The interpretation of radium : being the substance of six free popular experimental lectures delivered at the University of Glasgow, 1908

Preface

HE present-day interpretation of radium, that it is an element undergoing spontaneous disintegration, was put forward in a series of joint scientific communications to the Philosophical Magazine of 1902 and 1903 by Professor Rutherford, now of Manchester University, and myself. As its application is not confined to the physical sciences, but has a wide and general bearing on our whole outlook upon Nature, I have attempted in this book a presentation of the subject in non-technical language, so that the ideas involved, and their bearing upon current thought, may be within the reach of the lay reader. Although written in non-technical language, no effort has been spared to get to the root of the matter and to secure accuracy, so that possibly the book may prove serviceable to workers in other fields of science and investigation as well as to the general public.

The book contains the main substance of six

VI

PREFACE popular experimental lectures delivered in the University of Glasgow at the beginning of the year, but being relieved from the necessity, always present in lecturing, of co-ordinating the experimental and descriptive sides, I have, while adhering to the lecture form of address, entirely rearranged and very largely rewritten the subject matter in order to secure the greatest possible degree of continuity of treatment. Certain portions of the lectures, for example those dealing with the X-rays and the spectra of elements, have been omitted, and attention thereby concentrated upon radium, the chief topic. In addition, I have briefly embodied the results of important discoveries which have appeared since the date of the lectures, particularly the experiments of Professor Rutherford and Dr. Geiger in counting the number of a-particles expelled by radium. The book also contains some account of the arrangement by means of which I have recently succeeded in detecting and measuring the quantity of the helium generated from the common radio-elements uranium and thorium.

I have borrowed freely from numerous scattered lectures and addresses bearing on the subject which I have from time to time been invited to deliver,

PREFACE vn and may mention in particular the Wilde lecture to the Manchester Literary and Philosophical Society, 1904, the Presidential and other addresses to the Rontgen Society, 1906, the opening of the discussion on the evolution of the elements in Section A of the British Association Meeting in York, 1906, and the Watt lecture to the Greenock Philosophical Society, 1 908.

FREDERICK SODDY.

The University, Glasgow, November , 1908.

A FOREWORD

O those among my audiences who read this

-A- book I may address myself in the hope that it will renew their interest, not only in the actual scientific discoveries it deals with, but also in the necessary conditions under which alone can scientific discoveries be made. As I then pointed out, the prosecution of original scientific investigation nowadays in this country calls for continuous and considerable support from the public and the publicspirited. These lectures were intended as a slight return for the help afforded to the University by prominent citizens, particularly in providing the Physical Chemistry Laboratory with the appliances necessary for research work. Although there existed probably as much general interest in and public enthusiasm for new discoveries in science as for new works in art, literature, or music, and although the national importance of scientific discovery was becoming daily more recognised, little improvement

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X

A FOREWORD could be hoped for until the creative aspect of science was considered and provided for totally independently of the professional and educational aspects. Man may and often does create great works of art, literature, or music in a garret, but when it comes to making’ great discoveries in electricity or in radioactivity, in addition to the ability of the investigator there is the almost equally important consideration of laboratory facilities, of materials and of the means of mastery over the properties of materials, the lack of which not even the genius of a Faraday could entirely overcome. From time immemorial the claims of art, music, literature and scholarship for their own sake apart from professional ends have been esteemed. Science for its own sake is at least as worth fostering among us, but a necessary first step must be to provide in this city a permanent institution devoted to its advancement.

It might not be out of place to point out that in London such an institution — the Royal Institution — had been in existence and had flourished. It was at once a permanent home for scientific research, and a means for the diffusion of accurate and intelligible information on scientific subjects

A FOREWORD xi to the general public. Since its foundation by Count Rumford a distinguished succession of men, including Davy, Faraday, and Tyndall, had worked within its walls, and how much the public in turn have benefited by the work and discoveries of such men it would be hard to say. It would be an error to suppose that this band of men would have accomplished what they have done apart Irom the existence of a place in which to work. No doubt also this institution had done much in England to combat the view, still far too prevalent, that science was a very narrow and narrowing subject of study, of importance only to the specialist and the technical man, but of no interest or importance to the man of ordinary affairs and culture. The public of Glasgow might seriously consider the question whether they could not make similar provision for scientific research and establish a home for its advancement and diffusion in their midst.

