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, *J . M. .A.

HARVARD

COLLEGELIBRARY

FROM THE LIBRARY OF

ODIN ROBERTS

class or .886

IBRARY

fl

*

A A

HOME UNIVERSITY LIBRARYOF MODERN KNOWLEDGE

No. 48


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Eiitont

HBRBBRT PISHBR, M.A., F.B.A.Pbov. GILBERT MURRAY, Lrrr.D.,

LL.D., F.B.A.PKOV. J. ARTHUR THOMSON, M.A.Pkht. WILLIAM T. BRBWSTBR, M.A.

THE HOME UNIVEESITT LIBEAEYOF MODEEN KNOWLEDGE

i6mo cloth, 50 cents net, by mail 56 cents

SCIENCEAlready Published

ANTHROPOLOGY By R. R. Marett

AN INTRODUCTION TO

SCIENCE By J. Arthur Thomsow

EVOLUTION By J. Arthur Thomson and

Patrick Geddes

THE ANIMAL WORLD By F. W. Gamble

INTRODUCTION TO MATHE-MATICS By A. N. Whitehead

ASTRONOMY By A. R. Hikes

PSYCHICAL RESEARCH . . . . By W. F. BarrettTHE EVOLUTION OF PLANTS By D. H. ScottCRIME AND INSANITY .... By C. A. MercieeMATTER AND ENERGY .... By F. Soddy

PSYCHOLOGY By W. McDougall

PRINCIPLES OF PHYSIOLOGY By J. G. McKenduck

Future Issues

ELECTRICITY By Gisbert Kapp

CHEMISTRY ByR.MELDOLA

THE MAKING OF THE EARTH By J. W. GregoryTHE MINERAL WORLD .... By Sir T. H. HoixakdTHE HUMAN BODY By A. Keith

MATTERAND ENERGY


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BY

FREDERICK SODDY

LECTURER IN PHY8ICAL CHEMISTRY AND RADIOACTIVITY

UNIVERSITY OF GLASGOW

NEW YORKHENRY HOLT AND COMPANY

LONDONWILLIAMS AND NOR6ATE

\

'* ^ 7 s . ! '> -

*/ ' w HARVARD COLLEGE LIBRARY

6IFT OFK?8. ODIN R03ERTS

NOV 15 1934

COPYRIGHT, Iplft,

BY

HBNRY HOLT AND COMPANY

THE UNIVERSITY PRESS, CAMBRIDGE, U.S.A.

V

CONTENTS


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CHAP. TAG*

Periodic Tabus of thk Elements 6-7

I Physical History . . . 9

II Matter : I. Atoms and Molecules . ... 38

III Matter t II. Tax Elements 58

IV Heat and the Kinetic Theory of Matter 71V Potential and Chemical Energy . . . . . 105

VI Electrons and X-Rays ........ 144

VII Inertia 164

VIII Radiation . . 183

IX Radioactivity ... 197

X Cosmscal Energy 939

Bibliography 954

Index 955

PERIODIC TABLE

GteourO.

Gaoup L

Gbo-ofO.

GiourHI.

GbodtIY.

Group V.

HeliumHe 8*99

lithiumU6-94

Beryllium


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Be 91

BoronBll-0

CarbonC 12*00

NitrogenN 14*01

NeonHe 20*2.

SodiumNa 28*00

Magnesium

Mg 24*82

AluminiumAI27-1

8iIioon81288

PhosphorusP 31*04

AivonA 39*88

PotassiumK 39*10

CalciumCa 40-07

SO 44-1

Ti48'l

/ Vanadium\ V61-0


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CopperCu 63*67

ZinoZn 66*37

GalliumGa69*9

Germanium \Ge72*6 /

ArsenicAs 74*96

Krypton

Kr 62^92

Rubidium)Rb 58*45

StrontiumSr 87*63

YttriumYt89*0

ZirconiumZr90*6

/Niobium\Nb93'6

SQverAg 107*88

CadmiumCd 112*40


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IndiumIn 114*8

Tm \8n 119*0 /

AntimonySb 120*2

Xenon ^Xe 130*2

CaesiumCs 132*81

BariumBa 137*37

"Lanthanum CeriumL La 139*0 Ce 140*26

Europium Gadolinium TerbiumEu 152*0 Gd 157*3 Tb 169*2

Tbnlium Ytterbium Lutecium*!Tm 168*9 Yb 172*0 Lu 174*0 J

/Tantalum

\ Ta 181*6

GoldAu 197*2

* *

MercuryHg 200*6

ThalliumTl 204*0

Lead \Pb 20710 /


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BismuthBi 208*0

EadiumIbnanationI 222*

RadiumRa 226*4

ThoriumTh 232*4

OF THE ELEMENTS

Group VL

Group VII.

Group VIIL

Oxygen

oif-oo

FluorineF19*0

Sulphur882*07

ChlorineCI 86-4o

ChromiumCr52*Q


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Manganesettn*498

Iron Cobalt NickelFe 65-84 Co 68*97 Mi 68*68

Beknium8e79-2

BromineBr 79*92

Molybdenum

Ho 96D

Ruthenium Rhodium PalladiumRu 101*7 Rh 102*9 Pd 100*7

TelluriumTe 127 6

Iodine1126*92

Piaesodymlum Neodymium 8amariuraPr 140-6 Nd 144*8 8a 150*4

Itysproelum BrtnumDy 162*6 Er 107*7

TungstenWW4-0

r Otmium Iridium PlatinumOs 100*9 Irl981 Pt 195*2


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(Polonium)

-UraniumU 288-6

MATTER AND ENERGY

CHAPTER I

PHYSICAL HISTORY

The behaviour of matter and energyrepresents one aspect only of human knowl-edge, which is generally known by the nameof physical science. It seems well to stateat the outset that, throughout these pages,when the term science is employed it referssolely to this one branch. Physical scienceenjoys the distinction of being the mostfundamental of the experimental sciences,and its laws are obeyed universally, so faras is known, not merely by inanimate things,

but also by living organisms, in their minutestparts, as single individuals, and also as wholecommunities. It results from this that,however complicated a series of phenomenamay be and however many other sciences

may enter into its complete presentation,

o

10 MATTER AND ENERGY

the purely physical aspect, or the applicationof the known laws of matter and energy,can always be legitimately separated from theother aspects. This aspect comes first, notnecessarily in relative importance, but in theorder of the scientific definition of the phe-nomena and of the problems it presents fora solution. A great simplification therebyresults, which is too often neglected. Com-


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plete ignorance of these laws is, nowadays,rare, for they enter into the general commonsense of the age, and any flagrant violationof them is quickly exposed. But the neglectto give precedence to the purely physicalaspect of the complicated occurrences andevents of human experience in their orderlypresentation, has led to much confusedhistory and a general lack of clearness as tothe precise terms with Nature on which therace exists on this planet. There is a specialbranch of study known as physical geography,but the need for a similar branch of physicalhistory does not appear to have been widelyfelt. The laws expressing the relationsbetween energy and matter are, however,not solely of importance in pure science.They necessarily come first in order, in thefundamental sense described, in the wholerecord of human experience, and they control,

PHYSICAL HISTORY 11

in the last resort, the rise or fall of politicalsystems, the freedom or bondage of nations,the movements of commerce and industry,the origin of wealth and poverty, and thegeneral physical welfare of the race. If thishas been too imperfectly recognized in thepast, there is no excuse, now that thesephysical laws have become incorporated intoeveryday habits of thought, for neglectingto consider them first in questions relatingto the future. It is an interesting and byno means hackneyed side of the subject to

consider, so far as the operation of purelyphysical laws can teach, exactly what thefuture has in store for this world and thecomplicated civilisation that it contains. Isit a stable and permanent movement, or doesit carry in itself, like the life of the individualsthat comprise it, the seeds of its own inevi-table decay? Moreover, if, as will transpirewhen the nature of the controlling physicallaws has been made clear, it is ephemeral andwill decline the sooner the more rapid itsdevelopment and the more glorious the zenithit attains, what alteration of the existing con-

ditions would suffice to convert it into aphysically stable and permanent movement?On these great questions, rendered the morefascinating because of the disposition, since


the development of the doctrine of evolution,


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to consider the fate and future of the indi-vidual as of little importance compared withthe fate and future of the species, physicalscience in its later developments has much tosay that is of general interest. The proverbcounselling the cobbler to stick to his last is agood one; but since the province of physicalscience is the universe and all that movestherein, its right to be heard first, in order ofpresentation of the subject only, cannot bewithstood. It may or may not assist in dis-closing the fundamental bearings of anyquestion, but anything it has to say will ingeneral be definite and, in so far as the lawsare perfectly known, incapable of being inval-idated by any other considerations whatever.The laws may not be fully known and maygive rise to false deductions, a case of whicharose in the question of the duration of geo-logical time. In such a case, the discussion ofthe conflicting evidence can only result in theadvance of knowledge. Physical science, byreason of the universality of its laws, hassomething to say on almost every subject.

