THE The source of this uncorrected OCR text may be viewed as a digital facsimile at: http://fax.libs.uga.edu/ OF RADIUM THE INTERPRETATION OF RADIUM BEING THE SUBSTANCE OF SIX FREE POPULAR EXPERIMENTAL LECTURES DELIVERED AT THE UNIVERSITY OF GLASGOW, 1908 BY FREDERICK SODDY, M.A. INDEPENDENT LECTURER IN PHYSICAL CHEMISTRY AND RADIOACTIVITY IN THE UNIVERSITY OF GLASGOW WITH ILLUSTRATIONS NEW YORK : G. P. PUTNAM'S SONS LONDON: JOHN MURRAY 1909 PREFACE THE present-day interpretation of radium, that it is an element undergoing spontaneous dis integration, was put forward in a series of joint scientific communications to the Philosophical Mag azine 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 Uni versity of Glasgow at the beginning- of the year, but being relieved from the necessity, always pre sent 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 atten tion thereby concentrated upon radium, the chief topic. In addition, I have briefly embodied the results of important discoveries which have ap peared since the date of the lectures, particularly the experiments of Professor Rutherford and Dr. Geiger in counting the number of a-particles ex pelled 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 1 have from time to time been invited to deliver, PREFACE vu 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 discus sion 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, 1908. FREDERICK SODDY. THE UNIVERSITY, GLASGOW, November, 1908. A FOREWORD TO those among my audiences who read this book 1 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 scien tific 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 public- spirited. 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 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 elec tricity or in radioactivity, in addition to the ability of the investigator there is the almost equally im portant 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 re search, and a means for the diffusion of accurate and intelligible information on scientific subjects A FORE I ORD 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 from 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 techni cal 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. CONTENTS CHAPTER I i The new science of radioactivity—Its discovery—The four ex perimental effects of radioactivity—The rays of radioactive substances —The continuous emission of energy from the radio-elements—The arresting feature of radioactivity . CHAPTER II Mine. Curie's discovery of radium—Radioactivity an atomic property—Its unalterability—The radioactivity of thorium— Pitchblende—Quantity of radium in pitchblende—The small est 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-, /3-, 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 /3-rays clue to the expulsion of particles—Can a single a-particle be detected ?—The spinthariscope—Counting the a- particles expelled by radium . . • • 39 xiv CONTENTS CHAPTER IV PAGE 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 alterna tives 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 proper ties—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 no menclature—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 dis integration—Average life of a disintegrating atom—Its ex pectation of life—The " how," not the " why," of atomic dis integration explained—Determination of the period of aver age life of atoms—Primary radio-elements and ephemeral transition-forms—Radioactive equilibrium—Average life of radium—The total energy evolved in the complete disinte gration 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 ura nium—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 disinte gration 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 disintegra tion—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 Max well—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 trans mutation were possible—Primitive man and the art of kind ling fire—Modern man and the problem of transmutation— Cosmical evolution and its sinews of war—Atomic disintegra tion 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 radio activity— 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 pro spect . .... 223 INDEX 251 LIST OF ILLUSTRATIONS FIG. I. 9- 10. 11. 12. '5- 16. 17- 18. '9- Becquerel's uranium radiograph of an aluminium medallion t Welsbach mantle, taken by the rays from the thorium contained in it Photograph and radiograph of a piece of pitch blende (Sir William Crookes) Photograph of silk tassel electrified by friction . The same discharged by the rays of radium Radium writing on a photographic plate Box of compasses taken by 7-rays of radium Diagram of coated flask and radium-covered dish for showing a-rays Photograph of the same apparatus . Diagram of Spinthariscope of Sir William Crookes Photograph of the Spinthariscope Photograph of the electro-magnet for deviating the /3-rays . . . Diagram of magnetic deviation of /3-rays Diagram of Crookes' tube to show magnetic devia tion of cathode-rays Diagram of Strutt's radium clock Photograph of radium clock Photograph of tube containing willemite Photograph of the same tube by its own light when containing radium emanation Diagram of apparatus for showing the condensa tion of the radium emanation Diagram of the first disintegration of radium To face 10 „ 20 25 44 - 49 To face 49 . 60 To face 49 . 66 . 68 • 75 . 82 To face 66 „ 106 no 127 xviii LIST OF ILLUSTRATIONS 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 j helium produced from radium . . .j 23. Photograph of apparatus for detecting and measur ing helium produced from uranium and thor ium ..... 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 To face 135 '36 140 177 184 190 To face 192 192 200 THE INTERPRETATION OF RADIUM CHAPTER I. The new science of radioactivity—Its discovery—The four experi mental effects of radioactivity—The rays of radioactive substances vj —The continuous emission of energy from the radio-elements— The arresting feature of radioactivity. ONE of the main duties of science is the corre lation 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 gun powder. In each process there is the one common fact that oxygen enters into new chemical com binations. Similarly to the physicist, the fall of the traditional apple of Newton, the revolution of the earth and planets round the sun, the appari tions of comets, and the ebb and flow of the tides are all phases of the universal law of gravitation. 2 THE NE SCIENCE 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 con cerned in these lectures belong to the newly-born science of radioactivity and to the spontaneous disintegration of elements which the study of radio activity has revealed to us. It is a natural inquiry to ask—To what most nearly are these new pheno mena 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. 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 con cerned with what may be termed atomic and molecular architecture, the external qualities of THE NE} SCIENCE 3 atoms and the construction and study of com plexes 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 know ledge 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 super ficial 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 4 THE NE SCIENCE capable of entirely different 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 archi tect 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 connec tion 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 THE NEW SCIENCE 5 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 un expected 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 of 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 con cerned, 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. eep in mind that our know ledge 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 in vestigator can be maintained. The laws and prin- 6 THE NEW SCIENCE ciples 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 NEW SCIENCE ^ 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 per functorily 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 philo sophic thought. In a few years the elementary principles of radioactivity would be taught in all schools as belonging to the very beginnings 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 con nection was apparent. Indeed, this aspect of the advance was perhaps the most revolutionary. e 8 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 ex istence. 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 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 9 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 consider ing wherein lies its great significance. The memor able 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 them selves invisible to the unaided eye, but are able to affect the photographic plate. This led to experi ments 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 uraniu , and found as the result of the experiment that the photographic plate beneath the preparation was darkened. The llll IO DISCOVERY OF RADIOACTIVITY preparation had given out rays which, unlike sun light, 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 phos phorescent uranium preparation upon it were set aside in a dark drawer for some weeks, and M. Bec- querel, 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 iiraniiim. No other phos phorescent body would have darkened the plate even in the sunlight, while all preparations contain ing uranium do so, whether they are phos phorescent 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. M, H >;";i '»' J-' M$ .,',., .*** 1! ,>:< Dili ' it M " .' Ill -t » ' * ',"*' i »!' 11 FIG. 2. Welsbach Mantle imprinted by its own rays. To face p. 10. 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 radio activity 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 phos phorescence or fluorescence in certain substances when brought in their neighbourhood. Thirdly, radioactive bodies cause the air and other gases to lose the insulating power they normally possess and to become partial conductors of electricity. In consequence, any electrified object has its elec tricity 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 em ployed to detect this effect is the gold-leaf electro scope, 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 12 NAYS Of RADIOACTIVE SUB'STANCES 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 ex amined by men for centuries. Hence you will understand how it is that the discovery of radio activity could not under any circumstances have been indefinitely delayed. But only the more power fully radioactive substances, like radium, give ap preciable 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 un aided 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 considera tion. 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 RAYS OF RADIOACTIVE SUBSTANCES 13 when an electric current is forced by powerful appli ances to traverse a nearly vacuous space, a path which it much prefers not to take if it can avoid it. The first effects of most new things are old. Motor-cars and railways do the old work of horses. In commercial life a really new effect is generally valueless until it has ceased to be new, as many inventors know to their cost. In scientific dis covery a new effect does not usually proclaim itself from the housetops. It often needs new instru ments and the way must first be paved for its dis covery, while old effects are generally recognised first. It is natural that the first effects of radio activity to be discovered should be those more or less familiar, But for the development to perfec tion of that marvellous thing, the photographic plate, radioactivity would not have been discovered in the way it was, and we should still be without one of the readiest methods of detecting it. But for the work on the conduction of electricity through gases immediately following the discovery of the X-rays, the only other method of detecting radioactivity in the natural state would be unknown, and therefore also in all probability radioactivity itself. On the other hand, if radioactive substances exhibit any entirely new kind of properties—and it is quite possible that they do—it is very likely that their very novelty would delay their discovery. i4 ARRESTING FEATURE OF RADIOACTIVITY Why then, you may ask, if all of the effects of radioactivity are shown in other ways do I insist that radioactivity is a phenomenon unparalleled in science ? The distinctive feature of radioactivity is not, however, so much in the rays the radioactive substances emit, though we shall find upon a closer examination that these are distinctive and most remarkable. The main interest of the new pro perty consists in the spontaneous and continuous emission of energy of which the rays are but one manifestation. Heat and light may be obtained in numerous ways, but it is a new thing to find it being given out by a substance, as it is by radium, year in, year out, without apparent intermission or dimi nution, and without the substance being in any apparent way consumed or altered. This was the arresting fact. The radioactive substances apga^- ently were performing the scientifically impossible feat of evolving a store of energy presumably out of nothing. So long as radioactivity was known only on the scale and in the degree exhibited by uranium, it was perhaps possible to explain away this aspect of the question because of the minute ness of the amount of energy involved and the difficulty of proving that it was not in some way derived from the surroundings. But the work of M. and Mme. Curie, by their discovery of radium, made the world familiar with an element over a ARRESTING FEATURE OF RADIOACTIVITY 15 million times as radioactive as uranium. In this case the energy evolved is great enough to produce effects which are obvious to all and which cannot be explained away. In a strictly scientific sense there is no difference of principle between the radio activity of radium and that of uranium. The difference is one of degree only, but it is so great that radium, though, as we shall come to see, not so wonderful in reality as uranium, rapidly acquired a monopoly of public interest and attention. MME. CURIE'S FIRST STEP 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 com pared with that from the burning of coal—Source of cosmical energy — A quotation from Professor Tait — Radium and the " physically impossible." IT was worth while to stop to consider the starting-point of Mme. Curie's discovery. Chemistry analyses all known substances into their component constituents or elements, all of which are fundamentally different, the one from the other, and inconvertible the one into the other. Uranium is such an element, gold, silver, lead, and many of the common metals are others, but uranium is dis tinguished by having relatively the heaviest of all known atoms. The atom is the minimum unit quantity of an element. The relative atomic weight of an element is one of its most important characteristics, and as a first approximation the atom of hydrogen is chosen as the standard and is assigned unit value. For exact work it is more convenient to choose oxygen as the standard, with 16 the value 16. On this basis the atomic weight of hydrogen becomes 1.008. Now radioactivity is an intrinsic property of the element uranium, and therefore of the atom of uranium. This Mme. Curie first recognised, and it formed the starting-point of her work. In the case of uranium, the element itself and all its various compounds are radioactive, and the radioactivity of each compound is conditioned simply by the relative amount of uranium it contains. It does not matter where the uranium comes from—it is always to the same degree radioactive. Non-radioactive uranium is unknown. Not only so, but it is absolutely im possible really to affect the radioactivity of uranium or any other of the radioactive elements in the slightest degree. In this the process is utterly unlike any other process previously known in Nature. Radioactivity is part and parcel of the very nature of the element which possesses the property, and therefore of the atom or unit quantity of the element. The attempts that have been made artificially to alter or to stop the radioactivity of an element have met with signal failure. This is still an impossible feat—a thing modern science cannot do—and yet, as we shall come to see quite clearly in the sequel, a thing which science must do if mankind is to realise to the full the destiny these discoveries have for the first time unveiled. There i8 THORIUM is another still impossible feat, to the accomplish ment of which all the appliances of modern science have been directed in vain, as well as all the utmost power of man from the earliest time. It is trans mutation, or the conversion of one element into another. Radioactivity is the one process going on in malter we cannot influence or stop, while trans mutation is the one process in matter we have so far signally failed to effect. The juxtaposition of radioactivity and transmutation is not a fanciful one, because it will appear, as we proceed, that the two processes are most intimately connected. Radioactivity being a property of the element uranium, it was natural to ask whether uranium alone of all the eighty elements known possessed it. This was the starting-point of Mme. Curie's illustrious researches in the subject. She found only one other element among those known which possessed the property — the element thorium, which, at one time rare and little known, has come into industrial prominence of recent years in the manufacture of the Welsbach incandescent gas mantle,1 of which it forms the main constituent. To the electrical test—the power of discharging 1 The cause of the action of the gas-mantle in generating light is quite unconnected with the property of radioactivity. THORIUM 19 a gold-leaf electroscope—thorium preparations are of about the same degree of radioactivity as uranium; but to the photographic plate thorium is far less active than uranium, owing to the fact that the type of rays which affect the photographic plate most strongly are not those with most effect on the electroscope. The radioactivity of thorium is a fact which can be beautifully demonstrated by any one acquainted with the process of photo graphy. An incandescent mantle, after burning off the cotton, is cut open and pressed as flat as possible on a card. A photographic plate, which has first been wrapped in a light-tight envelope, is laid upon the flat mantle, and the whole is left undisturbed for ten days or a fortnight. On developing the plate it will be found that an image of the mantle has been formed on the plate in the dark by the rays from the thorium contained in the mantle. Any one can do this simple experi ment for himself. Fig. 2 (facing p. 10) shows the result I obtained with a very thin piece of aluminium foil between the film and the mantle. The foil, while quite opaque, allows the a- as well as the /3-rays to go through. Paper would stop the a-rays entirely. The radioactivity of thorium, though producing the same general effects as that of uranium, differs from it entirely in detail. Indeed, by a few simple zo PITCHBLENDE tests on the radioactivity, any one of the radio active elements can be recognised and distinguished far more quickly and certainly than by any of the other chemical or spectroscopic tests, even when present in very minute quantities. In the his torical development of the views now held in radioactivity thorium played a leading part. But, as it is quite foreign to my intention to give any thing approaching a detailed systematic account of the subject, and as radium lends itself more readily to experimental demonstrations, I shall confine myself chiefly to the properties of the latter substance. Although uranium and thorium were the only two known elements possessing- radioactivity, Mme. Curie found that the natural minerals containing uranium are more radioactive than can be accounted for by the uranium present. Certain minerals, called pitchblende, particularly the variety from the celebrated Joachimsthal mine in Austria, con tain often more than 50 per cent of uranium in the form of uranium oxide. The radioactivity of pitchblende to the photographic plate is beautifully shown by two photographs of Sir W. Crookes (Fig. 3). The lower figure shows the polished face of a piece of pitchblende photographed in the ordinary way by daylight. The upper figure « * * 4 y* FIG. 3. Sir William Crookes1 Pictures of Pitchblende. The lower figure is a daylight photograph. The upper was imprinted in the dark by the rays from the substance. To face p, 20. PITCHBLENDE 21 was taken by placing the polished face of the mineral on a photographic film wrapped in light- tight paper. The lighter portions of the figure indicate where the plate has been acted on by the rays from the radioactive matter in the pitchblende. Some pitchblendes are from three to four times as radioactive as pure uranium oxide. This could only be the case, Mme. Curie correctly ar ued, if there existed in the minerals one or more unknown elements more powerfully radioactive than uranium. By the ordinary process of chemical analysis it is easy to separate out the various constituent ele ments in pitchblende. There are a great number of elements in pitchblende, though most of them are present in very small amount. A fact that will be found significant later is that lead is always present in appreciable quantity. Mme. Curie found that of the elements so separated two in particular, the bismuth and the barium, were strongly radio active. Now ordinary bismuth and barium are not at all radioactive, and the radioactivity of these elements, when separated from pitchblende, is really due to the presence of two new elements in minute J amount mixed with them. The one associated | with bismuth was discovered first by Mme. Curie and named Polonium, after her native country. Its consideration is more profitably delayed till later. A he other, which was discovered very soon after- 22 QUANTITY OF RADIU IN PITCHBLENDE wards, is associated with the barium and is Radium. The exact quantity of radium in pitchblende and other uranium minerals is a fact of considerable importance. There is about one part of radium in five million parts of the best pitchblende, so that to extract an ounce of radium—and the total amount ever extracted probably does not exceed half an ounce—150 tons of this pitchblende would be required. But do not think when you have reduced the 150 tons of ore to one ounce you do not gain in quality—that quality we are interested in, namely, the radioactivity—what you lose in quantity. It is like the myriad of roses we are told go to make a single drop of the real attar, which is almost priceless. The radium that is extracted is a million times more radioactive than the mineral, and several million times more than pure uranium itself. Conversely, just as you can buy quite a large bottle of rose-water for a small sum, so quantity is not the only consideration to be taken into account in the buying of radium preparations. A very small quantity of radium is sufficient to confer on a large quantity of an inactive salt many of its own peculiar properties. Particularly is this the case with the property of glowing visibly in the dark. Weak radium preparations, which contain usually barium, shine by themselves in the- dark more strongly QUANTITY OF RADIU RECOGNISABLE 23 even than the pure radium salts, owing to a phos phorescent action of the barium salts, although they may hardly contain enough radium to affect an X-ray screen through a piece of metal. If you mix a very minute quantity of radium with a quantity of a very highly phosphorescent body, like sulphide of zinc, it will shine in the dark so brilliantly that an inexperienced person might well be deceived into believing that it must contain a large quantity of radium. It is an interesting digression to consider here the smallest absolute quantity of radium which can be detected and identified with certainty in the laboratory. Recently we have had occasion to determine this quantity in the Physical Chemistry- laboratory, and we have found that a quantity of one three-thousand-millionth of a grain of radium is easily recognisable. This is far less than could be detected in the case of any non-radioactive element by any method known, not excluding even the spectroscope. If the half of a grain of pure radium bromide, which is in this room to-night, were divided equally among every human being at present alive in the world, and one such portion were returned to us, it would prove sufficient for detection and identification by means of a gold-leaf electroscope with the greatest ease. With half 24 EXPERIMENTS ITH RADIUM a grain of a pure radium compound the main effects of radioactivity, which in the case of uranium or thorium would either be too feeble to show or would require the use of inconveniently delicate instruments, can be shown in a striking and con vincing manner to you all in the simplest possible way. Of the small amount of radium bromide, which by a labour of love certain chemists have succeeded in extracting from pitchblende, I am fortunate to possess about a grain, or sixty-five milligrams. Half of this quantity, which I shall use for most of my lecture experiments, is contained in a small ebonite capsule. The other half is dissolved in water and not brought into this lecture-room, but kept in the laboratory half a mile away. With the room dark the radium in the capsule is hardly visible to you, because the rays do not of them selves affect the unaided eye, but if I bring some crystals of the fluorescent substance, barium platino- cyanide, near to it, you will see that the crystals shine out at once with a beautiful green light. An ordinary X-ray fluorescent screen, which is simply a piece of card painted over with the same fluorescent substance in the form of powder, is very convenient for these experiments. When thin pieces of metal foil are placed between the radium I 1 w QJ I £ 33 EXPERIi ENTS WITH RADIUM 25 and the crystals you see their brightness is only slightly reduced, while several shillings can be interposed one above the other without altogether stopping the rays from the radium. Those in the front will see the crystals still shining faintly, although the rays from the radium have first to traverse more than half an inch of solid silver before reaching the crystals. The electrical effect of radioactivity can be shown in a very rough and simple way with this, comparatively speaking, large quantity of radium. A silk tassel is stroked with a rubber tobacco-pouch and so electrified. All the threads then repel one another and stand out as you see (Fig. 4). The moment the radium is brought near the threads collapse at once (Fig. 5). Lastly, the photographic action of the rays is seen in the photograph (Fig. 6, facing p. 44) which was obtained by slowly writing, with a small tube con taining a small fraction of a grain of radium bromide as if it were a pencil, over a photographic plate wrapped in black paper, and then developing the plate without exposure to light. By the aid of delicate thermometers it could also be shown that this small quantity of radium is always a few degrees hotter than the surrounding air. The one fact about radium, which every one is aware of, is its tremendous cost. When you con- To face p. 25. 26 VALUE OF RADIUM sider that there is only one mine in the world, the ore of which contains enough to make the extrac tion of radium practicable on a large scale, and that even from this ore several hundredweights must be worked up to obtain the small quantity here exhibited, you can understand that the price is necessarily very high. So far as radium can be given a monetary value, a grain is probably worth many hundreds of pounds at the present time. We shall see, as we proceed, that from its very nature any strongly radioactive body like radium must always be excessively rare. Indeed, in the degree of radioactivity we have a scientific standard of rarity, and therefore of "value." There are un fortunately some fields of scientific investigation, of which radioactivity is one, which cannot be thoroughly explored without continuous and con siderable expenditure. The old boast of science, that some of her grandest discoveries were made with very simple apparatus, largely built up of wire and sealing-wax, costing little or nothing, does not apply to any of the discoveries with which we are now concerned. The investigations of Mme. Curie naturally have cost many thousands of pounds, pro vided by the public-spiritedness of the Austrian Government and of the Rothschilds. This radium we are using to-night we owe to the work of a German chemist, Dr. Giesel, who undertook its A SCIENTIFIC MONOPOLY 27 extraction on a large scale in the early days when the raw material was to be obtained in the market, and who very unselfishly distributed much of the radium he prepared among workers in all parts of the world. To-day the Austrian Government, who hold the monopoly of the Joachimsthal mine, do not allow the ore to be exported, and there is a great dearth of the raw material. Even those who have borne the heat and burden of the day, who have given the waste raw material its value, and who have discovered the very methods of extrac tion, are unable any longer to obtain the raw material for their researches. A monopoly in science is an unfortunate innovation, but such, alas! so limited is the supply of radium, exists to-day. It is stated to have been through the influence of the Prince of Wales with the Austrian Court that the Royal Society recently obtained a thousand pounds' worth of the raw material. To-night it is not my intention to take you through the various phases of the new properties of radium. We have to face squarely the great general question which its simple existence has demanded of physical science, Last century will remain for ever memorable on account of the de velopment and establishment of the great doctrine of energy. Those were great days for physical 28 DOCTRINE OF ENERGY science in Scotland, for that doctrine, which lies at the root of all modern industry and enterprise, took its rise largely in Scotland, and was developed by Tait, of Edinburgh, and Kelvin, of Glasgow. For a full account of these stirring developments you should read Tail's Recent Advances in Physical Science, which, in spite of the fact that it is now over thirty years old, still continues fresh and inspiring. The first law, that of the conservation of energy, states that energy is a real entity, and has a real existence no less than matter, and no more than matter can energy be created or destroyed, although the forms it may assume are legion. The second law, that of the availability of energy, is sufficiently accurately stated for present purposes by saying that the same energy is available for useful work but once. To obtain useful work from any source of stored-up or potential energy, it is necessary to transform it into new forms which are kinetic, and by which some thing is made to move. As motion is invariably attended by friction or similar processes, ultimately the energy passes into heat. It is said to be de graded into low-grade or waste energy, for although all forms of energy tend, after assuming the kinetic form, to turn into heat, the transformation of the waste heat so produced back into useful forms cannot be practically effected. The conversion is not altogether impossible, but requires for its acconi- DOCTRINE OF ENERGY 29 plishment the degradation of more fresh energy than is gained, and so is practically out of the question. The practical aspect of the question may be summed up by saying that if you want useful energy you must pay for it like any other commodity, and the value of the energy, though not the energy itself, is destroyed by use. The up-to-date street car driven by the electric motor, which so long has been the pride of this city and which more recently in London has displaced the old horse-tram, although it has not the same obvious incentive to locomotion as its predecessor, nevertheless does not go by itself. It requires energy or power, which is bought and sold and has a value as strictly as the oats and hay which energised the now emancipated horse. The driving power of the machinery of the modern world is often mysterious, but the laws of energy state that nothing goes by itself, and our experience, in spite of all the perpetual motion machines which inventors have claimed to have constructed, bore this doctrine out, until we came face to face with radium. Nothing goes by itself in Nature, except apparently radium and the radio active substances. That is why, in radioactivity, science has broken fundamentally new ground. I cannot too plainly insist that available energy, though immaterial and intangible, has a definite 30 ENERGY EVOLVED BY RADIUM and real physical existence. Were it not so, coal would not be the very expensive commodity it unfortunately is this year. No one burns coal for the sake of polluting the atmosphere, but simply and solely because it gives out during combustion a certain amount of energy as light or heat. Last century civilisation may be said to have attained its majority and to have entered upon the control of an inheritance of energy stored up by the sun in fuel during the long ages of the past, and now it is dissipating that inheritance as quickly as it can. With the lightheartedness and irresponsibility of youth, it is taking no thought of the future, but confidently assumes that the supply of natural energy, upon which at every turn it is now entirely dependent, will continue indefinitely. Well! if it does not do so, new stores of energy cannot be created to order, and there will be an end to the age of energy in which we are living, and to civili sation as we have come to understand it. Energy is susceptible of exact measurement and, though it exists in many varieties, all forms of energy can be most readily and completely con verted into heat and measured as such. The energy given out by radium, although it is in nature new, is no exception to this rule. Practically the whole of the energy is transformed into heat ENERGY EVOLVED BY RADIUM 31 when the radium is kept in a leaden vessel, so that the rays are absorbed in the surrounding metal. The actual amount of heat given out, for instance, by this small quantity on the table is, of course, very small, but in comparison with the quantity of .sub- 'stance producing it, it is very great indeed. Exact experiments have proved that half a grain of radium gives out about three calories per hour. The amount of heat evolved by any quantity of radium every hour is just about as much as is required to raise a quantity of water equal in weight to the radium from the freezing-point to the boiling-point. Radium bromide, if it is pure, consists roughly of two-thirds by weight of the element radium and one-third of the element bromine. Haifa grain of radium bromide thus evolves about two calories every hour. This specimen of half a grain of radium bromide has been in my possession for four years, and the outpouring of energy has been going on ceaselessly day and night at a steady rate. A simple calculation shows that in this time about 70,000 calories have been evolved. To obtain an idea of what this means consider the amount of energy given out in the burning of coal. A weight of coal equal to the weight of this radium bromide would give out during complete combustion only about 250 calories, so that already, while it has been in my possession, this radium has evolved 32 ENERGY EVOLVED BY RADIUM nearly three hundred times the energy obtainable from the same weight of coal. I have chosen coal for the comparison because in the combustion of carbon we obtain more energy from a given weight of sub stance than in almost any other change previously known. During the last four years this radium, therefore, has given many hundred times as much energy as could be obtained from an equal weight of any other kind of substance in any way known. Coal is no longer coal when it is burnt and con sumed. Gunpowder and dynamite, once they have exploded and evolved their stored-up energy, dis appear as such, and there remain incombustible and non-explosive solids and gases, out of which no more energy can be drawn. But, as you see, this radium to-night is as active as ever. So far, careful measurements have failed to detect the least dimi nution in the radioactivity of radium with time. Rather it increases steadily, rapidly in the first month and slowly for the first few years after pre paration, for certain profound reasons we shall have to go into subsequently. In the face of a new fact of this character it is obvious that this doctrine of energy, which we thought so well founded, requires further considera tion. Based as it has always been on the results of our experience and the practical impossibility of COS ICAL ENERGY 33 achieving perpetual motion of any kind, it is con fronted with a natural example, going on apparently for an unlimited space of time under our very eyes, which not only does not come to a stop, but which cannot be stopped by any means whatever. Now, although the doctrine of energy accords well enough with our terrestrial experiences, the student of the physical sciences has only to turn his thoughts from the laboratory to the heavens to see there, in the larger laboratory of Nature, an example of practical perpetual motion on the grandest and most majestic scale. What, for ex ample, is the source of the apparently inexhaustible supply of energy from the sun, upon the receipt of a minute and insignificant fraction of which life on this planet absolutely depends for its continued existence from year to year ? This is a question which has been frequently asked and only im perfectly answered by physical science. It