F. S.

Chapter I Page

The new science of radioactivity — Its discovery — The four experimental effects of radioactivity — The rays of radioactive substances —The continuous emission of energy from the radio-elements — The arresting feature of radioactivity . . i

Chapter II

Mme. Curie’s discovery of radium — Radioactivity an atomic property — Its unalterability — The radioactivity of thorium — Pitchblende — Quantity of radium in pitchblende — The smallest quantity of radium that can be detected — Experiments with pure radium bromide — Doctrine of energy — Energy evolved by radium compared with that from the burning of coal— Source of cosmical energy — A quotation from Professor Tait — Radium and the “ physically impossible ” . 16

Chapter III

The radiations of the radio-elements — a-, and 7-rays — Test of penetrating power — Experiments with the penetrating |3- and 7-rays — The non-penetrating a-rays — Experiment to show the absorption of a-rays by air — Physical nature of radiations — Necessity for an ether — Corpuscular and wave radiations — a- and i3-rays due to the expulsion of particles — Can a single a-particle be detected? — The spinthariscope — Counting the aparticles expelled by radium .

xiii

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XIV

CONTENTS

Chapter IV Pack

The /3-rays — Their deviability by a magnet — The nature of the /3-particle — Analogy to cathode-rays or “Radiant Matter”

— The electron — Velocity of the /3-rays — The nature of the a-particle— Its velocity — Its power of passing through atoms of matter in its path — a-Particles might be expelled without their being detectable , . . . 66

Chapter V

From where does the energy of radium come? — The two alternatives and their consequences — The internal energy of matter — Atomic disintegration — Disintegration in cascade — The successive outbursts of energy from otherwise impalpable quantities of matter — The emanation of radium — Its properties— Experiments with the emanation — Its condensation by liquid air — The infinitesimal quantity of the emanation from radium — The chemical nature of the emanation — The energy evolved by the emanation — The decay of the emanation and its reproduction by radium— The facts, not the theories, of radioactivity are revolutionary — Unalterability of radioactive changes — Evanescent products of radioactive change — All products equally knowable whether short-lived or long . . 91

Chapter VI

The connection of the a-particle with radioactive changes — The a-particle and helium — Accumulation of helium in geological time — Discovery of helium in the sun and on the earth — Its connection with radioactivity— Production of helium from radium— Its production from uranium and thorium — Proof that the a-particle is an atom of helium — The nature of the first change of radium . . .... 126

Chapter VII

Atomic disintegration and the periodic law — Questions of nomenclature— Definition of the chemist’s atom — Difference between atoms and chemical compounds — The insufficiency of chemical methods in many radioactive problems — Hypo-

CONTENTS xv

PAGE

theses or mental pictures — The two possible pictures of atomic disintegration — Sudden explosive character of the disintegration — Law of radioactive changes — Chance of disintegration—Average life of a disintegrating atom — Its expectation of life — The “how,” not the “why,” of atomic disintegration explained— Determination of the period of average life of atoms — Primary radio-elements and ephemeral transition-forms — Radioactive equilibrium — Average life of radium — The total energy evolved in the complete disintegration of radium . . . . . . 141

Chapter VIII

How is it there is any radium left? — The parent of radium — Fixity of ratio between the quantity of uranium and radium in all minerals — Period of average life of uranium — Relation of uranium to radium — An analogy to the Glasgow water supply system — Age of pitchblende— Radioactivity of uranium— Uranium X — Uranium not the direct parent of radium — Growth of radium by uranium — Intermediate transition-forms of long life— The direct parent of radium — The stately procession of elementary evolution . . 165

Chapter IX

The subsequent changes of radium — The induced or excited radioactivity — The active deposit of radium — The disintegration of the emanation— Radium A, B, C — Experiments with the active deposit — Radium A gives only a-rays and has a very short life — Radium B gives no rays — Radium C gives a-, /3-, and 7-rays — The emanation only gives a-rays —