It need only be stated once for all, thatalthough the purely physical side can beconsidered separately, it does not renderother points of view less necessary, though, of

PHYSICAL HISTORY 18

course, it is only with the physical point ofview that the present volume is concerned.To adopt for the moment the language ofSpencer's Classification of the Sciences, referred

to in the Introductory volume of this Series(p. 89), physical science supplies subject-matter for every actual occurrence in theuniverse, but none of the truths outside ofphysical science can help in the solution ofphysical problems.

The recognition of the fundamental physi-cal conditions which control the destinies of arace, too often occurs too late in its develop-ment to be of service. History throws somestrange sidelights on this blindness to theobvious. The upward progress of the race

has, for example, been classified into succeed-ing eras, each designated by the name of amaterial. Thus are distinguished the StoneAge, the Bronze Age, the Iron Age, and theSteel Age. The names indicate that the erain question was associated with a certaindegree of mastery over a particular materialsufficient to enable new weapons to be forgedin the struggle for existence. Yet, when theearly records of these eras are examined,


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little or nothing is found about the pioneerswhose knowledge and craft effected thesebroad advances. Often were they held in


such contempt that it was considered almostbeneath the dignity of an educated man evento make himself superficially acquainted withthe technical processes to which, in the judg-ment of history, his era owed its initiation.To come to more recent times, how manypeople blessed with a liberal education wouldbe at a loss if asked offhand what steel is,and how it is distinguished from iron; orwould recognise even the names of the greatfounders of the modern era?

Fundamental as materials are in shapingthe broad lines of progress, it is necessary togo but very little deeper to come uponsomething equally fundamental but less ob-

viously so. Materials are employed merelyas weapons, tools, or instruments for theutilisation of power or energy. Even thefood we eat is not the end but the means ofliving. life is physically distinguished fromdeath by movement, and what food is tothe motion of living organisms, fuel is tothe motion of mechanical engines. Withthe advent of steel the utilisation of thenatural sources of energy has progressedwith enormous strides. Less than a hundredyears ago little was known about energy,and, indeed, the modern idea of energy as

a definite fundamental existence was not

PHYSICAL HISTORY 15

developed till well on in the last century.Isolated examples of energy, apart from thatof living beings, have been known necessarilyand some have been utilised from the re-motest times. The wind that propels thesailing ship was probably one of the firstforms to be harnessed to the affairs of life.

The phenomena of fire, and the thermalenergy derived from it, were known to allbut the most primitive races, though itsrecognition as one of the manifestations ofenergy is not yet a century old. Not untilthe law of the conservation of energy wasestablished, and it was shown that energy likematter is indestructible and uncreatable,could energy be regarded as one of thefundamental physical existences. Its recog-


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nition, as a separate entity, distinguishesthe present age from all its predecessors.This is the Age of Energy, or rather this isthe beginning of the ages of energy, theAge of the Energy of Coal. The triumphs ofthis age have been sung in season and out ofseason. Already, however, science has out-grown such immature jubilation. That thisstill is the age of the energy of coal is un-fortunately only too true, and the wholeearth is rendered the filthier thereby. More-over, the age will last just so long as the


coal supply lasts, and after that the laststate of the race will be worse than the first,unless it has learned better. Only ten yearsago the prospect was, in fact, anything buta cause for jubilation; but these last yearshave wrought a wonderful revolution in ourknowledge of energy, and therefore in the

future outlook of the race, now entirelybound up with that of energy. It is possibleto look forward to a time, which may awaitthe world, when this grimy age of fuel willseem as truly a beginning of the mastery ofenergy as the rude stone age of paleolithicman now appears as the beginning of themastery of matter. It may await the world,but by no means of necessity awaits it.The prospect is physically possible, but therealisation depends also upon man andwhether he can ever hope to rise to theheights of knowledge the problem demands.

The discoveries in connection with therecently explored field of radioactivity haveput an entirely different complexion on thequestion as to how long the energy resourcesof the world may be expected to last. Ithas transpired that there exists in matter,associated with its ultimate atoms, that is,by definition, with the smallest particlescapable of separate existence of the elements

PHYSICAL HISTORY 17

or most fundamental known forms of matter,sufficient potential energy to supply theuttermost ambitions of the race for cosmicalepochs of time. But, just in proportion asthe prizes to be won by the progressivemastery over the physical universe becomethe more magnificent, the more does theirachievement transcend in difficulty andseeming impossibility the older successes*


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Think of the ages that elapsed after mankindled his first fire, before the world hummedto the tune of the steam engine. Think ofthe ages that preceded even this remotediscovery, so ancient that no record of itsurvives, during which in natural conflagra-tions man must have been made aware ofthe energy in fuel before he had learned howto liberate it at will. With reference tothe newly revealed stores of atomic energy inmatter, and to the time, so lightly pictured inthe imagination, when it may raise the race tothe loftiest pinnacle of its ambitions, we arein the position that savage man bears to thepresent, aware, but in every other wayignorant. True we are thoroughly familiarwith the more superficial processes of nature,and bring to the task a trained and disciplinedintelligence. But the task increases in pro-portion to the knowledge which defines it.


Practically King Coal is as likely as ever todie naturally of exhaustion as to be deposedby another monarch; and, if so, he carriesaway with him the means of subsistence ofour boasted civilisation. But the recognitionof the boundless and inexhaustible energy ofNature, and the intellectual pleasure andgratification it affords, brightens the wholeoutlook of the twentieth century.

Mere accumulations of knowledge, sifted,classified, and reduced to their final most con-

cise expression in a series of text-books, arelittle more than the sepulchral monument ofscience. However complete and accurate,mere knowledge deadens rather than developsthe intellect. The history of the winning ofknowledge preserves a sparkle of life and maystimulate as well as instruct. But the realvalue of science is in the getting, and thosewho have tasted the pleasure of discoveryalone know what science is. A problemsolved is dead. A world without problemsto be solved would be devoid of sciencethough it might be full of scientific text-

books and dictionaries. Such is the prospectthat recent discoveries are opening up thatthere is no fear that science will yet awhilebe sighing, like Alexander, for fresh worldsto conquer.

PHYSICAL HISTORY 19


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Before the doctrine of its conservation wasestablished, energy was mysterious and un-accountable in its comings and goings.To-day it is no longer a mystery. Theunaccounted-for appearance or disappearanceof a quantity of energy in any process, how-ever complex, would rouse as much scientificinterest as the mysterious appearance ordisappearance of matter. When it appearsit must come from somewhere, and when itdisappears it must go somewhere. Graduallythis Law of Conservation has supplied thephysicist with an experimental test of realityin a changing universe. What appears anddisappears mysteriously, giving no clue ofits origin or destination, is outside of hisprovince. To him it has no physical exist-ence. What is conserved has physicalexistence, whether it is tangible and ponder-able like matter, or intangible and imponder-able like energy. Early writers, when theyreally meant what is now called energy, oftenused the term force; and the idea of force, aswill later be discussed, has confused the issues

and retarded the growth of science to an al-most incalculable extent. Carlyle says,meaning energy "Force, force, everywhereForce; we ourselves a mysterious Force inthe centre of that 'There is not a leaf rotting


on the highway but has Force in it: how elsecould it rot?*" The very idea of Force is,however, what would be termed an anthro-

pomorphism, that is to say, it ascribes theI behaviour of inanimate objects to causesderived from the behaviour of human beings.We have come to associate the motion ofmatter with somebody or something pullingor pushing it. When one body is observed tomove towards another, like a stone falling tothe ground, it has been supposed that, al-though no agent is visible, something mustbe pulling it. What, however, is actuallyobserved is a change of position of the body,which acquires at the same time motion orvelocity. The observation is correctly ex-

pressed by saying that energy, before asso-ciated with the position of one body withreference to another (potential energy), haschanged into energy of motion (kineticenergy). To suppose that the one bodyattracts or pulls the other with a certain"force" is to imagine a cause, which if itexisted would account for the effect. Forcesare not conserved, they have no physicalexistence, but they still survive even in


