QfotrtteU Htttoersitg Itthtarg iltljara, Jfem lurk BOUGHT WITH THE INCOME OF THE SAGE ENDOWMENT FUND THE GIFT OF HENRY W. SAGE 1891 DATE DUE D I8'» SEP 1 - iy43 yit^iztz?r Cornell university Librarv QC 721.S67 1920 Themterpr«tationo1radjum,a^ 3 1924 012 334 383 Cornell University Library The original of tiiis book is in tine Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924012334383 Front End Papers. THE INTERPRETATION OF RADIUM AND THE STRUCTURE OF THE ATOM THE INTERPRETATION OF RADIUM AND THE STRUCTURE OF THE ATOM BY FREDERICK SODDY, M.A., F.R.S. DR. lee's professor OF INORGANIC AND PHYSICAL CHEMISTRY, UNIVERSITY OF OXFORD WITH ILLUSTRATIONS FOURTH EDITION REVISED AND ENLARGED NEW YORK G. P. PUTNAM'S SONS 1920 ( (I i^!l: i- I K V ' I ^' 1 r. Y n /^ ^50 5^S4 First Edition Second Edition Third Edition Fourth Edition March, 1909. November, 1909. October, 1912. August, 1920, )' I ' : : Vini) PREFACE TO THE FOURTH EDITION In again revising this book I have conformed to the earlier plan of writing what I should have said if the lectures had been delivered in 1920 instead of 1908. The original statement has been amplified rather than modified. It has lost, long since, the appearance of challenge to existing theories which at first it may have presented. But the subject has now grown entirely beyond the power of being fully encompassed by the original very simple and popular mode of treatment. I have thought it best, therefore, not to compress the original part unduly, as it still may serve a useful purpose to those not familiar with scientific conceptions, but to add, as a second part, a more briefly written and less elementary account of the later developments, particularly those that bear upon the problem of the constitution of the atom. It is to be hoped that even those who are not chemists or physicists, who have followed the exposition in the first part, may not be entirely unable to profit by the second part. Though, naturally, the new subject-matter, by reasons of its more general and often more specu- lative character — much of it still being in the making — cannot but be more difficult to understand than the original work, which dealt with distinct and easily understood steps in the progress of knowledge, made once and for all time. FREDERICK SODDY. The University of Oxford, July, 1920. PREFACE The present-day interpretatioii of radium, that it is an element undexgoing spontaneous disintegration, was put forward in a series of joint scientific com- munications to the Philosophical Magazine of 1902 and 1903 by Professor Rutherford, now of Manchester University, and myself. As its apphcation 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 pubhc. The book contains the main substance of six popular experimental lectures deUvered in the University of Glasgow at the beginning of the year, but being reheved from the necessity, always present in lecturing, of co- ordinating the experimental and descriptive sides, I have, while adhering to the lecture form of address, entirely rearranged and very largely rewritten the subject matter in order to secure the greatest possible degree of continuity of treatment. Certain portions of the lectures, for example those dealing with the X-rays and the spectai of dements, have been omitted, and "attention thereby concentrated upon radimn, the chief topic. In addition, I have brieflj* embodied the viii PREFACE results of important discoveries which have appeared since the date of the lectures, particularly the experi- ments of Professor Rutherford and Dr. Geiger in count- ing the number of a-particles expelled by radium. The book also contains some account of the arrangement by means of which I have recently succeeded in detecting and measuring the quantity of the helium generated from the common radio-elements uranium and thorium. I have borrowed freely from numerous scattered lectures and addresses bearing on the subject which , I have from time to time been invited to deliver, and may mention in particular the Wilde lecture to the Manchester Literary and Philosophical Society, 1904, the Presidential and other addresses to the Rontgen Society, 1906, the opening of the discussion on the evolution of the elements in Section A of the British Association Meeting in York, 1906, and the Watt lecture to the Greenock Philosophical Society, 1908. FREDERICK SODDY. The University, Glasgow, November, 1908. CONTENTS PART I CHAPTER I PAGE THE DISCOVERY OF KADIOACTIVITY Radioactivity, a new science— Its discovery — The four experi- mental effects of radioactivity — Tlie rays of radioactive sub- stances — The continuous emission of energy from the radio- elements - - - - - - -1 CHAPTER II RADIUM Radioactivity an unalterable atomic property — The radioactivity of thorium — Pitchblende — Quantity of radium in pitchblende— The smallest quantity of radium detectable — Experiments with radium — Cost of radium — The doctrine of energy — Measure- ment of the energy emitted by radium — The source of cosmical energy — Radium and the " physically impossible " - - 12 CHAPTER III THE RAYS OF RADIOACTIVE SUESTANCES The radiations of the radio-elements — a-, §-, and y-rays — Test of penetrating power — Experiments with the penetrating §- and y-rays — The feebly penetrating a-rays — Experiment with a- rays — The range of a-rays in air— The physical nature of radiation — Corpuscular radiation — The wave theory of light a- and /3- rays due to the expulsion of particles — The individual atom of matter — The spinthariscope — The decay of a-radiation — Counting the a-partieles - - - - - 28 X CONTENTS CHAPTER IV I'AOK THE RAYS OP KADIOACTIVE SUBSTANCES {COtltinUed) The j3-rays — Deviation by a magnet — Electric charge carried by i8-rays — Tlie nature of electricity — Radiant matter or cathode- rays — ^The electron — Inertia or mass — Velocity of tlie ;8-rays — The radium clock — Magnetic deviation of the a-particle — Its velocity — Passage of ( J same change as that in which the ^"^ emanation is produced. The emana- Radium^^ Emanation. ^^^^^ -^ regarded, in fact, as radium that has lost one a-particle. This, which is a perfectly general point of view, was proved from the first 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 may be left for later treatment. The evidence that has since been accumulated enables the same deduc- tion 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. Helium and the a-P article. 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 8,000 to 12,000 miles per second. Long before the real nature of the a-particle was known, helium had been first pre- dicted 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 RADIOACTIVE EQUILIBRIUM 95 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 emanation, which it may be expected will be formed by 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 pro- ducts 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 emana- tion has reached a maximum and does not further appreciably increase with lapse of time. In the case of the emanation practical equilibrium results in the com- paratively 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 96 HELIUM AND RADIUM 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 dis- integration 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 iii a short period to obtain evidence of its pro- duction, by means of ordinary chemical tests, but, on the other hand, the quantity would go on accumulating indefinitely with lapse of time. The Ultimate Prodqcts. 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 ordinary way by the statical methods of chemistiy and physics, but they will accumu- late 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, accu- mulated in sufficient quantity to be capable of detection by the ordinary methods of chemistry. Now the radioactive minerals are always very com- plex, 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. Another definite but less unequivocal exception was the element lead. SOLAR AND TERRESTRIAL HELIUM 97 Discovery of Helium, Solar and Terrestrial. The history of our knowledge of helium is unsurpassed by that of any other in interest. Its very name (from j?^.^09, 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 spectroscope in the sun's chromosphere, by means of the characteristic bright yellow line in its spectrum, 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 syste- matic investigation of its physical and chemical nature. It is a gas, the second lightest known, only twice as dense as hydrogen, and for long was the only gas which successfully resisted all efforts made to liquefy it by extreme cold and pressure. In 1908, however, Kammer- lingh Onnes succeeded by the exercise of wonderful experimental skill and persistence in reducing helium to the liquid state, attaining thereby a far lower tem- perature (270° Centigrade, or only 3° from the absolute zero of temperature) than has ever before been reached. 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 substance known. Indeed, helium resembles argon perfectly in chemical nature, in that it is quite without 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=l). 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 speculation. In 98 HELIUM AND RADIUM certain cases the volume of helium evolved is nearly a hundred times as great as the volume of the mineral in which it is contained. Prediction of the Production of Helium. The disintegration theory enabled Professor Ruther- ford 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 disintegra- tion 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 products of radio- active changes, being formed in Nature from radium, uranium, and thorium, excessively 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. It is true that in certain uranium minerals — e.g., autunite and carnotite, the amount present is often excessively minute, but these also are just the minerals which it is believed are of extremely recent geological formation. Indeed, the ratio between helium and uranium or thorium in minerals is now one of the recognised methods of estimating their age. From this point the work proceeded along two separate lines. Rutherford, in an exhaustive examina- tion of the nature of the a-rays, which we have already considered, proved first that they consisted of positively charged atoms expelled with great velocity. At first their mass was given as twice that of hydrogen, on the assumption they carried one atomic charge. Then, as the sequel to the beautiful counting experiments we have considered, it was proved in 1908 that each «-particle carries two atomic charges of positive electricity. Therefore the mass of the a-particle is four, that is to Fig. 24. — Original Spfxtrum-Tube i\ which the Formation OF Helium from Radium was first observed. Helium Hydrogen Fig. 25. — Dr. Giesel's Photograph of the Spectrum ok the Gas from Radium. II 20 minutes', III 5 ininiites' evposure. I is the Spectrum of Helium, IV that of Hydrogen, for comparison. To face p. 99 PRODUCTION OF HELIUM FROM RAdIUM 99 say, it is the same as that of the atom of helium. This made it very probable, therefore, that the a-particle is an atom of helium. Production of Helium from Radium. The prediction that helium was a product of radio- active changes was proved directly by Sir William Ramsay and myself in 1903. We chose for the parti- cular case of radioactive change studied that of the emanation of radium, since it is rapid, and the emana- tion can readily be obtained, free from other gases, first by the action of suitable absorbents, and finally 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 characteristic spectrum of helium was given by the tube. Fig. 24 shows a photograph of one of the original spectrum tubes in which the pro- duction 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 confirmed 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 sub- stance 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. 25. It represents four separate spectra, one below the other in parallel strips. The uppermost (I) is ordinary helium. The 100 HELIUM AND RADIUM 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 hydrogen respec- tively clearly visible in photograph II. They refer to the wave-lengths in Angstrom units (10"^° metre). It must be remembered that the (visually) brilliant yellow line D3, owing to its colour, appears far less intense in the photograph than the blue and violet lines. Production of Helium from Uranium and Thorium. I was engaged for four years in an attempt to detect t he production of helium from the primary radio-elements uranium and thorium, and 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 disintegration. This quantity is about one five-hundred-thousand- millionth of the mass of the uranium or thorium per annum ! A photograph of the apparatus employed, as it stood in the Physical Chemistry Laboratory, is shown in Fig. 26. These are seven exactly similar arrangements side by side, each of which is quite separate and unconnected with the others. Each consists essentially 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 in- definite time, can at any moment be opened by sucking down the mercury in the barometer tubes, so that the X H o < <:3 a o o To face p. loo PRODUCTION OF HELIUM FROM URANIUM 101 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-fuU 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 ex- periments are commenced. 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 ab- sorbing the last traces of all gases except the gases of the heliym 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 D3 line of the helium spectrum can be detected with certainty 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 quantity of any element that has ever been detected by the spectro- scope. 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 production has been found to be of the same order 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. 102 HELIUM AND RADIUM Identity of the a-P article and Helium. The position is then this: helium has actually been found to be produced from the various radioactive 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 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. One further step in this long converging series of experiments clinches the argument. We have seen that the a-particle, though but feebly penetrating, 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 been performed by Rutherford and Royds with a large quantity of radium loaned 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 but still perfectly gas-tight RADIOACTIVE RECOIL 103 capillary tube, enclosed within a wider vessel. After some days the gas in the outer vessel was found to con- tain 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. The First Change of Radium. So we are justified in writing the first disintegration suffered by radium: Radiara. Ernanation. Helium. Fig. 27. There is a great deal of evidence which proves 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 presumably 222. This is the value obtained by direct experiment (Chapter V.). The above diagram is typical of no less than nineteen different radioactive changes, in all of which an atom of mass between 240 and 206 expels an a-particle, or helium atom, of mass 50 or 60 times less. By the usual dynamical law it is to be expected that the heavy residue of the original atom, whatever it is, should recoil in the direction opposite to that in which the a-particle is expelled with a velocity between 50 and 60 times less than the a-particle, that is to say, with a velocity between 150 and 250 miles a second. The kinetic energy of this recoiling atom, since it depends upon the mass multiplied by the square of the velocity, will also be between 50 and 60 times less than that of the a-particle. The velocity and kinetic energy pos- sessed by a recoiling atom, though greatly inferior 104 HELIUM AND RADIUM to that of an a-particle, are nevertheless greatly superior to that possessed by an ordinary gas molecule at any attainable temperature. Radioactive Recoil. The phenomenon of radioactive recoil comes into evidence in a very curious and interesting manner, which at the same time has proved of very great practical utility. Very many of the products resulting from the expulsion of a-rays, although after their formation they are either not at all volatile or can only be volatilised at a high temperature, yet at the moment of production behave like volatile substances, and are carried away under suitable circumstances from the preparation in which they are produced, and deposited on the nearest available surface. The best conditions are obtained by working in a good vacuum, and charging the preparation positively, and the surface, on which it is required to deposit the recoil product, negatively. The residual atom, after the a-particle is expelled, carries a positive charge, and so is attracted to the negatively charged surface. It is essential that the preparation should be in the form of a very thin layer in order to give the recoiling product a chance of escaping from it. In this way many products, of period of life too short to allow of their being separated by any other method, have been isolated and identified with ease. CHAPTER VII THEORY OF ATOMIC DISINTEGRATION Questions of Nomenclature. 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, how 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. 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 (Fig. 43, p. 214) — the most sudden and surprising differences appear between suc- ceeding elements. Chlorine, potassium, and argon are three succeeding elements in such an arrangement, and there is no resemblance whatever between them. In the nine successive transformations radium undergoes, the atom suffers, in most but not in all, a disintegration 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 105 106 THEORY OF ATOMIC DISINTEGRATION rather to be expected that the succeeding transition- substances produced one after the other should differ entirely from one another in their material character- istics. Further discoveries on this important question are dealt with in Chapter XV. Definition of the Atom. 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 atom is, of course, derived frota 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 mean- ing 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 compounds will be simple multiples or sub- multiples 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. Thus, one atom of carbon combines with either one or two atoms of oxygen, and for iron and oxygen the ratio is either one to one or two to three. These units of chemical combination of definite relative weight are the atoms of the chemist. In all the various changes of matter which chemistry 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. THE ATOM OF THE CHEMIST 107 In compounds the component atoms preserve their 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 com- ponent 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 meaning 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 uranium which exists. An atom 240 times lighter than this is known, but it is not uranium, it is hydrogen. Elements and Chemical Compounds. The discoveries in radioactivity have left this meaning of the word atom 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 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 com- pound, which may at will be formed from and decom- posed into its constituents. Were radium a chemical compound of helium it would, as Sir William Huggins has pointed out, 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 108 THEORY OF ATOMIC DISINTEGRATION 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 resolution 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 complexes, 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 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 chemical 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 dis- tinguish it from the chemical molecule, which has also its own definite meaning distinct from the chemical atom ? The Experimental Facts. These questions of nomenclature at first diverted attention from the experimental facts, and gave rise to much more or less random criticism of the younger workers in radioactivity. Another source of con- fusion has been the tendency to associate the discoveries in radioactivity with other entirely distinct discoveries made somewhat earlier with reference to the nature of the negative electron. It was thought at one time that it would be possible Nature of atomic disintegratio!n 109 to explain the atoms of matter as being built up entirely of electrons or atoms of electricity, which turned out to be as little in accord with actual evidence as it would be to regard the solar system as composed entirely of planets and to neglect the central sun. The problem of the real nature of the atoms of matter has not been completely solved by either of these independent scientific advances. Another objection to the validity of radioactive evidence has been the minuteness of the amounts of matter on which the evidence is based. It has been stated that it is impossible to come to any settled conclusions in regard to radioactivity, until enough of the materials can be obtained to suffice for the requirements of chemical investigation. But surely, this criticism puts weight on mere familiarity with the older methods rather than on their real in- trinsic value. 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. The Nature of Atomic Disintegration. It is my intention to give you, so far as I am able with accuracy, broad general mental pictures of radio- active 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, and these consequences in every known case agree with the facts. Without such mental 9 110 THEORY OF ATOMIC DISINTEGRATION 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 accurately, 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 alter- natives 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 sub- stance 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 entertain for a moment. Or, the change is slow and gradual with regard only to the mass of the substance 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 proceed 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 THE CHANCE OF DISINTEGRATION 111 into the emanation and helium atoms. Regarding 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 eaeh unit of time. The Chance of Disintegration. This, then, is the very vivid mental picture of atomic disintegration which the detailed researches in radio- activity have established. Any one radio-element like radium being considered at any instant, among its innumerable host of atoms, most of which are destined to last for hundreds, some for thousands 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 dis- integrate, and so the process goes on, a-particles being expelled as a continuous 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 la.r ger fraction of these disintegrate 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 mathe- matical 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. This follows from the consideration of the one 112 THEORY OF ATOMIC DISINTEGRATION 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 — that is, the quantity of the substance changing in the unit of time — is proportional to the quantity present of the substance which is changing, in others to some power of this quantity. Now, in radioactive change the rate of change is invariably simply proportional to the quan- tity 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. Hence you cannot get much further by simple arithmetic and algebra. Of course, in the case of a slow change like that of radiuin itself, when even in a lifetime the quan- tity 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. In most cases some 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 Period of Average Life of a Disintegrating Atom. 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 RADIOACTIVE CONSTANT 113 the symbol X, represent the fraction of the total changing per second, and to this symbol X. is given the special name " the radioactive constant." X may represent a small or a large fraction, according to the particular case, according as the disintegration process is slow or rapid. The important point is that it is a real con- stant 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, X, the radioactive constant, has the value 1/481,250, which signifies that in this case 1/481, 250th of the total amount of emanation in existence changes per second. The next step, skipping the mathematics,^ is that the average period of life of the atom of a radioactive substance — that is to say, the period of time in seconds it exists on the average before its turn comes to disintegrate — is simply the reciprocal of the radioactive constant, or 1/X. Thus the average life of the radium emanation is 481,250 seconds, or 5-57 days. Now as radioactive change proceeds during every instant at the rate proportional only to the total quan- tity of substance undergoing the change, which is present and remains unchanged at that instant, and as in this method of looking at the changes we do not consider at all the absolute quantities, only the fraction of the whole changing, it follows that \ is always of the same value throughout the process from start to finish. It also follows that l/\, 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, con- sidering only the atoms still remaining unchanged at that instant, then from that instant the average period of life is always l/\. 1 So far as I know, the period of average life was first dediired by Mr. J. K. H. Inglis, to whom I put the problem. 114 THEORY OF ATOMIC DISINTEGRATION Our own period of average life, of course, follows very different and far more complicated laws. The expectation of life at any age is a practical problem for the actuary. But every one knows, owing to the mor- tality among infants, that the expectation of life at birth is less than shortly afterwards, when it reaches a maximum and then gets less and less with increasing age. The " expectation of 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. That this is so can be directly proved in the simplest way, by comparing, for example, the rate of change of newly-born radium emanation, not in existence a few minutes before, with that of the residue of an originally much larger quantity that has survived a period several times greater than the period of average life. The Unknown Cause of Disintegration. This answers fully the general question, How does an element change ? You will probably wish to know why it changes in this particular way. That cannot be said, although the true answer would undoubtedly take us far. All that can be stated is that the immediate /j ^ cause of atomic disintegration appears to be due to ! ! chance. 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." 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 THE PERIOD OF HALF-CHANGE 115 ultimate cause." Atomic disintegration is assuredly t 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 radio- active changes shows clearly that the past history of an atom does not increase its chances of undergoing dis- integration 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 geometrical progres- sion. Suppose in a time of t seconds one-half of the total amount changes and one-half remains unchanged. In the next period of r 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 t the quantity is reduced to 1/2^. In any period of time represented by N r seconds, where N is any multiple or submultiple, the quantity of substance remaining is 1/2". It remains to state what relation the time t required for the half-change to occur, bears to the period of average life l/X. of the former way of considering the change. There is a fixed ratio between these two periods, the latter being always 1-45 times the former. In a time equal to the period of average life l/X, the quantity of substance present is reduced to l/e=0-368 of the initial quantity. Detkumination of the Period of Average Life. These considerations would have little interest to us but for the fact that they afford the means whereby 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 116 THEORY OF ATOMIC DISINTEGRATION 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, representing 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 transi- tion-form. If it is, as in the case of radium, a few thousand years we are inclined to look upon the sub- stance as a permanent and primary radio-element. There is really not this sharp difference. But it is con- venient 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 geometrical progression with the time to half its initial value in the course of 3-84 days. The period of average life is 1-45 times greater, or 5-57 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. The radioactivity of radium is several million times that of uranium when the a-rays of equal quantities of the two elements are compared. From this it may be con- THE PERIOD OF AVERAGE LIFE 117 eluded that the period of uranium is several million times longer than that of radium, and if the latter is known, that of uranium may be roughly estimated, although it is a period of some thousands of millions of years. As a matter of fact, there is a very beautiful generalisa- tion, 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 disintegration, the emanation of radium. I have already alluded to the fact that owing to the very 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. Thk Pekiod of Average Life of Radium. This state of things is known generally by the name of radioactive equilibrium. The importance of the existence of this state of radioactive equilibrium it is impossible to overrate. Many problems, as we shall come to see, which, to us with our limited period of life, might well appear absolutely 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 6nly giving you the main idea and one specific illustration of what is in fact a law of great generality. / 118 THEOEXJai^ATGMie-BISINTEGRATION By the law of radioactivg _chaiige ] if Xj is the radio- active 'cOTrstant^Primtnff^^., the fraction of the i whole changing per second — and N is the total number of radium atoms dealt with, then the number of radium atoms changing into the emanation per second, and therefore also the number of atoms of fresh emanation produced per second, is XiN. But in equilibrium this equals the number of emanation atoms disappearing. If the radioactive constant of the emanation is Xj; and the number of atoms of emanation present during equilibrium is denoted by X, the number of emanation atoms disappearing per second is XjX. Hence we have (x,N=X,Xandg=Jj. j This law, the most important in radioactivity, thus states that in successive disintegrations the product 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 proportional to their respective average lives. It is necessary for the law to hold true that the period of the parent body should be much longer than the periods of any of its products, and in this case the product selected need not necessarily be the first product, but may be any one of the successive products formed in the serieg.