has been the custom vaguely to connect the appar ently endless and inexhaustible outpourings of energy going on everywhere in the universe with its vast scale and dimensions. In the background there has always been the tacit assumption that the supply of fresh energy is only apparently inexhaustible, and that in some remote future a time will at length arrive when the supplies °f fresh energy are exhausted and all things 34 QUOTATION FROM PROFESSOR TAIT will come to a stop and remain at rest for ever. We have applied the teachings of the laboratory, our knowledge of the laws of energy and its conservation, and the impossibility of perpetual motion, without modification to the cosmos, only making allowance for its enormous scale. Astronomers, if one may judge from recent utterances, still continue to regard cosmical evolu tion as proceeding on these old conventional lines. That is, they seem to regard it still as only a question of time before the sun and planets cool down to a dead uniform temperature. In former days this point of view was the only possible one. A hot body radiating heat and light into space, even when all possible sources of energy, such as the accretion of meteorites, shrinkage, etc., had been allowed for, must ultimately radiate awav its energy. The same is still true but with a differ ence. Thus Professor Tait, in his Recent Advances in Physical Science (1876), says (p. 169): " If we were to trace the state of affairs back, instead of to ten millions, to a hundred millions of years, we should find that (if the earth then existed at all) if that collocation of matter which we call the earth was then actually formed, and if the physical laws which at present hold have been in operation during that hundred million years, then the surface QUOTATION FROM PROFESSOR TAIT 35 of the earth would undoubtedly have been liquid and at a high white heat, so that it would have been utterly incompatible with the existence of life of any kind such as we can conceive from what we are acquainted with. Thus we can say at once to geologists, that granting this premiss—that physical laws have remained as they now are, and that we know of all the physical laws which have been operating during that time—we cannot give more scope for their speculations than about ten or (say at most) fifteen millions of years. " But I dare say many of you are acquainted with the speculations of Lyell and others, especially of Darwin, who tell us that even for a comparatively brief portion of recent geological history three hundred millions of years will not suffice. " We say, so much the worse for geology as at present understood by its chief authorities, for, as you will presently see, physical considerations from various independent points of view render it utterly impossible that more than ten or fifteen millions of years can be granted." Again (p. 154): "Take (in mass equal to the sun's mass) the most energetic chemicals known to us, and in proper proportion for giving the greatest amount of heat by actual chemical combination, and, so far as we yet know their properties, we cannot see the means of supplying the sun's present II " 36 NEW STORES OF NATURAL ENERGY waste for even 5000 years. . . . This question is totally unanswerable, unless there be chemical agencies at work in the sun of a far more power ful order than anything that we meet with on the earth's surface." I do not quote these utterances with any wish to revive the old controversy between geologists and physicists, long since tacitly abandoned by both sides mutually as barren and unprofitable, but because of their present extraordinary aptness. To-day, science has come to know, by means of radioactivity, of agencies at work on the earth's surface of a far more powerful order than anything that was known in the time of Professor Tait. The discovery of radioactivity and the revelation it has given of unsuspected stores of energy in Nature available for cosmical purposes, of necessity put the whole question of the evolution, the past history and the future destiny of the universe in a new light. This is one of the conclusions of clearly general interest which follow from the recent discoveries. There is nothing of the vast scale and dimensions of the universe about this tiny scrap of radium. Yet it is giving out energy at a rate, relative to its mass, which no sun or star is doing. Suppose, for example, our sun, instead of being composed of the THE INDIFFERENCE OF RADIUM 37 materials it is, which we know by the spectroscope are practically the same as those of the earth, were made of pure radium. Provided only that every part of its mass gave out energy at the rate this radium on the table is doing, there would then be no difficulty in accounting for its outpourings of energy. Rather, the light and heat that would be given out from such a sun would be of the order of a million times greater than it actually is. On another count also one's thoughts almost uncon sciously revert from radium to the transcendental phenomena of the larger universe, for in no other phenomena are we so reduced to the position of onlookers, powerless alike to influence or control. All the powerful resources of the modern labora tory—extremes of heat and cold, and of pressure, violent chemical reagents, the action of powerful explosives and the most intense electrical dis charges—do not affect the radioactivity of radium or the rate at which it works in the slightest degree. It draws its supplies of energy from an hitherto unknown source and obeys as yet un discovered laws. There is something sublime about its aloofness from and its indifference to its external environment. It seems to claim lineage with the worlds beyond us, fed with the same inexhaustible fires, urged by the same un controllable mechanicism which keeps the great 38 "PHYSICAL IMPOSSIBILITY" suns alight in the heavens over endless periods of time. This tiny speck of matter we can hold in our hands exhibits in perfect miniature many ancient mysteries, forgotten almost in their familiarity, or mistakenly and too easily dismissed as belonging and appropriate to the infinitely great dimensions of the universe. The "physical im possibility " of one era becomes the commonplace of the next, and in the controversy between the geologists and the physicists we have a good illustration that no theory can claim a universal application. It is of necessity partial, and bounded on all sides by the unknown and unexplored. It is rarely proved false, so surely and truly are the foundations of modern science laid, but it is liable at any moment to be restricted in its appli cation to the particular cases for which it was formulated and found not to apply in new spheres at the time of its inception unsuspected. As we shall see, the law of the conservation of energy is not necessarily controverted by any of the new facts with reference to radium, but prior to these discoveries our knowledge of the avail able sources of energy in Nature has been partial and superficial to a degree. CHAPTER III. The radiations of the radio-elements—a-, /3-, 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—Neces sity for an ether—Corpuscular and wave radiations—a- and /3-rays due to the expulsion of particles—Can a single a-particle be de tected?—The spinthariscope—Counting the a-particles expelled by radium. IN the previous lectures we have considered the bare fact that radium and the radioactive sub stances are continually evolving from themselves a perennial supply of energy, and the fundamentally new ground which this discovery opens up in physical science. To-night our inquiries will be directed to one special portion of the subject, namely, the nature of the rays emitted by the radioactive elements, by means of which, or rather of the effects of which, the property was first discovered. These rays themselves, apart from their effects, we have hitherto scarcely considered, but they play an essential part in the theoretical scheme by which the activity of the radio-elements is now inter preted. The tracing back of the main effects of radio- 39 40 RAYS OF RADIOACTIVE SUBSTANCES activity, photographic, fluorescent, electrical, and thermal, to definite radiations emitted by the radio- elements came very early in the subj'ect, but it must not be forgotten that such tracing back is of the es sence of the discovery. Too frequently it is wrongly assumed without such evidence that any substance capable of simulating one or other of the various effects of radioactivity is therefore a radioactive substance. Naturally, the exact study of the new radiations has been mainly the work of physicists. They have succeeded, not only in clearly analysing into distinct classes the complex radiations involved and distinguishing the part played by each alone, but also they have advanced very far towards a solution of the real nature of each class of radiation emitted. Much of this latter work, however, is based upon reasoning of too specialised and in tricate character for general presentation, and as these lectures are intended primarily for the general public, and not for trained physicists, I propose con centrating attention for the most part on the con clusions which are universally accepted and of the greatest general interest. Although the reasoning is difficult, the chief conclusions are very simple and easily followed, and they fit in with the general scheme of the cause and nature of radioactivity in a way which makes the whole subject clearer and more easily visualised. a-, ft-, AND y-RAYS 41 The first analysis of the complex radiations emitted by each of the radio-elements—uranium, thorium, and radium—was done by Rutherford, and much of the work we are considering is his, and has called forth in their highest degree his well- known experimental genius and energy. He classed the rays into three main types, the a-, B-, and y-, distinguished from one another by enormous differences in their power of penetrating matter. I may say at once that the a-rays of radium, for instance, are readily distinguishable in penetrating power from the a-rays of uranium, and the latter again from those of thorium. Moreover, the a-rays of radium are themselves complex and consist of no less than four separate types readily distin guished. The same is true of the /3- and y-rays of radium, which are themselves complex and recognisably different from the /?- and y-rays of uranium or thorium. But the differences between the a-rays as a class, for example, are small and unimportant relatively compared to the enormous difference between any a-ray and any /3-ray or y-ray. The most penetrating a-ray known is not much more than twice as penetrating as the least penetrating known, whereas the /3-rays as a class may be considered to be approximately a hun dred times more penetrating than the a-, and the y-rays a hundred times more penetrating than 42 PENETRATION TEST the /3-.- Again, the kind of matter penetrated, although it has a certain influence which may be different for different types of rays, is only of secondary importance. For these rays, like the new X-rays, and unlike light, are absorbed by matter roughly in proportion to its density, and quite independently of its optical qualities of trans parency and opacity. The first result of these re searches was to bring into prominence the a-class of rays, which at first sight are of apparently little importance, and to diminish relatively the impor tance of the fi-class of rays which had been operative in the photographic effects hitherto mainly studied. For the a-rays are completely absorbed by very thin screens—even by a sheet of thin paper, or by three inches of ordinary gaseous air,—and they pro duce but little action on the photographic plate in comparison with the /3-rays, which are able to pass through a visiting card or piece of tinfoil with ease. To the electrical test—the discharge, for example, of an electrified silk tassel or electroscope—the «-rays are immensely more effective than the /3- and y-rays together, and from this fact Rutherford con cluded, and the conclusion has been wholly borne out by subsequent developments, that the energy possessed by these feebly penetrating, and not at first sight very striking, a-rays is always immensely greater than that of the other two types taken EXPERIMENTS WITH /3- AND TRA YS 43 together. In fact, the /3- and y-rays at most possess but a few per cent of the total energy of radiation, and therefore are in this fundamental respect rela tively of less consequence than the previously neglected a-class. Although less suited to lecture experiments than the other more penetrating types, the a-class have proved far the most instructive and important in the theory of radioactive change. The small capsule in which my radium is con tained is closed by a thin sheet of mica, which effectively stops all the a-rays, so that in working with the capsule only the /3- and y-rays are opera tive. The platinocyanide salts fluoresce most brilliantly under the /3-rays. On interposing suc cessive thicknesses of thin copper or aluminium foil the fluorescence is weakened, very rapidly at first, but a point is soon reached when the feeble fluor escence remaining is not much further weakened by additional thicknesses of foil. This is because the /3-rays have all been absorbed, and there re main only the relatively feeble but extraordinarily penetrating y-rays. These y-rays are always very feeble, and comparatively unimportant, but their chief interest lies in the fact that they are by far the most penetrating type of radiation at present known. If the capsule is completely closed in a box of steel, half an inch thick, and a platino- 44 -/-RAYS cyanide crystal laid on the top, those in front can readily see that the crystal still fluoresces, and stops the moment it is taken away from the radium. Through a pile of twelve shillings, or pennies, the effect can still be observed, while by means of a sensitive gold-leaf electroscope it can readily be shown that some of the rays still penetrate a foot thickness of solid iron. The rays from radium are not well adapted for the taking of radiographs of the kind produced by X-rays. The /3-rays are hardly sufficiently pene trating for this purpose, so that the flesh as well as the bones of the hand, for example, casts a heavy shadow. The y-rays, on the other hand, are far too penetrating, and the bones hardly cast a shadow at all. The picture (Fig. 7), however, is a good example of a radium radiograph taken by the y-rays of radium. A small box of compasses with the lid shut was placed on a table. Over it, film down, was placed an X-ray plate wrapped in a light- tight envelope. On the floor beneath, at a distance of twenty-five inches from the plate, was placed one-tenth of a grain of pure radium bromide sealed up in a tiny glass tube. The radium was placed between the poles of an electro-magnet, as recom mended by Mme. Curie, to deflect away the /3-rays which tend to blur the distinctness of the picture. In this way the y-rays of radium were FIG. 6. Written by Radium in the Dark. (From a Radiograph by R. Hill Crombie, Esq., Jo^t al of the Rijntgen Society, Dec., 1906.) FIG. 7. Closed Box of Compasses taken with the 7-Rays of Radium. To face p. 44. y-RA YS 45 alone used. The exposure was five clays. It will be seen that the shadow cast by the wooden box is scarcely noticeable, while even the metal compasses and fastenings of the box by no means entirely stop the rays. The metal parts appear in the negative only slightly darker than the unprotected portions of the plate. The negative was reduced and intensified before reproduction. The y-rays appear to be a secondary radiation produced by and accompanying the /3-rays, much as X-rays are produced by and accompany cathode- rays. The important point is that the ft- and y-rays always go together. Any variation of the /3-rays is accompanied by a similar variation of the y-rays. Their real nature is at present an open question as rival views are in the field, and as the rays are not of primary importance at the present time we may, with these experiments and remarks, dismiss the subject and pass on to the more detailed considera tion of the two more important types of rays. Before proceeding to show experiments with the «-rays, it is necessary to touch on certain considera tions which come into play on account of their very great absorption in passing through matter. In the first place, radioactivity is a mass or volume phenomenon. That is to say, every part, not the surface only but the inner portions also, of a 46 a-RA YS radium salt, for example, is giving out «-, /?-, and y-rays. All these rays are absorbed by the sub stance itself very considerably, for the salts of radium are dense or heavy. But this absorption naturally does not affect the more penetrating rays nearly so much as the feebly penetrating a-rays. That part of the latter, generated inside the salt, does not escape at all. Only a very thin surface film contributes to the a-radiation. The conse quence is that whereas, with the small quantities of radium that we have to work with, the strength of the penetrating rays are more or less proportional to the quantity of radium employed, with the a-rays this is no longer the case. The weight of the substance is less important than the amount of sur face exposed. A very small quantity, say a milli gram, of radium bromide, spread out as a thin film on a large plate, will give out immensely more a-rays than the same quantity in the form of a small crystal. In order to free the (3- and y-rays from the a-rays, or the y-rays from the /5-rays, it suffices to interpose screens of successively increasing thick ness until the more easily stopped type is completely absorbed. But it is not possible so easily to eliminate by physical methods the /3- and y-rays from the a-rays in order to leave the latter by themselves. For practical purposes, however, this result can be achieved very simply. If we take a very minute 0.-RA YS 47 quantity of a radium salt spread over a very large area, the /3- and y-rays from so small a quantity will be so feeble as to be practically negligible, whereas the a-rays under these circumstances will reach their greatest intensity. For practical pur poses a thin film of pure radium salt can be used to give a-rays by themselves, essentially free from /8- and y-rays. Such a thin film I have prepared for these ex periments. On this shallow platinum dish, about a square inch in area, I have evaporated down a solution containing about a milligram of pure radium bromide, and the dish, with its precious film open to the air, is carefully preserved when not in use in a special tube containing a desiccating agent to keep it dry, so that without undue risk of loss I can work with a bare film of radium salt and show you the a-rays. Over the bare film I bring the electri fied silk tassel. It collapses instantly, in fact, much faster than it does when brought over the whole thirty milligrams of radium bromide contained in the mica-covered capsule. The a-rays from one milligram of radium produce more electrical effect than the /8- and X-rays from thirty milligrams. Now I cover the bare film of radium with a single sheet of thin writing-paper, which stops the a-rays com pletely, the /8- and y-raysj scarcely at all. You 48 EXPERIMENTS WITH a-J?AYS observe the tassel remains now charged as if the radium were absent. The ft- and y-rays from so small a quantity hardly appreciably discharge it. I But if I displace the paper ever so slightly and expose a tiny part of the bare surface, the tassel instantly collapses. From these experiments, and the fact that it was the fashion at the time to cover radioactive substances when experimenting with them, you will have no difficulty in understanding how it was that these feebly penetrating but in tensely powerful a-rays remained at first neglected and almost unknown. I now have to show you a very striking experi ment indeed, suggested by some profound investi gations of Professor Bragg in Adelaide, on the a-rays, to which we shall again have occasion to refer. So readily are these a-rays stopped that a few inches of air suffice entirely to absorb them. But the a-rays show this remarkable peculiarity not exhibited by any other type known. Each individual a-ray of any one homogeneous type travels exactly the same distance in an absorbing medium, and is stopped sharply and completely when a certain thickness of matter has been penetrated. The con sequence is that if we work with a homogeneous beam of a-rays, just without the distance of com plete absorption, there is absolutely no effect, while FIG. 9. Apparatus to show Absorption of a-Rays by Air. ABSORPTION OF a-RAYS BY AIR 49 just within there is a very large effect. I have said that the a-rays derived from radium are complex, consisting of four different types, each with a definite " range," as it is termed, or distance, it will travel in any given absorbing medium. For the f RADIUM I V T / FIG. 11. The Spinthariscope of Sir William Crookes. To face p. 49. ——^ FlG. 8. purposes of this experiment, however, it is neces sary to consider only the most penetrating type, which Bragg found could travel in air at atmo spheric pressure and ordinary temperature, 71 millimeters (or just under three inches) and no more. Now this flask (Figs. 8 and 9) is a little more than six inches in diameter, and it has been coated on the 1 50 ABSORPTION OF a-RA YS BY AIR upper hemisphere of the inside surface with a phos phorescent film of zinc sulphide. For these a-rays the usual phosphorescers (e.g. the platinocyanides, willemite, etc.), employed for the /3- and y-rays, are far less sensitive than crystallised zinc sulphide, or, as it is called, Sidot's hexagonal blende. The coated flask is arranged so that I can plunge my platinum dish with its bare radium film upward inside the flask and hold it centrally by a cork. In the dark, the flask being full of air, you observe hardly any glow. The three inches of air surrounding the radium film on all sides suffices completely to stop all the a-rays, and the |6- and y-rays, from so small a quantity of radium, produce only a negligible effect on the zinc sulphide. But I have connected the flask to an air-pump and can pump out the air. At the very first stroke of the pump the whole globe flashes into luminescence, and as I continue pumping the glow gets stronger and fairly illuminates the im mediate neighbourhood with its soft white light. I now readmit the air, and the glow disappears as suddenly as it came. So that you see with some what carefully designed arrangements, and keeping in mind the peculiar properties of these a-rays which physicists have exactly worked out, it is possible even from a minute amount of pure radium bromide to obtain quite a fair amount of light, whereas the same quantity of radium less cunningly NATURE OF RADIATION 5* disposed would give very little effect. Most of those who are in possession of radium compounds hardly dare ever expose it bare to the air, as under these circumstances the substance attracts moisture and tends to liquefy, and so they miss altogether the effects produced by these a-rays. Problems connected with the real physical nature of radiations are, it is well recognised, among the most fundamental in physics, and they involve more deeply perhaps than any others the great under lying metaphysical relationships between the external world of physical fact and the subjective mental processes by which we attempt to visualise these facts and obtain some sort of a reasonable explana tion of them. Take, for example, the great problem that is always before us of the real nature of light. Is there anything more difficult of mental com prehension ? The difficulties are not minimised but rather increased by the very definite view we take to-day of energy as a separate entity having a real physical existence. Contemplate for a moment, if you can, the origin of the force which impels every moving thing in earth or sea or sky. With the exception of a very small and practically negligible movement contri buted by the tides and by volcanic agencies, and, it must not be forgotten, by the radioactive substances THE ETHER themselves, all things which move do so directly or indirectly by virtue of the energy reaching this earth as radiations in the form of the sun's light and heat. Great masses move hither and thither here because of happenings at some time past, re mote or recent, 90 millions of miles away in the sun. Inevitably, when we begin to contemplate radiation phenomena, we are driven to inquire into the medium filling the outer void of space by virtue of which this immaterial, but vital entity—energy— reaches us from far distant worlds. It is true we call it ether, and try to give to it all sorts of material, or pseudo-material, characteristics. Lord Kelvin seems to have spent a large part of his leisure time trying as it were to dematerialise matter into ether, that is, trying by all sorts of mechanically ingenious arrangements and analogy from material models— the only possible models our minds can yet grasp— to obtain a possible construction which would simu late the elusive but all-pervading ether. Others, on the well-known principle that topsy-turvydom, if only consistent and all-embracing enough, results finally in a system no less logical and rational than the original one, have given to the ether inconceiv ably great density, and to the atoms of matter the character of holes or voids in it. The necessity for the existence of a universal all-pervading medium, or ether, capable of transmitting energy, no one in THE ETHER 53 these days of wireless telegraphy would deny, but on the question of its real nature opinion is as divided as it well could be. The tendency, however, in modern physics to day is rather to derive and explain material pheno mena from the properties of the ether than to attempt to construct an ether on a material or pseudo-material model. As yet, however, we know little about the properties of the ether itself. One definite thing we do know, for certain, and have known for a very long time, namely the velocity at which influences are transmitted across the ether. It is 185,000 miles a second, the speed of light. So far as we yet know, all influences that are transmitted by the ether travel at this one definite velocity. Not only light, but also the electro magnetic radiations employed in wireless tele graphy, the magnetic storms, as they are termed, which reach us from the sun, and also, we believe, the X-rays, travel through the ether at this one definite speed. The great mind of Newton two centuries ago appreciated to the full the fundamental difficulty in the explanation of radiation, and proposed the only way oi escape from the more modern doctrine of an ether which, so far as I know, has ever been put forward. 54 CORPUSCULAR RADIATION Light, on the Newtonian hypothesis, consisted in the emission from the glowing body of exces sively minute material particles or corpuscles travel ling with immense velocity. This corpuscular theory, so far as light is concerned, failed when subjected to a closer examination, and gave way to the present undulatory theory that light consists in a transverse vibration of the ether, the existence of which, it was beginning to be recognised, was as great a necessity for the transmission of gravita tional, magnetic, and other forms of energy which reach us from outer space as it was for the trans mission of radiation itself. Though proved wrong so far as light is concerned, this idea of corpuscular radiation, strangely enough, will rank as one of the most suggestive flashes of Newton's genius, for it, in fact, anticipated by two centuries the march of experimental discovery. To-day, thanks to radioactivity, science has been enriched by the discovery of a-, (3-, and y-rays, and two, at least, out of these types, the a- and the /8-rays, are not, like light, vibrations of the ether, but consist of the emission of excessively minute material particles (atoms and corpuscles) travelling with immense velocity. This is one of two chief main lines of evidence that radioactivity is an accompanying manifestation of "atomic disintegration." Into this aspect of the matter, however, I do not WAVE THEORY OF LIGHT 55 propose entering to-night. Its consideration is more conveniently deferred. It is sufficient to say that the a- and /3-rays, or, as I shall henceforth also refer to them, «- and /3-particles, comprise the lighter fragments, as it were, of the disintegrating atoms of the radioactive substance. In ordinary circumstances radium appears to be expelling both a- and ^-particles together, but this as we shall come to see is due to the fact that several successive disintegrations are occurring, and the effect is a composite one. The nature of these rays is so utterly different from that of light that it is worth while to stop and examine the difference a little more closely. The wave theory of light has often been illus trated by what happens when a stone is dropped into a pool. Ripples extend outwards in concentric circles from the disturbance. The water, as the ripple reaches it, first rises above, then immediately afterwards falls below the normal level. The dis turbance is propagated transversely, that is, out wards horizontally by a vertical, or up and down, wave-movement of the water. The surface dis closes the nature of the disturbance, but the same type of disturbance is taking place below the surface, and each circular ripple is in reality the section of a hemispherical shell. It is not possible 56 DISCRETE THEORY OF RADIUM RAYS to get an ether surface like a water surface, since the ether is all-pervading. Light travels out from an incandescent point in all directions in spherical ripples, in which a to-and-fro motion of some kind is going on in the ether, transverse to the direction of propagation of the light. Contrast with this what is believed to be the nature of the a- and /3-rays given out from a radioactive substance. The rays are given out uniformly in all directions, not as a succession of spherical waves, but as the random flight of immense swarms of tiny projectiles ejected from the radioactive substance. For shortness I shall call this the " discrete theory," as contrasted with the wave theory, because the radiation is considered to be due to the flight, radially outward from the substances like the spokes of a wheel, of swarms of free-flying, independent discrete particles. You could hardly imagine two more different pheno mena, and yet that it is not easy to distinguish between their effects is shown by the fact that for a long time a controversy raged between the two views regarding the nature of light itself. I must anticipate a little here for the sake of clearness. It is now an old story that in the tiniest grain of matter there is a mentally inconceivable myriad of separate atoms. In this tiny quantity of radium bromide, weighing half a grain, we know with fair certainty there are fifty million billion DISCRETE THEORY OF RADIUM RAYS 57 (5xio19) separate atoms of radium, assuming that the compound is pure. It has been proved that, roughly, one two-thousandth of these disintegrate yearly. There are about 32,000,000 seconds in a year, so that in every second of time rather less than one thousand million of these radium atoms dis integrate, giving some small multiple of this number of a- and /3-particles. So mighty a host projected outwards in all directions at random, as you may suppose, fill the surrounding space with their trajectories to all intents and purposes as com pletely as if they advanced as one continuous spherical wave-front. In other words, if only the number of projected particles is sufficiently great a discrete radiation will be, in many of its general effects and laws of propagation, not different from a wave-radiation. It is true that such a radiation will show neither regular reflection, refraction, nor polarisation in the manner that light does, and the absence of these phenomena for the a- and /3-rays is part of the evidence in favour of their discrete nature. If, however, we continuously reduce the number of particles ejected, in other words, if we continuously diminish the quantity of radium em ployed, there should come a point when the discrete radiation should no longer simulate the wave-type. It should, as it were, break up and show discontinuity, much as some of those faint 58 RESOLUTION OF a-RAYS continuous light-patches in the heavens, known as the planetary nebula;, when investigated by more and more powerful telescopes, begin to break up and show discontinuity, and finally are resolved into an innumerable host of separate twinkling stars. Is it possible so to resolve a swarm of a-rays ? The older physicists who first deduced by accu rate computation the weight and measure of the single individual atom and evaluated the number of billions contained in the smallest portion of matter perceptible to the senses, had they been soberly asked whether it would be possible ever to observe a single atom of matter, would have scouted the bare possibility. A single atom of matter !—A single atom of matter!— I recall this one exclama tion, repeated over and over again with varying intonation by a distinguished foreign visitor, whose years had been spent at the microscope on the border land between the perceptible and the imperceptible worlds, when the question we are now considering was under discussion at a British Association meeting. Let us, however, now make a few calculations to see whether there is any hope whatever of being able to detect the effect of, say, a single a-particle expelled from radium, in the same sense as it has been found possible in astronomy to detect the indi vidual stars which go to make up a planetary nebula. THE SPINTHARISCOPE 59 In my first lecture (p. 23) I alluded to the smallest quantity of radium that could be detected by the aid of the gold-leaf electroscope, that is, therefore, by means of the a-rays emitted. It was one three-thousand-millionth of a grain. Half a grain, as we have seen, gives out a few thousand million a-particles every second. So that the smallest quantity of radium detectable by the ordinary electroscope must be giving out only a few individual a-particles per second. From a very early stage it appeared not inconceivable to Ruther ford that a discontinuity in the emission of a-rays might actually be detected by using a very minute quantity of radium. The Spinthariscope:—The problem was actually solved, almost unawares, by Sir William Crookes, by means of an instrument he devised and called the Spinthariscope. The instrument is the only genuine instrument worked by radium that it is at present possible to buy at the optician's in the ordinary way, and it can be bought—radium and all—for a few shillings. The reason for this apparent paradox is to be found in the fact that it is in the essence of the result to be attained to reduce the amount of radium to the smallest possible quantity, and this unusual condition allows of a practically unlimited number of spinthariscopes to 6o THE SPINTHARISCOPE be made out of an almost invisible quantity of radium bromide. The amount of radium in each instrument is absolutely unweighable and invisible. A needle, A, is made to touch a tiny phial which once contained radium, and is then mounted (Figs. 10 and 11, opposite p. 49) centrally in a little brass tube, the JB FIG. 10. size of a small reel of cotton, at the bottom of which is a phosphorescent screen, B, coated with zinc sulphide. At the other end of the tube is a lens, C, for magnify ing the screen and, by means of a little screw, D, out side, the needle point may be moved nearer to or away from the screen. If now in a dark room the screen is observed through the lens, it will be seen to be luminous, and this luminosity can be concentrated THE SPINTHARISCOPE 61 or spread out by screwing the needle point nearer to or farther from the screen. After the eye has become used to the darkness it will be seen that the luminosity is not just a quiet continuous glow. The light, like that of the planetary nebulae, has been resolved and shows discontinuity. It resembles most nearly a shower of shooting stars. Bright momentary flashes of light or scintillations, too numerous at any instant to count, are appearing and disappearing in the field of vision. These flashes are caused by the a-particles of radium. This minute insignificant trace of radium is positively belching forth a-particles. It seems incredible that the incessant bombardment of the screen can be caused by such an infinitesimal amount of radium. Yet so it is, and in a month's time, if the instrument is re-examined, it will be found that the scintillations are as numerous and as brilliant as formerly. After a time, perhaps a year, the phosphorescent screen itself will be worn out by the incessant bombardment, will become insensitive and need renewal. But replace it by a new one and the radium will be found to be as energetic as ever. The owner of the instrument will pass away, his heirs and successors, and even his race will probably have been forgotten before the radium shows any appreciable sign of exhaustion. 