Popular Science Monthly/Volume 64/April 1904/Recent Discoveries in Radiation and their Significance

(1904)
Recent Discoveries in Radiation and their Significance by R. A. Millikan

THE

POPULAR SCIENCE

MONTHLY

APRIL, 1904.

 RECENT DISCOVERIES IN RADIATION AND THEIR SIGNIFICANCE.

By Professor R. A. MILLIKAN,

UNIVERSITY OF CHICAGO.

THERE are times when the atmosphere seems to be fairly saturated with the spirit of scientific discovery. Such a time existed during the opening years of the nineteenth century when John Dalton was putting the atomic theory of matter upon an experimental rather than upon the purely speculative foundation upon which it had previously rested; when Count Rumford, an American by birth, was laying the corner-stone of the modern mechanical theory of heat, in accordance with which heat consists in the vibratory motion of the particles of which matter is composed; when Thomas Young was forging the final links in the chain of proof that light consists in the wave motion of some all-pervading medium, the ether.

It is not a little interesting that the opening years of the twentieth century have also been marked in no less a degree than those of its predecessor by epoch-making discoveries in physics. Most of this new activity has been grouped about the general subject of radiation, discoveries of new rays having followed one another in such rapid succession that it is difficult even for a physicist to keep posted about them all. As a result of these discoveries important progress has been made toward the solution of one of the most fundamental questions with which science has to deal, viz., the question as to the nature and the constitution of matter.

The Discovery of X-rays.

The discovery of X-rays is to be regarded as the starting point of this epoch of investigation upon radiation. It was in the Christmas season of 1895 that Professor Röntgen of Wurzburg, Germany, exhibited to the Physical Society of Berlin the first X-ray photographs. These photographs showed that from a vacuum bulb in which an electrical discharge was passing some sort of radiation was emitted, which was like light in that it produced an effect upon the photographic plate, but was unlike light, first, in that it was wholly invisible, and, second, in that it was able to pass easily through many substances which are perfectly opaque to ordinary light, such, for example, as cardboard, wood, leather and, notably, the flesh of the human hand. This discovery would probably have attracted little attention outside of scientific circles had it not been for this last-mentioned remarkable property, but the idea of obtaining photographs of the skeleton of a living being was so startling, so uncanny, at that time, to the average mind, that the discovery took to itself wings and within two weeks had set the whole world agog. Scores of scientists in all countries dropped at once their pending researches and began to experiment upon these strange new rays which Röntgen had named X-rays because they were such a completely unknown quantity. A surprisingly small amount of new knowledge concerning the nature of X-rays themselves resulted from all this research. The X-rays are almost as much of an unknown quantity to-day as they were when Röntgen made his first announcement. As is so often the case, it was in unexpected directions that this wave of experimentation upon X-rays bore fruit. The discovery of radio-activity was not the least important result of this activity. It came about in this way.

It was noticed that an exhausted bulb which is emitting X-rays under the influence of electrical discharges is always aglow with a peculiar greenish-yellow light which is commonly known as fluorescent light. Now it had long been known that there are some natural substances, notably the mineral uranium and its compounds, which possess a similar property of emitting this yellowish-green light not only when they are in a vacuum tube through which electrical discharges are passing, but also when they are exposed to the invisible radiation from the sun, that is, to the so-called actinic or ultra-violet rays which are chiefly responsible for the effects which sunlight produces upon photographic plates. It accordingly very naturally occurred to some scientists that the X-rays might perhaps be due to this fluorescent light which came from a vacuum bulb, rather than to any immediate influence of the electrical discharge, and, if so, that they ought to be emitted not simply by a vacuum tube, but also by uranium when exposed to sunlight. It was in 1896, within a year of the discovery of X-rays, that Henri Becquerel, the fourth illustrious possessor of that illustrious name, devised some experiments to test this inference. His method was to expose uranium to strong sunlight for a long time, and then to notice whether a photographic plate, which was wrapped up carefully in perfectly opaque paper and placed beneath the uranium, received any impression from it. He found that it did; but he further found that the exposure of the uranium to sunlight was altogether unnecessary; that the uranium itself in a perfectly dark room would affect, in the course of ten or twenty days, a photographic plate from which it was separated both by opaque black paper and by a thin sheet of metal. In fact he obtained in this way a radiograph of a metallic object similar in all respects to the pictures which Röntgen had obtained with X-rays. This showed, in the first place, that the fluorescent light had nothing whatever to do with the production of the photograph, but it showed also something much more important than this, namely, that the mineral uranium is all the time spontaneously emitting rays of some sort, which are capable of penetrating opaque objects in just the way the X-rays do.