-^ — X2 is well known by direct observation. Now if X /N, the ratio between the number of atoms of emana- tion and of radium in equilibrium together, can be found, then Xi, the radioactive constant and therefore 1/Xi, the period of average life of radium can be de- duced. 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 important consequences, can be deduced. For a slowly changing body like radium the second is an inconveniently 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 119 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. 82), this volume was first approximately 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 con- cluded about 1/I150th part of the radium changes annually, so that the period of average life on this estimate is 1,150 years. Owing to the excessive minute- ness 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. Later experiments by the same method with much larger quantities of radium have shown that the correct value is about double that first found. With the growth of the subject other methods, less direct but more accurate, have become available. Professor Rutherford recently, from a con- sideration 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 2,500 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 narrow limits the average life of radium may be taken to be 2,500 years. The Total Energy evolved in the Complete Disintegration of Radium. 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 120 THEORY OF ATOMIC DISINTEGRATION change— that is, during a period of some thousands of years. We saw (p. 22) that a gram of pure radium evolved about 133 calories of heat per hour. There are 8,760 hours in the year, so that in a "year a gram of radium evolves about 1,160,000 calories. In a year l/2500th part changes. Therefore in the complete change of one gram of radium no less than 2,900,000,000 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 radio- activity. By the burning of a gram of coal, for example, only about 8,000 calories are obtained. In this change, however, 2f grams of oxygen are also consumed, so that per gram of the two substances taken together the heat evolved is only 2,200 calories. On this basis of calculation the energy of radium is well over a million times that furnished from the combustion of coal. No wonder then that to account for the boundless energy displayed everywhere in the starry heavens proved a difficult problem for physicists, acquainted with no more energetic chemical process than the burning of coal ! CHAPTER VIII THE ORIGIN OF RADIUM Why has Radium Survived ? One of our chief duties will be to follow out this theory of the disintegration of atoms in radioactivity. The bare idea of elements spontaneously 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 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 disin- tegration been going on at its present rate. That is to say, even if the whole woi'ld 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 121 122 THE ORIGIN OF RADIUM 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 discovery 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 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 disintegration 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 quantity of emanation in existence over a period a hundred 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 ? The Reproduction of Radium. 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 REPRODUCTION OF RADIUM 123 a fresh crop of radium ? The answer is that we should. This was one of the first predictions made 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 pril^ciples 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 radioactive 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 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. The Ratio between the Quantities of Uranium AND Radium in All Minerals. By the law already formulated on p. 118, in two succes- sive, not necessarily consecutive, disintegrations of which the second is much 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, 124 THE ORIGIN OF RADIUM 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 parent 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 heHum and radium is to be ex- pected. But the case is different with radium and its parent. There must be a fixed ratio, independent of the age of the mineral examined. As the original first parent of radium 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 is 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 therefore in comparing uranium with radium it follows that iu:anium 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 QUANTITY OF RADIUM IN MINERALS 125 up for radium. So that from the very first there existed a strong presumption that uranium is the original parent of radium. The evidence in support of this view at the present time is complete and satisfactory. "We owe it to the careful work of McCoy, Strutt and Boltwood 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 con- stant of proportionality. They found that for every one part of radium there always exists 3,200,000 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 2,500 years. Hence it follows that that of uranium is 8,000,000,000 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. Hydraulic Analogy to Radioactive Change. It will help us considerably if we try to find some analogy to the important and intricate relations 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 Corporation to control for the purposes of our illustration. As you know, we in Glasgow are supplied ultimately from Loch Katrine through an intermediate 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, 10 126 THE ORIGIN OF RADIUM 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 approximately one eight-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 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 quan- tity 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 proportional 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 depends only on and is proportional to the amount present. Never- theless, we shall find that there is about 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 increase. In either case, intake THE AGE OF PITCHBLENDE 127 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 dis- connect 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 1,700 years. Whereas 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. The Age of Pitchblende. 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 continuous 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 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 re- plenishing the supplies of uranium, just as there are replenishing Loch Katrine, there is no difficulty in 128 THE ORIGIN OF RADIUM 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. Uranium X. At this stage it will be well to make a short digression into the radioactivity of uranium itself, and how it is explained on the theory of atomic disintegration. Uranium and its compounds in their normal state give out both a- and ,0-rays. As in all other cases, the y8-rays, being photographically the most active and being the more penetrating, were the first chiefly studied. Sir William Crookes and also M. Becquerel found that by certain chemical processes 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 ^S-rays of uranium belonged to the uranium X. Uranium freed from uranium X gave its normal amount of a-rays. Then it was found that the ;S-radia- tion 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 j3-rays, gradually and completely recovered its power of producing /3-rays. Uranium grows uranium X, in exactly the same way as radium grows the emanation. The activityof uranium X after separation from uranium, consisting entirely of jS-rays, steadily decays in a geo- URANIUM X 129 metrical progression with the time, falling to one half the initial value in 24-6 days. The average life of uranium X is thus 35-5 days. The disintegration of uranium up to the point so far discussed is represented on the following scheme: 0-^0 Uranium. Uranium X. 8,000,000,000 years. S5-.5 days. Fig. 28. This is as far as the methods of radioactivity enable us directly to trace the disintegration of uranium at the present time. The substance produced — uranium X — is only an ephemeral transition-form, lasting on the average 35-5 days, and when it disintegrates, the process appears to come to d stop so far as our experimental methods have yet been able to disclose. Now, on the view that has been developed that uranium is the parent of radium, it is natural to suppose that uranium X in the course of time turns 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 period of life of radium compared with that of uranium X. As already explained (p. 92), chemical 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. Attempts to detect the Growth of Radium. In 1903 I started a series of special experiments which have been continued ever since, partly in con- 130 THE ORIGIN OF RADIUM junction with Mr. T. D. Mackenzie and more lately with Miss A. F. Hitchins, to see whether uranium does, in fact, produce radium. The uranium, after being puri- fied as completely as possible by chemical methods from radium, is left sealed up in a flask and is periodically 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, allowing quantities of radium of only a few million-millionths part of a gram 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 radium, after standing sealed up in a glass flask for a period of at least a month to allow the equilibrium 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 first result of these experiments, while they furnished the first evidence of a growth of radium, withal in very minute amount, showed that this growth is not due to uranium. In the first 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. In later experiments more perfect methods of purify- ing the uranium initially were adopted, with the result that the growth now of radium occurred chiefly in the impurities separated, whilst the growth in the purified FIRST EXPERIMENTS 131 radium was reduced to an excessively minute amount. In these the greatest growth recorded was only one fifty- millionth of a milligram of radium after six years. At this rate, even at the present enormous price of radium, it would require sixty thousand years to produce one pennyworth. Now if uranium X, when it disintegrates, produced radium directly, then with the quantities of materials used in these later experiments, the amount formed in a single hour would be greater than has actually been formed in six 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 appar- ently conclusive negative result, it was practically certain that uranium is the original parent of radium, and that in the course of years our preparations would begin to grow radium. Existence of Intermediate Products. The natural explanation of this failure to detect a growth of radium from uranium is, that one or more intermediate bodies of long life exist 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. 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. 132 THE ORIGIN OF RADIUM I well remember one fact told me by the engineer in charge of the magnificent scheme of waterworks, supply- ing 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 uranimn 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 watched and waited seven 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 had not reached there yet and the radium reservoir remained practically as empty as at the start. But there was no doubt it would come, and there was good reason to expect that some of us, at least, would be still alive when it arrived. 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 100,000 years, that is, forty times that of radium itself. Also, that the radium, in this case, must be produced at a rate proportional to the square of the tinae from purifi- cation, the growth in a century being a hundred times as great as that in the fiist decade. On our analogy, then, between Loch Katrine and Milngavie, there must exist a reservoir of forty times the capacity of Milngavie, provided there is only one. Since the equilibrium quantity to which an intermediate body accumulates is proportional to its period of average life, then if there is only one intermediate parent of radivmi between radium IONIUM 133 and uranium, there must be forty times as much of it in minerals containing radium as there is of radium itself. Ionium. This leads me to the next step. The failure to detect a production of radium from uranium merely fore- shadowed the actual discovery of an intermediate sub- stance of long period of life. Boltwood in America suc- ceeded in isolating it from minerals containing radium, and it proves to be the direct parent of radium. It possesses the property of producing radium directly from itself by disintegration, and it has been called ionium. It expels a-rays during its disintegration into radium, and these a-rays possess a relatively low velocity. Their range is very little more than one inch of air. Chemically, ionium resembles thorium so completely 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 treat- ment, although all other substances are. The chemical resemblance between these two different elements is complete. Later we shall come to recognise many other cases of the same kind. Ionium and thorium are what are now called isotopes. The disintegration series thus reads : ^S:)'^ ^Tt^^'^l^ P" /)" p" f 238 J ^ f 234 J — ^ _ — — — ^ f 230 1 — > ( 226 J > f 222 j — Uranium T. Uranium X. Ionium. Radium. Emanation. 8,00,000,000 35-5 days. lOO.COO years. 2,.500 years. O'Bdays. years. Fig. 29. as far as we have yet considered it. In the centre is placed the known or presumed atomic weights of the various bodies. 134 THE ORIGIN OF RADIUM Production of Radium by Uranium. Going back to the purified uranium preparations, in 1915 with the help of Miss Hitehins, the measurements of the quantity of radium present first clearly estab- lished that there was a steady growth of radium, and, moreover, that it was proceeding proportionally to the square of the time, as the theory requires. This growth has continued regularly up to the present time (1919). The period of average life of ionium calculated from it is almost exactly 100,000 years. The amount of radium in some of the preparations is (1919) about ten times as great as initially. But the problem has taxed to the uttermost even the extraordinarily delicate tests for radium. For the preparation containing the largest quantity of uranium — namely, three kilograms calcu- lated as the element — the growth of radium after ten years has been only one five millionth of a milligram^ — • i.e., one part of radium from fifteen billion of uranium. The Stately Procession of Element Evolution. So far, then, as we have inquired, uranium, uranium X, ionium, 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 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 now known no less than thirteen, starting from the element uranium, but for our present purposes of illustration these four will suffice. But this new transformation scene on ELEMENTARY EVOLUTION 135 which the curtain of the twentieth century has been rung up, beginning as it has done with the transforma- tion of the most fundamental and permanent of the existences which physical science has recognised in the past, extends beyond physical science and trans- figures with new light some of the most fundamental and permanent ideas which in one form or another are deep-rooted in the world's philosophies. CHAPTER IX THE SUCCESSIVE CHANGES OF RADIUM The Later Changes of Radium. We have attempted to trace 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 exception 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 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 neigh- bouring objects. Thorium, which is very hke 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 radioactivity," 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 now confine ourselves to the case 136 THE ACTIVE DEPOSIT 137 of radium. Any object left in the immediate neigh- bourhood of a radium salt becomes radioactive, but after it is removed the radioactivity decays away rapidly and almost completely, abnormally at first, but sub- sequently more regularly, with a half-value period approaching thirty minutes. The temporary activity so " induced " consists of «-, I3-, and 7-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- 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 com- pletely closed vessel — it does not matter how thin-walled so long as it is air-tight — no radioactivity 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 disintegration of the emanation. Just as radium cannot exist without continuously producing the emana- tion, so in turn the emanation cannot exist without continuously 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 emana- tion expels a second a-particle and changes back again into a solid, or at least into a non-gaseous form of matter, the first of the " active deposit " group. The latter in turn expels more a- and also yQ-particles, and so the course of successive disintegrations goes on. In the active deposit itself at least three changes follow one another with great rapidity, so that the analysis of them proved a complicated task. 138 THE SUCCESSIVE CHANGES OF RADIUM The Active Deposit of Radium. 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 happening in an atmo- sphere containing the radium emanation. Every second two out of every million of the atoms of emanation dis- integrate, 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 con- centrated upon it. This enables me to show you more effectively the production of the active deposit from the emanation and some of its chief properties. The separation of the non-volatile product of a volatile parent or emanation by this use of a negatively charged surface is a very simple operation, much more so than when the parent substance is non-volatile and the recoil of the product is used to effect its separation and con- centration on a negatively charged surface, as discussed on p. 104. It would take us too long and too far 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 disintegration and then illustrate as many of the points in it as possible experimentally. I have said that after the disintegration of the emana- tion at least three successive disintegrations, following one another rapidly, occur. The bodies produced are RADIUM A, B AND C 139 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 sueh fleeting existence (Fig. 30) Radium. Emanation. Radium A. Eadium B. Badium C. Active deposit of rapid cliange. 2,500 3'ears. 5'6 days. 4-3 minutes. 3S-5 minutes. 28-1 minutes. Fig. 30. 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 symbol {^) here and throughout indicates that ^-rays are expelled, but that they are not the normal penetrating yS-rays, but rays akin to the cathode-rays in their low penetrating power and low velocity. They only come into evidence in special experiments, and are not of great general importance. The first body pro- duced from the emanation, radium A, changes with great rapidity with a period of average life of 4-3 minutes, expelling an a-particle. The body radium B resulting undergoes a change which was at first thought to be entirely " rayless." Neither a- nor /3-rays of the ordin- ary kind can be detected, although a very feebly pene- trating /3-ray is produced, which we need not further consider. The period of this substance is 38-5 minutes. The body produced, radium C, changes, expelling both a- and /8-particles and 7-rays also. The period is 28-1 minutes. It is probable that this change is complex and that the y8- and 7-rays are given off in a separate change to that in which the a-rays result. This point will be dealt with later. The Radiations from the Active Deposit. We started our description of the rays of radium with the statement that they consisted of a-, /3-, and 140 THE SUCCESSIVE CHANGES OF RADIUM 7-rays. One of the most interesting points of the above scheme is to show that the /S- and 7-rays do not come from radium itself, any more than they do from uranium itself, but from the later products. It is loose, but con- venient, to talk of the /3- and 7-rays of radium. Really we mean the /3- and 7-rays of radium C. 1 he emanation, like radium itself, gives only a-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 /S- and 7-rays make their appearance only after the subsequent pro- ducts have accumulated. EXPKUIMENTS WITH THE ACTIVE DEPOSIT. On the table there is a small glass vessel silvered internally (Figs. 31 and 32) containing the emanation 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 connected to the negative pole of the electric supply and the silver coating to the posi- tive pole. If only the point of the needle is made to project beyond 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. 1^ — 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 emana- tion. The disintegrations all going on together, the wire should give a-, /3-, and 7-rays, the /3- and 7-rays being as intense as those given from the half-grain of radium bromide from which the emanation was derived. Now I withdraw No. 1 needle from the emanation, and with the room darkened we will examine its active deposit. Fig. 32, — Apparatus for obtainixg the Active Detosit of Radium. To face p. 141 EXPERIMENTS WITH ACTIVE DEPOSIT l4l To detect the a-rays we will use a glass translucent screen, thinly coated with phosphorescent 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 practically Fig. 31. entirely ceases. The a-radiations producing this effect come both from radium A and from radium C. To detect the ,S-rays we will use an ordinary card- board X-ray screen of barium platinocyanide. Bring- ing 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 11 142 THt: SUCCESSIVE CHANGES OF RADIUM 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 inter- posed, 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 7-rays. In fact the active needle-point gives all the penetrating rays given by half a grain of radium bromide. It is now several minutes since the needle was removed from the emanation. If we now again examine the «-rays you will notice they already are very perceptibly less intense than at first. 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, and are only half as intense as at first. Radium A. Now if, instead of exposing the needle to the emana- tion for some hours so as to allow all the successive products time to be produced, we expose it to the emana- tion 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. 1, the two are now very similar in their a-ray-giving power. No. 1 being only slightly the better. The a-rays from No. 2 come almost entirely from radium A, for there has not yet been time for any appreciable quantity of radium C to be formed. The a-rays from No. 1 come entirely from radium C, and this EXPERIMENTS WITH ACTIVE DEPOSIT 143 radiation has not yet had time appreciably to decay. Let us, however, test their /3-rays. You observe that No. 2 gives no (8-rays worth considering, whereas No. 1 still gives /3-rays in practically undiminished intensity. Radium A gives no /3-rays, and as there is no appreciable quantity of radium C formed there yet, the consequence is that No. 2 wire gives no /3-rays. I can show you at this stage a very striking experi- ment 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 de- posit 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. Radium B and C. Now we will contrast the decay of the activity of the needles Nos. 1 and 2. The activity due to radium A by itself decays very rapidly, half disappearing every three minutes. The consequeirce is, if we now again test the a-rays of No. 2, we shall find they have already nearly disappeared, whereas No. 1 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 the conclusion that the first change of the active deposit is a very lapid 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 ^-rays are expelled. It is more difficult to give you in a lecture satisfactory evidence of the existence of radium B, a body not itself giving rays, intermediate between the first and second changes in which rays are expelled. If we examine carefully the decay of the a- and ^-rays of wire No. 1, in which at first all these pro- 144 THE SUCCESSIVE CHANGES OF RADIUM ducts co-existed in equilibrium, we shall find, as already shown, that for the first half-hour after removal from the emanation the yS-rays suffer very little change and then the regular decay begins. In the next half-hour the ^-rays decay approximately to one-half their original intensity, and the decay then goes on at this rate regu- larly 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 disintegration 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 removed from the emanation. Exactly the same pause occurs in the decay of the a-rays. As we saw with No. 1, within a very few minutes after the needle was removed from the emanation the a-rays had decayed to about one-half, owing to the disappear- ance of the a-ray-giving radium A. Then, however, little further change occurred. It is now about half an hour since No. 1 was first tested, and the a-activity is similar to what 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. 1, we should find, from now on, a rapid decay of both a- and yS-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 by these rough methods. The Radiation from the Emanation. 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 RAYS OF THE EMANATION 145 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 tube against the X-ray screen, you can see that no penetrating rays come from the tube. The emanation itself gives no /S-rays, only a-rays. By the end of the lecture, how- ever, sufl&cient radium C will probably have been formed inside the tube to give an appreciable /3-radia- tion. Owing to the existence of the intermediate body radium B, there occurs a similar pause in the growth of /8-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. The Later Slow Changes of Radium — Radium D, E, and F. This finishes this subject and brings us to the next. What happens to radium C when it disintegrates ? 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 persists 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 completion. The effect of these further changes is in consequence extremely small, but they last a very long time. Con- tinuing our diagram where it last ended at radium C, the next stage is represented in Fig. 33. 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 twenty-four 146 THE SUCCESSIVE CHANGES OF RADIUM years. No very important rays are given in its change. /^-rays, however, result from the body produced from it, which changes rapidly again with a period of only a Radium C. Radium 0. Radium E. Radium F. Radium G. (Pulouium.) (Lead?) Active deposit of slow change, 2S1 minutes. 24 years {?j 7-6 days. 202 days. Fig. 33. few days. We shall pass over these intermediate changes and consider the last known change of the series, that of radium F, which has a period of average life of 202 days, in which an a-particle is expelled. Radium F is the polonium of Madame Curie, having been separated by her from pitchblende first before she discovered radium. Polonium. A digression may here conveniently be made on what is known about polonium, before its connection with radium is considered. Chemically it resembles bismuth, and was separated first from pitchblende in association with the bismuth contained in the mineral. Its radio- activity, which consists entirely of a-rays, slowly and completely decays, so that a few years after it has been prepared, the most intensely active preparations of it lose practically all their activity. The work was carried on by Marckwald in Germany, who discovered new and simple methods of extracting polonium from the mineral andjWorked up many tons of pitchblende for this sub- stance. 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 bismuth, and he first proposed the name " radio- tellurium " for it, which, however, with the elucidation POLONIUM 147 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 radio- activity is correspondingly intense, and greatly exceeds, so far as the a-radiation is concerned, that of pure radium itself. The period of average life, 202 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 ^-vsiY^, which, however, increase at different rates. The a-rays are duetto 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 ^-rays, however, reach a maximum much more quickly. The /3-ray product (radium E) 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 merely a sketch of the evidence in favour of regarding polonium as the last radioactive substance produced in the disintegration of uranium. Thk Ultimate Peoduct of Radium. 