62 THE INDIVIDUAL a-PARTICLE The actual a-particle itself must, of course, be extremely small. How else could a mere speck of radium send out such an incessant and numerous swarm ? In all probability, as we have still to prove, the a-particle is an atom of helium, the second lightest atom of matter known. A grain of radium bromide expels every second about ten thousand million a-particles, and if we contemplate this mighty swarm expelled once every second of time throughout many centuries we may begin to have some idea of how many atoms there must be in a single grain of matter, and how small must be the single atom. The philosophers of only a decade ago would have ridiculed the hope that we should ever be able to look through a magnifying-glass to see the effect of a single atom of matter, yet each of the scintillations of the spinthariscope is nothing else. The spinthariscope was the original, but to-day it was only one of many lines of evidence which have established the discrete character of the a-radiation and the nature of the a-particle. We know of many radioactive substances—polonium is one—emitting a-radiations, which gradually and completely lose their radioactivity with the lapse of time. Anticipating, we may say that the dis integration of polonium proceeds so rapidly that it THE INDIVIDUAL a-PARTICLE 63 is complete in the course of a few years. Were the process at all similar, for example, to the case of a hot body cooling, one would expect a gradual alteration in the character of the radiation with the diminution of its intensity with lapse of time; whereas the character of the radiation is exactly the same at the end, when it has nearly all decayed, as it is at the beginning. This is explained simply on the view that the number of a-particles expelled grows less as the activity decays. The individual a-particles have the same velocity and other charac teristics, whether expelled at the end or at the beginning of the process. Professor Bragg's dis covery that each a-particle has a definite "range," characteristic of it, is quite inexplicable on a wave theory. The range of the a-particles emitted by polonium, for example, is thirty-eight millimetres of air, and though in the course of a few years the a-radiation of polonium decays always completely, the range of the a-particle expelled at the end is exactly the same as at the beginning. In this connection, finally, I may mention some really wonderful work recently done by Professor Rutherford and his co-worker Dr. Geiger, in which they have actually succeeded in counting directly the number of a-particles expelled from a given quantity of radium every second. As you may 64 COUNTING THE a-f>ARTICLES know, if two points are connected to an electrical machine, or other method of generating an electric force or tension, a spark will pass between them under suitable circumstances. Now suppose the distance apart of the two points is just so great that no spark will pass with the particular electrical tension applied, and that some radium is then brought near to the points. Then a spark will pass. The rays from radium by making the air a conductor of electricity facilitate the passage of the spark, so that under their influence the discharge will leap across a greater distance than it otherwise would. Substitute for the crude method of detect ing the discharge by means of a spark a highly refined electrical instrument, known as the electro meter, in which, as in the galvanometer, a spot of light is reflected from a mirror attached to a needle, which can be arranged to move when a discharge passes across the gap, and you have the essential principle of Rutherford's arrangement. Such an arrangement can be made so excessively sensitive that the passage of a single a-particle from radium through what corresponded to the "spark gap" of the first arrangement described, is sufficient to cause the spot of light from the needle of the electrometer to move with a sudden jerk. The experiment consists, then, in counting the number of these sudden jerks of the electrometer needle in i I COUNTING THE a-PARTICLES 65 a given time, when a known quantity of radium is placed at a known distance. The radium has to be placed many yards away from the apparatus, and the a-rays are fired along a long exhausted tube with a small window at the end to admit the passage of a very minute definite proportion of the total number of a-particles, which proportion can be calculated. In the actual experiments the distance of the radium and the size of the window through which the a-particles passed were such that, roughly, only one out of every 100 million a-particles ex pelled found their way into the apparatus. The total number of a-particles actually expelled per second by a grain of radium in its normal condition was found to be about ten thousand million.- Per milligram of radium the exact number per second is 136 million. These results were also checked by counting the number of scintillations per second in a special form of spinthariscope. There have always been scientific men who have regarded the atom and the atomic theory with suspicion and have never tired of insisting upon its "hypo thetical" character. It may therefore be rightly regarded as one of the greatest triumphs of science that an observer can now actually sit down in front of a vessel and with the aid of a watch count the number of atoms entering it every minute from a quantity of radium outside. CHAPTER IV. The /3-rays—Their deviability by a magnet—The nature of the /3-par- ticle—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. IN addition to their varying power of penetrating matter, there is another test which has proved of great service in analysing the three types of rays from radioactive bodies and in determining the real nature of each. The trajectories of some of the rays are powerfully influenced by a magnet, while others are hardly if at all affected. Thus the /3-rays of all radioactive substances if caused to traverse the space between the poles of a magnet are very strongly deflected, and if the magnet is a power ful one may be completely coiled up into closed circles or spirals. Faraday imagined that between the N-pole and S-pole of a magnet there existed actual lines of agneticforce. In the electro-magnet on the table (Fig. 12), which is formed so that the N- and S-poles are bent round so as to face one another, the lines of force between the opposite faces of the 66 (J a M3 w O S . esl o c ** tj -CrS ARTICLE When we speak of being able to detect the effect of a single a-particle, and therefore of a single atom of matter, we mean the detection of its energy, which is a quarter of a million times as great as that of any other kind of atom known to us. Similarly, when we speak of being able to detect in a few seconds by radioactive methods the course of a change which would have to pro ceed continuously for geological epochs before it produced an effect detectable by the most sensitive chemical test, it is because, firstly, we detect the energy evolved by the change, not the change itself; and, secondly, because the energy is at once so relatively enormous and at the same time so much more easily detected compared with any other kind of energy outburst previously known to us. Matter moving with the speed of 10,000 miles a second is so novel and strange to us at present that it is doubtful whether our ordinary concep tions afford much guide or analogy. The muzzle- velocity of a cannon-ball, for instance, is a small fraction of one mile per second. Now we have seen that the a-panicle of radium is capable of traversing very thin aluminium leaves and also several inches of gaseous air. It is extremely interesting to inquire what happens during the A QUOTATION FROM PROFESSOR JBRAGG 87 collision of an a-particle with a molecule of gas or metal. Some at least of these collisions must be full and direct, not simple grazing or glancing coincidence ; and it seems at first sight difficult to believe that an a particle striking a gas-molecule full and fair should not be stopped, however fast it is moving. Nevertheless, it is not so. Upon this matter the researches of Bragg and his colleagues have thrown a flood of light. His conclusions are as remarkable as they are definite. " Each a-particle pursues a rectilinear course, no matter what it encounters ; it passes through all the atoms it meets, whether they form part of a solid or a gas (or, in all probability, of a liquid), suffering no deflection1 on account of any encounter until, at any rate, very near the end of its course. ... A thin metal plate may be placed in the way of the stream, and so rob every particle of some of its energy, but not a single one is brought to rest by collision with the atoms of the metal, and the number of particles in the stream remains un changed." Surely this vivid picture of the flight of a swarm of ^-particles raises anew the old metaphysical conundrum of the schoolmen, whether two portions of matter could occupy the same space 1 A very slight deflection or scattering of the beam (about 3° at most) in its passage through matter has since been proved to take place. 88 INTERPENETRATlON OF ATOMS at the same time. For the only possible meaning of Professor Bragg's conclusion is that the a-particle must go clean throug the atoms of matter it pene trates as though they were not there, and therefore at the instant of collision the two atoms do occupy the same space at the same time. This power of the interpenetration of masses may be one of the peculiar properties of matter moving at these, what may be termed ultra-material, velocities. We know for certain it is not a normal property of matter. The only apparent consequence of the passage of the a-particle through the atoms it encounters is that it ionises them, that is, they become charged, some with + and some with - electricity, -after the collision. There is little doubt that the + charge or charges on the a-part:cle itself can be explained in the same way. Whereas in the case of the /3-particle the charge of electricity is the particle, in the case of the a-particle the charge is in all probability a secondary consequence of the velocity with which the particle is moving. At least it is certain that no atom moving at 10,000 miles a second would continue uncharged. The very first collision with an atom of matter would "knock out an electron," that is, charge the moving particle positively. The quotation from Professor Bragg pursued the question of what happens to the a-particle on col- FATE OF THE ^.-PARTICLE 89 lision only as far as the initial stages. The final fate of the a-particle is still enshrouded in mystery. Rutherford has followed the successive retardations of velocity of the a-particle produced by each suc cessive thickness of matter penetrated, and found that at the point at which the a-particle ceases to pro duce any longer a detectable effect it is still moving with a velocity of 5000 miles a second. All attempts to trace it when its speed is reduced below this critical limit have failed. Fluorescent, photo- graphical, and electrical actions all cease simul taneously. It may then suffer a sudden stop, although there is no evidence of this, or it may continue to move without ionising the gas atoms it collides with. For us, who are concerned for the most part with the broad limitations of our past and present know ledge, the most interesting feature of this pheno menon is that it indicates quite definitely that an a-particle expelled with an initial velocity below 5000 miles a second could not by any of the present known methods be detected. Any of the appa rently stable and non-radioactive elements might be disintegrating and expelling a-partides, but if these did not attain this limiting speed we should have no evidence of the fact. It is really by a somewhat slender margin of velocity that the a-particles have come within our knowledge at all. The light we 90 UNDISCOVERABLE PROCESSES have gained has but served to intensify the dark ness by which we are surrounded on all sides. Processes similar to and but little less energetic than those which produce radioactivity, may be going on unsuspected everywhere around us, without producing any yet detectable effects. Radioactivity was to be regarded rather as a benevolent hint given to us by Nature into secrets we might never have guessed, rather than as the necessary and invariable concomitant of the processes of atomic disintegration. 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 out bursts 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. IF we are to continue to regard energy in the modern way as something having a definite existence, we have to answer the question, " From where does the energy of radium come ?" That it comes from nowhere, or that it is being newly created out of nothing by radium, is a view it is not possible to entertain for a moment without destroying the basis upon which nineteenth-century physical science has largely been reared. " How has it got the energy in it to do it ?" is the first question that naturally arises in the mind with re gard to radium, but obviously we should first ask, "Has it the energy in it ? " 91 92 SOURCE OF THE ENERGY OF RADIUM If the doctrine of energy is true, there are for tunately only two possible alternatives to be con sidered. Either the energy must be derived from within the radium, which we shall call the first, and as we think the true, alternative, or it must be supplied from outside the radium, and this we shall call the second alternative. This simple narrowing clown of all the possible issues to two alternatives may appear to you somewhat trite, but in reality it carries with it far more than appears on the surface. In the first place, being an intrinsic property of the element, radioactivity is therefore a property of the atom, and if we take the first alternative and say the energy comes from within, it means from within the atom, and therefore that there must exist an enormous and not previously suspected store of energy in matter, or at least in radioactive matter, in some way associated with its atoms or smallest integral parts. On the second alternative, which has often been advanced, radium acts merely as a transform ing mechanism. There are electrical transformers dotted all over this city, receiving the economically transmitted but dangerous hmh-tension currents o o from the central power station and delivering the comparatively safe low-tension currents to your houses, which are wasteful to transmit for long distances. Are the atoms of radium acting as the THE TWO ALTERNATIVES 93 transformers of a mysterious and hitherto unknown source of external energy, first receiving it and then delivering it up again in a form which can be recog nised ? It may be said at once that so vague a view, postulating the existence of illimitable and mysterious supplies of energy from without, cannot be directly disproved. At first it seemed to provide a way of escape from some of the more unpalatable logical consequences of the first alternative and was eagerly adopted. In reality, instead of a way of escape, it proves to be a veritable will-of-the-wisp, luring on its followers beyond the limits of credulity into a quagmire of unsubstantial hypotheses, so bottomless and unreal that even the facts of radium are a wholly inadequate justification, and, even so, incapable of throwing any light on the facts when these are more nearly examined. Nevertheless, we must pursue both alternatives impartially, if only to leave no doubt that both have only to be fairly considered for one to be dismissed. On the second alternative the radium owes its activity to a supply of energy from outside. One has only to isolate the transformers which light this city from all connection with the outside central station to plunge the city in darkness. But we have seen that to quench radioactivity or to modify it in any way is one of the things science cannot do. Experiment has proved that even in the 94 SOURCE OF THE ENERGY OF RADIUM natural state in the mine, hundreds of feet deep down in the earth, pitchblende exhibits its normal radioactivity. So that if it derives its energy from without, this must be of a kind entirely different from any at present known, for it must be capable of traversing without loss hundreds of feet of solid rock. This is as far as we need pursue the second alternative for the moment. Provided we can call into existence a new kind of radiant energy un limited in amount, permeating all space and unim peded by passage through matter of any thickness, we may, but only so far as we have yet gone, seek a bare explanation of the energy of radium on the second alternative. Such a view would accord at first sight with the continuous and permanent activity of radium for an indefinite time, and there would be no reason why radioactivity, however intense and powerful, should decay or diminish with the lapse of time. But if the first alternative is true, and the energy comes from within, large as the store of energy in the atom must be to explain radioactivity it cannot be infinite, and therefore it is to be expected that the activity will slowly decay with the lapse of time. If two radioactive bodies, one much more power fully radioactive than the other, are compared together, it is to be expected on this view that the activity of the more powerful body will decay faster THE Tl O ALTERNATIVES 95 than that of the other. But for both a time will come, as soon as the internal stores of energy are ex hausted, when the radioactivity will come to an end. By far the most important consequence of the first alternative, however, has still to be considered. Radium, if we call by that name the substance con taining the unevolved store of energy, can no longer be radium when the energy is lost. Coal is not coal after it is burnt. When energy is ob tained from matter the matter changes, and before it can be regained in its former state the energy evolved must be put back. In no case is it possible for matter to part with its store of energy and re main the same, for otherwise you will readily see a perpetual motion machine would be easy enough to construct. Indeed, most of those attempted involved this impossible assumption. But we have seen that if the energy is stored up in the radium it must be within the atom, and, therefore, if radium changes, it must be a change of the atom and of the element itself. This change of an element would be transmutation, which is a more fundamental and deep-seated change than chemical change or any known kind of material change, and until the discovery of radioactivity such changes certainly had never been observed. If the energy of radium comes from within, radium must be suffering a spontaneous kind of transmutation 96 THE INTERNAL ENERGY OF MATTER into other elements. So that, if we would avoid the necessity of believing in the process of trans mutation, not as a vague possibility, for example, in the sun and stars, under some unattainable trans cendental condition, but as actually going on im- perturbably around us, which the first alternative demands, we must seek a way of escape on the second alternative which requires none of these bewildering heresies, but simply transfers the mystery from the radium to the great external unknown, and leaves it there in good company with many of a similar kind. At this stage it is well to ask the question, Is there anything opposed either to reason or to prob ability in the view that the energy evolved from radium is actually derived from an existing pre viously unsuspected internal store within the atom, and that in this process the element suffers a trans formation into other elements ? How is it that such enormous stores of energy in matter have remained so long unknown ? One of the most elusive features of energy is that you cannot say by mere observation, or by the use of any instrument, how much or how little is stored up in any kind of matter. For example, this flask contains a large quantity of an oily yellow liquid. We cannot tell by simple inspection the amount of energy stored up in this fluid. It may THE INTERNAL ENERGY OF MATTER 97 be some quiet and harmless oil, which can be shaken vigorously with impunity, or it may be nitro-glycerine, one of the most dangerous and powerful explosives. Something more than obser vation is necessary to tell us the amount of energy that may be stored within this substance, possibly only awaiting a slight shock to be evolved. The only way to find out is to try to explode it as thoroughly as we can, and then if it will not ex plode we may conclude that, as far as we know, it has no latent store of energy waiting to be loosed from prison. Explosion is merely a very rapid and violent type of chemical change, and the same general idea holds good for all the changes it is possible for matter to undergo. We may determine the energy evolved or absorbed in any change, that is, in the passage from one kind of matter to another kind. We have no means of telling the absolute amount of energy in any kind of matter. But the one thing of which the chemist is positive is that in all the material changes matter undergoes—radioactivity being ex- cepted—the elements do not change into one another, but remain in their various compounds essentially unaltered. If transmutation were pos sible, and one element could be changed into another, it would be easy to measure the difference in the amount of energy of the two elements. 98 THE STABILITY OF ELEMENTS As it is, the internal energy of the elements remains always unaffected by previously known material changes, and therefore till recently quite unknowable. Before we can find out how much or how little energy is internally associated with the atoms we must be able to study a case of transmutation. The great stability of all elements under all condi tions—even in the sun the identical elements which we know here persist, if we can rely on the evi dence of the spectroscope—is well in accord with the view that all the elements contain a very large store of internal energy, which is never released in ordinary changes, but which makes them indifferent to changes in their environment. Thus the internal kinetic energy of a torpedo containing a revolving gyrostat makes it successfully resist deflection from its course by the wind and waves. The internal energy of the solar system, taken as a whole, is the sole reason why it continues to exist as a system and does not drift apart. So far then from there being anything opposed to reason or probability in the view that the atom of the element contains a great and hitherto unknown store of internal energy, we see that if it possessed such a store we could not know of it until it changed, while the greater the store the more would it resist change from without, and therefore the less MINUTENESS OF THE EXPECTED CHANGES 99 likely should we be to suspect its existence. From this point forward we shall find that the more the apparent objections to the first alternative of internal energy are faced the less serious they appear, while with the second alternative of external energy the contrary is the case. Having with these preliminaries somewhat cleared the ground, I now wish to attempt to explain a series of experimental investigations which have thrown a flood of light upon the nature of radio activity. Though by a superficial or merely external observation of radium, even over the period of a whole lifetime, it would hardly be possible to detect the least change of any kind in the matter itself or any exhaustion of its output of energy, these investigations have proved that radium, and every element that is radioactive, is actually- changing in a very peculiar and definite way. These new changes in radioactivity are always excessively minute as regards the actual quantities of matter undergoing change in any period of time. Except in very special circumstances they are quite beyond the range of the most delicate methods of investigation previously known to the chemist. The methods employed in their investigation are in the first place wholly novel, but they are none the less trustworthy or definite on that account. 100 THE CASCADE OF CHANGES They depend on the important fact that when a radioactive element changes it does not as a le do so once only, producing in a single step the final product of its change. Usually there are several successive changes following one another, so to speak, in cascade. Just as a waterfall, instead of taking one plunge into a lake, may cascade in a series of successive leaps from pool to pool on the way down, so a radioactive element like radium passes in its change through a long series of inter mediate bodies, each produced from the one pre ceding and producing the one following. Whereas, however, the first change is and must be slow, the subsequent changes may be, and usually are, rela tively far more rapid. But for the existence of these ephemeral, rapidly changing, intermediate substances, continually being produced and as continually changing, it is safe to say the mystery of radium would to-day be still unsolved. Picture to yourselves exactly what this problem involves. Out of a remote, and so far as we know unlimited, past this world has gradually come into the state we find it to-day, and what we find is that there is a process known as radioactivity still spontaneously going on in matter in its natural state as it is dug out of the earth, which we cannot in any way stop or retard, and which we recog nise as the intrinsic property of certain chemical E. GOODWILLIE EXISTING METHODS elements. We must conclu evidence to the contrary, that radio ' """ ot a process which has started recently, or that it is confined to the particular epoch of the earth's history we are now living in. So long as the radioactive elements have existed this process must have been going on, and, if we are forced to the conclusion that the radioactive elements are changing, is it not obvious that the changes must be excessively slow for any of the radio active elements to have survived ? What could the methods of chemistry avail in such a search ? Delicate as these are to-day, beyond the limit of what was possible even a hundred years ago, infinitely finer and more sensitive methods are required. The geologists tell us, and we shall find in radio activity only confirmation, that the earth has existed in much the same physical condition as it exists to-day for hundreds if not thousands of millions of years. A chemist could probably in many cases detect the change of one thousandth part of one element into another, whereas we shall come to see that for even such a small fraction of a primary radioactive element to change a period of the order of a million years would almost certainly be necessary. You all know the stride that chemistry took for- 102 IMPONDERABLE QUANTITIES OF MATTER wards when it impressed into its service the spectro scope, and was able to detect with certainty quantities of new elements absolutely imperceptible in any other way. For example, Bunsen and Kirchofi detected by the spectroscope the unknown element ceesium in the natural waters of the Durkheim spring in the Palatinate, but to obtain enough caesium for their chemical investigations they had to boil down forty tons of this water. Coming nearer the present day, Madame Curie made an equal or even greater step forward when she impressed into the service of chemistry the property of radioactivity and dis covered the new element radium in pitchblende, though a ton of pitchblende contains only two grains of radium. But we must improve even on this. We have to detect the change in a minute amount of radium which is changing so slowly that it appears not to be changing at all. The actual amount of new matter which this half- grain of radium bromide would produce by its change in, say, a month or a year, is a quantity so small that one has only to attempt to conceive it to be ready to give up the search in despair, Yet in a moment I hope to show it to every one in this large room, and to demonstrate to you a few of its most striking properties in the clearest way. Were radium to change in one single step into, say, lead, which we believe may be the ultimate SUCCESSIVE OUTBURSTS OF ENERGY 103 product in the main line of descent, this would be impossible. Those of you in the back could hardly see a quantity of lead equal in quantity to the whole of this radium. How much less then could you hope to be shown the infinitesimal fraction of this small quantity which is produced in a month or a year ? No chemist has yet detected lead as the final product of radium, and our evidence on this point is at present only indirect, and not even very conclusive. But radium does not change all at once in one step. At least eight intermediate bodies intervene, each one of which is formed from the one preceding it with an outburst of energy, and changes into the next with another outburst of energy. A soldier on a battlefield knows without any doubt when he is being fired at, but it would take him a long and patient examination to find out, and it would be a matter of only secondary interest, whether the bullets are made, say, of lead or of nickel. The energy possessed by the flying bullets are their, to him, practically im portant feature. After the energy is all spent the bullet ceases to make its presence felt. So it is with radium. The energy possessed by the changing intermediate substances and evolved from them is the sole but sufficient evidence of their existence. After the energy is all spent and io4 ACHIEVEMENTS OF THE NEWER METHOD the change is complete, only a most minute and patient examination, which has still to be made complete, will reveal the chemical nature of the minute amount of dead products formed, but before this stage is reached, in the long succes sion of energy outbursts which accompany the change of one intermediate form into the next, we have a succession of most remarkable and obvious phenomena which enable us to detect the separate changes and to discover the whole nature and the periods of average life of all the intermediate bodies, although these all exist only in absolutely infinitesimal quantity, and not one of them is known, or probably ever can become known, to the chemist in the ordinary way. It is one of the most wonderful triumphs in the whole history of physical science that such changes should have ever been detected. Let us turn to the main evidence on which the view that radium is changing was first based. If this specimen of radium bromide was dis solved in water and the liquid evaporated down to dryness in order to get back the solid com pound, it would be found that as the result of this very simple operation the radium had lost the greater part of its radioactivity in the process. The penetrating /8- and y-rays would have com- THE EMANATION OF RADIUM 105 pletely disappeared, and the non-penetrating a-rays would only be one quarter as powerful as initially. Then a strange thing would happen. Left to itself the radium would spontaneously recover its lost activity, little by little from day to day, and at the end of a month it would be not appreciably less active than it at first was, or as it now is. This appears to be in direct conflict with the statement previously made that the radioactivity of radium cannot be affected by any known process, but it is only apparently so. If we study the process carefully we shall find that when the radium is dissolved in water "some thing" escapes into the air, and this "something" is intensely radioactive. It diffuses about in the air, but remains contained within a closed vessel, if it is gas-tight. In short, this "something" is a new gas possessing the property of radioactivity to a very intense degree. We owe the greater part of our knowledge of this new radioactive gas to Professor Rutherford, who has given to it a special name. He called it the emanation of radium, or, for short, simply the emanation. The vague term "emanation" is, with our present exact knowledge of its real nature, apt to mislead. Some, unfortunately, have used the term " emanation" or " ema nations " in speaking of the various radiations io6 THE EMANATION OF RADIUM which radium emits, and which we have already considered in some detail. I shall use the term " emanation" strictly to denote the intensely radioactive gaseous body generated by radium, and it is necessary to be absolutely clear upon this point. In the laboratory, half a mile from this lecture- room, I have a further quantity of about half a grain of pure radium bromide which has been dissolved in water. The solution is kept in a closed vessel. This morning I extracted the emanation from the vessel, and I have brought it here to show you. The radium from which it was derived is not in the room, it is still in the laboratory half a mile away. The emanation is contained, mixed with air, in a little glass tube (Fig. 17) provided with taps for its admission and extraction, and inside this tube are some frag ments of the mineral willemite, a silicate of zinc. This mineral has the appearance of an ordinary cold greenish-grey stone, quite undistinguished and not very different from many of the common pebbles of the road or sea-shore. It however possesses the power of fluorescing, under the action of X-rays and the rays from radium, with a brilliant greenish light, as you may see when I bring my capsule containing half a grain of solid radium bromide near to a block of the mineral FIG. 17. Tube containing Willemite used to exhibit the Radium Emanation. FIG. 18. The same tube photographed in the dark by its own phosphorescent light. To face p. 106. THE EMANATION OF RADIU 107 in the dark. Let us now in the dark examine the tube containing the emanation and willemite together. We find the willemite glowing with a most remarkable light. Even in ordinary lamp light or weak daylight the glow of the willemite is clearly visible. Fig. 18 shows the tube (Fig. 17), which has been placed in front of the camera in the dark room, and, as you can see, the pieces of glowing willemite have photo graphed themselves by their own light. In the negative the walls of the glass tube, which also are rendered feebly fluorescent by the emanation, are faintly visible. The photograph proved some what difficult to obtain, as the light, consisting almost wholly of green and yellow, is almost non- actinic to the photographic plate. An isochromatic plate must be employed and a long exposure given. Under these circumstances the B- and y-rays from the tube, as they are not refracted by the lens, themselves fog the plate uniformly to a consider able extent. The photograph gives no idea of the beauty of the original tube. Willemite glowing in the emanation of radium is one of the most beautiful sights 1 know, and considered with reference to the origin of its light and all that the phenomenon foreshadows for humanity, it raises feelings which only a poet adequately could express. io8 THE EMANATION OF RADIUM What is the emanation of radium ? I shall treat this question to-night solely as though the emana tion was a body with no connection whatever with radium, because a knowledge of its own nature is necessary before its real relation to radium can be appreciated. In the first place, it is intensely radio active on its own account—that is to say, it gives out the new kinds of rays very similar in character to those given by other radioactive bodies and capable of producing the same effects. What I am about to say refers only to a tube in which the radium ema nation has been confined for some hours. At first the emanation gives only a- but no /?