This discovery, which has been one of the most fruitful in the history of science, is immediately due to the accident of a few cloudy days in Paris, during which Becquerel, since he could not expose his uranium to sunlight, set away his plate with the uranium on the top of it, to wait for fair weather. When the fair weather returned and he was ready to continue his experiments, it fortunately occurred to him that it might be worth while to develop the plate upon which the uranium had rested to see if anything had happened to it. The discovery of radio-activity was the result. Those who recall the story of the discovery of photography will remember that it was made quite as accidentally and under quite similar circumstances.

Becquerel further found that the rays emitted by uranium are also emitted by all uranium compounds. He therefore named them uranium rays. Another property which he found that the rays possessed, in addition to that of affecting a photographic plate, was the important property of rendering a gas through which they pass a conductor of electricity, or, to state the same thing in another way, the property of discharging any electrified body which is brought into their neighborhood.

It was but a few months after this that Madame Curie, one of the few women who has attained eminence in the pursuit of science, and who together with her husband, with whom most of her work has been done, deserves a large share of the credit for our present knowledge of radium, set about investigating all the then known elements to see if any of the rest of them possessed this remarkable property which Becquerel discovered in uranium. She found that one, and but one, of the remainder of the elements, namely, thorium, the element which is one of the chief constituents of Welsbach mantles, was capable of producing precisely the same effects which Becquerel had discovered with uranium. After this discovery the rays from all this class of substances began to be called Becquerel rays, in honor of Becquerel, and all substances which emitted such rays were called radio-active substances.

But in connection with this investigation, Madame Curie noticed something which appeared to her very noteworthy. It was that pitchblend, which is the crude ore from which uranium is extracted and which consists chiefly of uranium oxide, would produce an effect upon a photographic plate, or would discharge an electrified body, in about one fourth the time in which the same weight of a pure uranium salt would produce the same effect. She inferred, therefore, that the activity of pitchblend in emitting rays could not be due solely to the uranium contained in it: that, on the contrary, pitchblend must contain some hitherto unknown element which had the property of emitting Becquerel rays more powerfully than uranium itself. She therefore immediately set about the task of separating as carefully as possible the dozen or so of substances which are contained in pitchblend, such for example, as uranium, barium, lead, copper, arsenic, antimony, and so on, and after each separation, testing the two portions separated to find which part carried with it the activity, that is, the ability to affect a photographic plate or to discharge an electrically charged body. The methods employed were the ordinary ones used in qualitative chemical analysis. The search was a long and difficult one, but ended triumphantly in the separation from several tons of pitchblend of two or three grains of the new element which has now become one of the wonders of the world.

The successive steps in this discovery were as follows: Madame Curie found, first, that in this process of separation of the constituents of pitchblend, the reagent which separated the barium out of the solution also brought down in the barium precipitate a large part of the activity. The barium chloride precipitate obtained in this way had about sixty times the activity of pure uranium chloride. She next found that when alcohol was added to a solution of this barium chloride, the first precipitate which was thus formed was more active than that which came down later. By retaining only this first precipitate and discarding the rest, and again redissolving and repeating the process over and over again (this process is called fractional precipitation) she succeeded in obtaining a sample of barium chloride which was 4,000 times as active as uranium chloride. Further, since the weight of the barium chloride for a given weight of contained chlorine was greater in the ratio 140 to 137 than the weight of ordinary inactive barium chloride for the same weight of contained chlorine, she concluded that the apparent activity of the barium chloride could not be due to barium at all, but must be due to this unknown element which was mixed with the barium in the precipitate. She therefore announced definitely the discovery of a new substance which she named radium.

There is a second process, which is now more commonly used than the above, for separating this active substance, that is, the radium, from the barium with which it is always found. It is called the process of fractional crystallization, and consists simply in retaining the first crystals which crystallize out from an active barium chloride solution and then redissolving these crystals and allowing some of them to crystallize out again, and so on. With each new crystallization the activity of the crystals per unit of weight increases. In this way the Curies have recently obtained samples of radium which are as much as 1,800,000 times as active as uranium, the activity being measured by comparing the rates at which equal weights of radium and uranium will discharge an electrified body.