The later slow changes of radium — Radium D, E, and F — Polonium — Its identity with radium F — The last disintegration— What is the ultimate product ? . . . . 186

Chapter X

Ratio of quantities of polonium and radium in minerals — Table of the ratio'of the quantities of all the products of uranium — Increase of activity of radium with time — Radioactivity a

XVI

CONTENTS

PAGE physical measure of value or rarity — The currency metals and their rarity — Are they changing like radium ? — Physical necessity for rarity of a changing element — One aspect of the ultimate nature of matter — A quotation from Clerk Maxwell— Evolution of the elements denied — Similarity of all the atoms of the same element — The atom a complex and perfect piece of mechanism — Professor Schuster’s analogy —

The atom true to its character at dissolution — Similarity in the velocity of all a-particles expelled from a radio-element — Survival of the fittest or most stable atoms — Universality of the conception of evolution to the material universe, animate and inanimate . . ... 206

Chapter XI

Why is radium unique among the elements? — Its rate of change only makes it remarkable — Uranium is more wonderful than radium — The energy stored up in a pound of uranium — Transmutation is the key to the internal energy of matter —

The futility of ancient alchemy — The consequences if transmutation were possible — Primitive man and the art of kindling fire — Modern man and the problem of transmutation — Cosmical evolution and its sinews of war — Atomic disintegration a sufficient, if not the actual primary source of natural energy — Radioactivity and geology — Quantity of radium in the earth’s crust — The earth probably not a cooling globe — Mountain formation by means of radium — The temperature of the moon and planets— Ancient mythology and radioactivity— The serpent “Ouroboros” — The “Philosopher’s Stone” and the “Elixir of Life” — The “Fall of Man” and the “Ascent of Man” — The great extension in the possible duration of past time— Speculations on possible forgotten races of men — Radium and the struggle for existence — Existence as a struggle for physical energy — The new prospect . . . . ... 223

Indkx

. 251

LIST OF ILLUSTRATIONS

FIG.

1. Becquerel’s uranium radiograph of an aluminium'

medallion . . ...

-

2. Welsbach mantle, taken by the rays from the

thorium contained in it . .

3. Photograph and radiograph of a piece of pitch-

blende (Sir William Crookes)

4. Photograph of silk tassel electrified by friction .4

5. The same discharged by the rays of radium ./

6. Radium writing on a photographic plate . .'j

7. Box of compasses taken by 7-rays of radium ./

8. Diagram of coated flask and radium-covered dish

for showing a-rays . ...

9. Photograph of the same apparatus .

10. Diagram of Spinthariscope of Sir William

Crookes . . ...

11. Photograph of the Spinthariscope

12. Photograph of the electro-magnet for deviating

13. Diagram of magnetic deviation of /3-rays

14. Diagram of Crookes’ tube to show magnetic devia-

tion of cathode-rays . ...

15. Diagram of Strutt’s radium clock

16. Photograph of radium clock ...

17. Photograph of tube containing willemite . . "1

18. Photograph of the same tube by its own light when -

containing radium emanation . . . J

19. Diagram of apparatus for showing the condensa-

tion of the radium emanation

20. Diagram of the first disintegration of radium l’AGE

3?

•

XVII xvm

LIST OF ILLUSTRATIONS

TAGE

FIG.

21. Photograph of the spectrum-tube in which the pro-'

duction of helium from radium emanation was observed . . . . -

22. Photograph by Dr. Giesel of the spectrum of

helium produced from radium

23. Photograph of apparatus for detecting and measur-

ing helium produced from uranium and thorium . . . ...

24. Diagram showing the first change of radium

25. Diagram for the first disintegration of uranium .

26. Diagram of the uranium-radium disintegration

series (initial changes) . ...

27. Diagram of the first four disintegrations of radium

28. Diagram of apparatus for obtaining the active

deposit of radium . ...

29. Photograph of the same apparatus .

30. Diagram of the later disintegrations of radium .

31. Diagram of the complete uranium disintegration

series . . ...

9?

192 1 92 200

THE INTERPRETATION OF RADIUM

Chapter I

The new science of radioactivity — Its discovery — The four experimental effects of radioactivity — The rays of radioactive substances — The continuous emission of energy from the radio-elements — The arresting feature of radioactivity.

