearing damage. Prolonged exposure can result in a coma or even death. Before these facts became widely known in the late twentieth century, lead was applied as an ingredient in paint. In addition, it was used in water pipes, and as an anti-knock agent in gasolines. Increased awareness of the health hazards involved have led to a discontinuation of these practices. (Note that pencil “lead” is actually graphite, a form of carbon.) Lead, however, is used for absorbing radiation—for instance, as a shield against X rays. Another important use for lead today is in storage batteries. Yet another “application” of lead relates to its three stable isotopes (lead-206, -207, and -208), the end result of radioactive decay on the part of various isotopes of uranium and other radioactive elements. BISMUTH. During the Middle Ages, when a number of unscientific ideas prevailed, scholars believed that lead eventually became tin, tin eventually turned into bismuth, and bismuth developed into silver. Hence they referred to bismuth as “roof of silver.” Only in the sixteenth century did German metallurgist Georgius Agricola (George Bauer; 1494-1555) put forward the idea that bismuth was a separate element.

Bismuth appears in nature not only in its elemental form, but also in a number of compounds and ores. It is also obtained as a by-product from the smelting of gold, silver, copper, and lead. Because bismuth expands dramatically when it cools, early printers used a bismuth alloy

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Metals

KEY TERMS Those transition metals

fill the f orbitals. For this reason, they are

that fill the 5f orbital. Because actinium—

usually treated as distinct from the transi-

which does not fill the f orbital—exhibits

tion metals.

ACTINIDES:

characteristics similar to those of the actinides, it is usually considered part of the actinides family. ALKALI METALS:

ION:

An atom or group of atoms that has

lost or gained one or more electrons, and thus has a net electric charge.

All members, ex-cept

hydrogen, of Group 1 on the periodic table of elements, with valence electron configurations of ns1.

ISOTOPES: Atoms that have an equal

number of protons, and hence are of the same element, but differ in their number of neutrons. This results in a difference of

ALKALINE EARTH METALS: Group 2

mass. Isotopes may be either stable or

on the periodic table of elements, with

unstable. The latter type, known as

valence electron configurations of ALLOY:

ns2.

A mixture containing more than

radioisotopes, are radioactive. IUPAC SYSTEM: A version of the peri-

one metal.

odic table of elements, authorized by the

CONDUCTIVITY: The ability to conduct

International Union of Pure and Applied

heat or electricity. Metals are noted for

Chemistry (IUPAC), which numbers all

their high levels of conductivity.

groups on the table from 1 to 18. Thus in

CRYSTALLINE: A term describing an

internal structure in which the constituent parts have a simple and definite geometric arrangement, repeated in all directions. Metals have a crystalline structure. DUCTILE:

Capable of being bent or

molded into various shapes without breaking.

the IUPAC system, in use primarily outside of North America, both the representative or main-group elements and the transition metals are numbered. LANTHANIDES: The transition metals

that fill the 4f orbital. Because lanthanum—which does not fill the f orbital—exhibits characteristics similar to those of the lanthanides, it is usually con-

ELECTRON SEA MODEL: A widely

accepted model of internal bonding within metals, which depicts metal atoms as floating in a “sea” of valence electrons.

sidered part of the lanthanide family. METALS: A collection of 87 elements

that includes numerous families—the alkali metals, alkaline earth metals, transi-

Columns on the periodic

tion metals, lanthanides, and actinides, as

table of elements. These are ordered

well as seven elements in groups 3 through

according to the numbers of valence elec-

5 of the North American system periodic

trons in the outer shells of the atoms for

table. Metals are lustrous or shiny in

the elements represented.

appearance, and ductile. They are excellent

GROUPS:

INNER TRANSITION METALS: The

conductors of heat and electricity, and tend

lanthanides and actinides, both of which

to form positive ions by losing electrons.

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Metals

KEY TERMS

CONTINUED

NORTH AMERICAN SYSTEM: A ver-

decays to form another isotope until even-

sion of the periodic table of elements that

tually (though this may take a long time) it

only numbers groups of elements in which

becomes stable.

the number of valence electrons equals the group number. Hence the transition metals are usually not numbered in this system. ORBITAL:

A pattern of probabilities

REACTIVITY: The tendency for bonds

between atoms or molecules to be made or broken in such a way that materials are transformed.

regarding the position of an electron for an atom in a particular energy state. The high-

REPRESENTATIVE OR MAIN-GROUP

er the principal energy level, the more

ELEMENTS:

complex the pattern of orbitals.

1 through 8 on the North American ver-

PERIODIC TABLE OF ELEMENTS: A

chart that shows the elements arranged in order of atomic number, along with chem-

sion of periodic table of elements, for which the number of valence electrons equals the group number. (The only excep-

ical symbol and the average atomic mass

tion is helium.) The alkali metals and alka-

for that particular element. Elements are

line earth metals are representative ele-

arranged in groups or columns, as well as

ments; all other metals are not.

in rows or periods, according to specific

SALT:

aspects of their valence electron config-

that brings together a metal and a non-

urations.

metal. Salts are produced, along with water,

PERIODS: Rows on the periodic table of

in the reaction of an acid and a base.

elements. These represent successive principal energy levels for the valence electrons in the atoms of the elements involved. PRINCIPAL ENERGY LEVEL: A value

indicating the distance that an electron may move away from the nucleus of an atom. This is designated by a whole-number integer, beginning with 1 and moving upward. The higher the principal energy level, the greater the energy in the atom, and the more complex the pattern of

RADIOACTIVITY:

Generally, a salt is any compound

TRANSITION METALS: A group of 40

elements (counting lanthanum and actinium), which are not assigned a group number in the version of the periodic table of elements known as the North American system. These are the only elements that fill the d orbital. In addition, the transition metals—unlike the representative or maingroup elements—have their valence electrons on two different principal energy levels. Though the lanthanides and actinides

orbitals. A term describing a

phenomenon whereby certain isotopes

160

The 44 elements in Groups

are known as inner transition metals, they are usually treated separately. Electrons

known as radioisotopes are subject to a

VALENCE

form of decay brought about by the emis-

that occupy the highest principal energy

sion of high-energy particles. “Decay” does

level in an atom. These are the electrons

not mean that the isotope “rots”; rather, it

involved in chemical bonding.

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ELECTRONS:

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to make metal type. When poured into forms cut with the shapes of letters and other characters, the alloy produced symbols with clear, sharp edges.

Kerrod, Robin. Matter and Materials. Illustrated by Terry Hadler. Tarrytown, NY: Benchmark Books, 1996. “Lead Programs.” Environmental Protection Agency (Web site). (May 28, 2001).

Wood’s metal is a bismuth alloy involving cadmium, tin, and lead, which melts at a temperature of 158°F (70°C). This makes it useful for automatic sprinkler systems inside a building: when a fire breaks out, it melts the Wood’s metal seal, and turns on the sprinklers. Bismuth compounds are applied in cosmetics, paints, and dyes, and prior to the twentieth century, compounds of bismuth were widely used for treating sexually transmitted diseases. Today, antibiotics have largely taken their place, but bismuth still has one well-known medicinal application: in bismuth subsalicylate, better known by the brand name Pepto-Bismol.

WebElements (Web site). (May 22, 2001).

WHERE TO LEARN MORE

Whyman, Kathryn. Metals and Alloys. Illustrated by Louise Nevett and Simon Bishop. New York: Gloucester Press, 1988.

Aluminum Association (Web site). (May 28, 2001).

Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

S C I E N C E O F E V E R Y D AY T H I N G S

Metals

Mebane, Robert C. and Thomas R. Rybolt. Metals. Illustrated by Anni Matsick. New York: Twenty-First Century Books, 1995. “Metals Industry Guide.” About.com (Web site). (May 28, 2001). “Minerals and Metals.” Natural Resources of Canada (Web site). (May 28, 2001). Oxlade, Chris. Metal. Chicago, IL: Heinemann Library, 2001. Snedden, Robert. Materials. Des Plaines, IL: Heinemann Library, 1999.

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A L K A L I M E TA L S Alkali Metals

CONCEPT Group 1 of the periodic table of elements consists of hydrogen, and below it the six alkali metals: lithium, sodium, potassium, rubidium, cesium, and francium. The last three are extremely rare, and have little to do with everyday life; on the other hand, it is hard to spend a day without encountering at least one of the first three—particularly sodium, found in table salt. Along with potassium, sodium is an important component of the human diet, and in compounds with other substances, it has an almost endless array of uses. Lithium does not have as many applications, but to many people who have received it as a medication for bipolar disorder, it is quite literally a life-saver.

HOW IT WORKS Electron Configuration of the Alkali Metals In the essay on Families of Elements, there is a lengthy discussion concerning the relationship between electron configuration and the definition of a particular collection of elements as a “family.” Here that subject will only be touched upon lightly, inasmuch as it relates to the alkali metals. All members of Group 1 on the periodic table of elements have a valence electron configuration of s1. This means that a single electron is involved in chemical bonding, and that this single electron moves through an orbital, or range of probabilities, roughly corresponding to a sphere.

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Most elements bond according to what is known as the octet rule, meaning that when two or more atoms are bonded, each has (or shares) eight valence electrons. It is for this reason that the noble gases, at the opposite side of the periodic table from the alkali metals, almost never bond with other elements: they already have eight valence electrons. The alkali metals, on the other hand, are quite likely to find “willing partners,” since they each have just one valence electron. This brings up one of the reasons why hydrogen, though it is also part of Group 1, is not included as an alkali metal. First and most obviously, it is not a metal; additionally, it bonds according to what is called the duet rule, such that it shares two electrons with another element.

Chemical and Physical Characteristics of the Alkali Metals The term “alkali” (essentially the opposite of an acid) refers to a substance that forms the negatively charged hydroxide ion (OH-) in contact with water. On their own, however, alkali metals almost always form positive ions, or cations, with a charge of +1. When alkali metals react with water, one hydrogen atom splits off from the water molecule to form hydrogen gas, while the other hydrogen atom joins the oxygen to form hydroxide. Where the heavier members of the alkali metal family are concerned, reactions can often be so vigorous that the result is combustion or even explosion. Alkali metals also react with oxygen to produce

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Alkali Metals

ONE

OF THE USES OF LITHIUM IS IN BATTERIES.

THE NISSAN HYPERMINI

IS AN ELECTRIC VEHICLE POWERED BY A

LITHIUM BATTERY. (Reuters NewMedia Inc./Corbis. Reproduced by permission.)

either an oxide, peroxide, or superoxide, depending on the particular member of the alkali metal family involved. Shiny and soft enough to be cut with a knife, the alkali metals are usually white (though cesium is more of a yellowish white). When placed in a flame, most of these substances produce characteristic colors: lithium, for instance, glows bright red, and sodium an intense yellow. Heated potassium produces a violet color, rubidium a dark red, and cesium a light blue. This makes it possible to identify the metals, when heated, by color—a useful trait, since they are so often inclined to be bonded with other elements. MELTING AND BOILING POINTS. As one moves down the rows or

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periods of the periodic table, the mass of atoms increases, as does the energy each atom possesses. Yet the amount of energy required to turn a solid alkali metal into a liquid, or to vaporize a liquid alkali metal, actually decreases with higher atomic number. In other words, the higher the atomic number, the lower the boiling and melting points. The list below lists the six alkali metals in order of atomic number, along with chemical symbol, atomic mass, melting point, and boiling point. Note that for francium, which is radioactive, the figures given are for its most stable isotope, francium-223 (223Fr)—which has a half-life of only about 21 minutes.

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Alkali Metals

ONE

1932, WITH THE DEVELOPMENT OF THE FIRST PARTISHOWN HERE ARE ITS INVENTORS, ERNEST WALTON (LEFT), ERNEST RUTHERFORD (CENTER), COCKCROFT. (Hulton-Deutsch Collection/Corbis. Reproduced by permission.)

OF THE MOST STRIKING USES OF LITHIUM OCCURRED IN

CLE ACCELERATOR. AND

JOHN

Atomic Number, Mass, and Melting and Boiling Points of the Alkali Metals: • 3. Lithium (Li), 6.941; Melting Point: 356.9°F (180.5°C); Boiling Point: 2,457°F (1,347°C) • 11. Sodium (Na), 22.99; Melting Point: 208°F (97.8°C); Boiling Point: 1,621.4°F (883°C) • 19. Potassium (K), 39.10; Melting Point: 145.9°F (63.28°C); Boiling Point: 1,398.2°F (759°C) • 37. Rubidium (Rb), 85.47; Melting Point: 102.8°F (39.31°C); Boiling Point: 1,270.4°F (688°C) • 55. Cesium (Cs), 132.9; Melting Point: 83.12°F (28.4°C); Boiling Point: 1,239.8°F (671°C) • 87. Francium (Fr), (223); Melting Point: 80.6°F (27°C); Boiling Point: 1,250.6°F (677°C)

Abundance of Alkali Metals Sodium and potassium are, respectively, the sixth and seventh most abundant elements on Earth, comprising 2.6% and 2.4% of the planet’s known elemental mass. This may not seem like much, but considering the fact that just two elements—

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oxygen and silicon—make up about 75%, and that just 16 elements make up most of the remainder, it is an impressive share. Lithium, on the other hand, is much less abundant, and therefore, figures for its part of Earth’s known elemental mass are measured in parts per million (ppm). The total lithium in Earth’s crust is about 17 ppm. Surprisingly, rubidium is more abundant, at 60 ppm; less surprisingly, cesium, with just 3 ppm, is very rare. Almost no francium is found naturally, except in very small quantities within uranium ores.

REAL-LIFE APPLICATIONS Lithium Swedish chemist Johan August Arfvedson (17921841) discovered lithium in 1817, and named it after the Greek word for “stone.” Four years later, another scientist named W. T. Brande succeeded in isolating the highly reactive metal. Most of the lithium available on Earth’s crust is bound up with aluminum and silica in minerals. Since the time of its discovery, lithium has been used in lubricants, glass, and in alloys of

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lead, aluminum, and magnesium. In glass, it acts as a strengthening agent; likewise, metal alloys that contain lithium tend to be stronger, yet less dense. In 1994, physicist Jeff Dahn of Simon Fraser University in British Columbia, Canada, developed a lithium battery. Not only was the battery cheaper to produce than the traditional variety, Dahn and his colleagues announced, but the disposal of used lithium batteries presented less danger to the environment. One of the most striking uses of lithium occurred in 1932, when English physicist John D. Cockcroft (1897-1967) and Irish physicist Ernest Walton (1903-1995) built the first particle accelerator. By bombarding lithium atoms, they produced highly energized alpha particles. This was the first nuclear reaction brought about by the use of artificially accelerated particles—in other words, without the need for radioactive materials such as uranium-235. Cockcroft’s and Walton’s experiment with lithium thus proved pivotal to the later creation of the atomic bomb. LITHIUM IN PSYCHIATRIC TREATMENT. The most important applica-

tion of lithium, however, is in treatment for the psychiatric condition once known as manic depression, today identified as bipolar disorder. Persons suffering from bipolar disorder tend toward mood swings: during some periods the patient is giddy (“manic,” or in a condition of “mania”), and during others the person is suicidal. Indeed, prior to the development of lithium as a treatment for bipolar disorder, as many as one in five patients with this condition committed suicide. Doctors do not know exactly how lithium does what it does, but it obviously works: between 70% and 80% of patients with the bipolar condition respond well to treatment, and are able to go on with their lives in such a way that their condition is no longer outwardly evident. Lithium is also administered to patients who suffer unipolar depression and some forms of schizophrenia. EARLY MEDICINAL USES OF LITHIUM. It is said that the great Greco-

Roman physician Galen ( 129-c. 199) counseled patients suffering from “mania” to bathe in, and even drink the water from, alkaline springs. If so, he was nearly 2,000 years ahead of his time. Even in the 1840s, not long after lithium was discovered, the mineral—mixed with carbonate or cit-

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rate—was touted as a cure for insomnia, gout, epilepsy, diabetes, and even cancer.

Alkali Metals

None of these alleged cures proved a success; nor did a lithium chloride treatment administered in the 1940s as a salt substitute for patients on low-sodium diets. As it turned out, when not enough sodium is present, the body experiences a buildup of sodium’s sister element, lithium. The result was poisoning, which in some cases proved fatal. CADE’S BREAKTHROUGH. Then in 1949, Australian psychiatrist John Cade discovered the value of lithium for psychiatric treatment. He approached the problem from an entirely different angle, experimenting with uric acid, which he believed to be a cause of manic behavior. In administering the acid to guinea pigs, he added lithium salts merely to keep the uric acid soluble—and was very surprised by what he discovered. The uric acid did not make the guinea pigs manic, as he had expected; instead, they became exceedingly calm.

Cade changed the focus of his research, and tested lithium treatment on ten manic patients. Again, the results were astounding: one patient who had suffered from an acute bipolar disorder (as it is now known) for five years was released from the hospital after three months of lithium treatment, and went on to lead a healthy, normal life. Encouraged by the changes he had seen in patients who received lithium, Cade published a report on his findings in the Medical Journal of Australia, but his work had little impact at the time. Nor did the idea of lithium treatment meet with an enthusiastic reception on the other side of the Pacific: in the aftermath of the failed experiments with lithium as a sodium substitute in the 1940s, stories of lithium poisoning were widespread in the United States. LITHIUM TODAY. Were it not for the efforts of Danish physician Mogens Schou, lithium might never have taken hold in the medical community. During the 1950s and 1960s, Schou campaigned tirelessly for recognition of lithium as a treatment for manic-depressive illness. Finally during the 1960s, the U.S. Food and Drug Administration began conducting trials of lithium, and approved its use in 1974. Today some 200,000 Americans receive lithium treatments.

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A non-addictive and non-sedating medication, lithium—as evidenced by the failed experiment in the 1940s—may still be dangerous in large quantities. It is absorbed quickly into the bloodstream and carried to all tissues in the brain and body before passing through the kidneys. Both lithium and sodium are excreted through the kidneys, and since sodium affects lithium excretion, it is necessary to maintain a proper quantity of sodium in the body. For this reason, patients on lithium are cautioned to avoid a low-salt diet.

Sodium Sodium compounds had been known for some time prior to 1807, when English chemist Sir Humphry Davy (1778-1829) succeeded in isolating sodium itself. The element is represented by a chemical symbol (Na), reflecting its Latin name, natrium. In its pure form, sodium has a bright, shiny surface, but in order to preserve this appearance, it must be stored in oil: sodium reacts quickly with oxygen, forming a white crust of sodium oxide. Pure sodium never occurs in nature; instead, it combines readily with other substances to form compounds, many of which are among the most widely used chemicals in industry. It is also highly soluble: thus whereas sodium and potassium occur in crystal rocks at about the same ratio, sodium is about 30 times more abundant in seawater than its sister element. OBTAINING SODIUM CHLORIDE. Though the extraction of sodium

involves the use of a special process, the metal is plentiful in the form of sodium chloride—better known as table salt. In fact, the term salt in chemistry refers generally to any combination of a metal with a nonmetal. More specifically, salts are (along with water) the product of reactions between acids and bases. Sodium chloride is so easy to obtain, and therefore so cheap, that most industries making other sodium compounds use it, simply separating out the chloride (as described below) before adding other elements. The United States is the world’s largest producer of sodium chloride, obtained primarily from brine, a term used to describe any solution of sodium chloride in water. Brine comes from seawater, subterranean wells, and desert lakes, such as the Great Salt Lake in Utah. Another source of sodium chloride is

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rock salt, created underground by the evaporation of long-buried saltwater seas. Other top sodium-chloride-producing nations include China, Germany, Great Britain, France, India, and various countries in the former Soviet Union. Salt may be cheap and plentiful for the world in general, but there are places where it is a precious commodity. One such place is the Sahara Desert, where salt caravans ply a brisk trade today, much as they have since ancient times. ISOLATING SODIUM. Modern methods for the production of sodium represent an improvement in the technique Davy used in 1807, although the basic principle is the same. Though several decades passed before electricity came into widespread public use, scientists had been studying its properties for years, and Davy applied it in a process called electrolysis.

Electrolysis is the use of an electric current to produce a chemical reaction—in this case, to separate sodium from the other element or elements with which it is combined. Davy first fused or melted a sample of sodium chloride, then electrolyzed it. Using an electrode, a device that conducts electricity and is used to emit or collect electric charge, he separated the sodium chloride in such a way that liquid sodium metal collected on the cathode, or negatively charged end. Meanwhile, the gaseous chlorine was released through the anode, or the positively charged end. The apparatus used for sodium separation today is known as the Downs cell, after its inventor, J. C. Downs. In a Downs cell, sodium chloride and calcium chloride are combined in a molten mixture in which the presence of calcium chloride lowers the melting point of the sodium chloride by more than 30%. When an electric current is passed through the mixture, sodium ions move to the cathode, where they pick up electrons to become sodium atoms. At the same time, ions of chlorine migrate to the anode, losing electrons to become chlorine atoms. Sodium is a low-density material that floats on water, and in the Downs cell, the molten sodium rises to the top, where it is drawn off. The chlorine gas is allowed to escape through a vent at the top of the anode end of the cell, and the resulting sodium metal—that is, the elemental form of sodium—is about 99.8% pure. USES FOR SODIUM CHLORIDE.

As indicated earlier, sodium chloride is by far the most widely known and commonly used sodium

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compound—and this in itself is a distinction, given the fact that so many sodium compounds are a part of daily life. Today people think of salt primarily as a seasoning to enhance the taste of food, but prior to the development of refrigeration, it was vital as a preservative because it kept microbes away from otherwise perishable food items. Salt does not merely improve the taste of food; it is an essential nutrient. Sodium compounds regulate transmission of signals through the nervous system, alter the permeability of membranes, and perform a number of other lifepreserving functions. On the other hand, too much salt can aggravate high blood pressure. Thus, since the 1970s and 1980s, food manufacturers have increasingly offered products low in sodium, a major selling point for health-conscious consumers. OTHER SODIUM COMPOUNDS.

In addition to its widespread use in consumer goods, sodium chloride is the principal source of sodium used in making other sodium compounds. These include sodium hydroxide, for manufacturing cellulose products such as film, rayon, soaps, and paper, and for refining petroleum. In its application as a cleaning solution, sodium hydroxide is known as caustic soda or lye. Another widely used sodium compound is sodium carbonate or, soda ash, applied in glassmaking, paper production, textile manufacturing, and other areas, such as the production of soaps and detergents. Sodium also can be combined with carbon to produce sodium bicarbonate, or baking soda. Sodium sulfate, sometimes known as salt cake, is used for making cardboard and kraft paper. Yet another widely used sodium compound is sodium silicate, or “water glass,” used in the production of soaps, detergents, and adhesives; in water treatment; and in bleaching and sizing of textiles. Still other sodium compounds used by industry and/or consumers include sodium borate, or borax; sodium tartrate, or sal tartar; the explosive sodium nitrate, or Chilean saltpeter; and the food additive monosodium glutamate (MSG). Perhaps ironically, there are few uses for pure metallic sodium. Once applied as an “anti-knock” additive in leaded gasoline, before those products were phased out for environmental reasons, metallic sodium is now used

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as a heat-exchange medium in nuclear reactors. But its widest application is in the production of the many other sodium compounds used around the world.

Alkali Metals

Potassium In some ways, potassium is a strange substance, as evidenced by its behavior in response to water. As everyone knows, water tends to put out a fire, and most explosives, when exposed to sufficient quantities of water, become ineffective. Potassium, on the other hand, explodes in contact with water and reacts violently with ice at temperatures as low as -148°F (-100°C). In a complete reversal of the procedures normally followed for most substances, potassium is stored in kerosene, because it might burst into flames if exposed to moist air! Many aspects of potassium mirror those already covered with regard to sodium. The two have a number of the same applications, and in certain situations, potassium is used as a sodium substitute. Like sodium, potassium is never found alone in nature; instead, it comes primarily from sylvinite and carnalite, two ores containing potassium chloride. Also, like sodium, potassium was first isolated in 1807 by Davy, using the process of electrolysis described above. A few years later, a German chemist dubbed the newly isolated element “kalium,” apparently a derivation of the Arabic qali, for “alkali”; hence the use of K as the chemical symbol for potassium. USES FOR POTASSIUM. Potassium

has another similarity with sodium; although it was not isolated until the early nineteenth century, its compounds have been in use for many centuries. The Romans, for instance, used potassium carbonate, or potash, obtained from the ashes of burned wood, to make soap. During the Middle Ages, the Chinese applied a form of saltpeter, potassium nitrate, in making gunpowder. And in colonial America, potash went into the production of soap, glass, and other products. The production of just one ton of potash required the burning of several acres’ worth of trees—a wasteful practice in more ways than one. Though there was no environmentalist movement in those days, financial concerns never go out of style. In order to save the money lost by using up vast acres of timber, American industry in the nineteenth century sought another means of making potash. The many similarities between

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IN THE SAHARA DESERT, SALT HERE, TRADERS IN MALI LOAD

CARAVANS PLY A BRISK TRADE TODAY, MUCH AS THEY HAVE SINCE ANCIENT TIMES. A CAMEL FOR A SALT CARAVAN. (Nik Wheeler/Corbis. Reproduced by permission.)

sodium and potassium provided a key, and the substitution of sodium carbonate for potassium carbonate saved millions of trees. In 1847, German chemist Justus von Liebig (1803-1873) discovered potassium in living tis-

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sues. As a result, scientists became aware of the role this alkali metal plays in sustaining life: indeed, potassium is present in virtually all living cells. In the human body, potassium—which accounts for only 0.4% of the body’s mass—is

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Alkali Metals

KEY TERMS The elements in

mass. Isotopes may be either stable or

Group 1 of the periodic table of elements,

unstable—that is, radioactive. Such is the

with the exception of hydrogen. The alkali

case with francium, a radioactive member

metals, which include lithium, sodium,

of the alkali metals family.

ALKALI

METALS:

potassium, rubidium, cesium, and francium, all have one valence electron in the s1 orbital, and are highly reactive.

ORBITAL:

A pattern of probabilities

regarding the position of an electron for an atom in a particular energy state. The six

ANODE: An electrode at the positively

alkali metals all have valence electrons in

charged end of a supply of electric current.

an s1 orbital, which describes a more or less

BRINE:

A term used to describe any

solution of sodium chloride in water.

spherical shape. PERIODIC TABLE OF ELEMENTS: A

CATHODE: An electrode at the negative-

chart that shows the elements arranged in

ly charged end of a supply of electric cur-

order of atomic number. Vertical columns

rent.

within the periodic table indicate groups The positive ion that results

or “families” of elements with similar

when an atom loses one or more electrons.

chemical characteristics; the alkali metals

All of the alkali metals tend to form +1

occupy Group 1.

CATION:

cations (pronounced KAT-ie-un).

RADIOACTIVITY:

A term describing a

ELECTRODE: A structure, often a metal

phenomenon whereby certain materials

plate or grid, that conducts electricity, and

are subject to a form of decay brought

which is used to emit or collect electric

about by the emission of high-energy par-

charge.

ticles. “Decay” in this sense does not mean

ELECTROLYSIS: The use of an electric

“rot”; instead, radioactive isotopes contin-

current to cause a chemical reaction.

ue changing into other isotopes until they

HALF-LIFE: The length of time it takes a

become stable.

substance to diminish to one-half its initial

SALT:

amount.

water, by the reaction of an acid and a base.

ION:

An atom or group of atoms that has

A compound formed, along with

Generally, salts are any compounds that

lost or gained one or more electrons, and

bring together a metal and a nonmetal.

thus has a net electric charge.

VALENCE

ISOTOPES: Atoms that have an equal

that occupy the highest energy levels in an

number of protons, and hence are of the

atom, and which are involved in chemical

same element, but differ in their number of

bonding. The alkali metals all have one

neutrons. This results in a difference of

valence electron, in the s1 orbital.

S C I E N C E O F E V E R Y D AY T H I N G S

ELECTRONS:

Electrons

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essential to the functioning of muscles. In larger quantities, however, it can be dangerous, causing a state of permanent relaxation known as potassium inhibition. Since plants depend on potassium for growth, it was only logical that potassium, in the form of potassium chloride, was eventually applied as a fertilizer. This, at least, distinguishes it from its sister element: sodium, or sodium chloride, which can kill plants if administered to the soil in large enough quantities. Another application of potassium is in the area pioneered by the Chinese about 800 years ago: the manufacture of fireworks and gunpowder from potassium nitrate. Like ammonium nitrate, made infamous by its use in the 1993 World Trade Center bombing and the Oklahoma City bombing in 1995, potassium nitrate doubles as a fertilizer.

Rubidium, Cesium, and Francium The three heaviest alkali metals are hardly household names, though one of them, cesium, does have several applications in industry. Rubidium and cesium, discovered in 1860 by German chemist R. W. Bunsen (1811-1899) and German physicist Gustav Robert Kirchhoff (1824-1887), were the first elements ever found using a spectroscope. Matter emits electromagnetic radiation along various spectral lines, which can be recorded using a spectroscope and then analyzed to discern the particular “fingerprint” of the substance in question. When Bunsen and Kirchhoff saw the bluish spectral lines emitted by one of the two elements, they named it cesium, after a Latin word meaning “sky blue.” Cesium, which is very rare, appears primarily in compounds such as pollucite. It is used today in photoelectric cells, military infrared lamps, radio tubes, and video equipment. During the 1940s, American physicist Norman F. Ramsey, Jr. (1915-) built a highly accurate atomic clock based on the natural frequencies of cesium atoms.

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Rubidium, by contrast, has far fewer applications, and those are primarily in areas of scientific research. On Earth it is found in pollucite, lepidolite, and carnallite. It is considerably more abundant than cesium, and vastly more so than francium. Indeed, it is estimated that if all the francium in Earth’s crust were combined, it would have a mass of about 25 grams. Francium was discovered in 1939 by French physicist Marguerite Perey (1909-1975), student of the famous French-Polish physicist and chemist Marie Curie (1867-1934). For about four decades, scientists had been searching for the mysterious Element 87, and while studying the decay products of an actinium isotope, actinium227, Perey discovered that one out of 100 such atoms decayed to form the undiscovered element. She named it francium, after her homeland. Though the discovery of francium solved a mystery, the element has no known uses outside of its applications in research.

WHERE TO LEARN MORE “Alkali Metals” (Web site). (May 24, 2001). “Alkali Metals” ChemicalElements.com (Web site). (May 24, 2001). “Hydrogen and the Alkali Metals.” University of Colorado Department of Physics (Web site). (May 24, 2001). Kerrod, Robin. Matter and Materials. Illustrated by Terry Hadler. Tarrytown, N.Y.: Benchmark Books, 1996. Mebane, Robert C. and Thomas R. Rybolt. Metals. Illustrated by Anni Matsick. New York: Twenty-First Century Books, 1995. Oxlade, Chris. Metal. Chicago, IL: Heinemann Library, 2001. Snedden, Robert. Materials. Des Plaines, IL: Heinemann Library, 1999. “Visual Elements: Group I—The Alkali Metals” (Web site). (May 24, 2001).

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Alkaline Earth Metals

ALK ALINE E A R T H M E TA L S

CONCEPT The six alkaline earth metals—beryllium, magnesium, calcium, strontium, barium, and radium—comprise Group 2 on the periodic table of elements. This puts them beside the alkali metals in Group 1, and as their names suggest, the two families share a number of characteristics, most notably their high reactivity. Also, like the alkali metals, or indeed any other family on the periodic table, not all members of the alkali metal family are created equally in terms of their abundance on Earth or their usefulness to human life. Magnesium and calcium have a number of uses, ranging from building and other structural applications to dietary supplements. In fact, both are significant components in the metabolism of living things—including the human body. Barium and beryllium have numerous specialized applications in areas from jewelry to medicine, while strontium is primarily used in fireworks. Radium, on the other hand, is rarely used outside of laboratories, in large part because its radioactive qualities pose a hazard to human life.

HOW IT WORKS Defining a Family The expression “families of elements” refers to groups of elements on the periodic table that share certain characteristics. These include (in addition to the alkaline earth metals and the alkali metals) the transition metals, halogens, noble gases, lanthanides, and actinides. (All of these are covered in separate essays within this book.) In addition, there are several larger categories with regard to shared traits that often cross

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family lines; thus all elements are classified either as metals, metalloids, and nonmetals. (These are also discussed in separate essays, which include reference to “orphans,” or elements that do not belong to one of the families mentioned above.) These groupings, both in terms of family and the broader divisions, relate both to external, observable characteristics, as well as to behaviors on the part of electrons in the elements’ atomic structures. For instance, metals, which comprise the vast majority of elements on the periodic table, tend to be shiny, hard, and malleable (that is, they can bend without breaking.) Many of them melt at fairly high temperatures, and virtually all of them vaporize (become gases) at high temperatures. Metals also form ionic bonds, the tightest form of chemical bonding. ELECTRON CONFIGURATIONS OF THE ALKALINE EARTH METALS.

Where families are concerned, there are certain observable properties that led chemists in the past to group the alkaline earth metals together. These properties will be discussed with regard to the alkaline earth metals, but another point should be stressed in relation to the division of elements into families. With the advances in understanding that followed the discovery of the electron in 1897, along with the development of quantum theory in the early twentieth century, chemists developed a more fundamental definition of family in terms of electron configuration. As noted, the alkaline earth metal family occupies the second group, or column, in the periodic table. All elements in a particular group, regardless of their apparent differences, have a common pattern in the configuration of their

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DURING WORLD WAR II,

MAGNESIUM WAS HEAVILY USED IN AIRCRAFT COMPONENTS. IN THIS

ERS POUR MOLTEN MAGNESIUM INTO A CAST AT THE

1941 PHOTO, WORKWRIGHT AERONAUTICAL CORPORATION. (Bettmann/Corbis. Reproduced

by permission.)

valence electrons—the electrons at the “outside” of the atom, involved in chemical bonding. (By contrast, the core electrons, which occupy lower regions of energy within the atom, play no role in the bonding of elements.) All members of the alkaline earth metal family have a valence electron configuration of s2. This means that two electrons are involved in chemical bonding, and that these electrons move through an orbital, or range of probabilities, roughly corresponding to a sphere. The s orbital pattern corresponds to the first of several sublevels within a principal energy level. Whatever the number of the principal energy level which corresponds to the period, or row,

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on the periodic table the atom has the same number of sublevels. Thus beryllium, on Period 2, has two principal energy levels, and its valence electrons are in sublevel 2s2. At the other end of the group is radium, on Period 7. Though radium is far more complex than beryllium, with seven energy levels instead of two, nonetheless it has the same valence electron configuration, only on a higher energy level: 7s2. HELIUM AND THE ALKALINE EARTH METALS. If one studies the valence

electron configurations of elements on the periodic table, one notices an amazing symmetry and order. All members of a group, though their principal energy levels differ, share characteristics in

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their valence shell patterns. Furthermore, for the eight groups numbered in the North American version of the periodic table, the group number corresponds to the number of valence electrons. There is only one exception: helium, with a valence electron configuration of 1s2, is normally placed in Group 8 with the noble gases. Based on that s2 configuration, it might seem logical to place helium atop beryllium in the alkaline earth metals family; but there are several reasons why this is not done. First of all, helium is obviously not a metal. More importantly, helium behaves in a manner quite different from that of the alkaline earth metals. Whereas helium, like the rest of the noble gases, is highly resistant to chemical reactions and bonding, alkaline earth metals are known for their high reactivity—that is, a tendency for bonds between atoms or molecules to be made or broken so that materials are transformed. (A similar relationship exists in Group 1, which includes hydrogen and the alkali metals. All have the same valence configuration, but hydrogen is never included as a member of the alkali metals family.)

Characteristics of the Alkaline Earth Metals Like the alkali metals, the alkaline earth metals have the properties of a base, as opposed to an acid. The alkaline earth metals are shiny, and most are white or silvery in color. Like their “cousins” in the alkali metal family, they glow with characteristic colors when heated. Calcium glows orange, strontium a very bright red, and barium an apple green. Physically they are soft, though not as soft as the alkali metals, many of which can be cut with a knife.

metal hydroxide, though their reactions are less pronounced than those of the alkali metals. Magnesium metal in its pure form is combustible, and when exposed to air, it burns with an intense white light, combining with the oxygen to produce magnesium oxide. Likewise calcium, strontium, and barium react with oxygen to form oxides. Due to their high levels of reactivity, the alkaline earth metals rarely appear by themselves in nature; rather, they are typically found with other elements in compound form, often as carbonates or sulfates. This, again, is another similarity with the alkali metals. But whereas the alkali metals tend to form 1+ cations (positively charged atoms), the alkaline earth metals form 2+ cations—that is, cations with a positive charge of 2. BOILING AND MELTING POINTS. One way that the alkaline earth met-

als are distinguished from the alkali metals is with regard to melting and boiling points—those temperatures, respectively, at which a solid metal turns into a liquid, and a liquid metal into a vapor. For the alkali metals, the temperatures of the boiling and melting points decrease with an increase in atomic number. The pattern is not so clear, however, for the alkaline earth metals. The highest melting and boiling points are for beryllium, which indeed has the lowest atomic number. It melts at 2,348.6°F (1,287°C), and boils at 4,789.8°F (2,471°C). These figures are much higher than for lithium, the alkali metal on the same period as beryllium, which melts at 356.9°F (180.5°C) and boils at 2,457°F (1,347°C).

Yet another similarity the alkaline earth metals have with the alkali metals is the fact that four of the them—magnesium, calcium, strontium, and barium—were either identified or isolated in the first decade of the nineteenth century by English chemist Sir Humphry Davy (1778-1829). Around the same time, Davy also isolated sodium and potassium from the alkali metal family.

Magnesium, the second alkali earth metal, melts at 1,202°F (650°C), and boils at 1,994°F (1,090°C)—significantly lower figures than for beryllium. However, the melting and boiling points are higher for calcium, third of the alkaline earth metals, with figures of 1,547.6°F (842°C) and 2,703.2°F (1,484°C) respectively. Melting and boiling temperatures steadily decrease as energy levels rise through strontium, barium, and radium, yet these temperatures are never lower than for magnesium.

REACTIVITY. The alkaline earth metals are less reactive than the alkali metals, but like the alkali metals they are much more reactive than most elements. Again like their “cousins,” they react with water to produce hydrogen gas and the

ABUNDANCE. Of the alkaline earth metals, calcium is the most abundant. It ranks fifth among elements in Earth’s crust, accounting for 3.39% of the elemental mass. It is also fifth most abundant in the human body, with a share

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MANY

ALKALINE EARTH METALS ARE USED IN THE PRODUCTION OF FIREWORKS. (Bill Ross/Corbis. Reproduced by permission.)

of 1.4%. Magnesium, which makes up 1.93% of Earth’s crust, is the eighth most abundant element on Earth. It ranks seventh in the human body, accounting for 0.50% of the body’s mass. Barium ranks seventeenth among elements in Earth’s crust, though it accounts for only 0.04% of the elemental mass. Neither it nor the other three alkali metals appear in the body in significant quantities: indeed, barium and beryllium are poisonous, and radium is so radioactive that exposure to it can be extremely harmful.

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Within Earth’s crust, strontium is present in quantities of 360 parts per million (ppm), which in fact is rather abundant compared to a number of elements. In the ocean, its presence is about 8 ppm. By contrast, the abundance of beryllium in Earth’s crust is measured in parts per billion (ppb), and is estimated at 1,900 ppb. Vastly more rare is radium, which accounts for just 0.6 parts per trillion of Earth’s crust—a fact that made its isolation by French-Polish physicist and chemist Marie Curie (1867-1934) all the more impressive.

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REAL-LIFE AP P LICATIONS Beryllium In the eighteenth century, French mineralogist René Just-Haüy (1743-1822) had observed that both emeralds and the mineral beryl had similar properties. French chemist Louis-Nicolas Vauquelin (1763-1829) in 1798 identified the element they had in common: beryllium (Be), which has an atomic number of 4 and an atomic mass of 9.01 amu. Some three decades passed before German chemist Friedrich Wöhler (18001882) and French chemist Antoine Bussy (17941882), working independently, succeeded in isolating the element. Beryllium is found primarily in emeralds and aquamarines, both precious stones that are forms of the beryllium alluminosilicate compound beryl. Though it is toxic to humans, beryllium nonetheless has an application in the health-care industry: because it lets through more x rays than does glass, beryllium is often used in x-ray tubes. Metal alloys that contain about 2% beryllium tend to be particularly strong, resistant to wear, and stable at high temperatures. Copperberyllium alloys, for instance, are applied in hand tools for industries that use flammable solvents, since tools made of these alloys do not cause sparks when struck against one another. Alloys of beryllium and nickel are applied for specialized electrical connections, as well as for high-temperature uses.

Magnesium English botanist and physician Nehemiah Grew (1641-1712) in 1695 discovered magnesium sulfate in the springs near the English town of Epsom, Surrey. This compound, called “Epsom salts” ever since, has long been noted for its medicinal value. Epsom salts are used for treating eclampsia, a condition that causes seizures in pregnant women. The compound is also a powerful laxative, and is sometimes used to rid the body of poisons—such as magnesium’s sister element, barium. For some time, scientists confused the oxide compound magnesia with lime or calcium carbonate, which actually involves another alkaline earth metal. In 1754, Scottish chemist and physi-

S C I E N C E O F E V E R Y D AY T H I N G S

cist Joseph Black (1728-1799) wrote “Experiments Upon Magnesia, Alba, Quick-Lime, and Some Other Alkaline Substances,” an important work in which he distinguished between magnesia and lime. Davy in 1808 declared magnesia the oxide of a new element, which he dubbed magnesium, but some 20 years passed before Bussy succeeded in isolating the element.

Alkaline Earth Metals

Magnesium (Mg) has an atomic number of 12, and an atomic mass of 24.31 amu. It is found primarily in minerals such as dolomite and magnesite, both of which are carbonates; and in carnallite, a chloride. Magnesium silicates include asbestos, soapstone or talc, and mica. Not all forms of asbestos contain magnesium, but the fact that many do only serves to show the ways that chemical reactions can change the properties an element possesses in isolation. AN IMPORTANT COMPONENT OF HEALTH. Whereas magnesium is flam-

mable, asbestos was once used in large quantities as a flame retardant. And whereas asbestos has been largely removed from public buildings throughout the United States due to reports linking asbestos exposure with cancer, magnesium is an important component in the health of living organisms. It plays a critical role in chlorophyll, the green pigment in plants that captures energy from sunlight, and for this reason, it is also used in fertilizers. In the human body, magnesium ions (charged atoms) aid in the digestive process, and many people take mineral supplements containing magnesium, sometimes in combination with calcium. There is also its use as a laxative, already mentioned. Epsom salts, as befits their base or alkaline quality, are exceedingly bitter—the kind of substance a person only ingests under conditions of the most dire necessity. On the other hand, milk of magnesia is a laxative with a far less unpleasant taste. MAGNESIUM GOES TO WAR. It is a hallmark of magnesium’s chemical versatility that the same element, so important in preserving life, has also been widely used in warfare. Just before World War I, Germany was a leading manufacturer of magnesium, thanks in large part to a method of electrolysis developed by German chemist R. W. Bunsen (1811-1899). When the United States went to war against Germany, American companies began producing magnesium in large quantities.

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Bunsen had discovered that powdered magnesium burns with a brilliant white flame, and in the war, magnesium was used in flares, tracer bullets, and incendiary bombs, which ignite and burn upon impact. The bright light produced by burning magnesium has also led to a number of peacetime applications—for instance, in fireworks, and for flashes used in photography. Magnesium saw service in another world war. By the time Nazi tanks rolled into Poland in 1939, the German military-industrial complex had begun using the metal for building aircraft and other forms of military equipment. America once again put its own war-production machine into operation, dramatically increasing magnesium output to a peak of nearly 184,000 tons (166,924,800 kg) in 1943. STRUCTURAL

APPLICATIONS.

Magnesium’s principal use in World War I was for its incendiary qualities, but in World War II it was primarily used as a structural metal. It is lightweight, but stronger per unit of mass than any other common structural metal. As a metal for building machines and other equipment, magnesium ranks in popularity only behind iron and aluminum (which is about 50% more dense than magnesium). The automobile industry is one area of manufacturing particularly interested in magnesium’s structural qualities. On both sides of the Atlantic, automakers are using or testing vehicle parts made of alloys of magnesium and other metals, primarily aluminum. Magnesium is easily cast into complex structures, which could mean a reduction in the number of parts needed for building a car—and hence a streamlining of the assembly process. Among the types of sports equipment employing magnesium alloys are baseball catchers’ masks, skis, racecars, and even horseshoes. Various brands of ladders, portable tools, electronic equipment, binoculars, cameras, furniture, and luggage also use parts made of this lightweight, durable metal.

Calcium Davy isolated calcium (Ca) by means of electrolysis in 1808. The element, whose name is derived from the Latin calx, or “lime,” has an atomic number of 20, and an atomic mass of 40.08. The principal sources of calcium are limestone and

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dolomite, both of which are carbonates, as well as the sulfate gypsum. In the form of limestone and gypsum, calcium has been used as a building material since ancient times, and continues to find application in that area. Lime is combined with clay to make cement, and cement is combined with sand and water to make mortar. In addition, when mixed with sand, gravel, and water, cement makes concrete. Marble—once used to build palaces and today applied primarily for decorative touches— also contains calcium. The steel, glass, paper, and metallurgical industries use slaked lime (calcium hydroxide) and quicklime, or calcium oxide. It helps remove impurities from steel, and pollutants from smokestacks, while calcium carbonate in paper provides smoothness and opacity to the finished product. When calcium carbide (CaC2) is added to water, it produces the highly flammable gas acetylene (C2H2), used in welding torches. In various compounds, calcium is used as a bleach; a material in the production of fertilizers; and as a substitute for salt as a melting agent on icy roads. The food, cosmetic, and pharmaceutical industries use calcium in antacids, toothpaste, chewing gum, and vitamins. To an even greater extent than magnesium, calcium is important to living things, and is present in leaves, bone, teeth, shells, and coral. In the human body, it helps in the clotting of blood, the contraction of muscles, and the regulation of the heartbeat. Found in green vegetables and dairy products, calcium (along with calcium supplements) is recommended for the prevention of osteoporosis. The latter, a condition involving a loss of bone density, affects elderly women in particular, and causes bones to become brittle and break easily.

Strontium Irish chemist and physician Adair Crawford (1748-1795) and Scottish chemist and surgeon William Cumberland Cruikshank (1790-1800) in 1790 discovered what Crawford called “a new species of earth” near Strontian in Scotland. A year later, English chemist Thomas Charles Hope (1766-1844) began studying the ore found by Crawford and Cruikshank, which they had dubbed strontia. In reports produced during 1792 and 1793, Hope explained that strontia could be distinguished from lime or calcium hydroxide on the

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Alkaline Earth Metals

IN

THIS

1960

PHOTO, A MOTHER TESTS HER SON’S MILK FOR SIGNS OF RADIOACTIVITY, THE RESULT OF NUCLEAR

WEAPONS TESTING DURING THE

1950S

THAT INVOLVED THE RADIOACTIVE ISOTOPE STRONTIUM-90.

THE

ISOTOPE

FELL TO EARTH IN A FINE POWDER, WHERE IT COATED THE GRASS, WAS INGESTED BY COWS, AND EVENTUALLY WOUND UP IN THE MILK THEY PRODUCED. (Bettmann/Corbis. Reproduced by permission.)

one hand, and baryta or barium hydroxide on the other, by virtue of its response to flame tests. Whereas calcium produced a red flame and barium a green one, strontia glowed a brilliant red easily distinguished from the darker red of calcium. Once again, it was Davy who isolated the new element, using electrolysis, in 1808. Subsequently dubbed strontium (Sr), its atomic number is 38, and its atomic mass 87.62. Silvery white, it oxidizes rapidly in air, forming a pale yellow oxide crust on any freshly cut surface. Though it has properties similar to those of calcium, the comparative rarity of strontium and the expense involved in extracting it offer no economic incentives for using it in place of its much more abundant sister element. Nonetheless, strontium does have a few uses, primarily because of its brilliant crimson flame. Therefore it is applied in the making of fireworks, signal flares, and tracer bullets—that is, rounds that emit a light as they fly through the air. One of the more controversial “applications” of strontium involved the radioactive isotope strontium-90, a by-product of nuclear weapons testing in the atmosphere from the late 1940s

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onward. The isotope fell to earth in a fine powder, coated the grass, was ingested by cows, and eventually wound up in the milk they produced. Because of its similarities to calcium, the isotope became incorporated into the teeth and gums of children who drank the milk, posing health concerns that helped bring an end to atmospheric testing in the early 1960s.

Barium Aspects of barium’s history are similar to those of other alkaline earth metals. During the eighteenth century, chemists were convinced that barium oxide and calcium oxide constituted the same substance, but in 1774, Swedish chemist Carl Wilhelm Scheele (1742-1786) demonstrated that barium oxide was a distinct compound. Davy isolated the element, as he did two other alkaline earth metals, by means of electrolysis, in 1808. Barium (Ba) has an atomic mass of 137.27 and an atomic number of 56. It appears primarily in ores of barite, a sulfate, and witherite, a carbonate. Barium sulfate is used as a white pigment in paints, while barium carbonate is applied in the production of optical glass, ceramics, glazed

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Alkaline Earth Metals

KEY TERMS ALKALINE EARTH METALS: Group 2

ION:

on the periodic table of elements, with

lost or gained one or more electrons, and

valence electron configurations of ns2. The

thus has a net electric charge.

six alkaline earth metals, all of which are

ISOTOPES: Atoms that have an equal

highly reactive chemically, are beryllium,

number of protons, and hence are of the

magnesium, calcium, strontium, barium,

same element, but differ in their number of

and radium.

neutrons. This results in a difference of

ALKALI

METALS:

The elements in

Group 1 of the periodic table of elements, with the exception of hydrogen. The alkali metals all have one valence electron in the s1 orbital, and are highly reactive.

An atom or group of atoms that has

mass. Isotopes may be either stable or unstable—that is, radioactive. Such is the case with the isotopes of radium, a radioactive member of the alkaline earth metals family. ORBITAL:

CATION:

The positive ion that results

when an atom loses one or more electrons.

regarding the position of an electron for an atom in a particular energy state. The six

All of the alkaline earth metals tend to

alkaline earth metals all have valence elec-

form 2+ cations (pronounced KAT-ie-

trons in an s2 orbital, which describes a

unz).

more or less spherical shape.

ELECTROLYSIS: The use of an electric

PERIODS: Rows of the periodic table of

current to cause a chemical reaction.

elements. These represent successive prin-

pottery, and specialty glassware. One of its most important uses is as a drill-bit lubricant—known as a “mud” or slurry—for oil drilling. Like a number of its sister elements, barium (in the form of barium nitrate) is used in fireworks and flares. Motor oil detergents for keeping engines clean use barium oxide and barium hydroxide. Beryllium is not the only alkaline earth metal used in making x rays, nor is magnesium the only member of the family applied as a laxative. Barium is used in enemas, and barium sulfate is used to coat the inner lining of the intestines to allow a doctor to examine a patient’s digestive system. (Though barium is poisonous, in the form of barium sulfate it is safe for ingestion because the compound does not dissolve in water or other bodily fluids.) Prior to receiving x rays, a patient may be instructed to drink a chalky barium sulfate liquid, which absorbs a great deal of the radiation emitted by the x-ray

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A pattern of probabilities

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machine. This adds contrast to the black-andwhite x-ray photo, enabling the doctor to make a more informed diagnosis.

Radium Today radium (Ra; atomic number 88; atomic mass 226 amu) has few uses outside of research; nonetheless, the story of its discovery by Marie Curie and her husband Pierre (1859-1906), a French physicist, is a compelling chapter not only in the history of chemistry, but of human endeavor in general. Inspired by the discovery of uranium’s radioactive properties by French physicist Henri Becquerel (1852-1908), Marie Curie became intrigued with the subject of radioactivity, on which she wrote her doctoral dissertation. Setting out to find other radioactive elements, she and Pierre refined a large quantity of pitchblende, an ore commonly found in uranium mines. Within a year, they had discovered

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KEY TERMS

Alkaline Earth Metals

CONTINUED

cipal energy levels in the atoms of the ele-

broken in such a way that materials are

ments involved.

transformed.

PRINCIPAL ENERGY LEVEL: A value

SALT:

indicating the distance that an electron

that brings together a metal and a non-

may move away from the nucleus of an

metal. More specifically, salts (along with

atom. This is designated by a whole-num-

water) are the product of a reaction

ber integer, beginning with 1 and moving

between an acid and a base.

upward. The higher the principal energy

SHELL:

level, the greater the energy in the atom,

same principal energy level.

and the more complex the pattern of

SUBLEVEL: A region within the princi-

orbitals.

pal energy level occupied by electrons in an

Generally speaking, a compound

A group of electrons within the

A term describing a

atom. Whatever the number n of the prin-

phenomenon whereby certain materials

cipal energy level, there are n sublevels. At

are subject to a form of decay brought

each principal energy level, the first sublev-

about by the emission of high-energy par-

el to be filled is the one corresponding to

ticles. “Decay” in this sense does not mean

the s orbital pattern—where the alkaline

“rot”; instead, radioactive isotopes contin-

earth metals all have their valence electrons.

ue changing into other isotopes until they

VALENCE

become stable.

that occupy the highest energy levels in an

REACTIVITY: The tendency for bonds

atom, and which are involved in chemical

between atoms or molecules to be made or

bonding.

RADIOACTIVITY:

the element polonium, but were convinced that another radioactive ingredient was present— though in much smaller amounts—in pitchblende. The Curies spent most of their savings to purchase a ton of ore, and began working to extract enough of the hypothesized Element 88 for a usable sample—0.35 oz (1 g). Laboring virtually without ceasing for four years, the Curies—by then weary and in financial difficulties—finally produced the necessary quantity of radium. Their fortunes were about to improve: in 1903 they shared the Nobel Prize in physics with Becquerel, and in 1911, Marie received a second Nobel, this one in chemistry, for her discoveries of polonium and radium. She is the only individual in history to win Nobels in two different scientific categories. Because the Curies failed to patent their process, however, they received no profits from

S C I E N C E O F E V E R Y D AY T H I N G S

ELECTRONS:

Electrons

the many “radium centers” that soon sprung up, touting the newly discovered element as a cure for cancer. In fact, as it turned out, the hazards associated with this highly radioactive substance outweighed any benefits. Thus radium, which at one point was used in luminous paint and on watch dials, was phased out of use. Marie Curie’s death from leukemia in 1934 resulted from her prolonged exposure to radiation from radium and other elements. WHERE TO LEARN MORE “Alkaline Earth Metals.” ChemicalElements.com (Web site). > (May 25, 2001). “The Alkaline Earth Metals” (Web site). (May 25, 2001). Ebbing, Darrell D.; R. A. D. Wentworth; and James P. Birk. Introductory Chemistry. Boston: Houghton Mifflin, 1995.

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Kerrod, Robin. Matter and Materials. Illustrated by Terry Hadler. Tarrytown, N.Y.: Benchmark Books, 1996. Mebane, Robert C. and Thomas R. Rybolt. Metals. Illustrated by Anni Matsick. New York: Twenty-First Century Books, 1995. Oxlade, Chris. Metal. Chicago, IL: Heinemann Library, 2001.

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Snedden, Robert. Materials. Des Plaines, IL: Heinemann Library, 1999. “Visual Elements: Group 1—The Alkaline Earth Metals” (Web site). (May 25, 2001).

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T R A N S I T I O N M E TA L S

CONCEPT By far the largest family of elements is the one known as the transition metals, sometimes called transition elements. These occupy the “dip” in the periodic table between the “tall” sets of columns or groups on either side. Consisting of 10 columns and four rows or periods, the transition metals are usually numbered at 40. With the inclusion of the two rows of transition metals in the lanthanide and actinide series respectively, however, they account for 68 elements—considerably more than half of the periodic table. The transition metals include some of the most widely known and commonly used elements, such as iron—which, along with fellow transition metals nickel and cobalt, is one of only three elements known to produce a magnetic field. Likewise, zinc, copper, and mercury are household words, while cadmium, tungsten, chromium, manganese, and titanium are at least familiar. Other transition metals, such as rhenium or hafnium, are virtually unknown to anyone who is not scientifically trained. The transition metals include the very newest elements, created in laboratories, and some of the oldest, known since the early days of civilization. Among these is gold, which, along with platinum and silver, is one of several precious metals in this varied family.

HOW IT WORKS Two Numbering Systems for the Periodic Table The periodic table of the elements, developed in 1869 by Russian chemist Dmitri Ivanovitch Mendeleev (1834-1907), is discussed in several places within this volume. Within the table, ele-

S C I E N C E O F E V E R Y D AY T H I N G S

ments are arranged in columns, or groups, as well as in periods, or rows. As discussed in the Periodic Table of Elements essay, two basic versions of the chart, differing primarily in their method of number groups, are in use today. The North American system numbers only eight groups—corresponding to the representative or main-group elements—leaving the 10 columns representing the transition metals unnumbered. On the other hand, the IUPAC system, approved by the International Union of Pure and Applied Chemistry (IUPAC), numbers all 18 columns. Both versions of the periodic table show seven periods. In general, this book applies the North American system, in part because it is the version used in most American classrooms. Additionally, the North American system relates group number to the number of valence electrons in the outer shell of the atom for the element represented. Hence the number of valence electrons (a subject discussed below as it relates to the transition metals) for Group 8 is also eight. Yet where the transition metals are concerned, the North American system is less convenient than the IUPAC system. ATTEMPTS TO ADJUST THE NORTH AMERICAN SYSTEM. In

addressing the transition metals, there is no easy way to apply the North American system’s correspondence between the number of valence electrons and the group number. Some versions of the North American system do, however, make an attempt. These equate the number of valence electrons in the transition metals to a group number, and distinguish these by adding a letter “B.”

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Transition Metals

SEVERAL

PRECIOUS METALS, INCLUDING GOLD, ARE AMONG THE TRANSITION METALS. (Charles O’Rear/Corbis. Reproduced

by permission.)

Thus the noble gases are placed in Group 8A or VIIIA because they are a main-group family with eight valence electrons, whereas the transition metal column beginning with iron (Group 8 in the IUPAC system) is designated 8B or VIIIB. But this adjustment to the North American system only makes things more cumbersome, because Group VIIIB also includes two other columns—ones with nine and 10 valence electrons respectively. VALUE OF THE IUPAC SYSTEM.

Obviously, then, the North American system— while it is useful in other ways, and as noted, is applied elsewhere in this book—is not as practical for discussing the transition metals. Of course, one could simply dispense with the term “group” altogether, and simply refer to the columns on the periodic table as columns. To do so, however, is to treat the table simply as a chart, rather than as a marvelously ordered means of organizing the elements. It makes much more sense to apply the IUPAC system and to treat the transition metals as belonging to groups 3 through 12. This is particularly useful in this context, because those group numbers do correspond to the numbers of valence electrons. Scandium, in group 3 (henceforth all group numbers refer to the IUPAC ver-

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sion), has three valence electrons; likewise zinc, in Group 12, has 12. Neither the IUPAC nor the North American system provide group numbers for the lanthanides and actinides, known collectively as the inner transition metals. THE TRANSITION METALS BY IUPAC GROUP NUMBER. Here, then, is

the list of the transition metals by group number, as designated in the IUPAC system. Under each group heading are listed the four elements in that group, along with the atomic number, chemical symbol, and atomic mass figures for each, in atomic mass units. Note that, because the fourth member in each group is radioactive and sometimes exists for only a few minutes or even seconds, mass figures are given merely for the most stable isotope. In addition, the last elements in groups 8, 9, and 10, as of 2001, had not received official names. For reasons that will be explained below, lanthanum and actinium, though included here, are discussed in other essays. Group 1 • • • •

21. Scandium (Sc): 44.9559 39. Yttrium (Y): 88.9059 57. Lanthanum (La): 138.9055 89. Actinium (Ac): 227.0278

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Group 2 • • • •

Transition Metals

22. Titanium (Ti): 47.9 40. Zirconium (Zr): 91.22 72. Hafnium (Hf): 178.49 104. Rutherfordium (Rf): 261 Group 3

• • • •

23. Vanadium (V): 50.9415 41. Niobium (Nb): 92.9064 73. Tantalum (Ta): 180.9479 105. Dubnium (Db): 262 Group 4

• • • •

24. Chromium (Cr): 51.996 42. Molybdenum (Mo): 95.95 74. Tungsten (W): 183.85 106. Seaborgium (Sg): 263 Group 5

• • • •

25. Manganese (Mn): 54.938 43. Technetium (Tc): 98 75. Rhenium (Re): 186.207 107. Bohrium (Bh): 262 AN

Group 6 • • • •

26. Iron (Fe): 55.847 44. Ruthenium (Ru): 101.07 76. Osmium (Os): 190.2 108. Hassium (Hs): 265 Group 7

• • • •

27. Cobalt (Co): 58.9332 45. Rhodium (Rh): 102.9055 77. Iridium (Ir): 192.22 109. Meitnerium (Mt): 266 Group 8

• • • •

28. Nickel (Ni): 58.7 46. Palladium (Pd): 106.4 78. Platinum (Pt): 195.09 110. Ununnilium (Uun): 271 Group 9

• • • •

29. Copper (Cu): 63.546 47. Silver (Ag): 107.868 79. Gold (Au): 196.9665 111. Unununium (Uuu): 272 Group 10

• • • •

30. Zinc (Zn): 65.38 48. Cadmium (Cd): 112.41 80. Mercury (Hg): 200.59 112. Ununbium (Uub): 277

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IRON ORE MINE IN

FOURTH

MOST

ACCOUNTING FOR

SOUTH AFRICA. IRON

ABUNDANT

4.71%

ELEMENT

ON

IS THE

EARTH,

OF THE ELEMENTAL MASS IN

THE PLANET ’S CRUST. (Charles O’Rear/Corbis. Reproduced by

permission.)

Electron Configurations and the Periodic Table We can now begin to explain why transition metals are distinguished from other elements, and why the inner transition metals are further separated from the transition metals. This distinction relates not to period (row), but to group (column)—which, in turn, is defined by the configuration of valence electrons, or the electrons that are involved in chemical bonding. Valence electrons occupy the highest energy level, or shell, of the atom—which might be thought of as the orbit farthest from the nucleus. However, the term “orbit” is misleading when applied to the ways that an electron moves. Electrons do not move around the nucleus of an atom in regular orbits, like planets around the Sun; rather, their paths can only be loosely defined in terms of orbitals, a pattern of probabilities indicating the regions that an electron may occupy. The shape or pattern of orbitals is determined by the principal energy level of the

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Transition Metals

THE AMERICAN

FIVE-CENT COIN, CALLED A

“NICKEL,”

IS ACTUALLY AN ALLOY OF NICKEL AND COPPER. (Layne

Kennedy/Corbis. Reproduced by permission.)

atom, which indicates a distance that an electron may move away from the nucleus. Principal energy level defines period: elements in Period 4, for instance, all have their valence electrons at principal energy level 4. The higher the number, the further the electron is from the nucleus, and hence, the greater the energy in the atom. Each principal energy level is divided into sublevels corresponding to the number n of the principal energy level: thus, principal energy level 4 has four sublevels, principal energy level 5 has five, and so on. ORBITAL PATTERNS. The four basic

types of orbital patterns are designated as s, p, d, and f. The s shape might be described as spherical, which means that in an s orbital, the total electron cloud will probably end up being more or less like a sphere. The p shape is like a figure eight around the nucleus; the d like two figure eights meeting at the nucleus; and the f orbital pattern is so complex that most basic chemistry textbooks do not even attempt to explain it. In any case, these orbital patterns do not indicate the exact path of an electron. Think of it, instead, in this way: if you could take millions of photographs of the electron during a period of a few seconds, the

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resulting blur of images in a p orbital, for instance, would somewhat describe the shape of a figure eight. Since the highest energy levels of the transition metals begin at principal energy level 4, we will dispense with a discussion of the first three energy levels. The reader is encouraged to consult the Electrons essay, as well as the essay on Families of Elements, for a more detailed discussion of the ways in which these energy levels are filled for elements on periods 1 through 3.

Orbital Filling and the Periodic Table If all elements behaved as they “should,” the pattern of orbital filling would be as follows. In a given principal energy level, first the two slots available to electrons on the s sublevel are filled, then the six slots on the p sublevel, then the 10 slots on the d sublevel. The f sublevel, which is fourth to be filled, comes into play only at principal energy level 4, and it would be filled before elements began adding valence electrons to the s sublevel on the next principal energy level. That might be the way things “should” happen, but it is not the way that they do happen. The list that follows shows the pattern by which

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orbitals are filled from sublevel 3p onward. Following each sublevel, in parentheses, is the number of “slots” available for electrons in that shell. Note that in several places, the pattern of filling defies the ideal order described in the preceding paragraph. Orbital Filling by Principal Energy Level and Sublevel from 3p Onward: • • • • • • • • • • • • • •

3p (6) 4s (2) 3d (10) 4p (6) 5s (2) 4d (10) 5p (6) 6s (2) 4f (14) 5d (10) 6p (6) 7s (2) 5f (14) 6d (10) The 44 representative elements follow a regular pattern of orbital filling through the first 18 elements. By atomic number 18, argon, 3p has been filled, and at that point element 19 (potassium) would be expected to begin filling row 3d. However, instead it begins filling 4s, to which calcium (20) adds the second and last valence electron. It is here that we come to the transition metals, distinguished by the fact that they fill the d orbitals. Note that none of the representative elements up to this point, or indeed any that follow, fill the d orbital. (The d orbital could not come into play before Period 3 anyway, because at least three sublevels—corresponding to principal energy level 3—are required in order for there to be a d orbital.) In any case, when the representative elements fill the p orbitals of a given energy level, they skip the d orbital and go on to the next principal energy level, filling the s orbitals. Filling of the d orbital on the preceding energy level only occurs with the transition metals: in other words, on period 4, transition metals fill the 3d sublevel, and so on as the period numbers increase.

Distinguishing the Transition Metals Given the fact that it is actually the representative elements that skip the d sublevels, and the transi-

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tion metals that go back and fill them, one might wonder if the names “representative” and “transition” (implying an interruption) should be reversed. But it is the transition metals that are the exception to the rule, for two reasons. First of all, as we have seen, they are the only elements that fill the d orbitals.

Transition Metals

Secondly, they are the only elements whose outer-shell electrons are on two different principal energy levels. The orbital filling patterns of transition metals can be identified thus: ns(n-1)d, where n is the number of the period on which the element is located. For instance, zinc, on period 4, has an outer shell of 4s23d10. In other words, it has two electrons on principal energy level 4, sublevel 4s—as it “should.” But it also has 10 electrons on principal energy level 3, sublevel 3d. In effect, then, the transition metals “go back” and fill in the d orbitals of the preceding period. There are further complications in the patterns of orbital filling for the transition metals, which will only be discussed in passing here as a further explanation as to why these elements are not “representative.” Most of them have two s valence electrons, but many have one, and palladium has zero. Nor do they add their d electrons in regular patterns. Moving across period 4, for instance, the number of electrons in the d orbital “should” increase in increments of 1, from 1 to 10. Instead the pattern goes like this: 1, 2, 3, 5, 5, 6, 7, 8, 10, 10. LANTHANIDES AND ACTINIDES. The lanthanides and actinides are set

apart even further from the transition metals. In most versions of the periodic table, lanthanum (57) is followed by hafnium (72) in the transition metals section of the chart. Similarly, actinium (89) is followed by rutherfordium (104). The “missing” metals—lanthanides and actinides respectively—are listed at the bottom of the chart. There are reasons for this, as well as for the names of these groups. After the 6s orbital fills with the representative element barium (56), lanthanum does what a transition metal does—it begins filling the 5d orbital. But after lanthanum, something strange happens: cerium (58) quits filling 5d, and moves to fill the 4f orbital. The filling of that orbital continues throughout the entire lanthanide series, all the way to lutetium (71). Thus lanthanides can be defined as those metals that fill the 4f orbital; however, because lanthanum

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exhibits similar properties, it is usually included with the lanthanides.

Earth’s crust, the percentages for elements outside of the top 18 tend to very small.

A similar pattern occurs for the actinides. The 7s orbital fills with radium (88), after which actinium (89) begins filling the 6d orbital. Next comes thorium, first of the actinides, which begins the filling of the 5f orbital. This is completed with element 103, lawrencium. Actinides can thus be defined as those metals that fill the 5f orbital; but again, because actinium exhibits similar properties, it is usually included with the actinides.

In the human body, iron is the 12th most abundant element, constituting 0.004% of the body’s mass. Zinc follows it, at 13th place, accounting for 0.003%. Again, these percentages may not seem particularly high, but in view of the fact that three elements—oxygen, carbon, and hydrogen—account for 93% of human elemental body mass, there is not much room for the other 10 most common elements in the body. Transition metals such as copper are present in trace quantities within the body as well.

REAL-LIFE APPLICAT ION S Survey of the Transition Metals The fact that the transition elements are all metals means that they are lustrous or shiny in appearance, and malleable, meaning that they can be molded into different shapes without breaking. They are excellent conductors of heat and electricity, and tend to form positive ions by losing electrons. Generally speaking, metals are hard, though a few of the transition metals—as well as members of other metal families—are so soft they can be cut with a knife. Like almost all metals, they tend to have fairly high melting points, and extremely high boiling points. Many of the transition metals, particularly those on periods 4, 5, and 6, form useful alloys— mixtures containing more than one metal—with one another, and with other elements. Because of their differences in electron configuration, however, they do not always combine in the same ways, even within an element. Iron, for instance, sometimes releases two electrons in chemical bonding, and at other times three. ABUNDANCE OF THE TRANSITION METALS. Iron is the fourth most

abundant element on Earth, accounting for 4.71% of the elemental mass in the planet’s crust. Titanium ranks 10th, with 0.58%, and manganese 13th, with 0.09%. Several other transition metals are comparatively abundant: even gold is much more abundant than many other elements on the periodic table. However, given the fact that only 18 elements account for 99.51% of

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DIVIDING THE TRANSITION METALS INTO GROUPS. There is no

easy way to group the transition metals, though certain of these elements are traditionally categorized together. These do not constitute “families” as such, but they do provide useful ways to break down the otherwise rather daunting 40-element lineup of the transition metals. In two cases, there is at least a relation between group number on the periodic table and the categories loosely assigned to a collection of transition metals. Thus the “coinage metals”— copper, silver, and gold—all occupy Group 9 on the periodic table. These have traditionally been associated with one another because their resistance to oxidation, combined with their malleability and beauty, has made them useful materials for fashioning coins. Likewise the members of the “zinc group”— zinc, cadmium, and mercury—occupy Group 10 on the periodic table. These, too, have often been associated as a miniature unit due to common properties. Members of the “platinum group”— platinum, iridium, osmium, palladium, rhodium, and ruthenium—occupy a rectangle on the table, corresponding to periods 5 and 6, and groups 6 through 8. What actually makes them a “group,” however, is the fact that they tend to appear together in nature. Iron, nickel, and cobalt, found alongside one another on Period 4, may be grouped together because they are all magnetic to some degree or another. This is far from the only notable characteristic about such metals, but provides a convenient means of further dividing the transition metals into smaller sections. To the left of iron on the periodic table is a rectangle corresponding to periods 4 through 6, groups 4 through 7. These 11 elements—titani-

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um, zirconium, hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, and rhenium—are referred to here as “alloy metals.” This is not a traditional designation, but it is nonetheless useful for describing these metals, most of which form important alloys with iron and other elements.

ity with gold, and jewelry found in Egyptian tombs indicates the use of sophisticated techniques among the goldsmiths of Egypt as early as 2600 B.C. Likewise the Bible mentions gold in several passages. The Romans called it aurum (“shining dawn”), which explains its chemical symbol, Au.

One element was left out of the “rectangle” described in the preceding paragraph. This is technetium, which apparently does not occur in nature. It is lumped in with a final category, “rare and artificial elements.”

Early chemistry had its roots among the medieval alchemists, who sought in vain to turn plain metals into gold. Likewise the history of European conquest in the New World is filled with bitter tales of conquistadors, lured by gold, who wiped out entire nations of native peoples.

It should be stressed that there is nothing hard and fast about these categories. The “alloy metals” are not the only ones that form alloys; nickel is used in coins, though it is not called a coinage metal; and platinum could be listed with gold and silver as “precious metals.” Nonetheless, the categories used here seem to provide the most workable means of approaching the many transition metals.

Coinage Metals GOLD. Gold almost needs no introduction: virtually everyone knows of its value, and history is full of stories about people who killed or died for this precious metal. Part of its value springs from its rarity in comparison to, say iron: gold is present on Earth’s crust at a level of about 5 parts per billion (ppb). Yet as noted earlier, it is more abundant than some metals. Furthermore, due to the fact that it is highly unreactive (reactivity refers to the tendency for bonds between atoms or molecules to be made or broken in such a way that materials are transformed), it tends to be easily separated from other elements.

This helps to explain the fact that gold may well have been the first element ever discovered. No ancient metallurgist needed a laboratory in which to separate gold; indeed, because it so often keeps to itself, it is called a “noble” metal— meaning, in this context, “set apart.” Another characteristic of gold that made it valuable was its great malleability. In fact, gold is the most malleable of all metals: A single troy ounce (31.1 g) can be hammered into a sheet just 0.00025 in (0.00064 cm) thick, covering 68 ft2 (6.3 m2). Gold is one of the few metals that is not silver, gray, or white, and its beautifully distinctive color caught the eyes of metalsmiths and royalty from the beginning of civilization. Records from India dating back to 5000 B.C. suggest a familiar-

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Transition Metals

At one time, gold was used in coins, and nations gauged the value of their currency in terms of the gold reserves they possessed. Few coins today—with special exceptions, such as the South African Krugerrand—are gold. Likewise the United States, for instance, went off the gold standard (that is, ceased to tie its currency to gold reserves) in the 1930s; nonetheless, the federal government maintains a vast supply of gold bullion at Fort Knox in Kentucky. Gold is as popular as ever for jewelry and other decorative objects, of course, but for the most part, it is too soft to have many other commercial purposes. One of the few applications for gold, a good conductor of electricity, is in some electronic components. Also, the radioactive gold-198 isotope is sometimes implanted in tissues as a means of treating forms of cancer. SILVER. Like gold, silver has been a part of human life from earliest history. Usually it is considered less valuable, though some societies have actually placed a higher value on silver because it is harder and more durable than gold. In the seventh century B.C., the Lydian civilization of Asia Minor (now Turkey) created the first coins using silver, and in the sixth century B.C., the Chinese began making silver coins. Succeeding dynasties in China continued to mint these coins, round with square holes in them, until the early twentieth century.

The Romans called silver argentum, and therefore today its chemical symbol is Ag. Its uses are much more varied than those of gold, both because of its durability and the fact that it is less expensive. Alloyed with copper, which adds strength to it, it makes sterling silver, used in coins, silverware, and jewelry. Silver nitrate compounds are used in silver plating, applied in mir-

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rors and tableware. (Most mirrors today, however, use aluminum.) Like gold, silver was once widely used for coinage, and silver coins are still more common than their gold counterparts. In 1963, however, the United States withdrew its silver certifications, paper currency exchangeable for silver. Today, silver coins are generally issued in special situations, such as to commemorate an event. A large portion of the world’s silver supply is used by photographers for developing pictures. In addition, because it is an excellent conductor of heat and electricity, silver has applications in the electronics industry; however, its expense has led many manufacturers to use copper or aluminum instead. Silver is also present, along with zinc and cadmium, in cadmium batteries. Like gold, though to a much lesser extent, it is still an important jewelry-making component. COPPER. Most people think of pennies as containing copper, but in fact the penny is the only American coin that contains no copper alloys. Because the amount of copper necessary to make a penny today costs more than $0.01, a penny is actually made of zinc with a thin copper coating. Yet copper has long been a commonly used coinage metal, and long before that, humans used it for other purposes.

Seven thousand years ago, the peoples of the Tigris-Euphrates river valleys, in what is now Iraq, were mining and using copper, and later civilizations combined copper with zinc to make bronze. Indeed, the history of prehistoric and ancient humans’ technological development is often divided according to the tools they made, the latter two of which came from transition metals: the Stone Age, the Bronze Age (c. 33001200 B.C.), and the Iron Age. Thus, by the time the Romans dubbed copper cuprum (from whence its chemical symbol, Cu, comes), copper in various forms had long been in use. Today copper is purified by a form of electrolysis, in which impurities such as iron, nickel, arsenic, and zinc are removed. Other “impurities” often present in copper ore include gold, silver, and platinum, and the separation of these from the copper pays for the large amounts of electricity used in the electrolytic process. Like the other coinage metals, copper is an extremely efficient conductor of heat and electricity, and because it is much less expensive than the other two, pure copper is widely used for

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electrical wiring. Because of its ability to conduct heat, copper is also applied in materials used for making heaters, as well as for cookware. Due to the high conductivity of copper, a heated copper pan has a uniform temperature, but copper pots must be coated with tin because too much copper in food is toxic. Copper is also like its two close relatives in that it resists corrosion, and this makes it ideal for plumbing. Its use in making coins resulted from its anti-corrosive qualities, combined with its beauty: like gold, copper has a distinctive color. This aesthetic quality led to the use of copper in decorative applications as well: many old buildings used copper roofs, and the Statue of Liberty is covered in 300 thick copper plates. Why, then, is the famous statue not coppercolored? Because copper does eventually corrode when exposed to air for long periods of time. Over time, it develops a thin layer of black copper oxide, and as the years pass, carbon dioxide in the air leads to the formation of copper carbonate, which imparts a greenish color. The human body is about 0.0004% copper, though as noted, larger quantities of copper can be toxic. Copper is found in foods such as shellfish, nuts, raisins, and dried beans. Whereas human blood has hemoglobin, a molecule with an iron atom at the center, the blood of lobsters and other large crustaceans contains hemocyanin, in which copper performs a similar function.

The Zinc Group ZINC. Together with copper, zinc appeared in another alloy that, like bronze, helped define the ancient world: brass. (The latter is mentioned in the Bible, for instance in the Book of Daniel, when King Nebuchadnezzar dreams of a statue containing brass and other substances, symbolizing various empires.) Used at least from the first millennium B.C. onward, brass appeared in coins and ornaments throughout Asia Minor. Though it is said that the Chinese purified zinc in about A.D. 1000, the Swiss alchemist Paracelsus (1493-1541) is usually credited with first describing zinc as a metal.

Bluish-white, with a lustrous sheen, zinc is found primarily in the ore sulfide sphalerite. The largest natural deposits of zinc are in Australia and the United States, and after mining, the metal is subjected to a purification and reduction

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process involving carbon. Zinc is used in galvanized steel, developed in the eighteenth century by Italian physicist Luigi Galvani (1737-1798). Just as a penny is not really copper but zinc, “tin” roofs are usually made of galvanized steel. Highly resistant to corrosion, galvanized steel finds application in everything from industrial equipment to garbage cans. Zinc oxide is applied in textiles, batteries, paints, and rubber products, while luminous zinc sulfide appears in television screens, movie screens, clock dials, and fluorescent light bulbs. Zinc phosphide is used as a rodent poison. Like several other transition metals, zinc is a part of many living things, yet it can be toxic in large quantities or specific compounds. For a human being, inhaling zinc oxide causes involuntary shaking. On the other hand, humans and many animals require zinc in their diets for the digestion of proteins. Furthermore, it is believed that zinc contributes to the healing of wounds and to the storage of insulin in the pancreas. In 1817, German chemist Friedrich Strohmeyer (1776-1835) was working as an inspector of pharmacies for the German state of Hanover. While making his rounds, he discovered that one pharmacy had a sample of zinc carbonate labeled as zinc oxide, and while inspecting the chemical in his laboratory, he discovered something unusual. If indeed it were zinc carbonate, it should turn into zinc oxide when heated, and since both compounds were white, there should be no difference in color. Instead, the mysterious compound turned a yellowish-orange. CADMIUM.

Strohmeyer continued to analyze the sample, and eventually realized that he had discovered a new element, which he named after the old Greek term for zinc carbonate, kadmeia. Indeed, cadmium typically appears in nature along with zinc or zinc compounds. Silvery white and lustrous or shiny, cadmium is soft enough to be cut with a knife, but chemically it behaves much like zinc: hence the idea of a “zinc group.” Today cadmium is used in batteries, and for electroplating of other metals to protect them against corrosion. Because the cost of cadmium is high due to the difficulty of separating it from zinc, cadmium electroplating is applied only in specialized situations. Cadmium also appears in the control rods of nuclear power plants, where

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its ready absorption of neutrons aids in controlling the rate at which nuclear fission occurs.

Transition Metals

Cadmium is highly toxic, and is believed to be the cause behind the outbreak of itai-itai (“ouch-ouch”) disease in Japan in 1955. People ingested rice oil contaminated with cadmium, and experienced a number of painful side effects associated with cadmium poisoning: nausea, vomiting, choking, diarrhea, abdominal pain, headaches, and difficulty breathing. MERCURY. One of only two elements—

along with bromine—that appears in liquid form at room temperature, mercury is both toxic and highly useful. The Romans called it hydragyrum (“liquid silver”), from whence comes its chemical symbol, Hg. Today, however, it is known by the name of the Romans’ god Mercury, the nimble and speedy messenger of the gods. Mercury comes primarily from a red ore called cinnabar, and since it often appears in shiny globules that form outcroppings from the cinnabar, it was relatively easy to discover. Several things are distinctive about mercury, including its bright silvery color. But nothing distinguishes it as much as its physical properties— not only its liquidity, but the fact that it rolls rapidly, like the fleet-footed god after which it is named. Its surface tension (the quality that causes it to bead) is six times greater than that of water, and for this reason, mercury never wets the surfaces with which it comes in contact. Mercury, of course, is widely used in thermometers, an application for which it is extremely well-suited. In particular, it expands at a uniform rate when heated, and thus a mercury thermometer (unlike earlier instruments, which used water, wine, or alcohol) can be easily calibrated. (Note that due to the toxicity of the element, mercury thermometers in schools are being replaced by other types of thermometers.) At temperatures close to absolute zero, mercury loses its resistance to the flow of electric current, and therefore it presents a promising area of research with regard to superconductivity. Even with elements present in the human body, such as zinc, there is a danger of toxicity with exposure to large quantities. Mercury, on the other hand, does not occur naturally in the human body, and is therefore extremely dangerous. Because it is difficult for the body to eliminate, even small quantities can accumulate and exert their poisonous effects, resulting in disor-

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ders of the nervous system. In fact, the phrase “mad as a hatter” refers to the symptoms of mercury poisoning suffered by hatmakers of the nineteenth century, who used a mercury compound in making hats of beaver and fur.

Magnetic Metals IRON. In its purest form, iron is relatively soft and slightly magnetic, but when hardened, it becomes much more so. As with several of the elements discovered long ago, iron has a chemical symbol (Fe) reflecting an ancient name, the Latin ferrum. But long before the Romans’ ancestors arrived in Italy, the Hittites of Asia Minor were purifying iron ore by heating it with charcoal over a hot flame.

The Hittites jealously guarded their secret, which gave them a technological advantage over enemies such as the Egyptians. But when Hittite civilization crumbled in the wake of an invasion by the mysterious “Sea Peoples” in about 1200 B.C., knowledge of ore smelting spread throughout the ancient world. Thus began the Iron Age, in which ancient technology matured greatly. (It should be noted that the Bantu peoples, in what is now Nigeria and neighboring countries, developed their own ironmaking techniques a few centuries later, apparently without benefit of contact with any civilization exposed to Hittite techniques.) Ever since ancient times, iron has been a vital part of human existence, and with the growth of industrialization during the eighteenth century, demand for iron only increased. But heating iron ore with charcoal required the cutting of trees, and by the latter part of that century, England’s forests had been so badly denuded that British ironmakers sought a new means of smelting the ore. It was then that British inventor Abraham Darby (1678?-1717) developed a method for making coke from soft coal, which was abundant throughout England. Adoption of Darby’s technique sped up the rate of industrialization in England, and thus advanced the entire Industrial Revolution soon to sweep the Western world. Symbolic of industrialism was iron in its many forms: pig iron, ore smelted in a cokeburning blast furnace; cast iron, a variety of mixtures containing carbon and/or silicon; wrought iron, which contains small amounts of various other elements, such as nickel, cobalt, copper,

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chromium, and molybdenum; and most of all steel. Steel is a mixture of iron with manganese and chromium, purified with a blast of hot air in the Bessemer converter, named after English inventor Henry Bessemer (1813-1898). Modern techniques of steelmaking improved on Bessemer’s design by using pure oxygen rather than merely hot air. The ways in which iron is used are almost too obvious (and too numerous) to mention. If iron and steel suddenly ceased to exist, there could be no skyscrapers, no wide-span bridges, no ocean liners or trains or heavy machinery or automobile frames. Furthermore, alloys of steel with other transition metals, such as tungsten and niobium, possess exceptionally great strength, and find application in everything from hand tools to nuclear reactors. Then, of course, there are magnets and electromagnets, which can only be made of iron and/or one of the other magnetic elements, cobalt and nickel. In the human body, iron is a key part of hemoglobin, the molecule in blood that transports oxygen from the lungs to the cells. If a person fails to get sufficient quantities of iron— present in foods such as red meat and spinach— the result is anemia, characterized by a loss of skin color, weakness, fainting, and heart palpitations. Plants, too, need iron, and without the appropriate amounts are likely to lose their color, weaken, and die. COBALT. Isolated in about 1735 by Swedish chemist Georg Brandt (1694-1768), cobalt was the first metal discovered since prehistoric, or at least ancient, times. The name comes from Kobald, German for “underground gnome,” and this reflects much about the early history of cobalt. In legend, the Kobalden were mischievous sprites who caused trouble for miners, and in real life, ores containing the element that came to be known as cobalt likewise caused trouble to men working in mines. Not only did these ores contain arsenic, which made miners ill, but because cobalt had no apparent value, it only interfered with their work of extracting other minerals.

Yet cobalt had been in use by artisans long before Brandt’s isolated the element. The color of certain cobalt compounds is a brilliant, shocking blue, and this made it popular for the coloring of pottery, glass, and tile. The element, which makes up less than 0.002% of Earth’s crust, is found today primarily in ores extracted from mines in

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Canada, Zaire, and Morocco. One of the most important uses of cobalt is in a highly magnetic alloy known as alnico, which also contains iron, nickel, and aluminum. Combined with tungsten and chromium, cobalt makes stellite, a very hard alloy used in drill bits. Cobalt is also applied in jet engines and turbines. NICKEL. Moderately magnetic in its pure form, nickel had an early history much like that of cobalt. English workers mining copper were often dismayed to find a metal that looked like copper, but was not, and they called it “Old Nick’s copper”—meaning that it was a trick played on them by Old Nick, or the devil. The Germans gave it a similar name: Kupfernickel, or “imp copper.”

Though nickel was not identified as a separate metal by Swedish mineralogist Axel Fredrik Cronstedt (1722-1765) until the eighteenth century, alloys of copper, silver, and nickel had been used as coins even in ancient Egypt. Today, nickel is applied, not surprisingly, in the American five-cent piece—that is, the “nickel”—made from an alloy of nickel and copper. Its anti-corrosive nature also provides a number of other applications for nickel: alloyed with steel, for instance, it makes a protective layer for other metals.

The Platinum Group PLATINUM. First identified by an Italian physician visiting the New World in the mid-sixteenth century, platinum—now recognized as a precious metal—was once considered a nuisance in the same way that nickel and cadmium were. Miners, annoyed with the fact that it got in the way when they were looking for gold, called it platina, or “little silver.” One of the reasons why platinum did not immediately catch the world’s fancy is because it is difficult to extract, and typically appears with the other metals of the “platinum group”: iridium, osmium, palladium, rhodium, and ruthenium.

Only in 1803 did English physician and chemist William Hyde Wollaston (1766-1828) develop a means of extracting platinum, and when he did, he discovered that the metal could be hammered into all kinds of shapes. Platinum proved such a success that it made Wollaston financially independent, and he retired from his medical practice at age 34 to pursue scientific research. Today, platinum is used in everything from thermometers to parts for rocket engines,

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both of which take advantage of its ability to withstand high temperatures.

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In the same year that Wollaston isolated platinum, three French chemists applied a method of extracting it with a mixture of nitric and hydrochloric acids. The process left a black powder that they were convinced was a new element, and in 1804 English chemist Smithson Tennant (1761-1815) gave it the name iridium. Iris was the Greek goddess of the rainbow, and the name reflected the element’s brilliant, multi-colored sheen. As with platinum, iridium is used today in a number of ways, particularly for equipment such as laser crystals that must withstand high temperatures. (In addition, one particular platinumiridium bar provides the standard measure of a kilogram.) IRIDIUM

AND

OSMIUM.

Tennant discovered a second element in 1804, also from the residue left over from the acid process for extracting platinum. This one had a distinctive smell when heated, so he named it osmium after osme, Greek for “odor.” In 1898, Austrian chemist Karl Auer, Baron von Welsbach (1858-1929), developed a light bulb using osmium as a filament, the material that is heated. Though osmium proved too expensive for commercial use, Auer’s creation paved the way for the use of another transition metal, tungsten, in making long-lasting filaments. Osmium, which is very hard and resistant to wear, is also used in electrical devices, fountain-pen tips, and phonograph needles. PALLADIUM, RHODIUM, AND RUTHENIUM. By the time Tennant isolated

osmium, Wollaston had found yet another element that emerged from the refining of platinum with nitric and hydrochloric acids. This was palladium, named after a recently discovered asteroid, Pallas. Soft and malleable, palladium resists tarnishing, which makes it valuable to the jewelry industry. Combined with yellow gold, it makes the alloy known as white gold. Because palladium has the unusual ability of absorbing up to 900 times its volume in hydrogen, it provides a useful means of extracting that element. Also in 1805, Wollaston discovered rhodium, which he named because it had a slightly red color. The density of rhodium is lower, and the melting point higher, than for most of the platinum group elements, and it is often used as an alloy to harden platinum. As with many elements

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in this group, it has a number of high-temperature applications. When electroplated, it is highly reflective, and this makes it useful as a material for optical instruments. The element that came to be known as ruthenium had been detected several times before 1844, when Russian chemist Carl Ernest Claus (1796-1864) finally identified it as a distinct element. Thus it was the only platinum group element in whose isolation neither Tennant nor Wollaston played a leading role. Since it had been found in Russia, the element was called ruthenium, in honor of that country’s ancient name, Ruthenia.

Alloy Metals The elements described here as “alloy metals” will be treated briefly, but the amount of space given to them here does not reflect their importance to civilization. Several of these are relatively abundant, and many—titanium, tungsten, manganese, chromium and others—are vital parts of daily life. However, due to space considerations, the treatment of these metals that follows is a regrettably abbreviated one. As suggested by the informal name given to them here, they are often applied in alloys, typically with other transition metals such as iron, or with other “alloy metals.” They are also used in a variety of compounds for a wide variety of applications.

Titanium, Zirconium, and Hafnium English clergyman and amateur scientist William Gregor (1761-1817) in 1791 noticed what he called “a new metallic substance” in a mineral called menchanite, which he found in the Menachan region of Cornwall, England. Four years later, German chemist Martin Heinrich Klaproth (1743-1817) studied menchanite and found that it contained a new element, which he named titanium after the Titans of Greek mythology. Only in 1910, however, was the element isolated. Due to its high strength-to-weight ratio, titanium is applied today in alloys used for making airplanes and spacecraft. It is also used in white paints and in a variety of other combinations, mostly in alloys containing other transition metals, as well as in compounds with nonmetals such as oxygen.

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Six years before he identified titanium, in 1789, Klaproth had found another element, which he named zirconium after the mineral zircon in which it was found. Zircon had been in use for many centuries, and another zirconiumcontaining mineral, jacinth, is mentioned in the Bible. As with titanium, there was a long period from identification to isolation, which did not occur until 1914. Zirconium alloys are used today in a number of applications, including superconducting magnets and as a construction metal in nuclear power plants. Compounds including zirconium also have a variety of uses, ranging from furnace linings to poison ivy lotion. Because it is almost always found with zirconium, hafnium—named after the ancient name of Copenhagen, Denmark—proved extremely difficult to isolate. Only in 1923 was it isolated, using x-ray analysis, and today it is applied primarily in light bulb filaments and electrodes. Sometimes it is accidentally “applied,” when zirconium is actually the desired metal, because the two are so difficult to separate. VANADIUM, TANTALUM, AND NIOBIUM. Vanadium, named after the Norse

goddess Vanadis or Freyja, was first identified in 1801, but only isolated in 1867. It is often applied in alloys with steel, and sometimes used for bonding steel and titanium. Tantalum and niobium are likewise named after figures from mythology—in this case, Greek. These two elements appear together so often that their identification as separate substances was a long process, involving numerous scientists and heated debates. For this reason, alloys involving tantalum—valued for its high melting point—usually include niobium as well. CHROMIUM, TUNGSTEN, AND MANGANESE, AND OTHER METALS. Because it displays a wide variety of col-

ors, chromium was named after the Greek word for “color.” It is used in chrome and tinted glass, and as a strengthening agent for stainless steel. Tungsten likewise strengthens steel, in large part because it has the highest melting point of any metal: 6,170°F (3,410°C). Its name comes from a Swedish word meaning “heavy stone,” and its chemical symbol (W) refers to the ore named wolframite, in which it was first discovered. Its high resistance to heat gives tungsten alloys applications in areas ranging from light bulb filaments to furnaces to missiles.

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Transition Metals

KEY TERMS Those transition metals

that fill the 5f orbital. Because actinium—

main-group elements and the transition metals are numbered.

which does not fill the 5f orbital—exhibits

LANTHANIDES: The transition metals

characteristics similar to those of the

that fill the 4f orbital. Because lanthanum—which does not fill the 4f orbital—exhibits characteristics similar to those of the lanthanides, it is usually considered part of the lanthanide family.

ACTINIDES:

actinides, it is usually considered part of the actinides family. ALLOY:

A mixture containing more than

one metal. ELECTROLYSIS: The use of an electric

current to cause a chemical reaction. ELECTRON CLOUD: A term used to

describe the pattern formed by orbitals. GROUPS:

Columns on the periodic

table of elements. These are ordered according to the numbers of valence electrons in the outer shells of the atoms for the elements represented. INNER TRANSITION METALS: The

lanthanides and actinides, both of which fill the f orbitals. For this reason, they are usually treated separately. ION:

An atom or group of atoms that has

lost or gained one or more electrons, and thus has a net electric charge. ISOTOPES: Atoms that have an equal

number of protons, and hence are of the same element, but differ in their number of neutrons. This results in a difference of mass. Isotopes may be either stable or unstable. The latter type, known as radioisotopes, are radioactive. IUPAC SYSTEM: A version of the peri-

odic table of elements, authorized by the International Union of Pure and Applied Chemistry (IUPAC), which numbers all groups on the table from 1-18. Thus in the IUPAC system, in use primarily outside of North America, both the representative or

S C I E N C E O F E V E R Y D AY T H I N G S

NORTH AMERICAN SYSTEM: A version of the periodic table of elements that only numbers groups of elements in which the number of valence electrons equals the group number. Hence the transition metals are usually not numbered in this system. Some North American charts, however, do provide group numbers for transition metals by including an “A” after the group numbers for representative or main-group elements, and “B” after those of transition metals.

A pattern of probabilities indicating the regions that may be occupied by an electron. The higher the principal energy level, the more complex the pattern of orbitals.

ORBITAL:

PERIODIC TABLE OF ELEMENTS: A

chart that shows the elements arranged in order of atomic number, along with chemical symbol and the average atomic mass (in atomic mass units) for that particular element. Elements are arranged in groups or columns, as well as in rows or periods, according to specific aspects of their valence electron configurations. PERIODS: Rows on the periodic table of

elements. These represent successive principal energy levels for the valence electrons in the atoms of the elements involved. Elements in the transition metal family occupy periods 4, 5, 6, or 7.

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Transition Metals

KEY TERMS PRINCIPAL ENERGY LEVEL: A value

elements are assigned group numbers 1, 2,

indicating the distance that an electron

and 13 through 18. (In these last six

may move away from the nucleus of an

columns on the IUPAC chart, there is no

atom. This is designated by a whole-num-

relation between the number of valence

ber integer, beginning with 1 and moving

electrons and group number.)

upward. The higher the principal energy

SHELL:

level, the greater the energy in the atom,

valence electrons at the outside of an atom.

The orbital pattern of the

and the more complex the pattern of

SUBLEVEL:

orbitals. Elements in the transition metal

pal energy level occupied by electrons in an

family have principal energy levels of 4, 5,

atom. Whatever the number n of the prin-

6, or 7.

cipal energy level, there are n sublevels. At

A region within the princi-

A term describing a

each principal energy level, the first sublev-

phenomenon whereby certain isotopes

el to be filled is the one corresponding to

known as radioisotopes are subject to a

the s orbital pattern, followed by the p and

form of decay brought about by the emis-

then the d pattern.

sion of high-energy particles. “Decay” does

TRANSITION METALS: A group of 40

not mean that the isotope “rots”; rather, it

elements, which are not assigned a group

decays to form another isotope until even-

number in the version of the periodic table

tually (though this may take a long time), it

of elements known as the North American

becomes stable.

system. These are the only elements that fill

REACTIVITY: The tendency for bonds

the dorbital. In addition, the transition

between atoms or molecules to be made or

metals—unlike the representative or main-

broken in such a way that materials are

group elements—have their valence elec-

transformed.

trons on two different principal energy lev-

REPRESENTATIVE OR MAIN-GROUP

els. Though the lanthanides and actinides

The 44 elements in Groups

are considered inner transition metals,

RADIOACTIVITY:

ELEMENTS:

1 through 8 on the periodic table of ele-

they are usually treated separately.

ments in the North American system, for

VALENCE

which the number of valence electrons

that occupy the highest principal energy

equals the group number. (The only excep-

level in an atom. These are the electrons

tion is helium.) In the IUPAC system, these

involved in chemical bonding.

Likewise manganese, first identified in the mid-eighteenth century, is extraordinarily strong. Its major use is in steelmaking. In addition, compounds such as manganese oxide are used in fertilizers. Other alloy metals include rhenium (identified only in 1925) and molybdenum. These two are often applied in alloys together, and with tungsten, for a variety of industrial purposes.

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ELECTRONS:

Electrons

Rare and Artificial Elements RARE EARTH-LIKE ELEMENTS.

Though they are not part of the lanthanide series, scandium and yttrium share many properties with those elements, once known as the “rare earths.” (In this context, “rarity” refers not so much to scarcity as to difficulty of extraction.) Furthermore, like most of the lan-

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thanides, these two elements were first discovered in Scandinavia. Their origin is reflected in the name given by Swedish chemist Lars Nilson (1840-1899) to scandium, which he discovered in 1876. As for yttrium, it came from the complex mineral known as ytterite, from whence emerged many of the lanthanides. Yttrium is used in alloys to impart strength, and has a wide if specialized array of applications. Scandium has no important uses. ARTIFICIAL ELEMENTS. Technetium, with an atomic number of 43, is unusual in that it is one of the few elements with an atomic number less than 92 that does not occur in nature. This is reflected in its name, from the Greek technetos, meaning “artificial.” The strange thing is that technetium, produced in a laboratory in 1936, does occur naturally in the universe— but it seems to be found only in the spectral lines from older stars, and not in those of younger stars such as our Sun.

The other elements in the transition metals family are referred to as “transuranium elements,” meaning that they have atomic numbers higher than that of uranium (92). These have all been created artificially; all are highly radioac-

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tive; few survive for more than a few minutes (at most); and few have applications in ordinary life.

Transition Metals

WHERE TO LEARN MORE The Copper Page (Web site). (May 26, 2001). Gold Institute (Web site). (May 26, 2001). Kerrod, Robin. Matter and Materials. Illustrated by Terry Hadler. Tarrytown, N.Y.: Benchmark Books, 1996. Mebane, Robert C. and Thomas R. Rybolt. Metals. Illustrated by Anni Matsick. New York: Twenty-First Century Books, 1995. Oxlade, Chris. Metal. Chicago, IL: Heinemann Library, 2001. “The Pictorial Periodic Table” (Web site). (May 22, 2001). Stwertka, Albert. A Guide to the Elements. New York: Oxford University Press, 1998. “Transition Metals” ChemicalElements.com (Web site). (May 26, 2001). “The Transition Metals.”University of Colorado Department of Physics (Web site). (May 26, 2001). WebElements (Web site). (May 22, 2001). Zincworld (Web site). (May 22, 2001).

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ACTINIDES Actinides

CONCEPT The actinides (sometimes called actinoids) occupy the “bottom line” of the periodic table—a row of elements normally separated from the others, placed at the foot of the chart along with the lanthanides. Both of these families exhibit unusual atomic characteristics, properties that set them apart from the normal sequence on the periodic table. But there is more that distinguishes the actinides, a group of 14 elements along with the transition metal actinium. Only four of them occur in nature, while the other 10 have been produced in laboratories. These 10 are classified, along with the nine elements to the right of actinium on Period 7 of the periodic table, as transuranium (beyond uranium) elements. Few of these elements have important applications in daily life; on the other hand, some of the lowernumber transuranium elements do have specialized uses. Likewise several of the naturally occurring actinides are used in areas ranging from medical imaging to powering spacecraft. Then there is uranium, “star” of the actinide series: for centuries it seemed virtually useless; then, in a matter of years, it became the most talked-about element on Earth.

HOW IT WORKS The Transition Metals Why are actinides and lanthanides set apart from the periodic table? This can best be explained by reference to the transition metals and their characteristics. Actinides and lanthanides are referred to as inner transition metals, because, although they belong to this larger family, they are usually considered separately—rather like grown chil-

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dren who have married and started families of their own. The qualities that distinguish the transition metals from the representative or main-group elements on the periodic table are explained in depth within the Transition Metals essay. The reader is encouraged to consult that essay, as well as the one on Families of Elements, which further places the transition metals within the larger context of the periodic table. Here these specifics will be discussed only briefly. ORBITAL PATTERNS OF THE TRANSITION METALS. The transition

metals are distinguished by their configuration of valence electrons, or the outer-shell electrons involved in chemical bonding. Together with the core electrons, which are at lower energy levels, valence electrons move in areas of probability referred to as orbitals. The pattern of orbitals is determined by the principal energy level of the atom, which indicates a distance that an electron may move away from the nucleus. Each principal energy level is divided into sublevels corresponding to the number n of the principal energy level. The actinides, which would be on Period 7 if they were included on the periodic table with the other transition metals, have seven principal energy levels. (Note that period number and principal energy level number are the same.) In the seventh principal energy level, there are seven possible sublevels. The higher the energy level, the larger the number of possible orbital patterns, and the more complex the patterns. Orbital patterns loosely define the overall shape of the electron cloud, but this does not necessarily define the paths along which the electrons move. Rather, it

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means that if you could take millions of photographs of the electron during a period of a few seconds, the resulting blur of images would describe more or less the shape of a specified orbital. The four basic types of orbital patterns are discussed in the Transition Metals essay, and will not be presented in any detail here. It is important only to know that, unlike the representative elements, transition metals fill the sublevel corresponding to the d orbitals. In addition, they are the only elements that have valence electrons on two different principal energy levels. LANTHANIDES AND ACTINIDES. The lanthanides and actinides are fur-

ther set apart even from the transition metals, due to the fact that these elements also fill the highly complex f orbitals. Thus these two families are listed by themselves. In most versions of the periodic table, lanthanum (57) is followed by hafnium (72) in the transition metals section of the chart; similarly, actinium (89) is followed by rutherfordium (104). The “missing” metals— lanthanides and actinides, respectively—are shown at the bottom of the chart. The lanthanides can be defined as those metals that fill the 4f orbital. However, because lanthanum (which does not fill the 4f orbital) exhibits similar properties, it is usually included with the lanthanides. Likewise the actinides can be defined as those metals that fill the 5f orbital; but again, because actinium exhibits similar properties, it is usually included with the actinides.

Isotopes One of the distinguishing factors in the actinide family is its great number of radioactive isotopes. Two atoms may have the same number of protons, and thus be of the same element, yet differ in their number of neutrons—neutrally charged patterns alongside the protons at the nucleus. Such atoms are called isotopes, atoms of the same element having different masses. Isotopes are represented symbolically in one of several ways. For instance, there is this format: ᎏmᎏS, where S is the chemical symbol of the elea ment, a is the atomic number (the number of protons in its nucleus), and m the mass number—the sum of protons and neutrons. For the

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isotope known as uranium-238, for instance, this 238 is shown as ᎏ9ᎏ 2 U.

Actinides

Because the atomic number of any element is established, however, isotopes are usually represented simply with the mass number thus: 238U. They may also be designated with a subscript notation indicating the number of neutrons, so that this information can be obtained at a glance without it being necessary to do the arithmetic. For the uranium isotope shown here, this is writ238 ten as ᎏ9ᎏ 2 U146.

Radioactivity The term radioactivity describes a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of highenergy particles, or radiation. Types of particles emitted in radiation include: • Alpha particles, or helium nuclei; • Beta particles—either an electron or a subatomic particle called a positron; • Gamma rays or other very high-energy electromagnetic waves. Isotopes are either stable or unstable, with the unstable variety, known as radioisotopes, being subject to radioactive decay. In this context, “decay” does not mean “rot”; rather, a radioisotope decays by turning into another isotope. By continuing to emit particles, the isotope of one element may even turn into the isotope of another element. Eventually the radioisotope becomes a stable isotope, one that is not subject to radioactive decay. This is a process that may take seconds, minutes, hours, days, years—and sometimes millions or even billions of years. The rate of decay is gauged by the half-life of a radioisotope sample: in other words, the amount of time it takes for half the nuclei (plural of nucleus) in the sample to become stable. Actinides decay by a process that begins with what is known as K-capture, in which an electron of a radioactive atom is captured by the nucleus and taken into it. This is followed by the splitting, or fission, of the atom’s nucleus. This fission produces enormous amounts of energy, as well as the release of two or more neutrons, which may in turn bring about further K-capture. This is called a chain reaction.

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Actinides

GLENN T. SEABORG HOLDS A SAMPLE CONTAINING IC TABLE. (Roger Ressmeyer/Corbis. Reproduced by permission.)

ARTIFICIAL ELEMENTS BETWEEN

REAL-LIFE APPLICAT ION S The First Three Naturally Occurring Actinides In the discussion of the actinides that follows, atomic number and chemical symbol will follow the first mention of an element. Atomic mass figures are available on any periodic table, and these will not be mentioned in most cases. The atomic mass figures for actinide elements are very high, as fits their high atomic number, but for most of these, figures are usually for the most stable isotope, which may exist for only a matter of seconds. Though it gives its name to the group as a whole, actinium (Ac, 89) is not a particularly significant element. Discovered in 1902 by German chemist Friedrich Otto Giesel (1852-1927), it is found in uranium ores. Actinium is 150 times more radioactive than radium, a highly radioactive alkaline earth metal isolated around the same time by French-Polish physicist and chemist Marie Curie (1867-1934) and her husband Pierre (1859-1906). THORIUM. More significant than actinium is thorium (Th, 90), first detected in 1815 by

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94

AND

102

ON THE PERIOD-

the renowned Swedish chemist Jons Berzelius (1779-1848). Berzelius promptly named the element after the Norse god Thor, but eventually concluded that what he had believed to be a new element was actually the compound yttrium phosphate. In 1829, however, he examined another mineral and indeed found the element he believed he had discovered 14 years earlier. It 1898, Marie Curie and an English chemist named Gerhard Schmidt, working independently, announced that thorium was radioactive. Today it is believed that the enormous amounts of energy released by the radioactive decay of subterranean thorium and uranium plays a significant part in Earth’s high internal temperature. The energy stored in the planet’s thorium reserves may well be greater than all the energy available from conventional fossil and nuclear fuels combined. Thorium appears on Earth in an abundance of 15 parts per million (ppm), many times greater than the abundance of uranium. With its high energy levels, thorium has enormous potential as a nuclear fuel. When struck by neutrons, thorium-232 converts to uranium-233, one of the few known fissionable isotopes—that is, isotopes that can be split to start nuclear reactions.

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It is perhaps ironic that this element, with its potential for use in some of the most high-tech applications imaginable, is widely applied in a very low-tech fashion. In portable gas lanterns for camping and other situations without electric power, the mantle often contains oxides of thorium and cerium, which, when heated, emit a brilliant white light. Thorium is also used in the manufacture of high-quality glass, and as a catalyst in various industrial processes. PROTACTINIUM. Russian chemist Dmitri Ivanovitch Mendeleev (1834-1907), father of the periodic table, used the table’s arrangement of elements as a means of predicting the discovery of new substances: wherever he found a “hole” in the table, Mendeleev could say with assurance that a new element would eventually be found to fill it. In 1871, Mendeleev predicted the discovery of “eka-tantalum,” an element that filled the space below the transition metal tantalum. (At this point in history, just two years after Mendeleev created the periodic table, the lanthanides and actinides had not been separated from the rest of the elements on the chart.)

Forty years after Mendeleev foretold its existence, two German chemists found what they thought might be Element 91. It had a half-life of only 1.175 minutes, and, for this reason, they named it “brevium.” Then in 1918, Austrian physicist Lise Meitner (1878-1968)—who, along with Curie and French physicist Marguerite Perey (1909-1975) was one of several women involved in the discovery of radioactive elements—was working with German chemist Otto Hahn (1879-1968) when the two discovered another isotope of Element 91. This one had a much, much longer half-life: 3.25 • 104 years, or about five times as long as the entire span of human civilization. Originally named protoactinium, the name of Element 91 was changed to protactinium (Pa), whose longest-lived isotope has an atomic mass of 231. Shiny and malleable, protactinium has a melting point of 2,861.6°F (1,572°C). It is highly toxic, and so rare that no commercial uses have been found for it. Indeed, protactinium could only be produced from the decay products of uranium and radium, and thus it is one of the few elements with an atomic number less than 93 that cannot be said to occur in nature.

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Uranium Actinides URANIUM’S

EARLY

HISTORY.

Chemistry books, in fact, differ as to the number of naturally occurring elements. Some say 88, which is the most correct figure, because protactinium, along with technetium (43) and two others, cannot be said to appear naturally on Earth. Other books say 92, a less accurate figure that nonetheless reflects an indisputable fact: above uranium (U, 92) on the periodic table, there are no elements that generally occur in nature. (However, a few do occur as radioactive by-products of uranium.) But uranium is much more than just the last truly natural element, though for about a century it apparently had no greater importance. When German chemist Martin Heinrich Klaproth (1743-1817) discovered it in 1789, he named it after another recent discovery: the planet Uranus. During the next 107 years, uranium had a very quiet existence, befitting a rather dulllooking material. Though it is silvery white when freshly cut, uranium soon develops a thin coating of black uranium oxide, which turns it a flat gray. Yet glassmakers did at least manage to find a use for it—as a coating for decorative glass, to which it imparted a hazy, fluorescent yellowish green hue. Little did they know that they were using one of the most potentially dangerous substances on Earth. THE DESTRUCTIVE POWER OF URANIUM. In 1895, German physicist Wil-

helm Röntgen (1845-1923) noticed that photographic plates held near a Crookes tube—a device for analyzing electromagnetic radiation—became fogged. He dubbed the rays that had caused this x rays. A year later, in 1896, French physicist Henri Becquerel (1852-1908) left some photographic plates in a drawer with a sample of uranium, and discovered that the uranium likewise caused a fogging of the photographic plates. This meant that uranium was radioactive. With the development of nuclear fission by Hahn and German chemist Fritz Strassman in 1938, uranium suddenly became all-important because of its ability to undergo nuclear fission, accompanied by the release of huge amounts of energy. During World War II, in what was known as the Manhattan Project, a team of scientists in Los Alamos, New Mexico, developed the atomic

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Actinides

GLOVED

HANDS HOLD A GRAY LUMP OF URANIUM.

REMOVED FROM A

TITAN II MISSILE,

THIS

MATERIAL HAS BEEN REMOLDED AFTER HAVING BEEN

PART OF THE DISARMAMENT AFTER THE END OF THE

COLD WAR. (Martin Mariet-

ta; Roger Ressmeyer/Corbis. Reproduced by permission.)

bomb. The first of the two atomic bombs dropped on Japan in 1945 contained uranium, while the second contained the transuranium element plutonium.

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OTHER

USES

FOR

URANIUM.

Though nuclear weapons have fortunately not been used against human beings since 1945, uranium has remained an important component of

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nuclear energy—both in the development of bombs and in the peaceful application of nuclear power. It has other uses as well, due to the fact that it is extremely dense. Indeed, uranium has a density close to that of gold and platinum, but is much cheaper, because it is more abundant on Earth. In addition, various isotopes of uranium are a by-product of nuclear power, which separates these isotopes from the highly fissionable 235U isotope. Thus, quantities of uranium are available for use in situations where a great deal of mass is required in a small space: in counterweights for aircraft control systems, for instance, or as ballast for missile reentry vehicles. Because 238U has a very long half-life—4.47 • 109 years, or approximately the age of Earth—it is used to estimate the age of rocks and other geological features. Uranium-238 is the “parent” of a series of “daughter” isotopes that geologists find in uranium ores. Uranium-235 also produces “daughter” isotopes, including isotopes of radium, radon, and other radioactive series. Eventually, uranium isotopes turn into lead, but this can take a very long time: even 235U, which lasts for a much shorter period than 238U, has a half-life of about 700 million years. The radiation associated with various isotopes of uranium, as well as other radioactive materials, is extremely harmful. It can cause all manner of diseases and birth defects, and is potentially fatal. The tiny amounts of radiation produced by uranium in old pieces of decorative glass is probably not enough to cause any real harm, but the radioactive fallout from Hiroshima resulted in birth defects among the Japanese population during the late 1940s.

Transuranium Elements Transuranium elements are those elements with atomic numbers higher than that of uranium. None of these occur in nature, except as isotopes that develop in trace amounts in uranium ore. The first such element was neptunium (Np, 93), created in 1940 by American physicist Edwin Mattison McMillan (1907-1991) and American physical chemist Philip Hauge Abelson. The development of the cyclotron by American physicist Ernest Lawrence (1901-1958) at the University of California at Berkeley in the 1930s made possible the artificial creation of new elements. A cyclotron speeds up protons or ions

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(charged atoms) and shoots them at atoms of uranium or other elements with the aim of adding positive charges to the nucleus. In the first two decades after the use of the cyclotron to create neptunium, scientists were able to develop eight more elements, all the way up to mendelevium (Md, 101), named in honor of the man who created the periodic table.

Actinides

Most of these efforts occurred at Berkeley under the leadership of American nuclear chemist Glenn T. Seaborg (1912-1999) and American physicist Albert Ghiorso. The pace of development in transuranium elements slowed after about 1955, however, primarily due to the need for ever more powerful cyclotrons and ionaccelerating machines. In addition to Berkeley, there are two other centers for studying these high-energy elements: the Joint Institute for Nuclear Research in Dubna, Russia, and the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany.

The Transuranium Actinides PLUTONIUM. Just as neptunium had been named for the next planet beyond Uranus, a second transuranium element, discovered by Seaborg and two colleagues in 1940, was named after Pluto. Among the isotopes of plutonium (Pu, 94) is plutonium-239, one of the few fissionable isotopes other than uranium-233 and uranium-235. For that reason, it was applied in the second bomb dropped on Japan.

In addition to its application in nuclear weapons, plutonium is used in nuclear power reactors, and in thermoelectric generators, which convert the heat energy it releases into electricity. Plutonium is also used as a power source in artificial heart pacemakers. Huge amounts of the element are produced each year as a by-product of nuclear power reactors. BERKELEY DISCOVERIES OF THE 1950S. Because the lanthanide ele-

ment above it on the periodic table was named europium after Europe, americium (Am, 95) was named after America. Discovered by Seaborg, Ghiorso, and two others in 1944, it was first produced in a nuclear reaction involving plutonium-239. Americium radiation is used in measuring the thickness of glass during production; in addition, the isotope americium-241 is used as an ionization source in smoke detectors, and

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Actinides

KEY TERMS ACTINIDES: Those elements that fill the

same element, but differ in their number of

5f orbital. Because actinium—which does

neutrons. This results in a difference of

not fill the 5f orbital—exhibits characteris-

mass. Isotopes may be either stable or

tics similar to those of the actinides, it is

unstable. The unstable type, known as

usually considered part of the actinides

radioisotopes, are radioactive.

family. Of the other 14 actinides, usually shown at the bottom of the periodic table, only the first three occur in nature.

LANTHANIDES: The transition metals

that fill the 4f orbital.

The number of

MASS NUMBER: The sum of protons

protons in the nucleus of an atom. Since

and neutrons in the atom’s nucleus. The

this number is different for each element,

designation

elements are listed on the periodic table of

that this particular isotope of uranium has

elements in order of atomic number.

a mass number of 238. Since uranium has

ELECTRON CLOUD: A term used to

an atomic number of 92, this means that

describe the pattern formed by orbitals.

uranium-238 has 146 neutrons in its

HALF-LIFE: The length of time it takes a

nucleus.

substance to diminish to one-half its initial

NEUTRON:

amount. For a sample of radioisotopes, the

has no electric charge. Neutrons are found

half-life is the amount of time it takes for

at the nucleus of an atom, alongside

half of the nuclei to become stable iso-

protons.

ATOMIC

NUMBER:

238U,

for uranium-238, means

A subatomic particle that

topes. Half-life can be a few seconds; on the other hand, for uranium-238, it is a matter of several billion years. INNER TRANSITION METALS: The

lanthanides and actinides, which are

region where protons and neutrons are located, and around which electrons spin. The plural of “nucleus” is nuclei.

unique in that they fill the f orbitals.

ORBITAL:

For this reason, they are usually treated

regarding the position of an electron for an

separately.

atom in a particular energy state. The high-

ISOTOPES: Atoms that have an equal

er the principal energy level, the more

number of protons, and hence are of the

complex the pattern of orbitals.

in portable devices for taking gamma-ray photographs. Above curium (Cm, 96) on the periodic table is the lanthanide gadolinium, named after Finnish chemist Johan Gadolin (1760-1852). Therefore, the discoverers of Element 96 also decided to name it after a person, Marie Curie. As with some of the other relatively low-number

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NUCLEUS: The center of an atom, a

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A region of probabilities

transuranium elements, this one is not entirely artificial: its most stable isotope, some geologists believe, may have been present in rocks many millions of years ago, but these isotopes have long since decayed. Because curium generates great amounts of energy as it decays, it is used for providing compact sources of power in remote locations on Earth and in space vehicles.

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Actinides

KEY TERMS

CONTINUED

PRINCIPAL ENERGY LEVEL: A value

RADIOISOTOPE:

indicating the distance that an electron

the decay associated with radioactivity. A

may move away from the nucleus of an

radioisotope is thus an unstable isotope.

atom. This is designated by a whole-num-

SUBLEVEL: A region within the princi-

ber integer, beginning with 1 and moving

pal energy level occupied by electrons in an

upward. The higher the principal energy

atom. Whatever the number n of the prin-

level, the greater the energy in the atom,

cipal energy level, there are n sublevels.

and the more complex the pattern of

Actinides are distinguished by the fact that

orbitals. Elements in the transition metal

their valence electrons are in a sublevel

family have principal energy levels of 4, 5,

corresponding to the 5f orbital.

6, or 7.

TRANSITION METALS: A group of 40

RADIATION:

In a general sense, radia-

tion can refer to anything that travels in a stream, whether that stream be composed of subatomic particles or electromagnetic waves. In a more specific sense, the term relates to the radiation from radioactive materials, which can be harmful to human

An isotope subject to

elements (counting lanthanum and actinium), which are the only elements that fill the d orbital. In addition, the transition metals have their valence electrons on two different principal energy levels. Though the lanthanides and actinides are considered inner transition metals, they are usually considered separately.

beings. TRANSURANIUM ELEMENTS:

Ele-

A term describing a

ments with an atomic number higher than

phenomenon whereby certain isotopes,

that of uranium (92). These have all have

known as radioisotopes, are subject to a

been produced artificially. The transurani-

form of decay brought about by the emis-

um elements include 11 actinides, as well

sion of high-energy particles. “Decay” does

as 9 transition metals.

not mean that the isotope “rots”; rather, it

VALENCE

decays to form another isotope until even-

that occupy the highest principal energy

tually (though this may take a long time) it

level in an atom. These are the electrons

becomes stable.

involved in chemical bonding.

RADIOACTIVITY:

When Seaborg, Ghiorso, and others created Element 97, berkelium (Bk), they again took the naming of lanthanides as their cue. Just as terbium, directly above it on the periodic table, had been named for the Swedish town of Ytterby, where so many lanthanides were discovered, they named the new element after the American city where so many transuranium elements had been developed. Berkelium has no known applications

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ELECTRONS:

Electrons

outside of research. The Berkeley team likewise named californium (Cf, 98) after the state where Berkeley is located. Researchers today are studying the use of californium radiation for treatment of tumors involved in various forms of cancer. THE REMAINING TRANSURANIUM ACTINIDES. The remaining transura-

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nium actinides were all named after famous people: einsteinium (Es, 99) for Albert Einstein (1879-1955); fermium (Fm, 100) after ItalianAmerican physicist Enrico Fermi (1901-1954); mendelevium after Mendeleev; nobelium (No, 102) after Swedish inventor and philanthropist Alfred Nobel (1833-1896); and lawrencium (Lr, 103) after Ernest Lawrence. Both einsteinium and fermium were byproducts of nuclear testing at Bikini Atoll in the south Pacific in 1952. For this reason, their existence was kept a secret for two years. Neither element has a known application. The same is true of mendelevium, produced by Seaborg, Ghiorso, and others with a cyclotron at Berkeley in 1955, as well as the other two transuranium actinides.

Beyond the Transuranium Actinides As noted earlier, there are nine additional transuranium elements, which properly belong to the transition metals. The first of these is rutherfordium, discovered in 1964 by the Dubna team and named after Ernest Rutherford (18711937), the British physicist who discovered the nucleus. The Dubna team named dubnium, discovered in 1967, after their city, just as berkelium had been named after the Berkeley team’s city. Both groups developed versions of Element 106 in 1974, and both agreed to name it seaborgium after Seaborg, but this resulted in a controversy that was not settled for some time. The name of bohrium, created at Dubna in 1976, honors Danish physicist Niels Bohr (1885-

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1962), who developed much of the model of electron energy levels discussed earlier in this essay. Hassium, produced at the GSI in 1984, is named for the German state of Hess. Two years earlier, the GSI team also created the last named element on the periodic table, meitnerium (109), named after Meitner. Beyond meitnerium are three elements, as yet unnamed, created at the GSI in the mid-1990s. WHERE TO LEARN MORE Cooper, Dan. Enrico Fermi and the Revolutions in Modern Physics. New York: Oxford University Press, 1999. “Exploring the Table of Isotopes” (Web site). (May 15, 2001). Kidd, J. S. and Renee A. Kidd. Quarks and Sparks: The Story of Nuclear Power. New York: Facts on File, 1999. Knapp, Brian J. Elements. Illustrated by David Woodroffe and David Hardy. Danbury, CT: Grolier Educational, 1996. “A Periodic Table of the Elements” Los Alamos National Laboratory (Web site). (May 22, 2001). Sherrow, Victoria. The Making of the Atom Bomb. San Diego, CA: Lucent Books, 2000. “Some Physics of Uranium.” The Uranium Institute (Web site). (May 27, 2001). Stwertka, Albert. A Guide to the Elements. New York: Oxford University Press, 1998. Uranium Information Center (Web site). (May 27, 2001).

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Lanthanides

LANTHANIDES

CONCEPT Along the bottom of the periodic table of elements, separated from the main body of the chart, are two rows, the first of which represents the lanthanides. Composed of lanthanum and the 14 elements of the lanthanide series, the lanthanides were once called the “rare earth” metals. In fact, they are not particularly rare: many of them appear in as much abundance as more familiar elements such as mercury. They are, however, difficult to extract, a characteristic that defines them as much as their silvery color; sometimes high levels of reactivity; and sensitivity to contamination. Though some lanthanides have limited uses, members of this group are found in everything from cigarette lighters to TV screens, and from colored glass to control rods in nuclear reactors.

HOW IT WORKS Defining the Lanthanides The lanthanide series consists of the 14 elements, with atomic numbers 58 through 71, that follow lanthanum on the periodic table of elements. These 14, along with the actinides—atomic numbers 90 through 103—are set aside from the periodic table due to similarities in properties that define each group. Specifically, the lanthanides and actinides are the only elements that fill the f-orbitals. The lanthanides and actinides are actually “branches” of the larger family known as transition metals. The latter appear in groups 3 through 12 on the IUPAC version of the periodic table, though

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they are not numbered on the North American version. The lanthanide series is usually combined with lanthanum, which has an atomic number of 57, under the general heading of lanthanides. As their name indicates, members of the lanthanide series share certain characteristics with lanthanum; hence the collective term “lanthanides.” These 15 elements, along with their chemical symbols, are: • • • • • • • • • • • • • • •

Lanthanum (La) Cerium (Ce) Praseodymium (Pr) Neodymium (Nd) Promethium (Pm) Samarium (Sm) Europium (Eu) Gadolinium (Gd) Terbium (Tb) Dysprosium (Dy) Holmium (Ho) Erbium (Er) Thulium (Tm) Ytterbium (Yb) Lutetium (Lu) Most of these are discussed individually in this essay. PROPERTIES OF LANTHANIDES. Bright and silvery in appearance,

many of the lanthanides—though they are metals—are so soft they can be cut with a knife. Lanthanum, cerium, praseodymium, neodymium, and europium are highly reactive. When exposed to oxygen, they form an oxide coating. (An oxide is a compound formed by metal with an oxygen.) To prevent this result, which tarnishes the

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Lanthanides

MISCH

METAL, WHICH INCLUDES CERIUM, IS OFTEN USED IN CIGARETTE LIGHTERS. (Massimo Listri/Corbis. Reproduced by

permission.)

metal, these five lanthanides are kept stored in mineral oil. The reactive tendencies of the other lanthanides vary: for instance, gadolinium and lutetium do not oxidize until they have been exposed to air for a very long time. Nonetheless, lanthanides tend to be rather “temperamental” as a class. If contaminated with other metals, such as calcium, they corrode easily, and if contaminated with nonmetals, such as nitrogen or oxygen, they become brittle. Contamination also alters their boiling points, which range from 1,506.2°F (819°C) for ytterbium to 3,025.4°F (1,663°C) for lutetium. Lanthanides react rapidly with hot water, or more slowly with cold water, to form hydrogen gas. As noted earlier, they also are quite reactive with oxygen, and they experience combustion readily in air. When a lanthanide reacts with another element to form a compound, it usually loses three of its outer electrons to form what are called tripositive ions, or atoms with an electric charge of +3. This is the most stable ion for lanthanides, which sometimes develop less stable +2 or +4 ions. Lanthanides tend to form ionic compounds, or compounds containing either positive or negative ions, with other substances—in particular, fluorine.

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Are They Really “Rare”? Though they were once known as the rare earth metals, lanthanides were so termed because, as we shall see, they are difficult to extract from compounds containing other substances— including other lanthanides. As for rarity, the scarcest of the lanthanides, thulium, is more abundant than either arsenic or mercury, and certainly no one thinks of those as rare substances. In terms of parts per million (ppm), thulium has a presence in Earth’s crust equivalent to 0.2 ppm. The most plentiful of the lanthanides, cerium, has an abundance of 46 ppm, greater than that of tin. If, on the other hand, rarity is understood not in terms of scarcity, but with regard to difficulty in obtaining an element in its pure form, then indeed the lanthanides are rare. Because their properties are so similar, and because they are inclined to congregate in the same substances, the original isolation and identification of the lanthanides was an arduous task that took well over a century. The progress followed a common pattern. First, a chemist identified a new lanthanide; then a few years later, another scientist came along and extracted another lanthanide from the

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sample that the first chemist had believed to be a single element. In this way, the lanthanides emerged over time, each from the one before it, rather like Russian matryoshka or “nesting” dolls. EXTRACTING

Lanthanides

LANTHANIDES.

Though most of the lanthanides were first isolated in Scandinavia, today they are found in considerably warmer latitudes: Brazil, India, Australia, South Africa, and the United States. The principal source of lanthanides is monazite, a heavy, dark sand from which about 50% of the lanthanide mass available to science and industry has been extracted. In order to separate lanthanides from other elements, they are actually combined with other substances—substances having a low solubility, or tendency to dissolve. Oxalates and fluorides are low-solubility substances favored for this purpose. Once they are separated from non-lanthanide elements, ion exchange is used to separate one lanthanide element from another. There is a pronounced decrease in the radii of lanthanide atoms as they increase in atomic number: in other words, the higher the atomic number, the smaller the radius. This decrease, known as the lanthanide contraction, aids in the process of separation by ion exchange. The lanthanides are mixed in an ionic solution, then passed down a long column containing a resin. Various lanthanide ions bond more or less tightly, depending on their relative size, with the resin. After this step, the lanthanides are washed out of the ion exchange column and into various solutions. One by one, they become fully separated, and are then mixed with acid and heated to form an oxide. The oxide is then converted to a fluoride or chloride, which can then be reduced to metallic form with the aid of calcium.

REAL-LIFE AP P LICATIONS The Historical Approach In studying the lanthanides, one can simply move along the periodic table, from lanthanum all the way to lutetium. However, in light of the difficulties involved in extracting the lanthanides, one from another, an approach along historical lines aids in understanding the unique place each lanthanide occupies in the overall family.

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SAMARIUM

IS USED IN NUCLEAR POWER PLANT CON-

TROL RODS, SUCH AS THE ONE SHOWN HERE. (Tim

Wright/Corbis. Reproduced by permission.)

The terms “lanthanide series” or even “lanthanides” did not emerge for some time—in other words, scientists did not immediately know that they were dealing with a whole group of metals. As is often the case with scientific discovery, the isolation of lanthanides followed an irregular pattern, and they did not emerge in order of atomic number. Cerium was in fact discovered long before lanthanum itself, in the latter half of the eighteenth century. There followed, a few decades later, the discovery of a mineral called ytterite, named after the town of Ytterby, Sweden, near which it was found in 1787. During the next century, most of the remaining lanthanides were extracted from ytterite, and the man most responsible for this was Swedish chemist Carl Gustav Mosander (1797-1858). Because Mosander had more to do with the identification of the lanthanides than any one individual, the middle portion of this historical overview is devoted to his findings. The recognition and isolation of lanthanides did not stop with Mosander, however; therefore another

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group of minerals is discussed in the context of the latter period of lanthanide discovery.

Early Lanthanides CERIUM. In 1751, Swedish chemist Axel Crönstedt (1722-1765) described what he thought was a new form of tungsten, which he had found at the Bastnäs Mine near Riddarhyttan, Sweden. Later, German chemist Martin Heinrich Klaproth (1743-1817) and Swedish chemist Wilhelm Hisinger (1766-1852) independently analyzed the material Crönstedt had discovered, and both concluded that this must be a new element. It was named cerium in honor of Ceres, an asteroid between Mars and Jupiter discovered in 1801. Not until 1875 was cerium actually extracted from an ore.

Among the applications for cerium is an alloy called misch metal, prepared by fusing the chlorides of cerium, lanthanum, neodymium, and praseodymium. The resulting alloy ignites at or below room temperature, and is often used as the “flint” in a cigarette lighter, because it sparks when friction from a metal wheel is applied. Cerium is also used in jet engine parts, as a catalyst in making ammonia, and as an antiknock agent in gasoline—that is, a chemical that reduces the “knocking” sounds sometimes produced in an engine by inferior grades of fuel. In cerium (IV) oxide, or CeO2, it is used to extract the color from formerly colored glass, and is also applied in enamel and ceramic coatings. GADOLINIUM. In 1794, seven years after the discovery of ytterite, Finnish chemist Johan Gadolin (1760-1852) concluded that ytterite contained a new element, which was later named gadolinite in his honor. A very similar name would be applied to an element extracted from ytterite, and the years between Gadolin’s discovery and the identification of this element spanned the period of the most fruitful activity in lanthanide identification.

During the next century, all the other lanthanides were discovered within the composition of gadolinite; then, in 1880, Swiss chemist JeanCharles Galissard de Marignac (1817-1894) found yet another element hiding in it. French chemist Paul Emile Lecoq de Boisbaudran (18381912) rediscovered the same element six years later, and proposed that it be called gadolinium.

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Silvery in color, but with a sometimes yellowish cast, gadolinium has a high tendency to oxidize in dry air. Because it is highly efficient for capturing neutrons, it could be useful in nuclear power reactors. However, two of its seven isotopes are in such low abundance that it has had little nuclear application. Used in phosphors for color television sets, among other things, gadolinium shows some promise for ultra hightech applications: at very low temperatures it becomes highly magnetic, and may function as a superconductor.

Mosander’s Lanthanides LANTHANUM. Between 1839 and 1848, Mosander was consumed with extracting various lanthanides from ytterite, which by then had come to be known as gadolinite. When he first succeeded in extracting an element, he named it lanthana, meaning “hidden.” The material, eventually referred to as lanthanum, was not prepared in pure form until 1923.

Like a number of other lanthanides, lanthanum is very soft—so soft it can be cut with a knife—and silvery-white in color. Among the most reactive of the lanthanides, it decomposes rapidly in hot water, but more slowly in cold water. Lanthanum also reacts readily with oxygen, and corrodes quickly in moist air. As with cerium, lanthanum is used in misch metal. Because lanthanum compounds bring about special optical qualities in glass, it also used for the manufacture of specialized lenses. In addition, compounds of lanthanum with fluorine or oxygen are used in making carbon-arc lamps for the motion picture industry. SAMARIUM. While analyzing an oxide formed from lanthanide in 1841, Mosander decided that he had a new element on his hands, which he called didymium. Four decades later, Boisbaudran took another look at didynium, and concluded that it was not an element; rather, it contained an element, which he named samarium after the mineral samarskite, in which it is found. Still later, Marignac was studying samarskite when he discovered what came to be known as gadolinium. But the story did not end there: even later, in 1901, French chemist EugèneAnatole Demarçay (1852-1903) found yet another element, europium, in samarskite.

Samarium is applied today in nuclear power plant control rods, in carbon-arc lamps, and in

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optical masers and lasers. In alloys with cobalt, it is used in manufacturing the most permanent electromagnets available. Samarium is also utilized in the manufacture of optical glass, and as a catalyst in the production of ethyl alcohol. ERBIUM AND TERBIUM. To return to Mosander, he was examining ytterite in 1843 when he identified three different “earths,” all of which he also named after Ytterby: yttria, erbia, and terbia. Erbium was the first to be extracted. A pure sample of its oxide was prepared in 1905 by French chemist Georges Urbain (1872-1938) and American chemist Charles James (18801928), but the pure metal itself was only extracted in 1934.

Soft and malleable, with a lustrous silvery color, erbium produces salts (which are usually combinations of a metal with a nonmetal) that are pink and rose, making it useful as a tinting agent. One of its oxides is utilized, for instance, to tint glass and porcelain with a pinkish cast. It is also applied, to a limited extent, in the nuclear power industry. Mosander also identified another element, terbium, in ytterite in 1839, and Marignac isolated it in a purer form nearly half a century later, in 1886. To repeat a common theme, it is silverygray and soft enough to be cut with a knife. When hit by an electron beam, a compound containing terbium emits a greenish color, and thus it is used as a phosphor in color television sets.

Later Isolation of Lanthanides YTTERBIUM, HOLMIUM, AND THULIUM. For many years after Mosander,

there was little progress in the discovery of lanthanides, and when it came, it was in the form of a third element, named after the town where so many of the lanthanides were discovered. In 1878, while analyzing what Mosander had called erbia, Marignac realized that it contained one or possibly two elements. A year later, Swedish chemist Lars Frederik Nilson (1840-1899) concluded that it did indeed contain two elements, which were named ytterbium and scandium. (Scandium, with an atomic number of 21, is not part of the lanthanide series.) Urbain is sometimes credited for discovering ytterbium: in 1907, he showed that the materials Nilson had studied were actually a mixture of two oxides. In any case, Urbain said that

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Lanthanides

KEY TERMS

ALLOY:

A mixture of two or more

metals. The number of protons in the nucleus of an atom. Since this number is different for each element, elements are listed on the periodic table of elements in order of atomic number. ATOMIC

NUMBER:

An atom or atoms that has lost or gained one or more electrons, and thus has a net electrical charge. ION:

A progressive decrease in the radius of lanthanide atoms as they increase in atomic number. LANTHANIDE

CONTRACTION:

LANTHANIDE SERIES: A group of 14 elements, with atomic numbers 58 through 71, that follow lanthanum on the periodic table of elements.

The lanthanide series, along with lanthanum. LANTHANIDES:

OXIDE: A compound formed by the chemical bonding of a metal with oxygen. PERIODIC TABLE OF ELEMENTS: A

chart showing the elements arranged in order of atomic number, grouping them according to common characteristics. RARE EARTH METALS: An old name for the lanthanides, reflecting the difficulty of separating them from compounds containing other lanthanides or other substances.

Groups 3 through 12 on the IUPAC or European version of the periodic table of elements. The lanthanides and actinides, which appear at the bottom of the periodic table, are “branches” of this family. TRANSITION

METALS:

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the credit should be given to Marignac, who is the most important figure in the history of lanthanides other than Mosander. As for ytterbium, it is highly malleable, like other lanthanides, but does not have any significant applications in industry. Swedish chemist Per Teodor Cleve (18401905) found in 1879 that erbia contained two more elements, which he named holmium and thulium. Thulium refers to the ancient name for Scandinavia, Thule. Rarest of all the lanthanides, thulium is highly malleable—and also highly expensive. Hence it has few commercial applications. DYSPROSIUM. Named for the Greek word dysprositos, or “hard to get at,” dysprosium was discovered by Boisbaudran. Separating ytterite in 1886, he found gallium (atomic number 31—not a lanthanide); samarium (discussed above); and dysprosium. Yet again, a mineral extracted from ytterite had been named after a previously discovered element, and, yet again, it turned out to contain several elements. The substance in question this time was holmium, which, as Boisbaudran discovered, was actually a complex mixture of terbium, erbium, holmium, and the element he had identified as dysprosium. A pure sample was not obtained until 1950.

Because dysprosium has a high affinity for neutrons, it is sometimes used in control rods for nuclear reactors, “soaking up” neutrons rather as a sponge soaks up water. Soft, with a lustrous silver color like other lanthanides, dysprosium is also applied in lasers, but otherwise it has few uses. EUROPIUM

AND

LUTETIUM.

Whereas many other lanthanides are named for regions in northern Europe, the name for europium refers to the European continent as a whole, and that of lutetium is a reference to the old Roman name for Paris. As mentioned earlier,

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Demarçay found europium in samarskite, a discovery he made in 1901. Actually, Boisbaudran had noticed what appeared to be a new element about a decade previously, but he did not pursue it, and thus the credit goes to his countryman. Most reactive of the lanthanides, europium responds both to cold water and to air. In addition, it is capable of catching fire spontaneously. Among the most efficient elements for the capture of neutrons, it is applied in the control systems of nuclear reactors. In addition, its compounds are utilized in the manufacture of phosphors for TV sets: one such compound, for instance, emits a reddish glow. Yet another europium compound is added to the glue on postage stamps, making possible the electronic scanning of stamps. Urbain, who discovered lutetium, named it after his hometown. James also identified a form of the lanthanide, but did not announce his discovery until much later. Except for some uses at a catalyst in the production of petroleum, lutetium has few industrial applications.

WHERE TO LEARN MORE Cotton, Simon. Lanthanides and Actinides. New York: Oxford University Press, 1991. Heiserman, David L. Exploring Chemical Elements and Their Compounds. Blue Ridge Summit, PA: Tab Books, 1992. “Luminescent Lanthanides” (Web site). (May 16, 2001). Snedden, Robert. Materials. Des Plaines, IL: Heinemann Library, 1999. Oxlade, Chris. Metal. Chicago: Heinemann Library, 2001. Stwertka, Albert. A Guide to the Elements. New York: Oxford University Press, 1996. Whyman, Kathryn. Metals and Alloys. Illustrated by Louise Nevett and Simon Bishop. New York: Gloucester Press, 1988.

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S C I E N C E O F E V E RY DAY T H I N G S r e a l - l i f e CHEMISTRY

N O N M E TA L S A N D M E TA L L O I D S N O N M E TA L S M E TA L L O I D S HALOGENS NOBLE GASES CARBON HYDROGEN

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Nonmetals

CONCEPT Nonmetals, as their name implies, are elements that display properties quite different from those of metals. Generally, they are poor conductors of heat and electricity, and they are not ductile: in other words, they cannot be easily reshaped. Included in this broad grouping are the six noble gases, the five halogens, and eight “orphan” elements. Two of these eight—hydrogen and carbon—are so important that separate essays are devoted to them. Two more, addressed in this essay, are absolutely essential to human life: oxygen and nitrogen. Hydrogen and helium, a nonmetal of the noble gas family, together account for about 99% of the mass of the universe, while Earth and the human body are composed primarily of oxygen, with important components of carbon, nitrogen, and hydrogen. Indeed, much of life—human, animal, and plant—can be summed up with these four elements, which together make Earth different from any other known planet. Among the other “orphan” nonmetals are phosphorus and sulfur, the “brimstone” of the Bible, as well as boron and selenium.

N O N M E TA L S

losing electrons. The vast majority of elements— 87 in all—are metals, and these occupy the left, center, and part of the right-hand side of the periodic table. With the exception of hydrogen, placed in Group 1 above the alkali metals, the nonmetals fill a triangle-shaped space in the upper right corner of the periodic table. All gaseous elements are nonmetals, a group that also includes some solids, as well as bromine, the only element other than mercury that is liquid at room temperature. In general, the nonmetals are characterized by properties opposite those of metals. They are poor conductors of heat and electricity (though carbon, a good electrical conductor, is an exception), and they tend to form negative ions by gaining electrons. They are not particularly malleable, and whereas most metals are shiny, nonmetals may be dull-colored, black (in the case of carbon in some forms or allotropes), or invisible, as is the case with most of the gaseous nonmetals.

Grouping the Nonmetals

The majority of elements on the periodic table are metals: solids (along with one liquid, mercury) which are lustrous or shiny in appearance. Metals are ductile, or malleable, meaning that they can be molded into different shapes without breaking. They are excellent conductors of heat and electricity, and tend to form positive ions by

Whereas the metals can be broken down into five families, along with seven “orphans”—elements that do not fit into a family grouping—the nonmetals are arranged in two families, as well as eight “orphans.” Hydrogen, because of its great abundance in the universe and its importance to chemical studies, is addressed in a separate essay within this book. The same is true of carbon: though not nearly as abundant as hydrogen, carbon is a common element of all living things, and therefore it is discussed in the essay on Carbons and Organic Chemistry.

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HOW IT WORKS Defining the Nonmetals

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Nonmetals

LIQUID

NITROGEN ACCOUNTS FOR ABOUT ONE-THIRD OF ALL COMMERCIAL USES OF NITROGEN.

HERE,

A MEDICAL

WORKER PUTS A SAMPLE OF BONE MARROW INTO A TANK OF LIQUID NITROGEN FOR PRESERVATION. (Leif Skoogfors/

Corbis. Reproduced by permission.)

The other six “orphan” nonmetals, which will be examined in detail later in this essay, are listed below by atomic number: • • • • • •

5. Boron (B) 7. Nitrogen (N) 8. Oxygen (O) 15. Phosphorus (P) 16. Sulfur (S) 34. Selenium (Se)

T H E N O B L E GAS E S . The noble gases, discussed in detail within an essay devoted to that subject, are listed below by atomic number:

• • • • • •

2. Helium (He) 10. Neon (Ne) 18. Argon (Ar) 36. Krypton (Kr) 54. Xenon (Xe) 86. Radon (Rn) Occupying Group 8 of the North American periodic table, the noble gases—with the exception of helium—have valence electron configurations of ns2np6. This means that they have two valence electrons (the electrons involved in chemical bonding) on the orbital designated as s, and six more on the p orbital. As for n, this des-

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ignates the energy level, a number that corresponds to the period or row that an element occupies on the periodic table. Most elements bond according to what is known as the octet rule, forming a shell composed of eight electrons. Since the noble gases already have eight electrons on their shell, they tend not to bond with other elements: hence the name “noble,” which in this context means “set apart.” Helium, on the other hand, has a valence shell of s2; however, it too is characterized by a lack of reactivity, and therefore is included in the noble gas family. Noble gases are all monatomic, meaning that they exist as individual atoms, rather than in molecules. To put it another way, their molecules are single atoms. (By contrast, atoms of oxygen, for instance, usually combine to form a diatomic molecule, designated O2.) Due to their apparent lack of reactivity, the noble gases—also known as the rare gases—were once called the inert (inactive) gases. Indeed, helium, neon, and argon have not been found to combine with other elements to form compounds. However, in 1962, English chemist Neil Bartlett (1932-) succeeded in preparing a compound of xenon with platinum and fluorine (XePtF6), thus overturning the idea that the

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AT

AN

“OXYGEN BAR,” YOU CAN RELAX BY BREATHING JAPAN. (AFP/Corbis. Reproduced by permission.)

OXYGEN.

HERE,

A WOMAN IS BREATHING FLAVORED OXYGEN

AT A BAR IN

noble gases were entirely “inert.” Since that time, numerous compounds of xenon with other elements, most notably oxygen and fluorine, have been developed. Fluorine has also been used to form simple compounds with krypton and radon. T H E H A LO G E N S . Next to the noble

gases, in Group 7, are the halogens, a family discussed in a separate essay. These are listed below, along with atomic number and chemical symbol. Note that astatine is sometimes included with the metalloids, elements that display characteristics both of metals and nonmetals. • • • • •

9. Fluorine (F) 17. Chlorine (Cl) 35. Bromine (Br) 53. Iodine (I) 85. Astatine (At) The halogens all have valence electron configurations of ns2np5: in other words, at any energy level n, they have two valence electrons in the s orbital, and 5 in the p orbital. In terms of phase of matter, the halogens are the most varied family on the periodic table. Fluorine and chlorine are gases, iodine is a solid, and bromine (as noted earlier) is one of only two elements existing at room temperature as a liquid. As for asta-

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tine, it is a solid too, but so highly radioactive it is hard to determine much about its properties. (The only other nonmetal that has a large number of radioactive isotopes is the noble gas radon, considered highly dangerous.) Though they are “next door” to the noble gases, the halogens could not be more different. Whereas their neighbors to the right are the least reactive elements on the periodic table, the halogens are the most reactive. Indeed, fluorine has the highest possible value of electronegativity, the relative ability of an atom to attract valence electrons. It is therefore one of the only elements that will bond with a noble gas. All of the halogens tend to form salts, compounds—formed, along with water, from the reaction of an acid and base—that bring together a metal and a nonmetal. Due to this tendency, the first of the family to be isolated—chlorine, in 1811—was originally named “halogen,” a combination of the Greek words halos, or salt, and gennan, “to form or generate.” In their pure form, halogens are diatomic, and in contact with other elements, they form ionic bonds, which are the strongest form of chemical bond. In the process of bonding, they become negatively charged ions, or anions.

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Abundance of the Nonmetals Nonmetals I N T H E U N I V E R S E . As is stated many times throughout this book, humans may be created equal, but the elements are not. Though 88 elements exist in nature, this certainly does not mean that each occupies a 1/88 share of the total pie. Just two elements—the nonmetals hydrogen and helium, which occupy the first two positions on the periodic table— account for 99.9% of the matter in the entire universe, yet the percentage of matter they make up on Earth is very small.

The reason for this disparity is that, whereas our own planet (as far as we know) is unique in sustaining life—requiring oxygen, carbon, nitrogen, and other elements—the vast majority of the universe is made up of stars, composed primarily of hydrogen and helium. O N E A RT H A N D I N T H E AT M O S P H E R E . Nonmetals account for a large

portion of Earth’s total known elemental mass— that is, the composition of the planet’s crust, waters, and atmosphere. Listed below are figures ranking nonmetals within the overall picture of elements on Earth. The numbers following each element’s name indicate the percentage of each within the planet’s total known elemental mass. • • • • • • • •

1. Oxygen (49.2%) 9. Hydrogen (0.87%) 11. Chlorine (0.19%) 12. Phosphorus (0.11%) 14. Carbon (0.08%) 15. Sulfur (0.06%) 17. Nitrogen (0.03%) 18. Fluorine (0.03%) Some of the figures above may seem rather small, but in fact only 18 elements account for all but 0.49% of the planet’s known elemental mass, the remainder being composed of numerous other elements in small quantities. In Earth’s atmosphere, the composition is all nonmetallic, as one might expect: • • • •

1. Nitrogen (78%) 2. Oxygen (21%) 3. Argon (0.93%) 4. Various trace gases, including water vapor, carbon dioxide, and ozone or O3 (0.07%) I N T H E H U M A N B O DY. As noted earlier, carbon is a component in all living things, and it constitutes the second-most abundant ele-

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ment in the human body. Together with oxygen and hydrogen, it accounts for 93% of the body’s mass. Listed below are the most abundant nonmetals in the human body, by ranking. • • • • • • •

1. Oxygen (65%) 2. Carbon (18%) 3. Hydrogen (10%) 4. Nitrogen (3%) 6. Phosphorus (1.0%) 9. Sulfur (0.26%) 11. Chlorine (0.14%)

REAL-LIFE A P P L I C AT I O N S Boron The first “orphan” nonmetal, by atomic number, is boron, named after the Arabic word buraq or the Persian burah. (It is thus unusual in being one of the only elements whose name is not based on a word from a European language, or— for the later elements—the name of a person or place.) It was discovered in 1808 by English chemist Sir Humphry Davy (1778-1829), a man responsible for identifying or isolating numerous elements; French physicist and chemist Joseph Gay-Lussac (1778-1850), known in part for the gas law named after him; and French chemist Louis Jacques Thénard (1777-1857). These scientists used the reaction of boric acid (H3BO3) with potassium to isolate the element. Sometimes classified as a metalloid because it is a semiconductor of electricity, boron is applied in filaments used in fiber optics research. Few of its other applications, however, have much to do with electrical conductivity. Boric or boracic acid is used as a mild antiseptic, and in North America, it is applied for the control of cockroaches, silverfish, and other pests. A compound known as borax is a water softener in washing powders, while other compounds are used to produce enamels for the coating of refrigerators and other appliances. Compounds involving boron are also present in pyrotechnic flares, because they emit a distinctive green color, and in the igniters of rockets.

Nitrogen Though most people think of air as consisting primarily of oxygen, in fact—as noted above— the greater part of the air we breathe is made up

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of nitrogen. Scottish chemist David Rutherford (1749-1819) is usually given credit for discovering the element: in 1772, he identified nitrogen as the element that remained when oxygen was removed from air. Several other scientists around the same time made a similar discovery. Because of its heavy presence in air, nitrogen is obtained primarily by cooling air to temperatures below the boiling points of its major components. Nitrogen boils (that is, turns into a gas) at a lower temperature than oxygen: -320.44°F (-195.8°C), as opposed to -297.4°F (-183°C). When air is cooled to -328°F (-200°C) and then allowed to warm slowly, the nitrogen boils first, and therefore evaporates first. The nitrogen gas is captured, cooled, and liquefied once more. Nitrogen can also be obtained from compounds such as potassium nitrate or saltpeter, found primarily in India; or sodium nitrate (Chilean saltpeter), which comes from the desert regions of Chile. To isolate nitrogen, various processes are undertaken in a laboratory—for instance, heating barium azide or sodium azide, both of which contain nitrogen. R E AC T I O N S W I T H O T H E R E L E M E N T S . Existing as diatomic molecules,

nitrogen forms very strong triple bonds, and as a result tends to be fairly unreactive at low temperatures. Thus, for instance, when a substance burns, it reacts with the oxygen in the air, but not with the nitrogen. At high temperatures, however, nitrogen combines with other elements, reacting with metals to form nitrides; with hydrogen to form ammonia; with O2 to form nitrites; and with O3 to form nitrates. Nitrogen and oxygen, in particular, react at high temperatures to form numerous compounds: nitric oxide (NO), nitrous oxide (N2O), nitrogen dioxide (NO2), dinitrogen trioxide (N2O3), and dinitrogen pentoxide (N2O5). In reaction with halogens, nitrogen forms unstable, explosive compounds such as nitrogen trifluoride (NF3), nitrogen trichloride (NCl3), and nitrogen triiodide (NI3). One thing is for sure: never mix ammonia with bleach, which involves the halogen chlorine. Together, these two produce chloramines, gases with poisonous vapors. U S E S F O R N I T R O G E N . One of the most striking scenes in a memorable film, “Terminator 2: Judgment Day” (1991), occurs near the end of the movie, when the villainous T1000 robot steps into a pool of liquid nitrogen

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and cracks to pieces. As noted, nitrogen must be at extremely low temperatures to assume liquid form. Liquid nitrogen, which accounts for about one-third of all commercial uses of the element, is applied for quick-freezing foods, and for preserving foods in transit. Liquid nitrogen also makes it possible to process materials, such as some forms of rubber, that are too pliable for machining at room temperature. These materials are first cooled in liquid nitrogen, and then become more rigid.

Nonmetals

In processing iron or steel, which forms undesirable oxides if exposed to oxygen, a blanket of nitrogen is applied to prevent this reaction. The same principle is applied in making computer chips and even in processing foods, since these items too are detrimentally affected by oxidation. Because it is far less combustible than air (magnesium is one of the few elements that burns nitrogen in combustion), nitrogen is also used to clean tanks that have carried petroleum or other combustible materials. As noted, nitrogen combines with hydrogen to form ammonia, used in fertilizers and cleaning materials. Ammonium nitrate, applied primarily as a fertilizer, is also a dangerous explosive, as shown with horrifying effect in the bombing of the Alfred P. Murrah Federal Building in Oklahoma City on April 19, 1995—a tragedy that took 168 lives. Nor is ammonium nitrate the only nitrogen-based explosive. Nitric acid is used in making trinitrotoluene (TNT), nitroglycerin, and dynamite, as well as gunpowder and smokeless powder. NITROGEN, THE ENVIRONM E N T, A N D H E A LT H . The nitrogen

cycle is the process whereby nitrogen passes from the atmosphere into living things, and ultimately back into the atmosphere. Both through the action of lightning in the sky, and of bacteria in the soil, nitrogen is converted to nitrates and nitrites—compounds of nitrogen and oxygen. These are then absorbed by plants to form plant proteins, which convert to animal proteins in the bodies of animals who eat the plants. When an animal dies, the proteins are returned to the soil, and denitrifying bacteria break down these compounds, returning elemental nitrogen to the atmosphere. Not all nitrogen in the atmosphere is healthful, however. Oxides of nitrogen, formed in the high temperatures of internal combustion

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engines, pass into the air as nitric oxide. This compound reacts readily with oxygen in the air to form nitrogen dioxide, a toxic reddish-brown gas that adds to the tan color of smog over major cities. Another health concern is posed by sodium nitrate and sodium nitrite, added to bacon, sausage, hot dogs, ham, bologna and other food products to inhibit the growth of harmful microorganisms. Many researchers believe that nitrites impair the ability of a young child’s blood to carry oxygen. Furthermore, nitrites often combine with amines, a form of organic compound, to create a variety of toxins known as nitrosoamines. Because of concerns about these dangers, scientists and health activists have called for a ban on the use of nitrites and nitrates as food additives.

Oxygen Discovered independently by Swedish chemist Carl W. Scheele (1742-1786) and English chemist Joseph Priestley (1733-1804) in the period 17731774, oxygen was named by a third scientist, French chemist Antoine Lavoisier (1743-1794). Believing (incorrectly) that all acids contained the newly discovered element, Lavoisier called it oxygen, which comes from a French word meaning “acid-former.” Like many elements, oxygen has been in use since the beginning of time. But this is quite different from saying, for instance, that iron has been in use since the early stages of human history. A person can live without iron (except for the necessary quantities in the human body), and can survive for weeks or even months without food. One can live for a few days without water (the most famous and plentiful of all oxygencontaining compounds); but one cannot survive for more than a few minutes without the oxygen in air. Oxygen appears in three allotropes (different versions of the same element, distinguished by molecular structure): monatomic oxygen (O); diatomic oxygen or O2; and triatomic oxygen (O3), better known as ozone. The diatomic form dominates the natural world, but in the upper atmosphere, ozone forms a protective layer that keeps the Sun’s harmful ultraviolet radiation from reaching Earth. Concerns that chlorofluorocarbons (CFCs) may be depleting the ozone layer by converting these triatomic molecules to

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O2 has led to a reduction in the output of CFCs by industrialized nations. O B TA I N I N G OX Y G E N . Oxygen, of

course, is literally “in the air,” mixed with larger quantities of nitrogen. Higher up in the atmosphere, it occurs as a free element. Through electrolysis,, it can be obtained from water; however, this process is prohibitively expensive for most commercial applications. Oxygen-containing compounds are also sources of oxygen for commercial use, but generally oxygen is obtained by the fractional distillation of liquid air, described above with regard to nitrogen. After the nitrogen has been separated, argon and neon (which also have lower boiling points than oxygen) also boil off, leaving behind an impure form of oxygen. This is further purified by a process of cooling, liquefying, and evaporation, which eliminates traces of noble gases such as krypton and xenon. Many millions of years ago, when Earth was first formed, there was no oxygen on the planet. The growth of oxygen on Earth coincided with the development of organisms that, as they evolved, increasingly needed oxygen. The present concentration of oxygen in the atmosphere, oceans, and the rocks of Earth’s crust was reached about 580 million years ago, and is sustained today by biological activity. When plants undergo photosynthesis, carbon dioxide and water react in the presence of chlorophyll to produce carbohydrates and oxygen. OXIDES AND OTHER COMP O U N D S . Though the bond in diatomic

oxygen is strong, once it is broken, monatomic oxygen reacts readily with other elements to form a seemingly limitless range of compounds: oxides, silicates, carbonates, phosphates, sulfates, and other more complex substances. The process known as oxidation results in the formation of numerous oxides. Sometimes oxygen and another element form several oxides, as for example in the case of nitrogen, whose five oxides are listed above. Water is an oxide; so too are carbon dioxide and carbon monoxide. When animals and plants die, the organic materials that make them up react with oxygen in the air, resulting in a complex form of oxidation known as decay—or, in common language, “rotting.” Oxygen, reacting with compounds such as hydrocarbons, produces carbon dioxide and water vapor at high temperatures. If the oxida-

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tion process is extremely rapid, and takes place at high temperatures, it is usually identified as combustion. In addition, oxygen reacts with iron and other metals to form oxides. Many of these oxides, commonly known as rust, are undesirable. Every year, millions upon millions of dollars are spent on painting metal structures, or for other precautions to protect against the formation of metallic oxides. On the other hand, metallic oxides may be produced deliberately for applications in materials such as mortar color, to enhance the appearance of a brick building. U S E S O F OX Y G E N . Aside from the obvious application of oxygen for breathing, there are four major fields that make use of this element: medicine, metallurgy, rocketry, and the field of chemistry concerned with chemical synthesis. The medical application is closest to how we normally use oxygen in our daily lives. In oxygen therapy, a patient having difficulty breathing is given doses of pure, or nearly pure, oxygen. This is used during surgical procedures, and to treat patients who have had heart attacks, as well as those suffering from various infectious or respiratory diseases.

The use of oxygen in metallurgy involves refining coke, which is almost pure carbon, to make carbon monoxide. Carbon monoxide, in turn, reduces iron oxides to pure metallic iron. Oxygen is also used in blast furnaces to convert pig iron to steel by removing excess carbon, silicon, and metallic impurities. In addition, oxygen is applied in torches for welding and cutting. In the form of liquefied oxygen, or LOX, oxygen is used in rockets and missiles. The space shuttle, for instance, carries a huge internal tank containing oxygen and hydrogen, which, when they react, give the vehicle enormous thrust. In chemical synthesis—the preparation of compounds (especially organic ones) from easily available chemicals—commercial chemists use oxygen, for instance, to loosen the bonds in hydrocarbons. If this is done too quickly, it results in combustion; but at a controlled rate, the chemical synthesis of hydrocarbons can generate products such as acetylene, ethylene, and propylene.

addition, divers carry tanks in which oxygen is mixed with helium, rather than nitrogen, to prevent the dangerous condition known as “the bends.”

Nonmetals

As early as the 1960s, smog-ridden cities such as Tokyo and Mexico City were equipped with coin-operated oxygen booths—a sort of “phone booth for the lungs.” After inserting the appropriate amount of money, a person received a dose of oxygen for inhaling. This idea, spawned by necessity, is the likely inspiration for a rather bizarre fad that took hold in the trendier cities of North America during the mid-1990s: oxygen bars. Popular in Los Angeles, New York, and Toronto, these are establishments in which patrons pay up to a dollar per minute to inhale pure or flavored oxygen. Enthusiasts have touted the health benefits of this practice, but some physicians have warned of oxygen toxicity and other dangers.

The Other “Orphan” Nonmetals P H O S P H O R U S . Some elements, such as iron or gold, were known from ancient or even prehistoric times—meaning that the identity of the discoverer is unknown. Phosphorus was the first element whose discoverer is known: German chemist Hennig Brand (c. 1630-c. 1692), who identified it in 1674.

Highly reactive with oxygen, phosphorus is used in the production of safety matches, smoke bombs, and other incendiary devices. It is also important in fertilizers, and in various industrial applications. Phosphorus forms a number of important compounds, most notably phosphates, on which animals and plants depend. Phosphorus pollution, created by the use of household detergents containing phosphates, raised environmental concerns in the 1960s and 1970s. It was feared that high phosphate levels in rivers and creeks would lead to runaway, detrimental growth of plants and algae near bodies of water, a condition known as eutrophication. These concerns led to a ban on the use of phosphates in detergents.

Oxygen can be used to produce synthetic fuels, as well as for water purification and sewage treatment. Airplanes carry oxygen supplies in case of depressurization at high altitudes; in

On its own, sulfur has no smell, but in combination with other elements, it often acquires a foul odor, which has given it an unpleasant reputation. The element’s smell, combined with its combustibility, led to the associa-

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KEY TERMS ALLOTROPES:

Different versions of the same element, distinguished by molecular structure.

gases are noted for their extreme lack of reactivity—in other words, they tend not to react to, or bond with, other elements.

A term describing an element that exists as molecules composed of two atoms. This is in contrast to monatomic elements.

Elements that have a dull appearance; are not malleable; are poor conductors of heat and electricity; and tend to gain electrons to form negative ions. They are thus opposite of metals in most regards, as befits their name. In addition to hydrogen, in Group 1 of the periodic table, the other 18 nonmetals occupy the upper right-hand side of the chart. They include the noble gases, halogens, and seven “orphan” elements: boron, carbon, nitrogen, oxygen, phosphorus, sulfur, and selenium.

DIATOMIC:

The use of an electric current to cause a chemical reaction. ELECTROLYSIS:

HALOGENS: Group 7 of the periodic table of elements, with valence electron configurations of ns2np5. In contrast to the noble gases, the halogens are known for high levels of reactivity.

An atom or group of atoms that has lost or gained one or more electrons, and thus has a net electric charge. ION:

Atoms that have an equal number of protons, and hence are of the same element, but differ in their number of neutrons. This results in a difference of mass. Isotopes may be either stable or unstable. The unstable type, known as radioisotopes, are radioactive. ISOTOPES:

Elements that are lustrous or shiny in appearance; malleable, meaning that they can be molded into different shapes without breaking; and excellent conductors of heat and electricity. Metals, which constitute the vast majority of all elements, tend to form positive ions by losing electrons. METALS:

A term describing an element that exists as single atoms. This in contrast to diatomic elements.

MONATOMIC:

Group 8 of the periodic table of elements, all of whom (with the exception of helium) have valence electron configurations of ns2np6. The noble NOBLE GASES:

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NONMETALS:

ORBITAL: A pattern of probabilities regarding the position of an electron for an atom in a particular energy state. The higher the principal energy level, the more complex the pattern of orbitals.

A term describing a phenomenon whereby certain isotopes known as radioisotopes are subject to a form of decay brought about by the emission of high-energy particles. “Decay” does not mean that the isotope “rots”; rather, it decays to form another isotope until eventually (though this may take a long time), it becomes stable. RADIOACTIVITY:

The tendency for bonds between atoms or molecules to be made or broken in such a way that materials are transformed.

REACTIVITY:

SHELL: The orbital pattern of the valence electrons at the outside of an atom.

Electrons that occupy the highest principal energy level in an atom. These are the electrons involved in chemical bonding.

VALENCE ELECTRONS:

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tion of “brimstone”—the ancient name for sulfur—with the fires of hell. Because it is not usually combined with other elements in nature, the discovery of sulfur was relatively easy. Pure, or nearly pure, sulfur is mined on the Gulf Coast of the United States, as well as in Poland and Sicily. Sulfur compounds also appear in a number of ores, such as gypsum (calcium sulfate), or magnesium sulfate, better known as Epsom salts. Applications of the aforementioned compounds are discussed in the Alkaline Earth Metals essay. In addition, sulfates are used as agricultural insecticides, and for killing algae in water supplies. Potassium aluminum sulfate, a gelatinous solid that sinks to the bottom when dropped into water, is also used in water purification. Sulfur is sometimes applied in pure form as a fungicide, or in matches, fireworks, and gunpowder. More often, however, it is found in compounds such as the sulfates or the sulfides— including sulfuric acid and the evil-smelling gas known as hydrogen sulfide. Rotten eggs and intestinal gas are two examples of hydrogen sulfide, which, though poisonous, usually poses little danger, because the smell keeps people away. Another sulfur compound is mercaptan, an ingredient in the skunk’s distinctive aroma. Tiny quantities of mercaptan are added to natural gas (which has no odor) so that dangerous gas leaks can be detected by smell. S E L E N I U M . When Swedish chemist Jons Berzelius (1779-1848) first discovered selenium in 1817, in deposits at the bottom of a tank in a sulfuric acid factory, he thought it was tellurium, a metalloid discovered in 1800. A few months later, he reconsidered the evidence, and realized he had found a new element. Because tellurium, which lies just below selenium on the periodic table, had been named for the Earth (tellus in Latin), he named his new discovery after the Greek word for the Moon, selene.

Found primarily in impurities from sulfide ores, selenium is often obtained commercially as

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a by-product of the refining of copper by electrolysis. It occurs in at least three allotropic forms, variously black and red in color. Plants and animals, including humans, need small amounts of selenium to survive, but larger quantities can be toxic. This was demonstrated in the late 1970s, when waterfowl in the area of Kesterson Reservoir in northern California began turning up with birth defects. The cause was later traced to the dumping of selenium from agricultural wastes and industrial plants.

Nonmetals

Because selenium is photovoltaic (able to convert light directly into electricity) and photoconductive (meaning that its resistance to the flow of electric current decreases in the presence of light), it has applications in photocells, exposure meters, and solar cells. It is also used for the conversion of alternating current to direct current, and is applied as a semiconductor in electronic and solid-state appliances. Photocopiers use selenium in toners, and compounds containing selenium are used to tint glass red, orange, or pink. WHERE TO LEARN MORE Beatty, Richard. Phosphorus. New York: Benchmark Books, 2001. Blashfield, Jean F. Nitrogen. Austin, TX: Raintree SteckVaughn, 1999. Fitzgerald, Karen. The Story of Oxygen. New York: F. Watts, 1996. “Group Trends: Selected Nonmetals” (Web site). (May 29, 2001). Knapp, Brian J. Elements. Illustrated by David Woodroffe and David Hardy. Danbury, CT: Grolier Educational, 1996. “Nonmetals” Rutgers University Department of Chemistry (Web site). (May 29, 2001). “Nonmetals, Semimetals, and Their Compounds” (Web site). (May 29, 2001). WebElements (Web site). (May 22, 2001).

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M E TA L L O I D S Metalloids

CONCEPT The term “metalloid” may sound like a reference to a heavy-metal music fan, but in fact it describes a small collection of elements on the right-hand side of the periodic table. Forming a diagonal between boron and astatine, which lies four rows down and four columns to the right of boron, the metalloids are six elements that display qualities of both metals and nonmetals. (Some classifications include boron and astatine as well, but in this book, they are treated as a nonmetal and a halogen respectively.) Of these six—silicon, germanium, arsenic, antimony, tellurium, and polonium—only a few are household names. People know that arsenic is poison, and they have some general sense that silicon is important in surgical implants. But most people do not know that silicon, also the principal material in sand and glass, is the second-most plentiful element on Earth. Without silicon, from which computer chips are made, our computerbased society simply could not exist.

HOW IT WORKS Families and “Orphans” Most elements fit into some sort of “family” grouping: the alkali metals, the alkaline earth metals, transition metals, lanthanides, actinides, halogens, and noble gases. These seven families, five metallic and two nonmetallic, account for 91 of 112 elements on the periodic table. The “orphan” elements, or those not readily classifiable within a family, occupy groups 3 through 6 on the North American version of the

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periodic table, which only numbers the “tall” columns. Twenty elements, occupying a rectangle that stretches across four groups or columns, and five periods or rows, are “orphans.” (This is in addition to hydrogen, on Period 1, Group 1.) These are best classified, not by family, but by crossfamily characteristics that unite large groups of elements. These cross-family groupings can be likened to nations: on one level, a person identifies with his or her family (“I’m a Smith”); but on another level, a person has a national identity (“I’m an American.”) Likewise gold, for instance, is both a member of the transition metals family, and of the larger metals grouping. The “orphan” elements, because they have no family, are best identified with characteristics that unite a large body of elements. (For this reason, the “orphan” metals are discussed in the Metals essay, and the “orphan” nonmetals in the Nonmetals essay.)

Between Metals and Nonmetals Of the twenty “orphan” elements in groups 3 through 6, seven are metals and seven nonmetals. Between them runs a diagonal, comprising the six metalloids. (As noted earlier, boron is sometimes considered a metalloid, but in this book, it is discussed in the Nonmetals essay, while astatine—also sometimes grouped with the metalloids—is treated in the Halogens essay.) What is a metalloid? The best way to answer that question is by evaluating the differences between a metal and a nonmetal. On the periodic table, metals fill the left, center, and part of the right-hand side of the

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WITHOUT SILICON, FROM WHICH COMPUTER CHIPS ARE MADE, OUR COMPUTER-BASED SOCIETY SIMPLY COULD NOT HERE, A WORKER HOLDS A SILICON WAFER WITH INTEGRATED CIRCUITS. (Charles O’Rear/Corbis. Reproduced by

EXIST.

permission.)

chart, as well as the two rows—corresponding to the lanthanides and actinides—placed separately at the bottom. Thus it should not come as a surprise that most elements (87, in fact) are metals. Metals are lustrous or shiny in appearance, and ductile, meaning that they can be molded into different shapes without breaking. They are excellent conductors of heat and electricity, and tend to form positive ions by losing electrons. Nonmetals, which occupy the upper righthand side of the periodic table, include the noble gases, halogens, and the seven “orphan” elements alluded to above. With the addition of hydrogen (which, as noted, is covered in a separate essay), they comprise 19 elements. Whereas all metals

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are solids, except for mercury—a liquid at room temperature—the nonmetals are a collection of gases, solids, and one other liquid, bromine. (Not surprisingly, all gaseous elements are nonmetals.) As their name suggests, nonmetals are opposite to metals in most regards: dull in appearance, they are not particularly ductile or malleable. With the exception of carbon, they are poor conductors of heat and electricity, and tend to gain electrons to form negative ions.

Survey of the Metalloids The metalloids can thus be defined as those elements which exhibit characteristics of both metals and nonmetals. They are all solids, but not

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wise known from ancient times, and both attracted the attention of alchemists—medieval mystics, in many regards the forerunners of chemists, who believed it was possible to change plain metals into gold.

Metalloids

Traces of the alchemical mindset remained a part of European science as late as the mid-eighteenth century, a tendency exemplified by the fact that tellurium, discovered around that time, was believed to be “unripe gold.” On the other hand, the discovery of germanium more than a century later—following predictions of its existence based on the periodic table—reflected a much greater degree of scientific rigor. Finally, the isolation of polonium, known for its radioactivity, was evidence of an even more advanced stage of development in chemistry and science as a whole.

POLONIUM WAS FIRST ISOLATED BY MARIE AND PIERRE CURIE. MARIE CURIE NAMED THE ELEMENT IN HONOR OF HER HOMELAND, POLAND.

lustrous, and conduct heat and electricity moderately well. The six metalloids treated here are listed below, with the atomic number and chemical symbol of each. • • • • • •

14. Silicon (Si) 32. Germanium (Ge) 33. Arsenic (As) 51. Antimony (Sb) 52. Tellurium (Te) 84. Polonium (Po) Silicon is the second most abundant element on Earth, comprising 25.7% of the planet’s known elemental mass. Together with oxygen, it accounts for nearly three-quarters of the known total. (The planet’s core is probably composed largely of iron, with significant deposits of uranium and other metals between the crust and the core, but geologists can only make educated guesses as to the elemental composition beneath the crust.) THE

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M E TA L LO I D S

IN

HISTO-

REAL-LIFE A P P L I C AT I O N S Silicon Swedish chemist Jons Berzelius (1779-1848) discovered silicon in 1823, but humans had been using the element—in the form of compounds known as silicates—for thousands of years. Indeed, silicon may have been one of the first elements formed, many millions of years before life appeared on this planet. Geologists believe that the Earth was once composed primarily of molten iron, oxygen, silicon, and aluminum, still the predominant elements in the planet’s crust. Because iron has a greater atomic mass, it settled toward the center, while the more lightweight elements rose to the surface. Silicon is found in everything from the Sun and other stars, as well as meteorites, to plants and animal bones. Many hundreds of minerals on Earth contain silicon and oxygen in various forms: for instance, silicon and oxygen form silica (SiO2), commonly known as sand. This sand is mixed with lime and soda (sodium carbonate), as well other substances, to make glass. The purest varieties of silica form quartz, known for its clear crystals, while impure quartz forms crystals of semiprecious gems such as amethyst, opal, and agate.

RY. By far the most important of the metalloids, silicon has been in use as a compound for centuries, but was only isolated in the early nineteenth century. Arsenic and antimony were like-

bon on the periodic table, and the elements in

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that group (which includes germanium, selenium, and lead) are sometimes called the “carbon family.” Like carbon, silicon forms a huge array of compounds, because it has four electrons to share in chemical bonding, and thus can form long strings of atoms. Because silicon atoms are much larger than carbon atoms, however, not as many other atoms can get close to silicon. Thus the number of possible compounds involving silicon—though still quite impressive—is less than the many millions of carbon-based compounds Often silicon is combined with oxygen in long chains that use the oxygen atoms as separators. Because oxygen has a valence of two, meaning that it can bond to two other atoms at a time, it bonds easily between two other silicon atoms. The silicon, meanwhile, has two other electrons to use in bonding, so it typically attaches to two more, non-bridging, oxygen atoms. Compounds of this type are known as silicates, and most of the rocks and clay on Earth—with the exception of lime, which is calcium oxide—are silicates made up of combinations of silicon, oxygen, and various metals such as aluminum, iron, sodium, and potassium. Silicones are another variety of compound involving silicon atoms strung together by bridging oxygen atoms. Instead of attaching to two other, non-bridging oxygen atoms, as in a silicate, the silicon atoms in a silicone attach to organic groups—that is, molecules containing carbon. Because silicones often resemble organic compounds such as oils, greases, and rubber, silicone oils are frequently used in place of organic petroleum as a lubricant because they can withstand greater variations in temperature. Silicone rubbers are used in everything from bouncing balls to space vehicles. And because the body tolerates the introduction of silicone implants better than it does organic ones, silicones are used in surgical implants as well. THE MANY OTHER USES OF S I L I C O N . Silicones are also present in elec-

trical insulators, rust preventives, fabric softeners, hair sprays, hand creams, furniture and automobile polishes, paints, adhesives (including bathtub sealers), and chewing gum. Yet even this list does not exhaust the many applications of elemental silicon, as opposed to silicone.

with as many as half a million microscopic and intricately connected electronic circuits. These chips manipulate voltages using binary codes, for which 1 means “voltage on” and 0 means “voltage off.” By means of these pulses, silicon chips perform multitudes of calculations in seconds—calculations that would take humans hours or months or even years.

Metalloids

A porous form of silica, known as silica gel, absorbs water vapor from the air, and is often packed alongside moisture-sensitive products such as electronics components, in order to keep them dry. Silicon carbine, an extremely hard crystalline material manufactured by fusing sand with coke (almost pure carbon) at high temperatures, has applications as an abrasive.

Arsenic Silicon is unquestionably the “star” of the metalloids, but several other elements in this broad grouping have been known since ancient times. Among these is arsenic, well-known from many a detective novel. Highly poisonous, it is often a fixture of such stories: in a typical plot, a greedy nephew drops some arsenic into the tea of an elderly aunt who has left money to him in her will, and it is up to the detective/hero/heroine to uncover the details behind this dastardly scheme. The use of arsenic as a prop in such tales had already become something of a cliché by 1941, when “Arsenic and Old Lace,” a play by Joseph Kesselring that satirized an old-fashioned “whodunit,” made its debut on Broadway. Known since ancient times, arsenic is named after the Greek word arsenikon, meaning “yellow orpiment.” (Orpiment is arsenic trisulfide, or As2S3.) But the Greeks were not the only peoples in antiquity who knew of arsenic and its toxic properties: the Egyptians and Chinese were likewise familiar with it. Alchemists took an interest in arsenic, and one of them, Albertus Magnus (1193-1280)—a German physician who contributed significantly to the rebirth of interest in science during the late Middle Ages—probably isolated the element from arsenic trisulfide in about 1250.

Due to its semi-metallic qualities, silicon is used as a semiconductor of electricity. Computer chips are tiny slices of ultra-pure silicon, etched

Arsenic is more than just a prop in a detective story. (And in fact it is not a very effective poison for a murderer who hopes to get away with his crime: traces of arsenic remain in the body of a murder victim—even in the person’s hair—for years after death.) Today, arsenic is

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used for purposes such as bronzing, and for hardening and improving the sphericity (roundness) of lead shot in shotgun shells. In solid-state devices such as transistors, arsenic or antimony is used for the purposes of “doping.” This may sound like another version of the whodunit-style poisoning described above, but actually doping involves the alteration of chemical properties by deliberately introducing impurities. Through the doping of silicon, its electrical conductivity is improved.

Antimony In ancient times, antimony was known by the Latin name stibium; hence its chemical symbol of Sb. The word, which can be deciphered as “a metal that does not occur by itself,” has roots much older than the Roman civilization, however. Many centuries before the rise of the Romans, the biblical prophet Jeremiah railed against women’s use of “stibic stone” to paint their faces. In fact, stibic stone is antimony (III) sulfide, or Sb2S3, a black mineral which has long been used as an eyeliner. During the Middle Ages, alchemists studied antimony and its compounds, but the first scientist to distinguish between the compounds and the element itself was French chemist Jean-Baptiste Buquet, in 1771. During the early modern era, antimony was used in making bells, tools, and type for printing presses, because its addition to an alloy strengthened the other metals. Today, antimony is added to lead in storage batteries to make the lead harder and stronger. It is still used in metal type, as well as bullets and cable sheathing. Antimony was a component in early varieties of matches, but by 1855, the less volatile phosphorus had replaced it. In altered form, however, antimony still appears in some matches. Oxides and sulfides of antimony are also applied in paints, ceramics, fireworks, pharmaceuticals, and other products. These are also utilized for dyeing cloth and fireproofing materials. In addition, as noted above with regard to arsenic, antimony is applied for the doping of silicon and other materials in semiconductors, particularly those that use infrared detectors.

Tellurium

226

century, he did so in alchemical terms, referring to it as “unripe gold.” This reflected the belief, still lingering from the Middle Ages, that metals “grow” to higher states. At least part of his confusion is understandable, since the element did appear in minerals that also contained gold. A few years later, Baron Franz Joseph Müller von Reichenstein, an Austrian mining inspector in Transylvania, analyzed the substance, which he concluded was bismuth sulfide. Later he changed his mind, then subjected the material to a series of tests over a period of many years, until finally he sent a sample to the distinguished German chemist, Martin Heinrich Klaproth (1743-1817). Klaproth concluded that it was indeed a new element, and suggested the Latin word tellus, for “earth,” as a name. Grayish-white and lustrous, tellurium looks like a metal, but it is much more brittle than most metallic elements. Furthermore, it is a semiconductor, which further separates it from the typically conductive metals. Its semiconductive properties have led to applications in building electronic components. Added to copper and stainless steel, tellurium improves the machinability of those metals; in addition, tellurium compounds are used for the coloring of glass, porcelain, enamel, and ceramics.

Germanium and Polonium Germanium. Both named after European countries, germanium and polonium were identified much later than the other metalloids. In 1871, two years after he created his famous periodic table, Russian chemist Dmitri Ivanovitch Mendeleev (1834-1907) predicted the existence of an element he called “eka-silicon.” This turned out to be germanium, which sits directly beneath silicon on the periodic table, and which was discovered in 1886 by German chemist Clemens Alexander Winkler (1838-1904). Germanium is principally used as a doping agent in making transistor elements. It is also used as a phosphor in fluorescent lamps. In addition, germanium and germanium oxide, transparent to the infrared portion of the electromagnetic spectrum, are present in infrared spectroscopes and infrared detectors.

When Hungarian chemist Joseph Ramacsaházy first described tellurium in the mid-eighteenth

P O LO N I U M . While the applications of germanium are limited compared to those of, say, silicon, polonium is useful primarily as a source

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Metalloids

KEY TERMS ION:

An atom or group of atoms that

has lost or gained one or more electrons,

and tend to form positive ions by losing electrons.

and thus has a net electric charge.

Elements that have a

NONMETALS:

Atoms that have an equal

dull appearance; are not malleable; are

number of protons, and hence are of the

poor conductors of heat and electricity;

same element, but differ in their number of

and tend to gain electrons to form negative

neutrons. This results in a difference of

ions. They are thus the opposite of metals

mass. Isotopes may be either stable or

in most regards, as befits their name.

ISOTOPES:

unstable. The latter type, known as radioisotopes, are radioactive.

NORTH AMERICAN SYSTEM:

A ver-

sion of the periodic table of elements that

Elements which exhibit

only numbers groups of elements in which

characteristics of both metals and non-

the number of valence electrons equals the

metals. Metalloids are all solids, but are not

group number—that is, the two “tall”

lustrous or shiny, and they conduct heat

columns to the left of the transition metals,

and electricity moderately well. The six

as well as the six “tall” columns to the right.

METALLOIDS:

metalloids occupy a diagonal region

“ORPHAN”:

between the metals and nonmetals on the

belong to any clearly defined family of ele-

right side of the periodic table. Sometimes

ments. The metalloids are all “orphans.”

An element that does not

boron and astatine are included with the metalloids, but in this volume, they are treated as an “orphan” nonmetal and a halogen, respectively.

RADIOACTIVITY:

A term describing a

phenomenon whereby certain isotopes known as radioisotopes are subject to a form of decay brought about by the emis-

Elements that are lustrous, or

sion of high-energy particles. “Decay” does

shiny in appearance, and malleable, mean-

not mean that the isotope “rots”; rather, it

ing that they can be molded into different

decays to form another isotope until even-

shapes without breaking. They are excel-

tually (though this may take a long time) it

lent conductors of heat and electricity,

becomes stable.

METALS:

of neutrons in atomic laboratories—hardly something with which a person comes in contact during the course of an ordinary day. Yet the isolation of polonium by Polish-French physicist and chemist Marie Curie (1867-1934), along with her husband Pierre (1859-1906), a French physicist, was a prelude to one of the greatest stories in the history of chemistry. Setting out to find radioactive elements other than uranium, the Curies refined a large quantity of pitchblende, an ore commonly found

in uranium mines. Within a year, they had discovered polonium, which Marie named after her homeland. Polonium is about 100 times as radioactive as uranium, but the Curies were certain that yet another element lay in the pitchblende, and they devoted the next four years to the exhausting process of isolating it. The story of radium, its isolation, and the great price Marie Curie paid for working with radioactive materials is discussed in the essay on Alkaline Earth Metals.

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WHERE TO LEARN MORE “The Arsenic Website Project.” Harvard University Department of Physics (Web site). (May 30, 2001). Knapp, Brian J. Elements. Illustrated by David Woodroffe and David Hardy. Danbury, CT: Grolier Educational, 1996.

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“Metalloids.” ChemicalElements.com (Web site). (May 30, 2001). “Metalloids” (Web site). (May 30, 2001). Pflaum, Rosalynd. Marie Curie and Her Daughter Irène. Minneapolis, MN: Lerner Publications, 1993. Thomas, Jens. Silicon. New York: Benchmark Books, 2001.

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HALOGENS

Halogens

CONCEPT Table salt, bleach, fluoride in toothpaste, chlorine in swimming pools—what do all of these have in common? Add halogen lamps to the list, and the answer becomes more clear: all involve one or more of the halogens, which form Group 7 of the periodic table of elements. Known collectively by a term derived from a Greek word meaning “saltproducing,” the halogen family consists of five elements: fluorine, chlorine, bromine, iodine, and astatine. The first four of these are widely used, often in combination; the last, on the other hand, is a highly radioactive and extremely rare substance. The applications of halogens are many and varied, including some that are dangerous, controversial, and deadly.

HOW IT WORKS The Halogens on the Periodic Table As noted, the halogens form Group 7 of the periodic table of elements. They are listed below, along with chemical symbol and atomic number: • • • • •

place. Many chemists outside the United States refer to these as 18 different groups of elements; however, within the United States, a somewhat different system is used. In many American versions of the chart, there are only eight groups, sometimes designated with Roman numerals. The 40 transition metals in the center are not designated by group number, nor are the lanthanides and actinides, which are set apart at the bottom of the periodic table. The remaining eight columns are the only ones assigned group numbers. In many ways, this is less useful than the system of 18 group numbers; however, it does have one advantage. E L E C T R O N C O N F I G U RAT I O N S A N D B O N D I N G . In the eight-group sys-

tem, group number designates the number of valence electrons. The valence electrons, which occupy the highest energy levels of an atom, are the electrons that bond one element to another. These are often referred to as the “outer shell” of an atom, though the actual structure is much more complex. In any case, electron configuration is one of the ways halogens can be defined: all have seven valence electrons.

Fluorine (F) 9 Chlorine (Cl): 17 Bromine (Br): 35 Iodine (I): 53 Astatine (At): 85 On the periodic table, as displayed in chemistry labs around the world, the number of columns and rows does not vary, since these configurations are the result of specific and interrelated properties among the elements. There are always 18 columns; however, the way in which these are labeled differs somewhat from place to

Because the rows in the periodic table indicate increasing energy levels, energy levels rise as one moves up the list of halogens. Fluorine, on row 2, has a valence-shell configuration of 2s22p5; while that of chlorine is 3s23p5. Note that only the energy level changes, but not the electron configuration at the highest energy level. The same goes for bromine (4s24p5), iodine (4s24p5), and astatine (5s25p5).

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All members of the halogen family have the same valence-shell electron configurations, and thus tend to bond in much the same way. As we

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Halogens

SALT,

LIKE THE MOUND SHOWN HERE, IS A SAFE AND COMMON SUBSTANCE THAT CONTAINS CHLORINE. (Charles

O’Rear/Corbis. Reproduced by permission.)

shall see, they are inclined to form bonds more readily than most other substances, and indeed fluorine is the most reactive of all elements. Thus it is ironic that they are “next door” to the Group 8 noble gases, the least reactive among the elements. The reason for this, as discussed in the Chemical Bonding essay, is that most elements bond in such a way that they develop a valence shell of eight electrons; the noble gases are already there, so they do not bond, except in some cases—and then principally with fluorine.

Characteristics of the Halogens In terms of the phase of matter in which they are normally found, the halogens are a varied group. Fluorine and chlorine are gases, iodine is a solid, and bromine is one of only two elements that exists at room temperature as a liquid. As for astatine, it is a solid too, but so highly radioactive that it is hard to know much about its properties.

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figurations. One of the first things scientists noticed about these five elements is the fact that they tend to form salts. In everyday terminology, “salt” refers only to a few variations on the same thing—table salt, sea salt, and the like. In chemistry, however, the meaning is much broader: a salt is defined as the result of bonding between an acid and a base. Many salts are formed by the bonding of a metal and a nonmetal. The halogens are all nonmetals, and tend to form salts with metals, as in the example of sodium chloride (NaCl), a bond between chlorine, a halogen, and the metal sodium. The result, of course, is what people commonly call “salt.” Due to its tendency to form salts, the first of the halogens to be isolated— chlorine, in 1811—was originally named “halogen.” This is a combination of the Greek words halos, or salt, and gennan, “to form or generate.”

Despite these differences, the halogens have much in common, and not just with regard to their seven valence electrons. Indeed, they were identified as a group possessing similar characteristics long before chemists had any way of knowing about electrons, let alone electron con-

In their pure form, halogens are diatomic, meaning that they exist as molecules with two atoms: F2, Cl2, and so on. When bonding with metals, they form ionic bonds, which are the strongest form of chemical bond. In the process, halogens become negatively charged ions, or anions. These are represented by the symbols F-, Cl-, Br-, and I-, as well as the names fluoride, chloride, bromide, and iodide. All of the halogens

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are highly reactive, and will combine directly with almost all elements.

Halogens

Due to this high level of reactivity, the halogens are almost never found in pure form; rather, they have to be extracted. Extraction of halogens is doubly problematic, because they are dangerous. Exposure to large quantities can be harmful or fatal, and for this reason halogens have been used as poisons to deter unwanted plants and insects—and, in one of the most horrifying chapters of twentieth century military history, as a weapon in World War I.

REAL-LIFE A P P L I C AT I O N S Chlorine Chlorine is a highly poisonous gas, greenishyellow in color, with a sharp smell that induces choking in humans. Yet, it can combine with other elements to form compounds safe for human consumption. Most notable among these compounds is salt, which has been used as a food preservative since at least 3000 B.C. Salt, of course, occurs in nature. By contrast, the first chlorine compound made by humans was probably hydrochloric acid, created by dissolving hydrogen chloride gas in water. The first scientist to work with hydrochloric acid was Persian physician and alchemist Rhazes (ar-Razi; c. 864-c. 935), one of the most outstanding scientific minds of the medieval period. Alchemists, who in some ways were the precursors of true chemists, believed that base metals such as iron could be turned into gold. Of course this is not possible, but alchemists in about 1200 did at least succeed in dissolving gold using a mixture of hydrochloric and nitric acids known as aqua regia. The first modern scientist to work with chlorine was Swedish chemist Carl W. Scheele (1742-1786), who also discovered a number of other elements and compounds, including barium, manganese, oxygen, ammonia, and glycerin. However, Scheele, who isolated it in 1774, thought that chlorine was a compound; only in 1811 did English chemist Sir Humphry Davy (1778-1829) identify it as an element. Another chemist had suggested the name “halogen” for the alleged compound, but Davy suggested that it

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THIS

WORKER

REFRIGERATOR.

IS

REMOVING

CFCS

FREON

FROM

AN

OLD

LIKE FREON MAY BE RESPONSI-

BLE FOR THE DEPLETION OF THE OZONE LAYER. (James A.

Sugar/Corbis. Reproduced by permission.)

be called chlorine instead, after the Greek word chloros, which indicates a sickly yellow color. U S E S O F C H LO R I N E . The dangers involved with chlorine have made it an effective substance to use against stains, plants, animals— and even human beings. Chlorine gas is highly irritating to the mucous membranes of the nose, mouth, and lungs, and it can be detected in air at a concentration of only 3 parts per million (ppm).

The concentrations of chlorine used against troops on both sides in World War I (beginning in 1915) was, of course, much higher. Thanks to the use of chlorine gas and other antipersonnel agents, one of the most chilling images to emerge from that conflict was of soldiers succumbing to poisonous gas. Yet just as it is harmful to humans, chlorine can be harmful to microbes, thus preserving human life. As early as 1801, it had been used in solutions as a disinfectant; in 1831, its use in hospitals made it effective as a weapon against a cholera epidemic that swept across Europe. Another well-known use of chlorine is as a bleaching agent. Until 1785, when chlorine was

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first put to use as a bleach, the only way to get stains and unwanted colors out of textiles or paper was to expose them to sunlight, not always an effective method. By contrast, chlorine, still used as a bleach today, can be highly effective—a good reason not to use regular old-fashioned bleach on anything other than white clothing. (Since the 1980s, makers of bleaches have developed all-color versions to brighten and take out stains from clothing of other colors.)

Fluorine

Calcium hydrocholoride (CaOCl), both a bleaching powder and a disinfectant used in swimming pools, combines both the disinfectant and bleaching properties of chlorine. This and the others discussed here are just some of many, many compounds formed with the highly reactive element chlorine. Particularly notable—and controversial—are compounds involving chlorine and carbon.

French chemist Edmond Fremy (1814-1894) very nearly succeeded in isolating fluorine, and though he failed to do so, he inspired his student Henri Moissan (1852-1907) to continue the project. One of the problems involved in isolating this highly reactive element was the fact that it tends to “attack” any container in which it is placed: most metals, for instance, will burst into flames in the presence of fluorine. Like the others before him, Moissan set about to isolate fluorine from hydrogen fluoride by means of electrolysis—the use of an electric current to cause a chemical reaction—but in doing so, he used a platinum-iridium alloy that resisted attacks by fluorine. In 1906, he received the Nobel Prize for his work, and his technique is still used today in modified form.

CHLORINE AND ORGANIC C O M P O U N D S . Chlorine bonds well with

organic substances, or those containing carbon. In a number of instances, chlorine becomes part of an organic polymer such as PVC (polyvinyl chloride), used for making synthetic pipe. Chlorine polymers are also applied in making synthetic rubber, or neoprene. Due to its resistance to heat, oxidation, and oils, neoprene is used in a number of automobile parts. The bonding of chlorine with substances containing carbon has become increasingly controversial because of concerns over health and the environment, and in some cases chlorinecarbon compounds have been outlawed. Such was the fate of DDT, a pesticide soluble in fats and oils rather than in water. When it was discovered that DDT was carcinogenic, or cancercausing, in humans and animals, its use in the United States was outlawed. Other, less well-known, chlorine-related insecticides have likewise been banned due to their potential for harm to human life and the environment. Among these are chlorine-containing materials once used for dry cleaning. Also notable is the role of chlorine in chlorofluorocarbons (CFCs), which have been used in refrigerants such as Freon, and in propellants for aerosol sprays. CFCs tend to evaporate easily, and concerns over their effect on Earth’s atmosphere have led to the phasing out of their use.

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Fluorine has the distinction of being the most reactive of all the elements, with the highest electronegativity value on the periodic table. Because of this, it proved extremely difficult to isolate. Davy first identified it as an element, but was poisoned while trying unsuccessfully to decompose hydrogen fluoride. Two other chemists were also later poisoned in similar attempts, and one of them died as a result.

P R O P E RT I E S A N D U S E S O F F LU O R I N E . A pale green gas of low density,

fluorine can combine with all elements except some of the noble gases. Even water will burn in the presence of this highly reactive substance. Fluorine is also highly toxic, and can cause severe burns on contact, yet it also exists in harmless compounds, primarily in the mineral known as fluorspar, or calcium fluoride. The latter gives off a fluorescent light (fluorescence is the term for a type of light not accompanied by heat), and fluorine was named for the mineral that is one of its principal “hosts”. Beginning in the 1600s, hydrofluoric acid was used for etching glass, and is still used for that purpose today in the manufacture of products such as light bulbs. The oil industry uses it as a catalyst—a substance that speeds along a chemical reaction—to increase the octane number in gasoline. Fluorine is also used in a polymer commonly known as Teflon, which provides a nonstick surface for frying pans and other cookingrelated products. Just as chlorine saw service in World War I, fluorine was enlisted in World War II to create a

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weapon far more terrifying than poison gas: the atomic bomb. Scientists working on the Manhattan Project, the United States’ effort to develop the bombs dropped on Japan in 1945, needed large quantities of the uranium-235 isotope. This they obtained in large part by diffusion of the compound uranium hexafluoride, which consists of molecules containing one uranium atom and six fluorine anions. F L U O R I D AT I O N

OF

W AT E R .

Long before World War II, health officials in the United States noticed that communities having high concentration of fluoride in their drinking water tended to suffer a much lower incidence of tooth decay. In some areas the concentration of fluoride in the water supply was high enough that it stained people’s teeth; still, at the turn of the century—an era when dental hygiene as we know it today was still in its infancy—the prevention of tooth decay was an attractive prospect. Perhaps, officials surmised, it would be possible to introduce smaller concentrations of fluoride into community drinking water, with a resulting improvement in overall dental health. After World War II, a number of municipalities around the United States undertook the fluoridation of their water supplies, using concentrations as low as 1 ppm. Within a few years, fluoridation became a hotly debated topic, with proponents pointing to the potential health benefits and opponents arguing from the standpoint of issues not directly involved in science. It was an invasion of personal liberty, they said, for governments to force citizens to drink water which had been supplemented with a foreign substance. During the 1950s, in fact, fluoridation became associated in some circles with Communism—just another manifestation of a government trying to control its citizens. In later years, ironically, antifluoridation efforts became associated with groups on the political left rather than the right. By then, the argument no longer revolved around the issue of government power; instead the concern was for the health risks involved in introducing a substance lethal in large doses.

tist and announced “Look, Ma! No cavities!” Within a few years, virtually all brands of toothpaste used fluoride; however, the use of fluoride in drinking water remained controversial.

Halogens

As late as 1993, in fact, the issue of fluoridation remained heated enough to spawn a study by the U.S. National Research Council. The council found some improvement in dental health, but not as large as had been claimed by early proponents of fluoridation. Furthermore, this improvement could be explained by reference to a number of other factors, including fluoride in toothpastes and a generally heightened awareness of dental health among the U.S. populace. CHLOROFLUOROCARBONS.

Another controversial application of fluorine is its use, along with chlorine and carbon, in chlorofluorocarbons. As noted above, CFCs have been used in refrigerants and propellants; another application is as a blowing agent for polyurethane foam. This continued for several decades, but in the 1980s, environmentalists became concerned over depletion of the ozone layer high in Earth’s atmosphere. Unlike ordinary oxygen (O2), ozone or O3 is capable of absorbing ultraviolet radiation from the Sun, which would otherwise be harmful to human life. It is believed that CFCs catalyze the conversion of ozone to oxygen, and that this may explain the “ozone hole,” which is particularly noticeable over the Antarctic in September and October. As a result, a number of countries signed an agreement in 1996 to eliminate the manufacture of halocarbons, or substances containing halogens and carbon. Manufacturers in countries that signed this agreement, known as the Montreal Protocol, have developed CFC substitutes, most notably hydrochlorofluorocarbons (HCFCs), CFC-like compounds also containing hydrogen atoms.

Fluoride had meanwhile gained application in toothpastes. Colgate took the lead, introducing “stannous fluoride” in 1955. Three years later, the company launched a memorable advertising campaign with commercials in which a little girl showed her mother a “report card” from the den-

The ozone-layer question is far from settled, however. Critics argue that in fact the depletion of the ozone layer over Antarctica is a natural occurrence, which may explain why it only occurs at certain times of year. This may also explain why it happens primarily in Antarctica, far from any place where humans have been using CFCs. (Ozone depletion is far less significant in the Arctic, which is much closer to the population centers of the industrialized world.)

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In any case, natural sources, such as volcano eruptions, continue to add halogen compounds to the atmosphere.

Bromine Bromine is a foul-smelling reddish-brown liquid whose name is derived from a Greek word meaning “stink.” With a boiling point much lower than that of water—137.84°F (58.8°C)—it readily transforms into a gas. Like other halogens, its vapors are highly irritating to the eyes and throat. It is found primarily in deposits of brine, a solution of salt and water. Among the most significant brine deposits are in Israel’s Dead Sea, as well as in Arkansas and Michigan. Credit for the isolation of bromine is usually given to French chemist Antoine-Jérôme Balard (1802-1876), though in fact German chemist Carl Löwig (1803-1890) actually isolated it first, in 1825. However, Balard, who published his results a year later, provided a much more detailed explanation of bromine’s properties. The first use of bromine actually predated both men by several millennia. To make their famous purple dyes, the Phoenicians used murex mollusks, which contained bromine. (Like the names of the halogens, the word “Phoenicians” is derived from Greek—in this case, a word meaning “red” or “purple,” which referred to their dyes.) Today bromine is also used in dyes, and other modern uses include applications in pesticides, disinfectants, medicines, and flame retardants. At one time, a compound containing bromine was widely used by the petroleum industry as an additive for gasoline containing lead. Ethylene dibromide reacts with the lead released by gasoline to form lead bromide (PbBr2), referred to as a “scavenger,” because it tends to clean the emissions of lead-containing gasoline. However, leaded gasoline was phased out during the late 1970s and early 1980s; as a result, demand for ethylene dibromide dropped considerably. H A L O G E N L A M P S . The name “halogen” is probably familiar to most people because of the term “halogen lamp.” Used for automobile headlights, spotlights, and floodlights, the halogen lamp is much more effective than ordinary incandescent light. Incandescent “heat-producing” light was first developed in the 1870s and improved during the early part of the

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twentieth century with the replacement of carbon by tungsten as the principal material in the filament, the area that is heated. Tungsten proved much more durable than carbon when heated, but it has a number of problems when combined with the gases in an incandescent bulb. As the light bulb continues to burn for a period of time, the tungsten filament begins to thin and will eventually break. At the same time, tungsten begins to accumulate on the surface of the bulb, dimming its light. However, by adding bromine and other halogens to the bulb’s gas filling—thus making a halogen lamp— these problems are alleviated. As tungsten evaporates from the filament, it combines with the halogen to form a gaseous compound that circulates within the bulb. Instead of depositing on the surface of the bulb, the compound remains a gas until it comes into contact with the filament and breaks down. It is then redeposited on the filament, and the halogen gas is free to combine with newly evaporated tungsten. Though a halogen bulb does eventually break down, it lasts much longer than an ordinary incandescent bulb and burns with a much brighter light. Also, because of the decreased tungsten deposits on the surface, it does not begin to dim as it nears the end of its life.

Iodine First isolated in 1811 from ashes of seaweed, iodine has a name derived from the Greek word meaning “violet-colored”—a reference to the fact it forms dark purple crystals. During the 1800s, iodine was obtained commercially from mines in Chile, but during the twentieth century wells of brine in Japan, Oklahoma, and Michigan have proven a better source. Among the best-known properties of iodine is its importance in the human diet. The thyroid gland produces a growth-regulating hormone that contains iodine, and lack of iodine can cause a goiter, a swelling around the neck. Table salt does not naturally contain iodine; however, sodium chloride sold in stores usually contains about 0.01% sodium iodide, added by the manufacturer. Iodine was once used in the development of photography: during the early days of photographic technology, the daguerreotype process used silver plates sensitized with iodine vapors.

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Halogens

KEY TERMS The negative ion that results

element is never called, for instance, the

A form of chemical bonding that results from attractions between ions with opposite electrical charges.

chlorine anion. Rather, an anion involving

ISOTOPES:

ANION:

when an atom gains one or more electrons. An anion (pronounced “AN-ie-un”) of an

IONIC BONDING:

The number of

Atoms that have an equal number of protons, and hence are of the same element, but differ in their number of neutrons. This results in a difference of mass. Isotopes may be either stable or unstable—that is, radioactive.

protons in the nucleus of an atom. Since

PERIODIC TABLE OF ELEMENTS:

this number is different for each element,

A chart that shows the elements arranged in order of atomic number. Vertical columns within the periodic table indicate groups or “families” of elements with similar chemical characteristics.

a single element is named by adding the suffix -ide to the name of the original element—in this case, “chloride.” Other rules apply for more complex anions. ATOMIC NUMBER:

elements are listed on the periodic table of elements in order of atomic number. CHEMICAL SYMBOL:

A one-or two-

letter abbreviation for the name of an element. DIATOMIC:

A term describing an ele-

ment that exists as molecules composed of two atoms. All of the halogens are diatomic. ELECTROLYSIS:

The use of an elec-

trical current to cause a chemical reaction. HALF-LIFE:

The length of time it takes

a substance to diminish to one-half its initial amount. HALOGENS:

Group 7 of the periodic

table of elements, including fluorine, chlorine, bromine, iodine, and astatine. The halogens are diatomic, and tend to form salts; hence their name, which comes from two Greek terms meaning “salt-forming.” ION:

A large molecule containing many small units that hook together. POLYMER:

An atom that has lost or gained

one or more electrons, and thus has a net electric charge.

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A term describing a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of high-energy particles. “Decay” in this sense does not mean “rot”; instead, radioactive isotopes continue to emit particles, changing into isotopes of other elements, until they become stable.

RADIOACTIVITY:

A compound formed by the reaction of an acid with a base. Salts are usually formed by the joining of a metal and a nonmetal.

SALT:

Electrons that occupy the highest energy levels in an atom, and are involved in chemical bonding. The halogens all have seven valence electrons.

VALENCE ELECTRONS:

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Iodine compounds are used today in chemical analysis and in synthesis of organic compounds.

Astatine Just as fluorine has the distinction of being the most reactive, astatine is the rarest of all the elements. Long after its existence was predicted, chemists still had no luck finding it in nature, and it was only created in 1940 by bombarding bismuth with alpha particles (positively charged helium nuclei). The newly isolated element was given a Greek name meaning “unstable.” Indeed, none of astatine’s 20 known isotopes is stable, and the longest-lived has a half-life of only 8.3 hours. This has only added to the difficulties involved in learning about this strange element, and therefore it is difficult to say what applications, if any, astatine may have. The most promising area involves the use of astatine to treat a condition known as hyperthyroidism, related to an overly active thyroid gland.

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WHERE TO LEARN MORE “The Chemistry of the Halogens.” Purdue University Department of Chemistry (Web site). (May 20, 2001). “Halogens.” Chemical Elements.com (Web site). (May 20, 2001). “Halogens.” Corrosion Source (Web site). (May 20, 2001). “Halogens” (Web site). (May 20, 2001). “The Halogens” (Web site). (May 20, 2001). Knapp, Brian J. Chlorine, Fluorine, Bromine, and Iodine. Henley-on-Thames, England: Atlantic Europe, 1996. Oxlade, Chris. Elements and Compounds. Chicago: Heinemann Library, 2001. Stwertka, Albert. A Guide to the Elements. New York: Oxford University Press, 1998. “Visual Elements: Group VII—The Halogens” (Web site). (May 20, 2001).

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Noble Gases

NOBLE GASES

CONCEPT Along the extreme right-hand column of the periodic table of elements is a group known as the noble gases: helium, neon, argon, krypton, xenon, and radon. Also known as the rare gases, they once were called inert gases, because scientists believed them incapable of reacting with other elements. Rare though they are, these gases are a part of everyday life, as evidenced by the helium in balloons, the neon in signs—and the harmful radon in some American homes.

HOW IT WORKS Defining the Noble Gases The periodic table of elements is ordered by the number of protons in the nucleus of an atom for a given element (the atomic number), yet the chart is also arranged in such a way that elements with similar characteristics are grouped together. Such is the case with Group 8, which is sometimes called Group 18, a collection of non-metals known as the noble gases. The six noble gases are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Their atomic numbers are, respectively, 2, 10, 18, 36, 54, and 86. Several characteristics, aside from their placement on the periodic table, define the noble gases. Obviously, all are gases, meaning that they only form liquids or solids at extremely low temperatures—temperatures that, on Earth at least, are usually only achieved in a laboratory. They are colorless, odorless, and tasteless, as well as monatomic—meaning that they exist as individual atoms, rather than in molecules. (By contrast,

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atoms of oxygen—another gas, though not among this group—usually combine to form a molecule, O2.)

Low Reactivity There is a reason why noble gas atoms tend not to combine: one of the defining characteristics of the noble gas “family” is their lack of chemical reactivity. Rather than reacting to, or bonding with, other elements, the noble gases tend to remain apart—hence the name “noble,” implying someone or something that is set apart from the crowd, as it were. Due to their apparent lack of reactivity, the noble gases—also known as the rare gases—were once known as the inert gases. Indeed, helium, neon, and argon have not been found to combine with other elements to form compounds. However, in 1962 English chemist Neil Bartlett (1932-) succeeded in preparing a compound of xenon with platinum and fluorine (XePtF6), thus overturning the idea that the noble gases were entirely “inert.” Since that time, numerous compounds of xenon with other elements, most notably oxygen and fluorine, have been developed. Fluorine has also been used to form simple compounds with krypton and radon. Nonetheless, low reactivity—instead of no reactivity, as had formerly been thought—characterizes the rare gases. One of the factors governing the reactivity of an element is its electron configuration, and the electrons of the noble gases are arranged in such a way as to discourage bonding with other elements.

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Noble Gases

A

SCIENTIST USING THE POTASSIUM-ARGON DATING PROCESS TO DETERMINE THE AGE OF MATERIALS. (Dean

Conger/Corbis. Reproduced by permission.)

REAL-LIFE A P P L I C AT I O N S Isolation of the Noble Gases H E L I U M . Helium is an unusual element in many respects—not least because it is the only element to have first been identified in the Solar System before it was discovered on Earth. This is significant, because the elements on Earth are the same as those found in space: thus, it is more than just an attempt at sounding poetic when scientists say that humans, as well as the world around them, are made from “the stuff of stars.”

In 1868, a French astronomer named Pierre Janssen (1824-1907) was in India to observe a total solar eclipse. To aid him in his observations, he used a spectroscope, an instrument for analyzing the spectrum of light emitted by an object. What Janssen’s spectroscope showed was surprising: a yellow line in the spectrum, never seen before, which seemed to indicate the presence of a previously undiscovered element. Janssen called it “helium” after the Greek god Helios, or Apollo, whom the ancients associated with the Sun.

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had a worldwide reputation for his work in analyzing light waves. Lockyer, too, believed that what Janssen had seen was a new element, and a few months later, he observed the same unusual spectral lines. At that time, the spectroscope was still a new invention, and many members of the worldwide scientific community doubted its usefulness, and therefore, in spite of Lockyer’s reputation, they questioned the existence of this “new” element. Yet during their lifetimes, Janssen and Lockyer were proven correct. NEON, ARGON, KRYPTON, A N D X E N O N . They had to wait a quarter

century, however. In 1893, English chemist Sir William Ramsay (1852-1916) became intrigued by the presence of a mysterious gas bubble left over when nitrogen from the atmosphere was combined with oxygen. This was a phenomenon that had also been noted by English physicist Henry Cavendish (1731-1810) more than a century before, but Cavendish could offer no explanation. Ramsay, on the other hand, had the benefit of observations made by English physicist John William Strutt, Lord Rayleigh (1842-1919).

Janssen shared his findings with English astronomer Sir Joseph Lockyer (1836-1920), who

Up to that time, scientists believed that air consisted only of oxygen, carbon dioxide, and water vapor. However, Rayleigh had noticed that when nitrogen was extracted from air after a

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process of removing those other components, it had a slightly higher density than nitrogen prepared from a chemical reaction. In light of his own observations, Ramsay concluded that whereas nitrogen obtained from chemical reactions was pure, the nitrogen extracted from air contained trace amounts of an unknown gas.

Noble Gases

Ramsay was wrong in only one respect: hidden with the nitrogen was not one gas, but five. In order to isolate these gases, Ramsay and Rayleigh subjected air to a combination of high pressure and low temperature, allowing the various gases to boil off at different temperatures. One of the gases was helium—the first confirmation that the element existed on Earth—but the other four gases were previously unknown. The Greek roots of the names given to the four gases reflected scientists’ wonder at discovering these hard-to-find elements: neos (new), argos (inactive), kryptos (hidden), and xenon (stranger). RA D O N . Inspired by the studies of Polish-French physicist and chemist Marie Curie (1867-1934) regarding the element radium and the phenomenon of radioactivity (she discovered the element, and coined the latter term), German physicist Friedrich Dorn (1848-1916) became fascinated with radium. Studying the element, he discovered that it emitted a radioactive gas, which he dubbed “radium emanation.” Eventually, however, he realized that what was being produced was a new element. This was the first clear proof that one element could become another through the process of radioactive decay.

Ramsay, who along with Rayleigh had received the Nobel Prize in 1904 for his work on the noble gases, was able to map the new element’s spectral lines and determine its density and atomic mass. A few years later, in 1918, another scientist named C. Schmidt gave it the name “radon.” Due to its behavior and the configuration of its electrons, chemists classified radon among what they continued to call the “inert gases” for another half-century—until Bartlett’s preparation of xenon compounds in 1962.

LAS VEGAS

HAS

NUMEROUS

EXAMPLES

OF

SIGNS

ILLUMINATED BY NEON. (Dave Bartruff/Corbis. Reproduced by

permission.)

released into the air long ago as a by-product of decay on the part of radioactive materials in the Earth’s crust. Within the atmosphere, argon is the most “abundant”—in comparative terms, given the fact that the “rare gases” are, by definition, rare. Nitrogen makes up about 78% of Earth’s atmosphere and oxygen 21%, meaning that these two elements constitute fully 99% of the air above the Earth. Argon ranks a distant third, with 0.93%. The remaining 0.07% is made up on water vapor, carbon dioxide, ozone (O3), and traces of the noble gases. These are present in such small quantities that the figures for them are not typically presented as percentages, but rather in terms of parts per million (ppm). The concentrations of neon, helium, krypton, and xenon in the atmosphere are 18, 5, 1, and 0.09 ppm respectively.

I N T H E AT M O S P H E R E . Though the rare gases are found in minerals and meteorites on Earth, their greatest presence is in the planet’s atmosphere. It is believed that they were

I N T H E S O I L . Radon in the atmosphere is virtually negligible, which is a fortunate thing, in light of its radioactive qualities. Few Americans, in fact, even knew of its existence until 1988, when the United States Environmental Protection Agency (EPA) released a report estimating that some ten million American homes had potentially harmful radon levels. This

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Presence of the Rare Gases on Earth

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Noble Gases

A RESEARCHER WORKS WITH LIQUID HELIUM AS PART OF AN EXPERIMENT ON COSMIC BACKGROUND RADIATION. CLOSE TO ABSOLUTE ZERO, HELIUM TRANSFORMS INTO A HIGHLY UNUSUAL LIQUID THAT HAS NO MEASURABLE RESISTANCE TO FLOW. (Roger Ressmeyer/Corbis. Reproduced by permission.)

set off a scare, and during the late 1980s and 1990s, sales of home radon detectors boomed. Meanwhile, the federal government increased concerns with additional reports, advising people to seal their basements and ventilate their homes if radon exceeded certain levels. A number of scientists have disputed the government’s claims, yet some regions of the United States appear to be at relatively high risk due to the presence of radon in the soil. The element seems to be most plentiful in soils containing high concentrations of uranium. If radon is present in a home that has been weather-sealed to improve the efficiency of heating and cooling systems, it is indeed potentially dangerous to the residents. Chinese scientists in the 1960s made an interesting discovery regarding radon and its application to seismography, or the area of the earth sciences devoted to studying and predicting earthquakes. Radon levels in groundwater, the Chinese reports showed, rise considerably just before an earthquake. Since then, the Chinese have monitored radon concentrations in water, and used this data to predict earthquakes.

Many of the noble gases are extracted by liquefying air—that is, by reducing it to temperatures at which it assumes the properties of a liquid rather than a gas. By controlling temperatures in the liquefied air, it is possible to reach the boiling point for a particular noble gas and thereby extract it, much as was done when these gases were first isolated in the 1890s. T H E U N I Q U E S I T UAT I O N O F H E L I U M . Helium is remarkable, in that it

Radon, in fact, is not the only rare gas that can be

only liquefies at a temperature of -457.6°F (-272°C), just above absolute zero. Absolute zero is the temperature at which the motion of atoms or molecules comes to a virtual stop, but the motion of helium atoms never completely ceases. In order to liquefy it, in fact, even at those low

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E X T RAC T I N G

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obtained as the result of radioactive decay: in 1903, Ramsay and British chemist Frederick Soddy (1877-1956) showed that the breakdown of either uranium or radium results in the production of helium atoms (beta particles). A few years later, English physicist Ernest Rutherford (1871-1937) demonstrated that radiation carrying a positive electrical charge (alpha rays) was actually a stream of helium atoms stripped of an electron.

RA R E

GASES.

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temperatures, it must be subjected to pressures many times that exerted by Earth’s atmosphere.

volcanic layers above and below fossil and artifact remains in east Africa.

Given these facts, it is difficult to extract helium from air. More often, it is obtained from natural gas wells, where it is present in relatively large concentrations—between 1% and 7% of the natural gas. The majority of the Earth’s helium supply belongs to the United States, where the greatest abundance of helium-supplying wells are in Texas, Oklahoma, and Kansas. During World War II, the United States took advantage of this supply of relatively inexpensive helium to provide buoyancy for a fleet of airships used for reconnaissance.

Krypton has a number of specialized applications—for instance, it is mixed with argon and used in the manufacture of windows with a high level of thermal efficiency. Used in lasers, it is often mixed with a halogen such as fluorine. In addition, it is also sometimes used in halogen sealed-beam headlights. Many fans of Superman, no doubt, were disappointed at some point in their lives to discover that there is no such thing as “kryptonite,” the fictional element that caused the Man of Steel to lose his legendary strength. Yet krypton—the real thing—has applications that are literally out of this world. In the development of fuel for space exploration, krypton is in competition with its sister element, xenon. Xenon offers better performance, but costs about ten times more to produce; thus krypton has become more attractive as a fuel for space flight.

There is one place with an abundant supply of helium, but there are no plans for a mining expedition any time soon. That place is the Sun, where the nuclear fusion of hydrogen atoms creates helium. Indeed, helium seems to be the most plentiful element of all, after hydrogen, constituting 23% of the total mass of the universe. Why, then, is it so difficult to obtain on Earth? Most likely because it is so light in comparison to air; it simply floats off into space.

Applications for the Noble Gases RA D O N , A R G O N , K RY P T O N , A N D X E N O N . Though radon is known pri-

marily for the hazards it poses to human life and well-being, it has useful applications. As noted above, its presence in groundwater appears to provide a possible means of predicting earthquakes. In addition, it is used for detecting leaks, measuring flow rates, and inspecting metal welds. One interesting use of argon and, in particular, the stable isotope argon-40, is in dating techniques used by geologists, paleontologists, and other scientists studying the distant past. When volcanic rocks are subjected to extremely high temperatures, they release argon, and as the rocks cool, argon-40 accumulates. Because argon-40 is formed by the radioactive decay of a potassium isotope, potassium-40, the amount of argon-40 that forms is proportional to the rate of decay for potassium-40. The latter has a half-life of 1.3 billion years, meaning that it takes 1.3 billion years for half the potassium-40 originally present to be converted to argon-40. Using argon-40, paleontologists have been able to estimate the age of

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Noble Gases

In addition to its potential as a space fuel, xenon is used in arc lamps for motion-picture film projection, in high-pressure ultraviolet radiation lamps, and in specialized flashbulbs used by photographers. One particular isotope of xenon is utilized for tracing the movement of sands along a coastline. Xenon is also applied in high-energy physics for detecting nuclear radiation in bubble chambers. Furthermore, neuroscientists are experimenting with the use of xenon in diagnostic procedures to clarify x-ray images of the human brain. N E O N . Neon, of course, is best-known for its application in neon signs, which produce an eye-catching glow when lit up at night. French chemist Georges Claude (1870-1960), intrigued by Ramsay’s discovery of neon, conducted experiments that led to the development of the neon light in 1910. That first neon light was simply a glass tube filled with neon gas, which glowed a bright red when charged with electricity.

Claude eventually discovered that mixing other gases with neon produced different colors of light. He also experimented with variations in the shapes of glass tubes to create letters and pictures. By the 1920s, neon light had come into vogue, and it is still popular today. Modern neon lamps are typically made of plastic rather than glass, and the range of colors is much greater than in Claude’s day: not only the gas filling, but the coating inside the tube, is varied, resulting in a variety of colors from across the spectrum.

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Though the neon sign is its best-known application, neon is used for many other things. Neon glow lamps are often used to indicate on/off settings on electronic instrument panels, and lightweight neon lamps are found on machines ranging from computers to voltage regulators. In fact, the first practical color television, produced in 1928, used a neon tube to produce the red color in the receiver. Green came from mercury, but the blue light in that early color TV came from another noble gas, helium. H E L I U M . Helium, of course, is widely known for its use in balloons—both for large airships and for the balloons that have provided joy and fun to many a small child. Though helium is much more expensive than hydrogen as a means of providing buoyancy to airships, hydrogen is extremely flammable, and after the infamous explosion of the airship Hindenburg in 1937, helium became the preferred medium for airships. As noted earlier, the United States military made extensive use of helium-filled airships during the World War II.

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Among the most fascinating applications of helium relate to its extraordinarily low freezing point. Helium has played a significant role in the low-temperature science known as cryogenics, and has found application in research concerning superconductivity: the use of very low temperatures to develop materials that conduct electrical power with vastly greater efficiency than ordinary conductors. Close to absolute zero, helium transforms into a highly unusual liquid unlike any known substance, in that it has no measurable resistance to flow. This means that it could carry an electrical current hundreds of times more efficiently than a copper wire.

WHERE TO LEARN MORE “The Chemistry of the Rare Gases” (Web site). (May 13, 2001). “Homework: Science: Chemistry: Gases” Channelone.com (Web site). (May 12, 2001). Knapp, Brian J.; David Woodroffe; David A. Hardy. Elements. Danbury, CT: Grolier Educational, 2000.

The use of helium for buoyancy is one of the most prominent applications of this noble gas, but far from the only one. In fact, not only have people used helium to go up in balloons, but divers use helium for going down beneath the surface of the ocean. In that situation, of course, helium is not used for providing buoyancy, but as a means of protecting against the diving-related condition known as “the bends,” which occurs when nitrogen in the blood bubbles as the diver rises to the surface. Helium is mixed with oxygen in diver’s air tanks because it does not dissolve in the blood as easily as nitrogen.

Taylor, Ron. Facts on Radon and Asbestos. Illustrated by Ian Moores. New York: F. Watts, 1990.

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Mebane, Robert C. and Thomas R. Rybolt. Air and Other Gases. Illustrations by Anni Matsick. New York: Twenty-First Century Books, 1995. “Noble Gases”Xrefer.com (Web site). (May 13, 2001). Rare Gases. Praxair (Web site). (May 13, 2001). Stwertka, Albert. Superconductors: The Irresistible Future. New York: F. Watts, 1991.

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CARBON

Carbon

CONCEPT The phrase “carbon-based life forms,” often used in science-fiction books and movies by aliens to describe the creatures of Earth, is something of a cliché. It is also a redundancy when applied to creatures on Earth, the only planet known to support life: all living things contain carbon. Carbon is also in plenty of things that were once living, which makes it useful for dating the remains of past settlements on Earth. Of even greater usefulness is petroleum, a substance containing carbon-based forms that died long ago, became fossilized, and ultimately changed chemically into fuels. Then again, not all materials containing carbon were once living creatures; yet because carbon is a common denominator to all living things on Earth, the branch of study known as organic chemistry is devoted to the study of compounds containing carbon. Among the most important organic compounds are the many carboxylic acids that are vital to life, but carbon is also present in numerous important inorganic compounds—most notably carbon dioxide and carbon monoxide.

HOW IT WORKS The Basics of Carbon Carbon’s name comes from the Latin word carbo, or charcoal—which, indeed, is almost pure carbon. Its chemical symbol is C, and it has an atomic number of 6, meaning that there are six protons in its nucleus. Its two stable isotopes are 12C, which constitutes 98.9% of all carbon found in nature, and 13C, which accounts for the other 1.1%.

for all other elements are measured: the amu is defined as exactly 1/12 the mass of a single 12C atom. The difference in mass between 12C and 13C, which is heavier because of its extra neutron, account for the fact that the atomic mass of carbon is 12.01 amu: were it not for the small quantities of 13C present in a sample of carbon, the mass would be exactly 12.00 amu. WHERE

CA R B O N

IS

FOUND.

Carbon makes up only a small portion of the known elemental mass in Earth’s crust, oceans, and atmosphere—just 0.08%, or 1/1250 of the whole—yet it is the fourteenth most abundant element on the planet. In the human body, carbon is second only to oxygen in abundance, and accounts for 18% of the body’s mass. Thus if a person weighs 100 lb (45.3 kg), she is carrying around 18 lb (8.2 kg) of carbon—the same material from which diamonds are made! Present in the inorganic rocks of the ground and in the living creatures above it, carbon is everywhere. Combined with other elements, it forms carbonates, most notably calcium carbonate (CaCO3), which appears in the form of limestone, marble, and chalk. In combination with hydrogen, it creates hydrocarbons, present in deposits of fossil fuels: natural gas, petroleum, and coal. In the environment, carbon—in the form of carbon dioxide (CO2)—is taken in by plants, which undergo the process of photosynthesis and release oxygen to animals. Animals breathe in oxygen and release carbon dioxide to the atmosphere.

Carbon and Bonding

The mass of the 12C atom is the basis for the atomic mass unit (amu), by which mass figures

Located in Group 4 of the periodic table of elements (Group 14 in the IUPAC system), carbon has a valence electron configuration of 2s22p2;

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likewise, all the members of Group 4—sometimes known as the “carbon family”—have configurations of ns2np2, where n is the number of the period or row that the element occupies on the table. There are two elements noted for their ability to form long strings of atoms and seemingly endless varieties of molecules: one is carbon, and the other is silicon, directly below it on the periodic table. Silicon, found in virtually all types of rocks except the calcium carbonates (mentioned above), is to the inorganic world what carbon is to the organic. Yet silicon atoms are about one and a half times as large as those of carbon; thus not even silicon can compete with carbon’s ability to form a seemingly limitless array of molecules in various shapes and sizes, and having various chemical properties. B AS I CS O F C H E M I CA L B O N D I N G . Carbon is further distinguished by its

high value of electronegativity, the relative ability of an atom to attract valence electrons. Electronegativity increases with an increase in group number, and decreases with an increase in period number. In other words, the elements with the highest electronegativity values lie in the upper right-hand corner of the periodic table. Actually, the previous statement requires one significant qualification: the extreme righthand side of the periodic table is occupied by elements with negligible electronegativity values. These are the noble gases, which have eight valence electrons each. Eight, as it turns out, is the “magic number” for chemical bonding: most elements follow what is known as the octet rule, meaning that when one element bonds to another, the two atoms have eight valence electrons. If the two atoms have an electric charge and thus are ions, they form strong ionic bonds. Ionic bonding occurs when a metal bonds with a nonmetal. The other principal type of bond is a covalent bond, in which two uncharged atoms share eight valence electrons. If the electronegativity values of the two elements involved are equal, they share the electrons equally; but if one element has a higher electronegativity value, the electrons will be more drawn to that element.

So why is fluorine—capable of forming multitudinous bonds—not as chemically significant as carbon? There are a number of answers, but a simple one is this: because fluorine is too strong, and tends to “overwhelm” other elements, precluding the possibility of forming long chains, it is less chemically significant than carbon. Carbon, on the other hand, has an electronegativity value of 2.5, which places it well behind fluorine. Yet it is still at sixth place (in a tie with iodine and sulfur) on the periodic table, behind only fluorine; oxygen (3.5); nitrogen and chlorine (3.0); and bromine (2.8). In addition, with four valence electrons, carbon is ideally suited to find other elements (or other carbon atoms) for forming covalent bonds according to the octet rule. M U LT I P L E B O N D S . Normally, an element does not necessarily have the ability to bond with as many other elements as it has valence electrons, but carbon—with its four valence electrons—happens to be tetravalent, or capable of bonding to four other atoms at once. Additionally, carbon is capable of forming not only a single bond, but also a double bond, or even a triple bond, with other elements.

Suppose a carbon atom bonds to two oxygen atoms to form carbon dioxide. Let us imagine these three atoms side by side, with the oxygen in the middle. (This, in fact, is how these bonds are depicted in the Couper and Lewis systems of chemical symbolism, discussed in the Chemical Bonding essay.) We know that the carbon has four valence electrons, that the oxygens have six, and that the goal is for each atom to have eight valence electrons—some of which it will share covalently.

periodic table, let us ignore the noble gases, which are the chemical equivalent of snobs.

Two of the valence electrons from the carbon bond with two valence electrons each from the oxygen atoms on either side. This means that the carbon is doubly bonded to each of the oxygen atoms. Therefore, the two oxygens each have four other unbonded valence electrons, which might bond to another atom. It is theoretically possible, also, for the carbon to form a triple

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(Hence the term “noble,” meaning that they are set apart.) To the left of the noble gases are the halogens, a wildly gregarious bunch—none more so than the element that occupies the top of the column, fluorine. With an electronegativity value of 4.0, fluorine is the most reactive of all elements, and the only one capable of bonding even to a few of the noble gases.

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bond with one of the oxygens by sharing three of its valence electrons. It would then have one electron free to share with the other oxygen.

cyanate, it does not. Thus, to reduce the specifics of organic chemistry even further, it can be said that this area of the field constitutes the study of carbon chains, and ways to rearrange them in order to create new substances.

REAL-LIFE A P P L I C AT I O N S

Rubber, vitamins, cloth, and paper are all organically based compounds we encounter in our daily lives. In each case, the material comes from something that once was living, but what truly makes these substance organic in nature is the common denominator of carbon, as well as the specific arrangements of the atoms. We have organic chemistry to thank for any number of things: aspirins and all manner of other drugs; preservatives that keep food from spoiling; perfumes and toiletries; dyes and flavorings, and so on.

Organic Chemistry We have stated that carbon forms tetravalent bonds, and makes multiple bonds with a single atom. In addition, we have mentioned the fact that carbon forms long chains of atoms and varieties of shapes. But how does it do these things, and why? These are good questions, but not ones we will attempt to answer here. In fact, an entire branch of chemistry is devoted to answering these theoretical questions, as well as to determining solutions to a host of other, more practical problems. Organic chemistry is the study of carbon, its compounds, and their properties. (There are carbon-containing compounds that are not considered organic, however. Among these are oxides such as carbon dioxide and monoxide; as well as carbonates, most notably calcium carbonate.) At one time, chemists thought that “organic” was synonymous with “living,” and even as recently as the early nineteenth century, they believed that organic substances contained a supernatural “life force.” Then, in 1828, German chemist Friedrich Wöhler (1800-1882) cracked the code that distinguished the living from the nonliving, and the organic from the inorganic. Wöhler took a sample of ammonium cyanate (NH4OCN), and by heating it, converted it into urea (H2N-CO-NH2), a waste product in the urine of mammals. In other words, he had turned an inorganic material into a organic one, and he did so, as he observed, “without benefit of a kidney, a bladder, or a dog.” It was almost as though he had created life. In fact, what Wöhler had glimpsed—and what other scientists who followed came to understand, was this: what separates the organic from the inorganic is the manner in which the carbon chains are arranged.

Carbon

Allotropes of Carbon G RA P H I T E . Carbon has several allotropes—different versions of the same element, distinguished by molecular structure. The first of these is graphite, a soft material with an unusual crystalline structure. Graphite is essentially a series of one-atom-thick sheets of carbon, bonded together in a hexagonal pattern, but with only very weak attractions between adjacent sheets. A piece of graphite is thus like a big, thick stack of carbon paper: on the one hand, the stack is heavy, but the sheets are likely to slide against one another.

Actually, people born after about 1980 may have little experience with carbon paper, which was gradually phased out as photocopiers became cheaper and more readily available. Today, carbon paper is most often encountered when signing a credit-card receipt: the signature goes through the graphite-based backing of the receipt, onto a customer copy.

Ammonium cyanate and urea have exactly the same numbers and proportions of atoms, yet they are different compounds. They are thus isomers: substances which have the same formula, but are different chemically. In urea, the carbon forms an organic chain, and in ammonium

In such a situation, one might notice that the copied image of the signature looks as though it were signed in pencil. This is not surprising, considering that pencil “lead” is, in fact, a mixture of graphite, clay, and wax. In ancient times, people did indeed use lead—the heaviest member of Group 4, the “carbon family”—for writing, because it left gray marks on a surface. Lead, of course, is poisonous, and is not used today in pencils or in most applications that would involve prolonged exposure of humans to the element. Nonetheless, people still use the word “lead” in reference to pencils, much as they still

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refer to a galvanized steel roof with a zinc coating as a “tin roof.” In graphite the atoms of each “sheet” are tightly bonded in a hexagonal, or six-sided, pattern, but the attractions between the sheets are not very strong. This makes it highly useful as a lubricant for locks, where oil would tend to be messy. A good conductor of electricity, graphite is also utilized for making high-temperature electrolysis cells. In addition, the fact that graphite resists temperatures of up to about 6,332°F (3,500°C) makes it useful in electric motors and generators. D I A M O N D . The second allotrope of carbon is also crystalline in structure. This is diamond, most familiar in the form of jewelry, but in fact widely applied for a number of other purposes. According to the Moh scale, which measures the hardness of minerals, diamond is a 10— in other words, the hardest type of material. It is used for making drills that bore through solid rock; likewise, small diamonds are used in dentists’ drills for boring through the ultra-hard enamel on teeth.

Neither diamonds nor graphite are, in the strictest sense of the term, formed of molecules. Their arrangement is definite, as with a molecule, but their size is not: they simply form repeating patterns that seem to stretch on forever. Whereas graphite is in the form of sheets, a diamond is basically a huge “molecule” composed of carbon atoms strung together by covalent bonds. The size of this “molecule” corresponds to the size of the diamond: a diamond of 1 carat, for instance, contains about 1022 (10,000,000,000,000,000,000,000 or 10 billion billion) carbon atoms. The diamonds used in industry look quite different from the ones that appear in jewelry. Industrial diamonds are small, dark, and cloudy in appearance, and though they have the same chemical properties as gem-quality diamonds, they are cut with functionality (rather than beauty) in mind. A diamond is hard, but brittle: in other words, it can be broken, but it is very difficult to scratch or cut a diamond—except with another diamond.

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monds—as well as the diamonds on an engagement ring—are cut to refract or bend light rays, and to disperse the colors of visible light. BUCKMINSTERFULLERENE.

Until 1985, carbon was believed to exist in only two crystalline forms, graphite and diamond. In that year, however, chemists at Rice University in Houston, Texas, and at the University of Sussex in England, discovered a third variety of carbon—and later jointly received a Nobel Prize for their work. This “new” carbon molecule composed of 60 bonded atoms in the shape of what is called a “hollow truncated icosahedron.” In plain language, this is rather like a soccer ball, with interlocking pentagons and hexagons. However, because the surface of each geometric shape is flat, the “ball” itself is not a perfect sphere. Rather, it describes the shape of a geodesic dome, a design created by American engineer and philosopher R. Buckminster Fuller (1895-1983). There are other varieties of buckminsterfullerene molecules, known as fullerenes. However, the 60-atom shape, designated as 60C, is the most common of all fullerenes, the result of condensing carbon slowly at high temperatures. Fullerenes potentially have a number of applications, particularly because they exhibit a whole range of electrical properties: some are insulators, while some are conductors, semiconductors, and even superconductors. Due to the high cost of producing fullerenes artificially, however, the ways in which they are applied remain rather limited. A M O R P H O U S CA R B O N . There is

a fourth way in which carbon appears, distinguished from the other three in that it is amorphous, as opposed to crystalline, in structure. An example of amorphous carbon is carbon black, obtained from smoky flames and used in ink, or for blacking rubber tires.

The cutting of fine diamonds for jewelry is an art, exemplified in the alluring qualities of such famous gems as the jewels in the British Crown or the infamous Hope Diamond in Washington, D.C.’s Smithsonian Institution. Such dia-

Though it retains some of the microscopic structures of the plant cells in the wood from which it is made, charcoal—wood or other plant material that has been heated without enough air present to make it burn—is mostly amorphous carbon. One form of charcoal is activated charcoal, in which steam is used to remove the sticky products of wood decomposition. What remains are porous grains of pure carbon with enormous microscopic surface areas. These are used in water purifiers and gas masks.

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Carbon

A

MONTREAL, QUEBEC, CANADA. SUCH DOMES ARE NAMED AFTER THEIR DESIGNER, R. BUCKFULLER, AND EVENTUALLY PROVIDED THE NAME FOR THE CARBON MOLECULES KNOWN AS BUCKMINSTERFULLERENES. (Lee Snider/Corbis. Reproduced by permission.) GEODESIC DOME IN

MINSTER

Coal and coke are particularly significant varieties of amorphous carbon. Formed by the decay of fossils, coal was one of the first “fossil fuels” (for example, petroleum) used to provide heat and power for industrial societies. Indeed, when the words “industrial revolution” are mentioned, many people picture tall black smokestacks belching smoke from coal fires. Fortunately— from an environmental standpoint—coal is not nearly so widely used today, and when it is (as for instance in electric power plants), the methods for burning it are much more efficient than those applied in the nineteenth century.

joins with two oxygens to produce a gas essential to plant life. In carbon monoxide (CO), a single oxygen joins the carbon, creating a toxic—but nonetheless important—compound.

Actually, much of what those smokestacks of yesteryear burned was coke, a refined version of coal that contains almost pure carbon. Produced by heating soft coal in the absence of air, coke has a much greater heat value than coal, and is still widely used as a reducing agent in the production of steel and other alloys.

Flemish chemist and physicist Johannes van Helmont (1579-1644) discovered in 1630 that air was not, as had been thought up to that time, a single element: it contained a second substance, produced in the burning of wood, which he called “gas sylvestre.” Thus he is recognized as the first scientist to note the existence of carbon dioxide.

Carbon Dioxide Carbon forms many millions of compounds, some families of which will be discussed below. Two others, formed by the bonding of carbon atoms with oxygen atoms, are of particular significance. In carbon dioxide, a single carbon

More than a century later, in 1756, Scottish chemist Joseph Black (1728-1799) showed that carbon dioxide—which he called “fixed air”— combines with other chemicals to form compounds. This and other determinations Black made concerning carbon dioxide led to enor-

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The first gas to be distinguished from ordinary air, carbon dioxide is an essential component in the natural balance between plant and animal life. Animals, including humans, produce carbon dioxide by breathing, and humans further produce it by burning wood and other fuels. Plants use carbon dioxide when they store energy in the form of food, and they release oxygen to be used by animals. D I S C O V E R Y.

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Carbon

SOFT

DRINKS ARE MADE POSSIBLE BY THE USE OF CARBONATED WATER. (Sergio Dorantes/Corbis. Reproduced by permission.)

mous progress in the discovery of gases by various chemists of the late eighteenth century. By that time, chemists had begun to arrive at a greater degree of understanding with regard to the relationship between plant life and carbon dioxide. Up until that time, it had been believed that plants purify the air by day, and poison it at night. Carbon dioxide and its role in the connection between animal and plant life provided a much more sophisticated explanation as to the ways plants “breathe.” U S E S . Around the same time that Black

made his observations on carbon dioxide, English chemist Joseph Priestley (1733-1804) became the first scientist to put the chemical to use. Dissolving it in water, he created carbonated water, which today is used in making soft drinks. Not only does the gas add bubbles to drinks, it also acts as a preservative.

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In the solid form of dry ice, carbon dioxide is used for chilling perishable food during transport. It is also one of the only compounds that experiences sublimation, or the instantaneous transformation of a solid to a gas without passing through an intermediate liquid state, at conditions of ordinary pressure and temperature. Dry ice has often been used in movies to generate “mists” or “smoke” in a particular scene.

Carbon Monoxide During the late eighteenth century, Priestley discovered a carbon-oxygen compound different from carbon dioxide: carbon monoxide. Scientists had actually known of this toxic gas, released in the incomplete combustion of wood, from the Middle Ages onward, but Priestley was the first to identify it scientifically.

Though the natural uses of carbon dioxide are by far the most important, the compound has numerous industrial and commercial applications. Used in fire extinguishers, carbon dioxide is ideal for controlling electrical and oil fires, which cannot be put out with water. Heavier than air, carbon dioxide blankets the flames and smothers them.

Industry uses carbon monoxide in a number of ways. By blowing air across very hot coke, the result is producer gas, which, along with water gas (made by passing hot steam over coal) is an important fuel. Producer gas constitutes a 6:1:18 mixture of carbon monoxide, carbon dioxide, and nitrogen, while water gas is 40% carbon monoxide, 50% hydrogen, and 10% carbon dioxide and other gases.

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Carbon

KEY TERMS Different versions of

ALLOTROPES:

the same element, distinguished by molecular structure.

ferent chemically due to differences in the arrangement of atoms. ISOTOPES:

Having no definite

AMORPHOUS:

structure.

Atoms that have an equal

number of protons, and hence are of the same element, but differ in their number of

Naturally occur-

neutrons. This results in a difference of

ring compounds of carbon, hydrogen, and

mass. Isotopes may be either stable or

oxygen. These are primarily produced by

unstable. The latter type, known as

green plants through the process of photo-

radioisotopes, are radioactive.

synthesis.

OCTET RULE:

CELLULAR

CARBOHYDRATES:

A term describing the

The

distribution of valence electrons that takes

process whereby nutrients from plants are

place in chemical bonding for most ele-

broken down in an animal’s body to create

ments, which usually end up with eight

carbon dioxide.

valence electrons.

RESPIRATION:

A type of

COVALENT BONDING:

ORGANIC CHEMISTRY:

The study of

chemical bonding in which two atoms

carbon, its compounds, and their proper-

share valence electrons.

ties. (Many carbon-containing oxides and

CRYSTALLINE:

A term describing a

carbonates are not considered organic,

type of solid in which the constituent

however.)

parts have a simple and definite geo-

PHOTOSYNTHESIS:

metric arrangement that is repeated in all

conversion of light energy (that is, electro-

directions.

magnetic energy) to chemical energy in

DOUBLE BOND:

A form of bonding

The biological

plants.

in which two atoms share two pairs of

RADIOACTIVITY:

valence electrons. Carbon is also capable of

phenomenon whereby certain isotopes

single bonds and triple bonds.

known as radioisotopes are subject to a

A term describing a

The relative

form of decay brought about by the emis-

ability of an atom to attract valence

sion of high-energy particles. “Decay” does

electrons.

not mean that the isotope “rots”; rather, it

ELECTRONEGATIVITY:

An atom or group of atoms that

decays to form another isotope until even-

has lost or gained one or more electrons,

tually (though this may take a long time) it

and thus has a net electric charge.

becomes stable.

ION:

IONIC BONDING:

A form of chemical

SINGLE BOND:

A form of bonding in

bonding that results from attractions

which two atoms share one pair of valence

between ions with opposite electric

electrons. Carbon is also capable of double

charges.

bonds and triple bonds.

ISOMERS:

Substances which have the

same chemical formula, but which are dif-

S C I E N C E O F E V E RY DAY T H I N G S

TETRAVALENT:

Capable of bonding

to four other elements.

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Carbon

KEY TERMS TRIPLE BOND:

A form of bonding in

VALENCE ELECTRONS:

Electrons

which two atoms share three pairs of

that occupy the highest principal energy

valence electrons. Carbon is also capable of

level in an atom. These are the electrons

single bonds and double bonds.

involved in chemical bonding.

Not only are producer and water gas used for fuel, they are also applied as reducing agents. Thus, when carbon monoxide is passed over hot iron oxides, the oxides are reduced to metallic iron, while the carbon monoxide is oxidized to form carbon dioxide. Carbon monoxide is also used in reactions with metals such as nickel, iron, and cobalt to form some types of carbonyls. Carbon monoxide—produced by burning petroleum in automobiles, as well as by the combustion of wood, coal, and other carbon-containing fuels—is extremely hazardous to human health. It bonds with iron in hemoglobin, the substance in red blood cells that transports oxygen throughout the body, and in effect fools the body into thinking that it is receiving oxygenated hemoglobin, or oxyhemoglobin. Upon reaching the cells, carbon monoxide has much less tendency than oxygen to break down, and therefore it continues to circulate throughout the body. Low concentrations can cause nausea, vomiting, and other effects, while prolonged exposure to high concentrations can result in death.

Carbon and the Environment Carbon is released into the atmosphere by one of three means: cellular respiration; the burning of fossil fuels; and the eruption of volcanoes. When plants take in carbon dioxide from the atmosphere, they combine this with water and manufacture organic compounds using energy they have trapped from sunlight by means of photosynthesis—the conversion of light to chemical energy through biological means. As a by-product of photosynthesis, plants release oxygen into the atmosphere.

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CONTINUED

carbon-based as well. Animals eat the plants, or eat other animals that eat the plants, and thus incorporate the fats, proteins, and sugars (a form of carbohydrate) from the plants into their bodies. Cellular respiration is the process whereby these nutrients are broken down to create carbon dioxide. Photosynthesis and cellular respiration are thus linked in what is known as the carbon cycle. Cellular respiration also releases carbon into the atmosphere through the action of decomposers—bacteria and fungi that feed on the remains of plants and animals. The decomposers extract the energy in the chemical bonds of the decomposing matter, thus releasing more carbon dioxide into the atmosphere. When creatures die and are buried in such a way that they cannot be reached by decomposers—for instance, at the bottom of the ocean, or beneath layers of rock—the carbon in their bodies is eventually converted to fossil fuels, including petroleum, natural gas, and coal. The burning of fossil fuels releases carbon (both monoxide and dioxide) into the atmosphere. Because the rate of such burning has increased dramatically since the late nineteenth century, this has raised fears that carbon dioxide in the atmosphere may create a greenhouse effect, leading to global warming. On the other hand, volcanoes release tons of carbon into the atmosphere regardless of whether humans burn fossil fuels or not.

Radiocarbon Dating

In the process of undergoing photosynthesis, plants produce carbohydrates, which are various compounds of carbon, hydrogen, and oxygen essential to life. The other two fundamental components of a diet are fats and proteins, both

Radiocarbon dating is used to date the age of charcoal, wood, and other biological materials. When an organism is alive, it incorporates a certain ratio of carbon-12 in proportion to the amount of the radioisotope (that is, radioactive isotope) carbon-14 that it receives from the

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atmosphere. As soon as the organism dies, however, it stops incorporating new carbon, and the ratio between carbon-12 and carbon-14 will begin to change as the carbon-14 decays to form nitrogen-14. Carbon-14 has a half-life of 5,730 years, meaning that it takes that long for half the isotopes in a sample to decay to nitrogen-14. Therefore a scientist can use the ratios of carbon-12, carbon-14, and nitrogen-14 to guess the age of an organic sample. The problem with radiocarbon dating, however, is that there is a good likelihood the sample can become contaminated by additional carbon from the soil. Furthermore, it cannot be said with certainty that the ratio of carbon-12 to carbon-14 in the atmosphere has been constant throughout time.

S C I E N C E O F E V E RY DAY T H I N G S

WHERE TO LEARN MORE

Carbon

Blashfield, Jean F. Carbon. Austin, TX: Raintree SteckVaughn, 1999. “Carbon.” Xrefer (Web site). (May 30, 2001). “Diamonds.” American Museum of Natural History (Web site). (May 30, 2001). Knapp, Brian J. Carbon Chemistry. Illustrated by David Woodroffe. Danbury, CT: Grolier Educational, 1998. Loudon, G. Marc. Organic Chemistry. Menlo Park, CA: Benjamin/Cummings, 1988. “Organic Chemistry” (Web site). (May 30, 2001). “Organic Chemistry.” Frostburg State University Chemistry Helper (Web site). (May 30, 2001). Sparrow, Giles. Carbon. New York: Benchmark Books, 1999. Stille, Darlene. The Respiratory System. New York: Children’s Press, 1997.

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HYDROGEN Hydrogen

The atomic number of hydrogen is 1, meaning that it has a single proton in its nucleus. With its

single electron, hydrogen is the simplest element of all. Because it is such a basic elemental building block, figures for the mass of other elements were once based on hydrogen, but the standard today is set by 12C or carbon-12, the most common isotope of carbon. Hydrogen has two stable isotopes—forms of the element that differ in mass. The first of these, protium, is simply hydrogen in its most common form, with no neutrons in its nucleus. Protium (the name is only used to distinguish it from the other isotopes) accounts for 99.985% of all the hydrogen that appears in nature. The second stable isotope, deuterium, has one neutron, and makes up 0.015% of all hydrogen atoms. Tritium, hydrogen’s one radioactive isotope, will be discussed below. The fact that hydrogen’s isotopes have separate names, whereas all other isotopes are designated merely by element name and mass number (for example, “carbon-12”) says something about the prominence of hydrogen as an element. Not only is its atomic number 1, but in many ways, it is like the number 1 itself—the essential piece from which all others are ultimately constructed. Indeed, nuclear fusion of hydrogen in the stars is the ultimate source for the 90-odd elements that occur in nature. The mass of this number-one element is not, however, 1: it is 1.008 amu, reflecting the small quantities of deuterium, or “heavy hydrogen,” present in a typical sample. A gas at ordinary temperatures, hydrogen turns to a liquid at -423.2°F (-252.9°C), and to a solid at –434.°F (–259.3°C). These figures are its boiling point and melting point respectively; only the figures for helium are lower. As noted earlier, these two elements make up all but 0.01% of the known

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CONCEPT First element on the periodic table, hydrogen is truly in a class by itself. It does not belong to any family of elements, and though it is a nonmetal, it appears on the left side of the periodic table with the metals. The other elements with it in Group 1 form the alkali metal family, but obviously, hydrogen does not belong with them. Indeed, if there is any element similar to hydrogen in simplicity and abundance, it is the only other one on the first row, or period, of the periodic table: helium. Together, these two elements make up 99.9% of all known matter in the entire universe, because hydrogen atoms in stars fuse to create helium. Yet whereas helium is a noble gas, and therefore chemically unreactive, hydrogen bonds with all sorts of other elements. In one such variety of bond, with carbon, hydrogen forms the backbone for a vast collection of organic molecules, known as hydrocarbons and their derivatives. Bonded with oxygen, hydrogen forms the single most important compound on Earth, and the most important complex substance other than air: water. Yet when it bonds with sulfur, it creates toxic hydrogen sulfide; and on its own, hydrogen is extremely flammable. The only element whose isotopes have names, hydrogen has long been considered as a potential source of power and transportation: once upon a time for airships, later as a component in nuclear reactions—and, perhaps in the future, as a source of abundant clean energy.

HOW IT WORKS The Essentials

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Hydrogen

AMONG

THE USES OF TRITIUM IS IN FUEL FOR HYDROGEN BOMBS.

STATE OF

WASHINGTON,

SHOWN

HERE IS THE

HANFORD

PLANT IN THE

WHERE SUCH FUEL IS PRODUCED. (Roger Ressmeyer/Corbis. Reproduced by permission.)

elemental mass of the universe, and are the principal materials from which stars are formed. Normally hydrogen is diatomic, meaning that its molecules are formed by two atoms. At the interior of a star, however, where the temperature is many millions of degrees, H2 molecules are separated into atoms, and these atoms become ionized. In other words, the electron separates from the proton, resulting in an ion with a positive charge, along with a free electron. The positive ions experience fusion—that is, their nuclei bond, releasing enormous amounts of energy as they form new elements. Because the principal isotopic form of helium has two protons in the nucleus, it is natural that helium is the element usually formed; yet it is nonetheless true—amazing as it may seem— that all the elements found on Earth were once formed in stars. On Earth, however, hydrogen ranks ninth in its percentage of the planet’s known elemental mass: just 0.87%. In the human body, on the other hand, it is third, after oxygen and carbon, making up 10% of human elemental body mass.

is to combine its electron with one from the atom of a nonmetallic element to make a covalent bond, in which the two electrons are shared. Hydrogen is unusual in this regard, because most atoms conform to the octet rule, ending up with eight valence electrons. The bonding behavior of hydrogen follows the duet rule, resulting in just two electrons for bonding. Examples of this first type of bond include water (H2O), hydrogen sulfide (H2S), and ammonia (NH3), as well as the many organic compounds formed on a hydrogen-carbon backbone. But hydrogen can form a second type of bond, in which it gains an extra electron to become the negative ion H-, or hydride. It is then able to combine with a metallic positive ion to form an ionic bond. Ionic hydrides are convenient sources of hydrogen gas: for instance, calcium hydride, or CaH2, is sold commercially, and provides a very convenient means of hydrogen generation. The hydrogen gas produced by the reaction of calcium hydride with water can be used to inflate life rafts.

Having just one electron, hydrogen can bond to other atoms in one of two ways. The first option

The presence of hydrogen in certain types of molecules can also be a factor in intermolecular bonding. Intermolecular bonding is the attraction between molecules, as opposed to the bond-

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Hydrogen

THE

USE OF HYDROGEN IN AIRSHIPS EFFECTIVELY ENDED WITH THE CRASH OF THE

HINDENBURG in 1937. (UPI/Cor-

bis-Bettmann. Reproduced by permission.)

ing within molecules, which is usually what chemists mean when they talk about “bonding.”

Hydrogen’s Early History Because it bonds so readily with other elements, hydrogen almost never appears in pure elemental form on Earth. Yet by the late fifteenth century, chemists recognized that by adding a metal to an acid, hydrogen was produced. Only in 1766, however, did English chemist and physicist Henry Cavendish (1731-1810) recognize hydrogen as a substance distinct from all other “airs,” as gases then were called. Seventeen years later, in 1783, French chemist Antoine Lavoisier (1743-1794) named the substance after two Greek words: hydro (water) and genes (born or formed). It was another two decades before English chemist John Dalton (1766-1844) formed his atomic theory of matter, and despite the great strides he made for science, Dalton remained convinced that hydrogen and oxygen in water formed “water atoms.” Around the same time, however, Italian physicist Amedeo Avogadro (1776-1856) clarified the distinction between atoms and molecules, though this theory would not be generally accepted until the 1850s.

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Contemporary to Dalton and Avogadro was Swedish chemist Jons Berzelius (1779-1848), who developed a system of comparing the mass of various atoms in relation to hydrogen. This method remained in use for more than a century, until the discovery of neutrons, protons, and isotopes pointed the way toward a means of making more accurate determinations of atomic mass. In 1931, American chemist and physicist Harold Urey (1893-1981) made the first separation of an isotope: deuterium, from ordinary water.

REAL-LIFE A P P L I C AT I O N S Deuterium and Tritium Designated as 2H, deuterium is a stable isotope, whereas tritium—3H—is unstable, or radioactive. Not only do these two have names; they even have chemical symbols (D and T, respectively), as though they were elements on the periodic table. Just as hydrogen represents the most basic proton-electron combination against which other atoms are compared, these two are respectively the most basic isotope containing a single neu-

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tron, and the most basic radioisotope, or radioactive isotope. Deuterium is sometimes called “heavy hydrogen,” and its nucleus is called a deuteron. In separating deuterium—an achievement for which he won the 1934 Nobel Prize—Urey collected a relatively large sample of liquid hydrogen: 4.2 qt (4 l). He then allowed the liquid to evaporate very slowly, predicting that the more abundant protium would evaporate more quickly than the heavier isotope. When all but 0.034 oz (1 ml) of the sample had evaporated, he submitted the remainder to a form of analysis called spectroscopy, adding a burst of energy to the atoms and then analyzing the light spectrum they emitted for evidence of differing varieties of atoms. With an atomic mass of 2.014102 amu, deuterium is almost exactly twice as heavy as protium, which has an atomic mass of 1.007825. Its melting points and boiling points, respectively –426°F (–254°C) and –417°F (–249°C), are higher than for protium. Often, deuterium is applied as a tracer, an atom or group of atoms whose participation in a chemical, physical, or biological reaction can be easily observed. DEUTERIUM IN WAR AND P E AC E . In nuclear power plants, deuterium

is combined with oxygen to form “heavy water” (D2O), which likewise has higher boiling and melting points than ordinary water. Heavy water is often used in nuclear fission reactors to slow down the fission process, or the splitting of atoms. Deuterium is also present in nuclear fusion, both on the Sun and in laboratories. During the period shortly after World War II, physicists developed a means of duplicating the thermonuclear fusion process. The result was the hydrogen bomb—more properly called a fusion bomb—whose detonating device was a compound of lithium and deuterium called lithium deuteride. Vastly more powerful than the “atomic” (that is, fission) bombs dropped by the United States over Japan (Nagaski and Hiroshima) in 1945, the hydrogen bomb greatly increased the threat of worldwide nuclear annihilation in the postwar years.

process involving the fusion of two deuterons. This fusion would result in a triton, the nucleus of tritium, along with a single proton. Theoretically, the triton and deuteron would then be fused to create a helium nucleus, resulting in the production of vast amounts of energy.

Hydrogen

T R I T I U M . Whereas deuterium has a single neutron, tritium—as its mass number of 3 indicates—has two. And just as deuterium has approximately twice the mass of protium, tritium has about three times the mass, or 3.016 amu. Its melting and boiling points are higher still than those of deuterium: thus tritium heavy water (T2O) melts at 40°F (4.5°C), as compared with 32°F (0°C) for H2O.

Tritium has a half-life (the length of time it takes for half the radioisotopes in a sample to become stable) of 12.26 years. As it decays, its nucleus emits a low-energy beta particle, which is either an electron or a subatomic particle called a positron, resulting in the creation of the helium3 isotope. Due to the low energy levels involved, the radioactive decay of tritium poses little danger to humans. In fact, there is always a small quantity of tritium in the atmosphere, and this quantity is constantly being replenished by cosmic rays. Like deuterium, tritium is applied in nuclear fusion, but due to its scarcity, it is usually combined with deuterium. Sometimes it is released in small quantities into the groundwater as a means of monitoring subterranean water flow. It is also used as a tracer in biochemical processes, and as an ingredient in luminous paints.

Hydrogen and Oxygen WAT E R. Water, of course, is the most well-known compound involving hydrogen. Nonetheless, it is worthwhile to consider the interaction between hydrogen and oxygen, the two ingredients in water, which provides an interesting illustration of chemistry in action.

Yet the power that could destroy the world also has the potential to provide safe, abundant fusion energy from power plants—a dream as yet unrealized. Physicists studying nuclear fusion are attempting several approaches, including a

Chemically bonded as water, hydrogen and oxygen can put out any type of fire except an oil or electrical fire; as separate substances, however, hydrogen and oxygen are highly flammable. In an oxyhydrogen torch, the potentially explosive reaction between the two gases is controlled by a gradual feeding process, which produces combustion instead of the more violent explosion that sometimes occurs when hydrogen and oxygen come into contact.

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H Y D R O G E N P E R O X I D E . Aside from water, another commonly used hydrogenoxygen compound is hydrogen peroxide, or H2O2. A colorless liquid, hydrogen peroxide is chemically unstable (not “unstable” in the way that a radioisotope is), and decomposes slowly to form water and oxygen gas. In high concentrations, it can be used as rocket fuel.

By contrast, the hydrogen peroxide used in homes as a disinfectant and bleaching agent is only a 3% solution. The formation of oxygen gas molecules causes hydrogen peroxide to bubble, and this bubbling is quite rapid when the peroxide is placed on cuts, because the enzymes in blood act as a catalyst to speed up the reaction.

Hydrogen Chloride Another significant compound involving hydrogen is hydrogen chloride, or HCl—in other words, one hydrogen atom bonded to chlorine, a member of the halogens family. Dissolved in water, it produces hydrochloric acid, used in laboratories for analyses involving other acids. Normally, hydrogen chloride is produced by the reaction of salt with sulfuric acid, though it can also be created by direct bonding of hydrogen and chlorine at temperatures above 428°F (250°C). Hydrogen chloride and hydrochloric acid have numerous applications in metallurgy, as well as in the manufacture of pharmaceuticals, dyes, and synthetic rubber. They are used, for instance, in making pharmaceutical hydrochlorides, water-soluble drugs that dissolve when ingested. Other applications include the production of fertilizers, synthetic silk, paint pigments, soap, and numerous other products. Not all hydrochloric acid is produced by industry, or by chemists in laboratories. Active volcanoes, as well as waters from volcanic mountain sources, contain traces of the acid. So, too, does the human body, which generates it during digestion. However, too much hydrochloric acid in the digestive system can cause the formation of gastric ulcers.

Hydrogen Sulfide It may not be a pleasant subject, but hydrogen— in the form of hydrogen sulfide—is also present in intestinal gas. The fact that hydrogen sulfide is an extremely malodorous substance once again illustrates the strange things that happen when

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elements bond: neither hydrogen nor sulfur has any smell on its own, yet together they form an extremely noxious—and toxic—substance. Pockets of hydrogen sulfide occur in nature. If a person were to breathe the vapors for very long, it could be fatal, but usually, the foul odor keeps people away. The May 2001 National Geographic included two stories relating to such natural hydrogen-sulfide deposits, on opposite sides of the Earth, and in both cases the presence of these toxic fumes created interesting results. In southern Mexico is a system of caves known as Villa Luz, through which run some 20 underground springs, many of them carrying large quantities of hydrogen sulfide. The National Geographic Society’s team had to enter the caves wearing gas masks, yet the area teems with strange varieties of life. Among these are fish that are red from high concentrations of hemoglobin, or red blood cells. The creatures need this extra dose of hemoglobin, necessary to move oxygen through the body, in order to survive on the scant oxygen supplies. The waters of the cave are further populated by microorganisms that oxidize the hydrogen sulfide and turn it into sulfuric acid, which dissolves the rock walls and continually enlarges the cave. Thousands of miles away, in the Black Sea, explorers supported by a grant from the National Geographic Society examined evidence suggesting that there indeed had been a great ancient flood in the area, much like the one depicted in the Bible. In their efforts, they had an unlikely ally: hydrogen sulfide, which had formed at the bottom of the sea, and was covered by dense layers of salt water. Because the Black Sea lacks the temperature differences that cause water to circulate from the bottom upward, the hydrogen sulfide stayed at the bottom. Under normal circumstances, the wreck of a 1,500-year-old wooden ship would not have been preserved; but because oxygen could not reach the bottom of the Black Sea—and thus woodboring worms could not live in the toxic environment—the ship was left undisturbed. Thanks to the presence of hydrogen sulfide, explorers were able to study the ship, the first fully intact ancient shipwreck to be discovered.

Hydrocarbons Together with carbon, hydrogen forms a huge array of organic materials known as hydrocar-

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bons—chemical compounds whose molecules are made up of nothing but carbon and hydrogen atoms. Theoretically, there is no limit to the number of possible hydrocarbons. Not only does carbon form itself into seemingly limitless molecular shapes, but hydrogen is a particularly good partner. Because it has the smallest atom of any element on the periodic table, it can bond to one of carbon’s valence electrons without getting in the way of the others.

functional groups include aldehydes, ketones, carboxylic acids, and esters. Products of these functional groups range from aspirin to butyric acid, which is in part responsible for the smell both of rancid butter and human sweat. Hydrocarbons also form the basis for polymer plastics such as Nylon and Teflon.

Hydrocarbons may either be saturated or unsaturated. A saturated hydrocarbon is one in which the carbon atom is already bonded to four other atoms, and thus cannot bond to any others. In an unsaturated hydrocarbon, however, not all the valence electrons of the carbon atom are bonded to other atoms.

We have already seen that hydrogen is a component of petroleum, and that hydrogen is used in creating nuclear power—both deadly and peaceful varieties. But hydrogen has been applied in many other ways in the transportation and power industries.

Hydrogenation is a term describing any chemical reaction in which hydrogen atoms are added to carbon multiple bonds. There are many applications of hydrogenation, but one that is particularly relevant to daily life involves its use in turning unsaturated hydrocarbons into saturated ones. When treated with hydrogen gas, unsaturated fats (fats are complex substances that involve hydrocarbons bonded to other molecules) become saturated fats, which are softer and more stable, and stand up better to the heat of frying. Many foods contain hydrogenated vegetable oil; however, saturated fats have been linked with a rise in blood cholesterol levels— and with an increased risk of heart disease. P E T R O C H E M I CA L S A N D F U N C T I O N A L G R O U P S . One important variety

of hydrocarbons is described under the collective heading of petrochemicals—that is, derivatives of petroleum. These include natural gas; petroleum ether, a solvent; naphtha, a solvent (for example, paint thinner); gasoline; kerosene; fuel for heating and diesel fuel; lubricating oils; petroleum jelly; paraffin wax; and pitch, or tar. A host of other organic chemicals, including various drugs, plastics, paints, adhesives, fibers, detergents, synthetic rubber, and agricultural chemicals, owe their existence to petrochemicals.

Hydrogen

Hydrogen for Transportation and Power

There are only three gases practical for lifting a balloon: hydrogen, helium, and hot air. Each is much less dense than ordinary air, and this gives them their buoyancy. Because hydrogen is the lightest known gas and is relatively cheap to produce, it initially seemed the ideal choice, particularly for airships, which made their debut near the end of the nineteenth century. For a few decades in the early twentieth century, airships were widely used, first in warfare and later as the equivalent of luxury liners in the skies. One of the greatest such craft was Germany’s Hindenburg, which used hydrogen to provide buoyancy. Then, on May 6, 1937, the Hindenburg caught fire while mooring at Lakehurst, New Jersey, and 36 people were killed—a tragic and dramatic event that effectively ended the use of hydrogen in airships. Adding to the pathos of the Hindenburg crash was the voice of radio announcer Herb Morrison, whose audio report has become a classic of radio history. Morrison had come to Lakehurst to report on the landing of the famous airship, but ended up with the biggest—and most horrifying—story of his career. As the ship burst into flames, Morrison’s voice broke, and he uttered words that have become famous: “Oh, the humanity!”

Then there are the many hydrocarbon derivatives formed by the bonding of hydrocarbons to various functional groups—broad arrays of molecule types involving other elements. Among these are alcohols—both ethanol (the alcohol in beer and other drinks) and methanol, used in adhesives, fibers, and plastics, and as a fuel. Other

Half a century later, a hydrogen-related disaster destroyed a craft much more sophisticated than the Hindenburg, and this time, the medium of television provided an entire nation with a view of the ensuing horror. The event was the explosion of the space shuttle Challenger on January 28, 1986, and the cause was the failure of a rubber seal in the shuttle’s fuel tanks. As a result,

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Hydrogen

KEY TERMS COVALENT BONDING:

A type of

chemical bonding in which two atoms share valence electrons.

radioactive. The center of an atom, a

NUCLEUS:

A term describing an ele-

region where protons and neutrons are

ment that exists as molecules composed of

located, and around which electrons spin.

two atoms.

The plural of “nucleus” is nuclei.

DIATOMIC:

A term describing the

DUET RULE:

A term describing the

OCTET RULE:

distribution of valence electrons when

distribution of valence electrons that takes

hydrogen atoms—which end up with only

place in chemical bonding for most ele-

two valence electrons—experience chemi-

ments, which end up with eight valence

cal bonding with other atoms. Most other

electrons. Hydrogen is an exception, and

elements follow the octet rule.

follows the duet rule.

ELECTROLYSIS:

The use of an elec-

tric current to cause a chemical reaction. FISSION:

A nuclear reaction involving

FUSION:

A nuclear reaction that

involves the joining of atomic nuclei. HYDROCARBON:

ORGANIC:

At one time, chemists used

the term “organic” only in reference to living things. Now the word is applied to most compounds containing carbon, with

the splitting of atoms.

Any chemical com-

the exception of calcium carbonate (limestone) and oxides such as carbon dioxide. RADIOISOTOPE:

An isotope subject

pound whose molecules are made up of

to the decay associated with radioactivity.

nothing but carbon and hydrogen atoms.

A radioisotope is thus an unstable isotope.

HYDROGENATION:

A chemical reac-

SATURATED:

A term describing a

tion in which hydrogen atoms are added to

hydrocarbon in which each carbon is

carbon multiple bonds, as in a hydro-

already bound to four other atoms.

carbon.

TRACER:

An atom or group of atoms

An atom or group of atoms that

whose participation in a chemical, physi-

has lost or gained one or more electrons,

cal, or biological reaction can be easily

and thus has a net electrical charge.

observed. Radioisotopes are often used as

ION:

IONIC BONDING:

A form of chemical

tracers. A term describing a

bonding that results from attractions

UNSATURATED:

between ions with opposite electric

hydrocarbon, in which the carbons

charges. The bonding of a metal to a non-

involved in a multiple bond are free to

metal such as hydrogen is ionic.

bond with other atoms.

ISOTOPES:

258

mass. An isotope may either be stable or

Atoms that have an equal

VALENCE ELECTRONS:

Electrons

number of protons, and hence are of the

that occupy the highest principal energy

same element, but differ in their number of

level in an atom. These are the electrons

neutrons. This results in a difference of

involved in chemical bonding.

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hydrogen gas flooded out of the craft and straight into the jet of flame behind the rocket. All seven astronauts aboard were killed. THE FUTURE OF HYDROGEN P O W E R. Despite the misfortunes that have

occurred as a result of hydrogen’s high flammability, the element nonetheless holds out the promise of cheap, safe power. Just as it made possible the fusion, or hydrogen, bomb—which fortunately has never been dropped in wartime, but is estimated to be many hundreds of times more lethal than the fission bombs dropped on Japan—hydrogen may be the key to the harnessing of nuclear fusion, which could make possible almost unlimited power. A number of individuals and agencies advocate another form of hydrogen power, created by the controlled burning of hydrogen in air. Not only is hydrogen an incredibly clean fuel, producing no by-products other than water vapor, it is available in vast quantities from water. In order to separate it from the oxygen atoms, electrolysis would have to be applied—and this is one of the challenges that must be addressed before hydrogen fuel can become a reality. Electrolysis requires enormous amounts of electricity, which would have to be produced before the benefits of hydrogen fuel could be realized. Furthermore, though the burning of hydrogen could be controlled, there are the dan-

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gers associated with transporting it across country in pipelines. Nonetheless, a number of advocacy groups—some of whose Web sites are listed below—continue to promote efforts toward realizing the dream of nonpolluting, virtually limitless, fuel.

Hydrogen

WHERE TO LEARN MORE American Hydrogen Association (Web site). (June 1, 2001). Blashfield, Jean F. Hydrogen. Austin, TX: Raintree SteckVaughn, 1999. Farndon, John. Hydrogen. New York: Benchmark Books, 2001. “Hydrogen” (Web site). (June 1, 2001). Hydrogen Energy Center (Web site). (June 1, 2001). Hydrogen Information Network (Web site). (June 1, 2001). Knapp, Brian J. Carbon Chemistry. Illustrated by David Woodroffe. Danbury, CT: Grolier Educational, 1998. Knapp, Brian J. Elements. Illustrated by David Woodroffe and David Hardy. Danbury, CT: Grolier Educational, 1996. National Hydrogen Association (Web site). (June 1, 2001). Uehling, Mark. The Story of Hydrogen. New York: Franklin Watts, 1995.

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BONDING AND REACTIONS CHEMICAL BONDING COMPOUNDS CHEMICAL REACTIONS O X I D AT I O N - R E D U C T I O N REACTIONS CHEMICAL EQUILIBRIUM C A T A LY S T S ACIDS AND BASES ACID-BASE REACTIONS

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CHEMICAL BONDING

CONCEPT Almost everything a person sees or touches in daily life—the air we breathe, the food we eat, the clothes we wear, and so on—is the result of a chemical bond, or, more accurately, many chemical bonds. Though a knowledge of atoms and elements is essential to comprehend the subjects chemistry addresses, the world is generally not composed of isolated atoms; rather, atoms bond to one another to form molecules and hence chemical compounds. Not all chemical bonds are created equal: some are weak, and some very strong, a difference that depends primarily on the interactions of electrons between atoms.

HOW IT WORKS Early Ideas of Bonding The theory that all of matter is composed of atoms did not originate in modern times: the atomic model actually dates back to the fifth century B.C. in Greece. The leading exponent of atomic theory in ancient times was Democritus (c. 460-370 B.C.), who proposed that matter could not be infinitely subdivided. At its deepest substructure, Democritus maintained, the material world was made up of tiny fragments he called atomos, a Greek term meaning “no cut” or “indivisible” Forward-thinking though it was, Democritus’s idea was not what modern scientists today would describe as a proper scientific hypothesis. His “atoms” were not purely physical units, but rather idealized philosophical constructs, and thus, he was not really approaching the subject from the perspective of a scientist. In any case,

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there was no way for Democritus to test his hypothesis even if he had wanted to: by their very nature, the atoms he described were far too small to observe. Even today, what scientists know about atomic behavior comes not from direct observation, but indirect means. Hence, Democritus and the few other ancients who subscribed to atomic theory went more on instinct than by scientific methods. Yet, some of them were remarkably prescient in their description of the bonding of atoms, in view of the primitive scientific methods they had at their disposal. No other scientist came close to the accuracy of their theory for about 2,000 years. AS C L E P I A D E S A N D LU C R E T I U S D I S C U SS B O N D S . The physician

Asclepiades of Prusa (c.130-40 B.C.) drew on the ideas of the Greek philosopher Epicurus (341270 B.C., another proponent of atomism. Asclepiades speculated on the ways in which atoms interact, and discussed “clusters of atoms,” though, of course, he had no idea what force attracted the atoms to one another. A few years after Asclepiades, the Roman philosopher and poet Lucretius (c.95-c.55 B.C. espoused views that combined atomism with the idea of the “four elements”—earth, air, fire, and water. In his great work De rerum natura (“On the Nature of Things”), Lucretius described atoms as tiny spheres attached to each other by fishhook-like appendages that became entangled with one another. LAC K O F P R O G R E SS U N T I L 1 8 0 0 . Unfortunately, the competing idea of

the four elements, handed down by the great philosopher Aristotle (384-322 B.C.), prevailed over the atomic model. As the Roman Empire

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Chemical Bonding

ELECTRON-DOT

DIAGRAMS OF SOME ELEMENTS IN THE PERIODIC TABLE. (Robert L. Wolke. Reproduced by permission.)

began to decline after A.D. 200, the pace of scientific inquiry slowed and—in Western Europe at least—eventually came to a virtual halt. Hence, the four elements theory, which had its own fanciful explanations as to why certain “elements” bonded with one another, held sway in Europe until the beginning of the modern era. During the seventeenth century, a mounting array of facts from the realms of astronomy and physics collectively disproved the Aristotelian model. In the area of chemistry, English physicist and chemist Robert Boyle (1627-1691) showed that the four elements were not elements at all, because they could be broken down into simpler substances. Yet, no one really understood what constituted an element until the very beginning of the nineteenth century, and until that question was addressed, it was difficult to move on to the mystery of why certain atoms bonded with one another.

Early Modern Advances in Bonding Theory

Though Dalton’s theory paved the way for enormous advances in the years that followed, there were a number of flaws in it. Mass alone, for instance, is not really what differentiates one atom from another: differences in mass reflect the presence of subatomic particles—protons and neutrons—of whose existence scientists were unaware at the time. Furthermore, the properties of atoms that cause them to bond relate to a third subatomic particle, the electron, which, though it contributes little to the mass of the atom, is allimportant to the energy it possesses. As for how atoms bond to one another, Dalton had little to say: in his conception of the atomic model, atoms simply sit adjacent to one another without forming true bonds, as such.

T H E O R Y.

AV O GA D R O A N D T H E M O L E C U L E . Though Dalton recognized that the

The birth of atomic theory in modern times occurred in 1803, when English chemist John Dalton (1766-1844) formulated the idea that all

structure of atoms in a particular element or compound is uniform, he maintained that compounds are made up of compound atoms: thus,

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D A LT O N ’ S

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elements are composed of tiny, indestructible particles. These he called by the name Democritus had given them nearly 23 centuries earlier: atoms. All known substances, he said, are composed of some combination of atoms, which differ from one another only in mass.

AT O M I C

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Chemical Bonding

HYDROGEN

BONDING IN

HF, H2O,

AND

NH3. (Robert L. Wolke. Reproduced by permission.)

water is a compound of “water atoms.” However, water is not an element, and therefore, there had to be some structure—still very small, but larger than the atom—in which atoms coalesced to form the basic materials of a compound.

the molecule resurrected by Italian chemist Stanislao Cannizzaro (1826-1910). Cannizzaro’s work was occasioned by disagreement among scientists regarding the determination of atomic mass; however, the establishment of the molecular model had far-reaching implications for theories of bonding.

That structure was the molecule, first described by Italian physicist Amedeo Avogadro (1776-1856). For several decades, Avogadro, who originated the idea of the mole as a means of comparing large groups of atoms or molecules, remained a more or less unsung hero. Only in 1860, four years after his death, was his idea of

In 1858, German chemist Friedrich August Kekulé (1829-1896) made the first attempt to define the concept of valency, or the property an atom of one element possesses that determines

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SYMBOLIZING ATOMIC BONDS.

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“PURE”

WATER FROM A MOUNTAIN STREAM IS ACTUALLY FILLED WITH TRACES OF THE ROCKS OVER WHICH IT

HAS FLOWED.

IN

FACT, WATER IS ALMOST IMPOSSIBLE TO FIND IN PURE FORM EXCEPT BY PURIFYING IT IN A LABO-

RATORY. (David Muench/Corbis. Reproduced by permission.)

266

its ability to bond with atoms of other elements. A pioneer in organic chemistry, which deals with chemical structures containing carbon, Kekulé described the carbon atom as tetravalent, mean-

ing that it can bond to four other atoms. (The Latin prefix tetra- means “four.”) He also speculated that carbon atoms are capable of bonding with one another in long chains.

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This was one of the first attempts to examine the subject of bonding using modern scientific terminology, complete with hypotheses that could be tested by experimentation. Kekulé also recognized that in order to discuss bonds understandably, there needed to be some means of representing those bonds with symbols. He even went so far as to develop a system for showing the arrangement of bonds in space; however, his system was so elaborate that it was replaced in favor of a simpler one developed by Scottish chemist Archibald Scott Couper (1831-1892). Couper, who also studied valency and the tetravalent carbon bond—he is usually given equal credit with Kekulé for these ideas—created an extremely straightforward schematic representation still in use by chemists today. In Couper’s system, short dashed lines serve to designate chemical bonds. Hence, the bond between two hydrogen atoms and an oxygen atom in a water molecule would be represented thus: H-OH. As the understanding of bonds progressed in modern times, this system was modified to take into account multiple bonds, discussed below.

REAL-LIFE A P P L I C AT I O N S Atoms, Electrons, and Ions Today, chemical bonding is understood as the joining of atoms through electromagnetic force. Before that understanding could be achieved, however, scientists had to unlock the secret of the electromagnetic interactions that take place within an atom.

and, like a bee, it buzzes to and fro, carrying a powerful “sting”—its negative electric charge, which attracts it to the positively charged proton.

Chemical Bonding

E L E C T R O N S A N D I O N S . Though the electron weighs much, much less than a proton, it possesses enough electric charge to counterbalance the positive charge of the proton. All atoms have the same number of protons as electrons, and hence the net electric charge is zero. However, as befits their highly active role, electrons are capable of moving from one atom to another under the proper circumstances. An atom that loses or acquires electrons has an electric charge, and is called an ion.

The atom that has lost an electron or electrons becomes a positively charged ion, or cation. On the other hand, an atom that gains an electron or electrons becomes a negatively charged ion, or anion. As we shall see, ionic bonds, such as those that join sodium and chlorine atoms to form NaCl, or salt, are extremely powerful. ELECTRON

C O N F I G U RAT I O N .

Even in covalent bonding, which does not involve ions, the configurations of electrons in two atoms are highly important. The basics of electron configuration are explained in the Electrons essay, though even there, this information is presented with the statement that the student should consult a chemistry textbook for a more exhaustive explanation.

The key to bonding is the electron, discovered in 1897 by English physicist J. J. Thomson (1856-1940). Atomic structure in general, and the properties of the electron in particular, are discussed at length elsewhere in this volume. However, because these specifics are critical to bonding, they will be presented here in the shortest possible form.

In the simplest possible terms, electron configuration refers to the distribution of electrons at various positions in an atom. However, because the behavior of electrons cannot be fully predicted, this distribution can only be expressed in terms of probability. An electron moving around the nucleus of an atom can be compared to a fly buzzing around some form of attractant (e.g., food or a female fly, if the moving fly is male) at the center of a sealed room. We can state positively that the fly is in the room, and we can predict that he will be most attracted to the center, but we can never predict his location at any given moment.

At the center of an atom is a nucleus, consisting of protons, with a positive electrical charge; and neutrons, which have no charge. These form the bulk of the atom’s mass, but they have little to do with bonding. In fact, the neutron has nothing to do with it, while the proton plays only a passive role, rather like a flower being pollinated by a bee. The “bee” is the electron,

As one moves along the periodic table of elements, electron configurations become ever more complex. The reason is that with an increase in atomic number, there is an increase in the energy levels of atoms. This indicates a greater range of energies that electrons can occupy, as well as a greater range of motion. Electrons occupying the highest energy level in an atom are

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called valence electrons, and these are the only ones involved in chemical bonding. By contrast, the core electrons, or the ones closest to the nucleus, play no role in the bonding of atoms.

Ionic and Covalent Bonds T H E G OA L O F E I G H T VA L E N C E E L E C T R O N S . The above dis-

cussion of the atom, and the electron’s place in it, refers to much that was unknown at the time Thomson discovered the electron. Protons were not discovered for several more years, and neutrons several decades after that. Nonetheless, the electron proved the key to solving the riddle of how substances bond, and not long after Thomson’s discovery, German chemist Richard Abegg (1869-1910) suggested as much. While studying noble gases, noted for their tendency not to bond, Abegg discovered that these gases always have eight valence electrons. His observation led to one of the most important principles of chemical bonding: atoms bond in such a way that they achieve the electron configuration of a noble gas. This has been shown to be the case in most stable chemical compounds. TWO DIFFERENT TYPES OF B O N D S . Perhaps, Abegg hypothesized, atoms

combine with one another because they exchange electrons in such a way that both end up with eight valence electrons. This was an early model of ionic bonding, which results from attractions between ions with opposite electric charges: when they bond, these ions “complete” one another. Ionic bonds, which occur when a metal bonds with a nonmetal are extremely strong. As noted earlier, salt is an example of an ionic bond: the metal sodium loses an electron, forming a cation; meanwhile, the nonmetal chlorine gains the electron to become an anion. Their ionic bond results from the attraction of opposite charges.

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In ionic bonding, two ions start out with different charges and end up forming a bond in which both have eight valence electrons. In the type of bond Lewis described, a covalent bond, two atoms start out as atoms do, with a net charge of zero. Each ends up possessing eight valence electrons, but neither atom “owns” them; rather, they share electrons. L E W I S S T R U C T U R E S . In addition to discovering the concept of covalent bonding, Lewis developed the Lewis structure, a means of showing schematically how valence electrons are arranged among the atoms in a molecule. Also known as the electron-dot system, Lewis structures represent the valence electrons as dots surrounding the chemical symbols of the atoms involved. These dots, which look rather like a colon, may be above or below, or on either side of, the chemical symbol. (The dots above or below the chemical symbol are side-by-side, like a colon turned at a 90°-angle.)

To obtain the Lewis structure representing a chemical bond, it is first necessary to know the number of valence electrons involved. One pair of electrons is always placed between elements, indicating the bond between them. Sometimes this pair of valence electrons is symbolized by a dashed line, as in the system developed by Couper. The remaining electrons are distributed according to the rules by which specific elements bond. M U LT I P L E BONDS. Hydrogen bonds according to what is known as the duet rule, meaning that a hydrogen atom has only two valence electrons. In most other elements—there are exceptions, but these will not be discussed here—atoms end up with eight valence electrons, and thus are said to follow the octet rule. If the bond is covalent, the total number of valence electrons will not be a multiple of eight, however, because the atoms share some electrons.

Ionic bonding, however, could not explain all types of chemical bonds for the simple reason that not all compounds are ionic. A few years after Abegg’s death, American chemist Gilbert Newton Lewis (1875-1946) discovered a very different type of bond, in which nonionic compounds share electrons. The result, once again, is eight valence electrons for each atom, but in this case, the nuclei of the two atoms share electrons.

When carbon bonds to two oxygen atoms to form carbon dioxide (CO2), it is represented in the Couper system as O-C-O. The Lewis structure also uses dashed lines, which stand for two valence electrons shared between atoms. In this case, then, the dashed line to the left of the carbon atom indicates a bond of two electrons with the oxygen atom to the left, and the dashed line to the right of it indicates a bond of two electrons with the oxygen atom on that side.

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The non-bonding valence electrons in the oxygen atoms can be represented by sets of two dots above, below, and on the outside of each atom, for a total of six each. Combined with the two dots for the electrons that bond them to carbon, this gives each oxygen atom a total of eight valence electrons. So much for the oxygen atoms, but something is wrong with the representation of the carbon atom, which, up to this point, is shown only with four electrons surrounding it, not eight.

values form a covalent bond, this is described as polar covalent bonding. Sometimes these are simply called “polar bonds,” but that is not as accurate: all ionic bonds, after all, are polar, due to the extreme differences in electronegativity. The term “polar covalent bond” is much more specific, describing a bond, for instance, between hydrogen (2.1) and sulfur (2.6). Because sulfur has a slightly greater electronegativity value, the valence electrons will be slightly more attracted to the sulfur atom than to the hydrogen atom.

In fact carbon in this particular configuration forms not a single bond, but a double bond, which is represented by two dashed lines—a symbol that looks like an equals sign. By showing the double bonds joining the carbon atom to the two oxygen atoms on either side, the carbon atom has the required number of eight valence electrons. The carbon atom may also form a triple bond (represented by three dashed lines, one above the other) with an oxygen atom, in which case the oxygen atom would have only two other valence electrons.

Another example of a polar covalent bond is the one that forms between hydrogen and oxygen (3.5) to form H2O or water, which has a number of interesting properties. For instance, the polar quality of a water molecule gives it a great attraction for ions, and thus ionic substances such as salt dissolve easily in water. “Pure” water from a mountain stream is actually filled with traces of the rocks over which it has flowed. In fact, water—sometimes called the “universal solvent”—is almost impossible to find in pure form, except when it is purified in a laboratory.

Electronegativity and Polar Covalent Bonds Today, chemists understand that most bonds are neither purely ionic nor purely covalent; rather, there is a wide range of hybrids between the two extremes. Credit for this discovery belongs to American chemist Linus Pauling (1901-1994), who, in the 1930s, developed the concept of electronegativity—the relative ability of an atom to attract valence electrons. Elements capable of bonding are assigned an electronegativity value ranging from a minimum of 0.7 for cesium to a maximum of 4.0 for fluorine. Fluorine is capable of bonding with some noble gases, which do not bond with any other elements or each other. The greater the electronegativity value, the greater the tendency of an element to draw valence electrons to itself. If fluorine and cesium bond, then, the bond would be purely ionic, because the fluorine exerts so much more attraction for the valence electrons. But if two elements have equal electronegativity values—for instance, cobalt and silicon, both of which are rated at 1.9—the bond is purely covalent. Most bonds, as stated earlier, fall somewhere in between these two extremes.

By contrast, molecules of petroleum (CH2) tend to be nonpolar, because carbon and hydrogen have almost identical electronegativity values—2.5 and 2.1 respectively. Thus, an oil molecule offers no electric charge to bond it with a water molecule, and for this reason, oil and water do not mix. It is a good thing that water molecules attract each other so strongly, because this means that a great amount of energy is required to change water from a liquid to a gas. If this were not so, the oceans and rivers would vaporize, and life on Earth could not exist as it does. B O N D E N E R GY. The last two paragraphs allude to attractions between molecules, which is not the same as (nor is it as strong as) the attraction between atoms within a molecule. In fact, the bond energy—the energy required to pull apart the atoms in a chemical bond—is low for water. This is due to the presence of hydrogen atoms, with their two (rather than eight) valence electrons. It is thus relatively easy to separate water into its constituent parts of hydrogen and oxygen, through a process known as electrolysis.

When substances of differing electronegativity

Covalent bonds that involve hydrogen are among the weakest bonds between atoms. (Again, this is different from bonds between molecules.) Stronger than hydrogen bonds are regular, octet-rule covalent bonds: as one might expect, double covalent bonds are stronger than

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Chemical Bonding

KEY TERMS ANION:

The negative ion that results

CHEMICAL SYMBOL:

A one-or two-

when an atom gains one or more electrons.

letter abbreviation for the name of an

An anion (pronounced “AN-ie-un”) of an

element.

element is never called, for instance, the

A substance made up of

COMPOUND:

chlorine anion. Rather, for an anion

atoms of more than one element. These

involving a single element, it is named by

atoms are usually joined in molecules.

adding the suffix -ide to the name of the original element—hence, “chloride.” Other rules apply for more complex anions. ATOM:

The smallest particle of an ele-

ment. An atom can exist either alone or in combination with other atoms in a molecule. The number of

protons in the nucleus of an atom. Since this number is different for each element, elements are listed on the periodic table of elements in order of atomic number. BOND ENERGY:

The energy required

to pull apart the atoms in a chemical bond. The positive ion that results

when an atom loses one or more electrons. A cation (pronounced “KAT-ie-un”) is named after the element of which it is an ion and thus is called, for instance, the aluminum ion or the aluminum cation. CHEMICAL BONDING:

The joining,

through electromagnetic force, of atoms

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chemical bonding in which two atoms share valence electrons. Atoms may bond by single, double, or triple covalent bonds, which, in representations of Lewis structures, are shown by single, double, or triple dashed lines. (The double dashed line

ATOMIC NUMBER:

CATION:

A type of

COVALENT BONDING:

looks like an equals sign.) When atoms have differing values of electronegativity, they form polar covalent bonds. A term describing the

DUET RULE:

distribution of valence electrons when hydrogen atoms—which end up with only two valence electrons—experience chemical bonding with other atoms. Most other elements follow the octet rule. ELECTRON:

Negatively charged parti-

cles in an atom. Electrons, which spin around the protons and neutrons that make up the atom’s nucleus, are essential to chemical bonding. The relative

representing different elements. The prin-

ELECTRONEGATIVITY:

cipal types of bonds are covalent bonding

ability of an atom to attract valence

and ionic bonding, though few bonds are

electrons.

purely one or the other. Rather, there is a

ELEMENT:

wide range of “hybrid” bonds, in accor-

only one kind of atom. Unlike compounds,

dance with the electronegativity values of

elements cannot be broken down chemi-

the elements involved.

cally into other substances.

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A substance made up of

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Chemical Bonding

KEY TERMS

CONTINUED

An atom that has lost or gained

place in chemical bonding for most ele-

one or more electrons, and thus has a net

ments, which end up with eight valence

electrical charge. Ions may either be anions

electrons. Hydrogen is an exception, and

or cations.

follows the duet rule. A few elements fol-

ION:

A form of chemical

low other rules, and some (most notably

bonding that results from attractions

the noble gases) do not typically bond with

between ions with opposite electrical

other elements.

IONIC BONDING:

charges.

PERIODIC TABLE OF ELEMENTS:

A means of

A chart showing the elements arranged in

showing schematically how valence elec-

order of atomic number. Vertical columns

trons are distributed among the atoms in a

within the periodic table indicate groups

molecule. Also known as the electron-dot

or “families” of elements with similar

system, Lewis structure represents pairs of

chemical characteristics.

LEWIS

STRUCTURE:

electrons with a symbol rather like a colon, which—depending on the situation—can be placed above, below, or on either side of the chemical symbol. In the Lewis structure, the pairs of electrons involved in chemical bonds are usually represented by a dashed line.

POLAR COVALENT BONDING:

The

type of chemical bonding between atoms that have differing values of electronegativity. If the difference is extreme, of course, the bond is not a covalent bond at all, but an ionic bond. Thus, although these are sometimes called polar bonds, they are

A group of atoms, usual-

MOLECULE:

ly, but not always, representing more

more properly identified as polar covalent bonds.

than one element, joined in a structure. Compounds are typically made of up

PROTON:

molecules.

in an atom.

NEUTRON:

A subatomic particle that

A positively charged particle

VALENCE ELECTRONS:

Electrons

has no electrical charge. Neutrons are

that occupy the highest energy levels in an

found at the nucleus of an atom, alongside

atom. These are the only electrons involved

protons.

in chemical bonding. By contrast, the core

NUCLEUS:

The center of an atom, a

electrons, or the ones at lower energy lev-

region where protons and neutrons are

els, play no role in the bonding of atoms.

located, and around which electrons spin.

VALENCY:

OCTET RULE:

A term describing the

distribution of valence electrons that takes

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The property of the atom of

one element that determines its ability to bond with atoms of other elements.

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single ones, and triple covalent bonds are stronger still. Strongest of all are ionic bonds, involved in the bonding of a metal to a metal, or a metal to a nonmetal, as in salt. The strength of the bond energy in salt is reflected by its boiling point of 1,472°F (800°C), much higher than that of water, at 212°F (100°C). WHERE TO LEARN MORE “Chemical Bond” (Web site). (May 18, 2001). “Chemical Bonding” Oklahoma State University (Web site). (May 19, 2001).

272

“Chemical Bonding” (Web site). (May 19, 2001). “The Colored Periodic Table and Chemical Bonding” (Web site). (May 19, 2001). Ebbing, Darrell D.; R. A. D. Wentworth; and James P. Birk. Introductory Chemistry. Boston: Houghton Mifflin, 1995. Linus Pauling and the Twentieth Century: An Exhibition (Web site). (May 19, 2001). “Valence Shell Electron Pair Repulsion (VSEPR).” (May 18, 2001). White, Florence Meiman. Linus Pauling, Scientist and Crusader. New York: Walker, 1980.

“Chemical Bonding” (Web site). (May 19, 2001).

Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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Compounds

CONCEPT A compound is a chemical substance in which atoms combine in such a way that the compound always has the same composition, unless it is chemically altered in some way. Elements make up compounds, and although there are only about 90 elements that occur in nature, there are literally many millions of compounds. A compound is not the same as a mixture, which has a variable composition, but until chemists understood the atomic and molecular substructure of compounds, the distinction was not always clear. When atoms of one element bond to atoms of another, they form substances quite different from either element. Sugar, for instance, is made up of carbon, the material in coal and graphite, along with two gases, hydrogen and oxygen. None of these is sweet, yet when they are brought together in just the right way, they make the compound that sweetens everything from breakfast cereals to colas. A compound cannot be understood purely in terms of its constituent elements; an awareness of the structure is also needed. Likewise, it is important to know just how to name a compound, using a uniform terminology, since there are far too many substances in the world to give each an individual name.

HOW IT WORKS

COMPOUNDS

composed of two hydrogen atoms bonded to a single oxygen atom. It can be frozen or boiled, but the molecules themselves are unchanged, because freezing and boiling are merely physical processes. To transform one compound into another, on the other hand, requires a chemical change or reaction. Likewise, a chemical change is required to break down a compound into its constituent elements. Water can be broken down by passing a powerful electric current through it. This process, known as electrolysis, separates water into hydrogen and oxygen, both of which are highly flammable gases. The fact that they can be joined to form water, a substance used for putting out most kinds of fires, illustrates the types of changes elements undergo when they join to form compounds. The atoms in a compound do not change into other atoms; or to put it another way, the elements do not change into other elements. Though two hydrogen atoms bond with an oxygen atom to form water, they are still hydrogens, and the oxygen is still an oxygen. The atoms’ elemental identity thus remains intact, and if these elements are separated, they can join with other elements to form entirely different compounds.

Letters and Elements; Words and Compounds

Elements and Compounds A compound is a substance made up of atoms representing more than one element, and these atoms are typically joined in molecules. The composition of a compound is always the same: for instance, water always contains molecules

The nature of a compound is almost paradoxical: the constituent elements remain the same, yet the compound typically bears little resemblance to the elements that form it. How can this be? Perhaps an analogy to language, and the symbols used to express it, will serve to show that this

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There are between 88 and 92 elements that appear in nature; figures vary, because a handful of the elements with atomic numbers less than 92 have not actually been found in nature, but were produced in laboratories. (All elements with an atomic number higher than that of uranium, which is 92, are artificial.) In any case, there are far more elements than there are letters in the English alphabet; yet even with 26 characters, it is possible to form an almost limitless vocabulary.

Compounds

Let us focus on a single letter, g. Phonetically, it can make a hard sound, as in great, or a soft one, as in geranium. It may be silent, as in light, or it may combine with another letter to make either a typical “g” sound (edge) or a totally unexpected one (rough). It can slide into a vowel sound smoothly, as in singer, or with an almost imperceptible pause, as in finger. The complexities multiply when we consider the range of possibilities for the letter g, and all the resultant meanings involved: from God to dog, or from glory to degeneration. W H AT T H I S A N A LO GY S H O W S A B O U T C H E M I S T RY. One could go on

endlessly in this vein, and all with just one letter; however, a few points need to be made regarding the analogy between letters of the alphabet and elements. First of all, in all of the examples given above, or any number of others, the g still remained a g. In the same way, an atom of hydrogen (another g-word!) is always a hydrogen, whether it combines with oxygen to form water, nitrogen to make ammonia, or carbon to produce petroleum.

A

MIXTURE VERSUS A COMPOUND. (Robert L. Wolke. Repro-

duced by permission.)

larger combination.

A third point to consider is the fact that there is nothing about g itself that provides any information as to the meaning of the word it helps to form. Here the analogy to elements is imperfect, because the nature of the element’s subatomic structure—particularly the configuration of electrons on the outer shell of the atom—

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Secondly, note that this illustration would have been more difficult if the letter chosen had been q or z, since these are used more rarely. Likewise, there are elements such as technetium or praseodymium that seldom come up in discussions of compounds. On the other hand, one cannot go far in the study of chemistry without running across compounds involving hydrogen, carbon, oxygen, nitrogen, various metals, or halogens such as fluorine.

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provides a great deal of information as to how it will combine with other atoms. Still, it is evident, as we have seen, that the properties of a compound cannot easily be predicted by studying the properties of its constituent elements—just as one cannot begin to define a word simply because it contains a g. On the other hand, knowledge of the sounds that g makes may help us to pronounce a word containing that letter. In the same way, the fact that a compound includes carbon provides a clue that the compound might be organic. Even when we have all the letters to form a word, we still need to know how they are ordered. Loge and ogle contain the same letters, but one is a noun referring to a theater box, whereas the other is a verb meaning the act of looking at someone in an improper fashion. In chemistry, these are called isomers: two substances having identical chemical formulas, but differing chemical structures. It so happens that g is also part of the suffix -ing, used in forming a gerund. Aside from being yet another g-word, a gerund is a substantive, or noun form of a verb: for instance, going. Once again, there is a similarity between words and compounds. Just as certain classes of words are formed by the addition of regular suffixes, so the naming of whole classes of compounds can be achieved by means of a uniform system of nomenclature.

Tea, whether iced or hot, is a mixture. So is coffee, or even wine. In each case, substances are added together and subjected to a process, but the result is not a compound. We know this because the composition varies. Depending on the coffee beans used, for instance, coffee can have a wide variety of flavors. If, in the brewing process, too much coffee is used in proportion to the water, the resulting mixture will be strong or bitter; on the hand, an insufficient coffee-towater ratio will produce coffee that is too weak.

Compounds

Note that a number of terms have been used here that, from a scientific standpoint at least, are vague. How weak is “too weak”? That all depends on the tastes of the person making the coffee. But as long as coffee beans and hot water are used, no matter what the proportion, the mixture is still coffee. On the other hand, when two oxygen atoms, rather than one, are chemically combined with two hydrogen atoms, the result is not “strong water.” Nor is it “oxygenated water”: it is hydrogen peroxide, a substance no one should drink.

REAL-LIFE A P P L I C AT I O N S

Three principal characteristics serve to differentiate a compound from a mixture. First, as we have seen, a compound has a definite and constant composition, whereas a mixture can exist with virtually any proportion between its constituent parts. Second, elements lose their characteristic elemental properties once they form a compound, but the parts of a mixture do not. (For example, when mixed with water, sugar is still sweet.) Third, the formation of a compound involves a chemical reaction, whereas a mixture can be created simply by the physical act of stirring items together.

Distinguishing Between Compounds and Mixtures

The Atomic and Molecular Keys

To continue the analogy used above just one step further, a word is not just a collection of letters; it has to have a meaning. Likewise, the fact that various substances are mixed together does not necessarily make them a compound. Actually, the difference between a word and a mere collection of letters is somewhat greater than the difference between a compound and a mixture—a substance in which elements are not chemically bonded, and in which the composition is variable. A nonsensical string of characters serves no linguistic purpose; on the other hand, mixtures are an integral part of life.

The means by which compounds are formed are discussed numerous times, and in various ways, throughout this book. A few of those particulars will be mentioned briefly below, in relation to the naming of compounds, but for the most part, there will be no attempt to explain the details of the processes involved in chemical bonding. The reader is therefore encouraged to consult the essays on Chemical Bonding and Electrons.

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It is important, nonetheless, to recognize that chemists’ knowledge is based on their understanding of the atom and the ways that electrons, negatively charged particles in the atom, bring

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about bonds between elements. Awareness of these specifics emerged only at the beginning of the twentieth century, with the discovery of subatomic particles. Another important threshold had been crossed a century before, with the development of atomic theory by English chemist John Dalton (1766-1844), and of molecular theory by Italian physicist Amedeo Avogadro (1776-1856). Around the same time, French chemists Antoine Lavoisier (1743-1794) and Joseph-Louis Proust (1754-1826), respectively, clarified the definitions of “element” and “compound.” Until then, the idea of a compound had little precise meaning for chemists, who often used the term to describe a mixture. Thus, French chemist Claude Louis Berthollet (1748-1822) asserted that compounds have variable composition, and for evidence he pointed to the fact that when some metals are heated, they form oxides, in which the percentage of oxygen increases with temperature. Proust, on the other hand, maintained that compounds must have a constant composition, an argument supported by Dalton’s atomic theory. Proust worked to counter Berthollet’s positions on a number of particulars, but was still unable to explain why metals form variable alloys, or combinations of metals; no chemist at the time understood that an alloy is a mixture, not a compound. Nonetheless, Proust was right in his theory of constant composition, and Berthollet was incorrect on this score. With the discovery of subatomic structures, it became possible to develop highly sophisticated theories of chemical bonding, which in turn facilitated understanding of compounds.

Types of Compounds O R GA N I C C O M P O U N D S . Though

there are millions of compounds, these can be grouped into just a few categories. Organic compounds, of which there are many millions, are compounds containing carbon. The only major groupings of carbon-containing compounds that are not considered organic are carbonates, such as limestone, and oxides, such as carbon dioxide. Organic compounds can be further subdivided into a number of functional groups, such as alcohols. Within the realm of organic compounds, whether natural or artificial, are petroleum and

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its many products, as well as plastics and other synthetic materials. The term “organic,” as applied in chemistry, does not necessarily refer to living things, since the definition is based on the presence of carbon. Nonetheless, all living organisms are organic, and the biochemical compounds in living things form an important subset of organic compounds. Biochemical compounds are, in turn, divided into four families: carbohydrates, proteins, nucleic acids, and lipids. I N O R GA N I C C O M P O U N D S . Inorganic compounds can be classified according to five major groupings: acids, bases, salts, oxides, and others. An acid is a compound which, when dissolved in water, produces positive ions (atoms with an electric charge) of hydrogen. Bases are substances that produce negatively charged hydroxide (OH–) ions when dissolved in water. An oxide is a compound in which the only negatively charged ion is an oxygen, and a salt is formed by the reaction of an acid with a base. Generally speaking, a salt is any combination of a metal and a nonmetal, and it can contain ions of any element but hydrogen.

The remaining inorganic compounds, classified as “others,” are those that do not fit into any of the groupings described above. An important subset of this broad category are the coordination compounds, formed when one or more ions or molecules contributes both electrons necessary for a bonding pair, in order to bond with a metallic ion or atom.

Naming Compounds In the early days of chemistry as a science, common names were applied to compounds. Water is an example of a common name; so is sugar, as well as salt. These names work well enough in everyday life, and in fact, chemists still refer to water simply as “water.” (The only other common name still used in chemistry is ammonia.) But as the number of compounds discovered and developed by chemists began to proliferate, the need for a systematic means of naming them became apparent. With millions of compounds, it would be nearly impossible to come up with names for every one. Furthermore, common names tell chemists nothing about the chemical properties of a particular substance.

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Today, chemists use a system of nomenclature that is rather detailed but fairly easy to understand, once the rules are understood. We will examine this system briefly, primarily as it relates to binary compounds—compounds containing just two elements. Binary compounds are divided into three groups. The first two are ionic compounds, involving metals that form positively charged ions, or cations (pronounced KAT-ieunz). The third consists of compounds that contain only nonmetals. These groups are:

of substances, its chemical name is sodium chloride.

• Type I: Ionic compounds involving a metal that always forms a cation of a certain electric charge. • Type II: Ionic compounds involving a metal (typically a transition metal) that forms cations with differing charges. • Type III: Compounds containing only nonmetals. CAT I O N S A N D A N I O N S . Cations are represented symbolically thus: H+, or Mg2+. The first, a hydrogen cation, has a positive charge of 1, but note that the 1 is not shown—just as the first power of a number is never designated in mathematics. In the second example, a magnesium cation, the superscript number, combined with the plus sign (which can either follow the number, as is shown here, or proceed it) indicates that the atom has a positive charge of two. Thus, even if one were not told that this is a cation, it would be easy enough to discern from the notation.

The chemical nomenclature for type II binary compounds is somewhat more complicated, because they involve metals that can have multiple positive charges. This is particularly true of the transition metals, a family of 40 elements in the middle of the periodic table distinguished from other elements by a number of characteristics. The name “transition” thus implies a break in the even pattern of the periodic table.

Anions (AN-ie-unz), or negatively charged ions, are represented in a similar way: H- for hydride, an anion of hydrogen; or O2– for oxide. Note, however, that the naming of anions and cations is different. Cations are always called, for example, a hydrogen cation, or a magnesium cation. On the other hand, names of simple anions (involving a single atom) are formed by taking the root of the element name and adding an -ide: for example, fluoride (F–). T Y P E I B I N A RY C O M P O U N D S .

In a binary ionic compound, a metal combines with a nonmetal. The metal loses one or more electrons to become a cation, while the nonmetal gains one or more electrons to become an anion. Thus, table salt is formed by the joining of a cation of the metal sodium (Na+) and an anion of the nonmetal chlorine (Cl–). Instead of the common name “salt,” which can apply to a range

S C I E N C E O F E V E RY DAY T H I N G S

Compounds

In naming Type I binary compounds, of which sodium chloride is an example, the cation is always represented first by the name of the element. The anion follows, with the root name of the element attached to the -ide suffix, as described above. Another example is calcium sulfide, formed by a cation of the metal calcium (Ca2+) and an anion of the nonmetal sulfur (S2–). T Y P E I I B I N A RY C O M P O U N D S .

Iron (Fe), for instance, is a transition metal, and it can form cations with charges of 2+ or 3+, while copper (Cu) can form cations with charges of 1+ or 2+. When encountering positively charged cations, it is not enough to say, for instance, “iron oxide,” or “copper sulfide,” because it is not clear which iron cation is involved. To solve the problem, chemists use a system of Roman numerals. According to this system, the cations referred to above are expressed in the name of a Type II binary compound thus: iron (II), iron (III), copper (I), and copper (II). This is followed with the name of the anion, as before, using the -ide suffix. Note that the Roman numeral is usually the same as the number of positive charges in the cation. It should be noted, also, that there is an older system for naming Type II binary compounds with terms that incorporate the element name— often the Latin original, reflected in the chemical symbol—with a suffix. For instance, this system uses the word “ferrous” for iron (II) and “ferric” for iron (III). However, this method of nomenclature is increasingly being replaced by the one we have described here. T Y P E I I I B I N A RY C O M P O U N D S .

In a Type III binary compound involving only nonmetals, the first element in the formula is referred to simply by its element name, as though it were a cation, while the second element is given an -ide suffix, as though it were an anion. If there

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is more than one atom present, prefixes are used to indicate the number of atoms. These prefixes are listed below. It should be noted that mono- is never used for naming the first element in a type III binary compound. • • • • • • • •

mono-: 1 di-: 2 tri-: 3 tetra-: 4 penta-: 5 hexa-: 6 hepta-: 7 octa-: 8 Thus CO2 is called carbon dioxide, indicating one carbon atom and two oxygens. Again, mono- is not used for the first element, but it is used for the second: hence, the name of the compound with the formula CO is carbon monoxide. It is also possible to know the formula for a compound simply from the name: if confronted with a name such as “dinitrogen pentoxide,” for instance, it is fairly easy to apply the rules governing these prefixes to discern that the substance is N2O5. Note that vowels at the end of a prefix are dropped when the name of the element that follows it also begins with a vowel: we say monoxide, not “monooxide”; and pentoxide, not “pentaoxide.” This makes pronunciation much easier. POLYATOMIC IONS AND ACIDS.

More complicated rules apply for polyatomic ions, which are charged groupings of atoms such as NH4NO3, or ammonium nitrate. The only way to learn how to name polyatomic ions is by memorizing the names of the constituent parts. In the above example, for instance, the first part of the formula, which has a positive charge, is always called ammonium, while the second part, which has a negative charge, is always called nitrate. A good chemistry textbook should provide a table listing the names of common polyatomic ions. A number of polyatomic ions are called oxyanions, meaning that they include varying numbers of oxygen atoms combined with atoms of other elements. There are rules for designating the names of these polyatomic ions, some of which are listed in the essay on Ions and Ionization. Still other rules govern the naming of acids; here again, the operative factor is the presence of oxygen. Thus, if the anion does not contain oxy-

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gen, the name of the acid is created by adding the prefix hydro- and the suffix -ic, as in hydrochloric acid. If the anion does contain oxygen, the root name of the principal element is joined to a suffix: depending on the relative numbers of oxygen atoms, this may be -ic or -ous.

Isomers As observed much earlier, isomers are like two words with the same letters, but arranged in different ways. Specifically, isomers are chemical compounds having the same formula, but in which the atoms are arranged differently, thereby forming different compounds. There are two principal types of isomer: structural isomers, which differ according to the attachment of atoms on the molecule, and stereoisomers, which differ according to the locations of the atoms in space. An example of structural isomerism is the difference between propyl alcohol and isopropyl (rubbing) alcohol: these two have differing properties, because their alcohol functional groups are not attached to the same carbon atom in the carbon chain to which the functional group is attached. In a stereoisomer, on the other hand, atoms are attached in the same order, but have different spatial relationships. If functional groups are aligned on the same side of a double bond between two carbon atoms, this is called a cis isomer, from a Latin word meaning “on this side.” If they are on opposite sides, it is called a trans (“across”) isomer. Hence the term “trans fats,” which are saturated fats that improve certain properties of foods—including taste—but which may contribute to heart disease. WHERE TO LEARN MORE “Chemical Compounds” (Web site). (June 2, 2001). “Chemtutor Compounds.” Chemtutor (Web site). (June 2, 2001). Fullick, Ann. Matter. Des Plaines, IL: Heinemann Library, 1999. “Glossary of Products with Hazards A to Z” Environmental Protection Agency (Web site). (June 2, 2001). Knapp, Brian J. Elements, Compounds, and Mixtures, Volume 2. Edited by Mary Sanders. Danbury, CT: Grolier

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Compounds

KEY TERMS An inorganic compound that,

electrons necessary for a bonding pair in

when dissolved in water, produces positive

order to bond with a metallic ion or atom.

ACID:

ions—that is, cations—of hydrogen, designated symbolically as H+ ions. (This is just

A negatively charged par-

ELECTRON:

ticle in an atom.

one definition; see Acids and Bases; AcidFor the

INORGANIC COMPOUND:

Base Reactions for more.)

most part, inorganic compounds are any The negatively charged ion

compounds that do not contain carbon.

that results when an atom gains one or

However, carbonates and carbon oxides are

more electrons. The word is pronounced

also inorganic compounds. Compare with

“AN-ie-un.”

organic compounds.

ANION:

BASE:

An inorganic compound that

ION:

An atom or group of atoms that

produces negative hydroxide ions when it

has lost or gained one or more electrons,

is dissolved in water. These anions are des-

and thus has a net electric charge.

ignated by the symbol OH-. (This is just one definition; see Acids and Bases; AcidBase Reactions for more.) BINARY COMPOUND:

A form of chemical

bonding that results from attractions between ions with opposite electric

A compound

that contains just two elements. For the purposes of establishing compound names, binary compounds are divided into Type I, Type II, and Type III. CATION:

IONIC BONDING:

charges. IONIC COMPOUND:

A compound in

which ions are present. Ionic compounds contain at least one metal joined to another element by an ionic bond.

The positively charged ion

ISOMERS:

Substances which have the

that results when an atom loses one or

same chemical formula, but which have

more electrons. The word “cation” is pro-

different chemical properties due to differ-

nounced “KAT-ie-un.”

ences in the arrangement of atoms.

CHEMICAL BONDING:

The joining,

MIXTURE:

A substance in which ele-

through electromagnetic force, of atoms

ments are not chemically bonded, and in

representing different elements.

which the composition is variable. A mix-

COMPOUND:

A substance made up of

ture is distinguished from a compound.

atoms of more than one element, which are

ORGANIC COMPOUND:

chemically bonded and usually joined in

speaking, a compound containing carbon.

molecules. The composition of a com-

The only exceptions are the carbonates (for

pound is always the same, unless it is

example, calcium carbonate or limestone)

changed chemically.

and oxides, such as carbon dioxide.

COORDINATION

COMPOUNDS:

OXIDE:

Generally

An inorganic compound in

Inorganic compounds formed when one

which the only negatively charged ion is an

or more ions or molecules contribute both

oxygen.

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Compounds

KEY TERMS SALT:

An inorganic compound formed

by the reaction of an acid with a base. Generally speaking, a salt is a combination of a metal and a nonmetal, and it can contain

TYPE

II

BINARY

COMPOUNDS:

Ionic compounds involving a metal (typically a transition metal) that forms cations with differing charges.

ions of any element but hydrogen.

TYPE

TYPE

VALENCE ELECTRONS:

I

BINARY

COMPOUNDS:

Ionic compounds involving a metal that always forms a cation of a certain electric charge.

Educational, 1998. “List of Compounds” (Web site). (June 2, 2001). Maton, Anthea. Exploring Physical Science. Upper Saddle River, N.J.: Prentice Hall, 1997.

280

CONTINUED

III

BINARY

COMPOUNDS:

Compounds containing only nonmetals. Electrons that occupy the highest energy levels in an atom. These are the only electrons involved in chemical bonding.

(Web site). (June 2, 2001). Oxlade, Chris. Elements and Compounds. Chicago, IL: Heinemann Library, 2001.

“Molecules and Compounds.” General Chemistry Online

Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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CHEMICAL REACTIONS

CONCEPT If chemistry were compared to a sport, then the study of atomic and molecular properties, along with learning about the elements and how they relate on the periodic table, would be like going to practice. Learning about chemical reactions, which includes observing them and sometimes producing them in a laboratory situation, is like stepping out onto the field for the game itself. Just as every sport has its “vocabulary”—the concepts of offense and defense, as well as various rules and strategies—the study of chemical reactions involves a large set of terms. Some aspects of reactions may seem rather abstract, but the effects are not. Every day, we witness evidence of chemical reactions—for instance, when a fire burns, or metal rusts. To an even greater extent, we are surrounded by the products of chemical reactions: the colors in the clothes we wear, or artificial materials such as polymers, used in everything from nylon running jackets to plastic milk containers.

HOW IT WORKS

tric current is run through a sample, separating it into oxygen and hydrogen gas. Chemical change requires a chemical reaction, a process whereby the chemical properties of a substance are altered by a rearrangement of the atoms in the substance. Of course we cannot see atoms with the naked eye, but fortunately, there are a number of clues that tell us when a chemical reaction has occurred. In many chemical reactions, for instance, the substance may experience a change of state or phase—as for instance when liquid water turns into gaseous oxygen and hydrogen as a result of electrolysis. HOW DO WE KNOW WHEN A C H E M I CA L R E AC T I O N H AS O C C U R R E D ? Changes of state may of course be

merely physical—as for example when liquid water is boiled to form a vapor. (These and other examples of physical changes resulting from temperature changes are discussed in the essays on Properties of Matter; Temperature and Heat.) The vapor produced by boiling water, as noted above, is still water; on the other hand, when liquid water is turned into the elemental gases hydrogen and oxygen, a more profound change has occurred.

What Is a Chemical Reaction? If liquid water is boiled, it is still water; likewise frozen water, or ice, is still water. Melting, boiling, or freezing simply by the application of a change in temperature are examples of physical changes, because they do not affect the internal composition of the item or items involved. A chemical change, on the other hand, occurs when the actual composition changes—that is, when one substance is transformed into another. Water can be chemically changed, for instance, when an elec-

Likewise the addition of liquid potassium chromate (K2CrO4) to a solution of barium nitrate (Ba[NO3]2 forms solid barium chromate (BaCrO4). In the reaction described, a solution is also formed, but the fact remains that the mixture of two solids has resulted in the formation of a solid in a different solution. Again, this is a far more complex phenomenon than the mere freezing of water to form ice: here the fundamental properties of the materials involved have changed.

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Chemical Reactions

A

MICROSCOPIC CLOSE-UP OF NYLON FIBERS.

NYLON

IS FORMED THROUGH A SYNTHESIS REACTION. (Science Pictures

Limited/Corbis. Reproduced by permission.)

The physical change of water to ice or steam, of course, involves changes in temperature; likewise, chemical changes are often accompanied by changes in temperature, the crucial difference being that these changes are the result of alterations in the chemical properties of the substances involved. Such is the case, for instance, when wood burns in the presence of oxygen: once wood is turned to ash, it has become an entirely different mixture than it was before. Obviously, the ashes cannot be simply frozen to turn them back into wood again. This is an example of an irreversible chemical reaction.

282

right conditions, carbon—which is black—can be combined with colorless hydrogen and oxygen to produce white sugar. This suggests another kind of change: a change in taste. (Of course, not every product of a chemical reaction should be tasted—some of the compounds produced may be toxic, or at the very least, extremely unpleasant to the taste buds.) Smell, too, can change. Sulfur is odorless in its elemental form, but when combined with hydrogen to form hydrogen sulfide (H2S), it becomes an evil-smelling, highly toxic gas.

Chemical reactions may also involve changes in color. In specific proportions and under the

The bubbling of a substance is yet another clue that a chemical reaction has occurred. Though water bubbles when it boils, this is mere-

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ly because heat has been added to the water, increasing the kinetic energy of its molecules. But when hydrogen peroxide bubbles when exposed to oxygen, no heat has been added. As with many of the characteristics of a chemical reaction described above, bubbling does not always occur when two chemicals react; however, when one of these clues is present, it tells us that a chemical reaction may have taken place.

Chemical Reactions

REAL-LIFE A P P L I C AT I O N S Chemical Equations In every chemical reaction, there are participants known as reactants, which, by chemically reacting to one another, result in the creation of a product or products. As stated earlier, a chemical reaction involves changes in the arrangement of atoms. The atoms in the reactants (or, if the reactant is a compound, the atoms in its molecules) are rearranged. The atomic or molecular structure of the product is different from that of either reactant. Note, however, that the number of atoms does not change. Atoms themselves are neither created nor destroyed, and in a chemical reaction, they merely change partners, or lose partners altogether as they return to their elemental form. This is a critical principle in chemistry, one that proves that medieval alchemists’ dream of turning lead into gold was based on a fallacy. Lead and gold are both elements, meaning that each has different atoms. To imagine a chemical reaction in which one becomes the other is like saying “one plus one equals one.” SYMBOLS IN A CHEMICAL E Q UAT I O N . In a mathematical equation,

the sums of the numbers on one side of the equals sign must be the same as the sum of the numbers on the other side. The same is true of a chemical equation, a representation of a chemical reaction in which the chemical symbols on the left stand for the reactants, and those on the right are the product or products. Instead of an equals sign separating them, an arrow, pointing to the right to indicate the direction of the reaction, is used.

A

CLOSE-UP OF RUST ON A PIECE OF METAL.

RUST

IS

AN EXAMPLE OF A COMBINATION REACTION, NOT A DECOMPOSITION REACTION. (Marko Modic/Corbis. Reproduced

by permission.)

reactants and products. These symbols are as follows: • • • •

(s): solid (l): liquid (g): gas (aq): dissolved in water (an aqueous solution) The fourth symbol, of course, does not indicate a phase of matter per se (though obviously it appears to be a liquid); but as we shall see, aqueous solutions play a role in so many chemical reactions that these have their own symbol. At any rate, using this notation, we begin to symbolize the reaction of hydrogen and oxygen to form water thus: H(g) + O(g) 4H2O(l).

Chemical equations usually include notation indicating the state or phase of matter for the

This equation as written, however, needs to be modified in several ways. First of all, neither hydrogen nor oxygen is monatomic. In other words, in their elemental form, neither appears as a single atom; rather, these form diatomic (two-atom) molecules. Therefore, the equation must be rewritten as H2(g) + O2(g) 4H2O(l). But this is still not correct, as a little rudimentary analysis will show.

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Balancing Chemical Equations When checking a chemical equation, one should always break it down into its constituent elements, to determine whether all the atoms on the left side reappear on the right side; otherwise, the result may be an incorrect equation, along the lines of “1 + 1 = 1.” That is exactly what has happened here. On the left side, we have two hydrogen atoms and two oxygen atoms; on the right side, however, there is only one oxygen atom to go with the two hydrogens. Obviously, this equation needs to be corrected to account for the second oxygen atom, and the best way to do that is to show a second water molecule on the right side. This will be represented by a 2 before the H2O, indicating that two water molecules now have been created. The 2, or any other number used for showing more than one of a particular chemical species in a chemical equation, is called a coefficient. Now we have H2(g) + O2(g) 42H2O(l). Is this right? Once again, it is time to analyze the equation, to see if the number of atoms on the left equals the number on the right. Such analysis can be done in a number of ways: for instance, by symbolizing each chemical species as a circle with chemical symbols for each element in it. Thus a single water molecule would be shown as a circle containing two H’s and one O. Whatever the method used, analysis will reveal that the problem of the oxygen imbalance has been solved: now there are two oxygens on the left, and two on the right. But solving that problem has created another, because now there are four hydrogen atoms on the right, as compared with two on the left. Obviously, another coefficient of 2 is needed, this time in front of the hydrogen molecule on the left. The changed equation is thus written as: 2H2(g) + O2(g) 4 2H2O(l). Now, finally, the equation is correct. T H E P R O C E SS O F B A LA N C I N G C H E M I CA L E Q UAT I O N S . What

284

In writing and balancing a chemical equation, the first step is to ascertain the identities, by formula, of the chemical species involved, as well as their states of matter. After identifying the reactants and product, the next step is to write an unbalanced equation. After that, the unbalanced equation should be subjected to analysis, as demonstrated above. The example used, of course, involves a fairly simple substance, but often, much more complex molecules will be part of the equation. In performing analysis to balance the equation, it is best to start with the most complex molecule, and determine whether the same numbers and proportions of elements appear in the product or products. After the most complicated molecule has been dealt with, the second-most complex can then be addressed, and so on. Assuming the numbers of atoms in the reactant and product do not match, it will be necessary to place coefficients before one or more chemical species. After this has been done, the equation should again be checked, because as we have seen, the use of a coefficient to straighten out one discrepancy may create another. Note that only coefficients can be changed; the formulas of the species themselves (assuming they were correct to begin with) should not be changed. After the equation has been fully balanced, one final step is necessary. The coefficients must be checked to ensure that the smallest integers possible have been used. Suppose, in the above exercise, we had ended up with an equation that looked like this: 12H2(g) + 6O2(g) 412H2O(l). This is correct, but not very “clean.” Just as a fraction such as 12/24 needs to be reduced to its simplest form, 1/2, the same is true of a chemical equation. The coefficients should thus always be the smallest number that can be used to yield a correct result.

Types of Chemical Reactions

we have done is to balance an unbalanced equation. An unbalanced equation is one in which the numbers of atoms on the left are not the same as the number of atoms on the right. Though an unbalanced equation is incorrect, it is sometimes a necessary step in the process of finding the balanced equation—one in which the number of atoms in the reactants and those in the product are equal.

Note that in chemical equations, one of the symbols used is (aq), which indicates a chemical species that has been dissolved in water—that is, an aqueous solution. The fact that this has its own special symbol indicates that aqueous solutions are an important part of chemistry. Examples of reactions in aqueous solutions are discussed, for instance, in the essays on Acid-Base Reactions; Chemical Equilibrium; Solutions.

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Another extremely important type of reaction is an oxidation-reduction reaction. Sometimes called a redox reaction, an oxidationreduction reaction occurs during the transfer of electrons. The rusting of iron is an example of an oxidation-reduction reaction; so too is combustion. Indeed, combustion reactions—in which oxygen produces energy so rapidly that a flame or even an explosion results—are an important subset of oxidation-reduction reactions. R E AC T I O N S T H AT F O R M WA T E R, S O L I D S , O R GAS E S . Another

type of reaction is an acid-base reaction, in which an acid is mixed with a base, resulting in the formation of water along with a salt. Other reactions form gases, as for instance when water is separated into hydrogen and oxygen. Similarly, heating calcium carbonate (limestone) to make calcium oxide or lime for cement also yields gaseous carbon dioxide: CaCO3(s) + heat 4CaO(s) + CO2(g). There are also reactions that form a solid, such as the one mentioned much earlier, in which solid BaCrO4(s) is formed. Such reactions are called precipitation reactions. But this is also a reaction in an aqueous solution, and there is another product: 2KNO3(aq), or potassium nitrate dissolved in water. SINGLE AND DOUBLE DISP LAC E M E N T. The reaction referred to in

the preceding paragraph also happens to be an example of another type of reaction, because two anions (negatively charged ions) have been exchanged. Initially K+ and CrO42- were together, and these reacted with a compound in which Ba2+ and NO3-were combined. The anions changed places, an instance of a double-displacement reaction, which is symbolized thus: AB + CD 4AD + CB.

als be elements or simple compounds. A basic example of this is the reaction described earlier in relation to chemical equations, when hydrogen and oxygen combine to form water. On the other hand, some extremely complex substances, such as the polymers in plastics and synthetic fabrics such as nylon, also involve synthesis reactions. When iron rusts (in other words, it oxidizes in the presence of air), this is both an oxidationreduction and a synthesis reaction. This also represents one of many instances in which the language of science is quite different from everyday language. If a piece of iron—say, a railing on a balcony—rusts due to the fact that the paint has peeled off, it would seem from an unscientific standpoint that the iron has “decomposed.” However, rust (or rather, metal oxide) is a more complex substance than the iron, so this is actually a synthesis or combination reaction. A true decomposition reaction occurs when a compound is broken down into simpler compounds, or even into elements. When water is subjected to electrolysis such that the hydrogen and oxygen are separated, this is a decomposition reaction. The fermentation of grapes to make wine is also a form of decomposition. And then, of course, there are the processes that normally come to mind when we think of “decomposition”: the decay or rotting of a formerly living thing. This could also include the decay of something, such as an item of food, made from a formerly living thing. In such instances, an organic substance is eventually broken down through a number of processes, most notably the activity of bacteria, until it ultimately becomes carbon, nitrogen, oxygen, and other elements that are returned to the environment. SOME

OTHER

PA RA M E T E R S .

C O M B I N AT I O N A N D D E C O M P O S I T I O N . A synthesis, or combination,

Obviously, there are numerous ways to classify chemical reactions. Just to complicate things a little more, they can also be identified as to whether they produce heat (exothermic) or absorb heat (endothermic). Combustion is clearly an example of an exothermic reaction, while an endothermic reaction can be exemplified by the process that takes place in a cold pack. Used for instance to prevent swelling on an injured ankle, a cold pack contains an ampule that absorbs heat when broken.

reaction is one in which a compound is formed from simpler materials—whether those materi-

Still another way to identify chemical reactions is in terms of the phases of matter involved.

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It is also possible to have a single-displacement reaction, in which an element reacts with a compound, and one of the elements in the compound is released as a free element. This can be represented symbolically as A + BC 4B + AC. Single-displacement reactions often occur with metals and with halogens. For instance, a metal (A) reacts with an acid (BC) to produce hydrogen (B) and a salt (AC).

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We have already seen that some reactions form gases, some solids, and some yield water as one of the products. If reactants in one phase of matter produce a substance or substances in the same phase (liquid, solid, or gas), this is called a homogeneous reaction. On the other hand, if the reactants are in different phases of matter, or if they produce a substance or substances that are in a different phase, this is called a heterogeneous reaction. An example of a homogeneous reaction occurs when gaseous nitrogen combines with oxygen, also a gas, to produce nitrous oxide, or “laughing gas.” Similarly, nitrogen and hydrogen combine to form ammonia, also a gas. But when hydrogen and oxygen form water, this is a heterogeneous reaction. Likewise, when a metal undergoes an oxidation-reduction reaction, a gas and a solid react, resulting in a changed form of the metal, along with the production of new gases. Finally, a chemical reaction can be either reversible or irreversible. Much earlier, we described how wood experiences combustion, resulting in the production of ash. This is clearly an example of an irreversible reaction. The atoms in the wood and the air that oxidized it have not been destroyed, but it would be impossible to put the ash back together to make a piece of wood. By contrast, the formation of water by hydrogen and oxygen is reversible by means of electrolysis. KEEPING

IT

ALL

S T RA I G H T.

The different classifications of reactions discussed above are clearly not mutually exclusive; they simply identify specific aspects of the same thing. This is rather like the many physical characteristics that describe a person: gender, height, weight, eye color, hair color, race, and so on. Just because someone is blonde, for instance, does not mean that the person cannot also be browneyed; these are two different parameters that are more or less independent. On the other hand, there is some relation between these parameters in specific instances: for example, females over six feet tall are rare, simply because women tend to be shorter than men. But there are women who are six feet tall, or even considerably taller. In the same way, it is unlikely that a reaction in an aqueous solution will be a combustion reaction—yet it does happen, as for instance when potassium reacts with water.

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Studying Chemical Reactions Several aspects or subdisciplines of chemistry are brought to bear in the study of chemical reactions. One is stoichiometry (stoy-kee-AH-muhtree), which is concerned with the relationships among the amounts of reactants and products in a chemical reaction. The balancing of the chemical equation for water earlier in this essay is an example of basic stoichiometry. Chemical thermodynamics is the area of chemistry that addresses the amounts of heat and other forms of energy associated with chemical reactions. Thermodynamics is also a branch of physics, but in that realm, it is concerned purely with physical processes involving heat and energy. Likewise physicists study kinetics, associated with the movement of objects. Chemical kinetics, on the other hand, involves the study of the collisions between molecules that produce a chemical reaction, and is specifically concerned with the rates and mechanisms of reaction. S P E E D I N G U P A C H E M I CA L R E AC T I O N . Essentially, a chemical reaction

is the result of collisions between molecules. According to this collision model, if the collision is strong enough, it can break the chemical bonds in the reactants, resulting in a rearrangement of the atoms to form products. The more the molecules collide, the faster the reaction. Increase in the numbers of collisions can be produced in two ways: either the concentrations of the reactants are increased, or the temperature is increased. In either case, more molecules are colliding. Increases of concentration and temperature can be applied together to produce an even faster reaction, but rates of reaction can also be increased by use of a catalyst, a substance that speeds up the reaction without participating in it either as a reactant or product. Catalysts are thus not consumed in the reaction. One very important example of a catalyst is an enzyme, which speeds up complex reactions in the human body. At ordinary body temperatures, these reactions are too slow, but the enzyme hastens them along. Thus human life can be said to depend on chemical reactions aided by a wondrous form of catalyst. WHERE TO LEARN MORE Bender, Hal. “Chemical Reactions.” Clackamas Community College (Web site). (June 3, 2001).

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Chemical Reactions

KEY TERMS A chemical

istry—whether it be an element, com-

reaction in which an acid is mixed with a

pound, mixture, atom, molecule, ion, and

base, resulting in the formation of water

so forth.

ACID-BASE REACTION:

along with a salt. CHEMICAL

THERMODYNAMICS:

A mixture

The study of the amounts of heat and

of water and any substance that is solvent

other forms of energy associated with

in it.

chemical reactions.

AQUEOUS SOLUTIONS:

A chemical

BALANCED EQUATION:

COEFFICIENT:

A number used to

equation in which the numbers of atoms in

indicate the presence of more than one

the reactants and those in the product are

unit—typically, more than one molecule—

equal. In the course of balancing an equa-

of a chemical species in a chemical equa-

tion, coefficients may need to be applied to

tion. For instance, 2H2O indicates two

one or more of the chemical species

water molecules. (Note that 1 is never used

involved; however, the actual formulas of

as a coefficient.)

the species cannot be changed. COLLISION MODEL:

The theory that

A substance that speeds up

chemical reactions are the result of colli-

a chemical reaction without participating

sions between molecules that are strong

in it either as a reactant or product.

enough to break bonds in the reactants,

Catalysts are thus not consumed in the

resulting in a rearrangement of atoms to

reaction.

form a product or products.

CATALYST:

A represen-

CHEMICAL EQUATION:

DECOMPOSITION

REACTION:

A

tation of a chemical reaction in which

chemical reaction in which a compound is

the chemical symbols on the left stand for

broken down into simpler compounds, or

the reactants, and those on the right for the

even into elements. This is the opposite of

product or products. On paper, a chemical

a synthesis or combination reaction.

equation looks much like a mathematical one; however, instead of an equals sign, a chemical equation uses an arrow to show the direction of the reaction.

the rate at which chemical reactions occur. CHEMICAL REACTION:

TION:

REAC-

A chemical reaction in which

the partners in two compounds change places. This can be symbolized as

the study of

CHEMICAL KINETICS:

DOUBLE-DISPLACEMENT

A process

AB + CD 4AD + CB. Compare singledisplacement reaction. ENDOTHERMIC:

A term describing a

whereby the chemical properties of a sub-

chemical reaction in which heat is

stance are changed by a rearrangement of

absorbed or consumed.

the atoms in the substance. EXOTHERMIC: CHEMICAL SPECIES:

A generic term

used for any substance studied in chem-

S C I E N C E O F E V E RY DAY T H I N G S

A term describing a

chemical reaction in which heat is produced.

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Chemical Reactions

KEY TERMS A term describ-

product is in a different phase from that of

element reacts with a compound, and one of the elements in the compound is released as a free element. This can be represented symbolically as A + BC 4B + AC. Compare double-displacement reaction.

the reactants.

STOICHIOMETRY:

HETEROGENEOUS:

ing a chemical reaction in which the reactants are in different phases of matter (liquid, solid, or gas), or one in which the

HOMOGENEOUS:

A term describing

a chemical reaction in which the reactants and the product are all in the same phase of matter (liquid, solid, or gas). OXIDATION-REDUCTION REACTION:

The study of the relationships among the amounts of reactants and products in a chemical reaction. Producing a balanced equation requires application of stoichiometry (pronounced “stoy-kee-AH-muh-tree”).

A chemical reaction involving the transfer

SYNTHESIS

of electrons.

REACTION:

PRECIPITATION

REACTION:

A

chemical reaction in which a solid is formed. PRODUCT:

The substance or sub-

stances that result from a chemical reaction. REACTANT:

A substance that inter-

acts with another substance in a chemical reaction, resulting in the formation of a product. SINGLE-DISPLACEMENT TION:

288

CONTINUED

REAC-

A chemical reaction in which an

OR

COMBINATION

A chemical reaction in which a compound is formed from simpler materials—either elements or simple compounds. It is the opposite of a decomposition reaction.

A chemical equation in which the sum of atoms in the product or products does not equal the sum of atoms in the reactants. Initial observations of a chemical reaction usually produce an unbalanced equation, which needs to be analyzed and corrected (by the use of coefficients) to yield a balanced equation.

UNBALANCED EQUATION:

“Catalysis, Separations, and Reactions.” Accelrys (Web site). (June 3, 2001). Goo, Edward. “Chemical Reactions” (Web site). (June 3, 2001). Knapp, Brian J. Oxidation and Reduction. Illustrated by David Woodroffe. Danbury, CT: Grolier Educational, 1998. Knapp, Brian J. Energy and Chemical Change. Illustrated by David Woodroffe. Danbury, CT: Grolier Educational, 1998. Newmark, Ann. Chemistry. New York: Dorling Kindersley, 1993.

Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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S C I E N C E O F E V E RY DAY T H I N G S

“Periodic Table: Chemical Reaction Data.” WebElements (Web site). (June 3, 2001). Richards, Jon. Chemicals and Reactions. Brookfield, CT: Copper Beech Books, 2000. “Types of Chemical Reactions” (Web site). (June 3, 2001).

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O X I D AT I O N - R E D U C T I O N REACTIONS

Oxidation-Reduction Reactions

CONCEPT Most people have heard the term “oxidation” at some point or another, and, from the sound of the word, may have developed the impression that it has something to do with oxygen. Indeed it does, because oxygen has a tendency to draw electrons to itself. This tendency, rather than the presence of oxygen itself, is actually what identifies oxidation, defined as a process in which a substance loses electrons. The oxidation of one substance is always accompanied by reduction, or the gaining of electrons, on the part of another substance—hence the term “oxidation-reduction reaction,” sometimes called a redox reaction. The world is full of examples of this highly significant form of chemical reaction. One such example is combustion, or an even more rapid form of combustion, explosion. Likewise the metabolism of food, as well as other biological processes, involves oxidation and reduction reactions. So, too, do a number of processes that take place on the surfaces of metals: when iron rusts; when copper turns green; or when aluminum forms a coating of aluminum oxide that prevents it from rusting. Oxidation-reduction reactions also play a major role in electrochemistry, which has a highly useful application to daily life in the form of batteries.

affect the fundamental properties of the substance itself. A piece of copper can be heated, melted, beaten into different shapes, and so forth, yet throughout all those changes, it remains pure copper, an element of the transition metals family. But suppose a copper roof is exposed to the elements for many years. Copper is famous for its highly noncorrosive quality, and this, combined with its beauty, has made it a favored material for use in the roofs of imposing buildings. (Because it is relatively expensive, few middle-class people today can afford a roof entirely made of copper, but sometimes it is used as a decorative touch— for instance, over the entryway of a house.) Eventually, however, copper does begin to corrode when exposed to air for long periods of time. Over the years, exposed copper develops a thin layer of black copper oxide, and as time passes, traces of carbon dioxide in the air contribute to the formation of greenish copper carbonate. This explains why the Statue of Liberty, covered in sheets of copper, is green, rather than having the reddish-golden hue of new, uncorroded copper. EXTERNAL The CHANGE.

VS.

INTERNAL

A chemical reaction is a process whereby the chemical properties of a substance are changed by a rearrangement its atoms. The change produced by a chemical reaction is quite different from a purely physical change, which does not

preceding paragraphs describe two very different phenomena. The first was a physical change in which the chemical properties of a substance—copper—remained unaltered. The second, on the other hand, involved a chemical change on the surface of the copper, as copper atoms bonded with carbon and oxygen atoms in the air to form something different from copper. The difference between these two types of changes can be likened to varieties of changes in a person’s life—an external change

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on the one hand, and a deeply rooted change on the other. A person may move to another house, job, school, or town, yet the person remains the same. Many sayings in the English language express this fact: for instance, “Wherever you go, there you are,” or “You can take the boy out of the country, but you can’t take the country out of the boy.” Moving is simply a physical change. On the other hand, if a person changes belief systems, overcomes old feelings (or succumbs to new ones), changes lifestyles in a profound manner, or in any other way changes his or her mind about something important—this is analogous to a chemical change. In these instances, the person, like the surface of the copper described above, has changed not merely in external properties, but in inner composition.

“LEO the Lion Says ‘GER’” Chemical reactions are addressed in depth within the essay devoted to that subject, which discusses—among other subjects—many ways of classifying chemical reactions. These varieties of chemical reaction are not all mutually exclusive, as they relate to different aspects of the reaction. As noted in the review of various reaction types, one of the most significant is an oxidationreduction reaction (sometimes called a redox reaction) involving the transfer of electrons. As its name implies, an oxidation-reduction reaction is really two processes: oxidation, in which electrons are lost, and reduction, in which electrons are gained. Though these are defined separately here, they do not occur independently; hence the larger reaction of which each is a part is called an oxidation-reduction reaction. In order to keep the two straight, chemistry teachers long ago developed a useful, if nonsensical, mnemonic device: “LEO the lion says ‘GER’.” LEO stands for “Loss of Electrons, Oxidation,” and “GER” means “Gain of Electrons, Reduction.” Many, though not all, oxidation-reduction reactions involve oxygen. Oxygen combines readily with other elements, and in so doing, it tends to grab electrons from those other elements’ atoms. As a result, the oxygen atom becomes an ion (an atom with an electric charge)—specifically, an anion, or negatively charged ion.

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In interacting with another element, oxygen becomes reduced, while the other element is oxidized to become a cation, or a positively charged ion. This, too, is easy to remember: oxygen itself, obviously, cannot be oxidized, so it must be the one being reduced. But since not all oxidationreduction reactions involve oxygen, perhaps the following is a better way to remember it. Electrons are negatively charged, and the element that takes them on in an oxidation-reduction reaction is reduced—just as a person who thinks negative thoughts are “reduced” if those negative thoughts overcome positive ones.

Oxidation Numbers An oxidation number (sometimes called an oxidation state) is a whole-number integer assigned to each atom in an oxidation-reduction reaction. This makes it easier to keep track of the electrons involved, and to observe the ways in which they change positions. Here are some rules for determining oxidation number. • 1. The oxidation number for an atom of an element not combined with other elements in a compound is always zero. • 2. For an ion of any element, the oxidation number is the same as its charge. Thus a sodium ion, which has a charge of +1 and is designated symbolically as Na+, has an oxidation number of +1. • 3. Certain elements or families form ions in predictable ways: • a. Alkali metals, such as sodium, always form a +1 ion; oxidation number = +1. • b. Alkaline earth metals, such as magnesium, always form a +2 ion; oxidation number = +2. • c. Halogens, such as fluorine, form –1 ions; oxidation number = –1. • d. Other elements have predictable ways to form ions; but some, such as nitrogen, can have numerous oxidation numbers. • 4. The oxidation number for oxygen is –2 for most compounds involving covalent bonds. • 5. When hydrogen is involved in covalent bonds with nonmetals, its oxidation number is +1. • 6. In binary compounds (compounds with two elements), the element having greater electronegativity is assigned a negative oxidation number that is the same as its charge

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Batteries use oxidation-reduction reactions to produce electrical current. (Lester V. Bergman/ Corbis. Reproduced by permission.)

when it appears as an anion in ionic compounds. • 7. When a compound is electrically neutral, the sum of its elements’ oxidation states is zero. • 8. In an ionic chemical species, the sum of the oxidation states for its constituent elements must equal the overall charge. These rules will not be discussed here; rather, they are presented to show some of the complexities involved in analyzing an oxidationreduction reaction from a structural standpoint—that is, in terms of the atomic or molecular reactions. For the most part, we will be observing oxidation-reductions phenomenologically, or in terms of their outward effects. A good chemistry textbook should provide a more detailed review of these rules, along with a table showing oxidation numbers of elements and binary compounds. Oxidation-reduction reactions are easier to understand if they are studied as though they were two half-reactions. Half the reaction involves what happens to the substances and electrons in the oxidizing portion, while the other half-reaction indicates the activities of substances and electrons in the reduction portion. S C I E N C E O F E V E RY DAY T H I N G S

REAL-LIFE A P P L I C AT I O N S Combustion and Explosions As with any type of chemical reaction, combustion takes place when chemical bonds are broken and new bonds are formed. It so happens that combustion is a particularly dramatic type of oxidation-reduction reaction: whereas we cannot watch iron rust, combustion is a noticeable event. Even more dramatic is combustion that takes place at a rate so rapid that it results in an explosion. Coal is almost pure carbon, and its combustion in air is a textbook example of oxidationreduction. Although there is far more nitrogen than oxygen in air (which is a mixture rather than a compound), nitrogen is very unreactive at low temperatures. For this reason, it can be used to clean empty fuel tanks, a situation in which the presence of pure oxygen is extremely dangerous. In any case, when a substance burns, it is reacting with the oxygen in air. As one might expect from what has already been said about oxidation-reduction, the oxygen is reduced while the carbon is oxidized. In terms of oxidation numbers, the oxidation number of VOLUME 1: REAL-LIFE CHEMISTRY

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tary application, incidentally, was “Greek fire,” created by the Byzantines in the seventh century A.D. A mixture of petroleum, potassium nitrate, and possibly quicklime, Greek fire could burn on water, and was used in naval battles to destroy enemy ships.

OxidationReduction Reactions

For the most part, however, the range of activities to which combustion could be applied was fairly narrow until the development of the steam engine in the period from the late seventeenth century to the early nineteenth century. The steam engine applied the combustion of coal to the production of heat for boiling water, which in turn provided the power to run machinery. By the beginning of the twentieth century, combustion had found a new application in the internal combustion engine, used to power automobiles. EXPLOSIONS AND EXPLOS I V E S . An internal combustion engine does

Oxidation-reduction reactions fuel the space-shuttle at take-off. (Corbis. Reproduced by permission.)

carbon jumps from 0 to 4, while that of oxygen is reduced to –2. As they burn, these two form carbon dioxide or CO2, in which the two –2 charges of the oxygen atoms cancel out the +4 charge of the carbon atom to yield a compound that is electrically neutral. COMBUSTION IN HUMAN EXP E R I E N C E . Combustion has been a signifi-

cant part of human life ever since our prehistoric ancestors learned how to harness the power of fire to cook food and light their caves. We tend to think of premodern times—to use the memorable title of a book by American historian William Manchester, about the Middle Ages—as A World Lit Only By Fire. In fact, our modern age is even more combustion-driven than that of our forebears. For centuries, burning animal fat—in torches, lamps, and eventually in candles—provided light for humans. Wood fires supplied warmth, as well as a means to cook meals. These were the main uses of combustion, aside from the occasional use of fire in warfare or for other purposes (including that ghastly medieval form of execution, burning at the stake). One notable mili-

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not simply burn fuel; rather, by the combined action of the fuel injectors (in a modern vehicle), in concert with the pistons, cylinders, and spark plugs, it actually produces small explosions in the molecules of gasoline. These produce the output of power necessary to turn the crankshaft, and ultimately the wheels. An explosion, in simple terms, is a sped-up form of combustion. The first explosives were invented by the Chinese during the Middle Ages, and these included not only fireworks and explosive rockets, but gunpowder. Ironically, however, China rejected the use of gunpowder in warfare for many centuries, while Europeans took to it with enthusiasm. Needless to say, Europeans’ possession of firearms aided their conquest of the Americas, as well as much of Africa, Asia, and the Pacific, during the period from about 1500 to 1900. The late nineteenth and early twentieth centuries saw the development of new explosives, such as TNT or trinitrotoluene, a hydrocarbon. Then in the mid-twentieth century came the most fearsome explosive of all: the nuclear bomb. A nuclear explosion is not itself the result of an oxidation-reduction reaction, but of something much more complex—either the splitting of atoms (fission) or the forcing together of atomic nuclei (fusion). Nuclear bombs release far more energy than any ordinary explosive, but the resulting blast also causes plenty of ordinary combustion. When the United States dropped atomic bombs on the

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OxidationReduction Reactions

If not polished regularly, silver responds to oxidation by tarnishing—forming a surface of silver sulfide, or Ag2S. (Mimmo Jodice/Corbis. Reproduced by permission.)

Japanese cities of Hiroshima and Nagasaki in August 1945, those cities suffered not only the effects of the immediate blast, but also massive fires resulting from the explosion itself. FUELING

THE

S PAC E

SHUT-

T L E . Oxidation-reduction reactions also fuel

the most advanced form of transportation known today, the space shuttle. The actual orbiter vehicle is relatively small compared to its

solid rocket boosters on either side, along with an external fuel tank. Inside the solid rocket boosters are ammonium perchlorate (NH4ClO4) and powdered aluminum, which undergo an oxidation-reduction reaction that gives the shuttle enormous amounts of extra thrust. As for the larger single external fuel tank, this contains the gases that power the rocket: hydrogen and oxygen.

external power apparatus, which consists of two

Because these two are extremely explosive, they must be kept in separate compartments.

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When they react, they form water, of course, but in doing so, they also release vast quantities of energy. The chemical equation for this is: 2H2 + O2 42H2O + energy. On January 28, 1986, something went terribly wrong with this arrangement on the space shuttle Challenger. Cold weather had fatigued the O-rings that sealed the hydrogen and oxygen compartments, and the gases fed straight into the flames behind the shuttle itself. This produced a powerful and uncontrolled oxidation-reduction reaction, an explosion that took the lives of all seven astronauts aboard the shuttle.

The Environment and Human Health Combustion, though it can do much good, can also do much harm. This goes beyond the obvious: by burning fossil fuels or hydrocarbons, excess carbon (in the form of carbon dioxide and carbon monoxide) is released to the atmosphere, with a damaging effect on the environment. In fact, oxidation-reduction reactions are intimately connected with the functioning of the natural environment. For example, photosynthesis, the conversion of light to chemical energy by plants, is a form of oxidation-reduction reaction that produces two essentials of human life: oxygen and carbohydrates. Likewise cellular respiration, which along with photosynthesis is discussed in the Carbon essay, is an oxidationreduction reaction in which living things break down molecules of food to produce energy, carbon dioxide, and water. Enzymes in the human body regulate oxidation-reduction reactions. These complex proteins, of which several hundred are known, act as catalysts, speeding up chemical processes in the body. Oxidation-reduction reactions also take place in the metabolism of food for energy, with substances in the food broken down into components the body can use.

To forestall the effects of oxidation, some doctors and scientists recommend antioxidants—natural reducing agents such as vitamin C and vitamin E. The vitamin C in lemon juice can be used to prevent oxidizing on the cut surface of an apple, to keep it from turning brown. Perhaps, some experts maintain, natural reducing agents can also slow the pace of oxidation in the human body.

Forming a New Surface on Metal Clearly, oxidization can have a corrosive effect, and nowhere is this more obvious than in the corrosion of metals by exposure to oxidizing agents—primarily oxygen itself. Most metals react with O2, and might corrode so quickly that they become useless, were it not for the formation of a protective coating—an oxide. Iron forms an oxide, commonly known as rust, but this in fact does little to protect it from corrosion, because the oxide tends to flake off, exposing fresh surfaces to further oxidation. Every year, businesses and governments devote millions of dollars to protecting iron and steel from oxidation by means of painting and other measures, such as galvanizing with zinc. In fact, oxidation-reduction reactions virtually define the world of iron. Found naturally only in ores, the element is purified by heating the ore with coke (impure carbon) in the presence of oxygen, such that the coke reduces the iron.

Oxidation may also be linked with the effects of aging in humans, as well as with other condi-

C O I N AG E M E TA L S . Copper, as we have seen, responds to oxidation by corroding in a different way: not by rusting, but by changing color. A similar effect occurs in silver, which tarnishes, forming a surface of silver sulfide, or Ag2S. Copper and silver are two of the “coinage metals,” so named because they have often been used to mint coins. They have been used for this purpose not only because of their beauty, but also due to their relative resistance to corrosion. This resistance has, in fact, earned them the nickname “noble metals.”

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O X I D AT I O N : S P O I L I N G A N D AG I N G . At the same time, oxidation-reduc-

tion reactions are responsible for the spoiling of food, the culprit here being the oxidation portion of the reaction. To prevent spoilage, manufacturers of food items often add preservatives, which act as reducing agents.

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tions such as cancer, hardening of the arteries, and rheumatoid arthritis. It appears that oxygen molecules and other oxidizing agents, always hungry for electrons, extract these from the membranes in human cells. Over time, this can cause a gradual breakdown in the body’s immune system.

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KEY TERMS ANION:

An ion with a negative charge;

pronounced “AN-ie-un.”

ION:

An atom or group of atoms that

has acquired an electric charge due to the A compound

BINARY COMPOUND:

involving two elements.

loss or gain of electrons. OXIDATION:

A chemical reaction in

A process

which a substance loses electrons. It is

whereby the chemical properties of a sub-

always accompanied by reduction; hence

stance are changed by a rearrangement of

the term oxidation-reduction reaction.

the atoms in the substance.

OXIDATION

CHEMICAL REACTION:

NUMBER:

A number

A generic term

assigned to each atom in an oxidation-

used for any substance studied in chem-

reduction reaction as a means of keeping

istry—whether it be an element, com-

track of the electrons involved. Another

pound, mixture, atom, molecule, ion, and

term for oxidation number is “oxidation

so forth.

state.”

CHEMICAL SPECIES:

A type of

OXIDATION-REDUCTION REACTION:

chemical bonding in which two atoms

A chemical reaction involving the transfer

share valence electrons.

of electrons.

COVALENT BONDING:

ELECTROCHEMISTRY:

The study of

the relationship between chemical and electrical energy.

REDOX REACTION:

Another name

for an oxidation-reduction reaction. REDUCTION:

A chemical reaction in

The use of an elec-

which a substance gains electrons. It is

tric current to produce a chemical change.

always accompanied by oxidation; hence

ELECTROLYSIS:

ELECTRONEGATIVITY:

The relative

the term oxidation-reduction reaction.

ability of an atom to attract valence

VALENCE ELECTRONS:

electrons.

that occupy the highest energy levels in an

CATION:

An ion with a positive charge;

pronounced “KAT-ie-un.”

The third member of this mini-family is gold, which is virtually noncorrosive. Wonderful as gold is in this respect, however, no one is likely to use it as a roofing material, or for any such large-scale application involving its resistance to oxidation. Aside from the obvious expense, gold is soft, and not very good for structural uses, even if it were much cheaper. Yet there is such a “wonder metal”: one that experiences virtually no corrosion, is cheap, and strong enough in alloys to be used for structural purposes. Its name is aluminum.

S C I E N C E O F E V E RY DAY T H I N G S

Electrons

atom. These are the only electrons involved in chemical bonding.

A LU M I N U M . There was a time, in fact,

when aluminum was even more expensive than gold. When the French emperor Napoleon III wanted to impress a dinner guest, he arranged for the person to be served with aluminum utensils, while less distinguished personages had to settle for “ordinary” gold and silver. In 1855, aluminum sold for $100,000 a pound, whereas in 1990, the going rate was about $0.74. Demand did not go down—in fact, it increased exponentially—but rather, supply increased, thanks to the development of an inex-

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pensive aluminum-reduction process. Two men, one American and one French, discovered this process at the same time: interestingly, their years of birth and death were the same. Aluminum was once a precious metal because it proved extremely difficult to separate from oxygen. The Hall-Heroult process overcame the problem by applying electrolysis—the use of an electric current to produce a chemical change—as a way of reducing Al3+ ions (which have a high affinity for oxygen) to neutral aluminum atoms. In the United States today, 4.5% of the total electricity output is used for the production of aluminum through electrolysis. The foregoing statistic is staggering, considering just how much electricity Americans use, and it indicates the importance of this once-precious metal. Actually, aluminum oxidizes just like any other metal—and does so quite quickly, as a matter of fact, by forming a coating of aluminum oxide (Al2O3). But unlike rust, the aluminum oxide is invisible, and acts as a protective coating. Chromium, nickel, and tin react to oxygen in a similar way, but these are not as inexpensive as aluminum.

Electrochemistry and Batteries Electrochemistry is the study of the relationship between chemical and electrical energy. Among its applications is the creation of batteries, which use oxidation-reduction reactions to produce an electric current. A basic battery can be pictured schematically as two beakers of solution connected by a wire. In one solution is the oxidizing agent; in the other, a reducing agent. The wire allows electrons to pass back and forth between the two solutions, but to ensure that the flow goes both ways, the two solutions are also connected by a “salt bridge.” The salt bridge contains a gel or solution that permits ions to pass back and forth, but a porous membrane prevents the solutions from actually mixing.

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oxide (PbO2) acts as the oxidizing agent. A highly efficient type of battery, able to withstand wide extremes in temperature, the lead storage battery has been in use since 1915. Along the way, features have been altered, but the basic principles have remained—a testament to the soundness of its original design. The batteries people use for powering all kinds of portable appliances, from flashlights to boom boxes, are called dry cell batteries. In contrast to the model described above, using solutions, a dry cell (as its name implies) involves no liquid components. Instead, it utilizes various elements in a range of combinations, including zinc, magnesium, mercury, silver, nickel, and cadmium. The last two are applied in the nickelcadmium battery, which is particularly useful because it can be recharged over and over again by an external current. The current turns the products of the chemical reactions in the battery back into reactants. WHERE TO LEARN MORE “Batteries.” Oregon State University Department of Chemistry (Web site). (June 4, 2001). “Batteries and Fuel Cells” (Web site). (June 4, 2001). Borton, Paula and Vicky Cave. The Usborne Book of Batteries and Magnets. Tulsa, OK: EDC Publishing, 1995. Craats, Rennay. The Science of Fire. Milwaukee, WI: Gareth Stevens Publishing, 2000. Knapp, Brian J. Oxidation and Reduction. Danbury, CT: Grolier Educational, 1998. “Oxidation Reduction Links” (Web site). (June 4, 2001). “Oxidation-Reduction Reactions.” General Chemistry (Web site). (June 4, 2001). “Oxidation-Reduction Reactions: Redox.” UNC-Chapel Hill Chemistry Fundamentals (Web site). (June 4, 2001). Yount, Lisa. Antoine Lavoisier: Founder of Modern Chemistry. Springfield, NJ: Enslow Publishers, 1997.

In the lead storage battery of an automobile, lead itself is the reducing agent, while lead (IV)

Zumdahl, Steven S. Introductory Chemistry: A Foundation, fourth edition. Boston: Houghton Mifflin, 2000.

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CHEMICAL EQUILIBRIUM Chemical Equilibrium

CONCEPT Reactions are the “verbs” of chemistry—the activity that chemists study. Many reactions move to their conclusion and then stop, meaning that the reactants have been completely transformed into products, with no means of returning to their original state. In some cases, the reaction truly is irreversible, as for instance when combustion changes both the physical and chemical properties of a substance. There are plenty of other circumstances, however, in which a reverse reaction is not only possible but an ongoing process, as the products of the first reaction become the reactants in a second one. This dynamic state, in which the concentrations of reactants and products remains constant, is referred to as equilibrium. It is possible to predict the behavior of substances in equilibrium through the use of certain laws, which are applied in industries seeking to lower the costs of producing specific chemicals. Equilibrium is also useful in understanding processes that preserve—or potentially threaten—human health.

HOW IT WORKS Chemical Reactions in Brief What follows is a highly condensed discussion of chemical reactions, and particularly the methods for writing equations to describe them. For a more detailed explanation of these principles, the reader is encouraged to consult the Chemical Reactions essay.

The changes produced by a chemical reaction are fundamentally different from physical changes, such as boiling or melting liquid water, changes that alter the physical properties of water without affecting its molecular structure. I N D I CAT I O N S T H AT A C H E M I CA L R E AC T I O N H AS O C C U R R E D .

Though chemical reactions are most effectively analyzed in terms of molecular properties and behaviors, there are numerous indicators that suggest to us when a chemical reaction has occurred. It is unlikely that all of these will result from any one reaction, and in fact chances are that a particular reaction will manifest only one or two of these effects. Nonetheless, these offer us hints that a reaction has taken place: Signs that a substance has undergone a chemical reaction: • • • • • • •

Water is produced A solid forms Gases are produced Bubbles are formed There is a change in color The temperature changes The taste of a consumable substance changes • The smell changes C H E M I CA L CHANGES CONTRASTED WITH PHYSICAL CHANGES OF TEMPERATURE. Many of these

A chemical reaction is a process whereby the chemical properties of a substance are altered by a rearrangement of the atoms in the substance.

effects can be produced simply by changing the temperature of a substance, but again, the mere act of applying heat from outside (or removing heat from the substance itself) does not constitute a chemical change. Water can be “produced” by melting ice, but the water was already there— it only changed form. By contrast, when an acid

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the early nineteenth and the early twentieth centuries, however, the entire character of chemistry changed, as did the terms in which chemists discussed reactions. Today, those reactions are analyzed primarily in terms of subatomic, atomic, and molecular properties and activities.

Chemical Equilibrium

Despite all this progress, however, chemists still do not know exactly what happens in a chemical reaction—but they do have a good approximation. This is the collision model, which explains chemical reactions in terms of collisions between molecules. If the collision is strong enough, it can break the chemical bonds in the reactants, resulting in a re-formation of atoms within different molecules. The more the molecules collide, the more bonds are being broken, and the faster the reaction.

MANY

MOUNTAIN CLIMBERS USE PRESSURIZED OXYGEN

AT VERY HIGH ALTITUDES TO MAINTAIN PROPER HEMOGLOBIN-OXYGEN EQUILIBRIUM. (Galen Rowell/Corbis. Repro-

duced by permission.)

and a base react to form water and a salt, that is a true chemical reaction. Similarly, the freezing of water forms a solid, but no new substance has been formed; in a chemical reaction, by contrast, two liquids can react to form a solid. When water boils through the application of heat, bubbles form, and a gas or vapor is produced; yet in chemical changes, these effects are not the direct result of applying heat. In this context, a change in temperature, noted as another sign that a reaction has taken place, is a change of temperature from within the substance itself. Chemical reactions can be classified as heat-producing (exothermic) or heatabsorbing (endothermic). In either case, the transfer of heat is not accomplished simply by creating a temperature differential, as would occur if heat were transferred merely through physical means.

Raising the temperature and the concentrations of reactants can increase the energy and hasten the reactions, but in some cases it is not possible to do either. Fortunately, the rate of reaction can be increased in a third way, through the introduction of a catalyst, a substance that speeds up the reaction without participating in it either as a reactant or product. Catalysts are thus not consumed in the reaction. Many chemistry textbooks discuss catalysts within the context of equilibrium; however, because catalysts play such an important role in human life, in this book they are the subject of a separate essay.

Chemical Equations Involving Equilibrium

At one time, chemists could only study reactions from “outside,” as it were—purely in terms of effects noticeable through the senses. Between

A chemical equation, like a mathematical equation, symbolizes an interaction between entities that produces a particular result. In the case of a chemical equation, the entities are not numbers but reactants, and they interact with each other not through addition or multiplication, but by

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Increase in the numbers of collisions can be produced in two ways: either the concentrations of the reactants are increased, or the temperature is increased. By raising the temperature, the speeds of the molecules themselves increase, and the collisions possess more energy. A certain energy threshold, the activation energy (symbolized Ea) must be crossed in order for a reaction to occur. A temperature increase raises the likelihood that a given collision will cross the activation-energy threshold, producing the energy to break the molecular bonds and promote the chemical reaction.

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chemical reaction. Yet just as a product is the result of multiplication in mathematics, a product in a chemical equation is the substance or substances that result from the reaction. Instead of an equals sign between the reactants and the product, an arrow is used. When the arrow points to the right, this indicates a forward reaction; conversely, an arrow pointing to the left symbolizes a reverse reaction. In a reverse reaction, the products of a forward reaction have become the reactants, and the reactants of the forward reaction are now the products. This is indicated by an arrow that points toward the left. Chemical equilibrium, which occurs when the ratio between the reactants and products is constant, and in which the forward and reverse reactions take place at the same rate, is symbolized thus: 1. Note that the arrows, the upper one pointing right and the lower one pointing left, are of the same length. There may be certain cases, discussed below, in which it is necessary to show these arrows as unequal in length as a means of indicating the dominance of either the forward or reverse reaction. Chemical equations usually include notation indicating the state or phase of matter for the reactants and products: (s) for a solid; (l) for a liquid; (g) for a gas. A fourth symbol, (aq), indicates a substance dissolved in water—that is, an aqueous solution. In the following paragraphs, we will apply a chemical equation to the demonstration of equilibrium, but will not discuss the balancing of equations. The reader is encouraged to consult the passage in the Chemical Reactions essay that addresses that process, vital to the recording of accurate data. A SIMPLE EQUILIBRIUM EQUATION. Let us now consider a simple equation,

involving the reaction between water and carbon monoxide (CO) at high temperatures in a closed container. The initial equation is written thus: H2O(g) + CO(g) 4H2(g) + CO2g). In plain English, water in the gas phase (steam) has reacted with carbon monoxide to produce hydrogen gas and carbon dioxide.

CO(g) ! H2(g) + CO2g). Assuming that the system is not disturbed (that is, that the container is kept closed and no outside substances are introduced), equilibrium will continue to be maintained, because the reverse reaction is occurring at the same rate as the forward one.

Chemical Equilibrium

Note what has been said here: reactions are still occurring, but the forward and rearward reactions balance one another. Thus equilibrium is not a static condition, but a dynamic one, and indeed, chemical equilibrium is sometimes referred to as “dynamic equilibrium.” On the other hand, some chemists refer to chemical equilibrium simply as equilibrium, but here the qualifier chemical has been used to distinguish this from the type of equilibrium studied in physics. Physical equilibrium, which involves factors such as center of gravity, does help us to understand chemical equilibrium, but it is a different phenomenon.

REAL-LIFE A P P L I C AT I O N S Homogeneous and Heterogeneous Equilibria It should be noted that the equation used above identifies a situation of homogeneous equilibrium, in which all the substances are in the same phase or state of matter—gas, in this case. It is also possible to achieve chemical equilibrium in a reaction involving substances in more than one phase of matter. An example of such heterogeneous equilibrium is the decomposition of calcium carbonate for the production of lime, a process that involves the application of heat. Here the equation would be written thus: CaCO3(s) ! CaO(s) + CO2(g). Both the calcium carbonate (CaCO3) and the lime (CaO) are solids, whereas the carbon dioxide produced in this reaction is a gas.

The Equilibrium Constant

As the reaction proceeds, the amount of reactants decreases, and the concentration of products increases. At some point, however, there will be a balance between the numbers of products and reactants—a state of chemical equilibrium represented by changing the right-pointing arrow to an equilibrium symbol: H2O(g) +

In 1863, Norwegian chemists Cato Maximilian Guldberg (1836-1902) and Peter Waage (18331900)—who happened to be brothers-in-law— formulated what they called the law of mass action. Today, this is called the law of chemical equilibrium, which states that the direction taken by a reaction is dependant not merely on the mass of the various components of the reaction,

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but also upon the concentration—that is, the mass present in a given volume. This can be expressed by the formula aA + bB 1 cC +dD, where the capital letters represent chemical species, and the italicized lowercase letters indicate their coefficients. The equation [C]c[D]d/[A]a[B]b yields what is called an equilibrium constant, symbolized K. The above formula expresses the equilibrium constant in terms of molarity, the amount of solute in a given volume of solution, but in the case of gaseous reactants and products, the equilibrium constant can also be expressed in terms of partial pressures. In the reaction of water and carbon monoxide to produce hydrogen molecules and carbon dioxide (H2O + CO ! H2 + CO2). In chemical reactions involving solids, however, the concentration of the solid—because it is considered to be invariant—does not appear in the equilibrium constant. In the reaction described earlier, in which calcium carbonate was in equilibrium with solid lime and gaseous carbon dioxide, K = pressure of CO2. We will not attempt here to explore the equilibrium constant in any depth, but it is important to recognize its usefulness. For a particular reaction at a specific temperature, the ratio of concentrations between reactants and products will always have the same value—the equilibrium constant, or K. Because it is not dependant on the amounts of reactants and products mixed together initially, K remains the same: the concentrations themselves may vary, but the ratios between the concentrations in a given situation do not.

Le Châtelier’s Principle Not all situations of equilibrium are alike: depending on certain factors, the position of equilibrium may favor one side of the equation or the other. If a company is producing chemicals for sale, for example, its production managers will attempt to influence reactions in such a way as to favor the forward reaction. In such a situation, it is said that the equilibrium position has been shifted to the right. In terms of physical equilibrium, mentioned above, this would be analogous to what would happen if you were holding your arms out on either side of your body, with a heavy lead weight in your left hand and a much smaller weight in the right hand.

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Your center of gravity, or equilibrium position, would shift to the left to account for the greater force exerted by the heavier weight. A value of K significantly above 1 causes a shift to the right, meaning that at equilibrium, there will be more products than reactants. This is a situation favorable to a chemical company’s managers, who desire to create more of the product from less of the reactants. However, nature abhors an imbalance, as expressed in Le Châtelier’s principle. Named after French chemist Henri Le Châtelier (1850-1936), this principle maintains that whenever a stress or change is imposed on a chemical system in equilibrium, the system will adjust the amounts of the various substances to reduce the impact of that stress. Suppose we add more of a particular substance to increase the rate of the forward reaction. In an equation for this reaction, the equilibrium symbol is altered, with a longer arrow pointing to the right to indicate that the forward reaction is favored. Again, the equilibrium position has shifted to the right—just as one makes physical adjustments to account for an imbalanced weight. The system responds by working to consume more of the reactant, thus adjusting to the stress that was placed on it by the addition of more of that substance. By the same token, if we were to remove a particular reactant or product, the system would shift in the direction of the detached component. Note that Le Châtelier’s principle is mathematically related to the equilibrium constant. Suppose we have a basic equilibrium equation of A + B 1 C, with A and B each having molarities of 1, and C a molarity of 4. This tells us that K is equal to the molarity of C divided by that of A multiplied by B = 4/(1 • 1). Suppose, now, that enough of C were added to bring its concentration up to 6. This would mean that the system was no longer at equilibrium, because C/(A • B) no longer equals 4. In order to return the ratio to 4, the numerator (C) must be decreased, while the denominator (A • B) is increased. The reaction thus shifts from right to left. CHANGES IN VOLUME AND T E M P E RAT U R E . If the volume of gases in

a closed container is decreased, the pressure increases. An equilibrium system will therefore shift in the direction that reduces the pressure; but if the volume is increased, thus reducing the pressure, the system will respond by shifting to

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increase pressure. Note, however, that not all increases in pressure lead to a shift in the equilibrium. If the pressure were increased by the addition of a noble gas, the gas itself—since these elements are noted for their lack of reactivity— would not be part of the reaction. Thus the species added would not be part of the equilibrium constant expression, and there would be no change in the equilibrium.

material in red blood cells responsible for transporting oxygen to the cells. Each hemoglobin molecule attaches to four oxygen atoms, and the equilibrium conditions of the hemoglobin-oxygen interaction can be expressed thus: Hb(aq) + 4O2(g) ! Hb(O2)4(aq), where “Hb” stands for hemoglobin. As long as there is sufficient oxygen in the air, a healthy equilibrium is maintained; but at high altitudes, considerable changes occur.

In any case, no change in volume alters the equilibrium constant K; but where changes in temperature are involved, K is indeed altered. In an exothermic, or heat-producing reaction, the heat is treated as a product. Thus, when nitrogen and hydrogen react, they produce not only ammonia, but a certain quantity of heat. If this system is at equilibrium, Le Châtelier’s principle shows that the addition of heat will induce a shift in equilibrium to the left—in the direction that consumes heat or energy.

At significant elevations above sea level, the air pressure is lowered, and thus it is more difficult to obtain the oxygen one needs. The result, in accordance with Le Châtelier’s principle, is a shift in equilibrium to the left, away from the oxygenated hemoglobin. Without adequate oxygen fed to the body’s cells and tissues, a person tends to feel light-headed.

The reverse is true in an endothermic, or heat-absorbing reaction. As in the process described earlier, the thermal decomposition of calcium carbonate produces lime and carbon dioxide. Because heat is used to cause this reaction, the amount of heat applied is treated as a reactant, and an increase in temperature will cause the equilibrium position to shift to the right.

Equilibrium and Health Discussions of chemical equilibrium tend to be rather abstract, as the foregoing sections on the equilibrium constant and Le Châtelier’s principle illustrate. (The reader is encouraged to consult additional sources on these topics, which involve a number of particulars that have been touched upon only briefly here.) Despite the challenges involved in addressing the subject of equilibrium, the results of chemical equilibrium can be seen in processes involving human health.

When someone not physically prepared for the change is exposed to high altitudes, it may be necessary to introduce pressurized oxygen from an oxygen tank. This shifts the equilibrium to the right. For people born and raised at high altitudes, however, the body’s chemistry performs the equilibrium shift—by producing more hemoglobin, which also shifts equilibrium to the right. HEMOGLOBIN AND CARBON M O N OX I D E . When someone is exposed to

carbon monoxide gas, a frightening variation on the normal hemoglobin-oxygen interaction occurs. Carbon monoxide “fools” hemoglobin into mistaking it for oxygen because it also bonds to hemoglobin in groups of four, and the equilibrium expression thus becomes: Hb(aq) + 4CO(g) ! Hb(CO)4(aq). Instead of hemoglobin, what has been produced is called carboxyhemoglobin, which is even redder than hemoglobin. Therefore, one sign of carbon monoxide poisoning is a flushed face.

Hemoglobin, a protein containing iron, is the

The bonds between carbon monoxide and hemoglobin are about 300 times as strong as those between hemoglobin and oxygen, and this means a shift in equilibrium toward the right side of the equation—the carboxyhemoglobin side. It also means that K for the hemoglobin-carbon monoxide reaction is much higher than for the hemoglobin-oxygen reaction. Due to the affinity of hemoglobin for carbon monoxide, the hemoglobin puts a priority on carbon monoxide bonds, and hemoglobin that has bonded with carbon monoxide is no longer available to carry oxygen.

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The cooling of food with refrigerators, along with means of food preservation that do not involve changes in temperature, maintains chemical equilibrium in the foods and thereby prevents or at least retards spoilage. Even more important is the maintenance of equilibrium in reactions between hemoglobin and oxygen in human blood. H E M O G LO B I N

AND

OX Y G E N .

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Chemical Equilibrium

KEY TERMS ACTIVATION ENERGY:

The minimal

A process

energy required to convert reactants into

whereby the chemical properties of a sub-

products, symbolized Ea.

stance are changed by a rearrangement of

AQUEOUS SOLUTION:

A mixture of

water and a substance that is dissolved in it. CATALYST:

A substance that speeds up

a chemical reaction without participating in it, either as a reactant or product. Catalysts are thus not consumed in the reaction. CHEMICAL EQUATION:

the atoms in the substance. CHEMICAL SPECIES:

A represen-

chemical symbols on the left stand for the

A generic term

used for any substance studied in chemistry—whether it be an element, compound, mixture, atom, molecule, ion, and so forth. COEFFICIENT:

tation of a chemical reaction in which the

a number used to indi-

cate the presence of more than one unit— typically, more than one molecule—of a chemical species in a chemical equation. The theory that

reactants, and those on the right are the

COLLISION MODEL:

product or products.

chemical reactions are the result of colli-

CHEMICAL EQUILIBRIUM:

A situa-

tion in which the ratio between the reactants and products in a chemical reaction is

sions between molecules that are strong enough to break bonds in the reactants, resulting in a reformation of atoms. A term describing a

constant, and in which the forward reac-

ENDOTHERMIC:

tions and reverse reactions take place at the

chemical reaction in which heat is

same rate.

absorbed or consumed.

Carbon monoxide in small quantities can cause headaches and dizziness, but larger concentrations can be fatal. To reverse the effects of the carbon monoxide, pure oxygen must be introduced to the body. It will react with the carboxyhemoglobin to produce properly oxygenated hemoglobin, along with carbon monoxide: Hb(CO)4(aq) + 4O2(g) ! Hb(O2)4(aq) + 4CO(g). The gaseous carbon monoxide thus produced is dissipated when the person exhales.

Challoner, Jack. The Visual Dictionary of Chemistry. New York: DK Publishing, 1996.

“Chemical Equilibrium.” Davidson College Department of Chemistry (Web site). (June 9, 2001). “Chemical Equilibrium in the Gas Phase.” Virginia Tech Chemistry Department (Web site). (June 9, 2001). “Chemical Sciences: Mechanism of Catalysis.” University of Alberta Department of Chemistry (Web site). (June 9, 2001). Ebbing, Darrell D.; R. A. D. Wentworth; and James P. Birk. Introductory Chemistry. Boston: Houghton Mifflin, 1995. Hauser, Jill Frankel. Super Science Concoctions: 50 Mysterious Mixtures for Fabulous Fun. Charlotte, VT: Williamson Publishing, 1996. “Mark Rosen’s Chemical Equilibrium Links” (Web site). (June 9, 2001).

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WHERE TO LEARN MORE “Catalysts” (Web site). (June 9, 2001).

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CHEMICAL REACTION:

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Chemical Equilibrium

KEY TERMS EXOTHERMIC:

A term describing a

chemical reaction in which heat is

CONTINUED

the various substances in such a way as to reduce the impact of that stress.

produced. PRODUCT: FORWARD REACTION:

A chemical

reaction symbolized by a chemical equa-

stances that result from a chemical reaction.

tion in which the reactants and product are separated by an arrow that points to the right, toward the products.

The substance or sub-

REACTANT:

A substance that interacts

with another substance in a chemical reaction, resulting in a product.

HETEROGENEOUS EQUILIBRIUM:

A chemical

Chemical equilibrium in which the sub-

REVERSE REACTION:

stances involved are in different phases of

reaction symbolized by a chemical equa-

matter (solid, liquid, gas.)

tion in which the products of a forward reaction have become the reactants, and

HOMOGENEOUS

EQUILIBRIUM:

the reactants of the forward reaction are

Chemical equilibrium in which the sub-

now the products. This is indicated by an

stances involved are in the same phase of

arrow that points toward the left.

matter. SYSTEM:

In chemistry and other sci-

A

ences, the term “system” usually refers to

statement, formulated by French chemist

any set of interactions isolated from the

Henri Le Châtelier (1850-1936), which

rest of the universe. Anything outside of

holds that whenever a stress or change is

the system, including all factors and forces

imposed on a system in chemical equilibri-

irrelevant to a discussion of that system, is

um, the system will adjust the amounts of

known as the environment.

LE CHÂTELIER’S PRINCIPLE:

Oxlade, Chris. Chemistry. Illustrated by Chris Fairclough. Austin, TX: Raintree Steck-Vaughn, 1999.

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Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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C A T A LY S T S Catalysts

CONCEPT In most of the processes studied within the physical sciences, the lesson again and again is that nature provides no “free lunch”; in other words, it is not possible to get something for nothing. A chemical reaction, for instance, involves the creation of substances different from those that reacted in the first place, but the number of atoms involved does not change. In view of nature’s inherently conservative tendencies, then, the idea of a catalyst—a substance that speeds up a reaction without being consumed—seems almost like a magic trick. But catalysts are very real, and their presence in the human body helps to sustain life. Similarly, catalysts enable the synthesis of foods, and catalytic converters in automobiles protect the environment from dangerous exhaust fumes. Yet the presence of one particular catalyst in the upper atmosphere poses such a threat to Earth’s ozone layer that production of certain chemicals containing that substance has been banned.

HOW IT WORKS Reactions and Collisions In a chemical reaction, substances known as reactants interact with one another to create new substances, called products. In the present context, our concern is not with the reactants and products themselves, but with an additional entity, an agent that enables the reaction to move forward at faster rates and lower temperatures.

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that are sufficiently energetic break the chemical bonds that hold molecules together; as a result, the atoms in those molecules are free to recombine with other atoms to form new molecules. Hastening of a chemical reaction can be produced in one of three ways. If the concentrations of the reactants are increased, this means that more molecules are colliding, and potentially more bonds are being broken. Likewise if the temperature is increased, the speeds of the molecules themselves increase, and their collisions possess more energy. Energy is an important component in the chemical reaction because a certain threshold, called the activation energy (Ea), must be crossed before a reaction can occur. A temperature increase raises the energy of the collisions, increasing the likelihood that the activationenergy threshold will be crossed, resulting in the breaking of molecular bonds.

Catalysts and Catalysis It is not always feasible or desirable, however, to increase the concentration of reactants, or the temperature of the system in which the reaction is to occur. Many of the processes that take place in the human body, for instance, “should” require high temperatures—temperatures too high to sustain human life. But fortunately, our bodies contain proteins called enzymes, discussed later in this essay, that facilitate the necessary reactions without raising temperatures or increasing the concentrations of substances.

According to the collision model generally accepted by chemists, chemical reactions are the result of collisions between molecules. Collisions

An enzyme is an example of a catalyst, a substance that speeds up a reaction without participating in it either as a reactant or product. Catalysts are thus not consumed in the reaction. The

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Catalysts

CATALYTIC

CONVERTERS EMPLOY A CATALYST TO FACILITATE THE TRANSFORMATION OF POLLUTION-CAUSING EXHAUSTS

TO LESS HARMFUL SUBSTANCES. (Ian Harwood; Ecoscene/Corbis. Reproduced by permission.)

catalyst does its work—catalysis—by creating a different path for the reaction, and though the means whereby it does this are too complex to discuss in detail here, the process of catalyst can at least be presented in general terms. Imagine a graph whose x-axis is labeled “reaction progress,” while the y-axis bears the legend “energy.” There is some value of y equal to the normal activation energy, and in the course of experiencing the molecular collisions that lead to a reaction, the reactants reach this level. In a catalyzed reaction, however, the level of activation energy necessary for the reaction is represented by a lower y-value on the graph. The catalyzed substances do not need to have as much energy as they do without a catalyst, and there-

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fore the reaction can proceed more quickly— without changing the temperature or concentrations of reactants.

REAL-LIFE A P P L I C AT I O N S A Brief History of Catalysis Long before chemists recognized the existence of catalysts, ordinary people had been using the process of catalysis for a number of purposes: making soap, for instance, or fermenting wine to create vinegar, or leavening bread. Early in the nineteenth century, chemists began to take note of this phenomenon.

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kata (“down”) and lyein (“loosen.”) As Berzelius defined it, catalysis involved an activity quite different from that of an ordinary chemical reaction. Catalysis induced decomposition in substances, resulting in the formation of new compounds—but without the catalyst itself actually entering the compound.

Catalysts

Berzelius’s definition assumed that a catalyst manages to do what it does without changing at all. This was perfectly adequate for describing heterogeneous catalysis, in which the catalyst and the reactants are in different phases of matter. In the platinum-catalyzed reactions that Davy and Faraday observed, for instance, the platinum is a solid, while the reaction itself takes place in a gaseous or liquid state. However, homogeneous catalysis, in which catalyst and reactants are in the same state, required a different explanation, which English chemist Alexander William Williamson (1824-1904) provided in an 1852 study. FRITZ HABER. (Austrian Archives/Corbis. Reproduced by permission.)

In 1812, Russian chemist Gottlieb Kirchhof was studying the conversion of starches to sugar in the presence of strong acids when he noticed something interesting. When a suspension of starch in water was boiled, Kirchhof observed, no change occurred in the starch. However, when he added a few drops of concentrated acid before boiling the suspension (that is, particles of starch suspended in water), he obtained a very different result. This time, the starch broke down to form glucose, a simple sugar, while the acid—which clearly had facilitated the reaction—underwent no change. Around the same time, English chemist Sir Humphry Davy (1778-1829) noticed that in certain organic reactions, platinum acted to speed along the reaction without undergoing any change. Later on, Davy’s star pupil, the great British physicist and chemist Michael Faraday (1791-1867), demonstrated the ability of platinum to recombine hydrogen and oxygen that had been separated by the electrolysis of water. The catalytic properties of platinum later found application in catalytic converters, as we shall see.

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In discussing the reaction observed by Kirchhof, of liquid sulfuric acid with starch in an aqueous solution, Williamson was able to show that the catalyst does break down in the course of the reaction. As the reaction takes place, it forms an intermediate compound, but this too is broken down before the reaction ends. The catalyst thus emerges in the same form it had at the beginning of the reaction.

Enzymes: Helpful Catalysts in the Body In 1833, French physiologist Anselme Payen (1795-1871) isolated a material from malt that accelerated the conversion of starch to sugar, as for instance in the brewing of beer. Payen gave the name “diastase” to this substance, and in 1857, the renowned French chemist Louis Pasteur (1822-1895) suggested that lactic acid fermentation is caused by a living organism.

A N I M P R O V E D D E F I N I T I O N . In 1835, Swedish chemist Jons Berzelius (17791848) provided a name to the process Kirchhof and Davy had observed from very different perspectives: catalysis, derived from the Greek words

In fact, the catalysts studied by Pasteur are not themselves separate organisms, as German biochemist Eduard Buchner (1860-1917) showed in 1897. Buchner isolated the catalysts that bring about the fermentation of alcohol from living yeast cells—what Payen had called “diastase,” and Pasteur “ferments.” Buchner demonstrated that these are actually chemical substances, not organisms. By that time, German physiologist Willy Kahne had suggested the name “enzyme” for these catalysts in living systems.

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Enzymes are made up of amino acids, which in turn are constructed from organic compounds called proteins. About 20 amino acids make up the building blocks of the many thousands of known enzymes. The beauty of an enzyme is that it speeds up complex, life-sustaining reactions in the human body—reactions that would be too slow at ordinary body temperatures. Rather than force the body to undergo harmful increases in temperature, the enzyme facilitates the reaction by opening up a different reaction pathway that allows a lower activation energy. One example of an enzyme is cytochrome, which aids the respiratory system by catalyzing the combination of oxygen with hydrogen within the cells. Other enzymes facilitate the conversion of food to energy, and make possible a variety of other necessary biological functions. Because numerous interactions are required in their work of catalysis, enzymes are very large, and may have atomic mass figures as high as 1 million amu. However, it should be noted that reactions are catalyzed at very specific locations—called active sites—on an enzyme. The reactant molecule fits neatly into the active site on the enzyme, much like a key fitting in a lock; hence the name of this theory, the “lock-andmodel.”

Catalysis and the Environment The exhaust from an automobile contains many substances that are harmful to the environment. As a result of increased concerns regarding the potential damage to the atmosphere, the federal government in the 1970s mandated the adoption of catalytic converters, devices that employ a catalyst to transform pollutants in the exhaust to less harmful substances. Platinum and palladium are favored materials for catalytic converters, though some nonmetallic materials, such as ceramics, have been used as well. In any case, the function of a catalytic converter is to convert exhausts through oxidation-reduction reactions. Nitric oxide is reduced to molecular oxygen and nitrogen; at the same time, the hydrocarbons in petroleum, along with carbon monoxide, are oxidized to form carbon dioxide and water. Sometimes a reducing agent, such as ammonia, is used to make the reduction process more effective.

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A DA N G E R O U S CATA LY S T I N T H E AT M O S P H E R E . Around the same

Catalysts

time that automakers began rolling out models equipped with catalytic converters, scientists and the general public alike became increasingly concerned about another threat to the environment. In the upper atmosphere of Earth are traces of ozone, a triatomic (three-atom) molecular form of oxygen which protects the planet from the Sun’s ultraviolet rays. During the latter part of the twentieth century, it became apparent that a hole had developed in the ozone layer over Antarctica, and many chemists suspected a culprit in chlorofluorocarbons, or CFCs. CFCs had long been used in refrigerants and air conditioners, and as propellants in aerosol sprays. Because they were nontoxic and noncorrosive, they worked quite well for such purposes, but the fact that they are chemically unreactive had an extremely negative side-effect. Instead of reacting with other substances to form new compounds, they linger in Earth’s atmosphere, eventually drifting to high altitudes, where ultraviolet light decomposes them. The real trouble begins when atoms of chlorine, isolated from the CFC, encounter ozone. Chlorine acts as a catalyst to transform the ozone to elemental oxygen, which is not nearly as effective as ozone for shielding Earth from ultraviolet light. It does so by interacting also with monatomic, or single-atom oxygen, with which it produces ClO, or the hypochlorite ion. The end result of reactions between chlorine, monatomic oxygen, hypochlorite, and ozone is the production of chlorine, hypochlorite, and diatomic oxygen—in other words, no more ozone. It is estimated that a single chlorine atom can destroy up to 1 million ozone molecules per second. Due to concerns about the danger to the ozone layer, an international agreement called the Montreal Protocol, signed in 1996, banned the production of CFCs and the coolant Freon that contains them. But people still need coolants for their homes and cars, and this has led to the creation of substitutes—most notably hydrochlorofluorocarbons (HCFCs), organic compounds that do not catalyze ozone.

Other Examples of Catalysts Catalysts appear in a number of reactions, both natural and artificial. For instance, catalysts are used in the industrial production of ammonia,

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Catalysts

KEY TERMS The minimal energy required to convert reactants into products, symbolized Ea ACTIVATION ENERGY:

AQUEOUS SOLUTIONS: A mixture of water and a substance that is dissolved in it.

A substance that speeds up a chemical reaction without participating in it, either as a reactant or product. Catalysts are thus not consumed in the reaction. CATALYST:

CHEMICAL REACTION: A process whereby the chemical properties of a substance are changed by a rearrangement of the atoms in the substance. COLLISION MODEL: The theory that chemical reactions are the result of collisions between molecules strong enough to break bonds in the reactants, resulting in a reformation of atoms.

A reaction in which the catalyst and the reactants are in different phases of matter. HETEROGENEOUS CATALYSIS:

CATALYSIS: A reaction in which catalyst and reactants are in the same phase of matter.

HOMOGENEOUS

The substance or substances that result from a chemical reaction. PRODUCT:

A substance that interacts with another substance in a chemical reaction, resulting in a product. REACTANT:

SYSTEM: In chemistry and other sciences, the term “system” usually refers to any set of interactions isolated from the rest of the universe. Anything outside of the system, including all factors and forces irrelevant to a discussion of that system, is known as the environment.

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nitric acid (produced from ammonia), sulfuric acid, and other substances. The ammonia process, developed in 1908 by German chemist Fritz Haber (1868-1934), is particularly noteworthy. Using iron as a catalyst, Haber was able to combine nitrogen and hydrogen under pressure to form ammonia—one of the world’s most widely used chemicals. Eighteen ninety-seven was a good year for catalysts. In that year, it was accidentally discovered that mercury catalyzes the reaction by which indigo dye is produced; also in 1897, French chemist Paul Sabatier (1854-1941) found that nickel catalyzes the production of edible fats. Thanks to Sabatier’s discovery, nickel is used to transform inedible plant oils to margarine and shortening. Another good year for catalysts—particularly those involved in the production of polymers—was 1953. That was the year when German chemist Karl Ziegler (1898-1973) discovered a resin catalyst for the production of polyethylene, which produced a newer, tougher product with a much higher melting point than polyethylene as it was produced up to that time. Also in 1953, Italian chemist Giulio Natta (19031979) adapted Ziegler’s idea, and developed a new type of plastic he called “isotactic” polymers. These could be produced easily, and in abundance, through the use of catalysts. One of the lessons of chemistry, or indeed of any science, is that there are few things chemists can do that nature cannot achieve on a far more wondrous scale. No artificial catalyst can compete with enzymes, and no use of a catalyst in a laboratory can compare with the grandeur of that which takes place on the Sun. As GermanAmerican physicist Hans Bethe (1906-) showed in 1938, the reactions of hydrogen that form helium on the surface of the Sun are catalyzed by carbon—the same element, incidentally, found in all living things on Earth.

WHERE TO LEARN MORE “Bugs in the News: What the Heck Is an Enzyme?” University of Kansas (Web site). (June 9, 2001). “Catalysis.” University of Idaho Department of Chemistry (Web site). (June 9, 2001).

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“Catalysts” (Web site). (June 9, 2001).

Oxlade, Chris. Chemistry. Illustrated by Chris Fairclough. Austin, TX: Raintree Steck-Vaughn, 1999.

Ebbing, Darrell D.; R. A. D. Wentworth; and James P. Birk. Introductory Chemistry. Boston: Houghton Mifflin, 1995.

“Ozone Depletion” (Web site). (June 9, 2001).

“Enzymes.” Strategis (Web site). (June 9, 2001). “Enzymes: Classification, Structure, Mechanism.” The Hebrew University (Web site). (June 9, 2001).

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Catalysts

“University Chemistry: Chemical Kinetics: Catalysis.” University of Alberta Department of Chemistry (Web site). (June 9, 2001). Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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ACIDS AND BASES Acids and Bases

CONCEPT The name “acid” calls to mind vivid sensory images—of tartness, for instance, if the acid in question is meant for human consumption, as with the citric acid in lemons. On the other hand, the thought of laboratory- and industrialstrength substances with scary-sounding names, such as sulfuric acid or hydrofluoric acid, carries with it other ideas—of acids that are capable of destroying materials, including human flesh. The name “base,” by contrast, is not widely known in its chemical sense, and even when the older term of “alkali” is used, the sense-impressions produced by the word tend not to be as vivid as those generated by the thought of “acid.” In their industrial applications, bases too can be highly powerful. As with acids, they have many household uses, in substances such as baking soda or oven cleaners. From a taste standpoint, (as anyone who has ever brushed his or her teeth with baking soda knows), bases are bitter rather than sour. How do we know when something is an acid or a base? Acid-base indicators, such as litmus paper and other materials for testing pH, offer a means of judging these qualities in various substances. However, there are larger structural definitions of the two concepts, which evolved in three stages during the late nineteenth and early twentieth centuries, that provide a more solid theoretical underpinning to the understanding of acids and bases.

The phenomenological distinctions between acids and bases, gathered by scientists from ancient times onward, worked well enough for many centuries. The word “acid” comes from the Latin term acidus, or “sour,” and from an early period, scientists understood that substances such as vinegar and lemon juice shared a common acidic quality. Eventually, the phenomenological definition of acids became relatively sophisticated, encompassing such details as the fact that acids produce characteristic colors in certain vegetable dyes, such as those used in making litmus paper. In addition, chemists realized that acids dissolve some metals, releasing hydrogen in the process. W H Y “ B AS E ” A N D N O T “A L KA L I ” ? The word “alkali” comes from the Arabic

Prior to the development of atomic and molecular theory in the nineteenth century, followed by

al-qili, which refers to the ashes of the seawort plant. The latter, which typically grows in marshy areas, was often burned to produce soda ash, used in making soap. In contrast to acids, bases— caffeine, for example—have a bitter taste, and many of them feel slippery to the touch. They also produce characteristic colors in the vegetable dyes of litmus paper, and can be used to promote certain chemical reactions. Note that today chemists use the word “base” instead of “alkali,”

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the discovery of subatomic structures in the late nineteenth and early twentieth centuries, chemists could not do much more than make measurements and observations. Their definitions of substances were purely phenomenological—that is, the result of experimentation and the collection of data. From these observations, they could form general rules, but they lacked any means of “seeing” into the atomic and molecular structures of the chemical world.

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the reason being that the latter term has a narrower meaning: all alkalies are bases, but not all bases are alkalies.

Acids and Bases

Originally, “alkali” referred only to the ashes of burned plants, such as seawort, that contained either sodium or potassium, and from which the oxides of sodium and potassium could be obtained. Eventually, alkali came to mean the soluble hydroxides of the alkali and alkaline earth metals. This includes sodium hydroxide, the active ingredient in drain and oven cleaners; magnesium hydroxide, used for instance in milk of magnesia; potassium hydroxide, found in soaps and other substances; and other compounds. Broad as this range of substances is, it fails to encompass the wide array of materials known today as bases—compounds which react with acids to form salts and water.

Toward a Structural Definition SVANTE ARRHENIUS. (Hulton-Deutsch Collection/Corbis. Repro-

The reaction to form salts and water is, in fact, one of the ways that acids and bases can be defined. In an aqueous solution, hydrochloric acid and sodium hydroxide react to form sodium chloride—which, though it is suspended in an aqueous solution, is still common table salt— along with water. The equation for this reaction is HCl(aq) + NaOH(aq) 4H2O + NaCl(aq). In other words, the sodium (Na) ion in sodium hydroxide switches places with the hydrogen ion in hydrochloric acid, resulting in the creation of NaCl (salt) along with water. But why does this happen? Useful as this definition regarding the formation of salts and water is, it is still not structural—in other words, it does not delve into the molecular structure and behavior of acids and bases. Credit for the first truly structural definition of the difference goes to the Swedish chemist Svante Arrhenius (18591927). It was Arrhenius who, in his doctoral dissertation in 1884, introduced the concept of an ion, an atom possessing an electric charge. His understanding was particularly impressive in light of the fact that it was 13 more years before the discovery of the electron, the subatomic particle responsible for the creation of ions. Atoms have a neutral charge, but when an electron or electrons depart, the atom becomes a positive ion or cation. Similarly, when an electron or electrons join a previously uncharged

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duced by permission.)

atom, the result is a negative ion or anion. Not only did the concept of ions greatly influence the future of chemistry, but it also provided Arrhenius with the key necessary to formulate his distinction between acids and bases.

The Arrhenius Definition Arrhenius observed that molecules of certain compounds break into charged particles when placed in liquid. This led him to the Arrhenius acid-base theory, which defines an acid as any compound that produces hydrogen ions (H+) when dissolved in water, and a base as any compound that produces hydroxide ions (OH-) when dissolved in water. This was a good start, but two aspects of Arrhenius’s theory suggested the need for a definition that encompassed more substances. First of all, his theory was limited to reactions in aqueous solutions. Though many acid-base reactions do occur when water is the solvent, this is not always the case. Second, the Arrhenius definition effectively limited acids and bases only to those ionic compounds, such as hydrochloric acid or sodium hydroxide, which produced either hydrogen or

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Acids and Bases

THE

TARTNESS OF LEMONS AND LIMES IS A FUNCTION OF THEIR ACIDITY—SPECIFICALLY THE WAY IN WHICH CITRIC

ACID MOLECULES FIT INTO THE TONGUE’S

“SWEET”

RECEPTORS. (Orion Press/Corbis. Reproduced by permission.)

hydroxide ions. However, ammonia, or NH3, acts like a base in aqueous solutions, even though it does not produce the hydroxide ion. The same is true of other substances, which behave like acids or bases without conforming to the Arrhenius definition. These shortcomings pointed to the need for a more comprehensive theory, which arrived with the formulation of the Brønsted-Lowry definition by English chemist Thomas Lowry (1874-1936) and Danish chemist J. N. Brønsted (1879-1947). Nonetheless, Arrhenius’s theory represented an important first step, and in 1903, he was awarded the Nobel Prize in Chemistry for his work on the dissociation of molecules into ions.

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The Brønsted-Lowry Definition The Brønsted-Lowry acid-base theory defines an acid as a proton (H+) donor, and a base as a proton acceptor, in a chemical reaction. Protons are represented by the symbol H+, and in representing acids and bases, the symbols HA and A-, respectively, are used. These symbols indicate that an acid has a proton it is ready to give away, while a base, with its negative charge, is ready to receive the positively charged proton. Though it is used here to represent a proton, it should be pointed out that H+ is also the hydrogen ion—a hydrogen atom that has lost its sole electron and thus acquired a positive charge.

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It is thus really nothing more than a lone proton, but this is the one and only case in which an atom and a proton are exactly the same thing. In an acid-base reaction, a molecule of acid is “donating” a proton, in the form of a hydrogen ion. This should not be confused with a far more complex process, nuclear fusion, in which an atom gives up a proton to another atom. A N AC I D - B AS E R E AC T I O N I N B R Ø N S T E D - LO W RY T H E O RY. The

most fundamental type of acid-base reaction in Brønsted-Lowry theory can be symbolized thus HA(aq) + H2O(l) 4H3O+(aq) + A-(aq). The first acid shown—which, like three of the four “players” in this equation, is dissolved in an aqueous solution—combines with water, which can serve as either an acid or a base. In the present context, it functions as a base. Water molecules are polar, meaning that the negative charges tend to congregate on one end of the molecule with the oxygen atom, while the positive charges remain on the other end with the hydrogen atoms. The Brønsted-Lowry model emphasizes the role played by water, which pulls the proton from the acid, resulting in the creation of H3O+, known as the hydronium ion. The hydronium ion produced here is an example of a conjugate acid, an acid formed when a base accepts a proton. At the same time, the acid has lost its proton, becoming A–, a conjugate base—that is, the base formed when an acid releases a proton. These two products of the reaction are called a conjugate acid-base pair, a term that refers to two substances related to one another by the donating of a proton. Brønsted and Lowry’s definition represents an improvement over that of Arrhenius, because it includes all Arrhenius acids and bases, as well as other chemical species not encompassed in Arrhenius theory. An example, mentioned earlier, is ammonia. Though it does not produce OH– ions, ammonia does accept a proton from a water molecule, and the reaction between these two (with water this time serving the function of acid) produces the conjugate acid-base pair of NH4+ (an ammonium ion) and OH-. Note that the latter, the hydroxide ion, was not produced by ammonia, but is the conjugate base that resulted when the water molecule lost its H+ atom or proton.

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The Lewis Definition Despite the progress offered to chemists by the Brønsted-Lowry model, it was still limited to describing compounds that contain hydrogen. As American chemist Gilbert N. Lewis (1875-1946) recognized, this did not encompass the full range of acids and bases; what was needed, instead, was a definition that did not involve the presence of a hydrogen atom.

Acids and Bases

Lewis is particularly noted for his work in the realm of chemical bonding. The bonding of atoms is the result of activity on the part of the valence electrons, or the electrons at the “outside” of the atom. Electrons are arranged in different ways, depending on the type of bonding, but they always bond in pairs. According to the Lewis acid-base theory, an acid is the reactant that accepts an electron pair from another reactant in a chemical reaction, while a base is the reactant that donates an electron pair to another reactant. As with the Brønsted-Lowry definition, the Lewis definition is reaction-dependant, and does not define a compound as an acid or base in its own right. Instead, the manner in which the compound reacts with another serves to identify it as an acid or base. AN IMPROVEMENT OVER ITS P R E D E C E S S O R S . The beauty of the

Lewis definition lies in the fact that it encompasses all the situations covered by the others— and more. Just as Brønsted-Lowry did not disprove Arrhenius, but rather offered a definition that covered more substances, Lewis expanded the range of substances beyond those covered by Brønsted-Lowry. In particular, Lewis theory can be used to differentiate the acid and base in bond-producing chemical reactions where ions are not produced, and in which there is no proton donor or acceptor. Thus it represents an improvement over Arrhenius and BrønstedLowry respectively. An example is the reaction of boron trifluoride (BF3) with ammonia (NH3), both in the gas phases, to produce boron trifluoride ammonia complex (F3BNH3). In this reaction, boron trifluoride accepts an electron pair and is therefore a Lewis acid, while ammonia donates the electron pair and is thus a Lewis base. Though hydrogen is involved in this particular reaction, Lewis theory also addresses reactions involving no hydrogen.

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REAL-LIFE A P P L I C AT I O N S pH and Acid-Base Indicators Though chemists apply the sophisticated structural definitions for acids and bases that we have discussed, there are also more “hands-on” methods for identifying a particular substance (including complex mixtures) as an acid or base. Many of these make use of the pH scale, developed by Danish chemist Søren Sørensen (18681939) in 1909.

At the alkaline end of the scale is borax, with a pH of 9, while household ammonia has a pH value of 11, or 10-11 mol/l. Sodium hydroxide, or lye, an extremely alkaline chemical with a pH of 14, has a value equal to 10-14 moles of hydronium per liter of solution.

The term pH stands for “potential of hydrogen,” and the pH scale is a means of determining the acidity or alkalinity of a substance. (Though, as noted, the term “alkali” has been replaced by “base,” alkalinity is still used as an adjectival term to indicate the degree to which a substance displays the properties of a base.) There are theoretically no limits to the range of the pH scale, but figures for acidity and alkalinity are usually given with numerical values between 0 and 14.

urements are made with electronic pH meters, which can provide figures accurate to 0.001 pH. However, simpler materials are also used. Best known among these is litmus paper (made from an extract of two lichen species), which turns blue in the presence of bases and red in the presence of acids. The term “litmus test” has become part of everyday language, referring to a makeor-break issue—for example, “views on abortion rights became a litmus test for Supreme Court nominees.”

T H E M E A N I N G O F p H VA LU E S .

L I T M U S PA P E R A N D O T H E R I N D I CAT O R S . The most precise pH meas-

A rating of 0 on the pH scale indicates a substance that is virtually pure acid, while a 14 rating represents a nearly pure base. A rating of 7 indicates a neutral substance. The pH scale is logarithmic, or exponential, meaning that the numbers represent exponents, and thus an increased value of 1 represents not a simple arithmetic addition of 1, but an increase of 1 power. This, however, needs a little further explanation.

Litmus is just one of many materials used for making pH paper, but in each case, the change of color is the result of the neutralization of the substance on the paper. For instance, paper coated with phenolphthalein changes from colorless to pink in a pH range from 8.2 to 10, so it is useful for testing materials believed to be moderately alkaline. Extracts from various fruits and vegetables, including red cabbages, red onions, and others, are also applied as indicators.

The pH scale is actually based on negative logarithms for the values of H3O+ (the hydronium ion) or H+ (protons) in a given substance. The formula is thus pH = -log[H3O+] or -log[H+], and the presence of hydronium ions or protons is measured according to their concentration of moles per liter of solution.

Some Common Acids and Bases

p H VA LU E S O F VA R I O U S S U B S TA N C E S . The pH of a virtually pure acid,

such as the sulfuric acid in car batteries, is 0, and this represents 1 mole (mol) of hydronium per liter (l) of solution. Lemon juice has a pH of 2, equal to 10-2 mol/l. Note that the pH value of 2 translates to an exponent of -2, which, in this case, results in a figure of 0.01 mol/l. Distilled water, a neutral substance with a pH of 7, has a hydronium equivalent of 10-7 mol/l. It is interesting to observe that most of the

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fluids in the human body have pH values in the neutral range blood (venous, 7.35; arterial, 7.45); urine (6.0—note the higher presence of acid); and saliva (6.0 to 7.4).

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The tables below list a few well-known acids and bases, along with their formulas and a few applications Common Acids • Acetic acid (CH3COOH): vinegar, acetate • Acetylsalicylic acid (HOOCC6H4OOCCH3): aspirin • Ascorbic acid (H2C6H6O6): vitamin C • Carbonic acid (H2CO3): soft drinks, seltzer water • Citric acid (C6H8O7): citrus fruits, artificial flavorings • Hydrochloric acid (HCl): stomach acid • Nitric acid (HNO3): fertilizer, explosives • Sulfuric acid (H2SO4): car batteries

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Common Bases • Aluminum hydroxide (Al[OH]3): antacids, deodorants • Ammonium hydroxide (NH4OH): glass cleaner • Calcium hydroxide (Ca[OH]2): caustic lime, mortar, plaster • Magnesium hydroxide (Mg[OH]2): laxatives, antacids • Sodium bicarbonate/sodium hydrogen carbonate (NaHCO3): baking soda • Sodium carbonate (Na2CO3): dish detergent • Sodium hydroxide (NaOH): lye, oven and drain cleaner • Sodium hypochlorite (NaClO): bleach Of course these represent only a few of the many acids and bases that exist. Selected substances listed above are discussed briefly below.

Acids AC I D S I N T H E H U M A N B O DY A N D F O O D S . As its name suggests, citric

acid is found in citrus fruits—particularly lemons, limes, and grapefruits. It is also used as a flavoring agent, preservative, and cleaning agent. Produced commercially from the fermentation of sugar by several species of mold, citric acid creates a taste that is both tart and sweet. The tartness, of course, is a function of its acidity, or a manifestation of the fact that it produces hydrogen ions. The sweetness is a more complex biochemical issue relating to the ways that citric acid molecules fit into the tongue’s “sweet” receptors. Citric acid plays a role in one famous stomach remedy, or antacid. This in itself is interesting, since antacids are more generally associated with alkaline substances, used for their ability to neutralize stomach acid. The fizz in Alka-Seltzer, however, comes from the reaction of citric acids (which also provide a more pleasant taste) with sodium bicarbonate or baking soda, a base. This reaction produces carbon dioxide gas. As a preservative, citric acid prevents metal ions from reacting with, and thus hastening the degradation of, fats in foods. It is also used in the production of hair rinses and low-pH shampoos and toothpastes.

acids combine to make up proteins, one of the principal components in human muscles, skin, and hair. Carboxylic acids are also applied industrially, particularly in the use of fatty acids for making soaps, detergents, and shampoos.

Acids and Bases

S U L F U R I C AC I D . There are plenty of acids found in the human body, including hydrochloric acid or stomach acid—which, in large quantities, causes indigestion, and the need for neutralization with a base. Nature also produces acids that are toxic to humans, such as sulfuric acid.

Though direct exposure to sulfuric acid is extremely dangerous, the substance has numerous applications. Not only is it used in car batteries, but sulfuric acid is also a significant component in the production of fertilizers. On the other hand, sulfuric acid is damaging to the environment when it appears in the form of acid rain. Among the impurities in coal is sulfur, and this results in the production of sulfur dioxide and sulfur trioxide when the coal is burned. Sulfur trioxide reacts with water in the air, creating sulfuric acid and thus acid rain, which can endanger plant and animal life, as well as corrode metals and building materials.

Bases The alkali metal and alkaline earth metal families of elements are, as their name suggests, bases. A number of substances created by the reaction of these metals with nonmetallic elements are taken internally for the purpose of settling gastric trouble or clearing intestinal blockage. For instance, there is magnesium sulfate, better known as Epsom salts, which provide a powerful laxative also used for ridding the body of poisons. Aluminum hydroxide is an interesting base, because it has a wide number of applications, including its use in antacids. As such, it reacts with and neutralizes stomach acid, and for that reason is found in commercial antacids such as Di-Gel™, Gelusil™, and Maalox™. Aluminum hydroxide is also used in water purification, in dyeing garments, and in the production of certain kinds of glass. A close relative, aluminum hydroxychloride or Al2(OH)5Cl, appears in many commercial antiperspirants, and helps to close pores, thus stopping the flow of perspiration.

The carboxylic acid family of hydrocarbon derivatives includes a wide array of substances— not only citric acids, but amino acids. Amino

Baking soda, known by chemists both as sodium bicar-

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Acids and Bases

KEY TERMS A substance that, in its edible

aqueous solution: an acid produces hydro-

form, is sour to the taste, and in non-edible

gen ions (H+), and a base hydroxide ions

forms, is often capable of dissolving met-

(OH–).

ACID:

als. Acids and bases react to form salts and water. These are all phenomenological definitions, however, in contrast to the three structural definitions of acids and bases— the Arrhenius, Br Ønsted-Lowry, and Lewis acid-base theories.

A substance that, in its edible

BASE:

form, is bitter to the taste. Bases tend to be slippery to the touch, and in reaction with acids they produce salts and water. Bases and acids are most properly defined, however, not in these phenomenological terms,

A term referring to the soluble

but by the three structural definitions of

hydroxides of the alkali and alkaline earth

acids and bases—the Arrhenius, Br Ønsted-

metals. Once “alkali” was used for the class

Lowry, and Lewis acid-base theories.

ALKALI:

of substances that react with acids to form salts; today, however, the more general

BRØNSTED-LOWRY ACID-BASE THE-

term base is preferred.

ORY:

ALKALINITY:

definitions of acids and bases. Formulated

An adjectival term used

to identify the degree to which a substance displays the properties of a base. ANION:

The second of three structural

The negatively charged ion

that results when an atom gains one or

by English chemist Thomas Lowry (18741936) and Danish chemist J. N. Br Ønsted (1879-1947),

defines an acid as a proton

AQUEOUS SOLUTION:

(H+)

theory

donor, and

a base as a proton acceptor.

more electrons. “Anion” is pronounced CATION:

“AN-ie-un”.

Br Ønsted-Lowry

The positively charged ion

that results when an atom loses one or A substance

in which water constitutes the solvent. A

more electrons. “Cation” is pronounced “KAT-ie-un”.

large number of chemical reactions take place in an aqueous solution.

CHEMICAL SPECIES:

A generic term

used for any substance studied in chemARRHENIUS ACID-BASE THEORY:

The first of three structural definitions of acids and bases. Formulated by Swedish chemist Svante Arrhenius (1859-1927), the

316

istry—whether it be an element, compound, mixture, atom, molecule, ion, and so forth. An acid formed

Arrhenius theory defines acids and bases

CONJUGATE ACID:

according to the ions they produce in an

when a base accepts a proton (H+).

bonate and sodium hydrogen carbonate, is another example of a base with multiple purposes. As noted earlier, it is used in Alka-Seltzer™, with the addition of citric acid to improve the flavor; in fact, baking soda alone can perform the

function of an antacid, but the taste is rather unpleasant. Baking soda is also used in fighting fires, because at high temperatures it turns into carbon dioxide, which smothers flames by obstructing

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Acids and Bases

KEY TERMS CONJUGATE

ACID-BASE

PAIR:

CONTINUED

A logarithmic scale for

PH SCALE:

The acid and base produced when an acid

determining the acidity or alkalinity of a

donates a single proton to a base. In the

substance, from 0 (virtually pure acid) to 7

reaction that produces this pair, the acid

(neutral) to 14 (virtually pure base).

and base switch identities. By donating a

PHENOMENOLOGICAL:

A

term

proton, the acid becomes a conjugate base,

describing scientific definitions based

and by receiving the proton, the base

purely on experimental phenomena. These

becomes a conjugate acid.

convey only part of the picture, however—

CONJUGATE BASE:

A base formed

primarily, the part a chemist can perceive either through measurement or through

when an acid releases a proton.

the senses, such as sight. A structural defiION:

An atom or atoms that has lost or

gained one or more electrons, and thus has a net electric charge. There are two types of ions: anions and cations. IONIC BONDING:

nition is therefore usually preferable to a phenomenological one. A substance that interacts

REACTANT:

with another substance in a chemical reac-

A form of chemical

tion, resulting in the creation of a product. Ionic compounds formed by

bonding that results from attractions

SALTS:

between ions with opposite electric

the reaction between an acid and a base. In

charges.

this reaction, one or more of the hydrogen ions of an acid is replaced with another

IONIC COMPOUND:

A compound in

which ions are present. Ionic compounds

positive ion. In addition to producing salts, acid-base reactions produce water.

contain at least one metal and nonmetal

A homogeneous mixture

SOLUTION:

joined by an ionic bond.

in which one or more substances (the The

solute) is dissolved in one or more other

third of three structural definitions of

substances (the solvent)—for example,

acids and bases. Formulated by American

sugar dissolved in water.

chemist Gilbert N. Lewis (1875-1946),

SOLVENT:

Lewis theory defines an acid as the reactant

another, called a solute, in a solution.

that accepts an electron pair from another

STRUCTURAL:

reactant in a chemical reaction, and a base

entific definitions based on aspects of

as the reactant that donates an electron

molecular structure and behavior rather

pair to another reactant.

than purely phenomenological data.

LEWIS ACID-BASE THEORY:

A substance that dissolves A term describing sci-

the flow of oxygen to the fire. Of course, baking soda is also used in baking, when it is combined with a weak acid to make baking powder. The reaction of the acid and the baking soda produces carbon dioxide, which causes dough and

batters to rise. In a refrigerator or cabinet, baking soda can absorb unpleasant odors, and additionally, it can be applied as a cleaning product.

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HYDROXIDE

( LY E ) .

Another base used for cleaning is sodium

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hydroxide, known commonly as lye or caustic soda. Unlike baking soda, however, it is not to be taken internally, because it is highly damaging to human tissue—particularly the eyes. Lye appears in drain cleaners, such as Drano™, and oven cleaners, such as Easy-Off™, which make use of its ability to convert fats to water-soluble soap. In the process of doing so, however, relatively large amounts of lye may generate enough heat to boil the water in a drain, causing the water to shoot upward. For this reason, it is not advisable to stand near a drain being treated with lye. In a closed oven, this is not a danger, of course; and after the cleaning process is complete, the converted fats (now in the form of soap) can be dissolved and wiped off with a sponge. WHERE TO LEARN MORE

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“Acids, Bases, and Salts.” Chemistry Coach (Web site). (June 7, 2001). “Acids, Bases, and Salts.” University of Akron, Department of Chemistry (Web site). (June 7, 2001). ChemLab. Danbury, CT: Grolier Educational, 1998. Ebbing, Darrell D.; R. A. D. Wentworth; and James P. Birk. Introductory Chemistry. Boston: Houghton Mifflin, 1995. Haines, Gail Kay. What Makes a Lemon Sour? Illustrated by Janet McCaffery. New York: Morrow, 1977. Oxlade, Chris. Acids and Bases. Chicago: Heinemann Library, 2001. Patten, J. M. Acids and Bases. Vero Beach, FL: Rourke Book Company, 1995. Walters, Derek. Chemistry. Illustrated by Denis Bishop and Jim Robins. New York: F. Watts, 1982.

“Acids and Bases Frequently Asked Questions.” General Chemistry Online (Web site). (June 7, 2001).

Zumdahl, Steven S. Introductory Chemistry A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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ACID-BASE REACTIONS

CONCEPT To an extent, acids and bases can be defined in terms of factors that are apparent to the senses: edible acids taste sour, for instance, while bases are bitter-tasting and slippery to the touch. The best way to understand these two types of substances, however, is in terms of their behavior in chemical reactions. Not only do the reactions of acids and bases result in the creation of salts and water, but acids and bases can be defined by the ways in which they participate in a reaction—for instance, by donating or accepting electron pairs. The reaction of acids and bases to form water and salts is called neutralization, and it has a wide range of applications, including the promotion of plant growth in soil and the treatment of heartburn in the human stomach. Neutralization also makes it possible to test substances for their pH level, a measure of the degree to which the substance is acidic or alkaline.

HOW IT WORKS

Acids are fairly easy to understand on the phenomenological level: the name comes from the Latin term acidus, or “sour,” and many sour substances from daily life—lemons, for instance, or vinegar—are indeed highly acidic. In fact, lemons and most citrus fruits contain citric acid (C6H8O7), while the acidic quality of vinegar comes from acetic acid (CH3COOH). In addition, acids produce characteristic colors in certain vegetable dyes, such as those used in making litmus paper. The word “base,” as it is used in this context, may be a bit more difficult to appreciate on a sensory level. It helps, perhaps, if the older term “alkali” is used, though even so, people tend to think of alkaline substances primarily in contrast to acids. “Alkali,” which serves to indicate the basic quality of both the alkali metal and alkaline earth metal families of elements, comes from the Arabic al-qili. The latter refers to the ashes of the seawort plant, which usually grows in marshy areas and, in the past, was often burned to produce soda ash for making soap.

Before studying the reactions of acids and bases, it is necessary to define exactly what each is. This is not as easy as it sounds, and the Acids and Bases essay discusses in detail a subject covered more briefly here: the arduous task chemists faced in developing a workable distinction. Let us start with the phenomenological differences between the two—that is, aspects relating to things that can readily be observed without referring to the molecule properties and behaviors of acids and bases.

The reason chemists of today use the word “base” instead of “alkali” is that the latter term has a narrower meaning: all alkalies are bases, but not all bases are alkalies. Originally referring only to the ashes of burned plants containing either sodium or potassium, alkali was eventually used to designate the soluble hydroxides of the alkali and alkaline earth metals. Among these are sodium hydroxide or lye; magnesium hydroxide (found in milk of magnesia); potassium hydroxide, which appears in soaps; and other compounds. Because these represent only a few of the substances that react with acids in the ways discussed in this essay, the term “base” is preferred.

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of his time: only in 1884 did his countryman Svante Arrhenius (1859-1927) introduce the concept of the ion. This, in turn, enabled Arrhenius to formulate the first structural distinction between acids and bases.

Acid-Base Reactions

The Arrhenius Acid-Base Theory Arrhenius acid-base theory defines the two substances with regard to their behavior in an aqueous solution: an acid is any compound that produces hydrogen ions (H+), and a base is one that produces hydroxide ions (OH–) when dissolved in water. This occurred, for instance, in the reaction discussed above: the hydrochloric acid produced a hydrogen ion, while the sodium hydroxide produced a hydroxide ion, and these two ions bonded to form water.

GILBERT N. LEWIS.

T H E F O R M AT I O N O F SA LT S . As chemistry evolved, and physical scientists became aware of the atomic and molecular substructures that make up the material world, they developed more fundamental distinctions between acids and bases. By the early twentieth century, chemists had applied structural distinctions between acids and bases—that is, definitions based on the molecular structures and behaviors of those substances.

An important intermediary step occurred as chemists came to the conclusion that reactions of acids and bases form salts and water. For instance, in an aqueous solution, hydrochloric acid or HCl(aq) reacts with the base sodium hydroxide, designated as NaOH(aq), to form sodium chloride, or common table salt (NaCl[aq]) and H2O. What happens is that the sodium (Na) ion (an atom with an electric charge) in sodium hydroxide switches places with the hydrogen ion in hydrochloric acid, resulting in the creation of NaCl and water. Ions themselves had yet to be defined in 1803, when the great Swedish chemist Jons Berzelius (1779-1848) added another piece to the foundation for a structural definition. Acids and bases, he suggested, have opposite electric charges. In this, he was about eight decades ahead

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Though it was a good start, Arrhenius’s theory was limited to reactions in aqueous solutions. In addition, it confined its definition of acids and bases only to those ionic compounds, such as hydrochloric acid or sodium hydroxide, that produced either hydrogen or hydroxide ions. But ammonia, or NH3, acts like a base in aqueous solutions, even though it does not produce the hydroxide ion. These shortcomings pointed to the need for a more comprehensive theory, which came with the formulation of the BrønstedLowry definition.

The Brønsted-Lowry AcidBase Theory Developed by English chemist Thomas Lowry (1874-1936) and Danish chemist J. N. Brønsted (1879-1947), the Brønsted-Lowry acid-base theory defines an acid as a proton (H+) donor, and a base as a proton acceptor, in a chemical reaction. Protons are represented by the symbol H+, a cation (positively charged ion) of hydrogen. Elemental hydrogen, called protium to distinguish it from its isotopes, has just one proton and one electron—no neutrons. Therefore, the hydrogen cation, which has to lose its sole electron to gain a positive charge, is essentially nothing but a proton. It is thus at once an atom, an ion, and a proton, but the ionization of hydrogen constitutes the only case in which this is possible. Thus when the term “proton donor” or “proton acceptor” is used, it does not mean that a proton is splitting off from an atom or joining

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another, as in a nuclear reaction. Rather, when an acid behaves as a proton donor, this means that the hydrogen proton/ion/atom is separating from an acidic compound; conversely, when a base acts as a proton acceptor, the positively charged hydrogen ion is bonding with the basic compound. REACTIONS IN LOWRY ACID-BASE

BRØNSTEDT H E O R Y. In

representing Brønsted-Lowry acids and bases, the symbols HA and A–, respectively, are used. These appear in the equation representing the most fundamental type of Brønsted-Lowry acidbase reaction: HA(aq) + H2O(l) 4H3O+(aq) + A–(aq). The symbols (aq), (l), and 4are explained in the Chemical Reactions essay. In plain English, this equation states that when an acid in an aqueous solution reacts with liquid water, the result is the creation of H3O+, known as the hydronium ion, along with a base. Both products of the reaction are dissolved in an aqueous solution. Because water molecules are polar, the negative charges tend to congregate on one end of the molecule with the oxygen atom, while the positive charges remain on the other end with the hydrogen atoms. The Brønsted-Lowry model emphasizes the role played by water, which pulls the proton from the acid, resulting in the creation of the hydronium ion. The hydronium ion, in this equation, is an example of a conjugate acid, an acid formed when a base accepts a proton. At the same time, the acid has lost its proton, becoming A–, a conjugate base—that is, the base formed when an acid releases a proton. These two products of the reaction are called a conjugate acid-base pair, a term that refers to two substances related to one another by the donating of a proton. Brønsted and Lowry’s definition includes all Arrhenius acids and bases, as well as other chemical species not encompassed in Arrhenius theory. As mentioned earlier, ammonia is a base, yet it does not produce OH– ions; however, it does accept a proton from a water molecule. Water can serve either as an acid or base; in this instance, it is an acid, and in reaction with ammonia, it produces the conjugate acid-base pair of NH4+ (an ammonium ion) and OH–. Ammonia did not produce the hydroxide ion here; rather, OH– is the conjugate base that resulted when the water molecule lost its H+ atom (i.e., a proton.)

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The Lewis Acid-Base Theory The Brønsted-Lowry model still had its limitations, in that it only described compounds containing hydrogen. American chemist Gilbert N. Lewis (1875-1946), however, developed a theory of acids and bases that makes no reference to the presence of hydrogen. Instead, it relates to something much more fundamental: the fact that chemical bonding always involves pairs of electrons.

Acid-Base Reactions

Lewis acid-base theory defines an acid as the reactant that accepts an electron pair from another reactant in a chemical reaction, while a base is the reactant that donates an electron pair to another reactant. Note that, as with the Brønsted-Lowry definition, the Lewis definition is reaction-dependant. Instead of defining a compound as an acid or base in its own right, it identifies these in terms of how the compound reacts with another. The Lewis definition encompasses all the situations covered by the others, as well as many other reactions not described in the theories of either Arrhenius or Brønsted-Lowry. In particular, Lewis theory can be used to differentiate the acid and base in chemical reactions where ions are not produced, something that takes it far beyond the scope of Arrhenius theory. Also, Lewis theory addresses situations in which there is no proton donor or acceptor, thus offering an improvement over Brønsted-Lowry. When boron trifluoride (BF3) and ammonia (NH3), both in the gas phases, react to produce boron trifluoride ammonia complex (F3BNH3), boron trifluoride accepts an electron pair. Therefore, it is a Lewis acid, while ammonia—which donates the electron pair—can be defined as a Lewis base. This particular reaction involves hydrogen, but since the operative factor in Lewis theory relates to electron pairs and not hydrogen, the theory can be used to address reactions in which that element is not present.

REAL-LIFE A P P L I C AT I O N S Dissociation Dissociation is the separation of a molecule into ions, and it is a key factor for evaluating the “strength” of acids and bases. The more a substance is prone to dissociation, the better it can

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conduct an electric current, because the separation of charges provides a “pathway” for the current’s flow. A substance that dissociates completely, or almost completely, is called a strong electrolyte, whereas one that dissociates only slightly (or not at all) is designated as a weak electrolyte. The terms “weak” and “strong” are also applied to acids and bases. For instance, vinegar is a weak acid, because it dissociates only slightly, and therefore conducts little electric current. By contrast, hydrochloric acid (HCl) is a strong acid, because it dissociates almost completely into positively charged hydrogen ions and negatively charged chlorine ones. Represented symbolically, this is: HCl 4H+ + Cl–. A R E A C T I O N I N V O LV I N G A S T R O N G AC I D . It may seem a bit back-

ward that a strong acid or base is one that “falls apart,” while the weak one stays together. To understand the difference better, let us return to the reaction described earlier, in which an acid in aqueous solution reacts with water to produce a base in aqueous solution, along with hydronium: HA(aq) + H2O(l) 4H3O+(aq) + A–(aq). Instead of using the generic symbols HA and A–, however, let us substitute hydrochloric acid (HCl) and chloride (Cl–) respectively. The reaction HCl(aq) + H2O(l) 4H3O+(aq) is a reversible one, and for that reason, + the symbol for chemical equilibrium (!) can be inserted in place of the arrow pointing to the right. In other words, the substances on the right can just as easily react, producing the substances on the left. In this reverse reaction, the reactants of the forward reaction would become products, and the products of the forward reaction serve as the reactants. Cl–(aq)

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hydrogen ions join the water (a stronger base) to form hydronium. The chloride, incidentally, is the conjugate base of the hydrochloric acid, and this illustrates another principal regarding the “strength” of electrolytes: a strong acid produces a relatively weak conjugate base. Likewise, a strong base produces a relatively weak conjugate acid. THE STRONG ACIDS AND B AS E S . There are only a few strong acids and

bases, which are listed below: Strong Acids • • • • • •

Hydrobromic acid (HBr) Hydrochloric acid (HCl) Hydroiodic acid (HI) Nitric acid (HNO3) Perchloric acid (HClO4) Sulfuric acid (H2SO4) Strong Bases

Barium hydroxide (Ba[OH]2) Calcium hydroxide (Ca[OH]2) Lithium hydroxide (LiOH) Potassium hydroxide (KOH) Sodium hydroxide (NaOH) Strontium hydroxide (Sr[OH]2) Virtually all others are weak acids or bases, meaning that only a small percentage of molecules in these substances ionize by dissociation. The concentrations of the chemical species involved in the dissociation of weak acids and bases are mathematically governed by the equilibrium constant K.. • • • • • •

Neutralization

However, the reaction described here is not perfectly reversible, and in fact the most proper chemical symbolism would show a longer arrow pointing to the right, with a shorter arrow pointing to the left. Due to the presence of a strong electrolyte, there is more forward “thrust” to this reaction.

Neutralization is the process whereby an acid and base react with one another to form a salt and water. The simplest example of this occurs in the reaction discussed earlier, in which hydrochloric acid or HCl(aq) reacts with the base sodium hydroxide, designated as NaOH(aq), in an aqueous solution. The result is sodium chloride, or common table salt (NaCl[aq]) and H2O. This equation is written thus: HCl(aq) + NaOH(aq) 4NaCl(aq) + H2O.

Because it is a strong acid, the hydrogen chloride in solution is not a set of molecules, but a collection of H+ and Cl– ions. In the reaction, the weak Cl– ions to the right side of the equilibrium symbol exert very little attraction for the H+ ions. Instead of bonding with the chloride, these

The human stomach produces hydrochloric acid, commonly known as “stomach acid.” It is generated in the digestion process, but when a person eats something requiring the stomach to work overtime in digesting it—say, a pizza—the stomach may generate excess hydrochloric acid,

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and the result is “heartburn.” When this happens, people often take antacids, which contain a base such as aluminum hydroxide (Al[OH]3) or magnesium hydroxide (Mg[OH]2). When a person takes an antacid, the reaction leads to the creation of a salt, but not the salt with which most people are familiar—NaCl. As shown above, that particular salt is the product of a reaction between hydrochloric acid and sodium hydroxide, but a person who ingested sodium hydroxide (a substance used to unclog drains and clean ovens) would have much worse heartburn than before! In any case, the antacid reacts with the stomach acid to produce a salt, as well as water, and thus the acid is neutralized. When land formerly used for mining is reclaimed, the acidic water in the area must be neutralized, and the use of calcium oxide (CaO) as a base is one means of doing so. Acidic soil, too, can be neutralized by the introduction of calcium carbonate (CaCO3) or limestone, along with magnesium carbonate (MgCO3). If soil is too basic, as for instance in areas where there has been too little precipitation, acid-like substances such as calcium sulfate or gypsum (SaSO4) can be used. In either case, neutralization promotes plant growth. T I T RAT I O N A N D p H . One of the most important applications of neutralization is in titration, the use of a chemical reaction to determine the amount of a chemical substance in a sample of unknown purity. In a typical form of neutralization titration, a measured amount of an acid is added to a solution containing an unknown amount of a base. Once enough of the acid has been added to neutralize the base, it is possible to determine how much base exists in the solution.

Titration can also be used to measure pH (“power of hydrogen”) level by using an acidbase indicator. The pH scale assigns values ranging from 0 (a virtually pure acid) to 14 (a virtually pure base), with 7 indicating a neutral substance. An acid-base indicator such as litmus paper changes color when it neutralizes the solution.

from red to yellow across a pH range of 4.4 to 6.2, so it is most useful for testing a substance suspected of being moderately acidic.

Acid-Base Reactions

BUFFERED SOLUTIONS. A buffered solution is one that resists a change in pH even when a strong acid or base is added to it. This buffering results from the presence of a weak acid and a strong conjugate base, and it can be very important to organisms whose cells can endure changes only within a limited range of pH values. Human blood, for instance, contains buffering systems, because it needs to be at pH levels between 7.35 and 7.45.

The carbonic acid-bicarbonate buffer system is used to control the pH of blood. The most important chemical equilibria (that is, reactions involving chemical equilibrium) for this system are: H+ + HCO3– ! H2CO3 ! H20 + CO2. In other words, the hydrogen ion (H+) reacts with the hydrogen carbonate ion (HCO3– ) to produce carbonic acid (H2CO3). The latter is in equilibrium with the first set of reactants, as well as with water and carbon dioxide in the forward reaction. The controls the pH level by changing the concentration of carbon dioxide by exhalation. In accordance with Le Châtelier’s principle, this shifts the equilibrium to the right, consuming H+ ions. In normal blood plasma, the concentration of HCO3– is about 20 times as great as that of H2CO3, and this large concentration of hydrogen carbonate gives the buffer a high capacity to neutralize additional acid. The buffer has a much lower capacity to neutralize bases because of the much smaller concentration of carbonic acid.

Water: Both Acid and Base Water is an amphoteric substance; in other words, it can serve either as an acid or a base. When water experiences ionization, one water molecule serves as a Brønsted-Lowry acid, donating a proton to another water molecule— the Brønsted-Lowry base. This results in the production of a hydroxide ion and a hydronium ion: H2O(l) + H2O(l) ! H3O+(aq) + OH–(aq).

The transition interval (the pH at which the color of an indicator changes) is different for different types of indicators, and thus various indicators are used to measure substances in specific pH ranges. For instance, methyl red changes

This equilibrium equation is actually one in which the tendency toward the reverse reaction is much greater; therefore the equilibrium symbol, if rendered in its most proper form, would show a much shorter arrow pointing toward the right. In water purified by distillation, the concentra-

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Acid-Base Reactions

KEY TERMS A substance that in its edible

acids they produce salts and water. Bases

form is sour to the taste, and in non-edible

and acids are most properly defined, how-

forms is often capable of dissolving metals.

ever, not in these phenomenological terms,

Acids and bases react to form salts and

but by the three structural definitions of

water. These are all phenomenological def-

acids and bases—the Arrhenius, Brønsted-

initions, however, in contrast to the three

Lowry, and Lewis acid-base theories.

ACID:

structural definitions of acids and bases— the Arrhenius, Brønsted-Lowry, and Lewis acid-base theories. ALKALI:

BASIC:

In the context of acids and

bases, the word is the counterpart to “acidic,” identifying the base-like quality of

A term referring to the soluble

a substance.

hydroxides of the alkali and alkaline earth metals. Once “alkali” was used for the class

BRØNSTED-LOWRY ACID-BASE THE-

of substances that react with acids to form

ORY:

salts; today, however, the more general term base is preferred.

The second of three structural

definitions of acids and bases. Formulated by English chemist Thomas Lowry (18741936) and Danish chemist J. N. Brønsted

ALKALINITY:

An adjectival term used

(1879-1947),

defines an acid as a proton

displays the properties of a base.

a base as a proton acceptor.

AMPHOTERIC:

A term describing a

substance that can serve either as an acid or a base. Water is the most significant amphoteric substance. AQUEOUS SOLUTION:

CHEMICAL SPECIES:

A substance

large number of chemical reactions take place in an aqueous solution.

theory

donor, and

A generic term

used for any substance studied in chemistry—whether it be an element, comso forth. CONJUGATE ACID:

The first of three structural definitions of acids and bases. Formulated by Swedish chemist Svante Arrhenius (1859-1927), the Arrhenius theory defines acids and bases according to the ions they produce in an aqueous solution an acid produces hydrogen ions (H+), and a base hydroxide ions (OH–).

ACID-BASE

donates a single proton to a base. In the reaction that produces this pair, the acid and base switch identities. By donating a proton, the acid becomes a conjugate base, and by receiving the proton, the base becomes a conjugate acid. A base formed

when an acid releases a proton.

form is bitter to the taste. Bases tend to be

DISSOCIATION:

slippery to the touch, and in reaction with

molecules into ions.

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PAIR:

The acid and base produced when an acid

CONJUGATE BASE:

A substance that in its edible

An acid formed

when a base accepts a proton (H+). CONJUGATE

ARRHENIUS ACID-BASE THEORY:

BASE:

(H+)

pound, mixture, atom, molecule, ion, and

in which water constitutes the solvent. A

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Brønsted-Lowry

to identify the degree to which a substance

The separation of

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Acid-Base Reactions

KEY TERMS ION:

An atom or atoms that has lost or

CONTINUED

A substance that interacts

REACTANT:

gained one or more electrons, and thus has

with another substance in a chemical reac-

a net electric charge. There are two types of

tion, resulting in the creation of a product.

ions: anions and cations.

SALTS:

Ionic compounds formed by

A form of chemical

the reaction between an acid and a base. In

bonding that results from attractions

this reaction, one or more of the hydrogen

between ions with opposite electric

ions of an acid is replaced with another

charges.

positive ion. In addition to producing salts,

IONIC BONDING:

IONIC COMPOUND:

A compound in

acid-base reactions produce water. A homogeneous mixture

which ions are present. Ionic compounds

SOLUTION:

contain at least one metal and nonmetal

in which one or more substances (the

joined by an ionic bond.

solute) is dissolved in another substance (the solvent)—for example, sugar dis-

LEWIS ACID-BASE THEORY:

The

solved in water.

third of three structural definitions of acids and bases. Formulated by American chemist Gilbert N. Lewis (1875-1946),

SOLVENT:

A substance that dissolves

another, called a solute, in a solution.

Lewis theory defines an acid as the reactant

STRONG ELECTROLYTE:

that accepts an electron pair from another

highly prone to dissociation. The terms

reactant in a chemical reaction, and a base

“strong acid” or “strong base” refers to those

as the reactant that donates an electron

acids or bases which readily dissociate.

pair to another reactant.

A substance

A term describing sci-

STRUCTURAL:

process

entific definitions based on aspects of

whereby an acid and base react with one

molecular structure and behavior rather

another to form a salt and water.

than purely phenomenological data.

NEUTRALIZATION:

PH SCALE:

The

A logarithmic scale for

TITRATION:

The use of a chemical

determining the acidity or alkalinity of a

reaction to determine the amount of a

substance, from 0 (virtually pure acid) to 7

chemical substance in a sample of

(neutral) to 14 (virtually pure base).

unknown purity. Testing pH levels is an example of titration.

PHENOMENOLOGICAL:

A

term

describing scientific definitions based purely on experimental phenomena. These only convey part of the picture, however—

TRANSITION INTERVAL:

The pH

level at which the color of an acid-base indicator changes.

primarily, the part a chemist can perceive

WEAK ELECTROLYTE:

either through measurement or through

that experiences little or no dissociation.

the senses, such as sight. A structural defi-

The terms “weak acid” or “weak base” re-

nition is therefore usually preferable to a

fer to those acids or bases not prone to

phenomenological one.

dissociation.

S C I E N C E O F E V E RY DAY T H I N G S

A substance

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tions of hydronium (H3O+) and hydroxide (OH–) ions are equal. When multiplied by one another, these yield the constant figure 1.0 • 10–14, which is the equilibrium constant for water. In fact, this constant—denoted as Kw—is called the ion-product constant for water.

WHERE TO LEARN MORE “Acids and Bases.” Vision Learning (Web site). (June 7, 2001). “Acids, Bases, and Chemical Reactions” Open Access College (Web site). (June 7, 2001). “Acids, Bases, pH.” About.com (Web site).
Because the product of these two concentrations is always the same, this means that if one of them goes up, the other one must go down in order to yield the same constant. This explains the fact, noted earlier, that water can serve either as an acid or base—or, if the concentrations of hydronium and hydroxide ions are equal—as a neutral substance. In situations where the concentration of hydronium is higher, and the hydroxide concentration automatically decreases, water serves as an acid. Conversely, when the hydroxide concentration is high, the hydronium concentration decreases correspondingly, and the water is a base.

Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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“Acids, Bases, and Salts” (Web site). (June 7, 2001). “Junior Part: Acids, Bases, and Salts” (Web site). (June 7, 2001). Knapp, Brian J. Acids, Bases, and Salts. Danbury, CT: Grolier Educational, 1998. Moje, Steven W. Cool Chemistry: Great Experiments with Simple Stuff. New York: Sterling Publishing Company, 1999. Walters, Derek. Chemistry. Illustrated by Denis Bishop and Jim Robins. New York: F. Watts, 1982.

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S C I E N C E O F E V E RY DAY T H I N G S r e a l - l i f e CHEMISTRY

S O LU T I O N S A N D MIXTURES MIXTURES SOLUTIONS OSMOSIS D I S T I L L A T I O N A N D F I LT R A T I O N

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Mixtures

CONCEPT Elements and compounds are pure substances, but much of the material around us—including air, wood, soil, and even (in most cases) water— appears in the form of a mixture. Unlike a pure substance, a mixture has variable composition: in other words, it cannot be reduced to a single type of atom or molecule. Mixtures can be either homogeneous—that is, uniform in appearance, and having only one phase—or heterogeneous, meaning that the mixture is separated into various regions with differing properties. Most homogeneous mixtures are also known as solutions, and examples of these include air, coffee, and even metal alloys. As for heterogeneous mixtures, these occur, for instance, when sand or oil are placed in water. Oil can, however, be dispersed in water as an emulsion, with the aid of a surfactant or emulsifier, and the result is an almost homogeneous mixture. Another interesting example of a heterogeneous mixture occurs when colloids are suspended in fluid, whether that fluid be air or water.

HOW IT WORKS Mixtures and Pure Substances A mixture is a substance with a variable composition, meaning that it is composed of molecules or atoms of differing types. These may have intermolecular bonds, or bonds between molecules, but such bonds are not nearly as strong as those formed when elements bond to form a compound.

S C I E N C E O F E V E RY DAY T H I N G S

MIXTURES

Examples of mixtures include milk, coffee, tea, and soft drinks—indeed, they include virtually any drink except for pure water, which is a rarity, because water is highly soluble and tends to contain minerals and other impurities. Air, too, is a mixture, because it contains oxygen, nitrogen, noble gases, carbon dioxide, and water vapor. Likewise metal alloys such as bronze, brass, and steel are mixtures. S T R U C T U RA L A N D P H E N O M E N O LO G I CA L D I S T I N C T I O N S . Be-

fore chemists accepted the atomic theory of matter, it was often difficult to distinguish a mixture, such as air or steel, from pure substances, which have the same composition throughout. Pure substances are either elements such as oxygen or iron, or compounds—for example, carbon dioxide or iron oxide. An element is made up of only one type of atom, while a compound can be reduced to one type of bonded chemical species—usually either a molecule or a formation of ions. A distinction between mixtures and pure substances based on atomic and molecular structures is called, fittingly enough, a structural definition. Prior to the development of atomic theory by English chemist John Dalton (1766-1844), and the subsequent identification of molecules by Italian physicist Amedeo Avogadro (17761856), chemists defined compounds and mixtures on the basis of phenomenological data garnered through observation and measurement. This could and did lead to incorrect suppositions. P R O U S T A N D T H E D E B AT E O V E R C O N S TA N T C O M P O S I T I O N .

Around the time of Dalton and Avogadro, French

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Mixtures

THE

WONDROUS INVENTION KNOWN AS SOAP IS A MIXTURE WITH SURFACTANT QUALITIES. (Chad Weckler/Corbis. Reproduced

by permission.)

chemist Antoine Lavoisier (1743-1794) correctly identified an element as a substance that could not be broken down into a simpler substance. Later, his countryman Joseph-Louis Proust (1754-1826) clarified the definition of a compound, stating it in much the same terms that we have used: the composition of a compound is the same throughout. But with the very rudimentary knowledge of atoms and molecules that chemists of the early nineteenth century possessed, the distinctions between compounds and mixtures remained elusive. Proust’s contemporary and intellectual rival Claude Louis Berthollet (1748-1822) observed that when metals such as iron are heated, they form oxides in which the percentage of oxygen increases with temperature. If constant composition is a fact, he challenged Proust, then how is this possible? Proust maintained that the proportions of elements in a compound are always the same, a hypothesis Berthollet countered by observing that metals can form variable alloys. Bronze, for instance, is an alloy of tin and copper at a ratio of about 25:75, but if the ratio were changed to, say, 30:70, it would still be called bronze.

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not exist whereby Berthollet’s assertions could be successfully proven wrong. Indeed, Berthollet, a student of Lavoisier who contributed to progress in the study of acids and reactants, was no antiscientific villain: he was simply responding to the evidence as he saw it. Only with the discovery of subatomic particles in the late nineteenth and early twentieth centuries, discoveries that changed the entire character of chemistry as a discipline, did it truly become possible to answer Berthollet’s challenges. Chemists now understand that the oxide formed on the surface of a metal is a compound separate from the metal itself, and that alloys (discussed later in this essay) are not compounds at all: they are mixtures.

Mixtures and Compounds

At the time, the equipment—both in terms of knowledge and tools for analysis—simply did

With our modern knowledge of atomic and molecular properties, we can more fully distinguish between a compound and a mixture. First, a mixture can exist in virtually any proportion among its constituent parts, whereas a compound has a definite and constant composition. Coffee, whether weak or strong, is still coffee, but if a second oxygen atom chemically bonds to the oxygen in carbon monoxide (CO), the resulting

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Mixtures

BRONZE—AN

ALLOY OF TIN AND COPPER—WAS ONE OF THE MOST IMPORTANT INVENTIONS IN HUMAN HISTORY.

STONE CARVING IS FROM THE PERIOD KNOWN AS THE

THIS

BRONZE AGE. (Kevin Schafer/Corbis. Reproduced by permission.)

compound is not simply “strong carbon monoxide” or even “oxygenated carbon monoxide”: it is carbon dioxide (CO2), an entirely different substance. The difference is enormous: carbon dioxide is a part of the interactions between plant and animal life that sustains the natural environment, whereas carbon monoxide is a toxic gas produced, for instance, by automobiles. Second, the parts of a mixture retain most of their properties when they join together, but elements lose their elemental characteristics once they form a compound. Sugar is still sweet, regardless of the substance into which it is dissolved. In coffee or another drink, it may no longer be dry and granular, though it could be returned to that state by evaporating the coffee. In any case, dryness and granularity are physical properties, whereas sweetness is a chemical one. On the other hand, when elemental carbon bonds with hydrogen and oxygen to form sugar itself, the resulting compound is nothing like the elements from which it is made.

much higher temperatures. On the other hand, the formation of a compound involves a chemical reaction, a vastly more complex interaction than the one required to create a mixture. Carbon, a black solid found in coal, can be mixed with hydrogen and oxygen—both colorless and odorless gases—and the resulting mess would be something; but it would not be sugar. Only the chemical reaction between the three elements, in specific proportions and under specific conditions, results in their chemical bonding to form table sugar, known to chemists as sucrose.

Homogeneous and Heterogeneous Mixtures HOMOGENEOUS MIXTURES A N D S O LU T I O N S . As we have seen, the

Third, a mixture can usually be created simply by the physical act of stirring items together, and/or by applying heat. As discussed below, relatively high temperatures are sometimes needed to dissolve sugar in tea, for instance; likewise, alloys are usually formed by heating metals to

composition of a mixture is variable. Coffee, for instance, can be weak or strong, and milk can be “whole” or low-fat. Beer can be light or dark in color, as well as “light” in terms of calorie content; furthermore, its alcohol content can be varied, as with all alcoholic drinks. Though their molecular composition is variable, each of the mixtures described here is the same throughout: in other words, there is no difference between one region and another in a glass of beer. Such a mixture is described as homogeneous.

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A homogeneous mixture is one that is the same throughout, but this is not the same as saying that a compound has a constant composition. Rather, when we say that a homogeneous mixture is the same in every region, we are speaking phenomenologically rather than structurally. From the standpoint of ordinary observation, a glass of milk is uniform, but as we shall see, milk is actually a type of emulsion, in which one substance is dispersed in another. There are no milk “molecules,” and structural analysis would reveal that milk does not have a constant molecular composition. Coffee is an example of a solution, a specific type of homogeneous mixture. Actually, most homogeneous mixtures can be considered solutions, but since a solution is properly defined as a homogeneous mixture in which one or more substances is dissolved in another substance, a 50:50 homogeneous mixture is technically not a solution. When particles are perfectly dissolved in a solution, the composition is uniform, but again, this is not a compound, because that composition can always be varied.

atoms of element y in such-and-such a structure), there is less of a sharp dividing line between homogeneous and heterogeneous solutions. When too much of a solute is added to the solvent, the solvent becomes saturated, and is no longer truly homogeneous. Such is the case when the sugar drifts to the bottom of the tea, or when coffee grounds form in the bottom of a coffee pot containing an otherwise uniform mixture of coffee and water. Milk comes to us as a homogeneous substance, thanks to the process of homogenization, but when it comes out of the cow, it is a more heterogeneous mixture of milk fat and water. In fact, milk is an example of an emulsion, the result of a process whereby two substances that would otherwise form a heterogeneous mixture are made to form a homogeneous one.

REAL-LIFE A P P L I C AT I O N S Colloids

HETEROGENEOUS MIXTURES.

Anyone who has ever made iced tea has observed that it is easy to sweeten when it is hot, because temperature affects the ability of a solvent such as water—in this case, water with particles from tea leaves filtered into it—to dissolve a solute, such as sugar. Cold tea, on the other hand, is much harder to sweeten with ordinary sugar. Usually what happens is that, instead of obtaining a homogeneous mixture, the result is heterogeneous.

FROM HOMOGENEOUS TO HETEROGENEOUS AND BACK AGA I N . The distinction between homoge-

What Brown had observed was the suspension of a colloid—a particle intermediate in size between a molecule and a speck of dust—in the water. When placed in water or another fluid (both liquids and gases are called “fluids” in the physical sciences), a colloid forms a mixture similar both to a solution and a suspension. As with a solution, the composition remains homogeneous—the particles never settle to the bottom of the container. At the same time, however, the particles remain suspended rather than dissolved, rendering a cloudy appearance to the dispersion.

neous and heterogeneous solutions only serves to further highlight the difference between compounds and mixtures. Whereas the composition of a compound is definite and quantitative (so many atoms of element x bonded with so many

Almost everyone, especially as a child, has been fascinated by the colloidal dispersion of dust particles in a beam of sunlight. They seem to be continually in motion, as indeed they are, and this movement is called “Brownian motion” in

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Whereas every region within a homogeneous mixture is more or less the same as every other region, heterogeneous mixtures contain regions that differ from one another. In the example we have used, a glass of cold tea with undissolved sugar at the bottom is a heterogeneous mixture: the tea at the top is unsweetened or even bitter, whereas at the bottom, there is an overly sweet sludge of tea and sugar.

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In 1827, Scottish naturalist Robert Brown (17731858) was studying pollen grains under a microscope when he noticed that the grains underwent a curious zigzagging motion in the water. At first, he assumed that the motion had a biological explanation—in other words, that it resulted from life processes within the pollen—but later, he discovered that even pollen from long-dead plants behaved in the same way.

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honor of the man who first observed it. Yet Brown did not understand what he was seeing; only later did scientists recognize that Brownian motion is the result of movement on the part of molecules in a fluid. Even though the molecules are much smaller than the colloid, there are enough of them producing enough collisions to cause the colloid to be in constant motion. Another remarkable aspect of dust particles floating in sunlight is the way that they cause a column of sunlight to become visible. This phenomenon, called the Tyndall effect after English physicist John Tyndall (1820-1893), makes it seem as though we are actually seeing a beam of light. In fact, what we are seeing is the reflection of light off the colloidal dispersion. Another example of a colloidal dispersion occurs when a puff of smoke hangs in the air; furthermore, as we shall see, milk is a substance made up of colloids—in this case, particles of fat and protein suspended in water.

Emulsions Miscibility is a qualitative term identifying the relative ability of two substances to dissolve in one another. Generally, water and water-based substances have high miscibility with regard to one another, as do oil and oil-based substances with one another. Oil and water, on the other hand (or substances based in either) have very low miscibility. The reason is that water molecules are polar, meaning that the positive electric charge is in one region of the molecule, while the negative charge is in another region. Oil, on the other hand, is nonpolar, meaning that charges are more evenly distributed throughout the molecule. Trying to mix oil and water is thus rather like attempting to use a magnet to attract a nonmagnetic metal such as gold.

All liquids have a certain degree of surface tension. This is what causes water on a hard surface to bead up instead of lying flat. (Mercury has an extremely high surface tension.) Surfactants break down this surface tension, enabling the marriage of two formerly immiscible liquids.

Mixtures

In an emulsion, millions of surfactants surround the dispersed droplets (known as the internal phase), shielding them from the other liquid (the external phase). Supposing oil is the internal phase, then the oil-soluble end of the surfactant points toward the oils, while the water-soluble side joins with the water of the external phase. P H Y S I CA L C H A RAC T E R I S T I CS O F A N E M U L S I O N . The resulting emul-

sion has physical (as opposed to chemical) properties different from those of the substances that make it up. Water and most oils are transparent, whereas emulsions are typically opaque and may have a lustrous, pearly gleam. Oil and water flow freely, but emulsions can be made to form thick, non-flowing creams. Emulsions are inherently unstable: they are, as it were, only temporarily homogeneous. A good example of this is an oil-and-vinegar salad dressing, which has to be shaken before putting it on salad; otherwise, what comes out of the bottle is likely to be mainly oil, which floats to the top of the water-based vinegar. It is characteristic of emulsions that eventually gravity pulls them apart, so that the lighter substances float to the top. This process may take only a few minutes, as with the salad dressing; on the other hand, it can take thousands of years.

H O W S U R FAC TA N T S A I D T H E E M U L S I O N P R O C E SS . An emulsion is

A P P L I CAT I O N S . Surfactants themselves are often used in laundry or dish detergent, because most stains on plates or clothes are oilbased, whereas the detergent itself is applied to the clothes in an aqueous solution. As for emulsions, these are found in a wide variety of products, from cosmetics to paint to milk.

a mixture of two immiscible liquids such as oil and water—we will use these simple terms, with the idea that these stand for all oil- and watersoluble substances—by dispersing microscopic droplets of one liquid in another. In order to achieve this dispersion, there needs to be a “middle man,” known as an emulsifier or surfactant. The surfactant, made up of molecules that are both water- and oil-soluble, acts as an agent for joining other substances in an emulsion.

In the pharmaceutical industry, for instance, emulsions are used as a means of delivering the active ingredients of drugs, many of which are not water soluble, in a medium that is. Pharmaceutical manufacturers also use emulsions to make medicines more palatable (easier to take); to control dosage and improve effectiveness; and to improve the usefulness of topical drugs such as ointments, which must contend with both oily and water-soluble substances on the skin.

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The oils and other conditioning agents used in hair-care products would leave hair limp and sticky if applied by themselves. Instead, these products are applied in emulsions that dilute the oils in other materials. Emulsions are also used in color photography, as well as in the production of pesticides and fungicides. Paints and inks, too, are typically emulsified substances, and these sometimes come in the form of dispersions akin to those of colloids. As we have seen, in a dispersion fine particles of solids are suspended in a liquid. Surfactants are used to bond the particles to the liquid, though paint must be shaken, usually with the aid of a high-speed machine, before it is consistent enough to apply. As mentioned earlier, milk is a form of emulsion that contains particles of milk fat or cream dispersed in water. Light strikes the microscopic fat and protein colloids, resulting in the familiar white color. Other examples of emulsified foods include not only salad dressings, but gravies and various sauces, peanut butter, ice cream, and other items. Not only does emulsion affect the physical properties of foods, but it may enhance the taste by coating the tongue with emulsified oils.

Soap The wondrous invention called soap, which has made the world a cleaner place, exhibits several of the properties we have discussed. It is a mixture with surfactant qualities, and in powdered form, it can form something close to a true solution with water. Discovered probably by the Phoenicians around 600 B.C., it was made in ancient times by boiling animal fat (tallow) or vegetable oils with an alkali or base of wood ashes. This was a costly process, and for many centuries, only the wealthy could afford soap—a fact that contributed to the notorious lack of personal hygiene that prevailed among Europeans of the Middle Ages. In fact, the situation did not change until late in the eighteenth century, when the work of two French chemists yielded a more economical manufacturing method. In 1790, Nicholas Leblanc (1742-1806) developed a process for creating sodium hydroxide (called caustic soda at the time) from sodium chloride, or ordinary table salt. This made for an inexpensive base or alkali, with which soap manufacturers could combine natural fats and oils.

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Then in 1823, Michel Eugène Chevreul (17861889) showed that fat is a compound of glycerol with organic acids, a breakthrough that led to the use of fatty acids in producing soaps. THE

CHEMISTRY

OF

S O A P.

Soap as it is made today comes from a salt of an alkali metal, such as sodium or potassium, combined with a mixture of carboxylic acids (“fatty acids”). These carboxylic acids are the result of a reaction called saponification, involving triglycerides and a base, such as sodium hydroxide. Saponification breaks the triglycerides into their component fatty acids; then, the base neutralizes these to salts. This reaction produces glycerin, a hydrocarbon bonded to a hydroxyl (-OH) group. (Hydrocarbons are discussed in the essays on Organic Chemistry; Polymers.) In general, the formula for soap is RCOOX, where X stands for the alkali metals, and R for the hydrocarbon chain. Because it is a salt (meaning that it is formed from the reaction of an acid with a base), soap partially separates into its component ions in water. The active ion is RCOO-, whose two ends behave in different fashions, making it a surfactant. The hydrocarbon end (R-) is said to be lipophilic, or “oil-loving,” which is appropriate, since most oils are hydrocarbons. On the other hand, the carboxylate end (-COO-) is hydrophilic, or “water-loving.” As a result, soap can dissolve in water, but can also clean greasy stains. When soap is mixed with water, it does not form a true homogeneous mixture or solution due to the presence of hydrocarbons. These attract one another, forming spherical aggregates called micelles. The lipophilic “tails” of the hydrocarbons are turned toward the interior of the micelle, while the hydrophilic heads remain facing toward the water that forms the external phase.

Alloys We have primarily discussed liquid mixtures, but in fact mixtures can be gaseous or even solid—as for example in the case of an alloy, a mixture of two or more metals. Alloys are usually created by melting the component metals, then mixing them together in specific proportions, but an alloy can also be created by bonding metal powders. The structure of an elemental metal includes tight “electron sea” bonds, which are discussed in the essay on Metals. These, combined with the

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Mixtures

KEY TERMS ALLOY:

A mixture of two or more

out; rather, it has various regions possessing different properties. An example of a

metals. A term describing a solu-

mixture is sand in a container of water.

tion in which a substance or substances are

Rather than dissolving to form a homoge-

dissolved in water.

neous mixture, as sugar would, the sand

AQUEOUS:

ATOM:

The smallest particle of an

sinks to the bottom. HOMOGENEOUS:

element. CHEMICAL SPECIES:

A generic term

used for any substance studied in chemistry—whether it be an element, compound, mixture, atom, molecule, ion, and

a mixture that is the same throughout, as for example when sugar is fully dissolved in water. A solution is a homogenous mixture. IMMISCIBLE:

so forth. A substance made up of

COMPOUND:

atoms of more than one element, which are chemically bonded and usually joined in molecules. The composition of a com-

A term describing

Possessing a negligible

value of miscibility. ION:

An atom or group of atoms that

has lost or gained one or more electrons, and thus has a net electric charge. A qualitative term iden-

pound is always the same, unless it is

MISCIBILITY:

changed chemically.

tifying the relative ability of two substances

DISPERSION:

A term describing the

distribution of particles in a fluid. ELEMENT:

A substance made up of

only one kind of atom. Unlike compounds, elements cannot be chemically broken down into other substances. EMULSIFIER: EMULSION:

A surfactant. A mixture of two immis-

to dissolve in one another. MIXTURE:

A substance with a variable

composition, meaning that it is composed of molecules or atoms of differing types. Compare with pure substance or compound. MOLECULE:

A group of atoms, usual-

ly but not always representing more than one element, joined in a structure.

cible liquids such as oil and water, made by

Compounds are typically made of up

dispersing microscopic droplets of one liq-

molecules.

uid in another. In creating an emulsion, surfactants act as bridges between the two liquids.

PHENOMENOLOGICAL:

A

term

describing scientific definitions based purely on experimental phenomena. These

A substance with the ability to

only convey part of the picture, however—

flow. In physical sciences such as chemistry

primarily, the part a chemist can perceive

and physics, “fluid” refers both to liquids

either through measurement or through

and gases.

the senses, such as sight. A structural defi-

FLUID:

HETEROGENEOUS:

A term describ-

ing a mixture that is not the same through-

S C I E N C E O F E V E RY DAY T H I N G S

nition is therefore usually preferable to a phenomenological one.

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Mixtures

KEY TERMS PURE SUBSTANCE:

A substance that

CONTINUED

STRUCTURAL:

A term describing sci-

has the same composition throughout.

entific definitions based on aspects of

Pure substances are either elements or

molecular structure rather than purely

compounds, and should not be confused

phenomenological data.

with mixtures. A homogeneous mixture is SURFACTANT:

although it has an unvarying composition,

of molecules that are both water- and oil-

it cannot be reduced to a single type of

soluble, which acts as an agent for joining

atom or molecule.

other substances in an emulsion.

SOLUTION:

A homogeneous mixture A term that refers to a

in which one or more substances is dis-

SUSPENSION:

solved in another substances—for exam-

mixture in which solid particles are sus-

ple, sugar dissolved in water.

pended in a fluid.

metal’s crystalline structure, create a situation in which internal bonding is very strong, but nondirectional. As a result, most metals form alloys, but again, this is not the same as a compound: the elemental metals retain their identity in these metal composites, forming characteristic fibers, beads, and other shapes. S O M E FA M O U S A L LOY S . One of the most important alloys in early human history was a 25:75 mixture of tin and copper that gave its name to an entire stage of technological development: the Bronze Age (c. 3300-1200 B.C.). This combination formed a metal much stronger than either copper or tin, and bronze remained dominant until the discovery of new iron-smelting methods ushered in the Iron Age in about 1200 B.C.

Another important alloy of the ancient world was brass, which is about one-third zinc to two-thirds copper. Introduced in the period c. 1400-c. 1200 B.C, brass was one of the metals that defined the world in which the Bible was written. Biblical references to it abound, as in the famous passage from I Corinthians 13 (read at many weddings), in which the Apostle Paul proclaims that “without love, I am as sounding brass.” Some biblical scholars, however, maintain that some of the references to brass in the Old Testament are actually mistranslations of “bronze.”

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A substance made up

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Tin, copper, and antimony form pewter, a soft mixture that can be molded when cold and beaten regularly without turning brittle. Though used in Roman times, it became most popular in England from the fourteenth to the eighteenth centuries, when it offered a cheap substitute for silver in plates, cups, candelabra, and pitchers. The pewter turned out by colonial American metalsmiths is still admired for its beauty and functionality. As for iron, it appears in nature as an impure ore, but even when purified, it is typically alloyed with other elements. Among the well-known forms of iron are cast iron, a variety of mixtures containing carbon and/or silicon; wrought iron, which contains small amounts of various other elements, such as nickel, cobalt, copper, chromium, and molybdenum; and steel. Steel is a mixture of iron with manganese and chromium, purified with a blast of hot air. Steel is also sometimes alloyed with aluminum in a one-third/twothirds mixture to make duraluminum, developed for the superstructures of German zeppelins during World War I. WHERE TO LEARN MORE “All About Colloids.” Synthashield (Web site). (June 6, 2001). Carona, Phillip B. Magic Mixtures: Alloys and Plastics. Englewood Cliffs, NJ: Prentice Hall, 1963.

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ChemLab. Danbury, CT: Grolier Educational, 1998. “Corrosion of Alloys.” Corrosion Doctors (Web site). (June 6, 2001). “Emulsions.” Pharmweb (Web site). (June 6, 2001). Hauser, Jill Frankel. Super Science Concoctions: 50 Mysterious Mixtures for Fabulous Fun. Charlotte, VT: Williamson Publishing, 1996.

S C I E N C E O F E V E RY DAY T H I N G S

Knapp, Brian J. Elements, Compounds, and Mixtures. Danbury, CT: Grolier Educational, 1998.

Mixtures

Maton, Anthea. Exploring Physical Science. Upper Saddle River, NJ: Prentice Hall, 1997. Patten, J. M. Elements, Compounds, and Mixtures. Vero Beach, FL: Rourke Book Company, 1995. The Soap and Detergent Association (Web site). (June 6, 2001).

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S O LU T I O N S Solutions

CONCEPT We are most accustomed to thinking of solutions as mixtures of a substance dissolved in water, but in fact the meaning of the term is broader than that. Certainly there is a special place in chemistry for solutions in which water—”the universal solvent”—provides the solvent medium. This is also true in daily life. Coffee, tea, soft drinks, and even water itself (since it seldom appears in pure form) are solutions, but the meaning of the term is not limited to solutions involving water. Indeed, solutions do not have to be liquid; they can be gaseous or solid as well. One of the most important solutions in the world, in fact, is the air we breathe, a combination of nitrogen, oxygen, noble gases, carbon dioxide, and water vapor. Nonetheless, aqueous or water-based substances are the focal point of study where solutions are concerned, and reactions that take place in an aqueous solution provide an important area of study.

HOW IT WORKS Mixtures Solutions belong to the category of substances identified as mixtures, substances with variable composition. This may seem a bit confusing, since among the defining characteristics of a solution are its uniformity and consistency. But this definition of a solution involves external qualities—the things we can observe with our senses—whereas a mixture can be distinguished from a pure substance not only in terms of external characteristics, but also because of its internal and structural properties.

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At one time, chemists had a hard time differentiating between mixtures and pure substances (which are either elements or compounds), primarily because they had little or no knowledge of those “internal and structural properties.” Externally, a mixture and compound can be difficult to distinguish, but the “variable composition” mentioned above is a key factor. Coffee can be almost as weak as water, or so strong it seems to galvanize the throat as it goes down—yet in both cases, though its composition has varied, it is still coffee. By contrast, if a compound is altered by the variation of just one atom in its molecular structure, it becomes something else entirely. When an oxygen atom bonds with a carbon and oxygen atom, carbon monoxide—a poisonous gas produced by the burning of fossil fuels—is turned into carbon dioxide, an essential component of the interaction between plants and animals in the natural environment. MIXTURES VS. COMPOUNDS: T H R E E D I S T I N C T I O N S . Air (a mix-

ture) can be distinguished from a compound, such as carbon dioxide, in three ways. First, a mixture can exist in virtually any proportions among its constituent parts, whereas a compound has a definite and constant composition. Air can be oxygen-rich or oxygen-poor, but if we vary the numbers of oxygen atoms chemically bonded to form a compound, the result is an entirely different substance. Second, the parts of a mixture retain most of their properties when they join together, but elements—once they form a compound—lose their elemental characteristics. If pieces of carbon were floating in the air, their character would be quite

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Solutions

WATER

AND OIL DROPLETS EXPOSED TO POLARIZED LIGHT.

WATER

AND OIL HAVE VERY LITTLE MISCIBILITY WITH

REGARD TO ONE ANOTHER, MEANING THAT THEY TEND TO REMAIN SEPARATE WHEN COMBINED. (Science Pictures

Limited/Corbis. Reproduced by permission.)

different from that of the carbon in carbon dioxide. Carbon, in its natural form, is much like coal or graphite (graphite is pure carbon, and coal an impure form of the element), but when a carbon atom combines with an oxygen atom, the two form a gas. Pieces of carbon in the air, on the other hand, would be solid, as carbon itself is. Third, a mixture can usually be created simply by the physical act of stirring items together, and/or by applying heat. Certainly this is true of many solutions, such as brewed coffee. On the other hand, a compound such as carbon dioxide can only be formed by the chemical reaction of atoms in specific proportions and at specific temperatures. Some such reactions consume heat, whereas others produce it. These interactions of heat are highly complex; on the other hand, in making coffee, heat is simply produced by an external source (the coffee maker) and transferred to the coffee in the brewing process.

said to be the same throughout. It is thus a heterogeneous mixture. The same is true of cold tea when table sugar (sucrose) is added to it: the sugar drifts to the bottom, and as a result, the first sip of tea from the glass is not nearly as sweet as the last. Wood is a mixture that typically appears in heterogeneous form, a fact that can be easily confirmed by studying wood paneling. There are knots, for instance—areas where branches once grew, and where the concentrations of sap are higher. Striations of color run through the boards of paneling, indicating complex variations in the composition of the wood. Much the same is true of soil, with a given sample varying widely depending on a huge array of factors: the concentrations of sand, rock, or decayed vegetation, for instance, or the activities of earthworms and other organisms in processing the soil.

basic types of mixtures. One of these occurs, for instance, when sand is added to a beaker of water: the sand sinks to the bottom, and the composition of the sand-water mixture cannot be

In a homogeneous mixture, by contrast, there is no difference between one area of the mixture and another. If coffee has been brewed properly, and there are no grounds at the bottom of the pot, it is homogeneous. If sweetener is added to tea while it is hot, it too should yield a homogeneous mixture.

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HETEROGENEOUS AND HOMOGENEOUS MIXTURES. There are two

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MISCIBILITY

Solutions

AND

S O LU B I L I -

T Y. Miscibility is a term describing the relative

ability of two substances to dissolve in one another. Generally, water and water-based substances have high miscibility with regard to one another, as do oil and oil-based substances. Oil and water, on the other hand (or substances based in either) have very low miscibility. The reasons for this will be discussed below. Miscibility is a qualitative term like “fast” or “slow”; on the other hand, solubility—the maximum amount of a substance that dissolves in a given amount of solvent at a specific temperature—can be either qualitative or quantitative. In a general sense, the word is qualitative, referring to the property of being soluble, or able to dissolve in a solution.

WATER

COMPLETELY MISCIBLE, 99% WATER AND 1% ETHANOL, 50% OF BOTH, OR 99% ETHANOL AND 1% WATER. SHOWN HERE IS A GLASS OF BEER—AN EXAMPLE OF A WATER -ETHANOL SOLUTION. (Owen AND

ETHANOL

ARE

WHETHER THE SOLUTION BE

Franken/Corbis. Reproduced by permission.)

Solutions and Solubility Virtually all varieties of homogeneous mixtures can be classified as a solution—that is, a homogeneous mixture in which one or more substances is dissolved in another substance. If two or more substances were combined in exactly equal proportions, this would technically not be a solution; hence the distinction between solutions and the larger class of homogeneous mixtures. A solution is made up of two parts: the solute, the substance or substances that are dissolved; and the solvent, the substance that dissolves the solute. The solvent typically determines the physical state of the solution: in other words, if the solvent is a liquid, the solution will most likely be a liquid. Likewise if the solvent is a solid such as a metal, melted at high temperatures to form a metallic solution called an alloy, then when the solution is cooled to its normal temperature, it will be solid again.

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At some points in this essay, “solubility” will be used in this qualitative sense; however, within the realm of chemistry, it also has a quantitative meaning. In other words, instead of involving an imprecise quality such as “fast” or “slow,” solubility in this sense relates to precise quantities more along the lines of “100 MPH (160.9 km/h).” Solubility is usually expressed in grams (g) (0.0022 lb) of solute per 100 g (0.22 lb) of solvent. O T H E R Q UA N T I TAT I V E T E R M S .

Another quantitative means of describing a solution is in terms of mass percent: the mass of the solute divided by the mass of the solution, and multiplied by 100%. If there are 25 g of solute in a solution of 200 g, for instance, 25 would be divided by 200 and multiplied by 100% to yield a 12.5% figure of solution composition. Molarity also provides a quantitative means of showing the concentration of solute to solution. Whereas mass percent is, as its name indicates, a comparison of the mass of the solute to that of the solvent, molarity shows the amount of solute in a given volume of solution. This is measured in moles of solute per liter of solution, abbreviated mol/l.

REAL-LIFE A P P L I C AT I O N S Saturation and Dilution The quantitative terms for describing solubility that we have reviewed are useful to a professional chemist, or to anyone performing work in a laboratory. For the most part, however, qualita-

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tive expressions will suffice for the present discussion. Among the most useful qualitative terms is saturation. When it is possible to dissolve solute in a solvent, the solution is said to be unsaturated— rather like a sponge that has not been filled to capacity with liquid. By contrast, a solution that contains as much solute as it can at a given temperature is like a sponge that has been filled with all the liquid it can possibly contain. It can no longer absorb more of the liquid; rather, it can only push liquid particles along, and the sponge is said to be saturated. In the same way, if tea is hot, it is easier to introduce sugar to it, but when the temperature is low, sugar will not dissolve as easily. SAT U RAT I O N A N D T E M P E RA T U R E . With tea and many other substances,

higher temperatures mean that a greater amount of solute can be added before the solution is fully saturated. The reason for this relationship between saturation and temperature, according to generally accepted theory, is that heated solvent particles move more quickly than cold ones, and as a result, create more space into which the solvent can fit. Indeed, “space” is a prerequisite for a solution: the molecules of solute need to find a “hole” between the molecules of solvent into which they can fit. Thus the molecules in a solution can be compared to a packed crowd: if a crowd is suddenly dispersed, it is easier to walk through it. Not all substances respond to rises in temperature in the manner described, however. As a rule, gases (with the exception of helium) are more soluble at lower temperatures than at higher ones. Higher temperatures mean an increase in kinetic energy, with more molecules colliding with one another, and this makes it easier for the gases to escape the liquid in which they are dissolved. Carbonated soft drinks get their “fizz” from carbon dioxide gas dissolved, along with sugar and other flavorings, in a solution of water. When the soft-drink container is opened, much of the carbon dioxide quickly departs, while a certain proportion stays dissolved in the cola. The remaining carbon dioxide will eventually escape as well, but it will do so more quickly if left in a warm environment rather than a cold one, such as the inside of a refrigerator.

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D I LU T I O N A N D C O N C E N T RA T I O N . Another useful set of qualitative terms

Solutions

for solutions also relates to the relative amount of solute that has been dissolved. But whereas saturation is an absolute condition at a certain temperature (in other words, the solution is definitely saturated at such-and-such a temperature), concentration and dilution are much more relative terms. When a solution contains a relatively small amount of solute, it is said to be dilute; on the other hand, a solution with a relatively large amount of solute is said to be concentrated. For instance, hydrogen peroxide (H2O2) as sold over the counter in drug stores, to be used in homes as a disinfectant of bleaching agent, is a dilute 3% solution in water. In higher concentrations, hydrogen peroxide can cause burns to human skin, and is used to power rockets. Chemists often keep on hand in their laboratories concentrated versions of various frequently used solutions. Maintaining them in concentrated form saves space; when more of a solution is needed, the chemist dilutes it in water according to desired molarity values. To do so, the chemist determines how much water needs to be added to a particular “stock solution” (as these concentrated solutions in a laboratory are called) to create a solution of a particular concentration. In the dilution with water, the number of moles of the solute does not change; rather, the particles of solute are dispersed over a larger volume.

“Like Dissolves Like” In the molecules of a compound, the atoms are held together by powerful attractions. The molecules within a compound, however, also have an intermolecular attraction, but this is much weaker than the interatomic bonds in a molecule. It is thus quite possible for the molecules in a solution to exert a greater attractive force than the one holding the molecules of solute together with one another. When this happens, the solute dissolves. A useful rule of thumb when studying solutions is “like dissolves like.” In other words, water dissolves water-based solutes, whereas oil dissolves oily substances. Or to put it another way, water and water-based substances are miscible with one another, as are oil and oil-based substances. Water and oil together, however, are largely immiscible.

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P O L A R W AT E R , N O N P O L A R O I L . Water and oil and immiscible due to their

respective molecule structures, and hence their inherent characteristics of intermolecular bonding. Water molecules are polar, meaning that the positive electric charge is at one end of the molecule, while the negative charge is at the other end. Oil, on the other hand, is nonpolar—charges are more evenly distributed throughout the molecule. The oxygen atom in a water molecule has a much greater electronegativity than the hydrogen atoms, so it pulls negatively charged electrons to its end of the molecule. Conversely, most oils are composed of carbon and hydrogen, which have similar values of electronegativity. As a result, positive and negative charges are distributed evenly throughout the oil molecule. Whereas a water molecule is like a magnet with a north and south pole, an oil molecule is like a nonmagnetic metal. Thus, as most people know, “oil and water don’t mix.” The immiscible quality of oil and water in relation to one another explains a number of phenomena from the everyday world. It is easy enough to wash mud from your hands under running water, because the water and the mud (which, of course, contains water) are highly miscible. But motor oil or oil-based paint will leave a film on the hands, as these substances bond to oily molecules in the skin itself, and no amount of water will wash the film off. To remove it, one needs mineral spirits, paint thinner, or soap manufactured to bond with the oils. Everyone has had the experience of eating spicy foods and then trying to cool down the mouth with cold water—and just about everyone has discovered that this does not work. This is because most spicy substances contain emulsified oils that coat the tongue, and these oils are not attracted to water. A much better “solution” (in more ways than one) is milk, and in particular, whole milk. Though a great deal of milk is composed of water, it also contains tiny droplets of fats and proteins that join with the oils in spicy substances, thus cooling down the mouth.

How does the soap manage to “connect” both with oils and water? Likewise, how does milk—which is water-soluble and capable of dilution in water—wash away the oils associated with spicy food? To answer these questions, we need to briefly consider a subject examined in more depth within the Mixtures essay: emulsions, or mixtures of two immiscible liquids. E M U L S I F I E R S A N D S U R FAC TA N T S . The dispersion of two substances in

an emulsion is achieved through the use of an emulsifier or surfactant. Made up of molecules that are both water- and oil-soluble, an emulsifier or surfactant acts as an agent for joining other substances in an emulsion. The two words are virtually synonymous, but “emulsifier” is used typically in reference to foods, whereas “surfactant” most often refers to an ingredient in detergents and related products. In an emulsion, millions of surfactants surround the dispersed droplets of solute, known as the internal phase, shielding them from the solvent, or external phase. Surfactants themselves are often used in laundry or dish detergent, because most stains on plates or clothes are oilbased, whereas the detergent itself is applied to the clothes in a water-based external phase. The emulsifiers in milk help to bond oily particles of milk fat (cream) and protein to the external phase of water that comprises the majority of milk’s volume.

The active ingredient in a dry-cleaning chemical usually has nonpolar molecules, because the toughest stains are made by oils and greases. But

As for soap, it is a mixture of an acid and a base—specifically, carboxylic acids joined with a base such as sodium hydroxide. Carboxylic acids, chains of hydrogen and carbon atoms, are just some of many hydrocarbons that form the chemical backbone of a vast array of organic substances. Because it is a salt, meaning that it is formed from the reaction of an acid with a base, soap partially separates into its component ions in water. The active ion is RCOO- (R stands for

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Crossing the Oil/Water Barrier

342

what about ordinary soap, which we use in water to wash both water- and oil-based substances from our bodies? Though, as noted earlier, certain kinds of oil stains require special cleaning solutions, ordinary soap is fine for washing off the natural oils secreted by the skin. Likewise, it can wash off small concentrations of oil that come from the environment and attach to the skin—for instance, from working over a deep fryer in a fast-food restaurant.

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the hydrocarbon chain), whose two ends behave in different fashions, making it a surfactant. The hydrocarbon end (R-) is said to be lipophilic, or “oil-loving”; on the other hand, the carboxylate end (-COO-) is hydrophilic, or “water-loving.” As a result, soap can dissolve in water, but can also clean greasy stains. When soap is mixed with water, it does not form a true solution, due to the presence of the hydrocarbons, which attract one another to form spherical aggregates called micelles. The lipophilic “tails” of the hydrocarbons are turned toward the interior of the micelle, while the hydrophilic “heads” remain facing toward the water that forms the external phase. E T H A N O L . When a hydrocarbon joins with other substances in one of the hydrocarbon functional groups, these form numerous hydrocarbon derivatives. Among the hydrocarbon derivatives is alcohol, and within this grouping is ethyl alcohol or ethanol, which includes a hydrocarbon bonded to the –OH functional group. The formula for ethanol is C2H6O, though this is sometimes rendered as C2H5OH to show that the alcohol functional group (–OH) is bonded to the hydroxyl radical (C2H5).

Ethanol, the same alcohol found in alcoholic beverages, is obviously miscible with water. Beer, after all, is mostly water, and if ethanol and water were not miscible, it would be impossible to mix alcohol with water or a water-based substance to make Scotch and water or a Bloody Mary (vodka and tomato juice.) In fact, water and ethanol are completely miscible, whether the solution be 99% water and 1% ethanol, 50% of both, or 99% ethanol and 1% water. How can this be, given the fact that ethanol is a hydrocarbon derivative—in other words, a cousin of petroleum? The addition of the term “derivative” gives us a clue: note that ethanol contains oxygen, just as water does. As in water, the oxygen and hydrogen form a polar bond, giving that portion of the ethanol molecule a high affinity for water. Molecules of water and the alcohol functional group are joined through an intermolecular force called hydrogen bonding.

reactions in aqueous solutions—though we can only touch on them briefly here—constitute a large and significant body of reactions studied by chemists. Most of the solutions we have discussed up to this point are aqueous. We have referred, at least in an external or phenomenological way, to the means by which sugar dissolves in an aqueous solution, such as tea. Now let us consider, from a molecular or structural standpoint, the means by which salt does so as well. SA LT WAT E R. Salt is formed of positively charged sodium ions, firmly bonded to negatively charged chlorine ions. To break these ionic bonds and dissolve the sodium chloride, water must exert a strong attraction. However, salt is not really composed of molecules but simply repeating series of sodium and chlorine atoms joined together in a simple face-centered cubic lattice which has each each sodium ion (Na+) surrounded by six chlorine ions (Cl–); at the same time, each chlorine ion is surrounded by six sodium ions.

In dissolving the salt, water molecules surround ions that make up the salt, holding them in place through the electrical attraction of opposite charges. Due to the differences in electronegativity that give the water molecule its high polarity, the hydrogen atoms are more positively charged, and thus these attract the negatively charged chlorine ion. Similarly, the negatively charged oxygen end of the water molecule attracts the positively charged sodium ion. As a result, the salt is “surrounded” or dissolved. P LAS M A A N D AQ U E O U S - S O LUTION REACTIONS IN THE B O DY. Whereas saltwater is an aqueous solu-

tion that can eventually kill someone who drinks it, plasma is an essential component of the life process—and in fact, it contains a small quantity of sodium chloride. Not to be confused with the phase of matter also called plasma, this plasma is the liquid portion of blood. Blood itself is about 55% plasma, with red and white blood cells and platelets suspended in it.

The term aqueous refers to water, and because water has extraordinary solvent qualities (discussed in the Osmosis essay) it serves as the medium for numerous solutions. Furthermore,

Plasma is in turn approximately 90% water, with a variety of other substances suspended or dissolved in it. Prominent among these substances are proteins, of which there are about 60 in plasma, and these serve numerous important functions—for instance, as antibodies. Plasma

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Aqueous Solutions

Solutions

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Solutions

KEY TERMS A term describing a solu-

out; rather, it has various regions possess-

tion in which a substance or substances are

ing different properties. One example of

dissolved in water.

this is sand in a container of water. Rather

AQUEOUS:

A substance made up of

than dissolving to form a homogeneous

atoms of more than one element, which are

mixture as sugar does, the sand sinks to the

chemically bonded and usually joined in

bottom.

COMPOUND:

molecules. The composition of a com-

HOMOGENEOUS:

A term describing

pound is always the same, unless it is

a mixture that is the same throughout, as

changed chemically.

for example when sugar is fully dissolved

CONCENTRATED:

A qualitative term

describing a solution with a relatively large

in water. A solution is a homogenous mixture.

ratio of solute to solvent. IMMISCIBLE: DILUTE:

A qualitative term describing

Possessing a negligible

value of miscibility.

a solution with a relatively small ratio of MASS

solute to solvent. A surfactant. (Usually

EMULSIFIER:

the term “emulsifier” is applied to foods, and “surfactant” to detergents and other inedible products.)

A quantitative

means of measuring solubility in terms of the mass of the solute divided by the mass of the solution, multiplied by 100%. MISCIBILITY:

A term identifying the

A mixture of two immis-

relative ability of two substances to dissolve

cible liquids, such as oil and water, made by

in one another. Miscibility is qualitative,

dispersing microscopic droplets of one liq-

like “fast” or “slow”; solubility, on the other

uid in another. In creating an emulsion,

hand, can be a quantitative term.

EMULSION:

surfactants act as bridges between the two liquids. HETEROGENEOUS:

MIXTURE:

A substance with a variable

composition, composed of molecules or A term describ-

ing a mixture that is not the same through-

also contains ions, which prevent red blood cells from taking up excess water in osmosis. Prominent among these ions are those of salt, or sodium chloride. Plasma also transports nutrients such as amino acids, glucose, and fatty acids, as well as waste products such as urea and uric acid, which it passes on to the kidneys for excretion. Much of the activity that sustains life in the body of a living organism can be characterized as a chemical reaction in an aqueous solution. When we breathe in oxygen, it is taken to the lungs and fed into the bloodstream, where it

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atoms of differing types. Compare with pure substance.

associates with the iron-containing hemoglobin in red blood cells and is transported to the organs. In the stomach, various aqueous-solution reactions process food, turning part of it into fuel that the blood carries to the cells, where the oxygen engages in complex reactions with nutrients.

A Miscellany of Solutions Aqueous-solution reactions can lead to the formation of a solid, as when a solution of potassi-

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Solutions

KEY TERMS

CONTINUED

A quantitative means of

apply the word in a quantitative sense, to

showing the concentration of solute to

indicate the maximum amount of a sub-

solution. This is measured in moles of

stance that dissolves in a given amount of

solute per liter of solution, abbreviated

solvent at a specific temperature. Solubility

mol/L.

is usually expressed in grams of solute per

MOLARITY:

PURE SUBSTANCE:

A substance—

100 g of solvent.

either an element or compound—that has

SOLUBLE:

an unvarying composition. This means

solvent.

that by changing the proportions of atoms,

SOLUTE:

Capable of dissolving in a

The substance or substances

the result would be an entirely different

that are dissolved in a solvent to form a

substance. Compare with mixture.

solution.

Involving a compari-

QUALITATIVE:

A homogeneous mixture

SOLUTION:

son between qualities that are not precisely

in which one or more substances (the

defined, such as “fast” and “slow” or

solute) is dissolved in a solvent—for exam-

“warm” and “cold.”

ple, sugar dissolved in water. Involving a compari-

QUANTITATIVE:

SOLVENT:

The substance or sub-

son between precise quantities—for

stances that dissolve a solute to form a

instance, 10 lb vs. 100 lb, or 50 MPH vs. 120

solution.

MPH. SURFACTANT:

term

of molecules that are both water- and oil-

describing a solution that contains as

soluble, which acts as an agent for joining

much solute as it can dissolve at a given

other substances in an emulsion.

SATURATED:

A

qualitative

A substance made up

temperature. UNSATURATED:

A term describing a

In a broad, qualitative

solution that is capable of dissolving

sense, solubility refers to the property of

additional solute, if that solute is intro-

being soluble. Chemists, however, usually

duced to it.

SOLUBILITY:

um chromate (K2CrO4) is added to an aqueous solution of barium nitrate (Ba[NO3]2 to form solid barium chromate (BaCrO4) and a solution of potassium nitrate (KNO3). This reaction is described in the essay on Chemical Reactions.

ments (those in which two atoms join to form a molecule of a single element); monatomic elements (those elements that exist as single atoms); one element in a triatomic molecule; and two compounds.

We have primarily discussed liquid solutions, and in particular aqueous solutions. It should be stressed, however, that solutions can also exist in the gaseous or solid phases. The air we breathe is a solution, not a compound: in other words, there is no such thing as an “air molecule.” Instead, it is made up of diatomic ele-

The “solvent” in air is nitrogen, a diatomic element that accounts for 78% of Earth’s atmosphere. Oxygen, also diatomic, constitutes an additional 21%. Argon, which like all noble gases is monatomic, ranks a distant third, with 0.93%. The remaining 0.07% is made up of traces of other noble gases; the two compounds men-

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tioned, carbon dioxide and water (in vapor form); and, high in the atmosphere, the triatomic form of oxygen known as ozone (O3). The most significant solid solutions are alloys of metals, discussed in the essay on Mixtures, as well as in essays on various metal families, particularly the Transition Metals. Some well-known alloys include bronze (three-quarters copper, one-quarter tin); brass (two-thirds copper, one-third zinc); pewter (a mixture of tin and copper with traces of antimony); and numerous alloys of iron—particularly steel—as well as alloys involving other metals. WHERE TO LEARN MORE

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Hauser, Jill Frankel. Super Science Concoctions: 50 Mysterious Mixtures for Fabulous Fun. Charlotte, VT: Williamson Publishing, 1996. “Introduction to Solutions” (Web site). (June 6, 2001). Knapp, Brian J. Air and Water Chemistry. Illustrated by David Woodroffe. Danbury, CT: Grolier Educational, 1998. Seller, Mick. Elements, Mixtures, and Reactions. New York: Shooting Star Press, 1995. “Solubility.” Spark Notes (Web site). (June 6, 2001). “Solutions and Solubility” (Web site). (June 6, 2001).

“Aqueous Solutions” (Web site). (June 6, 2001).

Watson, Philip. Liquid Magic. Illustrated by Elizabeth Wood and Ronald Fenton. New York: Lothrop, Lee, and Shepard Books, 1982.

Gibson, Gary. Making Things Change. Brookfield, CT: Copper Beech Books, 1995.

Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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Osmosis

CONCEPT The term osmosis describes the movement of a solvent through a semipermeable membrane from a less concentrated solution to a more concentrated one. Water is sometimes called “the perfect solvent,” and living tissue (for example, a human being’s cell walls) is the best example of a semipermeable membrane. Osmosis has a number of life-preserving functions: it assists plants in receiving water, it helps in the preservation of fruit and meat, and is even used in kidney dialysis. In addition, osmosis can be reversed to remove salt and other impurities from water.

HOW IT WORKS If you were to insert a hollow tube of a certain diameter into a beaker of water, the water would rise inside the tube and reach the same level as the water outside it. But suppose you sealed the bottom end of the tube with a semipermeable membrane, then half-filled the tube with salt water and again inserted it into the beaker. Over a period of time, the relative levels of the salt water in the tube and the regular water in the beaker would change, with the fresh water gradually rising into the beaker. This is osmosis at work; however, before investigating the process, it is necessary to understand at least three terms. A solvent is a liquid capable of dissolving or dispersing one or more other substances. A solute is the substance that is dissolved, and a solution is the resulting mixture of solvent and solute. Hence, when you mix a packet of sugar into a cup of hot coffee, the coffee—which is mostly water—acts as a solvent for the sugar, a solute, and the resulting sweetened coffee is a solution. (Indeed, people who need a

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OSMOSIS

cup of coffee in the morning might say that it is a “solution” in more ways than one!) The relative amount of solute in the solution determines whether it can be described as more or less concentrated.

Water and Oil: Molecular Differences In the illustrations involving the beaker and the hollow tube, water plays one of its leading roles, as a solvent. It is possible to use a number of other solvents for osmosis, but most of the ones that will be discussed here are water-based substances. In fact, virtually everything people drink is either made with water as its central component (soft drinks, coffee, tea, beer and spirits), or comes from a water-based plant or animal life form (fruit juices, wine, milk.) Then of course there is water itself, still the world’s most popular drink. By contrast, people are likely to drink an oily product only in extreme circumstances: for instance, to relieve constipation, holistic-health practitioners often recommend a mixture of olive oil and other compounds for this purpose. Oil, unlike water, has a tendency to pass straight through a person’s system, without large amounts of it being absorbed through osmosis. In fact, oil and water differ significantly at the molecular level. Water is the best example of a polar molecule, sometimes called a dipole. As everyone knows, water is a name for the chemical H2O, in which two relatively small hydrogen atoms bond with a large atom of oxygen. You can visualize a water molecule by imagining oxygen as a basketball with hydrogen as two baseballs fused to the

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A

SEMIPERMEABLE MEMBRANE PERMITS ONLY CERTAIN COMPONENTS OF SOLUTIONS, USUALLY THE SOLVENT, TO PASS

THROUGH.

basketball’s surface. Bonded together as they are, the oxygen tends to pull electrons from the hydrogen atoms, giving it a slight negative charge and the hydrogen a slight positive charge. As a result, one end of a water molecule has a positive electrical charge, and the other end a negative charge. This in turn causes the positive end of one molecule to attract the negative side of its neighbor, and vice versa. Though the electromagnetic force is weak, even in relative terms, it is enough to bond water molecules tightly to one another.