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scientific parlance, mainly because of thepoverty of the language, which hardly allowseffects to be expressed^ without some causal

PHYSICAL HISTORY 21

inference. They are bad gates throughwhich to approach the study of energy, asis evident from the fact that mechanicsexisted in a highly developed state for cen-turies before the discovery of the conserva-tion of energy. In mechanics, which is thescience of the motion or absence of motionof matter in bulk, forces have a definitemeaning, and in terms of energy they aremeasured by the change in the kinetic (orpotential) energy of a body when its positionchanges by the unit of length. Many of themost important changes of energy are due tochanges of position, too small to be measur-able, between the smallest particles of thesubstance. The energy changes are, however,

easily measurable. The attempt even toimagine forces to exist in such cases as thecauses of the changes of energy, in absenceof all knowledge, not only of the actualdistances involved but also of the variationof the imagined cause with the distance, isto invent an elaborate, perfectly vague andbefogging mode of expression for a very simpleeffect, jit is better to try to grasp the meaningof energy as a fundamental fact of experiencethan to begin, with totally inadequate knowl-edge, to derive from the action of livingbeings a shallow analogy which, if true,


would serve as a possible explanation of afew of the grosser manifestations in whichenergy plays a part.

Energy is recognised in two forms, kineticand potential. The first depends on motion,the second on the position of the body underconsideration, and the law of conservation

states that any loss of energy of motion isbalanced by a gain due to position, and viceversa. But it is possible to select cases inwhich the distribution of the kinetic energychanges, as among various moving bodies,without any abiding changes of potentialenergy. The effect of position can thus beeliminated, and the question reduced to itssimplest form. The law of conservation,then, has reference simply to kinetic energy


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or energy of motion. The question thatfirst has to be asked is, What is conserved?Neither motion as such, nor what Newtontermed quantity of motion, or momentum,the product of the mass of the moving bodyand its velocity, is conserved.: A sufficientlygood example is in the collision of two elasticballs. No material is perfectly elastic, itis true, and in all actual collisions some ofthe energy of motion of the body as a wholeis transformed into heat, or the energy ofmotion of its smallest parts with reference to

PHYSICAL HISTORY 33

one another. This part can be accuratelymeasured in practice, so that for the purposesof a simple illustration it is legitimate toconsider the balls chosen as perfectly elastic,colliding on a level plane for example, abilliard table. If two perfectly elastic ballscollide, no matter what the relative masses

of the balls, or what their relative velocities,there is only one quantity, involving thesemasses and velocities, which is the samebefore and after the collision. That quantityis the sum obtained by adding together theproduct of the mass of each ball and thesquare of its velocity. The measure ofkinetic energy adopted is half the massmultiplied by the square of the velocity.The numerical factor one-half is not of greatsignificance in the present connection. Theimportant fact is that it is the square of thevelocity and not the velocity itself which is

conserved. It is the same in the case of allphenomena in which pure motion uninfluencedin any lasting way by position is considered.No matter what changes have occurred in therelative motions of the moving parts, thesum of the products of the masses into thesquare of their velocities is conserved. Thistherefore answers the question as to howkinetic energy depends on motion. It might


be supposed that a truth deduced thus halt-^ingly from the behaviour of imaginary per-fectly elastic substances had no very greatapplication in the real world. As a matter offact, it is what we call imperfect elasticitywhich has no great range of application inthe: real world, the world of molecules, ascontrasted with the gross world of matterin bulk, which is all our unaided senses can


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perceive. The application of the principleto all cases of pure motion is universal, andthe reservation as to imperfect elasticityhad to be made simply because it is notimmediately obvious that loss of kineticenergy of motion and its transformationinto heat is merely subdivision of motionamong smaller particles of matter that canbe directly perceived. Molecules, if they arereally the smallest particles of matter thatexist, must be perfectly elastic, as later on* will be quite evident. It would be as absurdto postulate an inelastic molecule in purescience, as it is at present understood, as itwould be to assume for the motion of eachindividual in a surging crowd the generalchaotic aimlessness which appears to char-acterise the whole. The science of heat ismainly one grand general example of the verycase that has been postulated, namely, that of

PHYSICAL HISTORY 25

pure motion uninfluenced by position, notin the visible seeming world of gross masses,but in the invisible real world of molecules.

In physics, work and energy are inter-changeable terms. The simplest case ofdoing work is the lifting of a weight fromthe ground to a height. The amount of workdone, and the amount of energy spent indoing it, are simply proportional, first, to themass or quantity of the matter lifted; second,to the height it is lifted. Mass is practically

measured by weight, so long as the measure-ments refer to one part of the earth. Owingto the fact that the position of an object onthe surface of the earth relative to its centrevaries with the latitude, the distance apartbeing appreciably greater at the equatorthan at the poles, a given mass weighs slightlyless, and falls to the ground a little lessrapidly, in the tropics than elsewhere. Whatfollows refers to a single locality where weightis truly a measure of mass. It does not requireany appreciably different amount of work tolift a weight in the upper room of a house than

in the basement. When a weight is lifted,kinetic energy disappears and the equivalentquantity of potential energy, measured bythe weight of matter multiplied by theheight it is lifted, is produced. A foot-



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pound is one unit of potential energy due toheight. Twenty foot-pounds is practically thesame whether it refers to the work done on a20 lb. weight raised one foot, or on a 1 lb.weight raised 20 feet. When the weightfalls again, the potential energy disappearsand the equivalent quantity of kinetic energy,measured by half the mass of matter multi-plied by the square of the velocity it acquiresat the end of its descent, reappears. Thetruth of the statement, derived from experi-mental observation, that it is the square ofthe velocity, not the velocity itself, which is ameasure of the kinetic energy, may now bemade a little more obvious. The kineticenergy acquired by a falling weight is theequivalent of the potential energy it possessedprior to falling, and is therefore the productof the weight and the height of fall. Ifkinetic energy were proportional to thevelocity simply, just as potential energy isproportional to the height, it would follow,therefore, that the velocity of a falling bodyshould increase uniformly with the height it

falls. Whereas the velocity increases, asevery one knows, uniformly with the timetaken for the fall. As the speed gets fasterand faster a greater distance is traversed ineach succeeding second, and therefore for

PHYSICAL HISTORY 27

each succeeding foot fallen through thevelocity must increase by a less and lessamount. The velocity acquired is propor-

tional not to the height, but to the squareroot of the height of fall. The kinetic energyacquired is proportional to the height, andmust therefore increase according to thesquare of the velocity. If further illustra-tion were needed that the kinetic energyacquired by a falling body is proportionalsimply to the height of the fall, all that isnecessary is to carry out the fall in twoequal stages. The body falls the sameheight in each stage, and therefore acquiresthe same velocity and kinetic energy. Butin falling the whole way the velocity acquired

is not twice that acquired in falling half-way,but the square root of twice. The law ofdependence of kinetic energy on motion canthus be deduced from the observed laws offalling bodies.

Space does not permit more than a refer-ence to the early history of the doctrine ofenergy. The first conception had referenceto chemical energy, and was contained in


26/158

the Theory of Phlogiston which dominatedchemistry during the eighteenth century.The twin laws of conservation, that of matterand of energy, struggled competitively for


birth in the search of science after the un-^changing entities. Energy was always beingconfounded with matter. Even the greatchemist Boyle, who gave us the modernconception of elements, thought that heatwas ponderable. The search was intuitivelyafter conservation, and matter was lesselusive than energy. Hence everything in-tuitively believed to be real ran the risk ofbeing regarded as material. This was thefate of phlogiston. As first put forward,phlogiston was something which escapedduring fire, or combustion, with the lightand heat evolved. It was a pure anticipationof what is now called energy. Combustible

substances were regarded as rich in phlogis-ton. The lode-star of conservation appearedfirst in the following way. When various prod-ucts of processes, which were recognised asbeing analogous to combustion, like thecalces, or as we should say oxides, of themetals, or sulphuric acid and the sulphates,were heated with highly inflammable bodieslike coal, oil, or organic matter, the originalcombustible substances, that is, the metalsor the sulphur, as the case might be, wereregenerated. This view recognised that dur-ing combustion something (energy), which

manifested itself as light and heat, escaped;

i

PHYSICAL HISTORY 29

and that, before the original materials couldbe got back from the products of combustion,this something had to be put back. It was

only recognised much later that during com-bustion something material (oxygen) wasabsorbed from the air, and that before theproducts could be regenerated the compoundformed had to be decomposed and theoxygen liberated. The spirit of chemistrytended towards pure materialism. The laterfollowers of the phlogiston theory made thefatal mistake of materialising phlogiston.With the enthronement of the balance, and