- or y-rays, as we shall consider more nearly later (Chapter VIII.). This tube, in which the emanation is confined, glows in the dark because the phosphorescent willemite it contains is being bombarded by the rays from the emanation. Some of these rays penetrate the glass walls of the tube, as you may see if I bring the X-ray screen between your eyes and the tube. Moreover, if a very thin plate of metal is interposed at the back of the screen it does not perceptibly diminish the effect, for the rays from a tube containing the emanation, like the radium- rays themselves, are capable of penetrating a con siderable thickness of metal. They consist, in fact, of a-, /3- and y-rays together. Any of the other phosphorescent bodies—for example, zinc sulphide THE EMANATION OF RADIUM 109 -—would, if placed inside this vessel with the ema nation, glow in its characteristic way just as if exposed to radium itself. Similarly, a photographic plate would be fogged almost instantly, and an elec trified silk tassel would be discharged at once by the rays proceeding from the emanation confined in this tube. The similarity between the «-rays from the emanation and those from radium have been proved by exact physical experiments. The next point is that the emanation is not a solid form of matter dispersed like fine particles of smoke in the air which carries it. It is a true gas. This has been proved by innumerable ex periments ; but I wish to show you one which is particularly beautiful, and which has, I think, con vinced every one who has ever seen it performed that the emanation of radium is a true gas with the property of radioactivity. It was first per formed by Professor Rutherford and myself in Montreal in November, 1902. If the emanation is a gas there ought to be some temperature, though, perhaps a very low one, at which it loses its gaseous form and is condensed or frozen. All our attempts to effect such a condensation at tem peratures down to - 100° Centigrade had proved futile, and we had no means of obtaining the very low temperatures now daily employed in a modern no THE CONDENSATION OF THE EMANATION laboratory. But a liquid air machine was given to the laboratory by its generous founder, and on its first run the emanation of radium was success fully condensed. Exact experiments showed that the emanation is condensed quite sharply when FIG. 19. the temperature falls below - 150° Centigrade (or - 238° Fahrenheit), and it volatilises and again resumes its gaseous state quite sharply when the temperature rises above this. We shall perform the experiment in the following manner (Fig. 19). To one of the tubes of the vessel containing the emanation is attached a rubber blowing-ball, for blowing out the emanation. The other tube is THE CONDENSATION OF THE EMANATION in connected with a U-tube of lass containing some fragments of willemite, immersed in a vessel of liquid air and so kept at the very low temperature of about -183° Centigrade or -300° Fahrenheit, into which the emanation is blown. Exposed to this extreme cold the emanation instantly loses its gaseous state and condenses in the tube. To make the experiment more striking, between the tube containing the emanation and the cooled U-tube I have interposed several yards of narrow tubing which the emanation has to traverse before reach ing the tube in which it condenses. As you see, when I open the taps and gently blow a blast of air to sweep out the emanation into the cold U-tube, the willemite in the cold tube suddenly shines out brilliantly, at the point where the emanation con denses. So long as the U-tube is kept in the liquid air the emanation will remain there, though I continue to send a gentle blast of air from the bellows. But a few moments after taking the tube out of the liquid air, it warms up to the point (- 150° Centi grade) at which the emanation again resumes its gaseous form, and now we can blow it out with a single puff of air. See ! I blow it oat through the narrow tubing, which I have connected to the U-tube, into a large flask dusted over its inside surface with the phosphorescent sulphide of zinc. 112 THE RADIU EMANATION In the dark the globe shines out with a soft white light like some fairy lantern, and I can see to read my watch by its light. The physiological effects of the radium emanation are imperfectly investi gated and are probably potent. This is a field of investigation I personally have no desire to explore, so that we must not forget to cork the globe and so prevent the emanation from diffusing out into the air of the room. After this demonstration you may have some difficulty in really believing that the actual amount of gaseous emanation which has produced these beautiful effects is almost infinitesimal. By making use of the same property—its condensation by liquid air—the actual volume occupied by the radium emanation freed by freezing from all other gases was measured by Sir William Ramsay and myself. Imagine a bubble of air the volume of a good-sized pin's head, say, one cubic millimetre, or one fifteen-thousandth part of a cubic inch. It would require thirty times more emanation than was actually employed in the last experiment to fill a bubble of this size. Of course, in the experiments this small quantity of emanation was mixed with a considerable volume of air for convenience in manipulation. ITS INFINITESIMAL QUANTITY 113 It requires a distinct step for the mind to assimi late the important fact that the property of radio activity, which so far has been studied only in solid substances and minerals, could be shown equally by a gas, and this fact accounted for the true nature of the emanation remaining largely unrecog nised even after the conclusive experiment I have shown you. There is, of course, nothing contrary to the nature of radioactivity in the fact that it is shown by a gas. When we apply Mme. Curie's theory that radioactivity is an intrinsic property of the atom, and of the element in question, the difficulty is not that the emanation is a gas, for many elements are gases, but how it is that a new radioactive element, such as the emanation un doubtedly is, should result when radium compounds are dissolved in water, and this question we have purposely deferred. The emanation, as we have employed it in our experiments, is mixed with ordinary air, and in this way it can be dealt with and treated like any other gas. We have blown it through tubes from one end of the lecture table to the other. If it had been an ordinary gas, like air, no one could have seen it, or known what became of it. But being intensely radioactive, although its actual quantity is almost inconceivably small, the radioactivity ii4 CHEMICAL NATURE OF THE EMANATION serves as a sufficient evidence of its presence or absence, making it, as a matter of fact, far easier to work with and to investigate than an ordinary gas in ordinary quantity. If a mining engineer wished to know how the air he pumped into his mine got distributed among the various shafts and pits, he could not do better than to put a little radium emanation into the entering air, and then subsequently to take samples at various parts of the mine, and have them tested for content of radium emanation by a gold-leaf electroscope. Many other practical problems in the flow of gases, which are difficult to solve by ordinary methods, might be readily solved by the help of this new gas. It has even been found possible to settle the chemical nature of this new gas, and to place it in its proper family of elements in the periodic table. Almost all gases, according to their various natures, are absorbed when subjected to the action of various chemical reagents. Thus oxygen is ab sorbed by phosphorus, hydrogen by heated copper oxide, nitrogen by heated magnesium, and so on. The exceptions, namely, gases which are not ab sorbed by any reagents and which will not combine with anything, are the newly discovered gases of Lord Rayleigh and Sir \ illiam Ramsay—argon, helium, neon, etc.—which exist in atmospheric air. THE ARGON FAMILY 115 The quantity in the air of these gases is extremely minute except in the single case of argon, which is present to the extent of one per cent. The radium emanation, like argon, is not absorbed by any known reagent, and does not appear to possess any power of chemical combination. It may be passed unchanged through absorbents, or subjected to drastic chemical treatment which would suffice to absorb every known gas except those of the argon type, and therefore we say the emanation is probably an element of the same family nature as the argon gases. Radium, on the other hand, in its chemical nature is extremely similar to barium, strontium, and calcium, a family known as the alkaline-earth elements. None other of the argon elements or the alkaline-earth elements are radioactive, and yet the radioactive elements are quite normal in their chemical properties, closely resembling ordinary elements, and being associated in the clearest and closest way with one or other of the old well-known types or families. Quite recently, by using quantities of radium about fifteen times as great as those used to-night in our experi ments, it has been possible to obtain enough of the emanation for it to be possible to photograph its spectrum. This proves to be a new and character istic bright-line spectrum, resembling in general character the spectra of the other argon gases. . n6 ENERGY EVOLVED FROM THE EMANATION Finally, it has been found possible to obtain some idea of the density of the emanation of radium and, therefore of the weight of its atom, from experi ments on the rate of its diffusion from one place to another. These indicate that the gas is extremely dense—denser probably than mercury—and there fore that it has a very heavy atom. In 'all prob ability, although the evidence is not yet complete and is somewhat indirect, the atomic weight of the emanation is four units below that of radium, and therefore is the fourth heaviest known. The heat given out by a gram of radium, as we have seen, is 100 calories per hour, but it must be understood that this refers to radium in its normal condition containing its full quota of emanation. After solution in water, that is after the emanation is extracted, the radium gives out heat to the extent of only twenty-five calories per hour, while the emanation produces seventy-five calories per hour. That is to say, the emanation of radium gives three times as much energy as the radium from which it is derived, although the actual amount of matter in the emanation is itself practically imperceptible. Now, perhaps it is easier to understand how it is that the minuteness of the quantities of material ENERGY EVOLVED FROM THE EMANATION 117 offers no barrier in the investigation of radioactivity. Mass is not the only consideration. A very small bullet suffices to work terrible havoc, in spite of its smallness, by means of the kinetic energy with which it is impelled. A little torpedo, stuffed full of imprisoned energy in the form of explosives, suffices to sink an enormous battleship. A quantity of emanation, which certainly does not weigh a hundred-thousandth part of a grain, gives out enough energy to produce effects plainly visible to you all at the very back of the room. If, instead of the thirtieth part of a pin's head full, we could obtain a pint of this gas—and to obtain such a quantity half a ton of pure radium would be required—it would radiate the energy of a hundred powerful arc-lamps. Indeed, as Rutherford has said, no vessel would hold it. Such a quantity would instantly melt and dispel in vapour any material known. These new facts, which transpire the moment we begin to make a systematic investigation of the radioactivity of radium, make the second alterna tive, that the energy of radium is derived from outside, well-nigh incredible. For to account for the energy evolved from the emanation we must suppose all space to be everywhere traversed by new and mysterious forms of radiant energy of n8 THE DECAY OF THE EMANATION such tremendous and incredible power that the explanation is harder to believe than the fact it is supposed to explain. To avoid the necessity of supposing that the energy resides within the comparatively small amounts of radioactive matter in existence, we must fill the whole of external space with radiant energy of a similar order of magnitude. This is straining at a gnat and swallowing a camel. Fortunately there is a crucial test by which we are now in a position to decide between the two alternative views. Let us apply the theorem we have already deduced (p. 94) from general princi ples. If the energy comes from within the radio active matter, its radioactivity must in course of time diminish and decay—the more rapidly the more powerfully radioactive it is. Whereas, if the energy comes from the outside, however powerful the radioactivity may be, there is no reason why it should not continue indefinitely with undiminished power. We have seen that the emanation is, mass for mass, far more intensely radioactive even than radium, and, if the energy comes from within, it is to be expected that the activity of the emana tion will be short-lived in comparison with that of radium, whereas, if the energy is derived from outside, no such decay is to be anticipated. Does the radioactivity of the radium emanation diminish or decay, or does it continue permanently ? THE DECAY OF THE EMANATION 119 The answer to this question is that the radio activity of the emanation rapidly decays away from day to day. Four days hence the activity will be but one-half of what it now is. In eight days the activity will be reduced to one-fourth, in twelve days to one-eighth, in sixteen days to one-sixteenth, and so on, diminishing practically to zero at the end of a month in a descending geometrical progression with the lapse of time. The light from the glowing willemite in this tube, when it is left entirely to itself, will gradually fade, and at the end of a month will have died almost completely. Vast as is the store of energy in matter which is released in the radioactive process, it is not infinite, and in the radium emanation we have an example of a change proceeding so rapidly that only a few weeks are necessary for its com pletion. Half a mystery is usually greater than the whole, and in science when mysteries begin to appear on all sides, the explanation is often near at hand. We dissolved a compound of radium in water, and the greater part of its activity disappeared in the process. Then little by little the lost activity was spontaneously recovered, and at the end of a month the radium was not appreciably less active than at first. The disappearance of the greater part of the 120 THE REPRODUCTION OF THE EMANATION activity after solution was explained by the fact that an extremely radioactive gas—the emanation—was liberated during the act of solution, and this carried away with it the whole of the radioactivity which the radium had lost. But, lo! while the radium slowly recovered its original radioactivity, the emanation lost what it had at first possessed. A quantitative examination of these two processes of decay and recovery at once showed that the total radioactivity had not been affected, but had re mained constant in spite of the treatment to which the radium had been subjected. This is a funda mental law of universal application to all radioactive bodies, and it has been called the Law of the Con servation of Radioactivity. Whatever you do to any radioactive substance you cannot artificially alter the total radioactivity, though you may frequently, as in this example, divide it into several parts, for reasons that will soon be clear. It is easy enough on the first alternative to account for the comparatively rapid decay of the activity of the emanation of radium. It is dissipat ing its internal store of energy so rapidly that it is soon exhausted. It is a clear case of a short life and a merry one. But how is the gradual recovery of the radioactivity of the radium in the course of time to be explained ? This is the key to the whole problem, and on the second alternative no THE REPRODUCTION OF THE EMANATION 121 answer whatever can be given. The explanation that the energy of radioactive substance is derived from outside is not merely incredible. It is alto gether insufficient. Imagine that a month has elapsed, and that the radium, which has now recovered completely its lost activity, is again dissolved in water and evapo rated down to dryness exactly as before. Again you would find that in the process the radium had lost the same large proportion of its radioactivity, and again you would obtain from it a new amount of emanation no less than that which is on the table to-night. Repeat the experiment as often as you please and you will find the result always the same. While the emanation you separate from the radium is decaying away from day to day, a fresh crop is being spontaneously manufactured by the radium. The change of the radium into the emanation is, as a matter of fact, only the first of a long series of successive changes of a similar character. The gaseous emanation in turn rapidly changes into a third body, not a gas, called Radium A; this into a fourth, called Radium B ; and so on. Nine suc cessive changes are at present known, which we shall have to give some account of later. This explanation of radioactivity, which has come to be known as the theory of atomic disintegration, 122 ATOMIC DISINTEGRATION was put forward by Professor Rutherford and my self as the result of a long series of experimental investigations carried out in the Macclonald Physical and Chemical laboratories at M'Gill University, Montreal. It has, since, not only shown itself capable of interpreting all the very complicated known facts of radioactivity, but also of predicting and accounting for many new ones. Although on the surface a revolutionary addition to the theories of physical science, it must be remembered that it is the facts of radioactivity which are really re volutionary. While accommodating these strange new facts the disintegration theory conserves in a truly remarkable way the older established prin ciples of physical science. Without such a guiding hypothesis, reconciling the old and the new, it is safe to say that the facts of radioactivity would ultimately have wrought a far greater change in scientific theory than has actually taken place. Although the emanation of radium is not and, as we shall come to see, never can be obtained in palpable quantities—it is changing too rapidly for that—we know almost as much about its nature and properties as we do about any of the older gases. A very important point is that just as we cannot really alter the radioactivity of a body artificially in RADIOACTIVE EQUILIBRIUM 123 any way, we cannot and do not in any process in fluence the rate at which the emanation is being- formed from radium or the rate at which it in turn spontaneously changes. The same amount is always in existence whether you separate it or not. The apparent constancy of the radioactivity of radium is not the real constancy to be expected of a trans forming mechanism. It is the apparent constancy produced by the equilibrium between continuous and opposing changes, on the one hand the rapid decay of the part of the radioactivity due to the emanation, and on the other the regeneration of fresh emanation as fast as the old disappears. This process of regeneration is always going on at a perfectly definite and unalterable rate, and the property of producing a certain definite amount of emanation in a given time is as much a part and parcel of the very nature of radium—and indeed the best and most easily applied qualitative and quantitative test for the presence of radium in the minutest quantity that we possess—as is its power of giving the rays which lit up the X-ray screen and discharged the silk tassel, or as its power of gener ating heat. All of these properties are but the various aspects of a single primary cause. The element radium is changing, so slowly it is true, that at I24 OLD AND NEW METHODS CONTRASTED first sight it appears not to be changing at all, and yet with so tremendous and unparalleled an evolution of energy that the transformation of an otherwise imperceptible part of its mass is accom panied by an amount of energy so great that the change could not by any possibility have remained unknown. The emanation is the first main pro duct of the change of radium. If the emanation were like lead or any ordinary element it would take years of accumulation and the most minute and patient investigation to detect its production. But it is not. The emanation changes again into a third type of matter we have not yet considered (the nature of which does not yet concern us), but whereas it would take hundreds of years for any appreciable fraction of the radium itself to change, the change of the emanation is rapid and goes to practical completion within a single month. It is precisely on this account that we can work with and detect such almost infinitesimal quantities. What may be termed the material evidence of radioactive change, the detection, by purely chemi cal or spectroscopic methods, of the materials formed in the changes, is still scanty, although not altogether lacking. But the radioactive evidence, which depends not on the material produced, but upon the energy evolved, and on the way in which the energy is manifested, is abundant and suffi- OLD AND NE\ ETHODS CONTRASTED 125 cient. So long as the energy evolved is sufficient in quantity, and of a kind suitable for detection in any of the various ways I have illustrated, the actual quantity of matter producing the energy is of no consequence. But the amount of energy produced by any change depends not only on the quantity of matter changing, but also on the time the change lasts, that is, on the period of life of the changing matter. Chemical and spectroscopic methods of detecting matter depend on quantity, whereas radioactive methods depend on qiiantity divided by life. The shorter the life of the changing substance the less of it is necessary for its detection by means of radio activity. This is a merely preliminary and tenta tive indication of the operation of an exactly com pensating principle of great importance, which later it will be possible to formulate as a general law. Its result in the long run is this. Each of the ephemeral intermediate substances in the cascade of changes comes equally within our powers of investigation, whether it changes slowly or rapidly, whether it lasts long enough to accumulate in ponderable quantity, or whether it is changing so rapidly that it anon, Like snow upon the desert's dusty face, Lighting a little hour or two, is gone. CHAPTER VI. The connection of the a-particle with radioactive changes — The a-par- ticle 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 pro duction from uranium and thorium — Proof that the a-particle is an atom of helium — The nature of the first change of radium. LVST week we studied the first step in the evidence that radium is changing, and con sidered in some detail the chief practical reason why such changes have proved within our powers of discovery, namely, that the change is not single but proceeds in cascade from stage to stage, producing ephemeral intermediate transition-forms, of which the radium emanation is one, almost inconceivably minute in their actual quantity but evolving in their next change very large amounts of energy, by means of which it is possible to trace them and study their nature with ease. We considered the first product of the change of radium, namely, the emanation of radium, its nature and properties, and its continual production from radium. We reserved purposely the examination of the connection between radium and the emanation it produces. Now I wish to combine with the knowledge we have gained of 126 RADIOACTIVE CHANGE 127 the nature of the radium emanation that already considered (Chapters III. and IV.) with reference to the nature of the a-particle. A radium salt is dissolved in water, and the imprisoned emanation, which was formed but stored during the previous month throughout the whole mass of the substance, is thereby liberated and escapes. The radium left to itself continues to produce fresh emanation at a steady rate. The released stores of emanation begin to lose their radioactivity. We shall confine our attention at first solely to the case of the radium. When radium in this way is freed from all pre viously formed emanation it still gives out a-par- ticles, although only about one-fourth as many as it gives out when it contains its full quota of emana tion and other products. These a-particles we regard as produced from the radium atom in the same change as that in which . 0 t~\&- the emanation is produced. The emanation is regarded, in fact, as radium that has lost One «- Radium. Emanation. . . FIG. 20. particle. This, which is a perfectly general point of view, was proved five years ago by the consideration of a mass of evidence accumulated with reference to the similar changes going on in the element thorium, but much of this lies beyond the scope of the 1 128 HELIUM AND RADIOACTIVE CHANGE present course. The evidence that has since been accumulated enables the same deduction to be more simply made, and this alone need be considered. Henceforth the original reasoning as to the nature of atomic disintegration, although it was, when first put forward, very complete and convincing to those acquainted with the whole of the experimental facts, will be largely replaced by the more direct evidence since obtained. We have seen in considering the nature of the a-rays that they are now regarded as due to the flight of swarms of helium atoms expelled from the radioactive substance with an almost inconceivable speed of from 8000 to 12,000 miles per second. Long before the real nature of the a-particle was known, helium had been first predicted to be and then proved experimentally to be a product of the radioactive changes of radium, and this chapter in the development of the subject has something more than an historical interest. Before proceeding, one underlying consideration governing the view that an atom of helium and an atom of emanation are simultaneously formed when an atom of radium disintegrates, must be made clear. It refers to the relative quantities of each product, helium and emana tion, which it may be expected will be formed by RADIOACTIVE EQUILIBRIUM 129 the continuous operation of the process. Helium we know is not radioactive, and therefore there is no evidence that helium is changing in any way, and we may in this sense refer to it as one of the ultimate products of the change. The emanation, on the other hand, is changing so rapidly that the change may be regarded as complete in the course of a single month. The bodies it is changing into we have not yet dealt with, and they do not immediately concern us. Now a changing substance, like the emanation, cannot possibly accumulate in quantity with lapse of time beyond a certain very small extent. It is true it is constantly being formed from radium in the same way as helium, but whereas the helium, being a stable substance, may be expected to accumulate in a quantity that is proportional to the time that elapses, the quantity of emanation will not increase beyond a certain point. For in a very short time after the process of accumulation of- emanation from the radium begins, as much emana tion will itself change as is formed, and the quantity from that time on will remain constant. This condition is known generally as " radioactive equilibrium," and when we speak of the emanation being in equilibrium with the radium we mean that the quantity of emanation has reached a maximum and does not further accumulate with further lapse H 130 ACCUMULATION OF PRODUCTS of time. In the case of the emanation practical equilibrium results in the comparatively short time of a few weeks. That is to say, however long radium is left undisturbed to accumulate its emanation, the quantity of the latter never exceeds a practically almost infinitesimal one, for it is a quantity which is produced from the change of the radium in quite a short period of time. Its quantity is therefore excessively minute. It is so very minute that were it not changing and evolving energy it would not be detectable by any ordinary method. You will see that it follows at once from this point of view that if any element were produced in the disintegration of radium, which itself did not change but was permanent, then on the one hand, owing to the extreme smallness of the amount formed, it would not be easy in a short period to obtain evidence of its production, by means of ordinary chemical tests, but, on the other hand, the quantity would go on accumulating indefinitely with lapse of time. As we saw last week, the first evidence of atomic disintegration was dynamical and due solely to the energy which is evolved in the process. The answer to the question as to what are the ultimate products of atomic disintegration must be looked for on quite different lines. The ultimate products formed will be too small for detection in the ordi- HELIUM IN MINERALS 131 nary way by the statical methods of chemistry and physics, but they will accumulate indefinitely. Since the processes go on steadily, so far as we know, in the minerals in which the radioactive elements are found, the ultimate products, formed through past ages of disintegration, must accumulate therein from one geological epoch to the next. So that at the present day one ought to find in the radioactive minerals the ultimate products of the disintegration process, accumulated in sufficient quantity to be capable of detection by the ordinary methods of chemistry. Now the radioactive minerals are always very complex, and contain a very large proportion of the total number of elements known, so that in most cases it is impossible to deduce very much from this evidence. Nevertheless, there was one clear definite exception, and that was the element helium. The history of our knowledge of this element is unsurpassed by that of any other in interest. Its very name (from >5Aioy, the sun) stands witness to the fact that it was known to exist in the sun as an element before it was known to exist on the earth at all. It was discovered in 1868 by the spectro scope in the sun's chromosphere, by means of the characteristic bright yellow line in its spectrum, 132 PROPERTIES OF HELIUM which is technically known as " D3". Then, in 1895, Sir William Ramsay discovered it in certain minerals found in the earth's crust, and made a systematic investigation of its physical and chemical nature. It is a gas, the second lightest known, only twice as dense as hydrogen, and the only gas which has till now resisted all efforts which have been made to liquefy it by extreme cold and pressure.1 It is readily evolved from the minerals in which it is found, either by heating them or by dissolving them, but once evolved it cannot again be absorbed by the minerals or by any other sub stance known. Indeed, helium resembles argon perfectly in chemical nature, in that it is quite with out any combining power, and exists free as single atoms without being known to form compounds of any kind whatever. Its atomic weight is four (hydrogen = i). Sir William Ramsay drew attention to the fact that all the minerals in which he found helium contained either uranium or thorium. This was before the days of radioactivity, and for long the origin of the helium—a non-condensable, non-combining gas—in minerals containing uranium and thorium was a matter for comment and specula tion. In certain cases the volume of helium 1 Kammerleigh Onnes has by marvellous experimental skill and persistence just liquefied it, attaining thereby a far lower temperature (-270° C.)—only 3° C. from the absolute zero—than has ever before been reached on this earth. September, 1908. HELIUM AN ULTIMATE PRODUCT 133 present is nearly a hundred times as great as the volume of the mineral in which it is contained. The disintegration theory enabled Professor Rutherford and myself at once to give a probable explanation which has since proved to be correct. We regarded helium as one of the ultimate products of the disintegration of the radioactive elements, radium, uranium, and thorium. Forming during the long ages of the past throughout the mass of the mineral, which is often of a glassy nature, it is unable to escape until the mineral is heated or dissolved, and it steadily accumulates with the passage of geological time. We ventured to predict that helium was one of the ultimate pro ducts of radioactive changes, being formed in Nature from radium, uranium, and thorium, exces sively slowly, but still fast enough to ensure that all minerals containing these elements must contain helium also. This has since been proved to be the case. From this point the work proceeded along two separate lines. Rutherford, in an exhaustive ex amination of the nature of the a-rays, which we have already considered, proved first that they con sisted of positively charged atoms expelled with great velocity. At first their mass was given as 134 PRODUCTION OF HELIUM FROM RADIUM twice that of hydrogen, on the assumption they carried one atomic charge. Then, as the sequel to the beautiful counting experiments we have con sidered, quite recently (1908) it was proved that each a-particle carries two atomic charges of posi tive electricity, and therefore the mass of the a-particle is four, that is to say, it is the same as that of the atom of helium. This makes it very probable, therefore, that the a-particle is an atom of helium. The prediction that helium was a product of radioactive changes was proved directly by Sir William Ramsay and myself in 1903. We chose for the particular case of radioactive change studied that of the emanation of radium, since it is rapid, and the emanation can readily be obtained, free from other gases, by condensing it with liquid air and removing the gases not condensed with a pump. So purified, it was sealed up in a small spectrum tube, so that the spectrum of the gas could be examined at will, and then it was left to itself. At first no helium was present. Helium, not being condensable by liquid air, could not have been present in the tube as first prepared. But in the course of three or four days, as the emanation disintegrated, the spectrum of helium gradually made its appearance, and finally the whole char acteristic spectrum of helium was given by the PRODUCTION OF HELIUM FROM RADIUM 135 FIG. 21. Original Spectrum-Tube in which the formation of Helium from Radium was first observed. X I ffi I Helium Gas I from ^\ Radium I Hydrogen [' 'H i I II III IV Red I I Violet FIG 22. Dr. Giesel's Photograph of the Spectrum of the Gas from Radium TV ?? !m?U«S', IIJ S minutes exposure). I is the Spectrum of Helium, IV that of Hydrogen, for comparison. To face p. 135. tube. Fig. 21 shows a photograph of one of the original spectrum tubes in which the production of helium from radium was proved. This observation of the production of the element helium from the radium emanation, and therefore (since the emana tion in turn is produced from radium) from the element radium, has since been verified and con firmed by numerous investigators in various parts of the world. It has also been found by Debierne in a similar manner by the spectroscope that actinium, a radioactive substance found by him in pitchblende, produces helium. Dr. Giesel has actually succeeded in photographing the spectrum of the gases generated by radium, and one of his photographs is reproduced in Fig. 22. It repre sents four separate spectra, one below the other in parallel strips. The uppermost (I) is ordinary helium. The second and third (II and III) are two photographs obtained from the gas generated by radium. In the second an exposure of twenty minutes, and in the third one of five minutes were given. The lowest spectrum (IV) is that of hydrogen. It will be seen that many of the helium lines are present in the spectrum of the gas from radium. The other lines are those of hydrogen, due, no doubt, to the presence of a trace of moisture. The figures above and below the plate refer to the stronger lines of helium and 136 HELIUM FRO. URANIUM AND THORIUM hydrogen respectively clearly visible in photo graph III. They refer to the wave-lengths in Angstrom units (io~10 metre). It must be re membered that the (visually) brilliant yellow line D3, owing to its colour, appears far less intense in the photograph than the blue and violet lines. I have been engaged for four years in an attempt to detect the production of helium from the primary radio-elements uranium and thorium, and I have recently succeeded in proving in both cases that helium is produced, and, moreover, that the rate of production is almost exactly what is to be expected from the theory of atomic disintegra tion. This quantity is about one five-hundred- thousand-millionth of the mass of the uranium or thorium per annum! A photograph of the ap paratus employed, as it at present stands in the Physical Chemistry Laboratory, is shown in Fig. 23. These are seven exactly similar arrangements side by side, each of which is quite separate and unconnected with the others, Each consists essen tially of a large flask, capable of holding a con siderable quantity of the material experimented upon in the form of solution. Each is provided with a peculiar form of mercury tap, which, while it serves perfectly to keep out the atmosphere from the flask for an indefinite time, can at any moment i f» it- •a c a ffi •a o To face p. 136. Pu HELIUM FROM URANIUM AND THORIUM 137 be opened by sucking down the mercury in the barometer tubes, so that the accumulated gases from the flask can be extracted and tested for helium without admitting air. Air has been the great trouble. A pin's-head-full of air left in the whole of the large flask or in the solution, or leaking in during the periods of accumulation, would completely ruin the experiment. Most of the elaborations of the apparatus have to do with the preliminary thorough removal of the air from the apparatus before the experiments are com menced. The methods of testing for helium are also entirely new. They depend on the power I found was possessed by the metal calcium, when heated to a very high temperature in a vacuum, of absorbing the last traces of all gases except the gases of the helium and argon type. In this way the minute amount of helium produced (usually not more than a thousandth part of a cubic millimetre) is freed perfectly from every other trace of gas and water vapour. Finally, it is compressed by means of mercury into the smallest-sized spectrum tube that can be made and its spectrum examined. As shown in numerous special experiments, the Ds line of the helium spectrum can be detected with cer tainty if one millionth part of a cubic centimetre, or one five-thousand-millionth part of a gram of helium is present. This is certainly the smallest nil 138 HELIUi AND THE a-PARTlCLE quantity of any element that has ever been detected by the spectroscope. By frequently repeated experiments one can find for each flask a period of accumulation that must be allowed before helium can be detected in the expelled gases, and so one can obtain a measure of the rate of production of helium. In this way I have obtained helium repeatedly from both uranium and thorium salts, and the rate of produc tion, though the measurements are not yet finished, has been found to be practically the same as that previously calculated from the disintegration theory. For the case of uranium the rate of production is about two milligrams of helium from a thousand tons of uranium per year. The position is then this. Helium has actually been found to be produced from the various radio active substances — radium, thorium, uranium, actinium—which have in common the fact that they all expel a-particles. The mass of these particles has been measured and found to agree with the mass of the helium atom. All a-particles have been proved to have the same mass and to differ only in the initial velocity of expulsion, whether expelled from radium itself, from the emanation, from actinium, uranium, thorium, or any other of the bodies which expel them. Hence we are HELIUM AND THE a-PARTICLE 139 justified in concluding that the a-particle is an atom of helium, or at least becomes one after the velocity with which it is expelled is lost and it is brought to comparative rest. Even more recently there comes to hand one further step in this long converging series of ex periments, which clinches the argument. We have seen that the a-particle, though but feebly penetrat ing, has a very definite small penetrating power. Now glass is a substance that can be blown to an excessive degree of thinness and yet retain to the full its air-tight properties. I have succeeded in blowing small windows of glass thin enough to allow the a-particle to get through, and yet strong enough and tight enough to stand the pressure of the air on one side when there was an almost perfect vacuum on the other. So that it ought to be possible, if the a-particle is an atom of helium, by storing the radioactive substance in a very thin- walled air-tight glass vessel, to get helium produced outside the vessel, although no helium or other gas in the ordinary state confined inside the vessel could escape. This experiment has recently been performed by Rutherford, with the large quantity of radium loaned to him by the Austrian Govern ment. The emanation from the radium, which gives a-particles and has been shown to give helium, was stored in an excessively thin-walled i4o ATOMIC WEIGHT OF THE EMANATION but still perfectly gas-tight tube, enclosed within a wider vessel. After some days the gas in the outer vessel was found to contain helium. It was proved that when helium was stored in the inner tube, none got through into the outer vessel. This final experiment clinches the proof that the a-particle is an atom of helium. So we are justified in writing the first disintegra tion suffered by radium :— Radium. Emanation. Helium. FIG. 24. There is a great deal of evidence going to prove that one atom of a radioactive body expels but one a-particle at each disintegration. Hence, since the atomic weight of radium is 226, and that of helium 4, the atomic weight of the emanation is pre sumably 222. CHAPTER VII. Atomic disintegration and the periodic law—Questions of nomen clature—Definition of the chemist's atom—Difference between atoms and chemical compounds—The insufficiency of chemical methods in many radioactive problems—Hypotheses or mental pictures—The two possible pictures of atomic disintegration— Sudden explosive character of the disintegration—Law of radio active changes—Chance of disintegration—Average life of a dis integrating 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 disinte gration of radium. THE question, How can an element or the atom of an element change, has given rise to many arguments, of etymological rather than scientific importance. What we now certainly know, and what radioactivity has given us for the first time the opportunity of learning is, first, that some elements do change, and secondly, kozv they change. The element radium changes, by the loss of an atom of helium, into the emanation, which is about as different from radium in its chemical or material nature as two elements well could be. The one is a member of the group of alkaline- earth, the other of the argon family of elements. 