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across the surface of the molecule. Hence, the bond between oil molecules is much less tight than for water molecules. Clean motor oil in a car’s crankshaft behaves as though it were made of millions of tiny ball-bearings, each rolling through the engine without sticking. Water, on the other hand, has a tendency to stick to surfaces, since its molecules are so tightly bonded to one another.

By contrast, oily substances—whether the oil is animal-, vegetable-, or petroleum-based— are typically nonpolar, meaning that the positive and negative charges are distributed evenly

This tight bond gives water highly unusual properties compared to other substances close to its molecular weight. Among these are its high boiling point, its surprisingly low density when frozen, and the characteristics that make osmosis possible. Thanks to its intermolecular structure, water is not only an ideal solvent, but its closely

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Osmosis

OSMOSIS

EQUALIZES CONCENTRATION.

packed structure enables easy movement, as, for instance, from an area of low concentration to an area of high concentration. In the beaker illustration, the “pure” water is almost pure solvent. (Actually, because of its solvent qualities, water seldom appears in a pure state unless one distills it: even water flowing through a “pure” mountain stream carries all sorts of impurities, including microscopic particles of the rocks over which it flows.) In any case, the fact that the water in the beaker is almost pure makes it easy for it to flow through the semipermeable membrane in the bottom of the tube. By contrast, the solute particles in the saltwater solution have a much harder time passing through, and are much more likely to block the openings in the membrane. As a result, the movement is all in one direction: water in the beaker moves through the membrane, and into the tube. A few points of clarification are in order here. A semipermeable membrane is anything with a structure somewhere between that of, say, plastic on the one hand and cotton on the other. Were the tube in the beaker covered with Saran wrap, for instance, no water would pass through. On the other hand, if one used a piece of cotton in the bottom of the tube, the water would pass straight through without osmosis taking place. In contrast to cotton, Gore-tex is a fabric containing

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a very thin layer of plastic with billions of tiny pores which let water vapor flow out without allowing liquid water to seep in. This accounts for the popularity of Gore-tex for outdoor gear—it keeps a person dry without holding in their sweat. So Gore-tex would work well as a semipermeable membrane. Also, it is important to consider the possibilities of what can happen during the process of osmosis. If the tube were filled with pure salt, or salt with only a little water in it, osmosis would reach a point and then stop due to osmotic pressure within the substance. Osmotic pressure results when a relatively concentrated substance takes in a solvent, thus increasing its pressure until it reaches a point at which the solution will not allow any more solvent to enter.

REAL-LIFE A P P L I C AT I O N S Cell Behavior and Salt Water Cells in the human body and in the bodies of all living things behave like microscopic bags of solution housed in a semipermeable membrane. The health and indeed the very survival of a person, animal, or plant depends on the ability of

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Osmosis

IN

THIS

1966

PHOTO, A GIRL IS UNDERGOING KIDNEY DIALYSIS, A CRUCIAL MODERN MEDICAL APPLICATION THAT

RELIES ON OSMOSIS. (Bettmann/Corbis. Reproduced by permission.)

the cells to maintain their concentration of solutes. Two illustrations involving salt water demonstrate how osmosis can produce disastrous effects in living things. If you put a carrot in salty water, the salt water will “draw” the water from inside the carrot—which, like the human body and most other forms of life, is mostly made up of water. Within a few hours, the carrot will be limp, its cells shriveled. Worse still is the process that occurs when a person drinks salt water. The body can handle a little bit, but if you were to consume nothing but salt water for a period of a few days, as in the case of being stranded on the proverbial desert island, the osmotic pressure would begin drawing water from other parts of your body. Since a human body ranges from 60% water (in an adult male) to 85% in a baby, there would be a great deal of water available—but just as clearly, water is the essential ingredient in the human body. If you continued to ingest salt water, you would eventually experience dehydration and die.

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for them, salt water is an isotonic solution, or one that has the same concentration of solute—and hence the same osmotic pressure—as in their own cells.

Osmosis in Plants Plants depend on osmosis to move water from their roots to their leaves. The further toward the edge or the top of the plant, the greater the solute concentration, which creates a difference in osmotic pressure. This is known as osmotic potential, which draws water upward. In addition, osmosis protects leaves against losing water through evaporation. Crucial to the operation of osmosis in plants are “guard cells,” specialized cells dispersed along the surface of the leaves. Each pair of guard cells surrounds a stoma, or pore, controlling its ability to open and thus release moisture.

How, then, do fish and other forms of marine life survive in a salt-water environment? In most cases, a creature whose natural habitat is the ocean has a much higher solute concentration in its cells than does a land animal. Hence,

In some situations, external stimuli such as sunlight may cause the guard cells to draw in potassium from other cells. This leads to an increase in osmotic potential: the guard cell becomes like a person who has eaten a dry biscuit, and is now desperate for a drink of water to wash it down. As a result of its increased osmotic potential, the guard cell eventually takes on

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water through osmosis. The guard cells then swell with water, opening the stomata and increasing the rate of gas exchange through them. The outcome of this action is an increase in the rate of photosynthesis and plant growth. When there is a water shortage, however, other cells transmit signals to the guard cells that cause them to release their potassium. This decreases their osmotic potential, and water passes out of the guard cells to the thirsty cells around them. At the same time, the resultant shrinkage in the guard cells closes the stomata, decreasing the rate at which water transpires through them and preventing the plant from wilting.

Osmosis and Medicine Osmosis has several implications where medical care is concerned, particularly in the case of the storage of vitally important red blood cells. These are normally kept in a plasma solution which is isotonic to the cells when it contains specific proportions of salts and proteins. However, if red blood cells are placed in a hypotonic solution, or one with a lower solute concentration than in the cells themselves, this can be highly detrimental. Hence water, a life-giving and life-preserving substance in most cases, is a killer in this context. If red blood cells were stored in pure water, osmosis would draw the water into the cells, causing them to swell and eventually burst. Similarly, if the cells were placed in a solution with a higher solute concentration, or hypertonic solution, osmosis would draw water out of the cells until they shriveled. In fact, the plasma solution used by most hospitals for storing red blood cells is slightly hypertonic relative to the cells, to prevent them from drawing in water and bursting. Physicians use a similar solution when injecting a drug intravenously into a patient. The active ingredient of the drug has to be suspended in some kind of medium, but water would be detrimental for the reasons discussed above, so instead the doctor uses a saline solution that is slightly hypertonic to the patient’s red blood cells. One vital process closely linked to osmosis is dialysis, which is critical to the survival of many victims of kidney diseases. Dialysis is the process by which an artificial kidney machine removes waste products from a patients’ blood—performing the role of a healthy, normally function-

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ing kidney. The openings in the dialyzing membrane are such that not only water, but salts and other waste dissolved in the blood, pass through to a surrounding tank of distilled water. The red blood cells, on the other hand, are too large to enter the dialyzing membrane, so they return to the patient’s body.

Osmosis

Preserving Fruits and Meats Osmosis is also used for preserving fruits and meats, though the process is quite different for the two. In the case of fruit, osmosis is used to dehydrate it, whereas in the preservation of meat, osmosis draws salt into it, thus preventing the intrusion of bacteria. Most fruits are about 75% water, and this makes them highly susceptible to spoilage. To preserve fruit, it must be dehydrated, which—as in the case of the salt in the meat— presents bacteria with a less-than-hospitable environment. Over the years, people have tried a variety of methods for drying fruit, but most of these have a tendency to shrink and harden the fruit. The reason for this is that most drying methods, such as heat from the Sun, are relatively quick and drastic; osmosis, on the other hand, is slower, more moderate—and closer to the behavior of nature. Osmotic dehydration techniques, in fact, result in fruit that can be stored longer than fruit dehydrated by other methods. This in turn makes it possible to provide consumers with a wider variety of fruit throughout the year. Also, the fruit itself tends to maintain more of its flavor and nutritional qualities while keeping out microorganisms. Because osmosis alone can only remove about 50% of the water in most ripe fruits, however, the dehydration process involves a secondary method as well. First the fruit is blanched, or placed briefly in scalding water to stop enzymatic action. Next it is subjected to osmotic dehydration by dipping it in, or spreading it with, a specially made variety of syrup whose sugar draws out the water in the fruit. After this, air drying or vacuum drying completes the process. The resulting product is ready to eat; can be preserved on a shelf under most climatic conditions; and may even be powdered for making confectionery items. Whereas osmotic dehydration of fruit is currently used in many parts of the world, the salt-

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Osmosis

KEY TERMS HYPERTONIC:

Of higher osmotic

pressure. HYPOTONIC:

Of

lower

osmotic

pressure. ISOTONIC:

Of equal osmotic pressure.

The movement of a solvent through a semipermeable membrane from a less concentrated solution to a more concentrated one.

OSMOSIS:

A difference in osmotic pressure that draws water from an area of less osmotic pressure to an area of greater osmotic pressure. OSMOTIC POTENTIAL:

The pressure that builds in a substance as it experiences osmosis, and eventually stops that process. OSMOTIC PRESSURE:

A liquid capable of dissolving or dispersing one or more other substances. SOLVENT:

SOLUTE: A substance capable of being dissolved in a solvent. SOLUTION:

A mixture of solvent and

solute.

curing of meat in brine is largely a thing of the past, due to the introduction of refrigeration. Many poorer families, even in the industrialized world, however, remained without electricity long after it spread throughout most of Europe and North America. John Steinbeck’s Grapes of Wrath (1939) offers a memorable scene in which a contemporary family, the Joads, kill and cure a pig before leaving Oklahoma for California. And a Web site for Walton Feed, an Idaho company specializing in dehydrated foods, offers reminiscences by Canadians whose families were still salt-curing meats in the middle of the twentieth century. Verla Cress of southern Alberta, for instance, offered a recipe from which the following details are drawn.

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First a barrel is filled with a solution containing 2 gal (7.57 l) of hot water and 8 oz (.2268 kg) of salt, or 32 parts hot water to one part salt, as well as a small quantity of vinegar. The pig or cow, which would have just been slaughtered, should then be cut up into what Cress called “ham-sized pieces (about 10-15 lb [5-7 kg]) each.” The pieces are then soaked in the brine barrel for six days, after which the meat is removed, dried, “and put... in flour or gunny sacks to keep the flies away. Then hang it up in a cool dry place to dry. It will keep like this for perhaps six weeks if stored in a cool place during the Summer. Of course, it will keep much longer in the Winter. If it goes bad, you’ll know it!” Cress offered another method, one still used on ham today. Instead of salt, sugar is used in a mixture of 32 oz (.94 l) to 3 gal (11.36 l) of water. After being removed, the meat is smoked—that is, exposed to smoke from a typically aromatic wood such as hickory, in an enclosed barn—for three days. Smoking the meat tends to make it last much longer: four months in the summer, according to Cress. The Walton Feeds Web page included another brine-curing recipe, this one used by the women of the Stirling, Alberta, Church of the Latter-Day Saints in 1973. Also included were reminiscences by Glenn Adamson (born 1915): “...When we butchered a pig, Dad filled a wooden 45-gal (170.34 l) barrel with salt brine. We cut up the pig into maybe eight pieces and put it in the brine barrel. The pork soaked in the barrel for several days, then the meat was taken out, and the water was thrown away.... In the hot summer days after they [the pieces of meat] had dried, they were put in the root cellar to keep them cool. The meat was good for eating two or three months this way.” For thousands of years, people used salt to cure and preserve meat: for instance, the sailing ships that first came to the New World carried on board barrels full of cured meat, which fed sailors on the voyage over. Meat was not the only type of food preserved through the use of salt or brine, which is hypertonic—and thus lethal—to bacteria cells. Among other items packed in brine were fish, olives, and vegetables. Even today, some types of canned fish come to the consumer still packed in brine, as do olives. Another method that survives is the use of

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sugar—which can be just as effective as salt for keeping out bacteria—to preserve fruit in jam.

Reverse Osmosis Given the many ways osmosis is used for preserving food, not to mention its many interactions with water, it should not be surprising to discover that osmosis can also be used for desalination, or turning salt water into drinking water. Actually, it is not osmosis, strictly speaking, but rather reverse osmosis that turns salt water from the ocean—97% of Earth’s water supply—into water that can be used for bathing, agriculture, and in some cases even drinking. When you mix a teaspoon of sugar into a cup of coffee, as mentioned in an earlier illustration, this is a non-reversible process. Short of some highly complicated undertaking—for instance, using ultrasonic sound waves— it would be impossible to separate solute and solvent. Osmosis, on the other hand, can be reversed. This is done by using a controlled external pressure of approximately 60 atmospheres, an atmosphere being equal to the air pressure at sea level—14.7 pounds-per-square-inch (1.013 ⫻ 105 Pa.) In reverse osmosis, this pressure is applied to the area of higher solute concentration—in this case, the seawater. As a result, the pressure in the seawater pushes water molecules into a reservoir of pure water. If performed by someone with a few rudimentary tools and a knowledge of how to provide just the right amount of pressure, it is possible that reverse osmosis could save the life of a shipwreck victim stranded in a location without a fresh water supply. On the other hand, a person in such a situation may be able to absorb sufficient water from fruits and plant life, as Tom Hanks’s character did in the 2001 film Cast Away.

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Companies such as Reverse Osmosis Systems in Atlanta, Georgia, offer a small unit for home or business use, which actually performs the reverse-osmosis process on a small scale. The unit makes use of a process called crossflow, which continually cleans the semipermeable membrane of impurities that have been removed from the water. A small pump provides the pressure necessary to push the water through the membrane. In addition to an under-the-sink model, a reverse osmosis water cooler is also available.

Osmosis

Not only is reverse osmosis used for making water safe, it is also applied to metals in a variety of capacities, not least of which is its use in treating wastewater from electroplating. But there are other metallurgical methods of reverse osmosis that have little to do with water treatment: metal finishing, as well as recycling of metals and chemicals. These processes are highly complicated, but they involve the same principle of removing impurities that governs reverse osmosis. WHERE TO LEARN MORE Francis, Frederick J., editor-in-chief. Encyclopedia of Food Science and Technology. New York: Wiley, 2000. Gardner, Robert. Science Project Ideas About Kitchen Chemistry. Berkeley, N.J.: Enslow Publishers, 2002. Laschish, Uri. “Osmosis, Reverse Osmosis, and Osmotic Pressure: What They Are” (Web site). (February 20, 2001). “Lesson 5: Osmosis” (Web site). (February 20, 2001). Rosenfeld, Sam. Science Experiments with Water. Illustrated by John J. Floherty, Jr. Irvington-on-Hudson, NY: Harvey House, 1965. “Salt-Curing Meat in Brine.” Walton Feed (Web site). (February 20, 2001).

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D I S T I L L AT I O N A N D F I LT R A T I O N Distillation and Filtration

CONCEPT When most people think of chemistry, they think about joining substances together. Certainly, the bonding of elements to form compounds through chemical reactions is an integral component of the chemist’s study; but chemists are also concerned with the separation of substances. Some forms of separation, in which compounds are returned to their elemental form, or in which atoms split off from molecules to yield a compound and a separated element, are complex phenomena that require chemical reactions. But chemists also use simpler physical methods, distillation and filtration, to separate mixtures. Distillation can be used to purify water, make alcohols, or separate petroleum into various components that range from natural gas to gasoline to tar. Filtration also makes possible the separation of gases or liquids from solids, and sewage treatment—a form of filtration—separates liquids, solids, and gases through a series of chemical and physical processes.

HOW IT WORKS Mixtures: Homogeneous and Heterogeneous In contrast to a pure substance, a class that includes only elements and compounds, mixtures constitute a much broader category. Usually, a mixture consists of many compounds mixed together, but it is possible to have a mixture (air is an example) in which elemental substances are combined with compounds. Elements alone can be combined in a mixture, as when copper and zinc are alloyed to make brass. It is also possible to have a mixture of mixtures, as for example

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when milk (a particular type of mixture called an emulsion) is added to coffee. It is sometimes difficult, without studying the chemical aspects involved, to recognize the difference between a mixture and a pure substance. A mixture, however, can be defined as a substance with a variable composition, meaning that if more of one component is added, it does not change the essential character of the mixture. People unconsciously recognize this when they joke, “Would you like a little coffee with your milk?” This comment, made when someone puts a great quantity of milk into a cup of coffee, implies that we generally regard coffee and milk as a solution in which the coffee dissolves the milk. But even if someone made a cup of coffee that was half milk, so that the resulting substance had a vanilla-like color, we would still call it coffee. If the components of a pure substance are altered, however, it becomes something entirely different. Two hydrogen atoms bonded to an oxygen atom create water, but when two hydrogen atoms bond to two oxygen atoms, this is hydrogen peroxide, an altogether different substance. Water at ordinary temperatures does not bubble, whereas hydrogen peroxide does. Hydrogen peroxide boils at a temperature more than 1.5 times the boiling point of water, and potatoes boiled in hydrogen peroxide would be nothing like potatoes boiled in water. Eating them, in fact, could very well be fatal. HOMOGENEOUS VS. HETEROG E N E O U S . Unlike a compound or element,

a mixture can never be broken down to a single type of molecule or atom, yet there are mixtures

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that appear to be the same throughout. These are called homogeneous mixtures, in which the properties and characteristics are the same throughout the mixture. In a heterogeneous mixture, on the other hand, the composition is not the same throughout, and in fact the mixture is separated into regions with varying properties. Chemists of the past sometimes had difficulty distinguishing between homogeneous mixtures—virtually all of which can be described as solutions—and pure substances. Air, for example, appears to be a pure substance; yet there is no such thing as an “air molecule.” Instead, air is composed primarily of nitrogen and oxygen, which appear in molecular rather than atomic form, along with separated atoms of noble gases and two important compounds: carbon dioxide and water. (Water exists in air as a vapor.) There is less probability of confusing a heterogeneous mixture with a pure substance, because one of the defining aspects of a heterogeneous substance is the fact that it is clearly a mixture. When sand is added to a container of water, and the sand sinks to the bottom, it is obvious that there is no “sand-and-water” molecule pervading the entire mixture. Clearly, the upper portion of the container is mostly water, and the lower portion mostly sand.

Distillation may seem like a chemical process, but in fact it is purely physical, because the composition of neither compound—salt or water—has been changed. As for filtration, it is clearly a physical process, just as heterogeneous mixtures are more obviously mixtures and not compounds. Suppose we wanted to separate the heterogeneous mixture we described earlier, of sand in water: all we would need would be a mesh screen through which the liquid could be strained, leaving behind the sand. Despite the apparent simplicity of filtration, it can become rather complicated, as we shall see, in the treatment of sewage to turn it into water that poses no threat to the environment. Furthermore, it should be noted that sometimes these processes—distillation and filtration—are used in tandem. Suppose we had a mixture of sand and salt water taken from the beach, and we wanted to separate the mixture. The first step would involve the simpler process of filtration, separating the sand from the salt water. This would be followed by the separation of pure water and salt through the distillation process.

REAL-LIFE A P P L I C AT I O N S

Separating Mixtures

Distillation

There are two basic processes for separating mixtures, distillation and filtration. In general, these are applied for the separation of homogeneous and heterogeneous mixtures, respectively. Distillation is the use of heat to separate the components of a liquid and/or gas, while filtration is the separation of solids from a fluid (either a gas or a liquid) by allowing the fluid to pass through a filter.

In distillation, the more volatile component of the mixture—that is, the part that is more easily vaporized—is separated from the less volatile portion. With regard to the illustration used above, of separating water from salt, clearly the water is the more volatile portion: its boiling point is much, much lower than that of salt, and the heat required to vaporize the salt is so great that the water would be long since vaporized by the time the salt was affected.

In a solution such as salt water, there are two components: the solvent (the water) and the solute (the salt). These can be separated by distillation in a laboratory using a burner placed under a beaker containing the salt water. As the water is heated, it passes out of the beaker in the form of steam, and travels through a tube cooled by a continual flow of cold water. Inside the tube, the steam condenses to form liquid water, which passes into a second beaker. Eventually, all of the solvent will be distilled from the first beaker, leaving behind the salt that constituted the solute of the original solution.

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Distillation and Filtration

Once the volatile portion has been vaporized, or turned into distillate, it experiences condensation—the cooling of a gas to form a liquid, after which the liquid or condensate is collected. Note that in all these stages of distillation, heat is the agent of separation. This is especially important in the process of fractional distillation, discussed below. The process of removing water vapor from salt water, described earlier, is an example of what is called “single-stage differential distillation.” Sometimes, however—especially when a

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A

WATER FILTRATION PLANT IN

CONTRA COSTA COUNTY, CALIFORNIA. (James A. Sugar/Corbis. Reproduced by permission.)

high level of purity in the resulting condensate is desired—it may be necessary to subject the condensate to multiple processes of distillation to remove additional impurities. This process is called rectification. PU R I F I E D WAT E R A N D E T H A N O L . If distillation can be used to separate

water from salt, obviously it can also be used to separate water from microbes and other impurities through a more detailed process of rectification. Distilled water, purchased for drinking and other purposes, is just one of the more common applications of the distillation process. Another well-known use of this process is the production of ethyl alcohol, made famous (or perhaps infamous) by stories of bootleggers producing

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homemade whiskey from “stills” or distilleries in the woods. Ethanol, the alcohol in alcoholic beverages, is produced by the fermentation of glucose, a sugar found in various natural substances. The choice of substances is a function of the desired product: grapes for wine; barley and hops for beer; juniper berries for gin; potatoes for vodka, and so on. Glucose reacts with yeast, fermenting to yield ethanol, and this reaction is catalyzed by enzymes in the yeast. The reaction continues until the alcohol content reaches a point equal to about 13%. After that point, the yeast enzymes can no longer survive, and this is where the production of beer or wine usually ends. Fortified wine or

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FEW

INDUSTRIES EMPLOY DISTILLATION TO A GREATER DEGREE THAN DOES THE PETROLEUM INDUSTRY.

SHOWN

HERE

IS AN OIL REFINERY. (Mark L. Stephenson/Corbis. Reproduced by permission.)

malt liquor—variations of wine and beer, respectively, that are more than 13% alcohol, or 26 proof—are exceptions. These two products are the result of mixtures between undistilled beer and wine and products of distillation having a higher alcohol content. Distillation constitutes the second stage in the production of alcohol. Often, the fermentation products are subjected to a multistage rectification process, which yields a mixture of up to 95% ethanol and 5% water. This mixture is called an azeotrope, meaning that it will not change composition when distilled further. Usually the production of whiskey or other liquor stops well below the point of yielding an azeotrope, but in

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some states, brands of grain alcohol at 190 proof (95%) are sold. INDUSTRIAL

D I S T I L L AT I O N .

In contrast to ethanol, methanol or “wood alcohol” is a highly toxic substance whose ingestion can lead to blindness or death. It is widely used in industry for applications in adhesives, plastics, and other products, and was once produced by the distillation of wood. In this old production method, wood was heated in the absence of air, such that it did not burn, but rather decomposed into a number of chemicals, a fraction of which were methanol and other alcohols. The distillation of wood alcohol was abandoned for a number of reasons, not least of which was the environmental concern: thou-

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sands of trees had to be cut down for use in an inefficient process yielding only small portions of the desired product. Today, methanol is produced from synthesis gas, itself a product of coal gasification; nonetheless, numerous industrial processes use distillation. Distillation is employed, for instance, to separate the hydrocarbons benzene and toluene, as well as acetone from acetic acid. In nuclear power plants that require quantities of the hydrogen isotope deuterium, the lighter protium isotope, or plain hydrogen, is separated from the heavier deuterium by means of distillation. Few industries, however, employ distillation to a greater degree than the petroleum industry. Through a process known as fractional distillation, oil companies separate hydrocarbons from petroleum, allowing those of lower molecular mass to boil off and be collected first. For instance, at temperatures below 96.8°F (36°C), natural gas separates from petroleum. Other substances, including petroleum ether and naphtha, separate before the petroleum reaches the 156.2°F-165.2°F (69°C-74°C) range, at which point gasoline separates. Fractional distillation continues, with the separation of other substances, all the way up to 959°F (515°C), above which tar becomes the last item to be separated. Petroleum and petroleumproduct companies account for a large portion of the more than 40,000 distillation units in operation across the country. About 95% of all industrial separation processes involve distillation, which consumes large amounts of energy; for that reason, ever more efficient forms of distillation are continually being researched.

Filtration In the simplest form of filtration, described earlier, a suspension (solid particles floating in a liquid) is allowed to pass through filter paper supported in a glass funnel. The filter paper traps the solid particles, allowing a clear solution (the filtrate) to pass through the funnel and into a receiving container.

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An example of the latter occurs in sewage treatment. Here the solid left behind is feces—as undesirable as gold is desirable. Filtration can further be classified as either gaseous or liquid, depending on which of the fluids constitutes the filtrate. Furthermore, the force that moves the fluid through the filter—whether gravity, a vacuum, or pressure—is another defining factor. The type of filter used can also serve to distinguish varieties of filtration. L I Q U I D F I LT RAT I O N . In liquid filtration, such as that applied in sewage treatment, a liquid can be pulled through the filter by gravitational force, as in the examples given. On the other hand, some sort of applied pressure, or a pressure differential created by the existence of a vacuum, can force the liquid through the filter.

Water filtration for purification purposes is often performed by means of gravitation. Usually, water is allowed to run down through a thick layer of granular material such as sand, gravel, or charcoal, which removes impurities. These layers may be several feet thick, in which case the filter is known as a deep-bed filter. Water purification plants may include fine particles of charcoal, known as activated carbon, in the deep-bed filter to absorb unpleasant-smelling gases. For separating smaller volumes of solution, a positive-pressure system—one that uses external pressure to push the liquid through the filter—may be used. Fluid may be introduced under pressure at one end of a horizontal tank, then forced through a series of vertical plates covered with thick filtering cloths that collect solids. In a vacuum filter, a vacuum (an area virtually devoid of matter, including air) is created beneath the solution to be filtered, and as a result, atmospheric pressure pushes the liquid through the filter. When the liquid is virtually freed of impurities, it may be passed through a second variety of filter, known as a clarifying filter. This type of filter is constructed of extremely fine-mesh wires or fibers designed to remove the smallest particles suspended in the liquid. Clarifying filters may also use diatomaceous earth, a finely powdered material produced from the decay of marine organisms.

There are two basic purposes for filtration: either to capture the solid material suspended in the fluid, or to clarify the fluid in which the solid is suspended. An example of the former occurs, for instance, when panning for gold: the water is allowed to pass through a sieve, leaving behind rocks that—the prospector hopes—contain gold.

GAS F I LT RAT I O N . Gas filtration is used in a common appliance, the vacuum cleaner, which passes a stream of dust-filled air

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Distillation and Filtration

KEY TERMS A solution that has been

AZEOTROPE:

subjected through distillation and is still

ing different properties. An example is sand in a container of water.

not fully separated, but cannot be separat-

HOMOGENEOUS:

ed any further by means of distillation. An

a mixture that is the same throughout, as

example is ethanol, which cannot be dis-

for example when sugar is fully dissolved

tilled beyond the point at which it reaches

in water. A solution is a homogenous

a 95% concentration with water.

mixture.

CONDENSATE:

The liquid product

that results from distillation. The cooling of a

CONDENSATION:

gas to form a liquid or condensate. DISTILLATE:

The vapor collected from

the volatile material or materials in distillation. This vapor is then subjected to condensation.

MIXTURE:

A term describing

A substance with a variable

composition, composed of molecules or atoms of differing types. Compare with pure substance. PURE SUBSTANCE:

A substance—

either an element or compound—that has an unvarying composition. This means that by changing the proportions of atoms, the result would be an entirely different

DISTILLATION:

The use of heat to

separate the components of a liquid and/or

substance. Compare with mixture. SOLUTE:

The substance or substances

gas. Generally, distillation is used to sepa-

that are dissolved in a solvent to form a

rate mixtures that are homogeneous.

solution.

FILTRATE:

The liquid or gas separated

from a solid by means of filtration. FILTRATION:

The separation of solids

SOLUTION:

A homogeneous mixture

in which one or more substances is dissolved in an another substance—for exam-

from a fluid (either gas or liquid) by allow-

ple, sugar dissolved in water.

ing the fluid to pass through a filter. Gen-

SOLVENT:

erally, filtration is used to separate mix-

a solute to form a solution.

tures that are heterogeneous.

SUSPENSION:

HETEROGENEOUS:

A term describ-

The substance that dissolves A term that refers to a

mixture in which solid particles are sus-

ing a mixture that is not the same through-

pended in a fluid.

out; rather, it has various regions possess-

VOLATILE:

Easily vaporized.

through a filtering bag inside the machine. The bag traps solid particles, while allowing clean air to pass back out into the room. This is essentially the same principle applied in air filters and even air conditioning and heating systems, which, in addition to regulating temperature, also remove dust, pollen, and other impurities from the air.

Various industries use gas filtration, not only to purify the products released into the atmosphere, but also to filter the air in the workplace, as for instance in a factory room where fine airborne particles are a hazard of production. The gases from power plants that burn coal and oil are often purified by passing them through filtering systems that collect particles.

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Sewage Treatment Distillation and Filtration

Sewage treatment is a complex, multistage process whereby waste water is freed of harmful contents and rendered safe, so that it can be returned to the environment. This is a critical part of modern life, because when people congregate in cities, they generate huge amounts of wastes that contain disease-causing bacteria, viruses, and other microorganisms. Lack of proper waste management in ancient and medieval times led to devastating plagues in cities such as Athens, Rome, and Constantinople. Indeed, one of the factors contributing to the end of the Athenian golden age of the fifth century B.C. was a plague, caused by lack of proper sewage-disposal methods, that felled many of the city’s greatest figures. The horrors created by improper waste management reached their apogee in 1347-51, when microorganisms carried by rats brought about the Black Death, in which Europe’s population was reduced by one-third. AEROBIC AND ANAEROBIC D E CAY. Small amounts of sewage can be dealt

with through aerobic (oxygen-consuming) decay, in which microorganisms such as bacteria and fungi process the toxins in human waste. With larger amounts of waste, however, oxygen is depleted and the decay mechanism must be anaerobic, or carried out in the absence of oxygen. For people who are not connected to a municipal sewage system, and therefore must rely on a septic tank for waste disposal, waste products pass through a tank in which they are subjected to anaerobic decay by varieties of microorganisms that do not require oxygen to survive. From there, the waste goes through the drain field, a network of pipes that allow the water to seep into a deep-bed filter. In the drain field, the waste is subjected to aerobic decay by oxygen-dependant microorganisms before it either filters through the drain pipes into the ground, or is evaporated. M U N I C I PA L WASTE T R E AT M E N T. Municipal waste treatment is a much

360

described in the most general terms here. Essentially, large-scale sewage treatment requires the separation of larger particles first, followed by the separation of smaller particles, as the process continues. Along the way, the sewage is chemically treated as well. The separation of smaller solid particles is aided by aluminum sulfate, which causes these particles to settle to the bottom more rapidly. Through continual processing by filtration, water is eventually clarified of biosolids (that is, feces), but it is still far from being safe. At that point, all the removed biosolids are dried and incinerated; or they may be subjected to additional processes to create fertilizer. The water, or effluent, is further clarified by filtration in a deep-bed filter, and additional air may be introduced to the water to facilitate aerobic decay— for instance, by using algae. Through a long series of such processes, the water is rendered safe to be released back into the environment. However, wastes containing extremely high levels of toxins may require additional or different forms of treatment. WHERE TO LEARN MORE “Case Study: Petroleum Distillation” (Web site). (June 6, 2001). “Classification of Matter” (Web site). _98/smith/smith2. ht ml (June 6, 2001). “Fractional Distillation” (Web site). (June 6, 2001). “Introduction to Distillation” (Web site). (June 6, 2001). Kellert, Stephen B. and Matthew Black. The Encyclopedia of the Environment. New York: Franklin Watts, 1999. Oxlade, Chris. Chemistry. Illustrated by Chris Fairclough. Austin, TX: Raintree Steck-Vaughn, 1999. Seuling, Barbara. Drip! Drop! How Water Gets to Your Tap. Illustrated by Nancy Tobin. New York: Holiday House, 2000. “Sewage Treatment” (Web site). (June 6, 2001).

more involved process, and will only be

Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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S C I E N C E O F E V E RY DAY T H I N G S r e a l - l i f e CHEMISTRY

ORGANIC CHEMISTRY ORGANIC CHEMISTRY P O LY M E R S

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ORGANIC CHEMISTRY

CONCEPT There was once a time when chemists thought “organic” referred only to things that were living, and that life was the result of a spiritual “life force.” While there is nothing wrong with viewing life as having a spiritual component, spiritual matters are simply outside the realm of science, and to mix up the two is as silly as using mathematics to explain love (or vice versa). In fact, the “life force” has a name: carbon, the common denominator in all living things. Not everything that has carbon is living, nor are all the areas studied in organic chemistry—the branch of chemistry devoted to the study of carbon and its compounds—always concerned with living things. Organic chemistry addresses an array of subjects as vast as the number of possible compounds that can be made by strings of carbon atoms. We can thank organic chemistry for much of what makes life easier in the modern age: fuel for cars, for instance, or the plastics found in many of the products used in an average day.

HOW IT WORKS Introduction to Carbon As the element essential to all of life, and hence the basis for a vast field of study, carbon is addressed in its own essay. The Carbon essay, in addition to examining the chemical properties of carbon (discussed below), approaches a number of subjects, such as the allotropes of carbon. These include three crystalline forms (graphite, diamond, and buckminsterfullerene), as well as amorphous carbon. In addition, two oxides of carbon—carbon dioxide and carbon monoxide—are important, in the case of the former, to

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the natural carbon cycle, and in the case of the latter, to industry. Both also pose environmental dangers. The purpose of this summary of the carbon essay is to provide a hint of the complexities involved with this sixth element on the periodic table, the 14th most abundant element on Earth. In the human body, carbon is second only to oxygen in abundance, and accounts for 18% of the body’s mass. Capable of combining in seemingly endless ways, carbon, along with hydrogen, is at the center of huge families of compounds. These are the hydrocarbons, present in deposits of fossil fuels: natural gas, petroleum, and coal. A PROPENSITY FOR LIMITL E SS B O N D I N G . Carbon has a valence

electron configuration of 2s22p2. Likewise all the members of Group 4 on the periodic table (Group 14 in the IUPAC version of the table)— sometimes known as the “carbon family”—have configurations of ns2np2, where n is the number of the period or row the element occupies on the table. There are two elements noted for their ability to form long strings of atoms and seemingly endless varieties of molecules: one is carbon, and the other is silicon, directly below it on the periodic table. Just as carbon is at the center of a vast network of organic compounds, silicon holds the same function in the inorganic realm. It is found in virtually all types of rocks, except the calcium carbonates—which, as their name implies, contain carbon. In terms of known elemental mass, silicon is second only to oxygen in abundance on Earth. Silicon atoms are about one and a half times as large as those of carbon; thus not even silicon can compete with carbon’s ability to form

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Organic Chemistry

“I

JUST WANT TO SAY ONE WORD TO YOU...PLASTICS.”

THESE WORDS WERE UTTERED TO

DUSTIN HOFFMAN’S

SIGNIFYING

THAT PLASTICS WERE THE WAVE OF THE FUTURE,

CHARACTER IN THE

1967

FILM

THE GRADUATE. (The Kobal

Collection. Reproduced by permission.)

an almost limitless array of molecules in various shapes and sizes, and with various chemical properties.

Electronegativity Carbon is further distinguished by its high value of electronegativity, the relative ability of an atom to attract valence electrons. To mention a few basic aspects of chemical bonding, developed at considerably greater length in the Chemical Bonding essay, if two atoms have an electric charge and thus are ions, they form strong ionic bonds. Ionic bonding occurs when a metal bonds with a nonmetal. The other principal type of bond is a covalent bond, in which two uncharged atoms share eight valence electrons. If the electronegativity values of the two elements involved are equal, then they share the electrons equally; but if one element has a higher electronegativity value, the electrons will be more drawn to that element.

364

reactive and most of which have high electronegativity values. If one ignores the noble gases, which are virtually unreactive and occupy the extreme right-hand side of the periodic table, electronegativity values are highest in the upper right-hand side of the table—the location of fluorine—and lowest in the lower left. In other words, the value increases with group or column number (again, leaving out the noble gases in Group 8), and decreases with period or row number.

The electronegativity of carbon is certainly not the highest on the periodic table. That distinction belongs to fluorine, with an electronegativity value of 4.0, which makes it the most reactive of all elements. Fluorine is at the head of Group 7, the halogens, all of which are highly

With an electronegativity of 2.5, carbon ties with sulfur and iodine (a halogen) for sixth place, behind only fluorine; oxygen (3.5); nitrogen and chlorine (3.0); and bromine (2.8). Thus its electronegativity is high, without being too high. Fluorine is not likely to form the long chains for which is carbon is known, simply because its electronegativity is so high, it overpowers other elements with which it comes into contact. In addition, with four valence electrons, carbon is ideally suited to find other elements (or other carbon atoms) for forming covalent bonds according to the octet rule, whereby most elements bond so that they have eight valence electrons.

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Carbon’s Multiple Bonds Carbon—with its four valence electrons—happens to be tetravalent, or capable of bonding to four other atoms at once. It is not necessarily the case that an element has the ability to bond with as many other elements as it has valence electrons; in fact, this is rarely the case. Additionally, carbon is capable of forming not only a single bond, with one pair of shared valence electrons, but a double bond (two pairs) or even a triple bond (three pairs.) Another special property of carbon is its ability to bond in long chains that constitute strings of carbons and other atoms. Furthermore, though sometimes carbon forms a typical molecule (for example, carbon dioxide, or CO2, is just one carbon atom with two oxygens), it is also capable of forming “molecules” that are really not molecules in the way that the word is typically used in chemistry. Graphite, for instance, is just a series of “sheets” of carbon atoms bonded tightly in a hexagonal, or six-sided, pattern, while a diamond is simply a huge “molecule” composed of carbon atoms strung together by covalent bonds.

Organic Chemistry

IT IS VIRTUALLY IMPOSSIBLE FOR AMERICA TO SPEND AN ENTIRE

A PERSON IN MODERN DAY WITHOUT COMING

INTO CONTACT WITH AT LEAST ONE, AND MORE LIKELY

Organic Chemistry

DOZENS, OF PLASTIC PRODUCTS, ALL MADE POSSIBLE BY ORGANIC CHEMISTRY.

Organic chemistry is the study of carbon, its compounds, and their properties. The only major carbon compounds considered inorganic are carbonates (for instance, calcium carbonate, alluded to above, which is one of the major forms of mineral on Earth) and oxides, such as carbon dioxide and carbon monoxide. This leaves a huge array of compounds to be studied, as we shall see. The term “organic” in everyday language connotes “living,” but organic chemistry is involved with plenty of compounds not part of living organisms: petroleum, for instance, is an organic compound that ultimately comes from the decayed bodies of organisms that once were alive. It should be stressed that organic compounds do not have to be produced by living things, or even by things that once were alive; they can be produced artificially in a laboratory. The breakthrough event in organic chemistry came in 1828, when German chemist Friedrich Wöhler (1800-1882) heated a sample of ammonium cyanate (NH4OCN) and converted it to urea (H2N-CO-NH2). Ammonium cyanite is an inorganic material, whereas urea, a waste product in the urine of mammals, is an organic one. “Without benefit of a kidney, a bladder, or a

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HERE, A FASHION MODEL PVC. (Reuters NewMedia

PARADES A DRESS MADE OF

Inc./Corbis. Reproduced by permission.)

dog,” as Wöhler later said, he had managed to transform an inorganic substance into an organic one. Ammonium cyanate and urea are isomers: substances having the same formula, but possessing different chemical properties. Thus they have exactly the same numbers and proportions of atoms, yet these atoms are arranged in different ways. In urea, the carbon forms an organic chain, and in ammonium cyanate, it does not. Thus, to reduce the specifics of organic chemistry even further, this discipline can be said to constitute the study of carbon chains, and ways to rearrange them to create new substances.

REAL-LIFE A P P L I C AT I O N S Organic Chemistry and Modern Life At first glance, the term “organic chemistry” might sound like something removed from

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Organic Chemistry

ANOTHER

BYPRODUCT OF ORGANIC CHEMISTRY: PETROLEUM JELLY. (Laura Dwight/Corbis. Reproduced by permission.)

everyday life, but this could not be further from the truth. The reality of the role played by organic chemistry in modern existence is summed up in a famous advertising slogan used by E. I. du Pont de Nemours and Company (usually referred to as “du Pont”): “Better Things for Better Living Through Chemistry.” Often rendered simply as “Better Living Through Chemistry,” the advertising campaign made its debut in 1938, just as du Pont introduced a revolutionary product of organic chemistry: nylon, the creation of a brilliant young chemist named Wallace Carothers (1896-1937). Nylon, an example of a polymer (discussed below), started a revolution in plastics that was still unfolding three decades later, in 1967. That was the year of the film The Graduate, which included a famous interchange between the character of Benjamin Braddock (Dustin Hoffman) and an adult named Mr. McGuire (Walter Brooke): • Mr. McGuire: I just want to say one word to you... just one word. • Benjamin Braddock: Yes, sir. • Mr. McGuire: Are you listening? • Benjamin Braddock: Yes, sir, I am. • Mr. McGuire: Plastics.

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The meaning of this interchange was that plastics were the wave of the future, and that an intelligent young man such as Ben should invest his energies in this promising new field. Instead, Ben puts his attention into other things, quite removed from “plastics,” and much of the plot revolves around his revolt against what he perceives as the “plastic” (that is, artificial) character of modern life. In this way, The Graduate spoke for a whole generation that had become ambivalent concerning “better living through chemistry,” a phrase that eventually was perceived as ironic in view of concerns about the environment and the many artificial products that make up modern life. Responding to this ambivalence, du Pont dropped the slogan in the late 1970s; yet the reality is that people truly do enjoy “better living through chemistry”—particularly organic chemistry. A P P L I CAT I O N S O F O R GA N I C C H E M I S T RY. What would the world be like

without the fruits of organic chemistry? First, it would be necessary to take away all the various forms of rubber, vitamins, cloth, and paper made from organically based compounds. Aspirins and all types of other drugs; preservatives that keep food from spoiling; perfumes and toiletries; dyes

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and flavorings—all these things would have to go as well. Synthetic fibers such as nylon—used in everything from toothbrushes to parachutes— would be out of the picture if it were not for the enormous progress made by organic chemistry. The same is true of plastics or polymers in general, which have literally hundreds upon hundreds of applications. Indeed, it is virtually impossible for a person in twenty-first century America to spend an entire day without coming into contact with at least one, and more likely dozens, of plastic products. Car parts, toys, computer housings, Velcro fasteners, PVC (polyvinyl chloride) plumbing pipes, and many more fixtures of modern life are all made possible by plastics and polymers. Then there is the vast array of petrochemicals that power modern civilization. Best-known among these is gasoline, but there is also coal, still one of the most significant fuels used in electrical power plants, as well as natural gas and various other forms of oil used either directly or indirectly in providing heat, light, and electric power to homes. But the influence of petrochemicals extends far beyond their applications for fuel. For instance, the roofing materials and tar that (quite literally) keep a roof over people’s heads, protecting them from sun and rain, are the product of petrochemicals—and ultimately, of organic chemistry.

Hydrocarbons Carbon, together with other elements, forms so many millions of organic compounds that even introductory textbooks on organic chemistry consist of many hundreds of pages. Fortunately, it is possible to classify broad groupings of organic compounds. The largest and most significant is that class of organic compounds known as hydrocarbons—chemical compounds whose molecules are made up of nothing but carbon and hydrogen atoms.

Theoretically, there is no limit to the number of possible hydrocarbons. Not only does carbon form itself into apparently limitless molecular shapes, but hydrogen is a particularly good partner. It has the smallest atom of any element on the periodic table, and therefore it can bond to one of carbon’s valence electrons without getting in the way of the other three.

Organic Chemistry

There are two basic varieties of hydrocarbon, distinguished by shape: aliphatic and aromatic. The first of these forms straight or branched chains, as well as rings, while the second forms only benzene rings, discussed below. Within the aliphatic hydrocarbons are three varieties: those that form single bonds (alkanes), double bonds (alkenes), and triple bonds (alkynes.) A L KA N E S . The alkanes are also known as saturated hydrocarbons, because all the bonds not used to make the skeleton itself are filled to their capacity (that is, saturated) with hydrogen atoms. The formula for any alkane is CnH2n+2, where n is the number of carbon atoms. In the case of a linear, unbranched alkane, every carbon atom has two hydrogen atoms attached, but the two end carbon atoms each have an extra hydrogen.

What follows are the names and formulas for the first eight normal, or unbranched, alkanes. Note that the first four of these received common names before their structures were known; from C5 onward, however, they were given names with Greek roots indicating the number of carbon atoms (e.g., octane, a reference to “eight.”)

Every molecule in a hydrocarbon is built upon a “skeleton” composed of carbon atoms, either in closed rings or in long chains. The chains may be straight or branched, but in each case—rings or chains, straight chains or branched ones—the carbon bonds not used in tying the carbon atoms together are taken up by hydrogen atoms.

Methane (CH4) Ethane (C2H6) Propane (C3H8) Butane (C4H10) Pentane (C5H12) Hexane (C6H14) Heptane (C7H16) Octane (C8H18) The reader will undoubtedly notice a number of familiar names on this list. The first four, being the lowest in molecular mass, are gases at room temperature, while the heavier ones are oily liquids. Alkanes even heavier than those on this list tend to be waxy solids, an example being paraffin wax, for making candles. It should be noted that from butane on up, the alkanes have numerous structural isomers, depending on

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whether they are straight or branched, and these isomers have differing chemical properties. Branched alkanes are named by indicating the branch attached to the principal chain. Branches, known as substituents, are named by taking the name of an alkane and replacing the suffix with yl—for example, methyl, ethyl, and so on. The general term for an alkane which functions as a substituent is alkyl. Cycloalkanes are alkanes joined in a closed loop to form a ring-shaped molecule. They are named by using the names above, with cyclo- as a prefix. These start with propane, or rather cyclopropane, which has the minimum number of carbon atoms to form a closed shape: three atoms, forming a triangle. A L K E N E S A N D A L KY N E S . The names of the alkenes, hydrocarbons that contain one or more double bonds per molecule, are parallel to those of the alkanes, but the family ending is -ene. Likewise they have a common formula: CnH2n. Both alkenes and alkynes, discussed below, are unsaturated—in other words, some of the carbon atoms in them are free to form other bonds. Alkenes with more than one double bond are referred to as being polyunsaturated.

As with the alkenes, the names of alkynes (hydrocarbons containing one or more triple bonds per molecule) are parallel to those of the alkanes, only with the replacement of the suffix -yne in place of -ane. The formula for alkenes is CnH2n-2. Among the members of this group are acetylene, or C2H2, used for welding steel. Plastic polystyrene is another important product from this division of the hydrocarbon family. A R O M AT I C

HYDROCARBONS.

Aromatic hydrocarbons, despite their name, do not necessarily have distinctive smells. In fact the name is a traditional one, and today these compounds are defined by the fact that they have benzene rings in the middle. Benzene has a formula C6H6, and a benzene ring is usually represented as a hexagon (the six carbon atoms and their attached hydrogen atoms) surrounding a circle, which represents all the bonding electrons as though they were everywhere in the molecule at once.

368

the vast hydrocarbon network is trinitrotoluene, or TNT. Naphthalene is derived from coal tar, and used in the synthesis of other compounds. A crystalline solid with a powerful odor, it is found in mothballs and various deodorantdisinfectants. P E T R O C H E M I CA L S . As for petrochemicals, these are simply derivatives of petroleum, itself a mixture of alkanes with some alkenes, as well as aromatic hydrocarbons. Through a process known as fractional distillation, the petrochemicals of the lowest molecular mass boil off first, and those having higher mass separate at higher temperatures.

Among the products derived from the fractional distillation of petroleum are the following, listed from the lowest temperature range (that is, the first material to be separated) to the highest: natural gas; petroleum ether, a solvent; naphtha, a solvent (used for example in paint thinner); gasoline; kerosene; fuel for heating and diesel fuel; lubricating oils; petroleum jelly; paraffin wax; and pitch, or tar. A host of other organic chemicals, including various drugs, plastics, paints, adhesives, fibers, detergents, synthetic rubber, and agricultural chemicals, owe their existence to petrochemicals. Obviously, petroleum is not just for making gasoline, though of course this is the first product people think of when they hear the word “petroleum.” Not all hydrocarbons in gasoline are desirable. Straight-chain or normal heptane, for instance, does not fire smoothly in an internal-combustion engine, and therefore disrupts the engine’s rhythm. For this reason, it is given a rating of zero on a scale of desirability, while octane has a rating of 100. This is why gas stations list octane ratings at the pump: the higher the presence of octane, the better the gas is for one’s automobile.

Hydrocarbon Derivatives

In this group are products such as naphthalene, toluene, and dimethyl benzene. These last two are used as solvents, as well as in the synthesis of drugs, dyes, and plastics. One of the more famous (or infamous) products in this part of

With carbon and hydrogen as the backbone, the hydrocarbons are capable of forming a vast array of hydrocarbon derivatives by combining with other elements. These other elements are arranged in functional groups—an atom or group of atoms whose presence identifies a specific family of compounds. Below we will briefly discuss some of the principal hydrocarbon deriv-

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atives, which are basically hydrocarbons with the addition of other molecules or single atoms. Alcohols are oxygen-hydrogen molecules wedded to hydrocarbons. The two most important commercial types of alcohol are methanol, or wood alcohol; and ethanol, which is found in alcoholic beverages, such as beer, wine, and liquor. Though methanol is still known as “wood alcohol,” it is no longer obtained by heating wood, but rather by the industrial hydrogenation of carbon monoxide. Used in adhesives, fibers, and plastics, it can also be applied as a fuel. Ethanol, too, can be burned in an internalcombustion engine, when combined with gasoline to make gasohol. Another significant alcohol is cholesterol, found in most living organisms. Though biochemically important, cholesterol can pose a risk to human health. Aldehydes and ketones both involve a double-bonded carbon-oxygen molecule, known as a carbonyl group. In a ketone, the carbonyl group bonds to two hydrocarbons, while in an aldehyde, the carbonyl group is always at the end of a hydrocarbon chain. Therefore, instead of two hydrocarbons, there is always a hydrocarbon and at least one other hydrogen bonded to the carbon atom in the carbonyl. One prominent example of a ketone is acetone, used in nail polish remover. Aldehydes often appear in nature—for instance, as vanillin, which gives vanilla beans their pleasing aroma. The ketones carvone and camphor impart the characteristic flavors of spearmint leaves and caraway seeds. CA R B OX Y L I C AC I D S A N D E S T E R S . Carboxylic acids all have in common

what is known as a carboxyl group, designated by the symbol -COOH. This consists of a carbon atom with a double bond to an oxygen atom, and a single bond to another oxygen atom that is, in turn, wedded to a hydrogen. All carboxylic acids can be generally symbolized by RCOOH, with R as the standard designation of any hydrocarbon. Lactic acid, generated by the human body, is a carboxylic acid: when a person overexerts, the muscles generate lactic acid, resulting in a feeling of fatigue until the body converts the acid to water and carbon dioxide. Another example of a carboxylic acid is butyric acid, responsible in part for the smells of rancid butter and human sweat. When a carboxylic acid reacts with an alcohol, it forms an ester. An ester has a structure similar to that described for a carboxylic acid,

S C I E N C E O F E V E RY DAY T H I N G S

with a few key differences. In addition to its bonds (one double, one single) with the oxygen atoms, the carbon atom is also attached to a hydrocarbon, which comes from the carboxylic acid. Furthermore, the single-bonded oxygen atom is attached not to a hydrogen, but to a second hydrocarbon, this one from the alcohol. One well-known ester is acetylsalicylic acid—better known as aspirin. Esters, which are a key factor in the aroma of various types of fruit, are often noted for their pleasant smell.

Organic Chemistry

Polymers Polymers are long, stringy molecules made of smaller molecules called monomers. They appear in nature, but thanks to Carothers—a tragic figure, who committed suicide a year before Nylon made its public debut—as well as other scientists and inventors, synthetic polymers are a fundamental part of daily life. The structure of even the simplest polymer, polyethylene, is far too complicated to discuss in ordinary language, but must be represented by chemical symbolism. Indeed, polymers are a subject unto themselves, but it is worth noting here just how many products used today involve polymers in some form or another. Polyethylene, for instance, is the plastic used in garbage bags, electrical insulation, bottles, and a host of other applications. A variation on polyethylene is Teflon, used not only in nonstick cookware, but also in a number of other devices, such as bearings for low-temperature use. Polymers of various kinds are found in siding for houses, tire tread, toys, carpets and fabrics, and a variety of other products far too lengthy to enumerate. WHERE TO LEARN MORE Blashfield, Jean F. Carbon. Austin, TX: Raintree SteckVaughn, 1999. “Carbon.” Xrefer (Web site). (May 30, 2001). Chemistry Help Online for Students (Web site). May 30, 2001). Knapp, Brian J. Carbon Chemistry. Illustrated by David Woodroffe. Danbury, CT: Grolier Educational, 1998. Loudon, G. Marc. Organic Chemistry. Menlo Park, CA: Benjamin/Cummings, 1988.

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Organic Chemistry

KEY TERMS Hydrocarbons that form single bonds. Alkanes are also called saturated hydrocarbons.

ALKANES:

ALKENES:

Hydrocarbons that form

double bonds. A general term for an alkane that functions as a substituent.

ALKYL:

ALKYNES:

Hydrocarbons that form

triple bonds. Different versions of the same element, distinguished by molecular structure.

ALLOTROPES:

AMORPHOUS:

Having no definite

structure. A type of chemical bonding in which two atoms share valence electrons.

COVALENT BONDING:

A term describing the distribution of valence electrons that takes place in chemical bonding for most elements, which end up with eight valence electrons. OCTET RULE:

The study of carbon, its compounds, and their properties. (Some carbon-containing compounds, most notably oxides and carbonates, are not considered organic.)

ORGANIC CHEMISTRY:

A term describing a hydrocarbon in which each carbon is already bound to four other atoms. Alkanes are saturated hydrocarbons. SATURATED:

A form of bonding in which two atoms share one pair of valence electrons. Carbon is also capable of double bonds and triple bonds.

SINGLE BOND:

A term describing a type of solid in which the constituent parts have a simple and definite geometric arrangement repeated in all directions.

SUBSTITUENTS:

A form of bonding in which two atoms share two pairs of valence electrons. Carbon is also capable of single bonds and triple bonds.

TETRAVALENT:

CRYSTALLINE:

DOUBLE BOND:

The relative ability of an atom to attract valence electrons. ELECTRONEGATIVITY:

An atom or group of atoms whose presence identifies a specific family of compounds. FUNCTIONAL GROUPS:

Any chemical compound whose molecules are made up of nothing but carbon and hydrogen atoms. HYDROCARBON:

Families of compounds formed by the joining of hydrocarbons with various functional groups. HYDROCARBON DERIVATIVES:

Substances having the same chemical formula, but that are different

ISOMERS:

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chemically due to disparities in the arrangement of atoms.

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Branches of alkanes, named by taking the name of an alkane and replacing the suffix with yl—for example, methyl, ethyl, and so on.

Capable of bonding to four other elements. A form of bonding in which two atoms share three pairs of valence electrons. Carbon is also capable of single bonds and double bonds.

TRIPLE BOND:

A term describing a hydrocarbon in which the carbons involved in a multiple bond (a double bond or triple bond) are free to bond with other atoms. Alkenes and alkynes are both unsaturated. UNSATURATED:

Electrons that occupy the highest principal energy level in an atom. These are the electrons involved in chemical bonding.

VALENCE ELECTRONS:

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“Organic Chemistry” (Web site). (May 30, 2001). “Organic Chemistry.” Frostburg State University Chemistry Helper (Web site). (May 30, 2001).

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Sparrow, Giles. Carbon. New York: Benchmark Books, 1999.

Organic Chemistry

Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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P O LY M E R S Polymers

CONCEPT Formed from hydrocarbons, hydrocarbon derivatives, or sometimes from silicon, polymers are the basis not only for numerous natural materials, but also for most of the synthetic plastics that one encounters every day. Polymers consist of extremely large, chain-like molecules that are, in turn, made up of numerous smaller, repeating units called monomers. Chains of polymers can be compared to paper clips linked together in long strands, and sometimes cross-linked to form even more durable chains. Polymers can be composed of more than one type of monomer, and they can be altered in other ways. Likewise they are created by two different chemical processes, and thus are divided into addition and condensation polymers. Among the natural polymers are wool, hair, silk, rubber, and sand, while the many synthetic polymers include nylon, synthetic rubber, Teflon, Formica, Dacron, and so forth. It is very difficult to spend a day without encountering a natural polymer—even if hair is removed from the list—but in the twenty-first century, it is probably even harder to avoid synthetic polymers, which have collectively revolutionized human existence.

HOW IT WORKS

The similarities between these two are so great, in fact, that some chemists speak of Group 4 (Group 14 in the IUPAC system) on the periodic table as the “carbon family.” Both carbon and silicon have the ability to form long chains of atoms that include bonds with other elements. The heavier elements of this “family,” however (most notably lead), are made of atoms too big to form the vast array of chains and compounds for which silicon and carbon are noted. Indeed, not even silicon—though it is at the center of an enormous range of inorganic compounds—can compete with carbon in its ability to form arrangements of atoms in various shapes and sizes, and hence to participate in an almost limitless array of compounds. The reason, in large part, is that carbon atoms are much smaller than those of silicon, and thus can bond to one another and still leave room for other bonds.

Polymers can be defined as large, typically chainlike molecules composed of numerous smaller, repeating units known as monomers. There are numerous varieties of monomers, and since these can be combined in different ways to form polymers, there are even more of the latter.

Carbon is such an important element that an entire essay in this book is devoted to it, while a second essay discusses organic chemistry, the study of compounds containing carbon. In the present context, there will be occasional references to non-carbon (that is, silicon) polymers, but the majority of our attention will be devoted to hydrocarbon and hydrocarbon-derivative polymers, which most of us know simply as “plastics.”

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Polymers of Silicon and Carbon

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The name “polymer” does not, in itself, define the materials that polymers contain. A handful of polymers, such as natural sand or synthetic silicone oils and rubbers, are built around silicon. However, the vast majority of polymers center around an element that occupies a position just above silicon on the periodic table: carbon.

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Polymers

COTTON

IS AN EXAMPLE OF A NATURAL POLYMER. (Carl Corey/Corbis. Reproduced by permission.)

Organic Chemistry As explained in the essay on Organic Chemistry, chemists once defined the term “organic” as relating only to living organisms; the materials that make them up; materials derived from them; and substances that come from formerly living organisms. This definition, which more or less represents the everyday meaning of “organic,” includes a huge array of life forms and materials: humans, all other animals, insects, plants, microorganisms, and viruses; all substances that make up their structures (for example, blood, DNA, and proteins); all products that come from them (a list diverse enough to encompass everything from urine to honey); and all materials derived from the bodies of organisms that were once alive (paper, for instance, or fossil fuels). As broad as this definition is, it is not broad enough to represent all the substances addressed by organic chemistry—the study of carbon, its compounds, and their properties. All living or once-living things do contain carbon; however, organic chemistry is also concerned with carboncontaining materials—for instance, the synthetic plastics we will discuss in this essay—that have never been part of a living organism.

carbon itself is found in other compounds not considered organic: oxides such as carbon dioxide and monoxide, as well as carbonates, most notably calcium carbonate or limestone. In other words, as broad as the meaning of “organic” is, it still does not encompass all substances containing carbon.

Hydrocarbons As for hydrocarbons, these are chemical compounds whose molecules are made up of nothing but carbon and hydrogen atoms. Every molecule in a hydrocarbon is built upon a “skeleton” of carbon atoms, either in closed rings or in long chains, which are sometimes straight and sometimes branched. Theoretically, there is no limit to the number of possible hydrocarbons: not only does carbon form itself into seemingly limitless molecular shapes, but hydrogen is a particularly good partner. It is the smallest atom of any element on the periodic table, and therefore it can bond to one of carbon’s four valence electrons without getting in the way of the other three.

It should be noted that while organic chemistry involves only materials that contain carbon,

There are many, many varieties of hydrocarbon, classified generally as aliphatic hydrocarbons (alkanes, alkenes, and alkynes) and aromatic hydrocarbons, the latter being those that con-

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Polymers

MODERN

APPLIANCES CONTAIN NUMEROUS EXAMPLES OF SYNTHETIC POLYMERS, FROM THE FLOORING TO THE COUN-

TERTOPS TO VIRTUALLY ALL APPLIANCES. (Scott Roper/Corbis. Reproduced by permission.)

374

tain a benzene ring. By means of a basic alteration in the shape or structure of a hydrocarbon, it is possible to create new varieties. Thus, as noted above, the number of possible hydrocarbons is essentially unlimited.

REAL-LIFE A P P L I C AT I O N S

Certain hydrocarbons are particularly useful, one example being petroleum, a term that refers to a wide array of hydrocarbons. Among these is an alkane that goes by the name of octane (C8H18), a preferred ingredient in gasoline. Hydrocarbons can be combined with various functional groups (an atom or group of atoms whose presence identifies a specific family of compounds) to form hydrocarbon derivatives such as alcohols and esters.

Many polymers exist in nature. Among these are silk, cotton, starch, sand, and asbestos, as well as the incredibly complex polymers known as RNA (ribonucleic acid) and DNA (deoxyribonucleic acid), which hold genetic codes. The polymers discussed in this essay, however, are primarily of the synthetic kind. Artificial polymers include such plastics (defined below) as polyethylene, styrofoam, and Saran wrap; fibers such as nylon, Dacron (polyester), and rayon; and other materials such as Formica, Teflon, and PVC pipe.

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Types of Polymers and Polymerization

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As noted earlier, most polymers are formed from monomers either of hydrocarbon or hydrocarbon derivatives. The most basic synthetic monomer is ethylene (C2H4), a name whose -ene ending identifies it as an alkene, a hydrocarbon formed by double bonds between carbon atoms. Another alkene hydrocarbon monomer is butadiene, whose formula is C4H6. This is an example of the fact that the formula of a compound does not tell the whole story: on paper, the difference between these two appears to be merely a matter of two extra atoms each of carbon and hydrogen. In fact, butadiene’s structure is much more complex. Still more complex is styrene, which includes a benzene ring. Several other monomers involve other elements: chloride, in vinyl chloride; nitrogen, in acrylonitrile; and fluorine, in tetrafluoroethylene. It is not necessary, in the present context, to keep track of all of these substances, which in any case represent just some of the more prominent among a wide variety of synthetic monomers. A good high-school or college chemistry textbook (either general chemistry or organic chemistry) should provide structural representations of these common monomers. Such representations will show, for instance, the vast differences between purely hydrocarbon monomers such as ethylene, propylene, styrene, and butadiene. When combined into polymers, the monomers above form the basis for a variety of useful and familiar products. Once the carbon double bonds in tetrafluoroethylene (C2F4) are broken, they form the polymer known as Teflon, used in the coatings of cooking utensils, as well as in electrical insulation and bearings. Vinyl chloride breaks its double bonds to form polyvinyl chloride, better known as PVC, a material used for everything from plumbing pipe to toys to Saran wrap. Styrene, after breaking its double bonds, forms polystyrene, used in containers and thermal insulation. P O LY M E R I Z AT I O N . Note that several times in the preceding paragraph, there was a reference to the breaking of carbon double bonds. This is often part of one variety of polymerization, the process whereby monomers join to form polymers. If monomers of a single type join, the resulting polymer is called a homopolymer, but if the polymer consists of more than one type of monomer, it is known as a copolymer. This joining may take place by one of two

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Polymers

A

SYNTHETIC POLYMER KNOWN AS KEVLAR IS USED IN

THE CONSTRUCTION OF BULLETPROOF VESTS FOR LAWENFORCEMENT OFFICERS. (Anna Clopet/Corbis. Reproduced by

permission.)

processes. The first of these, addition polymerization, is fairly simple: monomers add themselves to one another, usually breaking double bonds in the process. This results in the creation of a polymer and no other products. Much more complex is the process known as condensation polymerization, in which a small molecule called a dimer is formed as monomers join. The specifics are too complicated to discuss in any detail, but a few things can be said here about condensation polymerization. The monomers in condensation polymerization must be bifunctional, meaning that they have a functional group at each end. When characteristic structures at the ends of the monomers react to one another by forming a bond, they create a dimer, which splits off from the polymer. The products of condensation polymerization are thus not only the polymer itself, but also a dimer, which may be water, hydrochloric acid (HCl), or some other substance.

A Plastic World A DAY I N T H E L I F E . Though “plastic” has a number of meanings in everyday life,

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and in society at large (as we shall see), the scientific definition is much more specific. Plastics are materials, usually organic, that can be caused to flow under certain conditions of heat and pressure, and thus to assume a desired shape when the pressure and temperature conditions are withdrawn. Most plastics are made of polymers. Every day, a person comes into contact with dozens, if not hundreds, of plastics and polymers. Consider a day in the life of a hypothetical teenage girl. She gets up in the morning, brushes her teeth with a toothbrush made of nylon, then opens a shower door—which is likely to be plastic rather than glass—and steps into a molded plastic shower or bathtub. When she gets out of the shower, she dries off with a towel containing a polymer such as rayon, perhaps while standing on tile that contains plastics, or polymers. She puts on makeup (containing polymers) that comes in plastic containers, and later blowdries her hair with a handheld hair dryer made of insulated plastic. Her clothes, too, are likely to contain synthetic materials made of polymers. When she goes to the kitchen for breakfast, she will almost certainly walk on flooring with a plastic coating. The countertops may be of formica, a condensation polymer, while it is likely that virtually every appliance in the room will contain plastic. If she opens the refrigerator to get out a milk container, it too will be made of plastic, or of paper with a thin plastic coating. Much of the packaging on the food she eats, as well as sandwich bags and containers for storing food, is also made of plastic. And so it goes throughout the day. The phone she uses to call a friend, the computer she sits at to check her e-mail, and the stereo in her room all contain electrical components housed in plastic. If she goes to the gym, she may work out in Gore-tex, a fabric containing a very thin layer of plastic with billions of tiny pores, so that it lets through water vapor (that is, perspiration) without allowing the passage of liquid water. On the way to the health club, she will ride in a car that contains numerous plastic molds in the steering wheel and dashboard. If she plays a compact disc—itself a thin wafer of plastic coated with metal—she will pull it out of a plastic jewel case. Finally, at night, chances are she will sleep in sheets, and with a pillow, containing synthetic polymers.

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A S I L E N T R E V O LU T I O N . The scenario described above—a world surrounded by polymers, plastics, and synthetic materials— represents a very recent phenomenon. “Before the 1930s,” wrote John Steele Gordon in an article about plastics for American Heritage, “almost everything people saw or handled was made of materials that had been around since ancient times: wood, stone, metal, and animal and plant fibers.” All of that changed in the era just before World War II, thanks in large part to a brilliant young American chemist named Wallace Carothers (1896-1937).

By developing nylon for E. I. du Pont de Nemours and Company (known simply as “DuPont” or “du Pont”), Carothers and his colleagues virtually laid the foundation for modern polymer chemistry—a field that employs more chemists than any other. These men created what Gordon called a “materials revolution” by introducing the world to polymers and plastics, which are typically made of polymers. Yet as Gordon went on to note, “It has been a curiously silent revolution.... When we think of the scientific triumphs of [the twentieth century], we think of nuclear physics, medicine, space exploration, and the computer. But all these developments would have been much impeded, in some cases impossible, without... plastics. And yet ‘plastic’ remains, as often as not, a term of opprobrium.” A M B I VA L E N C E T O WA R D P LAS T I CS . Gordon was alluding to a cultural atti-

tude discussed in the essay on Organic Chemistry: the association of plastics, a physical material developed by chemical processes, with the condition—spiritual, moral, and intellectual—of being “plastic” or inauthentic. This was symbolized in a famous piece of dialogue about plastics from the 1967 movie The Graduate, in which a nonplussed Ben Braddock (Dustin Hoffman) listens as one of his parents’ friends advises him to invest his future in plastics. As Gordon noted, “however intergenerationally challenged that half-drunk friend of Dustin Hoffman’s parents may have been... he was right about the importance of the materials revolution in the twentieth century.” One aspect of society’s ambivalence over plastics relates to very genuine concerns about the environment. Most synthetic polymers are made from petroleum, a nonrenewable resource;

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but this is not the greatest environmental danger that plastics present. Most plastics are not biodegradable: though made of organic materials, they do not contain materials that will decompose and eventually return to the ground. Nor is there anything in plastics to attract microorganisms, which, by assisting in the decomposition of organic materials, help to facilitate the balance of decay and regeneration necessary for life on Earth. Efforts are underway among organic chemists in the research laboratories of corporations and other institutions to develop biodegradable plastics that will speed up the decomposition of materials in the polymers—a process that normally takes decades. Until such replacement polymers are developed, however, the most environmentally friendly solution to the problem of plastics is recycling. Today only about 1% of plastics are recycled, while the rest goes into waste dumps, where they account for 30% of the volume of trash. Long before environmental concerns came to the forefront, however, people had begun almost to fear plastics as a depersonalizing aspect of modern life. It seemed that in a given day, a person touched fewer and fewer things that came directly from the natural environment: the “wood, stone, metal, and animal and plant fibers” to which Gordon alluded. Plastics seemed to have made human life emptier; yet the truth of the matter—including the fact that plastics add more than they take away from the landscape of our world—is much more complex.

The Plastics Revolution

developed from cellulose, a substance found in the cell walls of plants—a much less appealing name, “Parkesine.” Hyatt, who used celluloid to make smooth, hard, round billiard balls (thereby winning the contest) took out a patent for the process involved in making the material he had dubbed “Celluloid,” with a capital C.

Polymers

Though the Celluloid made by Hyatt’s process was flammable (as was Parkesine), it proved highly successful as a product when he introduced it in 1869. He marketed it successfully for use in items such as combs and baby rattles, and Celluloid sales received a powerful boost after photography pioneer George Eastman (1854-1932) chose the material for use in the development of film. Eventually, Celluloid would be applied in motion-picture film, and even today, the adjective “celluloid” is sometimes used in relation to the movies. Actually, Celluloid (which can be explosive in large quantities) was phased out in favor of “safety film,” or cellulose acetate, beginning in 1924. Two important developments in the creation of synthetic polymers occurred at the turn of the century. One was the development of Galalith, an ivory-like substance made from formaldehyde and milk, by German chemist Adolf Spitteler. An even more important innovation happened in 1907, when Belgian-American chemist Leo Baekeland (1863-1944) introduced Bakelite. The latter, created in a reaction between phenol and formaldehyde, was a hard, black plastic that proved an excellent insulator. It soon found application in making telephones and household appliances, and by the 1920s, chemists had figured out how to add pigments to Bakelite, thus introducing the public to colored plastics.

Though the introduction of plastics is typically associated with the twentieth century, in fact the “materials revolution” surrounding plastics began in 1865. That was the year when English chemist Alexander Parkes (1813-1890) produced the first plastic material, celluloid. Parkes could have become a rich man from his invention, but he was not a successful marketer. Instead, the man who enjoyed the first commercial success in plastics was—not surprisingly—an American, inventor John Wesley Hyatt (1837-1920).

S Y N T H E T I C R U B B E R. Throughout these developments, chemists had only a vague understanding of polymers, but by the 1930s, they had come to accept the model of polymers as large, flexible, chain-like molecules. One of the most promising figures in the emerging field of polymer chemistry was Carothers, who in 1926 left a teaching post at Harvard University to accept a position as director of the polymer research laboratory at DuPont.

Responding to a contest in which a billiardball manufacturer offered $10,000 to anyone who could create a substitute for ivory, which was extremely costly, Hyatt turned to Parkes’s celluloid. Actually, Parkes had given his creation—

Among the first problems Carothers tackled was the development of synthetic rubber. Natural rubber had been known for many centuries when English chemist Joseph Priestley (17331804) gave it its name because he used it to rub

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out pencil marks. In 1839, American inventor Charles Goodyear (1800-1860) accidentally discovered a method for making rubber more durable, after he spilled a mixture of rubber and sulfur onto a hot stove. Rather than melting, the rubber bonded with the sulfur to form a much stronger but still elastic product, and Goodyear soon patented this process under the name vulcanization. Natural rubber, nonetheless, had many undesirable properties, and hence DuPont put Carothers to the task of developing a substitute. The result was neoprene, which he created by adding a chlorine atom to an acetylene derivative. Neoprene was stronger, more durable, and less likely to become brittle in cold weather than natural rubber. It would later prove an enormous boost to the Allied war effort, after the Japanese seized the rubber plantations of Southeast Asia in 1941. N Y LO N . Had neoprene, which Carothers developed in 1931, been the extent of his achievements, he would still be remembered by science historians. However, his greatest creation still lay ahead of him. Studying the properties of silk, he became convinced that he could develop a more durable compound that could replicate the properties of silk at a much lower cost.

Carothers was not alone in his efforts, as Gordon showed in his account of events at the DuPont laboratories: One day, an assistant, Julian Hill, noticed that when he stuck a glass stirring rod into a gooey mass at the bottom of a beaker the researchers had been investigating, he could draw out threads from it, the polymers forming spontaneously as he pulled. When Carothers was absent one day, Hill and his colleagues decided to see how far they could go with pulling threads out of goo by having one man hold the beaker while another ran down the hall with the glass rod. A very long, silk-like thread was produced. Realizing what they had on their hands, DuPont devoted $27 million to the research efforts of Carothers and his associates at the lab, and in 1937, Carothers presented his boss with the results, saying “Here is your synthetic textile fabric.” DuPont introduced the material, nylon, to the American public the following year with one of the most famous advertising campaigns of all time: “Better Things for Better Living Through Chemistry.”

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The product got an additional boost through exposure at the 1939 World’s Fair. When DuPont put 4,000 pairs of nylon stockings on the market, they sold in a matter of hours. A few months later, four million pairs sold in New York City in a single day. Women stood in line to buy stockings of nylon, a much better (and less expensive) material for that purpose than silk—but they did not have long to enjoy it. During World War II, all nylon went into making war materials such as parachutes, and nylon did not become commercially available again until 1946. As Gordon noted, Carothers would surely have won the Nobel Prize in chemistry for his work—“but Nobel prizes go only to living recipients....” Carothers had married in 1936, and by early 1937, his wife Helen was pregnant. (Presumably, he was unaware of the fact that he was about to become a father.) Though highly enthusiastic about his work, Carothers was always shy and withdrawn, and in Gordon’s words, “he had few outlets other than work.” He was, however, a talented singer, as was his closest sibling, Isobel, a radio celebrity. Her death in January 1937 sent him into a bout of depression, and on April 29, he killed himself with a dose of cyanide. Seven months later, on November 27, Helen gave birth to a daughter, Jane. H O W P LAS T I CS H AV E E N H A N C E D L I F E . Despite his tragic end,

Carothers had brought much good to the world by sparking enormous interest in polymer research and plastics. Over the years that followed, polymer chemists developed numerous products that had applications in a wide variety of areas. Some, such as polyester—a copolymer of terephthalic acid and ethylene—seemed to fit the idea of “plastics” as ugly, inauthentic, and even dehumanizing. During the 1970s, clothes of polyester became fashionable, but by the early 1980s, there was a public backlash against synthetics, and in favor of natural materials. Yet even as the public rejected synthetic fabrics for everyday wear, Gore-tex and other synthetics became popular for outdoor and workout clothing. At the same time, the polyester that many regarded as grotesque when used in clothing was applied in making safer beverage bottles. The American Plastics Council dramatized this in a 1990s commercial that showed a few seconds in the life of a mother. Her child takes a soft-

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KEY TERMS ADDITION

form

of

POLYMERIZATION:

polymerization

in

A

which

monomers having at least one double bond or triple bond simply add to one another, forming a polymer and no other products. Compare to condensation polymerization. Hydrocarbons that contain

ALKENES:

double bonds. COPOLYMER:

tional groups. MONOMERS:

Small, individual sub-

units, often built of hydrocarbons, that join together to form polymers. ORGANIC:

A term referring to any

compound that contains carbon, except for oxides such as carbon dioxide, or car-

A polymer composed of

more than one type of monomer. DIMER:

joining of hydrocarbons with various func-

bonates such as calcium carbonate (i.e., limestone).

A molecule formed by the ORGANIC CHEMISTRY:

joining of two monomers. DOUBLE BOND:

The study

of carbon, its compounds, and their

A form of bonding

properties.

in which two atoms share two pairs of valence electrons. Carbon is noted for its ability to form double bonds, as for instance in many hydrocarbons. FUNCTIONAL GROUPS:

An atom or

group of atoms whose presence identifies a specific family of compounds. When combined with hydrocarbons, various

PLASTICS:

Materials, usually organic,

that can be caused to flow under certain conditions of heat and pressure, and thus to assume a desired shape when the pressure and temperature conditions are withdrawn. Plastics are usually made up of polymers. The

process

functional groups form hydrocarbon

POLYMERIZATION:

derivatives.

whereby monomers join to form polymers.

HOMOPOLYMER:

A polymer that

consists of only one type of monomer. HYDROCARBON:

Any chemical com-

POLYMERS:

Large, typically chain-like

molecules composed of numerous smaller, repeating units known as monomers.

pound whose molecules are made up of

VALENCE ELECTRONS:

nothing but carbon and hydrogen atoms.

that occupy the highest principal energy

HYDROCARBON

level in an atom. These are the electrons

DERIVATIVES:

Families of compounds formed by the

drink bottle out of the refrigerator and drops it, and the mother cringes at what she thinks she is about to see next: glass shattering around her child. But she is remembering the way things were when she was a child, when soft drinks still came in glass bottles: instead, the plastic bottle bounces harmlessly.

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Electrons

involved in chemical bonding.

Of course, such dramatizations may seem a bit self-serving to critics of plastic, but the fact remains that plastics enhance—and in some cases even preserve—life. Kevlar, for instance, enhances life when it is used in making canoes for recreation; when used to make a bulletproof vest, it can save the life of a law-enforcement officer. Mylar, a form of polyester, enhances life

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when used to make a durable child’s balloon— but this highly nonreactive material also saves lives when it is applied to make replacement human blood vessels, or even replacement skin for burn victims.

Recycling As mentioned above, plastics—for all their benefits—do pose a genuine environmental threat, due to the fact that the polymers break down much more slowly than materials from living organisms. Hence the need not only to develop biodegradable plastics, but also to work on more effective means of recycling. One of the challenges in the recycling arena is the fact that plastics come in a variety of grades. Different catalysts are used to make polymers that possess different properties, with varying sizes of molecules, and in chains that may be linear, branched, or cross-linked. Long chains of 10,000 or more monomers can be packed closely to form a hard, tough plastic known as highdensity polyethylene or HDPE, used for bottles containing milk, soft drinks, liquid soap, and other products. On the other hand, shorter, branched chains of about 500 ethylene monomers each produce a much less dense plastic, low-density polyethylene or LDPE. This is used for plastic food or garment bags, spray bottles, and so forth. There are other grades of plastic as well. In some forms of recycling, plastics of all varieties are melted down together to yield a cheap, low-grade product known as “plastic lumber,” used in materials such as landscaping timbers, or in making park benches. In order to achieve higher-grade recycled plastics, the materials need to be separated, and to facilitate this, recycling codes have been developed. Many plastic materials sold today are stamped with a recycling code number between 1 and 6, identifying specific varieties of plastic. These can be melted

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or ground according to type at recycling centers, and reprocessed to make more plastics of the same grade. To meet the environmental challenges posed by plastics, polymer chemists continue to research new methods of recycling, and of using recycled plastic. One impediment to recycling, however, is the fact that most state and local governments do not make it convenient, for instance by arranging trash pickup for items that have been separated into plastic, paper, and glass products. Though ideally private recycling centers would be preferable to government-operated recycling, few private companies have the financial resources to make recycling of plastics and other materials practical. WHERE TO LEARN MORE Bortz, Alfred B. Superstuff!: Materials That Have Changed Our Lives. New York: Franklin Watts, 1990. Ebbing, Darrell D.; R. A. D. Wentworth; and James P. Birk. Introductory Chemistry. Boston: Houghton Mifflin, 1995. Galas, Judith C. Plastics: Molding the Past, Shaping the Future. San Diego, CA: Lucent Books, 1995. Gordon, John Steele. “Plastics, Chance, and the Prepared Mind.” American Heritage, July-August 1998, p. 18. Mebane, Robert C. and Thomas R. Rybolt. Plastics and Polymers. Illustrated by Anni Matsick. New York: Twenty-First Century Books, 1995. Plastics.com (Web site). (June 5, 2001). “Plastics 101.” Plastics Resource (Web site). (June 5, 2001). “Polymers.” University of Illinois, Urbana/Champaign, Materials Science and Technology Department (Web site). (June 5, 2001). “Polymers and Liquid Crystals.” Case Western Reserve University (Web site). (June 5, 2001). Zumdahl, Steven S. Introductory Chemistry: A Foundation, 4th ed. Boston: Houghton Mifflin, 2000.

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SCIENCEOF

EVERYDAY THINGS

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SCIENCEOF

EVERYDAY THINGS volume 2: REAL-LIFE PHYSICS

edited by NEIL SCHLAGER written by JUDSON KNIGHT A SCHLAGER INFORMATION GROUP BOOK

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S C I E N C E O F E V E RY DAY T H I N G S VOLUME 2

Re a l - L i f e p h y s i c s

A Schlager Information Group Book Neil Schlager, Editor Written by Judson Knight Gale Group Staff Kimberley A. McGrath, Senior Editor Maria Franklin, Permissions Manager Margaret A. Chamberlain, Permissions Specialist Shalice Shah-Caldwell, Permissions Associate Mary Beth Trimper, Manager, Composition and Electronic Prepress Evi Seoud, Assistant Manager, Composition and Electronic Prepress Dorothy Maki, Manufacturing Manager Rita Wimberley, Buyer Michelle DiMercurio, Senior Art Director Barbara J. Yarrow, Manager, Imaging and Multimedia Content Robyn V. Young, Project Manager, Imaging and Multimedia Content Leitha Etheridge-Sims, Mary K. Grimes, and David G. Oblender, Image Catalogers Pam A. Reed, Imaging Coordinator Randy Bassett, Imaging Supervisor Robert Duncan, Senior Imaging Specialist Dan Newell, Imaging Specialist While every effort has been made to ensure the reliability of the information presented in this publication, Gale Group does not guarantee the accuracy of the data contained herein. Gale accepts no payment for listing, and inclusion in the publication of any organization, agency, institution, publication, service, or individual does not imply endorsement of the editors and publisher. Errors brought to the attention of the publisher and verified to the satisfaction of the publisher will be corrected in future editions. The paper used in the publication meets the minimum requirements of American National Standard for Information Sciences—Permanence Paper for Printed Library Materials, ANSI Z39.48-1984. This publication is a creative work fully protected by all applicable copyright laws, as well as by misappropriation, trade secret, unfair competition, and other applicable laws. The authors and editors of this work have added value to the underlying factual material herein through one or more of the following: unique and original selection, coordination, expression, arrangement, and classification of the information. All rights to this publication will be vigorously defended. Copyright © 2002 Gale Group, 27500 Drake Road, Farmington Hills, Michigan 48331-3535 No part of this book may be reproduced in any form without permission in writing from the publisher, except by a reviewer who wishes to quote brief passages or entries in connection with a review written for inclusion in a magazine or newspaper. ISBN

0-7876-5631-3 (set) 0-7876-5632-1 (vol. 1) 0-7876-5633-X (vol. 2)

0-7876-5634-8 (vol. 3) 0-7876-5635-6 (vol. 4)

Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

Library of Congress Cataloging-in-Publication Data Knight, Judson. Science of everyday things / written by Judson Knight, Neil Schlager, editor. p. cm. Includes bibliographical references and indexes. Contents: v. 1. Real-life chemistry – v. 2 Real-life physics. ISBN 0-7876-5631-3 (set : hardcover) – ISBN 0-7876-5632-1 (v. 1) – ISBN 0-7876-5633-X (v. 2) 1. Science–Popular works. I. Schlager, Neil, 1966-II. Title. Q162.K678 2001 500–dc21

2001050121

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CONTENTS

Introduction.............................................v Advisory Board ......................................vii

THERMODYNAMICS Gas Laws . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183

GENERAL CONCEPTS

Molecular Dynamics. . . . . . . . . . . . . . . . . . . 192

Frame of Reference. . . . . . . . . . . . . . . . . . . . . . . 3 Kinematics and Dynamics . . . . . . . . . . . . . . . 13 Density and Volume . . . . . . . . . . . . . . . . . . . . 21 Conservation Laws . . . . . . . . . . . . . . . . . . . . . 27

Structure of Matter . . . . . . . . . . . . . . . . . . . . 203

KINEMATICS AND PARTICLE DYNAMICS Momentum . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 Centripetal Force . . . . . . . . . . . . . . . . . . . . . . . 45 Friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 Laws of Motion . . . . . . . . . . . . . . . . . . . . . . . . 59 Gravity and Gravitation . . . . . . . . . . . . . . . . . 69 Projectile Motion . . . . . . . . . . . . . . . . . . . . . . . 78 Torque. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 FLUID MECHANICS

Thermodynamics . . . . . . . . . . . . . . . . . . . . . 216 Heat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227 Temperature . . . . . . . . . . . . . . . . . . . . . . . . . 236 Thermal Expansion. . . . . . . . . . . . . . . . . . . . 245 WAVE MOTION AND OSCILLATION Wave Motion . . . . . . . . . . . . . . . . . . . . . . . . . 255 Oscillation . . . . . . . . . . . . . . . . . . . . . . . . . . . 263 Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 Resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 Interference . . . . . . . . . . . . . . . . . . . . . . . . . . 286 Diffraction . . . . . . . . . . . . . . . . . . . . . . . . . . . 294 Doppler Effect . . . . . . . . . . . . . . . . . . . . . . . . 301

Fluid Mechanics. . . . . . . . . . . . . . . . . . . . . . . . 95 Aerodynamics . . . . . . . . . . . . . . . . . . . . . . . . 102 Bernoulli’s Principle . . . . . . . . . . . . . . . . . . . 112 Buoyancy . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

SOUND

STATICS

Ultrasonics. . . . . . . . . . . . . . . . . . . . . . . . . . . 319

Statics and Equilibrium . . . . . . . . . . . . . . . . 133 Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140 Elasticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148 WORK AND ENERGY

Acoustics . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311

LIGHT AND ELECTROMAGNETISM Magnetism. . . . . . . . . . . . . . . . . . . . . . . . . . . 331 Electromagnetic Spectrum . . . . . . . . . . . . . . 340 Light. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 354

Mechanical Advantage and Simple Machines . . . . . . . . . . . . . . . . . . 157 Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

S C I E N C E O F E V E RY DAY T H I N G S

Luminescence . . . . . . . . . . . . . . . . . . . . . . . . 365 General Subject Index ......................373

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INTRODUCTION

Overview of the Series Welcome to Science of Everyday Things. Our aim is to explain how scientific phenomena can be understood by observing common, real-world events. From luminescence to echolocation to buoyancy, the series will illustrate the chief principles that underlay these phenomena and explore their application in everyday life. To encourage cross-disciplinary study, the entries will draw on applications from a wide variety of fields and endeavors. Science of Everyday Things initially comprises four volumes: Volume 1: Real-Life Chemistry Volume 2: Real-Life Physics Volume 3: Real-Life Biology Volume 4: Real-Life Earth Science Future supplements to the series will expand coverage of these four areas and explore new areas, such as mathematics.

Arrangement of Real Life Physics This volume contains 40 entries, each covering a different scientific phenomenon or principle. The entries are grouped together under common categories, with the categories arranged, in general, from the most basic to the most complex. Readers searching for a specific topic should consult the table of contents or the general subject index.

• How It Works Explains the principle or theory in straightforward, step-by-step language. • Real-Life Applications Describes how the phenomenon can be seen in everyday events. • Where to Learn More Includes books, articles, and Internet sites that contain further information about the topic. Each entry also includes a “Key Terms” section that defines important concepts discussed in the text. Finally, each volume includes numerous illustrations, graphs, tables, and photographs. In addition, readers will find the comprehensive general subject index valuable in accessing the data.

About the Editor, Author, and Advisory Board Neil Schlager and Judson Knight would like to thank the members of the advisory board for their assistance with this volume. The advisors were instrumental in defining the list of topics, and reviewed each entry in the volume for scientific accuracy and reading level. The advisors include university-level academics as well as high school teachers; their names and affiliations are listed elsewhere in the volume. N E I L S C H LAG E R is the president of

• Concept Defines the scientific principle or theory around which the entry is focused.

Schlager Information Group Inc., an editorial services company. Among his publications are When Technology Fails (Gale, 1994); How Products Are Made (Gale, 1994); the St. James Press Gay and Lesbian Almanac (St. James Press, 1998); Best Literature By and About Blacks (Gale,

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Within each entry, readers will find the following rubrics:

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2000); Contemporary Novelists, 7th ed. (St. James Press, 2000); and Science and Its Times (7 vols., Gale, 2000-2001). His publications have won numerous awards, including three RUSA awards from the American Library Association, two Reference Books Bulletin/Booklist Editors’ Choice awards, two New York Public Library Outstanding Reference awards, and a CHOICE award for best academic book. Judson Knight is a freelance writer, and author of numerous books on subjects ranging from science to history to music. His work on science titles includes Science, Technology, and Society, 2000 B.C.-A.D. 1799 (U*X*L, 2002), as well as extensive contributions to Gale’s seven-volume Science and Its Times (2000-2001). As a writer on history, Knight has published Middle Ages Reference Library (2000), Ancient

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Civilizations (1999), and a volume in U*X*L’s African American Biography series (1998). Knight’s publications in the realm of music include Parents Aren’t Supposed to Like It (2001), an overview of contemporary performers and genres, as well as Abbey Road to Zapple Records: A Beatles Encyclopedia (Taylor, 1999). His wife, Deidre Knight, is a literary agent and president of the Knight Agency. They live in Atlanta with their daughter Tyler, born in November 1998.

Comments and Suggestions Your comments on this series and suggestions for future editions are welcome. Please write: The Editor, Science of Everyday Things, Gale Group, 27500 Drake Road, Farmington Hills, MI 48331.

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ADVISORY BO T IATR LD E

William E. Acree, Jr. Professor of Chemistry, University of North Texas Russell J. Clark Research Physicist, Carnegie Mellon University Maura C. Flannery Professor of Biology, St. John’s University, New York John Goudie Science Instructor, Kalamazoo (MI) Area Mathematics and Science Center Cheryl Hach Science Instructor, Kalamazoo (MI) Area Mathematics and Science Center Michael Sinclair Physics instructor, Kalamazoo (MI) Area Mathematics and Science Center Rashmi Venkateswaran Senior Instructor and Lab Coordinator, University of Ottawa Ottawa, Ontario, Canada

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S C I E N C E O F E V E RY DAY T H I N G S real-life Physics

GENERAL CONCEPTS FRAME OF REFERENCE K I N E M AT I C S A N D D Y N A M I C S DENSITY AND VOLUME C O N S E R VAT I O N L A W S

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FRAME OF REFERENCE

CONCEPT Among the many specific concepts the student of physics must learn, perhaps none is so deceptively simple as frame of reference. On the surface, it seems obvious that in order to make observations, one must do so from a certain point in space and time. Yet, when the implications of this idea are explored, the fuller complexities begin to reveal themselves. Hence the topic occurs at least twice in most physics textbooks: early on, when the simplest principles are explained—and near the end, at the frontiers of the most intellectually challenging discoveries in science.

HOW IT WORKS There is an old story from India that aptly illustrates how frame of reference affects an understanding of physical properties, and indeed of the larger setting in which those properties are manifested. It is said that six blind men were presented with an elephant, a creature of which they had no previous knowledge, and each explained what he thought the elephant was. The first felt of the elephant’s side, and told the others that the elephant was like a wall. The second, however, grabbed the elephant’s trunk, and concluded that an elephant was like a snake. The third blind man touched the smooth surface of its tusk, and was impressed to discover that the elephant was a hard, spear-like creature. Fourth came a man who touched the elephant’s legs, and therefore decided that it was like a tree trunk. However, the fifth man, after feeling of its tail, disdainfully announced that the elephant was nothing but a frayed piece of rope. Last of all, the sixth blind man, standing beside the elephant’s slowly flapping ear, felt of the ear itself and

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determined that the elephant was a sort of living fan. These six blind men went back to their city, and each acquired followers after the manner of religious teachers. Their devotees would then argue with one another, the snake school of thought competing with adherents of the fan doctrine, the rope philosophy in conflict with the tree trunk faction, and so on. The only person who did not join in these debates was a seventh blind man, much older than the others, who had visited the elephant after the other six. While the others rushed off with their separate conclusions, the seventh blind man had taken the time to pet the elephant, to walk all around it, to smell it, to feed it, and to listen to the sounds it made. When he returned to the city and found the populace in a state of uproar between the six factions, the old man laughed to himself: he was the only person in the city who was not convinced he knew exactly what an elephant was like.

Understanding Frame of Reference The story of the blind men and the elephant, within the framework of Indian philosophy and spiritual beliefs, illustrates the principle of syadvada. This is a concept in the Jain religion related to the Sanskrit word syat, which means “may be.” According to the doctrine of syadvada, no judgment is universal; it is merely a function of the circumstances in which the judgment is made. On a complex level, syadvada is an illustration of relativity, a topic that will be discussed later; more immediately, however, both syadvada and the story of the blind men beautifully illus-

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trate the ways that frame of reference affects perceptions. These are concerns of fundamental importance both in physics and philosophy, disciplines that once were closely allied until each became more fully defined and developed. Even in the modern era, long after the split between the two, each in its own way has been concerned with the relationship between subject and object. These two terms, of course, have numerous definitions. Throughout this book, for instance, the word “object” is used in a very basic sense, meaning simply “a physical object” or “a thing.” Here, however, an object may be defined as something that is perceived or observed. As soon as that definition is made, however, a flaw becomes apparent: nothing is just perceived or observed in and of itself—there has to be someone or something that actually perceives or observes. That something or someone is the subject, and the perspective from which the subject perceives or observes the object is the subject’s frame of reference. A M E R I CA A N D C H I N A : F RA M E O F R E F E R E N C E I N P RAC T I C E . An

old joke—though not as old as the story of the blind men—goes something like this: “I’m glad I wasn’t born in China, because I don’t speak Chinese.” Obviously, the humor revolves around the fact that if the speaker were born in China, then he or she would have grown up speaking Chinese, and English would be the foreign language. The difference between being born in America and speaking English on the one hand—even if one is of Chinese descent—or of being born in China and speaking Chinese on the other, is not just a contrast of countries or languages. Rather, it is a difference of worlds—a difference, that is, in frame of reference. Indeed, most people would see a huge distinction between an English-speaking American and a Chinese-speaking Chinese. Yet to a visitor from another planet—someone whose frame of reference would be, quite literally, otherworldly—the American and Chinese would have much more in common with each other than either would with the visitor.

A landing in San Francisco would create a falsely inflated impression regarding the number of Asian Americans or Americans of Pacific Island descent, who actually make up only a small portion of the national population. The same would be true if one first arrived in Arizona or New Mexico, where the Native American population is much higher than for the nation as a whole. There are numerous other examples to be made in the same vein, all relating to the visitors’ impressions of the population, economy, climate, physical features, and other aspects of a specific place. Without consulting some outside reference point—say, an almanac or an atlas—it would be impossible to get an accurate picture of the entire country. The principle is the same as that in the story of the blind men, but with an important distinction: an elephant is an example of an identifiable species, whereas the United States is a unique entity, not representative of some larger class of thing. (Perhaps the only nation remotely comparable is Brazil, also a vast land settled by outsiders and later populated by a number of groups.) Another important distinction between the blind men story and the United States example is the fact that the blind men were viewing the elephant from outside, whereas the visitor to America views it from inside. This in turn reflects a difference in frame of reference relevant to the work of a scientist: often it is possible to view a process, event, or phenomenon from outside; but sometimes one must view it from inside—which is more challenging.

Frame of Reference in Science

Now imagine that the visitor from outer space (a handy example of someone with no preconceived ideas) were to land in the United States. If

Philosophy (literally, “love of knowledge”) is the most fundamental of all disciplines: hence, most persons who complete the work for a doctorate receive a “doctor of philosophy” (Ph.D.) degree. Among the sciences, physics—a direct offspring of philosophy, as noted earlier—is the most fun-

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the visitor landed in New York City, Chicago, or Los Angeles, he or she would conclude that America is a very crowded, fast-paced country in which a number of ethnic groups live in close proximity. But if the visitor first arrived in Iowa or Nebraska, he or she might well decide that the United States is a sparsely populated land, economically dependent on agriculture and composed almost entirely of Caucasians.

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damental, and frame of reference is among its most basic concepts.

Frame of Reference

Hence, it is necessary to take a seemingly backward approach in explaining how frame of reference works, examining first the broad applications of the principle and then drawing upon its specific relation to physics. It makes little sense to discuss first the ways that physicists apply frame of reference, and only then to explain the concept in terms of everyday life. It is more meaningful to relate frame of reference first to familiar, or at least easily comprehensible, experiences—as has been done. At this point, however, it is appropriate to discuss how the concept is applied to the sciences. People use frame of reference every day— indeed, virtually every moment—of their lives, without thinking about it. Rare indeed is the person who “walks a mile in another person’s shoes”—that is, someone who tries to see events from the viewpoint of another. Physicists, on the other hand, have to be acutely aware of their frame of reference. Moreover, they must “rise above” their frame of reference in the sense that they have to take it into account in making calculations. For physicists in particular, and scientists in general, frame of reference has abundant “real-life applications.”

LINES

OF

LONGITUDE

ON

EARTH

ARE

MEASURED

AGAINST THE LINE PICTURED HERE: THE IAN” RUNNING THROUGH

“PRIME MERIDGREENWICH, ENGLAND. AN

IMAGINARY LINE DRAWN THROUGH THAT SPOT MARKS THE Y-AXIS FOR ALL VERTICAL COORDINATES ON

EARTH, 0° ALONG THE X-AXIS, WHICH IS THE EQUATOR. THE PRIME MERIDIAN, HOWEVER, IS AN ARBITRARY STANDARD THAT DEPENDS ON ONE’S FRAME OF REFERENCE. (Photograph by Dennis di Cicco/Corbis. ReproWITH A VALUE OF

duced by permission.)

REAL-LIFE A P P L I C AT I O N S Points and Graphs There is no such thing as an absolute frame of reference—that is, a frame of reference that is fixed, and not dependent on anything else. If the entire universe consisted of just two points, it would be impossible (and indeed irrelevant) to say which was to the right of the other. There would be no right and left: in order to have such a distinction, it is necessary to have a third point from which to evaluate the other two points. As long as there are just two points, there is only one dimension. The addition of a third point—as long as it does not lie along a straight line drawn through the first two points—creates two dimensions, length and width. From the frame of reference of any one point, then, it is possible to say which of the other two points is to the right. S C I E N C E O F E V E RY DAY T H I N G S

Clearly, the judgment of right or left is relative, since it changes from point to point. A more absolute judgment (but still not a completely absolute one) would only be possible from the frame of reference of a fourth point. But to constitute a new dimension, that fourth point could not lie on the same plane as the other three points—more specifically, it should not be possible to create a single plane that encompasses all four points. Assuming that condition is met, however, it then becomes easier to judge right and left. Yet right and left are never fully absolute, a fact easily illustrated by substituting people for points. One may look at two objects and judge which is to the right of the other, but if one stands on one’s head, then of course right and left become reversed. Of course, when someone is upside-down, the correct orientation of left and right is still VOLUME 2: REAL-LIFE PHYSICS

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fairly obvious. In certain situations observed by physicists and other scientists, however, orientation is not so simple. It then becomes necessary to assign values to various points, and for this, scientists use tools such as the Cartesian coordinate system. C O O R D I N AT E S

AND

AX E S .

Though it is named after the French mathematician and philosopher René Descartes (15961650), who first described its principles, the Cartesian system owes at least as much to Pierre de Fermat (1601-1665). Fermat, a brilliant French amateur mathematician—amateur in the sense that he was not trained in mathematics, nor did he earn a living from that discipline—greatly developed the Cartesian system. A coordinate is a number or set of numbers used to specify the location of a point on a line, on a surface such as a plane, or in space. In the Cartesian system, the x-axis is the horizontal line of reference, and the y-axis the vertical line of reference. Hence, the coordinate (0, 0) designates the point where the x- and y-axes meet. All numbers to the right of 0 on the x-axis, and above 0 on the y-axis, have a positive value, while those to the left of 0 on the x-axis, or below 0 on the y-axis have a negative value. This version of the Cartesian system only accounts for two dimensions, however; therefore, a z-axis, which constitutes a line of reference for the third dimension, is necessary in three-dimensional graphs. The z-axis, too, meets the x- and yaxes at (0, 0), only now that point is designated as (0, 0, 0). In the two-dimensional Cartesian system, the x-axis equates to “width” and the y-axis to “height.” The introduction of a z-axis adds the dimension of “depth”—though in fact, length, width, and height are all relative to the observer’s frame of reference. (Most representations of the three-axis system set the x- and y-axes along a horizontal plane, with the z-axis perpendicular to them.) Basic studies in physics, however, typically involve only the x- and y-axes, essential to plotting graphs, which, in turn, are integral to illustrating the behavior of physical processes. T H E T R I P L E P O I N T. For instance, there is a phenomenon known as the “triple point,” which is difficult to comprehend unless one sees it on a graph. For a chemical compound such as water or carbon dioxide, there is a point at which it is simultaneously a liquid, a solid, and

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a vapor. This, of course, seems to go against common sense, yet a graph makes it clear how this is possible. Using the x-axis to measure temperature and the y-axis pressure, a number of surprises become apparent. For instance, most people associate water as a vapor (that is, steam) with very high temperatures. Yet water can also be a vapor—for example, the mist on a winter morning—at relatively low temperatures and pressures, as the graph shows. The graph also shows that the higher the temperature of water vapor, the higher the pressure will be. This is represented by a line that curves upward to the right. Note that it is not a straight line along a 45° angle: up to about 68°F (20°C), temperature increases at a somewhat greater rate than pressure does, but as temperature gets higher, pressure increases dramatically. As everyone knows, at relatively low temperatures water is a solid—ice. Pressure, however, is relatively high: thus on a graph, the values of temperatures and pressure for ice lie above the vaporization curve, but do not extend to the right of 32°F (0°C) along the x-axis. To the right of 32°F, but above the vaporization curve, are the coordinates representing the temperature and pressure for water in its liquid state. Water has a number of unusual properties, one of which is its response to high pressures and low temperatures. If enough pressure is applied, it is possible to melt ice—thus transforming it from a solid to a liquid—at temperatures below the normal freezing point of 32°F. Thus, the line that divides solid on the left from liquid on the right is not exactly parallel to the y-axis: it slopes gradually toward the y-axis, meaning that at ultra-high pressures, water remains liquid even though it is well below the freezing point. Nonetheless, the line between solid and liquid has to intersect the vaporization curve somewhere, and it does—at a coordinate slightly above freezing, but well below normal atmospheric pressure. This is the triple point, and though “common sense” might dictate that a thing cannot possibly be solid, liquid, and vapor all at once, a graph illustrating the triple point makes it clear how this can happen.

Numbers In the above discussion—and indeed throughout this book—the existence of the decimal, or base-

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(y-axis) pressure

(liquid) (solid)

triple point

(x-axis)

THIS CARTESIAN

(vapor)

temperature

COORDINATE GRAPH SHOWS HOW A SUBSTANCE SUCH AS WATER COULD EXPERIENCE A TRIPLE

POINT—A POINT AT WHICH IT IS SIMULTANEOUSLY A LIQUID, A SOLID, AND A VAPOR.

10, numeration system is taken for granted. Yet that system is a wonder unto itself, involving a complicated interplay of arbitrary and real values. Though the value of the number 10 is absolute, the expression of it (and its use with other numbers) is relative to a frame of reference: one could just as easily use a base-12 system. Each numeration system has its own frame of reference, which is typically related to aspects of the human body. Thus throughout the course of history, some societies have developed a base2 system based on the two hands or arms of a person. Others have used the fingers on one hand (base-5) as their reference point, or all the fingers and toes (base-20). The system in use throughout most of the world today takes as its frame of reference the ten fingers used for basic counting. C O E F F I C I E N T S . Numbers, of course, provide a means of assigning relative values to a variety of physical characteristics: length, mass, force, density, volume, electrical charge, and so on. In an expression such as “10 meters,” the numeral 10 is a coefficient, a number that serves as a measure for some characteristic or property. A coefficient may also be a factor against which other values are multiplied to provide a desired result.

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For instance, the figure 3.141592, better known as pi (π), is a well-known coefficient used in formulae for measuring the circumference or area of a circle. Important examples of coefficients in physics include those for static and sliding friction for any two given materials. A coefficient is simply a number—not a value, as would be the case if the coefficient were a measure of something.

Standards of Measurement Numbers and coefficients provide a convenient lead-in to the subject of measurement, a practical example of frame of reference in all sciences— and indeed, in daily life. Measurement always requires a standard of comparison: something that is fixed, against which the value of other things can be compared. A standard may be arbitrary in its origins, but once it becomes fixed, it provides a frame of reference. Lines of longitude, for instance, are measured against an arbitrary standard: the “Prime Meridian” running through Greenwich, England. An imaginary line drawn through that spot marks the line of reference for all longitudinal measures on Earth, with a value of 0°. There is nothing special about Greenwich in any profound scientific sense; rather, its place of impor-

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tance reflects that of England itself, which ruled the seas and indeed much of the world at the time the Prime Meridian was established. The Equator, on the other hand, has a firm scientific basis as the standard against which all lines of latitude are measured. Yet today, the coordinates of a spot on Earth’s surface are given in relation to both the Equator and the Prime Meridian. C A L I B R AT I O N . Calibration is the process of checking and correcting the performance of a measuring instrument or device against the accepted standard. America’s preeminent standard for the exact time of day, for instance, is the United States Naval Observatory in Washington, D.C. Thanks to the Internet, people all over the country can easily check the exact time, and correct their clocks accordingly.

There are independent scientific laboratories responsible for the calibration of certain instruments ranging from clocks to torque wrenches, and from thermometers to laser beam power analyzers. In the United States, instruments or devices with high-precision applications—that is, those used in scientific studies, or by high-tech industries—are calibrated according to standards established by the National Institute of Standards and Technology (NIST). T H E VA L U E O F S TA N D A R D I Z AT I O N T O A S O C I E T Y. Standardiza-

tion of weights and measures has always been an important function of government. When Ch’in Shih-huang-ti (259-210 B.C.) united China for the first time, becoming its first emperor, he set about standardizing units of measure as a means of providing greater unity to the country—thus making it easier to rule.

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More importantly, it increased the cost of transporting freight from East to West. Russia also used the old Julian calendar, as opposed to the Gregorian calendar adopted throughout much of Western Europe after 1582. Thus October 25, 1917, in the Julian calendar of old Russia translated to November 7, 1917 in the Gregorian calendar used in the West. That date was not chosen arbitrarily: it was then that Communists, led by V. I. Lenin, seized power in the weakened former Russian Empire. METHODS S TA N DA R D S .

OF

DETERMINING

It is easy to understand, then, why governments want to standardize weights and measures—as the U.S. Congress did in 1901, when it established the Bureau of Standards (now NIST) as a nonregulatory agency within the Commerce Department. Today, NIST maintains a wide variety of standard definitions regarding mass, length, temperature, and so forth, against which other devices can be calibrated. Note that NIST keeps on hand definitions rather than, say, a meter stick or other physical model. When the French government established the metric system in 1799, it calibrated the value of a kilogram according to what is now known as the International Prototype Kilogram, a platinum-iridium cylinder housed near Sèvres in France. In the years since then, the trend has moved away from such physical expressions of standards, and toward standards based on a constant figure. Hence, the meter is defined as the distance light travels in a vacuum (an area of space devoid of air or other matter) during the interval of 1/299,792,458 of a second.

More than 2,000 years later, another empire—Russia—was negatively affected by its failure to adjust to the standards of technologically advanced nations. The time was the early twentieth century, when Western Europe was moving forward at a rapid pace of industrialization. Russia, by contrast, lagged behind—in part because its failure to adopt Western standards put it at a disadvantage.

M E T R I C V S . B R I T I S H . Scientists almost always use the metric system, not because it is necessarily any less arbitrary than the British or English system (pounds, feet, and so on), but because it is easier to use. So universal is the metric system within the scientific community that it is typically referred to simply as SI, an abbreviation of the French Système International d’Unités—that is, “International System of Units.”

Train travel between the West and Russia was highly problematic, because the width of railroad tracks in Russia was different than in Western Europe. Thus, adjustments had to be performed on trains making a border crossing, and this created difficulties for passenger travel.

The British system lacks any clear frame of reference for organizing units: there are 12 inches in a foot, but 3 feet in a yard, and 1,760 yards in a mile. Water freezes at 32°F instead of 0°, as it does in the Celsius scale associated with the metric system. In contrast to the English system, the

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metric system is neatly arranged according to the base-10 numerical framework: 10 millimeters to a centimeter, 100 centimeters to a meter, 1,000 meters to kilometer, and so on. The difference between the pound and the kilogram aptly illustrates the reason scientists in general, and physicists in particular, prefer the metric system. A pound is a unit of weight, meaning that its value is entirely relative to the gravitational pull of the planet on which it is measured. A kilogram, on the other hand, is a unit of mass, and does not change throughout the universe. Though the basis for a kilogram may not ultimately be any more fundamental than that for a pound, it measures a quality that—unlike weight—does not vary according to frame of reference.

Frame of Reference in Classical Physics and Astronomy Mass is a measure of inertia, the tendency of a body to maintain constant velocity. If an object is at rest, it tends to remain at rest, or if in motion, it tends to remain in motion unless acted upon by some outside force. This, as identified by the first law of motion, is inertia—and the greater the inertia, the greater the mass. Physicists sometimes speak of an “inertial frame of reference,” or one that has a constant velocity—that is, an unchanging speed and direction. Imagine if one were on a moving bus at constant velocity, regularly tossing a ball in the air and catching it. It would be no more difficult to catch the ball than if the bus were standing still, and indeed, there would be no way of determining, simply from the motion of the ball itself, that the bus was moving. But what if the inertial frame of reference suddenly became a non-inertial frame of reference—in other words, what if the bus slammed on its brakes, thus changing its velocity? While the bus was moving forward, the ball was moving along with it, and hence, there was no relative motion between them. By stopping, the bus responded to an “outside” force—that is, its brakes. The ball, on the other hand, experienced that force indirectly. Hence, it would continue to move forward as before, in accordance with its own inertia—only now it would be in motion relative to the bus.

powerful role in astronomy. At every moment, Earth is turning on its axis at about 1,000 MPH (1,600 km/h) and hurtling along its orbital path around the Sun at the rate of 67,000 MPH (107,826 km/h.) The fastest any human being— that is, the astronauts taking part in the Apollo missions during the late 1960s—has traveled is about 30% of Earth’s speed around the Sun.

Frame of Reference

Yet no one senses the speed of Earth’s movement in the way that one senses the movement of a car—or indeed the way the astronauts perceived their speed, which was relative to the Moon and Earth. Of course, everyone experiences the results of Earth’s movement—the change from night to day, the precession of the seasons—but no one experiences it directly. It is simply impossible, from the human frame of reference, to feel the movement of a body as large as Earth—not to mention larger progressions on the part of the Solar System and the universe. F R O M AS T R O N O M Y T O P H Y S I CS . The human body is in an inertial frame of

reference with regard to Earth, and hence experiences no relative motion when Earth rotates or moves through space. In the same way, if one were traveling in a train alongside another train at constant velocity, it would be impossible to perceive that either train was actually moving— unless one referred to some fixed point, such as the trees or mountains in the background. Likewise, if two trains were sitting side by side, and one of them started to move, the relative motion might cause a person in the stationary train to believe that his or her train was the one moving. For any measurement of velocity, and hence, of acceleration (a change in velocity), it is essential to establish a frame of reference. Velocity and acceleration, as well as inertia and mass, figured heavily in the work of Galileo Galilei (15641642) and Sir Isaac Newton (1642-1727), both of whom may be regarded as “founding fathers” of modern physics. Before Galileo, however, had come Nicholas Copernicus (1473-1543), the first modern astronomer to show that the Sun, and not Earth, is at the center of “the universe”— by which people of that time meant the Solar System.

A S T R O N O M Y A N D R E L AT I V E M O T I O N . The idea of relative motion plays a

In effect, Copernicus was saying that the frame of reference used by astronomers for millennia was incorrect: as long as they believed Earth to be the center, their calculations were bound to be wrong. Galileo and later Newton,

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through their studies in gravitation, were able to prove Copernicus’s claim in terms of physics. At the same time, without the understanding of a heliocentric (Sun-centered) universe that he inherited from Copernicus, it is doubtful that Newton could have developed his universal law of gravitation. If he had used Earth as the centerpoint for his calculations, the results would have been highly erratic, and no universal law would have emerged.

Relativity For centuries, the model of the universe developed by Newton stood unchallenged, and even today it identifies the basic forces at work when speeds are well below that of the speed of light. However, with regard to the behavior of light itself—which travels at 186,000 mi (299,339 km) a second—Albert Einstein (1879-1955) began to observe phenomena that did not fit with Newtonian mechanics. The result of his studies was the Special Theory of Relativity, published in 1905, and the General Theory of Relativity, published a decade later. Together these altered humanity’s view of the universe, and ultimately, of reality itself. Einstein himself once offered this charming explanation of his epochal theory: “Put your hand on a hot stove for a minute, and it seems like an hour. Sit with a pretty girl for an hour, and it seems like a minute. That’s relativity.” Of course, relativity is not quite as simple as that— though the mathematics involved is no more challenging than that of a high-school algebra class. The difficulty lies in comprehending how things that seem impossible in the Newtonian universe become realities near the speed of light.

Again, a full explanation—requiring reference to formulae regarding time dilation, and so on—would be a rather involved undertaking. The short answer, however, is that which was stated above: no measurement of space or time is absolute, but each depends on the relative motion of the observer and the observed. Put another way, there is no such thing as absolute motion, either in the three dimensions of space, or in the fourth dimension identified by Einstein, time. All motion is relative to a frame of reference. R E LAT I V I T Y A N D I T S I M P L I CA T I O N S . The ideas involved in relativity have

been verified numerous times, and indeed the only reason why they seem so utterly foreign to most people is that humans are accustomed to living within the Newtonian framework. Einstein simply showed that there is no universal frame of reference, and like a true scientist, he drew his conclusions entirely from what the data suggested. He did not form an opinion, and only then seek the evidence to confirm it, nor did he seek to extend the laws of relativity into any realm beyond that which they described.

An exhaustive explanation of relativity is far beyond the scope of the present discussion. What is important is the central precept: that no measurement of space or time is absolute, but depends on the relative motion of the observer (that is, the subject) and the observed (the object). Einstein further established that the movement of time itself is relative rather than absolute, a fact that would become apparent at speeds close to that of light. (His theory also showed that it is impossible to surpass that speed.)

Yet British historian Paul Johnson, in his unorthodox history of the twentieth century, Modern Times (1983; revised 1992), maintained that a world disillusioned by World War I saw a moral dimension to relativity. Describing a set of tests regarding the behavior of the Sun’s rays around the planet Mercury during an eclipse, the book begins with the sentence: “The modern world began on 29 May 1919, when photographs of a solar eclipse, taken on the Island of Principe off West Africa and at Sobral in Brazil, confirmed the truth of a new theory of the universe.”

Imagine traveling on a spaceship at nearly the speed of light while a friend remains station-

As Johnson went on to note, “...for most people, to whom Newtonian physics... were perfectly

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P LAY I N G T R I C K S W I T H T I M E .

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ary on Earth. Both on the spaceship and at the friend’s house on Earth, there is a TV camera trained on a clock, and a signal relays the image from space to a TV monitor on Earth, and vice versa. What the TV monitor reveals is surprising: from your frame of reference on the spaceship, it seems that time is moving more slowly for your friend on Earth than for you. Your friend thinks exactly the same thing—only, from the friend’s perspective, time on the spaceship is moving more slowly than time on Earth. How can this happen?

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Frame of Reference

KEY TERMS ABSOLUTE:

Fixed; not dependent on

anything else. The value of 10 is absolute, relating to unchanging numerical principles; on the other hand, the value of 10 dol-

FRAME OF REFERENCE:

spective of a subject in observing an object. Something that is perceived

OBJECT:

or observed by a subject.

lars is relative, reflecting the economy, inflation, buying power, exchange rates with other currencies, etc. CALIBRATION:

The process of check-

ing and correcting the performance of a measuring instrument or device against a commonly accepted standard.

RELATIVE:

tion to an x-axis, y-axis, and z-axis. The system is named after the French mathe-

Dependent on something

else for its value or for other identifying qualities. The fact that the United States has a constitution is an absolute, but the fact that it was ratified in 1787 is relative: that date has meaning only within the Western calendar.

CARTESIAN COORDINATE SYSTEM:

A method of specifying coordinates in rela-

The per-

SUBJECT:

Something (usually a per-

son) that perceives or observes an object and/or its behavior. The horizontal line of refer-

matician and philosopher René Descartes

X-AXIS:

(1596-1650), who first described its princi-

ence for points in the Cartesian coordinate

ples, but it was developed greatly by French

system.

mathematician and philosopher Pierre de

Y-AXIS:

Fermat (1601-1665).

for points in the Cartesian coordinate sys-

COEFFICIENT:

A number that serves

The vertical line of reference

tem.

as a measure for some characteristic or

Z-AXIS:

property. A coefficient may also be a factor

of the Cartesian coordinate system, the z-

against which other values are multiplied

axis is the line of reference for points in the

to provide a desired result.

third dimension. Typically the x-axis

In a three-dimensional version

A number or set of

equates to “width,” the y-axis to “height,”

numbers used to specify the location of a

and the z-axis to “depth”—though in fact

point on a line, on a surface such as a

length, width, and height are all relative to

plane, or in space.

the observer’s frame of reference.

COORDINATE:

comprehensible, relativity never became more than a vague source of unease. It was grasped that absolute time and absolute length had been dethroned.... All at once, nothing seemed certain in the spheres....At the beginning of the 1920s the belief began to circulate, for the first time at a popular level, that there were no longer any absolutes: of time and space, of good and evil, of knowledge, above all of value. Mistakenly but perhaps inevitably, relativity became confused with relativism.”

Certainly many people agree that the twentieth century—an age that saw unprecedented mass murder under the dictatorships of Adolf Hitler and Josef Stalin, among others—was characterized by moral relativism, or the belief that there is no right or wrong. And just as Newton’s discoveries helped usher in the Age of Reason, when thinkers believed it was possible to solve any problem through intellectual effort, it is quite plausible that Einstein’s theory may have had this negative moral effect.

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If so, this was certainly not Einstein’s intention. Aside from the fact that, as stated, he did not set out to describe anything other than the physical behavior of objects, he continued to believe that there was no conflict between his ideas and a belief in an ordered universe: “Relativity,” he once said, “teaches us the connection between the different descriptions of one and the same reality.” WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. Fleisher, Paul. Relativity and Quantum Mechanics: Principles of Modern Physics. Minneapolis, MN: Lerner Publications, 2002. “Frame of Reference” (Web site). (March 21, 2001).

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“Inertial Frame of Reference” (Web site). (March 21, 2001). Johnson, Paul. Modern Times: The World from the Twenties to the Nineties. Revised edition. New York: HarperPerennial, 1992. King, Andrew. Plotting Points and Position. Illustrated by Tony Kenyon. Brookfield, CT: Copper Beech Books, 1998. Parker, Steve. Albert Einstein and Relativity. New York: Chelsea House, 1995. Robson, Pam. Clocks, Scales, and Measurements. New York: Gloucester Press, 1993. Rutherford, F. James; Gerald Holton; and Fletcher G. Watson. Project Physics. New York: Holt, Rinehart, and Winston, 1981. Swisher, Clarice. Relativity: Opposing Viewpoints. San Diego, CA: Greenhaven Press, 1990.

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K I N E M AT I C S A N D DYNAMICS

CONCEPT Webster’s defines physics as “a science that deals with matter and energy and their interactions.” Alternatively, physics can be described as the study of matter and motion, or of matter inmotion. Whatever the particulars of the definition, physics is among the most fundamental of disciplines, and hence, the rudiments of physics are among the most basic building blocks for thinking about the world. Foundational to an understanding of physics are kinematics, the explanation of how objects move, and dynamics, the study of why they move. Both are part of a larger branch of physics called mechanics, the study of bodies in motion. These are subjects that may sound abstract, but in fact, are limitless in their applications to real life.

HOW IT WORKS The Place of Physics in the Sciences Physics may be regarded as the queen of the sciences, not because it is “better” than chemistry or astronomy, but because it is the foundation on which all others are built. The internal and interpersonal behaviors that are the subject of the social sciences (psychology, anthropology, sociology, and so forth) could not exist without the biological framework that houses the human consciousness. Yet the human body and other elements studied by the biological and medical sciences exist within a larger environment, the framework for earth sciences, such as geology.

one corner of the realm studied by astronomy; and before a universe can even exist, there must be interactions of elements, the subject of chemistry. Yet even before chemicals can react, they have to do so within a physical framework—the realm of the most basic science—physics.

The Birth of Physics in Greece T H E F I R S T H Y P O T H E S I S . Indeed, physics stands in relation to the sciences as philosophy does to thought itself: without philosophy to provide the concept of concepts, it would be impossible to develop a consistent worldview in which to test ideas. It is no accident, then, that the founder of the physical sciences was also the world’s first philosopher, Thales (c. 625?-547? B.C.) of Miletus in Greek Asia Minor (now part of Turkey.) Prior to Thales’s time, religious figures and mystics had made statements regarding ethics or the nature of deity, but none had attempted statements concerning the fundamental nature of reality.

For instance, the Bible offers a story of Earth’s creation in the Book of Genesis which was well-suited to the understanding of people in the first millennium before Christ. But the writer of the biblical creation story made no attempt to explain how things came into being. He was concerned, rather, with showing that God had willed the existence of all physical reality by calling things into being—for example, by saying, “Let there be light.”

Earth sciences belong to a larger grouping of physical sciences, each more fundamental in concerns and broader in scope. Earth, after all, is but

Thales, on the other hand, made a genuine philosophical and scientific statement when he said that “Everything is water.” This was the first hypothesis, a statement capable of being scientif-

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ically tested for accuracy. Thales’s pronouncement did not mean he believed all things were necessarily made of water, literally. Rather, he appears to have been referring to a general tendency of movement: that the whole world is in a fluid state. AT T E M P T I N G TO UNDERS TA N D P H Y S I CA L R E A L I T Y. While

we can respect Thales’s statement for its truly earth-shattering implications, we may be tempted to read too much into it. Nonetheless, it is striking that he compared physical reality to water. On the one hand, there is the fact that water is essential to all life, and pervades Earth—but that is a subject more properly addressed by the realms of chemistry and the biological sciences. Perhaps of more interest to the physicist is the allusion to a fluid nature underlying all physical reality. The physical realm is made of matter, which appears in four states: solid, liquid, gas, and plasma. The last of these is not the same as blood plasma: containing many ionized atoms or molecules which exhibit collective behavior, plasma is the substance from which stars, for instance, are composed. Though not plentiful on Earth, within the universe it may be the most common of all four states. Plasma is akin to gas, but different in molecular structure; the other three states differ at the molecular level as well. Nonetheless, it is possible for a substance such as water—genuine H2O, not the figurative water of Thales—to exist in liquid, gas, or solid form, and the dividing line between these is not always fixed. In fact, physicists have identified a phenomenon known as the triple point: at a certain temperature and pressure, a substance can be solid, liquid, and gas all at once! The above statement shows just how challenging the study of physical reality can be, and indeed, these concepts would be far beyond the scope of Thales’s imagination, had he been presented with them. Though he almost certainly deserves to be called a “genius,” he lived in a world that viewed physical processes as a product of the gods’ sometimes capricious will. The behavior of the tides, for instance, was attributed to Poseidon. Though Thales’s statement began the process of digging humanity out from under the burden of superstition that had impeded scientific progress for centuries, the road forward would be a long one.

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M AT H E M AT I C S , MEASUREM E N T, A N D M AT T E R. In the two cen-

turies after Thales’s death, several other thinkers advanced understanding of physical reality in one way or another. Pythagoras (c. 580-c. 500 B.C.) taught that everything could be quantified, or related to numbers. Though he entangled this idea with mysticism and numerology, the concept itself influenced the idea that physical processes could be measured. Likewise, there were flaws at the heart of the paradoxes put forth by Zeno of Elea (c. 495-c. 430 B.C.), who set out to prove that motion was impossible—yet he was also the first thinker to analyze motion seriously. In one of Zeno’s paradoxes, he referred to an arrow being shot from a bow. At every moment of its flight, it could be said that the arrow was at rest within a space equal to its length. Though it would be some 2,500 years before slow-motion photography, in effect he was asking his listeners to imagine a snapshot of the arrow in flight. If it was at rest in that “snapshot,” he asked, so to speak, and in every other possible “snapshot,” when did the arrow actually move? These paradoxes were among the most perplexing questions of premodern times, and remain a subject of inquiry even today. In fact, it seems that Zeno unwittingly (for there is no reason to believe that he deliberately deceived his listeners) inserted an error in his paradoxes by treating physical space as though it were composed of an infinite number of points. In the ideal world of geometric theory, a point takes up no space, and therefore it is correct to say that a line contains an infinite number of points; but this is not the case in the real world, where a “point” has some actual length. Hence, if the number of points on Earth were limitless, so too would be Earth itself. Zeno’s contemporary Leucippus (c. 480-c. 420 B.C.) and his student Democritus (c. 460-370 B.C.) proposed a new and highly advanced model for the tiniest point of physical space: the atom. It would be some 2,300 years, however, before physicists returned to the atomic model.

Aristotle’s Flawed Physics The study of matter and motion began to take shape with Aristotle (384-322 B.C.); yet, though his Physics helped establish a framework for the discipline, his errors are so profound that any praise must be qualified. Certainly, Aristotle was

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one of the world’s greatest thinkers, who originated a set of formalized realms of study. However, in Physics he put forth an erroneous explanation of matter and motion that still prevailed in Europe twenty centuries later.

Kinematics and Dynamics

Actually, Aristotle’s ideas disappeared in the late ancient period, as learning in general came to a virtual halt in Europe. That his writings— which on the whole did much more to advance the progress of science than to impede it—survived at all is a tribute to the brilliance of Arab, rather than European, civilization. Indeed, it was in the Arab world that the most important scientific work of the medieval period took place. Only after about 1200 did Aristotelian thinking once again enter Europe, where it replaced a crude jumble of superstitions that had been substituted for learning. T H E F O U R E L E M E N T S . According to Aristotelian physics, all objects consisted, in varying degrees, of one or more elements: air, fire, water, and earth. In a tradition that went back to Thales, these elements were not necessarily pure: water in the everyday world was composed primarily of the element water, but also contained smaller amounts of the other elements. The planets beyond Earth were said to be made up of a “fifth element,” or quintessence, of which little could be known.

The differing weights and behaviors of the elements governed the behavior of physical objects. Thus, water was lighter than earth, for instance, but heavier than air or fire. It was due to this difference in weight, Aristotle reasoned, that certain objects fall faster than others: a stone, for instance, because it is composed primarily of earth, will fall much faster than a leaf, which has much less earth in it. Aristotle further defined “natural” motion as that which moved an object toward the center of the Earth, and “violent” motion as anything that propelled an object toward anything other than its “natural” destination. Hence, all horizontal or upward motion was “violent,” and must be the direct result of a force. When the force was removed, the movement would end.

ARISTOTLE. (The Bettmann Archive. Reproduced by permission.)

revolving around it. Though in constant movement, these heavenly bodies were always in their “natural” place, because they could only move on the firmly established—almost groove-like— paths of their orbits around Earth. This in turn meant that the physical properties of matter and motion on other planets were completely different from the laws that prevailed on Earth. Of course, virtually every precept within the Aristotelian system is incorrect, and Aristotle compounded the influence of his errors by promoting a disdain for quantification. Specifically, he believed that mathematics had little value for describing physical processes in the real world, and relied instead on pure observation without attempts at measurement.

Moving Beyond Aristotle

ter is the destination of all “natural” motion, it is easy to comprehend the Aristotelian cosmology, or model of the universe. Earth itself was in the center, with all other bodies (including the Sun)

Faulty as Aristotle’s system was, however, it possessed great appeal because much of it seemed to fit with the evidence of the senses. It is not at all immediately apparent that Earth and the other planets revolve around the Sun, nor is it obvious that a stone and a leaf experience the same acceleration as they fall toward the ground. In fact, quite the opposite appears to be the case: as everyone knows, a stone falls faster than a leaf. Therefore, it would seem reasonable—on the

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Kinematics and Dynamics

Kinematics: How Objects Move By the sixteenth century, the Aristotelian worldview had become so deeply ingrained that few European thinkers would have considered the possibility that it could be challenged. Professors all over Europe taught Aristotle’s precepts to their students, and in this regard the University of Pisa in Italy was no different. Yet from its classrooms would emerge a young man who not only questioned, but ultimately overturned the Aristotelian model: Galileo Galilei (1564-1642.)

GALILEO. (Archive Photos, Inc. Reproduced by permission.)

surface of it, at least—to accept Aristotle’s conclusion that this difference results purely from a difference in weight. Today, of course, scientists—and indeed, even people without any specialized scientific knowledge—recognize the lack of merit in the Aristotelian system. The stone does fall faster than the leaf, but only because of air resistance, not weight. Hence, if they fell in a vacuum (a space otherwise entirely devoid of matter, including air), the two objects would fall at exactly the same rate.

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Challenges to Aristotle had been slowly growing within the scientific communities of the Arab and later the European worlds during the preceding millennium. Yet the ideas that most influenced Galileo in his break with Aristotle came not from a physicist but from an astronomer, Nicolaus Copernicus (1473-1543.) It was Copernicus who made a case, based purely on astronomical observation, that the Sun and not Earth was at the center of the universe. Galileo embraced this model of the cosmos, but was later forced to renounce it on orders from the pope in Rome. At that time, of course, the Catholic Church remained the single most powerful political entity in Europe, and its endorsement of Aristotelian views—which philosophers had long since reconciled with Christian ideas—is a measure of Aristotle’s impact on thinking. GALILEO’S REVOLUTION IN P H Y S I CS . After his censure by the Church,

As with a number of truths about matter and motion, this is not one that appears obvious, yet it has been demonstrated. To prove this highly nonintuitive hypothesis, however, required an approach quite different from Aristotle’s—an approach that involved quantification and the separation of matter and motion into various components. This was the beginning of real progress in physics, and in a sense may be regarded as the true birth of the discipline. In the years that followed, understanding of physics would grow rapidly, thanks to advancements of many individuals; but their studies could not have been possible without the work of one extraordinary thinker who dared to question the Aristotelian model.

Galileo was placed under house arrest and was forbidden to study astronomy. Instead he turned to physics—where, ironically, he struck the blow that would destroy the bankrupt scientific system endorsed by Rome. In 1638, he published Discourses and Mathematical Demonstrations Concerning Two New Sciences Pertaining to Mathematics and Local Motion, a work usually referred to as Two New Sciences. In it, he laid the groundwork for physics by emphasizing a new method that included experimentation, demonstration, and quantification of results.

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In this book—highly readable for a work of physics written in the seventeenth century— Galileo used a dialogue, an established format among philosophers and scientists of the past.

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The character of Salviati argued for Galileo’s ideas and Simplicio for those of Aristotle, while the genial Sagredo sat by and made occasional comments. Through Salviati, Galileo chose to challenge Aristotle on an issue that to most people at the time seemed relatively settled: the claim that objects fall at differing speeds according to their weight. In order to proceed with his aim, Galileo had to introduce a number of innovations, and indeed, he established the subdiscipline of kinematics, or how objects move. Aristotle had indicated that when objects fall, they fall at the same rate from the moment they begin to fall until they reach their “natural” position. Galileo, on the other hand, suggested an aspect of motion, unknown at the time, that became an integral part of studies in physics: acceleration.

Scalars and Vectors Even today, many people remain confused as to what acceleration is. Most assume that acceleration means only an increase in speed, but in fact this represents only one of several examples of acceleration. Acceleration is directly related to velocity, often mistakenly identified with speed. In fact, speed is what scientists today would call a scalar quantity, or one that possesses magnitude but no specific direction. Speed is the rate at which the position of an object changes over a given period of time; thus people say “miles (or kilometers) per hour.” A story problem concerning speed might state that “A train leaves New York City at a rate of 60 miles (96.6 km/h). How far will it have traveled in 73 minutes?” Note that there is no reference to direction, whereas if the story problem concerned velocity—a vector, that is, a quantity involving both magnitude and direction—it would include some crucial qualifying phrase after “New York City”: “for Boston,” perhaps, or “northward.” In practice, the difference between speed and velocity is nearly as large as that between a math problem and real life: few people think in terms of driving 60 miles, for instance, without also considering the direction they are traveling.

at the head of the previous one. Then it is possible to draw a vector from the tail of the first to the head of the last. This is the sum of the vectors, known as a resultant, which measures the net change. Suppose, for instance, that a car travels east 4 mi (6.44 km), then due north 3 mi (4.83 km). This may be drawn on a graph with four units along the x axis, then 3 units along the y axis, making two sides of a triangle. The number of sides to the resulting shape is always one more than the number of vectors being added; the final side is the resultant. From the tail of the first segment, a diagonal line drawn to the head of the last will yield a measurement of 5 units—the resultant, which in this case would be equal to 5 mi (8 km) in a northeasterly direction. V E LO C I T Y A N D AC C E L E RA T I O N . The directional component of velocity

makes it possible to consider forms of motion other than linear, or straight-line, movement. Principal among these is circular, or rotational motion, in which an object continually changes direction and thus, velocity. Also significant is projectile motion, in which an object is thrown, shot, or hurled, describing a path that is a combination of horizontal and vertical components. Furthermore, velocity is a key component in acceleration, which is defined as a change in velocity. Hence, acceleration can mean one of five things: an increase in speed with no change in direction (the popular, but incorrect, definition of the overall concept); a decrease in speed with no change in direction; a decrease or increase of speed with a change in direction; or a change in direction with no change in speed. If a car speeds up or slows down while traveling in a straight line, it experiences acceleration. So too does an object moving in rotational motion, even if its speed does not change, because its direction will change continuously.

Dynamics: Why Objects Move

One can apply the same formula with velocity, though the process is more complicated. To obtain change in distance, one must add vectors, and this is best done by means of a diagram. You can draw each vector as an arrow on a graph, with the tail of each vector

G A L I L E O ’ S T E S T. To return to Galileo, he was concerned primarily with a specific form of acceleration, that which occurs due to the force of gravity. Aristotle had provided an explanation of gravity—if a highly flawed one— with his claim that objects fall to their “natural” position; Galileo set out to develop the first truly scientific explanation concerning how objects fall to the ground.

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According to Galileo’s predictions, two metal balls of differing sizes would fall with the same rate of acceleration. To test his hypotheses, however, he could not simply drop two balls from a rooftop—or have someone else do so while he stood on the ground—and measure their rate of fall. Objects fall too fast, and lacking sophisticated equipment available to scientists today, he had to find another means of showing the rate at which they fell. This he did by resorting to a method Aristotle had shunned: the use of mathematics as a means of modeling the behavior of objects. This is such a deeply ingrained aspect of science today that it is hard to imagine a time when anyone would have questioned it, and that very fact is a tribute to Galileo’s achievement. Since he could not measure speed, he set out to find an equation relating total distance to total time. Through a detailed series of steps, Galileo discovered that in uniform or constant acceleration from rest—that is, the acceleration he believed an object experiences due to gravity—there is a proportional relationship between distance and time. With this mathematical model, Galileo could demonstrate uniform acceleration. He did this by using an experimental model for which observation was easier than in the case of two falling bodies: an inclined plane, down which he rolled a perfectly round ball. This allowed him to extrapolate that in free fall, though velocity was greater, the same proportions still applied and therefore, acceleration was constant. P O I N T I N G T H E WAY T O WA R D N E W T O N . The effects of Galileo’s system were

enormous: he demonstrated mathematically that acceleration is constant, and established a method of hypothesis and experiment that became the basis of subsequent scientific investigation. He did not, however, attempt to calculate a figure for the acceleration of bodies in free fall; nor did he attempt to explain the overall principle of gravity, or indeed why objects move as they do—the focus of a subdiscipline known as dynamics.

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he lived in England, where the pope had no power—was free to explore the implications of his physical studies without fear of Rome’s intervention. Born in the very year Galileo died, his name was Sir Isaac Newton (1642-1727.) N E W T O N ’ S T H R E E LAW S O F M O T I O N . In discussing the movement of the

planets, Galileo had coined the term inertia to describe the tendency of an object in motion to remain in motion, and an object at rest to remain at rest. This idea would be the starting point of Newton’s three laws of motion, and Newton would greatly expand on the concept of inertia. The three laws themselves are so significant to the understanding of physics that they are treated separately elsewhere in this volume; here they are considered primarily in terms of their implications regarding the larger topic of matter and motion. Introduced by Newton in his Principia (1687), the three laws are: • First law of motion: An object at rest will remain at rest, and an object in motion will remain in motion, at a constant velocity unless or until outside forces act upon it. • Second law of motion: The net force acting upon an object is a product of its mass multiplied by its acceleration. • Third law of motion: When one object exerts a force on another, the second object exerts on the first a force equal in magnitude but opposite in direction. These laws made final the break with Aristotle’s system. In place of “natural” motion, Newton presented the concept of motion at a uniform velocity—whether that velocity be a state of rest or of uniform motion. Indeed, the closest thing to “natural” motion (that is, true “natural” motion) is the behavior of objects in outer space. There, free from friction and away from the gravitational pull of Earth or other bodies, an object set in motion will remain in motion forever due to its own inertia. It follows from this observation, incidentally, that Newton’s laws were and are universal, thus debunking the old myth that the physical properties of realms outside Earth are fundamentally different from those of Earth itself.

At the end of Two New Sciences, Sagredo offered a hopeful prediction: “I really believe that... the principles which are set forth in this little treatise will, when taken up by speculative minds, lead to another more remarkable result....” This prediction would come true with the work of a man who, because he lived in a somewhat more enlightened time—and because

the principle of inertia, and the second law makes reference to the means by which inertia is measured: mass, or the resistance of an object to a

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M A S S A N D G R A V I TAT I O N A L AC C E L E RAT I O N . The first law establishes

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Kinematics and Dynamics

KEY TERMS A change in velocity.

ACCELERATION:

A quantity that possesses

SCALAR:

The study of why objects

only magnitude, with no specific direction.

move as they do; compare with kinematics.

Mass, time, and speed are all scalars. The

DYNAMICS:

FORCE:

The product of mass multi-

plied by acceleration. HYPOTHESIS:

A statement capable of

being scientifically tested for accuracy. INERTIA:

The tendency of an object in

opposite of a scalar is a vector. The rate at which the position

SPEED:

of an object changes over a given period of time. Space entirely devoid of

VACUUM:

motion to remain in motion, and of an

matter, including air.

object at rest to remain at rest.

VECTOR:

KINEMATICS:

The study of how

objects move; compare with dynamics.

A quantity that possesses

both magnitude and direction. Velocity, acceleration, and weight (which involves

A measure of inertia, indicating

the downward acceleration due to gravity)

the resistance of an object to a change in its

are examples of vectors. Its opposite is a

motion—including a change in velocity.

scalar.

MASS:

MATTER:

The material of physical real-

ity. There are four basic states of matter: solid, liquid, gas, and plasma. MECHANICS:

The study of bodies in

motion.

The speed of an object in a

VELOCITY:

particular direction. WEIGHT:

A measure of the gravitation-

al force on an object; the product of mass multiplied by the acceleration due to grav-

The sum of two or more

ity. (The latter is equal to 32 ft or 9.8 m per

vectors, which measures the net change in

second per second, or 32 ft/9.8 m per sec-

distance and direction.

ond squared.)

RESULTANT:

change in its motion—including a change in velocity. Mass is one of the most fundamental notions in the world of physics, and it too is the subject of a popular misconception—one which confuses it with weight. In fact, weight is a force, equal to mass multiplied by the acceleration due to gravity.

velocity is also increasing at a rate of 32 ft per second per second. Hence, after 2 seconds, its velocity will be 64 ft (per second; after 3 seconds 96 ft per second, and so on.

It was Newton, through a complicated series of steps he explained in his Principia, who made possible the calculation of that acceleration—an act of quantification that had eluded Galileo. The figure most often used for gravitational acceleration at sea level is 32 ft (9.8 m) per second squared. This means that in the first second, an object falls at a velocity of 32 ft per second, but its

Mass does not vary anywhere in the universe, whereas weight changes with any change in the gravitational field. When United States astronaut Neil Armstrong planted the American flag on the Moon in 1969, the flagpole (and indeed Armstrong himself) weighed much less than on Earth. Yet it would have required exactly the same amount of force to move the pole (or, again, Armstrong) from side to side as it would have on Earth, because their mass and therefore their inertia had not changed.

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Beyond Mechanics Kinematics and Dynamics

The implications of Newton’s three laws go far beyond what has been described here; but again, these laws, as well as gravity itself, receive a much more thorough treatment elsewhere in this volume. What is important in this context is the gradually unfolding understanding of matter and motion that formed the basis for the study of physics today. After Newton came the Swiss mathematician and physicist Daniel Bernoulli (1700-1782), who pioneered another subdiscipline, fluid dynamics, which encompasses the behavior of liquids and gases in contact with solid objects. Air itself is an example of a fluid, in the scientific sense of the term. Through studies in fluid dynamics, it became possible to explain the principles of air resistance that cause a leaf to fall more slowly than a stone—even though the two are subject to exactly the same gravitational acceleration, and would fall at the same speed in a vacuum. EXTENDING THE REALM OF P H Y S I CA L S T U DY. The work of Galileo,

Newton, and Bernoulli fit within one of five major divisions of classical physics: mechanics, or the study of matter, motion, and forces. The other principal divisions are acoustics, or studies in sound; optics, the study of light; thermodynamics, or investigations regarding the relationships between heat and other varieties of energy; and electricity and magnetism. (These subjects, and subdivisions within them, also receive extensive treatment elsewhere in this book.) Newton identified one type of force, gravitation, but in the period leading up to the time of Scottish physicist James Clerk Maxwell (18311879), scientists gradually became aware of a new fundamental interaction in the universe. Building on studies of numerous scientists, Maxwell hypothesized that electricity and magnetism are in fact differing manifestations of a second variety of force, electromagnetism. M O D E R N P H Y S I CS . The term classical physics, used above, refers to the subjects of study from Galileo’s time through the end of the nineteenth century. Classical physics deals primarily with subjects that can be discerned by the senses, and addressed processes that could be observed on a large scale. By contrast, modern

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physics, which had its beginnings with the work of Max Planck (1858-1947), Albert Einstein (1879-1955), Niels Bohr (1885-1962), and others at the beginning of the twentieth century, addresses quite a different set of topics. Modern physics is concerned primarily with the behavior of matter at the molecular, atomic, or subatomic level, and thus its truths cannot be grasped with the aid of the senses. Nor is classical physics much help in understanding modern physics. The latter, in fact, recognizes two forces unknown to classical physicists: weak nuclear force, which causes the decay of some subatomic particles, and strong nuclear force, which binds the nuclei of atoms with a force 1 trillion (1012) times as great as that of the weak nuclear force. Things happen in the realm of modern physics that would have been inconceivable to classical physicists. For instance, according to quantum mechanics—first developed by Planck—it is not possible to make a measurement without affecting the object (e.g., an electron) being measured. Yet even atomic, nuclear, and particle physics can be understood in terms of their effects on the world of experience: challenging as these subjects are, they still concern—though within a much more complex framework—the physical fundamentals of matter and motion. WHERE TO LEARN MORE Ballard, Carol. How Do We Move? Austin, TX: Raintree Steck-Vaughn, 1998. Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. Fleisher, Paul. Objects in Motion: Principles of Classical Mechanics. Minneapolis, MN: Lerner Publications, 2002. Hewitt, Sally. Forces Around Us. New York: Children’s Press, 1998. Measure for Measure: Sites That Do the Work for You (Web site). (March 7, 2001). Motion, Energy, and Simple Machines (Web site). (March 7, 2001). Physlink.com (Web site). (March 7, 2001). Rutherford, F. James; Gerald Holton; and Fletcher G. Watson. Project Physics. New York: Holt, Rinehart, and Winston, 1981. Wilson, Jerry D. Physics: Concepts and Applications, second edition. Lexington, MA: D. C. Heath, 1981.

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D E N S I T Y A N D VO LU M E

CONCEPT Density and volume are simple topics, yet in order to work within any of the hard sciences, it is essential to understand these two types of measurement, as well as the fundamental quantity involved in conversions between them—mass. Measuring density makes it possible to distinguish between real gold and fake gold, and may also give an astronomer an important clue regarding the internal composition of a planet.

mental quality, mass, in determining this significantly less fundamental property of weight. According to the second law of motion, weight is a force equal to mass multiplied by acceleration. Acceleration, in turn, is equal to change in velocity divided by change in time. Velocity, in turn, is equal to distance (a form of length) divided by time. If one were to express weight in terms of l, t, and m, with these representing, respectively, the fundamental properties of length, time, and mass, it would be expressed as

M

HOW IT WORKS There are four fundamental standards by which most qualities in the physical world can be measured: length, mass, time, and electric current. The volume of a cube, for instance, is a unit of length cubed: the length is multiplied by the width and multiplied by the height. Width and height, however, are not distinct standards of measurement: they are simply versions of length, distinguished by their orientation. Whereas length is typically understood as a distance along an x-axis in one-dimensional space, width adds a second dimension, and height a third. Of particular concern within this essay are length and mass, since volume is measured in terms of length, and density in terms of the ratio between mass and volume. Elsewhere in this book, the distinction between mass and weight has been presented numerous times from the standpoint of a person whose mass and weight are measured on Earth, and again on the Moon. Mass, of course, does not change, whereas weight does, due to the difference in gravitational force exerted by Earth as compared with that of its satellite, the Moon. But consider instead the role of the funda-

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•

t

D

2

—clearly, a much more complicated formula than that of mass!

Mass So what is mass? Again, the second law of motion, derived by Sir Isaac Newton (16421727), is the key: mass is the ratio of force to acceleration. This topic, too, is discussed in numerous places throughout this book; what is actually of interest here is a less precise identification of mass, also made by Newton. Before formulating his laws of motion, Newton had used a working definition of mass as the quantity of matter an object possesses. This is not of much value for making calculations or measurements, unlike the definition in the second law. Nonetheless, it serves as a useful reminder of matter’s role in the formula for density. Matter can be defined as a physical substance not only having mass, but occupying space. It is composed of atoms (or in the case of subatomic particles, it is part of an atom), and is

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Density and Volume

HOW

DOES A GIGANTIC STEEL SHIP, SUCH AS THE SUPERTANKER PICTURED HERE, STAY AFLOAT, EVEN THOUGH IT

HAS A WEIGHT DENSITY FAR GREATER THAN THE WATER BELOW IT?

THE

ANSWER LIES IN ITS CURVED HULL, WHICH

CONTAINS A LARGE AMOUNT OF OPEN SPACE AND ALLOWS THE SHIP TO SPREAD ITS AVERAGE DENSITY TO A LOWER LEVEL THAN THE WATER. (Photograph by Vince Streano/Corbis. Reproduced by permission.)

convertible with energy. The form or state of matter itself is not important: on Earth it is primarily observed as a solid, liquid, or gas, but it can also be found (particularly in other parts of the universe) in a fourth state, plasma.

ed by volume. It so happens that the three items represent the three states of matter on Earth: liquid (water), solid (iron), and gas (helium). For the most part, solids tend to be denser than liquids, and liquids denser than gasses.

Yet there are considerable differences among types of matter—among various elements and states of matter. This is apparent if one imagines three gallon jugs, one containing water, the second containing helium, and the third containing iron filings. The volume of each is the same, but obviously, the mass is quite different.

One of the interesting things about density, as distinguished from mass and volume, is that it has nothing to do with the amount of material. A kilogram of iron differs from 10 kilograms of iron both in mass and volume, but the density of both samples is the same. Indeed, as discussed below, the known densities of various materials make it possible to determine whether a sample of that material is genuine.

The reason, of course, is that at a molecular level, there is a difference in mass between the compound H2O and the elements helium and iron. In the case of helium, the second-lightest of all elements after hydrogen, it would take a great deal of helium for its mass to equal that of iron. In fact, it would take more than 43,000 gallons of helium to equal the mass of the iron in one gallon jug!

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Volume Mass, because of its fundamental nature, is sometimes hard to comprehend, and density requires an explanation in terms of mass and volume. Volume, on the other hand, appears to be quite straightforward—and it is, when one is describing a solid of regular shape. In other situations, however, volume is more complicated.

Rather than comparing differences in molecular mass among the three substances, it is easier to analyze them in terms of density, or mass divid-

As noted earlier, the volume of a cube can be obtained simply by multiplying length by width by height. There are other means for measuring

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Density and Volume

SINCE SCIENTISTS KNOW EARTH’S MASS AS WELL AS ITS VOLUME, TY—APPROXIMATELY 5 G/CM3. (Corbis. Reproduced by permission.)

the volume of other straight-sided objects, such as a pyramid. That formula applies, indeed, for any polyhedron (a three-dimensional closed solid bounded by a set number of plane figures) that constitutes a modified cube in which the lengths of the three dimensions are unequal— that is, an oblong shape.

THEY ARE EASILY ABLE TO COMPUTE ITS DENSI-

REAL-LIFE A P P L I C AT I O N S Measuring Volume

For a cylinder or sphere, volume measurements can be obtained by applying formulae involving radius (r) and the constant π, roughly equal to 3.14. The formula for volume of a cylinder is V = πr2h, where h is the height. A sphere’s volume can be obtained by the formula (4/3)πr3. Even the volume of a cone can be easily calculated: it is onethird that of a cylinder of equal base and height.

What about the volume of a solid that is irregular in shape? Some irregularly shaped objects, such as a scooter, which consists primarily of one round wheel and a number of oblong shapes, can be measured by separating them into regular shapes. Calculus may be employed with more complex problems to obtain the volume of an irregular shape—but the most basic method is simply to immerse the object in water. This procedure involves measuring the volume of the water before and after immersion, and calculating the difference. Of course, the object being

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measured cannot be water-soluble; if it is, its volume must be measured in a non-water-based liquid such as alcohol. Measuring liquid volumes is easy, given the fact that liquids have no definite shape, and will simply take the shape of the container in which they are placed. Gases are similar to liquids in the sense that they expand to fit their container; however, measurement of gas volume is a more involved process than that used to measure either liquids or solids, because gases are highly responsive to changes in temperature and pressure. If the temperature of water is raised from its freezing point to its boiling point (32° to 212°F or 0 to 100°C), its volume will increase by only 2%. If its pressure is doubled from 1 atm (defined as normal air pressure at sea level—14.7 poundsper-square-inch or 1.013 x 105 Pa) to 2 atm, volume will decrease by only 0.01%. Yet, if air were heated from 32° to 212°F, its volume would increase by 37%; and if its pressure were doubled from 1 atm to 2, its volume would decrease by 50%. Not only do gases respond dramatically to changes in temperature and pressure, but also, gas molecules tend to be non-attractive toward one another—that is, they do not tend to stick together. Hence, the concept of “volume” involving gas is essentially meaningless, unless its temperature and pressure are known.

Buoyancy: Volume and Density Consider again the description above, of an object with irregular shape whose volume is measured by immersion in water. This is not the only interesting use of water and solids when dealing with volume and density. Particularly intriguing is the concept of buoyancy expressed in Archimedes’s principle.

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and ran naked through the streets of Syracuse shouting “Eureka!” (I have found it). What Archimedes had discovered was, in short, the reason why ships float: because the buoyant, or lifting, force of an object immersed in fluid is equal to the weight of the fluid displaced by the object. H O W A S T E E L S H I P F LOAT S O N WAT E R. Today most ships are made of

steel, and therefore, it is even harder to understand why an aircraft carrier weighing many thousands of tons can float. After all, steel has a weight density (the preferred method for measuring density according to the British system of measures) of 480 pounds per cubic foot, and a density of 7,800 kilograms-per-cubic-meter. By contrast, sea water has a weight density of 64 pounds per cubic foot, and a density of 1,030 kilograms-per-cubic-meter. This difference in density should mean that the carrier would sink like a stone—and indeed it would, if all the steel in it were hammered flat. As it is, the hull of the carrier (or indeed of any seaworthy ship) is designed to displace or move a quantity of water whose weight is greater than that of the vessel itself. The weight of the displaced water—that is, its mass multiplied by the downward acceleration due to gravity—is equal to the buoyant force that the ocean exerts on the ship. If the ship weighs less than the water it displaces, it will float; but if it weighs more, it will sink. Put another way, when the ship is placed in the water, it displaces a certain quantity of water whose weight can be expressed in terms of Vdg— volume multiplied by density multiplied by the downward acceleration due to gravity. The density of sea water is a known figure, as is g (32 ft or 9.8 m/sec2); thus the only variable for the water displaced is its volume.

More than twenty-two centuries ago, the Greek mathematician, physicist, and inventor Archimedes (c. 287-212 B.C.) received orders from the king of his hometown—Syracuse, a Greek colony in Sicily—to weigh the gold in the royal crown. According to legend, it was while bathing that Archimedes discovered the principle that is today named after him. He was so excited, legend maintains, that he jumped out of his bath

For the buoyant force on the ship, g will of course be the same, and the value of V will be the same as for the water. In order for the ship to float, then, its density must be much less than that of the water it has displaced. This can be achieved by designing the ship in order to maximize displacement. The steel is spread over as large an area as possible, and the curved hull, when seen in cross section, contains a relatively large area of open space. Obviously, the density of this space is much less than that of water; thus, the average density of the ship is greatly reduced, which enables it to float.

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Density and Volume

KEY TERMS ARCHIMEDES’S PRINCIPLE:

A rule

of physics which holds that the buoyant force of an object immersed in fluid is

part of an atom), and is convertible into energy. SPECIFIC GRAVITY:

an object or substance relative to the densi-

by the object. It is named after the Greek

ty of water; or more generally, the ratio

mathematician, physicist, and inventor

between the densities of two objects or

Archimedes (c. 287-212 B.C.), who first

substances.

identified it. BUOYANCY:

The tendency of an object

immersed in a fluid to float. This can be explained by Archimedes’s principle. DENSITY:

VOLUME:

The

amount

ter within a given area.

of

three-

dimensional space an object occupies. Volume is usually expressed in cubic units of length.

The ratio of mass to vol-

ume—in other words, the amount of mat-

WEIGHT DENSITY:

The proper term

for density within the British system of weights and measures. The pound is a unit

According to the second law of

of weight rather than of mass, and thus

motion, mass is the ratio of force to acceler-

British units of density are usually ren-

ation. Mass may likewise be defined, though

dered in terms of weight density—that is,

much less precisely, as the amount of mat-

pounds-per-cubic-foot. By contrast, the

ter an object contains. Mass is also the

metric or international units measure mass

product of volume multiplied by density.

density (referred to simply as “density”),

MASS:

Physical substance that occu-

which is rendered in terms of kilograms-

pies space, has mass, is composed of atoms

per-cubic-meter, or grams-per-cubic-

(or in the case of subatomic particles, is

centimeter.

MATTER:

Comparing Densities As noted several times, the densities of numerous materials are known quantities, and can be easily compared. Some examples of density, all expressed in terms of kilograms per cubic meter, are: • • • • • • • • • •

The density of

equal to the weight of the fluid displaced

Hydrogen: 0.09 kg/m3 Air: 1.3 kg/m3 Oak: 720 kg/m3 Ethyl alcohol: 790 kg/m3 Ice: 920 kg/m3 Pure water: 1,000 kg/m3 Concrete: 2,300 kg/m3 Iron and steel: 7,800 kg/m3 Lead: 11,000 kg/m3 Gold: 19,000 kg/m3

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Note that pure water (as opposed to sea water, which is 3% denser) has a density of 1,000 kilograms per cubic meter, or 1 gram per cubic centimeter. This value is approximate; however, at a temperature of 39.2°F (4°C) and under normal atmospheric pressure, it is exact, and so, water is a useful standard for measuring the specific gravity of other substances. S P E C I F I C G RAV I T Y A N D T H E D E N S I T I E S O F P LA N E T S . Specific

gravity is the ratio between the densities of two objects or substances, and it is expressed as a number without units of measure. Due to the value of 1 g/cm3 for water, it is easy to determine the specific gravity of a given substance, which will have the same number value as its density. For example, the specific gravity of concrete, which has a density of 2.3 g/cm3, is 2.3. The speVOLUME 2: REAL-LIFE PHYSICS

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cific gravities of gases are usually determined in comparison to the specific gravity of dry air. Most rocks near the surface of Earth have a specific gravity of somewhere between 2 and 3, while the specific gravity of the planet itself is about 5. How do scientists know that the density of Earth is around 5 g/cm3? The computation is fairly simple, given the fact that the mass and volume of the planet are known. And given the fact that most of what lies close to Earth’s surface— sea water, soil, rocks—has a specific gravity well below 5, it is clear that Earth’s interior must contain high-density materials, such as nickel or iron. In the same way, calculations regarding the density of other objects in the Solar System provide a clue as to their interior composition. Closer to home, a comparison of density makes it possible to determine whether a piece of jewelry alleged to be solid gold is really genuine. To determine the answer, one must drop it in a beaker of water with graduated units of measure clearly marked. (Here, figures are given in cubic centimeters, since these are easiest to use in this context.) ALL

T H AT

GLITTERS.

Suppose the item has a mass of 10 grams. The density of gold is 19 g/cm3, and since V = m/d = 10/19, the volume of water displaced by the gold should be 0.53 cm3. Suppose that instead, the item displaced 0.91 cm3 of water. Clearly, it is not gold, but what is it?

26

pure gold and pure lead, one could calculate what portion of the item was gold and which lead. It is possible, of course, that it could contain some other metal, but given the high specific gravity of lead, and the fact that its density is relatively close to that of gold, lead is a favorite gold substitute among jewelry counterfeiters. WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. Chahrour, Janet. Flash! Bang! Pop! Fizz!: Exciting Science for Curious Minds. Illustrated by Ann Humphrey Williams. Hauppauge, N.Y.: Barron’s, 2000. “Density and Specific Gravity” (Web site). (March 27, 2001). “Density, Volume, and Cola” (Web site). (March 27, 2001). “The Mass Volume Density Challenge” (Web site). (March 27, 2001). “Metric Density and Specific Gravity” (Web site). (March 27, 2001). “Mineral Properties: Specific Gravity” The Mineral and Gemstone Kingdom (Web site). (March 27, 2001). Robson, Pam. Clocks, Scales and Measurements. New York: Gloucester Press, 1993.

Given the figures for mass and volume, its density would be equal to m/V = 10/0.91 = 11 g/cm3—which happens to be the density of lead. If on the other hand the amount of water displaced were somewhere between the values for

Willis, Shirley. Tell Me How Ships Float. Illustrated by the author. New York: Franklin Watts, 1999.

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“Volume, Mass, and Density” (Web site). (March 27, 2001).

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C O N S E R VAT I O N L A W S

CONCEPT The term “conservation laws” might sound at first like a body of legal statutes geared toward protecting the environment. In physics, however, the term refers to a set of principles describing certain aspects of the physical universe that are preserved throughout any number of reactions and interactions. Among the properties conserved are energy, linear momentum, angular momentum, and electrical charge. (Mass, too, is conserved, though only in situations well below the speed of light.) The conservation of these properties can be illustrated by examples as diverse as dropping a ball (energy); the motion of a skater spinning on ice (angular momentum); and the recoil of a rifle (linear momentum).

HOW IT WORKS The conservation laws describe physical properties that remain constant throughout the various processes that occur in the physical world. In physics, “to conserve” something means “to result in no net loss of ” that particular component. For each such component, the input is the same as the output: if one puts a certain amount of energy into a physical system, the energy that results from that system will be the same as the energy put into it. The energy may, however, change forms. In addition, the operations of the conservation laws are—on Earth, at least—usually affected by a number of other forces, such as gravity, friction, and air resistance. The effects of these forces, combined with the changes in form that take place within a given conserved property, sometimes make it difficult to perceive the working of the conservation laws. It was stated above that

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the resulting energy of a physical system will be the same as the energy that was introduced to it. Note, however, that the usable energy output of a system will not be equal to the energy input. This is simply impossible, due to the factors mentioned above—particularly friction. When one puts gasoline into a motor, for instance, the energy that the motor puts out will never be as great as the energy contained in the gasoline, because part of the input energy is expended in the operation of the motor itself. Similarly, the angular momentum of a skater on ice will ultimately be dissipated by the resistant force of friction, just as that of a Frisbee thrown through the air is opposed both by gravity and air resistance—itself a specific form of friction. In each of these cases, however, the property is still conserved, even if it does not seem so to the unaided senses of the observer. Because the motor has a usable energy output less than the input, it seems as though energy has been lost. In fact, however, the energy has only changed forms, and some of it has been diverted to areas other than the desired output. (Both the noise and the heat of the motor, for instance, represent uses of energy that are typically considered undesirable.) Thus, upon closer study of the motor—itself an example of a system—it becomes clear that the resulting energy, if not the desired usable output, is the same as the energy input. As for the angular momentum examples in which friction, or air resistance, plays a part, here too (despite all apparent evidence to the contrary) the property is conserved. This is easier to understand if one imagines an object spinning in outer space, free from the opposing force of friction. Thanks to the conservation of angular

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Conservation Laws

AS

THIS HUNTER FIRES HIS RIFLE, THE RIFLE PRODUCES A BACKWARD

“KICK”

AGAINST HIS SHOULDER.

THIS

KICK,

WITH A VELOCITY IN THE OPPOSITE DIRECTION OF THE BULLET ’S TRAJECTORY, HAS A MOMENTUM EXACTLY THE SAME AS THAT OF THE BULLET ITSELF: HENCE MOMENTUM IS CONSERVED. (Photograph by Tony Arruza/Corbis. Reproduced by

permission.)

momentum, an object set into rotation in space will continue to spin indefinitely. Thus, if an astronaut in the 1960s, on a spacewalk from his capsule, had set a screwdriver spinning in the emptiness of the exosphere, the screwdriver would still be spinning today!

and kinetic energy. Every system possesses a certain quantity of both, and the sum of its potential and kinetic energy is known as mechanical energy. The mechanical energy within a system does not change, but the relative values of potential and kinetic energy may be altered.

Energy and Mass

A S I M P L E E XA M P L E O F M E C H A N I CA L E N E R GY. If one held a base-

Among the most fundamental statements in all of science is the conservation of energy: a system isolated from all outside factors will maintain the same total amount of energy, even though energy transformations from one form or another take place. Energy is manifested in many varieties, including thermal, electromagnetic, sound, chemical, and nuclear energy, but all these are merely reflections of three basic types of energy. There is potential energy, which an object possesses by virtue of its position; kinetic energy, which it possesses by virtue of its motion; and rest energy, which it possesses by virtue of its mass.

28

ball at the top of a tall building, it would have a certain amount of potential energy. Once it was dropped, it would immediately begin losing potential energy and gaining kinetic energy proportional to the potential energy it lost. The relationship between the two forms, in fact, is inverse: as the value of one variable decreases, that of the other increases in exact proportion.

The last of these three will be discussed in the context of the relationship between energy and mass; at present the concern is with potential

The ball cannot keep falling forever, losing potential energy and gaining kinetic energy. In fact, it can never gain an amount of kinetic energy greater than the potential energy it possessed in the first place. At the moment before it hits the ground, the ball’s kinetic energy is equal to the potential energy it possessed at the top of the building. Correspondingly, its potential energy is zero—the same amount of kinetic energy it possessed before it was dropped.

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Then, as the ball hits the ground, the energy is dispersed. Most of it goes into the ground, and depending on the rigidity of the ball and the ground, this energy may cause the ball to bounce. Some of the energy may appear in the form of sound, produced as the ball hits bottom, and some will manifest as heat. The total energy, however, will not be lost: it will simply have changed form.

Conservation Laws

R E S T E N E R G Y. The values for mechanical energy in the above illustration would most likely be very small; on the other hand, the rest or mass energy of the baseball would be staggering. Given the weight of 0.333 pounds for a regulation baseball, which on Earth converts to 0.15 kg in mass, it would possess enough energy by virtue of its mass to provide a year’s worth of electrical power to more than 150,000 American homes. This leads to two obvious questions: how can a mere baseball possess all that energy? And if it does, how can the energy be extracted and put to use?

The answer to the second question is, “By accelerating it to something close to the speed of light”—which is more than 27,000 times faster than the fastest speed ever achieved by humans. (The astronauts on Apollo 10 in May 1969 reached nearly 25,000 MPH (40,000 km/h), which is more than 33 times the speed of sound but still insignificant when compared to the speed of light.) The answer to the first question lies in the most well-known physics formula of all time: E = mc2 In 1905, Albert Einstein (1879-1955) published his Special Theory of Relativity, which he followed a decade later with his General Theory of Relativity. These works introduced the world to the above-mentioned formula, which holds that energy is equal to mass multiplied by the squared speed of light. This formula gained its widespread prominence due to the many implications of Einstein’s Relativity, which quite literally changed humanity’s perspective on the universe. Most concrete among those implications was the atom bomb, made possible by the understanding of mass and energy achieved by Einstein.

AS SURYA BONALY

GOES INTO A SPIN ON THE ICE, SHE

DRAWS IN HER ARMS AND LEG, REDUCING THE MOMENT OF INERTIA. LAR

BECAUSE

MOMENTUM ,

OF THE CONSERVATION OF ANGU-

HER

ANGULAR

VELOCITY

WILL

INCREASE, MEANING THAT SHE WILL SPIN MUCH FASTER.

(Bolemian Nomad Picturemakers/Corbis. Reproduced by permission.)

mechanical energy, its relation to the other forms can be easily shown. All three are defined in terms of mass. Potential energy is equal to mgh, where m is mass, g is gravity, and h is height. Kinetic energy is equal to 1⁄2 mv2, where v is velocity. In fact—using a series of steps that will not be demonstrated here—it is possible to directly relate the kinetic and rest energy formulae.

In fact, E = mc2 is the formula for rest energy, sometimes called mass energy. Though rest energy is “outside” of kinetic and potential energy in the sense that it is not defined by the abovedescribed interactions within the larger system of

The kinetic energy formula describes the behavior of objects at speeds well below the speed of light, which is 186,000 mi (297,600 km) per second. But at speeds close to that of the speed of light, 1⁄2 mv2 does not accurately reflect the energy possessed by the object. For instance, if v were equal to 0.999c (where c represents the speed of light), then the application of the formula 1⁄2 mv2 would yield a value equal to less than 3% of the object’s real energy. In order to calculate the true energy of an object at 0.999c, it would be necessary to apply a different formula for total energy, one that takes into account the fact that, at such a speed, mass itself becomes energy.

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C O N S E R V AT I O N

OF

MASS.

Mass itself is relative at speeds approaching c, and, in fact, becomes greater and greater the closer an object comes to the speed of light. This may seem strange in light of the fact that there is, after all, a law stating that mass is conserved. But mass is only conserved at speeds well below c: as an object approaches 186,000 mi (297,600 km) per second, the rules are altered. The conservation of mass states that total mass is constant, and is unaffected by factors such as position, velocity, or temperature, in any system that does not exchange any matter with its environment. Yet, at speeds close to c, the mass of an object increases dramatically. In such a situation, the mass would be equal to the rest, or starting mass, of the object divided by 8(1 – (v2/c2), where v is the object’s speed of relative motion. The denominator of this equation will always be less than one, and the greater the value of v, the smaller the value of the denominator. This means that at a speed of c, the denominator is zero—in other words, the object’s mass is infinite! Obviously, this is not possible, and indeed, what the formula actually shows is that no object can travel faster than the speed of light. Of particular interest to the present discussion, however, is the fact, established by relativity theory, that mass can be converted into energy. Hence, as noted earlier, a baseball or indeed any object can be converted into energy—and since the formula for rest energy requires that the mass be multiplied by c2, clearly, even an object of virtually negligible mass can generate a staggering amount of energy. This conversion of mass to energy happens well below the speed of light, in a very small way, when a stick of dynamite explodes. A portion of that stick becomes energy, and the fact that this portion is equal to just 6 parts out of 100 billion indicates the vast proportions of energy available from converted mass.

Other Conservation Laws In addition to the conservation of energy, as well as the limited conservation of mass, there are laws governing the conservation of momentum, both for an object in linear (straight-line) motion, and for one in angular (rotational) motion. Momentum is a property that a moving body possesses by virtue of its mass and velocity, which determines the amount of force and time

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VOLUME 2: REAL-LIFE PHYSICS

required to stop it. Linear momentum is equal to mass multiplied by velocity, and the conservation of linear momentum law states that when the sum of the external force vectors acting on a physical system is equal to zero, the total linear momentum of the system remains unchanged, or conserved. Angular momentum, or the momentum of an object in rotational motion, is equal to mr2ω, where m is mass, r is the radius of rotation, and ω (the Greek letter omega) stands for angular velocity. According to the conservation of angular momentum law, when the sum of the external torques acting on a physical system is equal to zero, the total angular momentum of the system remains unchanged. Torque is a force applied around an axis of rotation. When playing the old game of “spin the bottle,” for instance, one is applying torque to the bottle and causing it to rotate. E L E C T R I C C H A R G E . The conservation of both linear and angular momentum are best explained in the context of real-life examples, provided below. Before going on to those examples, however, it is appropriate here to discuss a conservation law that is outside the realm of everyday experience: the conservation of electric charge, which holds that for an isolated system, the net electric charge is constant.

This law is “outside the realm of everyday experience” such that one cannot experience it through the senses, but at every moment, it is happening everywhere. Every atom has positively charged protons, negatively charged electrons, and uncharged neutrons. Most atoms are neutral, possessing equal numbers of protons and electrons; but, as a result of some disruption, an atom may have more protons than electrons, and thus, become positively charged. Conversely, it may end up with a net negative charge due to a greater number of electrons. But the protons or electrons it released or gained did not simply appear or disappear: they moved from one part of the system to another—that is, from one atom to another atom, or to several other atoms. Throughout these changes, the charge of each proton and electron remains the same, and the net charge of the system is always the sum of its positive and negative charges. Thus, it is impossible for any electrical charge in the universe to be smaller than that of a proton or electron. Likewise, throughout the universe, there is

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always the same number of negative and positive electrical charges: just as energy changes form, the charges simply change position. There are also conservation laws describing the behavior of subatomic particles, such as the positron and the neutrino. However, the most significant of the conservation laws are those involving energy (and mass, though with the limitations discussed above), linear momentum, angular momentum, and electrical charge.

REAL-LIFE A P P L I C AT I O N S Conservation of Linear Momentum: Rifles and Rockets F I R I N G A R I F L E . The conservation of linear momentum is reflected in operations as simple as the recoil of a rifle when it is fired, and in those as complex as the propulsion of a rocket through space. In accordance with the conservation of momentum, the momentum of a system must be the same after it undergoes an operation as it was before the process began. Before firing, the momentum of a rifle and bullet is zero, and therefore, the rifle-bullet system must return to that same zero-level of momentum after it is fired. Thus, the momentum of the bullet must be matched—and “cancelled” within the system under study—by a corresponding backward momentum.

When a person shooting a gun pulls the trigger, it releases the bullet, which flies out of the barrel toward the target. The bullet has mass and velocity, and it clearly has momentum; but this is only half of the story. At the same time it is fired, the rifle produces a “kick,” or sharp jolt, against the shoulder of the person who fired it. This backward kick, with a velocity in the opposite direction of the bullet’s trajectory, has a momentum exactly the same as that of the bullet itself: hence, momentum is conserved. But how can the rearward kick have the same momentum as that of the bullet? After all, the bullet can kill a person, whereas, if one holds the rifle correctly, the kick will not even cause any injury. The answer lies in several properties of linear momentum. First of all, as noted earlier, momentum is equal to mass multiplied by velocity; the actual proportions of mass and velocity,

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however, are not important as long as the backward momentum is the same as the forward momentum. The bullet is an object of relatively small mass and high velocity, whereas the rifle is much larger in mass, and hence, its rearward velocity is correspondingly small.

Conservation Laws

In addition, there is the element of impulse, or change in momentum. Impulse is the product of force multiplied by change or interval in time. Again, the proportions of force and time interval do not matter, as long as they are equal to the momentum change—that is, the difference in momentum that occurs when the rifle is fired. To avoid injury to one’s shoulder, clearly force must be minimized, and for this to happen, time interval must be extended. If one were to fire the rifle with the stock (the rear end of the rifle) held at some distance from one’s shoulder, it would kick back and could very well produce a serious injury. This is because the force was delivered over a very short time interval—in other words, force was maximized and time interval minimized. However, if one holds the rifle stock firmly against one’s shoulder, this slows down the delivery of the kick, thus maximizing time interval and minimizing force. R O C K E T I N G T H R O U G H S PAC E .

Contrary to popular belief, rockets do not move by pushing against a surface such as a launchpad. If that were the case, then a rocket would have nothing to propel it once it had been launched, and certainly there would be no way for a rocket to move through the vacuum of outer space. Instead, what propels a rocket is the conservation of momentum. Upon ignition, the rocket sends exhaust gases shooting downward at a high rate of velocity. The gases themselves have mass, and thus, they have momentum. To balance this downward momentum, the rocket moves upward—though, because its mass is greater than that of the gases it expels, it will not move at a velocity as high as that of the gases. Once again, the upward or forward momentum is exactly the same as the downward or backward momentum, and linear momentum is conserved. Rather than needing something to push against, a rocket in fact performs best in outer space, where there is nothing—neither launchpad nor even air—against which to push. Not only is “pushing” irrelevant to the operation of

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Conservation Laws

KEY TERMS A set of principles describing physical properties that remain constant—that is, are conserved—throughout the various processes that occur in the physical world. The most significant of these laws concerns the conservation of energy (as well as, with qualifications, the conservation of mass); conservation of linear momentum; conservation of angular momentum; and conservation of electrical charge.

CONSERVATION OF MASS:

CONSERVATION OF ANGULAR MO-

mass begins converting to energy.

CONSERVATION

LAWS:

A physical law stating that when the sum of the external torques acting on a physical system is equal to zero, the total angular momentum of the system remains unchanged. Angular momentum is the momentum of an object in rotational motion, and torque is a force applied around an axis of rotation.

A phys-

ical principle stating that total mass is constant, and is unaffected by factors such as position, velocity, or temperature, in any system that does not exchange any matter with its environment. Unlike the other conservation laws, however, conservation of mass is not universally applicable, but applies only at speeds significant lower than that of light—186,000 mi (297,600 km) per second. Close to the speed of light,

MENTUM:

In physics, “to conserve”

something means “to result in no net loss of ” that particular component. It is possible that within a given system, the component may change form or position, but as long as the net value of the component remains the same, it has been conserved. The force that resists

CONSERVATION OF ELECTRICAL

FRICTION:

A physical law which holds that for an isolated system, the net electrical charge is constant.

motion when the surface of one object

CHARGE:

A law of physics stating that within a system isolated from all other outside factors, the total amount of energy remains the same, though transformations of energy from one form to another take place.

CONSERVATION OF ENERGY:

CONSERVATION OF LINEAR MO-

A physical law stating that when the sum of the external force vectors acting on a physical system is equal to zero, the total linear momentum of the system remains unchanged—or is conserved.

MENTUM:

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CONSERVE:

VOLUME 2: REAL-LIFE PHYSICS

comes into contact with the surface of another. MOMENTUM:

A property that a mov-

ing body possesses by virtue of its mass and velocity, which determines the amount of force and time required to stop it. SYSTEM:

In physics, the term “system”

usually refers to any set of physical interactions isolated from the rest of the universe. Anything outside of the system, including all factors and forces irrelevant to a discussion of that system, is known as the environment.

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the rocket, but the rocket moves much more efficiently without the presence of air resistance. In the same way, on the relatively frictionless surface of an ice-skating rink, conservation of linear momentum (and hence, the process that makes possible the flight of a rocket through space) is easy to demonstrate. If, while standing on the ice, one throws an object in one direction, one will be pushed in the opposite direction with a corresponding level of momentum. However, since a person’s mass is presumably greater than that of the object thrown, the rearward velocity (and, therefore, distance) will be smaller. Friction, as noted earlier, is not the only force that counters conservation of linear momentum on Earth: so too does gravity, and thus, once again, a rocket operates much better in space than it does when under the influence of Earth’s gravitational field. If a bullet is fired at a bottle thrown into the air, the linear momentum of the spent bullet and the shattered pieces of glass in the infinitesimal moment just after the collision will be the same as that of the bullet and the bottle a moment before impact. An instant later, however, gravity will accelerate the bullet and the pieces downward, thus leading to a change in total momentum.

Conservation of Angular Momentum: Skaters and Other Spinners As noted earlier, angular momentum is equal to mr2ω, where m is mass, r is the radius of rotation, and ω stands for angular velocity. In fact, the first two quantities, mr2, are together known as moment of inertia. For an object in rotation, moment of inertia is the property whereby objects further from the axis of rotation move faster, and thus, contribute a greater share to the overall kinetic energy of the body. One of the most oft-cited examples of angular momentum—and of its conservation— involves a skater or ballet dancer executing a spin. As the skater begins the spin, she has one leg planted on the ice, with the other stretched behind her. Likewise, her arms are outstretched, thus creating a large moment of inertia. But when she goes into the spin, she draws in her

S C I E N C E O F E V E RY DAY T H I N G S

arms and leg, reducing the moment of inertia. In accordance with conservation of angular momentum, mr2ω will remain constant, and therefore, her angular velocity will increase, meaning that she will spin much faster.

Conservation Laws

C O N S TA N T O R I E N TAT I O N . The motion of a spinning top and a Frisbee in flight also illustrate the conservation of angular momentum. Particularly interesting is the tendency of such an object to maintain a constant orientation. Thus, a top remains perfectly vertical while it spins, and only loses its orientation once friction from the floor dissipates its velocity and brings it to a stop. On a frictionless surface, however, it would remain spinning—and therefore upright—forever.

A Frisbee thrown without spin does not provide much entertainment; it will simply fall to the ground like any other object. But if it is tossed with the proper spin, delivered from the wrist, conservation of angular momentum will keep it in a horizontal position as it flies through the air. Once again, the Frisbee will eventually be brought to ground by the forces of air resistance and gravity, but a Frisbee hurled through empty space would keep spinning for eternity. WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. “Conservation Laws: An Online Physics Textbook” (Web site). (March 12, 2001). “Conservation Laws: The Most Powerful Laws of Physics” (Web site). (March 12, 2001). “Conservation of Energy.” NASA (Web site). (March 12, 2001). Elkana, Yehuda. The Discovery of the Conservation of Energy. With a foreword by I. Bernard Cohen. Cambridge, MA: Harvard University Press, 1974. “Momentum and Its Conservation” (Web site). (March 12, 2001). Rutherford, F. James; Gerald Holton; and Fletcher G. Watson. Project Physics. New York: Holt, Rinehart, and Winston, 1981. Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996.

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S C I E N C E O F E V E RY DAY T H I N G S real-life Physics

K I N E M AT I C S A N D PA R T I C L E D Y N A M I C S MOMENTUM C E N T R I P E TA L F O R C E FRICTION LAWS OF MOTION GRAVITY PROJECTILE MOTION TORQUE

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Momentum

MOMENTUM

CONCEPT

Momentum and Inertia

The faster an object is moving—whether it be a baseball, an automobile, or a particle of matter— the harder it is to stop. This is a reflection of momentum, or specifically, linear momentum, which is equal to mass multiplied by velocity. Like other aspects of matter and motion, momentum is conserved, meaning that when the vector sum of outside forces equals zero, no net linear momentum within a system is ever lost or gained. A third important concept is impulse, the product of force multiplied by length in time. Impulse, also defined as a change in momentum, is reflected in the proper methods for hitting a baseball with force or surviving a car crash.

It might be tempting to confuse momentum with another physical concept, inertia. Inertia, as defined by the second law of motion, is the tendency of an object in motion to remain in motion, and of an object at rest to remain at rest. Momentum, by definition, involves a body in motion, and can be defined as the tendency of a body in motion to continue moving at a constant velocity.

HOW IT WORKS Like many other aspects of physics, the word “momentum” is a part of everyday life. The common meaning of momentum, however, unlike many other physics terms, is relatively consistent with its scientific meaning. In terms of formula, momentum is equal to the product of mass and velocity, and the greater the value of that product, the greater the momentum. Consider the term “momentum” outside the world of physics, as applied, for example, in the realm of politics. If a presidential candidate sees a gain in public-opinion polls, then wins a debate and embarks on a whirlwind speaking tour, the media comments that he has “gained momentum.” As with momentum in the framework of physics, what these commentators mean is that the candidate will be hard to stop—or to carry the analogy further, that he is doing enough of the right things (thus gaining “mass”), and doing them quickly enough, thereby gaining velocity. S C I E N C E O F E V E RY DAY T H I N G S

Not only does momentum differ from inertia in that it relates exclusively to objects in motion, but (as will be discussed below) the component of velocity in the formula for momentum makes it a vector—that is, a quantity that possesses both magnitude and direction. There is at least one factor that momentum very clearly has in common with inertia: mass, a measure of inertia indicating the resistance of an object to a change in its motion.

Mass and Weight Unlike velocity, mass is a scalar, a quantity that possesses magnitude without direction. Mass is often confused with weight, a vector quantity equal to its mass multiplied by the downward acceleration due to gravity. The weight of an object changes according to the gravitational force of the planet or other celestial body on which it is measured. Hence, the mass of a person on the Moon would be the same as it is on Earth, whereas the person’s weight would be considerably less, due to the smaller gravitational pull of the Moon. Given the unchanging quality of mass as opposed to weight, as well as the fact that scientists themselves prefer the much simpler metric VOLUME 2: REAL-LIFE PHYSICS

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Momentum

WHEN

BILLIARD BALLS COLLIDE, THEIR HARDNESS RESULTS IN AN ELASTIC COLLISION—ONE IN WHICH KINETIC ENER-

GY IS CONSERVED. (Photograph by John-Marshall Mantel/Corbis. Reproduced by permission.)

system, metric units will generally be used in the following discussion. Where warranted, of course, conversion to English or British units (for example, the pound, a unit of weight) will be provided. However, since the English unit of mass, the slug, is even more unfamiliar to most Americans than its metric equivalent, the kilogram, there is little point in converting kilos into slugs.

Velocity and Speed Not only is momentum often confused with inertia, and mass with weight, but in the everyday world the concepts of velocity and speed tend to be blurred. Speed is the rate at which the position of an object changes over a given period of time, expressed in terms such as “50 MPH.” It is a scalar quantity.

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Linear Momentum and Its Conservation Momentum itself is sometimes designated as p. It should be stressed that the form of momentum discussed here is strictly linear, or straight-line, momentum, in contrast to angular momentum, more properly discussed within the framework of rotational motion. Both angular and linear momentum abide by what are known as conservation laws. These are statements concerning quantities that, under certain conditions, remain constant or unchanging. The conservation of linear momentum law states that when the sum of the external force vectors acting on a physical system is equal to zero, the total linear momentum of the system remains unchanged—or conserved.

Velocity, by contrast, is a vector. If one were to say “50 miles per hour toward the northeast,” this would be an expression of velocity. Vectors are typically designated in bold, without italics; thus velocity is typically abbreviated v. Scalars, on the other hand, are rendered in italics. Hence, the formula for momentum is usually shown as mv.

The conservation of linear momentum is reflected both in the recoil of a rifle and in the propulsion of a rocket through space. When a rifle is fired, it produces a “kick”—that is, a sharp jolt to the shoulder of the person who has fired it—corresponding to the momentum of the bullet. Why, then, does the “kick” not knock a person’s shoulder off the way a bullet would? Because the rifle’s mass is much greater than that of the bullet, meaning that its velocity is much smaller.

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As for rockets, they do not—contrary to popular belief—move by pushing against a surface, such as a launch pad. If that were the case, then a rocket would have nothing to propel it once it is launched, and certainly there would be no way for a rocket to move through the vacuum of outer space. Instead, as it burns fuel, the rocket expels exhaust gases that exert a backward momentum, and the rocket itself travels forward with a corresponding degree of momentum.

Momentum

Systems Here, “system” refers to any set of physical interactions isolated from the rest of the universe. Anything outside of the system, including all factors and forces irrelevant to a discussion of that system, is known as the environment. In the pool-table illustration shown earlier, the interaction of the billiard balls in terms of momentum is the system under discussion. It is possible to reduce a system even further for purposes of clarity: hence, one might specify that the system consists only of the pool balls, the force applied to them, and the resulting momentum interactions. Thus, we will ignore the friction of the pool table’s surface, and the assumption will be that the balls are rolling across a frictionless plane.

WHEN

PARACHUTISTS LAND, THEY KEEP THEIR KNEES

BENT AND OFTEN ROLL OVER—ALL IN AN EFFORT TO LENGTHEN THE PERIOD OF THE FORCE OF IMPACT, THUS REDUCING ITS EFFECTS. (Photograph by James A. Sugar/Corbis.

Reproduced by permission.)

in velocity over a change or interval in time. Expressed as a formula, this is a

Impulse For an object to have momentum, some force must have set it in motion, and that force must have been applied over a period of time. Likewise, when the object experiences a collision or any other event that changes its momentum, that change may be described in terms of a certain amount of force applied over a certain period of time. Force multiplied by time interval is impulse, expressed in the formula F • δt, where F is force, δ (the Greek letter delta) means “a change” or “change in...”; and t is time.

= ∆v

∆t .

Thus, force is equal to

) )

m ∆v ∆t , an equation that can be rewritten as Fδt = mδv. In other words, impulse is equal to change in momentum.

As with momentum itself, impulse is a vector quantity. Whereas the vector component of momentum is velocity, the vector quantity in impulse is force. The force component of impulse can be used to derive the relationship between impulse and change in momentum. According to the second law of motion, F = m a; that is, force is equal to mass multiplied by acceleration. Acceleration can be defined as a change

This relationship between impulse and momentum change, derived here in mathematical terms, will be discussed below in light of several well-known examples from the real world. Note that the metric units of impulse and momentum are actually interchangeable, though they are typically expressed in different forms, for the purpose of convenience. Hence, momentum is usually rendered in terms of kilogram-meters-per-second (kg • m/s), whereas impulse is typically shown as newton-seconds (N • s). In the English system, momentum is shown in units of slug-feet per-

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another so that they collide head-on. Due to the properties of clay as a substance, the two lumps will tend to stick. Assuming the lumps are not of equal mass, they will continue traveling in the same direction as the lump with greater momentum.

Momentum

As they meet, the two lumps form a larger mass MV that is equal to the sum of their two individual masses. Once again, MV = m1v1 + m2v2. The M in MV is the sum of the smaller values m, and the V is the vector sum of velocity. Whereas M is larger than m1 or m2—the reason being that scalars are simply added like ordinary numbers—V is smaller than v1 or v2. This lower number for net velocity as compared to particle velocity will always occur when two objects are moving in opposite directions. (If the objects are moving in the same direction, V will have a value between that of v1 and v2.)

AS SAMMY SOSA’S

BAT HITS THIS BALL, IT APPLIES A

TREMENDOUS MOMENTUM CHANGE TO THE BALL. CONTACT WITH THE BALL,

SOSA

AFTER

WILL CONTINUE HIS

SWING, THEREBY CONTRIBUTING TO THE MOMENTUM CHANGE AND ALLOWING THE BALL TO TRAVEL FARTHER.

(AFP/Corbis. Reproduced by permission.)

second, and impulse in terms of the poundsecond.

REAL-LIFE A P P L I C AT I O N S When Two Objects Collide Two moving objects, both possessing momentum by virtue of their mass and velocity, collide with one another. Within the system created by their collision, there is a total momentum MV that is equal to their combined mass and the vector sum of their velocity. This is the case with any system: the total momentum is the sum of the various individual momentum products. In terms of a formula, this is expressed as MV = m1v1 + m2v2 + m3v3 +... and so on. As noted earlier, the total momentum will be conserved; however, the actual distribution of momentum within the system may change.

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To add the vector sum of the two lumps in collision, it is best to make a diagram showing the bodies moving toward one another, with arrows illustrating the direction of velocity. By convention, in such diagrams the velocity of an object moving to the right is rendered as a positive number, and that of an object moving to the left is shown with a negative number. It is therefore easier to interpret the results if the object with the larger momentum is shown moving to the right. The value of V will move in the same direction as the lump with greater momentum. But since the two lumps are moving in opposite directions, the momentum of the smaller lump will cancel out a portion of the greater lump’s momentum—much as a negative number, when added to a positive number of greater magnitude, cancels out part of the positive number’s value. They will continue traveling in the direction of the lump with greater momentum, now with a combined mass equal to the arithmetic sum of their masses, but with a velocity much smaller than either had before impact.

T W O LU M P S O F C LAY. Consider the behavior of two lumps of clay, thrown at one

B I L L I A R D B A L L S . The game of pool provides an example of a collision in which one object, the cue ball, is moving, while the other— known as the object ball—is stationary. Due to the hardness of pool balls, and their tendency not to stick to one another, this is also an example of an almost perfectly elastic collision—one in which kinetic energy is conserved.

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The colliding lumps of clay, on the other hand, are an excellent example of an inelastic collision, or one in which kinetic energy is not conserved. The total energy in a given system, such as that created by the two lumps of clay in collision, is conserved; however, kinetic energy may be transformed, for instance, into heat energy and/or sound energy as a result of collision. Whereas inelastic collisions involve soft, sticky objects, elastic collisions involve rigid, non-sticky objects. Kinetic energy and momentum both involve components of velocity and mass: p (momentum) is equal to mv, and KE (kinetic energy) equals 1⁄2 mv2. Due to the elastic nature of poolball collisions, when the cue ball strikes the object ball, it transfers its velocity to the latter. Their masses are the same, and therefore the resulting momentum and kinetic energy of the object ball will be the same as that possessed by the cue ball prior to impact. If the cue ball has transferred all of its velocity to the object ball, does that mean it has stopped moving? It does. Assuming that the interaction between the cue ball and the object ball constitutes a closed system, there is no other source from which the cue ball can acquire velocity, so its velocity must be zero. It should be noted that this illustration treats pool-ball collisions as though they were 100% elastic, though in fact, a portion of kinetic energy in these collisions is transformed into heat and sound. Also, for a cue ball to transfer all of its velocity to the object ball, it must hit it straighton. If the balls hit off-center, not only will the object ball move after impact, but the cue ball will continue to move—roughly at 90° to a line drawn through the centers of the two balls at the moment of impact.

Impulse: Breaking or Building the Impact

Impulse, again, is equal to momentum change—and also equal to force multiplied by time interval (or change in time). This means that the greater the force and the greater the amount of time over which it is applied, the greater the momentum change. Even more interesting is the fact that one can achieve the same momentum change with differing levels of force and time interval. In other words, a relatively low degree of force applied over a relatively long period of time would produce the same momentum change as a relatively high amount of force over a relatively short period of time.

Momentum

The conservation of kinetic energy in a collision is, as noted earlier, a function of the relative elasticity of that collision. The question of whether KE is transferred has nothing to do with impulse. On the other hand, the question of how KE is transferred—or, even more specifically, the interval over which the transfer takes place—is very much related to impulse. Kinetic energy, again, is equal to 1⁄2 mv2. If a moving car were to hit a stationary car head-on, it would transfer a quantity of kinetic energy to the stationary car equal to one-half its own mass multiplied by the square of its velocity. (This, of course, assumes that the collision is perfectly elastic, and that the mass of the cars is exactly equal.) A transfer of KE would also occur if two moving cars hit one another headon, especially in a highly elastic collision. Assuming one car had considerably greater mass and velocity than the other, a high degree of kinetic energy would be transferred—which could have deadly consequences for the people in the car with less mass and velocity. Even with cars of equal mass, however, a high rate of acceleration can bring about a potentially lethal degree of force. C R U M P L E Z O N E S I N CA R S . In a highly elastic car crash, two automobiles would bounce or rebound off one another. This would mean a dramatic change in direction—a reversal, in fact—hence, a sudden change in velocity and therefore momentum. In other words, the figure for mδv would be high, and so would that for impulse, Fδt.

When a cue ball hits an object ball in pool, it is safe to assume that a powerful impact is desired. The same is true of a bat hitting a baseball. But what about situations in which a powerful impact is not desired—as for instance when cars are crashing? There is, in fact, a relationship between impulse, momentum change, transfer of kinetic energy, and the impact—desirable or undesirable—experienced as a result.

On the other hand, it is possible to have a highly inelastic car crash, accompanied by a small change in momentum. It may seem logical to think that, in a crash situation, it would be better for two cars to bounce off one another than

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Momentum

KEY TERMS ACCELERATION:

A change velocity.

IMPULSE:

Acceleration can be expressed as a formula

time required to cause a change in

δv/δt—that is, change in velocity divided

momentum. Impulse is the product of

by change, or interval, in time.

force multiplied by a change, or interval, in

CONSERVATION

OF

LINEAR

time (Fδt):

the greater the momentum,

A physical law, which

the greater the force needed to change it,

states that when the sum of the external

and the longer the period of time over

force vectors acting on a physical system is

which it must be applied.

MOMENTUM:

equal to zero, the total linear momentum

INELASTIC COLLISION:

A collision

of the system remains unchanged—or is

in which kinetic energy is not conserved.

conserved.

(The total energy is conserved: kinetic In physics, “to conserve”

energy itself, however, may be transformed

something (for example, momentum or

into heat energy or sound energy.) Typical-

kinetic energy) means “to result in no net

ly, inelastic collisions involve non-rigid,

loss of ” that particular component. It is

sticky objects—for instance, lumps of clay.

possible that within a given system, one

At the other extreme is an elastic collision.

CONSERVE:

type of energy may be transformed into another type, but the net energy in the system will remain the same. ELASTIC COLLISION:

INERTIA:

The tendency of an object in

motion to remain in motion, and of an object at rest to remain at rest.

A collision in

which kinetic energy is conserved. Typically elastic collisions involve rigid, non-sticky

KINETIC ENERGY:

The energy an

object possesses by virtue of its motion. A measure of inertia, indicating

objects such as pool balls. At the other

MASS:

extreme is an inelastic collision.

the resistance of an object to a change in its

for them to crumple together. In fact, however, the latter option is preferable. When the cars crumple rather than rebounding, they do not experience a reversal in direction. They do experience a change in speed, of course, but the momentum change is far less than it would be if they rebounded. Furthermore, crumpling lengthens the amount of time during which the change in velocity occurs, and thus reduces impulse. But even with the reduced impulse of this momentum change, it is possible to further reduce the effect of force, another aspect of impact. Remember that mδv = Fδt: the value of force and time interval do not matter, as long as their product is equal to the momentum change. Because F and

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The amount of force and

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δt are inversely proportional, an increase in impact time will reduce the effects of force. For this reason, car manufacturers actually design and build into their cars a feature known as a crumple zone. A crumple zone—and there are usually several in a single automobile—is a section in which the materials are put together in such a way as to ensure that they will crumple when the car experiences a collision. Of course, the entire car cannot be one big crumple zone— this would be fatal for the driver and riders; however, the incorporation of crumple zones at key points can greatly reduce the effect of the force a car and its occupants must endure in a crash. Another major reason for crumple zones is to keep the passenger compartment of the car

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Momentum

KEY TERMS

CONTINUED

motion—including a change in velocity. A

all factors and forces irrelevant to a discus-

kilogram is a unit of mass, whereas a

sion of that system, is known as the envi-

pound is a unit of weight.

ronment.

MOMENTUM:

A property that a mov-

A quantity that possesses

VECTOR:

ing body possesses by virtue of its mass and

both magnitude and direction—as con-

velocity, which determines the amount of

trasted with a scalar, which possesses mag-

force and time (impulse) required to stop

nitude without direction. Vector quantities

it. Momentum—actually linear momen-

are usually expressed in bold, non-itali-

tum, as opposed to the angular momen-

cized letters, thus: F (force). They may also

tum of an object in rotational motion—is

be shown by placing an arrow over the let-

equal to mass multiplied by velocity.

ter designating the specific property, as for

SCALAR:

A quantity that possesses

instance v for velocity.

only magnitude, with no specific direc-

VECTOR

tion—as contrasted with a vector, which

yields the net result of all the vectors

possesses both magnitude and direction.

applied in a particular situation. In the case

Scalar quantities are usually expressed in

of momentum, the vector component is

italicized letters, thus: m (mass).

velocity. The best method is to make a dia-

SUM:

A calculation that

The rate at which the position

gram showing bodies in collision, with

of an object changes over a given period of

arrows illustrating the direction of velocity.

time.

On such a diagram, motion to the right is

SPEED:

SYSTEM:

In physics, the term “system”

usually refers to any set of physical interac-

assigned a positive value, and to the left a negative value.

tions isolated from the rest of the universe.

VELOCITY:

Anything outside of the system, including

particular direction.

intact. Many injuries are caused when the body of the car intrudes on the space of the occupants—as, for instance, when the floor buckles, or when the dashboard is pushed deep into the passenger compartment. Obviously, it is preferable to avoid this by allowing the fender to collapse.

The speed of an object in a

collision with part of the car. As it deflates, it is receding toward the dashboard even as the driver’s or passenger’s body is being hurled toward the dashboard. It slows down impact, extending the amount of time during which the force is distributed.

mizing force in a car accident, in this case by reducing the time over which the occupants move forward toward the dashboard or windshield. The airbag rapidly inflates, and just as rapidly begins to deflate, within the split-second that separates the car’s collision and a person’s

By the same token, a skydiver or paratrooper does not hit the ground with legs outstretched: he or she would be likely to suffer a broken bone or worse from such a foolish stunt. Rather, as a parachutist prepares to land, he or she keeps knees bent, and upon impact immediately rolls over to the side. Thus, instead of experiencing the force of impact over a short period of time, the parachutist lengthens the amount of time that force is experienced, which reduces its effects.

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The same principle applies if one were catching a water balloon. In order to keep it from bursting, one needs to catch the balloon in midair, then bring it to a stop slowly by “traveling” with it for a few feet before reducing its momentum down to zero. Once again, there is no way around the fact that one is attempting to bring about a substantial momentum change—a change equal in value to the momentum of the object in movement. Nonetheless, by increasing the time component of impulse, one reduces the effects of force. In old Superman comics, the “Man of Steel” often caught unfortunate people who had fallen, or been pushed, out of tall buildings. The cartoons usually showed him, at a stationary position in midair, catching the person before he or she could hit the ground. In fact, this would not save their lives: the force component of the sudden momentum change involved in being caught would be enough to kill the person. Of course, it is a bit absurd to quibble over scientific accuracy in Superman, but in order to make the situation more plausible, the “Man of Steel” should have been shown catching the person, then slowly following through on the trajectory of the fall toward earth. T H E C R A C K O F T H E B AT : I N C R E AS I N G I M PU L S E . But what if—

to once again turn the tables—a strong force is desired? This time, rather than two pool balls striking one another, consider what happens when a batter hits a baseball. Once more, the correlation between momentum change and impulse can create an advantage, if used properly. As the pitcher hurls the ball toward home plate, it has a certain momentum; indeed, a pitch thrown by a major-league player can send the ball toward the batter at speeds around 100 MPH (160 km/h)—a ball having considerable momentum). In order to hit a line drive or “knock the ball out of the park,” the batter must therefore cause a significant change in momentum.

44

sports—and it applies as much in tennis or golf as in baseball—as “following through.” By increasing the time of impact, the batter has increased impulse and thus, momentum change. Obviously, the mass of the ball has not been altered; the difference, then, is a change in velocity. How is it possible that in earlier examples, the effects of force were decreased by increasing the time interval, whereas in the baseball illustration, an increase in time interval resulted in a more powerful impact? The answer relates to differences in direction and elasticity. The baseball and the bat are colliding head-on in a relatively elastic situation; by contrast, crumpling cars are inelastic. In the example of a person catching a water balloon, the catcher is moving in the same direction as the balloon, thus reducing momentum change. Even in the case of the paratrooper, the ground is stationary; it does not move toward the parachutist in the way that the baseball moves toward the bat. WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. Bonnet, Robert L. and Dan Keen. Science Fair Projects: Physics. Illustrated by Frances Zweifel. New York: Sterling, 1999. Fleisher, Paul. Objects in Motion: Principles of Classical Mechanics. Minneapolis, MN: Lerner Publications, 2002. Gardner, Robert. Experimenting with Science in Sports. New York: F. Watts, 1993. “Lesson 1: The Impulse Momentum Change Theorem” (Web site) (March 19, 2001). “Momentum” (Web site). (March 19, 2001). Physlink.com (Web site). (March 7, 2001). Rutherford, F. James; Gerald Holton; and Fletcher G. Watson. Project Physics. New York: Holt, Rinehart, and Winston, 1981. Schrier, Eric and William F. Allman. Newton at the Bat: The Science in Sports. New York: Charles Scribner’s Sons, 1984.

Consider the momentum change in terms of the impulse components. The batter can only apply so much force, but it is possible to magnify impulse greatly by increasing the amount of time over which the force is delivered. This is known in

Zubrowski, Bernie. Raceways: Having Fun with Balls and Tracks. Illustrated by Roy Doty. New York: William Morrow, 1985.

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C E N T R I P E TA L F O R C E

CONCEPT Most people have heard of centripetal and centrifugal force. Though it may be somewhat difficult to keep track of which is which, chances are anyone who has heard of the two concepts remembers that one is the tendency of objects in rotation to move inward, and the other is the tendency of rotating objects to move outward. It may come as a surprise, then, to learn that there is no such thing, strictly speaking, as centrifugal (outward) force. There is only centripetal (inward) force and the inertia that makes objects in rotation under certain situations move outward, for example, a car making a turn, the movement of a roller coaster— even the spinning of a centrifuge.

HOW IT WORKS Like many other principles in physics, centripetal force ultimately goes back to a few simple precepts relating to the basics of motion. Consider an object in uniform circular motion: an object moves around the center of a circle so that its speed is constant or unchanging. The formula for speed—or rather, average speed—is distance divided by time; hence, people say, for instance, “miles (or kilometers) per hour.” In the case of an object making a circle, distance is equal to the circumference, or distance around, the circle. From geometry, we know that the formula for calculating the circumference of a circle is 2πr, where r is the radius, or the distance from the circumference to the center. The figure π may be rendered as 3.141592..., though in fact, it is an irrational number: the decimal figures continue forever without repetition or pattern. S C I E N C E O F E V E RY DAY T H I N G S

From the above, it can be discerned that the formula for the average speed of an object moving around a circle is 2πr divided by time. Furthermore, we can see that there is a proportional relationship between radius and average speed. If the radius of a circle is doubled, but an object at the circle’s periphery makes one complete revolution in the same amount of time as before, this means that the average speed has doubled as well. This can be shown by setting up two circles, one with a radius of 2, the other with a radius of 4, and using some arbitrary period of time—say, 2 seconds. The above conclusion carries with it an interesting implication with regard to speeds at different points along the radius of a circle. Rather than comparing two points moving around the circumferences of two different circles—one twice as big as the other—in the same period of time, these two points could be on the same circle: one at the periphery, and one exactly halfway along the radius. Assuming they both traveled a complete circle in the same period of time, the proportional relationship described earlier would apply. This means, then, that the further out on the circle one goes, the greater the average speed.

Velocity = Speed + Direction Speed is a scalar, meaning that it has magnitude but no specific direction; by contrast, velocity is a vector—a quantity with both a magnitude (that is, speed) and a direction. For an object in circular motion, the direction of velocity is the same as that in which the object is moving at any given point. Consider the example of the city of Atlanta, Georgia, and Interstate-285, one of several instances in which a city is surrounded by a “loop” highway. Local traffic reporters avoid givVOLUME 2: REAL-LIFE PHYSICS

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means that it is experiencing acceleration. As with the subject of centripetal force and “centrifugal force,” most people have a mistaken view of acceleration, believing that it refers only to an increase in speed. In fact, acceleration is a change in velocity, and can thus refer either to a change in speed or direction. Nor must that change be a positive one; in other words, an object undergoing a reduction in speed is also experiencing acceleration.

Centripetal Force

The acceleration of an object in rotational motion is always toward the center of the circle. This may appear to go against common sense, which should indicate that acceleration moves in the same direction as velocity, but it can, in fact, be proven in a number of ways. One method would be by the addition of vectors, but a “hands-on” demonstration may be more enlightening than an abstract geometrical proof.

TYPICALLY,

A CENTRIFUGE CONSISTS OF A BASE; A

ROTATING TUBE PERPENDICULAR TO THE BASE; AND VIALS ATTACHED BY MOVABLE CENTRIFUGE ARMS TO THE ROTATING TUBE.

THE

MOVABLE ARMS ARE HINGED AT

THE TOP OF THE ROTATING TUBE, AND THUS CAN MOVE UPWARD AT AN ANGLE APPROACHING

WHEN

90°

TO THE TUBE.

THE TUBE BEGINS TO SPIN, CENTRIPETAL FORCE

PULLS THE MATERIAL IN THE VIALS TOWARD THE CENTER. (Photograph by Charles D. Winters. National Audubon Society

Collection/Photo Researchers, Inc. Reproduced by permission.)

ing mere directional coordinates for spots on that highway (for instance, “southbound on 285”), because the area where traffic moves south depends on whether one is moving clockwise or counterclockwise. Hence, reporters usually say “southbound on the outer loop.” As with cars on I-285, the direction of the velocity vector for an object moving around a circle is a function entirely of its position and the direction of movement—clockwise or counterclockwise—for the circle itself. The direction of the individual velocity vector at any given point may be described as tangential; that is, describing a tangent, or a line that touches the circle at just one point. (By definition, a tangent line cannot intersect the circle.)

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It is possible to make a simple accelerometer, a device for measuring acceleration, with a lit candle inside a glass. The candle should be standing at a 90°-angle to the bottom of the glass, attached to it by hot wax as you would affix a burning candle to a plate. When you hold the candle level, the flame points upward; but if you spin the glass in a circle, the flame will point toward the center of that circle—in the direction of acceleration.

Mass ⫻ Acceleration = Force Since we have shown that acceleration exists for an object spinning around a circle, it is then possible for us to prove that the object experiences some type of force. The proof for this assertion lies in the second law of motion, which defines force as the product of mass and acceleration: hence, where there is acceleration and mass, there must be force. Force is always in the direction of acceleration, and therefore the force is directed toward the center of the circle.

It follows, then, that the direction of an object in movement around a circle is changing; hence, its velocity is also changing—and this in turn

In the above paragraph, we assumed the existence of mass, since all along the discussion has concerned an object spinning around a circle. By definition, an object—that is, an item of matter, rather than an imaginary point—possesses mass. Mass is a measure of inertia, which can be explained by the first law of motion: an object in motion tends to remain in motion, at the same speed and in the same direction (that is, at the same velocity) unless or until some outside force acts on it. This tendency to maintain velocity is inertia. Put another way, it is inertia that causes an object standing still to remain motionless, and

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Centripetal Force

A

CLOTHOID LOOP IN A ROLLER COASTER IN

HAINES CITY, FLORIDA. AT

THE TOP OF A LOOP, YOU FEEL LIGHTER

THAN NORMAL; AT THE BOTTOM, YOU FEEL HEAVIER. (The Purcell Team/Corbis. Reproduced by permission.)

likewise, it is inertia which dictates that a moving object will “try” to keep moving.

Centripetal Force Now that we have established the existence of a force in rotational motion, it is possible to give it a name: centripetal force, or the force that causes an object in uniform circular motion to move toward the center of the circular path. This is not a “new” kind of force; it is merely force as applied in circular or rotational motion, and it is absolutely essential. Hence, physicists speak of a “centripetal force requirement”: in the absence of centripetal force, an object simply cannot turn. Instead, it will move in a straight line.

“seeking the center.” What, then, of centrifugal, a word that means “fleeing the center”? It would be correct to say that there is such a thing as centrifugal motion; but centrifugal force is quite a different matter. The difference between centripetal force and a mere centrifugal tendency— a result of inertia rather than of force—can be explained by referring to a familiar example.

REAL-LIFE A P P L I C AT I O N S Riding in a Car

The Latin roots of centripetal together mean

When you are riding in a car and the car accelerates, your body tends to move backward against

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the seat. Likewise, if the car stops suddenly, your body tends to move forward, in the direction of the dashboard. Note the language here: “tends to move” rather than “is pushed.” To say that something is pushed would imply that a force has been applied, yet what is at work here is not a force, but inertia—the tendency of an object in motion to remain in motion, and an object at rest to remain at rest. A car that is not moving is, by definition, at rest, and so is the rider. Once the car begins moving, thus experiencing a change in velocity, the rider’s body still tends to remain in the fixed position. Hence, it is not a force that has pushed the rider backward against the seat; rather, force has pushed the car forward, and the seat moves up to meet the rider’s back. When stopping, once again, there is a sudden change in velocity from a certain value down to zero. The rider, meanwhile, is continuing to move forward due to inertia, and thus, his or her body has a tendency to keep moving in the direction of the now-stationary dashboard. This may seem a bit too simple to anyone who has studied inertia, but because the human mind has such a strong inclination to perceive inertia as a force in itself, it needs to be clarified in the most basic terms. This habit is similar to the experience you have when sitting in a vehicle that is standing still, while another vehicle alongside moves backward. In the first split-second of awareness, your mind tends to interpret the backward motion of the other car as forward motion on the part of the car in which you are sitting—even though your own car is standing still. Now we will consider the effects of centripetal force, as well as the illusion of centrifugal force. When a car turns to the left, it is undergoing a form of rotation, describing a 90°-angle or one-quarter of a circle. Once again, your body experiences inertia, since it was in motion along with the car at the beginning of the turn, and thus you tend to move forward. The car, at the same time, has largely overcome its own inertia and moved into the leftward turn. Thus the car door itself is moving to the left. As the door meets the right side of your body, you have the sensation of being pushed outward against the door, but in fact what has happened is that the door has moved inward.

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rants further discussion below. But while on the subject of riding in an automobile, we need to examine another illustration of centripetal force. It should be noted in this context that for a car to make a turn at all, there must be friction between the tires and the road. Friction is the force that resists motion when the surface of one object comes into contact with the surface of another; yet ironically, while opposing motion, friction also makes relative motion possible. Suppose, then, that a driver applies the brakes while making a turn. This now adds a force tangential, or at a right angle, to the centripetal force. If this force is greater than the centripetal force—that is, if the car is moving too fast—the vehicle will slide forward rather than making the turn. The results, as anyone who has ever been in this situation will attest, can be disastrous. The above highlights the significance of the centripetal force requirement: without a sufficient degree of centripetal force, an object simply cannot turn. Curves are usually banked to maximize centripetal force, meaning that the roadway tilts inward in the direction of the curve. This banking causes a change in velocity, and hence, in acceleration, resulting in an additional quantity known as reaction force, which provides the vehicle with the centripetal force necessary for making the turn. The formula for calculating the angle at which a curve should be banked takes into account the car’s speed and the angle of the curve, but does not include the mass of the vehicle itself. As a result, highway departments post signs stating the speed at which vehicles should make the turn, but these signs do not need to include specific statements regarding the weight of given models.

The Centrifuge To return to the subject of “centrifugal force”— which, as noted earlier, is really just centrifugal motion—you might ask, “If there is no such thing as centrifugal force, how does a centrifuge work?” Used widely in medicine and a variety of sciences, a centrifuge is a device that separates particles within a liquid. One application, for instance, is to separate red blood cells from plasma.

The illusion of centrifugal force is so deeply ingrained in the popular imagination that it war-

Typically a centrifuge consists of a base; a rotating tube perpendicular to the base; and two vials attached by movable centrifuge arms to the

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rotating tube. The movable arms are hinged at the top of the rotating tube, and thus can move upward at an angle approaching 90° to the tube. When the tube begins to spin, centripetal force pulls the material in the vials toward the center. Materials that are denser have greater inertia, and thus are less responsive to centripetal force. Hence, they seem to be pushed outward, but in fact what has happened is that the less dense material has been pulled inward. This leads to the separation of components, for instance, with plasma on the top and red blood cells on the bottom. Again, the plasma is not as dense, and thus is more easily pulled toward the center of rotation, whereas the red blood cells respond less, and consequently remain on the bottom. The centrifuge was invented in 1883 by Carl de Laval (1845-1913), a Swedish engineer, who used it to separate cream from milk. During the 1920s, the chemist Theodor Svedberg (18841971), who was also Swedish, improved on Laval’s work to create the ultracentrifuge, used for separating very small particles of similar weight. In a typical ultracentrifuge, the vials are no larger than 0.2 in (0.6 cm) in diameter, and these may rotate at speeds of up to 230,000 revolutions per minute. Most centrifuges in use by industry rotate in a range between 1,000 and 15,000 revolutions per minute, but others with scientific applications rotate at a much higher rate, and can produce a force more than 25,000 times as great as that of gravity. In 1994, researchers at the University of Colorado created a sort of super-centrifuge for simulating stresses applied to dams and other large structures. The instrument has just one centrifuge arm, measuring 19.69 ft (6 m), attached to which is a swinging basket containing a scale model of the structure to be tested. If the model is 1/50 the size of the actual structure, then the centrifuge is set to create a centripetal force 50 times that of gravity.

Industrial uses of the centrifuge include that for which Laval invented it—separation of cream from milk—as well as the separation of impurities from other substances. Water can be removed from oil or jet fuel with a centrifuge, and likewise, waste-management agencies use it to separate solid materials from waste water prior to purifying the water itself.

Centripetal Force

Closer to home, a washing machine on spin cycle is a type of centrifuge. As the wet clothes spin, the water in them tends to move outward, separating from the clothes themselves. An even simpler, more down-to-earth centrifuge can be created by tying a fairly heavy weight to a rope and swinging it above one’s head: once again, the weight behaves as though it were pushed outward, though in fact, it is only responding to inertia.

Roller Coasters and Centripetal Force People ride roller coasters, of course, for the thrill they experience, but that thrill has more to do with centripetal force than with speed. By the late twentieth century, roller coasters capable of speeds above 90 MPH (144 km/h) began to appear in amusement parks around America; but prior to that time, the actual speeds of a roller coaster were not particularly impressive. Seldom, if ever, did they exceed that of a car moving down the highway. On the other hand, the acceleration and centripetal force generated on a roller coaster are high, conveying a sense of weightlessness (and sometimes the opposite of weightlessness) that is memorable indeed. Few parts of a roller coaster ride are straight and flat—usually just those segments that mark the end of one ride and the beginning of another. The rest of the track is generally composed of dips and hills, banked turns, and in some cases, clothoid loops. The latter refers to a geometric shape known as a clothoid, rather like a teardrop upside-down.

The Colorado centrifuge has also been used to test the effects of explosions on buildings. Because the combination of forces—centripetal, gravity, and that of the explosion itself—is so great, it takes a very small quantity of explosive to measure the effects of a blast on a model of the building.

Because of its shape, the clothoid has a much smaller radius at the top than at the bottom—a key factor in the operation of the roller coaster ride through these loops. In days past, rollercoaster designers used perfectly circular loops, which allowed cars to enter them at speeds that were too high, built too much force and resulted in injuries for riders. Eventually, engineers recognized the clothoid as a means of providing a safe, fun ride.

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Centripetal Force

KEY TERMS ACCELERATION: CENTRIFUGAL:

A change in velocity. A term describing the

scalar is contrasted with a vector.

tendency of objects in uniform circular

SPEED:

motion to move away from the center of

of an object changes over a given period of

the circular path. Though the term “cen-

time.

trifugal force” is often used, it is inertia,

The rate at which the position

TANGENTIAL:

Movement along a tan-

rather than force, that causes the object to

gent, or a line that touches a circle at just

move outward.

one point and does not intersect the circle.

CENTRIPETAL

FORCE:

The force

UNIFORM

CIRCULAR

MOTION:

that causes an object in uniform circular

The motion of an object around the center

motion to move toward the center of the

of a circle in such a manner that speed is

circular path. INERTIA:

constant or unchanging. The tendency of an object in

motion to remain in motion, and of an object at rest to remain at rest.

VECTOR:

A quantity that possesses

both magnitude and direction. Velocity, acceleration, and weight (which involves

A measure of inertia, indicating

the downward acceleration due to gravity)

the resistance of an object to a change in its

are examples of vectors. It is contrasted

motion—including a change in velocity.

with a scalar.

MASS:

SCALAR:

A quantity that possesses

only magnitude, with no specific direction.

As you move into the clothoid loop, then up, then over, and down, you are constantly changing position. Speed, too, is changing. On the way up the loop, the roller coaster slows due to a decrease in kinetic energy, or the energy that an object possesses by virtue of its movement. At the top of the loop, the roller coaster has gained a great deal of potential energy, or the energy an object possesses by virtue of its position, and its kinetic energy is at zero. But once it starts going down the other side, kinetic energy—and with it speed—increases rapidly once again. Throughout the ride, you experience two forces, gravity, or weight, and the force (due to motion) of the roller coaster itself, known as normal force. Like kinetic and potential energy— which rise and fall correspondingly with dips and hills—normal force and gravitational force are locked in a sort of “competition” throughout the roller-coaster rider. For the coaster to have its

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Mass, time, and speed are all scalars. A

VOLUME 2: REAL-LIFE PHYSICS

VELOCITY:

The speed of an object in a

particular direction.

proper effect, normal force must exceed that of gravity in most places. The increase in normal force on a rollercoaster ride can be attributed to acceleration and centripetal motion, which cause you to experience something other than gravity. Hence, at the top of a loop, you feel lighter than normal, and at the bottom, heavier. In fact, there has been no real change in your weight: it is, like the idea of “centrifugal force” discussed earlier, a matter of perception. WHERE TO LEARN MORE Aylesworth, Thomas G. Science at the Ball Game. New York: Walker, 1977. Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. Buller, Laura and Ron Taylor. Forces of Nature. Illustrations by John Hutchinson and Stan North. New York: Marshall Cavendish, 1990.

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“Centrifugal Force—Rotational Motion.” National Aeronautics and Space Administration (Web site). (March 5, 2001). “Circular and Satellite Motion” (Web site). (March 5, 2001). Cobb, Vicki. Why Doesn’t the Earth Fall Up? And Other Not Such Dumb Questions About Motion. Illustrated by Ted Enik. New York: Lodestar Books, 1988. Lefkowitz, R. J. Push! Pull! Stop! Go! A Book About Forces

S C I E N C E O F E V E RY DAY T H I N G S

and Motion. Illustrated by June Goldsborough. New York: Parents’ Magazine Press, 1975. “Rotational Motion.” Physics Department, University of Guelph (Web site). (March 4, 2001). Schaefer, Lola M. Circular Movement. Mankato, MN: Pebble Books, 2000. Snedden, Robert. Forces. Des Plaines, IL: Heinemann Library, 1999. Whyman, Kathryn. Forces in Action. New York: Gloucester Press, 1986.

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FRICTION Friction

CONCEPT Friction is the force that resists motion when the surface of one object comes into contact with the surface of another. In a machine, friction reduces the mechanical advantage, or the ratio of output to input: an automobile, for instance, uses onequarter of its energy on reducing friction. Yet, it is also friction in the tires that allows the car to stay on the road, and friction in the clutch that makes it possible to drive at all. From matches to machines to molecular structures, friction is one of the most significant phenomena in the physical world.

HOW IT WORKS The definition of friction as “the force that resists motion when the surface of one object comes into contact with the surface of another” does not exactly identify what it is. Rather, the statement describes the manifestation of friction in terms of how other objects respond. A less sophisticated version of such a definition would explain electricity, for instance, as “the force that runs electrical appliances.” The reason why friction cannot be more firmly identified is simple: physicists do not fully understand what it is. Much the same could be said of force, defined by Sir Isaac Newton’s (1642-1727) second law of motion as the product of mass multiplied acceleration. The fact is that force is so fundamental that it defies full explanation, except in terms of the elements that compose it, and compared to force, friction is relatively easy to identify. In fact, friction plays a part in the total force that must be opposed in order for movement to take place in many situations. So, too, does grav-

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ity—and gravity, unlike force itself, is much easier to explain. Since gravity plays a role in friction, it is worthwhile to review its essentials. Newton’s first law of motion identifies inertia, a tendency of objects in the physical universe that is sometimes mistaken for friction. When an object is in motion or at rest, the first law states, it will remain in that state at a constant velocity (which is zero for an object at rest) unless or until an outside force acts on it. This tendency to remain in a given state of motion is inertia. Inertia is not a force: on the contrary, a very small quantity of force may accelerate an object, thus overcoming its inertia. Inertia is, however, a component of force, since mass is a measure of inertia. In the case of gravitational force, mass is multiplied by the acceleration due to gravity, which is equal to 32 ft (9.8 m)/sec2. People in everyday life are familiar with another term for gravitational force: weight. Weight, in turn, is an all-important factor in friction, as revealed in the three laws governing the friction between an object at rest and the surface on which it sits. According to the first of these, friction is proportional to the weight of the object. The second law states that friction is not determined by the surface area of the object— that is, the area that touches the surface on which the object rests. In fact, the contact area between object and surface is a dependant variable, a function of weight. The second law might seem obvious if one were thinking of a relatively elastic object—say, a garbage bag filled with newspapers sitting on the finished concrete floor of a garage. Clearly as more newspapers are added, thus increasing the

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weight, its surface area would increase as well. But what if one were to compare a large cardboard box (the kind, for instance, in which televisions or computers are shipped) with an ordinary concrete block of the type used in foundations for residential construction? Obviously, the block has more friction against the concrete floor; but at the same time, it is clear that despite its greater weight, the block has less surface area than the box. How can this be?

Friction

The answer is that “surface area” is quite literally more than meets the eye. Friction itself occurs at a level invisible to the naked eye, and involves the adhesive forces between molecules on surfaces pushed together by the force of weight. This is similar to the manner in which, when viewed through a high-powered lens, two complementary patches of Velcro™ are revealed as a forest of hooks on the one hand, and a sea of loops on the other. On a much more intensified level, that of molecular structure, the surfaces of objects appear as mountains and valleys. Nothing, in fact, is smooth when viewed on this scale, and hence, from a molecular perspective, it becomes clear that two objects in contact actually touch one another only in places. An increase of weight, however, begins pushing objects together, causing an increase in the actual—that is, the molecular—area of contact. Hence area of contact is proportional to weight. Just as the second law regarding friction states that surface area does not determine friction (but rather, weight determines surface area), the third law holds that friction is independent of the speed at which an object is moving along a surface— provided that speed is not zero. The reason for this provision is that an object with no speed (that is, one standing perfectly still) is subject to the most powerful form of friction, static friction. The latter is the friction that an object at rest must overcome to be set in motion; however, this should not be confused with inertia, which is relatively easy to overcome through the use of force. Inertia, in fact, is far less complicated than static friction, involving only mass rather than weight. Nor is inertia affected by the composition of the materials touching one another. As stated earlier, friction is proportional to weight, which suggests that another factor is involved. And indeed there is another factor, known as coefficient of friction. The latter, desig-

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THE

RAISED TREAD ON AN AUTOMOBILE’S TIRES, COU-

PLED WITH THE ROUGHENED ROAD SURFACE, PROVIDES SUFFICIENT FRICTION FOR THE DRIVER TO BE ABLE TO TURN THE CAR AND TO STOP. (Photograph by Martyn God-

dard/Corbis. Reproduced by permission.)

nated by the Greek letter mu (µ), is constant for any two types of surface in contact with one another, and provides a means of comparing the friction between them to that between other surfaces. For instance, the coefficient of static friction for wood on wood is 0.5; but for metal on metal with lubrication in between, it is only 0.03. A rubber tire on dry concrete yields the highest coefficient of static friction, 1.0, which is desirable in that particular situation. Coefficients are much lower for the second type of friction, sliding friction, the frictional resistance experienced by a body in motion. Whereas the earlier figures measured the relative resistance to putting certain objects into motion, the sliding-friction coefficient indicates the relative resistance against those objects once they are moving. To use the same materials mentioned above, the coefficient of sliding friction for wood on wood is 0.3; for two lubricated metals 0.03 (no change); and for a rubber tire on dry concrete 0.7. Finally, there is a third variety of friction, one in which coefficients are so low as to be neg-

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REAL-LIFE A P P L I C AT I O N S

Friction

Self-Motivation Through Friction Friction, in fact, always opposes movement; why, then, is friction necessary—as indeed it is—for walking, and for keeping a car on the road? The answer relates to the differences between friction and inertia alluded to earlier. In situations of static friction, it is easy to see how a person might confuse friction with inertia, since both serve to keep an object from moving. In situations of sliding or rolling friction, however, it is easier to see the difference between friction and inertia.

ACTOR JACK BUCHANAN PLE

DISPLAY

OF

LIT THIS MATCH USING A SIM-

FRICTION:

DRAGGING

THE

MATCH

AGAINST THE BACK OF THE MATCHBOX. (Photograph by Hul-

ton-Deutsch Collection/Corbis. Reproduced by permission.)

ligible: rolling friction, or the frictional resistance that a wheeled object experiences when it rolls over a relatively smooth, flat surface. In ideal circumstances, in fact, there would be absolutely no resistance between a wheel and a road. However, there exists no ideal—that is, perfectly rigid— wheel or road; both objects “give” in response to the other, the wheel by flattening somewhat and the road by experiencing indentation. Up to this point, coefficients of friction have been discussed purely in comparative terms, but in fact, they serve a function in computing frictional force—that is, the force that must be overcome to set an object in motion, or to keep it in motion. Frictional force is equal to the coefficient of friction multiplied by normal force—that is, the perpendicular force that one object or surface exerts on another. On a horizontal plane, normal force is equal to gravity and hence weight. In this equation, the coefficient of friction establishes a limit to frictional force: in order to move an object in a given situation, one must exert a force in excess of the frictional force that keeps it from moving.

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Whereas friction is always opposed to movement, inertia is not. When an object is not moving, its inertia does oppose movement—but when the object is in motion, then inertia resists stopping. In the absence of friction or other forces, inertia allows an object to remain in motion forever. Imagine a hockey player hitting a puck across a very, very large rink. Because ice has a much smaller coefficient of friction with regard to the puck than does dirt or asphalt, the puck will travel much further. Still, however, the ice has some friction, and, therefore, the puck will come to a stop at some point. Now suppose that instead of ice, the surface and objects in contact with it were friction-free, possessing a coefficient of zero. Then what would happen if the player hit the puck? Assuming for the purposes of this thought experiment, that the rink covered the entire surface of Earth, it would travel and travel and travel, ultimately going around the planet. It would never stop, because there would be no friction to stop it, and therefore inertia would have free rein. The same would be true if one were to firmly push the hockey player with enough force (small in the absence of friction) to set him in motion: he would continue riding around the planet indefinitely, borne by his skates. But what if instead of being set in motion, the hockey player tried to set himself in motion by the action of his skates against the rink’s surface? He would be unable to move even a hair’s breadth. The fact is that while static friction opposes the movement of an object from a position of rest to a state of motion, it may—assuming it can be overcome to begin motion at all—be indispensable to that movement. As with the skater in per-

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petual motion across the rink, the absence of friction means that inertia is “in control;” with friction, however, it is possible to overcome inertia.

Friction in Driving a Car The same principle applies to a car’s tires: if they were perfectly smooth—and, to make matters worse, the road were perfectly smooth as well— the vehicle would keep moving forward when the driver attempted to stop. For this reason, tires are designed with raised tread to maintain a high degree of friction, gripping the road tightly and dispersing water when the roadway is wet. The force of friction, in fact, pervades the entire operation of a car, and makes it possible for the tires themselves to turn. The turning force, or torque, that the driver exerts on the steering wheel is converted into forces that drive the tires, and these in turn use friction to provide traction. Between steering wheel and tires, of course, are a number of steps, with the engine rotating the crankshaft and transmitting power to the clutch, which applies friction to translate the motion of the crankshaft to the gearbox. When the driver of a car with a manual transmission presses down on the clutch pedal, this disengages the clutch itself. A clutch is a circular mechanism containing (among other things) a pressure plate, which lifts off the clutch plate. As a result, the flywheel—the instrument that actually transmits force from the crankshaft—is disengaged from the transmission shaft. With the clutch thus disengaged, the driver changes gears, and after the driver releases the clutch pedal, springs return the pressure plate and the clutch plate to their place against the flywheel. The flywheel then turns the transmission shaft. Controlled friction in the clutch makes this operation possible; likewise the synchromesh within the gearbox uses friction to bring the gearwheels into alignment. This is a complicated process, but at the heart of it is an engagement of gear teeth in which friction forces them to come to the same speed. Friction is also essential to stopping a car— not just with regard to the tires, but also with respect to the brakes. Whether they are disk brakes or drum brakes, two elements must come together with a force more powerful than the engine’s, and friction provides that needed force. In disk brakes, brake pads apply friction to both

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sides of the spinning disks, and in drum brakes, brake shoes transmit friction to the inside of a spinning drum. This braking force is then transmitted to the tires, which apply friction to the road and thus stop the car.

Friction

Efficiency and Friction The automobile is just one among many examples of a machine that could not operate without friction. The same is true of simple machines such as screws, as well as nails, pliers, bolts, and forceps. At the heart of this relationship is a paradox, however, because friction inevitably reduces the efficiency of machines: a car, as noted earlier, exerts fully one-quarter of its power simply on overcoming the force of friction both within its engine and from air resistance as it travels down the road. In scientific terms, efficiency or mechanical advantage is measured by the ratio of force output to force input. Clearly, in most situations it is ideal to maximize output and minimize input, and over the years inventors have dreamed of creating a mechanism—a perpetual motion machine—to do just that. In this idealized machine, one would apply a certain amount of energy to set it into operation, and then it would never stop; hence the ratio of output to input would be nearly infinite. Unfortunately, the perpetual motion machine is a dream every bit as elusive as the mythical Fountain of Youth. At least this is true on Earth, where friction will always cause a system to lose kinetic energy, or the energy of movement. No matter what the design, the machine will eventually lose energy and stop; however, this is not true in outer space, where friction is very small—though it still exists. In space it might truly be possible to set a machine in motion and let inertia do the rest; thus perhaps perpetual motion actually is more than a dream. It should also be noted that mechanical advantage is not always desirable. A screw is a highly inefficient machine: one puts much more force into screwing it in than the screw will exert once it is in place. Yet this is exactly the purpose of a screw: an “efficient” one, or one that worked its way back out of the place into which it had been screwed, would in fact be of little use. Once again, it is friction that provides a screw with its strangely efficient form of inefficiency. Nonetheless, friction, in spite of the

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advantages discussed above, is as undesirable as it is desirable. With friction, there is always something lost; however, there is a physical law that energy does not simply disappear; it just changes form. In the case of friction, the energy that could go to moving the machine is instead translated into sound—or even worse, heat.

When Sparks Fly In movement involving friction, molecules vibrate, bringing about a rise in temperature. This can be easily demonstrated by simply rubbing one’s hands together quickly, as a person is apt to do when cold: heat increases. For a machine composed of metal parts, this increase in temperature can be disastrous, leading to serious wear and damage. This is why various forms of lubricant are applied to systems subject to friction. An automobile uses grease and oil, as well as ball bearings, which are tiny uniform balls of metal that imitate the behavior of oil-based substances on a large scale. In a molecule of oil— whether it is a petroleum-related oil or the type of oil that comes from living things—positive and negative electrical charges are distributed throughout the molecule. By contrast, in water the positive charges are at one end of the molecule and the negative at the other. This creates a tight bond as the positive end of one water molecule adheres to the negative end of another. With oil, the relative absence of attraction between molecules means that each is in effect a tiny ball separate from the others. The ball-like molecules “roll” between metal elements, providing the buffer necessary to reduce friction. Yet for every statement one can make concerning friction, there is always another statement with which to counter it. Earlier it was noted that the wheel, because it reduced friction greatly, provided an enormous technological boost to societies. Yet long before the wheel— hundreds of thousands of years ago—an even more important technological breakthrough occurred when humans made a discovery that depended on maximizing friction: fire, or rather the means of making fire. Unlike the wheel, fire occurs in nature, and can spring from a number of causes, but when human beings harnessed the means of making fire on their own, they had to rely on the heat that comes from friction.

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using a little stick with a phosphorus tip. This stick, of course, is known as a match. In a strikeanywhere match, the head contains all the chemicals needed to create a spark. To ignite this type of match, one need only create frictional heat by rubbing it against a surface, such as sandpaper, with a high coefficient of friction. The chemicals necessary for ignition in safety matches, on the other hand, are dispersed between the match head and a treated strip, usually found on the side of the matchbox or matchbook. The chemicals on the tip and those on the striking surface must come into contact for ignition to occur, but once again, there must be friction between the match head and the striking pad. Water reduces friction with its heavy bond, as it does with a car’s tires on a rainy day, which explains why matches are useless when wet.

The Outer Limits of Friction Clearly friction is a complex subject, and the discoveries of modern physics only promise to add to that complexity. In a February 1999 online article for Physical Review Focus, Dana Mackenzie reported that “Engineers hope to make microscopic engines and gears as ordinary in our lives as microscopic circuits are today. But before this dream becomes a reality, they will have to deal with laws of friction that are very different from those that apply to ordinary-sized machines.” The earlier statement that friction is proportional to weight, in fact, applies only in the realm of classical physics. The latter term refers to the studies of physicists up to the end of the nineteenth century, when the concerns were chiefly the workings of large objects whose operations could be discerned by the senses. Modern physics, on the other hand, focuses on atomic and molecular structures, and addresses physical behaviors that could not have been imagined prior to the twentieth century.

By the early nineteenth century, inventors had developed an easy method of creating fire by

According to studies conducted by Alan Burns and others at Sandia National Laboratories in Albuquerque, New Mexico, molecular interactions between objects in very close proximity create a type of friction involving repulsion rather than attraction. This completely upsets the model of friction understood for more than a century, and indicates new frontiers of discovery concerning the workings of friction at a molecular level.

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Friction

KEY TERMS A change in velocity.

one another. On a plane without any

A figure, constant for a particular pair of surfaces in contact, that can be multiplied by the normal force between them to calculate the frictional force they experience.

incline (which would add acceleration in

The product of mass multiplied by acceleration.

when it rolls over a relatively smooth, flat

ACCELERATION:

COEFFICIENT OF FRICTION:

FORCE:

The force that resists motion when the surface of one object comes into contact with the surface of another. Varieties including sliding friction, static friction, and rolling friction. The degree of friction between two specific surfaces is proportional to coefficient of friction. FRICTION:

The force necessary to set an object in motion, or to keep it in motion; equal to normal force multiplied by coefficient of friction. FRICTIONAL FORCE:

The tendency of an object in motion to remain in motion, and of an object at rest to remain at rest. INERTIA:

addition to that of gravity) normal force is the same as weight. ROLLING FRICTION:

The frictional

resistance that a circular object experiences surface. With a coefficient of friction much smaller than that of sliding friction, rolling friction involves by far the least amount of resistance among the three varieties of friction. SLIDING FRICTION:

The frictional

resistance experienced by a body in motion. Here the coefficient of friction is greater than that for rolling friction, but less than for static friction. The rate at which the position

SPEED:

of an object changes over a given period of time. STATIC

FRICTION:

The frictional

resistance that a stationary object must overcome before it can go into motion. Its

A measure of inertia, indicating the resistance of an object to a change in its motion—including a change in velocity.

coefficient of friction is greater than that of

The ratio of force output to force input in a machine.

VELOCITY:

The perpendicular force with which two objects press against

force on an object; the product of mass mul-

MASS:

MECHANICAL

ADVANTAGE:

NORMAL FORCE:

WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. Buller, Laura and Ron Taylor. Forces of Nature. Illustrations by John Hutchinson and Stan North. New York: Marshall Cavendish, 1990.

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sliding friction, and thus largest among the three varieties of friction. The speed of an object in a

particular direction. WEIGHT:

A measure of the gravitational

tiplied by the acceleration due to gravity.

Dixon, Malcolm and Karen Smith. Forces and Movement. Mankato, MN: Smart Apple Media, 1998. “Friction.” How Stuff Works (Web site). (March 8, 2001). “Friction and Interactions” (Web site). (March 8, 2001).

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Levy, Matthys and Richard Panchyk. Engineering the City: How Infrastructure Works. Chicago: Chicago Review Press, 2000. Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998.

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Rutherford, F. James; Gerald Holton; and Fletcher G. Watson. Project Physics. New York: Holt, Rinehart, and Winston, 1981. Skateboard Science (Web site).
Mackenzie, Dana. “Friction of Molecules.” Physical Review Focus (Web site).
Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996.

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L AWS OF MOTION

Laws of Motion

CONCEPT In all the universe, there are few ideas more fundamental than those expressed in the three laws of motion. Together these explain why it is relatively difficult to start moving, and then to stop moving; how much force is needed to start or stop in a given situation; and how one force relates to another. In their beauty and simplicity, these precepts are as compelling as a poem, and like the best of poetry, they identify something that resonates through all of life. The applications of these three laws are literally endless: from the planets moving through the cosmos to the first seconds of a car crash to the action that takes place when a person walks. Indeed, the laws of motion are such a part of daily life that terms such as inertia, force, and reaction extend into the realm of metaphor, describing emotional processes as much as physical ones.

HOW IT WORKS The three laws of motion are fundamental to mechanics, or the study of bodies in motion. These laws may be stated in a number of ways, assuming they contain all the components identified by Sir Isaac Newton (1642-1727). It is on his formulation that the following are based:

exerts a force on another, the second object exerts on the first a force equal in magnitude but opposite in direction.

Laws of Man vs. Laws of Nature These, of course, are not “laws” in the sense that people normally understand that term. Human laws, such as injunctions against stealing or parking in a fire lane, are prescriptive: they state how the world should be. Behind the prescriptive statements of civic law, backing them up and giving them impact, is a mechanism—police, courts, and penalties—for ensuring that citizens obey. A scientific law operates in exactly the opposite fashion. Here the mechanism for ensuring that nature “obeys” the law comes first, and the “law” itself is merely a descriptive statement concerning evident behavior. With human or civic law, it is clearly possible to disobey: hence, the justice system exists to discourage disobedience. In the case of scientific law, disobedience is clearly impossible—and if it were not, the law would have to be amended.

• First law of motion: An object at rest will remain at rest, and an object in motion will remain in motion, at a constant velocity unless or until outside forces act upon it. • Second law of motion: The net force acting upon an object is a product of its mass multiplied by its acceleration. • Third law of motion: When one object

This is not to say, however, that scientific laws extend beyond their own narrowly defined limits. On Earth, the intrusion of outside forces—most notably friction—prevents objects from behaving perfectly according to the first law of motion. The common-sense definition of friction calls to mind, for instance, the action that a match makes as it is being struck; in its broader scientific meaning, however, friction can be defined as any force that resists relative motion between two bodies in contact.

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Laws of Motion

THE CARGO BAY OF THE SPACE SHUTTLE DISCOVERY, shown just after releasing a satellite. Once released into the frictionless vacuum around Earth, the satellite will move indefinitely around Earth without need for the motive power of an engine. The planet’s gravity keeps it at a fixed height, and at that height, it could theoretically circle Earth forever. (Corbis. Reproduced by permission.)

The operations of physical forces on Earth are continually subject to friction, and this includes not only dry bodies, but liquids, for instance, which are subject to viscosity, or internal friction. Air itself is subject to viscosity, which prevents objects from behaving perfectly in accordance with the first law of motion. Other forces, most notably that of gravity, also come into play to stop objects from moving endlessly once they have been set in motion.

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radius, it would continue travelling indefinitely. But even this craft would likely run into another object, such as a planet, and would then be drawn into its orbit. In such a case, however, it can be said that outside forces have acted upon it, and thus the first law of motion stands.

The vacuum of outer space presents scientists with the most perfect natural laboratory for testing the first law of motion: in theory, if they were to send a spacecraft beyond Earth’s orbital

The orbit of a satellite around Earth illustrates both the truth of the first law, as well as the forces that limit it. To break the force of gravity, a powered spacecraft has to propel the satellite into the exosphere. Yet once it has reached the frictionless vacuum, the satellite will move indefinitely around Earth without need for the motive power of an engine—it will get a “free ride,”

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thanks to the first law of motion. Unlike the hypothetical spacecraft described above, however, it will not go spinning into space, because it is still too close to Earth. The planet’s gravity keeps it at a fixed height, and at that height, it could theoretically circle Earth forever. The first law of motion deserves such particular notice, not simply because it is the first law. Nonetheless, it is first for a reason, because it establishes a framework for describing the behavior of an object in motion. The second law identifies a means of determining the force necessary to move an object, or to stop it from moving, and the third law provides a picture of what happens when two objects exert force on one another. The first law warrants special attention because of misunderstandings concerning it, which spawned a debate that lasted nearly twenty centuries. Aristotle (384-322 B.C.) was the first scientist to address seriously what is now known as the first law of motion, though in fact, that term would not be coined until about two thousand years after his death. As its title suggests, his Physics was a seminal work, a book in which Aristotle attempted to give form to, and thus define the territory of, studies regarding the operation of physical processes. Despite the great philosopher’s many achievements, however, Physics is a highly flawed work, particularly with regard to what became known as his theory of impetus— that is, the phenomena addressed in the first law of motion.

Aristotle’s Mistake According to Aristotle, a moving object requires a continual application of force to keep it moving: once that force is no longer applied, it ceases to move. You might object that, when a ball is in flight, the force necessary to move it has already been applied: a person has thrown the ball, and it is now on a path that will eventually be stopped by the force of gravity. Aristotle, however, would have maintained that the air itself acts as a force to keep the ball in flight, and that when the ball drops—of course he had no concept of “gravity” as such—it is because the force of the air on the ball is no longer in effect. These notions might seem patently absurd to the modern mind, but they went virtually unchallenged for a thousand years. Then in the sixth century A.D., the Byzantine philosopher Johannes Philoponus (c. 490-570) wrote a criS C I E N C E O F E V E RY DAY T H I N G S

tique of Physics. In what sounds very much like a precursor to the first law of motion, Philoponus held that a body will keep moving in the absence of friction or opposition.

Laws of Motion

He further maintained that velocity is proportional to the positive difference between force and resistance—in other words, that the force propelling an object must be greater than the resistance. As long as force exceeds resistance, Philoponus held, a body will remain in motion. This in fact is true: if you want to push a refrigerator across a carpeted floor, you have to exert enough force not only to push the refrigerator, but also to overcome the friction from the floor itself. The Arab philosophers Ibn Sina (Avicenna; 980-1037) and Ibn Bâjja (Avempace; fl. c. 1100) defended Philoponus’s position, and the French scholar Peter John Olivi (1248-1298) became the first Western thinker to critique Aristotle’s statements on impetus. Real progress on the subject, however, did not resume until the time of Jean Buridan (1300-1358), a French physicist who went much further than Philoponus had eight centuries earlier. In his writing, Buridan offered an amazingly accurate analysis of impetus that prefigured all three laws of motion. It was Buridan’s position that one object imparts to another a certain amount of power, in proportion to its velocity and mass, that causes the second object to move a certain distance. This, as will be shown below, was amazingly close to actual fact. He was also correct in stating that the weight of an object may increase or decrease its speed, depending on other circumstances, and that air resistance slows an object in motion. The true breakthrough in understanding the laws of motion, however, came as the result of work done by three extraordinary men whose lives stretched across nearly 250 years. First came Nicolaus Copernicus (1473-1543), who advanced what was then a heretical notion: that Earth, rather than being the center of the universe, revolved around the Sun along with the other planets. Copernicus made his case purely in terms of astronomy, however, with no direct reference to physics.

Galileo’s Challenge: The Copernican Model Galileo Galilei (1564-1642) likewise embraced a heliocentric (Sun-centered) model of the uniVOLUME 2: REAL-LIFE PHYSICS

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verse—a position the Church forced him to renounce publicly on pain of death. As a result of his censure, Galileo realized that in order to prove the Copernican model, it would be necessary to show why the planets remain in motion as they do. In explaining this, he coined the term inertia to describe the tendency of an object in motion to remain in motion, and an object at rest to remain at rest. Galileo’s observations, in fact, formed the foundation for the laws of motion. In the years that followed Galileo’s death, some of the world’s greatest scientific minds became involved in the effort to understand the forces that kept the planets in motion around the Sun. Among them were Johannes Kepler (15711630), Robert Hooke (1635-1703), and Edmund Halley (1656-1742). As a result of a dispute between Hooke and Sir Christopher Wren (16321723) over the subject, Halley brought the question to his esteemed friend Isaac Newton. As it turned out, Newton had long been considering the possibility that certain laws of motion existed, and these he presented in definitive form in his Principia (1687). The impact of the Newton’s book, which included his observations on gravity, was nothing short of breathtaking. For the next three centuries, human imagination would be ruled by the Newtonian framework, and only in the twentieth century would the onset of new ideas reveal its limitations. Yet even today, outside the realm of quantum mechanics and relativity theory—in other words, in the world of everyday experience— Newton’s laws of motion remain firmly in place.

REAL-LIFE A P P L I C AT I O N S The First Law of Motion in a Car Crash It is now appropriate to return to the first law of motion, as formulated by Newton: an object at rest will remain at rest, and an object in motion will remain in motion, at a constant velocity unless or until outside forces act upon it. Examples of this first law in action are literally unlimited.

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velocity. The latter term, though it is commonly understood to be the same as speed, is in fact more specific: velocity can be defined as the speed of an object in a particular direction. In a car moving forward at a fixed rate of 60 MPH (96 km/h), everything in the car—driver, passengers, objects on the seats or in the trunk— is also moving forward at the same rate. If that car then runs into a brick wall, its motion will be stopped, and quite abruptly. But though its motion has stopped, in the split seconds after the crash it is still responding to inertia: rather than bouncing off the brick wall, it will continue plowing into it. What, then, of the people and objects in the car? They too will continue to move forward in response to inertia. Though the car has been stopped by an outside force, those inside experience that force indirectly, and in the fragment of time after the car itself has stopped, they continue to move forward—unfortunately, straight into the dashboard or windshield. It should also be clear from this example exactly why seatbelts, headrests, and airbags in automobiles are vitally important. Attorneys may file lawsuits regarding a client’s injuries from airbags, and homespun opponents of the seatbelt may furnish a wealth of anecdotal evidence concerning people who allegedly died in an accident because they were wearing seatbelts; nonetheless, the first law of motion is on the side of these protective devices. The admittedly gruesome illustration of a car hitting a brick wall assumes that the driver has not applied the brakes—an example of an outside force changing velocity—or has done so too late. In any case, the brakes themselves, if applied too abruptly, can present a hazard, and again, the significant factor here is inertia. Like the brick wall, brakes stop the car, but there is nothing to stop the driver and/or passengers. Nothing, that is, except protective devices: the seat belt to keep the person’s body in place, the airbag to cushion its blow, and the headrest to prevent whiplash in rear-end collisions.

One of the best illustrations, in fact, involves something completely outside the experience of Newton himself: an automobile. As a car moves down the highway, it has a tendency to remain in motion unless some outside force changes its

Inertia also explains what happens to a car when the driver makes a sharp, sudden turn. Suppose you are is riding in the passenger seat of a car moving straight ahead, when suddenly the driver makes a quick left turn. Though the car’s tires turn instantly, everything in the vehicle—its frame, its tires, and its contents—is still respond-

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Laws of Motion

WHEN A VEHICLE HITS A WALL, AS SHOWN HERE IN A CRASH TEST, ITS MOTION WILL BE STOPPED, AND QUITE ABRUPTLY. BUT THOUGH ITS MOTION HAS STOPPED, IN THE SPLIT SECONDS AFTER THE CRASH IT IS STILL RESPONDING TO INERTIA: RATHER THAN BOUNCING OFF THE BRICK WALL, IT WILL CONTINUE PLOWING INTO IT. (Photograph by Tim Wright/Corbis. Reproduced by permission.)

ing to inertia, and therefore “wants” to move forward even as it is turning to the left.

From Parlor Tricks to Space Ships

As the car turns, the tires may respond to this shift in direction by squealing: their rubber surfaces were moving forward, and with the sudden turn, the rubber skids across the pavement like a hard eraser on fine paper. The higher the original speed, of course, the greater the likelihood the tires will squeal. At very high speeds, it is possible the car may seem to make the turn “on two wheels”— that is, its two outer tires. It is even possible that the original speed was so high, and the turn so sharp, that the driver loses control of the car.

It would be wrong to conclude from the carrelated illustrations above that inertia is always harmful. In fact it can help every bit as much as it can potentially harm, a fact shown by two quite different scenarios.

Here inertia is to blame: the car simply cannot make the change in velocity (which, again, refers both to speed and direction) in time. Even in less severe situations, you are likely to feel that you have been thrown outward against the rider’s side door. But as in the car-and-brick-wall illustration used earlier, it is the car itself that first experiences the change in velocity, and thus it responds first. You, the passenger, then, are moving forward even as the car has turned; therefore, rather than being thrown outward, you are simply meeting the leftward-moving door even as you push forward.

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The beneficial quality to the first scenario may be dubious: it is, after all, a mere parlor trick, albeit an entertaining one. In this famous stunt, with which most people are familiar even if they have never seen it, a full table setting is placed on a table with a tablecloth, and a skillful practitioner manages to whisk the cloth out from under the dishes without upsetting so much as a glass. To some this trick seems like true magic, or at least sleight of hand; but under the right conditions, it can be done. (This information, however, carries with it the warning, “Do not try this at home!”) To make the trick work, several things must align. Most importantly, the person doing it has to be skilled and practiced at performing the feat. On a physical level, it is best to minimize the friction between the cloth and settings on the one hand, and the cloth and table on the other. It is also important to maximize the mass (a property

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that will be discussed below) of the table settings, thus making them resistant to movement. Hence, inertia—which is measured by mass—plays a key role in making the tablecloth trick work. You might question the value of the tablecloth stunt, but it is not hard to recognize the importance of the inertial navigation system (INS) that guides planes across the sky. Prior to the 1970s, when INS made its appearance, navigation techniques for boats and planes relied on reference to external points: the Sun, the stars, the magnetic North Pole, or even nearby areas of land. This created all sorts of possibilities for error: for instance, navigation by magnet (that is, a compass) became virtually useless in the polar regions of the Arctic and Antarctic. By contrast, the INS uses no outside points of reference: it navigates purely by sensing the inertial force that results from changes in velocity. Not only does it function as well near the poles as it does at the equator, it is difficult to tamper with an INS, which uses accelerometers in a sealed, shielded container. By contrast, radio signals or radar can be “confused” by signals from the ground—as, for instance, from an enemy unit during wartime. As the plane moves along, its INS measures movement along all three geometrical axes, and provides a continuous stream of data regarding acceleration, velocity, and displacement. Thanks to this system, it is possible for a pilot leaving California for Japan to enter the coordinates of a halfdozen points along the plane’s flight path, and let the INS guide the autopilot the rest of the way.

Soviet airspace over the Kamchatka Peninsula and later Sakhalin Island. There a Soviet Su-15 shot it down, killing all the plane’s passengers. In the aftermath of the Flight 007 shootdown, the Soviets accused the United States and South Korea of sending a spy plane into their airspace. (Among the passengers was Larry McDonald, a staunchly anti-Communist Congressman from Georgia.) It is more likely, however, that the tragedy of 007 resulted from errors in navigation which probably had something to do with the INS. The fact is that the R-20 flight plan had been designed to keep aircraft well out of Soviet airspace, and at the time KAL 007 passed over Kamchatka, it should have been 200 mi (320 km) to the east—over the Sea of Japan. Among the problems in navigating a transpacific flight is the curvature of the Earth, combined with the fact that the planet continues to rotate as the aircraft moves. On such long flights, it is impossible to “pretend,” as on a short flight, that Earth is flat: coordinates have to be adjusted for the rounded surface of the planet. In addition, the flight plan must take into account that (in the case of a flight from California to Japan), Earth is moving eastward even as the plane moves westward. The INS aboard KAL 007 may simply have failed to correct for these factors, and thus the error compounded as the plane moved further. In any case, INS will eventually be rendered obsolete by another form of navigation technology: the global positioning satellite (GPS) system.

Understanding Inertia Yet INS has its limitations, as illustrated by the tragedy that occurred aboard Korean Air Lines (KAL) Flight 007 on September 1, 1983. The plane, which contained 269 people and crew members, departed Anchorage, Alaska, on course for Seoul, South Korea. The route they would fly was an established one called “R-20,” and it appears that all the information regarding their flight plan had been entered correctly in the plane’s INS.

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From examples used above, it should be clear that inertia is a more complex topic than you might immediately guess. In fact, inertia as a process is rather straightforward, but confusion regarding its meaning has turned it into a complicated subject.

This information included coordinates for internationally recognized points of reference, actually just spots on the northern Pacific with names such as NABIE, NUKKS, NEEVA, and so on, to NOKKA, thirty minutes east of Japan. Yet, just after passing the fishing village of Bethel, Alaska, on the Bering Sea, the plane started to veer off course, and ultimately wandered into

In everyday terminology, people typically use the word inertia to describe the tendency of a stationary object to remain in place. This is particularly so when the word is used metaphorically: as suggested earlier, the concept of inertia, like numerous other aspects of the laws of motion, is often applied to personal or emotional processes as much as the physical. Hence, you could say, for instance, “He might have changed professions and made more money, but inertia kept him at his old job.” Yet you could just as easily say, for

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example, “He might have taken a vacation, but inertia kept him busy.” Because of the misguided way that most people use the term, it is easy to forget that “inertia” equally describes a tendency toward movement or nonmovement: in terms of Newtonian mechanics, it simply does not matter. The significance of the clause “unless or until outside forces act upon it” in the first law indicates that the object itself must be in equilibrium—that is, the forces acting upon it must be balanced. In order for an object to be in equilibrium, its rate of movement in a given direction must be constant. Since a rate of movement equal to 0 is certainly constant, an object at rest is in equilibrium, and therefore qualifies; but also, any object moving in a constant direction at a constant speed is also in equilibrium.

The Second Law: Force, Mass, Acceleration As noted earlier, the first law of motion deserves special attention because it is the key to unlocking the other two. Having established in the first law the conditions under which an object in motion will change velocity, the second law provides a measure of the force necessary to cause that change. Understanding the second law requires defining terms that, on the surface at least, seem like a matter of mere common sense. Even inertia requires additional explanation in light of terms related to the second law, because it would be easy to confuse it with momentum. The measure of inertia is mass, which reflects the resistance of an object to a change in its motion. Weight, on the other hand, measures the gravitational force on an object. (The concept of force itself will require further definition shortly.) Hence a person’s mass is the same everywhere in the universe, but their weight would differ from planet to planet. This can get somewhat confusing when you attempt to convert between English and metric units, because the pound is a unit of weight or force, whereas the kilogram is a unit of mass. In fact it would be more appropriate to set up kilograms against the English unit called the slug (equal to 14.59 kg), or to compare pounds to the metric unit of force, the newton (N), which is equal to the acceleration of one meter per second per second on an object of 1 kg in mass.

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Hence, though many tables of weights and measures show that 1 kg is equal to 2.21 lb, this is only true at sea level on Earth. A person with a mass of 100 kg on Earth would have the same mass on the Moon; but whereas he might weigh 221 lb on Earth, he would be considerably lighter on the Moon. In other words, it would be much easier to lift a 221-lb man on the Moon than on Earth, but it would be no easier to push him aside.

Laws of Motion

To return to the subject of momentum, whereas inertia is measured by mass, momentum is equal to mass multiplied by velocity. Hence momentum, which Newton called “quantity of motion,” is in effect inertia multiplied by velocity. Momentum is a subject unto itself; what matters here is the role that mass (and thus inertia) plays in the second law of motion. According to the second law, the net force acting upon an object is a product of its mass multiplied by its acceleration. The latter is defined as a change in velocity over a given time interval: hence acceleration is usually presented in terms of “feet (or meters) per second per second”—that is, feet or meters per second squared. The acceleration due to gravity is 32 ft (9.8 m) per second per second, meaning that as every second passes, the speed of a falling object is increasing by 32 ft (9.8 m) per second. The second law, as stated earlier, serves to develop the first law by defining the force necessary to change the velocity of an object. The law was integral to the confirming of the Copernican model, in which planets revolve around the Sun. Because velocity indicates movement in a single (straight) direction, when an object moves in a curve—as the planets do around the Sun—it is by definition changing velocity, or accelerating. The fact that the planets, which clearly possessed mass, underwent acceleration meant that some force must be acting on them: a gravitational pull exerted by the Sun, most massive object in the solar system. Gravity is in fact one of four types of force at work in the universe. The others are electromagnetic interactions, and “strong” and “weak” nuclear interactions. The other three were unknown to Newton—yet his definition of force is still applicable. Newton’s calculation of gravitational force (which, like momentum, is a subject unto itself) made it possible for Halley to determine that the comet he had observed in

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1682—the comet that today bears his name— would reappear in 1758, as indeed it has for every 75–76 years since then. Today scientists use the understanding of gravitational force imparted by Newton to determine the exact altitude necessary for a satellite to remain stationary above the same point on Earth’s surface. The second law is so fundamental to the operation of the universe that you seldom notice its application, and it is easiest to illustrate by examples such as those above—of astronomers and physicists applying it to matters far beyond the scope of daily life. Yet the second law also makes it possible, for instance, to calculate the amount of force needed to move an object, and thus people put it into use every day without knowing that they are doing so.

The Third Law: Action and Reaction As with the second law, the third law of motion builds on the first two. Having defined the force necessary to overcome inertia, the third law predicts what will happen when one force comes into contact with another force. As the third law states, when one object exerts a force on another, the second object exerts on the first a force equal in magnitude but opposite in direction. Unlike the second law, this one is much easier to illustrate in daily life. If a book is sitting on a table, that means that the book is exerting a force on the table equal to its mass multiplied by its rate of acceleration. Though it is not moving, the book is subject to the rate of gravitational acceleration, and in fact force and weight (which is defined as mass multiplied by the rate of acceleration due to gravity) are the same. At the same time, the table pushes up on the book with an exactly equal amount of force—just enough to keep it stationary. If the table exerted more force that the book—in other words, if instead of being an ordinary table it were some sort of pneumatic press pushing upward—then the book would fly off the table.

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beams and the foundation of the building, and the building and the ground, and so on. These pairs of forces exist everywhere. When you walk, you move forward by pushing backward on the ground with a force equal to your mass multiplied by your rate of downward gravitational acceleration. (This force, in other words, is the same as weight.) At the same time, the ground actually pushes back with an equal force. You do not perceive the fact that Earth is pushing you upward, simply because its enormous mass makes this motion negligible—but it does push. If you were stepping off of a small unmoored boat and onto a dock, however, something quite different would happen. The force of your leap to the dock would exert an equal force against the boat, pushing it further out into the water, and as a result, you would likely end up in the water as well. Again, the reaction is equal and opposite; the problem is that the boat in this illustration is not fixed in place like the ground beneath your feet. Differences in mass can result in apparently different reactions, though in fact the force is the same. This can be illustrated by imagining a mother and her six-year-old daughter skating on ice, a relatively frictionless surface. Facing one another, they push against each other, and as a result each moves backward. The child, of course, will move backward faster because her mass is less than that of her mother. Because the force they exerted is equal, the daughter’s acceleration is greater, and she moves farther. Ice is not a perfectly frictionless surface, of course: otherwise, skating would be impossible. Likewise friction is absolutely necessary for walking, as you can illustrate by trying to walk on a perfectly slick surface—for instance, a skating rink covered with oil. In this situation, there is still an equally paired set of forces—your body presses down on the surface of the ice with as much force as the ice presses upward—but the lack of friction impedes the physical process of pushing off against the floor.

There is no such thing as an unpaired force in the universe. The table rests on the floor just as the book rests on it, and the floor pushes up on the table with a force equal in magnitude to that with which the table presses down on the floor. The same is true for the floor and the supporting beams that hold it up, and for the supporting

It will only be possible to overcome inertia by recourse to outside intervention, as for instance if someone who is not on the ice tossed out a rope attached to a pole in the ground. Alternatively, if the person on the ice were carrying a heavy load of rocks, it would be possible to move by throwing the rocks backward. In this situation, you are exerting force on the rock, and this

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Laws of Motion

KEY TERMS A change in velocity

ACCELERATION:

over a given time period. EQUILIBRIUM:

A situation in which

the forces acting upon an object are in

MECHANICS:

The study of bodies in

motion. MOMENTUM:

The product of mass

multiplied by velocity.

balance. FRICTION:

Any force that resists the

motion of body in relation to another with which it is in contact. INERTIA:

The tendency of an object in

motion to remain in motion, and of an object at rest to remain at rest. MASS:

A measure of inertia, indicating

The rate at which the position

SPEED:

of an object changes over a given period of time. VELOCITY:

The speed of an object in a

particular direction. VISCOSITY: The internal friction in a

fluid that makes it resistant to flow.

the resistance of an object to a change in its A measure of the gravitation-

motion—including a change in velocity. A

WEIGHT:

kilogram is a unit of mass, whereas a

al force on an object. A pound is a unit of

pound is a unit of weight. The mass of an

weight, whereas a kilogram is a unit of

object remains the same throughout the

mass. Weight thus would change from

universe, whereas its weight is a function of

planet to planet, whereas mass remains

gravity on any given planet.

constant throughout the universe.

backward force results in a force propelling the thrower forward. This final point about friction and movement is an appropriate place to close the discussion on the laws of motion. Where walking or skating are concerned—and in the absence of a bag of rocks or some other outside force—friction is necessary to the action of creating a backward force and therefore moving forward. On the other hand, the absence of friction would make it possible for an object in movement to continue moving indefinitely, in line with the first law of motion. In either case, friction opposes inertia. The fact is that friction itself is a force. Thus, if you try to slide a block of wood across a floor, friction will stop it. It is important to remember this, lest you fall into the fallacy that bedeviled Aristotle’s thinking and thus confused the world for many centuries. The block did not stop moving because the force that pushed it was no longer being applied; it stopped because an

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opposing force, friction, was greater than the force that was pushing it. WHERE TO LEARN MORE Ardley, Neil. The Science Book of Motion. San Diego: Harcourt Brace Jovanovich, 1992. Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. Chase, Sara B. Moving to Win: The Physics of Sports. New York: Messner, 1977. Fleisher, Paul. Secrets of the Universe: Discovering the Universal Laws of Science. Illustrated by Patricia A. Keeler. New York: Atheneum, 1987. “The Laws of Motion.” How It Flies (Web site). (February 27, 2001). Newton, Isaac (translated by Andrew Motte, 1729). The Principia (Web site). (February 27, 2001). Newton’s Laws of Motion (Web site). (February 27, 2001).

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“Newton’s Laws of Motion.” Dryden Flight Research Center, National Aeronautics and Space Administration (NASA) (Web site). (February 27, 2001).

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(Web site). (February 27, 2001). Roberts, Jeremy. How Do We Know the Laws of Motion? New York: Rosen, 2001.

“Newton’s Laws of Motion: Movin’ On.” Beyond Books

Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996.

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Gravity and Gravitation

G R AV I T Y A N D G R AV I TAT I O N

CONCEPT Gravity is, quite simply, the force that holds together the universe. People are accustomed to thinking of it purely in terms of the gravitational pull Earth exerts on smaller bodies—a stone, a human being, even the Moon—or perhaps in terms of the Sun’s gravitational pull on Earth. In fact, everything exerts a gravitational attraction toward everything else, an attraction commensurate with the two body’s relative mass, and inversely related to the distance between them. The earliest awareness of gravity emerged in response to a simple question: why do objects fall when released from any restraining force? The answers, which began taking shape in the sixteenth century, were far from obvious. In modern times, understanding of gravitational force has expanded manyfold: gravity is clearly a law throughout the universe—yet some of the more complicated questions regarding gravitational force are far from settled.

philosopher exerted an incalculable influence on the development of scientific thought. Aristotle’s contributions to the advancement of the sciences were many and varied, yet his influence in physics was at least as harmful as it was beneficial. Furthermore, the fact that intellectual progress began slowing after several fruitful centuries of development in Greece only compounded the error. By the time civilization had reached the Middle Ages (c. 500 A.D.) the Aristotelian model of physical reality had been firmly established, and an entire millennium passed before it was successfully challenged. Wrong though it was in virtually all particulars, the Aristotelian system offered a comforting symmetry amid the troubled centuries of the early medieval period. It must have been reassuring indeed to believe that the physical universe was as simple as the world of human affairs was complex. According to this neat model, all materials on Earth consisted of four elements: earth, water, air, and fire.

Greek philosophers of the period from the sixth to the fourth century B.C. grappled with a variety of questions concerning the fundamental nature of physical reality, and the forces that bind that reality into a whole. Among the most advanced thinkers of that period was Democritus (c. 460370 B.C.), who put forth a hypothesis many thousands of years ahead of its time: that all of matter interacts at the atomic level.

Each element had its natural place. Hence, earth was always the lowest, and in some places, earth was covered by water. Water must then be higher, but clearly air was higher still, since it covered earth and water. Highest of all was fire, whose natural place was in the skies above the air. Reflecting these concentric circles were the orbits of the Sun, the Moon, and the five known planets. Their orbital paths, in the Aristotelian model of the universe—a model developed to a great degree by the astronomer Ptolemy (c. 100170)—were actually spheres that revolved around Earth with clockwork precision.

Aristotle (384-322 B.C.), however, rejected the explanation offered by Democritus, an unfortunate circumstance given the fact that the great

On Earth, according to the Aristotelian model, objects tended to fall toward the ground in accordance with the admixtures of differing

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Gravity and Gravitation

BECAUSE

OF

EARTH’S

GRAVITY, THE WOMAN BEING SHOT OUT OF THIS CANNON WILL EVENTUALLY FALL

TO THE GROUND RATHER THAN ASCEND INTO OUTER SPACE. (Underwood & Underwood/Corbis. Reproduced by

permission.)

elements they contained. A rock, for instance, was mostly earth, and hence it sought its own level, the lowest of all four elements. For the same reason, a burning fire rose, seeking the heights that were fire’s natural domain. It followed from this that an object falls faster or slower, depending on the relative mixtures of elements in it: or, to use more modern terms, the heavier the object, the faster it falls.

rejected his heliocentric (Sun-centered) model in favor of the geocentric or Earth-centered one. In subsequent centuries, no less a political authority than the Catholic Church gave its approval to the Ptolemaic system. This system seemed to fit well with a literal interpretation of biblical passages concerning God’s relationship with man, and man’s relationship to the cosmos; hence, the heliocentric model of Copernicus constituted an offense to morality.

Galileo Takes Up the Copernican Challenge

For this reason, Copernicus was hesitant to defend his ideas publicly, yet these concepts found their way into the consciousness of European thinkers, causing a paradigm shift so fundamental that it has been dubbed “the Copernican Revolution.” Still, Copernicus offered no explanation as to why the planets behaved as they did: hence, the true leader of the Copernican Revolution was not Copernicus himself but Galileo Galilei (1564-1642.)

Over the centuries, a small but significant body of scientists and philosophers—each working independent from the other but building on the ideas of his predecessors—slowly began chipping away at the Aristotelian framework. The pivotal challenge came in the early part of the century, and the thinker who put it forward was not a physicist but an astronomer: Nicolaus Copernicus (1473-1543.)

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Based on his study of the planets, Copernicus offered an entirely new model of the universe, one that placed the Sun and not Earth at its center. He was not the first to offer such an idea: half a century after Aristotle’s death, Aristarchus (fl. 270 B.C.) had a similar idea, but Ptolemy

Initially, Galileo set out to study and defend the ideas of Copernicus through astronomy, but soon the Church forced him to recant. It is said that after issuing a statement in which he refuted the proposition that Earth moves—a direct attack on the static harmony of the Aristotelian/Ptolemaic model—he protested in pri-

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vate: “E pur si muove!” (But it does move!) Placed under house arrest by authorities from Rome, he turned his attention to an effort that, ironically, struck the fatal blow against the old model of the cosmos: a proof of the Copernican system according to the laws of physics.

Gravity and Gravitation

G R A V I TAT I O N A L A C C E L E R A T I O N . In the process of defending Copernicus,

Galileo actually inaugurated the modern history of physics as a science (as opposed to what it had been during the Middle Ages: a nest of suppositions masquerading as knowledge). Specifically, Galileo set out to test the hypothesis that objects fall as they do, not because of their weight, but as a consequence of gravitational force. If this were so, the acceleration of falling bodies would have to be the same, regardless of weight. Of course, it was clear that a stone fell faster than a feather, but Galileo reasoned that this was a result of factors other than weight, and later investigations confirmed that air resistance and friction, not weight, are responsible for this difference. On the other hand, if one drops two objects that have similar air resistance but differing weight—say, a large stone and a smaller one— they fall at almost exactly the same rate. To test this directly, however, would have been difficult for Galileo: stones fall so fast that, even if dropped from a great height, they would hit the ground too soon for their rate of fall to be tested with the instruments then available. Instead, Galileo used the motion of a pendulum, and the behavior of objects rolling or sliding down inclined planes, as his models. On the basis of his observations, he concluded that all bodies are subject to a uniform rate of gravitational acceleration, later calibrated at 32 ft (9.8 m) per second. What this means is that for every 32 ft an object falls, it is accelerating at a rate of 32 ft per second as well; hence, after 2 seconds, it falls at the rate of 64 ft (19.6 m) per second; after 3 seconds, at 96 ft (29.4 m) per second, and so on.

THIS

PHOTO SHOWS AN APPLE AND A FEATHER BEING

DROPPED

IN

A

VACUUM

TUBE.

BECAUSE

OF

THE

ABSENCE OF AIR RESISTANCE, THE TWO OBJECTS FALL AT THE SAME RATE. (Photograph by James A. Sugar/Corbis. Repro-

duced by permission.)

twentieth century, when Albert Einstein (18791955) challenged it on certain specifics. Even so, Einstein’s relativity did not disprove the Newtonian system as Copernicus and Galileo disproved Aristotle’s and Ptolemy’s theories; rather, it showed the limitations of Newtonian mechanics for describing the behavior of certain objects and phenomena. However, in the ordinary world of day-to-day experience—the world in which stones drop and heavy objects are hard to lift—the Newtonian system still offers the key to how and why things work as they do. This is particularly the case with regard to gravity and gravitation.

Building on the work of his distinguished forebear, Sir Isaac Newton (1642-1727)—who, incidentally, was born the same year Galileo died— developed a paradigm for gravitation that, even today, explains the behavior of objects in virtually all situations throughout the universe. Indeed, the Newtonian model reigned until the early

Like Galileo, Newton began in part with the aim of testing hypotheses put forth by an astronomer—in this case Johannes Kepler (15711630). In the early years of the seventeenth century, Kepler published his three laws of planetary motion, which together identified the elliptical (oval-shaped) path of the planets around the Sun. Kepler had discovered a mathematical relationship that connected the distances of the planets from the Sun to the period of their revolution

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Newton Discovers the Principle of Gravity

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around it. Like Galileo with Copernicus, Newton sought to generalize these principles to explain, not only how the planets moved, but also why they did. Almost everyone has heard the story of Newton and the apple—specifically, that while he was sitting under an apple tree, a falling apple struck him on the head, spurring in him a great intuitive leap that led him to form his theory of gravitation. One contemporary biographer, William Stukely, wrote that he and Newton were sitting in a garden under some apple trees when Newton told him that “...he was just in the same situation, as when formerly, the notion of gravitation came into his mind. It was occasion’d by the fall of an apple, as he sat in a contemplative mood. Why should that apple always descend perpendicularly to the ground, he thought to himself. Why should it not go sideways or upwards, but constantly to the earth’s centre?” The tale of Newton and the apple has become a celebrated myth, rather like that of George Washington and the cherry tree. It is an embellishment of actual events: Newton never said that an apple hit him on the head, just that he was thinking about the way that apples fell. Yet the story has become symbolic of the creative intellectual process that occurs when a thinker makes a vast intuitive leap in a matter of moments. Of course, Newton had spent many years contemplating these ideas, and their development required great effort. What is important is that he brought together the best work of his predecessors, yet transcended all that had gone before—and in the process, forged a model that explained a great deal about how the universe functions. The result was his Philosophiae Naturalis Principia Mathematica, or “Mathematical Principles of Natural Philosophy.” Published in 1687, the book—usually referred to simply as the Principia—was one of the most influential works ever written. In it, Newton presented his three laws of motion, as well as his law of universal gravitation. The latter stated that every object in the universe attracts every other one with a force proportional to the masses of each, and inversely proportional to the square of the distance between them. This statement requires some clarification with regard to its particulars, after

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which it will be reintroduced as a mathematical formula. M ASS A N D F O R C E . The three laws

of motion are a subject unto themselves, covered elsewhere in this volume. However, in order to understand gravitation, it is necessary to understand at least a few rudimentary concepts relating to them. The first law identifies inertia as the tendency of an object in motion to remain in motion, and of an object at rest to remain at rest. Inertia is measured by mass, which—as the second law states—is a component of force. Specifically, the second law of motion states that force is equal to mass multiplied by acceleration. This means that there is an inverse relationship between mass and acceleration: if force remains constant and one of these factors increases, the other must decrease—a situation that will be discussed in some depth below. Also, as a result of Newton’s second law, it is possible to define weight scientifically. People typically assume that mass and weight are the same, and indeed they are on Earth—or at least, they are close enough to be treated as comparable factors. Thus, tables of weights and measures show that a kilogram, the metric unit of mass, is equal to 2.2 pounds, the latter being the principal unit of weight in the British system. In fact, this is—if not a case of comparing to apples to oranges—certainly an instance of comparing apples to apple pies. In this instance, the kilogram is the “apple” (a fitting Newtonian metaphor!) and the pound the “apple pie.” Just as an apple pie contains apples, but other things as well, the pound as a unit of force contains an additional factor, acceleration, not included in the kilo. B R I T I S H V S . S I U N I T S . Physicists universally prefer the metric system, which is known in the scientific community as SI (an abbreviation of the French Système International d’Unités—that is, “International System of Units”). Not only is SI much more convenient to use, due to the fact that it is based on units of 10; but in discussing gravitation, the unequal relationship between kilograms and pounds makes conversion to British units a tedious and ultimately useless task.

Though Americans prefer the British system to SI, and are much more familiar with pounds than with kilos, the British unit of mass—called the slug—is hardly a household word. By conS C I E N C E O F E V E RY DAY T H I N G S

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trast, scientists make regular use of the SI unit of force—named, appropriately enough, the newton. In the metric system, a newton (N) is the amount of force required to accelerate 1 kilogram of mass by 1 meter per second squared (m/s2) Due to the simplicity of using SI over the British system, certain aspects of the discussion below will be presented purely in terms of SI. Where appropriate, however, conversion to British units will be offered. C A L C U L AT I N G G RAV I TAT I O N A L F O R C E . The law of universal gravitation

can be stated as a formula for calculating the gravitational attraction between two objects of a certain mass, m1 AND M2: Fgrav = G • (m1M2)/ R2. Fgrav is gravitational force, and r2 the square of the distance between m1 and m2. As for G, in Newton’s time the value of this number was unknown. Newton was aware simply that it represented a very small quantity: without it, (m1m2)/r2 could be quite sizeable for objects of relatively great mass separated by a relatively small distance. When multiplied by this very small number, however, the gravitational attraction would be revealed to be very small as well. Only in 1798, more than a century after Newton’s writing, did English physicist Henry Cavendish (1731-1810) calculate the value of G. As to how Cavendish derived the figure, that is an exceedingly complex subject far beyond the scope of the present discussion. Even to identify G as a number is a challenging task. First of all, it is a unit of force multiplied by squared area, then divided by squared mass: in other words, it is expressed in terms of (N • m2)/kg2, where N stands for newtons, m for meters, and kg for kilograms. Nor is the coefficient, or numerical value, of G a whole number such as 1. A figure as large as 1, in fact, is astronomically huge compared to G, whose coefficient is 6.67 • 10-11—in other words, 0.0000000000667.

REAL-LIFE A P P L I C AT I O N S Weight vs. Mass

framework of physics requires setting aside everyday notions.

Gravity and Gravitation

First of all, why the distinction between weight and mass? People are so accustomed to converting pounds to kilos on Earth that the difference is difficult to comprehend, but if one considers the relation of mass and weight in outer space, the distinction becomes much clearer. Mass is the same throughout the universe, making it a much more fundamental characteristic—and hence, physicists typically speak in terms of mass rather than weight. Weight, on the other hand, differs according to the gravitational pull of the nearest large body. On Earth, a person weighs a certain amount, but on the Moon, this weight is much less, because the Moon possesses less mass than Earth. Therefore, in accordance with Newton’s formula for universal gravitation, it exerts less gravitational pull. By contrast, if one were on Jupiter, it would be almost impossible even to stand up, because the pull of gravity on that planet—with its greater mass—would be vastly greater than on Earth. It should be noted that mass is not at all a function of size: Jupiter does have a greater mass than Earth, but not because it is bigger. Mass, as noted earlier, is purely a measure of inertia: the more resistant an object is to a change in its velocity, the greater its mass. This in itself yields some results that seem difficult to understand as long as one remains wedded to the concept— true enough on Earth—that weight and mass are identical. A person might weigh less on the Moon, but it would be just as difficult to move that person from a resting position as it would be to do so on Earth. This is because the person’s mass, and hence his or her resistance to inertia, has not changed. Again, this is a mentally challenging concept: is not lifting a person, which implies upward acceleration, not an attempt to counteract their inertia when standing still? Does it not follow that their mass has changed? Understanding the distinction requires a greater clarification of the relationship between mass, gravity, and weight.

Before discussing the significance of the gravitational constant, however, at this point it is appropriate to address a few issues that were raised earlier—issues involving mass and weight. In many ways, understanding these properties from the

F = m a . Newton’s second law of motion, stated earlier, shows that force is equal to mass multiplied by acceleration, or in shorthand form, F = ma. To reiterate a point already made, if one assumes that force is constant, then mass and

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acceleration must have an inverse relationship. This can be illustrated by performing a simple experiment. Suppose one were to apply a certain amount of force to an empty shopping cart. Assuming the floor had just enough friction to allow movement, it would be easy for almost anyone to accelerate the shopping cart. Now assume that the shopping cart were filled with heavy lead balls, so that it weighed, say, 1,102 lb (500 kg). If one applied the same force, it would not move. What has changed, clearly, is the mass of the shopping cart. Because force remained constant, the rate of acceleration would become very small—in this case, almost infinitesimal. In the first case, with an empty shopping cart, the mass was relatively small, so acceleration was relatively high. Now to return to the subject of lifting someone on the Moon. It is true that in order to lift that person, one would have to overcome inertia, and, in that sense, it would be as difficult as it is on Earth. But the other component of force, acceleration, has diminished greatly. Weight is, again, a unit of force, but in calculating weight it is useful to make a slight change to the formula F = ma. By definition, the acceleration factor in weight is the downward acceleration due to gravity, usually rendered as g. So one’s weight is equal to mg—but on the Moon, g is much smaller than it is on Earth, and hence, the same amount of force yields much greater results. These facts shed new light on a question that bedeviled physicists at least from the time of Aristotle, until Galileo began clarifying the issue some 2,000 years later: why shouldn’t an object of greater mass fall at a different rate than one of smaller mass? There are two answers to that question, one general and one specific. The general answer—that Earth exerts more gravitational pull on an object of greater mass—requires a deeper examination of Newton’s gravitational formula. But the more specific answer, relating purely to conditions on Earth, is easily addressed by considering the effect of air resistance.

Idealization of reality makes it possible to set aside the things people think they know about the real world, where events are complicated due to friction. The latter may be defined as a force that resists motion when the surface of one object comes into contact with the surface of another. If two balls are released in an environment free from friction—one of them simply dropped while the other is rolled down a curved surface or inclined plane—they will reach the bottom at the same time. This seems to go against everything that is known, but that is only because what people “know” is complicated by variables that have nothing to do with gravity. The same is true for the behavior of falling objects with regard to air resistance. If air resistance were not a factor, one could fire a cannonball over horizontal space and then, when the ball reached the highest point in its trajectory, release another ball from the same height—and again, they would hit the ground at the same time. This is the case, even though the cannonball that was fired from the cannon has to cover a great deal of horizontal space, whereas the dropped ball does not. The fact is that the rate of acceleration due to gravity will be identical for the two balls, and the fact that the ball fired from a cannon also covers a horizontal distance during that same period is irrelevant.

One of Galileo’s many achievements lay in using an idealized model of reality, one that does not take into account the many complex factors that affect the behavior of objects in the real world.

T E R M I N A L V E LO C I T Y. In the real world, air resistance creates a powerful drag force on falling objects. The faster the rate of fall, the greater the drag force, until the air resistance forces a leveling in the rate of fall. At this point, the object is said to have reached terminal velocity, meaning that its rate of fall will not increase thereafter. Galileo’s idealized model, on the other hand, treated objects as though they were falling in a vacuum—space entirely devoid of matter, including air. In such a situation, the rate of acceleration would continue to grow indefinitely.

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This permitted physicists to study processes that apparently defy common sense. For instance, in the real world, an apple does drop at a greater rate of speed than does a feather. However, in a vacuum, they will drop at the same rate. Since Galileo’s time, it has become commonplace for physicists to discuss specific processes such as gravity with the assumption that all non-pertinent factors (in this case, air resistance or friction) are nonexistent or irrelevant. This greatly simplified the means of testing hypotheses.

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By means of a graph, one can compare the behavior of an object falling through air with that of an object falling in a vacuum. If the x axis measures time and the y axis downward speed, the rate of an object falling in a vacuum describes a 60°-angle. In other words, the speed of its descent is increasing at a much faster rate than is the rate of time of its descent—as indeed should be the case, in accordance with gravitational acceleration. The behavior of an object falling through air, on the other hand, describes a curve. Up to a point, the object falls at the same rate as it would in a vacuum, but soon velocity begins to increase at a much slower rate than time. Eventually, the curve levels off at the point where the object experiences terminal velocity. Air resistance and friction have been mentioned separately as though they were two different forces, but in fact air resistance is simply a prominent form of friction. Hence air resistance exerts an upward force to counter the downward force of mass multiplied by gravity—that is, weight. Since g is a constant (32 ft or 9.8 m/sec2), the greater the weight of the falling object, the longer it takes for air resistance to bring it to terminal velocity. A feather quickly reaches terminal velocity, whereas it takes much longer for a cannonball to do the same. As a result, a heavier object does take less time to fall, even from a great height, than does a light one—but this is only because of friction, and not because of “elements” seeking their “natural level.” Incidentally, if raindrops (which of course fall from a very great height) did not reach terminal velocity, they would cause serious injury by the time they hit the ground.

Applying the Gravitational Formula Using Newton’s gravitational formula, it is relatively easy to calculate the pull of gravity between two objects. It is also easy to see why the attraction is insignificant unless at least one of the objects has enormous mass. In addition, application of the formula makes it clear why G (the gravitational constant, as opposed to g, the rate of acceleration due to gravity) is such a tiny number. If two people each have a mass of 45.5 kg (100 lb) and stand 1 m (3.28 ft) apart, m1m2 is equal to 2,070 kg (4,555 lb) and r2 is equal to 1 m2. Applied to the gravitational formula, this figure is rendered as 2,070 kg2/1 m2. This number is

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then multiplied by gravitational constant, which again is equal to 6.67 • 10-11 (N • m2)/kg2. The result is a net gravitational force of 0.000000138 N (0.00000003 lb)—about the weight of a singlecell organism!

Gravity and Gravitation

E A RT H , G RAV I T Y, A N D W E I G H T.

Though it is certainly interesting to calculate the gravitational force between any two people, computations of gravity are only significant for objects of truly great mass. For instance, there is the Earth, which has a mass of 5.98 • 1024 kg— that is, 5.98 septillion (1 followed by 24 zeroes) kilograms. And, of course, Earth’s mass is relatively minor compared to that of several planets, not to mention the Sun. Yet Earth exerts enough gravitational pull to keep everything on it—living creatures, manmade structures, mountains and other natural features—stable and in place. One can calculate Earth’s gravitational force on any one person—if one wants to take the time to do so using Newton’s formula. In fact, it is much simpler than that: gravitational force is equal to weight, or m • g. Thus if a woman weighs 100 lb (445 N), this amount is also equal to the gravitational force exerted on her. By dividing 445 N by the acceleration of gravity—9.8 m/sec2—it is easy to obtain her mass: 45.4 kg. The use of the mg formula for gravitation helps, once again, to explain why heavier objects do not fall faster than lighter ones. The figure for g is a constant, but for the sake of argument, let us assume that it actually becomes larger for objects with a greater mass. This in turn would mean that the gravitational force, or weight, would be bigger than it is—thus creating an irreconcilable logic loop. Furthermore, one can compare results of two gravitation equations, one measuring the gravitational force between Earth and a large stone, the other measuring the force between Earth and a small stone. (The distance between Earth and each stone is assumed to be the same.) The result will yield a higher quantity for the force exerted on the larger stone—but only because its mass is greater. Clearly, then, the increase of force results only from an increase in mass, not acceleration.

Gravity and Curved Space As should be clear from Newton’s gravitational formula, the force of gravity works both ways: not only does a stone fall toward Earth, but Earth

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Gravity and Gravitation

KEY TERMS FORCE:

The product of mass multi-

plied by acceleration.

comes into contact with the surface of

A measure of inertia, indicating the resistance of an object to a change in its motion.

another.

TERMINAL

FRICTION:

The force that resists

motion when the surface of one object

MASS:

the decrease in the other.

A term describing the rate of fall for an object experiencing the drag force of air resistance. In a vacuum, the object would continue to accelerate with the force of gravity, but in most real-world situations, air resistance creates a powerful drag force that causes a leveling in the object’s rate of fall.

LAW OF UNIVERSAL GRAVITATION:

VACUUM:

INERTIA:

The tendency of an object in

motion to remain in motion, and of an object at rest to remain at rest. INVERSE RELATIONSHIP:

A situa-

tion involving two variables, in which one of the two increases in direct proportion to

VELOCITY:

A principle, put forth by Sir Isaac Newton

Space entirely devoid of matter, including air.

(1642-1727), which states that every object

WEIGHT:

in the universe attracts every other one with a force proportional to the masses of

actually falls toward it. The mass of Earth is so great compared to that of the stone that the movement of Earth is imperceptible—but it does happen. Furthermore, because Earth is round, when one hurls a projectile at a great distance, Earth curves away from the projectile; but eventually gravity itself forces the projectile to the ground. However, if one were to fire a rocket at 17,700 MPH (28,500 km/h), at every instant of time the projectile is falling toward Earth with the force of gravity—but the curved Earth would be falling away from it at the same moment as well. Hence, the projectile would remain in constant motion around the planet—that is, it would be in orbit. The same is true of an artificial satellite’s orbit around Earth: even as the satellite falls toward Earth, Earth falls away from it. This same relationship exists between Earth and its great natural satellite, the Moon. Likewise, with the 76

each, and inversely proportional to the square of the distance between them.

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A measure of the gravitational force on an object; the product of mass multiplied by the acceleration due to gravity.

Sun and its many satellites, including Earth: Earth plunges toward the Sun with every instant of its movement, but at every instant, the Sun falls away. W H Y I S E A RT H R O U N D ? Note

that in the above discussion, it was assumed that Earth and the Sun are round. Everyone knows that to be the case, but why? The answer is “Because they have to be”—that is, gravity will not allow them to be otherwise. In fact, the larger the mass of an object, the greater its tendency toward roundness: specifically, the gravitational pull of its interior forces the surface to assume a relatively uniform shape. There is a relatively small vertical differential for Earth’s surface: between the lowest point and the highest point is just 12.28 mi (19.6 km)—not a great distance, considering that Earth’s radius is about 4,000 mi (6,400 km). It is true that Earth bulges near the equator, but this is only because it is spinning rapidly on S C I E N C E O F E V E RY DAY T H I N G S

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its axis, and thus responding to the centripetal force of its motion, which produces a centrifugal component. If Earth were standing still, it would be much nearer to the shape of a sphere. On the other hand, an object of less mass is more likely to retain a shape that is far less than spherical. This can be shown by reference to the Martian moons Phobos and Deimos, both of which are oblong—and both of which are tiny, in terms of size and mass, compared to Earth’s Moon. Mars itself has a radius half that of Earth, yet its mass is only about 10% of Earth’s. In light of what has been said about mass, shape, and gravity, it should not surprising to learn that Mars is also home to the tallest mountain in the solar system. Standing 15 mi (24 km) high, the volcano Olympus Mons is not only much taller than Earth’s tallest peak, Mount Everest (29,028 ft [8,848 m]); it is 22% taller than the distance from the top of Mount Everest to the lowest spot on Earth, the Mariana Trench in the Pacific Ocean (-35,797 ft [-10,911 m]) A spherical object behaves with regard to gravitation as though its mass were concentrated near its center. And indeed, 33% of Earth’s mass is at is core (as opposed to the crust or mantle), even though the core accounts for only about 20% of the planet’s volume. Geologists believe that the composition of Earth’s core must be molten iron, which creates the planet’s vast electromagnetic field. T H E F R O N T I E R S O F G RAV I T Y.

The subject of curvature with regard to gravity can be both a threshold or—as it is here—a point of closure. Investigating questions over perceived anomalies in Newton’s description of the behavior of large objects in space led Einstein to his General Theory of Relativity, which posited a curved four-dimensional space-time. This led to entirely new notions concerning gravity, mass, and light. But relativity, as well as its relation to gravity, is another subject entirely. Einstein offered a new understanding of gravity, and indeed of physics itself, that has changed the way

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thinkers both inside and outside the sciences perceive the universe. Here on Earth, however, gravity behaves much as Newton described it more than three centuries ago.

Gravity and Gravitation

Meanwhile, research in gravity continues to expand, as a visit to the Web site <www.Gravity.org> reveals. Spurred by studies in relativity, a branch of science called relativistic astrophysics has developed as a synthesis of astronomy and physics that incorporates ideas put forth by Einstein and others. The <www.Gravity.org> site presents studies—most of them too abstruse for a reader who is not a professional scientist— across a broad spectrum of disciplines. Among these is bioscience, a realm in which researchers are investigating the biological effects—such as mineral loss and motion sickness—of exposure to low gravity. The results of such studies will ultimately protect the health of the astronauts who participate in future missions to outer space. WHERE TO LEARN MORE Ardley, Neil. The Science Book of Gravity. San Diego, CA: Harcourt Brace Jovanovich, 1992. Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. Bendick, Jeanne. Motion and Gravity. New York: F. Watts, 1972. Dalton, Cindy Devine. Gravity. Vero Beach, FL: Rourke, 2001. David, Leonard. “Artificial Gravity and Space Travel.” BioScience, March 1992, pp. 155-159. Exploring Gravity—Curtin University, Australia (Web site). (March 18, 2001). The Gravity Society (Web site). (March 18, 2001). Nardo, Don. Gravity: The Universal Force. San Diego, CA: Lucent Books, 1990. Rutherford, F. James; Gerald Holton; and Fletcher G. Watson. Project Physics. New York: Holt, Rinehart, and Winston, 1981. Stringer, John. The Science of Gravity. Austin, TX: Raintree Steck-Vaughn, 2000.

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PROJECTILE MOTION Projectile Motion

CONCEPT A projectile is any object that has been thrown, shot, or launched, and ballistics is the study of projectile motion. Examples of projectiles range from a golf ball in flight, to a curve ball thrown by a baseball pitcher to a rocket fired into space. The flight paths of all projectiles are affected by two factors: gravity and, on Earth at least, air resistance.

HOW IT WORKS The effects of air resistance on the behavior of projectiles can be quite complex. Because effects due to gravity are much simpler and easier to analyze, and since gravity applies in more situations, we will discuss its role in projectile motion first. In most instances on Earth, of course, a projectile will be subject to both forces, but there may be specific cases in which an artificial vacuum has been created, which means it will only be subjected to the force of gravity. Furthermore, in outer space, gravity—whether from Earth or another body—is likely to be a factor, whereas air resistance (unless or until astronomers find another planet with air) will not be.

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at sea level, but since the value of g changes little with altitude—it only decreases by 5% at a height of 10 mi (16 km)—it is safe to use this number. When a plane goes into a high-speed turn, it experiences much higher apparent g. This can be as high as 9 g, which is almost more than the human body can endure. Incidentally, people call these “g-forces,” but in fact g is not a measure of force but of a single component, acceleration. On the other hand, since force is the product of mass multiplied by acceleration, and since an aircraft subject to a high g factor clearly experiences a heavy increase in net force, in that sense, the expression “g-force” is not altogether inaccurate. In a vacuum, where air resistance plays no part, the effects of g are clearly demonstrated. Hence a cannonball and a feather, dropped into a vacuum at the same moment, would fall at exactly the same rate and hit bottom at the same time.

The Cannonball or the Feather? Air Resistance vs. Mass

The acceleration due to gravity is 32 ft (9.8 m)/sec2, usually expressed as “per second squared.” This means that as every second passes, the speed of a falling object is increasing by 32 ft/sec (9.8 m). Where there is no air resistance, a ball will drop at a velocity of 32 feet per second after one second, 64 ft (19.5 m) per second after two seconds, 96 ft (29.4 m) per second after three seconds, and so on. When an object experiences the ordinary acceleration due to gravity, this figure is rendered in shorthand as g. Actually, the figure of 32 ft (9.8 m) per second squared applies

Naturally, air resistance changes the terms of the above equation. As everyone knows, under ordinary conditions, a cannonball falls much faster than a feather, not simply because the feather is lighter than the cannonball, but because the air resists it much better. The speed of descent is a function of air resistance rather than mass, which can be proved with the following experiment. Using two identical pieces of paper—meaning that their mass is exactly the same—wad one up while keeping the other flat. Then drop them. Which one lands first? The wadded piece will fall faster and land first, precisely because it is less air-resistant than the sail-like flat piece.

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Projectile Motion

BECAUSE

OF THEIR DESIGN, THE BULLETS IN THIS

.357

MAGNUM WILL COME OUT OF THE GUN SPINNING, WHICH

GREATLY INCREASES THEIR ACCURACY. (Photograph by Tim Wright/Corbis. Reproduced by permission.)

Now to analyze the motion of a projectile in a situation without air resistance. Projectile motion follows the flight path of a parabola, a curve generated by a point moving such that its distance from a fixed point on one axis is equal to its distance from a fixed line on the other axis. In other words, there is a proportional relationship between x and y throughout the trajectory or path of a projectile in motion. Most often this parabola can be visualized as a simple up-anddown curve like the shape of a domed roof. (The Gateway Arch in St. Louis, Missouri, is a steep parabola.)

ally force the projectile downward, and once it hits the ground, it can no longer continue on its horizontal trajectory. Not, at least, at the same velocity: if you were to thrust a bowling ball forward, throwing it with both hands from the solar plexus, its horizontal velocity would be reduced greatly once gravity forced it to the floor. Nonetheless, the force on the ball would probably be enough (assuming the friction on the floor was not enormous) to keep the ball moving in a horizontal direction for at least a few more feet.

In the case of a cannonball fired at a 45° angle—the angle of maximum efficiency for height and range together—gravity will eventu-

There are several interesting things about the relationship between gravity and horizontal velocity. Assuming, once again, that air resistance is not a factor, the vertical acceleration of a projectile is g. This means that when a cannonball is at the highest point of its trajectory, you could simply drop another cannonball from exactly the same height, and they would land at the same moment. This seems counterintuitive, or opposite to common sense: after all, the cannonball that was fired from the cannon has to cover a great deal of horizontal space, whereas the dropped ball does not. Nonetheless, the rate of acceleration due to gravity will be identical for the two balls, and the fact that the ball fired from a cannon also covers a horizontal distance during that same period is purely incidental.

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Instead of referring to the more abstract values of x and y, we will separate projectile motion into horizontal and vertical components. Gravity plays a role only in vertical motion, whereas obviously, horizontal motion is not subject to gravitational force. This means that in the absence of air resistance, the horizontal velocity of a projectile does not change during flight; by contrast, the force of gravity will ultimately reduce its vertical velocity to zero, and this will in turn bring a corresponding drop in its horizontal velocity.

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Gravity, combined with the first law of motion, also makes it possible (in theory at least) for a projectile to keep moving indefinitely. This actually does take place at high altitudes, when a satellite is launched into orbit: Earth’s gravitational pull, combined with the absence of air resistance or other friction, ensures that the satellite will remain in constant circular motion around the planet. The same is theoretically possible with a cannonball at very low altitudes: if one could fire a ball at 17,700 MPH (28,500 k/mh), the horizontal velocity would be great enough to put the ball into low orbit around Earth’s surface. The addition of air resistance or airflow to the analysis of projectile motion creates a number of complications, including drag, or the force that opposes the forward motion of an object in airflow. Typically, air resistance can create a drag force proportional to the squared value of a projectile’s velocity, and this will cause it to fall far short of its theoretical range. Shape, as noted in the earlier illustration concerning two pieces of paper, also affects air resistance, as does spin. Due to a principle known as the conservation of angular momentum, an object that is spinning tends to keep spinning; moreover, the orientation of the spin axis (the imaginary “pole” around which the object is spinning) tends to remain constant. Thus spin ensures a more stable flight.

REAL-LIFE A P P L I C AT I O N S Bullets on a Straight Spinning Flight One of the first things people think of when they hear the word “ballistics” is the study of gunfire patterns for the purposes of crime-solving. Indeed, this application of ballistics is a significant part of police science, because it allows lawenforcement investigators to determine when, where, and how a firearm was used. In a larger sense, however, the term as applied to firearms refers to efforts toward creating a more effective, predictable, and longer bullet trajectory.

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ted to the barrel of the musket. When fired, they bounced erratically off the sides of the barrel, and this made their trajectories unpredictable. Compounding this was the unevenness of the lead balls themselves, and this irregularity of shape could lead to even greater irregularities in trajectory. Around 1500, however, the first true rifles appeared, and these greatly enhanced the accuracy of firearms. The term rifle comes from the “rifling” of the musket barrels: that is, the barrels themselves were engraved with grooves, a process known as rifling. Furthermore, ammunitionmakers worked to improve the production process where the musket balls were concerned, producing lead rounds that were more uniform in shape and size. Despite these improvements, soldiers over the next three centuries still faced many challenges when firing lead balls from rifled barrels. The lead balls themselves, because they were made of a soft material, tended to become misshapen during the loading process. Furthermore, the gunpowder that propelled the lead balls had a tendency to clog the rifle barrel. Most important of all was the fact that these rifles took time to load—and in a situation of battle, this could cost a man his life. The first significant change came in the 1840s, when in place of lead balls, armies began using bullets. The difference in shape greatly improved the response of rounds to aerodynamic factors. In 1847, Claude-Etienne Minié, a captain in the French army, developed a bullet made of lead, but with a base that was slightly hollow. Thus when fired, the lead in the round tended to expand, filling the barrel’s diameter and gripping the rifling. As a result, the round came out of the barrel end spinning, and continued to spin throughout its flight. Not only were soldiers able to fire their rifles with much greater accuracy, but thanks to the development of chambers and magazines, they could reload more quickly.

Curve Balls, Dimpled Golf Balls, and Other Tricks with Spin

From the advent of firearms in the West during the fourteenth century until about 1500, muskets were hopelessly unreliable. This was because the lead balls they fired had not been fit-

In the case of a bullet, spin increases accuracy, ensuring that the trajectory will follow an expected path. But sometimes spin can be used in more

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complex ways, as with a curveball thrown by a baseball pitcher.

Projectile Motion

The invention of the curveball is credited to Arthur “Candy” Cummings, who as a pitcher for the Brooklyn Excelsiors at the age of 18 in 1867— an era when baseball was still very young—introduced a new throw he had spent several years perfecting. Snapping as he released the ball, he and the spectators (not to mention the startled batter for the opposing team) watched as the pitch arced, then sailed right past the batter for a strike. The curveball bedeviled baseball players and fans alike for many years thereafter, and many dismissed it as a type of optical illusion. The debate became so heated that in 1941, both Life and Look magazines ran features using stopaction photography to show that a curveball truly did curve. Even in 1982, a team of researchers from General Motors (GM) and the Massachusetts Institute of Technology (MIT), working at the behest of Science magazine, investigated the curveball to determine if it was more than a mere trick. In fact, the curveball is a trick, but there is nothing fake about it. As the pitcher releases the ball, he snaps his wrist. This puts a spin on the projectile, and air resistance does the rest. As the ball moves toward the plate, its spin moves against the air, which creates an airstream moving against the trajectory of the ball itself. The airstream splits into two lines, one curving over the ball and one curving under, as the ball sails toward home plate. For the purposes of clarity, assume that you are viewing the throw from a position between third base and home. Thus, the ball is moving from left to right, and therefore the direction of airflow is from right to left. Meanwhile the ball, as it moves into the airflow, is spinning clockwise. This means that the air flowing over the top of the ball is moving in a direction opposite to the spin, whereas that flowing under it is moving in the same direction as the spin. This creates an interesting situation, thanks to Bernoulli’s principle. The latter, formulated by Swiss mathematician and physicist Daniel Bernoulli (1700-1782), holds that where velocity is high, pressure is low—and vice versa. Bernoulli’s principle is of the utmost importance to aerodynamics, and likewise plays a significant role in the operation of a curveball. At the top of the S C I E N C E O F E V E RY DAY T H I N G S

GOLF

BALLS ARE DIMPLED BECAUSE THEY TRAVEL MUCH

FARTHER

THAN

NONDIMPLED

ONES.

(Photograph by D.

Boone/Corbis. Reproduced by permission.)

ball, its clockwise spin is moving in a direction opposite to the airflow. This produces drag, slowing the ball, increasing pressure, and thus forcing it downward. At the bottom end of the ball, however, the clockwise motion is flowing with the air, thus resulting in higher velocity and lower pressure. As per Bernoulli’s principle, this tends to pull the ball downward. In the 60-ft, 6-in (18.4-m) distance that separates the pitcher’s mound from home plate on a regulation major-league baseball field, a curveball can move downward by a foot (0.3048 m) or more. The interesting thing here is that this downward force is almost entirely due to air resistance rather than gravity, though of course gravity eventually brings any pitch to the ground, assuming it has not already been hit, caught, or bounced off a fence. A curveball represents a case in which spin is used to deceive the batter, but it is just as possible that a pitcher may create havoc at home plate by throwing a ball with little or no spin. This is called a knuckleball, and it is based on the fact that spin in general—though certainly not the deliberate spin of a curveball—tends to ensure a VOLUME 2: REAL-LIFE PHYSICS

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more regular trajectory. Because a knuckleball has no spin, it follows an apparently random path, and thus it can be every bit as tricky for the pitcher as for the batter. Golf, by contrast, is a sport in which spin is expected: from the moment a golfer hits the ball, it spins backward—and this in turn helps to explain why golf balls are dimpled. Early golf balls, known as featheries, were merely smooth leather pouches containing goose feathers. The smooth surface seemed to produce relatively low drag, and golfers were impressed that a well-hit feathery could travel 150-175 yd (137-160 m). Then in the late nineteenth century, a professor at St. Andrews University in Scotland realized that a scored or marked ball would travel farther than a smooth one. (The part about St. Andrews may simply be golfing legend, since the course there is regarded as the birthplace of golf in the fifteenth century.) Whatever the case, it is true that a scored ball has a longer trajectory, again as a result of the effect of air resistance on projectile motion. Airflow produces two varieties of drag on a sphere such as a golf ball: drag due to friction, which is only a small aspect of the total drag, and the much more significant drag that results from the separation of airflow around the ball. As with the curveball discussed earlier, air flows above and below the ball, but the issue here is more complicated than for the curved pitch. Airflow comes in two basic varieties: laminar, meaning streamlined; or turbulent, indicating an erratic, unpredictable flow. For a jet flying through the air, it is most desirable to create a laminar flow passing over its airfoil, or the curved front surface of the wing. In the case of the golf ball, however, turbulent flow is more desirable. In laminar flow, the airflow separates quickly, part of it passing over the ball and part passing under. In turbulent flow, however, separation comes later, further back on the ball. Each form of air separation produces a separation region, an area of drag that the ball pulls behind it (so to speak) as it flies through space. But because the separation comes further back on the ball in turbulent flow, the separation region itself is narrower, thus producing less drag.

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designs that included squares, rectangles, and hexagons. In time, they settled on the dimpled design known today. Golf balls made in Britain have 330 dimples, and those in America 336; in either case, the typical drive distance is much, much further than for an unscored ball—180250 yd (165-229 m).

Powered Projectiles: Rockets and Missiles The most complex form of projectile widely known in modern life is the rocket or missile. Missiles are unmanned vehicles, most often used in warfare to direct some form of explosive toward an enemy. Rockets, on the other hand, can be manned or unmanned, and may be propulsion vehicles for missiles or for spacecraft. The term rocket can refer either to the engine or to the vehicle it propels. The first rockets appeared in China during the late medieval period, and were used unsuccessfully by the Chinese against Mongol invaders in the early part of the thirteenth century. Europeans later adopted rocketry for battle, as for instance when French forces under Joan of Arc used crude rockets in an effort to break the siege on Orleans in 1429. Within a century or so, however, rocketry as a form of military technology became obsolete, though projectile warfare itself remained as effective a method as ever. From the catapults of Roman times to the cannons that appeared in the early Renaissance to the heavy artillery of today, armies have been shooting projectiles against their enemies. The crucial difference between these projectiles and rockets or missiles is that the latter varieties are self-propelled. Only around the end of World War II did rocketry and missile warfare begin to reappear in new, terrifying forms. Most notable among these was Hitler’s V-2 “rocket” (actually a missile), deployed against Great Britain in 1944, but fortunately developed too late to make an impact. The 1950s saw the appearance of nuclear warheads such as the ICBM (intercontinental ballistic missile). These were guided missiles, as opposed to the V-2, which was essentially a huge self-propelled bullet fired toward London.

Clearly, scoring the ball produced turbulent flow, and for a few years in the early twentieth century, manufacturers experimented with

More effective than the ballistic missile, however, was the cruise missile, which appeared in later decades and which included aerodynamic structures that assisted in guidance and

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Projectile Motion

IN

THE CASE OF A ROCKET, LIKE THIS

PATRIOT

MISSILE BEING LAUNCHED DURING A TEST, PROPULSION COMES BY

EXPELLING FLUID—WHICH IN SCIENTIFIC TERMS CAN MEAN A GAS AS WELL AS A LIQUID—FROM ITS REAR END.

MOST

OFTEN THIS FLUID IS A MASS OF HOT GASES PRODUCED BY A CHEMICAL REACTION INSIDE THE ROCKET ’S BODY, AND THIS BACKWARD MOTION CREATES AN EQUAL AND OPPOSITE REACTION FROM THE ATMOSPHERE, PROPELLING THE ROCKET FORWARD. (Corbis. Reproduced by permission.)

maneuvering. In addition to guided or unguided, ballistic or aerodynamic, missiles can be classified in terms of source and target: surface-to-surface, air-to-air, and so on. By the 1970s, the United States had developed an extraordinarily sophisticated surface-to-air missile, the Stinger. Stingers proved a decisive factor in the AfghanSoviet War (1979-89), when U.S.-supplied Afghan guerrillas used them against Soviet aircraft. In the period from the late 1940s to the late 1980s, the United States, the Soviet Union, and other smaller nuclear powers stockpiled these warheads, which were most effective precisely

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because they were never used. Thus, U.S. President Ronald Reagan played an important role in ending the Cold War, because his weapons buildup forced the Soviets to spend money they did not have on building their own arsenal. During the aftermath of the Cold War, America and the newly democratized Russian Federation worked to reduce their nuclear stockpiles. Ironically, this was also the period when sophisticated missiles such as the Patriot began gaining widespread use in the Persian Gulf War and later conflicts. Certain properties unite the many varieties of rocket that have existed across time and

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Projectile Motion

KEY TERMS ACCELERATION:

A change in velocity

parabola there is a proportional relation-

over a given time period. AERODYNAMIC: BALLISTICS:

a result, between any two points on the

Relating to airflow.

The study of projectile

ship between x and y values. PROJECTILE:

Any object that has

been thrown, shot, or launched.

motion. The force that opposes the for-

DRAG:

ward motion of an object in airflow. In

SPECIFIC IMPULSE:

A measure of

rocket fuel efficiency—specifically, the mass that can be lifted by a particular type

most cases, its opposite is lift.

of fuel for each pound of fuel consumer FIRST LAW OF MOTION:

A principle,

formulated by Sir Isaac Newton (16421727), which states that an object at rest

(that is, the rocket and its contents) per second of operation time. Figures for specific impulse are rendered in seconds.

will remain at rest, and an object in motion will remain in motion, at a constant velocity unless or until outside forces act upon it. FRICTION:

Any force that resists the

SPEED:

The rate at which the position

of an object changes over a given period of time. Unlike velocity, direction is not a component of speed.

motion of body in relation to another with which it is in contact.

A princi-

ple, which like the first law of motion was

The tendency of an object in

formulated by Sir Isaac Newton. The third

motion to remain in motion, and of an

law states that when one object exerts a

object at rest to remain at rest.

force on another, the second object exerts

INERTIA:

LAMINAR:

A term describing a stream-

on the first a force equal in magnitude but

lined flow, in which all particles move at

opposite in direction.

the same speed and in the same direction.

TRAJECTORY:

Its opposite is turbulent flow.

in motion, a parabola upward and across

LIFT:

An aerodynamic force perpendi-

The path of a projectile

space.

cular to the direction of the wind. In most

TURBULENT:

cases, its opposite is drag.

highly irregular form of flow, in which a

A term describing a

A measure of inertia, indicating

fluid is subject to continual changes in

the resistance of an object to a change in its

speed and direction. Its opposite is laminar

motion—including a change in velocity.

flow.

MASS:

PARABOLA:

84

THIRD LAW OF MOTION:

A curve generated by a

VELOCITY:

The speed of an object in a

point moving such that its distance from a

particular direction.

fixed point on one axis is equal to its dis-

VISCOSITY:

tance from a fixed line on the other axis. As

fluid that makes it resistant to flow.

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The internal friction in a

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space—including the relatively harmless fireworks used in Fourth of July and New Year’s Eve celebrations around the country. One of the key principles that makes rocket propulsion possible is the third law of motion. Sometimes colloquially put as “For every action, there is an equal and opposite reaction,” a more scientifically accurate version of this law would be: “When one object exerts a force on another, the second object exerts on the first a force equal in magnitude but opposite in direction.” In the case of a rocket, propulsion comes by expelling fluid—which in scientific terms can mean a gas as well as a liquid—from its rear. Most often this fluid is a mass of hot gases produced by a chemical reaction inside the rocket’s body, and this backward motion creates an equal and opposite reaction from the rocket, propelling it forward.

along the flight in part to increase specific impulse. This was the case with the Saturn 5 rockets that carried astronauts to the Moon in the period 1969-72, but not with the varieties of space shuttle that have flown regular missions since 1981. The space shuttle is essentially a hybrid of an airplane and rocket, with a physical structure more like that of an aircraft but with rocket power. In fact, the shuttle uses many rockets to maximize efficiency, utilizing no less than 67 rockets—49 of which run on liquid fuel and the rest on solid fuel—at different stages of its flight. WHERE TO LEARN MORE “Aerodynamics in Sports Equipment.” K8AIT Principles of Aeronautics—Advanced (Web site). (March 2, 2001).

Before it undergoes a chemical reaction, rocket fuel may be either in solid or liquid form inside the rocket’s fuel chamber, though it ends up as a gas when expelled. Both solid and liquid varieties have their advantages and disadvantages in terms of safety, convenience, and efficiency in lifting the craft. Scientists calculate efficiency by a number of standards, among them specific impulse, a measure of the mass that can be lifted by a particular type of fuel for each pound of fuel consumed (that is, the rocket and its contents) per second of operation time. Figures for specific impulse are rendered in seconds.

The Physics of Projectile Motion (Web site). (March 2, 2001).

A spacecraft may be divided into segments or stages, which can be released as specific points

Richardson, Hazel. How to Build a Rocket. Illustrated by Scoular Anderson. New York: F. Watts, 2001.

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Projectile Motion

Asimov, Isaac. Exploring Outer Space: Rockets, Probes, and Satellites. Revisions and updates by Francis Reddy. Milwaukee, WI: G. Stevens, 1995. How in the World? Pleasantville, N.Y.: Reader’s Digest, 1990. “Interesting Properties of Projectile of Motion” (Web site). (March 2, 2001). JBM Small Arms Ballistics (Web site). (March 2, 2001).

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TORQUE Torque

CONCEPT Torque is the application of force where there is rotational motion. The most obvious example of torque in action is the operation of a crescent wrench loosening a lug nut, and a close second is a playground seesaw. But torque is also crucial to the operation of gyroscopes for navigation, and of various motors, both internal-combustion and electrical.

HOW IT WORKS Force, which may be defined as anything that causes an object to move or stop moving, is the linchpin of the three laws of motion formulated by Sir Isaac Newton (1642-1727.) The first law states that an object at rest will remain at rest, and an object in motion will remain in motion, unless or until outside forces act upon it. The second law defines force as the product of mass multiplied by acceleration. According to the third law, when one object exerts a force on another, the second object exerts on the first a force equal in magnitude but opposite in direction.

On the other hand, the third law can be demonstrated when there is no apparent movement, as for instance, when a person is sitting on a chair, and the chair exerts an equal and opposite force upward. In such a situation, when all the forces acting on an object are in balance, that object is said to be in a state of equilibrium. Physicists often discuss torque within the context of equilibrium, even though an object experiencing net torque is definitely not in equilibrium. In fact, torque provides a convenient means for testing and measuring the degree of rotational or circular acceleration experienced by an object, just as other means can be used to calculate the amount of linear acceleration. In equilibrium, the net sum of all forces acting on an object should be zero; thus in order to meet the standards of equilibrium, the sum of all torques on the object should also be zero.

REAL-LIFE A P P L I C AT I O N S Seesaws and Wrenches

One way to envision the third law is in terms of an active event—for instance, two balls striking one another. As a result of the impact, each flies backward. Given the fact that the force on each is equal, and that force is the product of mass and acceleration (this is usually rendered with the formula F = ma), it is possible to make some predictions regarding the properties of mass and acceleration in this interchange. For instance, if the mass of one ball is relatively small compared to that of the other, its acceleration will be correspondingly greater, and it will thus be thrown backward faster.

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As for what torque is and how it works, it is best discuss it in relationship to actual objects in the physical world. Two in particular are favorites among physicists discussing torque: a seesaw and a wrench turning a lug nut. Both provide an easy means of illustrating the two ingredients of torque, force and moment arm. In any object experiencing torque, there is a pivot point, which on the seesaw is the balancepoint, and which in the wrench-and-lug nut combination is the lug nut itself. This is the area around which all the forces are directed. In each

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Torque

A

SEESAW ROTATES ON AND OFF THE GROUND DUE TO TORQUE IMBALANCE. (Photograph by Dean Conger/Corbis. Reproduced

by permission.)

case, there is also a place where force is being applied. On the seesaw, it is the seats, each holding a child of differing weight. In the realm of physics, weight is actually a variety of force. Whereas force is equal to mass multiplied by acceleration, weight is equal to mass multiplied by the acceleration due to gravity. The latter is equal to 32 ft (9.8 m)/sec2. This means that for every second that an object experiencing gravitational force continues to fall, its velocity increases at the rate of 32 ft or 9.8 m per second. Thus, the formula for weight is essentially the same as that for force, with a more specific variety of acceleration substituted for the generalized term in the equation for force. As for moment arm, this is the distance from the pivot point to the vector on which force is being applied. Moment arm is always perpendicular to the direction of force. Consider a wrench operating on a lug nut. The nut, as noted earlier, is the pivot point, and the moment arm is the distance from the lug nut to the place where the person operating the wrench has applied force. The torque that the lug nut experiences is the product of moment arm multiplied by force.

force that, when applied to 1 kg of mass, will give it an acceleration of 1 m/sec2). Hence if a person were to a grip a wrench 9 in (23 cm) from the pivot point, the moment arm would be 0.75 ft (0.23 m.) If the person then applied 50 lb (11.24 N) of force, the lug nut would be experiencing 37.5 pound-feet (2.59 N•m) of torque. The greater the amount of torque, the greater the tendency of the object to be put into rotation. In the case of a seesaw, its overall design, in particular the fact that it sits on the ground, means that its board can never undergo anything close to 360° rotation; nonetheless, the board does rotate within relatively narrow parameters. The effects of torque can be illustrated by imagining the clockwise rotational behavior of a seesaw viewed from the side, with a child sitting on the left and a teenager on the right.

In English units, torque is measured in pound-feet, whereas the metric unit is Newtonmeters, or N•m. (One newton is the amount of

Suppose the child weighs 50 lb (11.24 N) and sits 3 ft (0.91 m) from the pivot point, giving her side of the seesaw a torque of 150 pound-feet (10.28 N•m). On the other side, her teenage sister weighs 100 lb (22.48 N) and sits 6 ft (1.82 m) from the center, creating a torque of 600 poundfeet (40.91 N•m). As a result of the torque imbalance, the side holding the teenager will rotate clockwise, toward the ground, causing the child’s side to also rotate clockwise—off the ground.

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Torque

TORQUE, ALONG WITH ANGULAR MOMENTUM, IS THE LEADING FACTOR DICTATING THE MOTION OF A GYROSCOPE. HERE, A WOMAN RIDES INSIDE A GIANT GYROSCOPE AT AN AMUSEMENT PARK. (Photograph by Richard Cummins/Corbis. Reproduced by permission.)

In order for the two to balance one another

but a more likely remedy is a change in position,

and therefore, of moment arm. Since the teenager weighs exactly twice as much as the child, the moment arm on the child’s side must be exactly twice as long as that on the teenager’s.

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perfectly, the torque on each side has to be adjusted. One way would be by changing weight,

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Hence, a remedy would be for the two to switch positions with regard to the pivot point. The child would then move out an additional 3 ft (.91 m), to a distance of 6 ft (1.83 m) from the pivot, and the teenager would cut her distance from the pivot point in half, to just 3 ft (.91 m). In fact, however, any solution that gave the child a moment arm twice as long as that of the teenager would work: hence, if the teenager sat 1 ft (.3 m) from the pivot point, the child should be at 2 ft (.61 m) in order to maintain the balance, and so on. On the other hand, there are many situations in which you may be unable to increase force, but can increase moment arm. Suppose you were trying to disengage a particularly stubborn lug nut, and after applying all your force, it still would not come loose. The solution would be to increase moment arm, either by grasping the wrench further from the pivot point, or by using a longer wrench. For the same reason, on a door, the knob is placed as far as possible from the hinges. Here the hinge is the pivot point, and the door itself is the moment arm. In some situations of torque, however, moment arm may extend over “empty space,” and for this reason, the handle of a wrench is not exactly the same as its moment arm. If one applies force on the wrench at a 90°angle to the handle, then indeed handle and moment arm are identical; however, if that force were at a 45° angle, then the moment arm would be outside the handle, because moment arm and force are always perpendicular. And if one were to pull the wrench away from the lug nut, then there would be 0° difference between the direction of force and the pivot point—meaning that moment arm (and hence torque) would also be equal to zero.

Gyroscopes A gyroscope consists of a wheel-like disk, called a flywheel, mounted on an axle, which in turn is mounted on a larger ring perpendicular to the plane of the wheel itself. An outer circle on the same plane as the flywheel provides structural stability, and indeed, the gyroscope may include several such concentric rings. Its focal point, however, is the flywheel and the axle. One end of the axle is typically attached to some outside object, while the other end is left free to float.

downward, rotating it on an axis perpendicular to that of the flywheel. This should cause the gyroscope to fall over, but instead it begins to spin a third axis, a horizontal axis perpendicular both to the plane of the flywheel and to the direction of gravity. Thus, it is spinning on three axes, and as a result becomes very stable—that is, very resistant toward outside attempts to upset its balance.

Torque

This in turn makes the gyroscope a valued instrument for navigation: due to its high degree of gyroscopic inertia, it resists changes in orientation, and thus can guide a ship toward its destination. Gyroscopes, rather than magnets, are often the key element in a compass. A magnet will point to magnetic north, some distance from “true north” (that is, the North Pole.) But with a gyroscope whose axle has been aligned with true north before the flywheel is set spinning, it is possible to possess a much more accurate directional indicator. For this reason, gyroscopes are used on airplanes—particularly those flying over the poles— as well as submarines and even the Space Shuttle. Torque, along with angular momentum, is the leading factor dictating the motion of a gyroscope. Think of angular momentum as the momentum (mass multiplied by velocity) that a turning object acquires. Due to a principle known as the conservation of angular momentum, a spinning object has a tendency to reach a constant level of angular momentum, and in order to do this, the sum of the external torques acting on the system must be reduced to zero. Thus angular momentum “wants” or “needs” to cancel out torque. The “right-hand rule” can help you to understand the torque in a system such as the gyroscope. If you extend your right hand, palm downward, your fingers are analogous to the moment arm. Now if you curl your fingers downward, toward the ground, then your fingertips point in the direction of g—that is, gravitational force. At that point, your thumb (involuntarily, due to the bone structure of the hand) points in the direction of the torque vector.

Once the flywheel is set spinning, gravity has a tendency to pull the unattached end of the axle

When the gyroscope starts to spin, the vectors of angular momentum and torque are at odds with one another. Were this situation to persist, it would destabilize the gyroscope; instead, however, the two come into alignment. Using the right-hand rule, the torque vector on a gyroscope is horizontal in direction, and the vector of angular momentum eventually aligns with

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Torque

KEY TERMS A change in velocity over a given time period. ACCELERATION:

EQUILIBRIUM:

balance.

The rate at which the position

of an object changes over a given period of

The product of mass multi-

The tendency of an object in

TORQUE:

The product of moment

arm multiplied by force. VECTOR:

A quantity that possesses

motion to remain in motion, and of an

both magnitude and direction. By contrast,

object at rest to remain at rest.

a scalar quantity is one that possesses only

MASS:

A measure of inertia, indicating

magnitude, with no specific direction.

the resistance of an object to a change in its

VELOCITY:

motion—including a change in velocity.

particular direction.

MOMENT ARM:

For an object experi-

WEIGHT:

The speed of an object in a A measure of the gravitation-

encing torque, moment arm is the distance

al force on an object; the product of mass

from the pivot or balance point to the vec-

multiplied by the acceleration due to

tor on which force is being applied.

gravity.

it. To achieve this, the gyroscope experiences what is known as gyroscopic precession, pivoting along its support post in an effort to bring angular momentum into alignment with torque. Once this happens, there is no net torque on the system, and the conservation of angular momentum is in effect.

that the engine supply enough torque to keep the car running; otherwise it will stall. To allocate torque and speed appropriately, the engine may decrease or increase the number of revolutions per minute to which the rotors are subjected.

An automobile engine produces energy, which the pistons or rotor convert into torque for transmission to the wheels. Though torque is greatest at high speeds, the amount of torque needed to operate a car does not always vary proportionately with speed. At moderate speeds and on level roads, the engine does not need to provide a great deal of torque. But when the car is starting, or climbing a steep hill, it is important

Torque comes from the engine, but it has to be supplied to the transmission. In an automatic transmission, there are two principal components: the automatic gearbox and the torque converter. It is the job of the torque converter to transmit power from the flywheel of the engine to the gearbox, and it has to do so as smoothly as possible. The torque converter consists of three elements: an impeller, which is turned by the engine flywheel; a reactor that passes this motion on to a turbine; and the turbine itself, which turns the input shaft on the automatic gearbox. An infusion of oil to the converter assists the impeller and turbine in synchronizing movement, and this alignment of elements in the torque converter creates a smooth relationship between engine and gearbox. This also leads to an increase in the car’s overall torque—that is, its turning force.

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Torque in Complex Machines Torque is a factor in several complex machines such as the electric motor that—with variations—runs most household appliances. It is especially important to the operation of automobiles, playing a significant role in the engine and transmission.

90

SPEED:

time.

plied by acceleration. INERTIA:

the direction of force.

A situation in which

the forces acting upon an object are in

FORCE:

Moment arm is always perpendicular to

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Torque is also important in the operation of electric motors, found in everything from vacuum cleaners and dishwashers to computer printers and videocassette recorders to subway systems and water-pumping stations. Torque in the context of electricity involves reference to a number of concepts beyond the scope of this discussion: current, conduction, magnetic field, and other topics relevant to electromagnetic force. WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991.

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Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998.

Torque

“Rotational Motion.” Physics Department, University of Guelph (Web site). (March 4, 2001). “Rotational Motion—Torque.” Lee College (Web site). (March 4, 2001). Schweiger, Peggy E. “Torque” (Web site). (March 4, 2001). “Torque and Rotational Motion” (Web site). (March 4, 2001).

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S C I E N C E O F E V E RY DAY T H I N G S real-life Physics

F LU I D M E C H A N I C S FLUID MECHANICS AERODYNAMICS BERNOULLI’S PRINCIPLE B U OYA N CY

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Fluid Mechanics

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F LU I D M E C H A N I C S

CONCEPT The term “fluid” in everyday language typically refers only to liquids, but in the realm of physics, fluid describes any gas or liquid that conforms to the shape of its container. Fluid mechanics is the study of gases and liquids at rest and in motion. This area of physics is divided into fluid statics, the study of the behavior of stationary fluids, and fluid dynamics, the study of the behavior of moving, or flowing, fluids. Fluid dynamics is further divided into hydrodynamics, or the study of water flow, and aerodynamics, the study of airflow. Applications of fluid mechanics include a variety of machines, ranging from the waterwheel to the airplane. In addition, the study of fluids provides an understanding of a number of everyday phenomena, such as why an open window and door together create a draft in a room.

HOW IT WORKS The Contrast Between Fluids and Solids To understand fluids, it is best to begin by contrasting their behavior with that of solids. Whereas solids possess a definite volume and a definite shape, these physical characteristics are not so clearly defined for fluids. Liquids, though they possess a definite volume, have no definite shape—a factor noted above as one of the defining characteristics of fluids. As for gases, they have neither a definite shape nor a definite volume.

reduce the size or volume of an object. A solid is highly noncompressible, meaning that it resists compression, and if compressed with a sufficient force, its mechanical properties alter significantly. For example, if one places a drinking glass in a vise, it will resist a small amount of pressure, but a slight increase will cause the glass to break. Fluids vary with regard to compressibility, depending on whether the fluid in question is a liquid or a gas. Most gases tend to be highly compressible—though air, at low speeds at least, is not among them. Thus, gases such as propane fuel can be placed under high pressure. Liquids tend to be noncompressible: unlike a gas, a liquid can be compressed significantly, yet its response to compression is quite different from that of a solid—a fact illustrated below in the discussion of hydraulic presses. One way to describe a fluid is “anything that flows”—a behavior explained in large part by the interaction of molecules in fluids. If the surface of a solid is disturbed, it will resist, and if the force of the disturbance is sufficiently strong, it will deform—as for instance, when a steel plate begins to bend under pressure. This deformation will be permanent if the force is powerful enough, as was the case in the above example of the glass in a vise. By contrast, when the surface of a liquid is disturbed, it tends to flow. M O L E C U LA R B E H AV I O R O F F LU I D S A N D S O L I D S . At the molecu-

One of several factors that distinguishes fluids from solids is their response to compression, or the application of pressure in such a way as to

lar level, particles of solids tend to be definite in their arrangement and close to one another. In the case of liquids, molecules are close in proximity, though not as much so as solid molecules, and the arrangement is random. Thus, with a glass of water, the molecules of glass (which at

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Fluid Mechanics

IN

A WIDE, UNCONSTRICTED REGION, A RIVER FLOWS SLOWLY.

WALLS, AS WITH

WYOMING’S BIGHORN RIVER,

HOWEVER,

IF ITS FLOW IS NARROWED BY CANYON

THEN IT SPEEDS UP DRAMATICALLY. (Photograph by Kevin R. Morris/Corbis.

Reproduced by permission.)

relatively low temperatures is a solid) in the container are fixed in place while the molecules of water contained by the glass are not. If one portion of the glass were moved to another place on the glass, this would change its structure. On the other hand, no significant alteration occurs in the character of the water if one portion of it is moved to another place within the entire volume of water in the glass. As for gas molecules, these are both random in arrangement and far removed in proximity. Whereas solid particles are slow-moving and have a strong attraction to one another, liquid molecules move at moderate speeds and exert a moderate attraction on each other. Gas molecules are extremely fast-moving and exert little or no attraction. Thus, if a solid is released from a container pointed downward, so that the force of gravity moves it, it will fall as one piece. Upon hitting a floor or other surface, it will either rebound, come to a stop, or deform permanently. A liquid, on the other hand, will disperse in response to impact, its force determining the area over which the total volume of liquid is distributed. But for a gas, assuming it is lighter than air, the downward pull of gravity is not even required to disperse it: 96

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once the top on a container of gas is released, the molecules begin to float outward.

Fluids Under Pressure As suggested earlier, the response of fluids to pressure is one of the most significant aspects of fluid behavior and plays an important role within both the statics and dynamics subdisciplines of fluid mechanics. A number of interesting principles describe the response to pressure, on the part of both fluids at rest inside a container, and fluids which are in a state of flow. Within the realm of hydrostatics, among the most important of all statements describing the behavior of fluids is Pascal’s principle. This law is named after Blaise Pascal (1623-1662), a French mathematician and physicist who discovered that the external pressure applied on a fluid is transmitted uniformly throughout its entire body. The understanding offered by Pascal’s principle later became the basis for one of the most important machines ever developed, the hydraulic press. HYDROSTATIC PRESSURE AND BUOYANCY. Some nineteen centuries before

Pascal, the Greek mathematician, physicist, and inventor Archimedes (c. 287-212 B.C.) discovered a precept of fluid statics that had implications at S C I E N C E O F E V E RY DAY T H I N G S

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least as great as those of Pascal’s principle. This was Archimedes’s principle, which explains the buoyancy of an object immersed in fluid. According to Archimedes’s principle, the buoyant force exerted on the object is equal to the weight of the fluid it displaces.

pressure and velocity are inversely related—in other words, as one increases, the other decreases. Hence, he formulated Bernoulli’s principle, which states that for all changes in movement, the sum of static and dynamic pressure in a fluid remains the same.

Buoyancy explains both how a ship floats on water, and how a balloon floats in the air. The pressures of water at the bottom of the ocean, and of air at the surface of Earth, are both examples of hydrostatic pressure—the pressure that exists at any place in a body of fluid due to the weight of the fluid above. In the case of air pressure, air is pulled downward by the force of Earth’s gravitation, and air along the planet’s surface has greater pressure due to the weight of the air above it. At great heights above Earth’s surface, however, the gravitational force is diminished, and thus the air pressure is much smaller.

A fluid at rest exerts pressure—what Bernoulli called “static pressure”—on its container. As the fluid begins to move, however, a portion of the static pressure—proportional to the speed of the fluid—is converted to what Bernoulli called dynamic pressure, or the pressure of movement. In a cylindrical pipe, static pressure is exerted perpendicular to the surface of the container, whereas dynamic pressure is parallel to it.

Water, too, is pulled downward by gravity, and as with air, the fluid at the bottom of the ocean has much greater pressure due to the weight of the fluid above it. Of course, water is much heavier than air, and therefore, water at even a moderate depth in the ocean has enormous pressure. This pressure, in turn, creates a buoyant force that pushes upward. If an object immersed in fluid—a balloon in the air, or a ship on the ocean—weighs less that the fluid it displaces, it will float. If it weighs more, it will sink or fall. The balloon itself may be “heavier than air,” but it is not as heavy as the air it has displaced. Similarly, an aircraft carrier contains a vast weight in steel and other material, yet it floats, because its weight is not as great as that of the displaced water. B E R N O U L L I ’ S P R I N C I P L E . Archimedes and Pascal contributed greatly to what became known as fluid statics, but the father of fluid mechanics, as a larger realm of study, was the Swiss mathematician and physicist Daniel Bernoulli (1700-1782). While conducting experiments with liquids, Bernoulli observed that when the diameter of a pipe is reduced, the water flows faster. This suggested to him that some force must be acting upon the water, a force that he reasoned must arise from differences in pressure.

Fluid Mechanics

According to Bernoulli’s principle, the greater the velocity of flow in a fluid, the greater the dynamic pressure and the less the static pressure. In other words, slower-moving fluid exerts greater pressure than faster-moving fluid. The discovery of this principle ultimately made possible the development of the airplane.

REAL-LIFE A P P L I C AT I O N S Bernoulli’s Principle in Action As fluid moves from a wider pipe to a narrower one, the volume of the fluid that moves a given distance in a given time period does not change. But since the width of the narrower pipe is smaller, the fluid must move faster (that is, with greater dynamic pressure) in order to move the same amount of fluid the same distance in the same amount of time. Observe the behavior of a river: in a wide, unconstricted region, it flows slowly, but if its flow is narrowed by canyon walls, it speeds up dramatically.

Specifically, the slower-moving fluid in the wider area of pipe had a greater pressure than the portion of the fluid moving through the narrower part of the pipe. As a result, he concluded that

Bernoulli’s principle ultimately became the basis for the airfoil, the design of an airplane’s wing when seen from the end. An airfoil is shaped like an asymmetrical teardrop laid on its side, with the “fat” end toward the airflow. As air hits the front of the airfoil, the airstream divides, part of it passing over the wing and part passing under. The upper surface of the airfoil is curved, however, whereas the lower surface is much straighter.

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As a result, the air flowing over the top has a greater distance to cover than the air flowing under the wing. Since fluids have a tendency to compensate for all objects with which they come into contact, the air at the top will flow faster to meet the other portion of the airstream, the air flowing past the bottom of the wing, when both reach the rear end of the airfoil. Faster airflow, as demonstrated by Bernoulli, indicates lower pressure, meaning that the pressure on the bottom of the wing keeps the airplane aloft. C R E AT I N G A D RA F T. Among the most famous applications of Bernoulli’s principle is its use in aerodynamics, and this is discussed in the context of aerodynamics itself elsewhere in this book. Likewise, a number of other applications of Bernoulli’s principle are examined in an essay devoted to that topic. Bernoulli’s principle, for instance, explains why a shower curtain tends to billow inward when the water is turned on; in addition, it shows why an open window and door together create a draft.

Suppose one is in a hotel room where the heat is on too high, and there is no way to adjust the thermostat. Outside, however, the air is cold, and thus, by opening a window, one can presumably cool down the room. But if one opens the window without opening the front door of the room, there will be little temperature change. The only way to cool off will be by standing next to the window: elsewhere in the room, the air will be every bit as stuffy as before. But if the door leading to the hotel hallway is opened, a nice cool breeze will blow through the room. Why? With the door closed, the room constitutes an area of relatively high pressure compared to the pressure of the air outside the window. Because air is a fluid, it will tend to flow into the room, but once the pressure inside reaches a certain point, it will prevent additional air from entering. The tendency of fluids is to move from high-pressure to low-pressure areas, not the other way around. As soon as the door is opened, the relatively high-pressure air of the room flows into the relatively low-pressure area of the hallway. As a result, the air pressure in the room is reduced, and the air from outside can now enter. Soon a wind will begin to blow through the room.

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chamber built for the purpose of examining the characteristics of airflow in contact with solid objects, such as aircraft and automobiles. The wind tunnel represents a safe and judicious use of the properties of fluid mechanics. Its purpose is to test the interaction of airflow and solids in relative motion: in other words, either the aircraft has to be moving against the airflow, as it does in flight, or the airflow can be moving against a stationary aircraft. The first of these choices, of course, poses a number of dangers; on the other hand, there is little danger in exposing a stationary craft to winds at speeds simulating that of the aircraft in flight. The first wind tunnel was built in England in 1871, and years later, aircraft pioneers Orville (1871-1948) and Wilbur (1867-1912) Wright used a wind tunnel to improve their planes. By the late 1930s, the U.S. National Advisory Committee for Aeronautics (NACA) was building wind tunnels capable of creating speeds equal to 300 MPH (480 km/h); but wind tunnels built after World War II made these look primitive. With the development of jet-powered flight, it became necessary to build wind tunnels capable of simulating winds at the speed of sound—760 MPH (340 m/s). By the 1950s, wind tunnels were being used to simulate hypersonic speeds—that is, speeds of Mach 5 (five times the speed of sound) and above. Researchers today use helium to create wind blasts at speeds up to Mach 50.

Fluid Mechanics for Performing Work H Y D RAU L I C P R E SS E S . Though

applications of Bernoulli’s principle are among the most dramatic examples of fluid mechanics in operation, the everyday world is filled with instances of other ideas at work. Pascal’s principle, for instance, can be seen in the operation of any number of machines that represent variations on the idea of a hydraulic press. Among these is the hydraulic jack used to raise a car off the floor of an auto mechanic’s shop.

A W I N D T U N N E L . The above scenario of wind flowing through a room describes a rudimentary wind tunnel. A wind tunnel is a

Beneath the floor of the shop is a chamber containing a quantity of fluid, and at either end of the chamber are two large cylinders side by side. Each cylinder holds a piston, and valves control flow between the two cylinders through the channel of fluid that connects them. In accordance with Pascal’s principle, when one applies force by pressing down the piston in one cylinder

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(the input cylinder), this yields a uniform pressure that causes output in the second cylinder, pushing up a piston that raises the car.

Fluid Mechanics

Another example of a hydraulic press is the hydraulic ram, which can be found in machines ranging from bulldozers to the hydraulic lifts used by firefighters and utility workers to reach heights. In a hydraulic ram, however, the characteristics of the input and output cylinders are reversed from those of a car jack. For the car jack, the input cylinder is long and narrow, while the output cylinder is wide and short. This is because the purpose of a car jack is to raise a heavy object through a relatively short vertical range of movement—just high enough so that the mechanic can stand comfortably underneath the car. In the hydraulic ram, the input or master cylinder is short and squat, while the output or slave cylinder is tall and narrow. This is because the hydraulic ram, in contrast to the car jack, carries a much lighter cargo (usually just one person) through a much greater vertical range—for instance, to the top of a tree or building. PU M P S . A pump is a device made for

moving fluid, and it does so by utilizing a pressure difference, causing the fluid to move from an area of higher pressure to one of lower pressure. Its operation is based on aspects both of Pascal’s and Bernoulli’s principles—though, of course, humans were using pumps thousands of years before either man was born. A siphon hose used to draw gas from a car’s fuel tank is a very simple pump. Sucking on one end of the hose creates an area of low pressure compared to the relatively high-pressure area of the gas tank. Eventually, the gasoline will come out of the low-pressure end of the hose. The piston pump, slightly more complex, consists of a vertical cylinder along which a piston rises and falls. Near the bottom of the cylinder are two valves, an inlet valve through which fluid flows into the cylinder, and an outlet valve through which fluid flows out. As the piston moves upward, the inlet valve opens and allows fluid to enter the cylinder. On the downstroke, the inlet valve closes while the outlet valve opens, and the pressure provided by the piston forces the fluid through the outlet valve.

PUMPS

FOR

GROUND

DRAWING

ARE

USABLE

UNDOUBTEDLY

WATER

THE

FROM

OLDEST

THE

PUMPS

KNOWN. (Photograph by Richard Cummins/Corbis. Reproduced by

permission.)

by providing a series of controlled explosions created by the spark plug’s ignition of the gas. In another variety of piston pump—the kind used to inflate a basketball or a bicycle tire—air is the fluid being pumped. Then there is a pump for water. Pumps for drawing usable water from the ground are undoubtedly the oldest known variety, but there are also pumps designed to remove water from areas where it is undesirable; for example, a bilge pump, for removing water from a boat, or the sump pump used to pump flood water out of a basement.

One of the most obvious applications of the piston pump is in the engine of an automobile. In this case, of course, the fluid being pumped is gasoline, which pushes the pistons up and down

F LU I D P O W E R. For several thousand years, humans have used fluids—in particular water—to power a number of devices. One of the great engineering achievements of ancient times was the development of the waterwheel, which included a series of buckets along the rim that made it possible to raise water from the river below and disperse it to other points. By about 70 B.C., Roman engineers recognized that they could use the power of water itself to turn wheels and grind grain. Thus, the waterwheel became one of the first mechanisms in which an inanimate

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Fluid Mechanics

KEY TERMS An area of fluid

AERODYNAMICS:

dynamics devoted to studying the proper-

fluid mechanics are fluid statics and fluid dynamics.

ties and characteristics of airflow.

An area of fluid

FLUID STATICS:

A rule

ARCHIMEDES’S PRINCIPLE:

of physics stating that the buoyant force of

mechanics devoted to studying the behavior of stationary fluids.

an object immersed in fluid is equal to the weight of the fluid displaced by the object.

An area of fluid

It is named after the Greek mathematician,

dynamics devoted to studying the proper-

physicist, and inventor, Archimedes (c.

ties and characteristics of water flow.

287-212 B.C.), who first identified it.

HYDROSTATIC

BERNOULLI’S PRINCIPLE:

A prop-

osition, credited to Swiss mathematician and physicist Daniel Bernoulli (17001782), which maintains that slower-moving fluid exerts greater pressure than fastermoving fluid. BUOYANCY:

explained by Archimedes’s principle. COMPRESSION:

To reduce in size or

volume by applying pressure. FLUID:

PRESSURE:

The

pressure that exists at any place in a body of fluid due to the weight of the fluid above. PASCAL’S PRINCIPLE:

A statement,

formulated by French mathematician and physicist Blaise Pascal (1623-1662), which holds that the external pressure applied on

The tendency of an object

immersed in a fluid to float. This can be

a fluid is transmitted uniformly throughout the entire body of that fluid. PRESSURE:

The ratio of force to sur-

face area, when force is applied in a direction perpendicular to that surface.

Any substance, whether gas or A machine that converts the

liquid, that conforms to the shape of its

TURBINE:

container.

kinetic energy (the energy of movement)

FLUID DYNAMICS:

An area of fluid

mechanics devoted to studying of the behavior of moving, or flowing, fluids.

in fluids to useable mechanical energy by passing the stream of fluid through a series of fixed and moving fans or blades.

Fluid dynamics is further divided into

WIND TUNNEL:

hydrodynamics and aerodynamics.

the purpose of examining the characteris-

A chamber built for

The study of

tics of airflow in relative motion against

the behavior of gases and liquids at rest

solid objects such as aircraft and auto-

and in motion. The major divisions of

mobiles.

FLUID MECHANICS:

The water clock, too, was another ingenious use of water developed by the ancients. It did not use water for power; rather, it relied on gravity— a concept only dimly understood by ancient peo-

ples—to move water from one chamber of the clock to another, thus, marking a specific interval of time. The earliest clocks were sundials, which were effective for measuring time, provided the Sun was shining, but which were less useful for measuring periods shorter than an hour. Hence,

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source (as opposed to the effort of humans or animals) created power.

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the development of the hourglass, which used sand, a solid that in larger quantities exhibits the behavior of a fluid. Then, in about 270 B.C., Ctesibius of Alexandria (fl. c. 270-250 B.C.) used gearwheel technology to devise a constant-flow water clock called a “clepsydra.” Use of water clocks prevailed for more than a thousand years, until the advent of the first mechanical clocks. During the medieval period, fluids provided power to windmills and water mills, and at the dawn of the Industrial Age, engineers began applying fluid principles to a number of sophisticated machines. Among these was the turbine, a machine that converts the kinetic energy (the energy of movement) in fluids to useable mechanical energy by passing the stream of fluid through a series of fixed and moving fans or blades. A common house fan is an example of a turbine in reverse: the fan adds energy to the passing fluid (air), whereas a turbine extracts energy from fluids such as air and water. The turbine was developed in the mid-eighteenth century, and later it was applied to the extraction of power from hydroelectric dams, the first of which was constructed in 1894. Today, hydroelectric dams provide electric power to millions of homes around the world. Among the most dramatic examples of fluid mechanics in action, hydroelectric dams are vast in size and equally impressive in the power they can generate using a completely renewable resource: water. A hydroelectric dam forms a huge steel-andconcrete curtain that holds back millions of tons of water from a river or other body. The water

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nearest the top—the “head” of the dam—has enormous potential energy, or the energy that an object possesses by virtue of its position. Hydroelectric power is created by allowing controlled streams of this water to flow downward, gathering kinetic energy that is then transferred to powering turbines, which in turn generate electric power.

Fluid Mechanics

WHERE TO LEARN MORE Aerodynamics for Students (Web site). (April 8, 2001). Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. Chahrour, Janet. Flash! Bang! Pop! Fizz!: Exciting Science for Curious Minds. Illustrated by Ann Humphrey Williams. Hauppauge, N.Y.: Barron’s, 2000. “Educational Fluid Mechanics Sites.” Virginia Institute of Technology (Web site). (April 8, 2001). Fleisher, Paul. Liquids and Gases: Principles of Fluid Mechanics. Minneapolis, MN: Lerner Publications, 2002. Institute of Fluid Mechanics (Web site). (April 8, 2001). K8AIT Principles of Aeronautics Advanced Text (web site). (February 19, 2001). Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998. Sobey, Edwin J. C. Wacky Water Fun with Science: Science You Can Float, Sink, Squirt, and Sail. Illustrated by Bill Burg. New York: McGraw-Hill, 2000. Wood, Robert W. Mechanics Fundamentals. Illustrated by Bill Wright. Philadelphia: Chelsea House, 1997.

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AERODYNAMICS Aerodynamics

CONCEPT Though the term “aerodynamics” is most commonly associated with airplanes and the overall science of flight, in fact, its application is much broader. Simply put, aerodynamics is the study of airflow and its principles, and applied aerodynamics is the science of improving manmade objects such as airplanes and automobiles in light of those principles. Aside from the obvious application to these heavy forms of transportation, aerodynamic concepts are also reflected in the simplest of manmade flying objects—and in the natural model for all studies of flight, a bird’s wings.

HOW IT WORKS All physical objects on Earth are subject to gravity, but gravity is not the only force that tends to keep them pressed to the ground. The air itself, though it is invisible, operates in such a way as to prevent lift, much as a stone dropped into the water will eventually fall to the bottom. In fact, air behaves much like water, though the downward force is not as great due to the fact that air’s pressure is much less than that of water. Yet both are media through which bodies travel, and air and water have much more in common with one another than either does with a vacuum. Liquids such as water and gasses such as air are both subject to the principles of fluid dynamics, a set of laws that govern the motion of liquids and vapors when they come in contact with solid surfaces. In fact, there are few significant differences—for the purposes of the present discussion—between water and air with regard to their behavior in contact with solid surfaces.

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When a person gets into a bathtub, the water level rises uniformly in response to the fact that a solid object is taking up space. Similarly, air currents blow over the wings of a flying aircraft in such a way that they meet again more or less simultaneously at the trailing edge of the wing. In both cases, the medium adjusts for the intrusion of a solid object. Hence within the parameters of fluid dynamics, scientists typically use the term “fluid” uniformly, even when describing the movement of air. The study of fluid dynamics in general, and of air flow in particular, brings with it an entire vocabulary. One of the first concepts of importance is viscosity, the internal friction in a fluid that makes it resistant to flow and resistant to objects flowing through it. As one might suspect, viscosity is a far greater factor with water than with air, the viscosity of which is less than two percent that of water. Nonetheless, near a solid surface—for example, the wing of an airplane— viscosity becomes a factor because air tends to stick to that surface. Also significant are the related aspects of density and compressibility. At speeds below 220 MPH (354 km/h), the compressibility of air is not a significant factor in aerodynamic design. However, as air flow approaches the speed of sound—660 MPH (1,622 km/h)—compressibility becomes a significant factor. Likewise temperature increases greatly when airflow is supersonic, or faster than the speed of sound. All objects in the air are subject to two types of airflow, laminar and turbulent. Laminar flow is smooth and regular, always moving at the same speed and in the same direction. This type of airflow is also known as streamlined flow, and under these conditions every particle of fluid that

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passes a particular point follows a path identical to all particles that passed that point earlier. This may be illustrated by imagining a stream flowing around a twig.

Aerodynamics

By contrast, in turbulent flow the air is subject to continual changes in speed and direction—as for instance when a stream flows over shoals of rocks. Whereas the mathematical model of laminar airflow is rather straightforward, conditions are much more complex in turbulent flow, which typically occurs in the presence either of obstacles or of high speeds. Absent the presence of viscosity, and thus in conditions of perfect laminar flow, an object behaves according to Bernoulli’s principle, sometimes known as Bernoulli’s equation. Named after the Swiss mathematician and physicist Daniel Bernoulli (1700-1782), this proposition goes to the heart of that which makes an airplane fly. While conducting experiments concerning the conservation of energy in liquids, Bernoulli observed that when the diameter of a pipe is reduced, the water flows faster. This suggested to him that some force must be acting upon the water, a force that he reasoned must arise from differences in pressure. Specifically, the slowermoving fluid had a greater pressure than the portion of the fluid moving through the narrower part of the pipe. As a result, he concluded that pressure and velocity are inversely related. Bernoulli’s principle states that for all changes in movement, the sum of static and dynamic pressure in a fluid remain the same. A fluid at rest exerts static pressure, which is the same as what people commonly mean when they say “pressure,” as in “water pressure.” As the fluid begins to move, however, a portion of the static pressure—proportional to the speed of the fluid—is converted to what scientists call dynamic pressure, or the pressure of movement. The greater the speed, the greater the dynamic pressure and the less the static pressure. Bernoulli’s findings would prove crucial to the design of aircraft in the twentieth century, as engineers learned how to use currents of faster and slower air for keeping an airplane aloft. Very close to the surface of an object experiencing airflow, however, the presence of viscosity plays havoc with the neat proportions of the Bernoulli’s principle. Here the air sticks to the object’s surface, slowing the flow of nearby air and creating a “boundary layer” of slow-moving

S C I E N C E O F E V E RY DAY T H I N G S

A

TYPICAL PAPER AIRPLANE HAS LOW ASPECT RATIO

WINGS, A TERM THAT REFERS TO THE SIZE OF THE WINGSPAN COMPARED TO THE CHORD.

IN

SUBSONIC

FLIGHT, HIGHER ASPECT RATIOS ARE USUALLY PREFERRED. (Photograph by Bruce Burkhardt/Corbis. Reproduced by per-

mission.)

air. At the beginning of the flow—for instance, at the leading edge of an airplane’s wing—this boundary layer describes a laminar flow; but the width of the layer increases as the air moves along the surface, and at some point it becomes turbulent. These and a number of other factors contribute to the coefficients of drag and lift. Simply put, drag is the force that opposes the forward motion of an object in airflow, whereas lift is a force perpendicular to the direction of the wind, which keeps the object aloft. Clearly these concepts can be readily applied to the operation of an airplane, but they also apply in the case of an automobile, as will be shown later.

REAL-LIFE A P P L I C AT I O N S How a Bird Flies—and Why a Human Being Cannot Birds are exquisitely designed (or adapted) for flight, and not simply because of the obvious fact

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Aerodynamics

BIRDS

LIKE THESE FAIRY TERNS ARE SUPREME EXAMPLES OF AERODYNAMIC PRINCIPLES, FROM THEIR LOW BODY

WEIGHT AND LARGE STERNUM AND PECTORALIS MUSCLES TO THEIR LIGHTWEIGHT FEATHERS. (Corbis. Reproduced by per-

mission.)

that they have wings. Thanks to light, hollow bones, their body weight is relatively low, giving them the advantage in overcoming gravity and remaining aloft. Furthermore, a bird’s sternum or breast bone, as well as its pectoralis muscles (those around the chest) are enormous in proportion to its body size, thus helping it to achieve the thrust necessary for flight. And finally, the bird’s lightweight feathers help to provide optimal lift and minimal drag. A bird’s wing is curved along the top, a crucial aspect of its construction. As air passes over the leading edge of the wing, it divides, and because of the curve, the air on top must travel a greater distance before meeting the air that flowed across the bottom. The tendency of airflow, as noted earlier, is to correct for the presence of solid objects. Therefore, in the absence of outside factors such as viscosity, the air on top “tries” to travel over the wing in the same amount of time that it takes the air below to travel under the wing. As shown by Bernoulli, the fast-moving air above the wing exerts less pressure than the slowmoving air below it; hence there is a difference in pressure between the air below and the air above, and this keeps the wing aloft.

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When a bird beats its wings, its downstrokes propel it, and as it rises above the ground, the force of aerodynamic lift helps push its wings upward in preparation for the next downstroke. However, to reduce aerodynamic drag during the upstroke, the bird folds its wings, thus decreasing its wingspan. Another trick that birds execute instinctively is the moving of their wings forward and backward in order to provide balance. They also “know” how to flap their wings in a direction almost parallel to the ground when they need to fly slowly or hover. Witnessing the astonishing aerodynamic feats of birds, humans sought the elusive goal of flight from the earliest of times. This was symbolized by the Greek myth of Icarus and Daedalus, who escaped from a prison in Crete by constructing a set of bird-like wings and flying away. In the world of physical reality, however, the goal would turn out to be unattainable as long as humans attempted to achieve flight by imitating birds. As noted earlier, a bird’s physiology is quite different from that of a human being. There is simply no way that a human can fly by flapping his arms—nor will there ever be a man strong enough to do so, no matter how apparently well-

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designed his mechanical wings are. Indeed, to be capable of flying like a bird, a man would have to have a chest so enormous in proportion to his body that he would be hideous in appearance. Not realizing this, humans for centuries attempted to fly like birds—with disastrous results. An English monk named Eilmer (b. 980) attempted to fly off the tower of Malmesbury Abbey with a set of wings attached to his arms and feet. Apparently Eilmer panicked after gliding some 600 ft (about 200 m) and suddenly plummeted to earth, breaking both of his legs. At least he lived; more tragic was the case of Abul Qasim Ibn Firnas (d. 873), an inventor from Cordoba in Arab Spain who devised and demonstrated a glider. Much of Cordoba’s population came out to see him demonstrate his flying machine, but after covering just a short distance, the craft fell to earth. Severely wounded, Ibn Firnas died shortly afterward. The first real progress in the development of flying machines came when designers stopped trying to imitate birds and instead used the principle of buoyancy. Hence in 1783, the French brothers Jacques-Etienne and Joseph-Michel Montgolfier constructed the first practical balloon. Balloons and their twentieth-century descendant, the dirigible, had a number of obvious drawbacks, however. Without a motor, a balloon could not be guided, and even with a motor, dirigibles proved highly dangerous. At that stage, most dirigibles used hydrogen, a gas that is cheap and plentiful, but extremely flammable. After the Hindenburg exploded in 1937, the age of passenger travel aboard airships was over. However, the German military continued to use dirigibles for observation purposes, as did the United States forces in World War II. Today airships, the most famous example being the Goodyear Blimp, are used not only for observation but for advertising. Scientists working in rain forests, for instance, use dirigibles to glide above the forest canopy; as for the Goodyear Blimp, it provides television networks with “eye in the sky” views of large sporting events. The first man to make a serious attempt at creating a heavier-than-air flying machine (as opposed to a balloon, which uses gases that are lighter than air) was Sir George Cayley (17731857), who in 1853 constructed a glider. It is interesting to note that in creating this, the foreS C I E N C E O F E V E RY DAY T H I N G S

runner of the modern airplane, Cayley went back to an old model: the bird. After studying the physics of birds’ flight for many years, he equipped his glider with an extremely wide wingspan, used the lightest possible materials in its construction, and designed it with exceptionally smooth surfaces to reduce drag.

Aerodynamics

The only thing that in principle differentiated Cayley’s craft from a modern airplane was its lack of an engine. In those days, the only possible source of power was a steam engine, which would have added far too much weight to his aircraft. However, the development of the internalcombustion engine in the nineteenth century overcame that obstacle, and in 1903 Orville and Wilbur Wright achieved the dream of flight that had intrigued and eluded human beings for centuries.

Airplanes: Getting Aloft, Staying Aloft, and Remaining Stable Once engineers and pilots took to the air, they encountered a number of factors that affect flight. In getting aloft and staying aloft, an aircraft is subject to weight, lift, drag, and thrust. As noted earlier, the design of an airplane wing takes advantage of Bernoulli’s principle to give it lift. Seen from the end, the wing has the shape of a long teardrop lying on its side, with the large end forward, in the direction of airflow, and the narrow tip pointing toward the rear. (Unlike a teardrop, however, an airplane’s wing is asymmetrical, and the bottom side is flat.) This cross-section is known as an airfoil, and the greater curvature of its upper surface in comparison to the lower side is referred to as the airplane’s camber. The front end of the airfoil is also curved, and the chord line is an imaginary straight line connecting the spot where the air hits the front—known as the stagnation point— to the rear, or trailing edge, of the wing. Again in accordance with Bernoulli’s principle, the shape of the airflow facilitates the spread of laminar flow around it. The slower-moving currents beneath the airfoil exert greater pressure than the faster currents above it, giving lift to the aircraft. Another parameter influencing the lift coefficient (that is, the degree to which the aircraft experiences lift) is the size of the wing: the longer the wing, the greater the total force exerted VOLUME 2: REAL-LIFE PHYSICS

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beneath it, and the greater the ratio of this pressure to that of the air above. The size of a modern aircraft’s wing is actually somewhat variable, due to the presence of flaps at the trailing edge. With regard to the flaps, however, it should be noted that they have different properties at different stages of flight: in takeoff, they provide lift, but in stable flight they increase drag, and for that reason the pilot retracts them. In preparing for landing, as the aircraft slows and descends, the extended flaps then provide stability and assist in the decrease of speed. Speed, too, encourages lift: the faster the craft, the faster the air moves over the wing. The pilot affects this by increasing or decreasing the power of the engine, thus regulating the speed with which the plane’s propellers turn. Another highly significant component of lift is the airfoil’s angle of attack—the orientation of the airfoil with regard to the air flow, or the angle that the chord line forms with the direction of the air stream. Up to a point, increasing the angle of attack provides the aircraft with extra lift because it moves the stagnation point from the leading edge down along the lower surface; this increases the low-pressure area of the upper surface. However, if the pilot increases the angle of attack too much, this affects the boundary layer of slowmoving air, causing the aircraft to go into a stall. Together the engine provides the propellers with power, and this gives the aircraft thrust, or propulsive force. In fact, the propeller blades constitute miniature wings, pivoted at the center and powered by the engine to provide rotational motion. As with the wings of the aircraft, the blades have a convex forward surface and a narrow trailing edge. Also like the aircraft wings, their angle of attack (or pitch) is adjusted at different points for differing effects. In stable flight, the pilot increases the angle of attack for the propeller blades sharply as against airflow, whereas at takeoff and landing the pitch is dramatically reduced. During landing, in fact, the pilot actually reverses the direction of the propeller blades, turning them into a brake on the aircraft’s forward motion—and producing that lurching sensation that a passenger experiences as the aircraft slows after touching down.

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preparing to land—drag. This apparent inconsistency results from the fact that the characteristics of air flow change drastically from situation to situation, and in fact, air never behaves as perfectly as it does in a textbook illustration of Bernoulli’s principle. Not only is the aircraft subject to air viscosity—the air’s own friction with itself—it also experiences friction drag, which results from the fact that no solid can move through a fluid without experiencing a retarding force. An even greater drag factor, accounting for one-third of that which an aircraft experiences, is induced drag. The latter results because air does not flow in perfect laminar streams over the airfoil; rather, it forms turbulent eddies and currents that act against the forward movement of the plane. In the air, an aircraft experiences forces that tend to destabilize flight in each of three dimensions. Pitch is the tendency to rotate forward or backward; yaw, the tendency to rotate on a horizontal plane; and roll, the tendency to rotate vertically on the axis of its fuselage. Obviously, each of these is a terrifying prospect, but fortunately, pilots have a solution for each. To prevent pitching, they adjust the angle of attack of the horizontal tail at the rear of the craft. The vertical rear tail plays a part in preventing yawing, and to prevent rolling, the pilot raises the tips of the main wings so that the craft assumes a V-shape when seen from the front or back. The above factors of lift, drag, thrust, and weight, as well as the three types of possible destabilization, affect all forms of heavier-thanair flying machines. But since the 1944 advent of jet engines, which travel much faster than pistondriven engines, planes have flown faster and faster, and today some craft such as the Concorde are capable of supersonic flight. In these situations, air compressibility becomes a significant issue.

By this point there have been several examples regarding the use of the same technique alternately to provide lift or—when slowing or

Sound is transmitted by the successive compression and expansion of air. But when a plane is traveling at above Mach 1.2—the Mach number indicates the speed of an aircraft in relation to the speed of sound—there is a significant discrepancy between the speed at which sound is traveling away from the craft, and the speed at which the craft is moving away from the sound. Eventually the compressed sound waves build up, resulting in a shock wave.

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Down on the ground, the shock wave manifests as a “sonic boom”; meanwhile, for the aircraft, it can cause sudden changes in pressure, density, and temperature, as well as an increase in drag and a loss of stability. To counteract this effect, designers of supersonic and hypersonic (Mach 5 and above) aircraft are altering wing design, using a much narrower airfoil and sweptback wings. One of the pioneers in this area is Richard Whitcomb of the National Aeronautics and Space Administration (NASA). Whitcomb has designed a supercritical airfoil for a proposed hypersonic plane, which would ascend into outer space in the course of a two-hour flight—all the time needed for it to travel from Washington, D.C., to Tokyo, Japan. Before the craft can become operational, however, researchers will have to figure out ways to control temperatures and keep the plane from bursting into flame as it reenters the atmosphere. Much of the research for improving the aerodynamic qualities of such aircraft takes place in wind tunnels. First developed in 1871, these use powerful fans to create strong air currents, and over the years the top speed in wind tunnels has been increased to accommodate testing on supersonic and hypersonic aircraft. Researchers today use helium to create wind blasts at speeds up to Mach 50.

Thrown and Flown: The Aerodynamics of Small Objects Long before engineers began to dream of sending planes into space for transoceanic flight—about 14,000 years ago, in fact—many of the features that make an airplane fly were already present in the boomerang. It might seem backward to move from a hypersonic jet to a boomerang, but in fact, it is easier to appreciate the aerodynamics of small objects, including the kite and even the paper airplane, once one comprehends the larger picture.

intelligent than Europeans. In fact, as Diamond showed, an individual would actually have to be smarter to figure out how to survive on the limited range of plants and animals available in Australia prior to the introduction of Eurasian flora and fauna. Hence the wonder of the boomerang, one of the most ingenious inventions ever fashioned by humans in a “primitive” state.

Aerodynamics

Thousands of years before Bernoulli, the boomerang’s designers created an airfoil consistent with Bernoulli’s principle. The air below exerts more pressure than the air above, and this, combined with the factors of gyroscopic stability and gyroscopic precession, gives the boomerang flight. Gyroscopic stability can be illustrated by spinning a top: the action of spinning itself keeps the top stable. Gyroscopic precession is a much more complex process: simply put, the leading wing of the boomerang—the forward or upward edge as it spins through the air—creates more lift than the other wing. At this point it should be noted that, contrary to the popular image, a boomerang travels on a plane perpendicular to that of the ground, not parallel. Hence any thrower who knows what he or she is doing tosses the boomerang not with a side-arm throw, but overhand. And of course a boomerang does not just sail through the air; a skilled thrower can make it come back as if by magic. This is because the force of the increased lift that it experiences in flight, combined with gyroscopic precession, turns it around. As noted earlier, in different situations the same force that creates lift can create drag, and as the boomerang spins downward the increasing drag slows it. Certainly it takes great skill for a thrower to make a boomerang come back, and for this reason, participants in boomerang competitions often attach devices such as flaps to increase drag on the return cycle.

There is a certain delicious irony in the fact that the first manmade object to take flight was constructed by people who never advanced beyond the Stone Age until the nineteenth century, when the Europeans arrived in Australia. As the ethnobotanist Jared Diamond showed in his groundbreaking work Guns, Germs, and Steel: The Fates of Human Societies (1997), this was not because the Aborigines of Australia were less

Another very early example of an aerodynamically sophisticated humanmade device— though it is quite recent compared to the boomerang—is the kite, which first appeared in China in about 1000 B.C. The kite’s design borrows from avian anatomy, particularly the bird’s light, hollow bones. Hence a kite, in its simplest form, consists of two crossed strips of very light wood such as balsa, with a lightweight fabric stretched over them.

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Kites can come in a variety of shapes, though for many years the well-known diamond shape has been the most popular, in part because its aerodynamic qualities make it easiest for the novice kite-flyer to handle. Like birds and boomerangs, kites can “fly” because of the physical laws embodied in Bernoulli’s principle: at the best possible angle of attack, the kite experiences a maximal ratio of pressure from the slowermoving air below as against the faster-moving air above. For centuries, when the kite represented the only way to put a humanmade object many hundreds of feet into the air, scientists and engineers used them for a variety of experiments. Of course, the most famous example of this was Benjamin Franklin’s 1752 experiment with electricity. More significant to the future of aerodynamics were investigations made half a century later by Cayley, who recognized that the kite, rather than the balloon, was an appropriate model for the type of heavier-than-air flight he intended. In later years, engineers built larger kites capable of lifting men into the air, but the advent of the airplane rendered kites obsolete for this purpose. However, in the 1950s an American engineer named Francis Rogallo invented the flexible kite, which in turn spawned the delta wing kite used by hang gliders. During the 1960s, Domina Jolbert created the parafoil, an even more efficient device, which took nonmechanized human flight perhaps as far as it can go. Akin to the kite, glider, and hang glider is that creation of childhood fancy, the paper airplane. In its most basic form—and paper airplane enthusiasts are capable of fairly complex designs—a paper airplane is little more than a set of wings. There are a number or reasons for this, not least the fact that in most cases, a person flying a paper airplane is not as concerned about pitch, yaw, and roll as a pilot flying with several hundred passengers on board would be. However, when fashioning a paper airplane it is possible to add a number of design features, for instance by folding flaps upward at the tail. These become the equivalent of the elevator, a control surface along the horizontal edge of a real aircraft’s tail, which the pilot rotates upward to provide stability. But as noted by Ken Blackburn, author of several books on paper airplanes, it is not necessarily the case that an airplane must

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have a tail; indeed, some of the most sophisticated craft in the sky today—including the fearsome B-2 “Stealth” bomber—do not have tails. A typical paper airplane has low aspect ratio wings, a term that refers to the size of the wingspan compared to the chord line. In subsonic flight, higher aspect ratios are usually preferred, and this is certainly the case with most “real” gliders; hence their wings are longer, and their chord lines shorter. But there are several reasons why this is not the case with a paper airplane. First of all, as Blackburn noted wryly on his Web site, “Paper is a lousy building material. There is a reason why real airplanes are not made of paper.” He stated the other factors governing paper airplanes’ low aspect ratio in similarly whimsical terms. First, “Low aspect ratio wings are easier to fold....”; second, “Paper airplane gliding performance is not usually very important....”; and third, “Low-aspect ratio wings look faster, especially if they are swept back.” The reason why low-aspect ratio wings look faster, Blackburn suggested, is that people see them on jet fighters and the Concorde, and assume that a relatively narrow wing span with a long chord line yields the fastest speeds. And indeed they do—but only at supersonic speeds. Below the speed of sound, high-aspect ratio wings are best for preventing drag. Furthermore, as Blackburn went on to note, low-aspect ratio wings help the paper airplane to withstand the relatively high launch speeds necessary to send them into longer glides. In fact, a paper airplane is not subject to anything like the sort of design constraints affecting a real craft. All real planes look somewhat similar, because the established combinations, ratios, and dimensions of wings, tails, and fuselage work best. Certainly there is a difference in basic appearance between subsonic and supersonic aircraft—but again, all supersonic jets have more or less the same low-aspect, swept wing. “With paper airplanes,” Blackburn wrote, “it’s easy to make airplanes that don’t look like real airplanes” since “The mission of a paper airplane is [simply] to provide a good time for the pilot.”

Aerodynamics on the Ground The preceding discussions of aerodynamics in action have concerned the behavior of objects off the ground. But aerodynamics is also a factor in

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wheeled transport on Earth’s surface, whether by bicycle, automobile, or some other variation.

Aerodynamics

On a bicycle, the rider accounts for 65-80% of the drag, and therefore his or her position with regard to airflow is highly important. Thus, from as early as the 1890s, designers of racing bikes have favored drop handlebars, as well as a seat and frame that allow a crouched position. Since the 1980s, bicycle designers have worked to eliminate all possible extra lines and barriers to airflow, including the crossbar and chainstays. A typical bicycle’s wheel contains 32 or 36 cylindrical spokes, and these can affect aerodynamics adversely. As the wheel rotates, the airflow behind the spoke separates, creating turbulence and hence drag. For this reason, some of the most advanced bicycles today use either aerodynamic rims, which reduce the length of the spokes, three-spoke aerodynamic wheels, or even solid wheels. The rider’s gear can also serve to impede or enhance his velocity, and thus modern racing helmets have a streamlined shape—rather like that of an airfoil. The best riders, such as those who compete in the Olympics or the Tour de France, have bikes custom-designed to fit their own body shape. One interesting aspect of aerodynamics where it concerns bicycle racing is the phenomenon of “drafting.” Riders at the front of a pack, like riders pedaling alone, consume 30-40% more energy than do riders in the middle of a pack. The latter are benefiting from the efforts of bicyclists in front of them, who put up most of the wind resistance. The same is true for bicyclists who ride behind automobiles or motorcycles. The use of machine-powered pace vehicles to help in achieving extraordinary speeds is far from new. Drafting off of a railroad car with specially designed aerodynamic shields, a rider in 1896 was able to exceed 60 MPH (96 km/h), a then unheard-of speed. Today the record is just under 167 MPH (267 km/h). Clearly one must be a highly skilled, powerful rider to approach anything like this speed; but design factors also come into play, and not just in the case of the pace vehicle. Just as supersonic jets are quite different from ordinary planes, super high-speed bicycles are not like the average bike; they are designed in such a way that they must be moving faster than 60 MPH before the rider can even pedal.

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A

PROFESSIONAL BICYCLE RACER’S STREAMLINED HEL-

MET AND CROUCHED POSITION HELP TO IMPROVE AIRFLOW, THUS INCREASING SPEED. (Photograph by Ronnen

Eshel/Corbis. Reproduced by permission.)

With regard to automobiles, as noted earlier, aerodynamics has a strong impact on body design. For this reason, cars over the years have become steadily more streamlined and aerodynamic in appearance, a factor that designers balance with aesthetic appeal. Today’s Chrysler PT Cruiser, which debuted in 2000, may share outward features with 1930s and 1940s cars, but the PT Cruiser’s design is much more sound aerodynamically—not least because a modern vehicle can travel much, much faster than the cars driven by previous generations. Nowhere does the connection between aerodynamics and automobiles become more crucial than in the sport of auto racing. For race-car drivers, drag is always a factor to be avoided and counteracted by means ranging from drafting to altering the body design to reduce the airflow under the vehicle. However, as strange as it may seem, a car—like an airplane—is also subject to lift. It was noted earlier that in some cases lift can be undesirable in an airplane (for instance, when trying to land), but it is virtually always undesirable in an automobile. The greater the speed, the greater the lift force, which increases

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Aerodynamics

KEY TERMS AERODYNAMICS:

The study of air

flow and its principles. Applied aerodynamics is the science of improving manmade objects in light of those principles.

Its opposite is turbulent flow. LIFT: An aerodynamic force perpendicu-

lar to the direction of the wind. For an air-

AIRFOIL: The design of an airplane’s

craft, lift is the force that raises it off the

wing when seen from the end, a shape

ground and keeps it aloft.

intended to maximize the aircraft’s response to airflow. ANGLE OF ATTACK: The orientation of

the airfoil with regard to the airflow, or the angle that the chord line forms with the direction of the air stream. BERNOULLI’S PRINCIPLE: A proposi-

tion, credited to Swiss mathematician and physicist Daniel Bernoulli (1700-1782),

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the same speed and in the same direction.

PITCH: The tendency of an aircraft in

flight to rotate forward or backward; see also yaw and roll. ROLL: The tendency of an aircraft in

flight to rotate vertically on the axis of its fuselage; see also pitch and yaw. STAGNATION POINT: The spot where

airflow hits the leading edge of an airfoil.

which maintains that slower-moving fluid

SUPERSONIC: Faster than Mach 1, or

exerts greater pressure than faster-moving

the speed of sound—660 MPH (1,622

fluid.

km/h). Speeds above Mach 5 are referred to

CAMBER: The enhanced curvature on the

as hypersonic.

upper surface of an airfoil.

TURBULENT: A term describing a highly

CHORD LINE: The distance, along an

irregular form of flow, in which a fluid is

imaginary straight line, from the stagna-

subject to continual changes in speed and

tion point of an airfoil to the rear, or trail-

direction. Its opposite is laminar flow.

ing edge.

VISCOSITY: The internal friction in a

DRAG: The force that opposes the forward

fluid that makes it resistant to flow.

motion of an object in airflow.

YAW: The tendency of an aircraft in flight

LAMINAR: A term describing a stream-

to rotate on a horizontal plane; see also

lined flow, in which all particles move at

Pitch and Roll.

the threat of instability. For this reason, builders of race cars design their vehicles for negative lift: hence a typical family car has a lift coefficient of about 0.03, whereas a race car is likely to have a coefficient of -3.00.

not wrap around the vehicle’s rear end. Instead, the spoiler creates a downward force to stabilize the rear, and it may help to decrease drag by reducing the separation of airflow (and hence the creation of turbulence) at the rear window.

Among the design features most often used to reduce drag while achieving negative lift is a rear-deck spoiler. The latter has an airfoil shape, but its purpose is different: to raise the rear stagnation point and direct air flow so that it does

Similar in concept to a spoiler, though somewhat different in purpose, is the aerodynamically curved shield that sits atop the cab of most modern eighteen-wheel transport trucks. The purpose of the shield becomes apparent when the

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truck is moving at high speeds: wind resistance becomes strong, and if the wind were to hit the truck’s trailer head-on, it would be as though the air were pounding a brick wall. Instead, the shield scoops air upward, toward the rear of the truck. At the rear may be another panel, patented by two young engineers in 1994, that creates a dragreducing vortex between panel and truck. WHERE TO LEARN MORE Cockpit Physics (Department of Physics, United States Air Force Academy web site.). (February 19, 2001). K8AIT Principles of Aeronautics Advanced Text. (web site). (February 19, 2001).

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Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998.

Aerodynamics

Blackburn, Ken. Paper Airplane Aerodynamics. (web site). (February 19, 2001). Schrier, Eric and William F. Allman. Newton at the Bat: The Science in Sports. New York: Charles Scribner’s Sons, 1984. Smith, H. C. The Illustrated Guide to Aerodynamics. Blue Ridge Summit, PA: Tab Books, 1992. Stever, H. Guyford, James J. Haggerty, and the Editors of Time-Life Books. Flight. New York: Time-Life Books, 1965. Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996.

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BERNOULLI’S PRINCIPLE

Bernoulli’s Principle

CONCEPT Bernoulli’s principle, sometimes known as Bernoulli’s equation, holds that for fluids in an ideal state, pressure and density are inversely related: in other words, a slow-moving fluid exerts more pressure than a fast-moving fluid. Since “fluid” in this context applies equally to liquids and gases, the principle has as many applications with regard to airflow as to the flow of liquids. One of the most dramatic everyday examples of Bernoulli’s principle can be found in the airplane, which stays aloft due to pressure differences on the surface of its wing; but the truth of the principle is also illustrated in something as mundane as a shower curtain that billows inward.

HOW IT WORKS The Swiss mathematician and physicist Daniel Bernoulli (1700-1782) discovered the principle that bears his name while conducting experiments concerning an even more fundamental concept: the conservation of energy. This is a law of physics that holds that a system isolated from all outside factors maintains the same total amount of energy, though energy transformations from one form to another take place. For instance, if you were standing at the top of a building holding a baseball over the side, the ball would have a certain quantity of potential energy—the energy that an object possesses by virtue of its position. Once the ball is dropped, it immediately begins losing potential energy and gaining kinetic energy—the energy that an object possesses by virtue of its motion. Since the total energy must remain constant, potential and

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kinetic energy have an inverse relationship: as the value of one variable decreases, that of the other increases in exact proportion. The ball cannot keep falling forever, losing potential energy and gaining kinetic energy. In fact, it can never gain an amount of kinetic energy greater than the potential energy it possessed in the first place. At the moment before the ball hits the ground, its kinetic energy is equal to the potential energy it possessed at the top of the building. Correspondingly, its potential energy is zero—the same amount of kinetic energy it possessed before it was dropped. Then, as the ball hits the ground, the energy is dispersed. Most of it goes into the ground, and depending on the rigidity of the ball and the ground, this energy may cause the ball to bounce. Some of the energy may appear in the form of sound, produced as the ball hits bottom, and some will manifest as heat. The total energy, however, will not be lost: it will simply have changed form. Bernoulli was one of the first scientists to propose what is known as the kinetic theory of gases: that gas, like all matter, is composed of tiny molecules in constant motion. In the 1730s, he conducted experiments in the conservation of energy using liquids, observing how water flows through pipes of varying diameter. In a segment of pipe with a relatively large diameter, he observed, water flowed slowly, but as it entered a segment of smaller diameter, its speed increased. It was clear that some force had to be acting on the water to increase its speed. Earlier, Robert Boyle (1627-1691) had demonstrated that pressure and volume have an inverse relationship,

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and Bernoulli seems to have applied Boyle’s findings to the present situation. Clearly the volume of water flowing through the narrower pipe at any given moment was less than that flowing through the wider one. This suggested, according to Boyle’s law, that the pressure in the wider pipe must be greater. As fluid moves from a wider pipe to a narrower one, the volume of that fluid that moves a given distance in a given time period does not change. But since the width of the narrower pipe is smaller, the fluid must move faster in order to achieve that result. One way to illustrate this is to observe the behavior of a river: in a wide, unconstricted region, it flows slowly, but if its flow is narrowed by canyon walls (for instance), then it speeds up dramatically. The above is a result of the fact that water is a fluid, and having the characteristics of a fluid, it adjusts its shape to fit that of its container or other solid objects it encounters on its path. Since the volume passing through a given length of pipe during a given period of time will be the same, there must be a decrease in pressure. Hence Bernoulli’s conclusion: the slower the rate of flow, the higher the pressure, and the faster the rate of flow, the lower the pressure. Bernoulli published the results of his work in Hydrodynamica (1738), but did not present his ideas or their implications clearly. Later, his friend the German mathematician Leonhard Euler (1707-1783) generalized his findings in the statement known today as Bernoulli’s principle.

The Venturi Tube Also significant was the work of the Italian physicist Giovanni Venturi (1746-1822), who is credited with developing the Venturi tube, an instrument for measuring the drop in pressure that takes place as the velocity of a fluid increases. It consists of a glass tube with an inward-sloping area in the middle, and manometers, devices for measuring pressure, at three places: the entrance, the point of constriction, and the exit. The Venturi meter provided a consistent means of demonstrating Bernoulli’s principle.

viscosity, the internal friction in a fluid that makes it resistant to flow. In 1904, the German physicist Ludwig Prandtl (1875-1953) was conducting experiments in liquid flow, the first effort in well over a century to advance the findings of Bernoulli and others. Observing the flow of liquid in a tube, Prandtl found that a tiny portion of the liquid adheres to the surface of the tube in the form of a thin film, and does not continue to move. This he called the viscous boundary layer.

Bernoulli’s Principle

Like Bernoulli’s principle itself, Prandtl’s findings would play a significant part in aerodynamics, or the study of airflow and its principles. They are also significant in hydrodynamics, or the study of water flow and its principles, a discipline Bernoulli founded.

Laminar vs. Turbulent Flow Air and water are both examples of fluids, substances which—whether gas or liquid—conform to the shape of their container. The flow patterns of all fluids may be described in terms either of laminar flow, or of its opposite, turbulent flow. Laminar flow is smooth and regular, always moving at the same speed and in the same direction. Also known as streamlined flow, it is characterized by a situation in which every particle of fluid that passes a particular point follows a path identical to all particles that passed that point earlier. A good illustration of laminar flow is what occurs when a stream flows around a twig. By contrast, in turbulent flow, the fluid is subject to continual changes in speed and direction—as, for instance, when a stream flows over shoals of rocks. Whereas the mathematical model of laminar flow is rather straightforward, conditions are much more complex in turbulent flow, which typically occurs in the presence of obstacles or high speeds.

Like many propositions in physics, Bernoulli’s principle describes an ideal situation in the absence of other forces. One such force is

Turbulent flow makes it more difficult for two streams of air, separated after hitting a barrier, to rejoin on the other side of the barrier; yet that is their natural tendency. In fact, if a single air current hits an airfoil—the design of an airplane’s wing when seen from the end, a streamlined shape intended to maximize the aircraft’s response to airflow—the air that flows over the top will “try” to reach the back end of the airfoil at the same time as the air that flows over the

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Bernoulli’s Principle

A

KITE’S DESIGN, PARTICULARLY ITS USE OF LIGHTWEIGHT FABRIC STRETCHED OVER TWO CROSSED STRIPS OF VERY

LIGHT WOOD, MAKES IT WELL-SUITED FOR FLIGHT, BUT WHAT KEEPS IT IN THE AIR IS A DIFFERENCE IN AIR PRESSURE.

AT

THE BEST POSSIBLE ANGLE OF ATTACK, THE KITE EXPERIENCES AN IDEAL RATIO OF PRESSURE FROM THE SLOW-

ER-MOVING AIR BELOW VERSUS THE FASTER-MOVING AIR ABOVE, AND THIS GIVES IT LIFT. (Roger Ressmeyer/Corbis. Repro-

duced by permission.)

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bottom. In order to do so, it will need to speed up—and this, as will be shown below, is the basis for what makes an airplane fly.

When viscosity is absent, conditions of perfect laminar flow exist: an object behaves in complete alignment with Bernoulli’s principle. Of

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Bernoulli’s Principle

THE

BOOMERANG, DEVELOPED BY

AUSTRALIA’S ABORIGINAL PEOPLE, FLIES THROUGH THE AIR ON A PLANE PERAS IT FLIES, THE BOOMERANG BECOMES BOTH A GYROSCOPE ROLE GIVES IT AERODYNAMIC LIFT. (Bettmann/Corbis. Reproduced by permission.)

PENDICULAR TO THE GROUND, RATHER THAN PARALLEL. AND AN AIRFOIL, AND THIS DUAL

course, though ideal conditions seldom occur in the real world, Bernoulli’s principle provides a guide for the behavior of planes in flight, as well as a host of everyday things.

REAL-LIFE A P P L I C AT I O N S Flying Machines For thousands of years, human beings vainly sought to fly “like a bird,” not realizing that this is literally impossible, due to differences in physiognomy between birds and homo sapiens. No man has ever been born (or ever will be) who possesses enough strength in his chest that he could flap a set of attached wings and lift his body off the ground. Yet the bird’s physical structure proved highly useful to designers of practical flying machines. A bird’s wing is curved along the top, so that when air passes over the wing and divides, the curve forces the air on top to travel a greater distance than the air on the bottom. The tendency of airflow, as noted earlier, is to correct for the presence of solid objects and to return to its original pattern as quickly as possible. Hence, when

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the air hits the front of the wing, the rate of flow at the top increases to compensate for the greater distance it has to travel than the air below the wing. And as shown by Bernoulli, fast-moving fluid exerts less pressure than slow-moving fluid; therefore, there is a difference in pressure between the air below and the air above, and this keeps the wing aloft. Only in 1853 did Sir George Cayley (17731857) incorporate the avian airfoil to create history’s first workable (though engine-less) flying machine, a glider. Much, much older than Cayley’s glider, however, was the first manmade flying machine built “according to Bernoulli’s principle”—only it first appeared in about 12,000 B.C., and the people who created it had had little contact with the outside world until the late eighteenth century A.D. This was the boomerang, one of the most ingenious devices ever created by a stone-age society—in this case, the Aborigines of Australia. Contrary to the popular image, a boomerang flies through the air on a plane perpendicular to the ground, rather than parallel. Hence, any thrower who properly knows how tosses the boomerang not with a side-arm throw, but overhand. As it flies, the boomerang becomes

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both a gyroscope and an airfoil, and this dual role gives it aerodynamic lift. Like the gyroscope, the boomerang imitates a top; spinning keeps it stable. It spins through the air, its leading wing (the forward or upward wing) creating more lift than the other wing. As an airfoil, the boomerang is designed so that the air below exerts more pressure than the air above, which keeps it airborne. Another very early example of a flying machine using Bernoulli’s principles is the kite, which first appeared in China in about 1000 B.C. The kite’s design, particularly its use of lightweight fabric stretched over two crossed strips of very light wood, makes it well-suited for flight, but what keeps it in the air is a difference in air pressure. At the best possible angle of attack, the kite experiences an ideal ratio of pressure from the slower-moving air below versus the fastermoving air above, and this gives it lift. Later Cayley studied the operation of the kite, and recognized that it—rather than the balloon, which at first seemed the most promising apparatus for flight—was an appropriate model for the type of heavier-than-air flying machine he intended to build. Due to the lack of a motor, however, Cayley’s prototypical airplane could never be more than a glider: a steam engine, then state-of-the-art technology, would have been much too heavy. Hence, it was only with the invention of the internal-combustion engine that the modern airplane came into being. On December 17, 1903, at Kitty Hawk, North Carolina, Orville (1871-1948) and Wilbur (1867-1912) Wright tested a craft that used a 25-horsepower engine they had developed at their bicycle shop in Ohio. By maximizing the ratio of power to weight, the engine helped them overcome the obstacles that had dogged recent attempts at flight, and by the time the day was over, they had achieved a dream that had eluded men for more than four millennia.

Again, in accordance with Bernoulli’s principle, the shape of the airflow facilitates the spread of laminar flow around it. The slower-moving currents beneath the airfoil exert greater pressure than the faster currents above it, giving lift to the aircraft. Of course, the aircraft has to be moving at speeds sufficient to gain momentum for its leap from the ground into the air, and here again, Bernoulli’s principle plays a part. Thrust comes from the engines, which run the propellers—whose blades in turn are designed as miniature airfoils to maximize their power by harnessing airflow. Like the aircraft wings, the blades’ angle of attack—the angle at which airflow hits it. In stable flight, the pilot greatly increases the angle of attack (also called pitched), whereas at takeoff and landing, the pitch is dramatically reduced.

Drawing Fluids Upward: Atomizers and Chimneys A number of everyday objects use Bernoulli’s principle to draw fluids upward, and though in terms of their purposes, they might seem very different—for instance, a perfume atomizer vs. a chimney—they are closely related in their application of pressure differences. In fact, the idea behind an atomizer for a perfume spray bottle can also be found in certain garden-hose attachments, such as those used to provide a high-pressure car wash.

As noted earlier, an airfoil has a streamlined design. Its shape is rather like that of an elongat-

The air inside the perfume bottle is moving relatively slowly; therefore, according to Bernoulli’s principle, its pressure is relatively high, and it exerts a strong downward force on the perfume itself. In an atomizer there is a narrow tube running from near the bottom of the bottle to the top. At the top of the perfume bottle, it opens inside another tube, this one perpendicular to the first tube. At one end of the horizontal tube is a simple squeeze-pump which causes air to flow quickly through it. As a result, the pressure

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Within fifty years, airplanes would increasingly obtain their power from jet rather than internal-combustion engines. But the principle that gave them flight, and the principle that kept them aloft once they were airborne, reflected back to Bernoulli’s findings of more than 160 years before their time. This is the concept of the airfoil.

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ed, asymmetrical teardrop lying on its side, with the large end toward the direction of airflow, and the narrow tip pointing toward the rear. The greater curvature of its upper surface in comparison to the lower side is referred to as the airplane’s camber. The front end of the airfoil is also curved, and the chord line is an imaginary straight line connecting the spot where the air hits the front—known as the stagnation point— to the rear, or trailing edge, of the wing.

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toward the top of the bottle is reduced, and the perfume flows upward along the vertical tube, drawn from the area of higher pressure at the bottom. Once it is in the upper tube, the squeezepump helps to eject it from the spray nozzle.

true. There is little variation in tones on a Hoover bugle: increasing the velocity results in a frequency twice that of the original, but it is difficult to create enough speed to generate a third tone.

A carburetor works on a similar principle, though in that case the lower pressure at the top draws air rather than liquid. Likewise a chimney draws air upward, and this explains why a windy day outside makes for a better fire inside. With wind blowing over the top of the chimney, the air pressure at the top is reduced, and tends to draw higher-pressure air from down below.

Spin, Curve, and Pull: The Counterintuitive Principle

The upward pull of air according to the Bernoulli principle can also be illustrated by what is sometimes called the “Hoover bugle”—a name perhaps dating from the Great Depression, when anything cheap or contrived bore the appellation “Hoover” as a reflection of popular dissatisfaction with President Herbert Hoover. In any case, the Hoover bugle is simply a long corrugated tube that, when swung overhead, produces musical notes. You can create a Hoover bugle using any sort of corrugated tube, such as vacuum-cleaner hose or swimming-pool drain hose, about 1.8 in (4 cm) in diameter and 6 ft (1.8 m) in length. To operate it, you should simply hold the tube in both hands, with extra length in the leading hand—that is, the right hand, for most people. This is the hand with which to swing the tube over your head, first slowly and then faster, observing the changes in tone that occur as you change the pace. The vacuum hose of a Hoover tube can also be returned to a version of its original purpose in an illustration of Bernoulli’s principle. If a piece of paper is torn into pieces and placed on a table, with one end of the tube just above the paper and the other end spinning in the air, the paper tends to rise. It is drawn upward as though by a vacuum cleaner—but in fact, what makes it happen is the pressure difference created by the movement of air. In both cases, reduced pressure draws air from the slow-moving region at the bottom of the tube. In the case of the Hoover bugle, the corrugations produce oscillations of a certain frequency. Slower speeds result in slower oscillations and hence lower frequency, which produces a lower tone. At higher speeds, the opposite is

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Bernoulli’s Principle

There are several other interesting illustrations— sometimes fun and in one case potentially tragic—of Bernoulli’s principle. For instance, there is the reason why a shower curtain billows inward once the shower is turned on. It would seem logical at first that the pressure created by the water would push the curtain outward, securing it to the side of the bathtub. Instead, of course, the fast-moving air generated by the flow of water from the shower creates a center of lower pressure, and this causes the curtain to move away from the slower-moving air outside. This is just one example of the ways in which Bernoulli’s principle creates results that, on first glance at least, seem counterintuitive— that is, the opposite of what common sense would dictate. Another fascinating illustration involves placing two empty soft drink cans parallel to one another on a table, with a couple of inches or a few centimeters between them. At that point, the air on all sides has the same slow speed. If you were to blow directly between the cans, however, this would create an area of low pressure between them. As a result, the cans push together. For ships in a harbor, this can be a frightening prospect: hence, if two crafts are parallel to one another and a strong wind blows between them, there is a possibility that they may behave like the cans. Then there is one of the most illusory uses of Bernoulli’s principle, that infamous baseball pitcher’s trick called the curve ball. As the ball moves through the air toward the plate, its velocity creates an air stream moving against the trajectory of the ball itself. Imagine it as two lines, one curving over the ball and one curving under, as the ball moves in the opposite direction. In an ordinary throw, the effects of the airflow would not be particularly intriguing, but in this case, the pitcher has deliberately placed a “spin” on the ball by the manner in which he has thrown it. How pitchers actually produce spin is a complex subject unto itself, involving grip,

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Bernoulli’s Principle

KEY TERMS AERODYNAMICS:

The study of air-

INVERSE RELATIONSHIP:

A situa-

flow and its principles. Applied aerody-

tion involving two variables, in which one

namics is the science of improving man-

of the two increases in direct proportion to

made objects in light of those principles.

the decrease in the other.

AIRFOIL:

The design of an airplane’s

wing when seen from the end, a shape

KINETIC ENERGY:

The energy that

an object possesses by virtue of its motion.

intended to maximize the aircraft’s LAMINAR: A term describing a stream-

response to airflow. ANGLE OF ATTACK: The orientation of

the airfoil with regard to the airflow, or the angle that the chord line forms with the

lined flow, in which all particles move at the same speed and in the same direction. Its opposite is turbulent flow. LIFT: An aerodynamic force perpendicu-

direction of the air stream. BERNOULLI’S PRINCIPLE: A proposi-

lar to the direction of the wind. For an air-

tion, credited to Swiss mathematician and

craft, lift is the force that raises it off the

physicist Daniel Bernoulli (1700-1782),

ground and keeps it aloft.

which maintains that slower-moving fluid

MANOMETERS:

exerts greater pressure than faster-moving

ing pressure in conjunction with a Venturi

fluid.

tube.

CAMBER: The enhanced curvature on the

upper surface of an airfoil.

Devices for measur-

POTENTIAL ENERGY:

The energy

that an object possesses by virtue of its

CHORD LINE: The distance, along an

position.STAGNATION POINT: The spot

imaginary straight line, from the stagna-

where airflow hits the leading edge of an

tion point of an airfoil to the rear, or trail-

airfoil.

ing edge. CONSERVATION OF ENERGY:

A

law of physics which holds that within a system isolated from all other outside factors, the total amount of energy remains

irregular form of flow, in which a fluid is subject to continual changes in speed and direction. Its opposite is laminar flow. An instrument, con-

the same, though transformations of ener-

VENTURI TUBE:

gy from one form to another take place.

sisting of a glass tube with an inward-slop-

Any substance, whether gas or

ing area in the middle, for measuring the

liquid, that conforms to the shape of its

drop in pressure that takes place as the

container.

velocity of a fluid increases.

FLUID:

HYDRODYNAMICS:

The study of

water flow and its principles.

118

TURBULENT: A term describing a highly

VISCOSITY: The internal friction in a

fluid that makes it resistant to flow.

wrist movement, and other factors, and in any case, the fact of the spin is more important than the way in which it was achieved.

If the direction of airflow is from right to left, the ball, as it moves into the airflow, is spinning clockwise. This means that the air flowing

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over the ball is moving in a direction opposite to the spin, whereas that flowing under it is moving in the same direction. The opposite forces produce a drag on the top of the ball, and this cuts down on the velocity at the top compared to that at the bottom of the ball, where spin and airflow are moving in the same direction. Thus the air pressure is higher at the top of the ball, and as per Bernoulli’s principle, this tends to pull the ball downward. The curve ball— of which there are numerous variations, such as the fade and the slider—creates an unpredictable situation for the batter, who sees the ball leave the pitcher’s hand at one altitude, but finds to his dismay that it has dropped dramatically by the time it crosses the plate. A final illustration of Bernoulli’s often counterintuitive principle neatly sums up its effects on the behavior of objects. To perform the experiment, you need only an index card and a flat surface. The index card should be folded at the ends so that when the card is parallel to the surface, the ends are perpendicular to it. These folds should be placed about half an inch (about one centimeter) from the ends. At this point, it would be handy to have an unsuspecting person—someone who has not studied Bernoulli’s principle—on the scene, and challenge him or her to raise the card by blowing under it. Nothing could seem easier, of course: by

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blowing under the card, any person would naturally assume, the air will lift it. But of course this is completely wrong according to Bernoulli’s principle. Blowing under the card, as illustrated, will create an area of high velocity and low pressure. This will do nothing to lift the card: in fact, it only pushes the card more firmly down on the table.

Bernoulli’s Principle

WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. “Bernoulli’s Principle: Explanations and Demos.” (Web site). (February 22, 2001). Cockpit Physics (Department of Physics, United States Air Force Academy. Web site.). (February 19, 2001). K8AIT Principles of Aeronautics Advanced Text. (Web site). (February 19, 2001). Schrier, Eric and William F. Allman. Newton at the Bat: The Science in Sports. New York: Charles Scribner’s Sons, 1984. Smith, H. C. The Illustrated Guide to Aerodynamics. Blue Ridge Summit, PA: Tab Books, 1992. Stever, H. Guyford, James J. Haggerty, and the Editors of Time-Life Books. Flight. New York: Time-Life Books, 1965.

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B U O YA N C Y Buoyancy

CONCEPT The principle of buoyancy holds that the buoyant or lifting force of an object submerged in a fluid is equal to the weight of the fluid it has displaced. The concept is also known as Archimedes’s principle, after the Greek mathematician, physicist, and inventor Archimedes (c. 287-212 B.C.), who discovered it. Applications of Archimedes’s principle can be seen across a wide vertical spectrum: from objects deep beneath the oceans to those floating on its surface, and from the surface to the upper limits of the stratosphere and beyond.

HOW IT WORKS Archimedes Discovers Buoyancy There is a famous story that Sir Isaac Newton (1642-1727) discovered the principle of gravity when an apple fell on his head. The tale, an exaggerated version of real events, has become so much a part of popular culture that it has been parodied in television commercials. Almost equally well known is the legend of how Archimedes discovered the concept of buoyancy. A native of Syracuse, a Greek colony in Sicily, Archimedes was related to one of that city’s kings, Hiero II (308?-216 B.C.). After studying in Alexandria, Egypt, he returned to his hometown, where he spent the remainder of his life. At some point, the royal court hired (or compelled) him to set about determining the weight of the gold in the king’s crown. Archimedes was in his bath pondering this challenge when suddenly it occurred to him that the buoyant force of a sub-

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merged object is equal to the weight of the fluid displaced by it. He was so excited, the legend goes, that he jumped out of his bath and ran naked through the streets of Syracuse shouting “Eureka!” (I have found it). Archimedes had recognized a principle of enormous value—as will be shown—to shipbuilders in his time, and indeed to shipbuilders of the present. Concerning the history of science, it was a particularly significant discovery; few useful and enduring principles of physics date to the period before Galileo Galilei (1564-1642.) Even among those few ancient physicists and inventors who contributed work of lasting value—Archimedes, Hero of Alexandria (c. 65-125 A.D.), and a few others—there was a tendency to miss the larger implications of their work. For example, Hero, who discovered steam power, considered it useful only as a toy, and as a result, this enormously significant discovery was ignored for seventeen centuries. In the case of Archimedes and buoyancy, however, the practical implications of the discovery were more obvious. Whereas steam power must indeed have seemed like a fanciful notion to the ancients, there was nothing farfetched about oceangoing vessels. Shipbuilders had long been confronted with the problem of how to keep a vessel afloat by controlling the size of its load on the one hand, and on the other hand, its tendency to bob above the water. Here, Archimedes offered an answer.

Buoyancy and Weight Why does an object seem to weigh less underwater than above the surface? How is it that a ship

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made of steel, which is obviously heavier than water, can float? How can we determine whether a balloon will ascend in the air, or a submarine will descend in the water? These and other questions are addressed by the principle of buoyancy, which can be explained in terms of properties— most notably, gravity—unknown to Archimedes.

solid object of exactly the same size. The solid object will experience exactly the same degree of pressure as the imaginary “bag” did—and hence, it will also experience the same buoyant force pushing it up from the bottom. This means that buoyant force is equal to the weight—Vdg—of displaced fluid.

To understand the factors at work, it is useful to begin with a thought experiment. Imagine a certain quantity of fluid submerged within a larger body of the same fluid. Note that the terms “liquid” or “water” have not been used: not only is “fluid” a much more general term, but also, in general physical terms and for the purposes of the present discussion, there is no significant difference between gases and liquids. Both conform to the shape of the container in which they are placed, and thus both are fluids.

Buoyancy is always a double-edged proposition. If the buoyant force on an object is greater than the weight of that object—in other words, if the object weighs less than the amount of water it has displaced—it will float. But if the buoyant force is less than the object’s weight, the object will sink. Buoyant force is not the same as net force: if the object weighs more than the water it displaces, the force of its weight cancels out and in fact “overrules” that of the buoyant force.

To return to the thought experiment, what has been posited is in effect a “bag” of fluid—that is, a “bag” made out of fluid and containing fluid no different from the substance outside the “bag.” This “bag” is subjected to a number of forces. First of all, there is its weight, which tends to pull it to the bottom of the container. There is also the pressure of the fluid all around it, which varies with depth: the deeper within the container, the greater the pressure. Pressure is simply the exertion of force over a two-dimensional area. Thus it is as though the fluid is composed of a huge number of twodimensional “sheets” of fluid, each on top of the other, like pages in a newspaper. The deeper into the larger body of fluid one goes, the greater the pressure; yet it is precisely this increased force at the bottom of the fluid that tends to push the “bag” upward, against the force of gravity. Now consider the weight of this “bag.” Weight is a force—the product of mass multiplied by acceleration—that is, the downward acceleration due to Earth’s gravitational pull. For an object suspended in fluid, it is useful to substitute another term for mass. Mass is equal to volume, or the amount of three-dimensional space occupied by an object, multiplied by density. Since density is equal to mass divided by volume, this means that volume multiplied by density is the same as mass.

Buoyancy

At the heart of the issue is density. Often, the density of an object in relation to water is referred to as its specific gravity: most metals, which are heavier than water, are said to have a high specific gravity. Conversely, petroleumbased products typically float on the surface of water, because their specific gravity is low. Note the close relationship between density and weight where buoyancy is concerned: in fact, the most buoyant objects are those with a relatively high volume and a relatively low density. This can be shown mathematically by means of the formula noted earlier, whereby density is equal to mass divided by volume. If Vd = V(m/V), an increase in density can only mean an increase in mass. Since weight is the product of mass multiplied by g (which is assumed to be a constant figure), then an increase in density means an increase in mass and hence, an increase in weight—not a good thing if one wants an object to float.

REAL-LIFE A P P L I C AT I O N S Staying Afloat

We have established that the weight of the fluid “bag” is Vdg, where V is volume, d is density, and g is the acceleration due to gravity. Now imagine that the “bag” has been replaced by a

In the early 1800s, a young Mississippi River flatboat operator submitted a patent application describing a device for “buoying vessels over shoals.” The invention proposed to prevent a problem he had often witnessed on the river— boats grounded on sandbars—by equipping the boats with adjustable buoyant air chambers. The young man even whittled a model of his inven-

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tion, but he was not destined for fame as an inventor; instead, Abraham Lincoln (1809-1865) was famous for much else. In fact Lincoln had a sound idea with his proposal to use buoyant force in protecting boats from running aground. Buoyancy on the surface of water has a number of easily noticeable effects in the real world. (Having established the definition of fluid, from this point onward, the fluids discussed will be primarily those most commonly experienced: water and air.) It is due to buoyancy that fish, human swimmers, icebergs, and ships stay afloat. Fish offer an interesting application of volume change as a means of altering buoyancy: a fish has an internal swim bladder, which is filled with gas. When it needs to rise or descend, it changes the volume in its swim bladder, which then changes its density. The examples of swimmers and icebergs directly illustrate the principle of density—on the part of the water in the first instance, and on the part of the object itself in the second. To a swimmer, the difference between swimming in fresh water and salt water shows that buoyant force depends as much on the density of the fluid as on the volume displaced. Fresh water has a density of 62.4 lb/ft3 (9,925 N/m3), whereas that of salt water is 64 lb/ft3 (10,167 N/m3). For this reason, salt water provides more buoyant force than fresh water; in Israel’s Dead Sea, the saltiest body of water on Earth, bathers experience an enormous amount of buoyant force. Water is an unusual substance in a number of regards, not least its behavior as it freezes. Close to the freezing point, water thickens up, but once it turns to ice, it becomes less dense. This is why ice cubes and icebergs float. However, their low density in comparison to the water around them means that only part of an iceberg stays atop the surface. The submerged percentage of an iceberg is the same as the ratio of the density of ice to that of water: 89%.

For a ship to be seaworthy, it must maintain a delicate balance between buoyancy and stability. A vessel that is too light—that is, too much volume and too little density—will bob on the top of the water. Therefore, it needs to carry a certain amount of cargo, and if not cargo, then water or some other form of ballast. Ballast is a heavy substance that increases the weight of an object experiencing buoyancy, and thereby improves its stability. Ideally, the ship’s center of gravity should be vertically aligned with its center of buoyancy. The center of gravity is the geometric center of the ship’s weight—the point at which weight above is equal to weight below, weight fore is equal to weight aft, and starboard (right-side) weight is equal to weight on the port (left) side. The center of buoyancy is the geometric center of its submerged volume, and in a stable ship, it is some distance directly below center of gravity. Displacement, or the weight of the fluid that is moved out of position when an object is immersed, gives some idea of a ship’s stability. If a ship set down in the ocean causes 1,000 tons (8.896 • 106 N) of water to be displaced, it is said to possess a displacement of 1,000 tons. Obviously, a high degree of displacement is desirable. The principle of displacement helps to explain how an aircraft carrier can remain afloat, even though it weighs many thousands of tons.

Down to the Depths

Because water itself is relatively dense, a highvolume, low-density object is likely to displace a quantity of water more dense—and heavier— than the object itself. By contrast, a steel ball dropped into the water will sink straight to the bottom, because it is a low-volume, high-density object that outweighs the water it displaced.

A submarine uses ballast as a means of descending and ascending underwater: when the submarine captain orders the crew to take the craft down, the craft is allowed to take water into its ballast tanks. If, on the other hand, the command is given to rise toward the surface, a valve will be opened to release compressed air into the tanks. The air pushes out the water, and causes the craft to ascend.

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This brings back the earlier question: how can a ship made out of steel, with a density of 487 lb/ft3 (77,363 N/m3), float on a salt-water ocean with an average density of only about one-eighth that amount? The answer lies in the design of the ship’s hull. If the ship were flat like a raft, or if all the steel in it were compressed into a ball, it would indeed sink. Instead, however, the hollow hull displaces a volume of water heavier than the ship’s own weight: once again, volume has been maximized, and density minimized.

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Buoyancy

Ic 0°

4°

THE

MOLECULAR STRUCTURE OF WATER BEGINS TO EXPAND ONCE IT COOLS BEYOND

TO EXPAND UNTIL IT BECOMES ICE.

FOR

39.4°F (4°C)

AND CONTINUES

THIS REASON, ICE IS LESS DENSE THAN WATER, FLOATS ON THE SURFACE,

AND RETARDS FURTHER COOLING OF DEEPER WATER, WHICH ACCOUNTS FOR THE SURVIVAL OF FRESHWATER PLANT AND ANIMAL LIFE THROUGH THE WINTER.

FOR

THEIR PART, FISH CHANGE THE VOLUME OF THEIR INTERNAL SWIM BLAD-

DER IN ORDER TO ALTER THEIR BUOYANCY.

A submarine is an underwater ship; its streamlined shape is designed to ease its movement. On the other hand, there are certain kinds of underwater vessels, known as submersibles, that are designed to sink—in order to observe or collect data from the ocean floor. Originally, the idea of a submersible was closely linked to that of diving itself. An early submersible was the diving bell, a device created by the noted English astronomer Edmund Halley (1656-1742.) Though his diving bell made it possible for Halley to set up a company in which hired divers salvaged wrecks, it did not permit divers to go beyond relatively shallow depths. First of all, the diving bell received air from the surface: in Halley’s time, no technology existed for taking an oxygen supply below. Nor did it provide substantial protection from the effects of increased pressure at great depths.

ducing two different—but equally frightening— effects. Nitrogen is an inert gas under normal conditions, yet in the high pressure of the ocean depths it turns into a powerful narcotic, causing nitrogen narcosis—often known by the poeticsounding name “rapture of the deep.” Under the influence of this deadly euphoria, divers begin to think themselves invincible, and their altered judgment can put them into potentially fatal situations.

P E R I L S O F T H E D E E P. The most immediate of those effects is, of course, the tendency of an object experiencing such pressure to simply implode like a tin can in a vise. Furthermore, the human body experiences several severe reactions to great depth: under water, nitrogen gas accumulates in a diver’s bodily tissues, pro-

Nitrogen narcosis can occur at depths as shallow as 60 ft (18.29 m), and it can be overcome simply by returning to the surface. However, one should not return to the surface too quickly, particularly after having gone down to a significant depth for a substantial period of time. In such an instance, on returning to the surface nitrogen gas will bubble within the body, producing decompression sickness—known colloquially as “the bends.” This condition may manifest as itching and other skin problems, joint pain, choking, blindness, seizures, unconsciousness, and even permanent neurological defects such as paraplegia.

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French physiologist Paul Bert (1833-1886) first identified the bends in 1878, and in 1907, John Scott Haldane (1860-1936) developed a method for counteracting decompression sickness. He calculated a set of decompression tables that advised limits for the amount of time at given depths. He recommended what he called stage decompression, which means that the ascending diver stops every few feet during ascension and waits for a few minutes at each level, allowing the body tissues time to adjust to the new pressure. Modern divers use a decompression chamber, a sealed container that simulates the stages of decompression. B AT H Y S P H E R E , S C U B A , A N D B AT H Y S CA P H E . In 1930, the American

naturalist William Beebe (1877-1962) and American engineer Otis Barton created the bathysphere. This was the first submersible that provided the divers inside with adequate protection from external pressure. Made of steel and spherical in shape, the bathysphere had thick quartz windows and was capable of maintaining ordinary atmosphere pressure even when lowered by a cable to relatively great depths. In 1934, a bathysphere descended to what was then an extremely impressive depth: 3,028 ft (923 m). However, the bathysphere was difficult to operate and maneuver, and in time it was be replaced by a more workable vessel, the bathyscaphe. Before the bathyscaphe appeared, however, in 1943, two Frenchmen created a means for divers to descend without the need for any sort of external chamber. Certainly a diver with this new apparatus could not go to anywhere near the same depths as those approached by the bathysphere; nonetheless, the new aqualung made it possible to spend an extended time under the surface without need for air. It was now theoretically feasible for a diver to go below without any need for help or supplies from above, because he carried his entire oxygen supply on his back. The name of one of inventors, Emile Gagnan, is hardly a household word; but that of the other— Jacques Cousteau (1910-1997)—certainly is. So, too, is the name of their invention: the self-contained underwater breathing apparatus, better known as scuba. The most important feature of the scuba gear was the demand regulator, which made it possible for the divers to breathe air at the same pressure as their underwater surroundings. This

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in turn facilitated breathing in a more normal, comfortable manner. Another important feature of a modern diver’s equipment is a buoyancy compensation device. Like a ship atop the water, a diver wants to have only so much buoyancy— not so much that it causes him to surface. As for the bathyscaphe—a term whose two Greek roots mean “deep” and “boat”—it made its debut five years after scuba gear. Built by the Swiss physicist and adventurer Auguste Piccard (1884-1962), the bathyscaphe consisted of two compartments: a heavy steel crew cabin that was resistant to sea pressure, and above it, a larger, light container called a float. The float was filled with gasoline, which in this case was not used as fuel, but to provide extra buoyancy, because of the gasoline’s low specific gravity. When descending, the occupants of the bathyscaphe—there could only be two, since the pressurized chamber was just 79 in (2.01 m) in diameter—released part of the gasoline to decrease buoyancy. They also carried iron ballast pellets on board, and these they released when preparing to ascend. Thanks to battery-driven screw propellers, the bathyscaphe was much more maneuverable than the bathysphere had ever been; furthermore, it was designed to reach depths that Beebe and Barton could hardly have conceived. R E AC H I N G N E W D E P T H S . It took several years of unsuccessful dives, but in 1953 a bathyscaphe set the first of many depth records. This first craft was the Trieste, manned by Piccard and his son Jacques, which descended 10,335 ft (3,150 m) below the Mediterranean, off Capri, Italy. A year later, in the Atlantic Ocean off Dakar, French West Africa (now Senegal), French divers Georges Houot and Pierre-Henri Willm reached 13,287 ft (4,063 m) in the FNRS 3.

Then in 1960, Jacques Piccard and United States Navy Lieutenant Don Walsh set a record that still stands: 35,797 ft (10,911 m)—23% greater than the height of Mt. Everest, the world’s tallest peak. This they did in the Trieste some 250 mi (402 km) southeast of Guam at the Mariana Trench, the deepest spot in the Pacific Ocean and indeed the deepest spot on Earth. Piccard and Walsh went all the way to the bottom, a descent that took them 4 hours, 48 minutes. Coming up took 3 hours, 17 minutes. Thirty-five years later, in 1995, the Japanese craft Kaiko also made the Mariana descent and

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confirmed the measurements of Piccard and Walsh. But the achievement of the Kaiko was not nearly as impressive of that of the Trieste’s twoman crew: the Kaiko, in fact, had no crew. By the 1990s, sophisticated remote-sensing technology had made it possible to send down unmanned ocean expeditions, and it became less necessary to expose human beings to the incredible risks encountered by the Piccards, Walsh, and others.

Buoyancy

F I L M I N G T I T A N I C . An example of such an unmanned vessel is the one featured in the opening minutes of the Academy Award-winning motion picture Titanic (1997). The vessel itself, whose sinking in 1912 claimed more than 1,000 lives, rests at such a great depth in the North Atlantic that it is impractical either to raise it, or to send manned expeditions to explore the interior of the wreck. The best solution, then, is a remotely operated vessel of the kind also used for purposes such as mapping the ocean floor, exploring for petroleum and other deposits, and gathering underwater plate technology data.

The craft used in the film, which has “arms” for grasping objects, is of a variety specially designed for recovering items from shipwrecks. For the scenes that showed what was supposed to be the Titanic as an active vessel, director James Cameron used a 90% scale model that depicted the ship’s starboard side—the side hit by the iceberg. Therefore, when showing its port side, as when it was leaving the Southampton, England, dock on April 15, 1912, all shots had to be reversed: the actual signs on the dock were in reverse lettering in order to appear correct when seen in the final version. But for scenes of the wrecked vessel lying at the bottom of the ocean, Cameron used the real Titanic. To do this, he had to use a submersible; but he did not want to shoot only from inside the submersible, as had been done in the 1992 IMAX film Titanica. Therefore, his brother Mike Cameron, in cooperation with Panavision, built a special camera that could withstand 400 atm (3.923 • 107 Pa)—that is, 400 times the air pressure at sea level. The camera was attached to the outside of the submersible, which for these external shots was manned by Russian submarine operators.

THE

DIVERS PICTURED HERE HAVE ASCENDED FROM A

SUNKEN SHIP AND HAVE STOPPED AT THE

10-FT (3-M)

DECOMPRESSION LEVEL TO AVOID GETTING DECOMPRESSION SICKNESS, BETTER KNOWN AS THE “BENDS.” (Photograph, copyright Jonathan Blair/Corbis. Reproduced by permission.)

would have been too dangerous for the humans in the manned craft. Cameron had intended the remotely operated submersible as a mere prop, but in the end its view inside the ruined Titanic added one of the most poignant touches in the entire film. To these he later added scenes involving objects specific to the film’s plot, such as the safe. These he shot in a controlled underwater environment designed to look like the interior of the Titanic.

Into the Skies In the earlier description of Piccard’s bathyscaphe design, it was noted that the craft consisted of two compartments: a heavy steel crew cabin resistant to sea pressure, and above it a larger, light container called a float. If this sounds rather like the structure of a hot-air balloon, there is no accident in that.

Because the special camera only held twelve minutes’ worth of film, it was necessary to make a total of twelve dives. On the last two, a remotely operated submersible entered the wreck, which

In 1931, nearly two decades before the bathyscaphe made its debut, Piccard and another Swiss scientist, Paul Kipfer, set a record of a different kind with a balloon. Instead of going lower

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than anyone ever had, as Piccard and his son Jacques did in 1953—and as Jacques and Walsh did in an even greater way in 1960—Piccard and Kipfer went higher than ever, ascending to 55,563 ft (16,940 m). This made them the first two men to penetrate the stratosphere, which is the next atmospheric layer above the troposphere, a layer approximately 10 mi (16.1 km) high that covers the surface of Earth. Piccard, without a doubt, experienced the greatest terrestrial altitude range of any human being over a lifetime: almost 12.5 mi (20.1 km) from his highest high to his lowest low, 84% of it above sea level and the rest below. His career, then, was a tribute to the power of buoyant force—and to the power of overcoming buoyant force for the purpose of descending to the ocean depths. Indeed, the same can be said of the Piccard family as a whole: not only did Jacques set the world’s depth record, but years later, Jacques’s son Bertrand took to the skies for another record-setting balloon flight. In 1999, Bertrand Piccard and British balloon instructor Brian Wilson became the first men to circumnavigate the globe in a balloon, the Breitling Orbiter 3. The craft extended 180 ft (54.86) from the top of the envelope—the part of the balloon holding buoyant gases—to the bottom of the gondola, the part holding riders. The pressurized cabin had one bunk in which one pilot could sleep while the other flew, and up front was a computerized control panel which allowed the pilot to operate the burners, switch propane tanks, and release empty ones. It took Piccard and Wilson just 20 days to circle the Earth—a far cry from the first days of ballooning two centuries earlier. T H E F I R S T B A L LO O N S . The Piccard family, though Swiss, are francophone; that is, they come from the French-speaking part of Switzerland. This is interesting, because the history of human encounters with buoyancy— below the ocean and even more so in the air— has been heavily dominated by French names. In fact, it was the French brothers, Joseph-Michel (1740-1810) and Jacques-Etienne (1745-1799) Montgolfier, who launched the first balloon in 1783. These two became to balloon flight what two other brothers, the Americans Orville and Wilbur Wright, became with regard to the invention that superseded the balloon twelve decades later: the airplane.

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On that first flight, the Montgolfiers sent up a model 30 ft (9.15 m) in diameter, made of linen-lined paper. It reached a height of 6,000 ft (1,828 m), and stayed in the air for 10 minutes before coming back down. Later that year, the Montgolfiers sent up the first balloon flight with living creatures—a sheep, a rooster, and a duck— and still later in 1783, Jean-François Pilatre de Rozier (1756-1785) became the first human being to ascend in a balloon. Rozier only went up 84 ft (26 m), which was the length of the rope that tethered him to the ground. As the makers and users of balloons learned how to use ballast properly, however, flight times were extended, and balloon flight became ever more practical. In fact, the world’s first military use of flight dates not to the twentieth century but to the eighteenth—1794, specifically, when France created a balloon corps. HOW

A

BALLOON

F L O AT S .

There are only three gases practical for lifting a balloon: hydrogen, helium, and hot air. Each is much less than dense than ordinary air, and this gives them their buoyancy. In fact, hydrogen is the lightest gas known, and because it is cheap to produce, it would be ideal—except for the fact that it is extremely flammable. After the 1937 crash of the airship Hindenburg, the era of hydrogen use for lighter-than-air transport effectively ended. Helium, on the other hand, is perfectly safe and only slightly less buoyant than hydrogen. This makes it ideal for balloons of the sort that children enjoy at parties; but helium is expensive, and therefore impractical for large balloons. Hence, hot air—specifically, air heated to a temperature of about 570°F (299°C), is the only truly viable option. Charles’s law, one of the laws regarding the behavior of gases, states that heating a gas will increase its volume. Gas molecules, unlike their liquid or solid counterparts, are highly nonattractive—that is, they tend to spread toward relatively great distances from one another. There is already a great deal of empty space between gas molecules, and the increase in volume only increases the amount of empty space. Hence, density is lowered, and the balloon floats. A I R S H I P S . Around the same time the Montgolfier brothers launched their first balloons, another French designer, Jean-BaptisteMarie Meusnier, began experimenting with a

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Buoyancy

ONCE CONSIDERED OBSOLETE, BLIMPS ARE ENJOYING A RENAISSANCE AMONG SCIENTISTS AND GOVERNMENT AGENCIES. THE BLIMP PICTURED HERE, THE AEROSTAT BLIMP, IS EQUIPPED WITH RADAR FOR DRUG ENFORCEMENT AND INSTRUMENTS FOR WEATHER OBSERVATION. (Corbis. Reproduced by permission.)

more streamlined, maneuverable model. Early balloons, after all, could only be maneuvered along one axis, up and down: when it came to moving sideways or forward and backward, they were largely at the mercy of the elements. It was more than a century before Meusnier’s idea—the prototype for an airship— became a reality. In 1898, Alberto SantosDumont of Brazil combined a balloon with a propeller powered by an internal-combustion instrument, creating a machine that improved on the balloon, much as the bathyscaphe later improved on the bathysphere. Santos-Dumont’s airship was non-rigid, like a balloon. It also used hydrogen, which is apt to contract during descent and collapse the envelope. To counter this problem, Santos-Dumont created the ballonet, an internal airbag designed to provide buoyancy and stabilize flight.

tions. Yet Zeppelin’s earliest launches, in the decade that followed 1898, were fraught with a number of problems—not least of which were disasters caused by the flammability of hydrogen. Zeppelin was finally successful in launching airships for public transport in 1911, and the quarter-century that followed marked the golden age of airship travel. Not that all was “golden” about this age: in World War I, Germany used airships as bombers, launching the first London blitz in May 1915. By the time Nazi Germany initiated the more famous World War II London blitz 25 years later, ground-based anti-aircraft technology would have made quick work of any zeppelin; but by then, airplanes had long since replaced airships.

One of the greatest figures in the history of lighter-than-air flight—a man whose name, along with blimp and dirigible, became a synonym for the airship—was Count Ferdinand von Zeppelin (1838-1917). It was he who created a lightweight structure of aluminum girders and rings that made it possible for an airship to remain rigid under varying atmospheric condi-

During the 1920s, though, airships such as the Graf Zeppelin competed with airplanes as a mode of civilian transport. It is a hallmark of the perceived safety of airships over airplanes at the time that in 1928, the Graf Zeppelin made its first transatlantic flight carrying a load of passengers. Just a year earlier, Charles Lindbergh had made the first-ever solo, nonstop transatlantic flight in an airplane. Today this would be the equivalent of someone flying to the Moon, or perhaps even Mars, and there was no question of carrying pas-

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Buoyancy

KEY TERMS ARCHIMEDES’S PRINCIPLE:

A rule

MASS:

of physics which holds that the buoyant

the resistance of an object to a change in its

force of an object immersed in fluid is

motion. For an object immerse in fluid,

equal to the weight of the fluid displaced

mass is equal to volume multiplied by

by the object. It is named after the Greek

density.

mathematician, physicist, and inventor Archimedes (c. 287-212 B.C.), who first identified it.

The exertion of force

PRESSURE:

over a two-dimensional area; hence the formula for pressure is force divided by

A heavy substance that, by

area. The British system of measures typi-

increasing the weight of an object experi-

cally reckons pressure in pounds per

encing buoyancy, improves its stability.

square inch. In metric terms, this is meas-

BALLAST:

BUOYANCY:

The tendency of an object

immersed in a fluid to float. This can be explained by Archimedes’s principle. Mass divided by volume.

DENSITY:

ured in terms of newtons (N) per square meter, a figure known as a pascal (Pa.) SPECIFIC GRAVITY:

The density of

an object or substance relative to the densi-

A measure of the

ty of water; or more generally, the ratio

weight of the fluid that has had to be

between the densities of two objects or

moved out of position so that an object can

substances.

be immersed. If a ship set down in the

VOLUME:

ocean causes 1,000 tons of water to be dis-

dimensional space occupied by an object.

placed, it is said to possess a displacement

Volume is usually measured in cubic units.

DISPLACEMENT:

of 1,000 tons. WEIGHT:

The

amount

of

three-

A force equal to mass multi-

Any substance, whether gas or

plied by the acceleration due to gravity (32

liquid, that conforms to the shape of its

ft/9.8 m/sec2). For an object immersed in

container.

fluid, weight is the same as volume multi-

FLUID:

FORCE:

The product of mass multi-

plied by acceleration.

sengers. Furthermore, Lindbergh was celebrated as a hero for the rest of his life, whereas the passengers aboard the Graf Zeppelin earned no more distinction for bravery than would pleasureseekers aboard a cruise.

plied by density multiplied by gravitational acceleration.

the leading contender in the realm of flight technology.

few years, airships constituted the luxury liners of the skies; but the Hindenburg crash signaled the end of relatively widespread airship transport. In any case, by the time of the 1937 Hindenburg crash, lighter-than-air transport was no longer

Ironically enough, by 1937 the airplane had long since proved itself more viable—even though it was actually heavier than air. The principles that make an airplane fly have little to do with buoyancy as such, and involve differences in pressure rather than differences in density. Yet the replacement of lighter-than-air craft on the cutting edge of flight did not mean that balloons and airships were relegated to the museum; instead, their purposes changed.

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A measure of inertia, indicating

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The airship enjoyed a brief resurgence of interest during World War II, though purely as a surveillance craft for the United States military. In the period after the war, the U.S. Navy hired the Goodyear Tire and Rubber Company to produce airships, and as a result of this relationship Goodyear created the most visible airship since the Graf Zeppelin and the Hindenburg: the Goodyear Blimp. B L I M P S A N D B A L LO O N S : O N T H E C U T T I N G E D G E ? . The blimp,

known to viewers of countless sporting events, is much better-suited than a plane or helicopter to providing TV cameras with an aerial view of a stadium—and advertisers with a prominent billboard. Military forces and science communities have also found airships useful for unexpected purposes. Their virtual invisibility with regard to radar has reinvigorated interest in blimps on the part of the U.S. Department of Defense, which has discussed plans to use airships as radar platforms in a larger Strategic Air Initiative. In addition, French scientists have used airships for studying rain forest treetops or canopies. Balloons have played a role in aiding space exploration, which is emblematic of the relationship between lighter-than-air transport and more advanced means of flight. In 1961, Malcolm D. Ross and Victor A. Prother of the U.S. Navy set the balloon altitude record with a height of 113,740 ft (34,668 m.) The technology that enabled their survival at more than 21 mi (33.8 km) in the air was later used in creating life-support systems for astronauts. Balloon astronomy provides some of the clearest images of the cosmos: telescopes mounted on huge, unmanned balloons at elevations as high as 120,000 ft (35,000 m)—far above the dust

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and smoke of Earth—offer high-resolution images. Balloons have even been used on other planets: for 46 hours in 1985, two balloons launched by the unmanned Soviet expedition to Venus collected data from the atmosphere of that planet.

Buoyancy

American scientists have also considered a combination of a large hot-air balloon and a smaller helium-filled balloon for gathering data on the surface and atmosphere of Mars during expeditions to that planet. As the air balloon is heated by the Sun’s warmth during the day, it would ascend to collect information on the atmosphere. (In fact the “air” heated would be from the atmosphere of Mars, which is composed primarily of carbon dioxide.) Then at night when Mars cools, the air balloon would lose its buoyancy and descend, but the helium balloon would keep it upright while it collected data from the ground. WHERE TO LEARN MORE “Buoyancy” (Web site). (March 12, 2001). “Buoyancy” (Web site). (March 12, 2001). “Buoyancy Basics” Nova/PBS (Web site). (March 12, 2001). Challoner, Jack. Floating and Sinking. Austin, TX: Raintree Steck-Vaughn, 1997. Cobb, Allan B. Super Science Projects About Oceans. New York: Rosen, 2000. Gibson, Gary. Making Things Float and Sink. Illustrated by Tony Kenyon. Brookfield, CT: Copper Beeck Brooks, 1995. Taylor, Barbara. Liquid and Buoyancy. New York: Warwick Press, 1990.

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S C I E N C E O F E V E RY DAY T H I N G S real-life Physics

S TAT I C S S TAT I C S A N D E Q U I L I B R I U M PRESSURE ELASTICITY

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Statics and Equilibrium

S TAT I C S A N D EQUILIBRIUM

CONCEPT Statics, as its name suggests, is the study of bodies at rest. Those bodies may be acted upon by a variety of forces, but as long as the lines of force meet at a common point and their vector sum is equal to zero, the body itself is said to be in a state of equilibrium. Among the topics of significance in the realm of statics is center of gravity, which is relatively easy to calculate for simple bodies, but much more of a challenge where aircraft or ships are concerned. Statics is also applied in analysis of stress on materials—from a picture frame to a skyscraper.

HOW IT WORKS Equilibrium and Vectors Essential to calculations in statics is the use of vectors, or quantities that have both magnitude and direction. By contrast, a scalar has only magnitude. If one says that a certain piece of property has an area of one acre, there is no directional component. Nor is there a directional component involved in the act of moving the distance of 1 mi (1.6 km), since no statement has been made as to the direction of that mile. On the other hand, if someone or something experiences a displacement, or change in position, of 1 mi to the northeast, then what was a scalar description has been placed in the language of vectors. Not only are mass and speed (as opposed to velocity) considered scalars; so too is time. This might seem odd at first glance, but—on Earth at least, and outside any special circumstances posed by quantum mechanics—time can only move forward. Hence, direction is not a factor. By

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contrast, force, equal to mass multiplied by acceleration, is a vector. So too is weight, a specific type of force equal to mass multiplied by the acceleration due to gravity (32 ft or [9.8 m] / sec2). Force may be in any direction, but the direction of weight is always downward along a vertical plane. V E C T O R S U M S . Adding scalars is simple, since it involves mere arithmetic. The addition of vectors is more challenging, and usually requires drawing a diagram, for instance, if trying to obtain a vector sum for the velocity of a car that has maintained a uniform speed, but has changed direction several times.

One would begin by representing each vector as an arrow on a graph, with the tail of each vector at the head of the previous one. It would then be possible to draw a vector from the tail of the first to the head of the last. This is the sum of the vectors, known as a resultant, which measures the net change. Suppose, for instance, that a car travels north 5 mi (8 km), east 2 mi (3.2 km), north 3 mi (4.8 km), east 3 mi, and finally south 3 mi. One must calculate its net displacement—in other words, not the sum of all the miles it has traveled, but the distance and direction between its starting point and its end point. First, one draws the vectors on a piece of graph paper, using a logical system that treats the y axis as the north-south plane, and the x axis as the east-west plane. Each vector should be in the form of an arrow pointing in the appropriate direction. Having drawn all the vectors, the only remaining one is between the point where the car’s journey ends and the starting point—that is, the resultant. The number of sides to the

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resulting shape is always one more than the number of vectors being added; the final side is the resultant. In this particular case, the answer is fairly easy. Because the car traveled north 5 mi and ultimately moved east by 5 mi, returning to a position of 5 mi north, the segment from the resultant forms the hypotenuse of an equilateral (that is, all sides equal) right triangle. By applying the Pythagorean theorem, which states that the square of the length of the hypotenuse is equal to the sum of the squares of the other two sides, one quickly arrives at a figure of 7.07 m (11.4 km) in a northeasterly direction. This is the car’s net displacement.

Calculating Force and Tension in Equilibrium Using vector sums, it is possible to make a number of calculations for objects in equilibrium, but these calculations are somewhat more challenging than those in the car illustration. One form of equilibrium calculation involves finding tension, or the force exerted by a supporting object on an object in equilibrium—a force that is always equal to the amount of weight supported. (Another way of saying this is that if the tension on the supporting object is equal to the weight it supports, then the supported object is in equilibrium.) In calculations for tension, it is best to treat the supporting object—whether it be a rope, picture hook, horizontal strut or some other item— as though it were weightless. One should begin by drawing a free-body diagram, a sketch showing all the forces acting on the supported object. It is not necessary to show any forces (other than weight) that the object itself exerts, since those do not contribute to its equilibrium. R E S O LV I N G X A N D Y C O M P O N E N T S . As with the distance vector graph

discussed above, next one must equate these forces to the x and y axes. The distance graph example involved only segments already parallel to x and y, but suppose—using the numbers already discussed—the graph had called for the car to move in a perfect 45°-angle to the northeast along a distance of 7.07 mi. It would then have been easy to resolve this distance into an x component (5 mi east) and a y component (5 mi north)—which are equal to the other two sides of the equilateral triangle.

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This resolution of x and y components is more challenging for calculations involving equilibrium, but once one understands the principle involved, it is easy to apply. For example, imagine a box suspended by two ropes, neither of which is at a 90°-angle to the box. Instead, each rope is at an acute angle, rather like two segments of a chain holding up a sign. The x component will always be the product of tension (that is, weight) multiplied by the cosine of the angle. In a right triangle, one angle is always equal to 90°, and thus by definition, the other two angles are acute, or less than 90°. The angle of either rope is acute, and in fact, the rope itself may be considered the hypotenuse of an imaginary triangle. The base of the triangle is the x axis, and the angle between the base and the hypotenuse is the one under consideration. Hence, we have the use of the cosine, which is the ratio between the adjacent leg (the base) of the triangle and the hypotenuse. Regardless of the size of the triangle, this figure is a constant for any particular angle. Likewise, to calculate the y component of the angle, one uses the sine, or the ratio between the opposite side and the hypotenuse. Keep in mind, once again, that the adjacent leg for the angle is by definition the same as the x axis, just as the opposite leg is the same as the y axis. The cosine (abbreviated cos), then, gives the x component of the angle, as the sine (abbreviated sin) does the y component.

REAL-LIFE A P P L I C AT I O N S Equilibrium and Center of Gravity in Real Objects Before applying the concept of vector sums to matters involving equilibrium, it is first necessary to clarify the nature of equilibrium itself—what it is and what it is not. Earlier it was stated that an object is in equilibrium if the vector sum of the forces acting on it are equal to zero—as long as those forces meet at a common point. This is an important stipulation, because it is possible to have lines of force that cancel one another out, but nonetheless cause an object to move. If a force of a certain magnitude is applied to the right side of an object, and a line of force of equal magnitude meets it exactly from the left, then the object is in equilibrium. But if the line of

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force from the right is applied to the top of the object, and the line of force from the left to the bottom, then they do not meet at a common point, and the object is not in equilibrium. Instead, it is experiencing torque, which will cause it to rotate.

Statics and Equilibrium

VA R I E T I E S O F E Q U I L I B R I U M .

There are two basic conditions of equilibrium. The term “translational equilibrium” describes an object that experiences no linear (straightline) acceleration; on the other hand, an object experiencing no rotational acceleration (a component of torque) is said to be in rotational equilibrium. Typically, an object at rest in a stable situation experiences both linear and rotational equilibrium. But equilibrium itself is not necessarily stable. An empty glass sitting on a table is in stable equilibrium: if it were tipped over slightly— that is, with a force below a certain threshold— then it would return to its original position. This is true of a glass sitting either upright or upsidedown. Now imagine if the glass were somehow propped along the edge of a book sitting on the table, so that the bottom of the glass formed the hypotenuse of a triangle with the table as its base and the edge of the book as its other side. The glass is in equilibrium now, but unstable equilibrium, meaning that a slight disturbance—a force from which it could recover in a stable situation—would cause it to tip over. If, on the other hand, the glass were lying on its side, then it would be in a state of neutral equilibrium. In this situation, the application of force alongside the glass will not disturb its equilibrium. The glass will not attempt to seek stable equilibrium, nor will it become more unstable; rather, all other things being equal, it will remain neutral. C E N T E R O F G RAV I T Y. Center of gravity is the point in an object at which the weight below is equal to the weight above, the weight in front equal to the weight behind, and the weight to the left equal to the weight on the right. Every object has just one center of gravity, and if the object is suspended from that point, it will not rotate.

A

GLASS SITTING ON A TABLE IS IN A STATE OF STABLE

EQUILIBRIUM. (Photograph by John Wilkes Studio/Corbis. Repro-

duced by permission.)

ing bell in an Olympic competition, the swimmer’s center of gravity is to the front—some distance from his or her chest. This is appropriate, since the objective is to get into the water as quickly as possible once the race starts. By contrast, a sprinter’s stance places the center of gravity well within the body, or at least firmly surrounded by the body—specifically, at the place where the sprinter’s rib cage touches the forward knee. This, too, fits with the needs of the athlete in the split-second following the starting gun. The sprinter needs to have as much traction as possible to shoot forward, rather than forward and downward, as the swimmer does.

Tension Calculations

One interesting aspect of an object’s center of gravity is that it does not necessarily have to be within the object itself. When a swimmer is poised in a diving stance, as just before the start-

In the earlier discussion regarding the method of calculating tension in equilibrium, two of the three steps of this process were given: first, draw a free-body diagram, and second, resolve the forces into x and y components. The third step is to set the force components along each axis equal to zero—since, if the object is truly in equilibrium, the sum of forces will indeed equal zero. This makes it possible, finally, to solve the equations for the net tension.

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Statics and Equilibrium

IN

THE STARTING BLOCKS, A SPRINTER’S CENTER OF GRAVITY IS ALIGNED ALONG THE RIB CAGE AND FORWARD KNEE,

THUS MAXIMIZING THE RUNNER’S ABILITY TO SHOOT FORWARD OUT OF THE BLOCKS. (Photograph by Ronnen Eshel/Corbis.

Reproduced by permission.)

136

Imagine a picture that weighs 100 lb (445 N) suspended by a wire, the left side of which may be called segment A, and the right side segment B. The wire itself is not perfectly centered on the picture-hook: A is at a 30° angle, and B on a 45° angle. It is now possible to find the tension on both.

However, the y-force formula is somewhat different than for x: since weight is exerted along the y axis, it must be subtracted. Thus, the formula for the y component of force is Fy = TAy + TBy–w = 0. (Note that the y components of both A and B are positive: by definition, this must be so for an object suspended from some height.)

First, one can resolve the horizontal components by the formula Fx = TBx + TAx = 0, meaning that the x component of force is equal to the product of tension for the x component of B, added to the product of tension for the x component of A, which in turn is equal to zero. Given the 30°-angle of A, Ax = 0.866, which is the cosine of 30°. Bx is equal to cos 45°, which equals 0.707. (Recall the earlier discussion of distance, in which a square with sides 5 mi long was described: its hypotenuse was 7.07 mi, and 5/7.07 = 0.707.)

Substituting the value for TB obtained above, (1.22)TA, makes it possible to complete the equation. Since the sine of 30° is 0.5, and the sine of 45° is 0.707—the same value as its cosine—one can state the equation thus: TA(0.5) + (1.22)TA(0.707)–100 lb = 0. This can be restated as TA(0.5 + (1.22 • 0.707)) = TA(1.36) = 100 lb. Hence, TA = (100 lb/1.36) = 73.53 lb. Since TB = (1.22)TA, this yields a value of 89.71 lb for TB. Note that TA and TB actually add up to considerably more than 100 lb. This, however, is known as an algebraic sum—which is very similar to an arithmetic sum, inasmuch as algebra is simply a generalization of arithmetic. What is important here, however, is the vector sum, and the vector sum of TA and TB equals 100 lb.

Because A goes off to the left from the point at which the picture is attached to the wire, this places it on the negative portion of the x axis. Therefore, the formula can now be restated as TB(0.707)–TA(0.866) = 0. Solving for TB reveals that it is equal to TA(0.866/0.707) = (1.22)TA. This will be substituted for TB in the formula for the total force along the y component.

lengthy calculation for center of gravity, we will explain the principles behind such calculations.

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C A L C U L AT I N G CENTER OF G RAV I T Y. Rather than go through another

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It is easy to calculate the center of gravity for a regular shape, such as a cube or sphere—assuming, of course, that the mass and therefore the weight is evenly distributed throughout the object. In such a case, the center of gravity is the geometric center. For an irregular object, however, center of gravity must be calculated. An analogy regarding United States demographics may help to highlight the difference between geometric center and center of gravity. The geographic center of the U.S., which is analogous to geometric center, is located near the town of Castle Rock in Butte County, South Dakota. (Because Alaska and Hawaii are so far west of the other 48 states—and Alaska, with its great geographic area, is far to the north—the data is skewed in a northwestward direction. The geographic center of the 48 contiguous states is near Lebanon, in Smith County, Kansas.) The geographic center, like the geometric center of an object, constitutes a sort of balance point in terms of physical area: there is as much U.S. land to the north of Castle Rock, South Dakota, as to the south, and as much to the east as to the west. The population center is more like the center of gravity, because it is a measure, in some sense, of “weight” rather than of volume— though in this case concentration of people is substituted for concentration of weight. Put another way, the population center is the balance point of the population, if it were assumed that every person weighed the same amount. Naturally, the population center has been shifting westward ever since the first U.S. census in 1790, but it is still skewed toward the east: though there is far more U.S. land west of the Mississippi River, there are still more people east of it. Hence, according to the 1990 U.S. census, the geographic center is some 1,040 mi (1,664 km) in a southeastward direction from the population center: just northwest of Steelville, Missouri, and a few miles west of the Mississippi. The United States, obviously, is an “irregular object,” and calculations for either its geographic or its population center represent the mastery of numerous mathematical principles. How, then, does one find the center of gravity for a much smaller irregular object? There are a number of methods, all rather complex. To measure center of gravity in purely physical terms, there are a variety of techniques relat-

S C I E N C E O F E V E RY DAY T H I N G S

ing to the shape of the object to be measured. There is also a mathematical formula, which involves first treating the object as a conglomeration of several more easily measured objects. Then the x components for the mass of each “sub-object” can be added, and divided by the combined mass of the object as a whole. The same can be done for the y components.

Statics and Equilibrium

Using Equilibrium Calculations One reason for making center of gravity calculations is to ensure that the net force on an object passes through that center. If it does not, the object will start to rotate—and for an airplane, for instance, this could be disastrous. Hence, the builders and operators of aircraft make exceedingly detailed, complicated calculations regarding center of gravity. The same is true for shipbuilders and shipping lines: if a ship’s center of gravity is not vertically aligned with the focal point of the buoyant force exerted on it by the water, it may well sink. In the case of ships and airplanes, the shapes are so irregular that center of gravity calculations require intensive analyses of the many components. Hence, a number of companies that supply measurement equipment to the aerospace and maritime industries offer center of gravity measurement instruments that enable engineers to make the necessary calculations. On dry ground, calculations regarding equilibrium are likewise quite literally a life and death matter. In the earlier illustration, the object in equilibrium was merely a picture hanging from a wire—but what if it were a bridge or a building? The results of inaccurate estimates of net force could affect the lives of many people. Hence, structural engineers make detailed analyses of stress, once again using series of calculations that make the picture-frame illustration above look like the simplest of all arithmetic problems.

WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. “Determining Center of Gravity” National Aeronautics and Space Administration (Web site). (March 19, 2001).

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Statics and Equilibrium

KEY TERMS A change in velocity.

ACCELERATION:

SCALAR:

The point on

only magnitude, with no specific direction.

an object at which the total weights on

Mass, time, and speed are all scalars. The

either side of all axes (x, y, and z) are iden-

opposite of a scalar is a vector.

tical. Each object has just one center of

SINE:

gravity, and if it is suspended from that

in a right triangle, the sine (abbreviated

point, it will be in a state of perfect rota-

sin) is the ratio between the opposite leg

tional equilibrium.

and the hypotenuse. Regardless of the size

CENTER OF GRAVITY:

COSINE:

For an acute (less than 90°)

angle in a right triangle, the cosine (abbre-

For an acute (less than 90°) angle

of the triangle, this figure is a constant for any particular angle.

viated cos) is the ratio between the adjacent leg and the hypotenuse. Regardless of the size of the triangle, this figure is a constant for any particular angle.

EQUILIBRIUM:

STATICS:

The study of bodies at rest.

Those bodies may be acted upon by a variety of forces, but as long as the vector sum for all those lines of force is equal to zero,

Change in position.

DISPLACEMENT:

A state in which vector

the body itself is said to be in a state of equilibrium.

sum for all lines of force on an object is equal to zero. An object that experiences no linear acceleration is said to be in translational equilibrium, and one that experiences no rotational acceleration is referred

TENSION:

The force exerted by a sup-

porting object on an object in equilibrium—a force that is always equal to the amount of weight supported. A quantity that possesses

to as being in rotational equilibrium. An

VECTOR:

object may also be in stable, unstable, or

both magnitude and direction. Force is a

neutral equilibrium.

vector; so too is acceleration, a component

FORCE:

The product of mass multi-

FREE-BODY

of force; and likewise weight, a variety of force. The opposite of a vector is a scalar.

plied by acceleration. DIAGRAM:

A sketch

VECTOR SUM:

A calculation, made by

showing all the outside forces acting on an

different methods according to the factor

object in equilibrium.

being analyzed—for instance, velocity or

HYPOTENUSE:

In a right triangle, the

side opposite the right angle. RESULTANT:

The sum of two or more

force—that yields the net result of all the vectors applied in a particular situation. VELOCITY:

The speed of an object in a

vectors, which measures the net change in

particular direction. Velocity is thus a vec-

distance and direction.

tor quantity.

RIGHT TRIANGLE:

138

A quantity that possesses

A triangle that

WEIGHT:

A measure of the gravitation-

includes a right (90°) angle. The other

al force on an object; the product of mass

two angles are, by definition, acute, or less

multiplied by the acceleration due to

than 90°.

gravity.

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“Equilibrium and Statics” (Web site). (March 19, 2001). “Exploratorium Snack: Center of Gravity.” The Exploratorium (Web site). (March 19, 2001). Faivre d’Arcier, Marima. What Is Balance? Illustrated by Volker Theinhardt. New York: Viking Kestrel, 1986. Taylor, Barbara. Weight and Balance. Photographs by Peter Millard. New York: F. Watts, 1990.

S C I E N C E O F E V E RY DAY T H I N G S

“Where Is Your Center of Gravity?” The K-8 Aeronautics Internet Textbook (Web site). (March 19, 2001).

Statics and Equilibrium

Wood, Robert W. Mechanics Fundamentals. Illustrated by Bill Wright. Philadelphia, PA: Chelsea House, 1997. Zubrowski, Bernie. Mobiles: Building and Experimenting with Balancing Toys. Illustrated by Roy Doty. New York: Morrow Junior Books, 1993.

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PRESSURE Pressure

CONCEPT Pressure is the ratio of force to the surface area over which it is exerted. Though solids exert pressure, the most interesting examples of pressure involve fluids—that is, gases and liquids—and in particular water and air. Pressure plays a number of important roles in daily life, among them its function in the operation of pumps and hydraulic presses. The maintenance of ordinary air pressure is essential to human health and well-being: the body is perfectly suited to the ordinary pressure of the atmosphere, and if that pressure is altered significantly, a person may experience harmful or even fatal side-effects.

HOW IT WORKS Force and Surface Area When a force is applied perpendicular to a surface area, it exerts pressure on that surface equal to the ratio of F to A, where F is the force and A the surface area. Hence, the formula for pressure (p) is p = F/A. One interesting consequence of this ratio is the fact that pressure can increase or decrease without any change in force—in other words, if the surface becomes smaller, the pressure becomes larger, and vice versa. If one cheerleader were holding another cheerleader on her shoulders, with the girl above standing on the shoulder blades of the girl below, the upper girl’s feet would exert a certain pressure on the shoulders of the lower girl. This pressure would be equal to the upper girl’s weight (F, which in this case is her mass multiplied by the downward acceleration due to gravity) divided by the surface area of her feet. Suppose, then, that

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the upper girl executes a challenging acrobatic move, bringing her left foot up to rest against her right knee, so that her right foot alone exerts the full force of her weight. Now the surface area on which the force is exerted has been reduced to half its magnitude, and thus the pressure on the lower girl’s shoulder is twice as great. For the same reason—that is, that reduction of surface area increases net pressure—a welldelivered karate chop is much more effective than an open-handed slap. If one were to slap a board squarely with one’s palm, the only likely result would be a severe stinging pain on the hand. But if instead one delivered a blow to the board, with the hand held perpendicular—provided, of course, one were an expert in karate— the board could be split in two. In the first instance, the area of force exertion is large and the net pressure to the board relatively small, whereas in the case of the karate chop, the surface area is much smaller—and hence, the pressure is much larger. Sometimes, a greater surface area is preferable. Thus, snowshoes are much more effective for walking in snow than ordinary shoes or boots. Ordinary footwear is not much larger than the surface of one’s foot, perfectly appropriate for walking on pavement or grass. But with deep snow, this relatively small surface area increases the pressure on the snow, and causes one’s feet to sink. The snowshoe, because it has a surface area significantly larger than that of a regular shoe, reduces the ratio of force to surface area and therefore, lowers the net pressure. The same principle applies with snow skis and water skis. Like a snowshoe, a ski makes it possible for the skier to stay on the surface of the

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snow, but unlike a snowshoe, a ski is long and thin, thus enabling the skier to glide more effectively down a snow-covered hill. As for skiing on water, people who are experienced at this sport can ski barefoot, but it is tricky. Most beginners require water skis, which once again reduce the net pressure exerted by the skier’s weight on the surface of the water.

Pressure

Measuring Pressure Pressure is measured by a number of units in the English and metric—or, as it is called in the scientific community, SI—systems. Because p = F/A, all units of pressure represent some ratio of force to surface area. The principle SI unit is called a pascal (Pa), or 1 N/m2. A newton (N), the SI unit of force, is equal to the force required to accelerate 1 kilogram of mass at a rate of 1 meter per second squared. Thus, a Pascal is equal to the pressure of 1 newton over a surface area of 1 square meter. In the English or British system, pressure is measured in terms of pounds per square inch, abbreviated as lbs./in2. This is equal to 6.89 • 103 Pa, or 6,890 Pa. Scientists—even those in the United States, where the British system of units prevails—prefer to use SI units. However, the British unit of pressure is a familiar part of an American driver’s daily life, because tire pressure in the United States is usually reckoned in terms of pounds per square inch. (The recommended tire pressure for a mid-sized car is typically 30-35 lb/in2.) Another important measure of pressure is the atmosphere (atm), which the average pressure exerted by air at sea level. In English units, this is equal to 14.7 lbs./in2, and in SI units to 1.013 • 105 Pa—that is, 101,300 Pa. There are also two other specialized units of pressure measurement in the SI system: the bar, equal to 105 Pa, and the torr, equal to 133 Pa. Meteorologists, scientists who study weather patterns, use the millibar (mb), which, as its name implies, is equal to 0.001 bars. At sea level, atmospheric pressure is approximately 1,013 mb. T H E B A R O M E T E R. The torr, once known as the “millimeter of mercury,” is equal to the pressure required to raise a column of mercury (chemical symbol Hg) 1 mm. It is named for the Italian physicist Evangelista Torricelli (1608-1647), who invented the barometer, an instrument for measuring atmospheric pressure.

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IN

THE INSTANCE OF ONE CHEERLEADER STANDING ON

ANOTHER’S EXERT

SHOULDERS,

DOWNWARD

SHOULDERS.

THE

THE

PRESSURE

CHEERLEADER’S ON

HER

FEET

PARTNER’S

PRESSURE IS EQUAL TO THE GIRL’S

WEIGHT DIVIDED BY THE SURFACE AREA OF HER FEET.

(Photograph by James L. Amos/Corbis. Reproduced by permission.)

The barometer, constructed by Torricelli in 1643, consisted of a long glass tube filled with mercury. The tube was open at one end, and turned upside down into a dish containing more mercury: hence, the open end was submerged in mercury while the closed end at the top constituted a vacuum—that is, an area in which the pressure is much lower than 1 atm. The pressure of the surrounding air pushed down on the surface of the mercury in the bowl, while the vacuum at the top of the tube provided an area of virtually no pressure, into which the mercury could rise. Thus, the height to which the mercury rose in the glass tube represented normal air pressure (that is, 1 atm.) Torricelli discovered that at standard atmospheric pressure, the column of mercury rose to 760 millimeters. The value of 1 atm was thus established as equal to the pressure exerted on a column of mercury 760 mm high at a temperature of 0°C (32°F). Furthermore, Torricelli’s invention eventually became a fixture both of scientific labora-

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both cases, the force is always perpendicular to the walls.

Pressure

In each of these three characteristics, it is assumed that the container is finite: in other words, the fluid has nowhere else to go. Hence, the second statement: the external pressure exerted on the fluid is transmitted uniformly. Note that the preceding statement was qualified by the term “external”: the fluid itself exerts pressure whose force component is equal to its weight. Therefore, the fluid on the bottom has much greater pressure than the fluid on the top, due to the weight of the fluid above it.

THE

AIR PRESSURE ON TOP OF

MOUNT EVEREST,

THE

WORLD’S TALLEST PEAK, IS VERY LOW, MAKING BREATHING DIFFICULT.

MOST CLIMBERS WHO ATTEMPT TO SCALE EVEREST THUS CARRY OXYGEN TANKS WITH THEM. SHOWN HERE IS JIM WHITTAKER, THE FIRST AMERICAN TO CLIMB EVEREST. (Photograph by Galen Rowell/Corbis. Reproduced by permission.)

tories and of households. Since changes in atmospheric pressure have an effect on weather patterns, many home indoor-outdoor thermometers today also include a barometer.

Pressure and Fluids In terms of physics, both gases and liquids are referred to as fluids—that is, substances that conform to the shape of their container. Air pressure and water pressure are thus specific subjects under the larger heading of “fluid pressure.” A fluid responds to pressure quite differently than a solid does. The density of a solid makes it resistant to small applications of pressure, but if the pressure increases, it experiences tension and, ultimately, deformation. In the case of a fluid, however, stress causes it to flow rather than to deform.

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Third, the pressure on any small surface of the fluid is the same, regardless of that surface’s orientation. In other words, an area of fluid perpendicular to the container walls experiences the same pressure as one parallel or at an angle to the walls. This may seem to contradict the first principle, that the force is perpendicular to the walls of the container. In fact, force is a vector quantity, meaning that it has both magnitude and direction, whereas pressure is a scalar, meaning that it has magnitude but no specific direction.

REAL-LIFE A P P L I C AT I O N S Pascal’s Principle and the Hydraulic Press The three characteristics of fluid pressure described above have a number of implications and applications, among them, what is known as Pascal’s principle. Like the SI unit of pressure, Pascal’s principle is named after Blaise Pascal (1623-1662), a French mathematician and physicist who formulated the second of the three statements: that the external pressure applied on a fluid is transmitted uniformly throughout the entire body of that fluid. Pascal’s principle became the basis for one of the important machines ever developed, the hydraulic press.

There are three significant characteristics of the pressure exerted on fluids by a container. First of all, a fluid in a container experiencing no external motion exerts a force perpendicular to the walls of the container. Likewise, the container walls exert a force on the fluid, and in

A simple hydraulic press of the variety used to raise a car in an auto shop typically consists of two large cylinders side by side. Each cylinder contains a piston, and the cylinders are connected at the bottom by a channel containing fluid. Valves control flow between the two cylinders. When one applies force by pressing down the piston in one cylinder (the input cylinder), this yields a uniform pressure that causes output in

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the second cylinder, pushing up a piston that raises the car. In accordance with Pascal’s principle, the pressure throughout the hydraulic press is the same, and will always be equal to the ratio between force and pressure. As long as that ratio is the same, the values of F and A may vary. In the case of an auto-shop car jack, the input cylinder has a relatively small surface area, and thus, the amount of force that must be applied is relatively small as well. The output cylinder has a relatively large surface area, and therefore, exerts a relatively large force to lift the car. This, combined with the height differential between the two cylinders (discussed in the context of mechanical advantage elsewhere in this book), makes it possible to lift a heavy automobile with a relatively small amount of effort. T H E H Y D RAU L I C RA M . The car

jack is a simple model of the hydraulic press in operation, but in fact, Pascal’s principle has many more applications. Among these is the hydraulic ram, used in machines ranging from bulldozers to the hydraulic lifts used by firefighters and utility workers to reach heights. In a hydraulic ram, however, the characteristics of the input and output cylinders are reversed from those of a car jack. The input cylinder, called the master cylinder, has a large surface area, whereas the output cylinder (called the slave cylinder) has a small surface area. In addition—though again, this is a factor related to mechanical advantage rather than pressure, per se—the master cylinder is short, whereas the slave cylinder is tall. Owing to the larger surface area of the master cylinder compared to that of the slave cylinder, the hydraulic ram is not considered efficient in terms of mechanical advantage: in other words, the force input is much greater than the force output. Nonetheless, the hydraulic ram is as wellsuited to its purpose as a car jack. Whereas the jack is made for lifting a heavy automobile through a short vertical distance, the hydraulic ram carries a much lighter cargo (usually just one person) through a much greater vertical range— to the top of a tree or building, for instance.

Exploiting Pressure Differences

tainer, a pump allows the fluid to escape. Specifically, the pump utilizes a pressure difference, causing the fluid to move from an area of higher pressure to one of lower pressure. A very simple example of this is a siphon hose, used to draw petroleum from a car’s gas tank. Sucking on one end of the hose creates an area of low pressure compared to the relatively high-pressure area of the gas tank. Eventually, the gasoline will come out of the low-pressure end of the hose. (And with luck, the person siphoning will be able to anticipate this, so that he does not get a mouthful of gasoline!)

Pressure

The piston pump, more complex, but still fairly basic, consists of a vertical cylinder along which a piston rises and falls. Near the bottom of the cylinder are two valves, an inlet valve through which fluid flows into the cylinder, and an outlet valve through which fluid flows out of it. On the suction stroke, as the piston moves upward, the inlet valve opens and allows fluid to enter the cylinder. On the downstroke, the inlet valve closes while the outlet valve opens, and the pressure provided by the piston on the fluid forces it through the outlet valve. One of the most obvious applications of the piston pump is in the engine of an automobile. In this case, of course, the fluid being pumped is gasoline, which pushes the pistons by providing a series of controlled explosions created by the spark plug’s ignition of the gas. In another variety of piston pump—the kind used to inflate a basketball or a bicycle tire—air is the fluid being pumped. Then there is a pump for water, which pumps drinking water from the ground It may also be used to remove desirable water from an area where it is a hindrance, for instance, in the bottom of a boat. BERNOULLI’S

PRINCIPLE.

Though Pascal provided valuable understanding with regard to the use of pressure for performing work, the thinker who first formulated general principles regarding the relationship between fluids and pressure was the Swiss mathematician and physicist Daniel Bernoulli (1700-1782). Bernoulli is considered the father of fluid mechanics, the study of the behavior of gases and liquids at rest and in motion.

PU M P S . A pump utilizes Pascal’s principle, but instead of holding fluid in a single con-

While conducting experiments with liquids, Bernoulli observed that when the diameter of a pipe is reduced, the water flows faster. This suggested to him that some force must be acting

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upon the water, a force that he reasoned must arise from differences in pressure. Specifically, the slower-moving fluid in the wider area of pipe had a greater pressure than the portion of the fluid moving through the narrower part of the pipe. As a result, he concluded that pressure and velocity are inversely related—in other words, as one increases, the other decreases. Hence, he formulated Bernoulli’s principle, which states that for all changes in movement, the sum of static and dynamic pressure in a fluid remain the same. A fluid at rest exerts static pressure, which is commonly meant by “pressure,” as in “water pressure.” As the fluid begins to move, however, a portion of the static pressure—proportional to the speed of the fluid—is converted to what is known as dynamic pressure, or the pressure of movement. In a cylindrical pipe, static pressure is exerted perpendicular to the surface of the container, whereas dynamic pressure is parallel to it. According to Bernoulli’s principle, the greater the velocity of flow in a fluid, the greater the dynamic pressure and the less the static pressure: in other words, slower-moving fluid exerts greater pressure than faster-moving fluid. The discovery of this principle ultimately made possible the development of the airplane. As fluid moves from a wider pipe to a narrower one, the volume of that fluid that moves a given distance in a given time period does not change. But since the width of the narrower pipe is smaller, the fluid must move faster (that is, with greater dynamic pressure) in order to move the same amount of fluid the same distance in the same amount of time. One way to illustrate this is to observe the behavior of a river: in a wide, unconstricted region, it flows slowly, but if its flow is narrowed by canyon walls, then it speeds up dramatically. Bernoulli’s principle ultimately became the basis for the airfoil, the design of an airplane’s wing when seen from the end. An airfoil is shaped like an asymmetrical teardrop laid on its side, with the “fat” end toward the airflow. As air hits the front of the airfoil, the airstream divides, part of it passing over the wing and part passing under. The upper surface of the airfoil is curved, however, whereas the lower surface is much straighter.

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under the wing. Since fluids have a tendency to compensate for all objects with which they come into contact, the air at the top will flow faster to meet with air at the bottom at the rear end of the wing. Faster airflow, as demonstrated by Bernoulli, indicates lower pressure, meaning that the pressure on the bottom of the wing keeps the airplane aloft.

Buoyancy and Pressure One hundred and twenty years before the first successful airplane flight by the Wright brothers in 1903, another pair of brothers—the Montgolfiers of France—developed another means of flight. This was the balloon, which relied on an entirely different principle to get off the ground: buoyancy, or the tendency of an object immersed in a fluid to float. As with Bernoulli’s principle, however, the concept of buoyancy is related to pressure. In the third century B.C., the Greek mathematician, physicist, and inventor Archimedes (c. 287-212 B.C.) discovered what came to be known as Archimedes’s principle, which holds that the buoyant force of an object immersed in fluid is equal to the weight of the fluid displaced by the object. This is the reason why ships float: because the buoyant, or lifting, force of them is less than equal to the weight of the water they displace. The hull of a ship is designed to displace or move a quantity of water whose weight is greater than that of the vessel itself. The weight of the displaced water—that is, its mass multiplied by the downward acceleration caused by gravity—is equal to the buoyant force that the ocean exerts on the ship. If the ship weighs less than the water it displaces, it will float; but if it weighs more, it will sink. The factors involved in Archimedes’s principle depend on density, gravity, and depth rather than pressure. However, the greater the depth within a fluid, the greater the pressure that pushes against an object immersed in the fluid. Moreover, the overall pressure at a given depth in a fluid is related in part to both density and gravity, components of buoyant force.

As a result, the air flowing over the top has a greater distance to cover than the air flowing

The pressure that a fluid exerts on the bottom of its container is equal to dgh, where d is density, g the acceleration due to gravity, and h the depth of the container. For any portion of the fluid, h is equal to its depth within the container, meaning that

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P R E SS U R E

AND

DEPTH.

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Pressure

THIS

YELLOW DIVING SUIT, CALLED A

“NEWT

SUIT,” IS SPECIALLY DESIGNED TO WITHSTAND THE ENORMOUS WATER

PRESSURE THAT EXISTS AT LOWER DEPTHS OF THE OCEAN. (Photograph by Amos Nachoum/Corbis. Reproduced by permission.)

the deeper one goes, the greater the pressure. Furthermore, the total pressure within the fluid is equal to dgh + pexternal, where pexternal is the pressure exerted on the surface of the fluid. In a piston-and-cylinder assembly, this pressure comes from the piston, but in water, the pressure comes from the atmosphere. In this context, the ocean may be viewed as a type of “container.” At its surface, the air exerts downward pressure equal to 1 atm. The density of the water itself is uniform, as is the downward acceleration due to gravity; the only variable, then, is h, or the distance below the surface. At the deepest reaches of the ocean, the pressure is incredibly great—far more than any human being could endure. This vast amount of pressure pushes upward, resisting the downward pressure of objects on its surface. At the same time, if a boat’s weight is dispersed properly along its hull, the ship maximizes area and minimizes force, thus exerting a downward pressure on the surface of the water that is less than the upward pressure of the water itself. Hence, it floats.

water, but to float in the sky with a craft lighter than air. The particulars of this achievement are discussed elsewhere, in the context of buoyancy; but the topic of lighter-than-air flight suggests another concept that has been alluded to several times throughout this essay: air pressure. Just as water pressure is greatest at the bottom of the ocean, air pressure is greatest at the surface of the Earth—which, in fact, is at the bottom of an “ocean” of air. Both air and water pressure are examples of hydrostatic pressure—the pressure that exists at any place in a body of fluid due to the weight of the fluid above. In the case of air pressure, air is pulled downward by the force of Earth’s gravitation, and air along the surface has greater pressure due to the weight (a function of gravity) of the air above it. At great heights above Earth’s surface, however, the gravitational force is diminished, and, thus, the air pressure is much smaller.

A I R P R E SS U R E . The Montgolfiers used the principle of buoyancy not to float on the

In ordinary experience, a person’s body is subjected to an impressive amount of pressure. Given the value of atmospheric pressure discussed earlier, if one holds out one’s hand—assuming that the surface is about 20 in2 (0.129 m2)—the force of the air resting on it is nearly 300 lb (136 kg)! How is it, then, that one’s

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THE RESPONSE TO CHANGES I N A I R P R E SS U R E . The human body is,

Pressure

KEY TERMS ATMOSPHERE: A measure of pressure, abbreviated “atm” and equal to the average pressure exerted by air at sea level. In English units, this is equal to 14.7 pounds per square inch, and in SI units to 101,300 pascals.

An instrument measuring atmospheric pressure. BAROMETER:

for

BUOYANCY: The tendency of an object immersed in a fluid to float.

Any substance, whether gas or liquid, that conforms to the shape of its container. FLUID:

The study of the behavior of gases and liquids at rest and in motion. FLUID MECHANICS:

HYDROSTATIC PRESSURE: the pressure that exists at any place in a body of fluid due to the weight of the fluid above.

The principle SI or metric unit of pressure, abbreviated “Pa” and equal to 1 N/m2. PASCAL:

A statement, formulated by French mathematician and physicist Blaise Pascal (1623-1662), which holds that the external pressure applied on a fluid is transmitted uniformly throughout the entire body of that fluid. PASCAL’S PRINCIPLE:

PRESSURE: The ratio of force to surface area, when force is applied in a direction perpendicular to that surface. The formula for pressure (p) is p = F/A, where F is force and A the surface area.

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in fact, suited to the normal air pressure of 1 atm, and if that external pressure is altered, the body undergoes changes that may be harmful or even fatal. A minor example of this is the “popping” in the ears that occurs when one drives through the mountains or rides in an airplane. With changes in altitude come changes in pressure, and thus, the pressure in the ears changes as well. As noted earlier, at higher altitudes, the air pressure is diminished, which makes it harder to breathe. Because air is a gas, its molecules have a tendency to be non-attractive: in other words, when the pressure is low, they tend to move away from one another, and the result is that a person at a high altitude has difficulty getting enough air into his or her lungs. Runners competing in the 1968 Olympics at Mexico City, a town in the mountains, had to train in high-altitude environments so that they would be able to breathe during competition. For baseball teams competing in Denver, Colorado (known as “the Mile-High City”), this disadvantage in breathing is compensated by the fact that lowered pressure and resistance allows a baseball to move more easily through the air. If a person is raised in such a high-altitude environment, of course, he or she becomes used to breathing under low air pressure conditions. In the Peruvian Andes, for instance, people spend their whole lives at a height more than twice as great as that of Denver, but a person from a lowaltitude area should visit such a locale only after taking precautions. At extremely great heights, of course, no human can breathe: hence airplane cabins are pressurized. Most planes are equipped with oxygen masks, which fall from the ceiling if the interior of the cabin experiences a pressure drop. Without these masks, everyone in the cabin would die.

hand is not crushed by all this weight? The reason is that the human body itself is under pressure, and that the interior of the body exerts a pressure equal to that of the air.

B LO O D P R E SS U R E . Another aspect of pressure and the human body is blood pressure. Just as 20/20 vision is ideal, doctors recommend a target blood pressure of “120 over 80”—but what does that mean? When a person’s blood pressure is measured, an inflatable cuff is wrapped around the upper arm at the same level as the heart. At the same time, a stethoscope is placed along an artery in the lower arm to monitor the sound of the blood flow. The cuff is inflated to stop the blood flow, then the pressure

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is released until the blood just begins flowing again, producing a gurgling sound in the stethoscope. The pressure required to stop the blood flow is known as the systolic pressure, which is equal to the maximum pressure produced by the heart. After the pressure on the cuff is reduced until the blood begins flowing normally—which is reflected by the cessation of the gurgling sound in the stethoscope—the pressure of the artery is measured again. This is the diastolic pressure, or the pressure that exists within the artery between strokes of the heart. For a healthy person, systolic pressure should be 120 torr, and diastolic pressure 80 torr.

Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. “Blood Pressure” (Web site). (April 7, 2001). Clark, John Owen Edward. The Atmosphere. New York: Gloucester Press, 1992. Cobb, Allan B. Super Science Projects About Oceans. New York: Rosen, 2000. “The Physics of Underwater Diving: Pressure Lesson” (Web site). (April 7, 2001). Provenzo, Eugene F. and Asterie Baker Provenzo. 47 Easy-to-Do Classic Experiments. Illustrations by Peter A. Zorn, Jr. New York: Dover Publications, 1989.

WHERE TO LEARN MORE

“Understanding Air Pressure” USA Today (Web site). (April 7, 2001).

“Atmospheric Pressure: The Force Exerted by the Weight of Air” (Web site). (April 7, 2001).

Zubrowski, Bernie. Balloons: Building and Experimenting with Inflatable Toys. Illustrated by Roy Doty. New York: Morrow Junior Books, 1990.

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ELASTICITY Elasticity

CONCEPT Unlike fluids, solids do not respond to outside force by flowing or easily compressing. The term elasticity refers to the manner in which solids respond to stress, or the application of force over a given unit area. An understanding of elasticity—a concept that carries with it a rather extensive vocabulary of key terms—helps to illuminate the properties of objects from steel bars to rubber bands to human bones.

HOW IT WORKS Characteristics of a Solid A number of parameters distinguish solids from fluids, a term that in physics includes both gases and liquids. Solids possess a definite volume and a definite shape, whereas gases have neither; liquids have no definite shape. At the molecular level, particles of solids tend to be precise in their arrangement and close to one another. Liquid molecules are close in proximity (though not as much so as solid molecules), and their arrangement is random, while gas molecules are both random in arrangement and far removed in proximity. Gas molecules are extremely fast-moving, and exert little or no attraction toward one another. Liquid molecules move at moderate speeds and exert a moderate attraction, but solid particles are slow-moving, and have a strong attraction to one another.

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nal pressure uniformly. If one applies pressure to a quantity of water in a closed container, the pressure is equal everywhere in the water. By contrast, if one places a champagne glass upright in a vise and applies pressure until it breaks, chances are that the stem or the base of the glass will be unaffected, because the pressure is not distributed equally throughout the glass. If the surface of a solid is disturbed, it will resist, and if the force of the disturbance is sufficiently strong, it will deform—for instance, when a steel plate begins to bend under pressure. This deformation will be permanent if the force is powerful enough, as in the above example of the glass in a vise. By contrast, when the surface of a fluid is disturbed, it tends to flow.

Types of Stress Deformation occurs as a result of stress, whether that stress be in the form of tension, compression, or shear. Tension occurs when equal and opposite forces are exerted along the ends of an object. These operate on the same line of action, but away from each other, thus stretching the object. A perfect example of an object under tension is a rope in the middle of a tug-of-war competition. The adjectival form of “tension” is “tensile”: hence the term “tensile stress,” which will be discussed later.

One of several factors that distinguishes solids from fluids is their relative response to pressure. Gases tend to be highly compressible, meaning that they respond well to pressure. Liquids tend to be noncompressible, yet because of their fluid characteristics, they experience exter-

Earlier, stress was defined as the application of force over a given unit area, and in fact, the formula for stress can be written as F/A, where F is force and A area. This is also the formula for pressure, though in order for an object to be under pressure, the force must be applied in a direction perpendicular to—and in the same direction as—its surface. The one form of stress

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that clearly matches these parameters is compression, produced by the action of equal and opposite forces, whose effect is to reduce the length of a material. Thus compression (for example, crushing an aluminum can in one’s hand) is both a form of stress and a form of pressure.

dimension, and k, a constant whose value is related to the nature and size of the object under stress. The harder the material, the higher the value of k; furthermore, the value of k is directly proportional to the object’s cross-sectional area or thickness.

Note that compression was defined as reducing length, yet the example given involved a reduction in what most people would call the “width” or diameter of the aluminum can. In fact, width and height are the same as length, for the purposes of most discussions in physics. Length is, along with time, mass, and electric current, one of the fundamental units of measure used to express virtually all other physical quantities. Width and height are simply length expressed in terms of other planes, and within the subject of elasticity, it is not important to distinguish between these varieties of length. (By contrast, when discussing gravitational attraction—which is always vertical—it is obviously necessary to distinguish between “vertical length,” or height, and horizontal length.)

The elastic limit of a given solid is the maximum stress to which it can be subjected without experiencing permanent deformation. Elastic limit will be discussed in the context of several examples below; for now, it is important merely to know that Hooke’s law is applicable only as long as the material in question has not reached its elastic limit. The same is true for any modulus of elasticity, or the ratio between a particular type of applied stress and the strain that results. (The term “modulus,” whose plural is “moduli,” is Latin for “small measure.”)

The third variety of stress is shear, which occurs when a solid is subjected to equal and opposite forces that do not act along the same line, and which are parallel to the surface area of the object. If a thick hardbound book is lying flat, and a person places a finger on the spine and pushes the front cover away from the spine so that the covers and pages no longer constitute parallel planes, this is an example of shear. Stress resulting from shear is called shearing stress.

Hooke’s Law and Elastic Limit To sum up the three varieties of stress, tension stretches an object, compression shrinks it, and shear twists it. In each case, the object is deformed to some degree. This deformation is expressed in terms of strain, or the ratio between change in dimension and the original dimensions of the object. The formula for strain is δL/Lo, where δL is the change in length (δ, the Greek letter delta, means “change” in scientific notation) and Lo the original length.

Elasticity

Moduli of Elasticity In cases of tension or compression, the modulus of elasticity is Young’s modulus. Named after English physicist Thomas Young (1773-1829), Young’s modulus is simply the ratio between F/A and δL/Lo—in other words, stress divided by strain. There are also moduli describing the behavior of objects exposed to shearing stress (shear modulus), and of objects exposed to compressive stress from all sides (bulk modulus). Shear modulus is the relationship of shearing stress to shearing strain. This can be expressed as the ratio between F/A and φ. The latter symbol, the Greek letter phi, stands for the angle of shear—that is, the angle of deformation along the sides of an object exposed to shearing stress. The greater the amount of surface area A, the less that surface will be displaced by the force F. On the other hand, the greater the amount of force in proportion to A, the greater the value of φ, which measures the strain of an object exposed to shearing stress. (The value of φ, however, will usually be well below 90°, and certainly cannot exceed that magnitude.)

Hooke’s law, formulated by English physicist Robert Hooke (1635-1703), relates strain to stress. Hooke’s law can be stated in simple terms as “the strain is proportional to the stress,” and can also be expressed in a formula, F = ks, where F is the applied force, s, the resulting change in

With tensile and compressive stress, A is a surface perpendicular to the direction of applied force, but with shearing stress, A is parallel to F. Consider again the illustration used above, of a thick hardbound book lying flat. As noted, when one pushes the front cover from the side so that the covers and pages no longer constitute parallel planes, this is an example of shear. If one pulled the spine and the long end of the pages away

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Elasticity

THE

MACHINE PICTURED HERE ROLLS OVER STEEL IN ORDER TO BEND IT INTO PIPES.

BECAUSE

OF ITS ELASTIC

NATURE, STEEL CAN BE BENT WITHOUT BREAKING. (Photograph by Vince Streano/Corbis. Reproduced by permission.)

from one another, that would be tensile stress, whereas if one pushed in on the sides of the pages and spine, that would be compressive stress. Shearing stress, by contrast, would stress only the front cover, which is analogous to A for any object under shearing stress. The third type of elastic modulus is bulk modulus, which occurs when an object is subjected to compression from all sides—that is, volume stress. Bulk modulus is the relationship of volume stress to volume strain, expressed as the ratio between F/A and δV/Vo, where δV is the change in volume and Vo is the original volume.

REAL-LIFE A P P L I C AT I O N S Elastic and Plastic Deformation As noted earlier, the elastic limit is the maximum stress to which a given solid can be subjected without experiencing permanent deformation, referred to as plastic deformation. Plastic deformation describes a permanent change in shape or size as a result of stress; by contrast, elastic deformation is only a temporary change in dimension.

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A classic example of elastic deformation, and indeed, of highly elastic behavior, is a rubber band: it can be deformed to a length many times its original size, but upon release, it returns to its original shape. Examples of plastic deformation, on the other hand, include the bending of a steel rod under tension or the breaking of a glass under compression. Note that in the case of the steel rod, the object is deformed without rupturing—that is, without breaking or reducing to pieces. The breaking of the glass, however, is obviously an instance of rupturing.

Metals and Elasticity Metals, in fact, exhibit a number of interesting characteristics with regard to elasticity. With the notable exception of cast iron, metals tend to possess a high degree of ductility, or the ability to be deformed beyond their elastic limits without experiencing rupture. Up to a certain point, the ratio of tension to elongation for metals is high: in other words, a high amount of tension produces only a small amount of elongation. Beyond the elastic limit, however, the ratio is much lower: that is, a relatively small amount of tension produces a high degree of elongation. Because of their ductility, metals are highly malleable, and, therefore, capable of experienc-

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Elasticity

RUBBER BANDS, LIKE THE ONES SHOWN HERE FORMED INTO MATION. (Photograph by Matthew Klein/Corbis. Reproduced by permission.)

ing mechanical deformation through metallurgical processes, such as forging, rolling, and extrusion. Cold extrusion involves the application of high pressure—that is, a high bulk modulus—to a metal without heating it, and is used on materials such as tin, zinc, and copper to change their shape. Hot extrusion, on the other hand, involves heating a metal to a point of extremely high malleability, and then reshaping it. Metals may also be melted for the purposes of casting, or pouring the molten material into a mold. U LT I M AT E S T R E N GT H . The ten-

sion that a material can withstand is called its ultimate strength, and due to their ductile properties, most metals possess a high value of ultimate strength. It is possible, however, for a metal to break down due to repeated cycles of stress that are well below the level necessary to rupture it. This occurs, for instance, in metal machines such as automobile engines that experience a high frequency of stress cycles during operation. The high ultimate strength of metals, both in tension and compression, makes them useful in a number of structural capacities. Steel has an ultimate compressive strength 25 times as great as concrete, and an ultimate tensile strength 250 times as great. For this reason, when concrete is poured for building bridges or other large struc-

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A BALL, ARE A CLASSIC EXAMPLE OF ELASTIC DEFOR-

tures, steel rods are inserted in the concrete. Called “rebar” (for “reinforced bars”), the steel rods have ridges along them in order to bond more firmly with the concrete as it dries. As a result, reinforced concrete has a much greater ability than plain concrete to withstand tension and compression.

Steel Bars and Rubber Bands Under Stress CRYSTALLINE MATERIALS. Metals are crystalline materials, meaning that they are composed of solids called crystals. Particles of crystals are highly ordered, with a definite geometric arrangement repeated in all directions, rather like a honeycomb. (It should be noted, however, that the crystals are not necessarily as uniform in size as the “cells” of the honeycomb.) The atoms of a crystal are arranged in orderly rows, bound to one another by strongly attractive forces that act like microscopic springs.

Just as a spring tends to return to its original length, the highly attractive atoms in a steel bar, when it is stretched, tend to restore it to its original dimensions. Likewise, it takes a great deal of force to pull apart the atoms. When the metal is subjected to plastic deformation, the atoms move

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are flexible, and tend to rotate, producing kinks along the length of the molecule.

Elasticity

When a piece of rubber is subjected to tension, as, for instance, if one pulls a rubber band by the ends, the kinks and loops in the elastomers straighten. Once the stress is released, however, the elastomers immediately return to their original shape. The more “kinky” the polymers, the higher the elastic modulus, and hence, the more capable the item is of stretching and rebounding. It is interesting to note that steel and rubber, materials that are obviously quite different, are both useful in part for the same reason: their high elastic modulus when subjected to tension, and their strength under stress. But a rubber band exhibits behaviors under high temperatures that are quite different from that of a metal: when heated, rubber contracts. It does so quite suddenly, in fact, suggesting that the added energy of the heat allows the bonds in the elastomers to begin rotating again, thus restoring the kinked shape of the molecules. A

HUMAN BONE HAS A GREATER

“ULTIMATE

STRENGTH”

THAN THAT OF CONCRETE. (Ecoscene/Corbis. Reproduced by per-

Bones

mission.)

to new positions and form new bonds. The atoms are incapable of forming bonds; however, when the metal has been subjected to stress exceeding its ultimate strength, at that point, the metal breaks.

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The tensile strength in bone fibers comes from the protein collagen, while the compressive strength is largely due to the presence of inorganic (non-living) salt crystals. It may be hard to believe, but bone actually has an ultimate strength—both in tension and compression— greater than that of concrete!

The crystalline structure of metal influences its behavior under high temperatures. Heat causes atoms to vibrate, and in the case of metals, this means that the “springs” are stretching and compressing. As temperature increases, so do the vibrations, thus increasing the average distance between atoms. For this reason, under extremely high temperature, the elastic modulus of the metal decreases, and the metal becomes less resistant to stress.

The ultimate strength of most materials is rendered in factors of 108 N/m2—that is, 100,000,000 newtons (the metric unit of force) per square meter. For concrete under tensile stress, the ultimate strength is 0.02, whereas for bone, it is 1.3. Under compressive stress, the values are 0.2 and 1.7, respectively. In fact, the ultimate tensile strength of bone is close to that of cast iron (1.7), though the ultimate compressive strength of cast iron (5.5) is much higher than for bone.

P O LY M E R S A N D E L A S T O M E R S . Rubber is so elastic in behavior that in

everyday life, the term “elastic” is most often used for objects containing rubber: the waistband on a pair of underwear, for instance. The long, thin molecules of rubber, which are arranged side-byside, are called “polymers,” and the super-elastic polymers in rubber are called “elastomers.” The chemical bonds between the atoms in a polymer

Even with these figures, it may be hard to understand how bone can be stronger than concrete, but that is largely because the volume of concrete used in most situations is much greater than the volume of any bone in the body of a human being. By way of explanation, consider a piece of concrete no bigger than a typical bone: under relatively small amounts of stress, it would crumble.

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Elasticity

KEY TERMS ANGLE OF SHEAR:

The angle of

HOOKE’S LAW:

A principle of elastic-

deformation on the sides of an object

ity formulated by English physicist Robert

exposed to shearing stress. Its symbol is f

Hooke (1635-1703), who discovered that

(the Greek letter phi), and its value will

strain is proportional to stress. Hooke’s law

usually be well below 90°.

can be written as a formula, F = ks, where

The modulus of

BULK MODULUS:

elasticity for a material subjected to compression on all surfaces—that is, volume stress. Bulk modulus is the relationship of volume stress to volume strain, expressed as the ratio between F/A and dV/Vo, where

F is the applied force, s the resulting change in dimension, and k a constant whose value is related to the nature and size of the object being subjected to stress. Hooke’s law applies only when the elastic limit has

dV is the change in volume and Vo is the

not been exceeded.

original volume.

LENGTH:

In discussions of elasticity,

A form of stress

“length” refers to an object’s dimensions on

produced by the action of equal and oppo-

any given plane, thus, it can be used not

site forces, whose effect is to reduce the

only to refer to what is called length in

length of a material. Compression is a form

everyday language, but also to width or

COMPRESSION:

of pressure. When compressive stress is applied to all surfaces of a material, this is

MODULUS OF ELASTICITY:

known as volume stress. DUCTILITY:

height.

A property whereby a

The

ratio between a type of applied stress (that

material is capable of being deformed far

is, tension, compression, and shear) and

beyond its elastic limit without experienc-

the strain that results in the object to which

ing rupture—that is, without breaking.

stress has been applied. Elastic moduli—

Most metals other than cast iron are high-

including Young’s modulus, shearing mod-

ly ductile.

ulus, and bulk modulus—are applicable

ELASTIC DEFORMATION:

A tempo-

rary change in shape or size experienced by

only as long as the object’s elastic limit has not been reached.

a solid subjected to stress. Elastic deformation is thus less severe than plastic deformation.

PLASTIC DEFORMATION:

A perma-

nent change in shape or size experienced by a solid subjected to stress. Plastic defor-

ELASTIC LIMIT:

The maximum stress

to which a given solid can be subjected without experiencing plastic deforma-

mation is thus more severe than elastic deformation. The ratio of force to sur-

tion—that is, without being permanently

PRESSURE:

deformed.

face area, when force is applied in a direc-

ELASTICITY:

The response of solids to

stress.

S C I E N C E O F E V E RY DAY T H I N G S

tion perpendicular to, and in the same direction as, that surface.

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Elasticity

KEY TERMS A form of stress resulting

area. Thus, it is similar to pressure, and

from equal and opposite forces that do not

indeed, compression is a form of pressure.

SHEAR:

act along the same line. If a thick hard-

TENSION:

A form of stress produced

bound book is lying flat, and one pushes

by a force which acts to stretch a material.

the front cover from the side so that the

The adjectival form of “tension” is “ten-

covers and pages no longer constitute par-

sile”: hence the terms “tensile stress” and

allel planes, this is an example of shear.

“tensile strain.”

SHEAR MODULUS:

The modulus of

ULTIMATE STRENGTH:

The tension

elasticity for an object exposed to shearing

that a material can withstand without rup-

stress. It is expressed as the ratio between

turing. Due to their high levels of ductility,

F/A and φ, where φ (the Greek letter phi)

most metals have a high value of ultimate

stands for the angle of shear.

strength.

STRAIN:

The ratio between the change

VOLUME STRESS:

The stress that

in dimension experienced by an object that

occurs in a material when it is subjected to

has been subjected to stress, and the origi-

compression from all sides. The modus of

nal dimensions of the object. The formula

elasticity for volume stress is the bulk mod-

for strain is dL/Lo, where dL is the change

ulus.

in length and Lo the original length. Hooke’s law, as well as the various moduli of elasticity, relates strain to stress.

YOUNG’S MODULUS:

A modulus of

elasticity describing the relationship between stress to strain for objects under

In general terms, stress is any

either tension or compression. Named

attempt to deform a solid. Types of stress

after English physicist Thomas Young

include tension, compression, and shear.

(1773-1829), Young’s modulus is simply

More specifically, stress is the ratio of force

the ratio between F/A and δL/Lo—in other

to unit area, F/A, where F is force and A

words, stress divided by strain.

STRESS:

WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. “Dictionary of Metallurgy” Steelmill.com: The Polish Steel Industry Directory (Web site). (April 9, 2001).

154

CONTINUED

Gibson, Gary. Making Shapes. Illustrated by Tony Kenyon. Brookfield, CT: Copper Beech Books, 1996. “Glossary of Materials Testing Terms” (Web site). (April 9, 2001). Goodwin, Peter H. Engineering Projects for Young Scientists. New York: Franklin Watts, 1987.

“Engineering Processes.” eFunda.com (Web site). (April 9, 2001).

Johnston, Tom. The Forces with You! Illustrated by Sarah Pooley. Milwaukee, WI: Gareth Stevens Publishing, 1988.

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WORK AND ENERGY M E C H A N I C A L A D VA N TA G E A N D SIMPLE MACHINES ENERGY

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MECHANICAL A D VA N TA G E A N D SIMPLE MACHINES

CONCEPT When the term machine is mentioned, most people think of complex items such as an automobile, but, in fact, a machine is any device that transmits or modifies force or torque for a specific purpose. Typically, a machine increases either the force of the person operating it—an aspect quantified in terms of mechanical advantage—or it changes the distance or direction across which that force can be operated. Even a humble screw is a machine; so too is a pulley, and so is one of the greatest machines ever invented: the wheel. Virtually all mechanical devices are variations on three basic machines: the lever, the inclined plane, and the hydraulic press. From these three, especially the first two, arose literally hundreds of machines that helped define history, and which still permeate daily life.

HOW IT WORKS

Instead, the machines presented for consideration here depend purely on gravitational force and the types of force explainable purely in a gravitational framework. This is the realm of classical physics, a term used to describe the studies of physicists from the time of Galileo Galilei (1564-1642) to the end of the nineteenth century. During this era, physicists were primarily concerned with large-scale interactions that were easily comprehended by the senses, as opposed to the atomic behaviors that have become the subject of modern physics. Late in the classical era, the Scottish physicist James Clerk Maxwell (1831-1879)—building on the work of many distinguished predecessors— identified electromagnetic force. For most of the period, however, the focus was on gravitational force and mechanics, or the study of matter, motion, and forces. Likewise, the majority of machines invented and built during most of the classical period worked according to the mechanical principles of plain gravitational force.

There are four known types of force in the universe: gravitational, electromagnetic, weak nuclear, and strong nuclear. This was the order in which the forces were identified, and the number of machines that use each force descends in the same order. The essay that follows will make little or no reference to nuclear-powered machines. Somewhat more attention will be paid to electrical machines; however, to trace in detail the development of forces in that context would require a new and somewhat cumbersome vocabulary.

This was even true to some extent with the steam engine, first developed late in the seventeenth century and brought to fruition by Scotland’s James Watt (1736-1819.) Yet the steam engine, though it involved ordinary mechanical processes in part, represented a new type of machine, which used thermal energy. This is also true of the internal-combustion engine; yet both steam- and gas-powered engines to some extent borrowed the structure of the hydraulic press, one of the three basic types of machine. Then came the development of electronic power, thanks to Thomas Edison (1847-1931) and others, and machines became increasingly divorced from basic mechanical laws.

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Mechanical Advantage Mechanical Advantage and Simple Machines

A common trait runs through all forms of machinery: mechanical advantage, or the ratio of force output to force input. In the case of the lever, a simple machine that will be discussed in detail below, mechanical advantage is high. In some machines, however, mechanical advantage is actually less than 1, meaning that the resulting force is less than the applied force. This does not necessarily mean that the machine itself has a flaw; on the contrary, it can mean that the machine has a different purpose than that of a lever. One example of this is the screw: a screw with a high mechanical advantage—that is, one that rewarded the user’s input of effort by yielding an equal or greater output— would be useless. In this case, mechanical advantage could only be achieved if the screw backed out from the hole in which it had been placed, and that is clearly not the purpose of a screw. THE

LEVER, LIKE THIS HYDROELECTRIC ENGINE LEVER,

IS A SIMPLE MACHINE THAT PERFECTLY ILLUSTRATES THE CONCEPT OF MECHANICAL ADVANTAGE. (Photograph by E.O.

Hoppe/Corbis. Reproduced by permission.)

The heyday of classical mechanics—when classical studies in mechanics represented the absolute cutting edge of experimentation—was in the period from the beginning of the seventeenth century to the beginning of the nineteenth. One figure held a dominant position in the world of physics during those two centuries, and indeed was the central figure in the history of physics between Galileo and Albert Einstein (1879-1955). This was Sir Isaac Newton (16421727), who discerned the most basic laws of physical reality—laws that govern everyday life, including the operation of simple machines. Newton and his principles are essential to the study that follows, but one other figure deserves “equal billing”: the Greek mathematician, physicist, and inventor Archimedes (c. 287212 B.C.). Nearly 2,000 years before Newton, Archimedes explained and improved a number of basic machines, most notably the lever. Describing the powers of the lever, he is said to have promised, “Give me a lever long enough and a place to stand, and I will move the world.” This he demonstrated, according to one story, by moving a fully loaded ship single-handedly with the use of a lever, while remaining seated some distance away.