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the test of weight as the criterion of materialreality, the existence of phlogiston as amaterial substance was disproved, and thetheory itself fell into quite undeserved dis-repute. In its original form it anticipatedby more than a century the modern doctrineof energy. It is most wonderful to reflectthat the first idea of conservation in sciencearose not in connection with weighablematter, but with the elusive, imponderableenergy. The second or modern phase arosein connection with the nature of heat, afterthe law of the conservation of matter hadbeen established, when it was no longerpossible to regard heat as a material fluid.Davy and Bumford, at the commencement


of last century, both had the modern con-ception of heat as a mode of motion ofmatter, and both came very near to estab-

lishing it. The latter was engaged in theboring of cannon by means of horses, andobserved the large amount of heat con-tinuously generated during the operation.He records in one experiment that by thework of a single horse 19 lbs. of water wereboiled, and the cannon, drill, and all themachinery employed were heated up to theboiling point of water in 140 minutes. Butboring changes the state of the metal fromthe compact to the finely divided form ofborings and turnings, and it had to be provedthat the latter had not a less capacity for

heat than the former. Joule, who repeatedthe experiments in another form by merelychurning water, which suffers thereby nophysical change except rise of temperature,established the modern view that the sourceof the heat is in the power or energy ex-pended. He measured exactly how muchheat is produced by a given amount ofmechanical work, and found that 772 foot-pounds have to be spent to raise the tempera-ture of 1 lb. of water 1 Fahrenheit. Thewater of a waterfall, 772 feet high, tumblingover into a deep pool, so that practically

PHYSICAL HISTORY 31

all of its kinetic energy is converted intoheat, is 1 F. hotter at the bottom than atthe top. Expressed in the modern scientificunits, which are based on the gram (.035 oz.or 15.4 grains) as the unit of mass, the centi-


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metre (0.394 inch) as the unit of length, andthe second as the unit of time, Joule's equiva-lent is 42,650. That is, a weight of 42.65kilograms falling a centimetre has sufficientenergy to raise the temperature of 1 gramof water 1 Centigrade. The latter unit ofheat is known as the caloric. In the com-bustion of coal, considered as pure carbon,the heat evolved would raise the temperatureof a mass of water about 8000 times that ofthe coal 1 Centigrade, or a mass 14,000 times1 Fahrenheit.

Conversely, when work is produced by anyheat engine the equivalent quantity of heatdisappears. As is well known, the conversionof heat into mechanical work is a very waste-ful process. But if it were possible to convertthe chemical energy of coal completely intowork, without first burning it to liberate theenergy as heat, the energy of 1 ton of coalwould then be sufficient to lift one of thelargest liners, weighing 20,000 tons, 500feet high. In other words, the chemical

energy of coal is equivalent to that of a

S2 MATTER AND ENERGY

mass equal to the mass of the coal fallingunder gravity a distance of 2000 miles, orone quarter of the earth's diameter. Theengineer's unit the horae-power--

known as an electro-magnet. From such asolenoid, with or without its iron core, mag-netic lines extend from the ends just as in thecase of a bar magnet. The surrounding spacehas been changed, in that it has becomemagnetised by the motion of the electronsin the wire. It is impossible without a knowl-edge of electro-magnetism to go into thesematters very deeply, but it will be sufficientif we confine ourselves to one important


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general aspect of this change which takesplace when the current flows in the solenoidand the surrounding space becomes magnet-ised. When the current is flowing the sur-rounding space has been endowed with energy,known as electro-magnetic energy, which itdoes not possess when the current does notflow. So we arrive at the philosophicalexplanation of the inertia of an electron.The space around an electron at rest becomesendowed with energy when the electronmoves, and before the electron can again bestopped this energy must be withdrawn.This we have seen is what we mean when wespeak of the attribute inertia.

INERTIA 179

We have seen that a stream of electronsflowing at right angles across a magneticline experiences a sideways thrust, in adirection at right angles both to its original

path and to the magnetic line of force. In-stead of moving the electrons past the magnet,we may move the magnet past the stationaryelectrons, which will experience a thrust justas before, and if free to move if, for example,they are the electrons resident in a piece ofmetal forming part of a closed metal circuitwe shall get a flow of electrons in the metalin the stated direction. That is to say, wemay generate an electric current by thepassage of a wire through the poles of amagnet. Imagine a straight copper wirestretched horizontally, and a magnet moved

from above vertically downward so that thewire passes between the poles cutting themagnetic lines between them. The electronsin the wire will be urged along it as themagnet passes, and if the ends of the wireare connected to any circuit, through which acurrent is capable of flowing, a momentarycurrent will traverse the circuit. The moderndynamo works on this principle, and may belooked upon as an electron pump. The wireis arranged to lie parallel to the axis on thesurface of a cylinder spinning between the


poles of a magnet, and so in its revolution iscontinually crossing and recrossing the mag-netic lines. Usually a large number ofcopper wires wound in a peculiar way on aniron core the whole being termed an arma-ture rotates between the poles of a power-


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ful stationary electro-magnet. The thrustswhich the electrons in the wire experienceas the wires cut the magnetic lines are usuallyarranged, by means of a commutator, to actall in the same direction and so all uniteto drive the electrons out of one pole andto draw them in at the other pole of themachine. When the outer circuit is open,so that no current flows, the power necessaryto drive the machine is only that wasted infriction of the bearings, etc. But when thecircuit is closed, so that a certain currentflows, an added amount of power is necessaryto force the electrons against the resistanceof the circuit. The dynamo transforms thisadded mechanical energy the kinetic energyof moving matter nearly quantitatively intoelectric energy the kinetic energy of movingelectrons.

If now with the dynamo at rest, but withits magnet excited, we pass a current throughthe armature, the machine becomes anelectric motor. The electrons now urged by

INERTIA 181

outside energy through the wires in themagnetic field turn the wires and with themthe armature. Perfect reciprocity is thefeature of these mechanical, electric, andmagnetic actions. Very few mechanicalengines are reversible in this sense. If youdrive a steam engine you do not raise steamin the boiler. In the world of electricity and

the ether, however, nothing can move in onedimension of space without attendant con-sequences in the other two dimensions.

These relations between the electron andthe external field of energy which attendsits motion are perfectly reciprocal. On theone hand, the electron cannot move fromrest without this attendant field of energyaround it coming into existence, and cannotbe stopped without this attendant field ofenergy disappearing from the ether. On theother, the creation of a magnetic field in

space endows, or tends to endow, all theelectrons in that space with motion in onedirection, and the cessation of the magneticfield endows, or tends to endow, them withmotion in the opposite direction. A steadycurrent of electricity, or flow of electronsin a circuit, produces no change in the sur-rounding magnetic field. A steady magneticfield produces no movement of electrons


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within it, and so no electric current. Elec-trons changing the speed or direction of theirmotion produce attendant changes in thesurrounding magnetic field, and converselya change in the direction or strength of amagnetic field produces motion or tendencyto motion of the contained electrons. Thesephenomena, well known since the discoveriesof Faraday, who termed them electro-mag-netic induction, now lie at the very founda-tion of the modern science of electricalengineering. They have no mechanicalanalogies, for they belong to a more funda-mental world than that of matter. It hasbeen stated that space has three dimensions length, breadth, and depth rather than anyof the other numbers which mathematicianshave attempted to picture, because of thepeculiarities of the ether, in which motionin any direction is attended simultaneously

by influences acting in the two directions atright angles to it and to each other. Howeverthis may be, until it is possible to educatethe mind so that it apprehends intuitivelythe three dimensional aspects of motion inthe ether, the electro-magnetic world, whichunderlies the material world and which maycompletely embrace it, must remain a foreignelement as difficult to breathe as air is to a

RADIATION 183

fish, by those accustomed only to the grosserideas of matter and its motion. It is a timeof transition. The discovery of the electronhas to a certain extent rendered the subjectconcrete and picturable, but the pioneerseven have hardly yet cleared their waythrough the jungle of obsolete and confus-ing habits of thought which naturally stillsurround the subject. Only a few of themore important cases can here be attempted,and the first of these is one of the oldest.