NE of the main duties of science is the correlation of phenomena, apparently disconnected and even contradictory. For example, chemistry teaches us to regard under one aspect, as various types of combustion or oxidation, the burning- of a candle, the rusting of metals, the physiological process of respiration, and the explosion of gunpowder. In each process there is the one common fact that oxygen enters into new chemical combinations. Similarly to the physicist, the fall of the traditional apple of Newton, the revolution of the earth and planets round the sun, the apparitions of comets, and the ebb and Bow of the tides are all phases of the universal law of gravitation.

B

A race ignorant of the nature of combustion or of the law of gravitation, and ignorant of the need of such generalisations, could not be considered to have advanced far along the paths of scientific discovery. The phenomena with which I am concerned in these lectures belong to the newly-born science of radioactivity and to the spontaneous disintegration of elements which the study of radioactivity has revealed to us. It is a natural inquiry to ask — To what most nearly are these new phenomena correlated? Is it possible to give, by the help of an analogy to familiar phenomena, any correct idea of the nature of this new phenomenon “ Radioactivity ” ? The answer may surprise those who hold to the adage that there is nothing new under the sun. Frankly, it is not possible, because in these latest developments science has broken fundamentally new ground, and has delved one distinct step further down into the foundations of knowledge.

o

During the century which has just closed there occurred, it is true at an ever-increasing rate, a ceaseless extension of our knowledge of the nature of matter upon which physical science is largely based. Yet this advance was for the most part an expansion rather than a deepening. It was concerned with what may be termed atomic and molecular architecture, the external qualities of atoms and the construction and study of complexes built of atoms— that is to say, molecules. As buildings are built of bricks, so compounds can nowadays be built up out of atoms. The atoms are to the chemist and physicist what bricks are to the architect — the units supplied ready-made to a certain limited number of standard specifications and dimensions capable of an endless variety of combinations and arrangements, each with its own peculiarities and external relationships.

The century which has just begun has seen the first definite and considerable step taken into the ultimate nature of their units of matter or atoms, which is in one sense not merely an extension of existing knowledge or principles, but a radically new departure. Radioactivity is a new primary science owing allegiance to neither physics nor chemistry, as these sciences were understood before its advent, because it is concerned with a knowledge of the elementary atoms themselves of a character so fundamental and intimate that the old laws of physics and chemistry, concerned almost wholly with external relationships, do not suffice. This first step has indeed emphasised how superficial our knowledge of matter has really been. If one were to demonstrate to an architect that the bricks he habitually and properly employs in his constructions were under other circumstances capable of entirely ditferent uses — let us say, for illustration, that they could with effect be employed as an explosive incomparably more powerful in its activities than dynamite — the surprise of the architect would be no greater than the surprise of the chemist at the new and undreamt of possibilities of matter demonstrated by the mere existence of such an element as radium.

In this first lecture our attention will be mainly directed to the one outstanding feature in connection with radium, and the property of radioactivity which it exhibits to an extraordinary degree, in which the whole range of its remarkable features are epitomised. The radioactive substances evolve a perennial supply of energy from year to year without stimulus and without exhaustion. It would be idle to deny with regard to this that physical science was taken completely by surprise. Had any one twelve years ago ventured to predict radium he would have been told simply that such a thing was not only wildly improbable, but actually opposed to all the established principles of the science of matter and energy. So drastic an innovation was, it is true, unanticipated. Radium, however, is an undisputed fact to-day, and there is no question, had its existence conflicted with the established principles of science, which would have triumphed in the conflict. Natural conservatism and dislike of innovation appear in the ranks of science more strongly than most people are aware. Indeed, science is no exception. There was, however, never the slightest ground for assuming that because the new facts were startling and unexpected they must necessarily conflict with older knowledge. That would be to pay science a poor compliment. Some of the new facts we shall discuss in the lectures appeared at first, and may even yet appear to you, almost incredible, but that was only on account of the entire newness ol the whole region to which they belong.