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Here a machine offers an improvement in terms of direction rather than force; likewise with scissors or a fishing rod, both of which will be discussed below, an improvement with regard to distance or range of motion is bought at the expense of force. In these and many more cases, mechanical advantage alone does not measure the benefit. Thus, it is important to keep in mind what was previously stated: a machine either increases force output, or changes the force’s distance or direction of operation. Most machines, however, work best when mechanical advantage is maximized. Yet mechanical advantage—whether in theoretical terms or real-life instances—can only go so high, because there are factors that limit it. For one thing, the operator must give some kind of input to yield an output; furthermore, in most situations friction greatly diminishes output. Hence, in the operation of a car, for instance, one-quarter of the vehicle’s energy is expended simply on overcoming the resistance of frictional forces. For centuries, inventors have dreamed of creating a mechanism with an almost infinite mechanical advantage. This is the much-soughtafter “perpetual motion machine,” that would only require a certain amount of initial input; after that, the machine would simply run on its own forever. As output compounded over the years, its ratio to input would become so high that the figure for mechanical advantage would approach infinity.

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A number of factors, most notably the existence of friction, prevent the perpetual motion machine from becoming anything other than a pipe dream. In outer space, however, the nearabsence of friction makes a perpetual motion machine viable: hence, a space probe launched from Earth can travel indefinitely unless or until it enters the gravitational field of some other body in deep space. The concept of a perpetual motion machine, at least on Earth, is only an idealization; yet idealization does have its place in physics. Physicists discuss most concepts in terms of an idealized state. For instance, when illustrating the acceleration due to gravity experienced by a body in free fall, it is customary to treat such an event as though it were taking place under conditions divorced from reality. To consider the effects of friction, air resistance, and other factors on the body’s fall would create an impossibly complicated problem—yet real-world situations are just that complicated. In light of this tendency to discuss physical processes in idealized terms, it should be noted that there are two types of mechanical advantage: theoretical and actual. Efficiency, as applied to machines in its most specific scientific sense, is the ratio of actual to theoretical mechanical advantage. This in some ways resembles the formula for mechanical advantage itself: once again, what is being measured is the relationship between “output” (the real behavior of the machine) and “input” (the planned behavior of the machine).

will henceforth be used as a loose synonym for mechanical advantage—even though the technical definition is rather different.

Mechanical Advantage and Simple Machines

Types of Machines The term “simple machine” is often used to describe the labor-saving devices known to the ancient world, most of which consisted of only one or two essential parts. Historical sources vary regarding the number of simple machines, but among the items usually listed are levers, pulleys, winches, wheels and axles, inclined planes, wedges, and screws. The list, though long, can actually be reduced to just two items: levers and inclined planes. All the items listed after the lever and before the inclined plane—including the wheel and axle—are merely variations on the lever. The same goes for the wedge and the screw, with regard to the inclined plane. In fact, all machines are variants on three basic devices: the lever, the inclined plane, and the hydraulic press. Each transmits or modifies force or torque, producing an improvement in force, distance, or direction. The first two, which will receive more attention here, share several aspects not true of the third. First of all, the lever and the inclined plane originated at the beginning of civilization, whereas the hydraulic press is a much more recent invention.

As with other mechanical processes, the actual mechanical advantage of a machine is a much more complicated topic than the theoretical mechanical advantage. The gulf between the two, indeed, is enormous. It would be almost impossible to address the actual behavior of machines within an environment framework that includes complexities such as friction.

The lever appeared as early as 5000 B.C. in the form of a simple balance scale, and within a few thousand years, workers in the Near East and India were using a crane-like lever called the shaduf to lift containers of water. The shaduf, introduced in Mesopotamia in about 3000 B.C., consisted of a long wooden pole that pivoted on two upright posts. At one end of the lever was a counterweight, and at the other a bucket. The operator pushed down on the pole to fill the bucket with water, and then used the counterweight to assist in lifting the bucket.

Each real-world framework—that is, each physical event in the real world—is just a bit different from every other one, due to the many varieties of factors involved. By contrast, the idealized machines of physics problems behave exactly the same way in one imaginary situation after another, assuming outside conditions are the same. Therefore, the only form of mechanical advantage that a physicist can easily discuss is theoretical. For that reason, the term “efficiency”

The inclined plane made its appearance in the earliest days of civilization, when the Egyptians combined it with rollers in the building of their monumental structures, the pyramids. Modern archaeologists generally believe that Egyptian work gangs raised the huge stone blocks of the pyramids through the use of sloping earthen ramps. These were most probably built up alongside the pyramid itself, and then removed when the structure was completed.

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THE

HYDRAULIC PRESS, LIKE THE LIFT HOLDING UP THIS CAR IN A REPAIR SHOP, IS A RELATIVELY RECENT INVEN-

TION THAT IS USED IN MANY MODERN DEVICES, FROM CAR JACKS TO TOILETS. (Photograph by Charles E. Rotkin/Corbis. Repro-

duced by permission.)

In contrast to the distant origins of the other two machines, the hydraulic press seems like a mere youngster. Also, the first two clearly arose as practical solutions first: by the time Archimedes achieved a conceptual understanding of these machines, they had been in use a long, long time. The hydraulic press, on the other hand, first emerged from the theoretical studies of the brilliant French mathematician and physicist Blaise Pascal (1623-1662.) Nearly 150 years passed before the English inventor Joseph Bramah (1748-1814) developed a workable hydraulic press in 1796, by which time Watt had already introduced his improved steam engine. The Industrial Revolution was almost underway.

REAL-LIFE A P P L I C AT I O N S The Lever In its most basic form, the lever consists of a rigid bar supported at one point, known as the fulcrum. One of the simplest examples of a lever is a crowbar, which one might use to move a heavy object, such as a rock. In this instance, the fulcrum could be the ground, though a more rigid 160

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“artificial” fulcrum (such as a brick) would probably be more effective. As the operator of the crowbar pushes down on its long shaft, this constitutes an input of force, variously termed applied force, effort force, or merely effort. Newton’s third law of motion shows that there is no such thing as an unpaired force in the universe: every input of force in one area will yield an output somewhere else. In this case, the output is manifested by dislodging the stone—that is, the output force, resistance force, or load. Use of the lever gives the operator much greater lifting force than that available to a person who tried to lift with only the strength of his or her own body. Like all machines, the lever links input to output, harnessing effort to yield beneficial results—in this case, by translating the input effort into the output effort of a dislodged stone. Note, however, the statement at the beginning of this paragraph: proper use of a lever actually gives a person much greater force than he or she would possess unaided. How can this be? There is a close relationship between the behavior of the lever and the concept of torque, as, for example, the use of a wrench to remove a lug nut. A wrench, in fact, is a sort of lever (Class

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I—a distinction that will be explored below.) In any object experiencing torque, the distance from the pivot point (the lug nut, in this case), to the area where force is being applied is called the moment arm. On the wrench, this is the distance from the lug nut to the place where the operator is pushing on the wrench handle. Torque is the product of force multiplied by moment arm, and the greater the torque, the greater the tendency of the object to be put into rotation. As with machines in general, the greater the input, the greater the output. The fact that torque is the product of force and moment arm means that if one cannot increase force, it is still possible to gain greater torque by increasing the moment arm. This is the reason why, when one tries and fails to disengage a stubborn lug nut, it is a good idea to get a longer wrench. Likewise with a lever, greater leverage can be gained without applying more force: all one needs is a longer lever arm. As one might suspect, the lever arm is the distance from the force input to the fulcrum, or from the fulcrum to the force output. If a carpenter is using a nail-puller on the head of a hammer to extract a nail from a board, the lever arm of force input would be from the carpenter’s hand gripping the hammer handle to the place where the hammerhead rests against the board. The lever arm of force output would be from the hammerhead to the end of the nail-puller. With a lever, the input force (that of the carpenter’s hand pulling back on the hammer handle) multiplied by the input lever arm is always equal to the output force (that of the nail-puller pulling up the nail) multiplied by the output lever arm. The relationship between input and output force and lever arm then makes it possible to determine a formula for the lever’s mechanical advantage. Since FinLin = FoutLout (where F = force and L = lever arm), it is possible to set up an equation for a lever’s mechanical advantage. Once again, mechanical advantage is always Fout/Fin, but with a lever, it is also Lin/Lout. Hence, the mechanical advantage of a lever is always the same as the inverse ratio of the lever arm. If the input arm is 5 units long and the output arm is 1 unit long, the mechanical advantage will be 5, but if the positions are reversed, it will be 0.2.

tive positions of the input lever arm, the fulcrum, and the output arm or load. In a Class I lever, such as the crowbar and the wrench, the fulcrum is between the input arm and the output arm. By contrast, a Class II lever, for example, a wheelbarrow, places the output force (the load carried in the barrow itself) between the input force (the action of the operator lifting the handles) and the fulcrum, which in this case is the wheel.

Mechanical Advantage and Simple Machines

Finally, there is the Class III lever, which is the reverse of a Class II. Here, the input force is between the output force and the fulcrum. The human arm itself is an example of a Class III lever: if one grasps a weight in one’s hand, one’s bent elbow is the fulcrum, the arm raising the weight is the input force, and the weight held in the hand—now rising—is the output force. The Class III lever has a mechanical advantage of less than 1, but what it loses in force output in gains in range of motion. The world abounds with levers. Among the Class I varieties in common use are a nail puller on a hammerhead, described earlier, as well as postal scales and pliers. A handcart, though it might seem at first like a wheelbarrow, is actually a Class I lever, because the wheel or fulcrum is between the input effort—the force of a person’s hands gripping the handles—and the output, which is the lifting of the load in the handcart itself. Scissors constitute an interesting type of Class I lever, because the force of the output (the cutting blades) is reduced in order to create a greater lever arm for the input, in this case the handles gripped when cutting. A handheld bottle opener provides an excellent illustration of a Class II lever. Here the fulcrum is on the far end of the opener, away from the operator: the top end of the opener ring, which rests atop the bottle cap. The cap itself is the load, and one provides input force by pulling up on the opener handle, thus prying the cap from the bottle with the lower end of the opener ring. Nail clippers represent a type of combination lever: the handle that one operates is a Class II, while the cutting blades are a Class III.

C LASS E S O F L E V E R S . Levers are divided into three classes, depending on the rela-

Whereas Class II levers maximize force at the expense of range of motion, Class III levers operate in exactly the opposite fashion. When using a fishing rod to catch a fish, the fisherman’s left hand (assuming he is right-handed) constitutes the fulcrum as it holds the rod just below the reel assembly. The right hand supplies the effort, jerk-

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ing upward, while the fish is the load. The purpose here is not to raise a heavy object (one reason why a fishing rod may break if one catches too large a fish) but rather to use the increased lever arm for one’s advantage in catching an object at some distance. Similarly, a hammer, which constitutes a Class III lever, with the operator’s wrist as fulcrum, magnifies the motion of the operator’s hand with a hammerhead that cuts a much wider arc.

There is nothing obvious about the wheel, and in fact, it is not nearly as old an invention as most people think. Until the last few centuries, most peoples in sub-Saharan Africa, remote parts of central and northern Asia, the Americas, and the Pacific Islands remained unaware of it. This did not necessarily make them “primitive”: even the Egyptians who built the Great Pyramid of Cheops in about 2550 B.C. had no concept of the wheel.

Many machines that arose in the Industrial Age are a combination of many levers—that is, a compound lever. In a manual typewriter or piano, for instance, each key is a complex assembly of levers designed for a given task. An automobile, too, uses multiple levers—most notably, a special variety known as the wheel and axle.

What the Egyptians did have, however, were rollers—-most often logs, onto which a heavy object was hoisted using a lever. From rollers developed the idea of a sledge, a sled-like device for sliding large loads atop a set of rollers. A sledge appears in a Sumerian illustration from about 3500 B.C., the oldest known representation of a wheel-like object.

T H E W H E E L A N D AX L E . A wheel is a variation on a Class I lever, but it represents such a stunning technological advancement that it deserves to be considered on its own. When driving a car, the driver places input force on the rim of the steering wheel, whose fulcrum is the center of the wheel. The output force is translated along the steering column to the driveshaft.

The combination of wheel and axle overcomes one factor that tends to limit the effectiveness of most levers, regardless of class: limited range of motion. An axle is really a type of wheel, though it has a smaller radius, which means that the output lever arm is correspondingly smaller. Given what was already said about the equation for mechanical advantage in a lever, this presents a very fortunate circumstance. When a wheel turns, it has a relatively large lever arm (the rim), that turns a relatively small lever arm, the axle. Because the product of input force and input lever arm must equal output force multiplied by output lever arm, this means that the output force will be higher than the input force. Therefore, the larger the wheel in proportion to the axle, the greater the mechanical advantage.

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The transformation from the roller-andsledge assembly to wheeled vehicles is not as easy as it might seem, and historians still disagree as to the connection. Whatever the case, it appears that the first true wheels originated in Sumer (now part of Iraq) in about 3500 B.C. These were tripartite wheels, made by attaching three pieces of wood and then cutting out a circle. This made a much more durable wheel than a sawed-off log, and also overcame the fact that few trees are perfectly round. During the early period of wheeled transportation, from 3500 to 2000 B.C., donkeys and oxen rather than horses provided the power, in part because wheeled vehicles were not yet made for the speeds that horses could achieve. Hence, it was a watershed event when wheelmakers began fashioning axles as machines separate from the wheel. Formerly, axles and wheels were made up of a single unit; separating them made carts much more stable, especially when making turns—and, as noted earlier, greatly increased the mechanical advantage of the wheels themselves. Transportation entered a new phase in about 2000 B.C., when improvements in technology made possible the development of spoked wheels. By heat-treating wood, it became possible to bend the material slightly, and to attach spokes between the rim and hub of the wheel. When the wood cooled, the tension created a much stronger wheel—capable of carrying heavier loads faster and over greater distances.

It is for this reason that large vehicles without power steering often have very large steering wheels, which have a larger range of motion and, thus, a greater torque on the axle—that is, the steering column. Some common examples of the wheel-and-axle principle in operation today include a doorknob and a screwdriver; however, long before the development of the wheel and axle, there were wheels alone.

In China during the first century B.C., a new type of wheeled vehicle—identified earlier as a

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Class II lever—was born in the form of the wheelbarrow, or “wooden ox.” The wheelbarrow, whose invention the Chinese attributed to a semi-legendary figure named Ko Yu, was of such value to the imperial army for moving arms and military equipment that China’s rulers kept its design secret for centuries. In Europe, around the same time, chariots were dying out, and it was a long time before the technology of wheeled transport improved. The first real innovation came during the 1500s, with the development of the horsedrawn coach. By 1640, a German family was running a regular stagecoach service, and, in 1667, a new, light, two-wheeled carriage called a cabriolet made its first appearance. Later centuries, of course, saw the development of increasingly more sophisticated varieties of wheeled vehicles powered in turn by human effort (the bicycle), steam (the locomotive), and finally, the internal combustion engine (the automobile.) But the wheel was never just a machine for transport: long before the first wheeled carts came into existence, potters had been using wheels that rotated in place to fashion perfectly round objects, and in later centuries, wheels gained many new applications. By 500 B.C., farmers in Greece and other parts of the Mediterranean world were using rotary mills powered by donkeys. These could grind grain much faster than a person working with a hand-powered grindstone could hope to do, and in time, the Greeks found a means of powering their mills with a force more useful than donkeys: water. The first waterwheels, turned by human or animal power, included a series of buckets along the rim that made it possible to raise water from the river below and disperse it to other points. By about 70 B.C., however, Roman engineers recognized that they could use the power of water itself to turn wheels and grind grain. Thus, the waterwheel became one of the first two rotor mechanisms in which an inanimate source (as opposed to the effort of humans or animals) created power to spin a shaft. In this way, the waterwheel was a prototype for the engine developed many centuries later. Indeed, in the first century A.D., Hero of Alexandria—who discovered the concept of steam power some 1,700 years before anyone took up the idea and put it to use—proposed what has been considered a prototype for the turbine

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engine. However, for a variety of complex reasons, the ancient world was simply not ready for the technological leap portended by such an invention; and so, in terms of significant progress in the development of machines, Europe was asleep for more than a millennium.

Mechanical Advantage and Simple Machines

The other significant form of wheel powered by an “inanimate” source was the windmill, first mentioned in 85 B.C. by Antipater of Thessalonica, who commented on a windmill he saw in northern Greece. In this early version of the windmill, the paddle wheel moved on a horizontal plane. However, the windmill did not take hold in Europe during ancient times, and, in fact, its true origins lie further east, and it did not become widespread until much later. In the seventh century A.D., windmills began to appear in the region of modern Iran and Afghanistan, and the concept spread to the Arab world. Europeans in the Near East during the Crusades (1095-1291) observed the windmill, and brought the idea back to Europe with them. By the twelfth century Europeans had developed the more familiar vertical mill. Finally, there was a special variety of wheel that made its appearance as early as 500 B.C.: the toothed gearwheel. By 300 B.C., it was in use throughout Egypt, and by about 270 B.C., Ctesibius of Alexandria (fl. c. 270-250 B.C.) had applied the gear in devising a constant-flow water clock called a clepsydra. Some 2,100 years after Ctesibius, toothed gears became a critical component of industrialization. The most common type is a spur gear, in which the teeth of the wheel are parallel to the axis of rotation. Helical gears, by contrast, have curved teeth in a spiral pattern at an angle to their rotational axes. This means that several teeth of one gearwheel are always in contact with several teeth of the adjacent wheel, thus providing greater torque. In bevel gears, the teeth are straight, as with a spur gear, but they slope at a 45°-angle relative to their axes so that two gearwheels can fit together at up to 90°-angles to one another without a change in speed. Finally, planetary gears are made such that one or more smaller gearwheels can fit within a larger gearwheel, which has teeth cut on the inside rather than the outside. Similar in concept to the gearwheel is the V belt drive, which consists of two wheels side by side, joined with a belt. Each of the wheels has

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grooves cut in it for holding the belt, making this a modification of the pulley, and the grooves provide much greater gripping power for holding the belt in place. One common example of a V belt drive, combined with gearwheels, is a bicycle chain assembly.

rope runs from the support pulley down to the load-bearing wheel, wraps around that pulley and comes back up to a fixed attachment on the upper pulley. Whereas the upper pulley is fixed, the load-bearing pulley is free to move, and raises the load as the rope is pulled below.

P U L L E Y S . A pulley is essentially a grooved wheel on an axle attached to a frame, which in turn is attached to some form of rigid support such as a ceiling. A rope runs along the grooves of the pulley, and one end is attached to a load while the other is controlled by the operator.

The simplest kind of compound pulley, with just two wheels, has a mechanical advantage of 2. On a theoretical level, at least, it is possible to calculate the mechanical advantage of a compound pulley with more wheels: the number is equal to the segments of rope between the lower pulleys and the upper, or support pulley. In reality, however, friction, which is high as ropes rub against the pulley wheels, takes its toll. Thus mechanical advantage is never as great as it might be.

In several instances, it has been noted that a machine may provide increased range of motion or position rather than power. So, this simplest kind of pulley, known as a single or fixed pulley, only offers the advantage of direction rather than improved force. When using Venetian blinds, there is no increase in force; the advantage of the machine is simply that it allows one to move objects upward and downward. Thus, the theoretical mechanical advantage of a fixed pulley is 1 (or almost certainly less under actual conditions, where friction is a factor). Here it is appropriate to return to Archimedes, whose advancements in the understanding of levers translated to improvements in pulleys. In the case of the lever, it was Archimedes who first recognized that the longer the effort arm, the less effort one had to apply in raising the load. Likewise, with pulleys and related devices— cranes and winches—he explained and improved the way these machines worked. The first crane device dates to about 1000 B.C., but evidence from pictures suggests that pulleys may have been in use as early as seven thousand years before. Several centuries before Archimedes’s time, the Greeks were using compound pulleys that contained several wheels and thus provided the operator with much greater mechanical advantage than a fixed pulley. Archimedes, who was also the first to recognize the relationship between pulleys and levers, created the first fully realized block-and-tackle system using compound pulleys and cranes. In the late modern era, compound pulley systems were used in applications such as elevators and escalators.

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A block-and-tackle, like a compound pulley, uses just one rope with a number of pulley wheels. In a block-and-tackle, however, the wheels are arranged along two axles, each of which includes multiple pulley wheels that are free to rotate along the axle. The upper row is attached to the support, and the lower row to the load. The rope connects them all, running from the first pulley in the upper set to the first in the lower set, then to the second in the upper set, and so on. In theory, at least, the mechanical advantage of a block-and-tackle is equal to the number of wheels used, which must be an even number—but again, friction diminishes the theoretical mechanical advantage.

The Inclined Plane To the contemporary mind, it is difficult enough to think of a lever as a “machine”—but levers at least have more than one part, unlike an inclined plane. The latter, by contrast, is exactly what it seems to be: a ramp. Yet it was just such a ramp structure, as noted earlier, that probably enabled the Egyptians to build the pyramids—a feat of engineering so stunning that even today, some people refuse to believe that the ancient Egyptians could have achieved it on their own.

A compound pulley consists of two or more wheels, with at least one attached to the support while the other wheel or wheels lift the load. A

Surely, as anti-scientific proponents of various fantastic theories often insist, the building of the pyramids could only have been done with machines provided by super-intelligent, extraterrestrial beings. Even in ancient times, the Greek historian Herodotus (c. 484-c. 424 B.C.) speculated that the Egyptians must have used huge cranes that had long since disappeared.

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These bizarre guesses concerning the technology for raising the pyramid’s giant blocks serve to highlight the brilliance of a gloriously simple machine that, in essence, doubles force. If one needs to move a certain weight to a certain height, there are two options. One can either raise the weight straight upward, expending an enormous amount of effort, even with a pulley system; or one can raise the weight gradually along an inclined plane. The inclined plane is a much wiser choice, because it requires half the effort. Why half? Imagine an inclined plane sloping evenly upward to the right. The plane exists in a sort of frame that is equal in both length and height to the dimensions of the plane itself. As we can easily visualize, the plane takes up exactly half of the frame, and this is true whether the slope is more than, less than, or equal to 45°. For any plane in which the slope is more than 45°, however, the mechanical advantage will be less than 1, and it is indeed hard to imagine why anyone would use such a plane unless forced to do so by limitations on their horizontal space—for example, when lifting a heavy object from a narrow canyon. The mechanical advantage of an inclined plane is equal to the ratio between the distance over which input force is applied and the distance of output; or, more simply, the ratio of length to height. If a man is pushing a crate up a ramp 4 ft high and 8 ft long (1.22 m by 2.44 m), 8 ft is the input distance and 4 ft the output distance; hence, the mechanical advantage is 2. If the ramp length were doubled to 16 ft (4.88 m), the mechanical advantage would likewise double to 4, and so on. The concept of work, in terms of physics, has specific properties that are a subject unto themselves; however, it is important here only to recognize that work is the product of force (that is, effort) multiplied by distance. This means that if one increases the distance, a much smaller quantity of force is needed to achieve the same amount of work. On an everyday level, it is easy to see this in action. Walking or running up a gentle hill, obviously, is easier than going up a steep hill. Therefore, if one’s primary purpose is to conserve effort, it is best to choose the gentler hill. On the other hand, one may wish to minimize distance—or, if moving for the purpose of exercise,

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to maximize force input to burn calories. In either case, the steeper hill would be the better option.

Mechanical Advantage and Simple Machines

W E D G E S . The type of inclined plane discussed thus far is a ramp, but there are a number of much smaller varieties of inclined plane at work in the everyday world. A knife is an excellent example of one of the most common types, a wedge. Again, the mechanical advantage of a wedge is the ratio of length to height, which, in the knife, would be the depth of the blade compared to its cross-sectional width. Due to the ways in which wedges are used, however, friction plays a much greater role, therefore greatly reducing the theoretical mechanical advantage.

Other types of wedges may be used with a lever, as a form of fulcrum for raising objects. Or, a wedge may be placed under objects to stabilize them, as for instance, when a person puts a folded matchbook under the leg of a restaurant table to stop it from wobbling. Wedges also stop other objects from moving: a triangular piece of wood under a door will keep it from closing, and a more substantial wedge under the front wheels of a car will stop it from rolling forward. Variations of the wedge are everywhere. Consider all the types of cutting or chipping devices that exist: scissors, chisels, ice picks, axes, splitting wedges (used with a mallet to split a log down the center), saws, plows, electric razors, etc. Then there are devices that use a complex assembly of wedges working together. The part of a key used to open a lock is really just a row of wedges for moving the pins inside the lock to the proper position for opening the door. Similarly, each tooth in a zipper is a tiny wedge that fits tightly with the adjacent teeth. S C R E W S . As the wheel is, without a doubt, the greatest conceptual variation on the lever, so the screw may be identified as a particularly cunning adaptation of an inclined plane. The uses of screws today are many and obvious, but as with wheels and axles, these machines have more applications than are commonly recognized. Not only are there screws for holding things together, but there are screws such as those on vises, clamps, or monkey wrenches for applying force to objects.

A screw is an inclined plane in the shape of a helix, wrapped around an axis or cylinder. In order to determine its mechanical advantage, one must first find the pitch, which is the distance

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Mechanical Advantage and Simple Machines

A

SCREW, LIKE THIS CORKSCREW USED TO OPEN A BOTTLE OF WINE, IS AN INCLINED PLANE IN THE SHAPE OF A

HELIX, WRAPPED AROUND AN AXIS OR CYLINDER. (Ecoscene/Corbis. Reproduced by permission.)

between adjacent threads. The other variable is lever arm, which with a screwdriver is the radius, or on a wrench, the length from the crescent or clamp to the area of applied force. Obviously, the lever arm is much greater for a wrench, which explains why a wrench is sometimes preferable to a screwdriver, when removing a highly resistant material screw or bolt. When one rotates a screw of a given pitch, the applied force describes a circle whose area may be calculated as 2πL, where L is the lever arm. This figure, when divided by the pitch, is the same as the ratio between the distance of force input to force output. Either number is equal to the mechanical advantage for a screw. As suggested earlier, that mechanical advantage is usually low, because force input (screwing in the screw) takes place in a much greater range of motion than force output (the screw working its way into the surface). But this is exactly what the screw is designed to do, and what it lacks in mechanical advantage, it more than makes up in its holding power.

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cal Archimedes screw, a device for lifting water. The invention consists of a metal pipe in a corkscrew shape, which draws water upward as it revolves. It proved particularly useful for lifting water that had seeped into the lower parts of a ship, and in many countries today, it remains in use as a simple pump for drawing water out of the ground. Some historians maintain that Archimedes did not invent the screw-type pump, but rather saw an example of it in Egypt. In any case, he clearly developed a practical version of the device, and it soon gained application throughout the ancient world. Archaeologists discovered a screw-driven olive press in the ruins of Pompeii, destroyed by the eruption of Mt. Vesuvius in A.D. 79, and Hero of Alexandria later mentioned the use of a screw-type machine in his Mechanica.

As with the lever and pulley, Archimedes did not invent the screw, but he did greatly improve human understanding of it. Specifically, he developed a mathematical formula for a simple spiral, and translated this into the highly practi-

Yet, Archimedes is the figure most widely associated with the development of this wondrous device. Hence, in 1837, when the SwedishAmerican engineer John Ericsson (1803-1899) demonstrated the use of a screw-driven ship’s propeller, he did so on a craft he named the Archimedes.

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From screws planted in wood to screws that drive ships at sea, the device is everywhere in modern life. Faucets, corkscrews, drills, and meat grinders are obvious examples. Though many types of jacks used for lifting an automobile or a house are levers, others are screw assemblies on which one rotates the handle along a horizontal axis. In fact, the jack is a particularly interesting device. Versions of the jack represent all three types of simple machine: lever, inclined plane, and hydraulic press.

The Hydraulic Press As noted earlier, the hydraulic press came into existence much, much later than the lever or inclined plane, and its birth can be seen within the context of a larger movement toward the use of water power, including steam. A little more than a quarter-century after Pascal created the theoretical framework for hydraulic power, his countryman Denis Papin (1647-1712) introduced the steam digester, a prototype for the pressure cooker. In 1687, Papin published a work describing a machine in which steam operated a piston—an early model for the steam engine. Papin’s concept, which was on the absolute cutting edge of technological development at that time, utilized not only steam power but also the very hydraulic concept that Pascal had identified a few decades earlier. Indeed, the assembly of pistons and cylinders that forms the central component of the internal-combustion engine reflects this hydraulic rule, discovered by Pascal in 1653. It was then that he formulated what is known as Pascal’s principle: that the external pressure applied on a fluid is transmitted uniformly throughout the entire body of that fluid. Inside a piston and cylinder assembly, one of the most basic varieties of hydraulic press, the pressure is equal to the ratio of force to the horizontal area of pressure. A simple hydraulic press of the variety that might be used to raise a car in an auto shop typically consists of two large cylinders side by side, connected at the bottom by a channel in which valves control flow. When one applies force over a given area of input—that is, by pressing down on one cylinder—this yields a uniform pressure that causes output in the second cylinder.

S C I E N C E O F E V E RY DAY T H I N G S

Once again, mechanical advantage is equal to the ratio of force output to force input, and for a hydraulic press, this can also be measured as the ratio of area output to area input. Just as there is an inverse relationship between lever arm and force in a lever, and between length and height in an inclined plane, so there is such a relationship between horizontal area and force in a hydraulic pump. Consequently, in order to increase force, one should minimize area.

Mechanical Advantage and Simple Machines

However, there is another factor to consider: height. The mechanical advantage of a hydraulic pump is equal to the vertical distance to which the input force is applied, divided by that of the output force. Hence, the greater the height of the input cylinder compared to the output cylinder, the greater the mechanical advantage. And since these three factors—height, area, and force— work together, it is possible to increase the lifting force and area by minimizing the height. Consider once again the auto-shop car jack. Typically, the input cylinder will be relatively tall and thin, and the output cylinder short and squat. Because the height of the input cylinder is large, the area of input will be relatively small, as will the force of input. But according to Pascal’s principle, whatever the force applied on the input, the pressure will be the same on the output. At the output end, where the car is raised, one needs a large amount of force and a relatively large lifting area. Therefore, height is minimized to increase force and area. If the output area is 10 times the size of the input area, an input force of 1 unit will produce an output force of 10 units—but in order to raise the weight by 1 unit of height, the input piston must move downward by 10 units. This type of car jack provides a basic model of the hydraulic press in operation, but, in fact, hydraulic technology has many more applications. A hydraulic pump, whether for pumping air into a tire or water from a basement, uses very much the same principle as the hydraulic jack. So too does the hydraulic ram, used in machines ranging from bulldozers to the hydraulic lifts used by firefighters and utility workers to reach great heights. In a hydraulic ram, however, the characteristics of the input and output cylinders are reversed from those of a car jack. The input cylinder, called the master cylinder, is short and squat, whereas the output cylinder—the slave

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Mechanical Advantage and Simple Machines

KEY TERMS A machine that

a crowbar, input would be the energy one

combines multiple levers to accomplish its

expends by pushing down on the bar.

task. An example is a piano or manual

Input force is often called applied force,

typewriter.

effort force, or simply effort.

COMPOUND LEVER:

CLASS I LEVER:

A lever in which the

One of the three basic varieties

of machine, a lever consists of a rigid bar

output force. Examples include a crowbar,

supported at one point, known as the ful-

a nail puller, and scissors.

crum.

CLASS II LEVER:

A lever in which the

output force is between the input force and the fulcrum. Class II levers, of which wheelbarrows and bottle openers are

On a lever, the distance

LEVER ARM:

from the input force or the output force to the fulcrum. A device that transmits or

examples, maximize output force at the

MACHINE:

expense of range of motion.

modifies force or torque for a specific pur-

CLASS III LEVER:

A lever in which

pose. The

the input force is between the output force

MECHANICAL

and the fulcrum. Class III levers, of which

ratio of force output to force input for a

a fishing rod is an example, maximize

machine.

range of motion at the expense of output force.

MOMENT ARM:

ADVANTAGE:

For an object experi-

encing torque, moment arm is the distance The ratio of actual

from the pivot or balance point to the vec-

mechanical advantage to theoretical

tor on which force is being applied.

mechanical advantage.

Moment arm is always perpendicular to

EFFICIENCY:

FRICTION:

The force that resists

motion when the surface of one object

the direction of force. OUTPUT:

The results achieved from the

comes into contact with the surface of

operation of a machine. In a Class I lever

another.

such as a crowbar, output is the moving of

FULCRUM:

The support point of a

lever. INERTIA:

a stone or other heavy load dislodged by the crowbar. Output force is often called

The tendency of an object in

the load or resistance force.

motion to remain in motion, and of an

TORQUE:

object at rest to remain at rest.

turning force; in scientific terms, it is

INPUT:

The effort supplied by the oper-

ator of a machine. In a Class I lever such as

168

LEVER:

fulcrum is between the input force and

In general terms, torque is

the product of moment arm multiplied by force.

cylinder—is tall and thin. The reason for this change is that in objects using a hydraulic ram, height is more important than force output: they

are often raising people rather than cars. When the slave cylinder exerts pressure on the stabilizer ram above it (the bucket containing the firefight-

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er, for example), it rises through a much larger range of vertical motion than that of the fluid flowing from the master cylinder. As noted earlier, the pistons of a car engine are hydraulic pumps—specifically, reciprocating hydraulic pumps, so named because they all work together. In scientific terms, “fluid” can mean either a liquid or a gas such as air; hence, there is an entire subset of hydraulic machines that are pneumatic, or air-powered. Among these are power brakes in a car, pneumatic drills, and even hovercrafts. As with the other two varieties of simple machine, the hydraulic press is in evidence throughout virtually every nook and cranny of daily life. The pump in a toilet tank is a type of hydraulic press, as is the inner chamber of a pen. So too are aerosol cans, fire extinguishers, scuba tanks, water meters, and pressure gauges.

Bains, Rae. Simple Machines. Mahwah, N.J.: Troll Associates, 1985. Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. Canizares, Susan. Simple Machines. New York: Scholastic, 1999. Haslam, Andrew and David Glover. Machines. Photography by Jon Barnes. Chicago: World Book, 1998. “Inventors Toolbox: The Elements of Machines” (Web site). (March 9, 2001). Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998. “Machines” (Web site). (March 9, 2001). “Motion, Energy, and Simple Machines” (Web site). (March 9, 2001). O’Brien, Robert, and the Editors of Life. Machines. New York: Time-Life Books, 1964.

WHERE TO LEARN MORE

“Simple Machines” (Web site). (March 9, 2001).

Archimedes (Web site). (March 9, 2001.)

Singer, Charles Joseph, et al., editors. A History of Technology (8 vols.) Oxford, England: Clarendon Press, 1954-84.

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ENERGY Energy

CONCEPT As with many concepts in physics, energy—along with the related ideas of work and power—has a meaning much more specific, and in some ways quite different, from its everyday connotation. According to the language of physics, a person who strains without success to pull a rock out of the ground has done no work, whereas a child playing on a playground produces a great deal of work. Energy, which may be defined as the ability of an object to do work, is neither created nor destroyed; it simply changes form, a concept that can be illustrated by the behavior of a bouncing ball.

HOW IT WORKS In fact, it might actually be more precise to say that energy is the ability of “a thing” or “something” to do work. Not only tangible objects (whether they be organic, mechanical, or electromagnetic) but also non-objects may possess energy. At the subatomic level, a particle with no mass may have energy. The same can be said of a magnetic force field. One cannot touch a force field; hence, it is not an object—but obviously, it exists. All one has to do to prove its existence is to place a natural magnet, such as an iron nail, within the magnetic field. Assuming the force field is strong enough, the nail will move through space toward it—and thus the force field will have performed work on the nail.

A person straining, and failing, to pull a rock from the ground has performed no work (in terms of physics) because nothing has been moved. On the other hand, a child on a playground performs considerable work: as she runs from the slide to the swing, for instance, she has moved her own weight (a variety of force) across a distance. She is even working when her movement is back-and-forth, as on the swing. This type of movement results in no net displacement, but as long as displacement has occurred at all, work has occurred. Similarly, when a man completes a full pushup, his body is in the same position—parallel to the floor, arms extended to support him—as he was before he began it; yet he has accomplished work. If, on the other hand, he at the end of his energy, his chest is on the floor, straining but failing, to complete just one more push-up, then he is not working. The fact that he feels as though he has worked may matter in a personal sense, but it does not in terms of physics.

Work may be defined in general terms as the exertion of force over a given distance. In order

CA L C U LAT I N G W O R K . Work can be defined more specifically as the product of force and distance, where those two vectors are exerted in the same direction. Suppose one were to drag a block of a certain weight across a given distance of floor. The amount of force one exerts parallel to the floor itself, multiplied by the distance, is equal to the amount of work exerted. On the other hand, if one pulls up on the block in a

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for work to be accomplished, there must be a displacement in space—or, in colloquial terms, something has to be moved from point A to point B. As noted earlier, this definition creates results that go against the common-sense definition of “work.”

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Energy

WHILE

THIS PLAYER DRIBBLES HIS BASKETBALL, THE BALL EXPERIENCES A COMPLEX ENERGY TRANSFER AS IT HITS

THE FLOOR AND BOUNCES BACK UP. (Photograph by David Katzenstein/Corbis. Reproduced by permission.)

position perpendicular to the floor, that force does not contribute toward the work of dragging the block across the floor, because it is not par allel to distance as defined in this particular situation.

force must be multiplied by cos 30°, which is equal to 0.866. Note that the cosine is less than 1; hence when multiplied by the total force exerted, it will yield a figure 13.4% smaller than the total force. In fact, the larger the angle, the smaller the cosine; thus for 90°, the value of cos = 0. On the other hand, for an angle of 0°, cos = 1. Thus, if total force is exerted parallel to the floor—that is, at a 0°-angle to it—then the component of force that counts toward total work is equal to the total force. From the standpoint of physics, this would be a highly work-intensive operation.

Similarly, if one exerts force on the block at an angle to the floor, only a portion of that force counts toward the net product of work—a portion that must be quantified in terms of trigonometry. The line of force parallel to the floor may be thought of as the base of a triangle, with a line perpendicular to the floor as its second side. Hence there is a 90°-angle, making it a right triangle with a hypotenuse. The hypotenuse is the line of force, which again is at an angle to the floor.

G RAV I T Y A N D O T H E R P E C U L I A R I T I E S O F W O R K . The above dis-

The component of force that counts toward the total work on the block is equal to the total force multiplied by the cosine of the angle. A cosine is the ratio between the leg adjacent to an acute (less than 90°) angle and the hypotenuse. The leg adjacent to the acute angle is, of course, the base of the triangle, which is parallel to the floor itself. Sizes of triangles may vary, but the ratio expressed by a cosine (abbreviated cos) does not. Hence, if one is pulling on the block by a rope that makes a 30°-angle to the floor, then

cussion relates entirely to work along a horizontal plane. On the vertical plane, by contrast, work is much simpler to calculate due to the presence of a constant downward force, which is, of course, gravity. The force of gravity accelerates objects at a rate of 32 ft (9.8 m)/sec2. The mass (m) of an object multiplied by the rate of gravitational acceleration (g) yields its weight, and the formula for work done against gravity is equal to weight multiplied by height (h) above some lower reference point: mgh.

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Energy

HYDROELECTRIC

DAMS, LIKE THE

SHASTA

DAM IN

CALIFORNIA,

SUPERBLY ILLUSTRATE THE PRINCIPLE OF ENERGY

CONVERSION. (Photograph by Charles E. Rotkin/Corbis. Reproduced by permission.)

Distance and force are both vectors—that is, quantities possessing both magnitude and direction. Yet work, though it is the product of these two vectors, is a scalar, meaning that only the magnitude of work (and not the direction over which it is exerted) is important. Hence mgh can refer either to the upward work one exerts against gravity (that is, by lifting an object to a certain height), or to the downward work that gravity performs on the object when it is dropped. The direction of h does not matter, and its value is purely relative, referring to the vertical distance between one point and another.

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the truth that work, in the sense in which it is applied by physicists, is quite different from “work” as it understood in the day-to-day world. There is a highly personal quality to the everyday meaning of the term, which is completely lacking from its physics definition.

The fact that gravity can “do work”—and the irrelevance of direction—further illustrates

If someone carried a heavy box up five flights of stairs, that person would quite naturally feel justified in saying “I’ve worked.” Certainly he or she would feel that the work expended was far greater than that of someone who had simply allowed the the elevator to carry the box up those five floors. Yet in terms of work done against gravity, the work done on the box by the elevator is exactly the same as that performed by the per-

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son carrying it upstairs. The identity of the “worker”—not to mention the sweat expended or not expended—is irrelevant from the standpoint of physics.

Energy

Measurement of Work and Power In the metric system, a newton (N) is the amount of force required to accelerate 1 kg of mass by 1 meter per second squared (m/s2). Work is measured by the joule (J), equal to 1 newton-meter (N • m). The British unit of force is the pound, and work is measured in foot-pounds, or the work done by a force of 1 lb over a distance of one foot. Power, the rate at which work is accomplished over time, is the same as work divided by time. It can also be calculated in terms of force multiplied by speed, much like the force-multiplied-by-distance formula for work. However, as with work, the force and speed must be in the same direction. Hence, the formula for power in these terms is F • cos θ • v, where F=force, v=speed, and cos θ is equal to the cosine of the angle θ (the Greek letter theta) between F and the direction of v. The metric-system measure of power is the watt, named after James Watt (1736-1819), the Scottish inventor who developed the first fully viable steam engine and thus helped inaugurate the Industrial Revolution. A watt is equal to 1 joule per second, but this is such a small unit that it is more typical to speak in terms of kilowatts, or units of 1,000 watts. Ironically, Watt himself—like most people in the British Isles and America—lived in a world that used the British system, in which the unit of power is the foot-pound per second. The latter, too, is very small, so for measuring the power of his steam engine, Watt suggested a unit based on something quite familiar to the people of his time: the power of a horse. One horsepower (hp) is equal to 550 foot-pounds per second.

IN THIS 1957 PHOTOGRAPH, ITALIAN OPERA SINGER LUIGI INFANTINO TRIES TO BREAK A WINE GLASS BY SINGING A HIGH “C” NOTE. CONTRARY TO POPULAR BELIEF, THE NOTE DOES NOT HAVE TO BE A PARTICULARLY HIGH ONE TO BREAK THE GLASS: RATHER, THE NOTE SHOULD BE ON THE SAME WAVELENGTH AS THE GLASS’S

OWN

VIBRATIONS.

WHEN

THIS

OCCURS,

SOUND ENERGY IS TRANSFERRED DIRECTLY TO THE GLASS, WHICH IS SHATTERED BY THIS SUDDEN NET INTAKE OF ENERGY. (Hulton-Deutsch Collection/Corbis. Repro-

duced by permission.)

created quite deliberately over a matter of just a few years following the French Revolution, which broke out in 1789. The metric system was adopted ten years later.

course, is horridly cumbersome compared to the metric system, and thus it long ago fell out of favor with the international scientific community. The British system is the product of loosely developed conventions that emerged over time: for instance, a foot was based on the length of the reigning king’s foot, and in time, this became standardized. By contrast, the metric system was

During the revolutionary era, French intellectuals believed that every aspect of existence could and should be treated in highly rational, scientific terms. Out of these ideas arose much folly—especially after the supposedly “rational” leaders of the revolution began chopping off people’s heads—but one of the more positive outcomes was the metric system. This system, based entirely on the number 10 and its exponents, made it easy to relate one figure to another: for instance, there are 100 centimeters in a meter and 1,000 meters in a kilometer. This is vastly more convenient than converting 12 inches to a foot, and 5,280 feet to a mile.

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For this reason, scientists—even those from the Anglo-American world—use the metric system for measuring not only horizontal space, but volume, temperature, pressure, work, power, and so on. Within the scientific community, in fact, the metric system is known as SI, an abbreviation of the French Système International d’Unités— that is, “International System of Units.” Americans have shown little interest in adopting the SI system, yet where power is concerned, there is one exception. For measuring the power of a mechanical device, such as an automobile or even a garbage disposal, Americans use the British horsepower. However, for measuring electrical power, the SI kilowatt is used. When an electric utility performs a meter reading on a family’s power usage, it measures that usage in terms of electrical “work” performed for the family, and thus bills them by the kilowatt-hour.

Three Types of Energy KINETIC AND POTENTIAL ENE R GY F O R M U LA E . Earlier, energy was

defined as the ability of an object to accomplish work—a definition that by this point has acquired a great deal more meaning. There are three types of energy: kinetic energy, or the energy that something possesses by virtue of its motion; potential energy, the energy it possesses by virtue of its position; and rest energy, the energy it possesses by virtue of its mass. The formula for kinetic energy is KE = 1⁄2 mv2. In other words, for an object of mass m, kinetic energy is equal to half the mass multiplied by the square of its speed v. The actual derivation of this formula is a rather detailed process, involving reference to the second of the three laws of motion formulated by Sir Isaac Newton (1642-1727.) The second law states that F = ma, in other words, that force is equal to mass multiplied by acceleration. In order to understand kinetic energy, it is necessary, then, to understand the formula for uniform acceleration. The latter is vf2 = v02 + 2as, where vf2 is the final speed of the object, v02 its initial speed, a acceleration and s distance. By substituting values within these equations, one arrives at the formula of 1⁄2 mv2 for kinetic energy.

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within an object, one must perform an amount of work on it equal to Fs. Hence, kinetic energy also equals Fs, and thus the preceding paragraph simply provides a means for translating that into more specific terms. The potential energy (PE) formula is much simpler, but it also relates to a work formula given earlier: that of work done against gravity. Potential energy, in this instance, is simply a function of gravity and the distance h above some reference point. Hence, its formula is the same as that for work done against gravity, mgh or wh, where w stands for weight. (Note that this refers to potential energy in a gravitational field; potential energy may also exist in an electromagnetic field, in which case the formula would be different from the one presented here.) R E S T E N E R GY A N D I T S I N T R I G U I N G F O R M U LA . Finally, there is

rest energy, which, though it may not sound very exciting, is in fact the most intriguing—and the most complex—of the three. Ironically, the formula for rest energy is far, far more complex in derivation than that for potential or even kinetic energy, yet it is much more well-known within the popular culture. Indeed, E = mc2 is perhaps the most famous physics formula in the world—even more so than the much simpler F = ma. The formula for rest energy, as many people know, comes from the man whose Theory of Relativity invalidated certain specifics of the Newtonian framework: Albert Einstein (1879-1955). As for what the formula actually means, that will be discussed later.

REAL-LIFE A P P L I C AT I O N S Falling and Bouncing Balls

The above is simply another form of the general formula for work—since energy is, after all, the ability to perform work. In order to produce an amount of kinetic energy equal to 1⁄2 mv2

One of the best—and most frequently used— illustrations of potential and kinetic energy involves standing at the top of a building, holding a baseball over the side. Naturally, this is not an experiment to perform in real life. Due to its relatively small mass, a falling baseball does not have a great amount of kinetic energy, yet in the real world, a variety of other conditions (among them inertia, the tendency of an object to maintain its state of motion) conspire to make a hit on the head with a baseball potentially quite serious.

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If dropped from a great enough height, it could be fatal.

of another ball and another form of vertical motion.

When one holds the baseball over the side of the building, potential energy is at a peak, but once the ball is released, potential energy begins to decrease in favor of kinetic energy. The relationship between these, in fact, is inverse: as the value of one decreases, that of the other increases in exact proportion. The ball will only fall to the point where its potential energy becomes 0, the same amount of kinetic energy it possessed before it was dropped. At the same point, kinetic energy will have reached maximum value, and will be equal to the potential energy the ball possessed at the beginning. Thus the sum of kinetic energy and potential energy remains constant, reflecting the conservation of energy, a subject discussed below.

This time instead of a baseball, the ball should be one that bounces: any ball will do, from a basketball to a tennis ball to a superball. And rather than falling from a great height, this one is dropped through a range of motion ordinary for a human being bouncing a ball. It hits the floor and bounces back—during which time it experiences a complex energy transfer.

It is relatively easy to understand how the ball acquires kinetic energy in its fall, but potential energy is somewhat more challenging to comprehend. The ball does not really “possess” the potential energy: potential energy resides within an entire system comprised by the ball, the space through which it falls, and the Earth. There is thus no “magic” in the reciprocal relationship between potential and kinetic energy: both are part of a single system, which can be envisioned by means of an analogy. Imagine that one has a 20-dollar bill, then buys a pack of gum. Now one has, say, $19.20. The positive value of dollars has decreased by $0.80, but now one has increased “non-dollars” or “anti-dollars” by the same amount. After buying lunch, one might be down to $12.00, meaning that “anti-dollars” are now up to $8.00. The same will continue until the entire $20.00 has been spent. Obviously, there is nothing magical about this: the 20-dollar bill was a closed system, just like the one that included the ball and the ground. And just as potential energy decreased while kinetic energy increased, so “non-dollars” increased while dollars decreased.

Energy

As was the case with the baseball dropped from the building, the ball (or more specifically, the system involving the ball and the floor) possesses maximum potential energy prior to being released. Then, in the split-second before its impact on the floor, kinetic energy will be at a maximum while potential energy reaches zero. So far, this is no different than the baseball scenario discussed earlier. But note what happens when the ball actually hits the floor: it stops for an infinitesimal fraction of a moment. What has happened is that the impact on the floor (which in this example is assumed to be perfectly rigid) has dented the surface of the ball, and this saps the ball’s kinetic energy just at the moment when the energy had reached its maximum value. In accordance with the energy conservation law, that energy did not simply disappear: rather, it was transferred to the floor. Meanwhile, in the wake of its huge energy loss, the ball is motionless. An instant later, however, it reabsorbs kinetic energy from the floor, undents, and rebounds. As it flies upward, its kinetic energy begins to diminish, but potential energy increases with height. Assuming that the person who released it catches it at exactly the same height at which he or she let it go, then potential energy is at the level it was before the ball was dropped. WHEN A BALL LOSES ITS B O U N C E . The above, of course, takes little

B O U N C I N G B AC K . The example of the baseball illustrates one of the most fundamental laws in the universe, the conservation of energy: within a system isolated from all other outside factors, the total amount of energy remains the same, though transformations of energy from one form to another take place. An interesting example of this comes from the case

account of energy “loss”—that is, the transfer of energy from one body to another. In fact, a part of the ball’s kinetic energy will be lost to the floor because friction with the floor will lead to an energy transfer in the form of thermal, or heat, energy. The sound that the ball makes when it bounces also requires a slight energy loss; but friction—a force that resists motion when the surface of one object comes into contact with the surface of another—is the principal culprit where energy transfer is concerned.

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Of particular importance is the way the ball responds in that instant when it hits bottom and stops. Hard rubber balls are better suited for this purpose than soft ones, because the harder the rubber, the greater the tendency of the molecules to experience only elastic deformation. What this means is that the spacing between molecules changes, yet their overall position does not. If, however, the molecules change positions, this causes them to slide against one another, which produces friction and reduces the energy that goes into the bounce. Once the internal friction reaches a certain threshold, the ball is “dead”—that is, unable to bounce. The deader the ball is, the more its kinetic energy turns into heat upon impact with the floor, and the less energy remains for bouncing upward.

Varieties of Energy in Action The preceding illustration makes several references to the conversion of kinetic energy to thermal energy, but it should be stressed that there are only three fundamental varieties of energy: potential, kinetic, and rest. Though heat is often discussed as a form unto itself, this is done only because the topic of heat or thermal energy is complex: in fact, thermal energy is simply a result of the kinetic energy between molecules. To draw a parallel, most languages permit the use of only three basic subject-predicate constructions: first person (“I”), second person (“you”), and third person (“he/she/it.”) Yet within these are endless varieties such as singular and plural nouns or various temporal orientations of verbs: present (“I go”); present perfect (“I have gone”); simple past (“I went”); past perfect (“I had gone.”) There are even “moods,” such as the subjunctive or hypothetical, which permit the construction of complex thoughts such as “I would have gone.” Yet for all this variety in terms of sentence pattern—actually, a degree of variety much greater than for that of energy types—all subject-predicate constructions can still be identified as first, second, or third person.

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between two electromagnetically charged particles is analogous to the force of gravity. M E C H A N I CA L E N E R GY. One term

not listed among manifestations of energy is mechanical energy, which is something different altogether: the sum of potential and kinetic energy. A dropped or bouncing ball was used as a convenient illustration of interactions within a larger system of mechanical energy, but the example could just as easily have been a roller coaster, which, with its ups and downs, quite neatly illustrates the sliding scale of kinetic and potential energy. Likewise, the relationship of Earth to the Sun is one of potential and kinetic energy transfers: as with the baseball and Earth itself, the planet is pulled by gravitational force toward the larger body. When it is relatively far from the Sun, it possesses a higher degree of potential energy, whereas when closer, its kinetic energy is highest. Potential and kinetic energy can also be illustrated within the realm of electromagnetic, as opposed to gravitational, force: when a nail is some distance from a magnet, its potential energy is high, but as it moves toward the magnet, kinetic energy increases. E N E R GY C O N V E R S I O N I N A DA M . A dam provides a beautiful illustration

of energy conversion: not only from potential to kinetic, but from energy in which gravity provides the force component to energy based in electromagnetic force. A dam big enough to be used for generating hydroelectric power forms a vast steel-and-concrete curtain that holds back millions of tons of water from a river or other body. The water nearest the top—the “head” of the dam—thus has enormous potential energy.

One might thus describe thermal energy as a manifestation of energy, rather than as a discrete form. Other such manifestations include electromagnetic (sometimes divided into electrical and magnetic), sound, chemical, and nuclear. The principles governing most of these are similar: for instance, the positive or negative attraction

Hydroelectric power is created by allowing controlled streams of this water to flow downward, gathering kinetic energy that is then transferred to powering turbines. Dams in popular vacation spots often release a certain amount of water for recreational purposes during the day. This makes it possible for rafters, kayakers, and others downstream to enjoy a relatively fast-flowing river. (Or, to put it another way, a stream with high kinetic energy.) As the day goes on, however, the sluice-gates are closed once again to build up the “head.” Thus when night comes, and energy demand is relatively high as people retreat to their homes, vacation cabins, and hotels, the dam is ready to provide the power they need.

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O T H E R M A N I F E S TAT I O N S O F E N E R G Y. Thermal and electromagnetic

energy are much more readily recognizable manifestations of energy, yet sound and chemical energy are two forms that play a significant part as well. Sound, which is essentially nothing more than the series of pressure fluctuations within a medium such as air, possesses enormous energy: consider the example of a singer hitting a certain note and shattering a glass. Contrary to popular belief, the note does not have to be particularly high: rather, the note should be on the same wavelength as the glass’s own vibrations. When this occurs, sound energy is transferred directly to the glass, which is shattered by this sudden net intake of energy. Sound waves can be much more destructive than that: not only can the sound of very loud music cause permanent damage to the ear drums, but also, sound waves of certain frequencies and decibel levels can actually drill through steel. Indeed, sound is not just a by-product of an explosion; it is part of the destructive force. As for chemical energy, it is associated with the pull that binds together atoms within larger molecular structures. The formation of water molecules, for instance, depends on the chemical bond between hydrogen and oxygen atoms. The combustion of materials is another example of chemical energy in action. With both chemical and sound energy, however, it is easy to show how these simply reflect the larger structure of potential and kinetic energy discussed earlier. Hence sound, for instance, is potential energy when it emerges from a source, and becomes kinetic energy as it moves toward a receiver (for example, a human ear). Furthermore, the molecules in a combustible material contain enormous chemical potential energy, which becomes kinetic energy when released in a fire.

energy—to a much greater extent than thermal or chemical energy—involves not only kinetic and potential energy, but also the mysterious, extraordinarily powerful, form known as rest energy. Throughout this discussion, there has been little mention of rest energy; yet it is ever-present. Kinetic and potential energy rise and fall with respect to one another; but rest energy changes little. In the baseball illustration, for instance, the ball had the same rest energy at the top of the building as it did in flight—the same rest energy, in fact, that it had when sitting on the ground. And its rest energy is enormous. N U C L E A R WA R FA R E . This brings back the subject of the rest energy formula: E = mc2, famous because it made possible the creation of the atomic bomb. The latter, which fortunately has been detonated in warfare only twice in history, brought a swift end to World War II when the United States unleashed it against Japan in August 1945. From the beginning, it was clear that the atom bomb possessed staggering power, and that it would forever change the way nations conducted their affairs in war and peace.

Yet the atom bomb involved only nuclear fission, or the splitting of an atom, whereas the hydrogen bomb that appeared just a few years after the end of World War II used an even more powerful process, the nuclear fusion of atoms. Hence, the hydrogen bomb upped the ante to a much greater extent, and soon the two nuclear superpowers—the United States and the Soviet Union—possessed the power to destroy most of the life on Earth.

Nuclear energy is similar to chemical energy, though in this instance, it is based on the binding of particles within an atom and its nucleus. But it is also different from all other kinds of energy, because its force component is neither gravitational nor electromagnetic, but based on one of two other known varieties of force: strong nuclear and weak nuclear. Furthermore, nuclear

The next four decades were marked by a superpower struggle to control “the bomb” as it came to be known—meaning any and all nuclear weapons. Initially, the United States controlled all atomic secrets through its heavily guarded Manhattan Project, which created the bombs used against Japan. Soon, however, spies such as Julius and Ethel Rosenberg provided the Soviets with U.S. nuclear secrets, ensuring that the dictatorship of Josef Stalin would possess nuclear capabilities as well. (The Rosenbergs were executed for treason, and their alleged innocence became a celebrated cause among artists and intellectuals; however, Soviet documents released since the collapse of the Soviet empire make it clear that they were guilty as charged.)

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Energy

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Energy

KEY TERMS CONSERVATION OF ENERGY:

A

JOULE:

The SI measure of work. One

law of physics which holds that within a

joule (1 J) is equal to the work required

system isolated from all other outside fac-

to accelerate 1 kilogram of mass by 1

tors, the total amount of energy re-mains

meter per second squared (1 m/s2) over a

the same, though transformations of ener-

distance of 1 meter. Due to the small size

gy from one form to another take place.

of the joule, however, it is often replaced

COSINE:

For an acute (less than 90°)

in a right triangle, the cosine (abbreviated

by the kilowatt-hour, equal to 3.6 million (3.6 • 106) J.

cos) is the ratio between the adjacent leg

KINETIC ENERGY:

and the hypotenuse. Regardless of the size

an object possesses by virtue of its motion.

of the triangle, this figure is a constant for MATTER:

any particular angle.

The energy that

Physical substance that occu-

pies space, has mass, is composed of atoms

The ability of an object (or

(or in the case of subatomic particles, is

in some cases a non-object, such as a mag-

part of an atom), and is convertible into

netic force field) to accomplish work.

energy.

ENERGY:

FRICTION:

The force that resists

MECHANICAL ENERGY:

motion when the surface of one object

potential energy and kinetic energy within

comes into contact with the surface of

a system.

another. POTENTIAL ENERGY: HORSEPOWER:

The British unit of

power, equal to 550 foot-pounds per

position. POWER:

HYPOTENUSE:

In a right triangle, the

side opposite the right angle.

Both nations began building up missile arsenals. It was not, however, just a matter of the United States and the Soviet Union. By the 1970s, there were at least three other nations in the “nuclear club”: Britain, France, and China. There were also other countries on the verge of developing nuclear bombs, among them India and Israel. Furthermore, there was a great threat that a terrorist leader such as Libya’s Muammar alQaddafi would acquire nuclear weapons and do the unthinkable: actually use them.

The energy

that an object possesses by virtue of its

second.

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The sum of

The rate at which work is

accomplished over time, a figure rendered mathematically as work divided by time.

a sort of high-stakes chess game—to use a metaphor mentioned frequently during the 1970s. Soviet leaders and their American counterparts both recognized that it would be the end of the world if either unleashed their nuclear weapons; yet each was determined to be able to meet the other’s ever-escalating nuclear threat.

Though other nations acquired nuclear weapons, however, the scale of the two superpower arsenals dwarfed all others. And at the heart of the U.S.-Soviet nuclear competition was

United States President Ronald Reagan earned harsh criticism at home for his nuclear buildup and his hard line in negotiations with Soviet President Mikhail Gorbachev; but as a result of this one-upmanship, he put the Soviets into a position where they could no longer compete. As they put more and more money into nuclear weapons, they found themselves less and

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Energy

KEY TERMS

CONTINUED

The SI unit of power is the watt, while the

actions free from interference by outside

British unit is the foot-pound per second.

factors. An example is the baseball dropped

The latter, because it is small, is usually

from a height to illustrate potential energy

reckoned in terms of horsepower.

and kinetic energy the ball, the space

REST ENERGY:

The energy an object

possesses by virtue of its mass. RIGHT TRIANGLE:

A triangle that

includes a right (90°) angle. The other two

through which it falls, and the ground below together form a system. A quantity that possesses

VECTOR:

both magnitude and direction.

angles are, by definition, acute or less

WATT:

than 90°.

to 1 joule per second. Because this is such a

SCALAR:

A quantity that possesses

only magnitude, with no specific direction. SI:

small unit, scientists and engineers typically speak in terms of kilowatts, or units of 1,000 watts.

An abbreviation of the French

Système International d’Unités, which means “International System of Units.” This is the term within the scientific community for the entire metric system, as applied to a wide variety of quantities ranging from length, weight and volume to work and power, as well as electromagnetic units. SYSTEM:

The metric unit of power, equal

WORK:

The exertion of force over a

given distance. Work is the product of force and distance, where force and distance are exerted in the same direction. Hence the actual formula for work is F • cos θ • s, where F = force, s = distance, and cos θ is equal to the cosine of the angle θ (the Greek letter theta) between F and s. In the metric or SI system, work is measured by

In discussions of energy, the

term “system” refers to a closed set of inter-

less able to uphold their already weak economic system. This was precisely Reagan’s purpose in using American economic might to outspend the Soviets—or, in the case of the proposed multitrillion-dollar Strategic Defense Initiative (SDI or “Star Wars”)—threatening to outspend them. The Soviets expended much of their economic energy in competing with U.S. military strength, and this (along with a number of other complex factors), spelled the beginning of the end of the Communist empire.

the joule (J), and in the British system by the foot-pound.

war like it would ever happen again—but brought on the specter of global annihilation. It created a superpower struggle—yet it also ultimately helped bring about the end of Soviet totalitarianism, thus opening the way for a greater level of peace and economic and cultural exchange than the world has ever known. Yet nuclear arsenals still remain, and the nuclear threat is far from over.

historical brief is to illustrate the epoch-making significance of a single scientific formula: E = mc2. It ended World War II and ensured that no

So just what is this literally earth-shattering formula? E stands for rest energy, m for mass, and c for the speed of light, which is 186,000 mi (297,600 km) per second. Squared, this yields an almost unbelievably staggering number.

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E = m c 2 . The purpose of the preceding

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Hence, even an object of insignificant mass possesses an incredible amount of rest energy. The baseball, for instance, weighs only about 0.333 lb, which—on Earth, at least—converts to 0.15 kg. (The latter is a unit of mass, as opposed to weight.) Yet when factored into the rest energy equation, it yields about 3.75 billion kilowatthours—enough to provide an American home with enough electrical power to last it more than 156,000 years! How can a mere baseball possess such energy? It is not the baseball in and of itself, but its mass; thus every object with mass of any kind possesses rest energy. Often, mass energy can be released in very small quantities through purely thermal or chemical processes: hence, when a fire burns, an almost infinitesimal portion of the matter that went into making the fire is converted into energy. If a stick of dynamite that weighed 2.2 lb (1 kg) exploded, the portion of it that “disappeared” would be equal to 6 parts out of 100 billion; yet that portion would cause a blast of considerable proportions. As noted much earlier, the derivation of Einstein’s formula—and, more to the point, how he came to recognize the fundamental principles involved—is far beyond the scope of this essay. What is important is the fact, hypothesized by Einstein and confirmed in subsequent experiments, that matter is convertible to energy, a fact that becomes apparent when matter is accelerated to speeds close to that of light.

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energy. Instead, atomic energy—whether of the wartime or peacetime varieties (that is, in power plants)—involves the acceleration of mere atomic particles. Nor is any atom as good as another. Typically physicists use uranium and other extremely rare minerals, and often, they further process these minerals in highly specialized ways. It is the rarity and expense of those minerals, incidentally—not the difficulty of actually putting atomic principles to work—that has kept smaller nations from developing their own nuclear arsenals. WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. Berger, Melvin. Sound, Heat and Light: Energy at Work. Illustrated by Anna DiVito. New York: Scholastic, 1992. Gardner, Robert. Energy Projects for Young Scientists. New York: F. Watts, 1987. “Kinetic and Potential Energy” Thinkquest (Web site). (March 12, 2001). Snedden, Robert. Energy. Des Plaines, IL: Heinemann, Library, 1999. Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996. “Work and Energy” (Web site). (March 12, 2001). World of Coasters (Web site). (March 12, 2001).

Physicists do not possess a means for propelling a baseball to a speed near that of light— or of controlling its behavior and capturing its

Zubrowski, Bernie. Raceways: Having Fun with Balls and Tracks. Illustrated by Roy Doty. New York: Morrow, 1985.

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S C I E N C E O F E V E RY DAY T H I N G S real-life Physics

THERMODYNAMICS GAS LAWS MOLECULAR DYNAMICS S T R U C T U R E O F M AT T E R THERMODYNAMICS H E AT T E M P E RAT U R E T H E R M A L E X PA N S I O N

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Gas Laws

CONCEPT Gases respond more dramatically to temperature and pressure than do the other three basic types of matter (liquids, solids and plasma). For gases, temperature and pressure are closely related to volume, and this allows us to predict their behavior under certain conditions. These predictions can explain mundane occurrences, such as the fact that an open can of soda will soon lose its fizz, but they also apply to more dramatic, lifeand-death situations.

HOW IT WORKS Ordinary air pressure at sea level is equal to 14.7 pounds per square inch, a quantity referred to as an atmosphere (atm). Because a pound is a unit of force and a kilogram a unit of mass, the metric equivalent is more complex in derivation. A newton (N), or 0.2248 pounds, is the metric unit of force, and a pascal (Pa)—1 newton per square meter—the unit of pressure. Hence, an atmosphere, expressed in metric terms, is 1.013 ⫻ 105 Pa.

Gases vs. Solids and Liquids: A Strikingly Different Response Regardless of the units you use, however, gases respond to changes in pressure and temperature in a remarkably different way than do solids or liquids. Using a small water sample, say, 0.2642 gal (1 l), an increase in pressure from 1-2 atm will decrease the volume of the water by less than 0.01%. A temperature increase from 32° to 212°F (0 to 100°C) will increase its volume by only 2% The response of a solid to these changes is even

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GAS L AWS

less dramatic; however, the reaction of air (a combination of oxygen, nitrogen, and other gases) to changes in pressure and temperature is radically different. For air, an equivalent temperature increase would result in a volume increase of 37%, and an equivalent pressure increase will decrease the volume by a whopping 50%. Air and other gases also have a boiling point below room temperature, whereas the boiling point for water is higher than room temperature and that of solids is much higher. The reason for this striking difference in response can be explained by comparing all three forms of matter in terms of their overall structure, and in terms of their molecular behavior. (Plasma, a gas-like state found, for instance, in stars and comets’ tails, does not exist on Earth, and therefore it will not be included in the comparisons that follow.)

Molecular Structure Determines Reaction Solids possess a definite volume and a definite shape, and are relatively noncompressible: for instance, if you apply extreme pressure to a steel plate, it will bend, but not much. Liquids have a definite volume, but no definite shape, and tend to be noncompressible. Gases, on the other hand, possess no definite volume or shape, and are compressible. At the molecular level, particles of solids tend to be definite in their arrangement and close in proximity—indeed, part of what makes a solid “solid,” in the everyday meaning of that term, is the fact that its constituent parts are basically immovable. Liquid molecules, too, are close in proximity, though random in arrangement. Gas

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molecules, too, are random in arrangement, but tend to be more widely spaced than liquid molecules. Solid particles are slow moving, and have a strong attraction to one another, whereas gas particles are fast-moving, and have little or no attraction. (Liquids are moderate in both regards.) Given these interesting characteristics of gases, it follows that a unique set of parameters— collectively known as the “gas laws”—are needed to describe and predict their behavior. Most of the gas laws were derived during the eighteenth and nineteenth centuries by scientists whose work is commemorated by the association of their names with the laws they discovered. These men include the English chemists Robert Boyle (1627-1691), John Dalton (1766-1844), and William Henry (1774-1836); the French physicists and chemists J. A. C. Charles (1746-1823) and Joseph Gay-Lussac (1778-1850), and the Italian physicist Amedeo Avogadro (1776-1856).

Boyle’s, Charles’s, and GayLussac’s Laws Boyle’s law holds that in isothermal conditions (that is, a situation in which temperature is kept constant), an inverse relationship exists between the volume and pressure of a gas. (An inverse relationship is a situation involving two variables, in which one of the two increases in direct proportion to the decrease in the other.) In this case, the greater the pressure, the less the volume and vice versa. Therefore the product of the volume multiplied by the pressure remains constant in all circumstances. Charles’s law also yields a constant, but in this case the temperature and volume are allowed to vary under isobarometric conditions—that is, a situation in which the pressure remains the same. As gas heats up, its volume increases, and when it cools down, its volume reduces accordingly. Hence, Charles established that the ratio of temperature to volume is constant.

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In Gay-Lussac’s law, a third parameter, volume, is treated as a constant, and the result is a constant ratio between the variables of pressure and temperature. According to Gay-Lussac’s law, the pressure of a gas is directly related to its absolute temperature. Absolute temperature refers to the Kelvin scale, established by William Thomson, Lord Kelvin (1824-1907). Drawing on Charles’s discovery that gas at 0°C (32°F) regularly contracted by about 1/273 of its volume for every Celsius degree drop in temperature, Thomson derived the value of absolute zero (-273.15°C or -459.67°F). Using the Kelvin scale of absolute temperature, Gay-Lussac found that at lower temperatures, the pressure of a gas is lower, while at higher temperatures its pressure is higher. Thus, the ratio of pressure to temperature is a constant.

Avogadro’s Law Gay-Lussac also discovered that the ratio in which gases combine to form compounds can be expressed in whole numbers: for instance, water is composed of one part oxygen and two parts hydrogen. In the language of modern science, this would be expressed as a relationship between molecules and atoms: one molecule of water contains one oxygen atom and two hydrogen atoms. In the early nineteenth century, however, scientists had yet to recognize a meaningful distinction between atoms and molecules. Avogadro was the first to achieve an understanding of the difference. Intrigued by the whole-number relationship discovered by Gay-Lussac, Avogadro reasoned that one liter of any gas must contain the same number of particles as a liter of another gas. He further maintained that gas consists of particles—which he called molecules—that in turn consist of one or more smaller particles.

By now a pattern should be emerging: both of the aforementioned laws treat one parameter (temperature in Boyle’s, pressure in Charles’s) as unvarying, while two other factors are treated as variables. Both in turn yield relationships between the two variables: in Boyle’s law, pressure and volume are inversely related, whereas in Charles’s law, temperature and volume are directly related.

In order to discuss the behavior of molecules, it was necessary to establish a large quantity as a basic unit, since molecules themselves are very small. For this purpose, Avogadro established the mole, a unit equal to 6.022137 ⫻ 1023 (more than 600 billion trillion) molecules. The term “mole” can be used in the same way we use the word “dozen.” Just as “a dozen” can refer to twelve cakes or twelve chickens, so “mole” always describes the same number of molecules.

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Just as one liter of water, or one liter of mercury, has a certain mass, a mole of any given substance has its own particular mass, expressed in grams. The mass of one mole of iron, for instance, will always be greater than that of one mole of oxygen. The ratio between them is exactly the same as the ratio of the mass of one iron atom to one oxygen atom. Thus the mole makes if possible to compare the mass of one element or one compound to that of another.

Gas Laws

Avogadro’s law describes the connection between gas volume and number of moles. According to Avogadro’s law, if the volume of gas is increased under isothermal and isobarometric conditions, the number of moles also increases. The ratio between volume and number of moles is therefore a constant.

The Ideal Gas Law Once again, it is easy to see how Avogadro’s law can be related to the laws discussed earlier, since they each involve two or more of the four parameters: temperature, pressure, volume, and quantity of molecules (that is, number of moles). In fact, all the laws so far described are brought together in what is known as the ideal gas law, sometimes called the combined gas law. The ideal gas law can be stated as a formula, pV = nRT, where p stands for pressure, V for volume, n for number of moles, and T for temperature. R is known as the universal gas constant, a figure equal to 0.0821 atm • liter/mole • K. (Like most terms in physics, this one is best expressed in metric rather than English units.) Given the equation pV = nRT and the fact that R is a constant, it is possible to find the value of any one variable—pressure, volume, number of moles, or temperature—as long as one knows the value of the other three. The ideal gas law also makes it possible to discern certain relations: thus if a gas is in a relatively cool state, the product of its pressure and volume is proportionately low; and if heated, its pressure and volume product increases correspondingly. Thus

V T1

p

1 1

=

V T2 ,

p

2 2

A

FIRE EXTINGUISHER CONTAINS A HIGH-PRESSURE MIX-

TURE OF WATER AND CARBON DIOXIDE THAT RUSHES OUT OF THE SIPHON TUBE, WHICH IS OPENED WHEN THE RELEASE VALVE IS DEPRESSED. (Photograph by Craig Lovell/

Corbis. Reproduced by permission.)

p2V2 the product of its final volume and final pressure, and T2 its final temperature.

Five Postulates Regarding the Behavior of Gases Five postulates can be applied to gases. These more or less restate the terms of the earlier discussion, in which gases were compared to solids and liquids; however, now those comparisons can be seen in light of the gas laws. First, the size of gas molecules is minuscule in comparison to the distance between them, making gas highly compressible. In other words, there is a relatively high proportion of empty space between gas molecules. Second, there is virtually no force attracting gas molecules to one another.

where p1V1 is the product of its initial pressure and its initial volume, T1 its initial temperature,

Third, though gas molecules move randomly, frequently colliding with one another, their net effect is to create uniform pressure.

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Gas Laws

A

HOT-AIR BALLOON FLOATS BECAUSE THE AIR INSIDE IT IS NOT AS DENSE THAN THE AIR OUTSIDE.

THE

WAY IN

WHICH THE DENSITY OF THE AIR IN THE BALLOON IS REDUCED REFLECTS THE GAS LAWS. (Duomo/Corbis. Reproduced by

permission.)

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Fourth, the elastic nature of the collisions results in no net loss of kinetic energy, the energy that an object possesses by virtue of its motion. If a stone is dropped from a height, it

rapidly builds kinetic energy, but upon hitting a nonelastic surface such as pavement, most of that kinetic energy is transferred to the pavement. In the case of two gas molecules colliding, however,

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they simply bounce off one another, only to collide with other molecules and so on, with no kinetic energy lost. Fifth, the kinetic energy of all gas molecules is directly proportional to the absolute temperature of the gas.

Laws of Partial Pressure Two gas laws describe partial pressure. Dalton’s law of partial pressure states that the total pressure of a gas is equal to the sum of its par tial pressures—that is, the pressure exerted by each component of the gas mixture. As noted earlier, air is composed mostly of nitrogen and oxygen. Along with these are small components carbon dioxide and gases collectively known as the rare or noble gases: argon, helium, krypton, neon, radon, and xenon. Hence, the total pressure of a given quantity of air is equal to the sum of the pressures exerted by each of these gases. Henry’s law states that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of the gas above the surface of the solution. This applies only to gases such as oxygen and hydrogen that do not react chemically to liquids. On the other hand, hydrochloric acid will ionize when introduced to water: one or more of its electrons will be removed, and its atoms will convert to ions, which are either positive or negative in charge.

REAL-LIFE A P P L I C AT I O N S Pressure Changes O P E N I N G A S O DA CA N . Inside a can or bottle of carbonated soda is carbon dioxide gas (CO2), most of which is dissolved in the drink itself. But some of it is in the space (sometimes referred to as “head space”) that makes up the difference between the volume of the soft drink and the volume of the container.

At the bottling plant, the soda manufacturer adds high-pressure carbon dioxide to the head space in order to ensure that more CO2 will be absorbed into the soda itself. This is in accordance with Henry’s law: the amount of gas (in this case CO2) dissolved in the liquid (soda) is directly proportional to the partial pressure of

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the gas above the surface of the solution—that is, the CO2 in the head space. The higher the pressure of the CO2 in the head space, the greater the amount of CO2 in the drink itself; and the greater the CO2 in the drink, the greater the “fizz” of the soda.

Gas Laws

Once the container is opened, the pressure in the head space drops dramatically. Once again, Henry’s law indicates that this drop in pressure will be reflected by a corresponding drop in the amount of CO2 dissolved in the soda. Over a period of time, the soda will release that gas, and will eventually go “flat.” F I R E E X T I N G U I S H E R S . A fire extinguisher consists of a long cylinder with an operating lever at the top. Inside the cylinder is a tube of carbon dioxide surrounded by a quantity of water, which creates pressure around the CO2 tube. A siphon tube runs vertically along the length of the extinguisher, with one opening near the bottom of the water. The other end opens in a chamber containing a spring mechanism attached to a release valve in the CO2 tube.

The water and the CO2 do not fill the entire cylinder: as with the soda can, there is “head space,” an area filled with air. When the operating lever is depressed, it activates the spring mechanism, which pierces the release valve at the top of the CO2 tube. When the valve opens, the CO2 spills out in the “head space,” exerting pressure on the water. This high-pressure mixture of water and carbon dioxide goes rushing out of the siphon tube, which was opened when the release valve was depressed. All of this happens, of course, in a fraction of a second—plenty of time to put out the fire. A E R O S O L CA N S . Aerosol cans are

similar in structure to fire extinguishers, though with one important difference. As with the fire extinguisher, an aerosol can includes a nozzle that depresses a spring mechanism, which in turn allows fluid to escape through a tube. But instead of a gas cartridge surrounded by water, most of the can’s interior is made up of the product (for instance, deodorant), mixed with a liquid propellant. The “head space” of the aerosol can is filled with highly pressurized propellant in gas form, and in accordance with Henry’s law, a corresponding proportion of this propellant is dissolved in the product itself. When the nozzle is depressed,

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the pressure of the propellant forces the product out through the nozzle. A propellant, as its name implies, propels the product itself through the spray nozzle when the latter is depressed. In the past, chlorofluorocarbons (CFCs)—manufactured compounds containing carbon, chlorine, and fluorine atoms— were the most widely used form of propellant. Concerns over the harmful effects of CFCs on the environment, however, has led to the development of alternative propellants, most notably hydrochlorofluorocarbons (HCFCs), CFC-like compounds that also contain hydrogen atoms.

When the Temperature Changes A number of interesting things, some of them unfortunate and some potentially lethal, occur when gases experience a change in temperature. In these instances, it is possible to see the gas laws—particularly Boyle’s and Charles’s— at work. There are a number of examples of the disastrous effects that result from an increase in the temperature of a product containing combustible gases, as with natural gas and petroleum-based products. In addition, the pressure on the gases in aerosol cans makes the cans highly explosive—so much so that discarded cans at a city dump may explode on a hot summer day. Yet there are other instances when heating a gas can produce positive effects. A hot-air balloon, for instance, floats because the air inside it is not as dense than the air outside. By itself, this fact does not depend on any of the gas laws, but rather reflects the concept of buoyancy. However, the way in which the density of the air in the balloon is reduced does indeed reflect the gas laws. According to Charles’s law, heating a gas will increase its volume. Also, as noted in the first and second propositions regarding the behavior of gases, gas molecules are highly nonattractive to one another, and therefore, there is a great deal of space between them. The increase in volume makes that space even greater, leading to a significant difference in density between the air in the balloon and the air outside. As a result, the balloon floats, or becomes buoyant.

188

were to put a bag of potato chips into a freezer, thinking this would preserve their flavor, he would be in for a disappointment. Much of what maintains the flavor of the chips is the pressurization of the bag, which ensures a consistent internal environment in which preservative chemicals, added during the manufacture of the chips, can keep them fresh. Placing the bag in the freezer causes a reduction in pressure, as per GayLussac’s law, and the bag ends up a limp version of its earlier self. Propane tanks and tires offer an example of the pitfalls that may occur by either allowing a gas to heat up or cool down by too much. Because most propane tanks are made according to strict regulations, they are generally safe, but it is not entirely inconceivable that an extremely hot summer day could cause a defective tank to burst. Certainly the laws of physics are there: an increase in temperature leads to an increase in pressure, in accordance with Gay-Lussac’s law, and could lead to an explosion. Because of the connection between heat and pressure, propane trucks on the highways during the summer are subjected to weight tests to ensure that they are not carrying too much of the gas. On the other hand, a drastic reduction in temperature could result in a loss in gas pressure. If a propane tank from Florida were transported by truck during the winter to northern Canada, the pressure would be dramatically reduced by the time it reached its destination.

Gas Reactions That Move and Stop a Car In operating a car, we experience two examples of gas laws in operation. One of these, common to everyone, is that which makes the car run: the combustion of gases in the engine. The other is, fortunately, a less frequent phenomenon—but it can and does save lives. This is the operation of an air bag, which, though it is partly related to laws of motion, depends also on the behaviors explained in Charles’s law.

Although heating a gas can be beneficial, cooling a gas is not always a wise idea. If someone

With regard to the engine, when the driver pushes down on the accelerator, this activates a throttle valve that sprays droplets of gasoline mixed with air into the engine. (Older vehicles used a carburetor to mix the gasoline and air, but most modern cars use fuel-injection, which sprays the air-gas combination without requiring an intermediate step.) The mixture goes into the

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Gas Laws

IN

CASE OF A CAR COLLISION, A SENSOR TRIGGERS THE AIR BAG TO INFLATE RAPIDLY WITH NITROGEN GAS.

BEFORE

YOUR BODY REACHES THE BAG, HOWEVER, IT HAS ALREADY BEGUN DEFLATING. (Illustration by Hans & Cassidy. The Gale

Group.)

cylinder, where the piston moves up, compressing the gas and air.

and air are what move the piston, which turns a crankshaft that causes the wheels to rotate.

While the mixture is still compressed (high pressure, high density), an electric spark plug produces a flash that ignites it. The heat from this controlled explosion increases the volume of air, which forces the piston down into the cylinder. This opens an outlet valve, causing the piston to rise and release exhaust gases.

So much for moving—what about stopping? Most modern cars are equipped with an airbag, which reacts to sudden impact by inflating. This protects the driver and front-seat passenger, who, even if they are wearing seatbelts, may otherwise be thrown against the steering wheel or dashboard..

As the piston moves back down again, an inlet valve opens, bringing another burst of gasoline-air mixture into the chamber. The piston, whose downward stroke closed the inlet valve, now shoots back up, compressing the gas and air to repeat the cycle. The reactions of the gasoline

But an airbag is much more complicated than it seems. In order for it to save lives, it must deploy within 40 milliseconds (0.04 seconds). Not only that, but it has to begin deflating before the body hits it. An airbag does not inflate if a car simply goes over a bump; it only operates in sit-

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Gas Laws

KEY TERMS Tem-

1691), which holds that for gases in

perature in relation to absolute zero

isothermal conditions, an inverse relation-

(-273.15°C or -459.67°F). Its unit is the

ship exists between the volume and pres-

Kelvin (K), named after William Thomson,

sure of a gas. This means that the greater

Lord Kelvin (1824-1907), who created the

the pressure, the less the volume and vice

scale. The Kelvin and Celsius scales are

versa, and therefore the product of pres-

directly related; hence, Celsius tempera-

sure multiplied by volume yields a constant

tures can be converted to Kelvins (for

figure.

ABSOLUTE TEMPERATURE:

which neither the word or symbol for “degree” are used) by adding 273.15.

LAW:

A

statement,

derived by French physicist and chemist

A statement,

J. A. C. Charles (1746-1823), which holds

derived by the Italian physicist Amedeo

that for gases in isobarometric conditions,

Avogadro (1776-1856), which holds that as

the ratio between the volume and temper-

the volume of gas increases under isother-

ature of a gas is constant. This means that

mal and isobarometric conditions, the

the greater the temperature, the greater the

number of molecules (expressed in terms

volume and vice versa.

AVOGADRO’S

LAW:

of mole number), increases as well. Thus the ratio of volume to mole number is a constant.

DALTON’S LAW OF PARTIAL PRESSURE:

A statement, derived by the

English chemist John Dalton (1766-1844), A statement, derived

which holds that the total pressure of a gas

by English chemist Robert Boyle (1627-

is equal to the sum of its partial pres-

BOYLE’S LAW:

uations when the vehicle experiences extreme deceleration. When this occurs, there is a rapid transfer of kinetic energy to rest energy, as with the earlier illustration of a stone hitting concrete. And indeed, if you were to smash against a fully inflated airbag, it would feel like hitting concrete—with all the expected results. The airbag’s sensor contains a steel ball attached to a permanent magnet or a stiff spring. The spring holds it in place through minor mishaps in which an airbag would not be warranted—for instance, if a car were simply to be “tapped” by another in a parking lot. But in a case of sudden deceleration, the magnet or spring releases the ball, sending it down a smooth bore. It flips a switch, turning on an electrical circuit. This in turn ignites a pellet of sodium azide, which fills the bag with nitrogen gas.

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CHARLES’S

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The events described in the above illustration take place within 40 milliseconds—less time than it takes for your body to come flying forward; and then the airbag has to begin deflating before the body reaches it. At this point, the highly pressurized nitrogen gas molecules begin escaping through vents. Thus as your body hits the bag, the deflation of the latter is moving it in the same direction that your body is going—only much, much more slowly. Two seconds after impact, which is an eternity in terms of the processes involved, the pressure inside the bag has returned to 1 atm. WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. “Chemistry Units: Gas Laws.” (Web site). (February 21, 2001).

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Gas Laws

KEY TERMS

CONTINUED

sures—that is, the pressure exerted by each

expressed as the formula pV = nRT, where

component of the gas mixture.

p stands for pressure, V for volume, n for

A statement,

number of moles, and T for temperature. R

derived by the French physicist and

is known as the universal gas constant, a

chemist Joseph Gay-Lussac (1778-1850),

figure equal to 0.0821 atm • liter/mole • K.

which holds that the pressure of a gas is

INVERSE RELATIONSHIP:

directly related to its absolute temperature.

tion involving two variables, in which one

Hence the ratio of pressure to absolute

of the two increases in direct proportion to

temperature is a constant.

the decrease in the other.

GAY-LUSSAC’S LAW:

HENRY’S LAW:

A statement, derived

IONIZATION:

A situa-

A reaction in which an

by the English chemist William Henry

atom or group of atoms loses one or more

(1774-836), which holds that the amount

electrons. The atoms are then converted to

of gas dissolved in a liquid is directly pro-

ions, which are either wholly positive or

portional to the partial pressure of the gas

negative in charge.

above the solution. This holds true only for gases, such as hydrogen and oxygen, that are capable of dissolving in water without undergoing ionization. IDEAL GAS LAW:

ISOTHERMAL:

Referring to a situa-

tion in which temperature is kept constant. ISOBAROMETRIC:

A proposition, also

uation in which pressure is kept constant.

known as the combined gas law, that draws

MOLE:

on all the gas laws. The ideal gas law can be

molecules.

Laws of Gases. New York: Arno Press, 1981. Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998.

Referring to a sit-

A unit equal to 6.022137 ⫻ 1023

“Tutorials—6.” (February 21, 2001).

Mebane, Robert C. and Thomas R. Rybolt. Air and Other Gases. Illustrations by Anni Matsick. New York: Twenty-First Century Books, 1995.

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MOLECUL AR DYNAMICS Molecular Dynamics

CONCEPT Physicists study matter and motion, or matter in motion. These forms of matter may be large, or they may be far too small to be seen by the most high-powered microscopes available. Such is the realm of molecular dynamics, the study and simulation of molecular motion. As its name suggests, molecular dynamics brings in aspects of dynamics, the study of why objects move as they do, as well as thermodynamics, the study of the relationships between heat, work, and energy. Existing at the borders between physics and chemistry, molecular dynamics provides understanding regarding the properties of matter— including phenomena such as the liquefaction of gases, in which one phase of matter is transformed into another.

HOW IT WORKS Molecules The physical world is made up of matter, physical substance that has mass; occupies space; is composed of atoms; and is, ultimately, convertible to energy. On Earth, three principal phases of matter exist, namely solid, liquid, and gas. The differences between these three are, on the surface at least, easily perceivable. Clearly, water is a liquid, just as ice is a solid and steam a gas. Yet, the ways in which various substances convert between phases are often complex, as are the interrelations between these phases. Ultimately, understanding of the phases depends on an awareness of what takes place at the molecular level.

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in the universe; atoms are composed of subatomic particles, including protons, neutrons, and electrons. These subatomic particles are discussed in the context of the structure of matter elsewhere in this volume, where they are examined largely with regard to their electromagnetic properties. In the present context, the concern is primarily with the properties of atomic and molecular particles, in terms of mechanics, the study of bodies in motion, and thermodynamics. An atom must, by definition, represent one and only one chemical element, of which 109 have been identified and named. It should be noted that the number of elements changes with continuing research, and that many of the elements, particularly those discovered relatively recently—as, for instance, meitnerium (No. 109), isolated in the 1990s—are hardly part of everyday experience. So, perhaps 100 would be a better approximation; in any case, consider the multitude of possible ways in which the elements can be combined.

An atom is the smallest particle of a chemical element. It is not, however, the smallest thing

Musicians have only seven tones at their disposal, and artists only seven colors—yet they manage to create a seemingly infinite variety of mutations in sound and sight, respectively. There are only 10 digits in the numerical system that has prevailed throughout the West since the late Middle Ages, yet it is possible to use that system to create such a range of numbers that all the books in all the libraries in the world could not contain them. This gives some idea of the range of combinations available using the hundredodd chemical elements nature has provided—in other words, the number of possible molecular combinations that exist in the universe.

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Molecular Dynamics

THIS

HUGE LIQUEFIED NATURAL GAS CONTAINER WILL BE INSTALLED ON A SHIP.

THE

VOLUME OF THE LIQUEFIED GAS

IS FAR LESS THAN IT WOULD BE IF THE GAS WERE IN A VAPORIZED STATE, THUS ENABLING EASE AND ECONOMY IN TRANSPORT. (Photograph by James L. Amos/Corbis. Reproduced by permission.)

THE

STRUCTURE

OF

MOLE-

A molecule is a group of atoms joined in a single structure. Often, these atoms come from different elements, in which case the molecule represents a particular chemical compound, such as water, carbon dioxide, sodium chloride (salt), and so on. On the other hand, a molecule may consist only of one type of atom: oxygen molecules, for instance, are formed by the joining of two oxygen atoms.

CULES.

As much as scientists understand about molecules and their structure, there is much that they do not know. Molecules of water are fairly easy to understand, because they have a simple, regular structure that does not change. A water molecule is composed of one oxygen atom joined by two hydrogen atoms, and since the oxygen atom is much larger than the two hydrogens, its shape can be compared to a basketball with two softballs attached. The scale of the molecule, of course, is so small as to boggle the mind: to borrow an illustration from American physicist Richard Feynman (1918-1988), if a basketball were blown up to the size of Earth, the molecules inside of it would not even be as large as an ordinary-sized basketball.

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As for the water molecule, scientists know a number of things about it: the distance between the two hydrogen atoms (measured in units called an angstrom), and even the angle at which they join the oxygen atom. In the case of salt, however, the molecular structure is not nearly as uniform as that of water: atoms join together, but not always in regular ways. And then there are compounds far more complex than water or salt, involving numerous elements that fit together in precise and complicated ways. But, once that discussion is opened, one has stepped from the realm of physics into that of chemistry, and that is not the intention here. Rather, the purpose of the foregoing and very cursory discussion of molecular structure is to point out that molecules are at the heart of all physical existence— and that the things we cannot see are every bit as complicated as those we can. T H E M O L E . Given the tiny—to use an understatement—size of molecules, how do scientists analyze their behavior? Today, physicists have at their disposal electron microscopes and other advanced forms of equipment that make it possible to observe activity at the atomic and molecular levels. The technology that makes this possible is beyond the scope of the present dis-

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Molecular Dynamics

HOW

SMALL ARE MOLECULES? IF THIS BASKETBALL WERE BLOWN UP TO THE SIZE OF

EARTH,

THE MOLECULES INSIDE

IT WOULD NOT BE AS BIG AS A REAL BASKETBALL. (Photograph by Dimitri Iundt/Corbis. Reproduced by permission.)

cussion. On the other hand, consider a much simpler question: how do physicists weigh molecules? Obviously “a bunch” of iron (an element known by the chemical symbol Fe) weighs more than “a bunch” of oxygen, but what exactly is “a bunch”? Italian physicist Amedeo Avogadro (1776-1856), the first scientist to clarify the distinction between atoms and molecules, created a unit that made it possible to compare the masses of various molecules. This is the mole, also known as “Avogadro’s number,” a unit equal to 6.022137 ⫻ 1023 (more than 600 billion trillion) molecules.

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Molecular Attraction and Motion Molecular dynamics can be understood primarily in terms of the principles of motion, identified by Sir Isaac Newton (1642-1727), principles that receive detailed discussion at several places in this volume. However, the attraction between particles at the atomic and molecular level cannot be explained by reference to gravitational force, also identified by Newton. For more than a century, gravity was the only type of force known to physicists, yet the pull of gravitation alone was too weak to account for the strong pull between atoms and molecules.

The term “mole” can be used in the same way that the word “dozen” is used. Just as “a dozen” can refer to twelve cakes or twelve chickens, so “mole” always describes the same number of molecules. A mole of any given substance has its own particular mass, expressed in grams. The mass of one mole of iron, for instance, will always be greater than that of one mole of oxygen. The ratio between them is exactly the same as the ratio of the mass of one iron atom to one oxygen atom. Thus, the mole makes it possible to compare the mass of one element or compound to that of another.

During the eighteenth century and early nineteenth centuries, however, physicists and other scientists became increasingly aware of another form of interaction at work in the world—one that could not be explained in gravitational terms. This was the force of electricity and magnetism, which Scottish physicist James Clerk Maxwell (1831-1879) suggested were different manifestations of a “new” kind of force, electromagnetism. All subatomic particles possess either a positive, negative, or neutral electrical charge. An atom usually has a neutral charge, meaning that it is composed of an equal number of protons (positive) and electrons (negative). In

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certain situations, however, it may lose one or more electrons and, thus, acquire a net charge, making it an ion. Positive and negative charges attract one another, much as the north and south poles of two different magnets attract. (In fact, magnetism is simply an aspect of electromagnetic force.) Not only do the positive and negative elements of an atom attract one another, but positive elements in atoms attract negative elements in other atoms, and vice versa. These interactions are much more complex than the preceding discussion suggests, of course; the important point is that a force other than gravitation draws matter together at the atomic and molecular levels. On the other hand, the interactions that are critical to the study of molecular dynamics are primarily mechanical, comprehensible from the standpoint of Newtonian dynamics. M O L E C U LA R B E H AV I O R A N D P H AS E S O F M AT T E R. All molecules are

in motion, and the rate of that motion is affected by the attraction between them. This attraction or repulsion can be though of like a spring connecting two molecules, an analogy that works best for solids, but in a limited way for liquids. Most molecular motion in liquids and gases is caused by collisions with other molecules; even in solids, momentum is transferred from one molecule to the next along the “springs,” but ultimately the motion is caused by collisions. Hence molecular collisions provide the mechanism by which heat is transferred between two bodies in contact. The rate at which molecules move in relation to one another determines phase of matter—that is, whether a particular item can be described as solid, liquid, or gas. The movement of molecules means that they possess kinetic energy, or the energy of movement, which is manifested as thermal energy and measured by temperature. Temperature is really nothing more than molecules in motion, relative to one another: the faster they move, the greater the kinetic energy, and the greater the temperature. When the molecules in a material move slowly in relation to one another, they tend to be close in proximity, and hence the force of attraction between them is strong. Such a material is called a solid. In molecules of liquid, by contrast, the rate of relative motion is higher, so the molecules tend to be a little more spread out, and

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therefore the force between them is weaker. A material substance whose molecules move at high speeds, and therefore exert little attraction toward one another, is known as a gas. All forms of matter possess a certain (very large) amount of energy due to their mass; thermal energy, however, is—like phase of matter—a function of the attractions between particles. Hence, solids generally have less energy than liquids, and liquids less energy than gases.

Molecular Dynamics

REAL-LIFE A P P L I C AT I O N S Kinetic Theories of Matter English chemist John Dalton (1766-1844) was the first to recognize that nature is composed of tiny particles. In putting forward his idea, Dalton adopted a concept from the Greek philosopher Democritus (c. 470-380 B.C.), who proposed that matter is made up of tiny units he called atomos, or “indivisible.” Dalton recognized that the structure of atoms in a particular element or compound is uniform, and maintained that compounds are made up of compound atoms: in other words, that water, for instance, is composed of “water atoms.” Soon after Dalton, however, Avogadro clarified the distinction between atoms and molecules. Neither Dalton nor Avogadro offered much in the way of a theory regarding atomic or molecular behavior; but another scientist had already introduced the idea that matter at the smallest levels is in a constant state of motion. This was Daniel Bernoulli (1700-1782), a Swiss mathematician and physicist whose studies of fluids—a term which encompasses both gases and liquids—provided a foundation for the field of fluid mechanics. (Today, Bernoulli’s principle, which relates the velocity and pressure of fluids, is applied in the field of aerodynamics, and explains what keeps an airplane aloft.) Bernoulli published his fluid mechanics studies in Hydrodynamica (1700-1782), a work in which he provided the basis for what came to be known as the kinetic theory of gases. B R O W N I A N M O T I O N . Because he came before Dalton and Avogadro, and, thus, did not have the benefit of their atomic and molecular theories, Bernoulli was not able to develop his kinetic theory beyond the seeds of an idea. The

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subsequent elaboration of kinetic theory, which is applied not only to gases but (with somewhat less effectiveness) to liquids and solids, in fact, resulted from an accidental discovery. In 1827, Scottish botanist Robert Brown (1773-1858) was studying pollen grains under a microscope, when he noticed that the grains underwent a curious zigzagging motion in the water. The pollen assumed the shape of a colloid, a pattern that occurs when particles of one substance are dispersed—but not dissolved—in another substance. Another example of a colloidal pattern is a puff of smoke. At first, Brown assumed that the motion had a biological explanation—that is, that it resulted from life processes within the pollen—but later, he discovered that even pollen from long-dead plants behaved in the same way. He never understood what he was witnessing. Nor did a number of other scientists, who began noticing other examples of what came to be known as Brownian motion: the constant but irregular zigzagging of colloidal particles, which can be seen clearly through a microscope.

In 1905, Albert Einstein (1879-1955) analyzed the behavior of particles subjected to Brownian motion. His work, and the confirmation of his results by French physicist Jean Baptiste Perrin (1870-1942), finally put an end to any remaining doubts concerning the molecular structure of matter. The kinetic explanation of molecular behavior, however, remains a theory.

M AX W E L L , B O LT Z M A N N , A N D T H E M AT U R I N G O F K I N E T I C T H E O RY. A generation after Brown’s time, kinetic

Maxwell’s and Boltzmann’s work helped explain characteristics of matter at the molecular level, but did so most successfully with regard to gases. Kinetic theory fits with a number of behaviors exhibited by gases: their tendency to fill any container by expanding to fit its interior, for instance, and their ability to be easily compressed.

theory came to maturity through the work of Maxwell and Austrian physicist Ludwig E. Boltzmann (1844-1906). Working independently, the two men developed a theory, later dubbed the Maxwell-Boltzmann theory of gases, which described the distribution of molecules in a gas. In 1859, Maxwell described the distribution of molecular velocities, work that became the foundation of statistical mechanics—the study of large systems—by examining the behavior of their smallest parts. A year later, in 1860, Maxwell published a paper in which he presented the kinetic theory of gases: the idea that a gas consists of numerous molecules, relatively far apart in space, which interact by colliding. These collisions, he proposed, are responsible for the production of thermal energy, because when the velocity of the molecules increases—as it does after collision— the temperature increases as well. Eight years later, in 1868, Boltzmann independently applied statistics to the kinetic theory, explaining the behavior of gas molecules by means of what would come to be known as statistical mechanics.

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Kinetic theory offered a convincing explanation of the processes involved in Brownian motion. According to the kinetic view, what Brown observed had nothing to do with the pollen particles; rather, the movement of those particles was simply the result of activity on the part of the water molecules. Pollen grains are many thousands of times larger than water molecules, but since there are so many molecules in even one drop of water, and their motion is so constant but apparently random, the water molecules are bound to move a pollen grain once every few thousand collisions.

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Kinetic Theory and Gases

This, in turn, concurs with the gas laws (discussed in a separate essay titled “Gas Laws”)—for instance, Boyle’s law, which maintains that pressure decreases as volume increases, and vice versa. Indeed, the ideal gas law, which shows an inverse relationship between pressure and volume, and a proportional relationship between temperature and the product of pressure and volume, is an expression of kinetic theory. T H E GAS LAW S I L LU S T RAT E D .

The operations of the gas laws are easy to visualize by means of kinetic theory, which portrays gas molecules as though they were millions upon billions of tiny balls colliding at random. Inside a cube-shaped container of gas, molecules are colliding with every possible surface, but the net effect of these collisions is the same as though the molecules were divided into thirds, each third colliding with opposite walls inside the cube.

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If the cube were doubled in size, the molecules bouncing back and forth between two sets of walls would have twice as far to travel between each collision. Their speed would not change, but the time between collisions would double, thus, cutting in half the amount of pressure they would exert on the walls. This is an illustration of Boyle’s law: increasing the volume by a factor of two leads to a decrease in pressure to half of its original value. On the other hand, if the size of the container were decreased, the molecules would have less distance to travel from collision to collision. This means they would be colliding with the walls more often, and, thus, would have a higher degree of energy—and, hence, a higher temperature. This illustrates another gas law, Charles’s law, which relates volume to temperature: as one of the two increases or decreases, so does the other. Thus, it can be said, in light of kinetic theory, that the average kinetic energy produced by the motions of all the molecules in a gas is proportional to the absolute temperature of the gas. GAS E S A N D A B S O LU T E T E M P E RAT U R E . The term “absolute tempera-

ture” refers to the Kelvin scale, established by William Thomson, Lord Kelvin (1824-1907). Drawing on Charles’s discovery that gas at 0°C (32°F) regularly contracts by about 1/273 of its volume for every Celsius degree drop in temperature, Thomson derived the value of absolute zero (-273.15°C or -459.67°F). The Kelvin and Celsius scales are directly related; hence, Celsius temperatures can be converted to Kelvins by adding 273.15. The Kelvin scale measures temperature in relation to absolute zero, or 0K. (Units in the Kelvin system, known as Kelvins, do not include the word or symbol for degree.) But what is absolute zero, other than a very cold temperature? Kinetic theory provides a useful definition: the temperature at which all molecular movement in a gas ceases. But this definition requires some qualification.

Changes of Phase Kinetic theory is more successful when applied to gases than to liquids and solids, because liquid and solid molecules do not interact nearly as frequently as gas particles do. Nonetheless, the proposition that the internal energy of any substance—gas, liquid, or solid—is at least partly related to the kinetic energies of its molecules helps explain much about the behavior of matter.

Molecular Dynamics

The thermal expansion of a solid, for instance, can be clearly explained in terms of kinetic theory. As discussed in the essay on elasticity, many solids are composed of crystals, regular shapes composed of molecules joined to one another, as though on springs. A spring that is pulled back, just before it is released, is an example of potential energy: the energy that an object possesses by virtue of its position. For a crystalline solid at room temperature, potential energy and spacing between molecules are relatively low. But as temperature increases and the solid expands, the space between molecules increases—as does the potential energy in the solid. An example of a liquid displaying kinetic behavior is water in the process of vaporization. The vaporization of water, of course, occurs in boiling, but water need not be anywhere near the boiling point to evaporate. In either case, the process is the same. Speeds of molecules in any substance are distributed along a curve, meaning that a certain number of molecules have speeds well below, or well above, the average. Those whose speeds are well above the average have enough energy to escape the surface, and once they depart, the average energy of the remaining liquid is less than before. As a result, evaporation leads to cooling. (In boiling, of course, the continued application of thermal energy to the entire water sample will cause more molecules to achieve greater energy, even as highly energized molecules leave the surface of the boiling water as steam.)

The Phase Diagram

First of all, the laws of thermodynamics show the impossibility of actually reaching absolute zero. Second, the vibration of atoms never completely ceases: rather, the vibration of the average atom is zero. Finally, one element— helium—does not freeze, even at temperatures near absolute zero. Only the application of pressure will push helium past the freezing point.

The vaporization of water is an example of a change of phase—the transition from one phase of matter to another. The properties of any substance, and the points at which it changes phase, are plotted on what is known as a phase diagram. The latter typically shows temperature along the x-axis, and pressure along the y-axis. It is also possible to construct a phase diagram that plots

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volume against temperature, or volume against pressure, and there are even three-dimensional phase diagrams that measure the relationship between all three—volume, pressure, and temperature. Here we will consider the simpler twodimensional diagram we have described. For simple substances such as water and carbon dioxide, the solid form of the substance appears at a relatively low temperature, and at pressures anywhere from zero upward. The line between solids and liquids, indicating the temperature at which a solid becomes a liquid at any pressure above a certain level, is called the fusion curve. Though it appears to be a line, it is indeed curved, reflecting the fact that at high pressures, a solid well below the normal freezing point for that substance may be melted to create a liquid. Liquids occupy the area of the phase diagram corresponding to relatively high temperatures and high pressures. Gases or vapors, on the other hand, can exist at very low temperatures, but only if the pressure is also low. Above the melting point for the substance, gases exist at higher pressures and higher temperatures. Thus, the line between liquids and gases often looks almost like a 45° angle. But it is not a straight line, as its name, the vaporization curve, indicates. The curve of vaporization reflects the fact that at relatively high temperatures and high pressures, a substance is more likely to be a gas than a liquid. C R I T I CA L P O I N T A N D S U B L I M AT I O N . There are several other interesting

phenomena mapped on a phase diagram. One is the critical point, which can be found at a place of very high temperature and pressure along the vaporization curve. At the critical point, high temperatures prevent a liquid from remaining a liquid, no matter how high the pressure. At the same time, the pressure causes gas beyond that point to become more and more dense, but due to the high temperatures, it does not condense into a liquid. Beyond the critical point, the substance cannot exist in anything other than the gaseous state. The temperature component of the critical point for water is 705.2°F (374°C)—at 218 atm, or 218 times ordinary atmospheric pressure. For helium, however, critical temperature is just a few degrees above absolute zero. This is why helium is rarely seen in forms other than a gas.

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There is also a certain temperature and pressure, called the triple point, at which some substances—water and carbon dioxide are examples—will be a liquid, solid, and gas all at once. Another interesting phenomenon is the sublimation curve, or the line between solid and gas. At certain very low temperatures and pressures, a substance may experience sublimation, meaning that a gas turns into a solid, or a solid into a gas, without passing through a liquid stage. A wellknown example of sublimation occurs when “dry ice,” which is made of carbon dioxide, vaporizes at temperatures above (-109.3°F [-78.5°C]). Carbon dioxide is exceptional, however, in that it experiences sublimation at relatively high pressures, such as those experienced in everyday life: for most substances, the sublimation point occurs at such a low pressure point that it is seldom witnessed outside of a laboratory.

Liquefaction of Gases One interesting and useful application of phase change is the liquefaction of gases, or the change of gas into liquid by the reduction in its molecular energy levels. There are two important properties at work in liquefaction: critical temperature and critical pressure. Critical temperature is that temperature above which no amount of pressure will cause a gas to liquefy. Critical pressure is the amount of pressure required to liquefy the gas at critical temperature. Gases are liquefied by one of three methods: (1) application of pressure at temperatures below critical; (2) causing the gas to do work against external force, thus, removing its energy and changing it to the liquid state; or (3) causing the gas to do work against some internal force. The second option can be explained in terms of the operation of a heat engine, as explored in the Thermodynamics essay. In a steam engine, an example of a heat engine, water is boiled, producing energy in the form of steam. The steam is introduced to a cylinder, in which it pushes on a piston to drive some type of machinery. In pushing against the piston, the steam loses energy, and as a result, changes from a gas back to a liquid. As for the use of internal forces to cool a gas, this can be done by forcing the vapor through a small nozzle or porous plug. Depending on the temperature and properties of the gas, such an

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Molecular Dynamics

KEY TERMS The temperature,

ABSOLUTE ZERO:

DYNAMICS:

The study of why objects

defined as 0K on the Kelvin scale, at which

move as they do. Dynamics is an element

the motion of molecules in a solid virtu-

of mechanics.

ally ceases. Absolute zero is equal to

FLUID:

-459.67°F (-273.15°C).

liquid, which tends to flow, and which con-

Any substance, whether gas or

The smallest particle of a chem-

forms to the shape of its container. Unlike

ical element. An atom can exist either

solids, fluids are typically uniform in

alone or in combination with other atoms

molecular structure: for instance, one mol-

in a molecule.

ecule of water is the same as another water

ATOM:

The constant

molecule.

but irregular zigzagging of colloidal parti-

FUSION

cles, which can be seen clearly through a

between solid and liquid for any given sub-

microscope. The phenomenon is named

stance, as plotted on a phase diagram.

BROWNIAN MOTION:

CURVE:

The boundary

after Scottish botanist Robert Brown

GAS:

(1773-1858), who first witnessed it but was

cules exert little or no attraction toward

not able to explain it. The behavior exhib-

one another, and, therefore, move at high

ited in Brownian motion provides evi-

speeds.

dence for the kinetic theory of matter. HEAT: CHANGE OF PHASE:

The transition

A phase of matter in which mole-

Internal thermal energy that

flows from one body of matter to another.

from one phase of matter to another. KELVIN

SCALE:

Established

by

A sub-

William Thomson, Lord Kelvin (1824-

stance made up of atoms of more than one

1907), the Kelvin scale measures tempera-

chemical element. These atoms are usually

ture in relation to absolute zero, or 0K.

joined in molecules.

(Units in the Kelvin system, known as

CHEMICAL

COMPOUND:

CHEMICAL ELEMENT:

A substance

made up of only one kind of atom.

Kelvins, do not include the word or symbol for degree.) The Kelvin and Celsius scales

A pattern that occurs when

are directly related; hence, Celsius temper-

particles of one substance are dispersed—

atures can be converted to Kelvins by

but not dissolved—in another substance. A

adding 273.15.

puff of smoke in the air is an example of a

KINETIC ENERGY:

colloid, whose behavior is typically charac-

an object possesses by virtue of its motion.

terized by Brownian motion.

KINETIC THEORY OF GASES:

COLLOID:

The energy that The

A coordinate, plot-

idea that a gas consists of numerous mole-

ted on a phase diagram, above which a sub-

cules, relatively far apart in space, which

stance cannot exist in anything other than

interact by colliding. These collisions are

the gaseous state. Located at a position of

responsible for the production of thermal

very high temperature and pressure, the

energy, because when the velocity of the

critical point marks the termination of the

molecules increases—as it does after colli-

vaporization curve.

sion—the temperature increases as well.

CRITICAL POINT:

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Molecular Dynamics

KEY TERMS

CONTINUED

The

MOLE: A unit equal to 6.022137 ⫻ 1023

application of the kinetic theory of gases to

(more than 600 billion trillion) molecules.

all forms of matter. Since particles of liq-

Since their size makes it impossible to

uids and solids move much more slowly

weigh molecules in relatively small quanti-

than do gas particles, kinetic theory is not

ties; hence, the mole, devised by Italian

as successful in this regard; however, the

physicist Amedeo Avogadro (1776-1856),

proposition that the internal energy of any

facilitates comparisons of mass between

substance is at least partly related to the

substances.

kinetic energies of its molecules helps

MOLECULAR DYNAMICS:

explain much about the behavior of

and simulation of molecular motion.

KINETIC THEORY OF MATTER:

matter. MOLECULE: LIQUID:

A phase of matter in which

molecules exert moderate attractions

A group of atoms, usual-

ly of more than one chemical element, joined in a structure.

toward one another, and, therefore, move PHASE DIAGRAM:

at moderate speeds.

A chart, plotted

for any particular substance, identifying

Physical substance that has

the particular phase of matter for that sub-

mass; occupies space; is composed of

stance at a given temperature and pressure

atoms; and is ultimately convertible to

level. A phase diagram usually shows tem-

energy. There are several phases of matter,

perature along the x-axis, and pressure

including solids, liquids, and gases.

along the y-axis.

MATTER:

MECHANICS:

The study of bodies in

motion.

HISTORICAL

PHASES OF MATTER:

The various

forms of material substance (matter),

operation may be enough to remove energy sufficient for liquefaction to take place. Sometimes, the process must be repeated before the gas fully condenses into a liquid. BACKGROUND.

Like the steam engine itself, the idea of gas liquefaction is a product of the early Industrial Age. One of the pioneering figures in the field was the brilliant English physicist Michael Faraday (1791-1867), who liquefied a number of highcritical temperature gases, such as carbon dioxide.

200

The study

uefy gases with much lower critical temperatures, among them oxygen, nitrogen, and carbon monoxide. By the end of the nineteenth century, physicists were able to liquefy the gases with the lowest critical temperatures. James Dewar of Scotland (1842-1923) liquefied hydrogen, whose critical temperature is -399.5°F (-239.7°C). Some time later, Dutch physicist Heike Kamerlingh Onnes (1853-1926) successfully liquefied the gas with the lowest critical temperature of them all: helium, which, as mentioned earlier, becomes a gas at almost unbelievably low temperatures. Its critical temperature is -449.9°F (-267.7°C), or just 5.3K.

Half a century after Faraday, French physicist Louis Paul Cailletet (1832-1913) and Swiss chemist Raoul Pierre Pictet (1846-1929) developed the nozzle and porous-plug methods of liquefaction. This, in turn, made it possible to liq-

A P P L I CAT I O N S O F GAS L I Q U E FAC T I O N . Liquefied natural gas (LNG)

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Molecular Dynamics

KEY TERMS

CONTINUED

which are defined primarily in terms of the

all factors and forces irrelevant to a discus-

behavior exhibited by their atomic or

sion of that system, is known as the envi-

molecular structures. On Earth, three prin-

ronment.

cipal phases of matter exist, namely solid, TEMPERATURE:

liquid, and gas.

A measure of the

average kinetic energy—or molecular POTENTIAL ENERGY:

The energy an

object possesses by virtue of its position. SOLID:

A phase of matter in which

molecules exert strong attractions toward

translational energy in a system. Differences in temperature determine the direction of internal energy flow between two systems when heat is being transferred.

one another, and, therefore, move slowly. THERMAL ENERGY: STATISTICAL MECHANICS:

A realm

of the physical sciences devoted to the study of large systems by examining the behavior of their smallest parts. SUBLIMATION CURVE:

Heat energy, a

form of kinetic energy produced by the movement of atomic or molecular particles. The greater the movement of these particles, the greater the thermal energy.

The bound-

ary between solid and gas for any given substance, as plotted on a phase diagram.

THERMODYNAMICS:

The study of

the relationships between heat, work, and energy.

SYSTEM:

In physics, the term “system” The bound-

usually refers to any set of physical interac-

VAPORIZATION CURVE:

tions isolated from the rest of the universe.

ary between liquid and gas for any given

Anything outside of the system, including

substance as plotted on a phase diagram.

and liquefied petroleum gas (LPG), the latter a mixture of by-products obtained from petroleum and natural gas, are among the examples of liquefied gas in daily use. In both cases, the volume of the liquefied gas is far less than it would be if the gas were in a vaporized state, thus enabling ease and economy in transport. Liquefied gases are used as heating fuel for motor homes, boats, and homes or cabins in remote areas. Other applications of liquefied gases include liquefied oxygen and hydrogen in rocket engines, and liquefied oxygen and petroleum used in welding. The properties of liquefied gases also figure heavily in the science of producing and studying low-temperature environments. In addition, liquefied helium is used in studying the behavior of matter at temperatures close to absolute zero.

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A “New” Form of Matter? Physicists at a Colorado laboratory in 1995 revealed a highly interesting aspect of atomic motion at temperatures approaching absolute zero. Some 70 years before, Einstein had predicted that, at extremely low temperatures, atoms would fuse to form one large “superatom.” This hypothesized structure was dubbed the BoseEinstein Condensate after Einstein and Satyendranath Bose (1894-1974), an Indian physicist whose statistical methods contributed to the development of quantum theory. Because of its unique atomic structure, the Bose-Einstein Condensate has been dubbed a “new” form of matter. It represents a quantum mechanical effect, relating to a cutting-edge area of physics devoted to studying the properties of

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subatomic particles and the interaction of matter with radiation. Thus it is not directly related to molecular dynamics; nonetheless, the Bose-Einstein Condensate is mentioned here as an example of the exciting work being performed at a level beyond that addressed by molecular dynamics. Its existence may lead to a greater understanding of quantum mechanics, and on an everyday level, the “superatom” may aid in the design of smaller, more powerful computer chips. WHERE TO LEARN MORE Cooper, Christopher. Matter. New York: DK Publishing, 1999.

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“The Kinetic Theory Page” (Web site). (April 15, 2001). Medoff, Sol and John Powers. The Student Chemist Explores Atoms and Molecules. Illustrated by Nancy Lou Gahan. New York: R. Rosen Press, 1977. “Molecular Dynamics” (Web site). (April 15, 2001). “Molecular Simulation Molecular Dynamics Page” (Web site). (April 15, 2001). Santrey, Laurence. Heat. Illustrated by Lloyd Birmingham. Mahwah, NJ: Troll Associates, 1985. Strasser, Ben. Molecules in Motion. Illustrated by Vern Jorgenson. Pasadena, CA: Franklin Publications, 1967.

“Kinetic Theory of Gases: A Brief Review” University of Virginia Department of Physics (Web site). (April 15, 2001).

Van, Jon. “U.S. Scientists Create a ‘Superatom.’” Chicago Tribune, July 14, 1995, p. 3.

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S T R U C T U R E O F M AT T E R

Structure of Matter

CONCEPT The physical realm is made up of matter. On Earth, matter appears in three clearly defined forms—solid, liquid, and gas—whose visible and perceptible structure is a function of behavior that takes place at the molecular level. Though these are often referred to as “states” of matter, it is also useful to think of them as phases of matter. This terminology serves as a reminder that any one substance can exist in any of the three phases. Water, for instance, can be ice, liquid, or steam; given the proper temperature and pressure, it may be solid, liquid, and gas all at once! But the three definite earthbound states of matter are not the sum total of the material world: in outer space a fourth phase, plasma, exists—and there may be still other varieties in the physical universe.

HOW IT WORKS Matter and Energy Matter can be defined as physical substance that has mass; occupies space; is composed of atoms; and is ultimately convertible to energy. A significant conversion of matter to energy, however, occurs only at speeds approaching that of the speed of light, a fact encompassed in the famous statement formulated by Albert Einstein (18791955), E = mc2. Einstein’s formula means that every item possesses a quantity of energy equal to its mass multiplied by the squared speed of light. Given the fact that light travels at 186,000 mi (297,600 km) per second, the quantities of energy avail-

S C I E N C E O F E V E RY DAY T H I N G S

able from even a tiny object traveling at that speed are massive indeed. This is the basis for both nuclear power and nuclear weaponry, each of which uses some of the smallest particles in the known universe to produce results that are both amazing and terrifying. The forms of matter that most people experience in their everyday lives, of course, are traveling at speeds well below that of the speed of light. Even so, transfers between matter and energy take place, though on a much, much smaller scale. For instance, when a fire burns, only a tiny fraction of its mass is converted to energy. The rest is converted into forms of mass different from that of the wood used to make the fire. Much of it remains in place as ash, of course, but an enormous volume is released into the atmosphere as a gas so filled with energy that it generates not only heat but light. The actual mass converted into energy, however, is infinitesimal. C O N S E R V AT I O N A N D C O N V E R S I O N . The property of energy is, at all

times and at all places in the physical universe, conserved. In physics, “to conserve” something means “to result in no net loss of ” that particular component—in this case, energy. Energy is never destroyed: it simply changes form. Hence, the conservation of energy, a law of physics stating that within a system isolated from all other outside factors, the total amount of energy remains the same, though transformations of energy from one form to another take place. Whereas energy is perfectly conserved, matter is only approximately conserved, as shown with the example of the fire. Most of the matter from the wood did indeed turn into more mat-

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Structure of Matter

AN

ICEBERG FLOATS BECAUSE THE DENSITY OF ICE IS LOWER THAN WATER, WHILE ITS VOLUME IS GREATER, MAKING

THE ICEBERG BUOYANT. (Photograph by Ric Engenbright/Corbis. Reproduced by permission.)

The conservation of mass holds that total mass is constant, and is unaffected by factors such as position, velocity, or temperature, in any system that does not exchange any matter with its environment. This, however, is a qualified statement: at speeds well below c (the speed of light), it is essentially true, but for matter approaching c and thus, turning into energy, it is not.

Consider an item of matter moving at the speed of 100 mi (160 km)/sec. This is equal to 360,000 MPH (576,000 km/h) and in terms of the speeds to which humans are accustomed, it seems incredibly fast. After all, the fastest any human beings have ever traveled was about 25,000 MPH (40,000 km/h), in the case of the astronauts aboard Apollo 11 in May 1969, and the speed under discussion is more than 14 times greater. Yet 100 mi/sec is a snail’s pace compared to c: in fact, the proportional difference between an actual snail’s pace and the speed of a human

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ter—that is, vapor and ash. Yet, as also noted, a tiny quantity of matter—too small to be perceived by the senses—turned into energy.

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walking is not as great. Yet even at this leisurely gait, equal to 0.00054c, a portion of mass equal to 0.0001% (one-millionth of the total mass) converts to energy.

Matter at the Atomic Level

One of the most well-known molecular forms in the world is water, or H2O, composed of two hydrogen atoms and one oxygen atom. The arrangement is extremely precise and never varies: scientists know, for instance, that the two hydrogen atoms join the oxygen atom (which is much larger than the hydrogen atoms) at an angle of 105° 3’. Other molecules are much more complex than those of water—some of them much, much more complex, which is reflected in the sometimes unwieldy names required to identify their chemical components.

In his brilliant work Six Easy Pieces, American physicist Richard Feynman (1918-1988) asked his readers, “If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generations of creatures, what statement would contain the most information in the fewest words? I believe it is the atomic hypothesis (or the atomic fact, or whatever you wish to call it) that all things are made of atoms—little articles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied.” Feynman went on to offer a powerful series of illustrations concerning the size of atoms relative to more familiar objects: if an apple were magnified to the size of Earth, for instance, the atoms in it would each be about the size of a regular apple. Clearly atoms and other atomic particles are far too small to be glimpsed by even the most highly powered optical microscope. Yet, it is the behavior of particles at the atomic level that defines the shape of the entire physical world. Viewed from this perspective, it becomes easy to understand how and why matter is convertible to energy. Likewise, the interaction between atoms and other particles explains why some types of matter are solid, others liquid, and still others, gas. AT O M S A N D M O L E C U L E S . An atom is the smallest particle of a chemical element. It is not, however, the smallest particle in the universe; atoms are composed of subatomic particles, including protons, neutrons, and electrons. But at the subatomic level, it is meaningless to refer to, for instance, “an oxygen electron”: electrons are just electrons. An atom, then, is the fundamental unit of matter. Most of the substances people encounter in the world, however, are not pure elements, such as oxygen or iron; they are chemical compounds, in which atoms of more than one element join together to form molecules.

than 24 centuries ago, the Greek philosopher Democritus (c. 470-380 B.C.) proposed that matter is composed of tiny particles he called atomos, or “indivisible.” Democritus was not, however, describing matter in a concrete, scientific way: his “atoms” were idealized, philosophical constructs, not purely physical units. Yet, he came amazingly close to identifying the fundamental structure of physical reality— much closer than any number of erroneous theories (such as the “four elements” of earth, air, fire, and water) that prevailed until modern times. English chemist John Dalton (1766-1844) was the first to identify what Feynman later called the “atomic hypothesis”: that nature is composed of tiny particles. In putting forward his idea, Dalton adopted Democritus’s word “atom” to describe these basic units. Dalton recognized that the structure of atoms in a particular element or compound is uniform. He maintained that compounds are made up of compound atoms: in other words, that water, for instance, is a compound of “water atoms.” Water, however, is not an element, and thus, it was necessary to think of its atomic composition in a different way—in terms of molecules rather than atoms. Dalton’s contemporary Amedeo Avogadro (1776-1856), an Italian physicist, was the first scientist to clarify the distinction between atoms and molecules. T H E M O L E . Obviously, it is impractical to weigh a single molecule, or even several thousand; what was needed, then, was a number large enough to make possible practical comparisons of mass. Hence, the mole, a quantity equal to “Avogadro’s number.” The latter, named after Avogadro though not derived by him, is equal to 6.022137 x 1023 (more than 600 billion trillion) molecules.

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Structure of Matter

AT O M I C AND MOLECULAR T H E O RY. The idea of atoms is not new. More

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The term “mole” can be used in the same way that the word “dozen” is used. Just as “a dozen” can refer to twelve cakes or twelve chickens, so “mole” always describes the same number of molecules. A mole of any given substance has its own particular mass, expressed in grams. The mass of one mole of iron, for instance, will always be greater than that of one mole of oxygen. The ratio between them is exactly the same as the ratio of the mass of one iron atom to one oxygen atom. Thus, the mole makes it possible to compare the mass of one element or compound to that of another.

Jean Baptiste Perrin (1870-1942), finally put an end to any remaining doubts concerning the molecular structure of matter.

BROWNIAN MOTION AND KIN E T I C T H E O RY. Contemporary to both

Eventually, physicists identified protons and electrons, but the neutron, with no electrical charge, was harder to discover: it was not identified until 1932. After that point, scientists were convinced that just three types of subatomic particles existed. However, subsequent activity among physicists—particularly those in the field of quantum mechanics—led to the discovery of other elementary particles, such as the photon. However, in this discussion, the only subatomic particles whose behavior is reviewed are the proton, electron, and neutron.

Dalton and Avogadro was Scottish naturalist Robert Brown (1773-1858), who in 1827 stumbled upon a curious phenomenon. While studying pollen grains under a microscope, Brown noticed that the grains underwent a curious zigzagging motion in the water. At first, he assumed that the motion had a biological explanation—that is, it resulted from life processes within the pollen—but later he discovered that even pollen from long-dead plants behaved in the same way. Brown never understood what he was witnessing. Nor did a number of other scientists, who began noticing other examples of what came to be known as Brownian motion: the constant but irregular zigzagging of particles in a puff of smoke, for instance. Later, however, Scottish physicist James Clerk Maxwell (1831-1879) and others were able to explain it by what came to be known as the kinetic theory of matter. The kinetic theory, which is discussed in depth elsewhere in this book, is based on the idea that molecules are constantly in motion: hence, the water molecules were moving the pollen grains Brown observed. Pollen grains are many thousands of times as large as water molecules, but there are so many molecules in just one drop of water, and their motion is so constant but apparently random, that they are bound to move a pollen grain once every few thousand collisions.

Motion and Attraction in Atoms and Molecules At the molecular level, every item of matter in the world is in motion. This may be easy enough to imagine with regard to air or water, since both tend to flow. But what about a piece of paper, or a glass, or a rock? In fact, all molecules are in constant motion, and depending on the particular phase of matter, this motion may vary from a mere vibration to a high rate of speed. Molecular motion generates kinetic energy, or the energy of movement, which is manifested as heat or thermal energy. Indeed, heat is really nothing more than molecules in motion relative to one another: the faster they move, the greater the kinetic energy, and the greater the heat.

Maxwell died, published a series of papers in which he analyzed the behavior of particles subjected to Brownian motion. His work, and the confirmation of his results by French physicist

The movement of atoms and molecules is always in a straight line and at a constant velocity, unless acted upon by some outside force. In fact, the motion of atoms and molecules is constantly being interfered with by outside forces, because they are perpetually striking one another. These collisions cause changes in direction, and may lead to transfers of energy from one particle to another.

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G R O W T H I N U N D E R S TA N D I N G T H E AT O M . Einstein, who was born the year

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It may seem amazing that the molecular and atomic ideas were still open to question in the early twentieth century; however, the vast majority of what is known today concerning the atom emerged after World War I. At the end of the nineteenth century, scientists believed the atom to be indivisible, but growing evidence concerning electrical charges in atoms brought with it the awareness that there must be something smaller creating those charges.

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ELECTROMAGNETIC FORCE I N AT O M S . The behavior of molecules can-

not be explained in terms of gravitational force. This force, and the motions associated with it, were identified by Sir Isaac Newton (1642-1727), and Newton’s model of the universe seemed to answer most physical questions. Then in the late nineteenth century, Maxwell discovered a second kind of force, electromagnetism. (There are two other known varieties of force, strong and weak nuclear, which are exhibited at the subatomic level.) Electromagnetic force, rather than gravitation, explains the attraction between atoms. Several times up to this point, the subatomic particles have been mentioned but not explained in terms of their electrical charge, which is principal among their defining characteristics. Protons have a positive electrical charge, while neutrons exert no charge. These two types of particles, which make up the vast majority of the atom’s mass, are clustered at the center, or nucleus. Orbiting around this nucleus are electrons, much smaller particles which exert a negative charge. Chemical elements are identified by the number of protons they possess. Hydrogen, first element listed on the periodic table of elements, has one proton and is thus identified as 1; carbon, or element 6, has six protons, and so on. An atom usually has a neutral charge, meaning that it is composed of an equal number of protons or electrons. In certain situations, however, it may lose one or more electrons and thus acquire a net charge. Such an atom is called an ion. But electrical charge, like energy, is conserved, and the electrons are not “lost” when an atom becomes an ion: they simply go elsewhere. M O L E C U LA R B E H AV I O R A N D S TAT E S O F M AT T E R. Positive and neg-

ative charges interact at the molecular level in a way that can be compared to the behavior of poles in a pair of magnets. Just as two north poles or two south poles repel one another, so like charges—two positives, or two negatives—repel. Conversely, positive and negative charges exert an attractive force on one another similar to that of a north pole and south pole in contact. In discussing phases of matter, the attraction between molecules provides a key to distinguishing between states of matter. This is not to say one particular phase of matter is a particularly good conductor of electrical current, however.

S C I E N C E O F E V E RY DAY T H I N G S

For instance, certain solids—particularly metals such as copper—are extremely good conductors. But wood is a solid, too, and conducts electrical current poorly.

Structure of Matter

The properties of various forms of matter, viewed from the larger electromagnetic picture, are a subject far beyond the scope of this essay. In any case, the electromagnetic properties of concern in the present instance are not the ones demonstrated at a macroscopic level—that is, in view of “the big picture.” Rather, the subject of the attractive force operating at the atomic or molecular levels has been introduced to show that certain types of material have a greater intermolecular attraction. As previously stated, all matter is in motion. The relative speed of that motion, however, is a function of the attraction between molecules, which in turn defines a material according to one of the phases of matter. When the molecules in a material exert a strong attraction toward one another, they move slowly, and the material is called a solid. Molecules of liquid, by contrast, exert a moderate attraction and move at moderate speeds. A material substance whose molecules exert little or no attraction, and therefore, move at high speeds, is known as a gas. These comparisons of molecular speed and attraction, obviously, are relative. Certainly, it is easy enough in most cases to distinguish between one phase of matter and another, but there are some instances in which they overlap. Examples of these will follow, but first it is necessary to discuss the phases of matter in the context of their behavior in everyday situations.

REAL-LIFE A P P L I C AT I O N S From Solid to Liquid The attractions between particles have a number of consequences in defining the phases of matter. The strong attractive forces in solids cause its particles to be positioned close together. This means that particles of solids resist attempts to compress them, or push them together. Because of their close proximity, solid particles are fixed in an orderly and definite pattern. As a result, a solid usually has a definite volume and shape. A crystal is a type of solid in which the constituent parts are arranged in a simple, definite

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geometric arrangement that is repeated in all directions. Metals, for instance, are crystalline solids. Other solids are said to be amorphous, meaning that they possess no definite shape. Amorphous solids—clay, for example—either possess very tiny crystals, or consist of several varieties of crystal mixed randomly. Still other solids, among them glass, do not contain crystals. V I B RAT I O N S A N D F R E E Z I N G .

Because of their strong attractions to one another, solid particles move slowly, but like all particles of matter, they do move. Whereas the particles in a liquid or gas move fast enough to be in relative motion with regard to one another, however, solid particles merely vibrate from a fixed position. This can be shown by the example of a singer hitting a certain note and shattering a glass. Contrary to popular belief, the note does not have to be particularly high: rather, the note should be on the same wavelength as the vibration of the glass. When this occurs, sound energy is transferred directly to the glass, which shatters because of the sudden net intake of energy. As noted earlier, the attraction and motion of particles in matter has a direct effect on heat and temperature. The cooler the solid, the slower and weaker the vibrations, and the closer the particles are to one another. Thus, most types of matter contract when freezing, and their density increases. Absolute zero, or 0K on the Kelvin scale of temperature—equal to -459.67°F (-273°C)— is the point at which vibration virtually stops. Note that the vibration virtually stops, but does not stop entirely. In any event, the lowest temperature actually achieved, at a Finnish nuclear laboratory in 1993, is 2.8 • 10-10K, or 0.00000000028K—still above absolute zero. U N U S UA L C H A RAC T E R I S T I CS O F I C E . The behavior of water at the freez-

ing/melting point is interesting and exceptional. Above 39.2°F (4°C) water, like most substances, expands when heated. But between 32°F (0°C) and that temperature, however, it actually contracts. And whereas most substances become much denser with lowered temperatures, the density of water reaches its maximum at 39.2°F. Below that point, it starts to decrease again. Not only does the density of ice begin decreasing just before freezing, but its volume increases. This is the reason ice floats: its weight is less than that of the water it has displaced, and

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therefore, it is buoyant. Additionally, the buoyant qualities of ice atop very cold water explain why the top of a lake may freeze, but lakes rarely freeze solid—even in the coldest of inhabited regions. Instead of freezing from the bottom up, as it would if ice were less buoyant than the water, the lake freezes from the top down. Furthermore, ice is a poorer conductor of heat than water, and, thus, little of the heat from the water below escapes. Therefore, the lake does not freeze completely—only a layer at the top—and this helps preserve animal and plant life in the body of water. On the other hand, the increased volume of frozen water is not always good for humans: when water in pipes freezes, it may increase in volume to the point where the pipe bursts. M E LT I N G . When heated, particles begin to vibrate more and more, and, therefore, move further apart. If a solid is heated enough, it loses its rigid structure and becomes a liquid. The temperature at which a solid turns into a liquid is called the melting point, and melting points are different for different substances. For the most part, however, solids composed of heavier particles require more energy—and, hence, higher temperatures—to induce the vibrations necessary for freezing. Nitrogen melts at -346°F (-210°C), ice at 32°F (0°C), and copper at 1,985°F (1,085°C). The melting point of a substance, incidentally, is the same as its freezing point: the difference is a matter of orientation—that is, whether the process is one of a solid melting to become a liquid, or of a liquid freezing to become a solid.

The energy required to change a solid to a liquid is called the heat of fusion. In melting, all the heat energy in a solid (energy that exists due to the motion of its particles) is used in breaking up the arrangement of crystals, called a lattice. This is why the water resulting from melted ice does not feel any warmer than when it was frozen: the thermal energy has been expended, with none left over for heating the water. Once all the ice is melted, however, the absorbed energy from the particles—now moving at much greater speeds than when the ice was in a solid state— causes the temperature to rise.

From Liquid to Gas The particles of a liquid, as compared to those of a solid, have more energy, more motion, and less

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attraction to one another. The attraction, however, is still fairly strong: thus, liquid particles are in close enough proximity that the liquid resists compression. On the other hand, their arrangement is loose enough that the particles tend to move around one another rather than merely vibrating in place, as solid particles do. A liquid is therefore not definite in shape. Both liquids and gases tend to flow, and to conform to the shape of their container; for this reason, they are together classified as fluids. Owing to the fact that the particles in a liquid are not as close in proximity as those of a solid, liquids tend to be less dense than solids. The liquid phase of substance is thus inclined to be larger in volume than its equivalent in solid form. Again, however, water is exceptional in this regard: liquid water actually takes up less space than an equal mass of frozen water. B O I L I N G . When a liquid experiences an

increase in temperature, its particles take on energy and begin to move faster and faster. They collide with one another, and at some point the particles nearest the surface of the liquid acquire enough energy to break away from their neighbors. It is at this point that the liquid becomes a gas or vapor. As heating continues, particles throughout the liquid begin to gain energy and move faster, but they do not immediately transform into gas. The reason is that the pressure of the liquid, combined with the pressure of the atmosphere above the liquid, tends to keep particles in place. Those particles below the surface, therefore, remain where they are until they acquire enough energy to rise to the surface. The heated particle moves upward, leaving behind it a hollow space—a bubble. A bubble is not an empty space: it contains smaller trapped particles, but its small weight relative to that of the liquid it disperses makes it buoyant. Therefore, a bubble floats to the top, releasing its trapped particles as gas or vapor. At that point, the liquid is said to be boiling. T H E E F F E C T O F AT M O S P H E R I C P R E SS U R E . As they rise, the particles

thus have to overcome atmospheric pressure, and this means that the boiling point for any liquid depends in part on the pressure of the surrounding air. This is why cooking instructions often vary with altitude: the greater the distance from

S C I E N C E O F E V E RY DAY T H I N G S

sea level, the less the air pressure, and the shorter the required cooking time.

Structure of Matter

Atop Mt. Everest, Earth’s highest peak at about 29,000 ft (8,839 m) above sea level, the pressure is approximately one-third normal atmospheric pressure. This means the air is onethird as dense as it is as sea level, which explains why mountain-climbers on Everest and other tall peaks must wear oxygen masks to stay alive. It also means that water boils at a much lower temperature on Everest than it does elsewhere. At sea level, the boiling point of water is 212°F (100°C), but at 29,000 ft it is reduced by one-quarter, to 158°F (70°C). Of course, no one lives on the top of Mt. Everest—but people do live in Denver, Colorado, where the altitude is 5,577 ft (1,700 m) and the boiling point of water is 203°F (95°C). Given the lower boiling point, one might assume that food would cook faster in Denver than in New York, Los Angeles, or some other city close to sea level. In fact, the opposite is true: because heated particles escape the water so much faster at high altitudes, they do not have time to acquire the energy needed to raise the temperature of the water. It is for this reason that a recipe may contain a statement such as “at altitudes above XX feet, add XX minutes to cooking time.” If lowered atmospheric pressure means a lowered boiling point, what happens in outer space, where there is no atmospheric pressure? Liquids boil at very, very low temperatures. This is one of the reasons why astronauts have to wear pressurized suits: if they did not, their blood would boil—even though space itself is incredibly cold. L I Q U I D T O GAS A N D B AC K AGA I N . Note that the process of a liquid

changing to a gas is similar to what occurs when a solid changes to a liquid: particles gain heat and therefore energy, begin to move faster, break free from one another, and pass a certain threshold into a new phase of matter. And just as the freezing and melting point for a given substance are the same temperature, the only difference being one of orientation, the boiling point of a liquid transforming into a gas is the same as the condensation point for a gas turning into a liquid. The behavior of water in boiling and condensation makes possible distillation, one of the principal methods for purifying seawater in various parts of the world. First, the water is boiled,

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then, it is allowed to cool and condense, thus forming water again. In the process, the water separates from the salt, leaving it behind in the form of brine. A similar separation takes place when salt water freezes: because salt, like most solids, has a much lower freezing point than water, very little of it remains joined to the water in ice. Instead, the salt takes the form of a briny slush. Having reached the gaseous state, a substance takes on characteristics quite different from those of a solid, and somewhat different from those of a liquid. Whereas liquid particles exert a moderate attraction to one another, particles in a gas exert little to no attraction. They are thus free to move, and to move quickly. The shape and arrangement of gas is therefore random and indefinite—and, more importantly, the motion of gas particles give it much greater kinetic energy than the other forms of matter found on Earth. GAS

AND

ITS

LAWS.

The constant, fast, and random motion of gas particles means that they are always colliding and thereby transferring kinetic energy back and forth without any net loss. These collisions also have the overall effect of producing uniform pressure in a gas. At the same time, the characteristics and behavior of gas particles indicate that they will tend not to remain in an open container. Therefore, in order to maintain any pressure on a gas—other than the normal atmospheric pressure exerted on the surface of the gas by the atmosphere (which, of course, is also a gas)—it is necessary to keep it in a closed container. There are a number of gas laws (examined in another essay in this book) describing the response of gases to changes in pressure, temperature, and volume. Among these is Boyle’s law, which holds that when the temperature of a gas is constant, there is an inverse relationship between volume and pressure: in other words, the greater the pressure, the less the volume, and vice versa. According to a second gas law, Charles’s law, for gases in conditions of constant pressure, the ratio between volume and temperature is constant—that is, the greater the temperature, the greater the volume, and vice versa.

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versa. Gay-Lussac’s law is combined, along with Boyle’s and Charles’s and other gas laws, in the ideal gas law, which makes it possible to find the value of any one variable—pressure, volume, number of moles, or temperature—for a gas, as long as one knows the value of the other three.

Other States of Matter P L A S M A . Principal among states of matter other than solid, liquid, and gas is plasma, which is similar to gas. (The term “plasma,” when referring to the state of matter, has nothing to do with the word as it is often used, in reference to blood plasma.) As with gas, plasma particles collide at high speeds—but in plasma, the speeds are even greater, and the kinetic energy levels even higher.

The speed and energy of these collisions is directly related to the underlying property that distinguishes plasma from gas. So violent are the collisions between plasma particles that electrons are knocked away from their atoms. As a result, plasma does not have the atomic structure typical of a gas; rather, it is composed of positive ions and electrons. Plasma particles are thus electrically charged, and, therefore, greatly influenced by electrical and magnetic fields. Formed at very high temperatures, plasma is found in stars and comets’ tails; furthermore, the reaction between plasma and atomic particles in the upper atmosphere is responsible for the aurora borealis or “northern lights.” Though not found on Earth, plasma—ubiquitous in other parts of the universe—may be the most plentiful among the four principal states of matter. Q UAS I - S TAT E S . Among the quasistates of matter discussed by physicists are several terms that describe the structure in which particles are joined, rather than the attraction and relative movement of those particles. “Crystalline,” “amorphous,” and “glassy” are all terms to describe what may be individual states of matter; so too is “colloidal.”

In addition, Gay-Lussac’s law shows that the pressure of a gas is directly related to its absolute temperature on the Kelvin scale: the higher the temperature, the higher the pressure, and vice

A colloid is a structure intermediate in size between a molecule and a visible particle, and it has a tendency to be dispersed in another medium—as smoke, for instance, is dispersed in air. Brownian motion describes the behavior of most colloidal particles. When one sees dust floating in a ray of sunshine through a window, the light reflects off colloids in the dust, which are driven

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back and forth by motion in the air otherwise imperceptible to the human senses. DA R K M AT T E R. The number of states

or phases of matter is clearly not fixed, and it is quite possible that more will be discovered in outer space, if not on Earth. One intriguing candidate is called dark matter, so described because it neither reflects nor emits light, and is therefore invisible. In fact, luminous or visible matter may very well make up only a small fraction of the mass in the universe, with the rest being taken up by dark matter. If dark matter is invisible, how do astronomers and physicists know it exists? By analyzing the gravitational force exerted on visible objects when there seems to be no visible object to account for that force. An example is the center of our galaxy, the Milky Way, which appears to be nothing more than a dark “halo.” In order to cause the entire galaxy to revolve around it in the same way that planets revolve around the Sun, the Milky Way must contain a staggering quantity of invisible mass. Dark matter may be the substance at the heart of a black hole, a collapsed star whose mass is so great that its gravitational field prevents light from escaping. It is possible, also, that dark matter is made up of neutrinos, subatomic particles thought to be massless. Perhaps, the theory goes, neutrinos actually possess tiny quantities of mass, and therefore in huge groups—a mole times a mole times a mole—they might possess appreciable mass. In addition, dark matter may be the deciding factor as to whether the universe is infinite. The more mass the universe possesses, the greater its overall gravity, and if the mass of the universe is above a certain point, it will eventually begin to contract. This, of course, would mean that it is finite; on the other hand, if the mass is below this threshold, it will continue to expand indefinitely. The known mass of the universe is nowhere near that threshold—but, because the nature of dark matter is still largely unknown, it is not possible yet to say what effect its mass may have on the total equation. A “ N E W ” F O R M O F M AT T E R ?

ed that, at extremely low temperatures, atoms would fuse to form one large “superatom.” This hypothesized structure was dubbed the BoseEinstein Condensate (BEC) after Einstein and Satyendranath Bose (1894-1974), an Indian physicist whose statistical methods contributed to the development of quantum theory.

Structure of Matter

Cooling about 2,000 atoms of the element rubidium to a temperature just 170 billionths of a degree Celsius above absolute zero, the physicists succeeded in creating an atom 100 micrometers across—still incredibly small, but vast in comparison to an ordinary atom. The superatom, which lasted for about 15 seconds, cooled down all the way to just 20 billionths of a degree above absolute zero. The Colorado physicists won the Nobel Prize in physics in 1997 for their work. In 1999, researchers in a lab at Harvard University also created a superatom of BEC, and used it to slow light to just 38 MPH (60.8 km/h)—about 0.02% of its ordinary speed. Dubbed a “new” form of matter, the BEC may lead to a greater understanding of quantum mechanics, and may aid in the design of smaller, more powerful computer chips.

States and Phases and In Between At places throughout this essay, references have been made variously to “phases” and “states” of matter. This is not intended to confuse, but rather to emphasize a particular point. Solids, liquids, and gases are referred to as “phases,” because substances on Earth—water, for instance—regularly move from one phase to another. This change, a function of temperature, is called (aptly enough) “change of phase.” There is absolutely nothing incorrect in referring to “states of matter.” But “phases of matter” is used in the present context as a means of emphasizing the fact that most substances, at the appropriate temperature and pressure, can be solid, liquid, or gas. In fact, a substance may even be solid, liquid, and gas.

An Analogy to Human Life

Physicists at the Joint Institute of Laboratory Astrophysics in Boulder, Colorado, in 1995 revealed a highly interesting aspect of atomic behavior at temperatures approaching absolute zero. Some 70 years before, Einstein had predict-

The phases of matter can be likened to the phases of a person’s life: infancy, babyhood, childhood, adolescence, adulthood, old age. The transition between these stages is indefinite, yet it is

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Structure of Matter

THE RESPONSE OF LIQUID CRYSTALS ERS. (AFP/Corbis. Reproduced by permission.)

TO LIGHT MAKES THEM USEFUL IN THE DISPLAYS USED ON LAPTOP COMPUT-

easy enough to say when a person is at a certain stage. At the transition point between adolescence and adulthood—say, at seventeen years old—a young person may say that she is an adult, but her parents may insist that she is still an adolescent or a child. And indeed, she might qualify as either. On the other hand, when she is thirty, it would be ridiculous to assert that she is anything other than an adult. At the same time, a person at a certain age may exhibit behaviors typically associated with another age. A child, for instance, may behave like an adult, or an adult like a baby. One interesting example of this is the relationship between age two and late adolescence. In both cases, the person is in the process of individualizing, developing an identity separate from that of his or her parents—yet clearly, there are also plenty of differences between a two-year-old and a seventeenyear-old.

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clearly is what it is: water at very low temperature and pressure, for instance, is indisputably ice— just as an average thirty-year-old is obviously an adult. As for the second observation, that a person at one stage in life may reflect characteristics of another stage, this too is reflected in the behavior of matter. L I Q U I D C RY S TA L S . A liquid crystal

is a substance that, over a specific range of temperature, displays properties both of a liquid and a solid. Below this temperature range, it is unquestionably a solid, and above this range it is just as obviously a liquid. In between, however, liquid crystals exhibit a strange solid-liquid behavior: like a liquid, their particles flow, but like a solid, their molecules maintain specific crystalline arrangements.

As with the transitional phases in human life, in the borderline pressure levels and temperatures for phases of matter it is sometimes difficult to say, for instance, if a substance is fully a liquid or fully a gas. On the other hand, at a certain temperature and pressure level, a substance

Long, wide, and placed alongside one another, liquid crystal molecules exhibit interesting properties in response to light waves. The speed of light through a liquid crystal actually varies, depending on whether the light is traveling along the short or long sides of the molecules. These differences in light speed may lead to a change in the direction of polarization, or the vibration of light waves.

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Structure of Matter

KEY TERMS ATOM:

The smallest particle of a chem-

Negatively charged parti-

ELECTRON:

ical element. An atom can exist either alone

cles in an atom. Electrons, which spin

or in combination with other atoms in a

around the nucleus of protons and neu-

molecule. Atoms are made up of protons,

trons, constitute a very small portion of the

neutrons, and electrons. In most cases, the

atom’s mass. In most atoms, the number of

electrical charges in atoms cancel out one

electrons and protons is the same, thus

another; but when an atom loses one or

canceling out one another. When an atom

more electrons, and thus has a net charge,

loses one or more electrons, however—

it becomes an ion.

thus becoming an ion—it acquires a net

CHEMICAL

COMPOUND:

A sub-

stance made up of atoms of more than one chemical element. These atoms are usually joined in molecules. CHEMICAL ELEMENT:

electrical charge. FRICTION:

The force that resists

motion when the surface of one object comes into contact with the surface of

A substance

another.

made up of only one kind of atom. CONSERVATION OF ENERGY:

A

law of physics which holds that within a system isolated from all other outside factors, the total amount of energy remains the same, though transformations of energy from one form to another take place. CONSERVATION OF MASS:

A phys-

Any substance, whether gas or

FLUID:

liquid, that tends to flow, and that conforms to the shape of its container. Unlike solids, fluids are typically uniform in molecular structure for instance, one molecule of water is the same as another water molecule. A phase of matter in which mole-

ical principle which states that total mass is

GAS:

constant, and is unaffected by factors such

cules exert little or no attraction toward

as position, velocity, or temperature, in any

one another, and therefore move at high

system that does not exchange any matter

speeds.

with its environment. Unlike the other

ION:

conservation laws, however, conservation

one or more electrons, and thus has a net

of mass is not universally applicable, but

electrical charge.

An atom that has lost or gained

applies only at speeds significant lower than that of light—186,000 mi (297,600 km) per second. Close to the speed of light, mass begins converting to energy. CONSERVE:

In physics, “to conserve”

LIQUID:

A phase of matter in which

molecules exert moderate attractions toward one another, and therefore move at moderate speeds. Physical substance that has

something means “to result in no net loss

MATTER:

of ” that particular component. It is possi-

mass; occupies space; is composed of

ble that within a given system, the compo-

atoms; and is ultimately (at speeds

nent may change form or position, but as

approaching that of light) convertible to

long as the net value of the component

energy. There are several phases of matter,

remains the same, it has been conserved.

including solids, liquids, and gases.

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Structure of Matter

KEY TERMS

CONTINUED

A unit equal to 6.022137 ⫻ 1023

does not exist on Earth, but is found, for

(more than 600 billion trillion) molecules.

instance, in stars and comets’ tails. Con-

Their size makes it impossible to weigh

taining neither atoms nor molecules, plas-

molecules in relatively small quantities;

ma is made up of electrons and positive

hence the mole facilitates comparisons of

ions.

MOLE:

mass between substances. MOLECULE:

A group of atoms, usual-

ly of more than one chemical element,

A positively charged particle

in an atom. Protons and neutrons, which together form the nucleus around which

joined in a structure. A subatomic particle that

NEUTRON:

PROTON:

has no electrical charge. Neutrons are

electrons orbit, have approximately the same mass—a mass that is many times

found at the nucleus of an atom, alongside

greater than that of an electron.

protons.

SOLID:

PHASES OF MATTER:

The various

forms of material substance (matter),

A phase of matter in which

molecules exert strong attractions toward one another, and therefore move slowly.

which are defined primarily in terms of the SYSTEM:

molecular structures. On Earth, three prin-

usually refers to any set of physical interac-

cipal phases of matter exist, namely solid,

tions isolated from the rest of the universe.

liquid, and gas. Other forms of matter

Anything outside of the system, including

include plasma.

all factors and forces irrelevant to a discus-

PLASMA:

One of the phases of matter,

closely related to gas. Plasma apparently

214

In physics, the term “system”

behavior exhibited by their atomic or

sion of that system, is known as the environment.

mometer displays those colors, since people typically associate red with heat and blue with cold.

The cholesteric class of liquid crystals is so named because the spiral patterns of light through the crystal are similar to those which appear in cholesterols. Depending on the physical properties of a cholesteric liquid crystal, only certain colors may be reflected. The response of liquid crystals to light makes them useful in liquid crystal displays (LCDs) found on laptop computer screens, camcorder views, and in other applications.

T H E T R I P L E P O I N T. A liquid crystal exhibits aspects of both liquid and solid, and thus, at certain temperatures may be classified within the crystalline quasi-state of matter. On the other hand, the phenomenon known as the triple point shows how an ordinary substance, such as water or carbon dioxide, can actually be a liquid, solid, and vapor—all at once.

In some cholesteric liquid crystals, high temperatures lead to a reflection of shorter visible light waves, and lower temperatures to a display of longer visible waves. Liquid crystal thermometers thus show red when cool, and blue as they are warmed. This may seem a bit unusual to someone who does not understand why the ther-

Again, water—the basis of all life on Earth— is an unusual substance in many regards. For instance, most people associate water as a gas or vapor (that is, steam) with very high temperatures. Yet, at a level far below normal atmospheric pressure, water can be a vapor at temperatures as low as -4°F (-20 °C). (All of the pressure values

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in the discussion of water at or near the triple point are far below atmospheric norms: the pressure at which water would turn into a vapor at 4°F, for instance, is about 1/1000 normal atmospheric pressure.) As everyone knows, at relatively low temperatures, water is a solid—ice. But if the pressure of ice falls below a very low threshold, it will turn straight into a gas (a process known as sublimation) without passing through the liquid stage. On the other hand, by applying enough pressure, it is possible to melt ice, and thereby transform it from a solid to a liquid, at temperatures below its normal freezing point. The phases and changes of phase for a given substance at specific temperatures and pressure levels can be plotted on a graph called a phase diagram, which typically shows temperature on the x-axis and pressure on the y-axis. The phase diagram of water shows a line between the solid and liquid states that is almost, but not quite, exactly perpendicular to the x-axis: it slopes slightly upward to the left, reflecting the fact that solid ice turns into water with an increase of pressure. Whereas the line between solid and liquid water is more or less straight, the division between these two states and water vapor is curved. And where the solid-liquid line intersects the vaporization curve, there is a place called the triple point. Just below freezing, in conditions

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equivalent to about 0.7% of normal atmospheric pressure, water is a solid, liquid, and vapor all at once.

Structure of Matter

WHERE TO LEARN MORE Biel, Timothy L. Atom: Building Blocks of Matter. San Diego, CA: Lucent Books, 1990. Feynman, Richard. Six Easy Pieces: Essentials of Physics Explained by Its Most Brilliant Teacher. New introduction by Paul Davies. Cambridge, MA: Perseus Books, 1995. Hewitt, Sally. Solid, Liquid, or Gas? New York: Children’s Press, 1998. “High School Chemistry Table of Contents—Solids and Liquids” Homeworkhelp.com (Web site). (April 10, 2001). “Matter: Solids, Liquids, Gases.” Studyweb (Web site). (April 10, 2001). “The Molecular Circus” (Web site). (April 10, 2001). Paul, Richard. A Handbook to the Universe: Explorations of Matter, Energy, Space, and Time for Beginning Scientific Thinkers. Chicago: Chicago Review Press, 1993. “Phases of Matter” (Web site). (April 10, 2001). Royston, Angela. Solids, Liquids, and Gasses. Chicago: Heinemann Library, 2001. Wheeler, Jill C. The Stuff Life’s Made Of: A Book About Matter. Minneapolis, MN: Abdo & Daughters Publishing, 1996.

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THERMODYNAMICS Thermodynamics

CONCEPT Thermodynamics is the study of the relationships between heat, work, and energy. Though rooted in physics, it has a clear application to chemistry, biology, and other sciences: in a sense, physical life itself can be described as a continual thermodynamic cycle of transformations between heat and energy. But these transformations are never perfectly efficient, as the second law of thermodynamics shows. Nor is it possible to get “something for nothing,” as the first law of thermodynamics demonstrates: the work output of a system can never be greater than the net energy input. These laws disappointed hopeful industrialists of the early nineteenth century, many of whom believed it might be possible to create a perpetual motion machine. Yet the laws of thermodynamics did make possible such highly useful creations as the internal combustion engine and the refrigerator.

HOW IT WORKS Historical Context Machines were, by definition, the focal point of the Industrial Revolution, which began in England during the late eighteenth and early nineteenth centuries. One of the central preoccupations of both scientists and industrialists thus became the efficiency of those machines: the ratio of output to input. The more output that could be produced with a given input, the greater the production, and the greater the economic advantage to the industrialists and (presumably) society as a whole.

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At that time, scientists and captains of industry still believed in the possibility of a perpetual motion machine: a device that, upon receiving an initial input of energy, would continue to operate indefinitely without further input. As it emerged that work could be converted into heat, a form of energy, it began to seem possible that heat could be converted directly back into work, thus making possible the operation of a perfectly reversible perpetual motion machine. Unfortunately, the laws of thermodynamics dashed all those dreams. S N O W ’ S E X P LA N AT I O N . Some texts identify two laws of thermodynamics, while others add a third. For these laws, which will be discussed in detail below, British writer and scientist C. P. Snow (1905-1980) offered a witty, nontechnical explanation. In a 1959 lecture published as The Two Cultures and the Scientific Revolution, Snow compared the effort to transform heat into energy, and energy back into heat again, as a sort of game.

The first law of thermodynamics, in Snow’s version, teaches that the game is impossible to win. Because energy is conserved, and thus, its quantities throughout the universe are always the same, one cannot get “something for nothing” by extracting more energy than one put into a machine. The second law, as Snow explained it, offers an even more gloomy prognosis: not only is it impossible to win in the game of energy-work exchanges, one cannot so much as break even. Though energy is conserved, that does not mean the energy is conserved within the machine where it is used: mechanical systems tend toward increasing disorder, and therefore, it is impossi-

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Thermodynamics

A

WOMAN WITH A SUNBURNED NOSE.

SUNBURNS

ARE CAUSED BY THE

SUN’S

ULTRAVIOLET RAYS. (Photograph by Lester

V. Bergman/Corbis. Reproduced by permission.)

ble for the machine even to return to the original level of energy. The third law, discovered in 1905, seems to offer a possibility of escape from the conditions imposed in the second law: at the temperature of absolute zero, this tendency toward breakdown drops to a level of zero as well. But the third law only proves that absolute zero cannot be attained: hence, Snow’s third observation, that it is impossible to step outside the boundaries of this unwinnable heat-energy transformation game.

Work and Energy Work and energy, discussed at length elsewhere in this volume, are closely related. Work is the exertion of force over a given distance to displace or move an object. It is thus the product of force and distance exerted in the same direction. Energy is the ability to accomplish work.

which is the sum of potential energy—the energy that an object has due to its position—and kinetic energy, or the energy an object possesses by virtue of its motion. M E C H A N I CA L E N E R GY. Kinetic energy relates to heat more clearly than does potential energy, discussed below; however, it is hard to discuss the one without the other. To use a simple example—one involving mechanical energy in a gravitational field—when a stone is held over the edge of a cliff, it has potential energy. Its potential energy is equal to its weight (mass times the acceleration due to gravity) multiplied by its height above the bottom of the canyon below. Once it is dropped, it acquires kinetic energy, which is the same as one-half its mass multiplied by the square of its velocity.

There are many manifestations of energy, including one of principal concern in the present context: thermal or heat energy. Other manifestations include electromagnetic (sometimes divided into electrical and magnetic), sound, chemical, and nuclear energy. All these, however, can be described in terms of mechanical energy,

Just before it hits bottom, the stone’s kinetic energy will be at a maximum, and its potential energy will be at a minimum. At no point can the value of its kinetic energy exceed the value of the potential energy it possessed before it fell: the mechanical energy, or the sum of kinetic and potential energy, will always be the same, though the relative values of kinetic and potential energy may change.

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C O N S E R VAT I O N

OF

E N E R GY.

What mechanical energy does the stone possess after it comes to rest at the bottom of the canyon? In terms of the system of the stone dropping from the cliffside to the bottom, none. Or, to put it another way, the stone has just as much mechanical energy as it did at the very beginning. Before it was picked up and held over the side of the cliff, thus giving it potential energy, it was presumably sitting on the ground away from the edge of the cliff. Therefore, it lacked potential energy, inasmuch as it could not be “dropped” from the ground. If the stone’s mechanical energy—at least in relation to the system of height between the cliff and the bottom—has dropped to zero, where did it go? A number of places. When it hit, the stone transferred energy to the ground, manifested as heat. It also made a sound when it landed, and this also used up some of its energy. The stone itself lost energy, but the total energy in the universe was unaffected: the energy simply left the stone and went to other places. This is an example of the conservation of energy, which is closely tied to the first law of thermodynamics. But does the stone possess any energy at the bottom of the canyon? Absolutely. For one thing, its mass gives it an energy, known as mass or rest energy, that dwarfs the mechanical energy in the system of the stone dropping off the cliff. (Mass energy is the other major form of energy, aside from kinetic and potential, but at speeds well below that of light, it is released in quantities that are virtually negligible.) The stone may have electromagnetic potential energy as well; and of course, if someone picks it up again, it will have gravitational potential energy. Most important to the present discussion, however, is its internal kinetic energy, the result of vibration among the molecules inside the stone.

Heat and Temperature Thermal energy, or the energy of heat, is really a form of kinetic energy between particles at the atomic or molecular level: the greater the movement of these particles, the greater the thermal energy. Heat itself is internal thermal energy that flows from one body of matter to another. It is not the same as the energy contained in a system—that is, the internal thermal energy of the

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system. Rather than being “energy-in-residence,” heat is “energy-in-transit.” This may be a little hard to comprehend, but it can be explained in terms of the stone-and-cliff kinetic energy illustration used above. Just as a system can have no kinetic energy unless something is moving within it, heat exists only when energy is being transferred. In the above illustration of mechanical energy, when the stone was sitting on the ground at the top of the cliff, it was analogous to a particle of internal energy in body A. When, at the end, it was again on the ground—only this time at the bottom of the canyon—it was the same as a particle of internal energy that has transferred to body B. In between, however, as it was falling from one to the other, it was equivalent to a unit of heat. T E M P E RAT U R E . In everyday life, people think they know what temperature is: a measure of heat and cold. This is wrong for two reasons: first, as discussed below, there is no such thing as “cold”—only an absence of heat. So, then, is temperature a measure of heat? Wrong again.

Imagine two objects, one of mass M and the other with a mass twice as great, or 2M. Both have a certain temperature, and the question is, how much heat will be required to raise their temperature by equal amounts? The answer is that the object of mass 2M requires twice as much heat to raise its temperature the same amount. Therefore, temperature cannot possibly be a measure of heat. What temperature does indicate is the direction of internal energy flow between bodies, and the average molecular kinetic energy in transit between those bodies. More simply, though a bit less precisely, it can be defined as a measure of heat differences. (As for the means by which a thermometer indicates temperature, that is beyond the parameters of the subject at hand; it is discussed elsewhere in this volume, in the context of thermal expansion.) MEASURING T E M P E R AT U R E A N D H E AT. Temperature, of course, can be

measured either by the Fahrenheit or Centigrade scales familiar in everyday life. Another temperature scale of relevance to the present discussion is the Kelvin scale, established by William Thomson, Lord Kelvin (1824-1907). Drawing on the discovery made by French physicist and chemist J. A. C. Charles (1746-

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1823), that gas at 0°C (32°F) regularly contracts by about 1/273 of its volume for every Celsius degree drop in temperature, Thomson derived the value of absolute zero (discussed below) as -273.15°C (-459.67°F). The Kelvin and Celsius scales are thus directly related: Celsius temperatures can be converted to Kelvins (for which neither the word nor the symbol for “degree” are used) by adding 273.15. M E AS U R I N G H E AT A N D H E AT CA PAC I T Y. Heat, on the other hand, is meas-

ured not by degrees (discussed along with the thermometer in the context of thermal expansion), but by the same units as work. Since energy is the ability to perform work, heat or work units are also units of energy. The principal unit of energy in the SI or metric system is the joule (J), equal to 1 newton-meter (N • m), and the primary unit in the British or English system is the foot-pound (ft • lb). One foot-pound is equal to 1.356 J, and 1 joule is equal to 0.7376 ft • lb. Two other units are frequently used for heat as well. In the British system, there is the Btu, or British thermal unit, equal to 778 ft • lb. or 1,054 J. Btus are often used in reference, for instance, to the capacity of an air conditioner. An SI unit that is also used in the United States—where British measures typically still prevail—is the kilocalorie. This is equal to the heat that must be added to or removed from 1 kilogram of water to change its temperature by 1°C. As its name suggests, a kilocalorie is 1,000 calories. A calorie is the heat required to change the temperature in 1 gram of water by 1°C—but the dietary Calorie (capital C), with which most people are familiar is the same as the kilocalorie. A kilocalorie is identical to the heat capacity for one kilogram of water. Heat capacity (sometimes called specific heat capacity or specific heat) is the amount of heat that must be added to, or removed from, a unit of mass for a given substance to change its temperature by 1°C. this is measured in units of J/kg • °C (joules per kilogram-degree Centigrade), though for the sake of convenience it is typically rendered in terms of kilojoules (1,000 joules): kJ/kg • °c. Expressed thus, the specific heat of water 4.185—which is fitting, since a kilocalorie is equal to 4.185 kJ. Water is unique in many aspects, with regard to specific heat, in that it requires far more heat to raise the temperature of water than that of mercury or iron.

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REAL-LIFE A P P L I C AT I O N S

Thermodynamics

Hot and “Cold” Earlier, it was stated that there is no such thing as “cold”—a statement hard to believe for someone who happens to be in Buffalo, New York, or International Falls, Minnesota, during a February blizzard. Certainly, cold is real as a sensory experience, but in physical terms, cold is not a “thing”—it is simply the absence of heat. People will say, for instance, that they put an ice cube in a cup of coffee to cool it, but in terms of physics, this description is backward: what actually happens is that heat flows from the coffee to the ice, thus raising its temperature. The resulting temperature is somewhere between that of the ice cube and the coffee, but one cannot obtain the value simply by averaging the two temperatures at the beginning of the transfer. For one thing, the volume of the water in the ice cube is presumably less than that of the water in the coffee, not to mention the fact that their differing chemical properties may have some minor effect on the interaction. Most important, however, is the fact that the coffee did not simply merge with the ice: in transferring heat to the ice cube, the molecules in the coffee expended some of their internal kinetic energy, losing further heat in the process. C O O L I N G M AC H I N E S . Even cooling machines, such as refrigerators and air conditioners, actually use heat, simply reversing the usual process by which particles are heated. The refrigerator pulls heat from its inner compartment—the area where food and other perishables are stored—and transfers it to the region outside. This is why the back of a refrigerator is warm.

Inside the refrigerator is an evaporator, into which heat from the refrigerated compartment flows. The evaporator contains a refrigerant—a gas, such as ammonia or Freon 12, that readily liquifies. This gas is released into a pipe from the evaporator at a low pressure, and as a result, it evaporates, a process that cools it. The pipe takes the refrigerant to the compressor, which pumps it into the condenser at a high pressure. Located at the back of the refrigerator, the condenser is a long series of pipes in which pressure turns the gas into liquid. As it moves through the condens-

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er, the gas heats, and this heat is released into the air around the refrigerator. An air conditioner works in a similar manner. Hot air from the room flows into the evaporator, and a compressor circulates refrigerant from the evaporator to a condenser. Behind the evaporator is a fan, which draws in hot air from the room, and another fan pushes heat from the condenser to the outside. As with a refrigerator, the back of an air conditioner is hot because it is moving heat from the area to be cooled. Thus, cooling machines do not defy the principles of heat discussed above; nor do they defy the laws of thermodynamics that will be discussed at the conclusion of this essay. In accordance with the second law, in order to move heat in the reverse of its usual direction, external energy is required. Thus, a refrigerator takes in energy from a electric power supply (that is, the outlet it is plugged into), and extracts heat. Nonetheless, it manages to do so efficiently, removing two or three times as much heat from its inner compartment as the amount of energy required to run the refrigerator.

Transfers of Heat It is appropriate now to discuss how heat is transferred. One must remember, again, that in order for heat to be transferred from one point to another, there must be a difference of temperature between those two points. If an object or system has a uniform level of internal thermal energy—no matter how “hot” it may be in ordinary terms—no heat transfer is taking place. Heat is transferred by one of three methods: conduction, which involves successive molecular collisions; convection, which requires the motion of hot fluid from one place to another; or radiation, which involves electromagnetic waves and requires no physical medium for the transfer. Conduction takes place best in solids and particularly in metals, whose molecules are packed in relatively close proximity. Thus, when one end of an iron rod is heated, eventually the other end will acquire heat due to conduction. Molecules of liquid or nonmetallic solids vary in their ability to conduct heat, but gas—due to the loose attractions between its molecules—is a poor conductor. CONDUCTION.

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shoulder, passing a secret down the line. In this case, however, the “secret” is kinetic thermal energy. And just as the original phrasing of the secret will almost inevitably become garbled by the time it gets to the tenth or hundredth person, some energy is lost in the transfer from molecule to molecule. Thus, if one end of the iron rod is sitting in a fire and one end is surrounded by air at room temperature, it is unlikely that the end in the air will ever get as hot as the end in the fire. Incidentally, the qualities that make metallic solids good conductors of heat also make them good conductors of electricity. In the first instance, kinetic energy is being passed from molecule to molecule, whereas in an electrical field, electrons—freed from the atoms of which they are normally a part—are able to move along the line of molecules. Because plastic is much less conductive than metal, an electrician will use a screwdriver with a plastic handle. Similarly, a metal pan typically has a handle of wood or plastic. C O N V E C T I O N . There is a term, “convection oven,” that is actually a redundancy: all ovens heat through convection, the principal means of transferring heat through a fluid. In physics, “fluid” refers both to liquids and gases— anything that tends to flow. Instead of simply moving heat, as in conduction, convection involves the movement of heated material—that is, fluid. When air is heated, it displaces cold (that is, unheated) air in its path, setting up a convection current.

Convection takes place naturally, as for instance when hot air rises from the land on a warm day. This heated air has a lower density than that of the less heated air in the atmosphere above it, and, therefore, is buoyant. As it rises, however, it loses energy and cools. This cooled air, now more dense than the air around it, sinks again, creating a repeating cycle. The preceding example illustrates natural convection; the heat of an oven, on the other hand, is an example of forced convection—a situation in which some sort of pump or mechanism moves heated fluid. So, too, is the cooling work of a refrigerator, though the refrigerator moves heat in the opposite direction.

When conduction takes place, it is as though a long line of people are standing shoulder to

Forced convection can also take place within a natural system. The human heart is a pump, and blood carries excess heat generated by the body to the skin. The heat passes through the

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skin by means of conduction, and at the surface of the skin, it is removed from the body in a number of ways, primarily by the cooling evaporation of moisture—that is, perspiration.

Thermodynamics

RA D I AT I O N . If the Sun is hot—hot enough to severely burn the skin of a person who spends too much time exposed to its rays—then why is it cold in the upper atmosphere? After all, the upper atmosphere is closer to the Sun. And why is it colder still in the empty space above the atmosphere, which is still closer to the Sun? The reason is that in outer space there is no medium for convection, and in the upper atmosphere, where the air molecules are very far apart, there is hardly any medium. How, then, does heat come to the Earth from the Sun? By radiation, which is radically different from conduction or convection. The other two involve ordinary thermal energy, but radiation involves electromagnetic energy.

A great deal of “stuff ” travels through the electromagnetic spectrum, discussed in another essay in this book: radio waves, microwaves for television and radar, infrared light, visible light, x rays, gamma rays. Though the relatively narrow band of visible-light wavelengths is the only part of the spectrum of which people are aware in everyday life, other parts—particularly the infrared and ultraviolet bands—are involved in the heat one feels from the Sun. (Ultraviolet rays, in fact, cause sunburns.) Heat by means of radiation is not as “otherworldly” as it might seem: in fact, one does not have to point to the Sun for examples of it. Any time an object glows as a result of heat—as for example, in the case of firelight—that is an example of radiation. Some radiation is emitted in the form of visible light, but the heat component is in infrared rays. This also occurs in an incandescent light bulb. In an incandescent bulb, incidentally, much of the energy is lost to the heat of infrared rays, and the efficiency of a fluorescent bulb lies in the fact that it converts what would otherwise be heat into usable light.

The Laws of Thermodynamics Having explored the behavior of heat, both at the molecular level and at levels more easily perceived by the senses, it is possible to discuss the laws of thermodynamics alluded to throughout this essay. These laws illustrate the relationships between heat and energy examined earlier, and

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BENJAMIN THOMPSON, COUNT RUMFORD. (Illustration by H. Humphrey. UPI/Corbis-Bettmann. Reproduced by permission.)

show, for instance, why a refrigerator or air conditioner must have an external source of energy to move heat in a direction opposite to its normal flow. The story of how these laws came to be discovered is a saga unto itself, involving the contributions of numerous men in various places over a period of more than a century. In 1791, Swiss physicist Pierre Prevost (1751-1839) put forth his theory of exchanges, stating correctly that all bodies radiate heat. Hence, as noted earlier, there is no such thing as “cold”: when one holds snow in one’s hand, cold does not flow from the snow into the hand; rather, heat flows from the hand to the snow. Seven years later, an American-British physicist named Benjamin Thompson, Count Rumford (1753) was boring a cannon with a blunt drill when he noticed that this action generated a great deal of heat. This led him to question the prevailing wisdom, which maintained that heat was a fluid form of matter; instead, Thompson began to suspect that heat must arise from some form of motion. C A R N O T ’ S E N G I N E . The next major contribution came from the French physicist and engineer Sadi Carnot (1796-1832).

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Though he published only one scientific work, Reflections on the Motive Power of Fire (1824), this treatise caused a great stir in the European scientific community. In it, Carnot made the first attempt at a scientific definition of work, describing it as “weight lifted through a height.” Even more important was his proposal for a highly efficient steam engine. A steam engine, like a modern-day internal combustion engine, is an example of a larger class of machine called heat engine. A heat engine absorbs heat at a high temperature, performs mechanical work, and, as a result, gives off heat a lower temperature. (The reason why that temperature must be lower is established in the second law of thermodynamics.) For its era, the steam engine was what the computer is today: representing the cutting edge in technology, it was the central preoccupation of those interested in finding new ways to accomplish old tasks. Carnot, too, was fascinated by the steam engine, and was determined to help overcome its disgraceful inefficiency: in operation, a steam engine typically lost as much as 95% of its heat energy. In his Reflections, Carnot proposed that the maximum efficiency of any heat engine was equal to (TH-TL)/TH, where TH is the highest operating temperature of the machine, and TL the lowest. In order to maximize this value, TL has to be absolute zero, which is impossible to reach, as was later illustrated by the third law of thermodynamics. In attempting to devise a law for a perfectly efficient machine, Carnot inadvertently proved that such a machine is impossible. Yet his work influenced improvements in steam engine design, leading to levels of up to 80% efficiency. In addition, Carnot’s studies influenced Kelvin— who actually coined the term “thermodynamics”—and others. T H E F I R S T LAW O F T H E R M O D Y N A M I C S . During the 1840s, Julius

Robert Mayer (1814-1878), a German physicist, published several papers in which he expounded the principles known today as the conservation of energy and the first law of thermodynamics. As discussed earlier, the conservation of energy shows that within a system isolated from all outside factors, the total amount of energy remains the same, though transformations of energy from one form to another take place.

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The first law of thermodynamics states this fact in a somewhat different manner. As with the other laws, there is no definitive phrasing; instead, there are various versions, all of which say the same thing. One way to express the law is as follows: Because the amount of energy in a system remains constant, it is impossible to perform work that results in an energy output greater than the energy input. For a heat engine, this means that the work output of the engine, combined with its change in internal energy, is equal to its heat input. Most heat engines, however, operate in a cycle, so there is no net change in internal energy. Earlier, it was stated that a refrigerator extracts two or three times as much heat from its inner compartment as the amount of energy required to run it. On the surface, this seems to contradict the first law: isn’t the refrigerator putting out more energy than it received? But the heat it extracts is only part of the picture, and not the most important part from the perspective of the first law. A regular heat engine, such as a steam or internal-combustion engine, pulls heat from a high-temperature reservoir to a low-temperature reservoir, and, in the process, work is accomplished. Thus, the hot steam from the high-temperature reservoir makes possible the accomplishment of work, and when the energy is extracted from the steam, it condenses in the low-temperature reservoir as relatively cool water. A refrigerator, on the other hand, reverses this process, taking heat from a low-temperature reservoir (the evaporator inside the cooling compartment) and pumping it to a high-temperature reservoir outside the refrigerator. Instead of producing a work output, as a steam engine does, it requires a work input—the energy supplied via the wall outlet. Of course, a refrigerator does produce an “output,” by cooling the food inside, but the work it performs in doing so is equal to the energy supplied for that purpose. T H E S E C O N D LAW O F T H E R M O D Y N A M I C S . Just a few years after

Mayer’s exposition of the first law, another German physicist, Rudolph Julius Emanuel Clausius (1822-1888) published an early version of the second law of thermodynamics. In an 1850 paper, Clausius stated that “Heat cannot, of itself, pass from a colder to a hotter body.” He refined

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this 15 years later, introducing the concept of entropy—the tendency of natural systems toward breakdown, and specifically, the tendency for the energy in a system to be dissipated.

inevitably break down and die, or cease to function, someday.

The second law of thermodynamics begins from the fact that the natural flow of heat is always from a high-temperature reservoir to a low-temperature reservoir. As a result, no engine can be constructed that simply takes heat from a source and performs an equivalent amount of work: some of the heat will always be lost. In other words, it is impossible to build a perfectly efficient engine.

directly to the third law of thermodynamics, formulated by German chemist Hermann Walter Nernst (1864-1941) in 1905. The third law states that at the temperature of absolute zero, entropy also approaches zero. From this statement, Nernst deduced that absolute zero is therefore impossible to reach.

Though its relation to the first law is obvious, inasmuch as it further defines the limitations of machine output, the second law of thermodynamics is not derived from the first. Elsewhere in this volume, the first law of thermodynamics—stated as the conservation of energy law—is discussed in depth, and, in that context, it is in fact necessary to explain how the behavior of machines in the real world does not contradict the conservation law.

Thermodynamics

T H E T H I R D LAW O F T H E R M O DY N A M I CS . The subject of entropy leads

All matter is in motion at the molecular level, which helps define the three major phases of matter found on Earth. At one extreme is a gas, whose molecules exert little attraction toward one another, and are therefore in constant motion at a high rate of speed. At the other end of the phase continuum (with liquids somewhere in the middle) are solids. Because they are close together, solid particles move very little, and instead of moving in relation to one another, they merely vibrate in place. But they do move.

Even though they mean the same thing, the first law of thermodynamics and the conservation of energy law are expressed in different ways. The first law of thermodynamics states that “the glass is half empty,” whereas the conservation of energy law shows that “the glass is half full.” The thermodynamics law emphasizes the bad news: that one can never get more energy out of a machine than the energy put into it. Thus, all hopes of a perpetual motion machine were dashed. The conservation of energy, on the other hand, stresses the good news: that energy is never lost.

Absolute zero, or 0K on the Kelvin scale of temperature, is the point at which all molecular motion stops entirely—or at least, it virtually stops. (In fact, absolute zero is defined as the temperature at which the motion of the average atom or molecule is zero.) As stated earlier, Carnot’s engine achieves perfect efficiency if its lowest temperature is the same as absolute zero; but the second law of thermodynamics shows that a perfectly efficient machine is impossible. This means that absolute zero is an unreachable extreme, rather like matter exceeding the speed of light, also an impossibility.

In this context, the second law of thermodynamics delivers another dose of bad news: though it is true that energy is never lost, the energy available for work output will never be as great as the energy put into a system. A car engine, for instance, cannot transform all of its energy input into usable horsepower; some of the energy will be used up in the form of heat and sound. Though energy is conserved, usable energy is not. Indeed, the concept of entropy goes far beyond machines as people normally understand them. Entropy explains why it is easier to break something than to build it—and why, for each person, the machine called the human body will

This does not mean that scientists do not attempt to come as close as possible to absolute zero, and indeed they have come very close. In 1993, physicists at the Helsinki University of Technology Low Temperature Laboratory in Finland used a nuclear demagnetization device to achieve a temperature of 2.8 • 10-10 K, or 0.00000000028K. This means that a fragment equal to only 28 parts in 100 billion separated this temperature from absolute zero—but it was still above 0K. Such extreme low-temperature research has a number of applications, most notably with superconductors, materials that exhibit virtually no resistance to electrical current at very low temperatures.

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Thermodynamics

KEY TERMS The temperature,

either a gas or a liquid, and convection is

defined as 0K on the Kelvin scale, at which

the principal means of heat transfer, for

the motion of molecules in a solid virtual-

instance, in air and water.

ly ceases. The third law of thermodynamics

ENERGY:

establishes the impossibility of actually

work.

ABSOLUTE ZERO:

reaching absolute zero.

The ability to accomplish

ENTROPY:

The tendency of natural

A

systems toward breakdown, and specifical-

measure of energy or heat in the British

ly, the tendency for the energy in a system

system, often used in reference to the

to be dissipated. Entropy is closely related

capacity of an air conditioner. A Btu is

to the second law of thermodynamics.

BTU (BRITISH THERMAL UNIT):

equal to 778 foot-pounds, or 1,054 joules. FIRST LAW OF THERMODYNAMICS: CALORIE:

A measure of heat or energy

in the SI or metric system, equal to the heat that must be added to or removed from 1 gram of water to change its temperature by 33.8°F (1°C). The dietary Calorie (capital C) with which most people are familiar is the same as the kilocalorie. CONDUCTION:

The transfer of heat

by successive molecular collisions. Conduction is the principal means of heat transfer in solids, particularly metals. CONSERVATION OF ENERGY:

A

law of physics which holds that within a system isolated from all other outside factors, the total amount of energy remains the same, though transformations of ener-

a system remains constant, and therefore it is impossible to perform work that results in an energy output greater than the energy input. This is the same as the conservation of energy. FOOT-POUND:

The principal unit of

energy—and thus of heat—in the British or English system. The metric or SI unit is the joule. A foot-pound (ft • lb) is equal to 1.356 J. HEAT:

Internal thermal energy that

flows from one body of matter to another. Heat is transferred by three methods conduction, convection, and radiation. The amount of heat

gy from one form to another take place.

HEAT CAPACITY:

The first law of thermodynamics is the

that must be added to, or removed from, a

same as the conservation of energy.

unit of mass of a given substance to change

In physics, “to conserve”

its temperature by 33.8°F (1°C). Heat

something means “to result in no net loss

capacity is sometimes called specific heat

of ” that particular component. It is possi-

capacity or specific heat. A kilocalorie is

ble that within a given system, the compo-

the heat capacity of 1 gram of water.

nent may change form or position, but as

HEAT

long as the net value of the component

absorbs heat at a high temperature, per-

remains the same, it has been conserved.

forms mechanical work, and as a result

CONSERVE:

CONVECTION:

224

A law which states the amount of energy in

The transfer of heat

ENGINE:

A machine that

gives off heat at a lower temperature. The energy that

through the motion of hot fluid from one

KINETIC ENERGY:

place to another. In physics, a “fluid” can be

an object possesses by virtue of its motion.

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Thermodynamics

KEY TERMS

CONTINUED

The principal unit of energy—

therefore it is impossible to build a perfect-

and thus of heat—in the SI or metric sys-

ly efficient engine. This is a result of the

tem, corresponding to 1 newton-meter (N

fact that the natural flow of heat is always

• m). A joule (J) is equal to 0.7376 foot-

from a high-temperature reservoir to a

pounds.

low-temperature

JOULE:

fact

by

expressed in the concept of entropy. The

William Thomson, Lord Kelvin (1824-

second law is sometimes referred to as “the

1907), the Kelvin scale measures tempera-

law of entropy.”

ture in relation to absolute zero, or 0K.

SYSTEM:

(Units in the Kelvin system, known as

usually refers to any set of physical interac-

Kelvins, do not include the word or symbol

tions isolated from the rest of the universe.

for degree.) The Kelvin and Celsius scales

Anything outside of the system, including

are directly related; hence Celsius tempera-

all factors and forces irrelevant to a discus-

KELVIN

SCALE:

Established

reservoir—a

tures can be converted to Kelvins by adding 273.15.

In physics, the term “system”

sion of that system, is known as the environment.

KILOCALORIE:

A measure of heat or

energy in the SI or metric system, equal to the heat that must be added to or removed from 1 kilogram of water to change its temperature by 33.8°F (1°C). As its name suggests, a kilocalorie is 1,000 calories. The dietary Calorie (capital C) with which most people are familiar is the same as the

TEMPERATURE:

The direction of

internal energy flow between bodies when heat is being transferred. Temperature measures the average molecular kinetic energy in transit between those bodies. THERMAL ENERGY:

Heat energy, a

form of kinetic energy produced by the movement of atomic or molecular parti-

kilocalorie. MECHANICAL ENERGY:

The sum of

potential energy and kinetic energy in a

cles. The greater the movement of these particles, the greater the thermal energy. THERMODYNAMICS:

given system. POTENTIAL ENERGY:

The energy

that an object possesses due to its position.

The study of

the relationships between heat, work, and energy.

The transfer of heat by

THIRD LAW OF THERMODYNAMICS:

means of electromagnetic waves, which

A law of thermodynamics which states that

require no physical medium (e.g., water or

at the temperature of absolute zero,

air) for the transfer. Earth receives the Sun’s

entropy also approaches zero. Zero entropy

heat by means of radiation.

would contradict the second law of ther-

SECOND LAW OF THERMODYNAM-

modynamics, meaning that absolute zero is

RADIATION:

ICS:

A law of thermodynamics which

therefore impossible to reach. The exertion of force over a

states that no engine can be constructed

WORK:

that simply takes heat from a source and

given distance to displace or move an

performs an equivalent amount of work.

object. Work is thus the product of force

Some of the heat will always be lost, and

and distance exerted in the same direction.

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WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. Brown, Warren. Alternative Sources of Energy. Introduction by Russell E. Train. New York: Chelsea House, 1994. Encyclopedia of Thermodynamics (Web site). (April 12, 2001).

226

Fleisher, Paul. Matter and Energy: Principles of Matter and Thermodynamics. Minneapolis, MN: Lerner Publications, 2002. Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998. Moran, Jeffrey B. How Do We Know the Laws of Thermodynamics? New York: Rosen Publishing Group, 2001. Santrey, Laurence. Heat. Illustrated by Lloyd Birmingham. Mahwah, N.J.: Troll Associates, 1985. Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996.

Entropy and the Second Law of Thermodynamics (Web site). (April 12, 2001).

“Temperature and Thermodynamics” PhysLINK.com (Web site). (April 12, 2001).

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Heat

CONCEPT Heat is a form of energy—specifically, the energy that flows between two bodies because of differences in temperature. Therefore, the scientific definition of heat is different from, and more precise than, the everyday meaning. Physicists working in the area of thermodynamics study heat from a number of perspectives, including specific heat, or the amount of energy required to change the temperature of a substance, and calorimetry, the measurement of changes in heat as a result of physical or chemical changes. Thermodynamics helps us to understand such phenomena as the operation of engines and the gradual breakdown of complexity in physical systems—a phenomenon known as entropy.

HOW IT WORKS Heat, Work, and Energy Thermodynamics is the study of the relationships between heat, work, and energy. Work is the exertion of force over a given distance to displace or move an object, and is, thus, the product of force and distance exerted in the same direction. Energy, the ability to accomplish work, appears in numerous manifestations—including thermal energy, or the energy associated with heat.

H E AT

return to its original position, it begins gaining kinetic energy and losing potential energy. All manifestations of energy appear in both kinetic and potential forms, somewhat like the way football teams are organized to play both offense or defense. Just as a football team takes an offensive role when it has the ball, and a defensive role when the other team has it, a physical system typically undergoes regular transformations between kinetic and potential energy, and may have more of one or the other, depending on what is taking place in the system.

What Heat Is and Is Not Thermal energy is actually a form of kinetic energy generated by the movement of particles at the atomic or molecular level: the greater the movement of these particles, the greater the thermal energy. Heat is internal thermal energy that flows from one body of matter to another—or, more specifically, from a system at a higher temperature to one at a lower temperature. Thus, temperature, like heat, requires a scientific definition quite different from its common meaning: temperature measures the average molecular kinetic energy of a system, and governs the direction of internal energy flow between them.

Thermal and other types of energy, including electromagnetic, sound, chemical, and nuclear energy, can be described in terms of two extremes: kinetic energy, or the energy associated with movement, and potential energy, or the energy associated with position. If a spring is pulled back to its maximum point of tension, its potential energy is also at a maximum; once it is released and begins springing through the air to

Two systems at the same temperature are said to be in a state of thermal equilibrium. When this occurs, there is no exchange of heat. Though in common usage, “heat” is an expression of relative warmth or coldness, in physical terms, heat exists only in transfer between two systems. What people really mean by “heat” is the internal energy of a system—energy that is a property of that system rather than a property of transferred internal energy.

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means, convection. In fact, there are three methods heat is transferred: conduction, involving successive molecular collisions and the transfer of heat between two bodies in contact; convection, which requires the motion of fluid from one place to another; or radiation, which takes place through electromagnetic waves and requires no physical medium, such as water or air, for the transfer.

Heat

C O N D U C T I O N . Solids, particularly metals, whose molecules are packed relatively close together, are the best materials for conduction. Molecules of liquid or non-metallic solids vary in their ability to conduct heat, but gas is a poor conductor, because of the loose attractions between its molecules.

IF YOU HOLD A WHITE AND HER

SNOWBALL IN YOUR HAND, AS

VANNA

SON ARE DOING IN THIS PICTURE, HEAT

WILL MOVE FROM YOUR HAND TO THE SNOWBALL.

YOUR

HAND EXPERIENCES THIS AS A SENSATION OF COLDNESS. (Reuters NewMedia Inc./Corbis. Reproduced by permission.)

N O S U C H T H I N G AS “ C O L D . ”

Though the term “cold” has plenty of meaning in the everyday world, in physics terminology, it does not. Cold and heat are analogous to darkness and light: again, darkness means something in our daily experience, but in physical terms, darkness is simply the absence of light. To speak of cold or darkness as entities unto themselves is rather like saying, after spending 20 dollars, “I have 20 non-dollars in my pocket.” If you grasp a snowball in your hand, of course, your hand gets cold. The human mind perceives this as a transfer of cold from the snowball, but, in fact, exactly the opposite happens: heat moves from your hand to the snow, and if enough heat enters the snowball, it will melt. At the same time, the departure of heat from your hand results in a loss of internal energy near the surface of your hand, which you experience as a sensation of coldness.

Transfers of Heat

228

The qualities that make metallic solids good conductors of heat, as a matter of fact, also make them good conductors of electricity. In the conduction of heat, kinetic energy is passed from molecule to molecule, like a long line of people standing shoulder to shoulder, passing a secret. (And, just as the original phrasing of the secret becomes garbled, some kinetic energy is inevitably lost in the series of transfers.) As for electrical conduction, which takes place in a field of electric potential, electrons are freed from their atoms; as a result, they are able to move along the line of molecules. Because plastic is much less conductive than metal, an electrician uses a screwdriver with a plastic handle; similarly, a metal cooking pan typically has a wooden or plastic handle. C O N V E C T I O N . Wherever fluids are involved—and in physics, “fluid” refers both to liquids and gases—convection is a common form of heat transfer. Convection involves the movement of heated material—whether it is air, water, or some other fluid.

Convection is of two types: natural convection and forced convection, in which a pump or other mechanism moves the heated fluid. When heated air rises, this is an example of natural convection. Hot air has a lower density than that of the cooler air in the atmosphere above it, and, therefore, is buoyant; as it rises, however, it loses energy and cools. This cooled air, now denser than the air around it, sinks again, creating a repeating cycle that generates wind.

In holding the snowball, heat passes from the surface of the hand by one means, conduction, then passes through the snowball by another

Examples of forced convection include some types of ovens and even a refrigerator or air conditioner. These two machines both move warm

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air from an interior to an exterior place. Thus, the refrigerator pulls hot air from the compartment and expels it to the surrounding room, while an air conditioner pulls heat from a building and releases it to the outside.

Heat

But forced convection does not necessarily involve humanmade machines: the human heart is a pump, and blood carries excess heat generated by the body to the skin. The heat passes through the skin by means of conduction, and at the surface of the skin, it is removed from the body in a number of ways, primarily by the cooling evaporation of perspiration. RA D I AT I O N . Outer space, of course, is

cold, yet the Sun’s rays warm the Earth, an apparent paradox. Because there is no atmosphere in space, convection is impossible. In fact, heat from the Sun is not dependant on any fluid medium for its transfer: it comes to Earth by means of radiation. This is a form of heat transfer significantly different from the other two, because it involves electromagnetic energy, instead of ordinary thermal energy generated by the action of molecules. Heat from the Sun comes through a relatively narrow area of the light spectrum, including infrared, visible light, and ultraviolet rays. Every form of matter emits electromagnetic waves, though their presence may not be readily perceived. Thus, when a metal rod is heated, it experiences conduction, but part of its heat is radiated, manifested by its glow—visible light. Even when the heat in an object is not visible, however, it may be radiating electromagnetic energy, for instance, in the form of infrared light. And, of course, different types of matter radiate better than others: in general, the better an object is at receiving radiation, the better it is at emitting it.

A

REFRIGERATOR IS A TYPE OF REVERSE HEAT ENGINE

THAT USES A COMPRESSOR, LIKE THE ONE SHOWN AT THE

BACK

OF

THIS

REFRIGERATOR,

TO

COOL

THE

REFRIGERATOR’S INTERIOR. (Ecoscene/Corbis. Reproduced by

permission.)

Abbreviated “J,” a joule is equal to 1 newtonmeter (N • m). The newton is the SI unit of force, and since work is equal to force multiplied by distance, measures of work can also be separated into these components. For instance, the British measure of work brings together a unit of distance, the foot, and a unit of force, the pound. A foot-pound (ft • lb) is equal to 1.356 J, and 1 joule is equal to 0.7376 ft • lb.

The principal unit of work or energy in the metric system (known within the scientific community as SI, or the SI system) is the joule.

In the British system, Btu, or British thermal unit, is another measure of energy used for machines such as air conditioners. One Btu is equal to 778 ft • lb or 1,054 J. The kilocalorie in addition to the joule, is an important SI measure of heat. The amount of energy required to change the temperature of 1 gram of water by 1°C is called a calorie, and a kilocalorie is equal to 1,000 calories. Somewhat confusing is the fact that the dietary Calorie (capital C), with which most people are familiar, is not the same as a calorie (lowercase C)—rather, a dietary Calorie is the equivalent of a kilocalorie.

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Measuring Heat The measurement of temperature by degrees in the Fahrenheit or Celsius scales is a part of everyday life, but measurements of heat are not as familiar to the average person. Because heat is a form of energy, and energy is the ability to perform work, heat is, therefore, measured by the same units as work.

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REAL-LIFE A P P L I C AT I O N S Specific Heat Specific heat is the amount of heat that must be added to, or removed from, a unit of mass for a given substance to change its temperature by 1°C. Thus, a kilocalorie, because it measures the amount of heat necessary to effect that change precisely for a kilogram of water, is identical to the specific heat for that particular substance in that particular unit of mass. The higher the specific heat, the more resistant the substance is to changes in temperature. Many metals, in fact, have a low specific heat, making them easy to heat up and cool down. This contributes to the tendency of metals to expand when heated (a phenomenon also discussed in the Thermal Expansion essay), and, thus, to their malleability. M E AS U R I N G A N D CA L C U LAT I N G S P E C I F I C H E AT. The specific heat

of any object is a function of its mass, its composition, and the desired change in temperature. The values of the initial and final temperature are not important—only the difference between them, which is the temperature change. The components of specific heat are related to one another in the formula Q = mcδT. Here Q is the quantity of heat, measured in joules, which must be added. The mass of the object is designated by m, and the specific heat of the particular substance in question is represented with c. The Greek letter delta (δ) designates change, and δT stands for “change in temperature.” Specific heat is measured in units of J/kg • °C (joules per kilogram-degree Centigrade), though for the sake of convenience, this is usually rendered in terms of kilojoules (kJ), or 1,000 joules—that is, kJ/kg • °C. The specific heat of water is easily derived from the value of a kilocalorie: it is 4.185, the same number of joules required to equal a kilocalorie.

By the mid-1800s, a number of thinkers had come to the realization that—contrary to prevailing theories of the day—heat was a form of energy, not a type of material substance. Among these were American-British physicist Benjamin Thompson, Count Rumford (1753-1814) and English chemist James Joule (1818-1889)—for whom, of course, the joule is named. Calorimetry as a scientific field of study actually had its beginnings with the work of French chemist Pierre-Eugene Marcelin Berthelot (1827-1907). During the mid-1860s, Berthelot became intrigued with the idea of measuring heat, and by 1880, he had constructed the first real calorimeter. CALORIMETERS. Essential to calorimetry is the calorimeter, which can be any device for accurately measuring the temperature of a substance before and after a change occurs. A calorimeter can be as simple as a styrofoam cup. Its quality as an insulator, which makes styrofoam ideal for holding in the warmth of coffee and protecting the hand from scalding as well, also makes styrofoam an excellent material for calorimetric testing. With a styrofoam calorimeter, the temperature of the substance inside the cup is measured, a reaction is allowed to take place, and afterward, the temperature is measured a second time.

The measurement of heat gain or loss as a result of physical or chemical change is called calorimetry (pronounced kal-IM-uh-tree). Like the word “calorie,” the term is derived from a Latin root meaning “heat.”

The most common type of calorimeter used is the bomb calorimeter, designed to measure the heat of combustion. Typically, a bomb calorimeter consists of a large container filled with water, into which is placed a smaller container, the combustion crucible. The crucible is made of metal, having thick walls with an opening through which oxygen can be introduced. In addition, the combustion crucible is designed to be connected to a source of electricity.

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The foundations of calorimetry go back to the mid-nineteenth century, but the field owes much to scientists’ work that took place over a period of about 75 years prior to that time. In 1780, French chemist Antoine Lavoisier (17431794) and French astronomer and mathematician Pierre Simon Laplace (1749-1827) had used a rudimentary ice calorimeter for measuring the heats in formations of compounds. Around the same time, Scottish chemist Joseph Black (17281799) became the first scientist to make a clear distinction between heat and temperature.

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In conducting a calorimetric test using a bomb calorimeter, the substance or object to be studied is placed inside the combustion crucible and ignited. The resulting reaction usually occurs so quickly that it resembles the explosion of a bomb—hence, the name “bomb calorimeter.” Once the “bomb” goes off, the resulting transfer of heat creates a temperature change in the water, which can be readily gauged with a thermometer.

tion engine, a steam engine burns its fuel outside the engine. That fuel may be simply firewood, which is used to heat water and create steam. The thermal energy of the steam is then used to power a piston moving inside a cylinder, thus, converting thermal energy to mechanical energy for purposes such as moving a train.

To study heat changes at temperatures higher than the boiling point of water (212°F or 100°C), physicists use substances with higher boiling points. For experiments involving extremely large temperature ranges, an aneroid (without liquid) calorimeter may be used. In this case, the lining of the combustion crucible must be of a metal, such as copper, with a high coefficient or factor of thermal conductivity.

science and technology, the historical roots of the steam engine can be traced to the Greeks, who— just as they did with ideas such as the atom or the Sun-centered model of the universe—thought about it, but failed to develop it. The great inventor Hero of Alexandria (c. 65-125) actually created several steam-powered devices, but he perceived these as mere novelties, hardly worthy of scientific attention. Though Europeans adopted water power, as, for instance, in waterwheels, during the late ancient and medieval periods, further progress in steam power did not occur for some 1,500 years.

Heat Engines The bomb calorimeter that Berthelot designed in 1880 measured the caloric value of fuels, and was applied to determining the thermal efficiency of a heat engine. A heat engine is a machine that absorbs heat at a high temperature, performs mechanical work, and as a result, gives off heat at a lower temperature. The desire to create efficient heat engines spurred scientists to a greater understanding of thermodynamics, and this resulted in the laws of thermodynamics, discussed at the conclusion of this essay. Their efforts were intimately connected with one of the greatest heat engines ever created, a machine that literally powered the industrialized world during the nineteenth century: the steam engine. HOW A STEAM ENGINE WORKS.

Like all heat engines (except reverse heat engines such as the refrigerator, discussed below), a steam engine pulls heat from a high-temperature reservoir to a low-temperature reservoir, and in the process, work is accomplished. The hot steam from the high-temperature reservoir makes possible the accomplishment of work, and when the energy is extracted from the steam, the steam condenses in the low-temperature reservoir, becoming relatively cool water.

Heat

E V O LU T I O N O F S T E A M P O W E R. As with a number of advanced concepts in

Following the work of French physicist Denis Papin (1647-1712), who invented the pressure cooker and conducted the first experiments with the use of steam to move a piston, English engineer Thomas Savery (c. 1650-1715) built the first steam engine. Savery had abandoned the use of the piston in his machine, but another English engineer, Thomas Newcomen (1663-1729), reintroduced the piston for his own steam-engine design. Then in 1763, a young Scottish engineer named James Watt (1736-1819) was repairing a Newcomen engine and became convinced he could build a more efficient model. His steam engine, introduced in 1769, kept the heating and cooling processes separate, eliminating the need for the engine to pause in order to reheat. These and other innovations that followed—including the introduction of a high-pressure steam engine by English inventor Richard Trevithick (17711833)—transformed the world. CA R N O T P R O V I D E S T H E O R E T I CA L U N D E R S TA N D I N G . The men

A steam engine is an external-combustion engine, as opposed to the internal-combustion engine that took its place at the forefront of industrial technology at the beginning of the twentieth century. Unlike an internal-combus-

who developed the steam engine were mostly practical-minded figures who wanted only to build a better machine; they were not particularly concerned with the theoretical explanation for its workings. Then in 1824, a French physicist and engineer by the name of Sadi Carnot (17961832) published his sole work, the highly influ-

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ential Reflections on the Motive Power of Fire (1824), in which he discussed heat engines scientifically. In Reflections, Carnot offered the first definition of work in terms of physics, describing it as “weight lifted through a height.” Analyzing Watt’s steam engine, he also conducted groundbreaking studies in the nascent science of thermodynamics. Every heat engine, he explained, has a theoretical limit of efficiency related to the temperature difference in the engine: the greater the difference between the lowest and highest temperature, the more efficient the engine. Carnot’s work influenced the development of more efficient steam engines, and also had an impact on the studies of other physicists investigating the relationship between work, heat, and energy. Among these was William Thomson, Lord Kelvin (1824-1907). In addition to coining the term “thermodynamics,” Kelvin developed the Kelvin scale of absolute temperature and established the value of absolute zero, equal to -273.15°C or -459.67°F. According to Carnot’s theory, maximum effectiveness was achieved by a machine that could reach absolute zero. However, later developments in the understanding of thermodynamics, as discussed below, proved that both maximum efficiency and absolute zero are impossible to attain. R E V E R S E H E AT E N G I N E S . It is easy to understand that a steam engine is a heat engine: after all, it produces heat. But how is it that a refrigerator, an air conditioner, and other cooling machines are also heat engines? Moreover, given the fact that cold is the absence of heat and heat is energy, one might ask how a refrigerator or air conditioner can possibly use energy to produce cold, which is the same as the absence of energy. In fact, cooling machines simply reverse the usual process by which heat engines operate, and for this reason, they are called “reverse heat engines.” Furthermore, they use energy to extract heat.

A steam engine takes heat from a high-temperature reservoir—the place where the water is turned into steam—and uses that energy to produce work. In the process, energy is lost and the heat moves to a low-temperature reservoir, where it condenses to form relatively cool water. A refrigerator, on the other hand, pulls heat from a low-temperature reservoir called the evaporator,

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into which flows heat from the refrigerated compartment—the place where food and other perishables are kept. The coolant from the evaporator take this heat to the condenser, a high-temperature reservoir at the back of the refrigerator, and in the process it becomes a gas. Heat is released into the surrounding air; this is why the back of a refrigerator is hot. Instead of producing a work output, as a steam engine does, a refrigerator requires a work input—the energy supplied via the wall outlet. The principles of thermodynamics show that heat always flows from a high-temperature to a low-temperature reservoir, and reverse heat engines do not defy these laws. Rather, they require an external power source in order to effect the transfer of heat from a low-temperature reservoir, through the gases in the evaporator, to a high-temperature reservoir.

The Laws of Thermodynamics T H E F I R S T LAW O F T H E R M O DY N A M I CS . There are three laws of ther-

modynamics, which provide parameters as to the operation of thermal systems in general, and heat engines in particular. The history behind the derivation of these laws is discussed in the essay on Thermodynamics; here, the laws themselves will be examined in brief form. The physical law known as conservation of energy shows that within a system isolated from all outside factors, the total amount of energy remains the same, though transformations of energy from one form to another take place. The first law of thermodynamics states the same fact in a somewhat different manner. According to the first law of thermodynamics, because the amount of energy in a system remains constant, it is impossible to perform work that results in an energy output greater than the energy input. Thus, it could be said that the conservation of energy law shows that “the glass is half full”: energy is never lost. On the hand, the first law of thermodynamics shows that “the glass is half empty”: no machine can ever produce more energy than was put into it. Hence, a perpetual motion machine is impossible, because in order to keep a machine running continually, there must be a continual input of energy. T H E S E C O N D LAW O F T H E R M O DY N A M I CS . The second law of ther-

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Heat

KEY TERMS The temperature,

either a gas or a liquid, and convection is

defined as 0K on the Kelvin scale, at which

the principal means of heat transfer, for

the motion of molecules in a solid virtual-

instance, in air and water.

ly ceases. The third law of thermodynamics

ENERGY:

establishes the impossibility of actually

work.

ABSOLUTE ZERO:

The ability to accomplish

reaching absolute zero. ENTROPY: BTU (BRITISH THERMAL UNIT):

A

measure of energy or heat in the British system, often used in reference to the capacity of an air conditioner. A Btu is

The tendency of natural

systems toward breakdown, and specifically, the tendency for the energy in a system to be dissipated. Entropy is closely related to the second law of thermodynamics.

equal to 778 foot-pounds, or 1,054 joules. CALORIE:

A measure of heat or energy

in the SI or metric system, equal to the heat that must be added to or removed from 1 gram of water to change its temperature by 1°C. The dietary Calorie (capital C) with which most people are familiar is the same as the kilocalorie. CALORIMETRY:

The measurement of

heat gain or loss as a result of physical or chemical change. CONDUCTION:

The transfer of heat

by successive molecular collisions. Con-

FIRST LAW OF THERMODYNAMICS:

A law stating that the amount of energy in a system remains constant, and therefore, it is impossible to perform work that results in an energy, output greater than the energy input. This is the same as the conservation of energy. FOOT-POUND:

The principal unit of

energy—and, thus, of heat—in the British or English system. The metric or SI unit is the joule. A foot-pound (ft • lb) is equal to 1.356 J. Internal thermal energy that

duction is the principal means of heat

HEAT:

transfer in solids, particularly metals.

flows from one body of matter to another.

CONSERVATION OF ENERGY:

A

law of physics stating that within a system

Heat is transferred by three methods conduction, convection, and radiation. A machine that

isolated from all other outside factors, the

HEAT

total amount of energy remains the same,

absorbs heat at a high temperature, per-

though transformations of energy from

forms mechanical work, and, as a result,

one form to another take place. The first

gives off heat at a lower temperature.

law of thermodynamics is the same as the

JOULE:

conservation of energy.

and, thus, of heat—in the SI or metric sys-

ENGINE:

The principal unit of energy—

The transfer of heat

tem, corresponding to 1 newton-meter

through the motion of hot fluid from one

(N • m). A joule (J) is equal to 0.7376 foot-

place to another. In physics, a “fluid” can be

pounds.

CONVECTION:

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Heat

KEY TERMS KELVIN

SCALE:

Established

by

William Thomson, Lord Kelvin (18241907), the Kelvin scale measures tempera-

CONTINUED

POTENTIAL ENERGY:

The energy

that an object possesses due to its position. RADIATION:

The transfer of heat by

ture in relation to absolute zero, or 0K.

means of electromagnetic waves, which

(Units in the Kelvin system, known as

require no physical medium (for example,

Kelvins, do not include the word or symbol

water or air) for the transfer. Earth receives

for degree.) The Kelvin and Celsius scales

the Sun’s heat by means of radiation.

are directly related; hence, Celsius temperatures can be converted to Kelvins by adding 273.15.

ICS:

A law of thermodynamics stating

that no engine can be constructed that A measure of heat or

simply takes heat from a source and per-

energy in the SI or metric system, equal to

forms an equivalent amount of work.

the heat that must be added to or removed

Some of the heat will always be lost, and,

from 1 kilogram of water to change its

therefore, it is impossible to build a

KILOCALORIE:

temperature by 1°C. As its name suggests, a kilocalorie is 1,000 calories. The dietary Calorie (capital C) with which most people are familiar, is the same as the kilocalorie. KINETIC ENERGY:

The energy that

an object possesses by virtue of its motion.

modynamics begins from the fact that the natural flow of heat is always from a high-temperature to a low-temperature reservoir. As a result, no engine can be constructed that simply takes heat from a source and performs an equivalent amount of work: some of the heat will always be lost. In other words, it is impossible to build a perfectly efficient engine. In effect, the second law of thermodynamics compounds the “bad news” delivered by the first law with some even worse news: though it is true that energy is never lost, the energy available for work output will never be as great as the energy put into a system. Linked to the second law is the concept of entropy, the tendency of natural systems toward breakdown, and specifically, the tendency for the energy in a system to be dissipated. “Dissipated” in this context means that the highand low-temperature reservoirs approach equal

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SECOND LAW OF THERMODYNAM-

VOLUME 2: REAL-LIFE PHYSICS

perfectly efficient engine. This is a result of the fact that the natural flow of heat is always from a high-temperature reservoir to a low-temperature reservoir—a fact expressed in the concept of entropy. The second law is sometimes referred to as “the law of entropy.”

temperatures, and as this occurs, entropy increases. T H E T H I R D LAW O F T H E R M O DY N A M I CS . Entropy also plays a part in the

third law of thermodynamics, which states that at the temperature of absolute zero, entropy also approaches zero. This might seem to counteract the “worse news” of the second law, but in fact, what the third law shows is that absolute zero is impossible to reach. As stated earlier, Carnot’s engine would achieve perfect efficiency if its lowest temperature were the same as absolute zero; but the second law of thermodynamics shows that a perfectly efficient machine is impossible. Relativity theory (which first appeared in 1905, the same year as the third law of thermodynamics) showed that matter can never exceed the speed of light. In the same way, the collective effect of the second and third laws is to prove that absolute

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Heat

KEY TERMS SPECIFIC HEAT:

The amount of heat

that must be added to, or removed from, a unit of mass of a given substance to change its temperature by 1°C. A kilocalorie is the specific heat of 1 gram of water. SYSTEM:

In physics, the term “system”

usually refers to any set of physical interac-

CONTINUED

cles. The greater the movement of these particles, the greater the thermal energy. THERMAL EQUILIBRIUM:

The state

that exists when two systems have the same temperature. As a result, there is no exchange of heat between them. THERMODYNAMICS:

The study of

tions isolated from the rest of the universe.

the relationships between heat, work, and

Anything outside of the system, including

energy.

all factors and forces irrelevant to a discussion of that system, is known as the environment.

THIRD LAW OF THERMODYNAMICS:

A law of thermodynamics which states that at the temperature of absolute zero,

The direction of

entropy also approaches zero. Zero entropy

internal energy flow between two systems

would contradict the second law of ther-

when heat is being transferred. Tempera-

modynamics, meaning that absolute zero

ture measures the average molecular kinet-

is, therefore, impossible to reach.

TEMPERATURE:

ic energy in transit between those systems. WORK:

The exertion of force over a

Heat energy, a

given distance to displace or move an

form of kinetic energy produced by the

object. Work is, thus, the product of force

movement of atomic or molecular parti-

and distance exerted in the same direction.

THERMAL ENERGY:

zero—the temperature at which molecular motion in all forms of matter theoretically ceases—can never be reached. WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. Bonnet, Robert L and Dan Keen. Science Fair Projects: Physics. Illustrated by Frances Zweifel. New York: Sterling, 1999. Encyclopedia of Thermodynamics (Web site). (April 12, 2001). Friedhoffer, Robert. Physics Lab in the Home. Illustrated by Joe Hosking. New York: Franklin Watts, 1997.

S C I E N C E O F E V E RY DAY T H I N G S

Manning, Mick and Brita Granström. Science School. New York: Kingfisher, 1998. Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998. Moran, Jeffrey B. How Do We Know the Laws of Thermodynamics? New York: Rosen Publishing Group, 2001. Santrey, Laurence. Heat. Illustrated by Lloyd Birmingham. Mahwah, NJ: Troll Associates, 1985. Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996. “Temperature and Thermodynamics” PhysLINK.com (Web site). (April 12, 2001).

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T E M P E R AT U R E Temperature

CONCEPT Temperature is one of those aspects of the everyday world that seems rather abstract when viewed from the standpoint of physics. In scientific terms, it is not simply a measure of hot and cold, but an indicator of molecular motion and energy flow. Thermometers measure temperature by a number of means, including the expansion that takes place in a medium such as mercury or alcohol. These measuring devices are gauged in several different ways, with scales based on the freezing and boiling points of water—as well as, in the case of the absolute temperature scale, the point at which all molecular motion virtually ceases.

HOW IT WORKS Heat Energy appears in many forms, including thermal energy, or the energy associated with heat. Heat is internal thermal energy that flows from one body of matter to another—or, more specifically, from a system at a higher temperature to one at a lower temperature.

236

H E AT:

E N E R GY

IN

T RA N S I T.

Thus, heat cannot be said to exist unless there is one system in contact with another system of differing temperature. This can be illustrated by way of the old philosophical question: “If a tree falls in the woods when there is no one to hear it, does it make a sound?” From a physicist’s point of view, of course, sound waves are emitted whether or not there is an ear to receive their vibrations; but, consider this same scenario in terms of heat. First, replace the falling tree with a hypothetical object possessing a certain amount of internal energy; then replace sound waves with heat. In this case, if this object is not in contact with something else that has a different temperature, it “does not make a sound”—in other words, it transfers no internal energy, and, thus, there is no heat from the standpoint of physics. This could even be true of two incredibly “hot” objects placed next to one another inside a vacuum—an area devoid of matter, including air. If both have the same temperature, there is no heat, only two objects with high levels of internal energy. Note that a vacuum was specified: assuming there was air around them, and that the air was of a lower temperature, both objects would then be transferring heat to the air.

Two systems at the same temperature are said to be in a state of thermal equilibrium. When this occurs, there is no exchange of heat. Though people ordinarily speak of “heat” as an expression of relative warmth or coldness, in physical terms, heat only exists in transfer between two systems. It is never something inherently part of a system; thus, unless there is a transfer of internal energy, there is no heat, scientifically speaking.

energy in transfer, from whence does this energy originate? From the movement of molecules. Every type of matter is composed of molecules, and those molecules are in motion relative to one another. The greater the amount of relative motion between molecules, the greater the kinetic energy, or the energy of movement, which is manifested as thermal energy. Thus, “heat”—to

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R E LAT I V E M O T I O N B E T W E E N M O L E C U L E S . If heat is internal thermal

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use the everyday term for what physicists describe as thermal energy—is really nothing more than the result of relative molecular motion. Thus, thermal energy is sometimes identified as molecular translational energy.

Temperature

Note that the molecules are in relative motion, meaning that if one were “standing” on a molecule, one would see the other molecules moving. This is not the same as movement on the part of a large object composed of molecules; in this case, molecules themselves are not directly involved in relative motion. Put another way, the movement of Earth through space is an entirely different type of movement from the relative motion of objects on Earth—people, animals, natural forms such as clouds, manmade forms of transportation, and so forth. In this example, Earth is analogous to a “large” item of matter, such as a baseball, a stream of water, or a cloud of gas. The smaller objects on Earth are analogous to molecules, and, in both cases, the motion of the larger object has little direct impact on the motion of smaller objects. Hence, as discussed in the Frame of Reference essay, it is impossible to perceive with one’s senses the fact that Earth is actually hurling through space at incredible speeds. MOLECULAR MOTION AND P H A S E S O F M AT T E R. The relative

motion of molecules determines phase of matter—that is, whether something is a solid, liquid, or gas. When molecules move quickly in relation to one another, they exert a small electromagnetic attraction toward one another, and the larger material of which they are a part is called a gas. A liquid, on the other hand, is a type of matter in which molecules move at moderate speeds in relation to one another, and therefore exert a moderate intermolecular attraction. The kinetic theory of gases relates molecular motion to energy in gaseous substances. It does not work as well in relation to liquids and solids; nonetheless, it is safe to say that—generally speaking—a gas has more energy than a liquid, and a liquid more energy than a solid. In a solid, the molecules undergo very little relative motion: instead of bumping into each other, like gas molecules and (to a lesser extent) liquid molecules, solid molecules merely vibrate in place.

S C I E N C E O F E V E RY DAY T H I N G S

WILLIAM THOMSON, (BETTER

KNOWN AS

LORD KELVIN) KELVIN

ESTABLISHED WHAT IS NOW KNOWN AS THE SCALE.

Understanding Temperature As with heat, temperature requires a scientific definition quite different from its common meaning. Temperature may be defined as a measure of the average molecular translational energy in a system—that is, in any material body. Because it is an average, the mass or other characteristics of the body do not matter. A large quantity of one substance, because it has more molecules, possesses more thermal energy than a smaller quantity of that same substance. Since it has more thermal energy, it transfers more heat to any body or system with which it is in contact. Yet, assuming that the substance is exactly the same, the temperature, as a measure of average energy, will be the same as well. Temperature determines the direction of internal energy flow between two systems when heat is being transferred. This can be illustrated through an experience familiar to everyone: having one’s temperature taken with a thermometer. If one has a fever, one’s mouth will be warmer than the thermometer, and therefore heat will be transferred to the thermometer from the mouth until the two objects have the same temperature.

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Temperature

A

THERMOMETER WORKS BY MEASURING THE LEVEL OF

THERMAL

EXPANSION

EXPERIENCED

WITHIN THE THERMOMETER.

MERCURY

BY

A

MATERIAL

HAS BEEN A COM-

MON THERMOMETER MATERIAL SINCE THE

1700S. (Pho-

tograph by Michael Prince/Corbis. Reproduced by permission.)

At that point of thermal equilibrium, a temperature reading can be taken from the thermometer. T E M P E R AT U R E AND H E AT F LO W. The principles of thermodynamics—

the study of the relationships between heat, work, and energy, show that heat always flows from an area of higher temperature to an area of lower temperature. The opposite simply cannot happen, because coldness, though it is very real in terms of sensory experience, is not an independent phenomenon. There is not, strictly speaking, such a thing as “cold”—only the absence of heat, which produces the sensation of coldness.

238

The boiling water warms the tub of cool water, and due to the high ratio of cool water to boiling water in the bathtub, the boiling water expends all its energy raising the temperature in the bathtub as a whole. The greater the ratio of very hot water to cool water, on the other hand, the warmer the bathtub will be in the end. But even after the bath water is heated, it will continue to lose heat, assuming the air in the room is not warmer than the water in the tub. If the water in the tub is warmer than the air, it immediately begins transferring thermal energy to the lowtemperature air until their temperatures are equalized. As for the coffee and the ice cube, what happens is quite different from, indeed, opposite to, the common understanding of the process. In other words, the ice does not “cool down” the coffee: the coffee warms up the ice and presumably melts it. Once again, however, it expends at least some of its thermal energy in doing so, and as a result, the coffee becomes cooler than it was. If the coffee is placed inside a freezer, there is a large temperature difference between it and the surrounding environment—so much so that if it is left for hours, the once-hot coffee will freeze. But again, the freezer does not cool down the coffee; the molecules in the coffee respond to the temperature difference by working to warm up the freezer. In this case, they have to “work overtime,” and since the freezer has a constant supply of electrical energy, the heated molecules of the coffee continue to expend themselves in a futile effort to warm the freezer. Eventually, the coffee loses so much energy that it is frozen solid; meanwhile, the heat from the coffee has been transferred outside the freezer to the atmosphere in the surrounding room. T H E R M A L E X PA N S I O N A N D E Q U I L I B R I U M . Temperature is related to

One might pour a kettle of boiling water into a cold bathtub to heat it up; or put an ice cube in a hot cup of coffee “to cool it down.” These seem like two very different events, but from the standpoint of thermodynamics, they are exactly the same. In both cases, a body of high temperature is placed in contact with a body of low temperature, and in both cases, heat passes from the high-temperature body to the low-temperature one.

the concept of thermal equilibrium, and has an effect on thermal expansion. As discussed below, as well as within the context of thermal expansion, a thermometer provides a gauge of temperature by measuring the level of thermal expansion experienced by a material (for example, mercury) within the thermometer. In the examples used earlier—the thermometer in the mouth, the hot water in the cool bathtub, and the ice cube in the cup of coffee— the systems in question eventually reach thermal equilibrium. This is rather like averaging their temperatures, though, in fact, the equation

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involved is more complicated than a simple arithmetic average. In the case of an ordinary mercury thermometer, the need to achieve thermal equilibrium explains why one cannot get an instantaneous temperature reading: first, the mouth transfers heat to the thermometer, and once both mouth and thermometer reach the same temperature, they are in thermal equilibrium. At that point, it is possible to gauge the temperature of the mouth by reading the thermometer.

REAL-LIFE A P P L I C AT I O N S Development of the Thermometer A thermometer can be defined scientifically as a device that gauges temperature by measuring a temperature-dependent property, such as the expansion of a liquid in a sealed tube. As with many aspects of scientific or technological knowledge, the idea of the thermometer appeared in ancient times, but was never developed. Again, like so many other intellectual phenomena, it lay dormant during the medieval period, only to be resurrected at the beginning of the modern era. The Greco-Roman physician Galen (c. 129216) was among the first thinkers to envision a scale for measuring temperature. Of course, what he conceived of as “temperature” was closer to the everyday meaning of that term, not its more precise scientific definition: the ideas of molecular motion, heat, and temperature discussed in this essay emerged only in the period beginning about 1750. In any case, Galen proposed that equal amounts of boiling water and ice be combined to establish a “neutral” temperature, with four units of warmth above it and four degrees of cold below. T H E T H E R M O S C O P E . The great physicist Galileo Galilei (1564-1642) is sometimes credited with creating the first practical temperature measuring device, called a thermoscope. Certainly Galileo—whether or not he was the first—did build a thermoscope, which consisted of a long glass tube planted in a container of liquid. Prior to inserting the tube into the liquid—which was usually colored water, though Galileo’s thermoscope used wine—as much air as

S C I E N C E O F E V E RY DAY T H I N G S

possible was removed from the tube. This created a vacuum, and as a result of pressure differences between the liquid and the interior of the thermoscope tube, some of the liquid went into the tube.

Temperature

But the liquid was not the thermometric medium—that is, the substance whose temperature-dependent property changes the thermoscope measured. (Mercury, for instance, is the thermometric medium in most thermometers today.) Instead, the air was the medium whose changes the thermoscope measured: when it was warm, the air expanded, pushing down on the liquid; and when the air cooled, it contracted, allowing the liquid to rise. It is interesting to note the similarity in design between the thermoscope and the barometer, a device for measuring atmospheric pressure invented by Italian physicist Evangelista Torricelli (1608-1647) around the same time. Neither were sealed, but by the mid-seventeenth century, scientists had begun using sealed tubes containing liquid instead of air. These were the first true thermometers. E A R LY T H E R M O M E T E R S . Ferdinand II, Grand Duke of Tuscany (1610-1670), is credited with developing the first thermometer in 1641. Ferdinand’s thermometer used alcohol sealed in glass, which was marked with a temperature scale containing 50 units. It did not, however, designate a value for zero.

English physicist Robert Hooke (1635-1703) created a thermometer using alcohol dyed red. Hooke’s scale was divided into units equal to about 1/500 of the volume of the thermometric medium, and for the zero point, he chose the temperature at which water freezes. Thus, Hooke established a standard still used today; likewise, his thermometer itself set a standard. Built in 1664, it remained in use by the Royal Society— the foremost organization for the advancement of science in England during the early modern period—until 1709. Olaus Roemer (1644-1710), a Danish astronomer, introduced another important standard. In 1702, he built a thermometer based not on one but two fixed points, which he designated as the temperature of snow or crushed ice, and the boiling point of water. As with Hooke’s use of the freezing point, Roemer’s idea of the freezing and boiling points of water as the two parameters

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for temperature measurements has remained in use ever since.

Temperature Scales FA H R E N H E I T. Not only did he devel-

op the Fahrenheit scale, oldest of the temperature scales still used in Western nations today, but German physicist Daniel Fahrenheit (1686-1736) also built the first thermometer to contain mercury as a thermometric medium. Alcohol has a low boiling point, whereas mercury remains fluid at a wide range of temperatures. In addition, it expands and contracts at a very constant rate, and tends not to stick to glass. Furthermore, its silvery color makes a mercury thermometer easy to read. Fahrenheit also conceived the idea of using “degrees” to measure temperature in his thermometer, which he introduced in 1714. It is no mistake that the same word refers to portions of a circle, or that exactly 180 degrees—half the number in a circle—separate the freezing and boiling points for water on Fahrenheit’s thermometer. Ancient astronomers attempting to denote movement in the skies used a circle with 360 degrees as a close approximation of the ratio between days and years. The number 360 is also useful for computations, because it has a large quantity of divisors, as does 180—a total of 16 whole-number divisors other than 1 and itself. Though it might seem obvious that 0 should denote the freezing point and 180 the boiling point on Fahrenheit’s scale, such an idea was far from obvious in the early eighteenth century. Fahrenheit considered the idea not only of a 0to-180 scale, but also of a 180-to-360 scale. In the end, he chose neither—or rather, he chose not to equate the freezing point of water with zero on his scale. For zero, he chose the coldest possible temperature he could create in his laboratory, using what he described as “a mixture of sal ammoniac or sea salt, ice, and water.” Salt lowers the melting point of ice (which is why it is used in the northern United States to melt snow and ice from the streets on cold winter days), and, thus, the mixture of salt and ice produced an extremely cold liquid water whose temperature he equated to zero.

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French naturalist and physicist named Rene Antoine Ferchault de Reaumur (1683-1757) presented a scale for which 0° represented the freezing point of water and 80° the boiling point. Although the Reaumur scale never caught on to the same extent as Fahrenheit’s, it did include one valuable addition: the specification that temperature values be determined at standard sea-level atmospheric pressure. C E L S I U S . With its 32-degree freezing point and its 212-degree boiling point, the Fahrenheit system is rather ungainly, lacking the neat orderliness of a decimal or base-10 scale. The latter quality became particularly important when, 10 years after the French Revolution of 1789, France adopted the metric system for measuring length, mass, and other physical phenomena. The metric system eventually spread to virtually the entire world, with the exception of English-speaking countries, where the more cumbersome British system still prevails. But even in the United States and Great Britain, scientists use the metric system. The metric temperature measure is the Celsius scale, created in 1742 by Swedish astronomer Anders Celsius (17011744).

Like Fahrenheit, Celsius chose the freezing and boiling points of water as his two reference points, but he determined to set them 100, rather than 180, degrees apart. Interestingly, he planned to equate 0° with the boiling point, and 100° with the freezing point—proving that even the most apparently obvious aspects of a temperature scale were once open to question. Only in 1750 did fellow Swedish physicist Martin Strömer change the orientation of the Celsius scale. Celsius’s scale was based not simply on the boiling and freezing points of water, but, specifically, those points at normal sea-level atmospheric pressure. The latter, itself a unit of measure known as an atmosphere (atm), is equal to 14.7 lb/in2, or 101,325 pascals in the metric system. A Celsius degree is equal to 1/100 of the difference between the freezing and boiling temperatures of water at 1 atm.

With Fahrenheit’s scale, the ordinary freezing point of water was established at 32°, and the boiling point exactly 180° above it, at 212°. Just a few years after he introduced his scale, in 1730, a

The Celsius scale is sometimes called the centigrade scale, because it is divided into 100 degrees, cent being a Latin root meaning “hundred.” By international convention, its values were refined in 1948, when the scale was redefined in terms of temperature change for an ideal gas, as well as the triple point of water. (Triple

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point is the temperature and pressure at which a substance is at once a solid, liquid, and vapor.) As a result of these refinements, the boiling point of water on the Celsius scale is actually 99.975°. This represents a difference equal to about 1 part in 4,000—hardly significant in daily life, though a significant change from the standpoint of the precise measurements made in scientific laboratories. K E LV I N . In about 1787, French physicist

and chemist J. A. C. Charles (1746-1823) made an interesting discovery: that at 0°C, the volume of gas at constant pressure drops by 1/273 for every Celsius degree drop in temperature. This seemed to suggest that the gas would simply disappear if cooled to -273°C, which, of course, made no sense. In any case, the gas would most likely become first a liquid, and then a solid, long before it reached that temperature. The man who solved the quandary raised by Charles’s discovery was born a year after Charles—who also formulated Charles’s law— died. He was William Thomson, Lord Kelvin (1824-1907), and in 1848, he put forward the suggestion that it was molecular translational energy, and not volume, that would become zero at -273°C. He went on to establish what came to be known as the Kelvin scale. Sometimes known as the absolute temperature scale, the Kelvin scale is based not on the freezing point of water, but on absolute zero— the temperature at which molecular motion comes to a virtual stop. This is -273.15°C (-459.67°F), which in the Kelvin scale is designated as 0K. (Kelvin measures do not use the term or symbol for “degree.”) Though scientists normally use metric or SI measures, they prefer the Kelvin scale to Celsius, because the absolute temperature scale is directly related to average molecular translational energy. Thus, if the Kelvin temperature of an object is doubled, this means that its average molecular translational energy has doubled as well. The same cannot be said if the temperature were doubled from, say, 10°C to 20°C, or from 40°C to 80°F, since neither the Celsius nor the Fahrenheit scale is based on absolute zero. C O N V E R S I O N S . The Kelvin scale is, however, closely related to the Celsius scale, in that a difference of 1 degree measures the same amount of temperature in both. Therefore, Celsius temperatures can be converted to Kelvins by

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adding 273.15. There is also an absolute temperature scale that uses Fahrenheit degrees. This is the Rankine scale, created by Scottish engineer William Rankine (1820-1872), but it is seldom used today: scientists and others who desire absolute temperature measures prefer the precision and simplicity of the Celsius-based Kelvin scale.

Temperature

Conversion between Celsius and Fahrenheit figures is a bit more challenging. To convert a temperature from Celsius to Fahrenheit, multiply by 9/5 and add 32. It is important to perform the steps in that order, because reversing them will produce a wrong answer. Thus, 100°C multiplied by 9/5 or 1.8 equals 180, which, when added to 32 equals 212°F. Obviously, this is correct, since 100°C and 212°F each represent the boiling point of water. But, if one adds 32 to 100°, then multiplies it by 9/5, the result is 237.6°F—an incorrect answer. For converting Fahrenheit temperatures to Celsius, there are also two steps, involving multiplication and subtraction, but the order is reversed. Here, the subtraction step is performed before the multiplication step: thus, 32 is subtracted from the Fahrenheit temperature, then the result is multiplied by 5/9. Beginning with 212°F, if 32 is subtracted, this equals 180. Multiplied by 5/9, the result is 100°C—the correct answer. One reason the conversion formulae use fractions instead of decimal fractions (what most people simply call “decimals”) is that 5/9 is a repeating decimal fraction (0.55555....) Furthermore, the symmetry of 5/9 and 9/5 makes memorization easy. One way to remember the formula is that Fahrenheit is multiplied by a fraction— since 5/9 is a real fraction, whereas 9/5 is actually a whole number plus a fraction.

Thermometers As discussed earlier, with regard to the early history of the thermometer, it is important that the glass tube be kept sealed; otherwise, atmospheric pressure contributes to inaccurate readings, because it influences the movement of the thermometric medium. Also important is the choice of the thermometric medium itself. Water quickly proved unreliable, due to its unusual properties: it does not expand uniformly with a rise in temperature, or contract uniformly with a lowered temperature. Rather, it

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Temperature

KEY TERMS The temperature,

The Fahrenheit scale establishes the freez-

defined as 0K on the Kelvin scale, at which

ing and boiling points of water at 32° and

the motion of molecules in a solid virtual-

212° respectively. To convert a temperature

ly ceases.

from the Fahrenheit to the Celsius scale,

ABSOLUTE ZERO:

A scale of tempera-

subtract 32 and multiply by 5/9. Most Eng-

ture, sometimes known as the centigrade

lish-speaking countries use the Fahrenheit

scale, created in 1742 by Swedish

scale.

CELSIUS SCALE:

astronomer Anders Celsius (1701-1744). The Celsius scale establishes the freezing

HEAT:

Internal thermal energy that

flows from one body of matter to another.

and boiling points of water at 0° and 100°, respectively. To convert a temperature from the Celsius to the Fahrenheit scale, multiply by 9/5 and add 32. The Celsius scale is

SCALE:

Established

by

William Thomson, Lord Kelvin (18241907), the Kelvin scale measures tempera-

part of the metric system used by most

ture in relation to absolute zero, or 0K.

non-English speaking countries today.

(Units in the Kelvin system, known as

Though the worldwide scientific commu-

Kelvins, do not include the word or symbol

nity uses the metric or SI system for most

for degree.) The Kelvin and Celsius scales

measurements, scientists prefer the related

are directly related; hence, Celsius temper-

Kelvin scale.

atures can be converted to Kelvins by

FAHRENHEIT SCALE:

The oldest of

the temperature scales still used in Western

adding 273.15. The Kelvin scale is used almost exclusively by scientists. The energy that

nations today, created in 1714 by German

KINETIC ENERGY:

physicist Daniel Fahrenheit (1686-1736).

an object possesses by virtue of its motion.

reaches its maximum density at 39.2°F (4°C), and is less dense both above and below that temperature. Therefore, alcohol, which responds in a much more uniform fashion to changes in temperature, took its place. MERCURY

THERMOMETERS.

Alcohol is still used in thermometers today, but the preferred thermometric medium is mercury. As noted earlier, its advantages include a much higher boiling point, a tendency not to stick to glass, and a silvery color that makes its levels easy to gauge visually. Like alcohol, mercury expands at a uniform rate with an increase in temperature: hence, the higher the temperature, the higher the mercury stands in the thermometer.

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KELVIN

for a calibrated scale, usually in such a way as to allow easy conversions between Fahrenheit and Celsius. A thermometer is calibrated by measuring the difference in height between mercury at the freezing point of water, and mercury at the boiling point of water. The interval between these two points is then divided into equal increments—180, as we have seen, for the Fahrenheit scale, and 100 for the Celsius scale. ELECTRIC

THERMOMETERS.

In a typical mercury thermometer, mercury is placed in a long, narrow sealed tube called a capillary. The capillary is inscribed with figures

Faster temperature measures can be obtained by thermometers using electricity. All matter displays a certain resistance to electrical current, a resistance that changes with temperature. Therefore, a resistance thermometer uses a fine wire wrapped around an insulator, and when a change in temperature occurs, the resistance in the wire changes as well. This makes possible much quick-

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Temperature

KEY TERMS MOLECULAR TRANSLATIONAL EN-

CONTINUED

THERMAL EQUILIBRIUM:

The state

The kinetic energy in a system

that exists when two systems have the same

produced by the movement of molecules

temperature. As a result, there is no

in relation to one another.

exchange of heat between them.

ERGY:

SYSTEM:

In physics, the term “system”

THERMODYNAMICS:

The study of

usually refers to any set of physical interac-

the relationships between heat, work, and

tions, or any material body, isolated from

energy.

the rest of the universe. Anything outside of the system, including all factors and forces irrelevant to a discussion of that system, is known as the environment.

THERMOMETRIC MEDIUM:

A sub-

stance whose properties change with temperature. A mercury or alcohol thermometer measures such changes.

TEMPERATURE:

A measure of the

average kinetic energy—or molecular translational energy in a system. Differences in temperature determine the direction of internal energy flow between two systems when heat is being transferred. THERMAL ENERGY:

Heat energy, a

THERMOMETER:

A device that gauges

temperature by measuring a temperaturedependent property, such as the expansion of a liquid in a sealed tube, or resistance to electric current. TRIPLE POINT:

The temperature and

form of kinetic energy produced by the

pressure at which a substance is at once a

movement of atomic or molecular parti-

solid, liquid, and vapor.

cles. The greater the movement of these

VACUUM:

particles, the greater the thermal energy.

matter, including air.

er temperature readings than those offered by a thermometer containing a traditional thermometric medium. Resistance thermometers are highly reliable, but expensive, and are used primarily for very precise measurements. More practical for everyday use is a thermistor, which also uses the principle of electric resistance, but is much simpler and less expensive. Thermistors are used for providing measurements of the internal temperature of food, for instance, and for measuring human body temperature.

Space entirely devoid of

stant temperature, and a measurement junction. The measurement junction is applied to the item whose temperature is to be measured, and any temperature difference between it and the reference junction registers as a voltage change, which is measured with a meter connected to the system. OTHER TYPES OF THERMOMET E R. A pyrometer also uses electromagnetic

Another electric temperature-measurement device is a thermocouple. When wires of two different materials are connected, this creates a small level of voltage that varies as a function of temperature. A typical thermocouple uses two junctions: a reference junction, kept at some con-

properties, but of a very different kind. Rather than responding to changes in current or voltage, the pyrometer is a gauge that responds to visible and infrared radiation. Temperature and color are closely related: thus, it is no accident that greens, blues, and purples, at one end of the visible light spectrum, are associated with coolness, while reds, oranges, and yellows at the other end are associated with heat. As with the thermocou-

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ple, a pyrometer has both a reference element and a measurement element, which compares light readings between the reference filament and the object whose temperature is being measured. Still other thermometers, such as those in an oven that tell the user its internal temperature, are based on the expansion of metals with heat. In fact, there are a wide variety of thermometers, each suited to a specific purpose. A pyrometer, for instance, is good for measuring the temperature of an object that the thermometer itself does not touch.

WHERE TO LEARN MORE About Temperature (Web site). (April 18, 2001).

244

Gardner, Robert. Science Projects About Methods of Measuring. Berkeley Heights, N.J.: Enslow Publishers, 2000. Maestro, Betsy and Giulio Maestro. Temperature and You. New York: Macmillan/McGraw-Hill School Publishing, 1990. Megaconverter (Web site). (April 18, 2001). NPL: National Physics Laboratory: Thermal Stuff: Beginners’ Guides (Web site). (April 18, 2001). Royston, Angela. Hot and Cold. Chicago: Heinemann Library, 2001. Santrey, Laurence. Heat. Illustrated by Lloyd Birmingham. Mahwah, N.J.: Troll Associates, 1985. Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996.

About Temperature Sensors (Web site). (April 18, 2001).

Walpole, Brenda. Temperature. Illustrated by Chris Fairclough and Dennis Tinkler. Milwaukee, WI: Gareth Stevens Publishing, 1995.

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T H E R M A L E X PA N S I O N Thermal Expansion

CONCEPT Most materials are subject to thermal expansion: a tendency to expand when heated, and to contract when cooled. For this reason, bridges are built with metal expansion joints, so that they can expand and contract without causing faults in the overall structure of the bridge. Other machines and structures likewise have built-in protection against the hazards of thermal expansion. But thermal expansion can also be advantageous, making possible the workings of thermometers and thermostats.

HOW IT WORKS Molecular Translational Energy In scientific terms, heat is internal energy that flows from a system of relatively high temperature to one at a relatively low temperature. The internal energy itself, identified as thermal energy, is what people commonly mean when they say “heat.” A form of kinetic energy due to the movement of molecules, thermal energy is sometimes called molecular translational energy. Temperature is defined as a measure of the average molecular translational energy in a system, and the greater the temperature change for most materials, as we shall see, the greater the amount of thermal expansion. Thus, all these aspects of “heat”—heat itself (in the scientific sense), as well as thermal energy, temperature, and thermal expansion—are ultimately affected by the motion of molecules in relation to one another.

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MOLECULAR MOTION AND N E W T O N I A N P H Y S I CS . In general, the

kinetic energy created by molecular motion can be understood within the framework of classical physics—that is, the paradigm associated with Sir Isaac Newton (1642-1727) and his laws of motion. Newton was the first to understand the physical force known as gravity, and he explained the behavior of objects within the context of gravitational force. Among the concepts essential to an understanding of Newtonian physics are the mass of an object, its rate of motion (whether in terms of velocity or acceleration), and the distance between objects. These, in turn, are all components central to an understanding of how molecules in relative motion generate thermal energy. The greater the momentum of an object— that is, the product of its mass multiplied by its rate of velocity—the greater the impact it has on another object with which it collides. The greater, also, is its kinetic energy, which is equal to one-half its mass multiplied by the square of its velocity. The mass of a molecule, of course, is very small, yet if all the molecules within an object are in relative motion—many of them colliding and, thus, transferring kinetic energy— this is bound to lead to a relatively large amount of thermal energy on the part of the larger object. M O L E C U LA R AT T RAC T I O N A N D P H AS E S O F M AT T E R. Yet, precisely

because molecular mass is so small, gravitational force alone cannot explain the attraction between molecules. That attraction instead must be understood in terms of a second type of force—electromagnetism—discovered by Scottish physicist James Clerk Maxwell (1831-1879). The details of electromagnetic force are not

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Thermal Expansion

BECAUSE

STEEL HAS A RELATIVELY HIGH COEFFICIENT OF THERMAL EXPANSION, STANDARD RAILROAD TRACKS ARE

CONSTRUCTED SO THAT THEY CAN SAFELY EXPAND ON A HOT DAY WITHOUT DERAILING THE TRAINS TRAVELING OVER THEM. (Milepost 92 1/2/Corbis. Reproduced by permission.)

important here; it is necessary only to know that all molecules possess some component of electrical charge. Since like charges repel and opposite charges attract, there is constant electromagnetic interaction between molecules, and this produces differing degrees of attraction. The greater the relative motion between molecules, generally speaking, the less their attraction toward one another. Indeed, these two aspects of a material—relative attraction and motion at the molecular level—determine whether that material can be classified as a solid, liquid, or gas. When molecules move slowly in relation to one another, they exert a strong attraction, and the material of which they are a

246

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part is usually classified as a solid. Molecules of liquid, on the other hand, move at moderate speeds, and therefore exert a moderate attraction. When molecules move at high speeds, they exert little or no attraction, and the material is known as a gas.

Predicting Thermal Expansion COEFFICIENT OF LINEAR EXPA N S I O N . A coefficient is a number that

serves as a measure for some characteristic or property. It may also be a factor against which other values are multiplied to provide a desired result. For any type of material, it is possible to

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Thermal Expansion

A

MAN ICE FISHING IN

MONTANA. BECAUSE

OF THE UNIQUE THERMAL EXPANSION PROPERTIES OF WATER, ICE FORMS

AT THE TOP OF A LAKE RATHER THAN THE BOTTOM, THUS ALLOWING MARINE LIFE TO CONTINUE LIVING BELOW ITS SURFACE DURING THE WINTER. (Corbis. Reproduced by permission.)

calculate the degree to which that material will expand or contract when exposed to changes in temperature. This is known, in general terms, as its coefficient of expansion, though, in fact, there are two varieties of expansion coefficient. The coefficient of linear expansion is a constant that governs the degree to which the length of a solid will change as a result of an alteration in temperature For any given substance, the coefficient of linear expansion is typically a number expressed in terms of 10-5/°C. In other words, the value of a particular solid’s linear expansion coefficient is multiplied by 0.00001 per °C. (The °C in the denominator, shown in the equation below, simply “drops out” when the coefficient of

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linear expansion is multiplied by the change in temperature.) For quartz, the coefficient of linear expansion is 0.05. By contrast, iron, with a coefficient of 1.2, is 24 times more likely to expand or contract as a result of changes in temperature. (Steel has the same value as iron.) The coefficient for aluminum is 2.4, twice that of iron or steel. This means that an equal temperature change will produce twice as much change in the length of a bar of aluminum as for a bar of iron. Lead is among the most expansive solid materials, with a coefficient equal to 3.0. C A L C U L AT I N G LINEAR EXPA N S I O N . The linear expansion of a given

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solid can be calculated according to the formula δL = aLO∆T. The Greek letter delta (d) means “a change in”; hence, the first figure represents change in length, while the last figure in the equation stands for change in temperature. The letter a is the coefficient of linear expansion, and LO is the original length. Suppose a bar of lead 5 meters long experiences a temperature change of 10°C; what will its change in length be? To answer this, a (3.0 • 10-5/°C) must be multiplied by LO (5 m) and δT (10°C). The answer should be 150 & 10-5 m, or 1.5 mm. Note that this is simply a change in length related to a change in temperature: if the temperature is raised, the length will increase, and if the temperature is lowered by 10°C, the length will decrease by 1.5 mm. V O LU M E E X PA N S I O N . Obviously,

linear equations can only be applied to solids. Liquids and gases, classified together as fluids, conform to the shape of their container; hence, the “length” of any given fluid sample is the same as that of the solid that contains it. Fluids are, however, subject to volume expansion—that is, a change in volume as a result of a change in temperature. To calculate change in volume, the formula is very much the same as for change in length; only a few particulars are different. In the formula δV = bVOδT, the last term, again, means change in temperature, while δV means change in volume and VO is the original volume. The letter b refers to the coefficient of volume expansion. The latter is expressed in terms of 10-4/°C, or 0.0001 per °C. Glass has a very low coefficient of volume expansion, 0.2, and that of Pyrex glass is extremely low—only 0.09. For this reason, items made of Pyrex are ideally suited for cooking. Significantly higher is the coefficient of volume expansion for glycerin, an oily substance associated with soap, which expands proportionally to a factor of 5.1. Even higher is ethyl alcohol, with a volume expansion coefficient of 7.5.

REAL-LIFE A P P L I C AT I O N S

The behavior of gasoline pumped on a hot day provides an example of liquid thermal expansion in response to an increase in temperature. When it comes from its underground tank at the gas station, the gasoline is relatively cool, but it will warm when sitting in the tank of an already warm car. If the car’s tank is filled and the vehicle left to sit in the sun—in other words, if the car is not driven after the tank is filled—the gasoline might very well expand in volume faster than the fuel tank, overflowing onto the pavement. E N G I N E C O O LA N T. Another example of thermal expansion on the part of a liquid can be found inside the car’s radiator. If the radiator is “topped off ” with coolant on a cold day, an increase in temperature could very well cause the coolant to expand until it overflows. In the past, this produced a problem for car owners, because car engines released the excess volume of coolant onto the ground, requiring periodic replacement of the fluid.

Later-model cars, however, have an overflow container to collect fluid released as a result of volume expansion. As the engine cools down again, the container returns the excess fluid to the radiator, thus, “recycling” it. This means that newer cars are much less prone to overheating as older cars. Combined with improvements in radiator fluid mixtures, which act as antifreeze in cold weather and coolant in hot, the “recycling” process has led to a significant decrease in breakdowns related to thermal expansion. WAT E R. One good reason not to use pure water in one’s radiator is that water has a far higher coefficient of volume expansion than a typical engine coolant. This can be particularly hazardous in cold weather, because frozen water in a radiator could expand enough to crack the engine block.

Most liquids follow a fairly predictable pattern of gradual volume increase, as a response to an

In general, water—whose volume expansion coefficient in the liquid state is 2.1, and 0.5 in the solid state—exhibits a number of interesting characteristics where thermal expansion is concerned. If water is reduced from its boiling point—212°F (100°C) to 39.2°F (4°C) it will

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increase in temperature, and volume decrease, in response to a decrease in temperature. Indeed, the coefficient of volume expansion for a liquid generally tends to be higher than for a solid, and—with one notable exception discussed below— a liquid will contract when frozen.

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steadily contract, like any other substance responding to a drop in temperature. Normally, however, a substance continues to become denser as it turns from liquid to solid; but this does not occur with water. At 32.9°F, water reaches it maximum density, meaning that its volume, for a given unit of mass, is at a minimum. Below that temperature, it “should” (if it were like most types of matter) continue to decrease in volume per unit of mass, but, in fact, it steadily begins to expand. Thus, it is less dense, with a greater volume per unit of mass, when it reaches the freezing point. It is for this reason that when pipes freeze in winter, they often burst—explaining why a radiator filled with water could be a serious problem in very cold weather. In addition, this unusual behavior with regard to thermal expansion and contraction explains why ice floats: solid water is less dense than the liquid water below it. As a result, frozen water stays at the top of a lake in winter; since ice is a poor conductor of heat, energy cannot escape from the water below it in sufficient amounts to freeze the rest of the lake water. Thus, the water below the ice stays liquid, preserving plant and animal life.

Gases T H E GAS LAW S . As discussed, liq-

uids expand by larger factors than solids do. Given the increasing amount of molecular kinetic energy for a liquid as compared to a solid, and for a gas as compared to a liquid, it should not be surprising, then, to learn that gases respond to changes in temperature with a volume change even greater than that of liquids. Of course, where a gas is concerned, “volume” is more difficult to measure, because a gas simply expands to fill its container. In order for the term to have any meaning, pressure and temperature must be specified as well. A number of the gas laws describe the three parameters for gases: volume, temperature, and pressure. Boyle’s law, for example, holds that in conditions of constant temperature, an inverse relationship exists between the volume and pressure of a gas: the greater the pressure, the less the volume, and vice versa. Even more relevant to the subject of thermal expansion is Charles’s law.

between volume and temperature. As a gas heats up, its volume increases, and when it cools down, its volume reduces accordingly. Thus, if an air mattress is filled in an air-conditioned room, and the mattress is then taken to the beach on a hot day, the air inside will expand. Depending on how much its volume increases, the expansion of the hot air could cause the mattress to “pop.”

Thermal Expansion

VOLUME GAS THERMOMET E R S . Whereas liquids and solids vary signif-

icantly with regard to their expansion coefficients, most gases follow more or less the same pattern of expansion in response to increases in temperature. The predictable behavior of gases in these situations led to the development of the constant gas thermometer, a highly reliable instrument against which other thermometers— including those containing mercury (see below)—are often gauged. In a volume gas thermometer, an empty container is attached to a glass tube containing mercury. As gas is released into the empty container, this causes the column of mercury to move upward. The difference between the former position of the mercury and its position after the introduction of the gas shows the difference between normal atmospheric pressure and the pressure of the gas in the container. It is, then, possible to use the changes in volume on the part of the gas as a measure of temperature. The response of most gases, under conditions of low pressure, to changes in temperature is so uniform that volume gas thermometers are often used to calibrate other types of thermometers.

Solids Many solids are made up of crystals, regular shapes composed of molecules joined to one another as though on springs. A spring that is pulled back, just before it is released, is an example of potential energy, or the energy that an object possesses by virtue of its position. For a crystalline solid at room temperature, potential energy and spacing between molecules are relatively low. But as temperature increases and the solid expands, the space between molecules increases—as does the potential energy in the solid.

Charles’s law states that when pressure is kept constant, there is a direct relationship

In fact, the responses of solids to changes in temperature tend to be more dramatic, at least when they are seen in daily life, than are the behaviors of liquids or gases under conditions of

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thermal expansion. Of course, solids actually respond less to changes in temperature than fluids do; but since they are solids, people expect their contours to be immovable. Thus, when the volume of a solid changes as a result of an increase in thermal energy, the outcome is more noteworthy. JA R L I D S A N D P O W E R L I N E S .

An everyday example of thermal expansion can be seen in the kitchen. Almost everyone has had the experience of trying unsuccessfully to budge a tight metal lid on a glass container, and after running hot water over the lid, finding that it gives way and opens at last. The reason for this is that the high-temperature water causes the metal lid to expand. On the other hand, glass—as noted earlier—has a low coefficient of expansion. Otherwise, it would expand with the lid, which would defeat the purpose of running hot water over it. If glass jars had a high coefficient of expansion, they would deform when exposed to relatively low levels of heat. Another example of thermal expansion in a solid is the sagging of electrical power lines on a hot day. This happens because heat causes them to expand, and, thus, there is a greater length of power line extending from pole to pole than under lower temperature conditions. It is highly unlikely, of course, that the heat of summer could be so great as to pose a danger of power lines breaking; on the other hand, heat can create a serious threat with regard to larger structures. E X PA N S I O N J O I N T S . Most large bridges include expansion joints, which look rather like two metal combs facing one another, their teeth interlocking. When heat causes the bridge to expand during the sunlight hours of a hot day, the two sides of the expansion joint move toward one another; then, as the bridge cools down after dark, they begin gradually to retract. Thus the bridge has a built-in safety zone; otherwise, it would have no room for expansion or contraction in response to temperature changes. As for the use of the comb shape, this staggers the gap between the two sides of the expansion joint, thus minimizing the bump motorists experience as they drive over it.

250

of steel. Steel, as noted earlier, expands by a factor of 12 parts in 1 million for every Celsius degree change in temperature, and while this may not seem like much, it can create a serious problem under conditions of high temperature. Most tracks are built from pieces of steel supported by wooden ties, and laid with a gap between the ends. This gap provides a buffer for thermal expansion, but there is another matter to consider: the tracks are bolted to the wooden ties, and if the steel expands too much, it could pull out these bolts. Hence, instead of being placed in a hole the same size as the bolt, the bolts are fitted in slots, so that there is room for the track to slide in place slowly when the temperature rises. Such an arrangement works agreeably for trains that run at ordinary speeds: their wheels merely make a noise as they pass over the gaps, which are rarely wider than 0.5 in (0.013 m). A high-speed train, however, cannot travel over irregular track; therefore, tracks for high-speed trains are laid under conditions of relatively high tension. Hydraulic equipment is used to pull sections of the track taut; then, once the track is secured in place along the cross ties, the tension is distributed down the length of the track.

Thermometers and Thermostats MERCURY IN THERMOMETERS.

A thermometer gauges temperature by measuring a temperature-dependent property. A thermostat, by contrast, is a device for adjusting the temperature of a heating or cooling system. Both use the principle of thermal expansion in their operation. As noted in the example of the metal lid and glass jar above, glass expands little with changes in temperature; therefore, it makes an ideal container for the mercury in a thermometer. As for mercury, it is an ideal thermometric medium—that is, a material used to gauge temperature—for several reasons. Among these is a high boiling point, and a highly predictable, uniform response to changes in temperature.

Expansion joints of a different design can also be found in highways, and on “highways” of rail. Thermal expansion is a particularly serious problem where railroad tracks are concerned, since the tracks on which the trains run are made

In a typical mercury thermometer, mercury is placed in a long, narrow sealed tube called a capillary. Because it expands at a much faster rate than the glass capillary, mercury rises and falls with the temperature. A thermometer is calibrated by measuring the difference in height between mercury at the freezing point of water, and mercury at the boiling point of water. The interval

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Thermal Expansion

KEY TERMS COEFFICIENT:

A number that serves

as a measure for some characteristic or property. A coefficient may also be a factor

produced by the movement of molecules in relation to one another. The energy

POTENTIAL ENERGY:

against which other values are multiplied

that an object possesses by virtue of its

to provide a desired result.

position.

COEFFICIENT OF LINEAR EXPAN-

SYSTEM:

In physics, the term “system”

A figure, constant for any partic-

usually refers to any set of physical interac-

ular type of solid, used in calculating the

tions, or any material body, isolated from

amount by which the length of that solid

the rest of the universe. Anything outside

will change as a result of temperature

of the system, including all factors and

change. For any given substance, the coeffi-

forces irrelevant to a discussion of that sys-

cient of linear expansion is typically a

tem, is known as the environment.

SION:

number expressed in terms of 10-5/°C. TEMPERATURE: COEFFICIENT OF VOLUME EXPAN-

A measure of the

average kinetic energy—or molecular

A figure, constant for any partic-

translational energy in a system. Differ-

ular type of material, used in calculating

ences in temperature determine the direc-

the amount by which the volume of that

tion of internal energy flow between two

material will change as a result of tempera-

systems when heat is being transferred.

SION:

ture change. For any given substance, the coefficient of volume expansion is typically a number expressed in terms of 10-4/°C. HEAT:

Internal thermal energy that

flows from one body of matter to another. KINETIC ENERGY:

The energy that

an object possesses by virtue of its motion. MOLECULAR TRANSLATIONAL ENERGY:

The kinetic energy in a system

between these two points is then divided into equal increments in accordance with one of the well-known temperature scales.

THERMAL ENERGY:

Heat energy, a

form of kinetic energy produced by the movement of atomic or molecular particles. The greater the movement of these particles, the greater the thermal energy. THERMAL EXPANSION:

A property

in all types of matter that display a tendency to expand when heated, and to contract when cooled.

that is less responsive to temperature changes. As a result, the bimetallic strip will bend to one side.

tral component is a bimetallic strip, consisting of thin strips of two different metals placed back to back. One of these metals is of a kind that possesses a high coefficient of linear expansion, while the other metal has a low coefficient. A temperature increase will cause the side with a higher coefficient to expand more than the side

When the strip bends far enough, it will close an electrical circuit, and, thus, direct the air conditioner to go into action. By adjusting the thermostat, one varies the distance that the bimetallic strip must be bent in order to close the circuit. Once the air in the room reaches the desired temperature, the high-coefficient metal will begin to contract, and the bimetallic strip will straighten. This will cause an opening of the electrical circuit, disengaging the air conditioner.

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In cold weather, when the temperature-control system is geared toward heating rather than cooling, the bimetallic strip acts in much the same way—only this time, the high-coefficient metal contracts with cold, engaging the heater. Another type of thermostat uses the expansion of a vapor rather than a solid. In this case, heating of the vapor causes it to expand, pushing on a set of brass bellows and closing the circuit, thus, engaging the air conditioner.

Fleisher, Paul. Matter and Energy: Principles of Matter and Thermodynamics. Minneapolis, MN: Lerner Publications, 2002.

WHERE TO LEARN MORE

“Thermal Expansion Measurement” (Web site). (April 21, 2001).

Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991.

252

NPL: National Physics Laboratory: Thermal Stuff: Beginners’ Guides (Web site). (April 18, 2001). Royston, Angela. Hot and Cold. Chicago: Heinemann Library, 2001. Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996.

“Comparison of Materials: Coefficient of Thermal Expansion” (Web site). (April 21, 2001).

“Thermal Expansion of Solids and Liquids” (Web site). (April 21, 2001).

Encyclopedia of Thermodynamics (Web site). (April 12, 2001).

Walpole, Brenda. Temperature. Illustrated by Chris Fairclough and Dennis Tinkler. Milwaukee, WI: Gareth Stevens Publishing, 1995.

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S C I E N C E O F E V E RY DAY T H I N G S real-life Physics

W AV E M O T I O N A N D O S C I L L AT I O N WAVE MOTION O S C I L L AT I O N FREQUENCY RESONANCE INTERFERENCE DIFFRACTION DOPPLER EFFECT

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Wave Motion

W AV E M O T I O N

CONCEPT Wave motion is activity that carries energy from one place to another without actually moving any matter. Studies of wave motion are most commonly associated with sound or radio transmissions, and, indeed, these are among the most common forms of wave activity experienced in daily life. Then, of course, there are waves on the ocean or the waves produced by an object falling into a pool of still water—two very visual examples of a phenomenon that takes place everywhere in the world around us.

HOW IT WORKS Related Forms of Motion In wave motion, energy—the ability to perform work, or to exert force over distance—is transmitted from one place to another without actually moving any matter along the wave. In some types of waves, such as those on the ocean, it might seem as though matter itself has been displaced; that is, it appears that the water has actually moved from its original position. In fact, this is not the case: molecules of water in an ocean wave move up and down, but they do not actually travel with the wave itself. Only the energy is moved.

time that it takes an object orbiting another (as, for instance, a satellite going around Earth) to complete one cycle of orbit. With wave motion, a period is the amount of time required to complete one full cycle of the wave, from trough to crest and back to trough. H A R M O N I C M O T I O N . Harmonic motion is the repeated movement of a particle about a position of equilibrium, or balance. In harmonic motion—or, more specifically, simple harmonic motion—the object moves back and forth under the influence of a force directed toward the position of equilibrium, or the place where the object stops if it ceases to be in motion. A familiar example of harmonic motion, to anyone who has seen an old movie with a clichéd depiction of a hypnotist, is the back-and-forth movement of the hypnotist’s watch, as he tries to control the mind of his patient.

One variety of harmonic motion is vibration, which wave motion resembles in some respects. Both wave motion and vibration are periodic, involving the regular repetition of a certain form of movement. In both, there is a continual conversion and reconversion between potential energy (the energy of an object due to its position, as for instance with a sled at the top of a hill) and kinetic energy (the energy of an object due to its motion, as with the sled when sliding down the hill.) The principal difference between vibration and wave motion is that, in the first instance, the energy remains in place, whereas waves actually transport energy from one place to another.

A wave is an example of a larger class of regular, repeated, and/or back-and-forth types of motion. As with wave motion, these varieties of movement may or may not involve matter, but, in any case, the key component is not matter, but energy. Broadest among these is periodic motion, or motion that is repeated at regular intervals called periods. A period might be the amount of

O S C I L LAT I O N . Oscillation is a type of harmonic motion, typically periodic, in one or more dimensions. Suppose a spring is fixed in

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first spring is once again disturbed: the energy will pass through the rubber bands, from spring to spring, causing the entire row to oscillate. This is similar to what happens in the motion of a wave.

Wave Motion

Types and Properties of Waves There are some types of waves that do not follow regular, repeated patterns; these are discussed below, in the illustration concerning a string, in which a pulse is created and reflected. Of principal concern here, however, is the periodic wave, a series of wave motions, following one after the other in regular succession. Examples of periodic waves include waves on the ocean, sound waves, and electromagnetic waves. The last of these include visible light and radio, among others.

HEINRICH HERTZ. (Hulton-Deutsch Collection/Corbis. Reproduced by permission.)

place to a ceiling, such that it hangs downward. At this point, the spring is in a position of equilibrium. Now, consider what happens if the spring is grasped at a certain point and lifted, then let go. It will, of course, fall downward with the force of gravity until it comes to a stop—but it will not stop at the earlier position of equilibrium. Instead, it will continue downward to a point of maximum tension, where it possesses maximum potential energy as well. Then, it will spring upward again, and as it moves, its kinetic energy increases, while potential energy decreases. At the high point of this period of oscillation, the spring will not be as high as it was before it was originally released, but it will be higher than the position of equilibrium. Once it falls, the spring will again go lower than the position of equilibrium, but not as low as before—and so on. This is an example of oscillation. Now, imagine what happens if another spring is placed beside the first one, and they are connected by a rubber band. If just the first spring is disturbed, as before, the second spring will still move, because the energy created by the movement of the first spring will be transmitted to the second one via the rubber band. The same will happen if a row of springs, all side-by-side, are attached by multiple rubber bands, and the

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Electromagnetic waves involve only energy; on the other hand, a mechanical wave involves matter as well. Ocean waves are mechanical waves; so, too, are sound waves, as well as the waves produced by pulling a string. It is important to note, again, that the matter itself is not moved from place to place, though it may move in place without leaving its position. For example, water molecules in the crest of an ocean wave rotate in the same direction as the wave, while those in the trough of the wave rotate in a direction opposite to that of the wave, yet there is no net motion of the water: only energy is transmitted along the wave. F I V E P R O P E RT I E S O F WAV E S .

There are three notable interrelated characteristics of periodic waves. One of these is wave speed, symbolized by v and typically calculated in meters per second. Another is wavelength, represented as λ (the Greek letter lambda), which is the distance between a crest and the adjacent crest, or a trough and the adjacent trough. The third is frequency, abbreviated as f, which is the number of waves passing through a given point during the interval of 1 second. Frequency is measured in terms of cycles per second, or Hertz (Hz), named in honor of nineteenth-century German physicist Heinrich Rudolf Hertz (1857-1894). If a wave has a frequency of 100 Hz, this means that 100 waves are passing through a given point during the interval of 1 second. Higher frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per

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Wave Motion

TRANSVERSE

WAVES PRODUCED BY A WATER DROPLET PENETRATING THE SURFACE OF A BODY OF LIQUID. (Photograph

by Martin Dohrn/Science Photo Library, National Audubon Society Collection/Photo Researchers, Inc. Reproduced with permission.)

second) or megahertz (MHz; 106 or 1 million cycles per second.) Frequency is clearly related to wave speed, and there is also a relationship—though it is not so immediately grasped—between wavelength and speed. Over the interval of 1 second, a given number of waves pass a certain point (frequency), and each wave occupies a certain distance (wavelength). Multiplied by one another, these two properties equal the speed of the wave. This can be stated as a formula: v = fλ. Earlier, the term “period” was defined in terms of wave motion as the amount of time required to complete one full cycle of the wave. Period, symbolized by T, can be expressed in terms of frequency, and, thus, can also be related to the other two properties identified above. It is the inverse of frequency, meaning that T = 1/f. Furthermore, period is equal to the ratio of wavelength to wave speed; in other words, T = λ/v. A fifth property of waves—one not mathematically related to wavelength, wave speed, frequency, or period, is amplitude. Amplitude can be defined as the maximum displacement of oscillating particles from their normal position. For an ocean wave, amplitude is the distance

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from either the crest or the trough to the level that the ocean would maintain if it were perfectly still. WAV E S H A P E S . When most people think of waves, naturally, one of the first images that comes to mind is that of waves on the ocean. These are an example of a transverse wave, or one in which the vibration or motion is perpendicular to the direction the wave is moving. (Actually, ocean waves are simply perceived as transverse waves; in fact, as discussed below, their behavior is rather more complicated.) In a longitudinal wave, on the other hand, the movement of vibration is in the same direction as the wave itself.

Transverse waves are easier to visualize, particularly with regard to the aspects of wave motion—for example, frequency and amplitude—discussed above. Yet, longitudinal waves can be understood in terms of a common example. Sound waves, for instance, are longitudinal: thus, when a stereo is turned up to a high volume, the speakers vibrate in the same direction as the sound itself. A longitudinal wave may be understood as a series of fluctuations in density. If one were to take a coiled spring (such as the toy known as the “Slinky”) and release one end while holding the

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other, the motion of the springs would produce longitudinal waves. As these waves pass through the spring, they cause some portions of it to be compressed and others extended. The distance between each point of compression is the wavelength. Now, to return to the qualified statement made above: that ocean waves are an example of transverse waves. We perceive them as transverse waves, but, in fact, they are also longitudinal. In fact, all types of waves on the surface of a liquid are a combination of longitudinal and transverse, and are known as surface waves. Thus, if one drops a stone into a body of still water, waves radiate outward (longitudinal), but these waves also have a component that is perpendicular to the surface of the water, meaning that they are also transverse.

REAL-LIFE A P P L I C AT I O N S Pulses on a String There is another variety of wave, though it is defined in terms of behavior rather than the direction of disturbance. (In terms of direction, it is simply a variety of transverse wave.) This is a standing wave, produced by causing vibrations on a string or other piece of material whose ends are fixed in place. Standing waves are really a series of pulses that travel down the string and are reflected back to the point of the original disturbance. Suppose you hold a string in one hand, with the other end attached to a wall. If you give the string a shake, this causes a pulse—an isolated, non-periodic disturbance—to move down it. A pulse is a single wave, and the behavior of this lone wave helps us to understand what happens within the larger framework of wave motion. As with wave motion in general, the movement of the pulse involves both kinetic and potential energy. The tension of the string itself creates potential energy; then, as the movement of the pulse causes the string to oscillate upward and downward, this generates a certain amount of kinetic energy.

In accordance with the third law of motion, there should be an equal and opposite reaction once the pulse comes into contact with the wall. Assuming that you are holding the string tightly, this reaction will be manifested in the form of an inverted wave, or one that is upside-down in relation to the original pulse. In this case, the tension on the end attached to the support is equal and opposite to the tension exerted by your hand. As a result, the pulse comes back in the same shape as before, but inverted. If, on the other hand, you hold the other end of the string loosely; instead, once it reaches the wall, its kinetic energy will be converted into potential energy, which will cause the end of the string closest to the wall to move downward. This will result in sending back a pulse that is reversed in horizontal direction, but the same in vertical direction. In both cases, the energy in the string is reflected backward to its source—that is, to the place from which the pulse was originally produced by the action of your hand. If, however, you hold the string so that its level of tension is exactly between perfect rigidity and perfect looseness, then the pulse will not be reflected. In other words, there will be no reflected wave. T RA N S M I SS I O N A N D R E F L E C T I O N . If two strings are joined end-to-end,

and a pulse is produced at one end, the pulse would, of course, be transmitted to the second string. If, however, the second string has a greater mass per unit of length than the first one, the result would be two pulses: a transmitted pulse moving in the “right” direction, and a reflected, inverted pulse, moving toward the original source of energy. If, on the other hand, the first string has a greater mass per unit of length than the second one, the reflected pulse would be erect (right side up), not inverted.

The speed of the pulse is a function of the string and its properties, not of the way that the pulse

For simplicity’s sake, this illustration has been presented in terms of a string attached to a wall, but, in fact, transmission and reflection occur in a number of varieties of wave motion—

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was originally delivered. The tighter the string, and the less its mass per unit of length, the faster the pulse travels down it. The greater the mass per unit of length, however, the greater the inertia resisting the movement of the pulse. Furthermore, the more loosely you hold the string, the less it will respond to the movement of the pulse.

AND

REFLECTION.

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not just those involving pulses or standing waves. A striking example occurs when light hits an ordinary window. The majority of the light, of course, is transmitted through the window pane, but a portion is reflected. Thus, as one looks through the window, one also sees one’s reflection. Similarly, sound waves are reflected depending on the medium with which they are in contact. A canyon wall, for instance, will reflect a great deal of sound, and, thus, it is easy to produce an echo in such a situation. On the other hand, there are many instances in which the desire is to “absorb” sound by transmitting it to some other form of material. Thus, for example, the lobby of an upscale hotel will include a number of plants, as well as tapestries and various wall hangings. In addition to adding beauty, these provide a medium into which the sound of voices and other noises can be transmitted and, thus, absorbed.

Sound Waves P R O D U C T I O N . The experience of sound involves production, or the generation of sound waves; transmission, or the movement of those waves from their source; and reception, the principal example of which is hearing. Sound itself is discussed in detail elsewhere. Of primary concern here is the transmission, and to a lesser extent, the production of sound waves.

In terms of production, sound waves are, as noted, longitudinal waves: changes in pressure, or alternations between condensation and rarefaction. Vibration is integral to the generation of sound. When the diaphragm of a loudspeaker pushes outward, it forces nearby air molecules closer together, creating a high-pressure region all around the loudspeaker. The loudspeaker’s diaphragm is pushed backward in response, thus freeing up a volume of space for the air molecules. These, then, rush toward the diaphragm, creating a low-pressure region behind the highpressure one. As a result, the loudspeaker sends out alternating waves of high pressure (condensation) and low pressure (rarefaction).

the sound wave, a vibration that the ear’s inner mechanisms translate and pass on to the brain. The range of audibility for the human ear is from 20 Hz to 20 kHz. The lowest note of the eightyeight keys on a piano is 27 Hz and the highest 4.186 kHz. This places the middle and upper register of the piano well within the optimal range for audibility, which is between 3 and 4 kHz. Sound travels at a speed of about 1,088 ft (331 m) per second through air at sea level, and the range of sound audible to human ears includes wavelengths as large as 11 ft (3.3 m) and as small as 1.3 in (3.3 cm). Unlike light waves, which are very small, the wavelengths of audible sound are comparable to the sizes of ordinary objects. This creates an interesting contrast between the behaviors of sound and light when confronted with an obstacle to their transmission. It is fairly easy to block out light by simply holding up a hand in front of one’s eyes. When this happens, the Sun casts a shadow on the other side of one’s hand. The same action does not work with one’s ears and the source of a sound, however, because the wavelengths of sound are large enough to go right past a relatively small object such as a hand. However, if one were to put up a tall, wide cement wall between oneself and the source of a sound—as is often done in areas where an interstate highway passes right by a residential community—the object would be sufficiently large to block out much of the sound.

Radio Waves Radio waves, like visible light waves, are part of the electromagnetic spectrum. They are characterized by relatively long wavelengths and low frequencies—low, that is, in contrast to the much higher frequencies of both visible and invisible light waves. The frequency range of radio is between 10 KHz and about 2,000 MHz—in other words, from 10,000 Hz to as much as 2 billion Hz—an impressively wide range.

medium such as air, they create fluctuations between condensation and rarefaction. These result in pressure changes that cause the listener’s eardrum to vibrate with the same frequency as

AM radio broadcasts are found between 0.6 and 1.6 MHz, and FM broadcasts between 88 and 108 MHz. Thus, FM is at a much, much higher frequency than AM, with the lowest frequency on the FM dial 55 times as great as the highest on the AM dial. There are other ranges of frequency assigned by the FCC (Federal Communications Commission) to other varieties of radio trans-

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FREQUENCY AND WAV E L E N GT H . As sound waves pass through a

Wave Motion

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Wave Motion

KEY TERMS The maximum displace-

means that 20,000 waves are passing

ment of particles in oscillation from their

through a given point during the interval

normal position. For an ocean wave,

of one second. Higher frequencies are

amplitude is the distance from either the

expressed in terms of kilohertz (kHz; 103

crest or the trough to the level that the

or 1,000 cycles per second) or megahertz

ocean would maintain if it were per-

(MHz; 106 or 1 million cycles per second).

AMPLITUDE:

fectly still. KINETIC ENERGY:

The energy that

The ability to perform work,

an object possesses due to its motion, as

which is the exertion of force over a given

with a sled when sliding down a hill. This is

distance. Work is the product of force and

contrasted with potential energy.

ENERGY:

distance, where force and distance are exerted in the same direction. FREQUENCY:

The number of waves

passing through a given point during the interval of one second. The higher the fre-

A wave in

which the movement of vibration is in the same direction as the wave itself. This is contrasted to a transverse wave. Physical substance that has

quency, the shorter the wavelength. Fre-

MATTER:

quency can also be mathematically related

mass; occupies space; is composed of

to wave speed and period.

atoms; and is ultimately convertible to

HARMONIC MOTION:

The repeated

energy.

movement of a particle about a position of

MECHANICAL WAVE:

equilibrium, or balance.

that involves matter. Ocean waves are

A type of wave

A unit for measuring frequen-

mechanical waves; so, too, are the waves

cy, equal to one cycle per second. If a sound

produced by pulling a string. The matter

wave has a frequency of 20,000 Hz, this

itself may move in place, but, as with all

HERTZ:

mission: for instance, citizens’ band (CB) radios are in a region between AM and FM, ranging from 26.985 MHz to 27.405 MHz. Frequency does not indicate power. The power of a radio station is a function of the wattage available to its transmitter: hence, radio stations often promote themselves with announcements such as “operating with 100,000 watts of power....” Thus, an AM station, though it has a much lower frequency than an FM station, may possess more power, depending on the wattage of the transmitter. Indeed, as we shall see, it is precisely because of its high frequency that an FM station lacks the broadcast range of an AM station.

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A M P L I T U D E A N D F R E Q U E N CY M O D U LAT I O N S . What is the difference

between AM and FM? Or to put it another way, why is it that an AM station may be heard halfway across the country, yet its sound on a car radio fades out when the car goes under an overpass? The difference relates to how the various radio signals are modulated. A radio signal is simply a carrier: it may carry Morse code, or it may carry complex sounds, but in order to transmit voices and music, its signal must be modulated. This can be done, for instance, by varying the instantaneous amplitude of the radio wave, which is a function of the radio station’s power. These variations in amplitude are called amplitude modulation, or

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Wave Motion

KEY TERMS

CONTINUED

types of wave motion, there is no net

PULSE:

movement of matter—only of energy.

turbance that takes place in wave motion of

OSCILLATION:

A type of harmonic

motion, typically periodic, in one or more

a type other than that of a periodic wave. A type of trans-

STANDING WAVE:

verse wave produced by causing vibrations

dimensions. PERIOD:

An isolated, non-periodic dis-

For wave motion, a period is

the amount of time required to complete

on a string or other piece of material whose ends are fixed in place. A wave that exhibits

one full cycle of the wave, from trough to

SURFACE WAVE:

crest and back to trough. Period can be

the behavior of both a transverse wave and

mathematically related to frequency, wave-

a longitudinal wave.

length, and wave speed.

TRANSVERSE

PERIODIC MOTION:

Motion that is

which the vibration or motion is perpendi-

repeated at regular intervals. These inter-

cular to the direction in which the wave is

vals are known as periods.

moving. This is contrasted to a longitudi-

PERIODIC WAVE:

A wave in which a

WAVE:

A wave in

nal wave. The distance between

uniform series of crests and troughs follow

WAVELENGTH:

one after the other in regular succession.

a crest and the adjacent crest, or the trough

By contrast, the wave produced by apply-

and an adjacent trough, of a wave. Wave-

ing a pulse to a stretched string does not

length, abbreviated λ (the Greek letter

follow regular, repeated patterns.

lambda) is mathematically related to wave

POTENTIAL ENERGY:

The energy

speed, period, and frequency. Activity that carries

that an object possesses due to its position,

WAVE MOTION:

as for instance with a sled at the top of a

energy from one place to another without

hill. This is contrasted with kinetic energy.

actually moving any matter.

AM, and this was the first type of commercial radio to appear. Developed in the period before World War I, AM made its debut as a popular phenomenon shortly after the war. Ironically, FM (frequency modulation) was developed not long after AM, but it did not become commercially viable until well after World War II. As its name suggests, frequency modulation involves variation in the signal’s frequency. The amplitude stays the same, and this— combined with the high frequency—produces a nice, even sound for FM radio. But the high frequency also means that FM signals do not travel as far. If a person is listening to an FM station while moving away from the

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station’s signal, eventually the station will be below the horizon relative to the car, and the car radio will no longer be able to receive the signal. In contrast to the direct, or line-of-sight, transmissions of FM stations, AM signals (with their longer wavelengths) are reflected off of layers in Earth’s ionosphere. As a result, a nighttime signal from a “clear channel station” may be heard across much of the continental United States. WHERE TO LEARN MORE Ardley, Neil. Sound Waves to Music. New York: Gloucester Press, 1990. Berger, Melvin and Gilda Berger. What Makes an Ocean Wave?: Questions and Answers About Oceans and Ocean Life. New York: Scholastic, 2001.

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Catherall, Ed. Exploring Sound. Austin, TX: SteckVaughn Library, 1990. Glover, David. Sound and Light. New York: Kingfisher Books, 1993. “Longitudinal and Transverse Wave Motion” (Web site). (April 22, 2001).

262

Ruchlis, Hyman. Bathtub Physics. Edited by Donald Barr; illustrated by Ray Skibinski. New York: Harcourt, Brace, and World, 1967. “Wave Motion” (Web site). (April 22, 2001). “Wave Motion and Sound.” The Physics Web (Web site). (April 22, 2001).

“Multimedia Activities: Wave Motion.” ExploreScience.com (Web site). (April 22, 2001).

“Wave Motion Menu.”Carson City-Crystal High School Physics and Chemistry Departments (Web site). (April 22, 2001).

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Oscillation

O S C I L L AT I O N

CONCEPT When a particle experiences repeated movement about a position of stable equilibrium, or balance, it is said to be in harmonic motion, and if this motion is repeated at regular intervals, it is called periodic motion. Oscillation is a type of harmonic motion, typically periodic, in one or more dimensions. Among the examples of oscillation in the physical world are the motion of a spring, a pendulum, or even the steady back-andforth movement of a child on a swing.

and an adult caught the child at the point of maximum displacement—this would be an example of unstable equilibrium. The swing is in equilibrium because the forces on it are balanced: it is being held upward with a force equal to its weight. Yet, this equilibrium is unstable, because a disturbance (for instance, if the adult lets go of the swing) will cause it to move. Since the swing tends to oscillate, it will move back and forth across the position of stable equilibrium before finally coming to a rest in the stable position.

Properties of Oscillation

HOW IT WORKS Stable and Unstable Equilibrium When a state of equilibrium exists, the vector sum of the forces on an object is equal to zero. There are three varieties of equilibrium: stable, unstable, and neutral. Neutral equilibrium, discussed in the essay on Statics and Equilibrium elsewhere in this book, does not play a significant role in oscillation; on the other hand, stable and unstable equilibrium do.

There are two basic models of oscillation to consider, and these can be related to the motion of two well-known everyday objects: a spring and a swing. As noted below, objects not commonly considered “springs,” such as rubber bands, display spring-like behavior; likewise one could substitute “pendulum” for swing. In any case, it is easy enough to envision the motion of these two varieties of oscillation: a spring generally oscillates along a straight line, whereas a swing describes an arc.

If, on the other hand, the swing were raised to a certain height—if, say, a child were swinging

Either case involves properties common to all objects experiencing oscillation. There is always a position of stable equilibrium, and there is always a cycle of oscillation. In a single cycle, the oscillating particle moves from a certain point in a certain direction, then reverses direction and returns to the original point. The amount of time it takes to complete one cycle is called a period, and the number of cycles that take place during one second is the frequency of the oscillation. Frequency is measured in Hertz (Hz), with 1 Hz—the term is both singular and plural—equal to one cycle per second.

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In the example of a playground swing, when the swing is simply hanging downward—either empty or occupied—it is in a position of stable equilibrium. The vector sums are balanced, because the swing hangs downward with a force (its weight) equal to the force of the bars on the swing set that hold it up. If it were disturbed from this position, as, for instance, by someone pushing the swing, it would tend to return to its original position.

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Oscillation

THE

BOUNCE PROVIDED BY A TRAMPOLINE IS DUE TO ELASTIC POTENTIAL ENERGY. (Photograph by Kevin Fleming/Corbis.

Reproduced by permission.)

It is easiest to think of a cycle as the movement from a position of stable equilibrium to one of maximum displacement, or the furthest possible point from stable equilibrium. Because stable equilibrium is directly in the middle of a cycle, there are two points of maximum displacement. For a swing or pendulum, maximum displacement occurs when the object is at its highest point on either side of the stable equilibrium position. For example, maximum displacement in a spring occurs when the spring reaches the furthest point of being either stretched or compressed. The amplitude of a cycle is the maximum displacement of particles during a single period of oscillation, and the greater the amplitude, the greater the energy of the oscillation. When an object reaches maximum displacement, it reverses direction, and, therefore, it comes to a stop for an instant of time. Thus, the speed of movement is slowest at that position, and fastest as it passes back through the position of stable equilibrium. An increase in amplitude brings with it an increase in speed, but this does not lead to a change in the period: the greater the amplitude, the further the oscillating object has to move, and, therefore, it takes just as long to complete a cycle.

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Restoring Force Imagine a spring hanging vertically from a ceiling, one end attached to the ceiling for support and the other free to hang. It would thus be in a position of stable equilibrium: the spring hangs downward with a force equal to its weight, and the ceiling pulls it upward with an equal and opposite force. Suppose, now, that the spring is pulled downward. A spring is highly elastic, meaning that it can experience a temporary stress and still rebound to its original position; by contrast, some objects (for instance, a piece of clay) respond to deformation with plastic behavior, permanently assuming the shape into which they were deformed. The force that directs the spring back to a position of stable equilibrium—the force, in other words, which must be overcome when the spring is pulled downward—is called a restoring force. The more the spring is stretched, the greater the amount of restoring force that must be overcome. The same is true if the spring is compressed: once again, the spring is removed from a position of equilibrium, and, once again, the restoring force tends to pull it outward to its “natural” position. Here, the example is a spring,

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but restoring force can be understood just as easily in terms of a swing: once again, it is the force that tends to return the swing to a position of stable equilibrium. There is, however, one significant difference: the restoring force on a swing is gravity, whereas, in the spring, it is related to the properties of the spring itself.

Oscillation

Elastic Potential Energy For any solid that has not exceeded the elastic limit—the maximum stress it can endure without experiencing permanent deformation— there is a proportional relationship between force and the distance it can be stretched. This is expressed in the formula F = ks, where s is the distance and k is a constant related to the size and composition of the material in question. The amount of force required to stretch the spring is the same as the force that acts to bring it back to equilibrium— that is, the restoring force. Using the value of force, thus derived, it is possible, by a series of steps, to establish a formula for elastic potential energy. The latter, sometimes called strain potential energy, is the potential energy that a spring or a spring-like object possesses by virtue of its deformation from the state of equilibrium. It is equal to 1⁄2ks2. POTENTIAL AND KINETIC ENE R GY. Potential energy, as its name suggests,

involves the potential of something to move across a given interval of space—for example, when a sled is perched at the top of a hill. As it begins moving through that interval, the object will gain kinetic energy. Hence, the elastic potential energy of the spring, when the spring is held at a position of the greatest possible displacement from equilibrium, is at a maximum. Once it is released, and the restoring force begins to move it toward the equilibrium position, potential energy drops and kinetic energy increases. But the spring will not just return to equilibrium and stop: its kinetic energy will cause it to keep going. A

In the case of the “swing” model of oscillation, elastic potential energy is not a factor. (Unless, of course, the swing itself were suspended on some sort of spring, in which case the object will oscillate in two directions at once.) Nonetheless, all systems of motion involve potential and kinetic energy. When the swing is at a position of maximum displacement, its

S C I E N C E O F E V E RY DAY T H I N G S

BUNGEE JUMPER HELPS ILLUSTRATE A REAL-WORLD

EXAMPLE OF OSCILLATION. (Eye Ubiquitous/Corbis. Reproduced

by permission.)

potential energy is at a maximum as well. Then, as it moves toward the position of stable equilibrium, it loses potential energy and gains kinetic energy. Upon passing through the stable equilib-

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rium position, kinetic energy again decreases, while potential energy increases. The sum of the two forms of energy is always the same, but the greater the amplitude, the greater the value of this sum.

REAL-LIFE A P P L I C AT I O N S Springs and Damping Elastic potential energy relates primarily to springs, but springs are a major part of everyday life. They can be found in everything from the shock-absorber assembly of a motor vehicle to the supports of a trampoline fabric, and in both cases, springs blunt the force of impact. If one were to jump on a piece of trampoline fabric stretched across an ordinary table—one with no springs—the experience would not be much fun, because there would be little bounce. On the other hand, the elastic potential energy of the trampoline’s springs ensures that anyone of normal weight who jumps on the trampoline is liable to bounce some distance into the air. As a person’s body comes down onto the trampoline fabric, this stretches the fabric (itself highly elastic) and, hence, the springs. Pulled from a position of equilibrium, the springs acquire elastic potential energy, and this energy makes possible the upward bounce. As a car goes over a bump, the spring in its shock-absorber assembly is compressed, but the elastic potential energy of the spring immediately forces it back to a position of equilibrium, thus ensuring that the bump is not felt throughout the entire vehicle. However, springs alone would make for a bouncy ride; hence, a modern vehicle also has shock absorbers. The shock absorber, a cylinder in which a piston pushes down on a quantity of oil, acts as a damper—that is, an inhibitor of the springs’ oscillation.

H O W DA M P I N G W O R K S . When the spring is first released, most likely it will fly upward with so much kinetic energy that it will, quite literally, bounce off the ceiling. But with each transit within the position of equilibrium, the friction produced by contact between the metal spring and the air, and by contact between molecules within the spring itself, will gradually reduce the energy that gives it movement. In time, it will come to a stop.

If the damping effect is small, the amplitude will gradually decrease, as the object continues to oscillate, until eventually oscillation ceases. On the other hand, the object may be “overdamped,” such that it completes only a few cycles before ceasing to oscillate altogether. In the spring illustration, overdamping would occur if one were to grab the spring on a downward cycle, then slowly let it go, such that it no longer bounced. There is a type of damping less forceful than overdamping, but not so gradual as the slow dissipation of energy due to frictional forces alone. This is called critical damping. In a critically damped oscillator, the oscillating material is made to return to equilibrium as quickly as possible without oscillating. An example of a critically damped oscillator is the shock-absorber assembly described earlier.

occurs when a particle or object moves back and forth within a stable equilibrium position under the influence of a restoring force proportional to its displacement. In an ideal situation, where friction played no part, an object would continue to oscillate indefinitely.

Even without its shock absorbers, the springs in a car would be subject to some degree of damping that would eventually bring a halt to their oscillation; but because this damping is of a very gradual nature, their tendency is to continue oscillating more or less evenly. Over time, of course, the friction in the springs would wear down their energy and bring an end to their oscillation, but by then, the car would most likely have hit another bump. Therefore, it makes sense to apply critical damping to the oscillation of the springs by using shock absorbers.

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SIMPLE HARMONIC MOTION A N D DA M P I N G . Simple harmonic motion

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Of course, objects in the real world do not experience perpetual oscillation; instead, most oscillating particles are subject to damping, or the dissipation of energy, primarily as a result of friction. In the earlier illustration of the spring suspended from a ceiling, if the string is pulled to a position of maximum displacement and then released, it will, of course, behave dramatically at first. Over time, however, its movements will become slower and slower, because of the damping effect of frictional forces.

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Bungee Cords and Rubber Bands Many objects in daily life oscillate in a spring-like way, yet people do not commonly associate them with springs. For example, a rubber band, which behaves very much like a spring, possesses high elastic potential energy. It will oscillate when stretched from a position of stable equilibrium.

One type of pendulum is a metronome, which registers the tempo or speed of music. Housed in a hollow box shaped like a pyramid, a metronome consists of a pendulum attached to a sliding weight, with a fixed weight attached to the bottom end of the pendulum. It includes a number scale indicating the number of oscillations per minute, and by moving the upper weight, one can change the beat to be indicated.

Rubber is composed of long, thin molecules called polymers, which are arranged side by side. The chemical bonds between the atoms in a polymer are flexible and tend to rotate, producing kinks and loops along the length of the molecule. The super-elastic polymers in rubber are called elastomers, and when a piece of rubber is pulled, the kinks and loops in the elastomers straighten.

early nineteenth century, but, by then, the concept of a pendulum was already old. In the second century A.D., Chinese mathematician and astronomer Zhang Heng (78-139) used a pendulum to develop the world’s first seismoscope, an instrument for measuring motion on Earth’s surface as a result of earthquakes.

The structure of rubber gives it a high degree of elastic potential energy, and in order to stretch rubber to maximum displacement, there is a powerful restoring force that must be overcome. This can be illustrated if a rubber band is attached to a ceiling, like the spring in the earlier example, and allowed to hang downward. If it is pulled down and released, it will behave much as the spring did.

Zhang Heng’s seismoscope, which he unveiled in 132 A.D., consisted of a cylinder surrounded by bronze dragons with frogs (also made of bronze) beneath. When the earth shook, a ball would drop from a dragon’s mouth into that of a frog, making a noise. The number of balls released, and the direction in which they fell, indicated the magnitude and location of the seismic disruption.

The oscillation of a rubber band will be even more appreciable if a weight is attached to the “free” end—that is, the end hanging downward. This is equivalent, on a small scale, to a bungee jumper attached to a cord. The type of cord used for bungee jumping is highly elastic; otherwise, the sport would be even more dangerous than it already is. Because of the cord’s elasticity, when the bungee jumper “reaches the end of his rope,” he bounces back up. At a certain point, he begins to fall again, then bounces back up, and so on, oscillating until he reaches the point of stable equilibrium.

C LO C K S , S C I E N T I F I C I N S T R U M E N T S , A N D “FAX M AC H I N E ” . In

The Pendulum As noted earlier, a pendulum operates in much the same way as a swing; the difference between them is primarily one of purpose. A swing exists to give pleasure to a child, or a certain bittersweet pleasure to an adult reliving a childhood experience. A pendulum, on the other hand, is not for play; it performs the function of providing a reading, or measurement.

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Oscillation

ZHANG HENG’S SEISMOS C O P E . Metronomes were developed in the

718 A.D., during a period of intellectual flowering that attended the early T’ang Dynasty (618-907), a Buddhist monk named I-hsing and a military engineer named Liang Ling-tsan built an astronomical clock using a pendulum. Many clocks today—for example, the stately and imposing “grandfather clock” found in some homes—likewise, use a pendulum to mark time. Physicists of the early modern era used pendula (the plural of pendulum) for a number of interesting purposes, including calculations regarding gravitational force. Experiments with pendula by Galileo Galilei (1564-1642) led to the creation of the mechanical pendulum clock—the grandfather clock, that is—by distinguished Dutch physicist and astronomer Christiaan Huygens (1629-1695). In the nineteenth century, A Scottish inventor named Alexander Bain (1810-1877) even used a pendulum to create the first “fax machine.” Using matching pendulum transmitters and receivers that sent and received electrical

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Oscillation

KEY TERMS The maximum displace-

AMPLITUDE:

For a particle experi-

FREQUENCY:

ment of particles from their normal posi-

encing oscillation, frequency is the number

tion during a single period of oscillation.

of cycles that take place during one second.

One full repetition of oscilla-

CYCLE:

tion. In a single cycle, the oscillating particle moves from a certain point in a certain direction, then switches direction and moves back to the original point. Typically, this is from the position of stable equilibri-

Frequency is measured in Hertz. FRICTION:

The force that resists

motion when the surface of one object comes into contact with the surface of another. The repeated

um to maximum displacement and back

HARMONIC MOTION:

again to the stable equilibrium position.

movement of a particle within a position of

DAMPING:

The dissipation of energy

equilibrium, or balance.

during oscillation, which prevents an

HERTZ:

object from continuing in simple harmon-

cy. The number of Hertz is the number of

ic motion and will eventually force it to

cycles per second.

A unit for measuring frequen-

stop oscillating altogether. Damping is KINETIC ENERGY:

usually caused by friction.

The energy that

an object possesses due to its motion, as ELASTIC

POTENTIAL

ENERGY:

The potential energy that a spring or a spring-like object possesses by virtue of its

is contrasted with potential energy. For an

deformation from the state of equilibrium.

MAXIMUM DISPLACEMENT:

Sometimes called strain potential energy, it

object in oscillation, maximum displace-

is equal to

⁄2KS2,

WHERE S is the distance

ment is the furthest point from stable equi-

stretched and k is a figure related to the

librium. Since stable equilibrium is in the

size and composition of the material in

middle of a cycle, there are two points of

question.

maximum displacement. For a swing or

1

A state in which the

pendulum, this occurs when the object is at

vector sum for all lines of force on an

its highest point on either side of the stable

object is equal to zero.

equilibrium position. Maximum displace-

EQUILIBRIUM:

impulses, he created a crude device that, at the time, seemed to have little practical purpose. In fact, Bain’s “fax machine,” invented in 1840, was more than a century ahead of its time.

length, he was able to demonstrate that Earth rotates on its axis.

By far the most important experiments with pendula during the nineteenth century, however, were those of the French physicist Jean Bernard Leon Foucault (1819-1868). Swinging a heavy iron ball from a wire more than 200 ft (61 m) in

Foucault conducted his famous demonstration in the Panthéon, a large domed building in Paris named after the ancient Pantheon of Rome. He arranged to have sand placed on the floor of the Panthéon, and placed a pin on the bottom of the iron ball, so that it would mark the sand as the pendulum moved. A pendulum in oscillation maintains its orientation, yet the Foucault pen-

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268

with a sled, when sliding down a hill. This

F O U C A U LT

PENDULUM.

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Oscillation

KEY TERMS

CONTINUED

ment in a spring occurs when the spring is

position under the influence of a restoring

either stretched or compressed as far as it

force proportional to its displacement.

will go.

Simple harmonic motion is, in fact, an

OSCILLATION:

A type of harmonic

motion, typically periodic, in one or more

ideal situation; most types of oscillation are subject to some form of damping.

dimensions. STABLE EQUILIBRIUM:

A type of

The amount of time required

equilibrium in which, if an object were dis-

for one cycle in oscillating motion—for

turbed, it would tend to return to its origi-

instance, from a position of maximum dis-

nal position. For an object in oscillation,

placement to one of stable equilib-

stable equilibrium is in the middle of a

rium, and, once again, to maximum dis-

cycle, between two points of maximum

placement.

displacement.

PERIOD:

PERIODIC MOTION:

Motion that is

repeated at regular intervals. These inter-

VECTOR:

A quantity that possesses

both magnitude and direction.

vals are known as periods. POTENTIAL ENERGY:

The energy

that an object possesses due to its position, as for instance, with a sled at the top of a hill. This is contrasted with kinetic energy.

VECTOR

SUM:

A calculation that

yields the net result of all the vectors applied in a particular situation. Because direction is involved, it is necessary when calculating the vector sum of forces on an

A force that

object (as, for instance, when determining

directs an object back to a position of sta-

whether or not it is in a state of equilibri-

ble equilibrium. An example is the resist-

um), to assign a positive value to forces in

RESTORING FORCE:

ance of a spring, when it is extended.

one direction, and a negative value to Har-

forces in the opposite direction. If the

monic motion, in which a particle moves

object is in equilibrium, these forces will

back and forth about a stable equilibrium

cancel one another out.

SIMPLE HARMONIC MOTION:

dulum (as it came to be called) seemed to be shifting continually toward the right, as indicated by the marks in the sand.

Earth itself was moving beneath the pendulum, providing an additional force which caused the pendulum’s plane of oscillation to rotate.

The confusion related to reference point: since Earth’s rotation is not something that can be perceived with the senses, it was natural to assume that the pendulum itself was changing orientation—or rather, that only the pendulum was moving. In fact, the path of Foucault’s pendulum did not vary nearly as much as it seemed.

Ehrlich, Robert. Turning the World Inside Out, and 174 Other Simple Physics Demonstrations. Princeton, N.J.: Princeton University Press, 1990.

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WHERE TO LEARN MORE Brynie, Faith Hickman. Six-Minute Science Experiments. Illustrated by Kim Whittingham. New York: Sterling Publishing Company, 1996.

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“Foucault Pendulum” Smithsonian Institution FAQs (Web site). (April 23, 2001).

Shirley, Jean. Galileo. Illustrated by Raymond Renard. St. Louis: McGraw-Hill, 1967. Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996.

Kruszelnicki, Karl S. The Foucault Pendulum (Web site). (April 23, 2001).

Topp, Patricia. This Strange Quantum World and You. Nevada City, CA: Blue Dolphin, 1999.

Schaefer, Lola M. Back and Forth. Edited by Gail Saunders-Smith; P. W. Hammer, consultant. Mankato, MN: Pebble Books, 2000.

Zubrowski, Bernie. Making Waves: Finding Out About Rhythmic Motion. Illustrated by Roy Doty. New York: Morrow Junior Books, 1994.

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Frequency

CONCEPT Everywhere in daily life, there are frequencies of sound and electromagnetic waves, constantly changing and creating the features of the visible and audible world familiar to everyone. Some aspects of frequency can only be perceived indirectly, yet people are conscious of them without even thinking about it: a favorite radio station, for instance, may have a frequency of 99.7 MHz, and fans of that station knows that every time they turn the FM dial to that position, the station’s signal will be there. Of course, people cannot “hear” radio and television frequencies— part of the electromagnetic spectrum—but the evidence for them is everywhere. Similarly, people are not conscious, in any direct sense, of frequencies in sound and light—yet without differences in frequency, there could be no speech or music, nor would there be any variations of color.

HOW IT WORKS Harmonic Motion and Energy In order to understand frequency, it is first necessary to comprehend two related varieties of movement: oscillation and wave motion. Both are examples of a broader category, periodic motion: movement that is repeated at regular intervals called periods. Oscillation and wave motion are also examples of harmonic motion, or the repeated movement of a particle about a position of equilibrium, or balance.

FREQUENCY

conversion of energy from one form to another. On the one hand is potential energy, or the energy of an object due to its position and, hence, its potential for movement. On the other hand, there is kinetic energy, the energy of movement itself. Potential-kinetic conversions take place constantly in daily life: any time an object is at a distance from a position of stable equilibrium, and some force (for instance, gravity) is capable of moving it to that position, it possesses potential energy. Once it begins to move toward that equilibrium position, it loses potential energy and gains kinetic energy. Likewise, a wave at its crest has potential energy, and gains kinetic energy as it moves toward its trough. Similarly, an oscillating object that is as far as possible from the stable-equilibrium position has enormous potential energy, which dissipates as it begins to move toward stable equilibrium. V I B RAT I O N . Though many examples of periodic and harmonic motion can be found in daily life, the terms themselves are certainly not part of everyday experience. On the other hand, everyone knows what “vibration” means: to move back and forth in place. Oscillation, discussed in more detail below, is simply a more scientific term for vibration; and while waves are not themselves merely vibrations, they involve— and may produce—vibrations. This, in fact, is how the human ear hears: by interpreting vibrations resulting from sound waves.

types of periodic motion, there is a continual

Indeed, the entire world is in a state of vibration, though people seldom perceive this movement—except, perhaps, in dramatic situations such as earthquakes, when the vibrations of plates beneath Earth’s surface become too force-

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KINETIC AND POTENTIAL ENE R GY. In harmonic motion, and in some

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In oscillation, whether the oscillator be spring-like or swing-like, there is always a cycle in which the oscillating particle moves from a certain point in a certain direction, then reverses direction and returns to the original point. Usually a cycle is viewed as the movement from a position of stable equilibrium to one of maximum displacement, or the furthest possible point from stable equilibrium. Because stable equilibrium is directly in the middle of a cycle, there are two points of maximum displacement: on a swing, this occurs when the object is at its highest point on either side of the stable equilibrium position, and on a spring, maximum displacement occurs when the spring is either stretched or compressed as far as it will go.

Frequency

Wave Motion

GRANDFATHER VARIETIES

OF

CLOCKS ARE ONE OF THE BEST-KNOWN A

PENDULUM .

(Photograph by Peter

Harholdt/Corbis. Reproduced by permission.)

ful to ignore. All matter vibrates at the molecular level, and every object possesses what is called a natural frequency, which depends on its size, shape, and composition. This explains how a singer can shatter a glass by hitting a certain note, which does not happen because the singer’s voice has reached a particularly high pitch; rather, it is a matter of attaining the natural frequency of the glass. As a result, all the energy in the sound of the singer’s voice is transferred to the glass, and it shatters.

Wave motion is a type of harmonic motion that carries energy from one place to another without actually moving any matter. While oscillation involves the movement of “an object,” whether it be a pendulum, a stretched rubber band, or some other type of matter, a wave may or may not involve matter. Example of a wave made out of matter—that is, a mechanical wave—is a wave on the ocean, or a sound wave, in which energy vibrates through a medium such as air. Even in the case of the mechanical wave, however, the matter does not experience any net displacement from its original position. (Water molecules do rotate as a result of wave motion, but they end up where they began.) There are waves that do not follow regular, repeated patterns; however, within the context of frequency, our principal concern is with periodic waves, or waves that follow one another in regular succession. Examples of periodic waves include ocean waves, sound waves, and electromagnetic waves.

Oscillation Oscillation is a type of harmonic motion, typically periodic, in one or more dimensions. There are two basic types of oscillation: that of a swing or pendulum and that of a spring. In each case, an object is disturbed from a position of stable equilibrium, and, as a result, it continues to move back and forth around that stable equilibrium position. If a spring is pulled from stable equilibrium, it will generally oscillate along a straight path; a swing, on the other hand, will oscillate along an arc.

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Periodic waves may be further divided into transverse and longitudinal waves. A transverse wave is the shape that most people imagine when they think of waves: a regular up-and-down pattern (called “sinusoidal” in mathematical terms) in which the vibration or motion is perpendicular to the direction the wave is moving. A longitudinal wave is one in which the movement of vibration is in the same direction as the wave itself. Though these are a little harder to picture, longitudinal waves can be visualized as a series of concentric circles emanating

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from a single point. Sound waves are longitudinal: thus when someone speaks, waves of sound vibrations radiate out in all directions.

Frequency

Amplitude There are certain properties of waves, such as wavelength, or the distance between waves, that are not properties of oscillation. However, both types of motion can be described in terms of amplitude, period, and frequency. The first of these is not related to frequency in any mathematical sense; nonetheless, where sound waves are concerned, both amplitude and frequency play a significant role in what people hear. Though waves and oscillators share the properties of amplitude, period, and frequency, the definitions of these differ slightly depending on whether one is discussing wave motion or oscillation. Amplitude, generally speaking, is the value of maximum displacement from an average value or position—or, in simpler terms, amplitude is “size.” For an object experiencing oscillation, it is the value of the object’s maximum displacement from a position of stable equilibrium during a single period. It is thus the “size” of the oscillation. In the case of wave motion, amplitude is also the “size” of a wave, but the precise definition varies, depending on whether the wave in question is transverse or longitudinal. In the first instance, amplitude is the distance from either the crest or the trough to the average position between them. For a sound wave, which is longitudinal, amplitude is the maximum value of the pressure change between waves.

Period and Frequency Unlike amplitude, period is directly related to frequency. For a transverse wave, a period is the amount of time required to complete one full cycle of the wave, from trough to crest and back to trough. In a longitudinal wave, a period is the interval between waves. With an oscillator, a period is the amount of time it takes to complete one cycle. The value of a period is usually expressed in seconds. Frequency in oscillation is the number of cycles per second, and in wave motion, it is the number of waves that pass through a given point per second. These cycles per second are called Hertz (Hz) in honor of nineteenth-century Ger-

S C I E N C E O F E V E RY DAY T H I N G S

MIDDLE C—WHICH IS AT THE MIDDLE OF A PIANO KEYBOARD—IS THE STARTING POINT OF A BASIC MUSICAL SCALE. IT IS CALLED THE FUNDAMENTAL FREQUENCY, OR THE FIRST HARMONIC . (Photograph by Francoise Gervais/Corbis. Reproduced by permission.)

man physicist Heinrich Rudolf Hertz (18571894), who greatly advanced understanding of electromagnetic wave behavior during his short career. If something has a frequency of 100 Hz, this means that 100 waves are passing through a given point during the interval of one second, or that an oscillator is completing 100 cycles in a second. Higher frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second); megahertz (MHz; 106 or 1 million cycles per second); and gigahertz (GHz; 109 or 1 billion cycles per second.). A clear mathematical relationship exists between period, symbolized by T, and frequency (f): each is the inverse of the other. Hence,

T

= 1

f

= 1

f ,

and

T .

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If an object in harmonic motion has a frequency of 50 Hz, its period is 1/50 of a second (0.02 sec). Or, if it has a period of 1/20,000 of a second (0.00005 sec), that means it has a frequency of 20,000 Hz.

REAL-LIFE A P P L I C AT I O N S Grandfather Clocks and Metronomes One of the best-known varieties of pendulum (plural, pendula) is a grandfather clock. Its invention was an indirect result of experiments with pendula by Galileo Galilei (1564-1642), work that influenced Dutch physicist and astronomer Christiaan Huygens (1629-1695) in the creation of the mechanical pendulum clock—or grandfather clock, as it is commonly known. The frequency of a pendulum, a swing-like oscillator, is the number of “swings” per minute. Its frequency is proportional to the square root of the downward acceleration due to gravity (32 ft or 9.8 m/sec2) divided by the length of the pendulum. This means that by adjusting the length of the pendulum on the clock, one can change its frequency: if the pendulum length is shortened, the clock will run faster, and if it is lengthened, the clock will run more slowly. Another variety of pendulum, this one dating to the early nineteenth century, is a metronome, an instrument that registers the tempo or speed of music. Consisting of a pendulum attached to a sliding weight, with a fixed weight attached to the bottom end of the pendulum, a metronome includes a number scale indicating the frequency—that is, the number of oscillations per minute. By moving the upper weight, one can speed up or slow down the beat.

Harmonics As noted earlier, the volume of any sound is related to the amplitude of the sound waves. Frequency, on the other hand, determines the pitch or tone. Though there is no direct correlation between intensity and frequency, in order for a person to hear a very low-frequency sound, it must be above a certain decibel level. The range of audibility for the human ear is from 20 Hz to 20,000 Hz. The optimal range for hearing, however, is between 3,000 and 4,000 Hz.

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This places the piano, whose 88 keys range from 27 Hz to 4,186 Hz, well within the range of human audibility. Many animals have a much wider range: bats, whales, and dolphins can hear sounds at a frequency up to 150,000 Hz. But humans have something that few animals can appreciate: music, a realm in which frequency changes are essential. Each note has its own frequency: middle C, for instance, is 264 Hz. But in order to produce what people understand as music—that is, pleasing combinations of notes—it is necessary to employ principles of harmonics, which express the relationships between notes. These mathematical relations between musical notes are among the most intriguing aspects of the connection between art and science. It is no wonder, perhaps, that the great Greek mathematician Pythagoras (c. 580-500 B.C.) believed that there was something spiritual or mystical in the connection between mathematics and music. Pythagoras had no concept of frequency, of course, but he did recognize that there were certain numerical relationships between the lengths of strings, and that the production of harmonious music depended on these ratios. RAT I O S O F F R E Q U E N CY A N D P L E AS I N G T O N E S . Middle C—located,,

appropriately enough, in the middle of a piano keyboard—is the starting point of a basic musical scale. It is called the fundamental frequency, or the first harmonic. The second harmonic, one octave above middle C, has a frequency of 528 Hz, exactly twice that of the first harmonic; and the third harmonic (two octaves above middle C) has a frequency of 792 cycles, or three times that of middle C. So it goes, up the scale. As it turns out, the groups of notes that people consider harmonious just happen to involve specific whole-number ratios. In one of those curious interrelations of music and math that would have delighted Pythagoras, the smaller the numbers involved in the ratios, the more pleasing the tone to the human psyche. An example of a pleasing interval within an octave is a fifth, so named because it spans five notes that are a whole step apart. The C Major scale is easiest to comprehend in this regard, because it does not require reference to the “black keys,” which are a half-step above or below the “white keys.” Thus, the major fifth in the CMajor scale is C, D, E, F, G. It so happens that the

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Frequency

KEY TERMS For an object oscillation,

(1857-1894). Higher frequencies are

amplitude is the value of the object’s max-

expressed in terms of kilohertz (kHz; 103

imum displacement from a position of sta-

or 1,000 cycles per second); megahertz

ble equilibrium during a single period. In a

(MHz; 106 or 1 million cycles per second);

transverse wave, amplitude is the distance

and gigahertz (GHz; 109 or 1 billion cycles

from either the crest or the trough to the

per second.)

AMPLITUDE:

average position between them. For a sound wave, the best-known example of a longitudinal wave, amplitude is the maximum value of the pressure change between waves. CYCLE:

In oscillation, a cycle occurs

when the oscillating particle moves from a certain point in a certain direction, then switches direction and moves back to the

KINETIC ENERGY:

The energy that

an object possesses due to its motion, as with a sled when sliding down a hill. This is contrasted with potential energy. LONGITUDINAL WAVE:

A wave in

which the movement of vibration is in the same direction as the wave itself. This is contrasted to a transverse wave.

original point. Typically, this is from the

MAXIMUM DISPLACEMENT:

position of stable equilibrium to maxi-

object in oscillation, maximum displace-

mum displacement and back again to the

ment is the farthest point from stable equi-

stable equilibrium position.

librium.

FREQUENCY:

For a particle experi-

OSCILLATION:

For an

A type of harmonic

encing oscillation, frequency is the number

motion, typically periodic, in one or more

of cycles that take place during one second.

dimensions.

In wave motion, frequency is the number of waves passing through a given point during the interval of one second. In either case, frequency is measured in Hertz. Period (T) is the mathematical inverse of frequency (f) hence f=1/T.

In oscillation, a period is the

PERIOD:

amount of time required for one cycle. For a transverse wave, a period is the amount of time required to complete one full cycle of the wave, from trough to crest and back to trough. In a longitudinal wave, a period

The repeated

is the interval between waves. Frequency is

movement of a particle about a position of

the mathematical inverse of period (T):

equilibrium, or balance.

hence, T=1/f.

HARMONIC MOTION:

HERTZ:

A unit for measuring fre-

PERIODIC MOTION:

Motion that is

quency, named after nineteenth-century

repeated at regular intervals. These inter-

German physicist Heinrich Rudolf Hertz

vals are known as periods.

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Frequency

KEY TERMS that an object possesses due to its position,

between two points of maximum displacement.

as, for instance, with a sled at the top of

TRANSVERSE

POTENTIAL ENERGY:

The energy

a hill. This is contrasted with kinetic energy. STABLE EQUILIBRIUM:

A position

in which, if an object were disturbed, it would tend to return to its original position. For an object in oscillation, stable equilibrium is in the middle of a cycle,

ratio in frequency between middle C and G (396 Hz) is 2:3. Less melodious, but still certainly tolerable, is an interval known as a third. Three steps up from middle C is E, with a frequency of 330 Hz, yielding a ratio involving higher numbers than that of a fifth—4:5. Again, the higher the numbers involved in the ratio, the less appealing the sound is to the human ear: the combination E-F, with a ratio of 15:16, sounds positively grating.

The Electromagnetic Spectrum

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CONTINUED

A wave in which the vibration or motion is perpendicular to the direction in which the wave is moving. This is contrasted to a longitudinal wave. WAVE:

A type of harmonic motion that carries energy from one place to another without actually moving any matter.

WAVE MOTION:

of gamma rays, reaching to frequencies as high as 1025. The latter value is equal to 10 trillion trillion. Obviously, these ultra-ultra high-frequency waves must be very small, and they are: the higher gamma rays have a wavelength of around 10-15 meters (0.000000000000001 m). For frequencies lower than those of visible light, the wavelengths get larger, but for a wide range of the electromagnetic spectrum, the wavelengths are still much too small to be seen, even if they were visible. Such is the case with infrared light, or the relatively lower-frequency millimeter waves.

Everyone who has vision is aware of sunlight, but, in fact, the portion of the electromagnetic spectrum that people perceive is only a small part of it. The frequency range of visible light is from 4.3 • 1014 Hz to 7.5 • 1014 Hz—in other words, from 430 to 750 trillion Hertz. Two things should be obvious about these numbers: that both the range and the frequencies are extremely high. Yet, the values for visible light are small compared to the higher reaches of the spectrum, and the range is also comparatively small.

Only at the low end of the spectrum, with frequencies below about 1010 Hz—still an incredibly large number—do wavelengths become the size of everyday objects. The center of the microwave range within the spectrum, for instance, has a wavelength of about 3.28 ft (1 m). At this end of the spectrum—which includes television and radar (both examples of microwaves), short-wave radio, and long-wave radio—there are numerous segments devoted to various types of communication.

Each of the colors has a frequency, and the value grows higher from red to orange, and so on through yellow, green, blue, indigo, and violet. Beyond violet is ultraviolet light, which human eyes cannot see. At an even higher frequency are x rays, which occupy a broad band extending almost to 1020 Hz—in other words, 1 followed by 20 zeroes. Higher still is the very broad range

RA D I O A N D M I C R O WAV E F R E Q U E N C I E S . The divisions of these sections

of the electromagnetic spectrum are arbitrary and manmade, but in the United States—where they are administered by the Federal Communications Commission (FCC)—they have the force of law. When AM (amplitude modulation) radio first came into widespread use in the early

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1920s—Congress assigned AM stations the frequency range that they now occupy: 535 kHz to 1.7 MHz. A few decades after the establishment of the FCC in 1927, new forms of electronic communication came into being, and these too were assigned frequencies—sometimes in ways that were apparently haphazard. Today, television stations 2-6 are in the 54-88 MHz range, while stations 7-13 occupy the region from 174-220 MHz. In between is the 88 to 108 MHz band, assigned to FM radio. Likewise, short-wave radio (5.9 to 26.1 MHz) and citizens’ band or CB radio (26.96 to 27.41 MHz) occupy positions between AM and FM. In fact, there are a huge variety of frequency ranges accorded to all manner of other communication technologies. Garage-door openers and alarm systems have their place at around 40 MHz. Much, much higher than these—higher, in fact, than TV broadcasts—is the band allotted to deep-space radio communications: 2,290 to 2,300 MHz. Cell phones have their own realm, of course, as do cordless phones; but so too do radio controlled cars (75 MHz) and even baby monitors (49 MHz).

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WHERE TO LEARN MORE

Frequency

Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991. Allocation of Radio Spectrum in the United States (Web site). (April 25, 2001). DiSpezio, Michael and Catherine Leary. Awesome Experiments in Light and Sound. New York: Sterling Juvenile, 2001. Electromagnetic Spectrum (Web site). (April 25, 2001). “How the Radio Spectrum Works.” How Stuff Works (Web site). (April 25, 2001). Internet Resources for Sound and Light (Web site). (April 25, 2001). “NIST Time and Frequency Division.” NIST: National Institute of Standards and Technology (Web site). (April 25, 2001). Parker, Steve. Light and Sound. Austin, TX: Raintree Steck-Vaughn, 2000. Physics Tutorial System: Sound Waves Modules (Web site). (April 25, 2001). “Radio Electronics Pages” ePanorama.net (Web site). (April 25, 2001).

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RESONANCE Resonance

CONCEPT Though people seldom witness it directly, the entire world is in a state of motion, and where solid objects are concerned, this motion is manifested as vibration. When the vibrations produced by one object come into alignment with those of another, this is called resonance. The power of resonance can be as gentle as an adult pushing a child on a swing, or as ferocious as the force that toppled what was once the world’s third-longest suspension bridge. Resonance helps to explain all manner of familiar events, from the feedback produced by an electric guitar to the cooking of food in a microwave oven.

Due to the high rate of motion in gas molecules, gases possess enormous internal kinetic energy. The internal energy of solids and liquids is much less than in gases, yet, as we shall see, the use of resonance to transfer energy to these objects can yield powerful results.

HOW IT WORKS

Oscillation

Vibration of Molecules

In colloquial terms, oscillation is the same as vibration, but, in more scientific terms, oscillation can be identified as a type of harmonic motion, typically periodic, in one or more dimensions. All things that oscillate do so either along a more or less straight path, like that of a spring pulled from a position of stable equilibrium; or they oscillate along an arc, like a swing or pendulum.

The possibility of resonance always exists wherever there is periodic motion, movement that is repeated at regular intervals called periods, and/or harmonic motion, the repeated movement of a particle about a position of equilibrium or balance. Many examples of resonance involve large objects: a glass, a child on a swing, a bridge. But resonance also takes place at a level invisible to the human eye using even the most powerful optical microscope.

A substance in which molecules move at high speeds, and therefore hardly attract one

In the case of the swing or pendulum, stable equilibrium is the point at which the object is hanging straight downward—that is, the position to which gravitation force would take it if no other net forces were acting on the object. For a spring, stable equilibrium lies somewhere between the point at which the spring is stretched to its maximum length and the point at which it is subjected to maximum compression without permanent deformation.

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All molecules exert a certain electromagnetic attraction toward each other, and generally speaking, the less the attraction between molecules, the greater their motion relative to one another. This, in turn, helps define the object in relation to its particular phase of matter.

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another at all, is called a gas. Liquids are materials in which the rate of motion, and hence of intermolecular attraction, is moderate. In a solid, on the other hand, there is little relative motion, and therefore molecules exert enormous attractive forces. Instead of moving in relation to one another, the molecules that make up a solid tend to vibrate in place.

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CYC L E S A N D F R E Q U E N CY. A cycle of oscillation involves movement from a certain point in a certain direction, then a reversal of direction and a return to the original point. It is simplest to treat a cycle as the movement from a position of stable equilibrium to one of maximum displacement, or the furthest possible point from stable equilibrium.

Resonance

The amount of time it takes to complete one cycle is called a period, and the number of cycles in one second is the frequency of the oscillation. Frequency is measured in Hertz. Named after nineteenth-century German physicist Heinrich Rudolf Hertz (1857-1894), a single Hertz (Hz)— the term is both singular and plural—is equal to one cycle per second. A M P L I T U D E A N D E N E R GY. The

amplitude of a cycle is the maximum displacement of particles during a single period of oscillation. When an oscillator is at maximum displacement, its potential energy is at a maximum as well. From there, it begins moving toward the position of stable equilibrium, and as it does so, it loses potential energy and gains kinetic energy. Once it reaches the stable equilibrium position, kinetic energy is at a maximum and potential energy at a minimum. As the oscillating object passes through the position of stable equilibrium, kinetic energy begins to decrease and potential energy increases. By the time it has reached maximum displacement again—this time on the other side of the stable equilibrium position—potential energy is once again at a maximum. O S C I L L AT I O N I N WAV E M O T I O N . The particles in a mechanical wave (a

wave that moves through a material medium) have potential energy at the crest and trough, and gain kinetic energy as they move between these points. This is just one of many ways in which wave motion can be compared to oscillation. There is one critical difference between oscillation and wave motion: whereas oscillation involves no net movement, but merely movement in place, the harmonic motion of waves carries energy from one place to another. Nonetheless, the analogies than can be made between waves and oscillations are many, and understandably so: oscillation, after all, is an aspect of wave motion.

A

COMMON EXAMPLE OF RESONANCE: A PARENT PUSH-

ES HER CHILD ON A SWING. (Photograph by Annie Griffiths

Belt/Corbis. Reproduced by permission.)

other in regular succession. Two basic types of periodic waves exist, and these are defined by the relationship between the direction of oscillation and the direction of the wave itself. A transverse wave forms a regular up-and-down pattern, in which the oscillation is perpendicular to the direction in which the wave is moving. On the other hand, in a longitudinal wave (of which a sound wave is the best example), oscillation is in the same direction as the wave itself.

A periodic wave is one in which a uniform series of crests and troughs follow one after the

Again, the wave itself experiences net movement, but within the wave—one of its defining characteristics, as a matter of fact—are oscillations, which (also by definition) experience no net movement. In a transverse wave, which is usually easier to visualize than a longitudinal wave, the oscillation is from the crest to the trough and back again. At the crest or trough, potential energy is at a maximum, while kinetic energy reaches a maximum at the point of equilibrium between crest and trough. In a longitudinal wave, oscillation is a matter of density fluctuations: the greater the value of these fluctuations, the greater the energy in the wave.

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Resonance

THE POWER OF RESONANCE CAN DESTROY A BRIDGE. ON NOVEMBER 7, 1940, THE ACCLAIMED TACOMA NARROWS BRIDGE COLLAPSED DUE TO OVERWHELMING RESONANCE. (UPI/Corbis-Bettmann. Reproduced by permission.)

Parameters for Describing Harmonic Motion The maximum value of the pressure change between waves is the amplitude of a longitudinal wave. In fact, waves can be described according to many of the same parameters used for oscillation—frequency, period, amplitude, and so on. The definitions of these terms vary somewhat, depending on whether one is discussing oscillation or wave motion; or, where wave motion is concerned, on whether the subject is a transverse wave or a longitudinal wave.

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which we will be concerned is oscillation, and though wave motion will be mentioned, our principal concern is the oscillations within the waves, not the waves themselves. Second, the two parameters of importance in understanding resonance are amplitude and frequency.

Resonance and Energy Transfer

For the present purposes, however, it is necessary to focus on just a few specifics of harmonic motion. First of all, the type of motion with

Resonance can be defined as the condition in which force is applied to an oscillator at the point of maximum amplitude. In this way, the motion of the outside force is perfectly matched to that of the oscillator, making possible a transfer of energy.

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As its name suggests, resonance is a matter of one object or force “getting in tune with” another object. One literal example of this involves shattering a wine glass by hitting a musical note that is on the same frequency as the natural frequency of the glass. (Natural frequency depends on the size, shape, and composition of the object in question.) Because the frequencies resonate, or are in sync with one another, maximum energy transfer is possible. The same can be true of soldiers walking across a bridge, or of winds striking the bridge at a resonant frequency—that is, a frequency that matches that of the bridge. In such situations, a large structure may collapse under a force that would not normally destroy it, but the effects of resonance are not always so dramatic. Sometimes resonance can be a simple matter, like pushing a child in a swing in such a way as to ensure that the child gets maximum enjoyment for the effort expended.

REAL-LIFE A P P L I C AT I O N S A Child on a Swing and a Pendulum in a Museum Suppose a father is pushing his daughter on a swing, so that she glides back and forth through the air. A swing, as noted earlier, is a classic example of an oscillator. When the child gets in the seat, the swing is in a position of stable equilibrium, but as the father pulls her back before releasing her, she is at maximum displacement. He releases her, and quickly, potential energy becomes kinetic energy as she swings toward the position of stable equilibrium, then up again on the other side. Now the half-cycle is repeated, only in reverse, as she swings backward toward her father. As she reaches the position from which he first pushed her, he again gives her a little push. This push is essential, if she is to keep going. Without friction, she could keep on swinging forever at the same rate at which she begun. But in the real world, the wearing of the swing’s chain against the support along the bar above the swing will eventually bring the swing itself to a halt.

moment. That right moment is the point of greatest amplitude—the point, that is, at which the father’s pushing motion and the motion of the swing are in perfect resonance.

Resonance

If the father waits until she is already on the downswing before he pushes her, not all the energy of his push will actually be applied to keeping her moving. He will have failed to efficiently add energy to his daughter’s movement on the swing. On the other hand, if he pushes her too soon—that is, while she is on the upswing— he will actually take energy away from her movement. If his purpose were to bring the swing to a stop, then it would make good sense to push her on the upswing, because this would produce a cycle of smaller amplitude and hence less energy. But if the father’s purpose is to help his daughter keep swinging, then the time to apply energy is at the position of maximum displacement. It so happens that this is also the position at which the swing’s speed is the slowest. Once it reaches maximum displacement, the swing is about to reverse direction, and, therefore, it stops for a split-second. Once it starts moving again, now in a new direction, both kinetic energy and speed increase until the swing passes through the position of stable equilibrium, where it reaches its highest rate. THE

F O U C A U LT

PENDULUM.

Hanging from a ceiling in Washington, D.C.’s Smithsonian Institution is a pendulum 52 ft (15.85 m) long, at the end of which is an iron ball weighing 240 lb (109 kg). Back and forth it swings, and if one sits and watches it long enough, the pendulum appears to move gradually toward the right. Over the course of 24 hours, in fact, it seems to complete a full circuit, moving back to its original orientation.

T I M I N G T H E PU S H . Therefore, the father pushes her—but in order for his push to be effective, he must apply force at just the right

There is just one thing wrong with this picture: though the pendulum is shifting direction, this does not nearly account for the total change in orientation. At the same time the pendulum is moving, Earth is rotating beneath it, and it is the viewer’s frame of reference that creates the mistaken impression that only the pendulum is rotating. In fact it is oscillating, swinging back and forth from the Smithsonian ceiling, but though it shifts orientation somewhat, the greater component of this shift comes from the movement of the Earth itself.

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This particular type of oscillator is known as a Foucault pendulum, after French physicist Jean Bernard Leon Foucault (1819-1868), who in 1851 used just such an instrument to prove that Earth is rotating. Visitors to the Smithsonian, after they get over their initial bewilderment at the fact that the pendulum is not actually rotating, may well have another question: how exactly does the pendulum keep moving? As indicated earlier, in an ideal situation, a pendulum continues oscillating. But situations on Earth are not ideal: with each swing, the Foucault pendulum loses energy, due to friction from the air through which it moves. In addition, the cable suspending it from the ceiling is also oscillating slightly, and this, too, contributes to energy loss. Therefore, it is necessary to add energy to the pendulum’s swing. Surrounding the cable where it attaches to the ceiling is an electromagnet shaped like a donut, and on either side, near the top of the cable, are two iron collars. An electronic device senses when the pendulum reaches maximum amplitude, switching on the electromagnet, which causes the appropriate collar to give the cable a slight jolt. Because the jolt is delivered at the right moment, the resonance is perfect, and energy is restored to the pendulum.

Resonance in Electricity and Electromagnetic Waves Resonance is a factor in electromagnetism, and in electromagnetic waves, such as those of light or radio. Though much about electricity tends to be rather abstract, the idea of current is fairly easy to understand, because it is more or less analogous to a water current: hence, the less impedance to flow, the stronger the current. Minimal impedance is achieved when the impressed voltage has a certain resonant frequency. NUCLEAR MAGNETIC RESON A N C E . The term “nuclear magnetic reso-

nance” (NMR) is hardly a household world, but thanks to its usefulness in medicine, MRI—short for magnetic resonance imagining—is certainly a well-known term. In fact, MRI is simply the medical application of NMR. The latter is a process in which a rotating magnetic field is produced, causing the nuclei of certain atoms to absorb energy from the field. It is used in a range of areas, from making nuclear measurements to

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medical imaging, or MRI. In the NMR process, the nucleus of an atom is forced to wobble like a top, and this speed of wobbling is increased by applying a magnetic force that resonates with the frequency of the wobble. The principles of NMR were first developed in the late 1930s, and by the early 1970s they had been applied to medicine. Thanks to MRI, physicians can make diagnoses without the patient having to undergo either surgery or x rays. When a patient undergoes MRI, he or she is made to lie down inside a large tube-like chamber. A technician then activates a powerful magnetic field that, depending on its position, resonates with the frequencies of specific body tissues. It is thus possible to isolate specific cells and analyze them independently, a process that would be virtually impossible otherwise without employing highly invasive procedures. L I G H T A N D RA D I O WAV E S . One example of resonance involving visible and invisible light in the electromagnetic spectrum is resonance fluorescence. Fluorescence itself is a process whereby a material absorbs electromagnetic radiation from one source, then re-emits that radiation on a wavelength longer than that of the illuminating radiation. Among its many applications are the fluorescent lights found in many homes and public buildings. Sometimes the emitted radiation has the same wavelength as the absorbed radiation, and this is called resonance fluorescence. Resonance fluorescence is used in laboratories for analyzing phenomena such as the flow of gases in a wind tunnel.

Though most people do not realize that radio waves are part of the electromagnetic spectrum, radio itself is certainly a part of daily life, and, here again, resonance plays a part. Radio waves are relatively large compared to visible light waves, and still larger in comparison to higher-frequency waves, such as those in ultraviolet light or x rays. Because the wavelength of a radio signal is as large as objects in ordinary experience, there can sometimes be conflict if the size of an antenna does not match properly with a radio wave. When the sizes are compatible, this, too, is an example of resonance. M I C R O WAV E S . Microwaves occupy a

part of the electromagnetic spectrum with higher frequencies than those of radio waves. Examples of microwaves include television signals, radar—and of course the microwave oven, which

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cooks food without applying external heat. Like many other useful products, the microwave oven ultimately arose from military-industrial research, in this case, during World War II. Introduced for home use in 1955, its popularity grew slowly for the first few decades, but in the 1970s and 1980s, microwave use increased dramatically. Today, most American homes have microwaves ovens. Of course there will always be types of food that cook better in a conventional oven, but the beauty of a microwave is that it makes possible the quick heating and cooking of foods—all without the drying effect of conventional baking. The basis for the microwave oven is the fact that the molecules in all forms of matter are vibrating. By achieving resonant frequency, the oven adds energy—heat—to food. The oven is not equipped in such a way as to detect the frequency of molecular vibration in all possible substances, however; instead, the microwaves resonant with the frequency of a single item found in nearly all types of food: water. Emitted from a small antenna, the microwaves are directed into the cooking compartment of the oven, and, as they enter, they pass a set of turning metal fan blades. This is the stirrer, which disperses the microwaves uniformly over the surface of the food to be heated. As a microwave strikes a water molecule, resonance causes the molecule to align with the direction of the wave. An oscillating magnetron, a tube that generates radio waves, causes the microwaves to oscillate as well, and this, in turn, compels the water molecules to do the same. Thus, the water molecules are shifting in position several million times a second, and this vibration generates energy that heats the water. Microwave ovens do not heat food from the inside out: like a conventional oven, they can only cook from the outside in. But so much energy is transferred to the water molecules that conduction does the rest, ensuring relatively uniform heating of the food. Incidentally, the resonance between microwaves and water molecules explains why many materials used in cooking dishes—materials that do not contain water— can be placed in a microwave oven without being melted or burned. Yet metal, though it also contains no water, is unsafe.

ence of these free electrons means that the microwaves produce electric currents in the surfaces of metal objects placed in the oven. Depending on the shape of the object, these currents can jump, or arc, between points on the surface, thus producing sparks. On the other hand, the interior of the microwave oven itself is in fact metal, and this is so precisely because microwaves do bounce back and forth off of metal. Because the walls are flat and painted, however, currents do not arc between them.

Resonance

Resonance of Sound Waves A highly trained singer can hit a note that causes a wine glass to shatter, but what causes this to happen is not the frequency of the note, per se. In other words, the shattering is not necessarily because of the fact that the note is extremely high; rather, it is due to the phenomenon of resonance. The natural, or resonant, frequency in the wine glass, as with all objects, is determined by its shape and composition. If the singer’s voice (or a note from an instrument) hits the resonant frequency, there will be a transfer of energy, as with the father pushing his daughter on the swing. In this case, however, a full transfer of energy from the voice or musical instrument can overload the glass, causing it to shatter. Another example of resonance and sound waves is feedback, popularized in the 1960s by rock guitarists such as Jimi Hendrix and Pete Townsend of the Who. When a musician strikes a note on an electric guitar string, the string oscillates, and an electromagnetic device in the guitar converts this oscillation into an electrical pulse that it sends to an amplifier. The amplifier passes this oscillation on to the speaker, but if the frequency of the speaker is the same as that of the vibrations in the guitar, the result is feedback.

Metals have free electrons, which makes them good electrical conductors, and the pres-

Both in scientific terms and in the view of a music fan, feedback adds energy. The feedback from the speaker adds energy to the guitar body, which, in turn, increases the energy in the vibration of the guitar strings and, ultimately, the power of the electrical signal is passed on to the amp. The result is increasing volume, and the feedback thus creates a loop that continues to repeat until the volume drowns out all other notes.

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KEY TERMS AMPLITUDE:

The maximum displace-

ment of particles from their normal position during a single period of oscillation. CYCLE:

One full repetition of oscil-

lation. FREQUENCY:

For a particle experi-

encing oscillation, frequency is the number of cycles that take place during one second. Frequency is measured in Hertz. HARMONIC MOTION:

The amount of time required for one cycle in oscillating motion. PERIOD:

Motion that is repeated at regular intervals. These intervals are known as periods.

PERIODIC MOTION:

A wave in which a uniform series of crests and troughs follow one after the other in regular succession. PERIODIC WAVE:

The energy that an object possesses due to its position, as, for instance, with a sled at the top of a hill. This is contrasted with kinetic energy.

POTENTIAL ENERGY:

The repeated

movement of a particle about a position of equilibrium, or balance.

man physicist Heinrich Rudolf Hertz

The condition in which force is applied to an object in oscillation at the point of maximum amplitude.

(1857-1894). Higher frequencies are

RESONANT

HERTZ:

A unit for measuring frequen-

cy, named after nineteenth-century Ger-

expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second) or megahertz (MHz; 106 or 1 million cycles per second.)

RESONANCE:

FREQUENCY: A frequency that matches that of an oscillating object.

which the movement of vibration is in the

A position in which, if an object were disturbed, it would tend to return to its original position. For an object in oscillation, stable equilibrium is in the middle of a cycle, between two points of maximum displacement.

same direction as the wave itself. This is

TRANSVERSE

STABLE EQUILIBRIUM: KINETIC ENERGY:

The energy that

an object possesses due to its motion, as with a sled when sliding down a hill. This is contrasted with potential energy. LONGITUDINAL WAVE:

A wave in

ment is the furthest point from stable equi-

A wave in which the vibration or motion is perpendicular to the direction in which the wave is moving. This is contrasted to a longitudinal wave.

librium.

WAVE MOTION:

contrasted to a transverse wave. MAXIMUM DISPLACEMENT:

For an

object in oscillation, maximum displace-

OSCILLATION:

A type of harmonic

motion, typically periodic, in one or more dimensions.

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WAVE:

A type of harmonic motion that carries energy from one place to another without actually moving any matter.

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How Resonance Can Break a Bridge The power of resonance goes beyond shattering a glass or torturing eardrums with feedback; it can actually destroy large structures. There is an old folk saying that a cat can destroy a bridge if it walks across it in a certain way. This may or may not be true, but it is certainly conceivable that a group of soldiers marching across a bridge can cause it to crumble, even though it is capable of holding much more than their weight, if the rhythm of their synchronized footsteps resonates with the natural frequency of the bridge. For this reason, officers or sergeants typically order their troops to do something very unmilitary—to march out of step—when crossing a bridge. The resonance between vibrations produced by wind and those of the structure itself brought down a powerful bridge in 1940, a highly dramatic illustration of physics in action that was captured on both still photographs and film. Located on Puget Sound near Seattle, Washington, the Tacoma Narrows Bridge was, at 2,800 ft (853 m) in length, the third-longest suspension bridge in the world. But on November 7, 1940, it gave way before winds of 42 mi (68 km) per hour. It was not just the speed of these winds, but the fact that they produced oscillations of resonant frequency, that caused the bridge to twist and, ultimately, to crumble. In those few seconds of battle with the forces of nature, the bridge writhed and buckled until a large segment col-

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lapsed into the waters of Puget Sound. Fortunately, no one was killed, and a new, more stable bridge was later built in place of the one that had come to be known as “Galloping Gertie.” The incident led to increased research and progress in understanding of aerodynamics, harmonic motion, and resonance.

Resonance

WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991. Berger, Melvin. The Science of Music. Illustrated by Yvonne Buchanan. New York: Crowell, 1989. “Bridges and Resonance” (Web site). (April 23, 2001). “Resonance” (Web site). (April 26, 2001). “Resonance” (Web site). (April 23, 2001). “Resonance.” The Physics Classroom (Web site). (April 26, 2001). “Resonance Experiment” (Web site). (April 26, 2001). “Resonance, Frequency, and Wavelength” (Web site). (April 26, 2001). Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996. “Tacoma Narrows Bridge Disaster” (Web site). (April 23, 2001).

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INTERFERENCE Interference

CONCEPT When two or more waves interact and combine, they interfere with one another. But interference is not necessarily bad: waves may interfere constructively, resulting in a wave larger than the original waves. Or, they may interfere destructively, combining in such a way that they form a wave smaller than the original ones. Even so, destructive interference may have positive effects: without the application of destructive interference to the muffler on an automobile exhaust system, for instance, noise pollution from cars would be far worse than it is. Other examples of interference, both constructive and destructive, can be found wherever there are waves: in water, in sound, in light.

HOW IT WORKS Waves Whenever energy ripples through space, there is a wave. In fact, wave motion can be defined as a type of harmonic motion (repeated movement of a particle about a position of equilibrium, or balance) that carries energy from one place to another without actually moving any matter. A wave on the ocean is an example of a mechanical wave, or one that involves matter; but though the matter moves in place, it is only the energy in the wave that experiences net movement.

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individual waves themselves are oscillating even as the overall wave pattern moves. A transverse wave forms a regular up-anddown pattern in which the oscillation is perpendicular to the direction the wave is moving. Ocean waves are transverse, though they also have properties of longitudinal waves. In a longitudinal wave, of which a sound wave is the best example, oscillation occurs in the same direction as the wave itself. PA RA M E T E R S O F WAV E M O T I O N . Some waves, composed of pulses, do

not follow regular patterns. However, the waves of principal concern in the present context are periodic waves, ones in which a uniform series of crests and troughs follow each other in regular succession. Periodic motion is movement repeated at regular intervals called periods. In the case of wave motion, a period (represented by the symbol T) is the amount of time required to complete one full cycle of the wave, from trough to crest and back to trough.

Wave motion is related to oscillation, a type of harmonic motion in one or more dimensions. There is one critical difference, however: oscillation involves no net movement, only movement in place, whereas the harmonic motion of waves carries energy from one place to another. Yet,

Period can be mathematically related to several other aspects of wave motion, including wave speed, frequency, and wavelength. Frequency (abbreviated f) is the number of waves passing through a given point during the interval of one second. It is measured in Hertz (Hz), named after nineteenth-century German physicist Heinrich Rudolf Hertz (1857-1894), and a Hertz is equal to one cycle of oscillation per second. Higher frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second) or megahertz (MHz; 106 or 1 million cycles per second.) Wavelength (represented by the symbol abbreviated λ, the Greek letter lambda) is the distance between a crest and the adjacent crest, or a

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A

PIANO TUNER, USING A TUNING FORK SUCH AS THE ONES SHOWN ABOVE, UTILIZES INTERFERENCE TO TUNE THE

INSTRUMENT. (Bettmann/Corbis. Reproduced by permission.)

trough and an adjacent trough, of a wave. The higher the frequency, the shorter the wavelength. Another parameter for describing wave motion—one that is mathematically independent from the quantities so far described—is amplitude, or the maximum displacement of particles from a position of stable equilibrium. For an ocean wave, amplitude is the distance from either the crest or the trough to the level that the ocean would maintain if it were perfectly still.

Superposition and Interference S U P E R P O S I T I O N . The principle of

superposition holds that when several individual but similar physical events occur in close proximity, the resulting effect is the sum of the magnitude of the separate events. This is akin to the popular expression, “The whole is greater than the sum of the parts,” and it has numerous applications in physics. Where the strength of a gravitational field is being measured, for instance, superposition dictates that the strength of that field at any given point is the sum of the mass of the individual particles in that field. In the realm of electromag-

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netic force, the same statement applies, though the units being added are electrical charges or magnetic poles, rather than quantities of mass. Likewise, in an electrical circuit, the total current or voltage is the sum of the individual currents and voltages in that circuit. Superposition applies only in equations for linear events—that is, phenomena that involve movement along a straight line. Waves are linear phenomena, and, thus, the principle describes the behavior of all waves when they come into contact with one another. If two or more waves enter the same region of space at the same time, then, at any instant, the total disturbance produced by the waves at any point is equal to the sum of the disturbances produced by the individual waves. I N T E R F E R E N C E . The principle of superposition does not require that waves actually combine; rather, the net effect is as though they were combined. The actual combination or joining of two or more waves at a given point in space is called interference, and, as a result, the waves produce a single wave whose properties are determined by the properties of the individual waves.

If two waves of the same wavelength occupy the same space in such a way that their crests and

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Interference

IF

THIS BOAT ’S WAKE WERE TO CROSS THE WAKE OF ANOTHER BOAT, THE RESULT WOULD BE BOTH CONSTRUCTIVE

AND DESTRUCTIVE INTERFERENCE. (Photograph by Roger Ressmeyer/Corbis. Reproduced by permission.)

troughs align, the wave they produce will have an amplitude greater than that possessed by either wave initially. This is known as constructive interference. The more closely the waves are in phase—that is, perfectly aligned—the more constructive the interference. It is also possible that two or more waves can come together such that the trough of one meets the crest of the other, or vice versa. In this case, what happens is destructive interference, and the resulting amplitude is the difference between the values for the individual waves. If the waves are perfectly unaligned—in other words, if the trough of one exactly meets the crest of the other—their amplitudes cancel out, and the result is no wave at all.

Resonance It is easy to confuse interference with resonance, and, therefore, a word should be said about the latter phenomenon. The term resonance describes a situation in which force is applied to an oscillator at the point of maximum amplitude. In this way, the motion of the outside force is perfectly matched to that of the oscillator, making possible a transfer of energy. As with interference, resonance implies alignment

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between two physical entities; however, there are several important differences. Resonance can involve waves, as, for instance, when sound waves resonate with the vibrations of an oscillator, causing a transfer of energy that sometimes produces dramatic results. (See essay on Resonance.) But in these cases, a wave is interacting with an oscillator, not a wave with a wave, as in situations of interference. Furthermore, whereas resonance entails a transfer of energy, interference involves a combination of energy. T RA N S F E R V S . C O M B I N AT I O N .

The importance of this distinction is easy to see if one substitutes money for energy, and people for objects. If one passes on a sum of money to another person, a business, or an institution—as a loan, repayment of a loan, a purchase, or a gift—this is an example of a transfer. On the other hand, when married spouses each earn paychecks, their cash is combined. Transfer thus indicates that the original holder of the cash (or energy) no longer has it. Yet, if the holder of the cash combines funds with those of another, both share rights to an amount of money greater than the amount each originally owned. This is analogous to constructive interference.

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On the other hand, a husband and wife (or any other group of people who pool their cash) also share liabilities, and, thus, a married person may be subject to debt incurred by his or her spouse. If one spouse creates debt so great that the other spouse cannot earn enough to maintain the payments, this painful situation is analogous to destructive interference.

REAL-LIFE A P P L I C AT I O N S Mechanical Waves One of the easiest ways to observe interference is by watching the behavior of mechanical waves. Drop a stone into a still pond, and watch how its waves ripple: this, as with most waveforms in water, is an example of a surface wave, or one that displays aspects of both transverse and longitudinal wave motion. Thus, as the concentric circles of a longitudinal wave ripple outward in one dimension, there are also transverse movements along a plane perpendicular to that of the longitudinal wave. While the first wave is still rippling across the water, drop another stone close to the place where the first one was dropped. Now, there are two surface waves, crests and troughs colliding and interfering. In some places, they will interfere constructively, producing a wave—or rather, a portion of a wave—that is greater in amplitude than either of the original waves. At other places, there will be destructive interference, with some waves so perfectly out of phase that at one instant in time, a given spot on the water may look as though it had not been disturbed at all. One of the interesting aspects of this interaction is the lack of uniformity in the instances of interference. As suggested in the preceding paragraph, it is usually not entire waves, but merely portions of waves, that interfere constructively or destructively. The result is that a seemingly simple event—dropping two stones into a still pond—produces a dazzling array of colliding circles, broken by outwardly undisturbed areas of destructive interference.

when seen from the air, the boat appears to be at the apex of a triangle whose sides are formed by rippling eddies of water.

Interference

Now, another boat passes through the wake of the first, only it is going in the opposite direction and producing its own ever-widening wake as it goes. As the waves from the two boats meet, some are in phase, but, more often than not, they are only partly in phase, or they possess differing wavelengths. Therefore, the waves at least partially cancel out one another in places, and in other places, reinforce one another. The result is an interesting patchwork of patterns when seen from the air.

Sound Waves IN TUNE AND OUT OF TUNE.

The relationships between musical notes can be intriguing, and though tastes in music vary, most people know when music is harmonious and when it is discordant. As discussed in the essay on frequency, this harmony or discord can be equated to the mathematical relationships between the frequencies of specific notes: the lower the numbers involved in the ratio, the more pleasing the sound. The ratio between the frequency of middle C and that of its first harmonic—that is, the C note exactly one octave above it—is a nice, clean 1:2. If one were to play a song in the key of C-which, on a piano, involves only the “white notes” C-D-E-FG-A-B—everything should be perfectly harmonious and (presumably) pleasant to the ear. But what if the piano itself is out of tune? Or what if one key is out of tune with the others? The result, for anyone who is not tone-deaf, produces an overall impression of unpleasantness: it might be a bit hard to identify the source of this discomfort, but it is clear that something is amiss. At best, an out-of-tune piano might sound like something that belonged in a saloon from an old Western; at worst, the sound of notes that do not match their accustomed frequencies can be positively grating. HOW A TUNING FORK WORKS.

A similar phenomenon, though manifested by the interaction of geometric lines rather than concentric circles, occurs when two power boats pass each other on a lake. The first boat chops up the water, creating a wake that widens behind it:

To rectify the situation, a professional piano tuner uses a tuning fork, an instrument that produces a single frequency—say, 264 Hz, which is the frequency of middle C. The piano tuner strikes the tuning fork, and at the same time strikes the appropriate key on the piano. If their frequencies are perfectly aligned, so is the sound

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of both; but, more likely, there will be interference, both constructive and destructive. As time passes—measured in seconds or even fractions of seconds—the sounds of the tuning fork and that of the piano key will alternate between constructive and destructive interference. In the case of constructive interference, their combined sound will become louder than the individual sounds of either; and when the interference is destructive, the sound of both together will be softer than that produced by either the fork or the key. The piano tuner listens for these fluctuations of loudness, which are called beats, and adjusts the tension in the appropriate piano string until the beats disappear completely. As long as there are beats, the piano string and the tuning fork will produce together a frequency that is the average of the two: if, for instance, the out-oftune middle C string vibrates at 262 Hz, the resulting frequency will be 263 Hz.

Clearly, in a music hall, destructive interference is a problem; but there are cases in which it can be a benefit—situations, that is, in which the purpose, indeed, is to deaden the sound. One example is an automobile muffler. A car’s exhaust system makes a great deal of noise, and, thus, if a car does not have a proper muffler, it creates a great deal of noise pollution. A muffler counteracts this by producing a sound wave out of phase with that of the exhaust system; hence, it cancels out most of the noise. Destructive interference can also be used to reduce sound in a room. Once again, a machine is calibrated to generate sound waves that are perfectly out of phase with the offending noise— say, the hum of another machine. The resulting effect conveys the impression that there is no noise in the room, though, in fact, the sound waves are still there; they have merely canceled each other out.

Electromagnetic Waves Another interesting aspect of the interaction between notes is the “difference tone,” created by discord, which the human ear perceives as a third tone. Though E and F are both part of the C scale, when struck together, the sound is highly discordant. In light of what was said above about ratios between frequencies, this dissonance is fitting, as the ratio here involves relatively high numbers—15:16. DIFFERENCE

TONES.

When two notes are struck together, they produce a combination tone, perceived by the human ear as a third tone. If the two notes are harmonious, the “third tone” is known as a summation tone, and is equal to the combined frequencies of the two notes. But if the combination is dissonant, as in the case of E and F, the third tone is known as a difference tone, equal to the difference in frequencies. Since an E note vibrates at 330 Hz, and an F note at 352 Hz, the resulting difference tone is equal to 22 Hz. DESTRUCTIVE INTERFERENCE S O U N D WAV E S . When music is

In fact, Newton was partly right, but Young’s discovery helped advance the view of light as a wave, which is also partly right. (According to quantum theory, developed in the twentieth century, light behaves both as waves and as particles.) The interference in the visible spectrum that Young witnessed was manifested as bright and dark bands. These bands are known as fringes—variations in intensity not unlike the beats created in some instances of sound interference, described above.

played in a concert hall, it reverberates off the walls of the auditorium. Assuming the place is well designed acoustically, these bouncing sound waves will interfere constructively, and the auditorium comes alive with the sound of the music. In other situations, however, the sound waves may interfere destructively, and the result is a certain muffled deadness to the sound.

Many people have noticed the strangely beautiful pattern of colors generated when light interacts with an oily substance, as when light reflected on a soap bubble produces an astonishing array of shades. Sometimes, this can happen in situations not otherwise aesthetically pleasing: an oily film in a parking lot, left there by a car’s leaky

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IN

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In 1801, English physicist Thomas Young (17731829), known for Young’s modulus of elasticity became the first scientist to identify interference in light waves. Challenging the corpuscular theory of light put forward by Sir Isaac Newton (1642-1727), Young set up an experiment in which a beam of light passed through two closely spaced pinholes onto a screen. If light was truly made of particles, he said, the beams would project two distinct points onto the screen. Instead, what he saw was a pattern of interference.

O I LY F I L M S A N D RA I N B O W S .

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Interference

KEY TERMS The maximum displace-

through a given point during the interval

ment of particles in oscillation from a posi-

of one second. The higher the frequency,

tion of stable equilibrium. For an ocean

the shorter the wavelength. Frequency is

wave, amplitude is the distance from either

mathematically related to wave speed and

the crest or the trough to the level that the

period.

AMPLITUDE:

ocean would maintain if it were perfectly still.

HARMONIC MOTION:

The repeated

movement of a particle about a position of

CONSTRUCTIVE

INTERFERENCE:

A type of interference that occurs when

equilibrium, or balance. HERTZ:

A unit for measuring frequen-

two or more waves combine in such a way

cy, named after nineteenth-century Ger-

that they produce a wave whose amplitude

man physicist Heinrich Rudolf Hertz

is greater than that of the original waves. If

(1857-1894).

waves are perfectly in phase—in other words, if the crest and trough of one exactly meets the crest and trough of the

High

separate waves. CYCLE:

are

expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second) or megahertz (MHz; 106 or 1 million cycles per second.)

other—then the resulting amplitude is the sum of the individual amplitudes of the

frequencies

The combination of

INTERFERENCE:

two or more waves at a given point in space to produce a wave whose properties are

In oscillation, a cycle occurs

when the oscillating particle moves from a certain point in a certain direction, then switches direction and moves back to the original point. Typically, this is from the position of stable equilibrium to maximum displacement and back again to the stable equilibrium position. In a wave, a

determined by the properties of the individual waves. This accords with the principle of superposition. LONGITUDINAL WAVE:

A wave in

which the movement of vibration is in the same direction as the wave itself. This is contrasted to a transverse wave. For an

cycle is equivalent to the movement from

MAXIMUM DISPLACEMENT:

trough to crest and back to trough.

object in oscillation, maximum displace-

DESTRUCTIVE INTERFERENCE:

A

type of interference that occurs when two

ment is the furthest point from stable equilibrium. A type of

or more waves combine to produce a wave

MECHANICAL

whose amplitude is less than that of the

wave—for example, a wave on the ocean—

original waves. If waves are perfectly out of

that involves matter. The matter itself may

phase—in other words, if the trough of

move in place, but as with all types of wave

one exactly meets the crest of the other,

motion, there is no net movement of mat-

and vice versa—their amplitudes cancel

ter—only of energy.

out, and the result is no wave at all. FREQUENCY:

In wave motion, fre-

quency is the number of waves passing

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OSCILLATION:

WAVE:

A type of harmonic

motion, typically periodic, in one or more dimensions.

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Interference

KEY TERMS PERIOD:

For wave motion, a period is

CONTINUED

STABLE EQUILIBRIUM:

the amount of time required to complete

in which, if an object were disturbed, it

one full cycle. Period is mathematically

would tend to return to its original posi-

related to frequency, wavelength, and wave

tion. For an object in oscillation, stable

speed.

equilibrium is in the middle of a cycle,

PERIODIC MOTION:

Motion that is

repeated at regular intervals. These inter-

between two points of maximum displacement.

vals are known as periods. PERIODIC WAVE:

A wave in which a

uniform series of crests and troughs follow one after the other in regular succession. PHASE:

When two waves of the same

A wave that exhibits

SURFACE WAVE:

the behavior of both a transverse wave and a longitudinal wave. TRANSVERSE

WAVE:

A wave in

frequency and amplitude are perfectly

which the vibration or motion is perpendi-

aligned, they are said to be in phase.

cular to the direction in which the wave is

PRINCIPLE OF SUPERPOSITION:

moving. This is contrasted to a longitudi-

A physical principle stating that when sev-

nal wave.

eral individual, but similar, physical events occur in close proximity to one another, the resulting effect is the sum of the magnitude of the separate events. Interference is an example of superposition. PULSE:

An isolated, non-periodic dis-

turbance that takes place in wave motion of a type other than that of a periodic wave.

WAVELENGTH:

The distance between

a crest and the adjacent crest, or the trough and an adjacent trough, of a wave. Wavelength, abbreviated λ (the Greek letter lambda) is mathematically related to wave speed, period, and frequency. WAVE MOTION:

A type of harmonic

The condition in which

motion that carries energy from one place

force is applied to an object in oscillation at

to another, without actually moving any

the point of maximum amplitude.

matter.

RESONANCE:

crankcase, can produce a rainbow of colors if the sunlight hits it just right.

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A position

The phenomenon of light-wave interference with oily or filmy surfaces has the effect of filtering light, and, thus, has a number of applications in areas relating to optics: sunglasses, lenses for binoculars or cameras, and even visors for astronauts. In each case, unfiltered light could be harmful or, at least, inconvenient for the user, and the destructive interference eliminates certain colors and unwanted reflections.

This happens because the thickness of the oil causes a delay in reflection of the light beam. Some colors pass through the film, becoming delayed and, thus, getting out of phase with the reflected light on the surface of the film. These shades destructively interfere to such an extent that the waves are cancelled, rendering them invisible. Other colors reflect off the surface so that they are perfectly in phase with the light traveling through the film, and appear as an attractive swirl of color on the surface of the oil.

small part of the electromagnetic spectrum, whose broad range of wave phenomena are, likewise, subject to constructive or destructive inter-

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RA D I O WAV E S . Visible light is only a

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ference. After visible light, the area of the spectrum most people experience during an average day is the realm of relatively low-frequency, longwavelength radio waves and microwaves, the latter including television broadcast signals. People who rely on an antenna for their TV reception are likely to experience interference at some point. However, an increasing number of Americans use either cable or satellite systems to pick up TV programs. These are much less susceptible to interference, due to the technology of coaxial cable, on the one hand, and digital compression, on the other. Thus, interference in television reception is a gradually diminishing problem. Interference among radio signals continues to be a challenge, since most people still hear the radio via old-fashioned means rather than through new technology, such as the Internet. A number of interference problems are created by activity on the Sun, which has an enormously powerful electromagnetic field. Obviously, such interference is beyond the control of most radio listeners, but according to a Web page set up by WHKY Radio in Hickory, North Carolina, there are a number of things listeners can do to decrease interference in their own households. Among the suggestions offered at the WHKY Web site is this: “Nine times out of ten, if your radio is near a computer, it will interfere with your radio. Computers send out all kinds of signals that your radio ‘thinks’ is a real radio signal. Try to locate your radio away from comput-

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ers... especially the monitor.” The Web site listed a number of other household appliances, as well as outside phenomena such as power lines or thunderstorms, that can contribute to radio interference.

Interference

WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991. Bloomfield, Louis A. “How Things Work: Radio.” How Things Work (Web site). (April 27, 2001). Harrison, David. “Sound” (Web site). (April 27, 2001). Interference Handbook/Federal Communications Commission (Web site). (April 27, 2001). Internet Resources for Sound and Light (Web site). (April 25, 2001). “Light—A-to-Z Science.” DiscoverySchool.com (Web site). (April 27, 2001). Oxlade, Chris. Light and Sound. Des Plaines, IL: Heinemann Library, 2000. “Sound Wave—Constructive and Destructive Interference” (Web site). (April 27, 2001). Topp, Patricia. This Strange Quantum World and You. Nevada City, CA: Blue Dolphin, 1999. WHKY Radio and TV (Web site). (April 27, 2001).

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DIFFRACTION Diffraction

CONCEPT Diffraction is the bending of waves around obstacles, or the spreading of waves by passing them through an aperture, or opening. Any type of energy that travels in a wave is capable of diffraction, and the diffraction of sound and light waves produces a number of effects. (Because sound waves are much larger than light waves, however, diffraction of sound is a part of daily life that most people take for granted.) Diffraction of light waves, on the other hand, is much more complicated, and has a number of applications in science and technology, including the use of diffraction gratings in the production of holograms.

HOW IT WORKS Comparing Sound and Light Diffraction Imagine going to a concert hall to hear a band, and to your chagrin, you discover that your seat is directly behind a wide post. You cannot see the band, of course, because the light waves from the stage are blocked. But you have little trouble hearing the music, since sound waves simply diffract around the pillar. Light waves diffract slightly in such a situation, but not enough to make a difference with regard to your enjoyment of the concert: if you looked closely while sitting behind the post, you would be able to observe the diffraction of the light waves glowing slightly, as they widened around the post.

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door and hear the band. The sound quality would be far from perfect, of course, but you would still be able to hear the music well enough. And if you stood right in front of the doorway, you would be able to see light from inside the concert hall. But, if you moved away from the door and stood with your back to the building, you would see little light, whereas the sound would still be easily audible. WAV E L E N GT H A N D D I F F RAC T I O N . The reason for the difference—that is,

why sound diffraction is more pronounced than light diffraction—is that sound waves are much, much larger than light waves. Sound travels by longitudinal waves, or waves in which the movement of vibration is in the same direction as the wave itself. Longitudinal waves radiate outward in concentric circles, rather like the rings of a bull’s-eye. The waves by which sound is transmitted are larger, or comparable in size to, the column or the door—which is an example of an aperture— and, hence, they pass easily through apertures and around obstacles. Light waves, on the other hand, have a wavelength, typically measured in nanometers (nm), which are equal to one-millionth of a millimeter. Wavelengths for visible light range from 400 (violet) to 700 nm (red): hence, it would be possible to fit about 5,000 of even the longest visible-light wavelengths on the head of a pin!

Suppose, now, that you had failed to obtain a ticket, but a friend who worked at the concert venue arranged to let you stand outside an open

Whereas differing wavelengths in light are manifested as differing colors, a change in sound wavelength indicates a change in pitch. The higher the pitch, the greater the frequency, and, hence, the shorter the wavelength. As with light waves—though, of course, to a much lesser

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Diffraction

HOLOGRAMS

ARE MADE POSSIBLE THROUGH THE PRINCIPLE OF DIFFRACTION.

SPACE SHUTTLE ORBITING

SHOWN HERE EARTH. (Photograph by Roger Ressmeyer/Corbis. Reproduced by permission.)

extent—short-wavelength sound waves are less capable of diffracting around large objects than are long-wavelength sound waves. Chances are, then, that the most easily audible sounds from inside the concert hall are the bass and drums; higher-pitched notes from a guitar or other instruments, such as a Hammond organ, are not as likely to reach a listener outside.

IS A HOLOGRAM OF A

have already seen that wavelength plays a role in diffraction; so, too, does the size of the aperture relative to the wavelength. Hence, most studies of diffraction in light involve very small openings, as, for instance, in the diffraction grating discussed below.

Due to the much wider range of areas in which light diffraction has been applied by scientists, diffraction of light and not sound will be the principal topic for the remainder of this essay. We

But light does not only diffract when passing through an aperture, such as the concert-hall door in the earlier illustration; it also diffracts around obstacles, as, for instance, the post or pillar mentioned earlier. This can be observed by looking closely at the shadow of a flagpole on a bright morning. At first, it appears that the shadow is “solid,” but if one looks closely enough, it becomes clear that, at the edges, there is a blur-

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Observing Diffraction in Light

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Diffraction

THE

CHECKOUT SCANNERS IN GROCERY STORES USE HOLOGRAPHIC TECHNOLOGY THAT CAN READ A UNIVERSAL

PRODUCT CODE

(UPC)

FROM ANY ANGLE. (Photograph by Bob Rowan; Progressive Image/Corbis. Reproduced by permission.)

ring from darkness to light. This “gray area” is an example of light diffraction. Where the aperture or obstruction is large compared to the wave passing through or around it, there is only a little “fuzziness” at the edge, as in the case of the flagpole. When light passes through an aperture, most of the beam goes straight through without disturbance, with only the edges experiencing diffraction. If, however, the size of the aperture is close to that of the wavelength, the diffraction pattern will widen. Sound waves diffract at large angles through an open door, which, as noted, is comparable in size to a sound wave; similarly, when light is passed through extremely narrow openings, its diffraction is more noticeable.

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Newton also became embroiled in a debate as the nature of light itself—a debate in which diffraction studies played an important role. Newton’s view, known at the time as the corpuscular theory of light, was that light travels as a stream of particles. Yet, his contemporary, Dutch physicist and astronomer Christiaan Huygens (1629-1695), advanced the wave theory, or the idea that light travels by means of waves. Huygens maintained that a number of factors, including the phenomena of reflection and refraction, indicate that light is a wave. Newton, on the other hand, challenged wave theorists by stating that if light were actually a wave, it should be able to bend around corners—in other words, to diffract.

Early Studies in Diffraction

GRIMALDI IDENTIFIES DIFF RA C T I O N . Though it did not become

Though his greatest contributions lay in his epochal studies of gravitation and motion, Sir Isaac Newton (1642-1727) also studied the production and propagation of light. Using a prism, he separated the colors of the visible light spectrum—something that had already been done by other scientists—but it was Newton who discerned that the colors of the spectrum could be recombined to form white light again.

widely known until some time later, in 1648— more than a decade before the particle-wave controversy erupted—Johannes Marcus von Kronland (1595-1667), a scientist in Bohemia (now part of the Czech Republic), discovered the diffraction of light waves. However, his findings were not recognized until some time later; nor did he give a name to the phenomenon he had observed. Then, in 1660, Italian physicist Francesco Grimaldi (1618-1663) conducted an

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experiment with diffraction that gained widespread attention. Grimaldi allowed a beam of light to pass through two narrow apertures, one behind the other, and then onto a blank surface. When he did so, he observed that the band of light hitting the surface was slightly wider than it should be, based on the width of the ray that entered the first aperture. He concluded that the beam had been bent slightly outward, and gave this phenomenon the name by which it is known today: diffraction. F R E S N E L A N D F RAU N H O F E R D I F F RAC T I O N . Particle theory continued

to have its adherents in England, Newton’s homeland, but by the time of French physicist Augustin Jean Fresnel (1788-1827), an increasing number of scientists on the European continent had come to accept the wave theory. Fresnel’s work, which he published in 1818, served to advance that theory, and, in particular, the idea of light as a transverse wave. In Memoire sur la diffraction de la lumiere, Fresnel showed that the transverse-wave model accounted for a number of phenomena, including diffraction, reflection, refraction, interference, and polarization, or a change in the oscillation patterns of a light wave. Four years after publishing this important work, Fresnel put his ideas into action, using the transverse model to create a pencil-beam of light that was ideal for lighthouses. This prism system, whereby all the light emitted from a source is refracted into a horizontal beam, replaced the older method of mirrors used since ancient times. Thus Fresnel’s work revolutionized the effectiveness of lighthouses, and helped save lives of countless sailors at sea. The term “Fresnel diffraction” refers to a situation in which the light source or the screen are close to the aperture; but there are situations in which source, aperture, and screen (or at least two of the three) are widely separated. This is known as Fraunhofer diffraction, after German physicist Joseph von Fraunhofer (1787-1826), who in 1814 discovered the lines of the solar spectrum (source) while using a prism (aperture). His work had an enormous impact in the area of spectroscopy, or studies of the interaction between electromagnetic radiation and matter.

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Diffraction

Diffraction Studies Come of Age Eventually the work of Scottish physicist James Clerk Maxwell (1831-1879), German physicist Heinrich Rudolf Hertz (1857-1894), and others confirmed that light did indeed travel in waves. Later, however, Albert Einstein (1879-1955) showed that light behaves both as a wave and, in certain circumstances, as a particle. In 1912, a few years after Einstein published his findings, German physicist Max Theodor Felix von Laue (1879-1960) created a diffraction grating, discussed below. Using crystals in his grating, he proved that x rays are part of the electromagnetic spectrum. Laue’s work, which earned him the Nobel Prize in physics in 1914, also made it possible to measure the length of x rays, and, ultimately, provided a means for studying the atomic structure of crystals and polymers. S C I E N T I F I C B R E A KT H R O U G H S M A D E P O S S I B L E B Y D I F F RA C T I O N S T U D I E S . Studies in diffraction

advanced during the early twentieth century. In 1926, English physicist J. D. Bernal (1901-1971) developed the Bernal chart, enabling scientists to deduce the crystal structure of a solid by analyzing photographs of x-ray diffraction patterns. A decade later, Dutch-American physical chemist Peter Joseph William Debye (1884-1966) won the Nobel Prize in Chemistry for his studies in the diffraction of x rays and electrons in gases, which advanced understanding of molecular structure. In 1937, a year after Debye’s Nobel, two other scientists—American physicist Clinton Joseph Davisson (1881-1958) and English physicist George Paget Thomson (1892-1975)—won the Prize in Physics for their discovery that crystals can bring about the diffraction of electrons. Also, in 1937, English physicist William Thomas Astbury (1898-1961) used x-ray diffraction to discover the first information concerning nucleic acid, which led to advances in the study of DNA (deoxyribonucleic acid), the buildingblocks of human genetics. In 1952, English biophysicist Maurice Hugh Frederick Wilkins (1916-) and molecular biologist Rosalind Elsie Franklin (1920-1958) used x-ray diffraction to photograph DNA. Their work directly influenced

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a breakthrough event that followed a year later: the discovery of the double-helix or double-spiral model of DNA by American molecular biologists James D. Watson (1928-) and Francis Crick (1916-). Today, studies in DNA are at the frontiers of research in biology and related fields.

Diffraction Grating Much of the work described in the preceding paragraphs made use of a diffraction grating, first developed in the 1870s by American physicist Henry Augustus Rowland (1848-1901). A diffraction grating is an optical device that consists of not one but many thousands of apertures: Rowland’s machine used a fine diamond point to rule glass gratings, with about 15,000 lines per in (2.2 cm). Diffraction gratings today can have as many as 100,000 apertures per inch. The apertures in a diffraction grating are not mere holes, but extremely narrow parallel slits that transform a beam of light into a spectrum. Each of these openings diffracts the light beam, but because they are evenly spaced and the same in width, the diffracted waves experience constructive interference. (The latter phenomenon, which describes a situation in which two or more waves combine to produce a wave of greater magnitude than either, is discussed in the essay on Interference.) This constructive interference pattern makes it possible to view components of the spectrum separately, thus enabling a scientist to observe characteristics ranging from the structure of atoms and molecules to the chemical composition of stars. X - R AY D I F F R A C T I O N . Because they are much higher in frequency and energy levels, x rays are even shorter in wavelength than visible light waves. Hence, for x-ray diffraction, it is necessary to have gratings in which lines are separated by infinitesimal distances. These distances are typically measured in units called an angstrom, of which there are 10 million to a millimeter. Angstroms are used in measuring atoms, and, indeed, the spaces between lines in an x-ray diffraction grating are comparable to the size of atoms.

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between atoms by studying x-ray diffraction patterns. Bragg’s law—named after the father-andson team of English physicists William Henry Bragg (1862-1942) and William Lawrence Bragg (1890-1971)—describes x-ray diffraction patterns in crystals. Though much about x-ray diffraction and crystallography seems rather abstract, its application in areas such as DNA research indicates that it has numerous applications for improving human life. The elder Bragg expressed this fact in 1915, the year he and his son received the Nobel Prize in physics, saying that “We are now able to look ten thousand times deeper into the structure of the matter that makes up our universe than when we had to depend on the microscope alone.” Today, physicists applying x-ray diffraction use an instrument called a diffractometer, which helps them compare diffraction patterns with those of known crystals, as a means of determining the structure of new materials.

Holograms A hologram—a word derived from the Greek holos, “whole,” and gram, “message”—is a threedimensional (3-D) impression of an object, and the method of producing these images is known as holography. Holograms make use of laser beams that mix at an angle, producing an interference pattern of alternating bright and dark lines. The surface of the hologram itself is a sort of diffraction grating, with alternating strips of clear and opaque material. By mixing a laser beam and the unfocused diffraction pattern of an object, an image can be recorded. An illuminating laser beam is diffracted at specific angles, in accordance with Bragg’s law, on the surfaces of the hologram, making it possible for an observer to see a three-dimensional image.

When x rays irradiate a crystal—in other words, when the crystal absorbs radiation in the form of x rays—atoms in the crystal diffract the rays. One of the characteristics of a crystal is that its atoms are equally spaced, and, because of this, it is possible to discover the location and distance

Holograms are not to be confused with ordinary three-dimensional images that use only visible light. The latter are produced by a method known as stereoscopy, which creates a single image from two, superimposing the images to create the impression of a picture with depth. Though stereoscopic images make it seem as though one can “step into” the picture, a hologram actually enables the viewer to glimpse the image from any angle. Thus, stereoscopic images can be compared to looking through the plateglass window of a store display, whereas holo-

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Diffraction

KEY TERMS APERTURE:

An opening.

RADIATION:

In a general sense, radia-

The bending of waves

tion can refer to anything that travels in a

around obstacles, or the spreading of waves

stream, whether that stream be composed

by passing them through an aperture.

of subatomic particles or electromagnetic

DIFFRACTION:

waves. ELECTROMAGNETIC

SPECTRUM:

A phenomenon where-

The complete range of electromagnetic

REFLECTION:

waves on a continuous distribution from a

by a light ray is returned toward its source

very low range of frequencies and energy

rather than being absorbed at the interface.

levels, with a correspondingly long wavelength, to a very high range of frequencies and energy levels, with a correspondingly short wavelength. Included on the electro-

REFRACTION:

The bending of a light

ray that occurs when it passes through a dense medium, such as water or glass.

magnetic spectrum are long-wave and

SPECTRUM:

short-wave radio; microwaves; infrared,

tion of properties in an ordered arrange-

visible, and ultraviolet light; x rays, and

ment across an unbroken range. Examples

gamma rays.

of spectra (the plural of “spectrum”)

The continuous distribu-

The number of waves

include the colors of visible light, or the

passing through a given point during the

electromagnetic spectrum of which visible

interval of one second. The higher the fre-

light is a part.

FREQUENCY:

quency, the shorter the wavelength.

TRANSVERSE

WAVE:

A wave in

A wave in

which the vibration or motion is perpendi-

which the movement of vibration is in the

cular to the direction in which the wave is

same direction as the wave itself. A sound

moving.

LONGITUDINAL WAVE:

wave is an example of a longitudinal wave. WAVELENGTH: PRISM:

A three-dimensional glass

shape used for the diffusion of light rays. PROPAGATION:

The act or state of

traveling from one place to another.

grams convey the sensation that one has actually stepped into the store window itself. DEVELOPMENTS IN HOLOGRA P H Y. While attempting to improve the res-

The distance between

a crest and the adjacent crest, or the trough and an adjacent trough, of a wave. The shorter the wavelength, the higher the frequency.

lasers to create 3-D images, and in 1971, Gabor received the Nobel Prize in physics for the discovery he had made a generation before.

olution of electron microscopes in 1947, Hungarian-English physicist and engineer Dennis Gabor (1900-1979) developed the concept of holography and coined the term “hologram.” His work in this area could not progress by a great measure, however, until the creation of the laser in 1960. By the early 1960s, scientists were using

Today, holograms are used on credit cards or other identification cards as a security measure, providing an image that can be read by an optical scanner. Supermarket checkout scanners use holographic optical elements (HOEs), which can read a universal product code (UPC) from any angle. Use of holograms in daily life and scientific research is likely to increase as scientists find

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new applications: for instance, holographic images will aid the design of everything from bridges to automobiles. H O LO G RA P H I C M E M O RY. One of the most fascinating areas of research in the field of holography is holographic memory. Computers use a binary code, a pattern of ones and zeroes that is translated into an electronic pulse, but holographic memory would greatly extend the capabilities of computer memory systems. Unlike most images, a hologram is not simply the sum of its constituent parts: the data in a holographic image is contained in every part of the image, meaning that part of the image can be destroyed without a loss of data.

To bring the story full-circle, holographic memory calls to mind an idea advanced by a scientist who, along with Huygens, was one of Newton’s great professional rivals, German mathematician and philosopher Gottfried Wilhelm Leibniz (1646-1716). Though Newton is usually credited as the father of calculus, Leibniz developed his own version of calculus at around the same time. As a philosopher, Leibniz had apparently had a number of strange ideas, which made him the butt of jokes among some sectors of European intellectual society: hence, the French writer and thinker Voltaire (François-Marie Arouet; 1694-1778) satirized him with the character Dr. Pangloss in Candide (1759). Few of Leibniz’s ideas were more bizarre than that of the monad: an elementary particle of existence that reflected the whole of the universe. In advancing the concept of a monad, Leibniz was not making a statement after the manner of a scientist: there was no proof that monads existed, nor was it possible to prove this in any

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scientific way. Yet, a hologram appears to be very much like a manifestation of Leibniz’s imagined monads, and both the hologram and the monad relate to a more fundamental aspect of life: human memory. Neurological research in the late twentieth century suggested that the structure of memory in the human mind is holographic. Thus, for instance, a patient suffering an injury affecting 90% of the brain experiences only a 10% memory loss. WHERE TO LEARN MORE Barrett, Norman S. Lasers and Holograms. New York: F. Watts, 1985. “Bragg’s Law and Diffraction: How Waves Reveal the Atomic Structure of Crystals” (Web site). (May 6, 2001). Burkig, Valerie. Photonics: The New Science of Light. Hillside, N.J.: Enslow Publishers, 1986. “Diffraction of Sound” (Web site). (May 6, 2001). Gardner, Robert. Experimenting with Light. New York: F. Watts, 1991. Graham, Ian. Lasers and Holograms. New York: Shooting Star Press, 1993. Holoworld: Holography, Lasers, and Holograms (Web site). (May 6, 2001). Proffen, T. H. and R. B. Neder. Interactive Tutorial About Diffraction (Web site). (May 6, 2001). Snedden, Robert. Light and Sound. Des Plaines, IL: Heinemann Library, 1999. “Wave-Like Behaviors of Light.” The Physics Classroom (Web site). (May 6, 2001).

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DOPPLER EFFECT

CONCEPT Almost everyone has experienced the Doppler effect, though perhaps without knowing what causes it. For example, if one is standing on a street corner and an ambulance approaches with its siren blaring, the sound of the siren steadily gains in pitch as it comes closer. Then, as it passes, the pitch suddenly lowers perceptibly. This is an example of the Doppler effect: the change in the observed frequency of a wave when the source of the wave is moving with respect to the observer. The Doppler effect, which occurs both in sound and electromagnetic waves—including light waves—has a number of applications. Astronomers use it, for instance, to gauge the movement of stars relative to Earth. Closer to home, principles relating to the Doppler effect find application in radar technology. Doppler radar provides information concerning weather patterns, but some people experience it in a less pleasant way: when a police officer uses it to measure their driving speed before writing a ticket.

HOW IT WORKS Wave Motion and Its Properties

medium are oscillating even as the overall wave pattern moves. The term periodic motion, or movement repeated at regular intervals called periods, describes the behavior of periodic waves—waves in which a uniform series of crests and troughs follow each other in regular succession. A period (represented by the symbol T) is the amount of time required to complete one full cycle of the wave, from trough to crest and back to trough. Period is mathematically related to several other aspects of wave motion, including wave speed, frequency, and wavelength. Frequency (abbreviated f) is the number of waves passing through a given point during the interval of one second. It is measured in Hertz (Hz), named after nineteenth-century German physicist Heinrich Rudolf Hertz (1857-1894), and a Hertz is equal to one cycle of oscillation per second. Higher frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second); megahertz (MHz; 106 or 1 million cycles per second); and gigahertz (GHz; 109 or 1 billion cycles per second.) Wavelength (represented by the symbol λ, the Greek letter lambda) is the distance between a crest and the adjacent crest, or a trough and an adjacent trough, of a wave. The higher the frequency, the shorter the wavelength.

Sound and light are both examples of energy, and both are carried on waves. Wave motion is a type of harmonic motion that carries energy from one place to another without actually moving any matter. It is related to oscillation, a type of harmonic motion in one or more dimensions. Oscillation involves no net movement, only movement in place; yet individual points in the wave

Amplitude, though mathematically independent from the parameters discussed, is critical to the understanding of sound. Defined as the maximum displacement of a vibrating material, amplitude is the “size” of a wave. The greater the amplitude, the greater the energy the wave contains: amplitude indicates intensity, which, in the case of sound waves, is manifested as what people commonly call “volume.” Similarly, the

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THE DOPPLER

EFFECT.

amplitude of a light wave determines the intensity of the light.

Frame of Reference A knowledge of the fundamentals involved in wave motion is critical to understanding the Doppler effect; so, too, is an appreciation of another phenomenon, which is as much related to human psychology and perception as it is to physics. Frame of reference is the perspective of an observer with regard to an object or event. Things may look different for one person in one frame of reference than they do to someone in another. For example, if you are sitting across the table from a friend at lunch, and you see that he has a spot of ketchup to the right of his mouth, the tendency is to say, “You have some ketchup right here”—and then point to the left of your own mouth, since you are directly across the table from his right. Then he will rub the left side of his face with his napkin, missing the spot entirely, unless you say something like, “No— mirror image.” The problem is that each of you has a different frame of reference, yet only your friend took this into account.

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object in relation to another. For instance, the molecules in the human body are in a constant state of motion, but they are not moving relative to the body itself: they are moving relative to one another. On a larger scale, Earth is rotating at a rate of about 1,000 MPH (1,600 km/h), and orbiting the Sun at 67,000 MPH (107,826 km/h)—almost three times as fast as humans have ever traveled in a powered vehicle. Yet no one senses the speed of Earth’s movement in the way that one senses the movement of a car—or, indeed, the way the astronauts aboard Apollo 11 in 1969 perceived that their spacecraft was moving at about 25,000 MPH (40,000 km/h). In the case of the car or the spacecraft, movement can be perceived in relation to other objects: road signs and buildings on the one hand, Earth and the Moon on the other. But humans have no frame of reference from which to perceive the movement of Earth itself.

R E LAT I V E M O T I O N . Physicists often speak of relative motion, or the motion of one

If one were traveling in a train alongside another train at constant velocity, it would be impossible to perceive that either train was actually moving, unless one looked at a reference point, such as the trees or mountains in the background. Likewise, if two trains were sitting side by side, and one train started to move, the relative motion might cause a passenger in the unmoving train to believe that his or her train

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DOPPLER

RADAR, LIKE THE WIND SPEED RADAR SHOWN HERE, HAVE BECOME A FUNDAMENTAL TOOL FOR METE-

OROLOGISTS. (Photograph by Roger Ressmeyer/Corbis. Reproduced by permission.)

was the one moving. In fact, as Albert Einstein (1879-1955) demonstrated with his Theory of Relativity, all motion is relative: when we say that something is moving, we mean that it is moving in relation to something else.

common, but previously unexplained, phenomenon. When an observer is standing beside a railroad track and a train approaches, Doppler noticed, the train’s whistle has a high pitch. As it passes by, however, the sound of the train whistle suddenly becomes much lower.

Doppler’s Discovery Long before Einstein was born, Austrian physicist Christian Johann Doppler (1803-1853) made an important discovery regarding the relative motion of sound waves or light waves. While teaching in Prague, now the capital of the Czech Republic, but then a part of the Austro-Hungarian Empire, Doppler became fascinated with a

By Doppler’s time, physicists had recognized the existence of sound waves, as well as the fact a sound’s pitch is a function of frequency—in other words, the closer the waves are to one another, the higher the pitch. Taking this knowledge, he reasoned that if a source of sound is moving toward a listener, the waves in front of the source are compressed, thus creating a higher frequency. On the other hand, the waves behind

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the moving source are stretched out, resulting in a lower frequency. After developing a mathematical formula to describe this effect, Doppler presented his findings in 1842. Three years later, he and Dutch meteorologist Christopher Heinrich Buys-Ballot (1817-1890) conducted a highly unusual experiment to demonstrate the theory. Buys-Ballot arranged for a band of trumpet players to perform on an open railroad flatcar, while riding past a platform on which a group of musicians with perfect pitch (that is, a finely tuned sense of hearing) sat listening. The experiment went on for two days, the flatcar passing by again and again, while the horns blasted and the musicians on the platform recorded their observations. Though Doppler and Buys-Ballot must have seemed like crazy men to those who were not involved in the experiment, the result—as interpreted from the musicians’ written impressions of the pitches they heard—confirmed Doppler’s theory.

REAL-LIFE A P P L I C AT I O N S Sound Compression and the Doppler Effect As stated in the introduction, one can observe the Doppler effect in a number of settings. If a person is standing by the side of a road and a car approaches at a significant rate of speed, the frequency of the sound waves grows until the car passes the observer, then the frequency suddenly drops. But Doppler, of course, never heard the sound of an automobile, or the siren of a motorized ambulance or fire truck. In his day, the horse-drawn carriage still constituted the principal means of transportation for short distances, and such vehicles did not attain the speeds necessary for the Doppler effect to become noticeable. Only one mode of transportation in the mid-nineteenth century made it possible to observe and record the effect: a steam-powered locomotive. Therefore, let us consider the Doppler effect as Doppler himself did—in terms of a train passing through a s tation.

As the train begins to move, however, it is no longer at the center of the sound waves emanating from it. Instead, the circle of waves is moving forward, along with the train itself, and, thus, the locomotive compresses waves toward the front. If someone is standing further ahead along the track, that person hears the compressed sound waves. Due to their compression, these have a much higher frequency than the waves produced by a stationary train. At the same time, someone standing behind the train—a listener on the platform at the station, watching the train recede into the distance—hears the sound waves that emanate from behind the train. It is the same train making the same sound, but because the train has compressed the sound waves in front of it, the waves behind it are spread out, producing a sound of much lower frequency. Thus, the sound of the train, as perceived by two different listeners, varies with frame of reference. T H E S O N I C B O O M : A R E LAT E D E F F E C T. Some people today have had

the experience of hearing a jet fly high overhead, producing a shock wave known as a sonic boom. A sonic boom, needless to say, is certainly not something of which Doppler would have had any knowledge, nor is it an illustration of the Doppler effect, per se. But it is an example of sound compression, and, therefore, it deserves attention here.

leaving, it blows its whistle, but listeners standing

The speed of sound, unlike the speed of light, is dependant on the medium through which it travels. Hence, there is no such thing as a fixed “speed of sound”; rather, there is only a speed at which sound waves are transmitted through a given type of material. Its speed through a gas, such as air, is proportional to the square root of the pressure divided by the density. This, in turn, means that the higher the altitude, the slower the speed of sound: for the altitudes at which jets fly, it is about 660 MPH (1,622 km/h).

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THE SOUND OF A TRAIN WHISTLE. When a train is sitting in a station prior to

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nearby notice nothing unusual. There is no difference—except perhaps in degree of intensity— between the sound heard by someone on the platform, and the sound of the train as heard by someone standing behind the caboose. This is because a stationary train is at the center of the sound waves it produces, which radiate in concentric circles (like a bulls-eye) around it.

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As a jet moves through the air, it too produces sound waves which compress toward the front, and widen toward the rear. Since sound waves themselves are really just fluctuations in pressure, this means that the faster a jet goes, the greater the pressure of the sound waves bunched up in front of it. Jet pilots speak of “breaking the sound barrier,” which is more than just a figure of speech. As the craft approaches the speed of sound, the pilot becomes aware of a wall of high pressure to the front of the plane, and as a result of this high-pressure wall, the jet experiences enormous turbulence. The speed of sound is referred to as Mach 1, and at a speed of between Mach 1.2 and Mach 1.4, even stranger things begin to happen. Now the jet is moving faster than the sound waves emanating from it, and, therefore, an observer on the ground sees the jet move by well before hearing the sound. Of course, this would happen to some extent anyway, since light travels so much faster than sound; but the difference between the arrival time of the light waves and the sound waves is even more noticeable in this situation. Meanwhile, up in the air, every protruding surface of the aircraft experiences intense pressure: in particular, sound waves tend to become highly compressed along the aircraft’s nose and tail. Eventually these compressed sound waves build up, resulting in a shock wave. Down on the ground, the shock wave manifests as a “sonic boom”—or rather, two sonic booms—one from the nose of the craft, and one from the tail. People in the aircraft do not hear the boom, but the shock waves produced by the compressed sound can cause sudden changes in pressure, density, and temperature that can pose dangers to the operation of the airplane. To overcome this problem, designers of supersonic aircraft have developed planes with wings that are swept back, so they fit within the cone of pressure.

Doppler Radar and Other Sensing Technology The Doppler effect has a number of applications relating to the sensing of movement. For instance, physicians and medical technicians apply it to measure the rate and direction of blood flow in a patient’s body, along with ultrasound. As blood moves through an artery, its top speed is 0.89 MPH (0.4 m/s)—not very fast, yet fast enough, given the small area in which move-

S C I E N C E O F E V E RY DAY T H I N G S

ment is taking place, for the Doppler effect to be observed. A beam of ultrasound is pointed toward an artery, and the reflected waves exhibit a shift in frequency, because the blood cells are acting as moving sources of sound waves—just like the trains Doppler observed.

Doppler Effect

Not all applications of the Doppler effect fall under the heading of “technology”: some can be found in nature. Bats use the Doppler effect to hunt for prey. As a bat flies, it navigates by emitting whistles and listening for the echoes. When it is chasing down food, its brain detects a change in pitch between the emitted whistle, and the echo it receives. This tells the bat the speed of its quarry, and the bat adjusts its own speed accordingly. D O P P L E R RA DA R. Police officers may not enjoy the comparison—given the public’s general impression of bats as evil, bloodthirsty creatures—but in using radar as a basis to check for speeding violations, the police are applying a principle similar to that used by bats. Doppler radar, which uses the Doppler effect to calculate the speed of moving objects, is a form of technology used not only by law-enforcement officers, but also by meteorologists.

The change in frequency experienced as a result of the Doppler effect is exactly twice the ratio between the velocity of the target (for instance, a speeding car) and the speed with which the radar pulse is directed toward the target. From this formula, it is possible to determine the velocity of the target when the frequency change and speed of radar propagation are known. The police officer’s Doppler radar performs these calculations; then all the officer has to do is pull over the speeder and write a ticket. Meteorologists use Doppler radar to track the movement of storm systems. By detecting the direction and velocity of raindrops or hail, for instance, Doppler radar can be used to determine the motion of winds and, thus, to predict weather patterns that will follow in the next minutes or hours. But Doppler radar can do more than simply detect a storm in progress: Doppler technology also aids meteorologists by interpreting wind direction, as an indicator of coming storms.

The Doppler Effect in Light Waves So far the Doppler effect has been discussed purely in terms of sound waves; but Doppler

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Doppler Effect

KEY TERMS The maximum displace-

expressed in terms of kilohertz (kHz; 103

ment of a vibrating material. In wave

or 1,000 cycles per second); megahertz

motion, amplitude is the “size” of a wave,

(MHz; 106 or 1 million cycles per second);

an indicator of the energy and intensity of

and gigahertz (GHz; 109 or 1 billion cycles

the wave.

per second.)

AMPLITUDE:

CYCLE:

One complete oscillation. In

INTENSITY:

Intensity is the rate at

wave motion, this is equivalent to the

which a wave moves energy per unit of

movement of a wave from trough to crest

cross-sectional area. Where sound waves

and back to trough. For a sound wave,

are concerned, intensity is commonly

in particular, a cycle is one complete

known as “volume.”

vibration.

A type of harmonic

OSCILLATION:

The change in

DOPPLER EFFECT:

the observed frequency of a wave when the

motion, typically periodic, in one or more dimensions.

source of the wave is moving with respect to the observer. It is named after Austrian physicist Johann Christian Doppler (18031853), who discovered it.

PERIOD:

For wave motion, a period is

the amount of time required to complete one full cycle. Period is mathematically related to frequency, wavelength, and wave

FRAME OF REFERENCE:

The per-

speed.

spective an observer has with regard to an object or action. Frame of reference affects perception of various physical properties, and plays a significant role in the Doppler effect. In wave motion, fre-

quency is the number of waves passing

Motion that is

repeated at regular intervals. These intervals are known as periods. A wave in which a

PERIODIC WAVE:

FREQUENCY:

uniform series of crests and troughs follow one after the other in regular succession. The motion of

through a given point during the interval

RELATIVE MOTION:

of one second. The higher the frequency,

one object in relation to another.

the shorter the wavelength. Measured in

WAVELENGTH:

Hertz, frequency is mathematically related

a crest and the adjacent crest, or the trough

to wave speed, wavelength, and period.

and an adjacent trough, of a wave. Wave-

The distance between

The repeated

length, symbolized λ (the Greek letter

movement of a particle about a position of

lambda) is mathematically related to wave

equilibrium, or balance.

speed, period, and frequency.

HARMONIC MOTION:

HERTZ:

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PERIODIC MOTION:

A unit for measuring fre-

WAVE MOTION:

A type of harmonic

quency, named after nineteenth-century

motion that carries energy from one place

German physicist Heinrich Rudolf Hertz

to another without actually moving any

(1857-1894).

matter.

High

frequencies

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himself maintained that it could be applied to light waves as well, and experimentation conducted in 1901 proved him correct. This was far from an obvious point, since light is quite different from sound. Not only does light travel much, much faster—186,000 mi (299,339 km) a second—but unlike sound, light does not need to travel through a medium. Whereas sound cannot be transmitted in outer space, light is transmitted by radiation, a form of energy transfer that can be directed as easily through a vacuum as through matter. The Doppler effect in light can be demonstrated by using a device called a spectroscope, which measures the spectral lines from an object of known chemical composition. These spectral lines are produced either by the absorption or emission of specific frequencies of light by electrons in the source material. If the light waves appear at the blue, or high-frequency end of the visible light spectrum, this means that the object is moving toward the observer. If, on the other hand, the light waves appear at the red, or lowfrequency end of the spectrum, the object is moving away. H U B B L E A N D T H E R E D S H I F T.

In 1923, American astronomer Edwin Hubble (1889-1953) observed that the light waves from distant galaxies were shifted so much to the red end of the light spectrum that they must be moving away from the Milky Way, the galaxy in which Earth is located, at a high rate. At the same time, nearer galaxies experienced much less of a red shift, as this phenomenon came to be known, meaning that they were moving away at relatively slower speeds. Six years later, Hubble and another astronomer, Milton Humason, developed a mathemat-

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ical formula whereby astronomers could determine the distance to another galaxy by measuring that galaxy’s red shifts. The formula came to be known as Hubble’s constant, and it established the relationship between red shift and the velocity at which a galaxy or object was receding from Earth. From Hubble’s work, it became clear that the universe was expanding, and research by a number of physicists and astronomers led to the development of the “big bang” theory—the idea that the universe emerged almost instantaneously, in some sort of explosion, from a compressed state of matter.

Doppler Effect

WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991. Bryant-Mole, Karen. Sound and Light. Crystal Lake, IL: Rigby Interactive Library, 1997. Challoner, Jack. Sound and Light. New York: Kingfisher, 2001. Dispenzio, Michael A. Awesome Experiments in Light and Sound. Illustrated by Catherine Leary. New York: Sterling Publishing Company, 1999. “The Doppler Effect.” The Physics Classroom (Web site). (April 29, 2001). Maton, Anthea. Exploring Physical Science. Upper Saddle River, N.J.: Prentice Hall, 1997. Russell, David A. “The Doppler Effect and Sonic Booms” Kettering University (Web site). (April 29, 2001). Snedden, Robert. Light and Sound. Des Plaines, IL: Heinemann Library, 1999. “Sound Wave—Doppler Effect” (Web site). (April 29, 2001). “Wave Motion—Doppler Effect” (Web site). (April 29, 2001).

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S C I E N C E O F E V E RY DAY T H I N G S real-life Physics

SOUND ACOUSTICS U LT R A S O N I C S

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Acoustics

CONCEPT The area of physics known as acoustics is devoted to the study of the production, transmission, and reception of sound. Thus, wherever sound is produced and transmitted, it will have an effect somewhere, even if there is no one present to hear it. The medium of sound transmission is an all-important, key factor. Among the areas addressed within the realm of acoustics are the production of sounds by the human voice and various instruments, as well as the reception of sound waves by the human ear.

HOW IT WORKS Wave Motion and Sound waves Sound waves are an example of a larger phenomenon known as wave motion, and wave motion is, in turn, a subset of harmonic motion—that is, repeated movement of a particle about a position of equilibrium, or balance. In the case of sound, the “particle” is not an item of matter, but of energy, and wave motion is a type of harmonic movement that carries energy from one place to another without actually moving any matter. Particles in waves experience oscillation, harmonic motion in one or more dimensions. Oscillation itself involves little movement, though some particles do move short distances as they interact with other particles. Primarily, however, it involves only movement in place. The waves themselves, on the other hand, move across space, ending up in a position different from the one in which they started.

ACOUSTICS

dicular to the direction the wave is moving. This is a fairly easy type of wave to visualize: imagine a curve moving up and down along a straight line. Sound waves, on the other hand, are longitudinal waves, in which oscillation occurs in the same direction as the wave itself. These oscillations are really just fluctuations in pressure. As a sound wave moves through a medium such as air, these changes in pressure cause the medium to experience alternations of density and rarefaction (a decrease in density). This, in turn, produces vibrations in the human ear or in any other object that receives the sound waves.

Properties of Sound Waves CYC L E A N D P E R I O D . The term cycle has a definition that varies slightly, depending on whether the type of motion being discussed is oscillation, the movement of transverse waves, or the motion of a longitudinal sound wave. In the latter case, a cycle is defined as a single complete vibration.

A period (represented by the symbol T) is the amount of time required to complete one full cycle. The period of a sound wave can be mathematically related to several other aspects of wave motion, including wave speed, frequency, and wavelength. THE SPEED OF SOUND IN VA R I O U S M E D I A . People often refer to

A transverse wave forms a regular up-anddown pattern in which the oscillation is perpen-

the “speed of sound” as though this were a fixed value like the speed of light, but, in fact, the speed of sound is a function of the medium through which it travels. What people ordinarily mean by the “speed of sound” is the speed of sound through air at a specific temperature. For sound

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Acoustics

BECAUSE

THE SOUND GENERATED BY A JET ENGINE CAN DAMAGE A PERSON’S HEARING, AIRPORT GROUND CREWS

ALWAYS WEAR PROTECTIVE HEADGEAR. (Photograph by Patrick Bennett/Corbis. Reproduced by permission.)

traveling at sea level, the speed at 32°F (0°C) is 740 MPH (331 m/s), and at 68°F (20°C), it is 767 MPH (343 m/s). In the essay on aerodynamics, the speed of sound for aircraft was given at 660 MPH (451 m/s). This is much less than the figures given above for the speed of sound through air at sea level, because obviously, aircraft are not flying at sea level, but well above it, and the air through which they pass is well below freezing temperature. The speed of sound through a gas is proportional to the square root of the pressure divided by the density. According to Gay-Lussac’s law, pressure is directly related to temperature, meaning that the lower the pressure, the lower the temperature—and vice versa. At high altitudes, the temperature is low, and, therefore, so is the pressure; and, due to the relatively small gravitational pull that Earth exerts on the air at that height, the density is also low. Hence, the speed of sound is also low.

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only because we are just more accustomed to hearing sounds that travel through that medium. The speed of sound in water varies from about 3,244 MPH (1,450 m/s) to about 3,355 MPH (1500 m/s). Sound travels even faster through a solid—typically about 11,185 MPH (5,000 m/s)—than it does through a liquid. F R E Q U E N CY. Frequency (abbreviated f) is the number of waves passing through a given point during the interval of one second. It is measured in Hertz (Hz), named after nineteenthcentury German physicist Heinrich Rudolf Hertz (1857-1894) and a Hertz is equal to one cycle of oscillation per second. Higher frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second) or megahertz (MHz; 106 or 1 million cycles per second.)

It follows that the higher the pressure of the material, and the greater the density, the faster sound travels through it: thus sound travels faster through a liquid than through a gas. This might seem a bit surprising: at first glance, it would seem that sound travels fastest through air, but

The human ear is capable of hearing sounds from 20 to approximately 20,000 Hz—a relatively small range for a mammal, considering that bats, whales, and dolphins can hear sounds at a frequency up to 150 kHz. Human speech is in the range of about 1 kHz, and the 88 keys on a piano vary in frequency from 27 Hz to 4,186 Hz. Each note has its own frequency, with middle C (the “white key” in the very middle of a piano keyboard) at 264 Hz. The quality of harmony or dissonance when two notes are played together is a

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Acoustics

PIANO

STRINGS GENERATE SOUND AS THEY ARE SET INTO VIBRATION BY THE HAMMERS.

THE

HAMMERS, IN TURN,

ARE ATTACHED TO THE BLACK-AND -WHITE KEYS ON THE OUTSIDE OF THE PIANO. (Photograph by Bob Krist/Corbis. Reproduced

by permission.)

function of the relationship between the frequencies of the two. Frequencies below the range of human audibility are called infrasound, and those above it are referred to as ultrasound. There are a number of practical applications for ultrasonic technology in medicine, navigation, and other fields. W AV E L E N GT H . Wavelength (represented by the symbol λ, the Greek letter lambda) is the distance between a crest and the adjacent crest, or a trough and an adjacent trough, of a wave. The higher the frequency, the shorter the wavelength, and vice versa. Thus, a frequency of 20 Hz, at the bottom end of human audibility, has a very large wavelength: 56 ft (17 m). The top end frequency of 20,000 Hz is only 0.67 inches (17 mm).

There is a special type of high-frequency sound wave beyond ultrasound: hypersound, which has frequencies above 107 MHz, or 10 trillion Hz. It is almost impossible for hypersound waves to travel through all but the densest media, because their wavelengths are so short. In order to be transmitted properly, hypersound requires an extremely tight molecular structure; otherwise, the wave would get lost between molecules.

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Wavelengths of visible light, part of the electromagnetic spectrum, have a frequency much higher even than hypersound waves: about 109 MHz, 100 times greater than for hypersound. This, in turn, means that these wavelengths are incredibly small, and this is why light waves can easily be blocked out by using one’s hand or a curtain. The same does not hold for sound waves, because the wavelengths of sounds in the range of human audibility are comparable to the size of ordinary objects. To block out a sound wave, one needs something of much greater dimensions— width, height, and depth—than a mere cloth curtain. A thick concrete wall, for instance, may be enough to block out the waves. Better still would be the use of materials that absorb sound, such as cork, or even the use of machines that produce sound waves which destructively interfere with the offending sound. AMPLITUDE

AND

I N T E N S I T Y.

Amplitude is critical to the understanding of sound, though it is mathematically independent from the parameters so far discussed. Defined as the maximum displacement of a vibrating material, amplitude is the “size” of a wave. The greater the amplitude, the greater the energy the wave

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respond to sounds in a linear, or straight-line, progression. If the intensity of a sound is doubled, a person perceives a greater intensity, but nothing approaching twice that of the original sound. Instead, a different system—known in mathematics as a logarithmic scale—is applied.

Acoustics

In measuring the effect of sound intensity on the human ear, a unit called the decibel (abbreviated dB) is used. A sound of minimal audibility (10-12 W/m2) is assigned the value of 0 dB, and 10 dB is 10 times as great—10-11 W/m2. But 20 dB is not 20 times as intense as 0 dB; it is 100 times as intense, or 10-10 W/m2. Every increase of 10 dB thus indicates a tenfold increase in intensity. Therefore, 120 dB, the maximum decibel level that a human ear can endure without experiencing damage, is not 120 times as great as the minimal level for audibility, but 1012 (1 trillion) times as great—equal to 1 W/m2, referred to above as the highest safe intensity level.

THE

HUMAN VOICE, WHETHER IN SPEECH OR IN SONG,

IS A REMARKABLE SOUND -PRODUCING INSTRUMENT: AT ANY GIVEN MOMENT AS A PERSON IS TALKING OR SINGING, PARTS OF THE VOCAL CORDS ARE OPENED, AND

PARTS

SUPERSTAR

ARE CLOSED. SHOWN HERE IS OPERA JOAN SUTHERLAND. (Hulton-Deutsch Collec-

tion/Corbis. Reproduced by permission.)

contains: amplitude indicates intensity, commonly known as “volume,” which is the rate at which a wave moves energy per unit of a crosssectional area. Intensity can be measured in watts per square meter, or W/m2. A sound wave of minimum intensity for human audibility would have a value of 10-12, or 0.000000000001, W/m2. As a basis of comparison, a person speaking in an ordinary tone of voice generates about 10-4, or 0.0001, watts. On the other hand, a sound with an intensity of 1 W/m2 would be powerful enough to damage a person’s ears.

Production of Sound Waves M U S I CA L I N S T R U M E N T S . Sound waves are vibrations; thus, in order to produce sound, vibrations must be produced. For a stringed instrument, such as a guitar, harp, or piano, the strings must be set into vibration, either by the musician’s fingers or the mechanism that connects piano keys to the strings inside the case of the piano.

For measuring the intensity of a sound as experienced by the human ear, we use a unit other than the watt per square meter, because ears do not

In other woodwind instruments and horns, the musician causes vibrations by blowing into the mouthpiece. The exact process by which the vibrations emerge as sound differs between woodwind instruments, such as a clarinet or saxophone on the one hand, and brass instruments, such as a trumpet or trombone on the other. Then there is a drum or other percussion instrument, which produces vibrations, if not musical notes.

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Of course, sounds can be much louder than 120 dB: a rock band, for instance, can generate sounds of 125 dB, which is 5 times the maximum safe decibel level. A gunshot, firecracker, or a jet—if one is exposed to these sounds at a sufficiently close proximity—can be as high as 140 dB, or 20 times the maximum safe level. Nor is 120 dB safe for prolonged periods: hearing experts indicate that regular and repeated exposure to even 85 dB (5 less than a lawn mower) can cause permanent damage to one’s hearing.

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E L E C T R O N I C A M P L I F I CAT I O N .

Sound is a form of energy: thus, when an automobile or other machine produces sound incidental to its operation, this actually represents energy that is lost. Energy itself is conserved, but not all of the energy put into the machine can ever be realized as useful energy; thus, the automobile loses some energy in the form of sound and heat. The fact that sound is energy, however, also means that it can be converted to other forms of energy, and this is precisely what a microphone does: it receives sound waves and converts them to electrical energy. These electrical signals are transmitted to an amplifier, and next to a loudspeaker, which turns electrical energy back into sound energy—only now, the intensity of the sound is much greater. Inside a loudspeaker is a diaphragm, a thin, flexible disk that vibrates with the intensity of the sound it produces. When it pushes outward, the diaphragm forces nearby air molecules closer together, creating a high-pressure region around the loudspeaker. (Remember, as stated earlier, that sound is a matter of fluctuations in pressure.) The diaphragm is then pushed backward in response, freeing up an area of space for the air molecules. These, then, rush toward the diaphragm, creating a low-pressure region behind the high-pressure one. The loudspeaker thus sends out alternating waves of high and low pressure, vibrations on the same frequency of the original sound. T H E H U M A N V O I C E . As impressive as the electronic means of sound production are (and of course the description just given is highly simplified), this technology pales in comparison to the greatest of all sound-producing mechanisms: the human voice. Speech itself is a highly complex physical process, much too involved to be discussed in any depth here. For our present purpose, it is important only to recognize that speech is essentially a matter of producing vibrations on the vocal cords, and then transmitting those vibrations.

person is talking, parts of the vocal cords are opened, and parts are closed.

Acoustics

The sound of a person’s voice is affected by a number of factors: the size and shape of the sinuses and other cavities in the head, the shape of the mouth, and the placement of the teeth and tongue. These factors influence the production of specific frequencies of sound, and result in differing vocal qualities. Again, the mechanisms of speech are highly complicated, involving action of the diaphragm (a partition of muscle and tissue between the chest and abdominal cavities), larynx, pharynx, glottis, hard and soft palates, and so on. But, it all begins with the production of vibrations.

Propagation: Does It Make a Sound? As stated in the introduction, acoustics is concerned with the production, transmission (sometimes called propagation), and reception of sound. Transmission has already been examined in terms of the speed at which sound travels through various media. One aspect of sound transmission needs to be reiterated, however: for sound to be propagated, there must be a medium. There is an age-old “philosophical” question that goes something like this: If a tree falls in the woods and there is no one to hear it, does it make a sound? In fact, the question is not a matter of philosophy at all, but of physics, and the answer is, of course, “yes.” As the tree falls, it releases energy in a number of forms, and part of this energy is manifested as sound waves. Consider, on the other hand, this rephrased version of the question: “If a tree falls in a vacuum—an area completely devoid of matter, including air—does it make a sound?” The answer is now a qualified “no”: certainly, there is a release of energy, as before, but the sound waves cannot be transmitted. Without air or any other matter to carry the waves, there is literally no sound.

Before a person speaks, the brain sends signals to the vocal cords, causing them to tighten. As speech begins, air is forced across the vocal cords, and this produces vibrations. The action of the vocal cords in producing these vibrations is, like everything about the miracle of speech, exceedingly involved: at any given moment as a

Hence, there is a great deal of truth to the tagline associated with the 1979 science-fiction film Alien: “In space, no one can hear you scream.” Inside an astronaut’s suit, there is pressure and an oxygen supply; without either, the astronaut would perish quickly. The pressure and air inside the suit also allow the astronaut to hear sounds within the suit, including communica-

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Acoustics

KEY TERMS ACOUSTICS:

An area of physics

devoted to the study of the production,

exerted in the same direction.

transmission, and reception of sound.

FREQUENCY:

AMPLITUDE:

The maximum displace-

ment of a vibrating material. In wave motion, amplitude is the “size” of a wave, and for sound waves, amplitude indicates the intensity or volume of sound. CYCLE:

For a sound wave, a cycle is a

single complete vibration. DECIBEL:

A unit for measuring inten-

sity of sound. Decibels, abbreviated dB, are calibrated along a logarithmic scale whereby every increase of 10 dB indicates an increase in intensity by a factor of 10. Thus if the level of intensity is increased from 30 to 60 dB, the resulting intensity is not twice as great as that of the earlier sound—it is 1,000 times as great.

In wave motion, fre-

quency is the number of waves passing through a given point during the interval of one second. The higher the frequency, the shorter the wavelength. Measured in Hertz, frequency is mathematically related to wave speed, wavelength, and period. HARMONIC MOTION:

The repeated

movement of a particle about a position of equilibrium, or balance. HERTZ:

A unit for measuring frequen-

cy, named after nineteenth-century German physicist Heinrich Rudolf Hertz (1857-1894).

High

frequencies

are

expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second) or megahertz (MHz; 106 or 1 million cycles per second.) INTENSITY:

Intensity is the rate at

which a wave moves energy per unit of

The ability to perform work,

cross-sectional area. Where sound waves

which is the exertion of force over a given

are concerned, intensity is commonly

distance. Work is the product of force and

known as “volume.”

ENERGY:

tions via microphone from other astronauts. But, if there were an explosion in the vacuum of deep space outside the spacecraft, no one inside would be able to hear it.

Reception of Sound

316

distance, where force and distance are

These electrical pulses are processed and ultimately passed on to an electromagnetic “recording head.” The magnetic field of the recording head extends over the section of tape being recorded: what began as loud sounds now produce strong magnetic fields, and soft sounds produce weak fields. Yet, just as electronic means of sound production and transmission are still not as impressive as the mechanisms of the human voice, so electronic sound reception and recording technology is a less magnificent device than the human ear.

R E C O R D I N G . Earlier the structure of electronic amplification was described in very simple terms. Some of the same processes— specifically, the conversion of sound to electrical energy—are used in the recording of sound. In sound recording, when a sound wave is emitted, it causes vibrations in a diaphragm attached to an electrical condenser. This causes variations in the electrical current passed on by the condenser.

everyone has noticed, a change in altitude (and, hence, of atmospheric pressure) leads to a

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H O W T H E E A R H E A R S . As almost

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Acoustics

KEY TERMS LONGITUDINAL WAVE:

A wave in

which the movement of vibration is in the

CONTINUED

related to frequency, wavelength, and wave speed.

same direction as the wave itself. A sound wave is an example of a longitudinal wave. MATTER:

Physical substance that has

PERIODIC MOTION:

Motion that is

repeated at regular intervals. These intervals are known as periods.

mass; occupies space; is composed of atoms; and is ultimately convertible to energy. MEDIUM:

A decrease in density.

RAREFACTION:

Sound waves with a

ULTRASOUND :

Material through which

sound travels. (It cannot travel through a

frequency above 20,000 Hertz, which makes them inaudible to the human ear.

vacuum.) The most common medium (plural, media) of sound transmission experienced in daily life is air, but in fact

VACUUM:

An area entirely devoid of

matter, including air.

sound can travel through any type of

WAVELENGTH:

matter.

a crest and the adjacent crest, or the trough

OSCILLATION:

The vibration experi-

The distance between

and an adjacent trough, of a wave. Wave-

enced by individual waves even as the wave

length, symbolized by λ (the Greek letter

itself is moving through space. Oscillation

lambda) is mathematically related to wave

is a type of harmonic motion, typically

speed, period, and frequency.

periodic, in one or more dimensions.

WAVE MOTION:

A type of harmonic

For wave motion, a period is

motion that carries energy from one place

the amount of time required to complete

to another without actually moving any

one full cycle. Period is mathematically

matter.

PERIOD:

strange “popping” sensation in the ears. Usually, this condition can be overcome by swallowing, or even better, by yawning. This opens the Eustachian tube, a passageway that maintains atmospheric pressure in the ear. Useful as it is, the Eustachian tube is just one of the human ear’s many parts. The “funny” shape of the ear helps it to capture and amplify sound waves, which pass through the ear canal and cause the eardrum to vibrate. Though humans can hear sounds over a much wider range, the optimal range of audibility is from 3,000 to 4,000 Hz. This is because the structure of the ear canal is such that sounds in this frequency produce magnified pressure fluc-

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tuations. Thanks to this, as well as other specific properties, the ear acts as an amplifier of sounds. Beyond the eardrum is the middle ear, an intricate sound-reception device containing some of the smallest bones in the human body— bones commonly known, because of their shapes, as the hammer, anvil, and stirrup. Vibrations pass from the hammer to the anvil to the stirrup, through the membrane that covers the oval window, and into the inner ear. Filled with liquid, the inner ear contains the semicircular canals responsible for providing a sense of balance or orientation: without these, a person literally “would not know which way is up.” Also, in the inner ear is the cochlea, an organ

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shaped like a snail. Waves of pressure from the fluids of the inner ear are passed through the cochlea to the auditory nerve, which then transmits these signals to the brain. The basilar membrane of the cochlea is a particularly wondrous instrument, responsible in large part for the ability to discriminate between sounds of different frequencies and intensities. The surface of the membrane is covered with thousands of fibers, which are highly sensitive to disturbances, and it transmits information concerning these disturbances to the auditory nerve. The brain, in turn, forms a relation between the position of the nerve ending and the frequency of the sound. It also equates the degree of disturbance in the basilar membrane with the intensity of the sound: the greater the disturbance, the louder the sound.

318

Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991. Friedhoffer, Robert. Sound. Illustrated by Richard Kaufman and Linda Eisenberg; photographs by Timothy White. New York: F. Watts, 1992. Gardner, Robert. Science Projects About Sound. Berkeley Heights, NJ: Enslow Publishers, 2000. Internet Resources for Sound and Light (Web site). (April 25, 2001). “Music and Sound Waves” (Web site). (April 28, 2001). Oxlade, Chris. Light and Sound. Des Plaines, IL: Heinemann Library, 2000. Physics Tutorial System: Sound Waves Modules (Web site). (April 25, 2001).

WHERE TO LEARN MORE

“Sound Waves and Music.” The Physics Classroom (Web site). (April 28, 2001).

Adams, Richard C. and Peter H. Goodwin. Engineering Projects for Young Scientists. New York: Franklin Watts, 2000.

“What Are Sound Waves?” (Web site). (April 28, 2001).

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Ultrasonics

U LT R A S O N I C S

CONCEPT The word ultrasonic combines the Latin roots ultra, meaning “beyond,” and sonic, or sound. The field of ultrasonics thus involves the use of sound waves outside the audible range for humans. These sounds have applications for imaging, detection, and navigation—from helping prospective parents get a glimpse of their unborn child to guiding submarines through the oceans. Ultrasonics can be used to join materials, as for instance in welding or the homogenization of milk, or to separate them, as for example in extremely delicate cleaning operations. Among the broad sectors of society that regularly apply ultrasonic technology are the medical community, industry, the military, and private citizens.

HOW IT WORKS In the realm of physics, ultrasonics falls under the category of studies in sound. Sound itself fits within the larger heading of wave motion, which is in turn closely related to vibration, or harmonic (back-and-forth) motion. Both wave motion and vibration involve the regular repetition of a certain form of movement; and in both, potential energy (think of the energy in a sled at the top of a hill) is continually converted to kinetic energy (like the energy of a sled as it is sliding down the hill) and back again. Wave motion carries energy from one place to another without actually moving any matter. Waves themselves may consist of matter, as for instance in the case of a wave on a plucked string or the waves on the ocean. This type of wave is called a mechanical wave, but again, the matter itself does not undergo any net displacement over horizontal space: contrary to what our eyes

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tell us, molecules of water in an ocean wave move up and down, but they do not actually travel with the wave itself. Only the energy is moved.

Sound Waves Then there are waves of pulses, such as light, sound, radio, or electromagnetic waves. Sound travels by means of periodic waves, a period being the amount of time it takes a complete wave, from trough to crest and back again, to pass through a given point. These periodic waves are typified by a sinusoidal pattern. To picture a sinusoidal wave, one need only imagine an x-axis crossed at regular intervals by a curve that rises above the line to point y before moving downward, below the axis, to point y. This may be expressed also as a graph of sin x versus x. In any case, the wave varies by equal distances upward and downward as it moves along the x-axis in a regular, unvarying pattern. Periodic waves have three notable interrelated characteristics. One of these is speed, typically calculated in seconds. Another is wavelength, or the distance between a crest and the adjacent crest, or a trough and the adjacent trough, along a plane parallel to that of the wave itself. Finally, there is frequency, the number of waves passing through a given point during the interval of one second. Frequency is measured in terms of cycles per second, or Hertz (Hz), named in honor of the nineteenth-century German physicist Heinrich Hertz. If a wave has a frequency of 100 Hz, this means that 100 waves are passing through a given point during the interval of one second. Higher frequencies are expressed in terms of kilohertz

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Ultrasonics

SUBMARINES,

SUCH AS THE

U.S. NAVY

SUBMARINE PICTURED HERE, RELY HEAVILY ON SONAR. (Corbis. Reproduced by

permission.)

(kHz; 103 or 1,000 cycles per second) or megahertz (MHz; 106 or 1 million cycles per second.) Clearly, frequency is a function of the wave’s speed or velocity, and the same relationship— though it is not so obvious intuitively—exists between wavelength and speed. Over the interval of one second, a given number of waves pass a certain point (frequency), and each wave occupies a certain distance (wavelength). Multiplied by one another, these two properties equal the velocity of the wave. An additional characteristic of waves (though one that is not related mathematically to the three named above) is amplitude, or maximum displacement, which can be described as the distance from the x-axis to either the crest or the trough. Amplitude is related to the intensity or the amount of energy in the wave. These four qualities are easiest to imagine on a transverse wave, described earlier with reference to the x-axis—a wave, in other words, in which vibration or harmonic motion occurs perpendicular to the direction in which the wave is moving. Such a wave is much easier to picture, for the purposes of illustrating concepts such as frequency, than a longitudinal wave; but in fact,

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sound waves are longitudinal. A longitudinal wave is one in which the individual segments vibrate in the same direction as the wave itself. The shock waves of an explosion, or the concentric waves of a radio transmission as it goes out from the station to all points within receiving distance, are examples of longitudinal waves. In this type of wave pattern, the crests and troughs are not side by side in a line; they radiate outward. Wavelength is the distance between each concentric circle or semicircle (that is, wave), and amplitude the “width” of each wave, which one may imagine by likening it to the relative width of colors on a rainbow. Having identified its shape, it is reasonable to ask what, exactly, a sound wave is. Simply put, sound waves are changes in pressure, or an alternation between condensation and rarefaction. Imagine a set of longitudinal waves—represented as concentric circles—radiating from a sound source. The waves themselves are relatively higher in pressure, or denser, than the “spaces” between them, though this is just an illustration for the purposes of clarity: in fact the “spaces” are waves of lower pressure that alternate with higher-pressure waves. Vibration is integral to the generation of sound. When the diaphragm of a loudspeaker

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A FISHING REVOLUTION? IT’S POSSIBLE, THANKS TO THIS ELECTRONIC FISH-FINDER DEVELOPED THE DEVICE INCLUDES A DETACHABLE SONAR SENSOR. (AFP/Corbis. Reproduced by permission.)

pushes outward, it forces nearby air molecules closer together, creating a high-pressure region all around the loudspeaker. The diaphragm is pushed backward in response, thus freeing up a volume of space for the air molecules. These then rush toward the diaphragm, creating a low-pressure region behind the high-pressure one. As a result, the loudspeaker sends out alternating waves of high pressure (condensation) and low pressure (rarefaction). Furthermore, as sound waves pass through a medium—air, for the purposes of this discussion—they create fluctuations between density and rarefaction. These result in pressure changes that cause the listener’s eardrum to vibrate with the same frequency as the sound wave, a vibration that the ear’s inner mechanisms translate and pass on to the brain.

The Speed of Sound: Consider the Medium

BY

MATSUSHITA.

instance, sound travels at 19,680 ft per second (6,000 mps), whereas in air, the speed of sound is only 1,086 ft per second (331 mps). It follows that sound travels faster in water—5,023 ft per second (1,531 mps), to be exact—than in air. It should be clear, then, that there is a correlation between density and the ease with which a sound travels. Thus, sound cannot travel in a vacuum, giving credence to the famous tagline from the 1979 science fiction thriller Alien: “In space, no one can hear you scream.” When sound travels through a medium such as air, however, two factors govern its audibility: intensity or volume (related to amplitude and measured in decibels, or dB) and frequency. There is no direct correlation between intensity and frequency, though for a person to hear a very low-frequency sound, it must be above a certain decibel level. (At all frequencies, however, the threshold of discomfort is around 120 decibels.)

The speed of sound varies with the hardness of the medium through which it passes: contrary to what you might imagine, it travels faster through liquids than through gasses such as air, and faster through solids than through liquids. By definition, molecules are closer together in harder material, and thus more quickly responsive to signals from neighboring particles. In granite, for

In any case, when discussing ultrasonics, frequency and not intensity is of principal concern. The range of audibility for the human ear is from 20 Hz to 20,000 Hz, with frequencies below that range dubbed infrasound and those above it referred to as ultrasound. (There is a third category, hypersound, which refers to frequencies above 1013, or 10 trillion Hz. It is almost impos-

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Ultrasonics

TINY

LISTENING DEVICES, LIKE THE ONE SHOWN HERE, HAVE A HOST OF MODERN APPLICATIONS, FROM ELECTRON-

IC EAVESDROPPING TO ULTRASONIC STEREO SYSTEMS. (Photograph by Jeffrey L. Rotman/Corbis. Reproduced by permission.)

sible for hypersound waves to travel through most media, because its wavelengths are so short.)

What Makes the Glass Shatter The lowest note of the eighty-eight keys on a piano is 27 Hz and the highest 4,186 Hz. This places the middle and upper register of the piano well within the optimal range for audibility, which is between 3,000 and 4,000 Hz. Clearly, the higher the note, the higher the frequency—but it is not high frequency, per se, that causes a glass to shatter when a singer or a violinist hits a certain note. All objects, or at least all rigid ones, possess their own natural frequency of vibration or oscillation. This frequency depends on a number of factors, including material composition and shape, and its characteristics are much more complex than those of sound frequency described above. In any case, a musician cannot cause a glass to shatter simply by hitting a very high note; rather, the note must be on the exact frequency at which the glass itself oscillates. Under such conditions, all the energy from the voice or musical instrument is transferred to the glass, a sudden burst that overloads the object and causes it to shatter.

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To create ultrasonic waves, technicians use a transducer, a device that converts energy into ultrasonic sound waves. The most basic type of transducer is mechanical, involving oscillators or vibrating blades powered either by gas or the pressure of gas or liquids—that is, pneumatic or hydrodynamic pressure, respectively. The vibrations from these mechanical devices are on a relatively low ultrasonic frequency, and most commonly they are applied in industry for purposes such as drying or cleaning. An electromechanical transducer, which has a much wider range of applications, converts electrical energy, in the form of current, to mechanical energy—that is, sound waves. This it does either by a magnetostrictive or a piezoelectric device. The term “magnetostrictive” comes from magneto, or magnetic, and strictio, or “drawing together.” This type of transducer involves the magnetization of iron or nickel, which causes a change in dimension by forcing the atoms together. This change in dimension in turn produces a high-frequency vibration. Again, the frequency is relatively low in ultrasonic terms, and likewise the application is primarily industrial, for purposes such as cleaning and machining.

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Most widely used is a transducer equipped with a specially cut piezoelectric quartz crystal. Piezoelectricity involves the application of mechanical pressure to a nonconducting crystal, which results in polarization of electrical charges, with all positive charges at one end of the crystal and all negative charges at the other end. By successively compressing and stretching the crystal at an appropriate frequency, an alternating electrical current is generated that can be converted into mechanical energy—specifically, ultrasonic waves. Scientists use different shapes and materials (including quartz and varieties of ceramic) in fashioning piezoelectric crystals: for instance, a concave shape is best for an ultrasonic wave that will be focused on a very tight point. Piezoelectric transducers have a variety of applications in ultrasonic technology, and are capable of acting as receivers for ultrasonic vibrations.

REAL-LIFE A P P L I C AT I O N S Pets and Pests: Ultrasonic Behavior Modification Some of the simplest ultrasonic applications build on the fact that the upper range of audibility for human beings is relatively low among animals. Cats, by comparison, have an infrasound threshold only slightly higher than that of humans (100 Hz), but their ultrasound range of audibility is much greater—32,000 Hz instead of a mere 20,000. This explains why a cat sometimes seems to respond mysteriously to noises its owner is incapable of hearing. For dogs, the difference is even more remarkable: their lower threshold is 40 Hz, and their high end 46,000 Hz, giving them a range more than twice that of humans. It has been said Paul McCartney, who was fond of his sheepdog Martha, arranged for the Beatles’ sound engineer at Abbey Road Studios to add a short 20,000-Hz tone at the very end of Sgt. Peppers’ Lonely Hearts Club Band in 1967. Thus—if the story is true— the Beatles’ human fans would never hear the note, but it would be a special signal to Martha and all the dogs of England. On a more practical level, a dog whistle is an extremely simple ultrasonic or near-ultrasonic device, one that obviously involves no transduc-

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ers. The owner blows the whistle, which utters a tone nearly inaudible to humans but—like McCartney’s 20,000-Hz tone—well within a dog’s range. In fact, the Acme Silent Dog Whistle, which the company has produced since 1935, emits a tone that humans can hear (the listed range is 5,800 to 12,400 Hz), but which dogs can hear much better.

Ultrasonics

There are numerous products on the market that use ultrasonic waves for animal behavior modification of one kind or another; however, most such items are intended to repel rather than attract the animal. Hence, there are ultrasonic devices to discourage animals from relieving themselves in the wrong places, as well as some which keep unwanted dogs and cats away. Then there is one of the most well-known uses of ultrasound for pets, which, rather than keeping other animals out, is designed to keep one’s own animals in the yard. Many people know this item as an “Invisible Fence,” though in fact that term is a registered trademark of the Invisible Fence Company. The “Invisible Fence” and similar products literally create a barrier of sound, using both radio signals and ultrasonics. The pet is outfitted with a collar that contains a radio receiver, and a radio transmitter is placed in some centrally located place on the owner’s property—a basement, perhaps, or a garage. The “fence” itself is “visible,” though usually buried, and consists of an antenna wire at the perimeter of the property. The transmitter sends a signal to the wire, which in turn signals the pet’s collar. A tiny computer in the collar emits an ultrasonic sound if the animal tries to stray beyond the boundaries. Not all animals have a higher range of hearing than humans: elephants, for instance, cannot hear tones above 12,000 Hz. On the other hand, some are drastically more sensitive acoustically than dogs: bats, whales, and dolphins all have an upper range of 150,000 Hz, though both have a low-end threshold of 1,000. Mice, at 100,000 Hz, are also at the high end, while a number of other pests—rodents and insects—fall into the region between 40,000 and 100,000 Hz. This fact has given rise to another type of ultrasonic device, for repelling all kinds of unwanted household creatures by bombarding them with ear-splitting tones. An example of this device is the Transonic 1X-L, which offers three frequency ranges: “loud

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mode” (1,000-50,000 Hz); “medium mode” (10,000-50,000 Hz); and “quiet mode” (20,00050,000 Hz). The lowest of these can be used for repelling pest birds and small animals, the medium range for insects, and the “quiet mode” for rodents.

Ultrasonic Detection in Medicine Medicine represents one of the widest areas of application for ultrasound. Though the machinery used to provide parents-to-be with an image of their unborn child is the most well-known form of medical ultrasound, it is far from the only one. Developed in 1957 by British physician Ian Donald (1910-1987), also a pioneer in the use of ultrasonics to detect flaws in machinery, ultrasound was first used to diagnosis a patient’s heart condition. Within a year, British hospitals began using it with pregnant women. High-frequency waves penetrate soft tissue with ease, but they bounce off of harder tissue such as organs and bones, and thus send back a message to the transducer. Because each type of tissue absorbs or deflects sound differently, according to its density, the ultrasound machine can interpret these signals, creating an image of what it “sees” inside the patient’s body. The technician scans the area to be studied with a series of ultrasonic waves in succession, and this results in the creation of a moving picture. It is this that creates the sight so memorable in the lives of many a modern parent: their first glimpse of their child in its mother’s womb. Though ultrasound enables physicians and nurses to determine the child’s sex, this is far from being the only reason it is used. It also gives them data concerning the fetus’s size; position (for instance, if the head is in a place that suggests the baby will have to be delivered by means of cesarean section); and other abnormalities.

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Ultrasonic heart examination can locate tumors, valve diseases, and accumulations of fluid. Using the Doppler effect—the fact that a sound’s perceived frequency changes as its source moves past the observer—physicians observe shifts in the frequency of ultrasonic measurements to determine the direction of blood flow in the body. Not only can ultrasound be used to differentiate tumors from healthy tissue, it can sometimes be used to destroy those tumors. In some cases, ultrasound actually destroys cancer cells, making use of a principle called cavitation— a promising area of ultrasound research. Perhaps the best example of cavitation occurs when you are boiling a pot of water: bubbles—temporary cavities in the water itself—rise up from the bottom to the surface, then collapse, making a popping sound as they do. Among the research areas combining cavitation and ultrasonics are studies of light emissions produced in the collapse of a cavity created by an ultrasonic wave. These emissions are so intense that for an infinitesimal moment, they produce heat of staggering proportions—hotter than the surface of the Sun, some scientists maintain. (Again, it should be stressed that this occurs during a period too small to measure with any but the most sophisticated instruments.) As for the use of cavitation in attacking cancer cells, ultrasonic waves can be used to create microscopic bubbles which, when they collapse, produce intense shock waves that destroy the cells. Doctors are now using a similar technique against gallstones and kidney stones. Other medical uses of ultrasound technology include ultrasonic heat for treating muscle strain, or—in a process similar to some industrial applications— the use of 25,000-Hz signals to clean teeth.

Sonar and Other Detection Devices

The beauty of ultrasound is that it can provide this information without the danger posed by x rays or incisions. Doctors and ultrasound technicians use ultrasonic technology to detect body parts as small as 0.004 in (0.1 mm), making it possible to conduct procedures safely, such as locating foreign objects in the eye or measuring the depth of a severe burn. Furthermore, ultrasonic microscopes can image cellular structures to within 0.2 microns (0.002 mm).

Airplane pilots typically use radar, but the crew of a ocean-going vessel relies on sonar (SOund Navigation and Ranging) to guide their vessel through the ocean depths. This technology takes advantage of the fact that sound waves travel well under water—much better, in fact, than light waves. Whereas a high-powered light would be of limited value underwater, particularly in the murky realms of the deep sea, sonar provides excellent data on the water’s depth, as well as the location of shipwrecks, large obstacles—and, for

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commercial or even recreational fishermen—the presence of fish. At the bottom of the craft’s hull is a transducer, which emits an ultrasonic pulse. These sound waves travel through the water to the bottom, where they bounce back. Upon receiving the echo, the transducer sends this information to an onboard computer, which converts data on the amount of time the signal took, providing a reading of distance that gives an accurate measurement of the vessel’s clearance. For instance, it takes one second for sound waves from a depth of 2,500 ft (750 m) to return to the ship. The onboard computer converts this data into a rough picture of what lies below: the ocean floor, and schools of fish or other significant objects between it and the ship. Even more useful is a scanning sonar, which adds dimension to the scope of the ship’s ultrasonic detection: not only does the sonar beam move forward along with the vessel, but it moves from side-to-side, providing a picture of a wider area along the ship’s path. Sonar in general, and particularly scanning sonar, is of particular importance to a submarine’s crew. Despite the fact that the periscope is perhaps the most notable feature of these underwater craft, from the viewpoint of a casual observer, in fact, the purely visual data provided by the periscope is of limited value—and that value decreases as the sub descends. It is thanks to sonar (which produces the pinging sound one so often hears in movie scenes depicting the submarine control room), combined with nuclear technology, that makes it possible for today’s U.S. Navy submarines to stay submerged for months. Sonar is perhaps the most dramatic use of ultrasonic technology for detection; less wellknown—but equally intriguing, especially for its connection with clandestine activity—is the use of ultrasonics for electronic eavesdropping. Private detectives, suspicious spouses, and no doubt international spies from the CIA or Britain’s MI5, use ultrasonic waves to listen to conversations in places where they cannot insert a microphone. For example, an operative might want to listen in on an encounter taking place on the seventh floor of a building with heavy security, meaning it would be impossible to plant a microphone either inside the room or on the window ledge.

the room being monitored. If people are speaking inside the room, this will produce vibrations on the window the transducer can detect, although the sounds would not be decipherable as conversation by a person with unaided perception. Speech vibrations from inside produce characteristic effects on the ultrasonic waves beamed back to the transducer and the operative’s monitoring technology. The transducer then converts these reflected vibrations into electrical signals, which analysts can then reconstruct as intelligible sounds.

Ultrasonics

Much less dramatic, but highly significant, is the use of ultrasonic technology for detection in industry. Here the purpose is to test materials for faults, holes, cracks, or signs of corrosion. Again, the transducer beams an ultrasonic signal, and the way in which the material reflects this signal can alert the operator to issues such as metal fatigue or a faulty weld. Another method is to subject the material or materials to stress, then look for characteristic acoustic emissions from the stressed materials. (The latter is a developing field of acoustics known as acoustic emission.) Though industrial detection applications can be used on materials such as porcelain (to test for microscopic cracks) or concrete (to evaluate how well it was poured), ultrasonics is particularly effective on metal, in which sound moves more quickly and freely than any other type of wave. Not only does ultrasonics provide an opportunity for thorough, informative, but nondestructive testing, it also allows technicians to penetrate areas where they otherwise could not go—or, in the case of ultrasonic inspection of the interior of a nuclear reactor while in operation—would not and should not go.

Binding and Loosening: A Host of Industrial Applications Materials testing is but one among myriad uses for ultrasonics in industry, applications that can be described broadly as “binding and loosening”—either bringing materials together, or pulling them apart.

Instead, the operative uses ultrasonic waves, which a transducer beams toward the window of

For instance, ultrasound is often used to bind, or coagulate, loose particles of dust, mist, or smoke. This makes it possible to clean a factory smokestack before it exhales pollutants into the atmosphere, or to clear clumps of fog and

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mist off a runway. Another form of “binding” is the use of ultrasonic vibrations to heat and weld together materials. Ultrasonics provides an even, localized flow of molten material, and is effective both on plastics and metals. Ultrasonic soldering implements the principle of cavitation, producing microscopic bubbles in molten solder, a process that removes metal oxides. Hence, this is a case of both “binding” (soldering) and “loosening”—removing impurities from the area to be soldered. The dairy industry, too, uses ultrasonics for both purposes: ultrasonic waves break up fat globules in milk, so that the fat can be mixed together with the milk in the well-known process of homogenization. Similarly, ultrasonic pasteurization facilitates the separation of the milk from harmful bacteria and other microorganisms. The uses of ultrasonics to “loosen” include ultrasonic humidification, wherein ultrasonic vibrations reduce water to a fine spray. Similarly, ultrasonic cleaning uses ultrasound to break down the attraction between two different types of materials. Though it is not yet practical for home use, the technology exists today to use ultrasonics for laundering clothes without using water: the ultrasonic vibrations break the bond between dirt particles and the fibers of a garment, shaking loose the dirt and subjecting the fabric to far less trauma than the agitation of a washing machine does. As noted earlier, dentists use ultrasound for cleaning teeth, another example of loosening the bond between materials. In most of these forms of ultrasonic cleaning, a critical part of the process is the production of microscopic shock waves in the process of cavitation. The frequency of sound waves in these operations ranges from 15,000 Hz (15 kHz) to 2 million Hz (2 MHz). Ultrasonic cleaning has been used on metals, plastics, and ceramics, as well as for cleaning precision instruments used in the optical, surgical, and dental fields. Nor is it just for small objects: the electronics, automotive, and aircraft industries make heavy use of ultrasonic cleaning for a variety of machines.

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hand with the use of abrasive materials such as silicon carbide or aluminum oxide.

A World of Applications Scientists often use ultrasound in research, for instance to break up high molecular weight polymers, thus creating new plastic materials. Indeed, ultrasound also makes it possible to determine the molecular weight of liquid polymers, and to conduct other forms of investigation on the physical properties of materials. Ultrasonics can also speed up certain chemical reactions. Hence, it has gained application in agriculture, thanks to research which revealed that seeds subjected to ultrasound may germinate more rapidly and produce higher yields. In addition to its uses in the dairy industry, noted above, ultrasonics is of value to farmers in the related beef industry, who use it to measure cows’ fat layers before taking them to market. In contrast to the use of ultrasonics for electronic eavesdropping, as noted earlier, today ultrasonic technology is available to persons who think someone might be spying on them: now they can use ultrasonics to detect the presence of electronic eavesdropping, and thus circumvent it. Closer to home is another promising application of ultrasonics for remote sensing of sounds: ultrasonic stereo speakers. These make use of research dating back to the 1960s, which showed that ultrasound waves of relatively low frequency can carry audible sound to pinpointed locations. In 1996, Woody Norris had perfected the technology necessary to reduce distortion, and soon he and his son Joe began selling the ultrasonic speakers through the elder Norris’s company, American Technology Corporation of San Diego, California.

Ultrasonic “loosening” makes it possible to drill though extremely hard or brittle materials, including tungsten carbide or precious stones. Just as a dental hygienist cleaning a person’s teeth bombards the enamel with gentle abrasives, this form of high-intensity drilling works hand-in-

Eric Niiler in Business Week described a demonstration: “Joe Norris twists a few knobs on a receiver, takes aim with a 10-inch-square goldcovered flat speaker, and blasts an invisible beam.... Thirty feet away, the tinny but easily recognizable sound of Vivaldi’s Four Seasons rushes over you. Step to the right or left, however, and it fades away. The exotic-looking speaker emits `sound beams’ that envelop the listener but are silent to those nearby. `We use the air as our virtual speakers,’ says Norris....” Niiler went to note several other applications suggested by Norris: “Airline passengers could listen to their own

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Ultrasonics

KEY TERMS The number of waves

occurs perpendicular to the direction in

passing through a given point during the

which the wave is moving. Waves on the

interval of one second. The higher the fre-

ocean are an example of transverse waves;

quency, the shorter the wavelength.

by contrast, the shock waves of an explo-

FREQUENCY:

A unit for measuring frequen-

sion, the concentric waves of a radio trans-

cy, equal to one cycle per second. If a sound

mission, and sound waves are all examples

wave has a frequency of 20,000 Hz, this

of longitudinal waves.

means that 20,000 waves are passing

TRANSDUCER:

through a given point during the interval

verts energy into ultrasonic sound waves.

HERTZ:

of one second. Higher frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second) or megahertz (MHz; 106 or 1 million cycles per second.) Hence 20,000 Hz—the threshold of ultra-

ULTRASOUND:

A device that con-

Sound waves with a

frequency above 20,000 Hz, which makes them inaudible to the human ear. Its opposite is infrasound. The distance, meas-

sonic sound—would be rendered as 20

WAVELENGTH:

kHz.

ured on a plane parallel to that of the wave Sound of a frequency

itself, between a crest and the adjacent

between 20 Hz, which places it outside the

crest, or the trough and an adjacent trough.

range of audibility for human beings. Its

On a longitudinal wave, this is simply the

opposite is ultrasound.

distance between waves, which constitute a

INFRASOUND:

LONGITUDINAL WAVE:

A wave in

which the individual segments vibrate in

series of concentric circles radiating from the source. Activity that carries

the same direction as the wave itself. This is

WAVE MOTION:

in contrast to a transverse wave, or one in

energy from one place to another without

which the vibration or harmonic motion

actually moving any matter.

music channelsans headphones without disturbing neighbors. Troops could confuse the enemy with `virtual’ artillery fire, or talk to each other without having their radio communications picked up by eavesdroppers.”

WHERE TO LEARN MORE Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991.

Langone, John. National Geographic’s How Things Work. Washington, D.C.: National Geographic Society, 1999. Medical Ultrasound WWW Directory (Web site). (February 16, 2001). Niiler, Eric; edited by Alex Salkever. “Now Here This—If You’re in the Sweet Spot.” Business Week, October 16, 2000.

Crocker, Malcolm J. Encyclopedia of Acoustics. New York: John Wiley & Sons, 1997.

Meire, Hylton B. and Pat Farrant. Basic Ultrasound. Chichester, N.Y.: John Wiley & Sons, 1995.

Knight, David C. Silent Sound: The World of Ultrasonics. New York: Morrow, 1980.

Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996.

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S C I E N C E O F E V E RY DAY T H I N G S real-life Physics

LIGHT AND ELECTROMAGNETISM MAGNETISM ELECTROMAGNETIC SPECTRUM LIGHT LUMINESCENCE

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Magnetism

CONCEPT Most people are familiar with magnets primarily as toys, or as simple objects for keeping papers attached to a metal surface such as a refrigerator door. In fact the areas of application for magnetism are much broader, and range from security to health care to communication, transportation, and numerous other aspects of daily life. Closely related to electricity, magnetism results from specific forms of alignment on the part of electron charges in certain varieties of metal and alloy.

HOW IT WORKS Magnetism, along with electricity, belongs to a larger phenomenon, electromagnetism, or the force generated by the passage of an electric current through matter. When two electric charges are at rest, it appears to the observer that the force between them is merely electric. If the charges are in motion, however—and in this instance motion or rest is understood in relation to the observer—then it appears as though a different sort of force, known as magnetism, exists between them. In fact, the difference between magnetism and electricity is purely artificial. Both are manifestations of a single fundamental force, with “magnetism” simply being an abstraction that people use for the changes in electromagnetic force created by the motion of electric charges. It is a distinction on the order of that between water and wetness; nonetheless, it is often useful and convenient to discuss the two phenomena as though they were separate.

MAGNETISM

atomic particles, relative to one another. Rather like planets in a solar system, electrons both revolve around the atom’s nucleus and rotate on their own axes. (In fact the exact nature of their movement is much more complex, but this analogy is accurate enough for the present purposes.) Both types of movement create a magnetic force field between electrons, and as a result the electron takes on the properties of a tiny bar magnet with a north pole and south pole. Surrounding this infinitesimal magnet are lines of magnetic force, which begin at the north pole and curve outward, describing an ellipse as they return to the south pole. In most atomic elements, the structure of the atom is such that the electrons align in a random manner, rather like a bunch of basketballs bumping into one another as they float in a swimming pool. Because of this random alignment, the small magnetic fields cancel out one another. Two such self-canceling particles are referred to as paired electrons, and again, the analogy to bar magnets is an appropriate one: if one were to shake a bag containing an even number of bar magnets, they would all wind up in pairs, joined at opposing (north-south) poles. There are, however, a very few elements in which the fields line up to create what is known as a net magnetic dipole, or a unity of direction—rather like a bunch of basketballs simultaneously thrown from in the same direction at the same time. These elements, among them iron, cobalt, and nickel, as well as various alloys or mixtures, are commonly known as magnetic metals or natural magnets.

At the atomic level, magnetism is the result of motion by electrons, negatively charged sub-

It should be noted that in magnetic metals, magnetism comes purely from the alignment of

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Magnetism

THE

RECORDING HEAD IS A SMALL ELECTROMAGNET WHOSE MAGNETIC FIELD EXTENDS OVER THE SECTION OF TAPE

BEING RECORDED.

LOUD

SOUNDS PRODUCE STRONG MAGNETIC FIELDS, AND SOFT ONES WEAK FIELDS. (Illustration by

Hans & Cassidy. The Gale Group.)

forces exerted by electrons as they spin on their axes, whereas the forces created by their orbital motion around the nucleus tend to cancel one another out. But in magnetic rare earth elements such as cerium, magnetism comes both from rotational and orbital forms of motion. Of principal concern in this discussion, however, is the behavior of natural magnets on the one hand, and of nonmagnetic materials on the other. There are five different types of magnetism—diamagnetism, paramagnetism, ferromagnetism, ferrimagnetism, and antiferromagnetism. Actually, these terms describe five different types of response to the process of magnetization, which occurs when an object is placed in a magnetic field. A magnetic field is an area in which a magnetic force acts on a moving charged particle such that the particle would experience no force if it moved in the direction of the magnetic field—in other words, it would be “drawn,” as a ten-penny nail is drawn to a common bar or horseshoe (U-shaped) magnet. An electric current is an example of a moving charge, and indeed one of the best ways to create a magnetic field is with a current. Often this is done by means of a solenoid, a current-carrying wire coil

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through which the material to be magnetized is passed, much as one would pass an object through the interior of a spring. All materials respond to a magnetic field; they just respond in different ways. Some nonmagnetic substances, when placed within a magnetic field, slightly reduce the strength of that field, a phenomenon known as diamagnetism. On the other hand, there are nonmagnetic substances possessing an uneven number of electrons per atom, and in these instances a slight increase in magnetism, known as paramagnetism, occurs. Paramagnetism always has to overcome diamagnetism, however, and hence the gain in magnetic force is very small. In addition, the thermal motion of atoms and molecules prevents the objects’ magnetic fields from coming into alignment with the external field. Lower temperatures, on the other hand, enhance the process of paramagnetism. In contrast to diamagnetism and paramagnetism, ferro-, ferri-, and antiferromagnetism all describe the behavior of natural magnets when exposed to a magnetic field. The name ferromagnetism suggests a connection with iron, but in fact the term can apply to any of those materials in which the magnitude of the object’s magnetic

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Magnetism

THE

PERMANENT MAGNETIZATION OF A NATURAL MAGNET IS DIFFICULT TO REVERSE, BUT REVERSAL OF A TAPE’S MAG-

NETIZATION—IN OTHER WORDS, ERASING THE TAPE—IS EASY.

AN

ERASE HEAD, AN ELECTROMAGNET OPERATING AT

A FREQUENCY TOO HIGH FOR THE HUMAN EAR TO HEAR, SIMPLY SCRAMBLES THE MAGNETIC PARTICLES ON A PIECE OF TAPE. (Illustration by Hans & Cassidy. The Gale Group.)

field increases greatly when it is placed within an external field. When a natural magnet becomes magnetized (that is, when a metal or alloy comes into contact with an external magnetic field), a change occurs at the level of the domain, a group of atoms equal in size to about 5 ⫻ 10-5 meters across—just large enough to be visible under a microscope.

however, one of two things happens to the domains. Either they all come into alignment with the field or, in certain types of material, those domains in alignment with the field grow while the others shrink to nonexistence.

In an unmagnetized sample, there may be an alignment of unpaired electron spins within a domain, but the direction of the various domains’ magnetic forces in relation to one another is random. Once a natural magnet is placed within an external magnetic force field,

The first of these processes is called domain alignment or ferromagnetism, the second domain growth or ferrimagnetism. Both processes turn a natural magnet into what is known as a permanent magnet—or, in common parlance, simply a “magnet.” The latter is then capable of temporarily magnetizing a ferromagnetic item, as for instance when one rubs a paper clip against a permanent magnet and then uses the magnet-

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ized clip to lift other paper clips. Of the two varieties, however, a ferromagnetic metal is stronger because it requires a more powerful magnetic force field in order to become magnetized. Most powerful of all is a saturated ferromagnetic metal, one in which all the unpaired electron spins are aligned. Once magnetized, it is very hard for a ferromagnetic metal to experience demagnetization, or antiferromagnetism. Again, there is a connection between temperature and magnetism, with heat acting as a force to reduce the strength of a magnetic field. Thus at temperatures above 1,418°F (770°C), the atoms within a domain take on enough kinetic energy to overpower the forces holding the electron spins in alignment. In addition, mechanical disturbances—for instance, battering a permanent magnet with a hammer— can result in some reduction of magnetic force. Many of the best permanent magnets are made of steel, which, because it is an alloy of iron with carbon and other elements, has an irregular structure that lends itself well to the ferromagnetic process of domain alignment. Iron, by contrast, will typically lose its magnetization when an external magnetic force field is removed; but this actually makes it a better material for some varieties of electromagnet. The latter, in its simplest form, consists of an iron rod inside a solenoid. When a current is passing through the solenoid, it creates a magnetic force field, activating the iron rod and turning it into an electromagnet. But as soon as the current is turned off, the rod loses its magnetic force. Not only can an electromagnet thus be controlled, but it is often stronger than a permanent magnet: hence, for instance, giant electromagnets are used for lifting cars in junkyards.

A magnetic compass works because Earth itself is like a giant bar magnet, complete with vast arcs of magnetic force, called the geomagnetic field, surrounding the planet. The first scientist to recognize the magnetic properties of Earth was the English physicist William Gilbert (1544-1603). Scientists today believe that the source of Earth’s magnetism lies in a core of molten iron some 4,320 mi (6,940 km) across, constituting half the planet’s diameter. Within this core run powerful electric currents that ultimately create the geomagnetic field.

A north-south bar magnet exerts exactly the same sort of magnetic field as a solenoid. Lines of magnetic run through it in one direction, from “south” to “north,” and upon leaving the north pole of the magnet, these lines describe an ellipse as they curve back around to the south pole. In view of this model, it is also easy to comprehend why a pair of opposing poles attracts one anoth-

Just as a powerful magnet causes all the domains in a magnetic metal to align with it, a bar magnet placed in a magnetic field will rotate until it lines up with the field’s direction. The same thing happens when one suspends a magnet from a string: it lines up with Earth’s magnetic field, and points in a north-south direction. The Chinese of the first century B.C., though unaware of the electromagnetic forces that caused this to happen, discovered that a strip of magnetic metal always tended to point toward geographic north. This led ultimately to the development of the magnetic compass, which typically consists of a magnetized iron needle suspended over a card marked with the four cardinal directions. The needle is attached to a pivoting mechanism at its center, which allows it to move freely so that the “north” end will always point the user northward. The magnetic compass proved so important that it is typically ranked alongside paper, printing, and gunpowder as one of premodern China’s four great gifts to the West. Prior to the compass, mariners had to depend purely on the position of the Sun and other, less reliable, means of determining direction; hence the invention quite literally helped open up the world. But there is a somewhat irksome anomaly lurking in the seeming simplicity of the magnetic compass. In fact magnetic north is not the same thing as true north; or, to put it another way, if one continued to follow a compass northward, it would lead not to the Earth’s North Pole, but to a point identified in 1984 as 77°N, 102°18’ W— that is, in the Queen Elizabeth Islands of far

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er, and a pair of like poles—for whom the lines of force are moving away from each other—repels. This is a fact particularly applicable to the operation of MAGLEV trains, as discussed later.

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northern Canada. The reason for this is that Earth’s magnetic field describes a current loop whose center is 11° off the planet’s equator, and thus the north and south magnetic poles—which are on a plane perpendicular to that of the Earth’s magnetic field—are 11° off of the planet’s axis. The magnetic field of Earth is changing position slowly, and every few years the United States Geological Survey updates magnetic declination, or the shift in the magnetic field. In addition, Earth’s magnetic field is slowly weakening as well. The behavior, both in terms of weakening and movement, appears to be similar to changes taking place in the magnetic field of the Sun.

Magnets for Detection: Burglar Alarms, Magnetometers, and MRI A compass is a simple magnetic instrument, and a burglar alarm is not much more complex. A magnetometer, on the other hand, is a much more sophisticated piece of machinery for detecting the strength of magnetic fields. Nonetheless, the magnetometer bears a relation to its simpler cousins: like a compass, certain kinds of magnetometers respond to a planet’s magnetic field; and like a burglar alarm, other varieties of magnetometer are employed for security. At heart, a burglar alarm consists of a contact switch, which responds to changes in the environment and sends a signal to a noisemaking device. The contact switch may be mechanical— a simple fastener, for instance—or magnetic. In the latter case, a permanent magnet may be installed in the frame of a window or door, and a piece of magnetized material in the window or door itself. Once the alarm is activated, it will respond to any change in the magnetic field— i.e., when someone slides open the door or window, thus breaking the connection between magnet and metal. Though burglar alarms may vary in complexity, and indeed there may be much more advanced systems using microwaves or infrared rays, the application of magnetism in home security is a simple matter of responding to changes in a magnetic field. In this regard, the principle governing magnetometers used at security checkpoints is even simpler. Whether at an airport or at the entrance to some other high-secu-

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rity venue, whether handheld or stationary, a magnetometer merely detects the presence of magnetic metals. Since the vast majority of firearms, knife-blades, and other weapons are made of iron or steel, this provides a fairly efficient means of detection. At a much larger scale, magnetometers used by astronomers detect the strength and sometimes the direction of magnetic fields surrounding Earth and other bodies in space. This variety of magnetometer dates back to 1832, when mathematician and scientist Carl Friedrich Gauss (1777-1855) developed a simple instrument consisting of a permanent bar magnet suspended horizontally by means of a gold wire. By measuring the period of the magnet’s oscillation in Earth’s magnetic field (or magnetosphere), Gauss was able to measure the strength of that field. Gauss’s name, incidentally, would later be applied to the term for a unit of magnetic force. The gauss, however, has in recent years been largely replaced by the tesla, named after Nikola Tesla (1856-1943), which is equal to one newton/ampere meter (1 N/A•m) or 104 (10,000) gauss. As for magnetometers used in astronomical research, perhaps the most prominent—and certainly one of the most distant—ones is on Galileo, a craft launched by the U.S. National Aeronautics and Space Administration (NASA) toward Jupiter on October 15, 1989. Among other instruments on board Galileo, which has been in orbit around the solar system’s largest planet since 1995, is a magnetometer for measuring Jupiter’s magnetosphere and that of its surrounding asteroids and moons.

Magnetism

Closer to home, but no less impressive, is another application of magnetism for the purposes of detection: magnetic resonance imagining, or MRI. First developed in the early 1970s, MRI permits doctors to make intensive diagnoses without invading the patient’s body either with a surgical knife or x rays. The heart of the MRI machine is a large tube into which the patient is placed in a supine position. A technician then activates a powerful magnetic field, which causes atoms within the patient’s body to spin at precise frequencies. The machine then beams radio signals at a frequency matching that of the atoms in the cells (e.g., cancer cells) being sought. Upon shutting off the radio signals and magnetic field, those atoms

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emit bursts of energy that they have absorbed from the radio waves. At that point a computer scans the body for frequencies matching specific types of atoms, and translates these into threedimensional images for diagnosis.

Magnets for Projecting Sound: Microphones, Loudspeakers, Car Horns, and Electric Bells The magnets used in Galileo or an MRI machine are, needless to say, very powerful ones, and as noted earlier, the best way to create a superstrong, controllable magnet is with an electrical current. When that current is properly coiled around a magnetic metal, this creates an electromagnet, which can be used in a variety of applications. As discussed above, the most powerful electromagnets typically use nonpermanent magnets so as to facilitate an easy transition from an extremely strong magnetic field to a weak or nonexistent one. On the other hand, permanent magnets are also used in loudspeakers and similar electromagnetic devices, which seldom require enormous levels of power. In discussing the operation of a loudspeaker, it is first necessary to gain a basic understanding of how a microphone works. The latter contains a capacitor, a system for storing charges in the form of an electrical field. The capacitor’s negatively charged plate constitutes the microphone’s diaphragm, which, when it is hit by sound waves, vibrates at the same frequency as those waves. Current flows back and forth between the diaphragm and the positive plate of the capacitor, depending on whether the electrostatic or electrical pull is increasing or decreasing. This in turn produces an alternating current, at the same frequency as the sound waves, which travels through a mixer and then an amplifier to the speaker. A loudspeaker typically contains a circular permanent magnet, which surrounds an electrical coil and is in turn attached to a cone-shaped diaphragm. Current enters the speaker ultimately from the microphone, alternating at the same frequency as the source of the sound (a singer’s voice, for instance). As it enters the coil, this current induces an alternating magnetic field, which causes the coil to vibrate. This in turn vibrates

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the cone-shaped diaphragm, and the latter reproduces sounds generated at the source. A car horn also uses magnetism to create sound by means of vibration. When a person presses down on the horn embedded in his or her steering wheel, this in turn depresses an iron bar that passes through an electromagnet surrounded by wires from the car’s battery. The bar moves up and down within the electromagnetic field, causing the diaphragm to vibrate and producing a sound that is magnified greatly when released through a bell-shaped horn. Electromagnetically induced vibration is also the secret behind another noise-making device, a vibrating electric doorbell used in many apartments. The button that a visitor presses is connected directly to a power source, which sends current flowing through a spring surrounding an electromagnet. The latter generates a magnetic field, drawing toward it an iron armature attached to a hammer. The hammer then strikes the bell. The result is a mechanical reaction that pushes the armature away from the electromagnet, but the spring forces the armature back against the electromagnet again. This cycle of contact and release continues for as long as the button is depressed, causing a continual ringing of the bell.

Recording and Reading Data Using Magnets: From Records and Tapes to Disk Drives Just as magnetism plays a critical role in projecting the volume of sound, it is also crucial to the recording and retrieval of sound and other data. Of course terms such as “retrieval” and “data” have an information-age sound to them, but the idea of using magnetism to record sound is an old one—much older than computers or compact discs (CDs). The latter, of course, replaced cassettes in the late 1980s as the preferred mode for listening to recorded music, just as cassettes had recently made powerful gains against phonograph records. Despite the fact that cassettes entered the market much later than records, however, recording engineers from the mid-twentieth century onward typically used magnetic tape for master recordings of songs. This master would then be used to create a metal master record disk

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by means of a cutting head that responded to vibrations from the master tape; then, the record company could produce endless plastic copies of the metal record.

Magnetism

In recording a tape—whether a stereo master or a mere home recording of a conversation— the principles at work are more or less the same. As noted in the earlier illustration involving a microphone and loudspeaker, sound comes through a microphone in the form of alternating current. The strength of this current in turn affects the “recording head,” a small electromagnet whose magnetic field extends over the section of tape being recorded. Loud sounds produce strong magnetic fields, and soft ones weak fields. All of this information becomes embedded on the cassette tape through a process of magnetic alignment not so different from the process described earlier for creating a permanent magnet. But whereas the permanent magnetization of a natural magnet is difficult to reverse, reversal of a tape’s magnetization—in other words, erasing the tape—is easy. An erase head, an electromagnet operating at a frequency too high for the human ear to hear, simply scrambles the magnetic particles on a piece of tape. A CD, as one might expect, is much more closely related to a computer disk-drive than it is to earlier forms of recording technology. The disk drive receives electronic on-off signals from the computer, and translates these into magnetic codes that it records on the surface of a floppy disk. The disk drive itself includes two electric motors: a disk motor, which rotates the disk at a high speed, and a head motor, which moves the computer’s read-write head across the disk. (It should be noted that most electric motors, including the universal motors used in a variety of household appliances, also use electromagnets.)

MAGLEV TRAINS, LIKE THIS EXPERIMENTAL ONE IN MIYAZAKI, JAPAN, MAY REPRESENT THE FUTURE OF MASS TRANSIT. (Photograph by Michael S. Yamashita/Corbis. Reproduced by permission.)

MAGLEV Trains: The Future of Transport? One promising application of electromagnetic technology relates to a form of transportation that might, at first glance, appear to be old news: trains. But MAGLEV, or magnetic levitation, trains are as far removed from the old steam engines of the Union Pacific as the space shuttle is from the Wright brothers’ experimental airplane.

A third motor, called a stepper motor, ensures that the drive turns at a precise rate of speed. The stepper motor contains its own magnet, in this case a permanent one of cylindrical shape that sends signals to rows of metal teeth surrounding it, and these teeth act as gears to regulate the drive’s speed. Likewise a CD player, which actually uses laser beams rather than magnetic fields to retrieve data from a disc, also has a drive system that regulates the speed at which the disk spins.

As discussed earlier, magnetic poles of like direction (i.e., north-north or south-south) repel one another such that, theoretically at least, it is possible to keep one magnet suspended in the air over another magnet. Actually it is impossible to produce these results with simple bar magnets, because their magnetic force is too small; but an electromagnet can create a magnetic field powerful enough that, if used properly, it exerts enough repulsive force to lift extremely heavy objects. Specifically, if one could activate train tracks with a strong electromagnetic field, it might be possible to “levitate” an entire train. This in turn

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Magnetism

KEY TERMS

ELECTROMAGNET:

A type of magnet

in which an object is charged by an electrical current. Typically the object used is made of iron, which quickly loses magnetic force when current is reduced. Thus an electromagnet can be turned on or off, and its magnetic force altered, making it potentially much more powerful than a natural magnet. ELECTROMAGNETISM:

The unified

electrical and magnetic force field generated by the passage of an electric current through matter. ELECTRONS:

Negatively charged sub-

atomic particles whose motion relative to one another creates magnetic force. MAGNETIC FIELD:

Wherever a mag-

netic force acts on a moving charged particle, a magnetic field is said to exist. Magnetic fields are typically measured by a unit called a tesla. NATURAL MAGNET:

A chemical ele-

ment in which the magnetic fields created by electrons’ relative motion align uniformly to create a net magnetic dipole, or unity of direction. Such elements, among them iron, cobalt, and nickel, are also known as magnetic metals. PERMANENT MAGNET:

A magnetic

material in which groups of atoms, known as domains, are brought into alignment, and in which magnetization cannot be changed merely by attempting to realign the domains. Permanent magnetization is reversible only at very high temperatures— for example, 1,418°F (770°C) in the case of iron.

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would make possible a form of transport that could move large numbers of people in relative comfort, thus decreasing the environmental impact of automobiles, and do so at much higher speeds than a car could safely attain. Actually the idea of MAGLEV trains goes back to a time when trains held complete supremacy over automobiles as a mode of transportation: specifically, 1907, when rocket pioneer Robert Goddard (1882-1945) wrote a story describing a vehicle that traveled by means of magnetic levitation. Just five years later, French engineer Emile Bachelet produced a working model for a MAGLEV train. But the amount of magnetic force required to lift such a vehicle made it impractical, and the idea fell to the wayside. Then, in the 1960s, the advent of superconductivity—the use of extremely low temperatures, which facilitate the transfer of electrical current through a conducting material with virtually no resistance—made possible electromagnets of staggering force. Researchers began building MAGLEV prototypes using superconducting coils with strong currents to create a powerful magnetic field. The field in turn created a repulsive force capable of lifting a train several inches above a railroad track. Electrical current sent through guideway coils on the track allowed for enormous propulsive force, pushing trains forward at speeds up to and beyond 250 MPH (402 km/h). Initially, researchers in the United States were optimistic about MAGLEV trains, but safety concerns led to the shelving of the idea for several decades. Meanwhile, other industrialized nations moved forward with MAGLEVs: in Japan, engineers built a 27-mi (43.5-km) experimental MAGLEV line, while German designers experimented with attractive (as opposed to repulsive) force in their Transrapid 07. MAGLEV trains gained a new defender in the United States with now-retired Senator Daniel Patrick Moynihan (D-NY), who as chairman of a Senate subcommittee overseeing the interstate highway system introduced legislation to fund MAGLEV research. The 1998 transportation bill allocated $950 million toward the Magnetic Levitation Prototype Development Program. As part of this program, in January 2001 the U.S. Department of Transportation selected projects in Maryland and Pennsylvania as the two finalists in the com-

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petition to build the first MAGLEV train service in the United States. The goal is to have the service in place by approximately 2010. WHERE TO LEARN MORE

Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998. Molecular Expressions: Electricity and Magnetism: Interactive Java Tutorials (Web site). (January 26, 2001).

Barr, George. Science Projects for Young People. New York: Dover, 1964.

Topical Group on Magnetism (Web site). (January 26, 2001).

Beiser, Arthur. Physics, 5th ed. Reading, MA: Addison-Wesley, 1991.

VanCleave, Janice. Magnets. New York: John Wiley & Sons, 1993.

Hann, Judith. How Science Works. Pleasantville, NY: Reader’s Digest, 1991.

Wood, Robert W. Physics for Kids: 49 Easy Experiments with Electricity and Magnetism. New York: Tab, 1990.

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ELECTROMAGNETIC SPECTRUM Electromagnetic Spectrum

CONCEPT One of the most amazing aspects of physics is the electromagnetic spectrum—radio waves, microwaves, infrared light, visible light, ultraviolet light, x rays, and gamma rays—as well as the relationship between the spectrum and electromagnetic force. The applications of the electromagnetic spectrum in daily life begin the moment a person wakes up in the morning and “sees the light.” Yet visible light, the only familiar part of the spectrum prior to the eighteenth and nineteenth centuries, is also its narrowest region. Since the beginning of the twentieth century, uses for other bands in the electromagnetic spectrum have proliferated. At the low-frequency end are radio, short-wave radio, and television signals, as well as the microwaves used in cooking. Higher-frequency waves, all of which can be generally described as light, provide the means for looking deep into the universe—and deep into the human body.

F O U N DAT I O N S O F E L E C T R O M AG N E T I C T H E O RY. At the same time,

The ancient Romans observed that a brushed comb would attract particles, a phenomenon now known as static electricity and studied within the realm of electrostatics in physics. Yet, the Roman understanding of electricity did not extend any further, and as progress was made in the science of physics—after a period of more than a thousand years, during which scientific learning in Europe progressed very slowly—it developed in areas that had nothing to do with the strange force observed by the Romans.

a number of thinkers conducted experiments concerning the nature of electricity and magnetism, and the relationship between them. Among these were several giants in physics and other disciplines—including one of America’s greatest founding fathers. In addition to his famous (and highly dangerous) experiment with lightning, Benjamin Franklin (1706-1790) also contributed the names “positive” and “negative” to the differing electrical charges discovered earlier by French physicist Charles Du Fay (1698-1739). In 1785, French physicist and inventor Charles Coulomb (1736-1806) established the basic laws of electrostatics and magnetism. He maintained that there is an attractive force that, like gravitation, can be explained in terms of the inverse of the square of the distance between objects. That attraction itself, however, resulted not from gravity, but from electrical charge, according to Coulomb. A few years later, German mathematician Johann Karl Friedrich Gauss (1777-1855) developed a mathematical theory for finding the magnetic potential of any point on Earth, and his

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The fathers of physics as a serious science, Galileo Galilei (1564-1642) and Sir Isaac Newton (1642-1727), were concerned with gravitation, which Newton identified as a fundamental force in the universe. For nearly two centuries, physicists continued to believe that there was only one type of force. Yet, as scientists became increasingly aware of molecules and atoms, anomalies began to arise—in particular, the fact that gravitation alone could not account for the strong forces holding atoms and molecules together to form matter.

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contemporary, Danish physicist Hans Christian Oersted (1777-1851), became the first scientist to establish the existence of a clear relationship between electricity and magnetism. This led to the foundation of electromagnetism, the branch of physics devoted to the study of electrical and magnetic phenomena. French mathematician and physicist André Marie Ampère (1775-1836) concluded that magnetism is the result of electricity in motion, and, in 1831, British physicist and chemist Michael Faraday (1791-1867) published his theory of electromagnetic induction. This theory shows how an electrical current in one coil can set up a current in another through the development of a magnetic field. This enabled Faraday to develop the first generator, and for the first time in history, humans were able to convert mechanical energy systematically into electrical energy. M AX W E L L A N D E L E C T R O M AG N E T I C F O R C E . A number of other figures

contributed along the way; but, as yet, no one had developed a “unified theory” explaining the relationship between electricity and magnetism. Then, in 1865, Scottish physicist James Clerk Maxwell (1831-1879) published a groundbreaking paper, “On Faraday’s Lines of Force,” in which he outlined a theory of electromagnetic force— the total force on an electrically charged particle, which is a combination of forces due to electrical and/or magnetic fields around the particle. Maxwell had thus discovered a type of force in addition to gravity, and this reflected a “new” type of fundamental interaction, or a basic mode by which particles interact in nature. Newton had identified the first, gravitational interaction, and in the twentieth century, two other forms of fundamental interaction—strong nuclear and weak nuclear—were identified as well. In his work, Maxwell drew on the studies conducted by his predecessors, but added a new statement: that electrical charge is conserved. This statement, which did not contradict any of the experimental work done by the other physicists, was based on Maxwell’s predictions regarding what should happen in situations of electromagnetism; subsequent studies have supported his predictions.

netism. This understanding grew enormously in the late nineteenth and early twentieth centuries, thanks both to the theoretical work of physicists, and the practical labors of inventors such as Thomas Alva Edison (1847-1931) and SerbianAmerican electrical engineer Nikola Tesla (18561943). But our concern in the present context is with electromagnetic radiation, of which the waves on the electromagnetic spectrum are a particularly significant example. Energy can travel by conduction or convection, two principal means of heat transfer. But the energy Earth receives from the Sun—the energy conveyed through the electromagnetic spectrum—is transferred by another method, radiation. Whereas conduction of convection can only take place where there is matter, which provides a medium for the energy transfer, radiation requires no medium. Thus, electromagnetic energy passes from the Sun to Earth through the vacuum of empty space. ELECTROMAGNETIC

WAV E S .

The connection between electromagnetic radiation and electromagnetic force is far from obvious. Even today, few people not scientifically trained understand that there is a clear relationship between electricity and magnetism—let alone a connection between these and visible light. The breakthrough in establishing that connection can be attributed both to Maxwell and to German physicist Heinrich Rudolf Hertz (18571894). Maxwell had suggested that electromagnetic force carried with it a certain wave phenomenon, and predicted that these waves traveled at a certain speed. In his Treatise on Electricity and Magnetism (1873), he predicted that the speed of these waves was the same as that of light— 186,000 mi (299,339 km) per second—and theorized that the electromagnetic interaction included not only electricity and magnetism, but light as well. A few years later, while studying the behavior of electrical currents, Hertz confirmed Maxwell’s proposition regarding the wave phenomenon by showing that an electrical current generated some sort of electromagnetic radiation.

So far, what we have seen is the foundation for modern understanding of electricity and mag-

In addition, Hertz found that the flow of electrical charges could be affected by light under certain conditions. Ultraviolet light had already been identified, and Hertz shone an ultraviolet beam on the negatively charged side of a gap in a

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THE HUBBLE SPACE TELESCOPE INCLUDES AN ULTRAVIOLET LIGHT INSTRUMENT CALLED THE GODDARD HIGH RESOLUTION SPECTROGRAPH THAT IT IS CAPABLE OF OBSERVING EXTREMELY DISTANT OBJECTS. (Photograph by Roger Ressmeyer/Corbis. Reproduced by permission.)

current loop. This made it easier for an electrical spark to jump the gap. Hertz could not explain this phenomenon, which came to be known as the photoelectric effect. Indeed, no one else could explain it until quantum theory was developed in the early twentieth century. In the meantime, however, Hertz’s discovery of electromagnetic waves radiating from a current loop led to the invention of radio by Italian physicist and engineer Guglielmo Marconi (1874-1937) and others.

At this point, it is necessary to jump backward in history, to explain the progression of scientists’

understanding of light. Advancement in this area took place over a long period of time: at the end of the first millennium A.D., the Arab physicist Alhasen (Ibn al-Haytham; c. 965-1039) showed that light comes from the Sun and other self-illuminated bodies—not, as had been believed up to that time—from the eye itself. Thus, studies in optics, or the study of light and vision, were— compared to understanding of electromagnetism itself—relatively advanced by 1666, when Newton discovered the spectrum of colors in light. As Newton showed, colors are arranged in a sequence, and white light is a combination of all colors.

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Newton put forth the corpuscular theory of light—that is, the idea that light is made up of particles—but his contemporary Christiaan Huygens (1629-1695), a Dutch physicist and astronomer, maintained that light appears in the form of a wave. For the next century, adherents of Newton’s corpuscular theory and of Huygens’s wave theory continued to disagree. Physicists on the European continent began increasingly to accept wave theory, but corpuscular theory remained strong in Newton’s homeland.

rudiments of wave motion will be presented in short form.

Thus, it was ironic that the physicist whose work struck the most forceful blow against corpuscular theory was himself an Englishman: Thomas Young (1773-1829), who in 1801 demonstrated interference in light. Young directed a beam of light through two closely spaced pinholes onto a screen, reasoning that if light truly were made of particles, the beams would project two distinct points onto the screen. Instead, what he saw was a pattern of interference—a wave phenomenon.

The term periodic motion, or movement repeated at regular intervals called periods, describes the behavior of periodic waves: waves in which a uniform series of crests and troughs follow each other in regular succession. Periodic waves are divided into longitudinal and transverse waves, the latter (of which light waves are an example) being waves in which the vibration or motion is perpendicular to the direction in which the wave is moving. Unlike longitudinal waves, such as those that carry sound energy, transverse waves are fairly easy to visualize, and assume the shape that most people imagine when they think of waves: a regular up-and-down pattern, called “sinusoidal” in mathematical terms.

By the time of Hertz, wave theory had become dominant; but the photoelectric effect also exhibited aspects of particle behavior. Thus, for the first time in more than a century, particle theory gained support again. Yet, it was clear that light had certain wave characteristics, and this raised the question—which is it, a wave or a set of particles streaming through space? The work of German physicist Max Planck (1858-1947), father of quantum theory, and of Albert Einstein (1879-1955), helped resolve this apparent contradiction. Using Planck’s quantum principles, Einstein, in 1905, showed that light appears in “bundles” of energy, which travel as waves but behave as particles in certain situations. Eighteen years later, American physicist Arthur Holly Compton (1892-1962) showed that, depending on the way it is tested, light appears as either a particle or a wave. These particles he called photons.

Wave Motion and Electromagnetic Waves

A type of harmonic motion that carries energy from one place to another without actually moving any matter, wave motion is related to oscillation, harmonic—and typically periodic— motion in one or more dimensions. Oscillation involves no net movement, but only movement in place; yet individual waves themselves are oscillating, even as the overall wave pattern moves.

Electromagnetic Spectrum

PA RA M E T E R S O F WAV E M O T I O N . A period (represented by the symbol T)

is the amount of time required to complete one full cycle of the wave, from trough to crest and back to trough. Period is mathematically related to several other aspects of wave motion, including wave speed, frequency, and wavelength. Frequency (abbreviated f) is the number of waves passing through a given point during the interval of one second. It is measured in Hertz (Hz), named after Hertz himself: a single Hertz (the term is both singular and plural) is equal to one cycle of oscillation per second. Higher frequencies are expressed in terms of kilohertz (kHz; 103 or 1,000 cycles per second); megahertz (MHz; 106 or 1 million cycles per second); and gigahertz (GHz; 109 or 1 billion cycles per second.)

The particle behavior of electromagnetic energy is beyond the scope of the present discussion, though aspects of it are discussed elsewhere. For the present purposes, it is necessary only to view the electromagnetic spectrum as a series of waves, and in the paragraphs that follow, the

Wavelength (represented by the symbol λ, the Greek letter lambda) is the distance between a crest and the adjacent crest, or a trough and an adjacent trough, of a wave. The higher the frequency, the shorter the wavelength; and, thus, it is possible to describe waves in terms of either. According to quantum theory, however, electromagnetic waves can also be described in terms of

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photon energy level, or the amount of energy in each photon. Thus, the electromagnetic spectrum, as we shall see, varies from relatively longwavelength, low-frequency, low-energy radio waves on the one end to extremely short-wavelength, high-frequency, high-energy gamma rays on the other. The other significant parameter for describing a wave—one mathematically independent from those so far discussed—is amplitude. Defined as the maximum displacement of a vibrating material, amplitude is the “size” of a wave. The greater the amplitude, the greater the energy the wave contains: amplitude indicates intensity. The amplitude of a light wave, for instance, determines the intensity of the light. A R I G H T - H A N D R U L E . Physics textbooks use a number of “right-hand rules”: devices for remembering certain complex physical interactions by comparing the lines of movement or force to parts of the right hand. In the present context, a right-hand rule makes it easier to visualize the mutually perpendicular directions of electromagnetic waves, electric field, and magnetic field.

A field is a region of space in which it is possible to define the physical properties of each point in the region at any given moment in time. Thus, an electrical field and magnetic field are simply regions in which electrical and magnetic components, respectively, of electromagnetic force are exerted. Hold out your right hand, palm perpendicular to the floor and thumb upright. Your fingers indicate the direction that an electromagnetic wave is moving. Your thumb points in the direction of the electrical field, as does the heel of your hand: the electrical field forms a plane perpendicular to the direction of wave propagation. Similarly, both your palm and the back of your hand indicate the direction of the magnetic field, which is perpendicular both to the electrical field and the direction of wave propagation.

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mutually perpendicular electrical and magnetic fields that emanate from it. The electromagnetic spectrum is the complete range of electromagnetic waves on a continuous distribution from a very low range of frequencies and energy levels, with a correspondingly long wavelength, to a very high range of frequencies and energy levels, with a correspondingly short wavelength. Included on the electromagnetic spectrum are radio waves and microwaves; infrared, visible, and ultraviolet light; x rays, and gamma rays. Though each occupies a definite place on the spectrum, the divisions between them are not firm: as befits the nature of a spectrum, one simply “blurs” into another. F R E Q U E N CY RA N G E O F T H E E L E C T R O M AG N E T I C S P E C T R U M .

The range of frequencies for waves in the electromagnetic spectrum is from approximately 102 Hz to more than 1025 Hz. These numbers are an example of scientific notation, which makes it possible to write large numbers without having to include a string of zeroes. Without scientific notation, the large numbers used for discussing properties of the electromagnetic spectrum can become bewildering. The first number given, for extremely lowfrequency radio waves, is simple enough—100— but the second would be written as 1 followed by 25 zeroes. (A good rule of thumb for scientific notation is this: for any power n of 10, simply attach that number of zeroes to 1. Thus 106 is 1 followed by 6 zeroes, and so on.) In any case, 1025 is a much simpler figure than 10,000,000,000,000,000,000,000,000—or 10 trillion trillion. As noted earlier, gigahertz, or units of 1 billion Hertz, are often used in describing extremely high frequencies, in which case the number is written as 1016 GHz. For simplicity’s sake, however, in the present context, the simple unit of Hertz (rather than kilo-, mega-, or gigahertz) is used wherever it is convenient to do so. WAV E L E N GT H S O N T H E E L E C T R O M A G N E T I C S P E C T R U M . The

As stated earlier, an electromagnetic wave is transverse, meaning that even as it moves forward, it oscillates in a direction perpendicular to the line of propagation. An electromagnetic wave can thus be defined as a transverse wave with

range of wavelengths found in the electromagnetic spectrum is from about 108 centimeters to less than 10-15 centimeters. The first number, equal to 1 million meters (about 621 mi), obviously expresses a great length. This figure is for radio waves of extremely low frequency; ordinary radio waves of the kind used for actual radio

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broadcasts are closer to 105 centimeters (about 328 ft). For such large wavelengths, the use of centimeters might seem a bit cumbersome; but, as with the use of Hertz for frequencies, centimeters provide a simple unit that can be used to measure all wavelengths. Some charts of the electromagnetic spectrum nonetheless give figures in meters, but for parts of the spectrum beyond microwaves, this, too, can become challenging. The ultra-short wavelengths of gamma rays, after all, are equal to one-trillionth of a centimeter. By comparison, the angstrom—a unit so small it is used to measure the diameter of an atom—is 10 million times as large. ENERGY LEVELS ON THE ELECTROMAGNETIC SPECTRUM. Finally, in

terms of photon energy, the unit of measurement is the electron volt (eV), which is used for quantifying the energy in atomic particles. The range of photon energy in the electromagnetic spectrum is from about 10-13 to more than 1010 electron volts. Expressed in terms of joules, an electron volt is equal to 1.6 • 10-19 J. To equate these figures to ordinary language would require a lengthy digression; suffice it to say that even the highest ranges of the electromagnetic spectrum possess a small amount of energy in terms of joules. Remember, however, that the energy level identified is for a photon— a light particle. Again, without going into a great deal of detail, one can just imagine how many of these particles, which are much smaller than atoms, would fit into even the smallest of spaces. Given the fact that electromagnetic waves are traveling at a speed equal to that of light, the amount of photon energy transmitted in a single second is impressive, even for the lower ranges of the spectrum. Where gamma rays are concerned, the energy levels are positively staggering.

REAL-LIFE A P P L I C AT I O N S

lowest frequencies, the longest wavelengths, and the smallest levels of photon energy. Included in this broad sub-spectrum, with frequencies up to about 107 Hertz, are long-wave radio, short-wave radio, and microwaves. The areas of communication affected are many: broadcast radio, television, mobile phones, radar—and even highly specific forms of technology such as baby monitors. Though the work of Maxwell and Hertz was foundational to the harnessing of radio waves for human use, the practical use of radio had its beginnings with Marconi. During the 1890s, he made the first radio transmissions, and, by the end of the century, he had succeeded in transmitting telegraph messages across the Atlantic Ocean—a feat which earned him the Nobel Prize for physics in 1909. Marconi’s spark transmitters could send only coded messages, and due to the broad, longwavelength signals used, only a few stations could broadcast at the same time. The development of the electron tube in the early years of the twentieth century, however, made it possible to transmit narrower signals on stable frequencies. This, in turn, enabled the development of technology for sending speech and music over the airwaves.

Broadcast Radio THE DEVELOPMENT OF AM AND FM. A radio signal is simply a carrier: the

process of adding information—that is, complex sounds such as those of speech or music—is called modulation. The first type of modulation developed was AM, or amplitude modulation, which Canadian-American physicist Reginald Aubrey Fessenden (1866-1932) demonstrated with the first United States radio broadcast in 1906. Amplitude modulation varies the instantaneous amplitude of the radio wave, a function of the radio station’s power, as a means of transmitting information.

Among the most familiar parts of the electromagnetic spectrum, in modern life at least, is radio. In most schematic representations of the spectrum, radio waves are shown either at the left end or the bottom, as an indication of the fact that these are the electromagnetic waves with the

By the end of World War I, radio had emerged as a popular mode of communication: for the first time in history, entire nations could hear the same sounds at the same time. During the 1930s, radio became increasingly important, both for entertainment and information. Families in the era of the Great Depression would gather around large “cathedral radios”—so named for their size and shape—to hear comedy programs, soap operas, news programs, and

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speeches by important public figures such as President Franklin D. Roosevelt. Throughout this era—indeed, for more than a half-century from the end of the first World War to the height of the Vietnam Conflict in the mid-1960s—AM held a dominant position in radio. This remained the case despite a number of limitations inherent in amplitude modulation: AM broadcasts flickered with popping noises from lightning, for instance, and cars with AM radios tended to lose their signal when going under a bridge. Yet, another mode of radio transmission was developed in the 1930s, thanks to American inventor and electrical engineer Edwin H. Armstrong (1890-1954). This was FM, or frequency modulation, which varied the radio signal’s frequency rather than its amplitude. Not only did FM offer a different type of modulation; it was on an entirely different frequency range. Whereas AM is an example of a long-wave radio transmission, FM is on the microwave sector of the electromagnetic spectrum, along with television and radar. Due to its high frequency and form of modulation, FM offered a “clean” sound as compared with AM. The addition of FM stereo broadcasts in the 1950s offered still further improvements; yet despite the advantages of FM, audiences were slow to change, and FM did not become popular until the mid- to late 1960s. S I G N A L P R O PAGAT I O N . AM signals have much longer wavelengths, and smaller frequencies, than do FM signals, and this, in turn, affects the means by which AM signals are propagated. There are, of course, much longer radio wavelengths; hence, AM signals are described as intermediate in wavelength. These intermediatewavelength signals reflect off highly charged layers in the ionosphere between 25 and 200 mi (40-332 km) above Earth’s surface. Short-wavelength signals, such as those of FM, on the other hand, follow a straight-line path. As a result, AM broadcasts extend much farther than FM, particularly at night.

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osphere known as the F layer. (This is also sometimes known as the Heaviside layer, or the Kennelly-Heaviside layer, after English physicist Oliver Heaviside and British-American electrical engineer Arthur Edwin Kennelly, who independently discovered the ionosphere in 1902.) AM signals “bounce” off the F layer as though it were a mirror, making it possible for a listener at night to pick up a signal from halfway across the country. The Sun has other effects on long-wave and intermediate-wave radio transmissions. Sunspots, or dark areas that appear on the Sun in cycles of about 11 years, can result in a heavier buildup of the ionosphere than normal, thus impeding radio-signal propagation. In addition, occasional bombardment of Earth by charged particles from the Sun can also disrupt transmissions. Due to the high frequencies of FM signals, these do not reflect off the ionosphere; instead, they are received as direct waves. For this reason, an FM station has a fairly short broadcast range, and this varies little with regard to day or night. The limited range of FM stations as compared to AM means that there is much less interference on the FM dial than for AM.

Distribution of Radio Frequencies In the United States and most other countries, one cannot simply broadcast at will; the airwaves are regulated, and, in America, the governing authority is the Federal Communications Commission (FCC). The FCC, established in 1934, was an outgrowth of the Federal Radio Commission, founded by Congress seven years earlier. The FCC actually “sells air,” charging companies a fee to gain rights to a certain frequency. Those companies may in turn sell that air to others for a profit.

At a low level in the ionosphere is the D layer, created by the Sun when it is high in the sky. The D layer absorbs medium-wavelength signals during the day, and for this reason, AM signals do not travel far during daytime hours. After the Sun goes down, however, the D layer soon fades, and this makes it possible for AM signals to reflect off a much higher layer of the ion-

At the time of the FCC’s establishment, AM was widely used, and the federal government assigned AM stations the frequency range of 535 kHz to 1.7 MHz. Thus, if an AM station today is called, for instance, “AM 640,” this means that it operates at 640 kHz on the dial. The FCC assigned the range of 5.9 to 26.1 MHz to shortwave radio, and later the area of 26.96 to 27.41 MHz to citizens’ band (CB) radio. Above these are microwave regions assigned to television sta-

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tions, as well as FM, which occupies the range from 88 to 108 MHz. The organization of the electromagnetic spectrum’s radio frequencies—which, of course, is an entirely arbitrary, humanmade process—is fascinating. It includes assigned frequencies for everything from garage-door openers to deepspace radio communications. The FCC recognizes seven divisions of radio carriers, using a system that is not so much based on rational rules as it is on the way that the communications industries happened to develop over time. THE SEVEN FCC DIVISIONS.

Most of what has so far been described falls under the heading of “Public Fixed Radio Services”: AM and FM radio, other types of radio such as shortwave, television, various other forms of microwave broadcasting, satellite systems, and communication systems for federal departments and agencies. “Public Mobile Services” include pagers, air-to-ground service (for example, aircraft-to-tower communications), offshore service for sailing vessels, and rural radio-telephone service. “Commercial Mobile Radio Services” is the realm of cellular phones, and “Personal Communications Service” that of the newer wireless technology that began to challenge cellular for market dominance in the late 1990s. “Private Land Mobile Radio Service” (PMR) and “Private Operational-Fixed Microwave Services” (OFS) are rather difficult to distinguish, the principal difference being that the former is used exclusively by profit-making businesses, and the latter mostly by nonprofit institutions. An example of PMR technology is the dispatching radios used by taxis, but this is only one of the more well-known forms of internal electronic communications for industry. For instance, when a film production company is shooting a picture and the director needs to speak to someone at the producer’s trailer a mile away, she may use PMR radio technology. OFS was initially designated purely for nonprofit use, and is used often by schools; but banks and other profit-making institutions often use OFS because of its low cost.

Video and Data Services, or IVDS. Unlike other types of video technology, these will all be wireless, and, therefore, represent a telecommunications revolution all their own.

Electromagnetic Spectrum

Microwaves MICROWAVE COMMUNICATION.

Though microwaves are treated separately from radio waves, in fact, they are just radio signals of a very short wavelength. As noted earlier, FM signals are actually carried on microwaves, and, as with FM in particular, microwave signals in general are very clear and very strong, but do not extend over a great geographical area. Nor does microwave include only high-frequency radio and television; in fact, any type of information that can be transmitted via telephone wires or coaxial cables can also be sent via a microwave circuit. Microwaves have a very narrow, focused beam: thus, the signal is amplified considerably when an antenna receives it. This phenomenon, known as “high antenna gain,” means that microwave transmitters need not be highly powerful to produce a strong signal. To further the reach of microwave broadcasts, transmitters are often placed atop mountain peaks, hilltops, or tall buildings. In the past, a microwave-transmitting network such as NBC (National Broadcasting Company) or CBS (Columbia Broadcasting System) required a network of ground-based relay stations to move its signal across the continent. The advent of satellite broadcasting in the 1960s, however, changed much about the way signals are beamed: today, networks typically replace, or at least augment, ground-based relays with satellite relays.

Finally, there is the realm of “Personal Radio Services,” created by the FCC in 1992. This branch, still in its infancy, will probably one day include video-on-demand, interactive polling, online shopping and banking, and other activities classified under the heading of Interactive

The first worldwide satellite TV broadcast, in the summer of 1967, featured the Beatles singing their latest song “All You Need Is Love.” Due to the international character of the broadcast, with an estimated 200 million viewers, John Lennon and Paul McCartney wrote a song with simple, universal lyrics, and the result was just another example of electronic communication uniting large populations. Indeed, the phenomenon of rock music, and of superstardom as people know it today, would be impossible without many of the forms of technology discussed here. Long before the TV broadcast, the Beatles had come to fame through the playing of their music

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on the radio waves—and, thus, they owed much to Maxwell, Hertz, and Marconi. The same microwaves that transmit FM and television signals—to name only the most obviously applications of microwave for communication—can also be harnessed to cook food. The microwave oven, introduced commercially in 1955, was an outgrowth of military technology developed a decade before. M I C R O WAV E

OVENS.

During World War II, the Raytheon Manufacturing Company had experimented with a magnetron, a device for generating extremely short-wavelength radio signals as a means of improving the efficiency of military radar. While working with a magnetron, a technician named Percy Spencer was surprised to discover that a candy bar in his pocket had melted, even though he had not felt any heat. This led him to considering the possibilities of applying the magnetron to peacetime uses, and a decade later, Raytheon’s “radar range” hit the market. Those early microwave ovens had none of varied power settings to which modern users of the microwave—found today in two-thirds of all American homes—are accustomed. In the first microwaves, the only settings were “on” and “off,” because there were only two possible adjustments: either the magnetron would produce, or not produce, microwaves. Today, it is possible to use a microwave for almost anything that involves the heating of food that contains water—from defrosting a steak to popping popcorn. As noted much earlier, in the general discussion of electromagnetic radiation, there are three basic types of heat transfer: conduction, convection, and radiation. Without going into too much detail here, conduction generally involves heat transfer between molecules in a solid; convection takes place in a fluid (a gas such as air or a liquid such as water); and radiation, of course, requires no medium. A conventional oven cooks through convection, though conduction also carries heat from the outer layers of a solid (for example, a turkey) to the interior. A microwave, on the other hand, uses radiation to heat the outer layers of the food; then conduction, as with a conventional oven, does the rest. The difference is that the microwave heats only the food—or, more specifically, the water, which then transfers heat

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throughout the item being heated—and not the dish or plate. Thus, many materials, as long as they do not contain water, can be placed in a microwave oven without being melted or burned. Metal, though it contains no water, is unsafe because the microwaves bounce off the metal surfaces, creating a microwave buildup that can produce sparks and damage the oven. In a microwave oven, microwaves emitted by a small antenna are directed into the cooking compartment, and as they enter, they pass a set of turning metal fan blades. This is the stirrer, which disperses the microwaves uniformly over the surface of the food to be heated. As a microwave strikes a water molecule, resonance causes the molecule to align with the direction of the wave. An oscillating magnetron causes the microwaves to oscillate as well, and this, in turn, compels the water molecules to do the same. Thus, the water molecules are shifting in position several million times a second, and this vibration generates energy that heats the water.