CHAPTER Vm

RADIATION

There is still a gap in the chain of reason-ing to be supplied. When an electron isbeing urged on to move from rest at a con-tinually increasing speed, surrounding its


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path there is being built up, as the expressiongoes, a magnetic field of greater and greaterstrength. The energy being put into theelectron overcoming its inertia flows outcontinuously into the space surrounding itspath and travels along with it. When theelectron is being retarded and brought to


rest, the energy it, itself, is now supplying,by virtue of its inertia, flows in from thesurrounding space and the magnetic fieldgradually weakens and disappears. As weknow that the magnetic field extends fora considerable distance all round an electro-magnet, these outgoings and incomings ofenergy, between the electron and its en-vironment, involve the transference of energyover considerable distances, and as the airor surrounding matter plays no part in thephenomena, they must take place through

empty space. It is not too much to saythat the idea of an ether has been inventedby scientific men for the express purpose ofaccounting for the flow of energy acrossempty space, and is at present little morethan a term to express the medium in whichthese transferences occur. Action at a dis-tance, be it gravitation, electric pellationor tractation, or magnetic action, carrieswith it the necessity of supposing somethingto exist in the intervening space, and sciencetakes the simplest possible view when itsupposes that one such universal medium,

the ether, fills all space, alike between themolecules of a piece of matter as betweenthe most distant stars, and that in this onemedium all these various influences are

RADIATION 185

transmitted. Lest, however, it be supposedthat the ether is purely a philosophical wayof escape from the unknown, and that weknow nothing of this elusive and all-per-

vading medium, let us pause for a momentto consider the origin of the energy whichanimates nearly everything that moves onthis earth. The train that rushes on itsjourney bearing its hundreds of tons ofweight 1000 miles in the day, the linerbearing its tens of thousands of tons lightlyacross the seas, are animated with the energythat reached this earth, from a place90,000,000 miles away, as sunbeams during


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the forgotten ages of the past. Almost all theenergy, with which the modern world throbs,arrived through this medium which connectsus with the stars, and the thought on whichwe stumbled, considering the outflowingsand incomings of energy around a movingcharge, or current of electricity, underliesthe flow of all energy throughout the uni-verse. The luminiferous ether, as it wasfirst called, because it bore the light acrossintra-stellar space, has at least one verydefinite characteristic, which is quite suf-ficient to entitle it to be considered as aphysical reality. It transmits light and,so far as is known, every other influence


which traverses it, at a definite speed 185,000 miles in the second. This velocityis as characteristic of it as the velocity ofsound in air, 1200 feet per second, is char-

acteristic of the atmosphere. Radiations,be they light or heat, whatever their colouror wave-length, X-rays, the ether-wavesemployed in wireless telegraphy, magneticdisturbances, whether they reach us fromthe sun as the accompaniment of solarstorms, or whether, lastly, they circulatearound the space surrounding a wire in whichcurrent of electricity is being started orstopped all travel through space with thespeed of light. Sound is the vibration ofthe air, and all the gamut of sounds andnoises are essentially air disturbances of the

same type. Radiation is the vibration ofthe ether, and all the various phenomenajust enumerated are due to electro-magneticchanges accompanying the alteration eitherof the speed or direction of motion of elec-trons. The ether, so far as we know, vibratesonly in this one way, and the vibrations aretransmitted only with one velocity. Theexplanation of the inertia of the electronembraces also the phenomena of radiationin all its numerous forms.Electrons are a normal constituent . of

RADIATION 187

matter, and are found revolving round theatoms much as the planets of a solar systemdo around the central sun. The electricaltractation between the electron and thepositively charged atom takes the place ofgravitation. The electrons, like the planets,


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possess a period of revolution appropriateto their distance from the atom. The nearerthey are the faster they must revolve, thelaw regulating the period of revolution anddiameter of the orbit being completelyanalogous to Kepler's law for the planets.But owing to the minuteness of the atomand the relative greatness of the electricaltractation compared with gravitation, theperiods of revolution of electrons are almostinconceivably short. These electrons revolvethousands of millions of millions of timesper second, so that it might be supposedthat it would be quite impossible to measurethem. As a matter of fact, no magnitudesin science are known with greater exactitude.Consider the electron revolving round theatom. At any point of its revolution anelectro-magnetic field of energy extendsaround it, appropriate to its motion. Whenit has traversed half a revolution it is movingat the same speed as before, but in exactly theopposite direction. Its attendant magnetic


field is therefore exactly reversed in direc-tion. The reversal is not a sudden but arhythmic process occurring twice in therevolution. At any point in space themagnetic field attains a maximum, dimin-ishes to zero, reverses its direction, andattains a maximum again equal and oppositeto the former maximum in half a completerevolution. The ether transmits these rhyth-

matic changes in the field of energy sur-rounding a revolving electron outwardsthrough space, as always, at its own peculiarvelocity, the velocity of light. And theserhythmic movements of the ether, producedby the smallest entity known reversing itsdirection of motion regularly in its orbitround the relatively massive single atomsof matter thousands of billions of times inthe second, what inconceivably delicate in-strument can it be that science has inventedfor their detection and study? Ah! noinstrument maker can make such an instru-

ment. It is furnished ready made not onlyto man but to some of the lowliest organismsthat inhabit the world. The phenomenabeing described is radiation and the rhyth-matic vibrations of the ether, if they occurwithin certain limits of frequency, constitutelight.


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RADIATION 189

Instead of speaking of the periods of revolu-tion, it is more usual to speak of the wave-lengths. The velocity of transmission isalways the same, 185,000 miles per second,so that since in one second the number ofcomplete wave-lengths is the same as theperiod of revolution, and must cover a dis-tance of 185,000 miles when placed end toend, it follows that the length of the completewave is the velocity of light divided by theperiod. If the period is rapid the wave-lengthis short, and vice versa. For deep red-light, atone end of the visible scale of the spectrum,the wave-length is about 7/10,000th of a milli-metre (1 mm. = 1/25 inch) ; for deep violetlight, at the other end, about 4/10,000th mm.Waves shorter than this in the ultra-violet,down to 2/10,000th mm., affect the ordinaryphotographic plate, but not the eye. Tothese rays glass is opaque, but quartz andfluorspar remain transparent, whilst for stillshorter waves even the air and gases in

general are no longer transparent, but absorbthe rays, becoming "ionised" as by theX-rays. So that still shorter rays can onlybe studied in a vacuum, and fluorite is theonly transparent optical material available.These ultra-violet radiations are capable ofproducing brilliant fluorescence, f or example,


in barium platinocyanide. The retina of the

eye fluoresces faintly also, and this accountsfor the fact that far beyond the limit of theextreme violet of the spectrum, lines can beseen with a faint neutral or lavender tint.Longer waves up to 20/10,000th mm., in theinfra-red, can also be studied by photog-raphy with specially prepared emulsions.The ordinary emulsion is of course notaffected by red or infra-red light. Rocksalt is transparent to these long rays, butglass is more or less opaque, whilst ebonitein thin sheets is transparent even in thevisible red region. If the sun is viewed

through an ordinary camera shutter, madeof thin sheet ebonite, it may be distinctlyseen as a faint deep red object. There isone generalisation that can be made abouttransparent solid substances. They are allfine electrical insulators. Mica, ebonite,glass, quartz, amber, sulphur, etc. are amongthe most perfect insulators known, and allexist in transparent or translucent forms.Metals and good conductors of electricity are


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opaque. Moreover, in those cases where it ispossible to get a transparent film of metal,as can be done with gold and silver leaf byheating, the transparent form of the metal isfound to have lost its electric conductivity.

RADIATION 191

This, no doubt, is connected with the factthat in insulators the electrons are anchored tothe molecules and can vibrate with them butcannot move about, so that when the electro-magnetic wave of light crosses them theelectrons vibrate in unison with the light, butthe energy is not dissipated in moving thembodily.