141 I42 ATOMIC DISINTEGRATION After all, is not this rather to be anticipated ? When we arrange the elements in order of their atomic weights—an arrangement which led to the recognition of what is known as the Periodic Law—the most sudden and surprising differences appear between succeeding elements. Chlorine, potassium, and argon are three succeed ing elements in such an arrangement, and there is no resemblance whatever between them. In the nine successive transformations radium undero-oes. o * the atom suffers, in most but not in all, a disintegra tion in which a helium atom is expelled. The heavy residues of the original atom remaining after the successive loss of one, two, three and so on of these helium atoms constitute the intermediate bodies—the emanation, Radium A, Radium B, and Radium C—successively produced, each from the preceding. It is therefore rather to be expected that the succeeding transition-substances produced one after the other should differ entirely from one another in their material characteristics, and this, so far as we have been able to discover them, we find to be fully borne out. Let us from the point we have gained now face the question, which has proved a difficulty to so many, of how it is we find that the elements and the atoms are actually changing. The word ato QUESTIONS OF NOMENCLATURE 143 is, of course, derived from the Greek, and at first meant the indivisible or the undivided. For a long time it had a subjective meaning only, being the smallest particle imaginable, rather than the smallest particle obtainable, and as such it belongs to metaphysics, not to physical science. The idea of the atom was first given an objective meaning by Dalton. He showed that chemical change be tween two elements occurs in definite proportions by weight of the two elements. If unit weight of one is taken, the weight of the other will have a definite fixed value. But often the same two elements unite to form more than one compound in different proportions. Then, if unit weight of the one is still taken for reference throughout, the ratio of the weights of the other in various com pounds will be simple multiples or submultiples of one another, indicating that elements do not combine in haphazard proportions, but "atom for atom " by fixed increments or units of combination having definite relative weight. These units of chemical combination of definite relative weight are the atoms of the che ist. In all the various changes of matter which chei istry has investigated it has sufficed to regard all combination as taking place atom by atom, and fractions of an atom or the subdivision of atoms has not been necessary. In compounds the component atoms preserve their 144 THE ATOM NOT A CHEMICAL COMPOUND individuality and identity, because compounds can always be decomposed to give back the same elements out of which they are formed and not new ones. In none of these changes does any deep change of the component atoms themselves take place. As chemical changes till recently were the most fundamental material changes known, the chemist's atom fulfilled in a derived sense the ancient meaning of the smallest particle that exists. It did not suffer subdivision in the most funda mental changes known. But in this sense its mean ing was coupled with that of the particular element to which it referred. Thus the atom of uranium is about 240 times as massive as the atom of hydrogen. An atom of uranium is the smallest particle of itranium which exists. An atom 240 times lighter than this is known, but it is not uranium, it is hydrogen. The discoveries in radioactivity have left this meaning of the word ato unchanged. The atom of radium is the smallest particle of radium that exists, and is the unit of all the chemical changes radium undergoes. When, by new and more fundamental changes than those before known, it changes, it is no longer an atom of radium. The matter formed is as unlike radium as any body well could be. You may, if you like, regard the radium atom as a compound of the THE ATOM NOT A CHEMICAL COMPOm D 145 atom of emanation, and of the helium atom which result on its disintegration, as it certainly is such a compound, but you must make it quite clear that you do not mean a mere chemical compound, which may at will be formed from and decomposed into its constituents. Were radium a chemical com pound of helium it would, as Sir William Huggins recently pointed out to me, show the spectrum of helium. Instead, it shows an entirely new spectrum, clearly analogous to but distinct from that shown by barium, its nearest chemical relative. The spectrum of helium is not shown until after the radium has disintegrated. The radium spectrum does not contain a single helium line. The most vital distinction, however, between an element and a compound in the chemical sense is this. Both are ultimately compound. Of that there can be now no doubt. But the energy change which attends the resolution of an element into its constituent parts is of an order of a million times greater than in the case of the resolu tion of any chemical compound. Although this is a question of degree, it is of a degree of so entirely different an order of magnitude that it completely differentiates the two types of com plexes, and nothing but confusion can result from giving to each the same name. Radium is as much an element as any of the other eighty. If radium lilt I 146 WORDS AND FACTS is complex, so, almost certainly, are all to greater or less degree. If radium changes, so may (perhaps even so do) all. Their complexity is of a completely different character from that of chemi cal compounds, and it is best in the end to retain the old words "atom" and "element" in the sense they have had since the time of Dalton rather than attempt to meddle with this traditional, and to scientific men, well-understood nomenclature. The atom of the chemist remains exactly what it was. Why, therefore, alter its name ? If you call it a molecule, how are you to distinguish it from the chemical molecule, which has also its own definite meaning distinct from the chemical atom ? These questions of nomenclature are perhaps not of any great or lasting importance, but they have so diverted attention from the experimental facts, and have given rise to so much more or less random criticism of the younger workers in radio activity, that it seems proper to refer to them. In addition it has been the very great misfortune of this young and vigorous independent science of radioactivity to have been presented in the past, not as able to walk on its own feet, but too often in association with, and, if I may say so, under the wing of some extremely speculative and ambitious theories about the nature of matter. The workers QUOTATION FROM PROFESSOR SMITHELLS 147 in radioactivity have been openly accused of flying off into the wildest and most hare-brained of hypo theses before the facts had been ascertained, a criticism which had just this much of melancholy truth, that the particular speculations in question had nothing to do with the facts of radioactivity, although the latter were supposed to support them, and the facts of radioactivity had not then become generally known outside the ranks of the actual workers. Professor Smithells, not by any means an adverse or merely obstructive critic of the new science, in his Presidential Address to the Chemistry Section of the British Association at Leicester, 1907, said:— " There is an uneasy feeling that developments of great importance to the chemist are being made by experiments on quantities of matter of almost inconceivable minuteness. Spectrum analysis, of course, took chemistry beyond the limits of the balance, but the new materials which it disclosed could at least be accumulated in palpable quantity. With radioactivity we seem, in relation to the ponderable, almost to be creating a chemistry of phantoms, and this reduction in the amount of ex perimental materials, associated as it is with an exuberance of mathematical speculation of the most bewildering kind concerning the nature, or perhaps 148 THE CHEMIST AND RADIO ACTIVITY I should say, the want of nature, of matter, is calcu lated to perturb a stolid and earthy philosopher whose business has been hitherto confined to com paratively gross quantities of materials, and to a restricted number of crude mechanical ideas." We might reply that the balance of the chemist, so long the pride of his heart and the glory of his science, is about as useful to the worker in radio activity as a machine designed to weigh a locomotive would be to a chemist. The quantity of radium ema nation I have used in my experiments is the amount produced in the change of half a grain of radium bromide in only a few days, and therefore is a quantity so small that it would not turn the finest balance ever constructed. Yet even those of you at the back of the hall followed this quantity of matter backwards and forwards along the lecture- table, with your unaided eyes, as it was wafted about by gentle blasts of air, or captured, frozen and immobile, at the temperature of liquid air. We have even been told that it is impossible to come to any settled conclusions in regard to radio activity, until enough of the materials can be ob tained to suffice for the requirements of chemical investigation. But surely, this critic rather chal lenges the question whether he is not mistaking mere subjective familiarity with his own methods, and their interpretation for an objective directness ME TAL PICTURES 149 they do not in fact possess. The tests by which we can recognise and identify with ease, and measure with accuracy the amount of, say, one billionth of a milligram of the radium emanation, possess a philosophical foundation which would challenge comparison with any of the tests of the chemist on any kind of matter, in any quantity great or small. I do not wish to be misunderstood. Radioactivity has been developed as much by the chemist as by the physicist. The type of critic, however, who sneers at what he does not understand has obtained such a foothold in official'chemistry in this country that some protest is necessary. In physics he would not be tolerated,—hence the common im pression that radioactivity is a subject worked out by the physicist to the discomfiture of the chemist. It is my intention to give you, so far as I am able with accuracy, broad general mental pictures of radioactive processes, rather than the detailed technical investigations on which these pictures are based. Bear in mind exactly the relation of such mental pictures to the discovered facts. The pictures may not be true, but they are not demonstrably false at the present time. That is to say, you may in any case, without fear of being led into error, apply the picture you have to what is taking place, and the view will lead you to expect certain consequences, 150 NATURE OF ATOMIC DISINTEGRATION and these consequences in every known case agree with the facts. Without such mental pictures, or generalising hypotheses, no man could encompass even a small part of one science. So long as the deductions from the hypothesis are in agreement with facts and can be used to predict them accu rately, even when they are still unknown, thus saving the memory, the hypothesis or mental picture is not even supposed or expected to be the absolute truth. So long as all the known facts occur as though the hypothesis were true, the latter serves a very useful purpose, although at any time it may be replaced by a deeper view, one step nearer to absolute truth. In the early history of the subject two possible alternatives had to be taken into account with reference to the exact nature of radioactive changes. Radioactivity is an atomic phenomenon, and the radio-elements are slowly undergoing changes. What do we mean by " slowly " in this connection ? Two possibilities arise. Either the slow changes may result from a slow gradual alteration, through all the atoms of a radioactive substance gradually evolving their stores of internal energy and changing by slow degrees into new kinds of matter. This point of view it was never possible to enter tain for a moment. Or, the change is slow and gradual with regard only to the mass of the sub- NATURE OF ATOMIC DISINTEGRATION 151 stance as a whole, but sudden and explosive in character with regard to each individual atom as its turn to disintegrate arrives. This, from the first, the only possible point of view, is in accordance with all that has since been discovered with regard to the nature of the successive disintegrations and of the a-rays expelled. Radioactive changes pro ceed in cascade, from step to step, the accomplish ment of each successive step taking on the average a definite time. But as regards the individual atom disintegrating, the change is sudden in time and of the nature of an explosive disruption, in which an a-particle is expelled with enormous speed, and the old atom becomes ipso facto a new one, of atomic weight four units less. Regarding the individual radium atom, for example, there is no gradual change into the emanation and helium atoms. Re garding the whole mass of radium, there is a very gradual change in the sense that some definite small proportion of the whole suffers disintegration in each unit of time. This, then, is the very vivid mental picture of atomic disintegration which the detailed researches in radioactivity have established. Any one radio- element like radium being considered at any instant, among its innumerable host of atoms, most ol which are destined to last for hundreds, some for thousands 152 THE CHANCE OF DISINTEGRATION of years, a comparatively very small proportion every second fly apart, expelling a-particles and becoming emanation atoms. Next second the lot falls to a fresh set to disintegrate, and so the pro cess goes on, a-particles being expelled as a con tinuous swarm, and yet so small a fraction of the whole changing that the main part of the radium will remain unchanged even after hundreds of years. Now consider the emanation atoms formed. These are much less stable than the atoms of radium. A much larger fraction of these disinte grate every second, producing more a-particles and a new body not yet considered. It is now necessary to consider briefly the exact nature of radioactive change and the laws it follows. The deduction of these laws is a matter for the mathe matician. We are chiefly concerned with the general conclusions which have transpired. I will first state the most important of these in words divested of mathematical symbols. The chance at any instant whether any atom disintegrates or not in any particular second is fixed. It has nothing to do with any external or internal consideration we know of, and in particular it is not increased by the fact that the atom has already survived any period of past time. The events of the past in radioactive change have, so far as we can tell, no influence whatever on the progress of events in the future. LA W OF RADIOACTIVE CHANGE 153 This follows from the consideration of the one general mathematical law which all known cases of atomic disintegration so far investigated have been found to follow. Fortunately the law itself is simple. Its application in individual cases is often complicated, but I shall confine myself to the simplest, which are at the same time the most generally important, consequences. The chemist has to do with many types of change all following different laws. In some the rate of change is pro portional to the quantity of the substance changing, in others to some power of this quantity. Now, in radioactive change the rate of change is in variably simply proportional to the quantity of changing substance. This seems easy enough, but I would warn the uninitiated that they must not overlook the important fact that since the quantity of a changing substance itself changes as time goes on, owing to the progress of the change, the rate of change being proportional to the quantity also continuously changes, and at no time has a constant value.1 Hence you cannot get much further by simple arithmetic and algebra. Some 1 Of course, in the case of a slow change like that of radium Hself, when even in a lifetime the quantity of radium is not very appreciably reduced by the operation of the change, it is allowable to neglect the slow alteration of the rate of change with the time and to consider the rate of change as constant, since for short periods of time it essentially is so. Ill 154 THE AVERAGE LIFE OF AN ATOM knowledge, withal a slight one, of the mathematics of continuously varying quantities is essential for the complete deduction of the laws of radioactive change. However, as my intention is to avoid mathematics, I shall simply state these consequences ex cathedra. The rate of change in any single case of atomic disintegration is proportional to the quantity of the substance which is changing. The usual plan is to let the symbol X represent the fraction of the total changing per second, and to this symbol A is given the special name "the radioactive constant." A may represent a small or a large fraction, accord ing to the particular case, according as the dis integration process is slow or rapid. The important point is that it is a real constant of nature in every case, independent of the past and future history of the substance, its actual amount whether large or small, and of every other consideration whatever. Thus for the emanation of radium, A, the radio active constant, has the value 1/500,000, which signifies that in this case 1/500,oooth of the total amount of emanation in existence changes per second. The next step, skipping the mathematics,1 is that the average period of life of the atom of a radioactive substance, that is to say, the period of 1 So far as I know, the period of average life was first deduced by Mr. J. K. H. Inglis, to whom I put the problem. THE EXPECTATION OF FUTURE LIFE 155 time in seconds it exists on the average before its turn comes to disintegrate, is simply the reciprocal of the radioactive constant, or i/A. Thus the average life of the radium emanation is 500,000 seconds, or 5-3 days. Now as radioactive change proceeds during every instant at the rate proportional only to the total quantity of substance undergoing the change, which is present and remains unchanged at that instant, and as in this method of look ing at the changes we do not consider at all the absolute quantities, only the fraction of the whole changing, it follows that A is always of the same value throughout the process from start to finish. It also follows that i/A, the period of average life of the remaining atoms, does not, as you might be inclined to suppose, tend to lessen as time goes on. The atoms disintegrating first have a far shorter period of life, and those disintegrating last have a far longer total period than the average. But at any instant throughout, considering only the atoms still remaining unchanged at that instant, then from that instant the average period of life is always i/A. Our own period of average life, of course, follows very different and far more complicated laws. The expectation of future life at any age is a practical problem for the actuary. But every one knows, owing to the mortality among infants, that the ex- 156 "WHY" AND "HO " OF DISINTEGRATION pectation of life at birth is less than shortly after wards, when it reaches a maximum and then gets less and less with increasing age. The "expectation of future life " of a radioactive atom is independent of its age—as it happens the simplest possible law and one lending itself, as will appear, to some most beautiful deductions. This answers fully the general question, how does an element change ? You will probably wish to know why it changes in this particular way. I cannot tell you. In this matter I feel I must proceed cautiously, for I once got severely taken to task by a reviewer for venturing to point out in a book that the immediate cause of atomic disintegration " appeared to be due to chance—i.e. the orientation assumed by the atom at one instant has no deter mining influence on the orientation about to be assumed at the next instant." If the destroying angel selected out of all those alive on the world a fixed proportion to die every minute, independently of their age, whether young or old, if he regarded nothing but the number of victims and chose purely at random one here, and one there, to make up the required number, then our expectation of life would be that of the radioactive atoms. This, of course, is all that is meant by the statement that the course of atomic disintegration appears to be due to the operation of "chance." THE PERIOD OF HALF-CHANGE '57 It is natural to inquire why this particular law is followed. On this fundamental question no light is yet forthcoming. There is always " a cause of the ultimate cause." Atomic disintegration is assuredly the ultimate cause of radioactivity. It does not weaken this deduction that as yet we have not found the ultimate cause of atomic disintegration. Various possible causes have been discussed. Most of them, so far from helping the elucidation of the " why," do not conform even to the "how." The law of radioactive changes shows clearly that the past history of an atom does not increase its chances of undergoing disintegration in the future, which is a fundamental step gained, although it leaves the ultimate problem unsolved. There is another way of stating the law of radio active changes, and that is by saying that as the time increases in arithmetical progression the amount of substance remaining decreases in geo metrical progression. Suppose in a time of T seconds one half of the total amount changes and one half remains unchanged. In the next period of T seconds, 2 T altogether, one half of what is left, that is, one quarter, changes, and one quarter of the total remains unchanged. In 2 Tthe quantity is reduced to i/22. In any period of time represented by N T seconds, where N is any multiple or submultiple, the quantity of substance remaining is i/2N. It remains 158 EPHEMERAL AND PERMANENT to state what relation the time T required for the half-change to occur, bears to the period of average life i/A of the former way of considering the change. There is a fixed ratio between these two periods, the latter being always i '45 times the former. In a time equal to the period of average life i/A, the quantity of substance present is reduced to i 'e = 0*368 of the initial quantity. These considerations would have little interest to us but for the fact that they afford the means where by the period of average life of any radioactive element can by their aid be exactly determined, not only for those transition-bodies like the emanation, which change so rapidly that we can watch their complete transformation in the course of a few days or weeks, but also for the primary radio- elements, some of which we know require thousands of millions of years to run their course of change. The average life of a radioactive element, represent ing as it does a fundamental constant of nature, is one of its most important attributes. Our own period of average life being strictly limited, it naturally affects very much our way of looking at the various radioactive bodies. If, for example, the average life is a matter of a few days, as in the case of the radium emanation, we regard the body as an ephemeral transition-form. If it is, as in the RADIO-ELEMENTS '59 case of radium, a few thousand years we are in clined to look upon the substance as a permanent and primary radio-element. There is really not this sharp difference. But it is convenient to divide radioactive bodies into two classes, and in the one to put those for which the periods of average life are short compared to our own, and in the other to put those for which the periods are long. The method employed to determine the value of this fundamental and all-important constant is naturally quite different in the two cases. In the first, simple direct observation suffices. Thus if we measure the decay of the activity of any separated quantity of the emanation of radium with time, we shall find that it decays in a geo metrical progression with the time to half its initial value in the course of 3*7 days. The period of average life is 1-45 times greater or 5-3 days. But in the case of a body, of which one thousandth, or one thousand-millionth as the case may be, changes annually, simple direct observation does not help much. How are we to proceed ? In the first place, let us consider the cases of uranium and radium. We may determine how many times more powerfully radioactive radium is than uranium. In this case we can determine with fair certainty the relative numbers of «-particles expelled in the same time by equal quantities of iff 160 AVERAGE LIFE OF LO G-LIVED ATOMS uranium and of radium. The ratio is about i : 1,000,000, considering only the first change of radium. Since the view seems to be justified that only one a-particle is expelled from each atom at each disintegration, we may conclude that uranium is disintegrating 1,000,000 times more slowly than radium, and the period of average life of uranium therefore is 1,000,000 times that of radium. If the latter is known, then that of uranium can be thus roughly estimated also, although it is a period of some thousands of millions of years. As a matter of fact, there is a very beautiful generalisation, I have already referred to briefly, and which later on I shall try to develop further by the aid of an analogy, by means of which the periods of average life of the radio-elements of the second class, those, that is, which are long- lived compared with ourselves, have come into the region of exactly knowable quantities. If the period of average life of a single member of a series of successive atomic disintegrations is known the others can be calculated, provided certain data, not entirely impossible to obtain, are known. It will clear the ground considerably if I attempt to give you the main idea succinctly in the case of radium itself and of the first product of its dis integration, the emanation of radium. I have already alluded to the fact that owing to the very 'If AVERAGE LIFE OF LONG-LIVED ATOMS 161 rapid disintegration of the emanation its quantity does not continuously accumulate, but reaches an equilibrium ratio with respect to the radium pro ducing it, in which the amount of already formed emanation disappearing is exactly counterbalanced by the amount of new emanation formed. This state of things is known generally by the name of radioactive equilibrium. The importance of the existence of this state of radioactive equi librium it is impossible to overrate. Many prob lems, as we shall come to see, which, to us with our limited period of life, might well appear abso lutely insoluble, connected as they are with periods of time so vast that our little life by comparison appears a mere moment, are solved directly by the proper application of this principle. Now I am only giving you the main idea and one specific illus tration of what is in fact a law of great generality. By the law of radioactive change, if \, is the radioactive constant of radium, i.e. the fraction of the whole changing per second, and N is the total number of radium atoms dealt with, then the number of radium atoms changing into the emana tion per second, and therefore also the number of atoms of fresh emanation produced per second, is XjN. But in equilibrium this equals the number of emanation atoms disappearing. If the radio active constant of the emanation is X2, and the M iff 162 AVERAGE LIFE OF RADIUM number of atoms of emanation present during equi librium is denoted by X, the number of emanation atoms disappearing per second is A2X. Hence we have This law, the most important in radioactivity, thus states that in two successive (not necessarily con secutive changes) disintegrations of which the second is more rapid than the first, the more rapidly changing body accumulates in quantity until a fixed ratio with respect to the parent body is attained, and this ratio is inversely proportional to their respective radioactive constants or directly propor tional to their respective average lives. X2 is well known by direct observation. Now if X/N, the ratio between the number of atoms of emanation and of radium in equilibrium together, can be found, then AI, the radioactive constant and therefore il\, the period of average life of radium can be deduced. That is the important thing—the period of the average life of radium, the rate at which it is changing, and a host of vitally impor tant consequences, can be deduced. For a slowly changing body like radium the second is an incon veniently short unit of time to employ and it is better to take a year. What is wanted is the fraction of any quantity of radium which changes AVERAGE LIFE OF RADIUM 163 in a year. The quantity X/N, which is the ratio of the number of atoms of emanation and of radium in equilibrium together, can be deduced by ordinary physico-chemical laws if the actual volume of emanation in equilibrium with a given quantity of radium can be determined. As already mentioned (p. 112), this volume was first measured by Sir William Ramsay and myself in 1904. The actual volume of emanation is excessively minute, but it is just within the range of measurement. From our results we concluded about i/ii5Oth part of the radium changes annually, so that the period of average life on this estimate is 1150 years. Owing to the excessive minuteness of the volume, the method is not an accurate one, tending, since the volume of emanation is likely to be too great unless every trace of other gas is absent, to give too short a period. With the growth of the subject other methods, less direct but more accurate, have become available. Professor Rutherford recently, from a consideration of a large number of separate data accumulated by himself and others bearing on this question, came to the conclusion that the period of average life of radium is not very far removed from 255° years, and we shall take this value as.the most probable. It may suffer slight further alteration as fresh data are accumulated, but it is very improbable that it is seriously in error. Within 1(1 A ^ t64 TOTAL ENERGY EVOLVED BY RADIUM narrow limits the average life of radium may be taken to be 2500 years. " A knowledge of this important constant enables us at once to say how much energy any quantity of radium would evolve in the course of its complete change, that is, during a period of some thousands of years. We saw (p. 31) that a grain of pure radium evolved about six calories of heat per hour. There are 8760 hours in the year, so that in a year a grain of radium evolves about 50,000 calories. In a year i/25OOth part changes. Therefore in the complete change of one grain of radium no less than 125,000,000 calories would be evolved. The energy evolved in the change of radium is nearly a million times greater than that evolved from a similar weight of matter undergoing any change known previously to the discovery of radioactivity. By the burning of a grain of coal, for example, only about 500 calories are obtained. No wonder then that to account for the boundless energy dis played everywhere in the starry heavens proved a difficult problem for physicists, acquainted with rno more energetic chemical process than the . burning of coal! 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 min erals—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. ONE of our chief duties will be to follow out this theory of the disintegration of atoms in radioactivity. The bare idea of elements spon taneously changing raises so many obvious and apparently insurmountable difficulties that it will be interesting to consider them as they arise and to consider what answer can be made to them. To night we must concentrate on one of the chief of these—a difficulty which no doubt has already pre sented itself in many of your minds. If radium is changing at the rate of nearly one two-thousandth part every year, how is it that there is any radium left at the present time ? Even at the beginning of the time recorded in past history there must have existed several times as much radium as there is now, if the rate of disintegration has been constant 166 HOW IS RADIUM MAINTAINED1! over that period, while a hundred thousand years ago it can be calculated that there must have existed a thousand billion times as much as to-day, had the steady disintegration been going on at its present rate. That is to say, even if the whole world were originally pure radium, in a period of time brief compared to that which we know from geological evidence it has actually been in existence, there would be practically none left, and certainly not as much as actually exists to-day. Or, looking forward instead of backward, if we put this half- grain of radium bromide in a safe place, and then could revisit the earth say twenty-five thousand years hence, we should find less than one-thousandth part of it remaining. The slow disintegration would have done its work and changed the radium into the non-radioactive elements which are being formed from it. This question, apparently so insoluble, in reality admits of the most direct and satisfactory answer on the disintegration theory and serves as a good example of how a theory, if it is worth the name, must be able to predict future dis covery as well as to explain the existing facts. An analogy to facts we have already discussed will help us to find the solution of this difficulty. In the emanation of radium we have become acquainted with a body changing so rapidly that at the end of a month none of the original quantity HOW IS RADIUM AINTAINED? 