Radio Waves for Measurement and Ranging RA DA R. Radio waves can be used to send

communication signals, or even to cook food; they can also be used to find and measure things. One of the most obvious applications in this regard is radar, an acronym for RAdio Detection And Ranging. Radio makes it possible for pilots to “see” through clouds, rain, fog, and all manner of natural phenomena—not least of which is darkness. It can also identify objects, both natural and manmade, thus enabling a peacetime pilot to avoid hitting another craft or the side of a mountain. On the other hand, radar may help a pilot in wartime to detect the presence of an enemy. Nor is radar used only in the skies, or for military purposes, such as guiding missiles: on the ground, it is used to detect the speeds of objects such as automobiles on an interstate highway, as well as to track storms. In the simplest model of radar operation, the unit sends out microwaves toward the target, and the waves bounce back off the target to the unit. Though the speed of light is reduced somewhat, due to the fact that waves are traveling through air rather than through a vacuum, it is, nonetheless, possible to account for this difference. Hence, the distance to the target can be cal-

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culated using the simple formula d = vt, where d is distance, v is velocity, and t is time. Typically, a radar system includes the following: a frequency generator and a unit for controlling the timing of signals; a transmitter and, as with broadcast radio, a modulator; a duplexer, which switches back and forth between transmission and reception mode; an antenna; a receiver, which detects and amplifies the signals bounced back to the antenna; signal and data processing units; and data display units. In a monostatic unit—one in which the transmitter and receiver are in the same location—the unit has to be continually switched between sending and receiving modes. Clearly, a bistatic unit—one in which the transmitter and receiver antennas are at different locations—is generally preferable; but on an airplane, for instance, there is no choice but to use a monostatic unit. In order to determine the range to a target— whether that target be a mountain, an enemy aircraft, or a storm—the target itself must first be detected. This can be challenging, because only a small portion of the transmitted pulse comes back to the receiving antenna. At the same time, the antenna receives reflections from a number of other objects, and it can be difficult to determine which signal comes from the target. For an aircraft in a wartime situation, these problems are compounded by the use of enemy countermeasures such as radar “jamming.” Still another difficulty facing a military flyer is the fact that the use of radar itself—that is, the transmission of microwaves—makes the aircraft detectable to opposing forces. T E L E M E T RY. Telemetry is the process of making measurements from a remote location and transmitting those measurements to receiving equipment. The earliest telemetry systems, developed in the United States during the 1880s, monitored the distribution and use of electricity in a given region, and relayed this information back to power companies using telephone lines. By the end of World War I, electric companies used the power lines themselves as information relays, and though such electrical telemetry systems remain in use in some sectors, most modern telemetry systems apply radio signals.

body temperature, and so on) during a manned space flight. The transducer takes this information and converts it into an electrical impulse, which is then beamed to the space monitoring station on Earth. Because this signal carries information, it must be modulated, but there is little danger of interference with broadcast transmissions on Earth. Typically, signals from spacecraft are sent in a range above 1010 Hz, far above the frequencies of most microwave transmissions for commercial purposes.

Electromagnetic Spectrum

Light: Invisible, Visible, and Invisible Again Between about 1013 and 1017 Hz on the electromagnetic spectrum is the range of light: infrared, visible, and ultraviolet. Light actually constitutes a small portion of the spectrum, and the area of visible light is very small indeed, extending from about 4.3 • 1014 to 7.5 • 1014 Hz. The latter, incidentally, is another example of scientific notation: not only is it easier not to use a string of zeroes, but where a coefficient or factor (for example, 4.3 or 7.5) is other than a multiple of 10, it is preferable to use what are called significant figures—usually a single digit followed by a decimal point and up to 3 decimal places. Infrared light lies just below visible light in frequency, and this is easy to remember because of the name: red is the lowest in frequency of all the colors. Similarly, ultraviolet lies beyond the highest-frequency color, violet. Visible light itself, by far the most familiar part of the spectrum— especially prior to the age of radio communications—is discussed in detail elsewhere. I N F RA R E D L I G H T. Though we cannot see infrared light, we feel it as heat. GermanEnglish astronomer William Herschel (17381822), first scientist to detect infrared radiation from the Sun, demonstrated its existence in 1800 by using a thermometer. Holding a prism, a three-dimensional glass shape used for diffusing beams of light, he directed a beam of sunlight toward the thermometer, which registered the heat of the infrared rays.

An example of a modern telemetry application is the use of an input device called a transducer to measure information concerning an astronaut’s vital signs (heartbeat, blood pressure,

Eighty years later, English scientist Sir William Abney (1843-1920) developed infrared photography, a method of capturing infrared radiation, rather than visible light, on film. By the mid-twentieth century, infrared photography had come into use for a variety of purposes. Military forces, for instance, may use infrared to

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skin. A suntan, as a matter of fact, is actually the skin’s defense against these harmful UV rays. Due to the fact that Earth is largely opaque, or resistant, to ultraviolet light, the most significant technological applications of UV radiation are found in outer space.

Electromagnetic Spectrum

In 1978 the United States, in cooperation with several European space agencies, launched the International Ultraviolet Explorer (IUE), which measured the UV radiation from tens of thousands of stars, nebulae, and galaxies. Despite the progress made with IUE, awareness of its limitations—including a mirror of only 17 in (45 cm) on the telescope itself—led to the development of a replacement in 1992.

“SOFT”

X-RAY MACHINES, SUCH AS THIS ONE BEING

USED BY A DENTIST TO PHOTOGRAPH THE PATIENT ’S TEETH, OPERATE AT RELATIVELY LOW FREQUENCIES AND THUS DON’T HARM THE PATIENT. (Photograph by Richard T.

Nowitz/Corbis. Reproduced by permission.)

detect the presence of enemy troops. Medicine makes use of infrared photography for detecting tumors, and astronomers use infrared to detect stars too dim to be seen using ordinary visible light. The uses of infrared imaging in astronomy, as a matter of fact, are many. The development in the 1980s of infrared arrays, two-dimensional grids which produce reliable images of infrared phenomena, revolutionized infrared astronomy. Because infrared penetrates dust much more easily than does visible light, infrared astronomy makes it easier to see regions of the universe where stars—formed from collapsing clouds of gas and dust—are in the process of developing. Because hydrogen molecules emit infrared radiation, infrared astronomy helps provide clues regarding the distribution of this highly significant chemical element throughout the universe.

Ultraviolet astronomy is used to study the winds created by hot stars, as well as stars still in the process of forming, and even stars that are dying. It is also useful for analyzing the densely packed, highly active sectors near the centers of galaxies, where both energy and temperatures are extremely high.

X Rays Though they are much higher in frequency than visible light—with wavelengths about 1,000 times shorter than for ordinary light rays—x rays are a familiar part of modern life due to their uses in medicine. German scientist Wilhelm Röntgen (1845-1923) developed the first x-ray device in 1895, and, thus, the science of using xray machines is called roentgenology.

the Sun’s ultraviolet light penetrates Earth’s atmosphere—a fortunate thing, since ultraviolet (UV) radiation can be very harmful to human

The new invention became a curiosity, with carnivals offering patrons an opportunity to look at the insides of their hands. And just as many people today fear the opportunities for invasion of privacy offered by computer technology, many at the time worried that x rays would allow robbers and peeping toms to look into people’s houses. Soon, however, it became clear that the most important application of x rays lay in medicine.

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U LT RAV I O L E T L I G H T. Very little of

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This was the Extreme Ultraviolet Explorer (EUVE), which could observe UV phenomena over a much higher range of wavelengths than those observed by IUE. In addition, the Hubble Space Telescope, launched by the United States in 1990, includes a UV instrument called the Goddard High Resolution Spectrograph. With a mirror measuring 8.5 ft (2.6 m), it is capable of observing objects much more faint than those detected earlier by IUE.

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Electromagnetic Spectrum

KEY TERMS AMPLITUDE:

The maximum displace-

ment of a vibrating material. In wave motion, amplitude is the “size” of a wave, an indicator of the energy and intensity of the wave.

each point in the region at any given moment in time. FREQUENCY:

In wave motion, fre-

quency is the number of waves passing through a given point during the interval

One complete oscillation. In

of one second. The higher the frequency,

wave motion, this is equivalent to the

the shorter the wavelength. Measured in

movement of a wave from trough to crest

Hertz, frequency is mathematically related

and back to trough.

to wave speed, wavelength, and period.

CYCLE:

ELECTROMAGNETIC FORCE:

The

FUNDAMENTAL INTERACTION:

The

total force on an electrically charged parti-

basic mode by which particles interact.

cle, which is a combination of forces due to

There are four known fundamental inter-

electrical and/or magnetic fields around

actions in nature: gravitational, electro-

the particle. Electromagnetic force reflects

magnetic, strong nuclear, and weak

electromagnetic interaction, one of the

nuclear.

four fundamental interactions in nature. HARMONIC MOTION: ELECTROMAGNETIC

SPECTRUM:

The complete range of electromagnetic

movement of a particle about a position of equilibrium, or balance.

waves on a continuous distribution from a very low range of frequencies and energy levels, with a correspondingly long wavelength, to a very high range of frequencies

The repeated

HERTZ:

A unit for measuring frequen-

cy, named after nineteenth-century German physicist Heinrich Rudolf Hertz

and energy levels, with a correspondingly

(1857-1894). High frequencies are ex-

short wavelength. Included on the electro-

pressed in terms of kilohertz (kHz; 103 or

magnetic spectrum are long-wave and

1,000 cycles per second); megahertz (MHz;

short-wave radio; microwaves; infrared,

106 or 1 million cycles per second); and

visible, and ultraviolet light; x rays, and

gigahertz (GHz; 109 or 1 billion cycles per

gamma rays.

second.)

ELECTROMAGNETIC

WAVE:

A

INTENSITY:

Intensity is the rate at

transverse wave with electrical and mag-

which a wave moves energy per unit of

netic fields that emanate from it. The direc-

cross-sectional area.

tions of these fields are perpendicular to

OSCILLATION:

one another, and both are perpendicular to

motion, typically periodic, in one or more

the line of propagation for the wave itself.

dimensions.

ELECTROMAGNETISM:

The branch

PERIOD:

A type of harmonic

For wave motion, a period is

of physics devoted to the study of electrical

the amount of time required to complete

and magnetic phenomena.

one full cycle. Period is mathematically

FIELD:

A region of space in which it is

possible to define the physical properties of

S C I E N C E O F E V E RY DAY T H I N G S

related to frequency, wavelength, and wave speed.

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Electromagnetic Spectrum

KEY TERMS

CONTINUED

Motion that is

large numbers. This usually involves a coef-

repeated at regular intervals. These inter-

ficient, or factor, of a single digit followed

vals are known as periods.

by a decimal point and up to three decimal

PERIODIC MOTION:

PERIODIC WAVE:

A wave in which a

uniform series of crests and troughs follow one after the other in regular succession.

places, multiplied by 10 to a given exponent. Thus, instead of writing 75,120,000, the preferred scientific notation is 7.512 • 107. To visualize the value of very large

A particle of electromagnet-

multiples of 10, it is helpful to remember

ic radiation carrying a specific amount of

that the value of 10 raised to any power n is

energy, measured in electron volts (eV).

the same as 1 followed by that number of

For parts of the electromagnetic spectrum

zeroes. Hence 1025, for instance, is simply 1

with a low frequency and long wavelength,

followed by 25 zeroes.

PHOTON:

photon energy is relatively low; but for parts with a high frequency and short wavelength, the value of photon energy is very high.

A wave in

which the vibration or motion is perpendicular to the direction in which the wave is

The act or state of

traveling from one place to another. The transfer of energy by

WAVELENGTH:

The distance between

a crest and the adjacent crest, or the trough and an adjacent trough, of a wave. Wave-

means of electromagnetic waves, which

length, symbolized λ (the Greek letter

require no physical medium (for example,

lambda) is mathematically related to wave

water or air) for the transfer. Earth receives

speed, period, and frequency.

the Sun’s energy, via the electromagnetic spectrum, by means of radiation. SCIENTIFIC NOTATION:

A method

used by scientists for writing extremely

WAVE MOTION:

A type of harmonic

motion that carries energy from one place to another without actually moving any matter.

wavelengths, x rays can pass through substances of low density—for example, fat and other forms of soft tissue—without their movement being interrupted. But in materials of higher density, such as bone, atoms are packed closely together, and this provides x rays with less space through which to travel. As a result, x-ray images show dark areas where the rays traveled completely through the target, and light images of dense materials that blocked the movement of the rays.

Medical x-ray machines are typically referred to either as “hard” or “soft.” Soft x rays are the ones with which most people are more familiar. Operating at a relatively low frequency, these are used to photograph bones and internal organs, and provided the patient does not receive prolonged exposure to the rays, they cause little damage. Hard x rays, on the other hand, are designed precisely to cause damage—not to the patient, but to cancer cells. Because they use high voltage and high-frequency rays, hard x rays can be quite dangerous to the patient as well.

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H O W A M E D I CA L X - RAY M A C H I N E W O R K S . Due to their very short

352

WAVE:

moving.

PROPAGATION:

RADIATION:

TRANSVERSE

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O T H E R A P P L I C AT I O N S . X-ray crystallography, developed in the early twentieth century, is devoted to the study of the interference patterns produced by x rays passing through materials that are crystalline in Structure. Each of these discoveries, in turn, transformed daily life: insulin, by offering hope to diabetics, penicillin, by providing a treatment for a number of previously fatal illnesses, and DNA, by enabling scientists to make complex assessments of genetic information.

In addition to the medical applications, the scanning capabilities of x-ray machines make them useful for security. A healthy person receives an x ray at a doctor’s office only once in a while; but everyone who carries items past a certain point in a major airport must submit to x-ray security scanning. If one is carrying a purse or briefcase, for instance, this is placed on a moving belt and subjected to scanning by a lowpower device that can reveal the contents.

Gamma Rays At the furthest known reaches of the electromagnetic spectrum are gamma rays, ultra high-frequency, high-energy, and short- wavelength forms of radiation. Human understanding of gamma rays, including the awesome powers they contain, is still in its infancy. In 1979, a wave of enormous energy passed over the Solar System. Though its effects on Earth were negligible, instruments aboard several satellites provided data concerning an enormous quantity of radiation caused by gamma rays. As to the source of the rays themselves, believed to be a product of nuclear fusion on some other body in the universe, scientists knew nothing. The Compton Gamma Ray Observatory Satellite, launched by NASA (National Aeronautics and Space Administration) in 1991, detected a number of gamma-ray bursts over the next two years. The energy in these bursts was staggering:

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just one of these, scientists calculated, contained more than a thousand times as much energy as the Sun will generate in its entire lifetime of 10 billion years. Some astronomers speculate that the source of these gamma-ray bursts may ultimately be a distant supernova, or exploding star. If this is the case, scientists may have found the supernova; but do not expect to see it in the night sky. It is not known just how long ago it exploded, but its light appeared on Earth some 340,000 years ago, and during that time it was visible in daylight for more than two years. So great was its power that the effects of this stellar phenomenon are still being experienced.

Electromagnetic Spectrum

WHERE TO LEARN MORE Branley, Franklyn Mansfield. The Electromagnetic Spectrum: Key to the Universe. Illustrated by Leonard D. Dank. New York: Crowell, 1979. Electromagnetic Spectrum (Web site). (April 25, 2001). The Electromagnetic Spectrum (Web site). (April 30, 2001). The Electromagnetic Spectrum (Web site). (April 30, 2001). Electromagnetic Spectrum/NASA: National Aeronautics and Space Administration (Web site). (April 30, 2001). “How the Radio Spectrum Works.” How Stuff Works (Web site). (April 25, 2001). Internet Resources for Sound and Light (Web site). (April 25, 2001). Nassau, Kurt. Experimenting with Color. New York: F. Watts, 1997. “Radio Electronics Pages” ePanorama.net (Web site). (April 25, 2001). Skurzynski, Gloria. Waves: The Electromagnetic Universe. Washington, D.C.: National Geographic Society, 1996.

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LIGHT Light

CONCEPT Light exists along a relatively narrow bandwidth of the electromagnetic spectrum, and the region of visible light is more narrow still. Yet, within that realm are an almost infinite array of hues that quite literally give color to the entire world of human experience. Light, of course, is more than color: it is energy, which travels at incredible speeds throughout the universe. From prehistoric times, humans harnessed light’s power through fire, and later, through the invention of illumination devices such as candles and gas lamps. In the late nineteenth century, the first electric-powered forms of light were invented, which created a revolution in human existence. Today, the power of lasers, highly focused beams of high-intensity light, make possible a number of technologies used in everything from surgery to entertainment.

HOW IT WORKS Early Progress in Understanding of Light The first useful observations concerning light came from ancient Greece. The Greeks recognized that light travels through air in rays, a term from geometry describing that part of a straight line that extends in one direction only. Upon entering some denser medium, such as glass or water, as Greek scientists noticed, the ray experiences refraction, or bending. Another type of incidence, or contact, between a light ray and any surface, is reflection, whereby a light ray returns, rather than being absorbed at the interface.

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The Greeks worked out the basic laws governing reflection and refraction, observing, for instance, that in reflection, the angle of incidence is approximately equal to the angle of reflection. Unfortunately, they also subscribed to the erroneous concept of intromission—the belief that light rays originate in the eye and travel toward objects, making them visible. Some 1,500 years after the high point of Greek civilization, Arab physicist Alhasen (Ibn al-Haytham; c. 965-1039), sometimes called the greatest scientist of the Middle Ages, showed that light comes from a source such as the Sun, and reflects from an object to the eyes. The next great era of progress in studies of light began with the Renaissance (c. 1300-c. 1600.) However, the most profound scientific achievements in this area belonged not to scientists, but to painters, who were fascinated by color, shading, shadows, and other properties of light. During the early seventeenth century, Galileo Galilei (1564-1642) and German astronomer Johannes Kepler (1571-1630) built the first refracting telescopes, while Dutch physicist and mathematician Willebrord Snell (15801626) further refined the laws of refraction.

The Spectrum Sir Isaac Newton (1642-1727) was as intrigued with light as he was with gravity and the other concepts associated with his work. Though it was not as epochal as his contributions to mechanics, Newton’s work in optics, an area of physics that studies the production and propagation of light, was certainly significant. In Newton’s time, physicists understood that a prism could be used for the diffusion of light

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rays—in particular, to produce an array of colors from a beam of white light. The prevailing belief was that white was a single color like the others, but Newton maintained that it was a combination of all other colors. To prove this, he directed a beam of white light through a prism, then allowed the diffused colors to enter another prism, at which point they recombined as white light. Newton gave to the array of colors in visible light the term spectrum, (plural, “spectra”) meaning the continuous distribution of properties in an ordered arrangement across an unbroken range. The term can be used for any set of characteristics for which there is a gradation, as opposed to an excluded middle. An ordinary light switch provides an example of a situation in which there is an excluded middle: there is nothing between “on” and “off.” A dimmer switch, on the other hand, is a spectrum, because a very large number of gradations exist between the two extremes represented by a light switch. S E V E N C O LO R S . . . O R S I X ? The distribution of colors across the spectrum is as follows: red-orange-yellow-green-blue-violet. The reasons for this arrangement, explained below in the context of the electromagnetic spectrum, were unknown to Newton. Not only did he live in an age that had almost no understanding of electromagnetism, but he was also a product of the era called the Enlightenment, when intellectuals (scientists included) viewed the world as a highly rational, ordered mechanism. His Enlightenment viewpoint undoubtedly influenced his interpretation of the spectrum as a set of seven colors, just as there are seven notes on the musical scale.

In addition to the six basic colors listed above, Newton identified a seventh, indigo, between blue and violet. In fact, there is a noticeable band of color between blue and violet, but this is because one color fades into another. With a spectrum, there is a blurring of lines between one color and the next: for instance, orange exists at a certain point along the spectrum, as does yellow, but between them is a nearly unlimited number of orange-yellow and yellow-orange gradations.

Light

ISAAC

NEWTON.

(The Bettmann Archive. Reproduced by

permission.)

rization) device: the name “ROY G. BIV.” These letters form an acrostic (a word constructed from the first letters of other words) for the colors of the spectrum. Incidentally, there is something arbitrary even in the idea of six colors, or for that matter seven musical notes: in both cases, there exists a very large gradation of shades, yet also in both cases, the divisions used were chosen for practical purposes.

Waves, Particles, and Other Questions Concerning Light T H E WAV E - PA RT I C L E C O N T R O V E R S Y B E G I N S . Newton subscribed to

Indigo itself is not really a distinct color— just a deep, purplish blue. But its inclusion in the listing of colors on the spectrum has given generations of students a handy mnemonic (memo-

the corpuscular theory of light: the idea that light travels as a stream of particles. On the other hand, Dutch physicist and astronomer Christiaan Huygens (1629-1695) maintained that light travels in waves. During the century that followed, adherents of particle theory did intellectual battle with proponents of wave theory. “Battle” is not too strong a word, because the conflict was heated, and had a nationalistic element. Reflecting both the burgeoning awareness of the nation-state among Europeans, as well as Britons’ sense of their own island as an entity separate from the European continent, particle theory had its strongest defenders in Newton’s

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homeland, while continental scientists generally accepted wave theory. According to Huygens, the appearance of the spectrum, as well as the phenomena of reflection and refraction, indicated that light was a wave. Newton responded by furnishing complex mathematical calculations which showed that particles could exhibit the behaviors of reflection and refraction as well. Furthermore, Newton challenged, if light were really a wave, it should be able to bend around corners. Yet, in 1660, an experiment by Italian physicist Francesco Grimaldi (1618-1663) proved that light could do just that. Passing a beam of light through a narrow aperture, or opening, Grimaldi observed a phenomenon called diffraction, or the bending of light. In view of the nationalistic character that the wave-particle debate assumed, it was ironic that the physicist whose work struck a particularly forceful blow against corpuscular theory was himself an Englishman: Thomas Young (17731829), who in 1801 demonstrated interference in light. Directing a light beam through two closely spaced pinholes onto a screen, Young reasoned that if light truly were made of particles, the beams would project two distinct points onto the screen. Instead, what he saw was a pattern of interference—a wave phenomenon. THE QUESTION OF A MEDIUM.

As the nineteenth century progressed, evidence in favor of wave theory grew. Experiments in 1850 by Jean Bernard Leon Foucault (18191868)—famous for his pendulum —showed that light traveled faster in air than through water. Based on studies of wave motion up to that time, Foucault’s work added substance to the view of light as a wave. Foucault also measured the speed of light in a vacuum, a speed which he calculated to within 1% of its value as it is known today: 186,000 mi (299,339 km) per second. An understanding of just how fast light traveled, however, caused a nagging question dating back to the days of Newton and Huygens to resurface: how did light travel? All types of waves known to that time traveled through some sort of medium: for instance, sound waves were propagated through air, water, or some other type of matter. If light was a wave, as Huygens said, then it, too, must have some medium. Huygens and his followers proposed a

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weak theory by suggesting the existence of an invisible substance called ether, which existed throughout the universe and which carried light. Ether, of course, was really no answer at all. There was no evidence that it existed, and to many scientists, it was merely a concept invented to shore up an otherwise convincing argument. Then, in 1872, Scottish physicist James Clerk Maxwell (1831-1879) proposed a solution that must have surprised many scientists. The “medium” through which light travels, Maxwell proposed, was no medium at all; rather, the energy in light is transferred by means of radiation, which requires no medium.

Electromagnetism Maxwell brought together a number of concepts developed by his predecessors, sorting these out and adding to them. His work led to the identification of a “new” fundamental interaction, in addition to that associated with gravity. This was the mode of particle interaction associated with electromagnetic force. The particulars of electromagnetic force, waves, and radiation are a subject unto themselves—really, many subjects. As for the electromagnetic spectrum, it is treated at some length in an essay elsewhere in this volume, and the reader is encouraged to review that essay to gain a greater understanding of light and its place in the spectrum. In addition, some awareness of wave motion and related phenomena would also be of great value, and, for this purpose, other essays are recommended. In the present context, a number of topics relating to these larger subjects will be handled in short order, with a minimum of explanation, to enable a more speedy transition to the subject of principal importance here: light. ELECTROMAGNETIC

WAV E S .

There is, of course, no obvious connection between light and the electromagnetic force observed in electrical and magnetic interactions. Yet, light is an example of an electromagnetic wave, and is part of the electromagnetic spectrum. The breakthrough in establishing the electromagnetic quality of light can be attributed both to Maxwell and German physicist Heinrich Rudolf Hertz (1857-1894). In his Electricity and Magnetism (1873), Maxwell suggested that electromagnetic force

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might have aspects of a wave phenomenon, and his experiments indicated that electromagnetic waves should travel at exactly the same speed as light. This appeared to be more than just a coincidence, and his findings led him to theorize that the electromagnetic interaction included not only electricity and magnetism, but light as well. Some time later, Hertz proved Maxwell’s hypothesis by showing that electromagnetic waves obeyed the same laws of reflection, refraction, and diffraction as light. Hertz also discovered the photoelectric effect, the process by which certain metals acquire an electrical potential when exposed to light. He could not explain this behavior, and, indeed, there was nothing in wave theory that could account for it. Strangely, after more than a century in which acceptance of wave theory had grown, he had encountered something that apparently supported what Newton had said long before: that light traveled in particles rather than waves.

The Wave-Particle Debate Revisited One of the modern physicists whose name is most closely associated with the subject of light is Albert Einstein (1879-1955). In the course of proving that matter is convertible to energy, as he did with the theory of relativity, Einstein predicted that this could be illustrated by accelerating to speeds close to that of light. (Conversely, he also showed that it is impossible for matter to reach the speed of light, because to do so would—as he proved mathematically—result in the matter acquiring an infinite amount of mass, which, of course, is impossible.) Much of Einstein’s work was influenced by that of German physicist Max Planck (18581947), father of quantum theory. Quantum theory and quantum mechanics are, of course, far too complicated to explain in any depth here. It is enough to say that they called into question everything physicists thought they knew, based on Newton’s theories of classical mechanics. In particular, quantum mechanics showed that, at the subatomic level, particles behave in ways not just different from, but opposite to, the behavior of larger physical objects in the observable world. When a quantity is “quantized,” its values or properties at the atomic or subatomic level are separate from one another—meaning that some-

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thing can both be one thing and its opposite, depending on how it is viewed.

Light

Interpreting Planck’s observations, Einstein in a 1905 paper on the photoelectric effect maintained that light is quantized—that it appears in “bundles” of energy that have characteristics both of waves and of particles. Though light travels in waves, as Einstein showed, these waves sometimes behave as particles, which is the case with the photoelectric effect. Nearly two decades later, American physicist Arthur Holly Compton (1892-1962) confirmed Einstein’s findings and gave a name to the “particles” of light: photons.

Light’s Place in the Electromagnetic Spectrum The electromagnetic spectrum is the complete range of electromagnetic waves on a continuous distribution from a very low range of frequencies and energy levels, with a correspondingly long wavelength, to a very high range of frequencies and energy levels, with a correspondingly short wavelength. Included on the electromagnetic spectrum are radio waves and microwaves; infrared, visible, and ultraviolet light; x rays, and gamma rays. As discussed earlier, concerning the visible color spectrum, each of these occupies a definite place on the spectrum, but the divisions between them are not firm: in keeping with the nature of a spectrum, one band simply “blurs” into another. Of principal concern here is an area near the middle of the electromagnetic spectrum. Actually, the very middle of the spectrum lies within the broad area of infrared light, which has frequencies ranging from 1012 to just over 1014 Hz, with wavelengths of approximately 10-1 to 10-3 centimeters. Even at this point, the light waves are oscillating at a rate between 1 and 100 trillion times a second, and the wavelengths are from 1 millimeter to 0.01 millimeters. Yet, over the breadth of the electromagnetic spectrum, wavelengths get much shorter, and frequencies much greater. Infrared lies just below visible light in frequency, which is easy to remember because of the name: red is the lowest in frequency of all the colors, as discussed below. Similarly, ultraviolet lies beyond the highest-frequency color, violet. Neither infrared nor ultraviolet can be seen, yet we experience them as heat. In the case of ultraviolet (UV) light, the rays are so powerful that exposure

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to even the minuscule levels of UV radiation that enter Earth’s atmosphere can cause skin cancer. Ultraviolet light occupies a much narrower band than infrared, in the area of about 1015 to 1016 Hz—in other words, oscillations between 1 and 10 quadrillion times a second. Wavelengths in this region are from just above 10-6 to about 10-7 centimeters. These are often measured in terms of a nanometer (nm)—equal to one-millionth of a millimeter—meaning that the wavelength range is from above 100 down to about 10 nm. Between infrared and ultraviolet light is the region of visible light: the six colors that make up much of the world we know. Each has a specific range and frequency, and together they occupy an extremely narrow band of the electromagnetic spectrum: from 4.3 • 1014 to 7.5 • 1014 Hz in frequency, and from 700 down to 400 nm in wavelength. To compare its frequency range to that of the entire spectrum, for instance, is the same as comparing 3.2 to 100 billion.

REAL-LIFE A P P L I C AT I O N S Colors Unlike many of the topics addressed by physics, color is far from abstract. Numerous expressions in daily life describe the relationship between energy and color: “red hot,” for instance, or “blue with cold.” In fact, however, red—with a smaller frequency and a longer wavelength than blue— actually has less energy; therefore, blue objects should be hotter. The phenomenon of the red shift, discovered in 1923 by American astronomer Edwin Hubble (1889-1953), provides a clue to this apparent contradiction. As Hubble observed, the light waves from distant galaxies are shifted to the red end, and he reasoned that this must mean those galaxies are moving away from the Milky Way, the galaxy in which Earth is located. To generalize from what Hubble observed, when something shows red, it is moving away from the observer. The laws of thermodynamics state that where heat is involved, the movement is always away from an area of high temperature and toward an area of low temperature. Heated molecules that reflect red light are, thus, to use a colloquialism, “showing their tail end” as they

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move toward an area of low temperature. By contrast, molecules of low temperature reflect bluish or purple light because the tendency of heat is to move toward them. There are other reasons, aside from heat, that some objects tend to be red and others blue—or another color. Chemical factors may be involved: atoms of neon, for example, can be made to vibrate at a particular wavelength, producing a specific color. In any case, the color that an object reflects is precisely the color that it does not absorb: thus, if something is red, that means it has absorbed every color of the spectrum but red. W H Y I S T H E S KY B LU E ? The placement of colors on the electromagnetic spectrum provides an answer to that age-old question posed by generations of children to their parents: “Why is the sky blue?” Electromagnetic radiation is scattered as it enters the atmosphere, but all forms of radiation are not scattered equally. Those having shorter wavelengths—that is, toward the blue end of the spectrum—tend to scatter more than those with longer wavelengths, on the red and orange end.

Yet the longer-wavelength light becomes visible at sunset, when the Sun’s light enters the atmosphere at an angle. In addition, the dim quality of evening light means that it is easiest to see light of longer wavelengths. This effect is known as Rayleigh scattering, after English physicist John William Strutt, Lord Rayleigh (1842-1919), who discovered it in 1871. Thanks to Rayleigh’s discovery, there is an explanation not only for the question of why the sky is blue, but why sunsets are red, orange, and gold. RA I N B O W S . On the subject of color as children perceive it, many a child has been fascinated by a rainbow, seeing in them something magical. It is easy to understand why children perceive these beautiful phenomena this way, and why people have invented stories such as that of the pot of gold at the end of a rainbow. In fact, a rainbow, like many other “magical” aspects of daily life, can be explained in terms of physics.

A rainbow, in fact, is simply an illustration of the visible light spectrum. Rain drops perform the role of tiny prisms, dispersing white sunlight, much as scientists before Newton had learned to do. But if there is a pot of gold at the end of the rainbow, it would be impossible to find. In order for a rainbow to be seen, it must be viewed from

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a specific perspective: the observer must be in a position between the sunlight and the raindrops. Sunlight strikes raindrops in such a way that they are refracted, then reflected back at an angle so that they represent the entire visible light spectrum. Though they are beautiful to see, rainbows are neither magical nor impossible to reproduce artificially. Such rainbows can be produced, for instance, in the spiral of small water droplets emerging from a water hose, viewed when one’s back is to the Sun.

Perception of Light and Color People literally live and die for colors: the colors of a flag, for instance, present a rallying point for soldiers, and different colors are assigned specific political meanings. Blue, both in the American and French flags, typically stands for liberty. Red can symbolize the blood shed by patriots, or it can mean some version of fraternity or brotherhood. Such is the case with the red of the French tricolor (red-white-blue); likewise, the red in the flag of the former Soviet Union and other Communist countries stood for the alleged international brotherhood of all working peoples. In Islamic countries, by contrast, green stands for the unity of all Muslims. These are just a few examples, drawn from a specific realm—politics—illustrating the meanings that people ascribe to colors. Similarly, people find meanings in images presented to them by light itself. In his Republic, the ancient Greek philosopher Plato (c. 427-347 B.C.) offered a complex parable, intended to illustrate the difference between reality and illusion, concerning a group of slaves who do not recognize the difference between sunlight and the light of a torch in a cave. Modern writers have noted the similarities between Plato’s cave and a phenomenon which the ancient philosopher could hardly have imagined: a movie theatre, in which an artificial light projects images—images that people sometimes perceive as being all too real—onto a screen. People refer to “tricks of the light,” as, for instance, when one seems to see an image in a fire. One particularly well-known “trick of the light,” a mirage, is discussed below, but there are also manmade illusions created by light, shapes, and images. An optical illusion is something that produces a false impression in the brain, causing

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one to believe that something is as it appears, when, in fact, it is not. When two lines of equal length are placed side by side, but one has arrows pointed outward at either end while the other line has arrows pointing inward, it appears that the line with the inward-pointing arrows are shorter.

Light

This is an example of the ways in which human perception plays a role in what people see. That topic, of course, goes far beyond physics and into the realms of psychology and the social sciences. Nonetheless, it is worthwhile to consider, from a physical standpoint, how humans see what they see—and sometimes see things that are not there. A M I R A G E . Because they can be demonstrated in light waves as well as in sound waves, diffraction and interference are discussed in separate essays. As for refraction, or the bending of light waves, this phenomenon can be seen in the familiar example of a mirage. While driving down a road on a hot day, one may observe that there are pools of water up ahead, but by the time one approaches them, they disappear.

Of course, the pools were never there; light itself has created an optical illusion of sorts. As light moves from one material to another, it bends with a different angle of refraction, and, though, in this instance, it is traveling entirely through air, it is moving through regions of differing temperature. Light waves travel faster through warm air than through cool air, and, thus, when the light enters the area over the heated surface of the asphalt, it experiences refraction. The waves are thus bent, creating the impression of a reflection, which suggests to the observer that there is water up ahead. H O W T H E E Y E S E E S C O LO R.

White, as noted earlier, is the combination of all colors; black is the absence of color. Where ink, dye, or other forms of artificial pigmentation are concerned, of course, black is a “real” color, but in terms of light, it is not. In the same way, the experience of coldness is real, yet “cold” does not exist as a physical phenomenon: it is simply the absence of heat. The mixture of pigmentation is an entirely different matter from the mixture of light. In artificial pigmentation, the primary colors—the three colors which, when mixed, yield the remainder of the shades on the rainbow—are red, blue, and yellow. Red mixed with blue creates

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purple, blue mixed with yellow makes green, and red mixed with yellow yields orange. Black and white are usually created by using natural substances of that color—chalk for white, for instance, or various oxides for black. For light, on the other hand, blue and red are primary colors, but the third primary color is green, not yellow. From these three primary colors, all other shades of the visible spectrum can be made. The mechanism of the human eye responds to the three primary colors of the visible light spectrum: thus, the eye’s retina is equipped with tiny cones that respond to red, blue, and green light. The cones respond to bright light; other structures called rods respond to dim light, and the pupil regulates the amount of light that enters the eye. The eye responds with maximum sensitivity to light at the middle of the visible color spectrum—specifically, green light with a wavelength of about 555 nm. The optimal wavelength for maximum sensitivity in dim light is around 510 nm, on the blue end. It is difficult for the eye to recognize red light, at the far end of the spectrum, against a dark background. However, this can be an advantage in situations of relative darkness, which is why red light is often used to maintain vision for sailors, amateur astronomers, and the military on night maneuvers. Because there is not much difference between the darkness and the red light, the eye adjusts and is able to see beyond the red light into the darkness. A bright yellow or white light in such situations, on the other hand, would minimize visibility in areas beyond the light.

Artificial Light PREHISTORIC LIGHTING TECHNOLOGY. Prehistoric humans did not know

it, but they were making use of electromagnetic radiation when they lit and warmed their caves with light from a fire. Though it would seem that warmth was more essential to human survival than artificial light, in fact, it is likely that both functions emerged at about the same time: once humans began using fire for warmth, it would have been a relatively short time before they comprehended the power of fire to drive out both darkness and the fierce creatures (for instance, bears) that came with it.

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form of torches or rudimentary lamps. Torches were probably made by binding together resinous material from trees, while lamps were made either from stones with natural depressions, or from soft rocks—for example, soapstone or steatite—into which depressions were carved by using harder material. Most of the many hundreds of lamps found by archaeologists at sites in southwestern France are made of either limestone or sandstone. Limestone was a particularly good choice, since it conducts heat poorly; lamps made of sandstone, a good conductor of heat, usually had carved handles to protect the hands of the user. ARTIFICIAL LIGHT IN PREM O D E R N T I M E S . The history of lighting

is generally divided into four periods, each of which overlap, and which together illustrate the slow pace of change in illumination technology. First was the primitive, a period encompassing the torches and lamps of prehistoric human beings—though, in fact, French peasants used the same lighting methods depicted in nearby cave paintings until World War I. Next came the classical stage, the world of Greece and Rome. Earlier civilizations, such as that of Egypt, belong to the primitive era in lighting—before the relatively widespread adoption of the candle and of vegetable oil as fuel. Third was the medieval stage, which saw the development of metal lamps. Last came the modern or invention stage, which began with the creation of the glass lantern chimney by Leonardo da Vinci (1452-1519) in 1490, culminated with Thomas Edison’s (1847-1931) first practical incandescent bulb in 1879, and continues today. At various times, ancient peoples used the fat of seals, horses, cattle, and fish as fuel for lamps. (Whale oil, by contrast, entered widespread use only during the nineteenth century.) Primitive humans sometimes used entire animals—for example, the storm petrel, a bird heavy in fat—to provide light. Even without such cruel excesses, however, animal fat made for a smoky, dangerous, foul-smelling fire.

These distant forebears advanced to the fashioning of portable lighting technology in the

The use of vegetable oils, a much more efficient medium for lighting, did not take hold until Greek, and especially, Roman times. Animal oils remained in use, however, among the poor, whose homes often reeked with the odor of castor oil or fish oil. Because virtually all fuels came

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from edible sources, times of famine usually meant times of darkness as well. The candle, as well as the use of vegetable oils, dates back to earliest antiquity, but the use of candles only became common among the richest citizens of Rome. Because it used animal fat, the candle was apparently a return to an earlier stage, but its hardened tallow actually represented a much safer, more stable fuel than lamp oil. I N CA N D E S C E N T L I G H T. Lighting technology in the period from about 1500 to the late nineteenth century involved a number of improvements, but in one respect, little had progressed since prehistoric times: people were still burning fuel to provide illumination. This all changed with the invention of the incandescent bulb, which, though it is credited to Edison, was the product of experimentation that took place throughout the nineteenth century. As early as 1802, British scientist Sir Humphry Davy (17781829) showed that electricity running through thin strips of metal could heat them enough to cause them to give off light—that is, electromagnetic radiation.

Edison, in fact, was just one of several inventors in the 1870s attempting to develop a practical incandescent lamp. His innovation lay in his understanding of the parameters necessary for developing such a lamp—in particular, decreasing the electrical resistance in the lamp filament (the part that is heated) so that less energy would be required to light it. On October 19, 1879, using low-resistance filaments of carbon or platinum, combined with a high-resistance carbon filament in a vacuum-sealed glass container, Edison produced the first practical lightbulb. Much has changed in the design of lightbulbs during the decades following Edison’s ingenious invention, of course, but his design provided the foundation. There is just one problem with incandescent light, however—a problem inherent in the definition and derivation of the word incandescent, which comes from a Latin root meaning “to become hot.” The efficiency of a light is determined by the ratio of light, or usable energy, to heat—which, except in the case of a campfire, is typically not a desirable form of energy where lighting is concerned.

producing heat rather than light that people can use. The visible light tends to be in the red and yellow end of the spectrum—closer to infrared— but a blue-tinted bulb helps to absorb some of the red and yellow, providing a color balance. This, however, only further diminishes the total light output, and, hence, in many applications today, fluorescent light takes the place of incandescent light.

Light

Lasers A laser is an extremely focused, extremely narrow, and extremely powerful beam of light. Actually, the term laser is an acronym, standing for Light Amplification by Simulated Emission of Radiation. Simulated emission involves bringing a large number of atoms into what is called an “excited state.” Generally, most atoms are in a ground state, and are less active in their movements, but the energy source that activates a laser brings about population inversion, a reversal of the ratios, such that the majority of atoms within the active medium are in an excited rather than a ground state. To visualize this, picture a popcorn popper, with the excited atoms being the popping kernels, and the ground-state atoms the ones remaining unpopped. As the atoms become excited, and the excited atoms outnumber the ground ones, they start to cause a multiplication of the resident photons. This is simulated emission. A laser consists of three components: an optical cavity, an energy source, and an active medium. To continue the popcorn analogy, the “popper” itself—the chamber which holds the laser—is the optical cavity, which, in the case of a laser, involves two mirrors facing one another. One of these mirrors fully reflects light, whereas the other is a partly reflecting mirror. The light not reflected by the second mirror escapes as a highly focused beam. As with the popcorn popper, the power source involves electricity, and the active medium is analogous to the oil in a conventional popper.

Amazingly, only about 10% of the energy output from a typical incandescent light bulb is in the form of visible light; the rest comes through the infrared region of the spectrum,

T Y P E S O F LAS E R S . There are four types of lasers: solid-state, semiconductor, gas, and dye. Solid-state lasers are generally very large and extremely powerful. Having a crystal or glass housing, they have been implemented in nuclear energy research, and in various areas of industry. Whereas solid-state lasers can be as long as a city block, semiconductor lasers can be smaller than

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Light

THE

BEAM OF A MODE LOCKED, FREQUENCY DOUBLED

FILTERS AT

ND YLF LASER IS REFLECTED OFF MIRRORS AND THROUGH COLORADO STATE UNIVERSITY. (Photograph by Chris/RogersBikderberg. The Stock Market. Reproduced by permission.)

the head of a pin. Semiconductor lasers (involving materials such as arsenic that conduct electricity, but do not do so as efficiently as the metals typically used as conductors) are applied for the intricate work of making compact discs and computer microchips.

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gests, use different colored dyes. (Laser light itself, unlike ordinary light, is monochromatic.) Dye lasers can be used for medical research, or for fun—as in the case of laser light shows held at parks in the summertime.

Gas lasers contain carbon dioxide or other gases, activated by electricity in much the same way the gas in a neon sign is activated. Among their applications are eye surgery, printing, and scanning. Finally, dye lasers, as their name sug-

Laser beams have a number of other useful functions, for instance, the production of compact discs (CDs). Lasers etch information onto a surface, and because of the light beam’s qualities, can record far more information in much less space

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LASER

A P P L I C AT I O N S .

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Light

KEY TERMS An opening.

APERTURE:

Because light moves by radiation, it does

The bending of waves

not require a medium, and, in fact, move-

around obstacles, or the spreading of waves

ment through a medium slows the speed of

by passing them through an aperture.

light somewhat.

DIFFRACTION:

DIFFUSION:

A process by which the

concentration or density of something is decreased.

OPTICS:

An area of physics that studies

the production and propagation of light. PHOTOELECTRIC

EFFECT:

The

SPECTRUM:

phenomenon whereby certain metals

The complete range of electromagnetic

acquire an electrical potential when

waves on a continuous distribution from a

exposed to light.

ELECTROMAGNETIC

very low range of frequencies and energy levels, with a correspondingly long wavelength, to a very high range of frequencies and energy levels, with a correspondingly short wavelength. Included on the electro-

A particle of electromagnet-

PHOTON:

ic radiation—for example, light—carrying a specific amount of energy, measured in electron volts (eV). A three-dimensional glass

magnetic spectrum are long-wave and

PRISM:

short-wave radio; microwaves; infrared,

shape used for the diffusion of light rays.

visible, and ultraviolet light; x rays, and gamma rays.

PROPAGATION:

The act or state of

traveling from one place to another.

ELECTROMAGNETIC

WAVE:

A

transverse wave with electric and magnetic fields that emanate from it. The directions of these fields are perpendicular to one another, and both are perpendicular to the line of propagation for the wave itself. FREQUENCY:

The number of waves

passing through a given point during the

RADIATION:

The transfer of energy by

means of electromagnetic waves, which require no physical medium (for example, water or air) for the transfer. Earth receives the Sun’s energy (including its light), via the electromagnetic spectrum, by means of radiation. In geometry, a ray is that part of a

interval of one second. The higher the fre-

RAY:

quency, the shorter the wavelength.

straight line that extends in one direction

HERTZ:

A unit for measuring frequen-

cy, named after nineteenth—century German physicist Heinrich Rudolf Hertz (1857-1894).

only. The term “ray” is used to describe the directed line made by light as it moves through space. REFLECTION:

A type of incidence

Contact between a ray—

whereby a light ray is returned toward its

for example, a light ray—and a surface.

source rather than being absorbed at the

Types of incidence include reflection and

interface.

refraction.

REFRACTION:

INCIDENCE:

MEDIUM:

A substance through which

light travels, such as air, water, or glass.

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The bending of a light

ray that occurs when it passes through a dense medium, such as water or glass.

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Light

KEY TERMS SPECTRUM:

The continuous distribu-

CONTINUED

cular to the direction in which the wave is

tion of properties in an ordered arrange-

moving.

ment across an unbroken range. Examples

VACUUM:

of spectra (the plural of “spectrum”)

matter, including air.

An area of space devoid of

include the colors of visible light, or the electromagnetic spectrum of which visible light is a part. TRANSVERSE

The distance between

a crest and the adjacent crest, or the trough and an adjacent trough, of a wave. The

WAVE:

A wave in

which the vibration or motion is perpendi-

than the old-fashioned ways of producing phonograph records. Lasers used in the production of CD-ROM (Read-Only Memory) disks are able to condense huge amounts of information—a set of encyclopedias or the New York metropolitan phone book—onto a disk one can hold in the palm of one’s hand. Laser etching is also used to create digital videodiscs (DVDs) and holograms. Another way that lasers affect everyday life is in the field of fiber optics, which uses pulses of laser light to send information on glass strands.

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WAVELENGTH:

shorter the wavelength, the higher the frequency.

contractors ensure that walls and floors and ceilings are in proper alignment. WHERE TO LEARN MORE Burton, Jane and Kim Taylor. The Nature and Science of Colors. Milwaukee, WI: Gareth Stevens Publishing, 1998. Glover, David. Color and Light. New York: Dorling Kindersley Publishing, 2001. Kalman, Bobbie and April Fast. Cosmic Light Shows. New York: Crabtree Publishing, 1999. Kurtus, Ron. “Visible Light” (Web site). (May 2, 2001).

Before the advent of fiber-optic communications, telephone calls were relayed on thick bundles of copper wire; with the appearance of this new technology, a glass wire no thicker than a human hair now carries thousands of conversations. Lasers are also used in scanners, such as the price-code checkers at supermarkets and various kinds of tags that prevent thefts of books from libraries or clothing items from stores. In an industrial setting, heating lasers can drill through solid metal, or in an operating room, lasers can remove gallstones or cataracts. Lasers are also used for guiding missiles, and to help building

“Visible Light Waves” NASA: National Aeronautics and Space Administration (Web site). (May 2, 2001).

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“Light Waves and Color.” The Physics Classroom (Web site). (May 2, 2001). Miller-Schroeder, Patricia. The Science and Light of Color. Milwaukee, WI: Gareth Stevens Publishing, 2000. Nassau, Kurt. Experimenting with Color. New York: F. Watts, 1997. Riley, Peter D. Light and Color. New York: F. Watts, 1999. Taylor, Helen Suzanne. A Rainbow Is a Circle: And Other Facts About Color. Brookfield, CT: Copper Beech Books, 1999.

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Luminescence

LU M I N E S C E N C E

CONCEPT Luminescence is the generation of light without heat. There are two principal varieties of luminescence, fluorescence and phosphorescence, distinguished by the delay in reaction to external electromagnetic radiation. The ancients observed phosphorescence in the form of a glow emitted by the oceans at night, and confused this phenomenon with the burning of the chemical phosphor, but, in fact, phosphorescence has nothing at all to do with burning. Likewise, fluorescence, as applied today in fluorescent lighting, involves no heat—thus creating a form of lighting more efficient than that which comes from incandescent bulbs.

HOW IT WORKS Radiation Elsewhere in this volume, the term “radiation” has been used to describe the transfer of energy in the form of heat. In fact, radiation can also be described, in a more general sense, as anything that travels in a stream, whether that stream be composed of subatomic particles or electromagnetic waves.

Ionizing radiation can indeed cause a great deal of damage to matter—including the matter in a human body. Even radiation produced by parts of the electromagnetic spectrum possessing far less energy than gamma rays can be detrimental, as will be discussed below. In general, however, there is nothing inherently dangerous about radiation: indeed, without the radiation transmitted to Earth via the Sun’s electromagnetic spectrum, life simply could not exist.

The Electromagnetic Spectrum The electromagnetic spectrum is the complete range of electromagnetic waves on a continuous distribution from those with very low frequencies and energy levels, along with correspondingly long wavelengths, to those with very high frequencies and energy levels, with correspondingly short wavelengths. An electromagnetic wave is transverse, meaning that even as it moves forward, it oscillates in a direction perpendicular to the line of propagation. An electromagnetic wave can thus be defined as a transverse wave with mutually perpendicular electrical and magnetic fields that emanate from it. Though their shape is akin to that of waves on the ocean, electromagnetic waves travel much, much faster than any waves that human eyes can see. Their speed of propagation in a vacuum is equal to that of light: 186,000 mi (299,339 km) per second.

Many people think of radiation purely in terms of the harmful effects produced by radioactive materials—those subject to a form of decay brought about by the emission of highenergy particles or radiation, including alpha particles, beta particles, or gamma rays. These high-energy forms of radiation are called ionizing radiation, because they are capable of literally ripping through some types of atoms, removing electrons and leaving behind a string of ions.

electromagnetic spectrum are radio waves and microwaves; infrared, visible, and ultraviolet light; x rays, and gamma rays. Though each occu-

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trary about the order in which the different types of electromagnetic waves are listed above: this is their order in terms of frequency (measured in Hertz, or Hz) and energy levels, which are directly related. This ordering also represents the reverse order (that is, from longer to shorter) for wavelength, which is inversely related to frequency.

Luminescence

Extremely low-energy, long-wavelength radio waves have frequencies of around 102 Hz, while the highest-energy, shortest-wavelength gamma rays can have frequencies of up to 1025 Hz. This means that these gamma rays are oscillating at the rate of 10 trillion trillion times a second!

MODERN MUCH TO

UNDERSTANDING

OF

LUMINESCENCE

OWES

MARIE CURIE.

pies a definite place on the spectrum, the divisions between them are not firm: as befits the nature of a spectrum, one simply “blurs” into another. Though the Sun sends its energy to Earth in the form of light and heat from the electromagnetic spectrum, not everything within the spectrum is either “bright.” The “bright” area of the spectrum—that is, the band of visible light—is incredibly small, equal to about 3.2 parts in 100 billion. This is like comparing a distance of 16 ft (4.8 m) to the distance between Earth and the Sun: 93 million miles (1.497 • 109 km). When electromagnetic waves of almost any frequency are asborbed in matter, their energy can be converted to heat. Whether or not this happens depends on the absorption mechanism. However, the realm of “heat” as it is most experienced in daily life is much smaller, encompassing infrared, visible, and ultraviolet light. Below this frequency range are various types of radio waves, and above it are ultra high-energy x rays and gamma rays. Some of the heat experienced in a nuclear explosion comes from the absorption of gamma rays emitted in the nuclear reaction.

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The wavelengths of very low-energy, lowfrequency radio waves can be extremely long: 108 centimeters, equal to 1 million meters or about 621 miles. Precisely because these wavelengths are so very long, they are hard to apply for any practical use: ordinary radio waves of the kind used for actual radio broadcasts are closer to 105 cm (about 328 ft). At the opposite end of the spectrum are gamma rays with wavelengths of less than 10-15 centimeters—in other words, a decimal point followed by 14 zeroes and a 1. There is literally nothing in the observable world that can be compared to this figure, equal to one-trillionth of a centimeter. Even the angstrom—a unit so small it is used to measure the diameter of an atom— is 10 million times as large.

Emission and Absorption The electromagnetic spectrum is not the only spectrum: physicists, as well as people who are not scientifically trained, often speak of the color spectrum for visible light. The reader is encouraged to study the essays on both subjects to gain a greater understanding of each. In the present context, however, two other types of spectra (the plural of “spectrum”) are of interest: emission and absorption spectra.

FREQUENCY AND WAV E L E N GT H RA N G E . There is nothing arbi-

Emission occurs when internal energy from one system is transformed into energy that is carried away from that system by electromagnetic radiation. An emission spectrum for any given system shows the range of electromagnetic radiation it emits. When an atom has energy transferred to it, either by collisions or as a result of exposure to radiation, it is said to be experiencing excitation, or to be “excited.” Excited atoms

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Luminescence

COMPARED

TO STANDARD INCANDESCENT LIGHT BULBS (SHOWN ON THE LEFT), NEWER FLUORESCENT BULBS (SHOWN

ON THE RIGHT) USE FAR LESS ELECTRICITY AND LAST MUCH LONGER. (Photograph by Roger Ressmeyer/Corbis. Reproduced by per-

mission.)

will emit light of a given frequency as they relax back to their normal state. Atoms of neon, for instance, can be excited in such a way that they emit light at wavelengths corresponding to the color red, a property that finds application in neon signs.

heat. Even what premodern observers called “phosphorescence” was not phosphorescence as the term is used in modern science. Instead, the word was used to describe light given off in a fiery reaction that occurs when the element phosphorus is exposed to air.

As its name suggests, absorption has a reciprocal relationship with emission: it is the result of any process wherein the energy transmitted to a system via electromagnetic radiation is added to the internal energy of that system. Each material has a unique absorption spectrum, which makes it possible to identify that material using a device called a spectrometer. In the phenomenon of luminescence, certain materials absorb electromagnetic radiation and proceed to emit that radiation in ways that distinguish the materials as either fluorescent or phosphorescent.

There are, however, examples of luminescence in nature that had been observed from ancient times onward—for instance, the phosphorescent glow of the ocean, visible at night under certain conditions. At one time this, too, was mistakenly associated with phosphorus, which was supposedly burning in the water. In fact, the ocean’s phosphorescence comes neither from phosphorus nor water, but from living creatures called dinoflagellates. This is an example of a phenomenon known as bioluminescence— fireflies are another example—discussed below.

For the most part, prior to the nineteenth century, scientists had little concept of light without

Modern understanding of luminescence owes much to Polish-French physicist and chemist Marie Curie (1867-1934). Operating in fields that had been dominated by men since the birth of the physical sciences, Curie distinguished herself with a number of achievements, becoming the first scientist in history to receive two Nobel prizes (physics in 1903 and chemistry in 1911). While working on her doctoral thesis,

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REAL-LIFE A P P L I C AT I O N S Early Observations of Luminescence

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Luminescence

LIKE

MANY MARINE CREATURES, JELLYFISH PRODUCE THEIR OWN LIGHT THROUGH PHOSPHORESCENCE. (Photograph by

Mark A. Johnson. The Stock Market. Reproduced by permission.)

Curie noted that calcium fluoride glows when exposed to a radioactive material known as radium. Curie—who also coined the term “radioactivity”—helped spark a revolution in science and technology. As a result of her work and the discoveries of others who followed, interest in luminescence and luminescent devices grew. Today, luminescence is applied in a number of devices around the household, most notably in television screens and fluorescent lights.

Fluorescence As indicated in the introduction to this essay, the difference between the two principal types of luminescence relates to the timing of their reactions to electromagnetic radiation. Fluorescence is a type of luminescence whereby a substance absorbs radiation and almost instantly begins to re-emit the radiation. (Actually, the delay is 10-6 seconds, or a millionth of a second.) Fluorescent luminescence stops within 10-5 seconds after the energy source is removed; thus, it comes to an end almost as quickly as it begins.

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matician and physicist George Gabriel Stokes (1819-1903), who coined the term “fluorescence,” first discovered this difference in wavelength. However, in a special type of fluorescence known as resonance fluorescence, the wavelengths are the same. Applications of resonance include its use in analyzing the flow of gases in a wind tunnel. BLACK LIGHTS AND FLUOR E S C E N C E . A “black light,” so called

because it emits an eerie bluish-purple glow, is actually an ultraviolet lamp, and it brings out vibrant colors in fluorescent materials. For this reason, it is useful in detecting art forgeries: newer paint tends to fluoresce when exposed to ultraviolet light, whereas older paint does not. Thus, if a forger is trying to pass off a painting as the work of an Old Master, the ultraviolet lamp will prove whether the artwork is genuine or not.

Usually, the wavelength of the re-emitted radiation is longer than the wavelength of the radiation the substance absorbed. British mathe-

Another example of ultraviolet light and fluorescent materials is the “black-light” poster, commonly associated with the psychedelic rock music of the late 1960s and early 1970s. Under ordinary visible light, a black-light poster does not look particularly remarkable, but when exposed to ultraviolet light in an environment in which visible light rays are not propagated (that is, a darkened room), it presents a dazzling array

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of colors. Yet, because they are fluorescent, the moment the black light is turned off, the colors of the poster cease to glow. Thus, the poster, like the light itself, can be turned “on” and “off,” simply by activating or deactivating the ultraviolet lamp. R U B I E S A N D LAS E R S . Fluorescence has applications far beyond catching art forgers or enhancing the experience of hearing a Jimi Hendrix album. In 1960, American physicist Theodore Harold Maiman developed the first laser using a ruby, a gem that exhibits fluorescent characteristics. A laser is a very narrow, highly focused, and extremely powerful beam of light used for everything from etching data on a surface to performing eye surgery.

Crystalline in structure, a ruby is a solid that includes the element chromium, which gives the gem its characteristic reddish color. A ruby exposed to blue light will absorb the radiation and go into an excited state. After losing some of the absorbed energy to internal vibrations, the ruby passes through a state known as metastable before dropping to what is known as the ground state, the lowest energy level for an atom or molecule. At that point, it begins emitting radiation on the red end of the spectrum. The ratio between the intensity of a ruby’s emitted fluorescence and that of its absorbed radiation is very high, and, thus, a ruby is described as having a high level of fluorescent efficiency. This made it an ideal material for Maiman’s purposes. In building his laser, he used a ruby cylinder which emitted radiation that was both coherent, or all in a single direction, and monochromatic, or all of a single wavelength. The laser beam, as Maiman discovered, could travel for thousands of miles with very little dispersion—and its intensity could be concentrated on a small, highly energized pinpoint of space. F LU O R E S C E N T L I G H T S . By far

inefficient compared to fluorescent light: in an incandescent bulb, fully 90% of the energy output is wasted on heat, which comes through the infrared region.

Luminescence

A fluorescent bulb consuming the same amount of power as an incandescent bulb will produce three to five times more light, and it does this by using a phosphor, a chemical that glows when exposed to electromagnetic energy. (The term “phosphor” should not be confused with phosphorescence: phosphors are used in both fluorescent and phosphorescent applications.) The phosphor, which coats the inside surface of a fluorescent lamp, absorbs ultraviolet light emitted by excited mercury atoms. It then re-emits the ultraviolet light, but at longer wavelengths—as visible light. Thanks to the phosphor, a fluorescent lamp gives off much more light than an incandescent one, and does so without producing heat. P H O S P H O R E S C E N C E . In contrast

to the nearly instantaneous “on-off ” of fluorescence, phosphorescence involves a delayed emission of radiation following absorption. The delay may take as much as several minutes, but phosphorescence continues to appear after the energy source has been removed. The hands and numbers of a watch that glows in the dark, as well as any number of other items, are coated with phosphorescent materials. Television tubes also use phosphorescence. The tube itself is coated with phosphor, and a narrow beam of electrons causes excitation in a small portion of the phosphor. The phosphor then emits red, green, or blue light—the primary colors of light—and continues to do so even after the electron beam has moved on to another region of phosphor on the tube. As it scans across the tube, the electron beam is turned rapidly on and off, creating an image made up of thousands of glowing, colored dots.

the most common application of fluorescence in daily life is in the fluorescent light bulb, of which there are more than 1.5 billion operating in the United States. Fluorescent light stands in contrast to incandescent, or heat-producing, electrical light. First developed successfully by Thomas Edison (1847-1931) in 1879, the incandescent lamp quite literally transformed human life, making possible a degree of activity after dark that would have been impractical in the age of gas lamps. Yet, incandescent lighting is highly

PHOSPHORESCENCE IN SEA C R E AT U R E S . As noted above, one of the

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first examples of luminescence ever observed was the phosphorescent effect sometimes visible on the surface of the ocean at night—an effect that scientists now know is caused by materials in the bodies of organisms known as dinoflagellates. Inside the body of a dinoflagellate are the substances luciferase and luciferin, which chemically react with oxygen in the air above the water to produce light with minimal heat levels. Though

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Luminescence

KEY TERMS ABSORPTION:

The result of any

EMISSION:

The result of a process that

process wherein the energy transmitted to

occurs when internal energy from one sys-

a system via electromagnetic radiation is

tem is transformed into energy that is car-

added to the internal energy of that system.

ried away from it by electromagnetic radi-

Each material has a unique absorption

ation. An emission spectrum for any given

spectrum, which makes it possible to iden-

system shows the range of electromagnetic

tify that material using a device called a

radiation it emits. (Compare emission to

spectrometer. (Compare absorption to

absorption.)

emission.) EXCITATION: ELECTROMAGNETIC

SPECTRUM:

The complete range of electromagnetic

The transfer of energy to

an atom, either by collisions or due to radiation.

waves on a continuous distribution from a very low range of frequencies and energy levels, with a correspondingly long wave-

A type of lumines-

cence whereby a substance absorbs radia-

length, to a very high range of frequencies

tion and begins to re-emit the radiation

and energy levels, with a correspondingly

10-6 seconds after absorption. Usually the

short wavelength. Included on the electro-

wavelength of emission is longer than the

magnetic spectrum are long-wave and

wavelength of the radiation the substance

short-wave radio; microwaves; infrared,

absorbed. Fluorescent luminescence stops

visible, and ultraviolet light; x rays, and

within 10-5 seconds after the energy source

gamma rays.

is removed.

ELECTROMAGNETIC

WAVE:

A

FREQUENCY:

The number of waves

transverse wave with electric and magnetic

passing through a given point during the

fields that emanate from it. These waves are

interval of one second. The higher the fre-

propagated by means of radiation.

quency, the shorter the wavelength.

dinoflagellates are microscopic creatures, in large numbers they produce a visible glow. Nor are dinoflagellates the only bioluminescent organisms in the ocean. Jellyfish, as well as various species of worms, shrimp, and squid, all produce their own light through phosphorescence. This is particularly useful for creatures living in what is known as the mesopelagic zone, a range of depth from about 650 to 3,000 ft (2001,000 m) below the ocean surface, where little light can penetrate.

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FLUORESCENCE:

to a powder, a dead cypridina can continue to produce light, if mixed with water. Japanese soldiers in World War II used the powder of cypridina to illuminate maps at night, providing themselves with sufficient reading light without exposing themselves to enemy fire.

Processes that Create Luminescence

One interesting bioluminescent sea creature is the cypridina. Resembling a clam, the cypridina mixes its luciferin and luciferase with sea water to create a bright bluish glow. When dried

The phenomenon of bioluminescence actually goes beyond the frontiers of physics, into chemistry and biology. In fact, it is a subset of chemiluminescence, or luminescence produced by chemical reactions. Chemiluminescence is, in

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Luminescence

KEY TERMS

CONTINUED

A unit for measuring frequen-

decay brought about by the emission of

cy, named after ninetenth-century German

high-energy particles or radiation, includ-

physicist Heinrich Rudolf Hertz (1857-

ing alpha particles, beta particles, or

1894).

gamma rays.

HERTZ:

LUMINESCENCE:

The generation of

The continuous distribu-

SPECTRUM:

light without heat. There are two principal

tion of properties in an ordered arrange-

varieties of luminescence, fluorescence and

ment across an unbroken range. Examples

phosphorescence.

of spectra (the plural of “spectrum”) A type of

include the colors of visible light, the elec-

luminescence involving a delayed emission

tromagnetic spectrum of which visible

of radiation following absorption. The

light is a part, as well as emission and

delay may take as much as several minutes,

absorption spectra.

but phosphorescence continues to appear

TRANSVERSE

after the energy source has been removed.

which the vibration or motion is perpendi-

The act or state of

cular to the direction in which the wave is

PHOSPHORESCENCE:

PROPAGATION:

travelling from one place to another. RADIATION:

In a general sense, radia-

tion can refer to anything that travels in a stream, whether that stream be composed of subatomic particles or electromagnetic waves. RADIOACTIVE:

WAVE:

A wave in

moving. VACUUM:

An area of space devoid of

matter, including air. WAVELENGTH:

The distance between

a crest and the adjacent crest, or the trough and an adjacent trough, of a wave. The

A term describing

materials which are subject to a form of

turn, one of several processes that can create luminescence. Many of the types of luminescence discussed above are described under the heading of electroluminescence, or luminescence involving electromagnetic energy. Another process is triboluminescence, in which friction creates light. Though this type of friction can produce a fire, it is not to be confused with the heat-causing friction that occurs when flint and steel are struck together. Yet another physical process used to create luminescence is sonoluminescence, in which light is produced from the energy transmitted by sound waves. Sonoluminescence is one of the

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shorter the wavelength, the higher the frequency.

fields at the cutting edge in physics today, and research in this area reveals that extremely high levels of energy may be produced in small areas for very short periods of time. WHERE TO LEARN MORE Birch, Beverley. Marie Curie: Pioneer in the Study of Radiation. Milwaukee, WI: Gareth Stevens Children’s Books, 1990. Evans, Neville. The Science of a Light Bulb. Austin, TX: Raintree Steck-Vaughn Publishers, 2000. “Luminescence.” Slider.com (Web site). (May 5, 2001). “Luminescence.” Xrefer (Web site). (May 5, 2001).

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Macaulay, David. The New Way Things Work. Boston: Houghton Mifflin, 1998.

Skurzynski, Gloria. Waves: The Electromagnetic Universe. Washington, D.C.: National Geographic Society, 1996.

Pettigrew, Mark. Radiation. New York: Gloucester Press, 1986.

Suplee, Curt. Everyday Science Explained. Washington, D.C.: National Geographic Society, 1996.

Simon, Hilda. Living Lanterns: Luminescence in Animals. Illustrated by the author. New York: Viking Press, 1971.

“UV-Vis Luminescence Spectroscopy” (Web site). (May 5, 2001).

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SCIENCEOF

EVERYDAY THINGS

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SCIENCEOF

EVERYDAY THINGS volume 3: REAL-LIFE BIOLOGY

edited by NEIL SCHLAGER written by JUDSON KNIGHT A SCHLAGER INFORMATION GROUP BOOK

Detroit • New York • San Diego • San Francisco • Cleveland • New Haven, Conn. • Waterville, Maine • London • Munich

Science of Everyday Things Volume 3: Real-Life Biology A Schlager Information Group Book Neil Schlager, Editor Written by Judson Knight

Project Editor Kimberley A. McGrath

Permissions Lori Hines

Product Design Michelle DiMercurio, Michael Logusz

Editorial Mark Springer

Imaging and Multimedia Robert Duncan, Leitha Etheridge-Sims, Mary K. Grimes, Lezlie Light, Dan Newell, David G. Oblender, Robyn V. Young

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© 2002 by Gale. Gale is an imprint of The Gale Group, Inc., a division of Thomson Learning, Inc.

For permission to use material from this product, submit your request via the Web at http://www.gale-edit.com/permissions, or you may download our Permissions Request form and submit your request by fax or mail to: The Gale Group, Inc., Permissions Department, 27500 Drake Road, Farmington Hills, MI 48331-3535, Permissions hotline: 248699-8074 or 800-877-4253, ext. 8006, Fax: 248-699-8074 or 800-762-4058.

While every effort has been made to ensure the reliability of the information presented in this publication, The Gale Group, Inc. does not guarantee the accuracy of the data contained herein. The Gale Group, Inc. accepts no payment for listing; and inclusion in the publication of any organization, agency, institution, publication, service, or individual does not imply endorsement of the editors or the publisher. Errors brought to the attention of the publisher and verified to the satisfaction of the publisher will be corrected in future editions.

Gale and Design™ and Thomson Learning ™ are trademarks used herein under license. For more information contact The Gale Group, Inc. 27500 Drake Rd. Farmington Hills, MI 48331-3535 Or you can visit our Internet site at http://www.gale.com ALL RIGHTS RESERVED No part of this work covered by the copyright hereon may be reproduced or used in any form or by any means—graphic, electronic, or mechanical, including photocopying, recording, taping, Web distribution, or information storage retrieval systems—without the written permission of the publisher.

LIBRARY OF CONGRESS CATALOG-IN-PUBLICATION DATA Knight, Judson. Science of everyday things / written by Judson Knight, Neil Schlager, editor. p. cm. Includes bibliographical references and indexes. Contents: v. 1. Real-life chemistry – v. 2 Real-life physics. SBN 0-7876-5631-3 (set : hardcover) – ISBN 0-7876-5632-1 (v. 1) – ISBN 0-7876-5633-X (v. 2) 1. Science–Popular works. I. Schlager, Neil, 1966-II. Title. Q162.K678 2001 500–dc21

2001050121

ISBN 0-7876-5631-3 (set), 0-7876-5632-1 (vol. 1), 0-7876-5633-X (vol. 2), 0-7876-5634-8 (vol. 3), 0-7876-5635-6 (vol. 4)

Printed in the United States of America 10 9 8 7 6 5 4 3 2 1

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CONTENTS

Advisory Board . . . . . . . . . . . . . . . . . . . . . . vii

Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204 Speciation . . . . . . . . . . . . . . . . . . . . . . . . . . . 215

BIOCHEMISTRY

DISEASE

Carbohydrates. . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Amino Acids. . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Proteins. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 Noninfectious Diseases. . . . . . . . . . . . . . . . . 236 Infectious Diseases . . . . . . . . . . . . . . . . . . . . 244

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . v

IMMUNITY METABOLISM Metabolism . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 Digestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 Respiration . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

Immunity and Immunology . . . . . . . . . . . . 255 The Immune System. . . . . . . . . . . . . . . . . . . 262 INFECTION

NUTRITION

Parasites and Parasitology . . . . . . . . . . . . . . 273 Infection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283

Food Webs . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 Nutrients and Nutrition . . . . . . . . . . . . . . . . . 77 Vitamins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

BRAIN AND BODY

GENETICS Genetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Heredity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Genetic Engineering . . . . . . . . . . . . . . . . . . . 117 Mutation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

Chemoreception . . . . . . . . . . . . . . . . . . . . . . 295 Biological Rhythms. . . . . . . . . . . . . . . . . . . . 306 LEARNING AND BEHAVIOR Behavior. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 319 Instinct and Learning . . . . . . . . . . . . . . . . . . 327 Migration and Navigation . . . . . . . . . . . . . . 335

REPRODUCTION AND BIRTH Reproduction . . . . . . . . . . . . . . . . . . . . . . . . 135 Sexual Reproduction. . . . . . . . . . . . . . . . . . . 142 Pregnancy and Birth . . . . . . . . . . . . . . . . . . . 151 EVOLUTION Evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161 Paleontology . . . . . . . . . . . . . . . . . . . . . . . . . 176

THE BIOSPHERE AND ECOSYSTEMS The Biosphere . . . . . . . . . . . . . . . . . . . . . . . . 345 Ecosystems and Ecology . . . . . . . . . . . . . . . . 360 Biomes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 370 BIOLOGICAL COMMUNITIES

Taxonomy . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

Symbiosis. . . . . . . . . . . . . . . . . . . . . . . . . . . . 383 Biological Communities . . . . . . . . . . . . . . . . 391 Succession and Climax . . . . . . . . . . . . . . . . . 400 General Subject Index . . . . . . . . . . . . . 411

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BIODIVERSITY AND TAXONOMY

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INTRODUCTION

Overview of the Series Welcome to Science of Everyday Things. Our aim is to explain how scientific phenomena can be understood by observing common, real-world events. From luminescence to echolocation to buoyancy, the series will illustrate the chief principles that underlay these phenomena and explore their application in everyday life. To encourage cross-disciplinary study, the entries will draw on applications from a wide variety of fields and endeavors. Science of Everyday Things initially comprises four volumes: Volume 1: Real-Life Chemistry Volume 2: Real-Life Physics Volume 3: Real-Life Biology Volume 4: Real-Life Earth Science Future supplements to the series will expand coverage of these four areas and explore new areas, such as mathematics.

Arrangement of Real-Life Biology This volume contains 40 entries, each covering a different scientific phenomenon or principle. The entries are grouped together under common categories, with the categories arranged, in general, from the most basic to the most complex. Readers searching for a specific topic should consult the table of contents or the general subject index.

• How It Works: Explains the principle or theory in straightforward, step-by-step language. • Real-Life Applications: Describes how the phenomenon can be seen in everyday life. • Where to Learn More: Includes books, articles, and Internet sites that contain further information about the topic. In addition, each entry includes a “Key Terms” section that defines important concepts discussed in the text. Finally, each volume includes many illustrations and photographs throughout. In addition, readers will find the comprehensive general subject index valuable in accessing the data.

About the Editor, Author, and Advisory Board Neil Schlager and Judson Knight would like to thank the members of the advisory board for their assistance with this volume. The advisors were instrumental in defining the list of topics, and reviewed each entry in the volume for scientific accuracy and reading level. The advisors include university-level academics as well as high school teachers; their names and affiliations are listed elsewhere in the volume.

• Concept: Defines the scientific principle or theory around which the entry is focused.

Neil Schlager is the president of Schlager Information Group Inc., an editorial services company. Among his publications are When Technology Fails (Gale, 1994); How Products Are Made (Gale, 1994); the St. James Press Gay and Lesbian Almanac (St. James Press, 1998); Best Literature By and About Blacks (Gale, 2000); Contemporary Novelists, 7th ed. (St. James Press,

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Within each entry, readers will find the following rubrics:

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Introduction

2000); Science and Its Times (7 vols., Gale, 20002001); and Science in Dispute (Gale, 2002). His publications have won numerous awards, including three RUSA awards from the American Library Association, two Reference Books Bulletin/Booklist Editors’ Choice awards, two New York Public Library Outstanding Reference awards, and a CHOICE award for best academic book. Judson Knight is a freelance writer, and author of numerous books on subjects ranging from science to history to music. His work on science includes Science, Technology, and Society, 2000 B.C.-A.D. 1799 (U•X•L, 2002), as well as extensive contributions to Gale’s seven-volume Science and Its Times (2000-2001). As a writer on

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history, Knight has published Middle Ages Reference Library (2000), Ancient Civilizations (1999), and a volume in U•X•L’s African American Biography series (1998). Knight’s publications in the realm of music include Parents Aren’t Supposed to Like It (2001), an overview of contemporary performers and genres, as well as Abbey Road to Zapple Records: A Beatles Encyclopedia (Taylor, 1999).

Comments and Suggestions Your comments on this series and suggestions for future editions are welcome. Please write: The Editor, Science of Everyday Things, Gale Group, 27500 Drake Road, Farmington Hills, MI 483313535.

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ADVISORY BO T IATR LD E

William E. Acree, Jr. Professor of Chemistry, University of North Texas Russell J. Clark Research Physicist, Carnegie Mellon University Maura C. Flannery Professor of Biology, St. John’s University, New York John Goudie Science Instructor, Kalamazoo (MI) Area Mathematics and Science Center Cheryl Hach Science Instructor, Kalamazoo (MI) Area Mathematics and Science Center Michael Sinclair Physics instructor, Kalamazoo (MI) Area Mathematics and Science Center Rashmi Venkateswaran Senior Instructor and Lab Coordinator, University of Ottawa

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S C I E N C E O F E V E RY DAY T H I N G S real-life biology

BIOCHEMISTRY C A R B O H Y D RAT E S AMINO ACIDS PROTEINS ENZYMES

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Carbohydrates

C A R B O H Y D R AT E S

CONCEPT Carbohydrates are nutrients, along with proteins and other types of chemical compounds, but they are much more than that. In addition to sugars, of which there are many more varieties than ordinary sucrose, or table sugar, carbohydrates appear in the form of starches and cellulose. As such, they are the structural materials of which plants are made. Carbohydrates are produced by one of the most complex, vital, and amazing processes in the physical world: photosynthesis. Because they are an integral part of plant life, it is no wonder that carbohydrates are in most fruits and vegetables. And though they are not a dietary requirement in the way that vitamins or essential amino acids are, it is difficult to eat without ingesting some carbohydrates, which are excellent sources of quick-burning energy. Not all carbohydrates are of equal nutritional value, however: in general, the ones created by nature are good for the body, whereas those produced by human intervention—some forms of pasta and most varieties of bread, white rice, crackers, cookies,and so forth—are much less beneficial.

HOW IT WORKS What Carbohydrates Are Carbohydrates are naturally occurring compounds that consist of carbon, hydrogen, and oxygen, and are produced by green plants in the process of undergoing photosynthesis. In simple terms, photosynthesis is the biological conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants. It is an extremely complex process, and a

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thorough treatment of it involves a great deal of technical terminology. Although we discuss the fundamentals of photosynthesis later in this essay, we do so only in the most cursory fashion. Photosynthesis involves the conversion of carbon dioxide and water to sugars, which, along with starches and cellulose, are some of the more well known varieties of carbohydrate. Sugars can be defined as any of a number of water-soluble compounds, of varying sweetness. (What we think of as sugar—that is, table sugar—is actually sucrose, discussed later.) Starches are complex carbohydrates without taste or odor, which are granular or powdery in physical form. Cellulose is a polysaccharide, made from units of glucose, that constitutes the principal part of the cell walls of plants and is found naturally in fibrous materials, such as cotton. Commercially, it is a raw material for such manufactured goods as paper, cellophane, and rayon. M O N O S A C C H A R I D E S . The preceding definitions contain several words that also must be defined. Carbohydrates are made up of building blocks called monosaccharides, the simplest type of carbohydrate. Found in grapes and other fruits and also in honey, they can be broken down chemically into their constituent elements, but there is no carbohydrate more chemically simple than a monosaccharide. Hence, they are also known as simple sugars or simple carbohydrates.

Examples of simple sugars include glucose, which is sweet, colorless, and water-soluble and appears widely in nature. Glucose, also known as dextrose, grape sugar, and corn sugar, is the principal form in which carbohydrates are assimilated, or taken in, by animals. Other monosaccha-

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Another disaccharide is lactose, or milk sugar, the only type of sugar that is produced from animal (i.e., mammal) rather than vegetable sources. Maltose, a fermentable sugar typically formed from starch by the action of the enzyme amylase, is also a disaccharide. Sucrose, lactose, and maltose are all isomers, with the formula C12H22O11.

Carbohydrates

OLIGOSACCHARIDES AND P O LY SAC C H A R I D E S . The definitions

of oligosaccharide and polysaccharide are so close as to be confusing. An oligosaccharide is sometimes defined as a carbohydrate containing a known, small number of monosaccharide units, while a polysaccharide is a carbohydrate composed of two or more monosaccharides. In theory, this means practically the same thing, but in practice, an oligosaccharide contains 3-6 monosaccharide units, whereas a polysaccharide is composed of more than six. MICROGRAPH

OF PLANT CELL CHLOROPLASTS, WHERE

PHOTOSYNTHESIS, THE BIOLOGICAL CONVERSION OF LIGHT FROM THE

SUN

PLACE.

PLANTS

HIGHER

INTO CHEMICAL ENERGY, TAKES HAVE

THESE

STRUCTURES,

WHICH CONTAIN A CHEMICAL KNOWN AS CHLOROPHYLL

that absorbs light and speeds up the process of photosynthesis. (© Science Pictures Limited/Corbis. Reproduced by permission.)

rides include fructose, or fruit sugar, and galactose, which is less soluble and sweet than glucose and usually appears in combination with other simple sugars rather than by itself. Glucose, fructose, and galactose are isomers, meaning that they have the same chemical formula (C6H12O6), but different chemical structures and therefore different chemical properties. DISACCHARIDES. When two monosaccharide molecules chemically bond with each other, the result is one of three general types of complex sugar: a disaccharide, oligosaccharide, or polysaccharide. Disaccharides, or double sugars, are composed of two monosaccharides. By far the most well known example of a disaccharide is sucrose, or table sugar, which is formed from the bonding of a glucose molecule with a molecule of fructose. Sugar beets and cane sugar provide the principal natural sources of sucrose, which the average American is most likely to encounter in refined form as white, brown, or powdered sugar.

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Oligosaccharides are found rarely in nature, though a few plant forms have been discovered. Far more common are polysaccharides (“many sugars”), which account for the vast majority of carbohydrate types found in nature. (See Where to Learn More for the Nomenclature of Carbohydrates Web site, operated by the Department of Chemistry at Queen Mary College, University of London. A glance at the site will suggest something about the many, many varieties of carbohydrates.) Polysaccharides may be very large, consisting of as many as 10,000 monosaccharide units strung together. Given this vast range of sizes, it should not be surprising that there are hundreds of polysaccharide types, which differ from one another in terms of size, complexity, and chemical makeup. Cellulose itself is a polysaccharide, the most common variety known, composed of numerous glucose units joined to one another. Starch and glycogen are also glucose polysaccharides. The first of these polysaccharides is found primarily in the stems, roots, and seeds of plants. As for glycogen, this is the most common form in which carbohydrates are stored in animal tissues, particularly muscle and liver tissues.

Photosynthesis Photosynthesis, as we noted earlier, is the biological conversion of light or electromagnetic energy from the Sun into chemical energy. It occurs in green plants, algae, and some types of bacteria

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and requires a series of biochemical reactions. Higher plants have structures called chloroplasts, which contain a dark green or blue-black chemical known as chlorophyll. Light absorption by chlorophyll catalyzes, or speeds up, the process of photosynthesis. (A catalyst is a substance that accelerates a chemical reaction without participating in it.) In photosynthesis, carbon dioxide and water react with each other in the presence of light and chlorophyll to produce a simple carbohydrate and oxygen. This is one of those statements in the realm of science that at first glance sounds a bit dry and boring but which, in fact, encompasses one of life’s great mysteries—a concept far more captivating than any number of imaginary, fantastic, or pseudoscientific ideas one could concoct. Photosynthesis is one of the most essential life-sustaining processes, making possible the nutrition of all things and the respiration of animals and other oxygen-breathing organisms. In photosynthesis, plants take a waste product of human and animal respiration and, through a series of chemical reactions, produce both food and oxygen. The food gives nourishment to the plant, which, unlike an animal, is capable of producing its own nutrition from its own body with the aid only of sunlight and a few chemical compounds. Later, when the plant is eaten by an animal or when it dies and is consumed by bacteria and other decomposers, it will pass on its carbohydrate content to other creatures. (See Food Webs for more about plants as autotrophs and the relationships among primary producers, consumers, and decomposers.) A carbohydrate is not the only useful product of the photosynthetic reaction. The reaction produces an extremely important waste by-product—waste, that is, from the viewpoint of the plant, which has no need of oxygen. Yet the oxygen it generates in photosynthesis makes life possible for animals and many single-cell life-forms, which depend on oxygen for respiration. T H E P H O T O S Y N T H E S I S E Q UA T I O N . The photosynthesis reaction can be rep-

resented thus as a chemical equation: Light

6CO2 + 6H2O 4 C6H12O6 + 6O2 Note that the arrow indicates that a chemical reaction has taken place with the assistance of light and chlorophyll. In the same way, heat from a Bunsen burner may be required to initiate some other chemical reaction, without actually

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being part of the reactants to the left of the arrow. In the present equation, neither the added energy nor the catalyst appears on the left side, because they are not actual physical participants consumed in the reaction, as the carbon dioxide and water are. The catalyst does not participate in the reaction, whereas the energy, while it is consumed in the reaction, is not a material or physical participant—that is, it is energy, not matter.

Carbohydrates

One might also wonder why the equation shows six molecules of carbon dioxide and six of water. Why not one of each, for the sake of simplicity? To produce a balanced chemical equation, in which the same number of atoms appears on either side of the arrow, it is necessary to show six carbon dioxide molecules reacting with six water molecules to produce six oxygen molecules and a single glucose molecule. Thus, both sides contain six atoms of carbon, 12 of hydrogen, and 18 of oxygen. The equation gives the impression that photosynthesis is a simple, one-step process, but nothing could be further from the truth. In fact, the process occurs one small step at a time. It also involves many, many intricacies and aspects that require the introduction of scores of new terms and ideas. Such a discussion is beyond the scope of the present essay, and therefore the reader is encouraged to consult a reliable textbook for further information on the details of photosynthesis.

REAL-LIFE A P P L I C AT I O N S Fruits and Vegetables One of the principal ways in which people obtain carbohydrates from their diets is through fruits and vegetables. The distinctions between these two are based not on science but on custom. Traditionally, vegetables are plant tissues (which may be sweet, but usually are not), that are eaten as a substantial part of a meal’s main course. By contrast, fruits are almost always sweet and are eaten as desserts or snacks. It so happens, too, that people are much more likely to cook vegetables than they are fruits, though vegetables are nutritionally best when eaten raw. Fruits and vegetables are heavy in carbohydrate content, in the form of edible sugars and starches but also inedible cellulose, whose role in the diet will be examined later. In a fresh veg-

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the best part of all—the tender and fully edible “heart”—is enclosed beneath an intimidating shield of slender thistles. Whoever first discovered that an artichoke could be eaten must have been a brave person indeed, and whoever ascertained how to eat it was a wise one. Thanks to these adventurous souls, the world’s cuisine has an unforgettable delicacy.

Carbohydrates

T H E C A R B O H Y D R AT E C O N T E N T O F V E G E TA B L E S . In terms of

CELLULOSE,

SOMETIMES CALLED FIBER, IS AN IMPOR-

TANT DIETARY COMPONENT THAT AIDS IN DIGESTION.

IT

IS ABUNDANT IN FRUITS AND VEGETABLES, YET HUMANS LACK THE ENZYME NECESSARY TO DIGEST IT.

WITH

THE

HELP OF MICROBES IN THEIR GUT, TERMITES CAN DIGEST CELLULOSE. (© George D.Lepp/Corbis. Reproduced by permission.)

etable, for instance, water may account for about 70% of the volume, and proteins, fat, vitamins, and minerals may make up a little more than 5%, with nearly 25% taken up either by edible sugars and starches or by inedible cellulose fiber. T H E E XA M P L E O F T H E A RT I C H O K E . Every fruit or vegetable one could

conceivably eat—and there are hundreds—contains both edible carbohydrates, which are a good source of energy, and inedible ones, which provide fiber. An excellent example of this edibleinedible mixture is the globe, or French, artichoke—Cynara scolymus, a member of the family Asteraceae, which includes the sunflower. The globe artichoke (not to be confused with the Jerusalem artichoke, or Helianthus tuberosus) appears in the form of an inflorescence, or a cluster of flowers. This vegetable usually is steamed, and the bracts, or leaves, are dipped in butter or another sauce.

6

edible carbohydrate content, the artichoke has a low percentage. A few vegetables have a smaller percentage of carbohydrates, whereas others have vastly higher percentages, as the list shown here illustrates. In general, it seems that the carbohydrate content of vegetables (and in each of these cases we are talking about edible carbohydrates, not cellulose) is in the range of about 5–10%, somewhere around 20%, or a very high 60–80%. There does not seem to be a great deal of variation in these ranges. Water, Protein, and Carbohydrate Content of Selected Vegetables: • Artichoke: 85% water, 2.9% protein, 10.6% carbohydrate • Beets, red: 87.3% water, 1.6% protein, 9.9% carbohydrate • Celery: 94.1% water, 0.9% protein, 3.9% carbohydrate • Corn: 13.8% water, 8.9% protein, 72.2% carbohydrate • Lima bean: 10.3% water, 20.4% protein, 64% carbohydrate • Potato: 79.8% water, 2.1% protein, 17.1% carbohydrate • Red pepper: 74.3% water, 3.7% protein, 18.8% carbohydrate • Summer squash: 94% water, 1.1% protein, 4.2% carbohydrate

Starches

Not nearly all of the bract is edible, however; to consume the starchy “meat” of the artichoke, which has a distinctive, nutty flavor, one must draw the leaves between the teeth. Most of the artichoke’s best parts are thus hidden away, and

Not all the carbohydrates in these vegetables are the same. Some carbohydrates appear in the form of sugar and others in the form of inedible cellulose, discussed in the next section. In addition, some vegetables are high in starch content. As we noted earlier, starch is white and granular, and, unlike sugars, starches cannot be dissolved in cold water, alcohol, or other liquids that normally act as solvents.

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Manufactured in plants’ leaves, starch is the product of excess glucose produced during photosynthesis, and it provides the plant with an emergency food supply stored in the chloroplasts. Vegetables high in starch content are products of plants whose starchy portions happen to be the portions we eat. For example, there is the tuber, or underground bulb, of the potato as well as the seeds of corn, wheat, and rice. Thus, all of these vegetables, and foods derived from them, are heavy in the starch form of carbohydrate. In addition to their role in the human diet, starches from corn, wheat, tapioca, and potatoes are put to numerous commercial uses. Because of its ability to thicken liquids and harden solids, starch is applied in products (e.g., cornstarch) that act as thickening agents, both for foods and nonfood items. Starch also is utilized heavily in various phases of the garment and garment-care industries to impart stiffness to fabrics. In the manufacture of paper, starch is used to increase the paper’s strength. It also is employed in the production of cardboard and paper bags.

Carbohydrates

NINETEENTH-CENTURY

ADVERTISEMENT FOR STARCH. IN

ADDITION TO THEIR ROLE IN THE HUMAN DIET, STARCHES ARE PUT TO NUMEROUS COMMERCIAL USES, FOR EXAMPLE, AS THICKENING AGENTS FOR FOOD, IN THE

Cellulose

PRODUCTION OF CARDBOARD, AND IN VARIOUS PHASES OF THE GARMENT INDUSTRY TO IMPART STIFFNESS TO

One of the aspects of fruits and vegetables to which we have alluded several times is the high content of inedible material, or cellulose. (Actually, it is edible—just not digestible.) A substance found in the cell walls of plants, cellulose is chemically like starch but even more rigid, and this property makes it an excellent substance for imparting strength to plant bodies. Animals do not have rigid, walled cells, but plants do. The heavy cellulose content in plants’ cell walls gives them their erect, rigid form; in other words, without cellulose, plants might be limp and partly formless. Like human bone, plant cell walls are composed of fibrils (small filaments or fibers) that include numerous polysaccharides and proteins. One of these polysaccharides in cell walls is pectin, a substance that, when heated, forms a gel and is used by cooks in making jellies and jams. Some trees have a secondary cell wall over the primary one, containing yet another polysaccharide called lignin. Lignin makes the tree even more rigid, penetrable only with sharp axes.

FABRICS. (© BettmannCorbis. Reproduced by permission.)

and insect do not have an enzyme that digests this material. Instead, they harbor microbes in their guts that can do the digesting for them. (This is an example of symbiotic mutualism, a mutually beneficial relationship between organisms, discussed in Symbiosis.)

As we have noted, cellulose is abundant in fruits and vegetables, yet humans lack the enzyme necessary to digest it. Termites, cows, koalas, and horses all digest cellulose, but even these animals

Cows are ruminants, or animals that chew their cud—that is, food regurgitated to be chewed again. Ruminants have several stomachs, or several stomach compartments, that break down plant material with the help of enzymes and bacteria. The partially digested material then is regurgitated into the mouth, where it is chewed to break the material down even further. (If you have ever watched cows in a pasture, you have probably observed them calmly chewing their cud.) The digestion of cellulose by bacteria in the stomachs of ruminants is anaerobic, meaning that the process does not require oxygen. One of the by-products of this anaerobic process is methane gas, which is foul smelling, flammable, and toxic. Ruminants give off large amounts of methane daily, which has some environmentalists alarmed, since cow-borne methane may con-

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CELLULOSE

IN

DIGESTION.

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Carbohydrates

KEY TERMS Naturally occur-

hydrogen, and oxygen, whose primary

A protein material that speeds up chemical reactions in the bodies of plants and animals.

function in the body is to supply energy.

FRUCTOSE:

CARBOHYDRATES:

ring compounds, consisting of carbon,

Included in the carbohydrate group are sugars, starches, cellulose, and various other substances. Most carbohydrates are produced by green plants in the process of undergoing photosynthesis. CATALYST:

A substance that speeds up

a chemical reaction without participating in it. Catalysts, of which enzymes are a good example, thus are not consumed in the reaction. CELLULOSE:

A polysaccharide, made

from units of glucose, that is the principal material in the cell walls of plants. Cellulose also is found in natural fibers, such as cotton, and is used as a raw material in manufacturing such products as paper.

Fruit sugar, a monosaccharide that is an isomer of glucose.

A monosaccharide and isomer of glucose. Less soluble and sweet than glucose, galactose usually appears in combination with other simple sugars rather than by itself. GALACTOSE:

A monosaccharide that occurs widely in nature and is the form in which animals usually receive carbohydrates. Also known as dextrose, grape sugar, and corn sugar. GLUCOSE:

A white polysaccharide that is the most common form in which carbohydrates are stored in animal tissues, particularly muscle and liver tissues. GLYCOGEN:

composed of two monosaccharides. Exam-

A term that refers to all or part of the alimentary canal, through which foods pass from the mouth to the intestines and wastes move from the intestines to the anus. Although the word is considered a bit crude in everyday life, physicians and biological scientists concerned with this part of the anatomy use it regularly.

ples of disaccharides include the isomers

ISOMERS:

COMPLEX CARBOHYDRATE:

A dis-

accharide, polysaccharide, or oligosaccharide. Also called a complex sugar. DEXTROSE:

Another name for glu-

cose. DISACCHARIDE:

A double sugar,

sucrose, maltose, and lactose.

tribute to the destruction of the ozone high in Earth’s stratosphere. Alhough cellulose is indigestible by humans, it is an important dietary component in that it aids in digestion. Sometimes called fiber or roughage, cellulose helps give food bulk as it moves through the digestive system and aids the body in pushing out foods and wastes. This is particularly important inasmuch as it helps make possible regular bowel movements, thus ridding

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ENZYME:

VOLUME 3: REAL-LIFE BIOLOGY

GUT:

Two substances that have the same chemical formula but differ in

the body of wastes and lowering the risk of colon cancer. (See Digestion for more about the digestive and excretory processes.)

Overall Carbohydrate Nutrition A diet high in cellulose content can be beneficial for the reasons we have noted. Likewise, a healthy diet includes carbohydrate nutrients, but only under certain conditions. First of all, it should be

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Carbohydrates

KEY TERMS

CONTINUED

chemical structure and therefore in chemi-

dioxide and water are converted to carbo-

cal properties.

hydrates and oxygen.

LACTOSE:

Milk sugar. A disaccharide

isomer of sucrose and maltose, lactose is the only major type of sugar that is produced from animal (i.e., mammal) rather than vegetable sources. MALTOSE:

composed of more than six monosaccharides. A polysaccharide sometimes is defined as containing two or more monosaccharides, but this definition does little to distinguish it from an oligosaccharide.

A fermentable sugar gener-

A sugar.

SACCHARIDE:

ally formed from starch by the action of the enzyme amylase. Maltose is a disaccharide isomer of sucrose and lactose. MONOSACCHARIDE:

A carbohydrate

POLYSACCHARIDE:

The simplest

type of carbohydrate. Monosaccharides,

SIMPLE SUGAR:

STARCHES:

Complex carbohydrates,

without taste or odor, which are granular or powdery in physical form.

which cannot be broken down chemically into simpler carbohydrates, also are known

A monosaccharide,

or simple carbohydrate.

SUCROSE:

Common

table

sugar

(C12H22O11), a disaccharide formed from

as simple sugars. Examples of monosac-

the bonding of a glucose molecule with a

charides include the isomers glucose, fruc-

molecule of fructose. Sugar beets and cane

tose, and galactose.

sugar provide the principal natural sources

OLIGOSACCHARIDE:

A

carbohy-

drate containing a known, small number of monosaccharide units, typically between three and six. Compare with polysaccha-

of sucrose, which the average American is most likely to encounter in refined form as white, brown, or powdered sugar. SUGARS:

One of the three principal

types of carbohydrate, along with starches

ride.

and cellulose. Sugars can be defined as any The biological

of various water-soluble carbohydrates of

conversion of light energy (that is, electro-

varying sweetness. What we think of as

magnetic energy) from the Sun to chemical

“sugar” (i.e., table sugar) is actually

energy in plants. In this process, carbon

sucrose.

PHOTOSYNTHESIS:

understood that the human body does not have an essential need for carbohydrates in and of themselves—in other words, there are no “essential” carbohydrates, as there are essential amino acids or fatty acids.

mins, minerals, proteins, and dietary fiber that they contain. For these healthy carbohydrates, it is best to eat them in as natural a form as possible: for example, eat the whole orange, rather than just squeezing out the juice and throwing away the pulp. Also, raw spinach and other vegetables contain far more vitamins and minerals than the cooked versions.

On the other hand, it is very important to eat fresh fruits and vegetables, which, as we have seen, are heavy in carbohydrate content. Their importance has little do with their nutritional carbohydrate content, but rather with the vita-

SUGAR HIGHS AND FAT S T O RAG E . Carbohydrates can give people a

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Carbohydrates

short burst of energy, and this is why athletes may “bulk up on carbs” right before competition. But if the carbohydrates are not quickly burned off, they eventually will be stored as fat. This is the case even with healthy carbohydrates, but the situation is much worse with junk-food carbohydrates, which offer only empty calories stripped of vitamin and mineral content. One example is a particular brand of candy bar that, over the years, has been promoted in commercials as a means of obtaining a quick burst of energy. In fact, this and all other white-sugar-based candies give only a quick “sugar high,” followed almost immediately by a much lower energy “low”—and in the long run by the accumulation of fat. Fat is the only form in which the body can store carbohydrates for the long haul, meaning that the “fat-free” stickers on many a package of cookies or cakes in the supermarket are as meaningless as the calories themselves are empty. Carbohydrate consumption is one of the main reasons why the average American is so overweight. With an inactive lifestyle, as is typical of most adults in modern life, all those French fries, cookies, dinner rolls, and so on have no place to go but to the fat-storage centers in the abdomen, buttocks, and thighs. Of all carbohydrate-containing foods, the least fattening, of course, are natural nonstarches, such as fruits and vegetables (assuming they are not cooked in fat). Next on the least-fattening list are starchy natural foods, such as potatoes, and most fattening of all are processed starches, whether they come in the form of rice, wheat, or potato products. W H Y Y O U C A N E AT M O R E C A R B O H Y D R AT E S THAN PROT E I N S . One of the biggest problems with

starches is that the body can consume so many of them compared with proteins and fats. How many times have you eaten a huge plate of mashed potatoes or rice, mountains of fries, or piece after piece of bread? All of us have done it: with carbohydrates, and particularly starches, it seems we can never get enough. But how many

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times have you eaten a huge plate of nothing but chicken, steak, or eggs? Probably not very often, and if you have tried to eat too much of these protein-heavy foods at one time, you most likely started to get sick. The reason is that when you eat protein or fat, it triggers the release of a hormone called cholecystokinin (CCK) in the small intestine. CCK tells the brain, in effect, that the body is getting fed, and if enough CCK is released, it signals the brain that the body has received enough food. If one continues to consume proteins or fats beyond that point, nausea is likely to follow. Carbohydrates, on the other hand, do not cause a release of CCK; only when they enter the bloodstream do they finally send a signal to the brain that the body is satisfied. By then, most of us have piled on more mashed potatoes, which are destined to take their place in the body as fat stores. WHERE TO LEARN MORE Carbohydrates. Hardy Research Group, Department of Chemistry, University of Akron (Web site). . Dey, P. M., and R. A. Dixon. Biochemistry of Storage Carbohydrates in Green Plants. Orlando, FL: Academic Press, 1985. Carpi, Anthony. “Food Chemistry: Carbohydrates.” Visionlearning.com (Web site). . Food Resource, Oregon State University (Web site). . Kennedy, Ron. “Carbohydrates in Nutrition.” The Doctors’ Medical Library (Web site). . “Nomenclature of Carbohydrates.” Queen Mary College, University of London, Department of Chemistry (Web site). . Snyder, Carl H. The Extraordinary Chemistry of Ordinary Things. New York: John Wiley and Sons, 1998. Spallholz, Julian E. Nutrition, Chemistry, and Biology. Englewood Cliffs, NJ: Prentice-Hall, 1989. Wiley, T. S., and Bent Formby. Lights Out: Sleep, Sugar, and Survival. New York: Pocket Books, 2000.

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Amino Acids

AMINO ACIDS

CONCEPT Amino acids are organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations. Strings of amino acids make up proteins, of which there are countless varieties. Of the 20 amino acids required for manufacturing the proteins the human body needs, the body itself produces only 12, meaning that we have to meet our requirements for the other eight through nutrition. This is just one example of the importance of amino acids in the functioning of life. Another cautionary illustration of amino acids’ power is the gamut of diseases (most notably, sickle cell anemia) that impair or claim the lives of those whose amino acids are out of sequence or malfunctioning. Once used in dating objects from the distant past, amino acids have existed on Earth for at least three billion years— long before the appearance of the first true organisms.

HOW IT WORKS A “Map” of Amino Acids Amino acids are organic compounds, meaning that they contain carbon and hydrogen bonded to each other. In addition to those two elements, they include nitrogen, oxygen, and, in a few cases, sulfur. The basic structure of an amino-acid molecule consists of a carbon atom bonded to an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a fourth group that differs from one amino acid to another and often is referred to as the -R group or the side chain. The -R group, which can vary widely, is

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responsible for the differences in chemical properties. This explanation sounds a bit technical and requires a background in chemistry that is beyond the scope of this essay, but let us simplify it somewhat. Imagine that the amino-acid molecule is like the face of a compass, with a carbon atom at the center. Raying out from the center, in the four directions of the compass, are lines representing chemical bonds to other atoms or groups of atoms. These directions are based on models that typically are used to represent amino-acid molecules, though north, south, east, and west, as used in the following illustration, are simply terms to make the molecule easier to visualize. To the south of the carbon atom (C) is a hydrogen atom (H), which, like all the other atoms or groups, is joined to the carbon center by a chemical bond. To the north of the carbon center is what is known as an amino group (-NH2). The hyphen at the beginning indicates that such a group does not usually stand alone but normally is attached to some other atom or group. To the east is a carboxyl group, represented as -COOH. In the amino group, two hydrogen atoms are bonded to each other and then to nitrogen, whereas the carboxyl group has two separate oxygen atoms strung between a carbon atom and a hydrogen atom. Hence, they are not represented as O2. Finally, off to the west is the R- group, which can vary widely. It is as though the other portions of the amino acid together formed a standard suffix in the English language, such as -tion. To the front of that suffix can be attached all sorts of terms drawn from root words, such as educate or

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Amino Acids

COMPUTER-GENERATED MODEL OF A MOLECULE MADE UP OF THREE AMINO ACIDS—GLYCINE, CYSTEINE AND ALANINE. AMINO ACIDS FUNCTION AS MONOMERS, OR INDIVIDUAL UNITS, THAT JOIN TOGETHER TO FORM LARGE, CHAINLIKE MOLECULES CALLED POLYMERS; THREE AMINO ACIDS BONDED TOGETHER ARE CALLED TRIPEPTIDES. (Photo Researchers. Reproduced by permission.)

The name amino acid, in fact, comes from the amino group and the acid group, which are the

most chemically reactive parts of the molecule. Each of the common amino acids has, in addition to its chemical name, a more familiar name and a three-letter abbreviation that frequently is used to identify it. In the present context, we are not concerned with these abbreviations. Aminoacid molecules, which contain an amino group and a carboxyl group, do not behave like typical molecules. Instead of melting at temperatures hotter than 392°F (200°C), they simply decom-

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S C I E N C E O F E V E RY DAY T H I N G S

satisfy or revolt—hence, education, satisfaction, and revolution. The variation in the terms attached to the front end is extremely broad, yet the tail end, -tion, is a single formation. Likewise the carbon, hydrogen, amino group, and carboxyl group in an amino acid are more or less constant. A FEW ADDITIONAL POINTS.

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pose. They are quite soluble, or capable of being dissolved, in water but are insoluble in nonpolar solvents (oil- and all oil-based products), such as benzene or ether. RIGHT-HAND AND LEFT-HAND V E R S I O N S . All of the amino acids in the

human body, except glycine, are either righthand or left-hand versions of the same molecule, meaning that in some amino acids the positions of the carboxyl group and the R- group are switched. Interestingly, nearly all of the amino acids occurring in nature are the left-hand versions of the molecules, or the L-forms. (Therefore, the model we have described is actually the left-hand model, though the distinctions between “right” and “left”—which involve the direction in which light is polarized—are too complex to discuss here.) Right-hand versions (D-forms) are not found in the proteins of higher organisms, but they are present in some lower forms of life, such as in the cell walls of bacteria. They also are found in some antibiotics, among them, streptomycin, actinomycin, bacitracin, and tetracycline. These antibiotics, several of which are well known to the public at large, can kill bacterial cells by interfering with the formation of proteins necessary for maintaining life and for reproducing.

Amino Acids and Proteins A chemical reaction that is characteristic of amino acids involves the formation of a bond, called a peptide linkage, between the carboxyl group of one amino acid and the amino group of a second amino acid. Very long chains of amino acids can bond together in this way to form proteins, which are the basic building blocks of all living things. The specific properties of each kind of protein are largely dependent on the kind and sequence of the amino acids in it. Other aspects of the chemical behavior of protein molecules are due to interactions between the amino and the carboxyl groups or between the various Rgroups along the long chains of amino acids in the molecule.

acids are called dipeptides, whereas three amino acids bonded together are called tripeptides. If there are more than 10 in a chain, they are termed polypeptides, and if there are 50 or more, these are known as proteins. All the millions of different proteins in living things are formed by the bonding of only 20 amino acids to make up long polymer chains. Like the 26 letters of the alphabet that join together to form different words, depending on which letters are used and in which sequence, the 20 amino acids can join together in different combinations and series to form proteins. But whereas words usually have only about 10 or fewer letters, proteins typically are made from as few as 50 to as many as 3,000 amino acids. Because each amino acid can be used many times along the chain and because there are no restrictions on the length of the chain, the number of possible combinations for the formation of proteins is truly enormous. There are about two quadrillion different proteins that can exist if each of the 20 amino acids present in humans is used only once. Just as not all sequences of letters make sense, however, not all sequences of amino acids produce functioning proteins. Some other sequences can function and yet cause undesirable effects, as we shall see.

REAL-LIFE A P P L I C AT I O N S DNA (deoxyribonucleic acid), a molecule in all cells that contains genetic codes for inheritance, creates encoded instructions for the synthesis of amino acids. In 1986, American medical scientist Thaddeus R. Dryja (1940–) used amino-acid sequences to identify and isolate the gene for a type of cancer known as retinoblastoma, a fact that illustrates the importance of amino acids in the body.

or individual units, that join together to form large, chainlike molecules called polymers, which may contain as few as two or as many as 3,000 amino-acid units. Groups of only two amino

Amino acids are also present in hormones, chemicals that are essential to life. Among these hormones is insulin, which regulates sugar levels in the blood and without which a person would die. Another is adrenaline, which controls blood pressure and gives animals a sudden jolt of energy needed in a high-stress situation—running from a predator in the grasslands or (to a use a human example) facing a mugger in an alley or a bully on a playground. Biochemical studies of amino-acid sequences in hormones have made it

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NUMBERS AND COMBINAT I O N S . Amino acids function as monomers,

Amino Acids

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Amino Acids

NORMAL

RED BLOOD CELLS

(BOTTOM)

AND SICKLE CELL

(TOP). SICKLE CELL ANEMIA WHEN RED BLOOD CELLS

ABOUT BY A SINGLE MISTAKE IN AMINO ACID SEQUENCING.

IS A FATAL DISEASE BROUGHT RELEASE OXYGEN TO THE TIS-

SUES, THEY FAIL TO RE-OXYGENATE NORMALLY AND INSTEAD TWIST INTO THE SHAPE THAT GIVES SICKLE CELL ANEMIA ITS NAME, CAUSING OBSTRUCTION OF THE BLOOD VESSELS. (Photograph by Dr. Gopal Murti. National Audubon Society Col-

lection/Photo Researchers, Inc. Reproduced by permission.)

possible for scientists to isolate and produce artificially these and other hormones, including the human growth hormone.

Amino Acids and Nutrition Just as proteins form when amino acids bond together in long chains, they can be broken down by a reaction called hydrolysis, the reverse of the formation of the peptide bond. That is exactly what happens in the process of digestion, when special digestive enzymes in the stomach enable the breaking down of the peptide linkage. (Enzymes are a type of protein—see Enzymes.) The amino acids, separated once again, are

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released into the small intestine, from whence they pass into the bloodstream and are carried throughout the organism. Each individual cell of the organism then can use these amino acids to assemble the new and different proteins required for its specific functions. Life thus is an ongoing cycle in which proteins are broken into individual amino-acid units, and new proteins are built up from these amino acids. E SS E N T I A L A M I N O AC I D S . Out of the many thousands of possible amino acids, humans require only 20 different kinds. Two others appear in the bodies of some animal species, and approximately 100 others can be found in

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plants. Considering the vast numbers of amino acids and possible combinations that exist in nature, the number of amino acids essential to life is extremely small. Yet of the 20 amino acids required by humans for making protein, only 12 can be produced within the body, whereas the other eight—isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine—must be obtained from the diet. (In addition, adults are capable of synthesizing arginine and histidine, but these amino acids are believed to be essential to growing children, meaning that children cannot produce them on their own.) A complete protein is one that contains all of the essential amino acids in quantities sufficient for growth and repair of body tissue. Most proteins from animal sources, gelatin being the only exception, contain all the essential amino acids and are therefore considered complete proteins. On the other hand, many plant proteins do not contain all of the essential amino acids. For example, lysine is absent from corn, rice, and wheat, whereas corn also lacks tryptophan and rice lacks threonine. Soybeans are lacking in methionine. Vegans, or vegetarians who consume no animal proteins in their diets (i.e., no eggs, dairy products, or the like) are at risk of malnutrition, because they may fail to assimilate one or more essential amino acid.

lysine and tryptophan. Its symptoms often are described as the “three Ds”: diarrhea, dermatitis (or skin inflammation), and dementia. Thanks to a greater understanding of nutrition and health, pellagra has been largely eradicated, but there still exists a condition with almost identical symptoms: Hartnup disease, a genetic disorder named for a British family in the late 1950s who suffered from it. Hartnup disease is characterized by an inability to transport amino acids from the kidneys to the rest of the body. The symptoms at first seemed to suggest to physicians that the disease, which is present in one of about 26,000 live births, was pellagra. Tests showed that sufferers did not have inadequate tryptophan levels, however, as would have been the case with pellagra. On the other hand, some 14 amino acids have been found in excess within the urine of Hartnup disease sufferers, indicating that rather than properly transporting amino acids, their bodies are simply excreting them. This is a potentially very serious condition, but it can be treated with the B vitamin nicotinamide, also used to treat pellagra. Supplementation of tryptophan in the diet also has shown positive results with some patients.

Amino acids can be used as treatments for all sorts of medical conditions. For example, tyrosine may be employed in the treatment of Alzheimer’s disease, a condition characterized by the onset of dementia, or mental deterioration, as well as for alcohol-withdrawal symptoms. Taurine is administered to control epileptic seizures, treat high blood pressure and diabetes, and support the functioning of the liver. Numerous other amino acids are used in treating a wide array of other diseases. Sometimes the disease itself involves a problem with amino-acid production or functioning. In the essay Vitamins, there is a discussion of pellagra, a disease resulting from a deficiency of the B-group vitamin known as niacin. Pellagra results from a diet heavy in corn, which, as we have noted, lacks

S I C K L E C E L L A N E M I A . It is also possible for small mistakes to occur in the amino-acid sequence within the body. While these mistakes sometimes can be tolerated in nature without serious problems, at other times a single misplaced amino acid in the polymer chain can bring about an extremely serious condition of protein malfunctioning. An example of this is sickle cell anemia, a fatal disease ultimately caused by a single mistake in the amino acid sequence. In the bodies of sickle cell anemia sufferers, who are typically natives of sub-Saharan Africa or their descendants in the United States or elsewhere, glutamic acid is replaced by valine at the sixth position from the end of the protein chain in the hemoglobin molecule. (Hemoglobin is an iron-containing pigment in red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide from them.) This small difference makes sickle cell hemoglobin molecules extremely sensitive to oxygen deficiencies. As a result, when the red blood cells release their oxygen to the tissues, as all red blood cells do, they fail to re-oxygenate in a normal fashion and instead twist into the shape

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Amino Acids, Health, and Disease

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Amino Acids

KEY TERMS Organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations. Strings of amino acids make up proteins. AMINO ACIDS:

The chemical formation NH2, which is part of all amino acids. AMINO GROUP:

The area of the biological sciences concerned with the chemical substances and processes in organisms. BIOCHEMISTRY:

The formation COOH, which is common to all amino acids. CARBOXYL GROUP:

A substance in which atoms of more than one element are bonded chemically to one another. COMPOUND:

DIPEPTIDE:

A group of only two

amino acids. Deoxyribonucleic acid, a molecule in all cells and many viruses containing genetic codes for inheritance.

DNA:

A protein material that speeds up chemical reactions in the bodies of plants and animals.

ENZYME:

Amino acids that cannot be manufactured by the body, and which therefore must be obtained from the diet. Proteins that contain essential amino acids are known as complete proteins. ESSENTIAL AMINO ACIDS:

A unit of information about a particular heritable (capable of being inherited) trait that is passed from parent to offspring, stored in DNA molecules called chromosomes.

Molecules produced by living cells, which send signals to spots remote from their point of origin and induce specific effects on the activities of other cells. HORMONE:

A group of atoms, usually but not always representing more than one element, joined in a structure. Compounds typically are made up of molecules. MOLECULE:

At one time chemists used the term organic only in reference to living things. Now the word is applied to compounds containing carbon and hydrogen. ORGANIC:

A bond between the carboxyl group of one amino acid and the amino group of a second amino acid. PEPTIDE LINKAGE:

Large, chainlike molecules composed of numerous subunits known as monomers.

POLYMERS:

A group of between 10

POLYPEPTIDE:

and 50 amino acids. Large molecules built from long chains of 50 or more amino acids. Proteins serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body’s immune system, and making enzymes.

PROTEINS:

Ribonucleic acid, the molecule translated from DNA in the cell nucleus, the control center of the cell, that directs protein synthesis in the cytoplasm, or the space between cells. RNA:

GENE:

16

To manufacture chemically, as in the body.

SYNTHESIZE:

TRIPEPTIDE:

A group of three amino

acids.

that gives sickle cell anemia its name. This causes obstruction of the blood vessels. Before the development of a treatment with the drug

hydroxyurea in the mid-1990s, the average life expectancy of a person with sickle cell anemia was about 45 years.

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Amino Acids and the Distant Past The Evolution essay discusses several types of dating, a term referring to scientific efforts directed toward finding the age of a particular item or phenomenon. Methods of dating are either relative (i.e., comparative and usually based on rock strata, or layers) or absolute. Whereas relative dating does not involve actual estimates of age in years, absolute dating does. One of the first types of absolute-dating techniques developed was amino-acid racimization, introduced in the 1960s. As noted earlier, there are “left-hand” L-forms and “right-hand” Dforms of all amino acids. Virtually all living organisms (except some microbes) incorporate only the L-forms, but once the organism dies, the L-amino acids gradually convert to the mirrorimage D-amino acids. Numerous factors influence the rate of conversion, and though amino-acid racimization was popular as a form of dating in the 1970s, there are problems with it. For instance, the process occurs at different rates for different amino acids, and the rates are further affected by such factors as moisture and temperature. Because of the uncertainties with amino-acid racimization, it has been largely replaced by other absolute-dating methods, such as the use of radioactive isotopes. Certainly, amino acids themselves have offered important keys to understanding the planet’s distant past. The discovery, in 1967 and 1968, of sedimentary rocks bearing traces of amino acids as much as three billion years old had an enormous impact on the study of Earth’s

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biological history. Here, for the first time, was concrete evidence of life—at least, in a very simple chemical form—existing billions of years before the first true organism. The discovery of these amino-acid samples greatly influenced scientists’ thinking about evolution, particularly the very early stages in which the chemical foundations of life were established.

Amino Acids

WHERE TO LEARN MORE “Amino Acids.” Institute of Chemistry, Department of Biology, Chemistry, and Pharmacy, Freie Universität, Berlin (Web site). . Goodsell, David S. Our Molecular Nature: The Body’s Motors, Machines, and Messages. New York: Copernicus, 1996. “Introduction to Amino Acids.” Department of Crystallography, Birbeck College (Web site). . Michal, Gerhard. Biochemical Pathways: An Atlas of Biochemistry and Molecular Biology. New York: John Wiley and Sons, 1999. Newstrom, Harvey. Nutrients Catalog: Vitamins, Minerals, Amino Acids, Macronutrients—Beneficial Use, Helpers, Inhibitors, Food Sources, Intake Recommendations, and Symptoms of Over or Under Use. Jefferson, NC: McFarland and Company, 1993. Ornstein, Robert E., and Charles Swencionis. The Healing Brain: A Scientific Reader. New York: Guilford Press, 1990. Reference Guide for Amino Acids (Web site). . Silverstein, Alvin, Virginia B. Silverstein, and Robert A. Silverstein. Proteins. Illus. Anne Canevari Green. Brookfield, CT: Millbrook Press, 1992. Springer Link: Amino Acids (Web site). .

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PROTEINS Proteins

CONCEPT Most of us recognize the term protein in a nutritional context as referring to a class of foods that includes meats, dairy products, eggs, and other items. Certainly, proteins are an important part of nutrition, and obtaining complete proteins in one’s diet is essential to the proper functioning of the body. But the significance of proteins extends far beyond the dining table. Vast molecules built from enormous chains of amino acids, proteins are essential building blocks for living systems— hence their name, drawn from the Greek proteios, or “holding first place.” Proteins are integral to the formation of DNA, a molecule that contains genetic codes for inheritance, and of hormones. Most of the dry weight of the human body and the bodies of other animals is made of protein, as is a vast range of things with which we come into contact on a daily basis. In addition, a special type of protein called an enzyme has still more applications.

HOW IT WORKS

The Basics Amino acids react with each other to form a bond, called a peptide linkage, between the carboxyl group of one amino acid (symbolized as -COOH) and the amino group (-NH2) of a second amino acid. In this way they can make large, chainlike molecules called polymers, which may contain as few as two or as many as 3,000 aminoacid units. If there are more than 10 units in a chain, the chain is called a polypeptide, while a chain with 50 or more amino-acid units is known as a protein.

Protein is a foundational material in the structure of most living things, and as such it is rather like concrete or steel. Just as concrete is a mixture of other ingredients and steel is an alloy of iron and carbon, proteins, too, are made of something more basic: amino acids. These are organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations.

All the millions of different proteins in living things are formed by the bonding of only 20 amino acids into long polymer chains. Because each amino acid can be used many times along the chain, and because there are no restrictions on the length of the chain, the number of possible combinations for the creation of proteins is truly enormous: about two quadrillion, or 2,000,000,000,000,000. Just as not all sequences of letters make sense, however, not all sequences of amino acids produce functioning proteins. In fact, the number of proteins that have significance

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The Complexities of Biochemistry

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Amino acids are discussed in more depth within the essay devoted to that topic, though, as noted in that essay, it is impossible to treat such a subject thoroughly without going into an extraordinarily lengthy and technical discussion. Such is the case with many topics in biochemistry, the area of the biological sciences concerned with the chemical substances and processes in organisms: the deeper within the structure of things one goes, and the smaller the items under investigation, the more complex are the properties and interactions.

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in the functioning of nature is closer to about 100,000. This number is considerably smaller than two quadrillion—about 1/2,000,000,000th of that larger number, in fact—but it is still a very large number.

Proteins

COMPONENTS OTHER THAN A M I N O AC I D S . The specific properties of

each kind of protein are largely dependent on the kind and sequence of the amino acids in it, yet many proteins include components other than amino acids. For example, some may have sugar molecules (sugars are discussed in the essay on Carbohydrates) chemically attached. Exactly which types of sugars are attached and where on the protein chain attachment occurs vary with the specific protein. Other proteins may have lipid, or fat, molecules chemically bonded to them. Sugar and lipid molecules always are added when synthesis of the protein’s amino-acid chain is complete. Many other types of substance, including metals, also may be associated with proteins; for instance, hemoglobin, a pigment in red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide from them, is a protein that contains an iron atom. STRUCTURES AND SYNTHES I S . Protein structures generally are described

at four levels: primary, secondary, tertiary, and quaternary. Primary structure is simply the twodimensional linear sequence of amino acids in the peptide chain. Secondary and tertiary structures both refer to the three-dimensional shape into which a protein chain folds. The distinction between the two is partly historical: secondary structures are those that were first discerned by scientists of the 1950s, using the techniques and knowledge available then, whereas an awareness of tertiary structure emerged only later. Finally, quaternary structure indicates the way in which many protein chains associate with one another. For example, hemoglobin consists of four protein chains (spirals, actually) of two slightly different types, all attached to an iron atom.

SHEEP

SHEARING IN

NEW ZEALAND. THE

ENTIRE ANI-

MAL WORLD IS CONSTITUTED LARGELY OF PROTEIN, AS ARE A WHOLE HOST OF ANIMAL PRODUCTS, INCLUDING LEATHER AND WOOL. (© Adam Woolfitt/Corbis. Reproduced by

permission.)

process that we do not attempt to discuss here. Following synthesis, proteins fold up into an essentially compact three-dimensional shape, which is their tertiary structure.

Protein synthesis is the process whereby proteins are produced, or synthesized, in living things according to “directions” given by DNA (deoxyribonucleic acid) and carried out by RNA (ribonucleic acid) and other proteins. As suggested earlier, this is an extraordinarily complex

The steps involved in folding and the shape that finally results are determined by such chemical properties as hydrogen bonds, electrical attraction between positively and negatively charged side chains, and the interaction between polar and nonpolar molecules. Nonpolar molecules are called hydrophobic, or “water-fearing,” because they do not mix with water but instead mix with oils and other substances in which the electric charges are more or less evenly distributed on the molecule. Polar molecules, on the other hand, are termed hydrophilic, or “waterloving,” and mix with water and water-based substances in which the opposing electric charges occupy separate sides, or ends, of the molecule. Typically, hydrophobic amino-acid side chains tend to be on the interior of a protein, while hydrophilic ones appear on the exterior.

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Proteins

U.S. WANTED POSTER FOR A WORLD WAR II NAZI SABOTEUR (JULY 1942). FINGERPRINTS ARE AN EXPRESSION OF DNA, WHICH IS LINKED CLOSELY WITH THE OPERATION OF PROTEINS IN OUR BODIES. (© Bettmann/Corbis. Repro-

OUR

duced by permission.)

REAL-LIFE A P P L I C AT I O N S Proteins Are Everywhere Although it is very difficult to discuss the functions of proteins in simple terms, and it is similarly challenging to explain exactly how they function in everyday life, it is not hard at all to name quite a few areas in which these highly important compounds are applied. As we noted

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earlier, much of our bodies’ dry weight—that is, the weight other than water, which accounts for a large percentage of the total—is protein. Our bones, for instance, are about one-fourth protein, and protein makes up a very high percentage of the material in our organs (including the skin), glands, and bodily fluids. Humans are certainly not the only organisms composed largely of protein: the entire animal world, including the animals we eat and the microbes that enter our bodies (see Digestion

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and Parasites and Parasitology) likewise is constituted largely of protein. In addition, a whole host of animal products, including leather and wool, are nearly pure protein. So, too, are other, less widely used animal products, such as hormones for the treatment of certain conditions—for example, insulin, which keeps people with diabetes alive and which usually is harvested from the bodies of mammals. Proteins allow cells to detect and react to hormones and toxins in their surroundings, and as the chief ingredient in antibodies, which help us resist infection, they play a part in protecting our bodies against foreign invaders. The lack of specific proteins in the brain may be linked to such mysterious, terrifying conditions as Alzheimer and Creutzfeldt-Jakob diseases (discussed in Disease). Found in every cell and tissue and composing the bulk of our bodies’ structure, proteins are everywhere, promoting growth and repairing bone, muscles, tissues, blood, and organs. E N Z Y M E S . One particularly important type of protein is an enzyme, discussed in the essay on that topic. Enzymes make possible a host of bodily processes, in part by serving as catalysts, or substances that speed up a chemical reaction without actually participating in, or being consumed by, that reaction. Enzymes enable complex, life-sustaining reactions in the human body—reactions that would be too slow at ordinary body temperatures—and they manage to do so without forcing the body to undergo harmful increases in temperature. They also are involved in fermentation, a process with applications in areas ranging from baking bread to reducing the toxic content of wastewater. (For much more on these subjects, see Enzymes.)

Inside the body, enzymes and other proteins have roles in digesting foods and turning the nutrients in them—including proteins—into energy. They also move molecules around within our cells to serve an array of needs and allow healthful substances, such as oxygen, to pass through cell membranes while keeping harmful ones out. Proteins in the chemical known as chlorophyll facilitate an exceptionally important natural process, photosynthesis, discussed briefly in Carbohydrates.

present in each. This is only one of many key roles that proteins play where blood is concerned. If certain proteins are missing, or if the wrong proteins are present, blood will fail to clot properly, and cuts will refuse to heal. For sufferers of the condition known as hemophilia, caused by a lack of the proteins needed for clotting, a simple cut can be fatal. Similarly, proteins play a critical role in forensic science, or the application of medical and biological knowledge to criminal investigations. Fingerprints are an expression of our DNA, which is linked closely with the operation of proteins in our bodies. The presence of DNA in bodily fluids, such as blood, semen, sweat, and saliva, makes it possible to determine the identity of the individual who perpetrated a crime or of others who were present at the scene. In addition, a chemical known as luminol assists police in the investigation of possible crime scenes. If blood has ever been shed in a particular area, such as on a carpet, no matter how carefully the perpetrators try to conceal or eradicate the stain, it can be detected. The key is luminol, which reacts to hemoglobin in the blood, making it visible to investigators. This chemical, developed during the 1980s, has been used to put many a killer behind bars. D E S I G N E R P R O T E I N S . These are just a very few of the many applications of proteins, including a very familiar one, discussed in more depth at the conclusion of this essay: nutrition. Given the importance and complexity of proteins, it might be hard to imagine that they can be produced artificially, but, in fact, such production is taking place at the cutting edge of biochemistry today, in the field of “designer proteins.”

O) are differentiated on the basis of the proteins

Many such designs involve making small changes in already existing proteins: for example, by changing three amino acids in an enzyme often used to improve detergents’ cleaning power, commercial biochemists have doubled the enzyme’s stability in wash water. Medical applications of designer proteins seem especially promising. For instance, we might one day cure cancer by combining portions of one protein that recognizes cancer with part of another protein that attacks it. One of the challenges facing such a development, however, is the problem of designing a protein that attacks only cancer cells and not healthy ones.

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PROTEINS, BLOOD, AND C R I M E . The four blood types (A, B, AB, and

Proteins

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Proteins

KEY TERMS Organic compounds made of carbon, hydrogen, oxygen, nitrogen and (in some cases) sulfur bonded in characteristic formations. Strings of amino acids make up proteins. AMINO ACIDS:

The chemical formation -NH2, which is part of all amino acids. AMINO GROUP:

The area of the biological sciences concerned with the chemical substances and processes in organisms. BIOCHEMISTRY:

living cells, which send signals to spots remote from their point of origin and induce specific effects on the activities of other cells. An organism that eats

OMNIVORE:

both plants and other animals. ORGANIC:

At one time chemists used

the term organic only in reference to living things. Now the word is applied to com-

The formation -COOH, which is common to all amino acids.

pounds containing carbon and hydrogen.

Deoxyribonucleic acid, a molecule in all cells, and many viruses, containing genetic codes for inheritance.

the amino group of a second amino acid.

A protein material that speeds up chemical reactions in the bodies of plants and animals.

known as monomers.

CARBOXYL GROUP:

DNA:

ENZYME:

Amino acids that cannot be manufactured by the body and therefore must be obtained from the diet. Proteins that contain essential amino acids are known as complete proteins. ESSENTIAL AMINO ACIDS:

An iron-containing protein in red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide from them. Hemoglobin is known for its deep red color. HEMOGLOBIN:

HERBIVORE:

A plant-eating organism.

In the long term, scientists hope to design proteins from scratch. This is extremely difficult today and will remain so until researchers better understand the rules that govern tertiary structure. Nevertheless, scientists already have designed a few small proteins whose stability or instability has enhanced our understanding of those rules. Building on these successes, scientists

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Molecules produced by

HORMONE:

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PEPTIDE LINKAGE:

A bond between

the carboxyl group of one amino acid and

POLYMERS:

Large, chainlike mole-

cules composed of numerous subunits A group of between 10

POLYPEPTIDE:

and 50 amino acids. PROTEINS:

Large molecules built

from long chains of 50 or more amino acids. Proteins serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body’s immune system, and making enzymes. RNA:

Ribonucleic acid, the molecule

translated from DNA in the cell nucleus, the control center of the cell, that directs protein synthesis in the cytoplasm, or the space between cells.

hope that one day they may be able to design proteins to meet a host of medical and industrial needs.

Proteins in the Diet Proteins are one of the basic nutrients, along with carbohydrates, lipids, vitamins, and miner-

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als (see Nutrients and Nutrition). They can be broken down and used as a source of emergency energy if carbohydrates or fats cannot meet immediate needs. The body does not use protein from food directly: after ingestion, enzymes in the digestive system break protein into smaller peptide chains and eventually into separate amino acids. These smaller constituents then go into the bloodstream, from whence they are transported to the cells. The cells incorporate the amino acids and begin building proteins from them.

For a person who eats meat, it would be extremely difficult not to get enough protein. According to the U.S. Food and Drug Administration (FDA), protein should account for 10% of total calories in the diet, and since protein contains 4 calories per 0.035 oz. (1 g), that would be about 1.76 oz. (50 g) in a diet consisting of 2,000 calories a day. A pound (0.454 kg) of steak or pork supplies about twice this much, and though very few people sit down to a meal and eat a pound of meat, it is easy to see how a meat eater would consume enough protein in a day.

ANIMAL AND V E G E TA B L E P R O T E I N S . The protein content in plants is

For a vegetarian, meeting the protein needs may be a bit more tricky, but it can be done. By combining legumes or beans and grains, it is possible to obtain a complete protein: hence, the longstanding popularity, with meat eaters as well as vegetarians, of such combinations as beans and rice or peas and cornbread. Other excellent vegetarian combos include black beans and corn, for a Latin American touch, or the eastern Asian combination of rice and tofu, protein derived from soybeans.

very small, since plants are made largely of cellulose, a type of carbohydrate (see Carbohydrates for more on this subject); this is one reason why herbivorous animals must eat enormous quantities of plants to meet their dietary requirements. Humans, on the other hand, are omnivores (unless they choose to be vegetarians) and are able to assimilate proteins in abundant quantities by eating the bodies of plant-eating animals, such as cows. In contrast to plants, animal bodies (as previously noted) are composed largely of proteins. When people think of protein in the diet, some of the foods that first come to mind are those derived from animals: either meat or such animal products as milk, cheese, butter, and eggs. A secondary group of foods that might appear on the average person’s list of proteins include peas, beans, lentils, nuts, and cereal grains. There is a reason why the “protein team” has a clearly defined “first string” and “second string.” The human body is capable of manufacturing 12 of the 20 amino acids it needs, but it must obtain the other eight—known as essential amino acids—from the diet. Most forms of animal protein, except for gelatin (made from animal bones), contain the essential amino acids, but plant proteins do not. Thus, the nonmeat varieties of protein are incomplete, and a vegetarian who does not supplement his or her diet might be in danger of not obtaining all the necessary amino acids.

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Proteins

WHERE TO LEARN MORE “DNA and Protein Synthesis.” John Jay College of Criminal Justice, City University of New York (Web site). . Inglis, Jane. Proteins. Minneapolis, MN: Carolrhoda Books, 1993. Kiple, Kenneth F., and Kriemhild Coneè Ornelas. The Cambridge World History of Food. New York: Cambridge University Press, 2000. “Proteins and Protein Foods.” Food Resource, Oregon State University (Web site). . Silverstein, Alvin, Virginia B. Silverstein, and Robert A. Silverstein. Proteins. Brookfield, CT: Millbrook Press, 1992. Structural Classification of Proteins (Web site). . THINK: Teenage Health Interactive Network (Web site). .

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ENZYMES Enzymes

CONCEPT

Catalysts

Enzymes are biological catalysts, or chemicals that speed up the rate of reaction between substances without themselves being consumed in the reaction. As such, they are vital to such bodily functions as digestion, and they make possible processes that normally could not occur except at temperatures so high they would threaten the well-being of the body. A type of protein, enzymes sometimes work in tandem with nonproteins called coenzymes. Among the processes in which enzymes play a vital role is fermentation, which takes place in the production of alcohol or the baking of bread and also plays a part in numerous other natural phenomena, such as the purification of wastewater.

In a chemical reaction, substances known as reactants interact with one another to create new substances, called products. Energy is an important component in the chemical reaction, because a certain threshold, termed the activation energy, must be crossed before a reaction can occur. To increase the rate at which a reaction takes place and to hasten the crossing of the activation energy threshold, it is necessary to do one of three things.

HOW IT WORKS Amino Acids, Proteins, and Biochemistry

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The first two options are to increase either the concentration of reactants or the temperature at which the reaction takes place. It is not always feasible or desirable, however, to do either of these things. Many of the processes that take place in the human body, for instance, normally would require high temperatures—temperatures, in fact, that are too high to sustain human life. Imagine what would happen if the only way we had of digesting starch was to heat it to the boiling point inside our stomachs! Fortunately, there is a third option: the introduction of a catalyst, a substance that speeds up a reaction without participating in it either as a reactant or as a product. Catalysts thus are not consumed in the reaction. Enzymes, which facilitate the necessary reactions in our bodies without raising temperatures or increasing the concentrations of substances, are a prime example of a chemical catalyst.

Amino acids are organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations. Strings of 50 or more amino acids are known as proteins, large molecules that serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body’s immune system, and making enzymes. The latter are a type of protein that functions as a catalyst, a substance that speeds up a chemical reaction without participating in it. Catalysts, of which enzymes in the bodies of plants and animals are a good example, thus are not consumed in the reaction.

tence of catalysts, ordinary people had been using the chemical process known as catalysis for numerous purposes: making soap, fermenting wine to create vinegar, or leavening bread, for

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T H E D I S C O V E RY O F CATA LY S I S . Long before chemists recognized the exis-

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instance. Early in the nineteenth century, chemists began to take note of this phenomenon. In 1812 the Russian chemist Gottlieb Kirchhoff (1764–1833) was studying the conversion of starches to sugar in the presence of strong acids when he noticed something interesting. When a suspension of starch (that is, particles of starch suspended in water) was boiled, Kirchhoff observed, no change occurred in the starch. When he added a few drops of concentrated acid before boiling the suspension, however, he obtained a very different result. This time, the starch broke down to form glucose, a simple sugar (see Carbohydrates), whereas the acid—which clearly had facilitated the reaction—underwent no change. In 1835 the Swedish chemist Jöns Berzelius (1779–1848) provided a name to the process Kirchhoff had observed: catalysis, derived from the Greek words kata (“down”) and lyein (“loosen”). Just two years earlier, in 1833, the French physiologist Anselme Payen (1795–1871) had isolated a material from malt that accelerated the conversion of starch to sugar, for instance, in the brewing of beer. The renowned French chemist Louis Pasteur (1822–1895), who was right about so many things, called these catalysts ferments and pronounced them separate organisms. In 1897, however, the German biochemist Eduard Buchner (1860–1917) isolated the catalysts that bring about the fermentation of alcohol and determined that they were chemical substances, not organisms. By that time, the German physiologist Willy Kahne had suggested the name enzyme for these catalysts in living systems.

Substrates and Active Sites Each type of enzyme is geared to interact chemically with only one particular substance or type of substance, termed a substrate. The two parts fit together, according to a widely accepted theory introduced in the 1890s by the German chemist Emil Fischer (1852–1919), as a key fits into a lock. Each type of enzyme has a specific three-dimensional shape that enables it to fit with the substrate, which has a complementary shape.

enzyme that catalyzes the digestion of lactose, or milk sugar, and urease catalyzes the chemical breakdown of urea, a substance in urine. Enzymes bind their reactants or substrates at special folds and clefts, named active sites, in the structure of the substrate. Because numerous interactions are required in their work of catalysis, enzymes must have many active sites, and therefore they are very large, having atomic mass figures as high as one million amu. (An atomic mass unit, or amu, is approximately equal to the mass of a proton, a positively charged particle in the nucleus of an atom.)

Enzymes

Suppose a substrate molecule, such as a starch, needs to be broken apart for the purposes of digestion in a living body. The energy needed to break apart the substrate is quite large, larger than is available in the body. An enzyme with the correct molecular shape arrives on the scene and attaches itself to the substrate molecule, forming a chemical bond within it. The formation of these bonds causes the breaking apart of other bonds within the substrate molecule, after which the enzyme, its work finished, moves on to another uncatalyzed substrate molecule.

Coenzymes All enzymes belong to the protein family, but many of them are unable to participate in a catalytic reaction until they link with a nonprotein component called a coenzyme. This can be a medium-size molecule called a prosthetic group, or it can be a metal ion (an atom with a net electric charge), in which case it is known as a cofactor. Quite often, though, coenzymes are composed wholly or partly of vitamins. Although some enzymes are attached very tightly to their coenzymes, others can be parted easily; in either case, the parting almost always deactivates both partners.

The link between enzymes and substrates is so strong that enzymes often are named after the substrate involved, simply by adding ase to the name of the substrate. For example, lactase is the

The first coenzyme was discovered by the English biochemist Sir Arthur Harden (1865–1940) around the turn of the nineteenth century. Inspired by Buchner, who in 1897 had detected an active enzyme in yeast juice that he had named zymase, Harden used an extract of yeast in most of his studies. He soon discovered that even after boiling, which presumably destroyed the enzymes in yeast, such deactivated yeast could be reactivated. This finding led Harden to the realization that a yeast enzyme appar-

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Enzymes

THE CONVERSION OF CABBAGE TO SAUERKRAUT UTILIZES A PARTICULAR BACTERIUM THAT ASSISTS HERE WORKERS SPREAD SALT AND PACK CHOPPED CABBAGE IN BARRELS, WHERE IT WILL FERMENT

IN FERMENTATION. FOR FOUR WEEKS.

(© Bettmann/Corbis. Reproduced by permission.)

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ently consists of two parts: a large, molecular portion that could not survive boiling and was almost certainly a protein and a smaller portion that had survived and was probably not a pro-

tein. Harden, who later shared the 1929 Nobel Prize in chemistry for this research, termed the nonprotein a coferment, but others began calling it a coenzyme.

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REAL-LIFE A P P L I C AT I O N S

red blood cells are suspended. These enzymes in the blood assist the body in everything from growth to protection against infection.

The Body, Food, and Digestion

One digestive enzyme that should be in the body, but is not always present, is lactase. As we noted earlier, lactase works on lactose, the principal carbohydrate in milk, to implement its digestion. If a person lacks this enzyme, consuming dairy products may cause diarrhea, bloating, and cramping. Such a person is said to be “lactose intolerant,” and if he or she is to consume dairy products at all, they must be in forms that contain lactase. For this reason, Lactaid milk is sold in the specialty dairy section of major supermarkets, while many health-food stores sell lactaid tablets.

Enzymes enable the many chemical reactions that are taking place at any second inside the body of a plant or animal. One example of an enzyme is cytochrome, which aids the respiratory system by catalyzing the combination of oxygen with hydrogen within the cells. Other enzymes facilitate the conversion of food to energy and make possible a variety of other necessary biological functions. Enzymes in the human body fulfill one of three basic functions. The largest of all enzyme types, sometimes called metabolic enzymes, assist in a wide range of basic bodily processes, from breathing to thinking. Some such enzymes are devoted to maintaining the immune system, which protects us against disease, and others are involved in controlling the effects of toxins, such as tobacco smoke, converting them to forms that the body can expel more easily. A second category of enzyme is in the diet and consists of enzymes in raw foods that aid in the process of digesting those foods. They include proteases, which implement the digestion of protein; lipases, which help in digesting lipids or fats; and amylases, which make it possible to digest carbohydrates. Such enzymes set in motion the digestive process even when food is still in the mouth. As these enzymes move with the food into the upper portion of the stomach, they continue to assist with digestion. The third group of enzymes also is involved in digestion, but these enzymes are already in the body. The digestive glands secrete juices containing enzymes that break down nutrients chemically into smaller molecules that are more easily absorbed by the body. Amylase in the saliva begins the process of breaking down complex carbohydrates into simple sugars. While food is still in the mouth, the stomach begins producing pepsin, which, like protease, helps digest protein.

Enzymes

Fermentation Fermentation, in its broadest sense, is a process involving enzymes in which a compound rich in energy is broken down into simpler substances. It also is sometimes identified as a process in which large organic molecules (those containing hydrogen and carbon) are broken down into simpler molecules as the result of the action of microorganisms working anaerobically, or in the absence of oxygen. The most familiar type of fermentation is the conversion of sugars and starches to alcohol by enzymes in yeast. To distinguish this reaction from other kinds of fermentation, the process is sometimes termed alcoholic or ethanolic fermentation. At some point in human prehistory, humans discovered that foods spoil, or go bad. Yet at the dawn of history—that is, in ancient Sumer and Egypt—people found that sometimes the “spoilage” (that is, fermentation) of products could have beneficial results. Hence the fermentation of fruit juices, for example, resulted in the formation of primitive forms of wine. Over the centuries that followed, people learned how to make both alcoholic beverages and bread through the controlled use of fermentation.

Later, when food enters the small intestine, the pancreas secretes pancreatic juice—which contains three enzymes that break down carbohydrates, fats, and proteins—into the duodenum, which is part of the small intestine. Enzymes from food wind up among the nutrients circulated to the body through plasma, a watery liquid in which

A L C O H O L I C B E V E RA G E S . In fermentation, starch is converted to simple sugars, such as sucrose and glucose, and through a complex sequence of some 12 reactions, these sugars then are converted to ethyl alcohol (the kind of alcohol that can be consumed, as opposed to methyl alcohol and other toxic forms) and carbon dioxide. Numerous enzymes are needed to carry out this sequence of reac-

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Enzymes

BAKER’S

YEAST, SINGLE-CELL FUNGI THAT PRODUCE CARBON DIOXIDE IN THE OXIDATION OF SUGAR.

CARBON

DIOX-

IDE CAUSES BREAD TO RISE WHEN YEAST IS MIXED INTO DOUGH, A RESULT OF THE FERMENTATION OF THE SUGAR BY ENZYMES IN THE YEAST. (© Lester V. Bergman/Corbis. Reproduced by permission.)

tions, the most important being zymase, which is found in yeast cells. These enzymes are sensitive to environmental conditions, such that when the concentration of alcohol reaches about 14%, they are deactivated. For this reason, no fermentation product (such as wine) can have an alcoholic concentration of more than about 14%. Stronger alcoholic beverages, such as whisky, are the result of another process, distillation. The alcoholic beverages that can be produced by fermentation vary widely, depending primarily on two factors: the plant that is fermented and the enzymes used for fermentation. Depending on the materials available to them, various peoples have used grapes, berries, corn, rice, wheat, honey, potatoes, barley, hops, cactus juice, cassava roots, and other plant materials for fermentation to produce wines, beers, and other fermented drinks. The natural product used in making the beverage usually determines the name of the synthetic product. Thus, for instance, wine made with rice—a time-honored tradition in Japan—is known as sake, while a fermented beverage made from barley, hops, or malt sugar has a name very familiar to Americans: beer. Grapes make wine, but “wine” made from honey is known as mead.

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O T H E R F O O D S . Of course, ethyl alcohol is not the only useful product of fermentation or even of fermentation using yeast; so, too, are baked goods, such as bread. The carbon dioxide generated during fermentation is an important component of such items. When the batter for bread is mixed, a small amount of sugar and yeast is added. The bread then rises, which is more than just a figure of speech: it actually puffs up as a result of the fermentation of the sugar by enzymes in the yeast, which brings about the formation of carbon dioxide gas. The carbon dioxide gives the batter bulkiness and texture that would be lacking without the fermentation process. Another food-related application of fermentation is the production of one processed type of food from a raw, natural variety. The conversion of raw olives to the olives sold in stores, of cucumbers to pickles, and of cabbage to sauerkraut utilizes a particular bacterium that assists in a type of fermentation. INDUSTRIAL

A P P L I C AT I O N S .

There is even ongoing research into the creation of edible products from the fermentation of petroleum. While this may seem a bit far-fetched, it is less difficult to comprehend powering cars with an environmentally friendly product of fer-

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Enzymes

KEY TERMS ACTIVATION ENERGY:

A threshold

A protein that acts as a cata-

ENZYME:

that must be crossed to facilitate a chemical

lyst, a material that speeds up chemical

reaction. There are three ways to reach the

reactions in the bodies of plants and ani-

activation energy: by increasing the con-

mals without itself taking part in, or being

centration of reactants, by raising their

consumed by, these reactions.

temperature, or by introducing a catalyst,

FERMENTATION:

such as an enzyme.

enzymes in which a compound rich in Folds and clefts on the

ACTIVE SITES:

A process involving

energy is broken down into simpler sub-

surface of an enzyme that enable attach-

stances.

ment to its particular substrate.

METABOLISM:

The chemical process

Organic compounds

by which nutrients are broken down and

made of carbon, hydrogen, oxygen, nitro-

converted into energy or are used in the

gen, and (in some cases) sulfur bonded in

construction of new tissue or other materi-

characteristic formations. Strings of amino

al in the body.

acids make up proteins.

MOLECULE:

AMINO ACIDS:

A group of atoms, usual-

The area of the bio-

ly but not always representing more than

logical sciences concerned with the chemi-

one element, joined in a structure. Com-

cal substances and processes in organisms.

pounds typically are made up of mole-

BIOCHEMISTRY:

CARBOHYDRATES:

Naturally occur-

cules. At one time, chemists used

ring compounds, consisting of carbon,

ORGANIC:

hydrogen, and oxygen, whose primary

the term organic only in reference to living

function in the body is to supply energy.

things. Now the word is applied to com-

Included in the carbohydrate group are

pounds containing carbon and hydrogen.

sugars, starches, cellulose, and various

PROTEINS:

other substances. Most carbohydrates are

from long chains of 50 or more amino

produced by green plants in the process of

acids. Proteins serve the functions of pro-

undergoing photosynthesis.

moting normal growth, repairing damaged

CATALYSIS:

The act or process of cat-

Large molecules built

tissue, contributing to the body’s immune

alyzing, or speeding up the rate of reaction

system, and making enzymes.

between substances.

REACTANT:

A substance that interacts

A substance that speeds up

with another substance in a chemical reac-

a chemical reaction without participating

tion, resulting in the formation of a chem-

in it. Catalysts, of which enzymes are a

ical or chemicals known as the product.

good example, thus are not consumed in

STARCHES:

the reaction.

without taste or odor, which are granular

CATALYST:

COENZYME:

A nonprotein component

Complex carbohydrates

or powdery in physical form.

sometimes required to allow an enzyme to

SUBSTRATE:

set in motion a catalytic reaction.

is paired with a particular enzyme.

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A reactant that typically

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Enzymes

KEY TERMS Enzymes often are named after their respective substrates by adding the suffix

“sugar” (i.e., table sugar) is actually sucrose.

ase (e.g., the enzyme lactase is paired with

VITAMINS:

the substrate lactose). SUGARS:

One of the three principal

types of carbohydrate, along with starches and cellulose. Sugars can be defined as any of various water-soluble carbohydrates of varying sweetness. What we think of as

mentation known as gasohol. Gasohol first started to make headlines in the 1970s, when an oil embargo and resulting increases in gas prices, combined with growing environmental concerns, raised the need for a type of fuel that would use less petroleum. A mixture of about 90% gasoline and 10% alcohol, gasohol burns more cleanly that gasoline alone and provides a promising method for using renewable resources (plant material) to extend the availability of a nonrenewable resource (petroleum). Furthermore, the alcohol needed for this product can be obtained from the fermentation of agricultural and municipal wastes.

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CONTINUED

Organic substances that, in extremely small quantities, are essential to the nutrition of most animals and some plants. In particular, vitamins work with enzymes in regulating metabolic processes; however, they do not in themselves provide energy, and thus vitamins alone do not qualify as a form of nutrition.

converted to carbon dioxide, water, and mineral salts. WHERE TO LEARN MORE Asimov, Isaac. The Chemicals of Life: Enzymes, Vitamins, Hormones. New York: Abelard-Schulman, 1954. “Enzymes: Classification, Structure, Mechanism.” Washington State University Department of Chemistry (Web site). . “Enzymes.” HordeNet: Hardy Research Group, Department of Chemistry, The University of Akron (Web site). . Fruton, Joseph S. A Skeptical Biochemist. Cambridge, MA: Harvard University Press, 1992.

The applications of fermentation span a wide spectrum, from medicines that go into people’s bodies to the cleaning of waters containing human waste. Some antibiotics and other drugs are prepared by fermentation: for example, cortisone, used in treating arthritis, can be made by fermenting a plant steroid known as diosgenin. In the treatment of wastewater, anaerobic, or non-oxygen-dependent, bacteria are used to ferment organic material. Thus, solid wastes are

“Milk Makes Me Sick: Exploration of the Basis of Lactose Intolerance.” Exploratorium: The Museum of Science, Art, and Human Perception (Web site). .

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“Introduction to Enzymes.” Worthington Biochemical Corporation (Web site). . Kornberg, Arthur. For the Love of Enzymes: The Odyssey of a Biochemist. Cambridge, MA: Harvard University Press, 1989.

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S C I E N C E O F E V E RY DAY T H I N G S real-life biology

M E TA B O L I S M M E TA B O L I S M DIGESTION R E S P I RAT I O N

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Metabolism

CONCEPT The term metabolism refers to all of the chemical reactions by which complex molecules taken into an organism are broken down to produce energy and by which energy is used to build up complex molecules. All metabolic reactions fall into one of two general categories: catabolic and anabolic reactions, or the processes of breaking down and building up, respectively. The best example of metabolism from daily life occurs in the process of taking in and digesting nutrients, but sometimes these processes become altered, either through a person’s choice or through outside factors, and metabolic disorders follow. Such disorders range from anorexia and bulimia to obesity. These are all examples of an unhealthy, unnatural alteration to the ordinary course of metabolism; on the other hand, hibernation allows animals to slow down their metabolic rates dramatically as a means of conserving energy during times when food is scarce.

M E TA B O L I S M

Metabolism is indeed like a furnace, in that it burns energy, and that is the aspect most commonly associated with this concept. But metabolism also involves a function that a furnace does not: building new material. All metabolic reactions can be divided into either catabolic or anabolic reactions. Catabolism is the process by which large molecules are broken down into smaller ones with the release of energy, whereas anabolism is the process by which energy is used to build up complex molecules needed by the body to maintain itself and develop new tissue. D I G E S T I O N . One way to understand the metabolic process is to follow the path of a typical nutrient as it passes through the body. The digestive process is discussed in Digestion, while nutrients are examined in Nutrients and Nutrition as well as in Proteins, Amino Acids, Enzymes, Carbohydrates, and Vitamins. Here we touch on the process only in general terms, as it relates to metabolism.

The term metabolism, strangely enough, is related closely to devil, with which it shares the Greek root ballein, meaning “to throw.” By adding dia (“through” or “across”), one arrives at devil and many related words, such as diabolical; on the other hand, the replacement of that prefix with meta (“after” or “beyond”) yields the word metabolism. The connection between the two words has been obscured over time, but it might be helpful to picture metabolism in terms of an image that goes with that of a devil: a furnace.

The term digestion is not defined in the essay on that subject, because it is an everyday word whose meaning is widely known. For the present purposes, however, it is important to identify it as the process of breaking down food into simpler chemical compounds as a means of making nutrients absorbable by the body. This is a catabolic process, because the molecules of which foods are made are much too large to pass through the lining of the digestive system and directly into the bloodstream. Thanks to the digestive process, smaller molecules are formed and enter the bloodstream, from whence they are carried to individual cells throughout a person’s body.

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Metabolism

The smaller molecules into which nutrients are broken down make up the metabolic pool, which consists of simpler substances.The metabolic pool includes simple sugars, made by the breakdown of complex carbohydrates; glycerol and fatty acids, which come from the conversion of lipids, or fats; and amino acids, formed by the breakdown of proteins. Substances in the metabolic pool provide material from which new tissue is constructed—an anabolic process. The chemical breakdown of substances in the cells is a complex and wondrous process. For instance, a cell converts a sugar molecule into carbon dioxide and water over the course of about two dozen separate chemical reactions. This is what cell biologists call a metabolic pathway: an orderly sequence of reactions, with particular enzymes (a type of protein that speeds up chemical reactions) acting at each step along the way. In this instance, each chemical reaction makes a relatively modest change in the sugar molecule—for example, the removal of a single oxygen atom or a single hydrogen atom—and each is accompanied by the release of energy, a result of the breaking of chemical bonds between atoms.

ATP and ADP Cells capture and store the energy released in catabolic reactions through the use of chemical compounds known as energy carriers. The most significant example of an energy carrier is adenosine triphosphate, or ATP, which is formed when a simpler compound, adenosine diphosphate (ADP), combines with a phosphate group. (A phosphate is a chemical compound that contains oxygen bonded to phosphorus, and the term group in chemistry refers to a combination of atoms from two or more elements that tend to bond with other elements or compounds in certain characteristic ways.)

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be used to put simpler molecules together to make more complex molecules. For example, suppose that a cell needs to repair a rupture in its cell membrane. To do so, it will need to produce new protein molecules, which are made from hundreds or thousands of amino-acid molecules. These molecules can be obtained from the metabolic pool. The reactions by which a compound is metabolized differ for various nutrients. Also, energy carriers other than ATP may play a part. For example, the compound known as nicotinamide adenine dinucleotide phosphate (NADPH) also has a role in the catabolism and anabolism of various substances. The general outline described here, however, applies to all metabolic reactions.

Catabolism and Anabolism Energy released from organic nutrients (those containing carbon and hydrogen) during catabolism is stored within ATP, in the form of the highenergy chemical bonds between the second and third molecules of phosphate. The cell uses ATP for synthesizing cell components from simple precursors, for the mechanical work of contraction and motion, and for transport of substances across its membrane. ATP’s energy is released when this bond is broken, turning ATP into ADP. The cell uses the energy derived from catabolism to fuel anabolic reactions that synthesize cell components. Although anabolism and catabolism occur simultaneously in the cell, their rates are controlled independently. Cells separate these pathways because catabolism is a “downhill” process, or one in which energy is released, while anabolism is an “uphill” process requiring the input of energy.

ADP will combine with a phosphate group only if energy is added to it. In cells, that energy comes from the catabolism of compounds in the metabolic pool, including sugars, glycerol (related to fats), and fatty acids. The ATP molecule formed in this manner has taken up the energy previously stored in the sugar molecule, and thereafter, whenever a cell needs energy for some process, it can obtain it from an ATP molecule. The reverse of this process also takes place inside cells. That is, energy from an ATP molecule can

Catabolism and anabolism share an important common sequence of reactions known collectively as the citric acid cycle, the tricarboxylic acid cycle, or the Krebs cycle. Named after the German-born British biochemist Sir Hans Adolf Krebs (1900–1981), the Krebs cycle is a series of chemical reactions in which tissues use carbohydrates, fats, and proteins to produce energy; it is part of a larger series of enzymatic reactions known as oxidative phosphorylation. In the latter reaction, glucose is broken down to release energy, which is stored in the form of ATP—a catabolic sequence. At the same time, other molecules produced by the Krebs cycle are used as

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precursor molecules for reactions that build proteins, fats, and carbohydrates—an anabolic sequence. (A precursor is a substance, cellular component, or cell from which another substance, cellular component, or cell—different in kind from the precursor—is formed.)

Introduction to Lipids As noted earlier, many practical aspects of metabolism are discussed elsewhere, particularly in the essays Digestion and Nutrients and Nutrition. Also, two types of chemical compound, proteins and carbohydrates, are so important to a variety of metabolic processes that they are examined in detail within entries of their own. In the present context, let us focus on the third major kind of nutrient, lipids or fats. Lipids are soluble in nonpolar solvents, which is the reason why a gravy stain or other grease stain is difficult to remove from clothing without a powerful detergent or spot remover. Water molecules are polar, because the opposing electric charges tend to occupy opposite sides or ends of the molecule. In a molecule of oil, whether derived from petroleum or from animal or vegetable fat, electric charges are very small, and are distributed evenly throughout the molecule. Whereas water molecules tend to bond relatively well, like a bunch of bar magnets attaching to one another at their opposing poles, oil and fat molecules tend not to bond. (The “bond” referred to here is the fairly weak one between molecules. Much stronger is the chemical bond within molecules—a bond that, when broken, brings about a release of energy, as noted earlier.) Their functions are as varied as their structures, but because they are all fat-soluble, lipids share in the ability to approach and even to enter cells. The latter have membranes that, while highly complex in structure, can be identified in simple terms as containing lipids or lipoproteins (lipids attached to proteins). The behavior of lipids and lipid-like molecules, therefore, becomes very important in understanding how a substance may or may not enter a cell. Such a substance may be toxic, as in the case of some pesticides, but if they are lipid-like, they are able to penetrate the cell’s membrane. (See Food Webs for more about the biomagnification of DDT.) In addition to lipoproteins, there are glycolipids, or lipids attached to sugars, as well as lipids attached to alcohols and some to phosphoric

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acids. The attachment with other compounds greatly alters the behavior of a lipid, often making them bipolar—that is, one end of the molecule is water-soluble. This is important, because it allows lipids to move out of the intestines and into the bloodstream. In the digestive process, lipids are made water-soluble either by being broken down into smaller parts or through association with another substance. The breaking down usually is done via two different processes: hydrolysis, or chemical reaction with water, and saponification. The latter, a reaction in which certain kinds of organic compounds are hydrolyzed to produce an alcohol and a salt, is used in making soap.

Metabolism

REAL-LIFE A P P L I C AT I O N S Putting Lipids to Use Derived from living systems of plants, animals, or humans, lipids are essential to good health, not only for humans but also for other animals and even plants. Seeds, for example, contain lipids for the storage of energy. Because fat is a poor conductor of heat, lipids also can function as effective insulators, and for this reason, people living in Arctic zones seek fatty foods such as blubber. Some lipids function as chemical messengers in the body, while others serve as storage areas for chemical energy. There is a good reason why babies are born with “baby fat” and why children entering puberty often tend to become chubby: in both cases, they are building up energy reserves for the great metabolic hurdles that lie ahead, and within a few years, they will have used up those excessive fat stores. FAT S A N D O I L S . Fats and oils are both energy-rich compounds that are basic components of the normal diet. Both have essentially the same chemical structure—a mixture of fatty acids combined with glycerol—and are insoluble (do not dissolve) in water. While fats remain solid or at least semisolid at room temperature, however, most oils very quickly become liquid at increased temperatures. Animal fats and oils include butter, lard, tallow, and fish oil. Numerous other oils, such as cottonseed, peanut, and corn oils, are derived from plants.

Fats have two main functions: they provide some of the raw material for synthesizing (creat-

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Metabolism

CROSS-SECTION

MICROGRAPH OF A BLOOD VESSEL WITH ATHEROSCLEROTIC LESION, COMPOSED OF CHOLESTEROL

AND OTHER LIPIDS.

HIGH

LEVELS OF CERTAIN CIRCULATING FATS CAN LEAD TO A THICKENING OF THE ARTERY WALLS

AND HAVE BEEN LINKED TO VARIOUS ILLNESSES, INCLUDING HEART DISEASE AND CANCER. (© Lester V. Bergman/Corbis.

Reproduced by permission.)

ing) and repairing tissues, and they serve as a concentrated source of fuel energy. Fats, in fact, provide humans with roughly twice as much energy, per unit weight, as carbohydrates and proteins. Fats are not only an important source of day-to-day energy, but they also can be stored indefinitely as adipose (fat) tissue in case of future need. Fats also help by transporting fatsoluble vitamins, such as A and D (see Vitamins), throughout the system. They cushion and form protective pads around delicate organs, such as the heart, liver and kidneys, and the layer of fat under the skin helps insulate the body against too much heat loss. They even add to the flavor of foods that might otherwise be inedible. NOT

ALL

FAT

IS

C R E AT E D

E Q UA L . Although normal amounts of certain

kinds of fat in the diet are essential to good health, unnecessarily high amounts (especially of unhealthy fats) can lead to various problems. Healthy fats include those from fatty fish, such as salmon, mackerel, or tuna, or from fat-containing vegetables, such as the avocado. In addition, many vegetable oils, particularly olive oil, can be beneficial.

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Bad fats, on the other hand, are usually ones that have been tampered with through a process known as hydrogenation. This is a term describing any chemical reaction in which hydrogen atoms are added to fill in chemical bonds between carbon and other atoms, but in the case of fatty foods, hydrogenation involves the saturation of hydrocarbons, organic chemical compounds whose molecules are made up of nothing but carbon and hydrogen atoms. When they are treated with hydrogen gas, they become “saturated” with hydrogen atoms. Saturated fats, as they are called, are harder and more stable and stand up better to the heat of frying, which makes them more desirable for use in commercial products. For this reason, many foods contain hydrogenated vegetable oil; however, saturated fats have been linked to a rise in blood cholesterol levels— and to an increased risk of heart disease. Cholesterol is a variety of lipid, and, like other lipids, some of it is essential—but only some and only of the right kind. Most cholesterol is transported through the blood in low-density lipoproteins, or LDLs, which have been nicknamed bad cholesterol. These lipoproteins are received by LDL receptors on the cell mem-

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branes, but if there are more LDLs than LDL receptors, the excess LDLs will be deposited in the arteries. Thus, LDLs are not really “bad” unless there are too many of them. On the other hand, “good” cholesterol (HDLs, or high-density lipoproteins) help protect against damage to the artery walls by carrying excess LDLs back to the liver. HOW MUCH IS TOO MUCH? A

certain amount of excess adipose tissue can be valuable during periods of illness, overactivity, or food shortages. Too much, however, can be unsightly and also can overwork the heart and put added stress on other parts of the body. High levels of certain circulating fats may lead to atherosclerosis, which is a thickening of the artery walls, and they have been linked to various illnesses, including cancer. With fat, as with many things where the body is concerned, if a little is a good, this does not mean that a lot is better. In the past, nutritionists considered a diet that obtained 40% of its calories from fats a reasonable one; today, however, they recommend that no more than 30% of all calories (and preferably an even smaller percentage) come from fat. Agreement on this point, however, is far from universal. Some physicians and scientists maintain that dietary fat does not contribute as much to body fat as do carbohydrates. Carbohydrates are good for someone who needs a boost of energy that can be consumed easily by the body, such as an athlete going into competition. But for inactive people—and this includes a large portion of Americans—carbohydrates simply are stored as fat. Experts do not even agree on the answer to a question much simpler than “How much is too much fat in the diet?”—the question “How much is too much fat on the body?” Some doctors classify a person as obese whose weight is at least 20% more than the recommended weight for his or her height, but others say that standard height-and-weight charts are misleading. After all, muscle weighs more than fat, and it is conceivable that a very muscular athlete with very little body fat might qualify as “overweight” compared with the recommended weight for his or her height.

“pinch” test, is a better measure of obesity. (Being obese is not the same as being overweight: the muscular athlete described in the last paragraph is overweight but not obese, a term that implies an excess of body fat.) In healthy adults, fat typically should account for about 18–25% of the body weight in females and 15–20% in males. The reason for the difference between men and women is that fat naturally accumulates in a woman’s buttocks and thighs, because nature “assumes” that she will bear children, in which case such excess fat will be useful. This is why women over the age of about 25 often complain that when they and their husbands or boyfriends embark on a fitness program together, the men usually see results faster. The reason is that there is no genetic or evolutionary benefit to be gained from a man having fat around his waist, which is where men usually gain. If anything—since our genetic codes and makeup have changed little since prehistory—the well-being and propagation of the human species are best served by a lean, muscular male capable of killing animals to feed and protect his family. All of this means, of course, that men should not gloat if they see better results from a regular workout program; instead, they should just recognize that nature is at work in their wives’ or girlfriends’ bodies as in their own.

Metabolic Disorders Enzymes, as we noted earlier, are critical participants in metabolic reactions. They are like relay runners in a race, in this case a race along the metabolic pathways whereby nutrients are turned into energy or new bodily material. Therefore, if an enzyme is missing or does not function as it should, it can create a serious metabolic disorder. An example is phenylketonuria (PKU), caused by the lack of an enzyme known as phenylalanine hydroxylase. This enzyme is responsible for converting the amino acid phenylalanine to a second amino acid, tyrosine; when this does not happen, phenylalanine builds up in the body. It is converted to a compound called phenylpyruvate, which impairs normal brain development, resulting in severe mental retardation.

issue, many experts contend that the proportion of fat to muscle, measured by the skinfold

Other examples of metabolic disorders include alkaptonuria, thalassemia, porphyria, Tay-Sachs disease, Hurler syndrome, Gaucher disease, galactosemia, Cushing syndrome, dia-

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B O DY FAT, T H E S E X E S , A N D N AT U R E . Because of the complexity of the

Metabolism

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The word anorexia comes from the Greek for “lack of appetite,” but the problem for people with anorexia is not that they are not hungry. On the contrary, they are starving, but unlike poor people in the Third World, they are not starving as the result of a shortage of food but because they are denying themselves nutrition. They do this because they fear gaining weight, even when they are so severely underweight that they look like skeletons.

Metabolism

DIANA,

THE LATE

PRINCESS

OF

WALES,

SUFFERED

FROM BULIMIA, A DISORDER THAT PROMPTS A PERSON TO GO ON EATING BINGES AND THEN

“PURGE,”

EITHER

BY VOMITING OR BY TAKING LARGE AMOUNTS OF LAXATIVES. (© Corbis. Reproduced by permission.)

betes mellitus, hyperthyroidism, and hypothyroidism. Most of these conditions affect a small population; however, diabetes mellitus (discussed in Noninfectious Diseases) is one of the leading killers in America. At present, no cures for metabolic disorders exist. The best approach is to diagnose such conditions as early as possible and then arrange a person’s diet to deal as effectively as possible with that disorder.

ANOREXIA

AND

BULIMIA.

Young people are more likely than older people to suffer anorexia or bulimia, conditions that typically become apparent at about the age of 20 years. Although both men and women can experience the problem, in fact, only about 5% of people with these eating disorders are male. And though anorexia and bulimia are closely related—particularly inasmuch as they are psychological in origin but can exact a heavy biological toll—there are several important differences.

Anorexia nervosa, bulimia, and obesity are the most well known types of eating disorder.

People who have anorexia or bulemia often come from families with overprotective parents who have unrealistically high expectations of their children. Frequently, high expectations go hand in hand with a wealthy background, and certainly anorexia and bulimia are not conditions that typically affect the poor. Anorexia and bulimia often seem to develop after some stressful experience, such as moving to a new town, changing schools, or going through puberty. Low self-esteem, fear of losing control, and fear of growing up are common characteristics of people with these conditions. Their need for approval manifests in a quest to meet or exceed our culture’s idealized concept of extreme thinness. This quest is a part of our popular culture, promoted by waiflike models whose sunken eyes stare out of fashion magazines.

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Eating Disorders Eating disorders are a different matter, because they are psychological rather than physiological conditions. No one is sure what causes eating disorders, but researchers think that family dynamics, biochemical abnormalities, and modern American society’s preoccupation with thinness all may contribute. Eating disorders are virtually unknown in parts of the world where food is scarce, but in wealthy lands, such as the United States, problems of overeating, self-induced starvation, or forced purging have gained considerable attention.

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The name of a related condition, bulimia, literally means “hungry as an ox.” People with this problem go on eating binges, often gorging on junk food. Then they force their bodies to get rid of the food, either by vomiting or by taking large amounts of laxatives. A third type of eating disorder, obesity, also is characterized by uncontrollable overeating, but in this case the person does not force the body to eject the food that has been consumed. That, at least, makes obesity more healthy than bulimia, but there is nothing healthy about accumulating vast amounts of body fat, as severely obese people do.

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Like anorexia, bulimia results in starvation, but there are behavioral, physical, and psychological differences between the two. Bulimia is both less and more dangerous: on the one hand, people who have it tend to be of normal weight or are overweight, and unlike those with anorexia, they are aware of the fact that they have a problem. On the other hand, because the effects of their behavior are not so readily apparent, it is easier for a person with bulimia to persist in the pattern of bingeing and purging for much longer. Approximately one in five persons with bulimia has a problem with drug or alcohol use, and they pursue their binges in a way not unlike that of a guilty addict or alcoholic hiding the spent needles or empty bottles from family members. They may go from restaurant to restaurant to avoid being seen eating too much in any one place, or they may pretend to be shopping for a large dinner party when, in fact, they intend to eat all the food themselves. Because of the expense of consuming so much food, some resort to shoplifting. During a binge, people suffering from bulimia favor high-carbohydrate foods, such as doughnuts, candy, ice cream, soft drinks, cookies, cereal, cake, popcorn, and bread, and they consume many times the number of calories they would normally consume in one day. No matter what their normal eating habits, they tend to eat quickly and messily during a binge, stuffing the food into their mouths and gulping it down, sometimes without even tasting it. Some say they get a feeling of euphoria during binges, similar to the “runner’s high” that some people get from exercise. Then, when they have gorged themselves, they force the food back out, either by causing themselves to vomit or by taking large quantities of laxatives. Regular self-induced vomiting can cause all sorts of physical problems, such as damage to the stomach and esophagus, chronic heartburn, burst blood vessels in the eyes, throat irritation, and erosion of tooth enamel from the acid in vomit. Excessive use of laxatives can induce muscle cramps, stomach pains, digestive problems, dehydration, and even poisoning, while bulimia, in general, brings about vitamin deficiencies and imbalances of critical body fluids, which, in turn, can lead to seizures and kidney failure.

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The self-imposed starvation of people with anorexia likewise takes a heavy toll on the body. The skin becomes dry and flaky, muscles begin to waste away, bones stop growing and may become brittle, and the heart weakens. Seeking to protect itself in the absence of proper insulation from fat, the body sprouts downy hair on the face, back, and arms in response to lower body temperature. In women, menstruation stops, and permanent infertility may result. Muscle cramps, dizziness, fatigue, and even brain damage as well as kidney and heart failure are possible. An estimated 10% to 20% of people with anorexia die either as a direct result of starvation or by suicide.

Metabolism

To save people with anorexia, force-feeding may be necessary. Some 70% of anorexia patients who are treated for about six months return to normal body weight, but about 15-20% can be expected to relapse. Bulimia is not as likely as anorexia to reach life-threatening stages, so hospitalization typically is not necessary. Treatment generally calls for psychotherapy and sometimes the administration of antidepressant drugs. Unlike people with anorexia, those with bulimia usually admit they have a problem and want help overcoming it. O B E S I T Y. Unlike anorexia or bulimia, obesity is more of a problem among people from lower-income backgrounds. This probably relates to a lack of education concerning nutrition, combined with the fact that healthier food is more expensive; by contrast, unhealthy items, such as white sugar, corn meal, and fatty cuts of pork and other meats can fill or overfill a person’s stomach inexpensively. In addition, though men and women both tend to gain weight as they age, women are almost twice as likely as men to be obese.

Some cases of obesity relate to metabolic problems, while others stem from compulsive eating, which is psychologically motivated. Some studies suggest that obese people are much more likely than others to eat in response to stress, loneliness, or depression. And just as emotional pain can lead to obesity, obesity can lead to psychological scars. From childhood on, obese people are taunted and shunned, and throughout life they may face discrimination in school and on the job. Physically, obesity is a killer, especially for those who are morbidly obese—that is, people whose obesity endangers their health. Obesity is

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Metabolism

BEARS,

WHICH WE THINK OF AS THE CLASSIC HIBERNATING ANIMAL, ARE ACTUALLY JUST DEEP SLEEPERS.

A

HIBER-

NATING ANIMAL SHOWS A DRASTIC REDUCTION IN METABOLISM AND THEN AWAKES RELATIVELY SLOWLY, WHEREAS A SLEEPING ANIMAL DECREASES ITS METABOLISM ONLY SLIGHTLY AND CAN WAKE UP ALMOST INSTANTLY IF DISTURBED.

(© Dan Guravich/Corbis. Reproduced by permission.)

a risk factor for diabetes, high blood pressure, arteriosclerosis, angina pectoralis (chest pains due to inadequate blood flow to the heart), varicose veins, cirrhosis of the liver, and kidney disease. Obese people are about 1.5 times more likely to have heart attacks than are other people, and the overall death rate among people ages 20-64 is 50% higher for the obese than for people of ordinary weight.

Hibernation Having looked at several unnatural ways in which people alter their metabolisms, let us close with an example of a very natural way that animals sometimes temporarily change theirs. This is hibernation, a state of inactivity in which an animal’s heart rate, body temperature, and breathing rate are decreased as a way to conserve energy through the cold months of winter. A similar state, known as estivation, is adopted by some desert animals during the dry months of summer. Hibernation is a technique that animals have developed, as a result of natural selection over the generations (see Evolution), to adapt to harsh environmental conditions. When food is scarce, a

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nonhibernating animal would be like a business operating at a loss—that is, using more energy maintaining its body temperature and searching for food than it would receive from consuming the food. Hibernating animals use 70-100 times less energy than when they are active, allowing them to survive until food is once again plentiful. C O N T RAS T W I T H S L E E P. Many animals sleep more often when food is scarce, but only a few truly hibernate. Bears, which many people think of as the classic hibernating animal, are actually just deep sleepers. By contrast, true hibernation occurs only in small mammals, such as bats and woodchucks and a few birds, among them nighthawks. Some insects also practice a form of hibernation. Hibernation differs from sleep, in that a hibernating animal shows a drastic reduction in metabolism and then awakes relatively slowly, whereas a sleeping animal decreases its metabolism only slightly and can wake up almost instantly if disturbed. Also, hibernating animals do not show periods of rapid eye movement (REM), the stage of sleep associated with dreaming in humans. T H E P R O C E SS O F H I B E R N A T I O N . Animals prepare for hibernation in the

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Metabolism

KEY TERMS ADIPOSE:

Of or relating to animal fat. Organic compounds

AMINO ACIDS:

pounds as a means of making the nutrients absorbable by the body or organism.

made of carbon, hydrogen, oxygen, nitro-

ENZYME:

gen, and (in some cases) sulfur bonded in

speeds up chemical reactions in the bodies

characteristic formations. Strings of amino

of plants and animals without itself taking

acids make up proteins.

part in, or being consumed by, these reac-

ANABOLISM:

The metabolic process

A protein material that

tions.

by which energy is used to build up com-

GLUCOSE:

plex molecules that the body needs to

that occurs widely in nature and which is

maintain itself and develop new material.

the form in which animals usually receive

A monosaccharide (sugar)

The smallest particle of an ele-

carbohydrates. Also known as dextrose,

ment, consisting of protons, neutrons, and

grape sugar, and corn sugar. See also blood

electrons. An atom can exist either alone or in

sugar.

combination with other atoms in a molecule.

HYDROCARBON:

ATOM:

Any organic chemi-

Adenosine triphosphate, an ener-

cal compound whose molecules are made

gy carrier formed when a simpler com-

up of nothing but carbon and hydrogen

pound, adenosine diphosphate (ADP),

atoms.

ATP:

combines with a phosphate group. BLOOD SUGAR:

The glucose in the

blood.

LIPIDS:

Fats and oils, which dissolve in

oily or fatty substances but not in waterbased liquids. In the body, lipids supply

Naturally occur-

energy in slow-release doses, protect

ring compounds, consisting of carbon,

organs from shock and damage, and pro-

hydrogen, and oxygen, whose primary

vide insulation for the body, for instance

function in the body is to supply energy.

from toxins.

Included in the carbohydrate group are

METABOLIC PATHWAY:

sugars, starches, cellulose, and various

sequence of reactions, with particular

other substances. Most carbohydrates are

enzymes acting at each step along the way.

produced by green plants in the process of

Metabolic pathways may be either linear or

undergoing photosynthesis.

circular, and sometimes they are linked,

CARBOHYDRATES:

CATABOLISM:

The metabolic process

An orderly

meaning that the product of one pathway

by which large molecules are broken down

becomes a reactant in another.

into smaller ones with the release of ener-

METABOLIC POOL:

gy. Compare with anabolism.

tively simple substances (e.g., amino acids)

COMPOUND:

A substance in which

atoms of more than one element are bonded chemically to one another.

A group of rela-

formed by the breakdown of relatively complex nutrients. METABOLISM:

The chemical process

The process of breaking

by which nutrients are broken down and

food down into simpler chemical com-

converted into energy or are used in the

DIGESTION:

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Metabolism

KEY TERMS

CONTINUED

construction of new tissue or other material in the body. All metabolic reactions are either catabolic or anabolic.

phate, or a chemical compound that con-

A group of atoms, usually but not always representing more than one element, joined in a structure. Compounds typically are made up of molecules.

formed from the interaction of reactants in

MOLECULE:

Materials essential to the survival of organisms. They include proteins, carbohydrates, lipids (fats), vitamins, and minerals. NUTRIENT:

The series of processes by which an organism takes in nutrients and makes use of them for its survival, growth, and development. The term nutrition also can refer to the study of nutrients, their consumption, and their use in the organism’s body. NUTRITION:

At one time chemists used the term organic only in reference to living things. Now the word is applied to compounds containing carbon and hydrogen. ORGANIC:

A group (that is, a combination of atoms from two or more elements that tend to bond with other elements or compounds in certain characteristic ways) that includes a phos-

tains oxygen bonded to phosphorus. A substance or substances

PRODUCT:

a chemical reaction. PROTEINS:

Large molecules built

from long chains of 50 or more amino acids. Proteins serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body’s immune system, and making enzymes. REACTANT:

A substance that interacts

with another substance in a chemical reaction, resulting in the formation of a chemical or chemicals known as the product(s). SUGARS:

One of the three principal

types of carbohydrate, along with starches and cellulose. Sugars can be defined as any of various water-soluble carbohydrates of varying sweetness. What we think of as “sugar” (i.e., table sugar) is actually sucrose; “blood sugar,” on the other hand, is glucose.

PHOSPHATE GROUP:

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TISSUE:

A group of cells, along with

the substances that join them, that forms part of the structural materials in plants or animals.

fall by storing food; usually this storage is internal, in the form of fat reserves. A woodchuck in early summer may have only about 5% body fat, but as fall approaches, changes in the animal’s brain chemistry cause it to feel hungry and to eat constantly. As a result, the woodchuck’s body fat increases to about 15% of its total weight. In other animals, such as the dormouse, fat may constitute as much as 50% of the animal’s weight by the time hibernation begins. A short period of fasting follows the feeding frenzy, to ensure that

the digestive tract is emptied completely before hibernation begins.

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Going into hibernation is a gradual process. Over a period of days, an animal’s heart rate and breathing rate drop slowly, eventually reaching rates of just a few beats or breaths per minute. Their body temperatures also drop from levels of about 100°F (38°C) to about 60°F (15°C). The lowered body temperature makes fewer demands on metabolism and food stores. Electric activity in the brain ceases almost completely during

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hibernation, although some areas—those that respond to external stimuli, such as light, temperature, and noise—remain active. Thus, the hibernating animal can be aroused under extreme conditions. Periodically—perhaps every two weeks or so—the hibernating animal awakes and takes a few deep breaths to refresh its air supply. If the weather is particularly mild, some animals may venture from their lairs. An increase in heart rate signals that the time for arousal, or ending hibernation, is near. Blood vessels dilate, particularly around the heart, lungs, and brain, and this leads to an increased breathing rate. Eventually, the increase in circulation and metabolic activity spreads throughout the body, and the animal resumes a normal waking state. WHERE TO LEARN MORE

Center, Institute for Chemical Research, Kyoto University (Web site). . Medline Plus: Food, Nutrition, and Metabolism Topics. Medline, National Library of Medicine, National Institutes of Health (Web site). . Metabolic Pathways of Biochemistry. George Washington University (Web site). . Metabolism (Web site). . Michal, Gerhard. Biochemical Pathways: An Atlas of Biochemistry and Molecular Biology. New York: Wiley, 1999. Pasternak, Charles A. The Molecules Within Us: Our Body in Health and Disease. New York: Plenum, 1998. Pathophysiology of the Digestive System (Web site). .

Bouchard, Claude. Physical Activity and Obesity. Champaign, IL: Human Kinetics, 2000.

Spallholz, Julian E. Nutrition, Chemistry, and Biology. Englewood Cliffs, NJ: Prentice-Hall, 1989.

“KEGG Metabolic Pathways.” KEGG: Kyoto Encyclopedia of Genes and Genomes—GenomeNet, Bioinformatics

Wolinsky, Ira. Nutrition in Exercise and Sport. 3d ed. Boca Raton, FL: CRC Press, 1998.

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DIGESTION Digestion

CONCEPT Digestion is the process whereby the foods we eat pass through our bodies and are directed toward the purposes of either providing the body with energy or building new cellular material, such as fat or muscle. The parts of food that the body cannot use, along with other wastes from the body, are eliminated in the form of excrement. Aspects of digestion, particularly the production of waste and intestinal gas, are not exactly topics for polite conversation, yet without these and other digestive processes, life for humans and other organisms would be impossible. The functioning of digestion itself is like that of a wellorganized, cohesive sports team or even of a symphony orchestra: there are many parts and players, each with an indispensable role.

HOW IT WORKS Nutrients For digestion to occur, of course, it is necessary first to have something to digest—namely, nutrients. What follows is a cursory overview of nutrients and nutrition, subjects covered in much more depth within the essay of that name. Nutrients include proteins, carbohydrates, fats, minerals, and vitamins. In addition to these nutrients, animal life requires other materials, not usually considered nutrients, which include water, oxygen, and something that greatly aids the process of food digestion and elimination of wastes: fiber.

Carbohydrates are compounds that consist of carbon, hydrogen, and oxygen. Their primary function in the body is to supply energy. When a person ingests more carbohydrates than his or her body needs at the moment, the body converts the excess into a compound known as glycogen. It then stores the glycogen in the liver and muscle tissues, where it remains, a potential source of energy for the body to use in the future, though if it is not used soon, it may be stored as fat. The carbohydrate group comprises sugars, starches, cellulose (a type of fiber), and various other chemically related substances. L I P I D S , V I TA M I N S , A N D M I N E RA L S . Lipids include all fats and oils and are

from long chains of amino acids, which are

distinguished by the fact that they are soluble (i.e., capable of being dissolved) in oily or fatty substances but not in water. In the body, lipids supply energy much as carbohydrates do, only much more slowly. Lipids also protect the organs from shock and damage and provide the body with insulation from cold, toxins, and other threats. Processed, saturated fats (fats that have been enhanced artificially to make them more

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PROTEINS AND CARBOHYD RAT E S . Proteins are large molecules built

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organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfurbonded in characteristic formations. Proteins serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body’s immune system, and making enzymes. (An enzyme is a protein material that speeds up chemical reactions in the bodies of plants and animals.) Good examples of dietary proteins include eggs, milk, cheese, and other dairy products. Incomplete proteins, or ones lacking essential amino acids—those amino acids that are not produced by the human body—include peas, beans, lentils, nuts, and cereal grains.

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firm) are extremely unhealthy, and consumption of some types of animal fat (e.g., pork fat) is also inadvisable. On the other hand, vegetable fats, such as those in avocados and olive oil, as well as the animal fats in such fish as tuna, mackerel, and salmon can be highly beneficial. Vitamins are organic substances that, in extremely small quantities, are essential to the nutrition of most animals and some plants. In particular, they work with enzymes in regulating metabolic processes—that is, the chemical processes by which nutrients are broken down and converted into energy or used in the construction of new tissue or other material in the body. Vitamins do not in themselves provide energy, however, and thus they do not qualify as a form of nutrition. Much the same is true of minerals, except that these are inorganic substances, meaning that they do not contain chemical compounds made of carbon and hydrogen.

The Digestive System To supply the body with the materials it needs for energy and the building of new tissue, nutrients have to pass through the digestive system. The latter is composed of organs (an organ being a group of tissues and cells, organized into a particular structure, that performs a specific function within an organism) and other structures through which nutrients move. The nutrients pass first through the mouth and then through the esophagus, stomach, small intestine, and large intestine, or colon. Collectively, these structures are known as the alimentary canal. Nutrients advance through the alimentary canal to the stomach and small intestine, and waste materials continue from the small intestine to the colon (large intestine) and anus. Along the way, several glands play a role. A gland is a cell or group of cells that filters material from the blood, processes that material, and secretes it either for use again in the body or to be eliminated as waste. Among the glands that play a part in the digestive process are the salivary glands, liver, gallbladder, and pancreas. (The last three are examples of glands that are also organs.) The glands with a role in digestion secrete digestive juices containing enzymes that break down nutrients chemically into smaller molecules that are absorbed more easily by the body. There are also hormones involved in digestion-there are,

S C I E N C E O F E V E RY DAY T H I N G S

for example, glandular cells in the lining of the stomach that make the hormone gastrin.

Digestion

FROM THE MOUTH TO THE S T O M AC H . The first stage of digestion is

ingestion, in which food is taken into the mouth and then broken down into smaller pieces by the chewing action of the teeth. To facilitate movement of the food through the mouth and along the tongue, it is necessary for saliva to be present. Usually, the sensations of sight, taste, and smell associated with food set in motion a series of neural responses that induce the formation of saliva by the salivary glands in the mouth. Amylase, an enzyme in the saliva, begins the process of breaking complex carbohydrates into simple sugars. (The terms simple and complex in this context refer to chemical structures.) By the time it is ready to be swallowed, food is in the form of a soft mass known as a bolus. The action of swallowing pulls the food down through the pharynx, or throat, and into the esophagus, a tube that extends from the bottom of the throat to the top of the stomach. (Note that for the most part, we are using human anatomy as a guide, but many aspects of the digestive process described here also apply to other higher animals, particularly mammals.) The esophagus does not take part in digestion but rather performs the function of moving the bolus into the stomach. A wavelike muscular motion termed peristalsis, which consists of alternating contractions and relaxations of the smooth muscles lining the esophagus, moves the bolus through this passage. At the place where the esophagus meets the stomach, a powerful muscle called the esophageal sphincter acts as a valve to keep food and stomach acids from flowing back into the esophagus and mouth. (Although the most well-known sphincter muscle in the body is the one surrounding the anus, sometimes known simply as “the sphincter,” in fact, sphincter is a general term for a muscle that surrounds, and is able to control the size of, a bodily opening.) F R O M T H E S T O M AC H T O T H E S M A L L I N T E S T I N E . Chemical digestion

begins in the stomach, a large, hollow, pouchlike muscular organ. While food is still in the mouth, the stomach begins its production of gastric juice, which contains hydrochloric acid and pepsin, an enzyme that digests protein. Gastric juice is the material that breaks down the food.

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A

SIXTEENTH-CENTURY ANATOMICAL DIAGRAM OF THE INTERNAL ORGANS, SHOWING THE STOMACH, LIVER, INTESTINE,

AND GALLBLADDER. (© Corbis. Reproduced by permission.)

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Once nerves in the cheeks and tongue are stimulated by the food, they send messages to the brain, which, in turn, alerts nerves in the stomach wall, stimulating the secretion of gastric juice before the bolus itself arrives in the stomach.

Once the bolus touches the stomach lining, it triggers a second release of gastric juice, along with mucus that helps protect the stomach lining from the action of the hydrochloric acid. Three layers of powerful stomach muscles churn food

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into a thick liquid called chyme, which is pumped gradually through the pyloric sphincter, which connects the stomach small intestine. T H E S M A L L I N T E S T I N E . The names of the small and large intestines can be confusing, rather like those of Upper and Lower Egypt in ancient history. In both cases, the adjectives seem to refer to one thing but actually refer to something else entirely. Thus, it so happens that Upper Egypt was south of Lower Egypt (because it was “upper” in elevation, not latitude), while the small intestine is, in fact, much longer than the large intestine. The reason is that small refers to its diameter rather than its length: though it is about 23 ft. (7 m) long, the small intestine is only 1 in. (2.5 cm) in diameter, while the large intestine, only 5 ft. (1.5 m) in length, is 3 in. (7.6 cm) across.

The small intestine, which connects the stomach and large intestine, is in three sections: the duodenum, jejunum, and ileum. About 1 ft. (0.3 m) long, the duodenum breaks down chyme from the stomach with the aid of the pancreas and gallbladder. The pancreas, a large gland located below the stomach, secretes pancreatic juice, which contains three enzymes that break down carbohydrates, fats, and proteins, into the duodenum through the pancreatic duct. The gallbladder empties bile, a yellowish or greenish fluid from the liver, into the duodenum when chyme enters that portion of the intestine. Although bile does not contain enzymes, it does have bile salts that help dissolve fats. Digested carbohydrates, fats, proteins, and most of the vitamins, minerals, and iron in food are absorbed in the jejunum, which is about 4 ft. (1.2 m) long. Aiding this absorption are up to five million tiny finger-like projections called villi, which greatly increase the surface area of the small intestine, thus accelerating the rate at which nutrients are absorbed into the bloodstream. The remainder of the small intestine is taken up by the ileum, which is smaller in diameter and has thinner walls than the jejunum. It is the final site for absorption of some vitamins and other nutrients, which enter the circulatory system in plasma, a watery liquid in which red blood cells also are suspended.

energy and growth. Plasma also contains waste products from the breakdown of proteins, including creatinine, uric acid, and ammonium salts. These constituents are moved to the kidneys, where they are filtered from the blood and excreted in the urine. But, of course, urine is not the only waste product excreted by the body; there is also the solid waste, processed through the large intestine, or colon.

Digestion

T H E LA R G E I N T E S T I N E A N D B E YO N D . Like the small intestine, the large

intestine is in segments. It rises up on the right side of the body (the ascending colon), crosses over to the other side underneath the stomach (the transverse colon), descends on the left side, (the descending colon), and forms an S shape (the sigmoid colon) before reaching the rectum and anus. In addition to its function of pumping solid waste, the large intestine removes water from the waste products—water that, when purified, will be returned to the bloodstream. In addition, millions of bacteria in the large intestine help produce certain B vitamins and vitamin K, which are absorbed into the bloodstream along with the water. After leaving the sigmoid colon, waste passes through the muscular rectum and then the anus, the last point along the alimentary canal. In all, the movement of food through the entire length of the alimentary tract takes from 15 to 30 hours, with the majority of that time being taken up by activity in the colon. Food generally spends about three to five hours in the stomach, another four to five hours in the small intestine, and between five and 25 hours in the large intestine.

As it moves through the circulatory system, plasma takes with it amino acids, enzymes, glycerol (a form of alcohol found in fats), and fatty acids, which it directs to the body’s tissues for

The transit time, or the amount of time it takes for food to move through the system, is a function of diet: for a vegetarian who eats a great deal of fiber, it will be on the short end, while for a meat eater who has just consumed a dinner of prime rib, it will take close to the maximum time. People who eat diets heavy in red meat or junk foods are also likely to experience a buildup, over time, of partially digested material on the linings of their intestines. Obviously, this is not a healthy situation, and to turn it around, a person may have to change his or her diet and perhaps even undergo some sort of colon-cleansing program. There is an easy way to test transit time in one’s system: simply eat a large serving of corn or red beets, and measure how long it takes for these to fully work their way through the digestive system.

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Digestion

REAL-LIFE A P P L I C AT I O N S Digestive Disorders It is hard to watch more than a few minutes of commercial television without seeing advertisements for fast foods and other varieties of junk food or stomach-relief medicine or both. There is a connection, of course: a society glutted on greasy drive-through burgers and thick-crust pizzas needs something to cure the upset stomachs that result. Indigestion is a general condition that, as its name suggests, involves an inability to digest food properly. Heartburn, sometimes called acid indigestion, is a specific type of indigestion that occurs when the stomach produces too much hydrochloric acid. The latter is essential to digestion, but if a person eats a giant Polish sausage, a spicy-hot bowl of jambalaya, or some other hardto-digest food (as opposed to a healthy meal of baked fish with brown rice and spinach, for instance), the stomach may produce too much of the acid. Heartburn is so named because it causes a sharp pain behind the breastbone, which might feel like a heart attack. It also may produce acid reflux, in which the stomach acid backs up into the esophagus. If you have ever experienced what might be called a leap of vomit, in which a burp is associated with the rise of burning, foul-tasting bile through the esophagus, then you have firsthand knowledge of acid reflux and heartburn. Digestive tract diseases, such as dyspepsia, sometimes can cause chronic indigestion, but more often than not, people experience indigestion as a result of eating too quickly or too much, consuming high-fat foods, or eating in a stressful situation. (This is why you might feel sick to your stomach when eating lunch at school on the day of a difficult test, a fight, or a romantic trauma, such as a breakup or asking for a first date.) Smoking, excessive drinking, fatigue, and the consumption of medications that irritate the stomach lining also can contribute to indigestion. In addition, it is a good idea not to eat too soon before going to bed, since this can produce heartburn.

One famous commercial stomach remedy actually uses acid. This is Alka-Seltzer, but the presence of citric acid has more to do with marketing than with the chemistry of the stomach. The citric acid, often used as a sweetener, imparts a more pleasant flavor than the bitter taste of alkaline antacids, and, moreover, when AlkaSeltzer tablets are placed in water, the acid reacts chemically with the sodium bicarbonate to create the product’s trademark fizz. Ultimately, all antacids (Alka-Seltzer included) work because the bases in the product react with the acids in the stomach. This is a chemical process called neutralization, in which the acid and base cancel out each other, producing water and a salt in the process. (Table salt, or sodium chloride, is just one of many salts, all of which are formed by the chemical bonding of a metal with a nonmetal—in the case of table salt, sodium and chlorine, respectively.) Thanks to this process, acid in the stomach of a heartburn sufferer is neutralized. U L C E R S . There are some digestive disorders that cannot be cured by Alka-Seltzer or its many competitors, such as Rolaids, Maalox, Mylanta, Tums, Milk of Magnesia, or Pepto-Bismol. Instead of just occasional indigestion or heartburn, a person may be afflicted with a sore in one part of the digestive tract, which may be either a stomach ulcer or a duodenal ulcer. Stomach ulcers, which form in the lining of the stomach, are called peptic ulcers because they form with the help of stomach acid and pepsin. Duodenal ulcers, which are more common, tend to be smaller than stomach ulcers and heal more quickly. Any ulcer, whether a small sore or a deep cavity, leaves a scar in the alimentary canal.

nonprescription stomach-relief medicine is in the form of an antacid, which, as the term suggests, is a substance that works against acids in

Until the early 1990s, physicians generally maintained that personal behavior and conditions, such as stress and poor diet, were the principal factors behind ulcer. Medical researchers eventually came to believe, however, that the culprit was a certain bacterium, which can live

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the stomach. Chemically, the opposite of an acid is a base, or an alkaline substance, the classic example being sodium bicarbonate or sodium hydrogen carbonate (NaHCO3)—that is, baking soda. Baking soda alone can perform the function of an antacid, but the taste is rather unpleasant, and for this reason most antacid products combine it with other chemicals to enhance the flavor.

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Digestion

SCANNING

ELECTRON MICROGRAPH OF A STOMACH ULCER. INSTEAD OF JUST OCCASIONAL INDIGESTION, A PERSON

MAY BE AFFLICTED WITH AN ULCER, OR A SORE IN ONE PART OF THE DIGESTIVE TRACT.

STOMACH

ULCERS ARE CALLED

PEPTIC ULCERS BECAUSE THEY FORM WITH THE HELP OF STOMACH ACID AND PEPSIN. (Photo Researchers. Reproduced by per-

mission.)

undetected in the mucous lining of the stomach. This bacterium irritates and weakens the lining, making it more susceptible to damage by stomach acids. As many as 80% of all stomach ulcers may be caused by such a bacterial infection. With this newfound knowledge, ulcer patients today are more likely to be treated with antibiotics and antacids rather than special diets or expensive medicines.

eat only when you are really hungry and to eat slowly and chew food thoroughly. Drinking liquids with a meal is probably not a good practice; it is better to wait until you are finished, so as not to interfere with the action of digestive fluids. Certainly, smoking and excessive drinking have a negative impact on digestion, whereas regular exercise has a positive influence.

There are numerous ways to improve digestion, by changing either the way one eats or the things one eats. In the first category, it is important to

In the category of diet, it is a good idea to minimize one’s intake of red meat, such as steak. Although most people find red meat tasty, and it can be a good supplier of dietary iron in limited proportions, the digestion of red meat requires the production of much more stomach acid, and

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Digestion

thus it places a great burden on the digestive system. It is also wise to eliminate as many processed foods as possible, including sweets and junk foods, and to eat as many natural foods as one can manage. In general, one can hardly go wrong with raw vegetables, which are just about the best thing a person can eat—not only because of their digestive properties but also because many of them are packed so full of vitamins and minerals. (Note that vegetables are best when raw and fresh, since cooking removes many of the nutrients. Canned vegetables usually are both nutrient-poor and full of sodium or even synthetic chemicals. Frozen vegetables are much better than canned ones, but they still do not compare nutritionally with fresh vegetables.) A good diet includes a great deal of fiber, indigestible material that simply passes through the system, assisting in the peristaltic action of the alimentary canal and in the process of eliminating waste. Cellulose, found in most raw fruits and vegetables, is an example of fiber, also called bulk or roughage. Yogurt may be a beneficial food, because it includes “good” bacteria (a topic we discuss near the conclusion of this essay) that assist the digestive process. Some foods, such as raw bean sprouts, papaya, figs, and pineapple, contain enzymes that appear to assist the body in digesting them. In addition, one of the greatest “foods” for aiding digestion is not a food at all, but water, of which most people drink far too little. Some experts claim that a person should drink eight 8oz. (0.24 l) glasses a day, but others maintain that a person should drink half as many ounces of water as his or her weight in pounds. In other words, a person who weighed 100 lb. (45.36 kg) would drink 50 oz. (1.48 l) of water a day, whereas a person who weighed 150 lb. (68.04 kg) would drink 75 oz. (2.22 l). A good rule of thumb for metric users, instead of the 2:1 pounds-toounces ratio, would be 30:1 kilograms to liters. Note also that tap water may contain chemicals or other impurities, and therefore consuming it in large quantities is not advisable. A much better alternative is bottled or filtered water.

Human Waste

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Always Wanted to Know about Sex (But Were Afraid to Ask). Just as the title of David R. Reuben’s 1969 book inspired Woody Allen’s 1972 movie, which was very loosely based on it, the “Everything you always wanted to know ... ” motif inspired a whole array of imitators. Often such titles play on the very fact that hardly anyone wants to know, and certainly no one is afraid to ask, about the topic in question. A good example is an on-line article by central Asia authority Mark Dickens entitled “Everything You Always Wanted to Know about Tocharian But Were Afraid To Ask” (http://www.oxuscom.com/ eyawtkat.htm). The joke, of course, is that most people have never heard of Tocharian, a central Asian language. Nonetheless, one subject ranks with sex as something everyone wonders about but most are afraid to ask. Even the name of that topic creates problems, since people have so many euphemisms for it: “number two,” for instance, or BM (short for bowel movement). There are baby- and childoriented terms for this process and product, the bodily control of which can be a major problem for a very young human being, and, of course, there is at least one grown-up term for it that will not be mentioned here. People even have nicknames for animal dung, such as pies, patties, or chips. For the sake of convenience, let us call the process defecation, and the product human waste (or excrement or feces) and admit that everyone has wondered how something as pleasant as food can, after passing through the alimentary canal, turn into something as unpleasant as the final product. The average person excretes some 7 lb. (3.2 kg) of feces per day, an amount equal to a little more than 1 ton (0.91 tonnes) per year. This waste is made up primarily of indigestible materials as well as water, salts, mucus, cellular debris from the intestines, bacteria, and cellulose and other types of fiber. Like the human body itself, these waste products are mostly water: about 75%, compared with 25% solid matter. Much of what goes into producing excrement has nothing to do with what enters the digestive system, so even if a person were starving he or she would continue to excrete feces. C O LO RAT I O N . What about the color

Many years ago, both a serious book and a comedic movie had the title Everything You

and the smell? The color of feces comes from

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ESCHERICHIA COLI is a type of bacteria that lives in the intestinal tract, aiding the digestive process by suppressing the growth of harmful bacteria and synthesizing vitamins. (© 1997 Custom Medical Stock Photo. Reproduced by permission.)

bilirubin, a reddish-yellow pigment found in blood and bile, which passes through the liver and enters the small intestine via the gallbladder. Later, as it passes through the large intestine, it is degraded by the action of bacteria, a process that turns it brown and gives feces its characteristic color.

ing bleeding in the gastrointestinal tract (the stomach and intestines) may produce waste the color of black tar. In addition, foods with distinctive colors and textures also can affect the appearance of stools.

Not surprisingly, disorders involving the red blood cells, liver, or gallbladder can change the color of human waste. A person with gallstones or hepatitis (a disease characterized by inflammation of the liver) is likely to excrete grayishbrown feces, while anemia (a condition that involves a lack of red blood cells) may be associated with a yellowish stool. A person experienc-

Before addressing the smell of feces, which is the result of action by bacteria in the colon, it is worth saying a few words about those single-cell organisms themselves. This is especially important in light of the fact that bacteria have a bad reputation that is not entirely deserved. Without question, there are harmful microbes in the world, but a world completely free of these organisms

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BACTERIA.

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Digestion

KEY TERMS The entire length of tube that extends from the mouth to the anus, including the esophagus, stomach, and small and large intestines. Nutrients pass through the alimentary canal to the stomach and small intestine, and waste materials from these nutrients (and from other sites in the body) pass from the small intestine to the colon (large intestine) and anus.

ALIMENTARY

CANAL:

Organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations. Strings of amino acids make up proteins. AMINO ACIDS:

A yellowish or greenish digestive fluid excreted by the liver. BILE:

A term for a chewed mass of food making its way through the initial portions of the alimentary canal.

BOLUS:

Naturally occurring compounds, consisting of carbon, hydrogen, and oxygen, whose primary function in the body is to supply energy. Included in the carbohydrate group are sugars, starches, cellulose, and various other substances. CARBOHYDRATES:

A polysaccharide that is the principal material in the cell walls of plants. Cellulose also is found in such natural fibers as cotton and is used as a raw material in manufacturing such products as paper.

CELLULOSE:

The large intestine, through which waste materials pass on their way to excretion through the anus.

COLON:

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COMPOUND:

A substance in which

atoms of more than one element are bonded chemically to one another. A protein material that

ENZYME:

speeds up chemical reactions in the bodies of plants and animals. Indigestible material in food

FIBER:

that simply passes through the digestive system, assisting in the peristaltic action of the alimentary canal and in the processing of waste. Examples of fiber, also called bulk or roughage, include cellulose. GASTROINTESTINAL TRACT:

The

stomach and intestines. GLAND:

A cell or group of cells that fil-

ters material from the blood, processes that material, and secretes it either for use again in the body or to be eliminated as waste. GLUCOSE:

A type of sugar that occurs

widely in nature. Glucose is the form in which animals usually receive carbohydrates. GLYCOGEN:

A white polysaccharide

that is the most common form in which carbohydrates are stored in animal tissues, particularly muscle and liver tissues. GUT:

A term that refers to all or part of

the alimentary canal. Although the word is considered a bit crude in everyday life, physicians and biological scientists concerned with this part of the anatomy use it regularly.

would be one in which humans and other animals would be unable to live. In fact, we have a mutually beneficial relationship, a type of symbiosis (see Symbiosis) with the microorganisms

in our alimentary canals, particularly in the colon.

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Bacteria live in the guts—a term that refers to all or part of the alimentary canal—of most ani-

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KEY TERMS HEMOGLOBIN:

An iron-containing

CONTINUED

POLYSACCHARIDE:

A

complex

pigment in red blood cells that is responsi-

sugar, in which the molecules are com-

ble for transporting oxygen to the tissues

posed of many glucose subunits arranged

and removing carbon dioxide from them.

in a chain. Polysaccharides can be broken

LIPIDS:

Fats and oils, which dissolve in

down chemically to produce simple sugars,

oily or fatty substances but not in water-

or monosaccharides.

based liquids. In the body, lipids supply

PROTEINS:

energy in slow-release doses, protect

from long chains of amino acids. Proteins

organs from shock and damage, and pro-

serve the functions of promoting normal

vide insulation for the body, for instance, from toxins.

Large molecules built

growth, repairing damaged tissue, contributing to the body’s immune system,

METABOLISM:

The chemical process

and making enzymes.

by which nutrients are broken down and converted into energy or used in the construction of new tissue or other material in the body. Inorganic substances that,

MINERALS:

in a nutritional context, serve a function similar to that of vitamins. Minerals may include chemical elements, particularly metallic ones, such as calcium or iron, as well as some compounds.

A general term for a

SPHINCTER:

muscle that surrounds and is able to control the size of a bodily opening. A biological relationship

SYMBIOSIS:

in which (usually) two species live in close proximity to each other and interact regularly in such a way as to benefit one or both of the organisms. A group of cells, along with

TISSUE:

A group of tissues and cells,

the substances that join them, that forms

organized into a particular structure, that

part of the structural materials in plants or

performs a specific function within an

animals.

organism.

VITAMINS:

ORGAN:

Organic substances that, in

At one time, chemists used

extremely small quantities, are essential to

the term organic only in reference to living

the nutrition of most animals and some

things. Now the word is applied to com-

plants. In particular, vitamins work with

pounds containing carbon and hydrogen.

enzymes in regulating metabolic processes;

ORGANIC:

A series of involuntary

they do not in themselves provide energy,

muscle contractions that force bolus, and

however, and thus vitamins alone do not

later waste, through the alimentary canal.

qualify as a form of nutrition.

PERISTALSIS:

mals, where they assist in such difficult digestive activities as the processing of chewed grasses. The latter is heavy in cellulose, and to digest it, cows, sheep, deer, and other grass eaters (known as rumi-

nants) have stomachs with several compartments. The first of these compartments is called the rumen, and it serves as home to millions of bacteria, which assist in breaking down the heavy fibers.

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Humans’ bacterial symbiotic partners (actually, this is a type of symbiosis known as mutualism, in which both creatures benefit) include bacteria of the species Escherichia coli, or E. coli. The name is no doubt familiar to most readers from its appearance in the news in connection with horror stories involving E. coli poisoning in food or local water supplies. Certainly, E. coli can be extremely harmful when it is outside the human gut, but inside the gut it is humans’ friend. E. coli is a coprophile (literally, “excrement lover”), meaning that it depends on feces for survival. Fecal matter itself can contain all manner of harmful substances associated with the decomposition of foods or with the body’s efforts to rid itself of toxins (including pathogens, or disease-carrying parasites—see Parasites and Parasitology), so anything associated with feces is dirty and potentially dangerous. It is for this reason that E. coli can cause serious illness or death if it gets into other parts of the body. As long as it stays where it belongs, however, E. coli not only aids in the digestive process but also provides the body with vitamin K, essential for proper blood clotting, as well as vitamin B12, thiamine, and riboflavin. Every person carries millions and millions of these helpful fellow travelers; even though a single bacterium weighs almost nothing in human terms, the combined weight of all the helpful, “good” bacteria in our guts is a staggering 7 lb. (3.2 kg). I N T E S T I N A L GAS . As those bacteria

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Hydrogen sulfide is just one of many unpleasant-smelling chemical products that result from bacterial action on solids in the gut. Others include indole, skatole, ammonia, and mercaptans, though the most distinctivesmelling of all are indole and skatole, which come primarily from the digestion of an amino acid known as tryptophan. In addition, the particular foods a person eats, as well as the specific bacterial residents (some harmful) of his or her gut, can affect the odor of intestinal gas. When gases pass outside the rectum, the result is flatulence (of course, there are other, less polite words for it), which is the subject of much schoolboy humor. Even inside the body, intestinal gas can make noise and cause embarrassment, in the form of borborygmus—intestinal rumbling caused by moving gas. As for the flammability of intestinal gas, it probably results from the high proportion of hydrogen, an extremely flammable gas. WHERE TO LEARN MORE Avraham, Regina. The Digestive System. New York: Chelsea House Publishers, 1989. Ballard, Carol. The Stomach and Digestive System. Austin, TX: Raintree Steck-Vaughn, 1997. Digestive System Diseases. Karolinska Institutet (Web site). . The Human Body’s Digestive System Theme Page. Community Learning Network (Web site). . Medline Plus: Digestive System Topics. National Library of Medicine, National Institutes of Health (Web site). .

do their work, they generate vast quantities of gases, which are by-products of the chemical processes that play a part in breaking down the foods passing through the gut. Among these gaseous products are hydrogen sulfide, a foulsmelling substance that can be toxic in large quantities. Unlike carbon monoxide, which has no odor, few people are in danger of dying from inhalation of hydrogen sulfide—even though it is abundant in nature—because the smell is enough to dissuade anyone from inhaling it for long periods of time. (Incidentally, gas companies include traces of hydrogen sulfide with natural gas. By itself, natural gas is odorless, but when a leak occurs, a homeowner will smell hydrogen sulfide and alert the gas company.)

Richardson, Joy. What Happens When You Eat? Illus. Colin Maclean and Moira Maclean. Milwaukee, WI: Gareth Stevens Publishing, 1986.

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Morrison, Ben. The Digestive System. New York: Rosen Publishing Group, 2001. National Institute of Diabetes and Digestive and Kidney Diseases of the National Institutes of Health (Web site). .

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R E S P I R AT I O N

CONCEPT Respiration is much more than just breathing; in fact, the term refers to two separate processes, only one of which is the intake and outflow of breath. At least cellular respiration, the process by which organisms convert food into chemical energy, requires oxygen; on the other hand, some forms of respiration are anaerobic, meaning that they require no oxygen. Such is the case, for instance, with some bacteria, such as those that convert ethyl alcohol to vinegar. Likewise, an anaerobic process can take place in human muscle tissue, producing lactic acid—something so painful that it feels as though vinegar itself were being poured on an open sore.

cells with oxygen and receives carbon dioxide to transfer to the environment. This is just one meaning—albeit a more familiar one—of the word respiration. Respiration also can mean cellular respiration, a series of chemical reactions within cells whereby food is “burned” in the presence of oxygen and converted into carbon dioxide and water. This type of respiration is the reverse of photosynthesis, the process by which plants convert dioxide and water, with the aid of solar energy, into complex organic compounds known as carbohydrates. (For more about carbohydrates and photosynthesis, see Carbohydrates.)

How Gases Move Through the Body

HOW IT WORKS Forms of Respiration Respiration can be defined as the process by which an organism takes in oxygen and releases carbon dioxide, one in which the circulating medium of the organism (e.g., the blood) comes into contact with air or dissolved gases. Either way, this means more or less the same thing as breathing. In some cases, this meaning of the term is extended to the transfer of oxygen from the lungs to the bloodstream and, eventually, into cells or the release of carbon dioxide from cells into the bloodstream and thence to the lungs, from whence it is expelled to the environment. Sometimes a distinction is made between external respiration, or an exchange of gases with the external environment, and internal respiration, an exchange of gases between the body’s cells and the blood, in which the blood itself “bathes” the

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Later in this essay, we discuss some of the ways in which various life-forms breathe, but suffice it to say for the moment—hardly a surprising revelation!—that the human lungs and respiratory system are among the more complex mechanisms for breathing in the animal world. In humans and other animals with relatively complex breathing mechanisms (i.e., lungs or gills), oxygen passes through the breathing apparatus, is absorbed by the bloodstream, and then is converted into an unstable chemical compound (i.e., one that is broken down easily) and carried to cells. When the compound reaches a cell, it is broken down and releases its oxygen, which passes into the cell. On the “return trip”—that is, the reverse process, which we experience as exhalation— cells release carbon dioxide into the bloodstream, where it is used to form another unstable chemical compound. This compound is carried by the

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bloodstream back to the gills or lungs, and, at the end of the journey, it breaks down and releases the carbon dioxide to the surrounding environment. Clearly, the one process is a mirror image of the other, with the principal difference being the fact that oxygen is the key chemical component in the intake process, while carbon dioxide plays the same role in the process of outflow. HEMOGLOBIN AND OTHER C O M P O U N D S . In humans the compound

used to transport oxygen is known by the name hemoglobin. Hemoglobin is an iron-containing protein in red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide from them. In the lungs, hemoglobin, known for its deep red color, reacts with oxygen to form oxyhemoglobin. Oxyhemoglobin travels through the bloodstream to cells, where it breaks down to form hemoglobin and oxygen, and the oxygen then passes into cells. On the return trip, hemoglobin combines with carbon dioxide to form carbaminohemoglobin, an unstable compound that, once again, breaks down—only this time it is carbon dioxide that it releases, in this case to the surrounding environment rather than to the cells. In other species, compounds other than hemoglobin perform a similar function. For example, some types of annelids, or segmented worms, carry a green blood protein called chlorocruorin that functions in the same way as hemoglobin does in humans. And whereas hemoglobin is a molecule with an iron atom at the center, the blood of lobsters and other large crustaceans contains hemocyanin, in which copper occupies the central position. Whatever the substance, the compound it forms with oxygen and carbon dioxide must be unstable, so that it can break down easily to release oxygen to the cells or carbon dioxide to the environment.

Cellular Respiration Both forms of respiration involve oxygen, but cellular respiration also involves a type of nutrient—materials that supply energy, or the materials for forming new tissue. Among the key nutrients are carbohydrates, naturally occurring compounds that consist of carbon, hydrogen, and oxygen. Included in the carbohydrate group are sugars, starches, cellulose, and various other substances.

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Glucose is a simple sugar produced in cells by the breakdown of more complex carbohydrates, including starch, cellulose, and such complex sugars as sucrose (cane or beet sugar) and fructose (fruit sugar). In cellular respiration, an organism oxidizes glucose (i.e., combines it with oxygen) so as to form the energy-rich compound known as adenosine triphosphate (ATP). ATP, critical to metabolism (the breakdown of nutrients to provide energy or form new material), is the compound used by cells to carry out most of their ordinary functions. Among those functions are the production of new cell parts and chemicals, the movement of compounds through cells and the body as a whole, and growth. In cellular respiration, six molecules of glucose (C6H12O6) react with six molecules of oxygen (O2) to form six molecules of carbon dioxide (CO2), six molecules of water (H2O), and 36 molecules of ATP. This can be represented by the following chemical equation: 6C6H12O6 + 6 O2 4 6 CO2 + 6 H2O + 36 ATP The process is much more complicated than this equation makes it appear: some two dozen separate chemical reactions are involved in the overall conversion of glucose to carbon dioxide, water, and ATP.

The Mechanics of Breathing All animals have some mechanism for removing oxygen from the air and transmitting it into the bloodstream, and this same mechanism typically is used to expel carbon dioxide from the bloodstream into the surrounding environment. Types of animal respiration, in order of complexity, include direct diffusion, diffusion into blood, tracheal respiration, respiration with gills, and finally, respiration through lungs. Microbes, fungi, and plants all obtain the oxygen they use for cellular respiration directly from the environment, meaning that there are no intermediate organs or bodily chemicals, such as lungs or blood. More complex organisms, such as sponges, jellyfish, and terrestrial (land) flatworms, all of which have blood, also breathe through direct diffusion. The latter term describes an exchange of oxygen and carbon dioxide directly between an organism, or its bloodstream, and the surrounding environment. More complex is the method of diffusion into blood whereby oxygen passes through a moist layer of cells on the body surface and then

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ONE

FORM OF RESPIRATION IS DIFFUSION INTO BLOOD, WHEREBY OXYGEN PASSES THROUGH A MOIST LAYER OF

CELLS ON THE BODY SURFACE, THEN THROUGH CAPILLARY WALLS AND INTO THE BLOODSTREAM, WHERE IT MOVES ON TO TISSUES AND CELLS.

AMONG

THE ORGANISMS THAT RELY ON DIFFUSION INTO BLOOD ARE ANNELIDS, A GROUP

THAT INCLUDES LEECHES. (© Lester V. Bergman/Corbis. Reproduced by permission.)

through capillary walls (capillaries are small blood vessels that form a network throughout the body) and into the bloodstream. Once oxygen is in the blood, it moves throughout the body to different tissues and cells. Among the organisms that rely on diffusion into blood are annelids, a group that includes earthworms, various marine worms, and leeches. In tracheal respiration air moves through openings in the body surface called spiracles. It then passes into special breathing tubes called tracheae that extend into the body. The tracheae divide into many small branches that are in contact with muscles and organs. In small insects, air simply moves into the tracheae, while in large insects, body movements assist tracheal air movement. Insects and terrestrial arthropods (land-based organisms with external skeletons) use this method of respiration.

humans and other higher animals. A remnant of this chapter from humans’ evolutionary history can be seen in the way that an embryo breathes in its mother’s womb, not by drawing in oxygen through its lungs but through gill-like mechanisms that disappear as the embryo develops. LU N G S . Lungs are composed of many small chambers or air sacs surrounded by blood capillaries. Thus, they work with the circulatory system, which transports oxygen from inhaled air to all tissues of the body and also transports carbon dioxide from body cells to the lungs to be exhaled. After air enters the lungs, oxygen moves into the bloodstream through the walls of these capillaries. It then passes from the lung capillaries to the different muscles and organs of the body.

Much more complicated than tracheae, gills are specialized tissues with many infoldings. Each gill is covered by a thin layer of cells and filled with blood capillaries. These capillaries take up oxygen dissolved in water and expel carbon dioxide dissolved in blood. Fish and other aquatic animals use gills, as did the early ancestors of

Although they are common to amphibians, reptiles, birds, and mammals, lungs differ enormously throughout the animal kingdom. Frogs, for instance, have balloon-like lungs that do not have a very large surface area. By contrast, if the entire surface of an adult male human’s lungs were spread flat, it would cover about 750 sq. ft. (70 m2), approximately the size of a handball court. The reason is that humans have about 300

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million gas-filled alveoli, tiny protrusions inside the lungs that greatly expand the surface area for gas exchange. Birds have specialized lungs that use a mechanism called crosscurrent exchange, which allows air to flow in one direction only, making for more efficient oxygen exchange. They have some eight thin-walled air sacs attached to their lungs, and when they inhale, air passes through a tube called the bronchus and enters posterior air sacs—that is, sacs located toward the rear. At the same time, air in the lungs moves forward to anterior air sacs, or ones located near the bird’s front. When the bird exhales, air from the rear air sacs moves to the outside environment, while air from the front moves into the lungs. This efficient system moves air forward through the lungs when the bird inhales and exhales and makes it possible for birds to fly at high altitudes, where the air has a low oxygen content. Humans and other mammals have lungs in which air moves in and out through the same pathway. This is true even of dolphins and whales, though they differ from humans in that they do not take in nutrition through the same opening. In fact, terrestrial mammals, such as the human, horse, or dog, are some of the only creatures that possess two large respiratory openings: one purely for breathing and smelling and the other for the intake of nutrients as well as air (i.e., oxygen in and carbon dioxide out).

REAL-LIFE A P P L I C AT I O N S Anaerobic Respiration Activity that involves oxygen is called aerobic; hence the term aerobic exercise, which refers to running, calisthenics, biking, or any other form of activity that increases the heart rate and breathing. Activity that does not involve oxygen intake is called anaerobic. Weightlifting, for instance, will increase the heart rate and rate of breathing if it is done intensely, but that is not its purpose and it does not depend on the intake and outflow of breath. For that reason, it is called an anaerobic exercise—though, obviously, a person has to keep breathing while doing it.

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holding their breath. A buildup of carbon dioxide and hydrogen ions (electrically charged atoms) in the bloodstream stimulates the breathing centers to become active, no matter what we try to do. On the other hand, if a person were underwater, the lungs would draw in water instead of air, and though water contains air, the drowning person would suffocate. A N A E R O B I C B A C T E R I A . Some creatures, however, do not need to breathe air but instead survive by anaerobic respiration. This is true primarily of some forms of bacteria, and indeed scientists believe that the first organisms to appear on Earth’s surface were anaerobic. Those organisms arose when Earth’s atmosphere contained very little oxygen, and as the composition of the atmosphere began to incorporate more oxygen over the course of many millions of years, new organisms evolved that were adapted to that condition.

The essay on paleontology discusses Earth’s early history, including the existence of anaerobic life before the formation of oxygen in the atmosphere. The appearance of oxygen is a result of plant life, which produces it as a byproduct of the conversion of carbon dioxide that takes place in photosynthesis. Plants, therefore, are technically anaerobic life-forms, though that term usually refers to types of bacteria that neither inhale nor exhale oxygen. Anaerobic bacteria still exist on Earth and serve humans in many ways. Some play a part in the production of foods, as in the process of fermentation. Other anaerobic bacteria have a role in the treatment of sewage. Living in an environment that would kill most creatures—and not just because of the lack of oxygen—they consume waste materials, breaking them down chemically into simpler compounds. HUMANS AND ANAEROBIC R E S P I RAT I O N . Even in creatures, such as

In fact, a person cannot consciously stop breathing for a prolonged period, and for this reason, people cannot kill themselves simply by

humans, that depend on aerobic respiration, anaerobic respiration can take place. Most cells are able to switch from aerobic to anaerobic respiration when necessary, but they generally are not able to continue producing energy by this process for very long. For example, a person who exercises vigorously may be burning up glucose faster than oxygen is being pumped to the cells, meaning that cellular respiration cannot take place quickly enough to supply all the energy the body needs. In that case, cells switch over to

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COMPUTERIZED MAIN BRONCHI,

TOMOGRAPHIC VIEW OF THE LUNGS, SHOWING THE TRACHEA

(CENTER)

SPLITTING TO FORM THE TWO

WHICH LEAD TO THE LUNGS. INSIDE THE LUNGS, MANY BRANCHING BRONCHI TERMINATE IN ALVEOLI,

AIR SACS WHERE GAS EXCHANGE TAKES PLACE. (© BSIP/Gems Europe/Photo Researchers. Reproduced by permission.)

anaerobic respiration, which results in the production of lactic acid, or C3H6O3. One advantage of anaerobic respiration is that it can take place very quickly and in short bursts, as opposed to aerobic respiration, which is designed for slower and steadier use of muscles. The disadvantage is that anaerobic respiration produces lactic acid, which, when it builds up in muscles that are overworked, causes soreness and may even lead to cramps. LAC T I C

AC I D

IN

THE

B O DY.

Eventually, the buildup of lactic acid is carried away in the bloodstream, and the lactic acid is converted to carbon dioxide and water vapor, both of which are exhaled. But if lactic acid levels in the bloodstream rise faster than the body can neutralize them, a state known as lactic acidosis may ensue. Lactic acidosis rarely happens in healthy people and, more often than not, is a result of the body’s inability to obtain sufficient oxygen, as occurs in heart attacks or carbon monoxide or cyanide poisoning or in the context of diseases such as diabetes.

often have sore muscles from lactic acid buildup, even though they may not exercise. Lactic acid also can lead to a buildup of uric acid crystals in the joints, in turn causing gout, a very painful disease. LAC T I C AC I D I N F O O D A N D I N D U S T RY. Lactic acid is certainly not with-

out its uses, and it is found throughout nature. When lactose, or milk sugar, is fermented by the action of certain bacteria, it causes milk to sour. The same process is used in the manufacture of yogurt, but the reaction is controlled carefully to ensure the production of a consumable product. Lactic acid also is applied by the dairy industry in making cheese. Molasses contains lactic acid, a product of the digestion of sugars by various species of bacteria, and lactic acid also is used in making pickles and sauerkraut, foods for which a sour taste is desired.

The ability of the body to metabolize lactic acid is diminished significantly by alcohol, which impairs the liver’s ability to carry out normal metabolic reactions. For this reason, alcoholics

A compound made from lactic acid is used as a food preservative, but the applications of lactic acid extend far beyond food production. Lactic acid is important as a starting material for making drugs in the pharmaceutical industry. Additionally, it is involved in the manufacturing of lacquers and inks; is used as a humectant, or moisturizer, in some cosmetics; is applied as a

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mordant, or a chemical that helps fabrics accept dyes, to textiles; and is employed in tanning leather.

Respiratory Disorders In almost any bodily system, there are bound to be disorders, or at least the chance that disorders may occur. This is particularly the case with something as complex as the respiratory system, because the more complex the system, the more things that can go wrong. Among the respiratory disorders that affect humans is a whole range of ailments from the common cold to emphysema, and from the flu to cystic fibrosis. T H E C O M M O N C O L D . Colds are among the most common conditions that affect the respiratory system, though what we call the common cold is actually an invasion by one of some 200 different types of virus. Thus, it is really not one ailment but 200, though these are virtually identical, but the large number of viral causative agents has made curing the cold an insurmountable task.

When you get a cold, viruses establish themselves on the mucus membrane that coats the respiratory passages that bring air to your lungs. If your immune system is unsuccessful in warding off this viral infection, the nasal passages become inflamed, swollen, and congested, making it difficult to breathe. Coughing is a reflex action whereby the body attempts to expel infected mucus or phlegm. It is essential to removing infected secretions from the body, but of course it plays no role in actually bringing a cold to an end. Nor do antibiotics, which are effective against bacteria but not viruses (see Infection). Only when the body builds up its own defense to the cold— assuming the sufferer has a normally functioning immune system—is the infection driven away. I N F LU E N Z A A N D A L L E R G I E S .

Influenza, a group of viral infections that can include swine flu, Asian flu, Hong Kong flu, and Victoria flu, is often far more serious than the common cold. A disease of the lungs, it is highly contagious, and can bring about fever, chills, weakness, and aches. In addition, influenza can be fatal: a flu epidemic in the aftermath of World War I, spread to far corners of the globe by returning soldiers, killed an estimated 20 million people.

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Respiratory ailments often take the form of allergies such as hay fever, symptoms of which include sneezing, runny nose, swollen nasal tissue, headaches, blocked sinuses, fever, and watery, irritated eyes. Hay fever is usually aggravated by the presence of pollen or ragweed in the air, as is common in the springtime. Other allergy-related respiratory conditions may be aggravated by dust in the air, and particularly by the feces of dust mites that live on dust particles. B R O N C H I A L A I L M E N T S . Allergic reactions can be treated by antihistamines (see The Immune System for more about allergies), but simple treatments are not available for such complex respiratory disorders as asthma, chronic bronchitis, and emphysema. All three are characterized by an involuntary constriction in the walls of the bronchial tubes (the two divisions of the trachea or windpipe that lead to the right and left lungs), which causes the tubes to close in such a way that it becomes difficult to breathe.

Emphysema can be brought on by cigarette smoking, and indeed some heavy smokers die from that ailment rather than from lung cancer. On the other hand, a person can contract a bronchial illness without engaging in smoking or any other activity for which the sufferer could ultimately be blamed. Indeed, small children may have asthma. One treatment for such disorders is the use of a bronchodilator, a medicine used to relax the muscles of the bronchial tubes. This may be administered as a mist through an inhaler, or given orally like other medicine. TUBERCULOSIS AND PNEUM O N I A . More severe is tuberculosis, an infec-

tious disease of the lungs caused by bacteria. Tuberculosis attacks the lungs, leading to a chronic infection with such symptoms as fatigue, loss of weight, night fevers and chills, and persistent coughing that brings up blood. Without treatment, it is likely to be fatal. Indeed, it was a significant cause of death until the introduction of antibiotics in the 1940s, and it has remained a problem in underdeveloped nations. Additionally, thanks to mutation in the bacteria themselves, strains of the disease are emerging that are highly resistant to antibiotics. Another life-threatening respiratory disease is pneumonia, an infection or inflammation of the lungs caused by bacteria, viruses, mycoplasma (microorganisms that show similarities to

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KEY TERMS AEROBIC:

Oxygen-breathing.

ANAEROBIC: ATP:

the form in which animals usually receive

Non-oxygen-breathing.

Adenosine triphosphate, an ener-

carbohydrates. Also known as dextrose, grape sugar, and corn sugar. An iron-containing

gy carrier formed when a simpler com-

HEMOGLOBIN:

pound, adenosine diphosphate (ADP),

protein in human red blood cells that is

combines with a phosphate group.

responsible for transporting oxygen to the

CAPILLARY:

A very small blood vessel.

Capillaries form networks throughout the body. CARBOHYDRATES:

Naturally occur-

ring compounds, consisting of carbon, hydrogen, and oxygen, whose primary

tissues and removing carbon dioxide from them. Hemoglobin is known for its deep red color. LYMPH:

The portion of the blood that

includes white blood cells and plasma but not red blood cells. Masses of tissue, at

function in the body is to supply energy.

LYMPH NODES:

Included in the carbohydrate group are

certain places in the body, that act as filters

sugars, starches, cellulose, and various

for blood.

other substances. Most carbohydrates are

METABOLISM:

produced by green plants in the process of

by which nutrients are broken down and

undergoing photosynthesis.

converted into energy or are used in the

The chemical process

A

construction of new tissue or other materi-

process that, when it takes place in the

al in the body. All metabolic reactions are

presence of oxygen, involves the intake of

either catabolic or anabolic.

CELLULAR

RESPIRATION:

organic substances, which are broken

MONOSACCHARIDE:

The simplest

down into carbon dioxide and water, with

type of carbohydrate. Monosaccharides,

the release of considerable energy.

which cannot be broken down chemically

CIRCULATORY SYSTEM:

The parts

of the body that work together to move blood and lymph. They include the heart, blood vessels, blood, and the lymphatic glands, such as the lymph nodes. COMPOUND:

A substance in which

atoms of more than one element are bond-

into simpler carbohydrates, also are known as simple sugars. NUTRIENT:

Materials

that

supply

energy or the materials to form new tissue for organisms. They include proteins, carbohydrates, lipids (fats), vitamins, and minerals.

ed chemically to one another. PHOSPHATE GROUP:

A group (that

A process, involving

is, a combination of atoms from two or

enzymes, in which a compound rich in

more elements that tend to bond with

energy is broken down into simpler sub-

other elements or compounds in certain

stances.

characteristic ways) involving a phosphate,

FERMENTATION:

GLUCOSE:

A monosaccharide (sugar)

that occurs widely in nature and which is

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or a chemical compound that contains oxygen bonded to phosphorus.

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KEY TERMS The biological

dioxide and water are converted to carbo-

tinction is made between external respiration, or an exchange of gases with the external environment, and internal respiration, an exchange of gases between the body’s cells and the blood.

hydrates and oxygen.

SIMPLE SUGAR:

PHOTOSYNTHESIS:

conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants. In this process carbon

RESPIRATION:

A term that can refer

either to cellular respiration (see definition) or, more commonly, to the process by which an organism takes in oxygen and releases carbon dioxide. Sometimes a dis-

both viruses and bacteria), and fungi, as well as such inorganic agents as inhaled dust or gases. Symptoms include pleurisy (chest pain), high fever, chills, severe coughing that brings up small amounts of mucus, sweating, blood in the sputum (saliva and mucus expelled from the lungs), and labored breathing. In 1936, pneumonia was the principal cause of death in the United States. Since then, it has been controlled by antibiotics, but as with tuberculosis, resistant strains of bacteria have developed, and therefore the number of cases has increased. Today, pneumonia and influenza combined are among the most significant causes of death in the United States (see Diseases). LU N G CA N C E R A N D CY S T I C F I B R O S I S . Respiratory ailments may also

take the form of lung cancer, which may or may not be a result of smoking. Cigarette smoking and air pollution are considered to among the most significant causes of lung cancer, yet people have been known to die of the disease without being smokers or having been exposed to significant pollution.

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CONTINUED

A monosaccharide, or simple carbohydrate. A group of cells, along with the substances that join them, that forms part of the structural materials in plants or animals.

TISSUE:

annually. No cure for cystic fibrosis exists, and the disease is invariably fatal, with only about 50% of sufferers surviving into their thirties. Lung complications are the leading cause of death from cystic fibrosis, and most symptoms of the disease are related to the sticky mucus that clogs the lungs and pancreas. People with cystic fibrosis have trouble breathing, and are highly susceptible to bacterial infections of the lungs. Coughing, while it may be irritating and painful if you have a cold, is necessary for the expulsion of infected mucus, but mucus in the lungs of a cystic fibrosis is too thick to be moved. This makes it easy for bacteria to inhabit the lungs and cause infection. WHERE TO LEARN MORE Bryan, Jenny. Breathing: The Respiratory System. New York: Dillon Press, 1993. Cellular Metabolism and Fermentation. Estrella Mountain Community College (Web site). .

One particularly serious variety of respiratory illness is cystic fibrosis, a genetic disorder that causes a thick mucus to build up in the respiratory system and in the pancreas, a digestive organ. (For more about genetic disorders, see Heredity; for more on role of the pancreas, see Digestion.) In the United States, the disease affects about one in every 3,900 babies born

Kimball, Jim. “The Human Respiratory System.” Kimball’s Biology Pages (Web site). .

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Levesque, Mireille, Letitia Fralick, and Joni McDowell. “Respiration in Water: An Overview of Gills.” University of New Brunswick (Web site). .

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Llamas, Andreu. Respiration and Circulation. Milwaukee: Gareth Stevens, 1998. Paustian, Timothy. Anaerobic Respiration. Department of Bacteriology, University of Wisconsin–Madison (Web site). .

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Roca, Núria, and Marta Serrano. The Respiratory System, the Breath of Life. Illus. Antonio Tenllado. New York: Chelsea House Publishers, 1995.

Respiration

Silverstein, Alvin, and Virginia B. Silverstein. The Respiratory System. New York: Twenty-First Century Books, 1994.

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NUTRITION FOOD WEBS NUTRIENTS AND NUTRITION V I TA M I N S

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CONCEPT The idea of a food chain is common in everyday life, so much so that it has become a metaphor applied in many situations. A high achiever in business or other endeavors is said to be “at the top of the food chain,” and images of big fish eating little fish abound in cartoons. Yet in the study of the biological sciences, the concept of food chains is part of the much larger idea of a food web. Whereas a food chain is a linear series of organisms dependent on each other for food, a food web is an interconnected set of food chains in the same ecosystem. Food webs make possible the transfer of energy from plants through herbivores to carnivores and omnivores, and ultimately to the detritivores and decomposers that enrich the soil with organic waste. Just as a food web can transfer materials essential to the life of organisms, it is also a devastatingly efficient conduit for the transfer of poisons.

HOW IT WORKS Food Webs in Context An ecosystem is a community of independent organisms along with the inorganic components (chiefly soil, water, air, and rocks) that make up their environment. A biome is a large ecosystem, characterized by its dominant life-forms—for example, the Amazonian rain forest.

FOOD WEBS

whereas the term biota typically refers only to plant and animal life within a biological community or ecosystem. (For more on these subjects, see Ecosystems and Ecology and Biological Communities.)

Trophic Levels The organisms in a biological community are linked in their need to obtain energy from food, which derives from the Sun through plant life. (There are, however, some communities, in areas such as deep-ocean rifts, that are not dependent on sunlight at all.) The Sun’s energy is electromagnetic and travels in the form of radiation, which Earth receives as light and heat. Plants, known as primary producers, convert this electromagnetic energy into chemical energy through the process of photosynthesis. The plants are eaten by herbivores (planteating animals), known also as primary consumers, examples of which include squirrels, rabbits, mice, deer, cows, horses, sheep, and seedeating birds. These creatures, in turn, are eaten by secondary consumers, which are either carnivores, which are creatures that eat only meat, or omnivores—creatures, such as humans, that eat meat and plants.

That portion of an ecosystem composed only of living things, as opposed to the formerly living or never living components, is known as a biological community. This community includes creatures from all five kingdoms of living organisms (including bacteria, algae, and fungi),

There may even be tertiary, or third-level, consumers. These are animals that eat secondary consumers; examples are mountain lions and hawks, both of which eat such second-order consumers as snakes and owls. Human societies that eat dogs or cats, as well as those that engage in cannibalism, also behave as tertiary consumers. (See Biological Communities for a biological explanation of what otherwise is considered an abhorrent and immoral practice—not to men-

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tion a dangerous one, due to the risk of such diseases as kuru, a type of spongiform encephalopathy.) In any case, the further along in the chain of trophic levels or stages of the food web, the fewer consumers there are. E N E R GY A N D T R O P H I C L E V E L S . It is fairly obvious that when a creature is

“higher on the food chain” (to use the common expression), it has fewer natural predators. The reason for this is that at each successive trophic level, there are simply fewer organisms; this, in turn, is due to the fact that the energy available to each level is progressively smaller, and the organisms themselves progressively larger. This, in turn, stems from one of the most intriguing, maddening concepts in the entire universe: the second law of thermodynamics, which we discuss shortly. Because of the diminishing number of organisms at each trophic level, the food web often is depicted as a pyramid, a concept we explore further later in this essay. The number of organisms begins to increase again at the next trophic level beyond secondary or tertiary consumers, that of decomposers. Large omnivores and carnivores may not be prey for other creatures in life, but everything dies eventually, and anything that has ever lived is food for detritivores, or organisms that feed on waste matter. DETRITIVORES AND DECOMP O S E R S . Detritivores, which range in size

and complexity from maggots to vultures, may not be the most appealing creatures on Earth, but without them life itself would suffer. By consuming the remains of formerly living things, they break organic material down into inorganic substances. In other words, their internal systems chemically process compounds containing the element carbon in characteristic structures. They then release that carbon into the atmosphere and soil in such a way that what remains is inorganic material that enriches the soil for the growth of new plant life. But detritivores are not the last stop on the food web. The final trophic level, before the cycle comes back around to plants, contains the largest number of organisms in the entire food web— perhaps billions and billions, even in a space smaller than a coffee cup. These are decomposers, or organisms that, as with detritivores, obtain their energy from the chemical breakdown of dead organisms. The decomposers,

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however, break down the nutrients in decayed organic matter to a far greater extent than do detritivores. Typically, decomposers are microorganisms, including bacteria and fungi, and they process materials in such a way that complex compounds undergo the chemical reaction of decomposition. Through decomposition, compounds are broken down into simpler forms, or even into their constituent elements, which provide the environment with nutrients necessary to the growth of more plant life. CAT E G O R I Z I N G T H E T R O P H I C L E V E L S . The organisms in the food web can

be viewed in three groups: producers (plants), consumers (primary- and secondary-consuming animals, whether herbivores, carnivores, or omnivores), and decomposers (that is, both detritivores and true decomposers). Producers and consumers are part of a larger structure known as the grazing food web, in which food is “on its way up the food chain,” as it were. Decomposers and detritivores make up the decomposer food web, which brings food back “down” to the soil. Producers also are called autotrophs, from Greek roots meaning “self-feeders,” because they are not dependent on other organisms as a source of energy. Beyond the level of the primary producers, all consumers are known as heterotrophs, or “other-feeders.” These creatures feed on other organisms to obtain their energy and are classified according to the types of food they eat—herbivores, carnivores, omnivores, as we already have discussed. Detritivores and decomposers also are considered heterotrophs.

Organisms and Energy Rather than depending on other organisms for energy, autotrophs obtain energy from the Sun and carbon dioxide from the atmosphere. From these components, they build the large organic molecules that they need to survive. Green plants do this through the process of photosynthesis, a chemical reaction that can be represented as follows: solar energy + carbon dioxide (CO2) + water (H2O) 4 glucose (sugar: C6H12O6) + oxygen (O2) Actually, in order to produce what chemists call a balanced equation, it would be necessary to

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show this equation as a reaction between the energy and six molecules each of carbon dioxide and water, which would produce a glucose molecule and six oxygen molecules. In any case, what we have described here is an amazing thing and one of the great wonders of nature. Sunlight aids plants in converting carbon dioxide, which they receive from the respiration of animals, along with water (which also may come from animal respiration, though this is not necessarily the case), into a sugar molecule for the plant’s sustenance. Furthermore, oxygen, essential to the life of virtually all animals, also is produced—yet from the standpoint of the plant, it is simply a waste by-product! T H E S E C O N D LAW O F T H E R MODYNAMICS. The productivity of

plants, which is measured in terms of biomass (the combined mass of all organisms at a particular trophic level in a food web), determines the amount of “fixed,” or usable, energy available to other trophic levels on the food web. The amount of energy available always will be less for each successive trophic level, through the point where consumers end and decomposers begin—that is, through the level of the secondary or perhaps tertiary consumer. If there is any scientific equivalent of the curse in the Garden of Eden (the punishment for the sins of Adam and Eve, according to JudeoChristian belief), it is the second law of thermodynamics. Just as the expulsion from Paradise in the biblical story ensured that life would be much more difficult for humans than it would have been in Eden, so the second law thwarts all ambitions toward transcending the limits of physical reality. The first law of thermodynamics states that it is impossible to obtain more energy from a system than is put into it. Thus, for instance, a car will go only as far as is allowed by the amount of energy that is pumped into its tank. The first law, discovered in the mid–nineteenth century, effectively ruled out any hopes of a perpetual-motion machine, but the second law, derived a few decades later, delivered even worse news.

pated in the form of heat and sound, as a natural by-product of operating the engine. Even without running an air conditioner or other energyconsuming device, only about 30% of the energy from the gas goes to turning the wheels. THE

E C O LO G I CA L

Food Webs

P Y RA M I D .

What this means for the food web is that there is bound to be a loss of energy in the transfer from one trophic level to another. Organisms never manage to retrieve 100% of the energy from the materials they eat; in fact, the figure is more like 10%. A rabbit that eats a carrot gets only about 10% of the energy in it, and an owl that eats the rabbit gets only about 10% of the energy from the rabbit, or 1% of the energy in the carrot. Because of these diminishing returns, there are always fewer organisms at each successive trophic level on the grazing food web. This fact is expressed in a model known as the ecological pyramid, or energy pyramid, which shows that as the amount of total energy decreases with each trophic level, so does the biomass. As a result, it may take 1,000 carrots to support 100 rabbits, 10 owls, and one hawk. The picture changes as the shift is made from the grazing web to the decomposer web. Detritivores and decomposers are extraordinarily efficient feeders, reworking detritus over and over and extracting more fixed energy as they do. Eventually, they break the waste down into simple inorganic chemicals, which, as we have noted, then may be reused by the primary producers. The number of organisms in the decomposing food web dwarfs that of all others combined, though decomposers themselves are very small, and their combined population takes up very little physical space.

REAL-LIFE A P P L I C AT I O N S Keystone Species

Though it can be stated in a number of ways, the second law essentially means that it is impossible to extract as much energy from a system as one puts into it. Thus, in the case of an automobile, most of the energy contained in the gas does not go toward moving the car; rather, it is dissi-

The keystone in an archway is a wedge-shaped stone at the top of the arch. It’s position is extraordinarily important: if the keystone is removed, the arch will collapse. Thus, the keystone has become an often-used metaphor in other circumstances, as, for instance, in the nickname of Pennsylvania, the “Keystone State.” In the realm of ecology, the term keystone species refers to those organisms that, like a keystone in

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Food Webs

THE

TERM KEYSTONE SPECIES REFERS TO AN ORGANISM THAT PLAYS A CRITICAL ROLE IN ITS ENVIRONMENT, ONE

THAT MAY BECOME APPARENT ONLY ONCE IT IS REMOVED FROM AN ECOSYSTEM.

AMERICA,

ON

THE WEST COAST OF

NORTH

FOR INSTANCE, REMOVAL OF A CERTAIN SPECIES OF STARFISH CAUSED A RAPID GROWTH IN THE NUMBERS

AND BIOMASS OF THE MUSSEL UPON WHICH THE STARFISH FED. (© Stuart Westmorland/Corbis. Reproduced by permission.)

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architecture or engineering, play a critical role in their environments.

far out of proportion to its relative biomass or

Within a food web, for instance, a keystone species can have a powerful influence, one that is

predator (that is, a large secondary or even terti-

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productivity. Typically, a keystone species is a top ary consumer), though occasionally an herbivore

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can occupy the keystone position. Often, the role of the keystone species becomes apparent only once it is removed, either experimentally or by natural forces, from an ecosystem. S TA R F I S H A N D M U SS E L S . In temperate ecosystems on the west coast of North America, for instance, removal of a certain species of starfish (Pisaster ochraceous) was found to cause a rapid growth in the numbers and biomass of the mussel Mytilus californianus. The latter then forced out other species and proceeded to dominate the biological community. As it turned out, the starfish acted as a keystone predator by consuming these mussels.

Specifically, the starfish prevented the mussel from gaining dominance that it otherwise would have gained, owing to its competitive superiority in relation to other species within this particular coastal ecosystem. Yet the starfish could not eliminate the mussel, because it was incapable of feeding on larger individuals of that species. The result was that the community enjoyed a much greater degree of diversity and complexity than it would have if the mussel had been allowed to dominate. SEA OTTERS AND KELP F O R E S T S . Another keystone species in a

geographic area close to that of the starfish we have just described is the sea otter, native to western North America. Its principal food source is the sea urchin, an herbivore that, in turn, survives by consuming kelp, a large form of algae. By controlling the numbers and densities of sea urchins, sea otters allowed kelp to retain a relatively large biomass within the community, thus facilitating the growth of “kelp forests.” When humans began hunting sea otters for their fur during the late eighteenth and into the nineteenth centuries, however, the ecological effect soon was felt in the form of declining kelp forests. Fortunately, hunting did not render the species extinct, and since the 1930s, sea otters have been colonizing many of their former habitats. This colonization has resulted in a corresponding increase in the density of surrounding kelp forests.

Indicator Species A concept similar to that of the keystone species is the idea of an indicator species: plants or animals that, by their presence, abundance, or chemical composition, demonstrate a distinctive

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aspect of the character or quality of the environment. For instance, in an ecosystem affected by pollution, examination of indicator plant species may reveal the pollution patterns. By their presence, indicator species also may serve to show the quality or integrity of an ecosystem. Such is the case, for instance, with the spotted owl, or Strix occidentalis, and other species that depend on old-growth forests. (See Succession and Climax for more on this subject.) Because the needs of these species are so particular, their presence or absence can illustrate the health or lack thereof of the biome in question.

Food Webs

Other indicator plants also can be used to determine the presence of valuable mineral deposits in the soil, because those minerals make their way into the tissues of the plants themselves. Nickel concentrations as great as 10% have been found in the tissues of Russian plants from the mustard family, and a mintlike species called Becium homblei has proved useful for locating copper deposits in parts of Africa. Since the plant can tolerate more than 7% copper in soil (a great amount and many times the percentage of copper in the human body, for instance), it can and does live near enormous copper deposits. I N D I CAT I N G T OX I N S . Some plants can serve as indicators of serpentine minerals, varieties of compounds that can be toxic in large concentrations. In California, for instance, where serpentine soils are not uncommon, there plant species unique to specific ecosystems high in serpentine mineral content. Elsewhere, there are types of lichens that are sensitive to toxic gases, such as sulfur dioxide, and thus these lichens can be monitored as a way of keeping tabs on air pollution.

In semiarid regions where soils contain large quantities of the element selenium, plants can accumulate such large concentrations of the element that they become poisonous to primary consumers (for example, rabbits) who eat them. The result may be temporary or even permanent blindness. In such situations, legumes of the genus Astragalus, which can accumulate as much as 15,000 ppm (parts per million)—a comparatively enormous concentration—serve as indicators. Their heavy selenium concentration gives them a noticeably unpleasant smell. Aquatic invertebrates and fish often have been surveyed for what they can show as to the quality of water and the health of aquatic ecosys-

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BEACHGOERS

ARE SPRAYED WITH

TICIDE IN 1945.

ONCE

DDT

IN THE FIRST PUBLIC TEST OF A NEW MACHINE FOR DISTRIBUTING THE INSEC-

PESTICIDES SUCH AS

DDT

HAVE BEEN SPRAYED IN A REGION, RAIN CAN WASH THEM INTO

CREEKS, LAKES, AND OTHER BODIES OF WATER, WHERE THEY ARE ABSORBED BY CREATURES THAT EITHER DRINK OR SWIM IN THE WATER AND THUS ENTER THE FOOD WEB. (© Bettmann/Corbis. Reproduced by permission.)

tems. The presence of a creature known by the disgusting name sewage worm, or tubificid worm (Tubificidae), indicates the degradation of water quality by the introduction of sewage or other outside organic matter that consumes oxygen. Tubificid worms are virtually anaerobic, or non-oxygen-breathing, unlike the animals that would occupy a similar niche in a healthy aquatic environment.

Bioaccumulation and Biomagnification The subject of indicator species leads us, naturally enough, to the topic of pollution in the food web, which can be seen on a small scale in the phenomenon of bioaccumulation and which manifests on a much broader scale as biomagnification. One of the key concepts in ecological studies is the idea that a disturbance in one area can lead to serious consequences elsewhere. The interconnectedness of components in the environment thus makes it impossible for any event or phenomenon to be truly isolated.

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buildup of toxins, and particularly chemical pollutants, in the tissues of individual organisms. Part of the danger in these toxins is the fact that the organism cannot easily process them either by metabolizing them (i.e., incorporating them into the metabolic system, as one does food or water) or by excreting them through urine or other substances produced by the body. The only way for the organism to release toxins, in fact, is by passing them on to other members of the food web. Because organisms at each successive trophic level must consume more biomass to meet their energy requirements, they experience an increase in contamination, a phenomenon known as biomagnification. The following example illustrates how toxins enter the food web and gradually make their way down the line, growing in proportion as they do.

Nowhere is this principle better illustrated that in the processes of bioaccumulation and biomagnification. The first of these is the

Particles of pollutant may stick to algae, for instance, which are so small that the toxin does little damage at this level of the food web. But even a small herbivore, such as a zooplankton, takes in larger quantities of the pollutant when it consumes the algae, and so begins the cycle of biomagnification. By the time the toxin has passed from a zooplankton to a small fish, the

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amount of pollutant in a single organism might be 100 times what it was at the level of the algae. The reason, again, is that the fish can consume 10 zooplankton that each has consumed 10 algae. These particular numbers, of course, are used simply for the sake of convenience, as were those cited earlier in relation to the ecological pyramid. Note, incidentally, the similarity of the relationships between this “pyramid of poison” and the ecological pyramid, whereby energy, which is beneficial, passes between trophic levels. The higher the trophic level, the smaller the amounts of energy that the organism extracts from its food—but the higher the amount of toxic content. By the time the toxins have passed on to a few more levels in the food web, they might be appearing in concentrations as great as 10,000 times their original amount. DDT BIOMAGNIFICATION. Among the most prominent examples of chemical pollutants that are bioaccumulated are such pesticides as DDT (dichlorodiphenyltrichloroethane). DDT is an insecticide of the hydrocarbon family, a large group of chemical compounds of which the many varieties of petroleum are examples. Because it is based in hydrocarbons, DDT is hydrophobic (“water-fearing”) and instead mixes with oils—including the fat of organisms.

In the twenty or so years leading up to 1972, Americans used vast amounts of DDT for the purpose of controlling mosquitoes and other pests. DDT appeared to be a remarkably successful killer, and in fact it turned out to be a little too successful. As it found its way into water sources, DDT entered the bodies of fish, and then those of predatory birds such as osprey, peregrine falcons, brown pelicans, and even the bald eagle. The detrimental effect of DDT on America’s national symbol, a bird protected by law since 1940, aptly illustrates the ravages exacted by this powerful insecticide. DDT levels in birds became so high that the birds’ eggshells were abnormally thin, and adult birds sitting on the nest would accidentally break the shells of unhatched chicks. As a result, baby birds died, and populations of these species dwindled. Environmentalists in the late 1960s and early 1970s raised public awareness of this phenomenon, and this led to the banning of DDT spraying in 1972. The period since that time has seen dramatic increases in bird populations.

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T H E H U M A N FAC T O R. Because the species of bird affected were not ones that people normally consume for food, DDT biomagnification did not have a wide-ranging effect on human populations. However, tests showed that some DDT had made its way into the fat deposits of some members of the population. In any case, bioaccumulation and biomagnification have threatened humans, for instance in the late 1940s and early 1950s, when cows fed on grass that had been exposed to nuclear radiation, and this radioactive material found its way into milk.

Food Webs

Traces of the radioactive isotope strontium90, a by-product of nuclear weapons testing in the atmosphere from the late 1940s onward, fell to earth in a fine powder that coated the grass. Later the cows ate the grass, and strontium-90 wound up in the milk they produced. Because of its similarities to calcium, the isotope became incorporated into the teeth and gums of children who drank the milk, posing health concerns that helped bring an end to atmospheric testing in the early 1960s. Humans themselves can serve as repositories for contaminants, a particular concern for a mother nursing her baby. Assuming the mother’s own system has been contaminated by toxins, it is likely that her milk contains traces of the harmful chemical, which will be passed on to her child. Obviously, this is a very serious matter. Nursing mothers, babies, and their loved ones are not the only people affected by bioaccumulation and the storing of toxins in fatty tissues. In fact, some physicians and nutritionists maintain that one of the reasons for the buildup of fat on a person’s body (though certainly not the only reason) may be as a response to toxins, the idea being that the body produces fat cells as a means of keeping the toxins away from the bloodstream. Another case of large-scale bioaccumulation occurred during the 1970s and 1980s, when fish such as tuna were found to contain bioaccumulated levels of mercury. In the face of such concerns, governments have intervened in several ways, including the banning of DDT spraying, as mentioned earlier. The U.S. federal government and the governments of some states have issued warnings against the consumption of certain types of fish, owing to bioaccumulated levels of toxic pollutants. Bioaccumulation is particularly serious in the case of species that live a long time,

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KEY TERMS Primary producers.

insects, viruses, single-cell organisms, and

Autotroph means “self-feeder,” and these

so on—as well as all formerly living things

organisms are distinguished by the fact that

that have not yet decomposed.

AUTOTROPHS:

they do not depend on other organisms as

BIOTA:

A combination of all flora and

a source of energy. Instead, they obtain

fauna (plant and animal life, respectively)

energy from the Sun and carbon dioxide

in a region.

from the atmosphere, and from these constituents they build the large organic molecules that they need to survive. BIOACCUMULATION:

A meat-eating organ-

CARNIVORE:

ism, or an organism that eats only meat (as distinguished from an omnivore).

The buildup of

toxic chemical pollutants in the fatty tissues of organisms.

DECOMPOSER FOOD WEB:

portion of the food web occupied by detritivores and decomposers. (Compare with

BIOGEOCHEMICAL CYCLES:

The

grazing food web.)

changes that particular elements undergo as they pass back and forth through the various earth systems (e.g., the biosphere) and particularly between living and nonliving matter. The elements involved in biogeochemical cycles are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur.

DECOMPOSERS:

The liv-

ing components of an ecosystem. BIOMAGNIFICATION:

The increase in

bioaccumulated contamination at higher

consume larger quantities of food—and, hence, in the case of polluted materials, more toxins.

that

breakdown of dead organisms as well as from animal and plant waste products. The principal forms of decomposer are bacteria and fungi. REACTION:

A

chemical reaction in which a compound is broken down into simpler compounds, or into its constituent elements. In the biosphere, this often is achieved through the help of detritivores and decomposers.

levels of the food web. Biomagnification results from the fact that larger organisms

Organisms

obtain their energy from the chemical

DECOMPOSITION

BIOLOGICAL COMMUNITY:

DETRITIVORES:

Organisms that feed

on waste matter, breaking organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Their

The combined mass of all

function is similar to that of decomposers;

organisms at a particular trophic level in a

however, unlike decomposers—which tend

food web.

to be bacteria or fungi—detritivores are

BIOMASS:

BIOME:

A large ecosystem, character-

ized by its dominant life-forms. BIOSPHERE:

74

That

A combination of all liv-

relatively complex organisms, such as earthworms or maggots. ECOLOGY:

The study of the relation-

ing things on Earth—plants, mammals,

ships between organisms and their envi-

birds, amphibians, reptiles, aquatic life,

ronments.

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Food Webs

KEY TERMS

CONTINUED

A community of inter-

types of food they eat. Thus, they are

dependent organisms along with the inor-

known as herbivores, carnivores, and so

ganic components of their environment.

on.

ECOSYSTEM:

ENERGY TRANSFER:

The flow of

A plant or

INDICATOR SPECIES:

energy between organisms in a food web.

animal that, by its presence, abundance, or

FIRST LAW OF THERMODYNAMICS:

chemical composition, demonstrates a

A law of physics stating that the amount of

particular aspect of the character or quali-

energy in a system remains constant, and

ty of the environment.

therefore it is impossible to perform work

KEYSTONE SPECIES:

that results in an energy output greater

plays a crucial role in the functioning of its

than the energy input.

ecosystem or that has a disproportionate

FOOD CHAIN:

A series of singular

A species that

influence on the structure of its ecosystem.

organisms in which each plant or animal

NICHE:

depends on the organism that precedes it.

that a particular organism plays within its

Food chains rarely exist in nature; there-

biological community.

fore, scientists prefer the term food web. FOOD WEB:

A term describing the

interaction of plants, herbivores, carnivores, omnivores, decomposers, and detritivores in an ecosystem. Each of these organisms consumes nutrients and passes them along to other organisms (or, in the case of the decomposer food web, to the soil and environment). The food web may be thought of as a bundle or network of food chains, but since the latter rarely exist

A term referring to the role

An organism that eats

OMNIVORE:

both plants and other animals. ORGANIC:

At one time chemists used

the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals) and oxides, such as carbon dioxide. PHOTOSYNTHESIS:

The biological

conversion of light energy (that is, electro-

separately, scientists prefer the concept of a

magnetic energy) from the Sun to chemical

food web to that of a food chain.

energy in plants.

GRAZING FOOD WEB:

That portion

PRIMARY

CONSUMERS:

Animals

of the food web occupied by autotrophs,

that eat green plants. (Compare with sec-

herbivores, carnivores, and omnivores.

ondary consumers.)

(Compare with decomposer food web.) HERBIVORE:

A plant-eating organ-

PRODUCERS:

Green

plants that depend on photosynthesis for their nourishment.

ism. HETEROTROPHS:

PRIMARY

Secondary con-

SECONDARY CONSUMERS:

Ani-

sumers, or “other-feeders.” These creatures

mals that eat other animals.

feed on other organisms to obtain their

SECOND LAW OF THERMODYNAM-

energy and are classified according to the

ICS:

S C I E N C E O F E V E RY DAY T H I N G S

A law of physics, which can be stat-

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Food Webs

KEY TERMS ed in several ways, all of which mean the same thing. According to one version of the second law, it is impossible to transfer energy with perfect efficiency, because some energy always will be lost in the transfer. This is the same as saying that it is impossi-

because a longer life allows for much longer periods of bioaccumulation. For this reason, some governments warn against consuming fish over a certain age or size: the older and larger the creature, the more contaminated it is likely to be. WHERE TO LEARN MORE A to Z of Food Chains and Webs (Web site). . Bioaccumulation and Biomagnification (Web site). . Busch, Phyllis S. Dining on a Sunbeam: Food Chains and Food Webs. Photos Les Line. New York: Four Winds Press, 1973.

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CONTINUED

ble to extract from a system the same amount of energy that was put into it. Various stages within a food web. For instance, plants are on one trophic level, herbivores on another, and so on. TROPHIC

LEVELS:

Extension Toxicology Network (EXTOXNET): Toxicology Information Briefs (Web site). . Food Chains and Food Webs: An Introduction (Web site). . Food Webs: Build Your Own (Web site). . Fox, Nicols. Spoiled: The Dangerous Truth About a Food Chain Gone Haywire. New York: Basic Books, 1997. Introduction to Biogeography and Ecology: Trophic Pyramids and Food Webs. Fundamentals of Physical Geography (Web site). <www.geog.ouc.bc.ca/ physgeog/contents/9o.html>. Pimm, Stuart L. Food Webs. New York: Chapman and Hall, 1982. Wallace, Holly. Food Chains and Webs. Chicago: Heinemann Library, 2001.

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Nutrients and Nutrition

NUTRIENTS AND NUTRITION

CONCEPT In the modern world people are accustomed to hearing a great deal about nutrients and nutrition. Words such as protein, carbohydrate, vitamins, minerals, and fats are a regular part of daily life, yet few people who talk about these nutrients really know what they are. In fact, these are the basic building blocks of nutrition, whereby animal life is sustained. Whereas plants can get their energy directly from the Sun and the atmosphere, animals (including humans) depend on other organisms to provide them with nutrition. These other organisms include plants, which generate carbohydrates as a result of photosynthesis, as well as other animals that eat plants and thereby build proteins and fats. Plants also may contain proteins and fats, and both plants and animals contain vitamins and minerals. These nutrients, consumed in the proper forms and proportions, sustain life and prevent the miseries of malnutrition—a condition that can involve either undernourishment or overnourishment.

use of them for its survival, growth, and development. The term nutrition also can refer to the study of nutrients, their consumption, and their processing in the bodies of organisms. Here the general term organism has been used, but for the most part the present essay is concerned with animal nutrition, or at least the nutrition of primary consumers (animals that eat plants) and secondary consumers (animals that eat other animals). AUTOTROPHS AND THEIR N U T R I E N T S . By contrast, plants and a few

other types of organism are autotrophs, or primary producers in the food web. Autotroph means “self-feeder,” and these organisms are distinguished by the fact that they do not depend on other organisms as a source of energy. Instead, plants obtain energy from the Sun and carbon dioxide from the atmosphere, and from these materials they build the large organic molecules that they need to survive.

Nutrition itself is the series of processes by which an organism takes in nutrients and makes

Though plants are the most obvious example of an autotroph, they are not the only ones. In the deep oceans, far from any plant life, primary consumers depend on phytoplankton, which are microscopic organisms that encompass a range of bacteria and algae. Nonplant autotrophs may use means different from those employed by plants in generating their own food. For example, there are certain nonplant autotrophic organisms that live in the deep oceans near hydrothermal vents, which are cracks in the ocean floor caused by volcanic activity. These organisms, unlike most autotrophs, do not need sunlight to survive. Instead, they build their own nutrients in a sunless world, using sulfur compounds found near the vents.

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HOW IT WORKS Nutrients and Nutrition In order to live, animals must consume nutrients, of which there are five major classes: carbohydrates, proteins, lipids or fats, vitamins, and minerals. In addition to these constituents, of course, animal life requires other materials for its sustenance-water, oxygen, and fiber, which aids in the digestive processing of foods-but these components usually are not regarded as nutrients.

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Nutrients and Nutrition

Chemical Elements and Nutrition An element is a chemical substance made of only one kind of atom, whereas in a compound, atoms of more than one element are chemically bonded to one another. Unlike compounds, elements cannot be broken chemically into other substances. There are approximately 90 elements that occur in nature, and many of these elements—but not nearly all—are important to nutrition. ELEMENTS IN THE HUMAN BODY AND BIOGEOCHEMICAL CYC L E S . Even when we rule out obviously

harmful elements, such as lead or uranium, there are still numerous chemical elements that play a part in the nutrition of living things. This can be illustrated by a glance at the abundance of various chemical elements in the human body, which include oxygen, carbon, and hydrogen. Oxygen alone accounts for a whopping 65% of the human body’s mass, and carbon (18%), hydrogen (10%), and oxygen together make up 93% of the mass in the human body. A great deal of oxygen and hydrogen, of course, is found in that most useful of all chemical compounds, water. In this vein, it should be noted that all the elements that take part in biogeochemical cycles, which are essential to the functioning of Earth, appear in relatively large proportions within the human body. These elements are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur. (For more about biogeochemical cycles and the elements involved in them, including their proportion within the human body’s mass, see The Biosphere.) Carbon is present in all living things, and its presence in certain forms is key to distinguishing organic from inorganic substances. Contrary to popular belief, organic substances are not just living things, their parts, and their products. Something that has never been living still can be considered organic, provided that it contains compounds that include carbon. (The only exceptions would be carbonates and carbon oxides, two groups of carbon-based compounds that are excluded from the ranks of organic substances.) As we shall see, carbon, along with oxygen and hydrogen, plays a key role in nutrition.

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biogeochemical cycles, whose names are italicized: nitrogen (3%), calcium (1.4%), phosphorus (1.0%), magnesium (0.50%), potassium (0.34%), sulfur (0.26%), sodium (0.14%), chlorine (0.14%), iron (0.004%), and zinc (0.003%). Note that many of these elements are found in vitamin and mineral supplements that people might take on a daily basis to augment the essential nutrients in their bodies. There are exceptions, however, such as sodium, of which most people already ingest too much in the form of salt. T RAC E ELEMENTS. Generally speaking, it is safe to assume that any element that appears naturally in the human body is healthful as a nutritional component. This rule of thumb goes only so far, however: chlorine, for instance, is poisonous in large quantities, whereas in the very small proportions found in the human body, it can be essential to health and well-being. It is certainly possible to ingest some elements in unhealthy quantities, a fact that is particularly true of trace elements.

Copper is an example of a trace element, so named because only traces of them are present in the human body. In tiny quantities, copper is beneficial to human health, but if that small amount is exceeded, the effects can run the gamut from sneezing to diarrhea. In the proper proportions, however, trace elements are essential: without enough iodine, for instance, goiter, a large swelling of the thyroid gland in the neck area, can develop. Chromium helps the body metabolize sugars, which is why people concerned with losing weight or toning their bodies through exercise may take a chromium supplement. Even arsenic, which is lethal in large quantities, is a trace element in the human body, and medicines for treating such illnesses as “sleeping sickness” contain tiny amounts of arsenic. Other trace elements include cobalt, fluorine, manganese, molybdenum, nickel, selenium, silicon, and vanadium.

REAL-LIFE A P P L I C AT I O N S Nutrients

Most of the remaining 7% of the body’s mass is composed of ten other elements. Among these elements are the other three involved in

If you glance at the side of a cereal box, or virtually any other food product manufactured in the United States, chances are that you will see a table

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WHEN

A PERSON INGESTS MORE CARBOHYDRATES THAN THE BODY NEEDS, THE BODY CONVERTS THE EXCESS INTO

A COMPOUND KNOWN AS GLYCOGEN. IT THEN STORES THE GLYCOGEN IN THE LIVER (SHOWN HERE) AND MUSCLE TISSUES, WHERE IT REMAINS, A POTENTIAL SOURCE OF ENERGY FOR THE FUTURE. (© Lester V. Bergman/Corbis. Reproduced by

permission.)

listing proportions of nutrients per serving. This is the “Nutrition Facts” label, which replaced the old “Nutrition Information Per Serving” label in the early 1990s. Both forms of information label were administered by the United States Food and Drug Administration (FDA), with the newer labeling format being far more extensive in terms of the information it provides. Usually these tables show the amount of nutrients both in terms of mass (usually rendered in metric components, such as grams or milligrams) and as a proportion of recommended daily value according to the FDA. These listings must include information about some components such as calories, fat, sodium, sugars, certain vitamins, and so on. In addition to these mandatory listings, labels may contain information that the manufacturer chooses to provide concerning other food components, such as potassium or insoluble fiber.

have introduced offerings that represent a nod to nutritional concerns. These include products that are fat- or sodium-free, or are otherwise geared toward greater health consciousness. Clearly, diet is a significant concern to Americans, the most well-fed group of people that has ever existed, and terms from the Nutrition Facts label—proteins, carbohydrates, fats, minerals, and vitamins—are household words, known to almost everyone but understood by only a few. In much of the remainder of this essay, we explore these concepts, discussing what they mean in very basic scientific terms, as well as in terms of their significance in the diets of humans and other animals.

Today, even fast-food restaurants such as McDonald’s, which is probably not the first name that comes to mind when one thinks of healthy eating, provide extensive nutritional information to customers. Additionally, makers of fast food or supermarket “junk food” such as potato chips

P R O T E I N S . Proteins are large molecules built from long chains of amino acids, which are organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations. Proteins serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body’s immune system, and making enzymes. (An enzyme is a protein material that speeds up chemical reactions in the bodies of plants and animals.)

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Proteins in the human body contain about 20 different amino acids, of which the body is able to manufacture 12 from the foods we eat. The other eight, which the body requires for protein production but is unable to manufacture on its own, are known as the essential amino acids. When a protein contains all of the essential amino acids, it is known as a complete protein. Among the best forms of complete protein are fish, red meat, and poultry as well as eggs, milk, cheese, and other dairy products. Fittingly, a protein that lacks at least one of the essential amino acids is known as an incomplete protein. Examples include peas, beans, lentils, nuts, and cereal grains. These can, however, be combined in such a way as to make a complete protein, beans and rice being a good example. CA R B O H Y D RAT E S . Carbohydrates are natural compounds that consist of carbon, hydrogen, and oxygen and whose primary function in the body is to supply energy. When a person ingests more carbohydrates than his or her body needs at the moment, the body converts the excess into a compound known as glycogen. It then stores the glycogen in the liver and muscle tissues, where it remains, a potential source of energy for the body to use in the future.

Sugars, starches, cellulose, and various other chemically related substances are part of the carbohydrate group. Most carbohydrates are produced by green plants in the process of undergoing photosynthesis. Nutritionally, the carbohydrates include sugar in its various forms as well as another class of food that people do not always think of as carbohydrates: fruits. Additionally, such starchy foods as potatoes, rice, and wheat products (bread, pasta, and so on) rank as important carbohydrates, while cereal grains and corn are examples of foods that contain both starchy carbohydrates and proteins.

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tend to drift to opposite ends of the molecule. As a result, oil has the slippery texture for which it is known. With their affinity for nonpolar molecules, lipids are soluble, or capable of dissolving, in oily or fatty substances but not in water. In the body, lipids, like carbohydrates, supply energy, only in different ways and on a different timetable. When burned, a gram of lipid actually produces about three times as much energy as a gram of carbohydrate, but this energy release takes place much more slowly. Among the other functions performed by lipids in the body are protection of organs from shock and damage and the provision of insulation for the body, for instance, from toxins. (It is for this reason that toxins often are stored in fat cells. See Food Webs for more about DDT bioaccumulation in the fat cells of animals.) V I TA M I N S A N D M I N E RA L S . Vitamins are organic substances that, in extremely small quantities, are essential to the nutrition of most animals and some plants. In particular, they work with enzymes in regulating metabolic processes, but they do not in themselves provide energy; thus, vitamins alone do not qualify as a form of nutrition. Much the same is true of minerals, except that they are inorganic substances. And whereas vitamins are usually chemically complex (the formula for vitamin A, for instance, is C20H29OH), minerals may be as simple as a single element—for instance, iron or calcium.

Though the body can produce some vitamins, in general, vitamins and minerals are substances that the body is incapable of making for itself. Therefore, for optimal health, it is necessary to include them in the diet on a regular, if not daily, basis. They also have in common the fact that the body needs them only in very small quantities, for which reason they are sometimes known as micronutrients.

L I P I D S . All fats and oils are lipids; these substances are distinguished by the fact that they are soluble only in compounds made of nonpolar molecules. Water is an example of a polar molecule, because the oxygen and hydrogen atoms tend to occupy opposite “ends” of the molecule, with one end exerting a negative electric charge and the other end a positive one. Therefore, water molecules tend to stick closely together. On the other hand, oil molecules, which consist of carbon and hydrogen, are nonpolar, because the atoms of the two elements do not

Vitamin A, for example, is a substance necessary to the functioning of the eye’s retina in adjusting to light, and thus proper vitamin A levels are essential for night vision. Without vitamin A, a person can be afflicted with a condition known as night blindness, as well as with dryness of the skin. Vitamin A is also essential to bone growth. This vitamin occurs naturally in such foods as green and yellow vegetables, eggs, fruits, and liver and particularly in fish liver oils, such as cod liver oil.

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Calcium, a mineral, helps build strong bones and teeth. It also has a role in the normal functioning of nerve and muscle activity. Ninety-nine percent of the body’s calcium is stored in the skeleton and teeth, while the remainder circulates in the bloodstream, where it helps make possible muscle contractions. Bones are 70% calcium by weight, which gives them their strength and rigidity. Calcium, which is even more prevalent than iron, is the most abundant metallic element in the human body. Good dietary sources of calcium include milk and milk products, eggs, such leafy green vegetables as spinach, and sardines.

The U.S. Department of Agriculture Food Pyramid The U.S. Department of Agriculture (USDA) has developed a diagram, called the food pyramid, to illustrate the components needed in a healthy diet. The bottom and widest level of the pyramid contains the cereal foods, such as breads, pastas, and rice. Primarily carbohydrates, these foods are a major source of energy, and therefore the USDA recommends 6-11 servings of 1-2 oz (30–60 g) from this food group. As to the exact number of servings, this is a function of such variables as age, gender, weight, and degree of regular physical activity. The second level of the food pyramid, which is smaller than the first, consists of fruits and vegetables. These foods, which are also primarily carbohydrates, are especially important in supplying vitamins and minerals. A secondary function is the delivery of indigestible fiber, which improves the functioning and health of the large intestine, or colon. From this group, the USDA recommends 5-9 servings a day. At the third level of the pyramid are proteins, including meats, eggs, beans, nuts, and milk products. According to the USDA, the percentage of these foods in one’s diet should be much smaller than the percentage of carbohydrates. Smaller still is the quantity of servings at the top level, which contains the lipids. The small space allotted to this food emphasizes the fact that fats and oils should be consumed in small quantities for optimum health.

however, physicians, nutritionists, dieticians, and other specialists had begun to question the emphasis on carbohydrates in the USDA food pyramid and other mainstream diets. For a young person, whose body is still growing, the food pyramid is a good dietary plan. But for a person past the early twenties, particularly those who are overweight or suffering from a condition such as diabetes, other approaches may be needed.

Nutrients and Nutrition

The average American adult is considerably overweight for his or her height and age group, a fact for which a number of practices can be blamed. Among these practices are inactivity. Things have changed a great deal since our ancestors spent their whole lives in a flurry of physical activity, hunting animals for food and remaining ever on the move. Our bodies themselves—a product of natural selection that took place over countless generations (see Evolution)—know nothing about this change. They are still programmed to perform as they did 10,000 years ago, storing fat for use in lean times. Thus, inactivity breeds obesity, a condition that cannot be addressed successfully by diets aimed simply at reducing consumption. In such a situation, the body simply holds on to its fat more fiercely, and this is one reason why a starvation diet is less than useless as a means of bringing about healthy weight loss. Starving oneself also reduces lean muscle mass, which further slows the metabolism and makes it still harder to burn fat. In fact, one of the best ways to lose weight is by combining resistance exercise (i.e., weight lifting) with proper eating. Then, of course, there are the things Americans eat: junk food and fried foods, for instance. Eating junk food, pumped full of chemicals and white sugar, is like dumping garbage into a gas tank. As for fried foods, an American seldom realizes how much is in his or her diet until making a visit overseas. In Germany, for instance, virtually nothing is fried, and though people eat hearty meals with plenty of sausage, potatoes, bread, and beer, obesity is far less of a problem in Germany than in America. (Furthermore, the American traveler is likely to have far better bowel movements on the high-fiber German diet than on the greasy, fatty, salty American diet.)

Fat and the American Diet What we have just described is the orthodox view of nutrition in the United States as the nation entered the twenty-first century. By that time,

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The Case for Proteins As healthy as a diet based on the food pyramid is, an overweight adult who stuck religiously to it

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THE AVERAGE AMERICAN ADULT IS CONSIDERABLY OVERWEIGHT, A RESULT OF INACTIVITY COUPLED WITH POOR DIET. THINGS HAVE CHANGED A GREAT DEAL SINCE OUR ANCESTORS SPENT THEIR LIVES IN A FLURRY OF PHYSICAL ACTIVITY, HUNTING ANIMALS FOR FOOD AND EVER ON THE MOVE, BUT THE HUMAN BODY IS STILL PROGRAMMED TO PERFORM AS IT DID 10,000 YEARS AGO, STORING FAT FOR USE IN LEAN TIMES. (© Bettmann/Corbis. Reproduced by permission.)

most likely would remain overweight, even if he or she combined it with a regular program of aerobic exercise (e.g., jogging). Thus, by the late twentieth century, numerous experts across a wide spectrum of health professions began to challenge the old orthodoxy that held up carbohydrates as the central component of a healthy diet. Instead, a growing body of opinion favored diets that rely more on proteins and even healthy fats, such as those in broiled or grilled salmon or olive oil. Whereas carbohydrate consumption can help an athlete gain a burst of energy, for most people carbohydrates are simply the raw materials for fat, which the body will store when it discovers that it does not need the carbohydrates themselves as an immediate source of energy. Furthermore, the brain has a mechanism for signaling the body that it has consumed enough protein, whereas there is no such mechanism where carbohydrates are concerned. To test this, try eating a meal of just protein: chances are that you will feel you have had enough fairly quickly. On the other hand, try eating a meal of just starches; you will find that you can eat and eat and eat, piling on calories as you do.

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Malnutrition As we noted earlier, corn has both protein and carbohydrate components, but this does not mean that a diet heavy in corn and corn products is a healthy one. Such a diet was not uncommon among poor people in the American South during the late nineteenth and the early twentieth centuries and among even poorer people in Mexico and other parts of Latin America to the present day. With Southern foods, such as grits, hominy, and cornbread, and Latin American foods, such as corn tortillas and polenta, it is quite possible to eat cheaply from a diet that relies primarily on corn. Yet someone who does so is at risk of serious health problems, because corn is lacking in two essential amino acids, lysine and tryptophan. This is just one example of malnutrition, a condition that develops when the body does not obtain the right amount of the vitamins, minerals, and other nutrients it needs to maintain healthy tissues and organ function. Malnutrition occurs in people who are either undernourished or overnourished. Undernourishment is a consequence of consuming too few essential nutrients or using or excreting them more rapidly than

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KEY TERMS AMINO ACIDS:

Organic compounds

elements cannot be broken down chemi-

composed of carbon, hydrogen, oxygen,

cally into simpler substances.

nitrogen, and (in some cases) sulfur bond-

ENZYME:

ed in characteristic formations. Strings of

speeds up chemical reactions in the bodies

amino acids make up proteins.

of plants and animals.

AUTOTROPHS:

Primary producers.

A protein material that

ESSENTIAL AMINO ACIDS:

Amino

Autotroph means “self-feeder,” and these

acids that cannot be manufactured by the

organisms are distinguished by the fact

body and which therefore must be

that they do not depend on other organ-

obtained from the diet. Proteins that con-

isms as a source of energy. Instead, they

tain essential amino acids are known as

need only sunlight, water, and a few simple

complete proteins.

chemical compounds, from which they

FOOD WEB:

A term describing the

build the large organic molecules that they

interaction of plants, herbivores, carni-

need to survive.

vores, omnivores, decomposers, and detriThe

tivores in an ecosystem. Each of these

changes that particular elements undergo

organisms consumes nutrients and passes

as they pass back and forth through the

them along to other organisms (or, in the

various earth systems (e.g., the biosphere)

case of the decomposer food web, to the

and particularly between living and nonliv-

soil and environment). The food web may

ing matter. The elements involved in bio-

be thought of as a bundle or network of

geochemical cycles are hydrogen, oxygen,

food chains, but since the latter rarely exist

carbon, nitrogen, phosphorus, and sulfur.

separately, scientists prefer the concept of a

BIOGEOCHEMICAL CYCLES:

CARBOHYDRATES:

Naturally occur-

food web to that of a food chain. Fats and oils. With their affini-

ring compounds, consisting of carbon,

LIPIDS:

hydrogen, and oxygen, whose primary

ty for nonpolar molecules, lipids dissolve

function in the body is to supply energy.

in oily or fatty substances but not in water-

Included in the carbohydrate group are

based liquids. In the body, lipids supply

sugars, starches, cellulose, and various

energy in slow-release doses, protect

other substances. Most carbohydrates are

organs from shock and damage, and pro-

produced by green plants in the process of

vide insulation for the body, for instance,

undergoing photosynthesis.

from toxins.

COMPLETE PROTEIN:

A protein that

includes all eight essential amino acids.

MALNUTRITION:

Any one of several

conditions that develop when the body does not obtain the right amount of vita-

COMPOUND:

A substance in which

mins, minerals, and other nutrients it

atoms of more than one element are bond-

needs to maintain healthy tissues and

ed chemically to one another.

organ function. Malnutrition may result

ELEMENT:

A substance made up of

only one kind of atom. Unlike compounds,

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from undernourishment or overnourishment.

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KEY TERMS MINERALS:

Inorganic substances that,

CONTINUED

PRIMARY

Animals

in a nutritional context, serve a function

that eat green plants. Compare with sec-

similar to that of vitamins. Minerals may

ondary consumers.

include chemical elements—particularly

PRIMARY

metallic ones, such as calcium or iron—as

plants that depend on photosynthesis for

well as some compounds.

their nourishment.

NUTRIENT:

Materials essential to the

PROTEINS:

PRODUCERS:

Green

Large molecules built

survival of organisms. They include pro-

from long chains of amino acids. Proteins

teins, carbohydrates, lipids (fats), vitamins,

serve the functions of promoting normal

and minerals.

growth, repairing damaged tissue, con-

NUTRITION:

The series of processes by

which an organism takes in nutrients and makes use of them for its survival, growth, and development. The term nutrition also

tributing to the body’s immune system, and making enzymes. SECONDARY CONSUMERS:

Ani-

mals that eat other animals. A chemical element

can refer to the study of nutrients, their

TRACE ELEMENT:

consumption, and their use in the organ-

that appears within an organism or other

ism’s body.

natural system in very small quantities, or

ORGANIC:

At one time chemists used

the term organic only in reference to living things. Now the word is applied to most

traces. In the human body, for instance, such trace elements as copper or iodine are essential to health, though they comprise far less than 1% of the body’s mass.

compounds containing carbon, with the exception of carbonates (which are minerals), and oxides, such as carbon dioxide.

VITAMINS:

Organic substances that, in

extremely small quantities, are essential to the nutrition of most animals and some

The biological

plants. In particular, vitamins work with

conversion of light energy (that is, electro-

enzymes in regulating metabolic processes;

magnetic energy) from the Sun to chemical

however, they do not in themselves provide

energy in plants. In this process carbon

energy, and thus vitamins alone do not

dioxide and water are converted to sugars.

qualify as a form of nutrition.

PHOTOSYNTHESIS:

they can be replaced. This brings about the alltoo-familiar scenarios most of us associate with malnutrition: scenes of starving children in the third world, their bellies distended from kwashiorkor (which we discuss next) and thus abnormally large, far out of proportion to their bony arms and legs.

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CONSUMERS:

Such scenes of horror, and the virtually unimaginable misery of which they provide a hint, are almost beyond the comprehension of

the average American. In America even the poorest of people are reasonably well fed, certainly compared with the poor of Africa, Asia, or Latin America. But that does not mean that malnutrition is not a problem in the affluent Western world. In the United States undernourishment may not be nearly as much of a problem, but dietary imbalances and excesses are, and they have become associated with many of the leading causes of death and disability. Overnutrition

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results from eating too much, eating too many of the wrong things (including junk foods, especially those containing white sugar), not exercising enough, or taking too many vitamins or other dietary replacements. As with many things, where vitamins are concerned, it is not the case that if a little is good, a lot is better. Vitamins can only be absorbed by the body if ingested on a full stomach, and an excess of fat-soluble vitamins such as A or D could actually be toxic. (See Vitamins for more on this subject.) KWA S H I O R KO R . For any nutrient that the body requires, there is a corresponding disease, ailment, or condition (or many of them) that will develop if a person’s body is deprived of that nutrient. For instance, people whose diets lack protein may become susceptible to a condition known as kwashiorkor, characterized by apathy, exhaustion, fatigue, wasting of muscle tissue, and edema (swelling, a result of water collecting in the body.)

Kwashiorkor, in fact, is the condition that causes swelling in the stomachs of children suffering from malnutrition. When people eat a diet consisting mainly of starchy vegetables, as is common in parts of Africa, Latin America, and southern Asia, they consume an appropriate amount of calories, but they do not get certain essential amino acids that are important for growth. This is particularly serious for children. Nursing babies usually do not suffer from kwashiorkor, because they receive adequate protein from their mother’s milk. A child who is weaned, however, is at much greater risk. In much of the third world, where human populations are high relative to the number of animals raised for meat, meat itself is a luxury. A child of a poor family is unlikely to have meat in his or her diet, and without the mother’s milk the child is left virtually without sources of protein. This situation may have informed the choice of the word kwashiorkor to describe this condition. Derived from the Ga language of coastal Ghana in Africa, the word is based on kwàsìokó, a Ga term for “the influence a child is said to be under when a second child comes.” In regions where birth control is unavailable or simply is not used, it is easy to imagine how an older child would be forced to give up the mother’s milk to make room for a second child.

outward manifestation of the condition is, of course, the swollen, bloated abdomen, brought about by a decrease in the amounts of the protein albumin (also found in egg whites) in the bloodstream. Skin discoloration may occur, along with severe diarrhea, an enlarged liver, and atrophy, or withering, of muscles and glands. Kwashiorkor can bring about retarded mental and physical development as well, but at least there is a treatment: adding proteins to the diet. For this reason, aid organizations supply powdered milk, which, if administered to these undernourished children in time, can save them from further damage to their bodies and minds.

Nutrients and Nutrition

MALNUTRITION IN THE UNITE D S TAT E S . Malnutrition exists even in the

United States and not only in the form of overnourishment. Particularly among the nation’s poor, undernourishment is serious, though it is not necessarily because food is unavailable to poor families. Rather, the right food may be unavailable, because the health foods described at the conclusion of this essay tend to be expensive. Furthermore, lack of education and information ensures that poor families maintain destructive dietary practices that leave their bodies deprived of essential nutrients. Low-cost, high-bulk items, such as sugar, white flour, and corn meal, may provide immediate satisfaction to the stomach, but they leave the body undernourished in the long run. Nevertheless, the most prevalent form of malnutrition in America is still overnourishment. Implied in the term overnourishment, of course, is the idea that the person so afflicted is not eating the right kinds of things, because it is hard to be overnourished with a proper diet. Junk foods, such as one finds in the candy aisle of a convenience store or in a fast-food restaurant, contain carbohydrates and some proteins (not to mention plenty of fats); usually, these nutrients are presented in such a way as to maximize taste and minimize nutritional value. The result is “empty calories” that simply go to the waistline, buttocks, and abdomen of the consumer.

The results are devastating for the child deprived of protein sources. The most shocking

America is awash in food, but very little of it is the kind of food that sustains the health of the body or prolongs life. People have been conditioned by advertising to believe that white bread is the only kind of bread (even though the process of refining white flour leaches out most of the nutrients); that it is possible to have “zero

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fat” cookies sweetened with white sugar (which simply turns to fat as soon as the body digests it); and that a diet of burgers, fries, and soft drinks is “normal.” The result is an overnourished, overweight populace whose life spans are being cut short. There is another way, however, as a visit to a well-stocked health-food grocery store (or even the organic/health food section of a mainstream grocery store) will illustrate. After trying genuine whole wheat or sprouted wheat instead of white bread, fructose or cane-juice sweeteners in place of refined sugar, free-range chicken and grass-fed beef rather than meats pumped full of chemicals, many Americans might be surprised to learn just how good food can taste—and how good they can feel. WHERE TO LEARN MORE

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Everything Else We Eat and Drink. New York: William Morrow, 1995. Food and Nutrition Information Center, National Agricultural Library, U.S. Department of Agriculture (Web site). . Hands, Elizabeth S. Nutrients in Food. Philadelphia: Lippincott, Williams and Wilkins, 2000. Kiple, Kenneth F., and Kriemhild Coneè Ornelas. The Cambridge World History of Food. New York: Cambridge University Press, 2000. Nutrition.gov (Web site). . Nutrition and Healthy Eating Advice. About.com (Web site). . Patent, Dorothy Hinshaw. Nutrition: What’s in the Food We Eat. Illus. William Muñoz. New York: Holiday House, 1992. Renders, Eileen. Food Additives, Nutrients, and Supplements A–Z: A Shopper’s Guide. Santa Fe, NM: Clear Light Publishers, 1999. Schwarzbein, Diana, and Nancy Deville. The Schwarzbein Principle: The Truth About Losing Weight, Being Healthy, and Feeling Younger. Deerfield Beach, FL: Health Communications, 1999.

Anderson, Jean, and Barbara B. Deskins. The Nutrition Bible: A Comprehensive, No-Nonsense Guide to Foods, Nutrients, Additives, Preservatives, Pollutants, and

U.S. FDA Center for Food Safety and Applied Nutrition (Web site). .

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Vitamins

CONCEPT Most of us have been told to take our vitamins, but few people know why, and despite all the talk about them in modern culture, vitamins remain something of a mystery. Vitamins are organic substances, essential for maintaining life functions and preventing disease among humans and animals and even some plants. They are found in very small quantities in food; certain health specialists recommend taking vitamin supplements to augment the supplies in food, while others insist that a well-balanced diet provides all the vitamins that an ordinary person needs. Some vitamins, such as vitamin C and the B complex, are water-soluble, which means that they are excreted easily and must be ingested every day. Others, such as vitamins A, D, E, and K, are fatsoluble and therefore are retained in the body’s fatty tissues. With such vitamins, there may be a danger of taking too much, but in the case of most vitamins, the greatest harm comes from not receiving enough. Vitamin deficiencies can be the cause of rickets, pellagra, and other diseases that have plagued the poor in the Western world and the third world in the past and in the present.

V I TA M I N S

about vitamins per se until that time, folk wisdom certainly had taken account of the fact that certain foods are essential to the health and wellbeing of humans and animals. Vitamins may be defined as organic substances, found in food, that are essential in very small quantities for the health of most animals and some plants. Organic substances, discussed in The Biosphere, are compounds (substances in which atoms of more than one element are chemically bonded to one another) containing hydrogen and carbon. Primarily, vitamins work with enzymes (protein materials that speed up chemical reactions in the bodies of plants and animals) in regulating metabolic processes—that is, processes that convert food to energy. They do not in themselves provide energy, however, and thus vitamins alone do not qualify as a form of nutrition.

An Introduction to Vitamins

Organisms require vitamins only in very small amounts: the total amount of vitamin mass a person needs in one day, for instance, is only about 0.0011 lb. (0.5 g). Yet vitamins are absolutely essential to the maintenance of health and for disease prevention, and most animals are not capable of synthesizing or manufacturing vitamins on their own. Nonetheless, most animals can produce vitamin C, though there are exceptions—humans included.

Once they were called vitamines, but for reasons that we address later, the “e” was dropped, and they became known as vitamins. There is also a reason for the strange alphabet of vitamins (A, B, C, D, E, K), which, like the change in spelling, came out of the early days of scientific research into the subject during the first third of the twentieth century. Though people did not know

Animals depend on plants for their nutrition, either directly or indirectly (i.e., either by consuming the plant or by consuming an animal that has consumed the plant). Plants, on the other hand, are autotrophs, meaning that they can meet their nutritional needs with only sunlight, water, and a few chemical compounds. Among the nutrients plants produce are vita-

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mins, which they pass on to animals that consume them directly or indirectly. (See Food Webs for more about autotrophs and the relationship of animal consumers to them.)

Classifying Vitamins Numerous vitamin groups are necessary for the nutritional needs of humans, and though only minute amounts of each are required to achieve their purpose, without them life could not be maintained. Some vitamins, including A, D, E, and K, are fat-soluble, meaning that they are found in fattier foods and in body fat. Thus, they can be stored in the body; for this reason, it is not necessary to include them in the diet every day. In fact, it could be dangerous to do so, since it is possible that they would build up to toxic levels in the tissues. Other vitamins, the most notable of which are vitamin C and the many vitamins in what is known as the “B complex,” are water-soluble. They are found in the watery parts of food and body tissue, and because they are excreted regularly in the urine, they cannot be stored by the body. Instead, they must be consumed on a daily basis. This difference in solubility is extremely important to the way the vitamins function within an organism and in the ways and amounts in which they are consumed. T H E N A M E S O F V I TA M I N S . Vitamins originally were classified in terms of their solubility in water or in fat, and these distinctions remain important for the reasons outlined above. Today, vitamins are known primarily by letters of the alphabet, a fact that harks back to a naming system developed as more and more vitamins were discovered in the early years of the twentieth century.

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today the only alphabetical vitamin names are A, the B complex, C, D, E, and K.

REAL-LIFE A P P L I C AT I O N S Fat-Soluble Vitamins We are accustomed in modern life to being told that fat is bad for us, but to quote a much-cited line from the American composer George Gershwin’s opera Porgy and Bess, “It ain’t necessarily so.” Fat is not inherently bad for people; in fact, a certain amount in the diet is essential. The problem in America today is the type of fat that people consume. There is a big difference between the healthy, natural, unsaturated fats one might find, say, in fresh salmon, and the highly processed and saturated fats in a bag of potato chips. (The term saturated means that every gap in which a hydrogen atom could fit in a string of carbon and hydrogen atoms has been filled. This helps make fats firm, for use in such products as shortening.) Such fat is extremely harmful, because the body is not able to process it; even so, a certain amount of natural fat in the diet can be highly beneficial. This is true in large part because fat can serve as a medium for the fat-soluble vitamins A, D, E, and K, which are deposited in the body’s fat cells. But as we noted earlier, it is important not to overdose on fat-soluble vitamins, because then what is inherently healthy can become extremely unhealthy.

As scientists detected the existence of more vitamins, or what they thought were vitamins, they assigned to them successive letters of the alphabet: A, B, C, and so on. Eventually, however, they discovered that some substances originally thought to be vitamins were not vitamins, and they removed them from the roster. For example, what used to be called vitamin F is simply an essential fatty acid, a necessary component of the diet of a mammal but not the same thing as a vitamin. In other cases, what were once believed to be individual vitamins later were subsumed into the B complex. Among these substances are riboflavin, formerly termed vitamin G, and biotin, once called vitamin H. The result is that

V I TA M I N A . In 1596 the Dutch explorer Willem Barents (1550–1597) and his shipwrecked crew spent a grueling winter on the island of Novaya Zemlya in the Arctic Ocean north of Russia. They had sailed from Holland in search of the Northeast Passage, which, like the more famous Northwest Passage above Canada, offered the prospect of a short, relatively direct sea route from Europe to Asia and the Americas. The problem was that the ice made sailing the northern seas virtually impossible. It would be almost three centuries before a crew managed to negotiate the Northeast Passage, by which time the European powers had long since given up all hopes of using it as a viable sailing route. (The same was true of the Northwest Passage, which was not traversed until 1906.)

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THE DUTCH

EXPLORER

WILLEM BARENTS

A POISONING WHEN ARCTIC OCEAN. POLAR BEAR

AND HIS SHIPWRECKED CREW EXPERIENCED VITAMIN

THEY ATE POLAR BEARS TO STAY ALIVE ON THE ISLAND OF

NOVAYA ZEMLYA

LIVER CONTAINS ABOUT 450 TIMES THE RECOMMENDED DAILY DOSE OF VITAMIN

IN THE

A;

SYMPTOMS OF POISONING INCLUDE

PAINFUL JOINTS, BONE THICKENING, PEELING OF THE SKIN, AND LIVER DISEASE. (© Bettmann/Corbis. Reproduced by per-

mission.)

Barents and his men knew none of that, nor would they have cared in that miserable winter of 1596–1597. All they cared about was survival, the chances for which seemed slim—and not just because of the almost inhuman cold or the fact that their ship had been cracked to pieces by the ice. Men were dying of scurvy, a vitamin-deficiency disease we discuss later in the context of vitamin C, as well as from the cold. Yet there were a few blessings, mainly in the form of available wood for fuel and animals for food. The men killed polar bears and ate their meat, and no doubt they were thankful just to stay alive. They could not have guessed, however, that they were actually killing themselves with an overdose of vitamin A.

ents included—never lived to see Holland again, in part because the side effects of vitamin A toxicity had weakened them. So why take vitamin A at all? Because it is necessary for proper growth of bones and teeth, for the maintenance and functioning of skin and mucous membranes, and for the ability to see in dim light. There is some evidence that it can help prevent cataracts (a clouding of the lens in the eye) and cardiovascular disease, a condition of the heart and circulatory system. Furthermore, when taken at the onset of a cold, vitamin A can ward off the illness and fight its symptoms.

Just 1 lb. (0.454 kg) of polar bear liver contains about 450 times the recommended daily dose of vitamin A, and the men in Barents’s expedition were absorbing far more of the vitamin than they should have. In time, they began to experience the effects of vitamin A poisoning: painful joints, bone thickening, peeling of the skin over the entire body, and chronic liver disease. When spring came, the men managed to make it off the island, but many of them—Bar-

One of the first signs of vitamin A deficiency is “night blindness,” in which the rods of the eye (necessary for night vision) fail to function normally. Extreme cases of vitamin A deficiency can lead to total blindness. Other symptoms include dry and scaly skin, problems with the mucous linings of the digestive tract and urinary system, and abnormal growth of teeth and bones. The bodies of healthy adults who have an adequate diet can store several years’ supply of this vitamin, but young children, who have not had time to build up such a large reserve, suffer from

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DURING

GREAT DEPRESSION, A CHILD IN THE SOUTHERN UNITED STATES SHOWS SIGNS OF RICKETS, STEMD DEFICIENCY. UNDER THE INFLUENCE OF THIS DEBILITATING AND DISFIGURING DISEASE, THE BECOME BOWED BY THE WEIGHT OF THE BODY, AND THE WRISTS AND ANKLES THICKEN. (© Marion Post Wolcott/CorTHE

MING FROM VITAMIN LEGS

bis. Reproduced by permission.)

deprivation much more quickly if they do not consume enough.

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nearly impossible to ingest beta-carotene in toxic amounts, unlike vitamin A from animal sources, since the body will not convert excess amounts to toxic levels of vitamin A.

Vitamin A is present in meats (mainly liver), fish oil, egg yolks, butter, and cheese. Although plants do not have vitamin A, dark green leafy vegetables and yellow fruits and vegetables (e.g., carrots, sweet potatoes, cantaloupe, corn, and peaches) contain a substance called betacarotene, which is converted to vitamin A in the intestine and then absorbed by the body. It is

V I TA M I N D . Vitamin D is actually two different substances, D2 and D3. (There was no D1, since the substance designated thus at one time turned out to be a mixture of several compounds, including calciferol, or D2) Both forms of vitamin D are activated, or made effective, by

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sunlight, and for this reason vitamin D often is called the sunshine vitamin. It is hard to suffer a vitamin D deficiency if one gets enough sunshine in combination with consuming such foods as eggs (specifically, the yolk), such fatty fish as salmon, and enriched milk. (Milk does not naturally contain vitamin D, but the vitamin is sometimes included as an additive.) Vitamin D lets the body utilize calcium and phosphorus in bone and tooth formation, and a deficiency causes a bone disease called rickets. Under the influence of this physically debilitating and disfiguring disease, legs become bowed by the weight of the body, and the wrists and ankles thicken. The teeth are badly affected and, for a young child, take much longer to mature. Infants and children are most likely to suffer the effects of rickets, but since all milk and infant formulas have vitamin D added to them, the condition is seen rarely in the industrialized world today. In the brutal early days of the Industrial Revolution, however (i.e., in England ca. 1760–1830), crowded slum conditions in areas where there was little or no sunlight made possible many cases of rickets. Whereas rickets primarily affects children, adults may suffer from a disease called osteomalacia, caused by a deficiency of vitamin D, calcium, and phosphorous. Sometimes seen in the Middle East and other parts of Asia, osteomalacia brings with it rheumatic pain and causes the bones to become soft and deformed. As with rickets, the treatment for osteomalacia is a combination of calcium, phosphorous, and vitamin D. On the other hand, as with all fat-soluble vitamins, a person may take in excessive amounts of vitamin D, which has its own ill effects: nausea, diarrhea, weight loss, and pain in the bones and joints. Damage to the kidneys and blood vessels also can occur as calcium deposits build up in these tissues. V I TA M I N E . Composed of at least seven similar chemicals called the tocopherols, vitamin E is found in green leafy vegetables, wheat germ and other plant oils, egg yolks, and meat. The main function of this vitamin is to act as an antioxidant, to counteract the harmful effects oxygen can have on tissues. It may seem strange to speak of oxygen causing harm, since it is essential to life, but oxidation is an extremely powerful chemical reaction that, under various conditions, can manifest as rotting or putrefaction, rusting, or even combustion and explosion.

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When an apple turns brown a few minutes after you have cut it open, it is the result of oxidation.

Vitamins

Oxidation also may be linked to the effects of aging in humans as well as to other conditions, such as cancer, hardening of the arteries, and rheumatoid arthritis. It appears that oxygen molecules, which draw electrons to them, extract these electrons from the membranes in human cells. Over time, this can cause a gradual breakdown in the body’s immune system. Antioxidants, such as vitamin E or beta carotene, therefore may be important in preserving human health and well-being. Vitamin E is particularly important for counteracting oxidation in fats. When they are oxidized, fats form a highly reactive substance called peroxide, which is often very damaging to cells. Vitamin E is more reactive (i.e., more likely to form or break chemical bonds) than the fatty acid molecule, and, therefore, the vitamin reacts instead of the fat. Because cell membranes are composed partly of fat molecules, vitamin E is vitally important in maintaining the nervous, circulatory, and reproductive systems and in protecting the kidneys, lungs, and liver. Because vitamin E is so common in foods, it is very difficult to suffer from a deficiency of this vitamin unless a person avoids consuming fats altogether—another example of why a no-fat diet is not a healthy one. The effects of vitamin E deficiency, all of which are apparently linked to the loss of its antioxidant protection, include cramping in the legs, fibrocystic breast disease (a condition that involves the formation of lumps and cysts in the breasts), and even muscular dystrophy. The seriousness of the latter two diseases only serves to highlight the importance of vitamin E to the body. V I TA M I N K . Like vitamin D, vitamin K is composed of two groups of compounds, vitamins K1 and K2. There is also a substance called K3, but this vitamin is actually menadione, a synthetic compound from which the other forms of K are derived. You can find vitamin K in many plants, especially green leafy ones such as spinach, and in liver. Vitamin K is also made by the bacteria that live in the intestine—the “good” bacteria that help make possible the processing of food through the body.

Vitamin K appears to be critical to blood clotting, thanks to its role in assisting the formation of a chemical called prothrombin in the liver.

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names,” while biotin and folate, or folic acid, are not known by “B names” at all.

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Vitamin B1, present in whole grains, nuts, legumes (e.g., peas), pork, and liver, helps the body release energy from carbohydrates. More than 4,000 years ago, the Chinese described a disease we know today as beriberi, which affects the nervous and gastrointestinal systems and causes nausea, fatigue, and mental confusion. The cause of beriberi is a deficiency of thiamine, or B1, found in the husks or bran of rice and grains. White rice, which most people find more pleasing to the palate than brown rice, is the result of a milling and polishing process in which the husks—and along with them, this important nutrient—are removed. Manufacturers today produce “enriched” rice, flour, and other grain products by adding thiamine back in, but until scientists discovered the importance of thiamin in grain husks, many people, especially in the Far East, suffered the effects of beriberi. (Early research on beriberi will be discussed later.) THE AMERICAN

LINUS PAULING (SHOWN TOSSING AN ORANGE), WINNER OF THE NOBEL PRIZES IN CHEMISTRY AND IN PEACE, HELPED POPULARIZE VITAMIN C, ALSO KNOWN AS ASCORBIC ACID. (© Roger RessCHEMIST

meyer/ Corbis. Reproduced by permission.)

Deficiencies of this vitamin rarely occur as the result of an incomplete diet; instead, it is usually a consequence of liver damage and the blood’s inability to process the vitamin. The deficiency manifests in unusual bleeding or large bruises under the skin or in the muscles. Adults in the West seldom experience vitamin K deficiencies, but newborn infants have been known to suffer from brain hemorrhage owing to a lack of this vitamin.

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Vitamin B2 helps the body release energy from fats, proteins, and carbohydrates. It can be obtained from whole grains, organ meats (e.g., liver), and green leafy vegetables. Lack of this vitamin causes severe skin problems. Vitamin B6 is important in the building of body tissue as well as in protein metabolism and the synthesis of hemoglobin (an iron-containing pigment in red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide). A deficiency can cause depression, nausea, and vomiting. Vitamin B12 is necessary for the proper functioning of the nervous system and in the formation of red blood cells. It can be obtained from meat, fish, and dairy products. Anemia (a lack of red blood cells, which produces a lethargic condition), nervousness, fatigue, and even brain degeneration, can result from vitamin B12 deficiency.

B V I TA M I N S . The two water-soluble vitamins, as we shall see, have played a major part in medical history. Actually, there are more than two water-soluble vitamins, because vitamin B is really a complex of about a dozen vitamins— hence, the name B complex. Among them are vitamin B1 (thiamine), vitamin B2 (riboflavin), vitamin B6 (pyridoxine), and vitamin B12 (cobalamin). A few others—for example, niacin (vitamin B7) and pantothenic acid (vitamin B3), are known better by names other than their “B

Niacin is also highly important to human health, as we explain later in the context of the disease pellagra. Pantothenic acid helps release energy from fats and carbohydrates and is found in large quantities in egg yolks, liver, eggs, nuts, and whole grains. Deficiency of this vitamin causes anemia. Biotin, widely available from grains, legumes, and liver, plays a part in the release of energy from carbohydrates and in the formation of fatty acids. A lack of biotin causes dermatitis, or skin inflammation.

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V I TA M I N C . The American chemist and peace activist Linus Pauling (1901–1994), winner of the Nobel Prize in chemistry (1954) and peace (1962), helped popularize vitamin C, also known as ascorbic acid. It was Pauling who originated the idea, now widespread in society, that massive doses of vitamin C can ward off the common cold. Pauling went further, by maintaining that vitamin C offers protection against some forms of cancer. While scientific studies have been unable to prove this theory, they do suggest that the vitamin can at least reduce the severity of the symptoms associated with colds.

Most animals can synthesize this vitamin in the liver, where glucose (a type of sugar that occurs widely in nature) is converted to ascorbic acid. This is not the case with at least four types of animal: monkeys, guinea pigs, Indian fruit bats, and humans, all of which must obtain vitamin C from their diets. Citrus fruits, berries, and some vegetables (e.g., tomatoes and peppers) are good sources of vitamin C. It is a fragile vitamin, one that is oxidized or destroyed easily. Food storage or food processing can render it ineffective; so, too, can soaking vitamin C–containing fruits and vegetables in water for long periods.

Vitamin Deficiencies and History Most of the early history in the study of vitamins centered around what are now known as the water-soluble vitamins. Although vitamins as such were not discovered until early in the twentieth century, it was common knowledge long before that time that substances in certain foods were necessary for good health. An important turning point came in the mid–eighteenth century, with the work of the Scottish physician James Lind (1716–1794) on a vitamin deficiency condition that jeopardized England’s vast merchant and military navies.

vitamins was unknown, and sailors at sea continued to live on a diet that consisted primarily of salted meats and hard biscuits—items that could be stored easily without spoilage in an era before refrigeration.

Vitamins

In 1746, Lind, a ship’s doctor, observed that 80 of 350 seamen aboard his ship came down with scurvy during a 10-week cruise. Conducting a controlled experiment, he took 12 of the sailors in whom scurvy had developed and divided them into six groups. He gave each pair different substances, such as nutmeg, cider, seawater, and vinegar; the final pair was given lemons or oranges. The two men given the oranges and lemons both completely recovered in about a week. Not only was this a milestone in the history of vitamin research, but it also was the first example of a clinical trial, or the testing of a medication by careful and well-documented experimentation in which other variables or factors are kept unchanged. It would be another half-century before the British navy adopted Lind’s techniques. Another Scottish physician, Sir Gilbert Blane (1749– 1834), had long fought for the adoption of Lind’s methods, and finally, in 1796, he persuaded the navy to give each sailor a daily ration of lemons. At that time, the term lime was common for both lemons and limes, and, as a result, British sailors became known as limeys. Eventually, the treatment spread to the population as a whole, but outbreaks of scurvy continued until after World War I, when doctors isolated vitamin C as the controlling factor in scurvy prevention. BERIBERI IN THE DUTCH E AS T I N D I E S . In 1897 the Dutch govern-

At a time when England had emerged as the world’s leading sea power, even Her Majesty’s sailing crews were at the mercy of a condition known as scurvy. Common among crews who had been at sea too long, scurvy could result in swollen joints, bleeding gums, loose teeth, and an inability to recover from wounds. Scientists today recognize scurvy as resulting from a deficiency of vitamin C, available in such citrus fruits as oranges. At the time, however, the concept of

ment sent the physician Christiaan Eijkman (1858–1930) as part of a government commission to the Dutch East Indies (now Indonesia), which had been long afflicted by a condition known as beriberi. There are various forms of this disease, including infant beriberi, which can kill a breast-feeding baby after the fifth month, as well as various juvenile and adult forms. In the childhood and adult versions of the disease, there is a preliminary condition of fatigue, loss of appetite, and a numb, tingling feeling in the legs. This condition can lead to either wet beriberi, characterized by the accumulation of fluid throughout the body and a rapid heart rate that can bring about sudden death, or dry beriberi,

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KEY TERMS Organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations. Strings of amino acids make up proteins. AMINO ACIDS:

ANEMIA: A condition marked by a lack of red blood cells or hemoglobin or a shortage in total blood volume, any one of which can produce a lethargic condition.

An enzyme, or some other organic substance, that is capable of counteracting the negative impact of oxygen (which draws electrons to it) on living tissue.

ANTIOXIDANT:

Naturally occurring compounds, consisting of carbon, hydrogen, and oxygen, whose primary function in the body is to supply energy. Included in the carbohydrate group are sugars, starches, cellulose, and various other substances. CARBOHYDRATES:

A substance in which atoms of more than one element are bonded chemically to one another. COMPOUND:

The chemical process

by which nutrients are broken down and converted into energy or used in the construction of new tissue or other material in the body. MINERALS:

Inorganic substances that,

in a nutritional context, serve a function similar to that of vitamins. Minerals may include chemical elements, particularly metallic ones, such as calcium or iron, as well as some compounds. At one time chemists used

ORGANIC:

the term organic only in reference to living things. Now the word is applied to compounds containing carbon and hydrogen. PROTEINS:

Large molecules built

from long chains of amino acids. Proteins serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body’s immune system, and making enzymes. TISSUE:

A group of cells, along with

A protein material that speeds up chemical reactions in the bodies of plants and animals.

the substances that join them, that form

A type of sugar that occurs widely in nature. Glucose is the form in which animals usually receive carbohydrates.

VITAMINS:

An iron-containing pigment in red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide.

enzymes in regulating metabolic processes;

ENZYME:

GLUCOSE:

HEMOGLOBIN:

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METABOLISM:

part of the structural materials in plants or animals. Organic substances that, in

extremely small quantities, are essential to the nutrition of most animals and some plants. In particular, vitamins work with however, they do not in themselves provide energy, and thus vitamins alone do not qualify as a form of nutrition.

which is marked by a loss of sensation and weakness in the legs. The patient first needs to walk with the aid of a stick and then becomes bedridden and easy prey to infectious diseases.

Experimenting with birds, Eijkman noticed that some of the fowl experienced paralysis and polyneuritis (a disorder affecting several nerves at once), as in the dry form of beriberi. The director

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of the hospital had told Eijkman that he could not feed the birds with table scraps from the dining hall, where the diet was heavy in polished rice. The doctor thus was forced to feed his birds whole rice, and something amazing happened: the birds began to regain their ability to move and experienced no recurrence of paralysis. Eijkman’s colleagues rejected his claim that the birds had contracted some form of beriberi, though, in fact, he was correct. He was incorrect, however, in his supposition that the polished rice contained a toxin that was missing from the whole, unpolished rice. After Eijkman and the rest of the medical commission left the East Indies, another Dutch physician, Gerrit Grijns (1865–1944), stayed on to study the disease. He discovered that when the chickens were taken off the rice diet completely and fed meat instead, they did not show signs of the characteristic paralysis; if the meat was overcooked, however, the condition reappeared. In 1901, Grijns showed that beriberi could be cured by putting the rice polishings back into the rice. As it turned out, the husks and the meat contained vitamin B1, also present in wheat germ, whole grain and enriched bread, legumes, peanuts, and nuts. P E L LAG RA I N T H E A M E R I CAS .

A vitamin-deficiency disease often associated with poverty, pellagra produces symptoms known as the “three Ds”: diarrhea, dermatitis, and dementia, or mental deterioration. It was first identified in 1762 by the Spanish physician Gaspar Casal (ca. 1680–1759), who wrote about the “mal de la rosa”—so called because of the reddened dermatitis that appeared around the back of the victim’s neck—that afflicted sufferers in one particular region of Spain. Casal was far ahead of his time in maintaining that inadequate nutrition caused pellagra, but for many centuries the belief persisted that the disease resulted from infection. The breakthrough came with the work of the American physician Joseph Goldberger (1874–1929), a member of the U.S. Public Health Service who studied the high numbers of pellagra cases among poor blacks and whites in the southern United States. Goldberger established that pellagra stems from insufficient niacin, which is required to release energy from glucose.

happened that corn products constituted a major part of the diet in areas suffering from high rates of pellagra. The poor of Spain and Latin America subsisted on a diet heavy in corn products, such as tortillas and polenta, while their counterparts in the southern United States survived on cornbread, grits, hominy, and other variants of corn. Although such foods made it possible to fill the stomach cheaply, this diet was killing people in large numbers because it was not delivering the essential B vitamin niacin.

Vitamins

MODERN KNOWLEDGE OF VITA M I N S . As it turned out, corn, in fact, does

contain niacin, but to release the niacin from the large, fibrous molecules in corn, it is necessary to treat the corn with an alkaline solution, such as limewater. This is just one example of the vast knowledge that has accumulated since the time when the Polish-American biochemist Casimir Funk (1884–1967) coined the term vitamine in 1912. The first half of the word came from the Latin vita, or “life,” and the second half reflected Funk’s belief that all these substances belonged to a group of chemicals known as amines. Scientists later dropped the “e” when they discovered that not all vitamins contain an amine group. In the director Stephen Spielberg’s acclaimed 1987 film Empire of the Sun, one of the characters asks another, “Do you believe in vitamins?” The question reflects the relative newness of vitamins as an idea at the time when the movie was set, during World War II. After the war, interest in commercially produced vitamin supplements exploded, particularly among America’s middle classes. Pauling’s promotion of vitamin C, coming in the 1950s and 1960s, found a willing audience.

Niacin is present in whole grains, meat, fish, and dairy products. One of the foods from which niacin is not easily available is corn, and it so

Although vitamin supplements remain a big business today, opinions vary as to the importance of enhancing one’s diet with them. Many nutritionists insist that eating a well-balanced diet, consisting of the major food substances, is an effective and economical way to obtain nutrients for health. On the other hand, advocates of health foods and alternative medicine (medical practices that are not recognized officially by groups of university-trained and state-licensed medical doctors) insist that the recommended daily allowances established by the U.S. Food and Drug Administration do not provide sufficient vitamins for an average person.

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The American Dietetic Association (ADA) recommends that nutrient needs come from a variety of foods taken from different dietary sources rather than from self-prescribed vitamin supplements. The organization makes allowances, however, for supplement usage by people who need extra doses of key vitamins and minerals. Examples include the use of iron supplements by women experiencing heavy menstrual bleeding as well as supplements of iron, folic acid, and calcium by pregnant women. WHERE TO LEARN MORE Apple, Rima D. Vitamania: Vitamins in American Culture. New Brunswick, NJ: Rutgers University Press, 1996. Brody, Jane E., and Denise Grady. The New York Times Guide to Alternative Health: A Consumer Reference. New York: Times Books/Henry Holt, 2001.

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Kiple, Kenneth F., and Kriemhild Coneè Ornelas. The Cambridge World History of Food. New York: Cambridge University Press, 2000. Nardo, Don. Vitamins and Minerals. New York: Chelsea House Publishers, 1994. Reference Guide for Vitamins (Web site). . Snyder, Carl H. The Extraordinary Chemistry of Ordinary Things. New York: John Wiley and Sons, 1998. Vitamins and Coenzymes. Indiana State University (Web site). . The Vitamin Collection. Molecular Expressions: Exploring the World of Optics and Microscopy, Florida State University (Web site). . Vitamin-Deficiency Diseases. Medic Planet (Web site). .

Duyff, Roberta Larson. The American Dietetic Association’s Complete Food and Nutrition Guide. Minneapolis, MN: Chronimed Publishing, 1998.

Vitamins and Minerals Topic Page. Food and Nutrition Information Center National Agricultural Library, U.S. Department of Agriculture (Web site). .

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Genetics

CONCEPT Genetics is the area of biological study concerned with heredity and with the variations between organisms that result from it. It demands an understanding of numerous terms, such as DNA (deoxyribonucleic acid), a molecule in all cells that contains blueprints for genetic inheritance; genes, units of information about particular heritable traits, which are made from DNA; and chromosomes, DNA-containing bodies, located in the cells of most living things, that hold most of the organism’s genes. The vocabulary of genetics goes far beyond these three terms, as we shall see, but these are the core concepts. Among the areas in which genetics is applied is forensic science, or the application of science to matters of law—specifically, through “DNA fingerprinting,” whereby samples of skin, blood, semen, and other materials can be used to prove or disprove a suspect’s innocence. Another fascinating application of genetics is the Human Genome Project, an effort whose goals include the location and identification of every gene in the human body.

GENETICS

much as possible. For this reason, the Heredity essay is concerned with such issues as how traits are passed on and why they appear in a particular generation but not another. That essay addresses the topics of alleles, dominant and recessive genes, and so on. It also briefly discusses the history of studies in areas that encompass genetics, heredity, and the mechanics thereof. In general, the Heredity essay is concerned with the larger patterns of inheritance over the generations, while the present one examines inheritance at a level smaller than the microscopic—that is, from the molecular or biochemical level.

Somatic and Germ Cells Heredity begins with the cell, the smallest basic unit of all life. The information for heredity is carried within the cell nucleus, which is the control center not only in physical terms (it is usually located near the middle of the cell) but also because it contains the chromosomes. Within these threadlike structures is the genetic information organized in DNA molecules.

Genetics and heredity, the subject of another essay in this book, are closely related ideas. Whereas heredity is the transmission of genetic characteristics from ancestor to descendant through the genes, genetics is concerned with hereditary traits passed down from one generation to the next. It is very hard, if not impossible, to separate the two concepts completely, yet the entire body of knowledge encompassed by these topics is so large and so complex that it is best to separate them as

There are two basic types of cell in a multicellular organism: somatic, or body, cells, and germ, or reproductive, cells. The somatic cells are the primary components of most organisms, making up everything except some of the the cells in reproductive organs. The somatic cells of humans have 23 pairs of chromosomes, or 46 chromosomes overall, and are thus known as diploid cells. As the cells grow, they reproduce themselves by a process called mitosis, whereby a diploid cell splits to produce new diploids, each of which is a replica of the original. Thus cells grow and are replaced, making possible the formation of specific tissues and organs, such as

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muscles and nerves. Without mitosis, an organism’s cells would not regenerate, resulting not only in cell death but possibly even the death of the entire organism. Mitosis is also the means of reproduction for organisms that reproduce asexually (see Reproduction). A germ cell, by contrast, undergoes a process of cell division known as meiosis, whereby it becomes a haploid cell—a cell with half the basic number of chromosomes, which for a human would be 23 unpaired chromosomes. The sperm cells in a male and the egg cells in a female are both haploid germ cells: each contains only 23 chromosomes, and each is prepared to form a new diploid by fusion with another haploid. Sperm cells and egg cells are known as gametes, mature male or female germ cells that possess a haploid set of chromosomes and are prepared to form a new diploid by undergoing fusion with a haploid gamete of the opposite sex. When egg and sperm fuse, they form a zygote, in which the diploid chromosome number is restored, with the zygote possessing the same chromosomes as both the sperm and the egg. This cell carries all the genetic information needed to grow into an embryo and eventually a full-grown human, with the specific traits and attributes passed on by the parents. Not all offspring of the same parents are the same, of course, and this is because the sperm cells and egg cells vary in their genetic codes—that is, in their DNA blueprints.

The DNA Blueprint To understand genes and their biological function in heredity, it is necessary to understand the chemical makeup and structure of DNA. The complete DNA molecule often is referred to as the blueprint for life, because it carries all the instructions, in the form of genes, for the growth and functioning of organisms. This fundamental molecule is similar in appearance to a spiral staircase, which also is called a double helix. The sides of the DNA ladder are made up of alternate sugar and phosphate molecules, like links in a chain. The rungs, or steps, of DNA are made from a combination of four different chemical bases. Two of these, adenine and guanine, are known as purines, and the other two, cytosine and thymine, are pyrimidines. The four letters designating these bases—A, G, C, and T—are the alphabet of the genetic code, and each rung of

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the DNA molecule is made up of a combination of two of these letters. D N A S E Q U E N C E S . In this genetic code A always combines with T and C with G, to form what is called a base pair. Specific sequences of these base pairs make up the genes. Although a four-letter alphabet may seem rather small for constructing the extensive vocabulary that defines the myriad life-forms on Earth, in practice, the sequences of these base pairs make for almost limitless combinations. For any sequence, there are four possibilities as to the first two letters (AT, TA, CG, or GC) and four more possibilities for the second two letters. Thus, just for a four-letter sequence, there are 16 possibilities, and for each pair of letters added to the sequence, the total is multiplied by four. Given the long strings of base pairs that form DNA sequences, the numbers can be extremely large.

The more complex an organism, from bacteria to humans, the more rungs, or genetic sequences, appear on the ladder. The entire genetic makeup of a human, for example, may contain three billion base pairs, with the average gene unit being 2,000-200,000 base pairs long. Each one of these combinations has a different meaning, providing the code not only for the type of organism but also for specific traits, such as brown hair and blue eyes, dimples, detached earlobes, and so on and on. Except for identical twins, no two humans have exactly the same genetic information.

DNA Replication, Protein Synthesis, and RNA Genetic information is duplicated during the process of DNA replication, which is initiated by proteins in the cells. To produce identical genetic information during cell mitosis, the DNA hydrogen bonds between the two strands arebroken, splitting the DNA in half lengthwise. This process begins a few hours before the initiation of cell mitosis, and once it is completed, each half of the DNA ladder is capable of forming a new DNA molecule with an identical genetic code. It can do this because of specific chemical catalysts (a substance that enables a chemical reaction without taking part in it) that help synthesize the complementary strand. Catalysts formed from proteins are known as enzymes, and the functioning of specific cells and organisms is conducted by enzymes synthe-

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sized by the cells. Cells contain hundreds of different proteins, complex molecules that make up more than half of all solid body tissues and control most biological processes within and among these tissues. A cell functions in accordance with the particular protein—one of thousands of different types—it contains. It is the genetic basepair sequence in DNA that determines, or “codes for,” the specific arrangement of amino acids to build particular proteins.

because there are so many strands of DNA in the world, and so much material in the strands, mutation is bound to happen eventually—and, to an extent, at least, this is a good thing. Mutation is the engine that drives evolution, and a certain amount of genetic variation is necessary if species are to adapt by natural selection to a changing environment. If it were not for mutation, neither humans nor the many millions of other species that exist would ever have appeared.

Since the sites of protein production lie outside the cell nucleus, coded messages pass from the DNA in the nucleus to the cytoplasm, the material inside the cell that is external to the nucleus. This transfer of messages is achieved by RNA, or ribonucleic acid, and specifically by messenger RNA, or mRNA. Other types of RNA molecules are involved in linking the amino-acids together in a sequence form to shape the protein. (For more about amino acids, proteins, and enzymes, see the respective essays devoted to each subject.)

Mutation often occurs when chromosome segments from two parents physically exchange places with each other during the process of meiosis. This is known as genetic recombination. Genes also can change by mutations in the DNA molecule, which take place when a mutagen alters the chemical or physical makeup of DNA. The mutations that result are of two types, corresponding to the two basic varieties of cell: somatic mutations, which occur solely within the affected individual, and germinal mutations, which happen in the DNA of germ cells, producing altered genes that may be passed on to the next generation.

Mutation Once a protein has been created for a specific function, it cannot be changed. This is why the theory of acquired characteristics (the idea that changes in an organism’s overall anatomy, as opposed to changes in its DNA, can be passed on to offspring) is a fallacy. People may have genes that make it easier for them to acquire certain traits, such as larger muscles or the ability to play the piano through exercise or practice, respectively, but the traits themselves, if they are acquired during the life of the individual and are not encoded in the DNA, are not heritable. There is only one way in which changes that take place during the life of an organism can be passed on to its offspring, and that is if those changes are encoded in the organism’s DNA. This is known as mutation. Suppose lung cancer develops in a man as a result of smoking; unless a tendency to cancer is already a part of his genetic makeup, he cannot genetically pass the disease on to his unborn children. But if the tobacco has acted as a mutagen, a substance that brings about mutation, it is possible that his DNA can be altered in such a way as to pass on either the tendency toward lung cancer or some other characteristic.

Genetics

The odd thing about mutations is that while most of them are harmful, the few that are beneficial are, as we have noted, the driving force behind the evolution of life-forms that successfully adapt to their environments. Thus, while most germinal mutations bring about congenital disorders (birth defects) ranging from physical abnormalities to deficiencies in body or mind to diseases, every once in a while a germinal mutation results in an improvement, such as a change in body coloring that acts as camouflage. If the trait improves an individual organism’s chances for survival within a particular environment, it may become a permanent trait of the species, because the offspring with this gene have a greater chance of survival and thus will pass on the trait to succeeding generations. (For more about mutation, see the essay by that title. See also Evolution for a discussion of the role played by mutation and natural selection in the evolution of species.)

REAL-LIFE A P P L I C AT I O N S The Genetics Revolution

Because DNA is extremely stable chemically, it rarely mutates, or experiences an alteration in its physical structure, during replication. But

In the modern world genetics plays a part in more dramatic breakthroughs than any other

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A

SCIENTIST STUDIES THE ARRANGEMENT OF CHROMOSOME PAIRS, THE THREADLIKE

DNA-CONTAINING BODIES IN THE HUMAN GENOME PROJECT IS WORKING TO COMPLETE A MAP OUTTHE GENES IN HUMAN CELLS. (© BSIP/V & L /Photo Researchers. Reproduced by per-

CELLS THAT CARRY GENETIC INFORMATION. LINING THE LOCATION AND FUNCTION OF

mission.)

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field of biological study. These breakthroughs have an impact in a wide variety of areas, from curing diseases to growing better vegetables to catching criminals. The field of genetics is in the

midst of a revolution, and at the center of this exciting (and, to some minds, terrifying) phenomenon is the realm of genetic engineering: the alteration of genetic material by direct interven-

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tion in genetic processes. In agriculture, for instance, genes are transplanted from one organism to another to produce what are known as transgenic animals or plants. This approach has been used to reduce the amount of fat in cattle raised for meat or to increase proteins in the milk produced by dairy cattle. Fruits and vegetables also have been genetically engineered so that they do not bruise easily or have a longer shelf life. Not all of the work in genetics is genetic engineering per se; in the realm of law, for instance, the most important application of genetics is genetic fingerprinting. A genetic fingerprint is a sample of a person’s DNA that is detailed enough to distinguish it from the DNA of all others. The genetic fingerprint can be used to identify whether a man is the father of a particular child (i.e., to determine paternity), and it can be applied in the solving of crimes. If biological samples can be obtained from a crime scene—for example, skin under the fingernails of a murder victim, presumably the result of fighting against the assailant in the last few moments of life—it is possible to determine with a high degree of accuracy whether that sample came from a particular suspect. The use of DNA in forensic science is discussed near the conclusion of this essay. THE REVOLUTION IN MEDIC I N E . Some of the biggest strides in genetic

engineering and related fields are taking place, not surprisingly, in the realm of medicine. Genetic engineering in the area of health is aimed at understanding the causes of disease and developing treatments for them: for example, recombinant DNA (a DNA sequence from one species that is combined with the DNA of another species) is being used to develop antibiotics, hormones, and other disease-preventing agents. Vaccines also have been genetically re-engineered to trigger an immune response that will protect against specific diseases. One approach is to remove genetic material from a diseased organism, thus making the material weaker and initiating an immune response without causing the disease. (See Immunity and Immunology for more about how vaccines work.) Gene therapy is another outgrowth of genetics. The idea behind gene therapy is to introduce specific genes into the body either to correct a genetic defect or to enhance the body’s capabilities to fight off disease and repair itself. Since

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many inherited or genetic diseases are caused by the lack of an enzyme or protein, scientists hope one day to treat the unborn child by inserting genes to provide the missing enzyme. (For more about inherited disorders, see the essays Disease, Noninfectious Diseases, and Mutation.)

Genetics

THE HUMAN GENOME PROJECT.

One of the most exciting developments in genetics is the initiation of the Human Genome Project, designed to provide a complete genetic map outlining the location and function of the 40,000 or so genes that are found in human cells. (A genome is all of the genetic material in the chromosomes of a particular organism.) With the completion of this map, genetic researchers will have easy access to specific genes, to study how the human body works and to develop therapies for diseases. Gene maps for other species of animals also are being developed. The project had its origins in the 1990s, with the efforts of the United States Department of Energy (DOE) and the National Institutes of Health (NIH). The NIH connection is probably clear enough, but the DOE’s involvement at first might seem strange. What, exactly, does genetics have to do with electricity, petroleum, and other concerns of the DOE? The answer is that the DOE grew out of agencies, among them the Atomic Energy Commission (AEC), established soon after the explosion of the two atomic bombs over Japan in 1945. Even at that early date, educated nonscientists understood that the radioactive fallout produced from nuclear weaponry can act as a mutagen; therefore, Congress instructed the AEC to undertake a broad study of genetics and mutation and the possible consequences of exposure to radiation and the chemical by-products of energy production. Eventually, scientists in the AEC and, later, the DOE recognized that the best way to undertake such a study was to analyze the entire scope of the human genome. The project formally commenced on October 1, 1990, and is scheduled for completion in the middle of the first decade of the twenty-first century. Upon completion, the Human Genome Project will provide a vast store of knowledge and no doubt will lead to the curing of many diseases. Still, there are many who question the Human Genome Project in particular, and genetic engineering in general, on ethical grounds, fearing that it could give scientists or govern-

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THE

EXPLOSION OF AN ATOMIC BOMB OVER

NAGASAKI, JAPAN,

DURING

WORLD WAR II (SEPTEMBER 1, 1945). THE

RADIOACTIVE FALLOUT PRODUCED FROM NUCLEAR WEAPONRY CAN ACT AS A MUTAGEN, ALTERING THE CHEMICAL OR PHYSICAL MAKEUP OF

DNA. (© Bettmann/Corbis. Reproduced by permission.)

ments too much power, unleash a Nazi-style eugenics (selective breeding) program, or result in horrible errors, such as the creation of deadly new diseases. In fact, it is impossible to search “genetic engineering” on the World Wide Web without coming across the Web sites of literally dozens and dozens of agencies, activist groups, and individuals opposed to genetic engineering and the mapping of the human genome. For more about the Human Genome Project, genetic

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engineering, and their opponents, see Genetic Engineering.

Genetics in Forensic Science Forensic science, as we noted earlier, is the application of science to matters of law. It is based on the idea that a criminal always leaves behind some kind of material evidence that, through careful analysis, can be used to determine the identity of the perpetrator—and to exonerate

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someone falsely accused. Among those forms of material evidence of interest to forensic scientists working in the field of genetics are blood, semen, hair, saliva, and skin, all of which contain DNA that can be analyzed. In addition, there are areas of forensic science that rely on biological study, though not in the area of genetics: blood typing as well as the analysis of fingerprints or bite marks, both of which have patterns that are as unique to a single individual as DNA is. One of the first detectives to use science, including biology and medicine, in solving crimes was a fictional character: Sherlock Holmes, whose creator, the British writer Sir Arthur Conan Doyle (1859–1930), happened to be a physician as well. The first full-fledged (and real) police practitioner of forensic science was the French police official Alphonse Bertillon (1853–1914), who developed an identification system that consisted of a photograph and 11 body measurements, including dimensions of the head, arms, legs, feet, hands, and so on, for each individual. Bertillon claimed that the likelihood of two people having the same measurements for all 11 traits was less than one in 250 million. In 1894 fingerprints, which were easier to use and more unique than body measurements, were added to the Bertillon system. Fingerprints, unlike DNA, are unique to the individual; indeed, identical twins have the same DNA but different fingerprints. Mark Twain (1835–1910) could not have known this in 1894, when he published The Tragedy of Pudd’nhead Wilson, and the Comedy of Those Extraordinary Twins. Nonetheless, the story involves a murder committed by one man and blamed on his twin, who eventually is exonerated on the basis of fingerprint evidence—still a new concept at the time. In some situations, however, fingerprint evidence may be unavailable, and though lawenforcement agencies have developed extraordinary techniques for analyzing nearly invisible (i.e., latent) prints, sometimes this is still not enough.

Genetics

O. J. SIMPSON REACTS TO DESPITE DNA EVIDENCE THAT

THE JURY ’S VERDICT. LINKED BLOOD AT THE

CRIME SCENE WITH BLOOD IN HIS CAR, THE JURY FOUND HIM NOT GUILTY OF DOUBLE MURDER. (AP/WIde

World Photos. Reproduced by permission.)

the appearance of his prints at the scene of her murder in her Los Angeles home could be explained away easily, even though she had taken out a restraining order against her former husband (who she had accused of spousal abuse) some time before the murder. Rather than fingerprints, the prosecution in his murder trial used DNA evidence connecting blood at the crime scene with blood found in Simpson’s vehicle. (Some of this blood was apparently his own, since he had mysterious cuts on his hands that he could not explain to police officers.)

der of Nicole Brown Simpson and Ron Goldman on June 12, 1994, fingerprint evidence would have been ineffective in the case against the suspect, the former football star and actor O. J. Simpson. Since Nicole Simpson was his ex-wife,

A jury found Simpson not guilty on October 3, 1995, and jurors later claimed that the prosecution had failed to make a strong case using DNA evidence. Furthermore, they cited police contamination of the DNA evidence, which had been established in their minds by Simpson’s defense team, as a cause for reasonable doubt concerning Simpson’s guilt. In fact, assuming that the defense was fully justified in this claim, that would have meant only that the DNA samples would have been less (not more) likely to convict Simpson.

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T H E S I M P S O N CAS E A N D T H E CONTROVERSY OVER DNA EVID E N C E . For example, in the infamous mur-

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KEY TERMS ACQUIRED

CHARACTERISTICS:

Sometimes known as acquired characters or

CYTOPLASM:

cell that is external to the nucleus.

Lamarckism, after one of its leading proponents, the French natural philosopher Jean Baptiste de Lamarck (1744–1829), the theory of acquired characteristics is a fallacy that should not be confused with mutation. Acquired characteristics theory main-

A term for a cell that has the

DIPLOID:

basic number of doubled chromosome cells. In humans, somatic cells, which are diploid cells, have 23 pairs of chromosomes, for a total of 46 chromosomes. Deoxyribonucleic acid, a mole-

tains that changes that occur in an organ-

DNA:

ism’s overall anatomy (as opposed to

cule in all cells, and many viruses, that con-

changes in its DNA) can be passed on to

tains genetic codes for inheritance.

offspring.

DOMINANT:

AMINO ACIDS:

Organic compounds

trait that can manifest in the offspring

made of carbon, hydrogen, oxygen, nitro-

when inherited from only one parent. Its

gen, and (in some cases) sulfur bonded in

opposite is recessive.

characteristic formations. Strings of amino

ENZYME:

acids make up proteins.

speeds up chemical reactions in the bodies

In genetics, a term for a

A protein material that

A pair of chemicals that

of plants and animals without itself taking

form the “rungs” on a DNA molecule,

part in or being consumed by those reac-

which has the shape of a spiral staircase. A

tions.

base pair always consists of a type of chem-

FORENSIC SCIENCE:

ical called a purine on one side and a

tion of science to matters of law and legal

chemical termed a pyrimidine on the

or police procedure.

BASE PAIR:

other. This means that DNA base pairs

GAMETE:

The applica-

A mature male or female

always consist of adenine linked with

germ cell that possesses a haploid set of

thymine and guanine with cytosine.

chromosomes and is prepared to form a

BIOCHEMISTRY:

The area of the bio-

logical sciences concerned with the chemical substances and processes in organisms. BODY CELL:

See somatic cell.

new diploid by undergoing fusion with a haploid gamete of the opposite sex. GENE:

A unit of information about a

particular heritable trait. Usually stored on

A DNA-containing

chromosomes, genes contain specifications

body, located in the cells of most living

for the structure of a particular polypep-

things, that holds most of the organism’s

tide or protein.

genes.

GENETIC ENGINEERING:

CHROMOSOME:

CONGENITAL

106

The material inside a

DISORDER:

An

The alter-

ation of genetic material by direct inter-

abnormality of structure or function or a

vention in genetic processes.

disease that is present at birth. Congenital

GENETIC FINGERPRINT:

disorders also are called birth defects.

of a person’s DNA that is detailed enough

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KEY TERMS

CONTINUED

to distinguish it from all other people’s

MITOSIS:

DNA.

that produces diploid cells. Compare with

A process of cell division

A

meiosis.

process whereby chromosome segments

MRNA:

from two parents physically exchange

molecule of RNA that carries the genetic

places with each other during the process

information for producing proteins.

GENETIC

RECOMBINATION:

of meiosis. This is one of the ways that mutation occurs.

Messenger ribonucleic acid, a

MUTAGEN:

tor that increases the rate of mutation. The area of biological

GENETICS:

A chemical or physical fac-

study concerned with heredity, with hereditary traits passed down from one generation to the next through the genes, and

MUTATION:

Alteration in the physical

structure of an organism’s DNA, resulting in a genetic change that can be inherited. The process

with the variations between organisms that

NATURAL SELECTION:

result from heredity.

whereby some organisms thrive and others

GENOME:

All of the genetic material in

the chromosomes of a particular organism. GERM CELL:

One of two basic types

of cells in a multicellular organism. In contrast to somatic, or body, cells, germ cells are involved in reproduction. GERMINAL MUTATION:

perish, depending on their degree of adaptation to a particular environment. NUCLEIC ACIDS:

nucleotide chains. NUCLEOTIDE:

A mutation

Acids, including

DNA and RNA, that are made up of

A compound formed

from one of several types of sugar joined

that occurs in the germ cells, meaning that

with a base of purine or pyrimidine (see

the mutation can be passed on to the

base pair) and a phosphate group.

organism’s offspring.

Nucleotides are the basis for nucleic acids.

HAPLOID:

A term for a cell that has

NUCLEUS:

The control center of a

half the number of chromosome cells that

cell, where DNA is stored.

appear in a diploid, or somatic, cell. In

POLYPEPTIDE:

humans, germ cells, which are haploid

and 50 amino acids.

cells, have 23 unpaired chromosomes, as opposed to the 23 paired chromosomes (46 overall) that appear in a somatic cell.

PROTEINS:

A group of between 10

Large molecules built

from long chains of 50 or more amino acids. Proteins serve the functions of pro-

The transmission of genet-

moting normal growth, repairing damaged

ic characteristics from ancestor to descen-

tissue, contributing to the body’s immune

dant through the genes.

system, and making enzymes.

HEREDITY:

HERITABLE:

Capable of being inherited.

RECESSIVE:

In genetics, a term for a

The process of cell division

trait that can manifest in the offspring only

that produces haploid genetic material.

if it is inherited from both parents. Its

Compare with mitosis.

opposite is dominant.

MEIOSIS:

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KEY TERMS REPRODUCTIVE CELL: RNA:

See germ cell.

Ribonucleic acid, a molecule

translated from DNA in the cell nucleus that directs protein synthesis in the cytoplasm. See also mRNA. SOMATIC CELL:

One of two basic types

of cells in a multicellular organism. In con-

A mutation that occurs in cells other than the reproductive, or sex, cells. These mutations, as contrasted with germinal mutations, cannot be transmitted to the next generation. SOMATIC MUTATION:

To manufacture chemically, as in the body.

SYNTHESIZE:

as body cells) do not play a part in reproduc-

A mutation in which chromosomes exchange parts.

tion; rather, they make up the tissues, organs,

ZYGOTE:

trast to germ cells, somatic cells (also known

and other parts of the organism.

At the same time, a number of legitimate concerns regarding the use of DNA evidence were raised by experts for the defense in the Simpson trial. Samples can become contaminated and thus difficult to read; small samples are difficult for analysts to work with effectively; and results are often open to interpretation. Furthermore, the outcome of the Simpson case illustrates the fact that findings based on DNA evidence are not readily understood by non-specialists, and may not make the best basis for a caseparticularly in one so fraught with controversy. The prosecution based its case almost entirely on extremely technical material, explained in excruciating detail by experts who had devoted their lives to studying areas that are far beyond the understanding of the average person. Attempting to wow the jurors with science, the prosecution instead seemed to create the impression that DNA evidence was some sort of hocus-pocus invented to frame an innocent man. Simpson went free, though the jury in a 1996 civil trial (which took a much simpler approach, eschewing complicated DNA testimony) found him guilty.

TRANSLOCATION:

A diploid cell formed by the fusion of two gametes.

men. One of them, named Colin Pitchfork, paid another man to provide a sample in his place. This attracted the attention of the police, who tested his DNA and found their man. Since that time, DNA evidence has been used in more than 24,000 cases and has aided in the conviction of about 700 suspects. The DNA in such cases is not always obtained from a human subject. In the investigation of the May 1992 murder of Denise Johnson in Arizona, a homicide detective found two seed pods from a paloverde tree in the bed of a pickup truck owned by the suspect, Mark Bogan. The accused man admitted having known the victim but denied ever having been near the site where her body was found. It so happened that there was a paloverde tree at the site, and testing showed that the DNA in the pods on his truck bed matched that of the tree itself. Bogan became the first suspect ever convicted by a plant.

use of DNA evidence gained something of a bad name. Nonetheless, it has been successful in less high profile cases, beginning in 1986, when English police tracked down a rapist and murderer by collecting blood samples from some 2,000

On the other hand, in some cases, DNA evidence has cleared a suspect falsely accused. Such was the case with Kerry Kotler, convicted in 1981 for rape, robbery, and burglary and sentenced to 25–50 years in jail. In 1988, Kotler began petitioning for DNA analysis, which subsequently showed that his DNA did not match that of the rapist, who had left a semen sample in the victim’s underwear. Kotler was released in December 1992 and in March 1996 was awarded $1.5 million in damages for his wrongful imprisonment. The story does not end there, however.

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DNA EVIDENCE SUCCESS S T O R I E S . Because of the Simpson case, the

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Kotler’s case turned out to be one of the more bizarre in the annals of forensic DNA testing. Perhaps he did not commit the first rape, but a month after he received the damage award, he was on his way back to prison for the August 1995 rape of another victim. This time prosecutors showed that Kotler’s semen matched samples taken from his victim’s clothing—and to prove their case, they used DNA testing. WHERE TO LEARN MORE Department of Energy Human Genome Program (Web site). . The DNA Files/National Public Radio (Web site). . Fridell, Ron. DNA Fingerprinting: The Ultimate Identity. New York: Franklin Watts, 2001.

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Genetics Education Center, University of Kansas Medical Center (Web site). .

Genetics

Henig, Robin Marantz. The Monk in the Garden: The Lost and Found Genius of Gregor Mendel, the Father of Genetics. Boston: Houghton Mifflin, 2000. Lerner, K. Lee, and Brenda Wilmoth Lee. World of Genetics. Detroit: Gale Group, 2002. National Human Genome Research Institute (Web site). . Schwartz, Jeffrey H. Sudden Origins: Fossils, Genes, and the Emergence of Species. New York: John Wiley and Sons, 1999. Tudge, Colin. The Impact of the Gene: From Mendel’s Peas to Designer Babies. New York: Hill and Wang, 2001. Virtual Library on Genetics, Oak Ridge National Laboratory (Web site). .

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HEREDITY Heredity

CONCEPT Heredity is the transmission of genetic characteristics from ancestor to descendant through the genes. As a subject, it is tied closely to genetics, the area of biological study concerned with hereditary traits. The study of heritable traits helps scientists discern which are dominant and therefore are likely to be passed on from one parent to the next generation. On the other hand, a recessive trait will be passed on only if both parents possess it. Among the possible heritable traits are genetic disorders, but study in this area is ongoing, and may yield many surprises.

HOW IT WORKS

whose importance in the study of genetics is comparable to that of Charles Darwin (1809– 1882) in the realm of evolutionary studies—was the Austrian monk and botanist Gregor Mendel (1822–1884). G E N E S . For thousands of years, people have had a general understanding of genetic inheritance—that certain traits can be, and sometimes are, passed along from one generation to the next—but this knowledge was primarily anecdotal and derived from casual observation rather than from scientific study. The first major scientific breakthrough in this area came in 1866, when Mendel published the results of a study on the hybridization of plants in which he crossed pea plants of the same species that differed in only one trait.

Heredity and Genetics As discussed at the beginning of the essay on genetics, the subjects of genetics and heredity are inseparable from each other, but there are so many details that it is extremely difficult to wrap one’s mind around the entire concept. It is advisable, then, to break up the overall topic into more digestible bits. One way to do this is to study the biochemical foundations of genetics as a subject in itself, as is done in Genetics, and then to investigate the impact of genetic characteristics on inheritance in a separate context, as we do here. Also included in the present essay is a brief history of genetic study, which reveals something about the way in which these many highly complex ideas fit together. Many brilliant minds have contributed to the modern understanding of genetics and heredity; unfortunately, within the present context, space permits the opportunity to discuss only a few key figures. The first—a man

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Mendel bred these plants over the course of several successive generations and observed the characteristics of each individual. He found that certain traits appeared in regular patterns, and from these observations he deduced that the plants inherited specific biological units from each parent. These units, which he called factors, today are known as genes, or units of information about a particular heritable trait. From his findings, Mendel formed a distinction between genotype and phenotype that is still applied by scientists studying genetics. Genotype may be defined as the sum of all genetic input to a particular individual or group, while phenotype is the actual observable properties of that organism. We return to the subjects of genotype and phenotype later in this essay. M U TAT I O N A N D D N A . Although

Mendel’s theories were revolutionary, the scientific establishment of his time treated these new

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ideas with disinterest, and Mendel died in obscurity. Then, in 1900, the Dutch botanist Hugo De Vries (1848–1935) discovered Mendel’s writings, became convinced that his predecessor had made an important discovery, and proceeded to take Mendel’s theories much further. Unlike the Austrian monk, De Vries believed that genetic changes occur in big jumps rather than arising from gradual or transitional steps. In 1901 he gave a name to these big jumps: mutations. Today a mutation is defined as an alteration of a gene, which contains something neither De Vries nor Mendel understood: deoxyribonucleic acid, or DNA. Actually, DNA, a molecule that contains genetic codes for inheritance, had been discovered just four years after Mendel presented his theory of factors. In 1869 the Swiss biochemist Johann Friedrich Miescher (1844–1895) isolated a substance from the remnants of cells in pus. The substance, which contained both nitrogen and phosphorus, separated into a protein and an acid molecule and came to be known as nucleic acid. A year later he discovered DNA itself in the nucleic acid, but more than 70 years would pass before a scientist discerned its purpose. T H E D I S C O V E RY O F C H R O M O S O M E S . In the meantime, another major

step in the history of genetics was taken just two years after De Vries outlined his mutation theory. In 1903 the American surgeon and geneticist Walter S. Sutton (1877–1916) discovered chromosomes, threadlike structures that split and then pair off as a cell divides in sexual reproduction. Today we know that chromosomes contain DNA and hold most of the genes in an organism, but that knowledge still lay in the future at the time of Sutton’s discovery. In 1910 the American geneticist Thomas Hunt Morgan (1866–1945) confirmed the relationship between chromosomes and heredity through experiments with fruit flies. He also discovered a unique pair of chromosomes called the sex chromosomes, which determine the sex of offspring. From his observation that a sex-specific chromosome was always present in flies that had white eyes, Morgan deduced that specific genes reside on chromosomes. A later discovery showed that chromosomes could mutate, or change structurally, resulting in a change of characteristics that could be passed on to the next generation.

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DNA MAKES ITS APPEARA N C E . All this time, scientists knew about the

Heredity

existence of DNA without guessing its function. Then, in the 1940s, a research team consisting of the Canadian-born American bacteriologist Oswald Avery (1877–1955), the American bacteriologist Maclyn McCarty (1911–), and the Canadian-born American microbiologist Colin Munro MacLeod (1909–1972) discovered the blueprint function of DNA. By taking DNA from one type of bacteria and inserting it into another, they found that the second form of bacteria took on certain traits of the first. The final proof that DNA was the specific molecule that carries genetic information came in 1952, when the American microbiologists Alfred Hershey (1908–1997) and Martha Chase (1927–) showed that transferring DNA from a virus to an animal organ resulted in an infection, just as if an entire virus had been inserted. But perhaps the most famous DNA discovery occurred a year later, when the American biochemist James D. Watson (1928–) and the English biochemist Francis Crick (1916–) solved the mystery of the exact structure of DNA. Their goal was to develop a DNA model that would explain the blueprint, or language, by which the molecule provides necessary instructions at critical moments in the course of cell division and growth. To this end, Watson and Crick focused on the relationships between the known chemical groups that compose DNA. This led them to propose a double helix, or spiral staircase, model, which linked the chemical bases in definite pairs. Using this twisted-ladder model, they were able to explain how the DNA molecule could duplicate itself, since each side of the ladder contains a compound that fits with a compound on the opposite side. If separated, each would serve as the template for the formation of its mirror image.

Autosomes and Sex Chromosomes Genetic information is organized into chromosomes in the nucleus, or control center, of the cell. Human cells have 46 chromosomes each, except for germ, or reproductive, cells (i.e., sperm cells in males and egg cells in females), which each have 23 chromosomes. Each person receives 23 chromosomes from the mother’s egg and 23 chromosomes from the father’s sperm. Of these 23 chromosomes, 22 are called autosomes, or

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might say “unattached earlobe,” indicating a lobe that is not fully attached and therefore can be flapped.

Heredity

DOMINANT AND RECESSIVE A L L E L E S . Each person has two alleles of the

IF

TWO GROUPS OF THE SAME SPECIES ARE SEPARATED

FOR A LONG TIME, GENETIC DRIFT MAY LEAD TO THE FORMATION OF DISTINCT SPECIES, AS WHEN THE

COLRIVER CUT OPEN THE GRAND CANYON AND ISOLATED THE K AIBAB SQUIRREL OF THE NORTH RIM FROM THE ABERT SQUIRREL (SHOWN HERE) OF THE SOUTH. (© ORADO

W. Perry Conway/Corbis. Reproduced by permission.)

non-sex chromosomes, meaning that they do not determine gender. The remaining chromosome, the sex chromosome, is either an X or a Y. Females have two Xs (XX), and males have one of each (XY), meaning that females can pass only an X to their offspring, whereas males can pass either an X or a Y. (This, in turn, means that the sperm of the father determines the gender of the offspring.)

Alleles

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same gene—the genotype for a single locus. These can be written as uppercase or lowercase letters of the alphabet, with capital letters defining dominant traits and lowercase letters indicating recessive traits. A dominant trait is one that can manifest in the offspring when inherited from only one parent, whereas a recessive trait must be inherited from both parents in order to manifest. For instance, brown eyes are dominant and thus would be represented in shorthand with a capital B, whereas blue eyes, which are recessive, would be represented with a lowercase b. Genotypes are either homozygous (having two identical alleles, such as BB or bb) or heterozygous (having different alleles, such as Bb). The phenotype, however—that is, the actual eye color— must be one or the other, because both sets of genes cannot be expressed together. Unless there is some highly unusual mutation, a child will not have one brown eye and one blue eye; instead, the dominant trait will overpower the recessive one and determine the eye color of the child. If an individual’s genotype is BB or Bb, that person definitely will have brown eyes; the only way for the individual to have blue eyes is if the genotype is bb—meaning that both parents have blue eyes. Oddly, two parents with brown eyes could produce a child with blue eyes. How is that possible? Suppose both the mother and the father had the heterozygous alleles Bb— a dominant brown and a recessive blue. There is then a 25% chance that the child could inherit both parents’ recessive genes, for a bb genotype—and a blue-eyed phenotype.

The 44 autosomes have parallel coded information on each of the two sets of 22 autosomes, and this coding is organized into genes, which provide instructions for the synthesis (manufacture) of specific proteins. Each gene has a set locus, or position, on a particular chromosome, and for each locus, there are two slightly different forms of a gene. These differing forms, known as alleles, each represent slightly different codes for the same trait. One allele, for instance, might say “attached earlobe,” meaning that the bottom of the lobe is fully attached to the side of the head and cannot be flapped. Another allele, however,

LEARNING FROM HEREDITA RY LAW. What we have just described is

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called genetic dominance, or the ability of a single allele to control phenotype. This principle of classical Mendelian genetics does not explain everything. For example, where height is concerned, there is not necessarily a dominant or recessive trait; rather, offspring typically have a height between that of the parents, because height also is determined by such factors as diet. (Also, more than one pair of genes is involved.) Hereditary law does, however, help us predict everything from hair and eye color to genetic dis-

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orders. As with the blue-eyed child of browneyed parents, it is possible that neither parent will show signs of a genetic disorder and yet pass on a double-recessive combination to their children. Again, however, other factors—including genetic ones—may come into play. For example, Down syndrome (discussed in Mutation) is caused by abnormalities in the number of chromosomes, with the offspring possessing 47 chromosomes instead of the normal 46.

Heredity

REAL-LIFE A P P L I C AT I O N S Population Genetics Studies in heredity and genetics can be applied not only to an individual or family but also to a whole population. By studying the gene pool (the sum of all the genes shared by a population) for a given group, scientists working in the field of population genetics seek to explain and understand specific characteristics of that group. Among the phenomena of interest to population geneticists is genetic drift, a natural mechanism for genetic change in which specific traits coded in alleles change by chance over time, especially in small populations, as when organisms are isolated on an island. If two groups of the same species are separated for a long time, genetic drift may lead even to the formation of distinct species from what once was a single life-form. When the Colorado River cut open the Grand Canyon, it separated groups of squirrels that lived in the high-altitude pine forest. Over time, populations ceased to interbreed, and today the Kaibab squirrel of the north rim and the Abert squirrel of the south are different species, no more capable of interbreeding than humans and apes.

THERE

ARE SEVERAL THOUSAND GENETIC DISORDERS,

CLASSIFIED INTO AUTOSOMAL DOMINANT, AUTOSOMAL RECESSIVE, SEX-LINKED, AND MULTIFACTORIAL TYPES.

DWARFISM,

FOR EXAMPLE, CAN BE CAUSED BY ACHON-

DROPLASIA, TRANSMITTED BY A GENE INHERITED FROM ONE PARENT

(DOMINANT),

OR BY A GROWTH HORMONE

DEFICIENCY, TRANSMITTED BY GENES INHERITED FROM BOTH PARENTS (RECESSIVE). (© Photo Researchers. Reproduced

by permission.)

of peoples from Siberia to North America in about 11,000 B.C. took place in two distinct waves.

Genetic Disorders

Where humans are concerned, population genetics can aid, for instance, in the study of genetic disorders. As discussed in Mutation, certain groups are susceptible to particular conditions: thus, cystic fibrosis is most common among people of northern European descent, sickle cell anemia among those of African and Mediterranean ancestry, and Tay-Sachs disease among Ashkenazim, or Jews whose ancestors lived in eastern Europe. Studies in population genetics also can supply information about prehistoric events. As a result of studying the DNA in fossil records, for example, some scientists have reached the conclusion that the migration

There are several thousand genetic disorders, which can be classified into one of several groups: autosomal dominant disorders, which are transmitted by genes inherited from only one parent; autosomal recessive disorders, which are transmitted by genes inherited from both parents; sex-linked disorders, or ones associated with the X (female) and Y (male) chromosome; and multifactorial genetic disorders. If one parent has an autosomal dominant disorder, the offspring have a 50% chance of inheriting that disease. Approximately 2,000 autosomal dominant disorders have been identified, among them Huntington disease, achondroplasia (a type of

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Heredity

KEY TERMS For any locus, one of two (or more) slightly different forms of a gene. These differing forms mean that alleles code for different versions of the same trait. ALLELE:

AUTOSOMES:

The 22 non-sex chro-

One of two basic types

GERM CELL:

of cells in a multicellular organism. In contrast to somatic, or body, cells, germ cells are involved in reproduction. HEREDITY:

The transmission of genet-

mosomes.

ic characteristics from ancestor to descen-

A DNA-containing body, located in the cells of most living things, that holds most of the organism’s genes.

dant through the genes.

CHROMOSOME:

HETEROZYGOUS:

ent alleles—for example, Bb. Having two identical

HOMOZYGOUS:

Deoxyribonucleic acid, a molecule in all cells, and many viruses, that contains genetic codes for inheritance.

DNA:

In genetics, a term for a trait that can manifest in the offspring when inherited from only one parent. Its opposite is recessive.

alleles, such as BB or bb. LOCUS:

The position of a particular

gene on a specific chromosome.

DOMINANT:

A unit of information about a particular heritable trait. Usually stored on chromosomes, genes contain specifications for the structure of a particular polypeptide or protein. GENE:

The sum of all the genes shared by a population, such as that of a species. GENE POOL:

MUTATION:

Alteration in the physical

structure of an organism’s DNA, resulting in a genetic change that can be inherited. NUCLEUS:

The control center of a

cell, where DNA is stored. PHENOTYPE:

The actual observable

properties of an organism, as opposed to its genotype. RECESSIVE:

In genetics, a term for a

trait that can manifest in the offspring only if it is inherited from both parents. Its

A condition, such as a hereditary disease, that can be traced to an individual’s genetic makeup.

opposite is dominant.

The ability of a single allele to control phenotype.

females have two X chromosomes (XX),

The sum of all genetic input to a particular individual or group.

SYNTHESIZE:

GENETIC DISORDER:

GENETIC DOMINANCE:

GENOTYPE:

114

Having two differ-

SEX

CHROMOSOMES:

Chromo-

somes that determine gender. Human and males have an X and a Y (XY). To manufacture chemi-

cally, as in the body.

dwarfism), Marfan syndrome (extra-long limbs),

The first two are discussed in Mutation.

polydactyly (extra toes or fingers), some forms of

Marfan syndrome, or arachnodactyly (“spider

glaucoma (a vision disorder), and hypercholes-

arms”), is historically significant because it is

terolemia (high levels of cholesterol in the

believed that Abraham Lincoln suffered from

blood).

that condition. Some scientists even maintain

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that his case of Marfan, a disease sometimes accompanied by eye and heart problems, was so severe that he probably would have died six months or a year after the time of his actual death by assassination at age 56 in April 1865. RECESSIVE GENE DISORD E R S . Just as a person has a 25% chance of

inheriting two recessive alleles, so two parents who each have a recessive gene for a genetic disorder stand a 25% chance of conceiving a child with that disorder. Among the approximately 1,000 known recessive genetic disorders are cystic fibrosis, sickle cell anemia, Tay-Sachs disease, galactosemia, phenylketonuria, adenosine deaminase deficiency, growth hormone deficiency, Werner syndrome (juvenile muscular dystrophy), albinism (lack of skin pigment), and autism. Several of these conditions are discussed briefly elsewhere, and albinism is treated at length in Mutation. Note that all of the disorders mentioned earlier, in the context of population genetics, are recessive gene disorders. Phenylketonuria (see Metabolism) and galactosemia are examples of metabolic recessive gene disorders, in which a person’s body is unable to carry out essential chemical reactions. For example, people with galactosemia lack an enzyme needed to metabolize galactose, a simple sugar that is found in lactose, or milk sugar. If they are given milk and other foods containing galactose early in life, they eventually will suffer mental retardation. SEX-LINKED GENETIC DISORD E R S . Dominant sex-linked genetic disorders

affect females, are usually fatal, and—fortunately—are rather rare. An example is Albright hereditary osteodystrophy, which brings with it seizures, mental retardation, and stunted growth. On the other hand, several recessive sex-linked genetic disorders are well known, though at least one of them, color blindness, is relatively harmless. Among the more dangerous varieties of these disorders, which are passed on to sons through their mothers, the best known is hemophilia, discussed in Noninfectious Diseases. Many recessive sex-linked genetic disorders affect the immune, muscular, and nervous systems and are typically fatal. An example is severe combined immune deficiency syndrome (SCID), which is characterized by a very poor ability to combat infection. The only known cure for SCID is bone marrow transplantation from a close relative. Short of a cure, patients may be forced to live enclosed in a large plastic bubble that protects

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them from germs in the air. From this sad fact derives the title of an early John Travolta movie, The Boy in the Plastic Bubble (1976), based on the true story of the SCID victim Tod Lubitch. (The ending, in which Travolta, as Tod, leaves his bubble and literally rides off into the sunset with his beautiful neighbor Gina, is more Hollywood fiction than fact. Lubitch actually died in his early teens, shortly after receiving a bone marrow transplant.)

Heredity

M U LT I FA C T O R I A L GENETIC D I S O R D E R S . Scientists often find it diffi-

cult to determine the relative roles of heredity and environment in certain medical disorders, and one way to answer this question is with statistical and twin studies. Identical and fraternal twins who have been raised in different and identical homes are evaluated for multifactorial genetic disorders. Multifactorial genetic disorders include medical conditions associated with diet and metabolism, among them obesity, diabetes, alcoholism, rickets, and high blood pressure. Other such multifactorial conditions are a tendency toward certain infectious diseases, such as measles, scarlet fever, and tuberculosis; schizophrenia and some other psychological illnesses; clubfoot and cleft lip; and various forms of cancer. The tendency of a particular person to be susceptible to any one of these disorders is a function of that person’s genetic makeup, as well as environmental factors.

Breeding within the Family If there is one thing that most people know about heredity and breeding, it is that a person should never marry or conceive offspring with close relatives. Aside from moral restrictions, there is the fear of the genetic defects that would result from close interbreeding. How close is too close? Certainly, first cousins are off-limits as potential mates, though second or third cousins (people who share the same great-grandparents and the same great-great-grandparents, respectively) are probably far enough apart. Hence, the phrase “kissin’ cousins,” meaning a relative who is a distant enough to be considered a potential partner. What kind of defects? Hemophilia, mentioned earlier, is popularly associated with royalty because several members of European ruling houses around the turn of the nineteenth century had it. Common wisdom maintains that the tendency toward the disease resulted from the

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fact that royalty were apt to marry close relatives. In fact, hemophilia has nothing to do with royalty per se and certainly bears no relation to marriages between close relatives. Research findings gathered over the course of more than three decades, beginning in 1965, indicate that many views about first cousins marrying may be more a matter of tradition than of scientific fact. According to information published in the Journal of Genetic Counseling and reported in the New York Times in April 2002, first cousins who have children together face only a slightly higher risk than parents who are completely unrelated. For example, within the population as a whole, the risk that a child will be born with a serious defect, such as cystic fibrosis, is 3-4%, while first cousins who conceive a child typically add another 1.7-2.8 percentage points of risk. Although this represents nearly double the risk, it is still a very small factor.

win, who fathered many healthy children with his cousin, Emma Wedgwood. WHERE TO LEARN MORE Ackerman, Jennifer. Chance in the House of Fate: A Natural History of Heredity. Boston: Houghton Mifflin, 2001. Center for the Study of Multiple Birth (Web site). . Clark, William R., and Michael Grunstein. Are We Hardwired?: The Role of Genes in Human Behavior. New York: Oxford University Press, 2000. The Gene School (Web site). . Genetic Disorders (Web site). . Grady, Denise. “Few Risks Seen to the Children of First Cousins.” New York Times, April 4, 2002. Hawley, R. Scott, and Catherine A. Mori. The Human Genome: A User’s Guide. San Diego: Academic Press, 1999.

Researchers were quick to point out that mating should not take place between persons more closely related than first cousins. According to Denise Grady in the New York Times, “The report made a point of saying that the term ‘incest’ should not be applied to cousins, but only to sexual relations between siblings or between parents and children.” First cousins, on the other hand, are a quite different matter, a fact borne out by the long history of people who married their first cousins. One example was Charles Dar-

Wynbrandt, James, and Mark D. Ludman. The Encyclopedia of Genetic Disorders and Birth Defects. New York: Facts on File, 2000.

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Heredity and Genetics. The Biology Project at the University of Arizona (Web site). . Reproduction and Heredity (Web site). . Ridley, Matt. Genome: The Autobiography of a Species in 23 Chapters. New York: HarperCollins, 1999.

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Genetic Engineering

GENETIC ENGINEERING

CONCEPT Genetic engineering is the alteration of genetic material by direct intervention in genetic processes with the purpose of producing new substances or improving functions of existing organisms. It is a very young, exciting, and controversial branch of the biological sciences. On the one hand, it offers the possibility of cures for diseases and countless material improvements to daily life. Hopes for the benefits of genetic engineering are symbolized by the Human Genome Project, a vast international effort to categorize all the genes in the human species. On the other hand, genetic engineering frightens many with its potential for misuse, either in Nazi-style schemes for population control or through simple bungling that might produce a biological holocaust caused by a man-made virus. Symbolic of the alarming possibilities is the furor inspired by a single concept on the cutting edge of genetic engineering: cloning.

HOW IT WORKS DNA Any discussion of genetics makes reference to DNA (deoxyribonucleic acid), a molecule that contains genetic codes for inheritance. DNA resides in chromosomes, threadlike structures found in the nucleus, or control center, of every cell in every living thing. Chromosomes themselves are made up of genes, which carry codes for the production of proteins. The latter, of which there are many thousands of different varieties, make up the majority of the human body’s dry weight.

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Although it is central to the latest advances in modern genetic research, DNA was discovered more than 130 years ago. In 1869 the Swiss biochemist Johann Friedrich Miescher (1844–1895) isolated a substance, containing both nitrogen and phosphorus, that separated into a protein and an acid molecule. He called it nucleic acid, and in this material he discovered DNA. Some 74 years would pass, however, before scientists recognized the function of the nucleic acid Miescher had discovered. Then, in 1944, a research team led by the Canadian-born American bacteriologist Oswald Avery (1877–1955) found that by taking DNA from one type of bacterium and inserting it into another, the second bacterium took on certain traits of the first. This experiment, along with other experiments and research, proved that DNA serves as a blueprint for the characteristics and functions of organisms. T H E D O U B L E H E L I X . Nine years later, in 1953, the American biochemist James D. Watson (1928–) and the English biochemist Francis Crick (1916–) solved the mystery of DNA’s structure and explained the means by which it provides necessary instructions at critical moments in the course of cell division and growth. They proposed a double helix, or spiral staircase, model, which linked the chemical bases of DNA in definite pairs. Using this twisted-ladder model, they were able to explain how the DNA molecule could duplicate itself, since each side of the ladder is identical to the other; if separated, each would serve as the template for the formation of its mirror image.

The sides of the DNA ladder are composed of alternating sugar and phosphate molecules, like links in a chain, and consist of four different chemical bases: adenine, guanine, cytosine, and

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To see where this might lead, imagine that you started with a penny and tried to quadruple your funds every day. The first day there would not be a dramatic increase, since you would have to earn only $0.04, and even by day 4 you would need only $2.56 to meet your goal. But as the quadrupling process continued, day by day the sums of money would get bigger ($655.36 on day 8) and bigger ($16,772.16 on day 12) and bigger ($687,194,767.36 on day 18). Given the fact that the human body contains an almost unfathomable number of genes, each of which may be between 2,000 and 200,000 base pairs long, one can begin to imagine just how large the number of possibilities would become.

Genetic Engineering

Each one of these combinations has a different meaning, providing the code for all manner of specific traits, such as brown hair and blue eyes, dimples, unattached earlobes, and so on. Except for identical twins, no two humans have exactly the same genetic information. What follows are just a few facts about the human genome—that is, all of the genetic material in the chromosomes of the human organism: COMPUTER MODEL OF DNA, SHOWING ITS DOUBLE HELIX, OR SPIRAL STAIRCASE, FORM, WHICH LINKS THE CHEMICAL BASES OF DNA IN PAIRS. EACH SIDE OF THE LADDER IS IDENTICAL TO THE OTHER; IF SEPARATED, EACH WOULD SERVE AS THE TEMPLATE FOR THE FORMATION OF ITS MIRROR IMAGE. (© Kenneth Eward/Grafz/Science Source/Photo Researchers. Reproduced by permission.)

thymine. The four letters designating these bases—A, G, C, and T—are the alphabet of the genetic code, and each rung of the DNA molecule is made up of a combination of two of these letters. Owing to specific chemical affinities, A always combines with T and C with G, to form what is called a base pair. Specific sequences of these base pairs, which are bonded to each other by atoms of hydrogen, constitute the genes. E N D L E SS C O M B I N AT I O N S . A four-letter alphabet may seem rather small for constructing the extensive vocabulary that defines the myriad life-forms on Earth. If one stops to consider the exponential operations involved, however, it is easy to understand how large the range of possibilities can become. For any sequence, there are four possibilities for the first two letters (AT, TA, CG, or GC) and four more possibilities for the second two letters. Thus, just for a four-letter sequence, there are 16 possibilities, and for each pair of letters added to the sequence, the total is multiplied by four.

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Some Facts About the Human Genome • The human body contains about 100 trillion cells. • Each cell has a DNA code consisting of some 1.5 billion base pairs. • The DNA in each cell, if stretched to its full length, would be 6 ft. (1.8 m) long—yet it fits into a space about 0.0004 in. (0.0001 cm) across, smaller than the head of a pin. • If all of the DNA in the human body were stretched end to end, it would reach to the Sun and back more than 600 times. • If a person attempted to recite the entire human genome, with all its base pairs, at the rate of one letter per second, 24 hours a day, it would take a century. • Every second scientists working on the Human Genome Project are decoding some 12,000 letters of DNA. • Our DNA is 98% identical to that of chimpanzees. • Only 0.2% of all human DNA differs between individuals; in other words, people are 99.8% the same, and all the vast differences between people are a product of just 1/500th of the total DNA. • Despite all that scientists know about DNA, a staggering 97% of all human DNA has no known function.

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Principles of Genetic Engineering Just as DNA is at the core of studies in genetics, recombinant DNA (rDNA)—that is, DNA that has been genetically altered through a process known as gene splicing—is the focal point of genetic engineering. In gene splicing, a DNA strand is cut in half lengthwise and joined with a strand from another organism or perhaps even another species. Use of gene splicing makes possible two other highly significant techniques. Gene transfer, or incorporation of new DNA into an organism’s cells, usually is carried out with the help of a microorganism that serves as a vector, or carrier. Gene therapy is the introduction of normal or genetically altered genes to cells, generally to replace defective genes involved in genetic disorders. DNA also can be cut into shorter fragments through the use of restriction enzymes. (An enzyme is a type of protein that speeds up chemical reactions.) The ends of these fragments have an affinity for complementary ends on other DNA fragments and will seek those out in the target DNA. By looking at the size of the fragment created by a restriction enzyme, investigators can determine whether the gene has the proper genetic code. This technique has been used to analyze genetic structures in fetal cells and to diagnose certain blood disorders, such as sickle cell anemia. G E N E T RA N S F E R. Suppose that a particular base-pair sequence carries the instruction “make insulin”; if a way could be found to insert that base sequence into the DNA of bacteria, for example, those bacteria would be capable of manufacturing insulin. This, in turn, would greatly improve the lives of people with type 1 diabetes, who depend on insulin shots to aid their bodies in processing blood sugar. (See Noninfectious Diseases for more about diabetes.)

American biochemists Stanley Cohen (1922–) at Stanford University, and Herbert Boyer (1936–) at the University of California at San Francisco discovered an enzyme that greatly increased the efficiency of the Berg procedure. The gene-transfer technique developed by Berg, Boyer, and Cohen formed the basis for much of the ensuing progress in genetic engineering.

Genetic Engineering

REAL-LIFE A P P L I C AT I O N S Big Business in DNA Ever since the breakthrough discoveries of Watson, Crick, and others in the 1950s made genetic engineering a possibility, the new field has promised increasingly bigger payoffs. These payoffs take the form of improvements to human life and profits to those who facilitate those improvements. The possible applications of genetic engineering are virtually limitless—as are the profits to be made from genetic engineering as a business. As early as the 1970s, entrepreneurs (independent businesspeople) recognized the commercial potential of genetically engineered products, which promised to revolutionize life, technology, and commerce as computers also were doing. Thus was born one of the great buzzwords of the late twentieth century: biotechnology, or the use of genetic engineering for commercial purposes.

Although the concept of gene transfer is relatively simple, its execution presents considerable technical obstacles. The first person to surmount these obstacles was the American biochemist Paul Berg (1926–), often referred to as the “father of genetic engineering.” In 1973 Berg developed a method for joining the DNA from two different organisms, a monkey virus known as SV40 and a virus called lambda phage. Although the accomplishment was clearly a breakthrough, Berg’s method was difficult. Then, later that year, the

Several early biotechnology firms were founded by scientists involved in fundamental research: Boyer, for example, teamed up with the venture capitalist Robert Swanson in 1976 to form Genentech (Genetic Engineering Technology). Other pioneering companies, including Cetus, Biogen, and Genex, likewise were founded through the collaboration of scientists and businesspeople. Today biotechnology promises a revolution in numerous areas, such as agriculture. Recombinant DNA techniques enable scientists to produce plants that are resistant to freezing temperatures, that will take longer to ripen, that will develop their own resistance to pests, and so on. By 1988 scientists had tested more than two dozen kinds of plants engineered to have special properties such as these. Yet no field of biotechnology and genetic engineering is as significant as the applications to health and the cures for diseases.

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Genetic Engineering

M E D I C I N E S A N D C U R E S . The use of rDNA allows scientists to produce many products that were previously available only in limited quantities: for example, insulin, which we referred to earlier. Until the 1980s the only source of insulin for people with diabetes came from animals slaughtered for meat and other purposes. The supply was never high enough to meet demand, and this drove up prices. Then, in 1982, the U.S. Food and Drug Administration (FDA) approved the sale of insulin produced by genetically altered organisms—the first such product to become available. Since 1982 several additional products, such as human growth hormone, have been made with rDNA techniques.

One of the most exciting potential applications of genetic engineering is the treatment of genetic disorders, which are discussed in Heredity, through the use of gene therapy. Among the more than 3,000 such disorders, quite a few of which are quite serious or even fatal, many are the result of relatively minor errors in DNA sequencing. Genetic engineering offers the potential to provide individuals with correct copies of a gene, which could make possible a cure for that condition. In the 1980s scientists began clinical trials of a procedure known as human gene therapy to replace defective genes. The technique, still very much in the developmental stage, offers the hope of cures for diseases that medicine has long been powerless to combat. In 2001 scientists at the Weizmann Institute in Israel brought together two of the most exciting fields of research, biotechnology and computers, to produce the DNA-processing nanocomputer. It is an actual computer, but it is so small that a trillion of them would fit in a test tube. It consists of DNA and DNA-processing enzymes, both dissolved in liquid; thus its input, output, and software are all in the form of DNA molecules. The purpose of the nanocomputer is to analyze DNA, detecting abnormalities in the human body and creating remedies for them.

The purpose of the project is to locate each human gene and determine its specific structure and function. Such knowledge will provide the framework for studies in health, disease, biology, and medicine during the twenty-first century and no doubt will make possible the cures for countless diseases. Although great strides have been made in gene therapy in a relatively short time, its potential usefulness has been limited by lack of scientific data concerning the multitude of functions that genes control in the human body. For gene therapy to advance to its full potential, scientists must discover the biological role of each of these genes and locate each base pair of which they are comprised. A P R O G R E SS R E P O RT. Scientists participating in the project have identified an average of one new gene a day, and this rate of discovery has increased. At the time of its establishment in 1990 (under the leadership of James D. Watson, who served as director until 1992), HGP was expected to reach completion by 2005. In 2002, however, the project’s leadership predicted completion by some time in the following year. Along the way, they had discovered that the human genome, originally believed to include 100,000 to as many as 150,000 genes, actually consists of about 30,000 to 40,000 genes.

At the center of genetic studies, with vast potential applications to genetic engineering, is the Human Genome Project (HGP), an international effort to analyze and map the DNA of humans and several other organisms. As discussed in the essay Genetics, the HGP began with efforts by the Atomic Energy Commission, a predecessor to the

Both HGP and a private firm, Celera Genomics (founded 1998), had undertaken the study of the human genome, and in June 2000 the entities jointly reported that they had finished the initial sequencing of the three billionodd base pairs in the human genome. By that point, researchers also had completed thorough DNA sequences for many other organisms. The basis for the latter undertaking is that humans share many genes with other life-forms. With the completion of initial sequencing, scientists working on the HGP undertook the effort of determining the exact sequence of the base pairs that make them up all human genes. Long before completion, the project had yielded some infor-

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U.S. Department of Energy, to study the genetic effects of radioactive nuclear fallout. In 1990 the Department of Energy in cooperation with the National Institutes of Health (NIH), launched the project. At about the same time, the governments of the United Kingdom, Japan, Russia, France, and Italy initiated their own, similar undertakings, which are coordinated with American efforts.

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mation. Some of the genes identified through the HGP include one that guides reproduction of the human immunodeficiency virus (HIV), which causes the acquired immunodeficiency syndrome, better known as AIDS (see Infectious Diseases). Researchers also have located a gene that predisposes people to obesity as well as genes associated with such inherited disorders as Huntington disease, Lou Gehrig disease (also called amyotrophic lateral sclerosis, or ALS), and some colon and breast cancers.

Genetic Engineering

A World of Controversy The HGP has numerous implications—and not all of them, in the minds of some critics, are positive. The NIH inadvertently created cause for concern when, in 1991, it attempted to patent certain forms of DNA. While patenting is touted as a necessary financial incentive for research initiatives, critics maintain that it restricts access to the information generated and to the use of those discoveries. That places a great deal of control in the hands of the private firm that funded the research and may limit the spread of real benefits that result from discovery. Some scientists and politicians have raised the concern that the ability to produce detailed genetic information on people could give too much power to the people who possess that knowledge. Most states do not have laws protecting citizens against the misuse of genetic information, for instance, by employers and insurers. In the absence of effective legal remedies, genetic testing may be used to bar people from employment or insurance coverage. Insurers may even make mandatory testing a requirement for coverage. Existing laws may not be adequate to protect people’s privacy: whereas the individual may be protected from having to provide potentially damaging genetic information, such information still can be obtained by testing the individual’s relatives. N I G H T M A R I S H I M A G E S . The idea of genetic information being used to control a person’s destiny calls to mind all sorts of nightmarish images, such as those raised by the movie Gattaca (1997). Set in a dystopian, or anti-utopian, future, the film depicts a character who employs elaborate means to conceal his true identity in order to hold on to a job that otherwise would be forbidden to him because of his DNA. In fact, one does not have to go to fiction

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STARVING

PRISONERS

OF

A

NAZI

CONCENTRATION

CAMP, ONE FRIGHTENING EXAMPLE OF THE ATTEMPT TO USE GENETICS AS A FORM OF SOCIAL CONTROL.

GERMANY

PRACTICED MASS MURDER OF

OTHERS WHO WERE DEEMED TO HAVE

JEWS

NAZI AND

“UNDESIRABLE”

TRAITS. (© Corbis. Reproduced by permission.)

to find examples of societies and movements that have used genetics as a form of social control. The most frightening example of this was Nazi Germany, which practiced mass murder not only of Jews and other ethnic and social groups but also of people who suffered from mental retardation or other “undesirable” traits. The purpose of DNA was discovered only a year before the collapse of the Nazi empire in 1945, and one can only imagine what Adolf Hitler and his minions would have done with this knowledge if they had had access to it. (DNA is one of several scientific and technological concepts that came to fruition at the end of World War II but which Hitler, fortunately, was unable to use to his advantage. Others include rocketry, nuclear weaponry, radar, computers, and television.) Nazism was actually an especially repugnant version of a movement known as eugenics, which had its origins in the late nineteenth and early twentieth centuries. Based on the idea that populations could be improved by encouraging people with “positive” traits to reproduce while discour-

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body of opposition to genetic engineering, biotechnology, and the HGP. And no aspect of the larger subject is more upsetting to certain individuals, as well as special interest groups, as that of cloning.

Genetic Engineering

Cloning A clone is a cell, group of cells, or organism that contains genetic information identical to that of the parent cell or organism. It is a form of asexual reproduction (see Reproduction), and as such it is not as new as it seems; what is new, however, is humans’ ability to manipulate cloning at the genetic level. The first clones produced by humans as long as 2,000 years ago were plants developed from grafts and stem cuttings. By cloning—a process that calls into play complex laboratory techniques and the use of DNA replication—people usually mean a relatively recent scientific advance. Among these techniques is the ability to isolate and copy (that is, to clone) individual genes that direct an organism’s development. ON FEBRUARY

24, 1997, THE FIRST MAMMAL CLONED

FROM AN ADULT CELL, A LAMB NAMED HERE AS AN ADULT), WAS BORN IN

DOLLY (SHOWN EDINBURGH, SCOT-

LAND. (Photograph by Jeff Mitchell. Archive Photos. Reproduced by

permission.)

aging reproduction among those with less desirable traits, eugenics was at one time a mainstream movement whose adherents included the distinguished U.S. Supreme Court justice Oliver Wendell Holmes, Jr. (1841–1935). O T H E R C O N C E R N S . The specter of eugenics raises the threat that a single human, or a group of humans, could “play God” with the lives of others. Another dramatic fear associated with genetic engineering is the threat that a genetically re-engineered virus could turn out to be extremely virulent, or deadly, and spread. There are other, more mundane questions of ethics: for instance, is it appropriate for scientists to establish private, for-profit corporations to benefit from discoveries they made while working for public-sponsored research institutions? No wonder, then, that the budget for the HGP in the United States includes a small allocation (3% of its total) toward study of the ethical, legal, and social implications (ELSI) of the project. The ELSI Working Group is charged with studying the issues of fairness, privacy, delivery of health care, and education. Meanwhile, there is a vast

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THE

PROMISE

OF

C LO N I N G .

The cloning of specific genes can provide large numbers of copies of that gene for use in genetic and taxonomic research as well as in the practical areas of medicine and farming. In the latter field, the goal is to clone plants with specific traits that make them superior to naturally occurring organisms. For example, in 1985 scientists conducted field tests using clones of plants whose genes had been altered in the laboratory to generate resistance to insects, viruses, and bacteria. New strains of plants resulting from cloning could produce crops that can grow in poor soil or even underwater and fruits and vegetables with improved nutritional qualities and longer shelf lives. A cloning technique known as twinning could induce livestock to give birth to twins or even triplets, and on the environmental front cloning might help save endangered species from extinction. In the realm of medicine and health, cloning has been used to make vaccines and hormones. It has become possible, by combining two different kinds of cells (such as mouse and human cancer cells), to produce large quantities of specific antibodies, via the immune system, to fight off disease. When injected into the bloodstream, these cloned antibodies seek out and attack diseasecausing cells anywhere in the body. By attaching

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a tracer element to the cloned antibodies, scientists can locate hidden cancers, and by attaching specific cancer-fighting drugs, the treatment dose can be transported directly to the cancer cells. EXPERIMENTS

IN

CLONING.

The modern era of laboratory cloning began in 1958 when the British plant physiologist F. C. Steward (1904–1993) cloned carrot plants from mature single cells placed in a nutrient culture containing hormones. The first cloning of animal cells took place in 1964, when the British molecular biologist John B. Gurdon (1933–1989) took nuclei from intestinal cells of toad tadpoles and injected them into unfertilized eggs. The cell nuclei in the eggs had been destroyed with ultraviolet light, but when the eggs were incubated, Gurdon found that 1–2% of the eggs developed into fertile, adult toads. The first successful cloning of mammals occurred nearly 20 years later, when scientists in Switzerland and the United States successfully cloned mice using a method similar to Gurdon’s approach. Their method required one extra step, however: after taking the nuclei from the embryos of one type of mouse, they transferred them into the embryos of another type of mouse. The latter served as a surrogate, or replacement, mother. The cloning of cattle livestock was tried first in 1988, when embryos from prize cows were transplanted to unfertilized cow eggs whose own nuclei had been removed. An even greater breakthrough transpired on February 24, 1997, with the birth of a lamb named Dolly in Edinburgh, Scotland. Dolly was no ordinary sheep: she was the first mammal born from the cloning of an adult cell. Thus, she had been produced by asexual reproduction in the form of genetically engineered cloning rather than by anything resembling a normal process. Nonetheless, she proved her own ability to reproduce the old-fashioned way when, on April 23, 1998, she gave birth to a daughter named Bonnie.

however, did not develop to a stage that was suitable for transplantation into a human uterus. Then, on October 13, 2001, scientists at Advanced Cell Technology in Worcester, Massachusetts, successfully cloned a human embryo. They had not created human life, as it might sound; what they had developed instead was a source for nerve and other tissues that could be harvested for use in medicine and research. Still, the news—overshadowed though it was in America, where people were still reeling from the September 11 terrorist attacks—was earth-shattering. Human cells had been reproduced, and once again it appeared that the production of human clones might be possible.

Genetic Engineering

It is easy to understand how people might respond with alarm to such frightening news with alarm. Such fears have a great deal more to do with Hollywood than they do with science. In fact, the accomplishment of the Massachusetts firm, while impressive from a scientific standpoint, was fairly modest compared with the Frankenstein-like image presented by anti–genetic engineering scaremongers. “Cloned an embryo” actually sounds a great deal more dramatic than what the Massachusetts scientists achieved, with just one embryo reaching the size of six cells before the cells stopped dividing. This is hardly the beginnings of a clone army. At any rate, the cloning practiced at the Massachusetts firm was therapeutic cloning, involving the production of genetic material for the treatment of specific conditions. It is a far cry from reproductive cloning, which entails implanting a cloned embryo in a uterus—and even that is still a long way from the clichéd image of clones produced in a test tube without any parents other than the biological material used to create them.

A R E H U M A N S N E X T ? Though Dolly’s and Bonnie’s births excited hopes, they also inspired fears. If large mammals could be cloned, could humans? As early as 1993 an attempt had been made at cloning human embryos as part of studies on in vitro (out of the body) fertilization. The purpose was to develop fertilized eggs in test tubes and then to implant them into the wombs of women having difficulty becoming pregnant. These fertilized eggs,

Such ideas are related much more closely to those highlighted in Aldous Huxley’s 1932 novel Brave New World than they are to scientific realities. And even if humans wanted to develop such technology, it would be many, many years in the future. As for “creating life,” to do so is probably not even possible; if it is, such an achievement is about as far off as travel to another solar system. This is not to say that all fears of cloning and genetic engineering are unwarranted; on the contrary, it is good to have a healthy level of skepticism. But it is also good to be an equal-opportunity skeptic and therefore to question ideas in the

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Genetic Engineering

KEY TERMS AMINO ACIDS:

Organic compounds

made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations. Strings of amino acids make up proteins. A pair of chemicals that

BASE PAIR:

form the “rungs” on a DNA molecule, which has the shape of a spiral staircase. BIOTECHNOLOGY:

A name for the

industry built around the application of genetic engineering techniques. CHROMOSOME:

A DNA-containing

body, located in the cells of most living things, that holds most of the organism’s genes.

The introduction of normal or genetically altered genes to cells, typically to replace defective genes involved in genetic disorders.

GENE THERAPY:

Incorporation of new DNA into an organism’s cells, usually with the help of a microorganism that serves as a vector. GENE TRANSFER:

A condition, such as a hereditary disease, that can be traced to an individual’s genetic makeup.

GENETIC DISORDER:

The alteration of genetic material by direct intervention in genetic processes with the purpose of producing new substances or improving functions of existing organisms.

GENETIC ENGINEERING:

A cell, group of cells, or organ-

CLONE:

ism that contains genetic information identical to that of its parent cell or organism. process whereby clones are produced.

The control center of a cell, where DNA is stored.

Cloning is a form of asexual reproduction.

PROTEINS:

A

CLONING:

DNA:

specialized

genetic

Deoxyribonucleic acid, a mole-

cule in all cells, and many viruses, that contains genetic codes for inheritance. ENZYME:

A protein material that

speeds up chemical reactions in the bodies of plants and animals without itself taking part in or being consumed by those reactions.

NUCLEUS:

Large molecules built from long chains of 50 or more amino acids. Proteins serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body’s immune system, and making enzymes.

Recombinant DNA, or DNA that has been genetically altered through gene splicing. rDNA:

chromosomes, genes contain specifications

Enzymes that break DNA into fragments at particular sites.

for the structure of a particular polypep-

VECTOR:

GENE:

A unit of information about a

particular heritable trait. Usually stored on

tide or protein. GENE SPLICING:

A process whereby

recombinant DNA is formed by cutting a

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DNA strand in half lengthwise and joining it with a strand from another organism or perhaps even another species.

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RESTRICTION ENZYMES:

In the context of genetics, a vector is a microorganism or virus that is used to transfer DNA from one organism to another.

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popular culture—including opposition to genetic engineering. WHERE TO LEARN MORE Barash, David P. Revolutionary Biology: The New, GeneCentered View of Life. New Brunswick, NJ: Transaction Publishers, 2001. Chadwick, Ruth F. The Concise Encyclopedia of the Ethics of New Technologies. San Diego: Academic Press, 2001. “Cloning.” New Scientist (Web site). . Department of Energy Human Genome Program (Web site). . Genetic Engineering and Cloning: Improving Nature or Uncorking the Genie? (Web site). .

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Genetics Education Center, University of Kansas Medical Center (Web site). .

Genetic Engineering

Hyde, Margaret O., and John F. Setaro. Medicine’s Brave New World: Bioengineering and the New Genetics. Brookfield, CT: Twenty-First Century Books, 2001. Judson, Karen. Genetic Engineering: Debating the Benefits and Concerns. Berkeley Heights, NJ: Enslow Publishers, 2001. National Human Genome Research Institute (Web site). . Twenty Facts About the Human Genome. Wellcome Trust (Web site). . Wade, Nicholas. Life Script: How the Human Genome Discoveries Will Transform Medicine and Enhance Your Health. New York: Simon & Schuster, 2001.

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M U TAT I O N Mutation

CONCEPT A word familiar to all fans of science fiction, mutation refers to any sudden change in DNA— deoxyribonucleic acid, the genetic blueprint for an organism—that creates a change in an organism’s appearance, behavior, or health. Unlike in the sci-fi movies, however, scientists typically use the word mutant as an adjective rather than as a noun, as, for example, in the phrase “a mutant strain.” Mutation is a phenomenon significant to many aspects of life on Earth and is one of the principal means by which evolutionary change takes place. It is also the cause of numerous conditions, ranging from albinism to cystic fibrosis to dwarfism. Mutation indicates a response to an outside factor, and the nature of that factor can vary greatly, from environmental influences to drugsto high-energy radiation.

HOW IT WORKS DNA, Chromosomes, and Mutations Deoxyribonucleic acid, or DNA, is a molecule in the cells of all life-forms that contains genetic codes for inheritance. DNA, discussed elsewhere in this book, is as complex in structure as it is critically important in shaping the characteristics of the organism to which it belongs, and therefore it is not surprising that a subtle alteration in DNA can produce significant results. Alterations to DNA are called mutations, and they can result in the formation of new characteristics that are heritable, or capable of being inherited.

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chromosomes. Stretches of DNA that hold coded instructions for the manufacture of specific proteins are known as genes, of which the human race has approximately 40,000 varieties. If the DNA of a particular gene is altered, that gene may become defective, and the protein for which it codes also may be missing or defective. Just one missing or abnormal protein can have an enormous effect on the entire body: albinism, for instance, is the result of one missing protein. Mutations also can be errors in all or part of a chromosome. Humans normally have 23 pairs of chromosomes, and an extra chromosome can have a tremendous negative impact. For example, there should be two of chromosome 21, as with all other chromosomes, but if there are three, the result is Down syndrome. People with Down syndrome have a unique physical appearance and are developmentally disabled. Nor is an extra chromosome the only chromosomal abnormality that causes problems: if chromosomes 9 and 22 exchange materials, a phenomenon known as translocation, the result can be a certain type of leukemia. Down syndrome also results from translocation.

Every cell in the body of every living organism contains DNA in threadlike structures called

Germinal mutations are those that occur in the egg or sperm cells and therefore can be passed on to the organism’s offspring. Somatic mutations are those that happen in cells other than the sex cells, and they cannot be transmitted to the next generation. This is an important distinction to keep in mind in terms of both the causes and the effects of mutation. If only the somatic cells of the organism are affected, the mutation will not appear in the next generation; on the other hand, if a germinal mutation is involved, what was once an abnormality may

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become so common in certain populations that it emerges as the norm.

The Role of Mutation in Evolution Most of the forms of mutation we discuss in this essay appear suddenly (i.e., in a single generation) and affect just a few generations. Yet even such seemingly “normal” characteristics as our ten fingers and ten toes or our two eyes or our relatively hairless skin (compared with that of apes) are ultimately the product of mutations that took shape over the many hundreds of millions of years during which animal life has been evolving. Evolution, in fact, is driven by mutation, along with natural selection (see Evolution). Over the eons, advantageous mutations, examples of which we look at later, have allowed life to develop and diversify from primitive cells into the multitude of species—including Homo sapiens—that exist on Earth today. If DNA replicated perfectly every time, without errors, the only life-forms existing now would be those that existed about three billion years ago: single-cell organisms. Mutations, therefore, are critical to the development of diverse life-forms, a phenomenon known as speciation (see Speciation). Mutations that allow an organism to survive and reproduce better than other members of its species are always beneficial, though a mutation that may be beneficial in some circumstances can be harmful in others. Mutations become especially important when an organism’s environment is changing— something that has happened often over the course of evolutionary history. And though we cannot watch evolution taking place, we can see how mutations are used among domesticated plants and animals, as discussed later.

REAL-LIFE A P P L I C AT I O N S Ethnicity and Mutation Every single human trait—blue eyes, red hair, cystic fibrosis, a second toe longer than the big toe, and so on—is the result of some genetic mutation somewhere back down the line. Traits that are shared by all people must have arisen long ago, while other traits occur only in certain populations of people. Traits may be as innocuous as eye color or hair texture or as grave as a

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shared tendency toward a particular disease. Cystic fibrosis, for instance, is most common in people of northern European descent, while sickle cell anemia (see Amino Acids) occurs frequently in those of African and Mediterranean ancestry. A fatal disorder known as Tay-Sachs is found primarily in Jewish people whose ancestors came from Eastern Europe. In many cases, the particular mutation, while harmful in one regard, proved to be a useful one for that population. We know, for example, that while two copies of the mutant sickle cell anemia gene cause illness, one copy confers resistance to malaria—a very useful trait to people living in the tropics, where malaria is common.

Mutation

THE PIMA “FAT - S T O R A G E M U TAT I O N . ” Researchers have noted a high

incidence of obesity among the Pima, a Native American tribe whose ancestral homeland is along the Gila and Salt rivers in Arizona. The Pima tend to eat a diet that is no more fatty than that of the average American—which, of course, means that it is plenty fatty, complete with chips, bologna, ice cream, and all the other high-calorie, low-nutrient foods that most Americans consume. But whereas the average American is overweight, the average Pima is more dramatically so. This suggests that long ago, when the ancestors of the Pima had to face repeated periods of famine in the dry lands of the American Southwest, survival favored the individual or individuals who had a mutation for fat storage. It so happens that today, there is more than enough food at the local supermarket, but by now the Pima as a group has the fat-storage gene. Therefore, many members of the tribe have to undergo strict dietary and exercise regimens so as not to become grossly overweight and susceptible to heart disease and other ailments.

Favorable Mutations As with other mutations relating to ethnic groups, scientists have hypothesized that some advantage must be conferred upon people with single copies of the cystic fibrosis gene or the TaySachs disease gene. Though many mutations are harmful, others prove to be beneficial to a species by helping it adapt to a particular environmental influence. Useful mutations, in fact, are the driving force behind evolution. The processes of evolution are usually much too slow for people to discern, but it is possible to

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It produces shaking and a range of other symptoms, including depression, irritability, and apathy, and is usually fatal. The gene associated with Huntington’s is dominant.

Mutation

The horrible degenerative brain condition known as Creutzfeldt-Jakob disease, discussed in Diseases, is usually caused by another mutation. (Though it can be caused by infection, most cases of the disease are the result of heredity.) As with some of the other conditions we have mentioned, this one seems to affect particular groups more than others. Whereas the worldwide incidence of this rare condition is about one in one million, among Libyan Jews the rate is higher. The disease is a type of spongiform encephalopathy, so named because it produces characteristic spongelike patterns on the surface of the brain. Spongiform encephalopathies are caused by the appearance of a prion, a deviant form of protein whose production typically is caused by a mutation. PYGMIES,

A GROUP OF PEOPLE IN SOUTHERN

AFRICA,

APPEAR TO BE MIDGETS THROUGH A GERMINAL MUTATION, BUT IN MOST POPULATIONS THE MUTATION IS SOMATIC, OCCURRING ONLY OCCASIONALLY IN FAMILIES WHOSE

OTHER

MEMBERS

ARE

OF

ORDINARY

SIZE.

(© Bettmann/Corbis. Reproduced by permission.)

observe the effects of selective breeding when applied to domesticated animals and plants. The artificial selection of pigeons by breeders, in fact, provided the English naturalist Charles Darwin (1809–1882) with a model for his theory of natural selection, discussed in Evolution. Likewise, animal and plant breeders use mutations to produce new or improved strains of crops and livestock. Careful breeding in this manner has spawned the many different breeds of dogs, cats, and horses—each with their characteristic coloring, size, temperament, and so on—that we know today. It also has resulted in crops that are resistant to drought or insects or which have a high yield per acre. Likewise, goldfish, yellow roses, and Concord grapes are all descendants of ancestors with specific mutations.

Congenital Disorders

The majority of mutations, however, are less than favorable, and this is illustrated by the relationship between mutation and certain hereditary diseases. An example is Huntington disease, a condition that strikes people in their forties or fifties and slowly disables their nervous systems.

In the past, all manner of superstitions arose to explain why a child was born, for instance, with a cleft palate, a situation in which the two sides of the roof of the mouth fail to meet, causing a speech disorder that may be mild or severe. Once known as a harelip, the cleft palate was said to have formed as a result of the mother’s being frightened by a hare while she was carrying the child. In fact, it is just one example of a congenital disorder, an abnormality of structure or function or a disease that is present at birth. Congenital disorders, which also are called birth defects, may be the result of several different factors, mutation being one of the most significant. Among the many examples of congenital disorder are the hereditary diseases we have already mentioned, as well as dwarfism, Down syn-

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Most hereditary diseases are, by definition, linked with a mutation. Such is the case with hemophilia, for instance (see Noninfectious Diseases), and with cystic fibrosis, a lethal disorder that clogs the lungs with mucus and typically kills the patient before the age of 30 years. Cystic fibrosis, like Huntington, occurs when a person inherits two copies of a mutated gene. In 1989 researchers found the source of cystic fibrosis on chromosome 7, where an infinitesimal change in the DNA sequence leads to the production of an aberrant protein.

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Mutation

AN

EXTRA CHROMOSOME 21 CAUSES

DOWN

SYNDROME.

PEOPLE

WITH THIS CONGENITAL DISORDER HAVE UNIQUE

PHYSICAL FEATURES, SUCH AS A WIDE, FLAT FACE AND SLANTED EYES, AND ARE DEVELOPMENTALLY DISABLED. (©

Laura Dwight/Corbis. Reproduced by permission.)

drome, albinism, and numerous other conditions. D WA R V E S A N D M I D G E T S . The term dwarf has many associations from fairy tales—an example of the combined fascination and revulsion with which people with congenital disorders have long been treated—but it also is used to describe persons of abnormally short stature. A dwarf is distinguished from a midget in a number of ways, all of which indicate that the features of a midget are less removed from the norm. Midgets, while small, have bodies with proportions in the ordinary range. Likewise, the intelligence and sexual development of an adult midget are similar to those of other adults, and a midget or midget couple typically produces children of ordinary size. Pygmies, a group of people in southern Africa, appear to be midgets through a germinal mutation, but in many populations the mutation is somatic, occurring only occasionally in families whose other members are of ordinary size.

Dwarfs, by contrast, have several different disorders. One variety of dwarfism, known in the past as cretinism, is characterized by a small, abnormally proportioned body and an impaired mind. On the other hand, several forms of hered-

S C I E N C E O F E V E RY DAY T H I N G S

itary dwarfism carry with them no ill effect on the mental capacity. For example, people with the type of dwarfism known as achondroplasia have short limbs and unusually large heads, but the life span and intelligence of someone with this condition are quite normal. In the case of diastrophic dwarfism, the brain is fine, but the skeleton is deformed, and the risk of death from respiratory failure is high in infancy. Persons with diastrophic dwarfism who survive early childhood, however, are likely to enjoy a normal life span. D O W N S Y N D R O M E . Like people with many other congenital disorders, those with Down syndrome used to be called by a name that now is considered crude and insensitive: mongoloid. The term, when used with a capital M, refers to people of east Asian descent and is analogous to other broad racial groupings: Caucasoid, Negroid, and Australoid. In the case of people with Down syndrome, mongoloid referred to the unusual facial features that mark someone with that condition.

A person with Down syndrome (caused by an extra chromosome in the 21st chromosomal pair) is likely to have a wide, flat face and eyes that are slanted, sometimes with what is known

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AN ALBINO NORTHERN FUR SEAL. A CONDITION THAT RESULTS FROM AN INHERITED DEFECT IN MELANIN METABOLISM (MELANIN IS RESPONSIBLE FOR THE COLORING OF SKIN), ALBINISM IS MARKED BY AN ABSENCE OF PIGMENT FROM THE HAIR, SKIN, AND EYES. (© John Francis/Corbis. Reproduced by permission.)

as an inner epicanthal fold—all facial characteristics common among people who are racially Mongoloid. Numerous other facial features identify a person with Down syndrome as someone who suffers from a specific congenital disorder, including a short neck, ears that are set low, a small nose, large tongue and lips, and a chin that slopes. People with Down syndrome are apt to have poor muscle tone and possess abnormal ridge patterns on their palms and fingers and the soles of their feet. Heart and kidney problems are common with Down syndrome as well, but one feature is most common of all: mental retardation. The condition occurs in about one of 1,000 live births among women under age 40 but about one in 40 live births to older women. Overall, the incidence is about one in 800 live births. As noted earlier, the cause of Down syndrome is translocation, but the reason translocation occurs is not known. or Down syndrome, albinism is not nearly as severe in terms of its effect on a person’s functioning. A condition that results from an inherited defect in melanin metabolism (melanin is responsible for the coloring of skin), albinism is marked by an absence of pigment from the hair,

On the other hand, some ethnic groups experience enough albino births that another one causes no excitement. For example, among the San Blas Indians of Panama, one in approximately 130 births is an albino, compared with one in 17,000 for humans as a whole. Albinism comes about when melanocytes (melanin-producing cells) fail to produce melanin. In tyrosinase-negative albinism, the most common form, the enzyme tyrosinase (a catalyst in the conversion of tyrosine to melanin) is missing from the

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A L B I N I S M . Compared with dwarfism

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skin, and eyes. The hair of an albino tends to be whitish blond, the skin an extremely pale white, and the eyes pinkish. Albinism occurs among other animals: hence the white rats, rabbits, and mice almost everyone has seen. Domestic white chickens, geese, and horses are partial albinos that retain pigment in their eyes, legs, and feet. As was once true of people with other congenital disorders, human albinos once inspired fear and awe. Sometimes they were killed at birth, and in the mid–nineteenth century, albinos were exhibited in carnival sideshows. In these cruel spectacles, sometimes whole families were put on display, touted as a unique race of “night people” who lived underground and came out only when the light was dim enough not to hurt their eyes.

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KEY TERMS AMINO ACIDS:

Organic compounds

NATURAL SELECTION:

The process

made of carbon, hydrogen, oxygen, nitro-

whereby some organisms thrive and others

gen, and (in some cases) sulfur bonded in

perish, depending on their degree of adap-

characteristic formations. Strings of amino

tation to a particular environment.

acids make up proteins. ORGANIC:

At one time chemists used

DNA-containing

the term organic only in reference to living

bodies, located in the cells of most living

things. Now the word is applied to com-

things, that hold most of the organism’s

pounds containing carbon and hydrogen.

CHROMOSOME:

genes. CONGENITAL DISORDER:

An abnor-

mality of structure or function or a disease

A group of between 10

POLYPEPTIDE:

and 50 amino acids. Large molecules built

that is present at birth. Congenital disorders

PROTEINS:

also are called birth defects.

from long chains of 50 or more amino

DNA:

Deoxyribonucleic acid, a mole-

cule in all cells, and many viruses, containing genetic codes for inheritance. GENE:

A unit of information about a

acids. Proteins serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body’s immune system, and making enzymes. Ribonucleic acid, the molecule

particular heritable trait. Usually stored on

RNA:

chromosomes, genes contain specifications

translated from DNA in the cell nucleus,

for the structure of a particular polypep-

the control center of the cell, that directs

tide or protein.

protein synthesis in the cytoplasm, or the

GERMINAL MUTATION:

A mutation

space between cells. A mutation

that occurs in the egg or sperm cells, which

SOMATIC MUTATION:

therefore can be passed on to the organ-

that occurs in cells other than the repro-

ism’s offspring.

ductive, or sex, cells. These mutations, as

HERITABLE:

Capable of being inherit-

not be transmitted to the next generation.

ed. MUTAGEN:

A chemical or physical fac-

tor that increases the rate of mutation. MUTATION:

contrasted with germinal mutations, can-

Alteration in the physical

SPECIATION:

The divergence of evo-

lutionary lineages and creation of new species. A mutation in

structure of an organism’s DNA, resulting

TRANSLOCATION:

in a genetic change that can be inherited.

which chromosomes exchange parts.

melanocytes. When the enzyme is missing, no

tyrosinase-negative albinism. It is equally com-

melanin is produced. In tyrosinase-positive

mon among blacks and whites, while more blacks

albinism, a defect in the body’s tyrosine transport

than whites are affected by tyrosinase-positive

system impairs melanin production. One in

albinism. Native Americans have a particularly

every 34,000 persons in the United States has

high incidence of both forms of albinism.

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Mutagens and Other Causes Mutation

As might be expected, cells that divide many, many times in a lifetime are more at risk of errors and mutations than cells that divide less frequently. In a human female, egg cells are fully formed at birth, and they never divide thereafter. By contrast, sperm cells are being produced constantly, and the older a man is, the more frequently his sperm-producing cells have divided. By age 20 they will have divided 200 times and by age 45 about 770 times. This has led scientists to hypothesize that when a baby is born with a congenital disorder caused by an error in cell division, the father is the parent more likely to have contributed the gene with the mutation.

132

were linked with the chimney sweep’s cancer of late eighteenth-century England, discussed in Noninfectious Diseases. In fact, cancer itself is a kind of mutation, involving uncontrolled cell growth. Other environmental factors that are known to bring about mutations include exposure to pesticides, asbestos, and some food additives, many of which have been banned. WHERE TO LEARN MORE “Are Mutations Harmful?” Talk. Origins (Web site). . Human Gene Mutation Database, Institute of Medical Genetics, University of Wales College of Medicine (Web site). .

This is just one example of why mutation occurs. Many mutations are caused by mutagens—chemical or physical factors that increase the rate of mutation. Some mutagens occur naturally, and some are synthetic. Cosmic rays from space, for instance, are natural, but they are mutagenic. Some naturally occurring viruses are considered mutagenic, since they can insert themselves into host DNA. Hydrogen and atomic bombs are man-made, and they emit harmful radiation, which is a mutagen. Recreational drugs, tobacco, and alcohol also can be mutagens in the bodies of pregnant women. The first mutagens to be identified were carcinogens, or cancercausing substances. Carcinogens in chimney soot

Weinberg, Robert A. One Renegade Cell: How Cancer Begins. New York: Basic Books, 1998.

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S C I E N C E O F E V E RY DAY T H I N G S

Kimball, Jim. Mutations. Kimball’s Biology Pages (Web site). . “Mutations.” Brooklyn College, City University of New York (Web site). . Patterson, Colin. Evolution. Ithaca: Comstock Publishing Associates, 1999. Reilly, Philip. Abraham Lincoln’s DNA and Other Adventures in Genetics. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press, 2000. Twyman, Richard M. Advanced Molecular Biology: A Concise Reference. Oxford, UK: Bios Scientific Publishers, 1998.

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S C I E N C E O F E V E RY DAY T H I N G S real-life biology

REPRODUCTION AND BIRTH REPRODUCTION SEXUAL REPRODUCTION PREGNANCY AND BIRTH

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Reproduction

REPRODUCTION

CONCEPT The term reproduction encompasses the entire variety of means by which plants and animals produce offspring. Reproductive processes fall into two broad groupings: sexual and asexual, the latter being the means by which bacteria and algae reproduce. Many plants reproduce sexually by means of pollination, and some plants alternate between sexual and asexual forms of reproduction. Other creatures, such as bees and ants, reproduce through a form of reproduction called parthenogenesis, which is neither fully sexual nor asexual.

HOW IT WORKS Asexual Reproduction Asexual reproduction involves only one organism, as opposed to two in sexual reproduction. It occurs when a single cell divides to form two daughter cells that are genetically identical to the parent cell. This process is known as fission, and it may take the form either of binary fission, in which two new cells are produced, or multiple fission, which results in the creation of many new cells. Since there is no fusion of two different cells, the daughter cells produced by asexual reproduction are genetically identical to the parent cell. Asexual reproduction usually takes place by mitosis, a process during which the chromosomes in a cell’s nucleus are duplicated before cell division. (Mitosis, chromosomes, and many other topics referred to in this essay are discussed in considerably more detail in Genetics.)

much more so, freighted as it is with degrees of meaning that go far beyond mere biology—asexual reproduction is a fairly simple, cellular process. Of course, nothing in nature is really simple, and, in fact, the dividing and replication of DNA (deoxyribonucleic acid, the genetic blueprint material found in each cell) is a complicated subject; however, that subject, too, is discussed in the essay Genetics. DNA is located at the cell nucleus, which is the cell’s control center, and the nucleus is the first part of the cell to divide in asexual reproduction. After the nucleus splits, the cytoplasm, or the cellular material external to the nucleus, then divides. The result is the formation of two new daughter cells whose nuclei have the same number and kind of chromosomes as the parent. The adaptive advantage of asexual reproduction is that organisms can reproduce quickly and by doing so colonize favorable environments rapidly. (See Evolution for more about the importance of adaptation and environment in shaping species.) For example, some bacteria can double their numbers every 20 minutes. In addition to bacteria, which are discussed in more detail in Infection, other life-forms that reproduce asexually include protozoa (varieties of which are examined in Parasites and Parasitology), bluegreen algae, yeast, dandelions, and flatworms.

Sexual Reproduction

Whereas sexual reproduction is extremely complex—and human sexual reproduction is

Sexual reproduction involves the union of two organisms rather than the splitting of one. Like asexual reproduction, it is a process that takes place at the cellular level. In sexual reproduction it is not binary fission that occurs, but the fusion of two cells. Nor are the two cells identical;

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rather, the cells—known as gametes—can be identified as either male or female according to the makeup of their chromosomes. The male gamete is called a sperm cell, and the female gamete is termed an egg cell. In sexual reproduction, the sperm cell fuses, or bonds, with the egg cell to produce a cell that is genetically different from either of the parent cells. This process of fusion is known as fertilization, and the fertilized egg is called a zygote. Gametes are produced in the male testes and female ovaries by a splitting process called meiosis. (Meiosis and other terms mentioned briefly in these paragraphs are discussed in much more detail in Genetics.) Meiosis produces haploid cells, or ones that have half the number of chromosomes as are in a normal cell for that species. When the haploid sperm and egg cells fuse at fertilization, however, the chromosomes from both combine, so that the normal number of chromosomes appears in the zygote. The shuffling of the parents’ genetic material that happens during meiosis allows for new gene combinations in offspring that account for variations between offspring (which is why you don’t look just like your siblings) and which, over time, can improve a species’ chances of survival.

REAL-LIFE A P P L I C AT I O N S Examples of Asexual Reproduction As we noted earlier, bacteria, blue-green algae, most protozoa, yeast, and flatworms all reproduce asexually, as do mosses and starfish. (The last actually reproduce both sexually and asexually by means of alternation of generations, discussed later.) The products of asexual reproduction are known as clones—an example of the fact, discussed in Genetic Engineering, that cloning and the concept of clones are not as new as one might imagine. (See that essay for much more about artificial cloning.) A starfish can regenerate and eventually produce a whole new organism from a single severed appendage, while flatworms divide in two and regenerate to form two new flatworms. This formation of a separate organism is obviously much more complex than the simple splitting of single bacteria cells, but it is still a form of asexual reproduction.

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V E G E TAT I V E

P R O PA G AT I O N .

Strawberries reproduce by forming growths called runners, which grow horizontally and generate new stalks. At some point, the runner decomposes, leaving a new plant that is a clone of the original. This is an example of vegetative propagation, a term for a number of processes by which crop plants are produced asexually. Vegetative propagation is used for such crops as potatoes, bananas, raspberries, pineapples, and some flowering plants. Its advantage to farmers is that the crops will be more uniform than those grown from seed. Furthermore, some plants are difficult to cultivate from seed, and the vegetative propagation of those plants makes it possible to grow crops that otherwise would not be available for commercial marketing. In reproducing potatoes through vegetative propagation, farmers plant the so-called eyes to produce duplicates of the parent. With banana plants, the farmer separates the suckers that grow from the root of the plant and plants them. The farmer raising raspberry bushes bends the branches and covers them with soil, whereupon a process not unlike that of the runner growth of mosses takes place: the branches eventually grow into a separate plant, with their own root system, and ultimately can be detached from the parent plant.

Between Asexual and Sexual The example of vegetative propagation suggests that there is not a sharp dividing line between sexual and asexual reproduction—that is, that many organisms can reproduce either way. This is true even of humans, who, in theory, could be cloned, though the technology to do so—let alone resolution of the ethical issues of the procedure—lies in the far distant future. (See Genetic Engineering for more on this subject.) Even humans, however, can use external fertilization, which is sexual reproduction without sexual intercourse (see Sexual Reproduction). Plants go through a process known as alternation of generations, in which they alternate as sexual and asexual reproducers, or gametophytes and sporophytes, respectively. In the asexual stage, the sporophyte produces diploid reproductive cells called spores, which develop into gametophytes. These gametophytes produce haploid gametes, which then unite sexually to form a diploid zygote that grows into a sporophyte. In

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Reproduction

A

PINEAPPLE TOP PLACED ON SOIL CAN ROOT AND DEVELOP INTO A PLANT.

NUMEROUS

CROP PLANTS, AMONG THEM,

POTATOES, BANANAS, RASPBERRIES, AND PINEAPPLES, ARE PRODUCED ASEXUALLY, A PROCESS CALLED VEGETATIVE PROPAGATION. (© Corbis. Reproduced by permission.)

plain English, this means that the asexual “grandparent” generates a sexually reproducing “child,” which in turn produces a “grandchild” that is asexual, like its grandparent.

spores, which it releases into the air. These tiny spores, carried by the wind, float away from their point of origin until they come to rest, and soon the cycle begins once again.

At one phase in the alternation of generation for mosses, for instance, male and female moss plants grow from spores. Male moss plants produce sperm cells, which, when the moss receives rainfall, are able to propagate because they have a medium (water) in which to move. They fertilize the female plants, producing zygotes. The zygote grows on top of the female moss plant, which helps to store moisture and thus provides a hospitable environment in which the zygote can develop. The zygote eventually produces haploid

PA R T H E N O G E N E S I S . There are also organisms, including bees, ants, wasps, and other insects, that reproduce in a way that is neither fully sexual nor asexual. This is parthenogenesis, a type of reproduction in which a gamete develops without fertilization. In other words, a sex cell is reproduced without actual intercourse between male and female. The gamete is almost always female—a fact indicated in the name itself, which comes from parthenos, Greek for “maiden.”

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plants. Actually, cross-pollination, or the transfer of pollen from one plant to another, would perhaps be analogous to sexual intercourse in animals. Pollination occurs in seed-bearing plants, as opposed to the more primitive spore-producing plants, such as ferns and mosses. Gymnosperms, such as pines, firs, and spruces, produce male and female cones, whereas angiosperms produce flowers containing a male organ called the stamen and a female organ called the pistil. Both types of plants rely on insects and other creatures to aid in the pollen transfer.

Reproduction

THE

ACORN WORM UNDERGOES ASEXUAL REPRODUC-

TION BY BUDDING; SMALL PIECES FRAGMENT FROM THE TRUNK, AND EACH GROWS INTO A NEW WORM. (© Lester V.

Bergman/Corbis. Reproduced by permission.)

The Parthenon in Athens, like the city itself, is named after the goddess Athena (also called Minerva), who was known by the nickname Parthenos. She is said to have been born fully formed, having sprung from the head of her father, Zeus, dressed in armor and ready for battle. Thus, her own birth was a form of parthenogenesis, a word whose second half (a name well known from the Bible) means “beginning.”

Pollen and Pollination

138

D A R W I N ’ S M O T H . The German physician and botanist Rudolf Jakob Camerarius (1665–1721) was the first scientist to demonstrate that plants reproduce sexually, and he pioneered the study of pollination. One of the scientists influenced by his work was the English naturalist Charles Darwin (1809–1882), who discussed the subject in The Various Contrivances by which Orchids Are Fertilized by Insects (1862). Darwin wrote this book partly to support the ideas on evolution presented in his much more well known book Origin of Species (1859). In Various Contrivances, he suggested that orchids and their insect pollinators evolved by interacting with one another over many generations.

As an example, he discussed Angraecum sesquipedale, an orchid native to Madagascar. Darwin had not seen the plant in its native habitat, however; he had looked only at its dried leaves. The white flower of this orchid has a footlong (30 cm) tubular spur with a small drop of nectar at its base, and from observing this, he hypothesized that the orchid had been pollinated by an insect with a foot-long tongue. This hypothesis, he wrote, “has been ridiculed by some entomologists,” or scientists who study insects. After all, no such creature had been found in Madagascar. But then, around the turn of the nineteenth century—some two decades after Darwin’s death—it was found. A Madagascan moth was discovered that had a foot-long tongue that uncoils to sip the nectar of A. sesquipedale as it cross-pollinated the flowers.

Pollen is a fine, powdery substance consisting of microscopic grains containing the male gametophyte of certain plants that reproduce sexually. These plants include angiosperms, a type of plant that produces flowers during sexual reproduction, and gymnosperms, which reproduce sexually through the use of seeds that are exposed and not hidden in an ovary, as with an angiosperm. Pollen is designed for long-distance dispersal from the parent plant, so that fertilization can occur. Pollination is the transfer of pollen from the male reproductive organs to the female reproductive organs of a plant, and it precedes fertilization. In other words, pollination is the equivalent of sexual intercourse for seed-bearing

discussed in Ecosystems and Ecology, where each is compared in terms of its degree of adaptation to its environment. Angiosperms seem to be the hands-down winner: by enlisting the aid of

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P LA N T S A N D T H E I R P O L L I N A T O R S . Angiosperms and gymnosperms are

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Reproduction

KEY TERMS A protein material that

ALTERNATION OF GENERATIONS:

ENZYME:

A process whereby plant generations alter-

speeds up chemical reactions in the bodies

nate as sexual and asexual reproducers—

of plants and animals without itself taking

gametophytes and sporophytes, respectively.

part in or being consumed by those reac-

ASEXUAL REPRODUCTION:

One of

the two major varieties of reproduction (along with sexual reproduction), In contrast to sexual reproduction, which involves two organisms, asexual reproduction involves only one. Asexual reproduction occurs when a single cell divides

tions. The process of cellu-

FERTILIZATION:

lar fusion that takes place in sexual reproduction. The nucleus of a male reproductive cell, or gamete, fuses with the nucleus of a female gamete to produce a zygote. A reproductive cell—that is,

through mitosis to form two daughter

GAMETE:

cells, which are genetically identical to the

a mature male or female germ cell that pos-

parent cell.

sesses a haploid set of chromosomes and is

BINARY FISSION:

The process in

asexual reproduction whereby a single cell divides to form two daughter cells that are genetically identical to the parent cell. A cell, group of cells, or organ-

CLONE:

prepared to form a new diploid by undergoing fusion with a haploid gamete of the opposite sex. Sperm and egg cells are, respectively, male and female gametes. GAMETOPHYTE:

In alternation of

ism that contains genetic information iden-

generations, a gametophyte is a plant that

tical to that of its parent cell or organism.

reproduces sexually.

A

CLONING:

specialized

genetic

GERM CELL:

One of two basic types

process whereby clones are produced.

of cells in a multicellular organism. In con-

Cloning is a form of asexual reproduction.

trast to somatic or body cells, germ cells

CHROMOSOME:

A DNA-containing

play a part in reproduction. A term for a cell that has

body, located in the cells of most living

HAPLOID:

things, that holds most of the organism’s

half the number of chromosomes that

genes.

appear in a diploid or somatic cell.

CROSS-POLLINATION:

The transfer

of pollen from one plant to another. CYTOPLASM:

The material inside a

cell that is external to the nucleus. DIPLOID:

A term for a cell that has the

basic number of doubled chromosomes. DNA:

Deoxyribonucleic acid, a mole-

The process of cell division

MEIOSIS:

that produces haploid genetic material. Compare with mitosis. A process of cell division

MITOSIS:

that produces diploid cells, as in asexual reproduction. Compare with meiosis. NUCLEUS:

The control center of a

cule in all cells, and many viruses, that con-

cell, where DNA is stored.

tains genetic codes for inheritance.

OVARY:

EGG CELL:

A female gamete.

S C I E N C E O F E V E RY DAY T H I N G S

Female reproductive organ

that contains the eggs.

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KEY TERMS OVULE:

Female haploid gametophyte

CONTINUED

SEXUAL REPRODUCTION:

of seed plants, which develops into a seed

the two major varieties of reproduction

upon fertilization by a pollen grain.

(along with asexual reproduction). In con-

A type of

trast to asexual reproduction, which

reproduction that involves the develop-

involves a single organism, sexual repro-

ment of a gamete without fertilization. In

duction involves two. Sexual reproduction

other words, a sex cell (usually female) is

occurs when male and female gametes

reproduced without actual intercourse

undergo fusion, a process known as fertil-

between male and female.

ization, and produce cells that are geneti-

PARTHENOGENESIS:

POLLEN:

Male haploid gametophyte

cally different from those of either parent.

of seed plants (including angiosperms and

SOMATIC CELL:

gymnosperms), which unites with the

types of cells in a multicellular organism.

ovule to form a seed. Pollen is a fine, pow-

In contrast to germ cells, somatic cells (also

dery substance consisting of microscopic grains. POLLINATION:

One of two basic

known as body cells) are not involved in reproduction; rather, they make up the tis-

The transfer of pollen

from the male reproductive organs to the

sues, organs, and other parts of the organism.

female reproductive organs of a plant. Pollination precedes fertilization. See also

SPERM CELL:

cross-pollination.

SPOROPHYTE:

REGENERATION:

A

biological

process among some lower animals where-

140

One of

A male gamete. In alternation of gen-

erations, a sporophyte is a plant that reproduces asexually. A diploid cell formed by the

by a severed body part is restored by the

ZYGOTE:

growth of a new one.

fusion of two gametes.

insects and other pollinators, they manage to pollinate much more efficiently than gymnosperms, which have to produce vast quantities of pollen for each grain that reaches its target. Typically, pollination benefits the animal pollinator by supplying it with sweet nectar and, of course, benefits the plant by providing direct transfer of pollen from one plant to the pistil of another plant. For this reason, specific plant and animal species have developed a relationship of mutualism, a form of symbiosis in which each participant reaps benefits (see Symbiosis). In many cases, plant and pollinator have evolved together, and it is possible to determine which animal pollinates a certain flower species simply by studying the morphologic features (shapes), color, and odor of the flower.

For example, some flowers are pure red, or nearly pure red, and have very little odor. In most such situations, the pollinator is a bird species, since birds have excellent vision in the red region of the spectrum but a rather undeveloped sense of smell. It so happens that Europe, which has no pure red native flowers, also has no bird-pollinated native flower species. Not all bird-pollinated flowers are red, but they are all characterized by striking, and sometimes contrasting, colors that readily catch the eye. Examples of plants pollinated by birds include the cardinal flower, the red columbine, the hibiscus, the eucalyptus, and varieties of orchid, cactus, and pineapple.

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Some flowering plants have a very strong odor but are very dark, or at least drab, in color.

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These flowers and plants—examples include the saguaro cactus, century plant, or cup-and-saucer vine—are often pollinated by bats, which have very poor vision, are typically active during the night, and have a very well developed sense of smell. The flowers of many plant species are marked with special pigments called flavonoids, which absorb ultraviolet light and appear to direct the pollinator toward the pollen and nectar. These pigments are invisible to humans and most animals, but bees’ eyes have special ultraviolet photoreceptors that enable the bees to detect patterns and so pollinate these flowers. WHERE TO LEARN MORE “Asexual Reproduction Lab.” Lester B. Pearson College of the Pacific (Web site). . Canine Reproduction (Web site). . CRES: The Center for Reproduction of Endangered Species/San Diego Zoo (Web site). .

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Elia, Irene. The Female Animal. New York: Henry Holt, 1988.

Reproduction

“Flowering Plant Reproduction.” Estrella Mountain Community College (Web site). . Kevles, Bettyann. Females of the Species: Sex and Survival in the Animal Kingdom. Cambridge, MA: Harvard University Press, 1986. Kimball, Jim. “Asexual Reproduction.” Kimball’s Biology Pages (Web site). . Maxwell, Kenneth E. The Sex Imperative: An Evolutionary Tale of Sexual Survival. New York: Plenum, 1994. The Pollination Home Page (Web site). . Reproduction (Web site). . Topoff, Howard R. The Natural History Reader in Animal Behavior. New York: Columbia University Press, 1987. Walters, Mark Jerome. The Dance of Life: Courtship in the Animal Kingdom. New York: Arbor House, 1988.

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SEXUAL REPRODUCTION Sexual Reproduction

CONCEPT Sexual reproduction is one of the two major ways, along with asexual reproduction, that plants and animals create offspring and thus propagate the species. Critical to sexual reproduction is the process of fertilization, whereby the male and female sex cells fuse, or bond. Fertilization may be of two types, either internal or external, and though humans normally fertilize by the first of those means, they may use the second, in the form of in vitro fertilization. Humans, of course, have by far the most complicated reproductive process, inasmuch as it is surrounded by a vast societal, interpersonal, and moral framework that is not a factor in animal reproduction. Part of that framework is the activity that precedes sexual reproduction: attraction, courtship, and so forth. Although no other animal’s courtship rituals rival those of humans for sophistication, some of them are quite impressive in their complexity.

HOW IT WORKS The Reproductive System The contrast between sexual and asexual reproduction is examined in Reproduction, an essay that also provides examples of plant reproduction through pollination. The present essay is concerned primarily with human sexual reproduction and secondarily with animal sexual reproduction. Some technical aspects of reproduction at the cellular level require consultation of processes explained in Genetics; here we confine our technical discussion to reproduction at the level of organs, fluids, and other bodily components. Reproduction is facilitated by the repro-

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ductive system, a group of organized structures that can be subdivided into male and female reproductive systems. During puberty, which typically occurs between the ages of 10 and 14 years, the reproductive systems of both sexes mature. This phase is marked in part by the release of eggs (female sex cells) in the female ovary and the formation of sperm (male sex cells) in the male testes. Reproduction can take place only when a sperm unites with an egg, a process called fertilization. THE MALE REPRODUCTIVE S Y S T E M . The testes are the pair of male

reproductive glands located in the scrotum, a skin-covered sac that hangs from the groin. Each testis produces sperm cells, while the testes as a whole secrete testosterone. Testosterone is a hormone—a type of molecule that sends signals to spots remote from its point of origin to induce specific effects on the activities of other cells. Testosterone is associated with masculinity, though females secrete it in much smaller quantities as well. In males, testosterone secretion is critical to the development of secondary sexual characteristics—those unique traits that mark a person as a male or female, though they do not occur in the sexual organs themselves. A deepened voice is an example of a male secondary sex characteristic evident at puberty. Sperm cells produced in the testes move to the epididymis, a coiled tube at the base of the penis where they are stored and matured. During ejaculation, or the ejection of sperm from the penis during orgasm, sperm travel from the epididymis through a long tube called the vas deferens to the urethra. This single tube, which extends from the bladder to the tip of the penis, is also the means by which urine passes out of the

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body. Liquid secretions from various glands combine with sperm (itself a gooey substance that is barely liquid) to form the semen, or seminal fluid. Ejaculated semen may contain as many as 400 million sperm.

The Female Reproductive System The female system is much more complicated than the male version and has a role in all stages of reproduction. Whereas the male system primarily delivers semen to the vagina, the female system plays a critical part from fertilization until long after the birth of offspring. It produces ova, or eggs, receives sperm from the penis, houses and provides nutrients to the developing zygote (fertilized egg) and later the embryo and fetus, gives birth to offspring, and feeds those offspring after birth. The visible part of the female reproductive system, which, of course, is not even half of the entire picture, includes the opening of the vagina and the external genital organs, or vulva. The vagina, a muscular tube extending from the uterus to the outside of the body, is the receptacle for sperm ejaculated during sexual intercourse and also forms part of the birth canal that will be used later, when the offspring comes to term. The external genital organs, known collectively as the vulva, include the labia, folds of skin on both sides of the openings to the vagina and urethra; the clitoris, a small, sensitive organ that is comparable to the male penis inasmuch as it swells when stimulated; and the mons pubis, a mound of fatty tissue above the clitoris.

month by thickening, but if fertilization does not take place, the endometrium is shed during menstruation.

Fertilization During sexual intercourse, a man releases approximately 300 million sperm into a woman’s vagina, but only one of the sperm can fertilize the ovum. The successful sperm cell must enter the uterus, swim up the fallopian tube (a trumpetshaped passageway between the ovary and the uterus) to meet the ovum, and then pass through a thick coating, known as the zona pellucida, that surrounds the egg. The head of the sperm cell contains enzymes (a type of protein that speeds up chemical reactions—see Enzymes) that break through the zona pellucida and allow the sperm to penetrate the egg. Once the head of the sperm is inside the egg, the tail falls off, and the outside of the egg thickens to prevent another sperm from entering. Many variables affect whether fertilization occurs after intercourse among humans. One factor is a woman’s ovulatory, or menstrual, cycle. Human eggs can be fertilized for only a few days after ovulation, which typically occurs only once every 28 days. (To learn about what happens after fertilization, see Pregnancy and Birth.)

REAL-LIFE A P P L I C AT I O N S External Fertilization

oval-shaped organs in the groin that also generate sex hormones. At birth, a female’s ovaries contain hundreds of thousands of undeveloped eggs, each surrounded by a group of cells to form a follicle, or sac; however, only about 360-480 follicles reach full maturity. During puberty the action of hormones causes several follicles to develop each month. Normally, just one follicle fully matures, rupturing and releasing an ovum through the ovary wall in a process called ovulation. The mature egg enters one of the paired fallopian tubes, where it may be fertilized by a sperm and move on to the uterus to develop into a fetus. The lining of the uterus, called the endometrium, prepares for pregnancy each

Most land animals use some form of internal fertilization similar to that which we have described for humans. External fertilization, on the other hand, is more common among aquatic animals, who simply dump their sperm and eggs into the water and let currents mix the two male and female cells together. The sea urchin is a typical example: a male sea urchin releases several billion sperm into the water, and these sperm then swim toward eggs released in the same area. Fertilization occurs within seconds when sperm come into contact and fuse with eggs. As noted in Reproduction, external fertilization is essentially sexual reproduction without sexual intercourse. For humans the process of reproduction by external means may lack the intimacy of internal reproduction, but since 1978 a form of external fertilization has offered the opportunity of con-

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T H E O VA R I E S A N D M E N S T R U AT I O N . Eggs are produced in the ovaries,

Sexual Reproduction

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DURING IN VITRO fertilization, eggs are removed surgically from a female’s reproductive tract and fertilized in a test tube or petri dish with sperm (shown here). After the fertilized eggs have divided twice and an embryo is starting to form, they are reintroduced to the female’s body. (© Owen Franken/Corbis. Reproduced by permission.)

ceiving children to couples who otherwise might have remained childless. I N V I T R O Fe rt i l i z at i o n . This form of external fertilization is known as in vitro, or “in glass”—that is, in a glass test tube or petri dish. The term is contrasted with in vivo, meaning “in a living organism,” or in utero, “in the uterus.” During in vitro fertilization, eggs are removed surgically from a female’s reproductive tract. They then can be fertilized in a test tube or petri dish by sperm taken from the woman’s partner or another male. After the fertilized eggs have divided twice, indicating that the operation has “taken” and an embryo is starting to form, they are reintroduced to the female’s body. If all goes well, the embryo and fetus eventually result in a normal birth.

In vitro fertilization has been performed successfully on a variety of domestic animals since the 1950s but on humans only since the late 1970s. Two English physicians, Patrick Steptoe (1913–1988) and Robert G. Edwards (1925–), developed a method for stimulating ovulation with hormone treatment and then retrieving the nearly mature eggs and placing them in a petri dish to mature. In their breakthrough 1978 oper-

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ation, they added male sperm to the egg in the petri dish, where fertilization took place, and then implanted the eight-celled embryo in the mother’s body. The result of this extraordinary operation was a healthy baby named Louise Brown. A W O R L D W I T H O U T S E X ? The birth of Louise, whose twentieth birthday was celebrated in Britain with great fanfare, excited fears and controversy similar to those surrounding cloning (see Genetic Engineering). As with cloning, the fear was that the technology of in vitro fertilization would lead to the depersonalized manufacturing of human beings associated with some nightmarish future society, but this has not come to pass for several reasons. One is that in vitro fertilization is successful only about 15% of the time. Another, much more significant reason is that there are few women capable of producing a child through internal fertilization who would want to conceive without the intimacy of sexual intercourse.

There are exceptions, of course, in real life (two women in a same-sex relationship who want a child, for example) as well as in fiction. In the latter category, the character of Jenny Fields

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in the American novelist John Irving’s World According to Garp—published, ironically, in the same year as the birth of Louise Brown—would undoubtedly have conceived a child by in vitro means if those means had been available to her. As it was, Jenny, who conceived the book’s title character during World War II, manages to do so by having intercourse purely for the purpose of producing offspring. She does this by choosing a wounded, brain-damaged airman who will never remember having had sex with her and who dies shortly thereafter.

Sexual Reproduction

Courtship There is something far greater than mere biochemistry involved in sexual reproduction, which is why humans enjoy the intimacy of producing a baby. This is also why courtship and mating rituals are a significant part of human life, providing a means by which a male and female join as partners. Such is true even in modern America and the West in general, where the aftermath of the 1960s sexual revolution has left behind a world largely stripped of its former mystery. For better or worse, sex before marriage is a common part of life today in a way that it was not before the 1960s, with the advent of birth control and “free love,” and today there is little stigma attached to the conception of a child outside wedlock. That much has changed—and changed dramatically—but humans still practice courtship. Males still have to prove to females that they are suitable mates, usually by displaying their physical prowess or some other attribute associated with masculinity. Females still do most of the choosing, and despite all the changes in views toward male and female roles, the ideas of basic male and female differences seem to be hardwired into the minds of most people. This is particularly so of people who are heterosexual and especially those who either have children or intend to have them. Such attitudes are not surprising, since humans, while being something more than animals, are still animals as well.

AMONG

ANIMALS, COURTSHIP IS A COMPLEX SET OF

BEHAVIORS FRIGATE

THAT

BIRDS

LEADS DISPLAY

TO

MATING.

THEIR

HERE

INFLATED

MALE

SCARLET

POUCHES TO ATTRACT FEMALES. (© Wolfgang Kaehler/Corbis.

Reproduced by permission.)

animals use rituals, a series of behaviors for communication that is performed the same way by all the males or females in a species. They are governed by fixed-action patterns (FAPs), which are virtually identical with instinct (see Instinct and Learning). In courtship, some animals leap and dance, others sing, and still others ruffle their feathers or puff up pouches. The male peacock displays his glorious plumage to the female, and humpback whales advertise their presence under the sea by singing a song that can be heard hundreds of miles away. Courtship behavior enables an animal to find, identify, attract, and arouse a mate. Animals use signals, such as the release of pheromones, or scent signals, as well as visual displays to claim a particular mate or a territory.

mals, courtship is a complex set of behaviors that leads to mating. Courtship behavior communicates to each potential mate that the other is not a threat and serves to reveal to each that the species, gender, and physical condition of the other are suitable for mating. During courtship,

C H O O S I N G A M AT E . Usually, the females do the choosing. In some species of birds, males display themselves in a small communal area called a lek, where females select a mate from the parading males. Across the animal kingdom, males generally compete with one another for mates, either by fighting or by ritualized displays, and females pick the best quality of

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Sexual Reproduction

“THE THREE GRACES” by Peter Paul Rubens. A more amply formed female body is an ideal much closer to nature, since our evolutionary lineage has tended to favor women with large hips who are capable of bearing many children. (© Archivo Iconografico, S.A./Corbis. Reproduced by permission.)

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male available. Several basic factors influence a female in her choice of mate. First, she wants a male who can provide for her offspring, which is why the female tern (a type of bird) selects a

good fish catcher. As part of courtship among common terns, the male birds display fish to the females and may even feed them to the females as a way of demonstrating their ability to feed

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young. Among the long-jawed long-horned beetles that live in the Arizona desert, males battle each other for saguaro cactus fruit, and the females mate in exchange for access to the fruit. Genetic fitness, or the ability to survive— and to advance the survival of the species—is another important factor in mate selection. This is a large part of the reason why males may fight each other for a female—to show her that they are the most fit. Of course, the female animal does not know that she is choosing on this basis, but she is, and she is likely to select a partner with a striking appearance, capable of energetic displays—both of which are signs of good health.

Secondary Sex Characteristics, Evolution, and the Model of an Ideal Mate Humans, too, have deeply ingrained ideas regarding what makes an attractive potential mate, though to some extent, those ideas are cultural. Most societies regard a muscular male as attractive, for obvious evolutionary reasons: a physically fit male can provide for his mate and offspring, both by acquiring food and other resources and by protecting the nest. On the other hand, the modern American ideal of feminine beauty is more removed from nature. For one thing, this image of an attractive female is a thin body but large breasts—two things that seldom go together in nature but which are possible in the modern world through breast implantation surgery. That is, the achievement of such an ideal is available to a woman with a naturally thin body who also possesses the financial resources (and, of course, the desire) to undergo such surgery. A woman naturally gifted with large breasts is apt also to have large buttocks, hips, and thighs, and while some men may find such anatomical features (particularly the first) desirable, this more natural version of the female body is not the image usually promoted in advertising or other media. Therefore, if a woman with a full figure wishes to meet societal expectations regarding beauty, she must endure something much more strenuous than a mere operation—a strict low-fat diet and a great deal of exercise. Given these often unnatural expectations from society, it is no wonder that a great many women express frustration with their attempts to conform to them, nor is it any wonder that many women in the modern world simply stop trying

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to conform. In any case, the American ideal of a thin female is far from universal, in terms of either place or time. Many traditional cultures favor a more amply formed female body, as did Western civilization in the past—a fact exemplified by the paintings of seventeenth-century artists, such as Peter Paul Rubens, famous for his fleshy female subjects. This is an ideal much closer to nature, since our evolutionary lineage has tended to favor women with large hips who are capable of bearing many children.

Sexual Reproduction

E V O LU T I O N A N D T H E S E N S I T I V E M A N . As with much else about sexual

reproduction and the courtship rituals associated with it, the idea of beauty—itself an expression of secondary sex characteristics—is much simpler for men. As we noted earlier, physical fitness in men almost always is regarded as attractive, but women often value men for other traits, most notably intelligence and sense of humor. Whereas many men place an emphasis on physical appearance in choosing a mate, women’s expectations are much more complex and they are also likely to be much more forgiving toward members of the male population who do not look like Tom Cruise or some other Hollywood image of attractiveness. In defense of modern men, many have complained that modern women do not always want what they say they want. For example, from the early 1970s onward, it often was said in public discussions of sexuality on TV or in magazines that women wanted men to be more sensitive. That is, they wanted men who were more verbal and given to talking about their feelings and who were more aware of the woman and her needs. It is not surprising that men were failing to meet these expectations, since the idea of the “strong, silent type” is a masculine ideal in many cultures. Furthermore, not only civilization but also evolution has tended to favor men who are given more to actions than to words—men who can make war, either on the battlefield or in the business world. Such aggressive characteristics are all well and good among animals or in a human society just struggling to survive. But in the modern West, where most material needs are easily met, women have sought and expressed a desire for something more from men. Such was the situation that emerged in the wake of the sexual revolution, when people became more open not only

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Sexual Reproduction

KEY TERMS A DNA-containing

of its kind. An unborn human usually is

body, located in the cells of most living

called a fetus during the period from three

things, that holds most of the organism’s

months after fertilization to the time of

genes.

birth.

CHROMOSOME:

A term for a cell that has the

DIPLOID:

basic number of doubled chromosomes. EGG CELL: EMBRYO:

A female gamete. The stage of animal develop-

ment in the uterus before the animal is

A reproductive cell—that is,

a mature male or female germ cell that possesses a haploid set of chromosomes and is prepared to form a new diploid by undergoing fusion with a haploid gamete of the

considered a fetus. In humans this is equiv-

opposite sex. Sperm and egg cells are,

alent to the first three months.

respectively, male and female gametes.

FALLOPIAN TUBES:

A set of trum-

GENITALIA:

The sex organs, which

pet-like tubes that carries a fertilized egg

include the male penis and the female

from the ovary to the uterus.

vagina.

FAPS:

Fixed-action patterns of behav-

GESTATION:

The period between fer-

ior, or strong responses on the part of an

tilization and birth during which the

animal to particular stimuli. FAP is virtual-

unborn offspring develops in the uterus.

ly synonymous with instinct.

HAPLOID:

FERTILIZATION:

The process of cellu-

A term for a cell that has

half the number of chromosomes that

lar fusion that takes place in sexual repro-

appear in a diploid or somatic cell.

duction. The nucleus of a male reproduc-

HORMONE:

tive cell, or gamete, fuses with the nucleus

living cells, which send signals to spots

of an female gamete to produce a zygote.

remote from their point of origin and

FETUS:

An unborn or unhatched ver-

tebrate that has taken on the shape typical

about sex itself but about their feelings as well. Although the sexual revolution yielded a number of negative effects, including the loss of mystery associated with sex and an increase in out-ofwedlock pregnancies and cases of sexually transmitted disease, it also made it possible for people to be frank in a way that they had never been before. One outgrowth of this was the revelation, in many women’s magazines, that women were no longer satisfied or impressed with strong, silent men.

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GAMETE:

Molecules produced by

induce specific effects on the activities of other cells.

During the 1970s the model of the sensitive man emerged, and for the first time it became possible for men to talk about their feelings. And

they did, in books, in movies such as those of Woody Allen, and in therapist-led encounter groups or men’s groups. Gradually, however, a surprising fact emerged: women did not want men to be too sensitive and certainly not too nonmasculine. They wanted a man who would talk about his feelings but not a man who wore his heart on his sleeve. They wanted to be treated as equals in the workplace, but if there was a strange noise in the house at night, they wanted the man to go see what it was. They wanted a man who would respect them—but not a man who was so polite and predictable that he possessed no air of mystery.

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KEY TERMS Sloughing off of the lining of the uterus, which occurs monthly in nonpregnant females who have not reached menopause (the point at which menstrual cycles cease) and which is manifested as a discharge of blood. MENSTRUATION:

Female reproductive organ that contains the eggs.

Sexual Reproduction

CONTINUED

SEXUAL REPRODUCTION:

One of

the two major varieties of reproduction, along with asexual reproduction. In contrast to asexual reproduction, which involves a single organism, sexual reproduction involves two. Sexual reproduc-

OVARY:

OVUM:

An egg cell.

A stage in the maturation of humans and higher primates wherein the person first becomes capable of sexual reproduction. It is marked by the development of secondary sex characteristics, the maturation of the genital organs, and the beginning of menstruation in females. Puberty typically takes place somewhere between the ages of about 10 and 14 years. PUBERTY:

tion occurs when male and female gametes undergo fusion, a process known as fertilization, and produce cells that are genetically different from those of either parent. SPERM CELL: TESTES:

Those unique traits that mark an individual as a male or female but which are not manifested in the sexual organs themselves. Facial hair and a deep voice in males or breast development as well as hip and buttocks development in females are examples. TICS:

All of this reveals a great deal about humans and sexual reproduction. First of all, mating, or the interaction between males and females, is about far more than reproduction or even sex. It is an interaction that involves the whole person. Second, human sexuality is surrounded by webs of psychological, social, and spiritual complexity that make it a phenomenon quite different from animal sexuality, which tends to be about little more than procreation. And, third, for all the progress humans have made, and for all the levels of civilization that separate us from our evolutionary roots, there is still something in human sexuality that responds to very basic ideas about

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The pair of male reproduc-

tive glands located in the scrotum, a skincovered sac that hangs from the groin. UTERUS:

SECONDARY SEX CHARACTERIS-

A male gamete.

A reproductive organ, found

in most female mammals, in which an embryo and later a fetus grow and develop. VAGINA:

A passage from the uterus to

the outside of the body. ZYGOTE:

A diploid cell formed by the

fusion of two gametes.

sex, the sexes, and the need to propagate the species through mating and reproduction. WHERE TO LEARN MORE Avraham, Regina. The Reproductive System. Philadelphia: Chelsea House Publishers, 2001. Cool Nurse (Web site). . Francoeur, Robert T. Taking Sides: Clashing Views on Controversial Issues in Human Sexuality. Guilford, CT: Dushkin Publishing Group/Brown and Benchmark, 1996. “How Human Reproduction Works.” How Stuff Works (Web site). .

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Kimball, Jim. “Sexual Reproduction in Humans.” Kimball’s Biology Pages (Web site). . Parker, Steve. The Reproductive System. Austin, TX: Raintree Steck-Vaughn, 1997.

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“Puberty Information for Boys and Girls.” American Academy of Pediatrics (Web site). . Whitfield, Philip. The Human Body Explained: A Guide to Understanding the Incredible Living Machine. New York: Henry Holt, 1995.

“Puberty Guide in Adolescents.” Keep Kids Healthy (Web site). .

Winikoff, Beverly, and Suzanne Wymelenberg. The Whole Truth About Contraception: A Guide to Safe and Effective Choices. Washington, DC: Joseph Henry Press, 1997.

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PREGNANCY AND BIRTH

Pregnancy and Birth

CONCEPT One of the greatest dramas in the world of living things is that which takes place in pregnancy and birth. Pregnancy forms a bond between mother and offspring that, in humans at least, lasts throughout life. Humans and many other animals are viviparous, meaning that offspring develop inside the mother’s body and are delivered live. By contrast, birds and some other varieties of animal are oviparous, meaning that they deliver offspring in eggs that must develop further before hatching. In the modern world, human females experience birth in several ways—vaginal or cesarean, with anesthetics and without, at home or in a hospital—but just a few hundred years ago, there was little variety in an experience that was almost always painful and dangerous.

HOW IT WORKS Oviparity, Viviparity, and Ovoviviparity The birth of live offspring is a reproductive feature shared by mammals, some fishes, and selected invertebrates, such as scorpions, as well as various reptiles and amphibians. Animals who give birth to live offspring are called viviparous, meaning “live birth.” In contrast to viviparous animals, other animals—called oviparous, meaning “egg birth”—give birth to eggs that must develop before hatching. Finally, there are ovoviviparous animals, or ones that produce eggs but retain them inside the female body until hatching occurs, so that “live” offspring are born.

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Oviparous animals may fertilize their eggs either externally or internally, though all animals that fertilize their eggs externally in nature are oviparous. (See Sexual Reproduction for more about internal and external fertilization.) In cases of internal fertilization, male animals somehow pass their sperm into the female: for example, male salamanders deposit a sperm packet, or spermatophore, onto the bottom of their breeding pond and then induce an egg-bearing (or gravid) female to walk over it. The female picks up the spermatophore and retains it inside her body, where the eggs become fertilized. These fertilized eggs later are laid and develop externally. Oviparous offspring undergoing development before birth obtain all their nourishment from the yolk and the protein-rich albumen, or “white,” rather than from direct contact with the mother. Ovoviviparity is common in a wide range of animals, including certain insects, fish, lizards, and snakes, but it is much less typical than oviparity. Ovoviviparous insects do not supply oxygen or nourishment to their developing eggs; they merely give them a safe brooding chamber for development. Nonetheless, species of ovoviviparous fish, lizards, and snakes appear to provide some nutrition and oxygen to their growing offspring. Because nutrition is provided in these instances, some zoologists consider them examples of true live birth, or viviparity. VIVIPARITY. Viviparity is the type of birth process that takes place in most mammals and many other species. Viviparous animals give birth to living young that have been nourished in close contact with their mothers’ bodies. The offspring of both viviparous and oviparous animals develop from fertilized eggs, but the eggs of viviparous

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Pregnancy and Birth

OVIPAROUS ANIMALS GIVE BIRTH TO EGGS THAT MUST DEVELOP BEFORE HATCHING. THE SURINAM TOAD, FOR EXAMPLE, CARRIES HER EGGS ATTACHED TO HER BACK WHILE THEY MATURE. (© David A. Northcott/Corbis. Reproduced by permission.)

animals lack a hard outer covering, or shell. Viviparous young grow in the adult female until they are able to survive on their own outside her body. In many cases, the developing fetuses of viviparous animals are connected to a placenta, a special membranous organ with a rich blood supply that lines the uterus in pregnant mammals. It provides nourishment to the fetus through a supply line called an umbilical cord. All mammals, except for the platypus and the echidnas, are viviparous; only these two unusual mammals, called montremes, lay eggs. (See Speciation for more about mammal species.) Some snakes, such as the garter snake, are viviparous, as are certain lizards and even a few insects. Ocean perch, some sharks, and a few popular aquarium fish are also viviparous. Even certain plants, such as the mangrove and the tiger lily, are described as viviparous because they produce seeds that germinate, or sprout, before they become detached from the parent plant.

The zygote forms in one of the mother’s fallopian tubes, the tubes that connect the ovaries with the uterus. It then travels to the uterus, where it becomes affixed to the uterine lining. Along the way, the zygote divides a number of times, such that by the time it reaches the uterus it consists of about 100 cells and is called an embryoblast. The exact day on which the embryoblast implants on the uterine wall varies, but it is usually about the sixth day after fertilization. By the end of the first week, a protective sac, known as the amniotic cavity, begins to form around the embryoblast.

The essays on Reproduction and Sexual Reproduction discuss the basics of the reproductive process through the point of fertilization. A fertilized egg is called a zygote, but once it begins to develop in the uterus or womb, it is known as an

Changes then begin to take place at a rapid rate. As each week passes, the embryo takes on more and more necessary and distinctive features, such as blood vessels in week 3, internal organs in week 5, and finger and thumb buds on the hands in week 7. Unfortunately, miscarriages are not uncommon in the early weeks of pregnancy. The mother’s immune system (see Immune System) may react

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embryo and later, when it begins to assume the shape typical of its species, a fetus. In the uterus, the unborn offspring receives nutrients and oxygen during the period known as gestation, which extends from fertilization to birth. (In humans the gestation period is nine months.)

E M B RYO

AND

FETUS.

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to cells from the embryo that it classifies as “foreign” and begin to attack those cells. The embryo may die and be expelled. The first three months of embryonic development are known as the first trimester, or the first three-month period of growth. At the end of the first trimester, the embryo is about 3 in. (7.5 cm) long and looks like a tiny version of an adult human. Thereafter, the growing organism is no longer an embryo, but a fetus. Fetal development continues through the second and third trimesters until the baby is ready for birth at the end of ninth months.

Preparing for Birth At the end of the gestation period, the mother’s uterus begins to contract rhythmically, a process called labor. This is accompanied by the release of hormones, most notably oxytocin. From the time of fertilization, quantities of the hormone progesterone, which keeps the uterus from contracting, are high; but during the last weeks of gestation, maternal progesterone levels begin to drop, while levels of the female hormone estrogen rise. When progesterone levels drop to very low levels and estrogen levels are highest, the uterus begins to contract. Meanwhile, as birth approaches, the brain’s pituitary gland releases oxytocin, a hormone that stimulates uterine contractions and controls the production of milk in the mammary glands (a process called lactation). Synthetic oxytocin sometimes is given to women to induce labor. Scientists believe that the pressure of the fetus’s head against the cervix, the opening of the uterus, ultimately initiates the secretion of oxytocin. As the fetus’s head presses against the cervix, the uterus stretches and relays a message along nerves to the pituitary gland, which responds by releasing oxytocin. The more the uterus stretches, the more oxytocin is released.

moves into the birth canal and is expelled from the vagina. The uterus continues to contract even after the placenta is delivered, and it is thought that these contractions serve to control bleeding.

Pregnancy and Birth

After the baby is born, the umbilical cord that has attached the fetus to the placenta is clamped. The clamping cuts off the circulation of the cord, which eventually stops pulsing owing to the interruption of its blood supply. The baby now must breathe air through its own lungs, whereas before it has been breathing, fishlike, in the warm, wet environment of the mother’s amniotic fluid. The process of labor described here in a very cursory fashion (it is actually much more complicated) can take from less than one hour to 48 hours, but typically the entire birth process takes about 16 hours.

REAL-LIFE A P P L I C AT I O N S Changing Views on Childbirth Before modern times, the realm of childbirth was a world exclusive to women, and few men ever entered the birth chamber. It was a place of excruciating pain and serious danger to the mother giving birth, so filled with blood and screaming that few men would have dared enter even if they had wanted to do so. Women had to give birth without anesthesia and any number of other amenities of modern medical care, including sophisticated diagnostic techniques and equipment, such as ultrasound, as well as antiseptic environments and surgical techniques, such as cesarean section.

LA B O R A N D D E L I V E RY. Rhythmic contractions dilate the cervix, causing the fetus to move down the birth canal and to be expelled together with the placenta, which has supplied the developing fetus with nutrients from the mother during the gestation period. Before delivery, the placenta separates from the wall of the uterus. Since the placenta contains many blood vessels, its separation from the wall of the uterus causes bleeding. This bleeding is normal, assuming that it is not excessive. After the placenta separates from the uterine wall, it

In those days, birthing assistance was the work of midwives, women who lacked formal schooling in medicine (which was unavailable to most women in any case) but made up in experience for what they lacked in education. By about 1500, however, as medicine began to progress after many centuries of stagnation, male doctors increasingly forced midwives out of a job. In 1540 the European Guild of Surgeons declared that “no carpenter, smith, weaver, or woman shall practice surgery.” A major turning point in the male takeover of birthing assistance duties came with the invention of the forceps, tong-like instruments that could be used for extracting a baby during difficult births. The inventor was the

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English obstetrician (a physician concerned with childbirth) Peter Chamberlen the Elder (1560–1631), and he and his descendants for a century closely guarded the design of the brilliant invention. Even the mothers on whom it was used never saw the instrument, and midwives were prohibited from using forceps to assist during childbirth. O B S T E T R I C I A N S TA K E O V E R.

By the eighteenth century, however, Chamberlen’s descendants had released their exclusive claim over the forceps, and use of the instrument spread to other medical professionals. This gave male obstetricians a great technological advantage over female midwives and further ensured the separation of the midwives from the medical profession. By 1750 numerous physicians and surgeons had gained the status of “man-midwives,” and the growth of university courses on obstetrics established it as a distinct medical specialty. By the latter part of the 1700s, most women of the upper classes had come to rely on professionally trained doctors rather than midwives, yet in America, where doctors were scarcer than in Europe, the profession of midwife continued to flourish into the 1800s. Still, by the early twentieth century, childbirth had moved out of the home and into the hospital, and at mid-century it had become a completely medical process, attended by physicians and managed with medical equipment and procedures, such as fetal monitors, anesthesia, and surgical interventions. T H E R E AC T I O N I N T H E LAT E T W E N T I E T H C E N T U RY. Many women

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many years, the screams of women giving birth “naturally” once again filled the halls of hospital maternity wards and home birthing rooms.

Modern Childbirth The last few paragraphs represent an extreme reaction—a view not shared by many women, who have been more than happy to avail themselves of the benefits of childbirth in the modern world. Such benefits include an epidural, a type of anesthetic procedure that serves to alleviate the pain of parturition, or childbirth, while making it possible for the mother to remain conscious. Still, even for women who have no interest in giving birth at home or without the aid of drugs, much has changed in the world of childbirth. Women may choose a happy medium between the medical establishment and more traditional methods, for instance, by opting to consult with an obstetrician and a midwife. Today, many obstetricians are women. This has had an incalculable effect on making childbirth psychologically easier for many women: though some are happy to retain a male obstetrician, many others find themselves much more comfortable being cared for by a physician who, in all likelihood, has given birth herself. The increasingly important role of the female obstetrician, along with other factors, serves to symbolize the fact that the world has progressed beyond the old false dilemma between medical care from a male or a female, between medicine and nature, between hospital and home.

of the late twentieth century found themselves dissatisfied with this clinical approach to childbirth. Some believed that the medical establishment had taken control of a natural biological process, and women who wanted more command over labor and delivery helped popularize new ideas on childbirth that sought to reduce or eliminate medical interventions. Today, some women choose to deliver with the help of a nurse-midwife, who, like her premodern counterparts, is trained to deliver babies but is not a doctor. There are women who even choose home birth, attended by a doctor or midwife or sometimes both. There are even brave souls who, in the face of increasing concern about the effect of anesthesia on the fetus, refuse artificial means of controlling pain and instead rely on breathing and relaxation techniques. For the first time in

A H O S P I TA L AS H O M E . Hospital rooms, in fact, are starting to resemble rooms at home. Everywhere one looks in the modern maternity environment, there is evidence that much has changed, not only from the very old days, when male doctors were not involved in childbirth at all, but also from the more recent past, when males took over the process entirely. In a brilliant innovation, many hospitals have created a situation in which the woman gives birth in her own hospital room, which is outfitted with couches, cabinets, curtains, and rocking chairs to make it look like a home rather than a hospital. To emphasize the smooth transition between home life and the delivery room, fathers, once banished from the labor and delivery chambers, now are welcomed as partners in the birth process.

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ULTRASOUND

IMAGE OF A 30-WEEK-OLD FETUS.

BY

THE EIGHTEENTH WEEK OF PREGNANCY, ULTRASOUND TECHNOL-

OGY CAN DETECT MANY STRUCTURAL ABNORMALITIES, SUCH AS SPINA BIFIDA, HEART AND KIDNEY DEFECTS, AND HARELIP. (© BSIP/Kretz Technik/Photo Researchers. Reproduced by permission.)

A father may even cut the umbilical cord, and he is certainly likely to be in the delivery room with a video camera, recording the event for posterity—yet another change from the past. Fathers are not the only ones filming in the delivery room. Today, cable television networks, such as the Learning Channel, provide programming that offers a frank view of the delivery process, complete with candid footage that sometimes can be as dramatic as it is revealing. The maternity ward, once a closed place, has increasingly become an open book.

Saving and Improving Lives Many a mother and father alike can breathe a prayer (or at least a sigh) of thanks for all the innovations that today make birth much safer than it once was. Among them are a variety of techniques for embryo and fetal diagnosis, which help make parents aware of possible problems in the growing embryo. Ultrasound diagnosis, a technique similar to that applied on submarines for locating underwater structures, uses highpitched sounds that cannot be heard by the human ear. These sounds are bounced off the embryo, and the echoes received are used to identify embryonic size.

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By the eighteenth week of pregnancy, ultrasound technology can detect many structural abnormalities, such as spina bifida (various defects of the spine), hydrocephaly (water on the brain), anencephaly (no brain), heart and kidney defects, and harelip (in which the upper lip is divided into two or more parts). On a less dire and much more pleasant note, it can also give future parents an opportunity to gain their first glimpse of their child, and an experienced ultrasound technician usually can tell them the baby’s sex if they choose to learn it before the birth. P R E N ATA L T E S T I N G . Chorionic villi sampling is the most sophisticated modern technique used to assess possible inherited genetic defects. This test typically is performed between the sixth and eighth week of embryonic development. During the test, a narrow tube is passed through the vagina or the abdomen, and a sample of the chorionic villi (small hairlike projections on the covering of the embryonic sac) is removed while the physician views the baby via ultrasound.

Chorionic villi are rich in both embryonic and maternal blood cells. By studying them, genetic counselors can determine whether the baby will have any of several defects, including

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KEY TERMS The stage of animal development in the uterus before the point at which the animal is considered a fetus. In humans this is equivalent to the first three months. EMBRYO:

A set of trumpet-like tubes that carries a fertilized egg from the ovary to the uterus. FALLOPIAN TUBES:

The process of cellular fusion that takes place in sexual reproduction. The nucleus of a male reproductive cell, or gamete, fuses with the nucleus of a female gamete to produce a zygote.

remote from their point of origin and induce specific effects on the activities of other cells. OVARY:

Female reproductive organ

that contains the eggs. OVIPAROUS:

A term for an animal

that gives birth to eggs that must develop before hatching. Compare with viviparous.

FERTILIZATION:

An unborn or unhatched vertebrate that has taken on the shape typical of its kind. An unborn human usually is called a fetus during the period from three months after fertilization to the time of birth.

OVOVIVIPAROUS:

inside the body until hatching occurs, so that “live” offspring are born. Compare with oviparous and viviparous.

FETUS:

OVUM: UTERUS:

An egg cell. A reproductive organ, found

in most female mammals, in which an embryo and, later, a fetus grows and develops. A passage from the uterus to

The time between fertilization and birth, during which the unborn offspring develops in the uterus.

VAGINA:

Molecules produced by living cells, which send signals to spots

that gives birth to live offspring. Compare

GESTATION:

HORMONE:

Down syndrome (characterized by mental retardation, short stature, and a broadened face), cystic fibrosis (which affects the digestive and respiratory systems), and the blood diseases hemophilia, sickle cell anemia, and thalassemia. (Several of these disorders are discussed in different essays throughout this book; for instance, Down syndrome is examined in Mutation.) As with ultrasound, it also can show the baby’s gender.

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A term for an ani-

mal that produces eggs but retains them

the outside of the body. VIVIPAROUS:

A term for an animal

with oviparous.

ease, cystic fibrosis, and Down syndrome. However, amniocentesis involves the risk of fetal loss as a result of disruption of the placenta. Chorionic villi sampling is even more risky, with an even higher possibility of fetal loss than amniocentesis, probably because it is conducted at an earlier stage. In alpha-fetoprotein screening, which takes place somewhere between the 16th and 18th weeks, proteins from the amniotic sac and the fetal liver are taken as a means of screening for specific defects. Because of uncertainties involved in interpretation of the results, alpha-fetoprotein screening is not a common procedure.

Another important form of prenatal (before the birth of the child) testing is amniocentesis, performed around week 16, in which amniotic fluid is drawn from the uterus by means of a needle inserted through the abdomen. Amniocentesis, too, can reveal the sex of the child, as well as a host of genetic disorders such as Tay-Sachs dis-

C E S A R E A N S E C T I O N . Another extremely important technique that has saved the

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life of many babies and mothers is cesarean section. The normal position for a baby in delivery is head first; when a baby is in the breech position, with its bottom first, it poses grave dangers to both the mother and the child. Not only could the baby fail to emerge in time to begin breathing normally, thus running the risk of brain damage, but it also can become stuck, endangering the life of the mother. Today these dangers are overcome by such techniques as turning the baby and by cesarean section, an operation in which the baby is removed via surgery from the mother’s abdomen. Cesarean sections may also be performed due to other complications, including fetal and/or maternal distress. The term cesarean refers to the Roman emperor Julius Caesar (102–44 B.C.), who supposedly was delivered in this fashion. But the story of Caesar’s birth is undoubtedly a legend: until the early modern era, cesarean sections were performed only to save a living baby after the mother had died in childbirth. The reason is that cesareans were likely to be fatal to the mother. Only in the late nineteenth century, by which time doctors had come to understand the importance of providing an antiseptic or germ-free environment, did cesarean sections become prac-

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tical. Today the C-section, as it is called, has become a routine procedure—one that has saved literally hundreds of thousands, perhaps millions, of lives.

Pregnancy and Birth

WHERE TO LEARN MORE Assisted Reproduction Foundation (Web site). . Bainbridge, David. Making Babies: The Science of Pregnancy. Cambridge, MA: Harvard University Press, 2001. Facts About Multiples (Web site). . Midwifery, Pregnancy, Birth and Breastfeeding (Web site). . Pence, Gregory E. Who’s Afraid of Human Cloning? Lanham, MD: Rowman and Littlefield, 1998. Pregnancy and Birth (Web site). . Pregnancy and Reproduction Topics. Medline/National Library of Medicine, National Institutes of Health (Web site). . Rudy, Kathy. Beyond Pro-Life and Pro-Choice: Moral Diversity in the Abortion Debate. Boston: Beacon Press, 1996. Vaughan, Christopher C. How Life Begins: The Science of Life in the Womb. New York: Times Books, 1996.

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Evolution

CONCEPT Among the dominant concepts of the modern world in general, and biology in particular, few are as powerful—or as misunderstood—as evolution. Even the name is something of a misnomer, since it almost implies some sort of striving to reach a goal, as though the “purpose” of evolution were to produce the most intelligent species, human beings. In fact, what drives evolution is not a quest for biological greatness but something much more down to earth: the need for organisms to survive in their environments. Closely tied to evolution are two processes, mutation and natural selection. Natural selection is a process whereby survival is related directly to the ability of an organism to fit in with its environment, while mutation involves changes in the genetic instructions encoded in organisms. Although the English naturalist Charles Darwin (1809–1882) often is regarded as the father of evolutionary theory, he was not the first thinker to suggest the idea of evolution as such; however, by positing natural selection as a mechanism for evolution, he provided by far the most convincing theory of evolutionary biological change up to his time. In the years since Darwin, evolutionary theory has evolved, but the essential idea remains a sound one, and it is a “theory” only in the sense that it is impossible to subject it to all possible tests. The idea that evolution is somehow still open to question is another pervasive misconception, and it often appears hand in hand with the most pervasive misconception of all—that evolution is in some way anti-Christian, anti-religion, or anti-God.

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E VO LU T I O N

HOW IT WORKS The “Watch Analogy” and What It (Unintentionally) Teaches About Evolution Throughout this essay, we discuss misconceptions relating to evolution. Such misconceptions have had such a strong impact on modern civilization that it is important to begin by setting aside a few misguided ideas that strike at the very heart of the evolutionary process. Many of these misconceptions are embodied in a popular “argument” against evolution that goes something like this: Suppose you took a watch apart and laid the pieces on the ground. If you came back in a billion years, would you really expect the watch to have assembled itself? This argument is a virtual museum of all the fallacies associated with evolution. First of all, a watch (or any of the other variations used in similar arguments) is mechanical, not organic or biological, which is the class of objects under discussion within the framework of evolution. In that sense, the answer to this question is easy enough: No, a watch probably never would assemble itself, because it is not made of living material and it has no need for survival. Another problem with the watch argument is that it starts with impossibly large pieces. Let us assume that the watch is a living being; even so, one would not expect its dials and gears to assemble themselves. But evolution does not make such claims: there is nothing in the theory of evolution to lead one to believe that a collection of organs lying around on a beach eventually would piece themselves together to make a whale.

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be called the “fallacy of intention,” or the idea that evolution is driven by some overall purpose.

Evolution

T H E “FA L L A C Y O F I N T E N T I O N . ” Hidden in the watch analogy is the

idea of the watch itself, the finished product, as a “goal.” By the same analogy, the single-cell eukaryotes of a billion or two billion years ago were forming themselves for the purpose of later becoming pine trees or raccoons or people. This is not a valid supposition, as can be illustrated by analogies to human history.

CHARLES DARWIN (The Library of Congress.)

According to what paleontologists (see Paleontology) and other scientists can deduce, over the course of three billion years life-forms evolved from extremely simple self-replicating carbon-based molecules to single-cell organisms. This is hardly what one would call breakneck speed. The more visible or “exciting” part of evolution, with the proliferation of species that produced the dinosaurs and (much later) humans, took place in the past billion years. In fact, the pace of change was still very, very slow until about half a billion years ago, and it has been accelerating ever since. For the vast majority of evolutionary history, however, change has been so slow that, by contrast, watching paint dry would be like playing a high-speed video game.

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The history of human beings, of course, has taken place over a much, much shorter span than evolutionary history. (The Paleontology essay contains several comparisons between the span of human life on Earth and Earth’s entire existence.) Moreover, unlike cells, people do form goals and act on intentions, so if there were any good example of change with a goal in mind, it would have to come from human beings. Yet even in the few thousand years that humans have existed in organized societies, most trends have occurred not as part of a major plan but as a means of adapting to conditions. Consider the situation of a group of nomads who lived in what is now southern Russia about 5,000 years ago. At some point, this vast collection of tribes began to migrate outward, some moving into an area that is now central Asia and the Indian subcontinent and others migrating westward. No sane person would argue that the westward-traveling members of this group knew that in moving to the geographically advantageous territory of Europe, they were putting in place conditions that would help give their descendants dominance over most of the planet some 4,500 years later. Rather, they were probably just trying to find better land for grazing their horses.

Ironically, for the watch scenario to be truly analogous to anything in evolution, one would have to start with atoms and molecules not whole gears and dials. Opponents of evolutionary theory might take this fact as being favorable to their cause, but if the watch were made of living, organic material rather than metal, it is possible that the molecules would have some reason to join in the formation of organelles and, later, cells. Or perhaps they would not. Therein lies another problem with the watch analogy and, indeed, with many of the attempts to argue against evolution on a religious basis. This might

We cannot say what the Indo-Europeans, as they are known to history, were looking for. Our only evidence that they existed is the similarities between the languages of Europe, India, and Iran, first noted by the German philologist and folklorist Jacob Grimm (1785–1863) at about the same time that Darwin was formulating his theory of evolution. Grimm, in fact, used methods not unlike those of Darwin, but instead of fossils he studied words and linguistic structures. Along the way, he found remarkable links, such as the Sanskrit word agni, cousin to the Latin term ignis and such modern English words as ignite.

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In contrast to the Indo-Europeans, we know a great deal about another group of westwardmoving nomads, the Huns of around A.D. 300, who were indeed looking for better grazing lands. Dislocated from their native areas by the building of China’s Great Wall, the Huns crossed the Danube River, displacing the Ostrogoths. The Ostrogoths, in turn, moved westward, and this migration set in motion a domino effect that would bring an end to the Western Roman Empire in A.D. 476. Did the Huns intend to destroy the Roman Empire and bring about the Middle Ages? No reasonable person would adopt such a conspiratorial view of history. Even more absurd, did the Chinese build the Great Wall with the idea of precipitating this entire chain of events? Again, no one would assert such a premise. If those trends in the evolution of societies were not goaldirected, why would we assume that cells and organisms would have to be striving toward a particular end to obtain certain results? CONFUSING EVOLUTION W I T H G O D . In fact, there is no driving “pur-

pose” to evolution—no scientifically based substitute for God operating from behind the scenes and manipulating the evolutionary process to achieve its ultimate aims. Evolution is not guided by any one large aim but by a million or a billion small aims—the need for a particular species of mollusk to survive, for instance. As we discuss in the course of this essay, the idea of an underlying conflict between evolution and Christianity (or any other religion, for that matter) is almost entirely without merit. On the other hand, it is theoretically possible that all the processes of evolution took place without a creator—but this still should not pose a threat to anyone’s idea of God. There is nothing in evolution that would lead to the conclusion that there is no God, that the universe is not God’s handiwork, or that God does not continue to engage in a personal relationship with each human. Neither is there anything in evolution that would lead to the conclusion that God does exist. Rather, the matter of God is simply not relevant to the questions addressed by evolution. In other words, evolution leaves spiritual belief where it should be (at least, according to Christianity): in the realm of individual choice.

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Natural Selection Evolution

As we noted earlier, one of the principal mechanisms of evolutionary processes is natural selection. This in itself illustrates the lack of intention, or “goal orientation,” in evolution. Like the name evolution itself, the term natural selection can be deceptive, implying that nature selects certain organisms to survive and condemns others to extinction. In fact, something quite different is at work. Species tend to overproduce, meaning that the number of field mice, for instance, born in any year is so large that this entire population cannot possibly survive. The reason is that there is never enough of everything—food, water, or living space—for all members of the population to receive what they need. Therefore, only those best adapted to the environment are likely to survive. FAS T E R, F U R R I E R M I C E . Suppose, for instance, that the climate in the area where two field mice live is very cold, and suppose that some of the field mice have more protective fur than others; obviously, they are more likely to live. If there are many speedy predators around, judging purely on the basis of that factor alone, it would be easy to predict that the swiftest of the field mice would survive. Thus, faster-running, furrier mice would be “selected” over the slower or less furry mice.

Natural selection is not simply a matter of one particular mouse surviving in an environment. Instead, it involves the survival of specific strains, or lines of descent, that are more suited to the environment in question. Individuals adapted to an environment are more likely to live and reproduce and then pass on their genes to the next generation, while those less adapted are less likely to reproduce and pass on their traits. The genetic strains that survive are not “better” than those that do not—they are only better adapted. The process of natural selection is ongoing. For example, in generation A, the furrier field mice survive and pass on their “furriness” gene to their offspring. Some of the offspring may still not be furry, and these mice will be less likely to survive and reproduce. In addition, since there are almost always several survival factors affecting natural selection, it is likely that other traits also will determine the survivability of certain individual mice and their genes.

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Evolution

For instance, there may be furry but slow mice in generation B, which despite their adaptation to temperature conditions are simply not fast enough to get away from predators. Therefore, the mice in generation C are likely to be furrier and faster than their ancestors. Additional survival factors may come into the picture, to ensure that the average member of generation D has sharper teeth in addition to swifter feet and a furrier body. Although this illustration depicts evolutionary changes as taking place over the course of four generations, they are more likely to occur over the span of 400 or 4,000 or four million generations. In addition, the process is vastly more complicated than it has been portrayed here, because numerous factors are likely to play a part. The essential mechanism outlined here, however, prevails: certain traits are “naturally selected” because individuals possessing those traits are more capable of survival. THE “ S U R V I VA L OF THE F I T T E S T. ” The concept of natural selection

sometimes is rendered popularly as the “survival of the fittest.” Scientists are less likely to use this phrase for several reasons, including the fact that it has been associated with distasteful social philosophies or murderous political ideologies— for example, Nazism. Additionally, the word fittest is a bit confusing, because it implies “fitness,” or the quality of being physically fit. This implication, in turn, might lead a person to believe that natural selection entails the survival of the strongest, which is not the case. Yet this is precisely what proponents of a loosely defined philosophy known as social Darwinism claimed. Popular among a wide range of groups and people in the late nineteenth and early twentieth centuries, social Darwinism could be used in the service of almost any belief. Industrialists and men of wealth asserted that those who succeeded financially did so because they were the fittest, while Marxists claimed that the working class ultimately would triumph for the same reason. Across the political spectrum, social Darwinism confused the meaning of “fittest” with that of other concepts: “strongest,” “most advanced,” or even “most moral.” All of this, it need hardly be said, is misguided, not least because evolutionary theory has nothing to do with race, ethnicity, or social class.

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In fact, “survival of the fittest,” in a more accurate interpretation, means that individuals that “fit,” or “fit in with,” their environments are those most likely to survive. This is a far cry from any implication of strength or superiority. Imagine a group of soldiers in combat: Which type of soldier is most likely to survive? Is it the one who scores highest on physical training tests, looks the finest in a uniform, comes from a more socially upper-class home, and has the most advanced education? Or is it the one who keeps his head low, acts prudently, does not rush into dangerous situations without proper reconnaissance, and obeys instruction from qualified leaders? Clearly, the second set of characteristics has much more to do with survival, even though these qualities may seem less “noble” than the first set. Yet it is by adapting, or proving his or her adaptability, to the environment of war that a soldier survives—not by displays of strength or other types of “fitness” that simply appear impressive. In the same way, the fitness of a species does not necessarily have anything to do with strength: after all, the lion, the “king of beasts,” would die out in a polar climate or a desert or an aquatic environment.

Mutation Although natural selection is of principal importance in evolution, mutation also plays a pivotal role. Mutation is the process whereby changes take place in the genetic blueprint for an organism as a result of alterations in the physical structure of an organism’s DNA (deoxyribonucleic acid). DNA is a molecule in all cells and in many viruses that contains genetic codes for inheritance. DNA carries genetic information that is transmitted from parent to offspring; when a mutation occurs, this new genetic information— often quite different from the genetic code received by the parent from the grandparent—is passed on instead. Under normal conditions of reproduction, a copy of the DNA from the parent is replicated and transmitted to the offspring. The DNA from the parent normally is copied exactly, but every once in a while errors arise during replication. These errors usually originate in noncoding regions of the DNA and therefore have little effect on the observable traits of the offspring. On the other hand, some mutations may be lethal, and thus the offspring does not survive for

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the mutation to become apparent. In a very few cases, however, offspring with a slightly modified genetic makeup manage to survive. C O N T RA S T W I T H A C Q U I R E D C H A RAC T E R I S T I CS . Mutation is not to

be confused with the inheritance of acquired characteristics, a fallacious doctrine that had its adherents when Darwin was a young man. If acquired characteristics were taken to an extreme, a lumberjack who loses his arm cutting down a tree and later conceives a child with his wife would most likely father a child who is missing an arm. This notion is absurd, and attempts to put forward a workable theory of acquired characteristics in the late eighteenth and early nineteenth centuries involved much greater subtlety. Still, the idea is misguided. The French natural philosopher Jean Baptiste de Lamarck (1744–1829), one of the leading proponents of acquired characteristics, maintained that giraffes had gained their long necks from the need to stretch and reach leaves at the top of tall trees. In other words, if a giraffe parent had to stretch its neck, a giraffe baby would be born with a stretched neck as well. Later, Darwin’s natural selection provided a much more plausible explanation for how the giraffe might have acquired its long neck: assuming that the nutrients it needed were at the highest levels of the local trees, the traits of tallness, long necks, and the ability to stretch would be selected naturally among the giraffe population. M U TAT I O N S

AND

S U R V I VA L .

Unlike the idea of acquired characteristics, mutation does not entail the inheritance of anatomical traits acquired in the course of an organism’s life; rather, it is changes in the DNA that are passed on. For example, when mind-altering drugs became popular among young people in the 1960s, concerns were raised that the offspring of drug takers might suffer birth defects as a result of alterations in their DNA. For the most part, this did not happen. Conditions such as Huntington disease and cystic fibrosis, however, are the result of mutations in DNA; so, too, is albinism, which eliminates skin pigment.

bacteria. In a fraction of those who survive, however, a mutation may develop that makes them resistant to the medication. Eventually, these mutant bacteria will reproduce, creating more mutants and in time yielding an entire population resistant to the antibiotic.

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This is the reason why antibiotics can lose their effectiveness over time: bacteria with mutant genes will render every antibiotic useless eventually. The same often can happen with insect sprays, as roaches and other pests develop into mutant strains that are capable of surviving exposure to these pesticides. Such species, with their short cycles of birth, reproduction, and death, are extremely well equipped for survival as a group, which explains why many an unpleasant “bug” (whether a bacterium or an insect) has long been with us. (See Mutation for more on this subject.)

REAL-LIFE A P P L I C AT I O N S “Proving” Evolution Later in this essay, we look at examples of evolution in action and other phenomena that support the ideas of evolutionary theory. But before examining these many “proofs” of evolution, a few words should be said about the very fact that evolution seems to require so much more proof than most other scientific theories. All scientific ideas must be capable of being proved or disproved, of course, but the demand for proof in the case of evolution goes far beyond the usual rigors of science. In fact, at this point, the people demanding proof are not scientists but certain sectors of the population as a whole—in particular, religious groups or individuals who fear evolution as a challenge to their beliefs. QUANTUM MECHANICS: A M U C H M O R E D I F F I C U LT I D E A . By

Although mutations often are regarded as undesirable because they can affect the health of individuals adversely, they also can have positive effects for the population in question. Suppose a group of bacteria is exposed to an antibiotic, which rapidly kills off the vast majority of the

contrast, quantum mechanics, though it encompasses ideas completely opposed to common sense, has not sustained anything approaching the same challenge or the demand for proof that evolution has encountered from nonscientists. A theory in physics and chemistry that details the characteristics of energy and matter at a subatomic level, quantum mechanics goes against such common assumptions as the idea that we

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can know both the location and the speed of an object. It is as though science had proved that down was up and up was down. If there were ever a “dangerous” theory, inasmuch as it undermines all our assumptions about the world, it is quantum mechanics not evolution, which is a fairly straightforward idea by comparison. Quantum mechanics has gone virtually unchallenged (at least on a social or moral, as opposed to a scientific, basis), whereas even today there are many people who refuse to accept the idea of evolution. Granted, quantum mechanics is a much younger idea, having originated only in the 1920s, and it is vastly more difficult to understand. But the real reason why evolution has come under so much more challenge, of course, has to do with the fact that it is perceived (mistakenly) as challenging the primacy of God. J U S T A T H E O R Y ? One of the aspects of evolution often cited by opponents is the fact that it is, after all, the theory of evolution. The implication is that if it is still just a theory, it must be open to question. In a sense, this is accurate: for scientific progress to continue, ideas should never be accepted as absolute, unassailable truths. But this is not what opponents of evolution are getting at when they cite its status as a “mere” theory. In fact, their use of this point as a basis for attack only serves to illustrate a misunderstanding with regard to the nature of scientific knowledge.

The word theory in “theory of evolution” simply means that evolutionary ideas have not been and, indeed, cannot be tested in every possible circumstance. Most ideas in science are simply theories rather than laws because in few cases is it possible to say with absolute certainty that something always will be the case. One of the few actual scientific laws is the conservation of energy, which holds that for all natural systems the total amount of energy remains the same, though transformations of energy from one form to another take place. This has been tested in such a wide variety of settings and circumstances that there is no reason to believe that would it ever not be the case.

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discuss later, unfold within a short enough period of time that humans can observe them. In general, however, evolutionary processes take place over such extraordinarily long spans of time that it would be impossible to subject them to direct observation. None of this, however, does anything to discredit evolutionary theory. For that matter, the idea that the entire physical world is made of atoms is still technically a theory, though there is no significant movement of people attempting to discredit it. The reason, of course, is that atomic theory does not seem to contradict anyone’s idea of God. (This was not always the case, however. Almost 2,500 years ago, a Greek philosopher named Democritus developed the first atomic theory, but because his ideas were associated with atheism, atomic theory was largely rejected for more than two millennia.) FAC I N G T H E FAC T S . If people really understood the word theory, they would give it a great deal more respect. Unfortunately, the word so often is misused and applied to anything that has not been proved that it has begun to seem almost like an insult to call evolution a theory. After all, in the present essay, we refer to acquired characteristics as a theory, and in everyday life one often hears much less respectable ideas given the status of theory. For this reason, it is worth taking note of the process, from observation to hypothesis to the formulation of general statements, that goes into the development of a truly scientific theory.

In forming his theory of evolution, Darwin began with several observations about the natural world. Among the things he observed is the fact, which we noted earlier, that for a particular species, more individuals are born than can possibly survive with available resources. On the basis of this observation, he formed a hypothesis, or inference. His inference was that because populations are greater than resources, the members of a population must compete for resources.

By contrast, there probably never will be enough tests on evolution to advance it to the status of a law. The reason is quite simply that evolution takes a long time. Some examples, such as the instances of industrial melanism that we

A theory is made up of many hypotheses, but to proceed from a collection of hypotheses to a true theory, these inferences must be subjected to rigorous testing. Thus, Darwin, in effect, said to himself, “Is what I have said true? Are there more individuals of a species than there are available resources?” Then he began looking for examples, and like a true scientist, he did so with the attitude that if he found examples that con-

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tradicted his hypothesis, he would reject the hypothesis and not the facts.

are somehow linked, in this case using scientific facts as a basis for rejecting religion.

As it turns out, of course, there are always more members of a population than there are resources. This can be illustrated in a small way by observing a litter of puppies or piglets struggling to obtain milk from their mother. Chances are that the mother will not have enough teats for all her babies, and the “runt,” unless it is able to force its way through the others to the milk source, may die. Only after testing this hypothesis and other hypotheses, such as that of natural selection, did Darwin formulate his theory.

One such scientist was Darwin himself, who embraced agnosticism because his own findings had proved that the biblical account of creation cannot be literally true. In this religious choice, he was following in a family tradition: his grandfather, the physiologist Erasmus Darwin (1731–1802), belonged to the mechanist school, a muddle of atheism, bad theory, and genuine science.

Evolution and Religion The fact that some puppies or piglets die for lack of milk is not a nice or pleasant thought, but it is the truth. Again, like a true scientist, Darwin accepted reality, without attempting to mold it to fit his personal beliefs about how things should be. As a great thinker from the generation that preceded Darwin’s, the Scottish philosopher David Hume (1711–1776), wrote in his Enquiry Concerning Human Understanding: “There is no method of reasoning more common, and yet more blamable, than, in philosophical disputes, to endeavor the refutation of a hypothesis, by a pretense of its dangerous consequences to religion and morality.” In other words, there is an understandable, but nonetheless inexcusable, human tendency to evaluate ideas not on the basis of whether they are true but rather on the basis of whether they fit with our ideas about the world. A scientist may be a Christian, or an adherent of some other religion, and still approach the topic of evolution scientifically—as long as he or she does not allow religious convictions to influence acceptance or nonacceptance of facts. The scientist should start with no preconceived notions and no allegiance to anything other than the truth. If that person’s religious conviction is strong enough, it can weather any new scientific idea.

The mechanists claimed that humans were mere machines whose activities could be understood purely in terms of physical and chemical processes. Claims such as these ultimately led to the discrediting of their movement, whose ideas failed to explain such biological processes as growth. At the same time, such mechanist philosophers as the French physician and philosopher Julien de La Mettrie (1709–1751) went far beyond the territory of science, teaching that atheism was the only road to happiness and that the purpose of human life was to experience pleasure. The thinker who perhaps did the most to confuse science and atheism was one of Darwin’s most significant early followers, the German natural scientist and philosopher Ernst Haeckel (1834–1919). It was Haeckel, not Darwin, who first proposed an evolutionary explanation for the origin of human beings, which, of course, was a major step beyond even Darwin’s claim that all of life had evolved over millions of years. In the course of developing this idea, Haeckel, who was a practicing Christian until he read Darwin’s On the Origin of Species by Means of Natural Selection, renounced his faith and adopted a belief system he called monism, which is based on the idea that there is only a physical realm and no spiritual one. Technically, Haeckel was not an atheist but a pantheist, since his philosophy included the idea of a single spirit that lives in all things, both living and nonliving. Whatever the case, Haeckel’s monism is no more scientific than Christianity.

point regarding the alleged conflict between religion and science. Not all the blame for this belongs with religious groups or individuals who shut their minds to scientific knowledge. Many scientists over the years likewise have adopted the fallacy of maintaining that religion and science

HUMANS AND “MONKEYS.” It is interesting that the man who put forward the notorious idea that humans and apes are related also would attempt to turn evolution into a sort of “proof” of atheism. In fact, the evolutionary connection between humans and lower primates, or “monkeys,” has long been the most powerful point of contention between religion and evolution.

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This, in fact, remains one of the most challenging aspects of evolutionary theory—not because it is hard to see how the human body is similar to an ape’s body but because there is such a vast difference between a human mind and that of an ape. Whereas our physical similarity to primates is easy to establish, the fact is that no other animal—ape, dolphin, pig, or dog—comes close to humans in terms of reasoning ability. Nor is it reasoning ability alone that separates humans from other animals. Humans possesses a propensity for conceptualization and a level of selfawareness that sets them completely apart from other creatures, so much so that the brains of apes, cats, birds, and even frogs seem more or less alike compared with that of a human. Animals are concerned with a few things: eating, sleeping, eliminating waste, and procreating. Some mammals have the ability to engage in play, but there is still no comparison between even the most advanced mammalian brains and that of a human. Other primates have the ability to use sticks or stones as tools, but only humans—practically from the beginning of the species 2.5 million years ago—have the ability to fashion tools. Only humans are gifted, or cursed, with restless minds ever in search of new knowledge. Does any of this disprove evolution? It does not. Does it pose a significant challenge to the idea that humans and other primates evolved from a common ancestor? Not as it has been stated here. All that has been said in the preceding paragraphs is simply a matter of everyday observation, but it is not a scientific hypothesis, let alone a theory. Clearly, there are some questions still to be answered as to why and how humans developed brains so radically different from those of other primates, but the place for such questioning is within the realm of science not outside it. C R E AT I O N I S M . Another thing we can

say about the human mind is that it has a tendency to mold ideas toward its own preconceptions as to how things should be. As Hume observed, there is a great temptation, in the minds of all people, to demand that scientific facts conform to a particular set of religious or political beliefs. Such is the case with creationism and “intelligent design theory,” two scientific belief systems whose adherents have attempted to challenge evolutionary theory.

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Creationism, which sometimes goes by the name of creation science, is based on the belief that God created the universe and did so in a very short period of time. This claim, creationists maintain, can be supported by scientific evidence. Scientific evidence, however, is not really what drives creationism, which is based on a literal reading of the first two chapters of the Book of Genesis. Taken to an extreme, this means that God created the universe about 6,000 years ago in six days of 24 hours each. Adherents of creationism begin with the premise of a six-day Creation (or at least, a very young Earth) and then look for facts to support the premise—exactly the opposite of the approach taken by true science. The findings of creationists do not change much over the years, unlike evolutionary science, which has continued to develop with new discoveries. Sometimes creationists attempt to use the findings of evolutionary science against it. For instance, they may interpret industrial melanism (the adaptation of moths to discoloration in the environment caused by pollution, discussed later in this essay) as proof that organisms can change very quickly. This, of course, does not take into account the fact that moths have very short life spans compared with humans, for whom evolutionary change takes much longer. Creationists also point to areas of evolutionary theory where all scientists are not in agreement, citing these as “proof ” that the whole theory is unsound. INTELLIGENT DESIGN THEORY A N D T H E C O U RT B AT T L E . In

contrast to creationism, intelligent design theory is not based on any particular religious position. Instead, it begins with an observation that would find a great deal of agreement among many people, including those who support evolutionary theory. The idea is that evolution alone does not explain fully how life on Earth came to exist as it does, with all its complexity and order. According to intelligent design theory, there must have been some intelligence behind the formation of the universe. There is another contrast between intelligent design theory and creationism. Whereas it is hard to imagine a genuine scientist embracing creationism, it is not difficult at all to picture a scientific thinker adopting the viewpoint of intelligent design. In fact, this has happened, though long before the “movement” had a name.