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 148 THE SUCCESSIVE CHANGES OF RADIUM polonium expels its a-particle, what is produced ? The estimated atomic weight of polonium is 210, which is deduced by subtracting from the atomic weight of radium (226) the weight of the four atoms of helium known to be expelled in the form of a-particles. This 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, 208-5), and this element would be chemically 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 formation of which in proportion to the degree of slowness 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 contain- ing the elements of the uranium-radium series, and must therefore be a well-known common element. Lead has the atomic weight of 207-2, and bismuth, 208-0. The next known element is thallium (204), and then comes mercury (200). Lead is found in all the common minerals containing uranium in considerable quantity, and there is also evidence that the older the geological formation from which the niineral is obtained, the greater the percentage of lead present. Recently a uranium mineral, autunite, has been found containing no chemically detectable quantity of lead. But then the same mineral contains only an excessively minute trace of helium, and less than its full equilibrium amount of radium. There is every reason to believe that its formation as a mineral has occurred in quite recent times. This question has now been settled by indirect means, and there is no longer room for doubt that lead is the ultimate product of uranium. This evidence, however, may be deferred. The method of settling it directly is to study the change of polonium, separated from enor- THE TWO URANIUMS 149 mous quantities of pitchblende, by the aid of the spec- troscope, and on this tasli Mme. Curie and her colleagues have for long been engaged, but as yet without definite proof that lead is the product. Uranium I and Uranium II. A variety of evidence, some of which may be dealt with more profitably later, has lately established the conclusion that the change suffered by the uranium atoms when the a-particles are expelled, is not, as first supposed, a single change. The substance uranium, which chemists have hitherto considered an element, differs from every other known substance expelling a-rays, in that, per atom disintegrating, two a-particles are expelled instead of one. Moreover, these two a-particles are expelled at slightly different initial velocities, with the result that the " ranges " of the two sets of a-rays in air are slightly different (see p. 164). Most probably the two a-particles are not expelled from the uranium atom simultaneously but successively. In consequence, what chemists hitherto have accepted as a single element is, in reality, a mixture of two, chemi- cally so much alike that they have not yet been separ- ated, the first having the atomic weight 238-5, and which has been termed provisionally uranium I; the second, resulting after the expulsion of the first a-particle, having the atomic weight 234-5. It has been termed uranium II. It is probable that this uranium II is present in relatively very insignificant pj-oportion by weight, although it contributes one-half of the total a-radiation. Its period of life can only be estimated from very indirect and incomplete data at the present time. The more slowly a radioactive substance changes the shorter the range of the a-particles it expels, and so from the range of the rays an estimate of the period of average life may be formed. This estimate, such as it is, attri- butes a period to uranium II of about two million years. 150 THE SUCCESSIVE CHANGES OF RADIUM Numerous similar examples of elements identical chemically, but differing in radioactivity, are now known. These are called isotopes or isotopic elements. Uranium X and Uranium X^ (Brevium). Even more recently it was first predicted and then shown that uranium X is not a single substance. Ura- nium X gives two kinds of /3-rays, one of low velocity and comparatively non-penetrating — i.e., (^-) rays (compare p. 139) and ordinary high velocity /3-rays. (238 j ^ (234) ^(234) J^(234j - Uranium I UraniumX^ UraniumXg Uranium II Ionium Radium 8,000,000,000 35-5 days 1.65 minutes 3,000,000 100,000 2,500 years years years (?) years ^2) ^(2iaj >- (214 j ; >- (214J ^ (214J ^ Emanation ^Radium A Radium B Radium C R adium C^ '" ^ Active Deposit of Rapid Cliange 4-3mins. 38-5 mins. aS-imins, One milliontli ,^, , , ^ of a second (?) (j210j 5»-(210j >- (210) Sfc (2O6) Radium D Radium E Radium F Radium G (Polonium) (Lead) — • V •* Active Deposit of Slow Change 2.; years 7-25 days 202 days Fig. 34. These have been shown to originate from two distinct substances successively produced, and which are called uranium Xj and uranium X,. Uranium I expels an «-particle and changes into uranium Xj, which has a period of average life of 35-5 days, and expels, not the penetrating /S-rays of " uranium X," but the feeble and unimportant (/S)-radiation. Its product, uranium X,,, sometimes called brevium, has the very short period of average life of only 100 seconds, and, in its change, the powerful penetrating ;8-rays are expelled. Uranium Xa, after its change, is believed to become uranium II. COMPLETE URANIUM SERIES 151 The latter in its change expels an a-particle and is believed to produce ionium. Uranium X^ and uranium X„, unlike uranium I and uranium II, can be separated from one another by chemical methods. These new discoveries, although of highest theoretical importance, make very little practical difference to the results, which for almost all ordinary purposes are precisely what they would be were the simple scheme, shown in Fig. 29, actually the one followed. Radium C and Radium C. Lastly, there is indirect evidence that radium C consists of two successive products, distinguished as radium C and radium C, the first giving the /3- and 7-rays in its disintegration, and producing the second, which has a period of average life of only a millionth of a second, and changes, emitting an a-particle, into radium D (see p. 202). Fig. 34 shows so far as it is at present known the complete disintegration series of uranium. CHAPTER X RADIOACTIVITY AND THE NATURE OF MATTER Ratio of Quantities of Polonium and Radium IN Minerals. 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 4,500 times that of polonium, so that there must be 4,500 times more radium than polonium in minerals. A good pitchblende with 50 or 60 per cent, of uranium in it 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 of a grain of polonium. This is borne out by Marckwald's experiments, already referred to. Let us apply the law not only to radium and polo- nium, 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 np,me 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 milli- grams (1 milligram is about tV of a grain), then since 1,000,000,000 milligrams is roughly a ton, the quan- tities refer to an amount of mineral containing one ton of the clement uranium. X52 COMPOSITION OF A URANIUM MINERAL 153 Period. TABLE. Uranium I, 8,000,000,000 years. Uranium X . . 35-5 days. Uranium Xj Uranium II, 3 Ionium Radium Emanation Radium A Radium B Radium C Radium D Radium E Radium F (Polonium) 1-6 minutes. ,000,000 years (?). 100,000 years. 2,500 years. 5-6 days. 4-3 minutes. 38-5 minutes. 28-1 minutes. 24 years (?). 7'5 days. 202 days. Quantity. 1,000,000,000 mg. (=1 ton), One eightieth mg. l/250,000th mg. 400 grams (?). 12-5 grams. 312-5 mg. One five-hundredth mg. One millionth mg. Nine millionths mg. Seven millionths mg. 3 mg. (?). One four-thousandth mg. One fourteenth mg. These respective quantities in the last column emit a similar number of a-particles per second in the eight cases where a-particles are expelled at all, and so pro- duce similar radioactive effects. This is an illustration of the compensating 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 radio- active 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 comes into evidence through the energy of the a- particles expelled in its rapid disintegration no less than any of the other products. Impossibility of Concentrating Many of thk Products of Disintegration. The table brings out clearly that radium is but one of many radioactive substances in uranium minerals, Avhich would be of value if they could be extracted. Uranium II, ionium and radium D, all possess suffi- 154 RADIOACTIVITY AND NATURE OF MATTER ciently extended periods of life to repay recovery. Ionium gives only very feebly penetrating a-rays, and so would not be so generally '^useful as radium, whereas uranium II and radium D both, being followed by short- lived products which give y3-rays, would be of great general utility. The reason which has precluded the practical separation of these substances in the past is a general one, which has proved to be of the highest philosophical significance in the chemistry of these new ephemeral elements. They all so closely resemble one or other of the known elements that the separation is impossible. The resemblance between radium and barium is of great practical utility, because these two elements, though very closely alike in chemical nature, can be separated from each other after they have first been separated from every other element. Taking them in order, uranium II cannot yet be separated from uranium I, ionium cannot be separated from thorium, nor radium D from lead. Lead, as has been stated, is almost always present in considerable quantity in ura- nium minerals, and so usually is thorium, but to a much more variable extent. Hence, though it is easy to separate radium D from the mineral with the lead, it is at present useless practically, as it cannot be concen- trated from Lhe lead. By choosing suitable minerals like secondary pitchblendes, which do not contain ponderable quantities of thorium, intensely active pre- parations of ionium can however be separated. It is at present the only one in the uranium ^eries likely to become useful, and its lack of penetrating rays is a serious drawback. Polonium, with its period of less than a year and its absence of penetrating rays, hardly repays extraction, except for purely scientific investiga- tions. There is, however, another disintegration series, that of thorium, which offers a better chance of providing an efficient substitute for radium, and this series will therefore be briefly considered in a later chapter. THE RARITY OF ELEMENTS 155 Increask oj? Radioactivity of Radium ■with Time. 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 disintegration and consists of a-rays. After four weeks the first four products accu- mulate to their equilibrium, and the activity now consists of a-, /8-, and 7-rays, the a-rays being four times as great as initially. It is not difficult to see that the later slow changes must also cause a very slow further continuous increase of all these types of rays, due to the growth of radium E and polonium from radium D. These considerations are embodied in the following table giving an analysis of the total radio- activity of a radium preparation, kept in a sealed vessel so that none of the products escape, at different periods since preparation: a-PARTICLES. /3-PAETICLES. I. Freshly prepared. 1 (due to radium itself) II. After one month. 4 (1 due to radium) 1 or 2 (1 due to emanation) (due to Ra C) (1 due to radium A) (1 due to radium C) III. After a century. 5 (as in II and 1 due 2 or 3 to radium F) (1 due to Ra E2) The Rakity of Elements. The idea, which is a necessary consequence of the atomic disintegration theory, that fixed definite relation- ships must exist between the quantities of elements formed from one another — for example, between ura- nium, radium, and polonium — forms the first indication that physical laws may exist regulating 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 156 RADIOACTIVITY AND NATURE OF MATTER radium are rare because of the rapidity with which they are changing. The degree of radioactivity of an element being proportional 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 ques- tions 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 specu- late upon how far, if at all, the process of atomic disin- tegration, discovered 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 in- evitableness or independence from all known influences. But there is no reason why the process should be limited in its scope to the somewhat special phenomena which led to its discovery. The Currency Metals. It is, for example, natural to inquire whether the scarcity of elements like gold is fixed by the operation of similar physical laws to those which regulate the rarity of radium. The race has giown 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 CURRENCY METALS 157 violently in its degree of rarity — if, for example, in some hitherto unpenetrated fastness of the globe a mountain 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 discovery 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 8,000,000,000 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 that the elements are permanent* and unchanging is substantially correct. At the same time, we cannot but recognise that inevitably the effects of atomic dis- integration, too slow to be otherwise 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. Bvit 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. 1 Since the^' words were first written the whole social machinery of the Western World has been dislocated by violent alteration"! in th? purchasing power of gold, and it has been shown to be no longer a safe medium fur curreacy. (Compare A Fraudulent Slandird, A. T. ICitson. London: P. S. King and Son, Ltd., 1917.) 12 158 RADIOACTIVITY AND NATURE OF MATTER The Nature of Atoms. I now propose considering briefly another question of general philosophical interest in connection with the recent advances of physical science. Naturally the dis- coveries 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 of indi^adual 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 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, at least as far as was known up to the beginning of the present century, completely similar in character. There is, for example, ilot 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 em- ployed 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 A QUOTATION FROM CLERK MAXWELL 159 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, for example, whether in Sirius or in Arc- turub, executes its vibrations in precisely the same time. " Each molecule therefore throughout the universe 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 evolution 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 molecule. We are therefore unable to ascribe either the existence of the molecules or the identity of their properties to any of the causes whicJi we call natural. " On the other hand, the exact equality of each mole- cule 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 scientific path, very near to the point at which science must stop; not that science is debarred from studying the internal mechanism of a molecule which 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 trans- 160 RADIOACTIVITY AND NATURE OF MATTER pired in the forty-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 catas- trophes have occurred and may yet occur in the heavens, though ancient systems may be dissolved and new systems evolved out of their ruins, the molecules out of which these systems are built — the foundation-stones of the material universe — remain unbroken and un- worn." Before we dwell upon the modifications that have been made in this point of view^, let us rather consider 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 definite than it was in 1873. Recent developments in connection with isotopes have modified our point of view, but for the moment we may neglect this special advance. 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 Row- land, of Baltimore, once made the remark that a grand piano must be a very simple piece of mechanism com- pared 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 spectro- scope a difference in " tune " or period in the vibra- THE PERFECTION OF THE ATOMS 161 tions emitted by different .atoms of only one part in many millions would be easily detectable, and no such variation exists. In a similar vein Professor ' Schuster, referring to the broad teachings of the spectro- scope, has compared the atoms of the same element to an innumerable number of clocks all wound and regu- lated 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 many days. No clock- maker could make such clocks. Yet these almost infinitely complicated pieces of mechanism we call atoms are turned out by Nature with such undeviating accuracy and fidelity that in all the myriads in existence there are less than a hundred different kinds known. The Velocity of a-P articles. 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-par- ticles suffers little or 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 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 deviation, is expelled at the same velocity. In the table following, the approximate initial velo- cities of the a-particles from the changes in the uranium series have been collected, together with their " ranges " or distances in millimetres they will penetrate in air at 15° C. and 760 mm. of mercury pressure. 162 RADIOACTIVITY AND NATURE OF MATTER a-PARTICLE Period Velocity FROM (miles per second) Range Uranium I, . 8,000,000,000 years 8,800 . 25 Uranium II, . 3,000,000 years 9,800 . . 29 Ionium 100,000 years 9,400 . . 30 Radium 2,500 years 9,600 . 33 Emanation 5-6 days 10,400 . 42 Radium A 4-3 minutes 10,900 . 47-5 Radium C' . 1/1, 000,000th see. (?) 12,400 . 69-5 Radium F 202 days 10,200 . . 37-7 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 expelled is exactly the same in each case. We may liken the disintegration of an element to the bursting of shells, in which the fragments of the different shells all are expelled with the same velocity. Certainly no shells ever constructed would answer this require- ment. Truly, in the words of Sir John Herschel, the atom bears the essential character of a manufac- tured article, but of a degree of perfection humanly un- attainable. 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 Maxwell'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 favour- able 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. THE SURVIVAL OF ELEMENTS 163 Stability and Survival of Elements. Instead of regarding the hundred or less elements 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 in- stability or stability. Probably for every stable atom many unstable ones could be, even are being, formed. But only the stable forms can accumulate 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 -modifica- tions, in the latter the controlling factors still remain absolutely unknown. By the spectroscope a partial material survey of the whole universe has been ren- dered possible, and what we find is everj^where an essen- tial 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 not yet begun to probe. Yet this question, to us only of academic interest and possibly somewhat remote at that, will, as we shall soon come to see, be one of life and death to the inheritors of our civiUsation. 164 RADIOACTIVITY AND NATURE OF MATTER Connection between Range of k-Rays and Period. A very interesting development may now be men- tioned, which has resulted in a connection being estab- lished between the ranges or velocities of the various types of a-rays, and the periods of life of the atoms from which they are derived. As a general rule — not, it is true, entirely without exceptions, but possibly the exceptions may prove to be only apparent- — the more rapidly a radioactive substance disintegrates, or the shorter its period of average life, the greater is the velocity with which the a-particle is expelled from the atom, and the greater therefore is the range of the a-particle. Thus, the most stable radio-elements, ura- nium and thorium, give a-rays having the lowest ranges, and the low range of the a-rays of ionium was for long the only evidence that its period nmst be very long. The greatest ranges occur in the short-lived " active deposit " products. The very long ranges of the a-rays of radium C (69-5 mm.), and of the corresponding thorium C (86 mm.), is generally explained by the supposition that the real atoms giving these rays have periods of the order of only a millionth of a second, and therefore that it is impossibfe to separate them from their parents, which thus appear to be giving rays which in reality come from their products. This will be referred to again. Latterly, this generalisation has been put into stricter form by the discovery that if the logarithm of the period is plotted against the logarithm of the range or of the velocity, straight lines result for each of the three known disintegration series. The three straight lines are parallel to but not identical with one another. The reason for this is still obscure. Some mathematical connection exists between the two quantities, and that is all that can yet be said. On the other hand, it has been found possible to calculate approximately some of the unknown periods — like that of ionium, so estimated PLEOCHROIC HALOS 165 at 200,000 years, for example, from the ranges of the a-rays by means of this relation before it was directly determined to be 100,000 years. For long it was known that uranium was exceptional in that it appeared to give out two «-particles per atom disintegrating instead of one, as in all other cases. A very careful investigation revealed the fact that the ranges of these two sets of a-particles were not exactly alike. One set, those from uranium I, presumably, have a range of 25 mm., and the other set, those from its shorter-lived product, uranium II, presumably, a range of 29 mm. The period corresponding with 29 mm. of range is, in the uranium series, two million years, and this is the main evidence for believing that such a product, uranium II as it is called, exists, and that it has so far not been separated from uranium because of the identity of the chemical properties of the two elements. Pleochroic Halos. The account given in this chapter and in Chapter III. of the many extraordinary properties of the a-partiele would be incomplete if another natural phenomenon in a totally distinct field were omitted. The a-, in common with the other rays from radioactive substances, have the power of darkening glass and other transparent materials such as mica after long exposure. Indeed, the colours of many natural gems have been traced to the effect of such rays from naturally occurring radio- active materials in the earth, operating over immense periods. Sir William Crookes artificially coloured a large colourless diamond an intense green by exposing it for some weeks to the rays from a pure radium compound. Many other gems, usually found in a colourless state, can similarly be made to assume the most varied colours, the nature of which depend probably upon slight chemical impurities present in the gem. Mica under these circumstances becomes deeply stained and dark. 166 RADIOACTIVITY AND NATURE OF MATTER Now, occurring in various natural micas, there are sometimes found microscopic halos of darkening of perfect circular outline, called pleochroic halos. These have been very exhaustively studied by Professor Joly, and the microphotographs shown in Figs. 35 and 36 are taken from a paper by him and Mr. Fletcher in the Philosophical Magazine for 1910. Fig. 35 shows two of these halos in a specimen of mica. Sometimes the halos are made more visible by the use of polarised light, but this is not always necessary. It can be shown, by suit- ably sectioning the material, that the halos are true spheres, and often at the centre a minute microscopic nucleus is visible. Professor Joly measured exactly with the microscope the diameter of these halos, and found them to correspond perfectly correctly with the " range " of the a-particles from radium C, which in mica is 0-06 mm. He put forward the view that they were due to a-particles, from radioactive material in the central nucleus, darkening the mica over a sphere bounded by the range of the a-rays. This conclusion has been most brilliantly confirmed. It is possible to find halos in various stages of development. Young and incompletely developed halos often show only a central " pupil " of only 0-013 mm. in radius. This corresponds with the range of the shorter a-particles, due to uranium, ionium, and radium itself. In later stages a distinct " corona " appears of the full radius, 0-03 mm., which is the range of the a-particle from radium C in mica. And in particularly favourable cases it is possible to see between them an inner ring of dimensions corre- sponding with the intermediate range of the a-particles of radium A. A much enlarged micro-photograph of such a halo is shown in Fig. 36. Uranium and Thorium Hai.os. Moreover, a careful search revealed other halos of slightly greater radius than 0-03 mm. — viz., 0-038 mm. — which corresponds with the range of the fastest a-par- y f s k i% ...^^ ^ ^"^^H ^ ^^ ■ -^ij ^L i ^H :F'C^J^K ■ .. —i^jlSj^^- ^■■if ^ WPJ HP 1 kl ^^^^^Hlr Fig. 35. — Thorium and Radium Halos in Biotite. ( X 150 Diameters.) Fig. 36. — Halo in Biotite. ( x 450 Diameters. ) Showing ling due to Radium .V. To face p. URANIUM AND THORIUM HALOS 167 tide emitted in the thorium series. An examination of them showed a course of development totally different from that of the uranium halos. The successive states m this case correspond with the a-rays of the ranges that are emitted in the thorium series. As a matter of fact' the lower halo in Fig. 35 is due to uranium and the upper one due to thorium. The uranium halo is fully developed, so that the central "pupil," though visible in the microscope, cannot be seen in the reproduction. The thorium halo shows faintly but quite clearly the corona due to the long range rays of thorium C, the longest known. Still other halos attributed to radium emanation without the earlier members of the series ha.ve been observed. It may be concluded that the nucleus at the centre either contains uranium or thorium in minute quantity, or has the power of occluding radium emanation from water that has flowed through uranium minerals. But the actual amounts of radioactive materials so put into evidence are almost inconceivabh^ minute and far beyond the power of detection even by the most sensi- tive electrical method. It has been estimated that they are due to the expulsion of sometimes less than 100 ff-particles per year, continuing for several hundred million years. The mica integrates these infinitesimal effects throughout the ages so that at length they are able to ptoduce consequences visible to the eye. Until this explanation was forthcoming, they had remained a complete puzzle to the petrologist. CHAPTER XI RADIOACTIVITY AND THE EVOLUTION OF THE WORLD The Potentialities of Matter. 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 pre- dicting many, and perhaps even more unexpected, new ones. Let us from the point of vantage we iiave 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 peculiarities which raised it far above the ordinary run of common matter. The matter was the mere vehicle of ultra-material powers. If we now ask, why is radium so unique among the elements, the answer is not because it is dowered with any exceptional 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, 108 POTENTIALITIES OF MATTER 169 whereas the elements before known are either changing not at all or so slowly that the change has been unper- ceived. 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 matter. The aspect which matter has pre- sented to us in the past is but a consummate disguise, concealing 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. Why Radium is Unique. 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 con- tributed by a very small fraction of the whole, namely, the part which is actually disintegrating at the moment of observation. The whole of the rest of the radiiun is as quiescent and inactive as any other non-radio- active element. In its whole chemical nature it is an ordinary clement. The new properties are not con- tributed at all by the main part of the matter, but only by the minute fraction actually at the moment disintegrating. 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, dia- metrically 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, 170 RADIOACTIVITY AND EVOLUTION 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 com- pensated 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 disintegrat- ing. The common element uranium, well known to chemists for a century before its radioactivity was sus- pected, is in reality even more wonderful. It is only very feebly radioactive, and therefore is changing excessively slowly, but it changes into radium, expelling several a-particles and so evol\ang 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 teU, it will change, producing over thirteen ounces of radium, and, in that change into radium alone, energy is given out, as radioactive energy, aggregating 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, contains > all the energy contained in a but slightly smaller quantity of radium and more. It may be estimated that uranium evolves during complete disintegration some thirteen 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 INTERNAL ATOMIC ENERGY 171 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 radio- active elements, that is to the few which, however slowly, are actually changing ? Not at all, in all probability. Regarded merely as chemical elements between radioactive elements and non-radioactive ele- ments, 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 spec- trum, to the non-radioactive elements, barium, stron- tium, 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 aj^pears rather that this internal 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. The Total Energy evolved by Uranium. Let us, however, for the sake of conciseness, leave out of account altogether the non-radioactive elements, of which as yet we know nothing certainly. At least we cannot escape from the conclusion that the particular element uranium has relatively more energy stored up within it even than radium. Uranium is a compara- tively common element. The world's output per year is to be reckoned in tens of tons, whereas that of thorium, which we have still to consider, exceeds a thousand tons. I have already referred to the total amount of energy evolved by radium during the course of its complete change. It is about 360,000 times as much energy as is evolved from the same weight of coal in burning (p. 1 20). The energy evolved from uranium would be some thirteen 172 RADIOACTIVITY AND EVOLUTION 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 £1. Is it not wonderful to reflect that in this little bottle there lies asleep and waiting to be evolved the energy of at least one hundred and sixty 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 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 "7 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 kind. The difficulty is of quite another character. As we have seen, we cannot yet artificially accelerate or influence the rate of dis- integration 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 transmuta- tion. 