The distinction between radiant heat andlight is non-existent. In being absorbed byopaque objects, radiations of all wave-lengths, whether they belong to the visible orinvisible region of the spectrum, are trans-

formed into heat. Very small amounts ofradiant energy in the green and yellowregions are visible to the eye, and in theviolet and ultra-violet can affect the photo-graphic plate, whereas our experience of thelonger waves is usually confined to sufficientquantities to be detected by their heatingeffects, as radiant heat. In addition, when asolid substance is gradually heated, its radia-tion is confined to these dark infra-red orheat rays, and not until its temperature israised to "red-heat," about 500 to 600 C,does it begin to emit any visible light. In

passing to the temperature of a white heat,not only are waves of shorter and shorterlight emitted as the temperature rises, but


also the radiation of the longer waves isenormously increased at the same time, sothat even white-hot bodies give out far moreenergy as dark heat-rays than as visiblelight-rays. In astronomy the distance to

which the spectrum of a star extends into theviolet region is used as a measure of thetemperature of the star. Even the eyedistinguishes red or comparatively cool starslike Betelgeuze from blue or hot stars likeVega. For every temperature of a heatedsubstance, there is one particular wave-length of which more radiant energy isemitted than of any other. When glowingsubstances of higher and higher temperature


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are examined, it is found that the maximumemission of energy shifts regularly fartherand farther along the infra-red region towardsthe visible region of the spectrum.

An extraordinary case occurs in this con-nection of what the unsophisticated person,who commented upon the fact that largetowns always occur on navigable rivers,would term Providence. The sun is probablya red or comparatively cool star, but itstemperature, estimated at 6500 Centigrade,is incomparably greater than the highest,about 3500, attainable on earth even inthe electric furnace. For such high tern-

RADIATION 193

peratures the maximum amount of radiantenergy is no longer situated in the infra-redor dark heat region, but is for the solartemperature in the visible region between the

yellow and the green, at the point, that is tosay, of maximum sensitiveness of our eyes.What can this mean, but that the human eyehas adapted itself through the ages to thepeculiarities of the sun's light, so as to makethe most of that wave-length of which thereis most. For a star cooler than the sun themaximum of energy would be toward the red,for one hotter than the sun toward the violet.Hence if these suns have planets peopledwith inhabitants, their eyes, if they adaptthemselves, as ours seem to have done, tosuit the prevailing conditions, would be

most sensitive to red or violet respectively,and the yellow green of the spectrum whichappears so vivid to us would be to themrelatively dull. Let us indulge for a momentin those gloomy prognostications, as to theconsequences to this earth of the cooling ofthe sun with the lapse of ages, which used tobe in vogue, but which radioactivity has sorudely shaken. Picture the fate of theworld when the sun has become a dull red-hot ball, or even when it has cooled so farthat it would no longer emit light to us.


That does not at all mean that the worldwould be in inky darkness and that the sunwould not emit light to the people theninhabiting this world, if any had survivedand could keep themselves from freezing.To such, if the eye continued to adapt itself


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to the changing conditions, our blues andviolets would be ultra-violet and invisible,but our dark-heat would be light, and hotbodies would be luminous to them whichwould be dark to us. One can hardly emergefrom such thoughts without an intuitionthat, in spite of all, the universal LifePrinciple, which makes this world a teeminghive, may not be at the sport of every physicalcondition, may not be entirely confined to atemperature between freezing and boiling-point, to an oxygen atmosphere, to the mostfavourably situated planet of a sun at theright degree of incandescence, as we arealmost forced by our experience of life toconclude. Possibly the Great Organiser canoperate under conditions, where we couldnot for an instant survive, to produce beingswe should not, without a special education,recognise as being alive like ourselves.

The mathematician's way of expressing achange of velocity is to say that the velocityis accelerated, and this strictly scientific

RADIATION 195

use of the term deceleration includes thestopping or retardation of motion, and thechange of direction of motion, as well asthe mere increase of speed signified by thecurrent use of the word in ordinary speech.Radiation is the consequence of the accelera-tion of an electron in the scientific sense.An electron revolving round an atom, like

a planet round a sun, experiences a constantacceleration towards the centre, and theradiation is rhythmatic and regular so longas the orbit of the electron remains thesame, thousands of billions of preciselysimilar waves following each other out intospace every second. We have, however,already had to consider a far simpler casethan this. The electrons of the highlyexhausted X-ray tube suddenly, in fullflight, strike an obstacle made of the densestpossible material. Their course is suddenlyarrested and they are brought to rest. Let

us fix our attention on a single electron infull flight. We know that surrounding itthere is the appropriate field of energy.The next instant the electron has struckthe anti-cathode and has been brought torest. The field of energy around it suddenlychanges. The ether, however, cannot in-stantaneously transmit this change. If a


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long straight line of soldiers in open formationis sweeping regularly across a plain and anofficer at one end cries "Halt!" the soldiernext to him halts instantly at the word ofcommand, if he is an "ideal" soldier, thatis to say, and possesses no inertia. But theman at the other end of the line, say threehundred yards away, however instantane-ously he obeys the word of command whenhe receives it, does not receive it till aboutone second after it has been given, whichis the time taken for the sound to travel.In reviews of troops in open formation thesound wave may, as it were, be seen dis-tinctly travelling along the line. In exactlythe same way the order radiates from thesuddenly arrested electron, outward throughspace with the velocity of light in singlewave or pulse. But there is no rhythmalor periodic succession of waves as in light.The bombardment of the anti-cathode by

the electrons which produces the X-ray is,compared with light, like the noise of thepatter of hail on a roof compared with amusical note of sound. The next illustra-tion of the radiation attending the accelera-tion of electrons is the Hertz waves used inwireless telegraphy, but space forbids theirdetailed consideration.

RADIOACTIVITY 197

CHAPTER IX

RADIOACTIVITY

Until 1896, the observed facts of sciencewere in agreement with the view that theatom was the ultimate limit of material sub-division, and that in no known changes,chemical or physical, including in the latterterm electrical and electro-chemical, didthe atoms belie their designation as the

ultimate foundation-stones out of which thewhole material universe was built up. Nega-tive electrons came to be recognised asparticles smaller than the atom, and opinionswere expressed that in some unknown wayatoms were compounded out of these elec-trons. The philosophical explanation of theinertia, or mass, of electrons raised the ques-tion whether the mass or inertia of matter wasessentially different from that of electricity.


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The possibility was present, therefore, that,if by some means the overpowering pellationof electrons could be neutralised, and a verygreat number, many thousands or hundredsof thousands as the case may be, could becrowded together into the space occupied byan atom, that might be the atom and matter


as a separate existence might be referred toa condensed form of electricity. The progressof science has moved away from this simpleconception. Positive electrons, which werepostulated as the "cement," whereby thepellation of the negative electrons might beneutralised, has remained merely a term toexplain the supposed condensation of elec-tricity into matter, and has as yet no physicalor experimental basis of existence. In addi-tion two sorts even of negative electronshave had to be postulated. The one, free

electrons, which move freely among theatoms of matter, can be withdrawn fromor added to atoms, without the necessity ofsupposing that the atom, as a separateentity, has thereby been essentially altered,or has ceased to exist as such. The otherkind consists of the purely hypothetical" structural electrons " out of which the atomsthemselves, by hypothesis, are really builtup. The electric charges which make theirappearance on the rubber and the objectrubbed in frictional electricity, it would bealtogether far-fetched to regard as derived

from the disappearance from existence ofthe equivalent amount of matter. Graduallyall the known phenomena due to electrons,even the lines of the spectra of elements,

RADIOACTIVITY 199

which at one time it was thought revealedits most intimate internal construction, havebeen associated with the first kind of electron,which pursue a joint existence with the atoms,

much as the attendant satellites or planetsdo in reference to their central suns. Thesecond class, in other words, the atomsthemselves, remain, as they were, untouchedby these advances. There are still eighty ormore distinct types of elements, each with aspecific type of atom or smallest particle,which as yet can neither be expressed interms of one another, nor of anything morefundamental. On the other hand, a totally


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distinct experimental science, radioactivity,has grown up since 1896, which derivesfirst hand from Nature, most importantand astonishing evidence of the properties ofthese atoms, which till then had been entirelyunsuspected and unpredicted by the theoriesof physical science. Events have provedthat chemistry is not the most fundamentalknowable science of matter, and that changesare proceeding slowly and spontaneously incertain atoms, those of the elements exhibit-ing the new property of radioactivity, whichare totally distinct from the kinds of changewhich have hitherto been studied.The discovery of the property of radio^


activity, by Becquerel in Paris in 1896, was,experimentally, closely related to that ofthe X-rays by RSntgen in 1905. Becquerelexamined certain fluorescent substances, that

is, substances which have the power ofabsorbing light and other radiations and ofre-emitting it, changed to a colour char-acteristic of the fluorescent substance, ratherthan of the kind of radiation by which itis produced. By good fortune he includedcertain of the salts of uranium, which fluor-esce with a beautiful greenish yellow hue.He so discovered that these substances emitalso new kinds of rays, which, like the X-rays,traverse opaque substances like cardboardand thin metal foil and affect the photo-graphic plate. Continuing, he proved that

the new rays had nothing to do with theproperty of fluorescence, but were a constantentirely new property of the element ura-nium, exhibited under all circumstances in un-altering degree by all its compounds and bythe element itself. Uranium is distinguishedamong the elements as the last member ofthe Periodic System. It has the heaviestatom of all the elements and its atomicweight is 238.5, when that of oxygen istaken at 16. Mme. Curie made an examina-tion of all the known elements or their com-