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Darwin’s contemporary, the English naturalist Alfred Russel Wallace (1823–1913), who published his own theory of evolution at about the same time as Darwin’s Origin of Species, parted ways with Darwin because he maintained that there must be a spiritual force guiding evolution. Only such a force, he maintained, could explain the human soul. From a philosophical and theological standpoint, this idea has a great deal of merit, but because it cannot be tested, it cannot truly be regarded as science. Neither creationism nor intelligent design has received any support in the scientific community—nor, during court battles over the teaching of creationism in the public schools during the 1980s, did that idea receive the support of the United States justice system. Creationism, the courts ruled, is a religious and not a scientific doctrine. Evolutionary theory is based on an ever increasing body of evidence that is both observable and reproducible. To teach these other doctrines alongside evolution in the public schools would convey the impression that creationism and intelligent design had been subjected to the same kinds of rigorous tests that have been applied to evolution, and this is clearly not the case.

Evidence for Evolution A great deal of evidence for evolution appeared in the seminal text of evolutionary theory (mentioned previously), On the Origin of Species by Means of Natural Selection, which Darwin published in 1859. In fact, he had collected much of the evidence he discusses in this volume nearly three decades earlier, from 1831 to 1836, aboard a scientific research vessel off the coast of South America. (He delayed publication because he rightly feared the controversy that would ensue and resolved to present his ideas only when he learned that Wallace had developed his own theory of evolution.) Just 22 years old, Darwin traveled on the HMS Beagle, from which he collected samples of marine life. His most significant work was done on the Galápagos Islands some 563 mi. (900 km) west of Ecuador. As he studied organisms there, Darwin found that they resembled species in other parts of the world, but they were also unique and incapable of interbreeding with similar species on the mainland. He began to suspect that for any particular environment, certain traits

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came to the forefront, favored for survival by nature.

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Back in England, he already had seen such a mechanism at work in the artificial breeding of pigeons, whereby breeders favored certain gene pools—for instance, white-tailed birds—over others. (Breeders of dogs and other animals today still employ artificial-selection techniques to produce desirable strains.) Darwin posited a similar process of selection in nature, only this one was not artificial, directed by a goal-oriented human intelligence, but natural and guided by the need for survival. THE

SPREAD

OF

SPECIES.

Among the phenomena Darwin observed in the Galápagos was the differentiation among the 13 varieties of finch (a type of bird) on the islands as well as the contrasts among these finches and their counterparts on the mainland. As Darwin began to discover, they shared many characteristics, but each variety had its own specific traits (for instance, the ability to crack tough seeds for food) that allowed it to fill a particular niche in its own environment. From the beginning Darwin was influenced by the recent findings in geology, a newly emerging science whose leading figures maintained that Earth was very, very old. (These scientists included the Scottish geologist Charles Lyell [1797–1875], whose Principles of Geology, published between 1830 and 1833, Darwin read aboard the Beagle) The relationship between geology and evolution has persisted, and findings in the earth sciences continue to support evolutionary theory. Among the leading ideas in geology and other geosciences since the mid–twentieth century is plate tectonics, which indicates (among other things) that the continents of Earth are constantly moving. (See Paleontology for further discussion of this topic.) This idea of continental drift provided a mechanism for species differentiation of the kind Darwin had observed. It appears that in the past, when the landmasses were joined, organisms spread over all available land. Later, this land moved apart, and the organisms became isolated. Eventually, different forms evolved, and in time these distinct organisms became incapable of interbreeding. This is what occurred, for instance, when the Colorado River cut open the Grand Canyon, separating groups of squirrels who lived in the high-

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VAMPYRUM

BAT.

WIDELY

DIVERGENT ORGANISMS SOMETIMES POSSESS A COMMON STRUCTURE, ADAPTED TO THEIR

INDIVIDUAL NEEDS OVER COUNTLESS GENERATIONS YET REFLECTIVE OF A SHARED ANCESTOR.

THE

CAT ’S PAW, THE

DOLPHIN’S FLIPPER, THE BAT ’S WING, AND THE HUMAN HAND ARE ALL VERSIONS OF THE SAME ORIGINAL FIVE-DIGIT APPENDAGE, CALLED THE PENDTADACTYL LIMB. (© Gary Braasch/Corbis. Reproduced by permission.)

altitude pine forest. Eventually, populations ceased to interbreed, and today the Kaibab squirrel of the northern rim and the Abert squirrel of the south are separate species. C O M M O N A N C E S T RY. Darwin recognized that some of the best evidence for evolution lies hidden within the bodies of living creatures. If organisms have a history, he reasoned, then vestiges of that history will linger in their bodies—as studies in comparative anatomy show. An example is a phenomenon that sounds as if it is made up, but it is very real: snake hips. Though their ancestors ceased to walk on four legs many millions of years ago, snakes still possess vestigial hind limbs as well as reduced hip and thigh bones.

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an indication of a common four-footed ancestor that likewise had limbs with five digits at the end. The embryonic forms of animals also reflect common traits and shared evolutionary forebears. This is why most mammals look remarkably similar in early stages of development. In some cases animals in fetal form will manifest vestigial features reflective of what were once functional traits of their ancestors. Thus, fetal whales, while still in their mothers’ wombs, produce teeth after the manner of all vertebrates (creatures with an internal framework of bones), only to reabsorb those teeth, which they will not need in a lifetime spent filtering plankton through their jaws.

In some cases widely divergent organisms possess a common structure, adapted to their individual needs over countless generations yet reflective of a shared ancestor. A fascinating example of this is the pentadactyl limb, a fivedigit appendage common to mammals and found, in modified form, among birds. The cat’s paw, the dolphin’s flipper, the bat’s wing, and the human hand are all versions of the same original,

The molecular “language” of DNA also provides evidence of shared evolutionary lineage. When one studies the DNA of humans and chimpanzees, very close similarities rapidly become apparent. Likewise, there are common structures in the hemoglobin, or red blood cells, of different types of organisms. Comparisons of hemoglobin make it possible to pinpoint the date of the last common ancestor of differing species. For example, hemoglobin analysis reveals an ancestor common to humans and frogs dating

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LOBSTER

FOSSIL FROM THE LATE

JURASSIC

PERIOD.

THE

PRESERVED REMAINS OF SUCH PREHISTORIC LIFE-FORMS

APPEAR IN THE ORDER OF THEIR EVOLUTION IN THE STRATA, OR LAYERS, OF

EARTH’S

SURFACE, WHICH GEOLOGISTS

ARE ABLE TO DATE: THE AGE OF A STRATUM ALWAYS CORRELATES WITH THE FOSSILS DISCOVERED THERE. (© Layne

Kennedy/Corbis. Reproduced by permission.)

back 330 million years, whereas the common human and mouse ancestor lived 80 million years ago, and the ancestor we share with the rhesus monkey walked the earth “only” 26 million years ago. T H E F O SS I L R E C O R D . The fossil record also provides an amazing amount of evidence concerning common ancestors. Fossilized remains of invertebrates (animals without an internal skeleton), vertebrates, and plants appear in the strata or layers of Earth’s surface in the same order that the complexities of their anatomy suggest. The more evolutionarily distant organisms lie deeper, in the older layers, beneath the remains of the more recent organisms. Geologists are able to date rock strata with reasonable accuracy, and the age of a layer always correlates with the fossils discovered there. In other words, there would never be a stratum dating back 400 million years that contained fossils of mastodons, which evolved much later.

ally, fossilization involves changes in the hard portions, including bones, teeth, and shells. This series of changes, in which minerals are replaced by different minerals, is known as mineralization. Fossilized remains of single-cell organisms have been found in rock samples as old as 3.5 billion years, and animal fossils have been located in rocks that date to the latter part of Precambrian time, as long ago as one billion years. Certain fossil types, known as index fossils or indicator species, have been associated strongly with particular intervals of geologic time. An example is the ammonoid, a mollusk that proliferated for about 350 million years, from the late Devonian to the early Cretaceous periods, before experiencing mass extinction.

A fossil is the remains of any prehistoric lifeform, especially those preserved in rock before the end of the last ice age, about 10,000 years ago. The process by which a once living thing becomes a fossil is known as fossilization. Gener-

The fossil record is far from an open book, however, and interpreting fossil evidence requires a great deal of judgment. All manner of natural phenomena such as earthquakes can destroy fossil beds, rendering the evidence unreadable or at least unreliable. Nor is it a foregone conclusion that the animals who left behind fossils are fully representative of the species existing at a given time. Fossils are far more likely to be preserved in certain kinds of protected aquat-

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ic environments, for instance, than on land (particularly at higher elevations, where erosion is a significant factor), and therefore paleontologists’ knowledge of life forms in the distant past is heavily weighted toward marine creatures. FA U N A L S U C C E S S I O N A N D O T H E R F O R M S O F DAT I N G . Key to

the demonstration of evolution is the age of samples and the idea that many of the processes described took place a long, long time ago. This raises the question of how scientists know the age of things. In fact, they have at their disposal several techniques, both relative and absolute, for dating objects. One of the earliest ideas of dating in geology was faunal dating, or the use of bones from animals (fauna) to determine age. This was the brainchild of the English engineer and geologist William Smith (1769–1839), whose work is an example of the fact that evolutionary ideas were “in the air” long before Darwin. While excavating land for a set of canals near London, Smith discovered that any given stratum contains the same types of fossils, and therefore strata in two different areas can be correlated. Smith stated this in what became known as the law of faunal succession: all samples of any given fossil species were deposited on Earth, regardless of location, at more or less the same time. As a result, if a geologist finds a stratum in one area that contains a particular fossil and another in a distant area having the same fossil, it is possible to conclude that the strata are the same. Faunal succession is relative, meaning that it does not provide clues as to the actual age in years of a particular sample. Since the mid–twentieth century, however, scientists have had at their disposal several means for absolute dating, which make it possible to determine the rough age of samples in years. Most of these mechanisms for dating are based on the fact that over time, a particular substance converts to another, mirror substance. By comparing the ratios between them, it is possible to arrive at an estimate as to the amount of time that has elapsed since the organism died.

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and radioactive isotopes are ones that spontaneously eject various high-energy particles over time. Because chemists know how long it takes for half the isotopes in a given sample to stabilize (a half-life), they can judge the age of such a sample by examining the ratio of stable to radioactive isotopes. In the case of uranium, one isotopic form, uranium 238, has a half-life of 4,470 million years, which is very close to the age of Earth itself.

Evolution at Work Every creature that exists today is the result of an incredibly complex, lengthy series of changes brought about by mutation and natural selection, changes that influenced the evolution of that life-form. Take for instance the horse, whose evolutionary background is as well-documented as that of any creature. The horse family, or Equidae, had its origins at the beginnings of the Eocene epoch about 54 million years ago. This first ancestor, known as Hyracotherium or eohippus (“dawn horse”) was extremely small—only about the size of a dog. In addition, it had four hooves on its front feet and three on each rear foot, with all of its feet being padded, which is quite a contrast with the four unpadded, single-hoofed feet of the modern horse. These and other features, such as head size and shape, constitute such a marked difference from what we know about horses today that many scientists have questioned the status of eohippus as an equine ancestor. However, comparison with fossils from later, also extinct, horses shows a clear line of descent marked by an increase in body size, a decrease in the number of hooves, an elimination of foot pads, lengthening of the legs and fusion of the bones within, development of new teeth suited for eating grass, an increase in the length of the muzzle, and a growth in both the size and development of the brain.

Chief among the techniques for absolute dating is radiometric dating, which uses ratios between two different kinds of atoms for a given element: stable and radioactive isotopes. Isotopes are atoms that differ in their number of neutrons, or neutrally charged subatomic particles,

Of course, this was not a clear-cut, neat, and steadily unfolding process, and some features appeared abruptly; still, the progression is there to be observed in the fossil record. Over the course of the many millions of years since eohippus, species have emerged that were distinguished by a particular feature—for example, teeth size and shape—only to disappear if conditions favored species with other traits. Evolutionary lines have branched off, with some dead-ending, and others continuing.

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Thus, during the Miocene epoch, which lasted from about 26 million to 7 million years ago, various evolutionary branches competed for a time until the emergence of Parahippus. This species had teeth adapted for eating grass, in contrast to those of earlier horse ancestors, which grazed on leaves and other types of vegetation that did not require strong teeth. After Parahippus came Merychippus, which resembled a modern pony, and from which came numerous lateMiocene evolutionary lines. Most of these were three-toed, but Pliohippus had one toe per foot, and it was from this form that the genus Equus (which today includes horses, donkeys, and zebras) began to emerge in the late Pliocene epoch about 3 million years ago. I N D U S T R I A L M E LA N I S M A N D T H E P E P P E R M O T H . Despite the stag-

gering spans of time involved in evolution, one need not look back billions of years to see evolution at work. Both natural selection and mutation play a role in industrial melanism, a phenomenon whereby the processes of evolution can be witnessed within the scale of a human lifetime. Industrial melanism is the high level of occurrence of dark, or melanic, individuals from a particular species (usually insects) within a geographic region noted for its high levels of dark-colored industrial pollution. With so much pollution in the air, trees tend to be darkened, and thus a dark moth stands a much greater chance of surviving, because predators will be less able to see it. At the same time, there is a mutation that produces dark-colored moths, and in this particular situation, these melanic varieties are selected naturally. On the other hand, in a relatively unpolluted region, the lighter-colored individuals of the same species tend to have the advantage, and therefore natural selection does not favor the mutation. The best-known example of industrial melanism occurred in a species known as the pepper moth, or Biston betularia, which usually lives on trees covered with lichen. (An example of a lichen is reindeer “moss”; see Symbiosis.) Prior to the beginnings of the Industrial Revolution in England during the late eighteenth century, the proportion of light-colored pepper moths was much higher than that of dark-colored ones, both of which were members of the same species differentiated only by appearance.

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As the Industrial Revolution got into full swing during the 1800s, factory smokestacks put so much soot into the air in some parts of England that it killed the lichen on the trees, and by the 1950s, most pepper moths were dark-colored. It was at that point that Bernard Kettlewell (1907–1979), a British geneticist and entomologist (a scientist who studies insects), formed the hypothesis that the pepper moths’ coloration protected them from predators, namely birds.

Evolution

Kettlewell therefore reasoned that, before pollution appeared in mass quantities, light-colored moths had been the ones best equipped to protect themselves because they were camouflaged against the lichen on the trees. After the beginnings of the Industrial Revolution, however, the presence of soot on the trees meant that light-colored moths would stand out, and therefore it was best for a moth to be dark in color. This in turn meant that natural selection had favored the dark moths. In making his hypothesis, Kettlewell predicted that he would find more dark moths than light moths in polluted areas, and more light than dark ones in places that were unpolluted by factory soot. As it turned out, dark moths outnumbered light moths two-to-one in industrialized areas, while the ratios were reversed in unpolluted regions, confirming his predictions. To further test his hypothesis, Kettlewell set up hidden cameras pointed at trees in both polluted and unpolluted areas. The resulting films showed birds preying on light moths in the polluted region, and dark moths in the unpolluted one— again, fitting Kettlewell’s predictions. ANGIOSPERMS AND GYMN O S P E R M S . A final interesting example of

natural selection at work lies in the comparative success rates of angiosperms and gymnosperms. An angiosperm is a type of plant that produces flowers during sexual reproduction, whereas a gymnosperm reproduces sexually through the use of seeds that are exposed, for instance in a cone. Angiosperms are a beautiful example of how a particular group of organisms can adapt to its environment and do so in a much more efficient way than that of its evolutionary forebears. On the other hand, gymnosperms, with their much less efficient form of reproduction, perhaps one day will go the way of the dinosaur. Flowering plants evolved only about 130 million years ago, by which time Earth long since

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KEY TERMS ACQUIRED

CHARACTERISTICS:

Sometimes known as acquired characters or Lamarckism after one of its leading proponents, the French natural philosopher Jean Baptiste de Lamarck (1744–1829), acquired characteristics is a fallacy that should not be confused with mutation. Acquired characteristics theory maintains that changes that occur in an organism’s overall anatomy (as opposed to changes in its DNA) can be passed on to offspring.

FOSSIL:

any prehistoric life-form, especially those preserved in rock before the end of the last ice age. FOSSILIZATION:

Deoxyribonucleic acid, a molecule in all cells and in many viruses that contains genetic codes for inheritance.

DNA:

had been dominated by another variety of seedproducing plant, the gymnosperm, of which pines and firs are an example. Yet in a relatively short period of time, geologically speaking, angiosperms have become the dominant plants in the world. In fact, about 80% of all living plant species are flowering plants. Why did this happen? It happened because angiosperms developed a means whereby they coexist more favorably than gymnosperms with the insect and animal life in their environments.

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The

process

by

which a once living organism becomes a fossil. Generally, fossilization involves mineralization of the organism’s hard portions, such as bones, teeth, and shells. GENE:

Any effort directed toward finding the age of a particular item or phenomenon. Methods of dating are either relative (that is, comparative and usually based on rock strata, or layers) or absolute. The latter, based on methods such as the study of radioactive isotopes, typically is rendered in terms of actual years or millions of years.

DATING:

The mineralized remains of

A unit of information about a

particular heritable (capable of being inherited) trait that is passed from parent to offspring, stored in DNA molecules called chromosomes. GEOLOGIC TIME:

The vast stretch of

time over which Earth’s geologic development has occurred. This span (about 4.6 billion years) dwarfs the history of human existence, which is only about 2.5 million years. Much smaller still is the span of human civilization, only about 5,500 years.

angiosperm, by contrast, the seeds are tucked safely away inside the ovary. Furthermore, the evolution of the flower not only has added a great deal of beauty to the world but also has provided a highly successful mechanism for sexual reproduction. This sexual reproduction makes it possible for new genetic variations to develop, as genetic material from two individuals of differing ancestry come together to produce new offspring. (For more about angiosperms and gymnosperms, see Ecosystems and Ecology.)

Gymnosperms produce their seeds on the surface of leaflike structures, and this makes the seeds vulnerable to physical damage and drying as the wind whips the branches back and forth. Furthermore, insects and other animals view gymnosperm seeds as a source of nutrition. In an

Darwin, Charles, and Richard E. Leakey. The Illustrated Origin of Species. New York: Hill and Wang, 1979.

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WHERE TO LEARN MORE Campbell, Neil A., Lawrence G. Mitchell, and Jane B. Reece. Biology: Concepts and Connections. 2nd ed. Menlo Park, CA: Benjamin/Cummings, 1997.

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KEY TERMS HYPOTHESIS:

An unproven state-

ment regarding an observed phenomenon. INDUSTRIAL MELANISM:

The high

level of occurrence of dark, or melanic, individuals from a particular species (usually insects) within a geographic region noted for its high levels of dark-colored

An animal without

an internal skeleton. LAW:

MUTATION:

Alteration in the physical

structure of an organism’s DNA, resulting in a genetic change that can be inherited. The process

NATURAL SELECTION:

whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment. PALEONTOLOGY:

industrial pollution. INVERTEBRATE:

CONTINUED

The study of life-

forms from the distant past, primarily as revealed through the fossilized remains of plants and animals.

A scientific principle that is

SCIENTIFIC METHOD:

A set of prin-

shown always to be the case and for which

ciples and procedures for systematic study

no exceptions are deemed possible.

that includes observation; the formation of

MINERALIZATION:

A

series

of

hypotheses, theories, and laws; and contin-

changes experienced by a once living

ual testing and reexamination.

organism during fossilization. In mineral-

THEORY:

ization, minerals in the organism are

from a hypothesis that has withstood suffi-

replaced or augmented by different miner-

cient testing.

als or the hard portions of the organism

VERTEBRATE:

dissolve completely.

internal skeleton.

Dennett, Daniel Clement. Darwin’s Dangerous Idea: Evolution and the Meanings of Life. New York: Simon and Schuster, 1996. Evolution and Natural Selection (Web site). .

A general statement derived

An animal with an

Evolution. Public Broadcasting System (Web site). . Evolution. University of California, Berkeley, Museum of Paleontology (Web site). .

Evolution. British Broadcasting Corporation (Web site). .

Levy, Charles K. Evolutionary Wars: A Three-Billion-Year Arms Race. The Battle of Species on Land, at Sea, and in the Air. New York: W. H. Freeman, 1999.

“Evolution FAQs.” Talk Origins (Web site). .

Starr, Cecie, and Ralph Taggart. Biology: The Unity and Diversity of Life. 7th ed. Belmont, CA: Wadsworth, 1995.

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PA L E O N T O L O G Y Paleontology

CONCEPT Paleontology is the study of life-forms from the distant past, as revealed primarily through the record of fossils left on and in the earth. It is a subdiscipline of both the biological and the earth sciences, one that brings to bear the techniques of geologic study and several areas of biology, including botany and zoology. But the term paleontology, as used in the present context, also provides a convenient means of encompassing, in a single word, the study of the distant biological past. Such study, of course, is related intimately to the investigation of evolutionary processes and phenomena. In the present context, however, our concern is not so much with the theory and principles of evolution, discussed elsewhere in this book, but rather with a relatively short overview of biological history. This area of biological investigation calls upon such concepts as mass extinction and fossilization as well as the study of plant and animal forms that scientists know only from scientific reconstruction of the past rather than from direct experience. Chief among these life-forms are the dinosaurs, which dominated Earth for a period of more than 100 million years, ending about 65 million years ago. This span of time, impressive as it seems on the human scale, is minuscule compared with the entire history of life on Earth.

HOW IT WORKS Paleontology Among the Sciences Paleontology is the investigation of life-forms from the distant past, primarily through the study of fossilized plants and animals. Most peo-

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ple are familiar with fossils, a term that probably calls to mind an image of a flat piece of rock with a shadowy imprint of a leaf or animal on it. In fact, a fossil is the preserved remains of a once living organism that has undergone a process known as mineralization, in which the organic materials in hard parts of the organism (for example, teeth or bones) are replaced by minerals, which are inorganic. The difference between the organic and the inorganic, which we discuss later, is more than a difference between the living and nonliving, or even the living and formerly living. For now, though, it will suffice to say that all living and formerly living things, as well as their parts and products, are organic. The stress on formerly living things is important, since paleontology by definition encompasses plants, animals, and microscopic life-forms that lived a long time ago. (As for just how long “a long time ago” really is, that subject, too, is discussed later in this essay.) Among the fields related, or subordinate, to paleontology are paleozoology, which focuses on the study of prehistoric animal life; paleobotany, the study of past plant life; and paleoecology, the study of the relationships between prehistoric plants and animals and their environments. Despite the emphasis on past life-forms, however, paleontologists also avail themselves of evidence gleaned from observation of animals living today. In this approach, paleontology shows its relationship to geology, which, along with biology, is one of its two “parents.” G E O L O G Y A N D PA L E O N T O L O GY. One of the key concepts in geology is the

principle of uniformitarianism, which arose from the early history of the geologic sciences. At

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a time when the nascent science was embroiled in a debate over Earth’s origins and the age of the planet, the Scottish geologist James Hutton (1726–1797) transcended this debate by focusing on the processes at work on Earth in the present day. (In terms of geologic time or even the evolution of species, the eighteenth century and twenty-first century might as well be a few seconds apart, so the expression present day applies to Hutton’s time as much as to ours.) Rather than simply speculate as to how Earth had come into being, Hutton maintained that scientists could understand the processes that had formed it by studying the geologic phenomena they could see around them. The reason this approach works is that natural laws do not change over time, nor do the processes that are at work within nature. On the other hand, particular processes may not be in operation at all times, nor can it be assumed that the rate at which the processes take place is the same. The two foregoing sentences encompass some of the observations made by the modern American paleontologist Stephen Jay Gould (1941–2002) with his formulation of the “four types of uniformity” in Ever Since Darwin: Reflections in Natural History (1977). It is certainly appropriate that paleontology and geology interface in this concept of uniform change, because both are concerned with piecing together the distant past from the materials available in the present. In fact, from the standpoint of the earth sciences, paleontology belongs to a branch of geology known as historical geology, or the study of Earth’s physical history. (This field of study is contrasted to the other principal branch of the discipline, physical geology, the study of the material components of Earth and of the forces that have shaped the planet.) Other subdisciplines of historical geology are stratigraphy, the study of rock layers, or strata, beneath Earth’s surface; geochronology, the study of Earth’s age and the dating of specific formations in terms of geologic time; and sedimentology, the study and interpretation of sediments, including sedimentary processes and formations.

Before Life

Paleontology

A MUSEUM REPRODUCTION OF NEANDERTHAL MAN, OR HOMO SAPIENS NEANDERTHALENSIS. The span of time since the first appearance of the genus Homo (to which humans, or Homo sapiens, belong) is minuscule: 2.5 million years compared with 4.6 billion years, or about 0.04% of the planet’s history. (© Bettmann/Corbis. Reproduced by permission.)

prising to discover the large proportion of that time during which life existed on Earth; it is equally amazing to learn what a small portion of the biological history of the planet involved anything that modern humans would recognize as “life.” For the moment, let us go even further back, to the very beginning—an almost inconceivably long time ago. Scientists believe that the universe began between 10 billion and 20 billion years ago, with an explosion nicknamed the “big bang,” a cataclysm so powerful that it sent galaxies careening outward on a course they maintain even today. Among those galaxies moving outward from the universal point of origin was our Milky Way, which, about six billion years ago, began to develop a rotating cloud of cosmic gas somewhere between its center and its rim. That was the beginning of our solar system.

Later in this essay, we consider just what is meant by “geologic time,” the grand sweep of several billion years during which Earth evolved from a cloud of gases to its present form. It may be sur-

of the cloud, where the greatest amount of gases gathered, was naturally the densest and most

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massive portion as well as the hottest. There, hydrogen—the lightest of all elements—experienced extraordinary amounts of compression owing to the density of the clouded gases around it, and underwent nuclear fusion, or the bonding of atomic nuclei. This hot center became the Sun about five billion years ago, but there remained a vast nebula of gas surrounding it. As the fringes of this nebula began to cool, the gases condensed, forming solids around which particles began to accumulate. These were the future planets, including ours. The process of planetary formation took place over a span of about 500 million years. The planets contained various chemical elements, formed by nuclear fusion on the Sun—a fascinating aspect of life on Earth, since it means that the particles that make up the human body came from nuclear reactions on the stars. T H E E A R LY AT M O S P H E R E . The

brand-new Earth possessed an atmosphere that consisted primarily of elemental hydrogen, nitrogen, carbon monoxide, and carbon dioxide. This “air” would have been unbreathable to all but a very small portion of the life-forms that exist on Earth today. Yet for the development of life, it was absolutely essential that no oxygen be present in Earth’s atmosphere at that very early time. The reason is that oxygen is an extremely reactive element, which is why it is involved in an array of chemical reactions (including combustion and rusting) known collectively as oxidation-reduction reactions. If oxygen had been present at that time, it likely would have reacted with other elements immediately, rather than permitting the formation of what eventually became organic materials. WAT E R. Water appeared as the result of meteorite bombardment from space, which took place in the first half-billion years of Earth’s existence. It might seem strange to learn that water, a compound essential to life on Earth, came from the void of space—where, as far as we know, there are no other life-forms. But this is not as strange as it sounds: water itself is inorganic, and even today frozen water exists on several planets in our solar system.

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nings of life made their appearance, in the form of self-replicating molecules of carbon-based matter.

Life Begins One famous cliché of science-fiction movies is the use of the phrase “carbon-based life-forms” to describe humans. In fact, all living things contain carbon, and if we ever do find life, intelligent or otherwise, on other planets, chances are extremely high that it, too, will be “carbonbased.” Carbon, in fact, is almost synonymous with life, and hence the word organic, in its scientific meaning, refers to all substances that contain carbon. The only exceptions are the elemental carbon in diamonds or graphite, the carbonate forms that make up many of Earth’s rocks, and such oxides as carbon dioxide and monoxide, all of which are considered inorganic. It may sound as though a huge portion of carbon’s possible forms already have been eliminated from the list of organic substances, but, in fact, carbon is capable of forming an almost limitless array of compounds with other elements, particularly hydrogen. A class of molecule known as hydrocarbons, which are nothing but strings of carbon and hydrogen molecules bonded together, is the basis for literally millions of organic compounds, from petroleum to polymer plastics. It may sound odd to hear plastics referred to as organic,but this only highlights the difference between the popular and scientific meanings of that term. O R G A N I C M AT E R I A L S . At one time, organic referred only to living things, things that were once living, and materials produced by living things (for example, sap, blood, and urine). As recently as the early nineteenth century, scientists believed that organic substances contained a supernatural “life force,” but in 1828 the German chemist Friedrich Wöhler (1800–1882) made an amazing discovery.

In any case, water eventually accumulated on Earth’s surface in quantities sufficient for its condensation, with the result that clouds formed and rain fell on Earth more or less continually for many millions of years. It was then that the begin-

By heating a sample of ammonium cyanate, a material from a nonliving source, Wöhler converted it to urea, a waste product in the urine of mammals. As he later observed, “without benefit of a kidney, a bladder, or a dog,” he had turned an inorganic substance into an organic one. It was almost as though he had created life. Actually, what he had discovered was the distinction between organic and inorganic material, which results from the way in which the carbon chains are arranged.

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This explanation of the difference between organic and inorganic is pivotal to understanding how the beginnings of life first formed on Earth. It is an almost inconceivably large step from the nonliving to the living but not nearly so much of a jump from the inorganic to the organic. In fact, it appears that what happened on Earth in its distant past was that organic (but not living) substances underwent chemical reactions with inorganic ones to produce the rudiments of life. The American chemist Stanley Miller (1930–) illustrated this with an experiment that involved a mixture of hydrogen, methane (CH4), ammonia (NH3), and water. Subjected to a discharge of electric sparks intended to simulate lightning in Earth’s early atmosphere, the mixture eventually yielded amino acids, which are among the chief components of proteins. E A R LY F O R M S O F L I F E . The course that early forms of life followed during the first 800 million years of Earth’s existence was a lengthy one, and if a person could have glimpsed Earth at any interval of a few million years during this time, it might have seemed as though nothing at all was happening. In fact, however, life-forms were undergoing the most profound changes imaginable.

That span of 0.8 billion years saw a transition from elemental carbon to organic compounds and from organic compounds to organelles, which are discrete components of cells, and finally to cells themselves—the building blocks of life. The processes by which this happened were exceedingly complicated, and modern scientists have little to go on in forming their suppositions as to how these transitions came about. Among many key pieces of information missing from the picture, for instance, is the matter of how and when DNA first appeared in cells. (For more on DNA, see Cells.) The first cells to form were known as prokaryotic cells, or cells without a nucleus. (These cells, too, are discussed in the essay Cells.) Prokaryotic cells may have been little more than sacs of DNA that were capable of self-replication—much like bacteria today, which are themselves prokaryotic. These early forms of bacteria, which dominated Earth for many millions of years, were apparently anaerobic and eventually split into three branches: archaebacteria, eubacteria, and eukaryotes. Out of the last group grew all other forms of life, including fungi, plants, and animals.

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Geologic Time Marches On By about 2.5 billion years ago, bacteria had begun to undergo a form of photosynthesis, as plants do today. As a result, oxygen started to accumulate in Earth’s atmosphere, and this had two interesting implications for life on Earth. One of the results of oxygen formation was that formation of “new” cells—that is, spontaneously formed cells that did not come from already living matter—ceased altogether, because they were killed off by reactions with oxygen. Second, aerobic respiration thereafter became the dominant means for releasing energy among living organisms.

Paleontology

The real history of life-forms on Earth—the portion of that history about which paleontologists can learn a great deal from observation of fossils and other materials—dates from the beginning of the Cambrian period, about 550 million years ago. At this point, about 88% of Earth’s history already had passed, yet the remaining 12% contains virtually all the really dramatic events in the formation of life on Earth. Later in this essay, we examine a few analogies that help put these time periods into perspective as well as the concept of geologic time itself. P H AS E S I N E A RT H ’ S H I S T O RY. In the course of discussing these topics, it is

sometimes necessary to make reference to geologic time divisions—eons, eras, periods, and epochs. These are not units of a specific length in years, like a century or a millennium; instead, they are distinct phases in Earth’s history that historical geologists (including paleontologists) have pieced together from fossils and other materials. Their names usually refer to locations where fossils relevant to that phase of geologic time were found: for example, the Jurassic period, whose name became a household word after the release of the 1993 blockbuster movie Jurassic Park, is named after the Jura Mountains of Switzerland and France. The historical juncture mentioned in the paragraph before last—the beginning of the Cambrian period, some 550 million years ago, was also the end of the Proterozoic eon, the third of three eons in what is known as Precambrian time. The fourth and present eon is the Proterozoic, which has included three eras: Paleozoic (about 550–240 million years ago), Mesozoic (about 240–65 million years ago), and Cenozoic (about 65 million years ago to the present).

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CATAC LY S M S A N D C O N T I N E N TA L D R I F T. The divisions between these

phases have not been drawn arbitrarily; rather, they are based on evidence suggesting that those points in Earth’s history were marked by violent, cataclysmic events. Once life had come into the picture, these cataclysms brought about death on a vast scale, which we discuss later, in the context of mass extinctions. A number of phenomena caused mass extinctions at various points; most notable among them was the impact of meteorites. Also significant was continental drift, which, as its name implies, is the movement of the continents from distant origins. Just as evolutionary theory informs much of our modern thinking about biology, the theory of plate tectonics holds a dominant position in geology and related earth sciences. Plate tectonics involves the movement of large segments in the crust and the upper mantle of Earth, and one of the outcomes of such movement is continental drift. Even today, the continents we know are moving slowly away from or toward one another, but that movement is so slow that it would take millions of years for any change to be perceptible. At one time, however, the continents were distributed quite differently than they are today. For example, at the end of the Permian period and the beginning of the Triassic, when the dinosaurs began to appear on the scene, all of Earth’s landmasses were joined in a single continent, Pangea, that stretched between the North and South Poles and was surrounded by a vast ocean.

REAL-LIFE A P P L I C AT I O N S Our Place in the Grand Scheme of Things All areas of historical geology—including paleontology—are concerned with geologic time, a term that refers to the great sweep of Earth’s history. This is a timescale that dwarfs the span of human existence, and to study the history of life on Earth, a mental adjustment of monumental proportions is required. One must discard all notions used in studying the history of human civilization, including such concepts as modern, medieval, and ancient, which are essentially useless when discussing geologic time.

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Human civilization has existed for about 5,500 years, the blink of an eye in geologic terms. Even the span of time since the first appearance of the genus Homo (to which humans, or Homo sapiens, belong), is minuscule: 2.5 million years compared with 4.6 billion years, or about 0.04% of the planet’s history. A couple of analogies, one to a shorter span of time and another to a measurement of space, will help us understand the scope of time encompassed in the evolution of species. LIFE APPEARS IN JUNE—SO T O S P E A K . Suppose that the entire history

of Earth were likened to a single year of 365.25 days, starting with the formation of the planet from a cloud of dust and ending with the present. At what point in the year would the first aerobic life-forms appear? We are not talking about anything approaching a human or even a clam or a fern or a piece of algae—just a single-cell organism that depends on oxygen for energy. One might guess that such life-forms appeared sometime in January, or at least by the end of winter in late March. In fact, the first aerobic single-cell life-forms arose around June 15— nearly halfway through a year. Even at about the time of Thanksgiving, the most complex organisms would be fish and a few early amphibians. We tend to associate the dinosaurs with the early phases of Earth’s history, but this only illustrates our distorted view of geologic time. In fact, the Jurassic period would be analogous to a period of about five days from December 15–20. By Christmas Day, all remaining dinosaurs would have been headed toward extinction, their dead bodies eventually forming the fossil fuels that power present-day civilization. By this point we are within a few days of the year’s end, and yet nothing remotely resembling a human has appeared. Our own genus would not come on the scene until around 8:00 P.M. on December 31. The New Year’s Eve countdown would be nearing by the time human civilization began, at about 42 seconds before midnight. Christ’s birth would have occurred at about 14 seconds before midnight, and the final 10-second countdown would begin about the time the Roman Empire fell. The life span of the average person would correspond to approximately half a second or less. ANOTHER ANGELES TO

A N A LO GY: N E W YO R K .

LOS

To use another analogy, suppose we are driving from

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Los Angeles to New York City, and that this distance corresponds to Earth’s history. Once we reach western Nebraska, with about 46% of the distance behind us, we would come to the beginning of the Proterozoic era and the origins of aerobic life-forms. One might wonder why “nothing” happened on all those long miles from L.A. through the deserts, mountains, and plains of the western United States, but as we have seen, a great deal happened: Earth formed from a cloud of gas, was pounded by meteors, and gradually became the home to oceans. The end of the Proterozoic era (about 545 million years ago) would be at about 88% of the distance from Los Angeles to New York—somewhere around Pittsburgh, Pennsylvania. By this point, the continental plates have formed, oxygen has entered the atmosphere, and soft-bodied organisms have appeared. We are a long way from Los Angeles, and yet almost the entire history of life on Earth, at least in terms of relatively complex organisms, lies ahead of us. If we skip ahead by about 339 million years—a huge leap not only in time but also in biological development—we come to the point when the dinosaurs appeared. We are now 95% of the way from the beginning of Earth’s history to the present, and if it is measured against the distance from Los Angeles to New York, we would be somewhere around Scranton, in eastern Pennsylvania. Another 89 mi. (142 km) from the Scranton area would put us at a point about 65 million years ago, or the time when the dinosaurs became extinct. We then would have less than 40 mi. (64 km) to drive to reach the period where genus Homo appeared, by which time we would be in the middle of Manhattan. Compared with the distance from L.A. to New York, the span of time that Homo sapiens has existed would be much shorter than the drive from Central Park to the Empire State Building. The entire sweep of human civilization and history, from about a thousand years before the building of the pyramids to the beginning of the third millennium A.D., would be smaller than a city block. It is much smaller, in fact—close to the size of a modest storefront, or 15.54 ft. (5 m).

ably long, when one considers the diversity of forms that have evolved in the past half-billion years. What follows is an extremely brief overview, touching on a few points in the development of life on Earth. The discussion here is far from comprehensive, the purpose being not to provide a detailed overview of evolutionary processes but to illustrate the evolution of life-forms by a few examples. The reader is strongly encouraged to consult a textbook or other reliable information source for greater illumination of these particular topics. Along with this overview, we look at some of the more dramatic aspects of paleontologic study: dinosaurs and mass extinction. The fossilized history of life on Earth really began in earnest only with the Cambrian period, which saw an explosion of invertebrate (without an internal skeleton) marine forms. These dominated from about 550 to about 435 million years ago. At the latter point, the boundary between the Ordovician and Silurian periods of the Paleozoic era, there occurred the first of five major mass extinctions, as the “supercontinent” of Gondwana crossed the South Pole, freezing most life-forms that were then alive. As always happened with these mass extinctions, some lifeforms survived, and within a few million years life again was thriving. C O M I N G O U T O F T H E WAT E R.

One of the favorite subjects of modern cartoonists is the migration of creatures from the water to the land. For example, following the enactment of stiff airport security guidelines in the wake of the September 11 terrorist attacks, a New Yorker cartoon in late 2001 showed this evolution from fish to amphibian (a creature that can live both in the water and on the ground) and, ultimately, to the human being. Then, in the last evolutionary sequence, Homo sapiens is depicted in a business suit, going through an airport security scanner. Most cartoons based on this event in paleontologic history follow a common theme, which can be summed up thus: “We went through all that evolution just for this?”

On the one hand, the history of relatively complex life-forms is relatively short compared with Earth’s history; on the other hand, it is unbeliev-

Certainly, the transition from water to solid ground was a massive step—one of almost inconceivable importance. This invasion of the land, which probably took place about 400 million years ago, perhaps started when fishes of the Devonian period (named after Devonshire in England) began breathing oxygen near the sur-

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face of shallow waters. In time they made their way onto the land, and as the number of species grew, their lobed fins developed in different ways for different groups of fish, at length becoming the many limbs and appendages specific to large groups of species, such as birds or mammals. This may seem a bit far-fetched, but even today catfish are known to pull themselves out of the water and onto land if a specific need to do so arises. Why did the first fish leave the water? Perhaps because the water levels were receding, as they have done periodically in the course of Earth’s history as a result of massive freezing of global water supplies—that is, ice ages. Whatever the reason, we can be sure of the fact that the fish were not in any way attempting to reach some “higher” state of development, nor was any force pushing them to “evolve.” Therein lies one of the major misconceptions about evolution, even among many of its adherents. The very name evolution is somewhat unfortunate, because it suggests that life-forms are moving gradually toward a state of heightened development. Instead, they simply are adapting to circumstances that confront them; hence, the entire phenomenon might well be called by the much less dramatic-sounding name adaptation.

Mass Extinction Let us now pause in our narrative to discuss examples of a phenomenon that has been mentioned several times already: mass extinction. One of the truly amazing things about the history of life on Earth is the way in which forces have seemingly conspired to wipe out virtually all living things, not just once but at least five major times, along with a number of more limited instances. Over the course of Earth’s history—even its very recent history—numerous species have become extinct, usually as a result of their inability to adapt to changes in their natural environment. In the extremely recent past (recent geologically, that is—since the end of the last ice age about 10,000 years ago), some extinctions or endangerments of species have been attributed to human activities, including hunting and the disruption of natural habitats. For the most part, however, extinction is simply a part of Earth’s history, a result of the fact that organisms incapable of adaptation soon die out. This is the infamous “survival of the fittest,” an aspect of evolutionary theory that many a social or political ideologue

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has misappropriated for the purpose of insisting that one particular ethnic or social group is naturally superior to another (see Evolution). Why does mass extinction occur? As we noted earlier, one possible cause of mass extinction is a sudden and dramatic change in ocean levels. Other causes include volcanic eruptions or the effects of events or objects from space—the explosion of a star, perhaps, or the impact of a meteorite on Earth. Although scientists have a reasonable idea of the immediate causes of mass extinction in some cases, their understanding of the root causes still is limited. This fact was expressed by the University of Chicago paleobiologist David M. Raup, who wrote in Extinction: Bad Genes or Bad Luck?: “The disturbing reality is that for none of the thousands of well-documented extinctions in the geologic past do we have a solid explanation of why the extinction occurred.” THE G R E AT FIVE MASS E X T I N C T I O N S . The five largest known

mass extinctions occurred at intervals of between 50 million and 100 million years over a span of time from about 435 million to 65 million years ago. The first of them we have mentioned as perhaps being associated with the first migrations of organisms to land. This was at the end of the Ordovician period, about 435 million years ago, when a drop in the ocean level wiped out onefourth of all marine families. No wonder some of the fish adapted to life on land! Changes in sea level along with climate changes also appear to have brought about the second great mass extinction, which we also have mentioned, near the end of the Devonian period, about 370 million years ago. But no biological cataclysm in Earth’s history can compare to the mass extinction that attended the close of the Permian period some 240 million years ago—an event so traumatic to life on Earth that it has been dubbed the great dying. Imagine if 96% of all people alive today were killed—a staggering and terrifying concept. This would be far more fatalities than the combined death toll of both world wars; the Black Death of 1347–1351 and the influenza epidemic of 1918–1920; the various Nazi, Stalinist, and Maoist acts of genocide; and all major earthquakes and other natural disasters in history. Now imagine that along with all those people, 96% of all other life-forms—plants and trees, birds and fishes, single-cell organisms and the

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DINOSAURS DOMINATED EARTH FOR A PERIOD OF MORE THAN 100 MILLION YEARS, ENDING ABOUT 65 MILLION YEARS AGO. THE FLYING TRICERATOPS, a creature of the late Cretaceous period, may have been a link between dinosaurs and birds. (© Kevin Schafer/Corbis. Reproduced by permission.)

like, which vastly outnumber the relatively small human population of Earth—all ceased to exist as well. Such was the almost incomprehensible scale of the so-called great dying, an event whose cause has not been determined, though it may have resulted from a volcanic eruption in Siberia.

The Age of the Dinosaurs Now that we have mentioned only three of the five great mass extinctions, it is appropriate to discuss the creatures that died in the fourth and fifth: the dinosaurs, or “terrible lizards.” Though they were the most well-known creatures of the Mesozoic era, which lasted from about 240–65 million years ago, dinosaurs were far from the only notable species on Earth in that phase of geologic history. In fact, it was during the first period of the Mesozoic era, the Triassic, that a group of creatures destined to make a greater impact on Earth—mammals—first appeared. During this time, botanical life included grasses, flowering plants, and trees of both the deciduous (leaf-shedding) and coniferous (cone-bearing) varieties. Backtracking just a bit, at the beginning of the Devonian period, about 410 million years ago, the first vertebrates (animals with an internal skeleton) made their appearance in the form of

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jawless fishes. Plant life on land consisted of ferns and mosses. Then came the late-Devonian mass extinction we have mentioned, which removed many of the newly evolved invertebrates, including most fishes. By the end of the Devonian, about 360 million years ago, fish had evolved jaws, and life was thriving on land. Among these life-forms were reptiles in the animal kingdom, and gymnosperms, or plants that reproduce sexually by spreading exposed seeds—a primary example from our own time being pine trees. MONSTERS OF THE MESOZ O I C . Although the existence of dinosaurs has

been an established fact for as long as anyone alive can remember, we have had knowledge of them only since the mid–nineteenth century. It was then that early paleontologists began to piece together evidence from fossils, which revealed the record of a time when enormous reptiles had walked the earth. Not all of these dinosaurs were so large, however. Some were as small as chickens, while others—the sauropods, the largest terrestrial animals of all time—were the equivalent of half a dozen stories tall. Some dinosaurs were awesomely fierce predators, while others were mild-mannered plant eaters. Dinosaurs fell into two groups based on the

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shape of their hips, which were either lizardlike or birdlike. Though the lizardlike Saurischia emerged first, they lived alongside the birdlike Ornithischia throughout the late Triassic, Jurassic, and Cretaceous periods. Ornithischia were all herbivores, or plant eaters, whereas Saurischia included both herbivores and carnivores, or meat eaters. Naturally, the most fierce of the dinosaurs were carnivores, a group that included the largest carnivore ever to walk the Earth, Tyrannosaurus rex. Most dinosaurs had long tails and necks and walked on four legs, though some were bipedal, meaning either that they had only two legs or that they used only their two rear legs for movement or locomotion. Again, the shape of the creature correlated to some degree with its habits: all four-legged dinosaurs were relatively gentle, slow-moving herbivores, whereas most of the bipedal ones were fast-moving predators. Their teeth also reflected their diet: not surprisingly, carnivores had much sharper cutting surfaces shaped like serrated knives, which made it easy to rip their prey into smaller pieces before swallowing the pieces whole. As recently as the 1970s, scientists tended to believe that dinosaurs were cold-blooded creatures markedly lacking in brain power. This view is reflected in a number of popular-culture sources, such as the 1983 song by the Police, “Walking in Your Footsteps,” which suggests that if humans were to destroy themselves by nuclear warfare, they would seem even more “dumb” than the dinosaurs. By contrast, the film Jurassic Park, made a decade later, reflects changes in paleontologists’ attitudes toward dinosaurs, gained as a result of continued research. As it turns out, dinosaurs were far from dumb, and they may even have been warm-blooded—unlike most reptiles but like birds, to whom they may have been related.

Of course, T. rex is notable precisely because it was so unusual among dinosaurs in size and, for the most part, power. Deinonychus, for example, was much smaller—only about 10 ft. (3 m) and 220 lb. (100 kg)—but these “running lizards” were still fearsome predators, noted for their sickle-like claws. The phrase “sickle-like claws” might call to mind another creature, this one made famous by Jurassic Park: Velociraptor, or “swift plunderer,” a walking death machine that for all the terror it inspired in moviegoers (and, no doubt, in many a Mesozoic creature) attained a length of only about 6 ft. (2 m). Among the many lessons about dinosaurs to be learned from Jurassic Park was the striking contrast between herbivores and carnivores, a fact emphasized by the scene in which the two children take refuge with a herd of sauropods. These gentle giants possessed four legs the size of large pillars and had extraordinarily long necks and tails. Most famous among the sauropods was the Apatosaurus, which achieved a length of 65 ft. (20 m) and a weight of 30 tons (27 metric tons). Apatosaurus’s appearance in the scene with the children was not the great creature’s first time in the limelight of popular culture: known as Brontosaurus until the early 1990s, this dinosaur was a popular fixture of the Flintstones cartoon.

T. rex must have made a truly awesome sight running through the forests of its Mesozoic

In addition to land-based dinosaurs, there were such creatures as the flying Triceratops, which may have been a link between dinosaurs and birds. (The name raptor, incidentally, usually refers not to a type of dinosaur, but to a bird of prey.) It should be noted that Triceratops and Velociraptor were creatures of the late Cretaceous period, during which about a hundred different dinosaur genera (plural of genus) flourished. This is an important point, because many books picture all manner of dinosaurs coexisting, when, in fact, various ones existed at different times over a period of about 180 million years. Also, it is worth noting that the major nonhuman “star” of Jurassic Park did not even live during the Jurassic period!

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S O M E T Y P E S O F D I N O SAU R.

Few creatures have ever captured the imagination of humans as much as Tyrannosaurus rex— a particularly impressive feat, given the fact that the last T. rex died tens of millions of years before humans ever came on the scene. With a name that means “absolute ruler lizard” in Greek, this terrifying creature reached a maximum length of 45 ft. (14 m) and may have weighed as much as 9 tons (8 metric tons).

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world. Moving on its powerful hind legs with a gait that was at once swift and lumbering, it used its tail as a counterbalance and stabilizer. Once it had come abreast of its prey, T. rex probably attacked its victim with powerful head butts and then tore the animal apart with its massive jaws before ripping into it with some 60 daggershaped teeth as much as 6 in. (15 cm) long.

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Later Mass Extinctions—and New Life Today, of course, the dinosaurs are long gone— so distant in time, in fact, that the remains of many millions have become petroleum. This has happened in situations where specific conditions prevail: for example, the remains could not be allowed to rot, and decay had to be anaerobic and had to take place within certain types of rock. Yet oil reserves (for as long as they last) are not the only reason why the dinosaurs’ disappearance was beneficial from the human standpoint. At the time when dinosaurs controlled the world, mammals were small and hid from predators such as T. rex. Without the mass extinction of the dinosaurs, this class of creatures might never have come to the forefront, and human beings might not have evolved. Actually, two mass extinctions rocked the world of the dinosaurs. The first (fourth among the list of five major mass extinctions) happened at the end of the Triassic, the first period of the Mesozoic era. This was about 205 million years ago, when an asteroid may have hit Earth. Whatever the cause, the result was that creatures in the seas suffered major mass extinction. So, too, did those on the land, but many species of dinosaurs and mammals managed to survive the event, which marked the transition between the Triassic and Jurassic periods. THEORIES REGARDING THE D I N O SAU R S ’ D E M I S E . The dinosaurs

continued to flourish for another 140 million years, and their descendants might still be walking the earth were it not for the last great mass extinction, which took place about 65 million years ago. What killed the dinosaurs? Paleontologists and other scientists have proposed several theories: a rapid climate change; the emergence of new poisonous botanical species, eaten by herbivorous dinosaurs, that resulted in the passing of toxins along the food web; an inability to compete successfully with the rapidly evolving mammals; and even an epidemic disease to which the dinosaurs possessed no immunity.

Around the world, geologists have found traces of iridium deposited at a layer equivalent to the boundary between the Cretaceous and Tertiary periods, the Tertiary being the beginning of the present Cenozoic era. This is significant, because iridium seldom appears on Earth’s surface—but it is found in asteroids.

Paleontology

THE ASTEROID HITS. It appears that the asteroid smashed into what is now the northern tip of Mexico’s Yucatán peninsula. Today that area is home to a crater some 6 mi. (9.6 km) deep and 186 mi. (300 km) in diameter. Note, however, that 65 million years ago, the Yucatán was not exactly where it is now. It had begun to drift in the direction of its present location, but the map of the future continent of North America was quite different from what it is today and included large submerged areas. Among them was the region of impact, which only later rose to the surface as a result of tectonic activity.

The nearest large landmass at the time was a good distance away—equivalent to northern Louisiana. But it hardly mattered that there were no dinosaurs at the point of impact. Traveling at more than 100,000 MPH (160,000 km/h), which is almost four times as fast as the fastest spacecraft built by human beings, the asteroid probably brought about an explosion as intense as many thousands of hydrogen bombs. It may not have raised a mushroom cloud per se, but it probably produced an even more dramatic formation as it sent more than 48,800 cu. mi. (200,000 km3) of debris and gases into the atmosphere. This would be enough dust to cover the state of Mississippi to a depth of 1 mi. (1.6 km)

Interesting as many of these theories are, none has gained anything like the widespread acceptance achieved by another scenario. According to this highly credible theory, an asteroid hit Earth, hurtling vast quantities of debris into the atmosphere, blocking out the sunlight, and greatly lowering Earth’s surface temperature.

The sound of the impact must have been ear-shattering, even far away on what would one day become North America. Then came the tidal waves, some as tall as 394 ft. (120 m), with earthquakes soon to follow. But while these tidal waves undoubtedly caused massive localized death, or mass mortality, what really brought about the mass extinction of the dinosaurs was the aftermath of impact. All that dust in the atmosphere effectively blocked out the Sun’s light, bringing about a dramatic cooling on Earth’s surface. As a result, plants died, thus depriving herbivorous dinosaurs of an energy source. The herbivores died and then, in a domino effect brought about by the interdependence of components in the food web, the carnivores soon followed.

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ASTEROID

APPROACHING

EARTH. ACCORDING

TO ONE THEORY, THE MASS EXTINCTION OF THE DINOSAURS RESULT-

ED FROM THE IMPACT OF AN ASTEROID HITTING

EARTH,

HURTLING VAST QUANTITIES OF DEBRIS INTO THE ATMOS-

PHERE, BLOCKING OUT THE SUNLIGHT, AND GREATLY LOWERING

EARTH’S

SURFACE TEMPERATURE. (© D. Hardy. Photo

Researchers. Reproduced by permission.)

MAMMALS, HUMANS, AND M ASS E X T I N C T I O N . When that asteroid

hit, it brought an end to the dinosaurs and their Mesozoic world, ushering in the Cenozoic era, which saw the rise of mammals. Some examples of the creatures that proliferated in the course of the past 65 million years are tiny prehistoric horses with four toes as well as a giant rhinoceros-like herbivore, both of which lived in the Eocene epoch about 40 million years ago.

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a few seconds ago, in geologic terms), we emerged from the cold, ready to take over the world. Near the end of the last ice age, peoples from Siberia crossed a land bridge spanning what is today the Bering Strait between Russia and Alaska. Their descendants, of course, are the Native Americans, and native is a fitting term, since they arrived in the Americas about 12,000 years ago, or about 7,000 years before the Europeans’ ancestors arrived in Europe. What they found, as they poured into the Americas, was a range of species quite different from those known today. There were mammoth and mastodons; giant bears, beaver, and bison; and even saber-toothed “tigers” (which were not directly related to modern-day tigers), camels, and lions. Prehistoric America was also home to horses, but these creatures and many others were wiped out by hunting. Horses did not reappear in the New World until Europeans brought them after A.D. 1500, when they would prove an indispensable aid in European efforts to conquer the Native Americans’ lands.

As time went on, the range of species—not only animals and plants but also the much less complex organisms of the other three kingdoms (Protista, including protozoa and algae; Monera such as bacteria; and Fungi)—grew larger and larger. An ice age struck the planet about 1.65 million years ago, bringing about much smaller cases of mass extinction than the ones we have discussed. But other instances of mass extinction would occur at the hands of a creature that appeared about 2.5 million years ago: Homo sapiens. The ice age—which was only one of many and thus is referred to often as the “last ice age”— would prove a major turning point in the history of the human species. At the beginning, we were just embarking on the beginnings of the Stone Age, whereas at the end, just 10,000 years ago (or

Cadbury, Deborah. Terrible Lizard: The First Dinosaur Hunters and the Birth of a New Science. New York: Holt, 2001.

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KEY TERMS Oxygen-breathing.

AEROBIC:

be thought of as a bundle or network of

A creature capable of liv-

food chains, but since the latter rarely exist

ing either in the water or on solid ground.

separately, scientists prefer the concept of a

AMPHIBIAN:

ANAEROBIC:

Non-oxygen-breathing.

CARNIVORE:

A meat-eating organ-

ism.

food web to that of a food chain. The mineralized remains of

FOSSIL:

any prehistoric life-form, especially those preserved in rock before the end of the last

The longest phase of geologic

EON:

ice age.

time. Earth’s history has consisted of four eons, the Hadean, Archaean, Proterozoic, and Phanerozoic. The next-smallest subdivision of geologic time is the era. EPOCH:

The fourth-longest phase of

The

FOSSILIZATION:

about 10,000 years ago.

by

which a once living organism becomes a fossil. Generally, fossilization involves mineralization of the organism’s hard portions, such as bones, teeth, and shells.

geologic time, shorter than an era. The current epoch is the Holocene, which began

process

The vast stretch of

GEOLOGIC TIME:

time over which Earth’s geologic development has occurred. This span (about 4.6

The second-longest phase of geo-

billion years) dwarfs the history of human

logic time, after an eon. The current eon,

existence, which is only about 2.5 million

ERA:

the Phanerozoic, has had three eras, the Paleozoic, Mesozoic, and Cenozoic, which

years. Much smaller still is the span of human civilization, only about 5,500 years.

is the current era. The next-smallest subdivision of geologic time is the period. EUKARYOTE:

A cell that has a nucleus

and organelles (sections of the cell that

A type of plant that

GYMNOSPERM:

reproduces sexually through the use of seeds that are exposed, not hidden in an ovary as with an angiosperm.

perform specific functions) bound by HERBIVORE:

membranes. FOOD CHAIN:

A series of singular

organisms in which each plant or animal depends on the organism that precedes it.

A plant-eating organ-

ism. ICE AGE:

A period of massive and

widespread glaciation (the spread of gla-

Food chains rarely exist in nature; there-

ciers). Ice ages usually occur in series over

fore, scientists prefer the term food web.

stretches of several million or even several

FOOD WEB:

A term describing the

interaction of plants, herbivores, carnivores, omnivores, decomposers, and detritivores in an ecosystem. Each of these

hundred million years and have taken place periodically—often in conjunction with mass extinctions—throughout Earth’s history. An animal without

organisms consumes nutrients and passes

INVERTEBRATE:

them along to other organisms (or, in the

an internal skeleton.

case of the decomposer food web, to the

MASS EXTINCTION:

soil and environment). The food web may

in which numerous species cease to exist at

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A phenomenon

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KEY TERMS or about the same time, usually as the result of a natural calamity. A naturally occurring, typically inorganic substance with a specific chemical composition and a crystalline structure (that is, a structure in which the constituent parts have a simple and definite geometric arrangement that is repeated in all directions). MINERAL:

A series of changes experienced by a once living organism during fossilization. In mineralization, minerals in the organism are replaced or augmented by different minerals, or the hard portions of the organism dissolve completely.

CONTINUED

ships between prehistoric plants and animals and their environments.

forms from the distant past, primarily as revealed through the fossilized remains of plants and animals. PALEOZOOLOGY:

At one time chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals) and oxides, such as carbon dioxide.

animal life. The third-longest phase of

geologic time, after an era. The current eon, the Phanerozoic, has had 11 periods, and the current era, the Cenozoic, has consisted of three periods, of which the most recent is the Quaternary. The next-smallest subdivision of geologic time is the epoch. PHOTOSYNTHESIS:

The biological

conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants. PROKARYOTE:

An area of paleontology devoted to studying the relation-

VERTEBRATE:

PALEOECOLOGY:

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PERIOD:

An area of paleontology involving the study of past plant life. PALEOBOTANY:

An area of paleon-

tology devoted to the study of prehistoric

MINERALIZATION:

ORGANIC:

The study of life-

PALEONTOLOGY:

A cell without a nucle-

us or organelles bound by membranes. An animal with an

internal skeleton.

Gould, Stephen Jay. Ever Since Darwin: Reflections in Natural History. New York: W. W. Norton, 1977.

Palmer, Douglas. Atlas of the Prehistoric World. Bethesda, MD: Discovery Communications, 1999.

K–12: Paleontology—Dinos (Web site). .

Raup, David M. Extinction: Bad Genes or Bad Luck? New York: W. W. Norton, 1991.

Morris, S. Conway. The Crucible of Creation: The Burgess Shale and the Rise of Animals. New York: Oxford University Press, 1998.

Singer, Ronald. Encyclopedia of Paleontology. Chicago: Fitzroy Dearborn Publishers, 1999.

Munro, Margaret, and Karen Reczuch. The Story of Life on Earth. Toronto: Douglas and McIntyre, 2000.

Starr, Cecie, and Ralph Taggart. Biology: The Unity and Diversity of Life. 7th ed. Belmont, CA: Wadsworth, 1995.

Oceans of Kansas Paleontology (Web site). .

University of California, Berkeley Museum of Paleontology (Web site). .

Paleontology and Fossils Resources. University of Arizona Library, Tucson (Web site). .

USGS (United States Geological Survey) Paleontology Home Page (Web site). .

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Taxonomy

CONCEPT Taxonomy is the area of the biological sciences devoted to the identification, naming, and classification of living things according to apparent common characteristics. It is far from a simple subject, particularly owing to many disputes over the rules for classifying plants and animals. In terms of real-life application, taxonomy, on the one hand, is related to the entire world of life on Earth, but on the other hand, it might seem an ivory-tower discipline that it has nothing to do with the lives of ordinary people. Nonetheless, to understand the very science of life, which is biology, it is essential to understand taxonomy. Each discipline has its own form of taxonomy: people cannot really grasp politics, for instance, without knowing such basics of political classification as the difference between a dictatorship and a democracy or a representative government and one with an absolute ruler. In the biological sciences, before one can begin to appreciate the many varieties of organisms on Earth, it is essential to comprehend the fundamental ideas about how those organisms are related—or, in areas of dispute, may be related—to one another.

HOW IT WORKS Taxonomy in Context The term taxonomy is actually just one of several related words describing various aspects of classification in the biological sciences. In keeping with the spirit of order and intellectual tidiness that governs all efforts to classify, let us start with the most general concept, which happens to be classification itself. Classification is a very broad term, with applications far beyond the bio-

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TA X O N O M Y

logical sciences, that simply refers to the act of systematically arranging ideas or objects into categories according to specific criteria. While its meaning is narrower than that of classification, even taxonomy still has broader applications than the way in which it is used in the biological sciences. In a general sense, taxonomy refers to the study of classification or to methods of classification—for example, “political taxonomy,” as we used it in the introduction to this essay. Literary critics sometimes refer to a writer’s taxonomy of characters. Within the biological sciences, however, the term designates specifically a subdiscipline involving the process and study of the identification, naming, and classification of organisms according to apparent common characteristics. P H Y LO G E N Y A N D N O M E N C LA T U R E . Two other terms that one is likely to

run across in the study of taxonomy are phylogeny and nomenclature. Phylogeny is the evolutionary history of organisms, particularly as that history refers to the relationships between lifeforms and the broad lines of descent that unite them. Taxonomy is less fundamental a concept than phylogeny. Whereas taxonomy is a human effort to give order to all the data, phylogeny is the true evolutionary relationship between living organisms. Some scientists call phylogeny the tree of life, meaning that it represents the underlying hierarchical structure by which life-forms evolved and are related to one another. The word naming was used earlier in the definition of taxonomy because it is a familiar, easily understandable word. However, a more accurate term, and one that helps illuminate the distinction between taxonomy and systematics, is

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form, among birds. This is a homologous feature, indicating a common ancestor that likewise had limbs with five digits at the end. By contrast, there is no indication of a close evolutionary relationship in the fact that birds, butterflies, and bats all have wings that are similar in shape. Rather, the laws of physics require that a wing be of a certain shape in order to hold an object aloft, which is why the contour of an airplane wing, when viewed from the side, is remarkably like that of a bird’s wing where it joins the animal’s body.

Taxonomy

Cladistics and Numerical Taxonomy

THE GREEK

PHILOSOPHER

THE FATHER OF TAXONOMY.

ARISTOTLE IS REGARDED AS THE ARISTOTELIAN PRINCI-

PLES OF CLASSIFICATION WERE GOVERNED BY THE IDEA THAT THERE ARE CONSTANT, UNCHANGING

“ESSENCES”

THAT UNITE CLASSES OF ORGANISMS, WHICH IS COM-

Cladistics is a system of taxonomy that distinguishes taxonomic groups or entities on the basis of shared derived characteristics, hypothesizing evolutionary relationships to arrange them in a treelike, branching hierarchy. The expression derived characteristics in this definition means that the characteristics that unite two types of organism are not necessarily present in a shared evolutionary ancestor. Rather, they have developed over the course of evolutionary history since the time of that shared ancestor.

PLETELY AT ODDS WITH THE EMPIRICAL MENTALITY THAT GOVERNS TAXONOMY TODAY. (The Library of Congress.)

nomenclature. The latter can be defined as the act or process of naming as a well as a system of names, particularly one used in a specific science or discipline.

Homologous and Analogous Features Before going on to discuss methods of classification, it is important to note just which characteristics of an organism’s morphological aspect (i.e., structure or form) are important to scientists working in the field of taxonomy. In theorizing relationships between species, taxonomists are not interested in what are known as analogous features, those characteristics that are superficially similar but not as a result of any common evolutionary origin. Rather, they are interested in homologous features, or features that have a common evolutionary origin, even though they may differ in terms of morphological form.

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In explaining cladistics to the ordinary human being, the vast majority of science writers seem to be at a loss as to how to make the topic comprehensible. Thus, such terms as derived characteristics and its opposite, primitive characteristics, usually are left undefined. A welcome exception is Paul Willis, who, in an on-line article for the Australian Broadcasting Corporation (see Where to Learn More) gave a wonderful illustration that was an attempt to analyze the relationships between a mouse, a lizard, and a fish.

One example of a shared evolutionary characteristic, discussed briefly in the essay Evolution, is the pentadactyl limb, a five-digit appendage common to mammals and found, in modified

“They’ve all got backbones,” Willis wrote, “so the feature ‘backbone’ is useless [as an indication of evolutionary branching]; it’s a ‘primitive’ character that tells you nothing. But the [derived] feature ‘four legs’ is useful because it’s an evolutionary novelty shared only between the lizard and the mouse. This implies that the lizard and mouse are more closely related to each other than either is to the fish. Put another way, the lizard and the mouse share a common ancestor that had four legs.” Willis went on to note that “the more evolutionary novelties we can find that support a particular relationship, the greater our confidence that the relationship is correct. ‘Air breathing,’ ‘neck’ and ‘amniotic egg’ are another three evolutionary novelties that tie the lizard

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and the mouse together and leave the fish as a more distant relative.” N U M E R I CA L TAX O N O M Y. Cladistics, the most widely applied approach to taxonomy, has undergone considerable change since it was introduced by the German zoologist Willi Hennig (1913–1976) in the 1950s. Particularly important has been the marriage of cladistics with another taxonomic idea born in the mid–twentieth century, phenetics, or numerical taxonomy. Introduced by the Austrian biologist Robert Reuven Sokal (1926–) and the English microbiologist Peter Henry Andrews Sneath (1923–), numerical taxonomy is an approach in which specific morphological characteristics of an organism are measured and assigned numerical value, so that similarities between taxa (taxonomic groups or entities) can be compared mathematically. These mathematical comparisons are performed through the use of algorithms, or specific step-by-step mathematical procedures for computing the answer to a particular problem. The aim of numerical taxonomy is to remove all subjectivity (such as the taxonomist’s “intuition”) from the process of classification. Initially, many traditional taxonomists rejected numerical taxonomy, because its results sometimes contradicted their own decades-long studies of comparative morphological features. Nearly all modern taxonomists apply numerical methods in taxonomy, although there is often heated debate as to which particular algorithms should be used.

Identification, Classification, and Nomenclature Earlier, taxonomy was defined in terms of its relationship to the identification, classification, and nomenclature of taxa. Let us now briefly consider each in turn, with the understanding that they are exceedingly complex, technical subjects that can be treated here in the most cursory fashion. The process of identification is a particularly complex one. When an apparently new taxon is discovered, a taxonomist prepares an organized written description of the characteristics of similar species, which are referred to as a taxonomic key. Instead of using pictures, which often poorly convey the natural variations in morphological features, taxonomists prefer to use a taxonomic key in written form, which provides much more detail and exactitude.

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To put it in colloquial terms, by referring to a taxonomic key, a taxonomist may determine that if an organism “looks like a duck and quacks like a duck, it must be a duck”—only, in this instance, the taxa being compared are much more specific than the common term duck and the characteristics much more precisely described. (For one thing, there are several dozen species in the genus Anas, which includes all “proper” ducks, and many more species in the family Anatidae, or waterfowl, that are commonly called by “duck names”—including such amusingly named species as the ruddy duck, lack duck, freckled duck, and comb duck.) If there is no already established “duck” that the species in question resembles, the taxonomist may have discovered an entirely new genus, family, order, class, or even phylum.

Taxonomy

A taxonomist may use what is called a dichotomous key, which presents series of alternatives much like a flow chart. For example, if the flowers of a sample in question are white and the stem is woody, then (depending on additional alternatives) it could be either species A or species B. If the flowers are not white and the stem is herbaceous (non-woody), then, presented with another set of additional alternatives, it is possible that the plant is either species C or species D. C L A S S I F I C AT I O N . In discussing cladistics and phenetics, we touched briefly on the process of classification. Suffice it to say that this process is far more complex and technically elaborate than these few paragraphs can begin to suggest. We return later to specifics of classification as they relate to systems and innovations introduced by the Greek philosopher Aristotle (384–322 B.C.), the Swedish botanist Carolus Linnaeus (1707–1778), and the English naturalist Charles Darwin (1809–1882), the three most important men in the history of taxonomy before the twentieth century. For the present, our focus is on the overall ranking system.

There are many possible ranks of classification but only seven that are part of what is known as the obligatory taxonomy, or obligatory hierarchy. These ranks are kingdom, phylum, class, order, family, genus, and species. Listed here are all possible ranks, with obligatory ranks in italics. • Kingdom • Subkingdom

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• • • • • • • • • • • • • • • • • •

Phylum Subphylum Superclass Class Subclass Infraclass Cohort Superorder Order Suborder Superfamily Family Subfamily Tribe Genus Subgenus Species Subspecies The reader occasionally may come across nonobligatory ranks, most notably subphylum, but for the most part the only ranks referred to in this book are the obligatory ones. N O M E N C L AT U R E . In accordance with a tradition established by Linnaeus, all group names are in Latin, thus facilitating ease of communication. There are some rules concerning names of groups: for instance, those of families use the suffix -idae. In the world of taxonomy, however, few rules are accepted universally. Even as basic a term as phylum is not universal, since botanists prefer the word division.

The proper name of any ranking more general than species is capitalized (e.g., phylum Chordata), with species and subspecies names in lowercase. Genus, species, and subspecies names are rendered in italics (e.g., Homo sapiens, or “man the wise”), while proper names of the more general groupings are presented in ordinary type (e.g., class Mammalia). If the same name appears a second time in the same article, the genus name usually is abbreviated: thus, H. sapiens. Just as most people (with such rare exceptions as Cher and Madonna) are identified by two names, a personal and a family name, taxonomy makes use of a system called binomial nomenclature, in which each type of plant or animal is given a two-word name, with the first name identifying the genus and the second the species. In binomial nomenclature, the genus name is analogous to the family name, inasmuch as there are many species within a genus, and the

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species name is like a personal name. The difference is that whereas there may be thousands of boys and men named John Smith, there is only one species called Homo sapiens. Beyond the species name, there may be subspecies names: humans are subspecies sapiens, so our full species name with subspecies is Homo sapiens sapiens. Additional rules govern the inclusion of a name or an abbreviation, at the end of the species or subspecies name, to recognize the individual who first identified it.

REAL-LIFE A P P L I C AT I O N S The Urge to Classify One might ask what all the fuss is about. Why is classification so important? We attempt to answer that question from a few angles, including a brief look at the lengthy historical quest to develop a workable taxonomic system. But what was the original impulse that motivated that quest? One clue can be found in the Greek roots of the word taxonomy: taxis, or “arrangement,” and nomos, or “law.” The search for a taxonomic system represents humankind’s desire to make order out of the complexities with which nature presents us. When it comes to the organization of ideas (including ideas about the varieties of lifeforms), this desire for order is more than a mere preference. It is a necessity. T H E A N A LO GY T O A L I B RA RY.

Imagine a library without any organizational system, with books simply crammed willy-nilly on the shelves. Such a place would be totally chaotic, and if one happened to find a book one was looking for, it would be a case of pure luck. The odds would be weighted heavily against such luck, especially in a university library or a large municipal or regional one. Just as a good-size university library has upward of a million volumes, and many large university libraries have several million, so there are at least a couple of million identified species, and the total may be much larger. Some entomologists (scientists who study insects) speculate that there may be ten million species of insect alone. THE LURE OF A NEW SPECIES.

When a zoologist or botanist discovers what he or she believes to be a new species, the taxonomic system provides a standard against which to check

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it—rather as you would do if you thought you had discovered a book that was not in the library. If the “new” species matches an established one, that may be the end of the story—unless the scientist has discovered a new aspect of the species or a new subspecies. And if there is no match in the taxonomic “library,” the scientist has discovered an entirely new life-form, with all the grand and terrifying ramifications that may ensue. The new species might be an herb from which a cure can be synthesized for a devastating disease, or it could be a parasite that carries a new and previously unknown malady. Whatever it is, it is better to know about it than not to know, and though the vast majority of “new” species are not nearly as exciting as the preceding paragraph would imply, each has its part to play in the overall balance of life. Discovery of new species is particularly important when those species are endangered or might be in the process of disappearing even as they are identified.

Nonscientific Taxonomy Without knowing anything about scientific taxonomy, almost anyone can begin to classify animals and perhaps plants. If we limit the discussion purely to animals, there are many basic parameters according to which we could classify them, just off the tops of our heads, as it were. For example, there are aquatic and terrestrial animals, and these general groupings can be broken down further according to biome or habitat (see Biomes). There are animals that walk, fly, swim, slither, or move by some other means. Animals can be divided according to their forms of reproduction, whether asexual or sexual, oviparous or viviparous (expelling or retaining a fertilized egg, respectively), and so on. As discussed in Food Webs, animals may be classified as herbivores, carnivores, omnivores, or detritivores or as primary, secondary, or tertiary consumers. They may be endothermic or ectothermic (warm-blooded or cold-blooded), and they may be covered with scales, feathers, fur, or skin. (In the last case, that skin may be protected by either mucus or hair.)

stitute a form of classification, there is a great difference between this and scientific taxonomy.

Taxonomy

SCIENCE VERSUS “COMMON S E N S E . ” Taxonomy is tied closely to evolu-

tionary study, and Darwin’s theory of evolution was a turning point in the history of scientific classification. Thus, taxonomists are concerned more with the evolutionary patterns that link organisms than they are with what may be only superficial similarities. Habitat, for instance, is significant in studying biomes, but it seldom plays a role in taxonomy. Nor is the ability to fly, as we have noted, necessarily an indicator of taxonomic similarities. A striking example of the difference between scientific taxonomy and “common sense” classification is the fact that whales and dolphins are grouped along with other mammals (class Mammalia) rather than with fish and other creatures that most readily come to mind when thinking of aquatic organisms. In fact, whales and dolphins share not only a wide array of primitive characteristics with mammals (for example, the pentadactyl limb described earlier) but also the derived characteristic that defines mammal: the secreting of milk from mammary glands, by which a mother feeds her young. Not only is it impossible to get milk from a fish (even family Chanidae, known by the common name “milkfish”), but fish lack even that primitive characteristic, the pentadactyl limb, that links mammals, at least distantly, with nonmammalian creatures, such as birds (class Aves). COMMON TERMS AND FOLK TAX O N O M Y. For the sake of convenience, in

On and on go the categories, and if one is inclined toward a classifying mind, this kind of mental exercise can be fun. Certainly, little children enjoy it, and many educational programs and games call on the child to group animals thus. Although these kinds of groupings, and the efforts to place animals into one group or another, con-

many places throughout this book, common terms such as bird, horse, fish, and so forth are used. But common terms are far from adequate in a scientific context, because such terminology can be deceptive, as exemplified by the nonduck “ducks” mentioned earlier. Likewise, shellfish and starfish are not “fish” as that term is usually understood. But while common terminology can be misleading, sometimes correlations with scientific taxonomy can be found in what is known as folk taxonomy. The latter is a term for the taxonomic systems applied in relatively isolated non-Western societies. For example, the folk taxonomy of native peoples in New Guinea identified 136 bird species in the mountains of that island, a figure that came amazingly close to the 137 species identified by the German-born

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ILLUSTRATION OF A SEA MONSTER FROM “DE NATURA RERUM” by Albertus Magnus. During the Middle Ages, taxonomic writings consisted primarily of books of imaginary creatures; the first signs of scientific reawakening in taxonomy came with plant and animal catalogues by such medieval scholars as Magnus. (© Gianni Dagli Orti/Corbis. Reproduced by permission.)

American evolutionary biologist Ernst Mayr (1904–) when he studied New Guinea’s birds using scientific methods.

Aristotle, Linnaeus, Darwin, and Beyond Among his many other accomplishments as a thinker, Aristotle is regarded as the father of the biological sciences and of taxonomy. Among the dominant ideas in his work as a philosopher are the concepts of hierarchy and classification, and thus he took readily to the idea of classifying things. At his school in Athens, he put his students to work on all sorts of taxonomic pursuits,

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from listing the champions at the Pythian Games (a festival like the Olympics) to classifying the constitutions of various Greek city-states to analyzing the body parts of animals. Aristotle himself dissected hundreds of animals to understand what made them tick, and he proved to be some 2,000 years ahead of his time in recognizing that the dolphin is a mammal and not a fish. His system of classification, however, was a far cry from the ideas that developed in nineteenth-century taxonomy; rather than searching for evolutionary lines of descent, he ranked animals in order of their physical complexity. In most aspects of his other work, Aristotle established sharp distinctions between his own

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ideas and those of his teacher, Plato (427?–347 B.C.). For example, Aristotle rejected Plato’s position that every idea we can conceive is but a dim reflection of an essential concept—for example, that our idea of “red” is only a shadowy copy of the perfect notion of “redness.” Yet in his taxonomy, Aristotle seemed to hark back to his days as Plato’s star pupil. The Aristotelian principles of classification were governed by the idea that there are constant, unchanging “essences” that unite classes of organisms. This idea of essences is completely at odds with the empirical (experience-based) mentality that governs taxonomy today. Nonetheless, for two millennia, Aristotelian ideas represented the cutting edge in taxonomy and much else. THE MIDDLE AGES AND R E N A I SSA N C E . After Aristotle and his

brilliant student Theophrastus (371?–287? B.C.), the father of botany, there would be no Western biological theorists of remotely comparable stature until the time of the Renaissance. In the meantime, taxonomy, as with so many other areas of learning in Europe, declined badly. During the Middle Ages, what passed for taxonomic writings consisted primarily of bestiaries, books full of fanciful and imaginary creatures, such as the unicorn. The first signs of scientific reawakening in the biological sciences in general, and taxonomy in particular, came with plant and animal catalogues by such great medieval scholars as Peter Abelard (1079–1142) and Albertus Magnus (ca. 1200–1280). Even so, their work consisted primarily of summations of existing Aristotelian knowledge rather than new contributions. In the sixteenth century, the Swiss scientist Konrad von Gessner (1516–1565) wrote Historia animalium (1551–1558), a groundbreaking work that included descriptions of many animals never before seen by most Europeans. Gesner also denounced the practice of including fictitious animals in bestiaries. Around the same time, the discoveries of new plant and animal species in the New World began to point up the need for a taxonomy that went beyond Aristotle’s. The first scholar of the modern era to attack this problem was the Italian botanist Andrea Cesalpino (1519–1603), but nearly two centuries would pass before the development of a workable classification system.

Carl von Linné but adopted the Latinized name Carolus Linnaeus. Even that late in scientific history, scholars still wrote chiefly in Latin, not because they were trying to adhere to tradition but because it remained a common language between educated people of different countries. Thus, Linnaeus’s great work, which he first published in 1737 but revised numerous times, was named Systema naturae, or “The Natural System.” Thanks to Linnaeus, Latin became enshrined permanently as the language of taxonomy the world over, but this was far from his only accomplishment.

Taxonomy

It was Linnaeus who introduced binomial nomenclature, in a 1758 revision of his Systema, and also Linnaeus who established several of the obligatory rankings. Moreover, he instituted the first taxonomic keys, and his system, first applied in botany, became accepted in the zoological community as well. Others, including Baron Georges Cuvier (1769–1832), Michel Adanson (1727–1806), and Comte Georges Buffon (1707–1788), refined Linnaeus’s system, but he stands as a towering figure in the discipline. Later, the French natural philosopher Jean Baptiste de Lamarck (1744–1829) proposed a distinction between vertebrates, or animals with spinal columns, and invertebrates. Today this distinction is not considered as useful as it once was, since it is lopsided—that is, there are nine times as many invertebrates as vertebrates in the animal kingdom—but at the time, it represented an advancement. Less questionable were the distinctions introduced in 1866 by the German biologist Ernst Haeckel (1834–1919) between plants, animals, and single-cell organisms. As Haeckel reasoned, at the level of unicellular organisms, distinctions between plant and animal really make no sense. DA R W I N A N D T H E T W E N T I E T H C E N T U RY. By far the most influential figure

L I N N A E U S A N D O T H E R S . The man who revolutionized taxonomy was born

in taxonomy during the nineteenth century was the man also recognized as the most influential figure in all of biology during that era: Darwin. Whereas Linnaeus had retained the Aristotelian focus on the “essence” of the animal’s features, Darwin swept away such notions and, in his Origin of Species (1859), proposed that the “community of descent” is “the one known cause of close similarity in organic beings” and therefore the only reasonable basis for taxonomic classification systems. As result of Darwin’s work, taxonomists

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became much more oriented toward the representation of phylogeny in their classification systems. Therefore, instead of simply naming and cataloguing species, modern taxonomists also try to construct evolutionary trees showing the relationships between different species. Since Darwin’s time, taxonomy has seen numerous innovations, including the introduction of cladistics by Hennig and of numerical taxonomy by Sokal and Sneath. Taxonomists today make use of something unknown at the time of Darwin: DNA (deoxyribonucleic acid, a molecule that contains genetic codes for inheritance), which provides a wealth of evidence showing relationships between creatures. For example, a comparison of human and chimpanzee DNA reveals that we share more than 98% of the same genetic material, indicating that the two lines of descent are related more closely than either is to apes.

The Five Kingdoms There are several taxonomic systems, distinguished in part by the number of different kingdoms that each system recognizes. The system used in this book is that of five kingdoms, listed here, which is the result of modifications by the American biologists Lynn Margulis (1938–) and Karlene V. Schwartz (1936–) to the work of earlier taxonomists. (It should be noted that biologists are increasingly using a system of six kingdoms under three domains: eubacteria, arachaea, and eukaryotes. For the sake of simplicity, however, the five-kingdom system is used here.) These five kingdoms are as follows:

Fungi: fungi, molds, mushrooms, yeasts, mildews, and smuts (a type of fungus that afflicts certain plants). Fungi are multicellular, consisting of specialized eukaryotic cells arranged in a filamentous form (that is, a long, thin series of cells attached either to one another or to a long, thin cylindrical cell). There are some 100,000 varieties of fungi. Plantae: plants, of which there are upward of 250,000 species. Although plant is a common term, there is no universally accepted definition that includes all plants and excludes all nonplants. One of the most important characteristics of plants is the fact that they receive their nutrition almost purely through photosynthesis. Beyond the plant kingdom, this is true only of a few protests and bacteria. (For the most part, the three lower kingdoms obtain nutrition through absorption.) Other characteristics of plants include the fact that they are incapable of locomotion; have cells that contain a form of carbohydrate called cellulose, making their cell walls more or less rigid; are capable of nearly unlimited growth at certain localized regions (unlike most animals, which have set numbers of limbs and so forth); and have no sensory or nervous system.

Protista (or Protoctista): protozoans, slime molds (which resemble fungi), and algae other than the blue-green variety. Made up of more than 250,000 species, this kingdom is distinguished by the fact that its members are singlecell organisms, like the Monera. These organ-

Animalia: animals, of which there are more than 1,000,000 species. Like plants, animals are characterized by specialized eukaryotic cells, but also like plants, the comprehensive definition of animal is not as obvious as one might imagine. Mobility, or a means of locomotion, is not a defining characteristic, since sponges and corals are considered animals. The principal difference between animals and plants is at the cellular level: animals either lack cells walls entirely or have highly permeable walls, unlike the cellulose cell walls in plants. Another defining characteristic of animal is that they obtain nutrition by feeding on other organisms. Additionally, animals usually have more or less fixed morphological characteristics and possess a nervous system. The fact that most animals are mobile helps account for the large number of animal species compared with those of other kingdoms; over the course of evolutionary history, mobility brought about the introduction of animals to a wide range of environments, which required a wide range of adaptations.

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Monera: bacteria, blue-green algae, and spirochetes (spiral-shaped, undulating bacteria). Members of this kingdom, consisting of some 10,000 or more known species, are single-cell prokaryotes, meaning that the cell has no distinct nucleus. Some researchers have divided Monera into Eubacteria, or “true” bacteria, and Archaebacteria, which are bacteria-like organisms capable of living in extremely harsh and sometimes anaerobic (oxygen-lacking) environments, such as in acids, saltwater, or sewage.

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isms, however, are eukaryotes, or cells with a nucleus as well as organelles (sections of the cell that perform specific functions).

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A

DIVER INSIDE A BARREL SPONGE ALONG A CORAL REEF IN THE

TION, IS NOT A DEFINING CHARACTERISTIC OF KINGDOM

ANIMALIA;

CARIBBEAN. MOBILITY, OR A MEANS INDEED, SPONGES AND CORALS ARE

OF LOCOMOCONSIDERED

ANIMALS. (© Jeffrey L. Rotman/Corbis. Reproduced by permission.)

Space does not permit a discussion of the various phyla, let alone the smaller divisions, in anything like the detail we have accorded to kingdoms. Furthermore, the distinctions among

most phyla, apart from higher animals and some plants, makes for rather dry reading to a nonscientist. These divisions are discussed in further detail, however, within the essays Species and

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Speciation. The latter essays also address the definition of species, a great and continuing challenge that faces taxonomists.

Taxonomy in Action Two stories reported in National Geographic News online (see “Where to Learn More”) in 2001 and 2002 illustrate the fact that scientific classification is an ongoing process, and that the world of taxonomy is frequently home to controversies and surprises. Lee R. Berger of the Geographic reported the first story, on December 17, 2001, under the heading “How Do You Miss a Whole Elephant Species?” As it turns out, there are not just two species of elephant, as had long been believed, but three. In addition to the Asian elephant (Elephas maximus) scientists had long recognized the African savanna elephant, or Loxodonta africana, as a second species. However, DNA testing (see Genetics and Genetic Engineering) in 2001 revealed a second African variety, Loxodonta cyclotisare or the African forest elephant, formerly believed to constitute merely a subspecies. The news was not entirely new: as early as a century prior to the announcement of the “new” species, zoologists had begun to suspect that the forest elephant was a separate grouping distinguished by a number of characteristics. For example, the forest elephant is physically smaller, with males seldom measuring more than 8 ft. (2.5 m) at the shoulder, as compared to 13 ft. (4 m) for a large savanna male. Additionally, ivory samples confiscated from poachers or illegal hunters have revealed that the material in the tusks of the forest variety is pinker and harder than that of its savanna counterpart. Recognition of the third elephant species followed years of argument as to whether the two African varieties are capable of interbreeding, which would indicate that they are not separate species. That debate was rendered moot by the DNA studies, which showed that the African forest and savanna elephants are less closely related genetically than are lions and tigers, or horses and zebras. A “ N E W ” I N S E C T O R D E R. The

identification of the forest elephant in 2001 was a major taxonomic event, inasmuch as the elephant itself is a large and commonly known creature. However, it was still a matter only of identifying a new species, whereas in 2002, for the first

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time in 87 years, taxonomists identified an entirely new insect order. Actually, the order consists of a single known species, but this one is so different from others that it must be grouped separately. Discovered in Namibia, in southwestern Africa, the creature was given the nickname “the gladiator” in honor of the Academy Awardwinning 2000 film of that name. Entomologist Oliver Zompro of the Max Planck Institute of Limnology in Plön, Germany, described the creature as “a cross between a stick insect, a mantid, and a grasshopper,” according to the Geographic. Because its first body segment is the largest, it is distinguished from a stick insect, whereas it differs from a mantid inasmuch as it uses both fore and mid-legs to capture prey. And while it looks like a grasshopper, “the gladiator” cannot jump. Measuring as much as 1.6 in. (4 cm) long, the insect, whose order is designated as Mantophasmatodea, is a carnivorous, nocturnal creature. Its discovery raised the number of known insect orders to 31, a discovery that Piotr Naskrecki, director of the Conservation International Invertebrate Diversity Initiative, compared to finding a mastodon or saber-toothed tiger. Colorado State University ecologist Diana Wall described the discovery as “tremendously exciting” and told the Geographic, “This new order could be a missing link to determining relationships between insects and other groups. ... Every textbook discussing the orders of insects will now need to be rewritten.” WHERE TO LEARN MORE Classification—The Dinosaur FAQ (Web site). . The Germplasm Resources Information Network (GRIN), Agricultural Research Service (Web site). . Goto, H. E. Animal Taxonomy. London: Arnold, 1982. Lacey, Elizabeth A., and Robert Shetterley. What’s the Difference?: A Guide to Some Familiar Animal LookAlikes. New York: Clarion Books, 1993. Margulis, Lynn, and Karlene V. Schwartz. Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth. New York: W. H. Freeman, 1988. National Geographic News (Web site). . O’Neil, Dennis. Classification of Living Things/Palomar College (Web site). . Parker, Steve. Eyewitness Natural World. New York: Dorling Kindersley, 1994.

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Taxonomy

KEY TERMS A specific set of step-by-

ALGORITHM:

CLASSIFICATION:

A

very

broad

step procedures for computing answers to

term, with application far beyond the bio-

a mathematical problem.

logical sciences, that refers to the act of sysMorpho-

tematically arranging ideas or objects into

logical characteristics of two or more taxa

categories according to specific criteria. A

that are superficially similar but not as a

more specific term is taxonomy.

result of any common evolutionary origin.

EUKARYOTE:

For example, birds, bats, and butterflies all

as well as organelles (sections of the cell

have wings, but this is not because they are

that perform specific functions) bound by

closely related. Compare with homologous

membranes.

ANALOGOUS FEATURES:

features. FAMILY: BINOMIAL

NOMENCLATURE:

A

system of nomenclature in biological tax-

first name identifying the genus and the second the species. Genus name is always capitalized and abbreviated after the first use, and species name is lowercased. Both are always shown in italics; thus, Homo sapiens and, later in the same document, H.

order but before genus. FUNGI:

One of the five kingdoms of

living things, consisting of multicellular eukaryotic cells arranged in a filamentous form (that is, a long, thin series of cells attached either to one another or to a long, thin cylindrical cell.) Fungi include “true” fungi, molds, mushrooms, yeasts, mildews, and smuts (a type of fungus that afflicts

sapiens. BIOME:

The third most specific of the

seven obligatory ranks in taxonomy, after

onomy whereby each type of plant or animal is given a two-word name, with the

A cell that has a nucleus

A large ecosystem (community

certain plants). The second most specific of

of interdependent organisms and their

GENUS:

inorganic environment) characterized by

the obligatory ranks in taxonomy, after

its dominant life-forms. There are two

family but before species.

basic varieties of biome: terrestrial, or

HOMOLOGOUS FEATURES:

land-based, and aquatic.

phological characteristics of two or more

Mor-

A system of taxonomy

taxa that indicate a common evolutionary

that distinguishes taxonomic groups or

origin, even though the organisms may dif-

entities (i.e., taxa) on the basis of shared

fer in terms of other morphological fea-

derived characteristics, hypothesizing evo-

tures. An example is the pentadactyl limb,

lutionary relationships to arrange these in

common to many birds and most mam-

a treelike, branching hierarchy. Cladistics is

mals (e.g., the human’s four fingers and

one of several competing approaches to

thumb), which indicates a common ances-

taxonomic study.

tor. Compare with analogous features.

CLADISTICS:

CLASS:

The third most general obliga-

KINGDOM:

The highest or most gener-

tory of the taxonomic classification ranks,

al ranking in the obligatory taxonomic sys-

after phylum but before order.

tem. In the system used in this book, there

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Taxonomy

KEY TERMS are five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. MONERA:

One of the five kingdoms of

living things, consisting of single-cell prokaryotes, including bacteria, blue-green algae, and spirochetes (spiral-shaped undulating bacteria that may cause such diseases as syphilis). Structure or form, or

MORPHOLOGY:

the study thereof. NOMENCLATURE:

The act or process

of naming, or a system of names—particu-

CONTINUED

The biological conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants. In this process carbon dioxide and water are converted to sugars.

PHOTOSYNTHESIS:

The evolutionary history of organisms, particularly as that history refers to the relationships between lifeforms, and the broad lines of descent that unite them. PHYLOGENY:

The second most general of the obligatory taxonomic classification ranks, after kingdom and before class. PHYLUM:

larly one used in a specific science or disciPROKARYOTE:

pline. See also binomial nomenclature. NUMERICAL

An

TAXONOMY:

approach to taxonomy in which specific morphological characteristics of an organism are measured and assigned numerical value, so that similarities between two types of organism can be compared mathematically by means of an algorithm. Numerical taxonomy also is called phenetics. OBLIGATORY

TAXONOMY

(OR

OBLIGATORY

HIERARCHY):

The

seven taxonomic ranks by which all species

PROTISTA

The science of classifying and studying organisms with regard to their natural relationships. SYSTEMATICS:

tory categories, such as subphylum, cohort,

TAXONOMY:

The middle of the seven oblig-

atory ranks in taxonomy, more specific than class but more general than family. PHENETICS:

Another

name

numerical taxonomy.

202

A taxonomic group or entity.

TAXON:

class, order, family, genus, and species.

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for

PROTOCTISTA):

SPECIES: The most specific of the seven obligatory ranks in taxonomy.

also are identified according to nonobligaor tribe. These ranks are kingdom, phylum,

(OR

One of the five kingdoms of living things, consisting of single-cell eukaryotes. Protista include protozoans, slime molds (which resemble fungi), and algae other than the blue-green variety.

must be identified, whether or not they

ORDER:

A cell without a nucle-

us.

The area of the biological sciences devoted to the identification, nomenclature, and classification of organisms according to apparent common characteristics. The word taxonomy also can be used more generally to refer to the study of classification or to methods of classification (e.g., “the taxonomy of Dickens’s characters.”)

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Simpson, George Gaylord. Principles of Animal Taxonomy. New York: Columbia University Press, 1961. Taxonomy Browser, National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health (Web site). . The Tree of Life Web Project (Web site). .

S C I E N C E O F E V E RY DAY T H I N G S

Tudge, Colin. The Variety of Life: A Survey and a Celebration of All the Creatures That Have Ever Lived. London: Oxford University Press, 2000.

Taxonomy

Whyman, Kathryn. The Animal Kingdom: A Guide to Vertebrate Classification and Biodiversity. Austin, TX: Raintree Steck-Vaughn, 1999. Willis, Paul. “Dinosaurs and Birds: The Story.” Australian Broadcasting Corporation (Web site). .

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SPECIES Species

CONCEPT One of the challenges that faces a student of the biological sciences is the seemingly endless array of unfamiliar terms that one must learn. It is a relief to come across a relatively familiar one, such as species. Although it has a scientific sound to it, the word has entered everyday language, such that when people use it, most everyone understands what is meant. Or do they? As it turns out, there is no hard and fast definition for the word. Nonetheless, it is easy enough to find examples of species, since there are many millions of them in five kingdoms of living things—a product of a phenomenon know as speciation, whereby evolutionary lines of descent diverge and new species are created. In the world today, there are many interesting groups of species, distinguished neither by evolutionary line nor by taxonomy but instead by the ways in which they interact with their environments. Among these groups are endangered species, of whose existence most people are aware, owing to the spread of the environmentalist message through media and entertainment outlets since the early 1970s. Less familiar is another broad group that in many cases threaten humans: introduced or invasive species.

The concept of species falls under the heading of taxonomy, the area of the biological sciences devoted to the identification, naming, and classification of living things according to apparent common characteristics. Taxonomy is discussed in detail within the essay on that subject, but to appreciate the topic of species in context, it is

It is therefore possible for organisms in a particular environment to develop a common adaptive mechanism through generations of natural selection, even though those organisms themselves are not related to fish closely in terms of evolutionary line of descent. Thus, whales and dolphins, mammals that live underwater, evolved the ability to swim just as well as fish, but that does not mean they are connected closely. Conversely, organisms may be close, or relatively close, in terms of evolutionary lines of descent yet differ in significant morphological features. To use the whale and dolphin example again, these creatures are classified as mammals owing to certain particulars (discussed later), but they differ from the vast majority of mammals in that they have no legs. They do, however, have four appendages, just like the rest of the mammalian class; as a result of natural selection, however,

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HOW IT WORKS Taxonomy in Brief

204

helpful to have at least some knowledge of the larger subject. At one time taxonomists were concerned most with the morphological characteristics (i.e., the structure or form) of organisms as a basis for classifying many species within a larger grouping. Today, however, shared evolutionary lineage is much more important than morphological features in determining whether taxa (plural of taxon, meaning a taxonomic group or entity) can be classed together. Organisms may be linked closely in terms of evolutionary lines of descent but differ in a particular morphological aspect as a result of the adaptive changes that accompany natural selection. The latter, a key concept in the theory of evolution put forward by the English naturalist Charles Darwin (1809–1882), is a process whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment.

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theirs ceased to operate as legs (an encumbrance for life in the water) a long time ago, and today they function instead as fins.

Obligatory Ranks The classification system used today is an outgrowth of a system developed by the Swedish botanist Carolus Linnaeus (1707–1778) in the 1730s. The realms of zoology and botany, areas of biology devoted to the study of animal and plant life, respectively, differ somewhat with regard to their classification systems, but both use international codes of nomenclature with roots in the Linnaean system. There are many possible ranks of classification, but only seven are obligatory, meaning that all species must be assigned a place in these groupings. The obligatory ranks are listed here. The entire list of rankings, including versions of obligatory ranks with such prefixes as sub-, super-, and infra-, as well as such additional ranks as cohort or tribe, are given in Taxonomy. Note the difference between the zoological and botanical names for the second rank. Obligatory Taxonomic Ranks

specifics play no significant role in categorizing them within any of the more specific groupings to which they belong. Furthermore, the generic definitions of the categories—for example, class as opposed to class Mammalia, class Insecta, or some other class in the taxonomic system—are purely relative. In other words, class is simply the obligatory rank that is more specific than phylum but more general than order. DESIGNATING A SINGLE SPECIES WITHIN THE RANKS. When preparing an

outline for a paper, students are taught that no topic should have only one subheading; instead, that solitary subheading should be moved up one level. Such rules do not apply in taxonomy, and it is not necessary that there be more than one subgroup within a larger group. For example, there might be only one class in a phylum. Taxonomists use detailed definitions to single out particular groups, such as class Mammalia. The following list shows the placement of humans within the larger taxonomic universe, along with brief explanations of a few (though far from all) characteristics that define each group.

Below the level of kingdom, definitions become even more difficult. Organisms are grouped into phyla on the basis of body plan or organization, but there is no regular pattern for grouping within the smaller categories. For example (as noted later herein), humans are placed within their particular phylum and subphylum on the basis of their spinal columns and overall internal bone structure, but those

• Kingdom Animalia: Multicell eukaryotic (that is, possessing cells with a nucleus and specialized compartments called organelles) organisms that obtain their nutrition solely by feeding on other organisms. (Other defining characteristics of Animalia are discussed in Taxonomy.) • Phylum Chordata: Animals whose bodies, at least at some point in their life cycles, include a rudimentary internal skeleton with a stiff supporting rod known as a notochord. All chordates at some point also breathe through gills (in the case of a human, while still in the womb). Other characteristics set apart chordates, including a tail or the remnants of one. Humans belong to the subphylum Vertebrata, or chordates with a spinal column. • Class Mammalia: Vertebrates that feed their young from special milk-secreting glands, known as mammae, located on the mother’s body. Mammals have other distinguishing characteristics, such as a hinged lower jaw attached to the skull. • Order Primates: A group of mammals whose characteristics may include some version of an opposable digit (e.g., the human thumb) and other features that,

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• • • • • • •

Kingdom Phylum (Division in botany) Class Order Family Genus Species As discussed in Taxonomy, this book uses a system of five kingdoms, whose characteristics are defined in that essay. Even at the level of kingdom, not everything is delineated precisely (see the discussion in Taxonomy), and there are significant areas of dispute. For example, some taxonomic systems include viruses. Because viruses are not cellular in structure and are not universally regarded as true organisms, however, they are not included in the five-kingdom system used here.

Species

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Species

while they are prevalent among most primates, are not universal to them. Not every one of these traits is exclusive to primates, a group that includes prosimians (e.g., lemurs), monkeys, apes, and humans. • Family Hominidae: Primates noted for their erect posture, large brains, rounded skulls, small teeth, bipedal locomotion (i.e., they walk on two legs), and tendency to use language for communication. Humans are the only surviving species in the family, but extinct hominids include Homo habilis (about 1.6 million years ago) and H. erectus (about two million years ago) as well as the more distant Australopithecus (about eight million years ago). • Genus Homo: Hominids with especially large skulls as well as the features that characterize family Hominidae. Members of this genus, which included H. erectus and H. habilis as well as H. sapiens, also are known for their ability to fashion precise tools. • Species Homo sapiens: Members of the genus Homo (“man”) noted for, among other things, the ability to use symbols and writing. This category includes modern humans and the extinct Cro-Magnon and Neanderthal man. Note that the proper name of any ranking more general than species is capitalized (e.g., phylum Chordata), with species (and subspecies) names in lowercase. Genus, species, and subspecies names are rendered in italics (e.g., Homo sapiens, or “man the wise”), whereas proper names of the more general groupings are presented in ordinary type (e.g., class Mammalia). If the same name appears a second time in the same article, the genus name usually is abbreviated: thus, H. sapiens. Another important abbreviation is spp., implying several species within a genus— for example, Quercus spp. refers to more than one species of oak. Taxonomy makes use of a system called binomial nomenclature, in which each species is identified by a two-word name, designating genus and species proper. Beyond the species name, there may be subspecies names: humans are subspecies sapiens, so our full species name with subspecies is Homo sapiens sapiens. Additional rules govern the inclusion of a name or an abbreviation at the end of the species or subspecies name, to recognize the person who first identified it—in this case, Linnaeus. Hence the

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proper full name of our species is Homo sapiens sapiens Linneaus, 1758.

The Mystery of Species If one studies the delineation of humans’ place in the overall taxonomic structure, one may notice that for several groupings, the defining characteristics are a bit “fuzzy around the edges.” This is true even of the animal kingdom, as noted in Taxonomy: mobility and locomotion, seemingly so integral to the definition of animal, are not prevalent among all animal species. Given the many gray areas and areas of dispute in the larger taxonomic categories, it should come as no surprise that the smallest of the obligatory rankings, that of species, lacks a precise definition. The most widely accepted definition of species is the one put forward by the Germanborn American evolutionary biologist Ernst Mayr (1904–) in the 1940s. Mayr’s idea, known as the biological species concept, defines a species as a population of individual organisms capable of mating with one another and producing fertile offspring in a natural setting. Members of two different, but closely related species in some cases can mate with one another to produce infertile offspring, the most well-known example being the mule, a sterile hybrid produced by the union of a male donkey and a female horse. The definition offered by the biological species concept requires qualification. While many plants and animals reproduce sexually, many more do not; no single-cell life-forms reproduce in this way, yet there are certainly many different and distinct species of bacteria and protozoa. Thus, a further qualification typically is added to the definition of species: members of the same species share a gene pool, or a total sum of genes. Genes carry information about heritable traits, which are passed from parent to offspring. Whereas the gene pool is shared by members of a species, nonmembers of that species have genes that do not belong to that gene pool. To use a rudimentary example, let’s say that there is a gene pool containing genes x, y, and z. Individuals that have these genes fit within the gene pool, but an individual with gene w does not. The definition of species remains challenging, with special problems raised in the area of botany. It is also sometimes possible to confuse

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Species

INUIT

VILLAGERS BUTCHER A WHALE.

AMONG

ENDANGERED SPECIES IS THE RIGHT WHALE, SO CALLED BECAUSE

WHALERS OF THE NINETEENTH CENTURY CONSIDERED IT THE

“RIGHT”

WHALE TO HUNT: IT SWIMS SLOWLY AND CLOSE

TO SHORE AND CAN BE FOUND AND SLAUGHTERED EASILY. (© Lowell Georgia/Corbis. Reproduced by permission.)

species and race, a grouping that applies not only in the world of humans but also that of other animal and even plant species. Race is different from species inasmuch as races are not isolated genetically from one another; in other words, there are no biological barriers to interbreeding between races. (See Speciation for a discussion of the process whereby single species develop over time into more than one reproductively isolated species.)

REAL-LIFE A P P L I C AT I O N S Endangered Species An endangered species is any plant, animal, or microorganism that is at risk of becoming extinct or at least of disappearing from a particular local habitat. Over the course of Earth’s geological history, species have become extinct naturally— sometimes in large proportions, as discussed in the context of mass extinction in Paleontology. In modern times, however, species and their natural communities are threatened mostly by human activities.

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The number of endangered species worldwide is not known. In the United States—a country that, unlike most, expends considerable effort on keeping track of its endangered species— there were more than 750 species and subspecies listed by the late 1990s as endangered under the federal Endangered Species Act. Additional endangered species are being added at a rate of about 50 per year, and there is a “waiting list” of an estimated 3,500 candidate species. Efforts at monitoring endangered species in the United States have directed a disproportionate amount of attention toward larger organisms; consequently, smaller endangered species from such groups as arthropods, mosses, and lichens have received insufficient attention. The regions of the United States with the largest numbers of endangered species are in the humid southeast and the arid southwest. These areas tend to have the unfortunate combination of unique ecological communities alongside runaway urbanization and resource development. Overdevelopment and destruction of habitats is perhaps the most well-known ways that humans endanger the survival of species. For example, the habitat of the northern spotted owl is under threat from loggers in the Pacific North-

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west (see Succession and Climax). Another threat is the introduction of new species, particularly predators, to an area that is not their natural habitat—a topic we discuss in more depth later in this essay. HUNTING THE ESKIMO C U R L E W. Another way humans threaten

species is by excessive hunting. An example of a species thus threatened is the Eskimo curlew (Numenius borealis), a sandpiper (a type of bird) that was still abundant in North America during the nineteenth century. A large, friendly creature, it was hunted in vast numbers during its seasonal migrations over the prairies and coasts of Canada and the United States and during its winter seasons in South America. (See Migration and Navigation for more about birds’ winter migrations.) The Eskimo curlew became very rare by the end of the nineteenth century, and the last time an Eskimo curlew nest was seen (1866), the guns of the Civil War were practically still smoking. The last time a scientific team collected an Eskimo curlew specimen was in 1922. It might seem that the bird is extinct, but this is not the case. Although it is extremely rare, there have been a few reliable sightings of individuals and small flocks of this species, mostly during migration in Texas and elsewhere but also in its breeding habitat in the Canadian Arctic. Once abundant, the Eskimo curlew now hangs on by a thread. RIGHT WHALES AND BLUE W H A L E S . More familiar is the endanger-

ment of whales, a cause made popular by many a “Save the Whales” bumper sticker. Among endangered animals of this group are the blue whale (Balaenoptera musculus) and various species from the genus Balaena, or right whale. The latter species gained its common name because whalers of the nineteenth century considered it the “right” whale to hunt: it swims slowly and close to shore and so could be found and slaughtered easily. In addition, it yields a large amount of oil, used for lighting lamps in the era when Herman Melville’s Moby Dick (1851) was written. The estimated world population of right whales is currently about 2,000 individuals, much depleted from the historical high numbers; though it is now protected from whaling, it suffers an excessive mortality rate from ship collisions.

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As for the blue whale, it occurs virtually worldwide, and with a typical weight of 150 tons (136 tonnes) and a length of 100 ft. (30 m), it is the largest animal ever to have lived on Earth. Because it is such a fast swimmer, it could not be hunted effectively by whalers in sailing ships. Once steam-powered ships were invented, however, these whales were taken in tremendous numbers and became endangered. Because of its precarious status, this species has not been hunted for several decades, but it remains rare and endangered.

The Fate of the Dodo When a species becomes extinct, it is gone forever. It is like a family whose last member has died without leaving an heir, but in this case the impact is potentially much more profound. Several thousand species have become extinct as the result of human activities, mostly hunting, in the past few hundred years, and of these species perhaps none is more well known than Raphus cucullatus, or the dodo. Long before the application of the term clueless in the 1990s, a person out of touch or out of step was called a dodo. How did the bird’s name come to be a synonym for stupidity? Perhaps it is just the funny sound of the name, or perhaps it is the fact that the dodo looked a bit like a turkey, another bird name used for someone of less than exemplary capabilities. Or perhaps the application of the name dodo in this way carries a hint of blaming the victim—the implication that the dodo somehow played a part in its own extinction. In fact, the dodo’s only shortcoming was its inability to overcome the threat posed by an extremely dangerous predator: the human. A member of the dove or pigeon family, the dodo was flightless and lacked natural enemies until humans discovered its homeland, the Indian Ocean island of Mauritius, in the early sixteenth century. First came the Portuguese and then, in 1598, the Dutch, who made the island a colony in 1644. By 1681 the dodo had ceased to exist. Not only did sailors collect the birds for food, but introduced species, including dogs, cats, pigs, monkeys, and rats, also preyed on dodos. They were subjected to regular slaughter by sailors, but the species managed to breed and survive on the remote areas of the island for a time. After the establishment of their colony, however, Dutch

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Species

A

MEMBER OF THE DOVE OR PIGEON FAMILY, THE DODO WAS FLIGHTLESS AND LACKED NATURAL ENEMIES UNTIL

HUMANS DISCOVERED ITS HOMELAND, THE ISLAND OF

MAURITIUS,

IN THE EARLY SIXTEENTH CENTURY.

BY

1681,

THROUGH THE EFFECTS OF PREDATION, THE DODO HAD CEASED TO EXIST. (© Bettmann/Corbis. Reproduced by permission.)

settlers launched what amounted to an extermination campaign.

have been introduced to a region or continent, usually but not always through human activity.

No part of Earth’s living environment can be removed without repercussions, and the destruction of the dodo illustrates the ripple effect that occurs when one species is eliminated. As it turned out, the bird had a symbiotic relationship (see Symbiosis) with the dodo tree, or Calvaria major, whose fruit it ate, thus releasing the seeds to germinate. With the dodo gone, the dodo tree stopped being able to reproduce. Fortunately, it is a species with a long life, and some specimens of C. major continue to survive after some 300 years; when those die, however, this species, too, will be extinct.

In the case of species introduced by humans, some were introduced deliberately and were intended to improve conditions for some human activity (for example, in agriculture) or to achieve desired aesthetic results—for example, when colonists wanted to plant a flower or tree that reminded them of home. Other introductions have been accidental, as when plants were brought with soil transported as ballast in ships or insects were conveyed with timber or food. BENEFICIAL AND HARMFUL I N T R O D U C T I O N S . Some introduced

An introduced species is one that has been spread to a new environment or habitat as a result of human activity. An invasive species may or may not have been spread by humans (the ones we discuss were), but as the name suggests, it threatens an aspect of the habitat to which it has been introduced. Both introduced and invasive species are examples of exotic species, or species that

species have been wildly successful. In fact, most agricultural plants and animals are introduced species: for example, wheat (Triticum aestivum) was originally native only to a small region of the Middle East, but it now grows virtually anywhere conditions are suitable for its cultivation. Likewise, corn, or maize (Zea mays), has spread far beyond its home in Central America. The domestic cow (Bos taurus) once lived only in Eurasia and the turkey (Meleagris gallopavo) only in North America, but today these species can be found throughout the world. If all introduced species were like cows and corn, or turkeys and

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wheat, there would not be much cause for alarm. Many introduced species are invasive, however, and pose a wide variety of threats—threats to their environments or, in some cases, to human well-being. All manner of weeds and pests are among the nefarious roll-call of invasive species, a broad grouping that ranges from nuisances to serious dangers. A C C I D E N TA L A N D D E L I B E R AT E I N T R O D U C T I O N S . There are more

than 30,000 introduced species in the United States, and most of them enhance rather than diminish the quality of life. For example, there are the many species introduced by colonists to make them feel more comfortable in their new homes, among them, the Norway maple (Acer platanoides), linden (Tilia cordata), horse chestnut (Aesculus hippocastanum), and other trees as well as many exotic species of shrubs and herbaceous plants. The European settlers also introduced some species of birds and other animals with which they were familiar, such as the starling (Sturnus vulgaris), house sparrow (Passer domesticus), and pigeon, or rock dove (Columba livia). These are all deliberate introductions; on the other hand, accidental introductions are more likely to be undesirable. When cargo ships from Europe did not have a full load of goods, they had to carry other heavy material as ballast, to help the vessel maintain its stability on the ocean. Early ships to the New World often used soil as ballast, and upon arriving, sailors dumped this soil near the port. In this way, many European weeds and other soil-dwelling organisms arrived in the Americas. In addition, ships have used water as ballast since the late nineteenth century, and many aquatic species have become widely distributed by this practice. This is how two major pests, the zebra mussel (Dreissena polymorpha, discussed later) and the spiny water flea (Bythothrepes cederstroemii) were introduced to the Great Lakes from European waters. Several European weeds are toxic to cattle if eaten in large quantities, and when these plants become abundant in pastures, they represent a significant potential problem. Some examples of toxic introduced weeds in the pastures of North America include common Saint-John’s-wort (Hypericum perforatum), ragwort (Senecio jacobaea), and common milkweed (Asclepias syriaca). Several introduced insects have become troublesome pests in forests, as is the case with

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the gypsy moth (Lymantria dispar), which has defoliated many trees since its introduction to North America from Europe in 1869. Similarly, the introduced elm bark beetle (Scolytus multistriatus) has helped spread Dutch elm disease, itself caused by an introduced fungus, Ceratocystis ulmi. It would be interesting to note the irony inherent in this affliction, which at first glance seems to involve another apparently introduced species, the “Dutch elm.” There is no such tree, however; the name refers to the fact that the disease arrived in America from Holland, probably some time after World War I. Its principal victim is the American elm, or Ulmus americana. D E L I B E RAT E A N D H A R M F U L I N T R O D U C T I O N S . Not all harmful intro-

duced species were introduced accidentally. Settlers from Europe deliberately brought pets, such as the domestic dog (Canis familiaris) and cat (Felis catus); while these pets may add greatly to the quality of human life, they can cause problems, because they are wide-feeding predators. Such creatures threaten vulnerable animals in many places, especially isolated oceanic islands. Among other predators are mongooses (family Viverridae), often introduced to get rid of snakes, as well as omnivores, such as pigs (Sus scrofa) and rats (Rattus spp.) Meat-eating animals are not the only threat: herbivores such as sheep (Ovis aries) and goats (Capra hircus) also endanger plant life in some areas as a result of overgrazing. A particularly striking example of harmful, deliberate species introduction is the Nile perch (Lates niloticus). First introduced to Africa’s Lake Victoria in the 1950s, it has proved an economically important food source, with a large worldwide market. The problem is that the Nile perch is an extraordinarily active predator and has brought about a tragic mass extinction of native fishes. Until the 1980s, Lake Victoria supported an extremely diverse community of more than 400 species of fish, mostly cichlids (family Cichlidae), with 90% of those species being endemic, meaning that they exist only in one area. About one-half of the endemic species are now extinct in Lake Victoria because of predation by the Nile perch, although some species survive in captivity, and a few are still in the lake. K I L L E R B E E S , Z E B RA M U S S E L S , A N D KU D Z U . Three notable

examples of invasive species in America are

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THE AFRICANIZED,

OR

“KILLER,”

BEE IS ONE EXAMPLE OF AN INVASIVE SPECIES.

THESE

BEES HAVE NO MORE VENOM

THAN DOMESTICATED HONEYBEES, BUT THEY ATTACK MORE QUICKLY AND IN GREAT NUMBERS AND HAVE SPREAD THROUGHOUT MUCH OF

SOUTH

AND

CENTRAL AMERICA, TEXAS,

AND

CALIFORNIA. (© S. Camazine/Photo Researchers. Repro-

duced by permission.)

Africanized honeybees (Apis mellifera scutellata), better known as “killer bees”; the zebra mussel (Dreissena polymorpha); and kudzu (Pueraria lobata). The first “killer” bees were released accidentally by a Brazilian bee breeder in 1957. These aggressive insects have no more venom than domesticated honeybees (another A. mellifera subspecies, which is also an Old World import), but they attack more quickly and in great numbers. Interbreeding with resident bees and sometimes traveling with cargo shipments, Africanized bees have spread at a rate of up to 200 mi. (320 km) a year and now threaten humans, fruit orchards, and domestic bees throughout much of South and Central America and north to Texas and California. The zebra mussel was introduced to the Great Lakes in about 1985 in ballast water dumped by a ship or ships arriving from Europe. It colonizes any hard surface, including rocks, wharves, industrial water-intake pipes, and the shells of native bivalve mollusks. A bean-sized female zebra mussel can produce 50,000 larvae (an immature form of an animal) in a single year. Growing in masses with up to 70,000 individuals per square foot, zebra mussels clog pipes, suffo-

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cate native clams, and destroy the breeding habitats of other aquatic animals. These invaders have placed a great burden not only on the environment but also on the economy of the Great Lakes region: area industries spend hundreds of millions of dollars annually to unclog pipes and equipment. Kudzu is an integral part of culture in the southern United States, but it originated in Japan and did not arrive on American shores until 1876. In that year, numerous foreign governments sent exhibits to the Centennial Exposition, held in Philadelphia to honor the country’s 100th birthday. Two generations later, during the Great Depression, the U.S. Soil Conservation Service began promoting the use of kudzu for erosion control. At a time when work was scarce, young men in the government-sponsored Civilian Conservation Corps (CCC) earned a living by planting kudzu throughout the South. The federal government paid farmers as much as $8.00 an acre— a fabulous sum at the time—to plant kudzu fields. Before another generation had passed, in 1953, the federal government stopped promoting the use of kudzu. In 1972, just four years shy of a

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Species

KEY TERMS

first name identifying the genus and the

Species that have been introduced to a region or continent, usually but not always through human activity. See also introduced species and invasive species.

second the species. The genus name is

EXTINCTION:

BINOMIAL

NOMENCLATURE:

A

system of nomenclature in biological taxonomy whereby each type of plant or animal is given a two-word name, with the

always capitalized and abbreviated after the

A condition in which all members of a taxon have ceased to exist.

first use, and the species name is lower-

FAMILY:

cased. Both are always shown in italics— thus, Homo sapiens and, later in the same document, H. sapiens.

obligatory taxonomic classification ranks, after phylum but before order. DNA:

Deoxyribonucleic acid, a mole-

cule in all cells, and many viruses, containing genetic codes for inheritance. ENDANGERED

SPECIES:

Any

plant, animal, or microorganism that is at risk of becoming extinct or at least of disappearing from a particular local habitat. ENDEMIC

SPECIES:

Species that

exist in only one geographic region. EUKARYOTE:

The third most specific of the seven obligatory ranks in taxonomy, after order but before genus.

A unit of information about a particular heritable (capable of being inherited) trait that is passed from parent to offspring, stored in DNA molecules called chromosomes. GENE:

The third most general of the

CLASS:

A cell that has a nucleus

as well as organelles (sections of the cell that perform specific functions) bound by membranes.

century after its first introduction, kudzu was officially declared a weed by the U.S. Department of Agriculture. Obviously, something had gone wrong. The problem was that kudzu grew too well—so fast, in fact, that in the minds of many southerners, it began to possess some sort of mystical significance. This preoccupation with kudzu is reflected in the work of several Georgians, whose state has been particularly afflicted with the vine. There is the poem “Kudzu” by James Dickey as

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EXOTIC SPECIES:

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The sum of all the genes shared by a population, such as that of a species. GENE POOL:

The second most specific of the obligatory ranks in taxonomy, after family but before species. GENUS:

HYBRID: The product of sexual union between members of two species or other smaller and less genetically separate groups, such as two races. In the case of species hybrids, the process of hybridization involves genetic abnormalities that lead in most cases to sterility.

well as the cover of Murmur, the music group R.E.M.’s 1983 debut, which features a photograph of a kudzu-covered railroad trestle near the group’s hometown of Athens. Kudzu covered more than railroad tracks, and in the mid–twentieth century, it began to seem as though it would cover the entire South with its tangled vines. The plant is capable of growing by as much as 1 ft. (0.3 m) per day during the summer and can cover virtually anything that is not moving. Over the course of a good

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Species

KEY TERMS INTRODUCED SPECIES:

A species

CONTINUED

The second most general of

PHYLUM:

that has been spread to a new environment

the obligatory taxonomic classification

or habitat, whether deliberately or acciden-

ranks, after kingdom and before class.

tally, as a result of human activity. Introduced species, like invasive species, are considered exotic species. INVASIVE

SPECIES:

An

exotic

species that threatens some aspect of the habitat to which it has been introduced. KINGDOM:

The highest or most gener-

A cell without a nucle-

PROKARYOTE:

us. RNA:

Ribonucleic acid, the molecule

translated from DNA in the cell nucleus, the control center of the cell, that directs protein synthesis in the cytoplasm, or the space between cells.

al ranking in the obligatory taxonomic system. In the system used in this book there

SPECIATION:

The divergence of evo-

are five kingdoms: Monera, Protista, Fungi,

lutionary lineages and creation of new

Plantae, and Animalia.

species.

MORPHOLOGY:

Structure or form, or

The most specific of the

SPECIES:

seven obligatory ranks in taxonomy.

the study thereof. NATURAL SELECTION:

The process

whereby some organisms thrive and others

Species often are defined as a population of individual organisms capable of mating

perish, depending on their degree of adap-

with one another and producing fertile off-

tation to a particular environment.

spring in a natural setting. Also, members

NOMENCLATURE:

The act or process

of the same species share a gene pool. A taxonomic group or entity.

of naming or a system of names—particu-

TAXON:

larly one used in a specific science or disci-

TAXONOMY:

pline. See also binomial nomenclature.

sciences devoted to the identification,

The area of the biological

The middle of the seven oblig-

nomenclature, and classification of organ-

atory ranks in taxonomy, more specific

isms according to apparent common char-

than class but more general than family.

acteristics.

ORDER:

year, kudzu can grow by as much as 60 ft. (20 m), and it has proved impervious to many herbicides. One herbicide used in Auburn, Alabama, actually made it grow better! Thanks to the development of better chemical treatments, and the use of grazing animals, such as goats, kudzu no longer is perceived as such a great threat. Additionally, various entrepreneurs and scientists have set out to make use of the vine in weaving baskets or in preparing foods and medicines. Ground kudzu

root, called kuru, has long been used in foods and medications in China and Japan.

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VOLUME 3: REAL-LIFE BIOLOGY

One might wonder why Japan is not covered in kudzu and why kudzu is not crawling up the Great Wall of China. The answer is more than a little interesting from a biological standpoint. When kudzu was transplanted to America, it was taken out of its native environment and thus away from the local insects that threatened its growth. In its new home there were no threats to its spread, and with no obstacles in its way, it began to take over 213

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Species

the South. (For more about the development of species, see Speciation. See also the discussion of keystone and indicator species in Food Webs.) WHERE TO LEARN MORE All Species Foundation (Web site). . “Endangered Species on EE-Link.” EE-Link (Environmental Education Link), North American Association for Environmental Education (Web site). .

214

Levy, Charles K. Evolutionary Wars: A Three-Billion-Year Arms Race—The Battle of Species on Land, at Sea, and in the Air. New York: W. H. Freeman, 1999. Schilthuizen, Menno. Frogs, Flies, and Dandelions: Speciation—The Evolution of New Species. New York: Oxford University Press, 2001. Schwartz, Jeffrey H. Sudden Origins: Fossils, Genes, and the Emergence of Species. New York: John Wiley and Sons, 1999. Species 2000 (Web site). .

Integrated Taxonomic Information System (ITIS), United States Department of Agriculture (Web site). .

Van Driesche, Jason, and Roy Van Driesche. Nature out of Place: Biological Invasions in the Global Age. Washington, DC: Island Press, 2000.

Invasive Species, National Agricultural Library, U.S. Department of Agriculture (Web site). .

Vergoth, Karin, and Christopher Lampton. Endangered Species. New York: F. Watts, 1999.

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Speciation

CONCEPT One of the defining characteristics of a species is its reproductive isolation: the fact that among animals and plants that reproduce sexually, it is impossible for members of two different species to mate and produce fertile offspring. Speciation is the process whereby a single species develops over time into two distinct, reproductively isolated species. It is one of the key evolutionary processes and is responsible for the diversity of life that exists on Earth. In the following essay we explore not only the basic facts of speciation and biological diversity but also an example of adaptive radiation, in the form of the wide range of species within the mammalian order.

HOW IT WORKS Species and Speciation The concept of species, discussed in the article devoted to that subject, is an extraordinarily complex one. Owing to limitations of space, that essay only hints at the many details, the competing schools of thought, and the varying definitions of species. Likewise, in the present context, it is possible to examine the concept of speciation only in the most cursory fashion. In addition to consulting the essay on Species for more information, the reader is encouraged to review the article on Taxonomy. Taxonomy is the area of the biological sciences devoted to the identification, nomenclature, and classification of organisms according to apparent common characteristics. It uses a wide array of specialized rankings for grouping animals, but only seven of them are essential to most

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S P E C I AT I O N

biology students. These seven, known as the obligatory hierarchy, are kingdom, phylum, class, order, family, genus, and species. In the case of mammals, it is also useful to refer to subphylum, which in this case is Vertebrata (see the classification of humans in Species), but for the most part it is enough for the beginning student to attain at least some mastery of the obligatory ranks. Note that species is the most specific of these ranks, which is fitting, because species and specific come from the same Latin root, specie, or “kind.” Nonetheless, it is difficult to define species beyond a reference to its place among the categories of the obligatory taxonomy. According to the biological species concept, discussed briefly in Species, a species is any population of individual organisms capable of mating with one another and producing fertile offspring in a natural setting. This is far from the only definition, however. I N T E R S P E C I F I C M AT I N G . Occasionally, it is possible to produce an infertile hybrid, such as a mule, which is created by the mating of a male donkey and a female horse, or a hinny, the product of the less common union between a male horse and a female donkey. The infertility is due to genetic disorders that arise when mating takes place between distinct species, and even this imperfect product is possible only by mating two species that are very closely related. Donkeys and horses, for instance, both belong to family Equidae, which makes them very closely connected.

In the taxonomic ranking of humans, this would be equivalent to a human mating with a fellow hominid, or member of family Hominidae. If the long-extinct genus Australopithecus were

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group, along with all other mammalian orders, later in this essay.)

Speciation

THE PROBLEM OF DEFINING S P E C I E S . Although the biological species

concept is accepted widely, it has its shortcomings, not least of which is the fact that not all species reproduce sexually. Although sexual reproduction is the case with a wide array of animals and even plants, quite a few organisms reproduce by some asexual means: for example, single-cell organisms reproduce by splitting.

A

SPECIES IS A POPULATION OF INDIVIDUAL ORGANISMS

CAPABLE OF MATING WITH ONE ANOTHER AND PRODUCING FERTILE OFFSPRING.

OCCASIONALLY,

IT IS POSSI-

BLE TO PRODUCE AN INFERTILE HYBRID, SUCH AS A MULE, THROUGH THE MATING OF A MALE DONKEY AND

Among the competing definitions of species is the phenetic (or morphological) species concept, which relies in part on common sense. According to the phenetic species concept, a species is the smallest possible population of organisms that consistently and continually remains distinct and distinguishable by ordinary methods of observation. There are also a variety of definitions that fall under the heading “phylogenetic species concepts,” all of which maintain in one way or another that taxonomic classifications should incorporate the most widely recognized hypotheses regarding the evolutionary lines of descent that produced the organisms in question.

A FEMALE HORSE. (© D. Robert & Lori Franz/Corbis. Reproduced

by permission.)

still around, it is not inconceivable that humans could mate with them and produce at least sterile offspring. Of course, it is unlikely that many humans would want to mate with Australopithecus, the most famous example of which was named “Lucy” after the Beatles’ song “Lucy in the Sky with Diamonds.” Standing about 3.5-5 ft. (1–1.5 m) tall, Australopithecus was very close in appearance to a modern ape who lived about four million years ago. All of humans’ close relatives are extinct, and today our nearest relatives are members of the order Primates: apes, monkeys, and marsupials. It is impossible to imagine a human mating with one of these animals and producing offspring of any kind. Likewise, it is extremely unlikely that a horse or donkey could mate with a tapir or rhinoceros, which are about as distant in relation to them as other primates are to us. (These species all belong to the order Perissodactyla, herbivorous mammals possessing either one or three hoofed toes on each hind foot. We discuss this

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The Process of Speciation Clearly, there is no hard and fast definition of species, but in general terms, everyone who has some familiarity with the concept has at least a basic knowledge of what does and does not qualify as a species. We will leave finer distinctions to trained taxonomists and other biologists and move on to a fact regarding which there is no disagreement: a wide array of species exists in the world today. Some estimates calculate the number of species in the five kingdoms—animals, plants, monerans, protista, and fungi (see Taxonomy for a very brief identification of each)—at about 1.5 million. This is only the number of identified species, however. Other figures, based on the probable numbers of unidentified species in the world, put the sum total in the tens of millions. Whatever the case, it is obvious that over the course of evolutionary history (discussed in Evolution and Paleontology), there has been a widespread adaptive radiation—that is, a diversification of species as a result of specialized adaptations by particular populations of organisms.

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Speciation events are described as either allopatric or sympatric. Allopatric (“different places”) speciation occurs when a population of organisms is divided by a geographic barrier, a great example being the division of squirrel species caused by the formation of the Grand Canyon (see Evolution). Another example is the speciation of the black-throated green warbler, which today consists of one species in the eastern United States, along with three others in the western part of the country. Some scientists speculate that there may once have been a single species of black-throated green warbler, whose population was split by the formation of a glacier during the Pleistocene epoch. The latter was the period of the last ice age, which ended about 10,000 years ago, but the end of the ice age was a slow process. It may be that glaciers, formed in the latter part of that time, helped to separate what became three different western species. Species share the same gene pool, or the sum of all genetic codes possessed by members of that species. The isolation of two populations slowly results in differences between gene pools, until the two populations are unable to interbreed either because of changes in mating behavior or because of incompatibility of the DNA between the two populations. (Deoxyribonucleic acid, or DNA, contains genetic codes for inheritance. See Genetics for more on this subject.) More rare than allopatric speciation, sympatric (“same place”) speciation happens when a group of individuals becomes reproductively isolated from the larger population of the original species. This type of speciation typically results from mutation, or alterations in DNA that result in a genetic change. Studies of three-spined sticklebacks, a variety of freshwater fish, in British Columbia have revealed what appears to be a fascinating example of sympatric speciation. Evolutionary biologist Dolph Schluter and others have discovered that the region contains two species of stickleback, one with a large mouth that feeds on large prey close to shore, the other with a small mouth that feeds on plankton in open water. Both species jointly inhabit five different lakes. Through DNA analysis, scientists have determined that the lakes were colonized independently by common marine ancestors, meaning that the process of sympatric speciation between the two varieties had to have occurred independently at least five times. This seems to indicate a

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situation of competition for resources that favored stickleback species at either extreme of size, as opposed to those of medium size and medium-sized mouths.

Speciation

R AT E OF EVOLUTIONARY C H A N G E . Closely tied to speciation is the

rate of evolutionary change, or the speed at which new species arise. This is a long process, one that is usually not observable within a human lifetime or even the span of many lifetimes, though bacteria at least have shown some evolutionary change in their growing resistance to antiobiotics (see Infection). DNA analysis (see Genetics and Genetic Engineering for more about DNA) has been used to examine the rate of evolutionary change. To perform such analysis, it is necessary first to determine the percentage of similarity between the organisms under study: the greater the similarity, the more recently the organisms probably diverged from a common stock. Data obtained in this manner then must be corroborated by information obtained from other sources, such as the fossil record and comparative anatomy studies. At certain times the rate of evolutionary change can be very rapid, leaving little fossil evidence of intermediate forms, a phenomenon known as punctuated equilibrium. This is contrasted with phyletic (that is, evolutionary) gradualism. Of course, the term rapid in this context is relative, since we are talking about vast spans of time. Life on Earth has existed for about 3,000 million years, and the fossil record goes back some 1,000 million years. This is the case, in part, because to leave fossilized remains, an organism must have “hard parts” that can become mineralized to turn into fossils. (See Paleontology for more on these subjects.)

REAL-LIFE A P P L I C AT I O N S The Diversity of Mammals One of the most interesting examples of speciation is that which has produced the vast array of species, including humans, that fall within the mammalian class. Mammals began evolving before the dawn of the Cenozoic era about 65 million years ago. The Cenozoic era, which started with a catastrophic event that brought about the mass extinction of the dinosaurs and

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Speciation

the end of the Mesozoic era (see Paleontology), is truly the age of the mammal. Just as dinosaurs dominated the Mesozoic, today the world belongs to mammals as to no other class of creature.

combined aspects of both reptiles and mammals walked the earth.

Since its humble beginnings in the shadow of the dinosaurs, class Mammalia has undergone a massive radiation to the point that today some 4,625 species of mammal, in about 125 families and 24 orders, are recognized. (That number is changing, as noted later in the context of elephants.) This diversity is tied closely to mammals’ enormous mobility, which facilitated their spread throughout the world. Aside from much less complex life-forms, such as arachnids and insects (see Parasites and Parasitology), mammals are believed to be distributed more widely throughout the world than any other comparable taxonomic grouping. Insects may be the most diverse of all animal classes, with numbers of species that may be many times greater than the number of mammals, but considering mammals’ muchgreater level of physical development and complexity, the diversity of their species is astounding.

The listing of the 20 orders of living mammals that follows is arranged not alphabetically but in the probable order in which these groups evolved. (This is not to imply that the process was orderly or linear; it was not.) Very few dates are given, simply because there is much dispute in most cases. Numbers of species within each order are also a subject of debate among taxonomists, and therefore these numbers are not always precise.

MAMMALS’ E A R LY EVOLUT I O N . In the next section we list the orders of

mammals and give very brief descriptions of each. The purpose here is not to provide anything like a comprehensive discussion but rather to illustrate the enormous range of species in a class that includes anteaters, dolphins, humans, elephants, and bats. The fact that all these diverse creatures, and many more, emerged from a common evolutionary lineage is almost as amazing as the fact that this common ancestor was a reptile.

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Mammalian Orders

In the essay, Species, there is a taxonomic listing of the obligatory ranks for humans; included within that listing is a short description of the kingdom (Animalia), phylum (Chordata), and subphylum (Vertebrata) to which mammals belong. Mammal itself is defined as a vertebrate (an animal with a spinal column) that feeds its young from special milk-secreting glands, termed mammae, located on the mother’s body. Mammals are warm-blooded or endothermic, meaning that their internal temperatures remain relatively stable, and their bodies usually are covered with hair. They have other distinguishing characteristics as well, such as a relatively large cranium (skull) with a hinged lower jaw attached to it. M O N O T R E M E S . Order Monotrema consists of primitive, egg-laying mammals spread throughout parts of the region known as Oceania, which includes Australia, New Zealand, and islands of the southeastern Pacific. The habitat of this order lies specifically in Australia, Tasmania, and New Guinea. Monotremes, as they are called, are distinguished further by the fact that their mammary glands are without nipples, that teeth are present only in the young, and that adults have horny beaks.

Mammals are believed to have come from the reptilian order Therapsida, which emerged during the Triassic period (from about 245 to 208 million years ago) in the early part of the Mesozoic era. Over the course of many millions of years, these creatures began to develop a number of mammal-like qualities—in particular, endothermy, or the ability to maintain internal temperature regardless of environmental conditions. In other words, these cold-blooded creatures became warm-blooded. This evolutionary process was as complex as it was lengthy. Nor was there a clean break with the past—no moment when the therapsids faded away or when it would have been clear that mammals had taken the place of their reptilian ancestors. Rather, in what must have been a fascinating taxonomic situation, for many millions of years, species that

M A R S U P I A L S . The marsupials, or order Marsupialia, include two other famous ani-

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The monotremes illustrate the fact that to be constituted as an order or family, a taxon, or taxonomic group, need not have large numbers. The entire order consists of a single existing species, the duck-billed platypus, which constitutes a family of its own, and two species of echidnas, creatures that look like a cross between a platypus and a porcupine. (The “porcupine” look comes from the fact that their bodies are covered in spines, or spiky protrusions.)

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mal citizens of Oceania: the kangaroo and its close relative, the wallaby. Marsupials’ young are poorly developed at birth and must continue to grow while attached to their mothers’ nipples. For this reason, they must remain close to the mother, and therefore natural selection for marsupials favored those strains in which females possess a pouch bearing four teats.

Speciation

Immediately after birth, the young marsupial (a kangaroo baby is called a joey) installs itself in the mother’s pouch. Given this situation, marsupials can support only one offspring a year and thus are not given to the large litters that characterize another order, Carnivora, which we discuss later. Kangaroo offspring remain in the pouch until the age of about 7-10 months, by which time the mother has conceived again; the female kangaroo goes into heat just a few days after giving birth. The embryonic kangaroo remains in a state of dormancy, or arrested development, until the older sibling has left the pouch. The marsupial order (some authorities call it a superorder, with numerous subordinate orders) consists of some 240 species. The greatest number of these, including many species of kangaroo, wallaby, wombat, and koala, are found exclusively in Australia. Some 70 additional species are scattered across parts of Oceania, including Australia, Tasmania, New Guinea, and smaller islands. There are an additional 70 species in the Americas, including four species of the genus Didelphis—the large American opossum, better known in the southern United States as possums. Why the preponderance of marsupials in Australia and Oceania as a whole? The reason lies in Earth’s geologic history, which has seen regular collisions and divisions of the continents, which are even today shifting slowly under our feet. It appears that prior to about 70 million years ago, at a time when most of Earth was united in a single supercontinent called Pangaea, marsupials originated in what is now North America and migrated to the land masses that became Australia and Oceania. Because the bulk of marsupial species remained on Australia and nearby areas when Pangaea began to break apart, marsupials underwent much greater speciation there than in North America.

MARSUPIALS’

YOUNG ARE POORLY DEVELOPED AT BIRTH

AND MUST CONTINUE TO GROW WHILE ATTACHED TO THEIR MOTHERS’ NIPPLES. THE YOUNG KANGAROO

IMMEDIATELY AFTER BIRTH, (CALLED A JOEY) INSTALLS

ITSELF IN THE MOTHER’S POUCH AND REMAINS THERE UNTIL

THE

AGE

OF

7-10

MONTHS.

(© Michael S.

Yamashita/Corbis. Reproduced by permission.)

either lack teeth or have very small ones. Evolutionary development has adapted the forward limbs of these creatures for digging or for holding on to the branches of trees. Included in this order are some 30 species of sloth, anteater, and armadillo. Sloths are herbivores, armadillos are omnivores (i.e., they eat plants and small animals), and anteaters, as their name would suggest, are hard-core insectivores.

and edentates, members of order Xenarthra

The term insectivore can refer to any organism that lives by eating insects, but it is also the name for members of the order Insectivora, which includes shrews, hedgehogs, moles, and various other, less well known groups. Some 400 species, of which about 300 are shrews in a single family (Soricidae), make up this order. Not only their diet but also their pointed snouts and rodent-like appearance distinguishes this group. Many, but not all, are diggers, like the xenarthrans. Like all mammals, they have the pentadactyl limb (an appendage with five digits,

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X E N A RT H RA N S , INSECTIV O R E S , S CA N D E N T I A , A N D D E R M O P T E RA . Known variously as xenarthrans

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like the human arm and hand)—in their case, a foot with five toes. Order Scandentia, which is identical with the family Tupaiidae, or tree shrews, is sometimes grouped with order Insectivora. Despite their name, tree shrews, of which there are five genera and between 15 and 19 species, may live either on the ground or in the trees. Squirrel-like in appearance, they have strong claws on all their toes and are excellent climbers. Another very small order of tree-dwellers is Dermoptera, which consists of just two species of flying lemur. Found in Indonesia and the Philippines, these creatures are equipped with skin flaps adapted for gliding. This aspect of their morphological makeup calls to mind the “flying” squirrel, but their nocturnal habits are more like those of lemurs (discussed later, with other primates); hence their common name. C H I R O P T E RA N S . Among the most

fascinating of mammalian orders is Chiroptera, better known as bats. This order, which consists of about 900 species in some 175 genera, is the only group of truly flying mammals, as opposed to the “flying” lemurs we just discussed. Yet they, too, have the pentadactyl limb, only in their case the forelimb has been adapted as a wing. Among the intriguing features of bats is their use of acoustic orientation, or echolocation, to find their way through the dark caves and nocturnal exteriors that make up their world. Contrary to popular belief, they are not blind, but they do have very small eyes, simply because vision is not important for bat navigation. (See Migration and Navigation for more about this subject.) As befits an order with such a wide array of species, bats run the gamut with respect to their eating habits. The majority of bat species are insectivores that consume many thousands times their weight in insects each year. Many others are fruit bats, important members of the ecosystems they occupy, because they consume fruit and spread seeds, helping assist in seed dispersal. (Some bats also aid in pollination; see Reproduction). Then there are the three species of vampire bat, which are largely responsible for bats’ unfortunate reputation with humans.

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from the southwestern United States to the northwestern third of South America. As one would suspect of a creature named vampire, they live off blood, which they suck at night from birds or other mammals, including humans. These creatures are livestock pests and strike fear in humans both because of the imaginary association with vampires and for the quite real threat of contracting rabies from them. The vast majority of bats, however, are creatures that cause no harm to humans and often are unfairly persecuted as the result of human prejudices. Even in the case of the vampire bat, there is a strong possibility that it may one day help save human lives. Scientists have discovered that the saliva of Desmodus rotundus is better than any other known substance for keeping blood from clotting; therefore, vampire bat saliva may one day be adapted for use in treating heart attacks and strokes. P R I M AT E S . The order of humans, Primates, falls approximately in the middle of the mammalian class in terms of evolutionary order. This is an interesting aspect of speciation, evolution, and taxonomy: even though humans themselves are the most advanced of all creatures, it is not a logical necessity that we should come from the most recently evolved order. In fact, the opposite would seem to be the case. To produce a species whose intelligence dwarfs that of all other animals, the line of descent should be a long one. Where primates are concerned, that is certainly the case. The oldest primate samples date back some 75 million years, or long before the end of the Mesozoic.

Because it is from primates that humans draw their lineage, more has been written about primate evolution than on that of all other mammalian orders combined. The subject is such a vast one that we will not attempt to approach it here, except to encourage the reader to study in more detail elsewhere the process by which the human lineage emerged from order Primates, family Hominidae, and genus Homo.

Members of subfamily Desmodontinae, species of vampire bat include Diaemus youngi and Diphylla ecaudata. However, the best-known variety is the common vampire bat, Desmodus rotundus, which is native to an area that stretches

Primates consist of two broad groups, suborders Prosimii and Anthropoidea. The first, the prosimians, includes five families (or six, since tree shrews are sometimes included) of lemurs, lorises, and tarsiers. The other suborder, known as the higher primates, encompasses another six families: marmosets and tamarins; South American monkeys other than marmosets; African and Asian monkeys; lesser apes, or siamangs and gibbons; great apes, or orangutans, gorillas, and

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chimpanzees; and humans, both living and extinct. (Most orders contain extinct members, but for the most part they are not discussed here.) Most primates are tree dwellers, and among the approximately 230 species, there is enormous variation in eating habits. Many lemurs are insectivores, while great apes tend to be fruit eaters. Quite a few are omnivores, though no primate other than humans is known for eating large mammals, such as cows, sheep, and pigs. The pentadactyl limb (an appendage with five digits) is a significant feature for primates, which alone have the advantage of the opposable thumb for grasping. Humans and a few other primate species are also the only animals with four limbs who are not only capable of standing upright but also function best in this way. CA R N I V O R E S . As with insectivore, carnivore is a name both for an eating preference—in this case, meat—and for members of a primate order, Carnivora. Most members of this extraordinarily varied group eat meat, including, in some cases, the “meat” of insects. Bears and some other species are omnivorous, meaning that they also eat plants, and hyenas and jackals are classic examples of detritivores, or animals who feed on the remains of other creatures.

The distinction between detritivore and carnivore relates not to the materials each consumes but to their place in the food web. Rather than consume live creatures, hyenas and jackals feed on the carcasses of dead ones. Usually these creatures are artiodactyls (discussed later), such as antelopes, which have been killed by other carnivores—big cats, such as the lion or cheetah. After the big cats have fed on the fleshy parts of the prey, hyenas come to consume the flesh that remains, and they are followed by jackals and vultures, swoop in to pick the bones. These detritivores help process the remains of formerly living things, which ultimately return to the soil. (See Food Webs for more on this subject.) Clearly, all members of order Carnivora eat meat, though in different ways and sometimes in combination with fruit or other vegetation. Natural selection has equipped them for this purpose with sharp claws and teeth. Carnivora includes some 270 species grouped into ten families, listed here: • Canidae (dogs, wolves, jackals, and foxes) • Felidae (cats)

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• Hyaenidae (hyenas) • Mustelidae (skunks, mink, weasels, badgers, and otters) • Otariidae (eared seals) • Odobenidae (walrus) • Phocidae (earless seals) • Procyonidae (raccoons) • Ursidae (bears) • Viverridae (mongooses and civets). Note that Felidae is a particularly varied family of some 36 species: lions, lynxes, tigers, leopards, and even ordinary domesticated cats. Thirty-five species belong to a single subfamily, Felinae, which is native to most parts of the world other than Australia, Madagascar, most oceanic islands, and, of course, Antarctica. The last species, the cheetah (Acinonyx jubatus), is segregated into another subfamily, Acinonychinae, primarily because this cat, native to Africa and southwest Asia, is a daytime hunter, unlike its nocturnal cousins.

Speciation

C E TAC E A N S A N D S I R E N I A N S .

Orders Cetacea and Sirenia include the majority of aquatic mammals, as opposed to the many amphibious mammals, such as seals, sea lions, sea elephants, and walruses, that belong variously to families Otariidae, Odobenidae, and Phocidae of the order Carnivora. Cetaceans include whales, dolphins, and porpoises, while sirenians are made up of just three species of manatee and one of dugong. Sirenians are large, friendly creatures that inhabit the Atlantic coast and tributary rivers (manatee) or the Indian and Pacific coastlines (dugong), but cetaceans are much more familiar. With cetaceans, two questions, one specific and one general, often arise. The answer to the first of these, “What is the difference between a dolphin and a porpoise?,” is that a porpoise is smaller and more chubby and has a blunt snout, whereas a dolphin has a beaklike snout. Some taxonomists and marine biologists put porpoises in the same family as dolphins, whereas others treat them as two different families. The more general, and much more important, question is “Why are these mammals living in the water?” In fact, life itself first appeared in the sea, so perhaps the question should be “Why or how did anything start living on land?” The transition from water to land took place long before the age of the dinosaurs, much less the emergence of mam-

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mals, but later, some mammals began to return to the water, probably about 70 million years ago. Certainly, there is no question that cetaceans are mammals, a fact first recognized by Aristotle (384–322 B.C.), a Greek thinker who is noted not only as one of the greatest philosophers of all time but also as the father of the biological sciences. (See Taxonomy for more about Aristotle’s contributions.) As Aristotle observed, whales and dolphins bear live young and suckle them with milk-producing glands; their bodies have hair, albeit only very small strands; and they possess lungs, breathing air through a blowhole. As evidence of their terrestrial, or landbased, origins, consider that a whale fetus possesses the remnants of four limbs, each with five fingers (the pentadactyl limb), like any land mammal. Adult whales and dolphins have the streamlined, fishlike morphological appearance that is necessary for life underwater, but their resemblance to fishes is of the superficial, analogous variety discussed in Taxonomy. They have maintained and modified key terrestrial features; for example, a blowhole atop the head—one in dolphins, two in whales—replaces the nostrils, and thus the passageways for food and air are completely separate. This differs from the situation with most terrestrial mammals, which take in food and air through the same opening. PROBOSCIDEANS, PERISSODAC T Y L S , A N D H Y RAX E S . Moving

from the largest aquatic mammals, the whales, to the largest terrestrial variety, we come to the order Proboscidea, which includes elephants. Our discussions of most orders in class Mammalia have illustrated particular aspects of taxonomy and speciation, and so it is with proboscideans, which give evidence of the many species from the past that are gone forever. The order is a large one, with three suborders and some 300 species, but anyone who searches for most of those species will search in vain. All but three species are extinct. These three are the Asian elephant (Elephas maximus) and two varieties of African elephant (Loxodonta africana or African savanna elephant and Loxodonta cyclotisare, the African forest elephant). Until 2001, taxonomists believed that there were only two living species of elephant. (See Taxonomy for more on this subject.)

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one or three hoofed toes. Included among perissodactyls is another large animal from the grasslands of Africa and tropical Asia: the rhinoceros. The group also encompasses donkeys, zebras, and tapirs, but by far the most important in human terms is Equus caballus, the domesticated horse. Described by the French zoologist Comte Georges de Buffon (1707–1788) as “the proudest conquest of man,” the horse was domesticated (adapted so as to be useful and advantageous for humans) some 6,000 years ago. Nonetheless, feral or wild horses remain an important subspecies. Order Hyracoidea also consists of hoofed mammals: seven species of hyrax, a primarily herbivorous creature native to Africa and the far southwestern extremities of Asia. Hyraxes are sometimes lumped in with pikas under the term rock rabbit, but, in fact, pikas are lagomorphs, a group we discuss later. To further the confusion, hyraxes are probably the animal called a coney in the Bible, even though there is an animal called a cony (no “e”) that is actually a lagomorph. This is just one of many examples of confusion resulting from the complexities of the animal world and humans’ attempts to name and classify its members. T U B U L I D E N TAT E S . For most orders, the lowercase adjectival name (e.g., primates, carnivores, insectivores, and so on) is commonly used. On the other hand, the names hyracoidean and tubulidentate are seldom used for more obscure groups, such as Hyracoidea and Tubulidentata. If any order of mammal is obscure, it is Tubulidentata, which emerged some 60 million years ago and which consists of a single species: the African ant bear (Orycteropus afer). The latter creature is better known by the name aardvark, which in the Afrikaans language means “earth pig.”

Another 16 species belong to the order Perissodactyla, herbivores whose hind feet bear either

Aardvarks at first glance might seem to belong with anteaters, sloths, and armadillos in the order Edentata, and that is what taxonomists thought for a long time. The latter half of the name tubulidentate, however, suggests the area of differentiation: the teeth. Aardvarks’ teeth are unique among those of all mammals. Viewed from the top, the aardvark’s jawbone is V-shaped, with the teeth midway along either side of the V. The teeth themselves are not fixed to the jaw but rest in the flesh attached to it, and instead of being covered with enamel, they are protected with a cementlike substance. The substance comes from tubules that run under the teeth.

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ORDER PHOLIDOTA CONSISTS OF SEVEN SPECIES OF SCALY ANTEATER, OR PANGOLIN. THE WORD PANGOLIN comes from a Malay term meaning “rolling over,” a reference to the fact that when it is threatened, the animal curls into a little ball. (© Keren Su/Corbis. Reproduced by permission.)

A R T I O D A C T Y L S . Like many other mammalian orders we have discussed, members of order Artiodactyla are ungulates, or hoofed animals. Whereas perissodactyls have odd-numbered toes, artiodactyls have even-numbered toes—either two or four—on each foot. This large group, consisting of some 220 species, comprises a wide variety of well-known species in nine families. Among them are cows, pigs, sheep, goats, deer, antelope, bison, camels, giraffes, hippopotamuses, and numerous less well known varieties, such as okapi, pronghorn, peccaries, and deerlike chevrotains.

The camel family is particularly widespread geographically, including as it does many varieties—the llama, alpaca, and vicuña of South America—whose home is far from the habitats in the Near East that are associated with the camel. In this family is what may be a previously undiscovered species, whose existence the British Broadcasting Corporation (BBC) reported in early 2001. Living on a former nuclear weapons testing range in a remote region of Chinese central Asia, these creatures drink saltwater, which in itself is an unusual characteristic.

are threatened; fewer than 1,000 remain. As the BBC reported, this makes them more endangered than the more well known giant panda. The creatures survived nuclear testing in the area, which ceased in 1996, but they continue to be threatened by much less spectacular varieties of explosive: dynamite and land mines, planted by hungry locals. John Hare of the Wild Camel Protection Foundation told the BBC, “We found land mines put by the saltwater springs. So when the camels come to drink, they step on them. Bang! They are blown to pieces and picked up as meat.” As to whether the camels constitute a separate species, the molecular geneticist Olivier Hanotte told the BBC: “There are two possibilities here. One is that the domestic camel was bred from these wild ones some time back in history. The second is that the domestic camel we see today was bred from another species that has disappeared. This would mean that these wild camels are a totally separate species.” As of early 2002 the camels’ fate, both practically and taxonomically, remained undecided.

Although it is extraordinarily hardy, even by the standards of camels, the central Asian camels

P H O L I D O TA / PA N G O L I N S . Order Pholidota consists of seven species of scaly anteater, or pangolin. Members of this order are normally called pangolins, rather than an adjecti-

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KEY TERMS A diversifi-

specialized adaptations by particular pop-

To adapt an organism, whether plant or animal, so as to be useful and advantageous for humans.

ulations of organisms.

FAMILY:

ADAPTIVE RADIATION:

cation of species over time as a result of

DOMESTICATE:

of speciation that occurs when a popula-

The third most specific of the seven obligatory ranks in taxonomy, after order but before genus.

tion of organisms is divided by a geo-

GENE:

ALLOPATRIC SPECIATION:

A type

graphic barrier. ANALOGOUS FEATURES:

Morpho-

logic characteristics of two or more taxa that are superficially similar but not as a result of any common evolutionary origin. CARNIVORE:

A meat-eating organ-

ism, or an organism that eats only meat (as distinguished from an omnivore). CLASS:

The third most general of the

A unit of information about a particular heritable (capable of being inherited) trait that is passed from parent to offspring, stored in DNA molecules called chromosomes. The sum of all the genes shared by a population, such as that of a species. GENE POOL:

The second most specific of the obligatory ranks in taxonomy, after family but before species. GENUS:

obligatory taxonomic classification ranks, after phylum but before order. DETRITIVORES:

Organisms that feed

on waste matter, breaking organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Their function is similar to that of decomposers; however, unlike decomposers—which tend to be bacteria or fungi—detritivores are relatively complex organisms, such as earthworms or maggots. DNA:

Deoxyribonucleic acid, a mole-

cule in all cells, and many viruses, containing genetic codes for inheritance.

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HERBIVORE:

A plant-eating organism.

The product of sexual union between members of two species or other smaller and less genetically separate groups, such as two races. In the case of species hybrids, the process of hybridization involves genetic abnormalities that lead in most cases to sterility. HYBRID:

INSECTIVORE:

An

insect-eating

organism. The highest or most general ranking in the obligatory taxonomic system. In the system used in this book there are five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia.

KINGDOM:

order. Pangolins are also like aardvarks in the

val form of Pholidota. The word pangolin comes from a Malay term meaning “rolling over,” a reference to the fact that when it is threatened, the animal curls into a little ball. As with the aardvark, members of this order once were grouped with Edentata but now are considered a separate

MACROSCELIDEANS. Rodents, or mem-

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sense that their evolutionary relationship to other mammals is not clear. RODENTS, LAGOMORPHS, AND

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KEY TERMS Structure or form, or

MORPHOLOGY:

the study thereof. MUTATION:

CONTINUED

forms, and the broad lines of descent that unite them.

Alteration in the physical

The second most general of

PHYLUM:

structure of an organism’s DNA, resulting

the obligatory taxonomic classification

in a genetic change that can be inherited.

ranks, after kingdom and before class.

NATURAL SELECTION:

The process

whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment. NUMERICAL

TAXONOMY:

SPECIATION:

The divergence of evo-

lutionary lineages and creation of new species. See allopatric speciation and sympatric speciation.

An apThe most specific of the

proach to taxonomy in which specific mor-

SPECIES:

phological characteristics of an organism

seven obligatory ranks in taxonomy.

are measured and assigned numerical

Species often are defined as a population of

value, so that similarities between two types

individual organisms capable of mating

of organism can be compared mathemati-

with one another and producing fertile off-

cally by means of an algorithm. Numerical

spring in a natural setting. Also, members

taxonomy also is called phenetics.

of the same species share a gene pool.

OMNIVORE:

An organism that eats

both plants and other animals.

SYMPATRIC SPECIATION:

A type of

speciation that occurs when a group of

The middle of the seven oblig-

individuals becomes reproductively isolat-

atory ranks in taxonomy, more specific

ed from the larger population of the origi-

than class but more general than family.

nal species. This type of speciation typical-

ORDER:

PENTADACTYL LIMB:

An appendage

ly results from mutation.

with five digits, like the human arm and hand. This appendage is common to all

TAXON:

A taxonomic group or entity. The area of the biological

mammals, though it may take very different

TAXONOMY:

forms—for example, the dolphin’s flipper.

sciences devoted to the identification,

PHENETICS:

Another

name

for

numerical taxonomy. PHYLOGENY:

The evolutionary histo-

nomenclature, and classification of organisms according to apparent common characteristics.

ry of organisms, particularly as that histo-

VERTEBRATE:

ry refers to the relationships between life-

column.

An animal with a spinal

bers of order Rodentia, are familiar to us as both pests and pets as well as aids to research through their use as test subjects in laboratories. They are also the most abundant of all mammalian orders: about one-fourth of all families, 35% of all genera, and 50% of all living species of mammal are

rodents. The group consists of some 2,205 species, among them mice, rats, squirrels, beavers, gophers, and porcupines. The name rodent comes from the Latin rodere, meaning “to gnaw,” and, indeed, the defining characteristic of rodents is their chisel-like upper front teeth.

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Whereas rats typically are despised creatures, mice (distinguished from rats simply because they are smaller) often are considered cute—that is, if the mouse in question is a pet or a laboratory mouse, rather than a pest chewing up the insulation or electrical wiring in someone’s house. The fact that rodents are so often pests and pets arises in part from rodents’ close association with humans. This is a distinction in itself, since few mammal orders manage to live successfully in such close proximity to humans. Not only do squirrels often live around human dwellings, but other species (for better or worse) often enter structures where humans live or work. Particularly notorious in this regard are black rats (Rattus rattus) and Norway rats (R. norvegicus), which are just two of some 500 rat species. The two remaining orders of mammal also are composed of small, furry creatures. Lagomorphs, or members of the order Lagomorpha, are small mammals with large upper incisors (front teeth) but no canines or eyeteeth and with molars that lack roots. The 80-odd species of lagomorphs include rabbits, hares, and their lesser-known cousin the pika, or mouse-hare. The difference between rabbits and hares relates to their conditions at birth: rabbits are furless, blind, and helpless, whereas hares are furry, have open eyes, and are capable of hopping within minutes.

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others place them in another order, Mentophyla, with tree shrews. The latter often have been placed variously in orders Scandentia, Insectivora, or Primates, indicating that many areas of mammalian taxonomy remain in dispute. WHERE TO LEARN MORE Boxhorn, Joseph. “Observed Instances of Speciation.” Talk. Origins (Web site). . Kirby, Alex. “‘New’ Camel Lives on Salty Water.” British Broadcasting Corporation (Web site). . “Mammalia.” Animal Diversity Web, The University of Michigan Museum of Zoology http://animaldiversity.ummz.umich.edu/chordata/mammalia.html>. Mammal Species of the World (MSW). Smithsonian National Museum of Natural History, Department of Systematic Biology—Vertebrate Zoology (Web site). . Marks, Jonathan. Human Biodiversity: Genes, Race, and History. New York: Aldine de Gruyter, 1995. Norton, Bryan G. The Preservation of Species: The Value of Biological Diversity. Princeton, NJ: Princeton University Press, 1986. Patent, Dorothy Hinshaw. The Challenge of Extinction. Hillside, NJ: Enslow Publishers, 1991. ———. Biodiversity. Illus. William Muñoz. New York: Clarion Books, 1996. Schilthuizen, Menno. Frogs, Flies, and Dandelions: Speciation—The Evolution of New Species. New York: Oxford University Press, 2001.

Finally, 28 species of elephant shrew, or jumping shrew, make up the order Macroscelidea, a collection of species known for their long, flexible, sensitive snouts. Some authorities group macroscelideans with order Insectivora, whereas

UCMP Hall of Mammals, University of California, Berkeley, Museum of Paleontology (Web site). .

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Speciation (Web site). .

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S C I E N C E O F E V E RY DAY T H I N G S real-life biology

DISEASE DISEASE NONINFECTIOUS DISEASES INFECTIOUS DISEASES

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Disease

CONCEPT Disease is a term for any condition that impairs the normal functioning of an organism or body. Although plants and animals also contract diseases, by far the most significant disease-related areas of interest are those conditions that afflict human beings. They can be divided into three categories: intrinsic, or coming from within the body; extrinsic, or emerging from outside it; and of unknown origin. Until the twentieth century brought changes in the living standards and health care of industrialized societies, extrinsic diseases were the greater threat; today, however, diseases of intrinsic origin are much more familiar. Among them are stress-related diseases, autoimmune disorders, cancers, hereditary diseases, glandular conditions, and conditions resulting from malnutrition. There are also illnesses, such as Alzheimer’s disease, whose causes remain essentially unknown.

HOW IT WORKS Classifying Diseases Any condition that impairs the normal functioning of an organism can be called a disease. In the human organism, as in all others, there are certain basic requirements, which in the human body include the need for a certain proper amount of oxygen, acidity, salinity (salt content), nutrients, and so on. These conditions must all be maintained within a very narrow range, and any deviation can bring about disease.

DISEASE

infection through which a virus, bacterium, or other parasite enters the body. Infectious diseases, infections, and the immune system that usually protects us against them are discussed elsewhere in this book. Our attention in the present context will be devoted to the other two broad categories—noninfectious, or intrinsic, diseases and diseases of unknown origin. C LASS I F Y I N G I N T R I N S I C D I S E AS E S . There are several basic varieties of

intrinsic disease, or conditions that are neither contagious nor communicable. These varieties are listed in the next few paragraphs. The essay Noninfectious Diseases includes a discussion of other systems for classifying diseases of either the intrinsic or the extrinsic variety. Hereditary diseases: diseases that are genetic, meaning that they are passed down from generation to generation. An example, discussed in Noninfectious Diseases, is hemophilia. Heredity is not a “cause,” and some of the diseases of unknown origin may be transmitted from parent to offspring. Some forms of cancer are hereditary as well, as are other conditions discussed elsewhere in this book. (See Nonifectious Diseases, Mutation, and Heredity.)

Diseases can be classified into three general groups. There are conditions that are infectious, or extrinsic, meaning that they are caused by an

Glandular diseases: Conditions involving a gland—that is, a cell or group of cells that filters material from the blood, processes that material, and secretes it either for use again in the body or to be eliminated as waste. Examples include diabetes mellitus, examined in Noninfectious Diseases, as well as various kidney and liver diseases, among them, hepatitis and jaundice. Goiter, a swelling in the neck area caused by a diet poor in iodine, is both a glandular and a dietary condi-

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tion, a fact that illustrates the overlap between disease types. Dietary diseases: These are all illnesses that relate to nutrient deficiencies—either an overall lack of adequate nutrition (i.e., malnutrition) or the absence of a key nutrient. Examples include pellagra, scurvy, and rickets, all of which are vitamin deficiencies, as well as kwashiorkor, which brings about a swollen belly and is caused by a lack of protein. Vitamin deficiencies are discussed in Vitamins, and kwashiorkor and other varieties of malnutrition are examined in Nutrients and Nutrition. Cancers: Cancer is not just one disease but some 100 conditions. Its two main characteristics are uncontrolled growth of diseased cells in the human body and migration of the disease from the original site to distant sites within the body. If the spread is not controlled, cancer can result in death. (See Noninfectious Diseases for more.) Stress-related diseases: Some heart conditions are hereditary or glandular, but quite a few diseases of the heart and circulatory system are exacerbated by stress. Examples include heart murmurs, hardening of the arteries, and varicose veins. We will examine heart disease and the general effects of stress shortly. Autoimmune diseases: This is a particularly terrifying category of disease, because it involves a rejection of the body itself by the body’s own immune system. Autoimmune diseases, examples of which include lupus and rheumatoid arthritis, are discussed in The Immune System. D I S E AS E S O F U N K N O W N O R I G I N . Finally, there are diseases for which there

is no known cause. In some cases, it is possible that heredity, diet, or some other aspect of human existence has a role, but it is not certain. And even if, say, heredity plays a part, the exact hereditary factors are not established. In any case, many of the categories of disease we have listed do not amount to “causes,” but rather are types of disease. Moreover, some diseases classifiable in one of the listed categories also belong in the ranks of the diseases with unknown causes. For instance, many autoimmune diseases are mysterious to scientists. Likewise, chronic fatigue syndrome, considered a disease of unknown origin, is obviously a stress-related disorder, while fibromyalgia, characterized by sore muscles and tissues, may be stress-related as well. Two brain diseases of unknown origin, Creutzfeldt-Jakob

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and Alzheimer’s disease, are discussed near the conclusion of this essay.

REAL-LIFE A P P L I C AT I O N S A Changing Threat At one time the diseases that posed the greatest threat to human survival were infectious ones, such as the Black Death (actually a combination of bubonic and pneumonic plague), which killed about a third of Europe’s population during the period from 1347 to 1351. Plagues or epidemics, in fact, are among the persistent themes in history, punctuating the fall of empires and the rise of others. A plague that struck the eastern Roman (Byzantine) Empire in the sixth century, for instance, brought an end to a plan by the great Justinian I to reconquer the Italian peninsula and restore Roman rule in western Europe. It also spelled the beginning of the end of Byzantine glory (though the empire hung on until 1453) and opened the way for the rise of Islam and Muslim influence over the Mediterranean. Thus, the course of history up to the present day, including the events of the European Middle Ages, the Crusades, and even the modern-day conflict between the West and Islamic terrorists, can be traced in part back to a plague in about A.D. 540. Wherever people have gathered in large numbers, infectious diseases have arisen. Smallpox and chicken pox, cholera and malaria, diphtheria and scarlet fever, influenza and polio— these and many other diseases have threatened the very survival of whole populations, bringing about a collective death toll that dwarfs that of twentieth-century wars and genocide. Yet it was in the twentieth century—ironically, the era when humans discovered the capacity to kill themselves in truly frightening numbers through world wars, nuclear weaponry, and totalitarian social experiments—that the threat of infectious diseases began to recede. Thanks to successful vaccination programs, many infectious diseases are largely a thing of the past. This is true even of smallpox, a scourge that effectively ended in 1978 thanks to a United Nations inoculation program, but which reemerged as a potential threat of biological ter-

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ILLUSTRATION

FROM THE FIFTEENTH-CENTURY

TOGGENBERG BIBLE

OF PEOPLE AFFLICTED WITH BUBONIC PLAGUE.

AT

ONE TIME SUCH INFECTIOUS DISEASES POSED THE GREATEST THREAT TO HUMAN SURVIVAL; IN FACT, PLAGUES AND EPIDEMICS ARE AMONG THE PERSISTENT THEMES IN HISTORY, PUNCTUATING THE FALL OF EMPIRES AND THE RISE OF OTHERS. (© Bettmann/Corbis. Reproduced by permission.)

rorism in the hands of political terrorists, such as the Islamic terrorist Osama bin Laden. It would be difficult for bin Laden’s al-Qaeda organization to acquire samples of the virus, however; they are stored in only four or five laboratories worldwide (kept there for the purpose of making more vaccine if needed) and remain under heavy guard. THE RISE OF INTRINSIC DISE AS E S . Rather than infectious diseases, the

much greater threat today is in the form of intrinsic diseases, or ones that are neither communicable nor contagious. The leading causes of death in the United States are as follows:

Note that the one item on the list that is not an intrinsic disease and is not disease-related at all: accidents. After that is the first extrinsic disease entry on the list, number 6, pneumonia and the closely related condition influenza. Number 8, of course, is not related to disease—at least not physical disease. The high incidence of suicide, with 11.1 such deaths per 100,000 population, probably reflects the fact that the United States is an industrialized, wealthy nation. Ironically, people who are eking out a living, struggling for survival, are far less likely to end it all voluntarily.

Heart disease Cancer Stroke Chronic obstructive lung diseases (e.g., emphysema) Accidents (motor vehicle or other) Pneumonia and influenza Diabetes mellitus Suicide Kidney disease Chronic liver disease and cirrhosis

Also reflective of America’s high level of development is the overwhelming preponderance of intrinsic, noninfectious diseases on the list. Unquestionably, the greatest threat to human health today takes the form of noninfectious diseases, such as heart disease, cancer, and diseases of the circulatory system. This is true only in the industrialized world, however: whereas only about 25% of all patients who visit doctors in the United States do so because of infectious diseases, more than two-thirds of all deaths worldwide are caused by infectious diseases, such as malaria.

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Stress and Heart Disease Disease

Stress, simply put, is a condition of mental or physical tension brought about by internal or external pressures. Many events can cause stress: something as simple as taking a test or driving through rush-hour traffic or as traumatic as the death of a loved one or contracting a serious illness. Stress may be short-lived, as when facing a particular deadline, or it may be the ongoing, crippling stress related to a job that is slowly killing the victim. People who experience severe traumas, such as soldiers in combat, may experience a condition called post-traumatic stress disorder (PTSD). This condition first came to public attention after World War I, a war that completely dwarfed all preceding conflicts in its intensity and brutality. Formerly bright-eyed, optimistic youths came home behaving like madmen or nervous wrecks, and soon the condition gained the nickname shell shock. (Actually, shell shock dated back more than 50 years, to what might be regarded as the first modern war in the West— the first “total war” involving relatively sophisticated weaponry and a fully engaged citizenry: America’s Civil War, from which combatants returned home with a condition known as “soldier’s heart.”) E F F E C T S O F S T R E SS . Whereas PTSD has a distinct psychological dimension, in many stress-related diseases there is not as obvious a link between mental states and bodily disorders. Nonetheless, it is clear that stress kills. Some of the physical signs of stress are a dry mouth and throat, headaches, indigestion, tremors, muscle tics, insomnia, and a tightness of the muscles in the shoulders, neck, and back. Emotional signs of stress include tension, anxiety, and depression. During stress, heart rate quickens, blood pressure increases, and the body releases the hormone adrenaline, which speeds up the body’s metabolism. Stress may disrupt homeostasis, an internal bodily system of checks and balances, leading to a weakening of immunity.

Diseases and conditions associated with stress include adult-onset diabetes (see Noninfectious Diseases), ulcers, high blood pressure, asthma, migraine headaches, cancer, and even the common cold. The last, of course, is an infectious illness, but because stress impairs the immune system, it can leave a person highly susceptible to infection. Furthermore, medical

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researchers have determined that long-term stress causes the accumulation of fat, starch, calcium, and other substances in the linings of the blood vessels. This condition ultimately results in heart disease. H E A R T D I S E A S E S . The human heart weighs just 10.5 oz. (300 g), but it contracts more than 100,000 times a day to drive blood through about 60,000 mi. (96,000 km) of vessels. An average heart will pump about 1,800 gal. (6,800 l) of blood each day. With exercise, that amount may increase as much as six times. In an average lifetime the heart will pump about 100 million gal. (380 million l) of blood. The heart is divided into four chambers: the two upper atria and the two lower ventricles. The wall that divides the right and left sides of the heart is the septum. Movement of blood between chambers and in and out of the heart is controlled by valves that allow transit in only one direction.

Given its importance to human life, it follows that heart disease is an extremely serious condition. Among the many illnesses that fall under the general heading of heart disease is congenital heart disease, a term for any defect in the heart that is present at birth. About one of every 100 infants is born with some sort of heart abnormality, the most common form being the atrial septal defect, in which an opening in the septum allows blood from the right and left atria to mix. Coronary heart disease, also known as coronary artery disease, is the most common form of heart disease. A condition termed arteriosclerosis, in which there is a thickening of the artery walls, or a variety of arteriosclerosis known as atherosclerosis results when fatty material, such as cholesterol, accumulates on an artery wall. This forms plaque, which obstructs blood flow. When the obstruction occurs in one of the main arteries leading to the heart, the heart does not receive enough blood and oxygen, and its muscle cells begin to die.

Creutzfeldt-Jakob Disease A particularly frightening category of unexplained diseases includes those that attack and destroy the brain. Among them are two conditions named after German scientists: the psychiatrists Alfons Maria Jakob (1884–1931) and Hans Gerhard Creutzfeldt (1885–1964) and the neurologist Alois Alzheimer (1864–1915). Creutzfeldt-Jakob dis-

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COMPUTER GRAPHIC OF THE BRAIN OF AN ALZHEIMER PATIENT (LEFT) COMPARED WITH A NORMAL BRAIN (RIGHT). ALZHEIMER’S DISEASE SHRINKS THE BRAIN, WHICH SHOWS SIGNS OF THE DEGENERATION OF NERVE CELLS, TANGLED PROTEIN FILAMENTS, AND LESIONS CAUSED BY ACCUMULATION OF BETA-AMYLOID PROTEIN. (Photograph by Alfred Pasieka. Photo Researchers, Inc. Reproduced by permission.)

ease, fortunately, is a rare condition. The disease, first described by the two doctors in the 1920s, initially shows itself with the loss of memory, and within a few weeks it progresses to visual problems, loss of coordination, and seizure-like muscular jerking. Death usually follows within a year. It appears that Creutzfeldt-Jakob disease ensues when a certain protein in the brain, known as prion protein, changes into an abnormal form. As to what causes that change, scientists remain in the dark. The disease attacks about one of a million people worldwide, and victims are typically about 50-75 years of age. During the 1990s something strange happened: the disease began affecting relatively large numbers of young people in the United Kingdom. A 1996 report of British medical experts, however, linked the surge in Creutzfeldt-Jakob cases to what might be considered a dietary condition: bovine spongiform encephalopathy, or mad cow disease, contracted from eating cattle with a form of prion disease. The only way to contract such a condition, however, is by eating the brain or spinal cord of an affected cow, something that could only happen in the case of hamburger or sausage, in which one does not always know what one is getting. The cows themselves got the disease from eating feed

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tainted with by-products of other cows, and as a result of the outbreak, Great Britain issued wideranging controls prohibiting the production of feed containing any materials from cows. (These particular feed-production practices were never common in the United States.)

Alzheimer’s Disease Whereas Creutzfeldt-Jakob disease is a littleknown condition, Alzheimer’s disease is all too familiar to the families of the more than four million sufferers in America today. Note the reference to the families rather than the victims themselves: one of the most devastating aspects of Alzheimer’s disease is the patient’s progressive loss of contact with reality, such that a patient in an advanced stage does not even know that he or she has the disease. A progressive brain disease that brings about mental deterioration, Alzheimer’s disease is signaled by symptoms that include increasingly poor memory, personality changes, and a loss of concentration and judgment. Although most victims are older 65 years, Alzheimer’s is not a normal result of aging. Up until the 1970s people assumed that physical and mental decline were normal and unavoidable

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KEY TERMS A general term for any condition that impairs the normal functioning of an organism. DISEASE:

A term for a disease that is communicable or contagious and comes from outside the body. Compare with intrinsic. EXTRINSIC:

A cell or group of cells that filters material from the blood, processes that material, and secretes it either for use again in the body or to be eliminated as waste. GLAND:

A term for a disease that is not communicable or contagious and comes from inside the body. Compare with extrinsic. INTRINSIC:

features of old age and dismissed such cases of deterioration as “senility.” Yet as early as 1906, Alzheimer himself discovered evidence that pointed in a different direction. In that year Alzheimer was studying a 51year-old woman whose personality and mental abilities were obviously deteriorating. She forgot things, became paranoid, acted strangely, and just over four years after he began working with her, she died. Following an autopsy, Alzheimer examined sections of her brain under a microscope and noted deposits of an unusual substance in her cerebral cortex—the outer, wrinkled layer of the brain, where many of the higher brain functions, such as memory, speech, and thought, originate. The substance Alzheimer saw under the microscope is now known to be a protein called beta-amyloid. About 75 years later scientists and physicians began to recognize a strong link between “senility” and the condition Alzheimer had identified. Since then, the public has become more aware of the disease, especially since Alzheimer’s disease has stricken such wellknown figures as the former president Ronald Reagan (1911–) and the actress Rita Hayworth (1918–1987).

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T H E I M PAC T O F A L Z H E I M E R ’ S D I S E AS E . A slight decline in short-term

memory (as opposed to long-term memories of childhood and the like) is typical even in healthy elderly adults, but the memory loss seen in Alzheimer’s disease is much more severe. As years pass, memory loss becomes greater, and personality and behavioral changes occur. Later symptoms include disorientation, confusion, speech impairment, restlessness, irritability, and the inability to care for oneself. Although victims may remain physically healthy for years, the progressive decline of their mental faculties is ultimately fatal: eventually, the brain loses the ability to control basic physical functions, such as swallowing. Persons with Alzheimer’s disease typically live between five and ten years after diagnosis, although improvements in health care in recent years have enabled some victims to survive for 15 years or even longer. Improvements in health care also may help explain the fact that the numbers of Alzheimer victims are growing. Medical discoveries of the twentieth century served to prolong life greatly, such that there are far more people alive today who are 65 years of age or older than there were in 1900. More accurate reporting no doubt plays a part as well. Whereas about 2.5 million cases were reported throughout the 1970s, by the end of the twentieth century there were some four million living Alzheimer victims, and by the midtwenty-first century that number is expected to climb to the range of 13 million if physicians do not find a cure. Meanwhile, Alzheimer’s causes the deaths of more than 100,000 American adults each year and costs $80–90 billion annually in health-care expenses. U N D E R S T A N D I N G A L Z H E I M E R. It is not a simple procedure to

diagnose Alzheimer’s disease, and despite all the medical progress since the time of Alois Alzheimer, the “best” method for determining whether someone has the condition is hardly a good one. The only possible physical procedure for definitively diagnosing Alzheimer’s disease is to open the skull and remove a sample of brain tissue for microscopic examination. This is rarely done, of course, because brain surgery is far too drastic a procedure for simply obtaining a sample of tissue. The immediate cause of Alzheimer’s is the death of brain cells and a decrease in the connec-

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tions between those cells that survive. But what causes that? Many scientists today believe that the presence of beta-amyloid protein is a cause in itself, while others maintain that the appearance of the protein is simply a response to some other, still unknown phenomenon. Researchers have found that a small percentage of Alzheimer cases apparently are induced by genetic mutations, but most cases result from unknown factors. Various risk factors have been identified, but they are not the same as causes; rather, a risk factor simply means that if a person has x, he or she is more likely to have y. Risk factors for Alzheimer’s include exposure to toxins, head trauma (former president Reagan suffered a serious head injury before the onset of Alzheimer’s disease), Down syndrome (a genetic disorder that causes mental retardation), age, and even gender (women are more likely than men to suffer from Alzheimer’s disease). Familial Alzheimer’s disease, an inherited form, accounts for about 10% of cases. Approximately 100 families in the world are known to have rare genetic mutations that are linked with early onset of symptoms, and some of these families have an aggressive form of the disease in which symptoms appear before age 40. The remaining 90% of cases may be caused by various combinations of genetic and as yet undefined environmental factors.

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WHERE TO LEARN MORE

Disease

Centers for Disease Control and Prevention (Web site). . DeSalle, Rob. Epidemic!: The World of Infectious Disease. New York: New Press, 1999. Diseases, Disorders and Related Topics. Karolinska Institutet/Sweden (Web site). . Environmental Diseases from A to Z. National Institute of Environmental Health Sciences/National Institutes of Health (Web site). . Epidemiology. University of Minnesota, Crookston (Web site). . Ewald, Paul W. Plague Time: How Stealth Infections Cause Cancers, Heart Disease, and Other Deadly Ailments. New York: Free Press, 2000. Garrett, Laurie. The Coming Plague: Newly Emerging Diseases in a World out of Balance. New York: Farrar, Straus, Giroux, 1994. Moore, Pete. Killer Germs: Rogue Diseases of the TwentyFirst Century. London: Carlton Books, 2001. National Cancer Institute, National Institutes of Health (Web site). . Oncolink: University of Pennsylvania Cancer Center (Web site). . Oldstone, Michael B. A. Viruses, Plagues, and History. New York: Oxford University Press, 1998. “Plant and Animal Bacteria Diseases.” University of Texas Institute for Cellular and Molecular Biology (Web site). . World Health Organization (Web site). .

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NONINFECTIOUS DISEASES Noninfectious Diseases

CONCEPT In contrast to infectious, or extrinsic, diseases, noninfectious, or intrinsic, conditions are neither contagious nor communicable. They arise from inside the body as a result of hereditary conditions or other causes, such as dietary deficiencies. Although infectious forms historically have been the most life-threatening varieties of disease and remain so even today in much of the world, noninfectious disease is a far more serious concern in industrialized nations, such as the United States. Some categories of intrinsic diseases include stress-related, dietary, and autoimmune conditions as well as diseases of unknown origin. Additionally, there are several other categories we examine in this essay: hereditary diseases, such as hemophilia; glandular conditions, of which diabetes mellitus is a powerful example; and cancer, of which there are approximately 100 different varieties.

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infectious diseases: bacteria, viruses, and other parasites. Infectious diseases, discussed elsewhere in this book (see Infection, Infectious Diseases, and Parasites and Parasitology), continue to pose a threat to life in underdeveloped nations. In North America, Europe, the emerging capitalist democracies of eastern Asia, and a handful of other materially and technologically advanced lands, however, infectious diseases have taken a back seat to noninfectious ones as well as to other threats. CAU S E S O F D E AT H I N A M E R I CA . In the United States today, the four leading

causes of death (see list in Disease) are noninfectious conditions: heart disease, cancer, stroke, and chronic obstructive lung diseases. The first of these conditions is discussed in Disease, and the second is examined later in this essay. Stroke, which occurs when a clot obstructs the flow of blood to the brain, may be classified along with heart disease as a stress-related ailment of the circulatory system. Chronic obstructive lung diseases include emphysema, cystic fibrosis, and other conditions with widely differing causes that nonetheless all affect the same organ in the same way.

The world has long suffered under the threat of infectious diseases, some of which include smallpox, chicken pox, cholera, malaria, diphtheria, scarlet fever, influenza, polio, pneumonia, and even the common cold. Except for the last one, which is rarely fatal, such conditions have racked up a considerable death toll. For centuries people attributed these diseases to all manner of false causes, ranging from divine curses to an imbalance of bodily fluids. Only with the development of the microscope in the 1600s did scientists begin to identify the real cause behind most

The remainder of the list includes two nondisease-related conditions (accidents at no. 5 and suicide at no. 8), three more noninfectious diseases (diabetes mellitus at no. 7, kidney disease at no. 9, and chronic liver disease and cirrhosis at no. 10); and just one infectious-disease-related set of causes. The latter, no. 6, consists of two diseases, pneumonia and influenza, which may be the result of another type of infection, the only infectious condition that poses a serious threat in the Western world today: AIDS, or acquired immunodeficiency syndrome.

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MAGNETIC

RESONANCE IMAGING IS ONE TYPE OF DIAGNOSTIC TEST USED TO LOCATE TUMORS. (© Neal Preston/Corbis.

Reproduced by permission.)

Varieties of Disease The essay Disease classifies noninfectious diseases as follows: hereditary or genetic diseases (e.g., hemophilia, discussed later in this essay); glandular diseases, or conditions involving a group of cells that filters material from the blood (e.g., diabetes mellitus, also included in the present essay); dietary diseases (see the essays Vitamins and Nutrients and Nutrition); cancers (discussed here); stress-related diseases (see Disease); autoimmune diseases (see Immunity and Immunology); and diseases of unknown origin (see Immunity and Immunology).

tors and scientists use for grouping diseases are topographic; anatomic; physiological; pathologic; etiologic, or causal; and epidemiological classifications. Topographic classification refers to bodily region or system: for instance, the circulatory system, the neurological system, and so on. The second method, anatomic (by organ or tissue), also uses parts of the body as a criterion for classification. The designation of heart and lung diseases, used earlier in discussing leading causes of mortality in the United States, is an example of this system.

SOME OTHER CLASSIFICAT I O N S Y S T E M S . Among the systems doc-

Physiological classifications divide diseases in terms of function or effect (for example, metabolic disorders, some of which are discussed in Metabolism), while pathologic classifications separate diseases by the nature of the process that the disease takes: for example, inflammatory diseases. Etiologic, or causal, classifications are used most commonly in discussing infectious diseases, where broad types of causes can include viruses, bacteria, or other types of parasites. Likewise, epidemiological classifications usually refer to infectious diseases. Epidemiology is an area of the medical sciences devoted to the study of disease, including its incidence, distribution, and control within a population.

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This is one way of dividing up ailments, and it happens to be the method applied in the essays of this book that deal with diseases. This method has the advantage of illustrating the wide variation in noninfectious diseases, but it would not necessarily be the best model to use for an indepth professional study of disease. Scientists who study illnesses typically use one of several methods of classification that, while less broadly based than the one used here—and perhaps less interesting as well—are more efficient, because they group all diseases according to the same characteristics.

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REAL-LIFE A P P L I C AT I O N S Cancer Although we are accustomed to hearing of “cancer” as though it were one disease, it is actually many diseases, close to 100 in number. Some of the most common varieties include skin, lung, and colon cancer, as well as breast cancer in women and prostate cancer in men. Blood and lymph node cancers, known as leukemias and lymphomas, respectively, are widespread, whereas cancer of the kidneys, ovaries, uterus, pancreas, bladder, and rectum are included among the cancers that most often affect Americans. As this listing suggests, most cancers attack either body parts or systems and therefore are often classified anatomically or topographically; yet several characteristics unite these conditions. Cancer strikes the genes, which are carriers of genetic information that make up part of DNA (deoxyribonucleic acid), a molecule that appears in all cells. By gaining control at this level, the cancer is like a terrorist who has established a grip on all the communication or transportation systems in a country. Many genes produce proteins that play a part in controlling the processes of cell growth and division. An alteration, or mutation, to the DNA molecule can disrupt the genes and produce faulty proteins, causing the cells to become abnormal and multiply. The abnormal cell begins to divide uncontrollably and eventually forms a new growth, known as a tumor, or neoplasm. In a healthy person, the immune system can recognize the neoplastic cells and destroy them before they have a chance to divide. Some mutant cells may escape immune detection, however, and survive to become tumors or cancers. (The immune system is discussed in Immunity and Immunology.)

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CAU S E S A N D T Y P E S . A majority of cancers are caused by changes in the cell’s DNA because of damage from the environment. Environmental factors that are responsible for the initial mutation in DNA are called carcinogens, and there are many types, which we discuss shortly. Additionally, some cancers have a genetic basis: in other words, a person can inherit faulty DNA from his or her parents, which can predispose the patient to cancer of one kind or another. While there is scientific evidence that both factors (environmental and genetic) play a role, less than 10% of all cancers are purely hereditary.

There are several different types of cancers. In addition to leukemias and lymphomas, mentioned earlier, there are carcinomas, or cancers that arise in the epithelium (the layers of cells covering the body’s surface and lining the internal organs and various glands). These types alone account for about 90% of all cancers. Some forms of skin cancer are melanomas, which typically originate in the pigment cells. Other forms of cancer include sarcomas (cancers of the supporting tissues of the body, e.g., bone, muscle, and blood vessels), and gliomas, or cancers of the nerve tissue. D I A G N O S I N G C A N C E R . Many signs indicate the onset of cancer, among them, changes in the size, color, or shape of a wart or a mole; a sore that does not heal; or persistent cough, hoarseness, or sore throat. Many other diseases can produce similar symptoms, however, and for this reason it is important for a person to visit a doctor for regular checkups and diagnosis. Usually, diagnosis calls for fairly routine physical examination, though in the case of cancers of the reproductive organs, “routine” can still be plenty invasive.

Tumors can be either benign or malignant. A benign tumor is slow growing, does not spread or invade surrounding tissue, and, once removed, usually does not recur. A malignant tumor, on the other hand, invades surrounding tissue and spreads to other parts of the body. Therefore, even if the malignant tumor is removed, if the cancer cells have spread to the surrounding tissues, cancer will return. If the cancer cells are allowed to keep growing in number, migrating from the site of origin and spreading throughout the body, they eventually will kill the patient.

Doctors examining women for cancers of the ovaries, uterus, cervix, and vagina must palpate the internal organs—that is, examine them by touch. For males, inspection of the rectum and the prostate is included in the physical examination. The doctor inserts a gloved finger into the rectum and rotates it slowly to feel for any growths, tumors, or other abnormalities. The doctor also palpates the testicles to identify any lumps, thickening, or differences in the size, weight, or firmness. Such examinations, as well as diagnoses for certain other types of cancer in private parts (namely, colon cancer), can be less

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than pleasant, but they are certainly preferable to an early and painful death. If the patient has an abnormality that could be indicative of cancer, the doctor may order diagnostic tests. These tests may include laboratory studies of sputum or saliva, blood, urine, and stool (feces). To locate tumors, such imaging tests as computerized tomography (CT) scans, magnetic resonance imaging (MRI), ultrasound, or fiber-optic scope examinations may be used. The most definitive diagnostic test, however, is the biopsy, in which a piece of tissue is surgically removed for examination under a microscope. Besides confirming whether a patient has cancer, the biopsy also provides information about the type of cancer, the stage it has reached, the aggressiveness of the cancer, and the extent of its spread. Screening examinations, conducted regularly by health care professionals, can result in the detection of cancers at an early stage. In addition, advances in molecular biology (an area of biology concerned with the physical and chemical basis of living matter) and cancer genetics have led to the development of several tests for assessing one’s risk of developing cancers. These new techniques include genetic testing, in which molecular probes are used to identify mutations in certain genes that have been linked to particular cancers. At present, however, there are limitations to genetic testing, a fact that emphasizes the need for better strategies of early detection. Although there is as yet no cure for cancer, there are treatments designed to remove as much of the tumor or tumors as possible and to prevent the recurrence or spread of the cancer. Cancer treatment can take many different forms, including surgery, radiation, chemotherapy, immunotherapy, hormone therapy, and bone-marrow transplantation. Physicians recommend specific treatments based on the needs, condition, and illness of the particular patient. T R E AT I N G

highly effective, chemotherapy may be physically difficult for the patient and may have side effects, including temporary hair loss.

Noninfectious Diseases

Immunotherapy uses the body’s own immune system to destroy cancer cells. The various immunological agents being tested include substances produced by the body as well as vaccines. Unlike traditional vaccines, cancer vaccines do not prevent cancer; instead, they are designed to train the immune system of the cancer patient’s body to attack and destroy cancer cells. Hormone therapy is standard treatment for some types of cancers that are hormone-dependent and which grow faster in the presence of particular hormones. Among them are cancers of the prostate, breast, and uterus, and the therapy is designed to block the production or action of the hormones involved. A particularly aggressive form of treatment is bone-marrow transplantation, which involves taking tissue from within a donor’s bone cavities, where blood-forming cells are located, and transplanting it into the patient. In addition, cancer patients may use massage, reflexology, herbal remedies, and other forms of treatment dubbed “alternative,” meaning that they usually are not recognized by the mainstream of the medical profession. (Lack of official recognition does not necessarily mean anything: many people have benefited from alternative cancer treatments.)

Who Is at Risk?

CANCER.

Surgery, the most frequently used form of cancer treatment, involves the removal of the visible tumor. It is most effective when a cancer is small and confined to one area of the body. Radiation, which kills tumor cells by bombarding them with high-energy waves or particles, may be used alone in cases where a tumor is unsuitable for surgery. More often, however, it is used in conjunction with surgery and chemotherapy or with drugs to kill cancer cells. While it can be

One of every four deaths in the United States is from cancer, and each year more than a million Americans are diagnosed with some form of the disease. Of these people, about half will die of the disease. Cancer can attack anyone, even children, though, fortunately, cases of cancer in very young patients are the exception. Most cases are seen in middle-aged or older adults. Although scientists are a long way from being able to predict who will get cancer (much less effectively prevent it), they have identified numerous risk factors. A risk factor is not necessarily a cause per se (though it may be); rather, if there seems to be a link between a particular behavior and a specific disease, that behavior is referred to as a risk factor for that disease. Major risk factors for cancer are tobacco use, alcohol consumption, diet, certain types of sexual and reproductive behavior, infectious agents, family history, occupation, environment, and pollution.

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SPECIFIC

RISK

FA C T O R S .

Approximately two-fifths of all cancer deaths annually are linked to tobacco use and excessive alcohol consumption. In addition to the relationship between smoking and lung cancer (80–90% of all lung cancer patients are smokers), smoking also has been shown to be a contributory factor to a whole host of other cancers. Moreover, scientists have established that second-hand smoke (or passive smoking) can increase one’s risk of cancer. Nor is “smokeless tobacco” a safe alternative to cigarettes: snuff and chewing tobacco have been associated with countless cases of mouth cancer. Excessive drinking is a risk factor in liver cancer and other illnesses, and the deadly combination of tobacco and excessive alcohol use significantly increases the chances that a person will contract mouth, pharynx, larynx, or esophageal cancer. Tobacco and alcohol are not the only cancer-related agents that people take into their bodies: about one-third of all cancer deaths annually are related to the things people eat. For example, immoderate intake of fat, leading to obesity, has been associated with cancers of the breast, colon, rectum, pancreas, prostate, gallbladder, ovaries, and uterus. There are even varieties of cancer that are linked to contagious diseases. Since the mid1970s, scientists have obtained evidence showing that approximately 15% of all cancer deaths worldwide can be traced to viruses, bacteria, or parasites. One such pathogen, the human papilloma virus, is sexually transmitted. Having too many sex partners and becoming sexually active too early have been shown to increase one’s chances of contracting cancer of the cervix. (On the other hand, women who do not have children or those who have them late in life have a higher risk of both ovarian and breast cancer.) Certain cancers, including those of the breast, colon, ovaries, and uterus, recur generation after generation in some families, and therefore family history and genetics cannot be ruled out as risk factors. In addition, less well-known cancers, such as the eye condition known as retinoblastoma, have been traced to certain genes that can be tracked within a family. Thus, it is possible that inheriting particular genes makes a person susceptible to certain cancers.

In 1775 the English surgeon Percivall Pott (1714–1788) described the high incidence of cancer of the scrotum among former chimney sweeps, most of them men in their twenties. This was bad enough, but the fact that they had contracted the cancer much earlier hinted at a reality even more grim. In the harsh early days of the Industrial Revolution, before child labor laws had even been imagined, boys as young as four— orphans or children of desperately poor families—were put to work cleaning the insides of chimneys. The chimneys were so narrow that only children could clean them, and because of the tight fit, even small boys were unable to wear any clothes while doing their job. As a result, soot became embedded in their skin, and, since few people bathed, it stayed there, a silent killer whose effects became apparent only many years later. In addition to helping make English society aware of the injustices put upon members of its lowest classes, Pott’s research introduced the medical world to the concept of occupational health. Still, until the last third of the twentieth century, large numbers of industrial workers in the West labored at occupations with built-in cancer hazards. Thus, asbestos workers contracted lung cancer at a high rate, and a link became apparent between bladder cancer and dye and rubber industries. Likewise, connections were established between skin or lung cancer and the jobs of smelters, gold miners, and arsenic workers; between leukemia and glue and varnish workers’ occupations; between liver cancer and the business of PVC (polyvinyl chloride) manufacturers; and between lung, bone, and bone marrow cancer and the work of radiologists and uranium miners.

occupational hazards make up a particularly sig-

Radiation itself is an environmental hazard that affects not only workers in those specialized industries just named but also anyone who has been exposed to radioactive materials. Fortunately, this is less of a hazard today, thanks to numerous bans on nuclear testing; nonetheless, radiation—including ultraviolet radiation from

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nificant group. They account for 4% of all cancer deaths, and there is a certain poignancy in the fact that these cancer victims contracted their illness not by drinking or smoking or bad eating but simply by earning a living. Nowhere was this poignancy more evident than in some of the first documented cases of cancer arising from occupational causes.

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Noninfectious Diseases

PORTRAIT

OF YOUNG CHIMNEY SWEEPERS FROM THE 1890S.

BEFORE

CHILD LABOR LAWS, BOYS AS YOUNG AS FOUR

WERE PUT TO WORK CLEANING THE INSIDES OF CHIMNEYS; MANY LATER CONTRACTED CANCER OF THE SCROTUM FROM EXPOSURE TO SOOT. (© Bettmann/Corbis. Reproduced by permission.)

the Sun—causes 1–2% of all cancer deaths. Additionally, it has been estimated that 1% of cancer deaths are due to air, land, and water pollution, particularly as a result of chemical dumping in water supplies.

Hemophilia In contrast to the boys Pott treated, sufferers from hemophilia in the past often came from the highest echelons of society. A hereditary disease that primarily affects males, hemophilia was passed down through many royal lines, with the females acting as carriers and some of the males in the bloodline becoming victims of the disease. England’s Queen Victoria (1819–1901) had sev-

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eral sons with hemophilia who died before they had the opportunity to become king. Her nephew, Russia’s Czar Nicholas II, had a son who was hemophiliac as well. The boy’s affliction caused his mother to seek the help of the charismatic “healer” Grigori Rasputin, whose close involvement with the royal family fed Russian discontent and helped contribute to the overthrow of the czar in 1917. (Ironically, the boy died not from hemophilia but perished, along with his family, before a Bolshevik firing squad.) Thus, once again (as noted in Disease), disease affected the course of history. Its name taken from Greek and Latin words that together mean “love of bleeding,” hemophilia

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betes mellitus, a serious condition caused by an absence of, or an insufficient amount of, insulin. Insulin, a hormone produced by the pancreas in varying amounts, helps maintain a normal concentration of glucose, or blood sugar (see Carbohydrates). Diabetes in general is a glandular disease, the gland in this case being the pancreas, but type 1 diabetes also is considered an autoimmune disorder because the body’s own immune system destroys its insulin-producing cells. (See Immunity and Immunology and The Immune System for more about autoimmune disorders.)

Noninfectious Diseases

ALEXIS,

SON OF

CZAR NICHOLAS II

OF

RUSSIA,

HAD

HEMOPHILIA, A HEREDITARY DISEASE THAT IMPAIRS THE CLOTTING OF BLOOD.

THE

DISEASE PRIMARILY AFFECTS

MALES AND HAS BEEN PASSED DOWN THROUGH MANY ROYAL BLOODLINES. (© Bettmann/Corbis. Reproduced by permis-

sion.)

is caused by a genetic defect that prevents the body from developing proteins needed to help the blood clot. What would be a minor bruise or scratch for an ordinary person is therefore a lifethreatening situation to a hemophiliac, who runs a severe risk of bleeding to death from even the most minor cut. A hemophiliac must therefore live in a state of constant fearfulness: as a youngster, for instance, he cannot run and jump and get into mischief like other boys, for fear that he might skin his knee—a minor pain for most boys but a serious injury to him. For hemophiliacs today, there is good news and bad news. The good news is that the condition can be treated with transfusions containing the necessary proteins, and this has extended the life expectancy of some victims. The bad news, however, is that in this day and age the treatment itself might kill them: the transfusions originate from donated blood, which may contain the virus that causes AIDS, as well as pathogens linked to other diseases.

EXTENT AND TYPES OF DIAB E T E S . More than 12 million Americans,

Much more prevalent than the genetic disorder hemophilia, or any one form of cancer, is dia-

and some 100 million people worldwide, are affected by diabetes. That number is increasing by 5-6% annually, primarily the result of the population’s increased longevity, combined with other factors such as increasing obesity and consumption of rich, processed, and carbohydrateloaded foods (and drinks such as beer). Approximately 300,000 deaths each year are attributed to diabetes, which is of two principal varieties. Type 1, or insulin-dependent diabetes, is present at birth, is characterized by insulin deficiency, and normally is treated by taking insulin injections. Type 2, or non-insulin-dependent diabetes, arises not at birth, but somewhat later (though it can occur in childhood) and then among people who have normal insulin levels. (With type 2 diabetes, the problem is the body’s inability to use its insulin efficiently.) Type 2, which typically stems from dietary causes—is preventable, but it is not treatable by insulin injections. Type 2 diabetes also may temporarily affect pregnant women, who may experience heightened glucose levels in a condition known as gestational diabetes. (There is another kind of diabetes altogether, a rare condition known as diabetes insipidus,

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Diabetes prevents the body from putting glucose to use, and instead large amounts of it are excreted in the urine. The word diabetes means “siphon,” a reference to one of its major symptoms: frequent urination in an attempt to expel glucose. The urine itself is full of sugar; hence the term mellitus, meaning “honey.” (In this vein, it is worth mentioning Chen Chuan, an eighth-century Chinese physician who surely qualifies as one of history’s most dedicated scientists. He was the first to describe the sweetness of urine in patients suffering from diabetes, presumably as the result of firsthand research that went above and beyond the call of duty.)

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which involves inadequate production of another hormone, vasopressin.) One might wonder why problems with blood sugar could be so serious as to kill a million people every three years and to qualify diabetes as one of the leading killers in America. The reason is that the body depends on glucose as a source of immediate energy, and in the absence of usable glucose, it begins instead to use its fat cells. This rapid burning of fat produces a surplus of organic compounds known as ketones, and ketone accumulation brings about an accumulation of acids in the blood, a condition known as ketoacidosis. Severe ketoacidosis can cause nausea, vomiting, and a loss of consciousness, or diabetic coma. If the patient does not receive a shot of insulin, he or she can die. Thus, the symptoms displayed by Julia Roberts’s diabetic character in her first major movie, Steel Magnolias (1989), were not overdone. Diabetes can also bring about other conditions, including blindness, kidney diseases, and long-term organ damage. EFFECTS

OF

KEY TERMS

Noninfectious Diseases

DIABETES.

THE TRIUMPH OF BANTING A N D B E S T. As difficult as life is for diabet-

ics today, it is infinitely better than it was before 1921. That was the year when the Canadians Frederick Banting (1891–1941), a surgeon, and Charles Herbert Best (1899–1978), a physiologist, isolated insulin. Thanks to their work, and the subsequent development of insulin therapy—typically using insulin harvested from cows or pigs—deaths from ketoacidosis and diabetic coma declined. A person with type 1 diabetes, formerly consigned to a dramatically shortened life of misery, could hope to have something approaching a normal existence. Today diabetes remains a life-shortening illness, and doctors and scientists continue to search for a cure, but ever since 1921, the lot of those with diabetes has been improving steadily. (Banting was later knighted and shared the 1923 Nobel Prize in physiology and medicine for his achievement. Best, however, in one of history’s great snubs, received neither honor, because he had not earned his doctorate at the time of the discovery.) WHERE TO LEARN MORE American Cancer Society (Web site). . American Cancer Society’s Guide to Complementary and Alternative Cancer Methods. Atlanta, GA: American Cancer Society, 2000.

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A substance or agent that induces the development of cancer. CARCINOGEN:

A general term for any condition that impairs the normal functioning of an organism. DISEASE:

Deoxyribonucleic acid, a molecule in all cells, and many viruses, containing genetic codes for inheritance. DNA:

A term for a disease that is communicable or contagious and comes from outside the body. Compare with intrinsic. EXTRINSIC:

A unit of information about a particular heritable (capable of being inherited) trait that is passed from parent to offspring and stored in DNA molecules called chromosomes. GENE:

GENETICS: The study of hereditary traits passed down from one generation to the next through the genes.

A cell or group of cells that filters material from the blood, processes that material, and secretes it either for use again in the body or to be eliminated as waste. GLAND:

A term for a disease that is not communicable or contagious and comes from inside the body. Compare with extrinsic. INTRINSIC:

American Institute for Cancer Research (Web site). . CancerCare (Web site). . Centers for Disease Control and Prevention (Web site). . Izenberg, Neil, ed. Human Diseases and Conditions. New York: Scribner, 2000. Steen, R. Grant, and Joseph Mirro. Childhood Cancer: A Handbook from St. Jude Children’s Research Hospital with Contributions from St. Jude Clinicians and Scientists. Cambridge, MA: Perseus Publishing, 2000.

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INFECTIOUS DISEASES

CONCEPT The history of the human species, it has been said, is the history of infectious disease. Over the centuries, humans have been exposed to a vast amount and array of contagious conditions, including the Black Death and other forms of plague, typhoid fever, cholera, malaria, influenza, and the acquired immunodeficiency syndrome, or AIDS. Only in the past few hundred years have scientists begun to have any sort of accurate idea concerning the origin of such diseases, through the action of microorganisms and other parasites. Such understanding has led to the development of vaccines and methods of inoculation, yet even before they made these great strides in medicine, humans had an unseen protector: their own immune systems.

The background on scientists’ progressive understanding of the microorganisms that cause

The human body has numerous mechanisms for protecting itself from infectious disease, the first line of defense being the skin. Skin shields us all the time from unseen attackers and generally is able to prevent pathogens from entering the body; however, any break in the skin, such as a cut or scrape, provides an opening for microorganisms to invade the body. Germs that normally would be prevented from entering the body are able to invade the bloodstream through such openings. This is why it is so very important, in any situation involving potential contact with infection, to protect the skin. With the advent of AIDS, doctors and members of other professions who are likely to touch people carrying diseases—including officers arresting addicts or prostitutes—are much more likely to do their work wearing heavy plastic gloves.

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HOW IT WORKS Infection and Immunity There are two basic types of disease: ones that are infectious, or extrinsic, meaning that they are contagious or communicable and can be spread by contact between people, and ones that are intrinsic, or not infectious. Diseases in general and noninfectious diseases in particular are discussed in essays devoted to those subjects. So, too, is infection itself, a subject separate from infectious diseases: a person can get an infection, such as tetanus or salmonella, without necessarily having a disease that can be passed on through contact with others in the same way that colds, malaria, or syphilis is spread.

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disease and the means of fighting these microorganisms are discussed in Infection. Among the leading figures in that history were the French chemist and microbiologist Louis Pasteur (1822–1895) and the German bacteriologist Robert Koch (1843–1910), who contributed greatly to what is known today as germ theory— the idea that infection and infectious diseases are brought about by microorganisms. In most cases, the organisms are too small to be seen with the naked eye. They include varieties of amoeba and worm, discussed in the essay Parasites and Parasitology, as well as viruses and some forms of bacteria and fungi, which together are known as pathogens, or disease-carrying parasites. Other terms related to infectious diseases, their agents, and the prevention and study of them are defined in the essay Infection. IMMUNE

MECHANISMS.

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Suppose that a microorganism makes it through the barrier of skin, thanks to a cut or other opening; if so, the body puts into action a second defensive mechanism, the immune system. This system is a network of organs, glands, and tissues that protects the body from foreign substances. Without a properly functioning immune system, a person could die simply by walking out the door in the morning and coming into contact with an airborne infectant. Even in relatively healthy people, the immune system may be unable to react adequately to an invasion of microorganisms. In such cases, disease develops.

Transmission of Diseases Infectious diseases, by definition, are transmitted easily from one person to another. We have all been told, for instance, not to drink after someone who has a cold. On a much more serious level, persons who are sexually active or potentially sexually active, but not settled in a monogamous (one-partner) relationship, are advised to avoid unprotected sexual contact so as not to contract AIDS or some other sexually transmitted disease (STD). In these and many other cases, microorganisms travel from the carrier of the disease to the uninfected person. (Actually, in the case of AIDS, the pathogen is a virus, which is not, strictly speaking, an organism or even a living thing; however, viruses usually are lumped in with bacteria, amoeba, and some fungi as microorganisms.) Pathogens can be spread by many methods other than direct contact, including through water, food, air, and bodily fluids—blood, semen, saliva, and so on. For instance, any time a person with an infection coughs or sneezes, they may be transmitting illness. This is how diseases such as measles and tuberculosis are passed from person to person. AIDS and various STDs, as well as many other conditions, such as hepatitis, are transferred when one person comes into contact with the bodily fluids of another. This is the case not only with sexual intercourse but also with blood transfusions and any number of other interactions, including possibly drinking after someone. (Contrary to rumors that circulated in the early 1980s, when AIDS first made itself known, that particular syndrome cannot be transferred by saliva, but the common cold and other viral infections can be.)

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Cholera, caused by a bacterium found in dirty wells and rivers from India to England (in the 1800s, at least), is an example of a waterborne disease. Many foodborne pathogens tend to bring about what would be more commonly thought of as an illness than a disease, since in everyday language the latter term implies a longterm affliction, whereas food poisoning usually lasts for a week or so. (Still, some forms of food poisoning can be fatal.) Bacterial contamination may occur when food is not cooked thoroughly, is left unrefrigerated, is prepared by an infected food handler, or otherwise is handled in an unsanitary or improper fashion. (The case of Typhoid Mary, discussed near the conclusion of this essay, is an extreme example of this form of transmission.)

Infectious Diseases

Additionally, diseases may be transferred by vectors—animals (usually insects) that carry microorganisms from one person to another. Vectors may spread a disease either by mechanical or by biological means. Mechanical transmission occurs, for example, when flies transfer the germs for typhoid fever from the feces (stool) of infected people to food eaten by healthy people. Biological transmission takes place when an insect bites a person and takes infected blood into its own system. Once inside the insect’s gut, the disease-causing organisms may reproduce, increasing the number of parasites that can be transmitted to the next victim. This is how the Anopheles mosquito vector, for instance, transfers malaria.

REAL-LIFE A P P L I C AT I O N S A Tour of Diseases The range of infectious diseases, from conditions that merely cause discomfort to those that bring about death, is truly staggering. Some have brought about vast epidemics that have wiped out huge populations, and many have changed the course of history, while others are hardly known to anyone outside the ranks of epidemiologists and the victims of the disease. Some, such as smallpox, have been eradicated or largely eradicated through inoculation campaigns, while others, most notably AIDS, continue to elude efforts to defeat them. Diseases can be classified according to the systems or body parts affected. Some of those

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systems and parts, with examples of diseases relating to each, include the following. • Upper respiratory tract: common cold, sinusitis, croup • Lower respiratory tract: pneumonia, bronchitis • Cardiovascular system: rheumatic fever • Central nervous system: meningitis, encephalitis • Genitourinary tract: sexually transmitted diseases (i.e., venereal diseases, such as syphilis, gonorrhea, and the herpes simplex viral infection) • Gastrointestinal tract: cholera, salmonella, hepatitis • Bones and joints: septic arthritis • Skin: warts, candida • Eyes: conjunctivitis (pink eye) Another way to classify diseases is according to the types of organism that cause them: bacteria, viruses, or other forms of parasite, particularly worms, amoeba, and insects. The first two groups are discussed in further detail within Infection and the other varieties of parasite in Parasites and Parasitology. Bacterial infections include anthrax, botulism, tetanus (lockjaw), leprosy, tuberculosis, diphtheria, whooping cough, plague, and a variety of pneumococcal, staphylococcal, and streptococcal illnesses. Among viral illnesses and diseases are the common cold, influenza, infectious mononucleosis, smallpox, chicken pox, measles, mumps, rubella (or German measles), yellow fever, poliomyelitis (i.e., polio), rabies, herpes simplex, and AIDS. Diseases related to other varieties of parasite include malaria, Rocky Mountain spotted fever, trichinosis, scabies, and river blindness. Nonmicroscopic parasites, particularly such worms as hookworm and pinworm, bring about disease-like forms of parasitic infestation within the body.

Plagues From earliest times infectious diseases have wreaked havoc on the human species, and this was particularly so with the various plagues that struck Europe in ancient and medieval times. As noted in Infection, a plague in the fifth century B.C. helped bring an end to the golden age of Greek civilization. A thousand years later, another plague befell Greece, which by then dominated what remained of the Roman Empire. Based

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in Byzantium (Constantinople) this realm became known to history as the Byzantine (Eastern Roman), Empire, though its citizens saw themselves simply as “Romans” and thus as the inheritors of Roman civilization. Italy itself had fallen under the control of nomadic invaders, the Visigoths, but Emperor Justinian I (483–565) undertook a vast and costly campaign to wrest control of the Italian peninsula from the barbarians. Had he succeeded, the entire course of medieval history in Western Europe might have been different; he did not, largely because of a plague that swept Constantinople in 541. Through a series of interconnected events, the plague permanently weakened Byzantium and left the Mediterranean world ripe for conquest by a new power: Islam. Both directly and indirectly, the plague of 541 served to divide Eastern and Western Europe. Not only was the Roman Empire never truly reunited, meaning that the two halves of the continent grew increasingly separate, but the rise of Islam made possible the Crusades (1095–1291). The latter sowed further discord between the East and the West, owing to the fact that Western European crusaders overran Byzantium and incited trouble between the Byzantines and Arabs. Ultimately, the split between Eastern and Western Europe, which became particularly pronounced during the years of Communism and the Iron Curtain (1945–1990), can be traced to the plague of 541.

The Black Death The Byzantine plagues (there were several, occurring at intervals of a few generations), killed millions of people, yet for sheer scope of destruction—and, perhaps, historical impact—they were dwarfed by the plague that devastated Europe in the years 1347–1351. This one became known as the Plague (with a capital P) or by another name that gave some hint of the terror that was as much a part of the epidemic as the ghastly physical symptoms it brought on: the Black Death. It began in Asia and quickly made its way to the shores of the Black Sea, where it erupted in September 1346. The first outbreak in Western Europe occurred 13 months later, in October 1347, at the Sicilian port of Messina, from whence it was an easy jump to the Italian mainland. By the following April all of Italy was infected; meanwhile, the Plague had reached Paris in

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A

POLITICAL CARTOON FROM ABOUT 1870 ILLUSTRATES THE UNSANITARY CONDITIONS IN

POLITICIAN AND PUBLIC WORKS COMMISSIONER

BOSS TWEED

NEW YORK CITY,

WITH THE

WELCOMING A CHOLERA EPIDEMIC. (© Bettmann/Corbis.

Reproduced by permission.)

January 1348, and within a year, 800 people a day were dying in that city alone. Quickly it penetrated the entire European continent and beyond, from North Africa to Scandinavia and from England to the hinterlands of Russia. By 1351 it had spread so far and wide that sailors arriving in Greenland found its ports deserted.

black from respiratory failure—hence the name Black Death. SOCIAL I M PA C T OF THE P LAG U E . Lacking any modern concept of

The only merciful thing about the Black Death was that death came quickly. Victims typically died within four days—a hundred hours of agony. If they caught a strain of bubonic plague, their lymph glands swelled; if it was pneumonic plague, the lungs succumbed first. Either way, as the end approached, the victim turned purplish-

what causes disease, people looked for spiritual explanations. Some believed that the world was coming to an end, while others joined sects of flagellants, religious enthusiasts who wandered the countryside, beating themselves with lashes as a way of doing penance. The flagellants were tied closely tied to a rising trend toward anti-Semitism: searching for someone to blame, Europeans found a convenient scapegoat in the Jews, who, they claimed, had started the Plague by poisoning the wells of Europe.

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The Black Death aptly illustrates how infectious diseases can have an impact on history in ways both big and small. In just five years the disease killed about 30% of Europe’s population, which had been 100 million in 1300 but which would not reach that level again until 1500. All over the continent, farms were emptied and villages abandoned, leading to scarcity and higher prices. In the short run, these economic conditions spurred peasant revolts, but in the long run, the shortage of workers brought about higher wages and contributed to the emergence of the working and middle classes. Neither popes nor priests, neither kings nor noblemen, were any more equipped than the common people to confront the fearsome disease, and this, too, helped provoke the rise of competing classes and new centers of power in European society. THE ETIOLOGY OF THE P LAG U E . The Black Death, in short, may be

regarded as the beginning of the end of the Middle Ages—a hideously painful event that nevertheless carried positive consequences, which might hardly have been achieved without it. The irony was that the force at the center of all this devastation and change was too small to be seen by the naked eye. Although the disease was carried by rats, the cause of the Black Death was actually a bacillus known today as Pastuerella pestis or Yersinia pestis, which uses fleas as a vector. Modern medicines such as streptomycin, a variety of antibiotic developed after World War II, would have stopped the Plague, but such concepts were a long time in coming. Although the worst phase of the epidemic ended in 1351, it continued to spread, reaching Moscow by 1353; the next five centuries saw occasional outbreaks of the disease. As late as 1894 a strain of plague killed more than six million people in Asia over the course of 14 years.

The example of leprosy shows something about the many curiosities involved in diseases and their study: for example, the fact that a disease can be infectious without being significantly contagious. Leprosy is by definition infectious, inasmuch as it is caused by a pathogen known as Mycobacterium leprae, but the latter is unusual for a number of reasons, including the fact that it is extremely slow in dividing, unlike most bacteria. After years of study, researchers are still not clear as to how leprosy is transmitted, and many believe that genetics may play a role. Thanks to increased understanding of the disease, the stigma that used to go with leprosy—including the reference to people with the disease as “lepers”— has largely been lifted. Yet places such as the leprosy facilities at Carville, Louisiana, and Molokai, Hawaii, continued to exist for many years, if only because the disfigurement associated with the disease influenced the separation of leprosy sufferers from the rest of society. In 1998, with only about 6,000 victims of the disease left in the entire country, the federal government closed the facilities at Carville and Molokai.

The many biblical passages dealing with leprosy illustrate the role that infectious disease has played in human life from the earliest times. The fact that leprosy causes the victim’s skin to turn ghostly white and brings about a gradual withering away of body parts must certainly have seemed like a curse from God. In fact, leprosy, also known as Hansen disease, is caused by the bacillus Mycobacterium leprae, and despite the many fears throughout the ages associated with

Leprosy remains a threat, with some two million cases of the disease worldwide, primarily in nations of Asia, Africa, and Latin America that are both underdeveloped and located in tropical zones. It has, however, ceased to be the worldwide danger that it once was, and as such it joins ranks with numerous other afflictions that formerly held all of humankind in the grip of terror. For example, tuberculosis, caused by a bacillus that attacks the lungs, afflicted a huge population in the nineteenth century, bringing an end to the careers of figures that ranged from the great English poet John Keats to the American gunslinger Doc Holliday. Holliday, in fact, traveled to Tombstone, Arizona, where he and Wyatt Earp participated in the infamous shootout at the O.K. Corral, because he thought the climate would help his condition. Their story has been portrayed in countless films; for example, in Tombstone (1993), Val Kilmer gives an extremely convincing

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touching lepers, it is not very contagious. A scene in the 1973 blockbuster Papillon illustrates this fact. The title character, a prison escapee played by Steve McQueen, takes a drag from a cigar offered to him by a leper, who then asks him if he knew that leprosy is not contagious. Papillon says no, indicating that he simply intended to build a sense of shared risk with someone who he hoped would aid his escape.

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LEPROSY, CAUSED BY THE BACILLUS MYCOBACTERIUM LEPRAE, brings about a gradual withering away of body parts. There are some two million cases of the disease worldwide, primarily in the underdeveloped nations of Asia, Africa, and Latin America. (© Paul A. Souders/Corbis. Reproduced by permission.)

portrayal of the debilitating effects that Holliday’s tuberculosis (aggravated by his lifestyle) must have had on him. Today, tuberculosis is not nearly the scourge that it once was, though it remains a problem, particularly because of patients’ increasing resistance to the antibiotics used to treat it. (See Infection for more about antibiotics.) VAC C I N AT I O N A N D C O N T I N U I N G T H R E AT S . When Europeans invaded

the lands of Native Americans, they brought with them a host of microorganisms to which they had developed an immunity but to which the Indians were completely vulnerable. Although Europeans and their descendants had developed immunities to various diseases, thanks to generations of exposure to pathogens, they and the rest of the world remained vulnerable to a host of contagious disease, including cholera, smallpox, chicken pox, measles, mumps, yellow fever, polio, malaria, and many others. Today, vaccines have virtually eradicated many of these contagious diseases and keep others at bay. (Anyone who has ever had a cholera vaccine, which causes the patient’s body to become miserably sore, achy, and tender for about 48 hours, has some idea of just how awful the disease itself must be.) Polio,

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which once posed an enormous threat to American children and crippled one of America’s greatest leaders, President Franklin D. Roosevelt, is an artifact of history, thanks to vaccines developed after World War II. Yet some killers never really die. For instance, malaria, caused by a protozoan parasitic genus known as Plasmodium and spread by mosquito biological vectors, infects from 300 to 500 million people annually and kills up to 2.7 million people every year. Although the substance known as quinine showed some promise as a treatment during most of the nineteenth and twentieth centuries, Plasmodium has become increasingly resistant to it. In the search for a cure for what has been called “the most devastating disease in history,” some 100,000 drugs have been tested.

Some Other Killers The twentieth century saw its own version of the Plague, in the form of the 1918–1920 influenza epidemic. Carried to all corners of the globe by soldiers returning from World War I, “the Influenza,” as it came to be known (again with a capital letter to distinguish it as the greatest outbreak of a particular disease), killed 20 million

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AIDS. (For more about AIDS, see Immunity and Immunology.)

Infectious Diseases

T H E E B O LA V I R U S . AIDS was not the only infectious condition to come out of central Africa and terrorize the world in the late twentieth century. Beginning in about 1975, numerous viruses, previously unknown and terrifyingly lethal, emerged from tropical regions of Africa, South America, and Asia. So great was the rise of new infectious diseases that some epidemiologists believed this was tied with economic development: as humans cultivated previously undeveloped lands and delved into more isolated parts of the world, they might be exposing new viruses.

FOLLOWING

THE

ATTACKS ON THE

SEPTEMBER 11, 2001, TERRORIST UNITED STATES, A SERIES OF LETTERS

CONTAINING ANTHRAX SPORES SHOWED UP AROUND THE COUNTRY, AND EXPOSURE TO THE DISEASE LED TO A HANDFUL OF DEATHS.

IN

ONE INCIDENT HAZARDOUS

MATERIALS EXPERTS WERE CALLED TO

CAPITOL HILL

TO

INVESTIGATE. (© AFP/Corbis. Reproduced by permission.)

people—more than the war itself. Then there is the greatest epidemic of the latter part of the twentieth century and the early twenty-first century: AIDS. This disease is linked to the human immunodeficiency virus (HIV), a retrovirus (see Infection for an explanation of retrovirus) that causes a gradual breakdown of the victim’s immune system. People do not die of AIDS per se but of the illnesses—particularly pneumonia or Kaposi’s sarcoma, a cancer of the tissues—to which AIDS makes them susceptible. The disease is transmitted primarily by sexual contact and intravenous drug use. A smaller number of particularly tragic cases result from no actions on the part of the victim, who in this case is either the recipient of infected blood or the child of a mother with AIDS. Since the disease first came to public attention in 1981, 21.8 million people worldwide (and about 750,000 in the United States) have died from it. The vast majority of deaths have been in sub-Saharan Africa, and 90% of all AIDS cases are in developing countries. Worldwide, approximately 36.1 million people have either HIV or

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Few of these inspired as much terror as the Ebola virus, and the fear is understandable, given the effects of the disease. Three to nine days after the illness enters the body, the victim begins to experience fever and other flu-like symptoms, sudden exhaustion, sore throat, muscle pain, and headache. Vomiting and diarrhea soon follow, and the vomit and stools are black with blood. Soon hemorrhaging occurs, with blood flowing from the nose, ears, and even the eyes. Internal organs begin to liquefy, and within three weeks of contracting the virus, the victim is usually dead. An almost unbelievably hideous condition, Ebola might seem at first glance a great deal like the Black Death. Why, then, has it not ravaged whole populations the way the Plague did? It is certainly not because scientists have a cure for Ebola; the best doctors can hope to do, if they detect the disease early enough, is to provide supportive care, such as blood transfusions, that may save the patient’s life. Yet even the worst outbreaks of the disease have not occurred on anything like the scale of the Plague: the worst known outbreak of Ebola, in Uganda in 2000–2001, killed 425 people. Part of the reason Ebola is not capable of spreading rapidly is, ironically, because it is such an efficient killer: it kills its human victims before they have a chance to spread it to many other victims. Other than nonfatal incidents in laboratories in the United States, England, and Italy, as well as one case in a monkey export facility in the Philippines—various primates are carriers—all Ebola cases and outbreaks have been in Africa, primarily in Zaire (now Democratic Republic of the Congo), Sudan, and Gabon. Many times, local conditions, situations, and practices have

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exacerbated the spread of the disease. For example, in 1996, a group of people in Gabon found a dead chimpanzee in the forest and ate it; as a result, 37 people died. The Uganda outbreak became much worse than it might have been because locals, lacking education as to antiseptic procedures, failed to take proper precautions. Many died as a result of attending funerals of Ebola victims at which bodies were not disposed of properly. T Y P H O I D M A RY. Sometimes a single person can be a walking epidemic, as in the case of the Irish cook Mary Mallon (1869–1938), better known as “Typhoid Mary.” Mallon was an example of the fact that some people, because of genetic characteristics or other specifics, can act as carriers of a disease without ever contracting it themselves. Even though Typhoid Mary had Salmonella typhosa bacteria in her system, she did not get sick; still, she was highly contagious, and her profession as cook made her particularly dangerous. At least three deaths and 53 cases of typhoid fever were linked directly to her, with thousands of other probable cases of infection indirectly caused by this human vector.

Part of what made her so notorious—hence her nickname, given to her by the press—was the fact that Mallon did not seem to care how many people she infected. In the first decade of the twentieth century, authorities tracked her down as the cause of, or at least a contributing factor in, an outbreak of typhoid in the New York City area. Instead of cooperating with officials, Mallon repeatedly escaped before being caught and confined to Riverside Hospital on New York’s North Brother Island in 1910. She served three years in isolation there before her release, after which she promptly went back to work as a cook—despite explicit orders not to do so. It was this (and an outbreak of typhoid fever at her place of work, which happened to be a hospital) that earned her the nickname by which she became known to history. She was caught again in 1915 and spent the remainder of her life on North Brother Island. T H E T H R E AT O F B I O LO G I CA L WA R FA R E . Infinitely more despicable than

Typhoid Mary are terrorists and rogue nations that would willingly unleash infectious disease on large, unsuspecting civilian populations. One such pathogen is Bacillus anthracis, the cause of anthrax, a deadly bacterial disease of cattle and

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Infectious Diseases

KEY TERMS A term for a disease that is communicable or contagious and comes from outside the body. Compare with intrinsic. EXTRINSIC:

A theory in medicine, widely accepted today, that infections, contagious diseases, and other conditions are caused by the actions of microorganisms. GERM THEORY:

IMMUNE SYSTEM: A network of organs, glands, and tissues that protects the body from foreign substances.

The condition of being able to resist a specific disease, particularly through means that prevent the growth and development or counteract the effects of pathogens. IMMUNITY:

INFECTION: A state or condition in which parasitic organisms attach themselves to the body or to the inside of the body of another organism, causing contamination and disease in the host.

A term for a disease that is not communicable or contagious and comes from inside the body. Compare with extrinsic. INTRINSIC:

A disease-carrying parasite, usually a microorganism. PATHOGEN:

STD:

Sexually transmitted disease.

VECTOR: An organism, such as an insect, that transmits a pathogen to the body of a host.

other grazing animals. Under the right circumstances, anthrax can kill a human in about 36 hours, though a number of antibiotic treatments are effective in the early stages of the disease. During the late twentieth century, the United States and Soviet Union experimented with

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the use of anthrax in biological warfare, and an accidental release of anthrax spores at a Soviet lab in 1979 led to some 68 deaths. Following the September 11, 2001, terrorist attacks on the World Trade Center in New York City and on the Pentagon, a series of letters containing anthrax spores showed up around the United States, and exposure to the disease led to a handful of deaths. Although the attacks were linked initially to Osama bin Laden and his al-Qaeda organization, authorities increasingly began to suspect that home-grown terrorists were simply exploiting the September 11 attacks as cover for their own deeds.

rikyo attempted unsuccessfully to launch botulism attacks in Tokyo on three occasions in 1995. The Japanese government itself—that is, the Axis Japanese government of World War II—experimented with another biological agent, tularemia, or Francisella tularensis. The pathogen, which causes lung inflammation and death, is considered one of the most dangerous forms of biological weapon, because it is extremely efficient and easy to spread. America’s military, borrowing an idea from its former enemy, developed its own F. tularensis strain in the late 1960s but destroyed its stockpile in 1973.

Still, there was little doubt that bin Laden, the Iraqi dictator Saddam Hussein, or North Korea’s ruling clique would use biological agents if the opportunity arose. One threat that loomed in the aftermath of September 11 was the possibility that bin Laden’s followers would reintroduce the smallpox virus, which had been eradicated by worldwide vaccinations during the 1970s. The reason why smallpox could pose such a great threat is precisely that it has been eliminated, and few Americans born after 1973 have received vaccines. Unless they gained access to one of the two labs worldwide (one in the United States and one in Russia) where smallpox virus is stored for the purpose of making vaccines, however, terrorists would be unable to obtain a sample. (It is this matter of access that led authorities to suspect that the anthrax attacks were an “inside job.”)

WHERE TO LEARN MORE Centers for Disease Control and Prevention (Web site). . Cranmer, Hilarie. Anthrax Infection. Emedicine.com (Web site). . DeSalle, Rob. Epidemic!: The World of Infectious Disease. New York: New Press, 1999. Everything You Need to Know About Diseases. Springhouse, PA: Springhouse Corporation, 1996. Ewald, Paul W. Plague Time: How Stealth Infections Cause Cancers, Heart Disease, and Other Deadly Ailments. New York: Free Press, 2000. Hoff, Brent H., Carter Smith, and Charles H. Calisher. Mapping Epidemics: A Historical Atlas of Disease. New York: Franklin Watts, 2000. Infection and Immunity. University of Leicester Microbiology and Immunology (Web site). . Marr, Lisa. Sexually Transmitted Diseases: A Physician Tells You What You Need to Know. Baltimore, MD: Johns Hopkins University Press, 1998.

Another biological agent that poses a threat is Clostridium botulinum, which causes botulism, a toxic condition that can result in paralysis. Members of the fanatic Japanese cult Aum Shin-

Shein, Lori. AIDS. San Diego: Lucent Books, 1998.

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Oldstone, Michael B. A. Viruses, Plagues, and History. New York: Oxford University Press, 1998.

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IMMUNITY AND IMMUNOLOGY

CONCEPT Immunity is the condition of being able to resist a specific disease, particularly through means that prevent the growth and development of disease-carrying organisms or counteract their effects. It is regulated by the immune system, a network of organs, glands, and tissues that protects the body from foreign substances. Immunology is the study of the immune system, immunity, and immune responses. Progress in immunology over the past two centuries has made inoculation—the prevention of a disease by the introduction to the body, in small quantities, of the virus or other microorganism that causes the disease—widely accepted and practiced. Despite such progress, however, some diseases evade human efforts to counteract them through medicine or other forms of treatment. This is particularly the case with a disease in which the immune system shuts down entirely: a condition known as acquired immunodeficiency syndrome, or AIDS.

HOW IT WORKS Immunity and the Immune System

type of white blood cell. The principal types of lymphocyte are B cells and T cells. These cells recognize random antigens, or substances capable of requiring an immune response. Certain researchers believe that while some B cells and T cells are directed toward fighting an infection, others remain in the bloodstream for months or even years, primed to respond to another invasion of the body. Such “memory” cells may be the basis for immunities that allow humans to survive such plagues as the Black Death of 1347–1351 (see Infectious Diseases). Other immunologists, however, maintain that trace amounts of a pathogen persist in the body and that their continued presence keeps the immune response strong over time.

Immunology Immunology is the study of how the body responds to foreign substances and fights off infection and other disease-causing agents. Immunologists are concerned with the parts of the body that participate in this response, and this investigation takes them beyond looking merely at tissues and organs to studying specific types of cells or even molecules.

The functioning of the immune system is considered in a separate essay, along with the means by which that system responds to foreign invasion. Also included in that essay is a discussion of allergies, which arise when the body responds to ordinary substances as though they were pathogens, or disease-carrying parasites. The body cannot know in advance what a pathogen will look like and how to fight it, so it creates millions and millions of different lymphocytes, a

From ancient times, humans have recognized that some people survive epidemics, when the majority are dying. About 1,500 years ago in India, physicians even practiced a form of inoculation, as we discuss later. The modern science of immunology, however, had its beginnings only in 1798, when the English physician Edward Jenner (1749–1823) published a paper in which he maintained that people could be protected from the deadly disease smallpox by the prick of a needle dipped in the pus from a cowpox boil. (Cow-

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pox is a related, less-lethal disease that, as its name suggests, primarily affects cattle.) Later, the great French biologist and chemist Louis Pasteur (1822–1895) theorized that inoculation protects people against disease by exposing them to a version of the pathogen that is harmless enough not to kill them but sufficiently like the disease-causing organism that the immune system learns to fight it. Modern vaccines against such diseases as measles, polio, and chicken pox are based on this principle. H U M O RA L AND CELLULAR I M M U N I T Y. In the late nineteenth century, a

scientific debate raged between the German physician Paul Ehrlich (1854–1915) and the Russian zoologist Élie Metchnikoff (1845–1916) concerning the means by which the body protects against diseases. Ehrlich and his followers maintained that proteins in the blood, called antibodies, eliminate pathogens by sticking to them. This phenomenon and the theory surrounding it became known as humoral immunity. Metchnikoff and his students, on the other hand, had noted that certain white blood cells could swallow and digest foreign materials. This cellular immunity, they claimed, was the real way that the body fights infection. In fact, as modern immunologists have shown, both the humoral and cellular responses identified by Ehrlich and Metchnikoff, respectively, play a role in fighting disease.

REAL-LIFE A P P L I C AT I O N S Inoculation and Vaccines Inoculation is the prevention of a disease by the introduction to the body, in small quantities, of the virus or other microorganism that causes that particular ailment. It is a brilliant idea, yet one that seems to go against common sense. For that reason, it was a long time in coming: not until the time of Jenner, in about 1800, did the concept of inoculation become widely accepted in the West. Nonetheless, it had been applied more than 13 centuries earlier in India. In the period between about 500 B.C. and 500, Hindu physicians made extraordinary strides in a number of areas, pioneering such techniques as plastic surgery and the use of tourniquets to stop bleeding. Most impressive of

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all was their method of treating smallpox, which remained one of the world’s most deadly diseases until its eradication in the late 1970s. Indian physicians apparently took pus or scabs from the sores of a mildly infected patient and rubbed the material into a small cut made in the skin of a healthy person. The Indians’ method was risky, and there was always a chance that the patient would become deathly ill, but the idea survived and gradually made its way west over the ensuing centuries. SMALLPOX

V A C C I N AT I O N .

Smallpox, or variola, is carried by a virus that causes the victim’s body to break out in erupting, pus-filled sores. Eventually, these sores dry up, leaving behind scars that may alter the appearance of the victim permanently, depending on the intensity of the disease. Such was the case with Lady Mary Wortley Montagu (1689–1762), a celebrated English writer and noblewoman. Known for her passionate relationships, romantic and otherwise, Lady Montagu had been scarred from youth by smallpox, and no doubt this experience gave her heightened concern for the victims of the disease. While she was in Turkey with her husband, Edward, an ambassador, she became aware of an inoculation method, probably based on the Hindu practice of many centuries before, used by local women. Lady Montagu arranged for her three-year-old son to be inoculated against smallpox in 1717, and after returning home, initiated smallpox inoculations in England. Nonetheless, the problem remained that the inoculated person contracted a serious case of the disease and died, at least some of the time. More than 80 years later, in 1796, during a smallpox epidemic, Jenner decided to test a piece of folk wisdom to the effect that anyone who contracted cowpox became immune to human smallpox. He took cowpox fluid from the sores of a milkmaid named Sarah Nelmes and rubbed it into cuts on the arm of an eight-year-old boy, James Phipps, who promptly came down with a mild case of cowpox. Soon, however, James recovered, and six weeks later, when Jenner injected him with samples of the smallpox virus, the boy was unaffected.

A.D.

Jenner, who published his findings after conducting additional tests, coined a new term for the type of inoculation he had used: vaccination, from the Latin word for cowpox, vaccinia. (The

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THE

MODERN SCIENCE OF IMMUNOLOGY HAD ITS BEGINNINGS IN 1798, WHEN THE

ENGLISH

PHYSICIAN

EDWARD JEN-

NER PUBLISHED A PAPER IN WHICH HE MAINTAINED THAT PEOPLE COULD BE PROTECTED FROM THE DEADLY DISEASE SMALLPOX BY THE PRICK OF A NEEDLE DIPPED IN THE PUS FROM A COWPOX BOIL. (© Bettmann/Corbis. Reproduced by per-

mission.)

latter term comes from the Latin vacca, or “cow,” the source of such terms as the French vache.) With the success of his vaccine, Jenner was awarded a sum of money to continue his work, and he soon oversaw the vaccination of thousands of English citizens, including the royal family. The practice spread to Germany and Russia and then to the United States. In Lady Montagu’s time, the American clergyman Cotton Mather (1663–1728) had been an advocate of vaccination, and now President Thomas Jefferson (1743–1826) became an ardent proponent of Jenner’s methods.

for poliomyelitis (more commonly known as polio), in which the skeletal muscles waste away and paralysis and often permanent disability and deformity ensue. Although polio had been known for ages, the first half of the twentieth century had seen an enormous epidemic in the United States.

Exactly 70 years later, the American microbiologist Jonas Salk (1914–1995) created a vaccine

The most famous victim of this scourge was the future president Franklin D. Roosevelt (1882–1945), who contracted it while on vacation in 1921. Throughout the 1930s and 1940s, polio remained a threat, especially to children; at the peak of the epidemic, in 1952, it killed some 3,000 Americans in one year, while 58,000 new cases were reported. At the same time, Salk was working on his vaccine, which finally was declared safe after massive testing on schoolchildren. In 1961 an oral polio vaccine developed by the Polish-born American virologist Albert Sabin (1906–1993) was licensed in the United States. Whereas the Salk vaccine contained the killed versions of the three types of poliovirus that had been identified in the 1940s, the Sabin vaccine used weakened live poliovirus. Because it was taken by mouth, the Sabin vaccine proved more convenient and less expensive to administer than the Salk vaccine, and it soon overtook

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RA B I E S A N D P O L I O I N O C U LA T I O N . The next advancement in the study of

vaccines came almost 100 years after Jenner’s discovery. In 1885 Pasteur saved the life of Joseph Meister, a nine-year-old boy who had been attacked by a rabid dog, by using a series of experimental rabies vaccinations. Pasteur’s rabies vaccine, the first human vaccine created in a laboratory, was made from a version of the live virus that had been weakened by drying it over potash (sodium carbonate—burnt wood ashes).

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the latter in popularity. By the early 1990s health organizations reported that polio was close to extinction in the Western Hemisphere.

vaccines, many life-threatening infectious diseases have been forced into retreat. In the United States, children starting kindergarten typically immunized against polio, diphtheria, tetanus, measles, and several other diseases. Other vaccinations are used only by people who are at risk of contracting a disease, are exposed to a disease, or are traveling to an area (usually in the Third World) where particular diseases are common. Such vaccinations include those for influenza, yellow fever, typhoid, cholera, and hepatitis A.

Within two to four weeks of being infected with the virus that causes AIDS (HIV, human immunodeficiency virus), a patient will experience what at first seems like flu: high fever, headaches, sore throat, muscle and joint pains, nausea and vomiting, open ulcers in the mouth, swollen lymph nodes, and perhaps a rash. As the immune system begins to fight the invasion, some cells produce antibodies to neutralize the viruses that are floating free in the bloodstream. Killer T cells destroy many other cells infected with the AIDS virus, and the patient enters a phase of the disease in which no symptoms are evident.

Internationally, 80% of the world’s children had been inoculated as of 1990 for six of the primary infectious diseases: polio, whooping cough, measles, tetanus, diphtheria, and tuberculosis. Smallpox was no longer on the list, because efforts against it had proved overwhelmingly successful. (See Infectious Diseases for more on the threat, or nonthreat, of smallpox as a form of biological warfare.) Despite these successes, however, each year more than two million children who have not received any vaccinations die of infectious diseases. Even polio has continued to be a threat in some parts of the world: as many as 120,000 cases are reported around the world each year, most in developing regions. And as if the threat from age-old diseases were not enough, in the last quarter of the twentieth century a new killer entered the fray: AIDS.

Although at this point it seems as though the worst is over, in fact, the AIDS virus is at work on the immune system, quietly destroying the body’s protection by infecting those T cells that would protect it. With an immune system that gradually becomes more and more unresponsive, the patient is made vulnerable to any number of infections. Normally, the body would be able to fight off these attacks with ease, but with the immune system itself no longer functioning properly, infectious diseases and cancers are free to take over. The result is a long period of increasing misery and suffering, sometimes accompanied by dementia or mental deterioration caused by the ravaging of the brain by disease. Whatever the course it takes, the end result of AIDS is always the same: not just death but a miserable, excruciatingly painful death.

TRIUMPHS AND CONTINUING C H A L L E N G E S . Thanks to these and other

AIDS A viral disease that is almost invariably fatal, AIDS destroys the immune systems of its victims, leaving them vulnerable to a variety of illnesses. No cure has been found and no vaccine ever developed. The virus that causes AIDS has proved to be one of the most elusive pathogens in history, and so far the only effective way not to contract the disease is to avoid sharing bodily fluid with anyone who has it. This means not having sex without condoms (and, to be on the truly safe side, not having sex outside a committed, fully monogamous relationship) and not engaging in intravenous drug use. But there are some people who have contracted the AIDS virus through no actions or fault of their own: people who have received it in blood transfusions or,

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even worse, babies whose AIDS-infected mothers have passed the disease on to them.

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B I RT H O F A K I L L E R. Believed to have originated in Africa, where the majority of AIDS cases still are found (see Infectious Diseases for statistics on AIDS), the disease first appeared in the United States in 1981. In that year two patients were diagnosed with an unusual form of pneumonia and with Kaposi’s sarcoma, a type of cancer that previously had struck only people of Mediterranean origin aged 60 years and older. The appearance of that condition in younger persons of non-Mediterranean origin prompted an investigation by the United States Centers for Disease Control and Prevention (CDC).

Through the efforts of physicians both inside and outside the CDC, understanding of AIDS—the name and acronym appeared in 1982—gradually emerged. In 1983 scientists at

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the Pasteur Institute in Paris, as well as a separate team in the United States, identified the virus that causes AIDS, a pathogen that in 1986 was given the name human immunodeficiency virus (HIV). Further research showed that HIV, a retrovirus (see Infectious Diseases for an explanation of retrovirus), is subdivided into two types: HIV-1 and HIV-2. In people who have HIV-2, AIDS seems to take longer to develop; however, neither form of HIV carries with it a guarantee that a person will contract the disease. At first it was believed that if someone were HIVpositive, meaning that the person had the virus, it was a virtual death sentence. Therefore in 1991, when the basketball superstar Earvin “Magic” Johnson (1959–) announced that he was HIVpositive, it was an extremely melancholy event. Fans and admirers all over the world assumed that Johnson shortly would contract AIDS and begin to wither away in the process of suffering an exceedingly panful, dehumanizing death. The fact that Johnson was alive and healthy more than ten years after the diagnosis of his infection with HIV serves to indicate that there is a great deal of difference between being HIVpositive and having AIDS. It also says much about people’s emerging understanding of the disease and the virus that causes it. So, too, does Johnson’s experience as he attempted, twice, to make a return to the court after retiring in the wake of his HIV announcement. Before examining his experiences, let us look at the social climate engendered by this politically volatile immunodeficiency syndrome. CHANGING

VIEWS

ON

AIDS.

AIDS first was associated almost exclusively with the male homosexual community, which contracted the disease in large numbers. This had a great deal to do with the fact that male homosexuals were apt to have far more sexual partners than their heterosexual counterparts and because anal intercourse is more likely to involve bleeding and hence penetration of the skin shield that protects the body from infection. The association of AIDS with homosexuality led many who considered themselves part of the societal mainstream to dismiss AIDS as a “gay disease,” and the fact that intravenous drug users also contracted the disease seemed only to confirm the prejudice that AIDS had nothing to do with heterosexual non-junkies. Some so-called Christian ministers even went so far as to assert, sometimes with no

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Immunity and Immunology

A VIRAL DISEASE THAT IS ALMOST INVARIABLY FATAL, AIDS DESTROYS THE IMMUNE SYSTEMS OF ITS VICTIMS, LEAVING THEM VULNERABLE TO A VARIETY OF ILLNESSES, AMONG THEM, K APOSI’S SARCOMA (SHOWN HERE), A TYPE OF CANCER THAT BEFORE THE 1980S STRUCK ONLY PEOPLE OF

MEDITERRANEAN

ORIGIN AGED 60

YEARS AND OLDER. (© Roger Ressmeyer/Corbis. Reproduced by

small amount of satisfaction, that AIDS was God’s punishment for homosexuality. Then, during the mid-1980s, AIDS began spreading throughout much of society: to heterosexuals, hemophiliacs (see Noninfectious Diseases) and others who received blood, and even babies. The fact that AIDS could be transferred through heterosexual intercourse proved that it was not just a disease of homosexuals. Nor were all homosexuals necessarily susceptible to it. In fact, the safest of all sexual groups was homosexual women, who often tended toward monogamy and whose form of sexual contact was least invasive. As AIDS spread throughout society, so did paranoia. Rumors circulated that a person could catch the disease from a mosquito bite or from any contact with the bodily fluids of another person—not just semen or blood but even sweat or saliva. People with AIDS began to acquire the status lepers once had held (see Infectious Diseases). By the mid-1990s views had changed considerably, and society as a whole had a much more realistic view of AIDS. This came about to

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KEY TERMS The prevention of a

A change in bodily reactivity to an antigen as a result of a first exposure. Allergies bring about an exaggerated reaction to substances or physical states that normally would have little significant effect on a healthy person.

INOCULATION:

Proteins in the human immune system that help the body fight foreign invaders, especially pathogens and toxins.

cells, or B lymphocytes and T lymphocytes.

ALLERGY:

ANTIBODIES:

A substance capable of stimulating an immune response or reaction. ANTIGEN:

An antigen-presenting cell—a macrophage that has ingested a foreign cell and displays the antigen on its surface.

disease by the introduction to the body, in small quantities, of the virus or other microorganism that causes the disease. A type of white blood

LYMPHOCYTE:

cell, varieties of which include B cells and T

A type of phagocytic

MACROPHAGE:

cell derived from monocytes. A type of white blood cell

MONOCYTE:

that phagocytizes (engulfs and digests) foreign microorganisms.

APC:

A type of white blood cell that gives rise to antibodies. Also known as a B lymphocyte.

B CELL:

Having only one

mate. A disease-carrying para-

PATHOGEN:

site, usually a microorganism. PHAGOCYTE:

A cell that engulfs and

Affecting or potentially affecting a large proportion of a population (adj.) or an epidemic disease (n.)

digests another cell.

Of or relating to the antibodies secreted by B cells that circulate in bodily fluids.

role in the immune response. T cells

A network of organs, glands, and tissues that protects the body from foreign substances.

immune response; helper T cells, which are

The condition of being able to resist a particular disease, particularly through means that prevent the growth and development or counteract the effects of pathogens.

both the antibody and cell-mediated

EPIDEMIC:

HUMORAL:

IMMUNE

SYSTEM:

IMMUNITY:

The study of the immune system, immunity, and immune responses. IMMUNOLOGY:

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MONOGAMOUS:

T CELL:

A type of lymphocyte, also

known as a T lymphocyte, that plays a key include cytotoxic T cells, which destroy virus-infected cells in the cell-mediated key participants in specific immune responses that bind to APCs, activating immune responses; and suppressor T cells, which deactivate T cells and B cells. VACCINE:

A preparation containing

microorganisms, usually either weakened or dead, which are administered as a means of increasing immunity to the disease caused by those microorganisms.

some extent because of increased education and

Johnson, who was by far the most widely known

awareness—and in no small part because of

and admired HIV-positive celebrity.

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M AG I C C O M E S B AC K . After playing on the United States “Dream Team” that trounced all opponents at the 1992 Summer Olympics in Barcelona, Spain, Johnson attempted a comeback with the Lakers the following year. Owing to fears on the part of many other players that they might contract AIDS by coming into close contact with him on the court, however, he decided again to retire. In December 1991 Johnson had established the Magic Johnson Foundation to promote AIDS awareness, and he devoted himself to this and other AIDS-related causes as well as to other ventures. Raising money for AIDS led him out onto the basketball court again in October 1995, when he and the American All Stars faced an Italian team in a benefit game, with an unsurprisingly lopsided score of 135–81.

Then, in February 1996, Johnson made his second attempted comeback with the Lakers. He ended up retiring again four months later, this time for good, but because he had chosen to and not because he had been forced to do so. Thanks in part to his AIDS education programs, in his second comeback Johnson discovered that players realized that they were not likely to catch the virus on the court. As the New Jersey Nets’ player Jayson Williams told one reporter, “You’ve got a better chance of Ed McMahon knocking on

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your door with $1 million than you have of catching AIDS in a basketball game.”

Immunity and Immunology

WHERE TO LEARN MORE Aaseng, Nathan. Autoimmune Diseases. New York: Franklin Watts, 1995. American Autoimmune Related Diseases Association, Inc. (AARDA) (Web site). . Benjamini, Eli, and Sidney Leskowitz. Immunology: A Short Course. New York: Liss, 1988. Clark, William R. At War Within: The Double-Edged Sword of Immunity. New York: Oxford University Press, 1995. Dwyer, John M. The Body at War: The Miracle of the Immune System. New York: New American Library, 1989. Edelson, Edward. The Immune System. New York: Chelsea House, 1989. How Your Immune System Works. How Stuff Works (Web site). . “Infection and Immunity.” University of Leicester Microbiology and Immunology (Web site). . “The Lymphatic System and Immunity.” Estrella Mountain Community College (Web site). . “Magic Johnson Retires Again, Saying It’s on His Own Terms This Time.” Jet, June 3, 1996, p. 46. UNAids: The Joint UN Programme on HIV/AIDS (Web site). .

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THE IMMUNE SYSTEM The Immune System

CONCEPT The immune system is a network of organs, glands, and tissues that protects the body from foreign substances. These substances include bacteria, viruses, and other infection-causing parasites and pathogens. Usually, the immune system is extremely effective in performing its work of defending the body, but sometimes an error occurs in this highly complex system, and it can lead to terrible mistakes. The result can be an allergic reaction, which can be as simple as a case of the sniffles and as serious as a fatal condition. Or the error can manifest as an autoimmune disorder, such as lupus, in which the body rejects its own constituents as foreign invaders.

HOW IT WORKS The Immune System in General The human body is under near constant attack from pathogens, or disease-carrying parasites, of the type discussed in Infection, Infectious Diseases, and Parasites and Parasitology. No human would live very long without the immune system, which includes two levels or layers of protection, the nonspecific and the specific defenses. The nonspecific defenses, including the skin and mucous membranes, serve as a first defensive line for preventing pathogens from entering the body. The specific defenses are activated when these microorganisms get past the nonspecific defenses and invade the body.

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pathogens or toxins or by some other outside threat. Second, the immune response must be activated quickly, before the invaders destroy many body tissue cells. For the immune system to respond effectively, several conditions must be in place, including the proper interaction of nonspecific and specific defenses. The nonspecific defenses on the skin do not identify the antigen (a substance capable of stimulating an immune response or reaction) that is attacking or potentially attacking the body; instead, these defenses simply react to the presence of what it identifies as something foreign. Often, the nonspecific defenses effectively destroy microorganisms, but if these defenses prove ineffective and the microorganisms manage to infect tissues, the specific defenses go into action. The specific defenses function by detecting the antigen in question and mounting a response that targets it for destruction. T H E M A J O R H I S T O C O M PAT I B I L I T Y C O M P L E X . How does the specific

system “know” what is foreign and what is part of the body? The cell membrane of every cell is studded with various proteins, which together are known as the major histocompatibility complex, or MHC. The MHC is a kind of pass code, since all cells in the body must possess an identical pattern so that the body will identify those cells as belonging to the “self.” An invading microorganism, such as a bacterium, does not have the same MHC, and when the immune system encounters it, it alerts the body that it has been invaded by a foreign cell.

For the immune system to work properly, two things must happen: first, the body must recognize that it has been invaded, either by

Every person has his or her nearly unique MHC, and the response of the immune system to foreign MHC can pose a problem where organ transplants are concerned. Because the immune

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MARROW,

THE SOFT TISSUE AT THE CORE OF BONES, IS A KEY PRODUCER BOTH OF LYMPHOCYTES AND OF ANOTH-

ER COMPONENT OF BLOOD, THE HEMOGLOBIN-CONTAINING RED BLOOD CELLS. (© Lester V. Bergman/Corbis. Reproduced by per-

mission.)

system interprets the transplanted organ, with its foreign MHC, as an invader, the body may reject the transplant, and therefore organ recipients usually take immunosuppressant drugs to quell the immune response. Furthermore, doctors often attempt transplants only between close relatives, who are likely to have genetically similar MHCs, or try to find organs that match in the major histocompatibility antigens.

Parts of the Immune System The organs of the immune system include the lymphatic vessels, lymph nodes, tonsils, thymus, Peyer’s patch, and spleen. Each of these organs either produces the cells that participate in the immune response or serves as a site for immune function. Lymphocytes, a type of white blood cell, are concentrated in the lymph nodes, which are masses of tissue that act as filters for blood at various places throughout the body-most notably the neck, under the arms, and in the groin. As the lymph (white blood cells plus plasma) filters through the lymph nodes, foreign cells are detected and overpowered.

bacteria that might enter the body via the nose and mouth. Peyer’s patches, scattered throughout the small intestine and appendix, are lymphatic tissues that perform this same function in the digestive system. The thymus gland, located within the upper chest region, is another site of lymphocyte production, though it is most active during childhood. The thymus gland continues to grow until puberty, protecting a child through the critical years of early development, but in adulthood it shrinks almost to the point of vanishing. Marrow, the soft tissue at the core of bones, is a key producer both of lymphocytes and of another component of blood, the hemoglobincontaining red blood cells. Because of its critical role in the immune system, it is a very serious decision to allow marrow to be extracted (itself an extremely serious operation, of course) for use in a cancer treatment, as described in Noninfectious Diseases. The spleen, in addition to containing lymphatic tissue and producing lymphocytes, acts as a reservoir for blood and destroys worn-out red blood cells.

The tonsils, located at the back of the throat and under the tongue, contain large numbers of lymphocytes and filter out potentially harmful

ANTIBODIES, B CELLS, AND T C E L L S . The functioning of the immune sys-

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stances, most notably antibodies and the two significant varieties of lymphocyte: B cells and T cells. Antibodies, the most well known of the three, are proteins in the human immune system that help fight foreign invaders. B cells (B lymphocytes) are a type of white blood cell that gives rise to antibodies, whereas T cells (T lymphocytes), are a type of white blood cell that plays an important role in the immune response. T cells are a key component in the cell-mediated response, the specific immune response that utilizes T cells to neutralize cells that have been infected with viruses and certain bacteria. There are three types of T cells: cytotoxic, helper, and suppressor T cells. Cytotoxic T cells destroy virus-infected cells in the cell-mediated immune response, whereas helper T cells play a part in activating both the antibody and the cell-mediated immune responses. Suppressor T cells deactivate T cells and B cells when needed, and thus prevent the immune response from becoming too intense. The intricacies of the immune system’s functioning are far beyond the scope of this essay. The reader interested in a more in-depth review of the substances, organs, glands, and processes is encouraged to seek clarification from a textbook. On the other hand, a very basic and nontechnical example of how the body resists infection can help clarify, in general terms, how the immune system does its work.

REAL-LIFE A P P L I C AT I O N S Protecting the Body As discussed in Infection, not all bacteria are bad; in fact, many are helpful or even essential to humans. When the word bacteria is mentioned, however, most of us think of the “bad” bacteria, which is understandable, since there are so many of them and their effects can be so dramatic. Suppose such a bacterium enters the body, which is an easy situation to imagine—it happens all the time. Indeed, even as you are reading these words, literally trillions of bacteria the world over are attempting to invade human bodies, including your own. Their chances of success are determined by the immune system and response.

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skin is broken, it creates a pathway for invasion. Even a minor cut on a finger can serve as an opening for a microorganism that, once inside the body, will flourish in the body’s warm, bloodwashed interior. When it is established, the bacterium begins to divide rapidly, but already the specific immune system has begun to mount its resistance, and sometimes evidence of the battle can be seen on the outside—for example, in the form of a red, pus-exuding welt. In the bloodstream, lymphocytes engulf bacteria and carry them toward the lymph nodes. For this reason, when the body is under attack, it begins producing white blood cells at an accelerated rate, and for this reason doctors sometimes measure a patient’s white blood cell count. If the number is high, the physician knows that an infection is active somewhere in the patient’s body. Killer blood cells, known by the generic name phagocyte, engulf the bacteria and digestthem, but even as this is occurring, the rapid reproduction of the bacterium provides a challenge to the immune system. If the infectious agent reproduces at a rate beyond the control of the immune system, the physician may provide help in the form of an antibiotic. Alternatively, he or she may lance (cut open) a superficial infection to allow it to drain and to provide access for an antiseptic agent. If the bacterial invasion is minor, the immune system soon dispatches the invader, and the system returns to normal. Often, some of the white blood cells form antibodies against such invading bacteria, so that the immune system will be better armed to combat any future invasions by the same microorganism. The white blood cell count returns to its normal level, but still with the capability of mobilizing the immune defense on short notice. It is this response that is the basis for inoculations against certain infections, a topic discussed in Immunity. Sometimes, however, something goes wrong in the production of antibodies, and instead of properly protecting the body against invaders, the immune system creates an allergy.

Allergies

Most of the time, the skin provides us with sufficient protection from invaders, but if the

An allergy is a change in bodily reactivity to an antigen as a result of a first exposure. Allergies bring about an exaggerated reaction to substances or physical states that would normally have little significant effect on a healthy person. Although the immune system behaves as if it is

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ELECTRON

MICROGRAPH OF A SINGLE DUST MITE.

THEIR

DEAD BODIES AND EXCREMENT CAUSE ASTHMA AND ALLER-

GIC REACTIONS TO HOUSEHOLD DUST. (© Eye of Science/Photo Researchers. Reproduced by permission.)

fighting off a pathogen, in fact, it is launching a complex series of reactions against an irritant. The irritant, or allergen, may well be an otherwise innocuous substance that hardly bothers a person without the allergy. It could even be something that other people enjoy—for example, peanuts or bananas—or at least something, such as animal hair, that does not typically cause people undue discomfort. Allergies also may involve a substance, such as venom from a bee sting, that most people consider far from pleasant but which does not pose a serious threat to someone who is not allergic to it.

may be accompanied by a number of stressful symptoms, ranging from mild reactions, such as hives (the formation of red, swollen areas on the surface of the skin) to a life-threatening situation known as anaphylactic shock. The latter, a condition characterized by a sudden drop in blood pressure and difficulty in breathing, can be accompanied by acute skin irritation in the form of angry red boils all over the body. Collapse or coma can ensue and may result in death.

In extreme cases of allergic reaction, the situation that follows exposure to an allergen truly is one of life and death. The immune response

CAU S E S O F A L L E R GY. Pollens from grasses, trees, and weeds produce such allergic reactions as sneezing, runny nose, swollen nasal tissues, headaches, blocked sinuses, and watery, irritated eyes. Of the 46 million allergy sufferers in the United States, about 25 million

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KEY TERMS A change in bodily reactivity to an antigen as a result of a first exposure. Allergies bring about an exaggerated reaction to substances or physical states that normally would have little significant effect on a healthy person. ALLERGY:

Proteins in the human immune system that help fight foreign invaders, especially pathogens and toxins.

cells to neutralize cells that have been infected with viruses and certain bacteria. GLAND:

A cell or group of cells that fil-

ters material from the blood, processes that material, and secretes it either for use again in the body or to be eliminated as waste.

ANTIBODIES:

A substance capable of stimulating an immune response or reaction. ANTIGEN:

The practice of inhibiting the growth and multiplication of microorganisms, usually by ensuring the cleanliness of the environment.

HEMOGLOBIN:

An iron-containing

protein in red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide from them. Hemoglobin is known for its deep red color.

ANTISEPSIS:

An antigen-presenting cell—a macrophage that has ingested a foreign cell and displays the antigen on its surface.

APC:

A type of white blood cell that gives rise to antibodies. Also known as a B lymphocyte.

B CELL:

The specific immune response that utilizes T CELL-MEDIATED RESPONSE:

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HUMORAL:

Of or relating to the anti-

bodies secreted by B cells that circulate in bodily fluids. IMMUNE

SYSTEM:

A network of

organs, glands, and tissues that protects the body from foreign substances. IMMUNITY:

The condition of being

able to resist a particular disease, particularly through means that prevent the growth and development or counteract the effects of pathogens.

have this form of allergy, known to scientists as rhinitis but to the populace as hay fever. Other common allergens are dust and dust mites, pet hair and fur, insect bites, certain foods or drugs, and skin contact with specific chemical substances. About 12 million Americans are allergic to a variety of chemicals.

an allergen first enters the body, the lymphocytes make what are known as E antibodies. These antibodies attach to mast cells, large cells that are found in connective tissue and contain histamines. The histamines are chemicals released by basophils, a type of lymphocyte, during the inflammatory response.

Some people are allergic to a wide range of substances, while others are affected by only a few or none. Why the difference? The reasons can be found in the makeup of an individual’s immune system, which may produce several chemical agents that cause allergic reactions. The main immune system substances responsible for the symptoms of allergy are the histamines that are produced after exposure to an allergen. When

The second time a given allergen enters the body of a person who has an allergy, it becomes attached to the E antibodies. They stimulate the mast cells to discharge their histamines and other anti-allergen substances. One type of histamine travels to various receptor sites in the nasal passages, respiratory system, and skin, dilating smaller blood vessels and constricting airways. The results include some of the reactions associ-

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KEY TERMS The study of the immune system, immunity, and immune responses. IMMUNOLOGY:

CONTINUED

PATHOGEN:

site, usually a microorganism. PHAGOCYTE:

That portion of the blood that includes white blood cells and plasma but not red blood cells. LYMPH:

Masses of tissue at certain places in the body that act as filters for blood. LYMPH NODES:

A type of white blood cell, varieties of which include B cells and T cells, or B lymphocytes and T lymphocytes. LYMPHOCYTE:

A disease-carrying para-

A cell that engulfs and

digests another cell. T CELL:

A type of white blood cell, also

known as a T lymphocyte, that plays a key role in the immune response. T cells include cytotoxic T cells, which destroy virus-infected cells in the cell-mediated immune response; helper T cells, which are key participants in specific immune responses that bind to APCs, activating

A type of phagocytic cell derived from monocytes.

both the antibody and cell-mediated

Major histocompatibility complex, a group of proteins found on the membrane of each cell in an organism’s body. Since all cells in a particular body have the same MHC pattern, the MHC is a kind of pass code, identifying cells in the body as belonging to the “self.”

which deactivate T cells and B cells.

A type of white blood cell that phagocytizes (engulfs and digests) foreign microorganisms.

that are colorless, lack hemoglobin, have a

MACROPHAGE:

MHC:

MONOCYTE:

ated with allergies, for instance, sneezing or the formation of hives. Another type of histamine constricts the larger blood vessels and travels to the receptor sites found in the salivary and tear glands and in the stomach’s mucosal lining. These histamines stimulate the release of stomach acid, thus creating a stomach ulcer condition. T R E AT M E N T S . There are many treatments for allergy, including (obviously) avoidance of the substance to which the patient is allergic. Among these treatments are the administration of antihistamines, which either inhibit the production of histamine or block histamines at receptor sites. After the administration of antihistamines, E antibody receptor sites on the mast cells are blocked, thereby preventing the release

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immune responses; and suppressor T cells,

TISSUE:

A group of cells, along with

the substances that join them, which form part of the structural materials in plants or animals. WHITE BLOOD CELLS:

Blood cells

nucleus, and include the lymphocytes and other varieties.

of the histamines that cause the allergic reactions. The allergens are still there, but the body’s allergic reactions are suspended for the period of time that the antihistamines are active. Antihistamines, sold both in prescription and over-thecounter forms, also constrict the smaller blood vessels and capillaries, thereby removing excess fluids. Decongestants can bring relief as well, but they can be used for only a short time, since their continued use can irritate and intensify the allergic reaction. In cases of extreme allergic reaction leading to anaphylactic shock, the patient may require an injection of epinephrine (also sometimes called adrenaline), a hormone that the body produces for responding to situations of fear and danger.

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In the case of anaphylactic shock, which involves such severe constriction of the breathing passages that the patient runs a risk of suffocation, epinephrine causes the passages to open, making it possible to breathe again. It also constricts the blood vessels, increasing the pressure and making the blood move more rapidly throughout the body. The body’s own supply of epinephrine is not enough to counteract anaphylactic shock, however, and therefore a person experiencing that condition must receive an emergency injection containing many times the amount of the hormone naturally supplied by the body. It may be administered at a hospital, though doctors usually advise people with severe allergies to keep an emergency supply on hand.

Autoimmune Disorders Allergies are one example of an immune system gone awry, and though they can be fatal, they typically are a reaction to only one or two substances. An autoimmune disorder, on the other hand, is an entirely different class of phenomenon: it a condition in which a person’s body actually rejects itself. This condition comes about when the ability of the immune system to read MHCs becomes scrambled, such that it fails to recognize cells from within the body and instead rejects them as though they came from outside. As a result, the body sets in motion the same destructive operation against its own cells that it normally would carry out against bacteria, viruses, and other such harmful invaders.

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SOME AUTOIMMUNE DISE AS E S . Examples of autoimmune disorders

include lupus, rheumatoid arthritis, autoimmune hemolytic anemia, pernicious anemia, and type 1 diabetes mellitus. (The last of these diseases is discussed in Noninfectious Diseases.) Lupus, or systemic lupus erythematosus, is seen mainly in young and middle-aged women, and its symptoms include fever, chills, fatigue, weight loss, skin rashes (especially a “butterfly” rash on the face), patchy hair loss, sores in the mouth or nose, enlargement of the lymph nodes, stomach problems, and irregular menstrual cycles. Lupus also may induce problems in the cardiopulmonary, urinary, and central nervous systems and can cause seizures, depression, and psychosis. Rheumatoid arthritis, as its name suggests, is a type of both rheumatism and arthritis, which are general names for diseases associated with inflammation of connective tissue. Rheumatoid arthritis occurs when the immune system attacks and destroys the tissues that line bone joints and cartilage. The disease can affect any part of the body, although some joints may be more susceptible than others are. As it progresses, joint function diminishes sharply, and deformities arise. Like rheumatism and arthritis, anemia is a general term for several conditions. Forms of it are marked either by a lack of red blood cells (hemoglobin) or by a shortage in total blood volume, and these deficiencies can produce effects that range from lethargy or sluggishness to death. Autoimmune hemolytic anemia occurs when the body makes antibodies that coat red blood cells. Patients have been known to experience a variety of symptoms, including jaundice, characterized by a yellowish coloration, before dying—sometimes just a few weeks after showing the first signs of the disease.

The reasons why the immune system becomes dysfunctional are not well understood, but most researchers agree that a combination of genetic, environmental, and hormonal factors plays into autoimmunity. They also speculate that certain mechanisms may trigger it. First, a substance normally restricted to one part of the body, and therefore not usually exposed to the immune system, is released into other areas, where it is attacked. Second, the immune system may mistake a component of the body for a similar foreign component. Third, cells of the body may be altered in some way, by drugs, infection, or some other environmental factor, so that they are no longer recognizable as “self ” to the immune system. Fourth, the immune system itself may be dysfunctional, for instance, because of a genetic mutation.

Pernicious anemia was so named at a time when it, too, was almost always fatal (pernicious means “deadly”), though treatments developed in the twentieth century have changed that situation. A disorder in which the immune system attacks the lining of the stomach in such a way that the body cannot metabolize vitamin B12 (see Vitamins), pernicious anemia manifests symptoms that include weakness, sore tongue, bleeding gums, and tingling in the extremities. Because the disease leads to a decrease in stomach acid, nausea, vomiting, loss of appetite, weight loss, diarrhea, and constipation are also

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possible. Furthermore, since B12 is essential to the functioning of the nervous system, a deficiency can result in a host of neurological problems, including weakness, lack of coordination, blurred vision, loss of fine motor skills, impaired sense of taste, ringing in the ears, and loss of bladder control. WHERE TO LEARN MORE All About Allergies. About.com (Web site). . Clark, William R. At War Within: The Double-Edged Sword of Immunity. New York: Oxford University Press, 1995. Davis, Joel. Defending the Body: Unraveling the Mysteries of Immunology. New York: Atheneum, 1989.

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Deane, Peter M. G., and Robert H. Schwartz. Coping with Allergies. New York: Rosen Publishing Group, 1999.

The Immune System

Focus on Allergies (Web site). . “Infection and Immunity.” University of Leicester Microbiology and Immunology (Web site). . Joneja, Janice M. Vickerstaff, and Leonard Bielory. Understanding Allergy, Sensitivity, and Immunity: A Comprehensive Guide. New Brunswick, NJ: Rutgers University Press, 1990. The Vaccine Page: Vaccine News and Database (Web site). . Vaccine Safety (Web site). . Young, Stuart H., Bruce S. Dobozin, and Margaret Miner. Allergies: The Complete Guide to Diagnosis, Treatment and Daily Management. New York: Plume, 1999.

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Parasites and Parasitology

PA R A S I T E S A N D PA R A S I T O L O G Y

CONCEPT When people hear the word parasite, one of the first ideas or images that probably comes to mind is that of disease. Though many parasites do carry diseases, including some extremely deadly ones, “disease-carrying” is not necessarily a defining characteristic of a parasite. Rather, a parasite can be identified as any organism that depends on another organism, the host, for food, shelter, or some other benefit and which receives these benefits in such a way that the host experiences detrimental effects as a consequence. Theoretically, organisms from all across the kingdoms of living things can be characterized as parasites; in practice, however, the realm of organisms studied by parasitologists is confined to protozoa and various species within the animal kingdom, mostly worms and arthropods. Included among these organisms are countless varieties of tapeworm and roundworm as well as a parade of insects that have plagued humankind since the dawn of time: cockroaches, lice, bedbugs, flies, fleas, ticks, mites, and mosquitoes.

HOW IT WORKS Symbiosis

Organisms engaging in symbiotic relationships are called symbionts. There are three basic types of symbiosis, differentiated as to how the benefits (and the detriments, if any) are distributed. In the case of parasitism, of course, the arrangement benefits only one of the two: the parasite, an organism that obtains nourishment or other life support at the expense of the host. Though predation (the relationship of predator to prey) is technically a form of symbiosis, it usually is not considered in the context of symbiotic relationships; therefore, parasitism is really the only variety of symbiosis that is detrimental to one of the organisms. Unlike the predator killing its prey, however, the parasite allows its host to live as long as possible, since it depends on the host for support and sustenance. A second type of symbiosis, commensalism, likewise involves a benefit to only one of the two organisms, but in commensalism the beneficiary, or commensal, manages to receive its benefits without causing any detriment to the host. The other variety of symbiosis is mutualism, an arrangement from which both partners receive benefits. (Commensalism and mutualism, along with the overall concept of symbiosis, are discussed at much greater length in Symbiosis.)

Parasitism belongs within the context of symbiosis, a term for a biological relationship in which two species live in close proximity to each other and interact regularly in such a way as to benefit one or both of the organisms. In addition to the symbiosis of two species (that is, at least two individuals representing two different species), it is also possible for a symbiotic relationship to exist between two organisms of the same species.

In addition to the distinctions among parasitism, commensalism, and mutualism, symbiotic relationships are distinguished according to the participants’ ability to live without each other. In a facultative relationship, the partners can live apart successfully, whereas in an obligate one, the interacting species are incapable of living separately. Needless to say, for most parasites the relationship is obligate, whereas from the

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host’s viewpoint, the arrangement is more than facultative: not only could the host live without the parasite, but it actually would be better off without the tiny hanger-on.

Defining the Limits of Parasitology The realm of parasitism and parasitic creatures is far larger and more varied than that of parasitology. In other words, the range of organisms that exhibit parasitic behavior is much greater than the variety of species that are considered within the realm of parasitological study. Among the parasitic species excluded from ordinary parasitology are two named in the title of a book in the bibliography at the end of this essay: Despicable Species: On Cowbirds, Kudzu, Hornworms, and Other Scourges, by Janet Lembke. The cowbird is a species that exploits the instinctive tendency of other birds to care for their young. Rather than raise its offspring, it leaves its eggs with other birds, which mistake the eggs for their own and provide them with food and care. Meanwhile, the adult cowbird goes on about its business, freed from the responsibility of raising its own progeny. (See Instinct and Learning for more on this subject.) Kudzu is a plant that grows at an amazing rate, covering hillsides and virtually every surface to which it can attach itself. During the 1930s agricultural officials in the American South advocated the planting of kudzu, imported from China, as a means of controlling erosion on hillsides. The plant did help control erosion, but it virtually took over parts of Georgia, Alabama, Mississippi, and neighboring states. Only through successful, or at least partially successful, eradication campaigns did county and state governments, as well as private landowners, manage to stem the tide of kudzu’s onslaught. Cowbirds are clearly parasitic in their behavior, and kudzu is unquestionably a pest in plant form (i.e., a weed), but this does not truly make them parasites in the sense that parasitologists use the term. Nor, as we noted earlier, does the capacity to cause or carry disease necessarily define a parasite, though many parasites (known collectively as pathogens) do carry diseases. And even though many species of viruses, bacteria, and fungi exhibit parasitic behavior and can be transmitted by parasites, scientists usually study them separately in the context of infectious diseases.

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PROTOZOA, WORMS, AND A RT H R O P O D S . After making all the exclu-

sions indicated in the foregoing paragraph, the species studied within the realm of parasitology are only those creatures within certain phyla from two kingdoms: animals and protists. The latter category, which includes algae (other than bluegreen algae), slime molds, and protozoa, is made up primarily of unicellular, eukaryotic organisms. (A eukaryote is a cell that has a nucleus, as well as organelles, or sections of the cell that perform specific functions, enclosed in membranes.) Among protists, of principal concern is protozoa, which, unlike other protists, are capable of moving on their own. All the species we discuss in the remainder of this essay fall into one of three general categories: protozoa, worms, and arthropods, the latter two being groups of creatures within the animal kingdom.

The Taxonomy of Worms and Arthropods Taxonomy, or the area of the biological sciences devoted to the classification of species, is an exceedingly complex area of study. Within that realm, there are numerous matters that either are or have been areas of dispute, among them the classification of protozoa as protists rather than animals. Owing to differences between taxonomic systems and the complications involved in explaining the characteristics that unify members of particular phyla (the next-largest major taxonomic ranking after kingdom), we will dispense with any effort to delineate classifications of species rigorously. In other words, we will take a much simpler approach, a fact signaled by the use of the very general word worm. Whereas arthropods are a genuine phylum whose characteristics we examine shortly, worms is simply a broad term that encompasses numerous phyla: Platyhelminthes (flatworms), Nemertea (or Rhynchocoela, ribbon worms), Acanthocephala (spiny-headed worms), Aschelminthes, Priapulida (priapulids), and Annelida (annelid worms). The classification of the many species of worm serves to illustrate just how complex a subject taxonomy can be. The group Aschelminthes, for example, is divided into five classes—Rotifera, Gastrotricha, Kinorhyncha, Nematoda, and Nematomorpha—that sometimes are treated as separate phyla. Yet another classification system

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groups the worms into phyla completely different from the ones named here. Therefore, rather than hopelessly confusing the issue, in the present context we will call them all simply worms and not attempt to map the complex relationships between the species that fall under that heading. A RT H R O P O D S . In contrast to worms, arthropods, or members of the phylum Arthropoda, are much easier to identify. Arthropoda is the largest phylum in the animal kingdom, accounting for some 84% of all known animal species. Nearly 900,000 arthropod species have been identified, the vast majority of them being insects, but some zoologists maintain that this number represents but a tiny fraction of all species within this enormous phylum. In fact, there may be as many as ten million species of insect alone.

Arthropods are identified by a nonliving exoskeleton (an external skeleton), by segmented bodies, and by jointed appendages in pairs. The four subphyla of phylum Arthropoda are Trilobita, Crustacea, Chelicerata, and Uniramia. The first of these subphyla became extinct during the “Great Dying” that marked the end of the Permian period some 245 million years ago. (See Paleontology for more on this subject.) The second includes numerous organisms that humans eat, such as crabs and shrimp, as well as a few parasitic species. The most familiar parasites, however, belong to the other two subphyla. Within Chelicerata, by far the largest class is Arachnida, whose name might be familiar from that of the 1990 horror film about spiders, Arachnophobia. In addition to spiders, Arachnida includes scorpions, ticks, and mites, among which are many parasitic creatures. Finally, there is subphylum Uniramia, which includes several classes, such as Chilopoda and Diplopoda (centipedes and millipedes, respectively); the largest class is unquestionably Insecta. In the latter group are some of the most obnoxious creatures known, including the genera Anopheles (mosquitoes), Glossina (tsetse fly), and Climex (bedbugs) as well as the species Periplaneta americana (American cockroach), Phthirus pubis (pubic or crab louse), and Tunga penetrans (sand flea).

worms are endoparasites, or parasites that live inside a host’s body. As we shall see, insects are often vectors, or organisms that transmit pathogens, meaning that they serve as a delivery vehicle for the disease-causing protozoa and worms. Because endoparasites interact with the bodies of their hosts in much more complex ways, we devote more space to those species. In general, the life cycle of most worms begins in the body of a definitive host, a host that provides a setting for the sexual reproduction of parasites. (For most of the parasites we consider, humans are the definitive host.) The parasite’s eggs pass from the body of the definitive host, usually through the feces, and hatch in an inorganic medium, typically water. From there, the parasites enter the body of either a vector or an intermediate host. The latter is defined by the fact that sexual reproduction of parasites does not take place in its body; for the parasites to reproduce sexually, they must enter the body of a definitive host, where the cycle begins again. There may be more than one intermediate host, and they are identified, respectively, as first intermediate host, second intermediate host, and so on. For protozoa the life cycle is much as we have described; however, the concept of a definitive host, as we have defined it, does not usually enter in, since protozoa reproduce asexually. Nonetheless, they typically begin life in the intestinal tracts or other organs of a host, pass through the feces into the water supply, and enter the body either of a vector or a new host.

REAL-LIFE A P P L I C AT I O N S Protozoa

Most insects are ectoparasites, or parasites that live outside a host’s body, whereas protozoa and

They may be single-cell organisms, compared with the many millions of cells that make up the vastly more complex human body, but protozoa have been the cause of more human suffering and death than almost any category of diseasecarrying organism. Intestinal protozoa are common throughout the world, particularly in areas where food and water sources are subject to contamination from animal and human waste. Like soldiers sneaking into a city disguised as civilians, protozoa typically enter the body while still in an inactive state, thus getting around any defenses the immune system might put up against them.

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Protozoa in this inactive state, encased in a protective outer membrane, are known as cysts. As cysts, they enter the gastrointestinal tract of the host before developing into a mature form that feeds and reproduces. Other types of parasitic protozoa infect the blood or tissues of their hosts. These protozoa typically enter the host through a vector. Often the vector is an invertebrate (an animal without an internal skeleton), such as an insect, that feeds on the host and passes the protozoan on through the bite wound. The effects that protozoan parasites can have on human bodies range from no symptoms to extreme ones. Some victims of Trichomonas vaginalis, known colloquially as trick, are asymptomatic, meaning that they show no obvious symptoms of their condition. It is an STD, or sexually transmitted disease, for which men are typically carriers, passing it on to female sexual partners. Males do exhibit some symptoms, but usually it is the female who experiences the worst symptoms of the infection, including a burning sensation when urinating, genital itching, and a white discharge. When such pain is not present, the disease can be detected only by finding the trophozoites (protozoa in a feeding stage, as opposed to a reproductive or resting stage) in secretions within the victim’s genital tract. I N T E S T I N A L P R O T O Z OA . A classic intestinal protozoan is Giardia lamblia. Like all unicellular organisms, it is extremely small, with a length of about 15 microns, or micrometers, equal to about 0.00059 in. It has a teardrop shape with two nuclei on either side of a rod called an axostyle, and under a scanning electron microscope the nuclei look like eyes separated by a nose. For this reason, people who have glimpsed G. lamblia under such magnification have reported the decidedly unsettling sensation of a microorganism “staring back at them.”

These protozoa carry a condition known as giardiasis. As cysts, they pass through the feces of one host and enter the body of the next through contaminated food or water. The resulting symptoms can range from nothing to severe diarrhea. Not surprisingly, giardiasis is fairly common in developing countries, where open sewers commonly run through the city streets. Also, there are places in the third world where farmers use human feces as a fertilizer, another significant source of infection. Yet giardiasis is not simply an affliction of people in developing nations; rather,

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the ease with which the pathogen can contaminate water supplies makes it a condition known the world over. Even in the United States and other industrialized nations, G. lamblia may find its way into water supplies when waste-disposal systems are placed too close to wells. Campers who unwisely drink water from mountain streams also may contract giardiasis, which may come from beavers—hence the nickname “beaver fever,” which is sometimes given to giardiasis contracted in the wild. Far more notorious and destructive is Entamoeba histolytica, carrier of amoebic dysentery. The process of infection is much the same as with G. lamblia, but once E. histolytica enters the host’s body, it can wreak considerably more damage. In most cases, the parasite causes only diarrhea and relatively minor gastrointestinal problems, but it can bring about a rupture of the gastrointestinal tract (the stomach and intestines). Trophozoites may even pass into the circulatory system or other organs, such as the liver, and amoebic dysentery can be fatal. B LO O D I N F E C T I O N S . Among the

most significant protozoan blood infections is African trypanosomiasis, or “sleeping sickness.” The parasite enters humans through the bite of the tsetse fly, a bloodsucking pest that serves as vector for the Trypanosoma genus of parasites. The result is fever, inflammation of the lymph nodes (masses of tissue at certain places in the body that filter blood), and various negative effects on the brain and spinal cord that bring about extreme lethargy or tiredness—hence the name. Sleeping sickness, prevalent throughout central Africa, is frequently fatal. Still worse is the disease carried by the Plasmodium genera: malaria, which has been described at the leading health problem in the world today. It is estimated that more than two billion people live in regions where malaria is endemic (native), that the number of persons infected may be as high at 750 million, and that as many as three million people die of the disease each year. Four species of Plasmodium, borne by mosquitoes, are capable of infecting humans and causing malaria. P. falciparum is the most dangerous of the four strains; it can kill a healthy adult in 48 hours. Inside the human host, the parasite finds a home in the red blood cells, where it reproduces until the cell bursts, spreading more parasites

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throughout the blood. These new parasites infect and destroy more red blood cells. As a new generation of parasites bursts from the blood cells, the body tries to defend itself by causing a fever, which does destroy many (but not all) of the pathogens. Scientists have developed several chemical antidotes to malaria; unfortunately, strains of pathogen resistant to these antidotes are on the rise in various parts of the world.

Parasites and Parasitology

Worms The protozoa we have discussed up to this point, as well as the conditions that they carry, are far from pleasant. But the many species we examine under the general heading of “worms” are vastly more disgusting, if that is to be believed. It should be stated that much of what follows is not for those of weak stomach and probably should not be read while eating. In contrast to protozoa, worms are relatively complex, large creatures with fairly long life spans. For example, the genera Schistosoma, or blood flukes, average 0.4 in. (10 mm) in length, while Clonorchis sinensis (Chinese, or oriental, liver fluke) may attain lengths of about 1 in. (25 mm). Additionally, they can live for very long spans of time: a blood fluke (fluke is simply a more common way of designating a nematode, a type of worm) may last as long as 30 years. During that time, a female blood fluke can produce several hundred eggs a day. Blood flukes are one of the few worm species in which the sexes are separate; most worms are hermaphrodites, or both male and female. This brings up another distinction between worms and protozoa: the fact of sexual reproduction. From this follows yet another distinction, the involvement (typically) of one or more intermediate hosts as well as a definitive host, in whose body sexual reproduction takes place.

FLUKES

AND OTHER WORMS CAN BE AS DESTRUCTIVE AS

MANY OF THEIR PROTOZOAN COUNTERPARTS; ORIENTAL LIVER FLUKES, FOR EXAMPLE, PASS THROUGH THE BODIES OF SNAILS TO FISH AND THEN TO HUMANS, WHO ARE INFECTED BY EATING RAW OR UNDERCOOKED FISH.

THEY

BLOCK THE BILE DUCTS IN THE LIVER, A CONDI-

TION THAT CAN BE FATAL. (Photo Researchers. Reproduced by per-

mission.)

fish. They block the bile ducts in the liver, a condition that can be fatal. TA P E W O R M S

AND

THE

DA N -

G E R S O F U N D E R C O O K E D M E AT.

Flukes and other worms can be as destructive as many of their protozoan counterparts. Blood flukes, for instance, kill some one million people a year by inflaming tissues and causing organs, including the liver and small intestine, to cease functioning. Oriental liver flukes, so named because they are common throughout eastern Asia, pass through the bodies of snails (first intermediate host) to fish (second intermediate host) and ultimately to humans (definitive host), who are infected by eating raw or undercooked

There are numerous species of tapeworm, which may be as long as 50 ft. (15 m). Their bodies are made up of segments called proglottids, which contain male and female sexual organs and which house their eggs. Proglottids break off, usually into the feces of the host, thus enabling the spread of the tapeworm. One variety, Dipylidium caninum (cucumber tapeworm), uses dogs or cats as a definitive host, entering the body when the animal ingests a flea or louse, which serves as an intermediate host. Several varieties of worm can enter the body through improperly cooked pork. This is a testament to the wisdom of the injunctions against eating pork in the Old Testament and Koran, which were written for peoples in a world without refrigeration or sophisticated medical knowledge.

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New World, and was a widespread affliction throughout the southeastern United States during the early part of the twentieth century. Another common species, Ancylostoma duodenale, is found in southern Europe, northern Africa, northern Asia, and parts of South America.

Parasites and Parasitology

FOUND IN CENTRAL AFRICA AND PARTS OF ASIA, WUCHERERIA BANCROFTI is carried by mosquitoes and causes a condition known as elephantiasis. This parasite makes its way to the lymph nodes and brings about grotesque swelling. (© Sheldan Collins/Corbis. Reproduced by permission.)

The Taenia genus of tapeworm includes T. solium, the pork tapeworm, which has been known to crawl out of the anus of an infected human. Another worm often found in undercooked pork is Trichinella spiralis, which brings about the condition known as trichinosis. The latter disease is rarely fatal, but it can cause extreme discomfort, characterized by sore, tender muscles. Thanks to enhanced efforts at meat inspection, the incidence of trichinosis in U.S. pigs has dropped to less than 1%. Nonetheless, it is still quite possible to ingest the parasite from eating undercooked game, particularly bear. W O R M S T H AT A F F L I C T LA R G E P O P U L AT I O N S . Hookworms such as

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Pinworms, carriers of a condition known as enterobiasis, are also common in the United States, and in much of the world as a whole. They primarily afflict preschool and school-age children who live in crowded conditions, though adults can often contract enterobiasis as well. Pinworms infect about half as many people as hookworms, but the effects can be extremely painful, especially because they are felt in the perianal skin, or the skin around the anus, and (for adult women) in the genitals. The eggs hatch on the perianal skin, which causes it to itch, but if the victim scratches the area, this can cause additional bacterial infections. The affliction of pinworms can be especially severe for children, who often experience irritability, insomnia, restlessness, and other behavioral changes. They are highly contagious and can contaminate nonliving surfaces such as bed linens and carpets; therefore, if one family member is infected, the whole family usually must receive treatment. Drugs for treating pinworm are sold either as prescription or over-the-counter drugs, and treatment usually involves two doses over the course of two weeks. Ascaris lumbricoides, or intestinal roundworms, may infect as much as one-fourth of the world’s population. Affected areas are mostly in the third world, though in the United States, an estimated 10,000 cases of roundworm infection—frequent in dogs and cats—occur in humans. The roundworm pathogen goes through a strange life cycle inside the human body, hatching in the small intestine and making its way into the air passages, only to be swallowed and returned to the small intestine. It can cause a fluid buildup in the lungs, resulting in what is known as ascaris pneumonia, an often fatal disease. As with T. solium, the intestinal roundworm has been known to escape the body, though in an even more shocking way. Because it is very sensitive to anesthetics, the parasite may evacuate the body of a patient in a surgical recovery room by moving from the small intestine to the stomach and out the nose or mouth.

Necator americanus afflict more than a billion people as well as untold numbers of dogs and cats. In fact, one of the ways that the parasites spread is through human exposure to dog and cat feces. Some hookworms can produce as many as 25,000 eggs a day, wreaking havoc on the host, on whose blood the hookworms depend for sustenance. As its name implies, N. americanus is found in the

OTHER WORMS. Several other worms have been associated with various dis-

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SCANNING

ELECTRON MICROGRAPH OF A TICK FEEDING IN HUMAN SKIN.

CAN SERVE AS A VECTOR FOR BACTERIAL INFECTIONS, SUCH AS

A TICK IS A PARASITIC ARTHROPOD THAT ROCKY MOUNTAIN SPOTTED FEVER. (Photo Researchers.

Reproduced by permission.)

eases in parts of the third world. Among them are Loa loa, or the “eye worm,” found primarily in equatorial Africa. These worms live just under the skin and, as their name indicates, just under the surface of the eyeball. Onchocerca volvulus, also common in Africa as well as Arabia and Latin America, afflicts an estimated 30 million people. When it enters the human eye, it dies, eventually bringing about blindness. Because the vector, a black fly, breeds in rivers, the disease is commonly called river blindness, and in some towns it has blinded as much as 40% of the adult population.

and causes a condition known as elephantiasis. The parasite makes its way to the lymph nodes, particularly those that drain lower parts of the body, and brings about grotesque swelling, usually in the legs and genitals. (Visitors to the “Parasites and Parasitological Resources” Web site of Ohio State University’s College of Biological Sciences, will find links to photographs of elephantiasis victims. It should be noted that these images include grotesquely swollen male genitalia and may be too graphic for some viewers.)

Found in central Africa and parts of Asia, Wuchereria bancrofti is carried by mosquitoes

After the preceding discussion of worms, mere insects and other arthropods probably will

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KEY TERMS ARTHROPOD:

A term for members of

the phylum Arthropoda, largest in the animal kingdom. Arthropods are identified by a nonliving exoskeleton (an external skeleton), by segmented bodies, and by jointed appendages that appear in pairs. Among the classes within this phylum are Arachnida (spiders, scorpions, ticks, and mites) and Insecta. ASYMPTOMATIC:

Displaying

The third-largest major, or

obligatory, taxonomic classification rank, smaller than phylum but larger than order. COMMENSALISM:

A symbiotic rela-

tionship in which one organism, the commensal, benefits without causing any detriment to the other organism, the host. CYST:

A protozoan in an inactive state,

encased in a protective outer membrane. Protozoa usually enter the bodies of hosts in the form of cysts. DEFINITIVE HOST:

The host in whose

body a parasite reproduces sexually. A parasite enters a definitive host either through a vector, an intermediate host, or through some other source, such as a water supply. ECTOPARASITES:

Parasites that live

outside a host’s body.

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Parasites that live

inside a host’s body. A cell that has a nucleus, as well as organelles (sections of the cell that perform specific functions) that are bound by membranes.

EUKARYOTE:

A term for a symbiotic relationship in which partners are capable of living apart.

FACULTATIVE:

no

symptoms of a disease or other condition. CLASS:

ENDOPARASITES:

GASTROINTESTINAL TRACT:

The

stomach and intestines. The second-smallest major taxonomic classification rank, smaller than family and larger than species. GENUS:

The term for an organism that provides a benefit or benefits to another organism in a symbiotic relationship of commensalism or parasitism. See also intermediate host and definitive host.

HOST:

A creature that serves as host to a parasite, which it receives either directly (typically through the water supply) or from a vector before passing it on to a definitive host. Parasites do not reproduce sexually inside an intermediate host.

INTERMEDIATE HOST:

Masses of tissue at certain places in the body that act as filters for blood. LYMPH NODES:

not have the same power to disgust the reader as they otherwise might have had. This is particularly so because most arthropods are ectoparasites, and not as many of them are associated with diseases. This is not to say that arthropod parasites are anything pleasant or that they could be described as upstanding citizens of the biological world—only that, compared with worms, they are not quite so revolting.

Certainly many insect parasites, such as mosquitoes (genus Anopheles), are notorious vectors. Of the 400 or so mosquito species, about a dozen carry malaria, and many others transmit canine heartworm. Some parasites are known as mechanical, as opposed to biological, vectors. This means that they may happen to carry cysts of protozoa, or worm eggs, on their exteriors, but they do not serve as the biological carrier of any

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KEY TERMS OBLIGATIVE:

A term for a symbiotic

relationship in which the partners, if they were separated, would be incapable of continuing to live.

Parasites and Parasitology

CONTINUED

A term that refers to

PROTOZOAN:

some 50,000 species of mobile protists. A classification rank,

SUBPHYLUM:

smaller than that of phylum but larger than A general term for any

that of class, that is sometimes used to

organism that depends on another organ-

indicate a level between those two major

ism for support, which it receives at the

ranks.

PARASITE:

expense of the other organism. SYMBIOSIS: PARASITISM:

A symbiotic relation-

ship in which one organism, the parasite, benefits at the expense of the other organism, the host. A biological disci-

pline devoted to the study of parasites but primarily those among the animal and protist kingdoms. Parasitic bacteria, fungi, and viruses usually are studied within the context of infectious diseases.

larly in such a way as to benefit one or both between two or more individuals of the same species as well as between two or more individuals representing two different species. The three principal varieties of symbiosis are mutualism, commensalism, and parasitism. The area of the biological

TAXONOMY:

sciences devoted to the classification of

The second-largest major

taxonomic classification rank, smaller than kingdom but larger than class. PROTISTS:

proximity to each other and interact regu-

A disease-carrying para-

site, typically a microorganism. PHYLUM:

in which (usually) two species live in close

of the organisms. Symbiosis may exist

PARASITOLOGY:

PATHOGEN:

A biological relationship

A term for members of the

kingdom Protista, which includes algae (other than blue-green algae, which are

organisms according to apparent common characteristics. TROPHOZOITE:

A protozoan in a

feeding stage, as opposed to a reproductive or resting stage. An organism, such as an

monerans), slime molds (a group of about

VECTOR:

500 species that resemble fungi), and pro-

insect, that transmits a pathogen to the

tozoa.

body of a host.

disease. Such is the case with the American cockroach (Periplaneta americana). But one thing unites all insect parasites: they are pests, and if any of their species ever became endangered (something that is not likely to happen), very few people would be upset. I T C H I N G . The bedbug (genus Cimex) is an insect parasite that is not known to serve as a vec-

tor, either mechanical or biological, for any disease—yet it certainly gives rise to more than its share of misery. Like many another arthropod parasite, it causes its hosts to itch. Most familiar from the old saying “don’t let the bedbugs bite,” bedbugs live not only in beds but also in clothing, furniture, laundry, and numerous other areas around a house. And though they depend on human blood, it does no good simply to vacate a house for a few days in the hope of starving

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T H AT

CAUSE

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them; they can survive for up to year on the blood supplies they have stored in their bodies. Then there are body lice (Pediculus humanus) and pubic, or crab, lice (Phthirus pubis). Although people may tend to associate the problem of lice with the poor, lice afflict all socioeconomic classes. Body lice, as their name indicates, spread all over the body and particularly the head, while crab lice nest either in the pubic area or anywhere else they can find short, thick hair: armpits, eyebrows, eyelashes, beards, sideburns, and mustaches. Other than inducing itching, most lice do not pose a serious threat— unless they happen to be carrying typhus or other diseases. Other insect parasites known to cause itching are cat and dog fleas (Ctenocephalides felis and C. canis, respectively), which may carry the cucumber tapeworm we discussed earlier. Sand fleas (Tunga penetrans) are not associated with any disease, but by causing hosts to scratch, they can bring about secondary infections, and they become so firmly attached to the host that they usually require surgical removal.

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itching to severe neurological disorders. There are also several varieties of mange mites, among them Sarcoptes scabiei, which afflicts humans and animals with scabies, characterized by severe skin inflammation and itching. WHERE TO LEARN MORE Berenbaum, May. Bugs in the System: Insects and Their Impact on Human Affairs. Reading, MA: AddisonWesley, 1995. “Biology 160, Animal Behavior: Symbiosis and Social Parasitism.” Department of Biology, University of California at Riverside (Web site). . Knutson, Roger M. Furtive Fauna: A Field Guide to the Creatures Who Live on You. New York: Penguin Books, 1992. Lembke, Janet. Despicable Species: On Cowbirds, Kudzu, Hornworms, and Other Scourges. New York: Lyons Press, 1999. Parasites and Parasitism. University of Wales, Aberystwyth (Web site). . Parasites and Parasitological Resources. Ohio State University College of Biological Sciences (Web site). .

Like the insects we have described, most arachnid parasites also produce itching. Among these parasites are several varieties of tick, which can serve as vectors for bacterial infections, such as Rocky Mountain spotted fever. Tick species such as Ixodes dammini in the northeastern United States carry Lyme disease, named after the town in Connecticut were it was first identified in 1975. Effects of Lyme disease can range from

Zimmer, Carl. Parasite Rex: Inside the Bizarre World of Nature’s Most Dangerous Creatures. New York: Free Press, 2000.

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Trager, William. Living Together: The Biology of Animal Parasitism. New York: Plenum Press, 1986. The World of Parasites (Web site). . The World Wide Web Virtual Library: Parasitology (Web site). .

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Infection

CONCEPT Humans may hold dominance over most other life-forms on Earth, but a few varieties of organism have long held mastery over us. Ironically, these life-forms, including bacteria and viruses, are so small that they cannot be seen, and this, in fact, has contributed to their disproportionate influence in human history. For thousands of years, people attributed infection to spiritual causes or, at the very least, to imbalances of “humors,” or fluids, in the human body. Today germ theory and antisepsis—the ideas that microbes cause infection and that a clean body and environment can prevent infections—are ingrained so deeply that we almost take them for granted. Yet these concepts are very recent in origin, and for a much longer span of human history people quite literally wallowed in filth—with predictable consequences.

HOW IT WORKS What Is Infection? The term infection refers to a state in which parasitic organisms attach themselves to the body, or to the inside of the body, of another organism, causing contamination and disease in the host organism. Parasite refers generally to any organism that lives at the expense of another organism, on which it depends for support. Numerous parasites and the diseases they cause are discussed in the essay Parasites and Parasitology; in the present context, we are concerned primarily with infections that relate to bacteria and viruses.

INFECTION

Infections fall into two general categories: exogenous, or those that originate outside the body, and endogenous, which occur when the body’s resistance is lowered. Examples of exogenous infection include catching a cold by drinking after someone else from the same glass; coming down with salmonella after ingesting undercooked eggs, meat, or poultry; getting rabies from a dog bite; or contracting syphilis, AIDS (acquired immunodeficiency syndrome), or some other sexually transmitted disease from an infected partner. Any number of factors—lack of sleep, prolonged exposure to extreme cold or moisture, and so on—can lower the body’s resistance, opening the way for an endogenous infection. Malnutrition, illness, and trauma also can be factors in endogenous infection. Substance abuse, whether it be the use of tobacco in its many forms, excessive drinking, or drug use, lowers the body’s resistance. Furthermore, all of these behaviors tend to be coupled with poor eating habits, which invite infection by denying the body the nutrients it needs.

Some Terms

Almost all infections contracted by humans are passed along by other humans or animals.

A whole array of terminology attends the study of infection and infectious diseases, a subject that is touched upon in the present context but explored at length in its own essay as well. Among these terms are the names for the different branches of study relating to infection, its agents, and the resulting diseases. Although germ theory is a term (defined later) that is used widely in the context of infection, germ itself—a common word in everyday life—is not used as much as microorganism or pathogen. The latter word

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refers to disease-carrying parasites, which are usually microorganisms. Two of the principal types of pathogen, bacteria and viruses, are discussed later in this essay. Words relating to the effects of infectious agents include epidemic, an adjective meaning “affecting or potentially affecting a large proportion of a population”; as a noun, the word refers to an epidemic disease. Pandemic also doubles as an adjective, meaning “affecting an extremely high proportion of a population over a wide geographic area,” and a noun, referring to a disease of pandemic proportions. Areas of study relating to pathogens, their effects, and the prevention of those effects include the following. • Bacteriology: An area of the biological sciences concerned with bacteria, including their importance in medicine, industry, and agriculture • Epidemiology: An area of the medical sciences devoted to the study of disease, including its incidence, distribution, and control within a population • Etiology: A branch of medical study concerned with the causes and origins of disease. Also, a general term referring to all the causes of a particular disease or condition • Immunology: The study of the immune system, immunity, and immune responses • Pathology: The study of the essential nature of diseases • Virology: The study of viruses In addition, there are several terms relating to the prevention of infection. • Antibiotic: A substance produced by, or derived from, a microorganism, which in diluted form is capable of killing or at least inhibiting the action of another microorganism. Antibiotics typically are not effective against viruses. • Antisepsis: The practice of inhibiting the growth and multiplication of microorganisms • Germ theory: A theory in medicine, widely accepted today, that infections, contagious diseases, and other conditions are caused by the actions of microorganisms • Immunity: A condition of being able to resist a particular disease, particularly through means that prevent the growth and development of pathogens or counteract their effects

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• Inoculation: The prevention of a disease by the introduction to the body, in small quantities, of the virus or other microorganism that causes the disease • Vaccine: A preparation containing microorganisms, usually either weakened or dead, which are administered as a means of increasing immunity to the disease caused by those microorganisms Some of these words appear in this essay and others in related essays on infectious diseases and immunity.

Bacteria Five major groups of microorganisms are responsible for the majority of infections. They include protozoa and helminths, or worms— both of which are considered in Parasites and Parasitology—as well as bacteria and viruses. Bacteria and viruses often are discussed, along with fungi (the fifth major group), in the context of infection and infectious diseases. In the present context, however, we limit our inquiry to viruses and bacteria. Bacteria are very small organisms, typically consisting of one cell. They are prokaryotes, a term referring to a type of cell that has no nucleus. In eukaryotic cells, such as those of plants and animals, the nucleus controls the cell’s functions and contains its genes. Genes carry deoxyribonucleic acid (DNA), which determines the characteristics that are passed on from one generation to the next. The genetic material of bacteria is contained instead within a single, circular chain of DNA. Members of kingdom Monera, which also includes blue-green algae (see Taxonomy), bacteria generally are classified into three groups based on their shape: spherical (coccus), rodlike (bacillus), or spiral- or corkscrew-shaped (spirochete). Some bacteria also have a shape like that of a comma and are known as vibrio. Spirochetes, which are linked to such diseases as syphilis, sometimes are considered a separate type of creature; hence, Monera occasionally is defined as consisting of blue-green algae, bacteria, and spirochetes. The cytoplasm (material in the cell interior) of all bacteria is enclosed within a cell membrane that itself is surrounded by a rigid cell wall. Bacteria produce a thick, jellylike material on the surface of the cell wall, and when that material

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forms a distinct outer layer, it is known as a capsule. Many rod, spiral, and comma-shaped bacteria have whiplike limbs, known as flagella, attached to the outside of their cells. They use these flagella for movement by waving them back and forth. Other bacteria move simply by wiggling the whole cell back and forth, whereas still others are unable to move at all. Bacteria most commonly reproduce by fission, the process by which a single cell divides to produce two new cells. The process of fission may take anywhere from 15 minutes to 16 hours, depending on the type of bacterium. Several factors influence the rate at which bacterial growth occurs, the most important being moisture, temperature, and pH, or the relative acidity or alkalinity of the substance in which they are placed. Bacterial preferences in all of these areas vary: for example, there are bacteria that live in hydrothermal vents, or cracks in the ocean floor, where the temperature is about 660°F (350°C), and some species survive at a pH more severe than that of battery acid. Most bacteria, however, favor temperatures close to that of the human body—98.6°F (37°C)—and pH levels only slightly more or less acidic than water. Since they are composed primarily of water, they thrive in a moist environment.

asites, including bacteria and those species discussed in Parasites and Parasitology, depend on other organisms to serve as hosts, they can live when they are between hosts. They are rather like a person between jobs: without other means of support, the person eventually will go broke or starve, but typically such a person can hang on for a few months until he or she finds a new job. A virus without a host, on the other hand, is simply not alive—not dead, like a formerly living thing, but more like a machine that has been switched off.

Infection

Once a virus enters the body of a host, it switches on, and the result is truly terrifying. In order to produce new copies of itself, a virus must use the host cell’s reproductive “machinery”—that is, the DNA. The newly made viruses then leave the host cell, sometimes killing it in the process, and proceed to infect other cells within the organism. As for the organisms that viruses target, their potential victims include the whole world of living things: plants, animals, and bacteria. Viruses that affect bacteria are called bacteriophages, or simply phages. Phages are of special importance, because they have been studied much more thoroughly than most viruses; in fact, much of what virologists now know about viruses is based on the study of phages.

Viruses One of the interesting things about bacteria is their simplicity, coupled with the extraordinary complexity of their interactions with other organisms. As simple as bacteria are, however, viruses are vastly more simple. Furthermore, the diseases they can cause in other organisms are at least as complex as those of bacteria, and usually much more difficult to defeat. Whereas there are “good” bacteria, as we shall see, scientists have yet to discover a virus whose impact on the world of living things is beneficial. There is something downright creepy about viruses, which are not exactly classifiable as living things; in fact, a virus is really nothing more than a core of either DNA or RNA (ribonucleic acid), surrounded by a shell of protein. Two facts separate viruses from the world of the truly living. First, unlike all living things (even bacteria), viruses are not composed of even a single cell, and, second, a virus has no life if it cannot infect a host cell. When we say “no life” in this context, we truly mean no life. Although par-

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REAL-LIFE A P P L I C AT I O N S Bacteria and Humans Not all bacteria are harmful; in fact, some even are involved in the production of foods consumed by humans. For example, bacteria that cause milk to become sour are used in making cottage cheese, buttermilk, and yogurt. Vinegar and sauerkraut also are produced by the action of bacteria on ethyl alcohol and cabbage, respectively. Other bacteria, most notably Escherichia coli (E. coli) in the human intestines, make it possible for animals to digest foods and even form vitamins in the course of their work. (See Digestion for more on these subjects.) Others function as decomposers (see Food Webs), aiding in the chemical breakdown of organic materials, while still others help keep the world a cleaner place by consuming waste materials, such as feces. Despite its helpful role in the body, certain strains of E. coli are dangerous pathogens that

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can cause diarrhea, bloody stools, and severe abdominal cramping and pain. The affliction is rarely fatal, though in late 1992 and 1993 four people died during the course of an E. coli outbreak in Washington, Idaho, California, and Nevada. More often the outcome is severe illness that may bring on other conditions; for example, two teenagers among a group of 11 who became sick while attending a Texas cheerleading camp had to receive emergency appendectomies. The pathogen is usually transmitted through undercooked foods, and sometimes through other means; for example, a small outbreak in the Atlanta area in the late 1990s occurred in a recreational water park. B AC T E R I A L I N F E C T I O N S . Many bacteria attack the skin, eyes, ears, and various systems in the body, including the nervous, cardiovascular, respiratory, digestive, and genitourinary (i.e., reproductive and urinary) systems. The skin is the body’s first line of defense against infection by bacteria and other microorganisms, although it supports enormous numbers of bacteria itself. Bacteria play a major role in a skin condition that is the bane of many a young man’s (and, less frequently, a young woman’s) existence: acne. Pimples or “zits,” known scientifically as Acne vulgaris, constitute one of about 50 varieties of acne, or skin inflammation, which are caused by a combination of heredity, hormones, and bacteria—particularly a species known as Propionibacterium acnes. When a hair follicle becomes plugged by sebum, a fatty substance secreted by the sebaceous, or oil, glands, this forms what we know as a blackhead; a pimple, on the other hand, results when a bacterial infection, brought about by P. acnes, inflames the blackhead and turns it red. For this reason, antibiotics may sometimes cure acne or at least alleviate the worst symptoms.

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blood as the body tries to shunt blood to vital organs. If the syndrome is severe enough, gangrene may develop in the fingers and toes. In 1980, several women in the United States died from TSS, and several others were diagnosed with the condition. As researchers discovered, all of them had been menstruating and using highabsorbency tampons. It appears that such tampons provide an environment in which TSScausing bacteria can grow, and this led to recommendations that women use lower-absorbency tampons if possible, and change them every two to four hours. Since these guidelines were instituted, the incidence of toxic shock has dropped significantly, to between 1 and 17 cases per 100,000 menstruating women. Many bacteria produce toxins, poisonous substances that have effects in specific areas of the body. An example is Clostridium tetani, responsible for the disease known as tetanus, in which one’s muscles become paralyzed. A related bacterium, C. botulinum, releases a toxin that causes the most severe form of food poisoning, botulism. Salmonella poisoning comes from another genus, Salmonella, which includes S. typhi, the cause of typhoid fever.

Viral Infections With viruses, as we have noted, there is no need even to discuss “good” kinds, because there is no such thing—all viruses are harmful, and most are killers. The particular strains of virus that attack animals have introduced the world to a variety of ailments, ranging from the common cold to AIDS and some types of cancer. Other diseases related to viral infections are hepatitis, chicken pox, smallpox, polio, measles, and rabies.

Acne may seem like a life-and-death issue to a teenager, but it goes away eventually. On the other hand, toxic shock syndrome (TSS), caused by other bacteria at the surface of the skin— species of Staphylococcus and Streptococcus—can be extremely dangerous. The early stages of TSS are characterized by flulike symptoms, such as sudden fever, fatigue, diarrhea, and dizziness, but in a matter of a few hours or days the blood pressure drops dangerously, and a sunburn-like rash forms on the body. Circulatory problems arise as a result of low blood pressure, and some extremities, such as the fingers and toes, are deprived of

One reason why physicians and scientists have never found a cure for the common cold is that it can be caused by any one of about 200 viruses, including rhinoviruses, adenoviruses, influenza viruses, parainfluenza viruses, syncytial viruses, echoviruses, and coxsackie viruses. Each has its own characteristics, its favored method of transmission, and its own developmental period. These viruses can be transmitted from one person to another by sneezing on the person, shaking hands, or handling an object previously touched by the infected person. Surprisingly, some more direct forms of contact with an infected person, as in kissing, seldom spread viruses.

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A group of viruses called the orthomyxoviruses transmit influenza, an illness usually characterized by fever, muscle aches, fatigue, and upper respiratory obstruction and inflammation. The most common complication of influenza is pneumonia, a disease of the lungs that may be viral or bacterial. The viral form of pneumonia that goes hand in hand with influenza can be very severe, with a high mortality (death) rate; by contrast, bacterial pneumonia, which typically appears five to ten days after the onset of flu, can be treated with antibiotics. THE

EVER

ELUSIVE

Infection

VIRUS.

Viruses are tricky. Because their generations are very short and their structures extremely simple, they are constantly mutating (altering their DNA and hence their heritable traits) and thus becoming less susceptible to vaccines. This is the reason why flu vaccine has to be prepared anew each year to target the current strains, and even then the vaccine is far less than universally effective. On the other hand, vaccination has a high rate of success for strains of virus that undergo little mutation—for example, the smallpox virus. One particularly elusive type of virus is known as a retrovirus, which reverses the normal process by which living organisms produce proteins. Ordinarily, DNA in the cell’s nucleus carries directions for the production of new protein. Coded messages in the DNA molecules are copied into RNA molecules, which direct the manufacture of new protein. In retroviruses, that process is reversed, with viral RNA used to make new viral DNA, which then is incorporated into host cell DNA, where it is used to direct the manufacture of new viral protein. Among the diseases caused by retroviruses is AIDS, discussed in Infectious Diseases and The Immune System.

TWO

CHILDREN ARE CONFINED TO IRON LUNGS AS THE

RESULT OF INFECTION WITH POLIOVIRUS; SIX OF THE CHILDREN IN THIS ONE FAMILY WERE STRICKEN WITH THE VIRUS.

AN

EPIDEMIC DISEASE CAN AFFECT A LARGE

PROPORTION OF A POPULATION, AS HAPPENED IN THE CASE OF POLIO IN THE MIDDLE OF THE TWENTIETH CENTURY. (© Bettmann/Corbis. Reproduced by permission.)

trast, believed that disease was caused by evil spirits, cast upon individuals or populations by an angry God as punishment for disobedience.

Every day of our lives, we are at war with microorganisms, both individually and as a species. It is a war that has lasted for several million years, with billions of lives in the balance, yet it is an invisible war. Up until a few centuries ago, in fact, we had no idea what we were fighting. Before the advent of germ theory, the most scientific theories of disease blamed them either on an imbalance of “humors” (blood, phlegm, yellow bile, and green bile), or on inhaling bad air. These were the most advanced ideas, the ones held by men of learning; most of the populace, by con-

Personal hygiene and public health were completely foreign concepts: not only did people bathe infrequently, but they also thought nothing of throwing trash—including rotting food and even human excrement—into the city streets. This image of trash in the streets may call to mind a city of medieval Western Europe, a place and time widely known for its filth, squalor, and ignorance. Yet such an image also describes Athens during the fifth century B.C., when human imagination, wisdom, and appreciation for beauty reached perhaps their highest points in all of history. In the Athens of Socrates, Herodotus, Hippocrates, and Sophocles, the streets were piled with trash and crawling with vermin. In fact, this lack of concern for cleanliness contributed directly to the end of the Greek golden age, sometimes known as the Age of Pericles, after Athens’s great leader (495–425 B.C.)— who died in a great plague that swept the germridden city.

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patient. Semmelweis’s idea resulted in a drastic reduction of puerperal fever cases, but his colleagues denounced his outlandish notion as a useless and foolish waste of time. Six years later, in 1854, modern epidemiology was born when the English physician John Snow (1813–1858) determined that the source of a cholera epidemic in London could be traced to the contaminated water of the Broad Street pump. After he ordered the pump closed, the epidemic ebbed— and still many physicians refused to believe that invisible organisms could spread disease.

Infection

ANTON VAN LEEUWENHOEK’S PROTOTYPE MICROSCOPE. THE INVENTION OF THE MICROSCOPE MADE IT POSSIBLE TO SEE BACTERIA AND OTHER MICROORGANISMS, WHICH VAN LEEUWENHOEK, THE FIRST HUMAN BEING TO OBSERVE THEM, DUBBED ANIMALCULES, or “tiny animals.” (© Bettmann/Corbis. Reproduced by permission.)

BACTERIOLOGY AND ANTIS E P S I S . The first inkling of any etiology

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G E R M T H E O R Y. A major turning point came just three years later, in 1857, when the great French chemist and microbiologist Louis Pasteur (1822–1895) discovered that heating beer and wine to a certain temperature killed bacteria that caused these liquids to spoil or turn into vinegar. Thus was born the process of pasteurization, still used today to purify such foods as milk, because, as Pasteur observed, “There are similarities between the diseases of animals or man and the diseases of beer and wine.” Pasteur also dealt the final blow to spontaneous generation, a centuries-old belief that living organisms could originate from nonliving matter. As he showed in 1861, microorganisms present in the air can contaminate solutions that seem sterile.

other than that of imbalanced humors and demons was the work of the Italian physician Girolamo Fracastoro (ca. 1483–1553), who put forth the theory that disease is caused by particles so small they are almost imperceptible. The invention of the microscope in 1590 made it possible to glimpse those particles, which Holland’s Anton van Leeuwenhoek (1632–1723)—the first human being to observe bacteria and other microorganisms—dubbed animalcules, or “tiny animals.” The German scholar Athanasius Kircher (1601–1680) also observed “tiny worms” in the blood and pus of plague victims and theorized that they were the source of the infection. This was the first theory that dealt with microbial agents as infectious organisms.

Then, in 1876, the German physician Robert Koch (1843–1910) proved what Kircher had postulated two centuries earlier: that bacteria can cause diseases. Koch showed that the bacterium Bacillus anthracis was the source of anthrax in cattle and sheep and generalized the methodology he had used in that situation to form a specific set of guidelines for determining the cause of infectious diseases. Known as Koch’s postulates, these guidelines define a truly infectious agent as one that can be isolated from an infected animal, cultured in a laboratory setting, introduced into a healthy animal to produce the same infection as in the first animal, and isolated again from the second animal. These ideas formed the basis of research into bacterial diseases and are still dominant in the sciences devoted to the study of disease.

In 1848 Ignaz P. Semmelweis (1818–1865), a Hungarian physician working in German hospitals, came up with a novel idea: after examining the bodies of women who had died of puerperal (childbed) fever, he suggested that doctors should wash their hands in a solution of chlorinated lime water before touching a pregnant

Koch’s postulates helped usher in what has been called the golden era of medical bacteriology. Between 1879 and 1889 German microbiologists isolated the organisms that cause cholera, typhoid fever, diphtheria, pneumonia, tetanus, meningitis, and gonorrhea as well the Staphylococcus and Streptococcus organisms. Even as

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KEY TERMS A substance produced by

ANTIBIOTIC:

or derived from a microorganism, which in diluted form is capable of killing or at least

to offspring, stored in DNA molecules called chromosomes. GERM THEORY:

A theory in medi-

inhibiting the action of another microor-

cine, widely accepted today, that infections,

ganism. Antibiotics are not usually effec-

contagious diseases, and other conditions

tive against viruses.

are caused by the actions of microorgan-

The practice of inhibit-

ANTISEPSIS:

ing the growth and multiplication of

isms. IMMUNITY:

The condition of being

microorganisms, generally by ensuring the

able to resist a particular disease, particu-

cleanliness of the environment.

larly through means that prevent the

BACTERIOLOGY:

An area of the bio-

logical sciences concerned with bacteria, including their importance in medicine, industry, and agriculture. DNA:

Deoxyribonucleic acid, a mole-

cule in all cells, and many viruses, containing genetic codes for inheritance.

growth and development of pathogens or counteract their effects. IMMUNOLOGY:

The study of the

immune system, immunity, and immune responses. INFECTION:

A state or condition in

which parasitic organisms attach them-

A term for an infec-

selves to the body or to the inside of the

tion that occurs when the body’s resistance

body of another organism, producing con-

is lowered. Compare with exogenous.

tamination and disease in the host.

ENDOGENOUS:

Affecting or potentially

EPIDEMIC:

The prevention of a

INOCULATION:

affecting a large proportion of a popula-

disease by the introduction to the body, in

tion (adj.) or an epidemic disease (n.)

small quantities, of the virus or other

EPIDEMIOLOGY:

An area of the med-

microorganism that causes the disease.

ical sciences devoted to the study of dis-

MUTATION:

ease, including its incidence, distribution,

structure of an organism’s DNA, resulting

and control within a population.

in a genetic change that can be inherited.

ETIOLOGY:

A branch of medical study

PANDEMIC:

Alteration in the physical

Affecting an extremely

concerned with the causes and origins of

high proportion of a population over a

disease; also, a general term referring to all

wide geographic area (adj.) or a disease of

the causes of a particular disease or condi-

pandemic proportions (n.)

tion.

PARASITE:

A general term for any

A term for an infection

organism that depends on another organ-

that originates outside the body. Compare

ism for support, which it receives at the

with endogenous.

expense of the other organism.

EXOGENOUS:

GENE:

A unit of information about a

PARASITOLOGY:

A biological disci-

particular heritable (capable of being

pline devoted to the study of parasites, pri-

inherited) trait that is passed from parent

marily those among the animal and protist

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KEY TERMS kingdoms. Parasitic bacteria, fungi, and viruses usually are studied within the context of infectious diseases. PATHOGEN:

A disease-carrying para-

site, typically a microorganism.

Ribonucleic acid, the molecule translated from DNA in the cell nucleus, the control center of the cell, that directs protein synthesis in the cytoplasm, or the space between cells. RNA:

and methods for protecting and improving

VACCINE: A preparation containing microorganisms, usually either weakened or dead, which is administered as a means of increasing immunity to the disease caused by those microorganisms.

the health of a community through efforts

VECTOR:

PATHOLOGY:

The study of the essen-

tial nature of diseases. PUBLIC HEALTH:

A set of policies

that include disease prevention, health education, and sanitation.

Koch’s work was influencing the development of the germ theory, the influence of the English physician Joseph Lister (1827–1912) was being felt in operating rooms. Building on the work of both Semmelweis and Pasteur, Lister—for whom the well-known antiseptic mouthwash Listerine was named—began soaking surgical dressings in carbolic acid, or phenol, to prevent postoperative infection. A N T I B I O T I CS . Whereas antisepsis was the great battleground of the invisible war during the nineteenth century, in the twentieth century the most important struggle concerned the development of antibiotics. The first effective medications to fight bacterial infection in humans were sulfa drugs, developed in the 1930s. They work by blocking the growth and multiplication of bacteria and were initially effective against a broad range of bacteria, but many strains of bacteria have evolved resistance to them. Today, sulfa drugs are used most commonly in the treatment of urinary tract infections and for preventing infection of burn wounds.

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CONTINUED

An organism, such as an insect, that transmits a pathogen to the body of a host.

notatum, Fleming made a juice with it that he called penicillin. He administered it to laboratory mice and discovered that it killed bacteria in the mice without harming healthy body cells. It would be more than a decade before the development of a form of penicillin that could be synthesized easily. This drug arrived on the scene in 1941—just in time for the years of heaviest fighting in World War II—and after the war pharmaceutical companies began to manufacture numerous varieties of antibiotic. By the last decade of the twentieth century, however, a new problem emerged: bacteria were becoming resistant to antibiotics. This has been the case with medications used to treat conditions ranging from children’s ear infections to tuberculosis.

The importance of sulfa drugs was eclipsed by that of penicillin, first discovered in 1928 by the British bacteriologist Alexander Fleming (1881–1955). Working in his laboratory, Fleming noticed that a mold that had fallen accidentally into a bacterial culture killed the bacteria. Having identified the mold as the fungus Penicillium

An example is amoxicillin, a penicillin derivative developed in the late twentieth century. Many pediatricians found it a better treatment than penicillin for ear infections, because it did not tend to cause allergic reactions sometimes associated with the other antibiotic. However, by the late 1990s evidence surfaced indicating that certain types of bacteria had developed a protein that rendered amoxicillin ineffective against ear infections. Critics of amoxicillin (or of antibiotic treatments in general) maintained that widespread prescription of the antibiotic actually helped create that situation, because the bacteria developed the protein mutation defensively.

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Because of these and similar concerns associated with antibiotics, doctors have begun taking measures toward controlling the spread of antibiotic-resistant diseases, for instance by prescribing antibiotics only when absolutely necessary. Research into newer types and combinations of drugs is ongoing, as is research regarding the development of vaccines to prevent bacterial infections. WHERE TO LEARN MORE Biddle, Wayne. A Field Guide to Germs. New York: Henry Holt, 1995. The Big Picture Book of Viruses. Tulane University (Web site). . Cells Alive! (Web site). .

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Centers for Disease Control and Prevention (Web site). .

Infection

Infection Index. Spencer S. Eccles Health Sciences Library, University of Utah (Web site). . “Oral Health Topic: Infection Control.” American Dental Association (Web site). . The Race Against Lethal Microbes: Learning to Outwit the Shifty Bacteria, Viruses, and Parasites That Cause Infectious Diseases. Chevy Chase, MD: Howard Hughes Medical Institute, 1996. Virtual Museum of Bacteria. Bacteria Information from the Foundation for Bacteriology (Web site). . Weinberg, Winkler G. No Germs Allowed!: How to Avoid Infectious Diseases at Home and on the Road. New Brunswick, NJ: Rutgers University Press, 1996.

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Chemoreception

CHEMORECEPTION

CONCEPT Chemoreception is a physiological process whereby organisms respond to chemical stimuli. Humans and most higher animals have two principal classes of chemoreceptors: taste (gustatory receptors), and smell (olfactory receptors). Though our sense of smell assists us in distinguishing among tastes, the gustatory and olfactory receptors are different in many respects—not only in their locations but also in terms of their chemical and neurological makeup. Capabilities of taste and smell vary widely among people, as a function of genetics, age, and even personal habits. Likewise, culture influences attitudes toward taste and smell. As for the animal kingdom, certain creatures are gifted with exceedingly acute senses, particularly where smell is concerned, but for some invertebrates, such as worms, there is really little distinction between taste and smell. Among the most interesting aspects of chemoreception in animals is the use of smell for communication, particularly through the release of special chemicals called pheromones. As to whether pheromones, which function as sex attractants, play a role in human interaction, many scientists remain skeptical.

HOW IT WORKS The Senses The term sense, which also may be called sensory reception and sensory perception, refers to the means by which an organism (usually an animal) receives signals regarding physical or chemical changes or both in its environment. Sensory reception and perception entail the translation of these signals, which represent changes in the

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matter or energy of the environment, into processes within the body and brain. For example, if you eat a cookie, your sense of taste translates the chemical data from that cookie into the sensation of sweetness, a sensation your brain most likely perceives as a pleasing one. In everyday language, people are accustomed to speaking of five senses possessed by humans and, to a greater or lesser degree, by other animals: sight, touch, smell, hearing, and taste. The reality is rather more complex. For one thing, there are not really just five senses; rather, there are at least five others—and there may be more, depending on just how one defines and classifies the senses. These other senses include the kinesthetic sense, or the discernment of motion; the sensation of temperature, or distinguishing relative heat and cold; the awareness of pressure; the sense of equilibrium or balance; and the perception of pain. All of these senses involve a response to stimuli, which may be defined as any phenomenon (that is, an observable fact or event, such as an environmental change) that directly influences the activity or growth of a living organism. Each of these senses has a biological component as well as a physical or a chemical one. In the next few paragraphs, we briefly discuss what this means, first by considering the nervous system in general terms and then by looking at the physical and chemical receptors that transmit data through that system. T H E N E R V O U S S Y S T E M . All sensation is biological in the sense that an organism not only must be living to experience it but also must have a functioning nervous system. The latter is a network, found in the bodies of all verte-

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Chemoreception

CHEMORECEPTION ITS ENVIRONMENT,

IS THE MEANS BY WHICH AN ORGANISM RECEIVES SIGNALS REGARDING CHEMICAL CHANGES IN TRANSLATING THEM INTO PROCESSES WITHIN THE BODY AND BRAIN.

NERVE

CELLS, SUCH AS THE

ONES SHOWN HERE, MAKE UP A NETWORK OF PRIMARY RECEPTORS THAT RECEIVE AND INTERPRET STIMULI AND TRANSMIT MESSAGES BASED ON THESE SENSORY DATA TO THE BRAIN. (Photograph by Secchi-Lecague/Roussel-UCLAF/CNRI/Science Photo Library. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission.)

brates (animals with internal skeletons), whose purpose is to receive and interpret stimuli and to transmit impulses. Parts of the nervous system include the brain, spinal cord, nerves, and other components. To be experienced through the senses, all data must be transmitted to the brain through the nervous system. This happens through the conversion and transmission of physical or chemical information, which takes place in sensory nerves known as receptors. A receptor is a structure in the nervous system that receives specific stimuli and is affected in such a way that it sends particular messages to the brain. The brain interprets these messages as sensations corresponding to the stimuli. Primary receptors are those that directly convert stimuli to electronic signals, which they send to the brain. Neurons, or nerve cells, serve as primary receptors. In addition, there are secondary receptors, which simply transmit signals between neurons. Secondary receptors induce a response in an adjoining neuron, thus sending the signal down the line toward the brain.

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PHYSICAL RECEPTORS.

AND

CHEMICAL

As we have noted, the changes in the outside environment that the brain interprets as sensation are either physical or chemical in origin. By physical, we mean the types of data and phenomena that are studied in the science of physics: matter, energy, and the interactions between them. Likewise, chemical, refers to objects of study in the scientific realm of chemistry: elements, compounds, and mixtures. In the human body there are sensory nerves devoted to the interpretation of physical or chemical data. Among the receptors of physical data are photoreceptors, which respond to light and therefore play a part in the sense of sight; thermoreceptors, which respond to temperature and are concerned with the sense of heat and cold; and nociceptors, or pain receptors. In addition, there are mechanoreceptors, which respond to the mechanical properties of matter and are involved in the senses of touch, hearing, and equilibrium. As for chemical data, they are interpreted through chemoreceptors, which govern the senses of smell and touch. For this reason, chemore-

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ceptors sometimes are referred to as the chemical senses. Our focus in the present context, of course, is on chemoreception, the term for the physical process whereby organisms—not just humans, but virtually all animals—respond to chemical stimuli.

Distinguishing Gustation and Olfaction We tend to associate taste and smell, and indeed there is some relation between them, but there are also numerous distinctions. Not everyone with an acute sense of smell, for instance, has as finely honed a sense of taste. Conversely, people with poor senses of smell do not necessarily suffer a corresponding impairment in their taste buds. The neurological structures that have to do with taste and smell, at least in higher animals, are not the same. Whereas gustatory sensation travels via secondary receptors called epithelial cells, which pass messages to adjacent neurons, olfactory signals enter the nervous system through primary receptors. The principal sensory organs of gustation and olfaction in a human being are, respectively, the taste buds on the tongue and the olfactory receptors located within the olfactory epithelium in the nasal cavity. They are designed to respond to two entirely different types of chemical materials. Taste buds are best at receiving sensory data from water-soluble chemicals, or those that dissolve in water, while the olfactory receptors are most attuned to vapors that are water-insoluble. WAT E R A N D O I L . Almost everyone who has ever eaten a hot, spicy dish has tried to “put out the fire” in their mouths by drinking water, only to discover to their dismay that water only seemed to make the problem worse. On the other hand, milk is usually quite effective. The reason is that most spicy-hot substances tend to be oily, and therefore they do not readily form intermolecular bonds with water. On the other hand, milk, though it is largely composed of water, also contains oily fat particles. Thanks to the chemistry of water- and oil-based substances, it is true (as the old saying goes) that “oil and water don’t mix.”

cally bipolar, with all the negative charges on the end or side where the oxygen atom is located and all the positive charges on the end or side of the hydrogen molecules. In petroleum and most other oily substances, however, the molecules typically include some combination of carbon and hydrogen, which have similar electronegativity values. For that reason, the electric charges are distributed more or less evenly throughout the molecule. As a result, oily substances form very loose intermolecular bonds, and they tend not to bond with water-based substances.

Chemoreception

As for taste buds and olfactory membranes, it is fitting that the taste buds would be more receptive to water-based foods and liquids, since most foods (including jalapeño peppers) contain at least some water. Obviously, if we can taste spicy-hot substances, then there must be some receptivity to oil-based foods. Part of this receptivity involves the olfactory membranes, which, as we have noted, are more receptive to oily materials.

Animals and Chemoreception Just as taste and smell are sharply distinguished in humans (despite the fact that smell aids us in the process of tasting), the same is true of most vertebrates. In the case of invertebrates, such as worms, however, there is much less differentiation between gustatory and olfactory receptors. These animals, in fact, may have only one chemical sense, with only slight differences between what scientists call distance chemoreception, more or less the same as smell in vertebrates, and contact chemoreception, which corresponds to vertebrates’ sense of taste. Such analogies can be made because distance chemoreceptors appear to respond to nonwater-soluble substances in the same way that the olfactory receptors in vertebrates do, while contact chemoreceptors are more responsive to water-soluble chemicals. As with many other animals, these senses are linked to numerous behaviors—not just feeding—in invertebrates. Distance chemoreception enables invertebrates to sense the presence of chemicals that pose a danger, signaling the need to move away, while contact chemoreception assists the invertebrate in determining when to mate and when to lay eggs.

In water, hydrogen and oxygen have significantly different levels of electronegativity, or the relative ability of an atom to attract valence electrons, which are used in chemical bonding. Therefore, a water molecule tends to be electri-

land-based, animals whose skins secrete mucus

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TERRESTRIAL

ANIMALS WHOSE SKINS SECRETE MUCUS, LIKE THE SNAIL, HAVE WHAT SCIENTISTS CALL THE COMMON

CHEMICAL SENSE,

which makes them sensitive to the presence of foreign chemicals on the surface of their bodies. (JLM Visuals. Reproduced by permission.)

(e.g., snails and slugs) as well as aquatic animals have what scientists call the common chemical sense, which makes them sensitive to the presence of foreign chemicals anywhere on the surface of their bodies. Even humans and other animals whose skin does not secrete mucus across its entire surface have an evolutionary remnant of this sense. Thus, the mucous membranes in the eyes, mouth, nose, and genitals respond to chemical irritants. This is yet another example of the fact that, while chemoreception in animals is associated most readily with tasting and smelling, it is linked to myriad other functions as well. Some insects may use chemoreception to detect the presence of moisture, and many animals apply it for a variety of purposes. Such purposes include selection and courtship of a mate as well as the identification of friends or foes. A dogs’ sense of smell tells it everything it needs to know about a new animal or person it encounters; similarly, a cat may identify another cat by sniffing its rectum. Smell also can be used to mark territory, which is why dogs and cats mark their “turf ” by urinating. Both chemical senses, particularly smell, are important mediums of communication in the animal kingdom.

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Mechanisms of Taste and Smell Scientists in the nineteenth century believed that human tongues have receptors for four basic tastes: sweet, sour, salty, and bitter. More current research, however, shows that the taste receptors on the tongue are more complicated than was previously thought. Still, it does seem to be the case that specific kinds of taste buds are clustered in certain areas. Taste buds, so named because they look like plant buds when viewed under a microscope, cover the tongue and, to a lesser extent, can be found on the cheek, throat, and roof of the mouth. As we shall see, however, some people have a greater concentration of taste buds than do others. In the mouth, saliva breaks down the chemical components of substances, which travel through the pores in the papillae (small protuberances on the surface of the tongue) to reach the taste buds themselves. When specific proteins in food bind to receptors on the taste buds, these receptors send messages to the cerebral cortex, a surface layer on the brain that coordinates sensory information. And though taste buds in certain regions of the tongue have an affinity for particular flavors, as we discuss later, the intricacies of the neural and

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chemical networks tend to suggest that nothing is clear-cut about this highly complex biological process. O L FAC T I O N : A D I R E C T S E N S E .

Before modern times, several of the leading theories of vision maintained that the eye actually interacts with the objects it sees. Now we know that our eyes simply receive light reflected off those objects. By contrast, smelling is a direct experience, because we inhale microscopic portions of substances that have evaporated and make their way into the nasal cavity, where they chemically interact with sense receptors. Cells in the nose detect odors through receptor proteins on the cell surface, which bind to odor-carrying molecules. A specific odorant docks with an olfactory receptor protein in much the same way that a key fits into a lock. This, in turn, excites the nerve cell, causing it to send a signal to the brain. Although there are many tens of thousands of odor-carrying molecule types in the world, meaning that there are as many different smells, there are only hundreds (or at most about 1,000) different types of olfactory receptors in even the most sensitive animal species. This finding has led scientists to speculate that not every receptor recognizes a unique odorant molecule; rather, similar odorants can bind to the same receptor. Another way to put this, in light of the lock and key analogy, is that a few loose-fitting odorant “keys” of roughly similar structure can enter the same receptor “lock.” Just as we sense smells directly, the olfactory sense is also direct in the way that signals are transmitted to the brain. Vision, by contrast, puts into play several steps: a receptor cell detects light and passes the signal to a nerve cell, which passes it on to another nerve cell in the central nervous system, which then relays it to the visual center of the brain. In olfaction, on the other hand, the olfactory nerve cells perform all these functions.

ry and emotion, are coordinated with the sense of smell. Hence, as many people have observed, the sense of smell is linked strongly with longterm memory in a way that such senses as sight and touch are not.

Chemoreception

REAL-LIFE A P P L I C AT I O N S Taste Buds Humans have only about 10,000 taste buds, whereas rabbits have 17,000 and cows some 25,000. This seems more than a little ironic, since humans enjoy by far the most varied diet. Both of the other animals are herbivores, meaning that they do not eat meat, nor are they accustomed to sweets and the many other varieties of taste in the diet of the average well-fed American. If anything, cows, with about 50% more taste buds than rabbits, eat a diet even more plain than that of their furry, fleet-footed fellow mammals. Though our tasting equipment (that is, the chemoreceptors for taste in our tongues) may be much less sophisticated than that of cows or rabbits, the number of tastes our palate can recognize is as varied as a spectrum of color swatches at a paint store. Despite such variation, there are only a few basic tastes, most notably, the ones that once were thought to constitute primary tastes analogous to the primary colors: sweet, sour, salty, and bitter. T H E F O U R “ B AS I C TAS T E S . ”

In most animals these cells take scent messages directly to the nerve cells of the olfactory bulb in the brain. With insects and other invertebrates whose brains are relatively simple, functioning primarily as clearinghouses for sensations, the olfactory nerves send signals to the olfactory ganglia, a mass of nerve tissue that connects nerve cells external to the brain and spinal cord. In higher animals, such as humans, olfactory signals go to the olfactory cortex, a structure in the brain where higher functions, such as memo-

When you lick an ice cream cone, you may notice that you are experiencing the sweetness of it primarily at the tip of your tongue. This is not simply because you are licking it with the tip but also because there is a heavier concentration of sweetness receptivity in that area. On the other hand, if you eat a sour gumball, you experience the taste most notably on the sides of your tongue, where receptivity to sour tastes is strongest. Reception of salty tastes takes place near the front of the tongue, just behind the tip. As for bitter tastes, the focal point of receptivity appears to be near the back of the tongue. The latter may be a highly useful adaptive mechanism we have developed along the way, since many poisons are bitter, and the gagging reflex takes place near the back of the mouth. Not all bitter tastes are revolting, however: olives, which many people love, are

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bitter as well, as is coffee when it has no cream or sugar to alter its flavor. As we noted earlier, the organization of taste receptors on the tongue is not quite as simple as once was believed. For example, though our receptivity to sweetness seems to take place primarily at the tip, to a lesser extent we taste sweetness and other flavors all over the tongue. Furthermore, such flavors as sweet, sour, bitter, and salty are not the sum total of “taste” as we experience it; not only smell but also texture affects the taste of substances. Genetic and cultural factors also influence a person’s unique tastes and may explain why one person loves sweets while another cannot get enough of tangy tastes.

Taste and Other Senses Taste does not work alone; on the contrary, our sense of the smell, texture, and temperatures of foods affects our overall perception of its flavor and in some cases its desirability. When food is in the mouth, it produces a scent, which enters the nose through the nasopharynx, an opening that links the mouth and the nose. Because we experience smell more directly and our noses are more sensitive to olfactory sensations than our taste buds are to gustatory ones, people often experience the flavor of food first by its smell. This greatly affects our perception of what we eat. For example, while many people like blue cheese, many others despise it, and this probably has more to do with its smell than with its taste. While its taste is rather tangy, it is not quite as “radical” as the aroma of this cheese, which has been compared to everything from old socks to vomit. Other cheeses, such as gorgonzola and, particularly, Limburger, are even more pungent and therefore have more than their share of detractors—but again, the smell is more extreme than the taste. On a more pleasant note, anyone who has ever enjoyed a good steak or a hamburger cooked on an open grill will attest to the fact that a great deal of that enjoyment comes from the aromas of cooking. Usually it is the smell of a delectable food item, which we detect long before we taste it, that causes our salivary glands to begin operating, preparing us for the process of consuming and digesting the dish. Additionally, different types of cooking have particular smells and tastes associated with them, which people may find more or less appealing. For example, scrambled

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eggs cooked over an open campfire are likely (all other things being equal) to be more appealing to most people than eggs cooked in a skillet on an electric stove. But if the campfire is fueled with burning dung instead of wood, most Americans would choose the stove. In sparsely forested parts of the third world, however, animal dung is a principal form of fuel, and without it people might have to eat meals raw. T E X T U R E A N D T E M P E RAT U R E .

Texture and temperature may have less impact on taste than does smell, but these are still significant factors. Take the example of three plates of French fries. One is limp and soggy in consistency, and another is so crispy that the French fry crunches like potato chips. The third, however, is just a bit crispy on the outside and just a bit soft on the inside. Most people, though certainly not all, would judge the third plate of French fry the most delectable—purely on the basis of texture. By the same token, many people are less than enthusiastic about boiled okra, owing to its slimy consistency, whereas fried okra is more appealing to most Americans, since it lacks that gooey texture. Likewise, temperature plays more of a role than one might think. Many people, for example, find cold coffee unappealing, though others have a fondness for iced coffee, at least if it has milk and sweetener. Similarly, many Americans enjoy the combination of cold ice cream and hot pie or cobbler, partly because the contrast of temperatures adds to the overall flavor.

Different People, Different Sensations Not all people “taste” the same—that is, not all people have the same sense of taste or the same level of acuity for distinguishing different flavors. One person may have 10–1,100 taste buds per square inch (6.45 cm2) on the tongue, indicating a huge range of sensitivities with regard to gustatory data. Research also has shown that women, on average, have more taste buds than men, proving what many a woman has long asserted— that women have “better taste” than men. Though it does appear that the average female has a more acute sense of gustation than the typical male, taste buds are not the only biological factor involved in recognizing flavors. For example, the amount of saliva a person naturally generates and the amount of salt that appears in one’s saliva play a major role in determining an

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individual’s response to salty foods. A person whose mouth generates less saliva is more sensitive to the salt in foods, whereas a person prone to generating a greater quantity of saliva is less likely to taste the salt that has been added to a dish. That person is therefore more likely to add salt. Ability to smell also varies from person to person, though it appears that a less acute sense of smell may be a sign not merely of fewer olfactory receptors but also of an actual olfactory disorders. Just as some people may be color-blind, it appears that others are “smell-blind.” And just as being color-blind can have very serious consequences, for instance by causing a color-blind driver to miss a red light, much the same is true with an olfactory disorder. To a greater extent than one might immediately guess, smell serves a protective function. For example, without a sense of smell, one cannot tell if food is spoiled, unless, of course, it has reached such a state of putrefaction that it shows visible signs. For a person with an ordinary sense of smell, however, rotten food sends a signal through the olfactory receptors, which may cause a gag reflex when smelling food that has spoiled. AG E A N D S E N S E O F TAS T E . It

is an experience familiar to many people, and it goes something like this. Let us say that in his boyhood, a man enjoyed a particular brand of candy, of which he could never get enough. Left to his own devices, he probably would have eaten so much that he would have become sick. The fact that this never happened had more to do with his parents—and the fact that his allowance money had to go for other things as well—than it did with his own natural sense of restraint. So he dreamed of the day when he became a grown-up, when he could eat whatever he wanted. Eventually, he forgets this dream amid the many distractions of adolescence, but then one day many years later, as a grown man, he happens to see this particular item of candy in a store, and all his childhood memories come back to him. He buys several pieces, thrilled that he can enjoy in complete freedom what was once a rare treat. He can barely wait to get into his car, open the first piece, put it in his mouth—and then he recoils in disgust, thinking, How did I ever enjoy that? Disappointed, he throws away the rest of the candy.

mature, so do our taste buds, and their numbers increase, leading to greater sophistication of taste. Children tend to like very basic tastes, particularly sweet and sour, and respond much less favorably to the subtlety in more complex dishes. This fact, combined with an increased awareness of health issues as one ages, explains why an adult might relish a broccoli casserole but find cotton candy so sweet as to be repugnant, whereas a child’s reaction would probably be just the opposite.

Chemoreception

Just as taste buds mature along with the people who own them, they also age. Every 3-10 days, on average, our taste buds regenerate themselves, replacing old ones that have been worn out by foods that are too hot, too cold, or otherwise too taxing to the chemoreceptors in our tongues. But as people grow older, their taste buds replace themselves less frequently, and therefore their sense of taste becomes less finely tuned. An older person may require much more sweetness or spice to taste a particular food.

Chemoreception Impairment and Disorders Sense of smell also deteriorates with age; as we noted earlier, this can pose dangers, because a person depends on the sense of smell for protection more than one might imagine. For example, in addition to the inability to detect spoiled food, an elderly person would be far less likely to smell smoke if a building were on fire. Older people also are less likely to be cognizant of olfactory data that send messages concerning unpleasant smells of a less critical nature—body odor, for example. Many of the problems of gustation and olfaction suffered by the elderly are reflected, at a much younger age, in the bodies of smokers. In addition to its many other negative effects, smoking deadens taste buds and desensitizes the olfactory receptors. It is not uncommon to see a heavy smoker salting pizza or some other food that for people with ordinarily functioning taste buds would not seem to require any salt. As for olfactory sensation, a smoker becomes accustomed to the reek of stale smoke and ashes.

The candy, of course, has not changed, but the man—and his taste buds—have. As we

On a more temporary basis, many people find their senses of taste and smell impaired by illness. A person with a cold or flu, even at its final stages, usually has enough congestion that the senses of both smell and taste are limited, if not almost nonexistent. In this instance, the lack

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TASTE

AND SMELL ARE NOT PURELY BIOLOGICAL, BUT ALSO REFLECT CULTURAL FACTORS. AS LATE AS THE 1970S, AMERICANS WOULD HAVE SAID THAT THE IDEA OF EATING RAW FISH WAS REVOLTING. TODAY, AMERICA’S CITIES BRISTLE WITH JAPANESE RESTAURANTS THAT SERVE SUSHI, SHOWING THAT CULTURAL TASTES CAN CHANGE. (© Japack MOST

Company/Corbis. Reproduced by permission.)

of ability to taste serves to illustrate the strong link between gustation and olfaction: the taste buds themselves are working fine, but the lack of smell, resulting from congestion, hinders the brain’s ability to process flavor. TA S T E A N D S M E L L D I S O R D E R S . In addition to people whose olfactory

and gustatory senses are impaired by age, illness, or smoking, between two million and four million Americans suffer from some sort of taste or smell disorder. The inability to taste or smell not only robs an individual of certain sensory pleasures, it also can be dangerous to one’s mental health. Some psychiatrists believe that the lack of taste and smell can have a profoundly negative effect on a person’s quality of life, leading to depression or other psychological problems.

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cause more permanent damage to the intricate neural networks that process tastes and smells. Drugs such as lithium, used to treat bipolar disorder (what used to be called manic depression) also may cause taste and smell disorders. This occurs because certain drugs (lithium is just one example among many) inhibit the action of certain enzymes, affect the body’s metabolism, and interfere with the neural networks and receptors involved in tasting and smelling. Exposure to such environmental toxins as lead, mercury, insecticides, and solvents (e.g., paint thinner) also can damage taste buds and sensory cells in the nose or brain.

Culture and Chemoreception

Whereas impairments of smell and taste brought on by cold, flu, various viral and bacterial infections, and even allergies are usually temporary, some other illness-related taste and smell disorders are more long term. Such is the case with neurological disorders due to brain injury or diseases such as Parkinson’s or Alzheimer’s (conditions marked by tremors and mental deterioration, respectively). These conditions can

One of the favorite delicacies in the Philippines is known as dinuguan, or pork cooked in pork blood. Chances are that a visitor from England or northern Europe, when told the constituents of the dish, would feel right at home; an American, however, most likely would try to think of an excuse to pass up this Filipino delight. To most Americans, it would seem that eating dinuguan, or the many varieties of blood pudding or blood sausage common in England and Scandinavia, is

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simply “gross”—that it is objectively and unquestionably disgusting. But in the 1960s or even the 1970s, most Americans, even in large cities, would have said that the idea of eating raw fish was revolting. Today, however, America’s cities and suburbs bristle with Japanese restaurants that serve sushi, an indication of the fact that cultural tastes can change. As is discussed in Parasites and Parasitology, it should be noted that improperly cooked pork and fish (especially raw fish) are quite likely to serve as hosts for disease-carrying worms, so one should exercise care before sitting down to a plate of dinuguan or sushi. But then again, many Americans like their steaks on the rare side, and undercooked beef certainly has its share of pathogens (disease-carrying parasites) as well. It seems that it is not health concerns that explain our cultural double standards about certain food items. Of course, Americans are not the only people with quirky standards regarding tastes and smells; in fact, every culture has its idiosyncrasies. In China it is not considered at all offensive for one’s breath to smell heavily of onions; on the other hand, the smell of dairy products, virtually nonexistent in Chinese cuisine, is considered highly offensive. What does all of this prove? Only that taste and smell are not purely biological but also reflect cultural factors. During the nineteenth century, many European scientists embraced a racist theory concerning the olfactory capabilities of different peoples around the world. According to this highly unscientific “theory,” non-Europeans were more primitive than Europeans and therefore closer to animals, which meant that they had a stronger sense of smell. Completing the loop of this circular logic, subscribers to this nonsense maintained that because non-Europeans had a stronger sense of smell, it proved they were more primitive than Europeans!

Humans, Animals, and Smell

ble of detecting odors so faint that their proportion of the surrounding air is in the range of only a few parts per trillion. Many researchers are beginning to wonder whether smell does not play a greater role in human behavior and biology than previously was believed. For example, research has shown that only a few days after the birth of her baby, a human mother can smell the difference between a vest worn by her baby and one worn by another.

Chemoreception

Nevertheless, the fact remains that the olfactory abilities of many animals are far beyond those of humans. This is a fact that hardly needs scientific verification, since most of us have observed dogs’ reliance on their strong sense of smell. This explains why police use dogs to detect illegal drugs and explosives and to track runaway prisoners or the bodies of murder victims. Dogs are not the only animals gifted with acute senses of smell, which aid them in finding their way to specific targets. Salmon, for example, manage to find their way back to the streams where they were hatched, guided by their sense of smell. (For more about animals’ navigational abilities, see Migration and Navigation.) Most vertebrates other than humans have many more olfactory nerve cells in a proportionately larger olfactory epithelium, and this probably gives them much more sensitivity to odors. In addition, most land vertebrates have a specialized scent organ in the roof of their mouths called the vomeronasal organ, which gives them far more sensitivity to odors than humans have. P H E R O M O N E S . Some animals are known particularly for the odors they excrete, especially when it is an animal such as a skunk or stinkbug that puts off a repellent odor as a defense mechanism. But animals also send out much more subtle smells known as pheromones. Chemical substances produced and secreted by animals, pheromones serve as stimuli for behavioral responses on the part of other animals of the same species. Pheromones are common among insects as well as many vertebrates, but they are nonexistent among bird species.

By the early twentieth century, physiologists had begun to explore much more scientific ideas concerning olfactory and gustatory abilities in humans—abilities that, needless to say, are not a function of race or ethnicity. Only one assumption of the old-fashioned European scientists was correct: that animals have a stronger sense of smell than do humans. The human nose is capa-

Among so-called “social insects” such as bees and ants, pheromones play a particularly strong role. The queen honeybee gives off what is called the queen substance, a pheromone that acts to prevent the development of ovaries among workers, which are biologically unproductive females. Pheromones are vital for communication among

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KEY TERMS A physiological process whereby organisms respond to chemical stimuli. CHEMORECEPTION:

Of, or involving, the sense of taste (gustation). GUSTATORY:

An animal without

INVERTEBRATE:

an internal skeleton. A network, found in the bodies of all vertebrates, whose purpose is to receive and interpret stimuli and to transmit impulses. Parts of the nervous system include the brain, spinal cord, and nerves. NERVOUS

SYSTEM:

OLFACTORY: Of, or involving, the sense of smell (olfaction).

Chemical substances produced and secreted by animals, which serve as stimuli for one or more behavioral responses on the part of other animals of the same species. PHEROMONES:

A structure in the nervous system that receives specific stimuli and is affected in such a way that it sends particular messages to the brain. These messages are interpreted by the brain as sensations corresponding to the stimuli. RECEPTOR:

The means by which an organism (usually an animal) receives signals regarding physical or chemical changes or both in its environment. Sometimes called sensory reception or sensory perception. SENSE:

Any phenomenon (i.e., an observable fact or event, such as an environmental change) that directly influences the activity or growth of a living organism. STIMULUS:

VERTEBRATE:

social insects, which have little or no sense of sight. Ants, bees, and wasps send out smells to alarm others of danger, and ants may create a path of pheromones to guide others to a food source. The function for which pheromones are most widely known, however, is as a sex attractant. Male musk deer are noted for their excretion of musk, which, as the result of to its pleasant and powerful smell, is often an ingredient in perfume manufacture. Though musk is a sex attractant, it is not a pheromone, which is a much less obvious scent and which, as we have noted, likely has an effect only on animals of the same species. Experiments have shown that male mice who lack a gene for a pheromone receptor are likely to attempt to mate with males, simply because they cannot tell the difference. With humans, of course, it is easy to tell the difference between males and females on sight, but do humans also respond to pheromones? The fact that these chemicals theoretically could induce mating behavior led cologne and perfume makers long ago to embrace the idea of human pheromones, whose presence in a manufactured scent obviously would be a boon to many a single man or woman. Despite the enthusiastic claims of perfume manufacturers, however, many scientists have yet to be convinced that pheromones play a significant role for humans. Regarding the fact that the human anatomy includes the vestige of a vomeronasal organ, the olfactory researcher Charles Wysocki told Lee Bowman in an article published on the National Library of Medicine Web site, “It’s like the appendix—it’s there, but it doesn’t seem to do anything.” As for scent makers’ promises that pheromones will help users attract partners, Wysocki said, “Sure the claims are out there. ... ‘All you have to do is put this on and you’ll score.’ But there’s nothing in the published biomedical literature [to indicate] that we have any kind of pheromone that draws a partner.” Research does suggest that people give off chemical messages that correspond to certain moods, but it is a long way from this to the idea of a spray-on aphrodisiac.

An animal with an

internal skeleton.

WHERE TO LEARN MORE Ackerman, Diane. A Natural History of the Senses. New York: Vintage Books, 1991.

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Bowman, Lee. “A Nose for Romance?” U.S. National Library of Medicine/National Institutes of Health (Web site). . Chemical of the Week—Chemoreception: The Chemistry of Odors. Science Is Fun/University of Wisconsin–Madison (Web site). . Chemoreception Links. Leffingwell & Associates (Web site). . The ChemoReception Web (Web site). .

Finger, Thomas E., Wayne L. Silver, and Diego Restrepo. The Neurobiology of Taste and Smell. New York: Wiley-Liss, 2000.

Chemoreception

Monell Chemical Senses Center (Web site). . Pybus, David, and Charles Sell. The Chemistry of Fragrances. Cambridge, England: Royal Society of Chemistry, 1999. Rivlin, Robert, and Gravelle, Karen. Deciphering the Senses: The Expanding World of Human Perception. New York: Simon and Schuster, 1984.

Evans, David H. The Physiology of Fishes. Boca Raton, FL: CRC Press, 1998.

Whitfield, Philip, and D. M. Stoddart. Hearing, Taste and Smell: Pathways of Perception. Tarrytown, NY: Torstar Books, 1984.

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BIOLOGICAL RHYTHMS Biological Rhythms

CONCEPT People frequently talk about body clocks, a term that refers to the patterns of energy and exhaustion, functioning and resting, and wakefulness and sleep that characterize everyday life. In fact, the concept of the body clock, or circadian rhythm, is part of a larger picture of biological cycles, such as menstruation in mammalian females. Such cycles, which assume a variety of forms in a wide range of organisms, are known as biological rhythms. These rhythms may be defined as processes that occur periodically in an organism in conjunction with and often in response to periodic changes in environmental conditions—for example, a change in the amount of available light. Not all aspects of the body clock are part of day-to-day experience, and this is fortunate, since these interruptions in the healthy flow of biological rhythms can threaten the well-being of the human organism. Among these challenges to the ordered working of bodily “clocks” are jet lag, seasonal affective disorder (SAD), and other disorders linked to a range of causes, including drug use.

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Nor do all cycles involve sleep and wakefulness: menstruation, for instance, is a monthly cycle related to the sloughing off of the lining of the uterus, a reproductive organ found in most female mammals. Another biological rhythm is the beating of the heart, which, of course, takes place at very short intervals. Nonetheless, the circadian rhythm is the most universal of biological cycles, and it is the focus of our attention in this essay. B I O L O G I C A L C L O C K S . In discussing the operation of biological rhythms, the term biological clock often is used. A biological clock is any sort of mechanism internal to an organism that governs its biological rhythms. One such mechanism, which we examine in the next section, is the pineal gland. Internal clocks operate independently of the environment but also are affected by changes in environmental conditions.

Examples of such alterations of conditions include a decrease (or increase) in the hours of available light due to a change of seasons or a change in time alteration due to rapid travel from west to east or north to south. In the latter instance, a condition known as jet lag—increasingly familiar to humans since the advent of regular air travel in the mid–twentieth century—may ensue.

Understanding Biological Rhythms

The Pineal Gland

Among the many varieties of biological rhythm, the most well known are those relating to sleep and wakefulness, which are part of the circadian rhythm that we discuss later in this essay. Circadian, or daily, cycles are only one type of biological rhythm. Some rhythms take place on a cycle shorter than the length of a day, while others are based on a monthly or even an annual pattern.

Governing human biological cycles—the “computer” that operates our biological clocks—is the pineal gland, a cone-shaped structure about the size of a pea located deep inside the brain. At one time, the great French philosopher and mathematician René Descartes (1596–1650) held that the pineal gland was actually the seat of the soul. Though it might seem absurd now that a respect-

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ed thinker would seriously attempt to locate the soul in space, as though it were a physical object, Descartes’s claim resulted from hours of painstaking dissection conducted on animals. In searching for the human soul, Descartes sought that ineffable quality described some fifteen centuries earlier by the Roman emperor and philosopher Marcus Aurelius (121–180), who wrote, “This being of mine, whatever it really is, consists of a little flesh, a little breath, and the part which governs.” As it turns out, the pineal gland is, in a sense, “the part which governs”: it may not be the home of the soul (which, in any case, is not a question for science), but it does govern human circadian rhythms and thus has a powerful effect on the manner in which we experience the world. M E LAT O N I N . The pineal gland secretes two hormones (molecules that send signals to the body), melatonin and serotonin. During the late 1990s, melatonin became a popular over-thecounter treatment for persons afflicted with sleep disorders, because it is believed that the hormone is associated with healthful sleep. Scientists do not fully understand the role that melatonin plays in the body, although it appears that it regulates a number of diurnal, or daily, events.

In addition, melatonin seems to serve the function of controlling fat production, which is one reason why good sleep is associated not only with a healthy lifestyle but also with a healthy physique. Many health specialists maintain that for adults there is a close link between a “spare tire” (that is, fat accumulation around the waist) and stress, lack of sleep, and low melatonin levels. Among the many roles melatonin plays in the body is its job of regulating glucose levels in the blood, which, in turn, serve to govern the production of growth hormone, or somatotropin. Growth hormone is associated with the development of lean body mass, as opposed to fat, which is why athletes involved in the Olympics and other major sporting competitions sometimes have illegally “doped” with it as a means of increasing strength. It is not surprising, then, to learn that children—who clearly need and use more growth hormone and who also need more hours of sleep than adults—also have higher melatonin levels.

the body clock. Complementary to melatonin is serotonin, which is as important to waking functions as melatonin is to sleepiness. Like melatonin, serotonin serves several functions, including the regulation of attention. Serotonin is among the substances responsible for the ability of a human with a healthily functioning brain to filter out background noise and sensory data. Thanks in part to serotonin, you are able to read this book without having your attention diverted by other sensory data around you: the voice of someone talking nearby, the sunlight or a bird singing outside, the hum of a light or a fan in the room. By contrast, a person under the influence of the drug LSD (lysergic acid diethylamide) is not able to make those automatic filtering adjustments facilitated by serotonin. Instead, he or she is at the mercy of seemingly random intrusions of outside stimuli, such as the color of paint on a wall or the sound of music playing in the background. The secret of LSD’s powerful hallucinatory effect can be attributed in part to the fact that it apparently mimics the chemistry of serotonin in the brain, “tricking” the brain into accepting the LSD itself as serotonin. With regard to body clocks and biological rhythms, serotonin plays an even more vital governing role than does melatonin, since melatonin, in fact, is created by the chemical conversion of serotonin. On regular daily cycles the body converts serotonin to melatonin, thus influencing the organism to undergo a period of sleep. Then, as the sleeping period approaches its end, the body converts melatonin back into serotonin.

REAL-LIFE A P P L I C AT I O N S Circadian Rhythms The term circadian derives from the Latin circa (“about”) and dies (“day”), and, indeed, it takes “about” a day for the body to undergo its entire cycle of serotonin-melatonin conversions. In fact, the cycle takes almost exactly 25 hours. Why 25 hours and not 24? This is a fascinating and perplexing question.

Melatonin is not the only important hormone that is both secreted by the pineal gland and critical to the regulation of

It would be reasonable to assume that natural selection favors those organisms whose body clocks correspond to the regular cycles of Earth’s rotation on its axis, which governs the length of a

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day—or, more specifically, a solar day. Yet the length of the human daily cycle has been confirmed in countless experiments, for instance, with subjects in an environment such as a cave, where levels of illumination are kept constant for weeks on end. In each such case, the subject’s body clock adopts a 25-hour cycle. POSSIBLE E X P L A N AT I O N S F O R T H E 2 5 - H O U R CYC L E . One

might suggest that the length of the cycle has something to do with the fact that Earth’s rate of rotation has changed, as indeed it has. But the speed of the planet’s rotation has slowed, because—like everything else in the universe—it is gradually losing energy. (This is a result of the second law of thermodynamics.) About 650 million years ago, long before humans or even dinosaurs appeared on the scene, Earth revolved on its axis about 400 times in the interval required to revolve around the Sun. This means that there were 400 days in a year. By the time Homo sapiens emerged as a species about two million years ago, days were considerably longer, though still shorter than they are now. This only means that the 25-hour human body clock would have been even less compatible with the length of a day in the distant past of our species. One possible explanation of the 25-hour body clock is the length of the lunar day, or the amount of time it takes for the Moon to reappear in a given spot over the sky of Earth. In contrast to the 24hour solar day, the lunar day lasts for 24 hours and 50 minutes—very close in length to the natural human cycle. Still, the exact relationships between the Moon’s cycles and those of the human body have not been established fully: the idea that lunar cycles have an effect on menstruation, for instance, appears to be more rumor than fact. P E A K S A N D T R O U G H S . On the other hand, circadian rhythms do mirror the patterns of the Moon’s gravitational pull on Earth, which results in a high and low tide each day. Likewise, the human circadian rhythm has its highs and lows, or peaks and troughs. In the circadian trough, which occurs about 4:00 A.M., body temperature is at its lowest, whereas at the peak, around 4:00 P.M., it reaches a high. A person may experience a lag in energy after lunchtime, but usually by about 4:00 in the afternoon, energy picks up—a result of the fact that the body has entered a peak time in its cycle.

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This fact, by the way, points up the great wisdom of a practice common in Spanish-speaking countries and some other parts of the world: siestas. The siesta devotes one of the least productive parts of the day, the post-lunch lag, to rest, so that a person is equipped with energy for the rest of the afternoon and early evening—at precisely the time when energy is at a high. To compensate for the time “lost” on napping, many such societies maintain a later schedule, with offices closing in the early evening rather than late afternoon and with evening meals served at about 9:00 P.M. Note that even though our body clocks run on a 25-hour day, they readily adjust to the 24hour world in which we live. As long as a person is exposed to regular cycles of day and night, the pineal gland automatically adapts to the length of a 24-hour solar day. If a person has been living in a sunless cave, with no exposure to daylight for a length of time, it would take about three weeks for the pineal gland to reset itself, but thereafter it would track with Earth time consistently. The adjustment of the body clock is not simply a matter of sending signals for sleep and wakefulness. In fact, the pineal gland is at the center of a complex information network that controls sleep cycles, body temperature, and stress-fighting hormones. Hence the link that we noted earlier between body temperature and circadian rhythms: just as the body reaches its lowest temperature in the circadian trough, it also enters a period of extremely deep sleep. R E G U L AT I N G THE BODY C LO C K . Tied in with these sleep patterns are

many other bodily functions. For example, bodybuilders and others who work out with weights experience their greatest benefits not when lifting (which, in fact, tears muscles down rather than building them up) but when resting—and particularly when sleeping—after having worked out earlier in the day. Likewise, deep sleep is associated with growth, as we have noted. Furthermore, it appears that dreaming may be essential to the well-being of the psyche, providing an opportunity for the brain to “clean out” the signals and data it has been receiving for the preceding 16 hours of wakefulness. Given these and other important functions associated with deep sleep, it follows that the maintenance of the body clock is of great importance to the health of the human organism. For-

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VIEW

OF THE MIDNIGHT SUN IN

LYNGENFJORD, NORWAY. THE “BODY

CLOCK” CAN BE DISRUPTED BY CHANGES IN

THE AMOUNT OF AVAILABLE LIGHT, SUCH AS OCCUR IN REGIONS OF THE EXTREME NORTH THAT UNDERGO PERIODS OF ALMOST CONSTANT DAYLIGHT FROM MID -MAY TO LATE

tunately, animals’ brains are programmed to make adjustments of the body clock so as to accommodate the daily cycles of light and dark. We have discussed the means by which the human brain achieves this accommodation, but it is not the only animal brain thus equipped. “Bird brains” (quite literally) are similarly able to make an adjustment: whereas humans have a natural 25-hour clock, birds run on a 23-hour circadian cycle, but their pineal glands likewise assist them in adapting to the 24-hour solar day. The brains of birds, humans, and other animals respond to environmental features known collectively as zeitgebers (German for “time givers”), which aid in the adjustment to the solar schedule. The most obvious example is the change from day to night, but there are other zeitgebers of which we are less aware in our ordinary experience. For example, Earth’s magnetic field goes through its own 24-hour cycle, which subtly influences our biological rhythms.

Interfering with the Body Clock

JULY. (© Bettmann/Corbis. Reproduced by permission.)

person may drink coffee to stay awake at night, but he or she also may experience a sleep disorder as a result of some other situation, which may or may not be the result of purposeful action. Examples of sleep disorders that are the by-product of other activities include jet lag as well as the malfunctioning of the body clock that often stems from recreational drug use. The causes for interference with a person’s body clock may be outside that person’s control to one degree or another. Working at night, for instance, is a condition that almost never suits a human being, no matter how much a person may insist that he or she is a “night person.” Nevertheless, a person may be required by circumstances, such as schedule, economic necessity, or job availability, to take a night job. Another example of interference with the body clock would be narcolepsy (a condition characterized by brief attacks of deep sleep) or some other condition that is either congenital (something with which a person is born) or symptomatic (a symptom of some other condition rather than a condition in and of itself). W H I T E N I G H T S . At least one example

In modern life humans often interfere with their own body clocks, either deliberately and directly or indirectly and by accident. On the one hand, a

of human experience involving interference with the body clock relates to conditions completely

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outside people’s control. This is the situation of the “white nights” or “midnight sun,” whereby regions in the extreme north—Russia, Alaska, and Scandinavia—undergo periods of almost constant daylight from mid-May to late July. (These are matched by a much less pleasant phenomenon: near constant darkness from midNovember to late January.) During those times people often line their windows with dark material to make it easier to go to sleep in a world where the Sun is nearly as bright at 3:00 A.M. as it is at 3:00 P.M. The situation is even more pronounced in Antarctica, where researchers and adventurers may find themselves much closer to the South Pole than people in Saint Petersburg, Anchorage, or Oslo are to the North Pole. In Antarctica the human population is much higher in the summer, a period that coincides with the depth of winter in the Northern Hemisphere, and scientists or mountaineers trekking through remote regions may be forced to sleep in tents that keep out the cold but let in the light. Usually, however, the rugged conditions of life near the South Pole involve such exertions that by nighttime people are ready to sleep, light or no light. SOME

SLEEP

DISORDERS.

Few people ever get to experience the white nights, but almost everyone has suffered through a temporary bout of insomnia—a condition known specifically as transient insomnia. An unfortunate few suffer from chronic insomnia or some other sleep disorder. Insomnia, the inability to go to sleep or to stay asleep, is one of the two most common sleep disorders, the other being hypersomnia, or excessive daytime sleepiness. Transient forms of insomnia are usually treatable with short-term prescription drugs, but more serious conditions qualify as actual disorders and may require long-term treatment. These disorders may have as their cause drug use (either prescription or illegal) as well as medical or psychological problems. Among the most common of these more specialized disorders is apnea, the regular cessation of breathing whose most noticeable symptom is snoring.

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nia, which is a result of lost sleep due to the fact that the sufferer actually is waking up numerous times throughout the night. At the other extreme from apnea, in terms of prevalence among the population, is KleineLevin syndrome, which typically affects males in their late teens or twenties. The syndrome may bring about dramatic symptoms that range from excessive sleepiness, overeating, and irritability to abnormal behavior, hallucinations, and even loss of sexual inhibitions. Added to this strange mix is the fact that Kleine-Levin syndrome typically disappears after the person reaches the age of 40. J E T LAG . There are numerous classes of

sleep disorders, among them circadian rhythm disorders—those related to jet lag or work schedules. As we have seen, the pineal gland can adjust easily from a natural 25-hour cycle to a 24-hour one, but it can do so only gradually, and it cannot readily adapt to sudden changes of schedule, such as those brought about by air travel. Jet lag is a physiological and psychological condition in humans that typically includes fatigue and irritability; it usually follows a long flight through several time zones and probably results from disruption of circadian rhythms. The name is fitting, since jet lag is associated almost exclusively with jets: traveling great distances by ship, even at the speeds of modern craft, allows the body at least some time to adjust. Older modes of travel were too slow to involve jet lag; for this reason, the phenomenon is a relatively recent one. The only people who manage to experience jet lag without riding in a jet are those traveling in even faster craft—that is, astronauts. An astronaut orbiting Earth in a space shuttle experiences rapid shifts from day to night; if manned vessels ever go out into deep space, scientists will face a new problem: assisting the adjustment of circadian cycles to that sunless realm.

Apnea, which affects a large portion of the United States population, is a potentially very serious condition that can bring about suffocation or even death. More often its effects are less dramatic, however, and manifest in hypersom-

On a much more ordinary level, there is the jet lag of people who travel from the United States East Coast to Europe or between the East Coast and West Coast of the United States. The worst kinds of jet lag occur when a person flies from west to east across six or more time zones: anyone who flies to Europe from the East Coast is likely to spend much of the first day after arrival sleeping rather than sightseeing. Thereafter, it may take up to ten days (usually as long

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as or longer than most European vacations) for the body to adjust fully. By contrast, someone who has flown from the East Coast to the West Coast feels unexpected energy. The reason is that when it is 6:00 A.M. in the Pacific time zone, it is 9:00 A.M. in the eastern time zone, to which a person’s body clock (in this particular scenario) is still adapted. Therefore, at 6:00 in the morning, the newly arrived traveler will feel as good as he or she would normally feel at 9:00 A.M. back east. Conversely, at 9:00 P.M. in the west, it is midnight in the east. This means that the traveler is likely to feel tired long before his or her ordinary bedtime. There are steps one can take to avoid, or at least minimize the effects of, jet lag. One is to ensure a regular sleep schedule prior to traveling, so as to minimize the effects of sleep deprivation, if the latter does occur. It is even better if one can, in the days prior to leaving, adopt a schedule adjusted to the new time zone. For example, if one were traveling from the East Coast to California, one would start going to bed three hours earlier, and rising three hours earlier as well. Changing eating habits in the days prior to departure may also help. Some experts on the subject recommend a four-day period in which one alternates heavy eating (days one and three) and very light eating (days two and four.) It is believed that high-protein breakfasts stimulate the active, waking cycle, while high-carbohydrate evening meals stimulate the resting cycle; conversely, depriving the liver of carbohydrates may prepare the body clock to reset itself.

of working at night so unattractive, even though it is clear that in our modern society some nightshift positions are essential.

Biological Rhythms

People who have offices in their homes may find it beneficial to work at late hours, when the phone is not ringing and the world is quiet, but the “extra time” gained by working at night ultimately is counterbalanced by the body’s reaction to changes in its biological rhythms. Such is also the case with night-shift workers, who never really adjust to their schedules even after years on the job. There is such a thing as a “night person,” or someone with a chronic condition known as delayed sleep phase syndrome. A person with this syndrome is apt to feel most alert in the late evening and night, with a corresponding lag of energy in the late mornings and afternoons. Even so, given the role of sunlight in governing the body clock, the condition does not really lend itself to regular night work but rather merely causes a person to experience problems adapting to the schedule maintained by most of society. One possible means of dealing with this problem is to go to bed three hours later than would be normal for an ordinary 9-to-5 schedule, and wake up three hours later as well; unfortunately, that is not practical for most people. Another treatment applied with success is exposure of a person to artificial, high-intensity, full-spectrum light, which augments the effect of sunlight, between the hours of 7:00 and 9:00 A.M.

O N T H E N I G H T S H I F T. At least the body does adjust to jet lag; on the other hand, it may never become accustomed to working a night shift. If you stay up all night studying for a test, you will find that around 4:00 A.M. you hit a “lull” when you feel sleepy—and because of the lowered temperature at the circadian trough, you also feel cold. You might assume that this situation would improve if you worked regularly at night, but the evidence suggests that it does not.

C O LO N I Z I N G T H E N I G H T ? In this vein it is interesting to note that some of the optimistic predictions made in 1987 by Murray Melbin in his fascinating book Night as Frontier: Colonizing the World After Dark have not come to pass. Melbin, who explains circadian rhythms and the body clock in a highly readable and understandable fashion, makes a brilliant analysis of the means by which industrialized societies have extended their daily schedules into the nighttime hours. Thus, to use his analogy, such societies have “colonized” the night.

As long as a person lives in a sunlit world of 24-hour solar days, the body clock remains adapted to that schedule, and this will be true whether the person is at home and in bed or at work behind a desk or counter during the hours of night. In other words, the person always will hit the circadian trough about 4:00 A.M. This is one of the reasons why most people find the idea

Until the invention in 1879 of the first successful incandescent lamp by the American inventor Thomas Edison (1847–1931), activity at night was limited. Torches, crude lamps, and candles in ancient times; metal lamps in the Middle Ages; and the various oil-burning lamps that applied the glass lantern chimney devised in 1490 by the Italian scientist and artist Leonardo

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Before the events of September 11, 2001, when terrorists crashed hijacked planes into the World Trade Center in New York City and the Pentagon in Washington, D.C., the burden on America’s airports had become almost unbearable. The concourses of Hartsfield International in Atlanta, Georgia, the world’s busiest airport, were a nonstop melee of people, luggage, and noise, as travelers fought to change flights or pick up their bags. One obvious solution to the problem would have been to adopt a round-the-clock airport schedule, with flights regularly leaving at 3:00 or 4:00 in the morning.

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UNTIL THOMAS EDISON

INVENTED THE FIRST SUCCESS-

FUL INCANDESCENT LAMP IN 1879, ACTIVITY AT NIGHT WAS LIMITED.

WITH

ELECTRIC LIGHTING, INDUSTRIALIZED

SOCIETIES HAVE BEEN ABLE TO

“COLONIZE”

THE NIGHT,

EXTENDING DAYTIME ACTIVITIES INTO THE NIGHTTIME HOURS. (© Bettmann/Corbis. Reproduced by permission.)

da Vinci (1452–1519) all made it possible for a person to read at night and to perform other limited functions. After their introduction in the nineteenth century, street lamps in London, the first of their kind, made the streets safe for walking at late hours, but travel, large gatherings, and outdoor work after dark remained difficult before the advent of electric light. Since 1879 the Western world has indeed “colonized” the night with all-night eateries, roads that are never free of traffic, and roundthe-clock entertainment on radio, TV, and now the Internet. There are even hardware stores open all night in some major cities. Certainly today there are more gas stations, restaurants, television programs, and customer-service telephone lines that operate 24 hours than there were in 1987, when Melbin wrote his book, but it is unlikely that Americans will ever fully “colonize” the night in the thoroughgoing fashion that their ancestors colonized the New World. An example of the limits to night colonization is in air travel.

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No airport rushed to enact such a measure, however, and after September 11 heightened security concerns made it unlikely that any facility would adopt a 24-hour schedule, with the additional security threats it entailed. For a time at least, the volume of air traffic decreased dramatically, but even as it climbed back up in the months after the terrorist attacks, airports continued to operate on their ordinary schedules. The reason appears to be the difficulty of persuading people to adjust to a late-night schedule—that is, finding enough people willing to fly in the middle of the night and enough baggage handlers and ticket agents willing to service them. There are, it seems, limits to the extent to which nighttime can be colonized.

Other Examples of Biological Rhythms Although the circadian rhythms of sleep and wakefulness are particularly important examples of biological cycles, they are far from the only ones. Not all rhythms, in fact, are circadian. Some are ultradian, meaning that they occur more than once a day. Examples include the cycles of taking in fluid and forming urine as well as cell-division cycles and cycles related to hormones and the endocrine glands that release them. For instance, the pituitary gland in the brain of a normal male mammal secretes hormones about every one to two hours during the day. The overall cycle of sleeping and waking is circadian, but there is an ultradian cycle within sleep as the brain moves from drowsiness to REM (rapid eye movement, or dream, sleep) to dozing, then to light and deep sleep, and finally to slowwave sleep. Over the course of the night, this cycle, which lasts about 90 minutes, repeats itself several times. Among the functions affected by

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A ROCKY MOUNTAIN

GOAT SHEDS ITS THICK WINTER FUR.

THE

SHEDDING OF FUR, SKIN, OR ANTLERS IS ONE EXAM-

PLE OF A CIRCANNUAL CYCLE, WHICH TAKES A YEAR TO COMPLETE. (© W. Wayne Lockwood, M.D./Corbis. Reproduced by permis-

sion.)

this cycle are heart rate and breathing, which slow down in deep sleep. Additionally, heartbeat and respiration are themselves ultradian cycles of very short duration. M E N S T R UAT I O N A N D O T H E R I N F RA D I A N CYC L E S . In contrast to the

ultra-quick ultradian cycles of the beating heart and the lungs’ intake and outflow of oxygen, there are much longer infradian, or monthly, cycles. By far the most common is menstruation, which begins when a female mammal reaches a state of physical maturity and continues on a monthly basis until she is no longer able to conceive offspring. When she becomes pregnant, the menstrual cycle shuts down and, in some cases, does not resume until several months after delivery of the offspring. Assuming she is in good health, the human female will experience fairly regular menstrual periods at intervals of 28 days. Among human females, it has long been known that the menstrual cycles of women who live or work in close proximity to one another tend to come into alignment. For example, college girls on the same floor in a dormitory are likely to share menstrual cycles.

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The reasons for this alignment of menstrual cycles are not completely understood. Nor is the cause of the 28-day cycle evident. If it were the result of the Moon’s cycles, all women on Earth would have menstrual cycles that last 29.5 days, which is how long it takes the Moon to travel around Earth. Furthermore, if there were a clear connection between the Moon and menstruation, the periods of all menstruating females on Earth would be aligned with the Moon’s phases. Neither of these, of course, is the case. C I R CA N N UA L CYC L E S . Longer still than infradian cycles, circannual cycles, as their name suggests, take a year to complete. Among them is the cycle of dormancy and activity marked by the hibernation of certain species in the winter. There are also certain times of the year when animals shed things—fur, skin, antlers, or simply pounds. Likewise, at some points in the year animals gain weight.

People are affected strongly by the seasonal changes associated with the circannual cycle. There is almost no person who lives in a temperate zone (that is, one with four seasons) who is not capable of calling strong emotions to mind when imagining the sensations associated with winter, spring, summer, or fall. Some sensations,

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KEY TERMS A mechanism within an organism (for example, the pineal gland in the human brain) that governs biological rhythms.

JET LAG:

Processes that occur periodically in an organism in conjunction with and often in response to periodic changes in environmental conditions.

time zones and probably results from dis-

A subdiscipline of biology devoted to the study of biological rhythms.

female’s reproductive abilities.

BIOLOGICAL CLOCK:

BIOLOGICAL RHYTHMS:

CHRONOBIOLOGY:

A biological cycle that takes place over the course of approximately a day. In humans circadian rhythms run on a cycle of approximately 25 hours and govern states of sleep and wakefulness as well as core body temperature and other biological functions. CIRCADIAN RHYTHM:

Molecules produced by living cells, which send signals to spots remote from their point of origin and which induce specific effects on the activities of other cells. HORMONE:

A biological cycle that takes place over the course of a month. INFRADIAN RHYTHM:

however, are better than others, and though there can be negative associations with spring or summer, by far the season most likely to induce ill effects in humans is winter.

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A physiological and psycho-

logical condition in humans that typically includes fatigue and irritability; it usually follows from a long flight through several ruption of circadian rhythms. MENOPAUSE:

The point at which

menstrual cycles cease, a time that typically corresponds to the cessation of the

MENSTRUATION:

Sloughing off of

the lining of the uterus, which occurs monthly in nonpregnant females who have not reached menopause (the point at which menstrual cycles cease) and which manifests as a discharge of blood. PINEAL

GLAND:

A small, usually

cone-shaped portion of the brain, often located between the two lobes, that plays a principal role in governing the release of certain hormones, including those associated with human circadian rhythms. ULTRADIAN RHYTHM:

A biological

cycle that takes place over the course of less than a day. Compare with circadian rhythm.

ly, to the altered circadian rhythm) in wintertime.

The thirteen weeks between the winter solstice in late December and the vernal equinox in late March have such a powerful impact on the human psyche that scientists have identified a mental condition associated with it. It is SAD, or seasonal affective disorder, which seems to be related to the shortened days (and thus, ultimate-

As we have noted, the body responds to the onset of night and sleep by the release of melatonin, but when darkness lasts longer than normal, melatonin secretions become much more pronounced than they would be under ordinary conditions. The result of this hormone imbalance can be depression, which may be compounded by other conditions associated with winter. Among these conditions is “cabin fever,” or restlessness brought about by lengthy confine-

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ment indoors. An effective treatment for SAD is exposure to intense bright light.

Studying Biological Rhythms Treatment of SAD is just one example of the issues confronted by scientists working in the realm of chronobiology, a subdiscipline devoted to the study of biological rhythms. Naturally, a particularly significant area of chronobiological study is devoted to sleep research. The latter is a relatively new field of medicine stimulated by the discovery of REM sleep in 1953. In addition to studying such disorders as sleep apnea, sleep researchers are concerned with such issues as the effects of sleep deprivation and the impact on circadian rhythms brought about by isolation from sunlight. Note that the scientific study of biological rhythms has nothing to do with “biorhythms,” a fad that peaked in the 1970s but still has its adherents today. Biorhythms are akin to astrology in their emphasis on the moment of a person’s birth, and though biorhythms have a bit more scientific basis than astrology, that in itself is not saying much. As we have seen, biological rhythms do govern much of human life, but the study of these rhythms does not offer special insight into the fate or future of a person—one of the principal claims made by adherents of biorhythms. As with all pseudosciences, belief in biorhythms is maintained by emphasizing those examples that seem to correlate with the theory and ignoring or explaining away the many facts that contradict it. An example of scientific research in chronobiology and related fields is the work of the psychologist Stephany Biello at Glasgow University

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in Scotland, who in June 2000 announced findings linking the drug, ecstasy, to long-term damage to the body clock. As with LSD and many another drug, ecstasy plays havoc with serotonin and may exert such a negative impact on the pathways of serotonin release in the pineal gland that it permanently alters the brain’s ability to manufacture that vital hormone. Thus the drug, which induces a sense of euphoria in users, can induce serious sleep and mood disorders as well as severe depression.

Biological Rhythms

WHERE TO LEARN MORE Biological Rhythms (Web site). . Center for Biological Timing (Web site). . Circadian Rhythms (Web site). . “Ecstasy ‘Ruins Body Clock.’” British Broadcasting Corporation (Web site). . Hughes, Martin. Bodyclock: The Effects of Time on Human Health. New York: Facts on File, 1989. Melbin, Murray. Night as Frontier: Colonizing the World After Dark. New York: Free Press, 1987. Orlock, Carol. Inner Time: The Science of Body Clocks and What Makes Us Tick. Secaucus, NJ: Carol Publishing Group, 1993. Rose, Kenneth Jon. The Body in Time. New York: John Wiley and Sons, 1988. Sleep Disorders Information (Web site). . Waterhouse, J. M., D. S. Waters, and M. E. Waterhouse. Your Body Clock. New York: Oxford University Press, 1990. Winfree, Arthur T. The Timing of Biological Clocks. New York: Scientific American Library, 1987.

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LEARNING AND B E H AV I O R BEHAVIOR INSTINCT AND LEARNING M I G RAT I O N A N D N AV I G AT I O N

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Behavior

CONCEPT In biology the term behavior refers to the means by which living things respond to their environments. At first glance, this might seem to encompass only animal behavior, but, in fact, plants display observable behavior patterns as well. One of the principal manifestations of plant behavior is tropism, a response to a stimulus that acts in a particular direction, thus encouraging growth either toward or away from that stimulus. Behavior in plants is primarily a matter of response to stimuli, which may be any one of a variety of influences that derive either from inside or outside the organism. Response to stimuli is automatic, and even humans are capable of making these types of programmed responses. In most cases, behaviors in organisms are designed to ensure their survival. Such is the case, for instance, with the complex of behaviors known as territoriality, whereby animals defend what they perceive to be their own.

HOW IT WORKS Stimulus and Response A stimulus is any phenomenon that directly influences the activity or growth of a living organism. Phenomenon, meaning any observable fact or event, is a broad term and appropriately so, since stimuli can be of so many varieties. Chemicals, heat, light, pressure, and gravity all can serve as stimuli, as indeed can any environmental change. Nor are environmental changes limited to the organism’s external environment. In some cases its internal environment can act as a stimulus, as when an animal reaches the age of

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B E H AV I O R

courtship and mating and responds automatically to changes in its body. All creatures, even humans, are capable of automatic responses to stimuli. When a person inhales dust, pepper, or something to which he or she is allergic, a sneeze follows. The person may suppress the sneeze (which is not a good practice, since it puts a strain on blood vessels in the head), but this does not stop the body from responding automatically to the irritating stimulus by initiating a sneeze. Similarly, plants respond automatically to light and other stimuli in a range of behaviors known collectively as tropisms, which we explore later in this essay. INNATE AND LEARNED BEHAVIOR. Not all responses to stimuli are automat-

ic, however. Certainly not all behavior on the part of higher animals is automatic, though, as we have noted, even humans are capable of some automatic responses. In general, behavior can be categorized as either innate (inborn) or learned, but the distinction is frequently unclear. In many cases it is safe to say that behavior present at birth is innate, but this does not mean that behavior that manifests later in life is learned. (Later in this essay we look at an example of this behavior as it relates to chickens and pecking.) Behavior is considered innate when it is present and complete without any experience whereby it was learned. At the age of about four weeks, human babies, even blind ones, smile spontaneously at a pleasing stimulus. Like all innate behavior, babies’ smiling is stereotyped, or always the same, and therefore quite predictable. Plants, protista (single-cell organisms), and animals that lack a well-developed nervous system rely on innate behavior. Higher animals, on the

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other hand, use both innate and learned behavior. A fish is born knowing how to swim, whereas a human or a giraffe must learn how to walk.

Ethology Ethology is the study of animal behavior, including its mechanisms and evolution. The science dates back to the British naturalist Charles Darwin (1809–1882), who applied it in his research concerning evolution by means of natural selection (see Evolution). Darwin presented many examples to illustrate the fact that, in addition to other characteristics of an organism, such as its morphologic features or shape, behavior is an adaptation to environmental demands and can increase the chances of species survival. The true foundations of ethology, however, lie in the work of two men during the period between 1930 and 1950: the Austrian zoologist Konrad Lorenz (1903–1989) and the Dutch ethologist Nikolaas Tinbergen (1907–1988). Together with the Austrian zoologist Karl von Frisch (1886–1982), most noted for his study of bee communication and sensory perception, the two men shared the 1973 Nobel Prize in physiology or medicine. Lorenz and Tinbergen, who together are credited as founders of scientific ethology, contributed individually to the discipline and, during the mid–twentieth century, worked together on a theory that animals develop formalized, rigid sequences of action in response to specific stimuli. According to Lorenz and Tinbergen, animals show fixed-action patterns (FAPs) of behavior which are strong responses to particular stimuli. Later in this essay, we look at examples of FAPs in action. In addition, Lorenz put forward the highly influential theory of imprinting, discussed briefly in this essay and in more detail elsewhere (see Instinct and Learning).

Behaviorism and Conditioning The development of ethology by Lorenz and Tinbergen occurred against the backdrop of the rise of the behaviorist school in the realms of philosophy, psychology, and the biological sciences. This school of thought had its roots in the late nineteenth century, with the writings of a number of philosophers and psychologists as well as practical scientists, such as the Russian physiologist Ivan Pavlov (1849–1936). Pavlov showed that an animal can be trained to respond to a partic-

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ular stimulus even when that stimulus is removed, so long as the stimulus has been associated with a secondary one. Pavlov began his now famous set of experiments by placing powdered meat in a dog’s mouth and observing that saliva flowed into the mouth as a reflex reaction to the introduction of the meat. He then began ringing a bell before he gave the dog its food. After doing this several times, he discovered that the dog salivated merely at the sound of the bell. Many experiments of this type demonstrated that an innate behavior can be modified, and thus was born the scientific concept of conditioning, or learning by association with particular stimuli. The variety of conditioning applied by Pavlov, known as classical conditioning, calls for pairing a stimulus that elicits a specific response with one that does not, until the second stimulus elicits a response like the first. Classical conditioning is contrasted with operant conditioning, which involves administering or withholding reinforcements (that is, rewards) based on the performance of a targeted response. O P E RA N T

CONDITIONING.

During operant conditioning, a random behavior is rewarded and subsequently retained by an animal. According to operant conditioning theory, if we want to train a dog to sit on command, all we have to do is wait until the dog sits and then say, “Sit,” and give the dog a biscuit. After a few repetitions, the dog will sit on command because the reward apparently reinforces the behavior and fosters its repetition. Human parents apply operant conditioning when they admonish their offspring with such phrases as “You can’t watch TV until you’ve cleaned your room.” Likewise, young chimpanzees learn through a form of operant conditioning. By observing their parents, young chimps learn how to strip a twig and then use it to pick up termites (a tasty treat to a chimpanzee) from rotten logs. Their behavior thus is rewarded, an example of the way that operant conditioning enables animals to add new, noninherited forms of behavior to their range of skills. Though the theory of operant conditioning goes back to the work of the American psychologist Edward L. Thorndike (1874–1949), by far its most famous proponent was another American psychologist, B. F. Skinner (1904–1990). In applying operant conditioning to human beings,

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Skinner and his followers took the theory to extremes, maintaining that humans have no ideas of their own, only conditioned responses to stimuli. Love, courage, faith, and all the other emotions and attitudes that people hold in high esteem are, according to this school of thought, simply a matter of learned responses, rather like a parrot making human-like sounds to earn treats. This extreme form of behaviorism is no longer held in high regard within the scientific or medical communities.

Behavior

REAL-LIFE A P P L I C AT I O N S Behavior in Plants As noted earlier, the term behavior would seem at first glance to apply only to animals and not to plants. Certainly the majority of attention in behavioral studies, outside the realm of humans, is devoted to ethology, but plants are not without their observable behavioral characteristics. These features primarily manifest in the form of tropism, a response to a stimulus that acts in a particular direction, thus encouraging growth either toward or away from that stimulus. Tropism primarily affects members of the plant kingdom, though it has been observed in algae and fungi as well. Though the word tropism itself may be unfamiliar to most people, the phenomenon itself is not. There are plenty of opportunities in daily life to observe the response of plants to energy, substances, or forms of stimulation. For example, perhaps you have noticed the way that trees or flowers grow toward sunlight, even bending in their growth if it is necessary to reach the energy source. Similarly, plants in a parched region are likely to develop roots directed laterally toward a water source. Among the various forms of tropism are phototropism (response to light), geotropism (response to gravity), chemotropism (response to particular chemical substances), hydrotropism (response to water), thigmotropism (response to mechanical stimulation), traumatropism (response to wounds), and galvanotropism or electrotropism (response to electric current). Most of these types involve growth toward a stimulus, a phenomenon known as positive growth, or orthotropism. Plants tend to grow

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ONE

EXAMPLE OF INNATE ANIMAL BEHAVIOR IS THE

REFLEX, A SIMPLE, INBORN, AUTOMATIC RESPONSE TO A STIMULUS BY A PART OF AN ORGANISM’S BODY.

SUCH

A MECHANISM IS AT WORK, FOR INSTANCE, WHEN JELLYFISH

WITHDRAW

THEIR

TENTACLES.

(© Henry Horen-

stein/Corbis. Reproduced by permission.)

toward light or water, for instance. On the other hand, some kinds of stimuli tend to evoke diatropism, or growth away from the stimulus. Such is bound to be the case, for instance, with traumatropism and electrotropism. Tropism, along with movement due to changes in water content, is one of the two principal forms of innate behavior on the part of plants. In general, stems and leaves experience positive phototropism, as they grow in the direction of a light source, the Sun. At the same time, roots exhibit positive gravitropism, or growth toward the gravitational force of Earth, as well as positive hydrotropism, since they grow toward water sources below ground. On the other hand, a plant may move in a specific way regardless of the direction of the stimulus. Such movements are temporary, reversible, and result from changes in the water pressure inside the plant.

Animal Behavior An excellent example of an innate animal behavior, and one in which humans also take part, is

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it is a form of innate behavior that has undergone improvement as the organism matures.

Behavior

For example, chickens become more adept at pecking as they get older, but this does not mean that pecking is a learned behavior; on the contrary, it is innate. The improvement in pecking aim is not the result of learning and correction of errors but rather is due to a natural maturing of muscles and eyes and the coordination between them.

IN

CONTRAST TO SIMPLE FIXED -ACTION PATTERNS OF

BEHAVIOR,

OR

FAPS,

ARE

COMPLEX

PROGRAMMED

BEHAVIOR PATTERNS, WHICH COMPRISE SEVERAL STEPS AND ARE MUCH MORE COMPLICATED.

ONE

TYPE OF

COMPLEX BEHAVIOR IS THE BUILDING OF DAMS BY BEAVERS. (© Harry Engels/Nas. Photo Researchers. Reproduced by

permission.)

the reflex. A reflex is a simple, inborn, automatic response to a stimulus by a part of an organism’s body. The simplest model of reflex action involves a receptor and sensory neuron and an effector organ. Such a mechanism is at work, for instance, when certain varieties of coelenterate (a phylum that includes jellyfish) withdraw their tentacles. More complex reflexes require processing interneurons between the sensory and motor neurons as well as specialized receptors. These neurons send signals across the body, or to various parts of the body, as, for example, when food in the mouth stimulates the salivary glands to produce saliva or when a hand is pulled away rapidly from a hot object. Reflexes help animals respond quickly to a stimulus, thus protecting them from harm. By contrast, learned behavior results from experience and enables animals to adjust to new situations. If an animal exhibits a behavior at birth, it is a near certainty that it is innate and not learned. Sometimes later in life, however, a behavior may appear to be learned when, in fact,

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FA P S . In studying fixed-action patterns of behavior, or FAPs, Lorenz and Tinbergen observed numerous interesting phenomena. Male stickleback fish, for example, recognize potential competition—other breeding stickleback males—by the red stripe on their underside and thus engage in the FAP of attacking anything red on sight. Tinbergen discovered that jealous stickleback males were so attuned to the red stripe that they tried to attack passing British mail trucks, which were red, when they could see them through the glass of their tanks. Tinbergen termed the red stripe a behavioral releaser, or a simple stimulus that brings about a FAP.

Once a FAP is initiated, it continues to completion even if circumstances change. If an egg rolls out of a goose’s nest, the goose stretches her neck until the underside of her bill touches the egg. Then she rolls the egg back to the nest. If someone takes the egg away while she is reaching for it, the goose goes through the motions anyway, even without an egg. Not all animal behavior is quite so predictable, however. In contrast to FAPs are complex programmed behavior patterns, which comprise several steps and are much more complicated. Birds making nests or beavers building dams are examples of complex programmed behavior. I M P R I N T I N G . As we noted earlier, Lorenz initiated the study of a learning pattern that came to be known as imprinting. Witnessed frequently in birds, imprinting is the learning of a behavior at a critical period early in life, such that the behavior becomes permanent. The very young bird or other organism is like wet concrete, into which any pattern can be etched; once the concrete has dried, the pattern is set.

Newly hatched geese are able to walk. This is something they learn the moment they are hatched, and they do so by following their parents. But how, Lorenz wondered, do young geese distinguish their parents from all other objects in their environments? He discovered that if he

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ROCKY MOUNTAIN BIGHORN RAMS USING THEIR HORNS, THESE RAMS

MEET HEAD TO HEAD AT THE BOUNDARIES OF THEIR RESPECTIVE TERRITORIES. WILL STRONGLY DEFEND THEIR TERRITORIES AGAINST INVADERS. (© W. Perry Con-

way/Corbis. Reproduced by permission.)

removed the parents from view the first day after the goslings hatched and if he walked in front of the young geese at that point, they would follow him. This tactic did not work if he waited until the third day after hatching, however. Lorenz concluded that during a critical period following birth, the goslings follow their parents’ movement and learn enough about their parents to recognize them. But since he also had determined that young geese follow any moving object, he reasoned that they first identify their parents by their movement, which acts as a releaser for parental imprinting. (Imprinting is discussed further in Instinct and Learning.)

Interactive Behavior

which informs the other bees that food is near (about 250 ft., or 75 m, from the hive), and the other is a wagging dance, which conveys the fact that food is farther away. There are numerous other forms of communication using one or more sense organs. Birds hear each other sing, a dog sees and hears the spit and hiss of a cornered cat, and ants lay down scent signals, or pheromones, to mark a trail that leads to food. This is only one level of interactive behavior, however. Quite a different variety of interaction is courtship, discussed in Reproduction. Other forms of interactive behavior include the establishment of an animal’s territory, a subject we discuss at the conclusion of this essay.

Much of an animal’s behavior (this is true of the human animal as well) takes place in interaction with others. This interaction may include rudimentary forms of communication, such as bee dances, studied by Lorenz and Tinbergen’s colleague Frisch. As he showed in perhaps the most important research of his career, bees communicate information about food supplies, including their direction and the distance to them, by means of two different varieties of “dance,” or rhythmic movement. One is a circling dance,

L I F E I N C O M M U N I T I E S . Interactive behavior comes into play when animals live in close proximity to one another. Certainly there are benefits to group life for those species that practice it: the group helps protect individuals from predators and, through cooperation and division of labor, ensures that all are fed and sheltered. In order to be workable, however, a society must have a hierarchy. Thus, in a situation quite removed from the human ideals of freedom and democracy, insect and animal societies are ones in which every creature knows its place and sticks to it.

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Bees, ants, and termites live in complex communities in which some individuals are responsible for finding food, others defend the colony, and still others watch over the offspring. In such a highly organized society, a dominance hierarchy or ranking system helps preserve peace and discipline. Chickens, for example, have a pecking order from the most dominant to the most submissive. Each individual knows its place in the order and does not challenge individuals of higher rank. This, again, is quite unlike humans, who at least occasionally step out of line and challenge bullies; by contrast, that never happens with chickens (fittingly enough).

Territoriality Almost everyone has seen a dog “mark its territory” by urinating on a patch of ground or has watched a cat arch its back in fury at an intruder to what it perceives as its territory. In so doing, these household pets are participating in a form of behavior that cuts across the entire animal kingdom: territoriality, or the behavior by which an animal lays claim to and defends an area against others of its species and occasionally against members of other species as well. The physical size of the territory defended is extremely varied. It might be only slightly larger than the animal itself or it might be the size of a small United States county. The population of the territory might consist of the animal itself, the animal and its mate, an entire family, or an entire herd or swarm. Time is another variable: some animals maintain a particular territory year-round, using it as an ongoing source of food and shelter. Others establish a territory only at certain times of the year, when they need to do so for the purposes of attracting a mate, breeding, or raising a family. Territorial behavior offers several advantages to the territorial animal. An animal that has a “home ground” can react quickly to dangerous situations without having to seek hiding places or defensible ground. By placing potential competitors at spaced intervals, territoriality also prevents the depletion of an area’s natural resources and may even slow down the spread of disease. Furthermore, territorial behavior exposes weaker animals (which are unable to defend their territory) to attacks by predators and thus assists the process of natural selection in building a stronger, healthier population.

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E XA M P L E S O F T E R R I T O R I E S .

A territory established only for a single night, for the sole purpose of providing the animal or animals with a place to rest, is known as a roost. Even within the roost, there may be a battle for territory, since not all spots are created equal. Because roosting spots near the interior are the safest, they are the most highly prized. Another type of specialized territory is the lek, used by various bird and mammal species during the breeding season. Leks are the “singles bars” of the animal world: here animals engage in behavior known as lekking, in which they display their breeding ability in the hope of attracting a mate. Not surprisingly, leks are among the most strongly defended of all territories, since holding a good lek increases the chances of attracting a mate. Like the singles-only communities that they mimic, leks are no place for families: generally of little use for feeding or bringing up young, the lek usually is abandoned by the animal once it attracts a mate or mates. T H R E AT E N I N G D I S P LAY S . An animal has to be prepared to defend its territory by fighting off invaders, but naturally it is preferable to avoid actual fighting if a mere display of strength will suffice. Fighting, after all, uses up energy and can result in injury or even death. Instead, animals rely on various threats, through vocalizations, smells, or visual displays.

The songs of birds, the drumming of woodpeckers, and the loud calls of monkeys may seem innocuous to humans, but they are all warnings that carry for long distances, advertising to potential intruders that someone else’s territory is being approached. As noted earlier, many animals, such as dogs, rely on smells to mark their territories, spraying urine, leaving droppings, or rubbing scent glands around the territories’ borders. Thus, an approaching animal will be warned off the territory without ever encountering the territory’s defender. Or, if the invader is unfortunate enough to have trespassed on a skunk’s territory, it may get a big blast of scent when it is too late to retreat. Suppose an animal ignores these warnings, or suppose, for one reason or another, that two animals meet nose to nose at the boundaries of their respective territories. Usually there follows a threatening visual display, often involving exaggeration of the animals’ sizes by the fluffing up of feathers or fur. The animals may show off their

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KEY TERMS Learning by associa-

varieties of conditioning: classical condi-

ence to learn them. For example, fish have an innate ability to swim, whereas humans must learn how to walk.

tioning, which involves pairing a stimulus

NATURAL SELECTION:

CONDITIONING:

tion with particular stimuli. There are two

that elicits a specific response with one that does not until the second stimulus elicits a response like the first, and operant conditioning, which involves administering or withholding reinforcements (i.e., rewards) based on the performance of a targeted response. ETHOLOGY:

The study of animal

behavior, including its mechanisms and evolution. FAPs:

Fixed-action patterns of behav-

ior, or strong responses on the part of an animal to particular stimuli. IMPRINTING:

The learning of a behav-

ior at a critical period early in life, such that the behavior becomes permanent. INNATE:

A term to describe behaviors

that are present and complete within the individual and which require no experi-

The process whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment. An inborn, automatic response to a stimulus by a part of an organism’s body.

REFLEX:

Any phenomenon (for example, an environmental change) that directly influences the activity or growth of a living organism.

STIMULUS:

The behavior by which an animal lays claim to and defends an area against others of its species and occasionally against members of other species as well.

TERRITORIALITY:

A response to a stimulus that acts in a particular direction, thus encouraging growth either toward or away from that stimulus. TROPISM:

weapons, whether claws or fangs or other devices. Or the two creatures may go through all the motions of fighting without ever actually touching, a behavior known as ritual fighting.

does break out, it is an aberration. This typically happens only in overcrowded conditions, when resources are scarce—again, not unlike the situation with humans.

F I G H T I N G . The degree to which a creature engages in these displays of bravado helps define its territory. If the creature perceives that it is at the center of its own territory and is being attacked on home ground, it will go into as threatening a mode as it can muster. If, on the other hand, the animal is at the edge of its territorial boundaries, it will be much more halfhearted in its efforts at intimidation. As with humans, few animals want to fight when there is nothing really at stake. Also like humans, animals many times may seem to be spoiling for a fight without actually fighting, such that when a fight

Late in his career, Lorenz devoted himself to studying human fighting behavior. In Das sogenannte Böse (On Aggression, 1963), he maintained that fighting and warlike behavior are innate to human beings but that they can be unlearned through a process whereby humans’ basic needs are met in less violent ways. Just as fighting in animal communities has its benefits, Lorenz maintained, inasmuch as it helps keep competitors separated and enables the larger group to hold on to territory, so fighting among humans might be directed toward more useful means. As discussed in Biological Communities,

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it is possible that sports and business competition in the human community provides a more peaceful outlet for warlike instincts.

“Growth Movements, Turgor Movements, and Circadian Rhythmics.” Department of Biology, University of Hamburg (Germany) (Web site). .

WHERE TO LEARN MORE

Hart, J. W. Light and Plant Growth. Boston: Unwin Hyman, 1988.

Animal Behavior Resources on the Internet. Nebraska Behavioral Biology Group (Web site). .

Hauser, Marc D. Wild Minds: What Animals Really Think. New York: Henry Holt, 2000.

Applied Ethology (Web site). .

Hinde, Robert A. Individuals, Relationships, and Culture: Links Between Ethology and the Social Sciences. New York: Cambridge University Press, 1987.

Dugatkin, Lee Alan. Cheating Monkeys and Citizen Bees: The Nature of Cooperation in Animals and Humans. New York: Free Press, 1999.

Immelmann, Klaus, and Colin Beer. A Dictionary of Ethology. Cambridge, MA: Harvard University Press, 1989.

Ethology: Animal Behavior (Web site). .

Tropisms (Web site). .

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Instinct and Learning

INSTINCT AND LEARNING

CONCEPT Among the most fascinating areas in the biological sciences is ethology, or the study of animal behavior—in particular, the areas of ethology that deal with instinct and learning. Instinct is a stereotyped, or largely unvarying, behavior that is typical of a particular species. An instinctive behavior does not have to be learned; rather, it is fully functional the first time it is performed. On the other hand, learning, in an ethological context, is the alteration of behavior as the result of experience. Clearly, the distinction between instinct and learning revolves around the question of whether an animal, in responding to a specific situation, is acting on the basis of experience or instead is guided by instincts “hardwired” within its brain. The difference would seem to be a simple one, but nothing is simple in the study of instinct and learning. Plenty of gray area exists between pure instinct and genuine learning, and within that gray area is a fascinating concept known as imprinting, or the learning of a behavior at a critical period early in life, such that the behavior becomes permanent.

stimuli. These sequences they called fixed-action patterns of behavior, or FAPs. In studying male stickleback fish, Tinbergen discovered an excellent example of a FAP. Males of that species recognize potential competitors, in the form of other stickleback males capable of breeding, by the red stripe on their underside. Tinbergen termed the red stripe a behavioral releaser, or a simple stimulus that brings about a FAP. (A stimulus is any observable fact or event that directly influences the activity or growth of a living organism.) This particular FAP compels the male stickleback to attack anything red, even when it is not a competitor. Thus, as Tinbergen observed, jealous stickleback males actually would try to attack red British mail trucks when they could see them through the glass of their tanks. Clearly, then, a FAP such as the response to the red stripe is not something that an animal thinks through; rather, it is automatic, almost as though the animal were being acted upon, instead of acting in its own right.

The founding fathers of ethology (the study of animal behavior, including its mechanisms and evolution) were the German zoologist Konrad Lorenz (1903–1989) and the Dutch ethologist Nikolaas Tinbergen (1907–1988). In addition to their separate contributions to the field, which we discuss later, the two scientists together developed a theory that animals employ formalized, rigid sequences of action in response to specific

There have been stories and scientific studies of animals seemingly acting above and beyond the call of duty by raising the offspring of other individuals—even those of another species. Tinbergen, for example, observed and photographed a cardinal feeding baby minnows at water’s edge. Such behavior seems altruistic, touching, and even inspiring, yet for all the significance we might be inclined to place on it from our human perspective, it is nothing but a FAP. Most likely the bird had lost its own offspring, and was simply acting on a parental instinct, which caused it to respond to the sight of open, upturned, hungry mouths. It so happened that the open

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HOW IT WORKS Instinct

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mouths were not those of its offspring at all, but of fish, yet this made no difference in the behavior of the cardinal, acting as it was on an inborn, stereotypic pattern of response. S U R V I V A L - F R I E N D LY, CONT R O L L E D B E H AV I O R. This is one char-

acteristic of instinct, a concept virtually synonymous with that of the FAP: the creature acting on instinct is not thinking about what it does but is behaving almost as though it were controlled by some outside force. The use of the “outside force” concept perhaps makes instinct sound like something mystical when it is not. On the contrary, instinctive behavior is simply a survival mechanism in the brains of each member of a particular species, developed through countless generations of natural selection. If a particular behavior helps foster the survival of a species, natural selection favors it. In other words, those individuals of a species who possess the tendency toward a particular survival-friendly behavior are the ones that survive and pass on their genes to others, while those that do not possess this tendency do not. The survival-friendly instinct may be geared toward the survival of the individual, its offspring, or others. Closely tied to instinct is the innate animal behavior known as a reflex: a simple, inborn, automatic response to a stimulus by a part of an organism’s body. Reflexes help animals (including humans) respond quickly to a stimulus, thus protecting them from harm. Again, the animal does not think about what it is doing. If you touch a hot stove or receive an electric shock, you do not decide to pull your hand back: you whip your hand away from the painful stimulus faster than you can blink. As with instinct, a reflex is a survival-friendly behavior over which the animal (in this case, you) has little control.

Innate Versus Learned Behavior Instinct is innate, meaning that instinctive behaviors and responses are present and complete within the individual at birth. In other words, the individual does not have to undergo any experience to acquire such behaviors. For example, fish have an innate ability to swim, whereas most mammals must learn how to walk. It is fairly easy to identify innate behavior when an animal exhibits it at birth, but in some cases innate behavior manifests only later in life.

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In such situations improvements in the creatures’ ability to perform an innate behavior may seem to indicate that the animal is learning, when, in fact, another process is at work. For example, chickens exhibit the innate tendency toward pecking as a way of establishing and maintaining a dominance hierarchy. This is the “pecking order,” to which people often refer in everyday speech as a metaphor for various hierarchies in human experience, such as those at a workplace. Though pecking is innate, chickens’ ability to perform it actually improves as they grow older. Older chickens display a better aim when pecking than do younger ones, but this does not mean that they have learned from experience. Indeed, one clue that pecking is innate in older chickens is the fact that they uniformly improve in their ability to peck. On the other hand, if they were simply learning, with practice, how to peck more accurately, one could expect that some chickens would exhibit more dramatic improvement than others, in the same way that some humans play basketball (or sing or write poetry) better than others. In fact, what has happened is that the ability to perform an innate behavior simply has improved as a result of growth: as the chickens’ eyes and muscles mature, their aim improves, but this has nothing to do with experience per se.

Imprinting This is one example of the ways in which instinctive and learned behavior can become confused, though in the pecking example there is really no gray area; rather, what is actually an innate behavior merely seems to be a learned one. Yet there truly is a great deal of gray area between instinct and learning. Many behaviors that at first glance might appear purely instinctive can be shown to have an experiential component—that is, an aspect of the behavior has been modified through experience or learning. A fascinating example of how instinct and learning can be blurred or combined is imprinting, or the learning of a behavior at a critical period early in life, such that the behavior becomes permanent. Lorenz, who first developed the theory of imprinting, noted that newly hatched geese learn to walk by following their parents, but he wondered how they distinguished their parents from all other objects in their envi-

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INSTINCT

AND LEARNING CAN BE BLURRED IN THE CASE OF IMPRINTING, OR THE LEARNING OF A BEHAVIOR AT A CRIT-

ICAL PERIOD EARLY IN LIFE, SUCH THAT THE BEHAVIOR BECOMES PERMANENT.

NEWLY HATCHED GEESE LEARN TO IF THE GOSLINGS ARE REMOVED FROM THEIR PARENTS, THEY WILL IMPRINT OBJECT, EVEN A HUMAN BEING. (© Galen Rowell/Corbis. Reproduced by permission.)

WALK BY FOLLOWING THEIR PARENTS. INSTEAD ON ANOTHER MOVING

ronments. He discovered that if he removed the parents from view the first day after the goslings hatched and if he walked in front of the young geese at that point, they would follow him. From these experiments, Lorenz concluded two things. First of all, during a critical period following birth, goslings follow their parents’ movements and learn enough about their parents to recognize them. This critical period is short: if he walked in front of three-day old geese, they were already too old to imprint on him. Second, Lorenz determined that the parents’ movement must be a behavioral releaser for imprinting; thus, if the tiny goslings happened to fix on another moving object, they would imprint on that one. We can see in this example that imprinting has both innnate, or instinctive, and learned components. The tendency to imprint is innate, but if the act of imprinting itself were likewise innate, then it would be the same for all individuals within a species, and this is not the case. The vast majority of goslings will imprint on adult geese, of course, but in an experimental setting (or through some freak accident in nature), it is possible that some other object will come

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between the offspring and its parents, causing the offspring to imprint on it. Once imprinting is complete, it brings about numerous consequences that are more or less automatic or inevitable. Yet for all their automatic, inevitable qualities, these consequences are not the result of innate behaviors or instincts but of learned behaviors. Thus, learning is destiny, at least to an extent. The individual’s mind, at an early stage, is like wet concrete into which virtually any impression can be made. Once the individual has imprinted on something or someone, however, the concrete begins to set, and it hardens around the impressions made in it.

REAL-LIFE A P P L I C AT I O N S Instinct at Work When a kangaroo rat hears a rattling sound, even if that sound comes from a drum or a fan or some otherwise harmless contraption, it performs a lightning-quick escape jump maneuver. Why? Because its brain interprets the rattling sound as that of a rattlesnake ready to strike, and

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it acts automatically. In other words, the kangaroo rat does not reason out a course of action; it simply moves. Nor is it acting on experience in any way: even if it has never encountered a rattlesnake, it will respond in exactly the same way. Virtually all instinctive behavior, as we noted earlier, is geared toward preserving the life of the individual or of its offspring, and thus it serves to preserve the life of the species as well. Such is the case with the rat acting to protect itself from the snake, and so, too, with a mother when her young are threatened. People do not normally think of ducks as intimidating creatures, and in most cases they are not, but try stepping anywhere near a mother’s ducklings, and watch her response. She will start to hiss, spit, and waddle forward menacingly in such a way as to terrify any would-be intruder. If the outsider is still foolhardy enough to press forward, the duck or goose will readily become a squawking, flapping, biting, pecking army of one. This behavior does not vary from mother to mother but is the same in all instances of mother ducks who perceive a threat to their ducklings—a hallmark of an instinctive action. AU T O M AT I C B E H AV I O R S A N D R E L E A S E R S . There is an automatic,

almost robotlike character to many an example of instinctive behavior. If an egg rolls out of a goose’s nest, the goose stretches her neck until the underside of her bill touches the egg—an action that, like all instinctive ones, clearly is geared toward the survival of her offspring. Suppose someone takes the egg away while the goose is reaching for it: she continues to go through the motion of stretching to retrieve the egg. This may not seem very “smart,” but, of course, instinct has nothing to do with intelligence. Likewise, a spider preparing to lay its eggs spins a silk cocoon in a particular way, always the same and without any regard for outside factors. She begins by building a base plate, then constructs the walls of her cocoon before laying her eggs within it and sealing it with a lid. So rigid are these behavioral patterns that they cannot be altered, even if the spider needs to do so. (It is hard to imagine a spider “wanting” to do something, a term that implies decision-making abilities and a degree of self-awareness common only among higher mammals, particularly humans.)

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though there is no base plate to hold them. The eggs, therefore, will fall out of the bottom of the incomplete web, but the spider will continue working as before, building the lid for the top. Fortunately for the spider, she has several cocoons. If she is returned to her original completed base plate as she prepares to spin the next cocoon, she will not use the base plate she already has spun; rather, like a robot, she will start from the very beginning, spinning a new base plate as though the original were not there. Many of these automatic behaviors are triggered by a releaser. For example, a bright red spot on the bill of an adult gull serves as a releaser to its offspring, which peck on the parent’s bill to obtain food. For a female rat in heat, rubbing of her hindquarters acts as a releaser for an instinctive behavior pattern known as lordosis. In lordosis the female flexes her front legs, lowers her torso, raises her rump, and moves the tail to one side. This posture, in turn, acts as a releaser for a male rat, who initiates copulation—yet another example of instinctive behavior as life-preserving. In this case, however, what is being preserved is not the life of an individual but of the entire species, since intercourse yields offspring.

Challenging Situations of Instinct and Imprinting Instincts are ingrained so deeply that some animals possess what appears to be an instinct for exploiting, or taking advantage of, the instinctive behaviors of other animals. We typically think of parasites as microbes, or at least as no larger than insects, but there are species of bird regarded as parasites, inasmuch as they take advantage of other species. When people speak metaphorically of another person as a “parasite,” what they really mean is that the person in question exploits the good behavior of others. A human “parasite” never has to pay for dinner at a restaurant, for instance, because he or she can count on good, decent people always to pick up the tab. Likewise, parasitic bird species, such as the North American cowbird or the European cuckoo, can rely on other species’ instinctive tendency to do what all animals (and humans) should do: take care of their own offspring.

If the spider is moved physically after she has built the base plate, she nonetheless will set about spinning walls and depositing her eggs, even

Avian parasites lay their eggs in the nest of an unwitting host and then leave, “knowing” instinctively (which is not really the same thing as knowing in the way that humans think of it)

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that their victims will take care of the eggs. Cowbirds and cuckoos have developed the practice of laying their eggs in the nests of birds that are smaller than they, meaning that their hatchlings will be larger than the victims’ offspring. This only adds to the detriment caused by the interlopers: since the parasites’ children are larger than the hosts’, this triggers a more powerful release of the host parents’ instinctive feeding behavior. In other words, the parasites’ offspring get more food than the hosts’. As for the parasitic adults, they are long gone, having deposited their young in the care of the hosts. E R R O R S I N I M P R I N T I N G . The situation of the parasitic bird species is one example of the “dark side” to instinct and learning mechanisms in animals. Other examples include the many possible errors that can occur in imprinting. Such errors arise when an animal either fails to receive an imprint from an appropriate parent figure or receives an imprint from a creature of another species.

In their efforts to establish and maintain their territories and attract females, male birds of a given species learn a particular song. This takes place at a critical period when, as a nestling, the bird hears the song of its father and from this exposure eventually develops its own mature song. The process is a lengthy one: the immature bird does not begin singing until the following spring, when it starts trying to match its own, juvenile song with the one it heard from its father during the critical period. If during that early critical period, the nestling is prevented from hearing an adult song of its own species, it never will develop a speciestypical song, and thus its very life and its ability to propagate (which, in turn, affects the wellbeing of the entire species) are threatened. The bird may hear the songs of other species, but this does it little good. It appears that there is a strongly instinctive aspect to what the bird can learn during the critical period and also that birds are highly selective toward songs produced by other members of their species. Therefore, it learns either the right song or no song at all.

Why Imprinting Is Crucial Imprinting is crucial for an animal’s development and not only because the act of imprinting helps the animal learn one or another important early function, such as walking. Far beyond what

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happens to the animal in the first minutes of its life, imprinting will affect its entire destiny both as an individual and as a member of its species. Suppose an animal has imprinted on a creature of another species: this will haunt it for the rest of its life, for instance, determining its choice of a mate and its courtship behavior. Many species will avoid social contact with animals that are not similar to the one to which they have imprinted. In other words, a duckling that has imprinted properly on its duck parents will want to spend time with other ducks. From the larger perspective of nature and the continuing propagation of life-forms, this is a good thing, because it helps prevent attempts to breed between different species. If, however, through some accident (or as the result of exposure to artificial conditions, such as those in an experiment), an animal has imprinted on an individual of a different species, that animal will attempt to court a member of that other species later in life—usually with disastrous results. TA R Z A N T H E A P E M A N . In discussing the critical nature of imprinting, we could use all sorts of animal examples, some of which we have discussed. More compelling, however, is the example of a man, albeit a fictional one, who lived among the apes and attempted to become one of them. That man, of course, is Tarzan, creation of the American novelist Edgar Rice Burroughs (1875–1950). First appearing in Tarzan of the Apes (1914) and immortalized in countless novels, movies, comic books, and cartoons, Tarzan is the epitome of the noble savage, or the man at one with nature.

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The son of an English lord, the boy is left alone as an infant in Africa after his parents die and is raised by an ape who has lost her own child. The apes name him Tarzan, which, in their language, means “white skin,” and he grows up as one of them. Later, he discovers his parents’ cabin and the books left there, teaches himself to read, learns English, and begins to uncover the truth of his background. As a grown man, he meets and falls in love with Jane Porter, a beautiful young American who, like his parents when he was infant, was marooned nearby after a shipboard mutiny. Jane helps expose him to civilization, and Tarzan eventually travels to France, the United States, and other far-off lands. One can find in this astonishing tale antecedents in other rough-and-ready characters from American literature, most notably Buck, the VOLUME 3: REAL-LIFE BIOLOGY

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THERE

IS AN AUTOMATIC CHARACTER TO MANY EXAMPLES OF INSTINCTIVE BEHAVIOR.

A

SPIDER SPINS A COCOON IN

A PARTICULAR WAY, BEGINNING WITH A BASE PLATE AND THEN CONSTRUCTING WALLS BEFORE LAYING EGGS AND SEALING THE COCOON WITH A LID.

IF

THE SPIDER IS MOVED AFTER SHE HAS BUILT THE BASE PLATE, SHE WILL CONTIN-

UE TO SPIN WALLS AND DEPOSIT HER EGGS, LEAVING THEM TO FALL OUT THE BOTTOM OF THE INCOMPLETE WEB. (©

Science Pictures Limited/Corbis. Reproduced by permission.)

dog turned wolf in Jack London’s (1876–1916) novel The Call of the Wild (1903). Buck is a particularly interesting example from the standpoint of biological study, because London, an advocate of Darwin’s theory of evolution by natural selection, was attempting to show the common evolutionary thread linking dog and wolf and, by inference, ape and human. While Burroughs is not so didactic as London, he had a purpose as well, an aim opposite to that of London: to show that a human, thrust by circumstances beyond his control into a world of animals, would assert his humanness. There may even have been a class element to the Tarzan scenario, inasmuch as the young Tarzan is the offspring of nobility and ultimately proves his noble lineage by rising above his circumstances. (This, too, is a theme quite far removed from the heart of London, who was a socialist.) All literary analysis aside, is there any truth, from the standpoint of what we know about imprinting, instinct, and learning, to Burroughs’s portrayal of Tarzan? The answer is a resounding “no.” Assuming that a boy could be raised by apes, he would become socialized as an ape. Appropriately enough, in one story Burroughs shows 332

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Tarzan falling in love with a female ape, thinking that he is of the same species. The ape, for her part, recognizes the difference between them and rejects the human in favor of a male ape. A real-life Tarzan would be human only in a biological sense; otherwise, he would lack virtually all human characteristics. Not only would he be foul smelling (from our perspective) and covered with hair, but he also would be entirely ignorant of human speech or even human thought processes, which have to be learned. Most preposterous of all is the idea that he would find books and recognize them as a mode of communication, let alone teach himself to read them. Suppose that he had been raised as a human but that he had never seen a book or even so much as a written word. Even if he were the most brilliant human being who had ever lived, he would treat the books the way an ape would— as mere objects.

Comparing Humans and Animals The case of Tarzan provides an appropriate place to close this discussion, with a few words on humans and their place in the context of the largS C I E N C E O F E V E RY DAY T H I N G S

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er concepts of instinct, imprinting, and learning. Do humans possess instincts? It appears that we do, inasmuch as we are prone to certain automatic, innate responses. For example, human babies are capable of smiling at the age of four weeks, even though they do not know what a smile “means.” On the other hand, humans are not nearly as inclined to instinctive behavior as animals; in general, with higher mammals and especially humans, instinctive characteristics diminish in favor of the capacity to learn. Our methods of learning are quite different from those of lower animals, who “learn” in ways that do not involve conscious thought. For instance, a snail will pull its head back into its shell when touched, but when it is touched repeatedly with no subsequent harm, the withdrawal response ceases. The snail has experienced habituation, a type of behavior in which an animal develops a tendency to ignore a stimulus that is repeated over and over. Apparently, the snail’s nervous system has “learned” that the stimulus is not threatening and so stops the reflex. This is a far cry from learning as humans experience it, particularly as we move past the first few weeks and months of life. Even from the earliest moments of a human’s existence—that is, even in the womb—a human is self-aware in a way that few, if any, animals other than mammals are. Furthermore, the self-awareness of a human from about the age of two years old is far beyond any concept of self possessed by even the highest forms of mammal. From what we can discern, a house cat or even an ape does not experience any thought along the lines of Who am I? or What is my place in the order of things? (Of course, almost everyone who has had a pet has at one time or another believed that a cat or dog was embroiled in such types of philosophical inquiry, but this may be merely anthropomorphism—ascribing human qualities to animals.) By contrast, a twoyear-old human already is forming complex judgments about his or her role with regard to Mommy, Daddy, siblings, other relatives, and household pets. IMPRINTING

IN

HUMANS.

Whereas humans have fewer instinctive responses than most animals and are far more capable of learning than any other creature, in the area of imprinting we are not so different from cows or even birds. We have discussed the importance of imprinting in birds, but it should be noted that

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KEY TERMS A simple stimulus that brings about a fixed-action pattern of behavior, or FAP. BEHAVIORAL RELEASER:

The study of animal behavior, including its mechanisms and evolution. ETHOLOGY:

Fixed-action patterns of behavior, or strong responses on the part of an animal to particular stimuli. FAP is virtually synonymous with instinct. FAPs:

The learning of a behavior at a critical period early in life, such that the behavior becomes permanent. IMPRINTING:

INNATE: A term to describe behaviors that are present and complete within the individual and which require no experience to learn them. For example, fish have an innate ability to swim, whereas humans must learn how to walk.

A stereotyped, or largely unvarying, behavior that is typical of a particular species. An instinctive behavior does not have to be learned; rather, it is fully functional the first time it is performed. INSTINCT:

The alteration of behavior as the result of experience. LEARNING:

The process whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment. NATURAL SELECTION:

An inborn, automatic response to a stimulus by a part of an organism’s body. REFLEX:

RELEASER:

See Behavioral releaser.

Any phenomenon (that is, an observable fact or event, such as an environmental change) that directly influences the activity or growth of a living organism. STIMULUS:

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imprinting is at least as critical among hoofed mammals, such as cattle or sheep, since they tend to congregate in large herds wherein a young animal could be separated easily from its mother. Likewise, with humans imprinting of some sort is critical. Humans do not imprint as rigidly as geese do, but it is clear enough that some type of imprinting takes place in the mind of a baby. It is conceivable that Tarzan, having spent a year around his human parents, might retain a few human qualities—but only because it was the first year of his life, when the mind is by definition most impressionable. By the same token, a child that had spent six or seven years around humans would be socialized so thoroughly as a human that he or she probably would retain this human quality even if placed among apes. From the time of the Greeks, humans have understood that part of what makes us human is contact with other humans. Therefore, an infant separated from its mother for a prolonged period during its first year of life may experience serious mental retardation; irreparable damage and even death may result from a separation of several months. There are all too many terrible stories of people who, as a result of neglect, abuse, or mere misfortune, have been forced to grow up in some form of isolation and have been stunted as a result. Usually, the outcome of this isolation is not nearly as attractive as the noble savage portrayed in literature or in such movies as Nell

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(1994), in which Jodie Foster plays a young woman who has grown up with little exposure to other humans. WHERE TO LEARN MORE Animal Behaviour. University of Plymouth Department of Psychology (Web site). . Animal Cognition and Learning. Tufts University (Web site). . Dr. P’s Dog Training. University of Wisconsin—Stevens Point (Web site). . Gould, James L., and Carol Grant Gould. The Animal Mind. New York: Scientific American Library, 1994. Instinct (Web site). . Milne, Lorus Johnson, and Margery Joan Greene Milne. The Behavior and Learning of Animal Babies. Chester, CT: Globe Pequot Press, 1989. Rogers, Lesley J., and Gisela T. Kaplan. Songs, Roars, and Rituals: Communication in Birds, Mammals, and Other Animals. Cambridge, MA: Harvard University Press, 2000. Tinbergen, Niko. The Study of Instinct. New York: Clarendon Press, 1989. Topoff, Howard R. The Natural History Reader in Animal Behavior. New York: Columbia University Press, 1987. “Where’s My Mommy? Imprinting in the Wild and in Operation Migration—Journey North Whooping Cranes.” Annenberg/Corporation for Public Broadcasting (Web site). .

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M I G R AT I O N A N D N AV I G AT I O N

CONCEPT Among the most intriguing aspects of animal behavior and perception is the tendency to migrate long distances, coupled with the navigational ability that makes this possible. Most such migration is seasonal, a primary example being birds’ proverbial flight south for the winter. Sometimes, however, animals widely separated from their home environments nonetheless manage to find their way home. This fact has long fascinated humans, as reflected in a number of true and fictional stories on the subject that have circulated over the years. For example, The Incredible Journey, a 1963 Disney film remade in 1993, is a fictional tale, but there are numerous true stories of dogs and cats making their way home to their masters across thousands of miles. How do animals do this? Scientists do not fully understand the answers, but theories regarding animal navigation abound. In any case, there is no question that animals possess navigational abilities unavailable to humans, for example, echolocation, used by bats, whales, and dolphins for local navigation, requires an ability to hear sounds far beyond the range of the human ear.

HOW IT WORKS

put it another way, the physical “cost” of migrating must be less than the cost of staying home. Human beings would perform such calculations rationally, of course, by thinking through the options and weighing them. Animals, on the other hand, rely on instinct—a word that, like many terms in science, has a somewhat different meaning within the scientific community than it does for the world at large. People tend to think of instinct as a matter of “just knowing” something, as in “I just know he/she is a good/bad person,” but, in fact, instinct seems to have nothing to do with “knowing” at all. On the contrary, instinct can be defined as a stereotyped (that is, largely unvarying) behavior that is typical of a particular species. Instinctive behavior does not have to be learned; rather, it is fully functional the first time it is performed. Though animals do exhibit some problem-solving ability, when a bird flies south for the winter, it has not thought that process through in any way. Instead, it is on “autopilot.” This may seem almost magical, but it probably just reflects the processes of natural selection (see Evolution): for a particular bird species, those individuals “hardwired” with a tendency to fly south were those that survived harsh winters, and therefore this tendency became favored in the gene pool.

Why do animals migrate? Seasonal temperature changes, of course, are a factor, as in the wellknown instance of birds flying south for the winter. But to justify enduring the dangers and hardships of long-distance migration, there must be an underlying cost-benefit equation whereby the benefits of migration outweigh the costs. Or, to

Just as circumstances in the creature’s home environment present a compelling need for migration, so there are other circumstances in the wintering environment that force the animal to leave as spring approaches. The wintering environment, after all, has its own native species, which most likely remain in the area even as the influx of visitors from up north arrives. Competition for food and shelter thus can become

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rather intense. Over time, this increased competition creates a situation in which it is advantageous for the migrating creature to return home.

Types of Migration The idea of birds flying south en masse for an entire winter represents only one of four different types of migration: complete, as opposed to partial, differential, or irruptive migration. Complete migration involves the movement of all individuals within a population away from their breeding grounds at the conclusion of the breeding season. Usually this entails migration to a wintering site that may be thousands of miles away. Some species practice partial migration, whereby some individuals remain at the breeding ground year-round, while others migrate. Others employ differential migration, in which all members of the population migrate, but for periods of time and over distances that vary as a function of age or sex. For example, herring gulls migrate for increasingly shorter distances the older they get, and male American kestrels remain at the breeding grounds longer than females. Even when the male birds do set out on their journeys, they do not travel as far as their female counterparts. Finally, there is irruptive migration, whereby certain species do not migrate at all during some years but may do so during other years. The likelihood of migration seems to be tied to climate and resource availability: the colder the winter and the more scarce the food, the more likely migration will occur in species prone to irruptive migratory behavior.

Directions of Migration Though southward migration is the most widely known form of migratory behavior, not all migration is from the north to the south. Actually, this type of movement is more properly called latitudinal migration, since it also takes place in the Southern Hemisphere, where, of course, it is from south to north. (Also, winter in those latitudes occurs at the same time as summer in the Northern Hemisphere.) There is far less habitable land below the equator than above it, however, so latitudinal migration in the Southern Hemisphere is not nearly as significant as it is at northerly latitudes.

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movement probably is related to seasonal changes in the location, availability, and choice of prey. Nor does all migration involve movements across Earth’s surface; there is also elevational migration, which entails a change of altitude or depth beneath the sea. Animals that live on mountains, for instance, may take part in elevational migration, moving to lower elevations in winter just as other species move to lower latitudes. For zooplankton, tiny animals that float on the waters of the open ocean, migration is a matter of changing depths in the water. During the summertime, when populations of zooplankton are large, these organisms live on the surface and feed on the plant life there. During the cold months, however, zooplankton migrate to depths of about 3,300 ft. (1 km) and do not feed at all.

REAL-LIFE A P P L I C AT I O N S The Process of Migration When animals migrate, they move along more or less the same corridors or paths each year. For North American birds migrating south for the winter, one of the most commonly used “flyways” is across the Gulf of Mexico, a journey of 500-680 mi. (800–1,000 km). To make it across the open waters of the gulf, birds have to store up fat, on which they can live for some time while out of sight of food sources. Migrating birds are not like the proverbial parent (usually a father) who will not let the children stop to go to the bathroom on a long road trip. As they make their way south, birds stop regularly to rest and eat, sometimes for days at a time. These stops are particularly frequent and long just before crossing a large expanse of water. Only when the bird has stored sufficient quantities of body fat does it resume the journey.

There are, in fact, species of bird, such as the prairie falcon (Falco mexicanus), that travel longitudinally, or from east to west. This type of

DAY A N D N I G H T T RAV E L . In North America it is common to see flocks of birds apparently flying south during the daytime in the autumn months. Yet most migrating bird species travel at night. The fewer species that travel by day tend to follow paths that are slower and less direct than those of nocturnal migrants. The reason for this difference has to do with the differences in feeding opportunities.

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BIRDS

ARE NOT THE ONLY CREATURES CAPABLE OF GREAT NAVIGATIONAL FEATS.

SALMON

COME BACK FROM THE

OCEANS AND FIGHT THEIR WAY UPSTREAM TO SPAWN IN THE VERY SAME SPOT AT WHICH THEY WERE HATCHED. (©

Ralph A. Clevenger/Corbis. Reproduced by permission.)

Nocturnal migrants have the entire day in

that night. Daytime migrants, on the other hand,

have to forage at the same time that they are traveling. For this reason, they tend to stick close to the coastlines, which offer abundant supplies of insect life. This slows them down but offers a dependable food supply.

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which to rest, forage for food, and eat, thus building up reserves of energy for the nonstop flight

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In making their journeys, some creatures display navigational skills that would put such great mariners as Ferdinand Magellan and James Cook to shame. For example, the arctic tern (Sterna paradisaea) is a complete migrant in every sense of the word. Not only does the tern engage in complete migration, as defined earlier, but it also literally crosses the globe from pole to pole. In traveling from the Arctic to the Antarctic and back again in a single year, the arctic term completes a round-trip migration of more than 21,750 mi. (35,000 km). This distance, nearly the circumference of the planet, is the longest regular migratory path of any animal. A specimen of another bird species, the Manx shearwater, was taken via airplane to Boston from its home on an island off the coast of Wales. Released experimentally in Boston, the bird took only 13 days to return to its point of origin, a flight of some 3,050 mi. (4,880 km). In another experiment, an albatross from Midway Island, deep in the Pacific, was released in the Philippines and made its way to its home area, a distance of 4,120 mi. (6,592 km), in only 32 days. There are also many examples of swallows finding their way across great distances. For instance, swallows that winter in southern Africa still manage to get back to their homes in northern Europe each spring. Then there are the swallows of San Juan Capistrano, southwest of Los Angeles, which leave every year on October 23 and return like clockwork on March 19. Birds are not the only creatures capable of such great navigational feats. Monarch butterflies (Danas plexippus), which are born in Canada or the northern United States, winter each year in southern California, just as they have done for countless years. Then, when winter is over, they make their way back to their home regions. Likewise, salmon come back from the oceans and fight their way upstream to spawn in the very same spot at which they were hatched.

Several experiments have been performed on a variety of creature well known for its navigational abilities: the homing pigeon, which often can return across many hundreds of miles to its home. While they are undergoing training by humans, these pigeons are released from a series of sites, each just a little farther from the birds’ home area. This training seems to make them accustomed to traveling long distances and to finding their way back. Thus, once trained, a pigeon released 100 mi. (63 km) or more from its home will begin flying in the correct direction within a few minutes. BIOLOGICAL CLOCKS AND N AV I GAT I O N . Several theories regarding

pigeons’ homing skills cite internal or biological clocks (see Biological Rhythms). One such theory, which is no longer accepted widely, held that the pigeon’s perception of the Sun’s position in the sky, combined with its biological clock, helped it navigate. Experiments have not proved this to be the case, however. In one such trial pigeons were kept in a laboratory from which they could see the Sun for only very limited periods of time each day. After an extended period, the pigeons, with their eyes covered, were taken away about 40 mi. (64 km) to the south and released, so that the moment of release was their first unobstructed view of the Sun in weeks. Assuming the theory was correct, the pigeons would have been disoriented, but, in fact, they quickly took stock of their position and began flying north.

Based on these and other examples, one is left wondering, how do they do it? Numerous observations and theories have been put forward to answer this question. Salmon, for instance, seem to distinguish their home streams on the basis of smell, whereas some birds appear to use visual signals, primarily the position of the Sun or star

“ M AG N E T I C M A P S . ” One intriguing theory of animal navigation holds that creatures carry in their brains “magnetic maps,” or strong sensitivities to Earth’s magnetic field, that assist them in finding their way across distances. As reported in National Geographic Today on-line (October 12, 2001), research on loggerhead turtles has shown that hatchlings are sensitive to the strength and direction of Earth’s magnetic field and apparently use this in their migratory navigation.

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patterns. Auditory cues and the sensitivity of migratory species to these cues often have been advanced as a key to migration behavior. Finally, one theory, which we discuss shortly, holds that sensitivity to Earth’s magnetic field provides long-distance travelers with the navigational aid they need.

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A FASCINATING EXAMPLE OF ANIMAL NAVIGATION IS ECHOLOCATION, WHICH IS NOT NECESSARILY TIED TO MIGRATION. RATHER, IT PROVIDES A MEANS OF LOCAL NAVIGATION—AND ESCAPE FROM PREDATORS—FOR CREATURES THAT LACK THE ABILITY TO SEE IN THEIR ENVIRONMENTS, SUCH AS THIS WHALE IN THE CLOUDY REALM OF THE OCEAN. (© Terry Whittaker. Photo Researchers. Reproduced by permission.)

By rigging up harnesses attached to electronic tracking units and outfitting the turtles with these devices, researchers were able to follow the course of their migration. Because these are, after all, turtles (though seaborne rather than terrestrial or land-based), migration is no speedy affair. To complete the trip, an 8,000-mi. (12,900km) circuit from south Florida around the Saragasso Sea in the north Atlantic and back again, takes 5–10 years. In the experiment, baby loggerheads were tagged with tracking systems just after they came out of their underground nests on the eastern coast of Florida. Still babies, they would normally begin a journey across the Atlantic, past the Canary and Cape Verde islands on the west coast and Africa and then back to Florida. But instead of going on this journey, the turtles in the experiment were placed in a large circular water tank surrounded by an electric coil capable of generating specific magnetic fields.

these magnetic stimuli by turning in the appropriate direction—for instance, south when they perceived that they were in the magnetic field equivalent to that of Portugal and west in the field resembling that of Cape Verde. The research team, which published its results in the distinguished journal Science, concluded that the turtles were hardwired with magnetic sensitivities. The team leader Kenneth Lohmann, a biologist at the University of North Carolina in Chapel Hill, told National Geographic Today, “These turtles have never been exposed to water, yet they were able to process magnetic information and change their swimming direction accordingly. It seems they inherited some sort of magnetic map.”

Echolocation

By turns, the research team exposed the animals to fields simulating those in three key spots along the route: northern Florida, the area off the coast of Portugal, and the region near the Cape Verde Islands. In each case, turtles responded to

One final, fascinating example of animal navigation is echolocation, which differs from most of the navigational behaviors we have discussed so far in that this one is not necessarily tied to migration. Rather, echolocation provides a means of local navigation for creatures that lack the ability to see in their environments: bats flying through caves and dolphins, porpoises, and

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KEY TERMS A mechanism within an organism (for example, the pineal gland in the human brain) that governs biological rhythms. BIOLOGICAL CLOCK:

The use of sound waves, which are reflected back to the emitter, as a means of navigating. ECHOLOCATION:

A stereotyped, or largely unvarying, behavior that is typical of a particular species. An instinctive behavior does not have to be learned; rather, it is fully functional the first time it is performed. INSTINCT:

MIDDLE EAR: A small cavity that transmits sound waves, via a network of tiny bones, from the eardrum, which lies between it and the outer ear. Bats use the middle ear to separate transmission and reception signals in echolocation.

A pattern of movement, usually regular and seasonal, whereby animals travel (typically guided by instinct) to specific locations. MIGRATION:

The process whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment. NATURAL SELECTION:

toothed whales swimming beneath the ocean’s surface.

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with that of bats and oceangoing mammals, which can hear tones up to 150,000 Hz. Echolocation represents an evolutionary triumph in the form of adaptation to environments—the dark, nocturnal world of the bat and the cloudy realm of sea mammals. In the distant past, bats that hunted for insects during the day would have been at a disadvantage compared with birds, which are nimble of movement and extremely sharp-eyed in spotting their insect prey. Whales, porpoises, and dolphins were in an even worse situation with regard to sharks. Sharks, known for a finely tuned sense of smell, were not only competitors for prey but, as tertiary consumers (see Food Webs), they were and are also potential predators of sea mammals. I N B AT S . Only certain kinds of bats use echolocation. These are the carnivorous varieties that live on frogs, fish, and insects, as opposed to the herbivorous eaters of fruit and nectar. These carnivorous bats fly through the darkness, emitting extremely high frequency sounds and receiving the echoes from these sounds. Contained in the echo is a whole database of information regarding the object off which it has reflected: its distance, direction, size, surface texture, and even material composition.

Interestingly, the volume of these sounds is so great—as high as 100 decibels (dB)—that if people were able to hear them, the noises would be almost literally ear-splitting. (The upper range of safety is 120 dB.) As it is, a person would hear only clicks or chirps. Scientists have long wondered how the bats can both emit these sounds, which would be deafening to the bat, and hear them at the same time. Experimental evidence indicates that when the bat emits a sound, the middle ear (that is, the middle portion of the ear’s interior) adjusts in such a way as to momentarily deafen the bat. A split second later, the bat’s inner ear readjusts so as to permit it to hear the echo from the previous sound.

The frequency of a sound is related not to volume but to pitch: the higher the note, the higher the frequency. The frequency is measured in Hertz, or cycles per second. Human beings are capable of hearing sounds between 20 Hz and 20,000 Hz, whereas cats can hear sounds up to 32,000 Hz and dogs up to 46,000 Hz. This is why these creatures can hear dog whistles and other sounds inaudible to humans. Even the hearing ability of these household pets pales compared

I N O C E A N G O I N G M A M M A L S . It is true, as the tagline for the 1979 film Alien threatened, that “In space no one can hear you scream.” Another way of putting this is that sound requires a material medium through which to travel, and the more dense the medium, the more efficiently it moves. Thus, sound moves more effectively through water than through air, and for this reason echolocation is even more

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suited to the deep-sea environment than it is to the world above ground. As with bats, undersea mammals send out sounds and then listen for the echoes. Owing to their heightened sense of hearing, these creatures obtain far more information from sounds than a human would. Sound, in fact, does for them what sight would do for a human, providing a detailed, three-dimensional image of their surroundings, but the images it offers are even more precise because sound waves are less subject to interference and diffraction than light waves. (For example, sound waves can simply go around a building, whereas the light waves that hit one side of a building are not visible from the other side.)

it sends out noises. Some species have bony insulating structures, which separate the portion of the head where sounds are received from that part where sounds are generated. Likewise, structures in the middle ear assist the whale in distinguishing whether sounds come from the left or the right, thus facilitating the whale’s navigation. WHERE TO LEARN MORE Caras, Roger A. The Endless Migrations. New York: Dutton, 1985. Journey North: A Global Study of Wildlife Migration (Web site). . McDonnell, Janet. Animal Migration. Elgin, IL: Child’s World, 1989.

Toothed whales, dolphins, and porpoises, all of which normally would be disadvantaged by their weak senses of sight and smell, use more or less the same means to navigate by echolocation. A fatty deposit in the mammal’s head helps it direct the sound; then an area of the lower jaw called the acoustic window receives reflected noises. A fatty organ in the middle ear transmits vibrations from the echo, which are translated into neural impulses that go to the brain.

Trivedi, Bijal P. “‘Magnetic Map’ Found to Guide Animal Migration” National Geographic Today (Web site). .

As with the bat, the toothed whale has special structures in its head that help it hear even as

Waterman, Talbot H. Animal Migration. New York: Scientific American Library, 1989.

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Monarchs and Migration (Web site). . Neuroethology: Echolocation in the Bat (Web site). . Penny, Malcolm. Animal Migration. Illus. Vanda Baginska. New York: Bookwright Press, 1987.

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THE BIOSPHERE AND ECOSYSTEMS THE BIOSPHERE ECOSYSTEMS AND ECOLOGY BIOMES

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THE BIOSPHERE

CONCEPT The biosphere is simply “life on Earth”—the sum total, that is, of all living things on Earth. Yet the whole is more than the sum of the parts: not only is the biosphere an integrated system whose many components fit together in complex ways, but it also works, in turn, in concert with the other major earth systems. The latter include the geosphere, hydrosphere, and atmosphere, through which circulate the chemical elements and compounds essential to life. Among these elements is carbon, a part of all living things, which also cycles through the nonliving realms of soil, water, and air—just one of many vital biogeochemical cycles. As for the compounds on which life depends, none is more important than water, which, though it is the focal point of the hydrosphere, passes through the various earth systems as well. Organisms participate in the hydrologic cycle by providing moisture to the air through the process of transpiration, and they likewise benefit from the downward movement of moisture in the form of precipitation. These and many other interactions make it easy to see why scientists speak of Earth as a system—and why some go even further and call it a living thing.

that can be set apart mentally from the rest of the universe for the purposes of study, observation, and measurement. Thus, virtually by definition, a system is something in which the various parts fit together harmoniously, as though they were designed—or adapted over millions of years—to do so. Anything outside the system is known as the environment, and numerous qualifying terms identify the level of interaction between the system and its environment. An isolated system is one so completely sealed off from its environment that neither matter nor energy passes through its boundaries. This is a merely theoretical construct, however, because, in practice, some matter always flows between system and environment. For example, regardless of how tightly a vault or other interior chamber is sealed, there is always room for matter at the microscopic or atomic level to pass through the barrier; moreover, energy, which in many forms does not require any material medium for its transmission, will pass through as well.

Chances are that the mention of the word system calls to mind something mechanical or electrical, produced by humans: for example, a heating and cooling system. This application of the word is close to the scientific meaning, but in the sciences system identifies a wider range of examples. In scientific terms a system is any set of interactions

Earth itself is an approximation of a closed system, or a system in which, despite the sound of its name, exchanges of energy (but not matter) with the environment are possible. Earth absorbs electromagnetic energy from the Sun and returns that energy to space in a different form, but very little matter enters or departs Earth’s system. Earth is not a perfectly closed system, however, since meteorites can enter the atmosphere and hydrogen can escape. Without the intrusion of meteorites, in fact, it is unlikely that life could exist on the planet, because these projectiles from space first brought water (and possibly even the carbon-based rudiments of life) to the planet Earth more than four billion years ago.

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Nevertheless, Earth more closely resembles a closed system than it does an open system, or one that allows the full and free exchange of both matter and energy with its environment. The human circulatory system is an example of an open system, as are the various “spheres” of Earth (geosphere, hydrosphere, biosphere, and atmosphere) that we discuss later in this essay. The distribution of matter and energy within these earth systems may vary over time, but the total amount of energy and matter within the larger earth system is constant. T H E F O U R “ S P H E R E S . ” On the other hand, the four subsystems, or “spheres,” within the larger Earth system are very much open systems. Of these four subsystems, the name of only one, the atmosphere, is familiar from everyday life, whereas those of the other three sectors (geosphere, hydrosphere, and biosphere) may sound at first like scientific jargon. Yet each has a distinct identity and meaning, and each represents a part of Earth that is at once clearly defined and virtually inseparable from the rest of the planet.

The geosphere is the upper part of the planet’s continental crust, the portion of the solid earth on which human beings live and which provides them with most of their food and natural resources. It is also the oldest, followed by the hydrosphere, which had its beginnings with several hundred millions years’ worth of rains that took place about four billion years ago. Today the hydrosphere includes all water on Earth, except for water vapor in the atmosphere. The latter, incidentally, was probably the last of the four subsystems to take shape: though Earth in its early stages had a blanket of gases around it, there was no oxygen. (See Paleontology for more about the early atmosphere, oxygen, and early life.) T H E AT M O S P H E R E . Today the atmosphere is 78% nitrogen, 21% oxygen, and 0.93% argon. The remaining 0.07% is made up of water vapor, carbon dioxide, ozone (a form of oxygen in which three oxygen atoms bond chemically), and noble gases. The noble gases, including argon and neon, are noted for their lack of reactivity, meaning that they are extremely resistant to chemical bonding with other elements.

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the noble gases, is simply “hanging in the air” (literally), left over from the time when volcanoes hurled it into the atmosphere several billion years ago. By contrast, oxygen (both in O2 and O3, or ozone, molecules) and the other elements in air are vital to life. Furthermore, oxygen is one of two elements, along with hydrogen, that goes into the formation of water. O V E R LA P B E T W E E N S U B S Y S T E M S . The present atmosphere would not

exist without the biosphere. In order to put oxygen into the air, there had to be plants, which take in carbon dioxide and release oxygen in the process of photosynthesis. This resulted from an exceedingly complex series of evolutionary developments from anaerobic, or non-oxygenbreathing, single-cell life-forms to the appearance of algae. As plant life evolved, eventually it put more and more oxygen into the atmosphere, until the air became breathable for animal life. Thus, the atmosphere and biosphere have sustained one another. Such overlap is typical and indeed inevitable where the open earth subsystems are concerned, and examples of this overlap are everywhere. For instance, plants (biosphere) grow in the ground (geosphere), but to survive they absorb water (hydrosphere) and carbon dioxide (atmosphere). Nor are plants merely absorbing: they also give back oxygen to the atmosphere, and by providing nutrition to animals, they contribute to the biosphere. At the same time, the many components of the picture just described are involved in complex biogeochemical cycles, which we look at later. T H E B I O S P H E R E I N C O N T E X T.

The biosphere is, of course, integral to the functioning of earth systems. First of all, the present atmosphere, as we have noted, is the product of respiration on the part of plants, which receive carbon dioxide and produce oxygen. In addition, transpiration, a form of evaporation from living organisms (primarily plants), is a mechanism of fundamental importance for moving moisture from the hydrosphere through the biosphere to the atmosphere.

Nitrogen also tends to be unreactive, and the reason for its abundance in the atmosphere lies in the fact that it never attempted to bond with other elements. Therefore, nitrogen, along with

We examine transpiration later, within the larger context of evapotranspiration, along with another area in which the biosphere interacts closely with one or more of the other earth systems: soil. Though soil is part of the geosphere, its production and maintenance is an achievement of all spheres. The role of the biosphere in this

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instance is particularly important: the amount of decayed organic material (i.e., dead plants and animals) is critical to the quality of the soil for sustaining further life in the form of plants and other organisms that live underground. As important as the biosphere is, it may be surprising to learn just what a small portion of the overall earth system it occupies. As living beings, we tend to have a bias in favor of the living world, but the overwhelming majority of the planet’s mass and space is devoted to nonliving matter. The geosphere alone accounts for almost 82% of the combined mass of the four subsystems. (This is not the combined mass of Earth, which would be much larger; remember that the geosphere is only the extreme upper layer of Earth, and does not include the vast depths of the lower mantle and core.) Of the remaining mass that makes up the four earth systems, the hydrosphere is a little more than 18%, the atmosphere less than 1%, and the biosphere a tiny 0.00008%. Note just how much greater the amount of mass is in the air, which we tend to think of as being weightless (though, of course, it is not), than in the biosphere. Even within the biosphere’s almost infinitesimal fraction of total mass, the animal kingdom accounts for less than 2%, the remainder being devoted to other kingdoms: plants, fungi, monera (including bacteria), and protista, such as algae. (See Taxonomy and Species for more about the kingdoms of living things.) It need hardly be added that humans, in turn, are a very, very small portion of the animal world.

Water and the Hydrologic Cycle

of leaves. Scientists usually speak of the two as a single phenomenon, evapotranspiration. The atmosphere is just one of several “compartments” in which water is stored within the larger environment. In fact, the atmosphere is the only major reservoir of water on Earth that is not considered part of the hydrosphere.

The Biosphere

ACCOUNTING FOR EARTH’S WAT E R S U P P LY. The water that most of

us see or experience is only a very small portion of the total. Actually, that statement should be qualified: the oceans, parts of which most people have seen, make up about 5.2% of Earth’s total water supply. This may not sound like a large portion, but, in fact, the oceans are the secondlargest water compartment on Earth. If the oceans are such a small portion yet rank second in abundance, two things are true: there must be a lot of water on Earth, and most of it must be in one place. In fact, the vast majority of water on Earth is stored in aquifers, or underground rock formations, that hold 94.7% of the planet’s water. Thus, deep groundwater and oceans account for 99.9% of the total. Glaciers and other forms of permanent and semipermanent ice take third place, with 0.065%. Another 0.03% appears in the form of shallow groundwater, the source of most local water supplies. Next are the inland surface waters, including such vast deposits as the Great Lakes and the Caspian Sea as well as the Mississippi-Missouri, Amazon, and Nile river systems and many more, which collectively make up just 0.003% of Earth’s water. ATMOSPHERIC MOISTURE AND WEATHER. That leaves only 0.002%, which is

As any backyard horticulturist knows, plants need good soil and water. In the course of circulating throughout Earth, water makes its way through organisms in the biosphere as well as reservoirs housed within the geosphere. It also circulates continuously between the hydrosphere and the atmosphere. This movement, known as the hydrologic cycle, is driven by the twin processes of evaporation and transpiration.

the proportion taken up by moisture in the atmosphere: clouds, mist, and fog, as well as rain, sleet, snow, and hail. While it may seem astounding that atmospheric moisture is such a small portion of the total, this fact says more about the vast amounts of water on Earth than it does about the small amount in the atmosphere. That “small” amount, after all, weighs 1.433 ⫻ 1013 tons (1.3 ⫻ 1013 tonnes), or 28,659,540,000,000,000 pounds (12,999,967,344,000,002 kg).

The first of these processes, of course, is the means whereby liquid water is converted into a gaseous state and transported to the atmosphere, while the second one—a less familiar term—is the process by which plants lose water through their stomata, small openings on the undersides

This moisture in the atmosphere is the source of all weather, which clearly has an effect on Earth’s life-forms. (Weather is the condition of the atmosphere at a given time and in a given place, whereas climate is the pattern of weather in a particular area over an extended period of

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time.) On the one hand, rain is necessary to provide water to plants, and desert conditions can sustain only very specific life-forms; on the other hand, storms, icy precipitation, and flooding can be deadly.

Biogeochemical Cycles Water is not the only substance that circulates through the various earth systems. So, too, do six other substances or, rather, chemical elements. These elements are composed of a single type of atom, meaning that they cannot be broken down chemically to make a simpler substance, as is the case with such compounds as water. The six elements that cycle throughout Earth’s systems are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur. The two following lists provide rankings for their abundance. The first shows their ranking and share in the entire known mass of the planet, including the crust, living matter, the oceans, and atmosphere. The second list shows their relative abundance and ranking in the human body. Abundance of Selected Elements on Earth (Ranking and Percentage): • • • • • •

1. Oxygen (49.2%) 9. Hydrogen (0.87%) 12. Phosphorus (0.11%) 14. Carbon (0.08%) 15. Sulfur (0.06%) 16. Nitrogen (0.03%) Abundance of Selected Elements in the Human Body (Ranking and Percentage): • • • • • •

1. Oxygen (65%) 2. Carbon (18%) 3. Hydrogen (10%) 4. Nitrogen (3%) 6. Phosphorus (1%) 9. Sulfur (0.26%) Note that the ranking of all these elements (with the exception of oxygen) is relatively low in the total known elemental mass of Earth, whereas their relative abundance is much, much higher within the human body. This is significant, given the fact that these elements are all essential to the lives of organisms. All six of these elements take part in biogeochemical cycles, a term used to refer to the changes that a particular element undergoes as it passes back and forth through the various earth systems and particularly between living and nonliving matter.

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T H E CA R B O N A N D N I T R O G E N CYC L E S . Carbon, for instance, is present in

all living things and is integral to the scientific definition of the word organic. The latter term does not, as is popularly believed, refer only to living and formerly living things, their parts, and their products, such as sweat or urine. Organic refers to the presence of compounds containing carbon and hydrogen. The realm of organic substances encompasses not only the world of the living, the formerly living, and their parts and products, but also such substances as plastics that have never been living. The carbon cycle itself involves movement between the worlds of the living and nonliving, the organic and inorganic. This highly complex biogeochemical cycle circulates carbon from soil and carbonate rocks (which are inorganic because they do not contain carbon-hydrogen compounds) to plants and hence to animals, which put carbon dioxide into the air. Likewise, the nitrogen cycle moves that element among all these reservoirs. Because nitrogen is highly unreactive, the participation of microorganisms in the nitrogen cycle is critical to moving that element between the various earth systems. These organisms “fix” nitrogen, meaning that by processing the element through their bodies, they bring about a chemical reaction that makes nitrogen usable to plant life. Additionally, detritivores and decomposers in the soil are responsible for transforming nitrites and nitrates (compounds of nitrogen and oxygen) from the bodies of dead animals into elemental nitrogen that can be returned to the atmosphere.

REAL-LIFE A P P L I C AT I O N S Soil and the Life in It The soil is a sort of anchor to the biosphere. It teems with life like few other areas within the earth system, and, indeed, there are more creatures—plant, animal, monera, protista, and fungi—living in the soil than above it. Minerals from weathered rock in the soil provide plants with the nutrients they need to grow, setting in motion the first of several steps whereby organisms take root in and contribute to the soil.

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NUMEROUS

ANT SPECIES PERFORM A POSITIVE FUNCTION FOR THE ENVIRONMENT.

LIKE

EARTHWORMS, THEY AERATE

SOIL AND HELP BRING OXYGEN AND ORGANIC MATERIAL FROM THE SURFACE WHILE CIRCULATING SOILS FROM BELOW.

(© Ralph A. Clevenger/Corbis. Reproduced by permission.)

Plants provide food to animals, which, when they die, likewise become one with the soil. So, too, do plants themselves. Organisms from both the plant and animal kingdoms leave behind material to feed such decomposers as bacteria and fungi, which, along with detritivores, are critical to the functioning of food webs (see entry). Detritivores, of which earthworms are a great example, are much more complex organisms than the typically single-cell decomposers. We cannot see bacteria, but almost anyone who has ever dug in the dirt has discovered another type of organism: the Annelida phylum of the animal kingdom, which includes all segmented worms, among them, the earthworm. (Incidentally, the leech family also falls within phylum Annelida.) These slimy creatures at first might seem disgusting, but without the appropriately named earthworm, our world could not exist as it does.

that result in the breakdown of their components. As a result, simple and usable nutrients are made available to the soil. Earthworms, which are visible and relatively large, are not the only worms at work in the soil. There are also the colorless creatures of the phylum Nematoda, or roundworms. Included in this phylum are hookworms and pinworms, which can be extremely detrimental to the body when they live inside it as parasites. (See Parasites and Parasitology.) This is one good reason (among many) not to eat dirt. But nematodes in the soil, most of them only slightly larger than microorganisms, perform the vital function of processing organic material by feeding on dead plants. Even in a soil situation, however, some nematodes are parasites that live off the roots of such crops as corn or cotton.

Detritivores consume the remains of plant and animal life, which usually contain enzymes and proteins far too complex to benefit the soil in their original state. By feeding on organic remains, detritivores cycle these complex chemicals through their internal systems, thus causing the substances to undergo chemical reactions

In addition to earthworms, ants and other creatures are also significant inhabitants of the soil. Like earthworms, ants aerate soil and help bring oxygen and organic material from the surface while circulating soils from below. Among the larger creatures that call the soil home are moles, which live off earthworms, grubs (insect larvae), and the roots of plants. By burrowing under the ground, they help to loosen the soil,

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making it more porous and thus receptive to both moisture and air. Other large burrowing creatures include mice, ground squirrels, and, in some areas, even prairie dogs.

Soils and the Environments They Help Create Five different factors determine the quality of soil: parent material (the decayed organisms and weathered rock that make it up), climate, the presence of living organisms, topography (the shape of the land, including prominent natural features), and the passage of time. These factors influence the ability of the soil to sustain life. For example, in a desert, a place that obviously has a smaller abundance and complexity of life-forms (see Biological Communities), the soil itself is lacking in this life-sustaining quality. Desert soil, in fact, is usually referred to as immature soil. Healthy soil normally has a deep A horizon, the area in which decayed organic material appears and atop which humus sits. In general, the deeper the A horizon, the better the soil. Immature soil, on the other hand, has a very thin A horizon and no B horizon, which is the subsoil that typically separates the A horizon from a layer of weathered material that rests even lower, at the C horizon just above bedrock. In deserts, by definition, the water supply is very limited, and only those species that require very little water—for example, the varieties of cactus that grow in the American Southwest— are able to survive. But lack of water is not the only problem. Desert subsoils often contain heavy deposits of salts, and when rain or irrigation adds water to the topsoil, these salts rise. Thus, watering desert topsoil actually can make it a worse environment for growth. T R O P I CA L RA I N F O R E S T S . The soil in rain forests has just the opposite problem of desert soil: instead of being immature, it has gone beyond maturity and reached old age, a point at which plant growth and water percolation (the downward movement of water through the soil) have removed most of its nutrients. In environments located near the equator, whether these regions be desert or rain forest, soils tend to be “old.” This helps to explain the fact that equatorial regions are usually low in agricultural productivity, despite the fact that they enjoy an otherwise favorable climate.

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Given the poor quality of equatorial soil, one might wonder how it is that some of the most lush rain forests are located in the equatorial regions of Africa and South America. The answer is that a rain forest has such a wide assortment of life-forms that there is never a shortage of decayed organic material to “feed” the soil. Due to the sheer breadth and scope of the rain forest’s ecological diversity, there are bound to always be plants and animals dying, replenishing what would otherwise be poor soil. The rapid rate of decay common in warm, moist regions (which are extremely hospitable to bacteria and other microbes) further supports the process of renewing minerals in the ground. The fact that decayed organic material is critical to the life of soil helps to explain why many environmentalists have long been concerned about the destruction of tropical rain forests. For example, in Brazil, vast portions of the Amazon rain forest have been clear-cut, subjected to an extremely destructive form of slashand-burn agriculture that is motivated by modern economic concerns—the desire of the country’s leaders to create jobs and exports—but which resembles practices applied (albeit on a much smaller scale) by premodern peoples in Central and South America. By removing the heavy jungle canopy of tall trees, clear-cutting exposes the ground to the heat of the Sun and the pounding of monsoon rains. Sun and rain, thanks to the removal of this protection, fall directly on the ground, parching it in the first instance and eroding it in the second. Furthermore, when trees and other vegetation are removed, the animal life that these plants supported disappears as well, and this has a direct impact on the soil by removing organisms whose waste products and bodies eventually would have decayed and enriched it. W H AT M A K E S G O O D S O I L ? The soil of the Brazilian rain forest may be old and weak, but thankfully there are places in the world where the soil tells an entirely different story. Instead of being nutrient-poor, this soil is nutrient-rich, and instead of being red, such soil is a deep, rich black. Sometimes regions of good and bad soil exist in close proximity, as in ancient Egypt, where the Nile made possible a narrow strip of extraordinarily productive land, running the length of the country, flanked by deserts. The latter the Egyptians called “the red land” because

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of its nutrient-poor soil, whereas their own fertile region was “the black land.”

The Biosphere

Just as Egypt, after its annexation by the Romans in 30 B.C., became known as the breadbasket of the Roman Empire, there are other “black lands” that serve as the breadbaskets of today’s world. Unlike Egypt, however, most are located in temperate rather than equatorial regions. (See Biomes for more about temperate regions.) Examples include the midwestern United States, western Canada, and southern Russia, regions characterized by vast plains of fertile black soil. Below this rich topsoil is a thick subsoil that helps hold in moisture and nutrients. Rivers such as the Nile, the Mississippi-Missouri, or the Volga in Russia make possible the richest variety of soil on Earth, alluvial soil, a youngish sediment of sand, silt, and clay. A river pulls soil along with it as it flows, and with this comes nutrients from the regions through which the river has passed. The river then deposits these nutrients in the alluvial soil at the delta, the area where it enters a larger body, usually of salt water. (A delta is so named because, as it widens in the region near the sea, the shape of the river is like that of the Greek letter delta, or ∆.) Because they are depositories for accumulated alluvial soil, delta regions such as Mississippi and Louisiana in the United States (where the mighty Mississippi-Missouri, the largest river system in North America, empties into the Gulf of Mexico), are exceedingly fertile. The same is true in the Volga and Nile deltas, the deltas of the Danube and other major European rivers, and those of lesser-known rivers in Canada or Australia. F E RT I L I Z E R. It is also possible to artificially improve soil that is not in a river delta, or that has not otherwise been blessed by nature. The most significant way to achieve this is by using fertilizer, which augments the nutrients in the soil itself. As noted earlier, nitrogen is highly nonreactive, meaning that it tends not to bond chemically with other substances. However, because it is a necessary component of biogeochemical cycles, it is critical that nitrogen be introduced to the soil in such a way that it becomes useful, and typically this is done by combining it with a highly reactive element: oxygen.

DESERT TORTOISE AND MOJAVE DESERT. ONLY

BEAVERTAIL CACTUS IN THE THOSE PLANT AND ANIMAL

SPECIES THAT CAN ENDURE A LIMITED WATER SUPPLY AND IMMATURE SOIL WITH HEAVY DEPOSITS OF SALT IN THE LOWER LAYERS CAN SURVIVE A DESERT ENVIRONMENT. (© D. Suzio. Reproduced by permission.)

hydrogen compound. The latter may be ammonia (NH3) or ammonium (NH4+). Note that ammonia is much more than the product with which it is most readily associated: a household cleaner. In fact, ammonia is an extremely abundant substance, occurring naturally, for instance, in the atmospheres of Venus and other planets in our solar system. The importance of ammonia is reflected by the fact that it and water are the only two substances that chemists regularly refer to by their common names, as opposed to a scientific name such as carbon dioxide.

Fertilizers may contain nitrogen in the form of a nitrate, which is a compound of nitrogen and oxygen, or they may include a nitrogen–

As for ammonium, its extra hydrogen atom makes it a substance that dissolves in water and is attracted to negatively charged surfaces of clays and organic matter in soil. Therefore, it tends to become stuck in one place rather than to move around, as nitrate does. Plants in acidic soils typically receive their nitrogen from ammonium, but in nonacidic soils, nitrate is typically the more useful form of fertilizer. The two fertilizers are also combined to form ammonium nitrate, which is powerful both as a fertilizer and as an

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The Biosphere

explosive. (Ammonium nitrate was used both in the first World Trade Center bombing, in 1993, and in the even more devastating Oklahoma City bombing two years later.)

Erosion and Soil Conservation The mismanagement of agricultural lands, and/or the influence of natural forces, can produce devastating results, as illustrated by events during the years 1934 and 1935 in a region including Texas, Oklahoma, Kansas, and eastern Colorado. In just a few months, once-productive farmland turned into worthless fields of stubble and dust, good for virtually nothing. By the time it was over, the region had acquired a bitter nickname: the “dust bowl.” Ironically, in the years leading up to the early 1930s the future dust bowl farmlands had seemed remarkably productive. Farmers happily reaped abundant yields, year after year, not knowing that they were actually preparing the way for soil erosion on a grand scale. Farmers in the 1930s had long known about the principle of crop rotation as a means of giving the soil a rest and restoring its nutrients. But to be successful, crop rotation must include fallow years (i.e., no crops are planted), and must make use of crops that replenish the soil of nutrients. Cotton and wheat are examples of crops that deplete nutrient content in the soil, and in fact wheat was the crop of choice in the future dust bowl. In some places, farmers alternated between wheat cultivation and livestock grazing on the same plot of land. The hooves of the cattle further damaged the soil, already weakened by raising wheat. The land was ready to become the site of a full-fledged natural disaster, and in the depths of the Great Depression, that disaster came in the form of high winds. These winds scattered vast quantities of soil from the Great Plains of the Midwest to the Atlantic seaboard, and acreage that once had rippled with wheat turned into desert-like wastelands. The farmlands of the plains states have long since recovered from the dust bowl, and farming practices have changed considerably. Instead of alternating one year of wheat with a year of grazing livestock, farmers in the dust bowl region apply a three-year cycle of wheat, sorghum, and fallow land. They also have planted trees to serve as barriers against wind.

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Years after the dust bowl, the American West could have again become the site of another disaster, had not farmers and agricultural officials learned from the mistakes of an earlier generation. During the 1970s, American farms enjoyed such a great surplus that farmers increasingly began to sell their crops to the Soviet Union, and farmers were encouraged to cultivate even marginal croplands to increase profits. This alarmed environmental activists, who called attention to the flow of nutrients from croplands into water resources. As a result of public concerns over these and related issues, Congress in 1977 passed the Soil and Water Resource Conservation Act, mandating measures to conserve or protect soil, water, and other resources on private farmlands and other properties.

Leaching and Its Effect on Soil Like erosion, leaching moves substances through soil, only in this case it is a downward movement. Leached water can carry all sorts of dissolved substances, ranging from nutrients to contaminants. The introduction of manufactured contaminants to the soil, and hence the water table, is of course a serious threat to the environment. On the other hand, where human waste and other, more natural forms of toxin are concerned, nature itself is able to achieve a certain amount of cleanup on its own. In a septic tank system, used by people who are not connected to a municipal sewage system, anaerobic bacteria process wastes, removing a great deal of their toxic content in the tank itself. (These bacteria usually are not introduced artificially to the tank; they simply congregate in what is a natural environment for them.) The wastewater leaves the tank and passes through a drain field, in which the water leaches through layers of gravel and other filters that help remove more of its harmful content. In the drain field, the waste is subjected to aerobic decay by other forms of bacteria before it either filters through the drainpipes into the ground or is evaporated. In addition to purifying water, leaching also passes nutrients to the depths of the A horizon and into the B horizon—something that is not always beneficial. In some ecosystems, leaching removes large amounts of dissolved nitrogen from the soil, and it becomes necessary to fertilize the soil with nitrate. However, soil often has

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difficulty binding to nitrate, which tends to leach easily, and this leads to an overabundance of nitrogen in the lower levels of the soil and groundwater. This is a condition known as nitrogen saturation, which can influence the eutrophication of waters (a topic discussed later) and cause the decline and death of trees with roots in an affected area of ground.

How Deserts Are Formed Let us now consider what one might call an extreme ecosystem: a desert. Of the world’s deserts, by far the most impressive is the Sahara, which today spreads across some 3.5 million sq. mi. (9.06 million sq km), an area that is larger than the continental United States. Only about 780 acres (316 hectares) of it, a little more than 1 sq. mi. (2.6 sq km), is fertile. The rest is mostly stone and dry earth with scattered shrubs—and here and there the rolling sand dunes typically used to depict the Sahara in movies. Just 8,000 years ago—the blink of an eye in terms of Earth’s timescale—it was a region of flowing rivers and lush valleys. For thousands of years it served as a home to many cultures, some of them quite advanced, to judge from their artwork. Though they left behind an extraordinary record in the form of their rock-art paintings and carvings, which show an understanding of realistic representation that would not be matched until the time of the Greeks, the identity of the early Saharan peoples themselves remains largely a mystery. E V O LU T I O N O F T H E A N C I E N T SA H A RA N E C O S Y S T E M . The phases

in ancient Saharan cultures are identified by the names of the domesticated animals that dominated at given times, and collectively these names tell the story of the Saharan ecosystem’s transformation from forest to grassland to desert. First was the Hunter period, from about 6000 to about 4000 B.C., when a Paleolithic, or Old Stone Age, people survived by hunting the many wild animals then available in the region. Next came the Herder period, from about 4000 to 1500 B.C. As their name suggests, these people maintained herds of animals and also practiced basic agriculture.

stances that scientists today do not understand fully, desertification—the slow transformation of ordinary lands to desert—had begun to set in. As the Sahara became drier and drier, the herds disappeared.

The Biosphere

Eventually, the Egyptians began bringing in domesticated horses to cross the desert: hence the name of the Horse period (ca. 1500–ca. 600 B.C.), when the Sahara probably resembled the dry grasslands of western Texas or the sub-Saharan savanna in Africa today. By about 600 B.C., however, the climate had become so severe, the water supply so limited, and the ecosystem so depleted of supporting life-forms that not even horses could survive in the forbidding climate. There was only one mammal that could: the hardy, seemingly inexhaustible creature that gave its name to the Camel era, which continues to the present day. C O N T R O L L I N G D E S E RT I F I CA T I O N . What happened to the Sahara? The

answer is a complex one, as is the subject of desertification. Desertification does not always result in what people normally think of as a desert; rather, it is a process that contributes toward making a region more dry and arid, and because it is usually gradual, it can be reversed in some cases. Nor is it necessary for a society to undertake large-scale mechanized agricultural projects, such as those of the American dust bowl of the 1930s, to do long-term damage that can result in desertification. The Pueblan culture of what is now the southwestern United States depleted an already dry and vulnerable region after about A.D. 800 by removing its meager stands of mesquite trees.

A high point of ancient Saharan civilization came with the Herder period, which, not surprisingly, also marked the high point of the local ecosystem in terms of its ability to sustain varied life-forms. Yet through a complex set of circum-

And though human causes, either in the form of mismanagement or deliberate damage, certainly have contributed to desertification, sometimes nature itself is the driving force. Long-term changes in rainfall or general climate, as well as water erosion and wind erosion such as caused the dust bowl, can turn a region into a permanent desert. An ecosystem may survive short-term drought, but if soil is forced to go too long without proper moisture, it sets in motion a chain reaction in which plant life dwindles and, with it, animal life. Thus, the soil is denied the fresh organic material necessary to its continued sustenance, and a slow, steady process of decline begins.

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The Phosphorus Cycle The Biosphere

Having examined what can go wrong (and right) in soil, a key component of the biosphere, we will devote the remainder of this essay to two other aspects of the biosphere mentioned earlier: biogeochemical cycles and the hydrologic cycle. In the context of biogeochemical cycles, we will look at the phosphorus cycle, along with eutrophication—another instance of “what can go wrong.” Because of its high reactivity with oxygen, phosphorus is used in the production of safety matches, smoke bombs, and other incendiary devices. It is also important in various industrial applications and in fertilizers. In fact, ancient humans used phosphorus without knowing it when they fertilized their crops with animal bones. In the early 1800s chemists recognized that the critical component in the bones was the phosphorus, which plants use in photosynthesis—the biological conversion of energy from the Sun into chemical energy. With this discovery came the realization that phosphorus would make an even more effective fertilizer when treated with sulfuric acid, which makes it soluble, or capable of being dissolved, in water. This compound, known as superphosphate, can be produced from phosphate, a type of phosphorusbased mineral. Microorganisms in the biosphere absorb insoluble phosphorus compounds and, through the action of acids within the microorganisms, turn them into soluble phosphates. These soluble phosphates then are absorbed by algae and other green plants, which are eaten by animals. When they die, the animals, in turn, release the phosphates back into the soil. As with all elements, the total amount of phosphorus on Earth stays constant, but the distribution of it does not. Some of the phosphorus passes from the geosphere into the biosphere, but the vast majority of it winds up in the ocean. It may find its way into sediments in shallow waters, in which case it continues to circulate, or it may be taken to the deep parts of the seas, in which case it is likely to be deposited for the long term. Because fish absorb particles of phosphorus, some of it returns to dry land through the catching and consumption of seafood. Also, guano or dung from birds that live in an ocean environment (e.g., seagulls) returns portions of phosphorus to the terrestrial environment. Neverthe-

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less, scientists believe that phosphorus is steadily being transferred to the ocean, from whence it is not likely to return. It is for this reason that phosphorus-based fertilizers are important, because they feed the soil with nutrients that would otherwise be steadily lost. However, phosphorus still ends up making its way through the waters, and this creates a serious problem in the form of eutrophication. E U T R O P H I CAT I O N . Eutrophication (from a Greek term meaning “well nourished”) is a state of heightened biological productivity in a body of water. One of the leading causes of eutrophication is a high rate of nutrient input, in the form of phosphate or nitrate, a nitrogen-oxygen compound. As a result of soil erosion, fertilizers make their way into bodies of water, as does detergent runoff in wastewater. Excessive phosphates and nitrates stimulate growth in algae and other green plants, and when these plants die, they drift to the bottom of the lake or other body of water. There, decomposers consume the remains of the plants, and in the process of doing so, they also use oxygen that otherwise would be available to fish, mollusks, and other forms of life. As a result, those species die off, to be replaced by others that are more tolerant of lowered oxygen levels—for example, worms. Needless to say, the outcome of eutrophication is devastating to the lake’s ecosystem.

Lake Erie—one of the Great Lakes on the border of the United States and Canada— became an extreme example of eutrophication in the 1960s. As a result of high phosphate concentrations, Erie’s waters were choked with plant and algae growth. Fish were unable to live in the water, the beaches reeked with the smell of decaying algae, and Erie became widely known as a “dead” body of water. This situation led to the passage of new environmental standards and pollution controls by both the United States and Canada, whose governments acted to reduce the phosphate content in fertilizers and detergents drastically. Within a few decades, thanks to the new measures, the lake once again teemed with life. Thus, Lake Erie became an environmental success story.

Evapotranspiration Many of the phenomena and processes we have described tie together the biosphere with other “spheres” of Earth. Such is the case with evapo-

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CROSS

SECTION OF A LILAC LEAF.

A

KEY ELEMENT IN CIRCULATING LIFE-SUSTAINING MATERIALS AMONG THE VARI-

OUS EARTH SYSTEMS IS TRANSPIRATION, THE EVAPORATION OF MOISTURE FROM PLANTS. THROUGH MEMBRANES OF A TISSUE KNOWN AS SPONGY MESOPHYLL

(SHOWN

PLANTS

LOSE THEIR WATER

HERE), FOUND IN THE TINY CAVITIES

THAT LIE BENEATH THE MICROSCOPIC LEAF PORES CALLED STOMATA. (© Lester V. Bergman/Corbis. Reproduced by permission.)

transpiration, the sum total of evaporation and transpiration. The second of these terms is less well-known than the first, but the words in fact refer to the same process. The only difference is that evaporation involves the upward movement of water from nonliving sources, while transpiration is the evaporation of moisture from living sources. Transpiration is at least as important as evaporation when it comes to putting moisture into the atmosphere. It actually puts more water into the air than evaporation does: any large area of vegetation tends to transpire much larger quantities of moisture than an equivalent nonfoliated region, such as the surface of a lake or moist soil. Though animals can play a part in transpiration, plant transpiration has much greater environmental significance. Water in plants is lost through moist membranes of a tissue known as spongy mesophyll, found in the tiny cavities that lie beneath the microscopic leaf pores called stomata. Stomata remain open most of the time, but when they need to be closed, guard cells around their borders push them shut. Because plants depend on stomata to “breathe” by pulling in carbon dioxide,

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they keep them open—just as a human’s pores must remain open. Otherwise, the person would not take in enough oxygen, and would perish. The fact that the stomata are exposed in order to receive carbon dioxide for the plant’s photosynthesis also means that the stomata are open to allow the loss of moisture to the atmosphere. It can be said, then, that transpiration in plants— vital as it is to the functioning of our atmosphere—is actually an unavoidable consequence of photosynthesis, an unrelated process. (See Carbohydrates for more about photosynthesis.) A N I M A L T RA N S P I RAT I O N . Transpiration in animals (including humans) takes place for much the same reason as it does with plants: as a by-product of breathing. Animals have to keep their moist respiratory surfaces, such as the lungs, open to the atmosphere. We may not think of our own breathing as transferring moisture to the air, but the presence of moisture in our lungs can be proved simply by breathing on a piece of glass and observing the misty cloud that lingers there.

Transpiration can cause animals to become dehydrated, but it also can be important in cooling down their bodies. When human bodies

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KEY TERMS A HORIZON:

Topsoil, the uppermost

bedrock. The C horizon is made of

of the three major soil horizons. This layer

regolith, or weathered rock.

and the humus that lies above it house all

CANOPY:

the organic content in soil.

of the trees in a forest. A forest with a

AEROBIC:

closed canopy is one so dense with vegeta-

Oxygen-breathing.

ANAEROBIC: ATMOSPHERE:

Non-oxygen-breathing.

tion that the sky is not visible from the ground.

Earth’s atmosphere is

a blanket of gases that includes nitrogen (78%), oxygen (21%), argon (0.93%), and a combination of water vapor, carbon dioxide, ozone, and noble gases such as neon (0.07%). Most of these gases are contained in the troposphere, the lowest layer, which extends to about 10 mi. (16 km)

CLIMATE:

The pattern of weather con-

ditions in a particular region over an extended period. Compare with weather. CLOSED SYSTEM:

mits the exchange of energy with its external environment but does not allow matter to pass between the environment and the the one hand and open system on the other.

Subsoil, beneath topsoil

and above the C horizon. Though the B horizon contains no organic material, its presence is critical if the soil is to be suit-

COMPOUND:

A substance made up of

atoms, chemically bonded to one another, of more than one chemical element. DECOMPOSERS:

able for sustaining a varied ecosystem. BIOGEOCHEMICAL CYCLES:

A system that per-

system. Compare with isolated system on

above the planet’s surface. B HORIZON:

The upper portion or layer

The

changes that particular elements undergo as they pass back and forth through the various earth systems (e.g., the biosphere)

Organisms

obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. The principal forms of decomposer are bacteria and fungi.

and particularly between living and nonliving matter. The elements involved in biogeochemical cycles are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur. BIOSPHERE:

DECOMPOSITION

REACTION:

A

chemical reaction in which a compound is broken down into simpler compounds, or into its constituent elements. In the biosphere, this often is achieved through the

A combination of all liv-

help of detritivores and decomposers.

ing things on Earth—plants, animals, birds, marine life, insects, viruses, singlecell organisms, and so on—as well as all formerly living things that have not yet decomposed.

DETRITIVORES:

Organisms that feed

on waste matter, breaking organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Their

The bottommost of the

function is similar to that of decomposers;

soil horizons, between subsoil and

however, unlike decomposers—which tend

C HORIZON:

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KEY TERMS

CONTINUED

to be bacteria or fungi—detritivores are

it along to other organisms. Earth scientists

relatively complex organisms, such as

typically prefer this name to food chain, an

earthworms or maggots.

everyday term for a similar phenomenon.

The study of the relation-

A food chain is a series of singular organ-

ships between organisms and their envi-

isms in which each plant or animal

ronments.

depends on the organism that precedes it.

ECOLOGY:

ECOSYSTEM:

A community of inter-

dependent organisms along with the inorganic components of their environment. ELEMENT:

A substance made up of

only one kind of atom. Unlike compounds, elements cannot be broken down chemi-

A state of height-

ened biological productivity in a body of

The upper part of

GEOSPHERE:

Earth’s continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources. HUMUS:

cally into other substances. EUTROPHICATION:

Food chains rarely exist in nature.

Unincorporated, often par-

tially decomposed plant residue that lies at the top of soil and eventually will decay fully to become part of it.

water, which is typically detrimental to the ecosystem in which it takes place. Eutrophication can be caused by an excess of nitrogen or phosphorus, in the form of nitrates and phosphates, respectively. EVAPORATION:

The process whereby

liquid water is converted into a gaseous state and transported to the atmosphere.

HYDROCARBON:

Any organic chemi-

cal compound whose molecules are made up of nothing but carbon and hydrogen atoms. HYDROLOGIC CYCLE:

The continu-

ous circulation of water throughout Earth and between various earth systems. The entirety of

When discussing the atmosphere and pre-

HYDROSPHERE:

cipitation, usually evaporation is distin-

Earth’s water, excluding water vapor in the

guished from transpiration. In this context,

atmosphere but including all oceans, lakes,

evaporation refers solely to the transfer of

streams, groundwater, snow, and ice.

water from nonliving sources, such as the

ISOLATED SYSTEM:

soil or the surface of a lake.

separated so fully from the rest of the uni-

A system that is

The loss of

verse that it exchanges neither matter nor

water to the atmosphere via the combined

energy with its environment. This is an

(and related) processes of evaporation and

imaginary construct, since full isolation is

transpiration.

impossible.

EVAPOTRANSPIRATION:

FOOD WEB:

A term describing the

LEACHING:

The removal of soil mate-

interaction of plants, herbivores, carni-

rials that are in solution, or dissolved in

vores, omnivores, decomposers, and detri-

water.

tivores in an ecosystem. Each of these

MINERAL:

organisms consumes nutrients and passes

cally inorganic substance with a specific

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A naturally occurring, typi-

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KEY TERMS chemical composition and a crystalline structure. A crystalline structure is one in

A disease-carrying parasite, usually a microorganism.

which the constituent parts have a simple

PHOTOSYNTHESIS:

and definite geometric arrangement that is repeated in all directions. OPEN SYSTEM:

A system that allows

PATHOGEN:

The biological conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants.

complete, or near complete, exchange of

SOLUBLE:

matter and energy with its environment.

SYSTEM:

ORGANIC:

At one time, chemists used

the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals) and oxides, such as carbon dioxide.

become overheated, they produce perspiration, which cools the surface of the skin somewhat. If the air around us is too humid, however, then it already is largely saturated with water, and the perspiration has no place to evaporate. Therefore, instead of continuing to cool our bodies, the perspiration simply forms a sticky film on the skin. But assuming the air is capable of absorbing more moisture, the sweat will evaporate, cooling our bodies considerably. EFFECTS OF H E AT AND C O L D . When summer is at its height, air tem-

peratures are warm and the trees are fully foliated (i.e., covered in leaves), and a high rate of transpiration occurs. So much water is pumped into the atmosphere through foliage that the rate of evapotranspiration typically exceeds the input of water to the local environment through rainfall. The result is that soil becomes dry, some streams cease to flow, and by late summer in extremely warm temperate areas, such as the southern United States, there is a great threat of drought and related problems, such as forest fires. As trees drop their leaves in the autumn, transpiration rates decrease greatly. This makes it possible for the parched soil to become recharged by rainfall and for streams to flow again. Such is

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Capable of being dissolved.

Any set of interactions that can be set apart mentally from the rest of the universe for the purposes of study, observation, and measurement.

The condition of the atmosphere at a given time and place. Compare with climate. WEATHER:

the case in a temperate region, which, by definition, is one that has the four seasons to which most people in the United States (outside Hawaii, Alaska, and extreme southern Florida and Texas) are accustomed. In a tropical region, by contrast, there is a “dry season,” in which transpiration takes place, and a “rainy season,” in which moisture from the atmosphere inundates the solid earth. This rainy season may be so intense that it produces floods. WHERE TO LEARN MORE Bial, Raymond. A Handful of Dirt. New York: Walker, 2000. Biogeochemical Cycles (Web site). . Bocknek, Jonathan. The Science of Soil. Milwaukee, WI: Gareth Stevens, 1999. Global Hydrology and Climate Center (Web site). . Hancock, Paul L., and Brian J. Skinner. The Oxford Companion to the Earth. New York: Oxford University Press, 2000. Kump, Lee R., James F. Kasting, and Robert G. Crane. The Earth System. Upper Saddle River, NJ: Prentice Hall, 2000. Life and Biogeochemical Cycles (Web site). .

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Richardson, Joy. The Water Cycle. Illus. Linda Costello. New York: Franklin Watts, 1992. Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth Sys-

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tem Science. 2d ed. New York: John Wiley and Sons, 1999.

The Biosphere

Smith, David. The Water Cycle. Illus. John Yates. New York: Thomson Learning, 1993.

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ECOSYSTEMS AND ECOLOGY Ecosystems and Ecology

CONCEPT Composed of living organisms and the remains of living things as well as the nonliving materials in their surroundings, an ecosystem is a complete community. Its components include plants, animals, and microorganisms, both living and dead; soil, rocks, and minerals; water sources above and below ground; and the local atmosphere. An ecosystem can consist of an entire rain forest, geographically larger than many nations, or it can be as small as the body of an animal, which is likely to comprise far more microorganisms than there are people on Earth. Among the most significant areas of interest in the realm of ecosystems is ecology, or the study of the relationships between organisms and their environments. Forests, a broad category of ecosystem, provide a living laboratory in which to investigate the ways in which organisms interact with their environments. They also aptly illustrate the stresses placed on ecosystems by human activities.

The biosphere consists of all living things— microorganisms, plants, insects, birds, marine life, and all other forms of animals—as well as formerly living things that have not yet decomposed. (Decomposed remains of organic materials become part of the geosphere, specifically the soil.) Organisms in the biosphere, whether living or formerly living, are united by the interrelation of energy transfer that takes place through the food web. A food web is similar to the more well known expression food chain. In scientific terms, however, a food chain is defined as a series of singular organisms in which each plant or animal depends on the organism that precedes or follows it. This rarely exists in nature; instead, the feeding relationships between organisms in the real world are much more complex and are best described as a web rather than a chain. (For more on the biosphere and earth systems, see The Biosphere.)