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 interna] energy of matter, and the unlocking of the , internal stores of energy in matter would, strangely enough, be infinitely the most important and valuable consequence of transmutation. The Impoetance oe 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 TRANSMUTATION 173 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, so great an amount of energy would prob- ably be evolved that the gold in comparison would be of little account. The energy would be far more valuable than the gold. Although we are as ignorant as ever of how to set about transmutabion, it cannot be denied that the knowledge recently gained constitutes a very great help towards a proper understanding of the problem and its ultimate accomplishment. We see clearly the magnitude of the task and the insufficiency of even the most powerful of the means at our disposal in a way not before appreciated, and we have now a clear perception of the tremendous issues at stake. Looking backwards at the great things science has already accomplished, and at the steady growth in power and fruitfulness of scientific method, it can scarcely be doubted that one day we shall come to break down and build up elements in the laboratory as we now break down and build up compounds, and the pulses of the world will then throb with a new source of strength as immeasurably removed from any we at present control as they in turn are from the natural resources of the human savage. Primitive Man and Fire. It is, indeed, a sbrange situation we are confronted with. The first step in the long, upward journey out of barbarism to civilisation which man has accom- plished appears to have been the art of kindling fire. Those savage races who remain ignorant of this art are regarded as on the very lowest plane. The art of Idnd- ling fire is the first step towards the control and utilisa- tion of those natural stores of energy on which civilisa- tion even now absolutely depends. Primitive man 13 174 RADIOACTIVITY AND EVOLUTION 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 frorn naturally occurring conflagrations. W^ith 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 chmax of that civihsation 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 indefinitely 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 reahty 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 suppUes. The very existence of the latter till now have remained un- known 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. Source of Cosmical Energy. The question has frequently been discussed whether transmutation, so impossible to us, is not actually going on under the ti-anscendental conditions obtaining in the sun and the stars. We have seen that it is actually going on in the world under our eyes in a few special COSMlCAL ENERGY 175 cases and at a very slow rate. The possibility now under consideration, however, is rather that it may be going on vmiversally or at least much more generally, and at a 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 incessant dissipation of energy throughout the universe would disappear. Last century has wrought a great change in scientific thought as to the nature of the gigantic forces which have moulded the world to its present form and which regulated the march of events throughout the universe. At one time it was customary 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 eruptions of a Krakatoa or Pelee would be insignificant. Now, however, we regard the main process of moulding as due rather to ever-present, con- tinuous, 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 are but a passing incident of a continually shifting scene. Into the arena of these silent world-creating and destroying influences and pro- cesses has entered a new-comer — " Radioactivity " — and it has not required long before it has come to be recog- nised that in the discovery of radioactivitj', or rather of the sub-atomic powers and processes of which radio- activity is merely the outward and visible manifes- tation, we have penetrated one of Nature's innermost secrets. Whether or no the processes of continuous atomic disintegration bulk largely in the scheme of cosmical evolution, at least it cannot be gainsaid that these pro- cesses are 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 176 RADIOACTIVITY AND EVOLUTION of which the universe to-day is a going concern rather than a cold, hfeless collocation of extinct worlds. Slow, irresistible, incessant, unalterable, so apparently feeble that it has been reserved to the generation 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 slowprocesses 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. Radium in the Earth's Crust. It is 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. B\it this story would be incomplete if I did not refer, though it must be but briefly, to the labours of Pro- fessor Strutt^ Avho initiated the movement and to 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 common rocks is of course very small, although with the refined methods now at the disposal 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 by radiation has long been accurately known. Strutt concluded that if there existed only a comparatively 1 Now Lord Rayleigh. RADIOACTIVITY AND GEOLOGY 177 thin crust of rocks less than fifty miles thick of the same composition, as regards the content of radimn, 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. Otherwise 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 maintain 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 interesting work, Fadio- activity and Geology, has considered in detail some of the consequences of the existence of radioactive materials in the earth. One of the specific instances is the effect of the radium in the rocks of the Simplon Tunnel in producing the unexpectedly high temperatures there encountered. From a radioactive analysis of these rocks he came to the conclusion that without undue assumptions it is possible to explain the differences in the temperature of the rocks encountered in boring the tunnel by the differences in their radium content. Various Possible Fates of the Earth. The presence in the rock of a proportion amounting to a few million millionths of radium above the normal quantity very nearly wrecked the whole enterprise. From the importance of radioactivity in this instance, of a tunnel a few miles long bored through a mountain, some idea may be obtained of the significance of the new discoveries in the general problem of the thermal condition of the interior of the globe. Since Strutt's 178 RADIOACTIVITY AND EVOLUTION original work, it has been established that not only- radium, but all the other radioactive materials, includ- ing the whole thorium disintegration series, must con- tribute an important quantity of heat, so that his estimate of a crust only fifty miles thick is in reality too great, and a much thinner crust would suffice. Joly has had the courage to push the argument to its logical conclu- sion, and has supposed that the radioactive materials are not confined to a thin surface crust, but are equally distributed throughout the globe in nmch the same proportions as they are in the crust. If this is so, there is no escape from the conclusion that the interior of the earth, so far from gradually parting with its heat and cooling down, must actually be getting steadily hotter. The heat generated within, even after the lapse of hun- dreds of millions of years, would scarcely appreciably escape from the surface, for, as Lord Kelvin deduced, the central core of the earth must be almost insulated thermally from the surface, owingto the low conductivity of the rocks composing the crust. He assumes through- out an average composition of the globe of two parts of radium per million million, which is considerably below the average he found for the rocks of the crust, and he calculates that in the course of a hundred milUon years this minute quantity will produce a rise of the tempera- ture of the central core of no less than 1,800° C. Unless, therefore, this heat is utilised in some unknown way, or the disintegration of the radio-elements is prevented by the high temperature and pressure, the ultimate fate of the globe must be very much as depicted in the Biblical tradition. Sooner or later the crust must succumb to the ever-increasing pressure within, and the earth must become again, what it is supposed once to have been, a vastly swollen globe of incandescent gas. As Joly remarks, there is no evidence that this has not alreadv occurred more than once, nor assurance that it will not recur. So far as physical science yet can deduce, the accumulation of thermal energy within a world con- taining eleraenfs undergoing atomic disintegration during GEOLOGICAL AND INCANDESCENT AGES 179 the " geological age " must alternate with a state of things which might be termed " the incandescent age," in which this accumulated energy is dissipated by radia- tion. This periodic cj^cle of changes must continue until the elements in question have disintegrated — that is, over a period which radioactive measurements indicate is of the order of tens or hundreds of thousands of millions of years. During the incandescent age the loss of heat by radiation, which increases according to the fourth power of the temperature, is immensely greater than could be supplied even by atomic disintegration. Thus, if the known laws hold, it is certain that the present loss of heat of the sun 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 im- probable. Still, it is by no means to be concluded that the heat of the sun and stars is not in the first place of inte nal rather than, as has been the custom to regard it, of exfernal origin. As soon as sufficient of the heat energy of a world has been radiated away for a solid crust to form, the poor thermal conductivity of this crust at once reduces the radiation loss to a negligible figure again, a fresh geological age is inaugurated, and again the heat accu- mulates within. This view, that the elements contain within themselves the energy from which Nature obtains her primary supplies, and that in cosmical time " geo- logical age " and " incandescent age " alternate as the night and day, however imperfect it may still be, is at least more in harmony with existing knowledge than the older conventional view that the universe was wound up once for all in the beginning like a clock to go for a certain time, for the most part quietly and uneventfully, pursuing its allotted path towards ultimate physical stagnation and death. But what a picture it conjures up of life and of the precariousness of its tenure — from its lowest beginnings to its highest evolution, not a permanent accomplishment, but a process to be inaugu- 180 RADIOACTIVITY AND EVOLUTION rated and consummated afresh, if at all, between the ending and beginning of each new cosmical day ! To escape from this conclusion it is necessary to suppose that atomic disintegration is cosniically not the inevitable uncontrollable process it has hitherto been proved to be under all laboratory conditions, but that under conditions of pressure and temperature, such a§" exist in the interior of a world, it may either be stopped altogether, or compensated for by unknown comple- mentary processes of atomic synthesis in which energy is taken up. The Most Probable View. The balance of probability appears to rest with the view that the radioactivity of the materials comprising the earth is confined to a crust and that the central core is more or less free from radioactive matter. Our knowledge of earthquake phenomena, and particularly of the three distinct routes by which an earthquake wave travels from one point on the surface of the earth to another — (1) and (2) by circular paths clockwise and counter-clockwise through the crust, and (3), the " P3 " route, by a straight line joining the two points — has strongly supported the view that the core of the earth is of a totally different nature from the crust. On the P3 route, once the wave gets below the crust, it travels much faster than it does through the surface. This, especially, confirms the picture of the earth as a metallic sphere of nickel-steel within, surrounded with a thin surface layer of solidified slag, which its high specific gravity and the composition of meteorites first sug- gested. On this view, it is to be expected that the radioactive materials will be confined to the erust and be absent from the metallic core, and, therefore, that the crust may have reached a steady temperature, at which the loss of heat by radiation is exactly balanced by the heat evolved by its radioactive constituents. If this is so, the present state would continue without much change for hundreds of millions of years, RADIOACTIVITY AND MYTHOLOGY 181 Be that as it may, our outlook on the physical uni- verse has been permanently altered. We are no longer the inhabitants of a universe slowly dying from the physical exhaustion of its energy, Ijut of a universe which has in the internal energy of its material compo- nents the means to rejuvenate itself perennially over immense periods of time, intermittently and catastro- phically, which is the first possibility that presents itself, or continuously and in orderly fashion, if there exist compensating phenomena still outside the ken of science. Radioactivity and Mythology. The world probably being of much greater antiquity than physical science has thought to be possible, it is interesting and harmless to speculate whether man has shared with the world its more remote history. In this connection 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, knoAvn 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 evolu- tion of matter — 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 attractive and consistent explanation of the universe in the light of present know- ledge is, perhaps, that matter is breaking down and its energy being evolved and degraded in one part of a C3'^cle of evolution, and in another part, still unknown to us, the matter is being again built up with the utilisation of the waste energy. 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 imiversal and deep- 182 RADIOACTIVITY AND EVOLUTION rooted that we are accustomed to consider them almost as old as the race itself. One is tempted to inquire how far the unsuspected aptness 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 is 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 probably trace to its real source. The philosopher's stone 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 priraarj'' fountains of the physical life of the universe to-day is known to be transmutation. Is, 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 unrecorded 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 founda- tion affoided 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 the knowledge we have so recently won, but also to the power that is not yet ours ? Science has THE FALL OF MAN 183 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 tradi- tional 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 irreconcilable 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 favourable worlds as the superfluous to-day emigrate to more favourable continents. The legend of the Fall of Man, possibly, may be all that has survived of such a time before, for some unknown reason, • the whole world was plunged back again under the ' undisputed sway of Nature, to begin once more its upward toilsonie journey through the ages. The New Prospect. The vistas of new thought which have opened out in all directions in the physical sciences, to which man is merely incidental and external, must in turn react powerfully upon those departments of thought in which man is central and supreme. We find ourselves in con- sequence of the pi'ogress of physical science at the pin- nacle of one ascent of civilisation, taking the first step upwards out on to the lowest plane of the next. Above us still 1 ises indefinitely the ascent to physical power — • far bey -nd the dreams of mortals in any previous system of philosoph}^ 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 184 RADIOACTIVITY AND EVOLUTION for in the old familiar way, to be wrung from the grip of Nature, as all our achievements and civilisation have, in the past, been wrung by the labour of the collective brain of mankind guiding, directing, and multiplying the individual's puny power. This is the message of hope and inspiration to the race which radium has con- tributed 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. It is fitting to attempt to see how far purely physical considerations will take us in delimiting the major controlling influences which regu- late our existence. Surveying the long chequered, but on the whole con- tinuous, 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 generali- sation the most fundamental 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 continuous. 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 passing into the waste and useless unavailable kind, and this process, so far as Ave 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 aboriginal savage, The new prospect iss ignorant of agriculture and of the 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 susten- ance, 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 con- cerned, an entirely new prospect has been opened up. By these achievements of experimental science Man's inheritance has increased, his aspirations have been up- lifted, and his destiny has been ennobled to an extent beyond our present power to foretell. The real wealth of the world is its energy, and by these discoveries it, for the first time, transpires that the hard struggle for existence on the bare leavings of natural energy in which the race has evolved is no longer the only possible or enduring lot of Man. It is a legitimate aspiration to believe that one day he will 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 alters somewhat the relation of Man to his environment, and adds a dignity of its own to the actualities of existence. PART II CHAPTER XII THE THORIUM AND ACTINIUM DIS- INTEGRATION SERIES The Thorium Disintegration Series. Those who have mastered the intricacies of the uranium disintegration series may wish to know something of the important developments which have taken place since these lectures were first given in 1908, and of the other two great disintegration series known to science, the thorium and the actinium series. Space precludes a description as detailed and non-technical as that before aimed at, and in some of the more difficult sections it will be necessary to assume a considerable knowledge on the part of the reader of physical and chemical science. But the attempt seems worth making for the sake of completeness. The thorium disintegration series is becoming in- creasingly important, and its consideration does not involve any new principles. Thorium is an element which was at one time rare and little known even to chemists, but has come into prominence during the last twenty years, because of its use as a constituent of the incandescent gas-mantle, which is composed of about 99 per cent, of thorium oxide, and 1 per cent, of cerium oxide. Fairly abundant sources of thorium have been discovered in the sands of certain coasts in Brazil, North and South Carolina, etc., where a natural con- centration has taken place by the action of the sea- waves of the particles of the heavy mineral monazite, 186 THORIUM 187 which occurs as a itiinute constituent in many rocks, and in the sands derived from them by the action of weathering agencies. The monazite is concentrated from the sand usually by magnetic methods, until it contains 4 per cent, of thorium oxide. This constitutes the monazite sand of commerce, and from it every year hundreds' of tons of pure thorium salts are now manu- factured for the gas-mantle industry. More recently the find has been made of a very rich monazite in Central India, containing nearly 10 per cent, of thorium. Mesothorium and Radiothorium. As already described, the usual a-radioactivity of commercial thorium compounds is of about the same strength as that of pure uranium compounds, but the /3- and y-, or penetrating activity, is several times less intense. We have seen (p. 153) that in the uranium minerals, although several intermediate products of the disintegration of uranium are present, with periods of life sufficiently long, and radioactivity sufficiently im- portant, to repay extraction, it is practicable to extract only one of these — namely, radium. In the thorium minerals there are two such products, named meso- thorium and radiothorium, and though their periods of average life, about eight years and three years respec- tively, are very much less than that of radium, they are sufficiently long to make their extraction and utilisation practicable. Whereas the sources of radium are costly and comparatively limited in amount, the by-products of the thorium industry, after the extraction of the technically valuable thorium, are the source from which mesothorium and radiothorium are extracted. Much greater quantities of these by-products have to be handled, it is true, than in the extraction even of radium from pitchblende to produce similar results. The new substances must, on this account, always be costly to produce. But in the by-products of a single year's manufacture of thorium the new products capable of 1S8 THORIUM AND ACTINIUM being extracted possess as much radioactivity as at least an ounce of pure radium. They thus offer an abundant source of radioactive material, which at present is mostly wasted, and the product, while it lasts, is in every respect the equal of radium in its properties. The only disadvantage it possesses is its relatively much shorter period of life. The discoveries in the thorium series of these two technically valuable members were made by Otto Hahn, who has worked both with Sir William Ramsay and Sir Ernest Rutherford, comparatively recently, after the rest of the members had become quite well known. The historical development of the subject from the first dis- covery of the radioactivity of thorium compounds up to the present time is a most interesting chapter to the student, but would unduly complicate the subject if considered here. It is better to proceed in order through the thorium disintegration series as it is at present known, apart from historical considerations as to the order in which they were discovered, though, as in the case of the uranium series, the first members were the last to be separately recognised. Radioactivity of Thorium. Unlike pure uranium salts, which, a few months after preparation, have a definite constant radioactivity, consisting of all three types of rays, the a-activity being due to uranium, and the /3- and 7-activity to the short- lived uranium X in equilibrium with it, thorium salts, though chemically pure, vary continuously in their whole radioactivity for twenty or thirty years after manu- facture. Even after these periods, slight changes must still be going on, and probably fifty years would have to elapse before they became quite inappreciable. But in spite of the great apparent differences between the two elements, there is a very close analogy in their disin- tegration series, every one of the eleven known members of the thorium series having an analogue in the twelve RADIOACTIVITY OF THORIUM 189 members of the uranium series as far as radium D, at which point the thorium disintegration appears to come to an end. One a-ray giving product in the uranium series is not represented in the thorium series. The analogous members in the two series usually give out similar kinds of rays, and although their periods are often widely different, there is a rough correspondence in the two series between the relative periods of the suc- cessive members, the periods in the thorium series being, however, usually much less than in the uranium series. Thus uranium I, with its period of 8,000,000 years, gives a-rays, and is followed by uranium X^, giving (/S)-rays, of period 35-5 days, and by uranium Xj, or brevium, of very short period, which gives powerful and penetrat- ing /3-rays. Uranium II, which follows, is chemically identical with uranium I, and, like it, is of long period and gives a-rays. This produces ionium, which gives «-rays, and has the period of 100,000 years. Ionium, in turn, produces radium, which gives a-rays, and has a period of 2,500 years. Thorium itself is provisionally estimated to have a period about three times longer than uranium I, and gives only «-rays. It produces by its disintegration " mesothorium I," which does not give any important rays, and has a period of 7-9 years. It is identical in chemical character with radium, and corresponds with uranium X^, except that no yS-rays at all are expelled. Its product is called " mesothorium II," which corresponds very closely with uranium Xj, giving out powerful /3- and 7-rays, and having a period of only 8'9 hours. It produces in turn " radiothorium, " which corresponds perfectly with ionium, giving a-rays, and having a period of 2-9 years. These last two sub- stances are chemically identical with one another, and also with thorium itself, and caimot be separated by any known method when mixed together. This fact is of considerable importance, as thorium when separated from a mineral, always contains at first all the radio- thorium in the mineral and also all the ionium, if ura- nium was also present, as is almost invariably the case. 14 190 THORIUM AND ACTINIUM The product of radiothorium is thorium X, which corre- sponds with radium, giving a-rays, but having a period of only 5-6 days. Thorium X is chemically identical with radium, and also with mesothorium I. This chemical identity of radium and mesothorium I is the dominating fact in the separation of these new sub- stances, as will later be more clear. After thorium X, the thorium emanation results, corresponding perfectly in its whole nature as a member of the argon family of gases, with the radium emanation, and giving a-rays, but having the much shorter period of only 76 seconds. Its product is the thorium active deposit, of which the Meso- Meso- Radio- Thorium X. thorium I. thorium II. thorium. 26,000,000,000 7"9 years. 8'9 hours. 2-91 years. 5'35 days- (?) years 0« Qot. •(^ QaL •/Sf'"/) Emanation. Thorium A. Thorium B. Thorium C. Thorium D. Thorium E. 76 seconds. 0-2 second. 16'3 hours. 79 minutes. 4-6 minutes. (Lead.) Fig. 37. first three members, called thorium A, B, C, are almost precisely analogous to the corresponding radium mem- bers, except in period. The period of thorium A is only one-fifth of a second. Those of thorium B and C are 15'3 hours and 79 minutes respectively. These last are the only two, except thorium itself, possessing periods longer than the corresponding members of the uranium series. Lastly, there exists, as the product of thorium C, thorium D, the last active member known, which gives yS- and 7-rays, and has the short period of 4-5 minutes. It has little analogy to radium D. The ultimate pro- duct of thorium was till recently not even guessed. All that could be said is that its atomic weight, calculated THE THORIUM SERIES 191 from that of thorium and the number of a-particles ex- pelled, is 208, and this is the atomic weight of bismuth ! It cannot be bismuth, because in some ancient thorium minerals hardly a trace of bismuth can be found. The discovery of its nature came as a surprise, for in spite of the difference of atomic weight, it proves to be the same element as ends the uranium series — namely, lead. This has raised very deep issues. The complete thorium disintegration series is shown on p. 190 (Fig. 37), so far as we have yet considered it. But thorium C, there shown single, is like radium C complex (see p. 201). The extraordinary analogies between this series and the uranium series, on the one hand, and the actinium series, on the other, will later receive a very satisfying explanation. Mesothorium. It is clear that mesothorium I, with the period of average life of nearly eight years, being both the first and the longest lived of the successive products, is the centre of interest. The radioactivity of the element thorium itself, consisting onlj?