Having followed in this way the processes by which radium was discovered as early as 1898, let us turn to some of the other results which followed close upon the discovery of the X-rays, and which it is necessary to understand something about before we can intelligently discuss the nature of the radiation from radium and other radio-active substances.

The Nature of Cathode Rays.

1 have said that X-rays are emitted by an exhausted bulb in which an electrical discharge is passing, but the very existence of X-rays is found to depend upon another kind of rays which are also connected with the electrical discharge from an exhausted tube. These are called the cathode rays because they originate in the negative electrode or cathode, see Fig. 1, of a discharge tube when it is put into connection

Fig. 1. Illustrating Deflection of Cathode Rays by an Electrostatic Field.

with an induction coil or static machine. These cathode rays were discovered long before X-rays. Fig. 1 will give some idea of how they manifest themselves. If A and B are two diaphragms, in the middle of which are two horizontal slits, then, when an induction coil is connected to the points marked + and — and set into operation, a small spot of greenish-yellow light will appear on the glass at P, just as though some sort of rays were emitted in straight lines from C, and, passing through the two openings O, fell upon the point P. There are a great many substances which, if placed anywhere in the line OP so that these cathode rays from C can strike upon them, will light up with a characteristic glow. For example, if a screen coated with, zinc sulphide is placed within a discharge tube in the manner shown in Fig. 3, the cathode rays which pass through the slit in the mica diaphragm just opposite the cathode, light it up brilliantly in the parts along which they graze, and thus trace a distinct outline of their path from one end of the tube to the other.

The nature of these rays was the subject of much dispute between the years 1880, when they first began to be studied, and 1898. Some thought them to be streams of minute negatively charged particles shot off with enormous velocities from the cathode C, while others maintained that they did not consist of Fig. 2. Showing Cathode Beam of Rays.Fig. 3. Showing Deflection of Cathode Rays by a Magnet. particles at all, but were waves in the ether, just like light waves. The dispute was finally ended by two very conclusive experiments performed, the first by Perrin, a Frenchman, and the other by J. J. Thomson, professor of physics in Cambridge University, England. Perrin's experiment consisted in proving that under all circumstances a body which was placed along the path OP, so that the cathode rays could fall upon it, became charged with negative electricity, just as would be expected if the cathode rays consisted of negatively charged particles. J. J. Thomson's experiment consisted in showing that if a charge of positive electricity were placed upon the plate E (see Fig. 1), and a charge of negative electricity upon the plate D, the rays were deflected out of the line OP and into the path OP'. This, too, was to have been expected if the rays consist of negatively charged particles, for these particles would be repelled by the negative electricity upon D and attracted by the positive electricity upon E.

There is a further property of the rays, which, although it had long been known, adds powerful support to the projected particle theory. It is that when a magnet is brought near the cathode beam in the manner shown in Fig. 3, the beam is deflected by it also, just as would be expected if it consisted of a stream of negatively charged particles. These three experiments settled the question in favor of the projected particle theory, so that physicists are now all agreed in regarding the cathode rays as streams of minute, negatively charged corpuscles shot off in straight lines from the surface of the negative electrode and in a direction at right angles to this surface.

Cathode Ray Particles Much Smaller than the Smallest Known Atom.

But the most remarkable result of experiments upon cathode rays is the conclusion that while they consist of rapidly moving particles, these particles are not ordinary atoms or molecules, but are instead bodies whose mass is only about one one-thousandth of the mass of the smallest atom known, namely, the atom of hydrogen. The calculation by which this conclusion is obtained is based upon a comparison of the amount of deflection which is imparted to the rays by a magnet of known strength, and the amount of deflection which is produced by electric charges of known size on D and E. It can also be based upon other experiments which will not here be described. Suffice it to say that more than a dozen well-known physicists have made the observations and the calculations upon which they are based, and that, although they have worked by as many as three different methods, the results are all in substantial agreement.

A New Theory as to the Constitution of Matter.