Into this region the older chemistry and physics have, as we have seen, never before penetrated. It is concerned, to use one of the many prophetic phrases of Tait, with “matters of a higher order than common physics and chemistry.” Even so, it is not until we begin to apply to the new facts the established principles of science, which have served so well of old, that their full significance gradually becomes evident. Keep in mind that our knowledge of nature is always of necessity partial, and is bounded in all directions by certain inevitable but too often forgotten limitations connected, for example, with the briefness of human life and the physical impossibility of pursuing investigations except under conditions where the life of the investigator can be maintained. The laws and principles of physical science, old and new, are alike subject to these perpetual limitations, and are necessarily only true within these limits. From this point of view there is nothing- in the many surprising- properties of radium which conflicts with a single established principle of older science. Physics and chemistry remain almost unchanged where they were, and radioactivity, so far as it is concerned with the correctness of their principles, has, as a matter of fact, given to the old laws and theories a fuller and truer significance than they had before. The extension of the old theories which has been rendered necessary has not been revolutionary in any destructive sense. It is wonderful how accommodating a true theory is to new truth, apparently of a diametrically opposite character, and this not in any sense of mere ingenuity of explanation, but in a manner that arrests the investigator, and is his sign that he is on safe ground.

It may seem a paradox, but from the first the best proof of the newer views, to my mind, was in the completeness with which the strange, newly- won knowledge of radioactivity harmonised with the old views of the chemist about atoms and elements. On the other hand this gratifying harmony, where conflict might have been expected, is not a surrender. On every hand new vistas of thought are opening out. We see the simple and direct answer to many problems before deemed insoluble. We recognise now causes at work where before we only saw effects, many of them so familiar and ingrained in our consciousness that the necessity for a cause had been almost overlooked, or, if felt at all, met perfunctorily and wholly inadequately by existing knowledge. Highly technical and complicated as many of the researches on radioactivity were, the main conclusions of the science were as simple and certain as they were fundamental, and of general interest. It was the duty of every educated man to make himself aware of the chief bearings of these conclusions, for they touched human life strangely at many points, and were destined in the future to influence profoundly the course of philosophic thought. In a few years the elementary principles of radioactivity would be taught in all schools as belonorina to the very beodnninors of physical science. To-night, while all was strange and new and the very name of the science even unfamiliar, it might appear a far cry to attempt to foretell the effects these discoveries, concerned primarily with the ultimate nature of matter, were destined to exert on our conceptions of the ultimate destiny of man. But already the most direct connection was apparent. Indeed, this aspect of the advance was perhaps the most revolutionary. We

DISCOVERY OF RADIOACTIVITY shall be able to see more clearly at the end how this has come about. At present it is sufficient to indicate that radioactivity has introduced a new conception into the fundamental problems of existence. By its conclusion that there is imprisoned in ordinary common matter vast stores of energy, which ignorance alone at the present time prevents us from using for the purposes of life, radioactivity has raised an issue which it is safe to say will mark an epoch in the progress of thought. With all our mastery over the powers of Nature we have adhered to the view that the struggle for existence

<30

was a permanent and necessary condition of life. To-day it appears as though it might well be but a passing phase, to be altogether abolished in the future as it has to some extent been mitigated in the past by the unceasing, and as it now appears, unlimited ascent of man to knowledge, and through knowledge to physical power and dominion over Nature.

The first discovery of the property we now call radioactivity was made in the year 1896 by M. Henri Becquerel in Paris, and, like many other great discoveries, the actual experiment itself owed something to luck or chance or accident. Looking backward, however, it appears rather that only the particular day or month of the discovery was a

DISCOVERY OF RADIOACTIVITY matter of chance. The time was just ripe for the event, and it is certain that its coming could not long have been delayed. Some slight historical sketch of the conditions preceding and immediately following the discovery is necessary before considering wherein lies its oreat significance. The memorable discovery of the X-rays by Professor Rontgen, in 1895, which is known to all, familiarised scientific workers with a type of radiation able to traverse objects opaque to light. The X-rays are themselves invisible to the unaided eye, but are able to affect the photographic plate. This led to experiments being made in order to see if similar types of rays were not produced in other ways, As you all know, certain substances exposed to sunlight shine afterwards in the dark, and this property, which finds an application in the manufacture of luminous paint, is known as phosphorescence or fluorescence. Is phosphorescent light entirely stopped by opaque objects ? Or does it in part consist of invisible penetrating rays like the X-rays ? M. Becquerel wrapped a photographic plate in black paper and placed on it a phosphorescent substance which was then exposed to sunlight. By great good fortune M. Becquerel chose as the particular phosphorescent body a preparation of uranium , and found as the result of the experiment that the photographic plate beneath the preparation was darkened. The