- of low-range a-rays, of relatively feeble intensity because of the enormous period of the element, is technically and scientifically even of less interest than that of uranium. Mesothorium, how- ever, corresponds to radium in that it can be concen- trated, and the greater part of the radioactivity of many tons of minerals can be separated in a preparation weigh- ing less than a few milligrams. Just as when radium is first prepared it gives only the relatively ur important a-activity proper to itself, but in course of tinie develops enormously in all its activity due to the growth and accumulation of its products, so it is with mesothorium. Freshly prepared and free from its products, it has practically no activity. In the course of a few hours the strong penetrating activity of its short-lived product, mesothorium II, develops, and in two or three days this reaches a maximum or equilibrium value. This part of the activity then remains, so long as the preparation is 192 THORIUM AND ACTINIUM not chemically treated, apparently constant, but actually decaying Yery slowly. This decay is to half the initial value after 5-5 years, to a quarter after 11 years, and so on. But the product of this change is radiothorium, which gives a-rays; and, just as in the case of radium, this is followed by a small host of short-lived products, some of which give a- and others /S- and 7-rays. "What actually happens, therefore, is that in addition to the initial rapid growth of ^- and 7-rays already discussed, a slow steady increase of , the a-, /S-, and 7-activity of a meso- thoriuni preparation takes place for many years after its preparation, due to the growth and accumulation of radiothorium and its products. It is calculated that this increase will go on for about four and a half years, and then the activity of the preparation will reach a maximum, the penetrating activity (/3- and 7-rays) being then nearly twice that at two days after prepara- tion. From then onwards the regular slow decay of all the radioactivity will set in, and continue with the half- period of five and a half years, as already considered. Twenty years after preparation the activity will be some 12 per cent., whilst after a century it would be less than 1,000th per cent, of the maximum activity. In practice, however, the change is even more com- plicated than this on account of the invariable presence of radium in the mesothorium preparations. Radium and mesothorium form, as already remarked, an example of which now so many exist in radioactivity, of two different elements, having entirely different radioactive, but entirely identical, chemical character. For a long time the nature of the chemical processes used to extract mesothorium from the by-products of monazite was kept secret. It was thought that they were peculiarly diffi- cult and forbidding. I was therefore surprised and interested to find — and the same discovery was made at about the same time by Professor Marckwald in Berlin — that mesothorium and radium behaved in chemical processes identically. In consequence the extraction of mesothorium from monazite residues is entirely MESOTHORIUM 193 similar in principle to that of radium from pitchblende residues. Since monazite always contains a minute amount of uranium, and therefore the corresponding quantity of radium, the mesothorium separated alwa)'S contains the radium also. No successful separation has as yet been achieved, and it is most improbable that it ever will be. After a lengthy fractional crystallisation of the mixture I found the relative proportions of the two elements entirely unaltered. Technical meso- thorium owes about 12 per cent, of what has been termed its maximum activity (that after four and a half years) to radium. This activity will remain when all that due to mesothorium has completely decayed away. In practice, therefore, the decay of the preparations will be appreciably slower than if radium were absent. These discoveries have thus resulted in the provision of an effective substitute for radium, which for such purposes as medical application, or for general researches in the properties of the new radiations, are. while the activity lasts, its equal in every respect. Indeed, it is possible to obtain mesothorium preparations many times more concentrated in their activity than pure radium salts. There is no dearth of the raw material, which hitherto has been a wasted product. But, of course, from the strictly scientific point of view, the radioactivity of mesothorium is as distinct from that of radium as copper is from zinc, or as one flower is from another. It will be of interest to con- centrate upon some of the chief resemblances and differ- ences in the two disintegration series. The Thorium Emanation. The thorium emanation was the first of the three emanations to be discovered, and had been fairly com- pletely investigated by Rutherford before the others were known. It is given off in greater or less degree by all thorium compounds. If the radioactivity of the compound is measured by placing it in a closed electro- 194 THORIUM AND ACTINIUM scope, the activity is found to increase for about ten minutes, owing to the accumulation of the emanation, and then remains constant if the instrument is not dis- turbed. But if a current of air is blown through the instrument, it sweeps out the emanation, and the acti- vity is correspondingly reduced. On stopping the blast of air, it rises again precisely as at first. Uranium com- pounds show no trace of this behaviour, as they do not generate an emanation. The products of the disintegra- tion of the thorium emanation are known as the thorium active deposit, and they manifest themselves in much the same way as the radium active deposit, being attracted to the negatively charged surface in an electric field. They last much longer, however, the period of half- decay being about eleven hours instead of about half an hour, and, in consequence, they take longer to accu- mulate. In a vessel containing a thoiium or, better, a radiothorium preparation, which acts as a constant source of the evanescent thorium emanation, the active deposit on the walls of the vessel (or on the negative electrode, if an electric field is used), goes on increasing in amount for about two days, whereas in the radium emanation the active deposit reaches the maximum value in about three hours. Radiothorium. Radiothorium is the most powerful and convenient source of the emanation and active deposit of thorium. As already explained, radiothorium is not separable from thorium by any chemical process. Freshly pre- pared thorium compounds contain practically all the radiothorium of the original mineral, but its parent meso- thorium being absent, this radiothorium in the course of a few years decays. Before it decays completely, however, mesothorium has been regenerated by the thorium, and in time begins to produce fresh radio- thorium. The consequence is that commercial thorium compounds always contain more or less radiothorium, IlADIOTHORIUM 19S( and always, therefore, furnish more or less of the emana- tion and active deposit. But the amount is insignificant compared with what can now be obtained from a com- mercial radiothorium preparation. Mesothorium, after it is separated from the mineral and left to itself, pio- duces, as we have seen, radiothorium. After a year or more of accumulation these two substances may with advantage be separated. A trace of a thorium salt is added to the solution, and then precipitated by adding ammonia as thorium hydroxide, which carries with it the whole of the radiothorium, leaving the mesothorium in solution. This radiothorium preparation in turn gener- ates thorium X, and after a few weeks becomes a power- ful source of the thorium emanation during the few years it lasts. Apart from the intrinsic interest attaching to this method of " growing " radio-elements otherwise not separable from the raw material a point of great philosophical interest is involved. Were it not for the existence of mesothorium intermediate between and chemically distinct from thorium and radiothorium, the separate existence of the latter might not have been suspected, and they certainly could never be obtained as individuals. In the case of uranium I and uranium II, the evidence for the existence of two elements remains indirect, and they have never yet been separated. The intervening member, uranium X, is too short-lived and the product uranium II too long-lived for the quantity of the latter produced from the former to be detectable even by radioactive methods {vide pp. 129 and 150). One can hardly help wondering how many of the well-known common so-called elements may not be mixtures of more than one element with chemically identical properties. Experiments with the Thorium Emanation. Radiothorium may be used to show, in a striking way, by means of phosphorescent screens, many of the older classical experiments on the growth and decay of radio- 196 THORIUM AND ACTINIUM active substances on which the existing theory of atomic disintegration has been built up. For example, if a radiothorium preparation or old mesothorium prepara- tion containing radiothorium, is kept in a. tube through "which a puff of air can be sent from a rubber blower, and the accumulated emanation is thus blown out into a flask internally coated with zinc sulphide, as shown in Figs. 8 and 9, it will cause it to phosphoresce brilliantly in the dark. The decay of the emanation in the flask can then be watched from minute to minute with the eyes, and its concomitant reproduction in the radio- thorium tube can easily be demonstrated. For example, the radiothorium tube may first be thoroughly blown out, and then the effect observed of blowing through it into a zinc sulphide flask immediately, before any emana- tion has had time to accumulate, and then after waiting successive periods of, say, ten, twenty, thirty seconds, one, two, ten, or more minutes. For the shorter intervals the amount of emanation produced is very nearly pro- portional to the time, but for the longer ones the decay of that produced first during the period of accumulation begins to tell, and the increase with time gets less and less. So that after five or ten minutes no increase results, however long a time is allowed to elapse. The emanation is then in " equilibrium," as much decaying per second as is produced per second. In this way many of the simple laws of the decay and reproduction of the emanation, on which the whole super- structure of radioactive theory was at first largely based, may now be shown to a large audience. But all the original work was done with delicate electrical instru- ments long before anyone had ever observed a single visible effect, or had any other than indirect elec- trical evidence of the existence of the evanescent emanation. THORIUM A 197 Thorium A. The most recent member to be added to the thorium disintegration series is thorium A, the direct product of the emanation, which, on account of its short period of average Ufe — about one-fifth of a second only — had hitherto not been separately distinguished from the emanation. It was put in evidence by Rutherford and his colleagues in the following ingenious manner: An endless wire passed along the axis of a cylinder, con- taining a radiothorium preparation, through holes in ebonite stoppers closing the ends of the cylinder, and over suitable pulleys outside of the cylinder driven by an electric motor. In this way the wire was kept passing through a cylinder filled with thorium emanation. The wire was connected to the negative and the cylinder to the positive pole of a battery, so as to concentrate the active deposit on the wire. It was found that the wire iinmediately after leaving the cylinder was intensely active, giving out powerful a-rays, and capable of light- ing up a zinc sulphide screen brcjught near to the wire. This activity on the wire lasts only a small fraction of a second, so that after the wire has moved away a little from the cylinder its activity has practically disappeared. Thus, although the wire is being driven at a high speed all the time, it is only the part immediately issuing from the cylinder which is active, and which causes the sul- phide screen to glow. Thorium A is a non-volatile product of the gaseous emanation, and is attracted to the negative electrode. But almost as soon as it is deposited it breaks up, giving a-raj^s. On the principle of a short life and a merry one, the elTect of this product is far more marked, for short periods of exposure, than that of the longer-lived products it in turn produces. Though it would be easy to show that the wire, after the large activity due to thorium A is over, still possesses activity due to the products formed, this activity is, for short periods of exposure, far too small to light up a 198 THORIUM AND ACTINIUM phosphorescent screen. In this way the existence of this almost hopelessly unstable element has been demon- strated. In connection with the thorium active deposit and the complex character of thorimn C something has still to be said, but it may be deferred. The Actinium Disintegratiox Series. A few words may be said for the sake of completeness on the third and least important disintegration series, but one which is, however, just as interesting to the student as the others. In addition to the polonium and radium separated from pitchblende by M. and Mme. Curie, a colleague, M. Debierne, was successful in isolat- ing a third new radio-element, to which he gave the name Actinium. So far as is known, actinium is at least a fairly long-lived radio-element, for although it was discovered very shortly after radium, the original preparations have retained much, at least, of their activity. Recently it has been established that a slow decay, however, does occur which indicates a period of average life for this substance of only about thirty years. Actinium is separated with the " rare earths " in uranium minerals, and chemically it resembles most closely the rare-earth element, lanthanum, although it is not completely identical with it in chemical pro- perties. In radioactive properties its disintegration series is very closely analogous to that of thorium, and consists of eight members, in addition to itself, the first of which, known as radioactinium, corresponds with radiothorium. The next is actinium X, corresponding perfectly with thorium X, and after that the actinium emanation, actinium A, B, C, and D, follow in regular order, almost exactly as in the thorium series. The analogous products in the two series in each case give out the same kinds of rays, and are, so far as is known, chemically identical in character. But, almost without exception, the periods in the actinium series are uni- formly shorter than in the thorium series, the longest, ACTINIUM 199 that of radioactinium, being only twenty-eight days, and the shortest, that of actinium A, being only ^ioth of a second. The full series is shown in Fig. 38. QoL r)<^ 0<^ O"^ o-o-cf-o-o Actinium. Radio- Actinium X. Emanation. Actinium A. actinium. 30 years. 2S-1 days. 15 days. 5-6 seconds. 0' 003 second oo-o-o Actinium Actinium Actinium Actinium B. C. D. E. 52*1 minutes. 3'1 minutes. 6"S3 minutes, (unknown). Fig. 38. The Origin of Actinium. Whereas it is customary to regard the uranium and thorium series each as starting from a primary parent of so long life, that, old as the world is, some still survives unchanged, the problems connected with the real nature and origin of actinium are still not entirely cleared up. Its short period of life, recently established, proves that it cannot itself be a primary radio-element like the other two, and, in fact, its parent is now known. But it is not impossible that it may form part of a third independent primary series, though this has not been the view that has so far gained most support. So far as knowledge has been gained, actinium appears to be found only in the uranium minerals and in all of thete which have been examined for it. It is natural to conclude from this that it is a product of uranium. But here a difficulty arises. In the disintegration of actinium at least five a-particles are given out per atom disintegrat- ing, representing a loss in atomic weight of 20 units. There is certainly no room for the actinium sericb between uranium and polonium, and there is no evidence that it comes after polonium. 200 THORIUM AND ACTINIUM Multiple Atomic Disintegration. The imijortaut piece of evidence, however, Avhich shows conclusively that actinium cannot be in the main uranium-polonium series, and which at the same time serves to distinguish this series from the others, and to make it practically the most difficult to investigate, is the extraordinarily small relative quantity of actinium in uranium minerals. Although the actinium series gives out at least five a-particles per atom as compared with eight given out by the uranium-radium-polonium series, the a-radiation contributed by the whole actinium series in uranium minerals is only about one-fifteenth or one-sixteenth of that contributed by the uranium series. Whereas, if actinium were in the main line of descent from uranium, the a-activity of its series should be of the order of five-eights of that of the uranium series, in accordance with the principle discussed on p. 153. Two possibilities may be advanced. Either actinium is an entirely separate and independent primary radio- element, and, if so, its occurrence always in uranium minerals, and only in those minerals, is difficult to understand; or actinium may be derived from uranium as a branch, or offshoot, not in the main line of descent. One may suppose that at some stage of the disintegra- tion of the uranium atom a choice of two modes of dis- integration presents itself. The large majority of the atoms choose one way — the way leading to polonium — whilst a small minority choose a second way, the way leading through the actinium series. If this is true, it can be calculated that out of every twelve uranium atoms, eleven go through the main line of descent to- wards polonium, and one goes through the actinium line. This mode of explaining actinium is now supported by much new evidence and by the discovery very recently of actual cases of such a multiple disintegration at the ends of the thorium and radium series, among the active deposit products. BRANCH SERIES 201 Branch Series of Thorium and Radium. This may now be briefly dealt with. We have already "considered the evidence (p. 164) for supposing that, on account of the very high speed at which the a-particles are expelled from radium C and thorium C, the changes of these substances are complex, and that the a-rays in each case probably result from subsequent products, named radium C and thorium C, of excessively short life-period, which is estimated to be one millionth of a second in the case of the former and one hundred thousand millionth of a second in the case of the latter. In addition, the changes are complicated by branchings of the kind just considered, but especially instructive. Taking the ease of thorium C first, it is known that it breaks up in two ways. In the first mode an «-ray is expelled and the product formed then expels a /3-ray. In the second mode the order is reversed, the /3-ray being expelled first and the a-ray second. This may be represented (sec Fig. 39). Rangfe 4-£5Cir., Qa Qpandy 4-5 min. 121.1-, min. To-^'secs. Branching of the Thorium Series. Fig. 39. About 35 per cent, of the atoms disintegrating follow the first mode producing thorium D, and give out a-rays of range 4-55 cm. ; whereas 65 per cent, give out /S-rays in the second mode producing the hypothetical and hopelessly evanescent thorium C, which gives out 202 THORIUM AND ACTINIUM a-rays of range 8-16 cm. The two end products are of the same atomic weight, 208, and whether or not they are really identical cannot yet be said. The two separate periods of average life for thorium C shown in the figure, 225-7 and 121-5 minutes, are those calculated for the two kinds of change separately, assuming that the other did not occur. In the case of radium C, the same applies with the exception that only 0-03 per cent, of the atoms follow the " a- then /3-mode," the overwhelming preponder- ance, 99-97 per cent., following the " /8- then a-mode " (see Fig. 40). Bandy ^.ange6.57cni. , Etc. 24 years Calculated range Jo-oa'A . 6-5 days i'9 min. Branching of the Radium Series Fig. 40. The product produced in the first mode, called radium Cj is in such infinitesimal quantity, that little is known about it beyond the value of its period and the fact that it gives /3-rays. The Actinium Branch Series. Reverting now to actinium, the practical consequence of its being formed only in the minor mode of a dual disintegration, claiming only some 8 per cent, of the uranium atoms disintegrating, is that the substance is very much rarer and more difficult to obtain than the members of either of the other two series. If it were more common, it would lend itself to many demonstra- THE ACTINIUM EMANATION 203 tions and experiments similar to those detailed under radiothorium, but of an even more striking character. Actinium is relatively poor in penetrating rays, and even the most active preparations it is possible to procure are disappointing in this respect when compared with radium. The Actinium Emanation. The chief glory of actinium, however, is its emanation, a gaseous disintegration product, precisely analogous in every respect to those of radium and of thorium, but having a period of average life of only 5-6 seconds. The usual principle of a short life and a merry one applies. The dominating characteristic of the radioactivity of actinium preparations is the emanation that is given off. In the dark room, if a preparation is held over a zinc sulphide screen, the emanation diffusing away lights up the screen in patches, which are wafted from one part of the screen to another by draughts or any gentle puffs of air. The rapid decay of the emanation and corre- sponding rapid regeneration from the actinium prepara- tion makes it quite possible to experiment thus with the emanation in the open-air of the room. Whereas if the radium emanation were dealt with in this way, once it had been dissipated throughout the air of a room, some weeks would have to elapse before a fresh supply was available. Giesel, who rediscovered the substance subsequently to Debierne, actually named it " Ema- nium " before it was found to be identical with actinium. Actinium A. The only other product of actinium which calls for special mention is actinium A, the direct product of the emanation, which, like thorium A, has an extraordinarily short period of life. Indeed, actinium A is the most unstable element directly known, its period being only about ^dth of a second. It may be put into evidence by the same device as that described for thorium A (p. 197), but, of course, the endless wire has to be driven considerably more rapidly than is necessary to exhibit 204 THORIUM AND ACTINIUM the thorium product. As a matter of fact, a forgotten experiment of Giesel, eight years before the discovery of actinium A, clearly puts the existence of that sub- stance into evidence, when rightly interpreted. If a zinc sulphide screen is brought opposite to the open end of a tube containing an actinium preparation, and a little distance away from it, there is a diffuse luminosity produced on the screen in the dark, due to the emana- tion escaping from the tube. If the screen is now con- nected with tiie negative pole of an electrical machine, instantly there flashes out on the screen a sharply-defined bright spot of the same geometrical form as the opening of the tube. On discharging the screen this spot in- stantly disappears. Giesel thought, very naturally, that he was dealing with a new kind of radiation, attracted by a negatively charged surface, and called the supposed radiation the " E-ray," in brief for " emana- tion ray." However, it is not the ray, but the exces- sively short-lived product giving an ordinary a-ray, which is attracted to the negative surface; but owing to the infinitesimal time this product remains in existence it appears as if it is the ray, rather than the product, which is attracted by the electric field. Another way of showing the same experiment is to coat a wire with zinc sulphide, and to immerse it in a flask containing an actinium preparation. On charging the wire nega- tively to the flask, the zinc sulphide instantly flashes out and remains brilliantly luminous; but on discharging the wire, the luminosity disappears apparently instan- taneously. The same device can be used to show the existence of thorium A, but an appreciable, though small, time-lag occurs before tlie appearance and the decay of the luminosity. Eka-tantalum or Proto-actinium. In 1919 the main problem of the origin of actinium was cleared up by the discovery and isolation of its direct parent in uranium minerals by Cranston and the THE PARENT OF ACTINIUM 205 writer in this country, who named it "eka-tantahim," and by Otto Hahn and Miss Meitner in Germany, who named it " proto-actinium." In each case the search was helped by some wide and far-reaching generaUsa- tions, still to be dealt with, from which it was possible to predict the chemical character of the missing parent and its place in the periodic table. This place was the last and still vacant place in the niobimn-tantalum family, between uranium on the one side and thorium on the other. Meudelejeff, who was one of the dis- coverers of the Periodic Law, had called attention to three vacant places in the famiUes of boron, aluminium, and silicon respectively, which he assumed were occupied by three elements still to be discovered, and which he called eka-boron, eka-aluminium and eka-siUcon. In each case he was bold enough to predict their chemical character from their position in the table. Shortly afterwards the three missing elements were found, and named scandium, galhum, and germanium, and their properties were found to correspond very closely with what had been predicted. In the present case " eka- tantalum," a still unknown element, analogous in chemical character to tantalum, had been foreseen to be probably the missing parent of actinium. Beyond the fact that it has been isolated and that it produces actinium slowly and steadily with the lapse of time, just as ionium produces radium, not much work has yet been done on it. It gives a-rays, and from the range of these it is estimated that its period is of the order of from ten to a hundred thousand years. Uranium Y. One more member remains to be considered, and that is uranium Y, a radioactive product of short period of average life, 2-2 days, discovered by Antonoff in 1911 to be produced by uranium, and giving (/S)-rays some- what more penetrating than those of uranium X^. It is probable that this is the immediate parent of eka- 15 206 THORIUM AND ACTINIUM tantalum, and the first member of the actinium branch series. The branching is thought to occur either at uranium I or uranium II, probably the latter, and that in both branches an a-iay is expelled. So that the initial changes of the series, represented in Fig, 28 as a single change, has been gradually and with diffi- culty traced out to be something as follows : ^® ^'W ^•PC'^xdy) @ © © (233) ^(234) ^ (234) ^(234) ^(230) ^(226) ^ &c. Uranium I Uranium X. Uranium X, Uranium II Ionium Radium 8,000,000,000 SSSdays i.6s minutes 3>o~,°oo\, 'o"."" y«ars 2,5ooyears years years (?) \^^*(^^^^ (23OJ — >-f 230j — ^(^226j -T>~ &c. Uranium Y Eka Tantalum Actinium z-z days 10,000 to 30 years (?). xoo',000 years (?) Pig. 41. Considerations to be now dealt with have shown it to be of extreme importance that every change in the series should be separately and correctly recognised, and when this was sufficiently the case, a very great advance indeed resulted. Thus, with the discovery of these remaining sub- stances, the science of radioactivity now embraces thirty-six examples of elements in the course of spon- taneous change, with periods varying from tens of thousands of niilKons of years on the one hand, to a few billionths of a second on the other. It is unlikely that any more of these unstable elements remain to be discovered, unless some entirely unknown and un- suspected source of radioactive materials is found. The complete series are set out in detail in the table opposite (Fig. 42). The Unsolved Riddle of Matter. There remains unsolved that most fundamental and inaccessible problem, which at the same time appears to be the problem of ultimately the most practical signi- s 5 w 1/1 X u < m Chemical Character [Isotopic with] hes, and 8% How Thorium Ekatantalum Actinium Thorium Radium Emanation Polonium Lead Bismuth Thallium Lead in .SJ 'C 0) n x: O d 2 OJ :3 & 1 o rt 0) j: o ---? 9 _ 6 ■1 tj 1 a '•B ft; < Bismuth Thallium Lead i-, Hi n i J3 (L) ho t > 'o:j •a _o "u 0. 8,000,000,000 years 35-5 days 1-63 minutes 3,000,000 years (?) 100,000 years 2440 years 5.55 days 4.3 minutes 38-5 minutes 281 minutes i/i,ooo,ooothsec. (?) . 24 years 7.2 days ig6 days 1 8 25,000,000,000 years 9-67 years 8<9 hours 2. 75 years 5-25 days 78 seconds O'Z second 15.4 hours 87 minutes (-1. u w Is § 8" 1 .S c -a o E a 03.03. B a a a 03.02.°^ '; | 3IUJ0JV COrhTj-Tj-O'ONOOTfTl- ■3*000 NNNNNNNNNN IONMN N «00CO00*O\ON N MNNNnNmmm Nnnnnnnnn 2'3 i E 3 . . 1 4J S £ .2 0. h W ■ :;, • ■ -I _ '71'''?^ ■ ■ S 1; =q' -tJ q" ^ fe,' 1 |3 r! d d • • ctf ' 1 I All 1. 1-2 liiicBEEEE SEEES ■a -S -S -S 3 .3 W .3 .3 .3 .3 .2 .3 .3 £ i-u.L.uiCt(lrirtoj(,j rtcijdd DDDP°«KKKtti KKoiK 4J o I -a c Thorium . . Mesothorium-/ Mesothorium-// Radiothorium, . Thorium -A'' . . Th. Emanation. Thorium -W . . Thorium- 5 . . Thoriun^-C . 208 THORIUM AND ACTINIUM ficance — the real internal nature of matter. How is the atom of matter put together, and how can it be pulled apart ? These are the practical questions which the discoveries of radioactivity raise in a pressing form without as yet affording a hint of the answer. But in spite of that, our knowledge of the internal structure of 1;he atom continues to grow at a very rapid rate, and some of this more recent work may now be dealt Avith. CHAPTER XIII THE ULTIMATE STRUCTURE OF MATTER A Flood of Knowledge. Ever since the recognition of radioactivity, the dis- covery of radium, the estabhshment of the theory of atomic disintegration, and the independent proof by the spectroscope that the element helimn is actually being produced in a natural transmutation — discoveries which followed one another rapidly as the nineteenth century passed away and the present century succeeded it — it must have seemed to many that such a period of pioneering and fundamental reconstruction in science would soon exhaust itself and be succeeded by one of steady spade-work in the cultivation of the new terri- tory opened up. The development of the new territory and its detailed exploration have gone on steadily and rapidly, but, so far from the wave of original and fertile ideas having exhausted itself, the initial successes above mentioned have proved to be but the first indications of a continuously advancing tide. Already from many totally distinct directions the flood of knowledge has revealed many of the deeper secrets of the constitution of matter. Ignorance and impotence in this field still keeps the human race within its traditional boundaries, and Nature still holds the final citadel against all comers. But now it is being undermined from all sides, and changes its aspect almost from day to day, like an erstwhile impregnable barrier that is crumbling away before our eyes. The years 1911-13 witnessed a convergence of powerful new methods which, though their simultaneous 209 210 THE ULTIMATE STRUCTURE OF MATTER development must be regarded as largely fortuitous, all bear definite experimental testimony concerning the hidden internal construction of the atom of matter. In fact, we can now distinguish therein three distinct regions, one within the other, between which probably no interchange whatever of constituents occurs, but through which, in succession, the atom makes the par- ticular impression by which we recognise it in the external world, and by which, in turn, it is successively guarded from any direct influence from without. The first, outermost region is that which the older sciences of physics and chemistry have studied so minutely, and which is directly concerned in endowing the atom with most of those properties by which in the past it has been recognised and studied. The second is an intermediate region which can be reached and set into the vibration known to us as X-rays, by the purely artificially generated projectiles, the free-flying electrons or cathode-rays of the Crookes tube, dealt with in Chapter IV. The last and innermost region of the atom, or the nucleus, has never yet been reached save by methods which we owe solely to the study of natural radioactive changes and by the projectiles, of such inimitable swiftness and energy, which are spontaneously expelled during those changes. The Nature or Mass. Actually before the coming of radioactivity, the dis- covery of the electron, a particle more minute than the smallest individual atom of matter, had given, in the hands of Oliver Heaviside and Sir Joseph Thomson, a possible clue to the nature of mass (p. 57). Without any direct evidence that the mass of matter was, in fact, due to this cause, the reasoning indicated that, if the electron were sufficiently small — if the electric charge of which it consists were concentrated into the volume of a sphere of about 2 x 10"^^ cm. radius, which is about one-hundred thousandth of the usually accepted value for the radius of an atom — it would possess a mass ELECTRO-MAGNETIC INERTIA 211 equal to that found— namely xs^sirth part of the mass of the hydrogen atom, by virtue of thoroughly well-known and understood electro-magnetic principles. A charge of pure electricity, entirely unassociated with matter, as the negative electron is believed to be, cannot be moved from rest without an expenditure of energy, nor if moving can it be brought to rest without yielding up its energy. It, therefore, must possess inertia or mass. A moving charge of electricity is indistinguishable from a current of electricity. In the case of an ordinary electric current " self-induction " opposes both its starting and stopping. If we trace further the origin of the " self- induction " in the case of a flow of the electric current, or " electro-magnetic inertia " in the case of an indi- vidual electron, both terms being technical expressions for an identical action, we find it in a fundamental dis- tinction between electrostatic and electrodynamic pheno- mena — ^that is, betw^een a charge at rest and one in motion. The former has no magnetic properties, w'hereas the latter has. The space surrounding a current of elec- tricity, or a moving charge, is endowed with_ magnetic properties, and the change in the surrounding space when an electric charge, before at rest, is caused to move, demands the expenditure of energy. This change is believed to be transmitted outward from the electron with the velocit}'- of light. This endows a purely electric charge with inertia or mass. So that a charge of pure electricity must, if sufficiently small and concentrated, simulate matter in its most fundamental attribute. For the samecharge concentrated into spheres of different radius, the mass is inversely proportional to the radius. Are there, then, two kinds of inertia or mass, the one " material " and the other " electro-magnetic," the one for matter, still a fundamental, and the other for elec- tricity, a derived conception that can be fully explained by the phenomena known to attend its motion ? 