Energy Flow and Nutrients

HOW IT WORKS The Biosphere and Food Webs In the sciences, a system is any set of interactions that can be set apart mentally from the rest of the universe for the purposes of study, observation, and measurement. Modern earth scientists regard the planet as a massive ecosystem, the stage on which four extraordinarily complex earth systems interact. These systems are the atmosphere, the hydrosphere (all the planet’s waters, except for moisture in the atmosphere), the geosphere (the soil and the extreme upper portion of the continental crust), and the biosphere.

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Energy is the ability of objects or systems to accomplish work. (The latter is defined as the exertion of force over a given distance: for example, a plant growing from the ground, an insect or bird flying, or a human or pack animal moving an object.) Food webs are built around energy transfer, or the flow of energy between organisms, which begins with plant life. Hence the importance of plants to ecosystems, as we illustrate later in discussing various types of forest, which are defined by their dominant varieties of tree. Plants absorb energy in two ways. From the Sun they receive electromagnetic energy in the form of visible light and invisible infrared waves, which they convert to chemical energy by means

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of photosynthesis. In addition, plants take in nutrients from the soil, which contains energy in the forms of various chemical compounds. These compounds may be organic, which typically means that they came from living things, though, strictly speaking, organic refers to characteristic carbon-based chemical structures. Plants also receive inorganic compounds from minerals in the soil. (See Paleontology for an explanation of the scientific distinction between organic and inorganic.) Contained in these minerals are six chemical elements essential to the sustenance of life on planet Earth: hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur. These are the elements involved in biogeochemical cycles, through which they are circulated continually between the living and nonliving worlds—that is, between organisms on the one hand and the inorganic realms of rocks, minerals, water, and air on the other. (See The Biosphere for more about biogeochemical cycles.) FROM PLANTS TO M E AT E AT E R S . As plants take up nutrients from

the soil, they convert them into other forms. Eventually, the plants themselves become food either for herbivores (plant-eating organisms) or for omnivores (organisms that eat both plants and other animals), thus passing along these usable, energy-containing chemical compounds to other participants in the food web. It is likely that an herbivore will be eaten in turn either by an omnivore (for example, humans as well as a number of bird species and many others) or by a carnivore, an organism that eats only meat. Carnivores and omnivores are not usually prey for other carnivores or omnivores, but this does happen in the case of what are known as tertiary consumers (see Food Webs). There is also the matter of cannibalism, discussed in Biological Communities. For the most part, however, the only creatures that eat carnivores and omnivores do so after these organisms have died or been killed.

Nonetheless, detritivores are relatively large, complex organisms compared with another variety of species that occupies a level or position in the food web that comes “after” carnivores and omnivores: decomposers, including bacteria and fungi.

Ecosystems and Ecology

This illustrates the reason why ecological sciences treat the expression food chain with disfavor. There is no such thing as “the top of the food chain,” rather, there are simply stages, like a circular assembly line, with detritivores occupying a position between meat-eating animals and plants. Earthworms, in particular, help convert animal bodies into soil nutrients useful to the growth of plant life, yet even these and other detritivores must themselves eventually be converted to soil as well. This “final” stage of conversion—that is, the last stop after the animal portion of the food web and before the cycle comes back around to ordinary plants—is occupied by decomposers, such as bacteria and fungi. These decomposers obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. Like detritivores, they aid in decomposition, a chemical reaction in which a compound is broken down into simpler compounds, or into its constituent elements. Often an element such as nitrogen appears in forms that are not readily usable to organisms, and therefore such elements (which may appear individually or in compounds) need to be processed chemically through the body of a decomposer or detritivore. This process brings about a chemical reaction in which the substance (whether an element or compound) is transformed into a more usable version. By processing these chemical compounds, decomposers and detritivores provide nutrients necessary to plant growth.

REAL-LIFE A P P L I C AT I O N S

DETRITIVORES AND DECOMP O S E R S . An animal that obtains its energy

Forests and Ecology

in this way, from consuming the carcasses of carnivores and omnivores (as well as herbivores and perhaps even plants), is known as a detritivore. Large and notable examples include vultures and hyenas, though most detritivores—earthworms or maggots, for instance—are much smaller.

One easily understandable example of ecosystems and ecology in action is the forest. Virtually everyone has visited a forest at one time or another, and those who are enthusiasts for the great outdoors may spend a great deal of time in one. In the past, of course, people interacted with

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A

VARIETY OF ANGIOSPERM, MANGROVE TREES ARE FOUND IN LOW-LYING, MUDDY REGIONS NEAR SALTWATER, WHERE

THE CLIMATE IS HUMID.

A

MANGROVE FOREST IS POOR IN SPECIES: ONLY ORGANISMS THAT CAN TOLERATE FLOOD-

ING AND HIGH SALT LEVELS ARE CAPABLE OF SURVIVING. (© Wolfgang Kaehler/Corbis. Reproduced by permission.)

forests not so much out of choice, and certainly not with recreation as the foremost aim in mind, but simply because they depended on the forest for survival. Not only did the forest provide hunters and food-gatherers with an abundance of wildlife and fruit, but trees provided material for building dwellings. It is no wonder, then, that many early human settlements tended to be in, or at the edges of, forests. A forest is simply an ecosystem dominated by trees. There are many varieties of forest, however, because so many factors go into determining the character of a forest ecosystem. The fact that the forest is an ecosystem means that its qualities are defined by far more than just the varieties of trees, which are simply the most visible among many biological forms in the forest. Numerous abiotic, or nonbiological, factors also affect the characteristics of a forest as well. For instance, there is weather, defined as the condition of the atmosphere at a given time and place, and climate, the overall patterns of weather for extended periods.

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species of vegetation do not all shed at the same time, the rain forest canopy—the upper layer of trees in the forest—remains rich in foliage yearround. Hence, the tropical rain forest is an example of an evergreen forest. Climate can determine the type of life forms capable of surviving in the forest ecosystem. This can be illustrated by referring to a forest almost perfectly opposite in character to a rain forest: the taiga, or boreal forest, that spans much of northern Eurasia. The taiga is a deciduous forest, meaning that its trees shed their leaves seasonally; indeed, because of the very cold climate in taiga regions, where the temperature during winter is usually well below the freezing point, trees spend a great portion of the year bare. Rainfall is much, much lower than in a rain forest, of course: only about 10-20 in. (250–500 mm) per year, as compared with more than 70 in. (1,800 mm) for a typical rain forest. The dry, inhospitable climate of the taiga makes it a forbidding place for reptiles and amphibians, though the taiga is home to many endothermic (warm-blooded) creatures such as mammals or birds.

These play a clear role, for instance, in defining the tropical rain forest, a place where constant rainfall ensures that there are always plenty of plants in flower. Because the trees and other

L AT I T U D E , A LT I T U D E , AND F O R E S T S . Elevation or relief—that is,

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height above sea level—also determines the char-

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acter of a forest, as do latitude (distance north or south of the equator) and topography, or the overall physical configuration of Earth’s surface in a given area. Rain forests can exist anywhere, but by definition a tropical rain forest, such as those along the Amazon River in South America or the Congo River in Africa, must lie between the Tropic of Cancer in the north and the Tropic of Capricorn in the south. Naturally, in the tropical rain forest, temperatures are high—typically, about 86°F (30°C) during the day, cooling down to about 68°F (20°C) at night. By contrast, there are much cooler rain forests in the temperate zones. An example is the Cherokee National Forest on the border between North Carolina and Tennessee, which, though located in the southeastern United States, is chilly even in the summer months. Just as latitude affects a forest, so does altitude. Rain forests at relatively high elevations, such as the highlands of New Guinea, are known as montane forests. These forests, though they may be located at the same latitude as tropical rain forests—most montane forests are in eastern Brazil, southeastern Africa, northern Australia, and parts of southeast Asia—are much cooler. Lush by comparison to most non-rain forests, their vegetation is nonetheless much less dense than in a typical tropical rain forest. In addition to its role in defining the overall character of the forest, differences in relative altitude or elevation resulting from the great height of trees in the rain forest also influence the formation of differing biological communities. For example, monkeys, flying squirrels, and other animals capable of swinging, gliding, or otherwise moving from tree to tree inhabit the canopy, which is rich in well-watered leaves and other food sources. These top-dwellers seldom even need to come down to the ground for anything. The rain forest floor, by contrast, is mostly bare, since the trees above shade it. On this level live creatures such as chimpanzees and gorillas, who feed off of low-lying plant forms. Other biological communities exist above or below the forest floor. (For more on these subjects, including the various types of forests, see Biomes. See The Biosphere for a discussion of soil quality in the rain forest.)

modern times, a growing awareness of ecology, and of the distance that technology has placed between modern society and the forests, led to the movement for the establishment of national parks in general, and of national forests in particular. The first of these preserves—places where commercial development is forbidden and commercial activity is limited—was Yellowstone National Park in Wyoming, established by the administration of President Ulysses S. Grant in 1872. Though Yellowstone contains enormous areas of forest land, the first national forest reserve (as national forests were called at the time) was Sequoia National Park, established in 1891. Home to some of the largest, most aweinspiring trees in the world, Sequoia is part of a group of national forests and parks to the northeast of Bakersfield, California. The United States Forest Service was actually founded earlier (1905) than the National Park Service (1916), a fact that illustrates the importance of pristine forests to maintaining a proper balance between humans and their environment. Since the establishment of the forest service under the aegis of the U.S. Department of Agriculture, the lands controlled by the forest service have grown to encompass about 191 million acres (77.3 million hectares), an area larger than Texas. During the same period, the U.S. example of national parks and forests has inspired nations around the world to create their own preserves.

we noted the fact that humans’ early history kept them, like other primates, close to the forest. In

Coupled with the rise of national parks and forests at the turn of the nineteenth century was a growing interest in conservation and management of environmental resources. This interest manifested across a broad spectrum, from environmentalists who urged that the forests be left in their original state to industrial foresters who view the forest as a resource that can be utilized. Both sides have their merits, and both have their complaints about the other. The close historical ties between conservationism and the science of forestry (the management of forest ecosystems for purposes such as harvesting timber), both of which had their origins during the nineteenth century, suggest that there is no inherent reason that the two sides should be in conflict. If anything, responsible forestry goes hand-in-hand with an attitude of conserving as many resources as is feasible.

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H U M A N S A N D F O R E S T S . Earlier

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Flowering plants evolved only about 130 million years ago, by which time gymnosperms (of which modern pines are an example) had long since evolved and proliferated. Yet in a relatively short period of time, angiosperms have become the dominant plants in the world today. About 80% of all living plant species are flowering plants, and based on the record of angiosperms and gymnosperms heretofore, it is likely that the world of 100 million years from now will be one in which the forests are typified by angiosperms. Gymnosperms, meanwhile, may well become a dying, if not a dead, breed.

Ecosystems and Ecology

POLLINATION BY GYMNOSPERMS.

BY THE

EVOLVING BRIGHT COLORS, SCENTS, AND NECTAR, FLOWERS

OF

ANGIOSPERMS

ATTRACT

ANIMALS,

WHICH TRAVEL FROM ONE FLOWER TO ANOTHER, MOVING POLLEN AS THEY GO.

WHEREAS

INSECTS AND ANIMALS

POSE A THREAT TO GYMNOSPERMS, ANGIOSPERMS PUT BEES,

BUTTERFLIES,

HUMMINGBIRDS,

AND

OTHER

FLOWER-SEEKING CREATURES TO WORK ASSISTING THEIR REPRODUCTIVE PROCESS. (© Gallo Images/Corbis. Reproduced

by permission.)

Comparing Angiosperms and Gymnosperms Several times we have referred to angiosperms, a name that encompasses not just certain types of tree but also all plants that produce flowers during sexual reproduction. The name, which comes from Latin roots meaning “vessel seed,” is a reference to the fact that the plant keeps its seeds in a vessel whose name, the ovary, emphasizes the sexual quality of the reproductive process it undergoes.

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Gymnosperms reproduce sexually as well, but they do so by a much less efficient method than that of the angiosperm. Whereas the angiosperm keeps its seeds safely tucked away inside the ovary and coexists with its ecosystem most favorably by putting the insect and animal life to work, gymnosperm reproduction is an altogether less effective—and, indeed, less pleasant—process. For starters, gymnosperms produce their seeds on the surface of leaflike structures, and this makes the seeds vulnerable to physical damage and drying as the wind whips the trees’ branches back and forth. Furthermore, insects and other animals view gymnosperm seeds as a source of nutrition, as indeed they are. And in contrast to the angiosperm, which attracts bees and other creatures to it, gymnosperms package the male reproductive component in tiny pollen grains, which it releases into the wind.

Angiosperms are a beautiful example of how a particular group of organisms can adapt to specific ecosystems and do so in a highly efficient manner, such that the evolutionary future heralds only greater dominance for these species. This is all the more interesting in light of the contrast between the success of the angiosperm and the rather less impressive results achieved by another broad category of sexually reproducing plant, one that formerly dominated Earth’s forest: the gymnosperm.

Eventually, the grains make their way toward the female component of another individual within the same species, but the fact that they do is an example of the wonder inherent in life itself and not of the efficiency of gymnosperm reproduction. Gymnosperms shower their ecosystems with pollen, a fact familiar to anyone who lives in a place with a high gymnosperm population— and hence a high pollen count in the spring. In gymnosperm-heavy environments, yellowish dust forms on everything, and where humans interact with the natural world, this can create a great deal of discomfort in the form of hay fever and allergies. Meanwhile, cars, windowsills, mailboxes, and virtually every other available surface takes on a yellow film that usually is not relieved until a good rain falls or, more likely, pollination ends for the year.

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Though pollen is unpleasant to humans, it should be noted that like all natural mechanisms, it benefits the overall ecosystem. Packed with energy, pollen grains contain large quantities of nitrogen, making them a major boost to the nutrient content in the soil. But it costs the gymnosperm a great deal, in terms of chemical and biological energy and material, to produce pollen grains, and the benefits are uncertain.

Deforestation

POLLINATION BY ANGIOSPERMS.

Returning to the subject of forests in general, if a forest experiences significant disturbance, it may undergo deforestation. Despite the finality in the sound of the word, deforestation does not necessarily imply complete destruction of the forest. In fact, deforestation can describe any interruption in the ordinary progression of a forest’s life, including clear-cut harvesting—even if the forest fully recovers.

If the gymnosperm and angiosperm varieties of pollination were compared to marketing campaigns, gymnosperm reproduction would involve the client (i.e., the gymnosperms themselves) investing maximum capital for minimal returns. In a very real sense, gymnosperm pollination is like the marketing of a company that bombards a neighborhood with leaflets, such that advertising rapidly becomes another form of trash simply to be swept up and thrown away.

Deforestation can occur naturally, as a result of changes in the soil and climate, but the most significant cases of deforestation over the past few thousand years have been the consequence of human activities. Usually deforestation is driven by the need to clear land to harvest trees for fuel or, in some cases, to obtain building materials in the form of lumber. Though deforestation has been a problem the world over, since the 1970s it has become an issue primarily in developing countries.

By contrast, the “marketing” of angiosperms is like that of a company that uses carefully targeted, researched advertising, utilizing as many free means as possible for getting out word about itself. Just as a smart marketer sets in place the conditions to get consumers talking about a product—thus using advertising that is both free and extremely effective—the angiosperm enlists the aid of mobile organisms in its environment.

In developed nations such as the United States, environmental activism has raised public awareness concerning deforestation and led to curtailment of large-scale cutting in forests that are deemed important environmental habitats. By contrast, developing nations, such as Brazil, are cutting down their forests at an alarming rate. Generally, economics is the dominant factor, with the need for new agricultural land or the desire to obtain wood and other materials typically driving the deforestation process.

In addition, the angiosperm puts a great deal of its energy into producing reproductive structures, an effort that pays off bountifully. By evolving bright colors, scents, and nectar, the flowers of angiosperms attract animals, which travel from one flower to another, unintentionally moving pollen as they go. Thus, whereas insects and animals pose a threat to gymnosperms, angiosperms actually put bees, butterflies, hummingbirds, and other flower-seeking creatures to work assisting their reproductive process. Because of this remarkably efficient system, animal-pollinated species of flowering plants do not need to produce as much pollen as gymnosperms. They can put their resources into other important functions instead, such as growth and greater seed production. In this way, the angiosperm solves its own problem of reproduction—and, as a side benefit, adds enormously to the world’s beauty.

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Ecosystems and Ecology

CONSEQUENCES OF DEFORE S TAT I O N . The deforestation of valuable

reserves such as the Amazon rain forest is an environmental disaster in the making. As discussed in the essay The Biosphere, the soil in rain forests as a rule is “old,” and leached of nutrients. Without the constant reintroduction of organic material from the plants and animals of the rain forests, it would be too poor to grow anything. Therefore, when nations cut down their own rain forest lands, they are in effect killing the golden goose to get at the egg: once the rain forest is gone, the land itself is worthless. Deforestation has several other extremely serious consequences. From a biological standpoint, it greatly reduces biodiversity, or the range of species in the biota. In the case of tropical rain forests as well as old-growth forests (see Biological Communities), certain species cannot survive once the environmental structure has been rup-

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Ecosystems and Ecology

A

LONE TREE TRUNK STANDS IN AN AREA OF DEFORESTED GRASSLAND IN

OF VALUABLE RESERVES SUCH AS THE

AMAZON

MARANHÃO, BRAZIL. THE

DEFORESTATION

RAIN FOREST IS AN ENVIRONMENTAL DISASTER IN THE MAKING,

DEPLETING AND STARVING THE SOIL, REDUCING BIODIVERSITY, AND BRINGING ABOUT DANGEROUS CHANGES IN ATMOSPHERIC CARBON CONTENT. (© Barnabas Bosshart/Corbis. Reproduced by permission.)

tured. From an environmental perspective, it leads to dangerous changes in the carbon content of the atmosphere, discussed later in this essay. In the case of old-growth forests or rain forests, deforestation removes an irreplaceable environmental asset that contributes to the planet’s biodiversity—and to its oxygen supply. Even from a human standpoint, deforestation takes an enormous toll. Economically, it depletes valuable forest resources. Furthermore, deforestation in many developing countries often is accompanied by the displacement of indigenous peoples, while still other political and social horrors may lurk in the shadows. For example, Brazil’s forests are home to charcoal factories that amount to virtual slave-labor camps. Aboriginal peoples (i.e., “Indians”) are lured from cities with promises of high income and benefits, only to arrive and find that the situation is quite different from what was advertised. Having paid the potential employer for transportation to the work site, however, they are unable to afford a return ticket and must labor to repay the cost.

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The Greenhouse Effect The most potentially serious aspect of cutting down forests may well be the greenhouse effect, which some scientists and activists believe is causing an overall warming of the planet. Today, thanks to the popularity of environmental causes among entertainment figures and on college campuses, terms such as “the greenhouse effect” and “global warming” are commonplace. However, these phrases are used so frequently, and sometimes so confusingly or misleadingly, that it is worthwhile to address their meaning briefly; then, we can conclude our discussion by looking at what impact the steady reduction in forest lands has had on the increasing release of greenhouse gases. The greenhouse effect itself is not a consequence of any action on the part of human beings; rather, it is a part of life on Earth. In fact, without it, there could be no life on Earth. Though the planet receives an incredible amount of energy from the Sun, much of it is lost by being absorbed or reflected in the atmosphere or on the surface. So-called greenhouses gases such as carbon dioxide, however, help to trap this energy, keeping much more of the Sun’s warmth

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Ecosystems and Ecology

KEY TERMS A type of plant that

ANGIOSPERM:

produces flowers during sexual reproduction.

principal forms of decomposer are bacteria and fungi. DECOMPOSITION

REACTION:

A

Earth’s atmosphere is

chemical reaction in which a compound is

a blanket of gases that includes nitrogen

broken down into simpler compounds or

(78%), oxygen (21%), argon (0.93%), and

into its constituent elements. In the earth

a combination of water vapor, carbon

system, this often is achieved through the

dioxide, ozone, and noble gases such as

help of detritivores and decomposers.

ATMOSPHERE:

neon (0.07%). Most of these gases are contained in the troposphere, the lowest layer, which extends to about 10 mi. (16 km)

interruption in the ordinary progression of a forest’s life.

above the planet’s surface.

Organisms that feed

DETRITIVORES:

The degree of variety

BIODIVERSITY:

among the species represented in a particular ecosystem.

A term for any

DEFORESTATION:

on waste matter, breaking down organic material into inorganic substances that then can become available to the biosphere

BIOLOGICAL COMMUNITY:

The liv-

ing components of an ecosystem.

in the form of nutrients for plants. Their function is similar to that of decomposers;

The upper portion or layer

however, unlike decomposers—which tend

of the trees in a forest. A forest with a

to be bacteria or fungi—detritivores are

closed canopy is one so dense with vegeta-

relatively complex organisms, such as

tion that the sky is not visible from the

earthworms or maggots.

ground.

ECOLOGY:

CANOPY:

CARNIVORE:

A meat-eating organ-

The study of the relation-

ships between organisms and their envi-

ism, or an organism that eats only meat (as

ronments.

distinguished from an omnivore).

ECOSYSTEM:

COMPOUND:

A substance made up of

A community of inter-

dependent organisms along with the inor-

atoms, chemically bonded to one another,

ganic components of their environment.

of more than one chemical element.

ELEMENT:

CONIFER:

A type of tree that produces

only one kind of atom. Unlike compounds, elements cannot be broken chemically into

cones bearing seeds. DECIDUOUS:

A substance made up of

A term for a tree or

other substances. The ability of an object (or

other form of vegetation that sheds its

ENERGY:

leaves seasonally.

in some cases a nonobject, such as a mag-

DECOMPOSERS:

Organisms

that

netic force field) to accomplish work. The total amount

obtain their energy from the chemical

ENERGY BUDGET:

breakdown of dead organisms as well as

of energy available to a system or, more

from animal and plant waste products. The

specifically, the difference between the

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Ecosystems and Ecology

KEY TERMS energy flowing into the system and the energy lost by it.

vapor, in the atmosphere. These gases are

The flow of energy between organisms in a food web.

an even longer wavelength to space. (Wave-

ENERGY TRANSFER:

A term describing the interaction of plants, herbivores, carnivores, omnivores, decomposers, and detritivores in an ecosystem. Each of them consumes nutrients and passes it along to other organisms. Earth scientists typically prefer this name to food chain, an everyday term for a similar phenomenon. A food chain is a series of singular organisms in which each plant or animal depends on the organism that precedes it. Food chains rarely exist in nature.

FOOD WEB:

In general terms, a forest is simply any ecosystem dominated by treesize woody plants. A number of other characteristics and parameters (for example, weather, altitude, and dominant species) further define types of forests, such as tropical rain forests. FOREST:

Warming of the lower atmosphere and surface of Earth. This occurs because of the absorption of long-wavelength radiation from the planet’s surface by certain radiatively active gases, such as carbon dioxide and water GREENHOUSE EFFECT:

within Earth’s atmosphere, much as a greenhouse helps trap heat. Without the greenhouse effect, Earth would be so cold that the oceans would freeze.

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CONTINUED

heated and ultimately re-radiate energy at length and energy levels are related inversely; hence, the longer the wavelength, the less the energy.) GYMNOSPERM:

A type of plant that

reproduces sexually through the use of seeds that are exposed, not hidden in an ovary as with an angiosperm. HERBIVORE:

A plant-eating organ-

ism. An organism that eats

OMNIVORE:

both plants and other animals. ORGANIC:

At one time chemists used

the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals) and oxides, such as carbon dioxide. PHOTOSYNTHESIS:

The biological

conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants. SYSTEM:

Any set of interactions that

can be set apart mentally from the rest of the universe for the purposes of study, observation, and measurement.

Obviously, then, the greenhouse effect is a good thing—but only if greenhouse gases are kept at certain levels. Earth, after all, is not the only planet in the solar system that experiences a greenhouse effect; there is also Venus, a hellish place where surface temperatures are as high as

932°F (500°C). To many environmentalists, there is a grave danger that Earth could be slowly going the way of Venus, building up greenhouse gases such that the temperature is slowly increasing. This is the phenomenon of global warming, which threatens to melt the polar ice caps and submerge much of Earth’s land surface. At least, that is the opinion of environmentalists and others who subscribe to the idea that Earth is steadily warming as a result of human pollution and industrial activity.

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There is a considerable body of scientific knowledge that challenges the environmentalist position on global warming and the greenhouse effect, but it is not our purpose here to judge the various positions. Rather, our concern is the link between forests and the increase of greenhouses gases in the atmosphere. Old-growth or mature forests of the type discussed in Succession and Climax contain vast amounts of carbon—the basis for all living things—and when these forests are cut down, that carbon has to go somewhere. Specifically, carbon, in the form of carbon dioxide, will be released into the atmosphere, increasing the amount of greenhouse gases there. This release may occur quickly, as when wood is burned, or more slowly, if the timber from the forest is used over long periods of time—for instance, in the building of houses or other structures. Statistics suggest an alarming change in the amount of carbon in the forests as compared with that in the atmosphere: since about 1850, the amount of carbon stored in forests had dropped by about one-third, while the amount of carbon dioxide in the atmosphere has increased by a comparable factor. Thus, the effort to keep greenhouse gases at viable levels is inextricably tied to the movement to preserve forest ecosystems.

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WHERE TO LEARN MORE Ashworth, William, and Charles E. Little. Encyclopedia of Environmental Studies. New York: Facts on File, 2001.

Ecosystems and Ecology

Diamond, Jared M. Guns, Germs, and Steel: The Fates of Human Societies. New York: W. W. Norton, 1997. The Ecological Society of America: Issues in Ecology (Web site). . The Ecosystems Center, Marine Biological Laboratory, Woods Hole, Massachusetts (Web site). . Living Things: Habitats and Ecosystems (Web site). . Markley, O. W., and Walter R. McCuan. 21st Century Earth: Opposing Viewpoints. San Diego, CA: Greenhaven Press, 1996. Martin, Patricia A. Woods and Forests. Illus. Bob Italiano and Stephen Savage. New York: Franklin Watts, 2000. Nebel, Bernard J., and Richard T. Wright. Environmental Science: The Way the World Works. Upper Saddle River, NJ: Prentice Hall, 2000. Philander, S. George. Is The Temperature Rising?: The Uncertain Science of Global Warming. Princeton, NJ: Princeton University Press, 1998. Rybolt, Thomas R., and Robert C. Mebane. Environmental Experiments About Life. Hillside, NJ: Enslow Publishers, 1993. The State of the Nation’s Ecosystems (Web site). . Sustainable Ecosystems Institute (Web site). .

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BIOMES Biomes

CONCEPT On a political map of the world, Earth is divided into countries, of which there are almost 200. But nature, of course, knows no national boundaries, and therefore the natural divisions of the planet are quite different from those agreed upon by humans. While continents are a useful concept to geographers and earth scientists, in the worlds of biology, ecology, and biogeography, the concept of a biome makes much more sense. There are more than a dozen basic terrestrial and aquatic biomes or ecosystems, including boreal coniferous forests, deserts, tundra, and underwater environments. Each is a distinct “world” unto itself, with characteristic forms of plant life as well as animal species that congregate around the plants for food or shelter or both. Combined with these features of the biological community are aspects of the inorganic realm that likewise define a biome, for instance, climate and the availability of water.

HOW IT WORKS Ecosystems, Biomes, and Biological Communities

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types is divided further into marine and freshwater biomes. Within a biome or ecosystem, the sum of all living organisms is referred to as the biological community. Sometimes the term biota, which refers to all flora and fauna (plants and animals) in a region, is used instead. Thus, biological community is a larger concept, since it includes microorganisms, which are vital to the functioning of the food web. The food web, which may be thought of as an interconnected network of food chains, is the means by which energy is transferred through a biological community. Without microorganisms known as decomposers, a key link in the food web would be missing. (See Food Webs for more on this subject.) SUCCESSION

AND

C L I M AX .

Over the course of time, ecosystems experience a process known as succession, the progressive replacement of one biological community by another. This is rather like the series of changes one might witness if one were to record the activity on a major city block over the space of a few decades, as stores come in and shut down and buildings are erected and demolished. In the case of biological succession, a process akin to natural selection (see Evolution) is occurring: the ecosystem becomes home, in turn, to a number of different biological communities until (in the absence of outside interference) the one that is most suited or adapted to local conditions finally takes root. (That is, until it is replaced, and the process of succession continues.)

An ecosystem is a community of interdependent organisms along with the inorganic components of their environment, including water, soil, and air. Earth is the largest ecosystem, divided into biomes, large areas with similar climate and vegetation. A biome is a large ecosystem, extending over a wide geographic region, characterized by certain dominant life-forms—most notably, trees or the lack thereof. There are two basic varieties of biome: terrestrial, or land-based (of which there are six), and aquatic. The second of these

This most suited or adapted biological community is described as a climax community, one that has reached a stable point as a result of ongoing succession. In such a situation, the com-

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munity is at equilibrium with environmental conditions, and conditions are stable, such that the biota experiences little change thereafter. The most significant forms of climax vegetation are often the defining characteristics of terrestrial biomes. (See Succession and Climax for more about this subject.)

Defining Characteristics of a Biome The boreal coniferous forest often is cited by biogeographers as a classic example of a biome, for a number of reasons. First, like most other terrestrial biomes, this one is defined by specific latitudinal positions: the term boreal means “northern,” and these forests exist between 50 and 60 degrees north latitude. (Aside from the southernmost tip of South America and a few scattered islands, there is no significant landmass between 50 and 60 degrees south latitude.) In North America the region between 50 and 60 degrees north latitude is the southerly band of Canadian provinces (Alberta and Saskatchewan, for example). The Eurasian equivalent of this region is a band encompassing the British Isles; an area of continental Europe that includes northern Germany, Poland, and southern Sweden; and a vast swath that spans the width of Russia from Saint Petersburg and Moscow in the west across the nation’s wide expanse (10 time zones) to the Kamchatka peninsula north of Japan. The boreal coniferous forest thus illustrates a key fact about biomes: they can occur in widely separated geographic regions as long as the environmental conditions are the same. In each of these locales average temperatures are low; summers are short, moist, and of moderate warmth; and winters are long, cold, and dry. Most precipitation is in the form of snow, and the A horizon of the soil, home of the organic material in which plants grow, is thin. Moreover, the soil is acidic and poor in nutrients. (See The Biosphere for more about soil.) Most of the information conveyed in the preceding paragraph refers to the inorganic components of the boreal coniferous forest. (Organic does not necessarily mean “living,” but it does refer to carbon-based chemical compounds other than carbonates, which are rocks, and carbon oxides, such as carbon dioxide.) As noted earlier, inorganic components of a biome include water and air, which in turn are involved in pre-

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cipitation, weather, and climate. Although it does contain organic compounds from the decayed vegetable and animal matter that enriches it, soil, too, is largely inorganic, being formed from the weathering of rocks.

Biomes

F LO RA . Biomes are differentiated most clearly, however, on the basis of their organic components. The second term in the phrase boreal coniferous refers to a type of plant that produces cones containing seeds. Thus, the dominant plant life in the boreal coniferous forest includes evergreen conifers that can tolerate cold weather: pine, fir, and spruce.

The varieties that dominate may differ between geographic regions, however. The boreal coniferous forests of northeastern North America, for instance, are dominated by black spruce, while those in the northwest are characterized by stands of white spruce. In northeast Europe, Norway spruce is dominant, while species of pine and larch occupy the key positions in the forests of Siberia. Despite these differences in dominant species, the conditions are much the same, not only in terms of inorganic environment but also with regard to flora and fauna. In most boreal coniferous forests, the canopy or upper layer is so thick that it allows little light through. The result is that the understory, or lower layers of vegetation, is very limited. FAU N A . As for animal life, species in the boreal coniferous forest include bear, moose, wolf, lynx, deer, weasels, rabbits, beavers, and chipmunks. With a few local variations, this roster of animal life is typical in most such biomes, whether in British Columbia or western Europe or Siberia.

A biome constitutes a complex network of interactions among plants, animals, and their surroundings, such that certain animals depend, either directly or indirectly, on certain plants for their sustenance. An obvious example is the beaver’s use of coniferous tree limbs and even trunks for building shelter. Even more fundamental to the functioning of ecosystems is the role of plants as food. Although few animals actually feed off the needles or bark of conifers, they do eat from these trees in more indirect ways. The woodpecker, for instance, consumes bugs that live in a tree’s bark. Then there are the many insects that live off conifer seeds (see Ecosystems and Ecology for a discussion of conifer, or gymnosperm, reproduc-

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tion), and these bugs, in turn, serve as food for birds, which are the prey of larger carnivores. Furthermore, though the understory in boreal coniferous forests is not dense, it provides enough vegetation to meet the needs of deer, rabbits, and other herbivores.

Classifying Biomes Earlier it was stated that there are “almost” 200 countries on Earth. It might seem strange that something like the number of countries could be so inexact, when it would seem to be a matter of very exact quantities, like the number of states in the United States. But defining sovereign nations is a bit more challenging. Obviously, the United States, Switzerland, and Japan are sovereign nations, but many another political entity exists in a gray area. If the number of independent nations on Earth is so open to question, it would stand to reason that the number of basic biomes is as well. After all, nations typically are delineated by such things as borders, seats at the United Nations, currency, and so forth, whereas the boundaries between biomes are much less exact. Therefore, it would be futile to attempt to say exactly how many biomes there are on Earth, since the number varies according to interpretation. T E R R E S T R I A L , AQ UAT I C , A N D O T H E R CAT E G O R I E S . One of the more

useful methods for classifying biomes is that of the American ecologist Eugene Pleasants Odum (1913–), introduced in his Fundamentals of Ecology (1953). The classification scheme that follows is based on that of Odum, who divided biomes into terrestrial and aquatic. In the present context, biomes have been grouped into five categories: forest, nonforest, freshwater, marine, and anthropogenic. The last of these categories refers to biomes strongly influenced by humans and their activities, though it should be noted that to some degree at least, human activities have influenced all of Earth’s biomes. For example, many organisms carry in their fat cells trace amounts of human-manufactured contaminants, such as DDT. (See Food Webs for more on this subject.)

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rainfall patterns and other variables are considered. All climate zones, however, fall into one of three basic categories: tropical and subtropical, temperate, and polar and subpolar. The first of these categories is a term comprising the region along the equator, extending north and south by about 30 degrees in either direction. In North America this would include southern Florida, Texas, and Louisiana. Temperate zones reach from about 30 to 60 degrees on either side of the equator, thus taking in most of the United States and southern Canada. Finally, subpolar and polar regions lie between 60 degrees and the poles, which are at 90 degrees.

REAL-LIFE A P P L I C AT I O N S Forest Biomes The term forest, as used in the realm of ecology, is one of those rare words that means the same thing within a scientific context as it does in the everyday world. An ecologist or biogeographer would define forest in more or less the same way that a nonscientist would: as any ecosystem dominated by tree-size woody plants. Of course, numerous other characteristics and parameters, such as weather, altitude, and dominant species, further characterize types of forests. BOREAL CONIFEROUS FORESTS.

Starting with the most northerly of forest biomes, there is the boreal coniferous forest, which we have discussed. Called taiga in Russia, boreal coniferous forests often are bordered on the north by tundra, discussed later in the context of nonforest ecosystems. An important subset of the boreal coniferous grouping is the montane forest, which also is dominated by conifers but which most often is found on mountains, at subalpine altitudes where the climate is cool and moist.

Biomes are organized here in such a way as to take into account their relative latitudes and corresponding climate. (Distinctions of latitude and climate are mostly relevant where terrestrial biomes are concerned.) As with biomes, there are many possible climate zones, particularly when

In addition to the dominant conifers, boreal coniferous forests also have important broadleafed angiosperms (plants that flower during sexual reproduction), including aspen, birch, poplar, and willow species. These forests are typically subject to periodic catastrophes, which result in at least partial destruction of the dominant trees within stands if not across a given forest as a whole. Among these catastrophic events are wildfires as well as defoliation by such pests as the spruce budworm.

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TROPICAL

GRASSLAND AND SAVANNA BIOMES, PRIMARILY IN

AFRICA,

ARE BEST CHARACTERIZED BY THE EXTRAORDI-

NARILY ABUNDANT AND DIVERSE ANIMAL LIFE, WHICH INCLUDES SUCH LARGE MAMMALS AS THE CHEETAH AND THE GAZELLE. (© Kevin Schafer/Corbis. Reproduced by permission.)

TEMPERATE DECIDUOUS FORESTS.

Moving farther away from the poles, the next major forest biome is that of the temperate deciduous forest. The average American, especially on the East Coast, is likely to be more familiar with the temperate deciduous forest than with any other biome. This type of forest develops in a climate that is relatively moist, with winters that are fairly cold. The larger grouping of temperate deciduous forests is divided into smaller categories depending on the relative amount of annual rainfall.

include ash, basswood, birch, cherry, chestnut, dogwood, elm, hickory, magnolia, maple, oak, and walnut, among others. (Note that many of these species are angiosperms. Likewise, most coniferous trees are gymnosperms, or plants that reproduce sexually through exposed seeds as opposed to seeds hidden in a flower.) Among the varieties of animal life are squirrels, rabbits, skunks, opossums, deer, bobcat, timber wolves, foxes, and black bears. T E M P E RAT E

RA I N

FORESTS.

The term deciduous refers to a tree that sheds its leaves seasonally, and these forests are dominated by such trees, broad-leafed species that

Temperate rain forests are not necessarily farther from the poles than temperate deciduous forests, but they are subject to milder winters. For example, the temperate rain forests of Washington

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State are north of many a temperate deciduous forest on the East Coast, but owing to differences in climate patterns, they are subject to milder winters than those typical of the deciduous forests to the east. Characterized by abundant precipitation (most of it rain rather than snow, due to the milder temperatures), these systems are very moist—as the “rain forest” in their name implies. This, in turn, means that they are seldom subject to catastrophic wildfires, and therefore they often attain the climax stage of old-growth forests. In the temperate rain forest, coniferous trees are dominant, and many of these trees are extremely large and old. Among the tree species typical of this biome are Douglas fir, hemlock, cedar, redwood, spruce, and yellow cypress. Tropical forests are discussed at some length in the essays Ecosystems and Ecology as well as The Biosphere. Among the two most basic varieties of this biome are semi-evergreen tropical forests and evergreen tropical rain forests. Most of the Amazon rain forest in South America, for instance, is an evergreen tropical forest, while surrounding biomes are semi-evergreen. Much the same is true of biomes in central and southern Africa, such as that surrounding the Congo River, with evergreen forests closest to the river and semi-evergreen ones in nearby areas. The tropics, in general, are characterized not by the four seasons of the more temperate climate zones, but by a dry season and a wet season. The environment of a semi-evergreen tropical forest is one that is subject to great extremes of wet and dry, meaning that water is not available in abundance year-round. This means that most trees and shrubs in the biome are seasonally deciduous, shedding their leaves in anticipation of the drier season. In an evergreen tropical rain forest, on the other hand, rainfall is frequent and regular, so there is no seasonal drought. Deciduous trees may drop their leaves at various times of year, depending on the species, but with a wet climate and a wide range of trees, there is always something in bloom. As with the temperate rain forest, the tropical variety experiences little in the way of wildfire or other catastrophic disturbances, and therefore an old-growth, climax community often develops in this biome. For this reason, tropical rain forests usually contain a wide diversity of trees, an enormous richness of species, and an extraor-

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dinary range of animals and microorganisms. Though biogeographers and ecologists often use the boreal coniferous forest as an example when examining biomes, those northerly forests are hardly examples of biological diversity. On the other hand, the tropical rain forest represents such diversity to its greatest extent.

Nonforest Biomes Making an abrupt shift from the lush world of the tropical rain forest, let us look now at the tundra: a cold, treeless biome in the arctic and subarctic regions. (The Arctic Circle lies at approximately 66.5 degrees north latitude. Lands north of that line include northern Alaska and Canada, most of Greenland, extreme northern Scandinavia, and a northern strip of Russia and Siberia. The subarctic region, less clearly defined, comprises simply those lands that lie directly below the Arctic Circle.) Characterized by a short growing season, the tundra experiences very little precipitation in the form of liquid water. Yet the soil may well be marshy because temperatures are too low for significant evaporation and because the ground is usually frozen solid, preventing drainage. In the most northerly tundras the dominant plants are small, hardy species that grow no more than 2–4 in. (5–10 cm) tall. In subarctic regions the dominant shrub species may grow as tall as 3.28 ft. (1 m), and the marshiest subarctic tundra may be home to sedge and cotton grass meadows. Among the larger forms of animal life on the tundra are the caribou and musk ox as well as the wolf, one of the larger predatory species. G RASS LA N D S A N D C H A PA R RA L . Temperate grasslands are known as

prairies in North America and steppes in Eurasia, and these grasslands often are divided into smaller subgroups depending on the height of the dominant grasses. Fire, aided by the dry climate, acts as a curb to prevent the tall grass from giving way to larger trees and forests. In the United States, however, so much prairie has been converted to agricultural or other anthropogenic purposes that it constitutes an endangered biome. Much further south are the tropical grassland and savanna biomes that appear primarily in Africa. Although they do have scattered trees and shrubs, these biomes are dominated by grasses and other plants. In any case, the plant life

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ONE

TYPE OF FRESHWATER BIOME IS THE SWAMP FOREST, WHICH MAY BE FLOODED EITHER SEASONALLY OR PERMA-

NENTLY AND IS NOTABLE FOR ITS ANIMAL LIFE, INCLUDING HERON AND OTHER BIRD SPECIES. (© S. R. Maglione. Photo

Researchers. Reproduced by permission.)

is not what best characterizes this biome in the minds of most people. Rather, it is the extraordinarily abundant and diverse animal life, which includes such large mammals as the rhinoceros, elephant, hippopotamus, buffalo, cheetah, gazelle and other antelope, wild dog, and hyena. Then, of course, there is the lion, “king of beasts,” sometimes incorrectly portrayed (for instance, in many Tarzan movies) as a jungle creature. A biome typical of coastal southern California, the chaparral is distinguished by what often is described as a Mediterranean climate: dry, rainy in winter, and prone to drought in summers. The characteristic plant in a chaparral region has thick, leathery leaves that help it preserve moisture during the dry seasons. As with most other nonforest biomes, wildfire is a major controlling factor.

shadows,” areas separated from oceans by high mountains. The unavailability of water is the chief defining feature of the desert, a biome that receives less than 9.9 in. (25 cm) of precipitation per year. Extremely dry deserts support virtually no plant productivity and therefore little, if any, animal life either. Such is the case, for instance, with the extraordinarily forbidding desert known as Rub‘ al Khali (“The Empty Quarter”), which occupies the lower third of the Arabian peninsula. On the other hand, less dry deserts may support relatively diverse plant life, as is the case, for instance, in Arizona.

Freshwater Biomes

D E S E R T. Deserts, discussed in more detail within The Biosphere, constitute a biome that may be temperate or tropical and which usually appears near the center of a continent. Such is particularly the case with the Gobi and Taklimakan deserts in northwestern and southwestern China, respectively; both deserts are located almost as far away from ocean as it is possible to be on Earth. Deserts also may occur in “rain

Now we make another abrupt shift, in this case from the dry desert to aquatic biomes, beginning with the freshwater variety. Among these are lentic biomes, which appear in the area of lakes and ponds—any place where water is still. (This would include even a vast body of water such as Lake Superior, which, though it looks like a sea from the edge, and experiences waves and heavy swells, is nonetheless a freshwater body where water is not flowing. Hence, it is by definition a lentic environment.) The water in these bodies

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may take a few days to flush; on the other hand, it may take centuries. By sitting for such long periods of time, the water may accumulate large amounts of nutrients, and this is one of the variables whereby subgroups of the lentic biome are classified. In a lotic biome, such as that of a river or stream, water is flowing. A lotic biome may be as small as a babbling brook that runs for less than a mile, or as great as the Mississippi River, which drains much of the continental United States. Where lotic biomes are concerned, the greatest variables involve the strength of the flow, including its quantity, velocity, and seasonal variations. These characteristics influence other aspects of the ecosystem: for example, if water flows calmly and slowly, the bottom tends to gather silt. This provides a habitat for certain small species, such as the crustacean, known variously as a crayfish, crawfish, or crawdaddy. A lotic environment is not fully self-sustaining, and therefore it may not qualify as a true biome. Though they support some plant life, these ecosystems do not have a full complement of autotrophs, or life-forms (usually plants) that depend only on the Sun and the atmosphere, rather than other organisms, for sustenance. Usually, the lotic ecosystem relies on the input of organic matter from the nearby terrestrial environment or from lakes upstream or both. These other biomes provide the lotic biome with the nutrients necessary to feed its fish and other aquatic species. W E T L A N D S . Yet another freshwater biome is that of the wetlands. Just as rain forests were known as jungles until ecology and environmentalism entered the mainstream in the 1970s, so the term wetlands replaced a more bluntsounding word: swamp. That word and several other old-fashioned ones are preserved in the terms for the four major wetland types: marsh, swamp forest, bog, and fen. Regardless of the name, this is a biome found in shallow waters, often in regions known for their pronounced seasonal variations of water depth.

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cypress and silver maple and may be flooded either seasonally or permanently. The Okeefenokee is notable for its animal life: not just heron and other bird species but also some of the more terrifying reptilian forms, including alligators and water moccasins, for which swamps are notorious. Much less biologically productive than either marshes or swamps are bogs, which generally have acidic soil that supports only a limited range of vegetation. Characterized by a cool, wet climate (many of them are found in England), bogs often are dominated by sphagnum moss of one species or another. Later, when the sphagnum moss dies and the remains of several generations are compacted together with other plant debris, this becomes the basis for peat, which provides fuel for some homes in the British Isles and Europe. Finally, there is the fen, another wetland found throughout the British Isles. Fens resemble bogs in several ways, including the fact that the local climate is usually cold (unlike the swamp forest, where the climate is generally hot). The fen has a better nutrient supply than the bog, however, and consequently the soil is less acidic, meaning that the biome as a whole is more productive.

Marine Biomes The largest biome, geographically, is that of the open ocean, sometimes called a pelagic oceanic biome. Yet in terms of primary productivity— the first level in the food web—the ocean might as well be a desert. Tossed by waves and tides and heavily affected by the powerful salt content in its chemistry, the open ocean depends for its primary productivity not on plants but on phytoplankton, microscopic organisms that include a range of bacteria and algae.

The most biologically productive wetlands, marshes typically are dominated by relatively tall angiosperm varieties, such as the reed, cattail, and bulrush, as well as by floating flowers, such as the water lily and lotus. Swamp forests, such as the Okeefenokee on the Georgia-Florida border, are heavily populated with trees that include bald

Small crustaceans known as zooplankton eat the phytoplankton, only to be consumed, in turn, by small fish. Thus it goes up the trophic levels of the oceanic food web to the largest predators: bluefin tuna, sharks, squid, and whales. At the bottom of the ocean are other ecosystems, which depend on the slow rain of dead organic matter, or biomass, from the surface. Little is known about the deep-ocean biomes, but they appear to be diverse, if low in productivity (i.e., they have a relatively wide range of species but a small number of individuals).

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CORAL

REEFS APPEAR ONLY IN TROPICAL REGIONS.

THE

PRINCIPAL CHARACTERISTIC OF THIS BIOME IS ITS SUB-

STRATE OF CALCIUM CARBONATE, FORMED FROM THE EXOSKELETONS OF DEAD CORAL POLYPS AND OTHER CREATURES.

ON

THIS STRUCTURE IS BUILT A HIGHLY PRODUCTIVE BIOME IN WHICH CORAL, ALGAE, FISH, AND INVERTE-

BRATES THRIVE. (© Andrew Martinez. Photo Researchers. Reproduced by permission.)

Upwelling regions are relatively deep, nutrient-rich areas that sustain highly productive biomes. Among the large variety of species included in such biomes are fish and shark, marine mammals, and birds such as gulls. Upwellings off the west coast of South America and in the Antarctic Ocean provide some of the human world’s most abundant fisheries. C LO S E R T O S H O R E . Closer to the

major landmasses are the biomes of the continental shelves. There the water is warm compared with that of the open ocean, and the nutrient supply is relatively high. This flow of nutrients is fed partly by rivers that empty into the seas but also by the occasional rising of deeper, richer waters to the surface. Not surprisingly, then, phytoplankton and animal life are highly productive here, and continental shelf regions such as those of the Grand Banks off northeastern North America offer highly abundant and commercially important fisheries.

aries feature characteristics of both marine and freshwater biomes and offer highly productive ecosystems. Many a commercially important species of fish, shellfish, and crustacean makes its early home in an estuary before moving on to deeper waters after reaching maturity. Seashores constitute a variety of oceanic biome or, indeed, several varieties. Environmental factors such as the intensity of wave motion determine the characteristics of the seashore biome, as do latitude. For example, temperate seashore ecosystems can develop kelp “forests.” (See Succession and Climax for more about the interrelations of species in the kelp forest.) In other areas, where the bottoms are soft and covered with sand or mud, dominant species include mollusks, crustaceans, and marine worms.

Another ocean biome closer to shore is that of the estuary, an ecosystem that is enclosed by land on several sides but is still open to the sea. Because they typically experience substantial inflows of river water from the nearby land, estu-

As with seashores, coral reefs are those rare oceanic biomes affected by latitude; in fact, this type of biome appears only in tropical regions. The principal characteristic of the coral reef is its substrate of calcium carbonate, formed from the exoskeletons of dead coral polyps and other creatures. On this structure is built a highly productive biome in which coral, algae, fish, and invertebrates (animals without a backbone) thrive.

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KEY TERMS A measure of the degree to which an ecosystem possesses large numbers of particular species. An abundant ecosystem may or may not have a wide array of different species. Compare with complexity. ABUNDANCE:

A term that refers to a biogeographic zone that includes mountain slopes above the timberline. ALPINE:

A type of plant that produces flowers during sexual reproduction. ANGIOSPERM:

ANTHROPOGENIC:

Influenced

by

human activity.

The liv-

ing components of an ecosystem. A large ecosystem, characterized by its dominant life-forms. There are two basic varieties of biome: terrestrial, or land-based, and aquatic. BIOME:

A combination of all flora and fauna (plant and animal life, respectively) in a region. BIOTA:

The upper portion or layer of the trees in a forest. A forest with a closed canopy is one so dense with vegeta-

CANOPY:

CARNIVORE:

A meat-eating organ-

ism, or an organism that eats only meat (as distinguished from an omnivore). CLIMAX:

A theoretical notion intended

to describe a biological community that has reached a stable point as a result of ongoing succession. In such a situation, the community is at equilibrium with environmental conditions, and conditions are stable, such that the biota experiences little change thereafter. The range of ecological

niches within a biological community. The degree of complexity is the number of different species that theoretically could exist in a given biota, as opposed to its diversity, or actual range of existing species. CONIFER:

A type of tree that produces

cones bearing seeds. DECIDUOUS:

A term for a tree or

other form of vegetation that sheds its leaves seasonally. DECOMPOSERS:

Organisms

that

obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. The principal forms of decomposer are bacteria and fungi.

Finally, there are anthropogenic biomes, such as the urban-industrial techno-ecosystem found in many a large metropolitan area. Such an ecosystem may include many species in addition to humans, but these—pets, houseplants, and the like—are not always native to the region, and probably would not flourish unless returned to

their native biomes. New York City would be perhaps the ultimate example of an urban-industrial techno-ecosystem. Though it is far from a natural environment, it teems with life, from the oaks and elms in Central Park to the rats of the sewers, and from the pigeons that peck at crumbs on the sidewalks to the houseplants on the balconies and fire escapes of apartment buildings.

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ground.

COMPLEXITY:

The study of the geographic distribution of plants and animals, both today and over the course of extended periods. BIOGEOGRAPHY:

BIOLOGICAL COMMUNITY:

tion that the sky is not visible from the

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Biomes

KEY TERMS Organisms that feed

DETRITIVORES:

on waste matter, breaking organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Their function is similar to that of decomposers; however, unlike decomposers—which tend to be bacteria or fungi—detritivores are relatively complex organisms, such as earthworms or maggots. A measure of the number

DIVERSITY:

of different species within a biological community. ECOLOGY:

The study of the relation-

CONTINUED

but since the latter rarely exist separately, scientists prefer the concept of a food web to that of a food chain. In general terms, a forest is simply any ecosystem dominated by treesize woody plants. A number of other characteristics and parameters (for example, weather, altitude, and dominant species) further define types of forests, such as tropical rain forests. FOREST:

A type of plant that reproduces sexually through the use of seeds that are exposed, not hidden in an ovary, as with an angiosperm. GYMNOSPERM:

ships between organisms and their envi-

HERBIVORE:

ronments.

ism. A community of inter-

ECOSYSTEM:

dependent organisms along with the inorganic components of their environment. FAUNA:

Animals.

FLORA:

Plants.

FOOD WEB:

A term describing the

interaction of plants, herbivores, carnivores, omnivores, decomposers, and detritivores in an ecosystem. Each of these consumes nutrients and passes them along to other organisms (or, in the case of the

A plant-eating organ-

An organism that eats both plants and other animals. OMNIVORE:

At one time chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals), and oxides, such as carbon dioxide. ORGANIC:

The progressive replacement of earlier biological communities with others over time.

SUCCESSION:

Land-based.

decomposer food web, to the soil and envi-

TERRESTRIAL:

ronment). The food web may be thought

Layers of vegetation below the canopy in a forest.

of as a bundle or network of food chains,

UNDERSTORY:

Another anthropogenic biome is the rural techno-ecosystem, which is not as removed from human civilization as the “rural” in its name would imply. This type of biome appears in regions around transportation and transmission corridors, including highways, railways, canals, and aqueducts, as well as alongside power and telephone lines. Small towns are a characteristic

area for such a biome, as are the regions around coal mines and other industrial plants devoted to the extraction, processing, or manufacture of products from natural resources. This biome usually supports a mixture of introduced species and native species, the latter being those varieties that can survive the disturbances, pollution, and other stresses associated with the human pres-

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Biomes

ence. For example, in the woods along an interstate highway, there are bound to be omnivorous creatures such as raccoons, which thrive on litter thrown out of passing cars. Such creatures must be agile enough to survive the threat of becoming “road kill,” as well as other hazards associated with the environment. As with the city biome, the rural techno-ecosystem includes plenty of life-forms that have been introduced artificially, an example being the wildflowers planted on a median by state highway workers. Agro-ecosystems are ecosystems that are managed and harvested for human use: farms, orchards, fisheries, commercial forests, and other agricultural concerns. Here the defining characteristic is the level of management, or the degree of anthropogenic influence. Very heavily managed agro-ecosystems involve the planting of non-native crop species and the introducing of non-native plants, often to the exclusion of native species. The “crop” may be a herd of animals, as when western ranchers introduced nonnative cattle, and in the process killed off native predators such as coyotes. On the other hand, there are less heavily managed agro-ecosystems

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that do allow native wildlife species to thrive alongside those species introduced for commercial purposes. WHERE TO LEARN MORE Biomes of the World (Web site). . Earth Floor: Biomes (Web site). . Habitats/Biomes (Web site). . Johnson, Rebecca L. A Walk in the Boreal Forest. Illus. Phyllis V. Saroff. Minneapolis: Carolrhoda Books, 2001. ———. A Walk in the Desert. Illus. Phyllis V. Saroff. Minneapolis: Carolrhoda Books, 2001. ——— A Walk in the Rain Forest. Illus. Phyllis V. Saroff. Minneapolis: Carolrhoda Books, 2001. ———. A Walk in the Tundra. Illus. Phyllis V. Saroff. Minneapolis: Carolrhoda Books, 2001. Major Biomes of the World (Web site). . The World’s Biomes (Web site). .

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BIOLOGICAL COMMUNITIES SYMBIOSIS BIOLOGICAL COMMUNITIES S U C C E SS I O N A N D C L I M AX

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Symbiosis

CONCEPT Symbiosis is a biological relationship in which two species live in close proximity to each other and interact regularly in such a way as to benefit one or both of the organisms. When both partners benefit, this variety of symbiosis is known as mutualism. The name for a situation in which only one of the partners benefits is far more well known. Such an arrangement is known as parasitism, and a parasite is an organism that obtains nourishment or other life support from a host, usually without killing it. By their very nature, parasites are never beneficial, and sometimes they can be downright deadly. In addition to the extremes of mutualism and parasitism, there is a third variety of symbiosis, called commensalism. As with parasitism, in a relationship characterized by commensalism only one of the two organisms or species derives benefit, but in this case it manages to do so without causing harm to the host.

HOW IT WORKS Varieties of Symbiosis When two species—that is, at least two individuals representing two different species—live and interact closely in such a way that either or both species benefit, it is symbiosis. It is also possible for a symbiotic relationship to exist between two organisms of the same species. Organisms engaging in symbiotic relationships are called symbionts. There are three basic types of symbiosis, differentiated as to how the benefits (and the detriments, if any) are distributed. These are commensalism, parasitism, and mutualism. In the first two varieties, only one of the two creatures

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SYMBIOSIS

benefits from the symbiotic relationship, and in both instances the creature who does not benefit—who provides a benefit to the other creature—is called the host. In commensalism the organism known as the commensal benefits from the host without the host’s suffering any detriment. By contrast, in parasitism the parasite benefits at the expense of the host. MUTUALISM: HUMAN AND D O G . Mutualism is distinguished from the

other two types of symbiosis, because in this variety both creatures benefit. Thus, there is no host, and theoretically the partners are equal, though in practice one usually holds dominance over the other. An example of this inequality is the relationship between humans and dogs. In this relationship, both human and dog clearly benefit: the dog by receiving food, shelter, and care and the human by receiving protection and loving companionship—the last two being benefits the dog also receives from the human. Additionally, some dogs perform specific tasks, such as fetching slippers, assisting blind or disabled persons, or tracking prey for hunting or crime-solving purposes. For all this exchange of benefits, one of the two animals, the human, clearly holds the upper hand. There might be exceptions in a few unusual circumstances, such as dog lovers who are so obsessive that they would buy food for their dogs before feeding themselves. Such exceptions, however, are rare indeed, and it can be said that in almost all cases the human is dominant.

Obligate and Facultative Relationships Most forms of mutualism are facultative, meaning that the partners can live apart successfully.

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Some relationships of mutualism are so close that the interacting species are unable to live without each other. A symbiotic relationship in which the partners, if separated, would be unable to continue living is known as an obligate relationship. In commensalism or parasitism, the relationship is usually obligate for the commensal or the parasite, since by definition they depend on the host. At the same time, and also by definition, the host is in a facultative relationship, since it does not need the commensal or parasite—indeed, in the case of the parasite, would be much better off without it. It is possible, however, for an organism to become so adjusted to the parasite attached to its body that the sudden removal of the parasite could cause at least a short-term shock to the system.

Inquilinism A special variety of commensalism is inquilinism, in which the commensal species makes use of the host’s nest or habitat, without causing any inconvenience or detriment to the host. Inquilinism (the beneficiary is known as an inquiline) often occurs in an aquatic environment, though not always. In your own yard, which is your habitat or nest, there may be a bird nesting in a tree. Supposing you benefit from the bird, through the aesthetic enjoyment of its song or the pretty colors of its feathers—in this case the relationship could be said to be a mutualism. In any case, the bird still benefits more, inasmuch as it uses your habitat as a place of shelter. The bird example is an extremely nonintrusive case of inquilinism; more often than not, however, a creature actually uses the literal nest of another species, which would be analogous to a bird nesting in your attic or even the inside of your house. This is where the analogy breaks down, of course, because such an arrangement would no longer be one of commensalism, since you would be suffering a number of deleterious effects, not the least of which would be bird droppings on the carpet. Inquilinism sometimes is referred to as a cross between commensalism and parasitism and might be regarded as existing on a continuum between the two. Certainly, there are cases of a creature making use of another’s habitat in a parasitic way. Such is the case with the North American cowbird and the European cuckoo, both of which leave their offspring in the nests of other

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birds to be raised by them. (See Instinct and Learning for a discussion of how these species exploit other birds’ instinctive tendency to care for their young.)

REAL-LIFE A P P L I C AT I O N S Mycorrhizae One of the best examples of mutualism is known by the unusual name mycorrhiza, which is a “fungus root,” or a fungus living in symbiosis with the roots of a vascular plant. (A vascular plant is any plant species containing a vascular system, which is a network of vessels for moving fluid through the body of the organism.) The relationship is a form of mutualism because, while the fungus benefits from access to carbohydrates, proteins, and other organic nutrients excreted by or contained in the roots of the host plant, the host plant benefits from an enhanced supply of inorganic nutrients, especially phosphorus, that come from the fungus. The fungus carries out this function primarily by increasing the rate at which organic matter in the immediate vicinity of the plant root decomposes and by efficiently absorbing the inorganic nutrients that are liberated by this process-nutrients it shares with the plant. (The term organic refers to the presence of carbon and hydrogen together, which is characteristic not only of all living things but of many nonliving things as well.) The most important mineral nutrients that the fungus supplies to the plant are compounds containing either phosphorus or, to a lesser degree, nitrogen. (These elements are present in biogeochemical cycles—see The Biosphere.) So beneficial is the mycorrhizal mutualism that about 90% of all vascular plant families, including mustards and knotweeds (family Brassicaceae and Polygonaceae, respectively), enjoy some such relationship with fungi. S O M E E XA M P L E S O F M YC O R R H I Z A E . Many mycorrhizal fungi in the

Basiodiomycete group develop edible mushrooms, which are gathered by many people for use in gourmet cooking. Mushroom collectors have to be careful, of course, because some mycorrhizal fungi are deadly poisonous, as is the case with the death angel, or destroying angel— Amanita virosa.

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Perhaps the most famous of the edible mushrooms produced by mycorrhizae are the many varieties known by the name truffle. Among these mushrooms is Tuber melanosporum, which is commonly mycorrhizal on various species of oak tree. The spore-bearing bodies of the truffle fungi develop underground and are usually brown or black and covered with warts. Truffle hunters require the help of truffle-sniffing pigs or dogs, but their work is definitely worth the trouble: good truffles command a handsome price, and particularly in France the truffle industry is big business. Given the lucrative nature of the undertaking, one might ask why people do not cultivate truffles rather than hunting for them. To create the necessary conditions for cultivation, however, so much effort is required that it is difficult to make a profit, even at the high prices charged for truffles. The soil composition must be just right, and under conditions of cultivation this takes about five years. Orchids are an example of a plant in an obligate mutualism: they can thrive only in a mycorrhizal relationship. Tiny and dustlike, orchid seeds have virtually no stored energy to support the seedling when it germinates, or begins to grow. Only with the assistance of an appropriate mycorrhizal fungus can these seedlings begin developing. Until horticulturists discovered this fact, orchids were extremely difficult to propagate and grow in greenhouses; today, they are relatively easy to breed and cultivate. T H E I M P O RTA N C E O F M YC O R R H I Z A E . Some species of vascular plants do

not contain chlorophyll, the chemical necessary for photosynthesis, or the conversion of light energy from the Sun into usable chemical energy in a plant. Such a plant is like a person missing a vital organ, and under normal circumstances, it would be impossible for the plant to survive. Yet the Indian pipe, or Monotropa uniflora, has managed to thrive despite the fact that it produces no chlorophyll; instead, it depends entirely on mycorrhizal fungus to supply it with the organic nutrients it needs. This obligate relationship is just one example of the critical role mycorrhizae perform in the lives of plants throughout the world. Mycorrhizae are vital to plant nutrition, especially in places where the soil is poor in nutrients. Whereas many plant roots develop root hairs as a means of facilitating the extraction of water and nutrients from the soil, plant roots

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Symbiosis

ONE

OF THE BEST EXAMPLES OF MUTUALISM IS THE

MYCORRHIZA,

a fungus living in symbiosis with the roots of a vascular plant. Many mycorrhizal fungi develop edible mushrooms, such as the truffle, used in gourmet cooking. (© Owen Frnaken/Corbis. Reproduced by permission.)

that have a mycorrhizal fungus usually do not. Instead, these plants rely heavily on the fungus itself to absorb moisture and vital chemical elements from the ground. This means that it may be difficult or impossible for plants to survive if they are removed from an environment containing mycorrhizal fungus, a fact that indicates an obligate relationship. Often, when species of trees and shrubs grown in a greenhouse are transplanted to a nonforested outdoor habitat, they exhibit signs of nutritional distress. This happens because the soils in such habitats do not have populations of appropriate species of mycorrhizal fungi to colonize the roots of the tree seedlings. If, however, seedlings are transplanted into a clear-cut area that was once a forest dominated by the same or closely related species of trees, the plants generally will do well. This happens because the clearcut former forest land typically still has a population of suitable mycorrhizal fungi. Plants’ dependence on mycorrhizal fungi may be so acute that the plants do not do well in the absence of such fungi, even when growing in soil that is apparently abundant in nutrients.

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Although most mycorrhizal relationships are not so obligate, it is still of critical important to consider mycorrhizal fungi on a site before a natural ecosystem is converted into some sort of anthropogenic habitat (that is, an area dominated by humans—see Biomes). For example, almost all the tree species in tropical forests depend on mycorrhizae to supply them with nutrients from the soils, which are typically infertile. (See The Biosphere for more about the soil in rain forests.) If people clear and burn the forest to develop new agricultural lands, they leave the soil bereft of a key component. Even though some fungi will survive, they may not necessarily be the appropriate symbionts for the species of grasses and other crops that farmers will attempt to grow on the cleared land.

Interkingdom and Intrakingdom Partnerships Mycorrhizae are just one example of the ways that mutualism brings into play interactions between widely separated species—in that particular case, between members of two entirely different kingdoms, those of plant and fungi. In some cases, mutualism may bring together an organism of a kingdom whose members are incapable of moving on their own (plants, fungi, or algae) with one whose members are mobile (animals or bacteria). An excellent example is the relationship between angiosperm plants and bees, which facilitate pollination for the plants (see Ecosystems and Ecology.) Another plant-insect mutualism exists between a tropical ant (Pseudomyrmex ferruginea) and a shrub known as the bull’s horn acacia (Acacia cornigera). The latter has evolved hollow thorns, which the ants use as protected nesting sites. The bull’s horn acacia has the added benefit, from the ant’s perspective, of exuding proteins at the tips of its leaflets, thus providing a handy source of nutrition. In return, the ants protect the acacia both from competition with other plants (by removing any encroaching foliage from the area) and from defoliating insects (by killing herbivorous, or plant-eating, insects and attacking larger herbivores, such as grazing mammals).

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incorrectly called mosses (e.g., reindeer moss). Before the era of microscopy, botanists considered lichens to be single organisms, but they constitute an obligate mutualism between a fungus and an alga or a blue-green bacterium. The fungus benefits from access to photosynthetic products, while the alga or bacterium benefits from the relatively moist habitat that fungus provides as well as from enhanced access to inorganic nutrients. In contrast to these cross-kingdom or interkingdom types of mutualism, there may be intrakingdom (within the same kingdom) symbiotic relationships between two very different types of animal. Often, mutualism joins forces in such a way that humans, observing these interactions, see in them object lessons, or stories illustrating the concept that the meek sometimes provide vital assistance to the mighty. One example of this is purely fictional, and it is a very old story indeed: Aesop’s fable about the mouse and the lion. BIG

AND

SMALL.

In this tale a lion catches a mouse and is about to eat the little creature for a snack when the mouse pleads for its life; the lion, feeling particularly charitable that day, decides to spare it. Before leaving, the mouse promises one day to return the favor, and the lion chuckles at this offer, thinking that there is no way that a lowly mouse could ever save a fierce lion. Then one day the lion steps on a thorn and cannot extract it from his paw. He is in serious pain, yet the thorn is too small for him to remove with his teeth, and he suffers hopelessly—until the mouse arrives and ably extracts the thorn. Many real-life examples of this strong-weak or big-small symbiosis exist, one of the more well-known versions being that between the African black rhinoceros (Diceros bicornis) and the oxpecker, or tickbird. The oxpecker, of the genus Buphagus, appears in two species, B. africanus and B. erythrorhynchus. It feeds off ticks, flies, and maggots that cling to the rhino’s hide. Thus, this oddly matched pair often can be seen on the African savannas, the rhino benefiting from the pest-removal services of the oxpecker and the oxpecker enjoying the smorgasbord that the rhino’s hide offers.

A much less dramatic, though biologically quite significant, example of interkingdom mutualism is the lichen. Lichen is the name for about 15,000 varieties, including some that are

Humans engage in a wide variety of symbiotic relationships with plants, animals, and bacteria. Bacteria may be parasitic on humans, but far

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INTRAKINGDOM SYMBIOTIC RELATIONSHIPS CAN EXIST BETWEEN TWO VERY DIFFERENT TYPES OF ANIMAL. THE OXPECKER, OR TICKBIRD, FEEDS OFF THE TICKS, FLIES, AND MAGGOTS THAT CLING TO THE HIDE OF THE AFRICAN BLACK RHINOCEROS. THIS ODDLY MATCHED PAIR OFTEN CAN BE SEEN ON THE AFRICAN SAVANNAS. (© Joe McDonald/Corbis. Reproduced by permission.)

from all microorganisms are parasites: without the functioning of “good” bacteria in our intestines, we would not be able to process and eliminate food wastes properly. The relationship of humans to animals that provide a source of meat might be characterized as predation (i.e., the relationship of predator to prey), which is technically a form of symbiosis, though usually it is not considered in the same context. In any case, our relationship to the animals we have domesticated, which are raised on farms to provide food, is a mixture of predation and mutualism. For example, cows (Bos taurus) benefit by receiving food, veterinary services, and other forms of care and by protection from other predators, which might end the cows’ lives in a much more unpleasant way than a rancher will.

porting tissue. In other words, thanks to selective breeding, the corn that grows on farms is enclosed in a husk, and the kernels do not come off of the cob readily. Such corn may be desirable as a crop, but because of these characteristics, it is incapable of spreading its own seeds and thereby reproducing on its own. Obviously, agricultural corn is not on any endangered species list, the reason being that farmers continue to propagate the species through breeding and planting.

All important agricultural plants exist in tight bonds of mutualism with humans, because human farmers have bred species so selectively that they require assistance in reproducing. For example, over time, agricultural corn, or maize (Zea mays), has been selected in such a way as to favor those varieties whose fruiting structure is enclosed in a leafy sheath that does not open and whose seeds do not separate easily from the sup-

Another example of human-animal mutualism, to which we alluded earlier, is the relationship between people and their pets, most notably dogs (Canis familiaris) and house cats (Felis catus). Fed and kept safe in domestication, these animals benefit tremendously from their interaction with humans. Humans, in turn, gain from their pets’ companionship, which might be regarded as a mutual benefit—at least in the case of dogs. (And even cats, though they pretend not to care much for their humans, have been known to indulge in at least a touch of sentimentality.) In addition, humans receive other services from pets: dogs protect against burglars, and cats eradicate rodents.

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KEY TERMS A symbiotic relationship in which one organism, the commensal, benefits without causing any detriment to the other organism, the host. COMMENSALISM:

A term for a symbiotic relationship in which partners are capable of living apart. FACULTATIVE:

The term for an organism that provides a benefit or benefits for another organism in a symbiotic relationship of commensalism or parasitism. HOST:

A type of symbiosis in which one species, the inquiline, makes use of a host’s nest or habitat without causing any detriment to the host. Inquilinism is considered a variety of commensalism. INQUILINISM:

A term for a symbiotic relationship in which the partners, if they were separated, would be incapable of continuing to live. OBLIGATIVE:

A symbiotic relationship in which one organism, the parasite, benefits at the expense of the other organism, the host. PARASITISM:

A biological relationship in which (usually) two species live in close proximity to one another and interact regularly in such a way as to benefit one or both of the organisms. Symbiosis may exist between two or more individuals of the same species as well as between two or more individuals representing two different species. The three principal varieties of symbiosis are mutualism, commensalism, and parasitism. SYMBIOSIS:

In discussing the ant-aphid mutualism, scientists often compare the aphids to cattle, with the ants acting as protectors and “ranchers.” What aphids have that ants want is something called honeydew, a sweet substance containing surplus sugar from the aphid’s diet that the aphid excretes through its anus. In return, ants protect aphid eggs during the winter and carry the newly hatched aphids to new host plants. The aphids feed on the leaves, and the ants receive a supply of honeydew. In another mutualism involving a particular ant species, Formica fusca, two organisms appear to have evolved together in such a way that each benefits from the other, a phenomenon known as coadaptation. This particular mutualism involves the butterfly Glaucopsyche lygdamus when it is still a caterpillar, meaning that it is in the larval, or not yet fully developed, stage. Like the aphid, this creature, too, produces a sweet “honeydew” solution that the ants harvest as food. In return, the ants defend the caterpillar against parasitic wasps and flies. W H E N M U T UA L I S M A L S O CA N B E PA RAS I T I S M . As the old saying goes,

“One man’s meat is another man’s poison”—in other words, what is beneficial to one person may be harmful to another. So it is with symbiotic relationships, and often a creature that plays a helpful, mutualistic role in one relationship may be a harmful parasite in another interaction. Aphids, for instance, are parasitic to many a host plant, which experiences yellowing, stunting, mottling, browning, and curling of leaves as well as inhibiting of its ability to produce crops.

Where insects and symbiosis are involved, perhaps the ideas that most readily come to mind

One particular butterfly group, Heliconiinae (a member of the Nymphalidae, largest of the butterfly families) furnishes another example of the fact that a mutualistic symbiont, in separate interaction, can serve as a parasite. Moreover, in this particular case the heliconius butterfly can be a mutualistic symbiont and parasite for the very same plant. Heliconius butterflies scatter the

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are images of parasitism. Indeed, many parasites are insects, but insects often interact with other species in relationships of mutualism, such as those examples mentioned earlier (bees and angiosperms, ants and bull’s horn acacia plants). Additionally, there are numerous cases of mutualism between insect species. One of the most intriguing is the arrangements that exists between ants and aphids, insects of the order Homoptera, which also are known as plant lice.

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pollen from the flowers of passionflower vines (genus Passiflora), thus benefiting the plant, but their females also lay eggs on young Passiflora shoots, and the developing larva may eat the entire shoot. As an apparent adaptive response, several Passiflora species produce new shoots featuring a small structure that closely resembles a heliconius egg. A female butterfly that sees this “egg” will avoid laying her own egg there, and the shoot will be spared.

Commensalism Years ago a National Geographic article on the Indian city of Calcutta included a photograph that aptly illustrated the idea of commensalism, though in this case not between animals or plants but between people. The photograph showed a street vendor in a tiny wooden stall with a window, through which he sold his wares to passersby. It was a rainy day, and huddled beneath the window ledge (which also served as a countertop) was another vendor, protecting himself and his own tray of goods from the rain. The photograph provided a stunning example, in microcosm, of the overpopulation problem both in Calcutta and in India as a whole—a level of crowding and of poverty far beyond the comprehension of the average American. At the same time it also offered a beautiful illustration of commensalism (though this was certainly not the purpose of including the picture with the article). The vendor sitting on the ground acted in the role of commensal to the relatively more fortunate vendor with the booth, who would be analogous to the host. The relationship was apparently commensal, because the vendor on the ground received shelter from the other vendor’s counter without the other vendor’s suffering any detriment. If the vendor in the booth wanted to move elsewhere, and the vendor on the ground somehow prevented him from doing so, then the relationship would be one of parasitism. And, of course, if the vendor with the booth charged his less-fortunate neighbor rent, then the relationship would not be truly commensal, because the vendor on the ground would be paying for his shelter. To all appearances, however, the interaction between the two men was perfectly commensal. C O M M E N SA L I S M I N N AT U R E .

Plants that grow on the sides of other plants without being parasitic are known as epiphytic plants.

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Among these plants are certain species of orchids, ferns, and moss. By “standing on the shoulders of giants,” these plants receive enormous ecological benefits: the height of their hosts gives them an opportunity to reach a higher level in the canopy (the upper layer of trees in the forest) than they would normally attain, and this provides them with much greater access to sunlight. At the same time, the hosts are not affected either negatively or positively by this relationship.

Symbiosis

Another commensal relationship, known as phoresy, is a type of biological hitchhiking in which one organism receives access to transportation on the body of another animal, without the transporting animal being adversely affected by this arrangement. The burdock (Arctium lappa) is one of several North American plant species that produce fruit that adheres to fur and therefore is dispersed easily by the movement of mammals. The burdock is special from a human standpoint, however, inasmuch as the anatomical adaptation that makes possible its adhesion to fur provided designers with the model for that extremely useful innovation, Velcro. As with the illustration of the street vendors in Calcutta, it is always possible that commensalism, through a slight alteration, may yield a relationship in which the host is affected negatively. There are instances in which individual animals may become loaded heavily with sticky fruit from the burdock (or other plants that employ a similar mechanism), thus causing their fur to mat excessively and perhaps resulting in significant detriment. This is not common, however, and usually this biological relationship is truly commensal. Furthermore, phoresy should not be confused with parasitic relationships in which a creature such as a tick attaches itself to the body of another organism for transport or other purposes. (For much more about parasitism, see Parasites and Parasitology.) WHERE TO LEARN MORE “Biology 160, Animal Behavior: Symbiosis and Social Parasitism.” Department of Biology, University of California at Riverside (Web site). . Knutson, Roger M. Furtive Fauna: A Field Guide to the Creatures Who Live on You. New York: Penguin Books, 1992. Lanner, Ronald M. Made for Each Other: A Symbiosis of Birds and Pines. New York: Oxford University Press, 1996.

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Lembke, Janet. Despicable Species: On Cowbirds, Kudzu, Hornworms, and Other Scourges. New York: Lyons Press, 1999. Margulis, Lynn. Symbiotic Planet: A New Look at Evolution. New York: Basic Books, 1998.

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“Parasites and Parasitism.” University of Wales, Aberystwyth (Web site). . Sapp, Jan. Evolution by Association: A History of Symbiosis. New York: Oxford University Press, 1994. Symbiosis and Commensalism. The Sea Slug Forum (Web site). .

Mutualism and Commensalism. Neartica: The Natural World of North America (Web site). .

Trager, William. Living Together: The Biology of Animal Parasitism. New York: Plenum Press, 1986.

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BIOLOGICAL COMMUNITIES

CONCEPT An ecosystem is a complete community of interdependent organisms as well as the inorganic components of their environment; by contrast, a biological community is just the living members of an ecosystem. Within the study of biological communities there are a great number of complexities involved in analyzing the relationships between species as well as the characteristics of specific communities. Yet many of the concepts applicable to biological communities as a whole also apply to human communities in particular, and this makes these ideas easier to understand. For example, the competitive urge that motivates humans to war (and to less destructive forms of strife in the business or sports worlds) may be linked to the larger phenomenon of biological competition. Indeed, much of the driving force behind the development of human societies, as it turns out, has been biological in nature.

HOW IT WORKS The Biosphere, Ecosystems, and Ecology An ecosystem is a community of interdependent organisms along with the inorganic components of their environment—air, water, and the mineral content of the soil—and a biome is an ecosystem, such as a tundra, that extends over a large area. All living organisms are part of a larger system of life-forms, which likewise interact with large systems of inorganic materials in the operation of a still larger system called Earth. (The concepts of ecosystem, biosphere, and biome are discussed, respectively, in Ecosystems and Ecology, The Biosphere, and Biomes.)

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Note the importance of the distinctions between living and nonliving and organic and inorganic. Although in popular terms, organic means anything that is living as well as anything that was once living, along with their parts and products (bones, leaves, wood, sap, blood, urine, and so on), in fact, the scientific meaning of the word is both much broader and more targeted. A substance is organic if it contains carbon and hydrogen, and thus organic materials also include such items as plastics that have never been living. The study of the relationship between living things and their environment, pioneered by the German zoologist Ernst Haeckel (1834–1919) and others, is called ecology. Though the world scientific community was initially slow to accept ecology as a subject of study, the discipline has gained increasing respect since the mid–twentieth century. This change is due to growing acceptance for the idea that all of life is interconnected and that the living world is tied to the nonliving, or inorganic, world. On the other hand, there is also the gathering awareness that certain aspects of industrial civilization may have a negative impact on the environment, an awareness that has spurred further interest in the study of ecology.

Introduction to Biological Communities The term biological community refers to all the living components in an ecosystem. A slightly different concept is encompassed in the word biota, which refers to all flora and fauna, or plant and animal life, in a particular region.

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For the biological community to survive and thrive, a balance must be maintained between consumption and production of resources. Nature provides for that balance in numerous ways, but beginning in the late twentieth century students of ecology in the industrialized world have become more and more concerned with the possible negative impact their own societies exert on Earth’s biological communities and ecosystems. It should be noted, however, that nature itself sometimes replaces biological communities in a process called succession. This process involves the progressive replacement of earlier biological communities with others over time. Coupled with succession is the idea of climax, a theoretical notion intended to describe a biological community that has reached a stable point as a result of ongoing succession. (See Succession and Climax for more about these subjects.) N I C H E . Whereas climax and succession apply to broad biological communities, a niche refers to the role a particular organism or species plays within that community. Though the concept of niche is abstract, it is unquestionable that each organism plays a vital role and that the totality of the biological community (and, indeed, the ecosystem) would suffer stress if a large enough group of organisms were removed from it. Furthermore, given the apparent interrelatedness of all components in a biological community, every species must have a niche—even human beings.

An interesting idea, and one that is somewhat similar to a niche, is that of an indicator species. This is a plant or animal that, by its presence, abundance, or chemical composition, demonstrates a particular aspect of the character or quality of the environment. Indicator species, for instance, can be plants that accumulate large concentrations of metals in their tissues, thus indicating a preponderance of metals in the soil. This metal, in turn, could indicate the presence of valuable deposits nearby, or it could serve as a sign that the soil is being contaminated. (See Food Webs for more about indicator species.) Another concept closely tied to the concept of niche is that of symbiosis. The latter refers to a biological relationship in which (usually) two species live in close proximity to one another and interact regularly in such a way as to benefit one or both of the organisms. Symbiosis may exist between two or more individuals of the same

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species, as well as between two or more individuals representing two different species. The three principal varieties of symbiosis are mutualism, in which both participants benefit; commensalism, in which only one participant benefits, but at no expense to the other participant; and parasitism, in which one participant benefits at the expense of the other. These subjects are covered in much greater depth within the essays on Symbiosis and Parasites and Parasitology.

Evaluating Biological Communities A few billion years ago, Earth’s oceans and lands were populated with just a few varieties of singlecell organisms, but over time increasing differentiation of species led to the development of the much more complex ecosystems we know now. Such differentiation is essential, since the life forms in a particular region must adapt to that biome, whether it be forest or grassland, desert or aquatic environments, mountain setting or jungle. Diversity is a measure of the number of different species within a biological community, while complexity is the number of niches within it. Put another way, the complexity of a community is the number of species that could exist in it. Abundance is the measure of populations within individual species; thus, if a biological community has large numbers of individuals, even if it is not diverse in species, it is still said to be abundant. During its brief summer growing season, the arctic tundra has vast numbers of insects, migratory birds, and mammals, and thus its abundance is high, whereas its diversity is low. On the other hand, a rain forest might have several hundred or even a thousand different tree species, and an even larger number of insect species, in only a few hectares, but there may be only a few individuals representing each of those species in that area. Thus, the forest could have extremely high diversity but low abundance of any particular species. Needless to say, the rain forest is likely to have a much greater complexity than the tundra, meaning that it is theoretically likely to contain far more species. Another way to evaluate ecosystems is in terms of productivity. Productivity refers to the amount of biomass—potentially burnable energy—produced by green plants as they capture sunlight and use its energy to create new organic compounds that can be consumed by local ani-

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mal life. Once again, a forest, and particularly a rain forest, has a very high level of productivity, whereas a desert or tundra ecosystem does not. F O O D W E B S . Food web (in contrast to

the more popular, but less correct term, food chain) is the designation preferred by scientists to describe the means by which energy is transferred through a biological community. Within the food web are various stages, called trophic levels, that identify the position of various organisms in relation to the organisms they consume and the organisms that consume them. Green plants that depend for their nourishment on photosynthesis, or the biological conversion of electromagnetic energy from the Sun into chemical energy, are primary producers. Herbivores, or plant-eating creatures, are primary consumers, whereas the animals that eat herbivores (whether carnivores or omnivores) are secondary consumers. The largest carnivores and herbivores are usually not prey for any other creatures, but when they die, they, too, will be consumed by detritivores, or scavengers, as well as decomposers, such as bacteria and fungi. The second law of thermodynamics, one of several laws governing energy and the systems in which that energy is applied, holds that in each energy transfer some energy is lost. In the case of food webs, this means that much of the energy in each trophic level is unavailable to organisms at the next level. This, in turn, means that each successive trophic level generally has far fewer members than the prey on which they feed. While there might be thousands of primary producers in a particular community, there might be only a few top predators, including humans. (See Food Webs for more on these subjects.)

REAL-LIFE A P P L I C AT I O N S Competition

supply resources is smaller than the potential biological requirement for those resources.

Biological Communities

Scarcity of resources relative to the need for them is one of the governing facts of human life, reflected in such common expressions as “There is no such thing as a free lunch.” In fact, humans have spent most of their history in a modified form of biological competition, war. Furthermore, it may well be that our modern predilection for sports or business competition is simply a matter of transforming biological competition into a more refined form. In any case, competition prevails throughout the world of living things. I N T RA S P E C I F I C A N D I N T E R S P E C I F I C C O M P E T I T I O N . For exam-

ple, plants often compete for access to a limited supply of nutrients, water, sunlight, and space. Intraspecific competition occurs when individuals of the same species vie for access to essential resources (later we look at intraspecific competition between humans), or for mating partners, whereas interspecific competition takes place between different species. Individuals of the same species have virtually identical resource requirements: for example, all humans need food, water, air, and some protection from the natural elements. For this reason, whenever populations of a species are crowded together, intraspecific competition is intense. This also has been illustrated by experiments involving laboratory mice, which become increasingly brutal to one another when confronted with severely diminished resources. When intraspecific competition occurs in dense populations, the result is a process known as selfthinning, characterized by the mortality (death) of those individuals less capable of surviving coupled with the survival of individuals that are more competitive. If this sounds like the “survival of the fittest,” an idea associated with evolution that became the justification for a number of nefarious social movements and activities (see Evolution), it is no accident.

The word competition typically brings to mind images of a basketball court or football field, on one hand, or a Wall Street trading floor or boardroom, on the other. In biological terms, however, competition is the interaction among organisms of the same or different species vying for a common resource that appears in a limited supply relative to the demand. Put another way, this means that the capability of the environment to

Ideas related to intraspecific competition influenced the English naturalist Charles Darwin (1809–1882) in developing his theory of evolution by means of natural selection. Intraspecific competition is an important regulator of population size and can make the species as a whole more fit by ensuring that only the hardiest individuals survive. Likewise, interspecific competition, or competition between species, plays a

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A

BIOME IS AN ECOSYSTEM, SUCH AS A TUNDRA, THAT EXTENDS OVER A LARGE AREA. IN THE ARCTIC TUNDRA, COM-

PETITION IS LOW, BUT CREATURES LIKE THIS ARCTIC FOX FACE SUCH POWERFUL ENVIRONMENTAL STRESSES FROM THE LOCAL CLIMATE THAT COMPETITION IS NOT THE MOST SIGNIFICANT FACTOR LIMITING POPULATIONS. (Photo Researchers.

Reproduced by permission.)

strong role in shaping ecological communities. Furthermore, competition between species is also an agent of natural selection. C O M P E T I T I V E R E L E AS E . Environmental changes that affect a biological community may change the competitive relationships within it, leading to interesting results. During the early 1950s, a fungal pathogen (a disease-carrying parasite in the form of a fungus) known as chestnut blight, or Endothia parasitica, ended the dominance of the American chestnut in the eastern United States. Up to that time, the American chestnut (Castanea dentata) had been the leading species in the canopy, or uppermost layer, of the deciduous (prone to seasonal shedding of leaves) forests in the region. Thanks to the chestnut blight, it was as though the winner had been disqualified from a race, meaning that all the runners-up changed their standings.

By being relieved of the stresses associated with competition, other trees were allowed to become more successful and dominant in their habitat. They took advantage of this change to fill in the canopy gaps left by the demise of mature chestnut trees. In the same way, if a wildfire, storm, or other stress disturbs a mature forest,

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plants that previously have been suppressed by the higher-canopy trees find themselves with much greater access to such resources as light, moisture, and nutrients. As a result, they thrive. Given the distasteful aspects of biological competition, such as the destruction of the “weak” in favor of the “strong” (actually, less adapted and more adapted are much more accurate terms in this context), one might wish for a situation in which no competition exists. Indeed, there are such situations in nature, but they are far from pleasant. In such biomes as the arctic tundra, for instance, competition is low, but this is not because all nature lives in happiness and harmony; instead, organisms face such powerful environmental stresses from the local climate that competition is not the most significant factor limiting populations. THE T U N D RA BIOLOGICAL C O M M U N I T Y. Creatures on the tundra face

little stress from competition, but a great deal of stress in the form of very short growing seasons, thin soil, limited ground cover, low average temperatures and rainfall, high winds, and so on. If the density of individual plants of the tundra is decreased experimentally by thinning, the resid-

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ual plants do not thrive as a result, as they might in a less harsh climate. In the case of the tundra, it is not competition that constrains their productivity, and therefore the reduction of potential competition does little to improve conditions for the organisms that survive. It is interesting to observe what happens in a tundra environment if the intensity of environmental stress is artificially and experimentally alleviated by enclosing an area under a greenhouse and by fertilizing it with nutrients. Such experiments have been performed, and the results are fascinating: under these more favorable environmental conditions, competition actually increases, resulting in a biological community not unlike that of a more hospitable biome.

Biological Communities and Civilizations In his best-seller Guns, Germs, and Steel: The Fate of Human Societies, ethnobotanist Jared M. Diamond showed that local biological communities are among the leading determinants of the success or failure of human civilizations. The book had its beginnings, he wrote, during his many years of work with the native peoples of New Guinea. One day, a young man put a simple question to him: why do the societies of the West enjoy an abundance of material wealth and comforts, while those of New Guinea have so little? The question may have been simple, but the answer was not obvious. As a scientist, Diamond refused to give an answer informed by the politics of the Left or Right, which might have blamed the problem, respectively, on western exploitation or on the failures of the New Guineans themselves. Instead, he approached it as a question of environment, and the result was his thought-provoking analysis, contained in Guns, Germs, and Steel.

roughly chronological order, these locations were Mesopotamia, Egypt, India, and China. All were destined to emerge as civilizations, complete with written language, cities, and organized governments, between about 3000 and 2000 B.C. In the New World, by contrast, agriculture appeared much later and in a much smaller way. The same was true of Africa and the Pacific Islands. In seeking to find the reasons why this happened, Diamond noted a number of factors, including geography. The agricultural areas of the Old World were stretched across a wide area at similar latitudes. This meant that the climates were not significantly different and would support agricultural exchanges, such as the spread of wheat and other crops from one region or ecosystem to another. By contrast, the landmasses of the New World or Africa have a much greater north-to-south distance than they do east to west, resulting in great differences of climate. D I V E R S I T Y O F S P E C I E S . Today such places as the American Midwest support abundant agriculture, and one might wonder why that was not the case in the centuries before Europeans arrived. The reason is simple but subtle, and it has nothing to do, as many Europeans and their descendants believed, with the cultural “superiority” of Europeans over Native Americans. The fact is that the native North American biological communities were far less diverse than their counterparts in the Old World. Peoples of the New World successfully domesticated corn and potatoes, because those were available to them. But they could not domesticate emmer wheat, the variety used for making bread, when they had no access to that species (it originated in Mesopotamia and spread throughout the Old World).

Agriculture came into existence in four places during a period from about 8000 to 6000 B.C. In

The New World also possessed few animals that could be domesticated either for food or for labor. Every single plant or animal that is a part of human life today had to be domesticated— adapted in such a way that it becomes useful and advantageous for humans—and the range of species capable of domestication is far from limitless. In fact, it is safe to say that all major species capable of being domesticated have been, thousands of years ago. The list of animals that can be domesticated is a short one, much shorter than the list of animals that can be tamed. A bear, for instance, can be captured, or raised from birth in captivity, but it is unlikely that humans would ever be able to breed bears in such a way that

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FA V O R A B L E A N D U N FA V O R A B L E E C O S Y S T E M S . As Diamond

showed, the places where agriculture was born were precisely those blessed with favorable climate, soil, and indigenous plant and animal life. Of course, it is no accident that civilization was born in the societies where agriculture first developed. Before a civilization can evolve, a society must become settled, and for that to happen, it must have agriculture.

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those of the Native Americans. This was also true of the “biological communities” they could not see, and of which people were unaware in 1500: the world of microorganisms, or the “germs” in the title of Diamond’s book.

Biological Communities

A

SMALLPOX

VICTIM

SHOWS

LESIONS ON HIS LIMBS. THE

NATIVE AMERICANS

THE

EUROPEANS’

CHARACTERISTIC ADVANTAGE OVER

DERIVED FROM THE ECOLOGI-

CAL COMPLEXITY OF THEIR BIOLOGICAL COMMUNITIES, EVEN DOWN TO THE WORLD OF MICROORGANISMS. NATIVE PEOPLES OF THE

NEW WORLD

THE

HAD NO NATURAL

RESISTANCE TO SMALLPOX OR A HOST OF OTHER DISEASES. (© Corbis. Reproduced by permission.)

their wild instincts all but disappeared and they became reliable, useful companions. The animals that helped make possible the development of farms, villages, and ultimately empires in the Old World—cows and oxen, horses and donkeys, sheep and so on—were absent from the New World. (Actually, horses had once existed in the Americas, but they had been destroyed through overhunting, as discussed in the context of mass extinction within the Paleontology essay.) Many Indian tribes domesticated some types of birds and other creatures for food, but the only animal ever adapted for labor—by the most developed civilization of the preColumbian Americas, the Inca—was the llama, which is too small to carry heavy loads.

CA N N I B A L I S M . Even the practice of cannibalism in such remote locations as the New Guinea highlands is, according to Diamond, a consequence of a relatively limited biological community. In the past, westerners assumed that only very “primitive” societies engaged in cannibalism. However, events have shown that, when faced with starvation, even people from European and European-influenced civilizations may consume human flesh in order to survive.

For instance, in 1846 members of the Donner party, making the journey west across North America, resorted to eating the bodies of those who had died in the perilous crossing. Much the same happened in the 1970s, when a plane carrying Uruguayan athletes crashed in the Andes, and the survivors lived off the flesh of those who had died. Though their upbringing and cultural norms may have told them that cannibalism was immoral or at the very least disgusting, their bodies told them that if they did not consume the only available food, they would die.

advantage over the Native Americans derived ultimately from the ecological complexity of their biological communities compared with

Whereas these circumstances were unusual and temporary, peoples in some parts of the world have been faced with a situation in which the only sources of protein provided within the biological community are ones whose consump-

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The native peoples of the New World had no natural resistance to smallpox or a host of other diseases, including measles, chicken pox, influenza, typhoid fever, and the bubonic plague. As with many plants and animals of the Old World, they simply had no exposure to these microorganisms. In the Old World, however, close contact with farm animals exposed humans to diseases, as did close contact with other people in crowded, filthy cities. This exposure, of course, killed off large numbers in such plagues as the Black Death (1347–1351), but those who survived tended to be much stronger and possessed vastly greater immunities. Therefore, the vast majority of Native American deaths that followed the European invasion were not a result of warfare, enslavement, or massacre of villages (though all of these occurred as well), but of infection. (See Infection and Infectious Diseases for more on these subjects.)

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KEY TERMS ABUNDANCE:

A measure of the

CANOPY:

The upper portion or layer

degree to which an ecosystem possesses

of the trees in a forest. A forest with a

large numbers of particular species. An

closed canopy is one so dense with vegeta-

abundant ecosystem may or may not have

tion that the sky is not visible from the

a wide array of different species. Compare

ground.

with complexity.

CARNIVORE:

BIOENERGY:

Energy derived from

A meat-eating organ-

ism, or an organism that eats only meat (as

biological sources that are used directly as

distinguished from an omnivore).

fuel (as opposed to food, which becomes

CLIMAX:

fuel). Examples of bioenergy include wood

to describe a biological community that

or manure that can be burned. Usually,

has reached a stable point as a result of

petrochemicals, such as petroleum or nat-

ongoing succession.

ural gas, though they are derived from the

A theoretical notion intended

COMPLEXITY:

The range of ecological

bodies of dead organisms, are treated sepa-

niches within a biological community. The

rately from forms of bioenergy.

degree of complexity is the number of dif-

BIOLOGICAL COMMUNITY:

The liv-

ing components of an ecosystem. BIOMAGNIFICATION:

The increase in

ferent species that theoretically could exist in a given biota, as opposed to its diversity, or actual range of existing species. Organisms

that

bioaccumulated contamination at higher

DECOMPOSERS:

levels of the food web. Biomagnification

obtain their energy from the chemical

results from the fact that larger organisms

breakdown of dead organisms as well as

consume larger quantities of food—and,

from animal and plant waste products. The

hence, in the case of polluted materials,

principal forms of decomposer are bacteria

more toxins.

and fungi.

BIOMASS:

Materials that are burned

or processed to produce bioenergy. A large ecosystem, character-

BIOME:

ized by its dominant life-forms. BIOSPHERE:

A combination of all liv-

DECOMPOSITION

cell organisms, and so on—as well as all formerly living things that have not yet decomposed.

A

chemical reaction in which a compound is broken down into simpler compounds, or into its constituent elements. In the biosphere, this often is achieved through the help of detritivores and decomposers.

ing things on Earth—plants, animals, birds, marine life, insects, viruses, single-

REACTION:

DETRITIVORES:

Organisms that feed

on waste matter, breaking organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Their

A combination of all flora and

function is similar to that of decomposers;

fauna (plant and animal life, respectively)

however, unlike decomposers—which tend

in a region.

to be bacteria or fungi—detritivores are

BIOTA:

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KEY TERMS relatively complex organisms, such as earthworms or maggots. A measure of the number of different species within a biological community.

DIVERSITY:

The study of the relationships between organisms and their environments.

ECOLOGY:

A community of interdependent organisms along with the inorganic components of their environment. ECOSYSTEM:

The flow of energy between organisms in a food web.

ENERGY TRANSFER:

HERBIVORE:

A plant-eating organ-

ism. A plant or animal that, by its presence, abundance, or chemical composition, demonstrates a particular aspect of the character or quality of the environment. INDICATOR SPECIES:

The process whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment.

NATURAL SELECTION:

tion seems repugnant from the western viewpoint. Discussing the New Guinea highlands, Diamond noted that the area is virtually bereft of protein sources in the form of large, nonhuman mammals. Nor is bird life sufficient to support the local populace, and marine food sources are far away. For this reason, natives are prone not only to cannibalism but also to another culinary practice that most westerners find appalling: eating bugs, worms, grubs, caterpillars, and other creepy-crawly creatures. WHERE TO LEARN MORE

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CONTINUED

A term referring to the role that a particular organism plays within its biological community. NICHE:

An organism that eats both plants and other animals.

OMNIVORE:

PATHOGEN: A disease-carrying parasite, usually a microorganism.

The biological conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants.

PHOTOSYNTHESIS:

Green plants that depend on photosynthesis for their nourishment. PRIMARY

PRODUCERS:

The amount of biomass produced by green plants in a given biome. PRODUCTIVITY:

The progressive replacement of earlier biological communities with others over time.

SUCCESSION:

Various stages within a food web. For instance, plants are on one trophic level, herbivores on another, and so on. TROPHIC

LEVELS:

DeLong, J. Bradford. “Review of Diamond,” Guns, Germs, and Steel (Web site). . “Designing a Report on the State of the Nation’s Ecosystems.” U.S. Geological Survey, Biological Resources Division (Web site). . Diamond, Jared M. Guns, Germs, and Steel: The Fates of Human Societies. New York: W. W. Norton, 1997. Living Resources and Biological Communities (Web site). . Miller, Kenton, and Laura Tangley. Trees of Life: Saving Tropical Forests and Their Biological Wealth. Boston: Beacon Press, 1991.

Biota.org: The Digital Biology Project (Web site). .

Nebel, Bernard J. Environmental Science: The Way the World Works. Englewood Cliffs, NJ: Prentice Hall, 1990.

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NMITA: Neogene Marine Biota of Tropical America (Web site). .

Plant Communities of California (Web site). .

Patent, Dorothy Hinshaw. The Vanishing Feast: How Dwindling Genetic Diversity Threatens the World’s Food Supply. San Diego: Harcourt Brace, 1994.

Quinn, John R. Wildlife Survivors: The Flora and Fauna of Tomorrow. Blue Ridge Summit, PA: TAB Books, 1994.

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SUCCESSION AND CLIMAX Succession and Climax

CONCEPT Eventually almost everyone has the experience of watching an old neighborhood change. Sometimes we perceive that change for the better, sometimes for the worse, and the perception can have more to do with our individual desires or needs than it does with any qualities inherent in the change itself. For instance, one person might regard a new convenience store and gas station as an eyesore, while another might welcome it as a handy place to buy coffee, gasoline, or other items. Likewise, biological “neighborhoods” change, as when a complete or nearly complete community of living things replaces another. Once again, changes are not necessarily good or bad in any fundamental sense; rather, one community that happens to be better adapted to the changed environment replaces another. Sometimes a stress to the ecosystem brings about a change, such that life-forms that once were adapted to the local environment are no longer. Still, there appears to be a point when a community achieves near perfect adaptation to its environment, a stage in the levels of succession known as climax. This is the situation of old-growth forests, a fact that explains much about environmentalist opposition to logging in such situations.

Biogeography, which emerged in the nineteenth century amid efforts to explore and map the planet fully, draws on many fields. Among the areas that overlap with this interdisciplinary realm of study are the biological sciences of botany and zoology, the combined biological and earth sciences of oceanography and paleontology, as well as the earth sciences of geology and climatology. Not only do these disciplines contribute ideas to the growing field of biogeography, but they also make use of ideas developed by biogeographers. Biogeography is concerned with questions regarding local and regional variations in kinds and numbers of species and individuals. Among the issues addressed by biogeography are the reasons why particular species exist in particular areas, the physical and biotic (life-related) factors that influence the geographic range over which a species proliferates, changes in distribution of species over time, and so on.

Elsewhere in this book, there is considerable material about ecosystems, or communities of interdependent organisms along with the inorganic components of their environment, as well as about biological communities, or the living components of an ecosystem. There are also dis-

Species interact by three basic means: competition for resources, such as space, sunlight, water, or food (see Biological Communities for more about competition); predation, or preying, upon one another (see Food Webs); and symbiosis. An example of the latter form of interaction, discussed elsewhere (see Symbiosis), occurs when an insect pollinates a plant while the plant provides the insect with nourishment, for instance, in the form of nectar. These interac-

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cussions of biomes, or large ecosystems, and food webs, or the means by which energy transfer takes place across a biological community. Related to many of these ideas, as well as to succession and climax, is the realm of biogeography, or the study of the geographic distribution of plants and animals, both today and over the course of biological history.

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tions can and do affect the geographic distribution of species, and the presence or absence of a particular life-form may serve as a powerful control on the range of another organism. Other significant concepts in the realm of biogeography are dispersal (the spread of a species from one region to another), and barriers (environmental factors that act to block dispersal). A species may extend its geographic range by gradually colonizing, or taking over, adjacent areas, or it may cross a barrier (for instance, a mountain range, an ocean, or a desert) and colonize the lands beyond. Later, we briefly examine the case of a bird that managed to do both.

Succession Succession is the progressive replacement of earlier biological communities with others over time. It entails a process of ecological change, whereby new biotic communities replace old ones, culminating in a stable ecological system known as a climax community. In a climax community, climate, soil, and the characteristics of the local biota (the sum of all plants and animals) are all suited to one another. At the beginning of the succession process, a preexisting ecosystem undergoes some sort of disturbance—for example, a forest fire. This is followed by recovery, succession, and (assuming there are no further significant disturbances) climax. If the environment has not been modified previously by biological processes, meaning that succession takes place on a bare substrate, such as a sand dune or a dry riverbed, it is known as primary succession. Primary succession also occurs when a previous biological community has been obliterated. Secondary succession takes place on a substrate that has been home to other lifeforms and usually in the wake of disturbances that have not been so sweeping that they prevented the local vegetation from regenerating. THE

FA C I L I TAT I O N

MODEL.

Whether the conditions are those of primary or secondary succession, the outcome of the preceding disturbance is such that resources are now widely available, but there is little competition for them. One way of describing this situation is through what is known as the facilitation model, which identifies “pioneer species” as those lifeforms most capable of establishing a presence on the site of the disturbance.

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Pioneers modify a site by their presence, for instance, by regenerating the soil with organic material, thus making the area more attractive for invasion by other species. Eventually, new species move in, edging out the pioneers as they do so. This process may repeat itself several times, until the ecosystem reaches the climax stage, which we examine in greater depth a bit later in this essay. At the climax stage, there are few biological “openings” for further change, and change is only very slight and slow—at least until another disturbance arises and starts the process over again.

Succession and Climax

T H E T O L E RA N C E M O D E L . The tolerance model is another possible mechanism of succession. According to this concept, all species involved in succession are equally capable of establishing themselves on a recently disturbed site, but those capable of attaining a large population size quickly are most likely to become dominant. Unlike the facilitation model, the tolerance model does not depict earlier inhabitants as preparing the site biologically for new invader species; rather, this model is more akin to natural selection, discussed elsewhere (see Evolution).

According to the tolerance model, some species will prove themselves more tolerant of biological stresses that occur within the environment as succession proceeds. Among these stresses is competition, and those species less tolerant of competition may succeed earlier on, when there is little competition for resources. Later in the succession process, however, such species will be eliminated in favor of others more capable of competing. T H E I N H I B I T I O N M O D E L . Yet another model of succession is the inhibition model, which, like the tolerance model, starts with the premise of an open situation at the outset: in other words, all species have equal opportunity to establish populations after a disturbance. In the inhibition model, however, some of the early species actually make the site less suitable for the development of other species. An example of this is when plants secrete toxins in the soil, thus inhibiting the establishment and growth of other species. Nevertheless, in time the inhibitory species die, thus creating opportunities that can be exploited by later successional species.

There is evidence to support all three models—facilitation, tolerance, and inhibition—but just as each has a great deal of basis in fact, none of the three fully depicts the dynamics of a suc-

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max stage, resources have been allocated almost completely among the dominant life-forms.

Succession and Climax

Despite its slow rate of change, the climax community is not a perfectly static or unchanging one, because microsuccession (succession on the scale of a single tree, or a stand of trees) is always taking place. In fact, frequent enough events of disturbance within small sections of the biological community may prevent climax from even occurring. Once the biota does achieve a state of equilibrium with the environment, however, it is likely that change will slow down considerably, bringing an end to the stages of succession. Climax remains a somewhat theoretical notion, and in practice it may be difficult to identify a climax community.

REAL-LIFE A P P L I C AT I O N S Colonization and Island Biogeography CATTLE EGRET AND WILD HORSE IN SOUTHERN FRANCE. DURING THE NINETEENTH CENTURY, THE OLD WORLD CATTLE EGRET MANAGED TO CROSS THE ATLANTIC AND FOUND A BREEDING COLONY IN BRAZIL. SINCE THEN, IT HAS EXPANDED THE RANGE OF ITS HABITATS, SO THAT COLONIES CAN BE FOUND AS FAR NORTH AND EAST AS

ONTARIO AND AS FAR SOUTH AND WEST AS SOUTHERN CHILE. (© Hellio/Van Ingen. Photo Researchers. Reproduced by permission.)

cessional environment. Put another way, each is fully right in one particular instance, but none are correct in all circumstances. Facilitation seems to work best for describing primary succession, whereas the more intense, vigorous environment of secondary succession is best pictured by tolerance or inhibition models. All of this shows that succession patterns tend to be idiosyncratic, owing to the many variables that determine their character.

T H E B I O G E O G RA P H Y O F I S O L AT E D B I O M E S . Colonization is one

When a biological community reaches a position of stability and is in equilibrium with environmental conditions, it is said to have reached a state of climax. Often such communities are described as old growth, and in these situations change takes place slowly. Dominant species in a climax community are those that are highly tolerant of the biological stresses that come with competition. And well they might be, since by the cli-

example of the phenomena studied within the realm of biogeography. Other examples involve islands, and, indeed, island biogeography is a significant subdiscipline. The central idea of island biogeography, a discipline developed in 1967 by American biologists R. H. MacArthur (1930–1972) and Edward O. Wilson (1929–), is that for any landmass a certain number of species can coexist in a state of equilibrium. The larger the size of the landmass, the larger the number of species. Thus, reasonably enough, a large island should have a great number of species, whereas a small one should support only a few species.

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Earlier, in the context of biogeography, there was a reference to animals “colonizing.” This may sound like the behavior of humans only, but other animals are also capable of colonizing. Nor are humans the only creatures that crossed the Atlantic Ocean from the Old World to colonize parts of the New World. During the nineteenth century, an Old World bird species known as the cattle egret managed to cross the Atlantic, perhaps driven by a storm, and founded a breeding colony in Brazil. Since then, it has expanded the range of its habitats, so that cattle egret colonies can be found as far north and east as Ontario and as far south and west as southern Chile.

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These principles have helped in the study of other “island” ecosystems that are not necessarily on islands but rather in or on isolated lakes, mountain ranges surrounded by deserts, and patches of forest left behind by clear-cut logging. As a result of such investigations, loggers in the forests of the Pacific Northwest or the Amazon valley have been encouraged to leave behind larger stands of trees in closer proximity to one another.

Ardennes during the world wars nor logging in the Amazon valley in the late twentieth century managed to destroy those biomes, but it is quite conceivable that they could have. On the other hand, a disturbance can affect an individual lifeform, as when lightning strikes and kills a mature tree in a forest, creating a gap that will be filled through the growth of another tree—an example of microsuccession.

This makes possible the survival of species at higher trophic levels (positions on the food web) and of those with very specialized requirements as to food or habitat. Examples of the latter species include Amazonian monkeys and the northern spotted owl, which we discuss later. Because studies in island biogeography have made land-use planners more aware of the barriers posed by clear-cut foresting, it has become common practice to establish “forest corridors.” These long, thin lines of trees connecting sections of forest ensure that one section will not be isolated completely from another.

F O R M S O F S U C C E SS I O N . Once succession begins, it can take one of several courses. It may lead to the restoration of the ecosystem in a form similar to that which it took before the disturbance. Or, depending on environmental circumstances, a very different ecosystem may develop. For example, suppose that a forest fire has wiped out a biological community and secondary succession has begun. It is conceivable that this succession process will restore the forest to something approaching its former state. On the other hand, the wildfire itself may well have been a signal of a climate change, in this case, to a drier, warmer environment. In this instance, succession may bring about a community quite different from that which preceded the disturbance.

Succession in Action In discussing succession earlier, it was noted that a disturbance usually sets the succession process in motion. Examples of such disturbances can include seismic events (earthquakes, tidal waves, or volcanic eruptions) and weather events (hurricanes or tornadoes). Across larger geologic timescales, the movement of glaciers or even of plates in Earth’s crust (see Paleontology for more about plate tectonics and its effect on environments) can set succession processes in motion. There are also causes directly within the biosphere, or the realm of all life, that can bring about disturbances. Among them are wildfires as well as sudden infestations of insects that act to defoliate, or remove the leaf cover from, a mature forest. Quite a few disturbances can result from activities on the part of the biosphere’s most complex species: Homo sapiens. Humans can cause ecological disturbances by plowing up ground, by harvesting trees from forests, by bulldozing land for construction purposes—even by causing explosions on a military reservation or battlefield. Disturbances can take place on a grand scale or a small scale. It is theoretically possible for disturbances—even man-made ones—to wipe out forests as large as the Ardennes in northwestern Europe or the Amazon rain forest in South America. Fortunately, neither shelling in the

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The “disturbance” itself actually may be the alleviation of a long-term environmental stress that has plagued the community. Suppose that a biological community has suffered from a local source of pollution, for instance, from a factory dumping toxins into the water supply. Suppose, too, that pressure from state or federal authorities finally forces a cleanup. How does this affect the biotic environment? In all likelihood, species that are sensitive to pollution (i.e., ones that normally could not survive in polluted conditions) would invade the area. Removal of an environmental stress may not always be a matter of pollution and cleanup. For instance, a herd of cattle may be overgrazing a pasture, thus holding back the growth of plant species in the area. Imagine, then, that the cattle are moved elsewhere; as a result, new plant species will proliferate in the area, and, in all likelihood, the biological diversity of that particular ecosystem will increase.

Primary Succession As we noted earlier, primary succession occurs in an environment where there has never been a significant biological community or in the wake of

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GLACIER BAY, ALASKA,

IS AN EXAMPLE OF AN ECOSYSTEM THAT EXPERIENCED PRIMARY SUCCESSION IN THE WAKE

OF DEGLACIATION; AS THE GLACIERS MELTED, VARIOUS PLANTS MOVED IN, EACH TAKING ITS TURN AS DOMINANT SPECIES.

THE

HABITAT NOW HAS REACHED MATURITY, AND ACCESS TO RESOURCES IS ALLOCATED AS FULLY AS IT

CAN BE AMONG THE DOMINANT SPECIES. (© Pat O’Hara/Corbis. Reproduced by permission.)

404

disturbances that have been intense enough to wipe out all traces of a biological community. An example of primary succession in an area that

has not possessed a biological community would be an abandoned paved parking lot. Eventually, the asphalt would give way to plant life, and given

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ISRAELI

FARMERS TRY TO WARD OFF A SWARM OF LOCUSTS.

DEFOLIATION

BROUGHT ABOUT BY INSECTS IS ONE EXAM-

PLE OF THE TYPE OF DISTURBANCE THAT MAY SERVE AS A PRECURSOR TO SECONDARY SUCCESSION. (© Hulton-Deutsch

Collection/Corbis. Reproduced by permission.)

enough time, a wide-ranging biological community might develop around it.

soil. (See The Biosphere for more about nitrogen fixing and biogeochemical cycles.)

These statements should be qualified in two ways, however. When it is said that an area has not maintained a significant biological community, this refers only to the recent or relatively recent past. In the case of the parking lot area, there probably have been countless biological communities in that spot over the ages, each replaced by the other in a process of primary succession. Also, by significant biological community we mean a biological community that exists above ground; even in the instance of the parking lot, there would be an extensive biological community underground. (See The Biosphere for more about life in the soil.)

These were the pioneer species, and over time they were replaced by larger plants, such as a short version of the willow. Later, taller shrubs, such as the alder (also a nitrogen-fixing species), dominated the area for about half a century. In time, Sitka spruce (Picea sitchensis), western hemlock (Tsuga heterophylla), and mountain hemlock (T. mertensiana) each had its turn as dominant plant species. With the last group, Glacier Bay reached climax, meaning that the dominant species are not those most tolerant of stresses associated with competition. The habitat thus has reached maturity, and access to resources is allocated as fully as it can be among the dominant species. Accompanying these changes have been changes in nonliving parts of the ecosystem as well, including the soil and its acidity.

G LAC I E R B AY. Glacier Bay, in southern Alaska, is an example of an ecosystem that experienced primary succession in the wake of deglaciation, or the melting of a glacier. The glaciers there have been melting for at least the past few hundred years, and as this melting began to occur, plants moved in. The first were mosses and lichens, flowering plants such as the river-beauty (Epilobium latifolium), and the mountain avens (Dryas octopetala), noted for their ability to “fix” or transform nitrogen into forms usable by the

S C I E N C E O F E V E RY DAY T H I N G S

Secondary Succession When a disturbance has not been so intense or sweeping as to destroy all life within an ecosystem, regeneration may occur, bringing about secondary succession. But regeneration of existing species is not the only mechanism that makes secondary succession possible; invasions by new

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plant species typically augment the succession process. While much else changes in the environment of a secondary succession, the quality of the soil itself remains constant, as do other characteristics, such as climate. Because it is rare for a disturbance to be powerful enough to obliterate all preexisting lifeforms, secondary succession is much more common than primary succession. Examples of the type of disturbance that may serve as a precursor to secondary succession are windstorms, wildfires, and defoliation brought about by insects— provided, of course, that the destruction caused by these phenomena is less than total. The same is true of most disturbances associated with human activities, such as the abandonment of agricultural lands and the harvesting of forests by cutting down trees for lumber or pulp. In a forest of mixed species in the eastern United States, the dominant trees are a mixture of angiosperms and coniferous species (respectively, plants that reproduce by producing flowers and those that reproduce by producing cones bearing seeds), and there are plant species capable or surviving under the canopy, or “roof,” provided by these trees. Suppose that the forest has been clear-cut. This means that most or all of the large trees have been removed, but the entire biological community has not been wiped out, since loggers typically would not bother to cut down smaller plants that are not in their way. As soon as the clear-cutting is over, regeneration begins. One form that this takes is the formation of new sprouts from the stumps of the old angiosperms. These sprouts are likely to grow rapidly and then experience a process of self-thinning, in which only the hardiest shoots survive. Within half a century, a given tree will have only one to three mature stems growing from its stump. At the same time, other species regenerate seemingly from nowhere, though actually they are growing from a “seed bank” buried in the forest floor, where trees have dropped countless seeds over the generations. Species such as the pin cherry (Prunus pennsylvanica) and red raspberry (Rubus strigosus) are particularly adept at regeneration in this form. Therefore, these species are likely to feature prominently in the forest during the first several decades of secondary succession.

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large numbers; if they are to obtain a stake in the secondary succession, they must do so by a process of re-invasion. Such often happens in the case of coniferous trees. Other species may also invade when they have not previously been a part of the habitat, yet they enter now because the temporary conditions of resource availability and limited competition make the prospect for invasion attractive. A great number of species, from alders and white birch to various species of grasses, fit into this last category. Plants are not the only organisms involved in secondary succession. In a mature forest of the type described, the dominant bird forms probably include species of warblers, vireos, thrushes, woodpeckers, and flycatchers. When clear-cutting occurs, however, these birds are likely to be replaced by an entirely new avian community—one composed of birds more suited to the immature habitat that follows a disturbance. As time passes, however, and the forest regenerates fully, the bird species of the mature forest re-invade and resume dominance, a process that may well take three to four decades.

Old-Growth Forests Old-growth forests represent a climax ecosystem—one that has come to the end of its stages of succession. They are dominated by trees of advanced age (hence the name old-growth), and the physical structure of these ecosystems is extraordinarily complex. In some places, the forest canopy is dense and layered, whereas in others it has gaps. Tree sizes vary enormously, and the forest is littered with the remains of dead trees. An old-growth forest, by definition, takes a long time to develop. Not only must it have been free from human disturbance, but it also must have been spared various natural disturbances of the kind that we have mentioned, disturbances that bring about the conditions for succession. Typically, then, most old-growth forests are rain forests in tropical and temperate environments, where they are unlikely to suffer such stresses as drought and wildfire. Among North American old-growth forests are those of the United States Pacific Northwest as well as in adjoining regions of southwestern Canada.

On the other hand, some tree species simply do not survive clear-cutting, or at least not in

T H E S P O T T E D O W L . These oldgrowth forests of North America are home to a bird that became well known in the 1980s and 1990s to environmentalists and their critics: the

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KEY TERMS A measure of the

ongoing succession. In such a situation, the

degree to which an ecosystem possesses

community is at equilibrium with environ-

large numbers of particular species. An

mental conditions, and conditions are sta-

abundant ecosystem may or may not have

ble, such that the biota experiences little

a wide array of different species. Compare

change thereafter.

with complexity.

COMPETITION:

ABUNDANCE:

Interaction between

A type of plant that

organisms of the same or different species

produces flowers during sexual reproduc-

brought about by their need for a common

tion.

resource that is available in quantities

ANGIOSPERM:

BIOGEOGRAPHY:

The study of the

insufficient to meet the biological demand. The range of ecological

geographic distribution of plants and ani-

COMPLEXITY:

mals, both today and over the course of

niches within a biological community. The

extended periods.

degree of complexity is the number of dif-

BIOLOGICAL COMMUNITY:

The liv-

ing components of an ecosystem. A large ecosystem, character-

BIOME:

ized by its dominant life-forms.

ferent species that could exist, in theory, in a given biota, as opposed to its diversity, or the actual range of existing species. A type of tree that produces

CONIFER:

cones bearing seeds. BIOSPHERE:

A combination of all liv-

ing things on Earth—plants, animals, birds, marine life, insects, viruses, singlecell organisms, and so on—as well as all

A measure of the number

DIVERSITY:

of different species within a biological community. A community of inter-

formerly living things that have not yet

ECOSYSTEM:

decomposed.

dependent organisms along with the inor-

BIOTA:

A combination of all flora and

ganic components of their environment. A term describing the

fauna (plant and animal life, respectively)

FOOD WEB:

in a region.

interaction of plants, herbivores, carni-

BIOTIC:

Life-related.

vores, omnivores, decomposers, and detritivores in an ecosystem. Each of these

The upper portion or layer of

organisms consumes nutrients and passes

the trees in a forest. A forest with a closed

it along to other organisms. Earth scientists

canopy is one so dense with vegetation that

typically prefer this name to food chain, an

the sky is not visible from the ground.

everyday term for a similar phenomenon.

CANOPY:

The pattern of weather con-

A food chain is a series of singular organ-

ditions in a particular region over an

isms in which each plant or animal

extended period. Compare with weather.

depends on the organism that precedes it.

CLIMATE:

CLIMAX:

A theoretical notion intended

Food chains rarely exist in nature.

to describe a biological community that

FOREST:

has reached a stable point as a result of

simply any ecosystem dominated by tree-

S C I E N C E O F E V E RY DAY T H I N G S

In general terms, a forest is

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KEY TERMS size woody plants. Numerous other characteristics and parameters (for example, weather, altitude, and dominant species) further define types of forests, such as tropical rain forests. Succession on a very small scale within a larger ecosystem or biological community. Microsuccession can occur at the level of a stand of trees or even a single tree. MICROSUCCESSION:

The process whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment.

NATURAL SELECTION:

A term referring to the role that a particular organism plays within its biological community. NICHE:

northern spotted owl, or Strix occidentalis caurina. A nonmigratory bird, the spotted owl has a breeding pattern such that it requires large tracts of old-growth, moist-to-wet conifer forest as its habitat. These are the spotted owl’s environmental requirements, but given the potential economic value of old-growth forests in the region, the situation was bound to generate heated controversy as the needs of the spotted owl clashed with those of local humans. On the one hand, environmentalists insisted that the spotted owl’s existence would be threatened by logging, and, on the other, representatives of the logging industry and the local community maintained that prevention of logging in the old-growth forests would cost jobs and livelihoods. The question was not an easy one, pitting the interests of the environment against those of ordinary human beings. By the early 1990s the federal government had stepped in on the side of the environmentalists, having recognized the spotted owl as a threatened species under the terms of the U.S. Endangered Species Act of 1973. Even so, controversy over the spotted owl—and over the proper role of environmental,

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CONTINUED

OLD-GROWTH:

An adjective for a cli-

max community. SUCCESSION:

The progressive re-

placement of earlier biological communities with others over time. Succession, which can culminate in a climax community (see climax), is either primary, which occurs where there is no preexisting biological community (or no such community has survived), or secondary, in which a biological community regenerates in the wake of a disturbance, such as a forest fire. TROPHIC

LEVELS:

Various stages

within a food web. For instance, plants are on one trophic level, herbivores on another, and so on.

economic, and political concerns in such situations—continues. CONTINUING

C O N T R O V E R S Y.

Another concern raised by the logging of oldgrowth forests has been the need to preserve dead trees, which provide a habitat for woodpeckers and other varieties of species. This concern, too, has brought about conflict with loggers, who find that dead wood gets in the way of their work. Dead wood, after all, is an expression for something or someone that is not performing a useful function (as in, “We’re removing all the dead wood from the team”), and to loggers this literal dead wood is nothing more than a nuisance. Unfortunately, the United States logging industry typically has not pursued a strategy of attempting to manage old-growth forests as a renewable resource, which these forests could be, given enough time. Instead, logging companies—interested in immediate profits and not much else—have tended to treat old-growth forests as though they were more like coal mines, home of a nonrenewable resource. In this “mining” model of tree harvesting, the forest is allowed to experience a process of succession

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such that a younger, second-growth forest emerges. Over time, this might become an oldgrowth forest, but the need to turn a quick profit means that the forest likely will be cut down before that time comes.

removing only certain trees. Many environmentalists contend, however, that even the new forestry disturbs the essential character of oldgrowth forests.

The average citizen, who typically has no vested interest in the side of either the loggers or the environmentalists, might well find good and bad on both sides of the issue. Certainly, the image of radical environmentalists chaining themselves to trees is as distasteful as the idea of loggers removing valuable natural resources. There is also a class dimension to the struggle, since a person deeply concerned about environmental issues is probably someone from an economic level above mere survival. This results in another distasteful image: of upper-middle-class and upper-class environmentalists inhibiting the livelihood of working-class loggers.

WHERE TO LEARN MORE

On the other hand, as we have already suggested, the logging companies themselves are big business and hardly representative of the working class. Largely as a result of pressure from environmentalists, these companies have attempted to develop more environmentally responsible logging schemes under the framework of what is called new forestry. These practices involve leaving a forest largely intact and

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Browne, E. J. The Secular Ark: Studies in the History of Biogeography. New Haven: Yale University Press, 1983. Cox, C. Barry, and Peter D. Moore. Biogeography: An Ecological and Evolutionary Approach. Malden, MA: Blackwell Science, 2000. The Eastern Old Growth Clearinghouse (Web site). . Environmental Biology—Grasslands (Web site). . Forestry: Ecosystems: Forest Succession. Saskatchewan Interactive (Web site). . Introduction to Biogeography and Ecology: Plant Succession. Fundamentals of Physical Geography . Old-Growth Forests in the United States Pacific Northwest (Web site). . Reed, Willow. Succession: From Field to Forest. Hillside, NJ: Enslow Publishers, 1991. Succession (Web site). .

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LIBRARY OF CONGRESS CATALOG-IN-PUBLICATION DATA Knight, Judson. Science of everyday things / written by Judson Knight, Neil Schlager, editor. p. cm. Includes bibliographical references and indexes. Contents: v. 1. Real-life chemistry – v. 2 Real-life physics. SBN 0-7876-5631-3 (set : hardcover) – ISBN 0-7876-5632-1 (v. 1) – ISBN 0-7876-5633-X (v. 2) 1. Science–Popular works. I. Schlager, Neil, 1966-II. Title. Q162.K678 2001 500–dc21

2001050121

ISBN 0-7876-5631-3 (set), 0-7876-5632-1 (vol. 1), 0-7876-5633-X (vol. 2), 0-7876-5634-8 (vol. 3), 0-7876-5635-6 (vol. 4)

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CONTENTS

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . v

EARTH’S INTERIOR

Advisory Board . . . . . . . . . . . . . . . . . . . . . . vii

Earth’s Interior . . . . . . . . . . . . . . . . . . . . . . . 210 Plate Tectonics. . . . . . . . . . . . . . . . . . . . . . . . 219 Seismology. . . . . . . . . . . . . . . . . . . . . . . . . . . 230

UNDERSTANDING THE EARTH SCIENCES

GEOMORPHOLOGY Earth, Science, and Nonscience . . . . . . . . . . . . . 3 Geoscience and Everyday Life . . . . . . . . . . . . . 12 Earth Systems. . . . . . . . . . . . . . . . . . . . . . . . . . 23 THE STUDY OF EARTH Studying Earth . . . . . . . . . . . . . . . . . . . . . . . . . 35 Measuring and Mapping Earth. . . . . . . . . . . . 44 Remote Sensing . . . . . . . . . . . . . . . . . . . . . . . . 53 PLANETOLOGY

Geomorphology . . . . . . . . . . . . . . . . . . . . . . 245 Mountains . . . . . . . . . . . . . . . . . . . . . . . . . . . 253 Erosion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 264 Mass Wasting. . . . . . . . . . . . . . . . . . . . . . . . . 273 SEDIMENTOLOGY AND SOIL SCIENCE Sediment and Sedimentation . . . . . . . . . . . . 283 Soil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292 Soil Conservation . . . . . . . . . . . . . . . . . . . . . 301

Planetary Science . . . . . . . . . . . . . . . . . . . . . . . 61 Sun, Moon, and Earth . . . . . . . . . . . . . . . . . . . 72

GEOCHEMISTRY

GEOLOGY

Biogeochemical Cycles . . . . . . . . . . . . . . . . . 313 The Carbon Cycle . . . . . . . . . . . . . . . . . . . . . 323 The Nitrogen Cycle . . . . . . . . . . . . . . . . . . . . 331

Historical Geology Historical Geology . . . . . . . . . . . . . . . . . . . . . . 87 Geologic Time . . . . . . . . . . . . . . . . . . . . . . . . . 95 Stratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . 105 Paleontology . . . . . . . . . . . . . . . . . . . . . . . . . 114 Physical Geology Minerals. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129 Rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Economic Geology . . . . . . . . . . . . . . . . . . . . 154

THE BIOSPHERE Ecosystems. . . . . . . . . . . . . . . . . . . . . . . . . . . 341 Ecology and Ecological Stress . . . . . . . . . . . 351 WATER AND THE EARTH Hydrology . . . . . . . . . . . . . . . . . . . . . . . . . . . 361 The Hydrologic Cycle . . . . . . . . . . . . . . . . . . 369 Glaciology . . . . . . . . . . . . . . . . . . . . . . . . . . . 376

GEOPHYSICS Gravity and Geodesy. . . . . . . . . . . . . . . . . . . 169 Geomagnetism . . . . . . . . . . . . . . . . . . . . . . . 177 Convection . . . . . . . . . . . . . . . . . . . . . . . . . . 185 Energy and Earth . . . . . . . . . . . . . . . . . . . . . 192

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METEOROLOGY AND THE ATMOSPHERE Evapotranspiration and Precipitation . . . . . 387 Weather . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 395 Climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405 VOLUME 4: REAL-LIFE EARTH SCIENCE

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General Subject Index . . . . . . . . . . . . . 413 Cumulative Index by “Everyday Thing” . . . . . . . . . . . . . . . . . . . . . . . . . . 431 Cumulative General Subject Index . 449

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INTRODUCTION

Overview of the Series Welcome to Science of Everyday Things. Our aim is to explain how scientific phenomena can be understood by observing common, real-world events. From luminescence to echolocation to buoyancy, the series will illustrate the chief principles that underlay these phenomena and explore their application in everyday life. To encourage cross-disciplinary study, the entries will draw on applications from a wide variety of fields and endeavors. Science of Everyday Things initially comprises four volumes: Volume 1: Real-Life Chemistry Volume 2: Real-Life Physics Volume 3: Real-Life Biology Volume 4: Real-Life Earth Science Future supplements to the series will expand coverage of these four areas and explore new areas, such as mathematics.

Arrangement of Real-Life Earth Science This volume contains 40 entries, each covering a different scientific phenomenon or principle. The entries are grouped together under common categories, with the categories arranged, in general, from the most basic to the most complex. Readers searching for a specific topic should consult the table of contents or the general subject index.

• Concept: Defines the scientific principle or theory around which the entry is focused. • How It Works: Explains the principle or theory in straightforward, step-by-step language. • Real-Life Applications: Describes how the phenomenon can be seen in everyday life. • Where to Learn More: Includes books, articles, and Internet sites that contain further information about the topic. In addition, each entry includes a “Key Terms” section that defines important concepts discussed in the text. Finally, each volume includes many illustrations and photographs throughout. Included in this volume, readers will find (in addition to the volume-specific general subject index), a cumulative general index, as well as a cumulative index of “everyday things.” This latter index allows users to search the text of the series for specific everyday applications of the concepts.

About the Editor, Author, and Advisory Board Neil Schlager and Judson Knight would like to thank the members of the advisory board for their assistance with this volume. The advisors were instrumental in defining the list of topics, and reviewed each entry in the volume for scientific accuracy and reading level. The advisors include university-level academics as well as high school teachers; their names and affiliations are listed elsewhere in the volume.

Within each entry, readers will find the following rubrics:

Neil Schlager is the president of Schlager Information Group Inc., an editorial services company. Among his publications are When

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Technology Fails (Gale, 1994); How Products Are Made (Gale, 1994); the St. James Press Gay and Lesbian Almanac (St. James Press, 1998); Best Literature By and About Blacks (Gale, 2000); Contemporary Novelists, 7th ed. (St. James Press, 2000); Science and Its Times (7 vols., Gale, 20002001); and Science in Dispute (Gale, 2002). His publications have won numerous awards, including three RUSA awards from the American Library Association, two Reference Books Bulletin/Booklist Editors’ Choice awards, two New York Public Library Outstanding Reference awards, and a CHOICE award for best academic book.

extensive contributions to Gale’s seven-volume Science and Its Times (2000-2001). As a writer on history, Knight has published Middle Ages Reference Library (2000), Ancient Civilizations (1999), and a volume in U•X•L’s African American Biography series (1998). Knight’s publications in the realm of music include Parents Aren’t Supposed to Like It (2001), an overview of contemporary performers and genres, as well as Abbey Road to Zapple Records: A Beatles Encyclopedia (Taylor, 1999).

Comments and Suggestions

Judson Knight is a freelance writer, and author of numerous books on subjects ranging from science to history to music. His work on science includes Science, Technology, and Society, 2000 B.C.-A.D. 1799 (U•X•L, 2002), as well as

Your comments on this series and suggestions for future editions are welcome. Please write: The Editor, Science of Everyday Things, Gale Group, 27500 Drake Road, Farmington Hills, MI 483313535.

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ADVISORY BOARD

William E. Acree, Jr. Professor of Chemistry, University of North Texas Russell J. Clark Research Physicist, Carnegie Mellon University Maura C. Flannery Professor of Biology, St. John’s University, New York John Goudie Science Instructor, Kalamazoo (MI) Area Mathematics and Science Center Cheryl Hach Science Instructor, Kalamazoo (MI) Area Mathematics and Science Center Michael Sinclair Physics instructor, Kalamazoo (MI) Area Mathematics and Science Center Rashmi Venkateswaran Senior Instructor and Lab Coordinator, University of Ottawa

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S C I E N C E O F E V E RY DAY T H I N G S Real-Life Earth Science

U N D E R S TA N D I N G THE EARTH SCIENCES EARTH, SCIENCE, AND NONSCIENCE GEOSCIENCE AND E V E RY DAY L I F E EARTH SYSTEMS

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EARTH, SCIENCE, AND NONSCIENCE

CONCEPT To understand the composition and structure of Earth, one must comprehend the forces that shaped it. Much the same is true of the earth sciences themselves, which originated from attempts to explain the origins of Earth and the materials of which it is composed. Before the modern era, such explanations had roots in religion, mythology, or philosophy and drew from preconceived ideas rather than from observed data. A turning point came with the development of the scientific method, a habit of thinking that spread from astronomy and physics to chemistry and the earth sciences.

HOW IT WORKS

complex than these is a third variety of causeeffect relationship, the formal cause—that is, the design or blueprint on which something is modeled. The first three Aristotelian causes provide a pathway for explaining how; the fourth and last cause approaches the much more challenging question of why. This is the final cause, or the reason why a thing exists at all—in other words, the purpose for which it was made. Even in the case of the house, this is a somewhat complicated matter. A house exists, of course, to provide a dwelling for its occupants, but general contractors would not initiate the building process if they did not expect to make a profit, nor would the subcontractors and laborers continue to work on it if they did not earn an income from the project.

Aristotle’s Four Causes Though the Greek philosopher Aristotle (384–322 B.C.) exerted a negative influence on numerous aspects of what became known as the physical sciences (astronomy, physics, chemistry, and the earth sciences), he is still rightly regarded as one of the greatest thinkers of the Western world. Among his contributions to thought was the identification of four causes, or four approaches to the question of how and why something exists as it does.

Religion, Science, and Earth

In Aristotle’s system, which developed from ideas of causation put forward by his predecessors, the most basic of explanations is the material cause, or the substance of which a thing is made. In a house, for instance, the wood and other building materials would be the material cause. The builders themselves are the efficient cause, or the forces that shaped the house. More

There has always been a degree of tension between religion and the sciences, and nowhere has this been more apparent than in the earth sciences. As will be discussed later in this essay, most early theories concerning Earth’s structure and development were religious in origin, and even some modern explanations have theological roots. Certainly there is nothing wrong with a

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The matter of final cause is almost unimaginably more complex when applied to Earth rather than to a house. The question “Why does Earth exist?” or “What is the ultimate reason for Earth’s existence?” is not really a topic for science at all, but rather for theology and philosophy. Nor do the answers provided by religion and philosophical beliefs qualify as answers in the same sense that workable scientific theories do.

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animals, or for some other purpose. On the one hand, this seems like an example of conscious design by a loving creator, but as Charles Darwin (1809–1882) showed, it may simply be a matter of adaptability. According to Darwin, members of species unable to alter their appearance died out, leading to the dominance of those who could camouflage themselves.

Earth, Science, and Nonscience

In fact, science is not really capable of addressing the matter of a Designer (i.e., God), and thus, for scientists, the question of a deity’s role in nature is simply irrelevant. This is not because scientists are necessarily atheists (many are and have been dedicated men and women of faith) but because the concept of a deity simply adds an unnecessary step to scientific analysis.

ENGRAVING

AFTER A MARBLE BUST OF

ARISTOTLE.

(Library of Congress.)

scientist having religious beliefs, as long as those beliefs do not provide a filter for all data. If they do, the theologically minded scientist becomes rather like a mathematician attempting to solve a problem on the basis of love rather than reason. Most people would agree that love is higher and greater than mathematics; nonetheless, it has absolutely no bearing on the subject. SCIENTIFIC ANSWERS AND T H E S E A R C H F O R A D E S I G N E R.

The third, or formal, cause is less fraught with problems than the final cause when applied to the study of Earth, yet it also illustrates the challenges inherent in keeping science and theology separate. Does Earth have a “design,” or blueprint? The answer is yes, no, and maybe. Yes, Earth has a design in the sense that there is an order and a balance between its components, a subject discussed elsewhere with reference to the different spheres (geosphere, hydrosphere, biosphere, and atmosphere). The physical evidence, however, tends to suggest a concept of design quite different from the theistic notion of a deity who acts as creator.

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This is in line with Ockham’s razor, a principle introduced by the medieval English philosopher William of Ockham (ca. 1285?–1349). According to Ockham, “entities must not be unnecessarily multiplied.” In other words, in analyzing any phenomenon, one should seek the simplest and most straightforward explanation. Scientists are concerned with hard data, such as the evidence obtained from rock strata. The application of theological ideas in such situations would at best confuse and complicate the process of scientific analysis. THE ARGUMENT FROM DESIGN.

A few years before Ockham, the Italian philosopher Thomas Aquinas (1224 or 1225–1274) introduced a philosophical position known as the “argument from design.” According to Aquinas, whose idea has been embraced by many up to the present day, the order and symmetry in nature indicate the existence of God. Some philosophers have conceded that this order does indeed indicate the existence of a god, though not necessarily the God of Christianity. Science, however, cannot afford to go even that far: where spiritual matters are concerned, science must be neutral.

Consider, for example, the ability of an animal to alter its appearance as a means of blending in with its environment, to ward off predators, to disguise itself while preying upon other

Does any of this disprove the existence of God? Absolutely not. Note that science must be neutral, not in opposition, where spiritual matters are concerned. Indeed, one could not disprove God’s existence scientifically if one wanted to do so; to return to the analogy given earlier, such an endeavor would be akin to using mathematics to disprove the existence of love. Religious matters are simply beyond the scope of science,

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and to use science against religion is as misinformed a position as its opposite. SCIENCE AND THE FIRST T W O CAU S E S . To return to Aristotle’s

causes, let us briefly consider the material and efficient cause as applied to the subject of Earth. These are much simpler matters than formal and final cause, and science is clearly able to address them. An understanding of Earth’s material cause—that is, its physical substance—requires a brief examination of the chemical elements. The elements are primarily a subject for chemistry, though they are discussed at places throughout this book, inasmuch as they relate to the earth sciences and, particularly, geochemistry. Furthermore, the overall physical makeup of Earth, along with particular aspects of it, are subjects treated in much greater depth within numerous essays concerning specific topics, such as sedimentation or the biosphere. Likewise the efficient cause, or the complex of forces that have shaped and continue to shape Earth, is treated in various places throughout this book. In particular, the specifics of Earth’s origins and the study of these origins through the earth sciences are discussed in essays on aspects of historical geology, such as stratigraphy. Here the origins of Earth are considered primarily from the standpoint of the historical shift from mythological or religious explanations to scientific ones.

REAL-LIFE A P P L I C AT I O N S Mythology and Geology Most of what people believed about the origins and makeup of Earth before about 1700 bore the imprint of mythology or merely bad science. Predominant among these theories were the Creation account from the biblical Book of Genesis and the notion of the four elements inherited from the Greeks. These four elements—earth, air, fire, and water—were said to form the basis for the entire universe, and thus every object was thought to be composed of one or more of these elements. Thanks in large part to Aristotle, this belief permeated (and stunted) the physical sciences.

observed data but rather from religious principles. The concept of the four elements at least relates somewhat to observation, but specifically to untested observation; for this reason, it is hardly more scientific than the Genesis Creation story. The four elements were not, strictly speaking, a product of mythology, but they were mythological in the pejorative sense—that is, they had no real basis in fact.

Earth, Science, and Nonscience

The biblical explanation of Earth’s origins is but one of many creation myths, part of a larger oral and literary tradition that Dorothy B. Vitaliano, in her 1973 book Legends of the Earth, dubbed geomythology. Examples of geomythology are everywhere, and virtually every striking natural feature on Earth has its own geomythological backdrop. For instance, the rocky outcroppings that guard the western mouth of the Mediterranean, at Gibraltar in southern Spain and Ceuta in northern Morocco, are known collectively as the Pillars of Hercules because the legendary Greek hero is said to have built them. G E O M Y T H O L O G Y.

Geomythological stories can be found in virtually all cultures. For instance, traditional Hawaiian culture explains the Halemaumau volcano, which erupted almost continuously from 1823 to 1924, as the result of anger on the part of the Tahitian goddess Pele. Native Americans in what is now Wyoming passed down legends concerning the grooves along the sides of Devils Tower, which they said had been made by bears trying to climb the sides to escape braves hunting them. G R E E K G E O M Y T H O L O G Y. In Western culture, among the most familiar examples of geomythology, apart from those in the Bible, are the ones that originated in ancient Greece and Rome. The Pillars of Hercules represents but one example. In particular, the culture of the Greeks was infused with geomythological elements. They believed, for instance, that the gods lived on Mount Olympus and spoke through the Delphic Oracle, a priestess who maintained a trancelike state by inhaling intoxicating vapors that rose through a fault in the earth.

To call the biblical Creation story mythology is not, in this context at least, a value judgment. The Genesis account is not scientific, however, in the sense that it was not written on the basis of

Much of Greek mythology is actually geomythology. Most of the principal Greek deities ruled over specific aspects of the natural world that are today the province of the sciences, and many of them controlled realms now studied by the earth sciences and related disciplines. Certain branches of geology today are concerned

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with Earth’s interior, which the Greeks believed was controlled by Hades, or the Roman god Pluto. Volcanoes and thunderbolts were the work of the blacksmith god Hephaestus (the Roman deity Vulcan), while Poseidon (known to the Romans as Neptune) oversaw the area studied today by oceanographers. AT LA N T I S . Among the most persistent geomyths with roots in Greek civilization is the story of Atlantis, a continent that allegedly sank into the sea. Over the years, the myth grew to greater and greater dimensions, and in a blurring between the Atlantis myth and the biblical story of Eden, Atlantis came to be seen as a lost utopia. Even today, some people believe in Atlantis, and for scholarly endorsement they cite a passage in the writings of Plato (427?–347 B.C.). The great Greek philosopher depicted Atlantis as somewhere beyond the Pillars of Hercules, and for this reason its putative location eventually shifted to the middle of the Atlantic—an ocean in fact named for the “lost continent.”

Given the layers of mythology associated with Atlantis, it may come as a surprise that the story has a basis in fact and that accounts of it appear in the folklore of peoples from Egypt to Ireland. It is likely that the myth is based on a cataclysmic event, either a volcanic eruption or an earthquake, that took place on the island of Crete, as well as nearby Thíra, around 1500 B.C. This cataclysm, some eight centuries before the rise of classical Greek civilization, brought an end to the Minoan civilization centered around Knossos in Crete. Most likely it raised vast tidal waves, or tsunamis, that reached lands far away and may have caused other cities or settlements to disappear beneath the sea. B I B L I CA L G E O M Y T H O LO GY. As important as such Greek stories are, no geomythological account has had anything like the impact on Western civilization exerted by the first nine chapters of the Bible. These chapters contain much more than geomythology, of course; in fact, they introduce the central themes of the Bible itself: righteousness, sin, redemption, and God’s covenant with humankind. In these nine chapters (or, more properly, eight and a half chapters), which cover the period from Earth’s creation until the Great Flood, events are depicted as an illustration of this covenant. Thus, in 9 Genesis, when God introduces the rainbow after the Flood, he does so with the statement that it is

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a sign of his promise never again to attempt to destroy humanity. As with Atlantis, the story of the Great Flood appears in other sources as well. Its antecedents include the Sumerian Gilgamesh epic, which originated in about 2000 B.C., a millennium before the writing of the biblical account. Also as in the case of Atlantis, the biblical flood seems to have a basis in fact. Some modern scientists theorize that the Black Sea was once a freshwater lake, until floods covered the land barriers that separated it from saltwater. The Flood occupies chapters 6 through 9 of Genesis, while chapters 3 through 5 are concerned primarily with human rather than geologic events. The story of Adam, Eve, the serpent, and the fruit of the Tree of Knowledge is a beautiful, complex, and richly symbolic explanation of how humans, born innocent, are prone to sin. It is the first conflict between God and human, just as Cain’s murder of Abel is the first conflict between people. Both stories serve to illustrate the themes mentioned earlier: in both cases, God punishes the sins of the humans but also provides them with protection as a sign of his continued faithfulness. T H E B I B L E A N D S C I E N C E . In fact, the entire Creation story, source of centuries’ worth of controversy, occupies only two chapters, and this illustrates just how little attention the writers of the Bible actually devoted to “scientific” subjects. Certainly, many passages in the Bible describe phenomena that conflict with accepted scientific knowledge, but most of these fall under the classification of miracles—or, if one does not believe them, alleged miracles. Was Jesus born of a virgin? Did he raise the dead? People’s answers to those questions usually have much more to do with their religious beliefs than with their scientific knowledge.

Most of the biblical events related to the earth sciences appear early in the Old Testament, and most likewise fall under the heading of “miracles.” Certain events, such as the parting of the Red Sea by Moses, even have possible scientific explanations: some historians believe that there was actually an area of dry land in the Red Sea region and that Moses led the children of Israel across it. The account of Joshua causing the Sun to stand still while his men marched around the city of Jericho is a bit more difficult to square with science, but a believer might say that the Sun (or rather, Earth) seemed to stand still. S C I E N C E O F E V E RY DAY T H I N G S

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In any case, the Bible does not present itself as a book of science, and certainly the Israelites of ancient times had little concept of science as we know it today. Some of the biblical passages mentioned here have elicited controversy, but few have inspired a great deal of discussion, precisely because they are generally regarded as accounts of miracles. The same is not true, however, of the first two chapters of Genesis, which even today remain a subject of dispute in some quarters.

Earth, Science, and Nonscience

Actually, 2 Genesis concerns Adam’s life before the Fall as well as the creation of Eve from his rib, so the Creation story proper is confined to the first chapter. One of the most famous passages in Western literature, 1 Genesis describes God’s creation of the universe in all its particulars, each of which he spoke into being, first by saying, “Let there be light.” After six days of activity that culminated with the creation of the human being, he rested, thus setting an example for the idea of a Sabbath rest day. SIX

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As prose poetry, the biblical Creation story is among the great writings of all time. It is also a beautiful metaphoric description of creation by a loving deity; but it is not a guide to scientific study. Yet for many centuries, Western adherence to the Genesis account (combined with a number of other factors, including the general stagnation of European intellectual life throughout much of the medieval period) forced a virtual standstill of geologic study. The idea that Earth was created in 144 hours reached its extreme with the Irish bishop James Ussher (1581–1656), who, using the biblical genealogies from Adam to Christ, calculated that God finished making Earth at 9:00 A.M. on Sunday, October 23, 4004 B.C.

The Myth of the Four Elements Religion alone is far from the only force that has slowed the progress of science over the years. Sometimes the ideas of scientists or philosophers themselves, when formed on the basis of something other than scientific investigation, can prove at least as detrimental to learning. Such is the case when thinkers become more dedicated to the theory than to the pursuit of facts, as many did in their adherence to the erroneous concept of the four elements.

DEVILS TOWER, WITH THE BIG DIPPER VISIBLE IN THE NIGHT SKY. (© Jerry Schad/Photo Researchers. Reproduced by permission.)

broken down chemically into a simpler substance. This definition developed over the period from about 1650 to 1800, thanks to the British chemist Robert Boyle (1627–1691), who originated the idea of elements as the simplest substances; the French chemist Antoine Lavoisier (1743–1794), who first distinguished between elements and compounds; and the British chemist John Dalton (1766–1844), who introduced the atomic theory of matter. During the twentieth century, with the discovery of the atomic nucleus and the protons within it, scientists further refined their definition of an element. Today elements are distinguished by atomic number, or the number of protons in the atomic nucleus. Carbon, for instance, has an atomic number 6, meaning that there are six protons in the carbon nucleus; therefore, any element with six protons in its atomic nucleus must be carbon. AT O M I C T H E O RY V E R S U S T H E F O U R E L E M E N T S . Atomic, or corpuscu-

Today scientists understand an element as a substance made up of only one type of atom, meaning that unlike a compound, it cannot be

lar, theory had been on the rise for some 150 years before Dalton, who built on ideas of predecessors that included Galileo Galilei (1564–1642)

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and Sir Isaac Newton (1642–1727). In any case, the first thinker to conceive of atoms lived more than 2,000 years earlier. He was Democritus (ca. 460–ca. 370 B.C.), a Greek philosopher who described the world as being composed of indivisible particles—atomos in Greek. Democritus’s idea was far from modern scientific atomic theory, but it came much closer than any other theory before the Scientific Revolution (ca. 1550–1700). Why, then, did it take so long for Western science to come around to the atomic idea? The answer is that Aristotle, who exerted an almost incalculable impact on Muslim and Western thought during the Middle Ages, rejected Democritus’ atomic theory in favor of the four elements theory. The latter had its roots in the very beginnings of Greek ideas concerning matter, but it was the philosopher Empedocles (ca. 490–430 B.C.) who brought the notion to some kind of maturity. A

NONSCIENTIFIC

T H E O R Y.

According to the four elements theory, every object could be identified as a combination of elements: bone, for instance, was supposedly two parts earth, two parts water, and two parts fire. Of course, this is nonsense, and, in fact, none of the four elements are even really elements. Water comes the closest, being a compound of the elements hydrogen and oxygen. Earth and air are mixtures, while fire is the result of combustion, a form of oxidation-reduction chemical reaction. Nonetheless, the theory had at least some basis in observation, since much of the physical world seems to include liquids, things that grow from the ground, and so on. Such observations alone, of course, are not enough to construct a theory, as would have become apparent if the Greeks had attempted to test their ideas. The ancients, however, tended to hold scientific experimentation in low esteem, and they were more interested in applying their intellects to the development of ideas than they were in getting their hands dirty by putting their concepts to the test.

Much earlier, the philosopher and mathematician Pythagoras (ca. 580–ca. 500 B.C.), who held that all of nature could be understood from the perspective of numbers, first suggested the idea of four basic elements because, he maintained, the number four represents perfection. This concept influenced Greek thinkers, including Empedocles and even Aristotle, and is also probably the reason for the expression four corners of the world. That expression, which conveys a belief in a flat Earth, raises an important point that must be made in passing. Despite his many erroneous ideas, Aristotle was the first to prove that Earth is a sphere, which he showed by observing the circular shadow on the Moon during a lunar eclipse. This points up the fact that ancient thinkers may have been misguided in many regards, yet they still managed to make contributions of enormous value. In the same vein, Pythagoras, for all his strange and mystical ideas, greatly advanced scientific knowledge by introducing the concept that numbers can be applied to the study of nature. In any case, the emphasis on fours trickled down through classical thought. Thus, the great doctors Hippocrates (ca. 460–ca. 377 B.C.) and Galen (129–ca. 199) maintained that the human body contains four “humors” (blood, black bile, green bile, and phlegm), which, when imbalanced, caused diseases. Humoral theory would exert an incalculable toll on human life throughout the Middle Ages, resulting in such barbaric medical practices as the use of leeches to remove “excess” blood from a patient’s body. The idea of the four elements had a less clearly pernicious effect on human well-being, yet it held back progress in the sciences and greatly impeded thinkers’ understanding of astronomy, physics, chemistry, and geology.

The Showdown Between Myth and Science

Aristotle explained the four elements as combinations of four qualities, or two pairs of opposites: hot/cold and wet/dry. Thus, fire was hot and dry, air was dry and cold, water was cold and wet, and earth was wet and hot. It is perhaps not accidental that

Aristotle’s teacher Plato had accepted the idea of the four elements, but proposed that space is made up of a fifth, unknown element. This meant that Earth and the rest of the universe are fundamentally different, a misconception that prevailed for two millennia. Aristotle adopted that idea, as well as Plato’s concept of a Demiurge, or Prime Mover, as Aristotle called it. Cen-

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there were four elements, four qualities, or even perhaps four Aristotelian causes.

IN

FOURS.

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turies later Aquinas equated Aristotle’s Prime Mover with the Christian God. Building on these and other ideas, Aristotle proceeded to develop a model of the cosmos in which there were two principal regions: a celestial, or heavenly, realm above the orbit of the Moon and a terrestrial, or earthly, one in what was known as the sublunary (below the Moon) region. Virtually everything about these two realms differed. The celestial region never changed, whereas change was possible on Earth. Earth itself consisted of the four elements, whereas the heavens were made up of a fifth substance, which he called ether. If left undisturbed, Aristotle theorized, the four elements would completely segregate into four concentric layers, with earth at the center, surrounded by water, then air, and then fire, bounded at the outer perimeter by the ether. The motion of bodies above the Moon’s sphere caused the elements to behave unnaturally, however, and thus they remained mixed and in a constant state of agitation. The distinction between so-called natural and unnatural (or violent) motion became one of the central ideas in Aristotle’s physics, a scientific discipline whose name he coined in a work by the same title. According to Aristotle, all elements seek their natural position. Thus, the element earth tends to fall toward the center of the universe, which was identical with the center of Earth itself. THE SCIENTIFIC REVOLUT I O N . On these and other ideas, Aristotle built

a complex, systematic, and almost entirely incorrect set of principles that dominated astronomy and physics as well as what later became the earth sciences and chemistry. The influence of Aristotelian ideas on astronomy, particularly through the work of the Alexandrian astronomer Ptolemy (ca. 100–170), was especially pronounced. It was through astronomy, the oldest of the physical sciences, that the Aristotelian and Ptolemaic model of the physical world ultimately was overthrown. This revolution began with the proof, put forward by Nicolaus Copernicus (1473–1543), that Earth is not the center of the universe. The Catholic Church, which had controlled much of public life in Europe for the past thousand years, had long since accepted Ptolemy’s geocentric model on the reasoning that if the human being is created in God’s image, Earth

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must be at the center of the universe. Copernicus’ heliocentric (Sun-centered) cosmology therefore constituted a challenge to religious authority—a very serious matter at a time when the Church held the power of life and death.

Earth, Science, and Nonscience

Copernicus died before he suffered the consequences of his ideas, but Galileo, who lived much later, found himself in the middle of a debate between the Church and science. This conflict usually is portrayed in simplistic terms, with Galileo as the noble scientific genius defending reason against the powers of reaction, but the facts are much more complex. For centuries, the Church had preserved and encouraged learning, and the reactionary response to Copernican ideas must be understood in light of the challenges to Catholic authority posed by the Protestant Reformation. Furthermore, Galileo was far from diplomatic in his dealings, for instance, deliberately provoking Pope Urban VIII (1568–1644), who had long been a friend and supporter. In any case, Galileo made a number of discoveries that corroborated Copernicus’ findings while pointing up flaws in the ideas of Aristotle and Ptolemy. He also conducted studies on falling objects that, along with the laws of planetary motion formulated by Johannes Kepler (1571–1630), provided the basis for Newton’s epochal work in gravitation and the laws of motion. Perhaps most of all, however, Galileo introduced the use of the scientific method. T H E S C I E N T I F I C M E T H O D . The scientific method is a set of principles and procedures for systematic study using evidence that can be clearly observed and tested. It consists of several steps, beginning with observation. This creates results that lead to the formation of a hypothesis, an unproven statement about the way things are. Up to this point, we have gone no further than ancient science: Aristotle, after all, was making a hypothesis when he said, for instance, that heavy objects fall faster than light ones, as indeed they seem to do.

Galileo, however, went beyond the obvious, conducting experiments that paved the way for modern understanding of the acceleration due to gravity. As it turns out, heavy objects fall faster than light ones only in the presence of resistance from air or another medium, but in a vacuum a stone and a feather would fall at the same rate. How Galileo arrived at this idea is not important

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Earth, Science, and Nonscience

KEY TERMS The smallest particle of an ele-

electrons. An atom can exist either alone or

A scientific principle that is shown always to be the case and for which no exceptions are deemed possible.

in combination with other atoms in a mol-

PHYSICAL SCIENCES:

ATOM:

ment, consisting of protons, neutrons, and

ecule. The number of

ATOMIC NUMBER:

protons in the nucleus of an atom. A substance made up of

COMPOUND:

atoms of more than one element, chemically bonded to one another. COSMOLOGY:

A branch of astronomy

concerned with the origin, structure, and evolution of the universe.

LAW:

Astronomy, physics, chemistry, and the earth sciences.

PROTON:

A positively charged particle

in an atom. A set of principles and procedures for systematic study that includes observation; the formation of hypotheses, theories, and ultimately laws on the basis of such observation; and continual testing and reexamination.

SCIENTIFIC METHOD:

A period of accelerated scientific discovery that completely reshaped the world. Usually dated from about 1550 to 1700, the Scientific Revolution saw the origination of the scientific method and the introduction of such ideas as the heliocentric (Sun-centered) universe and gravity. SCIENTIFIC REVOLUTION:

COSMOS:

The universe.

ELEMENT:

A substance made up of

only one kind of atom. Unlike compounds, elements cannot be broken chemically into other substances. GEOCENTRIC:

Earth-centered. Folklore inspired

GEOMYTHOLOGY:

by geologic phenomena. HELIOCENTRIC: HYPOTHESIS:

Sun-centered. An unproven state-

ment regarding an observed phenomenon.

here; rather, his application of the scientific method, which requires testing of hypotheses, is the key point. If a hypothesis passes enough tests, it becomes a theory, or a general statement. An example of a theory is uniformitarianism, an early scientific explanation of Earth’s origins discussed elsewhere, in the context of historical geology. Many scientific ideas remain theories and are quite workable as such: in fact, much of modern physics is based on the quantum model of subatomic behavior, which remains a theory. But if something always has been observed to be the case and if, based on what scientists know, no 10

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A general statement derived from a hypothesis that has withstood sufficient testing.

THEORY:

VACUUM:

An area devoid of matter,

even air.

exceptions appear possible, it becomes a law. An example is Newton’s third law of motion: no one has ever observed or created a situation in which a physical action does not yield an equal and opposite reaction. Even laws can be overturned, however, and every scientific principle therefore is subjected to continual testing and reexamination, making the application of the scientific method a cyclical process. Thus, to be scientific, a principle must be capable of being tested. It should also be said that one of the hallmarks of a truly scientific theory is the attitude of its adherents. True scientists are S C I E N C E O F E V E RY DAY T H I N G S

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always attempting to disprove their own ideas by subjecting them to rigorous tests; the more such tests a theory survives, the stronger it becomes.

Creationism: Religion Under a Veil of Science During the twentieth century, a movement called creationism emerged at the fringes of science. Primarily American in origin, creationism is a fundamentalist Christian doctrine, meaning that it is rooted in a strict literal interpretation of the Genesis account of Creation. (For this reason, creationism has little influence among Christians and Christian denominations not prone to literalism.) From the 1960s onward, it has been called creation science, but even though creationism sometimes makes use of scientific facts, it is profoundly unscientific. Again, the reference to creationism as unscientific does not necessarily carry a pejorative connotation. Many valuable things are unscientific; however, to call creationism unscientific is pejorative in the sense that its adherents claim that it is scientific. The key difference lies in the attitude of creationists toward their theory that God created the Earth if not in six literal days, then at least in a very short time. If this were a genuine scientific theory, its adherents would be testing it constantly against evidence, and if the evidence contradicted the theory, they would reject the theory, not the evidence. Science begins with facts that lead to the development of theories, but the facts always remain paramount. The opposite is true of creationism and other nonscientific beliefs whose proponents simply look for facts to confirm what they have decided is truth. Conflicting evidence simply is dismissed or incorporated into the theory; thus, for instance, fossils are said to be the remains of animals who did not make it onto Noah’s ark. Creationism (for which The Oxford Companion to the Earth provides a cogent and balanced explanation) is far from the only unscientific theory that has pervaded the hard sciences, the social sciences, or society in general. Others, aside from the four elements, have included spontaneous generation and the phlogiston theory of fire as well as various bizarre modern notions, such as flat-Earth theory, Holocaust or Moon-landing

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denial, and Afrocentric views of civilization as a vast racial conspiracy. Compared with Holocaust denial, for instance, creationism is benign in the sense that its proponents seem to act in good faith, believing that any challenge to biblical literalism is a challenge to Christianity itself.

Earth, Science, and Nonscience

Still, there is no justification for the belief that Earth is very young; quite literally, mountains of evidence contradict this claim. Nor is the idea of an old Earth a recent development; rather, it has circulated for several hundred years—certainly long before Darwin’s theory of evolution, the scientific idea with which creationists take the most exception. For more about early scientific ideas concerning Earth’s age, see Historical Geology and essays on related subjects, including Paleontology and Geologic Time. These essays, of course, are concerned primarily with modern theories regarding Earth’s history, as well as the observations and techniques that have formed the basis for such theories. They also examine pivotal early ideas, such as the Scottish geologist James Hutton’s (1726–1797) principle of uniformitarianism. WHERE TO LEARN MORE Bender, Lionel. Our Planet. New York: Simon and Schuster Books for Young Readers, 1992. Elsom, Derek M. Planet Earth. Detroit: Macmillan Reference USA, 2000. Gamlin, Linda. Life on Earth. New York: Gloucester Press, 1988. Hancock, Paul L., and Brian J. Skinner. The Oxford Companion to the Earth. New York: Oxford University Press, 2000. Llamas Ruiz, Andrés. The Origin of the Universe. Illus. Luis Rizo. New York: Sterling Publishers, 1997. Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science. 2nd ed. New York: John Wiley and Sons, 1999. The Talk. Origins Archive: Exploring the Creation/Evolution Controversy (Web site). . Van der Pluijm, Ben A., and Stephen Marshak. Earth Structure (Web site). . Vitaliano, Dorothy B. Legends of the Earth. Bloomington: Indiana University Press, 1973. Web Elements (Web site). . Windows to the Universe (Web site). .

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GEOSCIENCE AND E V E R Y D AY L I F E Geoscience and Everyday Life

CONCEPT How can learning about rocks help us in our daily lives? The short answer is that geology and the related geologic sciences (sometimes referred to collectively as geoscience) give us a glimpse of the great complexity inherent in the natural world, helping us appreciate the beauty and order of things. This, in turn, makes us aware of our place in the scheme of things, so that we begin to see our own daily lives in their proper context. Beyond that, the study of geoscientific data can give us an enormous amount of information of practical value while revealing much about the world in which we dwell. The earth sciences are, quite literally, all around us, and by learning about the structures and processes of our planet, we may be surprised to discover just how prominent a place geoscience occupies in our daily lives and even our thought patterns.

HOW IT WORKS Why Study Geoscience? One of the questions students almost always ask themselves or their teachers is “How will I use this?” or “What does all this have to do with everyday life?” It is easy enough to understand the application of classes involved in learning a trade or practical skill—for example, wood shop or a personal finance course. But the question of applicability sometimes becomes more challenging when it comes to many mathematical and scientific disciplines. Such is the case, for instance, with the earth sciences and particularly geoscience. Yet if we think about these concerns for just a moment, it should become readily apparent just why they are applicable to our daily lives.

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After all, geoscience is the study of Earth, and therefore it relates to something of obvious and immediate practical value. We may think of a hundred things more important and pressing than studying Earth—romantic involvements, perhaps, or sports, or entertainment, or work (both inside and outside school)—yet without Earth, we would not even have those concerns. Without the solid ground beneath our feet, which provides a stage or platform on which these and other activities take place, life as we know it would be simply impossible. Our lives are bounded by the solid materials of Earth— rocks, minerals, and soil—while our language reflects the primacy of Earth in our consciousness. As we discuss later, everyday language is filled with geologic metaphors.

Defining Geoscience The geologic sciences—geology, geophysics, geochemistry, and related disciplines—are sometimes referred to together as geoscience. They are united in their focus on the solid earth and the mostly nonorganic components that compose it. In this realm of earth science, geology is the leading discipline, and it has given birth to many offshoots, including geophysics and geochemistry, which represent the union of geology with physics and chemistry, respectively. Geology is the study of the solid earth, especially its rocks, minerals, fossils, and land formations. It is divided into historical geology, which is concerned with the processes whereby Earth was formed, and physical geology, or the study of the materials that make up the planet. Geophysics addresses Earth’s physical processes as well as its gravitational, magnetic, and electric

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properties and the means by which energy is transmitted through its interior. Geochemistry is concerned with the chemical properties and processes of Earth—in particular, the abundance and interaction of chemical elements.

which geoscience and biology more or less overlap: sedimentology and soil science, since soil is a combination of rock fragments and organic material (see Soil).

These subjects are of principal importance in this book. Though geology takes the lion’s share of attention, geophysics and geochemistry each encompass areas of study essential to understanding our life on Earth: hence we look in separate essays at such geophysical subjects as Gravity and Geodesy or Geomagnetism as well as such geochemical topics as Biogeochemical Cycles, Carbon Cycle, and Nitrogen Cycle.

The Territory of Geoscience

OTHER AREAS OF GEOSCIENCE.

In addition to these principal areas of interest in geoscience, this book treats certain subdisciplines of geology as areas of interest in their own right. These include geomorphology and the studies of sediment and soil. Geomorphology is an area of physical geology concerned with the study of landforms, with the forces and processes that have shaped them, and with the description and classification of various physical features on Earth. In contrast to geology, which normally is associated with rocks and minerals, geomorphology is concerned more with larger configurations, such as mountains, or with the erosive and weathering forces that shape such landforms. (See, for instance, essays on Mountains, Erosion, and Mass Wasting.) Erosion and weathering also play a major role in creating sediment and soil, areas that are of interest in the subdisciplines of sedimentology and soil science. C O N T RAS T W I T H O T H E R D I S CIPLINES AND SUBDISCIPLINES.

Geoscience is distinguished sharply from the other branches of the earth sciences, namely, hydrologic sciences and atmospheric sciences. The first of these sciences, which is concerned with water, receives attention in essays on Hydrology and Hydrologic Cycle. The second, which includes meteorology (weather forecasting) and climatology, is the subject of the essays Weather and Climate.

Geoscience and Everyday Life

The organic material in soil—dead plants and animals and parts thereof—has ceased to be part of the biosphere and is part of the geosphere. The geosphere encompasses the upper part of Earth’s continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources. (For more about the “spheres,” see Earth Systems.) Later in this essay, we discuss several areas of geoscientific study that take place close to the surface of Earth. Yet the territory of geoscience extends far deeper, going well below the geosphere into the interior of the planet. (For more on this subject, see Earth’s Interior.) Geoscience even involves the study of “earths” other than our own; as discussed in such essays as Planetary Science and Sun, Moon, and Earth, there is considerable overlap between geoscience and astronomy.

REAL-LIFE A P P L I C AT I O N S The Primacy of Earth We may not think about geoscience or earth science much, or at least we may not think that we think about these topics very much—and yet we spend our lives in direct contact with these areas. Certainly in a given day, every person experiences physics (the act of getting out of bed is an example of the third law of motion, discussed in Gravity and Geodesy) and chemistry (eating and digesting food, for instance), but the experience of geoscience is more direct: we can actually touch the earth.

In addition to the hydrologic and atmospheric sciences, there are areas of earth sciences study that touch on biology. Essays in this book that treat biosphere-related topics include Ecosystems and Ecology and Ecological Stress. There is one area or set of areas, however, in

Before the late nineteenth century and the introduction of processed foods, everything a person ate clearly either was grown in the soil or was part of an animal that had fed on plants grown in the soil. Even today, the most grotesquely processed products, such as the synthetic cream puffs sold at a convenience store, still hold a connection to the earth, inasmuch as they contain sugar—a natural product. In any

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case, most of what we eat (especially in a healthconscious diet) has a close connection to the earth. GEOSCIENCE AND LANGUAGE.

No wonder, then, that a number of creation stories, including the one in Genesis, depict humankind as coming from the soil—an account of origins reflected in the well-known graveside benediction “Ashes to ashes, dust to dust.” Our language is filled with geoscientific metaphors, including such proverbs as “A rolling stone gathers no moss” or “Still waters run deep.” (The latter aphorism, despite its hydrologic imagery, actually refers to the fact that in deeper waters, rock formations are, by definition, not likely to be near the surface. By contrast, in order for a “babbling brook” to make as much noise as it does, it must be flowing over prominent rocks.) Then there are the countless geologic figures of speech: “rock solid,” “making mountains out of molehills,” “cold as a stone,” and so on. When the rock musician Bob Seger sang, in a 1987 hit, about being “Like a Rock” as a younger man, listeners knew exactly what he meant: solid, strong, dependable. So established was the metaphor that a few years later, Chevrolet used the song in advertising their trucks and sport-utility vehicles (including, ironically, a vehicle whose name uses a somewhat less reassuring geologic image: the Chevy Avalanche). THE GEOMORPHOLOGY OF R E L I G I O U S FA I T H . Rocks and other

geologic features have long captured the imagination of humans; hence, we have the many uses of mountains in, for instance, religious imagery. There was the mystic mountain paradise of Valhalla in Norse mythology as well as Mount Olympus in Greek myths and legends. Unlike Valhalla, Olympus is a real place; so, too, is Kailas in southwestern Tibet, which ancient adherents of the Jain religion called Mount Meru, the center of the cosmos, and which Sanskrit literature identifies as the paradise of Siva, one of the principal Hindu deities.

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ran aground, and Sinai (in the Sinai Desert between Egypt and Israel), where Moses was called by God and later received the Ten Commandments. The New Testament account of the life of Jesus Christ is punctuated throughout with geologic and geomorphologic details: the temptations in the desert, the Sermon on the Mount, and the Transfiguration, which probably took place atop Mount Tabor in Israel. He was crucified on a hill, buried in a cave, rolled a stone away at his Resurrection, and finally ascended to heaven from the Mount of Olives.

Arts, Media, and the Geosciences From ancient times rocks and minerals have intrigued humans, not only by virtue of their usefulness but also because of their beauty. On one level there is the purely functional use of rock as a building material, and on another level there is the aesthetic appreciation for the beauty imparted by certain types of rock, such as marble. Rock is an excellent building material when it comes to compression, as exerted by a great weight atop the rock; in the case of tension or stretching, however, rock is very weak. This shortcoming of stone, which was otherwise an ideal building material for the ancients (given its cheapness and relative abundance in some areas of the world), led to one of history’s great innovations in architecture and engineering: the arch. A design feature as important for its aesthetic value as for its strength, the arch owed its physical power to the principle of weight redistribution. Arched Roman structures two thousand or more years old still stand in Europe, a tribute to the interaction of art, functionality, and geoscience.

There is also Sri Pada, or Adam’s Peak, in Sri Lanka, a spot sacred to four religions. Buddhists believe the mountain is the footprint of the Buddha, while Hindus call it the footprint of Siva. Muslims and Christians believe it to be the footprint of Adam. Then there are the countless mountain locales of the Old Testament, including Ararat (in modern Turkey), where Noah’s ark

T H E V I S UA L A RT S . The Oxford Companion to the Earth contains a number of excellent entries on the relationship between geoscience and the arts. In the essay “Art and the Earth Sciences,” for instance, Andrew C. Scott notes four ways in which the earth sciences and the visual arts (including painting, sculpture, and photography) interact: through the depiction of such earth sciences phenomena as mountains or storms, through the use of actual geologic illustrations or even maps as forms of artwork, through the application of geologic materials in art (most notably, marble in sculpture), and

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Geoscience and Everyday Life

WHILE

STONE IS A STRONG BUILDING MATERIAL IN TERMS OF COMPRESSION, IT IS WEAK IN TERMS OF TENSION.

THE

ARCH OWES ITS STRENGTH TO THE PRINCIPLE OF WEIGHT DISTRIBUTION, WHICH OVERCOMES THIS SHORTCOMING OF STONE.

INDEED,

THE

ROMAN COLISEUM

HAS STOOD FOR MORE THAN TWO THOUSAND YEARS. (© John Moss/Photo

Researchers. Reproduced by permission.)

through the employment of geology to investigate aspects of art objects (for instance, determining the origins of materials in ancient sculpture). In the first category, visual depictions of geologic phenomena, Scott mentions works by unknown artists of various premodern civilizations (in particular, China and Japan) as well as by more recent artists whose names are hardly household words. On the other hand, some extremely well known figures produced notable works related to geoscience and the earth sciences. For example, the Italian artist and scientist Leonardo da Vinci (1452–1519), who happened to be one of the fathers of geology (see Studying

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Earth), painted many canvases in which he portrayed landscapes with a scientist’s eye. Another noteworthy example of earth sciences artwork and illustration is The Great Piece of Turf (1503), by Leonardo’s distinguished contemporary the German painter and engraver Albrecht Dürer (1471–1528). A life-size depiction of grasses and dandelions, Turf belongs within the realm of earth sciences or even biological sciences rather than geoscience, yet it is significant as a historical milestone for all natural sciences. In creating this work, Dürer consciously departed from the tradition, still strong even in

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Geoscience and Everyday Life

SCIENTIFIC

ILLUSTRATION BECAME POPULAR BETWEEN

SCIENCE AND ART.

THIS

1500

PITFALLS OF EXPLORATION, DATES TO

1700,

BRIDGING THE BOUNDARY BETWEEN EARTH

1689. (© G. Bernard/Photo Researchers. Reproduced by permission.)

the Renaissance, of representing “important” subjects, such as those of the Bible and classical mythology or history. By contrast, Dürer chose a simple scene such as one might find at the edge of any pond, yet his painting had a tremendous artistic and scientific impact. He set a new tone of naturalism in the arts and established a standard for representing nature as it is rather than in the idealized version of the artist’s imagination.

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AND

MAP OF THE WORLD, SURROUNDED BY ALLEGORICAL SCENES DEPICTING THE REWARDS AND

such geologists as England’s William Smith (1769–1839) would produce maps that are rightly regarded as works of art in their own right (see Measuring and Mapping Earth).

As a result of Dürer’s efforts, the period between about 1500 and 1700 saw the appearance of botanical illustrations whose quality far exceeded that of all previous offerings. Thus, he started a movement that spread throughout the world of scientific illustrations in general. Later,

Sometimes geologic phenomena have themselves become the basis for works of art, as Scott points out, observing that the modern American artist James Turrell once “set out to modify an extinct volcano, the Roden Crater [in northern Arizona], by excavating chambers and a tunnel to provide a visual experience of varying spatial relationships, the effects of light, and the perception of the sky.” Elsewhere in the Oxford Companion, other writers show how evidence of a

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geoscientific influence has appeared in other arts and media, including music. M U S I C . In “Music and the Earth Sciences,” D. L. Dineley and B. Wilcock offer a fascinating overview of natural formations or materials that have their own musical qualities: for example, the “singing sands” of the Arabian peninsula and other regions, which produce musical tones when millions of grains are rubbed together by winds. The authors also discuss the effect of geologic phenomena on the sound and production of music—for instance, the acoustic qualities of music played in an auditorium built of stone.

Then there is the subject of musical compositions inspired by geoscientific or earth sciences phenomena. Among them are The Hebrides; or, Fingal’s Cave by the German composer Felix Mendelssohn (1809–1847) as well as one the authors do not mention: The Planets, presented in 1918 by the German composer Gustav Holst (1874–1934). One also might list popular songs that refer to such phenomena, including “The White Cliffs of Dover.” Written by Walter Kent and Nat Burton in 1941, the song epitomizes the longing for peace in a world torn by war. The cliffs themselves, which guard the eastern approaches of Britain, sometimes are referred to incorrectly as “chalk,” though they are made of gypsum. Ironically, rock music has few significant songs that refer to rocks. Usually the language is metaphoric, as was the case with the Bob Seger song discussed earlier. Hence, we have the name of the rock group Rolling Stones (with its implicit reference to the proverbial saying mentioned earlier) as well as the title to one of their earliest hits, “Heart of Stone.” Jim Morrison’s lyrics for the Doors include several references to the ground and things underneath it, including a gold mine in “The End.” Coal mines have appeared in more than one song: “Working in the Coal Mine” was a hit for Lee Dorsey in the 1960s and was performed anew by the group Devo in 1981—not long after the Police song “Canary in a Coal Mine” appeared.

the actor Harrison Ford); however, the character of Jones is based on an American paleontologist, Roy Chapman Andrews (1884–1960). Earlier movies, Nield observes, had portrayed the typical scientist as an “egghead . . . an arrogant, unworldly, megalomaniac obsessive . . . But with Indiana Jones we saw the beginning of a reaction. Increasing audience sophistication is part of the reason.”

Geoscience and Everyday Life

Nield goes on to discuss the movie Jurassic Park (1993), which features three scientists, all of whom receive positive treatment. The actor Sam Neill, as a paleontologist, is described as “dedicated—perhaps a bit too educated—but also intuitive, a superb communicator, and above all, knowledgeable about dinosaurs.” Laura Dern, playing a paleobiologist, is “strong-willed, independent, feminist, and sexy,” while Jeff Goldblum’s mathematician is “weird, roguish, and cool.” Sparking a widespread interest in dinosaurs and paleontology, the film (a major box-office hit directed by Steven Spielberg) helped advance the cause of the geosciences. The positive trend in movie portrayals of geoscientists, Nield states, continued in Dante’s Peak (1997), in which even the casting of the ultra-handsome actor Pierce Brosnan as a geologist says a great deal about changing perceptions of scientists. Noting that audiences had come to differentiate between science and the misapplication thereof, Nield observes that “The heat seems to have come off those who are merely curious about Nature’s workings.” Additionally, “by being associated with the open air and fieldwork, [geoscientists] can take on some of the clichéd but healthy characteristics usually associated on film with oilmen and lumberjacks.”

F I L M . More significantly, the year 1981 marked the release of Raiders of the Lost Ark, a film cited as a major turning point by Ted Nield in the Oxford Companion’s “Geoscience in the Media” entry. The film is not about a geoscientist but an archaeologist, Indiana Jones (played by

In an entirely different category is another fascinating example of geoscience in film, Australian director Peter Weir’s Picnic at Hanging Rock (1975). Weir, who went on to make such well-known films as The Year of Living Dangerously (1982), Witness (1985), and Dead Poets’ Society (1989), established his reputation—and that of Australian cinema in general—with Picnic, which concerns the disappearance of a group of schoolgirls and their teacher on Valentine’s Day, 1900. The story itself is fictional, though it seems otherwise (Picnic later inspired The Blair Witch Project, which also presents fiction as fact); however, the rock in the title is very much a real place. In the film, Hanging Rock is by far the

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most striking character, a brooding presence whose foreboding features serve as a reminder of Earth’s vastness and great age in the face of human insignificance.

The Work of Geoscientists The work of the geoscientist indeed is associated with the open air to a much greater degree than that of the physicist or chemist; on the other hand, a geoscientist might very well work indoors, for instance, as a teacher. Prospective geoscientists who subscribe to a worldview of environmental utopianism can get a job “saving the world”—perhaps even working for starvation wages, so as to heighten the nobility of the undertaking. On the other hand, a pragmatist can go to work for an “evil” oil company and make a good living. The point is that there is a little of something for everyone in the world of geoscience. Geoscientists may work for educational institutions, governments, or private enterprise. They may be involved in the search for energy resources, such as coal or oil (or even uranium for nuclear power), or they may be put to work searching for valuable and precious metals ranging from iron to gold. They even may be employed in the mining of diamonds or other precious gems in South Africa, Russia, or other locales. Other, perhaps less glamorous but no less important resources for which geoscientists in various roles search are water as well as rocks, clay, and minerals for building. The majority of employed geoscientists work for industry but not always in the capacity of resource extraction. Some are involved in environmental issues; indeed, environmental geology—the application of geologic techniques to analyze, monitor, and control the environmental impact of natural and human phenomena—is a growing field. Among the areas of concern for environmental geologists are water management, waste disposal, and land-use planning. E N V I R O N M E N TA L A N D U R B A N G E O LO GY. Many environmental geologists,

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populations gather together. In fact, a growing area of specialization in environmental geology is urban geology. Urban geology can be defined as the application of geologic techniques to the study of the built environment. (The latter term is architectural and engineering jargon for any physical or geographic area containing human construction.) At first, “urban geology” might almost seem like an oxymoron, since the term geology usually calls to mind vast, unpopulated mountain ranges and rock formations—perhaps in South Dakota or Wyoming. In fact, geology is a major factor in the development of cities. Most are defined by their geomorphology: the hills of Athens and Rome, the mountains above Los Angeles, or the harbors of New York and other major ports, for instance. Most cities have natural barriers to growth, and this is precisely because geomorphology originally dictated the location at which the city was established. A rare exception is Atlanta, Georgia, which grew around the point where several rail lines met. (In the 1840s, when it was established, it bore the name Terminus, a reference to the fact that it lay at the end of the rail line.) Bounded by no ocean, significant rivers, mountains, or other natural barriers, such as deserts, Atlanta began a period of explosive growth in the latter part of the twentieth century and has never stopped growing. Today Atlanta is a textbook example of urban sprawl: lacking a vital city center, it is a settlement of some four million people spread over an area much larger than Rhode Island, with no end to growth in sight. Los Angeles often is cited as a case of urban sprawl, but its problems are quite different: it is rife with geomorphologic barriers, including oceans, mountains, and desert. The result is increasing growth within a limited area, resulting in heightened stress on existing resources. These are some of the issues confronted by urban geologists. Another example is the problem of determining the strength of bedrock, which dictates the viability of tall buildings. Urban geologists also are concerned with such issues as underground facilities for transportation, infrastructure, and even usable workspace—one possible solution to the problem of urban sprawl.

as one might expect, are employed by governments. They may be involved in soil studies before the commencement of a building project, in analyzing the necessary thickness and materials for a particular stretch of road, or in designing and establishing specifications for a landfill. Many such concerns come into play when large

in many ways, from urban geology is geoarchae-

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G E OA R C H A E O LO GY A N D R E LAT E D F I E L D S . At the opposite extreme,

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A

GOLD MINE IN

ZIMBABWE. GEOSCIENTISTS

WORK FOR EDUCATIONAL INSTITUTIONS, GOVERNMENTS, AND PRIVATE

ENTERPRISE IN SUCH FIELDS AS RESOURCE EXTRACTION, ENVIRONMENTAL STUDIES AND MANAGEMENT, AND EVEN ARCHAEOLOGY AND CRIMINOLOGY. (© Peter Bowater/Photo Researchers. Reproduced by permission.)

ology, or the application of geologic analysis to archaeology and related fields. Whereas urban geology is concerned with the here and now, geoarchaeology—like the larger field of historical geology—addresses the past. And whereas urban geologists are most likely to be employed by governments, geoarchaeologists and those in similar areas are typically on the payroll of universities.

er example of geoarchaeology would be the realm of ecclesiastical geology, which involves the study of old church masonry walls with the purpose of identifying areas from which rocks, bricks, and other materials were derived. Studies of medieval churches in England, for instance, show varieties of rock from sometimes unexpected locations, often placed alongside bricks taken from older Roman structures.

In a different sense, geoarchaeology also contrasts with archaeological geology, which is the study of archaeological sites for data relevant to the geosciences; thus, archaeological geology stands the approach of geoarchaeology on its head. An example of a study in archaeological geology can be found in the work conducted around the Roman ruins at Hierapolis in what is now Turkey. There, investigation of walls and gutters reveals the fact that the city was sitting astride an earthquake fault zone—a fact unknown to its residents, except when they experienced seismic tremors.

From the explanation and examples given here, it may be a bit hard to discern the difference between geoarchaeology and archaeological geology. Certainly there is a great deal of overlap, and in practice the difference comes down to a question of who is leading the fieldwork—a geologist or an archaeologist. In any case, both realms are concerned with the relatively recent human past, as opposed to the vast stretches of time that are the domain of historical geology (see Geologic Time).

By contrast, an example of geoarchaeology in action would be establishing an explanation for how people came to the Americas from Siberia near the end of the last ice age—by crossing a land bridge that existed at that time. Anoth-

F O R E N S I C G E O LO GY. On October 7, 2001, the United States launched air strikes against Afghanistan in retaliation for the refusal of that country’s Taliban regime to surrender Osama bin Laden, the suspected mastermind of the World Trade Center bombing on September 11. On the same day, bin Laden’s al-Qaeda ter-

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Geoscience and Everyday Life

KEY TERMS ATMOSPHERIC SCIENCES:

A major division of the earth sciences, distinguished from geoscience and the hydrologic sciences by its concentration on atmospheric phenomena. Among the atmospheric sciences are meteorology and climatology.

GEOCHEMISTRY:

A combination of all living things on Earth—plants, mammals, birds, reptiles, amphibians, aquatic life, insects, viruses, single-cell organisms, and so on—as well as all formerly living things that have not yet decomposed.

GEOLOGY:

BIOSPHERE:

The entire range of scientific disciplines focused on the study of Earth, including not only geoscience but also the atmospheric and hydrologic sciences.

EARTH SCIENCES:

A field of geology involved in the application of geologic techniques to analyze, monitor, and control environmental impact of both natural and human phenomena.

ENVIRONMENTAL GEOLOGY:

rorist organization released a videotape of their leader delivering a diatribe against the United States. Naturally, military and law-enforcement agencies involved in the hunt for bin Laden took an interest in the tape, and some specialists sought clues in an unexpected place: the rocks behind bin Laden, featured prominently in the tape. Although the efforts to trace bin Laden’s location by the rock formations in the area were not successful, the underlying premise—that geographic regions have their own specific types and patterns of rock—was both a fascinating and a plausible one. This was just another example of a specialty known as forensic geology, or the use of geologic and other geoscientific data in solving crimes. Forensic geology has it origins around the beginning of the twentieth century, but some

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A branch of the earth sciences, combining aspects of geology and chemistry, that is concerned with the chemical properties and processes of Earth—in particular, the abundance and interaction of chemical elements and their isotopes. The study of the solid earth, in particular, its rocks, minerals, fossils, and land formations.

GEOMORPHOLOGY: An area of physical geology concerned with the study of landforms, with the forces and processes that have shaped them, and with the description and classification of various physical features on Earth.

A branch of the earth sciences that combines aspects of geology and physics. Geophysics addresses the planet’s physical processes as well as its gravitational, magnetic, and electric properties and the means by which energy is transmitted through its interior. GEOPHYSICS:

historians cite Sherlock Holmes, the master sleuth created by the English physician and writer Sir Arthur Conan Doyle (1859–1930), as an early practitioner. In The Sign of Four, for instance, Holmes uses geologic data to ascertain that Watson has been to the Wigmore Street Post Office: “Observation tells me that you have a little reddish mould adhering to your instep,” he explains. “Just opposite the Wigmore Street Office they have taken up the pavement and thrown up some earth, which lies in such a way that it is difficult to avoid treading in it in entering. The earth is of this peculiar reddish tint which is found, as far as I know, nowhere else in the neighbourhood.” The true founder of forensic geology was probably the Austrian jurist and pioneer in

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KEY TERMS The geologic sciences

GEOSCIENCE:

(geology, geochemistry, geophysics, and related disciplines), as opposed to other

Geoscience and Everyday Life

CONTINUED

atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice. ORGANIC:

At one time chemists used

earth sciences—that is, atmospheric sci-

the term organic only in reference to living

ences, such as meteorology, and hydrologic

things. Now the word is applied to most

sciences, such as oceanography.

compounds containing carbon, with the

GEOSPHERE:

The upper part of

Earth’s continental crust, or that portion of the solid earth on which human beings live

exception of carbonates (which are minerals) and oxides, such as carbon dioxide. PHYSICAL GEOLOGY:

The study of

and which provides them with most of

the material components of Earth and of

their food and natural resources.

the forces that have shaped the planet.

The study

Physical geology is one of two principal

of Earth’s physical history. Historical geol-

branches of geology, the other being his-

ogy is one of two principal branches of

torical geology.

HISTORICAL GEOLOGY:

geology, the other being physical geology. HYDROLOGIC SCIENCES:

Areas of

the earth sciences concerned with the

PHYSICAL SCIENCES:

Astronomy,

physics, chemistry, and the earth sciences. SEDIMENT:

Material deposited at or

study of the hydrosphere. Among these

near Earth’s surface from a number of

areas are hydrology, glaciology, and

sources, most notably preexisting rock. Soil

oceanography.

is derived from sediment, particularly the

HYDROSPHERE:

The entirety of

Earth’s water, excluding water vapor in the

criminology Hans Gross (1847–1915), whose Handbuch für Untersuchungsrichter (Handbook for examining magistrates, 1898) was a pivotal work in the field. “Dirt on shoes,” wrote Gross, “can often tell us more about where the wearer of those shoes has last been than toilsome inquiries.” Near the turn of the nineteenth century, Germany’s Georg Popp, who operated a forensic laboratory in Frankfurt, used the new science effectively in two cases. The first of these cases involved the murder of a woman named Eva Disch in October 1904. Among the items found at the murder scene was a dirty handkerchief containing traces of coal, snuff, and hornblende, a mineral. Popp matched the handkerchief with a suspect who worked at two locations that used a great deal of hornblende. In addition, the suspect’s pants cuffs bore

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mixture of rock fragments and organic material.

soil both from the murder scene and the victim’s house. Four years later, in investigating the murder of Margaethe Filbert in Bavaria, Popp ascertained that the soil at the crime scene was characterized by red quartz and red clay rich in iron. By contrast, the chief suspect had a farm whose fields were notable for their porphyry, milky quartz, and mica content. As it turned out, the suspect’s shoes bore traces of quartz and red clay rather than those other minerals, even though he claimed he had been working in his fields when the crime occurred.

WHERE TO LEARN MORE Career Information for Geology Majors (Web site). .

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Careers in the Geosciences (Web site). . Geoarchaeology (Web site). . Geoarchaeology and Related Subjects (Web site). . Geology in the City (Web site). .

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Murray, Raymond C. Devil in the Details: The Science of Forensic Geology (Web site). . Sarah Andrews Forensic Geology Pages (Web site). . Sevil Atasoy’s Links of Forensic Geology (Web site). . Urban Geology of the National Capital (Web site). .

Hancock, Paul L., and Brian J. Skinner, eds. The Oxford Companion to the Earth. New York: Oxford University Press, 2000.

What Can I Do with a Degree in Geology? (Web site). .

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EARTH SYSTEMS

CONCEPT A system is any set of interactions set apart from the rest of the universe for the purposes of study, observation, and measurement. Theoretically, a system is isolated from its environment, but this is an artificial construct, since nothing is ever fully isolated. Earth is largely a closed system, meaning that it exchanges very little matter with its external environment in space, but the same is not true of the systems within the planet—geosphere, hydrosphere, biosphere, and atmosphere—which interact to such a degree that they are virtually inseparable. Together these systems constitute an intricate balance, a complex series of interrelations in which events in one sector exert a profound impact on conditions in another.

HOW IT WORKS Systems An isolated system is one so completely sealed off from its environment that neither matter nor energy passes through its boundaries. This is an imaginary construct, however, an idea rather than a reality, because it is impossible to create a situation in which no energy is exchanged between the system and the environment. Under the right conditions it is perhaps conceivable that matter could be sealed out so completely that not even an atom could pass through a barrier, but some transfer of energy is inevitable. The reason is that electromagnetic energy, such as that emitted by the Sun, requires no material medium in which to travel.

Despite its name, a closed system permits the exchange of energy with the environment but does not allow matter to pass back and forth between the external environment and the system. Thus, Earth absorbs electromagnetic energy, radiated from the Sun, yet very little matter enters or departs Earth’s system. Note that Earth is an approximation of a closed system: actually, some matter does pass from space into the atmosphere and vice versa. The planet loses traces of hydrogen in the extremities of its upper atmosphere, while meteorites and other forms of matter from space may reach Earth’s surface. Earth more closely resembles a closed system than it does an open one—that is, a system that allows the full and free exchange of both matter and energy with its environment. The human circulatory system is an example of an open system, as are the various “spheres” of Earth (geosphere, hydrosphere, biosphere, and atmosphere) discussed later. Whereas an isolated system is imaginary in the sense that it does not exist, sometimes a different feat of imagination is required to visualize an open system. It is intricately tied to its environment, and therefore the concept of an open system as a separate entity sometimes requires some imagination.

Using Systems in Science

In contrast to an isolated system is a closed system, of which Earth is an approximation.

To gain perspective on the use of systems in science as well as the necessity of mentally separating an open system from its environment, consider how these ideas are used in formulating problems and illustrating scientific principles. For example, to illustrate the principle of potential and kinetic energy in physics, teachers often use the example of a baseball dropping from a

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great height (say, the top of a building) to the ground. At the top of the building, the ball’s potential energy, or the energy it possesses by virtue of its position, is at a maximum, while its kinetic energy (the energy it possesses by virtue of its motion) is equal to zero. Once it is dropped, its potential energy begins to decrease, and its kinetic energy to increase. Halfway through the ball’s descent to the ground, its potential and kinetic energy will be equal. As it continues to fall, the potential energy keeps decreasing while the kinetic energy increases until, in the instant it strikes the ground, kinetic energy is at a maximum and potential energy equals zero. K E E P I N G O U T I R R E L E VA N T D E TA I L S . What has been described here is a

system. The ball itself has neither potential nor kinetic energy; rather, energy is in the system, which involves the ball, the height through which it is dropped, and the point at which it comes to a stop. Furthermore, because this system is concerned with potential and kinetic energy only in very simple terms, we have mentally separated it from its environment, treating it as though it were closed or even isolated, though in reality it would more likely be an open system. In the real world, a baseball dropping off the top of a building and hitting the ground could be affected by such conditions as prevailing winds. These possibilities, however, are not important for the purposes of illustrating potential and kinetic energy, and even if they were, they could be incorporated into the larger energy system.

Applying the System Principle to Earth In the baseball illustration, the distribution between types of energy varies, but the total amount is always the same. Likewise in the money-jar illustration, the total amount of money remains fixed even though the distribution according to various denominations may vary. The same is true of Earth, though here it is the total amount of matter. This includes valuable resources, among them materials that can be mined to produce energy—for instance, fossil fuels such as coal or petroleum—as well as waste products. Because Earth is a closed system, there are no additional resources, nor is there any dumping ground other than the one beneath our feet. Thus, the situation calls for prudence both in the use of the planet’s material wealth and in the processing of materials that will leave a byproduct of waste.

Since kinetic energy and potential energy are inversely related, the potential energy at the top of the building will always equal the kinetic energy at the point of maximum speed, just before impact. This is true whether the ball is dropped from 10 ft. (3 m) or 1,000 ft. (305 m). It may seem almost magical that the sum of potential and kinetic energy is always the same or that the two values are perfectly inverse. In fact, there is nothing magical here: the system has a certain total energy, and this does not change, though the distribution of that energy can and does vary.

The fact that a closed system is by definition finite leads to the principle that the relationships between its constituent parts are likewise finite, and therefore changes in one part of the system are liable to produce effects in another part. Conditions in the baseball or money-jar illustrations are so simple that it is easy to predict the effect of a change. For instance, if we substitute a basketball for a baseball, this will change the total energy, because the latter is a function of the ball’s mass. If the denominations making up the $20 in the money jar are replaced with a collection of two-dollar bills and dimes, this will make it impossible to reach in and pull out an odd-numbered value in dollars or cents.

Suppose one had a money jar known to contain $20. If one reaches in and grasps a five-dollar bill, two one-dollar bills, three quarters, a dime, and two nickels ($7.95), there must be $12.05 left in the jar. There is nothing magical in

What about the changes that result when one aspect of Earth’s system is altered? In some cases, it is easy to guess; in others, the interactions are so complex that prediction requires sophisticated mathematical models. It is perhaps

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this; rather, what has been illustrated is the physical principle of conservation. In physics and other sciences, “to conserve” something means “to result in no net loss of ” that particular component. It is possible that within a given system, the component may change form or position, but as long as the net value of the component remains the same, it has been conserved. Thus, the total energy is conserved in the situation involving the baseball, and the total amount of money is conserved in the money-jar.

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no accident that chaos theory was developed by a meteorologist, the American Edward Lorenz (1917–). Chaos theory, the study of complex systems that appear to follow no orderly laws, involves the analysis of phenomena that appear connected by something than an ordinary cause and effect relationship. The classic example of this is the “butterfly effect, ” the idea that a butterfly beating its wings in China can change the weather in New York City. This, of course, is a farfetched scenario, but sometimes changes in one sector of Earth’s system can yield amazing consequences in an entirely different part.

The Four “Spheres” The systems approach is relatively new to the earth sciences, themselves a group of disciplines whose diversity reflects the breadth of possible approaches to studying Earth (see Studying Earth). At one time, earth scientists tended to investigate specific aspects of Earth without recognizing the ways in which these aspects connect with one another; today, by contrast, the paradigm of the earth sciences favors an approach that incorporates the larger background. Given the complexities of Earth itself, as well as the earth sciences, it is helpful to apply a schema (that is, an organizational system) for dividing larger concepts and entities into smaller ones. For this reason, earth scientists tend to view Earth in terms of four interconnected “spheres. ” One of these terms, atmosphere, is a familiar one, while the other three (geosphere, hydrosphere, and biosphere) may sound at first like mere scientific jargon. UNDERSTANDING THE SPHERES.

In fact, each sphere represents a sector of existence on the planet that is at once clearly defined and virtually inseparable from the others. Each is an open system within the closed system of Earth, and overlap is inevitable. For example, the seeds of a plant (biosphere) are placed in the ground (geosphere), from which they receive nutrients for growth. In order to sustain life, they receive water (hydrosphere) and carbon dioxide (atmosphere). Nor are they merely receiving: they also give back oxygen to the atmosphere, and by providing nutrition to an animal, they contribute to the biosphere.

Earth system in much greater depth; what follows, by contrast, is the most cursory of introductions. It should be noted also that while these four subsystems constitute the entirety of Earth as humans know and experience it, they are only a small part of the planet’s entire mass. The majority of that mass lies below the geosphere, in the region of the mantle and core.

Earth Systems

A CURIOUS AND INSTRUCT I V E P O I N T. As a passing curiosity, it is

interesting to note that modern scientists have identified four subsystems and given them the name spheres. As discussed in the essay Earth, Science, and Nonscience, the ancient Greeks were inclined to divide natural phenomena into fours, a practice that reached its fullest expression in the model of the universe developed by the Greek philosopher Aristotle (384–322 B.C.) He even depicted the physical world as a set of spheres and suggested that the heaviest material would sink to the interior of Earth while the lightest would rise to the highest points. These points of continuity with ancient science are notable because almost everything about Aristotle’s system was wrong, and, indeed, the differences between his model of the physical world and the modern one are instructive. There are four spheres in the modern earth sciences because these four happen to be useful ways of discussing the larger Earth system—not, as in the case of the Greeks, because the number four represents spiritual perfection. Furthermore, scientists understand these spheres to be artificial constructs, at least to some extent, rather than a key to some deeper objective reality about existence, as the ancients would have supposed.

Each of the spheres, or Earth systems, is treated in various essays within this book. These essays examine these subsystems of the larger

Nor are the spheres of the modern earth sciences literally spheres, as Aristotle’s concentric orbits of the planets around Earth were. If anything, the use of the term sphere represents a holdover from the Greek way of viewing the material world. Finally, unlike such ancient notions as the concept of the four elements, four qualities, or four humors, the idea of the four spheres is not simply the result of pure conjecture. Instead, the concept of these four interrelated systems came about by application of the scientific method and entered the vocabulary of earth scientists because the ideas involved clearly reflected and illustrated the realities of Earth processes.

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Earth Systems

SANDSTONE

ERODED BY WAVES. (© Stephen Parker/Photo Researchers. Reproduced by permission.)

The Spheres in Brief The geosphere itself may be defined as the upper part of the planet’s continental crust, the portion of the solid earth on which human beings live, which provides them with most of their food and natural resources. Even with the exclusion of the mantle and core, the solid earth portion of Earth’s system is still by far the most massive. It is estimated that the continental and oceanic crust to a depth of about 1.24 mi. (2 km) weighs 6 1021 kg—about 13,300 billion billion pounds. The mass of the biosphere, by contrast, is about one millionth that figure. If the mass of all four spheres were combined, the geosphere would account for 81.57%, the hydrosphere 18.35%, the atmosphere 0.08%, and the biosphere a measly 0.00008%. (Of that last figure, incidentally, animal life—of which humans are, of course, a very small part—accounts for less than 2%.) Not only is the geosphere the largest, it is also by far the oldest of the spheres. Its formation dates back about four billion years, or within about 0.5 billion years of the planet’s formation. As Earth cooled after being formed from the gases surrounding the newborn Sun, its components began to separate according to density. The heaviest elements, such as iron and nickel, drift-

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ed toward the core, while silicon rose to the surface to form the geosphere. ATMOSPHERE, HYDROSPHERE, AND BIOSPHERE. In that distant time

Earth had an atmosphere in the sense that there was a blanket of gases surrounding the planet, but the atmospheric composition was quite different from today’s mixture of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%. The atmosphere then consisted largely of carbon dioxide from Earth’s interior as well as gases brought to Earth by comets. Elemental hydrogen and helium escaped the planet, and much of the carbon was deposited in what became known as carbonate rocks. What remained was a combination of hydrogen compounds, including methane, ammonia, nitrogen- and sulfur-rich compounds expelled by volcanoes, and (most important of all) H2O, or water. Simultaneous with these developments, the gases of Earth’s atmosphere cooled and condensed, taking the form of rains that, over millions of years, collected in deep depressions on the planet’s surface. This was the beginning of the oceans, the largest but far from the only component of Earth’s hydrosphere, which consists of

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all the planet’s water except for water vapor in the atmosphere. Thus, the hydrosphere includes not only saltwater but also lakes, streams, groundwater, snow, and ice. Water, of course, is necessary to life, and it was only after its widespread appearance that the first life-forms appeared. This was the beginning of the biosphere, which consists of all living organisms as well as any formerly living material that has not yet decomposed. (Typically, following decomposition an organism becomes part of the geosphere.) Over millions of years, plants formed, and these plants gradually began producing oxygen, helping to create the atmosphere as it is known today—an example of interaction between the open systems that make up the larger Earth system.

REAL-LIFE A P P L I C AT I O N S Earth As an Organism Clearly, a great deal of interaction occurs between spheres and has continued to take place for a long time. Earth often is described as a living organism, a concept formalized in the 1970s by the English meteorologist James Lovelock (1919–) and the American biologist Lynn Margulis (1938–), who developed the Gaia hypothesis. Sometimes called the Gaian hypothesis, this principle is named after the Greek earth goddess, a prototype for “Mother Earth,” and is based on the idea that Earth possesses homeostatic or selfregulating mechanisms that preserve life. (Lovelock’s neighbor William Golding [1911–1993], author of Lord of the Flies, suggested the name to him.)

The Gaia hypothesis is far from universally accepted, however, and remains controversial. One reason is that it seems to contain a teleologic, or goal-oriented, explanation of physical behaviors that does not fully comport with the findings of science. An animal responds to external conditions in such a way as to preserve life, but this is because it has instinctive responses “hardwired” into its brain. Clearly, if the Earth is an “organism, ” it is an organism in quite a different sense than an animal, since it does not make sense to describe Earth as having a “brain.”

Earth Systems

Homeostasis and Cycles Nonetheless, Lovelock, Margulis, and other supporters of the Gaia hypothesis have pointed to a number of anomalies that have yet to be explained fully and for which the Gaia hypothesis offers one possible solution. For example, it would have taken only about 80 million years for the present levels of salt in Earth’s oceans to have been deposited there from the geosphere; why, then, is the sea not many, many times more salty than it is? Could it be that Earth has somehow regulated the salinity levels in its own seas? Earth’s systems unquestionably display a homeostatic and cyclical behavior typical of living organisms. Just as the human body tends to correct any stresses imposed on it, Earth likewise seeks equilibrium. And just as blood, for instance, cycles through the body’s circulatory system, so matter and energy move between various spheres in the course of completing certain cycles of the Earth system. These include the energy and hydrologic cycles; a number of biogeochemical cycles, such as the carbon and nitrogen cycles; and a rock cycle of erosion, weathering, and buildup. (Each of these systems is discussed in a separate essay, or as part of a separate essay, in this book.)

Though the Gaia hypothesis seems very modern and even a bit “New Age” (that is, relating to a late twentieth-century movement that incorporates such themes as concern for nature and spirituality), it has roots in the ideas of the great Scottish geologist James Hutton (1726– 1797), who described Earth as a “superorganism.” A forward-thinking person, Hutton maintained that physiology provides the model for the study of Earth systems. Out of Hutton’s and, later, Lovelock’s ideas ultimately grew the earth science specialty of geophysiology, an interdisciplinary approach incorporating aspects of geochemistry, biology, and other areas.

F E E D B AC K . Though particulars of the Gaia hypothesis remain a matter of question, it is clear that Earth regulates these cycles and does so through a process of feedback and corrections. To appreciate the idea of feedback, consider a financial example. In the early 1990s, the U.S. Congress placed a steep tax on luxury boats, presumably with the aim of getting more money from wealthy taxpayers. The result, however, was exactly the opposite: boat owners sold their crafts, and many of those considering purchases cancelled their plans. Rather than redistributing

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Earth Systems

AN OIL-COVERED BIRD, VICTIM OF THE 1989 EXXON VALDEZ’S oil spill in Prince William Sound, Alaska. (AP/Wide World Photos. Reproduced by permission.)

wealth from the rich to those less fortunate, the tax resulted in the government’s actually getting less money from rich yacht owners. Whereas Congress expected the rich to provide positive feedback by giving up more tax money, instead the yacht owners responded by acting against the tax—a phenomenon known as negative feedback. Feedback itself is the return of output to a system, such that it becomes input which then produces further output. Feedback that causes the system to move in a direction opposite that of the input is negative feedback, whereas positive feedback is that which causes the system to move in the same direction as the input. The luxury tax would have made perfect sense if the purpose had been to halt the production and purchase of expensive boats, in which case the output would have been deemed positive. In the luxury-tax illustration, negative feedback is truly “negative” in the more common sense of the word, but this is not typically the case where nature in general or Earth systems in particular are concerned. In natural systems negative feedback serves as a healthy corrective and tends to stabilize a system. To use an example from physiology, if a person goes into a cold environment, the body responds by raising the internal temperature. Likewise, in chemical reactions the

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system tends to respond to any stress placed on it by reducing the impact of the stress, a concept known as Le Châtelier’s principle after the French chemist Henry Le Châtelier (1850–36). Positive feedback, on the other hand, is often far from “positive” and is sometimes described as a “vicious cycle.” Suppose rainwater erodes a portion of a hillside, creating a gully. Assuming the rains continue, the opening of this channel for the water facilitates the introduction of more water and therefore further erosion of the hillside. Given enough time, the rain can wash a deep gash into the hill or even wash away the hill entirely.

Far-Reaching Consequences Given the interconnectedness of systems on Earth, it is easy to see how changes in one part of the larger Earth system can have far-reaching impacts on another sector. For example, the devastating Alaska earthquake of March 1964 produced tsunamis felt as far away as Hawaii, while the Exxon Valdez oil spill that afflicted Alaska exactly 25 years later had an effect on the biosphere and hydrosphere over an enormous area. El Niño is a familiar example of far-reaching consequences produced by changes in Earth sys-

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Earth Systems

KEY TERMS A blanket of gases

ATMOSPHERE:

surrounding Earth and consisting of nitro-

surroundings—everything external to and separate from the system.

gen (78%), oxygen (21%), argon (0.93%),

FEEDBACK:

and other substances that include water

system, such that the output becomes

vapor, carbon dioxide, ozone, and noble

input that produces further output. Feed-

gases such as neon, which together com-

back that causes the system to move in a

prise 0.07%.

direction opposite to that of the input is

The return of output to a

A combination of all liv-

negative feedback, whereas positive feed-

ing things on Earth—plants, mammals,

back is that which causes the system to

birds, reptiles, amphibians, aquatic life,

move in the same direction as the input.

BIOSPHERE:

The concept,

insects, viruses, single-cell organisms, and

GAIA HYPOTHESIS:

so on—as well as all formerly living things

introduced in the 1970s, that Earth behaves

that have not yet decomposed. Typically,

much like a living organism, possessing

after decomposing, a formerly living

self-regulating mechanisms that preserve

organism becomes part of the geosphere.

life. Sometimes called the Gaian hypothesis, it is named after Gaia, the Greek god-

CLOSED SYSTEM:

A system that per-

dess of the earth.

mits the exchange of energy with its external environment but does not allow matter to pass between the environment and the system. Compare with isolated system, on the one hand, and open system, on the other. CONSERVATION:

In

physics

GEOSPHERE:

The upper part of

Earth’s continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources.

and

other sciences, “to conserve” something

HOMEOSTASIS:

equilibrium.

means “to result in no net loss of ” that particular component. It is possible that within a given system, the component may change form or position, but as long as the net value of the component remains the

A tendency toward

HOMEOSTATIC:

The quality of being

self-regulating. The entirety of

HYDROSPHERE:

Earth’s water, excluding water vapor in the atmosphere but including all oceans, lakes,

same, it has been conserved.

streams, groundwater, snow, and ice. ELECTROMAGNETIC

ENERGY:

A

ISOLATED SYSTEM:

A system that is

form of energy with electric and magnetic

so fully separated from the rest of the uni-

components, which travels in waves and,

verse that it exchanges neither matter nor

depending on the frequency and energy level,

energy with its environment. This is an

can take the form of long-wave and short-

imaginary construct, since full isolation is

wave radio; microwaves; infrared, visible, and

impossible.

ultraviolet light; x rays; and gamma rays. OPEN SYSTEM: ENVIRONMENT:

In discussing sys-

tems, the term environment refers to the

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A system that allows

complete, or near-complete, exchange of matter and energy with its environment.

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Earth Systems

KEY TERMS A set of principles and procedures for systematic study that includes observation; the formation of hypotheses, theories, and laws; and continual testing and reexamination.

SCIENTIFIC METHOD:

Any set of interactions that can be set apart mentally from the rest of

SYSTEM:

tems. Spanish for “child” (because it typically occurs around Christmastime), El Niño begins on the western coast of South America. There, every few years, trade winds slacken, allowing the wind from the west to push warm surface water eastward. Lacking vital nutrients, this warm water brings about a decline in the local marine life. It also causes heavy rains and storms. I M PAC T O F E L N I Ñ O A R O U N D T H E W O R L D . To the extent described, El

Niño is largely a local phenomenon. But it can affect the jet streams, or high-level winds, that push storms across the Western Hemisphere. This can result in milder weather for western Canada or the northern United States, as the winds push more severe storms into Alaska, but it also can bring about heavy rains in the Gulf of Mexico region. Nor are its effects limited to the Western Hemisphere. El Niño has been known to alter the pattern of monsoons, or rainy seasons, in India, Southeast Asia, and parts of Africa, thus producing crop failures that affect millions of people. Aside from the indirect effects, such as the famines in the Eastern Hemisphere, the direct effects of the El Niño phenomenon can be devastating. The El Niño of 1982–83, which affected the United States, the Caribbean, western South America, Africa, and Australia, claimed some 2,000 lives and cost about $13 billion in property damage. It returned with a vengeance 15 years later, in 1997–98, killing more than 2,100 people and destroying $33 billion worth of property.

the universe for the purposes of study, observation, and measurement. TSUNAMI:

A tidal wave produced by

an earthquake or volcanic eruption. The term comes from the Japanese words for “harbor” and “wave.”

and ultimately the biosphere, an even more terrifying phenomenon can begin with an eruption in the geosphere, which spreads to the atmosphere and then the hydrosphere and biosphere. This phenomenon might be called “years without summer”; an example occurred in 1815–16. In June of 1816 snow fell in New England, and throughout July and August temperatures hovered close to freezing. Frosts hit in September, and New Englanders braced themselves for an uncommonly cold winter, as that of 1816–17 turned out to be. It must have seemed as though the world were coming to an end, yet the summer of 1817 proved to be a normal one. The cause behind this year without summer in 1816 lay in what is now Indonesia, and it began a year earlier. In 1815, Mount Tambora to the east of Java had erupted, pouring so much volcanic ash into the sky that it served as a curtain against the Sun’s rays, causing a brutally cold summer in New England the following year. An eruption of Mount Katmai in Alaska in 1912 produced farreaching effects, including some lowering of temperatures, but its impact was nothing like that of Tambora. Nor did the 1980 Mount Saint Helens eruption in Washington State prove nearly as potent in the long run as the eruption of Tambora did (though it produced a devastating immediate impact). T H E CATAC LY S M O F

A.D.

535.

Whereas El Niño is an example of a disturbance in the hydrosphere that affects the atmosphere

Even the eruption of Mount Tambora may have been overshadowed by another, similar event, known simply as the catastrophe, or cataclysm, of A.D. 535. In the late twentieth century, the British dendrochronologist Mike Baillie discovered a pattern of severely curtailed growth in tree rings dating to the period A.D. 535–541. More or less

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simultaneous with Baillie’s work was that of the amateur archaeologist David Keys, who found a number of historical texts by Byzantine, Chinese, and Anglo-Saxon scholars of the era, all suggesting that something cataclysmic had happened in A.D. 535. For example, the Byzantine historian Procopius (d. 565) wrote, “The sun gave forth its light without brightness ... for the whole year.”

off from western Europe. Thus the Dark Ages, the split between Catholicism and Eastern Orthodoxy, the Crusades—even the Cold War, which reflected the old east-west split in Europe—may have been the results of a volcano on the other side of the world.

Some geologists have maintained that the cataclysm resulted from the eruption of another Indonesian volcano, the infamous Krakatau, which had a devastating eruption in 1883 and which could have produced enough dust to cause an artificial winter. Whatever the cause, the cataclysm had an enormous impact that redounds from that time perhaps up to the present. The temperature drop may have sparked a chain of events, beginning in southern Africa, that ultimately brought a plague to the Byzantine Empire, forcing Justinian I (r. A.D. 483–565) to halt his attempted reconquest of western Europe. At the same time, the cataclysm may have been responsible for food shortages in central Asia, which spawned a new wave of European invasions, this time led by the Avars.

WHERE TO LEARN MORE

The result was that the fate of Europe was sealed. For a few years it had seemed that Justinian could reconquer Italy, thus reuniting the Roman Empire, whose western portion had ceased to exist in A.D. 476. Forced to give up their reconquest, with the Avars and others overrunning Europe while the plague swept through Greece, the Byzantines turned their attention to affairs at home and increasingly shut themselves

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Earth Systems

Cox, Reg, and Neil Morris. The Natural World. Philadelphia: Chelsea House, 2000. Earth’s Energy Budget (Web site). . Farndon, John. Dictionary of the Earth. New York: Dorling Kindersley, 1994. The Gaia Hypothesis (Web site). . Geophysiology Online (Web site). . Hancock, Paul L., and Brian J. Skinner. The Oxford Companion to the Earth. New York: Oxford University Press, 2000. Knapp, Brian J. Earth Science: Discovering the Secrets of the Earth. Illus. David Woodroffe and Julian Baker. Danbury, CT: Grolier Educational, 2000. Kump, Lee R., James F. Kasting, and Robert G. Crane. The Earth System. Upper Saddle River, NJ: Prentice Hall, 2000. Lovelock, J. E. Gaia: A New Look at Life on Earth. New York: Oxford University Press, 2000. Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science. 2nd. ed. New York: John Wiley and Sons, 1999.

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S C I E N C E O F E V E RY DAY T H I N G S Real-Life Earth Science

THE STUDY OF EARTH STUDYING EARTH MEASURING AND MAPPING EARTH REMOTE SENSING

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STUDYING EARTH

CONCEPT The physical sciences include astronomy, physics, chemistry, and the earth sciences, but the last of these sciences is quite unlike the other three. Whereas the objects of study in physics and chemistry often seem abstract to the uninitiated and astronomy is concerned with faraway planets and other bodies, the earth sciences are devoted to things that are both concrete and immediate. The focus of study for the earth sciences is literally underneath our feet, a planet at once vast and tiny, a world that (as far as we know) stands alone in the universe as the sole supporter of intelligent life. The earth sciences also differ from other disciplines in that their boundaries are not always defined clearly. The study of Earth is a multifarious array of specialties that includes a range of geologic, hydrologic, and atmospheric sciences that overlap with the other physical sciences, biology, and even the social sciences.

HOW IT WORKS Introduction to the Earth Sciences

Usually the earth sciences are considered part of the physical sciences, as opposed to the biological sciences, such as biology, botany, and zoology. Yet the earth sciences clearly overlap with biological sciences in a variety of areas, such as oceanography and various studies of complex biological environments. There is even a new (and as yet not fully formalized) discipline called geophysiology, built on the premise that Earth has characteristics of a living organism. O V E R LA P W I T H O T H E R P H Y S I CA L S C I E N C E S . The earth sciences also

overlap with other physical sciences in several realms. There is geophysics, which addresses the planet’s physical processes, including its magnetic and electric properties and the means by which energy is transmitted through its interior. There is also geochemistry, which is concerned with the chemical properties and processes of Earth. And there are numerous areas of confluence between the earth sciences and astronomy (among them, planetary geology), which fall under the heading of planetary science (sometimes called planetology or planetary studies).

At the simplest level, the earth sciences can be divided into three broad areas: the geologic, hydrologic, and atmospheric sciences. These specialties fit neatly with three of the “spheres,” or subsystems within the larger Earth system: geosphere, hydrosphere, and atmosphere. (See Earth Systems for more about the spheres.) The fourth of these subsystems is the biosphere, and this illustrates the difficulty of stating exactly what is and what is not a part of the earth sciences.

These terms all refer to the same discipline, a branch of the earth sciences concerned with the study of other planetary bodies. This discipline, or set of disciplines, is concerned with the geologic, geophysical, and geochemical properties of other planets but also draws on aspects of astronomy, such as cosmology. Regardless of the name by which it is called, planetology is an example of the fact that the study of Earth is still very much an evolving set of disciplines. In many cases, the earth sciences are still in process of being defined.

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Scientific Paradigms Studying Earth

This last point is an important one to consider because of what it implies about the nature of scientific study. In the past, scientists tended to think that they were in the business of discovering some sort of objective truth that was waiting for them to discover it; in reality, however, the quest of the scientific thinker is much less guided. The natural world does not in any way speak to the scientist, telling him or her how to categorize data. In fact, the divisions of scientific knowledge with which we are familiar have come about not because they necessarily reflect an underlying truth, but because they have proved useful in separating certain aspects of the physical world from certain others. When science had its beginnings in ancient times, scientists were simply collecting observations (including a lot of incorrect ones) and sometimes forming theories of a sort, but they did not think in terms of developing models for viewing their objects of study. Today, however, scientific thinkers are acutely conscious of the model, or paradigm, that governs a particular discipline, school of thought, or theory. A paradigm may be likened to a lens. The lens does not change the actual object that is viewed through it; it can alter only the way in which it is viewed. As thinkers within a particular discipline or theory begin to define the governing paradigm, they are much like an eye doctor testing various lenses on a patient. In such a situation, there is no one lens that is right for all circumstances. Rather, it is a question of finding the lens that best suits the patient’s vision needs. All sciences are gradually changing, evolving models that better suit the data under their consideration. Chemistry, for instance, was once primarily a matter simply of mixing chemicals and observing their external processes. In fact, the definition of chemistry has expanded greatly since about 1800, and today it is more like what people tend to think of as physics; that is, it is concerned with atomic and subatomic structures and types of behavior. The earth sciences are in an even more transitional state, and the problem of defining the disciplines it comprises is a still more fundamental one.

To appreciate the way that these disciplines fit within the larger perspective, however, it is necessary to examine a few historical details. Most of these details concern the early history of the earth sciences, since much of the more modern history (for example, the development of plate tectonics theory during the 1960s) is treated in the relevant essays within this book. The purpose of this brief historical review, instead, is to impart an understanding about how the study of Earth emerged as a real science, as opposed to a merely descriptive undertaking concerned with recording observations. Important themes are the development of the scientific method as well as the search for proper ways of classifying the various studies under the heading of what became known as the earth sciences.

REAL-LIFE A P P L I C AT I O N S The Scientific Method As discussed later, the scientific method emerged during the seventeenth century and has remained in use ever since. It is a way of looking at facts and data, and its application is what truly separates science from nonscience. Nonscientific “theories” postulate answers based not on evidence but on pure conjecture, a habit of mind that was widespread before the development of the scientific method and is still all too common. A contemporary example would be the claim that intelligent extraterrestrial life-forms built the great pyramids of Egypt.

outline the broad parameters of the earth sciences, considering basic areas of study and spe-

The only real basis for this belief is the fact that the pyramids are extremely sophisticated architectural achievements for a civilization that had no metal tools, no wheel, and virtually no understanding of geometry, as was the case in Egypt in about 2500 B.C. But is a huge conceptual leap to go from the observation of these anomalies to the claim that visitors from outer space built the pyramids. The same is true of other strange artifacts from ancient or prehistoric

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cialties within them. This does not represent a definitive organizational scheme, nor does this brief review refer to every possible area of study in the wide-ranging earth sciences. To do so would require an entire book; rather, the purpose is to consider the most significant disciplines and subdisciplines.

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times, such as the great structures of Stonehenge in England or the Nazca lines in South America. They are curious, puzzling, and intriguing, and it may be fun to speculate about engineers from another world—but such speculation is not science. (For more about the methodological distinctions between science and its opposite, see Earth, Science, and Nonscience.) It is no mistake that here we are talking about the application of the scientific method to something outside the “hard sciences,” the study of the pyramids being the province of social sciences, such as archaeology and history. In fact, the method has had just as much impact in those areas as it has in the physical and biological sciences. The scientific method (along with the closely related philosophical principles of basic logic, handed down from the ancient Greeks) can be applied in many aspects of daily life, enabling a person to make sense of a complex world. Many of the controversies of the modern world, including those involving race, sex, religion, and politics, could be treated more constructively if people approached the topics with a genuine interest in understanding the facts rather than simply finding confirmation for their emotionally based preconceived notions. A P P LY I N G T H E S C I E N T I F I C M E T H O D . Scientists rigorously applying the

scientific method begin their quest for understanding by looking solely at the facts that can be garnered by observation. On the basis of these data, they form hypotheses, or unproven statements regarding observed phenomena. This is usually as far as many people go in their thinking, and it is not far enough: up to this point, for instance, the theory that claims that visitors from another planet built the pyramids is in accordance with the scientific method. But such fanciful notions, as well as kindred ideas, such as conspiracy theories, never go to the next step, which marks the dividing line between science and pure opinion. Having formed a hypothesis, the scientist subjects it to testing, the most critical component of the scientific method. By contrast, advocates of nonscientific ideas (which, of course, usually pose as scientific ideas) typically focus on searching for evidence that will confirm the hypothesis. Anything that supports the hypothesis is reported; anything that does not is simply ignored. By

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NICOLAUS COPERNICUS (Library of Congress.)

contrast, a true scientist is constantly trying to disprove his or her own hypotheses. If a hypothesis withstands enough repeated testing, it acquires the status of a theory, or a more general statement about nature. If the idea embodied in a theory is shown to be the case in every situation for which it is tested, it then becomes a law. The process is a bit like that involved in making metal stronger: the more abuse it endures, assuming it is able to recover, the more impervious it will be to further abuse. But every type of metal has its limits, a threshold of compression or tension beyond which it cannot retain its original shape, and likewise it is possible that a scientific law can be overturned. For this reason, all laws are subject to continual testing, and if a test disproves a scientific law, this opens the way for the development of a new paradigm.

The Historical Roots of the Earth Sciences The earth sciences are both old and new. On the one hand, they address matters of fundamental importance to human beings, and for this reason, the rudiments of the earth sciences probably made their appearance before any of the other fields of scientific study except perhaps astrono-

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my. From prehistoric times, societies have been concerned with obtaining metals from the ground to make tools and weapons, finding water to support human life and crops, and discerning the future of weather patterns that could greatly affect conditions for human populations. Thus were born, respectively, the geologic, hydrologic, and atmospheric sciences. Much of what took place in the earth sciences before about 1800, however, was a matter of superstition, legend, guesswork, and a smattering of real science. Much of it was dominated by religious belief, which relied on a strict interpretation of the Bible. Based on the amount of time that elapsed between Adam and Jesus, combined with the fact that Genesis 1 states that Adam was created at the end of the first week of Earth’s existence, the Catholic Church maintained that the planet could not possibly be more than a few thousand years old. (For more on this subject, see Earth, Science, and Nonscience.) THE ANCIENT EARTH SCIE N C E S . Despite the many impediments to

scientific study in ancient times, a few thinkers contributed significantly to our knowledge. For instance, the Greek philosopher Aristotle (384–322 B.C.) discovered that Earth is a sphere by noting the rounded shadow on the Moon during a lunar eclipse. His pupil, Theophrastus (372?–287? B.C.), wrote a highly competent work, Concerning Stones, that remained a guide to mineralogy for two millennia. A few centuries later, the Greek mathematician Eratosthenes of Cyrene (ca. 276–ca. 194 B.C.) made an astoundingly accurate measurement of Earth’s circumference. Much of what passed for science, however, was little more than entertaining, anecdotal misinformation. Such is the case, for instance, in the Historia Naturalis (Natural history) of the Roman scholar Pliny the Elder (A.D. 23–79), a work that, despite its many flaws, remained widely respected through the Renaissance. As for Eratosthenes’ measurement of Earth, the Alexandrian astronomer Ptolemy (ca. A.D. 100–170) rejected it in favor of a much smaller, much less correct figure. Thus, Ptolemy may deserve some of the credit for discovering the New World: if Christopher Columbus (1451–1506) had known just how far it was around Earth, he might not have been so confident about sailing off into the seas to the west of Europe.

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Ptolemy’s rejection of Eratosthenes’s measurement was far from his only negative contribution to the history of science. Influenced by highly misguided concepts handed down from Aristotle himself (see Earth, Science, and Nonscience), he developed a complex cosmology that depicted Earth as the center of the universe. By this he meant that Earth was the center of the solar system, because up until a few centuries ago, astronomers believed that space consisted only of Earth, Sun, Moon, the five planets visible to the naked eye, and the “fixed stars” in the night sky. DAWN OF THE SCIENTIFIC M E T H O D . What made Ptolemy’s cosmology

so complex, of course, was the fact that Earth is not anywhere near the center of the universe, and therefore his system required intricate mathematical acrobatics to remain workable. This posed little problem during the early Middle Ages, when learning in Europe all but ceased, and even in the much more scientifically progressive Muslim world of that time, Ptolemy’s word remained virtual holy writ. By the late Middle Ages, however, as scientific learning returned to Europe, thinkers began to notice increasing difficulties in using his system. The watershed event in what became known as the Scientific Revolution was the proof, by the Polish astronomer Nicolaus Copernicus (1473– 1543), that Earth and the other planets of the solar system revolve around the Sun. By that point, the Catholic Church had given its official approval to Ptolemy’s geocentric model, because it comported well with the idea that God had created humankind in his own image to fulfill a specific destiny. Therefore, Copernicus’s challenge to established teachings proved highly controversial, and the Italian astronomer Galileo Galilei (1564–1642) would be forced to recant his support of it or face punishment by death. Yet Galileo paved the way for the full acceptance of Copernicus’s work and for the Scientific Revolution that followed in its wake. His theories and experiments concerning gravitational acceleration greatly influenced the English natural philosopher Isaac Newton (1642–1727), leading to the latter’s epochal work on gravitation and motion. But at least as important as Galileo’s work was his methodology: Galileo virtually introduced the scientific method, providing a set of principles for systematic study.

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The Foundations of Modern Geology The scientific method had an enormous impact on all the sciences. Unlike earlier “scientific” principles, which were built on the teachings of religious prophets or the uninformed conjecture of philosophers, this one was established on a foundation of observation, and it opened the way for unprecedented progress in the sciences. Until late in the eighteenth century, however, the relatively young field of geology centered primarily on mere observation rather than the development of theories. Thus, the discipline was not all that different from what it had been in ancient times, or when the Anglo-Saxon historian known as the Venerable Bede (673–735) coined the term geology. The latter term, a combination of the Greek geo and logia, means “study of Earth,” and was intended to distinguish such pursuits from theology, or the study of heavenly things. H I S T O R I CA L A N D P H Y S I CA L G E O LO GY. In modern times, geology is

defined as the study of the solid earth, in particular, its rocks, minerals, fossils, and land formations. As for Bede’s putative opposition between geology and theology, it would become more pronounced in the period from about 1500 to 1800, as the findings of geologists began increasingly to contradict the teachings in the biblical book of Genesis. Among the first to consider the age of Earth in scientific terms, rather than by recourse to the Scriptures, was one of the world’s greatest thinkers: the Italian scientist and artist Leonardo da Vinci (1452–1519), who speculated that fossils might have been made by the remains of long-dead animals. Less famous was Leonardo’s German contemporary Georgius Agricola (1494–1555), the “father of mineralogy,” who wrote extensively on mining, metallurgy, and minerals. Together, these two men represent the two principal strains of geology: historical geology, or the study of Earth’s history, and physical geology. The latter discipline, of which Agricola was a key representative, is concerned with the material components of Earth and with the forces that have shaped the planet. All the areas of geology discussed here fall under one of those two headings.

ning with a key observation on strata, or layers of rock, made by the Danish geologist Nicolaus Steno (1638–1687). As Steno correctly hypothesized, the lower a layer of rock lies, the earlier the historical period it represents. These observations, later developed into a theory by the German geologist Johann Gottlob Lehmann (1719–1767), had several implications.

Studying Earth

First of all, the ideas of Steno and Lehmann provided geologists with a method for dating the age of rock formations not unlike the rings observed by dendrochronologists studying the biography of a tree. As a result of study based on these findings, scientists were confronted with the growing realization that Earth is much, much older than a strict interpretation of the Bible would suggest. This finding, in turn, led to the first theories concerning the shaping of Earth and thus to the foundation of geology as a modern scientific discipline. T H R E E I M P O RTA N T S C H O O L S O F T H O U G H T. In the wake of this break-

through, at least three schools of thought developed. One of them, catastrophism, centered around the foregone conclusion that Earth had been created in six literal days or, at the very least, in an extremely short time, through a series of catastrophes. Opposed to this view was the Neptunist stratigraphy of the German geologist Abraham Gottlob Werner (1750–1817), who maintained that Earth had been shaped by a vast ocean (hence the name Neptune) that once covered its entire surface. Finally, there was the Plutonist school of the Scottish geologist James Hutton (1726–1797). Named after the Greek god of the underworld, this theory held that volcanoes and other disturbances beneath Earth’s surface had been the principal forces in shaping the planet.

Most of the important developments in geology during the period from 1500 to 1800 fall under the heading of historical geology, begin-

Hutton’s theory would prevail, and today he is regarded as the father of modern geology. In Theory of the Earth (1795), he introduced one of the key concepts underlying the study of the planet’s history, the principle of uniformitarianism—the idea that the forces at work on Earth today have always been in operation and are the same ones that shaped it. Nonetheless, Neptunism and even catastrophism had their merits. Although Werner and his followers were incorrect, Neptunism was the first well-developed theory concerning Earth’s origins and helped pave the way for others.

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al diagram that enabled students to visualize the relationships between subdisciplines.

Studying Earth

Holmes’s model was concerned primarily with the solid earth sciences, or the geologic sciences, meaning that it did not include the hydrologic sciences. Within its purview, however, it used a method of classification so broad (yet still targeted) that it has been adapted in recent years to include subdisciplines developed since Holmes’s time. These changes serve to emphasize further the evolving nature of what came to be known as the earth sciences. The latter term came into use only during the 1960s and 1970s, when it became apparent that neither geology alone nor even a combination of geology, geophysics, and geochemistry could encompass all the areas of study devoted to Earth.

Overview of the Earth Sciences

DIAGRAM

SHOWING THE SOLAR SYSTEM AS

COPERNITHE

CUS ENVISIONED IT, WITH THE SUN AT THE CENTER.

COPERNICAN SYSTEM HERALDED THE START OF THE S CIENTIFIC REVOLUTION. (© Dr. Jeremy Burgess/Photo Researchers. Reproduced by permission.)

As for the advocates of catastrophism, they were correct inasmuch as they noted the role of sudden catastrophes in shaping the planet. These catastrophes (for instance, a comet about 66 million years ago, which may have destroyed the dinosaurs), however, can be explained within the framework of a very old Earth. Nor does the fact of the catastrophes themselves in any way suggest a planet that is only a few thousand years old.

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Throughout most of what remains of this essay, we very briefly sketch the outlines of the earth sciences. It should be reiterated that the organizational system used here is not necessarily definitive and is intended only to provide the reader with a general idea as to how the various earth sciences fit together. G E O LO GY. At the core of the earth sciences, of course, is geology itself, which focuses on the study of the solid earth. As noted earlier, geology can be subdivided into historical and physical geology. The principle subdisciplines of historical geology are as follows.

L AT E R DEVELOPMENTS. As noted earlier, the later history of the earth sciences is discussed more properly within the context of specific subjects. Instead, our focus here is on the array of disciplines that proliferated alongside geology and on the need for a disciplinary paradigm larger than that of geology alone. By the mid–twentieth century, the range of disciplines involved in the study of Earth had become so complex and varied that it was a major achievement when the English geologist Arthur Holmes (1890–1965) developed a model that incorporated most of them. Holmes’s system was not simply a “model” in the way that the term typically is used; Holmes also constructed a liter-

• Stratigraphy: the study of rock layers, or strata, beneath Earth’s surface • Geochronology: the study of Earth’s age and the dating of specific formations in terms of geologic time • Sedimentology: the study and interpretation of sediments, including sedimentary processes and formations • Paleontology: the study of fossilized plants and animals, or flora and fauna • Paleoecology: the study of the relationship between prehistoric plants and animals and their environments. Note that there are several other disciplines referred to by the prefix paleo- (or palaeo-), Greek for “very old.” Two of the more wellknown ones are paleobiology and paleobotany, but the subdisciplines can become very specialized, as evidenced by the existence of a field

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Studying Earth

KEY TERMS A combination of all liv-

the solid earth on which human beings live

ing things on Earth—plants, mammals,

and which provides them with most of

birds, reptiles, amphibians, aquatic life,

their food and natural resources.

BIOSPHERE:

insects, viruses, single-cell organisms, and

The study

HISTORICAL GEOLOGY:

so on—as well as all formerly living things

of Earth’s physical history. Historical geol-

that have not yet decomposed. Typically,

ogy is one of two principal branches of

after decomposing, a formerly living

geology, the other being physical geology.

organism becomes part of the geosphere. HYDROSPHERE:

The study of the origin,

COSMOLOGY:

structure, and evolution of the universe. ECONOMIC GEOLOGY:

The study of

fuels, metals, and other materials from Earth that are of interest to industry or the

Earth’s water, excluding water vapor in the atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice. ment regarding an observed phenomenon. LAW:

Earth-centered.

GEOCHEMISTRY:

A branch of the

earth sciences, combining aspects of geolo-

An unproven state-

HYPOTHESIS:

economy in general. GEOCENTRIC:

The entirety of

A scientific principle that is

shown to always be the case and for which no exceptions are deemed possible. MINERALOGY:

The study of minerals

gy and chemistry, that is concerned with

(crystalline structures that make up rocks),

the chemical properties and processes of

which includes several smaller subdisci-

Earth.

plines, such as crystallography.

GEOCHRONOLOGY:

The study of

PALEONTOLOGY:

The study of fos-

Earth’s age and the dating of specific for-

silized plants and animals, or flora and

mations in terms of geologic time.

fauna.

GEOLOGY:

The study of the solid

PHYSICAL GEOLOGY:

The study of

earth, in particular, its rocks, minerals, fos-

the material components of Earth and of

sils, and land formations.

the forces that have shaped the planet. The study of

Physical geology is one of two principal

landforms and of the forces and processes

branches of geology, the other being his-

that have shaped them.

torical geology.

GEOMORPHOLOGY:

GEOPHYSICS:

A branch of the earth

sciences that combines aspects of geology and physics. Geophysics addresses the

PHYSICAL SCIENCES:

Astronomy,

physics, chemistry, and the earth sciences. PLANETARY SCIENCE:

The branch

planet’s physical processes as well as its

of the earth sciences, sometimes called

magnetic and electric properties and the

planetology or planetary studies, that

means by which energy is transmitted

focuses on the study of other planetary

through its interior.

bodies. This discipline, or set of disciplines, The upper part of

is concerned with the geologic, geophysi-

Earth’s continental crust or that portion of

cal, and geochemical properties of other

GEOSPHERE:

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Studying Earth

KEY TERMS planets but also draws on aspects of astronomy, such as cosmology. SCIENTIFIC METHOD:

A set of prin-

ciples and procedures for systematic study that includes observation; the formation of hypotheses, theories, and laws; and continual testing and reexamination.

ideas such as the heliocentric (Sun-centered) universe and gravity. SEDIMENTOLOGY: The study and interpretation of sediments, including sedimentary processes and formations.

The study of rock layers, or strata, beneath Earth’s surface. STRATIGRAPHY:

completely reshaped the world. Usually

The study of rock structures, shapes, and positions in Earth’s interior.

dated from about 1550 to 1700, the Scien-

THEORY:

SCIENTIFIC REVOLUTION:

A peri-

od of accelerated scientific discovery that

tific Revolution saw the origination of the scientific method and the introduction of

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CONTINUED

STRUCTURAL GEOLOGY:

A general statement derived from a hypothesis that has withstood sufficient testing.

known as paleobiogeography, or the study of fossils’ geographic distribution. The principle subdisciplines of physical geology are:

include a third category of subdisciplines that overlap both historical and physical geology.

• Geomorphology: the study of landforms and of the forces and processes that have shaped them • Structural geology: the study of rock structures, shapes, and positions in Earth’s interior • Mineralogy: the study of minerals (crystalline structures that make up rocks), which includes several smaller subdisciplines, such as crystallography • Petrology: the study of rocks, which is divided into several smaller subdisciplines, most notably igneous, metamorphic, and sedimentary petrology • Economic geology: the study of fuels, metals, and other materials from Earth that are of interest to industry or the economy in general • Environmental geology: the study of the geologic impact of both natural and human activity on the environment. It should be noted that there is some overlap between historical and physical geology. For instance, sedimentology often is placed under the heading of physical geology, while some sources

Geology occupies a central place among the geologic sciences or geosciences, but also important are those disciplines and subdisciplines formed, as Holmes pointed out, at the intersections between geology and astronomy, physics, and chemistry, respectively. (Some sources, on the other hand, consider these disciplines to be a part of geology itself. In the present context, the term geologic sciences is used to encompass not only geology but also these related areas of study.)

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O T H E R G E O LO G I C S C I E N C E S .

Planetary science applies the earth sciences paradigm to other planets. Among its important subdisciplines is astrogeology or planetary geology, or the study of the rock record on the Moon, the planets, and other bodies. Also significant is cosmology, the study of the origin, structure, and evolution of the universe, which often is treated as part of astronomy. Geophysics, or an application of physics to the study of Earth, occupies a position of prominence within the earth sciences. Among the areas it addresses are the production, expenditure, and transmission of energy within Earth as well as the planet’s magnetic, electric, and gravitational properties. Geophysics encompasses such areas as geodesy, the science of measuring Earth’s

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shape and gravitational field. Seismology, or the study of the waves produced by earthquakes and volcanoes, is another important part of geophysics. (On the other hand, volcanology, or the study of volcanoes themselves, would fall more properly under physical geology.) Geochemistry, which is concerned with the chemical properties and processes of Earth, covers a wide array of natural phenomena—from radioactive isotopes in the ground to life-forms in the biosphere. Under the heading of geochemistry fall several biogeochemical processes, such as the carbon cycle, whose study brings together aspects of the physical sciences geology and chemistry as well as various life sciences. O T H E R E A RT H S C I E N C E S . The

hydrological sciences are concerned with the hydrosphere and its principal component, water. These disciplines include hydrology, the study of the water cycle; glaciology, the study of ice in general and glaciers in particular; and oceanography. Clearly, oceanography overlaps with the life sciences; likewise, hydrogeology (the study of groundwater), as its name implies, overlaps with geology. The atmospheric sciences, obviously, are devoted to the atmosphere. Most notable among these sciences is meteorology, the study of weather patterns, and climatology, the study of temperature and climate. (Paleoclimatology is an important subdiscipline of historical geology.) The atmospheric sciences also are concerned with phenomena ranging from pollution to the optical effects created by the interaction of the Sun’s rays with the atmosphere. Finally, there are miscellaneous areas of study that either are interdisciplinary or cross boundaries between the earth sciences and the social sciences. In the former category, for

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instance, would be environmental studies that involve aspects of the biosphere, atmosphere, geosphere, and hydrosphere. Examples of the second category are paleoarchaeology, the study of the earliest humans and humanoid forms, and, of course, geography. Also included in this group are such intriguing areas as urban geology, a branch of environmental geology concerned with human settlements.

Studying Earth

WHERE TO LEARN MORE Allaby, Ailsa, and Michael Allaby. A Dictionary of Earth Sciences. 2d ed. New York: Oxford University Press, 1999. Athro: Your Source for High School and College Level Biology, Earth Science, and Geology on the Web (Web site). . Cox, Reg, and Neil Morris. The Natural World. Philadelphia: Chelsea House Publishers, 2000. Dasch, E. Julius. Earth Sciences for Students. New York: Macmillan Reference USA, 1999. Geology Entrance (Web site). . Hancock, Paul L., and Brian J. Skinner. The Oxford Companion to the Earth. New York: Oxford University Press, 2000. Illustrated Glossary of Geologic Terms (Web site). . Jobs in Earth Sciences (Web site). . Knapp, Brian J., David Woodroffe, and Julian Baker. Earth Science: Discovering the Secrets of the Earth. Danbury, CT: Grolier Educational, 2000. Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science. 2d ed. New York: John Wiley and Sons, 1999. Stace, Alexa. Atlas of Earth. Milwaukee, WI: Gareth Stevens Publishing, 2000.

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MEASURING AND MAPPING EARTH Measuring and Mapping Earth

CONCEPT Today a sharp distinction exists between the earth sciences and geography, but this has not always been the case. In ancient times, when scientists lacked the theoretical or technological means to study Earth’s interior, the two disciplines were linked much more closely. Even in the centuries since these disciplines parted ways, the earth sciences have continued to benefit from a foundation established in part by early geographers, whose work informed the geophysical subdiscipline of geodesy. Like geographers, earth scientists are interested in measuring and mapping Earth, though their interests are quite different. Among the areas of concern to earth scientists are the location of underground resources and the obtaining of data on the planet’s gravitational and magnetic fields. In these and other pursuits, earth scientists use a number of techniques and technologies, ranging from the ancient discipline of surveying to the most modern forms of satellite-based remote sensing.

HOW IT WORKS

It is true that the divisions between geography and geology are clear, symbolized by the features that appear or do not appear on the maps used by either discipline. Geography is somewhat concerned with natural features, but its interests include man-made boundaries, points, and such formations as population centers, roads, and so on. Certainly, an atlas may include physical maps, which are dominated by natural features and contain little or no evidence of man-made demarcations or points of interest. Nonetheless, the purpose of a geographical atlas is to identify locations of interest to humans, among them, cities, roads from one place to another, and borders that must be crossed.

Whereas geology is the study of the solid earth, including its history, structure, and composition, geography is the study of Earth’s surface. Geologists are concerned with the grand sweep of the planet’s history over more than four billion years, whereas the work of geographers addresses the here and now—or at most, in the case of historical geography, a span of just a few thousand years.

By contrast, geologic maps contain detailed information about rock formations and other natural features, with virtually nothing to indicate the presence of humans except as it relates to natural features under study. An exception might be a map designed to be used by paleoarchaeologists, who study the earliest humans and humanoid forms. Their discipline, which combines aspects of the earth sciences and archaeology, is concerned with human settlements, but mostly only prehistoric human settlements.

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Geography and Geology

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Today the distinctions between these two disciplines are sharp, so much so that most books on the earth sciences barely even mention geography. In a modern university, chances are that the geography and geology/earth sciences departments will not even be located in the same building. Geography, after all, usually is classified among social sciences such as anthropology or archaeology, whereas geology is a “hard science,” along with physics or biology.

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Despite the depth and breadth of distinctions between them, it is significant that studies in both geography and geology make use of maps. Mapmaking, or cartography, is considered a subdiscipline of geography, yet it is actually an interdisciplinary pursuit (much like many of the earth sciences—see Studying Earth) and combines aspects of science, mathematics, technology, and even art. Although their interests are in most cases quite different from those of geographers, geologists rely heavily on the work of cartographers.

Early Geographic Studies The history of the sciences has been characterized by the continual specialization and separation of disciplines. Thus, it should not be surprising to discover that to the ancients, the lines were blurred between geography, mathematics, astronomy, and what people today would call earth sciences. Most of the early advances in the study of Earth involved all of those disciplines, an example being the remarkable estimate of Earth’s size made by Eratosthenes of Cyrene (ca. 276–ca. 194 B.C.). A mathematician and librarian at Alexandria, Egypt, Eratosthenes discovered that at Syene, several hundred miles south along the Nile near what is now Aswan, the Sun shone directly into a deep well and upright pillars cast no shadow at noon on the summer solstice (June 21). By using the difference in angles between the Sun’s rays in both locations as well as the distance between the two towns, he calculated Earth’s circumference at about 24,662 mi. (39,459 km). This figure is amazingly close to the one used today: 24,901.55 mi. (39,842.48 km) at the equator. Eratosthenes published his results in a book whose Greek name, Geographica (Geography), means “writing about Earth.” This was the first known use of the term. P T O L E M Y ’ S G E O G RA P H Y. Unfortunately, the Alexandrian astronomer Ptolemy (ca. A.D. 100–170), one of the most influential figures of the ancient scientific world, rejected Eratosthenes’ calculations and performed his own, based on faulty information. The result was a wildly inaccurate estimate of 16,000 mi. (25,600 km). More than thirteen centuries later, Christopher Columbus (1451–1506) relied on Ptolemy’s figures rather than those of Eratosthenes, whose work was probably unknown to

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him. Thinking that the circumference of Earth was two-thirds what it actually is, Columbus set sail westward from Spain—something he might not have done if he had known about the “extra” 8,000 mi. (12,874 km) that lay to the west of Europe.

Measuring and Mapping Earth

Nonetheless, in his Hyphegesis geographike (Guide to geography), Ptolemy did make useful contributions to geographical study. He helped popularize the use of latitude and longitude lines, first conceived by the Greek astronomer and mathematician Hipparchus (fl. 146–127 B.C.), and rejected the widespread belief that a vast ocean, known to the ancients as “the Ocean Sea,” surrounded the entire world. He also presented a set of workable mathematical principles for representing the spherical surface of Earth on a flat page, always a problem for cartographers. In addition, Ptolemy established the practice of orienting maps with north at the top of the page. Today this is taken for granted, but in his time cartographers depicted the direction of the rising Sun, east, at the top of their maps. Ptolemy used a northward orientation because the Mediterranean region that he knew extended twice as far east to west as it did north to south. To represent the area on a scroll, the form in which books appeared during his time, it was easier to make maps with north at the top. OTHER ANCIENT G E O G RA P H E R S . Most other ancient geographers of

note were primarily historians rather than scientists, preeminent examples being the Greek Herodotus (ca. 484–ca. 430–420 B.C.), the father of history, and Strabo (ca. 64 B.C.–ca. A.D. 24), whose work provides some of the earliest Western descriptions of India and Arabia. An exception was Pomponius Mela (fl. ca. A.D. 44), whose work would fit into the subdiscipline known today as physical geography, concerned with the exterior physical features and changes of Earth. In his three-volume De situ orbis, (A description of the world), Mela introduced a system of five temperature zones: northern frigid, northern temperate, torrid (very hot), southern temperate, and southern frigid. Unlike many scientific works of antiquity, Mela’s geography has remained influential well into modern times, and his idea of the five temperature zones remains in use. Mela and Ptolemy, who followed him by about a century, were among the last geographers

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Measuring and Mapping Earth

MERCATOR

CYLINDRICAL PROJECTION OF THE CONTINENTS OF

EARTH,

SHOWING THE CHARACTERISTIC EXAGGERATION

IN SCALE OF LAND MASSES NEAR THE POLES. (© R. Winter/Photo Researchers. Reproduced by permission.)

of note in the West for more than a thousand years. T H E S E PA RAT I O N O F G E O G RA P H Y A N D E A RT H S C I E N C E S .

During the first half of the Middle Ages significant work in cartography took place in the Muslim world and in the Far East, but not in Europe. Only in the course of the Crusades (1095–1291) did Europeans become interested in exploration again, and this interest grew as the Mongol invasions of the thirteenth century opened trade routes from Europe to China for the first time in a thousand years. The crusading and exploring spirits met in Henry the Navigator (1394–1460), the prince who, while he never actually took part in any voyages himself, quite literally launched the Age of Exploration from his navigation school at Sagres, Portugal.

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some of the first modern studies in the earth sciences. (See Studying Earth for more about Leonardo’s and Agricola’s contributions.) In the sixteenth century, the Flemish cartographer Gerhardus Mercator (1512–1594) greatly advanced the science of mapmaking with his development of the Mercator projection. The latter method, still used in many maps today, provided an effective means of rendering the spherical surface of Earth on a two-dimensional map. By that time, cartography had emerged as a vital subdiscipline, and over the ensuing two centuries the separation between geography and the earth sciences became more and more distinct.

In the two centuries that followed, European mariners and conquerors explored and mapped the continents of the world. Along the way, these nonscientists sometimes added knowledge to what would now be considered the earth sciences, as when the Italian explorer Christopher Columbus (1451–1506) became the first to notice magnetic declination. (See Geomagnetism for more on this subject.) Meanwhile, Columbus’s contemporary, the Italian artist and scientist Leonardo da Vinci (1452–1519), as well as Georgius Agricola (1494–1555) of Germany, known as the father of mineralogy, conducted

The full separation of disciplines took place in the eighteenth century, when the German geographer Anton Friedrich Büsching (1724– 1793) pioneered modern scientific geography. Beginning in 1754, Büsching published the 11volume Neue Erdbeschreibung (New description of the earth), which established a foundation for the study of geography in statistics rather than descriptive writing. During the same century geology was in the middle of its own paradigm shift. Until the time of the Scottish geologist James Hutton (1726–1797), a near exact contemporary of Büsching, geologists had been concerned primarily with explaining how the world began—a topic that almost inevitably led to conflict over religious questions (see Earth, Science, and Nonscience). Hutton, known as the father of

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geology, was the first to transcend this issue and instead offer a theory about the workings of Earth’s internal mechanisms.

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Measuring and Mapping Earth

Geologic Maps and Surveys Surveying The era of Büsching and Hutton coincided with that of the English astronomer Charles Mason (1730–1787) and the English surveyor Jeremiah Dixon (d. 1777), who in 1763 began their famous survey of the boundary between Pennsylvania and Maryland. It took them five years to survey the 233-mi. (373-km) Mason–Dixon Line, which eventually became known as the border between the free and slave states before the Civil War. Surveying, a profession practiced by such great Americans as the first president, George Washington (1732–1799), and the mathematician and astronomer Benjamin Banneker (1731–1806), eventually became associated with the United States, but it originated in Egypt as early as 2700 B.C. Surveying is a realm of applied mathematics devoted to measuring and mapping areas of land. Though it is obviously of value to the earth sciences and geography, it has a great deal of importance economically and politically as well. For this reason, the Romans, who were not nearly as inclined toward theoretical study as the Greeks, became preeminent surveyors whose land parcels still can be seen from the air over parts of western Europe. Owing to its great practical importance, surveying—unlike virtually all other forms of learning—continued to thrive in Europe during the early Middle Ages. Nonetheless, surveying improved in the Renaissance and thereafter, as the result of the introduction of new tools and mathematical techniques.

A geologic map shows the rocks beneath Earth’s surface, including their distribution according to type as well as their ages, relationships, and structural features. The first geologic map, made in 1743, depicted subterranean East Kent, England. Its creator, the English physician and geologist Christopher Packe (1686–1749), introduced the technique of hachuring, that is, representing relief (elevation) on a map by shading in short lines in the direction of the slopes. Three years later, the French geologist JeanEtienne Guettard (1715–1786) made the first geologic map that crossed national lines, thus illustrating the distinction between geology and geography. As Guettard discovered, the geologic features of the French and English coasts along the English Channel are identical, indicating that the areas are connected. At a time when France and England were still bitter political and military rivals, Guettard’s work showed that they literally shared some of the same land. Geologic mapmaking received a further boost in 1815, when the English geologist William Smith (1769–1839) produced what has been called the first geologic map based on scientific principles. Entitled A Delineation of the Strata of England and Wales with Part of Scotland, the map used different colors to indicate layers of sedimentation. Roughly 6 ft. by 9 ft. (1.8  2.7 m), the map linked paleontology with stratigraphy (the study of fossils and the study of rock layers, respectively) and proved to be a milestone in geologic cartography.

Among these tools were the theodolite, used for measuring horizontal and vertical angles, and the transit, a type of theodolite that employs a hanging plumb bob to determine a level sight line. Mathematical techniques included triangulation, whereby the third side of a triangle can be determined from measurements of the other two sides and angles. The German mathematician Karl Friedrich Gauss (1777–1855), regarded as the father of geodesy, introduced the heliotrope, a mechanism that aids in triangulation. Other tools included the compass, level, and measuring tapes; modern surveying benefits from remote sensing. (Both remote sensing and geodesy are discussed later in this essay.)

M O D E R N G E O LO G I C M A P M A K I N G . Geologic mapmaking changed consider-

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ably in the period after World War II. Before that time, geologists did most of their work with the use of topographical maps, or maps that showed only surface features. Following the war, however, aerial photography became much more common, giving rise to the technique of photogeology, the use of aerial photographic data to make determinations regarding the geologic characteristics of an area. Petroleum companies, which often have taken the lead in developing advanced methods of geologic study, introduced the practice of creating three-dimensional images from the air.

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RELIEF-SHADED

MAP OF

EARTH,

SHOWING THE CONTINENTS IN RAISED ELEVATION. (© M. Agliolo/Photo Researchers. Repro-

duced by permission.)

They did this by taking pairs of photographs which, when viewed through a stereoscope, provided images that could be studied in great detail for information about all manner of geologic features. Height proved a great advantage, revealing features that would not have been as clear to a geologist working on the ground. Of course, a great deal of work on the ground was still necessary for confirming the data revealed by aerial surveillance and for other purposes, such as measurement and sample collection. Nonetheless, aerial photography provided an enormous boost to geologic studies, as did the use of satellite imaging from the 1970s onward. Also helpful were such new devices as the handheld magnetometer, which made it relatively easy to separate rocks containing magnetite from other, nonmagnetic samples. One might wonder why any of this is important, aside from a purely academic interest in the structure of rocks under the ground. In fact, the need for precise geologic data goes far beyond “purely academic interests,” as the reference to oil companies suggests. Geologic mapmaking is critical to the location of oil as well as minerals and other valuable natural resources—including the most useful one of all, water.

G E O LO G I C S U R V E Y S . Geologic mapmaking is so vital, in fact, that national governments have undertaken large-scale and ongoing geologic studies since 1835. That was the year that Great Britain became the first country to establish a geologic survey, with the aim of preparing a geologic map of the entire British Isles. The project began with the mapping of Cornwall and southern Wales and has continued ever since, with the addition of new details as they have become available.

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Likewise it is necessary to have accurate geologic information before undertaking a large engineering project, such as the building of a road or bridge. In such situations, geologic studies can quite literally be a matter of life and death, and eventually such studies may save more lives by aiding in the prediction of earthquakes or volcanoes. Geologic data is also a critical part of studies directed toward environmental protection, both for areas designed to remain natural habitats and for those designated for development. And, finally, there are studies whose purpose is purely, or mostly, academic but that reveal a great deal of useful information about the history of the planet, the forces that shaped it, and perhaps even future events.

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In the ensuing years several other countries established their own geologic surveys, including the United States in 1889. (The Web site of the U.S. Geological Survey is listed in the bibliography.) Other important national geologic surveys include those of France, Canada, China, and Russia. The last of these surveys is of particular interest, dating as it does from the “Stone Department,” a mineralogical survey established in 1584. Today even much smaller nations, such as Uruguay, Slovakia, and Namibia, have their own national geologic surveys. Over the years, techniques of information gathering have evolved, particularly with the development of satellite remote-sensing technology. So, too, have the areas under the purview of various national geologic surveys, which since the 1950s have undertaken the mapping of the continental shelves adjacent to their own shorelines. (In addition, the nations claiming territories in Antarctica, including Britain and the United States, have mapped the geologic features of that continent extensively, though mining or other economic development there is forbidden.) The U.S. Geological Survey has also seen its scope extended to include studies on such issues as radioactive waste disposal and prediction of natural hazards, among them, earthquakes in urban areas.

Geophysical Measurements Geophysics is a branch of the earth sciences that combines aspects of geology and physics. Among the areas it addresses are Earth’s physical processes as well as its gravitational, magnetic, and electric properties and the means by which energy is transmitted through its interior. Areas of geophysics with a particular focus on measurement and mapping include the study of geomagnetism, or Earth’s magnetic field, and geodesy, which is devoted to the measurement of Earth’s shape and gravitational field. The measurement of gravitational fields involves the use of either weights dropped in a vacuum or mechanical force-balance instruments. The first of these techniques is much older than the other and provides an absolute measure of the gravitational field in a given area. As for force-balance instruments, they are similar in principle to scales and furnish a relative measure of the gravitational field. To compare the gravitational field at different positions, however,

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it is necessary to establish a frame of reference. This is known as the geoid, a surface of uniform gravitational potential covering the entire earth at a height equal to sea level. (See Gravity and Geodesy for more on these topics.) OF

Measuring and Mapping Earth

GEODETIC MEASUREMENTS E A RT H ’ S S U R FAC E . As noted,

geodesy is concerned not only with Earth’s gravitational field but also with its shape, and earth scientists working on this aspect of the subdiscipline employ many of the techniques and equipment described earlier with regard to geography and surveying. Eratosthenes’s measurement of Earth’s size is thought to be the first geodetic measurement, and in performing geodetic measurements today, earth scientists often employ concepts familiar to surveyors. Among these concepts is triangulation, which was developed in the sixteenth century by the Dutch mathematician Gemma Frisius (1508–1555). Triangulation remained an important method of geodetic measurement until the development of satellite geodesy made possible simpler and more accurate measurements through remote sensing. Even today triangulation is still used by geologists without access to satellite data. In performing measurements using triangulation, geologists employ the theodolite, and typically at least one triangulation point is highly visible—for instance, the top of a mountain. Until the 1950s scientists used a measuring tape of a material called Invar, a nickeliron alloy noted for its tendency not to expand or contract with changes in temperature. From that time, however, electronic distance measurement (EDM) systems, which employ microwaves or visible light, came into use. EDM helped overcome some of the possibilities for error inherent in using any kind of tape, for example, the likelihood that it would sag and thus render incorrect measurements. Furthermore, EDM tended to reduce errors caused by atmospheric refraction. With the advent of the United States program in the 1960s, increasingly more sophisticated forms of geodesic remote-sensing technology came into use. Among these techniques is satellite laser ranging, which relies on measurements of the amount of time required for a laser pulse to travel from a ground station to a satellite and back. Before the development of the global posi-

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Measuring and Mapping Earth

KEY TERMS The creation, production, and study of maps. Cartography is a subdiscipline of geography and involves not only science but also mathematics, technology, and even art.

CARTOGRAPHY:

The change in the observed frequency of a wave when the source of the wave is moving with respect to the observer. DOPPLER EFFECT:

A map showing the rocks beneath Earth’s surface, including their distribution according to type as well as their ages, relationships, and structural features. GEOLOGIC MAP:

FIELD:

The study of the solid earth, in particular, its rocks, minerals, fossils, and land formations.

An area of geophysics devoted to the measurement of Earth’s shape and gravitational field.

A branch of the earth sciences that combines aspects of geology and physics. Geophysics addresses the planet’s physical processes as well as its gravitational, magnetic, and electric properties and the means by which energy is transmitted through its interior.

A region of space in which it is possible to define the physical properties of each point in the region at any given moment in time. GEODESY:

A social science concerned with the description of physical, biological, and cultural aspects of Earth’s surface and with the distribution and interaction of these features. Compare with geology. GEOGRAPHY:

tioning system (GPS), discussed later, the use of satellite systems necessitated tracking through the Doppler effect, or the change in the observed frequency of a wave when the source of the wave is moving with respect to the observer. Thanks to GPS, put into operation by the U.S. Department of Defense, satellite tracking is much simpler and more accurate today.

GEOLOGY:

GEOPHYSICS:

GEOMAGNETISM: A term referring to the magnetic properties of Earth as a whole, rather than those possessed by a single object or place on Earth.

emphasize the importance of Earth’s magnetic field.

covered that pieces of lodestone (magnetite) tend to point north, mariners have used the compass for navigation. The compass was augmented by other navigational devices until it was supplanted by the gyroscope in modern times and still later by more sophisticated devices and methods, such as GPS. Yet a compass still works fine for many a hiker, and its use serves to

Earth has an overall geomagnetic field, and specific areas on the planet have their own local magnetic fields. Thanks in large part to the contributions of Gauss, who developed a standardized local magnetic coordinate system in the early nineteenth century, it became possible to perform reasonably accurate measurements of local magnetic data while correcting for the influence of Earth’s geomagnetic field. Indeed, one of the challenges in measuring magnetic fields is the fact that the Earth system possesses magnetic force from so many sources: the molten core, from whence originates the preponderance of Earth’s magnetic field; external fields, such as the magnetosphere and ionosphere; local materials, such as magnetite, hematite, or pyrrhotite;

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GEOMAGNETIC MEASUREM E N T S . Ever since the ancient Chinese dis-

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A surface of uniform gravitational potential covering the entire earth at a height equal to sea level. GEOID:

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KEY TERMS HACHURING:

A method of represent-

Measuring and Mapping Earth

CONTINUED

PHYSICAL GEOGRAPHY:

A subdis-

ing relief (elevation) on a map by shading

cipline of geography concerned with the

in short lines in the direction of the slopes.

exterior physical features and changes of

MAGNETOSPHERE:

An area sur-

Earth. Position in a field, such as

rounding Earth, reaching far beyond the

POTENTIAL:

atmosphere, in which ionized particles

a gravitational force field.

(i.e., ones that have lost or gained electrons

REFRACTION:

so as to acquire a net electric charge) are

it passes at an angle from one transparent

affected by Earth’s magnetic field.

material into a second transparent material.

An area of his-

PALEOMAGNETISM:

The bending of light as

REMOTE SENSING:

The gathering of

torical geology devoted to studying the

data without actual contact with the mate-

direction and intensity of magnetic fields

rials or objects being studied.

in the past, as discerned from the residual

STRATIGRAPHY:

magnetization of rocks.

layers, or strata, beneath Earth’s surface.

PALEONTOLOGY:

The study of fos-

SURVEYING:

The study of rock

An area of applied

silized plants and animals, or flora and

mathematics devoted to measuring and

fauna.

mapping areas of land.

PHOTOGEOLOGY:

The use of aerial

TRIANGULATION:

A technique in sur-

photographic data to make determinations

veying whereby the third side of a triangle

regarding the geologic characteristics of an

can be determined from measurements of

area.

the other two sides and angles.

and even man-made sources of magnetic or electric force. After making calculations that correct for interfering sources of magnetism, geophysicists study the remaining magnetic anomalies, which can impart extremely valuable information. Classic examples include the discovery that Earth’s magnetic polarity has reversed many times, a finding that led to the development of the geophysical subdiscipline known as paleomagnetism. Paleomagnetic studies, in turn, served as a highly significant confirmation of plate tectonics, which originated in the middle of the twentieth century and remains the dominant theory regarding geologic processes. (See Plate Tectonics. For more about geomagnetism, including some of the topics mentioned here, see Geomagnetism.)

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Knowing One’s Location In these and other types of studies that involve mapping and measurement, it is important for scientists conducting surveys to be aware of the frame of reference from which they are operating—that is, the perspective from which they view data. Simply put, one must know first where one is before one can measure and map geophysical or other data for surrounding areas. This requires knowledge of latitude and longitude, or east–west and north–south positions, respectively. From earliest times, mariners and scientists have been able to ascertain latitude with relative ease, simply by observing the angle of the Sun and other stars. Determination of longitude, however, proved much more difficult, because it required highly accurate timepieces. Only in the late eighteenth century, with the breakthroughs

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achieved by the British horologist John Harrison (1693–1776) did such calculations become possible. As a result, many a ship’s crew was saved from the misfortunes that could result from inaccurate estimates of location. GLOBAL POSITIONING SYST E M . By the latter part of the twentieth centu-

ry, navigational technology had become vastly more sophisticated than it was in Harrison’s day. From the 1957 launch of the Soviet satellite Sputnik 1, the skies over Earth became increasingly populated with satellites, such that within half a century dozens of countries had payloads in space. Aside from governments and scientific research establishments, even cable television companies used satellites to beam programming to homes all over the industrialized world, a fact that in itself says much about the spread of satellite technology. Among the most impressive uses of satellites is the GPS, developed by the U.S. Department of Defense to assist in surveillance. GPS consists of 24 satellites orbiting at an altitude of 12,500 mi. (20,000 km). They move in orbital paths such that an earthbound receiver can obtain signals from four or more satellites at any given moment. On board are atomic clocks, which provide exact time data with each signal and eliminate the necessity of the receiver’s having such an accurate clock. By receiving data from these satellites, persons on the ground can compute their own positions in terms of latitude and longitude as well as altitude. Not all receivers have access to the most accurate data possible: in line with the strategic mission for which it initiated GPS in the first place, the Defense Department ensures that only authorized personnel receive the most precise information. Thus, GPS has built-in errors so that civilian users can calculate locations with an accuracy of “only” 328-492 ft. (100–150 m). This, of course, is amazingly accurate, but not as accurate as the data available to those authorized to receive normally encrypted information on the P (Precise) code. The latter provides an accuracy of 3.2816.4 ft. (1–5 m) instantaneously, and more detailed measurements based on GPS data can be used to achieve accuracy of up to 0.2 in. (5 mm).

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Remote Sensing Many of the methods used by geologists and geophysicists to map and measure Earth make use of remote sensing, the gathering of data without actual contact with the materials or objects being studied. Without remote sensing, it would be impossible to discuss many physical phenomena intelligently, because it is unlikely that any technology ever will make it possible to explore many areas underneath the planet’s surface directly. An example of remote sensing is photogeology, described briefly earlier; so, too, is satellite imaging for data collection. The earth scientist of the twenty-first century likewise has other highly sophisticated forms of technology, such as radar systems or infrared imaging, at his or her disposal. As with many other aspects of geologic mapping and measurement, this one has value far beyond the classroom: remote-sensing studies make it possible, for instance, to observe the environmental impact of deforestation in large geographical areas. (For much more about this subject, see Remote Sensing.) WHERE TO LEARN MORE Brooks, Susan. The Geography of the Earth. New York: Oxford University Press, 1996. Erickson, John. Exploring Earth from Space. Blue Ridge Summit, PA: TAB Books, 1989. Geologic Maps (Web site). . Hancock, Paul L., and Brian J. Skinner. The Oxford Companion to the Earth. New York: Oxford University Press, 2000. Internet Resources for Geography and Geology (Web site). . Moffitt, Francis H., and Harry Bouchard. Surveying. 8th ed. New York: Harper and Row, 1987. Remote Sensing Data and Information (Web site). . U.S. Geological Survey (Web site). . Virtual Museum of Surveying (Web site). . Wilford, John Noble. The Mapmakers. New York: Knopf, 2000.

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REMOTE SENSING

CONCEPT Scientists of many disciplines are accustomed to studying data that cannot be observed through direct contact. Physicists and chemists, for instance, know a great deal about the structure of the atom, even though even the most high-powered microscope cannot make an atom visible to the human eye. The objects of study for earth scientists are often similarly remote, though not necessarily because they are small. In some cases, the problem is quite the opposite: an area selected for study is too large to provide understanding to geologists working only on the ground. Other areas are simply inaccessible to human beings or even their equipment. This has necessitated the development of remote sensing equipment and techniques, primarily involving views from the air or from space and utilizing electromagnetic radiation across a wide spectrum.

HOW IT WORKS An Introduction to Remote Sensing The work of geologists would be much easier if Earth were transparent and they could simply look down into the ground as they would into the sky. But the ground is not transparent; nor, for that matter, is the sky, to which meteorologists look for information regarding atmospheric and weather patterns. Some places are hard to see, and many are difficult or even impossible to visit physically. Some places, such as the Sun or the Earth’s core, could not be approached physically even by unmanned technology.

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Hence the need for remote sensing, or the gathering of data without actual contact with the materials or objects being studied. Some earth scientists define the term more narrowly, restricting “remote sensing” to the use of techniques involving radiation on the electromagnetic spectrum. The latter category includes visible, infrared, and ultraviolet light as well as lower-frequency signals in the microwave range of the spectrum. This definition excludes the study of force fields involving gravitational or electromagnetic force. In general, in this essay we abide by that more narrow definition, primarily because most forms of remote sensing in use today involve electromagnetic radiation. Remote sensing is used for a variety of measuring and mapping applications. The reader therefore is encouraged to consult the essay Measuring and Mapping Earth for more on this subject. Applications of remote sensing go far beyond cartography (mapmaking) and measurement, however. As suggested already, remote sensing makes it possible for earth scientists to collect data from places they could not possibly go. In addition, it allows for data collection in places where a human being would be “unable to see the forest for the trees”—which in places such as the Amazon valley is quite literally the case.

The Military Influence Scientists’ understanding of the electromagnetic spectrum was still in its infancy in 1849, when the French army engineer Aimé Laussedat (1819–1907) introduced what was then called iconometry, from the Greek words icon (“image”) and -metry (“measurement”). Laussedat, who experimented with aerial photography

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after the Hyksos invasion (ca. 1670 B.C.); the Assyrian introduction of logistics in an effort to supply imperial troops (ca. 800 B.C.); the Persian development of the postal service (ca. 600 B.C.); numerous Roman innovations, particularly in road building (ca. 200 B.C.–ca. A.D. 200); and the Chinese invention of the wheelbarrow (ca. 100 B.C.). And so the list goes, right up to such latterday American developments as the Internet and GPS, or global positioning system.

Remote Sensing

M I L I TA RY C O N T R I B U T I O N S T O R E M O T E S E N S I N G . Forms of technolo-

A

HAND -HELD GLOBAL POSITIONING DEVICE. (© Ken M.

Johns/Photo Researchers. Reproduced by permission.)

by means of cameras mounted on balloons or kites, is regarded as a pioneer of photogrammetry, the use of aerial or satellite photography to provide measurements of or between objects on the ground. A few years later, the United States armies of the Civil War adopted the use of aerial photography for surveillance purposes, mounting cameras on balloons to provide intelligence regarding federal or Confederate positions and troop strength. This fact, combined with Laussedat’s status as an army engineer, hints at one of the underlying themes in the history of remote sensing, and indeed of many another technological advance: the influence of the military. It is a fact of human existence that nations from at least the time of the Assyrians, if not the Egyptians of the New Kingdom, have devoted far more attention and resources to military applications than they have to peacetime activities. On the other hand, societies have benefited enormously from technological and organizational innovations with military origins, innovations whose application later spread to a variety of peacetime uses. Some examples include the adoption of the chariot by the Egyptian army

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gy pioneered by military forces and now used in remote sensing include infrared photography, thermal imagery, radar scanning, and satellites. The first of these types of technology makes use of light in the infrared portion of the electromagnetic spectrum—a region that, as its name suggests, is adjacent to the red portion of visible light. Red has the longest wavelength and the lowest frequency of all colors, and infrared has an even longer wavelength and lower frequency. Military forces use infrared photography to distinguish between vegetation and camouflage designed to look like vegetation: live plants reflect infrared radiation, whereas dead ones and camouflaged material absorb it. Whereas infrared photography measures reflection of infrared radiation, thermal imaging indicates the amount of such radiation that is emitted by the source. Its military origins lie in its use for reconnaissance during night bombing missions. Similarly, radar scanning makes it possible to view targets on the ground, regardless of lighting or cloud cover. Finally, there are satellites, which have extensive surveillance applications. Among the most important examples of military activity above Earth’s atmosphere are the 24 satellites of GPS, which make allow U.S. forces to plot positions with amazing accuracy. Less accurate GPS intelligence is also available to civilians. (See Measuring and Mapping Earth for more on GPS.)

REAL-LIFE A P P L I C AT I O N S Photogeology All of these innovations introduced by the military, of course, have found application for civilian purposes. Thanks in part to improvements in

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aircraft during World War II, for instance, photogeologic data gathering has increased dramatically in the years since then. Efforts at gaining information by means of airborne sensing devices underwent enormous improvements throughout the middle and latter part of the twentieth century, with the development of technology that made it possible for earth scientists to gather information using techniques beyond ordinary photography, visible light, and airplanes. Still, much of the remote-sensing activity that takes place today is performed aboard airplanes rather than satellites, using ordinary analogue photography within the visible spectrum. Stereoscopic techniques aid in the visualization of relief, or elevation and other inequalities on a land surface. Humans are used to seeing stereoscopically: the distance between the two eyes on our faces results in a difference between the two images each eye sees. The brain corrects for this difference, rendering a stereoscopic image that is more full and dimensional than anything a single eye could produce. The use of multiple cameras and stereoscopic technology replicates this activity of the human brain and thus provides earth scientists with much more information than they could gain simply by looking at “flat” photographs taken from an airplane. The materials studied by a geologist, of course, are primarily underground, but Earth’s surface furnishes many clues that a trained observer can interpret. Uplands and lowlands tend to suggest different types of rocks, while the direction of a dip in the land can supply volumes of information regarding the stratigraphic characteristics of the region. The presence of vegetation can make it harder to discern such clues, but a careful study of plant life can reveal much regarding minerals in the soil, local water resources, and so on.

Digital Photography Within both photogeology and the larger realm of remote sensing, several innovations from the 1960s onward have underpinned more effective methods of observation. One of these is digital photography, which is as much of an improvement over old-fashioned photography as compact discs are over phonograph records. In both cases, the contrast is between analog technology and digital technology. In analog photography,

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Remote Sensing

A COMMUNICATIONS SATELLITE IN ORBIT AROUND EARTH. (© ESA/Photo Researchers. Reproduced by permission.)

for instance, the image is recorded by a camera and stored on photosensitive materials in a film emulsion. In digital photography the image is recorded on a solid-state device called an image sensor and stored in the camera’s memory for transfer to a computer. An analogue (the preferred spelling for the word as a noun) is just that, a “close copy,” whereas digital methods make possible a more exact reproduction of images by assigning to each shade of color a number between 0 and 255. Instead of storing the image in a medium that can be destroyed or lost easily, as is the case with ordinary film, digital images can be saved on a computer, backed up, and sent anywhere in the world via the Internet. Furthermore, these images can be adjusted with the use of a computer, so as to make it easier to see certain features. Computers and digital photography aid in the creation of false-color imaging, a means of representing invisible electromagnetic data by assigning specific colors to certain wavelengths. An example would be the use of red to depict areas of high energy. This is certainly a false use of color, since red actually has the lowest energy

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Remote Sensing

AN

AERIAL PHOTOGRAPH SHOWS THE

COLORADO RIVER

DELTA IN THE

GULF

OF

CALIFORNIA. THE

RIVER ITSELF IS

THE DARK HEMISPHERE AT THE BOTTOM, WITH ITS WATERS BRANCHING OUT THROUGH SANDBARS LIKE THE BOUGHS OF A TREE. (© Photo Researchers. Reproduced by permission.)

in the visible spectrum, with purple possessing the highest energy. (The reason we associate red, orange, and yellow with heat and green, blue, and purple with coldness is that in either case, these are the colors objects reflect, not the ones they absorb.) RA DA R. Most remote-sensing technolo-

gy uses light, whether infrared or visible, that falls at the middle to high end of the electromagnetic spectrum. By contrast, at least one important means of remote detection uses microwaves, which are much lower in energy levels. Microwaves carry FM radio and television signals, as well as radar, or RAdio Detection And Ranging.

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Radar makes it possible for pilots to “see” through clouds, rain, fog, and all manner of natural phenomena—not least of which is darkness. It also can identify objects, both natural and man-made, on the ground. In addition to its application in remote sensing, radar using the Doppler effect (the change in the observed frequency of a wave when the source of the wave is moving with respect to the observer) helps meteorologists track storms. In the simplest model of radar operation, a sensing unit sends out microwaves toward the target, and the waves bounce back off the target to the unit. In a monostatic unit—one in which the transmitter and receiver are in the same loca-

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tion—the radar unit has to be switched continually between sending and receiving modes. Clearly, a bistatic unit—one in which the transmitter and receiver antennas are at locations remote from one another—is generally preferable, but on an airplane, for instance, there is no choice but to use a monostatic unit.

Satellite Data The term satellite refers to any object orbiting a larger one; thus, Earth’s Moon and all the other moons of the solar system are satellites, as are the many artificial satellites that orbit Earth. In practice, however, most people use the term to refer only to artificial satellites, of which there are many hundreds, launched by entities ranging from national governments to international associations to independent firms. Artificial satellites typically are intended for the purposes of gathering information (i.e., scientific research or military surveillance) or disseminating it (i.e., through satellite television broadcasting). In launching a satellite, it is necessary to overcome the enormous pull of Earth’s gravitational field. This is done by providing the satellite with power through rocket boosters that launch it far above Earth’s atmosphere. At a height of 200 mi. (320 km) or more, the satellite is far above the dense gases of the atmosphere yet well within the gravitational field of the planet. The craft is then in a position to orbit Earth indefinitely without the need for additional power from man-made sources; instead, Earth’s own gravitational energy keeps the satellite in orbit for as long as the satellite’s structure remains intact. (See Gravity and Geodesy for more about the mechanics of orbit.) The greater the altitude, the longer it takes a satellite to complete a single revolution. One of the most commonly used altitudes is at 22,500 mi. (36,000 km), at which height a satellite takes 24 hours to orbit Earth. Thus, it is said to be in geosynchronous orbit, meaning that it revolves at the same speed as the planet itself and therefore remains effectively stationary over a given area. Some satellites revolve at even higher altitudes— 25,000 mi. (40,225 km), which, while it is far beyond the atmosphere, is well within Earth’s gravitational field.

specifically for the use of earth scientists and resource managers. Conceived by the United States Department of the Interior in the mid1960s, the Landsat project soon came to involve the National Aeronautics and Space Administration (NASA) and the U.S. Geological Survey (USGS; see Measuring and Mapping Earth for more about geologic surveys.) Landsat 1 went into orbit on July 23, 1972.

Remote Sensing

Over the years, Landsat has gone into six subsequent generations. Landsat 6, launched in 1993, was unable to achieve orbit, but Landsat 1 lasted more than five times as long as its projected life expectancy of one year. Since 1972 at least one Landsat satellite has been in orbit over Earth, and as of early 2001 both Landsat 5 (launched in March 1984) and Landsat 7 (launched in April 1999) were on line. (Landsat 5 was decommissioned in June 2001.) Over the course of the years, the Landsat governing body has changed. In the 1980s, NOAA (National Oceanic and Atmospheric Administration) took over from NASA, and in October 1985 the Landsat system came under the direction of a commercial organization, the Earth Observation Satellite Company (EOSat). In contrast to communication satellites, which tend to maintain geosynchronous orbits, Landsat moves at a much lower altitude and therefore orbits Earth much more quickly. Landsat 7 takes approximately 99 minutes to orbit the planet, thus making 14 circuits in a 24-hour period. Though it never quite passes over the poles, it covers the rest of Earth in swaths 115 mi. (185 km) wide, meaning that eventually it passes over virtually all other spots on the planet. SAT E L L I T E S AT W O R K . Landsat and other satellites, such as France’s SPOT (Satellite Positioning and Tracking), provide data for governments, businesses, scientific institutions, and even the general public. Following the September 11, 2001, terrorist bombing of the World Trade Center in New York City, for instance, the SPOT U.S. Web site () provided viewers with “Images of Infamy”: views of downtown Manhattan before and just a few hours after the bombing.

LA N D SAT. One of the most impressive undertakings in the field of satellite research is Landsat, an Earth-monitoring satellite designed

Data from Landsat has been used to study disasters and potential disasters with particular application to the earth sciences. An example is the area of the tropical rainforest in Brazil’s Amazon River valley, a region of about 1.9 million sq.

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Remote Sensing

KEY TERMS CARTOGRAPHY:

The creation, pro-

A unit for measuring frequen-

duction, and study of maps. Cartography is

cy equal to one cycle per second. High fre-

a subdiscipline of geography and involves

quencies are expressed in terms of kilo-

not only science but also mathematics,

hertz (kHz; 103, or 1,000 cycles per sec-

technology, and even art.

ond), megahertz (MHz; 106, or one million

DOPPLER EFFECT:

The change in

cycles per second), and gigahertz (GHz;

the observed frequency of a wave when the

109, or one billion cycles per second).

source of the wave is moving with respect

PHOTOGEOLOGY:

to the observer.

photographic data to make determinations

ELECTROMAGNETIC

RADIATION:

See Electromagnetic spectrum and Radiation.

The use of aerial

regarding the geologic characteristics of an area. PHOTOGRAMMETRY:

The use of aer-

SPECTRUM:

ial or satellite photography to provide

The complete range of electromagnetic

measurements of or between objects on

waves on a continuous distribution from a

the ground.

ELECTROMAGNETIC

very low range of frequencies and energy levels, with a correspondingly long wavelength, to a very high range of frequencies and energy levels, with a correspondingly short wavelength. Included on the electromagnetic spectrum are long-wave and short-wave radio; microwaves; infrared, visible, and ultraviolet light; x rays; and gamma rays. FALSE-COLOR IMAGING:

A means

RADIATION:

The transfer of energy by

means of electromagnetic waves, which require no physical medium (for example, water or air) for the transfer. Earth receives the Sun’s energy, via the electromagnetic spectrum by means of radiation. RELIEF:

Elevation and other inequali-

ties on a land surface. The gathering of

of representing invisible electromagnetic

REMOTE SENSING:

data by assigning specific colors to certain

data without actual contact with the mate-

wavelengths.

rials or objects being studied.

FREQUENCY:

The number of waves,

STRATIGRAPHY:

The study of rock

measured in Hertz, passing through a given

layers, or strata, beneath Earth’s surface.

point during the interval of one second.

WAVELENGTH:

The higher the frequency, the shorter the

a crest and the adjacent crest or a trough

wavelength.

and the adjacent trough of a wave. Wave-

The distance between

An area of geophysics

length is inversely related to frequency,

devoted to the measurement of Earth’s

meaning that the shorter the wavelength,

shape and gravitational field.

the higher the frequency.

GEODESY:

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HERTZ:

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mi. (five million sq km), in which deforestation is claiming between 4,250 sq. mi. and 10,000 sq. mi. (11,000–26,000 sq km) a year. This is an extremely serious issue, because the Amazon basin represents approximately one-third of the total rainforest area on Earth. Earlier estimates, however, had suggested that deforestation was claiming up to three times as much as it actually is, and Landsat provided a more accurate figure.

Aldabra atoll in the Seychelles showed the world’s largest refuge for giant tortoises. And shots taken from Landsat over Lake Nasser in southern Egypt during the latter part of 2000 showed four lakes created by excess water from Nasser. As a result, that region of the Sahara had new lakes for the first time in 6,000 years.

Because of its acute spatial resolution (98 ft., or 30 m, compared with more than 0.6 mi., or 1 km), Landsat is much more effective for this purpose than other satellite systems operated by NOAA or other organizations. It is also cheaper to obtain images from it than from SPOT. Over the years, Landsat has provided data on urban sprawl in areas as widely separated as Las Vegas, Nevada, and Santiago, Chile. It has offered glimpses of disasters ranging from the eruption of Mount Saint Helens, Washington, in 1980 to some of the most potent recent examples of destruction caused by humans, including the nuclear disaster at Chernobyl, Ukraine, in 1986 and the fires and other effects of the Persian Gulf War of 1990–1991. (For more on this subject, see the Earthshots Web site, operated by USGS.)

Burtch, Robert. A Short History of Photogrammetry (Web site). .

Not all the news from Landsat is bad, as a visit to the Landsat 7 Web site () in late 2001 revealed. Certainly there were areas of concern, among them, flooding in Mozambique and runaway development in Denver, Colorado. But images taken over the

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Remote Sensing

WHERE TO LEARN MORE

“Earthshots: Satellite Images of Environmental Change,” U.S. Geological Survey (Web site). . Remote Sensing Data and Information (Web site). . Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science, 2d ed. New York: John Wiley and Sons, 1999. Smith, David G. The Cambridge Encyclopedia of Earth Sciences. New York: Cambridge University Press, 1981. Strain, Priscilla, and Frederick Engle. Looking at Earth. Atlanta: Turner Publishing, 1992. Visualization of Remote Sensing Data (Web site). . The WWW Virtual Library: Remote Sensing (Web site). .

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S C I E N C E O F E V E RY DAY T H I N G S Real-Life Earth Science

PLANETOLOGY P L A N E TA R Y S C I E N C E SUN, MOON, AND EARTH

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P L A N E TA R Y S C I E N C E

CONCEPT The term planetary science encompasses a whole range of studies involving a combination of earth sciences and astronomy. Sometimes known as planetology or planetary studies, these disciplines are concerned primarily with the geologic, geophysical, and geochemical properties of other planets. They also draw on areas of astronomy, such as cosmology, a fascinating discipline devoted to the study of the origin, structure, and evolution of the universe. As always when considering realms beyond our Earth, there are many surprises. Indeed, the more one learns about Earth’s relationship to the rest of the cosmos, the harder it is to say which is more intriguing: the many factors that make Earth different or the myriad ways that our home planet is just like the rest of the known universe.

ing around an average-sized star, the Sun, somewhere between the center and the edge of a galaxy called the Milky Way—itself just one of many galaxies in the universe. This awareness naturally makes a person feel small and almost inevitably raises questions about the nature of the soul, divinity, and the afterlife. R E L I G I O N , P H I LO S O P H Y, M Y S T I C I S M , A N D S C I E N C E . Such ques-

tions are a natural accompaniment to of our feeling that if one person is so truly insignificant in this vast cosmos, there must be something else that gives meaning to the structure of reality. These vast issues, of course, are properly addressed not by science but by theology and philosophy. Science, on the other hand, is concerned simply with the facts of how the universe emerged and how Earth fits into the larger picture.

Most of us spend our daily lives without devoting a great deal of thought to what lies beyond Earth. People who live outside cities are perhaps more attuned to the cosmos than are their urban counterparts, simply because they see the vast oceans of stars that cover the sky on a clear night. But a person who lives in the city, where bright lights and smog conspire to cover all but the brightest heavenly bodies, rarely finds a reason to look up into the night sky.

Yet it is easy to see how ancient peoples would have perceived no distinction between religion and science where the study of the cosmos was concerned. The Babylonians, for instance, had no concept of any difference between scientific astronomy and astrology, which today is recognized as a superstitious and thoroughly unscientific pursuit. The Greeks modeled the cosmos on their philosophical systems, which provided a hierarchy of material forms and an ordered arrangement of causes and substances. And the Judeo-Christian tradition depicts a universe fashioned by a loving, all-powerful creator who designed the human being in his own image.

One reason people spend little time thinking about the cosmos is that to do so ultimately fills one with a sense of awe bordering on dread. We know that Earth is but one planet of nine, revolv-

In the belief systems of Judaism and Christianity, handed down through the Bible, the cosmos is depicted as the setting of a vast spiritual drama centered around the themes of free will,

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sin, and redemption. The Bible never says that Earth is the center of the physical universe, but it clearly presents it as the center of the spiritual one. This is understandable enough, especially if human beings truly are the only intelligent lifeforms; unfortunately, these spiritual ideas eventually informed an erroneous cosmology that depicted Earth as the physical center of the cosmos.

Cosmology In fairness to Christianity, it should be said that most religious, philosophical, and even scientific traditions before about 1500 depicted Earth as the center of the universe. Indeed, it required a great feat of insight to discern that Earth is not the center. The same is true of many other discoveries about the cosmos, where nothing is as it appears when simply gazing into the night sky. In a scene from his great novel The Adventures of Huckleberry Finn (1884), Mark Twain aptly illustrated the impossibility of understanding the universe simply on the basis of unaided intellect. Huck and the runaway slave Jim have just finished supper and are lying on their backs and staring up at the stars, speculating as to their origins. One of them comes up with a theory that seems altogether plausible on the face of it: the Moon, because it looks larger than the stars, must have laid them like eggs. A similar scene occurs in the children’s movie The Lion King (1994), in which one character postulates that the stars have become stuck to the sky like flies on flypaper. When another character, the warthog Pumbaa, correctly suggests that the stars are actually great balls of burning gas billions of miles away, his companions laugh this off as preposterous. A R I S TA R C H U S AND HIPPA R C H U S . Although they lacked telescopes,

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later followers was the Alexandrian Ptolemy (ca. 100–170), destined to become the most influential astronomer of ancient and medieval times, who established geocentric cosmology as a guiding principle of astronomy. A.D.

T H E P T O L E M A I C S Y S T E M . The influence of Ptolemy’s erroneous ideas is partly an accident of history. He lived, as it turned out, in the last great era of civilization: ten years after his death came that of the Roman emperor Marcus Aurelius (A.D. 121–180), whose passing marked the beginning of Rome’s decline over the next three centuries. Learning in western Europe virtually ceased until about 1200, and even though the Muslim world produced several thinkers of note during this period, most of them worked within the tradition established by Ptolemy. Muslim thinkers’ respect for Ptolemy is reflected in the name that Arab translators gave to his most important writing: al-majisti or “majesty.” When this work made its way to Europe, it became known as the Almagest.

The Ptolemaic system proves that it is possible to prove anything, if one creates a methodology elaborate enough. Of course, as we know now, Earth is not the center of the universe, but pure observation alone did not reveal this, and Ptolemy’s cosmology worked because he developed mathematics and ideas of planetary motion that made it workable. For instance, not only did planets orbit around Earth in Ptolemy’s cosmology, but they also moved in circles around the paths of their own orbits. Of course, they do revolve on their axes, but that was not part of Ptolemy’s model. In fact, it is hard to find an analogy in the real world, with the exception of some bizarre amusement park ride, for the form of motion Ptolemy was describing.

the Greeks developed rather sophisticated (though in many cases wrong) ideas concerning the arrangement of the cosmos. Most notable among these early thinkers was the astronomer Aristarchus of Samos (ca. 320–ca. 250 B.C.), who proposed that Earth rotates on its axis once every day and revolves with other planets around the Sun. He also correctly suggested that the Sun is larger than Earth.

He was trying to explain retrograde motion, or the fact that other planets seem to speed up and slow down. Retrograde motion makes perfect sense once one understands that Earth is moving even as the other planets are moving, thus creating the optical illusion that the others are changing speeds. Since the Ptolemaic system depicted a still Earth in the middle of a moving universe, however, the explanation of retrograde motion required mental acrobatics.

Unfortunately, the astronomer Hipparchus (146–127 B.C.) rejected this heliocentric, or Suncentered, cosmology in favor of a geocentric, or Earth-centered, model. Among Hipparchus’s

Although it is incorrect, the Ptolemaic system was a creation of genius; otherwise, it could not have survived for as long as it did. Even with the

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CHALLENGING

P T O L E M Y.

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recovery of learning in Europe during the late Middle Ages, scientists continued to uphold Ptolemy’s ideas. Instead of discarding his system, or at least calling it into question, astronomers simply adjusted the mathematics and refined their ancient forebear’s physical model to account for any anomalies. The revolution against Ptolemy began quietly enough in the fifteenth century, when the Austrian astronomer and mathematician Georg Purbach (1423–1461) noted the inaccuracies of existing astronomical tables and the need for better translations of Greek texts. Purbach attempted to produce a revised and corrected version of the Almagest, but he died before completing it. The job fell to his student, Johann Müller, who was known as Regiomontanus (1436–1476). The Epitome of the Almagest (1463), begun by Purbach and completed by Regiomontanus, proved to be a turning point in astronomy. Like their medieval predecessors, the two men started out working in the Ptolemaic tradition, but by showing the errors in Ptolemy’s work, they actually were criticizing him. Their discoveries were not lost on a young Polish astronomer named Nicolaus Copernicus (1473–1543). THE COPERNICAN REVOLUT I O N . The story of the Copernican Revolu-

tion, the opening chapter in a larger movement known as the Scientific Revolution, is among the greatest sagas in the history of thought. It was a watershed event, marking the birth of modern science as such, but the change in thought patterns created by this revolution was not so much the work of Copernicus as it was of the Italian astronomer Galileo Galilei (1564–1642). Although he often is given less attention than Copernicus and the other most noted figure of the Scientific Revolution, the English natural philosopher Isaac Newton (1642–1727), Galileo was a thinker of the first order who took Copernicus’s discoveries much further.

greatest gift to science was his introduction of the scientific method.

Planetary Science

Thanks to Galileo and others who later refined the method, thinkers would no longer be content to let mere conjecture guide their work. Before his time, scientists generally had followed a pattern of absorbing the received wisdom of the ancients and then seeking evidence that confirmed those suppositions. The new scientific method, on the other hand, required rigorous work: detailed observation, the formation of hypotheses, testing of hypotheses, formation of theories, testing of theories, formation of laws, testing of laws—and always more observation and testing.

REAL-LIFE A P P L I C AT I O N S Gravity, the Sun, and Earth A fourth key figure in the Scientific Revolution was the German astronomer Johannes Kepler (1571–1630), whose laws of planetary motion directly influenced Newton’s laws of gravitation and motion. Thanks to Kepler, we know that planets do not make circular orbits around the Sun; rather, those orbits are elliptical. As Newton later showed, the reason for this is the gravitational pull exerted by the Sun. Gravitational force explains why Earth, the Sun, and all celestial bodies larger than asteroids are round—but also why they cannot be perfectly round. As to the latter issue—the fact that Earth bulges near the equator—it is a consequence of its motion around its axis. Because it is spinning rapidly, the mass of the planet’s interior responds to the centripetal (inward) force of its motion, producing a centrifugal, or outward, component. If Earth were standing still, it would be much nearer to the shape of a sphere.

Copernicus had been concerned with how the planets move as they do, and in the course of his work he showed that all of them (Earth included) move around the Sun. Galileo, on the other hand, set out to discover why the planets revolve around the Sun, and in so doing he discovered the principles of inertia and gravitational acceleration that would influence Newton. He made numerous other contributions, such as the discovery that Jupiter had moons, but by far his

M ASS A N D S P H E R I C I T Y. Now to the larger question: Why is Earth round? The answer is that the gravitational pull of its interior forces a planetary body to assume a more or less uniform shape. Furthermore, the larger the mass of an object, the greater its tendency toward roundness. Earth’s surface has a relatively small vertical differential: between the lowest point and the highest point is just 12.28 mi. (19.6 km), which is not a great distance, considering that Earth’s radius is about 4,000 mi. (6,400 km).

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the planet’s volume. Geologists believe that the composition of Earth’s core must be molten iron, which creates the planet’s vast electromagnetic field.

Planetary Science

Certain particulars of Earth’s core lead us to answering another great question about our home planet: Why is it alone capable of sustaining life—as far as we can tell—while the other planets of our solar system are either hellish worlds of fire or frigid, forbidding realms of ice crystals and liquefied gas? D E N S I T Y O F E A RT H ’ S I N T E R I O R. At first glance, Earth seems to have few dis-

JOHANNES KEPLER (The Bettmann Archive. Reproduced by permission.)

An object of less mass is more likely to retain a shape that is less than spherical. This can be shown by reference to the Martian moons Phobos and Deimos, both of which are oblong, and both of which are tiny, in terms of size and mass, compared with Earth’s Moon. Mars itself has a radius half that of Earth, yet its mass is only about 10% of Earth’s, and therefore it is capable of retaining a less perfectly spherical shape. There is also the possibility of more pronounced differences in elevation, and thus it should not be surprising to learn that Mars is also home to the tallest mountain in the solar system. Standing 15 mi. (24 km) high, the volcano Olympus Mons is not only much taller than Earth’s tallest peak, Mount Everest (29,028 ft., or 8,848 m), it is also 22% taller than the distance from the top of Mount Everest to the lowest spot on Earth, the Mariana Trench in the Pacific Ocean (–36,198 ft., or –10,911 m).

The only bodies that come close are Mercury and Venus, which along with Earth and Mars are designated as terrestrial planets. (Earth’s Moon often is considered along with the terrestrial planets because its composition is similar to them and because it is a relatively large satellite.) The terrestrial planets are small, rocky, and dense; have relatively small amounts of gaseous elements; and are composed primarily of metals and silicates. This is in contrast to the Jovian planets, which are large, low in density, and composed primarily of gases. (The Jovian planets usually are designated as the four giants Jupiter, Saturn, Uranus, and Neptune. Pluto, the smallest of all nine planets, has a density higher than any Jovian planet.)

With regard to gravitation, a spherical object behaves as though its mass were concentrated near its center. Indeed, 33% of Earth’s mass is at is core (as opposed to the crust or mantle), even though the core accounts for only about 20% of

Density is simply the ratio of mass to volume, meaning that Earth packs more mass into a given volume than any other body in the solar system. Saturn, least dense among the planets, has a mass 95.16 times as great as that of Earth, yet its volume is 764 times greater, meaning that its density is only about 12% of Earth’s. But whereas Saturn and other Jovian planets are composed primarily of gases surrounding small, dense cores, Earth—beneath its atmospheric layer and it waters—is a hard little ball. Its core, composed of iron, nickel, and traces of other elements, including uranium, is relatively heavy.

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tinctions other than its ability to support life: it is neither the largest nor the smallest planet in the solar system, positions held by Jupiter and Pluto, respectively. (Earth ranks fifth.) Earth has a moon, but that is hardly a distinction: Saturn has 18 moons. And not only does Olympus Mons tower over Everest, but the gaseous oceans of Jupiter also are much deeper than the Mariana Trench. In the lists of planetary superlatives, Earth has only one: it is the most dense.

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That gives it a strong gravitational pull and, in combination with the comparatively high speed of the planet’s rotation, causes Earth to have a powerful magnetic field. It is also important to note the significance of planetary mass in making possible the formation of an atmosphere. Because of their mass, larger planetary bodies exert enough gravitational pull to retain gases around their surfaces; by contrast, the Moon and Mercury are too small and have no atmosphere. Of course, Earth is the only planet whose atmosphere is capable of sustaining life as we know it, and this is a result of activity beneath the planet’s surface.

Planetary Science

A V O LAT I L E P LA N E T. Earth is the

only terrestrial planet on which the processes of plate tectonics, or the shifting of plates beneath the planetary surface, take place. The other terrestrial planets have crusts of fairly uniform thickness, suggesting that they have never experienced the internal shifting that has helped give our planet its unique topography. Earth also has a relatively thin lithosphere—the upper layer of the planetary surface, including the crust and the brittle portion at the top of the mantle—which helps make it a particularly volatile body. Of the terrestrial planets, the only ones still given to volcanic activity are Earth and Venus. Mars seems to have experienced volcanic activity at some point in the past billion years, while Mercury and the Moon have not had volcanoes for several billion years. This is also an important factor in determining Earth’s capacity to support living things, because volcanoes—which transport gases from the planet’s interior to its atmosphere—have been crucial to the creation of the conditions necessary for sustaining life. The heat generated by internal volatility is also a component influencing the sustainability of life on Earth. At the time Earth and other planets were formed, some 4.5 billion years ago, the planets experienced such heat that they melted, causing a separation of chemical compounds. The heavier compounds, mostly containing iron, sank to the core of the planet, where they remain today, while the lighter ones rose to the surface. Included in these lighter substances were oxygen and other elements essential to the sustenance of life. Even now Earth and Venus, because of their volcanic activity, are cooling at rates slower than

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THE SILHOUETTE OF A STARGAZER AGAINST THE MILKY WAY. (© F. Zullo/Photo Researchers. Reproduced by permission.)

those of the other planets, and this has facilitated the separation of elements. T H E C R E AT I O N A N D S U S T E N A N C E O F L I F E . Aside from the distinc-

tive features of its core, Earth’s position relative to the Sun has helped make it possible for life to take root on this planet. For decades scientists believed that Earth is unique in possessing that life-sustaining compound of hydrogen and oxygen, H2O or water; but now we know that even Jupiter—not to mention Venus and Mars—have water on their surfaces. The problem is that Venus’s water is too hot, existing as vapor in the upper atmosphere, while the water on Mars and Jupiter takes the form of ice crystals. Earth is uniquely placed to sustain liquid water. The existence of liquid water made it possible for the first microorganisms to form on Earth, leading over hundreds of millions of years to the development of the complex biosphere known today. The existence of life in simple forms promoted the development of the atmosphere and geosphere, because these life-forms took in carbon dioxide and water, processed them, and returned them to the environment as oxygen and organic materials.

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Planetary Science

COMPUTER

IMAGE OF THE NINE PLANETS OF OUR SOLAR SYSTEM. (© Photo Researchers. Reproduced by permission.)

The Solar System and Beyond The reader may have noticed that earlier in this essay, we ceased discussing progress in cosmology after about 1650. This is not because nothing happened after that time; on the contrary, the centuries that have elapsed since then have seen the greatest progress in astronomical study since the dawn of civilization. To give this topic the coverage it warrants, however, would require a lengthy discussion—one that would take us away from the earth sciences and toward the sister science of astronomy.

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concerning Earth and made sometimes unscientific speculations regarding its origin and composition. As the oldest of the physical sciences, astronomy was much more mature, but even it could progress only so far under the restrictions of the Ptolemaic system. Unfettered, it began to progress rapidly, and the result has been an unfolding vision of the universe that is at once more clear and more complex. THE SIZE OF THE UNIVERSE.

Up until the Scientific Revolution, the earth sciences hardly existed, except inasmuch as various people over the millennia had recorded data

One of the dominant themes in astronomy from Galileo’s time to the present day is astronomers’ quite literally expanding vision of the universe. Up until 1781, when the German-born English astronomer William Herschel (1738–1822) discovered Uranus, scientists had known only of the

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Planetary Science

KEY TERMS In general, an atmos-

energy level, can take the form of long-

phere is a blanket of gases surrounding a

wave and short-wave radio; microwaves;

planet. Unless otherwise identified, howev-

infrared, visible, and ultraviolet light; x

er, the term refers to the atmosphere of

rays, and gamma rays.

ATMOSPHERE:

Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%. BIOSPHERE:

A combination of all liv-

ELEMENT:

A substance made up of

only one kind of atom. Unlike compounds, elements cannot be chemically broken into other substances. GEOCENTRIC:

Earth-centered. A branch of the

ing things on Earth—plants, mammals,

GEOCHEMISTRY:

birds, reptiles, amphibians, marine life,

earth sciences, combining aspects of geolo-

insects, viruses, single-cell organisms, and

gy and chemistry, that is concerned with

so on—as well as all formerly living things

the chemical properties and processes of

that have not yet decomposed. Typically,

Earth.

after decomposing, a formerly living organism becomes part of the geosphere. CENTRIFUGAL:

A term describing the

GEOLOGY:

The study of the solid

earth, in particular, its rocks, minerals, fossils, and land formations.

tendency of objects in uniform circular motion to move outward, away from the center of the circle. Though the term centrifugal force often is used, it is inertia, rather than force, that causes the object to

A branch of the earth

sciences that combines aspects of geology and physics. Geophysics addresses the planet’s physical processes as well as its magnetic and electric properties and the

move outward. CENTRIPETAL

GEOPHYSICS:

FORCE:

The force

that causes an object in uniform circular

means by which energy is transmitted through its interior. The upper part of

motion to move inward, toward the center

GEOSPHERE:

of the circle.

Earth’s continental crust, or that portion of

COMPOUND:

A substance made up of

atoms of more than one element chemical-

The study of the origin,

structure, and evolution of the universe. COSMOS:

HELIOCENTRIC: HYDROSPHERE:

Sun-centered. The entirety of

Earth’s water, excluding water vapor in the

The universe.

ELECTROMAGNETIC ENERGY:

and which provides them with most of their food and natural resources.

ly bonded to one another. COSMOLOGY:

the solid earth on which human beings live

A

form of energy with electric and magnetic

atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice. An unproven state-

components that travels in waves and

HYPOTHESIS:

which, depending on the frequency and

ment regarding an observed phenomenon.

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Planetary Science

KEY TERMS The tendency of an object in

motion to remain in motion and of an

planets but also draws on aspects of astronomy, such as cosmology.

object at rest to remain at rest.

PROTON:

INERTIA:

JOVIAN

PLANETS:

The

planets

between Mars (the last terrestrial planet) and Pluto, all of which are large, low in density, and composed primarily of gases. LAW:

A scientific principle that is

shown always to be the case and for which no exceptions are deemed possible. LITHOSPHERE:

The upper layer of

Earth’s interior, including the crust and the brittle portion at the top of the mantle. The layer, approximately

MANTLE:

1,429 mi. (2,300 km) thick, between Earth’s crust and its core. MASS:

A measure of inertia, indicating

the resistance of an object to a change in its motion. (By contrast, weight—which people tend to think of as similar to mass—is a measure of gravitational force, or mass multiplied by the acceleration due to gravity.) PLANETARY SCIENCE:

The branch

of the earth sciences, sometimes known as planetology or planetary studies, that focuses on the study of other planetary bodies. This discipline, or set of disciplines, is concerned with the geologic, geophysical, and geochemical properties of other

A positively charged particle in the nucleus of an atom.

A set of principles and procedures for systematic study that includes observation; the formation of hypotheses, theories, and laws; and continual testing and reexamination.

SCIENTIFIC METHOD:

A period of accelerated scientific discovery that completely reshaped the world. Usually dated from about 1550 to 1700, the Scientific Revolution saw the origination of the scientific method and the introduction of such ideas as the heliocentric (Sun-centered) universe and gravity. SCIENTIFIC REVOLUTION:

The four inner planets of the solar system: Mercury, Venus, Earth, and Mars. These planets are all small, rocky, and dense; have relatively modest amounts of gaseous elements; and are composed primarily of metals and silicates. Compare with Jovian planets. TERRESTRIAL PLANETS:

A general statement derived from a hypothesis that has withstood sufficient testing.

THEORY:

The motion of an object around the center of a circle in such a manner that speed is constant or unchanging.

UNIFORM CIRCULAR MOTION:

In the seventeenth century, astronomers still regarded what we call the solar system as the entire universe, but Herschel was instrumental in

ascertaining that Earth is part of a bright band of stars called the Milky Way. Just as Earth once had been believed to be the center of the “universe,” or solar system, astronomers then came to believe it was at the center of the Milky Way. Only since 1920 has it been known that our solar system is, in fact, somewhere between the center and

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five other planets visible to the naked eye, all of which had been discovered in prehistoric times. (Neptune was discovered in 1846 and Pluto not until 1930.)

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the edge of the vast galaxy. Even the Milky Way, composed of several hundred billion stars and about 120,000 light-years in diameter, is not the entire universe; it is only one of many hundreds of galaxies or “island universes.” As discussed at the beginning of this essay, such a scale is almost too much for the human mind to comprehend, particularly inasmuch as Earth is the only planet known to sustain intelligent life. As the British science-fiction writer Arthur C. Clarke (1917–) has observed, either there are other intelligent life-forms out there in the universe, or there are not—and either possibility is mind-boggling. THE SOLAR

BIG BANG AND THE SYSTEM. Not only has

astronomers’ understanding of the universe expanded, along with their idea of its size; it also appears that the universe itself is expanding. Today the most widely accepted model regarding the formation of the universe is the big bang theory, first put forward by the Belgian astrophysicist Georges Édouard Lemaître (1894–1966) in 1927. According to this theory, an explosion 1020 billion years ago resulted in the rapid creation of all matter in the universe, and that matter is continuing to move outward, expanding the frontiers of the universe. Our own solar system appears to be about five billion years old, meaning that the Sun is a relatively young star. It seems that the future solar system was just one of many great balls of gas, rotating as they moved outward, that were scattered around the universe as a result of the big bang. Just as these balls of gas exploded from the center, the material of the various stars emerged from the center of the ball that became our solar system. F O R M AT I O N O F T H E P L A N ETS. The proto–solar system we have

described here was a great rotating cloud, and though it has long since ceased to be a cloud, it continues to rotate—only now it is in the form of planets turning around a sun at the center. The hottest portion of the cloud, at the center, became the Sun, while cooler portions at the fringes became planets. The Sun itself is composed primarily of hydrogen and helium, the two most plentiful elements in the universe. In the extraordinarily high temperatures on the Sun, atoms of hydrogen (which has one proton in its nucleus) experience nuclear fusion,

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becoming atoms of helium, which has two protons. It appears that continued fusion resulted in the creation of the heavier elements (for instance, nitrogen, carbon, oxygen, and silicon) of which the planets—in particular, our own— are composed.

Planetary Science

Earth’s elemental makeup is discussed elsewhere in this book, as is the structure of its interior. So, too, is the Sun’s effect on Earth. These matters are not unrelated. In studying the solar system and the planets that make it up, one is confronted again and again with the fact that a planet’s destiny is governed by its position relative to the Sun. Ultimately, the planets in our solar system are ruled by the same principle that drives the sale of real estate: location, location, location! This is true not only of the atmosphere and temperature of planets but also of their relative density. It is no mistake that the terrestrial planets are closer to the Sun: their internal composition is as it is because these bodies became the destination of most of the heavier elements that emanated from it. Many of the lighter elements continued to move outward, where they gathered around rocky centers to become the mostly gaseous Jovian planets. WHERE TO LEARN MORE Astronomy and Cosmology (Web site). . Beatty, J. Kelly. The New Solar System. New York: Sky, 1999. Cambridge Cosmology (Web site). . Canup, Robin M., and Kevin Righter. Origin of the Earth and Moon. Tucson: University of Arizona Press, 2000. The Cosmology: Explore the Largest Mystery (Web site). . Gallant, Roy A. Earth’s Place in Space. New York: Benchmark Books, 1999. Lambert, David, and Martin Redfern. The Kingfisher Book of the Universe. New York: Kingfisher, 2001. Llamas Ruiz, Andrés. The Origin of the Universe. Illus. Luis Rizo. New York: Sterling Publishers, 1997. Ned Wright’s Cosmology Tutorial (Web site). . Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science. 2d ed. New York: John Wiley and Sons, 1999.

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SUN, MOON, AND EARTH Sun, Moon, and Earth

CONCEPT Earth is intimately tied to the star around which it revolves, the Sun, and the satellite that revolves around Earth itself, the Moon. Without the Sun, of course, life on Earth simply could not exist, not just because of the need for light but to an even greater degree because of the energy it supplies. For that matter, Earth itself would not exist: our planet appears to have developed from the same cosmic cloud that formed the Sun, and without the Sun to hold it in place with its gravitational pull, Earth would go spinning off into space. For all its influence on human life, however, the Sun has less impact on the tides than the Moon, which is smaller but much closer. Together, these two bodies literally define time in human experience, which has been marked by the movements of the Sun and Moon from a time before civilization began.

HOW IT WORKS Formation of the Sun and Planets Scientists today believe that the universe began between 10 and 20 billion years ago, with an event nicknamed the “big bang,” The galaxies continued (and still continue) to move outward from that center, and among them was our Milky Way. Somewhere between the center and the rim of the Milky Way, a rotating cloud of cosmic gas formed about six billion years ago,

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ments—experienced extraordinary amounts of compression, owing to the density of the clouded gases around it, and underwent nuclear fusion, or the bonding of atomic nuclei. This hot center became the Sun about five billion years ago, but there remained a vast nebula of gas surrounding it. As the fringes of this nebula began to cool, the gases condensed, forming solids around which particles began to accumulate. These were the future planets. T H E P LA N E T S A N D T H E E L E M E N T S . Closest to the Sun, the planets and

other satellites were formed of elements that could condense at high temperatures: iron, silicon, aluminum, calcium, and magnesium. These elements, along with oxygen-containing compounds, constituted the material foundation around which other particles accumulated to form the terrestrial planets: Mercury, Venus, Earth, and Mars. Further from the Sun, where temperatures were lower, gaseous compounds— including methane, ammonia, and even water— could condense. These compounds became the basis around which the five outer planets (the four Jovian planets and Pluto) formed.

The center of the cloud, where the greatest amount of gases gathered, was naturally the densest and most massive portion as well as the hottest. There, hydrogen—the lightest of all ele-

The process of planetary formation involved additional steps and took place over a period of about 500 million years. Some of the factors that played a part in forming Earth are discussed in Planetary Science, but in the present context, let us consider the source of those elements mentioned here. Did they just magically form? Were they always there? In answer to the first question, there is nothing magical about the formation of “new” elements, though it almost seems so. As for the second question, the answer is yes and no.

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Sun, Moon, and Earth

EARTH

WITH THE

MOON

IN ECLIPSE. (© Chris Bjornberg/Photo Researchers. Reproduced by permission.)

The elements themselves were not always there in the universe or on Earth itself; however, the subatomic building blocks that make them up have indeed existed from the beginning of the universe. The basic atomic structure is as follows. There is a nucleus in which one or more protons (positively charged subatomic particles) may reside along with one or more neutrons, which have no charge. Spinning around the nucleus are one or more electrons, or negatively charged subatomic particles.

The number of protons and electrons in an atom is always the same, meaning that the atom has no electric charge. Atoms that have lost or gained electrons (in which case they would acquire a positive or negative charge, respectively) are called ions. Electrons, which move fast and possess very small mass compared with protons and neutrons, are very easy to dislodge from an atom; on the other hand, it takes an extraordinary event to change the number of protons in the nucleus.

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This fact is significant, because it points toward the defining characteristic of an element: the number of protons in the nucleus. Atoms of a particular element always have the same number of protons, called their atomic number. The atomic number of an element can be determined by consulting the periodic table of elements: for instance, iron, with an atomic number of 26, must have 26 protons in its nucleus. If an atom has 25, it is manganese, and if it is has 27, it is cobalt. FORMING

NEW

ELEMENTS.

The number of neutrons in the nucleus may vary for atoms within a given element. Atoms that have the same number of protons (and are thus of the same element) but differ in their number of neutrons are called isotopes. Most isotopes are stable, meaning that their chemical composition will remain as it is; however, some isotopes are radioactive, meaning that they experience the spontaneous emission of particles or energy over a given period of time. Radioactive decay is one of two ways that one element can become another. When a radioactive isotope emits an alpha particle, for example, its nucleus expels a positively charged nucleus consisting of two protons and two neutrons, which is the same thing as a helium atom stripped of its electrons. This obviously changes the number of protons in the nucleus of the isotope and may result in its stabilization. The other means of forming a new element is by nuclear fusion, in which two atomic nuclei fuse or bond. N U C L E A R F U S I O N . Note that the first of these means by which elements are formed is subtractive; in other words, with radioactive decay, a different element is formed by the expulsion of protons. Nuclear fusion, on the other hand, is additive, resulting in the creation of different elements by the addition of protons to an atomic nucleus. Radioactive decay takes place inside Earth (among other places), while nuclear fusion is the source of the Sun’s power.

Nuclear fusion involves the release of huge amounts of energy. On Earth, scientists have been able to bring about uncontrolled nuclear fusion in the form of the so-called hydrogen bomb, which is actually a “fusion bomb.” They have yet to succeed in creating controlled nuclear fusion. If and when they do, it would provide a safe, clean source of almost limitless power and

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probably would constitute the greatest scientific or technological discovery since fire. On the Sun, nuclear fusion has been taking place, and will continue to do so, for a long, long time. The 92 naturally occurring elements of the universe are the result of fusion reactions, meaning that all that we see around us was once part of a star. This represents a major break with the ancient belief that Earth is made of fundamentally different substances than are the bodies of space (see Earth, Science, and Nonscience). In fact, our world and everything in it—including our own bodies—is truly “the stuff of stars.”

The Moon and Earth In comparison to the Sun, the Moon is altogether less remarkable. Below, we review some statistics about the sizes of each, but as every elementary-school student today knows, the Sun is much, much larger and exerts far more impact on the fate of the solar system—including Earth. The Moon does not even have its own energy sources: its light comes from the Sun, and the absence of an atmosphere, of volcanic activity, or even of a significant magnetic field makes it a very dull place indeed. Yet the Moon has inspired at least as much fascination among humans over the ages as has the Sun. There is its physical beauty, though comparisons with the Sun are hardly fair, since we cannot look at the Sun without damage to our eyes. There is also its influence on earthly cycles ranging from the tides to the months themselves, though some claims of lunar influence have little basis in fact. During the Middle Ages, many believed that the Moon caused madness, a superstition still reflected in our word lunacy. Still, humans have long associated a spirit of mystery with the Moon, in part because of its ever-changing appearance and in part because it has always showed just one side to Earth. (We discuss the reason why later.) Only in 1959, when the Soviet space probe Luna traveled to the “dark side of the Moon,” did scientists gain a glimpse of it. Unmanned and later manned journeys, which culminated with the U.S. Moon landing in 1969, also changed astronomers’ understanding of the Moon’s origins. T H E “ B I G W H AC K . ” One of the curious things about the Moon is its size in relation to Earth. Nowhere in the universe is there such a small size differential between a satellite

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and the planet around which it orbits, the only possible exception being Pluto and its moon, Charon. Because our Moon is so close in size to Earth, scientists once speculated that they might have shared origins, and this speculation informed several theories concerning the formation of the Moon. According to the fission theory, the Moon was a piece of Earth that had been torn away, perhaps from the Pacific basin. The simultaneous creation theory likewise depicted Earth and the Moon as sharing origins, but in this case they literally had been formed together from the same materials. Finally, there was the capture theory, which, in contrast to the others, assumed quite different origins for the two bodies: the Moon had formed somewhere else in the solar system and had been captured by Earth’s gravitational field after it wandered too close to the planet. As it turned out, the capture theory was closest to the theory accepted today, though it was discarded along with the other two on the basis of data brought back from the Apollo Moon landings. According to the giant impact theory, sometimes called the Big Whack model or the ring ejection theory, at a young age Earth was sideswiped by a celestial object as large or larger than Mars. As a result of that collision, a ring of crustal matter was spewed into space, and over time the matter in this ring agglomerated to form the Moon.

Vital Statistics of the Sun and Moon The Moon is about 240,000 mi. (385,000 km) from Earth, meaning that it takes about 1.25 seconds for its light to reach Earth. By contrast, the Sun’s distance from Earth is so great that it takes eight minutes for sunlight to reach our planet, even though light travels through space at the speed of about 186,000 mi. (299,339 km) per second. The distance between Earth and the Sun is the basis for the astronomical unit (AU), a figure used for measuring the distance between bodies in the solar system. Equal to the average distance from Earth’s center to the center of the Sun, an AU is designated 1.49597870691  108 km, or approximately 92,955,807 mi. Usually we think of the solar system as the area encompassed by the orbit of the most remote planet, Pluto, but that is only 39.44 AU, a tiny figure compared with

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the diameter of the realm within the Sun’s gravitational pull, which is a staggering 100,000 AU.

Sun, Moon, and Earth

V O LU M E A N D M ASS . The Sun itself has a diameter of about 856,000 mi. (1,392,000 km), meaning that the distance across it is about 109 times that of Earth’s diameter. Another way to consider that figure is this: if one were to draw a circle as big as the Sun around Earth, the edge of that circle would be about twice as far away as the Moon. The Sun’s volume is so great that about 1.3 million Earths could fit inside it, and its mass is about 300,000 times that of Earth. In fact, it accounts for about 99.8% of the mass of the entire solar system.

By contrast, the Moon has a diameter of only about 2,160 mi. (3,475 km), a little less than the distance from New York to Los Angeles. Its mass is a little more than 1% of Earth’s, and as a result, its gravitational pull is too small to retain the gases that make up the atmosphere. That small mass, combined with an imbalance in its distribution, explains why the Moon shows only one face to Earth. The side of the Moon facing Earth is of greater mass than the other side and is therefore more strongly attracted by Earth’s gravitational force. The result is a phenomenon called gravitational locking, whereby the Moon rotates on its axis at exactly the same rate as it travels around Earth—once ever 29.5 days. Clearly, the Moon is tiny in both volume and mass, and were it as far away as the Sun, we would hardly pay it notice. Yet it should be pointed that even the Sun itself, while remarkable in our own solar system, is far from a standout in the universe as a whole. It is a youngish star, of average size, and not all that different from billions of other stars throughout the cosmos. It is not even unique in being the only star with its own solar system. In 1999 astronomers discovered an entire solar system some 44 light-years from Earth, in which three large planets were found to be circling the star Upsilon Andromedae.

REAL-LIFE A P P L I C AT I O N S Energy, Temperature, and Composition of the Moon Of course, the area in which the Sun and Moon differ most significantly is in terms of the energy they possess, which, in turn, affects both their

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potassium, and phosphorus have been found on the Moon, but it seems bereft of organic compounds, indicating that life never existed there. Moon rocks are primarily of basalt, or hardened lava, and breccia, soil, and rock fragments that have melted together.

Sun, Moon, and Earth

The formation of these rocks must have occurred a long, long time ago, because the Moon has not had volcanic activity for several billion years. Thus, it is almost entirely “dead,” lacking even a magnetic field of any significance. Moon rocks have only the faintest magnetic field, suggesting the absence of a significant molten core, which is what gives Earth its own magnetism. From the traces of magnetism found in these rocks, it is possible that the Moon had a magnetic field of some significance at one time, but that time is long past.

THE MOON’S

SURFACE IS POCKMARKED WITH CRATERS,

SHOWING ITS VULNERABILITY TO DAMAGE CAUSED BY PARTICLES FROM SPACE. (© NASA/Photo Researchers. Repro-

duced by permission.)

temperature and the composition of each body. With regard to the Moon’s temperature, little need be said, since it is entirely a function of the Moon’s exposure (or lack of exposure) to heat from the Sun. On the Moon figures range from 280°F (138°C) to –148°F (–100°C), with a mean temperature of –10°F (–23.33°C).

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T H E V U L N E RA B L E M O O N . Owing to the Moon’s lack of sufficient gravitational force, it retains no atmosphere, and this means that it is completely vulnerable to particles reaching it from space. Early in its life, the Moon was subjected to a 700-million-year-long meteor shower that formed its craters. The damage from these showers was so great that it melted the Moon’s crust, and eventually lava from the lunar interior surfaced to fill in cracks made by the meteorites.

In 1609 the Italian astronomer Galileo Galilei (1564–1642), the first scientist to gaze at the Moon through a telescope, observed dark patches that looked to him like bodies of water. He named these patches seas, a term that has remained in use among lunar cartographers, though Galileo’s “seas” long ago were identified as dark spots made by cooling lava in the cracked surface.

As for the Moon’s composition, it has similarities to and differences from that of Earth, and both these similarities and differences are instructive. The internal composition of the Moon is such that it often is treated along with the terrestrial planets. In addition, it is not much smaller than the smallest terrestrial planet, Mercury. But whereas Mercury has a proportionally large, extremely hot core, the Moon’s core is much smaller in proportion to its total size and much cooler. In all likelihood it is not even molten, or only a portion of it is.

Humans on the Moon

It appears that the Moon has an internal structure not dissimilar to that of Earth—that is, a crust, mantle, and core. The materials that make up the Moon, however, are quite different. Aluminum, calcium, iron, magnesium, titanium,

One such cold spot is at the Moon’s south pole, in a basin carved out by an asteroid. There, temperatures are as low as –387°F (–233°C), within 40°C of absolute zero, or the temperature at which all molecular motion virtually ceases. By

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As it is vulnerable to particles from space, the Moon also is susceptible to dramatic temperature changes brought about by the presence or absence of sunlight. For this reason, parts of the lunar surface are extremely hot at certain times, while other portions have never been penetrated by sunlight and are almost inconceivably cold.

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contrast, Pluto, which is so far from the Sun that the great star appears merely as a bright dot from there, has an average surface temperature that is warmer by 8°C. Yet in this inhospitable spot on the lunar surface, the U.S. probe Lunar Prospector in 1998 found something quite surprising: water. Mixed in with dirt in the South Pole–Aitken Basin are ice crystals, which scientists speculate make up about 10% of the material in the surrounding area. Apparently moisture residue from comets that struck the Moon over the past three billion years, the ice offers intriguing possibilities. If a cost-effective method for extracting ice from the soil can be developed, colonization and exploration of the Moon may become a reality. M O O N LA N D I N G S . For now, however, human presence on the Moon is more a thing of the past than of the future. The first man-made spacecraft to visit the Moon was the Soviet Luna 2 in 1959, and seven years later, Luna 9 became the first such craft to land on the lunar surface. Its landing dispelled long-held fears that the Moon was awash in thick layers of dust; in fact, the lunar surface is a grayish soil, primarily composed of rock fragments, from 5 ft. to 20 ft. (1.5 m to 6 m) deep.

Though the Soviets were the first to reach the Moon in unmanned craft, the United States was the first (and so far the only) nation to put a man on the Moon. On July 20, 1969, in one of the most dramatic events of human history, the U.S. astronauts Neil Armstrong (1930–) and Edwin (“Buzz”) Aldrin (1930–) walked on the lunar surface while Michael Collins (1930–) piloted Apollo 11 in its orbit around the Moon. Photographs from the Moon landing show the American flag extended, as though waving in a breeze as it would on Earth, but it actually was held in place mechanically, since there is no wind on the Moon. Aldrin and Armstrong demonstrated one of the most dramatic differences between the Moon and Earth: a decrease in weight. A man who weighs 200 lb. (90.72 kg) on Earth would weigh 33.04 lb. on the Moon and would therefore be much easier to lift. But he would be no easier to push from side to side, because his mass would not have changed.

universe. Thus, although 33.04 lb. is equal to 14.99 kg on Earth, the hypothetical astronaut described here would still have a mass of 90.72 kg.

Sun, Moon, and Earth

Physicists had long known these facts about weight and mass, but footage of Armstrong and Aldrin bouncing around on the lunar surface provided a much more vivid demonstration. As it turned out, however, their foray to the Moon was the beginning of an all-too-brief chapter. Over the next three years, the United States conducted five more lunar landings as well as the failed 1970 mission designated Apollo 13 (portrayed in the 1995 movie of that name), which never landed due to an onboard explosion. The last Moon mission took place in 1972, and soon afterward America was thrown into a recession spawned by the 1973 oil crisis. The next sustained effort at space flight, which began with the launch of the space shuttle in 1981, had an entirely different mission and destination—one that lay well within Earth’s gravitational field.

Solar Energy, Temperature, and Composition Needless to say, there never will be any space missions, manned or otherwise, to the Sun. Spacecraft have passed close to Mercury, but as for the Sun, even its corona, or outermost atmospheric layer, has a temperature of 2,000,000 on the Kelvin scale of absolute temperature, or 3,599,541°F. The Sun could not be more different from the Moon, most notably in terms of energy and temperature. It should be noted, however, that the Sun’s temperature does not increase uniformly from the corona to the core. The “coolest part,” at 5,800K (9,981°F), is the photosphere, a layer some 300 mi. (480 km) thick that constitutes the visible surface of the Sun. Above it, temperatures rise through the chromosphere, which is about 1,600 mi. (2,560 km) thick, becoming hotter still in the corona. Scientists do not understand the reasons for this temperature rise at the outer surface of the Sun.

Weight is the product of mass multiplied by the rate of gravitational acceleration and is therefore dependent on the gravitational force of the celestial body on which it is measured. Mass, on the other hand, does not vary anywhere in the

Less surprising is the continued increase of temperature beneath the photosphere. The deeper inside the Sun, the higher the temperatures, until it reaches a staggering 15,000,000K (27,000,000°F) at the core. It is so hot there that not even atoms can exist; instead, the Sun’s core is made up of subatomic particles—specifically, protons and electrons. Heat and movement are

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directly related in physics, and these particles are moving very fast—so fast that nuclear fusion can and does occur when these particles smash together. NUCLEAR FUSION IN THE S O LA R C O R E . When four protons fuse

and absorb two electrons, amazing things happen. The result is the creation of a helium nucleus, whose mass is slightly smaller than the combined mass of the separate particles. Where did that “missing” mass go? It was converted to energy—an amount of energy that is staggering when multiplied by the large numbers of hydrogen atoms being converted to helium at any given moment. That, in essence, is how the Sun creates energy—by converting hydrogen to helium. This energy is radiating from the Sun in electromagnetic waves at an amazing rate every second, and eventually it will be used up. Long before that happens, however, the Sun will begin to expand and cool. “Cooling,” of course, is a relative term; this expanding Sun, a red giant, will burn up Earth even as it absorbs Mercury entirely. Then, when it has used up all its hydrogen, nothing will remain but a glowing core called a white dwarf. There is no need to worry, however, because the events described will not happen anytime soon. The Sun has about as much life left in it as the amount of time it has lasted so far—approximately five billion years, or longer than Earth has existed. Given the fact that about 4.96 million tons (4.5  106 metric tons) of hydrogen are converted to helium every second, this gives some idea of the vast energy reserves on the Sun. THE E A RT H .

SUN’S

ENERGY

AND

Sunlight is more than just light, though that alone is a marvelous thing. The rays projected by the Sun are electromagnetic energy, of which visible light is only a small part. Also contained in the electromagnetic spectrum are long waves, short waves, and microwaves (including those used for transmitting radio and television signals); infrared and ultraviolet light; and x rays and gamma rays. Solar energy travels to Earth by means of radiation, a form of heat transfer that, unlike conduction or convection, requires no physical medium such as air. It can move through the vacuum of space to Earth’s atmosphere, where a portion of it is absorbed and becomes the fuel that powers spaceship Earth.

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It is a measure of the Sun’s vast energy that Earth receives only 0.00000005% of its total output at any given moment. Of that small fraction (equal to one part in two billion), a much smaller portion makes it through Earth’s atmosphere—yet that is enough to light the world and to facilitate the myriad other functions, such as photosynthesis, for which we depend on the Sun. Many effects of the Sun’s light and energy are less than desirable, of course: sunburns and sometimes even tans, bleaching of materials exposed to light for too long, temporary blindness caused by gazing at the Sun or even at something reflecting it, and so on. These effects, too, attest to the Sun’s awesome power. The Sun makes possible the operations of three of four Earth systems and indirectly affects the fourth (see Earth Systems). The hydrosphere and atmosphere are both affected, for instance, by the evaporation of water for eventual precipitation, a process powered by the Sun. Likewise, the biosphere could not exist without photosynthesis and the other biological processes dependent on sunlight. Even the geosphere, though not directly powered by the Sun, is influenced by Sun-powered phenomena from the other three spheres.

Curious Solar Phenomena Even traveling at the speed of light, a photon takes nearly 30,000 years to travel from the center of the Sun to the corona. If it were in a vacuum, it could make the journey in about four seconds, but it continually bumps into other particles, and this slows it down (to put it mildly). Once it finally escapes the solar surface, it travels rather quickly, transmitting the Sun’s light to the solar system. When that light reaches Earth’s atmosphere, it creates a number of strange optical effects. One of the best known is the rainbow, produced by the refraction, or bending, of light inside a raindrop. Because the colors of the visible spectrum are bent at different angles, they disperse as though in a prism; after being refracted a second time by the surrounding air, the billions of raindrops that fill the atmosphere after a storm produce a brilliant band of light. When sunlight strikes oxygen or nitrogen, the two most significant elements in our atmosphere, the shorter wavelengths of light—green, blue, and violet—are the ones reflected. When

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THE AURORA BOREALIS, OR NORTHERN LIGHTS. THIS PHENOMENON IS CAUSED BY SOLAR PARTICLES DRAWN TO EARTH’S ATMOSPHERE, WHERE THEY COLLIDE WITH OXYGEN AND NITROGEN MOLECULES AND BECOME ELECTRICALLY CHARGED, TAKING ON A COLORED GLOW. (© Michael Giannechini/Photo Researchers. Reproduced by permission.)

these wavelenths combine, they appear bluish. The combination of light and differing temperatures on the ground produces a mirage, while ice crystals in the air may bring about haloes around

the Sun or Moon and, in some cases, a sun dog, a reflected image of the Sun.

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T H E N O RT H E R N L I G H T S . One

of the most breathtaking displays of atmospheric

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optical phenomena are auroras (or aurorae), natural fireworks that appear in the night sky. Visible in the northern United States and Canada, the auroras there are known as aurora borealis, or northern lights. A similar phenomenon occurs in the Southern Hemisphere, where it is called the southern lights, or aurora australis. The source of the auroras is a stream of particles, called the solar wind, that emanates from the Sun. The solar wind is a consequence of the extreme heat on the Sun’s surface, which causes atoms there to move so rapidly that even the enormous gravitational force of the Sun itself cannot hold them. Solar particles pass through the entire solar system and out to space beyond, but few of them reach Earth’s atmosphere, because they are deflected by the planet’s magnetic field. The particles that cause auroras are electrons that instead of being deflected, are drawn toward Earth’s polar regions. Two great rings of charged particles, called the Van Allen belts, are located high above each polar region, and they catch the majority of charged particles flowing toward them from the northern and southern hemispheres, respectively. The Van Allen belts cannot trap all the charged particles, however, and some enter the atmosphere, where they collide with oxygen and nitrogen molecules at about 50 mi. to 600 mi. (80 km to 1,000 km) above sea level. Some of these molecules become electrically charged and take on a colored glow. At higher levels oxygen glows red, while oxygen at lower levels is yellowish-green. Nitrogen glows blue. S U N S P O T S A N D T H E S O LA R AC T I V I T Y CYC L E . Though auroras are

visually remarkable, a more significant solar phenomenon is the sunspot. Sunspots are cooler regions—many of them vast in size—on the photosphere. They tend to be about 2,700°F (1,500°C) cooler than the surrounding areas, and for this reason they appear darker. Sunspots seem to occur in 11-year cycles— a period known as a solar activity cycle—and are the result of strong magnetic fields. The latter, in turn, result from something called differential rotation: the Sun’s equator rotates once every 26 days, whereas its poles rotate every 36 days, resulting in massive twisting of magnetic fields. This can produce anomalies such as

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sunspots, which disrupt radio communications on Earth. Other solar anomalies, such as prominences, or hot spots formed by magnetic loops in the Sun’s atmosphere, also follow the 11-year solar activity cycle. Actually, the cycle as it is known and measured today is about 11 years long. Its length seems to have varied over time, producing past changes in global temperature: the shorter the cycle, the warmer the temperatures on Earth.

The Sun, Moon, and Events on Earth As noted earlier, the Sun and Moon have long inspired awe in humans, and in this the Moon has been a more than equal partner. Prehistoric and ancient humans regarded both lunar eclipses, in which Earth’s shadow covers the Moon, and solar eclipses, in which the Moon comes between Earth and the Sun, as portents from the gods. From at least the era of the Babylonians onward, the Sun and Moon played complementary roles in marking time. The marking of time according to the Sun relies on an objective reality, rather than a mere human construct. The year, which involves a complete cycle of seasons, is based on the amount of time Earth takes to revolve around the Sun. During this time, the planet also moves on its axis, causing the changes in orientation that bring about the seasons. Likewise a day is an objective reality, being the amount of time it takes Earth to revolve on its axis. The month and week, on the other hand, relate to the phases of the Moon, which are themselves dependent on the Moon’s position relative to Earth and the Sun. A lunar cycle lasts about 29.5 days, and this became the basis for the month, which lasts from 28 to 31 days. Within that lunar cycle are four phases—new, first quarter, full, and last quarter—which eventually became the basis for the idea that a month has about four seven-day weeks. DAY S O F T H E W E E K . Until about 1500, people thought of the Sun and Moon as two of the seven “planets” (including the five planets visible with the naked eye) that supposedly revolved around Earth. They further related these “planets” to the days of the week. As a result, virtually every culture that speaks a Euro-

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KEY TERMS The temperature

insects, viruses, single-cell organisms, and

at which all molecular motion virtually

so on—as well as all formerly living things

ceases.

that have not yet decomposed. Typically,

ABSOLUTE ZERO:

A fig-

ASTRONOMICAL UNIT (AU):

ure equal to the average distance from

after decomposing, a formerly living organism becomes part of the geosphere. The center of Earth, which

Earth’s center to the center of the Sun. The

CORE:

SI figure for an AU, adopted in 1996, is

appears to be of molten iron. For terrestri-

equal to 1.49597870691 

km, or

108

approximately 92,955,807 mi. ATMOSPHERE:

al planets in general, core refers to the center, which in most cases is probably molten

In general, an atmos-

metal of some kind. The study of the origin,

phere is a blanket of gases surrounding a

COSMOLOGY:

planet. Unless otherwise identified, howev-

structure, and evolution of the universe.

er, the term refers to the atmosphere of

COSMOS:

The universe.

Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%. ATOM:

The smallest particle of an ele-

ment, consisting of protons, neutrons, and electrons. An atom can exist either alone or in combination with other atoms in a molecule.

ELECTROMAGNETIC ENERGY:

A

form of energy with electric and magnetic components, which travels in waves. ELECTROMAGNETIC

SPECTRUM:

The complete range of electromagnetic waves on a continuous distribution from a very low range of frequencies and energy levels, with a correspondingly long wavelength, to a very high range of frequencies and energy levels, with a correspondingly

ATOMIC MASS UNIT:

An SI unit

(abbreviated amu), equal to 1.66 

10–24

g,

for measuring the mass of atoms. ATOMIC NUMBER:

short wavelength. Included on the electromagnetic spectrum are long-wave and short-wave radio; microwaves; infrared,

The number of

visible, and ultraviolet light; x rays, and

protons in the nucleus of an atom. Since

gamma rays.

this number is different for each element,

ELECTRON:

elements are listed on the periodic table in

ticle in an atom, which spins around the

order of atomic number.

nucleus.

AVERAGE ATOMIC MASS:

A figure

ELEMENT:

A negatively charged par-

A substance made up of

used by chemists to specify the mass—in

only one kind of atom. Unlike compounds,

atomic mass units—of the average atom in

elements cannot be broken chemically into

a large sample.

other substances.

BIOSPHERE:

A combination of all liv-

GEOSPHERE:

The upper part of

ing things on Earth—plants, mammals,

Earth’s continental crust, or that portion of

birds, reptiles, amphibians, aquatic life,

the solid earth on which human beings live

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KEY TERMS and which provides them with most of their food and natural resources.

scale; hence Celsius temperatures can be converted to Kelvin by adding 273.15.

HYDROSPHERE: The entirety of Earth’s water, excluding water vapor in the atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice.

LIGHT-YEAR: A unit of distance used by astronomers for measuring the extremely large expanses of space. Equal to the distance light travels in a year, a lightyear is 9.460528405  1012 km, or approximately 5.88 trillion mi.

INERTIA: The tendency of an object in motion to remain in motion and of an object at rest to remain at rest.

The upper layer of Earth’s interior, including the crust and the brittle portion at the top of the mantle.

LITHOSPHERE:

An atom that has lost or gained one or more electrons and thus has a net electric charge.

MANTLE:

Atoms that have an equal number of protons, and hence are of the same element, but differ in their number of neutrons. This results in a difference of mass. An isotope may be either stable or radioactive.

MASS:

ION:

ISOTOPES:

PLANETS: The planets between Mars (the last terrestrial planet) and Pluto, all of which are large, low in density, and composed primarily of gases.

JOVIAN

SCALE: Established by William Thomson, Baron Kelvin (1824– 1907), the Kelvin scale measures temperature in relation to absolute zero, or 0K. (Note that units in the Kelvin system, known as Kelvins, do not include the word or symbol for “degree.”) The Kelvin scale, which is the system usually favored by scientists, is directly related to the Celsius

KELVIN

pean language—not only in Europe itself but also among former European colonies in the Americas, Africa, Asia, and the Pacific—refers to the first day of the week as the Sun’s day and the second as the Moon’s day.

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CONTINUED

The layer, approximately 1,429 mi. (2,300 km) thick, between Earth’s crust and its core. In reference to the other terrestrial planets, mantle simply means the area of dense rock between the crust and core. A measure of inertia, indicating the resistance of an object to a change in its motion. (By contrast, weight—which people tend to think of as analogous to mass— is a measure of gravitational force, or mass multiplied by the acceleration due to gravity.) A subatomic particle that has no electric charge. Neutrons are found at the nucleus of an atom, alongside protons.

NEUTRON:

A nuclear reaction that involves the joining of atomic nuclei. NUCLEAR FUSION:

Today, in countries that speak Romance languages, or languages derived from Latin (most

notably Italian, French, Spanish, and Portuguese), variations on the original Latin names remain in use: Domingo and Lunes in Spanish, for instance, or Dimanche and Lundi in French. Germanic languages adopted the ideas of the Latin day names, for the most part, but translated them into their own tongues: hence, Sonntag

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Sun, Moon, and Earth

KEY TERMS The center of an atom, a region where protons and neutrons are located and around which electrons spin.

NUCLEUS:

At one time, chemists used the term “organic” only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of calcium carbonate (limestone) and oxides, such as carbon dioxide.

ORGANIC:

PERIODIC TABLE OF ELEMENTS:

CONTINUED

water or air) for the transfer. Earth receives the Sun’s energy, via the electromagnetic spectrum, by means of radiation. A term describing a

RADIOACTIVITY:

phenomenon whereby certain materials are subject to a form of decay brought about by the emission of high-energy particles or radiation. Forms of particles or energy include alpha particles (positively charged helium nuclei); beta particles (either electrons or

A chart that shows the elements arranged in order of atomic number, along with chemical symbol and the average atomic mass for that particular element.

subatomic particles called positrons); or

A particle of electromagnetic radiation carrying a specific amount of energy.

it passes at an angle from one transparent

The branch of the earth sciences, sometimes known as planetology or planetary studies, that focuses on the study of other planetary bodies. This discipline, or set of disciplines, is concerned with the geologic, geophysical, and geochemical properties of other planets but also draws on aspects of astronomy, such as cosmology.

objects under water appear to have a differ-

PHOTON:

PLANETARY SCIENCE:

PROTON:

A positively charged particle

gamma rays, which occupy the highest energy level in the electromagnetic spectrum. REFRACTION:

The bending of light as

material into a second transparent material. Refraction accounts for the fact that ent size and location than they have in air. SI:

An abbreviation of the French term

Système International d’Unités, or “International System of Units.” Based on the metric system, SI is the system of measurement units in use by scientists worldwide. TERRESTRIAL PLANETS:

The four

inner planets of the solar system: Mercury, Venus, Earth, and Mars. They are all small,

in an atom.

rocky, dense, have relatively small amounts

The transfer of energy by means of electromagnetic waves, which require no physical medium (for example,

of gaseous elements, and are composed

RADIATION:

and Montag in German, or Sunday and Monday in English. CYC L E S A N D T I D E S . One reason the ancients respected the Moon as much as the Sun is that it seemed to affect many aspects of human life—as indeed it does. Medieval ideas about “lunacy” were themselves more than a lit-

S C I E N C E O F E V E RY DAY T H I N G S

primarily of metals and silicates. Compare with Jovian planets.

tle off-kilter, and the belief that the Moon affects female menstrual cycles has come under significant challenge. Nevertheless, the Moon does seem to have some effect on human biological cycles. The human circadian rhythm, or cycle of sleep and wakefulness, stretches over a period of

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25 hours, meaning that a person living in a cave without any exposure to sunlight would eventually assume a 25-hour-a-day schedule. The reason for this disparity between the human circadian cycle and the length of a solar day is not known, but it is possible that the circadian cycle is based on the length of a lunar day, or the interval between periods of time when the Moon appears in the sky over a given spot on Earth. Because the Moon is moving even as Earth is rotating, this span of time is not 24 hours, the interval it takes Earth to rotate on its axis, but 24 hours and 50 minutes. It is also more than an old wives’ tale that people behave strangely around the time of a full moon: in fact, more deaths and accidents do occur at that point in the monthly cycle. One of the most significant lunar-influenced cycles, however, is not monthly but semidiurnal, or twice daily. These are the cycles of Earth’s tides, brought about by the gravitational attraction exerted on Earth by the nearest significant celestial object. (The Sun also affects tides, but much less so because of its greater distance from Earth.) Though the Moon’s effect on tides has been known since ancient times, the ancients did not understand the gravitational nature of its attraction, which actually causes a bulge to appear in the oceans. In fact, two bulges appear, one in the oceans on the side of Earth nearest the Moon and one on the opposite side. The latter is also a result of the Moon’s gravitation, which pulls Earth in the other direction, thus drawing the solid earth away from the water. These bulges are known as high tide, and the resulting displacement of water causes a low tide. In most places on Earth, tides are semidiurnal, meaning that there are two full cycles of high and low tide per day. Sometimes these variations can be very great, as at the Bay of Fundy in Canada, where the tidal range is as large as 46 ft. (14 m).

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At the new moon, the Moon is between Earth and the Sun, and during a full moon, the Moon is on the other side of Earth from the Sun. (Because of a 20° angle between Earth’s orbit and that of the Moon, Earth rarely casts a shadow on a full moon, except in the case of a lunar eclipse.) In the new-moon and full-moon phases, the Moon and Sun are aligned, and this combination of lunar and solar gravitation produces strong spring tides every 14 days. On the other hand, when the Moon and Sun are at angles to each other—during the first quarter and last quarter, sometimes known as half-moons—it produces a much weaker neap tide. WHERE TO LEARN MORE Beatty, J. Kelly, Carolyn Collins Petersen, and Andrew Chaikin. The New Solar System. 4th ed. New York: Sky Publishing, 1999. Earth and Moon Viewer (Web site). . Lewis, John S. Physics and Chemistry of the Solar System. San Diego: Academic Press, 1997. Lodders, Katharina, and Bruce Fegley. The Planetary Scientist’s Companion. New York: Oxford University Press, 1998. The Moon (Web site). . Moon Phases (Web site). Moore, Patrick. The Data Book of Astronomy. Philadelphia: Institute of Physics, 2000. Pérez, Miguel. The Earth and the Universe: How the Sun, Moon, and Stars Cause Changes on Earth. Illus. María Rius. Happage, NY: Barron’s Educational Series, 1998. Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science. 2d ed. New York: John Wiley and Sons, 1999. The Solar and Heliospheric Observatory (Web site). . The Sun (Web site). .

The tides are related closely to phases of the Moon, or, to put it another way, they are affected by the alignment between the Moon and the Sun.

The Sun (Web site). .

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S C I E N C E O F E V E RY DAY T H I N G S Real-Life Earth Science

GEOLOGY HISTORICAL GEOLOGY HISTORICAL GEOLOGY GEOLOGIC TIME STRATIGRAPHY PALEONTOLOGY

PHYSICAL GEOLOGY MINERALS ROCKS ECONOMIC GEOLOGY

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HISTORICAL GEOLOGY

CONCEPT Geologists are concerned primarily with two subjects: Earth’s physical features and the study of the planet’s history. These two principal branches of geology are known, appropriately enough, as physical geology and historical geology. Today they are of equal importance, but in the early modern era, geologists were most focused on topics related to historical geology, in particular, Earth’s age and the means by which Earth was formed. This debate pitted adherents of religion, which seemed to require a very young Earth, against adherents of science. A breakthrough came with the introduction of uniformitarianism, a still-influential principle based on the idea that the geologic processes at work today have always been at work. Opposing uniformitarianism was catastrophism, or the idea that Earth was formed in a short time by a series of cataclysmic events. Discredited at the time, catastrophism later gained acceptance, though this did not lead to support for the concept of a young Earth. In fact, the planet is very old—so old that all of human history is almost inconceivably short in comparison.

the challenge to mythological explanations put forward by earth scientists in modern times, are examined here. All religious explanations of the planet’s origins can be called myths, which is not necessarily a pejorative term: a myth is simply a story to explain how something came into being. So pervasive are myths about geology that a term, geomythology, has been coined to identify such myths. Geomythology in particular and mythology in general stand in sharp contrast to scientific explanations derived by using the scientific method of observation, hypothesis formation, testing of hypotheses, and the development and testing of theories. C O N T RAS T I N G S C I E N C E A N D R E L I G I O U S G E O M Y T H O LO GY. Sci-

ence and myth have in common the aim of explaining how things came to be, but the means by which they reach that explanation are quite different. So, too, are the reasons that drive science on the one hand and religion or myth on the other in seeking to develop such an explanation.

For thousands of years, humans were content to rely on religiously inspired stories, rather than scientific research, to provide an explanation regarding Earth’s origins. This topic, along with the scientific challenge to those early accounts of Earth’s formation, is discussed in considerable detail within the essay Earth, Science, and Nonscience. Other aspects of the subject, particularly

The most famous of all religious explanations of Earth’s origins, of course, is that found in the biblical book of Genesis. Probably written in the latter part of the second millennium B.C., it offered a compelling story of creation that virtually defined the Western view of Earth’s origins for more than 1,500 years, from about A.D. 300 to the beginning of the nineteenth century. Its purpose, of course, was not scientific, and it was not written as the result of research; rather, the Genesis account depicts nature as a vast stage on which a cosmic drama of love, sin, redemption, and salvation has been played out over the ages.

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HOW IT WORKS Explaining Origins

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By contrast, the scientific search for Earth’s origins is driven merely, or at least primarily, by curiosity to explain how things came to be as they are. Scientists certainly have their biases and are just as capable of error as anyone, yet at least they have a standard in the form of the scientific method. If a scientist’s findings, and the resulting theory, withstand the rigorous testing required by the scientific method, the theory is rewarded with increasing acceptance and new research designed to test further its ability to explain the world. If the theory fails those tests, its adherents may hold on to it for a time, but eventually they die off, and the theory is discarded. On the other hand, adherence to the religious explanation of Earth’s origins has proved more intractable, as we shall see.

The Religious War Concerning Earth’s Origins The great Italian artist and scientist Leonardo da Vinci (1452–1519) was among the first Western thinkers to speculate that fossils might have been made by the remains of long-dead animals. This was a daring supposition to make in the Renaissance, and it would become even more daring to uphold such an idea in the centuries that followed. The concept of fossils seemed to imply an Earth older than the biblical account suggested, and with the Catholic Church under attack by the forces of religious reformation (i.e., Protestantism) and other forms of “heresy,” church leaders became less and less inclined to tolerate any deviation from orthodoxy. The ecclesiastical view of Earth’s history reached a sort of extreme in the seventeenth century, with the Irish bishop James Ussher (1581–1656). The New Testament contains a thorough accounting of Jesus’ lineage, both through his mother and his earthly father, Joseph, all the way back to the time of Adam. Jesus was descended from David, whose lineage is provided in the Old Testament, complete with each ancestor’s life span and the age at which he fathered a successor in the Davidic line of descent. From these figures, Ussher concluded that God finished making Earth at 9:00 A.M. on Sunday, October 23, 4004 B.C. Accepted by the Church, Ussher’s calculation gave the idea of a very young Earth an aura of “scientific” justification.

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Henry Gellibrand (1597–1636). While researching Earth’s magnetic field, Gellibrand discovered that the field had changed over time (as indeed it has—for more on this subject, see Geomagnetism). This was one of the first indications that the planet’s history can be studied scientifically, even though humans have no direct information regarding the origins of Earth.

Early Stratigraphic Studies After Gellibrand came the Danish geologist Nicolaus Steno (1638–1687), who studied the age of rock beds. Thus was born the concept of stratigraphy, or the study of rock layers beneath Earth’s surface, which revealed a great deal about the planet’s age. (See Stratigraphy, which discusses many topics related to historical geology.) Along with the English physicist Robert Hooke (1635–1703) and others, Steno also became one of the first thinkers to confront the possibility that Earth must be much more than 6,000 years old. During the eighteenth century, the German geologist Johann Gottlob Lehmann (1719–1767) built on ideas introduced by Steno concerning the formation of rock beds. Lehmann put forward the theory that certain groups of rocks tend to be associated with each other and that each layer of rock is a sort of chapter in the history of Earth. Aspects of Lehmann’s theory were incorrect, but the general principle marked an advancement over previous ideas in geology and helped point the way toward a new view of the earth sciences. Previously, geologic studies had tended to be qualitative and descriptive, meaning that earth scientists used very generalized terminology and failed to possess a grasp of larger issues. Thanks to Lehmann and others who followed him, the earth sciences became more truly quantitative and predictive, offering explanations of what had happened in the past, along with justifiable theories concerning what might happen in the future.

Ironically, one of the first scientists to discover evidence that pointed toward an extremely old Earth was also a minister, the English astronomer

The German geologist Abraham Gottlob Werner (1750–1817) put forward a theory that was largely incorrect, yet one that nevertheless advanced the earth sciences. His “neptunist” theory was based on the idea that water had been the main force in shaping Earth’s surface. Though this theory was not accurate, his idea was significant, because it constituted the first well-ordered geologic theory of Earth’s origins and early history. At the same time, that history was turning out to be very long indeed.

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LEONARDO DA VINCI WAS THE FIRST EARTH IS OLDER THAN THE BIBLICAL

TO SUGGEST THAT FOSSILS ARE LONG-DEAD ANIMAL REMAINS, IMPLYING THAT ACCOUNT IN

GENESIS

WOULD SUGGEST. (AP/Wide World Photos. Reproduced by permis-

sion.)

In 1774 the French mathematician GeorgesLouis Leclerc, Comte de Buffon (1707–1788), applied new scientific ideas to the study of Earth and estimated its age at 75,000 years. Privately he

admitted that he actually thought Earth was billions of years old but did not think that such a figure would be understood. At the time, after all, the concept of a “billion” was hardly a familiar one, as it is today. More important, the idea of

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Earth being that old was shocking and downright frightening to people who accepted a strict interpretation of the biblical account. In an earlier century, the Italian astronomer Galileo Galilei (1564–1642) had been forced, on pain of death, to recant his support for the Polish astronomer Nicolaus Copernicus’s (1473–1543) discovery that Earth is not the center of the universe. In Buffon’s day, by contrast, few Europeans faced such dire threats for endorsing apparently unbiblical ideas. A scientist could still lose his job for supporting the wrong principles, and thus Buffon had to renounce his position on threat of losing his post at the University of Paris. AN

AT H E I S T I C

REACTION.

Other forces were at work in the sciences during the eighteenth century, and some were openly hostile to religious belief. An extreme example was the French physician and philosopher Julien de La Mettrie (1709–1751), a leading figure in the mechanist school of the biological sciences. La Mettrie maintained that humans are essentially a variety of monkey, to whom they were superior only by virtue of possessing the power of language. Moving far beyond the territory of science itself, he also taught that atheism is the only road to happiness and that the purpose of human life is to experience pleasure. In the physical sciences, an interesting example of reaction to religious belief can be found in the case of the French mathematician Pierre Simon de Laplace (1749–1827). Like those of La Mettrie, Laplace’s aims were not purely scientific; instead, he envisioned himself as a warrior against religious belief. Correctly enough, Laplace maintained that the origins of the universe as well as its workings could be explained fully without any reference to God. He also introduced a highly influential theory, widely accepted today, that the solar system originated from a cloud of gas. (See the entries Planetary Science and Sun, Moon, and Earth.) Like La Mettrie, Laplace took his ideas far beyond their justifiable purview in the realm of science, however, wielding them as a sword in a religious war. Laplace maintained that because it was possible to discuss the origins of the cosmos without reference to God, there must be no God—which is far from a logically necessary conclusion. Misguided as La Mettrie’s and Laplace’s atheistic crusades may have been, they are historically understandable: in France, far

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more than anywhere in western Europe except perhaps Spain, the Church had come to be seen as a force of political oppression, allied as it was with the French royalty. It is no wonder, then, that the French Revolution of 1789 was directed as much against the Church as against the king.

REAL-LIFE A P P L I C AT I O N S Uniformitarianism Late in the eighteenth century, the Scottish geologist James Hutton (1726–1797) put forward an idea that transcended the debate over Earth’s origins. Rather than speculate as to how Earth had come into being, Hutton analyzed the processes at work on the planet in his time and reasoned that they must be a key to understanding the means by which Earth was shaped. This was the principle of uniformitarianism, which is still a key concept in the study of Earth. Thanks to his introduction of this influential idea, Hutton today is regarded as the father of modern scientific geology. Uniformitarianism, in general, is the idea that the geologic processes at work today provide a key to understanding the geologic past. This means that the laws of nature have always been the same. The uniformitarianism promoted by Hutton and his fellow Scottish geologist Charles Lyell (1797–1875), however, has undergone some modification, namely, by the addition of the qualifying statement that the speed and intensity of those processes may not always be the same at any juncture in geologic history. For instance, land does not erode today at the same rate that it did before plants existed to hold rocks and soil in place. GOULD’S FOUR UNIFORMIT I E S . In the late twentieth century, the Amer-

ican paleontologist Stephen Jay Gould (1941– 2002) identified four different meanings of uniformity in science, not all of which are equally valid. Gould’s listing and analysis of these four meanings is as follows: • Uniformity of law: The assumption that natural laws do not change over time. This idea governs all sciences. • Uniformity of process: The idea embodied in the most well-known definition of geo-

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METEOR CRATER, ARIZONA. MOST NOTABLE AMONG CATASTROPHE THEORIES LISION OF METEORITES. (© Francois Gohier/Photo Researchers. Reproduced by permission.)

logic uniformitarianism, “The present is key to the past.” • Uniformity of rate: The incorrect assumption that the rate at which processes occur presently is the same as the rate at which they occurred in the past. • Uniformity of state: The incorrect assumption that the state of the universe always has been as it is today. As noted, uniformity of law is essential to all sciences. For instance, there is every reason to believe that the conservation of energy (a law stating that the total amount of energy in the universe remains constant) always has been the case. If the contrary were true, the conservation of energy could no longer properly be called a law, because it might cease to be the case at some time in the future. The statement “The present is key to the past” was formulated by yet another Scottish geologist, Sir Archibald Geikie (1835–1924). Geikie’s statement often has been criticized as an oversimplification, because processes that occurred in the past may not necessarily be occurring now, or vice versa, even though they could occur again. This idea has required modification of uniformitarianism, as noted earlier, to take into account the fact that the speed and

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OF

EARTH’S

FORMATION IS THE COL-

intensity of processes may not always be the same. Part of this modification has involved acceptance of a form of catastrophism, discussed later in this essay. Variations in the speed and intensity of processes also were addressed by Gould, with his observation that “uniformity of rate” is a fallacy. So, too, is “uniformity of state,” which is one of the few areas on which adherents of creationism (a strict interpretation of the Genesis account) would agree with their opponents. Even the Bible, after all, says “In the beginning ... the earth was without form, and void.”

Catastrophism In Theory of the Earth (1795), Hutton suggested that the weathering effects of water produced the sedimentary layers of Earth. Based on observation of river flow and mud content, he realized that this process would require much longer than 6,000 years. So, too, did Lyell, author of the highly influential Principles of Geology, which appeared in 12 editions from 1830 to 1875 and which presented a strict version of uniformitarianism. Aqueducts and other structures erected by the Romans had stood for a good one-fourth to

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one-third of the entire history of Earth, assuming that it was as young as Ussher’s biblical interpretation implied. Yet these Roman constructions had experienced very little weathering and certainly much less than mountains would have had to experience to leave behind the sediments observed by geologists. Surely, then, Earth must be millions upon millions of years old, not just a few thousand. CUVIER’S CATASTROPHIC THEORY. Not so, countered adherents of a move-

ment known as catastrophism, which arose in opposition to uniformitarianism during the late eighteenth and early nineteenth centuries. Catastrophism associates geologic phenomena with sudden, dramatic changes rather than ongoing and long-term processes, as in uniformitarianism. The leading proponent of catastrophism was the French geologist Baron Georges Cuvier (1769–1832), who used this theory to explain unconformities. These apparent gaps in the geologic record, revealed by observing rock layers, or strata, are discussed in the essay, Stratigraphy. Whereas Cuvier’s countryman (and fellow French noble) Buffon had asserted that Earth was 75,000 years old while actually believing that it was much older, Cuvier maintained that the planet is just 75,000 years old. The formation of mountains and other landforms, which should have taken millions of years, could be explained by sudden, violent changes, an example of which was Noah’s Flood in the Book of Genesis. As the ocean waters receded, they moved rocks far from their sources, carved out valleys, and left behind lakes and other bodies of fresh water. As more and more evidence for a very old Earth began to accumulate during the nineteenth century, catastrophism fell into disfavor. Discoveries from the 1970s onward, however, influenced a new look at catastrophism, and, as a result, the idea has received new attention in later years. C ATA S T R O P H I S M

T O D AY.

This has not led to a wholesale endorsement of creationism; rather, scientists have come to understand that the generally steady pace of processes on Earth periodically is broken by catastrophic events. Most notable among types of catastrophe is the collision of a meteorite with Earth, a remarkable example of which apparently occurred some 65 million years ago. That dramatic event seems to have forced so much dust and gas into the atmosphere that it blocked out

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the Sun, leading to the ultimate extinction of the dinosaurs.

Understanding Geologic Time So just how old is Earth? Modern earth scientists working in the realm of historical geology, and specifically geochronology, estimate its age at about 4.6 billion years. (The dating techniques used to determine the age of the planet are discussed in the essay Stratigraphy.) Such a vast span of time is more than a little difficult for humans to comprehend, given the fact that our lives last 70–80 years, on average, and the entire history of human civilization is only about 5,500 years long. For this reason, it is helpful to use scales of comparison, such as that offered at the Web site listed under the title Comprehending Geologic Time. Suppose that the entire geologic history of Earth were likened to a single year of 365.25 days, starting with the formation of the planet from a cloud of dust and ending with the present. More than two months would have been required simply for the accretion of Earth from a gas cloud to a planetesimal to something like its present form, but by about March 5 this evolution would have been accomplished. The entire spring would be analogous to a long, long period of time in which Earth was pounded by meteor showers and the oceans began to form. Not even the oldest known rocks date back this far, and many of our ideas about this phase in Earth’s history are based on conjecture. Much more is known about the second half of geologic history, beginning with the origins of the first single-cell life-forms on June 16. FROM SINGLE CELLS TO DIN O SAU R S . We are now almost halfway

through the year and still a long, long way from any sort of complex living beings. This is not surprising, given the fact that the formation of the continental plates and the development of oxygen in the atmosphere would have occurred only by about August 26. Even in the week after Thanksgiving, the most complex organisms would have been snails. Finally, a few days before the beginning of December, creatures would have begun to invade the land. We tend to associate the dinosaurs with the early phases of Earth’s history, but this only illustrates our distorted view of geologic time. In fact the Jurassic period, when dinosaurs roamed

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KEY TERMS The idea that geologic phenomena are brought about by sudden dramatic changes rather than ongoing and long-term processes, as in uniformitarianism. Although it was once used to promote the idea of a very young Earth, catastrophism today is accepted, in a very modified form, by many earth scientists.

CATASTROPHISM:

The study of Earth’s age and the dating of specific formations in terms of geologic time.

GEOCHRONOLOGY:

The study of the solid earth, in particular, its rocks, minerals, fossils, and land formations.

GEOLOGY:

GEOMYTHOLOGY:

Folklore inspired

by geologic phenomena. The study of Earth’s physical history. Historical geology is one of two principal branches of geology, the other being physical geology. HISTORICAL GEOLOGY:

Involving a comparison between qualities that are not defined precisely, such as “fast” and “slow” or “warm” and “cold.” QUALITATIVE:

Involving a comparison between precise quantities—for QUANTITATIVE:

Earth, would be parallel to a period of about five days, from December 15 to 20. By Christmas Day, the meteorite referred to earlier would have hit Earth, and the dinosaurs would be headed toward extinction, their dead bodies eventually forming the fossil fuels that have powered much of human civilization.

instance, 10 lb. versus 100 lb. or 50 mi. per hour versus 120 mi. per hour. A set of principles and procedures for systematic study that includes observation; the formation of hypotheses, theories, and laws; and continual testing and reexamination.

SCIENTIFIC METHOD:

Material deposited at or near Earth’s surface from a number of sources, most notably preexisting rock. SEDIMENT:

STRATIGRAPHY: The study of rock layers, or strata, beneath Earth’s surface.

An apparent gap in the geologic record, as revealed by observing rock layers, or strata. UNCONFORMITY:

The idea that the geologic processes at work today provide a key to understanding the geologic past. The speed and intensity of those processes, however, may not always be the same at any juncture in geologic history. Uniformitarianism usually is contrasted with catastrophism. UNIFORMITARIANISM:

The breakdown of rocks and minerals at or near the surface of Earth due to physical or chemical processes or both.

WEATHERING:

come on the scene until the last 0.16 days of the year—that is, at a few minutes after 8:00 p.m. on December 31. The New Year’s Eve countdown would be nearing by the time human civilization began, at about 42 seconds before midnight.

within a few days of the year’s end, and yet nothing remotely resembling a human has appeared. Our own species, Homo sapiens, would not have

Now we have come to a period about 6,000 years ago, or the point at which, according to Bishop Ussher, Earth was created. No wonder many people wanted to believe in a young Earth, and some even hold on to that belief today: when viewed against the backdrop of the planet’s true age, humanity seems very insignificant indeed.

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Christ’s birth would have occurred at about 14 seconds before midnight, and the final 10-second countdown would begin about the time the Roman Empire fell. The life span of the average person would correspond to about half a second or less.

How Do We Know Earth’s Age? What we know about Earth’s age comes, of course, not from direct observation but from the study of materials. One of the most important techniques for determining the age of samples taken from the earth is radiometric dating, discussed in more detail in Geologic Time. Radiometric dating involves ratios between two different kinds of atoms for a given element: stable and radioactive isotopes. Because chemists know how long it takes for half the isotopes in a given sample to stabilize (a half-life), they can judge the age of the sample by examining the ratio of stable to radioactive isotopes. In the case of uranium, one isotopic form, uranium-238, has a half-life of 4,470 million years, which is very close to the age of Earth itself. Use of uranium dating has detected rocks of an age between 3.8 and 3.9 billion years old, as well as even older crystal formations that suggest the earth had solid ground as early as 4.2 billion years ago.

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extensive study of the sample suggested that at the time of its formation, Earth was already covered in water—something that had supposedly happened many millions of years later. If this was the case, it could suggest the possibility that life appeared much earlier than has previously been supposed, and perhaps even that life disappeared and reappeared several times before finally taking hold. WHERE TO LEARN MORE Bishop, A. C., A. Woolley, and A. Hamilton. Cambridge Guide to Minerals, Rocks, and Fossils. New York: Cambridge University Press, 1992. Boggy’s Links to Stratigraphy and Geochronology (Web site). . Comprehending Geologic Time (Web site). . Harris, Nicholas, Alessandro Rabatti, and Andrea Ricciardi. The Incredible Journey to the Beginning of Time. New York: Peter Bedrick Books, 1998. “Historical Geology.” Georgia Perimeter College (Web site). . Lamb, Simon, and David Sington. Earth Story: The Shaping of Our World. Princeton, NJ: Princeton University Press, 1998. MacRae, Andrew. Radiometric Dating and the Geological Time Scale (Web site). .

A rock discovered in the Australian desert during the early 1980s appears to be the oldest rock sample in the world, according to data originally reported in Nature and included on the Scientific American Web site in early 2001. This zircon crystal, according to Simon Wilde of Curtin University in Western Australia, is 4.4 billion years old. Wilde and associates reported that

UCMP (University of California, Berkeley, Museum of Paleontology) Web Time Machine (Web site). .

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Reeves, Hubert. Origins: Cosmos, Earth, and Mankind. New York: Arcade, 1998. Spickert, Diane Nelson, and Marianne D. Wallace. Earthsteps: A Rock’s Journey Through Time. Golden, CO: Fulcrum Kids, 2000.

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Geologic Time

GEOLOGIC TIME

CONCEPT The expression geologic time refers to the vast span from Earth’s beginnings to the present, about 4.6 billion years. To examine the history of Earth, one must discard most familiar ideas about time. Instead of thinking in terms of years, centuries, or even millennia, the most basic unit is a million years, and even that is rather small when compared with the four eons into which geologic time is divided. Earth scientists’ knowledge of the first three eons is fairly limited. What they do know comes from a combination of absolute dating, mostly by the study of radioactive decay, and relative dating through the stratigraphic record of rock layers.

HOW IT WORKS Historical Geology The study of geologic time is encompassed within the larger subject of historical geology. The latter, the study of Earth’s physical history, is one of the two principal branches of geology, the other being physical geology, or the study of Earth’s physical components and the forces that have shaped them. The background of historical geology is discussed in some detail within the Historical Geology essay. Its principal subdisciplines include stratigraphy, the study of rock layers, or strata, beneath Earth’s surface; geochronology, the study of Earth’s age and the dating of specific formations in terms of geologic time; sedimentology, the study and interpretation of sediments, including sedimentary processes and formations; paleontology, the study of fossilized plants and

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animals; and paleoecology, the study of the relationship between prehistoric plants and animals and their environments. Several of these subjects are examined in essays within this book.

Divisions of Geologic Time Geologic time is divided according to two scales. The more well-known of these is the geologic scale, which divides time into named groupings according to six basic units: eon, era, period, epoch, age, and chron. In addition, the chronostratigraphic scale identifies successive layers of rock with specific units of time. As noted earlier, stratigraphy is the study of rock layers, or strata, beneath Earth’s surface, while chronostratigraphy is a subdiscipline devoted to studying the ages of rocks and what they reveal about geologic time. The chronostratigraphic scale likewise has six time units, analogous to those of the geologic scale: eonothem, erathem, system, series, stage, and chronozone. For the most part, we will not be concerned with the chronostratigraphic terms in the present context. R E L AT I V E AND ABSOLUTE T I M E . To discuss the divisions of geologic

time, it is necessary first to discuss the concepts of relative and absolute time. The term relative refers to a quality or quantity that is comparative, or dependent on something else. Its opposite is absolute, a term designating a quality or quantity that is independent and not defined in relation to another quality or quantity. If we say that Abraham Lincoln was born in 1809, it is an absolute designation of his birth year, whereas if we say that he was born 10 years after the death of George Washington (which

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terms of chronostratigraphic time divisions rather than millions of years.

Geologic Time

Relative Dating Given the meaning of relative age, it is easy enough to guess what relative dating would be, once one knows that dating, in a scientific context, usually refers to any effort directed toward finding the age of a particular item or phenomenon. Relative dating, then, assigns an age relative to that of other items, whereas absolute dating determines the age in actual years or millions of years. One of the principal means of relative dating is through stratigraphy, which is based on the assumption that the deeper a layer of rock lies beneath Earth’s surface, the earlier it was deposited. This holds true, however, for only one of the three major types of rock: sedimentary rock, which is formed by compression and deposition (i.e., formation of deposits) on the part of rock and mineral particles. (The other types of rock are igneous and metamorphic.) SUBATOMIC

PARTICLE TRACKS.

STUDY

OF RADIOACTIVE

DECAY, OR THE RELEASE OF SUBATOMIC PARTICLES, IS A METHOD OF ABSOLUTE DATING. (© John Giannicchi/Photo

Researchers. Reproduced by permission.)

occurred in 1799), that is an example of a relative time measurement. In actuality, of course, there is no truly absolute measure of time. For example, the reference to 1809 as Lincoln’s birth year is based on the system of time measurement developed in the West, which, in turn, is based on early ideas regarding the date of Christ’s birth. (As it turns out, Christ likely was born in about 6 B.C.) Since the B.C./A.D. system of dating is widely accepted and used, or at least recognized, by most of the non-Western world, a date rendered according to this system constitutes the closest possible approximation to an absolute measure of time. In any case, one knows the difference between absolute and relative when one sees it: thus, to say that Lincoln was born ten years after Washington died is obviously and unmistakably a relative statement.

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Aside from stratigraphy, discussed in a separate essay, other relative dating techniques include seriation, faunal dating, and pollen dating, or palynology. Used, for instance, in archaeological studies, seriation analyzes the abundance of a particular item (for instance, pieces of pottery) and assigns relative dates based on this abundance. The term faunal dating refers to fauna, or animal life, and faunal dating is the use of animal bones to determine age. Finally, pollen dating, or palynology, involves analysis of pollen deposits.

Absolute Dating

In terms of geology, the absolute age of a geologic phenomenon is its age in Earth years. On the other hand, its relative age is its age in comparison with other geologic phenomena, particularly the stratigraphic record of rock layers. Thus, references to relative age are given in

As dating technology has progressed, it has become increasingly possible for scientists to provide absolute dates for specimens. One such method, introduced in the 1960s, is amino-acid racimization. Amino acids exist in two forms, designated L-forms and D-forms, which are stereoisomers, or mirror images of each other. Virtually all living organisms (except some microbes) incorporate only the L-forms, but once the organism dies the L-amino acids gradually convert to D-amino acids. Several factors influence the rate of conversion, and though amino-acid racimization was popular in the 1970s, these uncertainties have led scientists to treat it with increasing disfavor.

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The principles that undergird amino-acid racimization, however, are essential to most forms of absolute dating. Generally, absolute dating uses ratios between the quantities of a particular substance (let us call it Substance A) and the quantities of a mirror substance (Substance B) to which it is converted over a period of time. The greater the ratio of Substance B to Substance A, the longer the time that has elapsed. The scale of time for various substances, however, differs greatly. Carbon-14 decay, for instance, takes place over a few thousand years, making it useful for measuring the age of human artifacts. On the other hand, uranium decay takes billions of years, and thus it is used for dating rocks. Cation-ratio dating, for instance, measures the amount of cations, or positively charged ions, that have formed on an exposed rock surface. (An ion is an atom or group of atoms that have lost or gained electrons, thus acquiring a net electric charge. Electron loss creates a cation, as opposed to a negatively charged anion, created when an atom or atoms gain electrons.) Cationratio dating is based on the idea that the ratio of potassium and calcium cations to titanium cations decreases with age. It is applicable only to rocks in desert areas, where the dry air stabilizes the cation “varnish.” Various forms of radiometric dating employ ratios as well. Every element has a particular number of protons, or positively charged particles, in its nucleus, but it may have varying numbers of neutrons, particles with a neutral electric charge but relatively great mass. (Neutrons and protons have approximately the same mass, which is more than 1,800 times greater than that of an electron.) When two or more atoms of the same element have a differing number of neutrons, they are called isotopes. RA D I OAC T I V E

D E C AY.

Some types of isotopes “fit” better with a particular element and tend to be most abundant. For instance, carbon has six protons, and it so happens that the most abundant carbon isotope has six neutrons. Because there are six protons and six neutrons, totaling 12, this carbon isotope is designated carbon-12, which accounts for 98.9% of the carbon in nature. Generally speaking, the most abundant isotope is also the most stable one, or the one least likely to release particles and thus change into something else.

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This release of particles is known as radioactive decay. In the context of radioactivity, “to decay” does not mean “to rot” rather, the isotope expels alpha particles (positively charged helium nuclei), beta particles (either electrons or subatomic particles called positrons), or gamma rays, which occupy the highest energy level in the electromagnetic spectrum. In so doing, it eventually will become another isotope, either of the same element or of a different element, and will stabilize. The amount of time it takes for half the isotopes in a sample to stabilize is called its halflife. This half-life varies greatly between isotopes, some of which have a half-life that runs into the billions of years.

Geologic Time

Determining Absolute Age When an organism is alive, it incorporates a certain ratio of carbon-12 in proportion to the amount of the radioisotope (that is, radioactive isotope) carbon-14 that it receives from the atmosphere. As soon as the organism dies, however, it stops incorporating new carbon, and the ratio between carbon-12 and carbon-14 will begin to change as the carbon-14 decays to form nitrogen-14. A scientist can use the ratios of carbon-12, carbon-14, and nitrogen-14 to ascertain the age of an organic sample. Carbon-14, known as radiocarbon, has a half-life of 5,730 years, meaning that it takes that long for half the isotopes in a sample to decay to nitrogen-14. Note that half-life is not half the amount of time it takes for the entire sample to decay, especially because the first half of the sample usually decays faster than the second half. Imagine, for instance, that you had 100 units and wanted to reduce it to zero units by continually halving it. At first, the results would be dramatic, as 100 became 50, then 25, then 12.5, and so on. Eventually you would be down to smaller and smaller fractions of 1, and each division by 2 would yield a smaller number—but never zero. Radioactive decay works that way as well, and, thus, while carbon-14 has a half-life of less than 6,000 years, it takes much longer than 6,000 years for the other half of the isotopes in a carbon-14 sample to decay. For this reason, the use of proper instrumentation makes it possible to judge the age of charcoal, wood, and other biological materials over a span of as long as 70,000 years. While this may be useful for archaeologists, it is not very helpful for measuring the vast spans

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of time encompassed in the earth sciences. Furthermore, there is a good likelihood that the sample will become contaminated by additional carbon from the soil. Moreover, it cannot be said with certainty that the ratio of carbon-12 to carbon-14 in the atmosphere has been constant throughout time. P O TA S S I U M - A R G O N

D AT I N G .

Much more useful, from the standpoint of geology, is potassium-argon dating. When volcanic rocks are subjected to extremely high temperatures, they release the element argon, a noble gas. As the rocks cool, the stable isotope argon-40 accumulates. Because argon-40 is formed by the radioactive decay of a potassium isotope, potassium-40, the amount of argon-40 that forms is proportional to the rate of decay for potassium-40. Potassium-40 has a half-life of 1.3 billion years, and with the help of argon-40, geologists have been able to estimate the age of volcanic layers above and below fossil and artifact remains in eastern Africa. Potassium-argon dating is most effective for rocks that are at least three million years old, because it takes about that long to accumulate enough argon-40 to make accurate measurements possible. This brings up a notable aspect of radiometric dating techniques. No one technique is most effective; rather, each technique is suited to a particular span of time. Thus, potassium-argon dating would be virtually useless for measuring the relatively short time scales for which radiocarbon dating is ideally suited. The converse is also true: as we have noted, radiocarbon dating simply does not cover a wide enough span of time to be useful in most geologic studies. U RA N I U M - S E R I E S DAT I N G . We now come to the element most useful for dating the age of material samples over a broad chronological spectrum: uranium, which has an atomic number of 92. This means that it has 92 protons in its nucleus, making uranium atoms typically the heaviest atoms that occur in nature. (There are about 20 elements with atomic numbers higher than 92, but all of them have been created artificially, either in laboratories or as the result of nuclear testing.)

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that range from just a few years to a few hundred thousand years, whereas the half-lives of the parent isotopes are much longer. That of uranium235, for instance, is 7.038  108 years, or more than 700 million years. On the other hand, the daughter isotope protactinium-231 has a half-life of 32,760 years. When uranium-235 is deposited in an area, over time it will decay to form daughter isotopes. Assuming that the sample has been left undisturbed (isotopes have neither entered nor exited the deposit since its initial formation), the age of certain types of sample may thus be determined. For mollusks and corals, for instance, the amount of protactinium-231, a daughter isotope that begins to accumulate only after the organism dies, makes it possible to date a sample. In some cases, large amounts of a daughter isotope may be deposited initially alongside samples of a parent, and if these are present in water, the quantities of each can be judged according to the amount that has dissolved. For example, the daughter isotope uranium-234 dissolves more readily in water than the parent, uranium-238.

REAL-LIFE A P P L I C AT I O N S How Do We Know Earth’s Age? We can now begin to answer a question almost inevitably raised when discussing geologic time: just how do we know that Earth is about 4.6 billion years old? A clue lies in the half-life of uranium-238, which is 4.47  109 years, or 4,470 million years. Geologists typically would abbreviate this as 4.47 Ga, the latter referring to “gigayears,” a unit of a billion years. As uranium atoms undergo fission, or splitting, this process releases energy that causes marks, called tracks, to form on the surface of volcanic minerals. In splitting, two daughter atoms shoot away from each other, forming tracks, and thus the rate of track formation is proportional to the rate of decay on the part of the parent isotope.

Both uranium and thorium, with an atomic number of 90, have unstable “parent” isotopes that decay into even more unstable “daughter” isotopes before eventually stabilizing as isotopes of lead. These daughter isotopes have half-lives

Incidentally, fission-track dating with uranium-238 defies the statement made earlier that certain types of dating are more suited to long periods of time, while others are best for shorter periods. When heated, the tracks disappear from

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Geologic Time

RELATIVE

DATING USES THE STRATIGRAPHIC RECORD OF ROCK LAYERS, AS SEEN IN THE WALL OF THIS GORGE, TO

DETERMINE THE AGE OF A GEOLOGIC FORMATION COMPARED WITH OTHERS. (© B. Bachman/Photo Researchers. Reproduced by

permission.)

a sample containing uranium-238, thus resetting the dating clock. As a result, if an object was heated just a few decades ago, it can be dated; so, too, can meteorites billions of years old. Most meteorites found in the solar system tend to be about 4.56 Ga; hence, the rough figure of 4.6 Ga is used for the point at which the solar system, including Earth, began to form. The oldest known materials on Earth are zircon crystals from western Australia, dated about 4.3 Ga. Small samples of gneiss in Canada’s Northwest Territories have been dated to about 4.0 Ga, but the oldest large-scale sample is a belt of 3.8 Ga gneiss in western Greenland.

A Question of Scale

Human civilization has existed for about 5,500 years, the blink of an eye in geologic terms. Even the span of time that the human species, Homo sapiens, has existed—about two million years—is negligible in the grand scheme of Earth’s history. The latter stretches back some 4,600 million years, meaning that human beings have existed on this Earth for just 0.043% of the planet’s history. As discussed in the essay Historical Geology, if the entire history of Earth were likened to a single year, humans would have appeared on the scene at a few minutes after 8:00 p.m. on December 31. Human civilization would date only from about 42 seconds before midnight, and the age of machinery and industrialization would not fill up even the final two seconds of the year.

Having discussed at least the rudiments of the dating system used by geologists, it is possible to examine geologic time itself. This requires a mental adjustment of monumental proportions, because one must discard all notions used in studying the history of human civilization. Concepts such as medieval, ancient, and prehistoric are practically useless when discussing geologic time, which dwarfs the scale of human events.

A N O T H E R A N A LO GY: LO S A N G E L E S T O N E W YO R K . When dis-

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cussing distances in space, astronomers dispense with miles, because they would be useless, given the vastness of the scale involved. The same is true of geologic time, in which the concept of years is hardly relevant. Instead, geologists speak in terms of millions of years, or megayears, abbreviated “Ma.” (Geologists also use the much larger unit of a gigayear, to which we have already

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referred.) To discuss the age of Earth in terms of years, in fact, would be rather like measuring the distance from Los Angeles to New York in feet; instead, of course, we use miles. Now let us consider geologic time in terms of the 2,462 mi. between Los Angeles and New York, with 1 mi. equal to 1.8684 Ma, or 1,868,400 years. Suppose we have left Los Angeles and driven a good deal of the distance to New York—46% of the way, in fact, to western Nebraska, a spot analogous to the beginning of the Proterozoic era. In the preceding miles, a duration equivalent to about 1,133 Ma, Earth was formed from a cloud of gas, pounded by meteors, and gradually became the home to oceans—but no atmosphere resembling the one we know now. The end of the Proterozoic era (about 545 Ma, or 545 million years ago) would be at about 88% of the distance from Los Angeles to New York—somewhere around Pittsburgh, Pennsylvania. By this point, the continental plates have been formed, oxygen has entered the atmosphere, and soft-bodied organisms have appeared. We are a long way from Los Angeles, and yet almost the entire history of life on Earth, at least in terms of relatively complex organisms, lies ahead of us. If we skip ahead by about 339 Ma (a huge leap in terms of biological development), we come to the time when the dinosaurs appeared. We are now 95% of the way from the beginning of Earth’s history to the present, and if measured against the distance from Los Angeles to New York, this would put us at a longitude equivalent to that of Baltimore, Maryland. Another 89 mi. would put us at about 65 million years ago, or the point when the dinosaurs became extinct.

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The Very, Very Distant Past So what happened for all those hundreds of millions of years before humans appeared on the scene? We will attempt to answer that question in an extremely cursory, abbreviated fashion, but for further clarification, the reader is strongly encouraged to consult a chart of geologic time. Such a chart can be found in virtually any earth sciences textbook; indeed, several versions (including a chronostratigraphic chart) may appear in a single book. In addition to showing geologic time in both absolute and relative terms, these charts typically provide information about the magnetic polarity over a given span, since that has changed many times since Earth came into existence. In other words, what is today the magnetic North Pole was once the magnetic South Pole, and vice versa. (For more on this subject, see the discussion of paleomagnetism in the entries Plate Tectonics and Geomagnetism.) As one might expect, disagreement between earth scientists is greatest with regard to the most distant phases of Earth’s geologic history. This encompasses nearly 90% of all geologic time, dating back to about 545 Ma, thus showing how little geologists know, even today, about the geologic events of the very distant past. For this reason, when discussing Precambrian time, it is usually necessary to consider only the three eons that composed it. Discussion of era and period, on the other hand, is reserved for the three eras, and 11 periods, of the Phanerozoic eon. The smaller division of epoch is generally only of concern with regard to the most recent era, the Cenozoic. As for divisions smaller than an epoch, these will not concern us here. T H E P R E CA M B R I A N E O N S . The last paragraph of the preceding section encompasses a number of ideas, which now need to be explained, in at least general terms. The term Precambrian encompasses about four billion years of Earth’s history, including three of the four eons (Hadean or Priscoan, Archaean, and Proterozoic) of the planet’s existence. The names of these eons are derived from Greek, with the first being taken from the name of the deity who ruled over the Underworld. The latter two are derived, respectively, from the Greek words for beginning and new life.

We would then have only 33.7 mi. to drive to reach the point where humans appeared, by which time we would be in the middle of Manhattan. Compared with the distance from Los Angeles to New York, the span of human existence would be much smaller than the cab ride from Central Park to the Empire State Building. The entire sweep of written human history, from about a thousand years before the building of the pyramids to the beginning of the third millennium A.D., would be much smaller than a city block. In fact, it would be about the width of a modest storefront, or 15.54 ft.

The Hadean eon (sometimes called the Priscoan) lasted from about 4,560 Ma to 4,000

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Ma ago, when the planet was being formed, or accreted, as pieces of solid matter floating around in the young solar system began to join one another. Meteorites showered the planet, bringing both solid matter and water, and thus forming the basis of the oceans. There was no atmosphere as such, but by the end of the eon, volcanic activity had ejected enough carbon dioxide and other substances into the air to form the beginnings of one. The oceans began to cool, making possible the beginnings of life—that is, molecules of carbon-based matter that were capable of replicating themselves. These appeared at the end of the Hadean eon, perhaps arriving from space in a meteorite. The boundaries of the Precambrian eons are far from certain, so it is possible only to say that the Archaean eon lasted from about 4,000 Ma to 2,500 Ma ago. The earliest known datable materials, described earlier, all come from this time; in fact, outcrops of Archaean rock have been found on all seven continents. The rocks of this eon contain the first clear evidence of life, in the form of microorganisms. Over the course of the Archaean eon, prokaryotes, or cells without a nucleus, made their appearance, and later they were followed by eukaryotes, or cells with a nucleus. During this great span of time, more than 20% of Earth’s history, the atmosphere and hydrosphere developed considerably, even as the biosphere had its true beginnings. As for the geosphere, it also matured enormously in the course of the Archaean eon. During the Hadean eon, Earth’s interior had begun to differentiate into core, mantle, and crust, and cooling in the two upper layers influenced the beginnings of the earliest plate-tectonic activity (see Plate Tectonics).

THE PHANEROZOIC E RAS A N D P E R I O D S . The end of the Protero-

Geologic Time

zoic eon, once again, is not sharply defined in the stratigraphic record, such that there is considerable dispute as to the time periods involved. In any case, it is clear that the pace of development in the biosphere increased dramatically in the Phanerozoic, the eon in which we are now living. During the beginning of the Phanerozoic eon, algae appeared, and there followed an acceleration in the development of living organisms that ultimately produced the varied biosphere we know today. As noted earlier, the only eras and periods that need concern most students of the earth sciences are those of the Phanerozoic eon. The three eras are as follows: Eras of the Phanerozoic Eon • Paleozoic (about 545 to 248.2 Ma) • Mesozoic (about 248.2 to 65 Ma) • Cenozoic (about 65 Ma to the present) Within these eras are the following periods: Periods of the Paleozoic Era • • • • • •

Cambrian (about 545 to 495 Ma) Ordovician (about 495 to 443 Ma) Silurian (about 443 to 417 Ma) Devonian (about 417 to 354 Ma) Carboniferous (about 354 to 290 Ma) Permian (about 290 to 248.2 Ma) Periods of the Mesozoic Era

• Triassic (about 248.2 to 205.7 Ma) • Jurassic (about 205.7 to 142 Ma) • Cretaceous (about 142 to 65 Ma) Periods of the Cenozoic Era

Even longer was the Proterozoic eon, which appears to have lasted from about 2,500 Ma to 545 Ma. This phase saw the beginnings of very basic forms of plant life, such that photosynthesis (the biological conversion of electromagnetic energy from the Sun into chemical energy in plants) began to take place. Plate-tectonic processes accelerated as well, with continents moving about over Earth’s surface and smashing against one another. Oxygen in the atmosphere assumed about 4% of its present levels, but animal life still consisted primarily of eukaryotes.

• Palaeogene (about 65 to 23.8 Ma) • Neogene (about 23.8 to 1.8 Ma) • Quaternary (about 1.8 Ma to present) These divisions, as well as the two most recent epochs of the Quaternary period (Pleistocene and Holocene), are discussed elsewhere in this book. It should be noted that there are variations for many of the eon, era, and period names given here; also, the Palaeogene and Neogene are often grouped together as a subera called the Tertiary. The latter nomenclature fits with a mnemonic device used by geology students memorizing the names of the 11 Phanerozoic periods: “Camels Ordinarily Sit Down Carefully; Perhaps Their Joints Creak Tremendously Quietly.”

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Geologic Time

KEY TERMS The absolute age of a geologic phenomenon is its age in Earth years. Compare with relative age. ABSOLUTE AGE:

A substance made up of

ELEMENT:

only one kind of atom. Unlike compounds, elements cannot be chemically broken into

In general, an atmosphere is a blanket of gases surrounding a planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%.

other substances.

The smallest particle of an element, consisting of protons, neutrons, and electrons. An atom can exist either alone or in combination with other atoms in a molecule.

longer than an age and a chron. The cur-

ATMOSPHERE:

ATOM:

A combination of all living things on Earth—plants, animals, birds, marine life, insects, viruses, singlecell organisms, and so on—as well as all formerly living things that have not yet decomposed. BIOSPHERE:

A subdiscipline of stratigraphy devoted to studying the ages of rocks and what they reveal about geologic time.

CHRONOSTRATIGRAPHY:

Any effort directed toward finding the age of a particular item or phenomenon. Methods of geologic dating are either relative (i.e., comparative and usually based on rock strata) or absolute. The latter, based on such methods as the study of radioactive isotopes, usually is given in terms of actual years or millions of years.

The longest phase of geologic

EON:

time. Earth’s history has consisted of four eons, the Hadean or Priscoan, Archaean, Proterozoic, and Phanerozoic. The nextsmallest subdivision of geologic time is the era. EPOCH:

The fourth-longest phase of

geologic time, shorter than an era and rent epoch is the Holocene, which began about 0.01 Ma (10,000 years) ago. ERA:

The second-longest phase of geo-

logic time, after an eon. The current eon, the Phanerozoic, has had three eras, the Paleozoic, Mesozoic, and Cenozoic, which is the current era. The next-smallest subdivision of geologic time is the period. GA:

An abbreviation meaning “giga-

years,” or “billion years.” The age of Earth is about 4.6 Ga. GEOCHRONOLOGY:

The study of

Earth’s age and the dating of specific formations in terms of geologic time.

DATING:

A negatively charged particle in an atom, which spins around the nucleus.

ELECTRON:

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GEOLOGIC TIME:

The vast stretch of

time over which Earth’s geologic development has occurred. This span (about 4.6 billion years) dwarfs the history of human existence, which is only about two million years. Much smaller still is the span of human civilization, only about 5,500 years. GEOSPHERE:

The upper part of

Earth’s continental crust, or that portion of the solid earth on which human beings live

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Geologic Time

KEY TERMS

CONTINUED

and which provides them with most of

Earth’s history, which lasted from about

their food and natural resources.

4,Ma to about 545 Ma ago.

HISTORICAL GEOLOGY:

The study

of Earth’s physical history. Historical geology is one of two principal branches of geology, the other being physical geology.

PROTON:

A positively charged particle

in an atom. RADIOACTIVITY:

A term describing a

phenomenon whereby certain materials

The entirety of

are subject to a form of decay brought

Earth’s water, excluding water vapor in the

about by the emission of high-energy par-

atmosphere but including all oceans, lakes,

ticles or radiation. Forms of particles or

streams, groundwater, snow, and ice.

energy include alpha particles (positively

HYDROSPHERE:

Atoms that have an equal

charged helium nuclei), beta particles

number of protons and hence are of the

(either electrons or subatomic particles

same element but differ in their number of

called positrons, or gamma rays, which

neutrons. This results in a difference of

occupy the highest energy level in the elec-

mass. An isotope may be either stable or

tromagnetic spectrum.

radioactive.

RADIOMETRIC DATING:

ISOTOPES:

A method

An abbreviation used by earth sci-

of absolute dating using ratios between

entists, meaning “million years” or

“parent” isotopes and “daughter” isotopes,

“megayears.” When an event is dated to, for

which are formed by the radioactive decay

instance, 160 Ma, it usually means that it

of parent isotopes.

took place 160 million years ago.

RELATIVE AGE:

MA:

The relative age of a

A subatomic particle that

geologic phenomenon is its age in compar-

has no electric charge. Neutrons are found

ison with other geologic phenomena, par-

at the nucleus of an atom, alongside pro-

ticularly the stratigraphic record of rock

tons.

layers. Compare with absolute age.

NEUTRON:

NUCLEUS:

The center of an atom, a

SEDIMENT:

Material deposited at or

region where protons and neutrons are

near Earth’s surface from a number of

located and around which electrons spin.

sources, most notably preexisting rock.

PERIOD:

The third-longest phase of

SEDIMENTARY ROCK:

Rock formed

geologic time, after an era. The current

by compression and deposition (i.e., for-

eon, the Phanerozoic, has had 11 periods,

mation of deposits) on the part of other

and the current era, the Cenozoic, has con-

rock and mineral particles.

sisted of three periods, of which the most

SEDIMENTOLOGY:

recent is the Quaternary. The next-smallest

interpretation of sediments, including sed-

subdivision of geologic time is the epoch.

imentary processes and formations.

PRECAMBRIAN TIME:

A term that

refers to the first three of four eons in

S C I E N C E O F E V E RY DAY T H I N G S

STRATIGRAPHY:

The study and

The study of rock

layers, or strata, beneath Earth’s surface.

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WHERE TO LEARN MORE Boggy’s Links to Stratigraphy and Geochronology (Web site). .

Lamb, Simon, and David Sington. Earth Story: The Shaping of Our World. Princeton, NJ: Princeton University Press, 1998.

Comprehending Geologic Time (Web site). .

MacRae, Andrew. Radiometric Dating and the Geological Time Scale (Web site). .

Hancock, Paul L., and Brian J. Skinner. The Oxford Companion to the Earth. New York: Oxford University Press, 2000.

Reeves, Hubert. Origins: Cosmos, Earth, and Mankind. New York: Arcade, 1998.

Harris, Nicholas, Alessandro Rabatti, and Andrea Ricciardi. The Incredible Journey to the Beginning of Time. New York: Peter Bedrick Books, 1998.

Spickert, Diane Nelson, and Marianne D. Wallace. Earthsteps: A Rock’s Journey Through Time. Golden, Colo.: Fulcrum Kids, 2000.

“Historical Geology.” Georgia Perimeter College (Web site). .

UCMP (University of California, Berkeley, Museum of Paleontology) Web Time Machine (Web site). .

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Stratigraphy

S T R AT I G R A P H Y

CONCEPT Stratigraphy is the study of rock layers (strata) deposited in the earth. It is one of the most challenging of geologic subdisciplines, comparable to an exacting form of detective work, yet it is also one of the most important branches of study in the geologic sciences. Earth’s history, quite literally, is written on the strata of its rocks, and from observing these layers, geologists have been able to form an idea of the various phases in that long history. Naturally, information is more readily discernible about the more recent phases, though even in studying these phases, it is possible to be misled by gaps in the rock record, known as unconformities.

HOW IT WORKS The Foundations of Stratigraphy Historical geology, the study of Earth’s physical history, is one of the two principal branches of geology, the other being physical geology, or the study of Earth’s physical components and the forces that have shaped them. Among the principal subdisciplines of historical geology is stratigraphy, the study of rock layers, which are called strata or, in the singular form, a stratum.

prehistoric plants and animals and their environments. Several of these subjects are examined in other essays within this book. E A R LY W O R K I N S T RAT I G RA P H Y. Among the earliest contributions to what

could be called historical geology came from the Italian scientist and artist Leonardo da Vinci (1452–1519), who speculated that fossils might have come from the remains of long-dead animals. Nearly two centuries later, stratigraphy itself had its beginnings when the Danish geologist Nicolaus Steno (1638–1687) studied the age of rock strata. Steno formulated what came to be known as the law of superposition, or the idea that strata are deposited in a sequence such that the deeper the layer, the older the rock. This, of course, assumes that the rock has been undisturbed, and it is applicable only for one of the three major types of rock, sedimentary (as opposed to igneous or metamorphic). Later, the German geologist Johann Gottlob Lehmann (1719–1767) put forward the theory that certain groups of rocks tend to be associated with each other and that each layer of rock is a sort of chapter in the history of Earth.

Other important subdisciplines include geochronology, the study of Earth’s age and the dating of specific formations in terms of geologic time; sedimentology, the study and interpretation of sediments, including sedimentary processes and formations; paleontology, the study of fossilized plants and animals; and paleoecology, the study of the relationship between

Thus, along with Steno, Lehmann helped pioneer the idea of the stratigraphic column, discussed later in this essay. The man credited as the “father of stratigraphy,” however, was the English engineer and geologist William Smith (1769– 1839). In 1815 Smith produced the first modern geologic map, showing rock strata in England and Wales. Smith’s achievement, discussed in Measuring and Mapping Earth, influenced all of geology to the present day by introducing the idea of geologic, as opposed to geographic, map-

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Stratigraphy

STRIATIONS

VISIBLE IN SANDSTONE FROM

NEON CANYON, UTAH. (© Rod Planck/Photo Researchers. Reproduced by permission.)

ping. Furthermore, by linking stratigraphy with paleontology, he formulated an important division of stratigraphy, known as biostratigraphy.

Areas of Stratigraphic Study Along with biostratigraphy, the major areas of stratigraphy include lithostratigraphy, chronostratigraphy, geochronometry, and magnetostratigraphy. The most basic type of stratigraphy, and the first to emerge, was lithostratigraphy, which is simply the study and description of rock layers. Earth scientists working in the area of lithostratigraphy identify various types of layers, which include (from the most specific to the most general), formations, members, beds, groups, and supergroups.

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Biostratigraphy involves the study of fossilized plants and animals to establish dates for and correlate relations between stratigraphic layers. Scientists in this field also identify categories of biostratigraphic units, the most basic being a biozone. Magnetostratigraphy is based on the investigation of geomagnetism and the reversals in Earth’s magnetic field that have occurred over time. (See Geomagnetism as well as the discussion of paleomagnetism in Plate Tectonics.) Chronostratigraphy is devoted to studying the ages of rocks and what they reveal about geologic time, or the vast stretch of history (approximately 4.6 billion years, abbreviated 4.6 Ga) over which Earth’s geologic development has occurred. It is concerned primarily with relative dating, whereas geochronometry includes the

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determination of absolute dates and time intervals. This typically calls for the use of radiometric dating.

The Stratigraphic Column The stratigraphic column is the succession of rock strata laid down over the course of time, each of which correlates to specific phases in Earth’s geologic history. The record provided by the stratigraphic column is most reliable for studying the Phanerozoic, the current eon of geologic history, as opposed to the Precambrian, which constituted the first three eons and hence the vast majority of Earth’s geologic history. The relatively brief span of time since the Phanerozoic began (about 545 million years, or Ma) has seen by far the most dramatic changes in plant and animal life. It was in this eon that the fossil record emerged, giving us far more detailed information about comparatively recent events than about a much longer span of time in the more distant past. RELATIVE AND ABSOLUTE DATI N G . Precambrian time is so designated

because it precedes the Cambrian period, one of 11 periods in the Phanerozoic eon. The Cambrian period extended for about 50 million years, from approximately 545 Ma to 495 Ma ago. This statement in terms of years, however inexact, is an example of absolute age. By contrast, if we say that the Cambrian period occurred at the beginning of the Paleozoic era, after the end of the Proterozoic eon and before the beginning of the Ordovician period, this is a statement of relative age. Both statements are true, and though it is obviously preferable to measure time in absolute terms, sometimes relative terms are the only ones available. Dating, in scientific terms, is any effort directed toward finding the age of a particular item or phenomenon. Relative dating methods assign an age relative to that of other items, whereas absolute dating determines age in actual years or millions of years. When geologists first embarked on stratigraphic studies, the only means of dating available to them were relative. Using Steno’s law of superposition, they reasoned that a deeper layer of sedimentary rock was necessarily older than a shallower layer. Advances in our understanding of atomic structure during the twentieth century, however, made possible a particularly useful absolute form

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of dating through the study of radioactive decay. Radiometric dating, which is explained in more detail in Geologic Time, uses ratios between “parent” and “daughter” isotopes. Radioactive isotopes decay, or emit particles, until they become stable, and as this takes place, parent isotopes spawn daughters. The amount of time that it takes for half the isotopes in a sample to stabilize is termed a half-life. Elements such as uranium, which has isotopes with half-lives that extend into the billions of years, make possible the determination of absolute dates for extremely old geologic materials.

Stratigraphy

D I V I S I O N S O F T H E S T RAT I G RA P H I C C O LU M N . Geologic time is

divided into named groupings according to six basic units, which are (in order of size from longest to shortest) eon, era, period, epoch, age, and chron. There is no absolute standard for the length of any unit; rather, it takes at least two ages to make an epoch, at least two epochs to compose a period, and so on. The dates for specific eons, eras, periods, and so on are usually given in relative terms, however; an example is the designation of the Cambrian period given earlier. Chronostratigraphy also uses six time units: the eonothem, erathem, system, series, stage, and chronozone. These time units are analogous to the terms in the geologic time scale, the major difference being that chronostratigraphic units are conceived in terms of relative time and are not assigned dates. The more distant in time a particular unit is, the more controversy exists regarding its boundary with preceding and successive units. This is true both of the geologic and the chronostratigraphic scales. For this reason, the International Union of Geological Sciences, the leading worldwide body of geologic scientists, has established a Commission on Stratigraphy to determine such boundaries. The commission selects and defines what are called Global Stratotype Sections and Points (GSGPs), which are typically marine fossil formations. Because it is believed that life has existed longest on Earth in its oceans, samples from the water provide the most reliable stratigraphic record.

Naming of Chronostratigraphic Units As noted, the chronostratigraphic divisions correspond to units of geologic time, even though

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chronostratigraphic units are based on relative dating methods and geologic ones use absolute time measures. Because attempts at relative dating have been taking place since the late eighteenth century, today’s geologic units originated as what would be called stratigraphic or chronostratigraphic units. Even today the names of the phases are the same, with the only difference being the units in which they are expressed. Thus, when speaking in terms of geologic time, one would refer to the Jurassic period, whereas in stratigraphic terms, this would be the Jurassic system. In 1759 the Italian geologist Giovanni Arduino (1714–1795) developed the idea of primary, secondary, and tertiary groups of rocks. Though the use of the terms primary and secondary has been discarded, vestiges of Arduino’s nomenclature survive in the modern designation of the Tertiary subera of the Cenozoic era (erathem in stratigraphic terminology) as well as in the name of the present period or system, the Quaternary. (Just as primary, secondary, and tertiary refer to a first, second, and third level, respectively, the term quaternary indicates a fourth level.) We are living in the fourth of four eons, or eonothems, the Phanerozoic, which is divided into three eras, or erathems: Paleozoic, Mesozoic, and Cenozoic. These eras, in turn, are divided into 11 periods, or systems, whose names (except for Tertiary and Quaternary) refer to the locations in which the respective stratigraphic systems were first observed. The names of these systems, along with their dates in millions of years before the present and the origin of their names, are as follows (from the most distant to the most recent): Periods/Systems of the Paleozoic Era/ Erathem • Cambrian (about 545 to 495 Ma): Cambria, the Roman name for the province of Wales • Ordovician (about 495 to 443 Ma): Ordovices, the name of a Celtic tribe in ancient Wales • Silurian (about 443 to 417 Ma): Silures, another ancient Welsh Celtic tribe • Devonian (about 417 to 354 Ma): Devonshire, a county in southwest England • Mississippian (a subperiod of the Carboniferous period, about 354 to 323 Ma): the Mississippi River

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• Pennsylvanian (a subperiod of the Carboniferous, about 323 to 290 Ma): the state of Pennsylvania • Permian (about 290 to 248.2 Ma): Perm, a province in Russia Periods/Systems of the Mesozoic Era/ Erathem • Triassic (about 248.2 to 205.7 Ma): a tripartite, or threefold, division of rocks in Germany • Jurassic (about 205.7 to 142 Ma): the Jura Mountains of Switzerland and France • Cretaceous (about 142 to 65 Ma): from a Latin word for “chalk,” a reference to the chalky cliffs of southern England and France Within the more recent Cenozoic era, or erathem, names of epochs (or “series” in stratigraphic terminology) become important. They are all derived from Greek words, whose meanings are given below: Epochs/Series of the Cenozoic Era/Erathem • Paleocene (about 65 to 54.8 Ma): “early dawn of the recent” • Eocene (about 54.8 to 33.7 Ma): “dawn of the recent” • Oligocene (about 33.7 to 23.8 Ma): “slightly recent” • Miocene (about 23.8 to 5.3 Ma): “less recent” • Pliocene (about 5.3 to 1.8 Ma): “more recent” • Pleistocene (about 1.8 to 0.01 Ma): “most recent” • Holocene (about 0.01 Ma to present): “wholly recent”

REAL-LIFE A P P L I C AT I O N S Correlation The geologist studying the stratigraphic record is a sort of detective, looking for clues. Just as detectives have their methods for solving crimes, geologists rely on correlation, or methods of establishing age relationships between various strata. There are two basic types of correlation: physical correlation, which requires comparison of the physical characteristics of the strata, and fossil correlation, the comparison of fossil types.

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Stratigraphy

A

POLARIZED -LIGHT MICROGRAPH SHOWS FOSSILS IN LIMESTONE DATING TO THE

EOCENE

AND

OLIGOCENE

EPOCHS.

(© A. Pasieka/Photo Researchers. Reproduced by permission.)

Actually, chronostratigraphic work is very similar some of the toughest cases confronted by police detectives, because more often than not the geologic detective has little evidence on which to operate. First of all, as noted earlier, only sedimentary rock can be used in making such determinations: for instance, igneous rock in its molten form, as when it is expelled from a volcano, could force itself underneath a rock stratum, thus confusing the stratigraphic record. POTENTIAL P I T FA L L S . Even when the rock is sedimentary, there is still plenty of room for error. The layers may be many feet or less than an inch deep, and it is up to the geologist to determine whether the stratum has been affected by such geologic forces as erosion. If ero-

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sion has occurred, it can cause a disturbance, or unconformity (discussed later), which tends to render inaccurate any reading of the stratigraphic record. Another possible source of disturbance is an earthquake, which could cause one part of Earth’s crust to shift over an adjacent section, making the stratigraphic record difficult, if not impossible, to read. Under the best of conditions, after all, the strata are hardly neat, easily defined lines. If one observes a horizontal section, there is likely to be a change in thickness, because as the stratum extends outward, it merges with the edges of adjacent deposits. Yet another potential pitfall in stratigraphic correlation involves one of the most useful tools

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KEY TERMS The absolute age of

longer than an age and a chron. An epoch

a geologic phenomenon is its age in Earth

is equivalent to a series in the stratigraphic

years. Compare with relative age.

time scale. The current epoch is the

ABSOLUTE AGE:

BIOSTRATIGRAPHY:

An

area

of

stratigraphy involving the study of fos-

(10,000 years) ago.

silized plants and animals in order to

ERA:

establish dates for and correlations

logic time, after an eon, and equivalent to

between stratigraphic layers.

an erathem in the stratigraphic time scale.

The second-longest phase of geo-

A subdis-

The current eon, the Phanerozoic, has had

cipline of stratigraphy devoted to studying

three eras, the Paleozoic, Mesozoic, and

the relative ages of rocks. Compare with

Cenozoic, which is the current era. The

geochronometry.

next-smallest subdivision of geologic time

CHRONOSTRATIGRAPHY:

CORRELATION:

A method of estab-

is the period. The movement of soil and

lishing age relationships between various

EROSION:

rock strata. There are two basic types of

rock due to forces produced by water,

correlation: physical correlation, which

wind, glaciers, gravity, and other influ-

requires comparison of the physical char-

ences.

acteristics of the strata, and fossil correla-

GA:

tion, the comparison of fossil types.

years” or “billion years.” The age of Earth is

DATING:

Any effort directed toward

An abbreviation meaning “giga-

about 4.6 Ga. An area of

finding the age of a particular item or phe-

GEOCHRONOMETRY:

nomenon. Methods of geologic dating are

stratigraphy devoted to determining

either relative (i.e., comparative and usual-

absolute dates and time intervals. Compare

ly based on rock strata) or absolute. The

with chronostratigraphy.

latter, based on such methods as the study

GEOLOGIC MAP:

A map showing the

of radioactive isotopes, usually is given in

rocks beneath Earth’s surface, including

terms of actual years or millions of years.

their distribution according to type as well

EON:

The longest phase of geologic

time, equivalent to an eonothem in the stratigraphic time scale. Earth’s history has

as their ages, relationships, and structural features. GEOLOGIC TIME:

The vast stretch of

consisted of four eons, the Hadean or

time over which Earth’s geologic develop-

Priscoan, Archaean, Proterozoic, and

ment has occurred. This span (about 4.6

Phanerozoic. The next-smallest subdivi-

billion years) dwarfs the history of human

sion of geologic time is the era.

existence, which is only about two million

The fourth-longest phase of

years. Much smaller still is the span of

geologic time, shorter than an era and

human civilization, only about 5,500 years.

EPOCH:

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Holocene, which began about 0.01 Ma

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Stratigraphy

KEY TERMS HISTORICAL GEOLOGY:

The study

CONTINUED

PERIOD:

The third-longest phase of

of Earth’s physical history. Historical

geologic time, after an era; it is equivalent

geology is one of two principal branches

to a system in the stratigraphic time scale.

of geology, the other being physical geol-

The current eon, the Phanerozoic, has had

ogy.

11 periods, and the current era, the CenoAtoms that have an equal

zoic, has consisted of three periods, of

number of protons, and hence are of the

which the most recent is the Quaternary.

same element, but differ in their number of

The next-smallest subdivision of geologic

neutrons. This results in a difference of

time is the epoch.

ISOTOPES:

mass. An isotope may be either stable or PRECAMBRIAN TIME:

radioactive. LAW

OF

A term that

refers to the first three of four eons in FAUNAL

SUCCESSION:

The principle that all samples of any given

Earth’s history, which lasted from about 4,560 Ma to about 545 Ma ago.

fossil species were deposited on Earth, A term describing a

regardless of location, at more or less the

RADIOACTIVITY:

same time. This makes it possible to corre-

phenomenon whereby certain materials

late widely separated strata.

are subject to a form of decay brought

LAW

OF

SUPERPOSITION:

The

principle that strata are deposited in a sequence such that the deeper the layer, the older the rock. This is applicable only or sedimentary rock, as opposed to igneous or metamorphic rock.

about by the emission of high-energy particles or radiation. Forms of particles or energy include alpha particles (positively charged helium nuclei), beta particles (either electrons or subatomic particles called positrons), or gamma rays, which occupy the highest energy level in the elec-

LITHOSTRATIGRAPHY:

An area of

tromagnetic spectrum.

stratigraphy devoted to the study and A method

description (but not the dating) of rock

RADIOMETRIC DATING:

layers.

of absolute dating using ratios between

MA:

An abbreviation used by earth sci-

entists, meaning “million years” or “megayears.” When an event is designated as, for instance, 160 Ma, it usually means 160 million years ago.

“parent” isotopes and “daughter” isotopes, which are formed by the radioactive decay of parent isotopes. RELATIVE AGE:

The relative age of a

geologic phenomenon is its age compared The study of fos-

with the ages of other geologic phenome-

silized plants and animals, or flora and

na, particularly the stratigraphic record of

fauna.

rock layers. Compare with absolute age.

PALEONTOLOGY:

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Stratigraphy

KEY TERMS SEDIMENT:

Material deposited at or

CONTINUED

STRATIGRAPHIC

COLUMN:

The

near Earth’s surface from a number of

succession of rock strata laid down over

sources, most notably preexisting rock.

the course of time, each of which correlates

SEDIMENTARY ROCK:

Rock formed

by compression and deposition (i.e., formation of deposits) on the part of other rock and mineral particles. Sedimentary

to specific junctures in Earth’s geologic history. STRATIGRAPHY:

The study of rock

layers, or strata, beneath Earth’s surface.

rock is one of the three major types of rock, along with igneous and metamorphic. SEDIMENTOLOGY:

The study and

interpretation of sediments, including sedimentary processes and formations.

An apparent gap in

the geologic record, as revealed by observing rock layers or strata. WEATHERING:

The breakdown of

Layers, or beds, of rocks

rocks and minerals at or near the surface of

beneath Earth’s surface. The singular form

Earth due to physical or chemical process-

is stratum.

es, or both.

STRATA:

available to a geologist attempting to find an absolute age for the materials he or she is studying: radiometric dating. Though this method can provide accurate absolute dates, it is quite possible that the age thus determined will be the age of the parent rock from which a sample is taken, not the age of the sample itself. The grains of sand in a piece of sandstone, for instance, are much older than the larger unit of sandstone, and for this reason, radiometric dating is useful only in specific circumstances. P H Y S I CA L A N D F O SS I L C O R R E LAT I O N . Given all these challenges, it is a

wonder that geologists manage to correlate strata successfully, yet they do. Physical correlations are achieved on the basis of several criteria, including color, the size of grains, and the varieties of minerals found within a stratum. By such means, it is sometimes possible to correlate widely separated strata. Particularly impressive feats of correlation can result from the study of fossils, whose stratigraphic implications, as we have noted, were first discovered by William Smith. Smith hit upon the idea of biostratigraphy while excavating land for a set of canals near London. As he discovered, any given stratum contains the same types of fossils,

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UNCONFORMITY:

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and strata in two different areas thus can be correlated. Long before his countryman Charles Darwin (1809–1882) developed the theory of evolution, Smith conceived his own law of faunal succession, which hints at the idea that species developed and disappeared over given phases in Earth’s past. According to the law of faunal succession, all samples of any given fossil species were deposited on Earth, regardless of location, at more or less the same time. As a result, if a geologist finds a stratum in one area that contains a particular fossil and another in a distant area containing the same fossil, it is possible to conclude that the strata are the same.

Unconformities In discussing the many challenges facing a geologist studying stratigraphic data, the role of erosion was noted. Let us return to that subject, because erosion is a source of what are known as unconformities, or gaps in the rock record. Unconformities are of three types: angular unconformities, disconformities, and nonconformities. Angular unconformities involve a tilting of the layers, such that an upper layer does not lie perfectly parallel to a lower one. Disconformities

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are more deceptive, because the layers are parallel, yet there is still an unconformity between them, and only a study of the fossil record can reveal the unconformity. Finally, a nonconformity arises when sedimentary rocks are divided from a type of igneous rock known as intrusive (meaning “cooled within Earth”). A N G U LA R U N C O N F O R M I T I E S .

Angular unconformities emerge as a by-product of the dramatic shifts and collisions that take place in plate tectonics (see Plate Tectonics). Sediment accumulates and then, as a result of plate movement, is moved about and eventually experiences weathering and erosion. Layers are tilted and then flattened by more erosion, and as the solid earth rises or sinks, they are shifted further. Such is the case, for instance, along the Colorado River at the Grand Canyon, where angular unconformities reveal a series of movements over the years. Another famous angular unconformity can be found at Siccar Point in Scotland, where nearly horizontal deposits of sandstone rest atop nearly vertical ones of graywacke, another sedimentary rock. Observations of this unconformity led the great geologist James Hutton (1726–1797) to the realization that Earth is much, much older than the 6,000 years claimed by theologians in his day (see Historical Geology).

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WHERE TO LEARN MORE

Stratigraphy

Bishop, A. C., A. Woolley, and A. Hamilton. Cambridge Guide to Minerals, Rocks, and Fossils. New York: Cambridge University Press, 1992. Boggy’s Links to Stratigraphy and Geochronology (Web site). . Harris, Nicholas, Alessandro Rabatti, and Andrea Ricciardi. The Incredible Journey to the Beginning of Time. New York: Peter Bedrick Books, 1998. Lamb, Simon, and David Sington. Earth Story: The Shaping of Our World. Princeton, NJ: Princeton University Press, 1998. MacRae, Andrew. Radiometric Dating and the Geological Time Scale (Web site). . Reeves, Hubert. Origins: Cosmos, Earth, and Mankind. New York: Arcade, 1998. Spickert, Diane Nelson, and Marianne D. Wallace. Earthsteps: A Rock’s Journey Through Time. Golden, CO: Fulcrum Kids, 2000. Stratigraphy and Earth History—West’s Geology Directory (Web site). . University of Georgia Stratigraphy Lab (Web site). . Web Time Machine. UCMP (University of California, Berkeley, Museum of Paleontology) (Web site). .

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PA L E O N T O L O G Y Paleontology

CONCEPT Thanks to a certain 1993 blockbuster, most people know the name of at least one period in geologic history. Jurassic Park spurred widespread interest in dinosaurs and, despite its fantastic plot, encouraged popular admiration and respect for the work of paleontologists. Paleontology is the study of life-forms from the distant past, as revealed primarily through the record of fossils left on and in the earth. It is a complex and varied subdiscipline of historical geology that is tied closely to the biological sciences. As with other types of historical geology, the work of a paleontologist is similar to that of a detective investigating a case with few and deceptive clues. Reliable fossil samples are more rare, compared with the vast number of species that have lived on Earth, than one might imagine. Furthermore, several factors pose challenges for paleontologists attempting to interpret the fossil record. Nonetheless, paleontologic research has led to a growing understanding of how life emerged, how Earth has changed, and how vast animal populations became extinct over relatively short periods of time.

In another example, geologic time is compared to the distance from Los Angeles to New York City. On this scale, the period of time in which humans have existed on the planet would be equivalent to the distance from New York’s Central Park to the Empire State Building, or less than 2 mi. (3.2 km). The history of human civilization, on the other hand, would be less than 16 ft. (4.9 m) long.

The term geologic time refers to the great sweep of Earth’s history, a timescale that dwarfs the span of human existence. The essays Historical Geology and Geologic Time offer several comparisons to emphasize the proportions involved and to illustrate the very short period during which human life has existed on this planet.

T H E “A BY SS O F T I M E . ” Needless to say, the scope of geologic time compared with the units with which we are accustomed to measuring our lives (or even the history of our civilization) is more than a little intimidating. This fact perhaps was best expressed by the Scottish geologist John Playfair (1748–1819), friend and countryman of the “father of geology,” James Hutton (1726–1797). At a time when many people were content to believe that the Earth had been around no more than 6,000 years (see Historical Geology), Hutton suggested that to undergo the complex processes that had shaped its landforms, the planet had to be much, much older. Commenting on Hutton’s discoveries, Playfair said, “The mind seemed to grow giddy by looking so far into the abyss of time.”

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HOW IT WORKS Thinking in Terms of Geologic Time

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As one example shows, if all of geologic time were compressed into a single year, the first Homo sapiens would have appeared on the scene at about 8:00 p.m. on December 31. Human civilization, which dates back about 5,500 years (a millennium before the building of Egypt’s great pyramids) would have emerged within the last minute of the year.

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Paleontology

THE HEAD OF A TYRANNOSAURUS REX (“king of the terrible lizards”) found in Montana, dating to the Mesozoic period. (© Tom McHugh/Photo Researchers. Reproduced by permission.)

Carbon: The Meaning of “Life” A discussion of life on Earth requires us to go deep into this “abyss,” though not nearly as far back as the planet’s origins. It does appear that life on Earth existed at a very early point, but in this context “life” refers merely to molecules of carbon-based matter capable of replicating themselves. Knowledge of these very early forms is extremely limited. Carbon appears in all living things, in things that were once living, and in materials produced by living things (for example, sap, blood, and urine). Hence, the term organic, which once meant only living matter, refers to almost all types of material containing carbon. The only carbon-containing materials that are not considered organic are oxides, such as carbon dioxide and carbon monoxide, and carbonates, a class of minerals that is extremely abundant on Earth.

major divisions of geologic time. Though extremely primitive life-forms existed before the Phanerozoic eon, the vast majority of species have evolved since it began, and consequently paleontological work is concerned primarily with the Phanerozoic eon. The divisions of geologic time are not arranged in terms of strict mathematical relationships of the type to which we are accustomed, for example, ten years in a decade, ten decades in a century, and so on. Instead, each era consists of two or more periods, each period consists of two or more epochs, and so on. The first 4,000 million years or so of Earth’s existence (abbreviated as 4,000 Ma, or 4 Ga) are known as Precambrian time. In discussing this period of time, the vast majority of the planet’s history, it is seldom necessary to speak of geologic time divisions smaller than the largest unit, the eon. Precambrian time consisted of three eons, the Hadean or Priscoan, Archaean, and Proterozoic.

We will return to the subject of carbon, which plays a role in one technique for dating relatively recent items or phenomena. For the present, however, let us set our bearings for a discussion of the Phanerozoic eon, the fourth and last of the

T H E F I R S T T H R E E E O N S . The Hadean (sometimes called the Priscoan and dating to about 4,560 Ma to 4,000 Ma ago) saw the formation of the planet and the beginnings of the oceans and an early form of atmosphere that consisted primarily of carbon dioxide. It was during this eon that the carbon-based matter

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referred earlier made its appearance, perhaps by means of the meteorites that bombarded the planet during that long-ago time. In the Archaean eon (about 4,000 Ma to 2,500 Ma ago) the first clear evidence of life appeared in the form of microorganisms. These were prokaryotes, or cells without a nucleus, which eventually were followed by eukaryotes, or cells with a nucleus. Many of the prerequisites for life as we know it were established during this time, though our present oxygen-containing atmosphere still lay far in the future. Longest of the four eons was the Proterozoic eon (about 2,500 Ma to 545 Ma). This phase saw the beginnings of very basic forms of plant life, while oxygen in the atmosphere assumed about 4% of its present levels. Animal life, meanwhile, still consisted primarily of eukaryotes.

The Phanerozoic Eon The majority of paleontologic history has taken place during the Phanerozoic eon. In the course of this essay, we discuss its eras and periods (the second- and third-longest spans of geologic time, respectively) as they relate to life on Earth. The three Phanerozoic eras are as follows: Eras of the Phanerozoic Eon • Paleozoic (about 545–248.2 Ma) • Mesozoic (about 248.2–65 Ma) • Cenozoic (about 65 Ma–present) Within these eras are the following periods: Periods of the Paleozoic Era • • • • • •

Cambrian (about 545–495 Ma) Ordovician (about 495–443 Ma) Silurian (about 443–417 Ma) Devonian (about 417–354 Ma) Carboniferous (about 354–290 Ma) Permian (about 290–248.2 Ma) Periods of the Mesozoic Era

EPOCHS OF THE CENOZOIC E RA . An epoch is the fourth-largest division of

geologic time and is, for the most part, the smallest one with which we will be concerned. (There are two smaller categories, the age and the chron.) Listed here are the epochs of the Cenozoic era from the most distant to the Holocene, in which we are now living. Their names are derived from Greek words whose meanings are provided: Epochs of the Cenozoic Era • Paleocene (about 65–54.8 Ma): “early dawn of the recent” • Eocene (about 54.8–33.7 Ma): “dawn of the recent” • Oligocene (about 33.7–23.8 Ma): “slightly recent” • Miocene (about 23.8–5.3 Ma): “less recent” • Pliocene (about 5.3–1.8 Ma): “more recent” • Pleistocene (about 1.8–0.01 Ma): “most recent” • Holocene (about 0.01 Ma to present): “wholly recent”

A Brief Overview of Paleontologic History

• Palaeogene (about 65–23.8 Ma) • Neogene (about 23.8–1.8 Ma) • Quaternary (about 1.8 Ma to the present) The Carboniferous period of the Paleozoic era usually is divided into two subperiods, the Mississippian (about 354 to 323 Ma) and the

The title “A Brief Overview of Paleontologic History” is almost a contradiction in terms, since virtually nothing about the history of Earth has been brief. Moreover, the history of life on Earth is so filled with detail and complexity that it could fill many books, as indeed it has. Owing to that complexity, anything approaching an exhaustive treatment of the subject would burden the reader with so much technical terminology that it would obscure the larger overview of paleontology and the materials of the paleontologist’s work. Therefore, only the most cursory of treatments is possible, or indeed warranted, in the present context. For additional detail, the reader is invited to consult other texts, including

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• Triassic (about 248.2–205.7 Ma) • Jurassic (about 205.7–142 Ma) • Cretaceous (about 142–65 Ma) Periods of the Cenozoic Era

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Pennsylvanian (about 323–290 Ma). In addition, the Palaeogene and Neogene periods of the Cenozoic era often are lumped together as a subera called the Tertiary. By substituting that name for those of the two periods, it is possible to use a time-honored mnemonic device by which geology students have memorized the names of the 11 Phanerozoic periods: “Camels Ordinarily Sit Down Carefully; Perhaps Their Joints Creak Tremendously Quietly.”

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A

DINOSAUR IS EXCAVATED AT

DINOSAUR NATIONAL MONUMENT

IN

COLORADO. (© James L. Amos/Photo Researchers. Repro-

duced by permission.)

those listed in the suggested reading section at the end of this essay.

Ma–410 Ma, life had appeared on land, in the form of algae and primitive insects.

As with many another process, the evolution of organisms was exceedingly slow in the beginning (and here the comparative term slow refers even to the standards of geologic time), but it sped up considerably over the course of Earth’s history. This is not to suggest that the development of life-forms has been a steady process; on the contrary, it has been punctuated by mass extinctions, discussed at the conclusion of this essay. Nonetheless, it is correct to say that during the first 80%–90% of Earth’s history, the few existing life-forms underwent an extremely slow process of change.

The beginning of the Devonian period (approximately 417 Ma) saw the appearance of the first vertebrates (animals with an internal skeleton), which were jawless fish. Plant life on land consisted of ferns and mosses. By the late Devonian (about 360 Ma), fish had evolved jaws, and amphibians had appeared on land. Reptiles emerged between about 320 Ma and 300 Ma, in the Pennsylvanian subperiod of the Carboniferous. In the last period of the Paleozoic era, the Permian (about 290–248.2 Ma), reptiles became the dominant land creatures.

P R E C A M B R I A N A N D PA L E O Z O I C L I F E - F O R M S . Life existed in Pre-

MESOZOIC AND CENOZOIC L I F E - F O R M S . The next era, the Mesozoic

cambrian time, as noted, but over the course of those four billion years, it evolved only to the level of single-cell microorganisms. Samples of these organisms have been found in the fossil record, but the fossilized history of life on Earth really began in earnest only with the Cambrian period at the beginning of the Paleozoic era and the Phanerozoic eon. The early Cambrian period saw an explosion of invertebrate (without an internal skeleton) marine forms, which dominated from about 545 Ma–417 Ma ago. By about 420

(about 248.2–65 Ma), belonged to a particularly impressive form of reptile, known as the terrible lizard: the dinosaur. These creatures are divided into groups based on the shape of their hips, which were either lizardlike or birdlike. Though the lizardlike Saurischia emerged first, they lived alongside the birdlike Ornithischia throughout the late Triassic, Jurassic, and Cretaceous periods. Ornithischia were all herbivores, or plant eaters, whereas Saurischia included both herbivores and carnivores, or meat eaters. Naturally, the most fierce of the dinosaurs were carnivores, a group

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that included the largest carnivore ever to walk the earth, Tyrannosaurus.

Classifying Plants and Animals

Though dinosaurs receive the most attention, the Mesozoic world was alive with varied forms, including flying reptiles and birds. (In fact, dinosaurs may have been related to birds, and, in the opinion of some paleontologists, they may have been warm-blooded, like birds and mammals, rather than cold-blooded, like other reptiles.) Botanical life included grasses, flowering plants, and trees of both the deciduous (leaf-shedding) and coniferous (cone-bearing) varieties.

Given the close relationship between paleontology and the biological sciences, it is necessary to discuss briefly the taxonomic system applied in biology, botany, zoology, and related fields. Taxonomy is an area of biology devoted to the identification, classification, and naming of organisms. Devised in the eighteenth century by the Swedish botanist Carolus Linnaeus (1707–1778) and improved in succeeding years by many others, the taxonomic system revolutionized biology.

A violent event, discussed in the context of mass extinction later in this essay, brought an end to the Mesozoic era. This cleared the way for the emergence of mammalian forms at the beginning of the Cenozoic, though it still would be a long time before anything approaching an ape, let alone a human, appeared on the scene. The earliest hominid, or humanlike creature, dates back to about four million years ago, in the Pliocene epoch of the Neogene period.

Historical Geology and Paleontology One of the two principal divisions of geology (along with physical geology) is historical geology, the study of Earth’s physical history. Other subdisciplines of historical geology are stratigraphy, the study of rock layers, or strata, beneath Earth’s surface; geochronology, the study of Earth’s age and the dating of specific formations in terms of geologic time; and sedimentology, the study and interpretation of sediments, including sedimentary processes and formations.

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Linnaeus’s taxonomy provided a framework for classifying known species not simply by superficial similarities but also by systemic characteristics. For example, worms and snakes have something in common on a surface level, because they are both without appendages and move by writhing on the ground. A worm is an invertebrate, however, whereas a snake is a vertebrate. The Linnaean system therefore would classify them in widely separated categories: they are not siblings or even first cousins but more like fourth cousins. Moreover, the system created by Linnaeus gave scientists a means for classifying and thereby potentially understanding much about the history and characteristics of species as yet undiscovered. Thus, it would prove of immeasurable significance to the English naturalist Charles Darwin (1809–1882) in formulating his theory of evolution. As Darwin showed, the varieties of different organisms have increased over time, as those organisms developed characteristics that made them more adaptable to their environments. Plants and animals that failed to adapt simply became extinct, though failure to adapt is only one of several causes for extinction, as we shall see.

Paleontology, the investigation of life-forms from the distant past (primarily through the study of fossilized plants and animals), is another subdiscipline of historical geology. Though it is rooted in the physical sciences, it obviously crosses boundaries into the biological or life sciences as well. Related or subordinate fields include paleozoology, which focuses on the study of prehistoric animal life; paleobotany, the study of past plant life; and paleoecology, the study of the relationship between prehistoric plants and animals and their environments.

system uses binomial nomenclature, or a twopart naming scheme (in Latin), to identify each separate type of organism. If a man is named John Smith, then “Smith” identifies his family, while John identifies him singularly. Likewise each variety of organism is identified by genus, equivalent to Smith, and species, analogous to John. In the Linnaean system, there are eight levels of classification, which, from most general to most specific, are kingdom, phylum, subphylum, class, order, family, genus, and species.

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These levels can be illustrated by identifying a species near and dear to all of us: Homo sapiens, commonly known as humans. We belong to the animal kingdom (Animalia), the Chordata (i.e., possessing some form of central nervous system) phylum, and the Vertebrata subphylum, indicating the existence of a backbone. Within the mammal (Mammalia) class we are part of the primate (Primata) order, along with apes. Humans are distinguished further as members of the hominid (Hominoidea), or “human-like” family; the genus Homo (“man”); and the species sapiens (“wise”).

Dating Materials From the Past In studying the past, paleontologists and other earth scientists working in the field of historical geology rely on a variety of dating techniques. “Dating,” in a scientific context, usually refers to any effort directed toward finding the age of a particular item or phenomenon. It may be relative, devoted to finding an item’s age in relation to that of other items; or absolute, involving the determination of age in actual years or millions of years. Among the methods of relative dating are stratigraphic dating, discussed in the essay Stratigraphy, as well as seriation, faunal dating, and pollen dating. Seriation entails analyzing the abundance of a particular item and assigning relative dates based on that abundance. Faunal dating is the use of bones from animals (fauna) to determine age, and pollen dating, or palynology, analyzes pollen deposits. FAU N A L DAT I N G A N D PA LY N O L O G Y. The concept of faunal dating

emerged from early work by the English engineer and geologist William Smith (1769–1839), widely credited as the “father of stratigraphy.” In particular, Smith established an important division of stratigraphy, known as biostratigraphy, that is closely tied to paleontology. While excavating land for a set of canals near London, he discovered that any given stratum, or rock layer, contains the same types of fossils, and therefore strata in two different areas can be correlated.

one area that contains a particular fossil and another in a distant area containing the same fossil, it is possible to conclude that the strata are the same.

Paleontology

Pollen dating, or palynology, is based on the fact that seed-bearing plants release large numbers of pollen grains each year. As a result, pollen spreads over the surrounding area, and in many cases pollen from the distant past has been preserved. This has occurred primarily in lake beds, peat bogs, and, occasionally, in areas with cool or acidic soil. By observing the species of pollen deposited in an area, scientists are able to develop a sort of “pollen calendar,” which provides information about such details as changes in climate. D E N D R O C H R O N O L O G Y. Scientists use relative dating when they must, but they would prefer to determine dates in an absolute sense wherever possible. Most methods of absolute dating rely on processes that are not immediately comprehensible to the average person, but there is one exception: dendrochronology, or the dating of tree rings. As almost everyone knows, trees produce one growth ring per year. There is nothing magical about this, since a year is not an abstract unit of time; rather, it is based on Earth’s revolution around the Sun, during which time the planet undergoes changes in orientation that result in the four seasons, which, in turn, affect the tree’s growth.

Though dendrochronology makes use of a principle familiar to most people, the work of the dendrochronologist requires detailed, often complex study. Just as the layers of rock beneath Earth’s surface reveal information about past geologic events (a matter discussed in the essay Stratigraphy), tree rings can tell us much about environmental changes. Thin rings, for instance, suggest climatic anomalies and may provide clues about cataclysmic events that were understood only vaguely by the ancient humans who experienced them. (An example of this is the apparent cataclysm of A.D. 535, which is discussed Earth Systems.) AMINO-ACID

R A C I M I Z AT I O N .

Smith stated this in what became known as the law of faunal succession: all samples of any given fossil species were deposited on Earth, regardless of location, at more or less the same time. As a result, if a geologist finds a stratum in

Dendrochronology is useful only for studying the relatively recent past, up to about 10,000 years— a span equivalent to the Holocene epoch, which began with the end of the last ice age. To investigate more distant phases of Earth’s history, it is necessary to use forms of radiometric dating, which we will discuss shortly. The principles of

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radiometric dating, however, are illustrated by another method, amino-acid racimization. With the exception of some microbes, living organisms incorporate only one of two forms of amino acids, known as L-forms. Once the organism dies, the L-amino acids gradually convert to D-amino acids. In the 1960s, scientists discovered that by comparing the ratios between the Land D-forms, it was possible to date organisms that were several thousand years old. Unfortunately, it has since come to light that because of the many factors affecting the rate of amino-acid conversion, this method is less reliable than once was believed. Moisture, temperature, and pH (the relative acidity and alkalinity of a substance) all play a part, and because these factors vary so widely, amino-acid racimization no longer is used commonly. Nonetheless, the basic principle behind amino-acid racimization plays a part in other, more reliable forms of absolute dating. Many of them are based on the fact that over time, a particular substance converts to another, mirror substance. By comparing the ratios between them, it is possible to arrive at some estimate of the amount of time that has elapsed since the organism died. R A D I O C A R B O N D AT I N G . The most significant method of absolute dating available to scientists today is radiometric dating, which is explained in detail in the essay Geologic Time. Each chemical element is distinguished by the number of protons (positively charged particles) in its atomic nucleus, but atoms of a particular element may have differing numbers of neutrons, or neutrally charged particles, in their nuclei. Such atoms are referred to as isotopes.

Certain isotopes are stable, whereas others are radioactive, meaning that they are likely to eject particles from the nucleus over time. The amount of time it takes for half the isotopes in a sample to stabilize is called its half-life. By analyzing the quantity of radioactive isotopes in a given sample that have converted to stable isotopes, it is possible to determine the age of the sample. In other situations, it is necessary to compare ratios of unstable “parent” isotopes to even more unstable “daughter” isotopes produced by the parent.

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tive form of carbon. All atoms of carbon have six protons, and the most stable and abundant carbon isotope is carbon-12, so designated because it has six neutrons. On the other hand, carbon14, with eight neutrons, is unstable. When an organism is alive, it incorporates a certain ratio of carbon-12 in proportion to the (very small) amount of carbon-14 that it receives from the atmosphere. Once the organism dies, however, it stops incorporating new carbon, and the ratio between carbon-12 and carbon-14 begins to change as the carbon-14 decays to form nitrogen-14. Therefore, a scientist can use the ratios of carbon-12, carbon-14, and nitrogen-14 to estimate the age of an organic sample. This method is known as radiocarbon dating. Carbon-14, or radiocarbon, has a half-life of 5,730 years, meaning that it is useful for analyzing only fairly recent samples. Nonetheless, it takes much longer than 5,730 years for the other half of the radiocarbon isotopes in a given sample to stabilize, and for this reason radiocarbon dating can be used with considerable accuracy for 30,000–40,000 years. Sophisticated instrumentation can extend this range even further, up to 70,000 years. THE LIMITS OF ABSOLUTE DAT I N G . While 70,000 years, or 0.07 Ma, may

be a long time in human terms, from the standpoint of the earth scientist, 0.07 Ma is only yesterday—the latter part of the last epoch, the Pleistocene. Other forms of radiometric dating, such as potassium-argon dating and uraniumseries dating, can be used to measure truly long spans of times, in the billions of years. These methods are discussed in Geologic Time. Potassium-argon dating and uranium-series dating can be useful to the paleontologist, inasmuch as they aid in determining the age of the geologic samples in which the remains of lifeforms are found. Nothing is simple, however, when it comes to dating specimens from the distant past. After all, geologists working in the realm of stratigraphy face numerous challenges in judging the age of samples, even with these sophisticated forms of radiometric dating. (For more on this subject, see Stratigraphy.)

As we noted earlier, carbon is present in all living things, and thus an important means of dating available to paleontologists uses a radioac-

In fact, the work of a paleontologist is much like that of the stratigrapher. The success of either type of scientist relies more on detailed and painstaking detective work than it does on sophisticated technology. Both must analyze lay-

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Paleontology

STINGRAY NO BONE,

FOSSIL, POSSIBLY DATING TO THE

JURASSIC

PERIOD.

THIS

IS A RARE FIND, BECAUSE STINGRAYS HAVE

ONLY CARTILAGE, WHICH MAKES IT HARDER FOR THEM TO UNDERGO MINERALIZATION AND BE PRESERVED

AS FOSSILS. (© Gary Retherford/Photo Researchers. Reproduced by permission.)

ered samples from the past to form a picture of the chronology in which the samples evolved, and oftentimes the apparent evidence can be deceptive. The principal difference between stratigraphic and paleontologic study relates to the materials: rocks in the first instance and fossils in the second.

REAL-LIFE A P P L I C AT I O N S Fossils and Fossilization The term fossil refers to the remains of any prehistoric life-form, especially those preserved in rock before the end of the last ice age. The process by which a once-living thing becomes a fossil is known as fossilization. Generally, fossilization refers to changes in the hard portions, including bones, teeth, shells, and so on. This series of changes, in which minerals are replaced by different minerals, is known as mineralization. Sometimes, soft parts may experience mineralization and thus be preserved as fossils. A deceased organism in the process of becoming a fossil is known as a subfossil.

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The majority of fossils come from invertebrates, such as mussels, that possess hard parts. Generally speaking, the older and smaller the organism, the more likely it is to have experienced fossilization, though other factors (which we will discuss later) also play a part. One of the most important factors involves location: for the most part, the lower the altitude, the greater the likelihood that a region will contain fossils. The best place of all is in the ocean, particularly the ocean floor. Nonetheless, fossils have been found on every continent of Earth, and the great distances that sometimes separate samples of the same species have aided earth scientists of many fields in understanding the processes that shaped our planet. FOSSILS AND GEOLOGIC H I S T O RY. The earth beneath our feet is not

standing still; rather, it is constantly moving, and over the great stretches of geologic time, the positions of the continents have shifted considerably. The details of these shifts are discussed in Plate Tectonics, an area of geologic study that explains much about the earth, from earthquakes and volcanoes to continental drift. Paleontology has contributed to the study of plate tectonics by revealing apparent anomalies,

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such as fossilized dinosaur parts in Antarctica. No dinosaur could have lived on that forbidding continent, so there must be some other explanation: the continental plates themselves have moved. Long ago, the present continents were united in a “supercontinent” called Pangaea. When Pangaea split apart to form the present continents, the remains of various species were separated from one another and from the latitudes to which they were accustomed in life. Fossilized remains of single-cell organisms have been found in rock samples as old as 3.5 Ga, and animal fossils have been located in rocks that date to the latter part of Precambrian time, as old as 1 Ga. Just as paleontologists have benefited from studies in chronostratigraphy and geochronometry, realms of stratigraphy concerned with the dating of rock samples, stratigraphers and other geologists have used fossil samples to date the rock strata in which they were found. Not all fossilized life-forms are equally suited to this purpose. Certain ones, known as index fossils or indicator species, have been associated strongly with particular intervals of geologic time. An example is the ammonoid, a mollusk that proliferated for about 350 Ma from the late Devonian to the early Cretaceous before experiencing mass extinction. M A K I N G T H E G RA D E A S A F O SS I L . Everything that is living eventually

dies, but not nearly all living things will become fossils. And even if they do, there are numerous reasons why fossils might not be preserved in such a way as to provide meaningful evidence for a paleontologist many millions of years later. In the potential pool of candidates for fossilization, as we have noted, organisms without hard structural portions are unlikely to become fossilized. Fossilization of soft-bodied creatures sometimes occurs, however, as, for instance, at Burgess Shale in British Columbia, where environmental conditions made possible the preservation of a wide range of samples. Furthermore, location is a powerful factor. Sedimentary rock, formed by compression and deposition (i.e., formation of deposits) on the part of other rock and mineral particles, provides the setting for many fossils. Best of all is sediment, such as sand or mud, that has not yet consolidated into harder sandstone, limestone, or other rocks. Organisms that die in upland locations are more likely to be disturbed either by

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wind or by scavengers, creatures that feed on the remains of living things. On the other hand, an organism at the bottom of an ocean is out of reach from most scavengers. Even at lesser depths, if the organism is in a calm, relatively scavenger-free marine environment, there is a good chance that it will be preserved. Assuming that all the conditions are right and the dead organism is capable of undergoing fossilization, it will experience mineralization of one type or another. Living things already contain minerals, which is the reason why people take mineral supplements to augment the substances nature has placed in their bodies to preserve and extend life. In the mineralization of a fossil, the minerals in the organism’s body may be replaced by other ones, or other minerals may be added to existing ones. It is also possible that both the hard and soft parts will dissolve and be replaced by a mineral cement that forms a mold that preserves the shape of the organism. F I N D I N G A N D S T U DY I N G F O S S I L S . Only about 30% of species are ever fos-

silized, a fact that scientists must take into account, because it could skew their reading of the paleontologic record. If a paleontologist judges the past only from the fossils that have been found in an area, it will result in a picture of a past environment that contained only certain species, when, in fact, others were present. Furthermore, there are many factors that contribute to the loss of fossils. For instance, if the area has been subjected to violent tectonic activity, it is likely that the sample will be destroyed partially or wholly. The removal of a fossil from its home in the rock is a painstaking process akin to restoring a valuable piece of art. Before removing it, the paleontologist photographs the fossil and surrounding strata and records details about the environment. Only when these steps have been taken is the fossil removed. This is done with a rock saw, which is used to cut out carefully a large area surrounding the fossil. The sample is then jacketed, or wrapped in muslin with an additional layer of wet plaster, and taken to a laboratory for study. Fossil research can reveal a great deal about the history of life on Earth, including the relationships between species or between species and their habitats. Studies of dinosaur bones have brought to light proteins that existed in the bod-

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ies of these long-gone creatures, while research on certain oxygen isotopes has aided attempts to discover whether dinosaurs were warm-blooded creatures. Thanks to advances in the understanding of DNA (deoxyribonucleic acid), which provides the genetic codes for all living things, it may be possible to make even more detailed studies in the future.

Mass Extinction The remains of dinosaurs, of course, have an importance aside from their significance to paleontology. The bodies of these giant lizards have been deposited in the earth, where over time they became coal, peat, petroleum, and other fossil fuels. The latter are discussed in Economic Geology, but the fact that the dinosaurs disappeared at all is of particular interest to paleontology. Why are there no dinosaurs roaming the earth today? The answer appears to be that they were wiped out in a dramatic event, perhaps brought about as the result of a meteorite impact. Numerous species have become extinct, typically as a result of their inability to adapt to changes in their natural environment. More recently, some extinctions or endangerments of species have been attributed to human activities, including hunting and the disruption of natural habitats. For the most part, however, extinction is simply a part of Earth’s history, a result of the fact that nature has a way of destroying organisms that do not adapt (the “survival of the fittest”). But there have been occasions in the course of the planet’s past in which vast numbers of individuals and species perished at once. A natural catastrophe may destroy a large population of individuals within a locality, a phenomenon known as mass mortality. Or mass mortality may take place on a global scale, destroying many species, in which case it is known as mass extinction. CAUSES OF MASS EXTINCT I O N . The Bible depicts an example of near

mass extinction, in the form of Noah’s flood, and, indeed, several instances of mass extinction have resulted from sudden and dramatic changes in ocean levels. Others have been caused by tectonic events, most notably vast volcanic eruptions that filled the atmosphere with so much dust that they caused a violent change in temperature. Scientific speculation concerning other such extinctions has pointed to events in or from

S C I E N C E O F E V E RY DAY T H I N G S

space—either the explosion of a star or the impact of a meteorite on Earth—as the cause of atmospheric changes and hence mass extinction.

Paleontology

Even though scientists have a reasonable idea of the immediate causes of mass extinction in some cases, their understanding of the ultimate or root causes is still limited. This fact was expressed by the University of Chicago paleobiologist David M. Raup, who wrote: “The disturbing reality is that for none of the thousands of well-documented extinctions in the geologic past do we have a solid explanation of why the extinction occurred.” E XA M P L E S O F M ASS E X T I N C T I O N . The five largest known mass extinctions

occurred at intervals of 50 Ma to 100 Ma over a span of time from about 435 to 65 million years ago. Most occurred at the end of a period, which is no accident, since geologists have used mass extinction as a factor in determining the parameters of a specific period. In the late Ordovician period, about 435 Ma ago, a drop in the ocean level wiped out onefourth of all marine families. Similarly, changes in sea level, along with climate changes, appear to have caused the destruction of one-fifth of existing marine families during the late Devonian period (about 357 Ma ago). Worst of all was the “great dying,” as the extinction at the end of the Permian period (about 250 Ma) is known. Perhaps caused by a volcanic eruption in Siberia, it eliminated a staggering 96% of all species over a period of about a million years. During the late Triassic period, about 198 million years ago, another catastrophe eliminated a quarter of marine families. Paleontologists know this, as they know about other mass extinctions, by the inordinate numbers of fossilized samples found in rock strata dating to that period. This reliance on the fossil record is also reflected in the fact that the scope of early mass extinctions usually is expressed in terms of marine life. As we have seen, the ocean environment provides the most reliable fossil record. Creatures died on land as well, but the terrestrial record is simply less reliable or less complete. Scientific disagreement over the late-Triassic mass extinction exemplifies the fact that our knowledge of these distant events is not firmly established, but rather is subject to much scientific conjecture and dispute. (This does not mean that just any old idea can compete on an equal

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Paleontology

KEY TERMS The absolute age of

correlation: physical correlation, which

a geologic phenomenon is its age in Earth

requires comparison of the physical char-

years. Compare with relative age.

acteristics of the strata, and fossil correla-

ABSOLUTE AGE:

ATMOSPHERE:

In general, an atmos-

Any effort directed toward

phere is a blanket of gases surrounding a

DATING:

planet. Unless otherwise identified, howev-

finding the age of a particular item or phe-

er, the term refers to the atmosphere of

nomenon. Methods of geologic dating are

Earth, which consists of nitrogen (78%),

either relative (i.e., comparative and usual-

oxygen (21%), argon (0.93%), and other

ly based on rock strata) or absolute. The

substances that include water vapor, car-

latter, based on such methods as the study

bon dioxide, ozone, and noble gases such

of radioactive isotopes, typically is given in

as neon, which together comprise 0.07%.

terms of actual years or millions of years.

BIOSPHERE:

A combination of all liv-

The longest phase of geologic

EON:

ing things on Earth—plants, mammals,

time. Earth’s history has consisted of four

birds, reptiles, amphibians, aquatic life,

eons, the Hadean or Priscoan, Archaean,

insects, viruses, single-cell organisms, and

Proterozoic, and Phanerozoic. The next-

so on—as well as all formerly living things

smallest subdivision of geologic time is the

that have not yet decomposed.

era.

BIOSTRATIGRAPHY:

An

area

of

EPOCH:

The fourth-longest phase of

stratigraphy involving the study of fos-

geologic time, shorter than an era and

silized plants and animals to establish dates

longer than an age or a chron. The current

for and correlations between stratigraphic

epoch is the Holocene, which began about

layers.

0.01 Ma (10,000 years) ago.

CHRONOSTRATIGRAPHY:

A subdis-

ERA:

The second-longest phase of geo-

cipline of stratigraphy devoted to studying

logic time, after an eon. The current eon,

the relative ages of rocks. Compare with

the Phanerozoic, has had three eras, the

geochronometry.

Paleozoic, Mesozoic, and Cenozoic, which

CONTINENTAL DRIFT:

The theory

is the current era. The next-smallest subdi-

that the configuration of Earth’s continents

vision of geologic time is the period.

was once different than it is today; that

FOSSIL:

some of the individual landmasses of today

any prehistoric life-form, especially those

once were joined in other continental

preserved in rock before the end of the last

forms; and that these landmasses later sep-

ice age.

arated and moved to their present locations.

The mineralized remains of

FOSSILIZATION:

The

process

by

which a once-living organism becomes a A method of estab-

fossil. Generally, fossilization involves min-

lishing age relationships between various

eralization of the organism’s hard portions,

rock strata. There are two basic types of

such as bones, teeth, and shells.

CORRELATION:

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tion, the comparison of fossil types.

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Paleontology

KEY TERMS GA:

An abbreviation meaning “gigayears,”

CONTINUED

A phenomenon

MASS EXTINCTION:

or “billion years.”The age of Earth is about 4.6

in which numerous species cease to exist at

Ga.

or around the same time, usually as the

GEOCHRONOMETRY:

An area of

result of a natural calamity.

stratigraphy devoted to determining

MINERALIZATION:

absolute dates and time intervals. Compare

changes experienced by a once-living

with chronostratigraphy.

organism during fossilization. In mineral-

The vast stretch of

GEOLOGIC TIME:

time over which Earth’s geologic development has occurred. This span (about 4.6 billion years) dwarfs the history of human

A

series

of

ization, minerals in the organism are either replaced or augmented by different minerals, or the hard portions of the organism dissolve completely. At one time, chemists used

existence, which is only about two million

ORGANIC:

years. Much smaller still is the span of

the term organic only in reference to living

human civilization, only about 5,500 years.

things. Now the word is applied to most

HISTORICAL GEOLOGY:

The study

of Earth’s physical history. Historical geology is one of two principal branches of geology, the other being physical geology. INVERTEBRATE:

An animal without

an internal skeleton.

compounds containing carbon, with the exception of carbonates (which are minerals), and oxides, such as carbon dioxide. PALEOBOTANY:

An area of paleontol-

ogy involving the study of past plant life. PALEOECOLOGY:

An area of paleon-

Atoms that have an equal

tology devoted to studying the relationship

number of protons, and hence are of the

between prehistoric plants and animals

same element, but differ in their number of

and their environments.

ISOTOPES:

neutrons. This results in a difference of

PALEONTOLOGY:

mass. An isotope may be either stable or

forms from the distant past, primarily as

radioactive.

revealed through the fossilized remains of

LAW

OF

FAUNAL

SUCCESSION:

The study of life-

plants and animals.

The principle that all samples of any given

PALEOZOOLOGY:

fossil species were deposited on Earth,

tology devoted to the study of prehistoric

regardless of location, at more or less the

animal life.

same time. This makes it possible to correlate widely separated strata.

PERIOD:

An area of paleon-

The third-longest phase of

geologic time, after an era. The current

An abbreviation used by earth sci-

eon, the Phanerozoic, has had 11 periods,

entists, meaning “million years,” or

and the current era, the Cenozoic, has con-

“megayears.” When an event is designated

sisted of three periods, of which the most

as, for instance, 160 Ma, it usually means

recent is the Quaternary. The next-smallest

160 million years ago.

subdivision of geologic time is the epoch.

MA:

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Paleontology

KEY TERMS A term that refers to the first three of four eons in Earth’s history, which lasted from about 4,560 to about 545 Ma ago. PRECAMBRIAN TIME:

A term describing a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of high-energy particles or radiation. Forms of particles or energy include alpha particles (positively charged helium nuclei), beta particles (either electrons or subatomic particles called positrons, or gamma rays, which occupy the highest energy level in the electromagnetic spectrum.

RADIOACTIVITY:

A method of absolute dating using ratios between “parent” isotopes and “daughter” isotopes, which are formed by the radioactive decay of parent isotopes. Radiometric dating also may involve ratios between radioactive isotopes and stable isotopes. RADIOMETRIC DATING:

The relative age of a geologic phenomenon is its age compared

RELATIVE AGE:

footing: we are talking here about differences of opinion among highly trained specialists.) At any rate, some scientists refer to the late-Triassic mass extinction as being one of the less exciting or eventful mass extinctions. Of course, it is hard to see how a mass extinction could be unexciting or uneventful, but they mean this in comparative terms; on the other hand, some paleontologists maintain that the late-Triassic was among the most devastating. As to the cause, some theorists point to a group of impact sites spread across Canada, the northern United States, and Ukraine, places that would have been more or less contiguous at the time of the mass extinction. Difficulties in analyzing the “signatures” left by the projectiles that made these impressions have prevented theorists

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CONTINUED

with other geologic phenomena, particularly the stratigraphic record of rock layers. Compare with absolute age. SEDIMENT:

Material deposited at or

near Earth’s surface from a number of sources, most notably preexisting rock. SEDIMENTARY ROCK:

Rock formed

by compression and deposition (i.e., formation of deposits) on the part of other rock and mineral particles. Sedimentary rock is one of the three major types of rock, along with igneous and metamorphic. SEDIMENTOLOGY:

The study and

interpretation of sediments, including sedimentary processes and formations. STRATA:

Layers, or beds, of rocks

beneath Earth’s surface. The singular form is stratum. STRATIGRAPHY:

The study of rock

layers, or strata, beneath Earth’s surface. VERTEBRATE:

An animal with an

internal skeleton.

from saying with any degree of certainty whether it was a comet or an asteroid that caused the impact. Others, in particular a team from the University of California at Berkeley led by geologist Paul R. Renne, cite a volcanic eruption as either the cause of the mass extinction, or at least a major abetting factor to an extinction already in progress. According to Renne and his team, basalt outcroppings scattered from New Jersey to Brazil to west Africa (again, areas that would have been contiguous then) suggest that a volcanic eruption of almost inconceivable magnitude occurred about 200 million years ago. Such an eruption would surely have destroyed vast quantities of living things. The last and best known mass extinction occurred about 65 million years ago, marking the

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end of the Cretaceous period—and the end of the dinosaurs. As to what happened, paleontologists and other scientists have proposed a number of theories: a rapid climate change; the emergence of new poisonous botanical species, eaten by herbivorous dinosaurs, that resulted in the passing of toxins along the food web (see Ecosystems); an inability to compete successfully with the rapidly evolving mammals; and even an epidemic disease to which the dinosaurs possessed no immunity. Interesting as many of these theories are, none has gained anything like the widespread acceptance achieved by another scenario. According to this highly credible theory, an asteroid hit Earth, hurtling vast quantities of debris into the atmosphere, blocking out the sunlight, and greatly lowering Earth’s surface temperature. Around the world, geologists have found traces of iridium deposited at a layer equivalent to the boundary between the Cretaceous and Tertiary periods, the Tertiary being the beginning of the present Cenozoic era. This is significant, because iridium seldom appears on Earth’s surface—but it is found in asteroids. H U M A N S A N D M ASS E X T I N C T I O N . There have been much more recent, if

less dramatic, examples of mass extinction, including those caused by the most highly developed of all life-forms: humans. Among these examples are the well-documented (and very recent) mass extinctions brought on by destruction of tropical rainforests. Such activities are killing off a vast array of organisms: according to the highly respected Harvard biologist Edward O. Wilson, some 17,500 species are disappearing each year. But cases of mass extinction are not limited to modern times. When prehistoric hunters (the ancestors of today’s Native Americans) crossed the Bering land bridge from Siberia to Alaska some 12,000 years ago, they found an array of species unknown in the Americas today. These species included mammoths and mastodons; giant bears, beaver, and bison; and even saber-toothed tigers, camels, and lions. Perhaps most remarkable of all, it appears that prehistoric America was once home to a creature that would prove to be of enormous benefit to humans until the beginning of the automotive age: the horse. Horses did not reappear in the Americas until

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Europeans arrived to conquer those lands after A.D. 1500.

Paleontology

Were Dinosaurs WarmBlooded? One of the most significant scientific debates of the later twentieth and early twenty-first centuries, not only in paleontology but in the earth sciences or even science itself, is the question of whether or not the dinosaurs were warm-blooded. In other words, were they like modern reptiles, which must adjust their temperature by moving into the sunlight when they are cold, and into the shade when they are too hot? Or were they more like modern birds and mammals, whose bodies generate their own heat? A warm-blooded animal always has a more or less constant body temperature, regardless of the temperature of its environment. This is due to the fact that it produces heat by the burning of food, as well as by physical activity, and stores that heat under a layer of fat just beneath the skin. Warm-blooded animals are also capable of cooling down their bodies by perspiring and panting. Birds and mammals are the only warmblooded animals; all others are cold-blooded. A cold-blooded creature, on the other hand, lacks control over its body temperature and therefore is warm when its environment is warm, and cold when its environment is cold. The difference between warm- and coldblooded animals is partly one of metabolic rate, or the rate at which nutrients are broken down and converted into energy. Cold-blooded creatures have slow metabolic rates; think of a python that swallows a medium-sized mammal whole and takes several days to digest it. The dinosaur debate is therefore often framed as a question of whether the dinosaurs’ bodies had a relatively high or relatively low metabolic rate. T H E D E B AT E . Until the 1960s, there was no debate: dinosaurs, whose existence had been known for about a century, were assumed to be big, dumb, slow, cold-blooded creatures. Then, in 1968, Robert T. Bakker—an undergraduate at Yale University, not a professor or a fullfledged paleontologist—revolutionized the world of paleontology with a paper called “The Superiority of Dinosaurs.”

In his article, Bakker described dinosaurs as “fast, agile, energetic creatures” whose physiology was so advanced that even the biggest and

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heaviest of them could outrun a human. Just a year later, John H. Ostrom, a professor of paleontology who also happened to be at Yale, wrote that a recently identified species of theropod dinosaur must have been “an active and very agile predator.” Thus began the great dinosaur debate, which rages even today. Jurassic Park reinforced the Bakker-Ostrom position, portraying Velociraptor as a cunning, fast-moving predator with clear links to birds. And indeed there are many arguments for endothermy (warm-bloodedness) in dinosaurs—arguments that relate to everything from brain size to rate of growth to the latitudes at which dinosaur fossils have been located. On the other hand, there is plenty of evidence for ectothermy (cold-bloodedness), based on the dinosaurs’ size, scaliness, the climate in the Mesozoic era, and so on. To explore, compare, and judge these many arguments, the reader is encouraged to consult the “Were Dinosaurs Warm-Blooded?” Web site listed in the “Where to Learn More” section at the conclusion of this essay. However, a word of warning, as noted on that site: “The issue is a tangled, complex one. There are not just two sides to the issue; there are numerous competing hypotheses. If you’re looking for a major controversy in science, look no further!”

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WHERE TO LEARN MORE Cadbury, Deborah. Terrible Lizard: The First Dinosaur Hunters and the Birth of a New Science. New York: Holt, 2001. K-12: Paleontology: Dinos (Web site). . Morris, S. Conway. The Crucible of Creation: The Burgess Shale and the Rise of Animals. New York: Oxford University Press, 1998. Munro, Margaret, and Karen Reczuch. The Story of Life on Earth. Toronto: Douglas and McIntyre, 2000. Oceans of Kansas Paleontology (Web site). . Paleontology and Fossils Resources—University of Arizona Library, Tucson (Web site). . Palmer, Douglas. Atlas of the Prehistoric World. Bethesda, MD: Discovery Communications, 1999. Sanders, Robert. “New Evidence Links Mass Extinction with Massive Eruptions That Split Pangea Supercontinent and Created the Atlantic 200 Million Years Ago.” University of California, Berkeley. (Web site). http://www.berkeley.edu/news/media/releases/ 99legacy/4-22-1999b.html>. Singer, Ronald. Encyclopedia of Paleontology. Chicago: Fitzroy Dearborn Publishers, 1999. University of California, Berkeley Museum of Paleontology (Web site). . USGS (United States Geological Survey) Paleontology Home Page (Web site). . “Were Dinosaurs Warm-Blooded?” (Web site). .

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Minerals

CONCEPT A mineral is a naturally occurring, typically inorganic substance with a specific chemical composition and structure. An unknown mineral usually can be identified according to known characteristics of specific minerals in terms of certain parameters that include its appearance, its hardness, and the ways it breaks apart when fractured. Minerals are not to be confused with rocks, which are typically aggregates of minerals. There are some 3,700 varieties of mineral, a handful of which are abundant and wide-ranging in their application. Many more occur less frequently but are extremely important within a more limited field of uses.

HOW IT WORKS Introduction to Minerals The particulars of the mineral definition deserve some expansion, especially inasmuch as mineral has an everyday definition somewhat broader than its scientific definition. In everyday usage, minerals would be the natural, nonliving materials that make up rocks and are mined from the earth. According to this definition, minerals would include all metals, gemstones, clays, and ores. The scientific definition, on the other hand, is much narrower, as we shall see. The fact that a mineral must be inorganic brings up another term that has a broader meaning in everyday life than in the world of science. At one time, the scientific definition of organic was more or less like the meaning assigned to it by nonscientists today, as describing all living or formerly living things, their parts, and substances

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that come from them. Today, however, chemists use the word organic to refer to any compound that contains carbon bonded to hydrogen, thus excluding carbonates (which are a type of mineral) and oxides such as carbon dioxide or carbon monoxide. Because a mineral must be inorganic, this definition eliminates coal and peat, both of which come from a wide-ranging group of organic substances known as hydrocarbons. A mineral also occurs naturally, meaning that even though there are artificial substances that might be described as “mineral-like,” they are not minerals. In this sense, the definition of a mineral is even more restricted than that of an element, discussed later in this essay, even though there are nearly 4,000 minerals and more than 92 elements. The number 92, of course, is not arbitrary: that is the number of elements that occur in nature. But there are additional elements, numbering 20 at the end of the twentieth century, that have been created artificially. PHYSICAL AND CHEMICAL P R O P E RT I E S O F M I N E RA L S . The

specific characteristics of minerals can be discussed both in physical and in chemical terms. From the standpoint of physics, which is concerned with matter, energy, and the interactions between the two, minerals would be described as crystalline solids. The definition of a mineral is narrowed further in terms of its chemistry, or its atomic characteristics, since a mineral must be of unvarying composition. A mineral, then, must be solid under ordinary conditions of pressure and temperature. This excludes petroleum, for instance (which, in any case, would have been disqualified owing to its organic origins), as well as all other liquids

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and gases. Moreover, a mineral cannot be just any type of solid but must be a crystalline one—that is, a solid in which the constituent parts have a simple and definite geometric arrangement that is repeated in all directions. This rule, for instance, eliminates clay, an example of an amorphous solid. Chemically, a mineral must be of unvarying composition, a stipulation that effectively limits minerals to elements and compounds. Neither sand nor glass, for instance, is a mineral, because the composition of both can vary. Another way of putting this is to say that all minerals must have a definite chemical formula, which is not true of sand, dirt, glass, or any other mixture. Let us now look a bit more deeply into the nature of elements and compounds, which are collectively known as pure substances, so as to understand the minerals that are a subset of this larger grouping.

Elements The periodic table of elements is a chart that appears in most classrooms where any of the physical sciences are taught. It lists all elements in order of atomic number, or the number of protons (positively charged subatomic particles) in the atomic nucleus. The highest atomic number of any naturally occurring element is 92, for uranium, though it should be noted that a very few elements with an atomic number lower than 92 have never actually been found on Earth. On the other hand, all elements with an atomic number higher than 92 are artificial, created either in laboratories or as the result of atomic testing. An element is a substance made of only one type of atom, meaning that it cannot be broken down chemically to create a simpler substance. In the sense that each is a fundamental building block in the chemistry of the universe, all elements are, as it were, “created equal.” They are not equal, however, in terms of their abundance. The first two elements on the periodic table, hydrogen and helium, represent 99.9% of the matter in the entire universe. Though Earth contains little of either, our planet is only a tiny dot within the vastness of space; by contrast, stars such as our Sun are composed almost entirely of those elements (see Sun, Moon, and Earth). A B U N DA N C E O N E A RT H . Of all

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the term mass refers to the known elemental mass of the planet’s atmosphere, waters, and crust; below the crust, scientists can only speculate, though it is likely that much of Earth’s interior consists of iron.) Together with silicon (25.7%), oxygen accounts for almost exactly three-fourths of the elemental mass of Earth. If we add in aluminum (7.5%), iron (4.71%), calcium (3.39%), sodium (2.63%), potassium (2.4%), and magnesium (1.93%), these eight elements make up about 97.46% of Earth’s material. Hydrogen, so plentiful in the universe at large, ranks ninth on Earth, accounting for only 0.87% of the planet’s known elemental mass. Nine other elements account for a total of 2% of Earth’s composition: titanium (0.58%), chlorine (0.19%), phosphorus (0.11%), manganese (0.09%), carbon (0.08%), sulfur (0.06%), barium (0.04%), nitrogen (0.03%), and fluorine (0.03%). The remaining 0.49% is made up of various other elements. Looking only at Earth’s crust, the numbers change somewhat, especially at the lower end of the list. Listed below are the 12 most abundant elements in the planet’s crust, known to earth scientists simply as “the abundant elements.” These 12, which make up 99.23% of the known crustal mass, together form approximately 40 different minerals that account for the vast majority of that 99.23%. Following the name and chemical symbol of each element is the percentage of the crustal mass it composes. Abundance of Elements in Earth’s Crust • • • • • • • • • • • •

Oxygen (O): 45.2% Silicon (Si): 27.2% Aluminum (Al): 8.0% Iron (Fe): 5.8% Calcium (Ca): 5.06% Magnesium (Mg): 2.77% Sodium (Na): 2.32% Potassium (K): 1.68% Titanium (Ti): 0.86% Hydrogen (H): 0.14% Manganese (Mn): 0.1% Phosphorus (P): 0.1%

Atoms, Molecules, and Bonding

elements, oxygen is by far the most plentiful on Earth, representing nearly half—49.2%—of the total mass of atoms found on this planet. (Here

As noted earlier, an element is identified by the number of protons in its nucleus, such that any atom with six protons must be carbon, since car-

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bon has an atomic number of 6. The number of electrons, or negatively charged subatomic particles, is the same as the number of protons, giving an atom no net electric charge. An atom may lose or gain electrons, however, in which case it becomes an ion, an atom or group of atoms with a net electric charge. An atom that has gained electrons, and thus has a negative charge, is called an anion. On the other hand, an atom that has lost electrons, thus becoming positive in charge, is a cation. In addition to protons and electrons, an atom has neutrons, or neutrally charged particles, in its nucleus. Neutrons have a mass close to that of a proton, which is much larger than that of an electron, and thus the number of neutrons in an atom has a significant effect on its mass. Atoms that have the same number of protons (and therefore are of the same element), but differ in their number of neutrons, are called isotopes. COMPOUNDS AND MIXTURES.

Whereas there are only a very few elements, there are millions of compounds, or substances made of more than one atom. A simple example is water, formed by the bonding of two hydrogen atoms with one oxygen atom; hence the chemical formula for water, which is H2O. Note that this is quite different from a mere mixture of hydrogen and oxygen, which would be something else entirely. Given the gaseous composition of the two elements, combined with the fact that both are extremely flammable, the result could hardly be more different from liquid water, which, of course, is used for putting out fires. The difference between water and the hydrogen-oxygen mixture described is that whereas the latter is the result of mere physical mixing, water is created by chemical bonding. Chemical bonding is the joining, through electromagnetic attraction, of two or more atoms to create a compound. Of the three principal subatomic particles, only electrons are involved in chemical bonding—and only a small portion of those, known as valence electrons, which occupy the outer shell of an atom. Each element has a characteristic pattern of valence electrons, which determines the ways in which the atom bonds.

the German chemist Richard Abegg (1869–1910) discovered that they all have eight valence electrons. His observation led to one of the most important principles of chemical bonding: atoms bond in such a way that they achieve the electron configuration of a noble gas. This concept, known as the octet rule, has been shown to be the case in most stable chemical compounds. Abegg hypothesized that atoms combine with one another because they exchange electrons in such a way that both end up with eight valence electrons. This was an early model of ionic bonding, which results from attractions between ions with opposite electric charges: when they bond, these ions “complete” each other. Metals tend to form cations and bond with nonmetals that have formed anions. The bond between anions and cations is known as an ionic bond, and is extremely strong. The other principal type of bond is a covalent bond. The result, once again, is eight valence electrons for each atom, but in this case, the nuclei of the two atoms share electrons. Neither atom “owns” them; rather, they share electrons. Today, chemists understand that most bonds are neither purely ionic nor purely covalent; instead, there is a wide range of hybrids between the two extremes, which are a function of the respective elements’ electronegativity, or the relative ability of an atom to attract valence electrons. If one element has a much higher electronegativity value than the other one, the bond will be purely ionic, but if two elements have equal electronegativity values, the bond is purely covalent. Most bonds, however, fall somewhere between these two extremes. I N T E R M O L E C U LA R B O N D I N G .

Chemical bonds exist between atoms and within a molecule. But there are also bonds between molecules, which affect the physical composition of a substance. The strength of intermolecular bonds is affected by the characteristics of the interatomic, or chemical, bond.

es, of which helium is an example, are noted for their lack of chemical reactivity, or their resistance to bonding. While studying these elements,

For example, the difference in electronegativity values between hydrogen and oxygen is great enough that the bond between them is not purely covalent, but instead is described as a polar covalent bond. Oxygen has a much higher electronegativity (3.5) than hydrogen (2.1), and therefore the electrons tend to gravitate toward the oxygen atom. As a result, water molecules have a strong negative charge on the side occu-

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pied by the oxygen atom, with a resulting positive charge on the hydrogen side. By contrast, molecules of petroleum, a combination of carbon and hydrogen, tend to be nonpolar, because carbon (with an electronegativity value of 2.5) and hydrogen have very similar electronegativity values. Therefore the electric charges are more or less evenly distributed in the molecule. As a result, water molecules form strong attractions, known as dipole-dipole attractions, to each other. Molecules of petroleum, on the other hand, have little attraction to each other, and the differences in charge distribution account for the fact that water and oil do not mix. Even weaker than the bonds between nonpolar molecules, however, are those between highly reactive elements, such as the noble gases and the “noble metals”—gold, silver, and copper, which resist bonding with other elements. The type of intermolecular attraction that exists in such a situation is described by the term London dispersion forces, a reference to the Germanborn American physicist Fritz Wolfgang London (1900–1954). The bonding between molecules of most other metals, however, is described by the electron sea model, which depicts metal atoms as floating in a “sea” of valence electrons. These valence electrons are highly mobile within the crystalline structure of the metal, and this mobility helps explain metals’ high electric conductivity. The ease with which metal crystals allow themselves to be rearranged explains not only metals’ ductility (their ability to be shaped) but also their ability to form alloys, a mixture containing two or more metals.

The Crystalline Structure of Minerals By definition, a solid is a type of matter whose particles resist attempts at compression. Because of their close proximity, solid particles are fixed in an orderly and definite pattern. Within the larger category of solids are crystalline solids, or those in which the constituent parts are arranged in a simple, definite geometric pattern that is repeated in all directions.

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cuss later in this essay. Glass, on the other hand, is an amorphous solid, meaning that its molecules are not arranged in an orderly pattern. C RY S TA L S Y S T E M S . Elsewhere in

this book (Earth, Science, and Nonscience and Planetary Science), there is considerable discussion of misconceptions originating with Aristotle (384–322 B.C.). Despite his many achievements, including significant contributions to the biological sciences, the great Greek philosopher spawned a number of erroneous concepts, which prevailed in the physical sciences until the dawn of the modern era. At least Aristotle made an attempt at scientific study, however; for instance, he dissected dead animals to observe their anatomic structures. His teacher, Plato (427?–347 B.C.), on the other hand, is hardly ever placed among the ranks of those who contributed, even ever so slightly, to progress in the sciences. There is a reason for this. Plato, in contrast to his pupil, made virtually no attempt to draw his ideas about the universe from an actual study of it. Within Plato’s worldview, the specific qualities of any item, including those in the physical world, reflected the existence of perfect and pure ideas that were more “real” than the physical objects themselves. Typical of his philosophy was his idea of the five Platonic solids, or “perfect” geometric shapes that, he claimed, formed the atomic substructure of the world. The “perfection” of the Platonic solids lay in the fact that they are the only five three-dimensional objects in which the faces constitute a single type of polygon (a closed shape with three or more sides, all straight), while the vertices (edges) are all alike. These five are the tetrahedron, octahedron, and icosahedron, composed of equilateral triangles (four, eight, and twenty, respectively); the cube, which, of course, is made of six squares; and the dodecahedron, made up of twelve pentagons. Plato associated the latter solid with the shape of atoms in outer space, while the other four corresponded to what the Greeks believed were the elements on Earth: fire (tetrahedron), earth (cube), air (octahedron), and water (icosahedron).

The term crystal is popularly associated with glass and with quartz, but only one of these is a crystalline solid. Quartz is a member of the silicates, a large group of minerals that we will dis-

All of this, of course, is nonsense from the standpoint of science, though the Platonic solids are of interest within the realm of mathematics. Yet amazingly, Plato in his unscientific way actually touched on something close to the truth, as applied to the crystalline structure of minerals.

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A

DODECAHEDRON, ONE OF THE

PLATONIC

SOLIDS. (© Richard Duncan/Photo Researchers. Reproduced by permission.)

Despite the large number of minerals, there are just six crystal systems, or geometric shapes formed by crystals. For any given mineral, it is possible for a crystallographer (a type of mineralogist concerned with the study of crystal structures) to identify its crystal system by studying a good, well-formed specimen, observing the faces of the crystal and the angles at which they meet.

tific preoccupations, or by simple lack of knowledge. Agricola’s De re metallica (On minerals, 1556), published after his death, constituted the first attempt at scientific mineralogy and mineral classification, but it would be two and a half centuries before the Swedish chemist Jöns Berzelius (1779–1848) developed the basics of the classification system used today.

An isometric crystal system is the most symmetrical of all, with faces and angles that are most clearly uniform. Because of differing types of polygon that make up the faces, as well as differing numbers of vertices, these crystals appear in 15 forms, several of which are almost eerily reminiscent of Plato’s solids: not just the cube (exemplified by halite crystals) but also the octahedron (typical of spinels) and even the dodecahedron (garnets).

Berzelius’s classification system was refined later in the nineteenth century by the American mineralogist James Dwight Dana (1813–1895) and simplified by the American geologists Brian Mason (1917–) and L. G. Berry (1914–). In general terms, the classification system accepted by mineralogists today is as follows:

Before the time of the great German mineralogist Georgius Agricola (1494–1555), attempts to classify minerals were almost entirely overshadowed by the mysticism of alchemy, by other nonscien-

Class 1: Native elements Class 2: Sulfides Class 3: Oxides and hydroxides Class 4: Halides Class 5: Carbonates, nitrates, borates, iodates • Class 6: Sulfates, chromates, molybdates, tungstates • Class 7: Phosphates, arsenates, vanadates • Class 8: Silicates N AT I V E E L E M E N T S . The first group, native elements, includes (among other

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• • • • •

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things) metallic elements that appear in pure form somewhere on Earth: aluminum, cadmium, chromium, copper, gold, indium, iron, lead, mercury, nickel, platinum, silver, tellurium, tin, titanium, and zinc. This may seem like a great number of elements, but it is only a small portion of the 87 metallic elements listed on the periodic table. The native elements also include certain metallic alloys, a fact that might seem strange for several reasons. First of all, an alloy is a mixture, not a compound, and, second, people tend to think of alloys as being man-made, not natural. The list of metallic alloys included among the native elements, however, is very small, and they meet certain very specific mineralogic criteria regarding consistency of composition. The native elements class also includes native nonmetals such as carbon, in the form of graphite or its considerably more valuable alter ego, diamond, as well as elemental silicon (an extremely important building block for minerals, as we shall see) and sulfur. For a full list of native elements and an explanation of criteria for inclusion, as well as similar data for the other classes of mineral, the reader is encouraged to consult the Minerals by Name Web site, the address of which is provided in “Where to Learn More” at the end of this essay. S U L F I D E S A N D H A L I D E S . Most important ores (a rock or mineral possessing economic value)—copper, lead, and silver— belong to the sulfides class, as does a mineral that often has been mistaken for a precious metal— iron sulfide, or pyrite. Better known by the colloquial term fool’s gold, pyrite has proved valuable primarily to con artists who passed it off as the genuine article. During World War II, however, pyrite deposits near Ducktown, Tennessee, became valuable owing to the content of sulfur, which was extracted for use in defense applications.

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just one group, that group would take up almost the entire list. For instance, under the present system, silicates account for the vast majority of minerals, but since those contain oxygen as well, a list that grouped all oxygen-based minerals together would consist of only four classes: native elements, sulfides, halides, and a swollen oxide category that would include 90% of all known minerals. Instead, the oxides class is limited only to noncomplex minerals that contain either oxygen or hydroxide (OH). Examples of oxides include magnetite (iron oxide) and corundum (aluminum oxide.) It should be pointed out that a single chemical name, such as iron oxide or aluminum oxide, is not limited to a single mineral; for example, anatase and brookite are both titanium oxide, but they represent different combinations. O T H E R N O N S I L I CAT E S . All the mineral classes discussed to this point, as well as several others to follow, are called nonsilicates, a term that stresses the importance of silicates among mineral classes.

Like the oxides, the carbonates, or carbonbased minerals, are a varied group. This class also contains a large number of minerals, making it the most extensive group aside from silicates and phosphates. Among these are limestones and dolostones, some of the most abundant rocks on Earth. The phosphates, despite their name, may or may not include phosphorus; in some cases, arsenic, vanadium, or antimony may appear in its place. The same is true of the sulfates, which may or may not involve sulfur; some include chromium, tungsten, selenium, tellurium, or molybdenum instead. T W O Q U E S T I O N A B L E C LASS E S . In addition to the seven formal classes just

Whereas the sulfides fit the common notion of a mineral as a hard substance, halides, which are typically soft and transparent, do not. Yet they are indeed a class of minerals, and they include one of the best-known minerals on Earth: halite, known chemically as NaCl or sodium chloride— or, in everyday language, table salt.

described, there are two other somewhat questionable classes of nonsilicate that might be included in a listing of minerals. They would be included, if at all, only with major reservations, since they do not strictly fit the fourfold definition of a mineral as crystalline in structure, natural, inorganic, and identifiable by a precise chemical formula. These two questionable groups are organics and mineraloids.

OX I D E S . Oxides, as their name suggests, are minerals containing oxygen; however, if all oxygen-containing minerals were lumped into

Organics, as their name suggests, have organic components, but as we have observed, “organic” is not the same as “biological.” This

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class excludes hard substances created in a biological setting—for example, bone or pearl—and includes only minerals that develop in a geologic setting yet have organic chemicals in their composition. By far the best-known example of this class, which includes only a half-dozen minerals, is amber, which is fossilized tree sap.

bon, is at the center of a vast array of compounds—organic in the case of carbon and inorganic in the case of silicon. Silicates, which, as noted earlier, account for nine-tenths of the mass of Earth’s crust (and 30% of all minerals), are to silicon and mineralogy what hydrocarbons are to carbon and organic chemistry.

Amber is also among the mineraloids, which are not really “questionable” at all—they are clearly not minerals, since they do not have the necessary crystalline structure. Nevertheless, they often are listed among minerals in reference books and are likely to be sold by mineral dealers. The other four mineraloids include two other well-known substances, opal and obsidian.

Whereas carbon forms it most important compounds with hydrogen—hydrocarbons such as petroleum—the most important silicon-containing compounds are those formed by bonds with oxygen. There is silica (SiO2), for instance, commonly known as sand. Aside from its many applications on the beaches of the world, silica, when mixed with lime and soda (sodium carbonate) and other substances, makes glass. Like carbon, silicon has the ability to form polymers, or long, chainlike molecules. And whereas carbon polymers are built of hydrocarbons (plastics are an example), silicon polymers are made of silicon and oxygen in monomers, or strings of atoms, that form ribbons or sheets many millions of units long.

Silicates Where minerals are concerned, the silicates are the “stars of the show”: the most abundant and most widely used class of minerals. That being said, it should be pointed out that there are a handful of abundant nonsilicates, most notably the iron oxides hematite, magnetite, and goethite. A few other nonsilicates, while they are less abundant, are important to the makeup of Earth’s crust, examples being the carbonates calcite and dolomite; the sulfides pyrite, sphalerite, galena, and chalcopyrite; and the sulfate gypsum. Yet the nonsilicates are not nearly as important as the class of minerals built around the element silicon. Though it was discovered by Jöns Berzelius in 1823, owing to its abundance in the planet’s minerals, silicon has been in use by humans for thousands of years. Indeed, silicon may have been one of the first elements formed in the Precambrian eons (see Geologic Time). Geologists believe that Earth once was composed primarily of molten iron, oxygen, silicon, and aluminum, which, of course, are still the predominant elements in the planet’s crust. But because iron has a greater atomic mass, it settled toward the center, while the more lightweight elements rose to the surface. After oxygen, silicon is the most abundant of all elements on the planet, and compounds involving the two make up about 90% of the mass of Earth’s crust.

S I L I CAT E S U B C LASS E S . There

are six subclasses of silicate, differentiated by structure. Nesosilicates include some the garnet group; gadolinite, which played a significant role in the isolation of the lanthanide series of elements during the nineteenth century; and zircon. The latter may seem to be associated with the cheap diamond simulant, or substitute, called cubic zirconium, or CZ. CZ, however, is an artificial “mineral,” whereas zircon is the real thing— yet it, too, has been applied as a diamond simulant.

below carbon, with which it shares an ability to form long strings of atoms. Because of this and other chemical characteristics, silicon, like car-

Just as silicon’s close relative, carbon, can form sheets (this is the basic composition of graphite), so silicon can appear in sheets as the phyllosilicate subclass. Included among this group are minerals known for their softness: kaolinite, talc, and various types of mica. These are used in everything from countertops to talcum powder. The kaolinite derivative known as kaolin is applied, for instance, in the manufacture of porcelain, while some people in parts of Georgia, a state noted for its kaolinite deposits, claim that it can and should be chewed as an antacid stomach remedy. (One can even find little bags of kaolin sold for this purpose at convenience stores around Columbus in southern Georgia.)

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CALCITE

WITH QUARTZ. (© Mark A. Schneider/Photo Researchers. Reproduced by permission.)

Included in another subclass, the tectosilicates, are the feldspar and quartz groups, which are the two most abundant types of mineral in Earth’s crust. Note that these are both groups: to a mineralogist, feldspar and quartz refer not to single minerals but to several within a larger grouping. Feldspar, whose name comes from the Swedish words for “field” and “mineral” (a reference to the fact that miners and farmers found the same rocks in their respective areas of labor), includes a number of varieties, such as albite (sodium aluminum silicate) or sanidine (potassium aluminum silicate). Other, more obscure silicate subclasses include sorosilicates and inosilicates. Finally, there are cyclosilicates, such as beryl or beryllium aluminum silicate.

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rates minerals from 1 to 10, with 10 being equivalent to the hardness of a diamond and 1 that of talc, the softest mineral. (See Economic Geology for other scales, some of which are more applicable to specific types of minerals.) Minerals sometimes can be identified by color, but this property can be so affected by the presence of impurities that mineralogists rely instead on streak. The latter term refers to the color of the powder produced when one mineral is scratched by another, harder one. Another visual property is luster, or the appearance of a mineral when light reflects off its surface. Among the terms used in identifying luster are metallic, vitreous (glassy), and dull. The term cleavage refers to the way in which a mineral breaks—that is, the planes across which the mineral splits into pieces. For instance, muscovite tends to cleave only in one direction, forming thin sheets, while halite cleaves in three directions, which are all perpendicular to one another, forming cubes. The cleavage of a mineral reveals its crystal system; however, minerals are more likely to fracture (break along something other than a flat surface) than they are to cleave.

Mineralogists identify unknown minerals by judging them in terms of various physical properties, including hardness, color and streak, luster, cleavage and fracture, density and specific gravity, and other factors, such as crystal form. Hardness, or the ability of one mineral to scratch another, may be measured against the Mohs scale, introduced in 1812 by the German mineralogist Friedrich Mohs (1773–1839). The scale

is the ratio of mass to volume, and specific grav-

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ity is the ratio between the density of a particular substance and that of water. Specific gravity almost always is measured according to the metric system, because of the convenience: since the density of water is 1 g per cubic centimeter (g/cm3), the specific gravity of a substance is identical to its density, except that specific gravity involves no units. For example, gold has a density of 19.3 g/cm3 and a specific gravity of 19.3. Its specific gravity, incidentally, is extremely high, and, indeed, one of the few metals that comes close is lead, which has a specific gravity of 11. By comparing specific gravity values and measuring the displacement of water when an object is set down in it, it is possible to determine whether an item purported to be gold actually is gold. In addition to these more common parameters for identifying minerals, it may be possible to identify certain ones according to other specifics. There are minerals that exhibit fluorescent or phosphorescent characteristics, for instance. The first term refers to objects that glow when viewed under ultraviolet light, while the second term describes those that continue to glow after being exposed to visible light for a short period of time. Some minerals are magnetic, while others are radioactive. Chemists long ago adopted a system for naming compounds so as to avoid the confusion of proliferating common names. The only compounds routinely referred to by their common names in the world of chemistry are water and ammonia; all others are known according to chemical nomenclature that is governed by specific rules. Thus, for instance, NaCl is never “salt,” but “sodium chloride.” NAMING

M I N E RA L S .

Geologists have not been able to develop such a consistent means of naming minerals. For one thing, as noted earlier, two minerals may be different from each other yet include the same elements. Furthermore, it is difficult (unlike the case of chemical compounds) to give minerals names that provide a great deal of information regarding their makeup. Instead, most minerals are simply named after people (usually scientists) or the locale in which they were found.

Abrasives

cussed, have an enormous impact on their usefulness and commercial value. Some minerals, such as diamonds and corundum, are prized for their hardness, while others, ranging from marble to the “mineral” alabaster, are useful precisely because they are soft. Others, among them copper and gold, are not just soft but highly malleable, and this property makes them particularly useful in making products such as electrical wiring.

Minerals

Diamonds, corundum, and other minerals valued for their hardness belong to a larger class of materials called abrasives. The latter includes sandpaper, which of course is made from one of the leading silicate derivatives, sand. Sandstone and quartz are abrasives, as are numerous variants of corundum, such as sapphire and garnets. In 1891, American inventor Edward G. Acheson (1856–1931) created silicon carbide, later sold under the trade name Carborundum, by heating a mixture of clay and coke (almost pure carbon). For 50 years, Carborundum was the second-hardest substance known, diamonds being the hardest. Today other synthetic abrasives, made from aluminum oxide, boron carbide, and boron nitride, have supplanted Carborundum in importance. Corundum, from the oxides class of mineral, can have numerous uses. Extremely hard, corundum, in the form of an unconsolidated rock commonly called emery, has been used as an abrasive since ancient times. Owing to its very high melting point—even higher than that of iron—corundum also is employed in making alumina, a fireproof product used in furnaces and fireplaces. Though pure corundum is colorless, when combined with trace amounts of certain elements, it can yield brilliant colors: hence, corundum with traces of chromium becomes a red ruby, while traces of iron, titanium, and other elements yield varieties of sapphire in yellow, green, and violet as well as the familiar blue. This brings up an important point: many of the minerals named here are valued for much more than their abrasive qualities. Many of the 16 minerals used as gemstones, including corundum (source of both rubies and sapphires, as we have noted), garnet, quartz, and of course diamond, happen to be abrasives as well. (See Economic Geology for the full list of precious gems.)