167 remains. How is it there is any emanation in existence at all ? Because it is being reproduced as fast as it disappears. Is there any reproduction of radium going on, balancing the effect of its disin tegration and maintaining its quantity from age to age ? Radium is the direct parent of the emana tion. Itself changing more than a hundred thousand times slower than its product, it maintains the quan tity of emanation in existence over a period a hun dred thousand times longer than would otherwise be the case. Is there then a parent of radium ? Does there exist any other element producing radium by its own disintegration as fast as that already in existence disappears ? Do not regard this thirty milligrams of radium bromide as something merely by itself. Consider its history. By infinite labour and patience this tiny quantity of radium has been separated from several hundredweights of the mineral pitchblende. Suppose in this operation all the rest of the mineral, after the extraction of the radium, were preserved and put in a safe place. When we revisited our specimen of radium twenty-five thousand years hence, and found practically none of it remaining, should we find that the mineral from which it was extracted had in the meantime grown a fresh crop of radium ? The answer is that we should. This was one of the first predictions made five 168 REPRODUCTION OF RADIUM years ago from the theory of atomic disintegration and one of the most recent to be confirmed by experiment. Long before the data were available which enabled an exact estimate of the life of radium to be calculated, it was recognised that radium, though at first sight a permanent and primary radio-element, is changing so rapidly that, had there existed no process in which fresh radium is supplied to replace that changing, none could possibly have survived till the present day, and from general principles it was possible to make a shrewd prediction as to which element was the parent of radium. We have already considered the general principles which enabled the prediction that helium was one of the ultimate products of radio active changes to be made. Ultimate products must co-exist with the radio-elements producing them in all the natural minerals in which the latter are found. Something of the same reasoning applies to the parent of radium, only in this case it is far more definite and elegant. The parent of radium must co-exist with radium in all minerals in which radium is present. Now it is at once obvious, if this explanation of the parent of radium is to meet the case, that such a body must be changing very much more slowly than radium, otherwise there would arise the same necessity to assume the existence of a parent of the parent REPRODUCTION OF RADIUM 169 as there is of a parent of radium. The original first parent of radium must be changing excessively slowly to maintain a steady supply of radium over long epochs of geological time. By the law already formulated on page 162, in two successive, not necessarily consecutive, dis integrations of which the second is more rapid than the first, the more rapidly changing- body accumulates in quantity until a fixed ratio with respect to the parent body is attained, and this ratio is inversely proportional to the ratio of their respective rates of change, or directly proportional to the ratio of their respective periods of average life. Let us apply this law. The parent body is the parent of radium. The quantity of radium in minerals must therefore attain a fixed ratio with respect to the quantity of the parent of radium, and this ratio is the ratio of the period of average life of radium to that of its parent. The quantity of helium that accumulates in a mineral continually increases as time goes on, assuming the helium does not succeed in escaping, and no definite proportion between helium and radium is to be expected. But the case is different with radium and its parent. There must be a fixed ratio, independent of the age of the mineral ex amined. As the original first parent of radium hi 170 URANIUM AND RADIUM URANIUM AND RADIUM 171 must be changing excessively slowly to survive geological epochs of past time, there must be always a very large quantity of it in the mineral. As the radium is changing, from the standpoint of geological epochs of time, very rapidly, there must always be a very small quantity of radium. Between these quantities great and small there must exist the same ratio as between the respective periods of average life of the two bodies. A very cursory examination of the minerals in which Mme. Curie found radium was sufficient to point strongly to the probability that uranium was the primary parent of radium. Uranium was, as we have seen, the original element for which the property of radioactivity was discovered, and its radioactivity is several million times more feeble than that of radium. Now the radioactivity depends only on the atoms actually breaking up, and there fore in comparing uranium with radium it follows that uranium must be disintegrating several million times more slowly even than radium, so that if uranium produces radium the quantity of uranium must be several million times greater than the quantity of radium in minerals. But this is exactly what Mme. Curie found to be the case in the minerals she worked up for radium. So that from the very first there existed a strong presumption that uranium was the original parent of radium. The evidence in support of this view at the present time is indirect, but quite satisfactory. We owe it to the careful work of McCoy, Strutt and Bolt- wood that the genetic relation between uranium and radium has been established. They determined the ratio between the quantities of uranium and of radium in a large number of minerals. In every mineral examined containing uranium there was found to exist a direct proportionality between the quantity of uranium and that of radium. To Rutherford and Boltwood together we owe the exact determinations of this important constant of proportionality. They found that for every one part of radium there always exists three million parts of uranium. This constant gives directly, unless other undetermined factors interfere, the ratio of the average lives of the two elements. As we have seen, that of radium is 2500 years. Hence it follows that that of uranium is 7,500,000,000 years. Enormous as this period is, it is not now merely a deduced or calculated value. I obtained the same result by direct experiment from the rate of production of helium from uranium. It will help us considerably if we try to find some analogy to the important and intricate rela- 172 URANIUM AND RADIUM tions that exist between uranium and radium. We may take for illustration the magnificent system of waterworks which supply this city, which we will suppose have been given over to us by the Cor poration to control for the purposes of our illustra tion. As you know, we in Glasgow are supplied ultimately from Loch Katrine through an inter mediate reservoir at Milngavie. We shall first cut off Loch Katrine from all fresh sources of supply of water, and from all outlets except to the intermediate reservoir at Milngavie, and we shall see to it also that the latter receives no water except from Loch Katrine, and delivers none except to Glasgow. We shall then issue to our engineers the instructions that there must be delivered every hour at Milngavie from Loch Katrine one seven-thousand-five-hundred-millionth part of the total store of water in Loch Katrine, and from Milngavie to Glasgow every hour one two-thousand-five-hundredth part of the total store of water at Milngavie. Then, if instead of hours we read years, the quantity of water in Loch Katrine represents the quantity of uranium, and the quantity of water in Milngavie that of radium. For the sake of brevity we shall term Loch Katrine the source and Milngavie the reservoir. First we shall suppose that our regulations have URANIUM AND RADIUM 173 been in operation already a considerable number of hours, as this is the condition in which, reading years for hours, we find uranium and radium together in minerals in Nature, for example, in a piece of pitchblende. What relation will the quantity of water in the source bear to the quantity in the reservoir, that is, the quantity of uranium to the quantity of radium ? The amount of water the reservoir receives is quite independent of the amount it contains, but the amount it delivers is propor tional to the amount it contains. Similarly the amount of radium produced from uranium does not depend at all on the amount of radium already present, while the amount that itself changes de pends only on and is proportional to the amount present. Nevertheless, we shall find that there is always three million times more water in the source than in the reservoir. Because only under this condition is the intake of the reservoir equal to the outflow from the reservoir, that is, the production of new radium equal to the disappear ance of the old. Imagine, for example, that there was just twice as much water as this ratio in the reservoir, then twice as much would flow out as flows in, and the supply in the reservoir would be rapidly depleted. Or, if there were but one half as much in the reservoir, twice as much would flow in as out, and the supply in the reservoir would 174 AGE OF PITCHBLENDE increase. In either case, intake and outflow would ultimately become equal, and no further change would then occur until both the source and the reservoir were empty. But let us now disconnect Loch Katrine from Milngavie reservoir, which is equivalent to separating, as Mme. Curie did, the radium from the uranium in pitchblende. Obviously the reservoir by itself will now be able to supply water for a very much shorter time than it did before, and, in general, with the conditions stated, source and reservoir together will last three million times longer than the source alone. The radium on the table will have half disintegrated, so that only half will remain, in about 1750 years. Where as had it remained in the mineral associated with its parent uranium, the quantity of radium in the mineral will not be reduced to one half what it is now until 5,000,000,000 years have elapsed. Thus we can say, following a cautious reservation once made by Professor Tait, provided the causes that are now at work have always been in con tinuous operation in the past as they are now, and that we know of all the causes that have been at work, 5,000,000,000 years ago there must have been about twice as much uranium and radium in this piece of pitchblende as there is to-night. Since, however, there is actually in this pitchblende MAINTENANCE OF RADIUM 175 now over 50 per cent of uranium, it is not possible that it can have been in existence in its present form more than 5,000,000,000 years. But, even from a geological point of view, this is a very long period of time indeed; longer, perhaps, than it would be profitable in the present state of science to push back our inquiries. That, then, is the position with regard to the maintenance of radium in Nature. Even when we deliberately leave out of account the possibility there may exist in Nature entirely unknown processes replenishing the sup plies of uranium, just as there are replenishing Loch Katrine, there is no difficulty in accounting for the continuous maintenance of radium over a period of the past as great as, or greater than, there is any reason to believe the earth has been in existence in its present condition. This is as far as we need pursue our analogy for the moment, but we shall again find it useful at a later period. We must pass on to another aspect of the question. At this stage it will be well to make a short digres sion into the radioactivity of uranium itself, and how it is explained on the theory of atomic disintegra tion. Uranium and its compounds in their normal state give out both a- and /3-rays. As in all other cases, the /3-rays, being photographically the most i76 URANIUM X active and being the more penetrating, were the first chiefly studied. Sir William Crookes and also M. Becquerel found that by certain chemical pro cesses a new substance in minute quantity could be separated from uranium, to which Crookes gave the name Uranium X, and this new body produced the whole of the photographic activity of uranium. The uranium after this treatment no longer affected a photographic plate. Crookes concluded that the radioactivity was due in reality to the presence of the foreign substance in minute amount, which he called uranium X, and that pure uranium was not radioactive. I repeated these experiments, and found that only the /3-rays of uranium belonged to the uranium X. Uranium freed from uranium X gave its normal amount of o-rays. Then it was found that the /3-radiation of uranium X decayed steadily in a geometrical progression with the time, whereas the uranium that had been freed from uranium X and at first gave no /3-rays, gradually and completely recovered its power of producing /3-rays. Uranium groius uranium X, in exactly the same way as radium grows the emanation. The activity of uranium X after separation from uranium, consisting entirely of /3-rays, steadily decays in a geometrical progression with the time, falling to one half the initial value in twenty-two days. The average life of uranium X is thus about thirty-two URANIUM X 177 days. These results are simply represented on the disintegration theory by the scheme Uranium. Uranium X. 7,500,000,000 32 days, years. FIG. 25. Since the atomic weight of uranium is about 238, and the rate of production of helium from uranium indicates that only one atom of helium is expelled from one atom of uranium, the atomic weight of uranium X presumably is about 234. This is as far as the methods of radioactivity enable us to trace the disintegration of uranium at the present time. The body first produced—uranium X —is only an ephemeral transition-form, lasting on the average thirty-two days, and when it disintegrates, the process appears to come to a stop so far as our experimental methods have yet been able to dis close. Now on the view that has been developed that uranium is the parent of radium, it was natural to suppose that uranium X in the course of time turned into radium. A little consideration will show that if this were the case it might easily be overlooked at first on account of the very long i78 ATTEMPTS TO DETECT period of life of radium compared with that of uranium X. As already explained (p. 125), chemi cal and spectroscopic methods of detecting matter depend only on quantity, but radioactive methods depend upon quantity divided by life. Assuming equal effects produced in the disintegration of an atom of uranium X and of an atom of radium, since the life of the latter is 30,000 times that of the former, it will be necessary to have 30,000 times as much radium as of uranium X to produce equal radioactive effects. In 1903 I started a series of special experiments which have been continued ever since I came to Glasgow, partly in conjunction with Mr. T. D. Mackenzie, to see if uranium did not produce radium. The uranium, after being purified as com pletely as possible by chemical methods from radium, is left sealed up in a flask and periodic ally tested to see if a growth of radium has occurred. The method of testing for minute traces of radium is a very simple and accurate one, allow ing quantities of radium of only a few million- millionths part of a grain to be detected with certainty and measured with exactitude. Use is made of the characteristic emanation generated by radium. Uranium does not generate any emana tion. The uranium solution to be tested for A GRO] TH OF RADIUM 179 radium, after standing sealed up in a glass flask for a period of at least a month to allow the equi librium quantity of emanation to accumulate, is boiled in a vacuum, and the gases expelled are collected and introduced into a sensitive gold-leaf electroscope. If radium is present in the solution, its emanation causes the leaf to lose its charge, and the rate at which the discharge occurs under defined conditions can be used accurately as a measure of the amount of radium present. The test is qualitative as well as quantitative, and there is no possibility of making a mistake as to the identity of the emanation and of the radium from which it is formed. The present result of these experiments, while they furnished the first evidence of a growth of radium, withal in very minute amount, have clearly proved that this growth was not due to uranium. In the later experiments with very carefully puri fied uranium no certain growth has occurred in three years. In the early experiments the uranium salt was only specially purified from radium, not from any other impurities that might have been present, derived from the minerals from which uranium is obtained, and a very slow growth of radium from the preparation was actually observed. Now if uranium X, when it disintegrates, pro duced radium directly, then with the quantities of 180 INTERMEDIATE PRODUCTS materials used in these later experiments, the amount formed in a single hour would be greater than has actually been formed in three years. In the earlier experiments, with not specially purified uranium, the growth of radium, although quite detectable, was still only one thousandth part of what would have occurred had uranium X changed directly into radium. In spite of this apparently conclusive negative result, it is practically certain at the present time that uranium is the original parent of radium, and that in the course of years our preparations will begin to grow radium. The natural explanation of this failure to detect a growth of radium from uranium is, that one or more intermediate bodies of long life exists in the disintegration series between uranium and radium. On the analogy proposed, this means that between Loch Katrine and Milngavie reservoir one or more large intermediate reservoirs exist, which have to fill up before the water reaches Milngavie. Uranium X represents the first of such a series of intermediate reservoirs, it is true, but owing to its short period of life and the large fraction of the total quantity always passing through on the way to the next, such a reservoir would be an extremely small one, and for periods such as we are considering its effect on the flow would be practically negligible. INTERMEDIATE PRODUCTS 181 It would be quite otherwise if one or more reservoirs as large as Milngavie — if one or more intermediate substances as long-lived as radium — existed in the series. I well remember one fact told me by the engineer in charge of the mag nificent scheme of waterworks, supplying the mines at Kalgurli, in Western Australia, from a source near the coast across three hundred miles of desert. There are several intermediate reservoirs on the way. The plant installed is capable of pumping five million gallons of water daily, and yet it took a period of many weeks since pumping operations began before the water appeared in Kalgurli. When uranium is carefully purified from all other substances one can be sure that one starts with all the intermediate reservoirs empty, that is, with none of the intermediate substances present. Water is flowing steadily from the source all the time, as the disintegration of uranium is always going on. We have been watching and waiting three years at the radium reservoir — strictly speaking, at the one beyond radium, since the emanation of radium, not radium itself, is actually employed for the test. But the flow has not reached there yet and the radium reservoir remains practically as empty as at the start. But there is no doubt it will come. I only hope we are still there when it arrives. T*"*'* M-w- „ 182 THE DIRECT PARENT OF RADIUj. It is not beyond the resources of mathematics to find out a good deal about these intermediate reservoirs. The present results indicate that if there is but one long-lived intermediate body between uranium and radium, then its period of average life must be at least 16,000 years, that is, six times that of radium itself. On our analogy then between Loch Katrine and Milngavie, there must exist a reservoir of six times the capacity of Milngavie, provided there is only one. There may be several. Several small ones would be much more effective than one large one. Since the equilibrium quantity to which an intermediate body accumulates is pro portional to its period of average life, then if there is only one intermediate parent of radium between radium and uranium, there must be six times as much of it in minerals containing radium as there is of radium itself. This leads me to the next step. Ever since the failure to detect a production of radium from uranium, the existence of one or more intermediate substances of long period of life has been fore shadowed. Recently Boltwood in America has succeeded in isolating one from minerals containing radium, and this one proves to be the direct parent of radium. Whether there are others it remains to be seen. The one found possesses the property of THE DIRECT PARENT OF RADIU 183 producing radium directly from itself by disintegra tion. It expels a-rays during its disintegration into radium, which are peculiar in that they possess the lowest velocity of expulsion known. The range of these a-rays is very little more than one inch of air. Chemically the body resembles thorium so com pletely that the two substances, if mixed, cannot be separated. This gives the means of separating the new body from minerals. Some thorium is added and separated by the well - known methods of chemical analysis. It is then purified as completely as possible. The parent of radium is not separated from the thorium by this treatment, although all other substances are. The chemical resemblance between the two substances is analogous to that between radium and barium or polonium and bismuth, and does not signify any genetic con nection between the substances. Boltwood has proposed the name Ionium for the new substance. There is no doubt that the small growth of radium I observed in 1904 in my first experi ments with uranium was due to the presence of a trace of this substance in the uranium derived from minerals. It seems a pity that the property by which the body was looked for and found, namely, its power of producing radium, has not been made the basis of its name. The name Subradmm, meaning " going up to" or " stand- 184 ELEMENTARY EVOLUTION ing immediately before" radium, seems very suit able. The disintegration series thus reads :— Uranium. Uranium X. 7,500,000,000 32 days, years. Parent of radium. Radium. Emanation. ? 2,500 5-3 days. years. FIG. 26. as far as we have yet considered it. In the centre is placed the known or presumed atomic weights of the various bodies. It is to be noticed that a gap still remains between uranium X and the parent of radium. As the mass of the /8-particle is practically negligible, the product of uranium X should still have an atomic weight of 234, so that there is presumably one more change in which an "-particle is expelled, still to be found here. There is little doubt that the gap will soon be filled up. So far, then, as we have inquired, uranium, uranium X, the parent of radium, radium, and the emanation represent respectively the starting-point and the four successive stopping-stations in the. long journey of continuous devolution from the heaviest and most complex atom known into less heavy and complex atoms which is going on around us, or, to preserve our original analogy, the source and four ELEMENTARY EVOLUTION 185 successive intermediate reservoirs in the flow of elementary evolution. " All things flow " was one of the dogmas of ancient philosophy, and in this, as in many others, the ancients guessed truer than they knew. Instead of four stopping-stations or inter mediate reservoirs in this stately procession of elements disclosed by radioactivity, there are already known no less than ten, starting from the element uranium, but for our present purposes of illustration these four will suffice. But this new transformation scene on which the curtain of the twentieth century has been rung up, beginning as it has done with the transformation of the most fundamental and permanent of the existences which physical science has recognised in the past, extends beyond physical science and transfigures with new light some of the most fundamental and permanent ideas which in one form or another are deep-rcoted in the world's philosophies. CHAPTER IX. The subsequent changes of radium—The induced or excited radio activity—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 «-, /3-, and y rays—The emana tion only gives « 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? WE have traced radium to its source. It remains to follow through its disintegration briefly to the end. This was a task to which Rutherford particularly devoted himself, after the main principles of atomic disintegration had become familiar, with the consequence that, with the excep tion of a lacuna here and there still to be supplied, our knowledge of the whole process from the start to finish is now tolerably complete. In addition, some new considerations have transpired which concern us nearly in the broad general application of the principles of atomic disintegration, so that for this reason, if for no other, the work claims our attention. Most of you who have read at all in the subject will be aware of one mysterious and extraordinary 186 SUBSEQUENT CHANGES OF RADIUM 187 power possessed by radium,, which I have hitherto carefully avoided all mention of, not wanting to have too many irons in the fire at once. Radium possesses the power of endowing with some of its own radioactivity neighbouring objects. Thorium, which is very like radium in many ways, particularly in giving a gaseous emanation (which, however, has the very short period of average life of only a little over a minute), also possesses a similar power. The phenomenon was discovered by the Curies for radium and termed " induced radio activity," and for thorium simultaneously by- Rutherford and termed " excited radioactivity." With the explanation of the property the original names have largely fallen into disuse. We shall, as usual, confine ourselves to the case of radium. Any object left in the immediate neighbourhood of a radium salt becomes radioactive, but after it is removed the radioactivity decays away rapidly and almost completely, abnormally at first, but subse quently in a geometrical progression with the time, with a half-value period of about thirty minutes. The temporary activity so " induced " consists of «-, /3-, and y- rays. The activity exists as an invisible film or deposit over the surface of the object rendered radioactive, for, by sand-papering, the activity can be rubbed off and then is found on the sand-paper. It is now customary in con- 188 ACTIVE DEPOSIT OF RADIUM sequence to refer to it as the "active deposit of radium." This power is, strictly speaking, not a property of radium itself, for if the radium is contained in a completely closed vessel—it does not matter how thin-walled so long as it is air-tight—no radio activity whatever is produced outside. The first step in understanding the nature of the phenomenon consisted in tracing it to the action of the emanation of radium. In the ordinary condition the emanation is always diffusing away to some extent from radium salts unless they are contained in air-tight vessels. The " active deposit" is the product of the disin tegration of the emanation. Just as radium cannot exist without continuously producing the emanation, so in turn the emanation cannot exist without con tinuously producing this active deposit. In any vessel containing radium emanation this body is being continuously deposited on the walls of the vessel, so that if the emanation is at any time blown out, the active deposit remains behind. Radium expels one a-particle and changes into the emanation. The emanation expels a second « par ticle and changes back again into a solid, or at least into a non-gaseous form of matter, the " active deposit." The latter in turn expels more a- and also /3-particles, and so the course of successive disintegrations goes on. In the active deposit ACTIVE DEPOSIT OF RADIUM 189 itself at least three changes follow one another with great rapidity, so that the analysis of them proved a complicated task. You know that if a moisture-laden atmosphere is sufficiently chilled, the vapour of water condenses directly into the solid form, and a snowstorm results. Something of this kind is always happen ing in an atmosphere containing the radium emanation. Every second two out of every million of the atoms of emanation disintegrate, expelling a-particles and leaving a solid residue, so that there is a sort of continuous snowstorm silently going on covering every available surface with this invisible, unweighable, but intensely radioactive deposit. Unlike snow, however, the particles of this active deposit are charged with positive electricity, so that if two surfaces are provided, one charged nega tively and the other positively, the deposit is attracted almost entirely to the negatively charged surface. The other surface repels the particles and so does not get coated. By making the negatively charged surface very small the active deposit can be almost entirely concentrated upon it. This enables me to show you more effectively the pro duction of the active deposit from the emanation and some of its chief properties. It would, however, take us too long and too far 190 RADIUM A, B, AND C if we attempted first to study these properties, and then tried from them to deduce their explanation. It must suffice if I give you first the explanation of the facts according to the theory of atomic dis integration and then illustrate as many of the points in it as possible experimentally, I have said that after the disintegration of the emanation at least three successive disintegrations, following one another rapidly, occur. The bodies produced are referred to as Radium A, Radium B, Radium C, in order to avoid the necessity of inventing a host of new names for bodies having such fleeting exist ence (Fig. 27). Radium. Emanation. Radium A. Radium B. Radium C. j Active deposit of rapid change. 2,500 years. 5-3 days. 4-3 minutes. 38 minutes. 30-5 minutes. FIG. 27. As before, the presumed atomic weights are placed inside the circles corresponding with the successive products. The periods of average life are placed below. The first body produced from the emanation, radium A, changes with great rapidity with a period of average life of 4-3 min utes, expelling an a-particle. The body radium B RADIUM A, B, AND C 191 resulting undergoes a change which was at first thought to be entirely "rayless." Neither a- nor /3-rays of the ordinary kind can be detected, although there is some recent evidence to show that a very feebly penetrating /3-ray is produced. This we need not further consider. The period of this substance is thirty-eight minutes. The body produced, Radium C, changes, expelling both a- and ^-particles (and y-rays also, since these always accompany the ordinary /3-rays). The period is 30*5 minutes. It is possible that this change is double, the first following the second so rapidly that it is impossible to disentangle their effects. We started our description of the rays of radium with the statement that they consisted of a-, /3-, and y-rays. One of the most interesting points of the above scheme is to show that the /3- and y-rays do not come from radium itself, any more than they do from uranium itself, but from the later products. It is loose, but convenient, to talk of the /3- and y-rays of radium. Really we mean the /3- and y-rays of radium C. The emanation, like radium itself, gives only «-rays. The whole of the /3-rays result in the later changes of the active deposit. We have seen that, freshly prepared from solution, radium salts give only a-rays. The j8- and y-rays , i92 EXPERIA ENTS WITH THE ACTIVE DEPOSIT make their appearance only after the subsequent products have accumulated. On the table there is a small glass vessel silvered internally (Figs. 28 and 29) containing the emana- FIG. 28. tion from half a grain of radium bromide. It is arranged so that steel knitting-needles can be inserted into the emanation and withdrawn through a glass tube held in a cork. The needle is con nected to the negative pole of the electric supply and the silver coating to the positive pole. If only the point of the needle is made to project beyond FIG. 29. Apparatus for obtaining the Active Deposit of Radium. To face p. 192. EXPERIMENTS WITH THE ACTIVE DEPOSIT 193 the glass tube, the whole of the active deposit can be concentrated on the point. Some hours before this lecture a needle—we will call it No. i—-was so inserted, and by now its point should be coated to its maximum degree of radioactivity with the products of the disintegration of the emanation. After some hours the products all arrive at the state of radioactive equilibrium, in which the quantity is at its maximum for all the products, radium A, radium B, and radium C, as much of each changing as is produced from the emanation. The disintegrations all going on together, the wire should give a-, /8-, and y-rays, the (3- and y-rays being as intense as those given from the half-grain of radium bromide from which the emanation was derived. Now I withdraw No. i needle from the emanation, and with the room darkened we will examine its active deposit. To detect the a-rays we will use a glass translucent screen, thinly coated with phos phorescent zinc sulphide on one side. I bring the point of the needle gradually near the coated side of the screen. As soon as it comes within a distance of three inches the screen lights up, and when the point is only a little distance removed from the screen a most brilliant phosphorescence is produced. Now if I interpose between the wire and the screen a single sheet of paper, the effect llflll 194 EX PERI ENTS WITH THE ACTIVE DEPOSIT practically entirely ceases. The a-radiations pro ducing this effect come both from radium A and from radium C. To detect the /3-rays we will use an ordinary cardboard X-ray screen of barium platinocyanide. Bringing the needle behind the screen, so that the rays have to penetrate the cardboard, you observe the screen lights up as brightly as with half a grain of radium bromide itself. In the dark I happened actually to touch the back of the screen with the active needle-point, and in so doing some of the active deposit has been transferred to the back of the screen. You can see where the back of the screen was touched, because this spot still glows though the needle has been removed. If now the needle is again presented to the back of the X-ray screen with thin pieces of metal foil interposed, you see that the rays are only slightly stopped by having to traverse the foil. When a piece of thick lead sheet is interposed, a faint luminosity on the screen still remains produced by the y-rays. In fact the active needle-point gives all the penetrating rays given by half a grain of radium. It is now several minutes since the needle was removed from the emanation. If we now again examine the a-rays you will notice they already are very perceptibly less intense than at first. EXPERIMENTS WITH THE ACTIVE DEPOSIT 195 Practically all the radium A, of which the period of average life is only 4-3 minutes, has already disintegrated, and in consequence the a-rays now come only from the radium C. Now if, instead of exposing the needle to the emanation for some hours so as to allow all the successive products time to be produced, we expose it to the emanation for a very short time, say for five minutes by the watch, we shall get quite a different set of effects. Here is a new needle, we will call it No. 2. Before putting it in I will test it with the screen to show you that at present it is an ordinary needle, not at all radioactive. We will let it stay in the emanation, connected to the negative pole as before, for five minutes and withdraw it, and test its a-rays immediately, exactly as before. You observe that it is already giving a-rays abundantly. Comparing it with No. I, the two are now very similar in their a-ray-giving power, No. i being only slightly the better. The a-rays from No. 2 come entirely from radium A, for there has not yet been time for any appre ciable quantity of radium C to be formed. The a-rays from No. i come entirely from radium C, and this radiation has not yet had time appre ciably to decay. Let us, however, test their /3-rays. You observe that No. 2 gives no /3-rays worth considering, whereas No, j still gives /3-rays in i96 EXPERIMENTS WITH THE ACTIVE DEPOSIT practically undiminished intensity. Radium A gives no /3-rays, and as there is no appreciable quantity of radium C formed there yet, the con sequence is that No. 2 wire gives no /3-rays. I can show you at this stage a very striking experiment with another needle, No. 3, which has been in the emanation a few minutes. I take it out and draw the point once through a piece of emery - cloth and expose the latter to the zinc sulphide screen. You observe that a single rub has removed a large part of the active deposit from the needle and transferred it to the emery- cloth, so that the latter makes the screen glow almost as brilliantly as the needles themselves. Now we will contrast the decay of the activity of the needles Nos. \ and 2. The activity due to radium A by itself decays very rapidly, half disappearing every three minutes. The con sequence is, if we now again test the a-rays of No. 2, we shall find they have already nearly disappeared, whereas No. i still continues to give a-rays at about the same strength as it did when last examined. In ten minutes the a-rays of No. 2 practically disappear. It is thus not difficult to give you a certain amount of experimental evidence in favour of EXPERIMENTS WITH THE ACTIVE DEPOSIT 197 the conclusion that the first change of the active deposit is a very rapid one in which a-, but no /3-rays are expelled, and that this is followed by a less rapid change in which both a- and /3-rays are expelled. It is more difficult to give you in a lecture satisfactory evidence of the exist ence of radium B, a body not itself giving rays, intermediate between the first and second changes in which rays are expelled. If we examine care fully the decay of the a- and /3-rays of wire No. i, in which at first all these products co-existed in equilibrium, we shall find, as already shown, that for the first half-hour after removal from the emanation the /3-rays suffer hardly any change and then quite definitely the decay begins. In the next half-hour the /3-rays decay to one half their original intensity, and the decay then goes on at this rate regularly and continuously to the end. After two hours they are only a few per cent of what they originally were, and in three or four hours they can no longer be detected. The initial pause before decay begins is due to the quantity of radium C being maintained, in spite of the fact that it is disintegrating all the time, expelling a- and /3-rays, by the disintegra tion of radium B. The latter continues to supply new radium C to replace that disappearing for the first half-hour or so after the needle is re- 198 THE RAYS FROM THE EMANATION moved from the emanation. Exactly the same pause occurs in the decay of the a-rays. As we saw with No. i, within a very few minutes after the needle was removed from the emanation the «-rays had decayed very perceptibly, owing to the dis appearance of the a-ray-giving radium A. Then, however, no further change occurred. It is now about half an hour since No. i was first tested, and the a-activity is about the same as it was when last tested twenty minutes ago. The a-rays of No. 2 have now almost completely disappeared. If we continued to examine No. i, we should find, from now on, a rapid decay of both a- and /3-rays at the same rate, so that at the end of the lecture both will be much enfeebled, and by midnight both will have ceased so far as we could tell. Now that we have finished with the emanation used in the preceding experiments, it is an interesting experiment to show that itself it gives no /3-rays. If we blow the emanation out into a U-tube of thin glass cooled in liquid air, it is condensed in the cold tube. The tube can then be sealed up to prevent the emanation from escaping. The tube contains some phosphorescent zinc sulphide and glows brightly owing to the a-rays from the emanation inside. But if we hold THE LATER SLOW CHANGES OF RADIUM 199 the tube against the X-ray screen, you can see that no penetrating rays come from the tube. The emanation itself gives no /3-rays, only a-rays. By the end of the lecture, however, sufficient radium C will probably have been formed inside the tube to give an appreciable ^-radiation. Owing to the existence of the intermediate body radium B, there occurs a similar pause in the growth of /3-rays from the emanation to that which, as we have seen, occurs in their decay, after the emanation is taken away. But in two or three hours the /3-rays from all the needles will have decayed, and that from the sealed U-tube will have reached a maximum. This finishes this subject and brings us to the next. What happens to radium C when it disinte grates? Is this the real or only the apparent end of the process ? It is, in fact, a very long way from the end. Madame Curie discovered that the rapid and almost complete decay of the active deposit, at the end of a few hours after removal from the emanation, is not in fact quite complete. A very small residual radioactivity remains and per sists for years. The series of changes have now entered on a stage which is as slow as the previous ones were rapid. The next change requires almost as many years as the last required minutes for com pletion. The effect of these further changes is in Ml II 2OO RADIUM D, £, AND F consequence extremely small, but they last a very long time. Continuing our diagram where it last ended at radium C, the next stage is represented in Fig. 30. Radium C. Radium D. Radium Er Radium E2. Radium F. Radium G. (Polonium.) (Lead?) Active deposit of slow change. 