212 THE ULTIMATE STRUCTURE OF MATTER Sir Joseph Thomson's Model Atom. From this the idea arose naturally and was developed by Sir Joseph Thomson, that atoms of matter might be compounded of electrons in sufficient numbers to account for their mass. For each atom nearly 2,000 electrons per unit of atomic mass would be required. The prob- lem of atomic constitution resolved itself into one of how to maintain such systems of electrons in permanently stable regime. The early attempts had little of reality to recommend them, because by no known means could such systems of electrons be held together without assuming the existence of positive electricity in some form. But positive electricity, existing like negative I electricity divorced from matter, refused to be dis- \ covered, and, in fact, still remains unknown. In Sir Joseph Thomson's model atom, the negative electrons were supposed to revolve in orbits within a uniform sphere of positive electrification of the same dimensions as the atom. It had one very great and suggestive merit, for it showed that the electrons would tend to arrange themselves in rings. If the number of the electrons were steadily increased, the newcomers would incorporate themselves into the existing outer ring until a certain number had been added, and then, if the numbers were further increased, these existing rings would become unstable, and the superfluous members would at a certain number suddenly rearrange them- selves into a new permanent outer ring concentric with those previously existing. The Periodic liAW. This simulates very well the known facts with regard to the elements as shown by the Periodic Table. Arrang- ing the elements in increasing order of atomic mass we get the well-known periodicity of chemical properties, the tenth element resembling closely the second, the eleventh the third, and so on, hydrogen being an exceptional THE PERIODIC TABLE 213 element without analogues. So that all the elements fall naturally into families, successive members in the same family being separated from one another by seven intervening elements in the early part of the table, and by seventeen in the latter part of the table. The Periodic Ta])le is shown in Fig. 43. The elements are set down successively in order of increasing atomic weight hori- zontally, the vertical columns then contain families of chemically allied elements. The separate places are numbered consecutively at the top of the place. These numbers are the so-called atomic numbers. Below the name of each element is its chemical symbol and its atomic weight. The families are numbered 0, I, II, etc., and these " Group Numbers " express, with certain reservations, the usual chemical valency of the element — that is, the number of units of affinity with which it enters into combination with other elements. Thus, aluminium is in the Ilird family and carbon is in the IVth. When these combine it takes four atoms of aluminium, each atom with three units of affinity, to combine with three of carbon, each with four units of affinity, the compound, aluminium carbide, being represented by AI4C3. After the IVth group, the ele- ments frequently combine to form compounds with many different valencies. But here it may be stated, though the simple rule is often not followed, that the most usual valencies are either the group mimber or eight minus the group number. Thus, nitrogen either has five valencies or three, chlorine one or seven, and so on. That elements possess units of combining power, or " bonds of affinity " as chemists call them, is one of the numerous facts which has been, at least partially, explained by the discovery of the electron and the fact that electricity exists in atoms no less than matter. Electrolytic Dissociation. During the last quarter of the nineteenth century, the theory of electrolytic dissociation, put forward by ELECTROLYTIC DISSOCIATION 215 Svante Arrhenius of Stockholm, became generally estab- lished. It asserted that compounds of the class which conduct the electric current in the state of solution, and known as electrolytes, exist in solution in a. more or less completely dissociated condition, as oppositely charged positive and negative ions, the migration of which to the opposite poles constitutes the electric current. It would be idle to pretend that complete clearness of interpretation has yet been attained, but the facts appear somewhat as follow: The elements, sodium and chlorine in Groups I and VII respectively, both act usually as elements with a single unit of valency, but they belong to, and are typical of, two distinct classes of elements. Sodium is a typical basic, metallic, or electropositive element, and chlorine is a typical acidic, non-metallic, or electro-negative element. They com- bine together with the utmost avidity to form common salt, NaCl. But in solution in water we are forced, by its behaviour to the electric current as an electrolyte, to recognise that the complex NaCl does not exist as such, at least for the most part. Rather, there are two new particles, " sodion " and " chlorion," which exist apart, and are called ions. The sodion — Na+ — is an atom of sodium carrying one atomic charge of positive elec- tricity, and the chlorion — Cl~ — is an atom of chlorine with one atomic charge of negative electricity. It is as though the act of chemical combination of metallic sodium with the element chlorine was essentially the transfer of an electron, or atom of negative electricity, from the sodium atom, to the chlorine atom. The sodium readily loses a constituent negative electron to another element, such as chlorine, which will take it up. But although equal numbers of positive and negative ions may exist as separate particles when mixed together, -neither kind can exist alone. The enormous forces of electrical repulsion between the similar charges, un- neutralised by the presence of the opposite kind, effec- tually prevent this being even conceivable. Whether 216 THE ULTIMATE STRUCTURE OF MATTER an element loses or gains one or more electrons, however, is not a self-contained property, but depends on the nature of the other element or elements in the com- pound formed, so that frequently elements in the later families, V, VI, and VII, which may usually tend to gain 3, 2, or 1 electrons and to act as acidic elements, may act like basic elements and lose 5, 6, or 7 electrons. Undoubtedly, in the broadest sense, though much is not yet so clear, the chemical combining power of an element is to be explained by the inherent tendency the atom possesses either to attract from, or to yield up to, another atom, one or more electrons. The act of chemical combination, in some of the best-known and typical cases, which, in an earlier day, was depicted as due to the powerful attraction of one atom for and by another, is primarily not exerted between the two atoms, but between one of the atoms and the constituent electron or electrons of the other. Between the two material components of such a stable compound as sodium chloride no cohesion or attraction probably exists. The molecule of sodium chloride, at least in the liquid state, either when fused or dissolved, consists essentially of two separate particles or ions, mixed together rather than combined, which being oppositely electrically charged, must always exist together in equal numbers in order that the whole may remain electrically neutral. But there is no definite bond of union other than this purely electrical requirement, and this refers merely to the aggregate number of each kind of particle rather than to the individuals. Apart from this limitation, the sodium and the chlorine in salt water exist separately and totally uncombined for the most part. The in- tensity of the electrical charge on an ion is almost incon- ceivably greater than any known for matter in the mass. It has been calculated that the mutual repulsion be- tween the charges carried, for example, by the hydrogen ions, would be sufficient to burst the strongest tube that THE OUTERMOST ATOMIC REGION 217 can be made, long before there was forced in as much hydrogen, in the form of ions, as would, in the ordinary state, showthe hydrogen spectruminavacuum tube. This assumes, of course, what is really quite impossible, that such free ions could be put into a tube without being discharged by contact with the walls of the tube. The " chemical combination " of the partners in a com- pound completely dissociated, as sodium chloride is in liquid form, is due to a purely electrical and statistical partnership of the otherwise completely independent ions, which, in the modern view, is practically as effec- tive in maintaining the combination as the rigid bonds linking each individual sodium atom to one chlorine atom which Dalton first pictured. This refers to the class of electrolytically dissociated substances, which comprises the acids, bases, and salts, and not to the very large class of non-electrolytes, which comprises all the organic compounds, where permanent individual unions between the atoms of the molecule undoubtedly exist. The Outermost Region of the Atom. Chemical changes and chemical properties, in general, deal only with the outermost region of the atomic structure, and we shall not probably do violence to the facts If, without at present attempting to review all the evidence for this conclusion, we picture it as con- taining a certain number of " valency " electrons. This number is the same for all the members in the same family or vertical row of the periodic table, and differs, literally, unit by unit in passing horizontally from one family to the next. For a certain number of electrons in the outer ring — namely, that possessed by the zero family — there is no tendency for the atom either to lose or gain electrons. The members of this family, which comprises the inert gases of the atmosphere, are totally devoid of chemical affinity. The next family, in Group I, which contains the alkali metals, has one 218 THE ULTIMATE STRUCTURE OF MATTER electron more than this number, which is relatively loosely held. In all. probability it moves in an orbit far external to all the rest. In the other direction, in Group VII, containing the halogen family, the number is one less than this number, and these elements readily take up an additional electron in the presence of an element of Group I which has such an electron in excess. The outer ring of electrons seems for all atoms to try and conform to a certain standard number. Atoms with less rob the ones with more, and this process probably constitutes, in the main, chemical combination. Whether the robber and the robbed entirely part company, as in the electrolytes, or remain interlocked, as in organic compounds, is a secondary consideration. We may suppose that, when the number of electrons in the outer ring exceeds a certain limit, which in the first part of the periodic table is seven, a complete new inner ring of eight electrons is formed. The chemical properties, however, depend only on the outermost ring directly, and the inner rings exert a subordinate effect. The valency of such an element and its general chemical nature resembles, therefore, the eighth preceding element. This holds in the early part of the periodic table. At the 22nd element, titanium, a new and more complicated dual periodicity commences, in which the number of elements separating the consecutive members of one family is eighteen instead of eight. A new group of three closely allied elements, the so-caUed Vlllth Group, now appears in the middle of the period, where previously an argon element would appear, and the next seven elements have a partial analogy to the seven preceding the Vlllth Group. The easiest way of regarding the matter is to suppose that ten metallic elements, indicated in Fig. 43 between { } are inter- polated into the old short periods. At the 57th element, lanthanum, the law suddenly and completely breaks down. A gfoup of seventeen elements, known as the rare-earth elements, and of which two remain to be discovered, is interpolated into the THE RARE-EARTH ELEMENTS 219 series at this point. They all resemble one another and lanthanum with such extreme closeness that their separa- tion and identification is one of the most laborious and difficult tasks that the chemist can undertake. At tantalum, the 73rd element, the series begins again almost as if it had not been interrupted, and continues normally to the end. CHAPTER XIV THE NUCLEAR ATOM The Innermost Region of the Atom. Now let us see what radioactivity can tell us of the insides of these atoms, for be it remembered that though the older chemical and physical properties of matter are concerned only with the outermost shell, the seat of government which impresses upon any atom its chemical character, and which conditions that chlorine should resemble bromine and differ from potassium, is inside the atom, in a region impenetrable to the methods of investigation known at the opening of the century. From such methods we could only guess what might be inside, and the guesses never even approached the truth. But now we can send a messenger right through the unknown territory, which perchance may, on re- emergence, tell us something of more interest and value to the race than any traveller who has ever struggled back again into being from the waste places of the earth. And this messenger, whose speed must be comparable with that of light and whose mass must be comparable with that of the atom it is to invade, is the a-particle (Chapter IV.). We owe to the genius of Sir Ernest Rutherford the recognition of the importance of this new method of attacking the most fundamental of all problems, that of the ultimate structure and constitution of the atom. Together with his students, he has made a close quantitative study of the effect on the a-particle of its passage through the various atoms of matter. Though, as Bragg showed, the a-particles pass straight 220 a-PARTICLES AS MESSENGERS 221 through the atoms, this is not the whole truth. Thou- sands of a-particles pass through thousands of atoms in their path, almost as if they were not there, suffering but slight retardation and hardly any appreciable deviation from their course at each encounter. But there occur also, and as an exception, large deflections (compare Fig. 19), and occasionally the «-particle is violently de- flected through a large angle by an exceptionally close encounter, like a comet passing round the sun. It may even emerge from the side it entered. This is termed " occasional large-angle scattering " to distinguish it from the incessant very slight deviations, first in one direction then in another, according to the laws of probability, which, as a more minute examination has shown, is continually happening to the a-particle as it ploughs its way through the atoms. Inevitably this makes us view the atom itself as consisting essentially of a very small dense nucleus at the centre of a relatively enormous and almost empty sphere of influence con- taining only electrons. The «-particles, being immensely more massive than the electrons, are not seriously dis- turbed by the rings or shells of electrons whose revolu- tions determine the apparent size of the atom as fixed by the older physical methods. Against all other invaders, these swiftly revolving satellites guard the interior of the atoms as efficiently as if the atom really occupies the space to the exclusion of everji;hing else. But such exclusive occupation of a definite volume of space by matter is an illusion. A material projectile, like the a-particle, moving at a speed the tenth of that of light, passes through all the electronic ring-systems as an errant sun might pass through the solar system. This happens many thousands of times without any serious consequences to the a-particles, or to the atomic system invaded. But an occasional a-particle finds its mark, and heads straight for the real atom — that is to say, the central nucleus in which the material as dis- tinguished from the e'ectrical constituents of the atom are concentrated. 16 222 THE NUCLEAR ATOM The a-particle we know to be a helium atom of mass 4 carrying two atomic charges of positive electricity. Or, more accurately, a helium atom is an a-partiele -Hftintts- two electrons. In all probability the a-particle is the simple nucleus of a helium atom, the central sun, as it were, alone and unattended by any electronic satellites or planets at all. The size of this central nucleus of the atom, in relation to the apparent size of the atom, is probably of the same order of magnitude as that of the earth to the whole solar system. Ruther- ford, for example, from these experiments, considers that practically the whole mass of the hydrogen or helium atom is contained in a central nucleus of dia- meter one hundred thousand times smaller than the accepted diameter of the atom. This central nucleus carries a positive charge, to the extent of about one unit, or atomic charge, of positive electricity, for every two units of atomic mass. For each unit of positive electricity resident in the nucleus a similar unit of negative electricity, or an electron, revolves in one or other of the outer shells, so that the negative charge on the electronic systems is neutrahsed by the positive charge on the nucleus or material core. This model of Rutherford's differs essentially from the earlier models in that it has been based on a careful and exhaustive experimental examination of the single large-angle scattering of a-particles. Now let us consider the exceptionally close encounters, when nucleus meets nucleus and large-angle deflection of the a-particle results. If the atom invaded by the a-particle is massive by comparison, the positively charged nucleus constituting the a-particle will be violently repelled as it impinges on the very much more intensely positively charged and much more massive nucleus of the heavy atom, and will be violently swung out of its path, much as a comet is at perihelion. It is true that the forces at work are repulsive rather than attractive, but this makes no essential difference. If the two nuclei happened to meet absolutely " head-on " the H-PARTICLES 223 tt-particle would be repelled the way it came almost at its original velocity. But when a-particles traverse atoms lighter than themselves — for example, atoms of the gas hydrogen — a different state of things must obtain. Here an abso- lutely " head-on " collision would result in the hydrogen atom being repelled in the same direction as that in which the a-particle was travelling, but with a velocity far in excess of that of the original a-particle. In fact, this hydrogen atom will then behave as a new kind of radiant particle, and by virtue of its smaller mass and charge and greater velocity it should travel through the hydrogen gas far further than the original a-particles before being stopped. Marsden has shown that when the a-particles are made to pass through hydrogen and their range examined by means of a zinc sulphide screen, in addition to the scintillations given by the a-particles themselves, a few weaker scintillations, which must be due to the repelled hydrogen atoms, can be observed at distances from the source some four times greater than the a-partieles themselves are able to pene- trate. These new particles may be termed " H-particles " for the sake of clearness. An Artificial Transmutation. In 1919, by the work of Sir Ernest Rutherford, a further important step in this advance was taken, which raises the question whether a beginning has not already been made in the achievement of actual artificial trans- mutation to an infinitesimal extent. It has been recog- nised, by the late Sir William Ramsay among others, that, of all known agencies likely to be able to transmute one element into another, the a-particle, on account of its unique kinetic energy, was the most likely to prove effective. The work described shows how exceedingly difficult it is to hit the real atom exactly with the a-particle. Later results have proved that only about one out of 100,000 a-particles, in passing through one 224 THE NUCLEAR ATOM centimetre of hydrogen gas at normal temperature and pressure, produced an H-particle. Since, in this path, the number of hydrogen atoms penetrated is 10,000, in only one out 6t one thousand million collisions is the nucleus of the atom of hydrogen really hit. In the rare case when the a-particle actually impinges upon the nucleus, it is to be anticipated that the latter, if not an exceedingly stable system, might sometimes be broken up. Of the common gases, hydrogen, oxygen, carbon dioxide, and nitrogen, which he exposed to the bom- bardment of the a-particles, Rutherford observed an anomaly in the case of nitrogen. These gases all gave the expected effects, namely, the production of " N- particles " and " 0-particles " — ^that is to say, new rays were observed, longer in range than the a-rays, which were first thought to be atoms of these ele- ments with a single positive charge put into violent motion by collisions of the a-particles with the nuclei of the oxygen and nitrogen atoms, always in the minute numbers to be expected from the results with hydrogen. In these cases the range of the new particle is only slightly longer than that of the a-particles them- selves. But in nitrogen there were observed, in addi- tion, particles of the long-range and other characteristics exactly similar to the H-particles produced in hydrogen gas. Only one such H-particle was observed for every twelve N-particles produced. These results strongly suggest, though they do not yet rigorously prove, that the nucleus of the nitrogen atom struck by an «-particle is occasionally shattered by the collision, and that hydrogen atoms are produced from it. It may be sur- mised, for example, as one possibility, that the nitrogen atom of mass 14 is converted into a carbon atom of mass 12 and two hydrogen atoms. The excessively small proportion of the nitrogen atoms penetrated by the «-particles, which are so shattered, must not be for- gotten. This makes it exceedingly unlikely that such a case of artificial transmutation, if it occurs, can ever ARTIFICIAL TRANSMUTATION 225 be directly confirmed by direct chemical analysis. It must also be remembered that in this case, even if it is correctly interpreted, transmutation has not been really artificially initiated. What has been done, at the most, is to use a naturally occurring transmutation, that can still be neither initiated artificially nor controlled, to produce a secondary transmutation. The real problem of how artificially to transmute one element into an- other at will remains still completely unsolved. While this book was passing through the press, Rutherford has published further results, in which the real nature of these particles, generated by the im- pact of the a-rays in different gases, has been examined by the method by which the nature of the electron and the a- and /3-particles has been established — that is to say, the particles were subjected to the action of electric and magnetic deviating fields, and, from the magnitude of the deflection, the mass, the charge,- and the velocity were determined. This established the correctness of the earlier conclusion that the H-particles generated in hydrogen, and also in nitrogen, consisted of singly positively charged hydrogen atoms. But it was found that what have been termed " N-particles " and " 0-particles " were not singly charged atoms of nitrogen and oxygen, as first surmised, but for each the same and an entirely new particle of mass 3, carrying two positive charges. On the views discussed in the next chapter they would appear to be atoms of an isotopic variety of helium, otherwise unknown. Thus the new results confirm the conclusion that the nitrogen atom is shattered during a close nuclear collision with an a-particle, but it appears to suffer disruption in two independent ways, giving, in one way, atoms of hydrogen of mass 1, and, in the other, atoms of a new kind, of mass 3. In the case of the oxygen atom the latter particles alone appear to be produced (Sir Ernest Rutherford, Bakerian Lecture, Royal Society, June 3, 1920). 