The physical properties of minerals, including many of the characteristics we have just dis-

D I A M O N D S . Diamonds, in fact, are so greatly prized for their beauty and their application in jewelry that their role as “working” min-

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erals—not just decorations—should be emphasized. The diamonds used in industry look quite different from the ones that appear in jewelry. Industrial diamonds are small, dark, and cloudy in appearance, and though they have the same chemical properties as gem-quality diamonds, they are cut with functionality (rather than beauty) in mind. A diamond is hard, but brittle: in other words, it can be broken, but it is very difficult to scratch or cut a diamond—except with another diamond. On the other hand, the cutting of fine diamonds for jewelry is an art, exemplified in the alluring qualities of such famous gems as the jewels in the British Crown or the infamous Hope Diamond in Washington, D.C.’s Smithsonian Institution. Such diamonds—as well as the diamonds on an engagement ring—are cut to refract or bend light rays and to disperse the colors of visible light.

Soft and Ductile Minerals At the other end of the Mohs scale are an array of minerals valued not for their hardness, but for opposite qualities. Calcite, for example, is often used in cleansers because, unlike an abrasive (also used for cleaning in some situations), it will not scratch a surface to which it is applied. Calcite takes another significant form, that of marble, which is used in sculpture, flooring, and ornamentation because of its softness and ease in carving—not to mention its great beauty. Gypsum, used in plaster of paris and wallboard, is another soft mineral with applications in building. Though, obviously, soft minerals are not much value as structural materials, when stud walls of wood provide the structural stability for gypsum sheet wall coverings, the softness of the latter can be an advantage. Gypsum wallboard makes it easy to put in tacks or nails for pictures and other decorations, or to cut out a hole for a new door, yet it is plenty sturdy if bumped. Furthermore, it is much less expensive than most materials, such as wood paneling, that might be used to cover interior walls.

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about as much as a raisin (0.0022 lb., or 1 g) can be pulled into the shape of a wire 1.5 mi. (2.4 km) long. This, along with its qualities as a conductor of heat and electricity, would give it a number of other applications, were it not for the high cost of gold. Therefore, gold, if it were a person, would have to be content with being only the most prized and admired of all metallic minerals, an element for which men and whole armies have fought and sometimes died. Gold is one of the few metals that is not silver, gray, or white in appearance, and its beautifully distinctive color caught the eyes of metalsmiths and royalty from the beginning of civilization. Hence it was one of the first widely used metals. Records from India dating back to 5000 B.C. suggest a familiarity with gold, and jewelry found in Egyptian tombs indicates the use of sophisticated techniques among the goldsmiths of Egypt as early as 2600 B.C. Likewise, the Bible mentions gold in several passages, and the Romans called it aurum (“shining dawn”), which explains its chemical symbol, Au. C O P P E R. Copper, gold, and silver are together known as coinage metals. They have all been used for making coins, a reflection not only of their attractiveness and malleability, but also of their resistance to oxidation. (Oxygen has a highly corrosive influence on metals, causing rust, tarnishing, and other effects normally associated with aging but in fact resulting from the reaction of metal and oxygen.) Of the three coinage metals, copper is by far the most versatile, widely used for electrical wiring and in making cookware. Due to the high conductivity of copper, a heated copper pan has a uniform temperature, but copper pots must be coated with tin because too much copper in food is toxic.

G O L D . Quite different sorts of minerals are valued not only for their softness but also their ductility or malleability. There is gold, for instance, the most ductile of all metals. A single troy ounce (31.1 g) can be hammered into a sheet just 0.00025 in. (0.00064 cm) thick, covering 68 sq. ft. (6.3 sq m), while a piece of gold weighing

Its resistance to corrosion makes copper ideal for plumbing. Likewise, its use in making coins resulted from its anticorrosive qualities, combined with its beauty. These qualities led to the use of copper in decorative applications for which gold would have been much too expensive: many old buildings used copper roofs, and the Statue of Liberty is covered in 300 thick copper plates. As for why the statue and many old copper roofs are green rather than copper-colored, the reason is that copper does eventually corrode when exposed to air for long periods of time. It develops a thin layer of black copper oxide, and

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as the years pass, the reaction with carbon dioxide in the air leads to the formation of copper carbonate, which imparts a greenish color. Unlike silver and gold, copper is still used as a coinage metal, though it, too, has been increasingly taken off the market for this purpose due to the high expense involved. Ironically, though most people think of pennies as containing copper, in fact the penny is the only American coin that contains no copper alloys. Because the amount of copper necessary to make a penny today costs more than one cent, a penny is actually made of zinc with a thin copper coating.

Insulation and Other Applications Whereas copper is useful because it conducts heat and electricity well, other minerals (e.g., kyanite, andalusite, muscovite, and silimanite) are valuable for their ability not to conduct heat or electricity. Muscovite is often used for insulation in electrical devices, though its many qualities make it a mineral prized for a number of reasons. Its cleavage and lustrous appearance, combined with its transparency and almost complete lack of color, made it useful for glass in the windowpanes of homes owned by noblemen and other wealthy Europeans of the Middle Ages. Today, muscovite is the material in furnace and stove doors: like ordinary glass, it makes it possible for one to look inside without opening the door, but unlike glass, it is an excellent insulator. The glass-like quality of muscovite also makes it a popular material in wallpaper, where ground muscovite provides a glassy sheen. In the same vein, asbestos—which may be made of chrysotile, crocidolite, or other minerals—has been prized for a number of qualities, including its flexibility and fiber-like cleavage. These factors, combined with its great heat resistance and its resistance to flame, have made it useful for fireproofing applications, as for instance in roofing materials, insulation for heating and electrical devices, brake linings, and suits for firefighters and others who must work around flames and great heat. However, information linking asbestos and certain forms of cancer, which began to circulate in the 1970s, led to a sharp decline in the asbestos industry.

give minerals value. Halite, or table salt, is an important—perhaps too important!—part of the American diet. Nor is it the only consumable mineral; people also take minerals in dietary supplements, which is appropriate since the human body itself contains numerous minerals. In addition to a very high proportion of carbon, the body also contains a significant amount of iron, a critical component in red blood cells, as well as smaller amounts of minerals such as zinc. Additionally, there are trace minerals, so called because only traces of them are present in the body, that include cobalt, copper, manganese, molybdenum, nickel, selenium, silicon, and vanadium.

Minerals

One mineral that does not belong in the human body is lead, which has been linked with a number of health risks. The human body can only excrete very small quantities of lead a day, and this is particularly true of children. Even in small concentrations, lead can cause elevation of blood pressure, and higher concentrations can effect the central nervous system, resulting in decreased mental functioning, hearing damage, coma, and possibly even death. The ancient Romans, however, did not know this, and used what they called plumbum in making water pipes. (The Latin word is the root of our own term plumber.) Many historians believe that plumbum in the Romans’ water supply was one of the reasons behind the decline and fall of the Roman Empire. Even in the early twentieth century, people did not know about the hazards associated with lead, and therefore it was applied as an ingredient in paint. In addition, it was used in water pipes, and as an antiknock agent in gasolines. Increased awareness of the health hazards involved have led to a discontinuation of these practices. G RA P H I T E . Pencil “lead,” on the other hand, is actually a mixture of clay with graphite, a form of carbon that is also useful as a dry lubricant because of its unusual cleavage. It is slippery because it is actually a series of atomic sheets, rather like a big, thick stack of carbon paper: if the stack is heavy, the sheets are likely to slide against one another.

M I N E RA L S F O R H E A LT H O R O T H E R W I S E . All sorts of other properties

Actually, people born after about 1980 may have little experience with carbon paper, which was gradually phased out as photocopiers became cheaper and more readily available. Today, carbon paper is most often encountered

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Minerals

KEY TERMS ALLOY:

A mixture of two or more

metals. ANION:

CRUST:

The uppermost division of the

solid earth, representing less than 1% of its The negative ion that results

volume and varying in depth from 3 mi. to

when an atom or group of atoms gains one

37 mi. (5–60 km).

or more electrons.

CRYSTALLINE

SOLID:

The smallest particle of an ele-

solid in which the constituent parts have a

ment, consisting of protons, neutrons, and

simple and definite geometric arrange-

electrons. An atom can exist either alone or

ment that is repeated in all directions.

ATOM:

in combination with other atoms in a molecule.

ELECTRON:

A negatively charged par-

ticle in an atom, which spins around the

ATOMIC NUMBER:

The number of

nucleus.

protons in the nucleus of an atom. Since this number is different for each element, elements are listed on the periodic table in order of atomic number. CATION:

The positive ion that results

when an atom or group of atoms loses one or more electrons.

ELECTRONEGATIVITY:

The relative

ability of an atom to attract valence electrons. ELEMENT:

A substance made up of

only one kind of atom. Unlike compounds, elements cannot be broken chemically into other substances.

CHEMICAL BONDING:

The joining,

through electromagnetic forces, of atoms

HARDNESS:

In mineralogy, the ability

representing different elements. The prin-

of one mineral to scratch another. This is

cipal methods of combining are through

measured by the Mohs scale.

covalent and ionic bonding, though few

HYDROCARBON:

bonds are purely one or the other.

pound whose molecules are made up of

CLEAVAGE:

A term referring to the

Any chemical com-

nothing but carbon and hydrogen atoms.

characteristic patterns by which a mineral

ION:

breaks and specifically to the planes across

has lost or gained one or more electrons

which breaking occurs.

and thus has a net electric charge. Positive-

COMPOUND:

A substance made up of

atoms of more than one element, chemically bonded to one another.

An atom or group of atoms that

ly charged ions are called cations, and negatively charged ones are called anions. IONIC BONDING:

A form of chemical

A type of

bonding that results from attractions

chemical bonding in which two atoms

between ions with opposite electric

share valence electrons.

charges.

COVALENT BONDING:

140

A type of

when signing a credit-card receipt: the signature

In such a situation, one might notice that the

goes through the graphite-based backing of the

copied image of the signature looks as though it

receipt onto a customer copy.

were signed in pencil, which of course is fitting

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Minerals

KEY TERMS The appearance of a mineral

LUSTER:

when light reflects off its surface. Among the terms used in identifying luster are metallic, vitreous (glassy), and dull.

CONTINUED

compounds containing carbon and hydrogen, thus excluding carbonates (which are minerals) and oxides such as carbon dioxide.

A naturally occurring, typi-

PERIODIC TABLE OF ELEMENTS:

cally inorganic substance with a specific

A chart that shows the elements arranged in order of atomic number, along with the chemical symbol and the average atomic mass for that particular element.

MINERAL:

chemical composition and a crystalline structure. Unknown minerals usually can be identified in terms of specific parameters, such as hardness or luster. which includes a number of smaller sub-

Large, typically chainlike molecules composed of numerous smaller, repeating units known as monomers.

disciplines, such as crystallography.

PROTON:

MINERALOGY:

The study of minerals,

A substance with a variable

MIXTURE:

composition, meaning that it is composed of molecules or atoms of differing types and in variable proportions. MOHS SCALE:

1812

by

the

A scale, introduced in German

mineralogist

Friedrich Mohs (1773–1839), that rates the hardness of minerals from 1 to 10. Ten is equivalent to the hardness of a diamond and 1 that of talc, an extremely soft mineral. Small, individual sub-

units that join together to form polymers. NUCLEUS:

The center of an atom, a

region where protons and neutrons are located and around which electrons spin. A rock or mineral possessing eco-

nomic value. ORGANIC:

At one time, chemists used

the term organic only in reference to living things. Now the word is applied to most

A positively charged particle

in an atom. PURE SUBSTANCE: A substance, whether an element or compound, that has the same chemical composition throughout. Compare with mixture.

A term referring to the ability of one element to bond with others. The higher the reactivity (and, hence, the electronegativity value), the greater the tendency to bond.

REACTIVITY:

ROCK:

MONOMERS:

ORE:

POLYMERS:

An aggregate of minerals.

The ratio between the density of a particular substance and that of water. SPECIFIC

GRAVITY:

The color of the powder produced when one mineral is scratched by another, harder one.

STREAK:

VALENCE ELECTRONS: Electrons that occupy the highest principal energy level in an atom. These are the electrons involved in chemical bonding.

due to the application of graphite in pencil

ments, along with carbon and silicon—for writ-

“lead.” In ancient times, people did indeed use

ing, because it left gray marks on a surface. Even

lead—which is part of the “carbon family” of ele-

today, people still use the word “lead” in refer-

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ence to pencils, much as they still refer to a galvanized steel roof with a zinc coating as a “tin roof.”

Minerals and Metals: A World to Discover. Natural Resources Canada (Web site). .

(For more about minerals, see Rocks. The economic applications of both minerals and rocks are discussed in Economic Geology. In addition, Paleontology contains a discussion of fossilization, a process in which minerals eventually replace organic material in long-dead organisms.)

Minerals by Name (Web site). .

WHERE TO LEARN MORE Atlas of Rocks, Minerals, and Textures (Web site). . Hurlbut, Cornelius, W. Edwin Sharp, and Edward Salisbury Dana. Dana’s Minerals and How to Study Them. New York: John Wiley and Sons, 1997.

142

Pough, Frederick H. A Field Guide to Rocks and Minerals. Boston: Houghton Mifflin, 1996. Roberts, Willard Lincoln, Thomas J. Campbell, and George Robert Rapp. Encyclopedia of Minerals. New York: Van Nostrand Reinhold, 1990. Sorrell, Charles A., and George F. Sandström. A Field Guide and Introduction to the Geology and Chemistry of Rocks and Minerals. New York: St. Martin’s, 2001. Symes, R. F. Rocks and Minerals. Illus. Colin Keates and Andreas Einsiedel. New York: Dorling Kindersley, 2000.

The Mineral and Gemstone Kingdom: Minerals A–Z (Web site). .

“USGS Minerals Statistics and Information.” United States Geological Survey (Web site). .

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Rocks

CONCEPT It might come as a surprise to learn that geologists regularly use an unscientific-sounding term, rocks. Yet as is almost always the case with a word used both in everyday language and within the realm of a scientific discipline, the meanings are not the same. For one thing, rock and stone are not interchangeable, as they are in ordinary discussion. The second of these two terms is used only occasionally, primarily as a suffix in the names of various rocks, such as limestone or sandstone. On the other hand, a rock is an aggregate of minerals or organic material. Rocks are of three different types: igneous, formed by crystallization of molten minerals, as in a volcano; sedimentary, usually formed by deposition, compaction, or cementation of weathered rock; and metamorphic, formed by alteration of preexisting rock.

HOW IT WORKS An Introduction to Rocks To expand somewhat on the definition of rock, the term may be said to describe an aggregate of minerals or organic material, which may or may not appear in consolidated form. Consolidation, which we will explore further within the context of sedimentary rock, is a process whereby materials become compacted, or experience an increase in density. It is likely that the image that comes to mind when the word rock is mentioned is that of a consolidated one, but it is important to remember that the term also can apply to loose particles.

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ROCKS

The role of organic material in forming rocks also belongs primarily within the context of sedimentary, as opposed to igneous or metamorphic, rocks. There are, indeed, a handful of rocks that include organic material, an example being coal, but the vast majority are purely inorganic in origin. The inorganic materials that make up rocks are minerals, discussed in the next section. Rocks and minerals of economic value are called ores, which are examined in greater depth elsewhere, within the context of Economic Geology.

Minerals Defined The definition of a mineral includes four components: it must appear in nature and therefore not be artificial, it must be inorganic in origin, it must have a definite chemical composition, and it must have a crystalline internal structure. The first of these stipulations clearly indicates that there is no such thing as a man-made mineral; as for the other three parts of the definition, they deserve a bit of clarification. At one time, the term organic, even within the realm of chemistry, referred to all living or formerly living things, their parts, and substances that come from them. Today, however, chemists use the word to describe any compound that contains carbon and hydrogen, thus excluding carbonates (which are a type of mineral) and oxides such as carbon dioxide or carbon monoxide. N O N VA RY I N G

COMPOSITION.

The third stipulation, that a mineral must be of nonvarying composition, limits minerals almost exclusively to elements and compounds—that is, either to substances that cannot be chemically

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broken down to yield simpler substances or to substances formed by the chemical bonding of elements. The chemical bonding of elements is a process quite different from mixing, and a compound is not to be confused with a mixture, whose composition is highly variable. Another way of putting this is to say that all minerals must have a definite chemical formula, which is not possible with a mixture such as dirt or glass. The Minerals essay, which the reader is encouraged to consult for further information, makes reference to certain alloys, or mixtures of metals, that are classified as minerals. These alloys, however, are exceptional and fit certain specific characteristics of interest to mineralogists. The vast majority of the more than 3,700 known varieties of mineral constitute either a single element or a single compound. CRYSTALLINE STRUCTURE. The fact that a mineral must have a crystalline structure implies that it must be a solid, since all crystalline substances are solids. A solid, of course, is a type of matter whose particles, in contrast to those of a gas or liquid, maintain an orderly and definite arrangement and resist attempts at compression. Thus, petroleum cannot be a mineral, nor is “mineral spirits,” a liquid paint thinner made from petroleum (and further disqualified by the fact that it is artificial in origin).

Crystalline solids are those in which the constituent parts are arranged in a simple, definite geometric pattern that is repeated in all directions. These solids are contrasted with amorphous solids, such as clay. Metals are crystalline in structure; indeed, several metallic elements that appear on Earth in pure form (for example, gold, copper, and silver) also are classified as minerals.

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mineral, observing the faces of the crystal and the angles at which they meet. Other characteristics by which minerals can be studied and identified visually are color, streak, and luster. The first of these features is not particularly reliable, because impurities in the mineral may greatly affect its hue. Therefore, mineralogists are much more likely to rely on streak, or the color of the powder produced when one mineral is scratched by a harder one. Luster, the appearance of a mineral when light reflects off its surface, is described by such terms as vitreous (glassy), dull, or metallic. H A R D N E S S . Minerals also can be identified according to what might be called tactile properties, or characteristics best discerned through the sense of touch. One of the most important among such properties is hardness, defined as the ability of one mineral to scratch another. Hardness is measured by the Mohs scale, introduced in 1812 by the German mineralogist Friedrich Mohs (1773–1839).

The scale rates minerals from 1 to 10, with 1 being equivalent to the hardness of talc, a mineral so soft that it is used for making talcum powder. A 2 on the Mohs scale is the hardness of gypsum, which is still so soft that it can be scratched by a human fingernail. Above a 5 on the scale, roughly equal to the hardness of a pocketknife or glass, are potassium feldspar (6), quartz (7), topaz (8), corundum (9), and diamond (10). O T H E R P R O P E RT I E S . Other tactile parameters are cleavage, the planes across which the mineral breaks, and fracture, the tendency to break along something other than a flat surface. Minerals also can be evaluated by their density (ratio of mass to volume) or specific gravity (ratio between the mineral’s density and that of water). Density and specific-gravity measures are particularly important for extremely dense materials, such as lead or gold.

The type of crystal that appears in a mineral is one of several characteristics that make it possible for a mineralogist to identify an unidentified mineral. Although, as noted earlier, there are nearly 4,000 known varieties of mineral, there are just six crystal systems, or geometric shapes formed by crystals. Crystallographers, or mineralogists concerned with the study of crystal structures, are able to identify the crystal system by studying a good, well-formed specimen of a

In addition to these specifics, others may be used for identifying some kinds of minerals. Magnetite and a few other minerals, for instance, are magnetic, while minerals containing uranium and other elements with a high atomic number may be radioactive, or subject to the spontaneous emission of high-energy particles. Still others are fluorescent, meaning that they glow when viewed under ultraviolet light, or phosphorescent, meaning that they continue to glow after

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being exposed to visible light for a short period of time.

Mineral Groups Minerals are classified into eight basic groups: • • • • •

Class 1: Native elements Class 2: Sulfides Class 3: Oxides and hydroxides Class 4: Halides Class 5: Carbonates, nitrates, borates, iodates • Class 6: Sulfates, chromates, molybdates, tungstates • Class 7: Phosphates, arsenates, vanadates • Class 8: Silicates The first group, native elements, includes metallic elements that appear in pure form somewhere on Earth; certain metallic alloys, alluded to earlier; and native nonmetals, semimetals, and minerals with metallic and nonmetallic elements. Sulfides include the most important ores of copper, lead, and silver, while halides are typically soft and transparent minerals containing at least one element from the halogens family: fluorine, chlorine, iodine, and bromine. (The most well known halide, table salt, is a good example of an unconsolidated mineral.) Oxides are noncomplex minerals that contain either oxygen or hydroxide (OH). Included in the oxide class are such well-known materials as magnetite and corundum, widely used in industry. Other nonsilicates (a term that stresses the importance of silicates among mineral classes) include carbonates, or carbon-based minerals, as well as phosphates and sulfates. The latter are distinguished from sulfides by virtue of the fact that they include a complex anion (a negatively charged atom or group of atoms) in which an atom of sulfur, chromium, tungsten, selenium, tellurium, or molybdenum (or a combination of these) is attached to four oxygen atoms.

ture, but they can be listed under the more loosely defined heading of “rocks.”

Rocks

S I L I CAT E S . Only a few abundant or

important minerals are nonsilicates, for example, the iron oxides hematite, magnetite, and goethite; the carbonates calcite and dolomite; the sulfides pyrite, sphalerite, galena, and chalcopyrite; and the sulfate gypsum. The vast majority of minerals, including the most abundant ones, belong to a single class, that of silicates, which accounts for 30% of all minerals. As their name implies, they are built around the element silicon, which bonds to four oxygen atoms to form what are called silica tetrahedra. Silicon, which lies just below carbon on the periodic table of elements, is noted, like carbon, for its ability to form long strings of atoms. Carbon-hydrogen formations, or hydrocarbons, are the foundation of organic chemistry, while formations of oxygen and silicon—the two most abundant elements on Earth—provide the basis for a vast array of geologic materials. There is silica, for instance, better known as sand, which consists of silicon bonded to two carbon atoms. Then there are the silicates, which are grouped according to structure into six subclasses. Among these subclasses, discussed in the Minerals essay, are smaller groupings that include a number of well-known mineral types: garnet, zircon, kaolinite, talc, mica, and the two most abundant minerals on Earth, feldspar and quartz. The name feldspar comes from the Swedish words feld (“field”) and spar (“mineral”), because Swedish miners tended to come across the same rocks that Swedish farmers found themselves extracting from their fields.

REAL-LIFE A P P L I C AT I O N S Rocks and Human Existence

There are two other somewhat questionable classes of nonsilicate that might be included in a listing of minerals—organics and mineraloids. Though they have organic components, organics—for example, amber—originated in a geologic and not a biological setting. Mineraloids, among them, opal and obsidian, are not minerals because they lack the necessary crystalline struc-

Rocks are all around us, especially in our building materials but also in everything from jewelry to chalk. Then, of course, there are the rocks that exist in nature, whether in our backyards or in some more dramatic setting, such as a national park or along a rugged coastline. Indeed, humans have a long history of involvement with rocks— a history that goes far back to the aptly named Stone Age.

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CHICHÉN-ITZÁ,

A

MAYAN

STONE PYRAMID IN THE STATE OF

YUCATÁN, MEXICO. (© Ulrike Welsch/Photo Researchers. Repro-

duced by permission.)

The latter term refers to a period in which the most sophisticated human tools were those made of rock—that is, before the development of the first important alloy used in making tools, bronze. The Bronze Age began in the Near East in about 3300 B.C. and lasted until about 1200 B.C., when the development of iron-making technology introduced still more advanced varieties of tools.

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icas did not enter the Bronze Age for almost 4,000 years, in about A.D. 1100. Nor did they ever develop iron tools before the arrival of the Europeans in about 1500. T H E S T O N E AG E . In any case, the

These dates apply to the Near East, specifically to such areas as Mesopotamia and Egypt, which took the lead in ancient technology, followed much later by China and the Indus Valley civilization of what is now Pakistan. The rest of the world was even slower in adopting the use of metal: for instance, the civilizations of the Amer-

Stone Age, which practically began with the species Homo sapiens itself, was unquestionably the longest of the three ages. The Stone Age is divided into two periods: Paleolithic and Neolithic, sometimes called Old and New Stone Age, respectively. (There was also a middle phase, called the Mesolithic, but this term is not used as widely as Paleolithic or Neolithic.) Throughout much of this time, humans lived in rock caves and used rock tools, including arrowheads for

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killing animals and (relatively late in prehistory) flint for creating fire. The Paleolithic, characterized by the use of crude tools chipped from pieces of stone, began sometime between 2.5 and 1.8 million years ago and lasted until last ice age ended (and the present Holocene epoch began), about 10,000 years ago. The Neolithic period that followed saw enormous advances in technology, so many advances that historians speak of a “Neolithic Revolution” that included the development of much more sophisticated, polished tools. The mining of gold, copper, and various other ores began long before the development of the first alloys (bronze is formed by the mixture of copper and tin). Yet even after humans discovered metals, they continued to use stone tools. THE STONE

P Y RA M I D S A N D O T H E R S T R U C T U R E S . Indeed, the

great pyramids of Egypt, built during the period from about 2600-2400 B.C., were constructed primarily with the use of stone rather than metal tools. The structures themselves, of course, also reflect the tight connection between humans and rocks. Built of limestone, the pyramids are still standing some 4,500 years later, even as structures of clay and mud built at about the same time in Mesopotamia (a region poor in stone resources) have long since dwindled to dust. Incidentally, the great pyramids once had surfaces of polished limestone, such that they gleamed in the desert sun. Centuries later, Arab invaders in the seventh century A.D. stripped this limestone facing to use it in other structures, and the only part of the facing that remains today is high atop the pyramid of Khafre. For this reason, Khafre’s pyramid is slightly taller than the structure known as the Great Pyramid, that of Cheops, or Khufu, which was originally the largest pyramid.

Still later, medieval Europe built its cathedrals and castles of stone, though it should be noted that the idea of the castle came from the Middle East, where the absence of lumber for fortresses caused Syrian castle builders to make use of abundant sandstone instead. Other societies left behind their own great stone monuments: the Great Wall of China, Angkor Wat in southeast Asia, the pyramids of Central America and Machu Picchu in South America, the great cliffside dwellings of what is now the southwestern United States, and the stone churches of medieval Ethiopia.

Rocks

Certainly there were civilizations that created great structures of wood, but these structures were simply not as durable. The oldest wood building, a Buddhist temple at Horyuji in Japan, dates back only to A.D. 607, which, of course, is quite impressive for a wooden structure. But it hardly compares to what may well be the oldest known human structure, a windbreak discovered by the paleobiologist Mary Leakey (1913–1996) in Tanzania in 1960. Consisting of a group of lava blocks that form a rough circle, it is believed to be 1.75 million years old.

Mineralogy and Petrology Not surprisingly, mineralogy is concerned with minerals—their physical properties, chemical makeup, crystalline structures, occurrence, distribution, and physical origins. Researchers whose work focuses on the physical origins of minerals study data and draw on the principles of physics and chemistry to develop hypotheses regarding the ways minerals form. Other mineralogical studies may involve the identification of a newly discovered mineral or the synthesis of mineral-like materials for industrial purposes.

The centuries that have followed the building of those great structures likewise are defined, at least in part, by their buildings of stone. The Bible is full of references to stones, whether those used in building Solomon’s temple or the precious gemstones said to form the gates of the New Jerusalem described in the Book of Revelation. Greece and Rome, too, are known for their structures of stone, ranging from marble (limestone that has undergone metamorphism) to unconsolidated stones in early forms of concrete, pioneered by the Egyptians.

The study of rocks is called petrology, from a Greek root meaning “rock.” (Hence also the words petroleum and petrify.) Its areas of interest with regard to rocks are much the same as those of mineralogy as they relate to minerals: physical properties, distribution, and origins. It includes two major subdisciplines, experimental petrology, or the synthesis of rocks in a laboratory as a means of learning the conditions under which rocks are formed in the natural world, and petrography, or the study of rocks observed in thin sections through a petrographic microscope, which uses polarized light.

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LAVA

FLOW AFTER THE

1992

ERUPTION OF

MOUNT ETNA

IN

SICILY. WHEN

IT COOLS, LAVA BECOMES IGNEOUS

ROCK. (© B. Edmaier/Photo Researchers. Reproduced by permission.)

Owing to the fact that most rocks contain minerals, petrology draws on and overlaps with mineralogical studies to a great extent. At the same time, it goes beyond mineralogy, inasmuch as it is concerned with materials that contain organic substances, which are most likely to appear within the realm of sedimentary rock. Petrologists also are concerned with the other two principal types of rock, igneous and metamorphic.

Igneous rock is rock formed by the crystallization of molten materials. It most commonly is associated with volcanoes, though, in fact, it comes into play in the context of numerous plate tectonic

When igneous rocks form deep within the Earth, they are likely to have large crystals, an indication of the fact that a longer period of time elapsed while the magma was cooling. On the other hand, volcanic rocks and others that form at or near Earth’s surface are apt to have very small crystals. Obsidian (which, as we have noted, is not truly a mineral owing to its lack of

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processes, such as seafloor spreading (see Plate Tectonics). The molten rock that becomes igneous rock is known as magma when it is below the surface of the earth and lava when it is at or near the earth’s surface. Its most notable characteristic is its interlocking crystals. For the most part, igneous rocks do not have a layered texture.

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crystals) is formed when hot lava comes into contact with water; as a result, it cools so quickly that crystals never have time to develop. Sometimes called volcanic glass, it once was used by prehistoric peoples as a cutting tool. C LASS I F Y I N G A N D I D E N T I F Y I N G I G N E O U S R O C K S . Igneous rocks

can be classified in several ways, referring to the means by which they were formed, the size of their crystals, and their mineral content. Extrusive igneous rocks, ejected by volcanoes to crystallize at or near Earth’s surface, have small crystals, whereas intrusive igneous rocks, which cooled slowly beneath the surface, have larger crystals. Sometimes the terms plutonic and volcanic, which roughly correspond to intrusive and extrusive, respectively, are used. Igneous rocks made of fragments from volcanic explosions are known as pyroclastic, or “fire-broken,” rocks. Those that consist of dense, dark materials are known as mafic igneous rocks. On the other hand, those made of lightly colored, less-dense minerals, such as quartz, mica, and feldspar, are called felsic igneous rocks. Among the most well known varieties of igneous rock is granite, an intrusive, felsic rock that includes quartz, feldspar, mica, and amphibole in its makeup. Also notable is basalt, which is mafic and extrusive.

Sedimentary Rocks Earlier, we touched on the subject of consolidation, which can be explained in more depth within the context of sedimentary rock. Consolidation is the compacting of loose materials by any number of processes, including recrystallization and cementation. The first of these processes is the formation of new mineral grains as a result of changes in temperature, pressure, or other factors. In cementation, particles of sediment (material deposited at or near Earth’s surface from several sources, most notably preexisting rock) are cemented together, usually with mud.

metamorphism, discussed later in the context of metamorphic rock.

Rocks

F O R M AT I O N O F S E D I M E N TA RY R O C K S . Sedimentary rock is formed by

the deposition, compaction, and cementation of rock that has experienced weathering (breakdown of rock due to physical, chemical, or biological processes) or as a result of chemical precipitation. The latter term refers not to “precipitation” in terms of weather but to the formation of a solid from a liquid, by chemical rather than physical means. (The freezing of water, a physical process, is not an example of precipitation.) Sedimentary rock usually forms at or near the surface of the earth, as the erosive action of wind, water, ice, gravity, or a combination of these forces moves sediment. Yet this formation also may occur when chemicals precipitate from seawater or when organic material, such as plant debris or animal shells, accumulate. Evaporation of saltwater, for instance, produces gypsum, a mineral noted for its lack of thermal conductivity; hence its use in drywall, the material that covers walls in most modern homes. (Ancient peoples made alabaster, a fine-grained ornamental stone, from gypsum.) C LASS I F I CAT I O N A N D S I Z E S .

Sedimentary rock is classified with reference to the size of the particles from which the rock is made as well as the origin of those particles. Clastic rock comes from fragments of preexisting rock (whether igneous, sedimentary, or metamorphic) and organic matter, while nonclastic sedimentary rock is formed either by precipitation or by organic means. Examples include gypsum, salts, and other rocks formed by precipitation of saltwater as well as those created from organic material or organic activity—coal, for example.

Compaction, recrystallization, and other processes, such as dehydration (which also may contribute to compaction), are collectively known as diagenesis. The latter term refers to all the changes experienced by a sediment sample under conditions of low temperature and low pressure following deposition. If the temperature and pressure increase, diagenesis may turn into

Ranging in size from fine clay (less than 0.00015 in., or 0.004 mm) to boulders (defined as any rock larger than 10 in., or 0.254 m), sedimentary rock bears a record of the environment in which the original sediments were deposited. This record lies in the sediment itself. For example, rocks containing conglomerate, material ranging in size from clay to boulders (including the intermediate categories of silt, sand, gravel, pebbles, and cobble), come from sediment that was deposited rapidly as the result of slides or slumps. (Slides and slumps are discussed in Mass Wasting.)

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Often formed in mountain environments, metamorphic rocks include such well-known varieties as marble, slate, and gneiss—metamorphosed forms of limestone, shale, and granite, respectively. Also notable is schist, composed of various minerals, such as talc, mica, and muscovite. There is not always a one-to-one correspondence between precursor rocks and metamorphic ones: increasing temperature and pressure can turn shale progressively into slate, phyllite, schist, and gneiss.

Rocks

METAMORPHIC

ROCK IS FORMED BY THE ALTERATION OF

PREEXISTING ROCK.

THE

PRESENCE OF MICA, SHOWN

HERE, IS A SIGN THAT ROCK MIGHT BE METAPHORPHIC.

(© C. D. Winters/Photo Researchers. Reproduced by permission.)

Sedimentary rocks are of particular interest to paleontologists, stratigraphers, and others working in the field of historical geology, because they are the only kinds of rock in which fossils are preserved. The pressure and temperature levels that produce igneous and metamorphic rock would destroy the organic remnants that produce fossils; on the other hand, sedimentary rock—created by much less destructive processes—permits the formation of fossils. Thus, the study of these formations has contributed greatly to geologists’ understanding of the distant past. (See the essays Historical Geology, Stratigraphy, and Paleontology. For more about sedimentary rock, see Sediment and Sedimentation.)

M E TA M O R P H I S M . Given the conditions described for metamorphism, one might conclude that in terms of violence, drama, and stress, it is a process somewhere between sedimentation and the formation of igneous rock. That, in fact, is precisely the case: the temperature and pressure conditions necessary for metamorphism lie between those of diagenesis, on the one hand, and the extreme conditions necessary for the production of igneous rock, on the other hand. Specifically, metamorphism occurs at temperatures between 392°F (200°C) and 1,472°F (800°C) and under levels of pressure between 1,000 and 10,000 bars. (A bar is slightly less than the standard atmospheric pressure at sea level. The latter, equal to 14.7 lb. per square inch, or 101,325 Pa, is equal to 1.01325 bars.)

Metamorphic rock is formed through the alteration of preexisting rock as a result of changes in temperature, pressure, or the activity of fluids (usually gas or water). These changes in temperature must be extreme (figures are given later), such that the preexisting rock—whether igneous, sedimentary, or metamorphic—is no longer stable.

There are several types of metamorphism: regional, contact, dynamic, and hydrothermal. Regional metamorphism results from a major tectonic event or events, producing widespread changes in rocks. Contact or thermal metamorphism results from contact between igneous intrusions and cooler rocks above them, which recrystallize as a result of heating. Dynamic metamorphism takes place in the high-pressure conditions along faults. Finally, hydrothermal metamorphism ensues from contact with fluids

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The presence of mica in a rock—or of other minerals, including amphibole, staurolite, and garnet—is a sign that the rock might be metamorphic. These minerals, typical of metamorphic rocks, are known as metamorphic facies. Also indicative of metamorphism are layers in the rock, more or less parallel lines along which minerals are laid as a result of the high pressures applied to the rock in its formation. Metamorphism, the process whereby metamorphic rock is created, also may produce characteristic formations, such as an alignment of elongate crystals or the separation of minerals into layers.

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Rocks

KEY TERMS ALLOY:

A mixture of two or more

metals.

EROSION:

The movement of soil and

rock as the result of forces produced by A process of consoli-

water, wind, glaciers, gravity, and other

dation whereby particles of sediment are

influences. In most cases, a fluid medium,

cemented together, usually with mud.

such as air or water, is involved.

CEMENTATION:

A substance made up of

COMPOUND:

IGNEOUS ROCK:

One of the three

atoms of more than one element, chemi-

principal types of rock, along with sedi-

cally bonded to one another.

mentary and metamorphic rock. Igneous

CONGLOMERATE:

Unconsolidated

rock material containing rocks ranging in size from very small clay (less than 0.00015 in., or 0.004 mm) to boulders (defined as

rock is formed by the crystallization of molten materials, for instance, in a volcano or other setting where plate tectonic processes take place. Molten rock at or near the sur-

any rock larger than 10 in., or 0.254 m).

LAVA:

Sedimentary rock often appears in the

face of the earth that becomes igneous

form of conglomerate.

rock. Below the surface, lava is known as

CONSOLIDATION:

A process whereby

magma. Molten rock beneath the sur-

materials become compacted, or experi-

MAGMA:

ence an increase in density. This takes place

face of the earth that becomes igneous

through several processes, including

rock. Once it is at or near the surface,

recrystallization and cementation.

magma is known as lava.

CRYSTALLINE

SOLID:

A type of

MINERAL:

A naturally occurring, typi-

solid in which the constituent parts have a

cally inorganic substance with a specific

simple and definite geometric arrange-

chemical composition and a crystalline

ment that is repeated in all directions.

structure.

DEPOSITION:

The process whereby

MINERALOGY:

An area of geology

sediment is laid down on the Earth’s sur-

devoted to the study of minerals. Mineral-

face.

ogy includes several subdisciplines, such as

DIAGENESIS:

A term referring to

all the changes experienced by a sedi-

crystallography, the study of crystal formations within minerals. A substance with a variable

ment sample under conditions of low

MIXTURE:

temperature and low pressure following

composition, meaning that it is composed

deposition. Higher temperature and

of molecules or atoms of differing types in

pressure conditions may lead to meta-

varying proportions.

morphism.

ORE:

ELEMENT:

A substance made up of

A rock or mineral possessing eco-

nomic value.

only one kind of atom. Unlike compounds,

ORGANIC:

elements cannot be broken chemically into

the term organic only in reference to living

other substances.

things. Now the word is applied to most

S C I E N C E O F E V E RY DAY T H I N G S

At one time, chemists used

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Rocks

KEY TERMS compounds containing carbon and hydrogen, thus excluding carbonates (which are minerals), and oxides such as carbon dioxide.

CONTINUED

though most likely it is silica (SiO2). Sand is also a term used for a size of rock ranging from very fine to very coarse. SEDIMENT:

An area of geology devoted to the study of rocks, including their physical properties, distribution, and origins. PETROLOGY:

In the context of chemistry, precipitation refers to the formation of a solid from a liquid. PRECIPITATION:

The formation of new mineral grains as a result of changes in temperature, pressure, or other factors. RECRYSTALLIZATION:

An aggregate of minerals or organic matter, which may be consolidated or unconsolidated.

ROCK:

near Earth’s surface from a number of sources, most notably preexisting rock. SEDIMENTARY ROCK:

One of the

three major types of rock, along with igneous and metamorphic rock. Sedimentary rock usually is formed by the deposition, compaction, and cementation of rock that has experienced weathering. It also may be formed as a result of chemical precipitation. UNCONSOLIDATED

ROCK:

Rock

that appears in the form of loose particles, such as sand. A process whereby the surface

The ongoing process whereby rocks continually change from one type to another, typically through melting, metamorphism, uplift, weathering, burial, or other processes.

UPLIFT:

A term that can have several meanings. The sand at a beach could be a variety of unconsolidated materials,

rocks and minerals at or near the surface of

ROCK CYCLE:

SAND:

heated by igneous rock. Reacting with minerals in the surrounding rock, the fluids produce different minerals, which, in turn, yield metamorphic rocks.

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Material deposited at or

of Earth rises, owing to either a decrease in downward force or an increase in upward force. WEATHERING:

The breakdown of

Earth as the result of physical, chemical, or biological processes.

TYPES OF M E TA M O R P H I C R O C K S . Metamorphic rocks that contain

unfoliated rocks. As a foliated metamorphic rock, slate is particularly good for splitting into thin layers—hence one of its most important applications is in making shingles for roofing. By contrast, marble, which is unfoliated, is valued precisely for its lack of tendency to split.

elongate or platy minerals, such as mica and amphibole, are called foliated rocks. These rocks have a layered texture, which may manifest as the almost perfect arrangement of materials in slate or as the alternating patterns of light and dark found in some other varieties of rock. Metamorphic rocks without visible layers are referred to as

Petrologists attempting to determine exactly which rocks or combinations of rocks metamorphosed to produce a particular sample often face a challenge. Many metamorphic rocks are stubborn about giving up their secrets; on the other hand, it is possible to match up precursor rocks with certain varieties. For example, as noted ear-

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lier, marble comes from limestone, while gneiss usually (but not always) comes from granite. Quartzite is metamorphosed sandstone. Nonetheless, it is not as easy to trace the history of a metamorphic rock as it is to say that a raisin was once a grape or that a pickle was once a cucumber.

Where to Find Rocks In general, one might find igneous rocks such as basalt in any place known for volcanic activity either in the recent or distant past. This would include such well-known areas of volcanism as Hawaii, the Philippines, and Italy, but also places where volcanic activity occurred in the distant past. (See, for instance, the discussion in the essay titled “Paleontology” regarding possible volcanic activity in what is now the continental United States at the conclusion of the Triassic period.) The best place for metamorphic rock would be in areas of mountain-building and powerful tectonic activity, as for instance in the Himalayas or the Alps of central Europe. Sedimentary rock is basically everywhere, but a good place to find large samples of it would include areas with large oil deposits, which are always found in sedimentary rock. Closer to home, a wide array of sedimentary rocks can be located in the plains and lowlands of the United States, particularly in the West and Midwest, where large samples are exposed. Igneous and metamorphic rocks can be found, predictably, in regions where mountains provide evidence of past tectonic activity: New England, the Appalachians, and the various mountain ranges of the western United States such as the Rockies, Cascades, and Sierra Nevada.

The Rock Cycle Given what we have seen about the characteristics of the three rock varieties—igneous, sedimentary, and metamorphic—it should be clear that there is no such thing as a rock that simply is what it is, without any possibility of changing.

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Rocks, in fact, are constantly changing, as is Earth itself. This process whereby rocks continually change from one type to another—typically through melting, metamorphism, uplift, weathering, burial, or other processes—is known as the rock cycle.

Rocks

The rock cycle can go something like this: Exposed to surface conditions such as wind and the activity of water, rocks experience weathering. The result is the formation of sediments that are eventually compacted to make sedimentary rocks. As the latter are buried deeper and deeper beneath greater amounts of sediment, the pressure and temperature builds. This process ultimately can result in the creation of metamorphic rock. On the other hand, the rock may undergo such extreme conditions of temperature that it recrystallizes to form igneous rock. Whatever the variety—igneous, sedimentary, or metamorphic—the rock likely will be in a position eventually to experience erosion, in which case the rock cycle begins all over again. WHERE TO LEARN MORE Atlas of Igneous and Metamorphic Rocks, Minerals, and Textures (Web site). . Bishop, A. C., Alan Robert Woolley, and William Roger Hamilton. Cambridge Guide to Minerals, Rocks, and Fossils. New York: Cambridge University Press, 1999. Busbey, Arthur Bresnahan. Rocks and Fossils. Alexandria, VA: Time-Life Books, 1996. Discovery Channel Rocks and Minerals: An Explore Your World Handbook. New York: Discovery Books, 1999. “The Essential Guide to Rocks.” BBC Education (Web site). . “Igneous, Sedimentary, and Metamorphic Rock Info.” University of British Columbia (Web site). . RocksForKids.com (Web site). . Rocks and Minerals (Web site). . Symes, R. F., Colin Keates, and Andreas Einsiedel. Rocks and Minerals. New York: Dorling Kindersley, 2000. Vernon, R. H. Beneath Our Feet: The Rocks of Planet Earth. New York: Cambridge University Press, 2000.

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ECONOMIC GEOLOGY Economic Geology

CONCEPT Economic geology is the study of fuels, metals, and other materials from the earth that are of interest to industry or the economy in general. It is concerned with the distribution of resources, the costs and benefits of their recovery, and the value and availability of existing materials. These materials include ore (rocks or minerals possessing economic value) as well as fossil fuels, which embrace a range of products from petroleum to coal. Rooted in several subdisciplines of the geologic sciences—particularly geophysics, structural geology, and stratigraphy—economic geology affects daily life in myriad ways. Masonry stones and gasoline, gypsum wallboard (sometimes known by the brand name Sheetrock) and jewelry, natural gas, and table salt—these and many more products are the result of efforts in the broad field known as economic geology.

HOW IT WORKS Background of Economic Geology

154

organic resources alongside valuable inorganic ones. The concept of economic geology as such is a relatively new one, even though humans have been extracting metals and minerals of value from the ground since prehistoric times. For all their ability to appreciate the worth of such resources, however, premodern peoples possessed little in the way of scientific theories regarding either their formation or the means of extracting them. The Greeks, for instance, believed that veins of metallic materials in the earth indicated that those materials were living things putting down roots after the manner of trees. Astrologers of medieval times maintained that each of the “seven planets” (Sun, Moon, and the five planets, besides Earth, known at the time) ruled one of the seven known metals—gold, copper, silver, lead, tin, iron, and mercury—which supposedly had been created under the influence of their respective “planets.” AG R I C O LA’ S

CONTRIBUTION.

Some sources of information in the geologic sciences use a definition of “economic geology” narrower than the one applied here. Rather than including nonmineral resources that develop in and are recovered from a geologic environment—a category that consists primarily of fossil fuels—this more limited definition restricts the scope of economic geology to minerals and ores. Given the obvious economic importance of fossil fuels such as petroleum and its many byproducts as well as coal and peat, however, it seems only appropriate to discuss these valuable

The first thinker who attempted to go beyond such unscientific (if imaginative) ideas was a German physician writing under the Latinized name Georgius Agricola (1494–1555). As a result of treating miners for various conditions, Agricola, whose real name was Georg Bauer, became fascinated with minerals. The result was a series of written works, culminating with De re metallica (On the nature of minerals, 1556, released postthumusly), that collectively initiated the modern subdiscipline of physical geology. (It is worth noting that the first translators of Metallica into English were Lou [d. 1944] and Herbert Clark Hoover [1874–1964]. The couple pub-

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lished their translation in 1912 in London’s Mining Magazine, and the husband went on to become the thirty-first president of the United States in 1929.) Rejecting the works of the ancients and all manner of fanciful explanations for geologic phenomena, Agricola instead favored careful observation, on the basis of which he formed verifiable hypotheses. Regarded as the father of both mineralogy and economic geology, Agricola introduced several ideas that provided a scientific foundation for the study of Earth and its products. In De ortu et causis subterraneorum (1546), he critiqued all preceding ideas regarding the formation of ores, including the Greek and astrological notions mentioned earlier as well as the alchemical belief that all metals are composed of mercury and sulfur. Instead, he maintained that subterranean fluids carry dissolved minerals, which, when cooled, leave deposits in the cracks of rocks and thus give rise to mineral veins. Agricola’s ideas later helped form the basis for modern theories regarding the formation of ore deposits. In De natura fossilium (On the nature of fossils, 1546), Agricola also introduced a method for the classification of “fossils,” as minerals were then known. Agricola’s system, which categorizes minerals according to such properties as color, texture, weight, and transparency, is the basis for the system of mineral classification in use today. Of all his works, however, the most important was De re metallica, which would remain the leading textbook for miners and mineralogists during the two centuries that followed. In this monumental work, he introduced many new ideas, including the concept that rocks contain ores that are older than the rocks themselves. He also explored in detail the mining practices in use during his time, itself an extraordinary feat in that miners of the sixteenth century tended to guard their trade secrets closely.

Metals, Minerals, and Rocks Of all known chemical elements, 87, or about 80%, are metals. The latter group is identified as being lustrous or shiny in appearance and malleable or ductile, meaning that they can be molded into different shapes without breaking. Despite their ductility, metals are extremely durable, have high melting and boiling points, and are excellent conductors of heat and electric-

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ity. Some, though far from all, register high on the Mohs hardness scale, discussed later in the context of minerals.

Economic Geology

The bonds that metals form with each other, or with nonmetals, are known as ionic bonds, the strongest type of chemical bond. Even within a metal, however, there are extremely strong, nondirectional bonds. Therefore, though it is easy to shape metals, it is very difficult to separate metal atoms. Obviously, most metal are solids at room temperature, though this is not true of all: mercury is liquid at ordinary temperatures, and gallium melts at just 85.6°F (29.76°C). Generally, however, metals would be described as crystalline solids, meaning that their constituent parts have a simple and definite geometric arrangement that is repeated in all directions. Crystalline structure is important also within the context of minerals as well as the rocks that contain them. M I N E RA L S . Whereas there are only 87 varieties of metal, there are some 3,700 types of mineral. There is considerable overlap between metals and minerals, but that overlap is far from complete: many minerals include nonmetallic elements, such as oxygen and silicon. A mineral is a substance that appears in nature and therefore cannot be created artificially, is inorganic in origin, has a definite chemical composition, and possesses a crystalline internal structure.

The term organic does not refer simply to substances with a biological origin; rather, it describes any compound that contains carbon, with the exception of carbonates (which are a type of mineral) and oxides, such as carbon dioxide or carbon monoxide. The fact that a mineral must be of nonvarying composition limits minerals almost exclusively to elements and compounds—that is, either to substances that cannot be broken down chemically to yield simpler substances or to substances formed by the chemical bonding of elements. Only in a few highly specific circumstances are naturally occurring alloys, or mixtures of metals, considered minerals. M I N E RA L

GROUPS.

Minerals are

classified into eight basic groups: • • • •

Class 1: Native elements Class 2: Sulfides Class 3: Oxides and hydroxides Class 4: Halides

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• Class 5: Carbonates, nitrates, borates, iodates • Class 6: Sulfates, chromates, molybdates, tungstates • Class 7: Phosphates, arsenates, vanadates • Class 8: Silicates The first group, native elements, includes metallic elements that appear in pure form somewhere on Earth; certain metallic alloys, alluded to earlier; as well as native nonmetals, semimetals, and minerals with metallic and nonmetallic elements. The native elements, along with the six classes that follow them in this list, are collectively known as nonsilicates, a term that emphasizes the importance of the eighth group. (For more about the nonsilicates, as well as other subjects covered in the present context, see Minerals.)

the Mohs scale, introduced in 1812 by the German mineralogist Friedrich Mohs (1773–1839), which rates minerals from 1 (talc) to 10 (diamond.) Though it is useful for geologists attempting to identify a mineral in the field, the Mohs scale is not considered helpful for the industrial testing of fine-grained materials, such as steel or ceramics. For such purposes, the Vickers or Knoop scales are applied. These scales (named, respectively, after a British company and an American official) also have an advantage over Mohs in that they offer a precise, proportional scale in which each increase of number indicates the same increase in hardness. By contrast, on the Mohs scale, an increase from 3 to 4 (calcite to fluorite) indicates an additional 25% in hardness, whereas a shift from 9 to 10 (corundum to diamond) marks an increase of 300%.

The vast majority of minerals, including the most abundant ones, belong to the silicates class, which is built around the element silicon. Just as carbon can form long strings of atoms, particularly in combination with hydrogen (as we discuss in the context of fossil fuels later in this essay), silicon also forms long strings, though its “partner of choice” is typically oxygen rather than hydrogen. Together with oxygen, silicon— known as a metalloid because it exhibits characteristics of both metals and nonmetals—forms the basis for an astonishing array of products, both natural and man-made, which we examine in brief later.

Other properties significant in identifying minerals are color; streak, or the appearance of the powder produced when one mineral is scratched by a harder one; luster, the appearance of a mineral when light reflects off its surface; cleavage, the planes across which a mineral breaks; fracture, the tendency to break along something other than a flat surface; density, or ratio of mass to volume; and specific gravity, or the ratio between the mineral’s density and that of water. Sometimes minerals can be identified in terms of qualities unique to a specific mineral group or groups: magnetism, radioactivity, fluorescence, phosphorescence, and so on. (For more about mineral characteristics, see Minerals.)

C H A RA C T E R I S T I C S O F M I N E RA L S . From the list of parameters first

developed by Agricola has grown a whole array of characteristics by which minerals are classified. These characteristics also can be used to evaluate an unknown mineral and thus to determine the mineral class within which it fits. One such parameter is the type of crystal of which a mineral is composed. Though there are thousands of minerals, there are just six crystal systems, or basic geometric shapes formed by crystals. Crystallographers, mineralogists concerned with the study of crystal structures, are able to identify the crystal system (the simplest being isometric, or cubic) by studying a good specimen of a mineral and observing the faces of the crystal and the angles at which they meet.

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R O C K S . A rock is an aggregate of minerals or organic material, which can appear in consolidated or unconsolidated form. Rocks are of three different types: igneous, formed by crystallization of molten minerals, as in a volcano; sedimentary, usually formed by deposition, compaction, or cementation of weathered rock; and metamorphic, formed by alteration of preexisting rock. Rocks made from organic material are typically sedimentary, an example being coal.

Minerals also can be identified by their hardness, defined as the ability of one mineral to scratch another. Hardness can be measured by

Rocks have possessed economic importance from a time long before “economics” as we know it existed—a time when there was nothing to buy and nothing to sell. That time, of course, would be the Stone Age, which dates back practically to the beginnings of the human species and overlapped with the beginnings of civilization some 5,500 years ago. In the hundreds of thousands of years when stone constituted the most advanced

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toolmaking material, humans developed an array of stone devices for making fire, sharpening knives, killing animals (and other humans), cutting food or animal skins, and so on. The Stone Age, both in the popular imagination and (with some qualifications) in actual archaeological fact, was a time when people lived in caves. Since that time, of course, humans have generally departed from the caves, though exceptions exist, as the United States military found in 2001 when attempting to hunt for terrorists in the caves of Afghanistan. In any case, the human attachment to stone dwellings has taken other forms, beginning with the pyramids and continuing through today’s masonry homes. Nor is rock simply a structural material for building, as the use of gypsum wallboard, slate countertops, marble finishes, and graveled walkways attests. And, of course, construction is only one of many applications to which rocks and minerals are directed, as we shall see.

Hydrocarbons As noted earlier, the focus of economic geology is on both rocks and minerals, on the one hand, and fossil fuels, on the other. The latter may be defined as fuel (specifically, coal, oil, and gas) derived from deposits of organic material that have experienced decomposition and chemical alteration under conditions of high pressure. Given this derivation from organic material, by definition all fossil fuels are carbon-based, and, specifically, they are built around hydrocarbons—chemical compounds whose molecules are made up of nothing but carbon and hydrogen atoms. Theoretically, there is no limit to the number of possible hydrocarbons. Carbon forms itself into apparently limitless molecular shapes, and hydrogen is a particularly versatile chemical partner. Hydrocarbons may form straight chains, branched chains, or rings, and the result is a variety of compounds distinguished not by the elements in their makeup or even (in some cases) by the numbers of different atoms in each molecule, but rather by the structure of a given molecule.

names as methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10). The first four, being the lowest in molecular mass, are gases at room temperature, while the heavier ones—including octane (C8H18)—are oily liquids. Alkanes even heavier than octane tend to be waxy solids, an example being paraffin wax, for making candles. With regard to octane, incidentally, there is a reason why its name is so familiar, while that of heptane (C7H16) is not. Heptane does not fire smoothly in an internal-combustion engine and therefore disrupts the engine’s rhythm. For this reason, it has a rating of zero on a scale of desirability, while octane has a rating of 100. This is why gas stations list octane ratings at the pump: the higher the content of octane, the better the gas is for one’s automobile. In a hydrocarbon chain, if one or more hydrogen atoms is removed, a new bond may be formed. The hydrocarbon chain is then named by adding the suffix yl—hence such names as methyl, ethyl, and so on. This indicates that the substance is an alkane, and that something other than hydrogen can be attached to the chain; for example, the attachment of a chlorine atom could yield methyl chloride. Two other large structural groups of hydrocarbons are alkenes and alkynes, which contain double or triple bonds between carbon atoms. Such hydrocarbons are unsaturated—in other words, if the double or triple bond is broken, some of the carbon atoms are then free to form other bonds. Among the products of these groups is the alkene known as acetylene, or C2H2, used for welding steel. In addition to alkanes, alkenes, and alkynes, all of which tend to form carbon chains, there are the aromatic hydrocarbons, a traditional name that actually has nothing to do with smell.

Among the various groups of hydrocarbons are alkanes or saturated hydrocarbons, so designated because all the chemical bonds are filled to their capacity (that is, “saturated”) with hydrogen atoms. Included among them are such familiar

All aromatic hydrocarbons contain what is known as a benzene ring, which has the chemical formula C6H6 and appears in characteristic ring shapes. In this group are such products as naphthalene, toluene, and dimethyl benzene. These last two are used as solvents as well as in the synthesis of drugs, dyes, and plastics. One of the more famous (or infamous) products in this part of the vast hydrocarbon network is trinitrotoluene, or TNT. Naphthalene is derived from coal tar and used in the synthesis of other compounds. A crystalline solid with a powerful odor,

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it is found in mothballs and various deodorant disinfectants.

REAL-LIFE A P P L I C AT I O N S Fossil Fuels The organic material that has decomposed to create the hydrocarbons in fossil fuels comes primarily from dinosaurs and prehistoric plants, though it just as easily could have come from any other organisms that died in large numbers a long, long time ago. To form petroleum, there must be very large quantities of organic material deposited along with sediments and buried under more sediment. The accumulated sediments and organic material are called source rock. What happens after accumulation of this material is critical and depends a great deal on the nature of the source rock. It is important that the organic material—for example, the vast numbers of dinosaurs that died in a mass extinction about 65 million years ago (see Paleontology)—not be allowed simply to rot, as would happen in an aerobic, or oxygen-containing, environment. Instead, the organic material undergoes transformation into hydrocarbons as a result of anaerobic chemical activity, or activity that takes place in the absence of oxygen. Good source rocks for this transformation are shale or limestone, provided the particular rocks are composed of between 1% and 5% organic carbon. The source rocks should be deep enough that the pressure heats the organic material, yet not so deep that the pressure and temperature cause the rocks to undergo metamorphism or transform them into graphite or other non-hydrocarbon versions of carbon. Temperatures of up to 302°F (150°C) are considered optimal for petroleum generation. Once generated, petroleum gradually moves from the source rock to a reservoir rock, or a rock that stores petroleum in its pores. A good reservoir rock is one in which the pore space constitutes more than 30% of the rock volume. Yet the rock must be sealed by another rock that is much less porous; indeed, for a seal or cap rock, as it is called, a virtually impermeable rock is preferred. Thus, the best kind of seal-forming rock is one made of very small, closely fitting pieces of sediment, for instance, shale. Such a

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rock is capable of holding petroleum in place for millions of years until it is ready to be discovered and used. HUMANS

AND

PETROLEUM.

People have known about petroleum from prehistory, simply because there were places on Earth where it literally seeped from the ground. The modern era of petroleum drilling, however, began in 1853, when an American lawyer named George Bissell (1821–1884) recognized its potential for use as a lamp fuel. He hired “Colonel” Edwin Drake (1819–1880) to oversee the drilling of an oil well at Titusville, Pennsylvania, and in 1859 Drake struck oil. The legend of “black gold,” of fortunes to be made by drilling holes in the ground, was born. In the wake of the development and widespread application of the internal-combustion engine during the latter part of the nineteenth and the early part of the twentieth centuries, interest in oil became much more intense, and wells sprouted up around the world. Sumatra, Indonesia, yielded oil from its first wells in 1885, and in 1901, successful drilling began in Texas— the source of many a Texas-sized fortune. An early form of the company known today as British Petroleum (BP) discovered the first Middle Eastern oil in Persia (now Iran) in 1908. Over the next 50 years, the economic importance and prospects of that region changed considerably. With the vast expansion in automobile ownership that began following World War I (1914–1918) and reached even greater heights after World War II (1939–1945), the value and importance of petroleum soared. The oil industry boomed, and, as a result, many geologists found employment in a sector that offered far more in the way of financial benefits than university or government positions ever could. Today geologists assist their employers in locating oil reserves, not an easy task because so many variables must line up to produce a viable oil source. Given the cost of drilling a new oil well, which may run to $30 million or more, it is clearly important to exercise good judgment in assessing the possibilities of finding oil. The oil industry has been fraught with environmental concerns over the impact of drilling (much of which takes place offshore, on rigs placed in the ocean); possible biohazards associated with spills, such as the one involving the Exxon Valdez in 1989; and the effect on the

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BLOCKS OF SHALE AT THE PARAHO OIL SHALE FACILITY IN COLORADO. SHALE IS KNOWN AS A SEAL or cap rock, a virtually impermeable rock made of small, closely fitting pieces of sediment that covers a more porous rock holding petroleum. (© U.S. Department of Energy/Photo Researchers. Reproduced by permission.)

atmosphere of carbon monoxide and other greenhouse gases produced by petroleum-burning internal-combustion engines. There is even more wide-ranging concern over United States

dependence on oil sources in foreign countries (some of which are openly hostile to the United States) as well as the possible dwindling of resources.

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OFFSHORE

OIL RIG AT

SABINE PASS, TEXAS. (© Garry D. McMichael/Photo Researchers. Reproduced by permission.)

At the present rate of consumption, oil reserves will be exhausted by about the year 2040, but this takes into account only reserves that are considered viable today. As exploration continues, the tapping of United States reserves, such as those in Alaska, will become more and more profitable, leading to increased exploitation of U.S. resources and decreased dependence on oil produced by Middle Eastern states, many of which openly or covertly support terrorist attacks against the United States. In the long run, however, it will be necessary to develop new means of fueling the industrialized world, because petroleum is a nonrenewable resource: there is only so much of it underground, and when it is gone, it will not be replaced for millions of years (if at all). P E T R O C H E M I CA L S . In the mean-

time, however, petroleum—a mixture of alkanes, alkenes, and aromatic hydrocarbons—makes the world (or at least the industrialized world) go ’round. Petroleum itself is a raw material from which numerous products, collectively known as petrochemicals or petroleum derivatives, are obtained. Through a process termed fractional distillation, the petrochemicals of the lowest molecular mass boil off first, and those having higher mass separate at higher temperatures.

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Natural gas separates from petroleum at temperatures below 96.8°F (36°C)—far lower than the boiling point of water. At somewhat higher temperatures, petroleum ether and naphtha, both solvents (naphtha is used in paint thinner), separate; then, in the region between 156.2°F and 165.2°F (69–74°C), gasoline separates. Still higher temperatures yield other substances, each thicker than the one before it: kerosene; fuel for heating and the operation of diesel engines; lubricating oils; petroleum jelly; paraffin wax; and pitch, or tar. A host of other organic chemicals, including various drugs, plastics, paints, adhesives, fibers, detergents, synthetic rubber, and agricultural chemicals, owe their existence to petrochemicals.

Silicon, Silicates, and Other Compounds It was stated earlier that both carbon and silicon have the tendency to produce long strings of atoms, usually in combination with hydrogen in the first case and oxygen in the second. This is no accident, since silicon lies just below carbon on the periodic table of elements and they share certain chemical features (see Minerals). Just as carbon is at the center of a vast world of hydrocarbons, so silicon is equally important to inorganic

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substances ranging from sand or silica (SiO2) to silicone (a highly versatile set of silicon-based products), to the rocks known as silicates.

Economic Geology

Silicates are the basis for several well-known mineral types, including garnet, topaz, zircon, kaolinite, talc, mica, and the two most abundant minerals on Earth, feldspar and quartz. (Note that most of the terms used here refer to a group of minerals, not to a single mineral.) Made of compounds formed around silicon and oxygen and comprising various metals, such as aluminum, iron, sodium, and potassium, the silicates account for 30% of all minerals. As such, they appear in everything from gemstones to building materials; yet they are far from the only notable products centered around silicon. SILICONE AND OTHER COMP O U N D S . Silicone is not a mineral; rather, it

is a synthetic product often used as a substitute for organic oils, greases, and rubber. Instead of attaching to oxygen atoms, as in a silicate, silicon atoms in silicone attach to organic groups, that is, molecules containing carbon. Silicone oils frequently are used in place of organic petroleum as a lubricant because they can withstand greater variations in temperature. And because the body tolerates the introduction of silicone implants better than it does organic ones, silicones are used in surgical implants as well. Silicone rubbers appear in everything from bouncing balls to space vehicles, and silicones are also present in electrical insulators, rust preventives, fabric softeners, hair sprays, hand creams, furniture and automobile polishes, paints, adhesives, and even chewing gum. Even this list does not exhaust the many applications of silicon, which (together with oxygen) accounts for the vast majority of the mass in Earth’s crust. Owing to its semimetallic qualities, silicon is used as a semiconductor of electricity. Computer chips are tiny slices of ultrapure silicon, etched with as many as half a million microscopic and intricately connected electronic circuits. These chips manipulate voltages using binary codes, for which 1 means “voltage on” and 0 means “voltage off.” By means of these pulses, silicon chips perform multitudes of calculations in seconds—calculations that would take humans hours or months or even years.

SILICONE, A USES, FROM

SYNTHETIC PRODUCT, HAS A VARIETY OF ELECTRICAL INSULATORS TO FABRIC SOF-

TENERS TO PAINTS AND ADHESIVES.

IT

IS TOLERATED

BETTER BY THE HUMAN BODY THAN ORGANIC COMPOUNDS AND OFTEN IS USED FOR SURGICAL IMPLANTS.

(© Michelle Del Guercio/Photo Researchers. Reproduced by permission.)

such as electronics components, to keep them dry. Silicon carbine, an extremely hard crystalline material manufactured by fusing sand with coke (almost pure carbon) at high temperatures, has applications as an abrasive.

Ores Earlier, it was stated that an ore is a rock or mineral that possesses economic value. This is true, but a more targeted definition would include the adjective metalliferous, since economically valuable minerals that contain no metals usually are treated as a separate category, industrial minerals. Indeed, it can be said that the interests of economic geology are divided into three areas: ores, industrial minerals, and fuels, which we have discussed already.

A porous form of silica known as silica gel absorbs water vapor from the air and is often packed alongside moisture-sensitive products,

The very word ore seems to call to mind one of the oldest-known metals in the world and probably the first material worked by prehistoric metallurgists: gold. Even the Spanish word for gold, oro, suggests a connection. When conquistadors from Spain arrived in the New World after about 1500, oro was their obsession, and it was

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said that the Spanish invaders of Mexico found every bit of gold or silver ore located at the surface of the earth. However, miners of the sixteenth century lacked much of the knowledge that helps geologists today find ore deposits that are not at the surface. LO CAT I N G A N D E X T RAC T I N G O R E S . The modern approach uses knowl-

edge gained from experience. As in Agricola’s day, much of the wealth possessed by a mining company is in the form of information regarding the means of best seeking out and retrieving materials from the solid earth. Certain surface geochemical and geophysical indicators help direct the steps of geologists and miners searching for ore. Thus, by the time a company in search of ore begins drilling, a great deal of exploratory work has been done. Only at that point is it possible to determine the value of the deposits, which may simply be minerals of little economic interest. It is estimated that a cubic mile (1.6 km3) of average rock contains about $1 trillion worth of metals, which at first sounds promising—until one does the math. A trillion dollars is a lot of money, but 1 cu. mi. (equal to 5,280  5,280  5,280 ft., or 1,609 km3) is a lot of space too. The result is that 1 cu. ft. (0.028 m3) is worth only about $6.79. But that is an average cubic foot in an average cubic mile of rock, and no mining company would even consider attempting to extract metals from an average piece of ground. Rather, viable ore appears only in regions that have been subjected to geologic processes that concentrate metals in such a way that their abundance is usually many hundreds of times greater than it would be on Earth as a whole. Ore contains other minerals, known as gangue, which are of no economic value but which serve as a telltale sign that ore is to be found in that region. The presence of quartz, for example, may suggest deposits of gold. Ore may appear in igneous, metamorphic, or sedimentary deposits as well as in hydrothermal fluids. The latter are emanations from igneous rock, in the form of gas or water, that dissolve metals from rocks through which they pass and later deposit the ore in other locations.

And, of course, there is the environmental stress created by mining—not just by the immediate impact of cutting a gash in Earth’s surface, which may disrupt ecosystems on the surface, but myriad additional problems, such as the seepage of pollutants into the water table. Abandoned mines present further dangers, including the threat of subsidence, which make these locations unsafe for the long term. Higher environmental and occupational safety standards, established in the United States during the last third of the twentieth century, have led to changes in the way mining is performed as well as in the way mines are left when the work is completed. For example, mining companies have experimented with the use of chemicals or even bacteria, which can dissolve a metal underground and allow it to be pumped to the surface without the need to create actual underground shafts and tunnels or to send human miners to work them.

Industrial Minerals and Other Products Industrial minerals, as noted earlier, are nonmetal-containing mineral resources of interest to economic geology. Examples include asbestos, a generic term for a large group of minerals that are highly resistant to heat and flame; boron compounds, which are used for making heatresistant glass, enamels, and ceramics; phosphates and potassium salts, used in making fertilizers; and sulfur, applied in a range of products, from refrigerants to explosives to purifiers used in the production of sugar.

not only ores but many industrial minerals and fuels, such as coal, is difficult work fraught with

Just one industrial mineral, corundum (from the oxides class of mineral), can have numerous uses. Extremely hard, corundum in the form of an unconsolidated rock commonly called emery has been used as an abrasive since ancient times. Owing to its very high melting point—even higher than that of iron—corundum also is employed in making alumina, a fire-

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numerous hazards. There are short-term dangers to the miners, such as cave-ins, flooding, or the release of gases in the mines, as well as long-term dangers that include such mining-related diseases as black lung (typically a hazard of coal miners). Then there is the sheer mental and emotional stress that comes from spending eight or more hours a day away from the sunlight, in claustrophobic surroundings.

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KEY TERMS A mixture of two or more met-

ALLOY:

experienced decomposition and chemical alteration under conditions of high pres-

als. The smallest particle of an ele-

sure. These nonrenewable forms of bioen-

ment, consisting of protons, neutrons, and

ergy include petroleum, coal, peat, natural

electrons. An atom can exist either alone

gas, and their derivatives.

ATOM:

Minerals of no economic

or in combination with other atoms in a

GANGUE:

molecule.

value, which appear in nature with ore. The joining

Recognition of certain characteristic com-

through electromagnetic force of atoms

binations can help geologists find ore on

that sometimes, but not always, represent

the basis of its attendant gangue. (The ue is

more than one chemical element.

silent, as in tongue.)

CHEMICAL BONDING:

A substance made up of

COMPOUND:

atoms of more than one element, chemi-

In mineralogy, the ability

of one mineral to scratch another. This can be measured by the Mohs scale.

cally bonded to one another. CONSOLIDATION:

HARDNESS:

A process whereby

materials become compacted, or experi-

HYDROCARBON:

Any organic chemi-

cal compound whose molecules are made up of nothing but carbon and hydrogen

ence an increase in density.

atoms. CRYSTALLINE

SOLID:

A type of

solid in which the constituent parts have a simple and definite geometric arrangement that is repeated in all directions. DEPOSITION:

The process whereby

sediment is laid down on the Earth’s surface. DUCTILE:

IGNEOUS ROCK:

One of the three

principal types of rock, along with sedimentary and metamorphic rock. Igneous rock is formed by the crystallization of molten materials, for instance, in a volcano or other setting where plate tectonic processes take place.

Capable of being bent or

molded into various shapes without break-

INDUSTRIAL MINERALS:

Nonme-

tallic minerals with uses for industry.

ing. LUSTER:

The appearance of a mineral

The study of

when light reflects off its surface. Among

fuels, metals, and other materials from the

the terms used in identifying luster

Earth that are of interest to industry or the

are metallic, vitreous (glassy), and dull.

ECONOMIC GEOLOGY:

economy in general. METALS:

Substances that are ductile,

A negatively charged par-

lustrous or shiny in appearance, extremely

ticle in an atom, which spins around the

durable, and excellent conductors of heat

nucleus.

and electricity. Metals have very high melt-

ELECTRON:

Fuel derived from

ing and boiling points, and some (though

deposits of organic material that have

far from all) have a high degree of hardness.

FOSSIL FUELS:

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KEY TERMS METAMORPHIC ROCK:

One of the

CONTINUED

three principal varieties of rock, along with

the term organic only in reference to living

sedimentary and igneous rock. Metamor-

things. Now the word is applied to most

phic rock is formed through the alteration

compounds containing carbon and hydro-

of preexisting rock as a result of changes in

gen, thus excluding carbonates (which are

temperature, pressure, or the activity of

minerals), and oxides such as carbon diox-

fluids. These changes are known as meta-

ide.

morphism.

The study of

PHYSICAL GEOLOGY:

A naturally occurring, typi-

the material components of Earth and of

cally inorganic substance with a specific

the forces that have shaped the planet.

chemical composition and a crystalline

Physical geology is one of two principal

structure. Unknown minerals usually can

branches of geology, the other being his-

be identified in terms of specific parame-

torical geology.

MINERAL:

ters, such as hardness or luster. An area of geology

MINERALOGY:

A positively charged particle

PROTON:

in an atom.

devoted to the study of minerals. Mineral-

ROCK:

ogy includes a number of subdisciplines,

organic matter, which may be consolidated

such as crystallography, or the study of

or unconsolidated.

An aggregate of minerals or

crystal formations within minerals. SEDIMENT: MOHS SCALE:

A scale introduced in

1812 by the German mineralogist Friedrich Mohs (1773–1839) that rates the hardness of minerals from 1 to 10. Ten is equivalent to the hardness of a diamond and 1 that of talc, an extremely soft mineral.

Material deposited at or

near Earth’s surface from a number of sources, most notably preexisting rock. SEDIMENTARY ROCK:

One of the

three major types of rock, along with igneous and metamorphic rock. Sedimentary rock usually is formed by the deposition,

A group of atoms, usual-

compaction, and cementation of rock that

ly but not always representing more than

has experienced weathering. It also may be

one element, joined in a structure. Com-

formed as a result of chemical precipitation.

MOLECULE:

pounds are typically made up of molecules. NUCLEUS:

The center of an atom, a

STREAK:

The color of the powder pro-

duced when one mineral is scratched by

region where protons and neutrons are

another, harder one.

located and around which electrons spin.

UNCONSOLIDATED

ORE:

A metalliferous rock or mineral

possessing economic value.

164

At one time chemists used

ORGANIC:

ROCK:

Rock

that appears in the form of loose particles, such as sand.

proof product used in furnaces and fireplaces. Though pure corundum is colorless, trace amounts of certain elements can yield brilliant

colors: hence, corundum with traces of chromium becomes a red ruby, while traces of iron, titanium, and other elements yield varieties of sap-

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phire in yellow, green, and violet as well as the familiar blue. A N A R RAY O F A P P L I CAT I O N S .

We have only begun to scratch the surface, as it were, of the uses to which minerals can be put: after all, everything—literally, every solid object—that people use is either organic in origin or a mineral. The wide array of applications of minerals is clear from the following list of mineral categories, classified by application: abrasives (corundum, diamond), ceramics (feldspar, quartz), chemical minerals (halite, sulfur, borax), and natural pigments (hematite, limonite). Lime, cement, and plaster comes from calcite and gypsum, while building materials—both structural and ornamental—are products of agate, as well as the two aforementioned minerals. Table salt is a mineral, and so is chalk, as are countless other products. There are rocks, such as granite and marble, used in building, decoration, or artwork, and then there are “rocks”—to use a word that is at once a geologic term and a slang expression—that appear in the form of jewelry. J E W E L RY. Out of all minerals, 16 are important for their use as gems: beryl, chrysoberyl, corundum, diamond, feldspar, garnet, jade, lazurite, olivine, opal, quartz, spinel, topaz, tourmaline, turquoise, and zircon. Not all forms of these minerals, of course, are precious. Furthermore, some minerals provide more than one type of gem: corundum, as we have noted, is a source of rubies and sapphires, while beryl produces both emeralds and aquamarines.

Note that many of the precious gems familiar to most of us are not minerals in their own right but versions of minerals. At least one, the pearl, is not on this list because, with its organic origin, it is not a mineral. Certainly not all minerals are created equal: even in the list of 16 just provided, the name diamond stands out, representing a worldwide standard of value. Yet a diamond is nothing but pure carbon, which also appears in the form of graphite and (with a very few impurities) as coke for burning.

S C I E N C E O F E V E RY DAY T H I N G S

A diamond is unusual, however, in many respects, including the fact that it is basically a huge “molecule” composed of carbon atoms strung together by chemical bonds. The size of this formation corresponds to the size of the diamond, such that a diamond of 1 carat is simply a gargantuan “molecule” containing about 1022 (10,000,000,000,000,000,000,000, or 10 billion trillion) carbon atoms. Not only is a diamond rare and (when properly selected, cut, and polished) extremely beautiful, it is also extraordinarily hard. At the top of the Mohs scale, it can cut any other substance, but nothing can cut a diamond except another diamond.

Economic Geology

WHERE TO LEARN MORE Atlas of Rocks, Minerals, and Textures (Web site). . Bates, Robert Latimer. Industrial Minerals: How They Are Found and Used. Hillside, NJ: Enslow Publishers, 1988. McGraw-Hill Encyclopedia of Science and Technology. 8th ed. New York: McGraw-Hill, 1997. The Mineral and Gemstone Kingdom: Minerals A–Z (Web site). . “Minerals and Metals: A World to Discover.” Natural Resources Canada (Web site). . Ohio Department of Natural Resources Division of Mineral Resources Management (Web site). . Spitz, Peter H. Petrochemicals: The Rise of an Industry. New York: John Wiley and Sons, 1988. Stevens, Paul. Oil and Gas Dictionary. New York: Nichols, 1988. Symes, R. F. Rocks and Minerals. Illus. Colin Keates and Andreas Einsiedel. New York: Dorling Kindersley, 2000. Western Australia Department of Mineral and Petroleum Resources (Web site). .

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S C I E N C E O F E V E RY DAY T H I N G S Real-Life Earth Science

GEOPHYSICS GRAVITY AND GEODESY GEOMAGNETISM CONVECTION ENERGY AND EARTH

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G R AV I T Y A N D G E O D E S Y

Gravity and Geodesy

CONCEPT Thanks to the force known as gravity, Earth maintains its position in orbit around the Sun, and the Moon in orbit around Earth. Likewise, everything on and around Earth holds its place— the waters of the ocean, the gases of the atmosphere, and so on—owing to gravity, which is also the force that imparts to Earth its nearly spherical shape. Though no one can really say what gravity is, it can be quantified in terms of mass and the inverse of the distance between objects. Earth scientists working in the realm of geophysics known as geodesy measure gravitational fields, as well as anomalies within them, for a number of purposes, ranging from the prediction of tectonic processes to the location of oil reserves.

HOW IT WORKS Gravitation Not only does gravity keep Earth and all other planets in orbit around the Sun, it also makes it possible for our solar system to maintain its position in the Milky Way, rather than floating off through space. Likewise, the position of our galaxy within the larger universe is maintained because of gravity. As for the universe itself, though many questions remain about its size, mass, and boundaries, it seems clear that the cosmos is held together by gravity.

as the greatest satellite of them all: the Moon. Even though people are accustomed to thinking of gravity in these large terms, with regard to vast bodies such as Earth or the Moon, every object in the universe, in fact, exerts some gravitational pull on another. This attraction is proportional to the product of the mass of the two bodies, and inversely related to the distance between them. Bodies have to be fairly large (i.e., larger than an asteroid) for this attraction to be appreciable, but it is there, and thus gravity acts as a sort of “glue” holding together the universe. As to what gravity really is or exactly why it works, both of which are legitimate questions, the answers have so far largely eluded scientists. Present-day scientists are able to understand how gravity works, however, inasmuch as it can be described as a function of mass and the gravitational constant (discussed later) and an inverse function of distance. They also are able to measure gravitational fields and anomalies within them. That, in fact, is the focus of geodesy, an area of geophysics devoted to the measurement of Earth’s shape and gravitational field.

Discovering Gravity

Thanks to gravity, all objects on Earth as well as those within its gravitational field remain fixed in place. These objects include man-made satellites, which have grown to number in the thousands since the first was launched in 1957, as well

As discussed in several places within this book (see the entries Earth, Science, and Nonscience and Studying Earth), the physical sciences made little progress until the early sixteenth century. For centuries, the writings of the Greek philosopher Aristotle (384–322 B.C.) and the Alexandrian astronomer Ptolemy (ca. A.D. 100–170) had remained dominant, reinforcing an almost entirely erroneous view of the universe. This Aristotelian/Ptolemaic universe had Earth at its

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later investigations confirmed that air resistance, rather than weight, is responsible for this difference. In other words, a stone falls faster than a feather not because it is heavier but because the feather encounters greater air resistance. In a vacuum, or an area devoid of all matter, including air, they would fall at the same rate.

Gravity and Geodesy

On the other hand, if one drops two objects that meet similar air resistance but differ in weight—say, a large stone and a smaller one— they fall at almost exactly the same rate. To test this hypothesis directly, however, would have been difficult for Galileo: stones fall so fast that even if dropped from a great height, they would hit the ground too soon for their rate of fall to be tested with the instruments then available.

GALILEO GALILEI (Corbis-Bettmann. Reproduced by permission.)

center, with the Sun, Moon, and other planets orbiting it in perfect circles. The discovery by the Polish astronomer Nicolaus Copernicus (1473–1543) that Earth rotates on its axis and revolves around the Sun ultimately led to the overturning of the Ptolemaic model. This breakthrough, which inaugurated the Scientific Revolution (ca. 1550–1700), opened the way for the birth of physics, chemistry, and geology as genuine sciences. Copernicus himself was a precursor to this revolution rather than its leader; by contrast, the Italian astronomer Galileo Galilei (1564–1642) introduced the principles of study, known as the scientific method, that govern the work of scientists to this day. GA L I L E O A N D G RAV I TAT I O N A L AC C E L E RAT I O N . Galileo applied his

scientific method in his studies of falling objects and was able to show that objects fall as they do, not because of their weight (as Aristotle had claimed) but as a consequence of gravitational force. This meant that the acceleration of all falling bodies would have to be the same, regardless of weight. Of course, everyone knows that a stone falls faster than a feather, but Galileo reasoned that this was a result of factors other than weight, and

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Instead, Galileo used the motion of a pendulum and the behavior of objects rolling or sliding down inclined planes as his models. On the basis of his observations, he concluded that all bodies are subject to a uniform rate of gravitational acceleration, later calibrated at 32 ft. (9.8 m) per second per second. What this means is that for every 32 ft. an object falls, it is accelerating at a rate of 32 ft. per second as well; hence, after two seconds it falls at the rate of 64 ft. per second, after three seconds it falls at 96 ft. per second, and so on. NEWTON’S

B R E A KT H R O U G H .

Building on the work of his distinguished forebear, Sir Isaac Newton (1642–1727), who was born the same year Galileo died, developed a paradigm for gravitation that even today explains the behavior of objects in virtually all situations throughout the universe. Indeed, the Newtonian model reigned supreme until the early twentieth century, when Albert Einstein (1879–1955) challenged it on certain specifics. Even so, Einstein’s relativity did not disprove the Newtonian system as Copernicus and Galileo had disproved Aristotle’s and Ptolemy’s theories; rather, it showed the limitations of Newtonian mechanics for describing the behavior of certain objects and phenomena. In the ordinary world of day-to-day experience, however, the Newtonian system still offers the key to how and why things work as they do. This is particularly the case with regard to gravity.

Understanding the Law of Universal Gravitation Like Galileo, Newton began in part with the aim of testing hypotheses put forward by an

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astronomer—in this case, Johannes Kepler (1571–1630). In the early years of the seventeenth century, Kepler published his three laws of planetary motion, which together identified the elliptical (oval-shaped) path of the planets around the Sun. Kepler had discovered a mathematical relationship that connected the distances of the planets from the Sun to the period of their revolution around it. Like Galileo with Copernicus, Newton sought to generalize these principles to explain not only how the planets moved but also why they did so. The result was Newton’s Philosophiae naturalis principia mathematica (Mathematical principles of natural philosophy, 1687). Usually referred to simply as the Principia, the book proved to be one of the most influential works ever written. In it, Newton presented his law of universal gravitation, along with his three laws of motion. These principles offered a new model for understanding the mechanics of the universe.

The Three Laws of Motion Newton’s three laws of motion may be summarized in this way: • An object at rest will remain at rest, and an object in motion will remain in motion at a constant velocity unless and until outside forces act upon it. • The net force acting upon an object is a product of its mass multiplied by its acceleration. • When one object exerts a force on another, the second object exerts on the first a force equal in magnitude but opposite in direction. The first law of motion identifies inertia, a concept introduced by Galileo to explain what kept the planets moving around the Sun. Inertia is the tendency of an object either to keep moving or to keep standing still, depending on what it is already doing. Note that the first law refers to an object moving at a constant velocity: velocity is speed in a certain direction, so a constant velocity would be the same speed in the same direction. Inertia is measured by mass, which—as the second law states—is a component of force and is inversely related to acceleration. The latter, as defined by physics, has a much broader meaning than it usually is given in ordinary life. Acceleration does not mean simply an increase of speed

S C I E N C E O F E V E RY DAY T H I N G S

for an object moving in a straight line; rather, it is a change in velocity—that is, a change of speed or direction or both.

Gravity and Geodesy

By definition, then, rotational motion (such as that of Earth around the Sun) involves acceleration, because any movement other than motion in a straight line at a constant speed requires a change in velocity. This further means that an object experiencing rotational motion must be under the influence of some force. That force is gravity, and as the third law shows, every force exerted in one direction is matched by an equal force in the opposing direction. (This law is sometimes rendered “For every action there is an equal and opposite reaction.”) NEWTON’S G R A V I TAT I O N A L F O R M U LA . The law of universal gravitation

can be stated as a formula for calculating the gravitational attraction between two objects of a certain mass, m1 and m2: Fgrav = G  (m1m2)/r2. In this equation, Fgrav is gravitational force, and r2 is the square of the distance between the two objects. As for G, in Newton’s time the value of this number was unknown. Newton was aware simply that it represented a very small quantity: without it, (m1m2)/r2 could be quite sizable for objects of relatively great mass separated by a relatively small distance. When multiplied by this very small number, however, the gravitational attraction would be revealed to be very small as well. Only in 1798, more than a century after Newton’s writing, did the English physicist Henry Cavendish (1731–1810) calculate the value of G using a precision instrument called a torsion balance. The value of G is expressed in units of force multiplied by distance squared, and then divided by,mass squared; in other words, G is a certain value of (N  m2)/kg2, where N stands for newtons, m for meters, and kg for kilograms. Nor is the numerical value of G a whole number such as 1. A figure as large as 1, in fact, is astronomically huge compared with G, whose value is 6.67  10–11—in other words, 0.0000000000667.

Physical Geodesy Within the realm of geodesy is that of physical geodesy, which is concerned specifically with the measurement of Earth’s gravitational field as well as the geoid. The latter may be defined as a surface of uniform gravitational potential covering the

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entire Earth at a height equal to sea level. (“Potential” here is analogous to height or, more specifically, position in a field. For a discussion of potential in a gravitational field, see Energy and Earth.) Thus, in areas that are above sea level, the geoid would be below ground—indeed, far below it in mountainous regions. Yet in some places (most notably the Dead Sea and its shores, the lowest point on Earth), it would be above the solid earth and waters. The geoid is also subject to deviations or anomalies, owing to the fact that the planet’s mass is not distributed uniformly; in addition, small temporary disturbances in the geoid may occur on the seas as a result of wind, tides, and currents. Generally speaking, however, the geoid is a stable reference platform from which to measure gravitational anomalies. It is a sort of imaginary gravitational “skin” covering the planet, and in the past, countries conducting geodetic surveys tended to choose a spot within their boundaries as the reference point for all measurements. With the development of satellites and their use for geodetic research, however, it has become more common for national geodetic societies to use global points of reference such as the planet’s center of mass. MEASUREMENTS FROM SPACE, LAND, AND SEA. The geoid can be deter-

mined by using such a satellite, equipped with a radar altimeter, but there is also the much older technique of terrestrial gravity measurement. The terrestrial method is much more difficult and prone to error, however, and calculations require detailed checking and correction to remove potential anomalies due to the presence of matter in areas above the points at which gravitational measurements were obtained.

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improve greatly the accuracy of seaborne measurements. H O W G RAV I T Y I S M E AS U R E D .

Scientists can obtain absolute terrestrial gravity measurements by measuring the amount of time it takes for a pellet to fall a certain distance within a vacuum—that is, a chamber from which all matter, including air, has been removed. This, of course, is the same technology Galileo used in making his observations more than 400 years ago. It is also possible to obtain relative gravity measurements with the use of mechanical balance instruments. As noted earlier, the acceleration due to gravity is 9.8 m/s2, or 9.8 m s-2. (Scientists sometimes use the latter notation, in which the minus sign is not meant to indicate a negative but rather is used in place of “per”.) This number is the measure of Earth’s gravitational field. In measuring gravitational anomalies, scientists may use the Gal, named after Galileo, which is equal to 0.01 m/s2. Typically, however, the milligal, equal to one-thousandth of a Gal, is used. Note that “Gal” sometimes is rendered in lowercase, but this can be confusing, because it looks like the abbreviation for “gallon. ”) W H Y M E AS U R E G RAV I T Y ? Why is it important to measure gravity and gravitational anomalies? One answer is that weight values can vary considerably, depending on one’s position relative to Earth’s gravitational field. A fairly heavy person might weigh as much as a pound less at the equator than at the poles and less still at the top of a high mountain. The value of the gravity field at sea level has a range from 9.78 to 9.83 m/s2, a difference of about 50,000 g.u., and it is likely to be much lower than 9.78 m/s2 at higher altitudes.

Also highly subject to error are measurements made from a vessel at sea. This has to do not only with the effect of the ship’s pitch and roll but also with something called the Eötvös effect. Named for the Hungarian physicist Baron Roland Eötvös (1848–1919), who conducted extensive studies on gravity, the effect is related to the Coriolis force, which causes the deflection of atmospheric and oceanic currents in response to Earth’s rotation. Measurements of gravity from the air are also subject to the Eötvös effect, though the use of GPS (global positioning system) information, obtained from satellites, can

Indeed, the higher one goes, the weaker Earth’s gravitational field becomes. At the same time, the gases of the atmosphere dissipate, which is the reason why it is hard to breathe on high mountains without an artificial air supply and impossible to do so in the stratosphere or above it. At the upper edge of the mesosphere, Earth’s gravitational field is no longer strong enough to hold large quantities of hydrogen, lightest of all elements, which constitutes the atmosphere at that point. Beyond the mesosphere, the atmosphere simply fades away, because there is not sufficient gravitational force to hold its particles in place.

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Back down on Earth, gravity measurements are of great importance to the petroleum industry, which uses them to locate oil-containing salt domes. Furthermore, geologists, in general, remain acutely interested in measurements of gravity, the force behind tectonics, or the deformation of Earth’s crust. Thus, gravity, responsible for fashioning Earth’s exterior into the nearly spherical shape it has, is key to the shaping of its interior as well.

32 ft. (9.8 m) per second squared. (Note, also, that this is a lowercase g, as opposed to the uppercase G that represents the gravitational constant.) Using the metric system, by multiplying the appropriate mass figure in kilograms by 9.8 m/s2, one would obtain a value in newtons (N). To perform the same calculation with the English system, used in America, it would be necessary first to calculate the value of mass in slugs (which, needless to say, is a little-known unit) and multiply it by 32 ft./s2 to yield a value in pounds.

REAL-LIFE A P P L I C AT I O N S

In both cases, the value obtained, whether in newtons or pounds, is a measure of weight rather than of mass, which is measured in kilograms or slugs. For this reason, it is not entirely accurate to say that 1 kg is equal to 2.2 lb. This is true on Earth, but it would not be true on the Moon. The kilogram is a unit of mass, and as such it would not change anywhere in the universe, whereas the pound is a unit of force (in this case, gravitational force) and therefore varies according to the rate of acceleration for the gravitational field in which it is measured. For this reason, scientists prefer to use figures for mass, which is one of the fundamental properties (along with length, time, and electric charge) of the universe.

Gravity on Earth Using Newton’s gravitational formula, it is relatively easy to calculate the pull of gravity between two objects. It is also easy to see why the attraction is insignificant unless at least one of the objects has enormous mass. In addition, application of the formula makes it clear why G is such a tiny number. Suppose two people each have a mass of 45.5 kg—equal to 100 lb. on Earth, though not on the Moon, a matter that will be explained later in this essay—and they stand 1 m (3.28 ft.) apart. Thus, m1m2 is equal to 2,070 kg (4,555 lb.), and r2 is equal to 1 m squared. Applied to the gravitational formula, this figure is rendered as 2,070 kg2/1 m2. This number then is multiplied by the gravitational constant, and the result is a net gravitational force of 0.000000138 N (0.00000003 lb.)— about the weight of a single-cell organism! W E I G H T. What about Earth’s gravitational force on one of those people? To calculate this force, we could apply the formula for universal gravitation, substituting Earth for m2, especially because the mass of Earth is known: 5.98  1024 kg, or 5.98 septillion (1 followed by 24 zeroes) kg. We know the value of that mass, in fact, through the application of Newton’s laws and the formulas derived from them. But for measuring the gravitational force between something as massive as Earth and something as small as a human body, it makes more sense to apply instead the formula embodied in Newton’s second law of motion: F = ma. (Force equals mass multiplied by acceleration.)

Gravity and Geodesy

Why Earth Is Round—and Not Round Everyone knows that Earth, the Sun, and all other large bodies in space are “round” (i.e., spherical), but why is that true? The reason is that gravity will not allow them to be otherwise: for any large object, the gravitational pull of its interior forces the surface to assume a relatively uniform shape. The most uniform of three-dimensional shape is that of a sphere, and the larger the mass of an object, the greater its tendency toward sphericity. Earth has a relatively small vertical differential between its highest and lowest surface points, Mount Everest (29,028 ft., or 8,848 m) on the Nepal-Tibet border and the Mariana Trench (–36,198 ft., –10,911 m) in the Pacific Ocean, respectively. The difference is just 12.28 mi. (19.6 km)—not a great distance, considering that Earth’s radius is about 4,000 mi. (6,400 km).

For a body of any mass on Earth, acceleration is figured in terms of g—the acceleration due to gravity, which, as noted earlier, is equal to

On the other hand, an object of less mass is more likely to retain a shape that is far less than spherical. This can be shown by reference to the Martian moons Phobos and Deimos, both of which are oblong—and both of which are tiny, in

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a sphere; however, it is not standing still but instead rotates on its axis, as does every other object of any significance in the solar system.

Gravity and Geodesy

Incidentally, if Earth were suddenly to stop spinning, the gases in the atmosphere would keep moving at their current rate of about 1,000 MPH (1,600 km/h). They would sweep over the planet with the force of the greatest hurricane ever known, ripping up everything but the mountains. As to why Earth spins at all, scientists are not entirely sure. It may well be angular momentum (the momentum associated with rotational motion) imparted to it at some point in the very distant past, perhaps because it and the rest of the solar system were once part of a vast spinning cloud.

THE FORCE OF GRAVITY IMPARTS A SPHERICAL SHAPE TO EARTH BECAUSE OF ITS LARGE MASS. AN OBJECT OF LESS MASS WILL HAVE A FAR LESS SPHERICAL SHAPE. THE MARTIAN MOON PHOBOS, SHOWN HERE, IS OBLONG OWING TO ITS TINY MASS. (© Julian Baum/Photo Researchers. Reproduced by permission.)

terms of size and mass, compared with Earth’s Moon. Mars itself has a radius half that of Earth, yet its mass is only about 10% of Earth’s. In light of what has been said about mass, shape, and gravity, it should not be surprising to learn that Mars is also home to the tallest mountain in the solar system, the volcano Olympus Mons, which stands 16 mi. (27 km) high. E A R T H ’ S ‘F L AT T O P ’ ( A N D B O T T O M ) . With regard to gravitation, a

spherical object behaves as though its mass were concentrated near its center, and indeed, 33% of Earth’s mass is at its core, even though the core accounts for only about 20% of the planet’s volume. Geologists believe that the composition of Earth’s core must be molten iron, which creates the planet’s vast electromagnetic field. It should be noted, however, that Earth is not really a perfect sphere, and the idea that its mass is concentrated at its center, while it works well in general, poses some problems in making exact gravitational measurements. If Earth were standing still, it would be much nearer to the shape of

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At any rate, the fact that Earth is spinning on its axis creates a certain centripetal, or inwardpulling, force, and this force produces a corresponding centrifugal (outward) component. To understand this concept, consider what happens to a sample of blood when it is rotated in a centrifuge. When the centrifuge spins, centripetal force pulls the material in the vial toward the center of the spin, but the material with greater mass has more inertia and therefore responds less to centripetal force. As a result, the heavier red blood cells tend to stay at the bottom of the vial (or, as it is spinning, on the outside), while the lighter plasma is pulled inward. The result is the separation between plasma and red blood cells. Where Earth is concerned, this centrifugal component of centripetal force manifests as an equatorial bulging. Simply put, Earth’s diameter around the equator is greater than at the poles, which are slightly flattened. The difference is small—the equatorial diameter of Earth is about 26.72 mi. (43 km) greater than the polar diameter—but it is not insignificant. In fact, as noted later, a person of fairly significant weight actually would notice a difference if he or she got on the scales at the equator (say, in Singapore) and then later weighed in near one of the poles (for instance, in the Norwegian possession of Svalbard, the northernmost human settlement on Earth). Owing to this departure from a perfectly spherical shape, the Sun and Moon exert additional torques on Earth, and these torques cause shifts in the position of the planet’s rotational axis in space. An imaginary line projected from

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Gravity and Geodesy

KEY TERMS A change in velocity

ACCELERATION:

The tendency of an object at

INERTIA:

over time. The acceleration due to gravity,

rest to remain at rest or an object in

for instance, is 32 ft. (9.8 m) per second per

motion to remain in motion, at a uniform

second, meaning that for every second an

velocity, at a uniform velocity, unless acted

object falls, its velocity is increasing as well.

upon by some outside force.

ATMOSPHERE:

In general, an atmos-

MASS:

A measure of inertia, indicating

phere is a blanket of gases surrounding a

the resistance of an object to a change in its

planet. Unless otherwise identified, howev-

motion.

er, the term refers to the atmosphere of

POTENTIAL:

Earth, which consists of nitrogen (78%),

a gravitational force field.

oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%. CENTRIFUGAL:

A term describing the

Position in a field, such as

SCIENTIFIC METHOD:

A set of prin-

ciples and procedures for systematic study that includes observation; the formation of hypotheses, theories, and laws; and continual testing and reexamination.

tendency of objects in uniform circular motion to move outward, away from the center of the circle. Though the term centrifugal force often is used, it is inertia, rather than force, that causes the object to move outward. CENTRIPETAL FORCE:

The force that

causes an object in uniform circular motion to move inward, toward the center of the circle. FORCE:

The product of mass multi-

plied by acceleration. GEODESY:

An area of geophysics

devoted to the measurement of Earth’s shape and gravitational field. GEOID:

A surface of uniform gravita-

tional potential covering the entire Earth at a height equal to sea level. GEOPHYSICS:

A branch of the earth

SCIENTIFIC REVOLUTION:

A peri-

od of accelerated scientific discovery that completely reshaped the world. Usually dated from about 1550 to 1700, the Scientific Revolution saw the origination of the scientific method and the introduction of ideas such as the heliocentric (Sun-centered) universe and gravity. TORQUE:

A force that produces, or

tends to produce, rotational motion. UNIFORM CIRCULAR MOTION:

The

motion of an object around the center of a circle in such a manner that speed is constant or unchanging. VACUUM:

An area devoid of matter,

even air. VELOCITY:

Speed in a certain direction. A measure of the gravitation-

sciences that combines aspects of geology

WEIGHT:

and physics. Geophysics addresses the

al force on an object. Weight thus would

planet’s physical processes as well as its

change from planet to planet, whereas

gravitational, magnetic, and electric prop-

mass remains constant throughout the

erties and the means by which energy is

universe. A pound is a unit of weight,

transmitted through its interior.

whereas a kilogram is a unit of mass.

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the North Pole and into space therefore, over a period of time, would appear to move. In the course of about 25,800 years, this point (known as the celestial north pole) describes the shape of a cone, a movement known as Earth’s precession.

Satellites Why, then, does Earth move around the Sun, or the Moon around Earth? As should be clear from Newton’s gravitational formula and the third law of motion, the force of gravity works both ways: not only does a stone fall toward Earth, but Earth also actually falls toward it. The mass of Earth is so great compared with that of the stone that the movement of Earth is imperceptible—but it does happen. Furthermore, because Earth is round, when one hurls a projectile at a great distance, Earth curves away from the projectile. Eventually, gravity itself forces the projectile to the ground. If one were to fire a rocket at 17,700 mi. per hour (28,500 km per hour), however, something unusual would happen. At every instant of time, the projectile would be falling toward Earth with the force of gravity—but the curved Earth would be falling away from it at the same rate. Hence, the projectile would remain in constant motion around the planet—that is, it would be in orbit.

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toward Earth, Earth falls away from it. Change the names of the players, and this same relationship exists between Earth and its great natural satellite, the Moon. Furthermore, it is the same with the Sun and its many satellites, including Earth: Earth plunges toward the Sun with every instant of its movement, but at every instant, the Sun falls away. WHERE TO LEARN MORE Ardley, Neil. The Science Book of Gravity. San Diego, CA: Harcourt Brace Jovanovich, 1992. Ask the Space Scientist (Web site). . Beiser, Arthur. Physics, 5th ed. Reading, MA: AddisonWesley, 1991. Exploring Gravity (Web site). . Geodesy for the Layman (Web site). . The Gravity Society (Web site). . Hancock, Paul L., and Brian J. Skinner. The Oxford Companion to the Earth. New York: Oxford University Press, 2000. Riley, Peter D. Earth. Des Plaines, IL: Heinemann Interactive Library, 1998. Smith, David G. The Cambridge Encyclopedia of Earth Sciences. New York: Cambridge University Press, 1981.

The same is true of an artificial satellite’s orbit around Earth: even as the satellite falls

Wilford, John Noble. The Mapmakers. New York: Knopf, 2000.

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Geomagnetism

GEOMAGNETISM

CONCEPT Scientists have long recognized a connection between electricity and magnetism, but the specifics of this connection, along with the recognition that electromagnetism is one of the fundamental interactions in the universe, were worked out only in the mid–nineteenth century. By that time, geologists had come to an understanding of Earth as a giant magnet. This was the principle that made possible the operation of compasses, which greatly aided mariners in navigating the seas: magnetic materials, it so happened, point northward. As it turns out, however, Earth’s magnetic North Pole is not the same as its geographic one, and even the pole’s northerly location is not a permanent fact. Once upon a time and, in fact, at many times in Earth’s history, the magnetic North Pole lay at the southern end of the planet.

As in so much else, studies in electromagnetism made little progress from the time of the Romans to the late Renaissance, a span of nearly 1,500 years. Yet it is worth noting that thefirst ideas scientists had about studying Earth’s history scientifically came from observing the planet’s magnetic field. In the course of his work on that subject, the English astronomer Henry Gellibrand (1597–1636) showed that the field has changed over time. This suggested that it would be possible to form hypotheses about the planet’s past, even though humans had no direct information regarding the origins of Earth. Thus, Gellibrand (who, ironically, was also a minister) helped make it possible for geologists to move beyond a strict interpretation of the Bible in studying the history of Earth. (See Earth, Science, and Nonscience for more on the Genesis account and its interpretations.) E L E C T R O M AG N E T I C S T U D I E S C O M E O F AG E . Beginning in the 1700s, a

HOW IT WORKS Electromagnetism The Greek philosopher Thales (640?–546 B.C.) was the first to observe that when amber is rubbed with certain types of materials, the friction imparts to it the ability to pick up light objects. The word electricity comes from the Greek word for amber, elektron, and, in fact, magnetism and electricity are simply manifestations of the same force. This concept of electric and magnetic interaction seems to have been established early in human history, though it would be almost 2,500 years before scientists came to a mature understanding of the relationship.

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number of thinkers conducted experiments concerning the nature of electricity and magnetism and the relationship between them. Among these thinkers were several giants in physics and other disciplines, including one of America’s greatest founding fathers, Benjamin Franklin (1706– 1790). In addition to his famous (and highly dangerous) experiment with lightning, Franklin also contributed the names positive and negative to the differing electric charges discovered earlier by the French physicist Charles Du Fay (1698–1739). In 1785 the French physicist and inventor Charles Coulomb (1736–1806) established the basic laws of electrostatics and magnetism. He maintained that there is an attractive force that, like gravitation, can be explained in terms of the

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Geomagnetism

THE

MAGNETIC FIELD AROUND TWO BAR MAGNETS. (© A. Pasieka/Photo Researchers. Reproduced by permission.)

inverse of the square of the distance between objects (see Gravity and Geodesy). That attraction itself, however, results not from gravity but from electric charge, according to Coulomb. A few years later, the German mathematician Karl Friedrich Gauss (1777–1855) developed a mathematical theory for finding the magnetic potential of any point on Earth. His contemporary, the Danish physicist Hans Christian Oersted (1777–1851), became the first scientist to establish a clear relationship between electricity and magnetism. This led to the formalization of electromagnetism, the branch of physics devoted to the study of electric and magnetic phenomena.

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that magnetism is the result of electricity in motion, and in 1831 the British physicist and chemist Michael Faraday (1791–1867) published his theory of electromagnetic induction. This theory shows how an electric current in one coil can set up a current in another through the development of a magnetic field, and it enabled Faraday’s development of the first generator. For the first time in history humans were able to convert mechanical energy systematically into electric energy. ELECTROMAGNETIC

FORCE.

The French mathematician and physicist André Marie Ampère (1775–1836) concluded

By this point scientists were convinced that a relationship between electricity and magnetism existed, yet they did not know exactly how the two related. Then, in 1865, the Scottish physicist James Clerk Maxwell (1831–1879) published a

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groundbreaking paper, “On Faraday’s Lines of Force,” in which he outlined a theory of electromagnetic force. The latter may be defined as the total force on an electrically charged particle, which is a combination of forces due to electric and magnetic fields around the particle. Maxwell thus had discovered a type of force in addition to gravity, and this reflected a “new” type of fundamental interaction, or a basic mode by which particles interact in nature. Nearly two centuries earlier Sir Isaac Newton (1642–1727) had identified the first, gravity, and in the twentieth century two other forms of fundamental interaction—strong nuclear and weak nuclear— were identified as well. In his work Maxwell drew on the studies conducted by his predecessors but added a new statement. According to Maxwell, electric charge is conserved, meaning that the sum total of electric charge in the universe does not change, though it may be redistributed. This statement, which did not contradict any of the experimental work done by other physicists, was based on Maxwell’s predictions regarding what should happen in situations of electromagnetism. Subsequent studies have supported his predictions.

Magnetism What, then, is the difference between electricity and magnetism? It is primarily a matter of orientation. When two electric charges are at rest, it appears to an observer that the force between them is merely electric. If the charges are in motion relative to the observer, it appears as though a different sort of force, known as magnetism, exists between them. An electromagnetic wave, such as that which is emitted by the Sun, carries both an electric and a magnetic component at mutually perpendicular angles. If you extend your hand, palm flat, with the fingers straight and the thumb pointing at a 90° angle to the fingers, the direction that the fingers are pointing would be that of the electromagnetic wave. Your thumb points in the direction of the electric field, and the flat of your palm indicates the direction of the magnetic field, which is perpendicular both to the electric field and to the direction of wave propagation.

magnetic field are simply regions in which the electric and magnetic components, respectively, of electromagnetic force are exerted.

Geomagnetism

M AG N E T I S M AT T H E AT O M I C L E V E L . At the atomic level magnetism is the

result of motion by electrons (negatively charged subatomic particles) in relation to one another. Rather like planets in a solar system, electrons revolve around the atom’s nucleus and rotate on their own axes. (In fact, the exact nature of their movement is much more complex, but this analogy is accurate enough for the present purposes.) Both types of movement create a magnetic force field between electrons, and as a result the electron takes on the properties of a tiny bar magnet with a north pole and south pole. Surrounding this infinitesimal magnet are lines of magnetic force, which begin at the north pole and curve outward, describing an ellipse as they return to the south pole. In most atoms, electrons are paired such that their magnetic fields cancel out one another. However, in certain cases, such as when there is an odd electron or when other factors become more significant, the fields line up to create what is known as a net magnetic dipole, or a unity of direction. These elements, among them, iron, cobalt, and nickel as well as various alloys or mixtures, are commonly known as magnetic metals, or natural magnets. M A G N E T I Z AT I O N . Magnetization occurs when an object is placed in a magnetic field. In this field magnetic force acts on a moving charged particle such that the particle would experience no force if it moved in the direction of the magnetic field. In other words, it would be “drawn,” as a ten-penny nail is drawn to a common bar or horseshoe (U-shaped) magnet. An electric current is an example of a moving charge, and, indeed, one of the best ways to create a magnetic field is with a current. Often this is done by means of a solenoid, a current-carrying wire coil through which the material to be magnetized is passed, much as one would pass a straight wire up through the interior of a spring.

A field, in this sense, is a region of space in which it is possible to define the physical properties of each point in the region at any given moment in time. Thus, an electric field and a

When a natural magnet becomes magnetized (that is, when a magnetic metal or alloy comes into contact with an external magnetic field), a change occurs at the level of the domain, a group of atoms equal in size to about 5  10–5 meters across—just large enough to be visible under a microscope.

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In an unmagnetized sample, there may be an alignment of unpaired electron spins within a domain, but the direction of the various domains’ magnetic forces in relation to one another is random. Once a natural magnet is placed within an external magnetic force field, however, one of two things happens to the domains. Either they all come into alignment with the field or, in certain types of material, those domains in alignment with the field grow, while the others shrink to nonexistence. The first of these processes is called domain alignment, or ferromagnetism, and the second is termed domain growth, or ferrimagnetism. Both processes turn a natural magnet into what is known as a permanent magnet—or, more simply, a magnet. The magnet is then capable of temporarily magnetizing a ferromagnetic item, as, for instance, when one rubs a paper clip against a permanent magnet and then uses the magnetized clip to lift other paper clips. Of the two varieties, however, a ferromagnet is stronger, because it requires a more powerful magnetic force field to become magnetized. Most powerful of all is a saturated ferromagnetic metal, one in which all the unpaired electron spins are aligned.

REAL-LIFE A P P L I C AT I O N S The Magnetic Compass A bar magnet placed in a magnetic field will rotate until it lines up with the field’s direction. The same thing happens when one suspends a magnet from a string: it lines up with Earth’s magnetic field and points in a north-south direction. The Chinese of the first century B.C. discovered that a strip of magnetic metal always tends to point toward geographic north, though they were unaware of the electromagnetic force that causes this to happen.

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ranked alongside paper, printing, and gunpowder as one of premodern China’s four great gifts to the West. Before the compass, mariners had to depend purely on the position of the Sun and other, less reliable means of determining direction; hence, the invention quite literally helped open up the world. O D D B E H AV I O R O F T H E C O M PASS . The compass, in fact, helped make pos-

sible the historic voyage of Christopher Columbus (1451–1506) in 1492. While sailing across the Atlantic, Columbus noticed something odd: his compass did not always point toward what he knew, based on the Sun’s position, to be geographic north. The further he traveled, the more he noticed this phenomenon, which came to be known as magnetic declination. When Columbus returned to Europe and reported on his observations of magnetic declination (along with the much bigger news of his landing in the New World, which he thought was Asia), his story perplexed mariners. Eventually, European scientists worked out tables of magnetic declination, showing the amount of deviation at various points on Earth, and this seemed to allay sailors’ concerns. Then, in 1544, the German astronomer Georg Hartmann (1489–1564) observed that a freely floating magnetized needle did not always stay perfectly horizontal and actually dipped more and more strongly as he traveled north. When he was moving south, on the other hand, the needle tended to become more closely horizontal. For many years, this phenomenon, along with magnetic declination, remained perplexing. Nor, for that matter, did scientists understand exactly how or why a compass works. Then, in 1600, the English physicist William Gilbert (1540–1603) became the first to suggest a reason.

Earth as a Giant Magnet

This led ultimately to the development of the magnetic compass, which typically consists of a magnetized iron needle suspended over a card marked with the four cardinal directions (north, south, east, and west). The needle is attached to a pivoting mechanism at its center, which allows it to move freely so that the tip of the needle will always point the user northward. The magnetic compass proved so important that it typically is

Gilbert coined the terms electric attraction, electric force, and magnetic pole. In De magnete (On the magnet), he became the first thinker to introduce the idea, now commonly accepted, that Earth itself is a giant magnet. Not only does it have north and south magnetic poles, but it also is surrounded by vast arcs of magnetic force, called the geomagnetic field. (The term geomagnetism, as opposed to magnetism, refers to the magnetic properties of Earth as a whole rather

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than those possessed by a single object or place on Earth.)

described earlier, were it not for the Sun’s influence.

In the paragraphs that follow, we discuss the shape of this magnetic field, including the positions of the magnetic north and south poles; the origins of the field, primarily in terms of the known or suspected physical forces that sustain it (as opposed to the original cause of Earth’s magnetic field, a more complicated and speculative subject); as well as changes in the magnetic field. Those changes, along with techniques for measuring the geomagnetic field, also are discussed at other places in this book.

The side of the magnetosphere closer to the Sun does indeed resemble the giant series of concentric loops described earlier. These loops are enormous, such that the forward, or sunlit, edge of the magnetosphere is located at a distance of some 10 Earth radii (about 35,000 mi., or 65,000 km). On the other side from the Sun—the rear, or dark, side—the shape of the magnetosphere is quite different. Instead of forming relatively small loops that curve right back around into Earth’s poles, the lines of magnetic force on this side shoot straight out into space a distance of some 40 Earth radii (about 140,000 mi., or 260,000 km).

EARTH’S

MAGNETIC

FIELD.

Hartmann’s compass phenomenon can be explained by the fact that Earth is a magnet and that its north and south magnetic poles are close to the geographic north and south. As for the phenomenon observed by Columbus, it is a result of the difference between magnetic and geographic north. If one continued to follow a compass northward, it would lead not to Earth’s North Pole but to a point identified in 1984 as 77°N, 102°18’ W—that is, in the Queen Elizabeth Islands of far northern Canada. Earlier we described the lines that make up the magnetic force field around a bar magnet. A field of similar shape, though, of course, of much larger size (yet still invisible), also surrounds Earth. From the magnetic north and south poles, lines of magnetic force rise into space and form giant curves that come back around and reenter Earth at the opposing pole, so that the planet is surrounded by a vast series of concentric loops. If one could draw a straight line through the center of all these loops, it would reach Earth at a point 11° from the equator. Likewise the north and south magnetic poles—which are on a plane perpendicular to that of Earth’s magnetic field—are 11° off the planet’s axis.

Geomagnetism

The shape of the magnetosphere, then, is a bit like that of a comet moving toward the Sun. Surrounding it is the magnetopause, a sort of magnetic dead zone about 62 mi. (100 km) thick, which shields Earth from most of the solar wind. In front of it (toward the Sun) is an area of magnetic turbulence known as the magnetosheath, and still closer to the Sun is a boundary called the bow shock, a shock-wave front that slows particles of solar wind considerably. Since Earth is turning, the side of the planet away from the Sun is continually changing, of course, but the shape of the magnetosphere remains more or less intact. It is, however, highly affected by solar activity, such that an increase in solar wind can cause a depression in the magnetosphere. (See Sun, Moon, and Earth for a discussion of auroras, produced by an interaction between the solar wind and the magnetosphere.)

The Source of Earth’s Magnetic Field

T H E M A G N E T O S P H E R E . Surrounding the planet is a vast region called the magnetosphere, an area in which ionized particles (i.e., ones that have lost or gained electrons so as to acquire a net electric charge) are affected by Earth’s magnetic field. The magnetosphere is formed by the interaction between our planet’s magnetic field and the solar wind, a stream of particles from the Sun. (See Sun, Moon, and Earth for more about the solar wind.) Its shape would be akin to that of Earth’s magnetic field, as

As noted earlier, scientists at present understand little with regard to the origins of Earth’s magnetic field—that is, the original action or actions that resulted in the creation of a geomagnetic field that has remained active for billions of years. No less a figure than Albert Einstein (1879–1955) identified this question as one of the great unsolved problems of science. On the other hand, scientists do have a good understanding of the geomagnetic field’s source, in terms of the physical conditions that make it possible. (This distinction is rather like that between efficient cause and material cause, as discussed in Earth, Science, and Nonscience.)

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Geomagnetism

THE

ELECTRIC DISCHARGE BETWEEN TWO METAL OBJECTS. (© P. Jude/Photo Researchers. Reproduced by permission.)

It is believed that the source of Earth’s magnetism lies in a core of molten iron some 4,320 mi. (6,940 km) across, constituting half the planet’s diameter. Within this core run powerful electric currents that create the geomagnetic field. Actually, the field seems to originate in the outer core, consisting of an iron-nickel alloy that is kept fluid owing to the exceedingly great temperatures. The materials of the outer core undergo convection, vertical circulation that results from variations in density brought about by differences in temperature. This process of convection (imagine giant spirals moving vertically through the molten metal) creates the equivalent of a solenoid, described earlier. Even so, there had to be an original source for the magnetic field, and it is possible that it came from the Sun. In any case, the magnetic field could not continue to exist if the fluid of the outer core were not in constant convective motion. If this convection stopped, within about 10,000 years (which, in terms of Earth’s life span, is like a few seconds to a human being), the geomagnetic field would decay and cease to operate. Likewise, if Earth’s core ever cooled and solidified, Earth would become like the Moon, a body whose magnetic field has dis-

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appeared, leaving only the faintest traces of magnetism in its rocks.

Changes in the Magnetic Field Though there is no reason to believe that anything so dramatic will happen, there are curious and perhaps troubling signs that Earth’s magnetic field is changing. According to data recorded by the U.S. Geological Survey, which updates information on magnetic declination, the field is shifting—and weakening. Over the course of about a century, scientists have recorded data suggesting a reduction of about 6% in the strength of the magnetic field. The behavior, in terms of both weakening and movement, appears to be similar to changes taking place in the magnetic field of the Sun. Indeed, as we have seen already, Earth’s magnetism is heavily affected by the Sun, and it is possible that a period of strong solar-flare activity could shut down Earth’s magnetic field. Even the present trend of weakening, if it were to continue for just 1,500 years, would wipe out the magnetic field. Some scientists believe that the planet is simply experiencing a fluctuation, however, and that the geomagnetic field will recover. Others

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Geomagnetism

KEY TERMS and

There are four known fundamental inter-

other sciences, “to conserve” something

actions in nature: gravitational, electro-

CONSERVATION:

In

physics

means “to result in no net loss of ” that particular component. It is possible that within a given system the component may

magnetic, strong nuclear, and weak nuclear. A term referring to

change form or position, but as long as the

GEOMAGNETISM:

net value of the component remains the

the magnetic properties of Earth as a whole

same, it has been conserved.

rather than those possessed by a single

A pair of equal and opposite

DIPOLE:

object or place on Earth.

electric charges, or an entire body having MAGNETIC

instance, a magnet with north and south

angle between magnetic north and geo-

poles.

graphic north. A

ELECTROMAGNETIC ENERGY:

form of energy with electric and magnetic components that travels in waves. ELECTROMAGNETIC FORCE:

DECLINATION:

The

the characteristics of a dipole—for

MAGNETOSPHERE:

An area sur-

rounding Earth, reaching far beyond the atmosphere, in which ionized particles

The

total force on an electrically charged particle, which is a combination of forces due to electric and magnetic fields around the

(i.e., ones that have lost or gained electrons so as to acquire a net electric charge) are affected by Earth’s magnetic field.

particle. Electromagnetic force reflects

PALEOMAGNETISM:

electromagnetic interaction, one of the

torical geology devoted to studying the

four fundamental interactions in nature. A negatively charged par-

ELECTRON:

ticle in an atom, which spins around the nucleus. ELEMENT:

A substance made up of

only one kind of atom. Unlike compounds, elements cannot be broken chemically into other substances. FIELD:

An area of his-

direction and intensity of magnetic fields in the past, as discerned from the residual magnetization of rocks. POTENTIAL:

Position in a field, such as

a gravitational force field. SOLAR WIND:

A stream of particles

continually emanating from the Sun and

A region of space in which it is

moving outward through the solar system.

possible to define the physical properties of Any set of interactions that

each point in the region at any given

SYSTEM:

moment in time.

can be set apart mentally from the rest of

FUNDAMENTAL INTERACTION:

The

basic mode by which particles interact.

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the universe for the purposes of study, observation, and measurement.

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maintain that the geomagnetic field is on its way to a reversal. A reversal? Odd as it may sound, the direction of the geomagnetic field has reversed itself many, many times in the past. Furthermore, the planet has attempted unsuccessfully to reverse its geomagnetic field many more times—as recently as 30,000 to 40,000 years ago. These reversals are among the interests of paleomagnetism, the area of geology devoted to the direction and intensity of magnetic fields in the past, as discerned from the residual magnetization of rocks.

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paleomagnetism and the shifting of plates beneath Earth’s surface.) WHERE TO LEARN MORE Campbell, Wallace Hall. Earth Magnetism: A Guided Tour Through Magnetic Fields. New York: Harcourt Academic Press, 2001. Dispezio, Michael A., and Catherine Leary. Awesome Experiments in Electricity and Magnetism. New York: Sterling, 1998. Geomagnetism (Web site). . The Great Magnet, the Earth (Web site). .

A compass works, of course, because the metal points toward Earth’s magnetic north pole, which is close to its geographic north pole. Likewise, the magnetic materials in the rocks of Earth point north—or rather, they would point north if the direction of the magnetic field had not changed over time. Around the turn of the nineteenth century, geologists noticed that whereas some magnetic rocks pointed toward Earth’s current North Pole, some were pointing in the opposite direction. This led to the realization that the magnetic field had reversed and to the development of paleomagnetism as a field of study. Studies in paleomagnetism, in turn, have provided confirmation of the powerful theory known as plate tectonics. (See Plate Tectonics for more on

Stern, David P. 400 Years of “De magnete” (Web site). .

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Hancock, Paul L., and Brian J. Skinner. The Oxford Companion to the Earth. New York: Oxford University Press, 2000. Laboratory of Earth’s Magnetism (Web site). . Lafferty, Peter. Magnets to Generators. Illus. Alex Pang and Galina Zolfanghari. New York: Gloucester Press, 1989. Lefkowitz, R. J., and J. Carol Yake. Forces in the Earth: A Book About Gravity and Magnetism. New York: Parents’ Magazine Press, 1976. Smith, David G. The Cambridge Encyclopedia of Earth Sciences. New York: Cambridge University Press, 1981.

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Convection

CONCEPT Convection is the name for a means of heat transfer, as distinguished from conduction and radiation. It is also a term that describes processes affecting the atmosphere, waters, and solid earth. In the atmosphere, hot air rises on convection currents, circulating and creating clouds and winds. Likewise, convection in the hydrosphere circulates water, keeping the temperature gradients of the oceans stable. The term convection generally refers to the movement of fluids, meaning liquids and gases, but in the earth sciences, convection also can be used to describe processes that occur in the solid earth. This geologic convection, as it is known, drives the plate movement that is one of the key aspects of plate tectonics.

HOW IT WORKS Introduction to Convection Some concepts and phenomena cross disciplinary boundaries within the earth sciences, an example being the physical process of convection. It is of equal relevance to scientists working in the geologic, atmospheric, and hydrologic sciences, or the realms of study concerned with the geosphere, atmosphere, and hydrosphere, respectively. The only major component of the earth system not directly affected by convection is the biosphere, but given the high degree of interconnection between different subsystems, convection indirectly affects the biosphere in the air, waters, and solid earth.

CONVECTION

ultimately brought about by differences in temperature, and it involves the transfer of heat through the motion of hot fluid from one place to another. In the physical sciences, the term fluid refers to any substance that flows and therefore has no definite shape. This usually means liquids and gases, but in the earth sciences it can refer even to slow-flowing solids. Over the great expanses of time studied by earth scientists, the net flow of solids in certain circumstances (for example, ice in glaciers) can be substantial.

Convection and Heat As indicated in the preceding paragraph, convection is related closely to heat and temperature and indirectly related to another phenomenon, thermal energy. What people normally call heat is actually thermal energy, or kinetic energy (the energy associated with movement) produced by molecules in motion relative to one another. Heat, in its scientific meaning, is internal thermal energy that flows from one body of matter to another or from a system at a higher temperature to a system at a lower temperature. Temperature thus can be defined as a measure of the average molecular kinetic energy of a system. Temperature also governs the direction of internal energy flow between two systems. Two systems at the same temperature are said to be in a state of thermal equilibrium; when this occurs, there is no exchange of heat, and therefore heat exists only in transfer between two systems.

Convection can be defined as vertical circulation that results from differences in density

There is no such thing as cold, only the absence of heat. If heat exists only in transit between systems, it follows that the direction of heat flow must always be from a system at a higher temperature to a system at a lower tempera-

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ture. (This fact is embodied in the second law of thermodynamics, which is discussed, along with other topics mentioned here, in Energy and Earth.) Heat transfer occurs through three means: conduction, convection, and radiation. CONDUCTION AND RA D I A T I O N . Conduction involves successive molec-

ular collisions and the transfer of heat between two bodies in contact. It usually occurs in a solid. Convection requires the motion of fluid from one place to another, and, as we have noted, it can take place in a liquid, a gas, or a near solid that behaves like a slow-flowing fluid. Finally, radiation involves electromagnetic waves and requires no physical medium, such as water or air, for the transfer. If you put one end of a metal rod in a fire and then touch the “cool” end a few minutes later, you will find that it is no longer cool. This is an example of heating by conduction, whereby kinetic energy is passed from molecule to molecule in the same way as a secret is passed from one person to another along a line of people standing shoulder to shoulder. Just as the original phrasing of the secret becomes garbled, some kinetic energy is inevitably lost in the series of transfers, which is why the end of the rod outside the fire is still much cooler than the one sitting in the flames. As for radiation, it is distinguished from conduction and convection by virtue of the fact that it requires no medium for its transfer. This explains why space is cold yet the Sun’s rays warm Earth: the rays are a form of electromagnetic energy, and they travel by means of radiation through space. Space, of course, is the virtual absence of a medium, but upon entering Earth’s atmosphere, the heat from the electromagnetic rays is transferred to various media in the atmosphere, hydrosphere, geosphere, and biosphere. That heat then is transferred by means of convection and conduction. H E AT T R A N S F E R T H R O U G H Like conduction and CONVECTION.

unlike radiation, convection requires a medium. However, in conduction the heat is transferred from one molecule to another, whereas in convection the heated fluid itself is actually moving. As it does, it removes or displaces cold air in its path. The flow of heated fluid in this situation is called a convection current.

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convection. Hot air has a lower density than that of the cooler air in the atmosphere above it and therefore is buoyant; as it rises, however, it loses energy and cools. This cooled air, now denser than the air around it, sinks again, creating a repeating cycle that generates wind. Forced convection occurs when a pump or other mechanism moves the heated fluid. Examples of forced-convection apparatuses include some types of ovens and even refrigerators or air conditioners. As noted earlier, it is possible to transfer heat only from a high-temperature reservoir to a low-temperature one, and thus these cooling machines work by removing hot air. The refrigerator pulls heat from its compartment and expels it to the surrounding room, while an air conditioner pulls heat from a room or building and releases it to the outside. Forced convection does not necessarily involve man-made machines: the human heart is a pump, and blood carries excess heat generated by the body to the skin. The heat passes through the skin by means of conduction, and at the surface of the skin it is removed from the body in a number of ways, primarily by the cooling evaporation of perspiration.

REAL-LIFE A P P L I C AT I O N S Convective Cells One important mechanism of convection, whether in the air, water, or even the solid earth, is the convective cell, sometimes known as the convection cell. The latter may be defined as the circular pattern created by the rising of warmed fluid and the sinking of cooled fluid. Convective cells may be only a few millimeters across, or they may be larger than Earth itself. These cells can be observed on a number of scales. Inside a bowl of soup, heated fluid rises, and cooled fluid drops. These processes are usually hard to see unless the dish in question happens to be one such as Japanese miso soup. In this case, pieces of soybean paste, or miso, can be observed as they rise when heated and then drop down into the interior to be heated again.

Convection is of two types: natural and forced. Heated air rising is an example of natural

On a vastly greater scale, convective cells are present in the Sun. These vast cells appear on the Sun’s surface as a grainy pattern formed by the

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A

CUMULONIMBUS CLOUD—THUNDERHEAD—IS A DRAMATIC EXAMPLE OF A CONVECTION CELL. (© Keith Kent/Photo

Researchers. Reproduced by permission.)

variations in temperature between the parts of the cell. The bright spots are the top of rising convection currents, while the dark areas are cooled gas on its way to the solar interior, where it will be heated and rise again.

A cumulonimbus cloud, or “thunderhead,” is a particularly dramatic example of a convection cell. These are some of the most striking cloud formations one ever sees, and for this reason the director Akira Kurosawa used scenes of

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sinks. And so it goes, with the heated air rising and the cooling air sinking, forming a convective cell that continually circulates air, creating a breeze.

Convection

CONVECTIVE CELLS UNDER O U R F E E T. Convective cells also can exist in

the solid earth, where they cause the plates (movable segments) of the lithosphere—the upper layer of Earth’s interior, including the crust and the brittle portion at the top of the mantle—to shift. They thus play a role in plate tectonics, one of the most important areas of study in the earth sciences. Plate tectonics explains a variety of phenomena, ranging from continental drift to earthquakes and volcanoes. (See Plate Tectonics for much more on this subject.)

CONVECTIVE

CELLS APPEAR ON THE

SUN’S

SURFACE AS

A GRAINY PATTERN FORMED BY VARIATIONS IN TEMPERATURE. (© Noao/Photo Researchers. Reproduced by permission.)

rolling thunderheads to add an atmospheric quality (quite literally) to his 1985 epic Ran. In the course of just a few minutes, these vertical towers of cloud form as warmed, moist air rises, then cools and falls. The result is a cloud that seems to embody both power and restlessness, hence Kurosawa’s use of cumulonimbus clouds in a scene that takes place on the eve of a battle. A S E A B R E E Z E . Convective cells, along with convection currents, help explain why there is usually a breeze at the beach. At the seaside, of course, there is a land surface and a water surface, both exposed to the Sun’s light. Under such exposure, the temperature of land rises more quickly than that of water. The reason is that water has an extraordinarily high specific heat capacity—that is, the amount of heat that must be added to or removed from a unit of mass for a given substance to change its temperature by 33.8°F (1°C). Thus a lake, stream, or ocean is always a good place to cool down on a hot summer day.

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Whereas the Sun’s electromagnetic energy is the source of heat behind atmospheric convection, the energy that drives geologic convection is geothermal, rising up from Earth’s core as a result of radioactive decay. (See Energy and Earth.) The convective cells form in the asthenosphere, a region of extremely high pressure at a depth of about 60–215 mi. (about 100–350 km), where rocks are deformed by enormous stresses. In the asthenosphere, heated material rises in a convection current until it hits the bottom of the lithosphere (the upper layer of Earth’s interior, comprising the crust and the top of the mantle), beyond which it cannot rise. Therefore it begins moving laterally or horizontally, and as it does so, it drags part of the lithosphere. At the same time, this heated material pushes away cooler, denser material in its path. The cooler material sinks lower into the mantle (the thick, dense layer of rock, approximately 1,429 mi. [2,300 km] thick, between Earth’s crust and core) until it heats again and ultimately rises up, thus propagating the cycle.

Subsidence: Fair Weather and Foul

The land, then, tends to heat up more quickly, as does the air above it. This heated air rises in a convection current, but as it rises and thus overcomes the pull of gravity, it expends energy and therefore begins to cool. The cooled air then

As with convective cells, subsidence can occur in the atmosphere or geosphere. The term subsidence can refer either to the process of subsiding, on the part of air or solid earth, or, in the case of solid earth, to the resulting formation. It thus is defined variously as the downward movement of air, the sinking of ground, or a depression in the earth. In the present context we will discuss atmospheric subsidence, which is more closely related to convection. (For more about geologic

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KEY TERMS ASTHENOSPHERE:

A

region

of

CORE:

The center of Earth, an area

extremely high pressure underlying the

constituting about 16% of the planet’s vol-

lithosphere, where rocks are deformed by

ume and 32% of its mass. Made primarily

enormous stresses. The asthenosphere lies

of iron and another, lighter element (possi-

at a depth of about 60–215 mi. (about

bly sulfur), it is divided between a solid

100–350 km).

inner core with a radius of about 760 mi.

ATMOSPHERE:

In general, an atmos-

(1,220 km) and a liquid outer core about

phere is a blanket of gases surrounding a

1,750 mi. (2,820 km) thick.

planet. Unless otherwise identified, howev-

CRUST:

er, the term refers to the atmosphere of

solid earth, representing less than 1% of its

Earth, which consists of nitrogen (78%),

volume and varying in depth from 3 to 37

oxygen (21%), argon (0.93%), and other

mi. (5 to 60 km). Below the crust is the

substances that include water vapor, car-

mantle.

bon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%.

FLUID:

The uppermost division of the

In the physical sciences, the

term fluid refers to any substance that

A combination of all liv-

flows and therefore has no definite shape.

ing things on Earth—plants, animals,

Fluids can be both liquids and gases. In the

BIOSPHERE:

birds, marine life, insects, viruses, singlecell organisms, and so on—as well as all formerly living things that have not yet decomposed. CONDUCTION:

The transfer of heat

by successive molecular collisions. Conduction is the principal means of heat transfer in solids, particularly metals. CONVECTION:

Vertical

earth sciences, occasionally substances that appear to be solid (for example, ice in glaciers) are, in fact, flowing slowly. FORCED CONVECTION:

that results from the action of a pump or other mechanism (whether man-made or natural), directing heated fluid toward a particular destination.

circulation

that results from differences in density ultimately brought about by differences in temperature. Convection involves the transfer of heat through the motion of hot fluid from one place to another and is of

Convection

GEOSPHERE:

The upper part of

Earth’s continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources. Internal thermal energy that

two types, natural and forced. (See natural

HEAT:

convection, forced convection.)

flows from one body of matter to another.

CONVECTION CURRENT:

The flow

HYDROSPHERE:

The entirety of

of material heated by means of convection.

Earth’s water, excluding water vapor in the

The circular

atmosphere but including all oceans, lakes,

CONVECTIVE

CELL:

pattern created by the rising of warmed

streams, groundwater, snow, and ice. The energy that

fluid and the sinking of cooled fluid. This is

KINETIC ENERGY:

sometimes called a convection cell.

an object possesses by virtue of its motion.

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Convection

KEY TERMS LITHOSPHERE:

The upper layer of

A term that refers

SUBSIDENCE:

Earth’s interior, including the crust and the

either to the process of subsiding, on the

brittle portion at the top of the mantle.

part of air or solid Earth, or, in the case of

The dense layer of rock,

MANTLE:

approximately 1,429 mi. (2,300 km) thick, between Earth’s crust and its core. NATURAL CONVECTION:

solid Earth, to the resulting formation. Subsidence thus is defined variously as the downward movement of air, the sinking of ground, or a depression in Earth’s crust.

Convec-

SYSTEM:

Any set of interactions that

tion that results from the buoyancy of

can be set apart mentally from the rest of

heated fluid, which causes it to rise.

the universe for the purposes of study,

PLATE TECTONICS:

The name both

of a theory and of a specialization of tectonics. As an area of study, plate tectonics deals with the large features of the litho-

observation, and measurement. TECTONICS:

The study of tectonism,

including its causes and effects, most notably mountain building. The deformation of the

sphere and the forces that shape them. As a

TECTONISM:

theory, it explains the processes that have

lithosphere.

shaped Earth in terms of plates and their

TEMPERATURE:

movement.

internal energy flow between two systems

PLATES:

Large, movable segments of

the lithosphere. RADIATION:

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CONTINUED

The direction of

when heat is being transferred. Temperature measures the average molecular kinetic energy in transit between those systems.

The transfer of energy by

THERMAL ENERGY:

Heat energy, a

means of electromagnetic waves, which

form of kinetic energy produced by the

require no physical medium (for example,

motion of atomic or molecular particles in

water or air) for the transfer. Earth receives

relation to one another. The greater the rel-

the Sun’s energy via the electromagnetic

ative motion of these particles, the greater

spectrum by means of radiation.

the thermal energy.

subsidence, see the entries Geomorphology and Mass Wasting.)

dence occurs, it results in the creation of a highpressure area known as an anticyclone.

In the atmosphere, subsidence results from a disturbance in the normal upward flow of convection currents. These currents may act to set up a convective cell, as we have seen, resulting in the flow of breeze. The water vapor in the air may condense as it cools, changing state to a liquid and forming clouds. Convection can create an area of low pressure, accompanied by converging winds, near Earth’s surface, a phenomenon known as a cyclone. On the other hand, if subsi-

Air parcels continue to rise in convective currents until the density of their upper portion is equal to that of the surrounding atmosphere, at which point the column of air stabilizes. On the other hand, subsidence may occur if air at an altitude of several thousand feet becomes denser than the surrounding air without necessarily being cooler or moister. In fact, this air is unusually dry, and it may be warm or cold. Its density then makes it sink, and, as it does, it compresses

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the air around it. The result is high pressure at the surface and diverging winds just above the surface.

Ocean waters fit the most common, everyday definition of fluid, but as noted at the beginning of this essay, a fluid can be anything that flows—including a gas or, in special circumstances, a solid. Solid rocks or solid ice, in the form of glaciers, can be made to flow if the materials are deformed sufficiently. This occurs, for instance, when the weight of a glacier deforms ice at the bottom, thus causing the glacier as a whole to move. Likewise, geothermal energy can heat rock and cause it to flow, setting into motion the convective process of plate tectonics, described earlier, which literally moves the earth.

The form of atmospheric subsidence described here produces pleasant results, explaining why high-pressure systems usually are associated with fair weather. On the other hand, if the subsiding air settles onto a cooler lay of air, it creates what is known as a subsidence inversion, and the results are much less beneficial. In this situation a warm air layer becomes trapped between cooler layers above and below it, at a height of several hundred or even several thousand feet. This means that air pollution is trapped as well, creating a potential health hazard. Subsidence inversions occur most often in the far north during the winter and in the eastern United States during the late summer.

Erickson, Jon. Plate Tectonics: Unraveling the Mysteries of the Earth. New York: Facts on File, 1992.

When a Non-Fluid Acts Like a Fluid

Hess, Harry. “History of Ocean Basins” (Web site). .

Up to this point we have spoken primarily of convection in the atmosphere and the geosphere, but it is of importance also in the oceans. The miso soup example given earlier illustrates the movement of fluid, and hence of particles, that can occur when a convective cell is set up in a liquid. Likewise, in the ocean convection—driven both by heat from the surface and, to a greater extent, by geothermal energy at the bottom— keeps the waters in constant circulation. Oceanic convection results in the transfer of heat throughout the depths and keeps the ocean stably stratified. In other words, the strata, or layers, corresponding to various temperature levels are kept stable and do not wildly fluctuate.

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Convection

WHERE TO LEARN MORE Educator’s Guide to Convection (Web site). .

Jones, Helen. Open-Ocean Deep Convection: A Field Guide (Web site). . Ocean Oasis Teacher’s Guide Activity 4 (Web site). . Santrey, Laurence, and Lloyd Birmingham. Heat. Mahwah, NJ: Troll Associates, 1985. Scorer, R. S., and Arjen Verkaik. Spacious Skies. Newton Abbot, England: David and Charles, 1989. Sigurdsson, Haraldur. Melting the Earth: The History of Ideas on Volcanic Eruptions. New York: Oxford University Press, 1999. Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science. 5th ed. New York: John Wiley and Sons, 1999. Smith, David G. The Cambridge Encyclopedia of Earth Sciences. New York: Cambridge University Press, 1981.

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ENERGY AND EARTH Energy and Earth

CONCEPT Earth is a vast flow-through system for the input and output of energy. The overwhelming majority of the input to Earth’s energy budget comes from the Sun in the form of solar radiation, with geothermal and tidal energy rounding out the picture. Each form of energy is converted into heat and re-radiated to space, but the radiation that leaves Earth travels in longer wavelengths than that which entered the planet. This is in accordance with the second law of thermodynamics, which shows that energy output will always be smaller than energy input and that energy which flows through a system will return to the environment in a degraded form. Yet what the Earth system does in processing that energy, particularly the portion that passes through the biosphere, is amazing. Some biological matter decays and, over the course of several hundred million years, produces fossil fuels that have given Earth a slight energy surplus. Human use of fossil fuels is rapidly depleting those sources, however, while posing new environmental problems, and this has encouraged the search for alternative forms of energy. Many of those forms, most notably geothermal energy, come from Earth itself.

HOW IT WORKS Energy, Work, and Power Physicists define energy as the ability of an object (and in some cases a nonobject, such as a magnetic force field) to accomplish work. “Work” in this context does not have the same meaning as it does in everyday life; along with the closely relat-

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ed concept of power, it is defined very specifically in a scientific context. Work is the exertion of force over a given distance, and therefore it is measured in units of force multiplied by units of length. In the English system used by most Americans, a pound is the unit of force, and the foot-pound (ft-lb) would be the unit of work. However, scientists worldwide use SI, or the International System, which applies metric units. The metric unit of force is the newton (N), and the metric unit of work is the joule (J), equal to 1 newton-meter (N  m). Power is the rate at which work is accomplished over time and therefore is measured in units of work divided by units of time. The metric unit of power is the watt (W), named after James Watt (1736–1819), the Scottish inventor who developed the first fully viable steam engine and thus helped inaugurate the Industrial Revolution. A watt is equal to 1 J per second, but this is such a small unit that kilowatts, or units of 1,000 W, are more frequently used. Discussing the vast energy budget of Earth itself, however, requires use of an even larger unit: the terawatt (TW), equal to 1012 (one trillion) W. Ironically, Watt himself—like most people in the British Isles and America—lived in a world that used the British system, in which the unit of power is the foot-pound per second. The latter unit, too, is very small, so for measuring the power of his steam engine, Watt suggested a unit based on something quite familiar to the people of his time: the power of a horse. One horsepower (hp) is equal to 550 ft-lb per second. In the present context, we will rely as much as possible on SI units, especially because the watt is widely used in America. Horsepower typ-

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ically is applied in the United States only for measuring the power of a mechanical device, such as an automobile or even a garbage disposal. For measuring electrical power, particularly in larger quantities, the SI kilowatt (kW) is used. When an electric utility performs a meter reading on a family’s power usage, for instance, it measures that usage in terms of electrical “work” performed for the family and thus bills them by the kilowatt-hour (kWh).

Varieties of Energy In the most fundamental sense, there are only three kinds of energy: kinetic, potential, and mass, or rest, energy. These types are, respectively, the energy an object possesses by virtue of its motion, its position (or its ability to perform work), and its mass. The first two are understood in relation to each other: for example, a ball held over the side of a building has a certain gravitational potential energy, but once it is dropped, it begins to lose potential energy and gain kinetic energy. The faster it moves, the greater the kinetic energy; but as it covers more distance, the less its potential energy is. (See Earth Systems for more about the kinetic-potential energy system.) As Earth moves around the Sun, the gravitational interaction between the two bodies is not unlike that of the baseball and the ground in the illustration just given. Earth makes an elliptical, or oval-shaped, path in its orbit, meaning that the distance between it and the Sun is not uniform. At its furthest distance, Earth’s potential energy is maximized, but as it comes closer to the Sun in its orbital path, its kinetic energy increases, with a corresponding decrease of potential energy. M ASS A N D E N E R GY. Mass, or rest, energy is identified in the famous formula E = mc2, derived by Albert Einstein (1879–1955). In simple terms Einstein’s formula means that every object possesses an amount of energy equal to its mass multiplied by the speed of light squared. Given that light travels at 186,000 mi. (299,339 km) per second, this is an enormous figure, even for a small object. A mere baseball, which weighs about 0.333 lb. (0.15 kg), possesses enough energy to yield about 3.75 billion kWh worth of power—enough to run all the lights and appliances in a typical American home for more than 156,000 years!

ball to a speed close to that of light. Even in ordinary experience, however, very small amounts of mass are converted to energy. For instance, when a fire burns, the mass of the ashes combined with that of the particles and gases sent into the atmosphere is smaller (by an almost imperceptible fraction) than the mass of the original wood. The “lost” mass is converted to energy. These mass-energy conversions occur on a much larger level in nuclear reactions, such as the nuclear fusion of hydrogen atoms to form helium in the solar core (see Sun, Moon, and Earth).

Energy and Earth

M A N I F E S TAT I O N S O F E N E R GY. In discussing kinetic and potential energy,

the example of dropping a baseball from a height illustrates these two types of energy in a gravitational field—that is, the gravitational field of Earth. Yet the concept of potential and kinetic energy translates to a situation involving an electromagnetic field as well. For instance, the positive or negative attraction between two electromagnetically charged particles is analogous to the force of gravity, and a system of two or more charges possesses a certain amount of kinetic and potential electromagnetic energy. Electromagnetic energy, which is the form in which solar power reaches Earth, is a type of energy that (as its name suggests) combines both electrical and magnetic energy. Another important form of energy in the Earth system is thermal, or heat, energy, which is the kinetic energy of molecules, since heat is simply the result of molecular motion. Other types of energy include sound, chemical, and nuclear energy. Sound waves, which require a physical medium such as air in which to travel, are simply pressure fluctuations that carry varying levels of energy, depending on the frequency (pitch) and amplitude (volume) of the waves. Chemical energy makes possible the forming and releasing of molecular bonds, and, for this reason, chemical reactions often are accompanied by the production of heat. Whereas chemical energy concerns the bonds between atoms, nuclear energy relates to the bonds within them. Nuclear fission reactions involve the splitting of an atomic nucleus, while nuclear fusion is the joining of nuclei.

Heat and Thermodynamics

To release this energy in significant quantities, it would be necessary to accelerate the base-

Thermodynamics is the study of the relationships between heat, work, and energy. As with

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work, energy, and power, heat and the related concept of temperature are terms that have special definitions in the physical sciences. Heat itself is not to be confused with thermal energy, which, as noted earlier, is the kinetic energy that arises from the motion of particles at the atomic or molecular level. The greater the movement of these particles relative to one another, the greater the thermal energy. Heat is internal thermal energy that flows from one body of matter to another. It is not the same as the energy contained in a system—that is, the internal thermal energy of the system. Rather than being “energy-in-residence,” heat is “energy-in-transit.” This may seem a little confusing, but all it means is that heat, in its scientific sense, exists only when internal energy is being transferred. As for temperature, it is not (as is commonly believed) a measure of heat and cold. Instead, temperature indicates the direction of internal energy flow between bodies and the average molecular kinetic energy in transit between those bodies. In any case, temperature could not be a measure of heat and cold, as though these two were equal and opposing entities, because, scientifically speaking, there is no such thing as cold— only an absence of heat. When we place an ice cube in a cup of coffee, we say that the ice is there to “cool the coffee down,” but, in fact, the opposite is happening: the coffee is warming up the ice cube, and in the process of doing so, it loses heat. This may seem like a difference of semantics, but it is not. It is a physical law that the flow of heat is always from a high-temperature reservoir to a low-temperature reservoir. Even air conditioners and refrigerators work by pulling heat out of a compartment rather than by bringing cold in. MEASURING T E M P E R AT U R E A N D H E AT. Temperature, of course, can be

measured by either the Fahrenheit or the Centigrade scales familiar in everyday life. Scientists, however, prefer the Kelvin (K) scale, established by William Thomson, Lord Kelvin (1824–1907). Drawing on the discovery made by the French physicist and chemist J. A. C. Charles (1746–1823) that gas at 0°C (32°F) regularly contracts by about 1/273 of its volume for every Celsius degree drop in temperature, Thomson derived the value of absolute zero (the temperature at which molecular motion virtually ceases) as –273.15°C (–459.67°F). The Kelvin and Cel-

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sius scales are thus directly related: Celsius temperatures can be converted to Kelvin units (for which neither the word nor the symbol for “degree” is used) by adding 273.15. Heat, on the other hand, is measured not by degrees but by the same units as work. Energy is the ability to perform work, so heat or work units are also units of energy. Aside from the joule, heat often is measured by the kilocalorie, or the amount of heat that must be added to or removed from 1 kg of water to change its temperature by 1°C. As its name suggests, a kilocalorie is 1,000 calories, a calorie being the amount of heat required to change the temperature in 1 g of water by 1°C. The dietary calorie with which most people are familiar, however, is the same as a kilocalorie.

The Laws of Thermodynamics The three laws of thermodynamics collectively show that it is impossible for a system to produce more energy than was put into it or even to produce an equal amount of usable energy. In other words, a perfectly efficient system—whether an engine or the entire Earth—is an impossibility. Derived during a period of about 60 years beginning in the 1840s, the laws of thermodynamics helped scientists and engineers improve the machines that powered the height of the Industrial Age. They also revealed the impossibility of constructing anything approaching a perpetualmotion machine, that great quest of dreamers over the ages, which the laws of thermodynamics proved to be an impossible dream. T H E F I R S T LAW O F T H E R M O DY N A M I CS . The first law of thermodynam-

ics is related to the conservation of energy, a physical law whereby the total energy in a system remains the same, though transformations of energy from one form to another take place. Such transformations occur frequently in the Earth system, as when a plant receives electromagnetic energy from the Sun and converts it to chemical potential energy in the form of carbohydrates. Likewise humans, by building dams, can harness the gravitational potential energy of flowing water and convert it into electromagnetic energy. The conservation of energy, in effect, states that “the glass is half full,” meaning that we can obtain as much energy from a system as we put into it. While saying the same thing, the first law of thermodynamics in effect states that “the glass

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is half empty,”: that is, that we can obtain no more energy from a system than we put into it. According to this law, because the amount of energy in a system remains constant, it is impossible to perform work that results in an energy output greater than the energy input. The term law in the physical sciences is no empty expression; it means that a principle has been shown to be the case always and may be expected to remain the case in all situations. It is possible, of course, for a physical law to be overturned in light of later evidence. It is not likely, however, that any set of circumstances in the universe will ever disprove the core truth behind this law, which may be stated colloquially as “You can’t get something for nothing.” T H E S E C O N D LAW O F T H E R M O DY N A M I CS . In a 1959 lecture pub-

lished as The Two Cultures and the Scientific Revolution, the British writer and scientist C. P. Snow (1905–1980) compared transfers of heat and energy to a game. The laws of thermodynamics are its rules, and, as Snow stated, the first law proves that it is impossible to win at this game, while the second law shows the impossibility of breaking even. The second law of thermodynamics is more complicated than the first and is stated in a number of ways, though they are all interrelated. According to this law, spontaneous or unaided transfers of energy are irreversible and impossible without an increase of entropy in the universe. Entropy is the tendency of natural systems toward breakdown, specifically, the tendency for the energy in a system to be dissipated or degraded. (Later in this essay, we discuss examples of energy that has been degraded—for instance, wood that has been burned to produce ashes.) The second law means that spontaneous processes are irreversible and that it is impossible, without the additional input of energy, to transfer heat from a colder to a hotter body or to convert heat into an equal amount of work. Whereas the first law showed engineers the impossibility of building a perpetual-motion machine, the second law proves that it is impossible to build even a perfectly efficient engine. Of all the energy we put into our automobiles in the form of gasoline (which is chemical potential energy in the form of hydrocarbons derived from the fossilized remains of dinosaurs in the earth), only about 30% of it goes into moving the car

S C I E N C E O F E V E RY DAY T H I N G S

forward. The rest is dissipated in a number of ways, chiefly through heat and sound. Entropy, as it turns out, is inescapable and as inevitable as death. In fact, death itself is a result of entropy in the systems of all living things.

Energy and Earth

T H E T H I R D LAW O F T H E R M O DY N A M I CS . The third law of thermody-

namics is not as well known as the other two and has little bearing on the discussion at hand, but it deserves at least brief mention. According to the third law, at the temperature of absolute zero entropy also approaches zero, which might sound like a way out of the restrictions imposed by the first two laws. All it really means is that absolute zero is impossible to reach—or, as Snow put it, the third law shows that “you can’t escape the game.” In 1824 the French physicist and engineer Nicolas Léonard Sadi Carnot (1796–1832) had shown that an engine could achieve maximum efficiency if its lowest operating temperature were absolute zero. His work influenced that of Kelvin, who established the absolute-temperature scale mentioned earlier. Additionally, Carnot’s discoveries informed the development of the third law. Whereas the second law is not derived from the first (though it is certainly consistent with it), the third law relies on the second: if it is impossible to build a perfectly efficient engine, as the second law states, it is likewise impossible to reach absolute zero. This, of course, has not stopped scientists from attempting to achieve absolute zero, most properly defined as the temperature at which the motion of the average atom or molecule is zero. Helium atoms, in fact, never fully cease their motion, even at temperatures very close to 0K— and scientists have come very, very close. In 1993 physicists at the Helsinki University of Technology Low Temperature Laboratory in Finland used a nuclear demagnetization device to achieve a temperature of 2.8  10–10K, or 0.00000000028K. This amounts to a difference of only 28 parts in 100 billion between that temperature and absolute zero.

Earth’s Energy Input Just as households have financial budgets, a system such as Earth (see Earth Systems) has an energy budget. The latter may be defined as the total amount of energy available to a system or, more specifically, the difference between the

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Energy and Earth

THE POHUTU

GEYSER AT THE

ROTORUA WHAKAREWAREWA THERMAL AREA

IN

NORTH ISLAND, NEW ZEALAND. (© A.

N. T./Photo Researchers. Reproduced by permission.)

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energy flowing into the system and the energy lost by it. Having reviewed the laws of thermodynamics, one might suspect that a great deal of energy is dissipated in the operation of the Earth system, and, of course, that is absolutely correct.

Earth receives 174,000 TW of energy, or 174 quadrillion J per second. Human civilization, by contrast, uses only 10 TW, or about 0.00574% as much as Earth’s total energy. Of that total, there are three principal sources, though one of these

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sources—the Sun—dwarfs the other two in importance. The breakdown of Earth’s energy input, along with the percentage of the total that each portion constitutes, is as follows: • Solar radiation: 99.985% • Geothermal energy: 0.013% • Tidal energy: 0.002% S O LA R RA D I AT I O N . The Sun radi-

ates electromagnetic energy, which, as mentioned previously, is a form of energy that produces both electric and magnetic fields. Electromagnetic energy travels in waves, and since waves follow regular patterns, it is possible to know that those waves with shorter wavelengths have a higher frequency and thus higher energy levels. The electromagnetic spectrum contains a variety of waves, each with progressively higher energy levels, including long-wave and shortwave radio; microwaves (used for TV transmissions); infrared, visible, and ultraviolet light; x rays; and ultra-high-energy gamma rays. Visible light is only a very small portion of this spectrum, and each color has its own narrow wavelength range. Red has the least energy and purple or violet the most; hence, the names infrared for light with less energy than red, and ultraviolet for light with more energy than violet. The order of these wavelengths of light, along with the colors between, is remembered easily by the mnemonic device ROY G. BIV (standing for red, orange, yellow, green, blue, indigo, and violet). (Actually, there are only six major color ranges, and the name really should be ROY G. BV.) Although it covers the entire electromagnetic spectrum, energy from the Sun is referred to by earth scientists as short-wavelength radiation. This is because the solar energy that enters the Earth system is shorter in wavelength (and thus higher in energy level) than the energy returned to space by Earth. (We discuss the degradation of energy in the Earth system later in this essay.) Without solar radiation, the life-giving processes of the hydrosphere, biosphere, and atmosphere would be impossible. An example is photosynthesis, the biological conversion of electromagnetic energy to chemical energy in plants. (See the later discussion of photosynthesis and the food web.)

internal heat energy. Much of this heat comes from Earth’s core, which has temperatures as high as 8,132°F (4,500°C) and from whence thermal energy circulates throughout the planet’s interior. Also significant is the heat from radioactive elements, most notably uranium and thorium, near Earth’s surface.

Energy and Earth

This thermal energy heats groundwater, and thus the principal visible sources of geothermal energy include geysers, hot springs, and fumaroles—fissures, created by volcanoes, from which hot gases pour. There are several types of geothermal energy reserves, among them dry and wet steam fields. The first of these reserves occurs when groundwater boils normally, whereas in the second type of reserve, groundwater is superheated, or prevented from boiling even though its temperature is above the boiling point. In both cases the waters have a much higher concentration of gases and minerals than ordinary groundwater. Another type of reserve can be found under the ocean floors, where natural gas mixes with very hot water. Geothermal energy powers seismic activity as well as volcanic eruptions and mountain building, which together have played a significant role in shaping Earth as we know it today. Aside from its obvious impact on the planet’s terrain, geothermal energy has had an indirect influence on the transfer of vital elements from beneath Earth’s surface, a benefit of volcanic activity. (See the later discussion of the human use of geothermal energy in this essay.) T I DA L E N E R GY. Whereas the principal form of energy in Earth’s budget comes from the Sun and the secondary source from Earth itself, the third type of energy input to the Earth system comes chiefly from the Moon. The Sun also affects tides, but because of its close proximity to Earth, the Moon has more influence over the movements of our planet’s ocean waters.

G E O T H E R M A L E N E R GY. A much smaller, but still significant component of Earth’s energy budget is geothermal energy, the planet’s

Though the Moon is much smaller than Earth, it is larger, in proportion to the planet it orbits, than any satellite in the solar system (with the possible exception of Pluto’s moon Charon). Given this fact, combined with its close proximity to Earth, it is understandable that the Moon would exert a powerful pull on its host planet. The gravitational pull of the Moon (and, to a lesser extent, that of the Sun) on Earth causes the oceans to bulge outward on the side of Earth closest to the Moon. At the same time, the oceans

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on the opposite side of the planet bulge in response. (See Sun, Moon, and Earth for more about tides and the bulges that result from the Moon’s gravitational pull.) This gravitational pull creates a torque that acts as a brake on Earth’s rotation, producing a relatively small amount of energy that is dissipated primarily within the waters of the ocean. Incidentally, the lunar-solar tidal torque, by increasing the amount of time it takes Earth to turn on its axis, is causing a gradual increase in the length of a day. Today, of course, there are 365.25 days in a year, but about 650 million years ago there were 400 days. In other words, Earth made 400 revolutions on its axis in the period of time it took it to revolve around the Sun. The change is a result of the fact that Earth’s rotation is being slowed by 24 microseconds a year.

Energy Profit and Loss Focusing now on solar radiation, since it is by far the greatest source of energy input to the Earth system, let us consider Earth’s energy budget in terms of “profit and loss.” In other words, how much useful energy output is denied to Earth owing to the laws of thermodynamics and other factors? First of all, a good 30% of the Sun’s energy input is reflected back into space unchanged, without entering Earth’s atmosphere. This results from our planet’s albedo, or reflective power. Albedo is the proportion of incoming radiation that is reflected by a body (e.g., a planet) or surface such as a cloud: the higher the proportion of incoming radiation that a planet deflects, the higher its albedo. The latter is influenced by such factors as solar angle, amount of cloud cover, particles in the atmosphere, and the character of the planetary surface.

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waves. A very small, but extremely significant portion of incoming solar radiation goes into plant photosynthesis, discussed later. The energy that enters the Earth system—not only solar radiation but also geothermal and tidal energy—ultimately leaves the system. As shown by the second law of thermodynamics, however, the energy that departs the Earth system will be in a degraded form compared with the energy that entered it. ENERGY

D E G RA D AT I O N .

In a steam engine, water in the form of steam goes to work to power gears or levers. In the process, it cools, and the resulting cool water constitutes a degraded form of energy. Likewise, the ashes that remain after a fire or the fumes that are a by-product of an internal combustion engine’s operation contain degraded forms of energy compared with that in the original wood or gasoline, respectively. In the same way, Earth receives short-wavelength energy from the Sun, but the energy it radiates to space is in a longwavelength form. All physical bodies with a temperature greater than absolute zero emit electromagnetic energy in accordance with their surface temperatures, and the hotter the body, the shorter the wavelength of the radiation. The sunlight that enters Earth’s atmosphere is divided between the visible portion of the spectrum and the high-frequency side of the infrared portion. (Note that the Sun emits energy across the entire electromagnetic spectrum, but only a small part gets through Earth’s atmospheric covering.) Earth, with an average surface temperature of 59°F (15°C), is much cooler than the Sun, with its average surface temperature of about 10,000°F (5,538°C). The radiation Earth sends back into space, then, is on the low-frequency, long-wavelength side of the infrared spectrum.

Another 25% of solar radiation is absorbed by the atmosphere, while about 45% is absorbed at the planetary surface by living and nonliving materials. Thus, electromagnetic energy from the Sun enters the atmosphere, biosphere, and hydrosphere, where it is converted to other forms of energy, primarily thermal. Some of this thermal energy, for instance, causes the evaporation of water, which cycles through the atmosphere and then reenters the hydrosphere as precipitation. In other cases, absorbed radiation drives atmospheric and hydrologic distribution mechanisms, including winds, water currents, and

N O E N E R GY LO SS . In accordance with the conservation of energy, no energy truly has been lost. In a relatively simple system, such as an automobile, chemical potential energy enters the vehicle in the form of gasoline and, after being processed by the engine, exits in a variety of forms. There is the kinetic energy that turns the wheels; the thermal energy of the engine and exhaust; electromagnetic energy from the battery for the headlights, dashboard lights, radio, air conditioning, and so on; and the sound energy dissipated in the noise of the car. If one

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could add all those energy components together, one would find that all the energy that entered the system left the system. Note, however, that once again the process is irreversible: one can use gasoline to power a battery and hence a car radio, but the radio or the battery cannot generate gasoline.

Energy and Earth

The Earth system is much more complex, of course, but the same principle applies: about 174,000 TW of energy enter the system, and about 174,000 TW are used in the form of heat. Of the portion that enters the atmosphere, some goes into warming the planet, some into moving the air and water, and a very small part into the all-important biological processes described later, but all of it is used. It should be noted that Earth has a very small energy surplus, owing to the accumulation of undecomposed biomass that ultimately becomes fossil fuels; however, the amount of energy involved is minor compared with the larger energy budget. (For more information about biomass and fossil fuels, see the following discussion.) A

THE

GREENHOUSE

E F F E C T.

Not only is there no net loss of energy in the universe, but Earth itself also possesses a remarkably efficient system for making use of the energy it receives. This is the greenhouse effect, whereby the planet essentially recycles the degraded energy it is in the process or returning to space. Water vapor and carbon dioxide, as well as methane, nitrous oxide, and ozone, all absorb long-wavelength radiated energy as the latter makes its way up through the atmosphere. When heated, these radiatively active gases (as they are called) re-radiate the energy, now at even longer wavelengths. In so doing, they slow the planet’s rate of cooling. Without the greenhouse effect, surface temperatures would be about 50°F (10°C) cooler than they are—that is, around 17.6°F (–8°C). This, of course, is well below the freezing temperature of water and much too cold for Earth’s biological processes. Thus, the greenhouse effect literally preserves life on the planet. It may be surprising to learn that the greenhouse effect, a term often heard in the context of dire environmental warnings, is a natural and healthful part of the Earth system. Like many useful things, the greenhouse effect is not necessarily better in larger doses, however, and that is the problem. It is believed that human activities have resulted in an increase of radiatively active

S C I E N C E O F E V E RY DAY T H I N G S

GERMINATING GARDEN PEA SEEKS SUNLIGHT FOR

PHOTOSYNTHESIS. (© J. Burgess/Photo Researchers. Reproduced by

permission.)

gases, which could lead to global warming. Among such gases are the chlorofluorocarbons (CFCs) used in aerosol cans, now banned by international treaty. Also of concern are the high levels of carbon dioxide produced by the burning of fossil fuels.

REAL-LIFE A P P L I C AT I O N S Energy and the Living World One of the most interesting components of the entire energy picture is the relationship of energy input, energy output, and the biosphere. Particularly important is the use of solar radiation for the purposes of photosynthesis, an activity that constitutes a small but vital sector of Earth’s energy budget. Though it accounts for only about 1% of the energy received from solar radiation, photosynthesis is essential to the sustenance of life. In photosynthesis, plants receive solar radiation, carbon dioxide, and water, which chemically react to produce carbohydrates and oxygen. Animals

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depend not only on the oxygen but also on carbohydrates, such as sugars, and thus photosynthesis makes possible the development of the food chains that constitute the world of living creatures. F O O D W E B S . Even though food chain

is a well-known expression, modern scientists prefer the term food web, because it more accurately portrays the complicated relationships involved. The term food chain, as it is commonly used, implies a strict hierarchical structure in which (to use another popular phrase) “the big fish eat the little fish.” In fact, the relationship between participants is not quite so neatly defined. It is, however, possible to describe a food web in terms of a few key players, or types of players. There are primary producers, which are green plants, and primary consumers—herbivores, or plant-eating animals. Secondary consumers (and those at further levels, such as tertiary and quaternary) are either carnivores—that is, meat-eating animals who eat the herbivores— or omnivores, which are both plant and animal eaters. For example, omnivorous humans eat herbivorous cows, who have eaten plants. Carnivores and omnivores, however, are not really at the top of the food chain, so to speak; rather, in line with the non-hierarchical idea of the food web, they represent points in an interlocking set of relationships. Materials from plants, herbivores, carnivores, and omnivores ultimately will all be consumed by the lowliest of creatures, that is, detritivores, or decomposers, including bacteria and worms. At each stage energy is transferred, and, as always, the second law of thermodynamics comes into play. The energy is degraded in transfer; specifically, the further away an organism is from the original plant source, the less a given quantity of fuel contributes to its growth. It is interesting to note the economy of energy use at the detritivore stage.

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ly) to providing fuel for carnivores, as when a bird eats a worm.

Bioenergy The food web is the mechanism whereby energy is cycled through the biosphere, as fuel in the form of food. Plant matter and other biological forms also can serve as direct sources of heat, providing fuel that can be either burned without processing or converted into gas or alcohol. In such situations the plant matter is described as bioenergy, or energy derived from biological sources that are used as fuel. Materials that are burned or processed to produce bioenergy are called biomass. Examples of the latter include wood logs burned on a fire, probably the oldest type of fuel known to humankind and still one of the principal forms of heating available in many developing countries. In fact, some of the least technologically advanced and most technologically advanced nations are alike in their use of another variety of biomass: waste. Dried animal dung provides heating material in many a third world village where electricity is unknown, while at the other end of the technological spectrum, some Western municipalities extract burnable biomass from processed sewage. Since Western countries have at their disposal plenty of other energy sources, they typically burn off the methane gas and dried waste material from treated sewage simply as a means of removing it, rather than as an energy source. Those materials could provide usable energy, however.

Worms and other decomposers are exceedingly efficient feeders, working the same food particles over and over and extracting more stored energy each time. They then produce waste products that increase the vitamin content of the soil, thus enabling the growth of plants and the continuation of the biological cycle. Also, detritivores may contribute directly (and, from the larger energy-cycle perspective, less efficient-

The products of sewage treatment, of course, are the result of processing, which involves the conversion of biomass to either gases (for example, methane, as mentioned previously) or alcohol. Farmers in rural China, for instance, often place agricultural waste and sewage in small closed pits, from whence they extract burnable methane gas. In the United States, Brazil, and other countries with abundant farmland, some of the agricultural output is directed not toward production of food but toward production of fuel in the form of ethanol, a type of alcohol made from sugarcane, corn, or sorghum grain. Ethanol can be mixed with gasoline to run an automobile or burned alone in specially modified engines. In either case, the fuel burns much

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more cleanly, producing less poisonous carbon monoxide than ordinary gasoline.

sary. For a time at least, these searches will continue to yield new (though increasingly harder to reach) deposits.

Fossil Fuels

Also, fears about Earth running out of fuel are built on the assumption that no one will develop other sources of energy, a few of which are discussed at the conclusion of this essay. Though most of the known alternative energy sources face their own challenges, it is thoroughly conceivable that scientists of the future will develop means of completely replacing fossil fuels as the source of electrical power, fuel for transport, and other uses. After all, there was once a time (during the second half of the nineteenth century) when Americans became increasingly anxious over dwindling reserves of a vital energy resource essential for powering the nation’s lamps: whale oil.

Biomass is potentially renewable, whereas another source of bioenergy is not. This is the bioenergy from fossil fuels—buried deposits of petroleum, coal, peat, natural gas, and other organic compounds. (Actually, fossil fuels typically are considered separate from other forms of bioenergy, because they are nonrenewable.) Fossil fuels are the product of plants and animals that lived millions of years ago, died, decomposed, and became part of Earth’s interior. Over the ages, as more sediment weighed down on these organic deposits, the weight applied more pressure, generated more heat, and led to the concentration of this decomposed material, which became a valuable source of energy.

Energy and Earth

Environmental Concerns

U S I N G U P R E S O U R C E S . Given

the vast spans of time that have passed, as well as the almost inconceivable numbers of plants and animals, the supplies of fossil-fuel energy stored under Earth’s surface are enormous. But they are not infinite, and, as noted earlier, they are nonrenewable; once they are gone, they might as well be gone forever, because it would take hundreds of millions of years to produce additional deposits. Furthermore, humans are using up these energy sources at an alarming rate. Up until about 1750, Earth’s fossil-fuel deposits were largely intact, but after that time industrialized societies began to extract coal for heating, transportation, and industrial uses. Today it remains a leading means of generating electrical power. During the twentieth century, petroleum increasingly was directed toward transportation, and this led to the extraction of still more of Earth’s fossil-fuel deposits. If civilization continues to consume these products at its current rate, reserves will be exhausted long before the end of the twenty-first century. SEARCHING FOR ENERGY S O U R C E S . There are several reasons not to

While the loss of energy reserves may not be an immediate cause for panic, the effects of fossilfuel burning on the environment have alarmed scientists and environmentalists alike. At the most basic level, there is the environmental impact posed by the extraction of fossil fuels. Coal mines, for instance, have been used up and now sit abandoned, their land worthless for any purpose. There is also the environmental danger created by hazards in misuse or transport of fossil fuels—for example, the vast oil spill caused by the grounding of the Exxon Valdez near Alaska’s Prince William Sound in 1989. By far the greatest environmental concern raised by fossil fuels, however, is the effect they produce in the atmosphere when burned. For instance, one of the impurities in coal is sulfur, and when coal burns, the sulfur reacts with oxygen in the combustion process to create sulfur dioxide and sulfur trioxide. Sulfur trioxide reacts with water in the air, creating sulfuric acid and thus acid rain, which can endanger plant and animal life as well as corrode metals and building materials.

panic over the loss of fossil-fuel resources. First, known reserves are just that—they are the ones that energy companies, and their geologists, know about at the present time. As long as plentiful resources are available, corporations and governments do not feel a pressing need to search for more, but as those resources are used up, such searches become economically neces-

release into the atmosphere of carbon monoxide and carbon dioxide, both of which are by-products in the burning of fossil fuels. The first is a poison, whereas the second is a vital part of the life cycle, yet carbon dioxide, in fact, may pose the greater threat.

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THE GREENHOUSE EFFECT R E V I S I T E D . Even greater fears center on the

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utilities, and transportation, many alternative energy sources are already in use.

Energy and Earth

Some of these energy forms are very old, for instance, burnable biomass, water power, or wind power, all of which date back to ancient times. Others are extremely new in concept, most notably, nuclear energy, which was developed in the twentieth century. Still others are new, hightech versions of old-fashioned energy sources, the best example being solar power. In fact, all three major contributors to Earth’s energy budget—solar radiation, tidal energy, and geothermal energy (discussed further later in this essay)— have been harnessed by human societies. HIGH-TECH ENERGY SOLUT I O N S . Several of the more ambitious ideas

AN

ARRAY

QUERQUE,

OF

SOLAR

NEW MEXICO,

REFLECTORS

NEAR

ALBU-

HARNESSES SOLAR POWER.

(© J. Mead/Photo Researchers. Reproduced by permission.)

Earlier we discussed the greenhouse effect, which, when it occurs naturally, is important to the preservation of life on the planet. The large number of internal-combustion engines in operation on Earth today produce an inordinate amount of carbon dioxide, which in turn provides the atmosphere with more radiatively active gas than it needs. According to many environmentalists, the result is, or will be, global warming. If it takes place over a long period of time, global warming could bring about serious hazards—in particular, the melting of the polar ice caps. It should be noted, however, that not all scientists are in agreement that global warming is occurring or that humans are the principal culprits inducing these environmental changes.

Alternative Energy Sources Whatever the merits of the various sides in the debate on global warming or the exhaustion of fuel resources, one need hardly be an environmentalist to agree that the world cannot forever rely only on existing fossil fuels and the technology that uses them. Today, even as scientists in laboratories around the world work to develop viable alternative means of powering industry,

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for energy creation, while they may capture people’s imaginations, have significant drawbacks. There is the proposal, for instance, to extract hydrogen gas from water by means of electrolysis, potentially providing an extremely clean, virtually limitless source of energy. Hydrogen gas, however, is highly flammable, as the 1937 explosion of the airship Hindenburg illustrated, and, in any case, the fuel to provide the electricity necessary for electrolysis would have to come from somewhere, presumably, the burning of fossil fuels. Nuclear energy, of course, has frightened many people in the wake of such well-known disasters as those that occurred at Three Mile Island, Pennsylvania, in 1979 and Chernobyl, in the former Soviet Union, in 1986. In fact, those two situations illustrate more about governments than they do about technology itself. No one died at Three Mile Island, and with an open society and media access to the site, the public outcry became so great that the plant was closed. By contrast, the Soviets’ outmoded technology helped bring about the Chernobyl disaster, and the communist dictatorship’s practice of censorship and suppression led to a massive cover-up that greatly increased the death toll. As a result, thousands died at Chernobyl, and thousands more died as the result of the indirect effects of nuclear pollution in the environment. There is no question, however, that nuclear energy does pose an enormous potential environmental threat from its waste products. Spent fuel rods, if simply buried, eventually leak radioactive waste into the water table and could kill or harm vast populations. This all relates to nuclear fission, the only type of peaceful nuclear

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Energy and Earth

KEY TERMS The temperature

tem isolated from all outside factors, the

at which all molecular motion virtually

total amount of energy remains the same,

ceases.

though transformations of energy from

ABSOLUTE ZERO:

ALBEDO:

The reflective power of a

surface or body or, more specifically, the proportion of incoming radiation that the surface or body reflects. ATMOSPHERE:

one form to another take place. The first law of thermodynamics is the same as the conservation of energy. ELECTROMAGNETIC ENERGY:

In general, an atmos-

phere is a blanket of gases surrounding a

A

form of energy with electric and magnetic components, which travels in waves.

planet. Unless otherwise identified, howev-

ELECTROMAGNETIC

er, the term refers to the atmosphere of

The complete range of electromagnetic

Earth, which consists of nitrogen (78%),

waves on a continuous distribution from a

oxygen (21%), argon (0.93%), and other

very low range of frequencies and energy

substances that include water vapor, car-

levels, with a correspondingly long wave-

bon dioxide, ozone, and noble gases such

length, to a very high range of frequencies

as neon, which together comprise 0.07%.

and energy levels, with a correspondingly

BIOENERGY:

Energy derived from

biological sources that are used directly as fuel (as opposed to food, which becomes fuel).

SPECTRUM:

short wavelength. Included on the electromagnetic spectrum are long-wave and short-wave radio; microwaves; infrared, visible, and ultraviolet light; x rays; and gamma rays.

Materials that are burned

BIOMASS:

or processed to produce bioenergy. BIOSPHERE:

ENERGY:

The ability of an object (or

in some cases a nonobject, such as a mag-

A combination of all liv-

ing things on Earth—plants, mammals,

netic force field) to accomplish work. ENERGY BUDGET:

The total amount

birds, reptiles, amphibians, aquatic life,

of energy available to a system or, more

insects, viruses, single-cell organisms, and

specifically, the difference between the

so on—as well as all formerly living things

energy flowing into the system and the

that have not yet decomposed.

energy lost by it.

CALORIE:

A measure of heat or energy

ENTROPY:

The tendency of natural

in the SI, or metric, system, equal to the

systems toward breakdown and, specifical-

heat that must be added to or removed

ly, the tendency for the energy in a system

from 1 g of water to change its temperature

to be dissipated. Entropy is related closely

by 1°C. The dietary calorie with which

to the second law of thermodynamics.

most people are familiar is the same as the kilocalorie, or 1,000 calories. CONSERVATION OF ENERGY:

ENVIRONMENT:

In discussing sys-

tems, the term environment refers to the A

law of physics that holds that within a sys-

S C I E N C E O F E V E RY DAY T H I N G S

surroundings—everything external to and separate from the system.

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Energy and Earth

KEY TERMS

CONTINUED

FIRST LAW OF THERMODYNAMICS:

atmosphere but including all oceans, lakes,

A law of physics stating that the amount of

streams, groundwater, snow, and ice.

energy in a system remains constant, and therefore it is impossible to perform work that results in an energy output greater than the energy input. This is the same as the conservation of energy. of bioenergy, including petroleum, coal, peat, natural gas, and other organic com-

accelerate 1 kg of mass by 1 m per second squared (1 m/s2) over a distance of 1 m. ever, it often is replaced by the kilowatthour, equal to 3.6 million (3.6  106) J. KELVIN

pounds usable as fuel. FREQUENCY:

joule (J) is equal to the work required to

Owing to the small size of the joule, how-

Nonrenewable forms

FOSSIL FUELS:

The number of waves,

measured in Hertz, passing through a given point during the interval of one second. The higher the frequency, the shorter the wavelength. Heat, or

thermal, energy from Earth’s interior. GREENHOUSE EFFECT:

Warming

Earth. This occurs because of the absorption of long-wavelength radiation from the

by

William Thompson, Lord Kelvin (1824– 1907), the Kelvin scale measures temperature in relation to absolute zero, or 0K. (Note that units in the Kelvin system, known as kelvins, do not include the word which is the system usually favored by sciscale; hence, Celsius temperatures can be converted to kelvins by adding 273.15. KINETIC ENERGY:

gases, such as carbon dioxide and water vapor, in the atmosphere. These gases are heated and ultimately re-radiate energy at an even longer wavelength to space.

LAW:

A scientific principle that is

shown always to be the case and for which no exceptions are deemed possible. MASS ENERGY:

Internal thermal energy that

flows from one body of matter to another. A unit for measuring frequen-

cy, equal to one cycle per second. High frequencies are expressed in terms of kilo-

The energy that

an object possesses by virtue of its motion.

planet’s surface by certain radiatively active

The energy an object

possesses by virtue of its mass. Sometimes called rest energy. NUCLEAR FISSION:

A nuclear reac-

tion that involves the splitting of an atomic nucleus. A nuclear reac-

hertz (kHz; 103 or 1,000 cycles per second),

NUCLEAR FUSION:

megahertz (MHz; 106 or one million cycles

tion that involves the joining of atomic

per second), and gigahertz (GHz;

109

or

one billion cycles per second.) HYDROSPHERE:

The entirety of

Earth’s water, excluding water vapor in the

204

Established

entists, is related directly to the Celsius

of the lower atmosphere and surface of

HERTZ:

SCALE:

or symbol for “degree.”) The Kelvin scale,

GEOTHERMAL ENERGY:

HEAT:

The SI measure of work. One

JOULE:

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nuclei. NUCLEUS:

The center of an atom, a

region where protons and neutrons are located and around which electrons spin.

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Energy and Earth

KEY TERMS PHOTOSYNTHESIS:

The biological

SI:

CONTINUED

An abbreviation of the French term

conversion of light energy (that is, electro-

Système International d’Unités, or Interna-

magnetic energy) to chemical energy in

tional System of Units. Based on the metric

plants.

system, SI is the system of measurement

POTENTIAL ENERGY:

The energy

units in use by scientists worldwide. Any set of interactions that

that an object possesses by virtue of its

SYSTEM:

position or its ability to perform work.

can be set apart mentally from the rest of

POWER:

The rate at which work is

accomplished over time, a figure rendered

the universe for the purposes of study, observation, and measurement. The direction of

mathematically as work divided by time.

TEMPERATURE:

The SI unit of power is the watt, while the

internal energy flow between two systems

British unit is the foot-pound per second.

when heat is being transferred. Tempera-

RADIATION:

The transfer of energy by

means of electromagnetic waves, which

ture measures the average molecular kinetic energy in transit between those systems. See watt.

require no physical medium (for example,

TERAWATT:

water or air) for the transfer. Earth receives

THERMAL ENERGY:

the Sun’s energy via the electromagnetic

form of kinetic energy produced by the

spectrum by means of radiation.

motion of atomic or molecular particles in

Heat energy, a

A term describing a

relation to one another. The greater the rel-

phenomenon whereby certain materials

ative motion of these particles, the greater

are subject to a form of decay brought

the thermal energy.

RADIOACTIVITY:

The metric unit of power, equal

about by the emission of high-energy par-

WATT:

ticles or radiation. Forms of particles or

to 1 J per second. Because this is such a

energy include alpha particles (positively

small unit, scientists and engineers typically

charged helium nuclei), beta particles

speak in terms of kilowatts, or units of 1,000

(either electrons or subatomic particles

W. Very large figures, such as those relating

called positrons), or gamma rays, which

to Earth’s energy budget, usually are given

occupy the highest energy level in the elec-

in terawatts, or 1012 (one trillion) W.

tromagnetic spectrum.

WAVELENGTH:

SECOND LAW OF THERMODYNAM-

a crest and the adjacent crest or the trough

A law of physics stating that spon-

and an adjacent trough of a wave. Wave-

taneous or unaided transfers of energy are

length is related inversely to frequency,

irreversible and impossible without an

meaning that the shorter the wavelength,

increase of entropy in the universe. It is

the higher the frequency.

therefore impossible, without the addition-

WORK:

al input of energy, to transfer heat from a

given distance. In the metric, or SI, system,

colder to a hotter body or to convert heat

work is measured by the joule (J) and in the

into an equal amount of work.

British system by the foot-pound (ft-lb).

ICS:

S C I E N C E O F E V E RY DAY T H I N G S

The distance between

The exertion of force over a

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power in use today. In building the hydrogen bomb in the mid-twentieth century, physicists and chemists used the much greater power of nuclear fusion, or the bonding of atomic nuclei. If nuclear fusion could be produced in controlled reactions, for peaceful use, it would provide safe, cheap, limitless power to the planet. G E O T H E R M A L E N E R GY. Unless or until nuclear fusion or some more advanced alternative energy source is developed, societies will continue to rely on fossil fuels for the bulk of their energy needs. At the same time, alternative sources will continue to supply energy in certain situations. An excellent example of such a source is geothermal energy, harnessed by peoples in areas as widespread as New Zealand and Iceland centuries ago.

Long before the first Europeans arrived in New Zealand, the native Maori people used geothermal energy from geysers to cook food. Modern applications of geothermal energy began with the creation of the first geothermal well, by workers who accidentally drilled into one in Hungary in 1867. Today Hungary is the world’s leading producer and consumer of geothermal energy, followed closely by Italy and Iceland. The word fumarole, used earlier in this essay, is Italian in origin, and it is said that the fumaroles near the town of Lardarello, used for the production of electricity since 1904, once inspired Dante Alighieri’s (1265–1321) vision of the Inferno, as captured in his celebrated work by that name. As for Iceland, more than 99% of the buildings in the capital city of Reykjavik use geothermal energy for heat. Heat is not the primary human application for geothermal energy. As noted earlier, in the context of the first law of thermodynamics, energy can be converted from one form to another. Thus, geothermal energy is applied for the creation of electromagnetic energy: steam or heated water from the ground runs turbines, which produce electricity.

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Geothermal energy has enormous advantages, including the fact that its raw materials (heated water and steam) are free and relatively inexpensive to extract. It is also inexpensive environmentally, causing virtually no air pollution. Will geothermal energy ever significantly compete with fossil fuels as a significant source of energy for humans? It is conceivable, but at present a number of barriers exist. Geothermal resources exist only in very specific parts of the world, and the extraction of the raw materials may release noxious gases, such as hydrogen sulfide (the same compound that gives intestinal gas its smell). Also, ironically, there are environmental concerns, not because of true damage but because geothermal mines often pose a threat of sight pollution in the midst of otherwise gorgeous natural settings. WHERE TO LEARN MORE Altman, Linda Jacobs. Letting Off Steam: The Story of Geothermal Energy. Minneapolis, MN: Carolrhoda Books, 1989. Brown, Warren. Alternative Sources of Energy. New York: Chelsea House, 1994. Cox, Reg, and Neil Morris. The Natural World. Philadelphia: Chelsea House, 2000. Earth’s Energy Budget (Web site). . Earth’s Energy Budget, or Can You Spare a Sun? (Web site). . Fowler, Allan. Energy from the Sun. New York: Children’s Press, 1997. Gutnik, Martin J., and Natalie Browne-Gutnik. The Energy Question: Thinking About Tomorrow. Hillside, NJ: Enslow Publishers, 1993. Hancock, Paul L., and Brian J. Skinner. The Oxford Companion to the Earth. New York: Oxford University Press, 2000. Knapp, Brian J. Earth Science: Discovering the Secrets of the Earth. Illus. David Woodroffe and Julian Baker. Danbury, CT: Grolier Educational, 2000. Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science. 2nd ed. New York: John Wiley and Sons, 1999.

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S C I E N C E O F E V E RY DAY T H I N G S Real-Life Earth Science

EARTH’S INTERIOR EARTH’S INTERIOR P L AT E T E C T O N I C S SEISMOLOGY

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EARTH’S INTERIOR

CONCEPT For the most part, this book is concerned with geologic, geophysical, and geochemical processes that take place on or near Earth’s surface. Even the essay Plate Tectonics, which takes up one of the central ideas in modern earth sciences, discusses only the lithosphere and crust but not the depths of the mantle or the core. Yet there are several good reasons to study Earth’s interior, even if it is not immediately apparent why this should be the case. At first glance it would seem that activities in Earth’s interior could hardly be removed further from day-to-day experience. By contrast, even the Moon seems more related to daily life. At least it is something we can see and a place to which humans have traveled; on the other hand, no human has ever seen the interior of our planet, nor is anyone likely to do so. What could Earth’s interior possibly have to do with everyday life? The answer may be a bit surprising. As it turns out, many factors that sustain life itself are the result of phenomena that take place far below our feet.

HOW IT WORKS

more or less uniform shape. Since there is no shape more perfectly uniform than that of a sphere, this is the typical form of bodies that possess large mass. In fact, the greater the mass, the greater the tendency toward roundness. Although they are less dense than Earth, Jupiter and Saturn are certainly more massive, and therefore they are more perfectly round. Mars and the Moon, on the other hand, are less so. Earth is not perfectly round, owing to the fact that its mass bulges at the equator because it is moving; if it were still, it would be quite round indeed, a result of the great mass at its core. A M ASS I V E C O R E . As we shall see, nearly a third of Earth’s mass is at its core, even though the core accounts for only about a fifth of its total volume. In other words, Earth’s core is exceptionally dense, and this has several implications. First of all, the planet has a powerful gravitational pull, which not only serves to keep people and other objects rooted on the surface of the solid earth but also holds our atmosphere in place.

In the essay on Planetary Science, there is a discussion of an age-old question: “Why is the earth round?” or rather “Why is Earth a sphere?” The answer, explained in more detail within the context of that essay, is that gravitational force dictates a spherical shape. As far as we know, there is no such thing as a planet or sun in the shape of a cube, because for every large object, the gravitational pull from the interior forces it to assume a

The gravitational attraction between any two objects is related directly to mass and inversely to the distance between them. Everything in the universe exerts some degree of gravitational pull on everything else, but unless at least one of the objects is of significant mass, the total gravitational force is negligible. The reason for this—as determined by the English mathematician and physicist Sir Isaac Newton (1642–1727)—is that gravitational force between two objects is the product of their mass divided by the distance between them and multiplied by

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about Pluto, which was discovered in the early twentieth century. It has a density higher than any Jovian planet, but there is little basis for classifying it as a terrestrial planet.)

Earth’s Interior

Saturn, which is the least dense among the planets, has a mass only about 100 times as great as that of Earth, while its volume is almost 800 times greater. Thus its density is only about 12% of Earth’s. And whereas Jovian planets, such as Saturn, are mostly gaseous and solid only in their small, dense cores, Earth is extremely solid. For a Jovian planet, there is little distinction between the “atmosphere” and the surface of the planet itself, whereas anyone who has ever jumped from a great height on Earth can attest to the sharp difference between thin air and solid ground.

SIR ISAAC NEWTON (Library of Congress.)

an extremely small quantity known as the gravitational constant. In the case of Earth, there is an extremely large amount of mass at the interior. Moreover, that mass is at a relatively short distance from objects on the planet’s surface—or, to put it another way, Earth has a relatively small radius. Hence its powerful gravitational pull—one of many ways that the interior of Earth affects the overall conditions of the planet. DENSITY OF TERRESTRIAL A N D J O V I A N P LA N E T S . Saying that a

large amount of mass is concentrated in a small area on Earth is another way of saying that the planet’s interior is extremely dense. As it turns out, Earth is, in fact, the densest planet in the solar system; indeed the only other planets that come close are Mercury and Venus.

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Beneath that solid ground is a planetary interior composed of iron, nickel, and traces of other elements. The vast mass of the interior not only gives Earth a strong gravitational pull but also, in combination with the comparatively high speed of the planet’s rotation, causes Earth to have a powerful magnetic field. Furthermore, Earth is distinguished even from most terrestrial planets (among which the Moon sometimes is counted) owing to the high degree of tectonic activity beneath its surface.

Plate Tectonics and the Interior Of all the terrestrial planets, Earth is the only one on which the processes of plate tectonics take place. Tectonism is the deformation of the lithosphere, the brittle area of Earth’s interior that includes the crust and upper mantle. (We take a closer look at these regions later in this essay.) The lithosphere is characterized by large, movable segments called plates, and plate tectonics is the name both of a theory and of a specialization of tectonics, or the study of tectonism.

Mercury, Venus, Earth, and Mars together are designated as the terrestrial planets: bodies that are small, rocky, and dense; have relatively small amounts of gaseous elements; and are composed primarily of metals and silicates. (See the essay Minerals for more on metals as well as the extremely abundant silicates.) By contrast, the Jovian planets—Jupiter, Saturn, Uranus, and Neptune—are large, low in density, and composed primarily of gases. (Scientists know little

As a realm of study, plate tectonics deals with the large features of the lithosphere and the forces that shape them. As a theory, it explains the processes that have shaped Earth in terms of plates and their movement. This theory, discussed in detail within the Plate Tectonics essay, brings together aspects of seismic (earthquake) and volcanic activity, the structures of Earth’s crust, and other phenomena to provide a unifying model of Earth’s evolution. It is one of the dominant concepts in the modern earth sciences.

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Earth’s Interior

THE

FOUR PLANETS OF THE INNER SOLAR SYSTEM, WITH THE

SUN. MERCURY, VENUS, EARTH,

AND

MARS

ARE DES-

IGNATED THE TERRESTRIAL PLANETS—SMALL, ROCKY, DENSE, AND COMPOSED OF METALS AND SILICATES WITH FEW GASEOUS ELEMENTS. (© Photo Researchers. Reproduced by permission.)

T H E I M P O R TA N C E O F T E C T O N I C AC T I V I T Y. As discussed in Plate

Tectonics, there is a difference in thickness between continental and oceanic plates on Earth. By contrast, the other terrestrial planets have crusts of fairly uniform thickness, suggesting that they have experienced little in the way of tectonic activity. Several other factors indicate that Earth is by far the most prone to tectonic activity. Earth’s core is enormous, larger than the entire planet Mercury. This means that there is a large area of high pressure and high heat driving tectonic processes, as we discuss later in this essay. In addition, Earth has a relatively thin lithosphere, meaning that the effects of heat below

S C I E N C E O F E V E RY DAY T H I N G S

the lithosphere are manifested dramatically above it in the form of shifting plates and the results of such shifts—for instance, mountain building.

REAL-LIFE A P P L I C AT I O N S “Digging to China” As children, many people growing up in the West heard something along these lines: “If you could dig a hole straight through the earth, you would end up in China.” This might be more or less literally true, since eastern China is on the opposite

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side of the planet from eastern North America. (Southeast Asia, however, is farther away, because it is more exactly opposite the eastern seaboard.) Even with the most sophisticated equipment imaginable, however, it is unlikely that anyone will put a hole straight through Earth. The idea of “digging to China” may have raised a new question in many a child’s mind. Suppose a person were to dig a hole through the Earth and jump down into it. What would happen? Gravity would carry the person to the center of the earth, but after that, would he or she just go on flying past the gravitational center of the planet? It is a good question, and the likelihood is that the powerful gravitational force at the center of the earth would hold the person there. Again, however, the likelihood of ever conducting such an experiment—for instance, with a steel ball that emitted a radio signal—is slim. H O W D E E P ? The reason for this slim likelihood can be illustrated by visiting some of the world’s deepest mines. There is, for instance, the Homestake Gold Mine in South Dakota, one of the deepest mines in the United States, which extends to about 8,500 ft. (2,591 m) below the surface. This is about 1.6 mi. (2.6 km), almost six times as deep as the height of the world’s tallest building, the Petronas Towers in Malaysia.

Impressive as the Homestake is, it is almost insignificant when compared with the Western Deeps Gold Mine, near Carletonville, South Africa, which reaches down about 13,000 ft. (3,962 m)! In a mine such as the Western Deeps, or even the Homestake, temperatures can reach 140°F (60°C), which makes working in such an environment extremely hazardous. Mines are airconditioned to make them bearable, but even so, there are other dangers associated with the great depth. For instance, the pressure caused by the rocks lying above the mine may become so great that rocks in the wall shatter spontaneously.

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Oklahoma, and the Kola Peninsula, Russia, where artificial holes extend to a staggering 7.5 mi. (12 km). This is more than three times as far down as the Western Deeps, and it is hard to imagine how any human could survive at such depths. H O W D O W E K N O W ? Even these deepest excavations represent only 0.2% of the distance from Earth’s surface to its core, which is about 3,950 mi. (6,370 km) below our feet. Given that fact, one might wonder exactly how it is that earth scientists—particularly geophysicists— claim to know so much about what lies beneath the crust. In fact, they have a number of fascinating tools and methods at their disposal.

Among these tools are such rocks as kimberlite and ophiolite, which originate deep in the crust and mantle but move upward to the surface. In addition, meteorites that have landed on Earth are believed to be similar to the rocks at the mantle and core, since the planet was originally a cloud of gas around which solid materials began to form as a result of bombardment from outer space (see the essays Sun, Moon, and Earth and Planetary Science). Most important of all are seismic, or earthquake, waves, whose speed, motion, and direction tell us a great deal about the materials through which they have passed and the distances over which they have traveled.

An Imaginary Journey Having established just how far humans would have to go to penetrate even just below Earth’s crust with existing technology, let us now pretend that such obstacles have been overcome. In this imaginary situation, through a miracle of science let us say that there really is a hole straight through the earth and an elevator that passes through it. This, of course, raises still more complications, aside from the gravitational problem mentioned earlier. Among other things, our elevator would have to be made of a heat-resistant material, given the temperatures we are likely to encounter in our descent. It may have sounded hot in the gold mines of South Africa and South Dakota, but that will seem cool by the time we reach Earth’s core, which is as hot as the Sun’s surface.

It is no wonder, then, that workers in extremely deep gold mines and diamond mines are well paid or that their insurance premiums are very expensive. Yet even the Western Deeps is not the deepest spot where humans have drilled holes on Earth. Scientists in Sweden and Russia have overseen the drilling of deep holes purely for research purposes, while in Louisiana and Oklahoma, a few such holes have been drilled in the process of exploring for petroleum. The deepest of these holes are at Andarko Basin,

S TA RT I N G O U T. For now, however, we will throw all those logistical problems out the window and begin our journey to the center of the earth. In so doing, we will pass through three

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major regions—crust, mantle, and core—as well as several subsidiary realms within. By far the smallest of these is the crust, which is also the only part about which we know anything from direct experience. Very quickly we find ourselves passing through the A, B, and C horizons of soil discussed in the Soil essay, and soon we are passing through bedrock into the main part of the crust. Bedrock might be only 5-10 ft. deep (1.5–3 m), or it might be half a mile deep (0.8 km) or perhaps even deeper. Although this is a long way for a person to dig, we still have barely scratched the surface. As noted earlier, there is a difference between continental crust and oceanic crust. We will ignore the details here, except to say that the continental crust is thicker but the oceanic crust is denser. Thus, the continents are at a higher elevation than the oceans around them. Depending on whether the crust is oceanic or continental, we have between 3 mi. and 40 mi. (5–70 km) to travel before we begin to pass out of the crust and into the mantle. THE LITHOSPHERE, SEISMOLO GY, A N D R E M O T E S E N S I N G . The

transition from crust to mantle is an abrupt one, marked by the boundary zone known as the Mohorovicic discontinuity. Sometimes called the M-discontinuity or, more commonly, the Moho, it was the discovery of the Croatian geologist Andrija Mohorovicic (1857–1936). On October 8, 1909, while studying seismic waves from an earthquake in southeastern Europe, Mohorovicic noticed that the speed of the waves increased dramatically at a depth of about 30 mi. (50 km). Since waves travel faster through denser materials, Mohorovicic reasoned that there must be an abrupt transition from the rocky material in the Earth’s crust to denser rocks below. His discovery is an excellent example of remote sensing (see Remote Sensing), whereby earth scientists are able to study places and phenomena that are impossible to observe directly. After the Moho, which is only about 0.1-1.9 mi. (0.2–3 km) thick, we enter the mantle—or, more specifically, the lithospheric mantle. This subregion may extend to depths between 30 mi. and 60 mi. (50–100 km) and is much more dense than the crust. Like the crust, it is brittle, solid, and relatively cool compared with the regions

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below; hence, the crust and lithospheric mantle are lumped together as the lithosphere.

Earth’s Interior

The Asthenosphere and Its Impact At the base of the lithosphere, we pass through another transition zone, known as the Gutenberg low-velocity zone (named after the Germanborn American seismologist Beno Gutenberg [1889–1960]), where the speed of seismic waves again increases dramatically. After that, we enter a layer of much softer material, known as the asthenosphere. The material in the asthenosphere is soft not because it is weak—on the contrary, it is made of rock—but because it is under extraordinarily high pressure. What happens in the asthenosphere plays a powerful role in life on the surface. The plates of the lithosphere float, as it were, atop the molten rock of the asthenosphere, which forces these plates against one another as though they were ice cubes floating in a bowl of water in constant motion. This motion is the phenomenon of plate tectonics, which, as we have discussed, quite literally shapes the world we know. V O L CA N I S M A N D T H E AT M O S P H E R E . Plate tectonics is responsible not

only for such phenomena as the creation of mountains but also, by influencing the development of volcanoes, indirectly for Earth’s atmosphere. In the first few billion years of the planet’s existence, the action of volcanoes brought water vapor, carbon dioxide, nitrogen, sulfur, and sulfur compounds from the planet’s interior to its surface. This was critical to the formation of the air we breathe today. Additionally, volcanic activity plays a significant role in the carbon cycle, whereby that vital element is circulated through various earth systems (see Biogeochemical Cycles and Carbon Cycle). Earth and Venus stand alone among terrestrial planets as the only two still prone to volcanic activity. (By contrast, Mercury and the Moon have long been dead volcanically, and volcanism on Mars seems to have ended at some point during the past billion years.) This is significant, because even though all the planets possess more or less the same chemical elements, volcanoes are critical to distributing those elements. In addition, volcanic activity, as well as the heat from Earth’s interior that drives it and other tectonic phenomena, is an important influence

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on the separation of chemical compounds. When Earth formed some 4.5 billion years ago, heavier compounds—among them, those containing iron—sank toward the planet’s core. At the same time, lighter ones began to rise into the atmosphere. Among these compounds was oxygen, which is clearly essential to the life of humans and other animals. This separation of compounds continues on Earth, owing to the large amount of heat that emanates from the interior. One would hardly guess that our atmosphere—or the circulation of carbon, a key component in all lifeforms—could be the indirect product of activity that takes place at least 60 mi.(100 km) below our feet. Nor is this the only illustration of the impact that Earth’s interior exerts on our world. The interior of Earth is also responsible for the action that produces geothermal energy, discussed in detail within Energy and Earth. GEOTHERMAL

E N E R G Y.

Geothermal energy provides heating and electricity for several countries and is responsible for the dramatic effect of such phenomena as “Old Faithful” at Yellowstone Park in Wyoming. It is also the source behind the soothing natural springs found in such well-known resorts as Warm Springs, Georgia (a favorite getaway for President Franklin D. Roosevelt, who died there in 1945), and Hot Springs, Arkansas, the hometown of another president, Bill Clinton.

Mesosphere to Inner Core: Geomagnetism and Gravity After we pass through the base of the asthenosphere, we are still only 155 mi. (250 km) deep. Now we are in the mesosphere, which extends to a depth of 1,800 mi. (2,900 km) and includes several other discontinuities, or thresholds of change. We will not discuss the details of these discontinuities here, except to note that they indicate changes in geochemical composition: for example, at 400 mi. (650 km) there appears to be a marked increase in the ratio of iron to magnesium.

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which, if this is true, would make the center of Earth about 50% hotter than the surface of the Sun! E A RT H ’ S F I E RY H E A RT. At such

temperatures, one would expect the rock of the outer core to be entirely molten, and indeed it is. In fact, it is this difference in phase or state of matter that marks the change from mantle (which is partially solid) to the liquid outer core. The region between the mantle and the outer core is one of undulating boundaries due to convection (see Convection), which may be the driving force behind the plate tectonic activity that occurs at a much higher level. In addition, the eddies and currents of molten iron in the core are ultimately responsible for the planet’s magnetic field (see Geomagnetism). The boundary between the mantle and core is lower today than it once was, a sign that our planet is slowly aging. If there ever comes a point when the heat is entirely dissipated, as may perhaps happen many billions of years from now, that could well be the end of Earth as a “living” planet. If we did not have a mantle and core, with all their heat, pressure, and resulting tectonic activity, Earth would be as dead as the Moon, whose interior is relatively cool. The distinction between the outer and inner cores, which starts at a depth of about 3,150 mi. (5,100 km), comes from the fact that here, too, there is a phase change—in this case from liquid, molten material back to solid. This has nothing to do with cooling, since, as we have noted, the inner core is almost unimaginably hot; rather, it is a result of the immense pressures apparent at this depth.

The Gutenberg discontinuity, or the coremantle boundary (CMB), marks our entrance to the core. By now it has become very, very hot. Whereas the lithospheric mantle is about 1,600°F (870°C), the bottom of the lithosphere is about 4,000–6,700°F (2,200–3,700°C). By the time we get to the inner core, we may be confronted with temperatures as high as 13,000°F (7,200°C)—

It is interesting to note that the core constitutes only about 16% of the planet’s volume but 32% of its mass. As we discussed near the beginning of this essay, enormous gravitational force exists between two objects when at least one of them has a relatively large amount of mass and the distance between them is great. Thus, Earth’s mass, concentrated deep in its interior, helps hold our world—people, animals, plants, buildings, and so forth—in place. It also keeps our atmosphere firmly rooted as well. Without an extensive gravitational field of the kind that Earth possesses, significantly less massive bodies, such as the Moon or Mercury, have no atmosphere. In this and many another way, it turns out that life on

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THE

BRUCE MCCANDLESS FLOATS FREELY IN SPACE, EARTH’S VAST INTERIOR MASS GIVES IT A STRONG

ASTRONAUT

TLE MISSION.

OUTSIDE

EARTH’S

ATMOSPHERE, DURING A SHUT-

GRAVITATIONAL PULL, HELPING ROOT THE PEOPLE

AND MATERIALS OF OUR WORLD AND HOLDING OUR ATMOSPHERE IN PLACE. (NASA. Reproduced by permission.)

Earth’s surface depends heavily on what goes on its ultra-hot, extremely pressurized interior.

A Bizarre Postscript Given the vast amount of power in Earth’s interior, it is no wonder that it has long fascinated humans—even before science possessed any sort of intelligent understanding with regard to the contents of that interior. The ancients offered all manner of fascinating speculation regarding the contents of Earth: it was hollow, some said, while others claimed that it contained one substance or another—perhaps even a heart of gold.

Dante Alighieri (1265–1321) described an allegorical journey through Earth’s interior in his epochal Divine Comedy. This epic poem depicts the inside of the planet as concentric circles of hell, descending toward the fiery core, where Satan himself resides. Beyond this lies Purgatory and further still—on the other side of Earth— Heaven, the New Jerusalem.

Such imaginative musings continued well into the Middle Ages, when the Italian poet

By the time the French writer Jules Verne (1828–1905) wrote Journey to the Center of the Earth almost six centuries later, scientific knowledge regarding Earth’s interior had increased dramatically, though many of the significant discoveries we have examined here—for example, the Moho—still lay in the future. In any case, the

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KEY TERMS ASTHENOSPHERE:

A

region

of

CRUST:

The uppermost division of the

extremely high pressure underlying the

solid Earth, representing less than 1% of its

lithosphere, where rocks are deformed by

volume and varying in depth from 3 mi. to

enormous stresses. The asthenosphere lies

37 mi. (5–60 km). Below the crust is the

at a depth of about 60 mi. to 215 mi.

mantle.

(about 100–350 km).

GEOCHEMISTRY:

A branch of the

earth sciences, combining aspects of geoloATMOSPHERE:

In general, an atmos-

phere is a blanket of gases surrounding a

gy and chemistry, that is concerned with the chemical properties and processes of

planet. Unless otherwise identified, howev-

Earth—in particular, the abundance and

er, the term refers to the atmosphere of

interaction of chemical elements and their

Earth, which consists of nitrogen (78%),

isotopes.

oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon (0.07%).

GEOLOGY:

The study of the solid

earth, in particular, its rocks, minerals, fossils, and land formations. GEOPHYSICS:

The center of Earth, an area

constituting about 16% of the planet’s vol-

and physics. Geophysics addresses the

ume and 32% of its mass. Made primarily

planet’s physical processes as well as its

of iron and another, lighter element (possi-

gravitational, magnetic, and electric prop-

bly sulfur), it is divided between a solid

erties and the means by which energy is

inner core with a radius of about 760 mi.

transmitted through its interior.

(1,220 km) and a liquid outer core about

GEOSPHERE:

1,750 mi. (2,820 km) thick. For terrestrial

Earth’s continental crust, or that portion of

planets, in general, core refers to the center,

the solid earth on which human beings live

which in most cases is probably molten

and which provides them with most of

metal of some kind.

their food and natural resources.

CORE:

tone of Verne’s work was that of a new literary style, science fiction. Pioneered by Verne and the British writer H. G. Wells, science fiction could not have been less like Dante’s poetry, infused as it was with spirituality and mystery.

The upper part of

crocodiles in sewers, ghostly hitchhikers, or the other usual fodder; instead, it concerned the center of the earth—where, it was claimed, hell had been discovered.

In the early 1990s, an urban legend of sorts brought together the science fiction of Verne, the religious vision of Dante, and a number of other, less pleasant strains—including ignorance and, on the part of its originators, the willingness to deceive. This “urban legend” did not involve

The full account appeared on Ship of Fools (see “Where to Learn More”), a Web site operated by Rich Buhler—himself a Christian minister and a debunker of what he has called “Christian urban legends.” As Buhler reported, the story gained so much support that it appeared on Trinity Broadcasting Network (TBN), a major evangelical television outlet. According to the TBN

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A branch of the earth

sciences that combines aspects of geology

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KEY TERMS planets

together aspects of continental drift,

between Mars (the last terrestrial planet)

seafloor spreading, seismic and volcanic

and Pluto, all of which are large, low in

activity, and the structures of Earth’s crust

density, and composed primarily of gases.

to provide a unifying model of Earth’s evo-

JOVIAN

PLANETS:

LITHOSPHERE:

The

CONTINUED

The upper layer of

lution. It is one of the dominant concepts

Earth’s interior, including the crust and the

in the modern earth sciences.

brittle portion at the top of the mantle.

PLATES:

MANTLE:

The thick, dense layer of

Large, movable segments of

the lithosphere.

rock, approximately 1,429 mi. (2,300 km)

SEISMIC WAVE:

thick, between Earth’s crust and its core. In

resulting from the disturbance that accom-

reference to the other terrestrial planets,

panies a strain on rocks in the lithosphere.

mantle simply means the area of dense rock between the crust and core. ORGANIC:

At one time chemists used

SEISMOLOGY:

compounds containing carbon, with the exception of carbonates (which are minerals) and oxides, such as carbon dioxide. PLATE TECTONICS:

The name both

The study of seismic

waves as well as the movements and vibrations that produce them.

the term organic only in reference to living things. Now the word is applied to most

A packet of energy

TECTONICS:

The study of tectonism,

including its causes and effects, most notably mountain building. TECTONISM:

The deformation of the

lithosphere. The four

of a theory and of a specialization of tec-

TERRESTRIAL PLANETS:

tonics. As an area of study, plate tectonics

inner planets of the solar system: Mercury,

deals with the large features of the litho-

Venus, Earth, and Mars. These are all small,

sphere and the forces that shape them. As a

rocky, and dense; have relatively small

theory, it explains the processes that have

amounts of gaseous elements; and are

shaped Earth in terms of plates and their

composed primarily of metals and silicates.

movement. Plate tectonics theory brings

Compare with Jovian planets.

report, Russian geologists had drilled a hole some 8.95 mi. (14.4 km) into Earth’s crust and heard screams, which supposedly came from condemned souls in the nether regions. As the embellished details of the story began to unfold, it turned out that the Russian geologists had found the temperatures to be much higher than expected: 2,000°F (1,093°C). Also, their drilling had unleashed a bat that flew out of hell with the words “I have conquered” inscribed in Russian on its wings. Buhler and his team traced this bizarre tale to Finland and then back

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to southern California. As to how the story originated, Buhler noted the drilling at the Kola Peninsula, which we mentioned earlier in this essay. The depth cited in the rumor, however, was greater than that which the drilling at Kola reached, and the temperatures claimed were much higher than what one actually would encounter at that depth. “It is possible that somewhere in the world there has been a spooky experience during deep drilling operations,” Buhler concluded. Nonetheless, “characteristic of many urban legends, this

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story was alleged to have occurred in an obscure part of the world where it would be virtually impossible to track down the facts. And once the story got started, people began quoting one another’s newsletters to validate their own. This is the stuff of which tabloid newspapers are made.” In the end, the “screams of hell” offered nothing of value in terms of either science or religion, but it proved to be an excellent example of human beings’ fascination with, and latent terror of, Earth’s interior.

De Bremaecker, Jean-Claude. Geophysics: The Earth’s Interior. New York: John Wiley and Sons, 1985.

WHERE TO LEARN MORE

Rockdoctors Guide: Earth’s Interior (Web site). .

Buhler, Rich. “Drilling for Hell,” Ship of Fools (Web site). .

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Earth’s Interior (Web site). . Earth’s Interior and Plate Tectonics (Web site). . Earth’s Interior and Plate Tectonics (Web site). . Hancock, Paul L., and Brian J. Skinner. The Oxford Companion to the Earth. New York: Oxford University Press, 2000. Physical Geology: Interior of the Earth (Web site). .

Darling, David J. Could You Ever Dig a Hole to China? Minneapolis, MN: Dillon Press, 1990.

Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science. 2d ed. New York: John Wiley and Sons, 1999.

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P L AT E T E C T O N I C S Plate Tectonics

CONCEPT The earth beneath our feet is not dead; it is constantly moving, driven by forces deep in its core. Nor is the planet’s crust all of one piece; it is composed of numerous plates, which are moving steadily in relation to one another. This movement is responsible for all manner of phenomena, including earthquakes, volcanoes, and the formation of mountains. All these ideas, and many more, are encompassed in the concept of plate tectonics, which is the name for a branch of geologic and geophysical study and for a powerful theory that unites a vast array of ideas. Plate tectonics works hand in hand with several other striking concepts and discoveries, including continental drift and the many changes in Earth’s magnetic field that have taken place over its history. No wonder, then, that this idea, developed in the 1960s but based on years of research that preceded that era, is described as “the unifying theory of geology.”

HOW IT WORKS

The first is the uppermost division of the solid earth, representing less than 1% of its volume and varying in depth from 3 mi. to 37 mi. (5–60 km). Below the crust is the mantle, a thick, dense layer of rock approximately 1,429 mi. (2,300 km) thick. The core itself is even more dense, as illustrated by the fact that it constitutes about 16% of the planet’s volume and 32% of its mass. Composed primarily of iron and another, lighter element (possibly sulfur), it is divided between a solid inner core with a radius of about 760 mi. (1,220 km) and a liquid outer core about 1,750 mi. (2,820 km) thick. Tectonism results from the release and redistribution of energy from Earth’s interior. There are two components of this energy: gravity, a function of the enormous mass at the core, and heat from radioactive decay. (For more about gravity, see Gravity and Geodesy. The heat from Earth’s core, the source of geothermal energy, is discussed in Energy and Earth.) Differences in mass and heat within the planet’s interior, known as pressure gradients, result in the deformation of rocks.

The interior of Earth itself is divided into three major sections: the crust, mantle, and core.

D E F O R M AT I O N O F R O C K S . Any attempt to deform an object is referred to as stress, and stress takes many forms, including tension, compression, and shear. Tension acts to stretch a material, whereas compression—a type of stress produced by the action of equal and opposite forces, whose effect is to reduce the length of a material—has the opposite result. (Compression is a form of pressure.) As for shear, this is a kind of stress resulting from equal and opposite forces that do not act along the same line. If a thick, hardbound book is lying flat and one pushes the front cover from the side so

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Tectonics and Tectonism The lithosphere is the upper layer of Earth’s interior, including the crust and the brittle portion at the top of the mantle. Tectonism is the deformation of the lithosphere, and the term tectonics refers to the study of this deformation, including its causes and effects, most notably mountain building. This deformation is the result of the release and redistribution of energy from Earth’s core.

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that the covers and pages are no longer perfectly aligned, this is an example of shear. Under the effects of these stresses, rocks may bend, warp, slide, or break. They may even flow, as though they were liquids, or melt and thus truly become liquid. As a result, Earth’s interior may manifest faults, or fractures in rocks, as well as folds, or bends in the rock structure. The effects of this activity can be seen on the surface in the form of subsidence, which is a depression in the crust, or uplift, which is the raising of crustal materials. Earthquakes and volcanic eruptions also may result. There are two basic types of tectonism: orogenesis and epeirogenesis. Orogenesis is taken from the Greek words oros (“mountain”) and genesis (“origin”) and involves the formation of mountain ranges by means of folding, faulting, and volcanic activity. The Greek word epeiros means “mainland,” and epeirogenesis takes the form of either uplift or subsidence. Of principal concern in the theory of plate tectonics, as we shall see, is orogenesis, which involves more lateral, as opposed to vertical, movement.

Continental Drift If one studies a world map for a period of time, one may notice something interesting about the shape of Africa’s west coast and that of South America’s east coast: they seem to fit together like pieces of a jigsaw puzzle. Early in the twentieth century, two American geologists, Frank Bursley Taylor (1860–1938) and Howard Baker, were among the first scientists to point out this fact. According to Taylor and Baker, Europe, the Americas, and Africa all had been joined at one time. This was an early version of continental drift, a theory concerning the movement of Earth’s continents. Continental drift is based on the idea that the configuration of continents was once different than it is today, that some of the individual landmasses of today once were joined in other continental forms, and that the landmasses later moved to their present locations. Though Taylor and Baker were early proponents, the theory is associated most closely with the German geophysicist and meteorologist Alfred Wegener (1880–1930), who made the case for continental drift in The Origin of Continents and Oceans (1915).

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PA N G A E A , L A U R A S I A , A N D G O N D WA N A LA N D . According to Wegen-

er, the continents of today once formed a single supercontinent called Pangaea, from the Greek words pan (“all”) and gaea (“Earth”). Eventually, Pangaea split into two halves, with the northern continent of Laurasia and the southern continent of Gondwanaland, sometimes called Gondwana, separated by the Tethys Sea. In time, Laurasia split to form North America, the Eurasian landmass with the exception of the Indian subcontinent, and Greenland. Gondwanaland also split, forming the major southern landmasses of the world: Africa, South America, Antarctica, Australia, and India. The Austrian geologist Eduard Suess (1831–1914) and the South African geologist Alexander du Toit (1878–1948), each of whom contributed significantly to continental drift theory, were responsible for the naming of Gondwanaland and Laurasia, respectively. Suess preceded Wegener by many years with his theory of Gondwanaland, named after the Gondwana region of southern India. There he found examples of a fern that, in fossilized form, had been found in all the modern-day constituents of the proposed former continent. Du Toit, Wegener’s contemporary, was influenced by continental drift theory and improved on it greatly. F O R M AT I O N O F T H E C O N T I N E N T S . Today continental drift theory is

accepted widely, in large part owing to the development of plate tectonics, “the unifying theory in geology.” We examine the evidence for continental drift, the arguments against it, and the eventual triumph of plate tectonics in the course of this essay. Before going on, however, let us consider briefly the now-accepted timeline of events described by Wegener and others. About 1,100 million years ago (earth scientists typically abbreviate this by using the notation 1,100 Ma), there was a supercontinent named Rodinia, which predated Pangaea. It split into Laurasia and Gondwanaland, which moved to the northern and southern extremes of the planet, respectively. Starting at about 514 Ma, Laurasia drifted southward until it crashed into Gondwanaland about 425 Ma. Pangaea, surrounded by a vast ocean called Panthalassa (“All Ocean”), formed approximately 356 Ma. In the course of Pangaea’s formation, what is now North America smashed into northwestern

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A

MAP OF PART OF THE

EAST PACIFIC RISE, A MID-OCEAN RIDGE TO THE WEST OF CENTRAL AMERICA THAT MARKS PACIFIC AND COCOS TECTONIC PLATES. (© Dr. Ken MacDonald/Photo Researchers. Reproduced by

THE BOUNDARY BETWEEN THE

permission.)

Africa, forming a vast mountain range. Traces of these mountains still can be found on a belt stretching from the southern United States to northern Europe, including the Appalachians. As Pangaea drifted northward and smashed into the ocean floor of Panthalassa, it formed a series of mountain ranges from Alaska to southern South America, including the Rockies and Andes. By about 200 Ma, Pangaea began to break apart, forming a valley that became the Atlantic Ocean. But the separation of the continents was not a “neat” process: today a piece of Gondwanaland lies sunken beneath the eastern United States, far from the other landmasses to which it once was joined.

By about 152 Ma, in the late Jurassic period, the continents as we know them today began to take shape. By about 65 Ma, all the present continents and oceans had been formed for the most part, and India was drifting north, eventually smashing into southern Asia to shape the world’s tallest mountains, the Himalayas, the Karakoram Range, and the Hindu Kush. This process is not finished, however, and geologists believe that some 250–300 million years from now, Pangaea will re-form.

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EVIDENCE AND ARGUMENTS.

As proof of his theory, Wegener cited a wide variety of examples, including the apparent fit between the coastlines of South America and

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western Africa as well as that of North America and northwestern Africa. He also noted the existence of rocks apparently gouged by glaciers in southern Africa, South America, and India, far from modern-day glacial activity. Fossils in South America matched those in Africa and Australia, as Suess had observed. There were also signs that mountain ranges continued between continents—not only those apparently linking North America and Europe but also ranges that seemed to extend from Argentina to South Africa and Australia. By measurements conducted over a period of years, Wegener even showed that Greenland was drifting slowly away from Europe, yet his theory met with scorn from the geoscience community of his day. If continents could plow through oceanic rock, some geologists maintained, then they would force up mountains so high that Earth would become imbalanced. As for his claim that matching fossils in widely separated regions confirmed his theory of continental drift, geologists claimed that this could be explained by the existence of land bridges, now sunken, that once had linked those areas. The apparent fit between present-day landmasses could be explained away as coincidence or perhaps as evidence that Earth simply was expanding, with the continents moving away from one another as the planet grew.

Introduction to Plate Tectonics Though Wegener was right, as it turned out, his theory had one major shortcoming: it provided no explanation of exactly how continental drift had occurred. Even if geologists had accepted his claim that the continents are moving, it raised more questions than answers. A continent is a very large thing simply to float away; even an aircraft carrier, which is many millions of times lighter, has to weigh less than the water it displaces, or it would sink like a stone. In any case, Wegener never claimed that continents floated. How, then, did they move? The answer is plate tectonics, the name both of a theory and of a specialization of tectonics. As an area of study, plate tectonics deals with the large features of the lithosphere and the forces that fashion them. As a theory, it explains the processes that have shaped Earth in terms of plates (large movable segments of the litho-

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sphere) and their movement. Plate tectonics theory brings together aspects of continental drift, seafloor spreading (discussed later), seismic and volcanic activity, and the structures of Earth’s crust to provide a unifying model of Earth’s evolution. It is hard to overemphasize the importance of plate tectonics in the modern earth sciences; hence, its characterization as the “unifying theory.” Its significance is demonstrated by its inclusion in the book The Five Biggest Ideas of Science, cited in the bibliography for this essay. Alongside plate tectonics theory in that volume are four towering concepts of extraordinary intellectual power: the atomic model, or the concept that matter is made up of atoms; the periodic law, which explains the chemical elements; big bang theory, astronomers’ explanation of the origins of the universe (see Planetary Science); and the theory of evolution in the biological sciences. THE PIECES COME TOGETHE R. In 1962 the United States geologist Harry

Hammond Hess (1906–1969) introduced a new concept that would prove pivotal to the theory of plate tectonics: seafloor spreading, the idea that seafloors crack open along the crest of mid-ocean ridges and that new seafloor forms in those areas. (Another American geologist, Robert S. Dietz [1914–1995], had published his own theory of seafloor spreading a year before Hess’s, but Hess apparently developed his ideas first.) According to Hess, a new floor forms when molten rock called magma rises up from the asthenosphere, a region of extremely high pressure underlying the lithosphere, where rocks are deformed by enormous stresses. The magma wells up through a crack in a ridge, runs down the sides, and solidifies to form a new floor. Three years later, the Canadian geologist John Tuzo Wilson (1908–1993) coined the term plates to describe the pieces that make up Earth’s rigid surface. Separated either by the mid-ocean rifts identified earlier by Heezen or by mountain chains, the plates move with respect to one another. Wilson presented a model for their behavior and established a global pattern of faults, a sort of map depicting the movable plates. The pieces of a new theory were forming (an apt metaphor in this instance!), but as yet it had no name. That name appeared in 1967, when D. P. Mackenzie of England and R. L. Parker of the

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United States introduced the term plate tectonics. They maintained that the surface of Earth is divided into six major as well as seven minor movable plates and compared the continents to enormous icebergs—much as Wegener had described them half a century earlier. Subsequent geologic research has indicated that there may be as many as nine major plates and as many as 12 minor ones. To test these emerging ideas, the U.S. National Science Foundation authorized a research voyage by the vessel Glomar Challenger in 1968. On their first cruise, through the Gulf of Mexico and the Atlantic, the Challenger’s scientific team collected sediment, fossil, and crust samples that confirmed the basics of seafloor spreading theory. These results led to new questions regarding the reactions between rocks and the heated water surrounding them, spawning new research and necessitating additional voyages. In the years that followed, the Challenger made more and more cruises, its scientific teams collecting a wealth of evidence for the emerging theory of plate tectonics.

REAL-LIFE A P P L I C AT I O N S Early Evidence of Plate Tectonics No single person has been as central to plate tectonics as Wegener was to continental drift or as the English naturalist Charles Darwin (1809–1882) was to evolution. The roots of plate tectonics lie partly in the observations of Wegener and other proponents of continental drift as well as in several discoveries and observations that began to gather force in the third quarter of the twentieth century. During World War II, submarine warfare necessitated the development of new navigational technology known as sonar (SOund Navigation And Ranging). Sonar functions much like radar (see Remote Sensing), but instead of using electromagnetic waves, it utilizes ultrasonic, or high-frequency, sound waves projected through water. Sonar made it feasible for geologists to study deep ocean basins after the war, making it possible for the first time in history to map and take samples from large areas beneath the seas. These findings raised many questions, particu-

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larly concerning the vast elevation differences beneath the seas.

Plate Tectonics

E W I N G A N D T H E M O U N TA I N S U N D E R T H E O C E A N . One of the first

earth scientists to notice the curious aspects of underwater geology was the American geologist William Maurice Ewing (1906–1974), who began his work long before the war. He had gained his first experience in a very practical way during the 1920s, as a doctoral student putting himself through school. Working summers with oil exploration teams in the Gulf of Mexico off the coast of Texas had given him a basic understanding of the subject, and in the following decade he went to work exploring the structure of the Atlantic continental shelf and ocean basins. His work there revealed extremely thick sediments covering what appeared to be high mountainous regions. These findings sharply contradicted earlier ideas about the ocean floor, which depicted it as a flat, featureless plain rather like the sandy-bottomed beaches found in resort areas. Instead, the topography at the bottom of the ocean turned out to be at least as diverse as that of the land above sea level. H E E Z E N A N D T H E R I F T VA L L E Y. During the 1950s, a team led by another

American geologist, Bruce Charles Heezen (1924–1977), worked on developing an overall picture of the ocean basin’s topography. Earlier work had identified a mountain range running the length of the Atlantic, but Heezen’s team discovered a deep valley down the middle of the chain, running parallel to it. They described it as a rift valley, a long trough bounded by two or more faults, and compared it to a similar valley in eastern Africa. Around the same time, a group of transatlantic telephone companies asked Heezen to locate areas of possible seismic or earthquake activity in the Atlantic. Phone company officials reasoned that if they could find the areas most likely to experience seismic activity, they could avoid placing their cables in those areas. As it turned out, earthquakes tended to occur in exactly the same region that Heezen and his team had identified as the rift valley.

The Plates and Their Interactions The most significant plates that make up Earth’s surface are as follows:

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Selected Major Plates • North American (almost all of North America and Mexico, along with Greenland and the northwestern quadrant of the Atlantic) • South American (all of South America and the southwestern quadrant of the Atlantic) • African (Africa, the southeastern Atlantic, and part of the Indian Ocean) • Eurasian (Europe and Asia, excluding the Indian subcontinent, along with surrounding ocean areas) • Indo-Australian (India, much of the Indian Ocean, Australia, and parts of the Indonesian archipelago and New Zealand) • Antarctic (Antarctica and the Antarctic Ocean) In addition to these plates, there are several plates that while they are designated as “major” are much smaller: the Philippine, Arabian, Caribbean, Nazca (off the west coast of South America), Cocos (off the west coast of Mexico), and Juan de Fuca (extreme western North America). Japan, one of the most earthquake-prone nations in the world, lies at the nexus of the Philippine, Eurasian, and Pacific plates. M O V E M E N T O F T H E P LAT E S .

One of the key principles of geology, discussed elsewhere in this book, is uniformitarianism: the idea that processes occurring now also occurred in the past. The reverse usually is also true; thus, as we have noted, the plates are still moving, just as they have done for millions of years. Thanks to satellite remote sensing, geologists are able to measure this rate of movement. (See Remote Sensing for more on this subject.) Not surprisingly, its pace befits the timescale of geologic, as opposed to human, processes: the fastest-moving plates are careening forward at a breathtaking speed of 4 in. (10 cm) per year. The ground beneath Americans’ feet (assuming they live in the continental United States, east of the Juan de Fuca) is drifting at the rate of 1.2 in. (3 cm) every year, which means that in a hundred years it will have shifted 10 ft. (3 m).

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depression known as an oceanic trench. Divergence results in the separation of plates and most often is associated either with seafloor spreading or the formation of rift valleys. There are three types of plate margins, or boundaries between plates, depending on the two types of crusts that are interacting: oceanic with oceanic, continental with continental, or continental with oceanic. The rift valleys of the Atlantic are an example of an oceanic margin where divergence has occurred, while oceanic convergence is illustrated by a striking example in the Pacific. There, subduction of the Philippine Plate by the Pacific Plate has created the Mariana Trench, which at 36,198 ft. (10,911 m) is the deepest depression on Earth. When continental plates converge, neither plate subducts; rather, they struggle against each other like two warriors in a fight to the death, buckling, folding, and faulting to create huge mountain ranges. The convergence of the IndoAustralian and Eurasian plates has created the highest spots on Earth, in the Himalayas, where Mount Everest (on the Nepal-Indian border) rises to 29,028 ft. (8,848 m). Continental plates also may experience divergence, resulting in the formation of seas. An example is the Red Sea, formed by the divergence of the African and Arabian plates. Given these facts about the interactions of oceanic and continental plates with each other, what occurs when continental plates meet oceanic ones is no surprise. In this situation, the oceanic plate meeting the continental plate is like a high-school football player squaring off against a National Football League pro tackle. It is no match: the oceanic plate easily subducts. This leads to the formation of a chain of volcanoes along the continental crust, examples being the Cascade Range in the U.S. Pacific Northwest (Juan de Fuca and Pacific plates) or the Andes (South American and Nazca plates).

W H E N P LAT E S I N T E RAC T. Plates interact by moving toward each other (convergence), away from each other (divergence), or past each other (transform motion). Convergence usually is associated with subduction, meaning that one plate is forced down into the mantle and eventually undergoes partial melting. This typically occurs in the ocean, creating a

Transform margins may occur with any combination of oceanic or continental plates and result in the formation of faults and earthquake zones. Where the North American Plate slides against the Pacific Plate along the California coast, it has formed the San Andreas Fault, the source of numerous earthquakes, such as the dramatic San Francisco quakes of 1906 and 1989 and the Los Angeles quake of 1994.

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SHUTTLE

EGYPT SHOWS THE RED SEA MEDITERRANEAN SEA. THE RED SEA WAS FORMED

PHOTOGRAPH OF EASTERN

ING IT TO THE

AT THE TOP, WITH THE

GULF

BY THE DIVERGENCE OF THE

SUEZ AFRICAN

OF

CONNECTAND

ARA-

BIAN PLATES. (© NASA/Photo Researchers. Reproduced by permission.)

Paleomagnetism As noted earlier, plate tectonics brings together numerous areas of study in the geologic sciences that developed independently but which came to be seen as having similar roots and explanations. Among these disciplines is paleomagnetism, an area of historical geology devoted to studying the direction and intensity of magnetic fields in the past, as discerned from the residual magnetization of rocks.

material in a compass points north; however, Earth’s magnetic north pole is not the same as its geographic north pole. It so happens that magnetic north lies in more or less the same direction as geographic north, but as geologists in the mid–nineteenth century discovered, this has not always been the case. (For more about magnetic north and other specifics of Earth’s magnetic field, see Geomagnetism.)

Earth has a complex magnetic field whose principal source appears to be the molten iron of the outer core. In fact, the entire planet is like a giant bar magnet, with a north pole and a south pole. It is for this reason that the magnetized

In 1849 the French physicist Achilles Delesse (1817–1881) observed that magnetic minerals tend to line up with the planet’s magnetic field, pointing north as though they were compass needles. Nearly 60 years later, however, another French physicist, Bernard Brunhes (1867–1910),

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Plate Tectonics

KEY TERMS of

Divergence results in the separation of

extremely high pressure underlying the

plates and is associated most often either

lithosphere, where rocks are deformed by

with seafloor spreading or with the forma-

enormous stresses. The asthenosphere lies

tion of rift valleys. It is one of the three

at a depth of about 60-215 mi. (about

ways, along with convergence and trans-

100–350 km).

form motion, that plates interact.

ASTHENOSPHERE:

COMPRESSION:

A

region

A form of stress

EPEIROGENESIS:

cipal forms of tectonism, the other being

site forces, the effect of which is to reduce

orogenesis. Derived from the Greek words

the length of a material. Compression is a

epeiros (“mainland”) and genesis (“ori-

form of pressure.

gins”), epeirogenesis takes the form of

CONTINENTAL DRIFT:

The theory

either uplift or subsidence.

that the configuration of Earth’s continents

FAULT:

was once different than it is today; that

rocks resulting from stress.

some of the individual landmasses of today once were joined in other continental forms; and that these landmasses later separated and moved to their present locations. CONVERGENCE:

A tectonic process

whereby plates move toward each other. Usually associated with subduction, convergence typically occurs in the ocean, creating an oceanic trench. It is one of the three ways, along with divergence and transform motion, that plates interact. CORE:

The center of Earth, an area

constituting about 16% of the planet’s volume and 32% of its mass. Made primarily

FOLD:

An area of fracturing between An area of rock that has been

bent by stress. GEOPHYSICS:

A branch of the earth

sciences that combines aspects of geology and physics. Geophysics addresses the planet’s physical processes as well as its gravitational, magnetic, and electric properties, and the means by which energy is transmitted through its interior. HISTORICAL GEOLOGY:

The study

of Earth’s physical history. Historical geology is one of two principal branches of geology, the other being physical geology. The upper layer of

of iron and another, lighter element (possi-

LITHOSPHERE:

bly sulfur), it is divided between a solid

Earth’s interior, including the crust and the

inner core with a radius of about 760 mi.

brittle portion at the top of the mantle.

(1,220 km) and a liquid outer core about

MA:

1,750 mi. (2,820 km) thick.

entists, meaning “million years.” When an

An abbreviation used by earth sci-

The uppermost division of the

event is designated as, for instance, 160 Ma,

solid earth, representing less than 1% of its

it means that it happened 160 million years

volume and varying in depth from 3-37 mi.

ago.

(5–60 km). Below the crust is the mantle.

MANTLE:

CRUST:

DIVERGENCE:

A tectonic process

whereby plates move away from each other.

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One of two prin-

produced by the action of equal and oppo-

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The thick, dense layer of

rock, approximately 1,429 mi. (2,300 km) thick, between Earth’s crust and its core.

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KEY TERMS MID-OCEAN

RIDGES:

Submarine

CONTINUED

RADIOACTIVITY:

A term describing a

mountain ridges where new seafloor is cre-

phenomenon whereby certain materials

ated by seafloor spreading.

are subject to a form of decay brought

OCEANIC TRENCH:

A deep depres-

sion in the ocean floor caused by the convergence of plates and the resulting subduction of one plate.

about by the emission of high-energy particles or radiation. Forms of particles or energy include alpha particles (positively charged helium nuclei), beta particles (either electrons or subatomic particles

One of two principal

called positrons), or gamma rays, which

forms of tectonism, the other being epeiro-

occupy the highest energy level in the elec-

genesis. Derived from the Greek words oros

tromagnetic spectrum.

OROGENESIS:

(“mountain”) and genesis (“origin”), orogenesis involves the formation of mountain ranges by means of folding, faulting, and volcanic activity. The processes of orogenesis play a major role in plate tectonics. PALEOMAGNETISM:

direction and intensity of magnetic fields in the past, as discerned from the residual magnetization of rocks.

The gathering of

data without actual contact with the materials or objects being studied. RIFT:

A split between two bodies (for

example, two plates) that once were joined.

An area of his-

torical geology devoted to studying the

PLATE

REMOTE SENSING:

RIFT VALLEY:

A long trough bounded

by two or more faults. SEAFLOOR SPREADING:

The theo-

ry that seafloors crack open along the crests of mid-ocean ridges and that new

MARGINS:

Boundaries

seafloor forms in those areas.

between plates. SHEAR:

A form of stress resulting

The name both

from equal and opposite forces that do not

of a theory and of a specialization of tec-

act along the same line. If a thick, hard-

tonics. As an area of study, plate tectonics

bound book is lying flat and one pushes

deals with the large features of the litho-

the front cover from the side so that the

sphere and the forces that fashion them. As

covers and pages are no longer perfectly

a theory, it explains the processes that have

aligned, this is an example of shear.

PLATE TECTONICS:

shaped Earth in terms of plates and their

STRESS:

In general terms, any attempt

movement. Plate tectonics theory brings

to deform a solid. Types of stress include

together aspects of continental drift,

tension, compression, and shear. More

seafloor spreading, seismic and volcanic

specifically, stress is the ratio of force to

activity, and the structures of Earth’s crust

unit area F/A, where F is force and A area.

to provide a unifying model of Earth’s evo-

SUBDUCTION:

A tectonic process that

lution. It is one of the dominant concepts

results when plates converge and one plate

in the modern earth sciences.

forces the other down into Earth’s mantle.

PLATES:

Large movable segments of

the lithosphere.

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As a result, the subducted plate eventually undergoes partial melting.

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Plate Tectonics

KEY TERMS SUBSIDENCE:

A

depression

in

Earth’s crust. TECTONICS:

CONTINUED

THEORY:

A general statement derived

from a hypothesis that has withstood suffiThe study of tectonism,

cient testing.

including its causes and effects, most notably mountain building. TECTONISM:

TRANSFORM MOTION:

The deformation of the A form of stress produced

by a force that acts to stretch a material.

convergence and divergence, that plates interact.

noted that in some rocks magnetic materials point south. This suggested one of two possibilities: either the planet’s magnetic field had reversed itself over time, or the ground containing the magnetized rocks had moved. Both explanations must have seemed far-fetched at the time, but as it turned out, both are correct.

called magnetometers, geologists have found that the orientation of magnetic minerals on one side of a rift mirrors that of materials on the other side. This suggests that the new rock on either side of the rift was formed simultaneously, as seafloor spreading theory indicates.

Earth’s magnetic field has shifted, meaning that the magnetic north and south poles have changed places many times over the eons. In addition, the magnetic poles have wandered around the southern and northern portions of the globe: for instance, whereas magnetic north today lies in the frozen islands to the north of Canada, at about 300 Ma it was located in eastern Siberia. The movement of magnetic rocks on Earth’s surface, however, has turned out to be too great to be explained either by magnetic shifts or by regional wandering of the poles. This is where plate tectonics and paleomagnetism come together.

Earthquakes and Volcanoes Several findings relating to earthquakes and volcanic activity also can be explained by plate tectonics. If one follows news stories of earthquakes, one may begin to wonder why such places as California or Japan have so many quakes, whereas the northeastern United States or western Europe have so few. The fact is that earthquakes occur along belts, and the vast majority of these belts coincide with the boundaries between Earth’s major tectonic plates.

Thus, paleomagnetic studies have served to confirm the ideas of continental drift and plate tectonics, while research conducted at sea bolsters seafloor spreading theory. Using devices

The same is true of volcanoes, and it is no mistake that places famous for earthquakes—the Philippines, say, or Italy—often also are known for their volcanoes. Although they are located near the center of the Pacific Plate, the islands of Hawaii are subject to plate movement, which has helped generate the volcanoes that gave those islands their origin. At the southern end of the island chain, many volcanoes are still active, while those at the northern end tend to be dormant. The reason is that the Pacific Plate as a whole is moving northward over a stationary lava source in the mantle below Hawaii. The southern islands remain poised above that source, while the northern islands have moved away from it.

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C O N F I R M AT I O N T E C T O N I C T H E O RY.

OF

P L AT E

Rocks in Alaska have magnetic materials aligned in such a way that they once must have been at or near the equator. In addition, the orientation of magnetic materials on South America’s east coast shows an affinity with that of similar materials on the west coast of Africa. In both cases, continental drift, with its driving mechanism of plate tectonics, seems the only reasonable explanation.

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process whereby plates slide past each other. It is one of the three ways, along with

lithosphere. TENSION:

A tectonic

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The Oceanic and Continental Crusts Given what we have seen about continental drift and seafloor spreading, it should come as no surprise to learn that, generally speaking, the deeper one goes in the ocean, the newer the crust. Specifically, the crust is youngest near the center of ocean basins and particularly along mid-ocean ridges, or submarine mountain ridges where new seafloor is created by seafloor spreading. It also should not be surprising to learn that oceanic and continental crusts differ both in thickness and in composition. Basalt, an igneous rock (rock formed from the cooling of magma), makes up the preponderance of ocean crust, whereas much of the continental crust is made up of granite, another variety of igneous rock. Whereas the ocean crust is thin, generally 3–6 mi. (5–10 km) in depth, the continental crust ranges in thickness from 12.5–55 mi. (20–90 km). This results in a difference in thickness for the lithosphere, which is only about 60 mi. (100 km) thick beneath the oceans but about 2.5 times as thick—150 mi. (250 km)—under the continents.

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WHERE TO LEARN MORE Erickson, Jon. Plate Tectonics: Unraveling the Mysteries of the Earth. New York: Facts on File, 1992.

Plate Tectonics

Gallant, Roy A. Dance of the Continents. New York: Benchmark Books, 2000. Geology: Plate Tectonics (Web site). . Kious, W. Jacquelyne, and Robert I. Tilling. This Dynamic Earth: The Story of Plate Tectonics. U.S. Geological Survey (Web site). . Miller, Russell. Continents in Collision. Alexandria, VA: Time-Life Books, 1987. Plate Tectonics (Web site). . Plate Tectonics (Web site). . Plate Tectonics, the Cause of Earthquakes (Web site). . Silverstein, Alvin, Virginia B. Silverstein, and Laura Silverstein Nunn. Plate Tectonics. Brookfield, CT: Twenty-First Century Books, 1998. Wynn, Charles M., Arthur W. Wiggins, and Sidney Harris. The Five Biggest Ideas in Science. New York: John Wiley and Sons, 1997.

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SEISMOLOGY Seismology

CONCEPT Disturbances within Earth’s interior, which is in a constant state of movement, result in the release of energy in packets known as seismic waves. An area of geophysics known as seismology is the study of these waves and their effects, which often can be devastating when experienced in the form of earthquakes. The latter do not only take lives and destroy buildings, but they also produce secondary effects, most often in the form of a tsunami, or tidal wave. Using seismographs and seismometers, seismologists study earthquakes and other seismic phenomena, including volcanoes and even explosions resulting from nuclear testing. They measure earthquakes according to their magnitude or energy as well as their intensity or human impact. Seismology also is used to study Earth’s interior, about which it has revealed a great deal.

HOW IT WORKS

Earth’s core possesses enormous energy, both gravitational and thermal. Gravitational energy is a result of the core’s great mass (see Gravity and Geodesy for more about the role of mass in gravity), while thermal energy results from the radioactive decay of elements. In the context of radioactivity, decay does not mean “rot” rather, it refers to the release of high-energy particles. The release of these particles results in the generation of thermal energy, commonly referred to as heat. (See Energy and Earth for more about the scientific definition of heat as well as a discussion of geothermal energy.)

The term tectonism refers to deformation of the lithosphere, the upper layer of Earth’s interior. Tectonics is the study of this deformation,

Differences in mass and temperature within the planet’s interior, known as pressure gradients, result in the deformation of rocks in the lithosphere. The lithosphere includes the brittle upper portion of the mantle, a dense layer of rock approximately 1,429 mi. (2,300 km) thick, as well as the crust, which varies in depth from 3 mi. to 37 mi. (5–60 km). Deformation is the result of stress—that is, tension (stretching), compression, or shear. (The last of these stresses results from equal and opposite forces that do not act along the same line. To visualize shear, one need only imagine a thick hardbound book with its front cover pushed from the side so that the covers and pages are no longer perfectly aligned.)

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Stress and Strain in Earth’s Interior Modern earth scientists’ studies in seismology, as in many other areas, are informed by plate tectonics, and to understand the causes of earthquakes and volcanoes, it is necessary to understand the basics of tectonics as well as plate tectonics theory. The latter subject is discussed in depth within a separate essay, which the reader is encouraged to consult for a more detailed explanation of concepts covered briefly here.

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which results from the release and redistribution of energy from Earth’s core. The core is an extremely dense region, composed primarily of iron and another, lighter element (possibly sulfur), and is divided between a solid inner core with a radius of about 760 mi. (1,220 km) and a liquid outer core about 1,750 mi. (2,820 km) thick.

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Seismology

A

CHASM ALONG A FAULT SCARP IN

SAN BERNARDINO COUNTY, CALIFORNIA. (© Ken M. Johns/Photo Researchers. Reproduced

by permission.)

Under the effects of these stresses, rocks experience strain, or a change in dimension as they bend, warp, slide, break, flow as though they were liquids, or melt. This strain, in turn, leads to a release of energy in the form of seismic waves. These waves may cause faults, or fractures, as well as folds, or bends in the rock structure, which manifest on the surface in the form of earthquakes, volcanoes, and other varieties of seismic activity. Seismology is the study of these waves as well as the movements and vibrations that produce them.

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Continental Drift and Plate Tectonics The theory of continental drift, discussed in Plate Tectonics, is based on the idea that the configuration of Earth’s continents was once different than it is today. Integral to this theory is the accompanying idea that some of the individual land masses of today once were joined in other continental forms and that the land masses later moved to their present locations.

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Continental drift theory was introduced in 1915 by the German geophysicist and meteorologist Alfred Wegener (1880–1930), but it failed to gain acceptance for half a century, in large part because it offered no explanation as to how the continents drifted. That explanation came in the 1960s with the development of plate tectonics, the name both of a theory and of a specialization of tectonics. As an area of study, plate tectonics deals with the large features of the lithosphere and the forces that shape them. As a theory, it explains the processes that have shaped Earth in terms of plates (large movable segments of the lithosphere) and their movement. T H E P L AT E S A N D S E I S M I C AC T I V I T Y. There are several major plates,

some of which are listed in Plate Tectonics. That essay also discusses modern theories regarding the means by which continents broke apart many millions of years ago and then drifted back together, slamming into one another to form a number of notable features, such as the high mountains between the Indian subcontinent and the Eurasian landmass. Nor have the continents stopped moving; they continue to do so, though at a rate too slow to be noticed in a lifetime or even over the course of several generations. Based on its current rate of movement, in another 6,000 years—approximately the span of time since human civilization began—North America will have drifted about 600 ft. (183 m). For the most part, the continents we know today are composed of single plates. For instance, South America sits on its own plate, which includes the southwestern quadrant of the Atlantic. But there are exceptions, an example being India itself, which is part of the Indo-Australian plate. Also notable is the Juan de Fuca Plate, a small portion of land attached to the North American continent and comprising the region from northern California to southern British Columbia.

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is moving northward over a hot spot, a region of high volcanic activity. The hot spot remains more or less stationary, while the Pacific Plate moves across it; this explains why the volcanoes of northern Hawaii are generally dormant, whereas many volcanoes in the southern part of the island chain are still active. Plates interact by moving toward each other (convergence), away from each other (divergence), or past each other (transform motion). When a continental plate converges with an oceanic plate (the differences between these types are discussed in Plate Tectonics), the much sturdier continental plate plows over the oceanic one. This is called subduction. The subducted plate undergoes partial melting, leading to the formation of volcanic chains, as in the nexus of the Juan de Fuca and Pacific or the South American and Nazca plates. The subduction of the Nazca Plate, which lies to the west of South America, helped form the Andes. Transform margins result in the formation of faults and earthquake zones, an example being the volatile San Andreas Fault.

Seismic Waves The first scientific description of seismic waves was that of John William Strutt, Baron Rayleigh (1842–1919), who in 1885 characterized them as having aspects both of longitudinal and of transverse waves. These are, respectively, waves in which the movement of vibration is in the same direction as the wave itself and those in which the vibration or motion is perpendicular to the direction in which the wave is moving. (Ocean waves, for example, are longitudinal, whereas sound waves are transverse.)

It so happens that this area is home to an unusual amount of volcanic activity. Southern California, where the North American and Pacific plates meet on the San Andreas fault, also is extremely prone to earthquakes, as is Japan, whose islands straddle the Philippine, Eurasian, and Pacific plates. Hawaii is another site of seismic activity in the form of volcanoes, but it does not lie at the nexus of any major plates. Instead, it is situated squarely atop the Pacific Plate, which

Rayleigh waves later would be distinguished from Love waves, named after the English mathematician and geophysicist Augustus Edward Hough Love (1863–1940). The motion of Love waves is entirely horizontal, or longitudinal. Both are examples of surface waves, or seismic waves whose line of propagation is along the surface of a medium, such as the solid earth. These waves tend to be slower and more destructive than body waves, defined as waves whose line of propagation is through the body of a medium. Body waves include P-waves (primary waves), which are extremely fast moving and longitudinal, and S-waves (secondary waves), which move somewhat less fast and are transverse. The respective

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SEISMOGRAPH READING FROM THE 1989

LOMA PRIETA, CALIFORNIA,

EARTHQUAKE. (© Russell D. Curtis/Photo Researchers.

Reproduced by permission.)

waves’ rates of propagation through the solid earth are as follows: • P-waves: about 4 mi. (6.4 km) per second • S-waves: about 2 mi. (3.2 km) per second • Rayleigh and Love waves: less than 2 mi. per second

REAL-LIFE A P P L I C AT I O N S The Lisbon Quake and Its Effects On November 1, 1755, the Portuguese capital of Lisbon became the site of one of the worst earthquakes in European history. The event had a number of aftereffects, natural and immediate as well as human and longer term. The results from nature were devastating; the earthquake caused a tsunami, or tidal wave, that flooded the Tagus River even as a fire, also caused by the earthquake, raged through the city. Estimates of the deaths related directly or indirectly to the Lisbon quake range from 10,000 to as many as 60,000, making it the worst European earthquake since 1531. That earlier quake, incidentally, also had occurred in Lisbon— another example of the fact that certain areas are

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more prone to seismic activity. It so happens that Portugal lies near the boundary between the Eurasian and African plates. As for the human response to the quake, it is best represented by the French writer Voltaire (1694–1778). Always a critic of religious faith, Voltaire saw in the incident evidence that called into question Christians’ belief in a loving God. He made this case both in the philosophical poem Le désastre de Lisbonne (The disaster of Lisbon, 1756) and, more memorably, in the satirical novel Candide (1759). M I C H E L L A N D T H E B I RT H O F Another, much less S E I S M O L O G Y.

famous, thinker responded to the Lisbon earthquake in quite a different fashion. This was English geologist and astronomer John Michell (ca. 1724–1793), who studied the event and concluded that quakes are accompanied by shock waves. In an article published in 1760, he noted that earthquakes are found to occur near volcanoes and suggested that they are caused by pressure produced by water that boils from volcanic heat. He also indicated that one can calculate the center of an earthquake by making note of the time at which the motions are felt. Today Michell is regarded as the father of seismology, a discipline that began to mature in

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the nineteenth century. The name itself was coined by the Irish engineer Robert Mallet (1810–1881), who in 1846 compiled the first modern catalogue of earthquakes. Eleven years after publishing the book, which listed all known quakes of any significance since 1606 B.C., Mallet conducted experiments with shock waves by exploding gunpowder and measuring the rate at which the waves travel through various types of material.

Detecting and Measuring Seismic Activity As noted earlier, seismology is concerned with seismic waves, which generally are caused by movements within the solid earth. These waves also may be produced by man-made sources. Seismologic studies assist miners in knowing how much dynamite to use for a quarry blast so as to be effective without destroying the mine itself or the resources being sought. In addition, seismology can be used to reveal the location of such materials as coal and oil. Thanks to seismometers (instruments for detecting seismic waves) and seismographs, which record information regarding those waves, seismologists are able to detect not only natural seismic activity but also the effects of underground nuclear testing. Underground testing is banned by international treaty, and if a “rogue nation” were to conduct such testing, it would come to the attention of the World-Wide Standardized Seismograph Network (WWSSN), which consists of 120 seismic stations in some 60 countries. Most of the remainder of this essay is devoted to a single type of seismic phenomenon: earthquakes. As noted, they are far from the only effect of seismic activity; however, they are the most prevalent and well documented. A close second would be volcanoes, which are discussed in the essay Mountains. E A R LY S E I S M O G RA P H I C I N S T R U M E N T S . In A.D. 132, the Chinese sci-

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The first crude seismograph was invented in 1703 by the French physicist Jean de Hautefeuille (1647–1724), long before Michell formally established a connection between shock waves and earthquakes. Historians date the starting point of modern seismographic monitoring, however, to an 1880 invention by the English geologist John Milne (1850–1913). Milne’s creation, the first precise seismograph, measured motion with a horizontal pendulum attached to a pen that recorded movement on a revolving drum. Milne used his device to record earthquakes from as far away as Japan and helped establish seismologic stations around the world. The first modern seismograph in the United States was installed at the University of California at Berkeley and proved its accuracy in recording the 1906 San Francisco quake, discussed later in this essay. MAGNITUDE: THE RICHTER S CA L E . An earthquake can be measured

according to either its magnitude or its intensity. The first refers to the amount of energy released by the earthquake, and its best-known scale of measurement is the Richter scale. Developed in 1935 by the American geophysicist Charles Richter (1900–1985), the Richter scale is logarithmic rather than arithmetic, meaning that increases in value involve multiplication rather than addition. The numbers on the Richter scale, from 1.0 to 10.0, should be thought of as exponents rather than integers. Each whole-number increase represents a tenfold increase in the amplitude (size from crest to trough) of the seismic wave. Therefore 2.0 is not twice as much as 1.0; it is 10 times as much. To go from 1.0 to 3.0 is an increase by a factor of 100, and to go from 1.0 to 4.0 indicates an increase by a factor of 1,000. The scales of magnitude thus become ever greater, and while a whole-number increase on the Richter scale indicates an increase of amplitude by a factor of 10, it represents an increase of energy by a factor of about 31. I N T E N S I T Y: T H E M E R CA L L I S CA L E . The amplitude and energy measured

entist Chang Heng (78–139) constructed what may have been the first seismographic instrument, which was designed to detect not only the presence of seismic activity but also the direction from which it came. His invention ultimately was discarded, however, and understanding of earthquakes progressed little for more than 1,600 years.

by the Richter scale are objective and quantitative, whereas intensity is more subjective and qualitative. Intensity, an indication of the earthquake’s effect on human beings and structures, is measured by the Mercalli scale, named after the Italian seismologist Giuseppe Mercalli (1850– 1914). The 12 levels on the Mercalli scale range

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from I, which means that few people felt the quake, to XII, which indicates total damage. A few comparisons serve to illustrate the scales’ relationship to each other. A score of I on the Mercalli scale equates to a value between 1.0 and a 3.0 on the Richter scale and indicates a tremor felt only by a very few people under very specific circumstances. At 5.0 to 5.9 on the Richter scale (VI to VII on the Mercalli scale), everyone feels the earthquake, and many people are frightened, but only the most poorly built structures are damaged significantly. Above 7.0 on the Richter scale and VIII on the Mercalli scale, wooden and then masonry structures collapse, as do bridges, while railways are bent completely out of shape. In populated areas, as we shall see, the death toll can be enormous.

Famous Quakes The great San Francisco earthquake, which struck on April 18, 1906, spawned a massive fire, and these events resulted in the deaths of some 700 people, including 270 inmates of a mental institution. Another 300,000 people were left homeless, and 490 city blocks were destroyed. Ultimately, the financial impact of the San Francisco quake proved to be one of the contributing factors in the March 13, 1907, stock market crash that played a key role in the panic of 1907. At 5:04 P.M. on October 17, 1989, another quake struck San Francisco. It lasted just 15 seconds, long enough to kill some 90 people and cause $6 billion in property damage. Though it was the biggest quake since the 1906 tremor, it was much smaller: 7.19 on the Richter scale, or about one-fifth of the 7.7 measured for the 1906 quake. The 1989 Loma Prieta quake cost much more than the earlier tragedy, which had caused $500 million in damage, but, of course, half a billion dollars in 1906 was worth a great deal more than $6 billion 83 years later.

The high incidence of earthquakes in Alaska is understandable enough, given the fact that its southern edge abuts a subduction zone and, along with the panhandle, sits astride the boundary between the North American and Pacific plates. Although this may not be much comfort to people in Alaska, it is fortunate that the most earthquake-prone state is also the most sparsely populated. Had the epicenter (the point on Earth’s surface directly above the hypocenter, or focal point from which a quake originates) of the 1964 earthquake been in New York City, the death toll would have been closer to 125,000 than 125.

Seismology

G R E AT E S T Q UA K E S I N T H E C O N T I N E N TA L U N I T E D S TAT E S .

Similarly, it is fortunate that the greatest quakes to strike the continental United States outside California have been in low-population centers. Of the 15 worst earthquakes in U.S. history, only one was outside Alaska, California, or Hawaii. In fact, it was the site of both the worst and the fifthworst earthquakes in the continental United States: New Madrid, Missouri, site of a 7.9 quake on February 7, 1812, and a 7.7 quake just two months earlier, on December 16, 1811. New Madrid lies at the extreme southeastern tip of Missouri, near the Mississippi River and within a few hundred miles of several major cities: St. Louis, Missouri; Memphis and Nashville, Tennessee; and Louisville, Kentucky. Had the 1811 and 1812 quakes occurred today, they undoubtedly would have taken a vast human toll owing to the resulting floods. As it was, some lakes rose by as much as 15 ft. (4.6 m), streams changed direction, and the Mississippi and Ohio rivers flowed backward. Fortunately, however, they occurred at a time when the Missouri Territory—it was not even a state yet—and surrounding areas were sparsely populated. The combined death toll was in the single digits.

Neither earthquake, however, was the greatest in American history; in fact, the 1989 quake does not rank among the top 15, even for the continental United States. The eight worst earthquakes in U.S. history all occurred in one state: Alaska. Greatest of all was the March 27, 1964, quake at Prince William Sound, which registered a staggering 9.2 on the Richter scale and took 125 lives. Of that number, 110 were killed in a tsunami resulting from the quake.

Of the top 15 earthquakes in the continental United States, all but the 1906 San Francisco quake (which ranks sixth) took place in areas with small populations. Ten were in California but generally in less populous areas or at times when there were fewer people there (e.g., no. 2: Fort Tejon, 1857; no. 3: Owens Valley, 1872; and no. 4: Imperial Valley, 1892). Other than the two New Madrid quakes, the remainder took place in Nevada (no. 12: Dixie Valley, 1954), Montana (no. 13: Hebgen Lake, 1959), and Idaho (no. 14:

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Seismology

EARTHQUAKE

236

DAMAGE IN

CALIFORNIA. (© David Weintraub/Photo Researchers. Reproduced by permission.)

Borah Peak, 1983). As of late 2001, the Idaho quake was the second most recent, after no. 9, at Landers, California, in 1992. (The 1994 Northridge quake, in the Los Angeles area, ranked 6.7 on the Richter scale, well below the 7.3 registered by no. 15, west of Eureka, California, in 1922.)

however, compares with the July 27, 1976, earthquake in T’ang-shan, China. The worst earthquake in modern history, it shattered some 20 sq. mi. (32 km sq.) near the capital city of Beijing

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THE WORLD’S MOST DESTRUCTIVE QUAKES. None of these U.S. quakes,

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and killed about 242,000 people while injuring an estimated 600,000 more. There are several interesting aspects to this quake, aside from its sheer scale. One is sociological, involving the human response to the quake. As in Portugal in 1755, people saw events in a cosmic light; in this case, though, they did not interpret the quake as evidence of divine unconcern but quite the opposite. Mao Tse-tung (1893–1976), by far the most influential Chinese leader of modern times, had just died, and the Chinese saw the natural disaster as fitting into a larger historical pattern. In the traditional Chinese view, earthquakes, floods, and other signs from the gods attend the change of dynasties. Also interesting is the fact that the T’angshan quake was merely the most destructive in a worldwide series of quakes that took place between February and November 1976. In the course of these events, 23,000 people died in Guatemala after a February 4 quake; 3,000 people were reported dead, and 3,000 more were missing in Indonesia, as a result of a series of quakes and landslides on June 26 (later, the U.S. Federal Emergency Management Agency, or FEMA, placed the number of dead from the Indonesia quake at just 443); as many as 8,000 people died in an earthquake and tsunami that hit the southern Philippines on August 16; and 4,000 more perished in a November 24 quake in eastern Turkey. Similarly, a few months before the 1755 Lisbon earthquake, a quake hit northern Iran. This is an aspect of seismology that cannot be explained readily by plate tectonics: Iran and Portugal are not on the same plate margins; in fact, northern Iran is not on a plate margin at all. Likewise, the areas hit in the 1976 quakes were not on the same plate margins, and T’ang-shan (unlike the other places affected) is not on a major plate margin at all. Nor is Shansi in northcentral China, site of history’s most destructive earthquake on January 24, 1556, which killed more than 830,000 people.

tude—in the twentieth century. Whereas the T’ang-shan quake registered 8.0, a quake in Chile on May 22, 1960, had a magnitude of 9.5, or about 50 times greater, yet the death toll was much smaller—2,000 people killed. Three thousand more were injured in the Chilean quake, and two million were rendered homeless. The last statistic perhaps best signifies the magnitude of the 1960 quake, which caused tsunamis that brought death and destruction as far away as Hawaii, Japan, the Philippines, and the west coast of the United States.

Seismology

Learning from Seismology As noted, plate tectonics does not explain every earthquake, but it does explain most, probably about 90%. Not that it is much help in predicting earthquakes, because the processes of plate tectonics take place on an entirely different time scale than the ones to which humans are accustomed. These processes happen over millions of years, so it is hard to say, for any particular year, just what will happen to a particular plate. Plate tectonics, then, tells us only areas of likelihood for earthquakes—specifically, plate boundaries of the types discussed near the end of Plate Tectonics. And even though the processes that create the conditions for an earthquake are extremely slow, usually the discernible indications that an earthquake is coming appear only seconds before the quake itself. Thus, as sophisticated as modern seismometers are, they generally do not provide enough advance notice of earthquakes to offer any lifesaving value. There are not just a few earthquakes each year but many thousands of tremors, most of them too small to register. Sometimes these tremors may be foreshocks, or indicators that a quake is coming to a particular area. In addition, studies of other phenomena, from tidal behavior to that of animals (probably a result of some creatures’ extremely acute hearing), may offer suggestions as to the locations of future quakes. EARTH’S CORE AND THE M O H O . Seismology is useful for learning

Note that the 1556 and 1976 Chinese quakes were the worst, respectively, of all history and of modern times—but worst in terms of intensity, not magnitude. One might say that they were the most destructive but not the worst in pure terms. The 1976 quake is not even on the list of the 10 worst earthquakes—those of the greatest magni-

about more than just earthquakes or volcanoes. During the early years of the twentieth century, the Irish geologist Richard Dixon Oldham (1858–1936) studied data from a number of recent earthquakes and noticed a difference in the behavior of compression waves and shear waves. (These terms merely express the differ-

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Seismology

KEY TERMS AMPLITUDE:

The maximum displace-

ment of a vibrating material, or the “size” of a wave from crest to trough.

(1,220 km) and a liquid outer core about 1,750 mi. (2,820 km) thick. CRUST:

Waves whose line of

solid earth, representing less than 1% of its

propagation is through the body of a medi-

volume and varying in depth from 3–37

um. These include P-waves (primary

mi. (5–60 km). Below the crust is the

waves), which move extremely fast and are

mantle.

BODY WAVES:

longitudinal, and S-waves (secondary

A tectonic process

DIVERGENCE:

waves), which are move somewhat less fast

whereby plates move away from each other.

and are transverse. Compare with surface

Divergence results in the separation of

waves.

plates and most often is associated either A form of stress

with seafloor spreading or the formation of

produced by the action of equal and oppo-

rift valleys. It is one of the three ways, along

site forces, the effect of which is to reduce

with convergence and transform motion,

the length of a material. Compression is a

that plates interact.

COMPRESSION:

form of pressure.

The response of solids to

ELASTICITY:

CONTINENTAL DRIFT:

The theory

that the configuration of Earth’s continents

stress. The point on Earth’s sur-

EPICENTER:

was once different than it is today, that

face directly above the hypocenter, or the

some of the individual landmasses of today

focal point from which an earthquake orig-

once were joined in other continental

inates.

forms, and that these landmasses later separated and moved to their present locations. CONVERGENCE:

A tectonic process

whereby plates move toward each other.

FAULT:

An area of fracturing, as a result

of stress, between rocks. FOLD:

An area of rock that has been

bent by stress. A branch of the earth

Usually associated with subduction, con-

GEOPHYSICS:

vergence typically occurs in the ocean, cre-

sciences that combines aspects of geology

ating an oceanic trench. It is one of the

and physics. Geophysics addresses the

three ways, along with divergence and

planet’s physical processes as well as its

transform motion, that plates interact.

gravitational, magnetic, and electric prop-

CORE:

The center of Earth, an area

constituting about 16% of the planet’s vol-

238

The uppermost division of the

erties and the means by which energy is transmitted through its interior. Internal thermal energy that

ume and 32% of its mass. Made primarily

HEAT:

of iron and another, lighter element (possi-

flows from one body of matter to another.

bly sulfur), it is divided between a solid

HOT SPOT:

inner core with a radius of about 760 mi.

activity.

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A region of high volcanic

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Seismology

KEY TERMS

CONTINUED

Where earthquakes are

tonics. As an area of study, plate tectonics

concerned, intensity refers to the amount

deals with the large features of the litho-

of damage to humans and buildings. Sub-

sphere and the forces that shape them. As a

jective and qualitative (as opposed to mag-

theory, it explains the processes that have

nitude, which is objective and quantita-

shaped Earth in terms of plates and their

tive), intensity is measured by the Mercalli

movement. Plate tectonics theory brings

scale.

together aspects of continental drift,

INTENSITY:

The energy that

seafloor spreading, seismic and volcanic

an object possesses by virtue of its motion.

activity, and the structures of Earth’s crust

KINETIC ENERGY:

LITHOSPHERE:

The upper layer of

Earth’s interior, including the crust and the brittle portion at the top of the mantle. LONGITUDINAL WAVE:

A wave in

which the movement of vibration is in the

to provide a unifying model of Earth’s evolution. It is one of the dominant concepts in the modern earth sciences. PLATES:

Large, movable segments of

the lithosphere. The act or state of

same direction as the wave itself. This is

PROPAGATION:

contrasted with a transverse wave.

traveling from one place to another.

LOVE WAVES: MAGNITUDE:

See surface waves. Where earthquakes are

P-WAVES:

See body waves.

RADIOACTIVITY:

A term describing a

concerned, magnitude refers to the

phenomenon whereby certain materials

amount of energy released by the quake as

are subject to a form of decay brought

well as the amplitude of the seismic waves.

about by the emission of high-energy par-

Objective and quantitative (as opposed to

ticles or radiation. Forms of particles or

intensity, which is subjective and qualita-

energy include alpha particles (positively

tive), magnitude is measured by the

charged helium nuclei); beta particles

Richter scale.

(either electrons or subatomic particles

MANTLE:

The thick, dense layer of

rock, approximately 1,429 mi. (2,300 km) thick, between Earth’s crust and its core. In

called positrons); or gamma rays, which occupy the highest energy level in the electromagnetic spectrum.

reference to the other terrestrial planets,

RAYLEIGH WAVES:

mantle simply means the area of dense

RICHTER SCALE:

rock between the crust and core. SEISMIC WAVE: MERCALLI SCALE: PLATE

MARGINS:

See magnitude. A packet of energy

See intensity.

resulting from the disturbance that accom-

Boundaries be-

panies a strain on rocks in the lithosphere.

tween plates. PLATE TECTONICS:

See surface waves.

SEISMOGRAPH:

The name both

of a theory and of a specialization of tec-

S C I E N C E O F E V E RY DAY T H I N G S

An

instrument

designed to record information regarding seismic waves.

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Seismology

KEY TERMS

CONTINUED

The study of seismic

transverse and longitudinal characteristics)

waves as well as the movements and vibra-

and Love waves (purely longitudinal).

tions that produce them.

Compare with body waves.

SEISMOLOGY:

SEISMOMETER:

An instrument for

detecting seismic waves. SHEAR:

S-WAVES:

See body waves. The study of tectonism,

TECTONICS:

A form of stress resulting

including its causes and effects, most

from equal and opposite forces that do not

notably mountain building.

act along the same line. If a thick, hard-

TECTONISM:

bound book is lying flat and one pushes

lithosphere.

the front cover from the side so that the covers and pages no longer constitute par-

TENSION:

The deformation of the A form of stress produced

by a force that acts to stretch a material.

allel planes, this is an example of shear. THERMAL ENERGY:

form of kinetic energy produced by the

in dimension experienced by an object that

motion of atomic or molecular particles in

has been subjected to stress and the origi-

relation to one another. The greater the rel-

nal dimensions of the object.

ative motion of these particles, the greater

STRESS:

In general terms, any attempt

the thermal energy.

to deform a solid. Types of stress include

TRANSFORM MOTION:

tension, compression, and shear.

process whereby plates slide past each

A tectonic

A tectonic process that

other. It is one of the three ways, along with

results when plates converge, and one plate

convergence and divergence, that plates

forces the other down into Earth’s mantle.

interact.

As a result, the subducted plate eventually

TRANSVERSE

undergoes partial melting.

which the vibration or motion is perpendi-

SUBDUCTION:

SURFACE

WAVES:

Seismic waves

WAVE:

A wave in

cular to the direction in which the wave is

whose line of propagation is along the sur-

moving. Compare with longitudinal wave.

face of a medium such as the solid earth.

TSUNAMI:

These waves tend to be slower and more

an earthquake or volcanic eruption. The

destructive than body waves. Examples

term comes from the Japanese words for

include Rayleigh waves (waves with both

“harbor” and “wave.”

A tidal wave produced by

Oldham’s findings, published in 1906—the same year as the great San Francisco quake— made him a pioneer in the application of seis-

mology to the study of Earth’s interior. Three years later, studies of earthquake waves by the Croatian geologist Andrija Mohorovicic (1857– 1936) revealed still more about the interior of the planet. Based on his analysis of wave speeds and arrival times, Mohorovicic was able to calculate the depth at which the crust becomes the mantle. This change is abrupt rather than gradual, and

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ences in stress produced by seismic waves.) As it turns out, shear waves are deflected as they pass through the center of Earth. Since liquid cannot experience shear, this finding told him that the planet’s core must be made of molten material.

240

Heat energy, a

The ratio between the change

STRAIN:

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the boundary on which it occurs is today known as the Mohorovicic discontinuity, or simply the Moho.

WHERE TO LEARN MORE Abbott, Patrick. Natural Disasters. Dubuque, IA: William C. Brown Publishers, 1996. National Earthquake Information Center—World Data Center for Seismology, Denver (Web site). . Prager, Ellen J., Kate Hutton, Costas Synolakis, et al. Furious Earth: The Science and Nature of Earthquakes, Volcanoes, and Tsunamis. New York: McGraw-Hill, 2000. Seismology Info Page—Netherlands (Web site). .

S C I E N C E O F E V E RY DAY T H I N G S

Seismology Research Centre—Australia (Web site). .

Seismology

Sigurdsson, Haraldur. Encyclopedia of Volcanoes. San Diego: Academic Press, 2000. Sleh, Keery E., and Simon LeVay. The Earth in Turmoil: Earthquakes, Volcanoes, and Their Impact on Humankind. New York: W. H. Freeman, 1998. University of Alaska Fairbanks Seismology (Web site). . University of Washington Seismology and Earthquake Information (Web site). . “Earthquake Hazards Program,” USGS (United States Geological Survey) (Web site). . Wade, Nicholas. The Science Times Book of Natural Disasters. New York: Lyons Press, 2000.

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Geomorphology

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GEOMORPHOLOGY

CONCEPT The surface of Earth is covered with various landforms, a number of which are discussed in various entries throughout this book. This essay is devoted to the study of landforms themselves, a subdiscipline of the geologic sciences known as geomorphology. The latter, as it has evolved since the end of the nineteenth century, has become an interdisciplinary study that draws on areas as diverse as plate tectonics, ecology, and meteorology. Geomorphology is concerned with the shaping of landforms, through such processes as subsidence and uplift, and with the classification and study of such landforms as mountains, volcanoes, and islands.

HOW IT WORKS An Evolving Area of Study Geomorphology is an area of geology concerned with the study of landforms, with the forces and processes that have shaped them, and with the description and classification of various physical features on Earth. The term, which comes from the Greek words geo, or “Earth,” and morph, meaning “form,” was coined in 1893 by the American geologist William Morris Davis (1850–1934), who is considered the father of geomorphology.

vailed throughout the late nineteenth and early twentieth centuries. By the mid-twentieth century, however, the concept of geomorphology inherited from Davis had fallen into disfavor, to be replaced by a paradigm, or model, oriented toward physical rather than historical geology. (These two principal branches of geology are concerned, in the first instance, with Earth’s past and the processes that shaped it and, in the second instance, with Earth’s current physical features and the processes that continue to shape it.) R E T H I N K I N G G E O M O R P H O LO GY. As reconceived in the 1950s and thereafter,

geomorphology became an increasingly exact science. As has been typical of many sciences in their infancy, early geomorphology focused on description rather than prediction and tended to approach its subject matter in a qualitative fashion. The term qualitative suggests a comparison between qualities that are not defined precisely, such as “fast” and “slow” or “warm” and “cold.” On the other hand, a quantitative approach, as has been implemented for geomorphology from the mid–twentieth century onward, centers on a comparison between precise quantities—for instance, 10 lb. (4.5 kg) versus 100 lb. (45 kg) or 50 MPH (80.5 km/h) versus 120 MPH (193 km/h).

During Davis’s time, geomorphology was concerned primarily with classifying different structures on Earth’s surface, examples of which include mountains and islands, discussed later in this essay. This view of geomorphology as an essentially descriptive, past-oriented area of study closely aligned with historical geology pre-

As part of its shift in focus, geomorphology began to treat Earth’s physical features as systems made up of complex and ongoing interactions. This view fell into line with a general emphasis on the systems concept in the study of Earth. (See Earth Systems for more about the systems concept.) As geomorphology evolved, it became more interdisciplinary, as we shall see. This, too,

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Geomorphology

MOUNT MACHHAPUCHHARE IN THE HIMALAYAS. THE HIMALAYAS WERE FORMED THROUGH OR RISING OF EARTH’S SURFACE. (© George Turner/Photo Researchers. Reproduced by permission.)

was part of an overall trend in the earth sciences toward an approach that viewed subjects in broad, cross-disciplinary terms as opposed to a narrow focus on specific areas of study.

Landforms and Processes

246

THE PROCESS OF UPLIFT,

These physical features are called landforms, examples of which include mountains, plateaus, and valleys. Geomorphology always has involved classification, and early scientists working in this subdiscipline addressed the classification of landforms. Other systems of classification, however, are not so concerned with cataloging topographical features themselves as with differentiating the processes that shaped them. This brings us to the other area of interest in geomorphology: the study of how landforms came into being.

Two concerns are foremost within the realm of geomorphology, and these concerns reflect the stages of its history. First, in line with Davis’s original conception of geomorphology as an area of science devoted to classifying and describing natural features, there is its concern with topography. The latter may be defined as the configuration of Earth’s surface, including its relief (elevation and other inequalities) as well as the position of physical features.

S H A P I N G T H E E A RT H . Among the processes that drive the shaping of landforms is plate tectonics, or the shifting of large, movable segments of lithosphere (the crust and upper layer of Earth’s mantle). Plate tectonics is dis-

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cussed in detail within its own essay and more briefly in other areas throughout this book, as befits its status as one of the key areas of study in the earth sciences.

REAL-LIFE A P P L I C AT I O N S

Other processes also shape landforms. Included among these processes are weathering, the breakdown of rocks and minerals at or near the surface of Earth due to physical or chemical processes; erosion, the movement of soil and rock due to forces produced by water, wind, glaciers, gravity, and other influences; and mass wasting or mass movement, the transfer of earth material, by processes that include flow, slide, fall, and creep, down slopes. Also of interest are fluvial and eolian processes (those that result from water flow and wind, respectively) as well as others related to glaciers and coastal formations.

Subsidence refers to the process of subsiding (settling or descending), on the part of either an air column or the solid earth, or, in the case of solid earth, to the resulting formation or depression. Subsidence in the atmosphere is discussed briefly in the entry Convection. Subsidence that occurs in the solid earth, known as geologic subsidence, is the settling or sinking by a body of rock or sediment. (The latter can be defined as material deposited at or near Earth’s surface from a number of sources, most notably preexisting rock.)

Human activity also can play a significant role in shaping Earth. This effect may be direct, as when the construction of cities, the building of dams, or the excavation of mines alters the landscape. On the other hand, it can be indirect. In the latter instance, human activity in the biosphere exerts an impact, as when the clearing of forest land or the misuse of crop land results in the formation of a dust bowl.

Interdisciplinary Studies As noted earlier, geomorphology is characteristic of the earth sciences as a whole in its emphasis on an interdisciplinary approach. As is true of earth scientists in general, those studying landforms and the processes that shape them do not work simply in one specialty. Among the areas of interest in geomorphology are, for example, deep-sea geomorphology, which draws on oceanography, and planetary geomorphology, the study of landscapes on other planets.

Geomorphology

Subsidence

As noted earlier, many geomorphologic processes can be caused either by nature or by human beings. An example of natural subsidence takes place in the aftermath of an earthquake, during which large areas of solid earth may simply drop by several feet. Another example can be observed at the top of a volcano some time after it has erupted, when it has expelled much of its material (i.e., magma) and, as a result, has collapsed. Natural subsidence also may result from cave formation in places where underground water has worn away limestone. If the water erodes too much limestone, the ceiling of the cave will subside, usually forming a sinkhole at the surface. The sinkhole may fill with water, making a lake; the formation of such sinkholes in many spots throughout an area (whether the sinkholes become lakes or not), is known as karst topography. In places where the bedrock is limestone— particularly in the sedimentary basins of rivers— karst topography is likely to develop. The United States contains the most extensive karst region in the world, including the Mammoth cave system in Kentucky. Karst topography is very pronounced in the hills of southern China, and karst landscapes have been a prominent feature of Chinese art for centuries. Other extensive karst regions can be found in southern France, Central America, Turkey, Ireland, and England.

When studying coastal geomorphology, a geologist may draw on realms as diverse as fluid mechanics (an area of physics that studies the behavior of gases and liquids at rest and in motion) and sedimentology. The investigation of such processes as erosion and mass wasting calls on knowledge in the atmospheric sciences as well as the physics and chemistry of soil. It is almost inevitable that a geomorphologic researcher will draw on geophysics as well as on such subspecialties as volcanology. These studies may go beyond the “hard sciences,” bringing in such social sciences as geography.

M A N - M A D E S U B S I D E N C E . Manmade subsidence often ensues from the removal of groundwater or fossil fuels, such as petroleum or coal. Groundwater removal can be perfectly safe, assuming the area experiences sufficient

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rainfall to replace, or recharge, the lost water. If recharging does not occur in the necessary proportions, however, the result will be the eventual collapse of the aquifer, a layer of rock that holds groundwater. In so-called room-and-pillar coal mining, pillars, or vertical columns, of coal are left standing, while the areas around them are extracted. This method maintains the ceiling of the “room” that has been mined of its coal. After the mine is abandoned, however, the pillar eventually may experience so much stress that it breaks, leading to the collapse of the mined room. As when the ceiling of a cave collapses, the subsidence of a coal mine leaves a visible depression above ground.

Uplift As its name implies, uplift describes a process and results opposite to those of subsidence. In uplift the surface of Earth rises, owing either to a decrease in downward force or to an increase in upward force. One of the most prominent examples of uplift is seen when plates collide, as when India careened into the southern edge of the Eurasian landmass some 55 million years ago. The result has been a string of mountain ranges, including the Himalayas, Karakoram Range, and Hindu Kush, that contain most of the world’s tallest peaks. Plates move at exceedingly slow speeds, but their mass is enormous. This means that their inertia (the tendency of a moving object to keep moving unless acted upon by an outside force) is likewise gargantuan in scale. Therefore, when plates collide, though they are moving at a rate equal to only a few inches a year, they will keep pushing into each other like two automobiles crumpling in a head-on collision. Whereas a car crash is over in a matter of seconds, however, the crumpling of continental masses takes place over hundreds of thousands of years. When sea floor collides with sea floor, one of the plates likely will be pushed under by the other one, and, likewise, when sea floor collides with continental crust, the latter will push the sea floor under. (See Plate Tectonics for more about oceanic-oceanic and continental-oceanic collisions.) This results in the formation of volcanic mountains, such as the Andes of South America or the Cascades of the Pacific Northwest, or vol-

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canic islands, such as those of Japan, Indonesia, or Alaska’s Aleutian chain. I S O S TAT I C

C O M P E N S AT I O N .

In many other instances, collision, compression, and extension cause uplift. On the other hand, as noted, uplift may result from the removal of a weight. This occurs at the end of an ice age, when glaciers as thick as 1.9 mi. (3 km) melt, gradually removing a vast weight pressing down on the surface below. This movement leads to what is called isostatic compensation, or isostatic rebound, as the crust pushes upward like a seat cushion rising after a person is longer sitting on it. Scandinavia is still experiencing uplift at a rate of about 0.5 in. (1 cm) per year as the after-effect of glacial melting from the last ice age. The latter ended some 10,000 years ago, but in geologic terms this is equivalent to a few minutes’ time on the human scale.

Islands Geomorphology, as noted earlier, is concerned with landforms, such as mountains and volcanoes as well as larger ones, including islands and even continents. Islands present a particularly interesting area of geomorphologic study. In general, islands have certain specific characteristics in terms of their land structure and can be analyzed from the standpoint of the geosphere, but particular islands also have unique ecosystems, requiring an interdisciplinary study that draws on botany, zoology, and other subjects. In addition, there is something about an island that has always appealed to the human imagination, as evidenced by the many myths, legends, and stories about islands. Some examples include Homer’s Odyssey, in which the hero Odysseus visits various islands in his long wanderings; Thomas More’s Utopia, describing an idealized island republic; Robinson Crusoe, by Daniel Defoe, in which the eponymous hero lives for many years on an island with no companion but the trusty native Friday; Treasure Island, by Robert Louis Stevenson, in which the island is the focus of a treasure hunt; and Mark Twain’s Adventures of Huckleberry Finn, depicting Jackson Island in the Mississippi River, to which Huckleberry Finn flees to escape “civilization.” One of the favorite subjects of cartoonists is that of a castaway stranded on a desert island, a mound of sand with no more than a single tree.

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Movies, too, have long portrayed scenarios, from the idyllic to the brutal, that take place on islands, particularly deserted ones, a notable example being Cast Away (2000). A famous line by the English poet John Donne (1572–1631) warns that “no man is an island,” implying that many wish they could enjoy the independence suggested by the concept of an island. Within the Earth system, however, nothing is fully independent, and, as we shall see, this is certainly the case where islands are concerned.

Geomorphology

T H E I S LA N D S O F E A RT H . Earth has literally tens of thousands of islands. Just two archipelagos (island chains), those that make up the Philippines and Indonesia, include thousands of islands each. While there are just a few dozen notable islands on Earth, many more dot the planet’s seas and oceans. The largest are these:

• Greenland (Danish, northern Atlantic): 839,999 sq. mi.(2,175,597 sq km) • New Guinea (divided between Indonesia and Papua New Guinea, western Pacific): 316,615 sq. mi. (820,033 sq km) • Borneo (divided between Indonesia and Malaysia, western Pacific): 286,914 sq. mi. (743,107 sq km) • Madagascar (Malagasy Republic, western Indian Ocean): 226,657 sq. mi. (587,042 sq km) • Baffin (Canadian, northern Atlantic): 183,810 sq. mi. (476,068 sq km) • Sumatra (Indonesian, northeastern Indian Ocean): 182,859 sq. mi. (473,605 sq km) The list could go on and on, but it stops at Sumatra because the next-largest island, Honshu (part of Japan), is less than half as large, at 88,925 sq. mi. (230,316 sq km). Clearly, not all islands are created equal, and though some are heavily populated or enjoy the status of independent nations (e.g., Great Britain at number eight or Cuba at number 15), they are not necessarily the largest. On the other hand, some of the largest are among the most sparsely populated. Of the 32 largest islands in the world, more than a third are in the icy northern Atlantic and Arctic, with populations that are small or practically nonexistent. Greenland’s population, for instance, was just over 59,000 in 1998, while that of Baffin Island was about 13,200. On both islands, then, each person has about 14 frozen sq. mi. (22 sq km) to himself or herself, making

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A

TINY ISLAND IN THE

TRUK

LAGOON,

MICRONESIA. (©

Stuart Westmorland/Photo Researchers. Reproduced by permission.)

them among the most sparsely populated places on Earth. CONTINENTS, OCEANS, AND I S LA N D S . Australia, of course, is not an

island but a continent, a difference that is not related directly to size. If Australia were an island, it would be by far the largest. Australia is regarded as a continent, however, because it is one of the principal landmasses of the Indo-Australian plate, which is among a handful of major continental plates on Earth. Whereas continents are more or less permanent (though they have experienced considerable rearrangement over the eons), islands come and go, seldom lasting more than 10 million years. Erosion or rising sea levels remove islands, while volcanic explosions can create new ones, as when an eruption off the coast of Iceland resulted in the formation of an island, Surtsey, in 1963. Islands are of two types, continental and oceanic. Continental islands are part of continental shelves (the submerged, sloping ledges of continents) and may be formed in one of two ways. Rising ocean waters either cover a coastal region, leaving only the tallest mountains exposed as islands or cut off part of a peninsula,

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Geomorphology

KEY TERMS BIOSPHERE:

A combination of all liv-

ing things on Earth—plants, animals,

tion and classification of various physical features on Earth.

birds, marine life, insects, viruses, singlecell organisms, and so on—as well as all formerly living things that have not yet decomposed.

A branch of the earth

GEOPHYSICS:

sciences that combines aspects of geology and physics. Geophysics addresses the planet’s physical processes as well as its

A tectonic process

gravitational, magnetic, and electric prop-

whereby plates move toward each other.

erties and the means by which energy is

Usually associated with subduction, con-

transmitted through its interior.

CONVERGENCE:

vergence typically occurs in the ocean, creating an oceanic trench. It is one of the

The upper part of

GEOSPHERE:

three ways, along with divergence and

Earth’s continental crust, or that portion of

transform motion, in which plates interact.

the solid earth on which human beings live and which provides them with most of

CRUST:

The uppermost division of the

their food and natural resources.

solid earth, representing less than 1% of its The study

volume and varying in depth from 3 to 37

HISTORICAL GEOLOGY:

mi. (5 to 60 km). Below the crust is the

of Earth’s physical history. Historical geol-

mantle.

ogy is one of two principal branches of geology, the other being physical geology.

EROSION:

The movement of soil and LANDFORM:

wind, glaciers, gravity, and other influ-

feature, such as a mountain, plateau, or

ences.

valley.

GEOLOGY:

The study of the solid

LITHOSPHERE:

The upper layer of

earth, in particular its rocks, minerals, fos-

Earth’s interior, including the crust and the

sils, and land formations.

brittle portion at the top of the mantle.

GEOMORPHOLOGY:

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A notable topographical

rock due to forces produced by water,

An area of geol-

MASS

WASTING:

The transfer of

ogy concerned with the study of land-

earth material, by processes that include

forms, with the forces and processes that

flow, slide, fall, and creep, down slopes.

have shaped them, and with the descrip-

Also known as mass movement.

which then becomes an island. Most of Earth’s significant islands are continental and are easily spotted as such, because they lie at close proximity to continental landmasses. Many other continental islands are very small, however; examples include the barrier islands that line the East Coast of the United States. Formed from mainland sand brought to the coast by rivers, these are

technically not continental islands, but they more clearly fit into that category than into the grouping of oceanic islands.

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Oceanic islands, of which the HawaiianEmperor island chain and the Aleutians off the Alaskan coast are examples, form as a result of volcanic activity on the ocean floor. In most cases, there is a region of high volcanic activity,

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Geomorphology

KEY TERMS The study of

CONTINUED

the forces that have shaped the planet.

The study and interpretation of sediments, including sedimentary processes and formations.

Physical geology is one of two principal

SUBSIDENCE:

PHYSICAL GEOLOGY:

the material components of Earth and of

branches of geology, the other being historical geology. PLATE TECTONICS:

The name both

of a theory and of a specialization of tectonics. As an area of study, plate tectonics deals with the large features of the lithosphere and the forces that shape them. As a theory, it explains the processes that have shaped Earth in terms of plates and their movement. PLATES:

SEDIMENTOLOGY:

A term that refers either to the process of subsiding (settling or descending), on the part of either air or solid earth or, in the case of solid earth, to the resulting formation. Subsidence thus is defined variously as the downward movement of air, the sinking of ground, or a depression in Earth’s crust.

Any set of interactions that can be set apart mentally from the rest of the universe for the purposes of study, observation, and measurement.

SYSTEM:

Large, movable segments of The study of tectonism, including its causes and effects, most notably mountain building. TECTONICS:

the lithosphere. Involving a compari-

QUALITATIVE:

son between qualities that are not defined precisely, such as “fast” and “slow” or “warm” and “cold.” QUANTITATIVE:

Involving a compari-

son between precise quantities—for instance, 10 lb. versus 100 lb. or 50 mi. per hour versus 120 mi. per hour. RELIEF:

Elevation and other inequali-

ties on a land surface. SEDIMENT:

Material deposited at or

near Earth’s surface from a number of sources, most notably preexisting rock.

TECTONISM:

The deformation of the

lithosphere. The configuration of Earth’s surface, including its relief, as well as the position of physical features.

TOPOGRAPHY:

A process whereby the surface of Earth rises, due to either a decrease in downward force or an increase in upward force. UPLIFT:

The breakdown of rocks and minerals at or near the surface of Earth due to physical or chemical processes.

WEATHERING:

called a hot spot, beneath the plates, which move across the hot spot. This is the situation in Hawaii, and it explains why the volcanoes on the southern islands are still active while those to the north are not: the islands themselves are moving north across the hot spot. If two plates converge and one subducts (see Plate Tectonics for an explanation of this process), a deep trench with a

parallel chain of volcanic islands may develop. Exemplified by the Aleutians, these chains are called island arcs.

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I S LA N D E C O S Y S T E M S . The ecosystem, or community of all living organisms, on islands can be unique owing to their separation from continents. The number of life-forms on an island is relatively small and can encompass some

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unusual circumstances compared with the larger ecosystems of continents. Ireland, for instance, has no native snakes, a fact “explained” by the legend that Saint Patrick drove them away. Hawaii and Iceland are also blessedly free of serpents. Oceanic islands, of course, tend to have more unique ecosystems than do continental islands. The number of land-based animal lifeforms is necessarily small, whereas the varieties of birds, flying insects, and surrounding marine life will be greater owing to those creatures’ mobility across water. Vegetation is relatively varied, given the fact that winds, water currents, and birds may carry seeds. Nonetheless, ecosystems of islands tend to be fairly delicate and can be upset by the human introduction of new predators (e.g., dogs) or new creatures to consume plant life (e.g., sheep). These changes sometimes can have disastrous effects on the overall balance of life on islands. Overgrazing may even open up the possibility of erosion, which has the potential of bringing an end to an island’s life.

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WHERE TO LEARN MORE Color Landform Atlas of the United States (Web site). . Erickson, Jon. Rock Formations and Unusual Geologic Structures: Exploring the Earth’s Surface. New York: Facts on File, 1993. Erickson, Jon. Making the Earth: Geologic Forces That Shape Our Planet. New York: Facts on File, 2000. Gerrard, John. Rocks and Landforms. Boston: Unwin Hyman, 1988. Image Gallery of Landforms (Web site). . Selby, Michael John. Earth’s Changing Surface: An Introduction to Geomorphology. New York: Clarendon Press, 1985. Tinkler, K. J. A Short History of Geomorphology. Totowa, NJ: Barnes and Noble, 1985. The Virtual Geomorphology (Web site). . Wells, Lisa. “Images Illustrating Principles of Geomorphology” (Web site). . Zoehfeld, Kathleen Weidner, and James Graham Hale. How Mountains Are Made. New York: HarperCollins, 1995.

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Mountains

CONCEPT Among the most striking of geologic features are mountains, created by several types of tectonic forces, including collisions between continental masses. Mountains have long had an impact on the human psyche, for instance by virtue of their association with the divine in the Greek myths, the Bible, and other religious or cultural traditions. One does not need to be a geologist to know what a mountain is; indeed there is no precise definition of mountain, though in most cases the distinction between a mountain and a hill is fairly obvious. On the other hand, the defining characteristics of a volcano are more apparent. Created by violent tectonic forces, a volcano usually is considered a mountain, and almost certainly is one after it erupts, pouring out molten rock and other substances from deep in the earth.

HOW IT WORKS Plate Tectonics Earth is constantly moving, driven by forces beneath its surface. The interior of Earth itself is divided into three major sections: the crust, mantle, and core. The lithosphere is the upper layer of Earth’s interior, including the crust and the brittle portion at the top of the mantle. Tectonism is the deformation of the lithosphere, and the term tectonics refers to the study of this deformation. Most notable among examples of tectonic deformation is mountain building, or orogenesis, discussed later in this essay.

M O U N TA I N S

ment is responsible for all manner of phenomena, including earthquakes, volcanoes, and mountain building. All these ideas and many more are encompassed in the concept of plate tectonics, which is the name for a branch of geologic and geophysical study and of a dominant principle often described as “the unifying theory of geology” (see Plate Tectonics). CONTENTS UNDER PRESSURE.

Tectonism results from the release and redistribution of energy from Earth’s interior. This energy is either gravitational, and thus a function of the enormous mass at the planet’s core, or thermal, resulting from the heat generated by radioactive decay. Differences in mass and heat within the planet’s interior, known as pressure gradients, result in the deformation of rocks, placing many forms of stress and strain on them. In scientific terms, stress is any attempt to deform an object, and strain is a change in dimension resulting from stress. Rocks experience stress in the form of tension, compression, and shear. Tension acts to stretch a material, whereas compression is a form of stress produced by the action of equal and opposite forces, whose effect is to reduce the length of a material. (Compression is a form of pressure.) Shear results from equal and opposite forces that do not act along the same plane. If a thick, hardbound book is lying flat, and one pushes the front cover from the side so that the covers and pages are no longer in alignment, is an example of shear.

The planet’s crust is not all of one piece: it is composed of numerous plates, which are steadily moving in relation to one another. This move-

Rocks manifest the strain resulting from these stresses by warping, sliding, or breaking. They may even flow, as though they were liquids, or melt and thus truly become liquid. As a result, Earth’s interior may manifest faults, or fractures in rocks, as

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Mountains

MOUNTAINS

SOMETIMES ARISE IN ISOLATION, AS WAS THE CASE WITH

MOUNT KILIMANJARO

IN

TANZANIA. (© T.

Davis/Photo Researchers. Reproduced by permission.)

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well as folds, or bends in the rock structure. The effects can be seen on the surface in the form of subsidence, which is a depression in the crust; or uplift, the raising of crustal materials. Earthquakes and volcanic eruptions also may result.

Orogenesis

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There are two basic types of tectonism: epeirogenesis and orogenesis. The first takes its name from the Greek words epeiros, meaning “main-

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land,” and genesis, or “origin.” Epeirogenesis, which takes the form of either uplift or subsidence, is a chiefly vertical form of movement and plays little role in either plate tectonics or mountain building. Orogenesis, on the other hand, is mountain building, as the prefix oros (“mountain”) shows. Orogenesis involves the formation of mountain ranges by means of folding, faulting, and volcanic activity—lateral movements as opposed to vertical ones. Geologists typically use the term orogenesis, instead of just “mountain building,” when discussing the formation of large belts of mountains from tectonic processes. P LAT E M A R G I N S . Plates may converge (move toward one another), diverge (move away from one another), or experience transform motion, meaning that they slide against one another. Convergence usually is associated with subduction, in which one plate is forced down into the mantle and eventually undergoes partial melting. This typically occurs in the ocean, creating a depression known as an oceanic trench.

There are three types of plate margins, or boundaries between plates, depending on the two types of crusts that interact: oceanic with oceanic, continental with continental, or continental with oceanic. Any of these margins may be involved in mountain formation. Orogenic belts, or mountain belts, typically are situated in subduction zones at convergent plate boundaries and consist of two types. The first type occurs when igneous material (i.e., rock from volcanoes) forms on the upper plate of a subduction zone, causing the surface to rise. This can take place either in the oceanic crust, in which case the mountains formed are called island arcs, or along continental-oceanic margins. The Aleutian Islands are an example of an island arc, while the Andes range represent mountains formed by the subduction of an oceanic plate under a continental one.

between India and Asia that happened some 30 million years ago. (See Plate Tectonics for more about continental drift and collisions between plates.)

Mountains

REAL-LIFE A P P L I C AT I O N S What Is a Mountain? In the 1995 film The Englishman Who Went Up a Hill But Came Down a Mountain, the British actor Hugh Grant plays an English cartographer, or mapmaker, sent in 1917 by his government to measure what is purportedly “the first mountain inside Wales.” He quickly determines that according to standards approved by His Majesty, the “mountain” in question is, in fact, a hill. Much of the film’s plot thereafter revolves around attempts on the part of the villagers to rescue their beloved mountain from denigration as a “hill,” a fate they prevent by piling enough rocks and dirt onto the top to make it meet specifications. This comedy aptly illustrates the somewhat arbitrary standards by which people define mountains. The British naturalist Roderick Peattie (1891–1955), in his 1936 book Mountain Geography, maintained that mountains are distinguished by their impressive appearance, their individuality, and their impact on the human imagination. This sort of qualitative definition, while it is certainly intriguing, is of little value to science; fortunately, however, more quantitative standards exist.

The second type of mountain belt occurs when continental plates converge or collide. When continental plates converge, one plate may “try” to subduct the other, but ultimately the buoyancy of the lower plate (which floats, as it were, on the lithosphere) pushes it upward. The result is the creation of a wide, unusually thick or “tall” belt. An example is the Himalayas, the world’s tallest mountain range, which is still being pushed upward as the result of a collision

In Britain and the United States, a mountain typically is defined as a landform with an elevation of 985 ft. (300 m) above sea level. This was the standard applied in The Englishman, but the Welsh villagers would have had a hard time raising their “hill” to meet the standards used in continental Europe: 2,950 ft. (900 m) above sea level. This seems to be a more useful standard, because the British and American one would take in high plains and other nonmountainous regions of relatively great altitude. On the other hand, there are landforms in Scotland that rise only a few hundred meters above sea level, but their morphologic characteristics or shape seem to qualify them as mountains. Not only are their slopes steep, but the presence of glaciers and snowcapped peaks, with their attendant severe weath-

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er and rocky, inhospitable soil, also seem to indicate the topography associated with mountains.

Mountain Geomorphology One area of the geologic sciences especially concerned with the study of mountains is geomorphology, devoted to the investigation of landforms. Geomorphologists studying mountains must draw on a wide variety of disciplines, including geology, climatology, biology, hydrology, and even anthropology, because, as discussed at the conclusion of this essay, mountains have played a significant role in the shaping of human social groups. From the standpoint of geology and plate tectonics, mountain geomorphology embraces a complex of characteristic formations, not all of which are necessarily present in a given orogen, or mountain. These include forelands and foredeeps along the plains; foreland fold-and-thrust belts, which more or less correspond to “foothills” in layperson’s terminology; and a crystalline core zone, composed of several types of rock, that is the mountain itself. ENVIRONMENTAL ZONES. Mountain geomorphology classifies various environmental zones, from lowest to highest altitude. Near the bottom are flood plains, river terraces, and alluvial fans, all areas heavily affected by rivers flowing from higher elevations. (In fact, many of the world’s greatest rivers flow from mountains, examples being the Himalayan Ganges and Indus rivers in Asia and the Andean Amazon in South America.) Farming villages may be found as high as the 9,845-ft. to 13,125ft. range (3,000–4,000 m), an area known as a submontane, or forested region.

The tree line typically lies at an altitude of 14,765 ft. (4,500 m). Above this point, there is little human activity but plenty of geologic activity, including rock slides, glacial flow, and, at very high altitudes, avalanches. From the tree line upward, the altitude levels that mark a particular region are differentiated for the Arctic and tropical zones, with much lower altitudes in the Arctic mountains. For instance, the tree line lies at about 330 ft. (100 m) in the much colder Arctic zone.

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region, below the tree line, the slope is about 30°, and above the subalpine, in the high alpine, the slope can become as sharp at 65°. In the subalpine, however, it is only about 20°, and because grass (if not trees) grows in this region, it is suited for grazing. It may seem surprising to hear of shepherds bringing sheep to graze at altitudes of 16,400 ft. (5,000 m), as occurs in tropical zones. This does not necessarily mean that people live at such altitudes; more often than not, mountain dwellers have their settlements at lower elevations, and shepherds simply take their flocks up into the heights for grazing. Yet the ancient Bolivian city of Tiahuanaco, which flourished in about A.D. 600—some four centuries before the rise of the Inca—lay at an almost inconceivable altitude of 13,125 ft. (4,000 m), or about 2.5 times the elevation of Denver, Colorado, America’s Mile-High City.

Classifying Mountains There are several ways to classify mountains and groups of mountains. Mountain belts, as described earlier, typically are grouped according to formation process and types of plates: island arcs, continental arcs (formed with the subduction of an oceanic plate by a continental plate), and collisional mountain belts. Sometimes a mountain arises in isolation, an example being Kilimanjaro in Tanzania, Africa. Another example is Stone Mountain outside Atlanta, an exposed pluton, or a mass of crystalline igneous rock that forms deep in Earth’s crust and rises. Many volcanoes, which we discuss later, arise individually, but mountains are most likely to appear in conjunction with other mountains. One such grouping, though far from the only one, is a mountain range, which can be defined as a relatively localized series of peaks and ridges. RANGES, CHAINS, AND MASSES.

Above the tree line is the subalpine, or montane, region. The mean slope angle of the mountain is less steep here than it is at lower or higher elevations: in the submontane, or forested

Some of the world’s most famous mountain ranges include the Himalayas, Karakoram Range, and Pamirs in central Asia; the Alps and Urals in Europe; the Atlas Mountains in Africa; the Andes in South America; and the Cascade Range, Sierra Nevada, Rocky Mountains, and Appalachians as well as their associated ranges in North America. Ranges affiliated with the Appalachians, for instance, include the Great Smokies in the south and the Adirondacks, Alleghenies, and Poconos in the north.

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Several of the examples given here illustrate the fact that ranges are not the largest groupings of mountains. Sometimes series of ranges stretch across a continent for great distances in what are called mountain chains, an example of which is the Mediterranean chain of Balkans, Apennines, and Pyrenees that stretches across southern Europe.

Mountains

There also may be irregular groupings of mountains, which lack the broad linear sweep of mountain ranges or chains and which are known as mountain masses. The mountains surrounding the Tibetan plateau represent an example of a mountain mass. Finally, ranges, chains, and masses of mountains may be combined to form vast mountain systems. An impressive example is the Alpine-Himalayan system, which unites parts of the Eurasian, Arabian, African, and Indo-Australian continental plates. O T H E R T Y P E S O F M O U N TA I N .

There are certain special types of orogeny, as when ocean crust subducts continental crust— something that is not supposed to happen but occasionally does. This rare variety of subduction is called obduction, and the mountains produced are called ophiolites. Examples include the uplands near Troodos in Cyprus and the Taconic Mountains in upstate New York. Fault-block mountains appear when two continental masses push against each other and the upper portion of a continental plate splits from the deeper rocks. A portion of the upper crust, usually several miles thick, begins to move slowly across the continent. Ultimately it runs into another mass, creating a ramp. This can result in unusually singular mountains, such as Chief Mountain in Montana, which slid across open prairie on a thrust sheet.

THE POPOCATEPETL VOLCANO ERUPTS, SPEWING ASH, ROCKS, AND GASES. (© Wesley Bocxe/Photo Researchers. Reproduced by permission.)

however, a volcano is not necessarily a mountain. A volcano may be defined as a natural opening in Earth’s surface through which molten (liquid), solid, and gaseous material erupts. The word volcano also is used to describe the cone of erupted material that builds up around the opening or fissure. Because these cones are often quite impressive in height, they frequently are associated with mountains.

Most volcanoes are mountains, and for this reason, it is appropriate to discuss them together;

Though volcanic activity has been the case of death and destruction, it is essential to the planet’s survival. Volcanic activity is the principal process through which chemical elements, minerals, and other compounds from Earth’s interior reach its surface. These substances, such as carbon dioxide, have played a major role in the development of the planet’s atmosphere, waters, and soils. Even today, soil in volcanic areas is among the richest on Earth. Volcanoes provide additional benefits in their release of geothermal energy, used for heating and other purposes in such countries as Iceland, Italy, Hungary, and New Zealand (see Energy and Earth). In addition, volcanic activity beneath the oceans promises to supply almost limitless geothermal energy,

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Under the ocean is the longest mountain chain on Earth, the mid-ocean ridge system, which runs down the center of the Atlantic Ocean and continues through the Indian and Pacific oceans. Lava continuously erupts along this ridge, releasing geothermal energy and opening up new strips of ocean floor. This brings us to a special kind of mountain, typically resulting from the sort of dramatic plate tectonic processes that also produce earthquakes: volcanoes.

Volcanoes

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once the technology for its extraction becomes available. F O R M AT I O N O F V O L CA N O E S .

As noted earlier, land volcanoes are formed in coastal areas where continental and oceanic plates converge. As the oceanic plate is subducted and pushed farther and farther beneath the continental surface, the buildup of heat and pressure results in the melting of rock. This molten rock, or magma, tends to rise toward the surface and collect in magma reservoirs. Pressure buildup in the magma reservoir ultimately pushes the magma upward through cracks in Earth’s crust, creating a volcano. Volcanoes also form underwater, in which case they are called seamounts. Convergence of oceanic plates causes one plate to sink beneath the other, creating an oceanic trench; as a result, magma rises from the subducted plate to fashion volcanoes. If the plates diverge, magma seeps upward at the ridge or margin between plates, producing more seafloor. This process, known as seafloor spreading, leads to the creation of volcanoes on either side of the ridge. In some places a plate slides over a stationary area of volcanic activity, known as a hot spot. These are extremely hot plumes of magma that well up from the crust, though not on the edge or margin of a plate. A tectonic plate simply drifts across the hot spot, and as it does, the area just above the hot spot experiences volcanic activity. Hot spots exist in Hawaii, Iceland, Samoa, Bermuda, and America’s Yellowstone National Park. CLASSIFYING

VOLCANOES.

Volcanoes can be classified in terms of their volcanic activity, in which case they are labeled as active (currently erupting), dormant (not currently erupting but likely to do so in the future), or extinct. In the case of an extinct volcano, no eruption has been noted in recorded history, and it is likely that the volcano has ceased to erupt permanently. In terms of shape, volcanoes fall into four categories: cinder cones, composite cones, shield volcanoes, and lava domes. These types are distinguished not only by morphologic characteristics but also by typical sizes and even angles of slope. For instance, cinder cones, built of lava fragments, have slopes of 30° to 40°, and are seldom more than 1,640 ft. (500 m) in height.

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Composite cones, or stratovolcanoes, are made up of alternating layers of lava (cooled magma), ash, and rock. (The prefix strato refers to these layers.) They may slope as little as 5° at the base and as much as 30° at the summit. Stratovolcanoes may grow to be as tall as 2–3 mi. (3.2–4.8 km) before collapsing and are characterized by a sharp, dramatic shape. Examples include Fuji, a revered mountain that often serves as a symbol of Japan, and Washington state’s Mount Saint Helens. A shield volcano, which may be a solitary formation and often is located over a hot spot, is built from lava flows that pile one on top of another. With a slope as little as 2° at the base and no more than 10° at the summit, shield volcanoes are much wider than stratovolcanoes, but sometimes they can be impressively tall. Such is the case with Mauna Loa in Hawaii, which at 13,680 ft. (4,170 m) above sea level is the world’s largest active volcano. Likewise, Mount Kilimanjaro, though long ago gone dormant, is the tallest mountain in Africa. Finally, there are lava domes, which are made of solid lava that has been pushed upward. Closely related is a volcanic neck, which often forms from a cinder cone. In the case of a volcanic neck, lava rises and erupts, leaving a mountain that looks like a giant gravel heap. Once it has become extinct, the lava inside the volcano begins to solidify. Over time the rock on the exterior wears away, leaving only a vent filled with solidified lava, usually in a funnel shape. A dramatic example of this appears at Shiprock, New Mexico. Volcanoes frequently are classified by the different ways in which they erupt. These types of eruption, in turn, result from differences in the material being disgorged from the volcano. When the magma is low in gas and silica (silicon dioxide, found in sand and rocks), the volcano erupts in a relatively gentle way. Its lava is thin and spreads quickly. Gas and silica–rich magma, on the other hand, brings about a violent explosion that yields tarlike magma. V O L CA N I C

ERUPTIONS.

There are four basic forms or phases of volcanic eruption: Hawaiian, Strombolian, Vulcanian, and Peleean. The Hawaiian phase is simply a fountain-like gush of runny lava, without any explosions. The Strombolian phase (named after a volcano on a small island off the Italian penin-

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CRATER LAKE IN OREGON IS THE RESULT OF THE COLLAPSE OF A MAGMA CHAMBER AFTER A VOLCANIC ERUPTION. THIS COLLAPSE FORMS A BOWL-LIKE CRATER CALLED A CALDERA, which fills with water. (© Francois Gohier/Photo Researchers. Reproduced by permission.)

sula) involves thick lava and mild explosions. In a Vulcanian phase, magma has blocked the volcanic vent, and only after an explosion is the magma released, with the result that tons of solid material and gases are hurled into the sky. Most violent of all is the Peleean, named after Mount Pelée on Martinique in the Caribbean (discussed later). In the Peleean phase, the volcano disgorges thick lava, clouds of gas, and fine ash, all at formidable velocities.

form) may fill with water, as was the case at Oregon’s Crater Lake. I N FA M O U S V O L CA N I C D I SAS T E R S . Volcanoes result from some of the

same tectonic forces as earthquakes (see Seismology), and, not surprisingly, they often have resulted in enormous death and destruction. Some remarkable examples include:

Not surprisingly, the eruption of a volcano completely changes the morphologic characteristics of the landform. During the eruption a crater is formed, and out of this flows magma and ash, which cool to form the cone. In some cases, the magma chamber collapses just after the eruption, forming a caldera, or a large, bowl-shaped crater. These caldera (the plural as well as singular

• Vesuvius, Italy, A.D. 79 and 1631: Situated along the Bay of Naples in southern Italy, Vesuvius has erupted more than 50 times during the past two millennia. Its most famous eruption occurred in A.D. 79, when the Roman Empire was near the height of its power. The first-century eruption buried the nearby towns of Pompeii and Herculaneum, where bodies and buildings were preserved virtually intact until excavation of the area in 1748. Another eruption, in 1631, killed some 4,000 people. • Krakatau, Indonesia, A.D. 535 (?) and 1883: The most famous eruption of Krakatau occurred in 1883, resulting in the loss of some 36,000 lives. The explosion, which was heard 3,000 mi. away, threw 70-lb. (32-kg) boulders as far as 50 mi. (80 km). It also produced a tsunami, or tidal wave, 130 ft.

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Accompanying a volcanic eruption in many cases are fierce rains, the result of the expulsion of steam from the volcano, after which the steam condenses in the atmosphere to form clouds. Gases thrown into the atmosphere are often volatile and may include hydrogen sulfide, fluorine, carbon dioxide, and radon. All are detrimental to human beings when present in sufficient quantity, and radon is radioactive.

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•

•

•

•

(40 m) high, which swept away whole villages. In addition, the blast hurled so much dust into the atmosphere that the Moon appeared blue or green for two years. It is also possible that Krakatau erupted in about A.D. 535, causing such a change in the atmosphere that wide areas of the world experienced years without summer. (See Earth Systems for more on this subject.) Tambora, Indonesia, 1815: Another Indonesian volcano, Tambora, killed 12,000 people when it erupted in 1815. As with Krakatau in 535, this eruption was responsible for a year without summer in 1816 (see Earth Systems). Pelée, Martinique, 1902: When Mount Pelée erupted on the Caribbean island of Martinique, it sent tons of poisonous gas and hot ash spilling over the town of SaintPierre, killing all but four of its 29,937 residents. Saint Helens, Washington, 1980: Relatively small compared with earlier volcanoes, the Mount Saint Helens blast is still significant because it was so recent and took place in the United States. The eruption sent debris flying upward 1,300 ft. (396 m) and caused darkness over towns as far as 85 mi. (137 km) away. Fifty-seven people died in the eruption and its aftermath. Pinatubo, Philippines, 1991: Dormant for 600 years, Mount Pinatubo began to rumble one day in 1991 and, after a few days, erupted in a cloud that spread ash 6 ft. (1.83 m) deep along a radius of 2 mi. (3.2 km). A U.S. air base 15 mi. (24 km) away was buried. The blast threw 20 million tons (18,144,000 metric tons) of sulfuric acid 12 mi. (19 km) into the stratosphere, and the cloud ultimately covered the entire planet, resulting in moderate cooling for a few weeks.

The Impact of Mountains Volcanic eruptions are among the most dramatic effects produced by mountains, but they are far from the only ones. Every bit as fascinating are the effects mountains produce on the weather, on the evolution of species, and on human society. In each case, mountains serve as a barrier or separator—between masses of air, clouds, and populations.

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Wind pushes air and moisture-filled clouds up mountain slopes, and as the altitude increases, the pressure decreases. As a result, masses of warm, moist air become larger, cooler, and less dense. This phenomenon is known as adiabatic expansion, and it is the same thing that happens when an aerosol can is shaken, reducing the pressure of gases inside and cooling the surface of the can. Under the relatively high-pressure and hightemperature conditions of the flatlands, water exists as a gas, but in the heights of the mountaintops, it cools and condenses, forming clouds. RA I N S H A D O W S . As the clouds rise along the side of the mountain, they begin to release heavy droplets in the form of rain and, at higher altitudes, snow. By the time the cloud crosses the top of the mountain, however, it will have released most of its moisture, and hence the other side of the mountain may be arid. The leeward side, or the side opposite the wind, becomes what is called a rain shadow.

Although they are only 282 mi. (454 km) apart, the cities of Seattle and Spokane, Washington, have radically different weather patterns. Famous for its almost constant rain, Seattle lies on the windward, or wind-facing, side of the Cascade Range, toward the Pacific Ocean. On the leeward side of the Cascades is Spokane, where the weather is typically warm and dry. Though it is only on the other side of the state, Spokane might as well be on the other side of the continent. Indeed, it is associated more closely with the arid expanses of Idaho, whereas Seattle belongs to a stretch of cold, wet Pacific terrain that includes San Francisco and Portland, Oregon. Much of the western United States consists of deserts formed by rain shadows or, in some cases, double rain shadows. Much of New Mexico, for instance, lies in a double rain shadow created by the Rockies in the west and Mexico’s Sierra Madres to the south. In southern California, tall redwoods line the lush windward side of the Sierra Nevadas, while Death Valley and the rest of the Mojave Desert lies in the rain shadow on the eastern side. The Great Basin that covers eastern Oregon, southern Idaho, much of Utah, and almost all of Nevada, likewise is created by the rain shadow of the Sierra Nevada-Cascade chain. M O U N TA I N S

AND

SPECIES.

One of the most intriguing subjects involved in the study of mountains is their effects on large

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KEY TERMS ACTIVE:

A term to describe a volcano

feature, such as a mountain, plateau, or

that is currently erupting. A form of stress

COMPRESSION:

A notable topographical

LANDFORM:

valley. The upper layer of

produced by the action of equal and oppo-

LITHOSPHERE:

site forces, the effect of which is to reduce

Earth’s interior, including the crust and the

the length of a material. Compression is a

brittle portion at the top of the mantle.

form of pressure.

MANTLE:

CONVERGENCE:

rock, approximately 1,429 mi. (2,300 km)

A tectonic process

whereby plates move toward each other.

The thick, dense layer of

thick, between Earth’s crust and its core. Structure or form or

The uppermost division of the

MORPHOLOGY:

solid earth, representing less than 1% of its

the study thereof.

volume and varying in depth from 3 mi. to

MOUNTAIN CHAIN:

37 mi. (5–60 km). Below the crust is the

stretching across a continent for a great

mantle.

distance.

CRUST:

DIVERGENCE:

A tectonic process

whereby plates move away from each other. DORMANT:

A term to describe a vol-

MOUNTAIN

A series of ranges

An irregular

MASS:

grouping of mountains, which lacks the broad linear sweep of a range or chain.

cano that is not currently erupting but is

MOUNTAIN

likely to do so in the future.

localized series of peaks and ridges.

EPEIROGENESIS:

One of two prin-

RANGE:

MOUNTAIN SYSTEM:

A relatively A combination

cipal forms of tectonism, the other being

of ranges, chains, and masses of mountains

orogenesis. Derived from the Greek words

that stretches across vast distances, usually

epeiros (“mainland”) and genesis (“ori-

encompassing more than one continent.

gins”), epeirogenesis takes the form of either uplift or subsidence.

OROS:

A Greek word meaning “moun-

tain,” which appears in such words as

A term to describe a volcano

orogeny, a variant of orogenesis; orogen,

for which no eruption has been known in

another term for “mountain” and orogenic,

recorded history. In this case, it is likely

as in “orogenic belt.”

EXTINCT:

that the volcano has ceased to erupt permanently.

OROGENESIS:

One of two principal

forms of tectonism, the other being epeiroAn area of phys-

genesis. Derived from the Greek words oros

ical geology concerned with the study of

(“mountain”) and genesis (“origin”), oro-

landforms, with the forces and processes

genesis involves the formation of moun-

that have shaped them, and with the

tain ranges by means of folding, faulting,

description and classification of various

and volcanic activity. The processes of oro-

physical features on Earth.

genesis play a major role in plate tectonics.

GEOMORPHOLOGY:

HOT SPOT:

A region of high volcanic

activity.

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PLATE

MARGINS:

Boundaries be-

tween plates.

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KEY TERMS

CONTINUED

The name both

part of air or solid earth, or, in the case of

of a theory and of a specialization of tec-

solid earth, to the resulting formation.

tonics. As an area of study, plate tectonics

Subsidence thus is defined variously as the

deals with the large features of the litho-

downward movement of air, the sinking of

sphere and the forces that shape them. As a

ground, or a depression in Earth’s crust.

PLATE TECTONICS:

theory, it explains the processes that have shaped Earth in terms of plates and their movement. PLATES:

notably mountain building. The deformation of the

TECTONISM:

lithosphere.

A form of stress resulting

from equal and opposite forces that do not act along the same line. If a thick, hard-

A form of stress produced

TENSION:

by a force that acts to stretch a material. The configuration of

bound book is lying flat, and one pushes

TOPOGRAPHY:

the front cover from the side so that the

Earth’s surface, including its relief as well as

covers and pages are no longer aligned, this

the position of physical features.

is an example of shear.

UPLIFT:

A process whereby the surface

The ratio between the change

of Earth rises, as the result of either a

in dimension experienced by an object that

decrease in downward force or an increase

has been subjected to stress and the origi-

in upward force.

STRAIN:

nal dimensions of the object.

VOLCANO:

A natural opening in

In general terms, any attempt

Earth’s surface through which molten (liq-

to deform a solid. Types of stress include

uid), solid, and gaseous material erupts.

tension, compression, and shear.

The word volcano is also used to describe

STRESS:

SUBSIDENCE:

A term that refers

either to the process of subsiding, on the

groups of plants, animals, and humans. Mountains may separate entire species, creating pockets of flora and fauna virtually unknown to the rest of the world. Thus, during the 1990s, huge numbers of species that had never been catalogued were discovered in the mountains of southeast Asia. The formation of mountains and other landforms may even lead to speciation, a phenomenon in which members of a species become incapable of reproducing with other members, thus creating a new species. When the Colorado River cut open the Grand Canyon, it separated

262

including its causes and effects, most

Large, movable segments of

the lithosphere. SHEAR:

The study of tectonism,

TECTONICS:

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the cone of erupted material that builds up around the opening or fissure.

groups of squirrels that lived in the high-altitude pine forest. Over time these populations ceased to interbreed, and today the Kaibab squirrel of the north rim and the Abert squirrel of the south are separate species, no more capable of interbreeding than humans and apes. HUMAN SOCIETIES AND M O U N TA I N S . Although the Appalachians

of the eastern United States are hundreds of millions of years old, most ranges are much younger. Most will erode or otherwise cease to exist in a relatively short time (short, that is, by geologic standards), yet to humans throughout the ages,

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mountains have seemed a symbol of permanence. This is just one aspect of mountains’ impact on the human psyche. In his 1975 study of symbolism in political movements, Utopia and Revolution, Melvin J. Lasky devoted considerable space to the mountain and its association with divinity through figures such as the Greek Olympians and Noah and Moses in the Bible. Clearly, mountains have proved enormously influential on human attitudes, and nowhere is this more obvious than in relation to the people who live in the mountains. Whether the person is a coal miner from Appalachia or a rancher from the Rockies, a Scottish highlander or a Quechua-speaking Peruvian, the mentality is similar, characterized by a combination of hardiness, fierce independence, and disdain for lowland ways. These characteristics, combined with the harsh weather of the mountains, have made mountain warfare a challenge to lowland invaders. This explains the fact that Switzerland has kept itself free from involvement in European wars since Napoleon’s time, and why the independent Scottish Highlands were long a thorn in England’s side. It also explains why neither the British nor the Russian empires could manage to control Afghanistan fully during their struggle over that mountainous nation in the late nineteenth and early twentieth centuries. Britain eventually pulled out of the “Great Game,” as this struggle was called, but Russia never really did. Many years later, the Soviets became bogged down in a war in Afghanistan that they could not win. The war, which lasted from 1979 to 1989, helped bring about the end of the Soviet Union and its system of satellite dictatorships. More than a decade later, as the United States launched strikes against Afghanistan in 2001, a superpower once again faced the chal-

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lenge posed by one of the poorest, most inhospitable nations on Earth.

Mountains

But the independence of the mountaineer is deceptive; in fact, mountains have little to offer, economically, other than their beauty and the resources deep beneath their surfaces. In other words, they are really of value only to flatland tourists and mining companies. Since few mountain environments offer much promise agriculturally, the people of the mountains are dependent on the flatlands for sustenance. Gorgeous and rugged as they are, such mountainous states as Colorado or Wyoming might be as poor as Afghanistan were it not for the fact that they belong to a larger political unit, the United States. WHERE TO LEARN MORE A Geological History of Rib Mountain, Wisconsin (Web site). . Gore, Pamela. “Physical Geology at Georgia Perimeter College” (Web site). . Kraulis, J. A., and John Gault. The Rocky Mountains: Crest of a Continent. New York: Facts on File, 1987. Michigan Technological University Volcanoes Page (Web site). . Oregon Geology—Cascade Mountains (Web site). . Prager, Ellen J., Kate Hutton, Costas Synolakis, et al. Furious Earth: The Science and Nature of Earthquakes, Volcanoes, and Tsunamis. New York: McGraw-Hill, 2000. Schaer, Jean-Paul, and John Rodgers. The Anatomy of Mountain Ranges. Princeton, NJ: Princeton University Press, 1987. Sigurdsson, Haraldur. Encyclopedia of Volcanoes. San Diego: Academic Press, 2000. Silver, Donald M., and Patricia Wynne. Earth: The EverChanging Planet. New York: Random House, 1989. Volcanoes, Glaciers, and Plate Tectonics: The Geology of the Mono Basin (Web site). .

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EROSION Erosion

CONCEPT Erosion is a broadly defined group of processes involving the movement of soil and rock. This movement is often the result of flowing agents, whether wind, water, or ice, which sometimes behaves like a fluid in the large mass of a glacier. Gravitational pull may also influence erosion. Thus, erosion, as a concept in the earth sciences, overlaps with mass wasting or mass movement, the transfer of earth material down slopes as a result of gravitational force. Even more closely related to erosion is weathering, the breakdown of rocks and minerals at or near the surface of Earth owing to physical, chemical, or biological processes. Some definitions of erosion even include weathering as an erosive process. Though most widely known as a by-product of irresponsible land use by humans and for its negative effect on landforms, erosion is neither unnatural nor without benefit. Far more erosion occurs naturally than as a result of land development, and a combination of weathering and erosion is responsible for producing the soil from which Earth’s plants grow.

Weathering, as the term is used in the geologic sciences, refers to these and other types of physical and chemical changes in rocks and minerals at or near the surface of Earth. A mineral is a substance that occurs naturally and is usually inorganic, meaning that it contains carbon in a form other than that of an oxide or a carbonate, neither of which is considered organic. It typically has a crystalline structure, or one in which the constituent parts have a simple and definite geometric arrangement repeated in all directions. Rocks are simply aggregates or combinations of minerals or organic material or both. TWO AND ONE-HALF KINDS O F W E AT H E R I N G . There are three kinds

The first step in the process of erosion is weathering. Weathering, in a general sense, occurs everywhere: paint peels; metal oxidizes, resulting in its tarnishing or rusting; and any number of products, from shoes to houses, begin to show the effects of physical wear and tear. The scuffing of a shoe, cracks in a sidewalk, or the chipping of glass in a gravel-spattered windshield are all examples of physical weathering. On the other

of weathering (or perhaps two and one-half, since the third incorporates aspects of the first two): physical or mechanical, chemical, and biological. Physical or mechanical weathering takes place as a result of such factors as gravity, friction, temperature, and moisture. Gravity may cause a rock to drop from a height, such that it falls to the ground and breaks into pieces, while the friction of wind-borne sand may wear down a rock surface. Changes in temperature and moisture cause expansion and contraction of materials, as when water seeps into a crack in a rock and then freezes, expanding and splitting the rock.

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HOW IT WORKS Weathering

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hand, the peeling of paint is usually the result of chemical changes, which have reduced the adhesive quality of the paint. Certainly oxidation is a chemical change, meaning that it has not simply altered the external properties of the item but also has brought about a change in the way that the atoms are bonded.

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Erosion

NICOLA RIVER CANYON

IN

BRITISH COLUMBIA

SHOWS THE EFFECTS OF FREEZE-THAW AND EROSION BY WIND AND

RAIN. (© K. Svensson/Photo Researchers. Reproduced by permission.)

Minerals are chemical compounds; thus, whereas physical weathering attacks the rock as a whole, chemical weathering effects the breakdown of the minerals that make up the rock. This breakdown may lead to the dissolution of the minerals, which then are washed away by water or wind or both, or it may be merely a matter of breaking the minerals down into simpler compounds. Reactions that play a part in this breakdown may include oxidation, mentioned earlier, as well as carbonation, hydrolysis (a reaction with water that results in the separation of a compound to form a new substance or substances), and acid reactions. For instance, if coal has been burned in an area, sulfur impurities in the air react with water vapor (an example of hydrolysis) to produce acid rain, which can eat away at rocks. Rainwater itself is a weak acid, and over the years it slowly dissolves the marble of headstones in old cemeteries. As noted earlier, there are either three or two and one-half kinds of weathering, depending on whether one considers biological weathering a third variety or merely a subset of physical and chemical weathering. The weathering exerted by organisms (usually plants rather than animals) on rocks and minerals is indeed chemical and physical, but because of the special circum-

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stances, it is useful to consider it individually. There is likely to be a long-term interaction between the organism and the geologic item, an obvious example being a piece of moss that grows on a rock. Over time, the moss will influence both physical and chemical weathering through its attendant moisture as well as its specific chemical properties, which induce decomposition of the rock’s minerals.

Unconsolidated Material The product of weathering in rocks or minerals is unconsolidated, meaning that it is in pieces, like gravel, though much less uniform in size. This is called regolith, a general term that describes a layer of weathered material that rests atop bedrock. Sand and soil, including soil mixed with loose rocks, are examples of regolith. Regolith is, in turn, a type of sediment, material deposited at or near Earth’s surface from a number of sources, most notably preexisting rock. Every variety of unconsolidated material has its own angle of repose, or the maximum angle at which it can remain standing. Piles of rocks may have an angle of repose as high as 45°, whereas dry sand has an angle of only 34°. The addition of water can increase the angle of repose, as anyone who has ever strengthened a sand castle by

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adding water to it knows. Suppose one builds a sand castle in the morning, sloping the sand at angles that would be impossible if it were dry. By afternoon, as wind and sunlight dry out the sand, the sand castle begins to fall apart, because its angle of repose is too high for the dry sand. Water gives sand surface tension, the same property that causes water that has been spilled on a table to bead up rather than lie flat. If too much water is added to the sand, however, the sand becomes saturated and will flow, a process called lateral spreading. On the other hand, with too little moisture, the material is susceptible to erosion. Unconsolidated material in nature generally has a slope less than its angle of repose, owing to the influence of wind and other erosive forces.

Introduction to Mass Wasting There are three general processes whereby a piece of earth material can be moved from a high outcropping to the sea: weathering, mass wasting, and erosion. In the present context, we are concerned primarily with the last of these processes, of course, and secondarily with weathering, inasmuch as it contributes to erosion. A few words should be said about mass wasting, however, which, in its slower forms (most notably, creep), is related closely to erosion. Mechanical or chemical processes, or a combination of the two, acting on a rock to dislodge it from a larger sample (e.g., separating a rock from a boulder) is an example of weathering, as we have seen. If the pieces of rock are swept away by a river in a valley below the outcropping, or if small pieces of rock are worn away by high winds, the process is erosion. Between the outcropping and the river below, if a rock has been broken apart by weathering, it may be moved farther along by mass-wasting processes, such as creep or fall.

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unstable equilibrium, like a soda can lying on its side rather than perpendicular to a table’s surface: in both cases, the object remains in place, yet a relatively small disturbance would be enough to dislodge it. Changes in temperature or moisture are among the leading factors that result in creep. A variation in either can cause material to expand or contract, and freezing or thawing may be enough to shake regolith from its position of unstable equilibrium. Water also can provide lubrication, or additional weight, that assists the material in moving. Though it is slow, over time creep can produce some of the most dramatic results of any mass-wasting process. It can curve tree trunks at the base, break or dislodge retaining walls, and overturn objects ranging from fence posts to utility poles to tombstones. O T H E R VA R I E T I E S O F F LO W.

Creep is related to another slow mass-wasting process, known as solifluction, that occurs in the active layer of permafrost—that is, the layer that thaws in the summertime. The principal difference between creep and solifluction is not the speed at which they take place (neither moves any faster than about 0.5 in. [1 cm] per year) but the materials involved. Both are examples of flow, a chaotic form of mass wasting in which masses of material that are not uniform move downslope. With the exception of creep and solifluction, most forms of flow are comparatively rapid, and some are extremely so. Because it involves mostly dry material, creep is an example of granular flow, which is composed of 0% to 20% water; on the other hand, solifluction, because of the ice component, is an instance of slurry flow, consisting of 20% to 40% water. If the water content is more than 40%, a slurry flow is considered a stream. Types of granular flow that move faster than creep range from earth flow to debris avalanche. Both earth flow and debris flow, its equivalent in slurry form, move at a broad range of speeds, anywhere from about 4 in. (10 cm) per year to 0.6 mi. (1 km) per hour. Grain flow can be as fast as 60 mi. (100 km) per hour, and mud flow is even faster. Fastest of all is debris avalanche, which may achieve speeds of 250 mi. (400 km) per hour.

One of the principal sources of erosion is gravity, which is also the force behind creep, the slow downward movement of regolith along a hill slope. The regolith begins in a condition of

slump, slide, and fall. Slump occurs when a mass of regolith slides over or creates a concave surface

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O T H E R T Y P E S O F M ASS WAS T I N G . Other varieties of mass wasting include

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ROCK

ARCHES FORMED BY EROSION. (© N. R. Rowan/Photo Researchers. Reproduced by permission.)

(one shaped like the inside of a bowl.) The result is the formation of a small, crescent-shaped cliff, known as a scarp, at the upper end—rather like the crest of a wave. Slump often is classified as a variety of slide, in which material moves downhill in a fairly coherent mass (i.e., more or less in

a section or group) along a flat or planar surface. These movements are sometimes called rock slides, debris slides, or, in common parlance, landslides.

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In contrast to most other forms of mass wasting, in which there is movement along slopes

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that are considerably less than 90°, fall occurs at angles almost perpendicular to the ground. The “Watch for Falling Rock” signs on mountain roads may be frightening, and rock or debris fall is certainly one of the more dramatic forms of mass wasting. Yet the variety of mass wasting that has the most widespread effects on the morphology or shape of landforms is the slowest one— creep. (For more about the varieties of mass wasting, see Mass Wasting.)

What Causes Erosion? As noted earlier, the influences behind erosion are typically either gravity or flowing media: water, wind, and even ice in glaciers. Liquid water is the substance perhaps most readily associated with erosion. Given enough time, water can wear away just about anything, as proved by the carving of the Grand Canyon by the Colorado River. Dubbed the universal solvent for its ability to dissolve other materials, water almost never appears in its pure form, because it is so likely to contain other substances. Even “pure” mountain water contains minerals and pieces of the rocks over which it has flowed, a testament to the power of water in etching out landforms bit by bit. Nor does it take a rushing mountain stream or crashing waves to bring about erosion; even a steady drip of water is enough to wear away granite over time. M O V I N G W AT E R . Along coasts, pounding waves continually alter the shoreline. The sheer force of those walls of water, a result of the Moon’s gravitational pull (and, to a lesser extent, the Sun’s), is enough to wear away cliffs, let alone beaches. In addition, waves carry pieces of pebble, stone, and sand that cause weathering in rocks. Waves even can bring about small explosions in pockmarked rock surfaces by trapping air in small cracks; eventually the pressure becomes great enough that the air escapes, loosening pieces of the rock.

In addition to the erosive power of saltwater waves on the shore, there is the force exerted by running water in creeks, streams, and rivers. As the river moves, pushing along sediment and other materials eroded from the streambed or riverbed, it carves out deep chasms in the bedrock beneath. These moving bodies of water continually reshape the land, carrying soil and debris downslope, or from the source of the river to its mouth or delta. A delta is a region of sedi-

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ment formed when a river enters a larger body of water, at which point the reduction in velocity on the part of the river current leads to the widespread deposition (depositing) of sediment. It is so named because its triangular shape resembles that of the Greek letter delta, ∆. Water at the bottom of a large body, such as a pond or lake, also exerts erosive power. Then there is the influence of falling rain. Assuming ground is not protected by vegetation, raindrops can loosen particles of soil, sending them scattering in all directions. A rain that is heavy enough may dislodge whole layers of topsoil and send them rushing away in a swiftly moving current. The land left behind may be rutted and scarred, much of its best soil lost for good. Just as erosion gives to the soil, it also can take away. Whereas erosion on the Nile delta acted to move rich, black soil into the region (hence, the ancient Egyptians’ nickname for their country, the “black land”), erosion also can remove soil layers. As is often the case, it is much easier to destroy than to create: 1 in. (2.5 cm) of soil may take as long as 500 years to form, yet a single powerful rainstorm or windstorm can sweep it away.

Glaciers Ice, of course, is simply another form of water, but since it is solid, its physical (not its chemical) properties are quite different. Generally, physical sciences, such as physics or chemistry, treat as fluid all forms of matter that flow, whether they are liquid or gas. Normally, no solids are grouped under the heading of “fluid,” but in the earth sciences there is at least one type of solid object that behaves as though it were fluid: a glacier. A glacier is a large, typically moving mass of ice either on or adjacent to a land surface. It does not flow in the same way that water does; rather, it is moved by gravity, as a consequence of its extraordinary weight. Under certain conditions, a glacier may have a layer of melted water surrounding it, which greatly enhances it mobility. Regardless of whether it has this lubricant, however, a glacier steadily moves forward, carrying pieces of rock, soil, and vegetation with it. These great rivers of ice gouge out pieces of bedrock from mountain slopes, fashioning deep valleys. Ice along the bottom of the glacier pulls away rocks and soil, which assist it in wearing

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A “DUST

DEVIL,” OR A SMALL WHIRLWIND, CARRIES WITH IT DEBRIS AND SAND. (© Clem Haagner/Photo Researchers. Repro-

duced by permission.)

away bedrock. The fjords of Norway, where high cliffs surround narrow inlets whose depths extend many thousands of feet below sea level, are a testament to the power of glaciers in shaping the Earth. The fact that the fjords came into existence only in the past two million years, a product of glacial activity associated with the last ice age, is evidence of something else remarkable about glaciers: their speed.

fjords, but also the active Franz Josef glacier in southern New Zealand. By contrast, in the dry, exceptionally cold, inland climate of Antarctica, the Meserve glacier moves at the rate of just 9.8 ft. (3 m) per year.

Wind

“Speed,” of course, is a relative term when speaking about processes involved in the shaping of the planet. A “fast” glacier, one whose movement is assisted by a wet and warm (again, relatively warm!) maritime climate, moves at the rate of about 980 ft. (300 m) per year. Examples include not only the glaciers that shaped the

The erosion produced by wind often is referred to as an eolian process, the name being a reference to Aeolus, the Greek god of the winds encountered in Homer’s Odyssey and elsewhere. Eolian processes include the erosion, transport, and deposition of earth material owing to the action of wind. It is most pronounced in areas that lack effective ground cover in the form of solidly rooted, prevalent vegetation.

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Eolian erosion in some ways is less forceful than the erosive influence of water. Water, after all, can lift heavier and larger particles than can the winds. Wind, however, has a much greater frictional component in certain situations. This is particularly true when the wind carries sand, every grain of which is like a cutting tool. In some desert regions the bases of rocks or cliffs have been sandblasted, leaving a mushroomshaped formation. The wind could not lift the fine grains of sand very high, but in places where it has been able to do its work, it has left an indelible mark.

The Dust Bowl and Human Contribution to Erosion Though human actions are not a direct cause of erosion, human negligence or mismanagement often has prepared the way for erosive action by wind, water, or other agents. Interesting, soil itself, formed primarily by chemical weathering and enhanced by biological activity in the sediment, is a product of nature’s erosive powers. Erosion transports materials from one place to another, robbing the soil in one place and greatly enhancing it in another. This is particularly the case where river deltas are concerned. By transporting sediment and depositing it in the delta, the river creates an area of extremely fertile soil that, in some cases, has become literally the basis for civilizations. The earliest civilizations of the Western world, in Egypt and Sumer, arose in the deltas of the Nile and the Tigris-Euphrates river systems, respectively. EROSION ON THE G R E AT P LA I N S . An extreme example of the negative

effects on the soil that can come from erosion (and, ultimately, from human mismanagement) took place in Texas, Oklahoma, Colorado, and Kansas during the 1930s. In the preceding years, farmers unwittingly had prepared the way for vast erosion by overcultivating the land and not taking proper steps to preserve its moisture against drought. In some places farmers alternated between wheat cultivation and livestock grazing on particular plots of land. The soil, already weakened by raising wheat, was damaged further by the hooves of livestock, and thus when a period of high winds began at the height of the Great Depression (1929–41), the land was particularly vulnerable. The winds

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carried dust to places as far away as the eastern seaboard, in some cases removing topsoil to a depth of 3–4 in. (7–10 cm). Dunes of dust as tall as 15–20 ft. (4.6–6.1 m) formed, and the economic blight of the Depression was compounded for the farmers of the plains states, many of whom lost everything. Out of the Dust Bowl era came some of the greatest American works of art: the 1939 film Wizard of Oz, John Steinbeck’s book The Grapes of Wrath and the acclaimed motion picture (1939 and 1940, respectively), as well as Dorothea Lange’s haunting photographs of Dust Bowl victims. The Dust Bowl years also taught farmers and agricultural officials a lesson about land use, and in later years farming practices changed. Instead of alternating one year of wheat growing with one year in which a field lay fallow, or unused, farmers discovered that a wheatsorghum-fallow cycle worked better. They also enacted other measures, such as the planting of trees to serve as windbreaks around croplands.

The Striking Landscape of Erosion Among the by-products of erosion are some of the most dramatic landscapes in the world, many of which are to be found in the United States. A particularly striking example appears in Colorado, where the Arkansas River carved out the Royal Gorge. Though it is not nearly as deep as the Grand Canyon, this one has something the more famous gorge does not: a bridge. Motorists with the stomach for it can cross a span 1,053 ft. (0.32 km) above the river, one of the most harrowing drives in America. Another, perhaps equally taxing, drive is that down California 1, a gorgeous scenic highway whose most dramatic stretches lie between Carmel and San Simeon. Drivers headed south find themselves pressed up against the edge of the cliffs, such that the slightest deviation from the narrow road would send an automobile and its passengers plummeting to the rocks many hundreds of feet below. These magnificent, terrifying landforms are yet another product of erosion, in this case, the result of the pounding Pacific waves. Also striking is the topography produced by the erosion of material left over from a volcanic eruption. As discussed in the Mountains essay, Devils Tower National Monument in Wyoming is

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KEY TERMS A form of mass wasting

CREEP:

A notable topographical

LANDFORM:

involving the slow downward movement

feature, such as a mountain, plateau, or

of regolith as a result of gravitational

valley.

force.

MASS

WASTING:

The transfer of

A region of sediment formed

earth material, by processes that include

when a river enters a larger body of water,

creep, slump, slide, flow, and fall, down

at which point the reduction in velocity on

slopes. Also known as mass movement.

DELTA:

the part of the river current leads to the

MORPHOLOGY:

widespread deposition of sediment.

the study thereof.

DEPOSITION:

The process whereby

REGOLITH:

Structure or form or

A general term describing

sediment is laid down on the Earth’s sur-

a layer of weathered material that rests atop

face.

bedrock.

EROSION:

The movement of soil and

rock due to forces produced by water, wind, glaciers, gravity, and other influences. In most cases, a fluid medium, such

SEDIMENT:

Material deposited at or

near Earth’s surface from a number of sources, most notably preexisting rock. SLIDE:

A variety of mass wasting in

which material moves downhill in a fairly

as air or water, is involved.

coherent mass (i.e., more or less in a section FLOW:

A form of mass wasting in

or group) along a flat or planar surface.

which a body of material that is not uniform moves rapidly downslope. GEOMORPHOLOGY:

An area of phys-

SLUMP:

A form of mass wasting that

occurs when a mass of regolith slides over or creates a concave surface (one shaped

ical geology concerned with the study of

like the inside of a bowl).

landforms, with the forces and processes

TOPOGRAPHY:

that have shaped them, and with the

Earth’s surface, including its relief as well as

description and classification of various

the position of physical features.

physical features on Earth.

WEATHERING:

The configuration of

The breakdown of

A large, typically moving

rocks and minerals at or near the surface of

mass of ice either on or adjacent to a land

Earth due to physical, chemical, or biolog-

surface.

ical processes.

GLACIER:

the remains of an extinct volcano whose outer surface long ago eroded, leaving just the hard lava of the volcanic “neck.” Erosion of lava also can produce mesas. Lava that has settled in a river valley may be harder than the rocks of the valley walls, such that the river eventually erodes the rocks, leaving only the lava platform. What was once the floor of the valley thus becomes the top of a mesa.

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Controlling Erosion The force that shapes valleys and coastlines is certainly enough to destroy hill slopes, often with disastrous consequences for nearby residents. Such has been the case in California, where, during the 1990s, areas were dealt a powerful onetwo punch of drought followed by rain. The drought killed off much of the vegetation that

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might have held the hillsides, and when rains came, they brought about mass wasting in the form of mudflows and landslides. Over the surface of the planet, the average rate of erosion is about 1 in. (2.2 cm) in a thousand years. This is the average, however, meaning that in some places the rate is much, much higher, and in others it is greatly lower. The rate of erosion depends on several factors, including climate, the nature of the materials, the slope and angle of repose, and the role of plant and animal life in the local environment.

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WHERE TO LEARN MORE Cherrington, Mark. Degradation of the Land. New York: Chelsea House, 1991. “Coastal and Nearshore Erosion.” United States Geological Survey (USGS) (Web site). . Dean, Cornelia. Against the Tide: The Battle for America’s Beaches. New York: Columbia University Press, 1999. Hecht, Jeff. Shifting Shores: Rising Seas, Retreating Coastlines. New York: Scribners, 1990. Middleton, Nick. Atlas of Environmental Issues. Illus. Steve Weston and John Downes. New York: Facts on File, 1989. Protecting Your Property from Erosion (Web site). .

Whereas many types of plants help prevent erosion, the wrong types of planting can be detrimental. The dangers of improper land usage for crops and livestock are illustrated by the Dust Bowl experience, which highlights the fact that the organism most responsible for erosion is humanity itself. On the other hand, people also can protect against erosion by planting vegetation that holds the soil, by carefully managing and controlling land usage, and by lessening slope angle in places where gravity tends to erode the soil.

Wind Erosion Research Unit. United States Department of Agriculture/Kansas State University (Web site). .

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Rybolt, Thomas R., and Robert C. Mebane. Environmental Experiments About Land. Hillside, NJ: Enslow Publishers, 1993. “Soil Erosion on Farmland.” New Zealand Ministry of Agriculture and Forestry (Web site). . Weathering and Erosion (Web site). .

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Mass Wasting

MASS WASTING

CONCEPT The term mass wasting (sometimes called mass movement) encompasses a broad array of processes whereby earth material is transported down a slope by the force of gravity. It is related closely to weathering, which is the breakdown of minerals or rocks at or near Earth’s surface through physical, chemical, or biological processes, and to erosion, the transport of material through a variety of agents, most of them flowing media, such as air or water. Varieties of mass wasting are classified according to the speed and force of the process, from extremely slow creep to very rapid, dramatic slide or fall. Examples of rapid mass wasting include landslides and avalanches, which can be the cause of widespread death and destruction when they occur in populated areas.

HOW IT WORKS Moving Earth and Rocks In discussing mass wasting, the area of principal concern is Earth’s surface rather than its interior. Thus, mass wasting is related most closely to the realm of geomorphology, a branch of physical geology concerned with the study of landforms, with the forces and processes that have shaped them, and with the description and classification of various physical features on Earth. Though plate tectonics (which involves the movement of giant plates beneath the earth’s surface) can influence mass wasting, plate tectonics entails interior processes that humans usually witness only indirectly, by seeing their effects. Mass wasting, on the other hand, often can be observed directly, particularly in its more rapid forms, such as rock fall.

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There are three general processes whereby a piece of earth material can be moved from a high outcropping to the sea: weathering, mass wasting, or erosion. If mechanical, biological, or chemical processes act on the material, dislodging it from a larger sample of material (e.g., separating a rock from a boulder), it is an example of weathering, which is discussed later in this essay. Supposing that a rock has been broken apart by weathering, it may be moved further by masswasting processes, such as creep or fall. Pieces of rock swept away by a river in a valley below the outcropping and small bits of rock worn away by high winds are examples of erosion. Erosion and weathering are examined in separate essays within this book. As for the relationships between erosion, weathering, and mass wasting, the lines are not clearly drawn. Some authors treat weathering and mass wasting as varieties of erosion, and some apply a strict definition of erosion as resulting only from flowing media. (In the physical sciences, fluid means anything that flows, not just liquids.) Weathering, mass wasting, and erosion also can be viewed as stages in a process, as described in the preceding paragraph. This broad array of approaches, while perhaps confusing, only serves to illustrate the fact that the earth sciences are relatively young compared with such ancient disciplines as astronomy and biology. Not all definitions in the earth sciences are, as it were, “written in stone.”

Weathering A mineral is a substance that occurs naturally, is usually inorganic, and typically has a crystalline structure. The term organic does not necessarily

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mean “living” rather, it refers to all carbon-containing compounds other than oxides, such as carbon dioxide, and carbonates, which are often found in Earth’s rocks. A crystalline solid is one in which the constituent parts have a simple and definite geometric arrangement repeated in all directions. Rocks, scientifically speaking, are simply aggregates or combinations of minerals or organic material or both, and weathering is the process whereby rocks and minerals are broken down into simpler materials. Weathering is the mechanism through which soil is formed, and therefore it is a geomorphologic process essential to the sustenance of life on Earth. There are three varieties of weathering: physical or mechanical, chemical, and biological. T H E T H R E E T Y P E S O F W E AT H E R I N G . Physical or mechanical weathering

involves such factors as gravity, friction, temperature, and moisture. Gravity, for instance, may cause a rock to drop from a height, such that it falls to the ground and breaks into pieces. If wind-borne sand blows constantly across a rock surface, the friction will have the effect of sandpaper, producing mechanical weathering. In addition, changes in temperature and moisture will cause expansion and contraction of materials, bringing about sometimes dramatic changes in their physical structure. Chemical weathering not only is a separate variety of weathering but also is regarded as a second stage, one that follows physical weathering. Whereas physical changes are typically external, chemical changes affect the molecular structure of a substance, bringing about a rearrangement in the ways that atoms are bonded. Important processes that play a part in chemical weathering include acid reactions, hydrolysis (a reaction with water that results in the separation of a compound to form a new substance or substances), and oxidation. The latter can be defined as any chemical reaction in which oxygen is added to or hydrogen is removed from a substance.

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beneficial organism called a lichen. (Reindeer moss is an example of a lichen.) Through a combination of physical and chemical processes, organisms ranging from lichen to large animals can wear away rock gradually.

Properties of Unconsolidated Material Regolith is a general term that describes a layer of weathered material that rests atop bedrock. It is unconsolidated, meaning that it is in pieces, like gravel, though much less uniform in size. Sand and soil, including soil mixed with loose rocks, are examples of regolith. Every variety of unconsolidated material has its own angle of repose, or the maximum angle at which it can remain standing. Everyone who has ever attempted to build a sand castle at the beach has experienced angle of repose firsthand, perhaps without knowing it. Imagine that you are trying to build a sand castle with a steep roof. Dry sand would not be good for this purpose, because it is loose and has a tendency to flow easily. Much better would be moist sand, which can be shaped into a sharper angle, meaning that it has a higher angle of repose. A certain amount of water gives sand surface tension, the same property that causes water to bead up on a table rather than lying flat. If too much water is added to the sand, however, the sand becomes saturated and will flow, a process called lateral spreading. Thus, to a point, the addition of water increases the angle of repose for sand, which is only about 34° when the sand is dry. (This is the angle of repose for sand in an hourglass.) On the other hand, piles of rocks may have an angle of repose as high as 45°. In practice, most aggregates of materials in nature have slopes less than their angle of repose, owing to the influence of wind and other erosive forces.

Types of Mass Wasting

An example of biological weathering occurs when a plant grows from a crevice in a rock. As the plant grows, it gradually forces the sides of the crevice apart even further, and it ultimately may tear the rock apart. Among the most notable agents of biological weathering are algae and fungi, which may be combined in a mutually

As noted earlier, there is some disagreement among writers in the geologic sciences regarding the types of mass wasting. Indeed, even the term mass wasting is not universal, since some writers refer to it as mass movement. Others do not even treat the subject as a category unto itself, preferring instead to address related concepts, such as weathering and erosion, as well as instances of mass wasting, such as avalanches and landslides.

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AN

AVALANCHE ON

MOUNT MCKINLEY

IN

ALASKA. (© W. Bacon/Photo Researchers. Reproduced by permission.)

For this reason, the classification of masswasting processes presented here is by no means universal and instead represents a composite of several schools of thought. Generally speaking, geologists and geomorphologists classify processes of mass wasting according to the rapidity with which they occur. Most sources recognize at least three types of mass wasting: flow, slide, and fall. Some sources include slump among the categories of relatively rapid masswasting process, as opposed to the slower, less dramatic (but ultimately more important) process known as creep. Some writers classify uplift and subsidence with mass wasting; howev-

er, in this book, uplift and subsidence are treated separately, in the Geomorphology essay.

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REAL-LIFE A P P L I C AT I O N S Creep Creep is the slow downward movement of regolith as a result of gravitational force. Before the initiation of the creeping process, the regolith is in what physicists call a condition of unstable equilibrium: it remains in place, yet a relatively small disturbance would be enough to dislodge

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A

MUDFLOW CAUSED BY HEAVY WINTER RAINS BRINGS DOWN THE HILLSIDE UNDER HOMES IN

MILLBRAE, CALIFOR-

NIA. (AP/Wide World Photos. Reproduced by permission.)

it. Though it is slow, creep can produce some of the most dramatic results over time. It can curve tree trunks at the base, break or overturn retaining walls, and cause objects from fence posts to utility poles to tombstones to be overturned. Changes in temperature or moisture are among the leading factors that result in the disturbance of regolith. A change in either can cause material to expand or contract, and freezing or thawing may be enough to shake regolith from its position of unstable equilibrium. In fact, some geomorphologists cite a distinct mass-wasting process, known as solifluction, that occurs in the active layer of permafrost, which thaws in the summertime. Water also can provide lubrication or additional weight that assists the material in moving. One of the only causes of creep not associated with changes in temperature or moisture is the burrowing of small animals.

Often, slump is classified as a variety of slide, in which material moves downhill in a fairly coherent mass (i.e., more or less in a section or group) along a flat or planar surface. These movements sometimes are called rock slides, debris slides, or, in common parlance, landslides. Among the most destructive types of mass wasting, they may be set in motion by earthquakes, which are caused by plate tectonic processes, or by hydrologic agents (i.e., excessive rain or melting snow and ice).

Flow

Slump occurs when a mass of regolith slides over or creates a concave surface (one shaped like the inside of a bowl). The result is the formation of a small, crescent-shaped cliff, known as a scarp, at the upper end—rather like the crest of a wave. Soil flow takes place at the bottom end of the slump. One is likely to see slumps in any place

When a less uniform, or more chaotic, mass of material moves rapidly downslope, it is called flow. Flow is divided into categories, depending on the amounts of water involved: granular flow (0-20% water) and slurry flow (20-40% water). Creep and solifluction often are classified as very slow forms of granular and slurry flow, respectively. In order of relative speed, these categories are as follows:

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where forces, whether man-made or natural, have graded material to a slope too steep for its angle of repose. This may happen along an interstate highway, where a road crew has cut the slope too sharply, or on a riverbank, where natural erosion has done its work.

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Granular Flow (0-20% Water) • • • •

Slowest: Creep Slower: Earth flow Faster: Grain flow Fastest: Debris avalanche Slurry Flow (20-40% Water)

• Slow: Solifluction • Medium: Debris flow • Fast: Mudflow Earth flow moves at a rate anywhere from 3.3 ft. (1 m) per year to 330 ft. (100 m) per hour. Grain flow can be nearly 60 mi. (100 km) per hour, and debris avalanche may achieve speeds of 250 mi. (400 km) per hour, making it extremely dangerous. Among types of slurry flow, debris flow is roughly analogous to earth flow, falling into a range from about 4 in. (10 cm) per year to 0.6 mi. (1 km) per hour. Mudflow is slightly faster than grain flow. If the water content is more than 40%, a slurry flow is considered a stream. Earth flows involve fine-grained materials, such as clay or silt, and typically occur in humid areas after heavy rains or the melting of snow. Debris flows usually result from heavy rains as well and may start with slumps before flowing downhill, forming lobes with a surface broken by ridges and furrows. Grain flows can be caused by a small disturbance, which forces the dry, unconsolidated material rapidly downslope. Debris avalanches are commonly the result of earthquakes or volcanic eruptions. Seismic disturbances or volcanic activity may cause the collapse of a mountain slope, sending debris avalanches moving swiftly even along the gentler slopes of the mountainside. Likewise, mudflows may be the result of volcanic activity, in which case they are known as lahars. In some situations, the material in a lahar is extremely hot. Mudflows tend to be highly fluid mixtures of sediment (material deposited at or near Earth’s surface from a number of sources, most notably preexisting rock) and water and typically flow along valley floors.

cliffs on either side, has seen signs that say “Watch for Falling Rock.” These warnings, which appear regularly on the drive through the Rockies in Colorado or on highways across the Blue Ridge and Great Smoky mountains in the southern United States, indicate the threat of rock fall.

Mass Wasting

The mechanism behind rock fall is simple enough. When a rock at the top of a slope is in unstable equilibrium, it can be dislodged such that it either falls directly downward or bounces and rolls. Usually, the bottom of the slope or cliff contains accumulated talus, or fallen rock material. Freezing and thawing as well as the growth of plant roots may cause fall. The latter is not limited to rock fall: debris fall, which is closely related, includes soil, vegetation, and regolith as well as rocks.

Mass Wasting and Natural Disasters Among the most dramatic and well-known varieties of mass wasting are avalanches, a variety of flow, and landslides, which (as their name suggests) are a type of slide. These can result, and have resulted, in enormous loss of life and property. Some notable modern occurrences of mass wasting, and the type of movement involved, are listed below. With each incident, the approximate number of fatalities is shown in parentheses. • China, 1920: Landslide caused by an earthquake (200,000) • Peru, 1970: Debris avalanche related to an earthquake (70,000) • Colombia, 1985: Mudflow related to a volcanic eruption (23,000) • Soviet Union, 1949: Landslide caused by an earthquake (12,000–20,000) • Italy and Austria, 1916: Landslide (10,000) • Peru, 1962: Landslide (4,000–5,000) • Italy, 1963: Landslide (2,000) • Japan, 1945: Landslide caused by a flood (1,200) • Ecuador, 1987: Landslide related to an earthquake (1,000) • Austria, 1954: Landslide (200)

Fall The Role of Plate Tectonics Most other forms of mass wasting entail movement along slopes that are considerably less than 90°, whereas fall takes place at angles almost perpendicular to the ground. Anyone who has driven through a wide mountain area, with steep

Note how many times an instance of mass wasting was either caused by or “related to” (meaning that geologists could not establish a full causal relationship) volcanic or seismic activity. Both, in

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KEY TERMS The maximum

amounts of water involved: granular flow

slope at which a relatively large sample of

(0-20% water) and slurry flow (20-40%

unconsolidated material can remain stand-

water). An avalanche is an example of flow

ing. Often, the addition of water increases

and may involve either rock (granular) or

the angle of repose, up to the point at

snow (slurry).

ANGLE OF REPOSE:

which the material becomes saturated. AVALANCHE:

See flow.

In the physical sciences, the

term fluid refers to any substance that

A form of mass wasting

flows and therefore has no definite shape—

involving the slow downward movement of

that is, both liquids and gases. In the earth

regolith as a result of gravitational force.

sciences, occasionally substances that

CREEP:

EROSION:

The movement of soil and

rock due to forces produced by water,

appear to be solid, for example, ice in glaciers, are, in fact, flowing slowly. An area of phys-

wind, glaciers, gravity, and other influ-

GEOMORPHOLOGY:

ences. In most cases, a fluid medium, such

ical geology concerned with the study of

as air or water, plays a part.

landforms, with the forces and processes

A form of mass wasting in which

that have shaped them, and with the

rock or debris moves downward along

description and classification of various

extremely steep angles.

physical features on Earth.

FALL:

FLOW:

A form of mass wasting in

LANDSLIDE:

See slide. The upper layer of

which a body of material that is not uni-

LITHOSPHERE:

form moves rapidly downslope. Flow is

Earth’s interior, including the crust and the

divided into categories, depending on the

brittle portion at the top of the mantle.

turn, are the result of plate movement in most instances, and thus it is not surprising that several of the locales noted here are either at plate margins or in mountainous regions where plate tectonic and other processes are at work. (For more on this subject, see the entries Plate Tectonics and Mountains.) To set mass wasting into motion, it is necessary to have a steep slope and some type of force to remove material from its position of unstable equilibrium. Plate tectonic processes provide both. Not only does an earthquake, for instance, jar rocks loose from the upper portion of a slope, but the movement of plates also helps create steep slopes, for example, the collision of the Indo-Australian and Eurasian belts that produced the Himalayas.

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FLUID:

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Some of the most vigorous plate tectonic activity occurs underwater, and, likewise, there are remarkable manifestations of mass wasting beneath the seas. Off Moss Landing, a research facility that serves a consortium of state universities in northern California, is an underwater canyon more than 0.6 mi. (1 km) deep. At one time, Monterey Canyon was thought to be the result of erosion by a river flowing into the ocean; however, today it is believed to be the result of underwater mass wasting.

Detecting and Preventing Mass Wasting The dramatic instances of mass wasting discussed here hardly require any effort at detection. Their effect is obvious and, to those unfortunate enough

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Mass Wasting

KEY TERMS MASS

The transfer of

WASTING:

SLIDE:

A variety of mass wasting in

earth material down slopes by processes

which material moves downhill in a fairly

that include creep, slump, slide, flow, and

coherent mass (i.e., more or less in a section

fall. Also known as mass movement.

or group) along a flat or planar surface.

PLATE MARGINS:

Boundaries between

A form of mass wasting that

occurs when a mass of regolith slides over

plates. PLATE TECTONICS:

The name both

of a theory and of a specialization of tectonics. As an area of study, plate tectonics deals with the large features of the lithosphere and the forces that shape them. As a theory, it explains the processes that have shaped Earth in terms of plates and their movement. PLATES:

SLUMP:

or creates a concave surface (one shaped like the inside of a bowl). SURFACE TENSION:

An attractive

force exerted by molecules in the interior of a liquid on molecules at the exterior. This force draws the material inward such that it occupies less than its maximum horizontal area. The surface tension of water is high, causing it to bead on most surfaces.

Large, movable segments of UNSTABLE EQUILIBRIUM:

the lithosphere. REGOLITH:

A situa-

tion in which an object remains in place, A general term describing

a layer of weathered material that rests atop bedrock.

yet a relatively small disturbance would be enough to dislodge it. WEATHERING:

The breakdown of

Material deposited at or

rocks and minerals at or near the surface of

near Earth’s surface from a number of

Earth due to physical, chemical, or biolog-

sources, most notably preexisting rock.

ical processes.

SEDIMENT:

to be nearby, inescapable. Other types of mass wasting occur so slowly that they do not invite immediate detection. This can be unfortunate, because in some cases slow mass wasting is a harbinger of much more rapid movements to follow. A dwelling atop a hill is subject to enormous gravitational force, and the more massive the dwelling, the greater the pull of gravity. (Weight is, after all, nothing but gravitational force.) If a homeowner adds a swimming pool or other items that contribute to the weight of the dwelling, it only increases the chances that it may experience mass wasting. Heavy rains can bring so much water that it saturates the soil, reducing its surface tension and causing it to slide—as occurred, for instance, in the area around Malibu, California, during the late 1990s.

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The California mud slides and landslides are a dramatic example of mass wasting, but more often than not mass wasting takes the form of creep, which is detectable only over a matter of years. When creep occurs, the upper layer of soil moves, while the layer below remains stationary. One way to keep the upper layer in place is to plant vegetation that will put down roots deep enough to hold the soil. This may create unintended consequences. During the 1930s, New Deal officials imported kudzu plants from China, intending to protect the hillsides of the American South from creep and erosion. The kudzu protected the slopes, but as it turned out, this voracious plant had a tendency to creep as well. Before communities began taking steps to eradicate it, or at least push

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it back, in the 1970s, kudzu seemingly threatened to cover the entire southern United States. To prevent some of the more dramatic varieties of mass wasting, such as landslides in a residential area, a homeowner or group of homeowners may commission an engineer’s study. The engineer can test the material of the slope, measure the stresses acting on it, and perform other calculations to predict the likelihood that a slope will succumb to a given amount of force. For this reason, zoning laws in areas with steep slopes are typically strict. These laws are geared toward preventing homeowners and builders from erecting structures likely to create a threat of mass wasting in a period of heavy rains.

WHERE TO LEARN MORE

280

Allen, Missy, and Michel Peissel. Dangerous Natural Phenomena. New York: Chelsea House, 1993. Armstrong, Betsy R., and Knox Williams. The Avalanche Book. Golden, CO: Fulcrum, 1986. Goodwin, Peter. Landslides, Slumps, and Creeps. New York: Franklin Watts, 1997. Gore, Pamela. “Mass Wasting” (Web site). . Mass Wasting (Web site). . “Mass Wasting Features of North Dakota.” North Dakota State University (Web site). . Murck, Barbara Winifred, Brian J. Skinner, and Stephen C. Porter. Dangerous Earth: An Introduction to Geologic Hazards. New York: John Wiley, 1996. Nelson, Stephen A. Mass-Wasting and Mass-Wasting Processes. Tulane University (Web site). .

Abbott, Patrick. Natural Disasters. Dubuque, IA: William C. Brown Publishers, 1996.

Weathering and Mass Wasting Learning Module (Web site). .

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S C I E N C E O F E V E RY DAY T H I N G S Real-Life Earth Science

SEDIMENTOLOGY AND SOIL SCIENCE S E D I M E N T A N D S E D I M E N TAT I O N SOIL S O I L C O N S E R VAT I O N

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Sediment and Sedimentation

SEDIMENT AND S E D I M E N TAT I O N

CONCEPT The materials that make up Earth are each products of complex cycles and interactions, as a study of sediment and sedimentation shows. Sediment is unconsolidated material deposited at or near Earth’s surface from a number of sources, most notably preexisting rock. There are three kinds of sediment: chemical, organic, and rock, or clastic sediment. Weathering removes this material from its source, while erosion and mass wasting push it along to a place where it is deposited. After deposition, the material may become a permanent part of its environment, or it may continue to undergo a series of cycles in which it experiences ongoing transformation.

HOW IT WORKS

rock from a boulder), this is an example of weathering. Assuming that a rock has been broken apart by weathering, it may be moved farther by mass-wasting processes, such as creep or fall, for which gravity is the driving factor. If the pieces of rock are swept away by a river, high winds, or a glacier (all of which are flowing media), this is an example of erosion. W E AT H E R I N G . Weathering is divided further into three different types: physical, chemical, and biological. Physical or mechanical weathering takes place as a result of such factors as gravity, friction, temperature, and moisture. Gravity may cause a rock to roll down a hillside, breaking to pieces at the bottom; friction from particles of matter borne by the wind may wear down a rock surface; and changes in temperature and moisture can cause expansion and contraction of materials.

Transporting Sediment There are three types of sediment: rocks, or clastic sediment; mineral deposits, or chemical sediment; and organic sediment, composed primarily of organic material. (In this context, the term organic refers to formerly living things and parts or products of living things; however, as discussed in Minerals, the term actually has a much broader meaning.) There are also three general processes involved in the transport of sediment from higher altitudes to lower ones, where they eventually are deposited: weathering, mass wasting, and erosion.

Whereas physical weathering attacks the rock as a whole, chemical weathering involves the breakdown of the minerals or organic materials that make up the rock. Chemical breakdown may lead to the dissolution of the materials in the rock, which then are washed away by water or wind or both, or it may be merely a matter of breaking the materials down into simpler compounds.

The lines between these three processes are not always clearly drawn, but, in general, the following guidelines apply. When various processes act on the material, causing it to be dislodged from a larger sample (for example, separating a

Biological weathering is not so much a third type of weathering as it is a manifestation of chemical and physical breakdown caused by living organisms. Suppose, for instance, that a plant grows within a crack in a rock. Over time, the plant will influence physical weathering through its moisture and the steady force of its growth pushing at the walls of the fissure in which it is rooted. At the same time, its specific chemical

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Sediment and Sedimentation

SEDIMENT

CAN BE TRANSPORTED BY WEATHERING, EROSION, AND MASS WASTING.

HERE

A WINTER STORM HAS

CAUSED COASTAL EROSION OF A BEACH. (© Rafael Macia/Photo Researchers. Reproduced by permission.)

properties likely will induce decomposition of the rock. M A S S W A S T I N G . Mass wasting, sometimes known as mass movement, comprises a number of types of movement of earth material, all of them driven by gravity. Creep is the slow downward drift of regolith (unconsolidated material produced by weathering), while slump occurs when a mass of regolith slides over, or creates, a concave surface—that is, one shaped like the inside of a bowl. Slump sometimes is classified as a variety of slide, in which material moves downhill in a fairly coherent mass along a flat or planar surface. Such movements, sometimes called rock slides, debris slides, or landslides, are among the most destructive types of mass wasting.

When a less uniform, or more chaotic, mass of material moves rapidly down a slope, it is called flow. Flow is divided into categories, depending on the specific amounts of water: granular flows (0–20% water) and slurry flows (20–40% water), the fastest varieties of which are debris avalanche and mudflow, respectively. Mudflows can be more than 60 mi. (100 km) per hour, while debris avalanches may achieve speeds of 250 mi. (400 km) per hour.

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considerably less than 90°, whereas a final variety of mass wasting, that is, fall, takes place at angles almost perpendicular to the ground. Typically the bottom of a slope or cliff contains accumulated talus, or fallen rock material. Nor is fall limited to rock fall: debris fall, which is closely related, includes soil, vegetation, and regolith as well as rocks. (For more on these subjects, see Mass Wasting.)

Erosion Erosion typically is caused either by gravity (in which case it is generally known as mass wasting, discussed earlier) or by flowing media, such as water, wind, and even ice in glaciers. It removes sediments in one of three ways: by the direct impact of the agent (i.e., the flowing media that is discussed in the following sections); by abrasion, another physical process; or by corrosion, a chemical process.

Even these high-speed varieties of mass wasting entail movement along slopes that are

In the case of direct impact, the wind, water, or ice removes sediment, which may or may not be loose when it is hit. On the other hand, abrasion involves the impact of solid earth materials carried by the flowing agent rather than the impact of the flowing agent itself. For example, sand borne by the wind, as discussed later, or pebbles carried by water may cause abrasion.

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Corrosion is chemical and is primarily a factor only in water-driven, as opposed to wind-driven or ice-driven, erosion. Streams slowly dissolve rock, removing minerals that are carried downstream by the water. WAT E R. Of the fluid substances driving erosion, liquid water is perhaps the one most readily associated in most people’s minds with erosion. In addition to the erosive power of waves on the seashore, there is the force exerted by running water in creeks, streams, and rivers. As a river moves, pushing along sediment eroded from the streambeds or riverbeds, it carves out deep chasms in the bedrock beneath.

Moving bodies of water continually reshape the land, carrying soil and debris down slopes, from the source of the river to its mouth, or delta. A delta is a region formed when a river enters a larger body of water, at which point the reduction in velocity on the part of the river current leads to the widespread deposition of sediment. It is so named because its triangular shape resembles that of the Greek letter delta, ∆. Water at the bottom of a large body, such as a pond or lake, also exerts erosive power; then there is the influence of falling rain. Assuming that the ground is not protected by vegetation, raindrops can loosen particles of soil, sending them scattering in all directions. A rain that is heavy enough may dislodge whole layers of topsoil and send them rushing away in a swiftly moving current. The land left behind may be rutted and scarred, much of its best soil lost for good. I C E . Ice, of course, is simply another form of water, but since it is solid, its physical properties are quite different. It is a solid rather than a fluid, such as liquid water or air (the physical sciences treat gases and liquids collectively as “fluids”), yet owing to the enormous volume of ice in glaciers, these great masses are capable of flowing. Glaciers do not flow in the same way as a fluid does; instead, they are moved by gravity, and like giant bulldozers made of ice, they plow through rock, soil, and plants.

Under certain conditions a glacier may have a layer of melted water surrounding it, which greatly enhances its mobility. Even without such lubricant, however, these immense rivers of ice move steadily forward, gouging out pieces of bedrock from mountain slopes, fashioning deep valleys, removing sediment from some regions

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and adding it to others. In unglaciated areas, or places that have never experienced any glacial activity, sediment is formed by the weathering and decomposition of rock. On the other hand, formerly glaciated areas are distinguished by layers of till, or glacial sediment, from 200 to 1,200 ft. (61–366 m) thick.

Sediment and Sedimentation

W I N D . The processes of wind erosion sometimes are called eolian processes, after Aeolus, the Greek god of the winds. Eolian erosion is in some ways less forceful than the erosive influence of water. Water, after all, can lift heavier and larger particles than can the winds. Wind, however, has a much greater frictional component in certain situations. This is particularly true when the wind carries sand, every grain of which is like a cutting tool.

Wind erosion, in fact, is most pronounced in precisely those places where sand abounds, in deserts and other areas that lack effective ground cover in the form of solidly rooted, prevalent vegetation. In some desert regions the bases of rocks or cliffs have been sandblasted, leaving a mushroom-shaped formation owing to the fact that the wind could not lift the fine grains of sand very high.

Sediment Load Eroded particles become part of what is called the sediment load transported by the fluid medium. Sediment load falls into three categories: dissolved load, suspended load, and bed load. The amount of each type of load that a fluid medium is capable of carrying depends on the density of the fluid medium itself: in other words, wind can carry the least of each and ice the most. The wind does not carry any dissolved load, since solid particles (unlike gases) cannot be dissolved in air. Ice or water, on the other hand, is able to dissolve materials, which become invisible within them. Typically, about 90% of the dissolved load in a river is accounted for by five different ions, or atoms that carry a net electric charge: the anions (negative ions) chloride, sulfate, and bicarbonate and the cations (positive ions) of sodium and calcium. Suspended load is sediment that is suspended, or floating, in the erosive medium. In this instance, wind is just as capable as water or ice of suspending particles of the sediment load, which are likely to color the medium that carries them. Hence, water or wind carrying suspended parti-

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cles is usually murky. The thicker the medium, the larger the particles it is capable of suspending. In other words, ice can suspend extremely large pieces of sediment, whereas water can suspend much more modest ones. Wind can suspend only tiny particles. Then there is bed load, large sediment that never becomes suspended but rather is almost always in contact with the substrate or bottom, whether “the bottom” is a streambed or the ground itself. Instead of being lifted up by the medium, bed load is nudged along, rolling, skipping, and sliding as it makes its way over the substrate. Once again, the density of the medium itself has a direct relationship to the size of the bed load it is capable of carrying. Wind rarely transports bed load thicker than fine sand, and water usually moves only pebbles, though under flood conditions it can transport boulders. As with suspended load, glaciers can transport virtually any size of bed load.

Sediment Sizes and Shapes Geologists and sedimentologists use certain terms to indicate sizes of the individual particles in sediment. Many of these terms are familiar to us from daily life, but whereas people typically use them in a rather vague way, within the realm of sedimentology they have very specific meanings. Listed below are the various sizes of rock, each with measurements or measurement ranges for the rock’s diameter: • Clay: Smaller than 0.00015 in. (0.004 mm) • Silt: 0.00015 in. (0.004 mm) to 0.0025 in. (0.0625 mm) • Sand: 0.0025 in. (0.0625 mm) to 0.08 in. (2 mm) • Pebble: 0.08 in. (2 mm) to 2.5 in. (64 mm) • Cobble: 2.5 in. (64 mm) to 10 in. (256 mm) • Boulder: Larger than 10 in. (256 mm). This listing is known as the Udden–Wentworth scale, which was developed in 1898 by J.A. Udden (1859–1932), an American sedimentary petrologist (a scientist who studies rocks). In 1922 the British sedimentary petrologist C. K. Wentworth) expanded Udden’s scale, adapting the definitions of various particle sizes to fit more closely with the actual usage and experience of researchers in the field. The scale uses modifiers to pinpoint the relative sizes of particles. In ascending order of size, these sizes are very fine, fine, medium, coarse, and very coarse.

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REAL-LIFE A P P L I C AT I O N S Sediments and Dust Bowls Sediment makes possible the formation of soil, which of course is essential for growing crops. Therefore it is a serious matter indeed when wind and other forces of erosion remove sediment, creating dust-bowl conditions. The term “Dust Bowl,” with capital letters, refers to the situation that struck the United States Great Plains states during the 1930s, devastating farms and leaving thousands of families without home or livelihood. (See Erosion for much more about the Dust Bowl.) During the late 1990s, some environmentalists became concerned that farming practices in the western United States were eroding sediment, putting in place the possibility of a return to the conditions that created the Dust Bowl. However, in August 1999, the respected journal Science reported studies showing that sediment in farmlands was not eroding at anything like the rate that had been feared. Soil scientist Stanley Trimble at the University of California, Los Angeles, studied Coon Creek, Wisconsin, and its tributaries, a watershed for which 140 years’ worth of erosion data were available. As Trimble discovered, the rate of sediment erosion in the area had dramatically decreased since the 1930s, and was now at 6% of the rate during the Dust Bowl years. Some studies from the 1970s onward had indicated that farming techniques, designed to improve the crop output from the soil, had created a situation in which sediment was being washed away at alarming rates. However, if such sediment removal were actually taking place, there would have to be some evidence—if nothing more, the sediment that had been washed away would have had to go somewhere. Instead, as Trimble reported, “We found that much of the sediment in Coon Creek doesn’t move very far, and that it moves in complex ways.” The sediment, as he went on to explain, was moving within the Coon Creek basin, but the amount that actually made it to the Mississippi River (which could be counted as true erosion, since it was removing sediment from the area) had stayed essentially the same for the past 140 years.

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Sediment and Sedimentation

SEDIMENTARY

STRUCTURES REMAINING IN A DRIED RIVER BED.

CLAY

SOILS CRACK AS THEY LOSE MOISTURE AND

CONTRACT WHEN TRAPPED WATER EVAPORATES AS THE RESULT OF DROUGHT. (© B. Edmaier/Photo Researchers. Reproduced by

permission.)

Deposition and Depositional Environments Eventually everything in motion—including sediment—comes to rest somewhere. A piece of sediment traveling on a stream of water may stop hundreds of times, but there comes a point when it comes to a complete stop. This process of coming to rest is known as deposition, which may be of two types, mechanical or chemical. The first of these affects clastic and organic sediment, while the second applies (fittingly enough) to chemical sediment.

These large pieces are followed by medium-size pieces and so on until both bed load and suspended load have been deposited. If the sediment has come to a full stop, as, for instance, in a stagnant pool of water, even the finest clay suspended in the water eventually will be deposited as well.

In mechanical deposition, particles are deposited in order of their relative size, the largest pieces of bed load coming to a stop first.

Unlike mechanical deposition, chemical deposition is not the result of a decrease in the velocity of the flow; rather, it comes about as a result of chemical precipitation, when a solid particle crystallizes from a fluid medium. This often happens in a saltwater environment, where waters may become overloaded with salt and other minerals. In such a situation, the water is

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unable to maintain the minerals in a dissolved state (i.e., in solution) and precipitates part of its content in the form of solids. DEPOSITIONAL ENVIRONM E N T S . The matter of sediment deposition

in water is particularly important where reservoirs are concerned, since in that case the water is to be used for drinking, cooking, bathing, and other purposes by humans. One of the biggest problems for the maintenance of clean reservoirs is the transport of sediment from agricultural areas, in which the soil is likely to contain pesticides and other chemicals, including the phosphorus found in fertilizer. A number of factors, including precipitation, topography, and land use, affect the rate at which sediment is deposited in reservoirs. The area in which sediment is deposited is known as its depositional environment, of which there are three basic varieties: terrestrial, marginal marine, and marine. These are, respectively, environments on land (and in landlocked waterways, such as creeks or lakes), along coasts, and in the open ocean. A depositional environment may be a large-scale one, known as a regional environment, or it may be a smaller subenvironment, of which there may be hundreds within a given regional environment. S E D I M E N TA RY

STRUCTURES.

There are many characteristic physical formations, called sedimentary structures, that sediment forms after it has reached a particular depositional environment. These formations include bedding planes and beds, channels, cross-beds, ripples, and mud cracks. A bed is a layer, or stratum, of sediment, and bedding planes are surfaces that separate beds. The bedding plane indicates an interruption in the regular order of deposition. (These are concepts that also apply to the field of stratigraphy. For more on that subject, see the essay Stratigraphy.) Channels are simply depressions in a bed that reflect the larger elongated depression made by a river as it flows along its course. Cross-beds are portions of sediment that are at an angle to the beds above and below them, as a result of the action of wind and water currents—for example, in a flowing stream. As for ripples, they are small sandbar-like protuberances that form perpendicular to the direction of water flow. At the beach, if you wade out into the water and look down at your feet, you are likely to see ripples perpendi-

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cular to the direction of the waves. Finally, mud cracks are the sedimentary structures that remain when water trapped in a muddy pool evaporates. The clay, formerly at the bottom of the pool, begins to lose its moisture, and as it does, it cracks.

The Impact of Sediment It is estimated that the world’s rivers carry as much as 24 million tons (21,772,800 metric tons) of sediment to the oceans each year. There is also the sediment carried by wind, glaciers, and gravity. Where is it all going? The answer depends on the type of sediment. Clastic and organic sediment may wind up in a depositional environment and experience compaction and cementation in the process of becoming sedimentary rock. (For more on sedimentary rock, see Rocks.) On the other hand, clastic and organic particles may be buried, but before becoming lithified (turned to rock), they once again may be exposed to wind and other forces of nature, in which case they go through the entire cycle again: weathering, erosion, transport, deposition, and burial. This cycle may repeat many times before the sediment finally winds up in a permanent depositional environment. In the latter case, particles of clastic and organic sediment ultimately may become part of the soil, which is discussed elsewhere in this book (See Soil). A chemical sediment also may become part of the soil, or it may take part in one or more biogeochemical cycles (also discussed elsewhere; see Biogeochemical Cycles). These chemicals may wind up as water in underground reservoirs, as ice at Earth’s poles, as gases in the atmosphere, as elements or compounds in living organisms, or as parts of rocks. Indeed, all three types of sediment—clastic, chemical, and organic—are part of what is known as the rock cycle, whereby rocks experience endlessly repeating phases of destruction and renewal. (See Rocks for more details.)

Sedimentary Mineral Deposits Among the most interesting aspects of sediment are the mineral deposits it contains—deposits that may, in the case of placer gold, be of significant value. A placer deposit is a concentration of heavy minerals left behind by the effect of gravity on moving particles, and since gold is the densest of all metals other than uranium (which

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Sediment and Sedimentation

KEY TERMS Sediment that is capable

BED LOAD:

In the physical sciences, the

FLUID:

of being transported by an erosive medi-

term fluid refers to any substance that

um (wind, water, or air) but only under

flows and therefore has no definite

conditions in which it remains in nearly

shape—that is, both liquids and gases.

constant contact with the substrate or bot-

Occasionally, substances that appear to be

tom (e.g., a streambed or the ground). Bed

solid (for example, ice in glaciers), in fact,

load, along with dissolved load and sus-

are flowing slowly; therefore, within the

pended load, is one of three types of sedi-

earth sciences, ice often is treated as

ment load.

another fluid medium. A substance made up of

COMPOUND:

ION:

An atom or group of atoms that

atoms of more than one element chemical-

has lost or gained one or more electrons

ly bonded to one another.

and thus has a net electric charge. Positive-

CONSOLIDATION:

A process where-

by materials become compacted, or expe-

ly charged ions are called cations, and negatively charged ones are called anions. The transfer of

rience an increase in density. This takes

MASS

place through a number of processes,

earth material down slopes by processes

including recrystallization and cementa-

that include creep, slump, slide, flow, and

tion.

fall. Also known as mass movement.

DEPOSITION:

The process whereby

WASTING:

MINERAL:

A naturally occurring, typi-

sediment is laid down on the Earth’s sur-

cally inorganic substance with a specific

face.

chemical composition and a crystalline

DIAGENESIS:

A term referring to all

structure. At one time, chemists used

the changes experienced by a sediment

ORGANIC:

sample under conditions of low tempera-

the term organic only in reference to living

ture and low pressure following deposi-

things. Now the word is applied to most

tion.

compounds containing carbon, with the

DISSOLVED LOAD:

Sediment load

that is absorbed completely by the erosive

exception of carbonates (which are minerals) and oxides, such as carbon dioxide. In the context of

medium (either water or ice) that carries it.

PRECIPITATION:

Dissolved load is one of three types of sed-

chemistry, precipitation refers to the for-

iment load, the others being suspended

mation of a solid from a liquid.

load and bed load.

REGOLITH:

EROSION:

The movement of soil and

A general term describing

a layer of weathered material that rests atop

rock due to forces produced by water,

bedrock.

wind, glaciers, gravity, and other influ-

ROCK:

ences. In most cases, a fluid medium, such

organic matter, which may be consolidated

as air or water, is involved.

or unconsolidated.

S C I E N C E O F E V E RY DAY T H I N G S

An aggregate of minerals or

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Sediment and Sedimentation

KEY TERMS SEDIMENT:

Material deposited at or

near Earth’s surface from a number of sources, most notably preexisting rock. There are three types of sediment: rocks, or clastic sediment; mineral deposits, or

CONTINUED

sediment load are dissolved load, suspended load, and bed load. SEDIMENTOLOGY:

The study and

interpretation of sediments, including sedimentary processes and formations.

chemical sediment; and organic sediment, composed primarily of organic material. SEDIMENTARY ROCK:

One of the

three major types of rock, along with igneous and metamorphic rock. Sedimentary rock usually is formed by the deposi-

Sediment that

is suspended, or floating, in the erosive medium (wind, water, or ice). Suspended load is one of three types of sediment load, along with dissolved load and bed load. A general term for the sediments

tion, compaction, and cementation of rock

TILL:

that has experienced weathering. It also

left by glaciers that lack any intervening

may be formed as a result of chemical pre-

layer of melted ice.

cipitation.

UNCONSOLIDATED

SEDIMENTATION:

The process of

erosion, transport, and deposition undergone by sediment.

ROCK:

Rock

that appears in the form of loose particles, such as sand. WEATHERING:

The breakdown of

A term for the par-

rocks and minerals at or near the surface of

ticles transported by a flowing medium of

Earth due to physical, chemical, or biolog-

erosion (wind, water, or ice). The types of

ical processes.

SEDIMENT LOAD:

290

SUSPENDED LOAD:

is even more rare), it is among the most notable of placer deposits.

holds every bit as much allure for many Americans as it did in 1848.

Of course, the fact that gold is valuable has done little to hurt, and a great deal to help, human fascination with placer gold deposits. Placer gold played a major role from the beginning of the famous California Gold Rush (1848–49), which commenced with discovery of a placer deposit by prospector James Marshall on January 24, 1848, along the American River near the town of Coloma. This discovery not only triggered a vast gold rush, as prospectors came from all over the United States in search of gold, but it also proved a major factor in the settlement of the West. Most of the miners who went to the West failed to make a fortune, of course, but instead they found something much better than gold: a gorgeous, fertile land like few places in the United States—California, a place that today

Despite the attention it naturally attracts, gold is far from the only placer mineral. Other placer minerals, all with a high specific gravity (density in comparison to that of water), include platinum, magnetite, chromite, native copper, zircon, and various gemstones. Nor are placer minerals found only in streams and other flowing bodies of water; wave action and shore currents can leave behind what are called beach placers. Among the notable beach placers in the world are gold deposits near Nome, Alaska, as well as zircon in Brazil and Australia, and marine gravel near Namaqualand, South Africa, which contains diamond particles.

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An entirely different process can result in the formation of evaporites, minerals that include carbonates, gypsum, halites, and magnesium and potassium salts. (These specific mineral types are

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discussed in Minerals.) Formed when the evaporation of water leaves behind ionic, or electrically charged, chemical compounds, evaporites sometimes undergo physical processes similar to those of clastic sediment. They may even have graded bedding, meaning that the heavier materials fall to the bottom. In addition to their usefulness in industry and commerce (e.g., the use of gypsum in sheetrock for building), physical and chemical aspects of evaporites also provide scientists with considerable information regarding the past climate of an area.

WHERE TO LEARN MORE Cherrington, Mark. Degradation of the Land. New York: Chelsea House, 1991. “Erosion—Fast or Slow or None?” (Web site). . Middleton, Nick. Atlas of Environmental Issues. Illus. Steve Weston and John Downes. New York: Facts on File, 1989.

S C I E N C E O F E V E RY DAY T H I N G S

Schneiderman, Jill S. The Earth Around Us: Maintaining a Livable Planet. New York: W. H. Freeman, 2000.

Sediment and Sedimentation

Snedden, Robert. Rocks and Soil. Illus. Chris Fairclough. Austin, TX: Raintree Steck–Vaughn, 1999. Soil Erosion and Sedimentation in the Great Lakes Region (Web site). . Sediment/Soil Quality Related Links (Web site). . State of the Land—Sedimentation and Water Quality (Web site). . USDA–ARS–National Sedimentation Laboratory. United States Department of Agriculture, Agriculture Research Laboratory National Sedimentation Laboratory (Web site). . USGS Sediment Database. United States Geological Survey (Web site). . Wyler, Rose. Science Fun with Mud and Dirt. Illus. Pat Ronson Stewart. New York: Julian Messner, 1986.

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SOIL Soil

CONCEPT If there is anything on Earth that seems simple and ordinary, it is the soil beneath our feet. Other than farmers, people hardly think of it except when tending to their lawns, and even when we do turn our attention to the soil, we tend to view it as little more than a place where grass grows and earthworms crawl. Yet the soil is a complex mixture of minerals and organic material, built up over billions of years, and without it, life on this planet would be impossible. It is home to a vast array of species that continually process it, enriching it as they do. Nor are all soils the same; in fact, there are a great variety of soil environments and a great deal of difference between the soil at the surface and that which lies further down, closer to the bedrock.

HOW IT WORKS The Beginnings of Soil Formation

All of these conditions—Earth itself, an atmosphere, waters, and life-forms—went into the creation of soil. Soil has its origins in the rocks that now lie below Earth’s surface, from which the rain washed minerals. For rain to exist, of course, it was necessary to have water on the planet, along with some form of atmosphere into which it could evaporate. Once these conditions had been established (as they were, over hundreds of millions of years) and the rains came down to cool the formerly molten rock of Earth’s surface, a process of leaching began.

After its formation from a cloud of hot gas some 4.5 billion years ago, Earth was pelted by meteorites. These meteorites brought with them solid matter along with water, forming the basis for the oceans. There was no atmosphere as such,

Leaching is the removal of soil particles that have become dissolved in water, but at that time, of course, there was no soil. There were only rocks and minerals, but these features of the geosphere, along with the chemical elements in the atmosphere and hydrosphere, were enough to set in motion the development of soil. While the atmosphere and hydrosphere supplied the falling rain, with its vital activity of leaching minerals from the rocks, the minerals themselves supplied additional chemical elements necessary to the formation of soil. (The chemical elements are discussed in several places, most notably Biogeochemical Cycles. See also Minerals and Rocks.)

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It has taken billions of years to yield the soil as we know it now. Over the course of these mind-boggling stretches of time, the chemical elements on Earth came into existence, and the uniformly rocky surface of the planet gradually gave way to deposits of softer material. This softer matter, the earliest ancestor of soil, became enriched by the presence of minerals from the rocks and, over a longer period, by decaying organic matter.

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but by about four billion years ago, volcanic activity had ejected enough carbon dioxide and other substances into the air to form the beginnings of one. The oceans began to cool, making possible the earliest forms of life—that is, molecules of carbon-based matter that were capable of replicating themselves. (For more on these subjects, see Sun, Moon, and Earth and Geologic Time. On the relationship between carbon and life-forms, see Carbon Cycle.)

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Soil

LIGHT

MICROGRAPH OF BLUE-GREEN ALGAE, AN EXAMPLE OF THE SIMPLEST PLANT ORGANISMS THAT WERE THE FORE-

RUNNERS OF LIFE ON THE EARTH.

PLANT

LIFE WAS MADE POSSIBLE BY THE LEACHING OF POTASSIUM, CALCIUM, AND

MAGNESIUM FROM ROCK, AND, IN TURN, PLANT DEATH LENT ORGANIC MATTER TO THE GROUND TO HELP FORM THE BASIS FOR SOIL. (© S. Stammers/Photo Researchers. Reproduced by permission.)

T H E F I R S T P LA N T S . Among the elements leached from the rock by the falling rains were potassium, calcium, and magnesium, all of which are essential for the growth of plant life. Thus, the foundation was laid for the first botanical forms, a fact that had several important consequences. First and most obviously, it helped set in motion the formation of the complex biosphere we have around us today. Not only did the simplest algae-like plants serve as forerunners for more complex varieties of plant and animal life to follow, but they also played a major role in the beginnings of an atmosphere breathable by animal life. As the plants absorbed carbon dioxide from their surroundings, there gradually evolved a process whereby the plant received carbon dioxide and, as a result of a chemical reaction, released oxygen.

In addition, plant life meant plant death, and as each plant died, it added just a bit more organic material—and with it nutrients and energy— to the ground. Notice the word ground as opposed to soil, which took a long, long time to form from the original rock and mineral material. Indeed, the processes we are describing here did not take shape over the course of centuries or

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millennia but over whole eons—the longest phases of geologic time, stretching for half a billion years or more (see Geologic Time). Only around the beginning of the present eon, the Phanerozoic, more than 500 million years ago, did soil as such begin to take shape.

What Is Soil? As the soil began to form, processes of weathering, erosion, and sedimentation (see the entries Erosion and Sediment and Sedimentation) slowly added to the soil buildup. Today the soil forms a sheath over much of the solid earth; just inches deep or nonexistent in some places, it is many feet deep in others. It separates the planet’s surface from its rocky interior and brings together a number of materials that contribute to and preserve life. Though its origins lie in pulverized rock and decayed organic material, soil looks and feels like neither. Whether brown, red, or black, moist or dry, sandy or claylike, it is usually fairly uniform within a given area, a fact for which the organisms living in it can be thanked. Under the surface of the soil live bacteria, fungi, worms, insects, and

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other creatures that continually churn through it and process its chemical contents.

The others are climate, living organisms, topography, and time.

A filter for water and a reservoir for air, soil provides a sort of stage on which the drama of an ecosystem (a community of mutually interdependent organisms) is played out. It receives rain and other forms of precipitation, which it filters through its layers, replenishing the groundwater supplies. This natural filtration system, sometimes augmented by a little human ingenuity, is amazingly efficient for leaching out harmful microorganisms and toxins at relatively low levels. (Thus, for instance, septic tank drainage systems process wastewater, with the help of soil, before returning it to the water table.)

PA R E N T M AT E R I A L , C L I M AT E , A N D O R GA N I S M S . Minerals, such as

By collecting rainwater, soil also gives the rain a place to go and thus helps prevent flooding. Water is not the only substance it stores; soil also collects air, which accounts for a large percentage of its volume. Thus, oxygen is made available to the roots of plants and to the large populations of organisms living underground. The creatures that live in the soil also die there, providing organic material that decays along with a vast collection of dead organisms from aboveground: trees and other plants as well as dead animals—including humans, whose decomposed bodies eventually become part of the soil as well.

Factors That Influence Soil The processes that formed soil over the eons and that continue to contribute to the soil under our feet today are similar to those by which sedimentary rock is formed. Sedimentary rocks, such as shale and sandstone, have their origins in the deposition, compaction, and cementation of rock that has experienced weathering. Added to this is organic material derived from its ecosystem—for example, fossilized remains of animals.

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feldspars and micas, react strongly to natural acids carried by rain and other forms of water; therefore, when these minerals are present in the rock that makes up the parent material, they break apart quite easily into small fragments. On the other hand, a mineral that is harder—for example, quartz—will break into larger pieces of clastic, or rock, sediment. Thus, the parent material itself has a great deal to do with the initial grain of the sediment that will become soil, and this in turn influences such factors as the rate at which water leaches through it. The release of chemical compounds and elements from minerals in weathering provides plants with the nutrients they need to grow, setting in motion the first of several steps whereby living organisms take root in, and ultimately contribute to, the soil. As the plant dies, it leaves behind material to feed decomposers, such as bacteria and fungi. The latter organisms play a highly significant role in the biogeochemical cycles whereby certain life-sustaining elements are circulated through the various earth systems. In addition, still-living plants provide food to animals, which, when they die, likewise will become one with the soil. This is achieved through the process of decomposition, aided not only by decomposers but by detritivores as well. The latter, of which earthworms are a great example, are much more complex organisms than the typically single-cell decomposers. Detritivores consume the remains of plant and animal life, which usually contains enzymes and proteins far too complex to benefit the soil in their original state. By feeding on organic remains, detritivores cycle these complex chemicals through their systems, causing them to undergo chemical reactions that result in the breakdown of their components. As a result, simple and usable nutrients are made available to the soil.

Both sedimentary rock and soil are made up of sediment, which originates from the weathering, or breakdown, of rock. Weathered remains of rocks ultimately are transported by forces of erosion to what is known as a depositional environment, a location where they are sedimented. (See Sediment and Sedimentation for more about these processes.) The nature of the “parent material,” or the rock from which the soil is derived, ranks among five key factors influencing the characteristics of soil in a given environment.

T O P O G RA P H Y A N D T I M E . Then there is the matter of topography, or what one might call landscape—the configuration of Earth’s surface, including its relief or elevation. Soil at the top of a hill, for instance, is liable to experience considerable leaching and loss of nutrients. On the other hand, if soil is located in

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a basin area, it is likely to benefit from the vitamins and minerals lost to soils at higher elevations, which lose these nutrients through leaching and erosion.

Soil

In addition, topography influences the presence or absence of organic material, which is vital if the soil is to sustain plant life. Organic matter in mountainous areas accounts for only 1% to 6% of the soil composition, while in wet lowland regions it may constitute as much as 90% of soil content. Because erosion tends to bring soil, water, and organic material from the highlands to the lowlands, it is no wonder that lowlands are almost always more fertile than the mountains that surround them. Finally, time is a factor in determining the quality of soil. As with everything else that either is living or contains living things, soil goes through a progression from immaturity to a peak to old age. In the earth sciences, age often is measured not in years, which is an absolute dating method, but by the relative dating technique of judging layers, beds, or strata of earth materials. (For more about studying rock strata as well as relative dating techniques, see Stratigraphy.)

LEAVES ON THE FOREST FLOOR ARE AN EXAMPLE OF HUMUS, A COMPONENT OF THE A HORIZON, OR TOPSOIL. (© Michael Hubrich/Photo Researchers. Reproduced by permission.)

REAL-LIFE A P P L I C AT I O N S Layers in the Soil If you dig down into the dirt of your backyard, you will see a miniature record of your regions’s geologic history over the past few million years. Actually, most homes in urban areas and suburbs today have yards made of what is called fill dirt— loose earth that has been moved into place by a backhoe or some other earthmoving mechanism. Even though the mixed quality of fill dirt makes it difficult to discern the individual strata, the soil itself tells a tale of the long ages of time that it took to shape it.

In any case, anyone with a shovel and a piece of ground that is reasonably untouched—that is, that has not been plowed up recently—can become an amateur soil scientist. Soil scientists study soil horizons, or layers of soil that lie parallel to the surface of Earth and which have built up over time. These layers are distinguished from one another by color, consistency, and composition. A cross-section combining all or most of the horizons that lie between the surface and bedrock is called a soil profile. The most basic division of layers is between the A, B, and C horizons, which differ in depth, physical and chemical characteristics, and age.

Better than a modern fill-dirt yard, of course, would be a sample taken from an older community. Here, too, however, human activities have intervened: people have dug in their yards and holes have been filled back up, for instance, thus altering the layers of soil from what they would have been in a natural state. To find a sample of soil layers that exists in a fully natural state, it might be necessary to dig in a woodland environment.

T O P S O I L . At the top is the A horizon, or topsoil, in which humus—unincorporated, often partially decomposed plant residue—is mixed with mineral particles. Technically, humus actually constitutes something called the O horizon, the topmost layer. Examples of humus would be leaves piled on a forest floor, pine straw that covers a bare-dirt area in a yard, or grass residue that has fallen between the blades of grass on a lawn. In each case, the passage of time will make the plant materials one with the soil.

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Owing to its high organic content, the soil of the A horizon may be black, or at least much darker than the soil below it. Between the A and B horizons is a noticeable layer called the E horizon, the depth of which is a function of the particulars in its environment, as discussed earlier. In rough terms, topsoil could be less than a foot (0.3 m) deep, or it could extend to a depth of 5 ft. (1.5 m) or more. In any case, the E horizon, known also as the eluviation or leaching layer, is composed primarily of sand and silt, built up as water has leached down through the soil. The sediment of the E horizon is nutrient-poor, because its valuable mineral content has drained through it to the B horizon. (The E horizon is just one of several layers aside from the principal A, B, and C layers. We will mention only a few of these here, but soil scientists include several other horizons in their classification system.) SUBSOIL, REGOLITH, BEDR O C K . The appearance and consistency of

the soil change dramatically again as we reach the B horizon. No longer is the earth black, even in the most organically rich environments; by this point it is more likely to exhibit shades of brown, since organic material has not reached this far below the surface. Yet subsoil, which is the consistency of clay, is certainly not poor in nutrients; on the contrary, it contains abundant deposits of iron, aluminum oxides, calcium carbonate, and other minerals, leached from the layers above it. The rock on the C horizon is called regolith, a general term for a layer of weathered material that rests atop bedrock. Neither plant roots nor any other organic material penetrate this deeply, and the deeper one goes, the more rocky the soil. At a certain depth, it makes more sense to say that there is soil among the rocks rather than rocks in the soil. Beneath the C horizon lies the R horizon, or bedrock. As noted earlier, depths can vary. Bedrock might be only 5–10 ft. deep (1.5–3 m), or it might be half a mile deep (0.8 km) or perhaps even deeper. Whatever the depth, it is here that the solid earth truly becomes solid, and for this reason builders of skyscrapers usually dig down to the bedrock to establish foundations there.

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Life Beneath the Surface The ground beneath our feet—that is, the topmost layer, the A horizon—is full of living things. In fact, there are more creatures below Earth’s surface than there are above it. The term creatures in this context includes microorganisms, of which there might be several billion in a sample as small as an acorn. These include decomposers, such as bacteria and fungi, which feed on organic matter, turning fresh leaves and other material into humus. In addition, both bacteria and algae convert nitrogen into forms usable by plants in the surrounding environment (see Nitrogen Cycle). W O R M S . We cannot see bacteria, of course, but almost anyone who has ever dug in the dirt has discovered another type of organism: worms. These slimy creatures might at first seem disgusting, but without them our world could not exist as it does. As they burrow through soils, earthworms mix organic and mineral material, which they make available to plants around them. They also may draw leaves deep into their middens, or burrows, thus furnishing the soil with nutrients from the surface. In addition, earthworms provide the extraordinarily valuable service of aerating the soil, or supplying it with air: by churning up the soil continuously, they expose it to oxygen from the surface and allow air to make its way down below as well.

Nor are these visible, relatively large worms the only ones at work in the soil. Colorless worms called nematodes, which are only slightly larger than microorganisms, also live in the soil, performing the vital function of processing organic material by feeding on dead plants. Some, however, are parasites that live off the roots of such crops as corn or cotton. ANTS AND LARGER CREAT U R E S . Likewise there are “bad” and “good”

ants. The former build giant, teeming mounds and hills that rise up like sores on the surface of the ground, and some species have the capacity to sting, causing welts on human victims. But a great number of ant species perform a positive function for the environment: like earthworms, they aerate soil and help bring oxygen and organic material from the surface while circulating soils from below. In some areas, much larger creatures call the soil home. Among these creatures are moles, who live off earthworms and other morsels to be

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found beneath the surface, including grubs (insect larvae) and the roots of plants. As with ants and earthworms, by burrowing under the ground, they help loosen the soil, making it more porous and thus receptive both to moisture and air. Other large burrowing creatures include mice, ground squirrels, and prairie dogs. They typically live in dry areas, where they perform the valuable function of aerating sandy, gravelly soil.

Soil

Soils and Environments In discussing our imaginary journey through the depths of the soil, it has been necessary to use vague terms concerning depths: “less than a foot,” for instance. The reason is that no solid figures can be given for the depth of the soil in any particular area, unless those figures are obtained by a soil scientist who has studied and measured the soil. Depth is just one of the ways that the soil may vary from one place to another. Earlier we mentioned five factors that affect the character of the soil: parent material, climate, living organisms, topography, and time. These factors determine all sorts of things about the soil—most of all, its ability to support varied life-forms. Collectively, these five factors constitute the environment in which a soil sample exists. P O O R S O I L S . A desert environment might be one of immature soil, defined as a sample that has only A and C horizons, with no B horizon between them. On the other hand, the soil in rainforests suffers from just the opposite condition: it has gone beyond maturity and reached old age, when plant growth and water percolation have removed most of its nutrients.

THE

BURROWING PRAIRIE DOG HELPS AERATE SANDY,

GRAVELLY

SOIL

IN

DRY

AREAS.

(© Rich Kirchner/Photo

Researchers. Reproduced by permission.)

support the dense, lush rain-forest ecosystems for which they are noted? The answer is that the abundance of organic material at the surface of the soil continually replenishes its nutrient content. The rapid rate of decay common in warm, moist regions further supports the process of renewing minerals in the ground.

Whether in the desert or in the rainforest, soils near the equator tend to be the “oldest,” and this helps explain why few equatorial regions are noted for their agricultural productivity, even though they enjoy otherwise favorable weather for growing crops. Soils there have been leached of nutrients and contain high levels of iron oxides that give them a reddish color. Moreover, red soil is never good for growing crops: the ancient Egyptians referred to the deserts beyond their realm as “the red land,” while their own fertile Nile valley was “the black land.”

This also explains why the clearing of tropical rainforests, an issue that environmentalists called to the world’s attention in the 1990s, is a serious problem. When the heavy jungle canopy of tall trees is removed, the heat of the sun and the pounding intensity of monsoon rains fall directly on ground that the canopy would normally protect. With the clearing of trees and other vegetation, the animal life that these plants support also disappears, thus removing organisms whose waste products and bodies would have decayed eventually and enriched the soil. Pounded by heat and water and without vegetation to resupply it, the soil in an exposed rainforest becomes hard and dry.

RA I N F O R E S T S . If soil is so poor at

D E S E RT S . In deserts the soil typically

the equator, why do equatorial regions such as the Congo or the Amazon River valley in Brazil

comes from sandstone or shale parent material, and the lack of abundant rainfall, vegetation, or

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Soil

KEY TERMS Topsoil, the uppermost

A HORIZON:

of the three major soil horizons. AERATE:

In general, an atmos-

phere is a blanket of gases surrounding a planet. Unless otherwise identified, howev-

that

breakdown of dead organisms as well as from animal and plant waste products. The principal forms of decomposer are bacteria and fungi. A

er, the term refers to the atmosphere of

DECOMPOSITION

Earth, which consists of nitrogen (78%),

chemical reaction in which a compound is

oxygen (21%), argon (0.93%), and other

broken down into simpler compounds or

substances that include water vapor, car-

into its constituent elements. In the earth

bon dioxide, ozone, and noble gases such

system, this often is achieved through the

as neon (0.07%).

help of detritivores and decomposers. Subsoil, beneath topsoil

B HORIZON:

DETRITIVORES:

REACTION:

Organisms that feed

on waste matter, breaking organic material

and above regolith. The solid rock that lies

down into inorganic substances that then

below the C horizon, the deepest layer of

can become available to the biosphere in

soil.

the form of nutrients for plants. Their

BEDROCK:

BIOGEOCHEMICAL CYCLES:

The

changes that particular elements undergo as they pass back and forth through the various earth systems and particularly between living and nonliving matter. The

function is similar to that of decomposers, but unlike decomposers—which tend to be bacteria or fungi—detritivores are relatively complex organisms, such as earthworms or maggots. A community of inter-

elements involved in biogeochemical cycles

ECOSYSTEM:

are hydrogen, oxygen, carbon, nitrogen,

dependent organisms along with the inor-

phosphorus, and sulfur.

ganic components of their environment.

BIOSPHERE:

A combination of all liv-

EROSION:

The movement of soil and

ing things on Earth—plants, mammals,

rock due to forces produced by water,

birds, reptiles, amphibians, aquatic life,

wind, glaciers, gravity, and other influ-

insects, viruses, single-cell organisms, and

ences. In most cases, a fluid medium, such

so on—as well as all formerly living things

as air or water, is involved.

that have not yet decomposed.

FILL DIRT:

Loose earth that has been

Regolith, which lies

moved into place by a backhoe or some

between subsoil and bedrock and consti-

other earthmoving machine, usually as

tutes the bottommost of the soil horizons.

part of a large construction project.

C

298

Organisms

obtain their energy from the chemical

To make air available to soil.

ATMOSPHERE:

DECOMPOSERS:

HORIZON:

animal life gives the soil little in the way of organic sustenance. For this reason, the A horizon level is very thin and composed of light-colored earth. Then, of course, there are desert

areas made up of sand dunes, where conditions are much worse, but even the best that deserts have to offer is not very good for sustaining abundant plant life.

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Soil

KEY TERMS

CONTINUED

The upper part of

sources, most notably preexisting rock.

Earth’s continental crust, or that portion of

There are three types of sediment: rocks, or

the solid earth on which human beings live

clastic sediment; mineral deposits, or

and which provides them with most of

chemical sediment; and organic sediment,

their food and natural resources.

composed primarily of organic material.

GEOSPHERE:

HUMUS:

Unincorporated, often par-

SEDIMENTARY ROCK:

One of the

tially decomposed plant residue that lies at

three major types of rock, along with

the top of soil and eventually will decay

igneous and metamorphic rock. Sedimen-

fully to become part of it.

tary rock typically has its basis in the deposition, compaction, and cementation of

The entirety of

rock that has experienced weathering,

Earth’s water, excluding water vapor in the

though it also may be formed as a result of

atmosphere but including all oceans, lakes,

chemical precipitation. Organic sediment

streams, groundwater, snow, and ice.

also may be a part of sedimentary rock.

HYDROSPHERE:

LEACHING:

The removal of soil mate-

SEDIMENTATION:

The process of

rials that are in solution, or dissolved in

erosion, transport, and deposition under-

water.

gone by sediment.

ORGANIC:

At one time chemists used

the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals), and oxides, such as carbon dioxide. PARENT MATERIAL:

Mineral frag-

ments removed from rocks by means of weathering. Along with organic deposits, these form the basis for soil.

SOIL HORIZONS:

Layers of soil, par-

allel to the surface of Earth, that have built up over time. They are distinguished from one another by color, consistency, and composition. SOIL PROFILE:

A cross-section com-

bining all or most of the soil horizons that lie between Earth’s surface and the bedrock below it. TOPOGRAPHY:

The configuration of

Earth’s surface, including its relief as well as REGOLITH:

A general term describing

the position of physical features.

a layer of weathered material that rests atop WEATHERING:

bedrock. SEDIMENT:

The breakdown of

rocks and minerals at or near the surface of Material deposited at or

near Earth’s surface from a number of

Earth due to physical, chemical, or biological processes.

Only those species that can endure a limited water supply—for example, the varieties of cactus that grow in the American Southwest—are able to survive. But lack of water is not the only problem.

Desert subsoils often contain heavy deposits of salts, and when rain or irrigation adds water to the topsoil, these salts rise. Thus, watering desert topsoil can make it a worse environment for growth.

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R I C H S O I L S . In striking contrast to

WHERE TO LEARN MORE

the barren soil of the deserts and the potentially barren soil of the rainforest is the rich earth that lies beneath some of the world’s most fertile crop-producing regions. On the plains of the midwestern United States, Canada, and Russia, the soil is black—always a good sign for growth. Below this rich topsoil is a thick subsoil that helps hold in moisture and nutrients.

Bial, Raymond. A Handful of Dirt. New York: Walker, 2000.

The richest variety of soil on Earth is alluvial soil, a youngish sediment of sand, silt, and clay transported by rivers. Large flowing bodies of water, such as the Nile or Mississippi, pull soil along with them as they flow, and with it they bring nutrients from the regions through which they have passed. These nutrients are deposited by the river in the alluvial soil at its delta, the place where it enters a larger body of water—the Mediterranean Sea and the Gulf of Mexico, respectively. Hence the delta regions of both rivers are extremely fertile.

Scheiderman, Jill S. The Earth Around Us: Maintaining a Livable Planet. New York: W. H. Freeman, 2000.

VOLUME 4: REAL-LIFE EARTH SCIENCE

Bocknek, Jonathan. The Science of Soil. Milwaukee, WI: Gareth Stevens, 1999. Canadian Soil Information System (Web site). . Gardner, Robert. Science Projects About the Environment and Ecology. Springfield, NJ: Enslow Publishers, 1999.

Snedden, Robert. Rocks and Soil. Illus. Chris Fairclough. Austin, TX: Raintree Steck–Vaughn, 1999. Soil Association (Web site). . Soil Science Society of America (Web site). . USDA-NRCS National Soil Survey Center (Web site). . World Soil Resources (Web site). .

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S O I L C O N S E R VAT I O N

CONCEPT With the rise of the environmentalist movement in the 1960s and afterward, it has become common to speak of conserving natural resources such as trees or fossil fuels. Yet long before humans recognized the need to make responsible use of things taken from the ground, they learned to conserve the ground itself—that is, the soil. This was a hard-won lesson: failure to conserve soil has turned many a fertile farmland into temporary dust bowl or even permanent desert. Techniques such as crop rotation aid in conservation efforts, but communities continue to face hazards associated with the soil. There is, for instance, the matter of leaching, the movement of dissolved substances through the soil, which, on the one hand, can benefit it but, on the other hand, can rob it of valuable nutrients. Issues of soil contamination also raise concerns that affect not just farmers but the population as a whole.

HOW IT WORKS Billions of Years in the Making Earth’s present wealth of soil is the result of hundreds of millions of years’ worth of weathering, erosion, and sedimentation. Once, long ago, there was no soil, only rock, and it took eons’ worth of weathering to dislodge particles of those rocks. These rocks, when combined with organic materials, became the basis for soil, but before the soil could even begin to take shape, a number of things had to fall into place. Chief among these was the formation of something that, at first glance, at least, does not seem to have

S C I E N C E O F E V E RY DAY T H I N G S

a great deal of bearing on the soil: the atmosphere. In combination with water in the hydrosphere (e.g., streams and rivers) as well as water in the form of evaporated moisture and precipitation in the air itself, the blanket of gases we call our atmosphere has been essential to the formation and sustenance of Earth’s soil. This importance goes beyond the obvious point that rain transports water to the soil, thus making possible the abundance of plant life that grows in it. Rain, of course, is of unquestionable importance, but it is only one of several factors associated with the atmosphere (including the water vapor it contains) that have a role in shaping soil as we know it. To move weathered rocks from highlands to lowlands, where they can become sediment and eventually begin to form soil, it is necessary to subject the rocks themselves to a process of erosion. And erosion—aside from erosion caused by gravity, which usually is considered weathering—can take place only when an atmosphere exists, along with water in the air and on the land. The chief agents of erosion are wind, water (both flowing and in the form of precipitation), and frozen water in the form of icy glaciers, all of which depend on an atmosphere or water or both (see Glaciology). Erosion transports not only rock sediment but organic material as well. Together, these two ingredients are as essential to making soil as tea bags and water are to making tea. Obviously, the greater the organic content, the richer the soil, and here again the air plays a part. It is important that deeper layers of soil receive a supply of air from the surface to sustain the life of subter-

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ranean organisms, who not only process nutrients through the soil but (by their burrowing activities) also aerate it, or make air available to it.

A Product of Its Environment Soil, like most people, is a product of the environment in which it was formed. That environment has five major influencing factors: the nature of the “parent material,” or the rock from which the soil was derived; the local climate; the presence of living organisms; local topography; and the passage of time. Specific classes of mineral break apart in characteristic ways, and the size of the pieces into which the original weathered rock is broken has a great deal to do with the character of the soil that it forms. This does not mean, however, that relatively large rock pieces necessarily will yield the worst soils, since erosive forces will continue to work on the rock, pulling out its nutrient-rich mineral wealth and gradually acting to break it apart. As for climate, it is clear that rain and sun are essential for the growth of plant matter, but, of course, too much of either or both is harmful. (See Soil for a discussion of soils in rainforests.) Plants aid the soil by dying and feeding it with more organic material, but they are not the only types of organism in the soil. Indeed, the soil constitutes an ecosystem in and of itself, a realm rich in biodiversity, in which various biogeochemical cycles are played out, and through which energy flows as part of the operation of the larger Earth system. The underground world teems with creatures ranging from bacteria to moles and prairie dogs (in some regions), each of which fulfills a function. These functions include aerating the soil by burrowing; processing material though ingestion and elimination of waste, thus converting compounds into nutrients that the soil can use; and mixing organic material with minerals. Organisms’ final contribution to the soil comes when they die, as their bodies become material that feeds the earth through decomposition. Topography, or elevation, plays a major role in making possible erosion, itself a process that can be either beneficial or detrimental. The question of whether it is one or the other may be a matter of perspective, or rather elevation. From the standpoint of lowland areas, which receive

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the wealth of the upland areas in the form of nutrient-rich runoff carried by gravity or flowing media, such as wind or water, erosion is a good thing. Matters do not look as good from the viewpoint of the mountains, which lose much of their best soil to low-lying areas. The influence of time in shaping soils—as well as much else about the soil itself—can be appreciated by studying soil horizons, the various strata, or layers, of soil that lie beneath the surface. The most basic division of layers is between the A, B, and C horizons, which differ in depth and physical and chemical characteristics as well as age. S O I L H O R I Z O N S . Above the A horizon, or topsoil, lies humus, decomposing organic material that eventually will become soil. The A horizon itself contains a large amount of organic matter, and thus it may be black, or at least much darker than the soil below it. Between the A and B horizons is a sandy, silty later called the E horizon. Then comes the B horizon, or subsoil, which starts at a depth as shallow as 1 ft. (0.3 m) or deeper than 5 ft. (1.5 m).

Lacking a great deal of organic material but still rich in nutrients, the B horizon has a sizable impact on the A horizon. Minerals—both healthful and harmful—may rise up from the B to the A horizon, and the ability of the B horizon to hold in moisture from above greatly affects the moisture of the A horizon soil. (See Soil for a discussion of how salt deposits in the B horizon affect topsoil in deserts.) Together, A and B horizons constitute what is called the solum, or true soil. The C horizon is called regolith. It is the home for the rocks of the parent material, which has given up much of its nutrient riches in fortifying the soil that lies above it. This far below the surface, there is no sign of plant or animal life, and below the C horizon is the R horizon, or bedrock—the top of the layers of rock and metal that descend all the way to the planet’s core. Once again depths vary, with bedrock as shallow as 5–10 ft. (1.5–3 m) or as deep as 0.5 mi. (0.8 km) or more.

Differences Between Soils The depth of the soil is a measure of wealth— wealth, that is, in terms of natural resources. A sheath over much of the solid earth, soil separates the planet’s surface from its rocky interior and preserves the lives of the plants and animals

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A

BOISE CITY, OKLAHOMA, DURING THE DUST BOWL OF (7.6-10.6 CM) OF TOPSOIL, TURNING ACREAGE THAT ONCE

CLOUD OF TOPSOIL IS PICKED UP BY THE WIND NEAR

1930S.

IN

SOME CASES, WIND REMOVED 3-4 IN.

THE RIP-

PLED WITH WHEAT INTO A DESERTLIKE WASTELAND. (AP/Wide World Photos. Reproduced by permission.)

that live on and in it. It receives rain and other forms of precipitation, which it filters through its layers, as we discuss later, in the context of leaching. Thus, it not only provides water to organisms above and below its surface but also helps prevent flooding by acting as a reservoir. A great deal of soil’s volume is air, for which it also acts as a reservoir. Underground creatures depend on this air and also help circulate it by burrowing. This circulation, in turn, provides oxygen to the roots of plants and makes the soil more hospitable to growth. Even though soil performs these and other life-preserving functions, it would be a mistake to assume that all soils are the same. In fact, the U.S. Department of Agriculture has identified 11 major soil orders, each of which is divided into suborders, groups, subgroups, families, and series. The specificity of soil types, as reflected in the identification and naming of soil series, illustrates the complexity of what at first seems a very simple thing. In fact, soils can be extremely specific, with names that reflect local landmarks. If soils share enough similarities, they are grouped together in a soil series, but it is safe to say that there are thousands of individual soil types on Earth.

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Conserving Soil On a broad level, there are certain types of environment more or less favorable to the formation of rich soil. Some of these types are discussed in the essay Soil, and specific examples of environmental problems are provided later in this essay. Yet almost any environment can become unfavorable to plant growth if proper soil-conservation procedures are not observed. The phrase soil conservation refers to the application of principles for maintaining the productivity and health of agricultural land by control of wind- and water-induced soil erosion. For the remainder of this essay, we examine the dangers involved in such erosion and the use of measures to prevent it. In so doing, we give the matter of soil conservation a somewhat larger scope than the preceding definition might suggest. Since soil affects the world far beyond farms, it seems only fitting to approach it not as a concern merely of agriculture but of the environment in general. Erosion is spoken of here in a general sense, but for a more in-depth discussion of erosive processes, see Erosion. Mass Wasting examines dramatic erosion-related phenomena, such as landslides. Biogeochemical Cycles contains some

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discussion of erosion, inasmuch as it helps circulate life-sustaining chemical elements throughout the various earth systems. Indeed, it is important to remember that erosion is not always negative in its results; on the contrary, it is a valuable process by which landforms are shaped. The erosive processes we explore here, however, generally contribute to the loss of soil health and productivity.

REAL-LIFE A P P L I C AT I O N S The Dust Bowl When people mismanage agricultural lands or when natural forces otherwise conspire to destroy soil, the results can be devastating. One of the most dramatic examples occurred in what came to be known as the dust bowl. This was the name given to a wide area covering Texas, Oklahoma, Kansas, and even agricultural parts of Colorado during the years 1934 and 1935. Over the course of a few months, once-productive farmlands turned into worthless fields of stubble and dust, good for almost nothing and highly vulnerable to violent wind erosion. And wind erosion came, scattering vast quantities of soil from the Great Plains of the Midwest to the Atlantic Seaboard. The classic 1939 film The Wizard of Oz sets its fantastic, otherworldly story against this backdrop, and to viewers in the late 1930s the tornado that swept Dorothy from her Kansas farmland into the world of Oz was all too real. The only difference was that no magical adventure awaited victims of the real-life tornadoes and other windstorms. The fate of the dust bowl farmers, many of whom lost everything, was dramatized in the novel The Grapes of Wrath by John Steinbeck in 1939 as well as in the acclaimed motion picture that followed a year later. A perhaps equally eloquent tribute appeared in the form of the American photographer Dorothea Lange’s photographs of dust bowl refugees. The images etched by Lange are unforgettable: in one a woman stares into the distance, her face a landscape of despair, as her children huddle next to her, their eyes hidden from the camera. In another a man, obviously exhausted from months or years of overwork, hardship, and fear, sits behind the wheel of a truck, gazing somewhere beyond the

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camera lens. Like the woman, he seems to be looking into a future that offers scant hope. CAU S E S O F T H E D U S T B O W L .

What happened? The sad fact is that in the years leading up to the early 1930s, the future dust bowl farmlands had seemed remarkably productive, and farmers continued to be pleasantly surprised, year after year, at the abundant yields they could draw from each field. In fact, farmers were unwittingly preparing the way for vast erosion by overcultivating the land and not taking proper steps to preserve its moisture against drought. This was particularly unfortunate because farmers in the 1930s had long known about the principle of crop rotation as a means of giving the soil a rest in order to restore its nutrients. Yet the farmers of the plains tried to push their crops to yield more and more, and for a time it worked, though at great future expense to the land. One is tempted to see in the agricultural world of the U.S. Midwest parallels to the foolhardy attitude that, just a few years earlier, created a boom on Wall Street, followed by the devastating stock market crash of October 29, 1929, that ushered in the Great Depression. Certainly the ravages of the dust bowl, when they came, were particularly unwelcome in a land already reeling from several years of widespread unemployment and a sagging economy. And though there was no cause-effect relationship between the Wall Street crash and the dust bowl, there is no question that both were brought about in large part by a lack of planning for the future and by a naive belief that it is possible to get “something for nothing”—that is, to get more out of the world (whether the world of finances or the natural world) than one puts into it. In some places farmers alternated between wheat cultivation and livestock grazing on particular plots of land. Thus, the hooves of the cattle damaged the soil, which had been weakened by raising wheat. The land was therefore ready to become the site of a full-fledged natural disaster, and, at the height of the depression, natural disaster came in the form of high winds. The winds in some cases removed topsoil as much as 3–4 in. (7–10 cm) thick. Dunes of dust as tall as 15–20 ft. (4.6–6.1 m) formed, turning acreage that once had rippled with wheat into desertlike wastelands.

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Erosion Control in Action Today the farmlands of the plains states long since have recovered, and American farmers have benefited from the lessons learned in the dust bowl. Out of the dust bowl years came the establishment, in 1935, of the Soil Conservation Service, a federal agency charged with implementing erosion-control practices. (The Soil Conservation Service was the predecessor of the modernday Natural Resources Conservation Service.) In the wake of the legislation creating the agency, signed into law by President Franklin D. Roosevelt (1882–1945), states passed laws creating nearly 3,000 local soil conservation districts. If one passes through agricultural lands today, one is likely to see signs identifying the local conservation district. Even more important, the lands themselves are a testament to principles put into practice as an outgrowth of the dust bowl years. For instance, instead of alternating one year of wheat with one year in which a field lies fallow, or unused, farmers in the dust bowl region discovered that a three-year cycle of wheat, sorghum, and fallow land worked much better. They also planted trees to serve as barriers against wind. E R O S I O N C O N T R O L L E G I S LA T I O N . Concerns over soil conservation in

America did not end with the dust bowl. As United States farm production soared in the 1970s, American farms enjoyed such a great surplus that U.S. farmers increasingly began to sell their crops overseas—most notably, to the Soviet Union. While some Americans were upset to see the farmers of the Midwest selling wheat to the Communists in Moscow, others saw in this act a testament to the failure of the Soviet agricultural system and to the strength of U.S. farming. In the wake of these increased exports, farmers were encouraged to cultivate even marginal croplands to increase profits, thus heightening the vulnerability of their lands to erosion.

lakes with eutrophication (see Biogeochemical Cycles). As a result of public concerns over these and related issues, Congress in 1977 passed the Soil and Water Resources Conservation Act, mandating the conservation of soil, water, and other resources on private farmlands and other properties.

Soil Conservation

In 1985 the Food Security Act further served to encourage steps toward the reduction of soil erosion. Some 45 million acres (18 million hectares) of land vulnerable to erosion were removed from intensive cultivation by the act. The legislation also forbade the conversion of rangelands into agricultural fields, which would have raised great potential for erosion and depletion of already vulnerable soil. In addition, the act required farmers to develop and maintain practices for the control of erosion on lands susceptible to that threat. B A R R I E R A N D C O V E R. Soil-conservation practices fall under two headings: barrier and cover. Under the barrier approach, various structures act as a wall against water runoff, wind, and the movement of soil. Among such structures are banks, hedgerows, walls of earth or other materials, and silt fences such as one sees at construction sites. The cover approach is devoted to the idea of maintaining a heavy soil cover of living and dead plant material. This is achieved through the use of mulch, cover crops, and other techniques.

Local governments and property owners in nonagricultural lands often apply both the cover and barrier approaches, planting trees as well as grass not simply to beautify the land but also to hold the soil in place. Land has to have some sort of vegetative protection to stand between it and the forces of wind and water erosion, and the two approaches together serve to protect soil against nature’s onslaught.

Leaching

What followed was not another dust bowl, however; instead, the experience of the 1970s and 1980s shows just how much American farmers, legislators, and others had learned from the 1930s. Environmental activists in the 1970s, concerned over water quality, helped return public interest to the problem of soil erosion. They called attention to the flow of nutrients from croplands into water resources, most notably leaching of nitrogen and phosphorus that choked

Like erosion, leaching—the movement of dissolved substances with water percolating through soil—can be both positive and negative. For any plot of land, assuming the rate of water input is greater than the rate of water loss through evaporation, water has to go somewhere, so it leaves the site by moving downward. Eventually it either reaches the deep groundwater or passes through subterranean springs to flow into the surface waters of streams, rivers, and lakes.

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Along the way, the leached water carries all sorts of dissolved substances, ranging from nutrients to contaminants. The threat of the latter has led to widespread concern in the United States over the leaching of toxins into water supplies, and in 1980 this concern spurred a massive piece of legislation called CERLA (Comprehensive Environmental Response, Compensation, and Liability Act), better known as Superfund. Six years later, in 1986, Congress updated CERLA with the Superfund Amendments and Reauthorization Act. These laws provided for a vast array of measures directed toward environmental cleanup, including the removal of chemicals and other toxins in soil. Drastic measures such as those outlined in CERLA and other legislation may be required for the cleanup of artificial materials introduced into soils and groundwater. But for human waste and other more natural forms of toxin, nature itself is able to achieve a certain amount of cleanup on its own. In a septic-tank system, used by people who are not connected to a municipal sewage system, bacteria process wastes, removing a great deal of their toxic content in the tank itself. The wastewater leaves the tank and passes through a filtration system, in which the water leaches through layers of gravel and other filters that help remove more of its harmful content. As the wastewater percolates from the filtration system through the soil (usually well below the A horizon by this point), it is purified further before it enters the groundwater supply. Not only does leaching help purify the water that passes through the soil, it also is an important part of the soil-formation process, inasmuch as it passes nutrients to the depths of the A horizon and into the B horizon. Its ability to pass along nutrients is not always beneficial, and in some ecosystems, large amounts of dissolved nitrogen are lost to soil as a result of leaching. In such a situation, soil typically is fertilized with nitrate, a form of the element with which soil often has difficulty binding (see Nitrogen Cycle). For this reason, nitrate tends to leach easily, leading to an overabundance of nitrogen in the lower levels of the soil and in the groundwater. This condition, known as nitrogen saturation, can influence the eutrophication of waters (see Biogeochemical Cycles for an explanation of eutrophication) and can cause the decline and death of trees on the surface.

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Desertification Much of North Africa lies under the cover of a vast desert, the Sahara. By far the world’s largest desert, the Sahara today spreads across some 3.5 million sq. mi. (9.06 million sq km), an area larger than the continental United States. Only about 780 acres (316 hectares) of it, or little more than 1 sq. mi. (2.6 sq km), is fertile. The rest is mostly stone and dry earth with scattered shrubs—and, here and there, the rolling sand dunes typically used to depict the Sahara in movies. Given the forbidding moonscape of the Sahara today, it might be surprising to learn that just 8,000 years ago—the blink of an eye in terms of geologic time—it was a region of flowing rivers and lush valleys. For thousands of years it served as a home to many cultures, some of them quite advanced, to judge from their artwork. Though they left behind an extraordinary record in the form of their rock-art paintings and carvings, which show an understanding of realistic representation that would not be matched until the time of the Greeks, the identity of the early Saharan peoples themselves remains largely a mystery. Instead of identifying them by the name of a nationality or empire, archaeologists divide the phases of the early Saharan culture according to a set of four names that collectively tell the story of the region’s progressive transformation into a desert. First was the Hunter period, from about 6000 to about 4000 B.C., when a Paleolithic, or Old Stone Age, people survived by hunting the many wild animals then available in the region. Next came the Herder period, from about 4000 to 1500 B.C. As their name suggests, these people maintained herds of animals and also practiced basic agriculture. As the Sahara became drier and drier, however, there were no more herds. Egyptians began bringing in domesticated horses to cross the desert: hence the name of the Horse period (ca. 1500–ca. 600 B.C.) By about 600 B.C., not even horses could survive in the forbidding climate. There was only one creature that could survive: the hardy, seemingly inexhaustible camel. Thus began the Camel era, which continues to the present day. AT T E M P T S T O C O N T R O L D E S E RT I F I CAT I O N . As with the dust bowl, the

first question one wants to ask when confronted

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CAMEL CARAVAN IN THE

SAHARA. THE

WORLD’S LARGEST DESERT, IT COVERS 3.5 MILLION SQ. MI. (9 MILLION SQ

KM), BUT 8,000 YEARS AGO THIS WAS A REGION OF LUSH VALLEYS AND FLOWING RIVERS. (© Tom Hollyman/Photo

Researchers. Reproduced by permission.)

with a story such as that of the Sahara, is “What happened?” The answer is much more complex, just as the effects of desertification—the slow transformation of ordinary lands to desert—are much more permanent than those of the erosion associated with the dust bowl. Desertification does not always result in what people normally think of as a desert. It is rather a process that contributes toward making a region more dry and arid, and because it is usually gradual, it can be reversed in some cases. But doing so represents a vast challenge. In 1977 the United Nations (UN), in the form of the UN Conference on Desertification in Nairobi, Kenya, set out to address the spread of the Sahara into the Sahel, an arid region that stretches south of the desert. Some 700 delegates from almost 100 countries adopted a number of measures designed to halt the spread of desertification in that region and others by the year 2000.

state of civil war in such countries as Ethiopia have played at least as important a role in spreading famine as nature itself. During the 1980s, in fact, the government of Ethiopia (at that time a Marxist-Leninist state) deliberately withheld food supplies, shipped to it from the West, as a way of exerting pressure on rebel factions and other groups it wished to subdue. T H E E XA M P L E O F I RAQ . The arid regions of Iraq provide another example of how human influences can result in desertification. Once that country, known in ancient times as Mesopotamia, was among the greenest and most lush places in the known world. For this reason, historians today use the name Fertile Crescent to describe an arc from the deltas of the Tigris and Euphrates rivers in Mesopotamia to the mouth of the Nile in Egypt. Today, of course, Iraq is mostly a dust-colored land of bare trees and brush.

Even though there have been some successes, the Sahel region today remains a blighted area where famine is common, and this state of affairs is not entirely the result of the natural causes addressed in the conference’s resolutions. Poor government management and a near-constant

What happened? Agricultural mismanagement certainly played a role, as did the simple exhaustion of the soil by some 6,000 years of human civilization. Indeed, since the Fertile Crescent was perhaps the first area settled by agricultural societies long before the beginning of full-fledged civilization as such in about 3500

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KEY TERMS Topsoil, the uppermost

A HORIZON:

help of detritivores and decomposers.

of the three major soil horizons. AERATE:

system, this often is achieved through the

To make air available to soil.

DETRITIVORES:

Organisms that feed

on waste matter, breaking organic material Subsoil, beneath topsoil

B HORIZON:

and above regolith.

down into inorganic substances that then can become available to the biosphere in

The solid rock that lies

the form of nutrients for plants. Their

below the C horizon, the deepest layer of

function is similar to that of decomposers;

soil.

however, unlike decomposers—which tend

BEDROCK:

BIOGEOCHEMICAL CYCLES:

The

changes that particular elements undergo as they pass back and forth through the various earth systems and particularly between living and nonliving matter. The elements involved in biogeochemical cycles

to be bacteria or fungi—detritivores are relatively complex organisms, such as earthworms or maggots. ECOSYSTEM:

A community of inter-

dependent organisms along with the inorganic components of their environment. The movement of soil and

are hydrogen, oxygen, carbon, nitrogen,

EROSION:

phosphorus, and sulfur.

rock due to forces produced by water,

C

HORIZON:

Regolith, which lies

between subsoil and bedrock and constitutes the bottommost of the soil horizons.

wind, glaciers, gravity, and other influences. In most cases, a fluid medium, such as air or water, is involved. EUTROPHICATION:

ened biological productivity in a body of

obtain their energy from the chemical

water, which is typically detrimental to the

breakdown of dead organisms as well as

ecosystem in which it takes place. Eutroph-

from animal and plant waste products. The

ication can be caused by an excess of nitro-

principal forms of decomposer are bacteria

gen or phosphorus in the form of nitrates

and fungi.

and phosphates, respectively.

DECOMPOSITION

Organisms

REACTION:

A

HUMUS:

Unincorporated, often par-

chemical reaction in which a compound is

tially decomposed plant residue that lies at

broken down into simpler compounds or

the top of soil and eventually will decay

into its constituent elements. In the earth

fully to become part of it.

B.C.,

308

A state of height-

that

DECOMPOSERS:

it is safe to say that the region has been under cultivation for several thousand years longer—perhaps 8,000 or even 10,000 years. Direct human action and malice also may have played a role: some historians believe that the Mongols, during their brutal invasion in the 1250s, so badly devastated the farmlands and

irrigation channels of Iraq that the land never recovered.

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S O M E CAU S E S O F D E S E RT I F I CAT I O N . With regard to human involve-

ment in the desertification process, it is not necessary for a society to be advanced agriculturally to do long-term damage to the soil. The Pueblan

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KEY TERMS

CONTINUED

A notable topographical

sources, most notably preexisting rock.

feature, such as a mountain, plateau, or

There are three types of sediment: rocks, or

valley.

clastic sediment; mineral deposits, or

LANDFORM:

LEACHING:

The removal of soil mate-

rials that are in solution, or dissolved in water. MASS

chemical sediment; and organic sediment, composed primarily of organic material. SEDIMENTATION:

WASTING:

The transfer of

earth material down slopes by processes

The process of

erosion, transport, and deposition undergone by sediment.

that include creep, slump, slide, flow, and fall. Also known as mass movement.

SOIL CONSERVATION:

The applica-

tion of principles for maintaining the proA naturally occurring, typi-

ductivity and health of agricultural land by

cally inorganic substance with a specific

control of wind- and water-induced soil

chemical composition and a crystalline

erosion. The term also may be applied more

structure.

broadly to encompass the maintenance and

MINERAL:

ORGANIC:

At one time chemists used

protection of nonagricultural soils.

the term organic only in reference to living things. Now the word is applied to most

Layers of soil, par-

SOIL HORIZONS:

compounds containing carbon, with the

allel to the surface of the earth, which have

exception of carbonates (which are miner-

built up over time. These layers are distin-

als) and oxides, such as carbon dioxide.

guished from one another by color, consistency, and composition.

PARENT MATERIAL:

Mineral frag-

ments removed from rocks by means of

SOIL PROFILE:

weathering. Along with organic deposits,

bining all or most of the soil horizons that

these fragments form the basis for soil.

lie between Earth’s surface and the bedrock

REGOLITH:

A general term describing

below it.

a layer of weathered material that rests atop WEATHERING:

bedrock. SEDIMENT:

A cross-section com-

The breakdown of

rocks and minerals at or near the surface of Material deposited at or

near Earth’s surface from a number of

Earth due to physical, chemical, or biological processes.

culture of what is now the southwestern United States depleted an already dry and vulnerable region after about A.D. 800 by removing its meager stands of mesquite trees. And though human causes, in the form of either mismanagement or deliberate damage, have contributed toward desertification, sometimes nature itself is the driving force.

Long-term changes in rainfall or general climate as well as water erosion and wind erosion such as caused the dust bowl can turn a region into a permanent desert. An ecosystem may survive short-term drought, but if soil is forced to go too long without proper moisture, it sets in motion a chain reaction in which plant life dwindles and, with it, animal life as well. Thus, the soil

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is denied the fresh organic material necessary to its continued sustenance, and a slow, steady process of decline begins.

“Desertification.” United States Geological Survey (Web site). . Gardner, Robert. Science Projects About the Environment and Ecology. Springfield, NJ: Enslow Publishers, 1999.

WHERE TO LEARN MORE Bear, Firman E., H. Wayne Pritchard, and Wallace E. Akin. Earth: The Stuff of Life. Norman: University of Oklahoma Press, 1986.

310

Natural Resources Conservation Service (Web site). . Pittman, Nancy P. From the Land. Washington, DC: Island Press, 1988.

Bocknek, Jonathan. The Science of Soil. Milwaukee, WI: Gareth Stevens, 1999.

Soil and Water Conservation Society (Web site). .

Bright Edges of the World: The Earth’s Evolving Drylands (Web site). . Cherrington, Mark. Degradation of the Land. New York: Chelsea House, 1991.

“Voices from the Dust Bowl: The Charles L. Todd and Robert Sonkin Migrant Worker Collection, 1940–1941.” Library of Congress (Web site). .

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GEOCHEMISTRY BIOGEOCHEMICAL CYCLES THE CARBON CYCLE THE NITROGEN CYCLE

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BIOGEOCHEMICAL C YCLES

Elements Involved in Biogeochemical Cycles

CONCEPT Of the 92 elements produced in nature, only six are critical to the life of organisms: hydrogen, carbon, nitrogen, oxygen, phosphorus, and sulfur. Though these elements account for 95% of the mass of all living things, their importance extends far beyond the biosphere. Hydrogen and oxygen, chemically bonded in the form of water, are the focal point of the hydrosphere, while oxygen and nitrogen form the bulk of the atmosphere. All six are part of complex biogeochemical cycles in which they pass through the biosphere, atmosphere, hydrosphere, and geosphere. These cycles circulate nutrients through the soil into plants, microbes, and animals, which return the elements to the earth system through chemical processes that range from respiration to decomposition.

HOW IT WORKS The Elements An element is a substance composed of a single type of atom, meaning that it cannot be broken down chemically to make a simpler substance. They are listed on the periodic table of elements, a chart that renders them in order of their atomic numbers, or the number of protons in the nucleus of the atom. The elements we want to discuss in the context of biogeochemical cycles are all low in atomic number, starting with hydrogen, which has just one proton in its nucleus. In the following list, the elements are cited by atomic number along with their chemical symbols, or the abbreviation by which they are known to chemists.

S C I E N C E O F E V E RY DAY T H I N G S

• • • • • •

1. Hydrogen (B) 6. Carbon (C) 7. Nitrogen (N) 8. Oxygen (O) 15. Phosphorus (P) 16. Sulfur (S) Given the fact, as noted earlier, that 92 elements appear in nature, it should come as no surprise that the highest atomic number for any naturally occurring element is 92, for uranium. Beyond uranium there are about two dozen artificially created elements, but they are of little interest outside the realm of certain specialties in chemistry and physics. The naturally occurring elements are the ones that matter to the earth sciences, and of these elements, only a handful play a significant role. In the essays Minerals and Economic Geology, other elements—most notably silicon—are discussed with regard to their importance in forming minerals, rocks, and ores. Though they are critical to Earth’s systems, elements other than the six discussed here play no role in biogeochemical cycles. Indeed, it is a fact of the physical sciences that not all elements are created equal: certainly, the universe is not divided evenly 92 ways, with equal amounts of all elements. In fact, hydrogen and helium account for 99% of the mass of the entire universe. A B U N DA N C E . On Earth the ratios are quite different, however. Oxygen and silicon constitute the preponderance of the known mass of Earth’s crust, while nitrogen and oxygen form the overwhelming majority of the atmosphere. Hydrogen is proportionally much, much less abundant on Earth than in the universe as a

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whole, but owing to its role in forming water, a substance essential to the sustenance of life, it is unquestionably of great significance. The two following lists provide rankings for the abundance of the six elements discussed in this essay. The first table shows their ranking and share in the entire known mass of the planet, including the crust, living matter, the oceans, and atmosphere. The second shows their relative abundance and ranking in the human body. Abundance of Selected Elements on Earth (Ranking and Percentage) • • • • • •

1. Oxygen (49.2%) 9. Hydrogen (0.87%) 12. Phosphorus (0.11%) 14. Carbon (0.08%) 15. Sulfur (0.06%) 16. Nitrogen (0.03%) Abundance of Selected Elements in the Human Body (Ranking and Percentage) • • • • • •

1. Oxygen (65%) 2. Carbon (18%) 3. Hydrogen (10%) 4. Nitrogen (3%) 6. Phosphorus (1%) 9. Sulfur (0.26%) Several things are interesting about these figures. First and most obviously, there is the fact that the ranking of all these elements (with the exception of oxygen) is relatively low in the total known elemental mass of Earth, whereas their ranking is much, much higher within the human body. This is significant, given the fact that these elements are all essential to the lives of organisms. Furthermore, note that it does not take a great percentage to constitute an “abundant” element: even nitrogen, with its 0.03% share of Earth’s total known mass, still is considered abundant. The presence of the vast majority of elements on Earth is measured in parts per million (ppm) or even parts per billion (ppb).

This change, in fact, should be described not in terms of a discovery so much as the development of a model. Long before chemists and physicists comprehended the structure of the atom, they developed an understanding of all chemical substances as composed of atomic units, each representing one and only one element. Until the development of this model— thanks to a number of chemists, most notably, John Dalton (1766–1844) of England, Antoine Lavoisier (1743–1794) of France, and Amedeo Avogadro (1776–1856) of Italy—chemistry was concerned primarily with mixing potions and observing their effects. Thanks to the atomic model, chemists never again would confuse mixtures with chemical compounds. C H E M I CA L R E AC T I O N S . The difference between a mixture and a compound goes to the heart of the distinction between physics and chemistry. A mixture, such as coffee, is the result of a physical process—in this case, the heating of water and coffee beans—and the result does not have a uniform chemical structure. On the other hand, a compound results from chemical reactions between atoms, which form enormously powerful bonds in the process of joining to create a molecule. A molecule is the basic particle of a compound, just as an atom is to an element. It should be noted that some elements, such as nitrogen, typically appear in diatomic form, that is, two atoms bond to form a molecule of nitrogen.

In a general sense, chemistry can be defined as an area of the physical sciences concerned with the composition, structure, properties, and changes of substances, including elements, compounds, and mixtures. This definition unites the phases in the history of the development of the discipline,

A substance may undergo physical changes without experiencing any alteration in its underlying structure; on the other hand, a chemical reaction makes a fundamental change to the substance. In a chemical reaction, a substance may experience a change of state (i.e., from solid to liquid or gas) without undergoing any physical process of being heated or cooled by an outside source. Chemical reactions involve the breaking of bonds between atoms in a molecule and the formation of new bonds. As a result, an entirely new

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Chemistry and Geochemistry

314

from early modern times—when it arose from alchemy, a set of mystical beliefs based on the idea that ordinary matter can be perfected—to modern times. Our modern understanding of chemistry, however, is quite different from the model of chemistry that prevailed until about 1800, a difference that relates to a key discovery: the atom.

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MOLECULAR

STRUCTURE.

A

COMPOUND IS FORMED BY CHEMICAL REACTIONS BETWEEN ATOMS, WHICH JOIN TOGETH-

ER TO MAKE MOLECULES. (© Blair Seitz/Photo Researchers. Reproduced by permission.)

substance is created—something that could never be achieved through mere physical processes.

REAL-LIFE A P P L I C AT I O N S

time, however, this subdiscipline has come to encompass many other concerns, particularly those discussed in the present context. Geochemistry today focuses on such issues as the recycling of elements between the various sectors of the earth system, especially between living and nonliving things.

Geochemistry Just as geochemistry is a branch of the geologic sciences that weds physics and geology, so there is a geologic subdiscipline, geochemistry, in which chemistry and the geologic sciences come together. Geochemistry is concerned with the chemical properties and processes of Earth—in particular, the abundance and interaction of chemical elements and their isotopes. (Isotopes are atoms that have an equal number of protons, and hence are of the same element, but differ in their number of neutrons. Isotopes may be either stable or unstable, in which case they are subject to the emission of high-energy particles. Some elements have numerous stable isotopes, others have only one or two, and some have none.) Before the mid–twentieth century, geochemistry had a relatively limited scope, confined primarily to the identification of elements in rocks and minerals and the determination of the relative abundance of those elements. Since that

S C I E N C E O F E V E RY DAY T H I N G S

Biogeochemical Cycles and Earth Systems The changes that a particular element undergoes as it passes back and forth through the various earth systems, and particularly between living and nonliving matter, are known as biogeochemical cycles. The four earth systems involved in these cycles are the atmosphere, the biosphere (the sum of all living things as well as formerly living things that have not yet decomposed), the hydrosphere (the entirety of Earth’s water except for vapor in the atmosphere), and the geosphere. The last of these spheres is defined as the upper part of Earth’s continental crust, or that portion of the solid earth on which human beings live and which provides them with most of their food and natural resources. Carbon, for instance, is present in all living things on Earth. Hence, the phrase carbon-based life-form, a cliché found in many an old science-

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BACTERIA

BEGIN THE PROCESS OF DECOMPOSITION OF ORGANIC WASTE, BREAKING DOWN PLANT MATTER AND CON-

VERTING IT INTO COMPOST. (© Scimat/Photo Researchers. Reproduced by permission.)

fiction movie, is actually a redundancy: all lifeforms contain carbon. In the context of the physical sciences, organic refers to all substances that contain carbon, with the exception of oxides, such as carbon dioxide and carbon monoxide, and carbonates, which are found in minerals. Still, carbon circulates between the organic world and the inorganic world, as when an animal exhales carbon dioxide. The carbon cycle is of such importance to the functioning of Earth that it is discussed separately (see Carbon Cycle). So, too, is the nitrogen cycle (see Nitrogen Cycle), whereby nitrogen passes between the soil, air, and biosphere as well as the hydrosphere. The hydrosphere, as noted earlier, is based on a single substance, water, created by the chemical bonding of hydrogen and oxygen, and it is likewise discussed in detail elsewhere (see Hydrologic Cycle). Despite the emphasis here on carbon in the biosphere, nitrogen in the geosphere, and hydrogen and oxygen in the hydrosphere, it should be noted that biogeochemical cycles involving these four elements take them through all four “spheres.” The same is true of sulfur, whose biogeochemical cycle is discussed later in this essay. On the other hand, phosphorus, also discussed

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later, is present in only three of Earth’s systems; it plays little role in the atmosphere.

Decomposers and Detritivores Most biogeochemical cycles involve a special type of chemical reaction known as decomposition, and for this to take place, agents of decomposition—known as decomposers and detritivores— are essential. Decomposition occurs when a compound is broken down into simpler compounds or into its constituent elements. This is achieved primarily by decomposers, organisms that obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. The principal forms of decomposer are bacteria and fungi. These creatures carry enzymes, which they secrete into the materials they consume, breaking them down chemically before taking in the products of this chemical breakdown. They thus take organic matter and render it in inorganic form, such that later it can be taken in again by plants and returned to the biosphere. Detritivores are much more complex organisms, but their role is similar to that of decom-

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posers. They, too, feed on waste matter, breaking this organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Examples of detritivores are earthworms and maggots. As discussed in Energy and Earth, detritivores are key players in the food web, the set of nutritional interactions—sometimes called a food chain—between living organisms.

Phosphorus and the Phosphorus Cycle There are a few elements that were known in ancient or even prehistoric times, examples being gold, iron, lead, and tin. The vast majority, on the other hand, have been discovered since the beginning of the modern era, and the first of them was phosphorus, which is also the first element whose discoverer is known. In 1674 the German alchemist Hennig Brand (ca. 1630–ca. 1692) was searching for the philosopher’s stone, a mythical substance that allegedly would turn common or base metals into gold. Convinced that he would find this substance in the human body, Brand evaporated water from a urine sample and burned the precipitate (the solid that remained) along with sand. The result was a waxy, whitish substance that glowed in the dark and reacted violently with oxygen. Brand named it phosphorus, a name derived from a Greek term meaning “lightbearer.” Owing to its high reactivity with oxygen, phosphorus is used in the production of safety matches, smoke bombs, and other incendiary devices. It is also important in various industrial applications and in fertilizers. In fact, ancient humans used phosphorus without knowing it when they fertilized their crops with animal bones. P H O S P H AT E S . In the early 1800s, chemists recognized that the critical component in bones was phosphorus, which plants use in photosynthesis—the biological conversion of energy from the Sun into chemical energy (see Energy and Earth). With this discovery came the realization that phosphorus would make an even more effective fertilizer when treated with sulfuric acid, which makes it soluble, or capable of being dissolved, in water. This compound, known as superphosphate, can be produced from phosphates, a type of mineral.

S C I E N C E O F E V E RY DAY T H I N G S

Phosphates represent one of the eight major classes of mineral (see Minerals). All phosphates contain a characteristic formation, PO4, which is bonded to other elements or compounds—for example, with aluminum in aluminum phosphate, or AlPO4. Phosphorus fertilizer is typically calcium phosphate, known as bone ash, the most important industrial mineral (see Economic Geology) produced from phosphorus. Another significant phosphate is sodium phosphate, used in dishwashing detergents. In fact, phosphates once played a much larger role in the detergent industry—with disastrous consequences, as we shall see. THE

PHOSPHORUS

Biogeochemical Cycles

CYCLE.

The majority of phosphorus in the earth system is located in rocks and deposits of sediment, from which it can be removed by one of three processes: weathering, the breakdown of rocks and minerals at or near the surface of Earth as the result of physical, chemical, or biological processes (see Erosion, Sediment and Sedimentation); leaching, the removal of soil materials that are in solution, or dissolved in water; and mining. Phosphorus is highly reactive, meaning that it is likely to bond with other elements, and for this reason it often is found in compounds. Microorganisms absorb insoluble phosphorus compounds (ones that are incapable of being dissolved) and, through the action of acids within the microorganisms, turn them into soluble phosphates. Algae and other green plants absorb these phosphates and, in turn, are eaten by animals. When they die, the animals release the phosphates back into the soil. As with all elements, the total amount of phosphorus on Earth stays constant, but the distribution of it does not. Some of the phosphorus passes from the geosphere into the biosphere, but the majority of it winds up in the ocean. It may find its way into sediments in shallow waters, in which case it continues to circulate, or it may be taken to the deep parts of the seas, in which case it is likely to be deposited for the long term. Fish absorb particles of phosphorus, and thus some of the element returns to dry land through the catching and consumption of seafood. In addition, guano, or dung, from birds that live in an ocean environment (e.g., seagulls) also returns portions of phosphorus to the terrestrial environment. Nonetheless, geochemists

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A

STAND OF FIR TREES SHOWS THE DEVASTATING EFFECTS OF ACID RAIN, WHICH IS CREATED WHEN SULFURIC ACID

MIXES WITH MOISTURE IN THE ATMOSPHERE. (© Will and Demi McIntyre/Photo Researchers. Reproduced by permission.)

believe that phosphorus is being transferred steadily to the ocean, from whence it is not likely to return. It is for this reason that phosphorusbased fertilizers are important, because they feed the soil with nutrients that otherwise would be continually lost.

otherwise would be available to fish, mollusks, and other forms of life. As a result, those species die off, to be replaced by others that are more tolerant of lowered oxygen levels—for example, worms. Needless to say, the outcome of eutrophication is devastating to the lake’s ecosystem.

To be sure, phosphorus, in the proper quantities, is good for the environment. But as with medicine or any other beneficial substance, if a little is good, that does not mean that a lot is necessarily better. In the case of phosphorus, an overabundance of the element in the environment can lead to a phenomenon called eutrophication, a state of heightened biological productivity in a body of water. One of the leading causes of eutrophication (from a Greek term meaning “well nourished”) is a high rate of nutrient input, in the form of phosphates or nitrates, a nitrogen-oxygen compound (see Nitrogen Cycle).

During the 1960s, Lake Erie—one of the Great Lakes on the U.S.-Canadian border— became an example of eutrophication gone mad. As a result of high phosphate concentrations, Erie’s waters were choked with plant and algae growth. Fish were unable to live in the water, the beaches reeked with the smell of decaying algae, and Erie became widely known as a “dead” body of water. This situation led to the passage of new environmental standards and pollution controls by both the United States and Canada, whose governments acted to reduce drastically the phosphate content in fertilizers and detergents. Lake Erie proved to be an environmental success story: within a few decades the lake once again teemed with life.

E U T R O P H I C AT I O N .

As a result of soil erosion, fertilizers make their way into bodies of water, as does detergent runoff in wastewater. Excessive phosphates and nitrates stimulate growth in algae and other green plants, and when these plants die, they drift to the bottom of the lake or other body of water. There, decomposers consume the remains of the plants and, in the process, also use oxygen that

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Sulfur and the Sulfur Cycle If there is any element that can be said to have a bad image—and a falsely bad one at that—it is sulfur. As everyone “knows,” sulfur has a foul smell, and this smell, combined with its com-

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KEY TERMS ALCHEMY:

A set of mystical beliefs

based on the idea that ordinary matter can be perfected. Though it was a pseudo-

more than one chemical element. The result is the formation of a molecule. CHEMICAL SYMBOL:

A one-letter or

science, alchemy, which flourished in the

two-letter abbreviation for the name of an

late Middle Ages, was a forerunner of sci-

element.

entific chemistry.

A substance made up of

COMPOUND: ATMOSPHERE:

In general, an atmos-

phere is a blanket of gases surrounding a

atoms of more than one element chemically bonded to one another.

planet. Unless otherwise identified, however, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon (0.07%). ATOMIC NUMBER:

The number of

protons in the nucleus of an atom. Since this number is different for each element, elements are listed on the periodic table in order of atomic number.

Organisms

that

obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. The principal forms of decomposers are bacteria and fungi. DECOMPOSITION

REACTION:

A

chemical reaction in which a compound is broken down into simpler compounds or into its constituent elements. In the earth system, this often is achieved through the help of detritivores and decomposers.

BIOGEOCHEMICAL CYCLES:

The

changes that particular elements undergo as they pass back and forth through the various earth systems and particularly between living and nonliving matter. The elements involved in biogeochemical cycles are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur. BIOSPHERE:

DECOMPOSERS:

Organisms that feed

DETRITIVORES:

on waste matter, breaking organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Their function is similar to that of decomposers; however, unlike decomposers—which tend to be bacteria or fungi—detritivores are

A combination of all liv-

ing things on Earth—plants, mammals, birds, amphibians, reptiles, aquatic life,

relatively complex organisms, such as earthworms or maggots. ECOSYSTEM:

A term referring to a

insects, viruses, single-cell organisms, and

community of interdependent organisms

so on—as well as all formerly living things

along with the inorganic components of

that have not yet decomposed.

their environment.

CARNIVORE:

A meat-eating organism.

ELEMENT:

A substance made up of

The joining

only one kind of atom. Unlike compounds,

through electromagnetic force of atoms

elements cannot be broken chemically into

that sometimes, but not always, represent

other substances.

CHEMICAL BONDING:

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KEY TERMS ened biological productivity in a body of

and which provides them with most of their food and natural resources.

water, which is typically detrimental to the

HERBIVORE:

EUTROPHICATION:

A state of height-

ecosystem in which it takes place. Eutrophication can be caused by an excess of nitrogen or phosphorus in the form of nitrates and phosphates, respectively. FOOD WEB:

A term describing the

interaction of plants, herbivores, carnivores, omnivores, decomposers, and detritivores, each of which consumes nutrients and passes it along to other organisms. GEOCHEMISTRY:

A branch of the earth

sciences, combining aspects of geology and chemistry, that is concerned with the chemical properties and processes of Earth—in particular, the abundance and interaction of chemical elements and their isotopes. GEOSPHERE:

The upper part of

Earth’s continental crust, or that portion of the solid earth on which human beings live

bustibility, led to the biblical association of brimstone—the ancient name for the element—with the fires of hell. It may come as a surprise, then, to learn that sulfur has no smell of its own. Only in combination with other elements does it acquire the offensive odor that has led to its unpleasant reputation. An example of such a compound is hydrogen sulfide, a poisonous substance present in intestinal gas. The May 2001 National Geographic included two stories relating to the presence of natural hydrogen sulfide deposits on opposite sides of the earth, and in both cases the presence of these toxic fumes created unusual ecosystems.

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CONTINUED

A plant-eating organism.

The entirety of Earth’s water, excluding water vapor in the atmosphere but including all oceans, lakes, streams, groundwater, snow, and ice. HYDROSPHERE:

The removal of soil materials that are in solution, or dissolved in water. LEACHING:

A naturally occurring, typically inorganic substance with a specific chemical composition and a crystalline structure. MINERAL:

A substance with a variable composition, meaning that it is composed of molecules or atoms of differing types. Compare with compound. MIXTURE:

A group of atoms, usually but not always representing more than one element, joined in a structure. Compounds typically are made up of molecules.

MOLECULE:

made Villa Luz their home are species of fish colored bright red; the pigmentation is a result of the fact that they have to produce high quantities of hemoglobin (a component in red blood cells) to survive on the scant oxygen. The waters of the cave also are populated by microorganisms that oxidize the hydrogen sulfide and turn it into sulfuric acid, which dissolves the rock walls and continually enlarges the cave.

A system of caves known as Villa Luz in southern Mexico contains some 20 underground springs that carry large quantities of hydrogen sulfide. Among the strange creatures that have

Thousands of miles away, in the Black Sea, explorers examining evidence of a great ancient flood like the one depicted in the Bible (see Earth, Science, and Nonscience) found an unexpected ally in the form of hydrogen sulfide. Because the Black Sea lacks the temperature differences that cause water to circulate from the bottom upward, hydrogen sulfide had gathered at the bottom and stayed there, covered by dense layers of saltwater. Oxygen could not reach the

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Biogeochemical Cycles

KEY TERMS OMNIVORE:

An organism that eats

both plants and other animals. ORGANIC:

At one time chemists

CONTINUED

The higher the reactivity, the greater the tendency to bond. SEDIMENT:

Material deposited at or

used the term organic only in reference to

near Earth’s surface from a number of

living things. Now the word is applied to

sources, most notably preexisting rock.

most compounds containing carbon,

There are three types of sediment: rocks,

with the exception of carbonates (which

or clastic sediment; mineral deposits, or

are minerals) and oxides, such as carbon

chemical sediment; and organic sedi-

dioxide.

ment, composed primarily of organic

PERIODIC TABLE OF ELEMENTS:

A chart that shows the elements arranged in order of atomic number along with their

material. SOLUBLE:

Capable of being dissolved.

SOLUTION:

A homogeneous mixture

chemical symbols and the average atomic

(i.e., one that is the same throughout) in

mass for each particular element.

which one or more substances is dissolved

PHOTOSYNTHESIS:

The biological

conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants. REACTIVITY:

in another substance—for example, sugar dissolved in water. WEATHERING:

The breakdown of

rocks and minerals at or near the surface of A term referring to the

ability of one element to bond with others.

bottom of the Black Sea, and thus wood-boring worms could not live in the toxic environment. As a result, a 1,500-year-old shipwreck had been virtually undisturbed. THE SULFUR CYCLE. Sulfur is removed from rock by weathering, at which point it reacts with oxygen in the air to form sulfate, or SO4. This sulfate is taken in by plants and microorganisms, which convert it to organic materials and pass it on to animals in the food web. Later, when these organisms die, decomposers absorb the sulfur from their bodies and return it to the environment. As with phosphorus, however, sulfur is being lost continually to the oceans as it drains through lakes and streams (and through the atmosphere) on its way to the sea.

Earth due to physical, chemical, or biological processes.

food webs, and some drifts to the bottom to bond with iron in the form of ferrous sulfide, or FeS. Ferrous sulfide contributes to the dark color of sediments at the bottom of the ocean. On the other hand, sulfur may be returned to the atmosphere, released by spray from saltwater. In addition, sulfur can pass into the atmosphere as the result of volcanic activity or through the action of bacteria, which release it in the form of hydrogen sulfide, the foul-smelling gas discussed earlier.

In the ocean ecosystem, sulfur can take one of three routes. Some of it circulates through

As with all biogeochemical cycles, humans play a part in the sulfur cycle, and the role of modern industrial society is generally less than favorable, as is true of most such cycles. A particularly potent example is the production of acid rain. Among the impurities in coal is sulfur, and when coal is burned (as it still is, for instance, in electric power plants), it results in the production of sulfur dioxide and sulfur trioxide—SO2

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and SO3, respectively. Sulfur trioxide reacts with water in the air to produce sulfuric acid, or H2SO4. This mixes with moisture in the atmosphere to create acid rain, which is hazardous to both plant and animal life.

Hancock, Paul L., Brian J. Skinner. The Oxford Companion to the Earth. New York: Oxford University Press, 2000.

WHERE TO LEARN MORE

Kump, Lee R., James F. Kasting, and Robert G. Crane. The Earth System. Upper Saddle River, NJ: Prentice Hall, 2000.

Beatty, Richard. Sulfur. New York: Benchmark Books, 2000.

Life and Biogeochemical Cycles (Web site). .

———. Phosphorus. New York: Benchmark Books, 2001.

Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science. 2d ed. New York: John Wiley and Sons, 1999.

Biogeochemical Cycles (Web site). .

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Geochemistry on the World Wide Web (Web site). .

General Chemistry Online (Web site). .

WebElements Periodic Table of the Elements (Web site). .

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THE CARBON C YCLE

CONCEPT If a person were asked to name the element most important to sustaining life, chances are he or she would say oxygen. It is true that many living things depend on oxygen to survive, but, in fact, carbon is even more fundamental to the sustenance of life. Indeed, in a very real sense, carbon is life, since every living thing contains carbon and the term organic refers to certain varieties present in all life-forms. Yet carbon, in the form of such oxides as carbon dioxide as well as carbonates like calcium carbonate, is a vital part of the inorganic realm as well. Hence, the carbon cycle, by which the element is circulated through the biosphere, geosphere, atmosphere, and hydrosphere, is among the most complex of biogeochemical cycles.

HOW IT WORKS Geochemistry Chemistry is concerned with the composition, structure, properties, and changes of substances, including elements, compounds, and mixtures. Central to the discipline is the atomic model, or the idea that all matter is composed of atoms, each of which represents one and only one chemical element. An element thus is defined as a substance made up of only one kind of atom, which cannot be broken chemically into other substances. A chemical reaction involves either the bonding of one atom with another or the breaking of chemical bonds between atoms.

has widened to take in aspects of other disciplines and subdisciplines. With its focus on such issues as the recycling of elements between the various sectors of the earth system, especially between living and nonliving things, geochemistry naturally encompasses biology, botany, and a host of earth science subdisciplines, such as hydrology. BIOGEOCHEMICAL

CYCLES.

Among the most significant areas of study within the realm of geochemistry are biogeochemical cycles. These are the changes that a particular element undergoes as it passes back and forth through the various earth systems—particularly between living and nonliving matter. As we shall see, this transition between the worlds of the living and the nonliving is particularly interesting where carbon is concerned. Along with carbon, five other elements— hydrogen, nitrogen, oxygen, phosphorus, and sulfur—are involved in biogeochemical cycles. With the exception of phosphorus, which plays little part in the atmosphere, these elements move through all four earth systems, including the atmosphere, the biosphere (the sum of all living things as well as formerly living things that have not yet decomposed), the hydrosphere (Earth’s water, except for water vapor in the atmosphere), and the geosphere, or the upper part of Earth’s continental crust.

Geochemistry brings together geology and chemistry, though as the subdiscipline has matured in the period since the 1940s, its scope

Earth systems and biogeochemical cycles are discussed in greater depth within essays devoted to those topics (see Earth Systems and Biogeochemical Cycles). Likewise, the nitrogen cycle is treated separately (see Nitrogen Cycle). The role of hydrogen and oxygen, which chemically bond

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THE NAME CARBON comes from the Latin word for charcoal, carbo. Coal has a wide variety of uses, from manufacturing steel to generating electricity. (© Andrew J. Martinez/Photo Researchers. Reproduced by permission.)

to form water, is discussed in the context of the hydrosphere (see Hydrologic Cycle).

Elements and Compounds We have referred to elements and compounds, which are essential to the study of chemistry; now let us examine them briefly before going on to the subject of a specific and very important element, carbon. An element is defined not by outward characteristics, though elements do have definable features by which they are known; rather, the true meaning of the term element is discernible only at the atomic level. Every atom has a nucleus, which contains protons, or subatomic particles of positive electric charge. The identity of an element is defined by the number of protons in the nucleus: for instance, if an atom has only a single proton, by definition it must be hydrogen. An atom with six protons in the nucleus, on the other hand, is always an atom of carbon. Thus, the elements are listed on the periodic table of elements by atomic number, or the number of protons in the atomic nucleus.

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the definition of an element, they play no role in the bonding between atoms, which usually produces chemical compounds. (The reason for this is qualified by the modifier usually, in that sometimes two atoms of the same element may bond as well.) Chemical bonding involves only the electrons, which are negatively charged subatomic particles that spin around the nucleus. In fact, only certain of these fast-moving particles take part in bonding. These are the valence electrons, which occupy the highest energy levels in the atom. One might say that valence electrons are at the “outside edge” of the atom, though the model of atomic structure, considered only in the briefest form here, is far more complex than that phrase implies. In any case, elements have characteristic valence electron patterns that affect their reactivity, or their ability to bond. Carbon is structured in such a way that it can form multiple bonds, and this feature plays a significant part in its importance as an element.

E L E C T R O N S A N D C H E M I CA L R E AC T I O N S . While protons are essential to

When an element reacts with another, they join together, generally in a molecule (we will examine some exceptions), to form a compound. Though the atoms themselves remain intact, and an element can be released from a compound, a

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compound quite often has properties quite unlike those of the original elements. Carbon and oxygen are essential to sustaining life, but when a single atom of one bonds with a single atom of the other, they form a toxic gas, carbon monoxide. And whereas carbon in its elemental form is a black powder and hydrogen and oxygen are colorless, odorless gases, when bonded in the proper proportions and structure, the three create sugar.

The Carbon Cycle

Carbon The name carbon comes from the Latin word for charcoal, carbo. In fact, charcoal—wood or other plant material that has been heated without enough air present to make it burn—is just one of many well-known substances that contain carbon. Others include coal, petroleum, and other fossil fuels, all of which contain hydrocarbons, or chemical compounds built around strings of carbon and hydrogen atoms. Graphite is pure carbon, and coke, a refined version of coal, is very nearly pure. Not everything made of carbon is black, however: diamonds, too, are pure carbon in another form. Though carbon makes up only a small portion of the known elemental mass in Earth’s crust, waters, and atmosphere—just 0.08%, or 1/1,250 of the whole—it is the fourteenth most abundant element on the planet. In the human body, carbon is second only to oxygen in abundance and accounts for 18% of the body’s mass. Present in the inorganic rocks of the ground and in the living creatures above it, carbon is everywhere in the earth system. CA R B O N B O N D I N G . There are two elements noted for their ability to form long strings of atoms and seemingly endless varieties of molecules: one is carbon, and the other is silicon, directly below it on the periodic table. Just as carbon forms a vast array of organic compounds, silicon, found in a huge variety of minerals, is at the center of a large number of inorganic compounds. Yet carbon is capable of forming an even greater number of bonds than silicon. (For more about silicon and the silicates, see the entries Minerals and Economic Geology.)

A

DIAMOND IS AN ALLOTROPE, A CRYSTALLINE FORM,

OF CARBON.

ESSENTIALLY,

IT IS A HUGE MOLECULE

COMPOSED OF CARBON ATOMS STRUNG TOGETHER BY COVALENT BONDS. (© V. Fleming/Photo Researchers. Reproduced

by permission.

atoms) with which to form chemical bonds. Normally, an element does not necessarily have the ability to bond with as many other elements as it has valence electrons, but carbon—with its four valence electrons—happens to be tetravalent, or capable of bonding to four other atoms at once. Additionally, carbon can form not just a single bond but also a double bond or even a triple bond with other elements. ALLOTROPES

OF

CARBON.

Carbon has several allotropes—different versions of the same element distinguished by their molecular structure. The first of them is graphite, a soft material that most of us regularly encounter in the form of pencil “lead.” Graphite is essentially a series of one-atom-thick sheets of carbon bonded together in a hexagonal pattern, but with only very weak attractions between adjacent sheets.

Carbon is distinguished further by its high value of electronegativity, the relative ability of an atom to attract valence electrons. In addition, with four valence electrons, carbon is ideally suited to finding other elements (or other carbon

Then there is that most alluring of all carbon allotropes, diamond. Neither diamonds nor graphite, strictly speaking, are formed of molecules. Their arrangement is definite, as with a molecule, but their size is not: they simply form repeating patterns that seem to stretch on forev-

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er. Whereas graphite is in the form of sheets, a diamond is basically a huge “molecule” composed of carbon atoms strung together by what are known as covalent chemical bonds. Graphite and diamond are both crystalline—solids in which the constituent parts have a simple and definite geometric arrangement that is repeated in all directions. (All minerals are crystalline in structure. See Minerals.) A third carbon allotrope, buckminsterfullerene, discovered in 1985 and named after the American engineer and philosopher R. Buckminster Fuller (1895–1983), is also crystalline in form. Carbon takes yet another form, distinguished from the other three allotropes in that it is amorphous in structure—lacking a definite shape—as opposed to crystalline. Though it retains some of the microscopic structures of the plant cells in the wood from which it is made, charcoal is mostly amorphous carbon. Coal and coke are particularly significant varieties of amorphous carbon. Formed by the decay of fossils, coal was the first important fossil fuel (discussed later in this essay) used to provide heat and power to human societies.

REAL-LIFE A P P L I C AT I O N S Organic Chemistry Organic chemistry is the study of carbon, its compounds (with the exception of the carbonates and oxides mentioned earlier), and their properties. At one time chemists thought that organic was synonymous with living, and even as recently as the early nineteenth century, they believed that organic substances contained a supernatural “life force.” Then, in 1828, the German chemist Friedrich Wöhler (1800–1882) made an amazing discovery. By heating a sample of ammonium cyanate, a material from a nonliving source, Wöhler converted it to urea, a waste product in the urine of mammals. As he later observed, “without benefit of a kidney, a bladder, or a dog,” he had turned an inorganic substance into an organic one. It was almost as though he had created life. Actually, what he had discovered was the distinction between organic and inorganic material, which results from the way in which the carbon chains are arranged.

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Organic chemistry encompasses the study of many things that people commonly think of as “organic”—living creatures, formerly living creatures, and the parts and products of their bodies—but it also is concerned with substances that seem quite far removed from the living world. Among these substances are rubber, vitamins, cloth, and paper, but even in these cases, it is easy to see the relationship to a formerly living organism: a rubber plant, or a tree that was cut down to make wood pulp. But it might come as a surprise to learn that plastics, which at first glance would seem completely divorced from the living world, also have an organic basis. All manner of artificial substances, such as nylon and polyester, are made from hydrocarbons. F O SS I L F U E L S . During the Mesozoic era, which began about 248.2 million years ago, dinosaurs ruled the earth; then, about 65 million years ago, a violent event brought an end to their world. The cause of this mass extinction is unknown, though it is likely that a meteorite hit the planet, sending so much dust into the atmosphere that it dramatically changed local climates, bringing about the destruction of the dinosaurs—along with a huge array of other animal and plant forms. (See Paleontology for more on this subject.)

The bodies of the dinosaurs, along with those of other organisms, were deposited in the solid earth and covered by sediment. They might well have simply rotted, and indeed many of them probably did. But many of these organisms were deposited in an anaerobic, or non-oxygencontaining, environment. Rather than simply rotting, this organic material underwent transformation into hydrocarbons and became the basis for the fossil fuels, the most important of which—from the standpoint of modern society—is petroleum. (See Economic Geology for more on this subject.)

Carbonates Carbonates are important forms of inorganic carbon in the geosphere. In chemical terms, a carbonate is made from a single carbon atom bonded to three oxygen atoms, but in mineralogical terms, carbonates are a class of mineral that may contain carbon, nitrogen, or boron in a characteristic molecular formation. Typically, a carbonate is transparent and light in color with a relatively high density.

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CALCIUM

CARBONATE, ONE OF THE MOST COMMON COMPOUNDS IN THE GEOSPHERE, IS FOUND IN SEASHELLS,

EGGSHELLS, PEARLS, AND CORAL

(PICTURED

HERE), BRIDGING THE BOUNDARY BETWEEN THE LIVING AND THE NON-

LIVING. (© Fred McConnaughey/Photo Researchers. Reproduced by permission.)

Among carbonate minerals, the most significant compound is calcium carbonate (CaCO3). One of the most common compounds in the entire geosphere, constituting 7% of the known crustal mass, it is found in such rocks as limestone, marble, and chalk. (Just as pencil “lead” is not really lead, the “chalk” used for writing on blackboards is actually gypsum, a form of calcium sulfate.) Additionally, calcium carbonate can combine with magnesium to form dolomite, and in caves it is the material that makes up stalactites and stalagmites. Yet calcium carbonate also is found in coral, seashells, eggshells, and pearls. This is a good example of how a substance can cross the chemical boundary between the worlds of the living and nonliving.

Carbon Dioxide and Carbon Monoxide Historically, carbon dioxide was the first gas to be distinguished from ordinary air, when in 1630 the Flemish chemist and physicist Jan Baptista van Helmont (1577?–1644) discovered that air was not a single element, as had been thought up to that time. The name perhaps most closely associated with carbon dioxide, however, is that of the English chemist Joseph Priestley (1733–1804), who created carbonated water, used today in making soft drinks. Not only does the gas add bubbles to drinks, it also acts as a preservative.

In the oceans, calcium reacts with dissolved carbon dioxide, forming calcium carbonate and sinking to the bottom. Millions of years ago, when oceans covered much of the planet, sea creatures absorbed calcium and carbon dioxide from the water, which reacted to form calcium carbonate that went into their shells and skeletons. After they died, their bodies became sedimented in the ocean floor, forming vast deposits of limestone.

By Priestley’s era, chemists had begun to glimpse a relationship between plant life and carbon dioxide. Up until that time, it had been believed that plants purify the air by day and poison it at night. Today we know that carbon dioxide is an essential component in the natural balance between plant and animal life. Animals, including humans, breathe in air, and, as a result of a chemical reaction in their bodies, the oxygen molecules (O2) bond with carbon to produce carbon dioxide. Plants “breathe” in this carbon dioxide (which is as important to their survival as air is to animals), and a reverse reaction leads

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KEY TERMS AMORPHOUS:

A term for a type of

solid that lacks a definite shape. Compare with crystalline. ATOMIC NUMBER:

The number of

protons in the nucleus of an atom. Since this number is different for each element, elements are listed on the periodic table in order of atomic number. BIOGEOCHEMICAL CYCLES:

The

changes that particular elements undergo as they pass back and forth through the various earth systems and particularly between living and nonliving matter. The elements involved in biogeochemical cycles are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur. CELLULAR

A

process that, when it takes place in the presence of oxygen, involves the intake of organic substances, which are broken down into carbon dioxide and water, with the release of considerable energy. The joining

through electromagnetic force of atoms that sometimes, but not always, represent

to the release of oxygen from the plants back into the atmosphere.

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COMPOUND: A substance made up of atoms of more than one element, chemically bonded to one another.

A type of solid in which the constituent parts have a simple and definite geometric arrangement that is repeated in all directions. CRYSTALLINE

SOLID:

Organisms that obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. The principal forms of decomposer are bacteria and fungi.

DECOMPOSERS:

A chemical reaction in which a compound is broken down into simpler compounds, or into its constituent elements. In the earth system, this often is achieved through the help of detritivores and decomposers.

DECOMPOSITION

RESPIRATION:

CHEMICAL BONDING:

more than one chemical element. The result is the formation of a molecule.

REACTION:

A term referring to a community of interdependent organisms along with the inorganic components of their environment. ECOSYSTEM:

When humans ingest carbon monoxide, it bonds with iron in hemoglobin, the substance in red blood cells that transports oxygen throughout the body. In effect, carbon monoxide fools the body into thinking that it is receiving oxygenated hemoglobin, or oxyhemoglobin. Upon reaching the cells, carbon monoxide has much less tendency than oxygen to break down, and therefore it continues to circulate throughout the body. Low concentrations can cause nausea, vomiting, and other effects, while prolonged exposure to high concentrations can result in death.

C A R B O N M O N O X I D E . Priestley discovered another carbon-oxygen compound quite different from carbon dioxide: carbon monoxide. The latter is used today by industry for several purposes, such as the production of certain fuels, proving that this toxic gas can be quite beneficial when used in a controlled environment. Nonetheless, carbon monoxide produced in an uncontrolled environment—generated by the burning of petroleum in automobiles as well as by the combustion of wood, coal, and other carbon-containing fuels—is extremely hazardous to human health.

Although we have referred to carbon monoxide

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THE

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E F F E C T.

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KEY TERMS ELECTRON:

A negatively charged par-

ticle in an atom, which spins around the nucleus. ELEMENT:

A substance made up of

only one kind of atom. Unlike compounds, elements cannot be broken chemically into other substances. FOSSIL FUELS:

At one time chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon and hydrogen, thus excluding carbonates (which are minerals) and oxides such as carbon dioxide.

ORGANIC:

PERIODIC TABLE OF ELEMENTS:

Fuel derived from

deposits of organic material that have experienced decomposition and chemical alteration under conditions of high pressure. These nonrenewable forms of bioenergy include petroleum, coal, peat, natural gas, and their derivatives. GEOCHEMISTRY:

CONTINUED

A branch of the

A chart that shows the elements arranged in order of atomic number along with the chemical symbol and the average atomic mass for each particular element. The biological conversion of light energy (that is, electromagnetic energy) from the Sun to chemical energy in plants. PHOTOSYNTHESIS:

A positively charged particle

earth sciences, combining aspects of geolo-

PROTON:

gy and chemistry, that is concerned with

in an atom.

the chemical properties and processes of

REACTIVITY:

Earth—in particular, the abundance and interaction of chemical elements and their isotopes. HYDROCARBON:

Any organic chemi-

cal compound whose molecules are made up of nothing but carbon and hydrogen atoms.

as toxic, it should be noted that carbon dioxide also would be toxic to a human or other animal—for instance, if one were trapped in a sealed compartment and forced to breathe in the carbon dioxide released from one’s lungs. On a global scale, both carbon dioxide and carbon monoxide in the atmosphere, produced in excessive amounts by the burning of fossil fuels, pose a potentially serious threat.

A term referring to the ability of one element to bond with others. The higher the reactivity, the greater the tendency to bond.

Electrons that occupy the highest principal energy level in an atom. These are the electrons involved in chemical bonding. VALENCE ELECTRONS:

in aerosol cans, however, has produced a much greater quantity of greenhouse gases than the atmosphere needs to maintain normal heat levels. As a result, some scientists believe, buildup of greenhouse gases in the atmosphere is causing global warming.

Cellular Respiration

Both gases are believed to contribute to the greenhouse effect, which, as discussed in Energy and Earth, is a mechanism by which the planet efficiently uses the heat it receives from the Sun. Human consumption of fossil fuels and use of other products, including chlorofluorocarbons

The burning of fossil fuels is one of three ways that carbon enters the atmosphere, the others being volcanic eruption and cellular respiration. When cellular respiration takes place in the presence of oxygen, there is an intake of organic substances, which are broken down into carbon

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dioxide and water, with the release of considerable energy. When plants take in carbon dioxide from the atmosphere, they combine it with water and manufacture organic compounds, using energy they have trapped from sunlight by means of photosynthesis—the conversion of light to chemical energy through biological means. As a byproduct of photosynthesis, plants release oxygen into the atmosphere, as we have noted earlier. In the process of photosynthesis, plants produce carbohydrates, which are various compounds of carbon, hydrogen, and oxygen that are essential to life. (The other two fundamental components of a diet are fats and proteins, both of which are carbon-based as well.) Animals eat the plants or eat other animals that eat the plants and thus incorporate the fats, proteins, and sugars (a form of carbohydrate) from the plants into their bodies. In cellular respiration, these nutrients are broken down to create carbon dioxide.

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Certain ecosystems, or communities of interdependent organisms, are better than others at producing carbon dioxide through decomposition. As one would expect, environments where heat and moisture are greatest—for example, a tropical rainforest—yield the fastest rates of decomposition. On the other hand, decomposition proceeds much more slowly in dry, cold climates such as that of a subarctic tundra. WHERE TO LEARN MORE Blashfield, Jean F. Carbon. Austin, TX: Raintree Steck–Vaughn, 1999. Carbon Cycle: Exploring the Environment (Web site). . Carbon Cycle Greenhouse Gases Group (CCGG) (Web site). . Chemical Carousel: A Trip Around the Carbon Cycle (Web site). . Frostburg State University Chemistry Helper (Web site). .

D E C O M P O S I T I O N . Cellular respiration also releases carbon into the atmosphere through the action of decomposers, organisms that obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. Bacteria and fungi, the principal forms of decomposer, extract energy contained in the chemical bonds of the organic matter they are decomposing and, in the process, release carbon dioxide.

“Global Carbon Cycle.” The Woods Hole Research Center (Web site). .

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Knapp, Brian J. Carbon Chemistry. Illus. David Woodroffe. Danbury, CT: Grolier Educational, 1998. Loudon, G. Marc. Organic Chemistry. Menlo Park, CA: Benjamin/Cummings, 1988. Sparrow, Giles. Carbon. New York: Benchmark Books, 1999. Stille, Darlene. The Respiratory System. New York: Children’s Press, 1997.

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THE NITROGEN C YCLE

CONCEPT Contrary to popular belief, the air we breathe is not primarily oxygen; by far the greatest portion of air is composed of nitrogen. A colorless, odorless gas noted for its lack of chemical reactivity— that is, its tendency not to bond with other elements—nitrogen plays a highly significant role within the earth system. Both through the action of lightning in the sky and of bacteria in the soil, nitrogen is converted to nitrites and nitrates, compounds of nitrogen and oxygen that are then absorbed by plants to form plant proteins. The latter convert to animal proteins in the bodies of animals who eat the plants, and when an animal dies, the proteins are returned to the soil. Denitrifying bacteria break down these compounds, returning elemental nitrogen to the atmosphere.

HOW IT WORKS Chemistry and Elements The concepts we discuss in this essay fall under the larger heading of geochemistry. A branch of the earth sciences that combines aspects of geology and chemistry, geochemistry is concerned with the chemical properties and processes of Earth. Among particular areas of interest in geochemistry are biogeochemical cycles, or the changes that particular elements undergo as they pass back and forth through the various earth systems and particularly between living and nonliving matter. The elements involved in biogeochemical cycles are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur (see Biogeochemical Cycles and Carbon Cycle).

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An element is a substance composed of a single type of atom, which cannot be broken down chemically into a simpler substance. Each element is distinguished by its atomic number, or the number of protons (positively charged subatomic particles) in the nucleus, or center, of the atom. On the periodic table of elements, these fundamental substances of the universe are listed in order of atomic number, from hydrogen to uranium—which has the highest atomic number (92) of any element that occurs in nature—and beyond. The elements with an atomic number higher than that of uranium, all of which have been created artificially, play virtually no role in the chemical environment of Earth and are primarily of interest only to specialists in certain fields of chemistry and physics. Everything that exists in the universe is an element, a compound formed by the chemical bonding of elements, or a mixture of compounds. In order to bond and form a compound, elements experience chemical reactions, which are the result of attractions on the part of electrons (negatively charged subatomic particles) that occupy the highest energy levels in the atom. These electrons are known as valence electrons. C H E M I CA L C H A N G E S . A chemical change is a phenomenon quite different from a physical change. If liquid water boils or freezes (both of which are examples of a physical change resulting from physical processes), it is still water. Physical changes do not affect the internal composition of an item or items; a chemical change, on the other hand, occurs when the actual composition changes—that is, when one substance is transformed into another. Chemical change requires a chemical reaction, a process whereby

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however, the only exceptions being the nonmetals as well as six “metalloids,” or elements that display characteristics of both metals and nonmetals.

The Nitrogen Cycle

Nitrogen is also one of eight “orphan” nonmetals—those nonmetals that do not belong to any family of elements, such as the halogens or noble gases. All six of the elements involved in biogeochemical cycles, in fact, are “orphan” nonmetals, with boron and selenium rounding out the list of eight orphans. Sometimes nitrogen is considered the head of a “family” of elements, all of which occupy a column or group on the periodic table. These five elements—nitrogen, phosphorus, arsenic, antimony, and bismuth—share a common pattern of valence electrons, but otherwise they share little in terms of physical properties or chemical behavior. By contrast, chemicals that truly are related all have a common “family resemblance”: all halogens are highly reactive, for instance, while all noble gases are extremely unreactive. LIQUID

NITROGEN. (© David Taylor/Photo Researchers. Repro-

duced by permission.)

the chemical properties of a substance are altered by a rearrangement of atoms. There are several clues that tell us when a chemical reaction has taken place. In many chemical reactions, for instance, the substance may experience a change of state or phase—as, for instance, when liquid water is subjected to an electric current through a process known as electrolysis, which separates it into oxygen and hydrogen, both of which are gases. Another clue that a chemical reaction has occurred is a change of temperature. Unlike the physical change of liquid water to ice or steam, however, this temperature change involves an alteration of the chemical properties of the substances themselves. Chemical reactions also may encompass changes in color, taste, or smell.

Nitrogen’s Place Among the Elements With an atomic number of 7, nitrogen (chemical symbol N) is one of just 19 elements that are nonmetals. Unlike metals, nonmetals are poor conductors of heat and electricity and are not ductile—in other words, they cannot be reshaped easily. The vast majority of elements are metallic,

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A B U N DA N C E . The seventeenth most abundant element on Earth, nitrogen accounts for 0.03% of the planet’s known elemental mass. This may seem very small, but at least nitrogen is among the 18 elements considered relatively abundant. These 18 elements account for all but 0.49% of the planet’s known elemental mass, the remainder being composed of numerous other elements in small quantities. The term known elemental mass takes account of the fact that scientists do not know with certainty the elemental composition of Earth’s interior, though it likely contains large proportions of iron and nickel. The known mass, therefore, is that which exists from the bottom of the crust to the top layers of the atmosphere.

Elemental proportions too small to be measured in percentage points are rendered in parts per million (ppm) or even parts per billion (ppb). Within the crust itself, nitrogen’s share is certainly modest: a concentration of 19 ppm, which ties it with gallium, a metal whose name is hardly a household word, for a rank of thirtythird. On the other hand, this still makes it more abundant in the crust than many quite familiar metals, including lithium, uranium, tungsten, silver, mercury, and platinum. In Earth’s atmosphere, on the other hand, the proportion of nitrogen is much, much high-

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er. The atmosphere is 78% nitrogen and 21% oxygen, while the noble gas argon accounts for 0.93%. The remaining 0.07% is taken up by various trace gases, including water vapor, carbon dioxide, and ozone, or O3.

The Nitrogen Cycle

In the human body, nitrogen’s share is much more modest than it is in the atmosphere but still 10 times greater than it is in relation to the planet’s total mass. The element accounts for 3% of the body’s mass, making it the fourth most abundant element in the human organism.

Properties and Applications of Nitrogen The Scottish chemist Daniel Rutherford (1749–1819) usually is given credit for discovering nitrogen in 1772, when he identified it as the element that remained when oxygen was removed from air. Several other scientists at about the same time made a similar discovery. Because of its heavy presence in air, nitrogen is obtained primarily by cooling air to temperatures below the boiling points of its major components. Nitrogen boils (that is, turns into a gas) at a lower temperature than oxygen: –320.44°F (–195.8°C), as opposed to –297.4°F (–183°C). If air is cooled to –328°F (–200°C), thus solidifying it, and then allowed to warm slowly, the nitrogen boils first and therefore evaporates first. The nitrogen gas is captured, cooled, and liquefied once more. Nitrogen also can be obtained from such compounds as potassium nitrate or saltpeter, found primarily in India, or from sodium nitrate (Chilean saltpeter), which comes from the desert regions of Chile. To isolate nitrogen chemically, various processes are undertaken in a laboratory—for instance, heating barium azide or sodium azide, both of which contain nitrogen. R E AC T I O N S W I T H O T H E R E L E M E N T S . Rather than appearing as single

NITROGEN COMBINES WITH HYDROGEN TO FORM AMMONIA. AMMONIUM NITRATE, A FERTILIZER, IS ALSO A DANGEROUS EXPLOSIVE. IT WAS USED IN APRIL OF 1995 TO BLOW UP THE ALFRED P. MURRAH FEDERAL BUILDING IN OKLAHOMA CITY, KILLING 168 PEOPLE. (© James H. Robinson/Photo Researchers. Reproduced by permission.)

air but not with the nitrogen. At very high temperatures, on the other hand, nitrogen combines with other elements, reacting with metals to form nitrides, with hydrogen to form ammonia, with O2 (oxygen as it usually appears in nature, two atoms bonded in a molecule) to form nitrites, and with O3 (ozone) to form nitrates. With the exception of the first-named group, all of these elements are important to our discussion of nitrogen. SOME USES FOR NITROGEN.

Even at the temperature of combustion, a burning substance reacts with the oxygen in the

In processing iron or steel, which forms undesirable oxides if exposed to oxygen, a blanket of nitrogen is applied to prevent this reaction. The same principle is applied in making computer chips and even in processing foods, since these items, too, are affected detrimentally by oxidation. Because it is far less combustible than air (magnesium is one of the few elements that burns nitrogen in combustion), nitrogen also is used to clean tanks that have carried petroleum or other combustible materials.