RADIOACTIVITY 201

pounds, to see if this new property waspossessed by any others than uranium.She found that thorium, the next heaviestelement, with atomic weight 232.5, possessesa similar property. None other of the knownelements are radioactive. But the natural


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ores of uranium, the minerals such as pitch-blende, in which this element is found in theearth, possess a greater degree of radio-activity than can be accounted for by theuranium therein contained. The same hassince been found true for the thorium min-erals. She proved that part only of the radio-activity is due to uranium, and that othernew radioactive elements, in excessivelyminute quantity, are present. One of these,happily given the name "radium," aftermany years of patient work was separatedfrom the mineral, and its compounds wereprepared in the pure state. Its atomicweight proved to be 226, the next highestto thorium, and in its whole chemical natureit was just what might have been expectedfrom the Periodic Classification of an elementwith this atomic weight. It resembles veryclosely barium and the other members ofthis family, strontium and calcium. Thecompounds of this group of elements arewell known; for example, lime is the oxide


of calcium, but the elements themselves arevery reactive metals which are difficult toisolate from their compounds. Last year,however, she succeeded in isolating theelement radium, and it proved to be a metalvery similar to barium, so far as its merechemical nature is concerned. The amountof radium in good pitchblende is little morethan one part in ten million. Ten tons of

pitchblende thus contain only 1 gram ofpure radium, and in spite of the interestawakened, the total amount of radium thathas ever been prepared probably does notexceed 10 grams or about |rd of an ounce.Yet, even in pitchblende, the radioactivitycontributed by the radium greatly exceedsthat due to the uranium. The pure radiumcompounds are, weight for weight, manymillions of times more radioactive thanthose of uranium or thorium, and manyremarkable properties are exhibited by themwhich uranium and thorium, on account of

the feebleness of their radioactivity, do notshow. Radium gives a characteristic spec-trum distinct from any other known sub-stance, and connected, by certain mathe-matical relations between the wave-lengthsof its lines, with the spectra given by barium,strontium, and calcium. This is of the


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RADIOACTIVITY 208

highest importance, for it establishes thetitle of radium to be considered a new elementbeyond all question. If radium, is no trueelement, then the word "element" has nomeaning.

At the outset it may make the matterclearer if it is stated that the chemistry ofthe radio-elements, uranium, thorium, rad-ium, etc., is in no way exceptional, but that,superimposed upon their chemical propertiesand totally unconnected with them, theelements exhibit an entirely new set ofproperties, which may be termed the radio-active properties. The radioactivity is aproperty of the atom, and neither the par-ticular compound in which the atom iscombined, the physical state or conditions,such as temperature, concentration, etc.,nor the past history of the substance, haveany real influence upon it.

In considering these radioactive properties,the nature of the rays, emitted by the radio-elements, first calls for remark. In this'department the pioneer was Rutherford.The general methods of studying the newradiations are similar to those employed forthe X-rays. First, the rays affect the photo-graphic plate; secondly, they, when suf-ficiently intense, excite visible fluorescence


in the well-known fluorescent substancesalready described; and lastly, they " ionise "the air. The air is normally an almostperfect insulator. A gold leaf electroscope,when charged, retains its charge for hoursor days, in spite of the fact that it is beingbombarded incessantly by the countlessmolecules of the air. But if traversed byX-rays or by any of the new rays, even alsoby light of exceptionally short wave-length,the air absorbs the rays and suffers a change.The neutral molecules are dissociated into

oppositely charged ions, and these ions carrythe electricity through the air, so that itbecomes a partial conductor. A gold-leafelectroscope is discharged by X-rays andthe rays from radioactive substances, andthis property has proved of the utmostservice, for upon it an accurate system ofmeasurement has been based. It may bestated that any of the new rays are easierto deal with quantitatively than common


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light is, because their intensity is readilyand accurately measurable by electrical in-struments, like the gold leaf electroscope.

Three kinds of rays are distinguishable,termed Alpha or a-rays, Beta or /8-rays, andGamma or 7-rays. The j8-rays are the onesmost obvious on first examination, for they

RADIOACTIVITY 205

affect the photographic plate powerfully andare capable of traversing metal foils. Theirpenetrating power is somewhat less thanthat of the average X-rays, but it is sufficientto be remarkable. The 7-rays are veryfeeble by comparison, arid very active prep-arations are necessary to exhibit them.But their penetrating power is by far thegreatest of any known kind of ray. The7-rays of radium traverse half an inch oflead before being half-absorbed, and other

substances, roughly in proportion to theirdensity. The fluorescent effects of the /3-and 7-rays are best shown with willemiteand the platinocyanides. The a-rays areamong the most feebly penetrating of thenew kinds of radiation, and are absorbed bya single sheet of paper or by a few inchesof air. Nevertheless they are by far themost important class, and possess over95% of the energy evolved from radio-active substances. They produce very power-ful ionising action and also brilliant fluor-escence in zinc sulphide, diamond, etc., but

their photographic action is relatively feeble,and their effect on the fluorescers, whichshow best the /S- and 7-rays, is small.

The y8- and 7-rays are believed to stand ina relationship similar to that of the cathode-


rays and X-rays already discussed. The/3-rays are free-flying single negative elec-

trons, but their velocity is, in some cases,almost that of light itself, the fastest velocityknown. They are deviated by a magnet justlike the cathode-rays, but less easily on ac-count of their much greater kinetic energy.

When these rays impinge upon atoms ofmatter, they are all, more or less, accordingto their nature, absorbed, and their kineticenergy, as always, is transferred to the


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molecules of matter and becomes heat. Theheat so generated by pure radium compoundsis extraordinary, considering the minutequantities of radium which are available.Every hour radium generates sufficient heatto raise the temperature of its own weightof water from the freezing-point to theboiling-point. In one day the energy gener-ated is sufficient to decompose its own weightof carbon dioxide into carbon and oxygen,and, in thirty-eight hours, its own weight ofwater into hydrogen and oxygen. Theseare among the most energetic chemicalreactions known. Yet, year after year, sincethe substance was discovered, radium hasbeen pouring forth this steady stream ofenergy and shows no sign of failing. Inten yearsthe energy generated equals that

RADIOACTIVITY 207

developed in the combustion of over a

thousand times its weight of pure carbon,and more than this of any ordinary fuel.Yet these supplies of energy continue un-abated and unaffected by any considerationswhatever. There is no way of turning thestream on and off as it is wanted. Thisproperty of continuously evolving energy is asmuch an essential part of the nature of radiumas their unalterability in the fire is a propertyof the noble metals.

How are these discoveries to be reconciledwith the law of the conservation of energy,

and with the view that energy is a definitephysical existence which must come fromsomewhere if continually generated? Theyhave been reconciled completely, but theexplanation involves the view that the atomof the chemist, although still the ultimatelimit of subdivision of matter in every arti-ficially engendered process, is not the naturallimit. This explanation was put forwardten years ago by Rutherford and the writer,and has since been adopted. In the naturallyoccurring phenomenon of radioactivity thereis a spontaneous process continuously going

on, in which the atoms themselves are theunits that change. The oft-quoted wordsof Clerk Maxwell, before the British Associa-


tion in 1873, are no longer true. He said:"Natural causes, as we know, are at work


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which tend to modify, if they do not at lengthdestroy, all the arrangements and dimensionsof the earth and the whole solar system. Butthough in the course of ages catastropheshave occurred and may yet occur in theheavens, though ancient systems may bedissolved and new systems evolved out oftheir ruins, the molecules out of which thesesystems are built the foundation-stones ofthe material universe remain unbroken andunworn." In present-day nomenclature theword "atoms" must be substituted for"molecules" in this quotation, for beforethe discovery of radioactivity even themost eminent physicists had not, like chem-ists, learned clearly to distinguish betweenatoms and molecules. The dissolution ofancient systems and the evolution of newones out of their ruins, referred to so elo-quently by Clerk Maxwell, are in all prob-ability controlled by the dissolution of theiratoms. The infinitely slow march of cos-mical evolution probably keeps pace withthe gigantic periods of time which these

atomic changes require.