226 THE NUCLEAR ATOM Atoms compared and contrasted with Solar Systems. Thus, inevitably as science proceeds, the solid tangible material universe dissolves before its touch into finer and still finer particles, the unit quantities or " atoms " of positive and negative electricity. The passive attri- butes of matter in occupying a definite volume of space to the exclusion of other matter resolves itself into an active dynamic occupation by virtue of the sweep of the electronic satellites in their orbits round the positive central sun. But whereas, in the solar system in which we live, the central sun is both large and massive in relation to the sizes and masses of its attendant planets, in the atomic solar systems there is a curious inversion. From the facts disclosed in reference to the passage of a-particles through hydrogen, it would appear that the centres of the two colliding nuclei, the hydrogen nucleus and the helium nucleus, approach to within a distance of less than the accepted diameter of the negative electron. The central material nucleus, in which all but a negligible part of the mass of the atom is concentrated, thus appears to be at least as small as, and probably smaller than, the negative electron, the smallest particle previously known to science. Since the smaller the volume in which a given electric charge is concentrated the^greater will be its mass, it may really be that the positive electron is very much more concen- trated and very much more massive than the electron, and that the nucleus of the hydrogen atom, the sim- plest of all atoms, is in reality the missing positive electron. But this, at present, is merely a suggestion. The positive charge is the same in amount as the nega- tive charge of the electron. For its mass to be that of the hydrogen atom, which is 1,830 times that of the elec- tron,its radius must bel,830 times less, or about 10~^^ cm. CHAPTER XV ISOTOPES Elements which are Chemically Identical. In another totally distinct direction, radioactivity has been the means of throwing a flood of light on the nature of matter and in particular on the periodic law of the elements, which epitomises the existing chemical know- ledge of matter. In the first chapter, the underlying limitations which attend all knowledge were empha- sised. Such an underlying limitation is revealed by the sequence of radioactive changes. In Chapter X. (p. 154) it was shown that many of the known radioelements resemble others so completely in their chemical nature that no separation can be effected once they have been mixed, and in Chapter XII. we came upon numerous further examples of the same resemblance among the members of the thorium disintegration series. No chemist could detect by chemical analysis the separate existence of the two uraniums, uranium I and II, or of thorium and radiothorium, or it\.esothorium and radium, or of lead and radiolead, in a mixture containing any of these pairs. Naturally the question was asked whether any of the common elements, for which radioactive methods of analysis are not available, are, as supposed, really homogeneous elements, and whether any are mixtures of different elements, with different atomic weights, but with identical chemical properties, so merely appearing to be homogeneous to chemical analysis. Matter is, in all probability, far more com- plex than chemical analysis alone is able to reveal, because radioactivity has shown us the existence 227 228 ISOTOPES of elements identical in their chemical behaviour, but, nevertheless, distinct in atomic weight and in stability. The Periodic Law and Radioactive Changes. In 1911 the writer pointed out that the products of a-ray changes have a certain definite relationship in chemical character to their parents. The chemical pro- perties of an a-ray product correspond with those of an element in the periodic table with group number two less than that of the parent. Thus, radium in Group II expels an a-particle and changes into the emanation in Group 0, ionium in Group IV changes by expulsion of an a-particle into radium in Group II, and so on. It was also noticed that the passage through the periodic table of the element undergoing change was frequently alter- nating, the products frequently reverting in chemical nature to that of an earlier parent. So radiothorium resembles thorium, thorium X mesothorium I, and so on. This curious atavism has now been very simply and fully explained, largely owing to the chemical in- vestigations of Alexander Fleck in the writer's labora- tory at Glasgow, who spent three years in the exhaustive study of the chemical nature of all the radioactive elements, which survive for a long enough period for their chemical nature to be determined, and many of which had previously been very imperfectly investigated from this standpoint. In consequence, the generalisa- tion already alluded to in preceding chapters has come to light. It was seen that the expulsion of a /8-particle was entirely analogous to that of the expulsion of the a-particle, but that, instead of the product possessing a chemical nature corresponding with an element in the periodic table with group number two less than the parent, it corresponded with an element of group number one greater. Hence if, in any order, one a- and two /3-rays are expelled, the product is chemically of the same nature as its parent, and the curious atavism THE a- AND y8-CHANGE GENERALISATION 229 referred to above is explained. Radioactive children frequently resemble their great-grandparents with such complete fidelity that no known means of separating them by chemical analysis exists. But, of course, the two intermediate parents are readily separated. By this means all the members of the family may be recognised severally, although, but for this means, that would be still impossible. The complete generalisation, which was put forward in 1913 independently during the same month by A. S. Russell, K. Fajans, and the writer, is illustrated by Fig. 44. The last twelve places of the periodic table, from uranium to thallium, are placed consecutively side by side, and the passage of the elements, in the uranium, thorium, and actinium series, from place to place, as the a- and ^-ray changes succeed one another, is indicated by arrows. The figure is to be read at 45°, so that the lines showing the atomic weights are horizontal. Every detail of the chemical nature of the members of the known sequences in the uranium, thorium, and actinium series, including the complicated branchings which occur towards the ends, bears out implicitly these two simple rules. Independently of their origin, atomic weights, and radioactive character — that is, of the kinds of change they are about to undergo — all the members of the three disintegration series, which, by the consistent application of these rules fall into the same place in the periodic table, are chemically com- pletely identical and non-separable from one another. Hence I have termed them isotopes or isotopic elements. The Atomic Number. Confining attention to the most generally important consequences of this embracing generalisation, we may at once connect the rules with the fact that the a-particle carries a double positive atomic charge and the /8- particle a single negative atomic charge. Each of the successive places in the periodic table thus corresponds 230 ISOTOPES Sequence of Changes of Uranium (U) and Thorium (Th) into various Isotopes cf Lead (Pb). Fio. 44. THE ATOMIC NUMBER 231 with unit difference of charge in the constitution of the atom. This suggestion was made tentatively by van der Broeck before it was first proved by these researches. The discovery of the atomic nucleus by Rutherford enables us to go further. It is hardly possible to doubt that both the a- and the ,S-particles are expelled from the nucleus. Hence this difference of charge in the constitution of the atom in passing from one place in the periodic table to the next must be a unit difference in the net positive charge of the nucleus of the atom, and a corresponding unit difference in the number of negative electrons external to the nucleus, which com- pensates the positive nuclear charge and renders the whole atom neutral. In his original theory Rutherford concluded that the magnitude of the positive charge of the nucleus was approximately one-half of the number representing the atomic weight of the elements. Now, from evidence still to be considered, it is known exactly to be equal to the number of the element in order of sequence in the periodic table, when the elements are arranged in order of atomic weight. This number is now always called " the atomic number." Usually it is rather less than one-half the atomic weight. Uranium, the last element, occupies the 92nd place in the periodic table; its atomic number is therefore 92, and its atomic weight is 238. So far as is known the atomic number of hydrogen is one, that of helium is two, of lithium three, and so on until we arrive at uranium, ninety-two. Gold is the 79th element, mercury the 80th, thallium the 81st, lead the 82nd, and thenceforward, as shown in Figs. 43 and 44, by the numbers at the head of each place of the periodic table. IsoTOPic Elements. The generalisation proves definitely that, as regards the last twelve places in the periodic table, between uranium and thallium, the successive places correspond 232 ISOTOPES with unit difference of nuclear charge and unit differ- ence in the number of external electrons as was pre- viously assumed. But it also shows that in the ten occupied places each place accommodates on the average no less than four distinct elements. The atomic masses of the various elements occupying the same place vary in some cases by as much as eight units, and there is nothing to show that the same may not occur through- out the whole periodic table. Such groups of isotopic elements, occupying the same place, possessing the same net nuclear positive charge and the same number of electrons in their external systems, are not merely chemically identical and indistinguishable. Many of their commoner purely physical characteristics, such as spectrum and volatility, have also been found to be identical. The existence of such isotopic elements would not have been suspected except for radioactive changes. What fixes the chemical and general material character of an element is a particular numerical charge, and this charge is not the total charge of the atom, not even the total charge of the nucleus of the atom, but is the net charge of the nucleus or the difference between the numbers of positive and negative charges which it con- tains. The same net charge may be, and, in the case of isotopes, is made up of different absolute numbers of posi- tive and negative charges differing by the same amount. When an a- and two /3-particles are successively expelled the net charge becomes again what it first was, and the position in the periodic table and whole chemical character also reverts to the initial state. But the atomic mass is different by four units, the mass of the a-particle expelled. The Problem of the Ancient Alchemist. There is one interesting point that may be referred to, which serves to show how nearly science has ap- proached to the ancient alchemical problem of turning THE PROBLEM OF THE ALCHEMIST 233 base metals into gold. In these spontaneous changes, if either actinium D or thorium D had elected to expel an a- instead of a /8-particle, the product would have been an isotope of gold instead of lead. Gold occupies a position in the periodic table two places removed from and before thallium, so that if thallium could be induced to part with an a-particle, the product would be an isotope of gold. If it was sufficiently stable it would be gold for all practical purposes. It is true its atomic weight and density would be somewhat greater, but otherwise it would be the same. Or, again, if bismuth could be made to expel two «-particles, or lead an a- and a /3-particle, gold again would be the product. This, then, is a list of recipes for the modern alchemist, one and all indubitable, but one and all awaiting a means of accomplishment. It remains for the future to show how the nucleus of an atom can be so influenced as to be caused to eject an a- or /8-particle at will. But it is a tremendous step gained to know for the first time in what transmutation really consists. CHAPTER XVI THE X-RAYS AND CONCLUDING EVIDENCE The X-R,ay Spectra of the Elements. We now have to turn to yet another great advance. Beginning with the case of ordinary light, it is well known that it may Jdc analysed into its component wave- lengths by the use of a " diffraction grating," as well as by an ordinary prism. In the Rowland diffraction grating some large known number, usually from ten to twenty thousand lines per inch, are accurately ruled by a diamond mounted on a dividing engine, upon a plane or concave surface of glass in such a manner that all the lines are exactly parallel and all precisely equally spaced apart. The light trans- mitted by such a grating is split up into a large number of parallel beams which " interfere " with one another, and the result is that the direct beam is more or less extinguished, but each different wave-length of light in the beam is bent, or diffracted, from its course through a definite angle which is different for each different wave-length. So the light is resolved, or spread out, into a pure spectrum much as when it passes through a prism. Now, if the distance between the rulings — one-ten- thousandth of an inch, for example — is exactly known, the actual wave-length of each line in the spectrum may be easily and exactly calculated. A beam of X-rays, as we now definitely know, consists of a radiation of precisely the same kind as light, but of wave-length some ten thousand times shorter. Hence, to resolve it, we would require the use of a " grating " at least a thousand or ten thousand times more finely ruled than 284 X-RAYS AND CRYSTALS 235 can be ruled by the most perfect dividing engine. Who could make such a grating ? But an infinitely more perfectly executed, and ten thousand times more closely packed, assemblage than the finest and most perfect Rowland grating ever made was found in 1912, by Laue, Friedrich, and Knipping, who discovered that the X-rays are regularly diffracted, like light is by the grating, when reflected from the surface of an ordinary crystal, such as rock salt, fluor spar, calcite, and the like. In this country the discovery was eagerly taken up, and we owe to the Professors Bragg, father and son, a clear insight into the whole subject. In the crystal, as the crystallographers have, with eyes of faith, long depicted, the atoms of the substance are naarshalled in a definite space-lattice of regular geometric form, so that each atom is fixed at a definite point in space at a definite distance from and in a definite angular direction to all the atoms surrounding it. The smallest number of atoms required completely to represent the pattern — so that the whole structure is made up simply by reduplicating this unit indefinitely in the three dimen- sions — is called the space-lattice of the crystal. More- over, the distances between the atoms, or points, of the space-lattice is, for common crystals, just of the right order of length to resolve the X-rays in a manner pre- cisely analogous to that in which light is resolved by the Rowland diffraction grating. If we know for any one crystal what this distance actually is, we can determine the wave-length of any X-ray from the angle at which it is reflected from the crystal. For the ordinary heterogeneous beam of X-rays given by an ordinary X-ray tube, which corresponds to white light, the beam is resolved by the crystal into an X-ray spectrum, and the wave-length of the component radiations may be found. If we know the wave-length for any one X-ray, we can find out for any crystal, in any plane or face we choose, the precise distance apart between the atoms 236 THE X-RAYS that make it up, and so we can construct its space- lattice. This has given crystallographers a powerful direct method of testing the reality of the space-lattices which have been arrived at by theoretical reasoning and the power of second sight of the mathematical mind. The results already have been gratifying and remarkable. The actual spacial arrangements of the individual atoms that go to make up the crystal are now being precisely measured and explored, and, as has so frequently hap- pened before, the patient theoretical conceptions of a generation less brilliantly equipped with experimental methods of inquiry are being triumphantly vindicated. But it is not with this field we are now most closely concerned. It is rather with the wave-length of the X-rays, and with their period or frequency, which can be so found by this method. If we consider the unit of time, one second, in this period light or X-rays travel 3x10^° cms., whatever the wave-length. But in the 3 X 10^" cms. of length, or second of time, there will be about twice as many separate waves of violet light as of red light, and many thousand times more waves of any X-ray than of either. From the wave-length we can at once find the frequency or " pitch " of the radiation, or the number of vibrations per second to which it corresponds. This frequency again, in atomic solar systems, corresponds with the number of revolutions made per second by the electron in its orbit within the atom, and this depends on the diameter" of its orbit. In much the same way we might speak of the frequency of a planet as the number of revolutions it makes round the sun in a century, and this depends on the distance of the planet from the sun. The rays that constitute the ordinary visible spectrum arise probably from the outermost electrons of the atom, the ones, that is, that are responsible for the chemical character and which traverse orbits of diameter of the order of 10"* cm., which is the diameter of the atom, meaning by that the whole atomic system. To get waves of a thousand to RESOLUTION OF y-RAYS 23f ten thousand times shorter length, and frequencies a thousand to ten thousand times greater than for visible light — to get X-rays, in fact — ^it is clear that we have to get much nearer to the centre of the atom, into a region intermediate between that in which the ordinary phenomena of physics and chemistry originate and the innermost nucleus disclosed by radioactivity. The 7-Rays. By the same method of reflection from crystal sur- faces some, at least, of the 7-rays have also been resolved and shown to be X-rays, but of very much shorter wave- length in general than those artificially produced. The wave-length of light is usually expressed in Angstrom units (written A). One Angstrom unit is equal to 10"' cm. The wave-lengths of visible light waves vary from 6,000 or 8,000 (A) in the red to 3,500 (A) in the violet, and to 2,000 in the extreme ultraviolet. The wave-length of the X-rays range from, perhaps, 8 A for very soft X-rays to 0-5 A for the most penetrating type that can be produced. But the wave-length of 7-rays is in general much less ranging from 1'2 A to as little as 0-07. Moreover, it is believed that for the most typical very penetrating 7-rays of radium and thorium the wave-length is far too short even for the crystal to be capable of resolving them, and they may have wave- lengths 100 times shorter than the shortest yet resolved. The existence of rays so short in wave-lpngth and high in frequency points to a revolution of electrons in the atom in orbits of excessively minute diameter, so minute that the question arises whether the 7-rays do not really originate from electrons actually contained within the atomic nucleus. These results furnish another and independent proof that radioactive pheno- mena occur entirely in the atomic nucleus. 17 ^38 THE K-RAYS The Intermediate Region of the Atomic Struc- ture. — The Homogeneous Characteristic X-Rays of Barkla. A Rontgen tube gives X-rays of all wave-lengths within limits which depend on a variety of conditions, such as the nature of the metal constituting the anti- cathode, the degree of vacuum, and the potential differ- ence between the electrodes. The very important dis- covery was made by Barkla that, when such X-rays impinge upon various metals, they will, if penetrating enough, produce new secondary homogeneous X-radia- tion, the properties of which are characteristic of the metal and not of the primary radiation. Ea h element, except those of less atomic weight than sodium, emits under such circumstances an X-ray of definite and characteristic spectrum, which differs from the ordinary light spectrum given by the same element in being ex- cessively simple. Often it consists of a single strong line together with one or more weaker ones. Such characteristic X-rays belong to various series, designated the K-, L-, M-series connected in the following way: Beginning with sodium, the 11th element in the Periodic Table, the X-ray, characteristic of the element sodium, belongs to the so-called K-series, and is extremely feebly penetrating and of long wave-length, as the wave- lengths of X-rays go. Going up through the elements in increasing order of atomic weight, as far as tin, the 50th element, the K-radiation produced steadily dimin- ishes in wave-length and increases in penetrating power, until, at tin, it is difficult artificially to generate a primary X-ray of sufficient penetrating power to excite the characteristic radiation. Hence this experimental limitation prevents this series being studied for elements of greater atomic weight. , Before this, however, beginning with the element zinc, the 30th element, in addition to the K-radiation, a new characteristic radiation of very feeble penetrating power BARKLA'S X-RAYS 239 belonging to the so-called L-series, makes its appearance. From zinc onward this new radiation increases in pene- trating power and decreases in wave-length until the last element uranium is reached. Again, at gold, the 79th element, another new series, the M-series, is first observed, very non-penetrating at first, but increasing in penetrating power to uranium. Moseley made a systematic determination of all the wave-lengths of the principal lines of these characteristic X-rays from aluminium to silver in the K-series, and from zirconium to gold in thcL-series, and discovered that they are connected together by a simple mathematical relation, involving the atomic number of the element. The square-root of the frequency (as we have seen the frequency is proportional to the reciprocal of the wave- length) is proportional to a number that increases by one in passing from any element in the periodic table to the next. In other words, the square root of the fre- quency is proportional to a number that increases in the same way as what we have termed the atomic number of the elements, when arranged in order according to the Periodic law. The practical value of this discovery was great. For the first time it was possible to call the roll of the chemical elements and to determine how many there were and how many remained to be discovered. There are between hydrogen and uranium ninety-two possible elements, of which only six remain to be found — namely, the two unknown heavier analogues of the element manganese, two rare-earth elements, and the two heaviest analogues of iodine and caesium re- spectively (see Fig. 43). It is curious that the first two should still and for so long elude discovery. They would in all probability be most useful metals, allied to the noble metals in char- acter, the first to the light platinum metals, ruthenium, rhodium, and palladium, and the second to the heavy platinum metals, osmium, iridium, and platinum. As is well known, the Periodic Table comprises certain 240 THE X-RAYS exceptions. Tellurium has an atomic weight higher than iodine, though in the periodic table it precedes it, and the same is true for argon and potassium, and for cobalt and nickel. The X-ray spectra of these elements con- firms the order in which they have been put by chemists in the periodic table on account of their chemical char- acter and despite their atonaic weights. This shows that it ig the atomic number — i.e., the net positive nuclear charge of the element, or the number of elec- trons external to the nucleus — which fixes the position in the periodic table, rather than, as hitherto supposed, the atomic weight. The existence of isotopic elements of identical chemical character but different atomic weight points to the same conclusion. In fact, this work on X-ray spectra dovetails perfectly into the con- clusions reached, independently, in the study of radio- active change, and extends them to all the elements in the periodic table. The Atomic Mass oe Weight. The chemical character, and even the spectrum of an element, at least to a degree of approximation attainable by common methods, depends upon the atomic shell and not upon the atomic nucleus, and the character of the shell is identical, whatever the nucleus, so long as the atomic number is the same. The atomic mass or weight, on the other hand, on the views adopted, is to all intents and purposes a property of the nucleus alone. Mass and radioactivity, the oldest and the newest pro- perties of matter, are in this respect allied and sharply to be distinguished from all the other properties. Iso- topes have in general nuclei of different mass but the same net positive charge, and therefore their outer electronic systems and all the properties which origin- ate therein — that is to say, all properties save mass and radioactivity — are practically identical and indis- tinguishable. We have seen that radioactive change afforded a very ATOMIC WEIGHT OF LEAD 241 subtle way of separately distinguishing between and of actually separating isotopes in favourable cases. In the disintegration sequence A>B>C>D>, A, B, C are neces- sarily elements completely distinct chemically and capable of easy separation by chemical analysis. But if in the three changes, one a- and two /8-particles are expelled, D is necessarily chemically identical with A, but of atomic mass four units less. Because of the change, D can be apprehended as an individual, and, since B and C are separable from A and in course of time turn into D, in cases when the periods are favourable, D can be separated from A. Except for the change, A and D would, in spite of the difference in their atomic weights, be mistaken by chemists, relying on the usual chemical and spectroscopic criteria of purity and homogeneity, for a single homogeneous element. Its atomic weight would be a mean of the atomic weights of its constituents, depending not only on the magni- tude of each, but on the proportions in which they were mixed. This would apply not merely to the radio- elements but equally to all. It is therefore, perhaps, not altogether surprising that all the many efforts made to find exact numerical relations between the atomic weights of the various elements should have proved fruitless. The Element Lead. These ideas have been put sharply to experimental test in the case of the element lead. As the generalisa- tion illustrated by Fig. 44 shows at once, the ultimate products of all the disintegration series in all branches, so far as they have been traced, end in the same place in the periodic table — namely, the place occupied by lead. Therefore, in spite of the differences of origin and of atomic weight, they must all be isotopes of lead, if the apparent ends of the series coincide with the actual ends and no further, as yet undetected, changes occur. The atomic weight of ordinary lead is 207-2, whereas that of the main branch of the uranium series is 206, 242 THE X-RAYS and that of both the branches of the thorium series is 208. The atomic weight of the end product of the actinium branch series is doubtful, but, as it is only present in small relative quantity, it may be, in the first place, neglected. Clearly, if this view is correct, the lead derived from a uranium mineral ought to have an atomic weight somewhat lower than that of ordinary lead, and the lead derived from a thorium mineral an atomic weight somewhat higher. The prediction, like so many that have been made in this subject, has been completely confirmed by experiment. Lead from care- fully selected uranium nainerals, not containing thorium in detectable quantity, has been found to have an atomic weight as low as 206-05. Lead from carefully selected thorium minerals containing only a small quantity of uranium has been found to have an atomic weight as high as 207-9. Chemically, they are identical and in- distinguishable from common lead, which, indeed, may well be a mixture of these two isotopes in the right proportion to give an atomic weight of 207-2 ! Their spectra, for all practical purposes, are identical with one another and with that of ordinary lead. But, quite recently, a minute difference of wave-length has been established in the case of one of the brightest lines, a difference that does not exceed one part in ten million or one-thousandth of the difference between the two sodium lines Dj and Dj. It is so minute that it can only with difficulty be established by the most refined measurements. Nevertheless this difference between the spectra of isotopes is likely to prove of great importance.^ Agreeably, however, with what is to be expected for isotopic atoms having identical shells but nuclei of 1 The ingenious suggestion has been made that it might be used to Separate the isotopes of chlorine (p. 248). A beam of hght filtered through chlorine will lose first the vibrations corresponding with those of the lighter isotope, since it is in predominant quantity, and may then be able to stimulate the heavier isotope only to react with hydrogen, thus effecting the separation. This is being tried at Oxford, and at the time of writing (July, 1920) the results appear most promising (T. R Merton and H. B. Hartley, Nature, March 25, 1920). THE ATOMIC WEIGHT OF IONIUM 243 different mass, the densities of the different kinds of lead are different just in proportion to the differences in atomic weight. In other words, the different isotopic atoms have the same volume. Another precisely parallel case has been established for the isotopic elements ionium and thorium. We have seen (p. 153) that, on account of its period being forty times longer than that of radium, the amount of ionium in a mineral must be something like 12-5 grams per ton of uranium, or 58 grams per gram of radium. Now all uranium minerals yet examined on a sufficiently large scale contain, probably, a larger quantity of thorium than this. It is a suggestive and unexplained point that the proportion is smallest in the secondary recent uranium minerals. In practically all the primary uranium minerals several per cent, of thorium is found. Thus, ionium can never be obtained pure free from its isotope, thorium, but from a suitable secondary uranium mineral, a preparation containing a considerable pro- portion of ionium, admixed only with thorium, may be separated. Such a preparation separated from 30 tons of Joachimsthal pitchblende by Auer von Welsbach, has been investigated by Honigschmid. For pure ionium an atomic weight, 230, is to be expected, since it changes into radium with expulsion of an a-particle. The atomic weight of the ionium-thorium mixture described was found to be 231-51, whereas that of pure thorium, by the same method, was 232-12. But the spectrum of the thorium-ionium preparation was, so far as could be seen, identical with that of the pure thorium preparation, and in both no impurities whatever could be detected. The elimination of everything but ionium from the thorium by the elaborate chemical purifications adopted in the treatment of the material had been effected, but these methods are incapable of affecting, to the slightest degree, the ratio of the ionium to the thorium. 244 THE X-RAYS Separation of Isotopks. It would be idle to deny that these new ideas, that different nuclei may exist in atoms which, to the chemist and spectroscopist, are indistinguishable and insepar- able, cuts far more deeply into the basis of chemical theory than did the discovery of the actual disintegra- tion of the radio-elements and of the spontaneous evolu- tion of one element from another. It is of interest to inquire into the possibilities of separating a mixture of isotopes, or, if this is impracticable, of detecting their separate existence in a mixture without separating them. It will be obvious that any property which involves directly the atomic mass could, theoretically, if not practically, be employed for their separation and separate detection. But it is remarkable how difficult such methods are to apply to this purpose, and how few of them ever have been used as practical aids to chemical analysis. The rate of diffusion of a gas, or, less suitably, of a substance dissolved in a liquid, depends directly on the molecular weight of the substance and therefore of the weight of the separate atoms the molecules contain. Theoretically, thorium and ionium, the two uraniums or common lead itself, if it is a mixture of isotopes as is possible, ought to be capable of resolution by diffusion methods. But this has not yet been practically achieved. Other methods, such as depend upon centrifuging the mixed material, or submitting it to the process of thermal diffusion, have been proposed but not yet successfully carried out. It would be an extraordinarily difficult and laborious piece of work, for example, to separate the constituents of the air in a pure state by diffusion, though a partial and incomplete separation by this means might easily be effected. It is not a method a chemist would employ unless he were obhged. On the other hand, though on the point there is a difference of opinion, any commonly used purely physical method other than those NEON AND METANEON 245 mentioned, such as fractional distillation, crystallisation, or adsorption, is not likely, even theoretically, to be effective in separating a mixture of isotopes. These certainly depend upon the chemical character of the element, rather than its atomic mass. Neon and Metaneon. Interesting, because it was discovered just at the time that the true interpretation of isotopes had been found, and also because it concerns an element very far removed from the heavy elements at the end of the periodic table undergoing radioactive change, is the case of neon and metaneon. The element neon is one of the inert gases, similar to argon, existing in the atmosphere to the extent of some twelve parts per million by volume. It is intermediate, in the zero family of elements, between helium with atomic weight 3-99 and argon with atomic weight 39-9, exactly ten times greater — both these being practically whole numbers. . The atomic weight of neon is 20-2, a number differing from the nearest integer by a fifth of a unit. As a sequel to his classical work (p. 57) in elucidating the charge and mass of the electron, which constitutes the cathode ray of the vacuum tube. Sir Joseph Thomson applied similar methods to the positively charged par- ticles, or " positive rays " as they are called, which under certain circumstances can also be detected in the vacuum tube discharge. Here in every case so far examined the mass of the particle is never less than that of the hydrogen atom, and often it is much greater. In fact, so was developed a novel method of determining the atomic mass of elements, such as hydrogen, oxygen, nitrogen, and other gases which are present as positive ions in the vacuum tube discharge, and the molecular weight of such particles as so exist in groups of more than one atom. One of the most interesting of the numerous discoveries made was that of the gas called X3, which has a mass three times that of the hydrogen 246 THE X-RAYS atom, and which is, in all probability, the molecule H3, analogous to ozone, the allotropic form of oxygen, O3, though chemists have never yet prepared or observed the existence of such an allotrope of hydrogen.''