Furthermore, since experiments of the kind mentioned above always lead to the same value for the mass of the cathode ray particle, no matter what be the nature of the gas which is used in the bulb and no matter what be the nature of the metal constituting the cathode C, physicists have found it necessary to conclude that these minute particles are constituents of each and every one of the different metallic elements at least, and probably of all the other elements also. In view of these discoveries, the suggestion has been put forward by several of the greatest living physicists, that these cathode particles are themselves the primordial atoms out of which the 70 odd atoms known to ordinary chemistry are built up. According to this suggestion, the chief difference between the different atoms of chemistry would consist simply in differences in the number of the primordial atoms which enter into them. Thus the hydrogen atom would be composed of about a thousand of these minute corpuscles, or electrons, as they have been called, the oxygen atom of 16,000, the mercury atom of 200,000, and so on. It is necessary to assume, however, that these electrons are half plus and half minus, for otherwise we can not account for the uncharged condition of ordinary atoms. Since, however, no evidence has as yet appeared to show that positively charged electrons ever become detached from atoms, J. J. Thomson has brought forward the hypothesis that perhaps the positive charges constitute the nucleus of the atom, while the negative electrons are on the outside and are therefore more easily detachable. It is too early to assert this theory as correct; it is introduced here merely as a profoundly interesting speculation brought forward by men high in authority in the scientific world. It differs radically from most other speculations of the same general nature, in that it is based upon a certain amount of experimental evidence. However, the experiments can not be said to have gone so far as to render its correctness even probable. This much, however, it is safe to say: the experiments upon cathode rays have proved conclusively that under some circumstances particles do exist which are smaller than the ordinary atoms of chemistry. It was the study of cathode rays, then, which first sounded the death-knell of the indivisible atom of our earlier chemistry and prepared the way for the discoveries, which were soon to follow, of subatomic transmutations which involve the liberation of stored-up energies, the very existence of which had never before been dreamed of.

The Nature of X-rays.

I have already said that cathode rays are very intimately connected with X-rays, for both are associated with the discharge of electricity in exhausted tubes. In fact, at the time of Röntgen's discovery, many physicists thought that the X-rays were nothing more nor less than cathode rays which had passed through the walls of the tube into the outside air. But Professor Röntgen demonstrated that the X-rays are wholly different from the cathode rays in these two important respects, namely: (1) they are not deflected in the slightest degree, either by a magnet or by bodies charged with static electricity; (2) they do not impart negative charges to objects upon which they fall. X-rays are therefore not cathode rays. They originate at the point at which the cathode rays strike against the walls of the tube, or against any object placed in their path inside the tube. In the ordinary X-ray tube a little plate of platinum is commonly placed in the middle of the tube, just opposite the cathode, for the purpose of receiving the stream of cathode rays. It thus becomes the source from which the X-rays proceed. This is about all that we know with certainty concerning X-rays. Most physicists, however, now believe them to be ethereal rather than material in their nature, that is, they believe them to be some sort of waves or pulses in the ether, not very dissimilar from light waves.

We are now in a position to understand the experiments which were performed with radio-active substances, namely, uranium, thorium and radium, in order to discover the nature of their radiation. It was at first suspected that these rays were similar to X-rays, because, like them, they possessed the power of penetrating opaque objects and of affecting photographic plates. But as soon as the test which distinguished X-rays from cathode rays was applied, that is, as soon as a magnet was placed so that it could distort the photograph produced with the aid of Becquerel rays, in case these rays like cathode rays were deflected by it, it was found indeed that these photographs did indicate such deflection. It was further found that they could be bent out of their course by electric charges just like the cathode rays, and, lastly, that, also like them, they imparted negative charges of electricity to objects upon which they fell. Further, when the mass of these particles was calculated by comparing the amount of deflection produced by a magnet with that produced by an electric charge, it proved to be, strangely enough, the same as that of the cathode ray particles. It seems certain, therefore, that radio-active substances spontaneously emit rays which are identical in all respects with the cathode rays, i. e., which consist of minute negatively charged particles of about one one-thousandth the size of the hydrogen atom. The velocity with which these minute particles are shot off from the radio-active substances is found to be even more enormous than the velocity of the same particles in the cathode rays. The latter were found to move with a velocity which is sometimes as high as 20,000 miles per second. Now, the velocity with which light travels from the sun to the earth or from star to star is 186,000 miles per second. Hence, the cathode ray particles sometimes move with a tenth the velocity of light. But the velocity of the particles shot off from radio-active substances is still more surprising, for it sometimes reaches the stupendous figure of 175,000 miles per second, only a trifle less than that of light.