30-5 minutes. 17 years (?) 9-5 days. 7 days. 203 days. FIG. 30. The body produced from radium C, radium D, has a period of many years. It is too early yet to state it exactly. One recent estimate makes it seventeen years. No detectable rays are given in its change. /3-rays, however, result from the body produced from it, which changes rapidly again with a period of only a few days. The present view, which is more or less provisional, is that two rayless changes intervene before the /3-rays change, but into this matter it is not necessary to enter. We shall pass over these intermediate changes and con sider the last known change of the series, that of radium F, which has a period of average life of 203 days, in which an a-particle is expelled. Radium F is the polonium of Madame Curie, POLONIUM 201 having been separated by her from pitchblende first before she discovered radium. A digression may here conveniently be made on what is known about polonium, before its connec tion with radium is considered. Chemically it re sembles bismuth, and was separated first from pitchblende in association with the bismuth con tained in the mineral. Its radioactivity, which consists entirely of a-rays, slowly and completely decays, so that a few years after it has been pre pared, the most intensely active preparations of it lose practically all their activity. The work was carried on by Marckwald in Germany, who dis covered new and simple methods of extracting polonium from the mineral and worked up many tons of pitchblende for this substance. His careful chemical investigations of the nature of the body made it clear that it was quite as nearly allied in chemical nature to the element tellurium as to bis muth, and he first proposed the name "radio- tellurium " for it, which, however, with the elucidation of its identity with polonium, has fallen into disuse. He proved that there is far less polonium in the mineral even than radium. In a ton of mineral there is less than a thousandth part of a grain of polonium, but the radioactivity is correspondingly intense, and greatly exceeds, so far as the a-radiation 2O2 POLONIU is concerned, that of pure radium itself. The period of average life, 203 days, is deduced by direct observation from the rate of decay of the radio activity. Returning now to the consideration of radium C, we saw that after its activity had decayed there existed still a residual activity which is very feeble. This steadily increases with time, and consists both of a- and /3-rays, which, however, increase at dif ferent rates. The a-rays are due to polonium, or radium F. These go on increasing for the first two years and then a maximum is reached, the amount of the radium F formed being in equilibrium. The /3-rays. however, reach a maximum much more quickly. The /3-ray product (radium E2) having a much shorter period, equilibrium is reached in a few weeks. If at any time the active matter is subjected to the chemical processes worked out by Marckwald for the separation of polonium, the a-ray body radium F can be separated from the other products, and its activity then decays away completely at exactly the same rate as in the case of polonium. Moreover, it shows the property of being volatile at a temperature of a bright red heat, which is the basis of one of the methods originally used by Madame Curie in separating polonium from the bismuth in pitchblende. This is the WHAT IS THE ULTIMATE. PRODUCT? 203 merest sketch of the evidence in favour of regarding polonium as the last radioactive substance produced in the disintegration of uranium. One more step remains to be discussed, and then this long story of continuous transformation is at an end. What is the ultimate product? When radium F or polonium expels its a-particle, what is produced ? The estimated atomic weight of polonium, 210, agrees well with its chemical nature, for there is a vacant place in the periodic table for an element, the next heavier than bismuth (atomic weight, 2o8'5), and this element would be chemi cally analogous to tellurium. The expulsion of an a-particle would further reduce the atomic weight four units, leaving a residue of atomic weight 206. What is it ? Now, if this is really the final product and not merely a very slowly changing substance, the forma tion of which in proportion to the degree of slow ness of the change would be difficult experimentally to detect, then it follows that the ultimate product must accumulate in quantity indefinitely with time in the minerals containing the elements of the uranium-radium series, and must therefore be a well-known common element. There seems to be only a possible choice of two such elements. Lead has the atomic weight of 207, and bismuth, 208-5. " 'J, 204 LEAD IN RADIUM MINERALS The next known element is thallium (204), and then comes mercury (200). For a long lime it looked as if lead would prove to be the ultimate product. It must be remembered the atomic weight of helium is not very accurately known, and is probably a little below, rather than above, 4. So that the atomic weight of lead, 207, agrees well enough with the estimate 206, obtained by subtracting from the atomic weight of radium 226, the weight of the five a-particles, or helium atoms, known to be expelled. Lead is found in all the common minerals containing uranium in con siderable quantity, and there is some evidence that the older the geological formation from which the mineral is obtained, the greater the percentage of lead present. Recently, however, a uranium mineral, autunite, has been found containing no detectable quantity of lead. So this question is still unsettled. We may rest assured that the patient application of the methods which have already resulted in the brilliant solution of many problems in radioactivity, at first apparently insoluble except by researches extending over im possible periods of time, will cause this last strong- hole! to capitulate. DISINTEGRATION SERIES OF URANIUM 205 Uranium. Uranium X. 7,500,000,000 32 clays, years. Parent of radium. Radium. ? 2,500 years. ^ x->/ Emanation. Radium A. Radium B. Radium C. Active deposit of rapid change. 5-3 days. 4-3 minutes. 38 minutes. 30-5 minutes. Radium D. Radium Et. Radium E2. Radium F. Radium G. (Poloninm.) (Lead?) Active deposit of slow change. 17 years (?) 9-5 days. 7 days. 203 days. FIG. 31. Fig. 31 shows, so far as it is at present known, the complete disintegration series of uranium. 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 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 quota tion from Clerk Maxwell—Evolution of the elements denied— Similarity of all the atoms of the tame 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. FROM the law, which has already been found so useful, we can calculate the ratio of the quantities of radium and polonium that exist together in a mineral from their periods of average life. The period of average life of radium is 4600 times that of polonium, so that there must be 4600 times more radium than polonium in minerals. A good pitchblende contains about an ounce of radium in 150 tons. The same quantity of polonium would therefore be contained in about 700,000 tons. The whole output of the Joachimsthal mine per annum, reckoned as 15 tons, contains about one hundredth 306 THE LA OF PROPORTIONALITY 207 of a grain of polonium. This is borne out by Marckwalcl's experiments, already referred to. Let us apply the law not only to radium and polonium, but to the whole list of known transition- forms existing as products of uranium. In the table this has been done. The first column gives the name of the substance, the second its period of average life, and the third its relative quantity in minerals, the quantity of uranium being considered 1,000,000,000. If these numbers are taken to refer throughout to milligrams (i milligram is about Tlff of a grain), then since 1,000,000,000 milligrams is roughly a ton, the quantities refer to an amount of mineral containing1 one ton of the element uranium. TABI PERIOD. Uranium, 7,500,000,000 years. 2,500 years. 5-3 days. 4/3 minutes. 38 minutes. 30*5 minutes. 17 years. 9'5 days. 7 days. Radium Emanation Radium A Radium B Radium C Radium D Radium El Radium E2 Radium F (Polonium) 203 days. .E. QUANTITY. 1,000,000,000 mg. (=i ton). 333'3 rag- Cine five-hundredth mg. One millionth mg. Nine millionths mg. Seven millionths mg. 2-3 mg. About four thousandths mg. About four thousandths mg. One fourteenth mg. These respective quantities in the last column emit a similar number of a-particles per second in 208 INCREASE OF ACTIVITY the six cases where a-particles are expelled at all, and so produce similar radioactive effects, although only radium of all the products is present in ponder able quantity. This is an illustration of the compen sating principle I spoke of earlier, that the quantity of a radioactive substance divided by its life, not the quantity only, gives a measure of its radioactive effects. It can readily be calculated that the actual amount of radium A used in our experiments, which produced powerful and striking effects on the phosphorescent screen, was much below one ten- millionth of a milligram, or below one thousand- millionth of a grain. For it was derived from 30 mg., i.e. half a grain of radium bromide. Yet while it lasts it conies into evidence through the energy of the a-particles expelled in its rapid disintegration no less than any of the other products. The increase of the radioactivity of radium after it is prepared is due to the steady growth of the products undergoing further disintegration. As we know, when freshly prepared from solution, the activity of radium is due solely to its own disin tegration and consists of a-rays. After four weeks the first four products accumulate to their equi librium, and the activity now consists of a-, /3-, and y-rays, the a-rays being four times as great as initially. It is not difficult to see that the later OF R DIUM WITH AGE 209 slow changes must also cause a very slow further continuous increase of all these types of rays. There is some reason to believe that the change of radium C is double, two /3-particles and one a-particle being expelled in the double change, but this is not yet settled. These considerations are embodied in the following table giving an analysis of the total radioactivity of a radium preparation, kept in a sealed vessel so that none of the products escape, at different periods since preparation :— O-PARTICLES. ^-PARTICLES. O i or 2 I. Freshly prepared, i (due to radium itself) II. After one month. 4 (i due to radium) (i due to emanation) (due to Ra C) (i due to radium A) (i due to radium C) 5 (as in II and i due to radium F) III. After fifty years. 2 or 3 (i due to Ra E2) The idea, which is a necessary consequence of the atomic disintegration theory, that fixed definite relationships must exist between the quantities of elements formed from one another—for example, between uranium, radium, and polonium—forms the first indication that physical laws may exist regulat ing the relative abundance and scarcity of elements in Nature. Gold and platinum, for example, are valuable or rare metals, and we do not know why. Radioactive bodies like radium are rare beca se of 210 A PHYSICAL MEASURE OF RARITY the rapidity with which they are changing. The degree of radioactivity of an element being propor tional to the rate at which it is changing, it follows that radioactive elements are scarce and valuable in proportion to their radioactivity. In this case degree of radioactivity is a physical measure of value or rarity. It is, for example, so far as we can see, an impossibility that an element like radium will ever be found in greater abundance in any minerals than in those already known. Naturally, in the consideration of some of these questions of general interest upon which we are now entering, we are, be it said, in sharp contrast to almost everything we have dealt with in the subject up to now, frankly speculating. But it is helpful and legitimate to speculate upon how far, if at all, the process of atomic disintegration, dis covered for the radio-elements, applies to the case of elements not radioactive, of which there is as yet no positive evidence that they are changing at all. The workers in radioactivity have within their province explored thoroughly the process of atomic disintegration. They have made clear the laws it follows, they have measured the rates at which it occurs, and they have established what may be termed its inevitableness or independence from all known influences. But there is no reason why the THE GOLD CURRENCY 211 process should be limited in its scope to the some what special phenomena which led to its discovery. It is, for example, natural to inquire whether the scarcity of elements like gold is fixed by the opera tion of similar physical laws to those which regulate the rarity of radium. The race has grown used from the earliest times to the idea that gold is a metal possessing a certain fixed degree of value, enabling it to be used safely for the purposes of currency and exchange. It is no exaggeration to say that the whole social machinery of the Western world would be dislocated if gold altered violently in its degree of rarity—if, for example, in some hitherto unpenetrated fastness of the globe a moun tain of gold came to be discovered. Is there not at least a strong presumption that this is really as contrary to the operation of natural law as the dis covery of a mountain of pure radium would be ? It may, I think, be taken for granted that an element changing more rapidly than uranium, for example,—that is with a period of average life of less than 7,500,000,000 years—is not likely to be much more plentiful in Nature than uranium, and therefore that all the common elements—lead, copper, iron, oxygen, silicon, etc. etc.—have periods of average life of many thousands of millions of years. So far, the traditional view 2i2 RELATIVE ABUNDANCE OF ELEMENTS THE NATURE OF ATOMS 213 that the elements are permanent and unchanging is substantially correct. At the same time, we cannot but recognise that inevitably the effects of atomic disintegration, too slow to be other wise detectable, would result in the accumulation of the more stable and longest-lived elements at the expense of the others, resulting- in some sort of equilibrium in which the relative abundance of the elements was proportional to their respective periods of average life. For example, the ratio between the relative abundance of gold and silver is roughly but pretty certainly known, owing to these metals being- employed for currency purposes from the earliest times. It is at least a possible view to take that the elements gold and silver belong to the same disintegration series, both changing very slowly, but the gold many times more rapidly than the silver. Obviously we are only at the beginning. But already it cannot be gainsaid that the interest and importance of this process of atomic disintegration is not confined to radioactivity only or even to physical science. It extends into almost every region of thought. I am convinced that the general nature of the reasoning has only to be divested of technicalities and made clear to workers in other fields, for new evidence to flow in from the most widely separated sciences, from archaeology and palaeontology to political economy. .- This is the chief reason, and must be my excuse for being so bold as to attempt to bring before a general audience so much of the, in one sense, highly technical discoveries of radioactivity. It would be a pity if mere technicalities and strange words barred the progress of ideas and their application to everyday thought. Otherwise the scientific discoverer would, so far as his own times were concerned, labour half in vain, and worse, the accurate and complete application of his work to other fields of thought would be delayed by erro neous and partial ideas, mere half-truths, springing up in their place. I now propose considering briefly another ques tion of general philosophical interest in connection with the recent advances of physical science. Naturally the discoveries in radioactivity have not been made without influencing considerably our ideas on the ultimate nature of atoms. In some points older conceptions have had to be modified, while in others these conceptions have been strangely confirmed. It has always been a matter for remark, considering the myriads ot individual atoms which go to make up the smallest perceptible quantity of matter, that there are so few different kinds. The number of atoms which go to make up this world, for example, would run into at least /HI 214 SIMILARITY OF ATOMS fifty-four figures, yet among them all there are less than a hundred different varieties. Moreover, it has come to be regarded as one of the greatest philosophical generalisations of physical science that all the atoms of one kind, that is to say of one element, are exactly and completely similar in character. There is, for example, not the shadow of distinction between gold found in the Klondyke, in Australia, or in S. Africa. Not only so, but we have learned from the spectroscope that this similarity of nature extends throughout the whole universe. In this connection, both to set forth the idea and to illustrate the deductions which have been drawn from it, I cannot do better than to quote a celebrated utterance of Clerk Maxwell to the British Association in 1873. I may remark that Clerk Maxwell throughout used the word molecule in the sense of "atom" as this word is employed by the chemist, and throughout these lectures. "In the heavens we discover by their light, and by their light alone, stars so far distant from each other that no material thing can ever have passed from one to another ; and yet this light, which is to us the sole evidence of the existence of these distant worlds, tells us also that each of them is built up of molecules of the same kinds as those which we find on earth. A molecule of hydrogen, A QUOTATION FRO CLERK MAXWELL 215 for example, whether in Sirius or in Arcturus, executes its vibrations in precisely the same time. " Each molecule therefore throughout the uni verse bears impressed upon it the stamp of a metric system as distinctly as does the metre of the Archives at Paris, or the double royal cubit of the temple of Karnac. " No theory of evolution can be formed to account for the similarity of molecules, for evolu tion necessarily implies continuous change, and the molecule is incapable of growth or decay, of generation or destruction. " None of the processes of Nature, since the time when Nature began, have produced the slightest difference in the properties of any mole cule. We are therefore unable to ascribe either the existence of the molecules or the identity of their properties to any of the causes which we call natural. " On the other hand, the exact equality of each molecule to all the others of the same kind gives it, as Sir John Herschel has well said, the essential character of a manufactured article, and precludes the idea of its being eternal and self-existent. "Thus we have been led, along a strictly scien tific path, very near to the point at which science must stop ; not that science is debarred from study ing the internal mechanism of a molecule which 216 FOUNDATION-STONES OF THE UNIVERSE she cannot take to pieces, any more than from investigating an organism which she cannot put together. But in tracing back the history of matter, Science is arrested when she assures herself, on the one hand, that the molecule has been made, and on the other, that it has not been made by any of the processes we call natural. " Science is incompetent to reason upon the creation of matter itself out of nothing. We have reached the utmost limits of our thinking faculties when we have admitted that because matter cannot be eternal and self-existent it must have been created." You will admit that, in the light of all that has transpired in the thirty-five years since Maxwell used these words, science has advanced far. The concluding words of the address are even more striking from this point of view. " Natural causes, as we know, are at work, which tend to modify, if they do not at length destroy, all the arrangements and dimensions of the earth and the whole solar system. But though in the course of ages catastrophes have occurred and may yet occur in the heavens, though ancient systems may be dissolved and new svstems evolved out of their j ruins, the molecules out of which these systems are built—the foundation-stones of the material universe •—remain unbroken and unworn." COMPLEXITY OF ATOMS 217 Before we dwell upon the modifications that have been made in this point of view, let us rather con sider the chief basis of the argument, namely, that all the atoms of any one element are exactly alike. On this fundamental question the evidence to-day is far more complete and striking than it was in 1873, and we believe more firmly than ever in the absolute similarity of all the atoms of the same element. We no longer regard the atom as a simple thing. On the contrary, we now look upon it as an almost infinitely complex piece of mechanism. The late Professor Rowland, of Baltimore, once made the remark that a grand piano must be a very simple piece of mechanism compared with an atom of iron. For in the spectrum of iron there is an almost innumerable wealth of separate bright lines, each one of which corresponds to a sharp definite period of vibration of the iron atom. Instead of the hundred-odd sound vibrations which a grand piano can emit, the single iron atom appears to emit many thousands of definite light vibrations. Two pianos would be regarded as in perfect tune together when there was a comparatively rough approximation of period between the various notes. Whereas by the spectroscope a difference in " tune " or period in the vibrations emitted by different atoms of only one part in many millions would be 2l8 VELOCITIES OF THE o.-1'ARTICLES easily detectable, and no such variation exists. In a similar vein Professor Schuster, referring to the broad teachings of the spectroscope, has compared the atoms of the same element to an innumerable number of clocks all wound and regulated to go at the same period. If all these clocks were set at the same time, not one of them would vary by a single second even after many days. No clockmaker could make such clocks. Yet these almost infinitely complicated pieces of mechanism we call atoms are turned out by Nature with such undeviating accu racy and fidelity that in all the myriads in existence there are less than a hundred different kinds known. We can, however, from the point of view of recent researches in radioactivity, push this idea even one step further, to the case of atoms actually in the condition of breaking up. We have seen that it is a property of the a-rays to possess a very sharp and definite range. In a beam of homogeneous a-rays passing through a homogeneous absorbing medium the number of a-particles suffers no diminution until the extreme end of the path is reached, and then they cease altogether. Just without the extreme range, there is absolutely no effect perceptible, while just within VELOCITIES OF TtiE v.-PARTICLES 219 this range, the effect, per small element of path, is at the maximum. Every a-particle expelled from the radio-element in the same change travels exactly the same distance before it ceases to be detectable, and, as Rutherford has shown by direct measurement of the magnetic and electric devia tion, is expelled at the same velocity. In the table following, the approximate initial velocities of the a-particles from the changes in the uranium series have been collected. VELOCITY a-PARTiCLE FROM (miles per second). Uranium ... ... .. ... 9,600 Parent of radium ... ... ... 8,800 Radium ... ... ... ... 9,600 Emanation ... ... ... ... 10,400 Radium A ... ... ... ... 11,000 Radium C ... ... ... ... 12,800 Radium F ... ... ... ... 10,000 The atom thus retains its role of a perfect piece of mechanism even up to and during the moment of its dissolution. So exactly alike are all the atoms of the same radioactive element, that when the break-up occurs the velocity with which the fragments of the atom, or a-particles, are ex pelled is exactly the same in each case. We may liken the disintegration of an element to the burst ing of shells, in which the fragments of the different shells all are expelled with the same velocity. 220 MATERIAL EVOLUTION Certainly no shells ever constructed would answer this requirement. Truly, in the words of Sir John Herschel, the atom bears the essential character of a manufactured article, but of a degree of per fection humanly unattainable. But with regard to the process of manufacture and of the cause of this undeviating fidelity to a few types, what a revolution of thought has taken place in the last few years! The evolution, or rather devolution, of matter, its continuous change, the generation and destruction of atoms—all of the things which seemed impossible in Clerk Max well's day—we know to be going on before our eyes. It is true the processes call for periods of time so vast, even in the most favourable cases, that the physicist of a generation ago would have dismissed them as physically inconceivable. Yet these periods are to-day actually determined by direct measurement in the laboratory. Instead of regarding the hundred or less ele ments which exist to-day as manufactured, created, once for all time, we rather regard them as existing because they have survived. All other forms less stable than those we recognise as elements have been weeded out. Over sufficiently great periods of time the rarity or abundance of an element must be controlled by its degree of MATERIAL EVOLUTION 221 instability or stability. Probably for every stable atom many unstable ones could be, even are being, formed. But only the stable forms can accumu late in quantity and become known to us as ordinary chemical elements. We have seen that the rarest of such in all probability must have a period of thousands of millions of years, while for the more common elements, if they are chang ing at all, periods of billions of years may be anticipated. At first glance only, the material universe gives the impression of a permanent and finished creation. In reality the now familiar remorseless operation of slow, continuous change moulds even " the foundation-stones " themselves. By this last step the doctrine of evolution has become universal, embracing alike the animate and inanimate worlds. But whereas in the former slight changes of environment effect the profoundest modifications, in the latter the controlling factors still remain absolutely unknown. By the spectroscope a partial material survey of the whole universe has been rendered possible, and what we find is every where an essential similarity of composition. For example, there is no evidence that in the sun or stars large quantities of elements unknown to us exist. The reason why some atoms are stable and others are not is a mystery we have 222 MATERIAL EVOLUTION not yet begun to probe. Yet this question, to us only of academic interest and possibly some what remote at that, will, as we shall soon come to see, be one of life and death to the inheritors of our civilisation. 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 disintegi'ation 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 tempera ture of the moon and planets—Ancient mythology and radio activity—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. THIS interpretation of radium is drawing to a close, but perhaps the more generally interesting part of it remains to be dealt with. We have steadily followed out the idea of atomic disintegration to its logical conclusions, so far as they can at present be drawn, and we have found it able to account for all the surprising discoveries that have been made in radioactivity, and capable of predicting many, and perhaps even 223 . 224 RADIUM AND more unexpected, new ones. Let us from the point of vantage we have gained return to the starting - point of our inquiries and see what a profound change has come over it since the riddle has been read. Radium, a new element, giving out light and heat like Aladdin's lamp, apparently defying the law of the conservation of energy, and raising questions in physical science which seemed unanswerable, is no longer the radium we know. But although its mystery has vanished, its significance and importance have vastly gained. At first we were compelled to regard it as unique, dowered with potentialities and exhibiting pecu liarities which raised it far above the ordinary run of common matter. The matter was the mere vehicle of ultra-mate rial powers. If we now ask, why is radium so unique among the elements, the answer is not because it is dowered with any ex ceptional potentialities or because it contains any abnormal store of internal energy which other elements do not possess, but simply and solely because it is changing comparatively rapidly, whereas the elements before known are either changing not at all or so slowly that the change has been unperceived. At first sight this might seem an anti-climax. Yet it is not so. The truer view is that this one element has clothed with its own dignity the whole empire of common ORDINARY ELEMENTS CONTRASTED 225 matter. The aspect which matter has presented to us in the past is but a consummate disguise, con cealing latent energies and hidden activities beneath an hitherto impenetrable mask. The ultra-material potentialities of radium are the common possession of all that world to which in our ignorance we used to refer as mere inanimate matter. This is the weightiest lesson the existence of radium has taught us, and it remains to consider the easy but remorseless reasoning by which the conclusion, is arrived at. Two considerations will make the matter clear. In the first place, the radioactivity of radium at any moment is, strictly speaking, not a property of the mass of the radium at all, although it is proportional to the mass. The whole of the new set of properties is contributed by a very small fraction of the whole, namely, the part which is actually disintegrating at the moment of observa tion. The whole of the rest of the radium is as quiescent and inactive as any other non-radioactive element. In its whole chemical nature it is an ordinary element. The new properties are not contributed at all by the main part of the matter, but only by the minute fraction actually at the moment disintegrating. I .Ill III1" 226 THE ENERGY IN URANIUM Let us next compare and contrast radium with its first product, the emanation, and with its original parent, uranium. Uranium on the one hand, and the emanation on the other, represent, compared with radium, diametrically opposed extremes. Uranium is changing so slowly that it will last for thousands of millions of years, the emanation so rapidly that it lasts only a few weeks, while radium is intermediate with a period of average life of two thousand five hundred years. We have seen that in many ways the emanation is far more wonderful than radium, as the rate its energy is given out is relatively far greater. But this is compensated for by the far shorter time its activity lasts. Also, if we compared uranium with radium, we should say at once that radium is far more wonderful than the uranium, whereas in reality it is not so, as the uranium, changing almost infinitely more slowly, lasts almost infinitely longer. The arresting character of radium is to be ascribed solely to the rate at which it happens to be disintegrating. The common element uranium, well known to chemists for a century before its radioactivity was suspected, is in reality even more wonderful. It is only very feebly radioactive, and therefore is changing excessively slowly, but it changes, we believe, into radium, THE ENERGY IN URANIUM 227 expelling several a-particles and so evolving large amounts of energy in the process. Uranium is a heavier element than radium, and the relative weights of the two atoms, which is a measure of their complexity, is as 238 is to 226. This bottle contains about a pound of an oxide of uranium which contains about seven-eighths of its weight of the element uranium. In the course of the next few thousand million years, so far as we can tell, it will change, producing over thirteen ounces of radium, and, in that change into radium alone, energy is given out, as radioactive energy, aggre gating of itself an enormous total, while the radium produced will also change, giving out a further enormous aggregate quantity of energy. So that uranium, since it produces radium, con tains all the energy contained in a but slightly smaller quantity of radium and more. It may be estimated that uranium evolves during complete disintegration at least some fourteen per cent more energy than is evolved from the same weight of radium. But what are we to say about the other heavy elements — lead, bismuth, mercury, gold, platinum, etc.