^ But the same is true of many groups, such as CH, CHj, CH3, for which this new and exceedingly delicate method of gas analysis indicates at least a passing existence. The interest of this method, depending as it does directly upon the mass of the atom or molecule, from the present point of view is that, undoubtedly, it would be capable of revealing, if they existed, in any gaseous element, the separate individual components of a mix- ture of isotopes of different atomic mass. It is, in fact, almost the only practical method that could do so without ambiguity. Now, in examining the positive rays produced in neon by the electric discharge, Sir Joseph Thomson and Mr. Aston found in addition to the neon atom carrying a single positive charge, Ne+, of mass 20, a much fainter indication of another atom with a single + charge, of mass 22, which provisionally, as it could not be ascribed to a known element, they attributed to a new gas which they named metaneon. The question at once arose whether this was a case of the isotopism with which we have become familiar in the case of lead and the radio-elements. An attempt to separate neon and metaneon from ordinary neon, by a prolonged series of fractional absorptions of the gas in cooled charcoal, effected no separation whatever. The density of the fractions separated by the process were identical and the same as before the treatment, whereas metaneon, with atomic weight 22, should have a density 10 per cent, greater than neon with atomic mass 20. But this, as we have seen, is to be expected of isotopes, for in all probability the ordinary physical properties, such as volatility, etc., are, like the chemical 1 This diifers from the new particle of mass three more recently obtained by Rutherford in the bombardment of oxygen and nitrogen atoms by a-particles, in that it carries a single instead of a double positive charge. ISOTOPES GALORE 247 properties, indistinguishable. Neon remains still un- resolved into its two components, though after a long series of fractional diffusion experiments some in- dication of a partial separation was obtained. But the latest information confirms the existence of metaneon in the gas. Aston has developed the positive ray method of analysis considerably, so that it is capable of fixing with great precision the atomic or molecular weight of the particle causing the positive ray. His measurements showed neon to be a mixture of two gases of atomic weight 20-00 and 22-00 to within an error of one part in a thousand. So we may conclude with considerable probability that these two isotopic gases, in proportion of about 90 per cent, of the first and 10 per cent, of the second, constitute the ordinary element neon derived from the atmosphere. The General Prevalence of Isotopism. At the time of correcting the proofs of this book (July, 1920), this work of Aston has developed into one of the most important contributions of recent times to our knowledge of the chemical elements. The new methods, a brilliant outcome of combined mathematical and experimental ability, have proved themselves to be of extraordinary power and accuracy in the detection of isotopes and the measurement of their separate atomic weights. By altering the mode of application of the electric and magnetic deviating fields, an effect of the utmost practical service, analogous to the focussing effect of an ordinary lens on light, was secured, whereby all the particles of the same mass and charge in a narrow diverging cone of positive-rays are brought to a focus at a point, the foci for different particles lying on a straight line, in the plane of which the photograhhic plate is put. Each particle thus records its position as a spot or line on the plate, and there results an analysis of the beam into its different constituent particles, quite analogous to the resolution of light into constituent 248 THE X-RAYS lines in a spectrum. From the position of the lines on the photographic plate, the mass of the atom producing it can be determined with an accuracy scarcely, if at all, inferior to that attained by chemical rhethods in the finest atomic weight determinations. But the method has the added inestimable advantage that mixtures of isotopes show their several atomic weights rather than the mean value, which is all that can be got from chemical determinations. The results of this new method so far announced are sufficiently startling. Eighteen elements have, as yet, been examined. Of these, nine only were found to be homogeneous. The other nine consist of mixtures of from two to as many as five or more isotopes. More- over, in every case, except hydrogen, the true atomic weight is found to be an exact integer (in terms of the atomic weight of oxygen as 16, taken as the standard of comparison) to an accuracy of one part in a thousand. For hydrogen, the atomic weight on this basis, 1-008, deduced by chemists from some of the finest atomic weight work ever performed, has been exactly con- firmed. The results are collected in the table below. " Pure " Atomic "Mixed'' Number of Atomic Elements. Weight. Elements. Isotopes. Weights. Hydrogen 1-008 Boron Two 10-00 and 11-00 Helium 400 Neon Two 20-00 and 2200 Carbon . . . 12-00 Silicon Two or three 28-0, 290, and (?) 30-0 Nitrogen 1400 Argon Two 360 and 400 Oxygen 16.00 Chlorine Two 350 and 37-0 Fluorine 1900 Bromine Two 79-0 and 81-0 Phosphorus 310 Krypton Five or six 78 (?), 80, 82, 83, 84, and 86 Sulphur 320 Xenon Five (?) 128,130,131,133, and 135 Arsenic . . 750 Mercury Five or more 202, 204, and three or four imresolved be- tween 197 and 200 As shown by the intensities of the different lines, the proportion in which the isotopes are present accord PROBLEM OF TRANSMUTATION 249 well in each case with the value of the mean atomic weight as determined chemically. Thus the two isotopes of bromine are in similar proportion, but the lighter isotope of argon is barely detectable. It is thus not too much to suppose that all the atomic weights, except hydrogen, are exact integers, and that the fractional values found by chemists for some of the elements are due to their being mixtures of several isotopes. The Problem of Transmutation. From the picture we have formed of the general structure of the atom and the view we have of what exactly would constitute a transmutation, we may attempt, in conclusion, to consider the kind of methods by which its accomplishment might practically be attempted. It is clear that it is the nucleus of the atom that has to be changed, either by adding to or sub* tracting from it positive or negative charges. The sub- traction or addition of electrons, so far as the outermost shell of the atom is concerned, in no sense constitutes a transmutation, but is what occurs in ordinary chemical changes. In the free state of the element the atom is electrically neutral. The number of external electrons is equal to the net positive charge of the nucleus. Subtraction of one " valency " electron or more from the outermost shell produces the positive ion, which is characteristic, not of the free element, but of it when combined with other elements to form chemical com- pounds. But such additions and subtractions are con- fined to the outermost shell. There is no exchange yet capable of being effected between the electrons in the inner completed rings and either the electrons in the outermost ring or the electrons inside the nucleus. When, however, the nucleus spontaneously ejects posi- tive or negative charges, as it does in the «- and yS-ray changes, a complete and instantaneous rearrangement of the electrons both in the completed rings and the outer shell appears to follow. In brief, to transmute an atom, 250 THE X-RAYS the change has to be effected from within, outwards from the central nucleus. It cannot, at least as yet, be impressed upon the nucleus by any changes in the exterior electronic shell, imposed from without. But the comparative ease with which the outer shell of the atom may be altered by chemical and also by electrical forces imposes in itself a formidable practical barrier to any more deep-seated change. We have seen that the a-particle may be regarded as the agent most likely to break up the nucleus of an atom if it impinges upon it, and that this actually may occur in the case of the nucleus of the nitrogen atom. Is it possible artificially to generate an a-particle or one possessing a similar amount of kinetic energy ? It may be calculated that the energy of the a-particle, over the range of velocity so far studied, is such as it would acquire in passing between two points differing in electric potential by from two to four million volts. This gives a quantitative idea of the strength of the electric field likely to be required before particles anal- ogous to the a-particle could be successfully produced. We may be fairly certain that the only influences likely to be effective in transmuting matter will be electrical in character, and that very much higher poten- tials at present known or utilised in electrical engineering will have to be developed before there is much chance of success. Along this road much that is new and impor- tant will first have to be made clear. So far as it has been followed, a barrier to further progress has been reached, which may or may not prove to be fundamental. The attainment of very high potentials at present seems to be limited by the failure of the insulation. Even a practically perfect vacuum, it appears, fails to insulate, and transmits a discharge across it when the potential exceeds a certain limit. Moseley hit upon the very ingenious idea of using the radium clock (Fig. 15, p. 59), as a method of arriving simply at otherwise unattainable potentials. If the clock there depicted is deprived of its leaves, if the CONCLUSION 251 insulating support of the radium can be made good enough and the vacuum sufficiently nearly perfect, there ought, theoretically, to be no limit to the extent the radium would become positively charged, and therefore to the difference of potential between it and the sur- roimding wall, unless, thereby, the radium products were prevented from further disintegrating and emitting their ;8-rays. In practice Moseley could not, with his particular apparatus, attain a potential much above 150,000 volts. A discharge through the vacuum always occurred at this point. The reason probably is that the loosely held " va- lency " electrons in the outermost shell of the atoms constituting the surfaces are dragged out of the atom by the electric field so causing the discharge. Such a change is not transmutational, but is allied to or identical with that produced by ordinary chemical agencies. It indicates that there is a definite limit to the extent to which matter can be charged, and at present this rather closes the door to further progress. The outer regions of the atom effectively guard the inner from being attacked. If a perfect vacuum is unable to withstand the electric forces without trans- mitting the discharge, it may be expected that any material insulator is even less likely to do so. Conclusion. This must conclude the attempt to deal with the numerous and important advances made since these lectures were first given. The field of work has opened out in a number of directions previously unsuspected. The problem of transmutation and the liberation of atomic energy to carry on the labour of the world is no longer surrounded with mystery and ignorance, but is daily being reduced to a form capable of exact quanti- tative reasoning. It may be that it will remain for ever imsolved. But we are advancing along the only road 252 THE X-RAYS likely to bring success at a rate which makes it probable that one day will see its achievement. Should that day ever arrive, let no one be blind to the magnitude of the issues at stake, or suppose that such an acquisition to the physical resources of humanity can safely be entrusted to those who in the past have con- verted the blessings already conferred by science into a curse. As suddenly and unexpectedly as the discovery of radioactivity itself, at any moment some fortunate one among the little group of researchers engrossed in these inquiries might find the clue and foUow it up. So would be diverted into the channels of human consciousness and purpose the full primary fountain of natural energy at its source, for use or misuse by men, according as to whether the long and bitter lessons of the painful past and present have even yet been really learned. INDEX a-particles, Collision of, with matter, 62-67, 223, 224 — Bombardment of gases with, 224 — Coloration of mica and gems by, 165 — Connection of, with helium, 44, 60, 93-104, — Energy of, 61 — from radium itself, 94 — from the emanation, 79, 144 — from uranium, 149 — Individual, 42, 44-46, 61 — -Limiting velocity of, 61, 66 — Mass of, 60, 98 — Number of, expelled by radium, 40, 42, 45 — passage through atoms, 220- 223 — Positive charges carried by, 60, 63 — Proof of identity with helium of, 102 — Scattering of, 63, 222 — Tracks left by, 65 — Velocity of, 61, 66, 94, 161, 221, 249 o-ray product, chemical properties of, 228 a-rays, 41-67 — Absorption of, 33 by air, 34 — Connection between range of, and period of substance, 164 — Magnetic deflection of, 60 — Making paths of visible, 64 — Range of, 34, 43, 133, 161, 164 in mica, 166 — Resolution of, 41-46 Accumulation of products, 93-98, 123 Actinium, disintegration series, 186, 198-204, 207, 214, 229 — Emanation, 198, 203 — Origin of, 199, 204 — Parent of, 205 Actinium, period of life of, 198 — Production of helium from, 99, 102 — A, 198, 203 — B, C, and D, 198 — X, 198 Active deposit of actinium, 198- 204 of thorium, 194-197 of radiimi, 137-144 Residual activity from, 145 Age of the earth, 25, 75, 98, 177-183 Ages, The geological and incan- descent, 179 Alchemist, The problem of the, 232 Alkaline-earth elements, 85, 105 Alternative theories of radio- active energy, 68, 89 Aluminium, 205, 213, 214 - — carbide, 213 Analogies between the disinte- gration series, 188-191, 198 Angstrom units, 237 Antimony, 214 Anlonoff,G.N., 205 Argon, 84, 85, 97, 105, 214, 240 — atomic weight, 248 Arrhenius, Svante, 215 Arsenic, 214 — Atomic weight of, 248 Aston, F. W., 246,247 Atom, Definition of, 105-108 — Innermost region of, 220 — Intermediate region of, 238 — Model, 212 — Nuclear, 220 — Outermost region of, 217 — Structure of, 210 Atomic disintegration, 39, 58, 67, 89, 94, 96, 98, 105, 109, 112, 155, 157, 168, 209 Cause of, 14 Multiple, 200 — mass or weight, 240, 248 253 18 254 INDEX Atomic number, 231, 239, 240 — property, Radioactivity an, 12, 13, 15, 68, 74, 83, 110 — synthesis, 180, 208 Atoms, 2, 12, 40, 46, 60, 63, 66, 84, 105, 158-161 — Interpenetration of, 63 — Passage through, of a-particles, 220-223 Atoms, Solar systems compared and contrasted with, 226 Autunite. 98, 148 Average life, Determination of, 115 of common elements, 157 of emanation, 113 of ionium, 134 of radium, 117, 125, 207 of thorium, 207 of uranium, 116, 125, 207 Period of, 112, 207 /3-particles, 49-59 — Charge of, 50, 51, 58 — Mass of, 57 — Tracks left by, 65 — Velocity of, 58 /S-ray product. Chemical proper- ties of, 228 /S-rays, 29-66, 228, 250 — Magnetic deilection of, 48, 60 — Making paths of visible, 165 jS-rays, Definition of, 139 Barium, 15, 85, 214 Barkla, C. G., 288 Beeguerel, Henri, 6, 7, 128 Berylliimi, 214 Bismuth, 15, 146, 148, 191, 214, 233 Boliwood, B., 125, 133 Bonds of afiinity, 213 Boron, 205, 214 — Atomic weight of, 248 Bragg, Sir William, 34, 35, 45, 62, 63, 64, 220, 235 Branch Series, 201 Breviimi, 150, 214 Broeck, van der, 231 Bromine, 214 — Atomic weight of, 248 Bunsen, H. W., 75 7-rays, 29-32, 66, 237 — Radiograph by, 31 Cadmium, 214 Csesium, 75, 214, 239 Calcium, 214 — absorption of gases by, 52, 101 Carbon, 106, 213, 214, 224 — Atomic weight of, 248 Carnotite, 20, 98 Cascade of changes, 74 Cathode-rays, 52-58, 210, 245 Cause of atomic disintegration, 114 Cerium, 214 Chance of disintegration. 111 Change, Law of radioactive, 112 — of radio-elements, 71, 74, 91, 92 el seq. Chemical combination. Nature of, 216 — elements, bonds of affinity, 213 Nimiber of, 239 Order of, 212 Table of, 214, 231, 239 Chemists and radioactivity, 109 Chlorine, 105, 213, 214, 215 — atomic weight, 248 Chlorion, 213 Chromium, 214 Cloud method of making paths of rays visible, 64 Cobalt, 214, 240 Conservation of radioactivity, 88 Constancy of radioactivity, 10, 13, 23, 24, 27, 43, 69, 70, 77, 90, 172 Control of natural energy, 5, 13, 173, 184 Copper, 214 Corpusciilar theory of radiation, 38 Cosmical aspect of life, 179 — energy, 24, 120, 174, 178 Cost of scientific investigations, 19 Cranston, J. A., 204 Crookes, Sir William, 15, 42, 52, 67, 128, 165 Crookes' tubes, 52, 58, 210 Crystal, space-lattice, 235 Curie, M. and Mme., 10, 12, 13, 15,19, 75,83, 124, 136,145, 198 D " Dg" line, 97, 100, 101 Dalton, John, 106, 108, 217 Debieme, A., 99, 198, 203 Decay of radioactivity, 70, 87 Definition of the atom, 105-108 Detection of infinitesimal quan- tities, 17, 75, 77, 82, 85, 90, 91, 95, 109 Determination of average life, 115 Dewar, Sir James, 52 Diffraction grating, 234 INDEX 255 Discovery of radioactivity, 6 Discrete theory of radium rays, 40, 44 Disintegration, see Atomic dis- integration — , Chance of, 111 — series. Analogies between, 188- 191, 198 of actinium, 186, 198-204 of thorium, 178, 186-198 of uranium, 121-151 Doctrine of energy, 20, 27, 37, 68, 178, 185 Dysprosium, 214 •" E-ray," 204 Earth, Age of the, 25, 75, 177-183 — Internal heat of, 178 Earthquake routes, 180 Effects of radioactivity, 8-11, 28 Eka-tantalum or proto-actinium, 204 Electric current. Action of magnet on, 49 T51ectrieity, Discharge of, 8, 14, 18, 34, 45, 64, 211 — Nature of, 50 Electrolytic dissociation. Theory of, 213-217 Electro-magnet, 47 Electro -magnetic inertia, 211 Electrometer, 46 Electron theory of matter, 109, 212 Electrons, 55-58, 63, 109, 210, 212, 216, 247 — period of revolution, 236 — valency, 217, 249 Electroscope, Gold-leaf, 8, 17, 42, 59, 84 Electrostatic and electromagnetic deflection methods, 55, 57, 60, 225, 245, 247 Elements, Chemical, bonds of affinity, 213 Number of, 289 Order of, 212 Stability of, 72, 157, 163 Table of, 214, 231, 239 Unchanging character of, 72, 73, 163, 227 — Isotopic, 229, 231 ■ — Rare-earth, 218 — Rarity of, 155 :Elixir of life, 182 Emanation of radium, 68, 77-94, 214 — a-particles from, 79, 145 Emanation of radium. Atomic weight of, 85, 103 — Average life of, 113, 116, 122 — Chemical nature of, 84, 105 — Condensation of, 80-82 — Density of, 85 — Heat generated by, 85, 86, 170 — Physiological action of, 82 — Rate of decay of, 87, 88 — Reproduction of, 88, 89, 90. 122 — Spectrum of, 85 — Volume of, 82, 119 — of actinium, 198, 203 — of thorium, 136, 190, 193-198 Emanations and radiations con- trasted, 78 Emanium, 203 Energy, cosmical, Source of, 174 — Doctrine of, 20, 27, 37, 68, 178, 185 — Internal, of matter, 68, 71-73, 86, 87, 91, 96, 108, 168-176 — Measurement of, 22, 71 Energy of coal, 22, 23, 70, 120 — of radioactive substances, 3, 5, 10, 58, 62, 68, 91, 92, 172 — of radium, 22, 68, 86, 119, 171 — • of uranium, 170-172 — Transformers of, 69, 90 Ephemeral transition-forms, 74, 92, 116, 121, 129, 203 Equilibrium, Radioactive, 90, 95, 117, 196 Ether, The, 37, 38, 56 Erbium, 214 Europium, 214 Evolution of elements, 134, 162, 163 — of universe, 26, 120, 175 Existence, Struggle for, 6, 184 F Facts and theories of radio- activity, 89, 108 Fajans, K., 229 Faraday, Michael, 47, 55 Fleck, Alexander, 228 Fletcher, A. L., 166 Fluorescence, 6, 18, 31, 53, 66, 78, 79, 195 Fluorine, 214 — Atomic weight of, 248 Friedrich, M., 235 G Gadolinium, 214 Gallium, 205, 214 256 INDEX Gas, A radioactive, 77, 80, 83 Gases, bombarded by o-partieles, 224 Geiger, Dr., 45 Geological bearing of radioac- tivity, 26, 75, 175-180 Geology, Controversy between physics and, 26 Germanium, 205, 214 Giesel, F. O., 20, 99, 203, 204 Gold, 214, 231, 233, 239 — currency, 156 H H-particles, 223, 224, 225 Hahn, Otto, 188, 205 Halogen family, 218 Halos, Pleochroic, 165 Hartley, II. B., 242 Heat generated by radium, 18, 19, 22, 85, 119, 178 -in the earth, 178 Heaviside, Oliver, 210 Helium, 44, 60, 84, 94-104, 209, 214 — atomic number, 231 weight, 245, 243 — Discovery of, 97 — Liquefaction of, 97 — Possible isotope of, 225 — Prediction concerning the origin of, 98 — Production of, by radium, 94, 99 by actinium, 99, 102 by thorium and uranium, 100 — in radioactive minerals, 96, 97 Volume of, 98 — Spectrum of, 99 Hersckell, Sir John, 159, 162 High vacua, 50, 52 Hitchins, Miss A. F., 130, 134 Holmlum, 214 Homogeneous Characteristic X- rays, 238 Hdnigschmid, O., 243 Huggins, Sir William, 107 Hydrogen, 107, 214, 224, 239 — atomic number, 231 — — weight, 248 Incandescent age, 179 ■ — gas-mantle, 14, 187 Increase of activity of radium with time, 16, 155 Indifference of radium to its en- vironment, 27, 77 Indium, 214 " Induced radioactivity," 136 Inertia, 56, 211 . ,-.„to^tinn Sawfueto/itomic weights, Intermediate substances, 74, 76, Intonaf energy of matter, 68, 70, 71-73, 86, 87, 168 — heat of earth, 178 Interpenetration of atoms, bd Iodine, 214, 239 lonisation of gases, 8, 63, 64, 66 — of liquids, 215 Ionium, 133, 151, 153, 154, 164, 189, 205 — atomic weight, 243 — Average life of, 134 — Estimated period of, 134, 165 — and uranium X, Connection between, 133 Iridium, 214, 239 Iron, 106, 214 Isotopes, 133, 150, 160, 229, 231- 233, 240, 248 — Separation of, 243, 248 Joachimsthal mine, 15, 152 .Joly, John, 166, 176-178 — "Radioactivity and geology," 177 K K-Series of X-rays, 288 Kalgurli, mines at, 132 Katrine, Loch, 125, 131 Kelvin , Lm-d, 20, 37, 178 Kirchoff, 75 Knipping, P., 235 Krypton, 214 L-Series of X-rays, 238 Lanthanum, 198, 214, 218 Laue, M., 235 Law of proportionality, 118, 123, 152 • — of radioactive change. 111 Lead, 214, 231 — Atomic weight of, 241 Lead and radium. Connection be- tween, 15, 76, 148, 241 — and thorium. Connection be- tween, 191, 241 Life from the cosmical standpoint, 179 INDEX 257 Life of radio-elements, 92 — Period of average, 113 Light, Nature of, 36, 39 — Velocity of, 38, 58, 211 Limitations of knowledge, 4, 6, 66, 173, 178-180, 227 Lithium, 214 • — atomic number, 231 Lutecium, 214 M M-Series of X-rays, 238 Macdonald laboratories of M'Gill University, 89 Mackenzie, T. D.,130 Magnesium, 214 Magnetic deflection of cathode- rays, 53 Maintenance of radium, 121 — — sun's energy, 24, 120, 179 Manganese, 214, 239 Marckwald, W., 146, 147, 192 Marsden.E., 223 Mass of the electron, 55-57, 210 Matter, Electron theory of, 109 — Ultimate structure of, 209 — Unsolved problems of, 109, 206 Maxwell, J. Clerk, 158, 162, 181 McCoij, H. N.,125 Measurement of energy, 22, 71 Meitner, Miss L., 205 Mendelejeff, V., 205 Mental pictures, 109 Mercury, 85, 148, 214, 231 — atomic weight, 247 Mi'Tlnn, T. R., 242 Mesothorium, 187-193, 227 Metaneon, 244 — atomic weight, 248 Mica, Coloration of by a-rays, 166 Milngavie, reservoir at, 125, 131 Minerals, Helium, in radioactive 96, 97 — Lead in radioactive, 15, 148 — Quantity of radium in, 16, 75, 123, 152 — Ratio between quantities of uranium and its products in, 152 Minimum quantity of helium de- tectable, 101 radium detectable, 17, 42 Molecules, 2, 108, 158 Molybdenum, 214 Monazite sand, 186, 192 Moseley, II. G. J., 239, 249 Multiple atomic disintegration, 200 N N-particles, 224, 225 Negative and positive electricity, 50 Neodymium, 214 Neon, 84, 214, 245 — atomic weight, 245, 248 Newton, Sir Isuic, 38 Nickel, 214, 240 Niobium, 214 Nitrogen, 213, 214, 224 — Atomic weight, 248 "Niton," 78 Nomenclature concerning atoms and molecules, 106-108 Non - separable radio - elements, 154, 187, 195, 227 Nuclear atom, 61, 210, 220 O O-particles, 224, 225 Orines, K., 97 Osmium, 214, 239 Ouroboros, 181 Oxygen, 106, 214, 224 — Atomic weight, 24 5 P-3 route of earthquakes, 180 Palladium, 214, 239 Parent of ionium, 133 — of radium, 122-134 Penetration test of rays, 7, 29, 30, 31, 80 Period of average life, 113 connection with range of a-rays, 164 — half change, 115 Periodic law, 105, 171, 205, 212, 227-229 — table of the chemical elements, 214, 228, 231, 239 Perpetual motion, 21, 24, 59 Phosphorescence, see Fluores- cence Phosphorus, 214 — Atomic weight of, 248 Photographic effects of radio- activity, 8, 14, 18, 66, 80 Physical impossibility, 25 Pitchblende, 15, 75, 127, 152, 187, 243 Planet, number of revolutions, 236 Platino-cyanides, 31, 35, 141 Platinum, 214, 239 Pleochroic halos, 165 Polonium, 16, 44, 146, 154,199,214 258 INDEX Positive and negative electricity, 50 • — rays, 245, 246 Potassium, 105, 214, 240 Praseodymium, 214 Prediction of origin of helium, 98 Proportionality, Law of, 118, 123, 152 Proto-actinium, 205 Q Quantity of helium detectable by spectroscope, 101 in minerals, 98 — of radium in minerals, 16, 75, 124-127, 152 R Radiant matter, 52, 57, 211 Radiation, IJature of, 36-39 Radiations, Complex, 28 Radioactivity, a new science, 1 — discovery, 6, 26, 175 — • Four experimental effects of, 8 — an unalterable atomic pro- perty, 12 Radiograph by 7-rays, 31 Radio-tellurium, 146, 148 Radio-thorium, 187-196, 227 Radium and uranium, connection between, 124-127 — Active deposit of, 137 — Average life of, 117, 125 — Changes of, 136 — Chemical nature of, 15 — clock, 59, 249 — Cost of, 19 — A changing element, 73 — emanation. See Emanation of Radium — Experiments with, 18 — Growth of, 134 — Maintenance of, 121-135 — " physically impossible," 26 — Quantity of, in pitchblende, 16 — Radiations from, 139 — Reproduction of, 122 — series, 207 — Substitute for, 154, 187, 193 — - War uses of, 19 Radium A, 89, 105, 139-144, 153 — B, 89, 105, 139-144 — C, 105, 139-144, 151, 164, 201 — C, 151, 202 — D, 153, 190 — D, E, and F, 145-147 Radium F, Identity of, with polonimn, 147 Ramsay, Sir William, 78, 82, 84, 85, 97, 99, 119, 188, 223 Ratio between uranium and its products, 152 Rayleigh, Lord, 59, 84 Rays of radioactive substances, 9, 28 et seq. Recoil, Radioactive, 103, 104 Recovery of radioactivity of radium, 77, 88 Rhodium, 214, 239 Rontgen, Wilhelm K., discovery of X-rays, 6 Rowland diffraction grating, 234 Rowland, Professor, 160 Royds,T., 102 Rubidium, 214 Russell, A. S., 229 Ruthenium, 214, 239 Rutherford, Sir Ernest, 29, 30, 43, 46, 60, 78, 80, 86, 89, 98, 102, 119, 125, 136, 161, 188, 197, 220, 222, 223, 224, 225, 231 Samarium, 214 Scandium, 205, 214 Scattering of a-particles, 63, 222 Schuster, Arthur, 161 Selenium, 214 Self-induction, 211 Sidot's hexagonal blende, 36 Silicon, 205, 214 — Atomic weight of, 248 Silk tassel experiment, 18, 34 Silver, 214, 239 Simplon Tunnel, Radium in rocks of, 177 Sodion, 215 Sodium, 214, 215, 238 Solar systems, compared and con- trasted with atoms, 226 Spectra of isotopes, 242 Spectroscope, 75, 91, 92, 97, 99, 101, 129, 160, 163, 179, 209 Spinthariscope, 42, 44, 65 Stability of elements, 72, 157, 163 Standard, The International radium, 17 Strontium, 214 Struggle for existence, 6, 184, Sirutt, Hon. R. J. (now Lord Rayleigh), 59, 125, 176 Substitute for radium, 154, 187, 193 Successive changes of radio ele- ments, 74, 77, 89, 110, 116, 129-133, 138, 145-149 INDEX 259 Sulphur, 214 — Atomic weight of, 248 Sun's energy. Maintenance of, 24, 120, 178-180 Synthesis of atoms, 180, 204 Table of atomic weights of " pure " and " mixed " elements (Aston), 248 disintegration series com- plete, 207 periods and quantities, uran- ium series, 153 velocities and ranges of a-rays, uranium series, 162 — Periodic, of the elements, 214 Chart showing sequence of a- and (3-changes through, 230 Tantalum, 205, 214, 219 Tail, Professor, Recent Advances in Physical Science, 20, 25, 26 Tellurium, 214, 240 Terbium, 214 Thallium, 148, 214, 231, 233 Theories and facts of radio- activity, 89, 108 Thomson, Sir Joseph, 55, 57, 210, 212, 245, 246 Thorium, 13, 94, 97, 98, 100, 102, 133, 136, 154, 186-198, 214 — Active deposit of, 194-198 — atomic weight, 243 — halos, 166 — disintegration series, 178, 186- 198, 207, 227, 229 — Production of helium from, 100 — Ultimate product of, 190, 242 Thorium A, 190, 197 — B, C, D, 164, 190, 201 — C, 201 — Emanation, 190, 193-196 — X, 190, 195 Thulium, 214 Tin, 214, 238 Titanium, 214, 218 Total energy in radium, 119 in uranium, 170-172 Transcendental character of radio- activity, 27, 58 Transformers of energy, 69 Transmutation, 13, 71, 72, 172, 182, 209, 223-225, 233, 248-250 Tungsten, 214 U Ultimate product of thorium, 190, 242 — products of radium, 76, 96, 123, 147, 148, 242 Ultra-material velocities, 63, 221 Unchanging character of elements, 72, 73, 163 Unsolved problem of matter, 109, 206 Uranium, 7, 12, 97, 98, 102, 107, 116, 124, 148, 169-172, 188, 189, 194, 207, 214, 229, 289 ■ — atomic number, 214, 231 — Average life of, 116, 125 — halos, 166 — Production of helium from, 100 — and radivun. Connection be- tween, 124-134 — I and II, 149, 165, 189, 195, 206, 227 — Y, 205 — X, 128-131, 133, 150, 188, 206, 214 ^XiandXj, 150 — ■ — and ionium. Connection be- tween, 133 Vacua, High, 50, 52, 249 Valency electrons, 217, 249 Value of gold, physical explanation to account for the unchang- ing, 156 — of radium, 19, 156 Vanadium, 214 Velocities, Ultra-material, 63 Velocity of cathode-ray particle, 58 — of light, 38, 58, 211 Visible, Making the paths of rays, 64 Volume of helium in minerals, 98 — emanation in equilibrium yyith radium, 82, 119 W Wave-length of 7-rays, 237 — of X-rays, 234-238 Wave theory of light, 39 Welsbach, Auer von, 13, 243 Whytlaw-Gray, R., 85 Willemite, 35, 53, 78, 79, 81, 87 260 Wilson, C. T. R., 64 Writing by radium, 19 INDEX X-ravs, 6, 30, 31, 38, 78, 210, 234 — Diffraction of, 234 — wave-length, 234-238 Xj gas, 245 Xenon, 214 — Atomic weight of, 248 Ytterbium, 214 Yttrium, 214 Zero family, 217, 245 Zinc, 214, 238 — sulphide, 35, 80, 197, 203, 204, 223 Zirconium, 214, 239 141, J 96 PRINTED IN GREAT BRITAIN BY BILLING AND S0N3, LTD., GUILDFORD AND G9HER Back End Papers.