DISCOVERY OF RADIOACTIVITY preparation had given out rays which, unlike sunlight, were capable of penetrating the black paper. It was soon found that these rays, like the X-rays, even penetrated thin plates of metal, for when such a thin plate was interposed between the preparation and the film darkening still occurred. But one day, the sun being obscured, the plate and the phosphorescent uranium preparation upon it were set aside in a dark drawer for some weeks, and M. Becquerel, wishing to see if any darkening had occurred without the sunlight, developed the plate as it was. It was found that darkening had gone on just as much in the darkness as in the light. Further experiments soon established that neither sunlight nor phosphorescence had anything to do with the experiment. The action is an entirely new inherent property of the element uranium. No other phosphorescent body would have darkened the plate even in the sunlight, while all preparations containing uranium do so, whether they are phosphorescent or not, in total darkness as well as in the light. Fig. i shows one of the photographs by uranium rays obtained by M. Becquerel. Between the patch of the uranium preparation and the plate was placed an aluminium medallion, stamped with a head of a figure in relief, which partially shielded the plate beneath from the rays. The impression under the thinner portions of the medallion is

Fig. i. Becquerel’s Uranium Picture.

r i \ gs»

BaS?

rf.vo 'I!

f**i i| if

''MM'wWfiwSfSSS *•* ■ ■- ■

********* rvw*

*f>|n>ti ft X i' if

Fig. 2. Welsbach Mantle imprinted by its own rays.

To face p. io.

•* 4

EFFECTS OF RADIOACTIVITY n

darker than under the thicker portions, thus causing the head of the figure to be clearly apparent in the photograph.

Although the radioactive process is itself without analogy in science, the main effects which it produces can almost all be more or less nearly imitated, and were all more or less perfectly studied prior to its discovery. The main effects of radioactivity with which we are most concerned are four. Firstly then, radioactive substances affect a photographic plate in the same manner as light and many other agencies. Secondly, they excite phosphorescence or fluorescence in certain substances when brought in their neighbourhood. Thirdly, radioactive bodies cause the air and other o-ases to

o

lose the insulating power they normally possess and to become partial conductors of electricity. In consequence, any electrified object has its electricity rapidly discharged in the neighbourhood of a radioactive substance. But the same effect is produced by X-rays, by incandescent bodies, and even by a lighted match. The instrument employed to detect this effect is the gold-leaf electroscope, the first and simplest electrical instrument to be invented, and for this purpose capable of so great refinement that it affords the most delicate and sensitive test it is possible to employ in i2 RAYS OF RADIOACTIVE SUBSTANCES the detection of radioactivity. Lastly, radioactive bodies generate heat, as does coal or any other substance burning*. The photographic action and the discharge of electricity from insulated charged bodies are clearly shown by radioactive substances even in the form in which they occur in Nature, as all unsuspected they have been handled and examined by men for centuries. Hence you will understand how it is that the discovery of radioactivity could not under any circumstances have been indefinitely delayed. But only the more powerfully radioactive substances, like radium, give appreciable phosphorescence or heat effects. In the naturally occurring* radioactive substances these effects are far too small to be readily detectable.

Exact physical experiments have demonstrated that all these effects of radioactivity owe their origin to the fact that the radioactive substances emit “rays.” These rays are invisible to the unaided eye it is true. In this they resemble Rontgen’s X-rays. There are three different types of rays given out by the radioactive substances, which are known respectively as the a-, ,8-, and y-rays. Each will require detailed future consideration. But they all bear less resemblance to light than to the recently discovered types of rays, of which the X-rays of Rontgen are typical, produced