—although their atoms are not quite so heavy as uranium or radium, and although none of them, so far as we yet know, are disintegrating at all ? Is this enormous internal store of energy confined to the radioactive elements, that is to 228 THE INTERNAL ENERGY OF ALL MATTER the few which, however slowly, are actually chang ing-? Not - at all, in all probability. Regarded merely as chemical elements between radioactive elements and non-radioactive elements, there exists so complete a parallelism that we cannot regard the radioactive elements as peculiar in possessing this internal store of energy, but only as peculiar in evolving it at a perceptible rate. Radium especially is so completely analogous in its whole chemical nature, and even in the character of its spectrum, to the non-radioactive elements, barium, strontium, and calcium, that chemists at once placed radium in the same family as these latter, and the value of its atomic weight confirms the arrangement in the manner required by the Periodic Law. It appears rather that this in ternal store of energy we learned of for the first time in connection with radium is possessed to greater or lesser degree by all elements in common and is part and parcel of their internal structure. Let us, however, for the sake of conciseness, leave out of account altogether the non-radioactive elements, of which as yet we know nothing cer tainly. At least we cannot escape from the conclusion that the particular element uranium has relatively more energy stored up within it USEFUL AND USELESS ENERGY 229 even than radium. Uranium is a comparatively common element. The mines of Cornwall last year produced, I believe, over ten tons of uranium. I have already referred to the total amount of energy evolved by radium during the course of its complete change. It is about two million times as much energy as is evolved from the same weight of coal in burning. The energy evolved from uranium would be some fourteen per cent greater than from the same weight of radium. This bottle contains about one pound of uranium oxide, and therefore about fourteen ounces of uranium. Its value is about £i. Is it not wonderful to reflect that in this little bottle there lies asleep and waiting to be evolved the energy of about nine hundred tons of coal ? The energy in a ton of uranium would be sufficient to light London for a year. The store of energy in uranium would be worth a thousand times as much as the uranium itself, if only it were under our control and could be harnessed to do the world's work in the same way as the stored energy in coal has been harnessed and controlled. There is, it is true, plenty of energy in the world which is practically valueless. The energy of the tides and of the waste heat from steam fall into this category as useless and low-grade energy. But the internal energy of uranium is .not of this llll I 230 THE PROBLEM Of TRANSMUTATION kind. The difficulty is of quite another character. As we have seen, we cannot yet artificially ac celerate or influence the rate of disintegration of an element, and therefore the energy in uranium, which requires a thousand million years to be evolved, is practically valueless. On the other hand, to increase the natural rate, and to break down uranium or any other element artificially, is simply transmutation. If we could accomplish the one so we could the other. These two great problems, at once the oldest and the newest in science, are one. Transmutation of the elements carries with it the power to unlock the internal energy of matter, and the unlocking of the internal stores of energy in matter would, strangely enough, be infinitely the most important and valuable con sequence of transmutation. Let us consider in the light of present knowledge the problem of transmutation, and see what the attempt of the alchemist involved. To build up an ounce of a heavy element like gold from a lighter element like silver would require in all probability the expenditure of the energy of some hundreds of tons of coal, so that the ounce of gold would be dearly bought. On the other hand, if it were possible artificially to disintegrate an element with a heavier atom than gold and produce gold from it, THE PROBLEM OF TRANSMUTATION 231 so great an amount of energy would probably be evolved that the gold in comparison would be of little account. The energy would be far more valuable than the gold. Although we were as ignorant as ever of how to set about transmutation, it could not be denied that the knowledge recently gained constituted a very great help towards a proper understanding of the problem and its ulti mate accomplishment. We saw clearlythe magnitude of the task and the insufficiency of even the most powerful of the forces at our disposal in a way not before appreciated, and we had now a clear percep tion of the tremendous issues at stake. Looking backwards at the great things science had already accomplished, and at the steady growth in power and fruitfulness of scientific method, it could scarcely be doubted that one day we should come to break clown and build up elements in the laboratory as we now break clown and build up compounds, and the pulses of the world would then throb with a new force, of a strength as immeasurably removed from any we at present control as they in turn are from the natural resources of the human savage. It is, indeed, a strange situation we are confronted with. The first step in the long, upward journey out of barbarism to civilisation which man has ac complished appears to have been the art of kindling THE ART OF KINDLING FIRE fire. Those savage races who remained ignorant of this art are regarded as on the very lowest plane. The art of kindling fire is the first step towards the control and utilisation of those natural stores of energy on which civilisation even now absolutely depends. Primitive man existed entirely on the day-to-day supply of sunlight for his vital energy, before he learned how to kindle fire for himself. One can imagine before this occurred that he became acquainted with fire and its properties from naturally occurring conflagrations. With reference to the newly recognised internal stores of energy in matter we stand to-day where primitive man first stood with regard to the energy liberated by fire. We are aware of its existence solely from the naturally occurring manifestations in radioactivity. At the climax of that civilisation the first step of which was taken in forgotten ages by primitive man, and just when it is becoming apparent that its ever-increasing needs cannot in definitely be borne by the existing supplies of energy, possibilities of an entirely new material civilisation are dawning with respect to which we find ourselves still on the lowest plane—that of on lookers with no power to interfere. The energy which we require for our very existence, and which Nature supplies us with but grudgingly and in none too generous measure for our needs, is in KEY TO THE NEW STORES OF ENERGY 233 reality locked up in immense stores in the matter all around us, but the power to control and use it is not yet ours. What sources of energy we can and do use and control, we now regard as but the merest leavings of Nature's primary supplies. The very existence of the latter till now have remained unknown and unsuspected. When we have learned how to transmute the elements at will the one into the other, then, and not till then, will the key to this hidden treasure-house of Nature be in our hands. At present we have no hint of how even to begin the quest. The question has frequently been discussed whether transmutation, so impossible to us, is not actually going on under the transcendental condi tions obtaining in the sun and the stars. We have seen it is actually going on in the world under our eyes in a few special cases and at a very slow rate. The possibility now under consideration, however, is rather that it may be going on universally or at least much more generally, and at much more rapid rate under celestial than under terrestrial conditions. From the new point of view it may be said at once that if it were so, many of the difficulties previously experienced in accounting for the enormous and in cessant dissipation of energy throughout the uni verse would disappear. 234 WORLD-CREATING INFLUENCES Last century had wrought a great change in scientific thought as to the nature of the gigantic forces which had moulded the world to its present form and which regulated the march of events throughout the universe. At one time it was cus tomary to regard the evolution of the globe as the result of a succession in the past times of mighty cataclysms and catastrophes beside which the erup tions of a Krakatoa or Pelee would be insignificant. Now, however, we regard the main process of moulding as due rather to ever-present, continuous, and irresistible actions, which, though operating so slowly that over short periods of time their effect is imperceptible, yet in the epochs of the cosmical calendar effected changes so great and complete that the present features of the globe were but a passing incident of a continually shifting scene. Into the arena of these silent world-creating and destroying influences and processes had entered a new-comer—" Radioactivity "—and it did not re quire long before it came to be recognised that in the discovery of radioactivity, or rather of the sub atomic powers and processes of which radioactivity was merely the outward and visible manifestation, we had penetrated one of Nature's innermost secrets. Whether or no the processes of continuous atomic disintegration bulked largely in the scheme ULTIMATE SOURCE OF COSMICAL ENERGY 235 of cosmical evolution, at least it could not be gain said that these processes were at once powerful enough and slow enough to furnish a sufficient and satisfactory explanation of the origin of those perennial outpourings of energy by virtue of which the universe to-day is a going concern rather than a cold, lifeless collocation of extinct worlds. Slow, irresistible, incessant, unalterable, so apparently feeble that it has been reserved to the genera- o tion in which we live to discover, the processes of radioactivity, when translated in terms of a more extended scale of space and time, appear already as though they well may be the ultimate controlling factors of physical evolution. For slow processes of this kind do the effective work of Nature, and the occasional intermittent displays of Plutonic activity correspond merely to the creaking now and again of an otherwise silent mechanism that never stops. It was one of the most pleasing features of this new work that geologists have been among the very first to recognise the applicability and importance of it in their science. I am not competent to deal adequately with or discuss the geological problems that it has raised. But this story would be incom plete if I did not refer, though it must be but briefly, to the labours of Professor Strutt who 236 GEOLOGICAL IMPORTANCE OF RADIOACTIVITY 337 u initiated the movement and those of Professor Joly who has carried it on. These workers carried out careful analyses of the representative rocks in the earth's crust for the amount of radium they con tained. Absolutely, the quantity of radium in com mon rocks is of course very small, although with the refined methods now at the djsposal of investigators it is quite measurable. The important fact which has transpired, however, is that the rocks examined contain on the average much larger quantities of radium, and therefore necessarily of its original parent uranium, than might be expected. The amount of heat which finds its way in a given time from the interior of the globe to the surface and thence outwards into external space has long been accurately known. Strutt concluded that if there existed only a comparatively thin crust of rocks less than fifty miles thick of the same composition, as regards the content of radium, as the average of those he examined, the radium in them would supply the whole of the heat lost by the globe to outer space. He concluded that the surface rocks must form such a thin crust, and that the interior of the globe must be an entirely different kind of material, free from the presence of radium. Other wise the world would be much hotter inside than is known to be the case. So far then as the earth is concerned, a quantity of radium less than in all probability actually exists would supply all the heat lost to outer space. So that there is no difficulty in- accounting for the necessary source of heat to main tain the existing conditions of temperature on the earth over a period of past time as long as the uranium which produces the radium lasts, that is to say, for a period of thousands of millions of years. Professor Joly in his Presidential Address to the Geology section of the British Association at Dublin this year has considered in detail the effect of the radium in the rocks of the Simplon Tunnel in producing the unexpectedly high temperatures there encountered, and has come to the conclusion that without undue assumptions it is possible to explain the differences in the temperature of the rocks by the differences in their radium content. He went on to propound a new theory of mountain formation on the lines that local concen trations of radium, brought about by sedimenta tion, cause local increases of temperature in the earth's crust. At these places the strength of the crust to stress is weakened, conditioning its up heaval and folding and even over-thrusting for many miles, with the foundation of mountain ranges. The rhythmic succession of periods of sedimentation followed by upheaval many times repeated is the common theory of mountain forma tion. In the concentration of radium in the 238 THE TEMPERATURE OF THE PLANETS sedimentary deposit Joly finds a sufficient explana tion of and cause for the next subsequent upheaval. Leaving this globe and taking a survey of the solar system, it has always struck me as remarkable that the temperature of the constituent worlds so far as we know them seems to be roughly in pro portion to their size. The moon we regard as quite cold. The Earth and Mars have similar tempera tures, while Jupiter and Saturn are probably nearly red-hot. Of course this agrees well enough with the old idea that these bodies were steadily cooling, the process being the slower the greater the mass. But it agrees also with the newer idea that the temperature is probably more or less constant, as the result of an equilibrium in which the heat lost by radiation is counterbalanced by new internal sources of heat provided by slow atomic disintegra tions, With regard to the sun itself, it is certain that the loss of heat cannot be supplied by the presence of radium. For this to be the case a very large part of the sun's mass must consist of uranium, and this we know from the spectroscope is very improbable. Still it is by no means to be concluded that the heat of the sun and stars is not in the first place of internal rather than, as had been the custom to regard it, of external origin. Obviously we were only at the beginning of our knowledge of the THE EXTENSION OF THE LIMITS OF TIME 239 internal stores of energy in matter, and the mere fact that these stores existed, and in a few actual cases within our knowledge were slowly evolved and became available for the purposes of cosmical evolution, justified us in regarding them as the probable, as they were certainly the sufficient, first source from which the available energy of all Nature was derived. There was one other sphere in which these dis coveries touched human life strangely into which I cannot forbear altogether from entering, although I am all unfitted to act as guide. Radioactivity has accustomed us in the laboratory to the matter- of-fact investigation of processes which require for their completion thousands of millions of years. In one sense the existence of such processes may be said largely to have annihilated time. That is to say, at one bound the limits of the possible extent of past and future time have been enormously extended. We are no longer merely the dying inhabitants of a world itself slowly dying, for the world, as we have seen, has in itself, in the internal energy of its own material constituents, the means, if not the ability, to rejuvenate itself perennially. It is, of course, true, pan present existing knovoledge, that the extent of the possible duration of time is merely increased, and that on the new scale exactly J 24o THE EXTENSION OF THE LIMITS OF TI E the same principles apply as before. Yet the in crease is so extensive that it practically constitutes a reversal of the older views. At the same time, it will be admitted that physical science can no longer, as at one time she felt justified in doing, impose a definite limit to the continuance of the existing conditions of things. The idea that evolu tion is proceeding in continuous cycles, without beginning and without end, in which the waste energy of one part of the cycle is transformed in another part of the cycle back into available forms, is at least as possible and conceivable in the present state of knowledge as the older idea, which was based on a too wide application of those laws of the availability of energy we have found to hold within our own experience. It remains for the future to decide whether what happens to be at present our sole experience of the laws of energy does apply, as has hitherto been quite definitely assumed, to the universe as a whole, and to all the conditions therein within which it is impossible for us to per form our experiments. This reservation is one legitimate consequence of the recent ideas, for we have learnt from them how easy it is to give to the generalisations of physical science a universal appli cation they do not in fact possess. If, then, the world is no longer slowly dying from exhaustion, but bears within itself its own OLD LEGENDS RECALLED 741 means of regeneration, so that it may continue to exist in much the same physical condition as at present for thousands of millions of years, what about Man ? The revelations of radio activity have removed the physical difficulties connected with the sufficiency of the supply of natural energy, which previously had been supposed to limit the duration of man's existence on this planet, but it adds of itself nothing new to our knowledge as to whether man has shared with the world its more remote history. Here again it is interesting and harmless to indulge in a little specu lation, and I may mention one rather striking- point. It is curious how strangely some of the old myths and legends about matter and man appear in the light of the recent knowledge. Consider, for example, the ancient mystic symbol of matter, known as Ouroboros—" the tail devourer "—which was a serpent, coiled into a circle with the head devouring the tail, and bearing the central motto " The whole is one." This symbolises evolution, moreover it is evolution in cycle—the latest possi bility—and stranger still it is evolution of matter— again the very latest aspect of evolution—the existence of which was strenuously denied by Clerk: Maxwell and others of only last century. The idea which arises in one's mind as the most attrac- I 242 THE PHILOSOPHERS STONE live and consistent explanation of the universe in light of present knowledge, is perhaps that matter is breaking down and its energy being evolved and degraded in one part of a cycle of evolution, and in another part still unknown to us, the matter is being again built up with the utilisation of the waste energy. The consequence would be that, in spite of the incessant changes, an equilibrium con dition would result, and continue indefinitely. If one wished to symbolise such an idea, in what better way could it be done than by the ancient tail- devouring serpent ? Some of the beliefs and legends which have come down to us from antiquity are so universal arid deep-rooted that we are accustomed to consider them almost as old as the race itself. One is tempted to inquire how far the unsuspected apt ness of some of these beliefs and sayings to the point of view so recently disclosed is the result of mere chance or coincidence, and how far it may be evidence of a wholly unknown and unsuspected ancient civilisation of which all other relic has dis appeared. It was curious to reflect, for example, upon the remarkable legend of the philosopher's stone, one of the oldest and most universal beliefs, the origin of which, however far back we penetrate into the records of the past, we do not seem to be able to trace to its source. The philosopher's stone THE ELIXIR OF LIFE 243 was accredited the power not only of transmuting the metals, but of acting as the elixir of life. Now, whatever the origin of this apparently meaningless jumble of ideas may have been, it is really a perfect and but very slightly allegorical expression of the actual present views we hold to-day. It does not require much effort of the imagination to see in energy the life of the physical universe, and the key to the primary fountains of the physical life of the universe to-day is known to be transmuta tion. Was then this old association of the power of transmutation with the elixir of life merely a coincidence ? I prefer to believe it may be an echo from one of many previous epochs in the unre corded history of the world, of an age of men which have trod before the road we are treading to-day, in a past possibly so remote that even the very atoms of its civilisation literally have had time to disintegrate. Let us give the imagination a moment's further free scope in this direction, however, before closing. What if this point of view that has now suggested itself is true, and we may trust ourselves to the slender foundation afforded by the traditions and superstitions which have been handed down to us from a prehistoric time ? Can we not read into them some justification for the belief that some former forgotten race of men attained not only to 244 THE FALL AND ASCENT OF MAN the knowledge we have so recently won, but also to the power that is not yet ours ? Science has reconstructed the story of the past as one of a con tinuous Ascent of Man to the present-day level of his powers. In face of the circumstantial evidence existing- of this steady upward progress of the race, the traditional view of the Fall of Man from a higher former state has come to be more and more difficult to understand. From our new standpoint the two points of view are by no means so irrecon cilable as they appeared. A race which could transmute matter would have little need to earn its bread by the sweat of its brow. If we can judge from what our engineers accomplish with their comparatively restricted supplies of energy, such a race could transform a desert continent, thaw the frozen poles, and make the whole world one smiling Garden of Eden. Possibly they could explore the outer realms of space, emigrating to more favour able worlds as the superfluous to-day emigrate to more favourable continents. One can see also that such dominance may well have been short-lived. By a single mistake, the relative positions of Nature and man as servant and master would, as now, become reversed, but with infinitely more disastrous consequences, so that even the whole world might be plunged back again under the undisputed sway of Nature, to begin once more its MAN AND HIS ENVIRONMENT 245 upward toilsome journey through the ages. The legend of the Fall of Man possibly may indeed be the story of such a past calamity. I cannot fittingly conclude this series of lectures without, however inadequately, directing attention to one further outstanding feature of general interest, which this interpretation of radium will in the course of time bring home to all thoughtful minds. The vistas of new thought which had opened out in all directions in the physical sciences, to which man was merely incidental and external, in turn reacted powerfully upon those departments of thought in which man was central and supreme. I am aware that in this field, concerned with the most profound of all questions — the relation of man to his external environment — it has lately been the custom for the physicist not to intrude. This phase of opinion is perhaps somewhat of the nature of a reaction from the other extreme of an earlier generation, in which science arrogated to itself the right to pronounce the final judgment upon the questions in dispute. At least it will be admitted that if the progress of physical science completely transforms, as it has recently so transformed, our notions of the outer world in which we live, its claim to be heard upon the relations of this world to its 246 THE NEW PROSPECT THE PROSPECT 24? inhabitants cannot be resisted. Another reason why perhaps the physicist has hesitated to encroach too directly upon the eternal problems of life has been that he could contribute little of hope or com fort for the race from his philosophy. In the past his conclusions concerning physical evolution and destiny have intensified rather than lightened the existing gloom. To what purpose is the incessant upward struggle of civilisation which history and the biological sciences has made us aware of, if its arena is a slowly dying world, destined to carry ultimately all it bears to one inevitable doom? At least this reason for silence no longer exists. We find ourselves in consequence of the progress of physical science at the pinnacle of one ascent of civilisation, taking the first step upwards out on to the lowest plane of the next. Above us still rises indefinitely the ascent to physical power—far be yond the dreams of mortals in any previous system of philosophy. These possibilities of a newer order of things, of a more exalted material destiny than any which have been foretold, are not the promise of another world. They exist in this, to be fought and struggled for in the old familiar way, to be wrung from the grip of Nature, as all our achieve ments and civilisation have, in the past, been wrung by the labour of the collective brain of mankind guiding, directing, and multiplying the individual's J puny power. This is the message of hope and in spiration to the race which radium has contributed to the great problems of existence. No attempt at presentation of this new subject could be considered complete which did not, however imperfectly, suggest something of this side. Released as physical science now is from the feel ing of hopelessness in dealing with such matters, and at the same time in possession of vast general isations concerning matter and energy of more than mere abstract significance to the race, it is fitting to attempt to see how far purely physical considerations will take us in delimiting the major controlling influ ences which regulate our existence. It is possible, without breaking any of the new ground, to go a long way. Just as you must feed a child at school before it can be educated, as you must provide a man with the possibility of some thing more than a brute struggle for life before he can be civilised, so generally in the same sense the physical conditions which encircle existence of necessity take precedence over every other con sideration. Whatever other aspect of life is con sidered, and they are many and as yet but little dealt with by science for the most part, the physical aspect comes first, in the sense that if the physical conditions of life are unfavourable, nothing can be expected of any higher aspect. 248 THE, NE PROSPECT Surveying the long chequered, but on the whole continuous, ascent of man from primeval conditions to the summit of his present-day powers, what has it all been at bottom but a fight with Nature for energy—for that ordinary physical energy of which we have said so much ? Physical science sums up accurately in that one generalisation the most funda mental aspect of life in the sense already defined. Of course life depends also on a continual supply of matter as well as on a continual supply of energy, but the struggle for physical energy is probably the more fundamental and general aspect of existence in all its forms. The same matter, the same chemical elements, serve the purposes of life over and over again, but the supply of fresh energy must be con tinuous. By the law of the availability of energy, which, whether universal or not, applies universally within our own experience, the transformations of energy which occur in Nature are invariably in the one direction, the more available forms pass ing into the waste and useless unavailable kind, and this process, so far as we yet know, is never reversed. The same energy is available but once. The struggle for existence is at the bottom a continuous struggle for fresh physical energy. This is as far as the knowledge available last century went. What is now the case ? The ab original savage, ignorant of agriculture and of the THE NE PROSPECT 249 means of kindling fire, perished from cold and hunger unless he subsisted as a beast of prey and succeeded in plundering and devouring other animals. Although the potentialities of warmth and food existed all round him, and must have been known to him from natural processes, he knew not yet how to use them for his own purposes. It is much the same to-day. With all our civilisation, we still subsist, struggling among ourselves for a sufficiency of the limited supply of physical energy available, while all around are vast potentialities of the means of sustenance, we know of from naturally occurring processes, but do not yet know how to use or control. Radium has taught us that there is no limit to the amount of energy in the world available to support life, save only the limit imposed by the boundaries of knowledge. It cannot be denied that, so far as the future is concerned, an entirely new prospect has been opened up. By these achievements of experimental science Man's inheritance had been increased, his aspira tions had been uplifted, and his destiny had been ennobled to an extent beyond our present power to foretell. The real wealth of the world was its energy, and by these discoveries it, for the first time, transpired that the hard struggle for existence on the bare leavings of natural energy in which the race had evolved was no longer the only possible or 25° THE NEW PROSPECT enduring lot of Man. It was a legitimate aspira tion to believe that one day he would attain the power to regulate for his own purposes the primary fountains of energy which Nature now so jealously conserves for the future. The fulfilment of this aspiration is, no doubt, far off, but the possibility altered somewhat the relation of Man to his environ ment, and added a dignity of its own to the actuali ties of existence. INDEX A a-particles, Number of expelled by radium, 57, 59, 62-5 — Individual, 58, 62-5. 86 — Mass of, 83, 134, 138 — Velocity of, 83, 84-6, 128, 138, 219 — Limiting velocity of, 89 — Energy of, 85 — Collision of with matter, 86-9 — Scattering of, 87 — Positive charges carried by, 83,88 — from radium itself, 127 — from the emanation, 108, 198 — Connection of with helium, 62, 84, 126-40 — Proof of identity with helium of, 129 a-rays, 39-90 — Absorption of, 46 — — by air, 49-50 — Range of, 49, 63, 183, 219 — Magnetic deflection of, 83, 84 — Resolution of, 58-65 Accumulation of products, I25; 128-31, 133, 139, 170 Actinium, Production of helium from, 135, 138 Active deposit of radium, 188-98 — — — Residual activity from, 199- 204 Age of the earth, 35, 101, 239-41 Alternative theories of radio active energy, 92, 121 Alkaline-earth elements, 115, 141 Argon, 114, 115, 132 Atom, Definition of, 143-6 Atoms, 3, 16, 57, 65, 84, 88, 116, 142, 213-20 — Interpenetration of, 88 Atomic disintegration, 54, 82, 90, 121, 128, 133, 156, 212 — — Cause of, 157 Atomic property.Radioactivity an, 17, 18,21,92, 100, 113, 150 Average life, Period of, 154 — — Determination of, 159 — — of radium, 163, 171 — — of emanation, 155 — — of uranium, 163, 171 — — of common elements, 211 B /3-particles, 70-83 — Velocity of, 80-1 — Charge of, 70, 72, 82 — Mass of, 80 /6-rays, 39-90 — Magnetic deflection of, 66, 82 Barium, 21, 113, 183 Becquerd, Henri, 8, 9, 10, 176 Bismuth, 21, 183, 203 Bolt-wood, B., 171, 182, 183 Bragff, Professor, 48, 49, 63, 87, 88 I Bunse/i, 102 251 252 INDEX 7-rays, 43-45 — Radiograph by, 44 Caesium, 102 Calcium, Absorption of gases by, 74, 137 Cascade of changes, 100 Cathode-rays, 73-82 Cause of atomic disintegration, 157 Chance of disintegration, 153 Change, Law of radioactive, 153 Change of radio-elements, 95, lor, 123, 125 et seg. Chemical nature of radium, 115 Chemists and radioactivity, 148 Conservation of radioactivity, 120 Constancy of radioactivity, 14, 17 32, 33, 37, 61, 93, 94, 105, 123, 230 Control of natural energy, 8, 17, 232, 244, 250 Corpuscular theory of radiation 54, 56-65 Cosmical energy, 33, 164, 233-5, 239 Cost of scientific investigations, 26,27 Crookes, Sir William, 20, 59, 73, 80, 176 Crookes1 tubes, 73, 81 Curie, M. and Mine., 16, 18, 21, 102, 113, 170, 187, 199 D "D3"line, 132, 136, 137 Dalton, John, 143, 146 Debierne, 135 Decay of radioactivity, 94, 118 Definition of the atom, 143-6 Detection of infinitesimal quan tities, 102, 104, 112. 116, 122, 124, 128, 147-9 Determination of average life, 159 Dewar, Sir James, 74 Discovery of radioactivity, 8 Discrete theory of radium rays, 56,62 Disintegration, see Atomic dis integration —, Chance of, 153 Doctrine of energy, 26, 38, 51, 91, 240, 248 E Earth, Age of the, 35, 101, 239-41 Effects of radioactivity, 11-13 Electric current, Action of mag net on, 70 Electricity, Discharge of, n, 18, 25, 47, 64, 89 Electro-magnet, 66 Electrons, 77-82, 88 Electroscope, Gold leaf, tr, 23, 59, i M Elements, Unchanging character of, 97, 99, 22°-2 —, Stability of, 98, 211, 220-2 Elixir of life, 243 Emanation of thorium, 187 — — radium, 105-28 — Condensation of, 109-112 — Physiological action of, 112 — Volume of, 112, 163 — Chemical nature of, 114, 115, 141 — Spectrum of, 115 — Density of, 116 — Heat generated by, 116, 117 226 — Rate of decay of, 119, 121 — Reproduction of, 120, 121, 123, 167 — a-particles from, iSS, 198 — Atomic weight of, 140 INDEX Emanation, Average life of, 155, 253 159, 167 Emanations and radiations con trasted, 106 Energy of radioactive substances, 4, 8, 14, 81, 87, 91, 124, 125, 229 — of coal, 30, 31, 91, 164 — of radium, 31, 116, 164, 229 — of uranium, 227-30 — Transformers of, 92, 93, 123 — Internal, of matter, 92, 95, 96- 9, 118, 119, 124, 130, 145,225- 35 — Measurement of, 30, 96 Ephemeral transition-forms, 100, 125, 158-64, 167, 177 Equilibrium, Radioactive, 123, 129, 161 Ether, The, 52, 53, 78 Evolution of elements, 184, 220- 22 — of universe, 36, 164, 239-41 Existence, Struggle for, 8,240-50 Facts and theories of radio activity, 122, 146 Faraday, 66, 77 Fluorescence, 9, 24, 43, 74, 89, 106, 107 G Gas, A radioactive, 105, 109, H3 Geiger, 63 Geological bearing of radioac tivity, 36, 101, 235-8 Geology, Controversy between physics and, 36 Giesel, 26, 135 Gold currency, 212 H Heat generated by radium, 12, 25, 31, 116, 164 Helium, 62, 84, 114, 126-40 — Discovery of, 131 — Liquefaction of, 132 — Prediction concerning the origin of, 133 — Spectrum of, 135 — in radioactive minerals, 131, 133 — — — Volume of, 133 — Production of by radium, 128, 134 — — — actinium, 135, 138 — — — thorium and uranium, Herschell, Sir John, 215, 220 High vacua, 72, 74 Huggins, Sir WUliam, 145 I Increase of activity of radium with time, 22, 208 " Induced radioactivity," 187 Indifference of radium to its en vironment, 37, 105, 221 Inertia, 79 Infinitesimal quantities, Detection of, 102, 104, 112, 116, 122, 124, 128 Inglis,J. K. H., 154 lonisation, 88, 89 "Ionium," 183 Intermediate substances, 100, 103, 104, 180-5 Internal energy of matter, 92,95, 96-9, 118, 119,225 Interpenetration of atoms, 88 J Joachimsthal mine, 20,27, 206 Joly, Professor, 236, 237, 238 254 INDEX K Kelvin, Lord, 28, 52 Kirchoff, 102 Law of proportionality, 162, 169, 206-8 Law of radioactive change, 153 Lead and radium, Connection be tween, 21, 103, 203, 204 Life of radio-elements, 125 —, Period of average, 154 Light, Nature of, 51, 55 — Velocity of, 53, Si Limitations of knowledge, 5, 8,_ ; 90,231,239 M Macdonald laboratories of M'Gill University, 122 Mackenzie, T. D., 178 Magnetic deflection of cathode- rays, 76 Maintenance of radium, 165-75 — sun's energy, 33, 164, 238 Marcknvald, Professor, 201, 202 Mass of the electron, 77-80 M ell,J. Clerk, 214, 220, 241 Measurement of energy, 30, 96 McCoy, H. N.,\T\ Mental pictures, 149 Mercury, 116, 204 Minerals, Quantity of radium in, 22, 102, 169-75, 207 —, Helium in radioactive, 131, 133 —, Lead in radioactive, 21, 204 —, Ratio between quantities of uranium and its products in, 207 Minimum quantity of radium de tectable, 23, 59 Minimum helium detectable, 137 Molecules, 3, 146, 214 Monopoly, Radium a scientific, 27 N Negative and positive electricity, 71 Neon, 114 Newton, 53, 54 Nomenclature concerning atoms and molecules, 143-6 O Onnes, K., 132 Ouroboros, 241 Parent of radium, 168-184 Penetration test of rays, 10, 41, 42, 43, 108 Period of average life, 154 — half change, 157 Periodic law, 142 Perpetual motion, 29, 33, 82 Phosphorescence, see Fluor escence Photographic effects of radio activity, n, 19, 25, 89, 109 Physical impossibility, 38 Pitchblende, 20, 102, 174 Planetary temperatures, 238 Platino-cyanides, 24, 50, 194 Polonium, 21, 62, 183, 200-3 Positive and negative electricity, 7' Prediction of origin of helium, 133 Parent of radium, 168 Proportionality, Law of, 162, 169, 206-8 Q Quantity of radium in minerals, 21, 102, 169-75 INDEX 255 Quantity of helium in minerals,i33 — detectable by spectroscope, 137 R Radiation, Nature of, 51-6 Radiant matter, 73, So Radiograph by 7-rays, 44 Radio-tellurium, 207 Radium clock, 82 —, Chemical nature of, 165 —, Average life of, 163, 171 —, Maintenance of, 163-75 — and uranium, connection between, 170-85 Radium A, 121, 142, 190-8, 208 — B, 121, 142, 190-8 — C, 142, 190-8, 209 — D, E and F, 200-4 — F', Identity of with polonium, 200-3 Ramsay, Sir William, 112, 114, 132, 134, 163 Rays of radioactive substanceSj 12, 39 et scf. Rayleigh. Lord, 114 Ratio between uranium and its products, 207 Recovery of radioactivity of radium, 105, 119 Rowland, Professor, 217 Rutherford, Professor, 41, 42, 63, 64, 83, 89, 105, 109, 117, 122, 'S3. 139, 163, 171, "86 S He/luster, Professor, 218 Sidot's hexagonal blende, 50 Silk tassel experiment, 25, 47 Smit/iells, Professor, 147 Spectroscope, 102, 124, 125, 132, '34, 138, 178,238 Spinthariscope, 59, 63 Struggle for existence, 8, 240-50 Strutt, 82, 171, 235 Stability of elements, 98, 211, 220-2 "Subradium," 183 Successive changes of radio elements, ico, 104, 121, 151, 160, 177-85, 190, 199-205 Sun's energy, Maintenance of, 33, 164, 238 T Tait, 5, 28, 34, 36, 124 Thallium, 204 Theories and facts of radio activity, 122, 146 Thomson, Sir J. /., 80 Thorium, 18, 127, 132, 133, 136-8, 183, 187 —, Production of helium from, 136-8 Total energy in radium, 164 — in uranium, 227, 229 Transcendental character of radio activity, 37, Si Transformers of energy, 92, 93, 123 Transmutation, 18, 95, 96, 98, 230-3, 242 r Ultimatc products of radium, 103, 129, 168, 203, 204 Ultra-material velocities, 88 Unchanging character of ele ments, 97, 99, 220-2 Uranium, 9, 17, 132, 133, 159, 170-85, 205-7, 226-30 —, Average life of, no, 171 —, Production of helium from 136-8 — and radium, connection be tween, 170-85 Uranium X, 176-80 1 1 1 1 1 1 1 ' " : III 11 \ 1' 1 ill 1 1 1 1 I'l H 256 INDEX \racua, High, 72, 74 Value of radium, 25, 26, 210 — gold, physical explanation to account for the unchanging, 211 Velocity of cathode-ray particle, 81 — of light, 53, 81 Velocities, Ultra-material, 88 Volume of helium in minerals, 133 — emanation in equilibrium with radium, 112, 163 W Wave theory of light, 55 Willemite, 50. 74, 76, 106, 107, in, 179 Writing by radium, 25 X X-rays, 9, 42, 44, 53, 106, 136-8, 160, 175 Zinc sulphide, 50, 108, 193 WILLIAM BRENUON AND SON, LTD. PK1NTEKS, I'LYMOUTH