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atoms, nitrogen is diatomic, meaning that two nitrogen atoms typically bond with each other to form dinitrogen, or N2. Nor do these atoms form single chemical bonds, as is characteristic of most elements; theirs is a triple bond, which effectively ties up the atoms’ valence electrons, making nitrogen an unreactive element at relatively low temperatures.

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As noted, nitrogen combines with hydrogen to form ammonia, used in fertilizers and cleaning materials. Ammonium nitrate, applied primarily as a fertilizer, is also a dangerous explosive, as shown with horrifying effect in the bombing of the Alfred P. Murrah Federal Building in Oklahoma City on April 19, 1995—a tragedy that took 168 lives. Nor is ammonium nitrate the only nitrogen-based explosive. Nitric acid is used in making trinitrotoluene (TNT), nitroglycerin, and dynamite as well as gunpowder and smokeless powder.

Introduction to the Nitrogen Cycle The nitrogen cycle is the process whereby nitrogen passes from the atmosphere into living things and ultimately back into the atmosphere. In the process, it is converted to nitrates and nitrites, compounds of nitrogen and oxygen that are absorbed by plants in the process of forming plant proteins. These plant proteins, in turn, are converted to animal proteins in the bodies of animals who eat the plants, and when the animal dies, the proteins are returned to the soil. Denitrifying bacteria break down these organic compounds, returning elemental nitrogen to the atmosphere. Note what happens in the nitrogen cycle and, indeed, in all biogeochemical cycles: organic material is converted to inorganic material through various processes, and inorganic material absorbed by living organisms eventually is turned into organic material. In effect, the element passes back and forth between the realms of the living and the nonliving. This may sound a bit mystical, but it is not. To be organic, a substance must be built around carbon in certain characteristic chemical structures, and by inducing the proper chemical reaction, it is possible to break down or build up these structures, thus turning an organic substance into an inorganic one, or vice versa. (For more on this subject, see Carbon Cycle.) S T E P S I N T H E CYC L E . Plants depend on biologically useful forms of nitrogen, the availability of which greatly affects their health, abundance, and productivity. This is particularly the case where plants in a saltwater ecosystem (a community of interdependent organisms) are concerned. Regardless of the specific ecosystem, however, fertilization of the soil

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with nitrogen has an enormous impact on the growth yield of plant life, which can be critical in the case of crops. Therefore, nitrogen is by far the most commonly applied nutrient in an agricultural setting. There are several means by which plants receive nitrogen. They may absorb it as nitrate or ammonium, dissolved in saltwater and taken up through the roots, or as various nitrogen oxide gases. In certain situations, plants have a symbiotic, or mutually beneficial, relationship with microorganisms capable of “fixing” atmospheric dinitrogen into ammonia. In any case, plants receive nitrogen and later, when they are eaten by animals, pass these nutrients along the food chain—or rather, to use a term more favored in the earth and biological sciences, the food web. When herbivorous or omnivorous animals consume nitrogen-containing plants, their bodies take in the nitrogen and metabolize it, breaking it down to generate biochemicals, or chemicals essential to life processes. At some point, the animal dies, and its body experiences decomposition through the activity of bacteria and other decomposers. These microorganisms, along with detritivores such as earthworms, convert nitrates and nitrites from organic sources into elemental nitrogen, which ultimately reenters the atmosphere.

REAL-LIFE A P P L I C AT I O N S Important Forms of Nitrogen As noted earlier, dinitrogen, or N2, is the form in which nitrogen typically appears when uncombined with other elements. This is also the form of nitrogen in the atmosphere, but it is so chemically unreactive that unlike oxygen, it plays little actual part in sustaining life. Indeed, because nitrogen in the air is essentially “filler” as far as humans are concerned, it can be substituted with helium, as is done in air tanks for divers. This prevents them from experiencing decompression sickness, or “the bends,” which occurs when the diver returns too quickly to the surface, causing nitrogen in the blood to boil. The dinitrogen in the air is a holdover from long ago in Earth’s development, when volcanoes expelled elements from deep in the planet’s interior to its atmosphere. Owing to its lack of reac-

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The Nitrogen Cycle

SMOG BLANKETS LOS ANGELES IN A HAZE. NITRIC OXIDE REACTS WITH OXYGEN IN THE AIR TO FORM NITROGEN OXIDE, A REDDISH BROWN GAS THAT COLORS SMOG. (Photograph by Walter A. Lyons. FMA Productions. Reproduced by permission.)

tivity, dinitrogen never went anywhere. For it to play a role in the functioning of Earth cycles, it must be “fixed,” as discussed later in this essay. In addition to dinitrogen, nitrogen appears in a number of other important inorganic compounds, including nitrite and nitrate; ammonia and ammonium; and nitric oxide, nitrogen dioxide, and nitrous oxide. Nitrite and nitrate are two ionic forms of nitrogen. An ion is an atom or group of atoms that has lost or gained electrons, thus acquiring a net electric charge. Both nitrite and nitrate are anions, or negatively charged ions, designated by the use of superscript minus signs that indicate that each has a net charge of negative 1. Thus, nitrite, in which nitrogen is chemically bonded with two atoms of oxygen, is rendered as NO2–, while the formula for nitrate (nitrogen with three oxygen atoms), is designated as NO3–.

NH3. The latter, which is probably familiar to most people in the form of a household cleaner, is actually an extremely abundant compound, both in natural and artificial forms. Ammonium is soluble, or capable of being dissolved, in water and often is used as a fertilizer. It is attracted to negatively charged surfaces of clays and organic matter in soil and therefore tends to become stuck in one place rather than moving around, as nitrate does. In acidic soils, typically plants receive their nitrogen from ammonium, but most nonacidic soils can use only nitrate. As noted earlier, ammonium may be combined with nitrate to form ammonium nitrate—both a powerful fertilizer and a powerful explosive.

Nitrification is a process in which nitrite is produced, whereupon it undergoes a chemical reaction to form nitrate, the principal form of nitrogen nutrition for most plant species. The chemical from which the nitrite is created in the nitrification reaction is ammonium (NH4+), which is formed by the addition of a hydrogen cation, or a positively charged ion (H+), to ammonia, or

OX I D E S . Nitrogen reenters the atmosphere in the form of the gas nitric oxide (NO), emitted primarily as the result of combustion reactions. This may occur in one of two ways. Organic nitrogen in bioenergy sources, such as biomass (organisms, their waste products, and their incompletely decomposed remains) or fossil fuels (e.g., coal or oil), may be oxidized. The latter term means that a substance undergoes a chemical reaction with oxygen: combustion itself, which requires the presence of oxygen, is an example of oxidation.

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AMMONIA

AND

AMMONIUM.

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The Nitrogen Cycle

KEY TERMS The smallest particle of an element, consisting of protons, neutrons, and electrons. An atom can exist either alone or in combination with other atoms in a molecule.

DECOMPOSERS:

The number of protons in the nucleus of an atom. Since this number is different for each element, elements are listed on the periodic table in order of atomic number.

and fungi.

ATOM:

ATOMIC NUMBER:

Energy derived from biological sources that are used directly as fuel (as opposed to food, which becomes fuel). BIOENERGY:

that

obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. The principal forms of decomposer are bacteria

DECOMPOSITION

REACTION:

A

chemical reaction in which a compound is broken down into simpler compounds or into its constituent elements. In the earth system, this often is achieved through the help of detritivores and decomposers. Organisms that feed

DETRITIVORES:

The changes that particular elements undergo as they pass back and forth through the various earth systems and specifically between living and nonliving matter. The elements involved in biogeochemical cycles are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur.

on waste matter, breaking organic material

The joining, through electromagnetic force, of atoms that sometimes, but not always, represent more than one chemical element. The result is usually the formation of a molecule.

earthworms or maggots.

BIOGEOCHEMICAL CYCLES:

CHEMICAL BONDING:

A substance made up of atoms of more than one element chemically bonded to one another. COMPOUND:

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Organisms

down into inorganic substances that then become available to the biosphere in the form of nutrients for plants. Their function is similar to that of decomposers; however, unlike decomposers—which tend to be bacteria or fungi—detritivores are relatively complex organisms, such as

DIATOMIC:

A term describing a chem-

ical element that typically exists as molecules composed of two atoms. Nitrogen and oxygen are both diatomic. ECOSYSTEM:

A term referring to a

community of interdependent organisms along with the inorganic components of their environment.

On the other hand, nitric oxide may enter the atmosphere when atmospheric dinitrogen is combined with oxygen under conditions of high temperature and pressure, as, for instance, in an internal-combustion engine. In the atmosphere, nitric oxide reacts readily with oxygen in the air to form nitrogen dioxide (NO2), a reddishbrown gas that adds to the tan color of smog over major cities.

Yet nitric oxide and nitrogen dioxide, usually designated together as NOx, are also part of the life-preserving nitrogen cycle. Gaseous NOx is taken in by plants, or oxidized to make nitrate, and circulated through the biosphere or else cycled directly to the atmosphere. In addition, denitrification, discussed later in this essay, transports nitrous oxide (N2O) into the atmosphere from nitrate-rich soils.

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The Nitrogen Cycle

KEY TERMS ELECTRON:

A negatively charged par-

ticle in an atom, which spins around the

ly charged ions are called cations, and negatively charged ones are called anions.

nucleus.

The removal of soil mate-

LEACHING: ELEMENT:

A substance made up of

only one kind of atom. Unlike compounds,

rials that are in solution, or dissolved in water.

elements cannot be chemically broken into A group of atoms, usual-

MOLECULE:

other substances. EUTROPHICATION:

A state of height-

ened biological productivity in a body of water, which is typically detrimental to the ecosystem in which it takes place. Eutroph-

ly but not always representing more than one element, joined in a structure. Compounds are typically made up of molecules.

ication can be caused by an excess of nitro-

PERIODIC TABLE OF ELEMENTS:

gen or phosphorus in the form of nitrates

A chart that shows the elements

and phosphates, respectively.

arranged in order of atomic number

A term describing the

along with their chemical symbols and

interaction of plants, herbivores, carni-

the average atomic mass for each partic-

vores, omnivores, decomposers, and detri-

ular element.

tivores, each of which consumes nutrients

PROTON:

and passes it along to other organisms.

in an atom.

FOOD WEB:

GEOCHEMISTRY:

A positively charged particle

A branch of the

earth sciences combining aspects of geology and chemistry, that is, concerned with the chemical properties and processes of Earth—in particular, the abundance and

REACTIVITY:

A term referring to the

ability of one element to bond with others. The higher the reactivity, the greater the tendency to bond. Capable of being dissolved.

interaction of chemical elements and their

SOLUBLE:

isotopes.

VALENCE ELECTRONS:

Electrons

An atom or group of atoms that

that occupy the highest principal energy

has lost or gained one or more electrons

level in an atom. These are the electrons

and thus has a net electric charge. Positive-

involved in chemical bonding.

ION:

Nitrogen Processes In order for most organisms to make use of atmospheric dinitrogen, it must be “fixed” into inorganic forms that a plant can take in through its roots and leaves. Nonbiological processes, such as a lightning strike, can bring about dinitrogen fixation. The high temperatures and pressures associated with lightning lead to the chem-

S C I E N C E O F E V E RY DAY T H I N G S

ical bonding of atmospheric nitrogen and oxygen (both of which appear in diatomic form) to create two molecules of nitric oxide. More often than not, however, dinitrogen fixation comes about through biological processes. Microorganisms are able to synthesize an enzyme that breaks the triple bonds in dinitrogen, resulting in the formation of two molecules

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of ammonia for every dinitrogen molecule thus reacted. This effect is achieved most commonly by bacteria or algae in wet or moist environments that offer nutrients other than nitrate or ammonium. In some instances, plants enjoy a symbiotic, or mutually beneficial, relationship with microorganisms capable of fixing dinitrogen. A M M O N I F I CAT I O N , N I T R I F I CA T I O N , A N D D E N I T R I F I C AT I O N .

Dinitrogen fixation is just one example of a process whereby nitrogen is processed through one or more earth systems. Another is ammonification, or the process whereby nitrogen in organisms is recycled after their death. Enabled by microorganisms that act as decomposers, ammonification results in the production of either ammonia or ammonium. Thus, the soil is fertilized by the decayed matter of formerly living things. Ammonium, as we noted earlier, also plays a part in nitrification, a process in which it first is oxidized to produce nitrite. Then the nitrite is oxidized to become nitrate, which fertilizes the soil. As previously mentioned, nitrate is useful as a fertilizer only in non-acidic soils; acidic ones, by contrast, require ammonium fertilizer. In contrast to nitrification is denitrification, in which nitrate is reduced to the form of either nitrous oxide or dinitrogen. This takes place under anaerobic conditions—that is, in the absence of oxygen—and on the largest scale when concentrations of nitrate are highest. Flooded fields, for example, may experience high rates of denitrification.

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When the soil has taken in all the nitrogen it can hold, a process of leaching—the removal of soil materials dissolved in water—eventually takes place. Nitrate, in particular, leaches from agricultural sites into groundwater as well as streams and other forms of surface water. This can lead to eutrophication, a state of heightened biological productivity that is ultimately detrimental to the ecosystem surrounding a lake or other body of water. (See Biogeochemical Cycles for more about eutrophication.) Yet another problem associated with overly nitrate-rich soils is an excessive rate of denitrification. This happens when soils that have been loaded down with nitrates become wet for long periods of time, leading to a dramatic increase in the denitrification rate. As a result, fixed nitrogen is lost, and nitrous oxide is emitted to the air. In the atmosphere nitrous oxide may contribute to the greenhouse effect, possibly helping increase the overall temperature of the planet (see Carbon Cycle and Energy and Earth). WHERE TO LEARN MORE Blashfield, Jean F. Nitrogen. Austin, TX: Raintree Steck–Vaughn, 1999. Farndon, John. Nitrogen. New York: Benchmark Books, 1999. Fitzgerald, Karen. The Story of Nitrogen. New York: Franklin Watts, 1997. The Microbial World: The Nitrogen Cycle and Nitrogen Fixation (Web site). . The Nitrogen Cycle (Web site). . The Nitrogen Cycle (Web site). .

Humans are involved in the nitrogen cycle in several ways, not all of them beneficial. One of the most significant roles people play in the nitrogen cycle is by the introduction of nitrogen-containing fertilizers to the soil. Because nitrogen has a powerful impact on plant growth, farmers are tempted to add more and more nitrate or ammonium or both to their crops, to the point that the soil becomes saturated with it and therefore unable to absorb more.

Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science. 2d ed. New York: John Wiley and Sons, 1999.

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The Nitrogen Cycle (Web site). . Nutrient Overload: Unbalancing the Global Nitrogen Cycle (Web site). . Postgate, J. R. The Outer Reaches of Life. New York: Cambridge University Press, 1994.

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S C I E N C E O F E V E RY DAY T H I N G S Real-Life Earth Science

THE BIOSPHERE ECOSYSTEMS ECOLOGY AND ECOLOGICAL STRESS

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Ecosystems

ECOSYSTEMS

CONCEPT An ecosystem is a complete community of living organisms and the nonliving materials of their surroundings. Thus, its components include plants, animals, and microorganisms; soil, rocks, and minerals; as well as surrounding water sources and the local atmosphere. The size of ecosystems varies tremendously. An ecosystem could be an entire rain forest, covering a geographical area larger than many nations, or it could be a puddle or a backyard garden. Even the body of an animal could be considered an ecosystem, since it is home to numerous microorganisms. On a much larger scale, the history of various human societies provides an instructive illustration as to the ways that ecosystems have influenced civilizations.

HOW IT WORKS The Biosphere Earth itself could be considered a massive ecosystem, in which the living and nonliving worlds interact through four major subsystems: the atmosphere, hydrosphere (all the planet’s waters, except for moisture in the atmosphere), geosphere (the soil and the extreme upper portion of the continental crust), and biosphere. The biosphere includes all living things: plants (from algae and lichen to shrubs and trees), mammals, birds, reptiles, amphibians, aquatic life, insects, and all manner of microscopic forms, including bacteria and viruses. In addition, the biosphere draws together all formerly living things that have not yet decomposed.

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Several characteristics unite the biosphere. One is the obvious fact that everything in it is either living or recently living. Then there are the food webs that connect organisms on the basis of energy flow from one species to another. A food web is similar to the more familiar concept food chain, but in scientific terms a food chain—a series of singular organisms in which each plant or animal depends on the organism that precedes or follows it—does not exist. Instead, the feeding relationships between organisms in the real world are much more complex and are best described as a web rather than a chain.

Food Webs Food webs are built around the flow of energy between organisms, known as energy transfer, which begins with plant life. Plants absorb energy in two ways. From the Sun, they receive electromagnetic energy in the form of visible light and invisible infrared waves, which they convert to chemical energy through a process known as photosynthesis. In addition, plants take in nutrients from the soil, which contain energy in the forms of various chemical compounds. These compounds may be organic, which typically means that they came from living things, though, in fact, the term organic refers strictly to characteristic carbon-based chemical structures. Plants also receive inorganic compounds from minerals in the soil. (See Minerals. For more about the role of carbon in inorganic compounds, see Carbon Cycle.) Contained in these minerals are six chemical elements essential to the sustenance of life on planet Earth: hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur. These are the ele-

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ments involved in biogeochemical cycles, through which they continually are circulated between the living and nonliving worlds—that is, between organisms, on the one hand, and the inorganic realms of rocks, minerals, water, and air, on the other (see Biogeochemical Cycles).

soil, they convert them into other forms, which provide usable energy to organisms who eat the plants. (An example of this conversion process is cellular respiration, discussed in Carbon Cycle.) When an herbivore, or plant-eating organism, eats the plant, it incorporates this energy.

Decomposers, which include bacteria and fungi, obtain their energy from the chemical breakdown of dead organisms as well as from animal and plant waste products. Detritivores perform a similar function: by feeding on waste matter, they break organic material down into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. The principal difference between detritivores and decomposers is that the former are relatively complex organisms, such as earthworms or maggots.

Chances are strong that the herbivore will be eaten either by a carnivore, a meat-eating organism, or by an omnivore, an organism that consumes both herbs and herbivores—that is, both plants and animals. Few animals consume carnivores or omnivores, at least by hunting and killing them. (Detritivores and decomposers, which we discuss presently, consume the remains of all creatures, including carnivores and omnivores.) Humans are an example of omnivores, but they are far from the only omnivorous creatures. Many bird species, for instance, are omnivorous.

Both decomposers and detritivores aid in decomposition, a chemical reaction in which a compound is broken down into simpler compounds or into its constituent elements. Often an element such as nitrogen appears in forms that are not readily usable by organisms, and therefore such elements (which may appear individually or in compounds) need to be chemically processed through the body of a decomposer or detritivore. This processing involves chemical reactions in which the substance—whether an element or compound—is transformed into a more usable version.

As nutrients pass from plant to herbivore to carnivore, the total amount of energy in them decreases. This is dictated by the second law of thermodynamics (see Energy and Earth), which shows that energy transfers cannot be perfectly efficient. Energy is not “lost”—the total amount of energy in the universe remains fixed, though it may vary with a particular system, such as an individual ecosystem—but it is dissipated, or directed into areas that do not aid in the transfer of energy between organisms. What this means for the food web is that each successive level contains less energy than the levels that precede it.

By processing chemical compounds from the air, water, and geosphere, decomposers and detritivores deposit nutrients in the soil. These creatures feed on plant life, thus making possible the cycle we have described. Clearly this system, of which we have sketched only the most basic outlines, is an extraordinarily complex and wellorganized one, in which every organism plays a specific role. In fact, earth scientists working in the realm of biosphere studies use the term niche to describe the role that a particular organism plays in its community. (For more about the interaction of species in a biological community, see Ecology and Ecological Stress.)

FROM PLANTS TO CARNIV O R E S . As plants take up nutrients from the

DETRITIVORES AND DECOMP O S E R S . In the case of a food web, some-

thing interesting happens with regard to energy efficiency as soon as we pass beyond carnivores and omnivores to the next level. It might seem at first that there could be no level beyond carnivores or omnivores, since they appear to be “at the top of the food chain,” but this only illustrates why the idea of a food web is much more useful. After carnivores and omnivores, which include some of the largest, most powerful, and most intelligent creatures, come the lowliest of all

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organisms: decomposers and detritivores, an integral part of the food web.

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REAL-LIFE A P P L I C AT I O N S The Fate of Human Civilizations An interesting place to start in investigating examples of ecosystems is with a species near and dear to all of us: Homo sapiens. Much has been written about the negative effect industrial civi-

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lization has, or may have, on the natural environment—a topic discussed in Ecology and Ecological Stress—but here our concern is somewhat different. What do ecosystems, and specifically the availability of certain plants and animals, teach us about specific societies?

today refer to ancient Mesopotamia as “the Fertile Crescent.” (For a very brief analysis regarding possible reasons why modern Mesopotamia— that is, Iraq—does not fit this description, see the discussion of desertification in Soil Conservation.)

In his 1997 bestseller Guns, Germs, and Steel: The Fate of Human Societies, the ethnobotanist Jared M. Diamond (1937-) explained how he came to approach this question. While he was working with native peoples in New Guinea, a young man asked him why the societies of the West enjoyed an abundance of material wealth and comforts while those of New Guinea had so little. It was a simple question, but the answer was not obvious.

In the New World, by contrast, agriculture appeared much later and in a much more circumscribed way. The same was true of Africa and the Pacific Islands. In seeking the reasons for why this happened, Diamond noted a number of factors, including geography. The agricultural areas of the Old World were stretched across a wide area at similar latitudes. This meant that the climates were not significantly different and would support agricultural exchanges, such as the spread of wheat and other crops from one region or ecosystem to another. By contrast, the land masses of the New World or Africa have a much greater north-south distance than they do east to west.

Diamond refused to give any of the usual pat responses offered in the past—for example, the Marxist or socialist claim that the West prospers at the expense of native peoples. Nor, of course, could he accept the standard answer that a white descendant of Europeans might have given a century earlier, that white Westerners are smarter than dark-skinned peoples. Instead, he approached it as a question of environment, and the result was his thought-provoking analysis contained in Guns, Germs, and Steel. A D V A N TA G E S O F G E O G R A P H Y. As Diamond showed, those places where

agriculture was first born were precisely those blessed with favorable climate, soil, and indigenous plant and animal life. Incidentally, none of these locales was European, nor were any of the peoples inhabiting them “white.” Agriculture came into existence in four places during a period from about 8000 to 6000 B.C. In roughly chronological order, they were Mesopotamia, Egypt, India, and China. All were destined to emerge as civilizations, complete with written language, cities, and organized governments, between about 3000 and 2000 B.C.

Ecosystems

D I V E R S I T Y O F S P E C I E S . Today such places as the American Midwest support abundant agriculture, and one might wonder why that was not the case in the centuries before Europeans arrived. The reason is simple but subtle, and it has nothing to do with Europeans’ “superiority” over Native Americans. The fact is that the native North American ecosystems enjoyed far less biological diversity, or biodiversity, than their counterparts in the Old World. Peoples of the New World successfully domesticated corn and potatoes, because those were available to them. But they could not domesticate emmer wheat, the variety used for making bread, when they had no access to that species, which originated in Mesopotamia and spread throughout the Old World.

Of course, it is no accident that civilization was born first in those societies that first developed agriculture: before a civilization can evolve, a society must become settled, and in order for that to happen, it must develop agriculture. Each of these societies, it should be noted, formed along a river, and that of Mesopotamia was born at the confluence of two rivers, the Tigris and Euphrates. No wonder, then, that the spot where these two rivers met was identified in the Bible as the site for the Garden of Eden or that historians

Similarly, the New World possessed few animals that could be domesticated either for food or labor. A number of Indian tribes domesticated some types of birds and other creatures for food, but the only animal ever adapted for labor was the llama. The llama, a cousin of the camel found in South America, is too small to carry heavy loads. Why did the Native Americans never harness the power of cows, oxen, or horses? For the simple reason that these species were not found in the Americas. After horses in the New World went extinct at some point during the last Ice Age (see Paleontology), they did not reappear in the

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Ecosystems

THE

LLAMA WAS ONE OF THE FEW DOMESTICATED ANIMALS ADAPTED FOR WORK IN THE

NEW WORLD,

A PLACE WITH

A SMALL NUMBER OF ANIMAL AND PLANT SPECIES AND LACK OF ECOLOGICAL COMPLEXITY BEFORE THE ARRIVED IN ABOUT 1500

Americas until Europeans brought them after A.D. 1500.

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EUROPEANS

a.d. (© Francois Gohier/Photo Researchers. Reproduced by permission.)

GREATER EXPOSURE TO MICROORGANISMS. Ultimately, these societies

Diamond also noted the link between biodiversity and the practice, common among peoples in New Guinea and other remote parts of the world, of eating what Westerners would consider strange cuisine: caterpillars, insects—even, in some cases, human flesh. At one time, such practices served only to brand these native peoples further as “savages” in the eyes of Europeans and their descendants, but it turns out that there is a method to the apparent madness. In places such as the highlands of New Guinea, a scarcity of animal protein sources compels people to seek protein wherever they can find it.

came to dominate their physical environments and excel in the development of technology; hence the “steel” and “guns” in Diamond’s title. But what about “germs”? It is a fact that after Europeans began arriving in the New World, they killed vast populations without firing a shot, thanks to the microbes they carried with them. Of course, it would be centuries before scientists discovered the existence of microorganisms. But even in 1500, it was clear that the native peoples of the New World had no natural resistance to smallpox or a host of other diseases, including measles, chicken pox, influenza, typhoid fever, and bubonic plague.

By contrast, from ancient times the Fertile Crescent possessed an extraordinary diversity of animal life. Among the creatures present in that region (the term sometimes is used to include Egypt as well as Mesopotamia) were sheep, goats, cattle, pigs, and horses. With the help of these animals for both food and labor—people ate horses long before they discovered their greater value as a mode of transportation—the lands of the Old World were in a position to progress far beyond their counterparts in the New.

Once again the Europeans’ advantage over the Native Americans derived from the ecological complexity of their world compared with that of the Indians. In the Old World, close contact with farm animals exposed humans to germs and disease. So, too, did close contact with other people in crowded, filthy cities. This exposure, of course, killed off large numbers of people, but those who survived tended to be much hardier and possessed much stronger immune systems. Therefore, when Europeans came into contact with

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Ecosystems

WITH

ITS SAUNA-LIKE ENVIRONMENT AND CLOSED CANOPY, A TROPICAL CLOUD FOREST PRODUCES LUSH VEGETATION

AND IS ONE OF THE MOST BIODIVERSE ECOSYSTEMS ON THE EARTH. (© G. Dimijian/Photo Researchers. Reproduced by permis-

sion.)

native Americans, they were like walking biological warfare weapons.

Evaluating Ecosystems

of the much more complex ecosystems we know now. Such differentiation is essential, given the many basic types of ecosystem that the world has to offer: forests and grasslands, deserts and aquatic environments, mountains and jungles. Among the many ways that these ecosystems can be evaluated, aside from such obvious parameters as relative climate, is in terms of abundance and complexity of species.

The ease with which Europeans subdued Native Americans fueled the belief that Europeans were superior, but, as Diamond showed, if anything was superior, it was the ecosystems of the Old World. This “superiority” relates in large part to the diversity of organisms an ecosystem possesses. Many millions of years ago, Earth’s oceans and lands were populated with just a few varieties of single-cell organisms, but over time increasing differentiation of species led to the development

fauna, or plant and animal life, respectively) in a desert or the Arctic tundra is much less complex than that of a tropical rain forest or, indeed,

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A B U N DA N C E A N D C O M P L E X I T Y. The biota (a combination of all flora and

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almost any kind of forest, because far fewer species can live in a desert or tundra environment. For this reason, it is said that a desert or tundra ecosystem is less complex than a forest one. There may be relatively large numbers of particular species in a less complex ecosystem, however, in which case the ecosystem is said to be abundant though not complex in a relative sense. Another way to evaluate ecosystems is in terms of productivity. This concept refers to the amount of biomass—potentially burnable energy—produced by green plants as they capture sunlight and use its energy to create new organic compounds that can be consumed by local animal life. Once again, a forest, and particularly a rain forest, has a very high level of productivity, whereas a desert or tundra ecosystem does not.

Forests Now let us look more closely at a full-fledged ecosystem—that of a forest—in action. It might seem that all forests are the same, but this could not be less the case. A forest is simply any ecosystem dominated by tree-sized woody plants. Beyond that, the characteristics of weather, climate, elevation, latitude, topography, tree species, varieties of animal species, moisture levels, and numerous other parameters create the potential for an almost endless diversity of forest types. In fact, the United Nations Educational, Scientific, and Cultural Organization (UNESCO) defines 24 different types of forest, which are divided into two main groups. On the one hand, there are those forests with a closed canopy at least 16.5 ft. (5 m) high. The canopy is the upper portion of the trees in the forest, and closedcanopy forests are so dense with vegetation that from the ground the sky is not visible. On the other hand, the UNESCO system encompasses open woodlands with a shorter, more sparse, and unclosed canopy. The first group tends to be tropical and subtropical (located at or near the equator), while the second typically is located in temperate and subpolar forests—that is, in a region between the two tropical latitudes and the Arctic and Antarctic circles, respectively. In the next paragraphs, we examine a few varieties of forest as classified by UNESCO.

Naturally, the creatures that have evolved in and adapted to a tropical rain forest environment are those capable of enduring high humidity, but they are tolerant of neither extremely cool conditions nor drought. Within those parameters, however, exists one of the most biodiverse ecosystems on Earth: the tropical rain forest is home to an astonishing array of animals, plants, insects, and microorganisms. Indeed, without the tropical rain forest, terrestrial (land-based) animal life on Earth would be noticeably reduced. In the tropics, by definition the four seasons to which we are accustomed in temperate zones—winter, spring, summer, and fall—do not exist. In their place there is a rainy season and a dry season, but there is no set point in the year at which trees shed their leaves. In a tropical and subtropical evergreen forest conditions are much drier than in the rain forest, and individual trees or tree species may shed their leaves as a result of dry conditions. All trees and species do not do so at the same time, however, so the canopy remains rich in foliage year-round—hence the term evergreen. As with a rain forest, the evergreen forest possesses a vast diversity of species. In contrast to the two tropical forest ecosystems just described, a mangrove forest is poor in species. In terms of topography and landform, these forests are found in low-lying, muddy regions near saltwater. Thus, the climate is likely to be humid, as in a rain forest, but only organisms that can tolerate flooding and high salt levels are able to survive there. Mangrove trees, a variety of angiosperm, are suited to this environment and to the soil, which is poor in oxygen. T E M P E RAT E A N D S U B A R C T I C F O R E S T S . Among the temperate and sub-

ecosystems with a wide array of species. The

arctic forest types are temperate deciduous forests, containing trees that shed their leaves seasonally, and temperate and subarctic evergreen conifer forests, in which the trees produce cones bearing seeds. These are forest types familiar to most people in the continental United

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dominant tree type is an angiosperm (a type of plant that produces flowers during sexual reproduction), known colloquially as tropical hardwoods. The climate and weather are what one would expect to find in a place called a tropical rain forest, that is, rainy and warm. When the rain falls, it cools things down, but when the sun comes back out, it turns the world of the tropical rain forest into a humid, sauna-like environment.

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States. The first variety is dominated by such varieties as oak, walnut, and hickory, while the second is populated by pine, spruce, or fir as well as other types, such as hemlock. Less familiar to most Americans outside the West Coast are temperate winter-rain evergreen broadleaf forests. These forests are dominated by evergreen angiosperms and appear in regions that have both a pronounced wet season and a summer drought season. Such forests can be found in southern California, where an evergreen oak of the Quercus genus is predominant. Even less familiar to Americans is the temperate and subpolar evergreen rain forest, which is found in the Southern Hemisphere. Occurring in a wet, frost-free ocean environment, these forests are dominated by such evergreen angiosperms as the southern beech and southern pine.

Angiosperms vs. Gymnosperms Several times we have referred to angiosperms, a name that encompasses not just certain types of tree but also all plants that produce flowers during sexual reproduction. The name, which comes from Latin roots meaning “vessel seed,” is a reference to the fact that the plant keeps its seeds in a vessel whose name emphasizes these plants’ sexual-type reproduction: an ovary. Angiosperms are a beautiful example of how a particular group of organisms can adapt to specific ecosystems and do so in a way much more efficient than did their evolutionary forebear. Flowering plants evolved only about 130 million years ago, by which time Earth had long since been dominated by another variety of seed-producing plant, the gymnosperm, of which pines and firs are an example. Yet in a relatively short period of time, from the standpoint of the earth sciences, angiosperms have gone on to become the dominant plants in the world. Today, about 80% of all living plant species are flowering plants.

mals view gymnosperm seeds as a source of nutrition.

Ecosystems

In an angiosperm, by contrast, the seeds are tucked away safely inside the ovary. Furthermore, the evolution of the flower not only has added a great deal of beauty to the world but also has provided a highly successful mechanism for sexual reproduction. This sexual reproduction makes it possible to develop new genetic variations, as genetic material from two individuals of differing ancestry come together to produce new offspring. GY M N O S P E R M

P O L L I N AT I O N .

Gymnosperms reproduce sexually as well, but they do so by a less efficient method. In both cases, the trees have to overcome a significant challenge: the fact that sexual reproduction normally requires at least one of the individual plants to be mobile. Gymnosperms package the male reproductive component in tiny pollen grains, which are released into the wind. Eventually, the grains are blown toward the female component of another individual plant of the same species. This method succeeds well enough to sustain large and varied populations of gymnosperms but at a terrific cost, as is evident to anyone who lives in a region with a high pollen count in the spring. A yellow dust forms on everything. So much pollen accumulates on window sills, cars, mailboxes, and roofs that only a good rain (or a car wash) can take it away, and one tends to wonder what good all this pollen is doing for the trees.

ANGIOSPERM VS. GYMNOSPERM SEEDS. How did they do this? They did it by

The truth is that pollination is wasteful and inefficient. Like all natural mechanisms, it benefit the overall ecosystem, in this case, by making nutrient-rich pollen grains available to the soil. Packed with energy, pollen grains contain large quantities of nitrogen, making them a major boost to the ecosystem if not to the human environment. But it costs the gymnosperm a great deal, in terms of chemical and biological energy and material, to produce pollen grains, and the benefits are much more uncertain.

developing a means to coexist more favorably than gymnosperms with the insect and animal life in their ecosystems. Gymnosperms produce their seeds on the surface of leaflike structures, making the seeds vulnerable to physical damage and drying as the wind whips the branches back and forth. Furthermore, insects and other ani-

Pollen might make it to the right female component, and, in fact, it will, given the huge amounts of pollen produced. Yet the overall system is rather like trying to solve an economic problem by throwing a pile of dollar bills into the air and hoping that some of the money lands in the right place. For this reason, it is no surprise

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Ecosystems

KEY TERMS ABUNDANCE:

A measure of the

A measure of the

COMPLEXITY:

degree to which an ecosystem possesses

degree to which an ecosystem possesses a

large numbers of particular species. An

wide array of species. These species may or

abundant ecosystem may or may not have

may not appear in large numbers. Com-

a wide array of different species. Compare

pare with abundance.

with complexity. ANGIOSPERM:

COMPOUND:

A type of plant that

produces flowers during sexual reproduction.

A substance made up of

atoms of more than one element chemically bonded to one another. DECOMPOSERS:

Organisms

that

obtain their energy from the chemical The

breakdown of dead organisms as well as

changes that particular elements undergo

from animal and plant waste products. The

as they pass back and forth through the

principal forms of decomposer are bacteria

various earth systems and particularly

and fungi.

BIOGEOCHEMICAL CYCLES:

between living and nonliving matter. The elements involved in biogeochemical cycles are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur. BIOTA:

A combination of all flora and

REACTION:

broken down into simpler compounds or into its constituent elements. On Earth, this often is achieved through the help of detritivores and decomposers.

in a region.

DETRITIVORES:

The upper portion of the

trees in a forest. In a closed-canopy forest the canopy (which may be several hundred feet, or well over 50 meters, high) protects the soil and lower areas from sun and torrential rainfall. CARNIVORE:

A

chemical reaction in which a compound is

fauna (plant and animal life, respectively)

CANOPY:

Organisms that feed

on waste matter, breaking down organic material into inorganic substances that then can become available to the biosphere in the form of nutrients for plants. Their function is similar to that of decomposers, but unlike decomposers—which tend to be bacteria or fungi—detritivores are relative-

A meat-eating organ-

ly complex organisms, such as earthworms or maggots.

ism.

that angiosperms gradually are overtaking gymnosperms. P O L L I N AT I O N .

and nectar, the flowers of angiosperms attract animals, which travel from one flower to another, accidentally moving pollen as they do.

The angiosperm overcomes its own lack of mobility by making use of mobile organisms. Whereas insects and animals pose a threat to gymnosperms, angiosperms actually put bees, butterflies, hummingbirds, and other flowerseeking creatures to work aiding their reproductive process. By evolving bright colors, scents,

Because of this remarkably efficient system, animal-pollinated species of flowering plants do not need to produce as much pollen as gymnosperms. Instead, they can put their resources into other important functions, such as growth and greater seed production. In this way, the angiosperm solves its own problem of reproduc-

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DECOMPOSITION

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Ecosystems

KEY TERMS ECOSYSTEM:

A community of inter-

dependent organisms along with the inorganic components of their environment. ELEMENT:

A substance made up of

CONTINUED

seeds that are exposed, not hidden in an ovary, as with an angiosperm. HERBIVORE:

A plant-eating organ-

ism.

only one kind of atom. Unlike compounds,

HYDROSPHERE:

elements cannot be broken chemically into

Earth’s water, excluding water vapor in the

other substances.

atmosphere but including all oceans, lakes,

ENERGY TRANSFER:

The flow of

energy between organisms in a food web. FOOD WEB:

A term describing the

interaction of plants, herbivores, carni-

The entirety of

streams, groundwater, snow, and ice. NICHE:

A term referring to the role

that a particular organism plays within its biological community. An organism that eats

vores, omnivores, decomposers, and detri-

OMNIVORE:

tivores in an ecosystem. Each consumes

both plants and other animals.

nutrients and passes it along to other

ORGANIC:

organisms. Earth scientists typically prefer

the term organic only in reference to living

this name to food chain, an everyday term

things. Now the word is applied to most

for a similar phenomenon. A food chain is

compounds containing carbon and hydro-

a series of singular organisms in which

gen, thus excluding carbonates (which

each plant or animal depends on the

are minerals) and oxides, such as carbon

organism that precedes or follows it. Food

dioxide.

At one time, chemists used

chains rarely exist in nature. PHOTOSYNTHESIS:

The biological

The upper part of

conversion of light energy (that is, electro-

Earth’s continental crust, or that portion of

magnetic energy) from the Sun to chemical

the solid earth on which human beings live

energy in plants.

GEOSPHERE:

and which provides them with most of their food and natural resources. GYMNOSPERM:

A type of plant that

reproduces sexually through the use of

tion—and as a side benefit adds enormously to the world’s beauty.

The Complexity of Ecosystems

SYSTEM:

Any set of interactions that

can be set apart mentally from the rest of the universe for the purposes of study, observation, and measurement.

in the natural world, and while there is some dispute as to how delicate that balance is—nature shows an amazing resilience in recovering from the worst kinds of damage—there is no question that a balance of some kind exists.

The relationships between these two types of seed-producing plant and their environments illustrate, in a very basic way, the complex interactions between species in an ecosystem. Environmentalists often speak of a “delicate balance”

To put it another way, an ecosystem is an extraordinarily complex environment that brings together biological, geologic, hydrologic, and atmospheric components. Among these components are trees and other plants; animals, insects,

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and microorganisms; rocks, soil, minerals, and landforms; water in the ground and on the surface, flowing or in a reservoir; wind, sun, rain, moisture; and all the other specifics that make up weather and climate. In the present context, we have not attempted to provide anything even approaching a comprehensive portrait of an ecosystem, drawing together all or most of the aspects described in the preceding paragraph. A full account of even the simplest ecosystem would fill an entire book. Given that level of complexity, it is safe to say that one should be very cautious before tampering with the particulars of an ecosystem. The essay on Ecology and Ecological Stress concerns what happens when such tampering occurs.

WHERE TO LEARN MORE Beattie, Andrew J., and Paul Ehrlich. Wild Solutions: How Biodiversity Is Money in the Bank. New Haven, CT: Yale University Press, 2001.

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Diamond, Jared M. Guns, Germs, and Steel: The Fates of Human Societies. New York: W. W. Norton, 1997. The Ecosystems Center. Marine Biological Laboratory, Woods Hole, Massachusetts (Web site). . Ecotopia (Web site). . Jordan, Richard N. Trees and People: Forestland, Ecosystems, and Our Future. Lanham, MD: Regnery Publishing, 1994. Living Things: Habitats and Ecosystems (Web site). . Martin, Patricia A. Woods and Forests. Illus. Bob Italiano and Stephen Savage. New York: Franklin Watts, 2000. Nebel, Bernard J., and Richard T. Wright. Environmental Science: The Way the World Works. Upper Saddle River, NJ: Prentice Hall, 2000. Rybolt, Thomas R., and Robert C. Mebane. Environmental Experiments About Life. Hillside, NJ: Enslow Publishers, 1993. The State of the Nation’s Ecosystems (Web site). . Sustainable Ecosystems Institute (Web site). .

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ECOLOGY AND ECOLOGICAL STRESS

CONCEPT Ecology is the study of the relationships between organisms and their environments. As such, it is subsumed into the larger subject of ecosystems, which encompasses both living and nonliving components of the environment. As a study of the biological aspect of ecosystems, ecology is properly a part of the biological sciences rather than the earth sciences; however, in practice it is difficult to draw a line between the disciplines. This is especially the case inasmuch as the study of the environment involves such aspects as soil science, where earth sciences and ecology meet. This fact, combined with increasing concerns over ecological stresses, such as the increase of greenhouse gases in the atmosphere, warrants the consideration of ecology in an earth sciences framework.

HOW IT WORKS Ecosystems, Biological Communities, and Ecology An ecosystem is the complete community of living organisms and the nonliving materials of their surroundings. It therefore includes components that represent the atmosphere, the hydrosphere (all of Earth’s waters, except for moisture in the atmosphere), the geosphere (the soil and extreme upper portion of the continental crust), and the biosphere. The biosphere includes all living things: plants (from algae and lichen to shrubs and trees); mammals, birds, reptiles, amphibians, aquatic life, and insects as well as all manner of microscopic forms, including bacteria and viruses. In addition, the biosphere draws

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together all formerly living things that have not yet decomposed. The components of the biosphere are united not only by the fact that all of them are either living or recently living but also by the food web. The food web, discussed in much detail within the context of Ecosystems, is a complex network of feeding relationships and energy transfers between organisms. At various levels and stages of the food web are plants; herbivores, or planteating organisms; carnivores (meat-eating organisms); omnivores (organisms that eat both meat and plants); and, finally, decomposers and detritivores, which obtain their energy from the chemical breakdown of dead organisms. E C O LO GY. When discussing the living components of an ecosystem—that is, those components drawn from the biosphere—the term biological community is used. This also may be called biota, which refers to all flora and fauna, or plant and animal life, respectively, in a particular region. The relationship between these living things and their larger environment, as we have noted, is called ecology. Pioneered by the German zoologist Ernst Haeckel (1834–1919), ecology was long held in disdain by the world scientific community, in part because it seemed to defy classification as a discipline. Though its roots clearly lie in biology, its broadly based, multidisciplinary approach seems more attuned to the earth sciences.

In any case, ecology long since has gained the respect it initially failed to receive, and much of that change has to do with a growing acceptance of two key concepts. On the one hand, there is the idea that all of life is interconnected and that the living world is tied to the nonliving, or

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inorganic, world. This is certainly a prevailing belief in the modern-day earth sciences, with its systems approach (see Earth Systems). On the other hand, there is the gathering awareness that certain aspects of industrial civilization may have a negative impact on the environment. Clearly, the ecosystem as a whole is held together by tight bonds of interaction, but where the biological community is concerned, those bonds are even tighter. For the biological community to survive and thrive, a balance must be maintained between consumption and production of resources. Nature provides for that balance in numerous ways, but beginning in the late twentieth century, environmentalists in the industrialized world became increasingly concerned over the possibly negative effects their own societies exert on Earth’s ecosystems and ecological communities.

Climax and Succession One of the concerns raised by environmentalists is the issue of endangered species, or varieties of animal whose existence is threatened by human activities. In fact, nature itself sometimes replaces biological communities in a process called succession. Succession involves the progressive replacement of earlier biological communities with others over time. Coupled with succession is the idea of climax, a theoretical notion intended to describe a biological community that has reached a stable point as a result of ongoing succession. Succession typically begins with a disturbance exerted on the preexisting ecosystem, and this disturbance usually is followed by recovery. This recovery may constitute the full extent of the succession process, at which point the community is said to have reached its climax point. Whether or not this happens depends on such particulars as climate, the composition of the soil, and the local biota. There are two varieties of succession, primary and secondary. Primary succession occurs in communities that have never experienced significant modification of biological processes. In other words, the community affected by primary succession is “virgin,” and primary succession typically involves enormous stresses. On the other hand, secondary succession happens after disturbances of relatively low intensity, such that the regenerative capacity of the local biota has not been altered significantly. Secondary succes-

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sion takes place in situations where the biological community has experienced alteration.

Niche Whereas climax and succession apply to broad biological communities, the term niche refers to the role a particular organism or species plays within the larger community. Though the concept of niche is abstract, it is unquestionable that each organism plays a vital role and that the totality of the ecosystem would suffer stress if a large enough group of organisms were removed from it. Furthermore, given the apparent interrelatedness of all components in a biological community, every species must have a niche—even human beings. An interesting idea related to the niche is the concept of an indicator species: a plant or animal that by its presence, abundance, or chemical composition demonstrates a particular aspect of the character or quality of the environment. Indicator species can, for instance, be plants that accumulate large concentrations of metals in their tissues, thus indicating a preponderance of metals in the soil. This metal could indicate valuable deposits nearby, or it could serve as a sign that the soil is being contaminated. In the rest of this essay, we explore a few examples of ecological stress—situations in which the relationship between organisms and environment has been placed under duress. We do not attempt to explore the ideas of succession, climax, niche, or indicator species with any consistency or depth; rather, our purpose in briefly discussing these terms is to illustrate a few of the natural mechanisms observed or hypothesized by ecologists in studying natural systems. The vocabulary of ecology, in fact, is as complex and varied as that of any natural science, and much of it is devoted to the ways in which nature responds to ecological stress.

REAL-LIFE A P P L I C AT I O N S Deforestation In Ecosystems, we discuss a number of forest types, whose makeup is determined by climate and the dominant tree varieties. Here let us consider what happens to a forest—particularly an old-growth forest—that experiences significant

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RAIN

FOREST DESTRUCTION BY FIRE IN

MADAGASCAR. SUCH DEFORESTATION AFFECTS THE CARBON BALANCE IN EARTH. (© Daniel Heuclin/Photo Researchers. Reproduced by permission.)

THE

ATMOSPHERE AND THE DIVERSITY OF SPECIES ON

disturbance. Actually, the term deforestation can describe any interruption in the ordinary progression of the forest’s life, including clear-cut harvesting, even if the forest fully recovers. Deforestation can take place naturally, as a result of changes in the soil and climate, but the most significant cases of deforestation over the past few thousand years have been the result of human activities. Usually, deforestation is driven by the need to clear land or to harvest trees for fuel and, in some cases, building. Though deforestation has been a problem the world over, since the 1970s it has become more of an issue in developing countries.

as the United States, environmental activism has raised public awareness concerning deforestation and has led to curtailment of large-scale cutting in forests deemed important environmental habitats. By contrast, developing nations, such as Brazil, are cutting down their forests at an alarming rate. Generally, economics is the driving factor, with the need for new agricultural land or the desire to obtain wood and other materials driving the deforestation process.

DEVELOPED AND DEVELOPI N G N AT I O N S . In developed nations such

Yet the deforestation of such valuable reserves as the Amazon rain forest is an environmental disaster in the making: as noted in Soil Conservation, the soil in rain forests is typically “old” and leached of nutrients. Without the constant reintroduction of organic material from the

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deforestation in many developing countries often is accompanied by the displacement of indigenous peoples. Other political and social horrors sometimes lurk in the shadows: for example, Brazil’s forests are home to charcoal plants that amount to virtual slave-labor camps. Indians are lured from cities with promises of high income and benefits, only to arrive and find that the situation is quite different from what was advertised. Having paid the potential employer for transportation to the work site, however, they are unable to afford a return ticket and must labor to repay the cost.

Ecology and Ecological Stress

Old-Growth Forests

OLD-GROWTH SPOTTED

FORESTS ARE HOME TO THE NORTHERN

OWL,

RECOGNIZED

AS

AN

ENDANGERED

SPECIES BECAUSE OF THE DESTRUCTION OF ITS HABITAT.

(© T. Davis/Photo Researchers. Reproduced by permission.)

plants and animals of the rain forests, it would be too poor to grow anything. Therefore, when nations cut down their own rain forest lands, in effect, they are killing the golden goose to get the egg. Once the rain forest is gone, the land itself is worthless. CONSEQUENCES OF DEFORE S TAT I O N . Deforestation has several

extremely serious consequences. From a biological standpoint, it greatly reduces biodiversity, or the range of species in the biota. In the case of tropical rain forests as well as old-growth forests, certain species cannot survive once the environmental structure has been ruptured. From an environmental perspective, it leads to dangerous changes in the carbon content of the atmosphere, discussed later in this essay. In the case of oldgrowth forests or rain forests, deforestation removes an irreplaceable environmental asset that contributes to the planet’s biodiversity—and to its oxygen supply.

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Old-growth forests represent a climax ecosystem—one that has come to the end of its stages of succession. They are dominated by trees of advanced age (hence the name old-growth), and the physical structure of these ecosystems is extraordinarily complex. In some places the canopy, or “rooftop,” of the forest is dense and layered, while in others it has gaps. Tree sizes vary enormously, and the forest is littered with the remains of dead trees. An old-growth forest, by definition, takes a long time to develop. Not only must it have been free from human disturbance, but it also must have been spared various natural types of disturbance that bring about succession: catastrophic storms or wildfire, for instance. For this reason, most old-growth forests are rain forests in tropical and temperate environments. Among North American old-growth forests are those of the United States Pacific Northwest as well as those in adjoining regions of southwestern Canada. T H E S P O T T E D O W L . These oldgrowth forests are home to a bird that, in the 1980s and 1990s, became well known both to environmentalists and to their critics: the northern spotted owl, or Strix occidentalis caurina. A nonmigratory bird, the spotted owl has a breeding pattern such that it requires large tracts of old-growth, moist-to-wet conifer forest—that is, a forest dominated by cone-producing trees—as its habitat. Given the potential economic value of old-growth forests in the region, the situation became one of heated controversy.

Even from a human standpoint, deforestation takes an enormous toll. Economically, it depletes valuable forest resources. Furthermore,

On the one hand, environmentalists insisted that the spotted owl’s existence would be threatened by logging, and, on the other hand, representatives of the logging industry and the local

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community maintained that prevention of logging in the old-growth forests would cost jobs and livelihoods. The question was not an easy one, pitting the interests of the environment against those of ordinary human beings. By the early 1990s, the federal government had stepped in on the side of the environmentalists, having recognized the spotted owl as a threatened species under the terms of the U.S. Endangered Species Act of 1973. Nonetheless, controversy over the spotted owl—and over the proper role of environmental, economic, and political concerns in such situations—continues.

The Greenhouse Effect Deforestation and other activities pose potential dangers to our atmosphere. In particular, such activities have led to an increasing release of greenhouse gases, which may cause the warming of the planet. As discussed in Energy and Earth, the greenhouse effect, in fact, is a natural process. Though it is typically associated, in the popular vocabulary at least, with the destructive impact of industrial civilization on the environment, it is an extremely effective mechanism whereby Earth makes use of energy from the Sun. Rather than simply re-radiating solar radiation, Earth traps some of this heat in the atmosphere with the help of greenhouse gases, such as carbon dioxide. As in the case of most natural processes, however, if a little bit of carbon dioxide in the atmosphere is good, this does not mean that a lot is better. As noted in the essay Carbon Cycle, all living things contain carbon in certain characteristic structures; hence, the term organic refers to this type of carbon content. Though carbon dioxide is not an organic compound, it is emitted by animals: they breathe in oxygen, which undergoes a chemical reaction in their carbon-based bodies, and, as a result, carbon dioxide is released. Plants, on the other hand, receive this carbon dioxide and, through a chemical process in their own cellular structures, take in the carbon while releasing the oxygen.

tion occurs in a mature forest ecosystem, the mature forest will be replaced by an ecosystem that contains much smaller amounts of carbon.

Ecology and Ecological Stress

Ultimately, the carbon from the former ecosystem will be released to the atmosphere in the form of carbon dioxide. This will happen quickly, if the biomass of the forest is burned, or more slowly, if the timber from the forest is used for a long periods of time, for instance, in the building of houses or other structures. Before humans began cutting down forests, Earth’s combined vegetation stored some 990 billion tons (900 billion metric tons) of carbon, 90% of it appeared in forests. Today only about 616 billion tons (560 billion metric tons) of carbon are stored in Earth’s vegetation, and the amount is growing smaller as time passes. At the same time, the amount of carbon dioxide in the atmosphere has increased from about 270 parts per million (ppm) in 1850 to about 360 ppm in 2000, and, again, the increase continues. SHOULD

WE

BE

WORRIED?

Given this rise in atmospheric carbon dioxide as a result of deforestation—not to mention the more well-known cause, burning of fossil fuels— it is no wonder that atmospheric scientists and environmentalists are alarmed. Some of these scientists hypothesize that larger concentrations of carbon dioxide in the atmosphere will lead to increased intensity of the greenhouse effect. If this is true, it is possible that global warming will ensue, an eventuality that could have enormous implications for human survival. As a worst-case scenario, the polar ice cap (see Glaciology) could melt, submerging the cities of Earth.

T H E R E S U LT O F C U T T I N G M A T U R E F O R E S T S . Mature forests, such as

Before succumbing to the sort of doomsday thinking and scaremongering for which many environmentalists are criticized, however, it is important to recognize that several contingencies are involved: if carbon dioxide in the atmosphere causes an increase in the intensity of the greenhouse effect, it could cause global warming. The fact is that despite a few mild winters at the end of the twentieth century, it is far from clear that the planet is warming. The winter of 1993, for instance, produced one of the worst blizzards that the eastern United States has ever seen.

those of the old-growth variety, contain vast amounts of carbon in the form of living and dead organic material: plants, animals, and material in the soil. Because this quantity is much greater than in a younger forest, when deforesta-

As recently as the mid-1970s some environmentalists claimed that Earth actually is cooling—a response to a spate of cold winters in that period. The fact of the matter is that climate cycles are difficult to determine and require the

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Ecology and Ecological Stress

KEY TERMS BIOACCUMULATION:

The buildup of

A theoretical notion intended

toxic chemical pollutants in the tissues of

to describe a biological community that

individual organisms.

has reached a stable point as a result of

BIOLOGICAL COMMUNITY:

The liv-

ing components of an ecosystem.

ongoing succession. DECOMPOSERS:

Organisms

that

The increase in

obtain their energy from the chemical

bioaccumulated contamination at higher

breakdown of dead organisms as well as

levels of the food web. Biomagnification

from animal and plant waste products. The

results from the fact that larger organisms

principal forms of decomposer are bacteria

consume larger quantities of food—and,

and fungi.

hence, in the case of polluted materials,

DECOMPOSITION

more toxins.

chemical reaction in which a compound is

BIOMAGNIFICATION:

REACTION:

A

A combination of all liv-

broken down into simpler compounds or

ing things on Earth—plants, mammals,

into its constituent elements. On Earth,

birds, reptiles, amphibians, aquatic life,

this often is achieved through the help of

insects, viruses, single-cell organisms, and

detritivores and decomposers.

BIOSPHERE:

so on—as well as all formerly living things that have not yet decomposed.

DETRITIVORES:

Organisms that feed

on waste matter, breaking organic material

A combination of all flora and

down into inorganic substances that then

fauna (plant and animal life, respectively)

can become available to the biosphere in

in a region.

the form of nutrients for plants. Their

BIOTA:

The upper portion of the

function is similar to that of decomposers;

trees in a forest. In a closed-canopy forest,

however, unlike decomposers—which tend

the canopy (which may be several hundred

to be bacteria or fungi—detritivores are

feet, or well over 50 meters, high) protects

relatively complex organisms, such as

the soil and lower areas from sun and tor-

earthworms or maggots.

rential rainfall.

ECOLOGY:

CANOPY:

CARNIVORE:

A meat-eating organ-

ism.

perspective of several centuries’ worth of data (at least), rather than just a few years’ worth. (See Glaciology for a discussion of the Little Ice Age, which took place just a few centuries ago.) Nonetheless, it is important to be aware of the legitimate environmental concerns raised by the increased presence of carbon dioxide in the atmosphere due to human activities. Atmospheric scientists continue to monitor levels of greenhouse gases and to form hypotheses regarding

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CLIMAX:

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The study of the relation-

ships between organisms and their environments.

the ultimate effect of such activities as deforestation and the burning of fossil fuels.

Bioaccumulation and Biomagnification As we have seen in a number of ways, one of the key concepts of ecological studies is also a core principle in the modern approach to the earth sciences. In both cases, there is the idea that a disturbance in one area can lead to serious conse-

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KEY TERMS ECOSYSTEM:

A community of inter-

dependent organisms along with the inorganic components of their environment. ENERGY TRANSFER:

The flow of

energy between organisms in a food web. FOOD WEB:

A term describing the

interaction of plants, herbivores, carnivores, omnivores, decomposers, and detritivores in an ecosystem. Each of these organisms consumes nutrients and passes

CONTINUED

gases, such as carbon dioxide and water vapor, in the atmosphere. These gases are heated and ultimately re-radiate energy to space at an even longer wavelength. HERBIVORE:

A plant-eating organ-

ism. The entirety of Earth’s water, excluding water vapor in the atmosphere, but including all oceans, lakes, streams, groundwater, snow, and ice. HYDROSPHERE:

chain, an everyday term for a similar phe-

A term referring to the role that a particular organism plays within its biological community.

nomenon. A food chain is a series of singu-

OMNIVORE:

them along to other organisms. Earth scientists typically prefer this name to food

lar organisms in which each plant or animal depends on the organism that pre-

NICHE:

An organism that eats both plants and other animals.

the solid earth on which human beings live

At one time chemists used the term organic only in reference to living things. Now the word is applied to most compounds containing carbon, with the exception of carbonates (which are minerals) and oxides, such as carbon dioxide.

and which provides them with most of

SUCCESSION:

cedes or follows it. Food chains rarely exist in nature. GEOSPHERE:

The upper part of

Earth’s continental crust, or that portion of

their food and natural resources. GREENHOUSE EFFECT:

Warming

of the lower atmosphere and surface of Earth. This occurs because of the absorption of long-wavelength radiation from the planet’s surface by certain radiatively active

quences elsewhere. The interconnectedness of components in the environment thus makes it impossible for any event or phenomenon to be truly isolated.

Ecology and Ecological Stress

ORGANIC:

The progressive replacement of earlier biological communities with others over time. Any set of interactions that can be set apart from the rest of the universe for the purposes of study, observation, and measurement.

SYSTEM:

either by metabolizing them (i.e., incorporating them into the metabolic system, as one does food or water) or by excreting them. Yet the organism ultimately does release some toxins—by passing them on to other members of the food web. This increase in contamination at higher levels of the food web is known as biomagnification.

A good example of this is biomagnification. Biomagnification is the result of bioaccumulation, or the buildup of toxic chemical pollutants in the tissues of individual organisms. Part of what makes these toxins dangerous is the fact that the organism cannot process them easily

examples of chemical pollutants that are bioac-

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T H E P R O C E SS O F B I O M AG N I F I C AT I O N . Among the most prominent

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cumulated are such pesticides as DDT (dichlorodiphenyl-trichloroethane). DDT is a chlorinated hydrocarbon (see Economic Geology) used as an insecticide. Because of its hydrocarbon base, DDT is highly soluble in oils—and in the fat of organisms. Once pesticides such as DDT have been sprayed, rain can wash them into creeks and, finally, lakes and other bodies of water, where they are absorbed by creatures that drink or swim in the water. Atmospheric deposition, for instance, from industrial smokestacks or automobile emissions, is another source of toxins. Sludge from a sewage treatment plant can make its way into water sources, spreading all sorts of pollutants to the food web. Whatever the case, these toxins usually enter the food web by attaching to the smallest components. Particles of pollutant may stick to algae, which are so small that the toxin does little damage at this level of the food web. But even a small herbivore, such as a zooplankton, when it consumes the algae, takes in larger quantities of the pollutant, and thus begins the cycle of biomagnification. By the time the toxin has passed from a zooplankton to a small fish, the amount of pollutant in a single organism might be 100 times what it was at the level of the algae. The reason, again, is that the fish can consume 10 zooplankton that each has consumed 10 algae. (These particular numbers, of course, are used simply for the sake of convenience.) By the time the toxins have passed on to a few more levels in the food web, they might be appearing in concentrations as great as 10,000 times their original amount. D D T B I O M AG N I F I CAT I O N . For a

period of about two decades before 1972, DDT was used widely in the United States to help control the populations of mosquitoes and other insects. Eventually, however, it found its way into water sources and fish species through the process we have described. Predatory birds, such as osprey, peregrine falcons, and brown pelicans, consumed these fish. So, too, did the bald eagle, which has long been a protected species owing to its role as America’s national symbol. DDT levels became so high that the birds’ eggshells became abnormally thin, and adult birds sitting on nests accidentially would break the shells of unhatched eggs. As a result, baby birds died, and populations of these species also died. Public awareness of this phenomenon,

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raised by environmentalists in the late 1960s and early 1970s, led to the banning of DDT spraying in 1972. Since that time, populations of many predatory birds have increased dramatically. ADDRESSING ECOLOGICAL C O N C E R N S . In the case of DDT biomagni-

fication, humans were not directly involved, because the species of birds affected were not ones that people consume for food. Yet bioaccumulation and biomagnification have threatened humans. For example, in the 1950s, cows fed on grass that had been exposed to nuclear radiation and this radioactive material found its way into milk. Another example occurred during the 1970s and 1980s, when fish, such as tuna, were found to contain abnormally high levels of mercury. This led the federal government and some states to issue warnings against the consumption of certain types of fish, owing to bioaccumulated levels of toxic pollutants. Obviously, such measures, however well intentioned, are just cosmetic fixes for larger problems. In the long run, what is needed is a systemic ecological approach that attempts to address problems such as biomagnification and the accumulation of greenhouse gases by approaching the root causes. WHERE TO LEARN MORE Ashworth, William, and Charles E. Little. Encyclopedia of Environmental Studies. New York: Facts on File, 2001. The Ecological Society of America: Issues in Ecology (Web site). . Ecology.com—An Ecological Source of Information (Web site). . Ecology WWW Page (Web site). . Endangered Species on EE-Link (Web site). . Markley, O. W., and Walter R. McCuan. 21st Century Earth: Opposing Viewpoints. San Diego, CA: Greenhaven Press, 1996. The Marshall Cavendish Illustrated Encyclopedia of Plants and Earth Sciences. New York: Marshall Cavendish, 1988. The Need to Know Library—Ecology and Environment Page (Web site). . Philander, S. George. Is the Temperature Rising?: The Uncertain Science of Global Warming. Princeton, NJ: Princeton University Press, 1998. Schultz, Warren. The Organic Suburbanite: An Environmentally Friendly Way to Live the American Dream. Emmaus, PA: Rodale, 2001.

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S C I E N C E O F E V E RY DAY T H I N G S Real-Life Earth Science

W AT E R A N D THE EARTH HYDROLOGY THE HYDROLOGIC CYCLE GLACIOLOGY

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Hydrology

CONCEPT Hydrology is among the principal disciplines within the larger framework of hydrologic sciences, itself a subcategory of earth sciences study. Of particular importance to hydrology is the hydrologic cycle by which water is circulated through various earth systems above and below ground. But the hydrologic cycle is only one example of the role water plays in the operation of earth systems. Outside its effect on living things, a central aspect of the hydrologic cycle, water is important for its physical and chemical properties. Its physical influence, exerted through such phenomena as currents and floods, can be astounding but no less amazing than its chemical properties, demonstrated in the fascinating realm of karst topography.

HOW IT WORKS The Systems Approach The modern study of earth sciences looks at the planet as a large, complex network of physical, chemical, and biological interactions. This is known as a systems approach to the study of Earth. The systems approach treats Earth as a combination of several subsystems, each of which can be viewed individually or in concert with the others. These subsystems are the geosphere, atmosphere, hydrosphere, and biosphere. The geosphere is that part of the solid earth on which people live and from which are extracted the materials that make up our world: minerals and rocks as well as the organic products of the soil. In the latter area, the geosphere overlaps with the biosphere, the province of all living and recently living things; in fact, once a formerly liv-

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HYDROLOGY

ing organism has decomposed and become part of the soil, it is no longer part of the biosphere and has become a component of the geosphere. Overlap occurs between all spheres in one way or another. Thus, the hydrosphere includes all of the planet’s waters, except for water that has entered the atmosphere in the form of evaporation. From the time moisture is introduced to the blanket of gases that surrounds the planet until it returns to the solid earth in the form of precipitation, water is a part of the atmosphere. This aspect of the planet’s water is treated in the essay on Evapotranspiration and Precipitation. THE HYDROSPHERE AND THE H Y D R O LO G I C CYC L E . All aspects of

water on Earth, other than evaporation and precipitation, fall within the hydrosphere. This includes saltwater and freshwater, water on Earth’s surface and below it, and all imaginable bodies of water, from mountain streams to underground waterways and from creeks to oceans. One of the fascinating things about water is that because it moves within the closed system of Earth, all the planet’s water circulates endlessly. Thus, there is a chance that the water in which you take your next bath or shower also bathed Cleopatra or provided a drink to Charlemagne’s horse. On a less charming note, there is also a good chance that the water with which you brush your teeth once passed through a sewer system. Lest anyone panic, however, this has always been the case and always will be; as we have noted, water circulates endlessly, and one particular molecule may serve a million different functions. Furthermore, as long as water continues to circulate through the various earth systems— that is, as long as it is not left to stagnate in a

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pond—it undergoes a natural cleansing process. Modern municipal and private water systems provide further treatment to ensure that the water that people use for washing is at least reasonably clean. In any case, it is clear that the movement of water through the hydrologic cycle is a subject complex enough to warrant study on its own (see Hydrologic Cycle).

The Hydrologic Sciences As noted in Studying Earth, the earth sciences can be divided into three broad areas: the geologic, hydrologic, and atmospheric sciences. Each of these areas corresponds to one of the “spheres,” or subsystems within the larger earth system, that we have discussed briefly: geosphere, hydrosphere, and atmosphere. The hydrologic sciences are concerned with the hydrosphere and its principal component, water. These disciplines include glaciology—the study of ice in general and glaciers in particular— and oceanography. Glaciology is discussed in a separate essay, and oceanography is examined briefly in the present context. Aside from these two areas of study, the central component of the hydrologic sciences is hydrology—its most basic discipline, as geology is to the geologic sciences. O C E A N O G RA P H Y. Oceanography is the study of the world’s saltwater bodies—that is, its oceans and seas—from the standpoint of their physical, chemical, biological, and geologic properties. These four aspects of oceanographic study are reflected in the four basic subdisciplines into which oceanography is divided: physical oceanography, chemical oceanography, marine geology, and marine ecology. Each represents the application of a particular science to the study of the oceans.

Physical oceanography, as its name implies, involves the study of physics as applied to the world’s saltwater bodies. In general, it concerns the physical properties of the oceans and seas, including currents and tides, waves, and the physical specifics of seawater itself—that is, its temperature, pressure at particular depths, density in specific areas, and so on.

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ocean plays in the biogeochemical cycles whereby certain chemical elements circulate between the organic and inorganic realms (see Biogeochemical Cycles, Carbon Cycle, and Nitrogen Cycle). The biogeochemical focus of chemical oceanography implies an overlap with geochemistry. Likewise, marine geology exists at the nexus of oceanography and geology, involving, as it does, such subjects as seafloor spreading (see Plate Tectonics), ocean topography, and the formation of ocean basins. Finally, there is the realm where oceanography overlaps with biology, a realm known as marine ecology or biological oceanography. This subdiscipline is concerned with the wide array of life-forms, both plant and animal, that live in the oceans as well as the food webs whereby they interact with one another.

Introduction to Hydrology As noted earlier, hydrology is the central field of the hydrologic sciences, dealing with the most basic aspects of Earth’s waters. Among the areas of focus in hydrology are the distribution of water on the planet, its circulation through the hydrologic cycle, the physical and chemical properties of water, and the interaction between the hydrosphere and other earth systems. Among the subdisciplines of hydrology are these: • Groundwater hydrology: The study of water resources below ground. • Hydrography: The study and mapping of large surface bodies of water, including oceans and lakes. • Hydrometeorology: The study of water in the lower atmosphere, an area of overlap between the hydrologic and atmospheric sciences • Hydrometry: The study of surface water— in particular, the measurement of its flow and volume. THE WORK OF HYDROLOG I S T S . Bringing together aspects of geology,

Just as physical oceanography weds physics to the study of seawater, chemical oceanography is concerned with the properties of the ocean as viewed from the standpoint of chemistry. These properties include such specifics as the chemical composition of seawater as well as the role the

chemistry, and soil science, hydrology is of enormous practical importance. Local governments, for instance, require hydrologic studies before the commencement of any significant building project, and hydrology is applied to such areas as the designation and management of flood plains. Hydrologists also are employed in the management of water resources, wastewater systems, and irrigation projects. The public use of water for

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WHIRLPOOLS ARE CREATED WHERE TWO CURRENTS NORTHERN HEMISPHERE AND COUNTERCLOCKWISE

MEET. IN THE

WATER TENDS TO ROTATE IN CIRCLES, CLOCKWISE IN THE SOUTHERN HEMISPHERE. (© B. Tharp/Photo Researchers. Repro-

duced by permission.)

recreation and power generation also calls upon the work of hydrologists, who assist governments and private companies in controlling and managing water supplies. Hydrologists in the field use a variety of techniques, some of them simple and time-honored and others involving the most cutting-edge modern technology. They may make use of highly sophisticated computer models and satellite remote-sensing technology, or they may apply relatively uncomplicated methods for the measurement of snow depth or the flow of rivers and streams. Local hydrologists searching for water may even avail themselves of the services of quasimystics who employ a nonscientific practice called dowsing. The latter method, which involves the sensing of underground water with a “magic” divining rod, sometimes is used, with varying degrees of success, to find water in rural areas.

gravitational pull—see Sun, Moon, and Earth) but also in the form of currents. These are patterns of oceanic flow, many of them regular and unchanging and others susceptible to change as a result of shifts in atmospheric patterns and other parameters. Among the factors that affect the flow of currents are landmasses, wind patterns, and the Coriolis effect, or the deflection of water caused by the turning of Earth. Landmasses on either side of the Atlantic, Pacific, and Indian Oceans act as barriers to the paths of currents. For instance, if Africa were not placed as it is, between the Atlantic and Indian Oceans, water in the equatorial region would flow uniformly from east to west, or from the Indian Ocean to the Atlantic. Likewise, the movement would be uniformly west to east at the poles, as it would be at the equator.

Ocean waters are continually moving, not only as waves hitting the shore (a function of the Moon’s

Such is the case just off the shores of Antarctica, where the Antarctic Circumpolar Current, without the obstruction of land barriers, consistently circles the globe in a west-to-east direction. On the other hand, at the southern extremity of Africa, far from the equator, the movement of water also would be uniform, but in this case from west to east. Because of these landmasses,

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REAL-LIFE A P P L I C AT I O N S Currents

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however, the movement of currents is much more complex. Wind patterns also drive currents. These patterns work in tandem with the Coriolis effect, a term that generally describes a phenomenon that occurs with all particles on a rotating sphere such as Earth. The result of the Coriolis effect is that water tends to rotate in circles, clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. (At the equator and poles, by contrast, the Coriolis effect is nonexistent.) Combined with prevailing winds, the Coriolis effect creates vast elliptical (oval-shaped) circulating currents called gyres. S U R FA C E CURRENTS AND T H E I R E F F E C T O N C L I M AT E .

Among the basic types of currents are surface, tidal, and deep-water currents. In addition, a fourth type of current, a turbidity current, is of interest to oceanographers, hydrologists, and underwater geologists. Surface currents such as the Gulf Stream are the most well known variety, being the major form of current by which water circulates on the ocean’s surface. Caused by the friction of atmospheric patterns—another type of current—moving over the sea, these currents largely are driven by winds. (Winds, in turn, are caused by differences in temperature between packets of air at differing altitudes—see Convection.) Running as deep as 656 ft. (200 m), surface currents can be a powerful force. As a result of the Gulf Stream, for instance, a craft that sets sail eastward from the Caribbean is likely to be pulled quickly toward England. Of even greater importance is the impact of the Gulf Stream on climate. By moving warm waters in a northeasterly direction, it causes the European climate to be much warmer than it would be normally. For instance, Boston, Massachusetts, and Rome, Italy, are on the same latitude, but whereas Boston is known for its icy winters, the mention of Rome rightly conjures images of sunshine and warmth. Likewise, London lies north of the 50th parallel, far above any city in the continental United States—including such places as International Falls, Minnesota, and Buffalo, New York, which are noted for their cruel winters. On the other hand, London, while it is far from balmy in wintertime, is many degrees warmer, thanks to the Gulf Stream.

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O T H E R T Y P E S O F C U R R E N T.

Among the other types of currents are tidal currents, which are horizontal movements of water associated with the changing tides of the ocean. Their effect is felt primarily in the area between the continental shelf and the shore, where tidalcurrent phenomena such as riptides can pose a serious danger to swimmers. Deep-water currents, while they are less noticeable to people, are responsible for 90% of the water circulation that takes place in the ocean. Caused by variations in water density, which is a function of salt content (salinity) and temperature, these are slow-moving currents that move colder, denser water toward the depths of the ocean. Then there are turbidity currents, which result from the mixing of relatively light water with water that has been made heavy by its sediment content. Earthquakes may cause these currents, which are local, fast movements of water along the ocean floor. Another cause of turbidity currents is the piling of sediment on underwater slopes. Turbidity currents play a major role in shaping the terrain of the ocean floor.

Flooding Whereas currents arise in areas of Earth where water is “supposed” to be, floods, by definition, do not. They often occur in valleys or on coastlines and can be caused by various natural and man-made factors. Among natural causes are rains and the melting of snow and ice, while human-related causes can include poor engineering of irrigation or other water-management systems as well as the bursting of dams. In addition, the building of settlements too close to rivers and other bodies of water that are prone to flooding has resulted in the increase of human casualties from flooding over the centuries. In terms of natural causes, changes in weather patterns typically are involved—but not always. For example, a low-lying coastal area may be susceptible to flooding at times when the ocean reaches high tide. (On the other hand, such weather conditions as low barometric pressure and high winds also can bring about heightened high tides.) Additionally, floods can be caused by earthquakes and other geologic phenomena that have no relation to the weather. From ancient times people have located settlements near water. This settlement pattern resulted from the obvious benefits that accrued

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THE FLOODWATERS OF THE MISSISSIPPI RISE TO 4 FT. (1.2 M), SURROUNDING THE PUMPING STATION IN HANNIBAL, MISSOURI. APART FROM NATURAL CAUSES, FLOODS CAN RESULT FROM INCONSISTENT FLOOD MANAGEMENT, POOR CIVIL ENGINEERING DESIGN, AND UNWISE AGRICULTURAL PRACTICES. (AP/Wide World Photos. Reproduced by permission.)

from access to water, and even though flooding was naturally a hazard, in some cases flooding itself was found to be beneficial. For the ancient Egyptians, the yearly cycles of flooding on the part of the Nile caused the deposition of rich soil, which played a major part in the fertility of the farmlands that, in turn, made possible the brilliant civilization of the pharaohs. Along with these benefits, however, ancient peoples learned to fear the changes in weather and other circumstances that could bring about sudden flooding. This feeling is reflected, for instance, in Jesus’ parable about the wise and foolish house builders. In the parable, a favorite Sunday school topic, the foolish man builds his house upon the sand, so that when the floods come, they sweep away his household. On the other hand, the wise man builds his house on rock, so that his household withstands the inevitable flood—an illustration about spiritual values that likewise reflects a reality of daily life in the ancient Near East.

rivers and other bodies of water from flowing over onto the land. In addition, without vegetation to absorb rain, ground becomes saturated and thus susceptible to flooding. Not surprisingly, these unwise agricultural practices have helped bring about other disasters, such as the massive erosion of soil in the United States plains states that culminated in the dust bowl of the mid-1930s (see Soil Conservation). Less well known than the dust bowl but still massive in its impact was the 1927 flood of the Mississippi River, which left more than a million people homeless. It, too, was in part the result of unwise practices, in this case, inconsistent flood management and civil engineering design, according to John M. Barry, the author of Rising Tide: The Great Mississippi Flood of 1927 and How It Changed America. As Barry indicated in his subtitle, the flood’s impact went far beyond its direct effect on human lives or the landscape.

disastrous practices as clear-cutting of land and runaway grazing. Such activities remove vegetation, which holds soil in place and, in turn, keeps

As Barry discusses in the book, the flood was a major cause behind the rise of the poor-white discontent in Louisiana that led to the governorship (and, in the opinion of some people, the dictatorship) of the notorious Huey P. Long (1893–1935). Long, who won election on promises to ease the suffering of the underclass, ulti-

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HUMAN CAUSES AND EFF E C T S . Humans can cause floods by such

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mately became the virtual ruler of his state, with a degree of power that in the opinions of some pundits rivaled that of President Franklin D. Roosevelt—if not that of his other contemporaries, Adolf Hitler and Benito Mussolini. Of even greater long-term significance, Barry maintained, the flood brought about the large-scale flight of African Americans to the north and the shift of black political allegiance from the Republican Party to the Democratic Party. Few people alive today remember the 1927 flooding, but plenty recall the devastating floods of 1993, which killed 52 people and left over 70,000 homeless. Human mismanagement could not be blamed for the flooding itself, an outgrowth of exceptionally high soil moisture levels remaining from the fall of 1992, as well as heavy precipitation that continued in early 1993. However, once again, human attempts to control the flooding were less than successful: of some 1,300 levees or embankments that had been built (partly as a result of the 1927 flood) to keep flood waters back, all but about 200 failed. The floods, which lasted from late June to mid-August, destroyed nearly 50,000 homes and rendered over 12,000 sq mi. (31,000 sq km) of farmland useless. The overall damage estimate was in the range of $15 to $20 billion. P O W E R A N D P R E V E N T I O N . It is no wonder that a flood can have such a far-reaching impact, given the enormous power of water running wild in nature. Water is extremely heavy: just a bathtub full of water can weigh as much 750 lb. (340 kg). And it can travel as fast as 20 MPH (32 km/h), giving it tremendous physical force. Under certain conditions, a flood just 1 in. (2.54 cm) in depth can have as much potential energy as 60,000 tons (54,400 metric tons) of TNT. A U.S. study of persons killed in natural disasters during the 20-year period that ended in 1967 found that of 443,000 victims, nearly 40%, or about 173,000, were killed in floods. The other 60% was made up of people killed in 18 different types of other natural disaster, including hurricanes, earthquakes, and tornadoes.

Given this great destructive potential, communities—often with the help of hydrologists— have devised several means to control floods. Among such methods are the construction of dams and the diversion of floodwaters away from populated areas to flood-control reservoirs. These reservoirs then release the water at a

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slower rate than it would be released in the situation of a flood, thus giving the soil time to absorb the excess water. About one-third of all reservoirs in the United States are used for this purpose. Hydrologists are particularly important in helping communities protect against flooding by methods known as hazard zoning and minimizing encroachment. By studying historical records, along with geologic maps and aerial photographs, hydrologists and other planners can make recommendations regarding the zoning laws for a particular area, so that builders will take special precautions. In addition, they can help minimize encroachment—that is, ensure that new buildings are not located in such a way that they restrict the flow of water or cause water to pool up excessively.

Karst Topography In contrast to the dramatic action of flooding or currents, karst topography is a more subtle but no less intriguing aspect of the ways in which water affects the dynamics of Earth. In this case, however, the effect is chemical rather than physical in origin. Karst topography is a particular variety of landscape created where water comes into contact with extremely soluble (easily dissolved in water) varieties of bedrock. Karst is the German name for Kras, a region of Slovenia noted for its unusual landscape of strangely shaped white rock. In addition to the odd, funhouse forms of nightmarishly steep hills and twisting caves, much of it like something from a Dr. Seuss book, karst topography is noted for its absence of surface water, topsoil, or vegetation. The reason is that the bedrock comprises extremely soluble calcium carbonate minerals, such as limestone, gypsum, or dolomite. Karst regions form as a result of chemical reactions between groundwater and bedrock. In the atmosphere and on the surface of the solid earth, water combines with carbon dioxide in the air, and this combination acts as a corrosive on calcium carbonate rocks. This corrosive or acidic material seeps into all crevices of the rock, developing into sinkholes and widening fissures over time. Gradually, it carves out enormous underground drainage systems and caves. Sometimes the underground drainage structure collapses, leaving behind more odd shapes in the form of natural bridges and sink-

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KEY TERMS BIOGEOCHEMICAL CYCLES:

The

and which provides them with most of

changes that particular elements undergo

their food and natural resources.

as they pass back and forth through the

HYDROLOGIC CYCLE:

various earth systems and particularly

ous circulation of water throughout Earth

between living and nonliving matter. The

and between various Earth systems.

The continu-

elements involved in biogeochemical cycles are hydrogen, oxygen, carbon, nitrogen, phosphorus, and sulfur. BIOSPHERE:

HYDROLOGIC SCIENCES:

Areas of

the earth sciences concerned with the study of the hydrosphere. Among these disci-

A combination of all liv-

ing things on Earth—plants, mammals, birds, reptiles, amphibians, aquatic life, insects, viruses, single-cell organisms, and

plines are hydrology, glaciology, and oceanography. HYDROLOGY:

The study of the hydros-

phere, including the distribution of water

so on—as well as all formerly living things

on Earth, its circulation through the hydro-

that have not yet decomposed.

logic cycle, the physical and chemical prop-

CORIOLIS EFFECT:

The deflection of

water caused by the rotation of Earth. The Coriolis effect causes water currents to

erties of water, and the interaction between the hydrosphere and other earth systems. HYDROSPHERE:

The entirety of

move in circles—clockwise in the North-

Earth’s water, excluding water vapor in the

ern Hemisphere and counterclockwise in

atmosphere but including all oceans, lakes,

the Southern Hemisphere.

streams, groundwater, snow, and ice.

GEOCHEMISTRY:

A branch of the

ORGANIC:

At one time, chemists used

earth sciences, combining aspects of geolo-

the term organic only in reference to living

gy and chemistry, that is concerned with

things. Now the word is applied to most

the chemical properties and processes of

compounds containing carbon, with the

Earth—in particular, the abundance and

exception of carbonates (which are miner-

interaction of chemical elements and their

als) and oxides, such as carbon dioxide.

isotopes.

SYSTEM:

Any set of interactions that

The upper part of

can be set apart mentally from the rest of

Earth’s continental crust, or that portion of

the universe for the purposes of study,

the solid earth on which human beings live

observation, and measurement.

GEOSPHERE:

holes. This is a variety of karst topography known as doline karst. Another type is cone or tower karst, which produces tall, jagged limestone peaks, such as the sharp hills that characterize the river landscape in many parts of China. The United States is home to the world’s largest karst region, which includes the Mammoth cave system in Kentucky.

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WHERE TO LEARN MORE Abbott, Patrick. Natural Disasters. Dubuque, IA: William C. Brown Publishers, 1996. Arnell, Nigel. Hydrology and Global Environmental Change. Upper Saddle River, NJ: Prentice Hall, 2001. Barry, John M. Rising Tide: The Great Mississippi Flood of 1927 and How It Changed America. New York: Simon and Schuster, 1997.

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Dingman, S. Lawrence. Physical Hydrology. 2d ed. Upper Saddle River, NJ: Prentice Hall, 2002.

Karst Link Page (Web site). .

Gallant, Roy A. Earth’s Water. New York: Benchmark Books, 2002.

Karst Waters Institute (Web site). .

Global Hydrology and Climate Center (Web site). .

Skinner, Brian J., Stephen C. Porter, and Daniel B. Botkin. The Blue Planet: An Introduction to Earth System Science. 2d ed. New York: John Wiley and Sons, 1999.

Hydrology, Marine Science, Freshwater Science and Aquaculture Journals (Web site). .

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The World-Wide Web Virtual Library: Oceanography (Web site). .

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THE HYDROLOGIC C YCLE

The Hydrologic Cycle

CONCEPT The hydrologic cycle is the continuous circulation of water throughout Earth and between Earth’s systems. At various stages, water—which in most cases is synonymous with the hydrosphere—moves through the atmosphere, the biosphere, and the geosphere, in each case performing functions essential to the survival of the planet and its life-forms. Thus, over time, water evaporates from the oceans; then falls as precipitation; is absorbed by the land; and, after some period of time, makes its way back to the oceans to begin the cycle again. The total amount of water on Earth has not changed in many billions of years, though the distribution of water does. The water that we see, though vital to humans and other living things, makes up only about 0.0001% of the total volume of water on Earth; far more is underground and in other compartments of the environment.

HOW IT WORKS Water and the Hydrosphere As we have noted, water and the hydrosphere are practically synonymous, but not completely so. The hydrosphere is the sum total of water on Earth, except for that portion in the atmosphere. This combines all water underground—which, as we shall see, constitutes the vast majority of water on the planet—as well as all freshwater in streams, rivers, and lakes; saltwater in seas and oceans; and frozen water in icebergs, glaciers, and other forms of ice (see Glaciology).

are almost entirely made of water, and without water we would die much sooner than we would if we were denied food. Humans are not the only organisms dependent on water; whereas there are forms of life designated as anaerobic, meaning that they do not require oxygen, virtually nothing that lives can survive independent of water. Thus, the biosphere, which combines all living things and all recently deceased things, is connected intimately with the hydrosphere. WAT E R O N E A RT H . Throughout most of the modern era of scientific study—from the 1500s, which is to say most of the era of useful scientific study in all of human history—it has been assumed that water is unique to Earth. Presumably, if and when we found life on another planet, that planet also would contain water. But until that time, so it was assumed, we could be assured that the only planet with life was also the only planet with water.

In the latter part of the twentieth century, however, as evidence began to gather that Mars contains ice crystals on its surface, this exclusive association of water with Earth has been challenged. As it turns out, frozen water exists in several places within our solar system—as well it might, since water on Earth had to arrive from somewhere. It is believed, in fact, that water arrived on Earth at a very early stage, carried on meteors that showered the planet from space (see the entries Planetary Science and Sun, Moon, and Earth).

It is almost unnecessary to point out that water is essential to life. Human bodies, after all,

Since about three billion years ago, the amount of water on Earth has remained relatively constant. The majority of that water, however, is not in the biosphere, the atmosphere, or what we normally associate with the hydrosphere—

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Still, the atmosphere is just one of several “compartments” in which water is stored within the larger environment. Among the other important places in which water is found are the oceans and other surface waters, ice in its many forms, and aquifers. The latter are underground rock formations in which groundwater—water resources that occupy pores in bedrock—is stored.

The Hydrologic Cycle

IMBALANCES IN THE SYST E M . The total amount of water in all these

compartments is fixed, but water moves readily between various compartments through the processes of evaporation, precipitation, and surface and subsurface flows. The hydrologic cycle is thus a system all its own, a “system” (in scientific terms) being any set of interactions that can be set apart mentally from the rest of the universe for the purposes of study, observation, and measurement. Its net input and output balance each other. There may be imbalances of input and output in particular areas, which will manifest as drought or flooding. WATER

MOLECULES EVAPORATING FROM A SOLUTION.

(© K. Eward/Photo Researchers. Reproduced by permission.)

that is, the visible rivers, lakes, oceans, and ice formations on Earth’s surface. Rather, the largest portion of water on Earth is hidden away in the geosphere—that is, the upper portion of Earth’s crust, on which humans live and from which we obtain the minerals and grow the plants that constitute much of the world we know.

Water Compartments in the Environment In the course of circulating throughout Earth, water passes from the hydrosphere to the atmosphere. It does so through the processes of evaporation and transpiration. The first of these processes, of course, is the means whereby liquid water is converted into a gaseous state and transported to the atmosphere, while the second one—a less familiar term—is the process by which plants lose water through their stomata, small openings on the undersides of leaves. Earth scientists sometimes speak of the two as a single phenomenon, evapotranspiration.

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Flooding, as well as other aspects of the hydrosphere and its study, is discussed in the essay Hydrology. As for drought, its immediate cause is a lack of precipitation, though other causes can be responsible for the removal of water from the local environment. For instance, at present a large portion of Earth’s water is tied up in glaciers and other ice formations, but at other times in the planet’s history this ice has been melted, leaving much of the continental mass that we know today submerged under water (see Glaciology). ACCOUNTING FOR EARTH’S WAT E R. Earth’s total water supplies are so

large that instead of being measured by gallons or other units of volume, they are measured in terms of tons or metric tons, designated as tonnes. Nonetheless, for comparison’s sake, consider the following figures in light of the fact that a gallon (3.8 l) of water weighs 8.4 lb. (3.8 kg). A ton contains 238 gal., and a tonne has 1,000 l.

Evaporation and transpiration, as well as the process whereby such moisture is returned to the solid earth—that is, precipitation—are discussed in the essay Evapotranspiration and Precipitation.

Just as heat from the Sun accounts for the lion’s share of Earth’s total energy budget (see Energy and Earth), the vast majority of water on Earth comes from the deep lithosphere, the upper layer of Earth’s interior, comprising the crust and the brittle portion at the top of the mantle. In this vast region are contained 2.76  1019 tons (2.5  1019 tonnes). This figure, equal to 27.6 billion billion tons, is about 94.7% of the global total.

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The next largest compartment is the oceans, which contain 1.41  1018 tons (1.38  1018 tonnes), or 5.2% of the total. Ice caps, glaciers, and icebergs contain 1.74  1016 tons (0.017  1016 tonnes), thus accounting for most of the remaining 0.1% of Earth’s water. Beyond these amounts are much smaller quantities representing shallow groundwater (2.76  1014 tons, or 2.5  1014 tonnes); inland surface waters, such as lakes and rivers (2.76  1013 tons, or 2.5  1013 tonnes); and the atmosphere (1.43  1013 tons, or 1.3  1013 tonnes).

REAL-LIFE A P P L I C AT I O N S The Life of a Water Droplet Now let us follow the progress of a single water droplet as it passes through the water cycle. This particular droplet, like all others, has passed through the cycle countless times over the course of the past few billion years, and in its various incarnations it has existed as groundwater, as moisture in the atmosphere, and as ice. For the short span of Earth’s existence that humans have occupied the planet, it is conceivable that our droplet has been consumed—either directly, as liquid water, or indirectly, as part of the water content in animal or vegetable material. That would mean that it also has been excreted, after which it will have continued the cycle of circulation. In theory, it might well be part of the water in which humans bathe, brush their teeth, or wash their clothes. WAT E R A N D F O R E I G N M AT E R I A L . Of course, personal hygiene as we know it

today is an extremely recent development: for instance, regular toothbrushing as a practice among the whole population began in the United States only around the turn of the nineteenth century. Still, it is a bit disconcerting to think that the water in which we brush our teeth today may have floated down a sewer pipe at another time. Nonetheless, by moving water through so many locales, the hydrologic cycle has a built-in cleansing component. This cleansing component can be illustrated by the experience of saltwater, which despite its presence in the ocean is actually a small portion of Earth’s total water supply. The reason is that the salt seldom travels with the water; as soon as

S C I E N C E O F E V E RY DAY T H I N G S

the water evaporates, the salt is left behind. This is why people on the proverbial desert island or in other survival situations use evaporation to make saltwater drinkable.

The Hydrologic Cycle

Likewise, saltwater as such cannot survive the transition from liquid water to ice: as the water freezes, the ice (which has a much lower freezing point) simply is precipitated and left behind. Just as salt does not travel with water as it makes its way through the various stages of the hydrologic cycle, so other varieties of foreign matter are left behind as well; as long as water is not allowed to stagnate, it usually is cleansed in the course of traveling between the ground and the atmosphere. This is not to say that water typically exists in a pure form. Often called the universal solvent, water has such a capacity to absorb other substances that it is unlikely ever to appear in pure form unless it is distilled under laboratory conditions. Water in mountain streams, for instance, absorbs fragments of rock as it travels downhill, slowly eroding the surrounding rock and soil.

From the Watershed to Points Beyond On a particular watershed—an area of terrain from which water flows into a stream, river, or lake—our hypothetical water droplet may enter from a number of directions. In the simplest model of water flow, it comes from precipitation, including rain, snow, or even mist from clouds. The water has to go somewhere, and it may go either up or down. It may return to the atmosphere as evapotranspiration; it may enter the ground; or, if it reaches the solid earth at some elevation above sea level, it may enter a stream and flow ultimately to the ocean. For water to enter the atmosphere generally requires an extensive surface area of vegetation, which supports high rates of transpiration. This transpiration, combined with evaporation from such inorganic surfaces as moist soil or bodies of water, puts a great deal of water into the atmosphere. Without significant evapotranspiration, however, it is necessary for the water to drain from the watershed, either as seepage to deep groundwater or as flow in the form of a stream. T H E F I V E S TA G E S HYDROLOGIC CYCLE.

OF

THE

The overall process of the hydrologic cycle can be divided into five parts: condensation, precipitation, infil-

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The Hydrologic Cycle

THE ARKANSAS RIVER,

PART OF THE

TION PLACES A BIG DEMAND ON THE

OGALLALA AQUIFER, OGALLALA’S

RUNS THROUGH IRRIGATED FIELDS IN

K ANSAS. IRRIGA-

DIMINISHING WATER SUPPLY. (AP/Wide World Photos. Reproduced by per-

mission.)

372

tration, runoff, and evaporation. Water vapor in the atmosphere condenses, forming clouds, which eventually become so saturated that they release the water to the solid earth in the form of

precipitation. When precipitation enters the ground, it is known as infiltration.

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Infiltration can be great or small, depending on the permeability of the ground. The soil of a

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rainforest, for instance, has so much organic matter that it is likely to be highly permeable. On the other hand, cities have large amounts of what land developers call impervious surface: roads, buildings, and other areas in which concrete and other materials prevent water from infiltrating the ground. Assuming that water is unable to infiltrate, it becomes runoff. Runoff is simply surface water, which may take the form of streams, rivers, lakes, and oceans. If runoff occurs in an area that is not already a body of water, flood conditions may ensue. Thus, water may either infiltrate or become runoff, but as long as it remains close to the surface, it will experience evaporation. In evaporation energy from the Sun changes liquid water into gaseous form, transporting it as a vapor into the atmosphere. Thus, the water is returned to the air, where it condenses and resumes the cycle we have described. As noted earlier, the water on or near Earth’s surface is a small portion of the total. What about groundwater far below the surface? Let us now examine a particularly notable example of an aquifer, or groundwater reservoir.

The Ogallala Aquifer Located beneath the central United States, the Ogallala Aquifer provides a vast store of groundwater that supports a large portion of American agriculture. The Ogallala, also known as the High Plains Regional Aquifer, was discovered in the early years of the nineteenth century. It did not become a truly significant economic resource, however, until the second half of the twentieth century, when advanced pumping technology made possible large-scale irrigation from the aquifer’s supplies. By 1980 the Ogallala supported some 170,000 wells and accounted for fully one-third of all water used for irrigation purposes in the United States.

In Nebraska, the aquifer is between 400 ft. and 1,200 ft. (130–400 m) deep, while at the southern edges its depth extends no more than 100 ft. (30 m). Composed of porous sand, silt, and clay formations deposited by wind and water from the Rocky Mountains, the Ogallala is made up of several sections, called formations. The largest of these is the Ogallala formation, which accounts for about 77% of its total volume. USING

UP

THE

The Hydrologic Cycle

OGALLALA.

The Ogallala is particularly important to local agriculture because the states that it serves are home to numerous dry areas. Yet high-volume pumping of the underground reserves has reduced the available groundwater, much as the pumping of oil gradually is consuming Earth’s fossil-fuel reserves. Indeed, the water of the Ogallala is known as fossil water, meaning that it has been stored underground for millions of years, just as the coal, oil, and gas that runs modern industrial civilization has. Of course, the water from the Ogallala is not simply used up in an irreversible process, as is the case with fossil fuels; nonetheless, the rapidly accelerating reduction of its water supplies is cause for some alarm. Less than 0.5% of the water removed from this aquifer is being returned to the ground, and if the current rate of pumping increases, the supplies will be 80% depleted by 2020. The consequences of this depletion are already being felt in Kansas, where streams and rivers, dependent on groundwater to feed their flow, are running dry. In that state alone, more than 700 mi. (1,126 km) of rivers that formerly flowed year-round have been reduced to dry channels. In New Mexico and Texas, use of center-pivot irrigation, which requires a well capable of pumping 750 gal. (2,839 l) per minute, is disappearing because the local aquifer can no longer sustain such volumes.

Centered in Nebraska, the aquifer underlies parts of seven other states: South Dakota, Wyoming, Colorado, Kansas, Oklahoma, Texas, and New Mexico. It stretches 800 mi. (1,287 km) from north to south and 200 mi. (322 km) from east to west at its widest point. All told, the Ogallala covers some 175,000 sq. mi. (453,250 sq km), an area larger than Germany—all of it underground.

In addition to the problem of diminishing supplies, contamination is an issue. As more and more agricultural chemicals seep into an ever shrinking reservoir, the towns of the high plains—places once known for their pure, clean groundwater—now have tap water that is considered unsafe for children and pregnant women. Overuse of the Ogallala is also exacting a financial toll, as more and more wells run dry and farms go bankrupt.

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The Hydrologic Cycle

KEY TERMS AQUIFER:

An underground rock for-

mation in which groundwater is stored.

The

HYDROLOGY:

of

the

hydrosphere, including the distribution of

The solid rock that lies

water on Earth, its circulation through the

below the C horizon, the deepest layer of

hydrologic cycle, the physical and chemical

soil.

properties of water, and the interaction

BEDROCK:

BIOSPHERE:

A combination of all liv-

ing things on Earth—plants, mammals,

between the hydrosphere and other earth systems.

birds, reptiles, amphibians, aquatic life,

HYDROSPHERE:

insects, viruses, single-cell organisms, and

Earth’s water, excluding water vapor in the

so on—as well as all formerly living things

atmosphere but including all oceans, lakes,

that have not yet decomposed.

streams, groundwater, snow, and ice.

EVAPORATION:

The process whereby

liquid water is converted into a gaseous state and transported to the atmosphere. The loss of

EVAPOTRANSPIRATION:

The entirety of

The upper layer of

LITHOSPHERE:

Earth’s interior, including the crust and the brittle portion at the top of the mantle. When discussing the

water to the atmosphere via the processes

PRECIPITATION:

of evaporation and transpiration.

hydrologic cycle or meteorology, precipita-

The upper part of

tion refers to the water, in liquid or solid

Earth’s continental crust, or that portion of

form, that falls to the ground when the

the solid earth on which human beings live

atmosphere has become saturated with

and which provides them with most of

moisture.

GEOSPHERE:

their food and natural resources.

SYSTEM:

Any set of interactions that

Underground water

can be set apart mentally from the rest of

resources that occupy the pores in bedrock.

the universe for the purposes of study,

GROUNDWATER:

HYDROLOGIC CYCLE:

The continu-

observation, and measurement.

ous circulation of water throughout Earth and between various earth systems. HYDROLOGIC SCIENCES:

Areas of

the earth sciences concerned with the study

TRANSPIRATION:

The process where-

by plants lose water through their stomata, small openings on the undersides of leaves. An area of terrain from

of the hydrosphere. Among these areas of

WATERSHED:

study are hydrology, glaciology, and

which water flows into a stream, river, lake,

oceanography.

or other large body.

Despite the environmental challenges posed by such situations as the exhaustion of the Ogallala, the hydrologic cycle continues to roll on. As it does, it is sustained in large part by processes we cannot see: the movement of groundwater from

the aquifer into streams or the evapotranspiration of surface waters to the atmosphere. Yet the movement of waters along the surface, because it is visible and recognizable to humans, attracts human attention in a way that many of these other components of the hydrologic cycle do not.

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Rivers

374

study

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Rivers and other forms of surface water actually account for a relatively small portion of the planet’s water supply, but they loom large in the human imagination as the result of their impact on our lives. The first human civilizations developed along rivers in Egypt, Mesopotamia, India, and China, and today many a great city lies along a river. Rivers provide us with a means of transportation and recreation, with hydroelectric power, and even—after the river water has been treated—with water for drinking and bathing. F O R M AT I O N O F R I V E R S . Rivers usually form from tributaries, such as springs. As the river flows, it is fed by more tributaries and by groundwater and continues on its way at various speeds, depending on the terrain. Finally, the river discharges into an ocean, a lake, or a desert basin.

River waters typically begin with precipitation, whether in the form of rainwater or melting snow. They also are fed by groundwater exuding from bedrock to the surface. When precipitation falls on ground that is either steeply sloped or already saturated, the runoff moves along Earth’s surface, initially in an even, paper-thin sheet. As it goes along, however, it begins to form parallel rills, and its flow becomes turbulent. As the rills pass over fine soil or silt, they dig shallow channels, or runnels. At some point in their flow, the runnels merge with one another, until there are enough of them to form a stream. Once enough streams have converged to create a continuously flowing body of water, it becomes a brook, and once the volume of water carried reaches a certain level, the brook becomes a river. As we have already noted, however, a river is really the sum of its tributaries, and thus hydrologists speak of river systems rather than single rivers. R I V E R S Y S T E M S . A particularly impressive example of a river system is the vast Mississippi-Missouri, which drains the central United States. Most of the rivers between the

S C I E N C E O F E V E RY DAY T H I N G S

Rockies and the Appalachians that do not empty directly into the Gulf of Mexico feed this system. This system includes the Ohio, itself an impressive river that divides the eastern United States. Indeed, just as the Mississippi separates east from west in America, the Ohio separates north from south.

The Hydrologic Cycle

After the Ohio and the Mississippi converge, at the spot where Illinois, Missouri, and Kentucky meet, they retain their separate identities for many miles. A strip of clear water runs along the river’s eastern side, while to the west of this strip the water is a cloudy yellow—indicating a heavier amount of sediment in the Mississippi than in the Ohio. A similar phenomenon occurs where the “Blue Nile” and “White Nile,” tributaries of another great river—both named for the appearance of the water—meet at Khartoum in Sudan. WHERE TO LEARN MORE “Groundwater Primer: Hydrologic Cycle.” U.S. Environmental Protection Agency (Web site). . Hamilton, Kersten. This Is the Ocean. Honesdale, PA: Caroline House/Boyds Mills Press, 2001. “Hydrologic Cycle.” The Groundwater Foundation (Web site). . The Hydrologic Cycle (Web site). . Hydrologic/Water Cycle (Web site). . Pidwirny, Michael J. “Introduction to Hydrology: The Hydrologic Cycle.” Fundamentals of Physical Geography (Web site). . Richardson, Joy. The Water Cycle. Illus. Linda Costello. New York: F. Watts, 1992. Smith, David. The Water Cycle. Illus. John Yates. New York: Thomson Learning, 1993. Schaefer, Lola M. This Is the Ocean. Illus. Jane Wattenberg. New York: Greenwillow Books, 2001. Walker, Sally M. Water Up, Water Down: The Hydrologic Cycle. Minneapolis: Carolrhoda Books, 1992.

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GLACIOLOGY Glaciology

CONCEPT Glaciology is the study of ice and its effects. Since ice can appear on or in the earth as well as in its seas and other bodies of water and even its atmosphere, the purview of glaciologists is potentially very large. For the most part, however, glaciologists’ attention is direction toward great moving masses of ice called glaciers, and the intervals of geologic history when glaciers and related ice masses covered relatively large areas on Earth. These intervals are known as ice ages, the most recent of which ended on the eve of human civilization’s beginnings, just 11,000 years ago. The last ice age may not even be over, to judge from the presence of large ice masses on Earth, including the vast ice sheet that covers Antarctica. On the other hand, evidence gathered from the late twentieth century onward indicates the possibility of global warming brought about by human activity.

HOW IT WORKS Ice Ice, of course, is simply frozen water, and though it might appear to be a simple subject, it is not. Glaciologists classify differing types of ice, for instance, with regard to their levels of density, designating them with Roman numerals. The ice to which most of us are accustomed is classified as ice I. We will not be concerned with the other varieties of ice in the present context, but it should be noted that the ice in glaciers is quite different from the ice in an ice cube or even the ice on a pond in winter. These differences are a result of massive pressure, which reduces the air content of the ice in glaciers.

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By definition, ice is composed of fresh water rather than saltwater. This is true even of icebergs, though they may float on the salty oceans. The reason is that water has a much higher freezing point than salt, and, therefore, when water freezes, very little of the salt remains joined to the water. Most of the salt is left behind in the form of a briny slush, and so much of Earth’s fresh water supply actually is contained in great masses of ice, such as the glaciers of Antarctica. Glaciology is defined as the study of ice, its forms, and its effects. This means that the glaciologist has a much wider scope than a geologist, meteorologist, or oceanographer, each of whom is concerned primarily with the geosphere, atmosphere, and hydrosphere, respectively. Though ice commonly is associated with the hydrosphere, where it appears on Earth’s oceans, rivers, and lakes, it also is found on and even under the solid earth. There are even situations in which ice is found in the atmosphere.

Glaciology and Glaciers Despite the wide distribution of ice on Earth and the many forms it takes, the work of most glaciologists is concerned primarily with ice as it appears in glaciers. A glacier is a large, typically moving mass of ice on or adjacent to a land surface. It does not flow, as water does; rather, it is moved by gravity, a consequence of its extraordinary weight. Obviously, a glacier can form only in an extremely cold region—one so cold that the temperature never becomes warm enough for snow to melt completely. Some snow may melt as a result of contact with the ground, which is likely to be warmer than the snow itself, but when tem-

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Glaciology

AN

INFRARED SATELLITE IMAGE OF

LARGEST ICE SHELF, THE

ROSS,

ANTARCTICA,

SHOWING ICE SHELVES PROJECTING FROM THE COASTLINE.

SITS TO THE LEFT OF THE

TRANSANTARCTIC MOUNTAINS (LOWER

THE

CENTER).

(© USGS/Photo Researchers. Reproduced by permission.)

peratures drop, it refreezes. A glacier starts with a layer of ice, on which snow gathers until refreezing gradually creates compacted layers of snow and ice.

Glacial Temperature and Morphologic Characteristics

As anyone who has ever held a snowball in his or her hand knows, snow is fluffy, or, to put it in more scientific terms, it is much less dense than ice. A sample of snow is about 80% air, but as ice accumulates over a layer of snow, the weight of the ice squeezes out most of the air. As the layers grow thicker and thicker, the weight reduces the air further, creating an extremely dense, thick layer of ice. Ultimately, the ice becomes so heavy that its weight begins to pull it downhill, at which point it becomes a glacier.

Glaciers may be classified according to either relative temperature or morphologic characteristics (i.e., in terms of its shape). In terms of temperature, a glacier may be “warm,” meaning that it is close to the pressure melting point. Pressure melting point is defined as the temperature at which ice begins to melt under a given amount of pressure. It is commonly known that water melts at 32°F (0°C), but only under conditions of ordinary atmospheric pressure at sea level. At higher pressures, the melting point of water is lower, which means that it can remain liquid at temperatures below its ordinary freezing point. (The

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melting point and freezing point of a substance are always the same.) A “warm” glacier, such as those that appear in the Alps, is relatively mobile, because it is at the pressure melting point. This kind of glacier contrasts with a “cold,” or polar, glacier, in which the temperatures are well below the pressure melting point; in other words, despite the extremely high pressure, the temperature is so low that the ice will not melt. As their name suggests, polar glaciers are found at Earth’s poles, which effectively means Antarctica, since the area of the North Pole is not a land surface. A third category of glacier, in terms of temperature, is a subpolar glacier, found (not surprisingly) in regions near the poles. Examples of subpolar glaciers, or ones in which the fringes of the glacier are colder than the interior, are found in Spitsbergen, islands belonging to Norway that sit in the Arctic Ocean, well to the north of Scandinavia. MORPHOLOGIC CLASSIFICAT I O N S . In the classification of geologic sci-

ences, glaciology often is grouped with geomorphology. The latter field of study is devoted to landforms, or notable topographical features, and the forces and processes that have shaped them. Among those forces and processes are glaciers, which can be viewed in terms of their shape, the locale in which they form, and their effect on the contour of the land. Alpine or mountain glaciers flow down a valley from a high mountainous region, typically following a path carved out by rivers or melting snow in warmer periods. They move toward valleys or the ocean, and in the process they exert considerable impact on the surrounding mountains, increasing the sharpness and steepness of these landforms. The rugged terrain in the vicinity of the Himalayas and the Andes, as well as the alpine regions of the Cascade Range and Rocky Mountains in the United States, are partly the result of weathering caused by these glaciers.

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move out of the basinlike areas in which they are formed.

Other Ice Formations There are several other significant varieties of ice formation, including ice caps, ice fields, and ice sheets. An ice cap, though much bigger than a glacier, typically has an area of less than 19,300 sq. mi. (50,000 sq km). Nonetheless, its mass is such that it exerts enormous weight on the land surface, and this exertion of force allows it to flow. At the center of an ice cap or an ice sheet is an ice dome, and at the edges are ice shelves and outlet glaciers. Symmetrical and convex (i.e., like the outside of a bowl), an ice dome is a mass of ice often thicker than 9,800 ft. (3,000 m). An outlet glacier is a rapidly moving stream of ice that extends from an ice dome. Ice shelves, at the far outer edges, extend into the oceans, typically ending in cliffs as high as 98 ft. (30 m). Ice fields are similar to ice caps; the main difference is that the ice field is nearly level and lacks an ice dome. There are enormous variations in size for ice fields. Some may be no larger than 1.9 sq. mi. (5 sq km), while at different times in Earth’s history, others have been as large as continents. The most physically impressive of all ice formations, an ice sheet is a vast expanse of ice that gradually moves outward from its center. Ice sheets are usually at least 19,300 sq. mi. (50,000 sq km) and, like ice caps, consist of ice domes and outlet glaciers, with outlying ice shelves. Given their even greater size compared with ice caps, ice sheets exert still more force on the solid earth beneath them. They cause the rock underneath to compress, and, therefore, if an ice sheet ever melts, Earth’s crust actually will rise upward in that area.

REAL-LIFE A P P L I C AT I O N S

The glacial forms found in Alaska, Greenland, Iceland, and Antarctica are often piedmont glaciers, large mounds of ice that slope gently. Iceland, Greenland, and Antarctica as well as Norway are also home to cirque glaciers, which are relatively small and wide in proportion to their length. Though they experience considerable movement in place, they usually do not

Antarctica

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An example of an ice sheet is the Antarctic ice sheet, which is permanently frozen—at least for the foreseeable future. The Antarctic ice sheet covers most of Antarctica, an area of about five million sq. mi. (12.9 million sq km), the size of the United States, Mexico, and Central America combined. Within it lies 90% of the world’s ice

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and more fresh water than in all the planet’s rivers and lakes combined. By contrast, the impressive Greenland ice sheet, at 670,000 sq. mi. (1,735,000 sq km), is dwarfed, as are smaller ice sheets in Iceland, northern Canada, and Alaska. The Antarctic ice sheet is the Sahara of ice masses, though, in fact, it is almost 50% larger than the Sahara desert and a good deal more inhospitable. Whereas the Sahara is scattered with towns and oases and has a steady population of isolated villagers, nomads, and merchants in caravans, no one lives on the Antarctic ice sheet except scientists on temporary missions. And whereas people have lived in the Sahara for thousands of years (it became a desert only somewhat recently, during the span of human civilization), scientific missions to Antarctica became possible only in the twentieth century. As it is, researchers spend only short periods of time on the continent and then in heavily protected environments. I C E S H E LV E S A N D G LAC I E R S .

Just as the Antarctic ice sheet is the largest in the world, one of its attendant shelves also holds first place among ice shelves. The continent is shaped somewhat like a baby chick, with its head and beak pointing northward toward the Falkland Islands off the coast of South America and its two greatest ice shelves lying on either side of the “neck.” Facing the Weddell Sea, and the southern Atlantic beyond it, is the Ronne Ice Shelf, which extends about 400 mi. (640 km) over the water. The world record–holder, however, is the Ross Ice Shelf on the other side of the “neck,” near Marie Byrd Land. About the same size as Texas or Spain, the Ross shelf extends some 500 mi. (800 km) into the sea and is the site of several permanent research stations. A N TA R C T I C T O P O G RA P H Y. The Antarctic is also home to a vast mountain range, the Transantarctic, which stretches some 3,000 mi. (4,828 km) across the “neck” between the Ross and Weddell seas. Included in the Transantarctic Mountains is Vinson Massif, which at 16,860 ft. (5,140 m) is the highest peak on the continent. The continent as a whole is largely covered with mountain ranges, between which lie three great valleys called the Wright, Taylor, and Victoria valleys. Each is about 25 mi. (40 km) long and 3 mi. (5 km) wide.

glimpses of the rocks that form the solid-earth surface deep beneath Antarctica. They are also among the strangest places on the planet, forbidding lands even by Antarctic standards. The three are known as the “dry valleys,” owing to their lack of precipitation; indeed, if they lay in a more temperate zone, they would be deserts far more punishing than the Sahara. Geologists estimate that it has not rained or snowed in these three valleys for at least one million years. The reason is that ceaseless winds keep the air so dry that any falling snow evaporates before it reaches the ground. In this arid, brutally cold climate, nothing decomposes, and seal carcasses a millennium old remain fully intact.

Glaciology

T H E T H I C K N E SS O F T H E I C E .

The dry valleys are exceptional, because most of Antarctica lies under so much ice that the rocks cannot be seen. The ice in Antarctica has an average depth of more than a mile: the depth averages about 6,600 ft. (2,000 m), but in places on the continent it is as thick as 2 mi. (3.2 km). Thus, “ground level” on Antarctica is equivalent to a fairly high elevation in the inhabitable portion of the planet. Denver, Colorado, for instance, touts itself as the “Mile-High City,” and its elevation has enough effect on a visiting flatlander that rival sports teams usually spend a few days in Denver before a game, adjusting themselves to the altitude. The thickness of the ice has allowed glaciologists to take deep ice-core samples from Antarctica. An ice core is simply a vertical section of ice that, when studied with the proper techniques and technology, can reveal past climatic conditions in much the same way that the investigation of tree rings does. (See Paleontology for more about tree-ring research, or dendrochronology.) Ice-core samples from the Antarctic provide evidence regarding Earth’s climate for the past 160,000 years and show a pattern of warming and cooling that is related directly to the presence of carbon dioxide and methane in the atmosphere. These core samples also reveal the warming effects of increases in both gases over the past two centuries.

These are the largest continuous areas of icefree land on the continent, and they offer rare

Because of the great thickness of its ice, Antarctica has the highest average elevation of any continent on the planet. Yet beneath all that ice, the actual landmass is typically well below sea level. The reason for this is that the ice weighs it down so much; by contrast, if the ice were to

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melt, the land would begin to spring upward. The melting of the Antarctic ice shelf would be a disaster of unparalleled proportions. If all that ice were to melt at once, it would raise global sea levels by some 200 ft. (65 m). This would be enough to flood all the world’s ports, along with vast areas of low-lying land. For instance, waters would swell over New York City and all ports on America’s eastern seaboard and probably would cover an area extending westward to the Appalachian Mountains. Even if only 10% of Antarctica’s ice were to melt, the world’s sea level would rise by 20 ft. (6 m), enough to cause considerable damage.

What Glaciers Do to Earth’s Surface Periodically over the past billion years, Earth’s sea levels have advanced or retreated dramatically in conjunction with the beginning and end of ice ages. The latter will be discussed at the conclusion of this essay; in the present context, let us consider simply the geomorphologic effects of glaciers and ice masses. For example, as suggested in the discussion of Antarctica’s ice sheet, when glaciers melt, thus redistributing their vast weight, Earth’s crust rebounds. At the end of the last ice age, the crust rose upward, and in parts of North America and Europe this process of crustal rebounding is still occurring. Glaciers move at the relatively slow speeds one would expect of massive objects made from ice: only a few feet or even a few inches per day. Friction with Earth’s surface may melt the layer of ice that comes in contact with it, however, and, as a result, this layer of meltwater becomes like a lubricated surface, allowing the glacier to move much faster. The entire body of ice experiences a sudden increase of speed, called surging.

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midable, giving it greater weight, cutting ability, and erosive power. The sediments left by glaciers that lack any intervening layer of melted ice are known by the general term till. In unglaciated areas, or places that have never experienced any glacial activity, sediment is formed by the weathering and decomposition of rock. On the other hand, formerly glaciated areas are distinguished by layers of till from 200 to 1,200 ft. (61–366 m) thick. Piles of till left behind by glaciers form hills called moraines, and the depressions left by these land-scouring ice masses are called kettle lakes. North America abounds with examples of moraines and kettle lakes. Illinois, for instance, is covered with ridges, called end moraines, left behind by the melting near the conclusion of the last ice age. Visitors can take in a splendid view of moraine formations at Moraine View State Recreation Area, located astride the Bloomington moraine in central Illinois. Likewise Minnesota, Wisconsin, the Dakotas, and Wyoming are home to many a moraine. As with the Illinois recreation area, Kettle Lakes Provincial Park, near Timmins, Ontario, provides an opportunity to glimpse gorgeous natural wonders left behind by the retreat of glaciers—in this case, more than 20 deep kettle lakes. Park literature invites visitors to boat, fish, or swim in the lakes, though it would take a hardy soul indeed to brave those icy waters.

PLOWING THROUGH THE LA N D . A glacier is like a huge bulldozer, plow-

The glacier transports material from the solid earth as long as it is frozen, but wholly or partially melted glaciers leave behind sedimentary forms with their own specific names. In addition to moraines, there are piles of sediment, called eskers, left by rivers flowing under the ice. In addition, deposits of sediment may wash off the top of a glacier to form steep-sided hills called kames. If the glacier runs over moraines, eskers, or kames left by another glacier, the resulting formation is called a drumlin.

ing though rock, soil, and plants and altering every surface with which it comes into contact. It erodes the bottoms and sides of valleys, changing their V shape to a U shape. The rate at which it erodes the land is directly proportional to the depth of the glacier: the thicker the ice, the more it bears down on the land below it. As it moves, the ice pulls along rocks and soil, which are incorporated into the glacier itself. These components, in turn, make the glacier even more for-

Just as rivers consist of main bodies formed by the flow of tributaries (for example, the many creeks and smaller rivers that pour into the Mississippi), so there are tributary glaciers. When a glacial tributary flows into a larger glacier, their top elevations become the same, but their bottoms do not. As a result, they carve out “hanging valleys,” often the site of waterfalls. Examples include Yosemite and Bridalveil in California’s Yosemite Valley.

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Glaciology

THE KENNICOTT

GLACIER IN THE

WRANGELL MOUNTAINS

OF

ALASKA. (© Pat and Tom Leeson/Photo Researchers. Reproduced by

permission.)

Ice Ages The glaciers that exist today are simply the remnants of the last ice age, a time in which the size of the ice masses on Earth dwarfed even the great Antarctic ice sheet. When people speak of “the Ice Age,” what they mean is the last ice age, which ended about 11,000 years ago. Yet it is one of only about 20 ice ages that have taken place over the past 2.5 million years, roughly coinciding with the late Pliocene and Pleistocene epochs. Actually, periods of massive glaciation (the covering of the landscape with large expanses of ice) have occurred at intervals over the past billion years. Their distribution over time has not been random; rather, they are concentrated at specific junctures in Earth’s history.

S C I E N C E O F E V E RY DAY T H I N G S

Like the great mass extinctions of the past (see Paleontology), ice ages are among the markers geologists use in separating one interval of geologic time from another. In fact, there have been connections between ice ages and mass extinctions, particularly those that resulted from a recession of the seas. For example, the mass extinction that took place near the end of the Ordovician period (about 435 million years, or Ma, ago) came about as a result of a drop in the ocean level, which was caused, in turn, by an increase of glaciation that coincided with that phase in Earth’s history. The late Ordovician/early Silurian ice ages (between 460 to 430 Ma ago) are among four major phases of glaciation during the past 800

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KEY TERMS ATMOSPHERE:

In general, an atmos-

phere is a blanket of gases surrounding a

geology devoted to the study of ice, its

planet. Unless otherwise identified, howev-

forms, and its effects.

er, the term refers to the atmosphere of

HYDROSPHERE:

Earth, which consists of nitrogen (78%),

Earth’s water, excluding water vapor in the

oxygen (21%), argon (0.93%), and other

atmosphere but including all oceans, lakes,

substances that include water vapor, car-

streams, groundwater, snow, and ice.

bon dioxide, ozone, and noble gases such as neon, which together comprise 0.07%.

The entirety of

A period of massive and

ICE AGE:

widespread glaciation. Ice ages usually

An upward

occur in series over stretches of several mil-

movement by Earth’s crust in response to the

lion years, or even several hundred million

melting of a glacier, which redistributes its

years.

vast weight and causes Earth to rebound.

ICE CAP:

CRUSTAL REBOUNDING:

An ice formation bigger than

An area of phys-

a glacier but smaller than an ice sheet. An

ical geology concerned with the study of

ice cap typically has an area of less than

landforms, with the forces and processes

19,300 sq. mi. (50,000 sq km) and, like an

that have shaped them, and with the

ice sheet, consists of an ice dome, with ice

GEOMORPHOLOGY:

description and classification of various physical features on Earth. GEOSPHERE:

The upper part of

Earth’s continental crust, or that portion of the solid Earth on which human beings live and that provides them with most of their food and natural resources. GLACIATION:

during an ice age. GLACIER:

shelves and outlet glaciers at the edges. ICE CORE:

A large, typically moving

mass of ice on or adjacent to a land surface.

A vertical section of ice,

usually taken from a deep ice sheet such as that in Antarctica. When studied with the proper techniques and technology, ice cores can reveal past climatic conditions in much the same way that the investigation of tree rings does.

The covering of the

landscape with large expanses of ice, as

382

An area of physical

GLACIOLOGY:

ICE DOME:

A symmetrical, convex

(i.e., like the outside of a bowl) mass of ice, often thicker than 9,800 ft. (3,000 m), usually found at the center of an ice cap or an ice sheet.

living, beginning in the late Neogene period and extending into the current Quaternary.

million years. The first of these occurred during the late Proterozoic eon, toward the end of Precambrian time (between 800 and 600 Ma ago). Another happened during the Pennsylvanian subperiod of the Carboniferous and extended throughout the Permian period, thus lasting from about 350 to 250 million years ago. The last period of glaciation is the one in which we are

In fact, many scientists question whether the last ice age has ended and whether we are merely living in an interglacial period. Certainly crustal rebounding is still taking place, as noted earlier.

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H U M A N S A N D T H E I C E AG E S .

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KEY TERMS ICE FIELD:

A large ice formation, sim-

ilar to an ice cap except that it is nearly

CONTINUED

A hill-like pile of till left

MORAINE:

behind by a glacier.

level and lacks an ice dome. There are

MORPHOLOGY:

enormous variations in size for ice fields.

the study thereof.

Structure or form, or

Some may be no larger than 1.9 sq. mi. (5 sq km), while at different times in Earth’s history, some have been as large as conti-

OUTLET GLACIER:

A rapidly moving

stream of ice that extends from an ice dome.

nents. PHYSICAL GEOLOGY:

The study of

A vast expanse of ice, usu-

the material components of Earth and of

ally at least 19,300 sq. mi. (50,000 sq km),

the forces that have shaped the planet.

that moves outward from its center. Like

Physical geology is one of two principal

the smaller ice caps, ice sheets consist of ice

branches of geology, the other being his-

domes and outlet glaciers, with outlying ice

torical geology.

ICE SHEET:

shelves. PRESSURE MELTING POINT:

The

An ice formation at the

temperature at which ice begins to melt

edge of an ice cap or ice sheet that extends

under a given amount of pressure. The

into the ocean, typically ending in cliffs as

higher the pressure, the lower the tempera-

high as 98 ft. (30 m).

ture at which water can exist in liquid

ICE SHELF:

LANDFORM:

A notable topographical

form.

feature, such as a mountain, plateau, or

RELIEF:

valley.

ties on a land surface.

MA:

entists,

An abbreviation used by earth scimeaning

million

years

or

Elevation and other inequali-

SEDIMENT:

Material deposited at or

near Earth’s surface from a number of

megayears. When an event is designated as,

sources, most notably preexisting rock.

for instance, 160 Ma, it usually means 160

TILL:

million years ago.

left by glaciers that lack any intervening

MASS EXTINCTION:

A phenomenon

A general term for the sediments

layer of melted ice.

in which numerous species cease to exist at

TOPOGRAPHY:

or around the same time, usually as the

Earth’s surface, including its relief as well as

result of a natural calamity.

the position of physical features.

The configuration of

It seems as though human existence has been bounded by ice, both in its onset and its recession. Ice ages have been a regular feature of

the two million years since Homo sapiens came into existence, and the species had much of its formative experience in times of glaciation. The latter part of the last ice age created a land bridge that made possible the migration of Siberian peoples to the Americas, so that they are known now as Native Americans. (The name is well deserved: the ancestors of the Native Americans

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Inasmuch as “glacial period” refers to a time when glaciers cover significant portions of Earth and when the oceans are not at their maximum levels, we are indeed still in a glacial period.

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moved east from Siberia about 12,000 years ago, whereas less than half that much time has elapsed since the Indo-European ancestors of Caucasian Europeans moved west from what is now Russia. Certainly no one today questions whether Germans, Italians, British, French, and other groups are “native” Europeans.) As an indication that ice ages have not ceased to occur, there is the Little Ice Age, which lasted from as early as 1250 to about 1850. This was a period of cooling and expansion of glaciers in the temperate latitudes on which Europe is located. Glaciers destroyed farmlands and buildings in the Alps, Norway, and Iceland, while Norse settlements in Greenland became uninhabitable. Europe as a whole suffered widespread crop failures, with a resulting loss of life. Evidence for this ice age, and indeed for all ice ages, can be discerned from the “footprints” left by glaciers from that time. Another telling sign is the transport of materials, such as rocks and fossils, from one part of Earth’s surface to another. U N D E R S TA N D I N G

ICE

In the meantime, ice offers a great deal of potential for understanding our own planet and others. It may yet turn out that the ice sheets covering Mars contain single-cell life-forms. Furthermore, in August 1996, the National Aeronautics and Space Administration (NASA) reported that a meteorite found on the Antarctic ice may provide evidence of life on Mars. It seems that the 4.1-lb. (2 kg) meteorite contains polycyclic aromatic hydrocarbons that may have existed on that planet several billion years ago.

AG E S .

What caused the Little Ice Age? The answer, or rather the attempt at an answer, goes to the core question of what causes ice ages in general. Earth scientists have cited both extraterrestrial and terrestrial factors. Among the leading extraterrestrial causes are an increase in sunspot activity (see Sun, Moon, and Earth for more about sunspots) as well as changes in Earth’s orientation with respect to the Sun. Contenders for a terrestrial explanation include changes in ocean circulation, as well as meteorites and volcanism. Either of these last two could have caused the atmosphere to become glutted with dust, choking out the Sun’s light and cooling the planet considerably. (Such a calamity has been blamed for several instances of mass extinction, most notably, the one that wiped out the dinosaurs some 65 million years ago. See Paleontology.)

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even more frightening prospect of humaninduced warming. As noted earlier, the Antarctic ice core reveals an increase of carbon dioxide and methane in the atmosphere during the past two centuries. Though these gases can be produced naturally, this excess in recent times appears to be a by-product of industrialized society. The glaciers of Europe are receding, and whether this could be the result of human activity or simply part of Earth’s natural change as it comes out of the last ice age remains to be seen.

WHERE TO LEARN MORE Alley, Richard B. The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future. Princeton, NJ: Princeton University Press, 2000. Bolles, Edmund Blair. The Ice Finders: How a Poet, a Professor, and a Politician Discovered the Ice Age. Washington, DC: Counterpoint, 1999. Charlotte the Vermont Whale: Glaciers and the Glacial Ages (Web site). . Fagan, Brian M. The Little Ice Age: How Climate Made History, 1300–1850. New York: Basic Books, 2000. Gallant, Roy A. Glaciers. New York: Franklin Watts, 1999. “Home of Deep Glaciology.” California Institute of Technology Division of Geological and Planetary Sciences (Web site). . Ice Ages (Web site). . National Oceanic and Atmospheric Administration (NOAA) at the National Snow and Ice Data Center/World Data Center for Glaciology (Web site). .

T H E F U T U R E . Though it appears that we are living in an interglacial period and that Earth could undergo significant cooling again thousands of years from now, there is also an

Virtually the Ice Age (Web site). .

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Stone, Richard. Mammoth: The Resurrection of an Ice Age Giant. Cambridge, MA: Perseus, 2001.

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S C I E N C E O F E V E RY DAY T H I N G S Real-Life Earth Science

METEOROLOGY AND T H E AT M O S P H E R E E VA P O T RA N S P I RAT I O N A N D P R E C I P I TAT I O N W E AT H E R C L I M AT E

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E VA P O T R A N S P I R AT I O N A N D P R E C I P I TAT I O N

CONCEPT Evaporation, along with the less well known process of transpiration, is the means by which water enters the atmosphere in the form of moisture. In evaporation liquid water from nonliving sources, such as the soil and surface waters, is converted into a gas. This conversion is driven by the power of the Sun, whose energy is also behind the process of transpiration, whereby plants lose water through their leaves. As with evaporation, transpiration places water in the atmosphere, and because the two processes work in tandem, they usually are spoken of together under the name evapotranspiration. Both make possible the formation of clouds, which, when they become saturated with moisture, produce the forms of precipitation by which water returns to the solid earth.

HOW IT WORKS The Movement of Water The hydrosphere is the sum total of Earth’s water, with the exception of water in the atmosphere. The hydrologic cycle is the continuous circulation between these two earth systems, hydrosphere and atmosphere, as well as the two other principal earth systems, biosphere and geosphere (see Earth Systems). Evapotranspiration and precipitation are principal components of this cycle, which is discussed in depth within the Hydrologic Cycle essay.

atmosphere from nonliving sources, transpiration is the movement of water from living organisms to the atmosphere. This is achieved by the release of water through the plants’ stomata, small openings on the undersides of leaves. In both cases, the Sun’s electromagnetic energy, experienced as heat, is the driving mechanism, and because these phenomena are so closely related, they are normally treated together as evapotranspiration.

Earth’s Water Budget Our present concern is primarily with the atmosphere; nonetheless, it is important to consider the other “compartments” in which water is stored. Some of them are regular way stations in the hydrologic cycle, but in the case of groundwater at least, the compartment is just that—a storage area from which it is conceivable that water might not be moved for millions of years. The vast majority of water on Earth is indeed stored away, deep beneath the planet’s surface in the form of groundwater in the lithosphere. This accounts for a staggering 94.7% of Earth’s water. (For figures on the mass of this and other components in the water supply, see Hydrologic Cycles.) Much of the remainder is made up of the oceans, which account for 5.2% of the water on Earth, while glaciers and other forms of permanent and semipermanent ice make up 0.065%.

In evaporation, heat converts liquid water to a gaseous state, thus allowing it to be transferred to the atmosphere in the form of vapor. Whereas evaporation involves the loss of water to the

T H E LAS T 0 . 0 3 5 % . We have now identified 99.965% of all water, yet we have not even approached any of the forms of water with which most of us typically come into contact. Of the remaining 0.035%, shallow groundwater, the source of most local water supplies, makes up the

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SCANNING

ELECTRON MICROGRAPH OF THE STOMATA, OR PORES, ON THE UNDERSIDE OF AN APPLE TREE LEAF.

LEAF

PORES PULL IN CARBON DIOXIDE FOR PHOTOSYNTHESIS. (© Scimat/Photo Researchers. Reproduced by permission.)

bulk, 0.30% of the total. Next are the inland surface waters. All of them combined, including such vast deposits as the Great Lakes and the Caspian Sea as well as the Mississippi-Missouri, Amazon, and Nile river systems—account for just 0.03% of Earth’s water. Now we are left with only 0.02% of the total, which is the proportion occupied by moisture in the atmosphere: clouds, mist, and fog as well as rain, sleet, snow, and hail. While it may seem astounding that atmospheric moisture is such a small portion of the total, this fact says more about the vast amounts of water on Earth than it does about the small amount in the atmosphere. That “small” amount, after all, weighs 1.433  1013 tons (1.3  1013 tonnes), or 28,659,540,000,000,000 lb.

Water Turnover The smaller the compartment of water, the greater the amount of turnover—that is, the exchange of “new” water for “old”—and the shorter the turnover time. Groundwater may stay put in aquifers, underground rock formations, for millions of years; on the other hand, atmospheric water experiences an enormous amount of turnover in just a year’s time.

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The atmosphere receives vast inputs of evaporation from the oceans as well as evapotranspiration from terrestrial ecosystems, or land-based communities of organisms. In the course of the year, the water in the atmosphere turns over about 34 times. Thus, the inputs of evaporation and transpiration are balanced almost perfectly by outputs of precipitation, which return more than 75% of atmospheric moisture to the oceans. The remainder falls on the land, where it contributes to brooks, streams, and rivers. The land receives 67% more water as precipitation than it loses through evaporation. The difference is made up by runoff to the oceans, primarily through rivers and in much smaller portions by subterranean channels.

How Does the Water Move? Clearly, water is moving through our atmosphere, but how? The answer is through gradients, or differences, of energy. When you hold a stone over the side of a cliff, preparing to drop it, the stone possesses a large quantity of gravitational potential energy. This potential energy is a function of the large gradient between the position of the stone and that of the ground. In the same way, rivers and streams are driven by the gravitational potential that exists by

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virtue of the gradient from their source to the place where they empty into a lake or ocean. By definition, water flows downhill, and therefore the source of the river must be at a higher elevation than its delta, or the place where it discharges. This matter of elevation is reflected in the confusing names for the two kingdoms that united in about 3100 B.C. to form Egypt: Upper Egypt was actually to the south of Lower Egypt. The names reflect the importance of the Nile, which flowed from the higher elevations of Upper Egypt into the fertile lowlands of Lower Egypt, home of Egyptian civilization’s greatest farmlands. E VA P O RAT I O N . Evaporation likewise is driven by energy gradients; in this case, however, the energy is not gravitational but electromagnetic. Specifically, it is the energy from the Sun, which we experience in the form of light and heat. (See Sun, Moon, and Earth for more about the Sun’s energy.) As surfaces on Earth absorb solar electromagnetic energy, it increases their heat content and provides a source of energy to drive evaporation.

The second law of thermodynamics, discussed in Energy and Earth, tells us that the flow of heat is always from a high-temperature reservoir or area to a low-temperature one. Thus, if you hold a snowball in your hand, you may think that the snowball is cooling your hand, but, in fact, the opposite is happening: heat is passing from your hand into the snowball and warming it. As this happens, your hand loses heat, which you perceive as cold, though no coldness has been transferred—only heat. In the same way, the energy difference between a heated surface and the atmosphere, manifested as a difference in temperature, makes it possible for water to be vaporized and transported into the air. Though the water has risen rather than fallen, its rise is brought about by the same principle that makes it possible for objects to fall from a great height, that is, a difference in potential. Thus it can be said that in evaporating, water “falls” from an area of high heat and high energy to one of low heat and low energy.

as evaporation. In fact, it puts more water into the air than evaporation does: any large area of vegetation tends to evaporate much larger quantities of moisture than an equivalent nonfoliated region, such as the surface of a lake or moist soil. Physically, however, the process of transpiration is the same as that of evaporation. The only difference is that in the case of transpiration, the source of the evaporation is a biological one—leaves, for instance, as well as skin or lungs. From an environmental standpoint, the most important kind of transpiration is that which occurs through leaves. The loss of water from foliage puts an enormous amount of moisture into the atmosphere, and for this reason, areas where foliage appears in high concentration (i.e., forests) are vital to the cycling of water through the atmosphere. Plants lose their water through moist membranes of a tissue known as spongy mesophyll, found in the tiny cavities that lie beneath the microscopic leaf pores called stomata. Stomata remain open most of the time, but when they need to be closed, guard cells around their borders push them shut. Because plants depend on them to “breathe” by pulling in carbon dioxide (see Carbon Cycle), however, they keep their stomata open—just as a human’s pores must remain open, or the person would die of suffocation. Because stomata are exposed in order to receive carbon dioxide for the plant’s photosynthesis, this also means that the stomata are open to allow the loss of moisture to the atmosphere. It can be said, then, that transpiration—vital as it is to the functioning of our atmosphere—is actually an unavoidable consequence of photosynthesis, an unrelated process. T R A N S P I R AT I O N AND ANIM A L S . As we have suggested, transpiration in

When it comes to putting moisture into the atmosphere, transpiration is at least as important

animals (including humans) takes place for much the same reason as it does with plants, as a by-product of breathing. Animals have to keep their moist respiratory surfaces, such as the lungs, open to the atmosphere. We may not think of our own breathing as transferring moisture to the air, but with just a little consideration of the subject it becomes clear that this is the case. The presence of moisture in our lungs can be proved simply by breathing on a piece of glass and observing the misty cloud that lingers there.

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REAL-LIFE A P P L I C AT I O N S Transpiration

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On the one hand, transpiration can cause animals to become dehydrated, but it also can be important in cooling down their bodies. When human bodies become overheated, they produce perspiration, which cools the surface of the skin somewhat. If the air around us is too humid, however, it already is saturated with water, and the perspiration has no place to evaporate. Therefore, instead of continuing to cool our bodies, the perspiration simply forms a sticky film on our skin. Assuming that the air is capable of absorbing more moisture, however, the sweat will evaporate, cooling our bodies considerably.

Heat, Cold, and Evapotranspiration The preceding discussion brings up several more points about the relationship between heat and evapotranspiration. First of all, everything that is living has some degree of heat; if it did not, it would be at a temperature known as absolute zero, or 0K (–459.67°F or –273.15°C), which is impossible to reach. Therefore, even in Greenland there is a small amount of molecular motion in plant tissues, even when they appear to be completely frozen. Naturally, however, there is very little evapotranspiration when temperatures are extremely cold, which is the reason why it can sometimes be “too cold to snow”: temperatures are too low for sufficient moisture to be moved to the atmosphere. Nonetheless, there still can be some physical evaporation as the result of the direct vaporization of solid water, a process called sublimation. H O T, D R Y S U M M E R S . At the height of summer, when air temperatures are warm and the trees are fully foliated (i.e., covered in leaves), a high rate of transpiration occurs. So much water is pumped into the atmosphere through foliage that the rate of evapotranspiration typically exceeds the input of water to the local environment through rainfall. The result is that soil becomes dry, some streams cease to flow, and by late summer, in extremely warm temperate areas such as the southern United States, there is a great threat of drought and related problems, such as forest fires.

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again. Such is the case in a temperate region, which by definition is one that has the four seasons to which most people in the United States (outside Hawaii, Alaska, and extreme southern Florida and Texas) are accustomed. In a tropical region, by contrast, there is a “dry season,” in which transpiration takes place, and a “rainy season,” in which moisture from the atmosphere inundates the solid earth. This rainy season may be so intense that it produces floods.

Clouds So far, we have talked mostly about the means by which water moves from the solid earth into the atmosphere. Now let us consider what happens when it gets there, at which point—assuming there is sufficient moisture—it will coalesce to form a cloud. Though most people are inclined to romanticize clouds, seeing in them shapes and colors and sometimes even reflections of their own moods, clouds are nothing more than atmospheric moisture that has condensed to form tiny droplets of water or crystals of ice. Heated, moist air that rises from the ground is much more dense than the air that lies above it, but as it rises, it expands and becomes less dense. This expansion cools the air, so that the water vapor condenses into tiny droplets. As a result, a cloud forms, and the relative intensity of the energy gradients that brought about the formation of the cloud creates different shapes. For example, if there is a vigorous uplift of air, resulting from sharp differences in temperature and pressure between the ground and the atmosphere, the resulting clouds will have a tall, stacked appearance. On the other hand, clouds formed by the gentle uplift of air currents have a flat, cottony appearance. Thus, the shapes of the clouds themselves, as well as the ways in which they change, assist meteorologists in predicting the weather.

Cloud Classifications

Once the trees drop their leaves in the autumn, transpiration rates decline greatly. This makes it possible for the parched soil to become recharged by rainfall and for streams to flow

Thanks to a system developed in 1803 by the English pharmacist and amateur naturalist Luke Howard (1772–1864), it is possible to classify clouds according to three basic shapes. These shapes are known by the Latin names cumulus (piled heaps and puffs), cirrus (curly, fibrous shapes), and stratus (stretched and layered).

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Howard combined these names with adjectival terms, such as alto (“high”) and nimbus (“rain”), to describe variations on the three basic cloud types. Today, the International Cloud Classification used by meteorologists worldwide applies Howard’s system to the identification of ten basic cloud types, or genera. They are divided into three high-altitude genera, two midlevel ones, three low-level genera, and two varieties of rain cloud.

Evapotranspiration and Precipitation

H I G H - L E V E L C LO U D S . The three genera of high-level clouds appear at a range between 16,500 ft. and 45,000 ft. (5,032–13,725 m), though they usually form in a belt between 20,000 ft. and 25,000 ft. (6,000–7,500 m). All are cirrus clouds, of which the highest are pure cirrus, made of ice crystals.

Then there are cirrocumulus, the least common variety of cloud. Composed of either supercooled water droplets (i.e., water that continues to exist in liquid form even though the temperature has dropped below the freezing point) or ice crystals, these are small and white or pale gray, with a rippled appearance like oatmeal. Finally, cirrostratus clouds—which, like pure cirrus clouds, are made of ice crystals—often form a halo around the Sun or Moon in the wintertime. M I D L E V E L C LO U D S . The two gen-

era of midlevel clouds, which appear at 6,50023,000 ft. (2,000–7,000 m), are named with the prefix “alto.” Altostratus clouds are usually a uniform bluish or gray sheet covering large portions of the sky and either completely or nearly obscure the Sun and Moon. These complex clouds are composed of layers, with ice crystals at the top, ice and snow in the middle, and water droplets in the lower layers. Altocumulus are oval-shaped, dense, fluffy balls that may appear either as singular units or as closely bunched groups. When sunlight or moonlight shines through these clouds, the light often is perceived from the ground in the form of rays. There are three genera in the low level, which extends from the surface to 6,500 ft. (2,000 m). Pure stratus clouds, which are generally the lowest, blanket the sky and typically appear gray. They are formed when a large mass of air rises slowly or when cool air moves in over an area close to ground level. Stratus clouds may produce mist or LO W - L E V E L

C LO U D S .

S C I E N C E O F E V E RY DAY T H I N G S

WHEN

MOISTURE COALESCES IN THE ATMOSPHERE, IT

FORMS A CLOUD.

ALTOCUMULUS

CLOUDS, WHICH LOOK

LIKE OVAL-SHAPED, DENSE, PUFFY BALLS, ARE MIDLEVEL CLOUDS. (© Mark A. Schneider/Photo Researchers. Reproduced

by permission.)

drizzle, or, when they form at ground level, they may appear as fog. Pure cumulus clouds are flat on the base and vertically thick, with a puffy appearance on top. This puffiness is the result of updrafts. Occurring primarily in warm weather, cumulus clouds are made of water droplets and look brilliant white because of the sunlight’s reflection off the droplets. Last, stratocumulus clouds are large, grayish, and puffy, often looking like dark rolls. RA I N C LO U D S . Stratocumulus clouds can transform into nimbostratus clouds, while cumulus can develop into cumulonimbus clouds. These two—nimbostratus and cumulonimbus—are the last of the ten varieties of cloud, designated not by altitude but by the fact that they give rise to precipitation. (Note that the light, airy variety known as cirrus never portends rain.)

Nimbostratus usually appear at midlevel altitudes. Made of water droplets that produce either rain or snow, they are thick, dark, and gray.

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teardrop shape is popularly associated with raindrops, they assume that shape only when they are falling.

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Snowflakes are formed by the aggregation, or physical bonding, of solid ice crystals inside a cloud, while hailstones are made of a combination of supercooled water droplets and pellets of ice. Not only are they more dense than snowflakes, but they are also more spherical. Similar to hail is sleet, pellets of pure ice that usually are much smaller than hail.

MICROGRAPH

OF

A

SNOWFLAKE.

SNOWFLAKES

ARE

F O R M I N G P R E C I P I TAT I O N . The type of precipitation formed depends on the warmth of the cloud from which it comes. “Warm” clouds are those whose temperatures are above freezing—32°F (0°C)—and “cold” clouds are those that are at least partially below freezing temperature. These temperature values are themselves a function of altitude, since temperature in the lower atmosphere (where all precipitation occurs) decreases by about 5.32°F for every mile (6°C per kilometer) of altitude gained.

FORMED BY THE BONDING OF SOLID ICE CRYSTALS INSIDE A CLOUD. (© Kent Wood/Photo Researchers. Reproduced by

permission.)

Sometimes they are seen with virga, which are skirts of rain trailing down their sides. Cumulonimbus, or thunderstorm, clouds arise from cumulus clouds that have reached a great height. At a certain height, the cloud flattens out, and powerful updrafts and downdrafts create a great deal of unrest inside the cloud, which contains all phases of water: gas, liquid, and solid. These clouds can cause violent storms.

Rain, Snow, Hail, and Sleet

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As we have suggested earlier, humidity relates to the ability of air to evaporate, and, therefore, when the air has all the water it can hold, it is said to have a relative humidity of 100%. The cooler air is, the less moisture is required for it to become saturated. Once saturation occurs, moisture molecules form around cloud condensation nuclei, such as nitrates or extremely fine particles of sea salt that have managed to evaporate. Cold clouds form around ice crystals. Before freezing, the water in these clouds may be supercooled,. Ice nuclei are much more rare than cloud condensation nuclei and are less well understood by meteorologists.

Eventually, the amount of moisture in the cloud becomes so great that it has to fall earthward in the form of precipitation—usually rain, snow, hail, or sleet. These types of precipitation are distinguished by the form they take: liquid in the first case, lightly frozen particles in the second case, and hard, frozen nuggets in the latter two cases.

M I X E D C LO U D S . Clouds that contain both liquid water and ice are called mixed clouds. Supercooled water will freeze when it strikes an ice crystal, and if it freezes immediately, it forms what is known as opaque or rime ice. If it freezes slowly, the result is called clear ice. Eventually the ice forms a thick coating, which is the origin of hail.

Liquid precipitation includes drizzle and raindrops, which are distinguished from each other on the basis of size. Raindrops have a radius of about 0.04 in. (1 mm), while drops of drizzle are about one-tenth that size. Note the use of the term radius, implying a sphere. Drops of liquid precipitation are spherical, and though a

Not all mixed clouds produce frozen precipitation, however. Thunderstorms, involving electric charges imparted to precipitation particles, which leads to the eventual discharge of lightning, are also the products of mixed clouds. (For more about storms and precipitation, see Weather and Climate.)

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KEY TERMS AQUIFER:

An underground rock for-

mation in which groundwater is stored. ATMOSPHERE:

In general, an atmos-

Underground water

GROUNDWATER:

resources that occupy the pores in bedrock. The continu-

HYDROLOGIC CYCLE:

phere is a blanket of gases surrounding a

ous circulation of water throughout Earth

planet. Unless otherwise identified, howev-

and between various earth systems.

er, the term refers to the atmosphere of Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such

study

of

the

hydrosphere, including the distribution of water on Earth, its circulation through the hydrologic cycle, the physical and chemical properties of water, and the interaction

as neon (0.07%). BIOSPHERE:

The

HYDROLOGY:

A combination of all liv-

ing things on Earth—plants, mammals,

between the hydrosphere and other earth systems. The entirety of

birds, amphibians, reptiles, aquatic life,

HYDROSPHERE:

insects, viruses, single-cell organisms, and

Earth’s water, excluding water vapor in the

so on—as well as all formerly living things

atmosphere but including all oceans, lakes,

that have not yet decomposed.

streams, groundwater, snow, and ice.

ECOSYSTEM:

A community of inter-

METEOROLOGY:

The study of the

dependent organisms along with the inor-

atmosphere, weather, and weather predic-

ganic components of their environment.

tion.

EVAPORATION:

The process whereby

liquid water is converted into a gaseous

POTENTIAL:

Position in a field, such as

a gravitational force field.

state and transported to the atmosphere. When discussing the atmosphere and precipitation, usually evaporation is distinguished from transpiration. In this context,

POTENTIAL ENERGY:

The energy

that an object possesses by virtue of its position or its ability to perform work. When discussing the

evaporation refers solely to the transfer of

PRECIPITATION:

water from nonliving sources, such as the

hydrologic cycle or meteorology, precipita-

soil or the surface of a lake.

tion refers to the water, in liquid or solid The loss of

form, that falls to the ground when the

water to the atmosphere via the combined

atmosphere has become saturated with

(and related) processes of evaporation and

moisture. In the context of chemistry, pre-

transpiration.

cipitation refers to the formation of a solid

EVAPOTRANSPIRATION:

GEOSPHERE:

The upper part of

from a liquid. A term for water

Earth’s continental crust, or that portion of

SUPERCOOLED:

the solid earth on which human beings live

that continues to exist in liquid form even

and which provides them with most of

though its temperature has dropped below

their food and natural resources.

the freezing point.

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KEY TERMS SYSTEM:

Any set of interactions that can be set apart mentally from the rest of the universe for the purposes of study, observation, and measurement.

tion of environmental moisture for precipitation comes from plants, which lose water through their stomata, small openings on the undersides of leaves.

The process whereby moisture is transferred from living organisms to the atmosphere. A major por-

An area of terrain from which water flows into a stream, river, lake, or other large body.

TRANSPIRATION:

WHERE TO LEARN MORE Bad Meteorology (Web site). . Global Hydrology and Climate Center (Web site). . Glover, David. Flying and Floating. New York: Kingfisher Books, 1993. ISCCP: International Satellite Cloud Climatology Project (Web site). .

394

CONTINUED

WATERSHED:

Parker, Steve. Weather. Illus. Kuo Kang Chen and Peter Bull. New York: Warwick Press, 1990. Schmid, Eleonore. The Water’s Journey. New York: North-South Books, 1990. Tropical Rainfall Measuring Mission/TRMM (Web site). . The World-Wide Web Virtual Library: Meteorology (Web site). .

Kahl, Jonathan D. Wet Weather: Rain Showers and Snowfall. Minneapolis: Lerner Publishing, 1992.

Wyman, Richard L. Global Climate Change and Life on Earth. New York: Routledge, Chapman and Hall, 1991.

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W E AT H E R

Weather

CONCEPT Though people are quite accustomed to experiencing weather—sunshine and rain, wind and storms, hot and cold, fair days and foul—most have little idea how weather originates or even really what weather is. In fact, it is a condition of the atmosphere produced by one or more of six factors: air temperature, air pressure, humidity in the air, the density and variety of cloud cover, the amount and type of precipitation, and the speed and direction of the wind. Those six variables can produce the sunny, beautiful days that everyone treasures, or they can conspire to create the sort of rainy, windy, and cold day that some people despise and others (especially creative types, who do not have to be out in the elements) adore. The variables of weather also can align to manifest in the form of brutal storms, including hurricanes and tornadoes. Yet as complex as weather is, its behavior can still be forecast—with at least some accuracy—and sometimes even manipulated.

HOW IT WORKS Introduction to Weather Weather and climate are not the same thing: weather is the condition of the atmosphere at any given time and place, while climate is the pattern of weather conditions in a particular region over an extended period. We do not speak therefore of “forecasting climate,” though it is possible, on the basis of long-term climate patterns, to make some predictions concerning future climatic trends. (Climate is discussed in a separate essay.)

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Nevertheless, the type of atmospheric prediction that applies much more to our daily lives is weather forecasting. The latter is the work of meteorologists, or scientists involved in studying the atmosphere and weather and in making weather predictions. In the 1970s most local television news programs still included a “weatherman” on their lineup. This was a television personality, much like the anchors and the specialists in sports or other topics, whose job it was to report the weather. During the late 1970s and early 1980s, however, most stations and networks changed their designation of weatherman to that of meteorologist. In part this change had to do with gender politics, since the title weatherman seemed sexist but weatherwoman or weatherperson sounded a bit absurd. On the other hand, it also was made in recognition of the work that weather forecasters perform: unlike ordinary reporters, they are not so much journalists as they are scientists. (Though a nice smile, charming personality, and good looks never hurt any TV meteorologist!) And while a meteorologist, like every member of the TV news team, reports what has happened— for instance, a record snowfall—his or her job also is concerned heavily with explaining what will happen. S I X FAC T O R S . As we have noted, weather is determined by six factors, all of which involve the atmosphere, and several of which also relate to water in the atmosphere. These factors are as follows:

• • • •

Air temperature Air pressure Wind Humidity

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• Cloud cover • Precipitation The role of air temperature in weather is fairly obvious, since that is one factor of which we are used to taking note on a regular basis. Air pressure, though much less familiar to most of us, has a powerful influence on air currents, and air pressure is linked closely with wind. Likewise, there are clear linkages among humidity (moisture in the air), the amount and type of cloud cover (formed, of course, from moisture in the air), and the amount and type of precipitation, which begins to fall when the clouds are saturated with moisture.

What Makes the Weather? So much for the factors involved in weather, which go into making the weather we experience at a given time. But if we want to search for the ultimate causes behind the weather, we have to look toward larger phenomena: the Sun and Earth as well as Earth’s surface and atmosphere and the interactions among all of these factors. At the heart of weather is the Sun and the energy it transmits to Earth. As discussed in Energy and Earth, the Sun sends out vast quantities of energy, only a small portion of which comes anywhere near Earth—and a much smaller portion reaches the surface of the planet as usable energy. Yet that “tiny” amount, which is the vast majority of the energy available to Earth in any form, is enough to light and warm the planet and to drive a variety of processes in the biosphere and atmosphere. MOVING W AT E R THROUGH T H E AT M O S P H E R E . The Sun’s most

direct and obvious influence on weather is its impact on temperature, but this is only one aspect of a larger and more complex picture. As discussed in the essay Evapotranspiration and Precipitation, differences in temperature—a result, primarily, of the Sun’s heat—make possible the conditions for both evaporation and transpiration, or the loss of moisture from living organisms to the atmosphere. The amount of water that evaporates into the atmosphere in a given area is its humidity, which exerts a powerful impact on the human experience of weather. As most people who have traveled widely will attest, high temperatures in a dry climate, such as that of the American Southwest, are far easier to endure than equally high

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temperatures (or even lower ones) in Florida or other parts of the Deep South, a much more humid region. As water moves from Earth’s surface into the atmosphere through evapotranspiration (a combination of evaporation and the related process of transpiration), the air through which it passes becomes increasingly cooler. Eventually, the vaporized water comes to an altitude at which the air around it is cold enough to cause condensation—that is, the formation of a liquid. When enough vapor has condensed into tiny droplets of water, or tiny ice crystals, the result is the formation of clouds. Clouds, discussed in detail within the context of Evapotranspiration and Precipitation, are integral to the formation of weather patterns, which is why TV meteorologists almost always show their audiences satellite maps illustrating cloud movements. Not only do clouds reflect sunlight into space, meaning that a sufficient accumulation of cloud cover will bring about a decrease in temperature, but clouds also manufacture precipitation. Forms of precipitation include rain in all its variations, from a fine mist to a downpour, as well as snow, sleet, and hail.

Air Pressure and Wind We have seen how the Sun affects the movement of water through the atmosphere, thus driving three of the six key weather factors we named earlier: humidity, cloud cover, and precipitation. In addition to its effect on these factors and on temperature itself, the Sun and its energy are also behind the other two important factors—air pressure and the movement of air through the atmosphere, that is, winds. Because it does not change as frequently or as dramatically as temperature (fortunately!), air pressure is something of which we are hardly aware. But if a person accustomed to the sea-level air pressure of a place such as New York City suddenly had to travel to a high-altitude locale such as Denver, Colorado, he or she immediately would become aware of the difficulty in breathing and the other changes that attend a change in pressure. For instance, the higher the altitude (and, hence, the lower the air pressure), the lower the temperature at which water boils. For this reason, cake mixes and similar products have special instructions for kitchens at high altitudes. Taken

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to an extreme, this means that with absolutely no pressure, liquid could boil—that is, turn into a vapor—at extremely low temperatures. This is one of the reasons, along with lack of oxygen and the presence of harmful rays, that an astronaut wears a protective suit; without it, the pressureless environment of space would cause a person’s blood to boil! In the English system, normal atmospheric pressure at sea level is 14.7 lb. per sq. in., or 1.013  105 pascals (Pa) in the metric or SI (Scientific International) system. This amount of pressure constitutes a unit in its own right, an atmosphere (atm). Atmospheric pressure, however, usually is measured in terms of the bar, an SI unit equal to 105 Pa. Thus, 1 atm = 1.013 bars. Meteorologists often use the millibar (mb), which, as its name implies, is equal to 0.001 bars—roughly 1/1,000 of ordinary air pressure at sea level. W I N D : F R O M H I G H P R E SS U R E T O LO W. Heat from the Sun does not fall

evenly on all places on Earth. Aside from the obvious fact that it makes a limited impact on polar regions, there are the differences in color and texture between various areas, even in the most tropical latitudes—that is, those closest to the direct path of sunlight. For example, soil, since it is almost always darker than water, tends to attract more sunlight than bodies of water do. When an area is heated, the air above it heats up as well, and this makes that region of air less dense. As a result, the air rises, a phenomenon known as convection. Convection currents also carry other masses of air downward from the upper atmosphere toward Earth’s surface. In regions where warm air moves upward, the atmospheric pressure tends to be low, whereas downward air movements are associated with higher atmospheric pressures. These higher or lower atmospheric pressures can be measured by a barometer, an instrument that registers pressure just as a thermometer measures temperature. The fact that there are differences in pressure between areas brings about wind, which is the movement of air from a region of high pressure to one of lower pressure. (Later in this essay, we discuss a way to “create” wind and thus test this statement.)

Other Influences on Weather

should be noted that certain aspects of Earth itself determine weather patterns. Among these factors, as noted earlier, are color and texture, whereby certain places on the planet are more apt to receive the Sun’s energy than others.

Weather

Also important is Earth’s position in space. First of all, there is the tilt of its axis relative to the plane of its rotation around the Sun, which accounts for the seasons. In addition, the planet follows an elliptical, or oval-shaped, orbital pattern around the Sun, which means that there are certain times when the planet is closer to the Sun and its energy than at others. Earth’s rotation brings about complicated patterns of movement on the part of air masses heated at the equator. If the planet were not rotating, these masses of air would simply move from the equator toward the poles, where they would be cooled. Because the planet is rotating, however, the movement of global winds is much more complex and is characterized by circular patterns known as cells. In addition to these factors that relate to the planet’s movement in space, there is also the matter of irregularities on Earth’s surface. An example of this is the effect mountains can have on cloud movements, creating a perpetually rainy climate on one side and a dry “rain shadow” on the other. Rain shadows are discussed in the essay on Mountains.

REAL-LIFE A P P L I C AT I O N S Creating Wind Earlier we discussed the fact that wind results from differences in pressure. This is a statement you can test for yourself—and perhaps you already have, without knowing it. Suppose you are in a room where the heat is on too high and there is no way to adjust the thermostat. Outside the air is cold, so you open a window, hoping to cool down the room. Does it do the trick? Not likely. But if you open the door leading from the room to the hallway, a nice cool breeze will blow through. You have, in effect, created wind or at least manipulated the conditions to make wind possible.

We have focused primarily on the influence that the Sun and its energy exerts on weather, but it

With the door closed, the room constitutes an area of high pressure in comparison with the area outside the window. Once the window alone

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is opened, it is theoretically possible for air to flow into the room, but that does not mean the air actually will flow. The reason is that fluids such as air—whether in a room or in the sky— tend to move from areas of high pressure to areas of low pressure. Because the room is of relatively high pressure compared with the outside, there is no reason for the air to move into the room. Furthermore, in line with the second law of thermodynamics (see Energy and Earth), the flow of heat always will be from an area of relatively high temperature to one of relatively low temperature. Therefore, if there is going to be any air movement in this situation, it will have to be movement of hot air out of the room rather than cool air into the room. (Even an air conditioner works by taking out the heat, not by bringing in “cold”—even though we may perceive it otherwise.) In the case of the overheated room, there is only one way to solve the problem: by opening the door into the hallway, or whatever else lies outside the door. As soon as the door is opened, the relatively high-pressure air of the room flows into the relatively low-pressure area of the hallway, just as the laws of physics say it should. This is exactly the same principle whereby wind blows from high-pressure regions into low pressure ones. By setting up a cross-draft, in effect, we have “created” wind.

Thunderstorms and Worse Wind and clouds combined with rain (discussed, like clouds, in Evapotranspiration and Precipitation) and a few other factors create a thunderstorm. Those other factors, of course, are lightning and that key difference between an ordinary storm and a thunderstorm: thunder. Inside a thunderstorm are updrafts and downdrafts of air, which bring about a buildup of static electric charges within the thunderstorm clouds. Over time, there is a large buildup of separate electric charges inside the cloud, with positive charges near the top and negative charges in the middle. This separation of charges creates huge voltage differences, which require something to equalize them: a bolt of lightning, which suddenly passes between the areas of differing charge.

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moisture in the cloud experience an extremely sudden expansion, and this expansion is accompanied by a release of energy. The energy in this situation takes the form of sound, which we experience as thunder. A N AT O M Y O F A T H U N D E R S T O R M . Let us go back now to the point

when the thunderstorm came into being. First solar energy, or some other influence, such as the presence of a mountain range, causes water to enter the atmosphere as vapor. As this air rises, it expands and cools, eventually condensing and coalescing to form a cloud. The cloud thus formed is a cumulus cloud, which may appear as the puffy, benign cloud of a clear summer’s day or may turn into a cumulonimbus—a thunderstorm—cloud. On its way to becoming a thunder cloud, a cumulus undergoes a transformation into a convective cloud, or one formed by convection. It may never become a thunder cloud at all, assuming that vertical movement stops, in which case fair weather prevails. But if the atmosphere is unstable, meaning that the air temperature drops rapidly with altitude, packets of air that begin rising and cooling inevitably will be warmer than the air around them. The rising air packet is like a balloon, weighing less per unit of volume than the surrounding area, and thus it continues to rise. As we have noted, in order to evaporate, water needs to receive a certain infusion of energy, or heat, from the Sun. Once it condenses and forms a cloud, it releases that heat, warming the cloud and causing it to rise still higher. As long as these updrafts can support the cloud, it continues to grow and to rise, until, in the case of a thunderstorm, it reaches very great atmospheric heights—about 40,000 ft. (12 km) above the surface. In the course of rising and growing, large raindrops and hailstones can form. TWO STORM.

TYPES

OF

THUNDER-

Lightning produces a spark, which heats the air in an instant to more than 54,000°F (30,000°C). As a result, the molecules of air and

Over western Florida and other areas around the Gulf of Mexico, thunderstorms grow as we have described, with cumulus clouds rising and cooling. These are called air-mass storms. Once the cloud reaches a certain point of moisture saturation, rain begins to fall from the upper part of the cloud, producing precipitation and with it downdrafts. Because the downdraft in such a situation is usually stronger than the updraft, the cloud is likely to dissipate before the

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thunderstorm has caused much damage. Certainly there will be rain showers, thunder, and lightning, but the storm is unlikely to produce extreme wind damage or hail.

Weather

On the other hand, parts of the central and eastern United States are prone to what are called frontal thunderstorms, and these storms are much more severe. They typically form just ahead of a cold air mass, or cold front. The cold air, much more dense than the warm, humid, and unstable air of the cloud system, pushes the clouds ahead of it, which rise and form convective clouds. Eventually, these clouds produce rain, which causes downdrafts. But instead of dissipating the storm’s impact (as in the case of the air-mass storm), the downdrafts only increase its intensity. In a frontal thunderstorm, strong downdrafts hit the ground, spreading out and sending more warm humid air rising into the storm. This gives the storm clouds more latent heat, increasing the updrafts, which in turn gives more speed to the wind and improves the chances of heavy rain or even hail. The latter can appear even in summertime or in warm climates, since the cloud forms at a great height. When ice crystals form in the cloud, the updrafts and downdrafts circulate them back and forth, and as this happens, the crystals pick up more and more moisture. Eventually, tiny ice crystals gather rainwater around them, until they have become hailstones. Depending on the conditions of the storm, hailstones can become extremely large—as big as 5.5 in. (14 cm) in diameter.

Tornadoes Some forms of weather make thunderstorms seem minor by comparison. Among these are tornadoes, a rapidly spinning column of air formed in a severe thunderstorm. The vortex, or rotating center, of the column, forms inside the storm cloud and begins to grow downward until it touches Earth’s surface. The United States is particularly prone to tornadoes, owing to specific factors of its location and its larger climate patterns. (See Climate for more about the distinction between weather and climate.) The type of severe frontal thunderstorm that can produce a tornado typically is associated with an extremely unstable atmosphere and with moving systems of low pressure, which bring

S C I E N C E O F E V E RY DAY T H I N G S

AS

CLOUDS RISE AND GROW, ICE CRYSTALS FORM AND

GATHER RAINWATER AROUND THEM UNTIL THEY BECOME HAILSTONES.

DEPENDING

ON THE CONDITIONS OF THE

STORM, HAILSTONES CAN GROW EXTREMELY LARGE— MORE THAN TWICE THE SIZE OF THE GOLF BALL–SIZE ONES SHOWN HERE. (© Gary Meszaros/Photo Researchers. Repro-

duced by permission.)

masses of cold air into contact with warmer, more humid air masses. It so happens that such storms occur frequently across a wide swath of North America, from the plains states to the eastern seaboard (i.e., more or less from Kansas to North Carolina) during a period from about June to October each year. As updrafts in a severe thunderstorm cloud become stronger, they pull more air into the base of the cloud to replace the rising air. As the air from the surface moves into a smaller area, it begins to rotate faster, owing to what physicists call the conservation of angular momentum. The latter can be illustrated by the example of an ice skater who, while spinning with her arms outstretched, pulls her arms inward. As she does so, the speed of her rotation increases. The same happens with rotating air as it moves from the large space of the ground to the smaller space of the cloud. Thus, a funnel cloud is produced and grows, and if it becomes large enough, it may touch

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ground—with devastating results. Within the vortex of a tornado, which is typically about 328 ft. (100 m) in diameter, wind speeds may be greater than 220 MPH (100 m/s). The tornado itself travels at speeds of 10–30 MPH (15–45 km/h), making a sound like a freight train or a jet engine and wreaking havoc along a path as long as 200 mi. (321 km). Nothing can stand in the way of a tornado with enough force: buildings shatter, roofs and whole houses take to the air, and pieces of straw can be blown through solid wood. T O R N A D O A L L E Y. No country in the world is more prone to tornado activity than the United States, a large portion of which is known as “Tornado Alley.” The latter area has no specific boundaries, though it is more or less contiguous with the wide Kansas-to-Carolina swath mentioned earlier. Some authorities describe Tornado Alley as including northern Texas, Oklahoma, Kansas, and southern Nebraska. However, the American Meteorological Society’s Glossary of Weather and Climate defines Tornado Alley as “The area of the United States in which tornadoes are most frequent. It encompasses the great lowland areas of the Mississippi, the Ohio, and lower Missouri River valleys. Although no state is entirely free of tornadoes, they are most frequent in the plains area between the Rocky Mountains and the Appalachians.”

It is no accident that a wide, flat region, over which heavy winds can blow from numerous directions—including winds off of the Gulf of Mexico and Atlantic Ocean—would be home to such enormous tornado activity. Tornado Alley, or parts of it, has seen numerous extraordinary weather events in which not one or two tornadoes struck, but many dozens—a phenomenon known as a tornado outbreak, or a super outbreak.

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Midwest and South, including the Ohio and Tennessee River valleys. The result was a series of nearly 150 tornadoes, including 48 that were classified as F4 or F5, the highest levels on the Fujita-Pearson scale used to rate the intensity or tornadoes. Some other significant tornado events since that time include the Carolina outbreak, which killed 57 people, injured 1,248, and caused $200 million in damage, on March 28, 1984. A year later, on May 31, 1985, an outbreak of 41 tornadoes in Ohio, Pennsylvania, New York, and Ontario killed almost 90 people and caused over $450 million in damage.

Cyclones Another form of extreme weather is a cyclone, a general term for what is sometimes called a hurricane or a typhoon. These are vast circulating storm systems characterized by bands of showers, thunderstorms, and winds. They develop over warm tropical oceans, generally as isolated thunderstorms, and turn into monsters. A cyclone may have a diameter as great as 403 mi. (650 km) and be more than 7.5 mi. (12 km) in height. Near the center of the cyclone, winds may be as high as 110 MPH (50 m/s), yet the very center is an area of complete calm, known as the eye. This is a region of descending air surrounded by the updrafts that characterize the cyclone, making the eye a little funnel of peace in the middle of a terrible storm. If it were possible to stay in the eye of the cyclone as it moves—at speeds comparable to that of a tornado—it is conceivable one could come away unscathed. THE

I M PAC T

OF

CYC LO N E S .

The worst super outbreak in recent memory occurred on April 3–4, 1974, when 148 rampaged through 14 states. It began on the morning of the 3rd, when a low-pressure system moved over central Kansas, spreading a cold front as far south as Texas. Meanwhile, a warm front clung to the lower Ohio River Valley, and various unstable patterns covered the South. Into this motley mix came strong winds, which soon took hold over much of the eastern United States. By afternoon, thunderstorms began raging over much of the

Once the cyclone blows inland and reaches population centers, it can cause massive death and destruction. The southern United States witnessed this with hurricanes such as Hugo, which devastated the Carolinas and nearby regions in 1989. The damage on Puerto Rico alone (one of the places Hugo touched down before moving north) amounted to 12 lives lost, and $2 billion in property. Thanks to evacuation measures and a bit of good fortune (the hurricane missed Charleston, South Carolina), loss of life was minimal on the continental United States, and loss of property was kept to $5 billion. By contrast, Hurricane Andrew, which struck Florida and other parts of the southern United States in August

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Weather

A

GLOBAL NETWORK OF WEATHER SATELLITES MAKES IT POSSIBLE TO IDENTIFY AND TRACK TROPICAL CYCLONES FROM

THEIR EARLIEST APPEARANCE AS DISTURBANCES OVER THE OCEAN.

A

CYCLONE CAN GENERATE WINDS AS HIGH AS

110 MPH (50 M/SEC), YET THE VERY CENTER IS AN AREA OF COMPLETE CALM, KNOWN AS THE EYE. (U.S. National Aero-

nautics and Space Administration [NASA].)

1992, was the costliest natural disaster in U.S. history, with over $25 billion in damage reported. But the impact of storms in the United States is dwarfed by the destruction of hurricanes and typhoons in third world countries. Bangladesh, for instance, is a country where a population half that of the United States is crammed into an area the size of Wisconsin. (And, it should be noted, a country so poor that the income of the average full-time working man is far less than that of an American teenager working a part-time job.) In such a situation, the impact of cyclones is bound to be devastating. Though dollar figures of property damage from the country’s many cyclones are not available, the human toll is better known. Since 1963, when it was still called East Pakistan, Bangladesh, has seen seven cyclones in which 10,000 or more died. The worst, in 1970, killed half a million. Government estimates of deaths from a cyclone on April 10, 1991, were 150,000, though it is likely that many more died from disease, starvation, and exposure.

erty. In part this is because a number of these countries are located in or near tropical zones that are especially susceptible to cyclones, but it is also a matter of preparedness. R E S P O N D I N G T O CYC LO N E S .

In the United States, with its greater material wealth and technological sophistication, it is possible to prepare people for extreme weather in a way that is simply beyond the reach of many less prosperous or powerful nations. It is not so much that Americans are capable of developing structures such as seawalls that can withstand the force of hurricanes, though this certainly helps. The seawall in Charleston, South Carolina, for instance—a massive bulwark of concrete more than 10 ft. (3 m) high—is intended to protect homes in that city’s historic Battery when hurricanes produce powerful ocean swells.

Likewise, typhoons in such countries as the Philippines cause massive power outages, flooding, landslides, and destruction of life and prop-

Much of the effectiveness of the American response to hurricanes, however, is attributable to the ability to react to circumstances rather than to prevent them. With a large amount of the population possessing electronic communication, it is possible to circulate word quickly regarding the coming deluge. Furthermore, people in the United States are much more mobile

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that century, however, the establishment of hurricane-watch services offered the hope of early warning, and by the 1930s ships and weather balloons were used to provide readings on atmospheric conditions that might portend cyclones. In the 1940s airplanes and, later, radar further increased meteorologists’ ability to monitor the atmosphere. Today a global network of weather satellites makes it possible to identify and track tropical cyclones from their earliest appearance as disturbances over the remote ocean.

Weather

DRY

ICE

(SOLID

CARBON DIOXIDE), SEEN HERE BEING

CONVERTED INTO A VAPOR, OFTEN IS USED TO CHANGE SUPERCOOLED WATER IN CLOUDS INTO ICE CRYSTALS IN THE WEATHER MODIFICATION TECHNIQUE CALLED CLOUD SEEDING. (© M. Meadow/Photo Researchers. Reproduced by permis-

sion.)

than their counterparts in developing countries and can evacuate hurricane regions more easily. Additionally, a wealthier and more powerful government is able to administer relief more quickly and in greater quantities than the leadership of small, poor nations. Even so, hurricanes and typhoons have a vast impact wherever they strike, and perhaps for this reason a certain mystery and lore have developed around them. Reflective of this fascination is the practice of personalizing hurricanes. Originally, the U.S. National Weather Service gave them women’s names, but in response to protests from feminist groups, by the late 1970s it had adopted a practice of using an alphabetic list of alternating male and female first names. The Weather Service draws up new lists each year to name the cyclones of the western Pacific and the Caribbean/Gulf regions.

A turning point came in the twentieth century, with the development of such monitoring systems as the cyclone-monitoring techniques we have mentioned. It says a great deal about just how young meteorology is as a science that one of its two founders as a modern discipline died only in the 1970s. This was Jakob Bjerknes (1897–1975), a Norwegian-born American meteorologist who, in the 1920s, established a network of weather stations with his father, the Norwegian physicist Vilhelm Bjerknes (1862–1951). Vilhelm proposed the idea of air masses, a pivotal concept in meteorology, while Jakob discovered the origin of cyclones. MODERN W E AT H E R FORECAS T I N G . Weather forecasting in the United

Until the twentieth century, people had little warning when a cyclone was coming. Early in

States is the responsibility of the National Weather Service (NWS), which is part of the National Oceanic and Atmospheric Administration (NOAA) within the Department of Commerce. The NWS maintains a vast network of field offices and weather stations as well as nine National Centers for Environmental Prediction, each of which is focused on specific weatherrelated responsibilities. The complexity of the NWS organizational system hints at the greater

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Forecasting and Controlling Weather

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From the time of the Greeks, at least, people have tried to forecast the weather. They attempted to do so with varying degrees of success, using means that included folklore, superstition, old wives’ tales, traditional wisdom, instinct, intuition, and even a little bit of science. Sometimes this mixed bag yielded valuable information, such as the old and essentially accurate saying “Red sky at morning, sailors take warning; red sky at night, sailor’s delight.” Yet without true scientific methods of forecasting, would-be meteorologists were just shooting in the dark—as the sometimes inaccurate results of today’s much more scientific forecasting shows.

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Weather

KEY TERMS ATMOSPHERE:

In general, an atmos-

EVAPOTRANSPIRATION:

The loss of

phere is a blanket of gases surrounding a

water to the atmosphere via the combined

planet. Unless otherwise identified, howev-

(and related) processes of evaporation and

er, the term refers to the atmosphere of

transpiration.

Earth, which consists of nitrogen (78%), oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such

The amount of water

The study of the

METEOROLOGY:

atmosphere, weather, and weather predic-

as neon (0.07%). CLIMATE:

HUMIDITY:

vapor in the air.

The pattern of weather con-

tion. When discussing the

ditions in a particular region over an

PRECIPITATION:

extended period. Compare with weather.

hydrologic cycle or meteorology, precipita-

CONDENSATION:

The formation of a

liquid from a vapor, usually as a result of a reduction in temperature. CONVECTION:

Vertical

tion refers to the water, in liquid or solid form, that falls to the ground when the atmosphere has become saturated with moisture.

circulation

that results from differences in density ultimately brought about by differences in

TRANSPIRATION:

The process where-

by moisture is transferred from living

temperature. Convection involves the

organisms to the atmosphere. A major por-

transfer of heat through the motion of hot

tion of environmental moisture for precip-

fluid (e.g., air) from one place to another.

itation comes from plants, which lose

CONVECTION CURRENT:

The flow

of material heated by means of convection. EVAPORATION:

The process whereby

liquid water is converted into a gaseous state and transported to the atmosphere. When discussing the atmosphere and precipitation, evaporation generally is distinguished from transpiration. In this context,

water through their stomata, small openings on the undersides of leaves. UNSTABLE ATMOSPHERE:

A term

describing a situation in which air temperature drops rapidly with altitude. As a result, a packet of air continues to move upward owing to the temperature difference between it and the surrounding air. The condition of the

evaporation refers solely to the transfer of

WEATHER:

water from nonliving sources, such as the

atmosphere at a given time and place.

soil or the surface of a lake.

Compare with climate.

complexity of weather forecasting itself, which we touch on here in only the most cursory fashion. A number of useful techniques and forms of technology are available to the modern meteorologist. Perhaps the best known of these is Doppler radar, used to track the movement of

storm systems. By detecting the direction and velocity of raindrops or hail, Doppler radar can help the meteorologist determine the direction of winds, and thus predict weather patterns that will follow in the next minutes or hours. But Doppler radar can do more than simply detect a

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storm in progress: Doppler technology also aids meteorologists by interpreting wind direction. Other forms of weather-forecasting technology include NEXRAD (Next Generation Radar) and GOES (Geostationary Operational Environmental Satellite). S O M E T Y P E S O F W E AT H E R F O R E CAS T. The simplest (and usually least

accurate) type of forecast is called a persistent forecast and starts with the assumption that existing patterns will continue into the future. The problem with this notion, of course, is that with the complexity of weather, patterns are always changing. More reliable is the so-called trend method, based on the relationship between the movement of air masses and the larger weather patterns. This is a type of prediction familiar to most of us from weather maps displayed by TV meteorologists. Similar to the trend method is the analogue method, which uses analogies (hence the name) between current weather maps and similar maps from the past. If a weather map for today closely matches the map of patterns that prevailed on a particular day three years ago, it is possible to make some predictions about the weather now by referring to conditions that developed at that time. Meteorology may employ statistical probability, and when it comes to long-range forecasting, the mathematics may become considerably more complex. This is illustrated by the fact that chaos theory, one of the most challenging branches of modern mathematics, was the brainchild not of a mathematician but of the American meteorologist Edward Lorenz (1917–). An outgrowth of Lorenz’s studies in atmospheric patterns, chaos theory is applied in studying extremely complex systems whose behavior appears random. (On a pop-culture note, chaos theory was the specialization of Ian Malcolm, the mathematician played by Jeff Goldblum in the film, Jurassic Park.)

Weather modification includes techniques such as cloud-seeding, which originated in the 1940s. By the beginning of the twenty-first century, several other methods of weather modification existed, among them frost prevention, fog and cloud dispersal, hurricane modification, hail suppression, and lighting suppression. Cloud seeding necessitates the conversion of supercooled water in clouds—that is, water whose temperature has dropped below the freezing point without it actually freezing—into ice crystals. Dry ice (solid carbon dioxide) and silver iodide are the substances most commonly employed for this purpose. The result is, or at least can be, snow clouds. On the other hand, cloud-seeding techniques can be used for fog dispersal, another form of weather modification useful, for instance, in the skies over airports. WHERE TO LEARN MORE Allaby, Michael. DK Guide to the Weather. New York: Dorling Kindersley, 2000. ———. Tornadoes and Other Dramatic Weather Systems. New York: Dorling Kindersley, 2001. The Atmosphere (Web site). . Folk Lore Weather Forecasting (Web site). . National Weather Service/National Oceanic and Atmospheric Administration (Web site). . Reynolds, Ross, ed. Cambridge Guide to the Weather. New York: Cambridge University Press, 2000. Silverstein, Alvin, Virginia B. Silverstein, and Laura Nunn. Weather and Climate. Brookfield, Conn.: Twenty-First Century Books, 1998. Stein, Paul, ed. The Macmillan Encyclopedia of Weather. New York: Macmillan Reference USA, 2001.

People at the turn of the nineteenth century would have been flabbergasted to know that it would be possible by the twenty-first century to provide a reasonably accurate 24-hour weather forecast. They

Weather Underground (Web site). .

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would have been even more astounded at the idea of a five-day forecast, which is common (though with varying degrees of accuracy) on most local and national weather reports. And, no doubt, they would have been “blown away”—to use a popular weather metaphor—at the concept that modern technology makes it possible even to exert some control over the weather.

W E AT H E R.

The Weather Channel (Web site). .

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C L I M AT E Climate

CONCEPT Many people think of weather and climate as concepts that are very nearly synonymous, but this is far from the truth. Whereas weather is a term referring to the atmospheric conditions for a particular place at a particular time, climate describes the overall weather pattern of a region over an extended period. A spot on the South Carolina coast, for instance, is likely to be humid and prone to hurricanes; another place, in the Rocky Mountains, might be dry and windy; while yet another locale, in Hawaii, most likely is inclined to extremely mild and equable or unvarying temperatures. In each case, what has been described is climate; for the weather in any of those particular places, one would need to check the weather report on a specific day. Despite the fact that climate is a concept that encompasses the weather in a region over an extended period, it is still possible for climate itself to vary.

HOW IT WORKS The Atmosphere All weather takes place in the atmosphere, the uppermost of the four earth systems. A blanket of gases created in large part by the expulsion of elements from the geosphere through volcanic eruptions, the atmosphere sustains the biosphere through its components of oxygen and carbon dioxide. But it also recirculates water from the hydrosphere, a key function in its role as a weather-generating system. (See Earth Systems for more about the interactions among geosphere, biosphere, hydrosphere, and atmosphere.)

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C O M P O S I T I O N O F T H E AT M O S P H E R E . Though we think of the

atmosphere as being composed primarily of oxygen, the vast majority of it (78%) is nitrogen. The latter is a highly unreactive gas, meaning that it does not tend to bond chemically with other elements. (Nitrogen itself is discussed in considerably greater detail within the essay Nitrogen Cycle.) Therefore, the nitrogen in our air is really just “filler,” the result of volcanic eruptions billions of years ago: unlike carbon dioxide, which dissolved in the water of Earth’s oceans, the nitrogen simply stuck around in the atmosphere. Air is not a chemical compound but a mixture, of which nitrogen and oxygen together account for 99%. Another 0.93% is argon, which is a noble gas, meaning that it, too, is highly unreactive. The last 0.07% comes from trace gases (i.e., gases in very small quantities), including two that are extremely important to the operation of Earth: carbon dioxide and water vapor. The first of these is a key component in the biosphere and in biogeochemical cycles (see Carbon Cycle), while the second is of enormous importance in weather and climate.

The Troposphere and Air It may be surprising to learn that water vapor is such a small portion of the atmosphere; even more shocking is the tiny fraction that this amount of water (equivalent to many billions of gallons) constitutes in proportion to Earth’s entire water supply. Furthermore, all our weather, which plays such an integral role in our lives, is generated in a very small portion of the atmosphere.

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Climate

FOR

EARTH, THERE ARE PARTICULAR TYPES OF PLANTS AND ANIMALS THAT HAVE ADAPTANIMALS THAT LIVE IN POLAR REGIONS, LIKE THIS BEAR, TYPICALLY HAVE VERY THEM LESS SURFACE AREA TO EXPOSE TO THE COLD; THEY ALSO TEND TO HIBERNATE AS A THE WINTER. (© John Shaw/Photo Researchers. Reproduced by permission.)

EVERY CLIMATE REGION ON

ED TO THE PREVAILING CONDITIONS. SMALL EARS, GIVING MEANS OF ENDURING

This is the troposphere, the lowest layer, which extends to about 10 mi. (16 km) above the surface of Earth. The height of the troposphere is about one-fifth the combined height of the troposphere, stratosphere, and mesosphere, the three atmospheric layers that contain air. (Beyond these layers are the thermosphere, which eventually dissolves into the exosphere and the emptiness of space, which is characterized precisely by its lack of an atmosphere.) A H I G H C O N C E N T RAT I O N O F A I R. Small as the troposphere is within the

larger atmosphere, however, it contains 80% of the atmosphere’s mass. The reason is that at higher altitudes, Earth exerts less gravitational attraction on the gas molecules that make up air. Thus, if one were to travel up through the outer layers, the amount of air in the atmosphere would simply shrink to nothingness because at that height, Earth does not exert enough gravitational force to hold in place those ultralight particles. By contrast, in the troposphere, the gravitational force—a function of the distance between two objects, in this case, a gas molecule and Earth—is extremely strong. This results in a high

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concentration of gas near the surface. (Note the term gas: remember that air is a mixture, not a compound, so there is no such thing as an “air molecule.” Instead, there are only molecules of oxygen [O2], ozone [O3], nitrogen [N2], carbon dioxide, and water, as well as atoms of argon and other noble gases.) In any case, the high concentration of gas creates pressure, which, along with several other factors, causes air to move. The result is weather, and weather patterns over a long enough period of time yield what we call climate. (See Weather for more about the components that make up weather as well as their interactions.)

The Atmospheric Sciences Weather is like a daily newspaper, whereas climate is like a book of history: whereas weather reflects the atmospheric conditions for a particular place at a particular time, climate is the overall pattern of weather over an extended period. This difference is reflected in the organization of the atmospheric sciences, the area of earth sciences concerned with atmospheric phenomena. Principal among the atmospheric sciences are meteorology, which is the study of weather,

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and climatology, or the study of climate. Meteorology focuses on daily or even hourly changes in conditions within the troposphere and the lower stratosphere, the region just above it. By contrast, climatology is concerned with analyzing the weather in a particular area over periods as short as a month or as long as several million years. Whereas meteorology involves daily weather forecasts and reports, efforts in climatology are directed toward explaining differences in climate around the earth. Climatologists also investigate how these differences are related to other aspects of the natural environment. Paleoclimatology is a specialized branch of climatology devoted to the study of climatic conditions in the distant past. Such study may require investigation of fossils and other materials that provide physical or chemical clues regarding the past climate and changes it experienced.

REAL-LIFE A P P L I C AT I O N S Climates Zones and Organisms One of the few ancient scientific thinkers whose work is still held in high regard was the Roman geographer Pomponius Mela (fl. ca. A.D. 44). In De situ orbis, Mela introduced the idea of five climate zones on Earth: northern frigid or cold, northern temperate or mild, torrid (very hot), southern temperate, and southern frigid. In a world that knew of no lands further north than Scandinavia (a semi-mythical place the Greeks had called “Thule”) or further south than the lower Nile, Mela’s designation of climate zones was extraordinarily accurate. Only in the 1910s did the Russian climatologist Wladimir Köppen (1846–1940) improve on Mela’s system, developing his own five-part climate designation. By Köppen’s time, it was clear that there was not necessarily any climatic difference between northern and southern seasons, except inasmuch as the seasons in the Southern Hemisphere took place at opposite times of the year from those in the Northern Hemisphere. Therefore, he dispensed with all references to latitude except insofar as they related to position relative to the equator.

humid tropical, dry, humid mid-latitude with mild winters, humid mid-latitude with cold winters, and polar. Each of these larger categories is then subdivided into smaller climate types: for example, among the dry-climate group, there is a distinction between deserts and steppes, which are arid but not completely barren plains of a type found in Russia and central Asia. Two factors, the average annual temperature and amount of precipitation, serve to differentiate climate types. Humid tropical, dry, and polar zones are fairly self-explanatory; as for the two humid mid-latitude types, they are distinguished by their distance from the equator. For instance, the southern United States would be an example of a humid mid-latitude region with mild winters, while the northeastern United States—New York and New England—would be considered a humid mid-latitude region with cold winters. For every climate region on Earth, there are particular types of plants and animals that have adapted to the prevailing conditions. Animals that live in desert regions, for example, might have large ears to add a greater surface area for perspiration. On the other hand, animals in polar regions typically have very small ears, giving them less surface area to expose to the cold. Both camels and cacti are organisms well adapted to a desert climate: the camel can store large amounts of water and food in its hump, while the cactus requires little moisture to survive. Many a desert animal is nocturnal, allowing it to survive the heat, while most polar animals tend to hibernate as a means of enduring winter.

Microclimates Not all climates necessarily spread across a whole desert or mountain range—or, in the case of Antarctica, with its decidedly polar climate, a whole continent. A very specific area either on Earth’s surface or just a few feet or meters above or below it (i.e., up in the trees or in the soil) likewise can have its own microclimate. The very existence of a microclimate, with all its complexity, serves to show just how complex the larger Earth system is.

The Köppen system recognizes the zones of

A particular microclimate, such as that of a forest or even a particular spot within a forest, has its own specific weather conditions: temperature, humidity, wind patterns, dew, frost, and evaporation or transpiration. Soil is a major factor influencing the quality of the microclimate: if the soil

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LARGE

C L I M AT E

REGIONS.

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BENI ABBES DUNES, SAHARA DESERT. SOIL

IS A MAJOR FACTOR INFLUENCING THE QUALITY OF A MICROCLIMATE:

SOIL THAT IS SANDY IN TEXTURE AND LIGHT IN COLOR IS LIKELY TO REFLECT MORE LIGHT AND HEAT. (© V. Engelbert/Photo

Researchers. Reproduced by permission.)

is sandy in texture, it is likely to reflect more light and heat. The same is true if it is light in color. Also important is topography, an example being the rain shadow created by mountains. (See Mountains for an explanation of rain shadows.) M I C R O C L I M AT E S

IN

AC T I O N .

A beautiful example of microclimate in action can be found when hiking on Mount Santo Tomas outside Baguio in the northern Philippines. Nestled in the mountains, with still higher regions such as Santo Tomas nearby, Baguio has long been a popular resort owing to its cool temperatures. It is one of the few places in the Philippines where one can find evergreen conifers such as pine trees, and the semi-alpine climate makes

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Baguio a welcome relief from the heat of Manila and other regions further south. Even though it lies in the northern portion of Luzon, the northernmost of the major islands in the Philippine archipelago, it is not latitude that gives Baguio its cool climate; instead, it is altitude. The same is true of other places around the world: Quito, Ecuador, for instance, which has an extremely cool climate because it is high in the Andes, even though it lies on the equator. (The 2001 movie Proof of Life, which portrays climatic conditions ranging from mild to cold, was filmed in and around Quito.) On Santo Tomas, however, where the altitude is even greater than that of Baguio, it is pos-

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sible to experience both the heat that characterizes most of the Philippines and the cool conditions of the mountains in northern Luzon. Standing at a particular spot along the mountainside, one can feel the heat that falls directly on the mountain, owing to its tropical latitude— which means that the sunlight reaches the surface at a more or less perpendicular angle. At the same time, one can feel the cool breeze that blows up the mountain as the result of its altitude. It is possible, in fact, to stand in such a way that one’s back, to the mountain itself, is hot and sweaty while one’s face enjoys the cool, moist highland breeze.

Can Climate Change? Though we have established that climate is a long-term weather pattern, that does not mean that climate itself is fixed and unchanging. Any number of factors can affect it. For example, the “fog” in London was once such an established fact that it became associated with that city in the way that wind is with Chicago. But just as Chicago’s status as the “Windy City” is something of a myth (in fact, there are more than a dozen cities more windy, though Chicago does experience powerful winds as a result of its proximity to Lake Michigan), the London fog was not actually what it seemed to be. The fog was not really fog at all but pollution produced by the burning of coal for heat. As coal gave way to gas and other types of heat during the twentieth century, the fog in London’s climate changed. H E AT I N G U P O R C O O L I N G D O W N ? A less advantageous condition in the

global climate may be a result of other technological changes, according to some scientists. The burning of fossil fuels, such as coal, natural gas, and petroleum, during the past century or more has put large amounts of carbon dioxide into the atmosphere. That part of the situation is fairly cut and dried; as to the potential result, the scientific community is divided. Some scientists believe that higher concentrations of carbon dioxide will lead to a greater retention of heat in the atmosphere, resulting in a higher annual average global temperature. This phenomenon, known as global warming, has attracted considerable media attention owing to its promotion by environmental activists, including celebrities who embrace environmental caus-

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es. Yet the global warming position is far from the only interpretation that the facts suggest.

Climate

Other experts contend that high levels of carbon dioxide in the atmosphere will have the opposite effect. According to this view, the concentration of carbon dioxide may heat the atmosphere in the short term, but this will result only in a larger amount of evaporation from the oceans. This evaporation, in turn, will result in the formation of much larger cloud masses, which would have the effect of reflecting sunlight back into space—thus, in fact, lowering Earth’s temperature. The global warming position, with its media and celebrity support, has held the lead since the 1980s. For this reason, it is easy to forget that in the mid- to late 1970s, many environmentalists held an exactly opposite position. At that time, a series of extremely cold winters led to the claim that Earth was cooling and that in a few more centuries, a new ice age would ensue. In fact, this is a considerably more plausible position, given the fact that Earth has experienced countless ice ages in the past. B LO C K I N G O U T T H E S U N . Of course, it could be maintained (as advocates of global warming do) that the present situation of massive fossil-fuel burning has never occurred in Earth’s history. This is certainly true, but there is less credibility in the claim that never before has so much carbon dioxide been introduced to the atmosphere. In fact, billions of years ago, volcanoes belched vast quantities of the gas into the region above Earth’s surface, but because carbon dioxide is highly water-soluble, most of it dissolved into the oceans’ waters.

This points to one of the means by which a rapid climate change, in particular, a sudden cooling, is brought about: by a volcanic eruption or other phenomenon that places enormous amounts of dust and ash in the air. In so doing, it reduces the amount of solar radiation that reaches Earth’s surface, causing temperatures to drop. An extreme example of this may have occurred about 65 million years ago, as the result of a meteorite hitting Earth and causing rapid climatic changes that brought about the mass extinction of the dinosaurs and other life-forms (see Paleontology). A more benign example of atmospheric blockage occurred in 1991, after Mount Pinatubo in the Philippines erupted. The eruption appar-

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The difference, of course, lies in the fact that New York City is one of the planet’s greatest municipalities, a massive concentration of humans, machines, and concrete in a very small area. Heat-sensitive satellite imaging equipment routinely produces images of the United States that show great areas of heat around the major cities, particularly New York. Concrete, buildings, roads, and the like constitute what developers call impervious surface, meaning that rain is not supposed to leach through these surfaces to the soil; rather, it will remain on the surface as runoff. But sunlight cannot gather in puddles or run off into storm drains: it can only reflect, and thus a big city is a huge mirror to the Sun’s rays.

Climate

NEW YORK CITY

HAS A MASSIVE CONCENTRATION OF

HUMANS, MACHINES, AND CONCRETE IN A VERY SMALL AREA.

CITIES WITHOUT ADEQUATE GREEN SPACE, SUCH AS NEW YORK’S CENTRAL PARK (SHOWN HERE), BECOME GIANT REFLECTORS, AND THE PRESENCE OF SMOG AND HEAT FROM CARS AND OTHER MACHINES ADDS

TO

THE

UNHEALTHY

ENVIRONMENT.

(© Rafael

Macia/Photo Researchers. Reproduced by permission.)

ently released so much ash and other material into the atmosphere that it temporarily reversed a general warming trend that had prevailed during the 1980s. By the mid-1990s, however, the materials from Mount Pinatubo appeared to have settled out of the atmosphere, thus causing a return to early climate trends. H E AT I N G

UP

A

MICROCLI-

Despite the questionable nature of some environmentalists’ claims concerning the global climate, environmentalists are absolutely correct in noting the effects of human civilization, development, and technology on microclimates. An excellent example is New York City. Though it is located in the humid temperate climate zone described earlier, with cold winters, the city has less snowfall—and hotter summers—than less populated areas of New York State or even areas in Pennsylvania that lie on the same latitude. M AT E .

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A major city, from the standpoint of climate, is rather like a person who places a mirrored piece of metal around his or her face to “soak up the Sun’s rays”—an extremely unhealthy, dangerous, and inadvisable practice. Cities without adequate green space, such as parks, become giant reflectors, and the presence of smog and heat from cars and other machines only adds to the unhealthy, artificially heated environment. It is particularly unfortunate to see such changes in a city such as Atlanta, which until about 1980 was a fairly sleepy town noted for its abundance of trees. Atlanta has long since become a boomtown, and the trees of its metropolitan area are being massacred at an alarming rate. Not only is this denuding the city and suburbs (and in the process, some would say, stripping its last vestiges of charm), but it is raising Atlanta’s average temperature—which was already high as a result of the city’s latitude.

Earth, Space, and Ice Ages If Earth were a horse on a carousel, with the Sun at the spinning center of the merry-go-round, the horse representing our planet would not be sitting upright, at a 90° angle to the carousel floor. Rather, it would be tilted off the perpendicular angle by 23.5°, a fact responsible for many features of the global climate and microclimates of Earth. At certain times of the year, rays from the Sun strike either the Northern or Southern hemisphere more directly than at other times. Summer occurs in the Northern Hemisphere during the middle of the year, because, at that point in the planet’s movement around the Sun, the rays of the Sun strike the Northern Hemisphere at an

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Climate

KEY TERMS ATMOSPHERE:

In general, an atmos-

phere is a blanket of gases surrounding a

over periods as short as a month or as long as several million years.

planet. Unless otherwise identified, howev-

METEOROLOGY:

er, the term refers to the atmosphere of

atmosphere, weather, and weather predic-

Earth, which consists of nitrogen (78%),

tion.

oxygen (21%), argon (0.93%), and other substances that include water vapor, carbon dioxide, ozone, and noble gases such as neon (0.07%).

MICROCLIMATE:

The study of the

The climate of a

very specific region a few feet or meters above or below Earth’s surface. The size of microclimates is undefined and variable,

A major

but they are most definitely smaller than

division of the earth sciences, distin-

the regions (a state, a country, even a con-

guished from geoscience and the hydrolog-

tinent) over which a particular climate is

ic sciences by its concentration on atmos-

said to prevail.

ATMOSPHERIC SCIENCES:

pheric phenomena. Among the atmospheric sciences are meteorology and climatology.

TROPOSPHERE:

The atmospheric

layer closest to the surface, extending upward approximately 10 mi. (16 km). The

The pattern of weather con-

troposphere contains about 80% of the air

ditions in a particular region over an

in the atmosphere and is the region where

extended period. Compare with weather.

weather occurs.

CLIMATE:

CLIMATOLOGY:

An area of the atmos-

WEATHER:

The condition of the

pheric sciences devoted to studying the

atmosphere at a given time and place.

weather in a particular region or regions

Compare with climate.

angle close to the perpendicular. The same happens in the Southern Hemisphere around the end of the year, when that section of the planet receives solar radiation at a nearly perpendicular angle. Another important factor that relates Earth’s position in space to its climate and microclimates is its pattern of movement around the Sun. The carousel analogy is a flawed one, because a carousel is round; Earth’s orbit, in fact, describes an ellipse, or oval. The reason is that the Sun exerts a greater gravitational pull on Earth at different parts of its orbital path, and the result is that at its closest approach to the Sun, Earth receives more solar energy than it does when it is farthest away from it. This has little do with seasons: the closest approach occurs in January, winter in the Northern Hemisphere. Conversely,

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in July, the hottest time of year for the Northern Hemisphere, Earth is at its furthest point from the Sun. E X P LA I N I N G I C E AG E S . It is possible that Earth’s position relative to the Sun may explain those dramatic periods of cooling known as ice ages, when much of Earth’s water freezes, larger amounts of land are exposed, and some life-forms die off while new ones develop. There have been numerous ice ages in Earth’s history, and what we call the Ice Age, which ended about 10,000 years ago, was just the last ice age.

That one was particularly significant, of course, not only because it was most recent, occurring as it did on the eve of civilization’s beginnings, but because it was a formative juncture in human development. During those thousands of years, human hunter-gatherer society

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and Paleolithic technology developed to its highest point. Also, many of the significant migrations of people took place, most notably the movement of Siberian tribes eastward across the land bridge of what is now the Bering Strait. What causes ice ages and other large-scale climate changes? In the 1930s the Serbian astrophysicist Milutin Milankovitch (1879–1958) put forward a theory maintaining that changes in the pattern of Earth’s orbit around the sun, as well as in Earth’s inclination to the plane of its orbit, could explain these changes in climate. The planet’s axis of inclination (that is, its angle in relation to the plane of orbit) changes over a period of about 22,000 years, a cycle known as the precession of the equinox. As a result, first one hemisphere and then the other is be pointed toward the Sun. Milankovitch evolved a complex theory that took into account precession of the equinoxes, changes in the shape of Earth’s orbit, and changes in its orbital tilt. The theory has been improved and modified numerous times since the 1930s, but, in general, Milankovitch’s ideas still provide climatologists with a basic understanding as to how large changes in Earth’s climate take place.

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WHERE TO LEARN MORE The Atmosphere (Web site). . Climate Prediction Center—National Oceanic and Atmospheric Administration/NOAA (Web site). . Elsner, James B., and A. Birol Kara. Hurricanes of the North Atlantic: Climate and Society. New York: Oxford University Press, 1999. National Climatic Data Center—National Oceanic and Atmospheric Administration/NOAA (Web site). . Philander, S. George. Is the Temperature Rising?: The Uncertain Science of Global Warming. Princeton, NJ: Princeton University Press, 1998. Resnick, Abraham. Due to the Weather: Ways the Elements Affect Our Lives. Westport, CT: Greenwood Press, 2000. Silverstein, Alvin, Virginia B. Silverstein, and Laura Nunn. Weather and Climate. Brookfield, CT: TwentyFirst Century Books, 1998. Stevens, William K. The Change in the Weather: People, Weather, and the Science of Climate. New York: Delacorte Press, 1999. United Nations Framework Convention on Climate Change/UNFCCC (Web site). . World Climate: Weather Rainfall and Temperature Data (Web site). .

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How to go to your page

INDEX OF E V E R Y D AY T H I N G S The “everyday” items discussed in this series (volumes 1-4) have been categorized into 26 general subject areas, and are listed below. Boldface type indicates main entry volume and page numbers. Italic type indicates photo and illustration volume and page numbers.

A G R I C U LT U R E Ammonia fertilizers, 3:351 Ammonium nitrate, 3:351–352, 4:333 Angiosperm reproduction, 3:138, 3:140–141 Angiosperms, 3:173–174, 3:362, 3:364, 3:364–365, 4:347–349 Aphids, 3:388 Apple tree stomata, 4:388 Bull’s horn acacia, 3:386 Burdock (plant), 3:389 Cabbage, 3:26 Cacti, 3:351 Cattle, 3:7–8, 3:123, 3:233 Chestnut blight, 3:394 Chickens, 3:322, 3:324, 3:328 Chlorophyll, 3:4, 3:5 Chloroplasts, 3:4 Crops, 3:136, 3:209, 3:350–352, 3:380 Delta soils, 3:351 Desert soil, 3:350, 3:351, 4:297–300 Domesticated animals, 3:387, 3:395–396 Dust bowls, 3:352, 4:270, 4:286, 4:303, 4:304–305 Epiphytic plants, 3:389 Fertilizers, 3:351 Greenhouse effect, 4:355–356 Gymnosperms, 3:138, 3:140, 3:173–174, 3:364, 3:364–365, 4:347–349 Humus, 4:295 Kudzu, 3:211–214, 3:274, 4:279–280 Lemons, 3:93 Limes, 3:93 Mad cow disease, 3:233 Minerals, 4:294 Mushrooms, 3:384–385, 3:385 Nitrogen cycle, 4:338 Oranges, 3:93 Orchids, 3:138, 3:385 Peas, 4:199 Pineapples, 3:137 Rice, 3:95

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Rubber, 2:152, 2:267 Salt, 3:350 Seeds and seed-bearing plants, 3:138 Soil, 3:351–352, 4:270, 4:292–300, 4:304–305 Soil conservation, 4:304–305 Soil formation, 4:294, 4:296 Spruce trees, 3:371 Topsoil, 4:295–296 Trees, 3:371–372, 3:374 Wheat, 3:352

ANIMALS Aardvarks, 3:222 African black rhinoceros, 3:386, 3:387 Albatross, 3:338 Anteaters, 3:223, 3:223–224 Ants, 3:349, 3:349–350, 3:386, 3:388–389 Apes, 3:220–221, 3:274–275 Aphids, 3:388 Arachnida, 3:275 Arctic fox, 3:394 Arctic tern, 3:338 Artiodactyls, 3:223 Bats, 2:305, 3:141, 3:170, 3:220, 3:340 Bears, 3:40, 3:89 Beavers, 3:322 Bedbugs, 3:281–282 Bees, 3:211, 3:211, 3:303–304, 3:323–324 Bird colonies, 3:402, 3:402, 3:406 Bird parasites, 3:274, 3:330–331 Birds, 3:73, 3:309, 3:322–324, 3:327–331, 3:329 Birds, aerodynamics of, 2:103–105, 2:115 Black-throated green warblers, 3:217 Butterflies, 3:338, 3:388–389 Camels, 3:223, 4:307 Cardinals, 3:327 Carnivores, 3:221, 3:361, 4:200, 4:342 Cats, 2:323, 3:298, 3:387 Cattle, 3:7–8, 3:123, 3:233

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Cetaceans, 3:221–222 Cheetahs, 3:373 Chickens, 3:322, 3:324, 3:328 Chiropterans, 3:220 Class Insecta, 3:275 Cold-blooded animals, 3:218 Cowpox, 3:256–257, 3:257 DDT (pesticide), 3:72, 3:73, 4:358 Dermoptera, 3:220 Desert tortoises, 3:351 Detritivores, 3:69, 3:221, 3:361, 4:294, 4:316–317 Detritivores in food webs, 3:68, 3:349, 4:200, 4:342 Dodo (bird), 3:208–209, 3:209 Dog whistles, 2:323 Dogs, 2:323, 3:298, 3:303, 3:383, 3:387 Dolly (cloned sheep), 3:122, 3:123 Dolphins, 3:195, 3:204–205, 3:221–222, 3:340–341 Domesticated animals, 3:387, 3:395–396 Donkeys, 3:215 Ducks, 3:193, 3:330 Dust mites, 3:265 Earthworms, 3:349 Egg-laying mammals, 3:218 Elephants, 3:200, 3:222 Endangered or extinct bird species, 3:208–209, 3:209, 3:408 Endangered species, 3:208 Eskimo curlews, 3:208 Felidae, 3:221 Field mice, 3:163–164 Fish, 2:122 Fleas, 3:282 Frigate birds, 3:145 Gazelles, 3:373 Geese, 3:322, 3:328–330, 3:329 Gills, 3:57 Hares, 3:226 Herons, 3:375 Hinnies, 3:215 Hookworms, 3:278 Horses, 3:172–173, 3:215, 3:222, 3:402, 4:343–344 Insectivores, 3:219–220 Insects, 3:165, 3:275, 3:279–282 Invisible fences, 2:323 Jellyfish, 3:321 Kangaroo rats, 3:329–330 Kangaroos, 3:219, 3:219 Kingdom animalia, 3:198, 3:205, 3:206 Lemurs, 3:220 Lice, 3:282 Lift (aerodynamics), 2:104–105 Llamas, 4:344 Loa loa, 3:279 Lobsters, 3:171 Locusts, 3:405 Loggerhead turtles, 3:338–339

Mad cow disease, 3:233 Mammals, 3:58, 3:195, 3:204–205, 3:217–226 Manx shearwaters (bird), 3:338 Marsupials, 3:218–219, 3:219 Mating rituals, 3:145, 3:145–147 Methane, 3:7–8 Mice, 3:123, 3:163–164, 3:226 Minnows, 3:327–328 Moles, 4:296–297 Moths, 3:138, 3:173 Mules, 3:215, 3:216 Mussels, 3:71, 3:211 Nematoda (phylum), 3:349 Nile perch, 3:210 Northern fur seal, 3:130 Northern spotted owls, 3:406, 3:408, 4:354, 4:354–355 Omnivores, 3:361 Owls, 3:406, 3:408 Oxpeckers, 3:386, 3:387 Parasites, 3:274, 3:279–282, 3:330–331 Pepper moths, 3:173 Pesticides, 3:72, 3:73 Pigeons, 3:338 Pinworms, 3:278 Polar bears, 3:89, 4:406 Pollinating plants, 3:140–141 Porpoises, 3:221, 3:340–341 Prairie dogs, 4:297 Pregnancy and birth, 3:151–153 Primates, 3:167–168, 3:205–206, 3:216, 3:220–221 Rabbits, 3:226 Rabies, 3:257 Rats, 3:226, 3:330 Rhinoceros, 3:386, 3:387 Right whales, 3:208 Rocky Mountain bighorn sheep, 3:323 Rodents, 3:224–226 Roundworms, 3:278 Ruminants, 3:7–8, 3:53 Salmon, 3:337 Sea otters, 3:71 Seals, 3:130 Sewage worms, 3:72 Shedding (fur or skin), 3:313, 3:313 Sheep, 3:122, 3:123, 3:323 Shrews, 3:219–220, 3:226 Snails, 3:298, 3:333 Soil, 4:297 Spiders, 3:330, 3:332 Sponges, 3:199 Spotted owls, 3:406, 3:408, 4:354, 4:354–355 Squirrels, 3:112, 3:113 Starfish, 3:70, 3:71 Stickleback fish, 3:217, 3:322, 3:327 Stingrays, 4:121 Suriname toad, 3:152

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How to go to your page Swallows, 3:338 Tapeworms, 3:277–278 Taxonomic keys, 3:193 Teeth, 3:222 Termites, 3:6, 3:6 Tickbirds, 3:386, 3:387 Ticks, 3:279, 3:282 Toads, 3:123, 3:152 Tortoises, 3:351 Training birds, 3:338 Trees, 3:363 Tubificid worms, 3:72 Turtles, 3:338–339 Vampire bats, 3:220 Whales, 3:195, 3:204–205, 3:207, 3:208, 3:222 Whales, echolocation by, 3:339, 3:340–341 Wings, 2:103–105, 2:115, 3:192 Worms, 3:138, 3:274–275, 3:277–279, 3:349, 4:296 Zebra mussels, 3:211 Zooplankton, 3:336

ART Beauty, 3:146, 3:147 Colors, 2:355, 2:359–360 Diffraction, 2:294–300, 2:356 Holograms, 2:295, 2:296, 2:298–300 Laser etching, 2:364 Light, 2:358–360 Primary colors, 2:360

AUTOMOBILES Aerodynamics, 2:109–111 Airbags, 1:56, 1:58–59, 2:43, 2:189–190 Automobile industry, 1:176 Axles, 2:162–164 Brakes, 2:55, 2:62 Car horns, 2:336 Car jacks, 2:142–143, 2:167–168 Car lifts, 2:160 Carburetors, 2:117 Catalytic converters, 1:305, 1:307 Centripetal force, 2:46–48 Chrysler PT Cruiser, 2:109 Clutches, 2:55 Collisions, 2:41–43, 2:62–63 Combustion engines, 1:51, 1:292 Conservation of energy, 2:27 Crash tests, 2:63 Crumple zones in cars, 2:41–43 Drafting (aerodynamics), 2:109 Drag (aeronautics), 2:109–110 Electric vehicles, 1:163

S C I E N C E O F E V E RY DAY T H I N G S

Engine coolant, 2:248 Engine torque, 2:90 Ethylene dibromide, 1:234 Fossil fuels, 4:201–202 Friction, 2:53, 2:55 Fuel-injected automobiles, 1:56 Gas laws, 2:188–190 Gasohol, 3:30 Highways, 3:380 Lift (aerodynamics), 2:109–110 Nissan Hypermini, 1:163 Racing cars, 2:109–110 Radiators, 2:248 Roads, 2:48 Shock absorbers, 2:266–267 Tires, 2:53, 2:55 Torque, 2:90

Index of Everyday Things

A V I AT I O N Aerodynamics, 2:102–103, 2:102–111 Aeronautics industry, 1:172 Aerostat Blimp, 2:127 Air pressure, 2:146 Air resistance, 2:78–80 Airfoils, 2:105–107 Airplanes, 2:105–107 Airports, 3:312 Airships, 1:254, 1:257, 2:126–129 Balloons, 1:58–59, 2:105, 2:125–127, 2:129 Blimps, 2:105, 2:127, 2:129 Dirigibles, 2:105, 2:127–128 Drafting (aerodynamics), 2:109 Drag (aeronautics), 2:106 Flaps (airplanes), 2:106 Fluid pressure, 2:144 Gliders, 2:105, 2:115–116 Goodyear Blimp, 2:105, 2:129 Graf Zeppelin (dirigible), 2:127–128 Helium, 2:126 Hindenburg (dirigible), 1:254, 1:257, 2:105, 2:126, 2:128 Hot-air balloons, 1:55–56, 2:126, 2:186, 2:188 Hydrogen balloons and airships, 1:254, 2:105, 2:126–128 Jet engines, 2:312 Korean Air Lines Flight 007 shootdown (1983), 2:64 Lift (aerodynamics), 2:105–106 Mach numbers, 2:106–107, 2:305 Parafoils, 2:108 Propellers, 2:106, 2:116 Radar, 2:348–349 Radar ranges, 2:348 Spoilers, 2:110 Wind tunnels, 2:98, 2:107

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Wings, 2:105–108 Zeppelins, 2:105, 2:127–128

Television tubes, 2:369 Train whistles, 2:302, 2:303–304

CIVIL ENGINEERING

COMPUTERS

Abrasive minerals, 4:137 Airports, 3:312 Aluminum, 1:152, 1:153, 1:157 Aqueducts, 4:91–92 Arches, 4:14, 4:15 Archimedes screws, 2:166 Bridges, 2:250, 2:280, 2:285 Building materials, 1:176 Centrifuges, 2:46, 2:48–49 Chichén-Itzá (Mayan pyramid), 4:146 Chimneys, 2:116–117 Concrete, 2:151, 2:152 Doorbells, 2:336 Doors, 2:89 Egyptian pyramids, 2:162, 2:164–165 Expansion joints, 2:250 Fill dirt, 4:295 Galvanized steel, 1:188–189 Gateway Arch (St. Louis, MO), 2:79 Geodesic domes, 1:247 Homestake Gold Mine (SD), 4:212 Iron mines, 1:183 Mining, 4:19, 4:161–162 The Parthenon (Greece), 3:138 Pipes, 2:97, 2:112–113, 2:144 Pyramids, 2:162, 2:164–165, 4:36, 4:146, 4:147 Rebar, 2:151 Roman Coliseum (Italy), 4:15 Rust, 1:283, 1:285, 1:294 Seawalls, 4:401 Statue of Liberty (Ellis Island, NY), 1:174 Tacoma Narrows Bridge (WA) collapse, 2:280, 2:285

Algorithms, 3:193 CD-ROMs, 2:364 Disk drives, 2:337 DNA (deoxyribonucleic acid) nanocomputer, 3:120 Grocery store scanners, 2:296 Landsat (satellite program), 4:57, 4:59 Magnetic recording devices, 2:332, 2:333, 2:336–337 Magnetic tape, 2:336–337 Nanocomputers, 3:120 Open systems, 3:345–346 Silicon, 1:372 Silicon wafers, 1:223

C O M M U N I C AT I O N S AM radio broadcasts, 2:276–277, 2:345–346 Broadcasting, radio, 2:345–346 Communications satellites, 4:55 Dipoles, 1:47, 1:113 Eavesdropping devices, 2:325 FM radio broadcasts, 2:345–346 Listening devices, 2:322 Microwaves, 2:276–277, 2:282–283, 2:347–349 Radio broadcasting, 2:345–346 Radio waves, 2:259–261, 2:276–277, 2:282, 2:346–347, 2:366 Remote sensing, 4:57, 4:59 Satellites, 2:347–348, 4:55, 4:57, 4:59 Shortwave radio broadcasts, 2:346–347 Sunspots, 4:80

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CRIME AND POLICE WORK Bulletproof vests, 1:375 Bullets, 2:79, 2:80 DNA, 3:20, 3:21, 3:103–105, 3:105 DNA evidence, 3:108 Explosives, 3:351–352, 4:333 Fingerprints, 3:20, 3:105 Forensic geology, 4:19–21 Gunpowder, 1:292 Holmes, Sherlock (fictional detective), 4:20–21 Incendiary devices, 1:175–176 Kevlar, 1:375 Murder cases, 3:105, 3:108 Murrah Federal Building bombing (1995), 4:333 Oklahoma City (OK) bombing (1995), 4:333 Plants, 3:108 Rape cases, 3:108–109 Simpson, O.J., 3:105, 3:105, 3:108 Terrorism, 3:231, 3:251–252 World Trade Center (New York, NY), 1:153

ELECTRONICS Amplification, 2:315 Amplitude, 2:313–314 Calibration, 1:9, 1:81 Conduction, heat, 2:220, 2:228, 4:186 Conductivity, electrical, 2:228 Conservation of electric charge, 2:30–31 Electric current, 2:21 Electrical conductivity, 2:228 Electromagnetic induction, 2:341 Electromagnetic sound devices, 2:336 Electromagnetism, 2:334, 2:341–342 Electron tubes, 2:345

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How to go to your page Electron volts (unit of measure), 2:345 Holograms, 2:295, 2:296, 2:298–300 Lasers, 2:361–362, 2:363 Loudspeakers, 2:315, 2:320–321, 2:336 Magnetic recording devices, 2:332, 2:333, 2:336–337 Magnetic sound devices, 2:336 Magnetic tape, 2:336–337 Magnetrons, 2:348 Microphones, 2:336 Nanotechnology, 2:56 Silicon, 1:244, 1:372 Silicon wafers, 1:223 Solenoids, 2:332, 2:334 Solid-state lasers, 2:361–362 Television tubes, 2:369 Thermistors, 2:243 Thermocouples, 1:17, 2:243 Transducers, 2:322–323, 2:349

ENERGY Energy, 2:170–180 Alternative energy sources, 4:202, 4:206 Aromatic hydrocarbons, 1:368 Atmosphere, 4:198 Batteries, 1:163, 1:164–165, 1:291, 1:296 Biomass, 4:200–201 Biosphere, 3:347–348, 4:199–201, 4:351–358 Bouncing balls, 2:175–176 Calorie (unit of measure), 2:219, 2:229 Calorimeters, 1:18–19, 2:230–231 Candles, 2:46, 2:360–361 Chernobyl nuclear disaster (1986), 1:98, 1:103 Coal, 4:324, 4:326 Coal gasification, 1:35, 1:43, 1:46 Conduction, heat, 2:220, 2:228, 4:186 Conservation of electric charge, 2:30–31 Conservation of energy, 2:27–29, 2:174–176, 2:179–180 Cyclotrons, 1:201 Earth, 4:192–206 Electromagnetic spectrum, 2:345 Entropy, 1:13–14, 2:222–223 Fossil fuels, 4:201 Gasohol, 3:30 Gasoline, 2:27, 2:248 Heat, 2:228–229, 4:185–186 Heavy water, 1:96 Human body, 3:10, 3:36, 3:308 Hydroelectric dams, 2:101, 2:176 Hydrogen, 4:202 Hydrogen fuel, 1:259 Hydrogen sulfide, 1:256, 4:320–321 Lithium batteries, 1:163, 1:164–165 Metabolism, 3:33–34, 3:44, 3:80

S C I E N C E O F E V E RY DAY T H I N G S

Nuclear energy, 2:177, 4:202, 4:206 Oil refineries, 1:357 Particle accelerators, 1:72 Petrochemicals, 1:257, 1:367–368, 4:160 Petroleum, 1:368, 3:28, 3:30, 4:158–160 Petroleum industry, 1:357, 1:358 Photosynthesis, 3:4–5, 3:68, 3:360–361 Plutonium, 1:201 Potential and kinetic energy, 2:174–177, 4:193 Power, 2:173–174, 2:260 Power lines, 2:250 Propane, 1:56 Propane tanks, 2:188 Radioactive waste, 1:98 Radium, 1:178–179 Ruby lasers, 2:369 Solar power, 4:202 Solar radiation, 4:197 Space shuttle fuels, 1:241, 1:292, 1:293–294, 4:85 Sun, 3:67–69, 4:74, 4:78 Temperature and heat, 1:11, 4:193–195 Thermodynamics, 2:217–218, 2:222, 2:223 Tides, 4:197–198

Index of Everyday Things

ENVIRONMENT Aerosol cans, 1:55, 2:187–188 Amazon rain forest, 3:365, 3:366 American chestnut trees, 3:394 American elm, 3:210 Angiosperms, 3:138–140, 3:173–174, 3:362, 3:364, 3:364–365, 4:347–349 Biodegradable plastics, 1:377 Biological communities, 3:391–399, 3:400–409 Biological rhythms, 3:306–315 Bioluminescence, 2:367, 2:369–370 Biomass ecosystems, 4:346 Biomes, 3:370–380 Biosphere, 3:345–359, 3:348, 3:360, 4:27, 4:341, 4:351–358 Birds, 3:73 Blue-green algae, 4:293 Bogs, 3:376 Boreal forest, 3:362, 3:371–372 Cacti, 3:351 Carbon, 1:243–251, 4:325–326 Carbon dioxide, 1:247–248, 1:268–269, 2:200, 3:28, 3:28, 3:369 Carbon dioxide in carbon cycle, 3:55–56, 4:326–329 Carbon monoxide, 1:248, 1:250, 1:301–302, 4:326–329, 4:328–329 Catalysts, 1:304–309, 1:305 Chaparral, 3:375 Chernobyl nuclear disaster (1986), 1:103 Chestnut blight, 3:394 Chlorine, 1:231–232

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Chlorofluorocarbons (CFCs), 1:233–234, 2:188 Chlorophyll, 3:4, 3:5 Chloroplasts, 3:4 Circadian rhythms, 3:306, 3:307–312, 4:83–84 Circannual cycles, 3:313 Climax biological communities, 3:370–371, 3:400–409 Climax (ecology), 4:352 Cloud forests, 4:345 Colorado River delta, 4:56 Communities (animals), 3:323–324, 3:331 Coniferous forests, 3:371–372, 3:374 Conservation laws, 2:27–33 Coral reefs, 3:377, 3:377, 4:327 Corrosion, 1:294 Cosmic background radiation, 1:240 Crater Lake (OR), 4:259 Currents, 4:363, 4:363–364 DDT (pesticide), 3:72, 3:73, 4:358 Deciduous trees, 3:362, 3:373 Decomposition, 3:68, 3:69 Deforestation, 3:365–366, 4:57, 4:59, 4:352–356, 4:353 Dendochronology, 4:119 Desertification, 3:353, 4:306–310 Desmodus rotundus, 3:220 Dinoflagellates, 2:369–370 Dodo bird and dodo tree, 3:208–209, 3:209 Dutch elm disease, 3:210 Ecology, 3:360–369, 3:391, 4:351–358 Endangered species, 3:207–208, 3:208, 3:223 Erosion, 3:352, 4:264–272, 4:284 Erosion prevention, 4:270, 4:271–272, 4:305 Estuaries, 3:377 Fens, 3:376 Fir trees, 4:318 Forest conservation, 3:363, 3:408–409 Forest ecology, 3:362–363 Fungi, 3:198, 3:361, 3:384–386 Gas laws, 2:188–190 Grasslands, 3:374–375 Great Lakes (North America), 3:211, 3:354 Greenhouse effect, 3:366, 3:368–369, 4:355–356 Gymnosperms, 4:347–349 Ice ages, 4:381–384, 4:411–412 Ice domes, 4:378 Ice sheets, 4:378–379 Ice shelves, 4:377, 4:379 Indian pipe (plant), 3:385 Inuit, 3:207 Islands, 4:251–252 Kelp forests, 3:71 Lake Erie, 3:354, 4:318 Lakes, 3:210–211, 3:354, 3:375–376 Leaves, 4:295 Lichen, 3:386 Lilac, 3:355

Litmus paper, 1:314 Logging and forestry, 3:363, 3:365–366, 3:403, 3:406–408 Madagascar, 3:138, 4:353 Mangrove forests, 3:362, 4:346 Mariana Trench, 2:124–125 Meteorites, 3:185–186, 3:186 Mid-ocean ridges, 4:221, 4:222–223, 4:257 Mississippi River, 4:365, 4:365–366 Mohave Desert, 3:351 Moles (animals), 4:296–297 Montane forests, 3:363 Moss, 3:137 Mud cracking, 4:287 Mycorrhizae, 3:384–386 National parks, 3:363 Native Americans, 3:127, 3:130–131, 3:186, 3:207, 3:396 Northern spotted owls, 4:354, 4:354–355 Oceans, 3:182, 3:185, 3:336 Old-growth forests, 4:354–355 Orchids, 3:138, 3:385 Passionflower, 3:389 Pesticides, 3:72, 3:73 Phosphorus, 3:348, 3:384 Phytoplankton, 3:77, 3:376 Plants, 4:355 Pollen, 3:138–141, 3:364, 3:364–365, 4:347–349 Pollution, 3:240–241, 3:403 Ponds, 3:375–376 Porpoises, 3:221, 3:340–341 Protozoa, 3:275–277 Radioactive waste, 1:98 Rain forests, 3:350, 3:392–393, 4:297, 4:346 Rain forests and climate, 3:362–363, 3:373–374 Rain forests, destruction of, 3:365, 3:366, 4:353 Rivers, 2:96, 2:97, 2:113, 3:376, 4:56, 4:268, 4:285, 4:300, 4:374–375 Sahara Desert, 3:353, 4:306–307 Sahel region, 4:307 Savannas, 3:373, 3:374–375 Sequoia National Park, 3:363 Ships, 3:210 Soil, 3:385–386, 4:268, 4:292–300, 4:301–302 Soil conservation, 3:352, 4:301–310 Spotted owls, 3:406, 3:408, 4:354, 4:354–355 Spruce trees, 3:371 Streams, 3:376 Subpolar forests, 4:346–347 Subpolar glaciers, 4:378 Subsoil, 4:296 Sulfur, 3:348, 4:318, 4:318, 4:320–322 Summer, 3:358 Sun, 4:384 Swamps, 3:375, 3:376 Temperate deciduous forests, 3:373

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How to go to your page Temperate forests, 4:346–347 Temperate rain forests, 3:373–374 Thermal expansion, 2:245–246, 2:249 Trees, 2:315, 3:362, 3:385, 3:406, 3:408 Trees, conifer, 3:371–372, 3:374 Trees, deciduous, 3:373 Trees and ecosystems, 3:209–210, 3:362 Tundra, 3:374, 3:392–395, 3:394 Volcanoes, 4:123, 4:126–127 Wetlands, 3:376 Worms, 3:275, 3:277–279, 3:349, 4:296 Yeast, 3:28, 3:28 Yellowstone National Park, 3:363 Zebra mussels, 3:211

FOOD AND DRINK Acid reflux, 3:48 ADA (American Dietetic Association), 3:96 Alcohol, 1:356–357, 1:369, 3:27–28, 3:59 American diet, 3:49–50, 3:79 Amino acids, 3:11–17 Antacids, 1:322–323, 3:48 Artichokes, 3:6 B vitamins, 3:92 Bacteria, 3:285 Bags, potato chip, 2:188 Baking soda, 1:315–317, 3:48 Beer, 1:340, 1:356–357 Beta-carotene, 3:90 Bingeing, 3:39 Boiling points, 1:39 Bowel movements, 3:50 Bread, 3:28, 3:28 Breast-feeding, 3:73 Brine-curing recipes, 1:352 Cabbage, 3:26 Calcium carbonate, 3:377, 4:327, 4:327 Calorie (unit of measure), 2:219, 2:229 Candy bars, 3:10 Carbohydrates, 3:3–10, 3:44, 3:79, 3:80, 3:81, 3:82 Carbonated water, 1:248 Cereal foods, 3:81 Cholesterol, 3:36, 3:36–37 Citric acid, 1:312, 1:315 Citrus fruits, 3:93 Convection cooking, 2:348 Corn, 3:82, 3:387 Cream, 2:49 Dehydrating fruits, 1:351 Dextrose, 3:3–4 Digestive system, 3:51, 3:51–54 Dinuguan (Filipino delicacy), 3:302–303 Disaccharides, 3:4 Distillation, 1:40, 1:354–360, 1:357, 2:209–210

S C I E N C E O F E V E RY DAY T H I N G S

Distilled water, 1:356 Drinking water and fluoridation, 1:233 Dry ice, 4:402 Eating habits, 3:39, 3:48, 3:242, 3:311 Emulsifiers, 1:333–334, 1:342–343 Enzymes, 3:24 Ethanol, 1:340, 1:343, 1:356–357 Fast food, 3:79, 3:81 Fats and oils, 3:35–37, 3:44–45, 3:81, 3:88, 3:297 Fiber in human diet, 3:50 Fried foods, 3:81 Health or organic foods, 3:86 Junk food, 3:10, 3:79 Lactose, 3:4 Lemons, 3:93 Limes, 3:93 Malnutrition, 3:82–86, 3:89 Maltose, 3:4 Meat, 3:23, 3:49–50, 3:277–278, 3:303 Meat preservation, 1:351–353 Milk, 1:177 Minerals, 3:80–81 Monosaccharides, 3:3–4 Mushrooms, 3:384–385, 3:385 Mussels, 3:71, 3:211 Oil emulsions, 1:333–334, 1:339, 1:342, 1:347–348 Oligosaccharides, 3:4 Oranges, 3:93 Organic and health foods, 3:86 Overweight Americans, 3:10, 3:37, 3:81, 3:82, 3:85–86 Phosphorus, 3:91 Pineapples, 3:137 Plants, 3:6–8, 3:23 Polysaccharides, 3:4 Pork, 3:277–278 Potato chip bags, 2:188 Preservation, 1:351–353 Proteins, 3:6, 3:13, 3:15, 3:18–23, 3:44, 3:100–101 Proteins in nutrition, 3:10, 3:79–82 Recommended daily allowances (RDA), 1:125–126 Rice, 3:95 Salts, 1:352–353 Saturated fats, 3:36, 3:88 Sauerkraut, 3:26 Sodium, 1:166–167 Sodium substitutes, 1:165 Soft drinks, 1:248 Starches, 3:6–7, 3:7, 3:25 Sucrose, 1:109, 3:4 Sugar, 1:109 Sugars, 3:3–4, 3:25 Sushi, 3:302, 3:303 Table sugar, 3:4 Trichinosis, 3:278 Truffles, 3:385, 3:385 USDA food pyramid, 3:81

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Abrasive minerals, 4:137 Alkali metals, 1:153, 1:162–170 Alkalies, 1:310–311, 1:319 Alkaline earth metals, 1:153–154, 1:171–180, 1:174 Alpine glaciers, 4:378 Aluminum, 1:101, 1:121, 1:152, 1:157, 1:295–296 Antimony, 1:226 Aquifers, 3:347 Arctic Ocean, 3:88 Arkansas River, 4:372 Aromatic hydrocarbons, 1:368 Arsenic, 1:225–226 Asbestos, 4:139 Barium, 1:177–178 Bedrock, 4:296 Berkelium, 1:203 Beryllium, 1:175 Biogeochemistry, 4:313–322 Biosphere, 4:27 Biostratigraphy, 4:106 Bismuth, 1:158, 1:161 Boron, 1:216 Brimstone, 4:320 Bromine, 1:234 Cadmium, 1:189 Calcite, 4:136, 4:138 Calderas, 4:259, 4:259 Californium, 1:203 Canyons, 4:106, 4:265, 4:270 Cap rocks, 4:159 Carbonates (minerals), 4:134 Carborundum, 4:137 Cerium, 1:208 Cesium, 1:170 Chemical deposition, 4:287–288 Chilean earthquake (1960), 4:237 Chlorine, 1:231–232 Chromium, 1:192 Chronostratigraphy, 4:95, 4:106–108 Cinder cones, 4:258 Cirque glaciers, 4:378 Clay, 2:40–41, 4:287 Coal, 4:324, 4:326 Cobalt, 1:190–191 Composite cones (volcanoes), 4:258 Compound minerals, 4:131 Continental crust, 4:229 Continental shelves, 3:377

Coon Creek (WI) erosion, 4:286 Copper, 1:188, 1:289–290, 4:138–139 Crystaline minerals, 4:132–133, 4:144 Curium, 1:202 Deuterium, 1:95–97, 1:254–255 Diamond, 1:246, 4:137–138, 4:165, 4:325–326 Digital photography, 4:55–56 Divergence (plate tectonics), 4:224 Dysprosium, 1:210 Earthquakes, 1:240, 4:235, 4:236 Elements, 1:119–126 Erbium, 1:209 Erosion, 4:284 Europium, 1:210 Evaporites, 4:290–291 Fossil fuels, 4:158–160 Geysers, 4:196 Gold, 1:25, 1:28, 1:30, 1:182, 1:187, 1:295, 4:138 Gold mining, 4:19, 4:161–162, 4:288, 4:290 Grand Canyon, 3:113 Great Lakes (North America), 3:211, 3:354 Gulf of California, 4:56 Gypsum, 4:138 Hafnium, 1:192 Halemaumau volcano (Hawaii), 4:5 Halogens, 1:229 Hanging valleys, 4:380 Hawaii, 4:5 Himalayas (mountain range), 4:224, 4:246 Homestake Gold Mine (SD), 4:212 Hydrocarbons, 4:157–158 Ice, 2:208, 2:247, 2:248–249, 4:285, 4:376–377, 4:381–384 Ice caps, 4:378, 4:379–380 Ice domes, 4:378 Ice sheets, 4:378–379 Ice shelves, 4:377, 4:379 Icebergs, 2:204 Igneous rocks, 4:148–149 Indium, 1:157–158 Industrial minerals, 4:162, 4:164–165 Infrared photography, 4:54 Iran earthquake (1755), 4:237 Iridium, 1:191 Iron, 1:190, 1:336, 2:332 Iron mines, 1:183 Islands, 3:402–403, 4:248–252, 4:249 Jewels, 4:165 Kennicott Glacier (AK), 4:381 Kettle lakes, 4:380 Krakatau (Indonesia), 4:259–260 Lake Erie (North America), 3:354, 4:318 Lake Victoria (Africa), 3:210 Landforms, 4:245–247 Landsat (satellite program), 4:57, 4:59 Lava, 4:148, 4:258

VOLUME 4: REAL-LIFE EARTH SCIENCE

S C I E N C E O F E V E RY DAY T H I N G S

Vitamins, 3:95–96 Wheat, 3:352 Yeast, 3:28, 3:28 Zinc, 1:154, 1:188–189

GEOLOGY

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How to go to your page Lead, 1:158, 4:139 Lisbon earthquake (1755), 4:233–234 Lithium, 1:164–166, 3:302 Lithosphere, 4:188, 4:212–213, 4:229 Little Ice Age (1250-1850), 4:384 Loma Prieta (CA) earthquake (1989), 4:233, 4:235, 4:236 Los Angeles (CA), 4:18 Magnesium, 1:172, 1:175 Magnetic metals, 1:190–191, 2:331–332 Manganese, 1:194 Maps, 4:44, 4:47–49 Metal crystals, 2:151–152 Metal elasticity, 2:150–151 Metalloids, 1:147, 1:222, 1:222–228, 1:223 Metals, 1:151–161, 1:152, 1:153 Metamorphic rocks, 4:150, 4:150, 4:152–153 Meteor Crater (AZ), 4:91 Meteorites, 3:345, 4:91 Mica, 4:150 Mid-ocean ridges, 4:221 Millbrae (CA) mudflows, 4:276 Mineral cleavage, 4:136 Mineraloids, 4:135 Minerals, 3:71, 4:129–142, 4:143–145, 4:155–156, 4:165, 4:288–291 Mining, 4:162 Moraines, 4:380 Mount Etna (Italy), 4:148 Mount Everest (Nepal), 2:142 Mount Katmai (AK), 4:30 Mount Kilimanjaro (Tanzania), 4:254 Mount Machhapuchhare (Himalayas), 4:246 Mount McKinley (AK), 4:275 Mount Pelée (Martinique) eruption (1815), 4:260 Mount Pinatubo (Philippines) eruption (1991), 4:260 Mount Saint Helens (WA), 4:30, 4:260 Mount Santo Tomas (Philippines), 4:407–408 Mount Tambora (Indonesia), 4:30 Mountain ranges, 4:256–257 Mountains, 4:253–263 Muscovite, 4:139 Music, 4:17 Natural magnets, 2:331–332 Neon Canyon (UT), 4:106 Nesosilicates (minerals), 4:135 New Guinea, 3:396, 3:398 New Madrid (MO) earthquakes, 4:235 New York City (NY), 3:247, 3:378, 3:410, 3:410 Nickel, 1:184, 1:191 Nicola River Canyon (British Columbia), 4:265 Niobium, 1:192 Noble metals, 1:122 Nonmetals, 1:213–221 Nonsilicate minerals, 4:133–135 Northeast Passage, 3:88

Novaya Zemlya (Russia), 3:88–89, 3:89 Ocean convection, 4:191 Oceans, 3:347, 3:376–377, 4:191, 4:363–364 Ogallala Aquifer, 4:373 Oil industry, 4:158–159 Ophiolites, 4:257 Ores, 4:161–162 Orphan metals, 1:156–158, 1:161, 1:222 Orphan nonmetals, 1:214, 1:219, 1:221 Osmium, 1:191 Palladium, 1:191 Petroleum, 1:368, 3:28, 3:30, 4:158–160 Petroleum industry, 1:357, 1:358 Phosphates (minerals), 4:134, 4:317 Phosphor, 2:369 Phosphorus, 1:219, 4:317–318 Photography, 4:55–56 Piedmont glaciers, 4:378 Pillars of Hercules (Mediterranean Sea), 4:5 Plate tectonics, 4:228 Platinum, 1:191–192 Plutonium, 1:201 Pohutu Geyser (New Zealand), 4:196 Polar glaciers, 4:378 Polonium, 1:226–227 Popocatepetl (volcano), 4:257 Potassium, 1:167, 1:170 Prince William Sound (AK) earthquake (1964), 4:235 Propane, 1:56 Quartz, 4:136 Radar, 4:56–57 Radium, 1:178–179 Radon, 1:239–240, 1:241 Rain shadows, 4:260 Red Sea, 4:224, 4:225 Reservoir rocks, 4:158 Rhodium, 1:191–192 River deltas, 3:351, 4:56, 4:268, 4:300 Rivers, 2:96, 2:97, 2:113, 3:376, 4:285, 4:374–375 Rocks, 4:143–153, 4:159, 4:219–220, 4:267, 4:286, 4:292 Rocks and compression, 4:14, 4:15 Rocks, dating of, 4:39, 4:99 Rocks and minerals, 4:155–158 Roden Crater (AZ), 4:16 Ronne Ice Shelf, 4:379 Ross Ice Shelf, 4:379 Royal Gorge (CO), 4:270 Rust, 1:283, 1:285, 1:294 Ruthenium, 1:192 Sabine Pass (TX), 4:160 Sahara Desert, 3:353, 4:407 Sahel region, 4:307 Salt, 1:111, 1:230, 1:230 Samarium, 1:207, 1:208–209 San Andreas Fault (CA), 4:224

S C I E N C E O F E V E RY DAY T H I N G S

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Index of Everyday Things

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San Francisco (CA) earthquake (1906), 4:235 Sandstone, 4:26 Seal rocks, 4:158 Seamounts, 4:258 Seashores, 3:377 Sedimentary rocks, 4:149–150, 4:294 Sedimentary structures, 4:288 Sediments, 4:283–291 Seismology, 4:233–237 Selenium, discovery of, 1:221 Shale, 4:159 Shansi (China) earthquake (1556), 4:237 Shield volcanoes, 4:258 Shores, sea, 3:377 Silicates, 1:224–225, 4:135–136, 4:145, 4:160–161 Silicon, 1:224–225, 1:363–364, 4:160–161 Silver, 1:187–188, 1:293 Sinkholes, 4:247 Slides, 4:266–267, 4:271–272, 4:276 Sliding friction, 2:53 Sodium, 1:166–167 Stone compression, 4:15 Stratovolcanoes, 4:258 Striations, 4:106 Strontium, 1:176–177 Subpolar glaciers, 4:378 Subsidence convection, 4:188, 4:190–191 Sulfur, 1:219, 1:221 Tambora (Indonesia) eruption (1815), 4:260 T’ang-shan (China) earthquake (1976), 4:236–237 Tectosilicates (minerals), 4:135 Tellurium, 1:226 Terbium, 1:209 Thallium, 1:158 Thorium, 1:198–199 Tidal waves, 3:185 Till (sediment), 4:380 Tin, 1:158 Titanium, 1:192 Transantarctic Mountains (Antarctica), 4:379 Transition metals, 1:136, 1:154–155, 1:181, 1:181–195, 1:182 Transuranium actinides, 1:201–204 Transuranium elements, 1:133, 1:155, 1:201 Tributary glaciers, 4:380 Tritium, 1:97, 1:253, 1:254–255 Truk Lagoon (Micronesia), 4:249 Uplift, 4:248 Upwelling regions (oceans), 3:377 Uranium, 1:199–201, 1:200 Valleys, 4:379, 4:380 Vanadium, 1:192 Vesuvius (Italy) eruptions, 4:259 Villa Luz caves (Mexico), 4:320 Volcanoes, 4:148, 4:213–214, 4:228, 4:257, 4:257–260, 4:259

Aerosol cans, 1:55, 2:187–188 Air conditioners, 2:219–220 Alkanes, 1:367–368 Alkenes, 1:368 Alkynes, 1:368 Appliances, 1:374 Bases, 1:310–318 Biodegradable plastics, 1:377 Burglar alarms, 2:335 Calendars, 2:8 Caustic soda, 1:317–318 Chimney sweeps, 3:240, 3:240 Chimneys, 2:116–117 Chlorofluorocarbons (CFCs), 1:233–234, 1:307, 2:188 Cigarette lighters, 1:206 Containers and fluid pressure, 2:142 Filaments (light bulbs), 2:361 Fire extinguishers, 1:55, 2:185, 2:187 Fluorescent bulbs, 2:367, 2:369 Freon, 1:231 Grandfather clocks, 2:267, 2:272, 2:274 Halogen lamps, 1:234 Hydrochlorofluorocarbons (HCFCs), 1:55, 2:188 Hydrogen chloride, 1:256 Hydrogen peroxide, 1:109, 1:256 Incandescent bulbs, 2:360–361, 2:367, 2:369 Index cards, 2:119 Jar lids and thermal expansion, 2:250 Lamps, 2:360–361 Lanterns, 2:360–361 Lids and thermal expansion, 2:250 Light, artificial, 2:360–361, 3:311–312, 3:312 Light bulbs, 2:360–361, 2:367, 2:369 Liquefied natural gas (LNG), 2:193, 2:200–201 Liquefied petroleum gas (LPG), 2:200–201 Liquid crystals, 1:43, 2:212, 2:214 Lye, 1:317–318 Matches, 2:54, 2:56 Mercury thermometers, 1:16, 1:189, 2:250–251 Microwave ovens, 2:348 Mirrors, 2:361 Oil lights, 2:360–361 Ovens, microwave, 2:348 Pendulum clocks, 2:267, 2:272, 2:274 Petroleum jelly, 1:366 Pewter, 1:336

VOLUME 4: REAL-LIFE EARTH SCIENCE

S C I E N C E O F E V E RY DAY T H I N G S

Volcanoes and climate, 4:30–31, 4:409–410 Western Deeps Gold Mine (South Africa), 4:212 Wind, 4:269, 4:269–270, 4:285 Yellowstone National Park (U. S.), 3:363 Zinc group metals, 1:188–190 Zirconium, 1:192

HOUSEHOLD PRODUCTS

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How to go to your page Phosphor, 2:369 Pianos, 2:273, 2:313 Picture frames, 2:136 Propane, 1:56 Propane tanks, 2:188 Propellants in aerosol cans, 1:55 Radiators, 2:248 Refrigerators, 2:219–220, 2:222, 2:229, 2:232 Septic tanks, 3:352, 4:306 Shopping carts, 2:74 Shower curtains, 2:117 Soap, 1:330, 1:334, 1:342–343 Soda cans, 1:54–55, 2:187 Thermostats, 2:251–252 Torches, 2:360 Ultraviolet lamps, 2:368–369 Velcro, 2:53 Venetian blinds, 2:164 Washing machines, 2:49 Wheelbarrows, 2:161–163 Wrenches, 2:86–89

MACHINES Machines, 2:157–169 Air conditioners, 2:219–220 Axles, 2:162–164 Bevel gears, 2:163 Block-and-tackle pulleys, 2:164 Compound levers, 2:162 Compound pulleys, 2:164 Cranes, 2:164 Crystals, 2:322–323 Electric engines, 2:90 Electric thermometers, 1:17, 2:242–244 Electrical generators, 2:341 Engine coolant, 2:248 Fax machines, 2:267–268 Flywheels, 2:55, 2:89–90 Fossil fuels, 4:201–202 Fulcrums, 2:160–162 Gears, 2:163–164 Generators, electrical, 2:341 Gyroscopes, 2:88, 2:89–90 Helical gears, 2:163 Hydraulic presses, 2:98–99, 2:142–143, 2:160 Inclined planes, 2:18, 2:71, 2:159, 2:164–165 Levers, 2:158, 2:159–164 Lubrication, 2:56 Mechanical advantage, 2:157–169 Microscopes, 3:288 Moment arm, 2:87–88, 2:160–162, 2:166 Motors, 2:27 Nanotechnology, 2:56 Pendula, 2:267–269, 2:281–282 Pendulum clocks, 2:267, 2:272, 2:274

S C I E N C E O F E V E RY DAY T H I N G S

Perpetual motion machines, 2:55, 2:158–159, 2:216, 2:223 Piezoelectric devices, 2:322–323 Pistons, 2:99, 2:167–169, 2:188–189 Pivot points, 2:86–87 Planetary gears, 2:163 Pulleys, 2:163–164 Pumps, 2:99, 2:166 Recording devices, 2:332, 2:333, 2:336–337 Screws, 2:55, 2:158, 2:165–167 Siphon hoses, 2:99 Sledges, 2:162 Springs, 2:263–264, 2:266 Spur gears, 2:163 Steam engines, 2:120, 2:163, 2:221–222, 2:231–232 Stepper motors, 2:337 Thermometers, 2:239–240, 2:249–251 Thermoscopes, 2:239 Toothed gears, 2:163 Torque converters, 2:90 Turbines, 2:101 V-belt drives, 2:163–164 Venturi tubes, 2:113 Waterwheels, 2:99–100, 2:163 Wedges, 2:165 Wheelbarrows, 2:161–163 Wheels, 2:162–164 Windmills, 2:163

Index of Everyday Things

M A N U FA C T U R I N G Alloy metals, 1:192 Alloys, 1:334, 1:336 Aluminum, 1:121, 1:152, 1:157, 1:295–296 Argon, 1:241 Atomizers and chimneys, 2:116–117 Automobile industry, 1:176 Bags, potato chip, 2:188 Brass, 1:188, 1:336 Bronze, 1:336 Carbon polymers, 1:372 Carboxylic acids, 1:315, 1:369 Catalysts, 1:304–309, 1:305 Cation resins, 1:106 Cations, 1:101, 1:103 Celluloid, 1:377 Cerium, 1:208 Cesium, 1:170 Chemical bonding, 1:112–113, 1:232, 1:244–245, 1:263–272 Cholesteric liquid crystals, 1:43, 2:213 Cigarette lighters, 1:206 Clay, 2:40–41, 4:287 Coinage metals, 1:187–188, 1:294–295 Crystal lasers, 2:369

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Dry ice, 1:248, 4:402 Dye lasers, 2:361–362 Elastomers, 2:152 Industrial distillation, 1:357–358 Iron ore, 1:190 Liquid crystals, 1:43, 2:212, 2:214 Magnetic metals, 1:190–191, 2:331–332 Mills, 2:163 Misch metal, 1:206, 1:208 Neoprene, 1:378 Nylon, 1:282, 1:366, 1:378 Petrochemicals, 1:257, 1:367–368, 4:160 Petroleum industry, 1:357, 1:358 Pewter, 1:336 Plastics, 1:364, 1:365, 1:366–367, 1:375–380 Polyester, 1:378–380 Polymers, 1:372–380, 1:373, 1:379, 2:152, 3:13, 3:18–19 Potash, 1:167, 1:170 Potato chip bags, 2:188 Raytheon Manufacturing Company, 2:348 Rubber, 2:151, 2:152, 2:267 Ruby lasers, 2:369 Silicon, 1:244, 1:372 Silicon wafers, 1:223 Silicones, 1:225, 4:161 Soda cans, 1:54–55, 2:187 Synthetic polymers, 1:374, 1:375, 1:377–379 Synthetic rubber, 1:377–378 Tungsten, 1:192

Accidents, 3:231 Acid reflux, 3:48 Acne, 3:286 Adrenaline, 3:267–268 African AIDS epidemic, 3:250 African trypanosomiasis, 3:276 AIDS (Acquired immunodeficiency syndrome), 3:245, 3:250, 3:258–261, 3:259 Air pressure, 2:145–146 Alcohol, 3:59, 3:240 Allergies, 3:60, 3:264–268, 3:265, 3:364–365 Alternative medicine, 3:239 Aluminum, 1:124–125 Alzheimer’s disease, 3:233, 3:233–235 American diet, 3:49–50, 3:79 American Dietetic Association (ADA), 3:96 Amino acids, 3:11–17 Amniocentesis, 3:156 Amoebic dysentery, 3:276 Amoxicillin, 3:290–291 Anaerobic bacteria, 3:58 Anaerobic respiration, 3:58–59

Anaphylactic shock, 3:265, 3:267–268 Anemias, 3:14, 3:15–16, 3:268–269 Anesthetics, 3:154 Anorexia nervosa, 3:38–39 Antacids, 1:322–323, 3:48 Anthrax, 3:250, 3:251–252 Antibiotics, 3:13, 3:165, 3:290–291 Antibodies, 3:122–123, 3:263–264 Antihistamines, 3:267 Antioxidants, 1:294, 3:91 Apnea, 3:310 Arsenic, 3:78 Asbestos, 4:139 Atherosclerosis, 3:36 Attention (brain function), 3:307 Autoimmune diseases, 3:230, 3:242, 3:268–269 B vitamins, 3:92 Bacteria, 3:165, 3:284–285 Bacterial infection, 3:284–286 Bacterial ulcers, 3:48–49, 3:49 Basilar membrane, 2:318 Beriberi, 3:92–95 Beta-carotene, 3:90 Bile, 3:47 Bilirubin, 3:51 Bingeing, 3:39 Biological rhythms, 3:306–315 Biorhythms (1970’s fad), 3:315 Biotechnology, 3:119 Bipolar disorder and lithium treatment, 1:165 Blood, 3:56–57 Blood components, 3:19, 3:21, 3:47, 3:56, 3:263–264 Blood infections, 3:276–277 Blood pressure, 2:146–147, 2:305 Blood vessels, 3:36 Bone marrow, 3:263, 3:263 Bones, 2:152 Botulism, 3:252 Bowel movements, 3:50 Brain, 2:318, 3:168, 3:299, 3:306–307 Brain diseases, 3:232–235, 3:233, 3:302 Breast-feeding, 3:73 Breathing, 2:145–146, 3:56–58 Bronchial disorders, 3:60 Bulimia, 3:38, 3:38–39 Calcium, 1:176, 3:81, 3:91 Calcium carbonate, 3:377, 4:327, 4:327 Cancer, 3:21, 3:62, 3:132, 3:230, 3:238–241 Cancer biopsy, 3:239 Cancer treatments, 3:21, 3:239 Carbohydrates, 3:3–10, 3:44, 3:79, 3:80–82 Carbon, 1:123, 3:78 Carbon dioxide, 3:55–56 Carcinogens, 3:238 Cardiovascular disease, 3:230, 3:232 Catabolism, 3:33, 3:34–35

VOLUME 4: REAL-LIFE EARTH SCIENCE

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MEDICINE

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How to go to your page CCK (Cholecystokinin), 3:10 Cellulose, 3:8 Centrifuges, 2:48–49 Cervix, 3:153 Cesarean section, 3:156–157 Chemotherapy, 3:239 Childbirth, 3:151–157, 3:154 Children, 3:258, 3:301, 3:334 Cholera, 3:247 Cholesterol, 3:36, 3:36–37 Chromium, 3:78 Chromosomes, 3:99–100, 3:102, 3:111–112, 3:117, 3:126, 3:135 Chromosomes and DNA, 3:99–100, 3:135 Circadian rhythms, 3:306, 3:307–312, 4:83–84 Citric acid, 1:312, 1:315 Cleft palate, 3:128 Cloning, 3:121, 3:122–123 Cochlea, 2:317–318 Coenzymes, 3:25–26 Cold, common, 3:60, 3:286–287, 3:301–302 Colloids, 1:42, 1:69, 1:332–333, 2:210–211 Color perception, 2:359–360 Congenital disorders, 3:128–131, 3:129, 3:155–156, 3:232 Copper trace elements, 3:78 Cousins, 3:115–116 Cowpox, 3:256–257, 3:257 Creutzfeldt-Jakob disease, 3:128, 3:232–233 Cystic fibrosis, 3:62, 3:128 Dehydration, 1:350 Delayed sleep phase syndrome, 3:311 Dental health, 1:233 Designer proteins, 3:21–22 Dextrose, 3:3–4 Diabetes mellitus, 3:242–243 Diaphragms (anatomy), 2:315, 2:320–321 Diastolic pressure, 2:147 Digestion, 3:44–54 Digestive disorders, 3:48–50 Digestive system, 3:44–54, 3:46, 3:51, 3:276, 3:278, 3:285–286 Diploid cells, 3:99 Disaccharides, 3:4 Disease risk factors, 3:239–241 Diseases, 3:229–235 Diseases of ethnic groups, 3:113, 3:127, 3:131, 4:344–345 Diseases, social impact, 3:230, 3:246–248 Diseases, unknown causes, 3:230, 3:232–235, 3:233 DNA evidence, 3:108 DNA in genes, 3:100–101, 3:117–119, 3:118, 3:126, 3:164–165 Dominant genes, 3:112–113 Doping agents, 1:226 Down syndrome, 3:126, 3:129, 3:129

S C I E N C E O F E V E RY DAY T H I N G S

Drugs, 3:120, 3:154, 3:242–243, 3:302, 3:307, 3:315 Duodenum, 3:47, 3:48 Dwarfism, 3:129 Ear infections, 3:290 Eardrums, 2:317 Ears, 2:316–318 Eating disorders, 3:38, 3:38–40 Eating habits, 3:39, 3:48, 3:49, 3:242, 3:311 Ebola virus, 3:250–251 Elephantiasis, 3:278, 3:279 Embryos, 3:152–153 Endogenous infection, 3:283 Enterobiasis, 3:278 Enzymes, 1:306–307, 3:21, 3:24–30, 3:37, 3:119 Epinephrine, 3:267–268 Epsom salts, 1:175 Escherichia coli bacteria, 3:51, 3:285–286 Esophagus, 3:45 Eustachian tubes, 2:317 Exhalation (human breathing), 3:55–56 Exogenous infection, 3:283 Eye diseases, 3:279 Eyes, 2:359–360 Facial characteristics, 3:129, 3:129–130 Fat, human body, 3:10, 3:35–37, 3:36, 3:88, 3:127, 3:146, 3:147, 3:307 Feces, human, 3:50–54 Flatulence, 3:54 Fluoride, 1:233 Fluorine, 1:232–234, 1:269 Forceps, 3:153–154 Fruits and vegetables, 3:93 Genetic disorders, 3:113, 3:113–116, 3:121, 3:127 Genitals, human, 3:142–143, 3:238–239, 3:240 Germs, 3:244, 3:283–284, 3:288, 3:290, 3:396 Goiters, 1:123 Hartnup disease, 3:15 Hearing, 2:318 Hearts, 2:324 Hemophilia, 3:115–116, 3:241–242 Histamines, 3:266–267 HIV (human immunodeficiency virus), 3:250, 3:258–261, 3:259, 3:259 Hospitals, 3:154 Human eggs, 3:143 Huntington disease, 3:128 Hydrochloric acid, 1:231, 1:256 Hydrogen sulfide, 3:54 Hypersomnia, 3:310 Immune system, 3:262–269 Immunotherapy, 3:239 In vitro fertilization, 3:144, 3:144 Indigestion, 3:48 Infection, 3:283–291 Infectious diseases, 3:230, 3:240, 3:244–252, 3:264, 3:278, 3:396

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Influenza, 3:60, 3:249–250, 3:287 Inner ear, 2:317–318 Insulin, 3:120, 3:242–243 Intestinal gas, 3:54 Iodine, 1:123, 1:234, 1:236 Iron lungs, 3:287 Jet lag, 3:310–311 Kaposi’s sarcoma, 3:258, 3:259 Ketoacidosis, 3:243 Ketones, 1:369 Kidney dialysis, 1:350, 1:351 Kleine-Levin syndrome, 3:310 Kwashiorkor, 3:85 Labor (birth), 3:153 Lactic acid, 3:59 Lactose intolerance, 3:27 Large intestine, 3:47 Leprosy, 3:248–249, 3:249 Lipids, 3:19, 3:35, 3:44–45, 3:80 Lithium, 1:165 Liver, human, 3:79, 3:91–92 Loa loa, 3:279 Low-density lipoproteins, 3:36–37 LSD (lysergic acid diethylamide), 3:307 Lung cancer, 3:62 Lungs, 3:56–58, 3:59 Lupus (systemic lupus erythematosus), 3:268 Lymph nodes, 3:263 Lymphocytes, 3:255, 3:263, 3:263–264 Mad cow disease, 3:233 Malaria, 3:249, 3:276–277 Male reproductive system, 3:142–143 Malnutrition, 3:82–86, 3:89 Manic depression, 1:165 Marfan syndrome, 3:114–115 Marrow (bone), 3:263, 3:263 Melanin, 3:130–131, 3:173 Melatonin, 3:307, 3:314–315 Menstruation, 3:286, 3:313 Mental development, 3:334 Mercury thermometers, 1:16, 1:189, 2:250–251 Metabolic enzymes, 3:27 Metabolism, 3:33–43 Microscopes, 3:288 Middle ear, 2:317 Midgets, 3:129 Midwives, 3:153–154 Miscarriage, 3:152–153 Mothers and babies, 3:73 Narcolepsy, 3:309 Native Americans, 4:344–345 Nervous system, 3:295–297, 3:296 Neurological disorders, 3:302 Niacin, 3:15, 3:95 Noninfectious diseases, 3:236–243 Noses, 2:217

Nursing mothers, 3:73 Obstetricians, 3:154 Organ transplants, 3:262–263 Osteomalacia, 3:91 Overweight Americans, 3:10, 3:37, 3:81, 3:82, 3:85–86 Oxytocin, 3:153 Pancreas, human, 3:47 Parasites, 3:275–282 Pathogens, 3:245, 3:255, 3:262, 3:285–286 Pellagra, 3:15, 3:95 Penicillin, 3:290 Peptide linkage, 3:13, 3:18 Pheromones, 3:303–304 Phosphorus, 3:91 Physicians, 3:153–154, 3:238–239 Pineal gland, 3:306–307 Placenta, 3:153 Plagues, 1:360, 3:230, 3:231, 3:246–248 Plasma, 1:343–344, 2:14, 2:210, 3:47 Pneumonia, 3:60, 3:62, 3:287 Poliomyelitis, 3:257–258, 3:287 Pollution, 3:240–241 Pregnancy, 2:324, 3:151–157 Proteins, 3:10, 3:13, 3:18–19, 3:79–82 Rabies, 3:257 Recommended daily allowances (RDA), 1:125–126 Red blood cells, 1:351, 3:263, 3:276–277 Respiratory disorders, 3:60, 3:62 Retroviruses, 3:287 Rickets, 3:90, 3:91 Scurvy, 3:93 Senses, 2:318, 3:295–305 Sickle-cell anemia, 3:14, 3:15–16 Silicone implants, 4:161, 4:161 Skin, 3:130, 3:244–245, 3:248, 3:262 Sleep cycles, 3:310 Sleep disorders, 3:309–311 Smallpox, 3:230–231, 3:252, 3:256, 3:257, 3:396 Soot, 3:173, 3:240, 3:240 Sperm cells, 3:100, 3:132, 3:136, 3:142–143 Starches, 3:7 Stethoscopes, 2:146–147 Stomach, human, 3:46–48, 3:49 Sugars, 3:9–10, 3:19, 3:34 Sulfa drugs, 3:3–4, 3:290 Sunlight, 3:91 Surgical silicone implants, 4:161 Taste, 3:301 Taste buds, 3:298–300 Thermometers, 1:14, 1:16–17, 2:238, 2:241–244 Thiamine (vitamin B1), 3:92, 3:95 Thymus gland, 3:263 Tobacco use, 3:240, 3:301–302 Tongue (human), 3:299–301 Trace elements, 1:123–126, 3:78 Trichinosis, 3:278

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How to go to your page Tuberculosis, 3:60, 3:248–249 Tumors, 3:238 Twins, 3:115 Typhoid fever, 3:251 Ulcers, 3:48–49, 3:49 Ultrasonics, 2:324 Undercooked meat, 3:277–278, 3:303 Urine, 1:200 Uterus, 3:152–153 Vaccinations, 3:258 Vegetables, 3:93 Viruses, 3:60, 3:250–251, 3:259–261, 3:285–287 Vitamin A, 3:80, 3:88–90 Vitamin B1 (thiamine), 3:92, 3:95 Vitamin B2, 3:92 Vitamin B6, 3:92 Vitamin B12, 3:92, 3:268–269 Vitamin C, 3:92, 3:93 Vitamin D, 3:90, 3:90–91 Vitamin E, 3:91 Vitamin K, 3:91–92 Vitamins, 3:45, 3:80–81, 3:87–96 X-rays, 2:298, 2:350, 2:352–353 Zinc, 1:154, 1:188–189

M I L I TA R Y Afghanistan, 4:263 Biological warfare, 3:251–252 Cruise missiles, 2:82–83 Eavesdropping devices, 2:325 Explosives, 3:351–352, 4:333 Guided missiles, 2:82–85 Gunpowder, 1:292 Hiroshima bombardment (1945), 1:72 Incendiary devices, 1:175–176 Kevlar, 1:375 Landsat (satellite program), 4:57, 4:59 Lasers, 2:361–362 Listening devices, 2:322 Manhattan Project, 1:72, 1:97, 1:199–200, 2:177 Mountain warfare, 4:263 Nagasaki bombardment (1945), 1:72, 3:104 Nuclear bombs, 1:72, 1:292–293 Radar ranges, 2:348 Remote sensing, 4:53–54 Rockets, 2:31, 2:33, 2:39 Sirenians, 3:221–222 Strategic Defense Initiative (SDI), 2:178–179 Sumer, 2:162

MUSIC Music, 2:274, 2:276 Acoustics, 2:311–314

S C I E N C E O F E V E RY DAY T H I N G S

Amplification, 2:315 Cassette tapes, 2:336–337 Decibels, 2:314 Electromagnetic sound devices, 2:336 Intervals, 2:274, 2:276 Magnetic recording devices, 2:332, 2:333, 2:336–337 Magnetic sound devices, 2:336 Magnetic tape, 2:336–337 Metronomes, 2:267, 2:274 Microphones, 2:336 Middle C, 2:273, 2:274 Musical instruments, 2:314 Octaves, 2:274, 2:276 Pianos, 2:273, 2:313 Recording devices, 2:332, 2:333, 2:336–337 Sound production, 2:314–315 Sound reception, 2:316–318 Tuning forks, 2:287, 2:289–290

Index of Everyday Things

N A V I G AT I O N Navigation, 2:334 Animals, 3:335–341 Cartography, 4:45, 4:46 Compasses, magnetic, 2:334–335, 4:180 Geography, 4:44–52 Global positioning system, 4:52, 4:54 Greenwich meridian, 2:5 Inertial navigation systems, 2:64 Latitude and longitude, 2:7–8 Lighthouses, 2:297 Longitude, 2:5, 4:51–52 Magnetic compasses, 2:334–335, 4:180 Magnetic fields, 2:332, 4:178 Maps, 4:44–52 Mercator projections, 4:46 Prime meridian, 2:5 Water clocks, 2:100–101, 2:163

SPORTS AND HOBBIES Aqualungs, 2:124 Baseball, 2:40, 2:44, 2:80–82, 2:117–119 Basketball, 3:261 Bicycle aerodynamics, 2:109 Bicycles, 2:109 Billiard balls, 2:38, 2:40–41 Boomerangs, 2:107, 2:115–116 Bouncing balls and energy, 2:175–176 Bungee jumping, 2:265, 2:267 Centripetal force, 2:45–50, 2:46 Cheerleaders, 2:140, 2:141 Curve balls, 2:80–82, 2:117–119 Decompression, 1:50, 2:123–124 Delta wing kites, 2:108

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Diving bells, 2:123 Fireworks, 1:174 Fish finders, 2:321 Fishing rods, 2:161–162 Fishing sonar, 2:321 Frisbees, 2:33 Golf, 2:81, 2:82 Handlebars (bicycles), 2:109 Helmets, bicycle, 2:109 Hockey, 2:54–55 Human cannonballs, 2:70 Ice fishing, 2:247 Ice skating, 2:27, 2:33 Karate chops, 2:140 Kite aerodynamics, 2:107–108 Kites, 2:107–108, 2:114, 2:116 Knuckle balls, 2:81–82 Mexico City Olympics (1968), 2:146 Mountain climbing, 1:298 Olympics (Mexico City, 1968), 2:146 Optical illusions, 2:359 Paper airplanes, 2:108 Parafoils, 2:108 Projectile motion, 2:78–85 Racing cars, 2:109–110 Rapture of the deep, 2:123–124 Rockets, 2:31, 2:33, 2:82–85 Roller coasters, 2:46–47, 2:47, 2:49–50 Sand castles, 4:265–266, 4:274 Scuba diving, 1:50, 1:54, 1:242, 2:124 Seesaws, 2:86–89 Self-contained underwater breathing apparatus, 2:124 Skating, 2:27, 2:29, 2:33 Skis, 2:140–141 Skydiving, 2:39, 2:43–44 Sledges, 2:162 Snowballs, 2:228 Snowshoes, 2:140–141 Sonar fishing, 2:321 Spinning tops, 2:33 Swimmers, 2:122 Swings, 2:263–264, 2:279, 2:281 Trampolines, 2:264 Underwater diving, 1:50, 1:54, 1:242, 2:124 Water balloons, 2:44 Weightlifting, 3:308 Wheels, bicycle, 2:109 Wings, 2:108 Yachts, 4:27–28

T R A N S P O R TAT I O N

Carbon polymers, 1:372 Cellulose, 3:7–8 Cotton, 1:373

Airplanes, 2:128 Airports, 3:312 Axles, 2:162–164 Ballast, 2:122 Bicycles, 2:109 Boat wakes, 2:288 Car aerodynamics, 2:109–111 Carburetors, 2:117 Carriages, horsedrawn, 2:163 Cartography, 4:45, 4:46 Challenger space shuttle explosion (1986), 1:257, 1:259, 1:294 Coaches, horsedrawn, 2:163 Displacement of ships, 2:122 Doppler radar, 2:303, 2:305, 4:403–404 Highways, 3:380 Interstate-285 (Atlanta, GA), 2:45–46 Lighthouses, 2:297 Liquefied natural gas (LNG), 2:200–201 Liquefied petroleum gas (LPG), 2:200–201 Mach numbers, 2:106–107, 2:305 MAGLEV trains, 2:337–339 Magnetic compasses, 2:334–335, 4:180 Magnetic levitation trains, 2:337–339 Northeast Passage, 3:88 Propellers, 2:106, 2:116, 2:166–167 Radar, 2:348–349 Railroad tracks, 2:246, 2:250 Roads, 2:48 Sahara Desert, 4:307 Sailors, 3:93 Salt caravans, 1:168 Ships, 2:22, 2:24–25, 2:121–122, 2:137, 2:144 Shock absorbers, 2:266–267 Sonar, 2:320 Space shuttles, 2:60, 2:85 Spokes (wheels), 2:162 Stagecoaches, 2:163 Steamships, 2:121–122 Submarines, 2:122–123, 2:320, 2:325 Titanic (ship), 2:125 Trains and magnetic levitation, 2:337–339 Trieste (bathyscaphe), 2:124–125 Trucks, 2:110–111 Unmanned underwater vessels, 2:124–125 Wakes, boat, 2:288, 2:289 Whirlpools, 4:363 Yachts, 4:27–28

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Indigo, 2:355 Nylon, 1:282, 1:366, 1:378 Polyester, 1:378–380

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How to go to your page WEAPONS Biological warfare, 3:251–252 Bombs, 1:71–72, 1:93, 2:230–231 Bullets, 1:375, 2:79, 2:80 Cruise missiles, 2:82–83 Crystal lasers, 2:369 Guided missles, 2:82–85 Gunpowder, 1:292 Heavy water, 1:96 Hiroshima bombardment (1945), 1:72 Hydrogen bombs, 1:93, 1:97, 1:255, 2:177–179 Incendiary devices, 1:175–176 Intercontinental ballistic missiles (ICBMs), 2:82 Manhattan Project, 1:72, 1:97, 1:199–200, 2:177 Muskets, 2:80 Nuclear weapons, 1:72, 1:98, 1:292–293, 2:177–179, 3:103, 3:104 Nuclear weapons testing, 1:177, 4:234 Patriot missiles, 2:83 Projectile motion, 2:78–80, 2:81–82 Rifles, 2:28, 2:31, 2:38, 2:80 Rockets, 2:31, 2:33, 2:82–85, 2:85 Stinger missiles, 2:83 Surface-to-air missiles, 2:83 V-2 rockets, 2:82 Wars, 1:71–72, 1:93, 1:175–176, 1:231

W E AT H E R Weather, 4:395–404 Acid rain, 4:318, 4:321–322 Air-mass storms, 4:398–399 Air pressure, 2:183, 4:396–398, 4:399 Altocumulus clouds, 4:391, 4:391 Altostratus clouds, 4:391 Atmosphere, 4:396–397, 4:405–407 Atmospheric pressure, 1:39, 2:239, 2:241 Atmospheric water, 4:387–394 Aurora borealis, 4:79, 4:79–80 Barometers, 1:50–51, 2:141–142 Biological weathering, 4:265, 4:274 Central Park (New York, NY) microclimate, 4:410 Cirrus, cirrocumulus, and cirrostratus clouds, 4:391 Climate, 3:353, 4:260, 4:405–407, 4:405–412, 4:407, 4:409–411 Climate changes, 4:410–411 Climatology, 4:407 Cloud seeding, 4:404 Clouds, 4:390–392, 4:391, 4:391–392, 4:404 Cold climate, 3:374, 3:394, 3:394–395 Condensation, 2:209–210, 4:372 Corrosion, 1:294 Crosscurrent exchange, 3:58 Cumulonimbus clouds, 4:187, 4:187–188, 4:391–392 Cumulous clouds, 4:391

S C I E N C E O F E V E RY DAY T H I N G S

Cyclones, 4:400–402, 4:401 Desertification, 4:309–310 Doppler radar, 2:303, 2:305, 4:403–404 Drafts (air currents), 2:98 Drizzle, 4:392 Drought, 4:287 Dry ice, 1:248, 4:402 Dust bowls, 3:352, 4:270, 4:286, 4:303, 4:304–305 Dust devils, 4:269 El Niño consequences, 4:28, 4:30 Electric thermometers, 1:17, 2:242–244 Erosion, 4:284 Evapotranspiration, 4:390 Greenhouse effect, 4:355–356 Gulf Stream, 4:364 Hail, 4:392, 4:399, 4:399 Heaviside layer, 2:346 High-level clouds, 4:391 Hurricane Andrew (1989), 4:400–401 Hurricane Hugo (1989), 4:400 Ice ages, 4:381–384, 4:411–412 Ice caps, 4:379–380 Ice-core samples, 4:379 Kennelly-Heaviside layer, 2:346 Krakatau (Indonesia), 4:31 Lightning, 4:398 Litmus paper, 1:314 Little Ice Age (1250-1850), 4:384 Low-level clouds, 4:391 Mercury barometers, 2:141 Mercury thermometers, 1:16, 1:189, 2:250–251 Meteorological radar, 2:303, 2:305 Meteorology, 4:402–404, 4:406–407 Microclimates, 4:407, 4:407–409, 4:410 Mid-level clouds, 4:391 Mirages, 2:359 Mount Pinatubo eruption (1991), 4:409–410 Mountain rain shadows, 4:260 National Weather Service, 4:402–403 New York (NY) microclimate, 4:410, 4:410 Nimbostratus clouds, 4:391–392 Northern latitudes, 3:310 Ocean currents, 4:363–364 Philippine microclimate, 4:407–408 Precipitation, 3:358, 3:374–375, 4:372, 4:387–394 Rain, 4:279, 4:392 Rain clouds, 4:391–392 Rain shadows, 4:260 Rainbows, 2:358–359, 4:78 Seasons, 3:313–315 Sky, 2:358 Sleet, 4:392 Snow, 4:377, 4:392 Snowflakes, 4:392 Solar wind, 4:80 Stratocumulus clouds, 4:391

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Stratosphere, 2:127 Stratus clouds, 4:391 Sulfur, 4:318, 4:321–322 Sun, 4:384, 4:396, 4:409–410 Thermometers, 1:14, 1:16–17, 2:238, 2:241–244 Thunderstorms, 4:187, 4:187–188, 4:391–392, 4:398–399 Tides, 4:84, 4:364 Tornado Alley (U.S.), 4:400

VOLUME 4: REAL-LIFE EARTH SCIENCE

Tornadoes, 4:399–400 Trees, 3:362 Troposphere, 4:405–406 Van Allen belts, 4:80 Volcanoes, 4:30–31, 4:409–410 Water, 3:347–348, 3:358, 4:387–394 Whirlpools, 4:363 Wind, 4:188, 4:396–398

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C U M U L AT I V E G E N E R A L SUBJECT INDEX This index contains items from volumes 1-4 of the series. Boldface type indicates main entry volume and page numbers. Italic type indicates photo and illustration volume and page numbers.

A Aardvarks, 3:222 Abegg, Richard, 1:103–104, 1:113, 1:268, 4:131 Abney, William, 2:349–350 Aborigines, Australian, 2:107 Abrasive minerals, 4:137 Absolute dating amino acid racimization, 3:17, 4:96–97, 4:119–120 carbon ratio dating, 1:98, 1:250–251, 4:97–98, 4:120 potassium-argon dating, 1:238, 4:98 uranium series dating, 3:172, 4:98 Absolute temperature scale. See Kelvin temperature scale Absolute zero Carnot’s engine, 2:222 heat engines, 2:232 Kelvin scale, 2:241 solids, 2:208 third law of thermodynamics, 2:223, 4:195 Absorption spectrums, 2:366–367 Acceleration centripetal force, 2:46–47 gravity, 2:19, 2:71 laws of motion, 2:18–20 roller coasters, 2:49–50 second law of motion, 2:65, 4:171 speed and velocity, 2:17–18 Accelerometers, 2:46 Accidents as cause of death, 3:231 Acheson, Edward G., 4:137 Acid-base reactions, 1:285, 1:319–326, 1:324t–325t Acid rain, 4:318, 4:321–322 Acid reflux, 3:48 Acids, 1:278, 1:310–318, 1:312, 1:316t–317t Acne, 3:286 Acoustics, 2:311–318, 2:316t–317t classical physics, 2:20 diffraction, 2:294–295

S C I E N C E O F E V E RY DAY T H I N G S

Doppler effect, 2:303–305 interference, 2:289–290 properties, 2:256 resonance, 2:283 ultrasonics, 2:319–321 See also Sound; Sound waves Acquired characteristics, 3:165 Actinides, 1:156, 1:196–204, 1:202t–203t electron configuration, 1:136 orbital patterns, 1:146, 1:186 Actinium, 1:198 Active sites (enzymes), 3:25 ADA (American Dietetic Association), 3:96 Adenosine diphosphate (ADP), 3:34 Adenosine triphosphate (ATP), 3:34, 3:56 Adrenaline, 3:267–268 Advanced Cell Technology (Worcester, MA), 3:123 Adventures of Huckleberry Finn (Twain), 4:64 Aerial photography Colorado River delta, 4:56 geography, 4:47–48 history, 4:53–54 Aerobic decay, 1:360 Aerodynamics, 2:102–111, 2:110t Bernoulli’s principle, 2:98 bullets, 2:79 See also Air resistance; Airflow; Dynamics Aeronautics industry, 1:172 Aerosol cans, 1:55, 2:187–188 Aerostat blimp, 2:127 Afghanistan and mountain warfare, 4:263 Africa and AIDS epidemic, 3:250 African black rhinoceros, 3:386, 3:387 African trypanosomiasis, 3:276 Agricola, Georgius economic geology, 4:154–155 mineral classification, 4:133 mineralogy, 4:39

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Agriculture biomes, 3:380 bred crop species, 3:387 ecosystems, 4:343–344 flooding, 4:365–366 history, 3:395–396 nitrogen cycle, 4:338 slash-and-burn, 3:350 soil, 4:300 soil conservation, 3:352, 4:304–305 See also Crops AIDS (Acquired immunodeficiency syndrome), 3:245, 3:250, 3:258–261, 3:259 Air conditioners, 2:219–220 Air-mass storms, 4:398–399 Air pressure fluid pressure, 2:142 human body, 2:145–146 measurement, 2:183 Mount Everest, 2:142 tornadoes, 4:399 weather, 4:396–398 Air resistance conservation of angular momentum, 2:27 gravity and acceleration, 2:20, 2:74–75 projectile motion, 2:78–80 See also Friction; Terminal velocity Airbags, 1:56, 1:58–59 gas laws, 2:189–190 linear momentum, 2:43 Airflow, 2:144 See also Aerodynamics; Laminar flow Airfoils airplanes, 2:105–107 Bernoulli’s principle, 2:97–98, 2:116 fluid pressure, 2:144 Airplanes air pressure, 2:146 Bernoulli’s principle, 2:116 center of gravity, 2:137 flight, 2:105–107 jet lag, 3:310–311 paper airplanes, 2:108 transportation, 2:128 Airports, 3:312 Airships, 1:254, 1:257, 2:126–129 See also Balloons; Blimps; Dirigibles; Hot-air balloons Alaska earthquakes, 4:235 ecosystems, 3:404, 3:405 Albatrosses, 3:338 Albertus Magnus, 3:196 Albinism, 3:130, 3:130–131 Alchemy, 1:112, 1:224 Alcohol, 1:369

cancer, 3:240 distillation, 1:356–357 fermentation process, 3:27–28 thermometric medium, 2:242 use and abuse, 3:59 Aldehydes, 1:369 Aldrin, Buzz, 1:24 Alexis (son of Czar Nicholas II), 3:242 Alfred A. Murrah Federal Building bombing (1995), 4:333 Algorithms, 3:193 Al-hasen (Arab physicist), 2:342, 2:354 Alien (motion picture), 2:315–316 Aligheri, Dante, 4:215 Alkali metals, 1:153, 1:162–170, 1:169t Alkalies, 1:310–311, 1:319 Alkaline earth metals, 1:153–154, 1:171–180, 1:174, 1:178t–179t Alkanes, 1:367–368 Alkenes, 1:368 Alkynes, 1:368 Alleles, 3:112 Allergies, 3:60, 3:264–268, 3:265, 3:364–365 Allopatric species, 3:217 Allotropes, carbon, 1:245–247 Alloy metals, 1:192 Alloys, 1:334, 1:336 Alluvial soil, 3:351 Alpha-fetoprotein screening, 3:156 Alpine glaciers, 4:378 Alternative energy sources, 4:202, 4:206 Alternative treatments for cancer, 3:239 Altitude and forest ecology, 3:362–363 See also Tree lines Altocumulus clouds, 4:391, 4:391 Altostratus clouds, 4:391 Aluminum, 1:152, 1:157 atomic structure, 1:101 covering of World Trade Center (New York, NY), 1:153 human health, 1:124–125 ions and ionization, 1:121 oxidation-reduction reactions, 1:295–296 Aluminum hydroxide, 1:315 Alzheimer’s disease, 3:233, 3:233–235 AM radio broadcasts, 2:276–277, 2:345–346 Amazon River valley (Brazil) deforestation, 3:365, 3:366, 4:57, 4:59 American chestnut trees, 3:394 American diet, 3:49–50, 3:79 overweight Americans, 3:10, 3:37, 3:81, 3:82, 3:85–86 recommended daily allowances (RDA), 1:125–126 American Dietetic Association (ADA), 3:96 American elm, 3:210

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How to go to your page Americium, 1:201–202 Amino acid racimization, 3:17, 4:96–97, 4:119–120 Amino acids, 3:11–17, 3:16t enzymes, 3:24 proteins, 3:18–19, 3:79–80 racimization, 4:96–97, 4:119–120 Ammonia in fertilizers, 3:351 nitrogen cycle, 4:335 Ammonification, 4:338 Ammonium, 4:335 Ammonium nitrate, 3:351–352, 4:333 Amniocentesis, 3:156 Amoebic dysentery, 3:276 Amorphous carbon, 1:246–247, 4:326 Amorphous matter, 2:210 Amoxicillin, 3:290–291 Ampère, André Marie, 2:341, 4:178 Amplification, electronic, 2:315 Amplitude acoustics, 2:313–314 Doppler effect, 2:301–302 electromagnetism, 2:344 frequency, 2:273 modulation of radio waves, 2:260–261 oscillations, 2:264 resonance, 2:279 ultrasonics, 2:320 waves, 2:257 Anabolism, 3:33, 3:34–35 Anaerobic decay, 1:360 Anaerobic respiration, 3:58–59 Anaphylactic shock, 3:265, 3:267–268 Ancestral record and evolution, 3:170–171 Andes Mountains (Peru), 2:146 Andrews, Roy Chapman, 4:17 Anemia, 3:268–269 Anesthetics and childbirth, 3:154 Angiosperms, 3:173–174, 3:362, 3:364, 3:364–365 gymnosperms vs., 4:347–349 reproduction, 3:138, 3:140–141 Angle of attack, 2:106 Angle of repose, 4:265–266, 4:274 Angstroms in atomic measurements, 1:76–77 Angular momentum conservation, 2:27, 2:30, 2:33 projectile motion, 2:80 torque, 2:89 See also Conservation of angular momentum; Momentum Angular unconformities (geography), 4:112–113 Animals behavior, 3:321–326 biomes, 3:375 carbon dioxide, 4:327 chemoreception, 3:297–298

S C I E N C E O F E V E RY DAY T H I N G S

cloning, 3:123 domestication, 4:343–344 evidence of evolution, 3:169–171 hibernation, 3:40–43 human intelligence vs., 3:167–168 instinct and learning, 3:327–334 kingdom animalia, 3:198, 3:205, 3:206 mating rituals, 3:145–147 migration, 3:338–341 natural selection, 3:163–165 pregnancy and birth, 3:151–153 presence of proteins, 3:20–21, 3:23 respiration, 3:58, 4:329–330 selective breeding, 3:128, 3:169 sense of smell, 3:303 shedding (fur or skin), 3:313, 3:313 symbiotic relationships, 3:386–387 transpiration, 3:355, 3:358, 4:389–390 ultrasonics, 2:323–324 See also Mammals; specific species Anion resins, 1:106 Anions, 1:101–102 aluminum, 1:121 naming system, 1:277 nonmetals, 1:103 Annelids asexual reproduction, 3:138 detritivores, 3:349 respiration, 3:57, 3:57 Anorexia nervosa, 3:38–39 Antacids, 1:322–323, 3:48 Antarctica, 4:377 glaciology, 4:378–380 midnight sun, 3:310 Anteaters, 3:223, 3:223–224 Anthrax, 3:250, 3:251–252 Anthropogenic biomes, 3:378–380 Anthropoidea, 3:220–221, 3:274–275 Antibiotics amino acids in, 3:13 bacterial resistance to, 3:165, 3:290–291 discovery, 3:290 Antibodies, 3:122–123, 3:263–264 Antiferromagnetism, 2:332, 2:334 See also Magnetism Antihistamines, 3:267 Antimony, 1:226 Antioxidants, 1:294, 3:91 Antipater of Thessalonica, 2:163 Ants, 3:349, 3:349–350, 3:386, 3:388 Apatosaurus, 3:184 Aphids, 3:388 Apis mellifera scutellata, 3:211 Apnea, 3:310 Apple tree stomata, 4:388 Appliances, 1:374

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Aqualung, 2:124 Aquatic animals behavior, 3:321 bioaccumulation, 3:72–73 echolocation, 3:340–341 endangered species, 3:208 evolutionary history, 3:195, 3:204–205, 3:221–222 food webs, 3:77, 3:376–377 migration, 3:336 mussels, 3:211 reproduction, 3:143–144 respiration, 3:57 taxonomy, 3:199 See also Fish; Oceans Aquatic biomes, 3:370 Aqueducts, 4:91–92 Aqueous solutions, 1:284, 1:320, 1:343–344 Aquifers, 3:347 Arabic language in chemical symbols, 1:133 Arabic numerals, 1:4 Arachnida (class), 3:275 Archaean eon, 4:116 Archaeological geology, 4:19 Arches, 4:14, 4:15 Archimedes (scientist) buoyancy, 2:24, 2:96–97, 2:120 fluid pressure, 2:144 machines, 2:158 pulleys, 2:164 Archimedes screws, 2:166 Archimedes (ship), 2:166 Archimedes’s principle, 2:120 Arco Sag River (ship), 2:22 Arctic fox, 3:394 Arctic Ocean, 3:88 Arctic tern, 3:338 Arduino, Giovanni, 4:108 Argon, 1:241 Aristarchus of Samos, 4:64 Aristotelian physics, 2:14–16, 4:7–9 development, 2:14–15 Earth’s spheres, 4:25 flaws, 2:15–16 four elements, 2:15, 2:69–70 gravity, 2:69–70 motion, 2:14–16, 2:18 theory of impetus, 2:61 See also Greek thought Aristotle, 1:67–68, 4:4 causation, 4:3–5 founder of taxonomy, 3:192, 3:196–197, 3:222 See also Aristotelian physics Arkansas River, 4:372 Armstrong, Edwin H., 2:346 Armstrong, Neil, 1:24

Aromatic hydrocarbons, 1:368 Arrhenius, Svante, 1:311 Arrhenius’s acid-base theory, 1:311–312, 1:320 Arsenic, 1:225–226, 3:78 Arteries. See Circulatory system Arthropoda (phylum), 3:275, 3:279–282 See also Insects Artichokes, 3:6 Artificial elements, 1:195, 1:198 Artificial polymers. See Synthetic polymers Artificial satellites. See Satellites Artiodactyls, 3:223 Asbestos, 4:139 Ascaris lumbricoides, 3:278 Ascorbic acid. See Vitamin C Asexual reproduction, 3:135, 3:136–138, 3:138, 3:285 See also Reproduction Aspect ratio of paper airplanes, 2:108 Astatine, 1:236 Astbury, William Thomas, 2:297 Asteroids catastrophism, 4:92 mass extinctions, 3:185–186, 3:186, 4:126–127 Meteor Crater (AZ), 4:91 See also Meteorites Asthenosphere convection, 4:188 structure, 4:212–214 Astronauts sleep cycles, 3:310 space walks, 4:215 Astronomical clocks, 2:267 Astronomical units, 4:75 Astronomy gamma rays, 2:353 infrared imaging, 2:350 magnetism, 2:335 Newton’s three laws of motion, 2:18–20 relative motion, 2:9–10 science and religion, 4:9, 4:63–64 specific gravity, 2:26 ultraviolet imaging, 2:350 See also Cosmology; Planetary science Atheism, 3:167, 4:90 Athena (goddess), 3:138 Athens (ancient Greece), 3:287 Atherosclerosis, 3:36 Atlanta (GA) geomorphology, 4:18 microclimate, 4:410 traffic, 2:45–46 Atlantis (mythical continent), 4:6 Atmosphere (Earth’s) atmospheric sciences, 4:43 carbon cycle, 4:328–330 climate, 4:405–407

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How to go to your page compared to water, 1:49 composition, 4:405 deforestation, 4:355–356 dust following massive asteroid strike, 3:185 early Earth, 3:178 elements of, 3:346 energy, 4:198 geoscience, 4:26 plant evolution, 4:293 role in biosphere, 3:347–348 subsidence, 4:188, 4:190–191 water, 4:387–394 weather, 4:396–397 wind, 4:396–398 See also Earth sciences Atmosphere (unit of measure), 2:141–142 Atmospheric pressure, 1:39 boiling, 2:209 temperature, 2:239, 2:241 Atomic bombs, 2:177–179 fluorine, 1:232–233 nuclear fission, 1:71–72, 1:93 radioactive fallout, 3:103, 3:104 Atomic bonding carbon, 1:244–245 octet rule, 1:103–104 Atomic Energy Commission (AEC), 3:103 Atomic hypothesis. See Atomic theory Atomic mass, 1:76–83, 1:82t–83t, 1:128, 1:130–131 Atomic mass units (amu), 1:25–26, 1:77 calibration, 1:81 hydrogen standard, 1:131–132 molar mass, 1:27 Atomic models, 1:64–65 Atomic numbers, 1:65, 1:71, 1:80, 4:74 element’s energy, 1:130 isotopes, 1:95 Atomic size main-group elements, 1:141 periodic table of elements, 1:136, 1:138 Atomic structure, 1:64–65, 1:84–85 bonding, 1:267 carbon, 1:363–364 electrons, 1:86–87 elements, 1:119–120 silicon, 1:363–364 Atomic theory, 1:34, 1:63, 1:66, 1:68–74, 1:128 Berzelius, Jons, 1:79 Dalton, John, 1:264–265 development, 4:7–8 discovery, 2:195 origins in Greece, 1:127, 1:263, 2:14 proof of theories, 3:166 structure of matter, 2:205 Atomic weight. See Atomic mass Atomizers and Bernoulli’s principle, 2:116–117

S C I E N C E O F E V E RY DAY T H I N G S

Atoms, 1:34–35, 1:36, 1:63–75, 1:74t–75t, 1:86 Avogadro’s number, 1:53 chemical equations, 1:283–284 electromagnetic force, 2:207 excitement, 2:366–367 lasers, 2:361 magnetism, 2:331, 4:179 minerals, 4:130–131 molecules, 2:192 structure, 4:73–74 ATP (adenosine triphosphate), 3:34, 3:56 Attention (brain function), 3:307 Aurora australis, 4:79, 4:79–80 Aurora borealis, 4:79, 4:79–80 Australia Aborigines and boomerangs, 2:107 marsupials, 3:219 Australopithecus, 3:215–216 Automobile industry, 1:176 Automobiles. See Cars Automotive engine torque, 2:90 Autosomes, 3:111–112 Autotrophs, 3:77, 3:87–88 Avalanches, 4:275 See also Flow (geology) Average atomic mass, 1:77, 1:131 Avery, Oswald, 3:111, 3:117 Avogadro, Amedeo, 1:53 atomic theory, 1:68–69 molecular theory, 1:79, 1:112, 1:265, 2:205 Avogadro’s law, 2:184–185 Avogadro’s number, 1:26–27, 1:53, 1:78, 2:184–185, 2:193–194, 2:205–206 atomic mass units, 1:131 molecule measurement, 1:36 Axes frame of reference, 2:6 statics and equilibrium, 2:134 tension calculations, 2:135–136 See also Cartesian system; Graphs Axles, 2:162–164

Cumulative General Subject Index

B B cells, 3:255, 3:263–264 B vitamins, 3:92 Bachelet, Emile, 2:338 Bacillus anthracis, 3:251 Bacteria anaerobic, 3:58 antibiotics and, 3:165 decomposers, 3:361 in digestive system, 3:51, 3:51–54 infection, 3:284–285, 3:286 infectious diseases, 3:245, 3:248, 3:250, 3:252,

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3:264 origin of life, 3:179 ulcers, 3:48–49, 3:49 See also Germs Bags, potato chip, 2:188 Baillie, Mike, 4:30–31 Bain, Alexander, 2:267–268 Baker, Howard, 4:220 Baking soda, 1:315–317, 3:48 Bakker, Robert T., 4:127–128 Balaena, 3:208 Balard, Antoine-Jérôme, 1:234 Ballast, 2:122 Ballonet, 2:127 Balloons, 2:105, 2:125–126 See also Airships; Hot-air balloons Bangladesh and cyclones, 4:401 Banting, Frederick, 3:243 Bar magnets, 2:334, 4:178 See also Magnets Barents, Willem, 3:88–89 Barium, 1:177–178 Barometers, 1:50–51, 2:141–142, 2:239 Barrel sponge (animal), 3:199 Barton, Otis, 2:124 Base-10 numbers, 2:7 Baseball Bernoulli’s principle, 2:117–119 linear momentum, 2:40, 2:44 projectile motion, 2:80–82 Bases, 1:310–318, 1:316t–317t alkaline earth metals, 1:154 used as antacids, 3:48 See also Acid-base reactions Basilar membrane, 2:318 Basketballs molecules, 2:194 work, 2:171 Bathyscaphe, 2:124–125 Bathysphere, 2:124 Bats (animals) Doppler effect, 2:305 echolocation, 3:340 order Chiroptera, 3:220 pentadactyl limb, 3:170 pollinating plants, 3:141 Batteries, 1:296 lithium, 1:163, 1:164–165 oxidation-reduction reactions, 1:291 Bauer, Georg. See Agricola, Georgius BBC (British Broadcasting Corp.), 3:223 Beaches breezes, 4:188 erosion, 4:284 Bears, 3:40, 3:89 Beatles (musical group), 2:347–348

Beauty (human perception), 3:146, 3:147 Beavers, 3:322 Beavertail cactus, 3:351 BEC (Bose-Einstein Condensate), 1:42–43, 2:201–202, 2:211 Becquerel, Henri, 1:70 Bed load, 4:286 Bedbugs, 3:281–282 Bedrock, 4:296 Beebe, William, 2:124 Beer, 1:340, 1:356–357 Bees behavior, 3:323–324 killer bees, 3:211, 3:211 pheromones, 3:303–304 Behavior, 3:319–326, 3:325t See also Instinct; Learning and learned behavior Behaviorism, 3:320–321 Bends (illness), 2:123–124, 4:334 Beni Abbes Dunes (Sahara Desert), 4:407 Berg, Paul, 3:119 Beriberi, 3:92, 3:93–95 Berkelium, 1:203 Bernal, J. D., 2:297 Bernal chart, 2:297 Bernoulli, Daniel fluid dynamics, 2:20 Hydrodynamica, 2:113 kinetic theory of gases, 2:195 projectile motion, 2:81–82 See also Bernoulli’s principle Bernoulli’s principle, 2:97, 2:112–119, 2:118t airplanes, 2:105–106 boomerangs, 2:107 fluid mechanics, 2:97–98 fluid pressure, 2:143–144 projectile motion, 2:81–82 Bert, Paul, 2:124 Berthelot, Pierre-Eugene Marcelin, 1:18, 2:230 Berthollet, Claude Louis, 1:276, 1:330 Bertillon, Alphonse, 3:105 Beryllium, 1:175 Berzelius, Jons, 4:135 atomic theory, 1:68–69, 1:79, 1:128 catalysts, 1:306 mineral classification, 4:133 selenium, discovery of, 1:221 thorium, discovery of, 1:198 Best, Charles Herbert, 3:243 Bestiaries, 3:196, 3:197 Beta-carotene, 3:90 “Better living through chemistry” (marketing campaign), 1:366 Bevel gears, 2:163 Biblical geomythology, 4:5–7 Bicycles, 2:109

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How to go to your page Biello, Stephany, 3:315 Big bang theory, 3:177, 4:71 Bighorn River (WY), 2:96 Bile, human, 3:47 Bilirubin, 3:51 Billiard balls, 2:38, 2:40–41 Binary ionic compounds, 1:277 Bingeing, 3:39 See also Bulimia Bioaccumulation, 3:71, 3:72–73, 3:76, 4:356–358 Biodegradable plastics, 1:377 Biodiversity, 3:365–366, 3:392 civilization, 4:343–344 deforestation, 4:354 ecosystems, 4:345–346, 4:349–350 tropical cloud forests, 4:345 See also Ecosystems Bioenergy biomass, 4:200–201 fossil fuels, 4:201 Biogeochemistry, 4:313–322, 4:319t–320t, 4:323–324 See also Geochemistry Biogeography, 3:400–401, 3:402–403 Biological communities, 3:391–399, 3:397t–398t, 3:400–409 See also Food webs Biological rhythms, 3:306–315, 3:314t Biological warfare, 3:251–252 Biological weathering, 4:265, 4:274 Bioluminescence, 2:367, 2:369–370 Biomagnification, 3:72–73, 4:356–358 Biomass bioenergy, 4:200–201 ecosystems, 4:346 Biomes, 3:370–380, 3:378t–379t Biopsies in cancer diagnosis, 3:239 Biorhythms (1970’s fad), 3:315 Biosphere, 3:345–359, 3:356t–358t, 3:360 ecology, 4:351–358 ecosystems, 4:341 energy, 4:199–201 geoscience, 4:27 Biostratigraphy, 4:106 Biotechnology, 3:119 See also Genetic engineering Bipolar disorder and lithium treatment, 1:165 Birds aerodynamics, 2:103–105 behavior, 3:322–323, 3:324, 3:327, 3:328–329, 3:329, 3:330–331 biomes, 3:375 circadian cycle, 3:309 colonization, 3:402, 3:402, 3:406 endangered or extinct species, 3:208–209, 3:209, 3:408 evolutionary history, 3:192

S C I E N C E O F E V E RY DAY T H I N G S

experiments in beriberi, 3:94–95 impact of DDT, 3:73 mating rituals, 3:145, 3:146–147 migration, 3:336–338 parasites, 3:274, 3:330–331 pollinating plants, 3:140–141 respiration, 3:58 speciation, 3:217 symbiotic relationships, 3:384, 3:386, 3:387 taxonomic keys, 3:193 training, 3:338 wings, 2:115 See also specific species Birth defects. See Congenital disorders Bismuth, 1:158, 1:161 Biston betularia, 3:173 Bjerknes, Jacob, 4:402 Bjerknes, Vilhelm, 4:402 Black Death (1347-1351), 3:247–248 Black, Joseph, 1:247, 2:230 Black light. See Ultraviolet light Black Sea, 4:320–321 Blackburn, Ken, 2:108 Black-throated green warblers, 3:217 Blane, Sir Gilbert, 3:93 Bleaching, 1:231–232 Blimps buoyancy, 2:105 usage, 2:127, 2:129 See also Airships Block-and-tackle pulleys, 2:164 Blood anemia, 3:268–269 hemoglobin, 3:19, 3:56 hemophilia, 3:115–116, 3:241–242 importance of bone marrow, 3:263 importance of vitamin K, 3:91–92 infections, 3:276–277 lymphocytes, 3:263, 3:264 oxygen diffusion in lungs, 3:56–57 plasma, 3:47 proteins in, 3:21 sickle-cell anemia, 3:14, 3:15–16 Blood pressure Doppler effect, 2:305 measurement, 2:146–147 Blood vessels, 3:36 Blue color of sky, 2:358 Blue-green algae, 4:293 Blue whales, 3:208 Boat wakes, 2:288 Body clock. See Biological rhythms Bogs, 3:376 Bohr, Neils, 1:65, 1:73–74, 1:86 Boiling, 2:209 Boiling points

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alkali metals, 1:163–164 alkaline earth metals, 1:173 liquids, 1:39 Boise City (OK) dust storm, 4:303 Boltzmann, Ludwig E., 2:196 Bomb calorimeters, 2:230–231 Bombs, 1:71–72, 1:93 See also Explosives; Nuclear weapons Bond energy, 1:113, 1:269, 1:272 Bone marrow, 3:263, 3:263 Bones, 2:152 Boomerangs aerodynamics, 2:107 Bernoulli’s principle, 2:115–116 Boreal forest, 3:362, 3:371–372 Boron, 1:216 Bose, Satyendranath, 2:201–202, 2:211 Bose-Einstein Condensate, 1:42–43, 2:201–202, 2:211 Botulism, 3:252 Bouncing balls and energy, 2:175–176 Bowel movements, human, 3:50 Boyer, Herbert, 3:119 Boyle, Robert, 1:51–52, 1:68, 1:128, 2:112–113, 2:184 Boyle’s law, 1:52, 2:184, 2:197, 2:249 Bragg, William Henry, 2:298 Bragg, William Lawrence, 2:298 Bragg’s law, 2:298 Brain, 3:168 Alzheimer’s disease, 3:233, 3:233–235 Creutzfeldt-Jakob disease, 3:232–233 hearing, 2:318 neurological disorders, 3:302 pineal gland, 3:306–307 processing sensory data, 3:299 Brakes (automotive), 2:55, 2:62 See also Cars Bramah, Joseph, 2:160 Branched alkanes, 1:368 Brand, Hennig, 4:317 Brass, 1:188, 1:336 Bread, 3:28, 3:28 Breast-feeding, 3:73 Breathing, 2:145–146, 3:56–58 See also Respiration Breeding. See Selective breeding Breezes. See Wind Breitling Orbiter 3, 2:126 Bridges resonance, 2:280, 2:285 thermal expansion, 2:250 Brimstone, 4:320 Brine-curing recipes, 1:352 British Broadcasting Corp. (BBC), 3:223 British Imperial System (measurement), 1:8, 2:8–9 See also Measurements; Metric system Broadcasting, radio, 2:276–277, 2:345–346

VOLUME 4: REAL-LIFE EARTH SCIENCE

Bromine, 1:234 Bronchial disorders, 3:60 Bronsted-Lowry acid-base theory, 1:312–313, 1:320–321 Bronze, 1:336 Brown, Robert, 1:69, 1:332–333, 2:196, 2:206 Brownian motion, 1:69, 1:332–333, 2:195–196, 2:206 Brunhes, Bernard, 4:225, 4:228 Buchanan, Jack, 2:54 Buchner, Eduard, 1:306, 3:25 Buckminsterfullerene, 1:246, 1:247, 4:326 Buffered solutions, 1:323 Buffon, Georges-Louis Leclerc de, 4:89–90 Buhler, Rich, 4:216–218 Building materials, 1:176 Bulimia, 3:38, 3:38–39 Bulk modulus for elasticity, 2:149 Bulletproof vests, 1:375 Bullets aerodynamics, 2:79 projectile motion, 2:80 Bull’s horn acacia, 3:386 Bungee jumping, 2:265, 2:267 Buoyancy, 2:120–129 Archimedes, 2:24 fluid mechanics, 2:96–97 fluid pressure, 2:144–145 hull displacement of ships, 2:22, 2:24–25 Burdock (plant), 3:389 Bureau of Standards, 1:7, 2:8 Burglar alarms, 2:335 Buridan, Jean, 2:61 Burns, Alan, 2:56 Burroughs, Edgar Rice, 3:331–332 Büsching, Anton Friedrich, 4:46 Business application. See Economic geology; Industrial uses Butte County (SD), 2:137 Butterflies, 3:338, 3:388–389 Butterfly effect, 4:25 Buys-Ballot, Christopher Heinrich, 2:304 Byzantine Empire, 3:230, 3:246

C Cabbage, 3:26 Cabriolets, 2:163 Cacti, 3:351 Cade, John, 1:165 Cadmium, 1:189 Caesar, Julius, 3:157 Cailletet, Louis Paul, 2:200 Calcite, 4:136, 4:138 Calcium, 1:176, 3:81, 3:91 Calcium carbonate, 3:377, 4:327, 4:327

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How to go to your page Calderas, 4:259, 4:259 Calendars, 2:8 Calibration, 1:9, 1:81 California gold rush, 4:290 Route 1, 4:270 slides, 4:271–272 Californium, 1:203 Calorie (unit of measure), 2:219, 2:229 Calorimeters, 1:18–19, 2:230–231 Calorimetry, 1:18–19, 2:230–231 Calvaria major, 3:209 Cambrian period, 3:179 Camels, 3:223, 4:307 Camerarius, Rudolf Jakob, 3:138 Cameron, James, 2:125 Cameron, Mike, 2:125 Cancer, 3:230, 3:238–241 lung cancer, 3:62 mutation, 3:132 treatment with designer proteins, 3:21 Candles accelerometers, 2:46 light, 2:360–361 Candy bars, 3:10 Cannibalism, 3:396, 3:398 Canyons erosion, 4:270 Neon Canyon (UT), 4:106 Nicola River Canyon (British Columbia), 4:265 Cap rocks, 4:159 Capillaries (thermometers), 2:242 Car horns, 2:336 Car jacks fluid pressure, 2:142–143 hydraulic presses, 2:167–168 Car lifts, 2:160 Caravans, 4:307 Carbohydrates, 3:3–10, 3:8t–9t, 3:44, 3:79, 3:80–82 Carbon, 1:243–251, 1:249t–250t allotropes, 4:325–326 in amino acids, 3:11–13 biogeochemistry, 4:315–316 carbon bonding, 4:325 carbon cycle, 4:323–330 chlorine, bonding with, 1:232 human body composition, 1:123 in old-growth biological communities, 3:369 organic chemistry, 1:363–370 organic compounds, 1:276 origin of life, 3:178–179 paleontology, 4:115 percentage of biosphere, 3:348 percentage of human body mass, 3:78 polymers, 1:372 Carbon cycle, 4:316, 4:323–330, 4:328t–329t, 4:355

S C I E N C E O F E V E RY DAY T H I N G S

Carbon dioxide, 1:247–248, 1:268–269, 2:200, 3:28, 3:28, 3:369 carbon cycle, 4:326–329 cloud seeding, 4:404 dry ice, 4:402, 4:404 exhaled, 3:55–56 Carbon monoxide, 1:248, 1:250, 1:301–302, 4:328–329 Carbon ratio dating, 1:98, 1:250–251, 4:97–98, 4:120 Carbonated water, 1:248 Carbonates (minerals), 4:134, 4:326–327 Carborundum, 4:137 Carboxylic acids, 1:315, 1:369 Carburetors, 2:117 Carcinogens, 3:238 Cardinals (birds), 3:327 Careers in geoscience, 4:18 Carnivores dinosaurs, 3:184 food webs, 3:361, 4:200, 4:342 order Carnivora, 3:221 Carnot, Sadi heat engines, 2:231–232 thermodynamics, 2:221–222 Carnot steam engine, 2:221–222, 2:234 Carothers, Wallace, 1:376, 1:377–378 Carriages, horsedrawn, 2:163 Cars aerodynamics, 2:109–111 centripetal force, 2:47–48 collisions, 2:41–43 friction, 2:53, 2:55 gas laws, 2:188–190 torque, 2:90 Cartesian system, 2:6, 2:7 See also Axes; Graphs; Points Cartography, 4:45, 4:46 See also Maps Casal, Gaspar, 3:95 Cassette tapes, 2:336–337 Castle Rock (SD), 2:137 Catabolism, 3:33, 3:34–35 Catalysts, 1:304–309, 1:308t chemical reactions, 1:286, 1:298 enzymes, 3:24–25 photosynthesis, 3:5 Catalytic converters, 1:305, 1:307 Catastrophism historical geology, 4:91–92 history, 4:39–40 Catholic Church. See Roman Catholic Church Cation resins, 1:106 Cations, 1:101 aluminum, 1:121 metals, 1:103 naming system, 1:277

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Cats human interaction with, 3:387 sense of smell, 3:298 taxonomy, 3:221 ultrasonics, 2:323 Cattle, 3:7–8, 3:123, 3:233 Cattle egrets, 3:402 Causation, 4:3–5 Caustic soda, 1:317–318 Cavendish, Henry, 4:171 Cavitation, 2:324, 2:326 Cayley, George Bernoulli’s principle, 2:115 glider flight, 2:105 kites, 2:108 CCK (Cholecystokinin), 3:10 CDC (Centers for Disease Control and Prevention), 3:258–259 CD-ROMs, 2:364 Cellular biology amino acids, 3:14 characteristics of bacteria, 3:284–285 chromosomes and DNA, 3:99–100, 3:135 metabolism, 3:34 origins of life, 3:179 photosynthesis, 3:4 starches, 3:7 taxonomy, 3:198, 3:206 Cellular respiration, 1:250, 3:56, 3:57, 3:58–59 Celluloid, 1:377 Cellulose, 3:6, 3:7–8 Celsius, Anders, 2:240–241 Celsius temperature scale, 1:15, 2:240–241 Cenozoic era, 3:186, 4:108, 4:116, 4:117–118 Center of buoyancy, 2:122 Center of geography (U.S.), 2:137 Center of gravity buoyancy, 2:122 calculations, 2:136–137 equilibrium, 2:135, 2:136 Center of population (U.S.), 2:137 Centers for Disease Control, 3:258–259 Centigrade scale (temperature), 2:240–241 Central Park (New York, NY) microclimate, 4:410, 4:410 Centrifugal force, 2:45–50, 2:46, 2:47, 2:50t See also Force Centrifuges, 2:46, 2:48–49 Centripetal force, 2:45–50, 2:46, 2:47, 2:50t See also Force CERCLA (Comprehensive Environmental Response, Compensation, and Liability) Act (1980), 4:306 Cereal foods, 3:81 Cerium, 1:208 Cervix, 3:153 Cesarean section, 3:156–157

Cesium, 1:170 Cetaceans, 3:221–222 CFCs. See Chlorofluorocarbons Chadwick, James, 1:71, 1:80, 1:93 Challenger space shuttle explosion (1986), 1:257, 1:259, 1:294 Chang Heng (Chinese scientist), 4:234 Chaos theory, 4:24–25 Chaparral, 3:375 Charles, J. A. C., 1:52, 2:184, 2:241 Charles’s law airbags, 1:58–59, 56 buoyancy of balloons, 2:126 hot-air balloons, 1:55–56 molecular dynamics, 2:197 pressure, 1:52 thermal expansion, 2:249 Chase, Martha, 3:111 Cheerleaders, 2:140, 2:141 Cheetahs, 3:373 Chelicerata (subphylum), 3:275 Chemical bonding, 1:263–272, 1:270t–271t carbon, 1:244–245 chlorine, 1:232 compounds vs., 1:112–113 enzymes, 3:25 hydrogen, 1:253, 1:265 lipids, 3:35 minerals, 4:131 peptide linkage, 3:13, 3:18 Chemical deposition, 4:287–288 Chemical energy, 1:11, 2:177, 3:4–5, 3:360–361, 3:361 See also Kinetic energy; Potential energy Chemical equations, 1:283–284 cellular respiration, 3:56 equilibrium, 1:298–299 photosynthesis, 3:5, 3:68 Chemical equilibrium, 1:19, 1:297–303, 1:298, 1:302t–303t Chemical ionization, 1:105 Chemical kinetics, 1:286 Chemical oceanography, 4:362 Chemical reactions, 1:34, 1:281–288, 1:287t–288t, 1:289–290 biogeochemistry, 4:314–315 catalysts, 1:304–309, 3:24–25 equations, 1:297 fertilizers, 3:351 human digestion, 3:45–46 nitrogen, 4:331–332 peptide linkage, 3:13, 3:18–19 physical changes vs., 1:297–298 spicy foods, 3:297 Chemical symbols Berzelius, Jons, 1:68–69 language, 1:122–123, 1:132–133

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How to go to your page Chemical thermodynamics, 1:286 Chemical weathering, 4:265, 4:274 Chemiluminescence, 2:370–371 Chemoreception, 3:295–305, 3:304t Chemotherapy, 3:239 Chernobyl nuclear disaster (1986), 1:98, 1:103 Chestnut blight, 3:394 Chicago (IL), 1:13 Chichén-Itzá (Mayan pyramid), 4:146 Chickens, 3:322, 3:324, 3:328 Childbirth, 3:151–157, 3:156t Children mental development, 3:334 sense of taste, 3:301 vaccinations, 3:258 Chilean earthquake (1960), 4:237 Chimney sweeps, 3:240, 3:240 Chimneys, 2:116–117 Ch’in Shih-huang-ti, 2:8 China earthquakes, 4:236–237 measurement standardization, 2:8 wheelbarrows, 2:162–163 Chiropterans, 3:220 Chlorine, 1:231–232 Chlorofluorocarbons (CFCs), 1:233–234 aerosol cans, 1:55, 2:188 ozone depletion, 1:307 Chlorophyll, 3:4, 3:5 Chloroplasts, 3:4 Cholecystokinin (CCK), 3:10 Cholera, 3:247 Cholesteric liquid crystals, 1:43, 2:213 Cholesterol, 3:36, 3:36–37 Chordata (phylum), 3:205 Chorionic villi, 3:155–156 Chromium, 1:192, 3:78 Chromosomes, 3:99–100, 3:102, 3:111–112, 3:117, 3:126, 3:135 Chronobiological study, 3:315 Chronostratigraphy, 4:95, 4:106–108 Chrysler PT Cruiser, 2:109 Cigarette lighters, 1:206 Cinder cones, 4:258 Circadian rhythms, 3:306, 3:307–312, 4:83–84 Circannual cycles, 3:313 Circular motion. See Rotational motion Circulatory system, 2:324, 3:230, 3:232 See also Blood Cirque glaciers, 4:378 Cirrocumulus clouds, 4:391 Cirrostratus clouds, 4:391 Cirrus clouds, 4:391 Cities. See specific cities Cities as anthropogenic biomes, 3:378–379 Citric acid, 1:312, 1:315

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Citrus fruits, 3:93 Cladistics, 3:192 Class I-III levers. See Levers Classical physics five major divisions, 2:20 frame of reference, 2:9–10 friction, 2:56 machines, 2:157–158 relationship to modern physics, 2:20 thermal expansion, 2:245–246 See also Aristotelian physics; Newtonian physics Classification. See Taxonomy Claude, Georges, 1:241 Clausius, Rudolph Julius Emanuel, 2:222–223 Clay, 2:40–41, 4:287 Cleavage of minerals, 4:136 Cleft palate, 3:128 Clepsydras, 2:100–101, 2:163 Climate, 4:405–412, 4:411t changes, 3:353 classifying biomes, 3:372 desertification, 4:309–310 evapotranspiration, 4:390 greenhouse effect, 4:355–356 ice-core samples, 4:379 mountains, 4:260 See also Weather Climatology, 4:407 Climax biological communities, 3:370–371, 3:400–409, 3:407t–408t Climax (ecology), 4:352 Cloning, 3:121, 3:122–123 See also Genetic engineering Clonorchis, 3:277 Closed systems (physics). See Systems Clostridium botulinum, 3:252 Clothoid loops, 2:49–50 Cloud forests, 4:345 Cloud seeding, 4:404 Clouds altocumulus clouds, 4:391 evapotranspiration, 4:390 precipitation, 4:391–392 seeding, 4:404 types, 4:390–392 Clutches, 2:55 See also Cars Coaches, horsedrawn, 2:163 Coal, 4:324, 4:326 See also Fossil fuels Coal gasification, 1:35, 1:43, 1:46 Cobalt, 1:190–191 Cochlea, 2:317–318 Cockcroft, John, 1:164, 1:165 Coefficient of linear expansion, 2:246–248 Coefficient of volume expansion, 2:248

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Coefficients, 2:7 coefficients of friction, 2:52–54 coefficients of thermal expansion, 2:246–248 G (gravitational coefficient), 2:73 lift, 2:105–106 pi, 2:7, 2:45 See also Numbers Coenzymes, 3:25–26 Cohen, Stanley, 3:119 Coinage metals, 1:187–188, 1:294–295 Cold. See Heat Cold, common caused by virus, 3:60, 3:286–287 sense of taste and smell, 3:301–302 Cold climate tundra, 3:374, 3:394, 3:394–395 See also Antarctica; Climate Cold extrusion of metals, 2:151 Cold-blooded animals, 3:218 Collisions cars, 2:41–43, 2:62–63 kinetic and potential energy, 2:175–176 linear momentum, 2:40–44 models, 1:298, 1:304 See also Elastic collisions; Inelastic collisions Colloids, 1:42, 1:69, 1:332–333, 2:210–211 Colonization (movement of species), 3:402, 3:402–403 Colorado River delta, 4:56 Colors interference, 2:290, 2:292 light, 2:358–360 perception, 2:359–360 spectrum, 2:355 See also Light; Primary colors Columbus, Christopher Earth’s circumference, 4:45 magnetic declination, 4:46, 4:180 Combination reactions, 1:283, 1:285 Combustion, 1:291–292 Combustion engines, 1:51, 1:292 Commensalism (symbiosis), 3:273, 3:383–384, 3:389 Common chemical sense, 3:298, 3:298 Communications satellites, 4:55 Communities (animals), 3:323–324, 3:331 Compasses, magnetic, 2:334–335, 4:180 Competition in biological communities, 3:393–395 Complete migration, 3:336 Composite cones (volcanoes), 4:258 Compound levers, 2:162 Compound pulleys, 2:164 Compounds, 1:273–280, 1:279t–280t carbon cycle, 4:324–325 chemical bonding vs., 1:112–113 definition, 1:329–330 formation, 1:111

minerals, 4:131 mixtures vs., 1:274, 1:330–331, 1:338–339 molecular structure, 4:315 nitrogen, 4:334–336 noble gases, 1:237 Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) (1980), 4:306 Compressibility aerodynamics, 2:102–103 airplanes, 2:106–107 gases, 2:183–184 See also Sonic booms Compression elasticity, 2:148 fluids and solids, 2:95–96 modulus, 2:148–149 stone, 4:15 Compton, Arthur Holly, 2:343 Compton Gamma Ray Observatory Satellite, 2:353 Concentration, 1:341, 1:349 Concentration camps, 3:121 Concrete, 2:151, 2:152 Condensation, 2:209–210, 4:372 Conditioning (behavior), 3:320–321 Conduction (heat), 2:220, 2:228, 4:186 Conductivity, electrical, 2:228 Congenital disorders, 3:128–131, 3:129, 3:155–156, 3:232 Coniferous forests, 3:371–372, 3:374 Conservation of angular momentum, 2:30 constant orientation, 2:33 skating, 2:27, 2:29, 2:33 tops, 2:33 tornadoes, 4:399 See also Angular momentum Conservation of electric charge, 2:30–31 Conservation of energy, 2:28–29, 4:194–195 Bernoulli’s principle, 2:112 Earth and energy, 4:198–199 gasoline and motors, 2:27 hydroelectric dams, 2:176 kinetic and potential energy, 2:174–176 matter, 2:203–205 mechanical energy, 2:28–29 rest energy, 2:29, 2:179–180 thermodynamics, 2:218, 2:222, 2:223 See also First law of thermodynamics Conservation laws, 2:27–33, 2:32t See also specific laws Conservation of linear momentum, 2:30 rifles, 2:28, 2:31, 2:38 rockets, 2:31, 2:33, 2:39 See also Linear momentum Conservation of mass, 2:30, 2:203–205 Conservation of matter, 2:179–180, 2:203–205

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How to go to your page Constant composition, 1:68, 1:329–330 Contagious diseases. See Infectious diseases Containers and fluid pressure, 2:142 Continental crust, 4:229 Continental drift evolution and, 3:169, 3:180 impact of massive asteroid, 3:185 plate tectonics, 4:220–222 seismology, 4:231–232 Continental shelves, 3:377 See also Plate tectonics Controversies cloning, 3:123 DNA evidence, 3:108 evolution, 3:165–169 genetic engineering, 3:103–104, 3:121–122 in vitro fertilization, 3:144 logging, 3:408–409 Convection, 4:185–191, 4:189t–190t cooking, 2:348 heat, 2:228–229 thermodynamics, 2:220–221 thunderstorms, 4:398 wind, 4:397 Convective cells, 4:186–188 Convergence (plate tectonics), 4:224 Conversion of mass to energy, 1:33 Coon Creek (WI) erosion, 4:286 Coordinates, 2:6 See also Axes; Cartesian system; Graphs; Points Copernicus, Nicholas, 4:37 frame of reference, 2:9–10 gravity, 2:70–71 heliocentric system, 4:9, 4:38, 4:40, 4:65 laws of motion, 2:61 See also Heliocentric universe Copper, 1:188, 1:289–290, 3:78, 4:138–139 Coral reefs, 3:377, 3:377, 4:327 Core (Earth), 4:182, 4:209–210, 4:214–215 Corn, 3:82, 3:387 Corpuscular theory of light, 2:290, 2:296, 2:342–343, 2:355–356 See also Light; Photons Correlation in stratigraphy, 4:108–109, 4:112 Corrosion, 1:294 Corundum, 4:137, 4:162, 4:164–165 Cosine. See Trigonometry Cosmic background radiation, 1:240 Cosmology, 4:63–65, 4:68, 4:71 Cotton, 1:373 Coulomb, Charles, 2:340, 4:177–178 Count Rumford. See Thompson, Benjamin Couper, Archibald Scott, 1:267 Courtship. See Mating rituals Cousins, 3:115–116 Cousteau, Jacques, 2:124

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Covalent bonding, 1:113, 1:268, 1:269, 1:364 Cowbird, 3:274 Cowpox, 3:256–257, 3:257 Cows. See Cattle Cranes (machines), 2:164 Crash tests, 2:63 Crater Lake (OR), 4:259 Cream, 2:49 Creation science, 3:163, 3:168, 4:11 Creation story (Bible), 4:5, 4:6–7, 4:11 Creationism, 3:163, 3:168, 4:11 Creep (geology), 4:266, 4:275–276 Creutzfeldt, Hans Gerhard, 3:232–233 Creutzfeldt-Jakob disease, 3:128, 3:232–233 Crick, Francis, 2:298, 3:111, 3:117 Criminal investigations. See Forensics Critical damping, 2:266 Critical point, 2:198 Crookes, William electromagnetism, 1:69–70 electrons, experiments with, 1:86 thallium, discovery of, 1:158 Crops, 3:136, 3:209, 3:350–351, 3:352, 3:380 See also Agriculture; Plants Crosscurrent exchange, 3:58 Cruise missiles, 2:82–83 Crumple zones in cars, 2:41–43 Crust (Earth). See Lithosphere Crystalline solids, 1:104, 1:111 Crystals calcite with quartz, 4:136 carbon, 4:325–326 crystalline matter, 2:210 elasticity, 2:151–152 igneous rocks, 4:148–149 jewels, 4:165 lasers, 2:369 melting, 2:207–208 minerals, 4:132–133, 4:144 piezoelectric devices, 2:322–323 snowflakes, 4:392 thermal expansion, 2:249–250 x-ray diffraction, 2:298 x-rays, 2:353 Ctesibius of Alexandria, 2:101, 2:163 Cumulonimbus clouds, 4:187, 4:187–188, 4:391–392 See also Thunderstorms Cumulus clouds, 4:391 Curie, Marie, 1:224, 2:366, 2:367–368 polonium, discovery of, 1:226–227 radioactivity, 1:70 radium, discovery of, 1:178–179 Curie, Pierre, 1:70 Curium, 1:202 Currents hydrology, 4:363–364

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Dahn, Jeff, 1:164–165 Dalton, John, 1:64 atomic mass, 1:78–79 atomic theory, 1:68, 1:128, 1:264–265, 2:205 atoms, discovery of, 1:112, 2:195 Dalton’s law of partial pressure, 1:54 Damping (energy), 2:266 Dante’s Peak (movie), 4:17 Dark matter, 1:42, 2:211 Darwin, Charles, 3:162 Darwin’s moth, 3:138 ethology, 3:320 evolution, 4:4 introduces theory of evolution, 3:161, 3:169 taxonomy, 3:197–198 Darwin, Erasmus, 3:167 Dating techniques. See Absolute dating; Relative dating Davis, William Morris, 4:245 Davisson, Clinton Joseph, 2:297 Davy, Humphry, 1:166, 2:361 DDT (dichlorodiphenyltrichloroethane), 3:72, 3:73, 4:358 Debye, Peter Joseph William, 2:297 Decibels, 2:314 Deciduous trees, 3:362, 3:373 Decomposers, 3:68, 3:69, 3:349, 3:361 biogeochemistry, 4:316–317 carbon cycle, 4:330 food webs, 4:200, 4:342 soil formation, 4:294, 4:296 Decomposition, 3:68, 3:69 See also Decomposers Decomposition reactions, 1:285 Decompression, 1:50 Decompression chambers, 2:124

Decompression sickness, 2:123–124, 4:334 Deflagration of airbags, 1:59 Deforestation, 4:353 ecology, 3:365–366, 4:352–356 remote sensing, 4:57, 4:59 Deformation elasticity, 2:150 stress, 2:148–149 Dehydration fruits, 1:351 humans, 1:350 Deinonychus, 3:184 Delayed sleep phase syndrome, 3:311 Delesse, Achilles, 4:225 Delta wing kites, 2:108 Deltas, river, 3:351, 4:56, 4:268, 4:300 Democritus, 1:67–68, 1:263, 2:205, 4:7–8 Dendochronology, 4:119 Denitrification, 4:338 Density, 1:23–30, 1:29t, 2:21–26, 2:25t aerodynamics, 2:102–103 buoyancy, 2:121 Earth, 2:23 gold, 1:25 minerals, 4:136–137 planets, 4:66–67, 4:210 Saturn, 1:26 See also Viscosity Dental health and fluoride, 1:233 Denver (CO), 2:146 Deoxyribonucleic acid. See DNA (deoxyribonucleic acid) Department of Energy (U.S.), 3:103, 3:120 Deposition of sediments, 4:287–288, 4:290–291 Depositional environments, 4:288 Depth, 2:144–145 Dermoptera, 3:220 Descartes, René, 2:6, 3:306–307 Desert tortoises, 3:351 Desertification, 3:353, 4:306–310 Deserts biomes, 3:375 formation of, 3:353 soil, 3:350, 3:351, 4:297–300 Designer proteins, 3:21–22 Desmodus rotundus, 3:220 Destructive interference, 2:290 Detritivores biogeochemistry, 4:316–317 energy, 3:69, 3:361 food webs, 3:68, 3:349, 4:200, 4:342 soil formation, 4:294, 4:296 speciation, 3:221 Deuterium, 1:95–97, 1:254–255 atomic mass, 1:80, 1:131 hydrogen bombs, 1:93, 1:255

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whirlpools, 4:363 Curve balls Bernoulli’s principle, 2:117–119 projectile motion, 2:80–82 Curves in roads. See Roads Cuvier, Georges, 4:92 Cycles (harmonic motion) acoustics, 2:311 resonance, 2:279 See also Harmonic motion; Oscillations; Waves Cycloalkanes, 1:368 Cyclones, 4:400–402, 4:401 See also Natural disasters Cyclotrons, 1:201 Cypridina, 2:370 Cystic fibrosis, 3:62, 3:128

D

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How to go to your page Developing nations deforestation, 4:353–354 starvation, 3:84–85 Devils Tower (WY), 4:5, 4:270–271 Devonian period, 3:182, 3:183 Dewar, James, 2:200 Dextrose, 3:3–4 Diabetes mellitus, 3:242–243 Diamagnetism, 2:332 See also Magnetism Diamond, Jared, 2:107, 3:395, 4:343 Diamonds, 1:246, 4:137–138, 4:165, 4:325–326 Diana (Princess of Wales), 3:38 Diaphragm (anatomy), 2:315, 2:320–321 Diastolic pressure, 2:147 Diet. See American Diet; Fitness; German Diet; Nutrition and nutrients Differential migration, 3:336 Diffraction, 2:294–300, 2:299t, 2:356 Diffraction gratings, 2:297, 2:298 Diffractometers, 2:298 Diffusion and respiration, 3:56–57, 3:57 Digestion, human, 3:44–54, 3:52t–53t breakdown of amino acids, 3:14–15 digestive system, 3:45–47, 3:46, 3:276, 3:278, 3:285–286 enzymes, 3:27 importance of cellulose, 3:8 metabolism, 3:33–34 Digital photography in geoscience, 4:55–56 Dilution, 1:341 Dinitrogen. See Nitrogen Dinoflagellates, 2:369–370 Dinosaur National Monument (CO), 4:117 Dinosaurs, 3:183, 3:183–185 endothermy, 4:127–128 fossil excavations, 4:117 tyrannosaurus rex fossil, 4:115 Dinuguan (Filipino delicacy), 3:302–303 Diploid cells, 3:99 Dipole-dipole attraction, 1:113 Dipoles, 1:47 Dirigibles, 2:105, 2:127–128 See also Airships Disaccharides, 3:4 Discordance (acoustics), 2:290 Discourses and Mathematical Demonstrations Concerning Two New Sciences (Galileo), 2:16–18 Diseases, human, 3:229–235, 3:234t beriberi, 3:92, 3:93–95 cancer, 3:21 congenital disorders, 3:128–131, 3:129, 3:155–156, 3:232 digestive disorders, 3:48–50 eating disorders, 3:38–40 ecosystems, 4:344–345

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ethnic groups, 3:113, 3:127, 3:131 genetic disorders, 3:113, 3:113–116, 3:121, 3:127 Hartnup disease, 3:15 infectious diseases, 3:244–252, 3:396 kwashiorkor, 3:85 neurological disorders, 3:302 noninfectious diseases, 3:236–243 parasites, 3:275–282 pellagra, 3:15, 3:95 respiratory disorders, 3:60, 3:62 rickets, 3:90, 3:91 risk factors, 3:239–241 scurvy, 3:93 sickle-cell anemia, 3:14, 3:15–16 sleep disorders, 3:309–311 social impact, 3:246, 3:247–248 unknown causes, 3:233 Vitamin A poisoning, 3:89–90 Disk drives, 2:337 Displacement ships, 2:122 volume measurement, 2:23–24 Dissociation, 1:321–322 Dissolved load, 4:285 Distillation, 1:40, 1:354–360, 1:357, 1:359t, 2:209–210 Distilled water, 1:356 Distortion (acoustics), 2:326 Divergence (plate tectonics), 4:224, 4:225 Divine Comedy (poem), 4:215 Diving, underwater. See Underwater diving Diving bells, 2:123 Dixon, Jeremiah, 4:47 DNA (deoxyribonucleic acid) asexual reproduction, 3:135 cancer, 3:238 effect of viruses on, 3:285, 3:287 forensics and criminal investigations, 3:20, 3:21, 3:103, 3:104–105, 3:105, 3:108 genes, 3:100–101, 3:117–118, 3:118, 3:119, 3:126, 3:164–165 history, 3:111 molecules, 1:111 nanocomputer, 3:120 phylogeny, 3:198 synthesis of amino acids, 3:13 Doctors, medical, 3:153–154, 3:238–239 Dodecahedrons, 4:133 Dodo (bird), 3:208–209, 3:209 Dog whistles, 2:323 Dogs human interaction with, 3:383, 3:387 Pavlov’s dog, 3:320 sense of smell, 3:298, 3:303 ultrasonics, 2:323 Dolly (cloned sheep), 3:122, 3:123 Dolphins, 3:195, 3:204–205, 3:221–222, 3:340–341

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See also Aerodynamics; Fluid dynamics; Hydrodynamics; Molecular dynamics; Thermodynamics Dysprosium, 1:210

Domain alignment, 2:332–334 Domain growth, 2:332–334 Domesticated animals, 3:387, 3:395–396 Dominant genes, 3:112–113 Donald, Ian, 2:324 Donkeys, 3:215 Doorbells, 2:336 Doors, 2:89 Doping agents, 1:226 Doppler, Christian Johann, 2:303–304 Doppler effect, 2:301–307, 2:306t, 2:324 Doppler radar, 2:303, 2:305, 4:403–404 Dorn, Friedrich, 1:239 Double-displacement reaction, 1:285 Double-helix model (DNA), 3:111, 3:117–118, 3:118 See also DNA (deoxyribonucleic acid) Down syndrome, 3:126, 3:129, 3:129 Downs cells, 1:166 Drafting (aerodynamics), 2:109 Drafts (air currents), 2:98 Drag (aeronautics) airplanes, 2:106 birds, 2:104–105 cars and trucks, 2:109–110 See also Induced drag Dreissena polymorpha, 3:211 Drinking water and fluoridation, 1:233 Drizzle, 4:392 Drought and mud cracks, 4:287 Drugs and medicines ecstasy, 3:315 impact on taste and smell, 3:302 insulin, 3:120, 3:242–243 LSD (lysergic acid diethylamide), 3:307 used in childbirth, 3:154 Dry ice, 1:248, 4:402, 4:404 Dry valleys (Antarctica), 4:379 Dryja, Thaddeus R., 3:13 Du Fay, Charles, 2:340 du Pont de Nemours and Company, E. I. nylon, 1:378 organic chemistry, 1:365–366 Ducks, 3:193, 3:330 Duodenum, 3:47, 3:48 Dürer, Albrecht, 4:15–16 Dust bowls, 3:352 dust storms, 4:303 erosion, 4:270 sediment, 4:286 soil conservation, 4:304–305 Dust devils, 4:269 Dust mites, 3:265 Dutch elm disease, 3:210 Dwarfism, 3:129 Dye lasers, 2:361–362 Dynamics, 2:13–20, 2:19t

E. I. du Pont de Nemours and Company. See du Pont de Nemours and Company, E. I. E. coli bacteria, 3:51, 3:285–286 Eardrums, 2:317 Ears bats, 3:340 hearing, 2:316–318 human ear infections, 3:290 whales, 3:341 Earth age, 4:89–90, 4:94, 4:98–99 alkali metals, 1:164 alkaline earth metals, 1:174 asthenosphere, 4:212–214 biological communities, 3:391–399, 3:400–409 biomes, 3:370–380 biosphere, 3:345–359 circumference, historical estimates, 4:38 climate changes, 4:410–411 core, 4:182, 4:209–210, 4:214–215 density, 1:26, 2:23, 4:66–67 Earth-Moon system, 4:73, 4:74–75 element abundance, 4:130, 4:313–314 energy, 4:192–206 energy output, 4:198–199 energy sources, 4:195–198 geologic time periods, 3:179 geomagnetic field, 2:334–335, 3:338–339 geomagnetism, 4:180–182, 4:184 glossaries, 4:81t–83t, 4:216t–217t gravity, 2:75–77, 4:65–66, 4:173–174, 4:176 greenhouse effect, 3:366, 3:368–369 historical map, 4:16 interior, 4:209–218, 4:237, 4:240–241 islands, 4:249 lithosphere, 4:212–213 Mercator map, 4:46 mesosphere, 4:214 metals, 1:151–152 natural systems, 4:23–31 noble gases, 1:239–240 nonmetals, 1:216, 1:224 origin of life, 3:17, 3:177–178, 3:180–181, 3:217 origins, 4:87–88 planetary science, 4:65–67 plate tectonics, 4:209–211 plates, 4:223–224 relative motion, 2:9–10

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How to go to your page relief map, 4:48 rotation, 2:268–269, 3:308 roundness, 2:76–77 soil formation, 4:292–293 solar system, 4:72–84 spheres, 4:25–27 struck by massive asteroid, 3:185–186, 3:186 transition metals, 1:186 water, 4:369–371, 4:387–388 weather, 4:397 Earth sciences Earth systems, 4:23–31 Earth’s spheres, 4:25–27 geography, 4:44–52 scientific method, 4:3–11 See also Geoscience; Hydrology; Weather Earthquakes damage, 4:236 plate tectonics, 4:228 predictions, 1:240 seismology, 4:233–237 See also Natural disasters; Seismology Earthworms, 3:349 East Pacific Rise, 4:221 Eating disorders, 3:38, 3:38–40 Eating habits, human, 3:39, 3:48, 3:49, 3:242, 3:311 Eavesdropping devices, 2:325 Ebola virus, 3:250–251 Echolocation, 3:339, 3:339–341 Ecology, 3:360–369, 3:367t–368t, 3:391, 4:351–358, 4:356t–357t Economic geology, 4:154–165, 4:163t–164t See also Geology Ecosystems, 4:341–350 biological communities, 3:391–399 biomes, 3:370–380 climate zones, 4:407 ecology, 3:360–369, 4:351–352 glossaries, 3:367t–368t, 4:348t–349t islands, 4:251–252 microclimates, 4:407–409 mountains, 4:256, 4:260, 4:262 soil, 4:297–300 tropical cloud forests, 4:345 See also Food webs Ecstasy (drug), 3:315 Ectothermy in dinosaurs, 4:127–128 Edison, Thomas, 2:360, 2:361, 2:369, 3:312 Edwards, Robert G., 3:144 Efficiency, machine. See Machine efficiency Efficient causes (philosophy), 4:3–5 Egg cells, 3:100, 3:136 Eggs bird parasites, 3:274, 3:330–331 egg-laying mammals, 3:218 human, 3:143

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Egypt fertile soil, 3:350–351 pyramids, 2:162, 2:164–165 Ehrlich, Paul, 3:255 Eijkman, Christiaan, 3:93–95 Eilmer, 2:105 Einstein, Albert, 1:72–73 Bose-Einstein Condensate, 2:201–202, 2:211 electromagnetism, 2:343 gravity, 2:77 light, 2:297 molecular dynamics, 2:196, 2:206 optics, 2:357 relativity, 2:10–12, 2:29, 2:303 El Niño consequences, 4:28, 4:30 Elastic collisions, 2:38, 2:40–44 Elastic deformation, 2:150 Elastic limit, 2:148 Elastic potential energy, 2:264–266 Elasticity, 2:148–154, 2:153t–154t See also Elastic deformation Elastomers elasticity, 2:152 oscillations, 2:267 Elderly Alzheimer’s disease, 3:234 sense of taste, 3:301 Electric charges atoms, 1:65–66 ions, 1:101–108 Electric current electromagnetism, 2:341–342 measurement, 2:21 Electric discharge, 4:182 Electric engines, 2:90 Electric thermometers, 1:17, 2:242–244 Electric vehicles, 1:163 Electrical conductivity, 2:228 Electrical generators, 2:341 Electricity and Magnetism (Maxwell), 2:356–357 Electrochemistry, 1:296 Electromagnetic induction, 2:341 Electromagnetic radiation, 2:341–344 electron behavior, 1:87 ionization, 1:105–106 photosynthesis, 3:4–5 Electromagnetic spectrum, 2:340–353 light, 2:357–358 luminescence, 2:365–366 solar radiation, 4:197 See also Electromagnetism Electromagnetic waves, 2:341–342 frequency, 2:276–277 interference, 2:290, 2:292–293 light, 2:356–357

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properties, 2:256 resonance, 2:282–283 Electromagnetism, 2:340–353, 2:351t–352t atomic theory, 1:69–70 classical physics, 2:20 discovery, 2:20 electric discharge, 4:182 electromagnetic energy, 4:193 electromagnetic radiation, 2:221, 2:229 gases, 1:48–49 geomagnetism, 4:177–180 light, 2:356–358 luminescence, 2:365–367 machines, 2:157 magnetism, 2:331–334 molecular dynamics, 2:194–195 molecules, 2:207 Electromagnets iron, 2:334 sound devices, 2:336 See also Magnets Electron-dot diagrams, 1:264 Electron impact ionization, 1:105 Electron tubes, 2:345 Electron volts (unit of measure), 2:345 Electronegativity, 1:113, 3:297 carbon, 1:244, 1:364 minerals, 4:131 Pauling, Linus, 1:269 Electronic amplification, 2:315 Electronic distance measurement, 4:49 Electrons, 1:84–91, 1:85, 1:90t–91t atomic structure, 1:35, 1:66–67, 1:92, 1:130, 1:135 behavior, 1:63–65 chemical bonding, 1:267–268 diffraction, 2:297 discovery, 2:206 identified, 1:79 magnetism, 2:331 orbitals, 1:142 Electrostatics, 2:340 Element symbols. See Chemical symbols Elements, 1:119–126, 1:120t, 1:124t–125t abundance, 4:313–314, 4:332–333 Aristotle, 1:67–68 atomic numbers, 1:65 atoms, 1:63–64 biogeochemistry, 4:313–314 Boyle, Robert, 1:68 carbon cycle, 4:324–325 in the human body, 3:78 minerals, 4:130 models, 1:110 native elements, 4:133–134

nitrogen, 4:332–333 periodic table, 1:69, 1:132 solar system formation, 4:72–74 See also Families of elements Elements, Aristotelian. See Aristotelian physics Elephantiasis, 3:278, 3:279 Elephants, 3:200, 3:222 Elevation. See Altitude Embryo, 3:152–153 E=mc2. See Relativity; Rest energy Emission (luminescence), 2:366–367 Empedocles, 4:8 Emulsifiers, 1:333–334, 1:342–343 Endangered species, 3:207–208, 3:223 See also Extinction Endogenous infection, 3:283 Endothermy in dinosaurs, 4:127–128 Energy, 2:170–180, 4:203t–205t alternative energy, 4:202, 4:206 earth, 4:192–206 Earth systems, 4:23–31 electromagnetic spectrum, 2:345 food webs, 4:342 human body, 3:10, 3:36, 3:308 hydrologic cycle, 4:388–389 interference, 2:288–289 matter, 1:33 metabolism, 3:33–34, 3:44, 3:80 moves from environment to system, 3:345, 3:360–361 produced in chemical reactions, 3:24 Sun, 4:78 Sun’s energy used by plants, 3:67–69 temperature and heat, 1:11, 4:193–195 thermodynamics, 2:217–218 transfer, 2:171, 3:69, 3:393 work, 4:192–193 See also Kinetic energy; Mechanical energy; Potential energy; Rest energy Energy conservation. See Conservation of energy Engine coolant, 2:248 Engines combustion, 1:51 gas laws, 2:188–189 torque, 2:90–91 See also Cars; Electric engines English measurement system, 1:7–8, 2:8–9 See also Measurements; Metric system The Englishman Who Went Up a Hill But Came Down a Mountain (movie), 4:255 Entamoeba histolytica, 3:276 Enterobiasis, 3:278 Entropy, 1:13–14, 2:222–223 Earth and energy, 4:198

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How to go to your page second law of thermodynamics, 2:222–223, 2:232, 2:234, 4:195 third law of thermodynamics, 2:223, 2:234–235 See also Second law of thermodynamics Environment, 3:345–346 Environmental concerns biomagnification, 4:356–358 desertification, 3:353, 4:306–310 forest conservation, 3:363, 3:408–409 fossil fuels, 4:201–202 mining, 4:162 Ogallala Aquifer (U.S.), 4:373 oil industry, 4:158–159 plastics, 1:376–377 recycling, 1:380 soil conservation, 3:352 tropical rain forests, 3:350 See also Dust bowls; Global warming; Pollution Environmental geology, 4:18 Enzymes, 1:306–307, 3:21, 3:24–30, 3:29t–30t, 3:37, 3:119 Eohippus (horse ancestor), 3:172 Eolian processes. See Wind erosion Eötvös effect, 4:172 Eötvös, Roland, 4:172 Epidemiology, 3:237 Epinephrine, 3:267–268 Epiphytic plants, 3:389 Epirogenesis. See Tectonism Epsom salts, 1:175 Equilibrium, 2:133–138, 2:138t, 2:263–264 See also Dynamics; Statics Eratosthenes of Cyrene, 4:38, 4:45 Erbium, 1:209 Ericsson, John, 2:166 Erosion, 4:264–272, 4:271t beaches, 4:284 dust bowls, 3:352 Nicola River Canyon (British Columbia), 4:265 prevention, 4:305 rock arches, 4:267 sandstone, 4:26 sedimentation, 4:284–285 Escherichia coli bacteria, 3:51, 3:285–286 Eskimo curlews, 3:208 Esophagus, 3:45 Esters, 1:369 Estuaries, 3:377 Ethanol, 1:340, 1:343, 1:356–357 Ether (optics), 2:356 Ethics of genetic engineering, 3:103–104, 3:122 Ethology, 3:320 Ethylene dibromide, 1:234 Eugenics, 3:121–122 Euler, Leonard von, 2:113

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Europe diseases, 4:344–345 early humans, 3:162 Europium, 1:210 Eustachian tubes, 2:317 Eutrophication, 3:354, 4:318 Evaporation, 2:208–209, 4:370 evaporite minerals, 4:290–291 evapotranspiration, 4:389 hydrologic cycle, 4:370, 4:371, 4:373 See also Evapotranspiration Evaporites, 4:290–291 Evapotranspiration, 3:346, 3:347, 3:354–355, 3:355, 4:387–394 climate, 4:390 clouds, 4:390–392 evaporation, 4:389 glossary, 4:393t–394t hydrologic cycle, 4:370, 4:371, 4:387–389 transpiration, 4:389–390 See also Evaporation Evergreen forests. See Coniferous forests Evolution, 3:161–175, 3:174t–175t amino acids dating, 3:17 choosing the ideal mate, 3:147–149 humans and food intake, 3:82 mammals, 3:221–222 moving from water to land, 3:181–182 primates, 3:220 role of mutation, 3:101, 3:127 trees, 3:364–365 See also Natural selection; Phylogeny Ewing, William Maurice, 4:223 Exhalation (human breathing), 3:55–56 Exogenous infection, 3:283 Expansion joints, 2:250 Explorers and exploration Barents, Willem, 3:88–89 Columbus, Christopher, 4:45, 4:46, 4:180 introduced species, 3:210, 3:395–396 Explosives, 1:292–294 ammonium nitrate, 3:351–352, 4:333 explosions, 1:292–294 impact of massive asteroid, 3:185 Exposure to lead, 1:158 Extinction dodo bird and dodo tree, 3:208–209, 3:209 mass extinctions, 3:181, 3:182–183, 3:185–186 See also Mass extinctions Extreme Ultraviolet Explorer, 2:350 Eyes color vision, 2:359–360 diseases, 3:279 heredity, 3:112–113

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F layer (ionosphere), 2:346 Facial characteristics, 3:129, 3:129–130 Facilitation model (biological communities), 3:401 Facultative relationships (symbiosis), 3:273–274, 3:383–384 Fahrenheit, Daniel, 2:240 Fahrenheit temperature scale, 1:14–15, 2:240–241 Fall (geology), 4:267–268, 4:277 Families of elements, 1:140–147, 1:145t–146t alkaline earth metals, 1:171–172 halogens, 1:229 metalloids, 1:222 metals, 1:153 nonmetals, 1:213–215 transition metals, 1:181 See also Elements Faraday, Michael, 1:85, 2:200, 2:341, 4:178 Farming nitrogen cycle, 4:338 soil, 4:300 soil conservation, 4:304–305 See also Agriculture Fast food, 3:79, 3:81 Fat, human body, 3:10, 3:35–37, 3:36, 3:88, 3:127, 3:146, 3:147, 3:307 Fats and oils, 3:35–37, 3:44–45, 3:81, 3:88, 3:297 See also Lipids; Oils Fault-block mountains, 4:257 Faults (geology), 4:224, 4:231 Faunal dating, 3:172, 4:119 Fax machines, 2:267–268 FCC (Federal Communications Commission), 2:276–277, 2:346–347 FDA (Food and Drug Administration), 3:79 Feces, human, 3:50–54 Federal Communications Commission (FCC), 2:276–277, 2:346–347 Feedback, 2:283, 4:27–28 Felidae, 3:221 Female reproductive system, 3:143, 3:152–153 Fens, 3:376 Ferdinand II (Grand Duke of Tuscany), 1:14, 2:239 Fermat, Pierre de, 2:6 Fermentation, 3:25, 3:26, 3:27–30, 3:28 Fermi, Enrico, 1:94, 1:97 Ferrimagnetism, 2:332–334 See also Magnetism Ferromagnetism, 2:332–334 See also Magnetism Fertilization (sexual reproduction), 3:136, 3:143–144, 3:144 Fertilizers, 3:351 Fetus development, 3:152–153, 3:155, 3:155–156 similarities among animals, 3:170

Feynman, Richard, 1:34, 2:205 Fiber in human diet, 3:50 See also Cellulose Fiber-optic communications, 2:364 Field ionization, 1:102, 1:105 Field mice, 3:163–164 Fighting (behavior) defending territory, 3:323 displays, 3:324–326 mating rituals, 3:145–146 Filaments (light bulbs), 2:361 Fill dirt, 4:295 Filtration, 1:354–360, 1:356, 1:359t Final causes (philosophy), 4:3–5 Fingerprints, 3:20, 3:105 Fir trees, 4:318 Fire friction, 2:56 light, 2:360 Fire extinguishers, 1:55, 2:185, 2:187 Fireworks, 1:174 Firnas, Abul Qasim Ibn, 2:105 First harmonic, 2:273, 2:274 First law of motion, 2:18–19 centripetal force, 2:46–47 friction, 2:59–61 gravity, 2:72 See also Inertia First law of thermodynamics, 2:216, 2:222, 2:232, 4:194–195 Earth and energy, 4:198–199 food webs, 3:69 See also Conservation of energy Fischer, Emil, 3:25 Fish behavior, 3:322, 3:327–328 bioaccumulation, 3:73, 3:76 buoyancy, 2:122, 2:123 evolutionary history, 3:195 food webs, 3:70, 3:71 introduced species, 3:210 migraton, 3:337 origin of life, 3:181–182, 3:183 respiration, 3:57 speciation, 3:217 sushi, 3:302, 3:303 See also Aquatic animals Fish finders, 2:321 Fishing rods, 2:161–162 Fishing sonar, 2:321 Fission (asexual reproduction), 3:135, 3:285 Fitness exercise, 3:37 ideals, 3:147 weight loss programs, 3:37, 3:81 The Five Biggest Ideas in Science (book), 4:222

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How to go to your page Fixed-action patterns, 3:322, 3:322, 3:327–328 Flaps (airplanes), 2:106 Flatulence, 3:54 Flavonoids, 3:141 Flavored oxygen, 1:215 Fleas, 3:282 Fleming, Alexander, 3:290 Flood, biblical, 4:6 Flooding, 4:364–366, 4:365 Flow (geology), 4:266, 4:276, 4:276–277 Flowering plants. See Angiosperms; Plants Fluid dynamics aerodynamics, 2:102–103 Bernoulli’s investigations, 2:20 fluid mechanics, 2:95 See also Aerodynamics; Dynamics; Fluid mechanics; Fluids; Hydrodynamics Fluid mechanics, 2:95–101, 2:100t See also Fluid dynamics; Fluids Fluid pressure, 2:140–147 Bernoulli’s principle, 2:143–144 buoyancy, 2:121, 2:144–145 characteristics, 2:142 force and surface area, 2:140–141 glossary, 2:146t human body, 2:145–147 measurements, 2:141–142 Pascal’s principle, 2:142–143 water pressure, 2:142 Fluids, 2:95–96 See also Fluid dynamics; Fluid mechanics Flukes, 3:277 Fluorescence, 2:368–370 Fluorescent bulbs, 2:367, 2:369 Fluoridation of water supplies, 1:233 Fluorine, 1:232–234, 1:269 Flywheels friction, 2:55 torque, 2:89–90 See also Cars; Gyroscopes FM radio broadcasts, 2:345–346 Foliation (rocks), 4:152 Folk science and taxonomy, 3:195–196 Food chains. See Food webs Food and Drug Administration (FDA), 3:79 Food preservation, 1:351–353 Food Security Act (1985), 4:305 Food webs, 3:67–76, 3:74t–76t, 4:351 competition, 3:393 ecosystems, 3:360–361, 4:341–342 energy, 4:200 marine, 3:376–377 Foods bacteria, 3:285 cultural attitudes, 3:302, 3:302–303

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improper cooking and handling, 3:245, 3:251, 3:277–278 nutrition labels, 3:79 produced by lactic acid, 3:59–60 spoilage, 3:27 taste, 3:297, 3:299–301 USDA food pyramid, 3:81 See also Crops; Meat Force, 2:18–19, 2:46–47, 2:65–66 centripetal force, 2:45–50 gravitational force, 2:75–77, 2:171–173 impulse, 2:39–44 machines, 2:157–169 paired forces, 2:66–67 strong nuclear force, 2:157 weak nuclear force, 2:157 See also Acceleration; Energy; Mass; Second law of motion; Torque; Vector measurements; Work Forced convection, 2:228–229, 4:186 Forceps, 3:153–154 Forensics criminal investigation, 3:20, 3:21, 3:103, 3:104–105, 3:105, 3:108 forensic geology, 4:19–21 Forestry. See Logging and forestry Forests biological communities, 3:371–374, 3:403 carbon content, 3:369 deforestation, 3:365–366, 4:352–356 ecology, 3:361–363 ecosystems, 4:346–347 old-growth, 3:406, 3:408–409 specialized climate, 3:362 tropical cloud forests, 4:345 Formal causes (philosophy), 4:3–5 Fossil fuels bioenergy, 4:201 carbon cycle, 4:326 economic geology, 4:158–160 environmental concerns, 4:201–202 global warming, 4:409 Fossils correlation, 4:112 excavation, 4:122–123 fossil record, 3:171, 3:171–172, 3:217 fossilization, 4:121–122 geologic history, 4:121–122 limestone, 4:109 mineralization, 3:176 stingray, 4:121 tyrannosaurus rex, 4:115 Vinci, Leonardo da 4:88, 4:105 Foucault, Jean Bernard Leon, 2:268–269, 2:281–282, 2:356 Foucault pendula, 2:268–269, 2:281–282

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“Four causes,” 4:3–5 Four elements (Greek physics), 2:15, 2:69–70, 4:7–8 Fractional distillation, 1:358 Frame of reference, 2:3–12, 2:11t Doppler effect, 2:302–303 geography, 4:51–52 Francisella tularensis, 3:252 Francium, 1:170 Franklin, Benjamin, 2:108, 2:340, 4:177 Franklin, Rosalind Elsie, 2:297–298 Fraunhofer, Joseph von, 2:297 Fraunhofer diffraction, 2:297 Freon, 1:231 Frequency, 2:271–277, 2:275t–276t acoustics, 2:312–313 Doppler effect, 2:301 electromagnetic spectrum, 2:344 electromagnetism, 2:343–344 luminescence, 2:366 modulation, 2:260–261 oscillations, 2:263–264 radio broadcasting, 2:345–347 radio waves, 2:259–260 resonance, 2:279 sound waves, 2:259 ultrasonics, 2:319–320, 2:321–322 waves, 2:256–257 See also Wavelength Fresnel, Augustin Jean, 2:297 Fresnel diffraction, 2:297 Friction, 2:52–57, 2:57t air resistance, 2:74–75 conservation of angular momentum, 2:27 conservation of linear momentum, 2:33 first law of motion, 2:59–61 kinetic and potential energy, 2:175–176 luminescence, 2:371 third law of motion, 2:66–67 See also Air resistance Frictionless surfaces, 2:54–55 Fried foods, 3:81 Frigate birds, 3:145 Frisbees, 2:33 Frisch, Karl von, 3:320, 3:323 Frisius, Gemma, 4:49 Frontal thunderstorms, 4:398–399 Fruits and vegetables artichokes, 3:6 cabbage, 3:26 healthy eating, 3:50, 3:81 pineapples, 3:137 preservation, 1:351–353 source of carbohydrates, 3:5–9 source of protein, 3:23 source of vitamin C, 3:93 Fuel-injected automobiles, 1:56

Fulcrums, 2:160–162 Fullerenes, 1:246 Fundamental frequency, 2:273, 2:274 Fungi decomposers, 3:361 kingdom, 3:198 mycorrhizae, 3:384–386 Funnel clouds, 4:399–400 See also Natural disasters Fusion bombs. See Hydrogen bombs Fusion (sexual reproduction), 3:135–136

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G G (gravitational coefficient), 2:73, 4:171 Gabor, Dennis, 2:299 Gadolinium, 1:208 Gagnan, Emile, 2:124 Gaia hypothesis, 4:27 Gal (unit of measurement), 4:172 Galactic movement, 2:307 Galactose, 3:115 Galápagos Islands, 3:169 Galaxies, 4:70–71 Galen (Greco-Roman physician), 2:239 Galilei, Galileo, 1:12, 4:170 censure by Roman Catholic Church, 2:16, 2:18, 2:70–71 frame of reference, 2:9–10 gravity, 2:17–18, 2:70–71, 4:65, 4:170 idealized models, 2:74 kinematics and dynamics, 2:16–18 laws of motion, 2:61–62 pendula, 2:267, 2:274 scientific method, 4:9–10, 4:38, 4:65 telescopes, 2:354 thermoscope, 1:14, 2:239 Two New Sciences, 2:16–17 Galileo (space probe), 2:335 Gallium, 1:157 Galvanized steel, 1:188–189 Gametes, 3:100, 3:136 Gamma ray astronomy, 2:353 Gamma rays, 2:353, 2:366 Gas filtration, 1:358–359 Gas lasers, 2:361–362 Gas laws, 1:19, 1:51–52, 2:183–191, 2:190t–191t fire extinguishers, 2:185 hot-air balloons, 2:186 molecular dynamics, 2:196–197, 2:210 thermal expansion, 2:249 Gases, 1:48–59, 1:57t–58t acoustics, 2:312, 2:321 behavior, 2:183–184, 2:185–186 boiling, 2:209

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How to go to your page characteristics, 2:148 diffraction, 2:297 evaporation, 2:208–209 kinetic theory, 2:196–197 laws, 2:183–190 liquification, 1:39–40, 1:43 properties, 1:49t resonance, 2:278 thermal expansion, 2:249 triple point, 2:6 volume, 1:27–28 volume measurement, 2:24 Gasohol, 3:30 Gasoline conservation of energy, 2:27 thermal expansion, 2:248 Gastric juices, human, 3:45–46 Gateway Arch (St. Louis, MO), 2:79 Gauss, Carl Friedrich, 2:335, 2:340–341, 4:178 Gauss (unit of measure), 2:335 Gay-Lussac, Joseph, 1:78–79, 2:184 Gay-Lussac’s law, 1:52–53, 1:56, 2:184, 2:312 Gazelles, 3:373 Gears, 2:163–164 Geese, 3:322, 3:328–329, 3:329, 3:330 Geike, Archibald, 4:91 Gellibrand, Henry, 4:88, 4:177 Gender determination, 3:111, 3:112 Gender differences attitudes towards ideal mates, 3:145–149 body fat, 3:37 desirable characteristics, 3:146 eating disorders, 3:38 genetic disorders, 3:115 hemophilia, 3:241–242 reproductive system, 3:142–143 taste buds, 3:300–301 Generators, electrical, 2:341 Genes, 3:99–109, 3:117–125 alleles, 3:112 cancer, 3:238 gene therapy, 3:103 Human Genome Project, 3:103–104 propensity to gain weight, 3:37, 3:127 single-cell life-forms, 3:206 speciation, 3:217 Genesis, Book of, 4:5–7, 4:11, 4:87–88 Genetic engineering, 3:102–103, 3:117–125, 3:124t Genetic recombination, 3:101 Genetics, 3:99–109, 3:106t–108t current research, 3:102, 3:118 in evolution, 3:164–165 history, 3:110–111 Genitals, human, 3:142–143, 3:238–239, 3:240 Genotype, 3:110, 3:112 Geoarchaeology, 4:18–19

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Geocentric universe, 2:15, 4:64–65 Geochemistry, 4:43 See also Biogeochemistry; Geoscience Geodesic domes, 1:247 Geodesy geography, 4:49–50 gravity, 4:171–173 Geography, 4:44–52, 4:50t–51t See also Earth sciences; Surveying Geoid, 4:171–172 Geologic maps, 4:44, 4:47–49 Geologic scale, 4:95 Geologic time, 4:95–103, 4:102t–103t, 4:114–118 Geology disciplines, 4:40, 4:42 geography, 4:44–45 history, 4:39–40 See also Economic geology; Geoscience Geomagnetism, 4:50–51, 4:177–184, 4:183t geography, 4:50–51 magnetic fields, 2:334–335 Geomorphology, 4:245–252, 4:250t–251t erosion, 4:270–271 glaciology, 4:378, 4:380 mountains, 4:256 soil formation, 4:294–295 speciation, 4:262 See also Geoscience Geomythology, 4:5–8, 4:87–88 Geophysics, 4:42–43 Geoscience, 4:12–21, 4:20t, 4:41t–42t disciplines, 4:35–43 Earth systems, 4:23–31 Earth’s spheres, 4:25–27 history, 4:37–40 mythology, 4:5–8 remote sensing, 4:53–59 scientific method, 4:3–11 See also Earth sciences; Geochemistry; Geology; Geomorphology; Geophysics Geosphere, 3:346, 4:26 See also Earth; Lithosphere Geothermal energy, 4:197, 4:206 convection, 4:188 Earth’s interior, 4:214 Germ (reproductive) cells, 3:99, 3:126–127 German diet, 3:81 German marks, 1:4, 1:6 Germanium, 1:226 Germs bacteriology, 3:288, 3:290 disease-causing, 3:244, 3:283–284, 3:396 See also Bacteria; Viruses Gessner, Konrad von, 3:197 Gestation, 3:152–153 Geysers, 4:196

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Giardia lamblia, 3:276 Gilbert, William, 2:334, 4:180–181 Gills, 3:57 Glacier Bay (AK), 3:404, 3:405 Glaciers, 3:405 characteristics, 4:377–378 erosion, 4:268–269 glaciology, 4:376–384 Kennicott Glacier (AK), 4:381 sedimentation, 4:285 See also Glaciology; Ice Glaciology, 4:376–384, 4:382t–383t See also Glaciers Glands, human, 3:45 diseases, 3:229–230, 3:242 pineal gland, 3:306–307 thymus gland, 3:263 Gliders Bernoulli’s principle, 2:115–116 flight, 2:105 Global positioning system, 4:52, 4:54 Global warming, 4:201–202, 4:355–356, 4:384, 4:409 See also Greenhouse effect Glomar Challenger (ship), 4:223 Glossaries acid-base reactions, 1:324t–325t acids and bases, 1:316t–317t acoustics, 2:316t–317t actinides, 1:202t–203t aerodynamics, 2:110t alkali metals, 1:169t alkaline earth metals, 1:178t–179t amino acids, 3:16t atomic mass, 1:82t–83t atoms, 1:74t–75t behavior, 3:325t Bernoulli’s principle, 2:118t biogeochemistry, 4:319t–320t biological communities, 3:397t–398t biological rhythms, 3:314t biosphere, 3:356t–358t carbohydrates, 3:8t–9t carbon cycle, 4:328t–329t catalysts, 1:308t centripetal force, 2:50t chemoreception, 3:304t childbirth, 3:156t climate, 4:411t climax biological communities, 3:407t–408t compounds, 1:279t–280t conservation laws, 2:32t convection, 4:189t–190t density, 2:25t diffraction, 2:299t digestion, human, 3:52t–53t diseases, human, 3:234t

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How to go to your page magnetism, 2:338t mass, density, and volume, 1:29t mass wasting, 4:278t–279t measurement, 1:9t metabolism, 3:41t–42t metalloids, 1:227t migration, 3:340t minerals, 4:140t–141t mixtures, 1:335t–336t molecular dynamics, 2:199t–201t molecules, 1:114t–115t mountains, 4:261t–262t mutation, 3:131t navigation, 3:340t nitrogen cycle, 4:336t noninfectious diseases, 3:243t nonmetals, 1:220t organic chemistry, 1:370t oscillations, 2:268t–269t osmosis, 1:352t oxidation-reduction reactions, 1:295t paleontology, 4:124t–126t parasites and parasitology, 3:280t–281t periodic table of elements, 1:137t–138t planetary science, 4:69t–70t plate tectonics, 4:226t–228t polymers, 1:379t precipitation, 4:393t–394t pregnancy, 3:156t pressure, 2:146t projectile motion, 2:84t properties of matter, 1:44t–46t proteins, 3:22t remote sensing, 4:58t reproduction, 3:139t–140t resonance, 2:284t respiration, human, 3:61t–62t rocks, 4:151t–152t scientific method, 4:10t sediments, 4:289t–290t seismology, 4:238t–240t sexual reproduction, 3:148t–149t soil, 4:298t–299t soil conservation, 4:308t–309t solar system, 4:81t–83t solutions, 1:344t–345t species and speciation, 3:212t–213t, 3:224t–225t statics, 2:138t stratigraphy, 4:110t–112t succession (biological communities), 3:407t–408t sun, 4:81t–83t symbiosis, 3:388t systems (physics), 4:29t–30t taxonomy, 3:202t–203t temperature, 2:242t–243t

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temperature and heat, 1:20t–21t thermal expansion, 2:251t thermodynamics, 2:224t–225t torque, 2:90t transition metals, 1:193t–194t ultrasonics, 2:327t vitamins, 3:94t volume, 2:25t wave motion, 2:260t–261t weather, 4:403t Glucose, 3:3–4, 3:56, 3:243 Glycogen, 3:44, 3:79, 3:80 God and science. See Religion and science Goddard High Resolution Spectrograph, 2:342, 2:350 Goddard, Robert, 2:338 Goiters, 1:123 Gold, 1:182, 4:138 coinage, 1:187, 1:295 density, 1:25, 1:28, 1:30 mining, 4:19, 4:161–162 placer deposits, 4:288, 4:290 Goldberger, Joseph, 3:95 Golf, 2:81, 2:82 Gondwanaland, 4:220–221 Goodyear Blimp, 2:105, 2:129 Goodyear, Charles, 1:377–378 Gorbachev, Mikhail, 2:178–179 Gordon, John Steele, 1:376–377 Gould, Stephen Jay, 3:177, 4:90–91 GPS (global positioning system), 4:52, 4:54 The Graduate (movie), 1:364, 1:366 Graf Zeppelin (dirigible), 2:127–128 Grand Canyon, 3:113 Grandfather clocks, 2:267, 2:272, 2:274 See also Pendula Graphite, 1:245–246, 4:139–142, 4:325 Graphs frame of reference, 2:5–6 statics and equilibrium, 2:134 See also Axes; Coordinates Grasslands, 3:374–375 Gravitational force, 1:30, 4:169 calculations, 2:75–77 Galilean tests, 2:71 geodesy, 4:171–173 mass vs. weight, 1:24–25, 1:30 See also Force; Gravity Gravity, 2:69–77, 2:76t, 4:169–176, 4:175t conservation of linear momentum, 2:33 Earth, 4:65–66, 4:214–215 Earth’s interior, 4:209–210 Galilean theories, 2:17–18 human cannonballs, 2:70 laws of motion, 2:18–20 machines, 2:157 mass wasting, 4:279

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Haber, Fritz, 1:306 Habitats and human encroachment, 3:207–209 See also Ecosystems

Habituation (behavior), 3:333 Hachuring, 4:47 Hadean eon, 4:115–116 Haeckel, Ernst von, 3:167, 3:197, 3:391, 4:351 Hafnium, 1:192 Hahn, Otto, 1:199 Hail, 4:392, 4:399, 4:399 Haldane, John Scott, 2:124 Halemaumau volcano (Hawaii), 4:5 Halides, 4:134 Halley, Edmund diving bell, 2:123 Halley’s comet, 2:65–66 laws of motion, 2:62 Hall-Héroult process, 1:157, 1:295–296 Halogen lamps, 1:234 Halogens, 1:215, 1:229–236, 1:235t Handlebars (bicycles), 2:109 Hanging valleys, 4:380 Haploid cells, 3:100 Harden, Sir Arthur, 3:25–26 Hardness of minerals, 4:136, 4:144, 4:156 Hares, 3:226 Harmonic motion damping, 2:266 Doppler effect, 2:301 electromagnetism, 2:343 frequency, 2:271–274, 2:273–274 resonance, 2:280 wave motion, 2:255 See also Oscillations; Wave motion Harmonics (acoustics) frequency, 2:274, 2:276 interference, 2:289 Hartmann, Georg, 4:180 Hartnup disease, 3:15 Harvard University, 2:211 Hauteville, Jean de, 4:234 Hawaiian geomythology, 4:5 Hay fever. See Allergies Hazardous materials and anthrax scare, 3:250 HCFCs (hydrochlorofluorcarbons), 1:55, 1:233–234, 2:188 Health, human. See Human health Health or organic foods, 3:86 Heart (human). See Circulatory system Heat, 2:227–235, 2:233t–234t convection, 4:185–186 friction, 2:56 luminescence, 2:366 measurement, 2:219 temperature, 4:193–194 thermal energy, 2:218–219, 2:236–237 transfer, 2:348 See also Temperature; Thermodynamics Heat capacity, 2:219, 2:230

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projectile motion, 2:78–80 second law of motion, 2:65–66 vacuum, 2:71 See also Gravitational force Great Flood (biblical event), 4:6 Great Lakes (North America), 3:211, 3:354, 4:318 The Great Piece of Turf (painting), 4:15–16 Greek language in chemical symbols, 1:133 Greek thought causation, 4:3–4 cosmology, 4:64 geography, 4:45–46 geomythology, 4:5–6 See also Aristotelian physics Greenhouse effect, 3:366, 3:368–369, 4:199 carbon cycle, 4:328–329 ecology, 4:355–356 fossil fuels, 4:201–202 See also Global warming Greenwich meridian, 2:5 Gregorian calendar, 2:8 Grimaldi, Francesco, 2:296–297, 2:356 Grimm, Jacob, 3:162 Grocery store scanners, 2:296 Gross, Hans, 4:20–21 Groundwater aquifers, 4:373 distribution, 4:387–388 leaching, 4:305–306 soil, 4:294 subsidence, 4:247–248 See also Water Groups of the periodic table of elements, 1:127, 1:134, 1:135 Guettard, Jean-Etienne, 4:47 Guided missiles, 2:82–85 Gulf of California, 4:56 Gulf Stream, 4:364 Gunpowder, 1:292 Guns, Germs, and Steel (Diamond), 4:343 Gurdon, John B., 3:123 Gutenberg discontinuity, 4:213 Gymnosperms, 3:138, 3:140, 3:173–174, 3:364, 3:364–365, 4:347–349 Gypsum, 4:138 Gyroscopes, 2:88, 2:89–90 Gyroscopic precession, 2:107 See also Gyroscopes Gyroscopic stability, 2:107 See also Gyroscopes

H

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How to go to your page Heat conduction, 2:220, 2:228 Heat engines, 2:231–232 Heat transfer, 2:220–221, 2:348 conduction, 2:228 convection, 2:228–229 radiation, 2:229 See also Heat Heaviside layer, 2:346 Heaviside, Oliver, 2:346 Heavy water, 1:96 Heezen, Bruce Charles, 4:223 Heisenberg, Werner, 1:73–74 Heisenberg Uncertainty Principle, 1:73–74 Helical gears, 2:163 Heliocentric universe, 4:40 frame of reference, 2:9–10 gravity, 2:70–71 history, 4:65 laws of motion, 2:61–62, 2:65 Helium, 1:238, 1:240, 1:240–241, 1:242 balloons and dirigibles, 2:126 component of universal matter, 1:123 liquefaction, 2:198, 2:200 mass to volume ratio, 1:28, 1:36–37 valence electron configuration, 1:173 Hell, alleged location of, 4:216–218 Helmets, bicycle, 2:109 Helmont, Jan Baptista van, 4:327 Helmont, Johannes van, 1:247 Helsinki University of Technology Low Temperature Laboratory, 2:223 Hemoglobin, 1:301, 3:15–16, 3:19, 3:56, 3:170–171, 4:328 Hemophilia, 3:115–116, 3:241–242, 3:242 Hennig, Willi, 3:193 Henry’s law, 1:54–55, 2:187 Herbivores dinosaurs, 3:184 food webs, 4:200, 4:342 place in food web, 3:361 Heredity, 3:99, 3:110–116, 3:114t congenital disorders, 3:128–131, 3:129, 3:232 disorders, 3:113, 3:121, 3:229, 3:235, 3:240, 3:241–242 mutation, 3:126, 3:127 Hero of Alexandria, 2:120, 2:163, 2:231 Herodotus (Greek historian), 2:164 Herons, 3:375 Herschel, William, 2:349, 4:68, 4:70 Hershey, Alfred, 3:111 Hertz, Heinrich Rudolf electromagnetism, 2:341–342 light, 2:356–357 wave properties, 2:256–257 Hertz (unit of measure), 2:256–257 Hess, Harry Hammond, 4:222

S C I E N C E O F E V E RY DAY T H I N G S

Heterogeneous equilibrium, 1:299 Heterogeneous mixtures, 1:332, 1:339, 1:354–355 Heterogeneous reactions, 1:286 Hibernation, 3:40, 3:40–43 See also Sleep Hierarchical behavior (animals), 3:323–324 High Plains Regional Aquifer (U.S.). See Ogallala Aquifer (U.S.) High-level clouds, 4:391 Highways, 3:380 Himalayas (mountain range) Mount Machhapuchhare, 4:246 plate tectonics, 4:224 Hindenburg (dirigible), 1:254, 1:257, 2:105, 2:126, 2:128 Hindu-Arabic notation system used in measurement, 1:4 Hinnies, 3:215 Hipparchus (Greek astronomer), 4:64 Hiroshima bombardment (1945), 1:72 Histamines, 3:266–267 Historical geology, 3:177, 4:87–94, 4:92t, 4:118 Hittites, 1:190 HIV (human immunodeficiency virus), 3:259 Hockey, 2:54–55 See also Ice skating Holmes, Arthur, 4:40 Holmes, Sherlock (fictional detective), 4:20–21 Holograms, 2:295, 2:296, 2:298–300 Holographic memory, 2:300 Holographic optical elements, 2:299 Homestake Gold Mine (SD), 4:212 Hominidae (family), 3:206 Homo (genus), 3:206 Homo sapiens, 3:206, 3:308 Homo sapiens neanderthalensis, 3:177 Homogeneous mixtures, 1:331–332, 1:339–340, 1:354–355 Homogeneous reactions, 1:286 Homosexual community, 3:259 Hooke, Robert, 1:14, 2:148, 2:239 Hooke’s law, 2:148 Hooke’s scale (temperature), 2:239 Hookworms, 3:278 Hoover bugles, 2:117 Horizontal motion. See Projectile motion Hormones amino acids in, 3:13–14 biological rhythms, 3:307, 3:314–315 insulin, 3:120, 3:242–243 therapy, fighting cancer, 3:239 Horsepower (unit of measure), 2:173 Horses, 3:402 domestication, 4:343–344 fossil record, 3:172–173

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mating with donkeys, 3:215 species, 3:222 Hospitals, 3:154 Hot-air balloons, 1:55–56, 2:126, 2:186, 2:188 See also Balloons Hot extrusion of metals, 2:151 Hot spots (geology), 4:258 Houot, Georges, 2:125 Housing, 1:154 Howard, Luke, 4:390–391 Hubble, Edwin, 2:307, 2:358 Hubble Space Telescope, 2:342, 2:350 Human behavior competition, 3:393 fighting, 3:325–326 learning, 3:331–334 operant conditioning, 3:320–321 symbiotic relationships, 3:383, 3:386–387 Human body acids and bases, 1:315 carbon content, 4:325 elements in, 3:348 nitrogen content, 4:333 nonmetals, 1:216 saltwater, 1:350 transition metals, 1:186 Human cannonballs, 2:70 Human Genome Project, 3:103–104, 3:118, 3:120–121 Human health, 1:124–126, 1:177 causes of death in U.S., 3:231, 3:236 impact of nuclear radiation, 3:73 impact of obesity, 3:39–40 nutrition, 3:8–10 presence of proteins, 3:20–23 See also Diseases, human Human history and development causing ecological disturbances, 3:403 early civilizations, 3:186, 3:353, 3:395–396 early migrations, 3:162–163 ecosystems, 4:342–345 encroaching on animal habitats, 3:207–209 evolution of primates, 3:167–169, 3:177, 3:220–221 first Homo sapiens, 3:181, 3:308 geologic time, 4:93–94 ice ages, 4:382–384 mass extinctions, 4:127 Human immunodeficiency virus (HIV), 3:259 Human intelligence, 3:168, 3:333 Human sexuality, 3:145–149, 3:146 Humason, Milton, 2:307 Humboldt, Alexander von, 1:78–79 Hume, David, 3:167 Humus, 4:295 Huns, 3:163 Hunting of endangered species, 3:208

Huntington disease, 3:128 Hurricane Andrew (1989), 4:400–401 Hurricane Hugo (1989), 4:400 Hurricanes, 4:400–402, 4:401 See also Natural disasters Hutton, James, 3:177, 4:27, 4:39, 4:90, 4:91 Huygens, Christiaan pendulum clocks, 2:267, 2:274 wave theory of light, 2:296, 2:343, 2:355–356 Hyatt, John Wesley, 1:377 Hybridization (genetics), 3:110 Hydraulic presses, 2:167–169 fluid mechanics, 2:98–99 origins, 2:160 Pascal’s principle, 2:142–143 Hydraulic rams. See Hydraulic presses Hydrocarbon derivatives, 1:368–369, 1:375 Hydrocarbons, 1:256–257, 1:367–368, 1:373–374, 1:375, 3:178, 4:157–158 Hydrochloric acid, 1:231, 1:256 Hydrochlorofluorocarbons (HCFCs), 1:55, 1:233–234, 2:188 Hydrodynamica (Bernoulli), 2:113, 2:195 Hydrodynamics, 2:96–97 See also Dynamics Hydroelectric dams conservation of energy, 2:176 fluid mechanics, 2:101 Hydrofluoric acid, 1:232 Hydrogen, 1:252–259, 1:258t alternative energy, 4:202 in amino acids, 3:11–13 atomic structure, 1:142–143 balloons and airships, 1:254, 2:105, 2:126–128 bonding, 1:265 component of universal matter, 1:123 infrared astronomy, 2:350 isotopes, 1:95–97 liquefaction, 2:200 percentage of biosphere, 3:348 percentage of human body mass, 3:78 Hydrogen bombs, 1:93, 1:97, 1:255, 2:177–179 See also Nuclear weapons Hydrogen chloride, 1:256 Hydrogen fuel, 1:259 Hydrogen peroxide, 1:109, 1:256 Hydrogen sulfide, 1:256, 3:54, 4:320–321 Hydrogenation, 3:36 Hydrologic cycle, 3:347–348, 4:369–375, 4:374t evapotranspiration, 4:387–394 hydrology, 4:361–362 precipitation, 4:387–394 weather, 4:396 Hydrology, 4:43, 4:361–367, 4:367t hydrologic cycle, 4:369–375

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How to go to your page whirlpools, 4:363 See also Earth sciences; Water Hydrolysis, 3:14 Hydrosphere, 3:346 geoscience, 4:26–27 hydrologic cycle, 4:369–370 hydrology, 4:361–362 See also Water Hypersomnia, 3:310 Hypersonic flight. See Supersonic flight Hypersound, 2:313 Hypoteneuse. See Trigonometry Hypotheses. See Scientific method Hyracotherium (early horse ancestor), 3:172 Hyraxes, 3:222

I I-hsing, 2:267 Ibn al-Haytham, 2:342, 2:354 Ibn Bâjja, 2:61 Ibn Sina, 2:61 ICBMs. See Intercontinental ballistic missiles (ICBMs) Ice buoyancy, 2:123 characteristics, 2:208 floating characteristics, 2:204 glaciology, 4:376–377 sedimentation, 4:285 snowflakes, 4:392 thermal expansion, 2:247, 2:248–249, 2:249 See also Glaciers; Water Ice ages climate, 4:411–412 glaciology, 4:381–384 Ice caps, 4:378, 4:379–380 Ice domes, 4:378 Ice fishing, 2:247 Ice sheets, 4:378–379 Ice shelves, 4:377, 4:379 Ice skating, 2:27, 2:33 See also Hockey Icebergs, 2:204 Iconometry. See Photogrammetry Ideal gas law, 1:53, 2:185 Igneous rocks, 4:148–149 Ileum, 3:47 Immune system, 3:262–269, 3:266t–267t AIDS, 3:245, 3:250, 3:258–261 autoimmune diseases, 3:230, 3:242 fighting infectious diseases, 3:244–245 Immunity and immunology, 3:255–261, 3:260t Immunotherapy, 3:239

S C I E N C E O F E V E RY DAY T H I N G S

Imprinting (behavior), 3:322–323, 3:328–329, 3:329, 3:333–334 Impulse (force) collisions, 2:41–44 linear momentum, 2:39–40 In vitro fertilization, 3:144, 3:144 Inbreeding, 3:115–116 Incandescent bulbs, 2:360–361, 2:367, 2:369 Incendiary devices, 1:175–176 Incest, 3:116 Inclined planes, 2:164–165 Galilean tests, 2:18, 2:71 origins, 2:159 screws, 2:165–167 wedges, 2:165 Index cards and Bernoulli’s principle, 2:119 Indian pipe (plant), 3:385 Indian vaccine research, 3:256 Indicator species, 3:71–72, 3:392, 4:352 Indigestion, 3:48 Indigo, 2:355 Indium, 1:157–158 Indo-European development, 3:162 Induced drag, 2:106 See also Drag Induction, electromagnetic, 2:341 Industrial distillation, 1:357–358 Industrial melanism, 3:173 Industrial minerals, 4:162, 4:164–165 Industrial Revolution, 3:173, 3:240 Industrial uses genetic engineering, 3:119 lactic acid in food production, 3:59–60 minerals, 4:162, 4:164–165 nighttime activity, 3:312 Industrialized nations causes of death, 3:231 deforestation, 3:365–366 Inelastic collisions, 2:40–44 Inert gases. See Noble gases Inertia cars, 2:62–63 centripetal force, 2:46–48 first law of motion, 2:18–19, 2:62–65, 4:171 frame of reference, 2:9–10 friction, 2:52–53 Galilean observations, 2:62 gravity, 2:72 linear momentum, 2:37 tablecloths, 2:63–64 See also First law of motion; Momentum Inertial navigation systems, 2:64 Infection, 3:283–291, 3:289t–290t Infectious diseases, 3:244–252 ecosystems, 4:344–345 glossary, 3:251t

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parasites, 3:278 plague, 3:230 relation to cancer, 3:240 See also Diseases, human Infiltration in hydrologic cycle, 4:372–373 Inflation, monetary, 1:4, 1:6 Influenza, 3:60 caused by virus, 3:287 epidemic 1918-1920, 3:249–250 Infradian cycles, 3:313 Infrared light, 2:349–350, 2:357–358 Infrared photography, 2:349–350, 4:54 Infrasound, 2:313 Inhibition model (biological communities), 3:401–402 Innate behavior, 3:319–320, 3:322, 3:328 Inner ear, 2:317–318 Inorganic compounds, 1:276 Inorganic substances. See Organic substances Inquilinism, 3:384 Insecticides. See Pesticides Insectivores, 3:219–220 Insects ants, 3:349, 3:386, 3:388–389 aphids, 3:388 bedbugs, 3:281–282 bees, 3:211, 3:211, 3:303–304, 3:323–324 butterflies, 3:338, 3:388–389 class Insecta, 3:275 evolution, 3:165 fleas, 3:282 infectious diseases, 3:245 lice, 3:282 locusts, 3:405 moths, 3:138, 3:173 parasites, 3:279–282 respiration, 3:57 symbiotic relationships, 3:386, 3:388–389 taxonomy, 3:200, 3:275 termites, 3:6, 3:6 ticks, 3:279, 3:282 Insomnia, 3:310 Instinct, 3:327–334, 3:333t, 3:335 See also Behavior; Learning and learned behavior Insulin, 3:120, 3:242–243 Intelligence, 3:168 Intelligent design theory, 3:168–169 Intercontinental ballistic missiles (ICBMs), 2:82 Interference (wave mechanics), 2:286–293, 2:291t–292t, 2:356 Intermolecular bonds, 1:47, 1:113–114 hydrogen, 1:253–254 metals, 1:151 minerals, 4:131–132 International Prototype Kilogram, 2:8

International System of Units, 1:7. See also Metric system International Ultraviolet Explorer, 2:350 International Union of Pure and Applied Chemistry (IUPAC). See IUPAC (International Union of Pure and Applied Chemistry) Interstate-285 (Atlanta, GA), 2:45–46 Intervals (music), 2:274, 2:276 Intestinal gas, 3:54 Intrinsic diseases, 3:229 Introduced species, 3:209–214 Intromission, 2:342, 2:354 Inuit, 3:207 Invisible fences, 2:323 Iodine, 1:234, 1:236 Ion exchange, 1:106 Ionic bonding, 1:268 carbon, 1:364 metals, 1:151 octet rule, 1:103–104 valence electrons, 1:113 Ionic compounds, 1:104 Ionization energy, 1:104–105 Ionizing radiation, 1:106, 1:108, 2:365 See also Ionosphere; Radiation (electromagnetism) Ionosphere, 2:346 Ions and ionization, 1:35, 1:84, 1:101–108, 1:107t, 1:121 chemical bonding, 1:267 static electricity, 1:102 Iranian earthquake (1755), 4:237 Iraqi desertification, 4:307–308 Iridium, 1:191, 3:185 Iron, 1:190, 1:336, 2:332 mass to volume ratio, 1:28, 1:36–37 rust, 1:285 Iron lung, 3:287 Iron ore mines, 1:183 Iron ore smelting, 1:190 Irruptive migration, 3:336 Islam, 3:246 Islands, 4:249 biogeography, 3:402–403 continental islands, 4:249–250 ecosystems, 4:251–252 geomorphology, 4:248–252 hot spots, 4:258 oceanic islands, 4:250–251 Isomers, 1:278 Isostatic compensation, 4:248 Isotopes, 1:35, 1:92–100, 1:99t–100t, 1:120–121, 4:74 actinides, 1:197 atomic mass, 1:65, 1:130 carbon, 1:243 hydrogen, 1:252

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J Jakob, Alfons Maria, 3:232–233 Janssen, Pierre, 1:238 Jar lids and thermal expansion, 2:250 Jejunum, 3:47 Jellyfish, 3:321 Jenner, Edward, 3:255–257, 3:257 Jet engines, 2:312 Jet lag, 3:310–311 Jewels, 4:165 Jews, 3:121 Johannes Philoponus, 2:61 Johnson, Earvin “Magic,” 3:259–261 Johnson, Paul, 2:10–12 Joint Institute of Laboratory Astrophysics (Boulder, CO), 2:211 Jolbert, Domina, 2:108 Jones, Indiana (fictional archaeologist), 4:17 Joule, James, 2:230 Joule (unit of measure), 2:219, 2:229, 2:345 Journey to the Center of the Earth (novel), 4:215–216 Jovian planets, 4:210 Julian calendar, 2:8 Junk food, 3:10, 3:79 Jurassic Park (movie), 3:184, 4:17 Justinian I (Roman emperor), 4:31

K Kaiko (unmanned underwater vessel), 2:124–125 Kangaroo rats, 3:329–330 Kangaroos, 3:219, 3:219 Kaposi’s sarcoma, 3:258, 3:259 Karate chops, 2:140 Karst topography, 4:247, 4:366–367 Kekulé, Friedrich August, 1:265–267 Kelp forests, 3:71 Kelvin, Lord. See Thomson, William, Lord Kelvin Kelvin scale, 2:184, 2:197 Kelvin temperature scale, 1:15–16, 1:52 Kennelly, Arthur Edwin, 2:346 Kennelly-Heaviside layer, 2:346 Kennicott Glacier (AK), 4:381 Kepler, Johannes, 4:66

S C I E N C E O F E V E RY DAY T H I N G S

laws of planetary motion, 2:71–72, 4:65 telescopes, 2:354 Ketoacidosis, 3:243 Ketones, 1:369 Kettle lakes, 4:380 Kettlewell, Bernard, 3:173 Kevlar, 1:375 Keys, David, 4:31 Keystone species, 3:69–71, 3:70 Kidney dialysis, 1:350, 1:351 Kilocalorie (unit of measure), 2:219, 2:229 Kilograms, 1:9 Kilohertz (unit of measure), 2:256–257 Kilowatt (unit of measure), 2:173–174 Kinematics, 2:13–20, 2:19t Kinetic energy, 1:11–12, 4:23–24 Bernoulli’s principle, 2:112 conservation, 2:28–29 conservation in elastic collisions, 2:41 falling objects, 2:174–175 formula, 2:29, 2:174 frequency, 2:271 heat, 2:227–228 molecular dynamics, 2:195 oscillations, 2:265–266 resonance, 2:279 roller coasters, 2:50 thermal expansion, 2:245 thermodynamics, 2:217–218 See also Energy Kinetic theory of gases, 1:53–54 Kinetic theory of matter Brownian motion, 1:69 gases, 2:196–197 molecular dynamics, 2:195–197 Kingdoms (taxonomy), 3:198–200 Kipfer, Paul, 2:126–127 Kircher, Athanasius, 3:288 Kirchhoff, Gottlieb, 1:306, 3:25 Kites, 2:107–108, 2:114, 2:116 Kleine-Levin syndrome, 3:310 Knives, 2:165 Knoop scale, 4:156 Knuckle balls, 2:81–82 Ko Yu, 2:162–163 Köppen, Wladimir, 4:407 Korean Air Lines Flight 007 shootdown (1983), 2:64 Kotler, Kerry, 3:108–109 Krakatau (Indonesia) eruptions, 4:259–260 meteorological effects, 4:31 Krebs cycle, 3:34–35 Krebs, Sir Hans Adolf, 3:34–35 Kronland, Johannes Marcus von, 2:296 Krypton, 1:241

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La Mettrie, Julien de, 4:90 Labels, food, 3:79 Labor (birth), 3:153 Laborers diseases, 3:240, 3:240 forced labor, 3:366 Lactic acid, 3:59 Lactose, 3:4 Lactose intolerance, 3:27 Laden, Osama bin, 4:19–20 Lagomorphs, 3:224–226 Lake Erie, 3:354, 4:318 Lake Victoria (Africa), 3:210 Lakes biomes, 3:375–376 eutrophication, 3:354 introduced species, 3:210–211 Lamarck, Jean Baptiste de, 3:165, 3:197 Laminar flow aerodynamics, 2:102–103 airplanes, 2:105–106 Bernoulli’s principle, 2:113–115 See also Airflow Lamps, 2:360–361 Landforms (geomorphology), 4:245–247 Landsat (satellite program), 4:57, 4:59 Lange, Dorothea, 4:304 Language used in chemical symbols, 1:122–123, 1:132–133 Languages, 3:162 Lanterns, 2:360–361 Lanthanide contraction, 1:207 Lanthanides, 1:155–156, 1:205–210, 1:209t electron configuration, 1:136 orbital patterns, 1:144, 1:146, 1:185–186, 1:197 Lanthanum, 1:208 Laplace, Pierre Simon de, 2:230, 4:90 Large intestine, 3:47 Las Vegas (NV), 1:239 Laser etching, 2:364 Lasers, 2:361–362, 2:363 fluorescence, 2:369 holograms, 2:298–300 Lasky, Melvin J., 4:263 Lates niloticus, 3:210 Latin used in element names, 1:122–123 Laue, Max Theodor Felix, 2:297 Laurasia, 4:220 Laussedat, Aimé, 4:53–54 Lava, 4:148

Lava domes, 4:258 Laval, Carl de, 2:49 Lavoisier, Antoine, 1:68, 1:112, 2:230 Law of chemical equilibrium, 1:299–300 Law of universal gravitation, 4:170–171 Laws of conservation. See Conservation laws Laws of friction, 2:52–54 Laws of motion, 2:18–20, 2:59–67, 2:68t gravity, 4:171 molecules, 2:206 potential and kinetic energy, 2:174 torque, 2:86 See also Conservation of angular momentum; Conservation of linear momentum; First law of motion; Laws of planetary motion; Motion; Second law of motion; Third law of motion Laws of planetary motion, 2:71–72, 4:65 See also Laws of motion Laws (Science). See Scientific method; specific laws Laws of thermodynamics, 2:216–217, 2:221–223, 4:194–195 Le Châtelier’s law, 1:19, 1:300 Leaching, 4:292 nitrogen cycle, 4:338 soil conservation, 4:305–306 See also Groundwater Lead, 1:158, 4:139 Learning and learned behavior, 3:319–320, 3:322, 3:327–334, 3:333t See also Behavior; Instinct Leaves, 4:295 Lebanon, Kansas, 2:137 Leewenhoek, Anton van, 3:288 Left-hand or right-hand amino acids, 3:13, 3:17 Legality of teaching evolution, 3:169 Lehmann, Johann Gottlob, 4:39, 4:88, 4:105 Leibniz, Gottfried Wilhelm, 2:300 Leks (animal territory), 3:324 Lemaître, Georges Édouard, 4:71 Lemons, 3:93 Lemurs, 3:220 Length (measurement), 1:23, 2:21 Lentic biomes, 3:375–376 Leonardo da Vinci, 2:360, 4:89 fossils, 4:88, 4:105 geology, 4:39 Leprosy, 3:248–249, 3:249 Leptons, 1:66 Leucippus, 1:67, 2:14 Lever arm. See Moment arm Levers, 2:160–162 classes, 2:161–162 mechanical advantage, 2:158 origins, 2:159 pulleys, 2:164

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How to go to your page wheels and axles, 2:162–164 See also Torque Lewis, Gilbert Newton, 1:268, 1:320 Lewis structure, 1:268–269 Lewis’s acid-base theory, 1:313, 1:321 Liang Ling-tsan, 2:267 Libraries and taxonomy, 3:194 Lice, 3:282 Lichen, 3:386 Lids and thermal expansion, 2:250 Life on Mars, 4:384 Lift (aerodynamics) airplanes, 2:105–106 birds, 2:104–105 cars, 2:109–110 Lift coefficient, 2:105–106 Light, 2:354–364, 2:363t–364t artificial light, 3:311–312, 3:312 Bose-Einstein Condensate, 2:211 electromagnetism, 2:349–350 luminescence, 2:366–367 source, 2:342 See also Corpuscular theory of light; Sunlight Light amplification by stimulated emission of radiation. See Lasers Light bulbs, 2:360–361, 2:367, 2:369 Light spectrum, 2:354–355 diffraction, 2:296–297 luminescence, 2:366–367 rainbows, 2:358–359 Light waves diffraction, 2:295–296 Doppler effect, 2:305, 2:307 frequency, 2:276 reflection, 2:259 resonance, 2:282 See also Waves Lighthouses, 2:297 Lightning, 4:398 Lilac, 3:355 Limes, 3:93 Lincoln, Abraham, 2:121–122, 3:114–115 Lind, James, 3:93 Lindbergh, Charles, 2:127–128 Linear momentum, 2:37–44 airbags, 2:43 baseball, 2:40, 2:44 conservation, 2:30–31, 2:33, 2:38–39 glossary, 2:42t–43t mass, 2:37–38 relationship to inertia, 2:37 second law of motion, 2:65 skydiving, 2:39, 2:43–44 Superman (fictional superhero), 2:44 water balloons, 2:44

S C I E N C E O F E V E RY DAY T H I N G S

See also Conservation of linear momentum; Momentum Linear motion, 2:16–18 See also Motion Linnaean system, 4:118–119 Linnaeus, Carolus, 3:197, 3:205, 4:118 The Lion King (movie), 4:64 Lipids, 3:44–45 importance in nutrition, 3:80 metabolism, 3:35 in proteins, 3:19 Liquefaction, 2:198, 2:200–201 Liquefied gases, 1:43 Liquefied natural gas (LNG) containers, 2:193 usage, 2:200–201 Liquefied petroleum gas (LPG), 2:200–201 Liquid crystals, 1:43, 2:212, 2:214 See also Crystals; Liquids Liquid filtration, 1:358 Liquid nitrogen, 1:214, 4:332 Liquids, 1:39 acoustics, 2:312, 2:321 behavior, 2:183 boiling, 2:209 characteristics, 2:148 evaporation, 2:208–209 liquid crystals, 2:212, 2:214 melting, 2:208 molecular behavior, 2:95–96 properties, 1:49t thermal expansion, 2:248–249 triple point, 2:6 volume measurement, 1:27–28, 2:23–24, 2:24 Lisbon earthquake (1755), 4:233–234 Listening devices, 2:322 Lister, Joseph, 3:290 Lithium, 1:164–166, 3:302 Lithium batteries, 1:163, 1:164–165 Lithosphere convection, 4:188 plate tectonics, 4:229 structure, 4:212–213 Lithostratigraphy, 4:106 Litmus paper, 1:314 Little Ice Age (1250-1850), 4:384 Liver, human, 3:79, 3:91–92 Living organisms and carbon, 1:243 Llamas, 4:344 LNG. See Liquefied natural gas Loa loa, 3:279 Lobsters, 3:171 Lockyer, Sir Joseph, 1:238 Locusts, 3:405 Loggerhead turtles, 3:338–339

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MacArthur, R. H., 3:402 Mach numbers

airplanes, 2:106–107 Doppler effect, 2:305 Machine efficiency, 2:159 friction, 2:55–56 thermodynamics, 2:216–218 See also Machines Machines, 2:157–169, 2:168t See also Force; Machine efficiency; Mechanical advantage; Work Mackenzie, D. P., 4:222–223 MacLeod, Colin Munro, 3:111 Macroscelideans, 3:224–226 Mad cow disease, 3:233 Madagascar, 3:138, 4:353 MAGLEV trains, 2:337–339 Magnesium, 1:172, 1:175 Magnetic compasses, 2:334–335, 4:180 Magnetic levitation trains, 2:337–339 Magnetic metals, 1:190–191, 2:331–332 Magnetic poles bar magnets, 2:334 Earth, 2:334–335, 3:338–339 electromagnets, 2:334 repulsion, 2:337–338 Magnetic repulsion, 2:337–338 Magnetic resonance imaging, 2:335–336, 3:237 Magnetic tape, 2:336–337 Magnetism, 2:331–339, 2:338t, 4:179–180 laws, 2:340–341 magnetic fields, 4:178 recording devices, 2:332, 2:333 See also Antiferromagnetism; Diamagnetism; Ferrimagnetism; Ferromagnetism; Geomagnetism; Paramagnetism Magnetometers, 2:335 Magnetorestrictive devices, 2:322 Magnetosphere, 4:181 Magnetron, 2:348 Magnets ferromagnetism, 2:332–334 magnetic fields, 2:332, 4:178 natural magnets, 2:331–332 See also Bar magnets; Electromagnets; Magnetism Maiman, Theodore Harold, 2:369 Main-group elements. See Representative elements Major histocompatibility complex, 3:262–263 Malaria, 3:249, 3:276–277 Male reproductive system, 3:142–143 Mallet, Robert, 4:234 Mallon, “Typhoid” Mary, 3:251 Malnutrition, 3:82–86, 3:89 Maltose, 3:4 Mammals class mammalia, 3:205, 3:218–226

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Logging and forestry, 3:363, 3:365–366, 3:403, 3:406, 3:407–408 Lohmann, Kenneth, 3:339 Loma Prieta (CA) earthquake (1989), 4:235 damage, 4:236 seismograph reading, 4:233 London dispersion forces, 1:47, 1:114–115 Long, Huey P., 4:365–366 Longitude, 2:5, 4:51–52 See also Prime meridian Longitudinal waves, 2:257–258 frequency, 2:272–273 ultrasonics, 2:320 See also Waves Lord Kelvin. See Thomson, William, Lord Kelvin Lord Rayleigh, 1:238–239, 2:358, 4:232 Lordosis (behavior), 3:330 Lorenz, Edward, 4:24–25, 4:404 Lorenz, Konrad, 3:320, 3:322–323, 3:325–326, 3:327, 3:328–329 Los Angeles (CA) geomorphology, 4:18 Lotic biomes, 3:375–376 Loudspeakers acoustics, 2:315, 2:320–321 magnetism, 2:336 Love, Augustus Edward Hough, 4:232 Love waves, 4:232–233 Lovelock, James, 4:27 Low-density lipoproteins, 3:36–37 Low-level clouds, 4:391 Loxodonta africana, 3:200 Loxodonta cyclotisare, 3:200 LPG (liquefied petroleum gas), 2:200–201 LSD (lysergic acid diethylamide), 3:307 Lubrication, 2:56 Lucretius (Roman philosopher), 1:263 Lucy (australopithecus), 3:216 Luminescence, 2:365–371, 2:370t Luminol, 3:21 Lunar cycle. See Moon Lung cancer, 3:62 Lungs, 3:57–58, 3:59 See also Respiration Lupus (systemic lupus erythematosus), 3:268 Lutetium, 1:210 Luxury taxes, 4:27–28 Lye, 1:317–318 Lyell, Charles, 4:90, 4:91 Lymph nodes, 3:263 Lymphocytes, 3:255, 3:263, 3:263, 3:264

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How to go to your page evolutionary history, 3:195, 3:204–205, 3:217–218 respiration, 3:58 Management of natural resources, 3:363 Manganese, 1:194 Mangrove forests, 3:362, 4:346 Manhattan Project, 1:72, 1:97, 1:199–200, 2:177 Manic depression, 1:165 Mantle (Earth). See Asthenosphere Mantophasmatodea, 3:200 Manx shearwaters (bird), 3:338 Mapmaking, 4:45, 4:46 Maps geography, 4:44–52 geologic maps, 4:44, 4:47–49 historical map of world, 4:16 Mercator projection of Earth, 4:46 relief map of Earth, 4:48 See also Cartography Marconi, Guglielmo, 2:342, 2:345 Marfan syndrome, 3:114–115 Margulis, Lynn, 4:27 Mariana Trench, 2:124–125 Marignac, Jean-Charles Galissard de, 1:208–209 Marine animals and phosphorescence, 2:369–370 Marrow (bone), 3:263, 3:263 Mars (planet) balloon exploration, 2:129 life, 4:384 Phobos, 4:174 planetary science, 4:66 roundness, 2:76–77, 4:173–174 Marsupials, 3:218–219, 3:219 Mason, Charles, 4:47 Mason-Dixon Line, 4:47 Mass, 1:23–30, 1:29t centripetal force, 2:46–47 definition, 2:21–22 gravity, 2:72–74, 4:173 laws of motion, 2:18–20 linear momentum, 2:37–38 measurement, 2:21 second law of motion, 2:65 weight vs., 1:76 See also Weight Mass energy. See Rest energy Mass extinctions, 4:123, 4:126–127 Mass movement (geology). See Mass wasting Mass number, 1:65 Mass spectrometry, 1:80–81, 1:106 Mass wasting, 4:273–280, 4:278t–279t, 4:284 Matches, 2:54, 2:56 Material causes (philosophy), 4:3–5 Material processing and ultrasonics, 2:325–326 Mathematical Principles of Natural Philosophy (Newton). See Principia

S C I E N C E O F E V E RY DAY T H I N G S

Mathematics, 2:14, 2:18 Mating rituals animal, 3:145, 3:145–147 human, 3:145 pheromones, 3:304 Matter, 1:44t–1:46t, 2:203–215 definition, 2:21–22 molecular dynamics, 2:195 phases, 2:14, 2:22, 2:197–198 properties, 1:33–47, 1:49t See also Phases of matter Maxwell, James Clerk, 4:178–179 electromagnetism, 2:20, 2:157, 2:207, 2:341, 2:356–357 kinetic theory of matter, 2:206 molecular dynamics, 2:194, 2:196 thermal expansion, 2:245–246 Mayer, Julius Robert, 2:222 Mayr, Ernst, 3:206 McCandless, Bruce, 4:215 McCartney, Paul, 2:323 McCarty, Maclyn, 3:111 M-discontinuity, 4:213, 4:240–241 Measurements, 1:3–10, 1:9t, 2:7–9 air pressure, 2:183 calibration, 2:8 establishment, 2:8 latitude and longitude, 2:7–8 metric system vs. British system, 2:8–9 standards, 2:21 temperature and heat, 1:17 value to societies, 2:8 See also English measurement system; Metric system Meat preservation, 1:351–353 protein content, 3:23 red meat in diet, 3:49–50 undercooked meat, 3:277–278, 3:303 See also Carnivores Mechanical advantage, 2:157–169, 2:168t See also Machines Mechanical deposition, 4:287 Mechanical energy conservation, 2:28–29 kinetic and potential energy, 2:174–177 thermodynamics, 2:217–218 See also Energy Mechanical waves interference, 2:289 properties, 2:256 See also Waves Mechanical weathering, 4:264 Mechanics (physics) classical physics, 2:20 machines, 2:158–160

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Mechanist school, 3:167 Medical treatments and research bacteriology, 3:288, 3:290 brain, 3:234 cancer, 3:239 childbirth, 3:155–157 cloning cells, 3:123 designer proteins, 3:21 genetic engineering, 3:103, 3:120, 3:122–123 immunology, 3:255–261 in vitro fertilization, 3:144 insulin, 3:120, 3:243 lithium, 1:165 ultrasonics, 2:324 use of amino acids, 3:15 x-rays, 2:350, 2:352 Medicines. See Drugs and medicines Megahertz (unit of measure), 2:256–257 Meiosis, 3:100, 3:136 Meitner, Lise, 1:199 Melanin, 3:130–131, 3:173 Melatonin, 3:307, 3:314–315 Melting, 2:207–208 Melting points alkali metals, 1:163–164 alkaline earth metals, 1:173 water, 1:38 Memoire sur la diffraction de la lumiere (Fresnel), 2:297 Mendel, Gregor, 3:110–111 Mendeleev, Dmitri, 1:69, 1:127, 1:128–130 Menstruation, 3:286, 3:313 Mercalli scale, 4:234–235 Mercator, Gerhardus, 4:46 Mercator projections, 4:46 Mercury (element), 1:189–190 barometers, 2:141 thermometers, 1:16, 1:189, 2:250–251 thermometric medium, 2:242 Merychippus (horse ancestor), 3:173 Mesosphere, 214 Mesozoic era, 3:183–184, 4:108, 4:117–118 Metabolic enzymes, 3:27 Metabolism, 3:33–43 disorders, 3:37–38 glossary, 3:41t–42t Metalloids, 1:147, 1:222–228, 1:227t Metals, 1:151–161, 1:152, 1:159t–160t cations, 1:103 conductivity, 2:228 crystals, 2:151–152 elasticity, 2:150–151 families of elements, 1:147 ferromagnetism, 2:332–334 housing, 1:154 metalloids, 1:222–223

microwave resonance, 2:283, 2:348 rust, 1:283 Metamorphic rocks, 4:150, 4:150, 4:152–153 Metchnikoff, Élie, 3:255 Meteor Crater (AZ), 4:91 Meteorites, 3:345 cause of mass extinction, 3:185–186, 3:186 life on Mars, 4:384 Meteor Crater (AZ), 4:91 See also Asteroids Meteorological radar, 2:303, 2:305 Meteorology, 4:402–404, 4:406–407 See also Weather Meters, 1:9 Methane produced by ruminants, 3:7–8 Metric system, 1:7–8 Celsius temperature scale, 1:15 comparison to British system, 2:8–9 establishment, 2:8 gravity, 2:72–74 pressure measurements, 1:50 See also Measurements Metronomes, 2:267, 2:274 Mettrie, Julien de La, 3:167 Meusnier, Jean-Baptiste-Marie, 2:126–127 Mexico City Olympics (1968), 2:146 Mica, 4:150 Mice, 3:123, 3:163–164, 3:226 Michell, John, 4:233–234 Microclimates, 4:407–409 Central Park (New York, NY), 4:410 sand dunes, 4:407 Microphones, 2:336 Microscopes, 3:288 Microwave ovens, 2:348 Microwaves communications, 2:347–348 frequency, 2:276–277 ovens, 2:348 radar, 2:348–349 resonance, 2:282–283 Mid-level clouds, 4:391 Mid-ocean ridges, 4:221, 4:222–223, 4:257 See also Rift valleys Middle Ages bestiaries, 3:196 plague (1347-1351), 3:247–248 Middle C, 2:273, 2:274 Middle ear, 2:317 Midgets, 3:129 Midwives, 3:153–154 Miescher, Johann Friedrich, 3:111, 3:117 Migration animal behavior, 3:335–341 early humans, 3:162 glossary, 3:340t

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How to go to your page Milankovitch, Milutin, 4:411 Military research into remote sensing, 4:53–54 Milk, 1:177 Milky Way (galaxy), 2:211, 4:67, 4:70–71 Millbrae (CA) mudflows, 4:276 Miller, Stanley, 3:179 Milligal (unit of measurement), 4:172 Mills, 2:163 Milne, John, 4:234 Mineraloids, 4:135 Minerals, 4:129–142, 4:140t–141t applications, 4:165 deposits, 4:288, 4:290–291 economic geology, 4:155–156 evaporites, 4:290–291 groups, 4:145 identification, 4:144–145 importance in nutrition, 3:80–81 jewels, 4:165 rocks, 4:143–145 serpentine, 3:71 soil, 4:294 Minié, Claude-Etienne, 2:80 Mining, 1:183 economic geology, 4:161–162 gold mines, 4:19 subsidence, 4:248 Minnows, 3:327–328 Mirages, 2:359 Mirrors, 2:361 Miscarriage, 3:152–153 Misch metal, 1:206, 1:208 Miscibility, 1:340 Mississippi River flooding, 4:365, 4:365–366 Mitosis, 3:99–100, 3:135 Mixtures, 1:329–337, 1:335t–336t, 1:354–355 compounds vs., 1:274, 1:275, 1:338–339 distillation, 1:355–358 filtration, 1:358–359, 355 minerals, 4:131 solutions, 1:338–339 Mobility (mammals), 3:198, 3:218 Modern physics, 2:20, 2:56 See also Physics Mohave Desert, 3:351 Moho (Mohorovicic discontinuity), 4:213, 4:240–241 Mohorovicic, Andrija, 4:213, 4:240–241 Mohorovicic discontinuity, 4:213, 4:240–241 Mohs, Friedrich, 4:136, 4:144 Mohs scale, 4:136, 4:144, 4:156 Moissan, Henri, 1:232 Molar mass, 1:26–27, 1:36, 1:81–82 Molarity, 1:340 Molecular dynamics, 2:192–202, 2:199t–201t gas laws, 2:209–210 matter, 2:205–207

S C I E N C E O F E V E RY DAY T H I N G S

resonance, 2:278 thermal expansion, 2:245–246 thermodynamics, 2:236–237 See also Dynamics; Molecules Molecular mass, 1:112 Molecular motion gases, 1:48–49 matter, 1:37 See also Brownian motion Molecular structure, 1:109–111, 1:115–116 acids and bases, 1:320 amino acids, 3:11–13, 3:12 proteins, 3:19 water, 1:347–348 Molecular translational energy, 1:19, 1:22 Molecules, 1:35–36, 1:109–116, 1:114t–115t Avogadro’s number, 1:53 electromagnetic force, 2:207 gases, 2:183–184 molecular dynamics, 2:192–202 phases of matter, 2:207 structure, 2:192–193, 4:315 structure of matter, 2:205 water, 4:370 See also Atoms; Molecular dynamics Moles (animals), 4:296–297 Moles (measurement), 1:53, 1:81–82 atomic theory, 2:205–206 Avogadro’s law, 2:184–185 See also Avogadro’s number Moment arm levers, 2:160–162 screws, 2:166 torque, 2:87–88 Momentum, 2:37–44 See also Angular momentum; Inertia; Linear momentum Monad, 2:300 Monera, 3:198 Monetary inflation, 1:4, 1:6 Monism, 3:167 Monomers, 1:372, 1:375 Monosaccharides, 3:3–4 Monotreme order, 3:218 Montagu, Lady Mary Wortley, 3:256 Montane forests, 3:363 Montgolfier, Jacques-Etienne, 2:105, 2:126, 2:144 Montgolfier, Joseph-Michel, 2:105, 2:126, 2:144 Moon, 4:74–77 astronomy and measurement, 4:80, 4:82–84 composition, 4:75–76 Earth-Moon system, 4:73 exploration, 4:76–77 glossary, 4:81t–83t lunar cycles, 3:308, 3:313

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statistics, 4:75 tidal energy, 4:197–198 Moon walk, 1:24 Moraines, 4:380 Morgan, Thomas Hunt, 3:111 Morrison, Herb, 1:257 Mosander, Carl Gustav, 1:207–209 Moseley, Henry atomic numbers, 1:130 x rays, 1:71 Moss, 3:137 Mothers and babies, 3:73 Moths, 3:138, 3:173 Motion Aristotelian, 2:15–16 Newton’s three laws of motion, 2:18–20 Zeno’s paradoxes, 2:14 See also Laws of motion; Linear motion; Projectile motion; Rotational motion Motion pictures and geoscience, 4:17–18 Motors, 2:27 Mount Etna (Italy), 4:148 Mount Everest (Nepal), 2:142 Mount Katmai (AK), 4:30 Mount Kilimanjaro (Tanzania), 4:254 Mount Machhapuchhare (Himalayas), 4:246 Mount McKinley (AK), 4:275 Mount Pelée (Martinique) eruption (1815), 4:260 Mount Pinatubo (Philippines) eruption (1991), 4:260, 4:409–410 Mount Saint Helens (WA), 4:30, 4:260 Mount Santo Tomas (Philippines), 4:407–408 Mount Tambora (Indonesia), 4:30 Mountain climbing, 1:298 Mountain ranges, 4:256–257 Mountain warfare, 4:263 Mountains, 4:253–263, 4:261t–262t animal migration, 3:336 microclimates, 4:407–408 Transantarctic Mountains (Antarctica), 4:379 See also Volcanoes; specific mountains and ranges Moynihan, Daniel Patrick, 2:338 Mud cracks, 4:287 Mudflows. See Flow (geology) Mules, 3:215, 3:216 Müller, Johann, 4:65 Municipal waste treatment, 1:360 Murrah Federal Building bombing (1995), 4:333 Muscovite, 4:139 Mushrooms, 3:384–385, 3:385 Music, 2:274, 2:276, 4:17 Musical instruments, 2:314 Muskets, 2:80 Mussels, 3:71, 3:211 Mutagens, 3:132 Mutation, 3:126–132, 3:131t

DNA, 3:101 early genetics research, 3:111 importance in evolution, 3:164–165 Mutualism (symbiosis), 3:273, 3:383–386, 3:388 Mycobacterium leprae, 3:248 Mycorrhizae, 3:384–386 Mythology and geoscience, 4:5–8

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N Nagasaki bombardment (1945), 1:72, 3:104 Naming conventions amino acids, 3:12–13 binomial nomenclature, 3:206 compounds, 1:276–277 elements, 1:133 geologic time periods, 3:179 germs, 3:283–284 vitamins, 3:88 worms, 3:274 Nanocomputers, 3:120 Nanotechnology, 2:56 Narcolepsy, 3:309 National Geographic Society, 1:256 National Institute of Standards and Technology (NIST), 1:7, 2:8 National Institutes of Health (NIH), 3:103, 3:120 National parks, 3:363 National Weather Service, 4:402–403 Native Americans, 3:127, 3:130–131, 3:186, 3:207, 3:396, 4:344–345 Native elements, 4:133–134 Natural convection, 2:228, 4:186 Natural disasters cyclones, 4:400–402 earthquakes, 4:233–237 mass wasting, 4:277 tornadoes, 4:399–400 volcanoes, 4:257–260 Natural magnets, 2:331–332 See also Magnets “Natural motion,” 1:67–68 Natural polymers, 1:373 Natural selection, 3:163–165, 3:204 animal migration, 3:335 behavior, 3:328, 3:393–394 choosing the ideal mate, 3:147–149 See also Evolution Navigation animal behavior, 3:335–341, 3:340t magnetic compass, 2:334 Nazis, 3:20, 3:121, 3:121–122 Nd YLF lasers, 2:362 Neanderthal man, 3:177 Necator americanus, 3:278

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How to go to your page Negative feedback. See Feedback Nematoda (phylum), 3:349 Neon, 1:239, 1:241–242 Neon Canyon (UT), 4:106 Neoprene, 1:378 Neptunist stratigraphy, 4:39, 4:88 Nernst, Hermann Walter, 2:223 Nerve cells, 3:296, 3:296–297, 3:299, 3:303 Nervous system, 3:295–297, 3:296 See also Brain Nesosilicates (minerals), 4:135 Net magnetic dipoles, 2:331 Neutralization, 1:322–323 Neutrons, 2:206 atomic mass, 1:131 atomic structure, 1:35, 1:66, 1:92 Chadwick, James, 1:71 electric charge of atoms, 1:93 New Guinea, 3:396, 3:398 New Madrid (MO) earthquakes, 4:235 New World. See Explorers and exploration New York City (NY), 3:247, 3:378, 4:410, 4:410 Newcomen, Thomas, 2:231 Newt suit, 2:145 Newton, Isaac, 4:210 apples, falling, 2:72 corpuscular theory of light, 2:290, 2:296, 2:342–343, 2:355–356 frame of reference, 2:9–10 gravity, 2:71–77, 4:170 laws of motion, 2:18–20, 2:59–67 machines, 2:158 molecular dynamics, 2:194 optics, 2:354–355 Principia, 2:18–20, 2:62 thermal expansion, 2:245 Newton (unit of measure), 2:73, 2:141, 2:219, 2:229 Newtonian physics frame of reference, 2:9–10 laws of motion, 2:18–20, 2:59–67 See also Aristotelian Physics; Classical physics; Physics Newton’s three laws of motion. See Laws of motion Niacin, 3:15, 3:95 Niche (ecology), 3:392, 4:352 Nickel, 1:184, 1:191 Nicola River Canyon (British Columbia), 4:265 Night, 3:311–312 Nile perch, 3:210 Nimbostratus clouds, 4:391–392 Niobium, 1:192 Nissan Hypermini, 1:163 NIST (National Institute of Standards and Technology), 1:7, 2:8 Nitrate, 4:335 Nitric oxide, 4:335–336

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Nitrification, 4:338 Nitrite, 4:335 Nitrogen, 1:216–218, 2:123–124 abundance, 4:332–333 applications, 4:333–334 biogeochemistry, 4:316 compounds, 4:334–336 depletion by leaching soil, 3:352–353 liquid nitrogen, 4:332 nitrogen cycle, 4:331–338 percentage of biosphere, 3:346, 3:348 properties, 4:333 used in fertilizers, 3:351 Nitrogen cycle, 3:348, 4:331–338, 4:336t Nitrogen fixation, 4:334, 4:337–338 Nitrogen gas, 1:59 Nitrogen narcosis, 2:123–124 Noah’s Flood (Bible), 4:6 Noble gases, 1:122, 1:214–215, 1:237–242 Noble metals, 1:122 Nocturnal activities, 3:336–337 Nomenclature. See Naming conventions; Taxonomy Noninfectious diseases, 3:236–243, 3:243t See also Diseases, human Nonmetals, 1:213–221, 1:220t anions, 1:103 compounds, 1:277–278 families of elements, 1:147 metalloids, 1:223 Nonsilicate minerals, 4:133–135 Norris, Joe, 2:326–327 Norris, Woody, 2:326–327 North American system of the periodic table of elements, 1:134 families of elements, 1:140 transition metals, 1:181–182 See also Periodic table of elements Northeast Passage, 3:88 Northern fur seals, 3:130 Northern latitudes and midnight sun, 3:310 Northern lights, 4:79, 4:79–80 Northern spotted owls, 3:406, 3:408, 4:354, 4:354–355 Norway, 3:309 Noses, 2:217 Novaya Zemlya (Russia), 3:88–89, 3:89 Nozzle method of liquefaction, 2:200 Nuclear bombs. See Nuclear weapons Nuclear energy, 2:177, 4:202, 4:206 Nuclear fission, 1:71–72, 1:199–200 Nuclear fusion, 1:72, 1:96–97, 1:255, 4:74, 4:78 Nuclear magnetic resonance, 2:282 Nuclear power, 1:71–72, 1:93, 1:207 Chernobyl nuclear disaster, 1986, 1:98, 1:103 plutonium, 1:201 uranium, 1:200–201 Nuclear radiation, 3:73

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Nuclear weapons, 1:72, 1:98, 1:292–293, 2:177–179, 3:103, 3:104 See also Hydrogen bombs Nuclear weapons testing, 1:177, 4:234 Nucleons, 1:65–66 Nuclides. See Isotopes Numbers, 1:3, 2:6–7 See also Coefficients Numerical taxonomy, 3:192–193 See also Taxonomy Nursing mothers, 3:73 Nutrition and nutrients, 3:44–45, 3:77–86, 3:83t–84t amino acids, 3:14–15 carbohydrates, 3:8–10 diseases, 3:230 fats, 3:37 iodine intake, 1:123 proteins, 3:22–23 recommended daily allowances (RDA), 1:125–126 vitamins, 3:95–96 See also American diet; German diet Nylon, 1:282, 1:366, 1:378

Obduction, 4:257 Obesity, 3:37, 3:39–40, 3:81, 3:127 See also American diet; German diet Obligate relationships (symbiosis), 3:273–274, 3:383–386 Obligatory taxonomy, 3:193–194, 3:205 See also Taxonomy Observation, 2:3–4 See also Scientific method Obstetricians, 3:154 Occupational health cancer, 3:240 nightshift, 3:309, 3:311 Oceania and marsupials, 3:219 Oceanic crust, 4:221, 4:229 Oceanic trenches, 4:224 Oceanography, 4:362 Oceans biomes, 3:376–377 convection, 4:191 currents, 4:363–364 food webs, 3:77, 3:376–377 Gaia hypothesis, 4:27 ice caps and sea levels, 4:379–380 mass extinctions, 3:182, 3:185 migrations, 3:336 oceanography, 4:362 percentage of Earth’s water in, 3:347 phosphorus in, 3:354

water pressure, 2:145 waves, 2:256, 2:258 Ockham’s razor, 4:4 Octaves, 2:274, 2:276 Octet rule, 1:103–104, 1:113, 1:214 Odum, Eugene Pleasants, 3:372 Oersted, Hans Christian, 2:340–341, 4:178 Offshore oil drilling, 4:160 Ogallala Aquifer (U.S.), 4:372, 4:373 Oil films, 2:290, 2:292 Oil industry, 4:158–160 Oil lights, 2:360–361 Oil refineries, 1:357 Oils, 1:333–334, 1:339, 1:342, 1:347–348 See also Fats and oils Oklahoma City (OK) bombing (1995), 4:333 Old-growth biological communities, 3:369, 3:402, 3:406, 3:408–409 Old-growth forests, 4:354–355 Old people. See Elderly Oldham, Richard, 4:237, 4:240 Oligosaccharides, 3:4 Olivi, Peter John, 2:61 Olympics (Mexico City, 1968), 2:146 Omnivores, 3:361, 4:200 On the Origin of the Species by Means of Natural Selection (Darwin), 3:169 Onnes, Heike Kamerlingh, 2:200 Open systems. See Systems Operant behavior, 3:320 Ophiolites, 4:257 Optical cavities (lasers), 2:361 Optical illusions, 2:359 Optics, 2:20 See also Light Oranges, 3:93 Orbital filling. See Orbital patterns Orbital patterns, 1:142 actinides, 1:196–197 irregularities, 1:136 principle energy levels, 1:135 representative elements, 1:143–144 transition metals, 1:144, 1:146, 1:184–185 Orbitals, 1:85, 1:87–89 electrons, 1:142 Schrödinger, Erwin, 1:74 Orbits (astronomy), 2:75–76, 4:55, 4:57 Orchids, 3:138, 3:385 Orders (taxonomy), 3:200, 3:218–226 Ordovician period, 3:182 Ores, 4:161–162 Organ transplants, 3:262–263 Organic and health foods, 3:86 Organic chemistry, 1:245, 1:363–371, 1:370t, 1:373, 4:326 Organic compounds, 1:276, 1:367–368

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How to go to your page Organic minerals, 4:134–135 Organic substances biogeochemical cycles, 3:348 biomes, 3:371 carbon and nutrition, 3:78 distinguished from inorganic, 3:176, 3:391 early life on Earth, 3:178 food webs, 3:361 history, 3:178 Origin of life, 3:178–182 amino acids dating, 3:17 theories of evolution, 3:162 Origin of the universe, 3:177–178 Ornithischia, 3:184 Orogenesis, 4:254–255 See also Tectonism Orphan metals, 1:156–158, 1:222, 161 Orphan nonmetals, 1:214, 1:219, 1:221 Oscillations, 2:263–269, 2:268t–269t acoustics, 2:311 electromagnetism, 2:343 frequency, 2:272 resonance, 2:278, 2:280–281 See also Harmonic motion; Vibrations; Wave motion Osmium, 1:191 Osmosis, 1:347–353, 1:348, 1:350, 1:352t Osmotic potential, 1:350–351 Osteomalacia, 3:91 Ostrom, John M., 4:128 Outer space. See Vacuum Ovens, microwave, 2:348 Overdamping, 2:266 Overproduction and natural selection, 3:163 Oviparity, 3:151, 3:152 Oviviparity, 3:151 Ovulation, 3:143 Owls, 3:406, 3:408 Oxidation, 1:293 effect on human body, 3:91 prevention, 4:333 Oxidation numbers, 1:290–291 Oxidation process, 1:218–219 Oxidation-reduction reactions, 1:285, 1:289–296, 1:292, 1:295t Oxidation states, 1:290–291 Oxides (chemicals), 1:218–219 Oxides (minerals), 4:134 Oxpecker, 3:386, 3:387 Oxygen, 1:218–219 absorbed in lungs, 3:55 early Earth, 3:178, 3:179 equilibrium, 1:301 mountain climbing, 1:298 oxidation-reduction reactions, 1:290 percentage of biosphere, 3:346, 3:348

S C I E N C E O F E V E RY DAY T H I N G S

percentage of human body mass, 3:78 produced in photosynthesis, 3:5, 3:58 quantity on Earth, 1:123 used in fertilizers, 3:351 Oxygen bars, 1:215, 1:219 Oxytocin, 3:153 Ozone, 1:233–234, 1:307

Cumulative General Subject Index

P Packe, Christopher, 4:47 Paired forces, 2:66–67 Paleomagnetism, 4:184, 4:225, 4:228 Paleontology, 3:176–188, 3:187t–188t, 4:114–128, 4:124t–126t Paleozoic era, 4:108, 4:116, 4:117 Palladium, 1:191 Palynology, 4:119 Pancreas, human, 3:47 Pangaea, 3:180, 4:220–221 Pangolins, 3:223, 3:223–224 Panthalassa, 4:220 Paper airplanes, 2:108 Papin, Denis hydraulic presses, 2:167 steam engines, 2:231 Parabolas, 2:79 Parachuting. See Skydiving Paradigms, 4:36 Parafoils, 2:108 Parahippus (early horse ancestor), 3:173 Paraho Oil Shale Facility (CO), 4:159 Paramagnetism, 2:332 See also Magnetism Parasites and parasitology, 3:273–282, 3:280t–281t, 3:330–331, 3:383–384 Parker, R. L., 4:222–223 Parkes, Alexander, 1:377 Parthenogenesis, 3:137–138 Parthenon (Greece), 3:138 Partial migration, 3:336 Partial pressure (gases), 1:54–55, 2:184–185, 2:187 Particle accelerators, 1:72 Particle-wave hypothesis, 1:87–88 Pascal, Blaise hydraulic press, 2:160, 2:167 Pascal’s principle, 2:96–97, 2:142–143 Pascal (unit of measure), 2:141 Pascal’s principle fluid mechanics, 2:96–97 fluid pressure, 2:142–143 hydraulic presses, 2:98–99, 2:167 Passionflower, 3:389 Pasteur, Louis lactic acid fermentation, 1:306

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pasteurization, 3:288 vaccinations, 3:256, 3:257 Pastuerlla pestis, 3:248 Patented research, 3:121 Pathogens cause infection, 3:285–286 glossary, 3:284 infectious diseases, 3:245 targeted by immune system, 3:255, 3:262 Patriot missiles, 2:83 Pauli exclusion principle, 1:89, 1:142 Pauli, Wolfgang, 1:89 Pauling, Linus, 1:269, 3:92, 3:93 Pavlov, Ivan, 3:320 Payen, Anselme, 1:306 Peas, 4:199 Peattie, Roderick, 4:255 Pelagic ocean biomes, 3:376 Pellagra, 3:15, 3:95 Pendula oscillations, 2:267–269 resonance, 2:281–282 See also Pendulum clocks Pendulum clocks, 2:267, 2:272, 2:274 See also Pendula Pendulums. See Pendula Penicillin, 3:290 Pentadactyl limb, 3:170, 3:170 Pepper moths, 3:173 Peptide linkage, 3:13, 3:18 Periodic table of elements, 1:121–122, 1:127–139, 1:129t, 1:137t–138t atomic mass units (amu), 1:81 carbon group, 1:244 electron configurations, 1:89–90, 1:364 families of elements, 1:140–147 halogens, 1:229–230 ion formation, 1:102–103 isotopes, 1:71 Mendeleev, Dmitri, 1:69 metals, 1:152–156 noble gases, 1:237 nonmetals, 1:213–215 transition metals, 1:181–185 Periods of the periodic table of elements, 1:127, 1:134, 1:135, 1:142 Periods (wave mechanics) acoustics, 2:311 Doppler effect, 2:301 electromagnetism, 2:343 frequency, 2:273–274 interference, 2:286–287 resonance, 2:279 ultrasonics, 2:319 waves, 2:257 Perissodactyls, 3:222

Permeability and hydrologic cycle, 4:373 Permian period, 3:182–183 Perpetual motion machines friction, 2:55 mechanical advantage, 2:158–159 thermodynamics, 2:216, 2:223 See also Machines Perrin, Jean Baptiste, 2:196, 2:206 Pest control, 2:323–324 Pesticides, 3:72, 3:73, 4:358 Petrochemicals, 1:257, 1:367, 1:368, 4:160 Petroleum economic geology, 4:158–160 fermentation, 3:28, 3:30 fractional distillation, 1:368 remains of dinosaurs, 3:185 See also Fossil fuels Petroleum industry, 1:357, 1:358 Petroleum jelly, 1:366 Petrology, 4:147–148 See also Rocks Pewter, 1:336 pH levels, 1:314, 1:323 Phanerozoic era, 4:108 geologic time, 4:100–101 paleontology, 4:116 Phase diagrams, 1:40–42, 1:41 Phases of matter analogy to human life, 2:211–212 changes, 2:211 molecules, 2:207 temperature, 2:237 thermal expansion, 2:245–246 triple point, 2:214–215 See also Matter Phenetics, 3:192–193 Phenotype, 3:110 Phenylalanine hydroxylase, 3:37 Phenylketonuria (PKU), 3:37 Pheromones, 3:303–304 Philippine microclimates, 4:407–408 Philosophiae Naturalis Principia Mathematica (Newton). See Principia Phobos (moon), 4:174 Pholidota, 3:223, 3:223–224 Phoresy (symbiosis), 3:389 Phosphates (minerals), 4:134, 4:317 Phosphor, 2:369 Phosphorescence, 2:367, 2:369–370 Phosphorus, 1:219 biogeochemistry, 4:317–318 importance in nutrition, 3:91 percentage of biosphere, 3:348 phosphorus cycle, 3:354 plants and fungi, 3:384 Phosphorus cycle, 4:317–318

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How to go to your page Photoelectric effect light, 2:343, 2:357 ultraviolet radiation, 2:341–342 Photogeology, 4:54–56 Photogrammetry, 4:53–54 Photography in geoscience, 4:55–56 Photoionization, 1:105–106 Photons, 2:343–344, 2:345, 2:357 See also Corpuscular theory of light; Light; Light waves Photosynthesis, 1:250, 4:199 carbon cycle, 4:330 creation of oxygen, 3:346 energy, 4:199–200 energy transfer, 3:360–361 micorrhizae, 3:385 production of carbohydrates, 3:3, 3:4, 3:4–5 stomata, 4:388 Phototropism, 3:321 Phyllosilicates (minerals), 4:135 Phylogeny, 3:191–192, 3:195, 3:197–198, 3:204, 3:216, 3:217 Phylum chordata, 3:205 nematoda, 3:349 Physical changes to matter, 1:34, 1:281–283, 1:289–290, 1:297–298 Physical fitness. See Fitness Physical geodesy, 4:171–172 Physical geology, 4:39–40 Physical oceanography, 4:362 Physical weathering, 4:264, 4:274 Physicians, 3:153–154, 3:238–239 Physics (Aristotle), 2:14–16, 2:61 Physics vs. chemistry, 1:119 Physics, classical. See Classical physics Physics (science), 2:13–16 See also Aristotelian physics; Classical physics; Modern physics; Newtonian physics Phytoplankton, 3:77, 3:376 Pi (coefficient), 2:7, 2:45 Pianos, 2:273, 2:313 Piccard, Auguste, 2:124, 2:126–127 Piccard, Bertrand, 2:127 Piccard, Jacques, 2:124–125 Picnic at Hanging Rock (movie), 4:17–18 Pictet, Raoul Pierre, 2:200 Picture frames, 2:136 Piedmont glaciers, 4:378 Piezoelectric devices, 2:322–323 Pigeons, 3:338 Pigmentation, 2:359–360 Pillars of Hercules (Mediterranean Sea), 4:5 Pima (Native American tribe), 3:127 Pineapples, 3:137 Pinworms, 3:278

S C I E N C E O F E V E RY DAY T H I N G S

Pipes Bernoulli’s principle, 2:97, 2:112–113 fluid pressure, 2:144 Pistons gas laws, 2:188–189 hydraulic presses, 2:167–169 pumps, 2:99 Pitch (orientation), 2:106 Pivot points, 2:86–87 Place-value numerical system, 1:3 Placenta, 3:153 Placer deposits, 4:288, 4:290 Plagues, 1:360, 3:230, 3:231, 3:246–248 Planck, Max, 1:73, 2:343, 2:357 Planetary gears, 2:163 Planetary science, 4:63–71, 4:69t–4:70t density, 4:210 geoscience, 4:35, 4:42 Planetology. See Planetary science Planets density, 4:210 origin, 3:178 spherical shape, 4:173–174, 4:174 Plants angiosperms vs. gymnosperms, 4:347–349 behavior, 3:321 biomes, 3:372–375 blue-green algae, 4:293 carbon dioxide, 4:327–328 erosion control, 4:272 evapotranspiration, 3:346, 3:347, 3:354–355, 3:355 evolution, 3:173–174 fermentation, 3:28 food webs, 4:200, 4:342 forensics, 3:108 genetic engineering, 3:119 greenhouse effect, 4:355 hybridization, 3:110 introduced species, 3:210–214 kingdom plantae, 3:198 mass wasting, 4:279–280 osmosis, 1:350–351 photosynthesis, 3:4–5, 3:58, 3:77, 3:87–88, 3:346, 3:360–361 protein content, 3:23 reproduction, 3:136–141, 4:347–349 respiration, 4:329–330 selective breeding, 3:128, 3:387 soil formation, 4:293, 4:294 starches and cellulose, 3:6–8 symbiotic relationships, 3:384–386, 3:389 transpiration, 4:389 vegetative propagation, 3:136 See also Fruits and Vegetables; Trees Plasma (blood), 1:343–344, 3:47

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Plasma (matter), 1:42, 2:14, 2:210 Plasmodium, 3:249, 3:276–277 Plastic deformation, 2:150 See also Deformation Plastics, 1:364, 1:365, 1:366–367, 1:375–380 Plate tectonics, 4:219–229, 4:226t–228t convection, 4:188 Earth’s interior, 4:210–211 evolution, and, 3:169, 3:180 mass wasting, 4:277–278 mid-ocean ridges, 4:221 mountains, 4:253–255 seismology, 4:228, 4:231–232, 4:237 uplift, 4:248 volcanoes, 4:213–214 Platinum, 1:191 Platinum group metals, 1:191–192 Plato (Greek philosopher), 2:359, 3:197, 4:132–133 Platonic solids, 4:132–133, 4:133 Pliohippus (horse ancestor), 3:173 “Plum pudding model” (atomic structure), 1:70, 1:86 Plutonism, 4:39 Plutonium, 1:201 Pneumonia, 3:60, 3:62, 3:287 Pohutu Geyser (New Zealand), 4:196 Points frame of reference, 2:5–6 Zeno’s paradoxes, 2:14 See also Cartesian system; Coordinates; Graphs Polar bears, 3:89, 4:406 Polar covalent bonding, 1:269 Polar glaciers, 4:378 Polarity of molecules, 3:19 Poliomyelitis, 3:257–258, 3:287 Pollen and pollination, 3:138–141, 3:364, 3:364–365, 4:119, 4:347–349 Pollution cancer-causing, 3:240–241 impact on biological communities, 3:403 Polonium, 1:226–227 Polyatomic ions, 1:278 Polyester, 1:378–380 Polymerization, 1:375 Polymers, 1:372–380, 1:373, 1:379t elasticity, 2:152 made of amino acids, 3:13, 3:18–19 oscillations, 2:267 ultrasonics, 2:326 Polysaccharides, 3:4 Pomponius Mela, 4:407 Ponds, 3:375–376 Popocatepetl (volcano), 4:257 Popp, Georg, 4:20–21 Pork, 3:277–278 Porous-plug method of liquefaction, 2:200 Porpoises, 3:221, 3:340–341

Positive feedback. See Feedback Post-traumatic stress disorder, 3:232 Potash, 1:167, 1:170 Potassium, 1:167, 1:170 Potassium-argon dating, 1:238, 4:98 Potato chip bags, 2:188 Potential energy, 1:11–12, 4:23–24 Bernoulli’s principle, 2:112 conservation, 2:28–29 falling objects, 2:174–175 formulas, 2:29, 2:174 frequency, 2:271 oscillations, 2:265–266 resonance, 2:279 roller coasters, 2:50 thermodynamics, 2:217–218 See also Energy Pott, Percivall, 3:240 Poverty and the poor forced labor, 3:366 malnutrition, 3:85, 3:95 occupational health, 3:240 Power, 2:173–174, 2:260 See also Energy; Force; Work Power lines, 2:250 Prairie dogs, 4:297 Prandtl, Ludwig, 2:113 Precambrian eon geologic time, 4:100–101 life forms, 4:117 paleontology, 4:115–116 Precession and ice ages, 4:411 Precipitation, 4:387–394, 4:393t–394t deserts, 3:375 hydrologic cycle, 4:372 rain, 3:358, 3:374 Pregnancy, 2:324, 3:151–157, 3:156t Preservation (food), 1:351–353 Pressure, 1:50–51, 1:300–301, 2:140–147, 2:146t See also Air pressure; Fluid pressure Pressurized oxygen, 1:298 Prevost, Pierre, 2:221 Priestley, Joseph, 4:327 carbon monoxide, identification of, 1:248 carbonated water, 1:248 Primary colors, 2:360 See also Colors Primary succession (ecology), 4:352 Primates, 3:167–168, 3:205–206, 3:216, 3:220–221 Prime Meridian, 2:5 See also Longitude Prince William Sound earthquake (1964), 4:235 Principia (Newton) gravity, 2:72 laws of motion, 2:18–20, 2:62 Principle energy levels, 1:88–89, 1:135

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How to go to your page actinides, 1:196 orbitals, 1:142 transition metals, 1:184 Prisms diffraction, 2:296–297 infrared light, 2:349 spectrum, 2:354–355 Probability amino acids in proteins, 3:18–19 base pairs in genes, 3:100, 3:118 Proboscideans, 3:222 Producers (food webs), 3:68 See also Food webs Projectile motion, 2:78–85, 2:84t bullets, 2:79 Galilean theories, 2:17 human cannonballs, 2:70 See also Motion Proofs. See Scientific method Propagation. See Reproduction; Vegetative propagation Propane, 1:56 Propane tanks, 2:188 Propellants in aerosol cans, 1:55 Propellers airplanes, 2:106 Bernoulli’s principle, 2:116 boat screws, 2:166–167 Properties of matter, 1:33–47, 1:44t–46t, 1:49t Prosimii, 3:220–221 Protactinium, 1:199 Proteins, 3:18–23, 3:22t, 3:44 complete proteins, 3:80 content of vegetables, 3:6 importance in nutrition, 3:10, 3:79–80, 3:81–82 made of amino acids, 3:13 synthesis, 3:100–101 Proterozoic eon, 4:116 Protista, 3:198 Protons, 2:206 acids and bases, 1:312–313 atomic structure, 1:35, 1:64, 1:92, 1:130 identified, 1:70–71 Protozoa, 3:275–277 Proust, Joseph-Louis compounds, 1:112, 1:276, 1:329–330 constant composition, 1:68 Psychological disorders Alzheimer’s disease, 3:233, 3:233–235 eating disorders, 3:38–40 mental retardation, 3:334 seasonal affective disorder, 3:314–315 Ptolemy (Greek scientist) cosmology, 4:38, 4:64–65 geography, 4:45 Pueblo people and desertification, 4:308–309

S C I E N C E O F E V E RY DAY T H I N G S

Pulleys, 2:163–164 Pulses (wave motion), 2:258–259 Pumps Archimedes screws, 2:166 fluid mechanics, 2:99 Purbach, Georg, 4:65 Pygmies, 3:128 Pyramids construction, 4:36, 4:147 Egyptian, 2:162, 2:164–165 Mayan, 4:146 Pyrometers, 1:17, 2:243–244 Pythagoras, 2:14, 4:8

Cumulative General Subject Index

Q Quantum mechanics, 2:201–202, 3:165–166 Quantum theory electromagnetism, 2:343–344 electrons, 1:87 light, 2:357 Planck, Max, 1:73 Quarks in electric charges, 1:65–66 Quartz, 4:136 Quasi-states of matter, 1:42

R Rabbits, 3:226 Rabies, 3:257 Race (humans), 3:207 Racing cars, 2:109–110 Radar, 2:348–349, 4:56–57 Radar ranges, 2:348 Radiation (electromagnetism), 2:229, 4:186 cancer-causing, 3:240–241 cancer treatments, 3:239 effects of exposure, 1:98, 3:103, 3:104 fire, 2:360 light, 2:356 luminescence, 2:365 microwave ovens, 2:348 nuclear, 3:73 thermodynamics, 2:221 Radiators, 2:248 Radio broadcasting, 2:276–277, 2:345–346 Radio waves AM and FM transmissions, 2:260–261 discovery, 2:345 FCC spectrum divisions, 2:346–347 frequency, 2:259–260, 2:276–277 luminescence, 2:366 resonance, 2:282 See also Radio broadcasting

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Radioactive decay, 1:95, 1:240 absolute dating, 4:97 particle tracks, 4:96 Radioactive waste, 1:98 Radioactivity, 1:70–72, 1:94–95, 1:97–98, 1:177 actinides, 1:197 thorium, 1:198 uranium, 1:199–200 Radiocarbon dating. See Carbon ratio dating Radioisotopes, 1:95, 1:197 Radiometric dating. See Absolute dating Radium, 1:178–179 Radon, 1:239–240, 1:241 Raiders of the Lost Ark (movie), 4:17–18 Railroad tracks, 2:246, 2:250 Rain, 4:279, 4:392 See also Precipitation Rain clouds, 4:391–392 Rain forests, 4:346 biodiversity, 3:392–393 climate, 3:362–363, 3:373–374 deforestation, 3:365, 3:366, 4:353 soil, 3:350, 4:297 Rain shadows, 4:260 Rainbows, 2:358–359, 4:78 Ramsay, Sir William, 1:238–239 Rape cases, 3:108–109 Raphus cucullatus, 3:208–209 Rapture of the deep, 2:123–124 Rare Earth-like elements, 1:194–195 Rare gases. See Noble gases Rats, 3:226, 3:330 Rayleigh, John William Strutt, Lord, 1:238–239, 2:358, 4:232 Rayleigh scattering, 2:358 Rayleigh waves, 4:232 Raytheon Manufacturing Company, 2:348 RDA (recommended daily allowances), 1:125–126 Reactants, 1:283, 1:299 Reactivity halogens, 1:231 noble gases, 1:237 Reagan, Ronald, 2:178–179 Reasoning ability, 3:168 Reaumur, Antoine Ferchault de, 1:15, 2:240 Rebar, 2:151 Receptors (senses), 3:296–297, 3:298–299 Recessive genes, 3:112–113, 3:115 Recommended daily allowances (RDA), 1:125–126 Recording devices, 2:332, 2:333, 2:336–337 Recycling, 1:380 Red blood cells, 1:351 malaria, 3:276–277 produced by bone marrow, 3:263 Red Sea, 4:224, 4:225 Red shift, 2:307, 2:358

Redox reactions. See Oxidation-reduction reactions Reflection optics, 2:354 waves, 2:258–259, 2:356 Reflections on the Motive Power of Fire (Carnot), 2:221–222, 2:231–232 Reflexes, 3:321, 3:322 Refraction optics, 2:354 waves, 2:356 Refrigerators first law of thermodynamics, 2:222 reverse heat engines, 2:229, 2:232 thermodynamics, 2:219–220 Regiomontanus, 4:65 Regolith, 4:265, 4:274, 4:296 Reich, Ferdinand, 1:157–158 Relative dating geologic time, 4:95–96 paleontology, 3:172, 4:119 stratigraphic column, 4:107 Relative motion astronomy, 2:8–9 Doppler effect, 2:302–303 relativity, 2:10 See also Relativity Relativity atomic theory vs., 1:72–73 conservation of rest energy, 2:29 frame of reference, 2:10 gravity, 2:77 social implications, 2:10–12 syadvada, 2:3–4 See also Relative motion; Rest energy Religion and science, 3:166–167, 4:3–11, 4:38 creationism, 3:163 historical geology, 4:87–90 planetary science, 4:63–64 See also Roman Catholic Church Remote sensing, 4:53–59, 4:58t Earth’s interior, 4:212 geodesy, 4:172 geography, 4:52 weather forecasting, 4:402–404 Renne, Paul R., 4:126 Replication of DNA, 3:100–101 Representative elements atomic size, 1:141 electron configuration, 1:136 valence electrons, 1:143–144 Reproduction, 3:135–141, 3:139t–140t See also Asexual reproduction; Sexual reproduction Reproductive system (human), 3:142–143 Republic (Plato), 2:359 Repulsion, magnetic, 2:337–338

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How to go to your page Reservoir rocks, 4:158 Resins in electric charges, 1:106 Resistance thermometers, 2:242–243 Resonance, 2:278–285, 2:284t, 2:288–289 Respiration, 3:55–63, 3:59, 4:329–330 glossary, 3:61t–62t See also Cellular respiration Respiratory system, 3:56–58, 3:59 Rest energy conservation of energy, 2:177 conservation of matter, 2:203–205 conservation of rest energy, 2:29 formula, 2:29, 2:174, 2:179–180 thermodynamics, 2:218 Restoring force (oscillations), 2:264–265 Resultants (vector mathematics), 2:17 Retroviruses, 3:287 Reverse heat engines, 2:229, 2:232 Reverse osmosis, 1:353 Rheumatoid arthritis, 3:268 Rhinoceroses, 3:386, 3:387 Rhodium, 1:191–192 Ribonucleic acid. See RNA (ribonucleic acid) Rice, 3:95 Richter, Hieronymus Theodor, 1:157–158 Richter, John, 4:234 Richter scale, 4:234–235 Rickets, 3:90, 3:91 Rifles, 2:31, 2:80 Rift valleys divergence, 4:224 oceanic, 4:223 Red Sea, 4:225 See also Mid-ocean ridges Right-hand or left-hand amino acids, 3:13, 3:17 Right-hand rules electromagnetism, 2:344 torque, 2:89–90 Right whales, 3:208 River blindness, 3:279 Rivers, 4:374–375 Bernoulli’s principle, 2:96, 2:97, 2:113 biomes, 3:376 deltas, 3:351, 4:56, 4:268, 4:300 sedimentation, 4:285 soil, 3:351, 4:300 RNA (ribonucleic acid), 3:101, 3:287 Roads, 2:48 Rock cycle, 4:152–153, 4:288 Rockets conservation of linear momentum, 2:31, 2:33 projectile motion, 2:82–85 third law of motion, 2:85 Rocks, 4:143–153, 4:151t–152t arches, 4:267 cap rocks, 4:159

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compression, 4:14, 4:15 dating, 4:39, 4:99 deformation, 4:219–220 sediment sizes, 4:286 soil formation, 4:292 See also Petrology Rocky Mountain bighorn sheep, 3:323 Roden Crater (AZ), 4:16 Rodents, 3:224–226 Rodinia, 4:220 Roemer, Olaus, 1:14, 2:239–240 Roentgenology, 2:350, 2:352–353 Rogallo, Francis, 2:108 Roll (orientation), 2:106 Roller coasters, 2:47, 2:49–50 Rolling friction, 2:53–54 Roman Catholic Church, 2:16, 2:18, 2:70–71, 4:38 Roman Coliseum, 4:15 Roman Empire, 3:230, 3:246, 4:31 Roman numerals used in measurement, 1:3–4 Ronne Ice Shelf, 4:379 Röntgen, Wilhelm, 1:70, 2:350 Roosts (animal territory), 3:324 Rosenberg, Ethel, 2:177 Rosenberg, Julius, 2:177 Ross Ice Shelf, 4:379 Rotational equilibrium, 2:135 See also Equilibrium Rotational motion, 2:17 See also Motion Roundworms, 3:278 Rowland, Henry Augustus, 2:298 Royal Gorge (CO), 4:270 Royalty and hemophilia, 3:115–116, 3:241 Rozier, Jean-François Pilatre de, 2:126 Rubber, 2:151, 2:152, 2:267 Rubidium, 1:170 Ruby lasers, 2:369 Ruminants, 3:7–8, 3:53 Runnels, 4:375 Runoff, 4:373 Rural techno-ecosystems, 3:379–380 Russia lag in industrialization, 2:8 royalty, 3:241, 3:242 Rust, 1:283, 1:285, 1:294 Ruthenium, 1:192 Rutherford, Daniel, 4:333 Rutherford, Ernest, 1:70–71, 1:77, 1:79–80, 1:93, 1:130, 1:164

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Sahara Desert caravans, 4:307 desertification, 3:353, 4:306–307 sand dunes, 4:407 Sahel region desertification, 4:307 Sailors, 3:93 Saliva, human, 3:45 Salk, Jonas, 3:257–258 Salmon, 3:337 Salmonella typhosa, 3:251 Salt caravans, 1:168 Salt water, 1:343 distillation, 1:355 human body’s reaction to, 1:350 reverse osmosis, 1:353 Salts, 1:230 formation, 1:111, 1:230 ionic bonding, 1:104 meat curing, 1:352–353 in soil, 3:350 Samarium, 1:207, 1:208–209 San Andreas Fault, 4:224 San Blas Indians, 3:130–131 San Francisco earthquake (1906), 4:235 Sand castles, 4:265–266, 4:274 Sandia National Laboratories, 2:56 Sandstone erosion, 4:26 Sanitariums for leprosy, 3:248 Sanitary conditions, 3:287–288, 3:288, 3:290 Santos-Dumont, Alberto, 2:127 Satellites communications satellites, 4:55 first law of motion, 2:59–61 gravity, 2:75–76, 4:176 microwave communications, 2:347–348 remote sensing, 4:57, 4:59 Saturated fats, 3:36, 3:88 Saturated hydrocarbons, 1:367–368 Saturation, 1:340–341 Saturn (planet), 1:26 Saturn V rockets, 2:85 Sauerkraut, 3:26 Saurischia, 3:184 Savannas, 3:373, 3:374–375 Savery, Thomas, 2:231 Scalar measurements speed, 2:17–18 statics and equilibrium, 2:133 work, 2:170–171 See also Measurements; Vector measurements Scandentia (animal order), 3:220 Scandium (element), 1:194–195 Schistosoma, 3:277 Schools and evolution, 3:169 Schou, Mogens, 1:165 Schrödinger, Erwin, 1:73–74

SCID (Severe combined immune deficiency syndrome). See Severe combined immune deficiency syndrome Science and religion. See Religion and science Scientific laws. See Scientific method; specific laws Scientific method application, 4:36–37 evolution, 3:166–167 glossary, 4:10t origins, 4:3–5, 4:9–11, 4:38 theories and proofs, 3:167 Scientific notation in measurement, 1:4–5 Scientific theories. See Scientific method Screws, 2:165–167 friction, 2:55 mechanical advantage, 2:158 Scrotum cancer, 3:240 Scuba diving. See Underwater diving Scurvy, 3:93 SDI. See Strategic Defense Initiative Sea otters, 3:71 Seaborg, Glenn T., 1:198 Seaborgium, 1:133 Seafloor spreading, 4:222 Seal rocks, 4:158 Seals, 3:130 Seamounts, 4:258 Seashores, 3:377 See also Beaches Seasonal affective disorder (SAD), 3:314 Seasons, 3:313–315 Seawalls, 4:401 Second law of motion, 2:18–19, 2:65–66 centripetal force, 2:46 friction, 2:52 gravity, 2:72, 2:73–74 mass, 2:20–21 shopping carts, 2:74 weight, 2:20 See also Force Second law of thermodynamics, 2:216–217, 2:222–223, 4:195 Earth and energy, 4:198 food webs, 3:69, 3:393 heat, 2:232, 2:234 hydrologic cycle, 4:389 wind, 4:398 See also Entropy Secondary succession (ecology), 4:352 Seconds (time), 1:9–10 Sediment load, 4:285–286 Sedimentary rocks, 4:149–150, 4:294 Sedimentary structures, 4:288 Sediments and sedimentation, 4:283–291, 4:289t–290t, 4:380 Seeds and seed-bearing plants, 3:138

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How to go to your page Seesaws, 2:86–89 Seismic waves, 4:232–233 Earth’s interior, 4:237, 4:240–241 seismograph reading, 4:233 Seismographs, 4:233, 4:234 Seismology, 4:228, 4:230–241, 4:238t–240t See also Earthquakes; Volcanoes Seismoscopes, 2:267 Selective breeding, 3:128, 3:169 Selenium, 1:221, 3:71 Self-contained underwater breathing apparatus (SCUBA), 2:124 Semiconductor lasers, 2:361–362 Semipermeable membranes, 1:106, 1:348 Senses, 3:295–305 Separating isotopes, 1:97 Septic tanks, 3:352, 4:306 Sequoia National Park, 3:363 Serotonin, 3:307 Serpentine minerals, 3:71 Severe combined immune deficiency syndrome (SCID), 3:115 Sewage treatment, 1:360 Sewage worms, 3:72 Sexual reproduction, 3:135–141, 3:142–150, 3:148t–149t, 3:206–207, 3:215–216 Sexual revolution, 3:147–149 Sexuality. See Human sexuality Sexually transmitted diseases, 3:240, 3:245, 3:258, 3:276 “Sgt. Pepper’s Lonely Hearts Club Band” (song), 2:323 Shale, 4:159 Shansi earthquake (1556), 4:237 Shear, 2:148 Shedding (fur or skin), 3:313, 3:313 Sheep, 3:122, 3:123, 3:323 Shell shock, 3:232 Shield volcanoes, 4:258 Ships buoyancy, 2:22, 2:24–25, 2:121–122 center of gravity, 2:137 fluid pressure, 2:144 introduced species, 3:210 Shock absorbers, 2:266–267 Shopping carts, 2:74 Shores, sea, 3:377 See also Beaches Shortwave radio broadcasts, 2:346–347 Shower curtains, 2:117 Shrews, 3:219–220, 3:226 Sickle-cell anemia, 3:14, 3:15–16 Siesta, 3:308 Signal propagation, 2:346 Significant figures in measurement, 1:5–6

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Silicates (minerals), 1:224–225, 4:135–136, 4:145, 4:160–161 Silicon, 1:224–225, 1:363–364 economic geography, 4:160–161 molecular compounds, 1:244 polymers, 1:372 Silicon wafers, 1:223 Silicone surgical implants, 4:161 Silicones, 1:225, 4:161 Silver, 1:187–188, 1:293 Simmelweis, Ignaz P., 3:288 Simple machines. See Machines Simple sugars, 3:3–4 See also Sugars Simpson, O. J., 3:105, 3:105, 3:108 Simulated emission of light. See Stimulated emission of light Sine. See Trigonometry Single-displacement reaction, 1:285 Sinkholes, 4:247 Siphon hoses, 2:99 Sirenians, 3:221–222 Skating. See Ice skating Skin, 3:130, 3:244–245, 3:248, 3:262 Skinner, B. F., 3:320–321 Skis, 2:140–141 Sky, color of, 2:358, 4:78–79 Skydiving, 2:39, 2:43–44 Sledges, 2:162 Sleep, 3:40, 3:307, 3:308, 3:312–313 See also Hibernation; Sleep disorders Sleep disorders, 3:309–311, 3:315 See also Sleep Sleet, 4:392 Slides (geology), 4:266–267, 4:276 Sliding friction, 2:53 Slump (geology), 4:266–267, 4:276 Small intestine, human, 3:47 Smallpox, 3:230–231, 3:252, 3:256, 3:257, 3:396 Smell (olfaction), 3:297, 3:299, 3:301–304 Smith, William (geologist), 4:47 faunal dating, 4:119 fossil correlation, 4:112 stratigraphy, 4:105–106 Smith County (KS), 2:137 Smithsonian Institution (Washington, DC), 2:281–282 Snails, 3:298, 3:333 Sneath, Peter Henry Andrews, 3:193 Snell, Willebrord, 2:354 Snow, 4:377, 4:392, 4:392 Snow, C. P., 2:216–217 Snow, John, 3:288 Snowballs, 2:228 Snowshoes, 2:140–141 Soap, 1:330, 1:334, 1:342–343

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Social Darwinism, 3:164 Societal views AIDS, 3:259–261 childbirth, 3:154 evolution, 3:165–169 in vitro fertilization, 3:147–149 Societies and evolution, 3:162–163 Soda cans, 1:54–55, 2:187 Soddy, Frederick, 1:71, 1:78, 1:79–80, 1:130 Sodium, 1:166–167 Sodium azide, 1:58–59 Sodium chloride, 1:166–167 Sodium hydrogen carbonate, 1:315–317 Sodium hydroxide, 1:317–318 Sodium substitutes, 1:165 Soft drinks, 1:248 Soil, 4:292–300, 4:298t–299t dust bowls, 4:270 erosion, 4:268 formation, 4:301–302 fungi, 3:385–386 layers, 4:302 prairie dogs, 4:297 role in biosphere, 3:346–347, 3:348–353 See also Soil conservation Soil conservation, 4:301–310, 4:303, 4:305–306, 4:308t–309t Soil Conservation Service, 4:305 Soil and Water Resources Conservation Act (1977), 4:305 Sokal, Robert Reuven, 3:193 Solar power, 4:202 Solar radiation, 4:197 Solar reflectors, 4:202 Solar system, 4:68 formation, 4:71, 4:72 glossary, 4:81t–83t origin, 3:177–178 terrestrial planets, 4:211 See also Cosmology; Geocentric universe; Heliocentric universe; Planetary science; Sun; individual planets Solar wind, 4:80 Solenoids, 2:332, 2:334 Solids acoustics, 2:321 behavior, 2:183 characteristics, 2:148 contrasted with fluids, 2:95–96 freezing, 2:208 liquid crystals, 2:212, 2:214 melting, 2:207–208 properties, 1:49t thermal expansion, 2:249–250 types, 1:37 Solid-state lasers, 2:361–362

Solifluction, 4:266 Solubility, 1:340, 3:35 Solutions, 1:338–346, 1:344t–345t Somatic (body) cells, 3:99, 3:126–127 Somatotropin, 3:307 Sonar fishing, 2:321 ocean floors, 4:223 submarines, 2:320 ultrasonics, 2:324–325 Sonic booms airplanes, 2:106–107 Doppler effect, 2:304–305 See also Compressibility Sonoluminescence, 2:371 Soot cause of cancer, 3:240, 3:240 industrial melanism, 3:173 Sosa, Sammy, 2:40 Sound compression, 2:304–305 conservation of energy, 2:177 echolocation in animals, 3:339–341 kinetic and potential energy, 2:175–176 luminescence, 2:371 recording, 2:316 speed, 2:311–312, 2:321 See also Acoustics Sound waves, 2:259 Soup and convection, 4:186 Source rocks, 4:158 South Pole and midnight sun, 3:310 Soviet Union, 2:177–179 Space exploration, 2:129 Space shuttles fuel, 1:241, 1:292, 1:293–294, 2:85 orbit, 2:60 Space telescopes, 2:342 Space walks, 4:215 Spark transmitters, 2:345 Species and speciation, 3:127, 3:204–214, 3:212t–213t, 3:215–226, 3:224t–225t competition in communities, 3:393–395 discovering new species, 3:194–195 genetic drift, 3:112 geomorphology, 4:262 Homo sapiens, 3:206 Specific gravity, 1:28, 1:30, 2:25–26 Specific heat, 1:17–18, 2:219, 2:230 Spectroscopy Doppler effect, 2:307 helium, discovery of, 1:238 indium, discovery of, 1:157–158 Spectrum, electromagnetic. See Electromagnetic spectrum Spectrum, light. See Light spectrum

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How to go to your page Speed acceleration, 2:17–18 centripetal force, 2:45–46 light, 2:356 sound, 2:259, 2:311–312, 2:321 velocity, 2:17–18, 2:38 Spencer, Percy, 2:348 Sperm cells, 3:100, 3:132, 3:136, 3:142–143 Spiders, 3:330, 3:332 Spoilage, foods, 3:27 Spoilers (aerodynamics), 2:110 Spokes (wheels), 2:162 Sponges (animal), 3:199 Spongiform encephalopathy, 3:128 Spotted owls, 3:406, 3:408, 4:354, 4:354–355 Springs (mechanical) damping, 2:266 oscillations, 2:263–264 Spruce trees, 3:371 Spur gears, 2:163 Squirrels, 3:112, 3:113 Stable equilibrium, 2:263–264 See also Equilibrium Stagecoaches, 2:163 Stagnation point, 2:105–106 Standards of measurement. See Measurements Staphylococcus, 3:286 Starches, 3:6–7, 3:7, 3:25 Starfish, 3:70, 3:71 Starvation children, 3:84–85 dieting technique, 3:81 Static electricity, 1:102, 2:340, 4:177 Static friction, 2:53 Statics, 2:133–138, 2:138t See also Equilibrium Statue of Liberty (New York, NY), 1:174 Steam engines Carnot’s engine, 2:221–222 heat, 2:231–232 Hero of Alexandria, 2:120, 2:163, 2:231 Steamships, 2:121–122 Steel buoyancy, 2:122 elasticity, 2:150 ferromagnetism, 2:334 Steelville (MO), 2:137 Steno, Nicolaus rock dating, 4:39 stratigraphy, 4:88, 4:105 Stepper motors, 2:337 Steptoe, Patrick, 3:144 Stereochemistry, 1:110–111 Stereoisomers, 1:278 Stereoscopy, 2:298–299, 4:55 Stethoscopes, 2:146–147

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Steward, F. C., 3:123 Stickleback fish, 3:217, 3:322, 3:327 Stimulated emission of light, 2:361 Stimuli (living organisms), 3:295, 3:296–297, 3:319, 3:320, 3:321, 3:327–328 Stinger missiles, 2:83 Stingray fossils, 4:121 Stoichiometry, 1:286 Stomach, human, 3:46–47, 3:48, 3:49 Stomata, 4:388 Stone Age, 4:145–147, 4:156–157 Stone carvings, 1:331 Stones. See Rocks Strategic Defense Initiative (SDI), 2:178–179 Stratigraphy, 4:105–113, 4:110t–112t fossil record, 3:171, 3:171–172 history, 4:88 relative dating, 4:96, 4:99 Stratocumulus clouds, 4:391 Stratosphere, 2:127 Stratovolcanoes, 4:258 Stratus clouds, 4:391 Streamlined flow. See Turbulent flow Streams (water), 3:376 Strength, ultimate, 2:151 Streptococcus, 3:286 Stress (mechanics), 2:148–149 Stress (psychology), 3:230, 3:232 Striations, 4:106 Strohmeyer, Friedrich, 1:189 Strömer, Martin, 2:240 Strong acids and bases, 1:322 Strong nuclear force, 2:157 Strontium, 1:176–177 Structural isomerism, 1:278 Strutt, John William, Lord Rayleigh. See Rayleigh, John William Strutt, Lord Subarctic forests, 4:346–347 Subatomic particles, 4:96 Sublimation, 1:41, 2:198 Submarines buoyancy, 2:122–123 sonar, 2:320, 2:325 Subpolar forests, 4:346–347 Subpolar glaciers, 4:378 Subsidence convection, 4:188, 4:190–191 geomorphology, 4:247–248 Subsoil, 4:296 Substrate, 3:25 Subsurface life, 4:296–297 Succession (biological communities), 3:370–371, 3:392, 3:400–409, 3:407t–408t, 4:352 Sucrose, 1:109, 3:4 Suess, Eduard, 4:220 Sugars

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See also Geography Survival of the fittest. See Evolution; Natural selection Sushi, 3:302, 3:303 Suspended load, 4:285–286 Sutherland, Joan, 2:314 Sutton, Walter S., 3:111 Svedberg, Theodor, 2:49 Swallows, 3:338 Swamps, 3:375, 3:376 Swimmers, 2:122 Swings oscillations, 2:263–264 resonance, 2:279, 2:281 Syadvada, 2:3–4 Syat, 2:3 Symbiosis, 3:383–390, 3:388t, 3:392 dodo bird and dodo tree, 3:209 parasites, 3:273–274 pollination, 3:140 Sympatric species, 3:217 Synthesis of proteins, 3:19, 3:21–22 Synthesis reactions, 1:285 Synthetic polymers, 1:374, 1:375, 1:377, 1:378–379 Synthetic rubber, 1:377–378 Systema naturae (Linnaeus), 3:197 Système International d’Unités (SI). See Metric system Systems biology, 3:345–347 closed systems, 3:345–346 Earth, 4:23–31, 4:29t–30t physics, 2:39 Systolic pressure, 2:147

chemical reactions, 3:25 metabolism, 3:34 molecules, 1:109 nutrition in carbohydrates, 3:9–10 in proteins, 3:19 simple sugars, 3:3–4 Suicide, 3:231 Sulfa drugs, 3:290 Sulfides, 4:134 Sulfur, 1:219, 1:221 acid rain, 4:318, 4:321–322 biogeochemistry, 4:318, 4:320–322 percentage of biosphere, 3:348 Sulfur cycle, 4:321–322 Sulfuric acid, 1:315 Sumerian invention of wheel, 2:162 Summer, 3:358 Sun astronomy and measurement, 4:80, 4:82–84 climate change, 4:409–410 composition, 4:77–78 convection, 4:186–187 glossary, 4:81t–83t ice ages, 4:384 nuclear fusion, 4:74 origin of the solar system, 3:177–178 phenomena, 4:78–80 solar radiation, 4:197 statistics, 4:75 weather, 4:396 See also Solar system Sunburns, 2:217 Sunlight after massive asteroid strike, 3:185 impact on sleep, 3:308, 3:309, 3:309–310 in photosynthesis, 3:4–5, 3:360–361 phototropism in plants, 3:321 vitamin D deficiency, 3:91 Sunspots, 4:80 Superatoms, 1:42–43 Superconductivity of helium, 1:242 Superconductors in MAGLEV trains, 2:338 Superfund Act (1980), 4:306 Superman (fictional superhero), 2:44 Superposition, 2:287–288 Supersonic flight, 2:106–107 Surface area, 2:140–141 Surface-to-air missiles, 2:83 Surface water, 4:387–388 Surfactants, 1:333–334, 1:342–343 Surgery brain, 3:234 fighting cancer, 3:239 Surgical silicone implants, 4:161 Suriname toad, 3:152 Surveying, 4:47

T cells, 3:255, 3:263–264 Table sugar, 3:4 Tacoma Narrows Bridge (WA) collapse, 2:280, 2:285 Taiga. See Boreal forest Tambora eruption (1815), 4:260 T’ang-shan (China) earthquake (1976), 4:236–237 Tantalum, 1:192 Tapeworms, 3:277–278 Tarzan (fictional character), 3:331–332 Taste buds, 3:298–299, 3:298–300 Taste (gustation), 3:297, 3:298–304 Taxonomy, 3:202t–203t biology, 3:191–203, 3:204–206, 3:215 biomes, 3:372 history, 3:192, 3:196, 3:196–198 Linnaean system, 4:118–119 taxonomic keys, 3:193, 3:197 worms and anthropods, 3:274–275 See also Naming conventions

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How to go to your page Taylor, Frank Bursley, 4:220 Technetium, 1:195 Tectonism mountains, 4:253–254 plate tectonics, 4:219–220 seismology, 4:230–231 Tectosilicates (minerals), 4:135 Teeth, 3:222 Telemetry, 2:349 Telescopes, space, 2:342 Television broadcasting, 2:346–347 Television tubes, 2:369 Tellurium, 1:226 Temperate forests, 3:373, 4:346–347 Temperate rain forests, 3:373–374 Temperature, 2:236–244, 2:242t–243t foods, impact on taste, 3:300 heat, 4:185–186, 4:193–194 See also Thermal energy; Thermodynamics Temperature and heat, 1:11–22, 1:13, 1:20t–21t Gay-Lussac’s law, 1:56 saturation, 1:341 Temperature scales, 1:14–16 Tennant, Smithson, 1:191 Tension elasticity, 2:148 modulus, 2:148–149 statics and equilibrium, 2:134, 2:135–136 waves, 2:258 Terbium, 1:209 Terminal velocity, 2:74–75 Termites, 3:6 Terrestrial biomes, 3:370 Terrestrial planets, 4:210 Territory (animal behavior), 3:324–326 Terrorism, biological, 3:231, 3:251–252 Tesla (unit of measure), 2:335 Testosterone, 3:142 Tetravalent bonds of carbon, 1:245, 1:365 Texture and taste, 3:300 Thales of Miletus (Greek philosopher), 1:67, 2:13–14, 4:177 Thallium, 1:158 Theories. See Scientific method Thermal energy, 1:12–13 conservation of energy, 2:177 heat, 2:227–228 kinetic and potential energy, 2:175–176 magnetism, 2:332 molecular dynamics, 2:195 temperature, 2:236–238 thermal expansion, 2:245 thermodynamics, 2:218–219 See also Temperature Thermal equilibrium, 2:238–239 Thermal expansion, 2:245–252, 2:251t

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ice, 2:247 railroad tracks, 2:246 temperature, 2:238–239 See also Thermodynamics Thermistors, 2:243 Thermocouples, 1:17, 2:243 Thermodynamics, 1:13–14, 2:216–225, 2:224t–225t chemical thermodynamics, 1:286 classical physics, 2:20 laws, 3:69 red shift, 2:358 See also Dynamics; Heat; Temperature; Thermal expansion Thermometers, 2:238 development, 1:14, 2:239–240 infrared light, 2:349 mercury, 1:189 modern, 1:16–17 temperature, 2:241–244 thermal expansion, 2:250–251 volume gas thermometers, 2:249 Thermometric mediums, 2:239–244 Thermoscopes, 2:239 Thermostats, 2:251–252 Thiamine (vitamin B1), 3:92, 3:95 Third law of motion, 2:18, 2:66–67 gravity, 4:171 levers, 2:160 projectile motion, 2:85 torque, 2:86 waves, 2:258 Third law of thermodynamics, 2:217, 2:223, 2:234–235, 4:195 Third world nations. See Developing nations Thomas Aquinas, St., 4:4 Thompson, Benjamin, Count Rumford, 2:217, 2:230 Thomson, George Paget, 2:297 Thomson, J. J., 1:70, 1:79, 1:86, 1:130 Thomson, William, Lord Kelvin, 2:237 absolute zero, 1:52, 2:184, 2:197 heat engines, 2:232 Kelvin scale, 2:241 plum pudding model, 1:70 Thorium, 1:198–199 Thunderheads. See Cumulonimbus clouds Thunderstorms, 4:187, 4:187–188, 4:391–392, 4:398–399 Thymus gland, 3:263 Tickbirds, 3:386, 3:387 Ticks, 3:279, 3:282 Tidal energy, 4:197–198 Tidal waves, 3:185 Tides currents, 4:364 Moon, 4:84 tidal energy, 4:197–198

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Till (sediment), 4:380 Time astronomy and measurement, 4:80, 4:82–84 geologic time, 4:92–94, 4:95–103 as measurement, 1:9–10 relativity, 2:10 Tin, 1:158 Tinbergen, Nikolaas, 3:320, 3:322–323, 3:327 Tires, 2:53, 2:55 See also Cars Titanic (film), 2:125 Titanic (ship), 2:125 Titanium, 1:192 Titration, 1:323 Toads, 3:123, 3:152 Tobacco use cancer, 3:240 sense of taste and smell, 3:301–302 Toit, Alexander du, 4:220 Tolerance model (biological communities), 3:401 Tongue (human), 3:299–301 Tooth decay and fluoride, 1:233 Toothed gears, 2:163 Topsoil, 4:295–296 Torches, 2:360 Tornado Alley (U.S.), 4:400 Tornadoes, 4:399–400 See also Natural disasters Torque, 2:86–91, 2:88, 2:160–161 See also Force; Levers Torque converters, 2:90 Torricelli, Evangelista, 1:50–51, 2:141–142, 2:239 Tortoises, 3:351 Toxic shock syndrome (TSS), 3:286 Toxins bioaccumulation, 3:72–73 indicator species, 3:71–72 targeted by immune system, 3:262 Trace elements in human body, 1:123–126, 3:78 Tracheal respiration, 3:57 Train whistles, 2:302, 2:303–304 Trains and magnetic levitation, 2:337–339 Trampolines, 2:264 Transantarctic Mountains (Antarctica), 4:379 Transducers, 2:322–323, 2:349 Transform motion (plate tectonics), 4:224 Transgenic animals or plants, 3:103 Transition metals, 1:136, 1:154–155, 1:181–195, 1:182, 1:193t–194t compounds, 1:277 orbital patterns, 1:144, 1:146 Translational equilibrium, 2:135 See also Equilibrium Transmission (wave motion), 2:258–259 Transmissions (automotive) friction, 2:55

torque, 2:90–91 See also Cars Transpiration, 3:354–355, 3:358, 4:370, 4:371, 4:389–390 See also Evapotranspiration Transuranium actinides, 1:201–204 Transuranium elements, 1:133, 1:155, 1:201 Transverse waves, 2:257–258 acoustics, 2:311 diffraction, 2:297 electromagnetism, 2:343–344 frequency, 2:272 luminescence, 2:365 resonance, 2:279 See also Waves Travel jet lag, 3:310–311 nighttime air travel, 3:312 Treatise on Electricity and Magnetism (Maxwell), 2:341 Tree lines, 4:256 Tree ring dating, 4:119 Trees conifers, 3:371–372, 3:374 dead trees, 3:408 deciduous trees, 3:373 dodo tree, 3:209 dominate forest ecosystem, 3:362 falling in forests, 2:315 introduced species, 3:210 micorrhizae, 3:385 recovery after logging, 3:406 specialized climate, 3:362 tree-dwellers, 3:363 See also Forests Trevithick, Richard, 2:231 Triangulation in geodesy, 4:49 Triboluminescence, 2:371 Tributary glaciers, 4:380 Triceratops, 3:183, 3:184 Trichinosis, 3:278 Trichomonas vaginalis, 3:276 Trieste (bathyscaphe), 2:124–125 Trigonometry statics and equilibrium, 2:134 tension calculations, 2:136 work, 2:170–171 Trimble, Stanley, 4:286 Trinity Broadcasting Network, 4:216–217 Triple point graphing, 2:6, 2:7 molecular dynamics, 2:198 phases of matter, 2:14, 2:214–215 water, 1:41–42 Tritium, 1:97, 1:253, 1:254–255 Trophic levels, 3:68–69, 3:73

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How to go to your page Tropical cyclones, 4:400–402, 4:401 See also Natural disasters Tropical forests, 4:345, 4:346 Tropics coral reefs, 3:377, 3:377 savannas, 3:373 tropical rain forests, 3:350, 3:362–363, 3:365, 3:366, 3:374 Tropism, 3:321 Troposphere, 4:405–406 Trucks, 2:110–111 Truffles, 3:385, 3:385 Truk Lagoon (Micronesia), 4:249 Tryptophan, 3:15 Tuberculosis, 3:60, 3:248–249 Tubificid worms, 3:72 Tubulidentates, 3:222 Tumors, 3:238 Tundra, 3:374, 3:392–393, 3:394, 3:394–395 Tungsten, 1:192 Tuning forks, 2:287, 2:289–290 Turbidity currents, 4:364 Turbines, 2:101 Turbulent flow aerodynamics, 2:102–103 Bernoulli’s principle, 2:113–115 Turrell, James, 4:16 Turtles, 3:338–339 Twain, Mark, 4:64 Twin studies (genetics), 3:115 Two Cultures and the Scientific Revolution (Snow), 2:216–217 Two New Sciences (Galileo), 2:16–18 Type I-III binary compounds, 1:277 Typhoid fever, 3:251 “Typhoid” Mary Mallon, 3:251 Typhoons, 4:400–402, 4:401 See also Natural disasters Tyrannosaurus rex, 3:184, 4:115

U Udden-Wentworth scale, 4:286 Ulcers, 3:48–49, 3:49 Ulmus americana, 3:210 Ultimate strength, 2:151 Ultracentrifuges. See Centrifuges Ultradian rhythms, 3:312–313 Ultrasonic images, 2:324 Ultrasonic speakers, 2:326–327 Ultrasonics, 2:319–327, 2:327t, 3:155, 3:155–156 Ultraviolet astronomy, 2:342, 2:350 Ultraviolet lamps, 2:368–369 Ultraviolet light, 2:350 astronomy, 2:342

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electromagnetic spectrum, 2:357–358 fluorescence, 2:368–369 harmful effects, 2:217 photoelectric effect, 2:341–342 Unconformities (stratigraphy), 4:112–113 Unconsolidated material erosion, 4:265–266 mass wasting, 4:274 Undersea diving. See Underwater diving Underwater diving bends, 1:50, 4:334 helium, 1:242 newt suit, 2:145 nitrogen, 4:334 partial pressure laws, 1:54 scuba, 2:124, 3:199 Unified atomic mass unit. See Atomic mass units (amu) Uniformitarianism, 4:90–92 Uniramia (subphylum), 3:275 United Kingdom Creutzfeldt-Jakob disease, 3:233 early vaccinations, 3:256–257 royalty, 3:115–116, 3:241 United States arms race, 2:177–179 center of geography, 2:137 center of population, 2:137 earthquakes, 4:235–236 See also specific government agencies United States Atomic Energy Commission (AEC), 3:103 United States Bureau of Standards, 2:8 United States Centers for Disease Control and Prevention (CDC), 3:258–259 United States Department of Agriculture (USDA), 3:81, 3:363 United States Department of Energy (DOE), 3:103, 3:120 United States Food and Drug Administration (FDA), 3:79 United States Forest Service, 3:363 United States Naval Observatory, 1:5 Units of measure atmosphere, 2:141–142 calorie, 2:219, 2:229 celsius degrees, 2:240–241 centigrade degrees, 2:240–241 electron volts, 2:345 fahrenheit degrees, 2:240–241 gauss, 2:335 hertz, 2:256–257 horsepower, 2:173 joule, 2:219, 2:229, 2:345 kelvin degrees, 2:184, 2:197 kilocalorie, 2:219, 2:229

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kilohertz, 2:256–257 kilowatt, 2:173–174 megahertz, 2:256–257 mole, 2:184–185, 2:193–194, 2:205–206 newton, 2:73, 2:141, 2:219, 2:229 pascal, 2:141 tesla, 2:335 torr, 2:141–142 watt, 2:173 See also Measurements Universal product code scanners, 2:296, 2:299 University of Colorado (CO), 2:49 Unmanned underwater vessels, 2:124–125 Unnamed elements, 1:133 Unsaturated fats. See Saturated fats Unstable equilibrium, 2:263–264 See also Equilibrium UPC (universal product code) scanners, 2:296, 2:299 Uplift and subsidence, 4:248 Upwelling regions (oceans), 3:377 Uranium, 1:199–201, 1:200 Uranium series dating, 3:172, 4:98 Uranus (planet), 4:68 Urbain, Georges, 1:209 Urban geology, 4:18 Urban legend of sounds from Hell, 4:216–218 Urey, Harold Clayton, 1:96 Urine and diabetes, 3:242 U.S. dollar, 1:4 USDA (U.S. Department of Agriculture), 3:81, 3:363 Ussher, James, 4:7, 4:88 Uterus, 3:152, 3:153 Utopia and Revolution (study), 4:263

V-2 rockets, 2:82 Vaccines and vaccination cancer, 3:239 genetic engineering, 3:103 history, 3:255–256 infectious diseases, 3:249 viruses, 3:287 Vacuum acoustics, 2:315–316 first law of motion, 2:59–61 gravity, 2:71 Valence electrons and configurations, 1:113 actinides, 1:196–197 alkali metals, 1:162 alkaline earth metals, 1:171–173 carbon, 1:243–244, 1:364–365, 4:324 chemical bonding, 1:267–268 halogens, 1:229–230 helium, 1:173

ionic bonding, 1:104 metals, 1:151 minerals, 4:131 periodic table of elements, 1:134, 1:135–136, 1:141–143 principle energy levels, 1:135 representative elements, 1:143–144 transition metals, 1:183–184 Valleys dry valleys (Antarctica), 4:379 hanging valleys, 4:380 Vampire bats, 3:220 Van Allen belts, 4:80 Vanadium, 1:192 Vaporization curve, 1:40 Variola. See Smallpox V-belt drives, 2:163–164 Vector measurements collisions, 2:40–41 graphing, 2:133–134 impulse, 2:39–40 linear momentum, 2:37–44 resultants, 2:17 statics and equilibrium, 2:133–135 work, 2:170–171 See also Scalar measurements Vectors. See Vector measurements Vegetables. See Fruits and vegetables Vegetarians, 3:23 Vegetative propagation, 3:136, 3:137 Velcro, 2:53 Velociraptor, 3:184 Velocity acceleration, 2:17–18 centripetal force, 2:45–46 second law of motion, 2:65 speed, 2:17–18, 2:38 Venerable Bede, 4:39 Venetian blinds, 2:164 Venturi, Giovanni, 2:113 Venturi tubes, 2:113 Venus (planet), 2:129 Verne, Jules, 4:215–216 Vertical motion. See Projectile motion Vestiges, 3:170 Vesuvius (Italy) eruptions, 4:259 Vibrations frequency, 2:271–272 in matter, 1:37–38 solids, 2:208 ultrasonics, 2:320–321 wave motion, 2:255 See also Oscillations Vickers scale, 4:156 Villa Luz caves (Mexico), 4:320 Vinci, Leonardo da, 2:360, 4:89

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How to go to your page fossils, 4:88, 4:105 geology, 4:39 Viruses cellular activity, 3:285 human immunodeficiency virus (HIV), 3:259–261 infections, 3:286–287 infectious diseases, 3:250–251 respiratory disorders, 3:60 Viscosity aerodynamics, 2:102–103 airplanes, 2:106 Bernoulli’s principle, 2:113 See also Density Vitamin A, 3:80, 3:88–90 Vitamin B1 (thiamine), 3:92, 3:95 Vitamin B2, 3:92 Vitamin B6, 3:92 Vitamin B12, 3:92, 3:268–269 Vitamin C, 3:92, 3:93 Vitamin D, 3:90, 3:90–91 Vitamin E, 3:91 Vitamin K, 3:91–92 Vitamins, 3:45, 3:80–81, 3:87–96, 3:94t Viviparity, 3:151–152 Voices, 2:314, 2:315 Volcanic eruptions, 4:258–260 Volcanoes climate change, 4:409–410 Crater Lake (OR), 4:259 mass extinctions, 4:123, 4:126–127 meteorological effects, 4:30–31 Mount Etna (Italy), 4:148 Mount Katmai (AK), 4:30 plate tectonics, 4:213–214, 4:228 Popocatepetl, 4:257 See also Mountains; Natural disasters; Seismology Voltaire (French author), 4:233 Volume, 1:23–30, 2:21–26, 2:25t Volume gas thermometers, 1:16–17, 2:249 Vomiting, 3:39 Vries, Hugo De, 3:111 VSEPR model (valence shell electron pair repulsion), 1:115–116

W Wakes, boat, 2:288, 2:289 Wallace, Alfred Russel, 3:169 Walsh, Don, 2:124–125 Walton, Ernest, 1:164, 1:165 Warm-blooded animals, 3:218 Wars bombs, 1:71–72, 1:93

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chlorine gas, 1:231 magnesium in incendiary devices, 1:175–176 Washing machines, 2:49 Waste management, 1:360, 2:49 Wastes, human, 3:50–54 “Watch analogy” (evolution), 3:161–162 Water, 1:339, 1:340, 1:342 acids and bases, 1:323, 1:326 atomic mass, 1:78–79 biomes, 3:375–377 buoyancy, 2:122 cause of giardiasis, 3:276 characteristics, 1:38 chemical equations, 1:283–284 compared to air, 1:49 content of vegetables, 3:6 Earth, 3:178, 4:369–370 erosion, 4:268 eutrophication, 3:354 filtration, 1:358 fluoridation, 1:233 in healthy diet, 3:50 in human digestion, 3:47, 3:88 hydrogen, 1:255 hydrologic cycle, 3:347–348, 4:361–362, 4:369–375 life, 4:67 molecular polarity, 3:19 molecules, 1:347–348, 4:370 percentage of human body mass, 3:78 phase diagram, 1:41 pollution, 3:71–72 purity, 1:266 red blood cells, 1:351 sedimentation, 4:285 in soil, 3:350, 3:352–353 solvents, 1:347–349 specific heat capacity, 1:17–18 spicy foods, 3:297 Thales of Miletus, 2:13–14 thermometric medium, 2:241–242 triple point, 1:41–42, 2:6 See also Hydrology; Hydrosphere; Ice Water balloons, 2:44 Water clocks, 2:100–101, 2:163 Water cycle. See Hydrologic cycle Water filtration plants, 1:356 Water pressure. See Fluid pressure Waterwheels, 2:99–100, 2:163 Watson, James D., 2:298, 3:111, 3:117 Watt, James, 2:231 Watt (unit of measure), 2:173 Wave mechanical model (atomic structure), 1:87–88 Wave motion, 2:255–261, 2:260t–261t acoustics, 2:311 Doppler effect, 2:301–302

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electromagnetism, 2:343–344 frequency, 2:272–273 interference, 2:286–287 light, 2:356–357 resonance, 2:279 See also Harmonic motion; Oscillations Wavelength, 2:256 diffraction, 2:294–295 electromagnetic waves, 2:343–345 frequency, 2:313 light, 2:357–358 luminescence, 2:366 radio broadcasting, 2:346 See also Frequency Waves electromagnetism, 2:341–342 erosion, 4:26, 4:268, 4:284 interference, 2:286–287 light, 2:355–356 properties, 2:256–258 seismic waves, 4:232–233, 4:233, 4:237, 4:240–241 superposition, 2:287–288 See also Light waves; Longitudinal waves; Mechanical waves; Transverse waves Weak acids and bases, 1:322 Weak nuclear force, 2:157 Weather, 4:395–404, 4:403t chaos theory, 4:24–25 mountains, 4:260 El Niño, 4:28, 4:30 volcanoes, 4:30–31 water in the atmosphere, 3:347–348, 3:358 See also Climate; Meteorology Weathering erosion, 4:264–266 mass wasting, 4:273–274 sedimentation, 4:283–284 Wedges, 2:165 Wegener, Alfred, 4:220–222 Weight buoyancy, 2:120–121 friction, 2:52–53 gravity, 2:72–74, 4:173 mass, 1:24–25, 1:76, 2:19–20 second law of motion, 2:65 See also Mass Weight loss. See Fitness Weightlifting, 3:308 Weizmann Institute, 3:120 Werner, Abraham Gottlob, 4:39, 4:88 Western Deeps Gold Mine (South Africa), 4:212 Wetlands, 3:376 Whales echolocation, 3:339, 3:340–341 endangered species, 3:207, 3:208

taxonomy, 3:195, 3:204–205, 3:222 Wheat, 3:352 Wheelbarrows, 2:161–163 Wheels, 2:109, 2:162–164 Whirlpools, 4:363 Whitcomb, Richard, 2:107 White, Vanna, 2:228 Whittaker, Jim, 2:142 WHKY radio station (Hickory, NC), 2:293 Wilkins, Maurice Hugh Frederick, 2:297–298 William of Ockham, 4:4 Williamson, William, 1:306 Willm, Pierre-Henri, 2:124 Wilson, Brian, 2:127 Wilson, Edward O., 3:402 Wilson, John Turzo, 4:222 Wind convection, 4:188 weather, 4:397–398 Wind erosion, 4:269–270 dust devils, 4:269 sedimentation, 4:285 Wind tunnels, 2:98, 2:107 Windmills, 2:163 Wings airplanes, 2:105–107 birds, 2:103–105, 2:115, 3:192 paper airplanes, 2:108 Winter animal migration, 3:335–336 impact on biological rhythms, 3:314–315 The Wizard of Oz (movie), 4:304 Wöhler, Friedrich, 1:245, 1:365, 3:178, 4:326 Wollaston, William Hyde, 1:191 Word origin kwashiorkor, 3:85 metabolism, 3:33 vitamins, 3:95 See also Naming conventions Work (labor). See Laborers; Occupational health Work (physics), 2:170–174 definition, 2:222, 2:232 gravity, 2:171–173 power, 2:173–174 thermodynamics, 2:217–218 See also Force; Power World Trade Center (New York, NY), 1:153 World War I, 2:127 World War II airships, 2:127, 2:129 concentration camps, 3:121 Nagasaki bombardment (1945), 3:104 World-Wide Standardized Seismograph Network, 4:234 Worms acorn worm, 3:138

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Yaw (orientation), 2:106 Yeast, 3:28, 3:28 Yellowstone National Park (U.S.), 3:363 Yersinia pestis, 3:248 Young, Thomas interference, 2:290, 2:356 wave theory of light, 2:343 Young’s modulus, 2:148 Young’s modulus of elasticity, 2:148 Ytterbium, 1:209–210 Yttrium, 1:194–195

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X X-axis. See Axes X-ray diffraction, 2:298 X-rays, 2:350 applications, 2:352–353 beryllium, 1:175 diffraction, 2:298 Moseley, Henry, 1:71 Röntgen, Wilhelm, 1:70 Xenarthrans, 3:219 Xenon, 1:241

Y Y-axis. See Axes Yachts, 4:27–28

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Z Z-axis. See Axes Zebra mussels, 3:211 Zeitgebers, 3:309 Zeno of Elea, 2:14 Zeppelin, Ferdinand von, 2:127 Zeppelins, 2:105, 2:127–128 See also Airships Zhang Heng, 2:267 Zinc, 1:154, 1:188–189 Zinc group metals, 1:188–190 Zirconium, 1:192 Zooplankton, 3:336 Zygotes, 3:100, 3:152

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