If radium, the element, is the source of theenergy it pours out in a continuous stream,

RADIOACTIVITY 209

radium, the element, must change, and thechange of an element is transmutation. Theposition of the parts constituting the atommust alter, and the energy associated with

the atom must suffer conversion into theenergy of motion, which ultimately appearsand can be measured as heat. Evidence ofthese changes was soon forthcoming, and now,complicated as some of them are, every detailalmost of the process, whereby the energy isevolved, is known to a degree of accuracyunsurpassed in many of the older examples ofmaterial change. Several reasons exist forthis. Radioactivity is an inevitable process,which is quite independent of the conditionsand circumstances, and indeed is not knownto be really affected in the slightest degree

by any circumstance whatever. Whereaschemical changes, notoriously, are far lesssimple, and are affected and sometimesreversed by a great variety of conditions,many of which are still only very imperfectlyunderstood. In the second place, the elec-trical methods of measurement employed areof unsurpassed delicacy and certainty, andthe changes occurring in a quantity of radio-active matter, which in a few minutes or


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seconds is easily detectable by these methods,might have to continue for geological epochs


of time before they produced any effect thatcould be detected by the ordinary methods ofchemistry. But the most important reasonof all is that the changes of radium and theother radio-elements are, on the one hand,neither gradual changes of the atom in whichall the atoms slowly evolve their energy andslowly change into new forms, nor, on theother, are they completely sudden processesin which the individual atoms give up theirstore of energy, changing at one step into theproduct or products. The changes in radio-activity are of the latter type exclusively,but they are not single. If the atoms ofradium changed suddenly into their finalproducts evolving their store of energy in onestage, such a process would be difficult to

identify. True, the energy would sufficientlyindicate the change, but it would remainthe sole indication. The proportion of theradium changing in a year or in ten yearsis altogether too small to be detectable byordinary methods with the minute quantitiesso far available, whilst in uranium andthorium, the radioactivity of which is millionsof times feebler than that of radium, the rateof the change is correspondingly smaller.This is of reality a necessity, as a little con-sideration will show. Radioactivity is a

RADIOACTIVITY Sll

natural spontaneous process occurring inknown materials at a constant rate. Theearth has existed, according to geologicalevidence, for hundreds of millions of years inmuch the same state as at present, and ifradioactivity is a change occurring in certainelements, these elements must long ago havedisappeared from the earth altogether, unlessthe changes were slow even as compared with

the progress of geological time. There is onlyone way by which such changes could comewithin the range of experimental science, andthat is the way in which the radio-elementsactually have been proved to be changing.

There is usually a long succession of sepa-rate sudden changes, part of the energy beingevolved at each change. In consequencethere exist, intermediate between the initial


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element, radium, for example, and its finalproduct, the element lead, as is generallysupposed but not yet proved, a number ofintermediate forms of matter having anexistence more or less transitory, but, inspite of their infinitesimal quantity, evolvingso much energy in their further changes thatthey can readily be detected and accuratelystudied. Nearly thirty of these new transi-tional forms of radioactive matter have beenrecognised in the changes of the elements

S12 MATTER AND ENERGY

uranium, thorium, radium, and actinium,and considerations of space alone wouldprevent the detailed consideration of theenormous number of important investigationsthat have been published upon them in thelast ten years. It must suffice to take oneexample, the first change of radium itself,in some detail, as all the others are strictly

analogous in their general character.

The radioactivity of a radium compoundappears to consist under ordinary circum-stances of all three types of rays in unchang-ing proportions. It is sufficient to dissolvethe compound in water and to evaporate thesolution to dryness again, or even, moresimply, strongly to heat the compoundwithout dissolving it, to remove by far thegreater part of this radioactivity. A fewhours after this treatment, the activity of theradium is at a minimum and no further

chemical or physical treatment, howeverelaborate, further alters it. At this stagethe - and 7-rays have been entirely removed,whilst the a-rays have been diminished toone-fourth of their initial amount. Thesubstance radium has not been at all alteredby the process. In the course of time thesolid compound of radium or its solution, ifkept in an air-tight vessel, recovers the

RADIOACTIVITY 213

activity it has lost. All the three types ofrays are regenerated at characteristic rates,and in a month it is as radioactive as initially.These operations may be repeated with thesame result any number of times. A closerexamination reveals the fact that during thesolution, or heating, a gaseous substance,called the radium emanation, escapes. Ifarrangements are made to collect this gas, it


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will be found that, generally speaking, thewhole of the radioactivity the radium haslost is possessed by the gas. The gas itselfis in almost absolutely infinitesimal quantity.Yet its radioactivity is so powerful that nodifficulty whatever is experienced in detect-ing and working with it, for it may be mixed,if necessary, with air and then dealt withby ordinary methods. In the solid radiumcompound many of the rays, particularly thepowerful a-rays, are absorbed by the materialitself, but in the gas they have full scope, andthe fluorescent action which the emanationproduces on bodies such as zinc sulphide, isremarkably brilliant.

The radium emanation itself shares withthe argon gases the property of not enteringinto chemical combination or being absorbedby any known reagent. It is also condensedto the non-gaseous form at the temperature


of liquid air, and in these ways it is possibleto separate it from all known substances andto study it pure. Then the extraordinaryminuteness of its actual quantity and thepower of its radioactivity become evident.From a gram of pure radium the gaseousemanation obtained occupies a volume,measured under standard conditions of tem-perature and pressure, of only 0.6 of a cubicmillimetre, the volume of an ordinary pin'shead. Yet the rays from far less than a

thousandth part of this quantity will causezinc sulphide to fluoresce in a way that willbe plainly visible in an absolutely dark hallto an audience of a thousand people. Indeed,if one-thousandth of the emanation obtain-able from a gram of radium were mixeduniformly with the air of a very large hall,say with 100,000 cubic feet, or over S tonsby weight, of air, no delicate instrumentsuch as is customarily employed in themeasurement of radioactivity could be workedin the hall, and the amount in a single cubicinch of the air could still be detected by a

sensitive gold leaf electroscope. The unitadopted in certain scientific work is theamount of emanation produced by one mil-lion-millionth of a gram of radium, a quantitywhich itself has a volume of less than a

RADIOACTIVITY 215


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million-millionth of a cubic millimetre, andweighs a million million times less than anexceptionally delicate chemical balance willturn to. Such a quantity contains less thana million single atoms. It is almost incredible,but nothing in science is better established.

The heat evolved from this emanation ofradium is in proportion to its radioactivity.The quantity of emanation derived from1 gram of radium evolves three-fourths ofthe total given by the radium, and when it isremoved from the radium, the latter onlygives one-fourth of what it gave before.Now it is almost incredible in any case that aquantity of less than a cubic millimetre ofgas can evolve spontaneously enough heat toraise the temperature of three-fourths of agram of water from freezing-point to boiling-point in an hour. To obtain a single cubicinch of this gas, measured under standardconditions, would require 26 kilograms ofpure radium. This, therefore, is almost animpossibly large quantity practically to

obtain, but it is interesting to note that theenergy such a quantity would emit wouldbe equal to that of a powerful electric arclamp. The mystery of the source of theenergy of radium is increased a million-foldwhen the nature of its emanation is studied.


How long can such an unparalleled evolutionof energy last?

This is just where the interesting pointcomes in. The emanation of radium doesnot last. It is not an apparently permanentsource of energy like the radium from whichit originates. If the activity of some radiumemanation, sealed up in a tube, is examinedfrom day to day, it will be found steadily todecay. In four days its activity is onlyhalf the initial. In eight days it is onequarter, and so on. In a month it has allpractically disappeared. But while thesechanges are taking place a concomitant set

are going on in the radium from which theemanation was derived. It recovers theactivity it lost, when the emanation wasremoved, just as fast as the activity of theremoved emanation decays. If at the endof the month, when the radium has fullyregained its activity, it is redissolved in water,a new quantity of emanation is obtained justas great as at first. Radium is producingthe emanation. The emanation of radium


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is the first product of the change of theradium atom. This emanation in its turn ischanging comparatively quickly. The changeis complete in a month. One-fourth of theenergy is derived from the change of the

RADIOACTIVITY 217

radium, and three-fourths from the sub-sequent changes suffered by the emanation.

Thus, when a quantity of radium is observedto be apparently pouring forth in an un-diminished stre

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