But it was discovered in 1899 by Rutherford, of McGill University, Canada, that uranium, thorium and radium all emit other rays besides cathode rays, which are distinguishable from them, first by their very much smaller penetrating power and, second, by the fact that they are not ordinarily deviated either by a magnet or by an electrically charged body. He named these rays the alpha rays, while he designated the cathode rays emitted by radio-active substances as the beta rays. In order to separate the alpha from the beta rays, it was only necessary to lay over the radio-active substance, that is, the uranium, the thorium or the radium, a very thin sheet of aluminum; for example, a sheet .005 centimeter thick. This opposed almost no obstruction to the passage of the beta rays, but it cut off entirely the alpha rays. Another mark of difference between the two kinds of rays was that, while the beta rays were very much more effective than the alpha rays in penetrating opaque objects and in affecting a photographic plate, their influence in rendering a gas electrically conducting was very small in comparison with that of the alpha rays; so that if the thin sheet of aluminum were taken away, the gas above the radio-active substance became a hundred times as good a conductor as when the alpha rays were screened off.

There is also a third kind of ray given off by radio-active substances, which has been given the name of gamma rays. These are very much more penetrating even than the beta rays; but, so far, little is known about their nature. Since, however, the energy carried by them is very insignificant as compared with that in the alpha and beta rays, we can leave them entirely out of account in most of the computations which we make upon the energy of radiations of radio-active substances. It is now conjectured that the gamma rays are ethereal pulses like the X-rays.

The Nature of the Alpha Rays.

It was at first conjectured that possibly the alpha rays might be X-rays, since, like them, they are not deflected by a magnet, and since, also like them, they are very effective in rendering a gas electrically conducting. But only last year Professor Rutherford contrived a very ingenious experiment by which he showed conclusively that the alpha rays are deflected very slightly by a magnet if the magnet is sufficiently powerful. He also succeeded in showing that they are deflected by a very strong electrical field. But in both of these cases the direction of the deflection is opposite to that obtained under the same conditions with beta rays. These results of Professor Rutherford's are of the utmost importance, and they have been recently confirmed both by Becquerel in Paris, and by a German physicist by the name of Des Coudres. The only possible interpretation which can be put upon them is that the alpha rays also consist of particles of matter shot off from the radio-active substances, but that, while the beta ray particles carry charges of negative electricity, the alpha ray particles carry charges of positive electricity.

Further, when from the amounts of the deflections produced by the magnet and by the electric charge, the size and velocity of the alpha particles are calculated, the results are again most interesting. For these particles are found to have a mass not one one-thousandth that of the hydrogen atom, like the cathode rays, but approximately twice as great as that of the hydrogen atom, or about the size of the atom of helium. (The atomic weight of helium is 4.) They are therefore about 2,000 times as heavy as the cathode ray particles. This explains why they do not pass through ordinary matter as readily as do the smaller beta particles. But despite this comparatively great mass, their velocity is found to be as much as 20,000 miles per second, more than a tenth that of the smaller particles. It will be seen, therefore, that the energy of the blows which they strike against the bodies upon which they fall is much greater than that of the beta particles. This explains why they knock the gas to pieces, or dissociate it and thus render it conducting, so much more energetically than do the beta particles.

The Crookes Spinthariscope.

We have attempted to follow, thus far, the evidence upon which we base the conclusion that the radiations from radioactive substances consist, largely at least, of projected particles of matter expelled with enormous velocities from the active substance. But no amount of reasoning of the sort thus far given will be found half as convincing to the ordinary mind as the sight of a bit of radium at work. Radium itself, in the dark, glows with a light which resembles that of a glowworm, and when placed near certain substances like willemite (zinc silicate) or zinc sulphide, it causes them to light up with a glow which is more or less brilliant according to the amount of the radium at hand. Last spring Sir William Crookes first exhibited the following most beautiful and wonderful experiment at the soirée of the Royal Society in London. A small bit of radium is placed about a millimeter above a zinc sulphide screen, and the latter is then viewed through a microscope of from ten to twenty diameters magnification. The continuous soft glow of the screen, which is all that one sees with the naked eye, is resolved by the microscope into a thousand tiny flashes of light. It is as though one were viewing a swamp full of fire flies, or, better still, a sky full of shooting stars. The appearance is as though the screen were being fiercely bombarded by an incessant rain of projectiles, each impact being marked by a flash of light, just as sparks fly off from an iron when it is struck with a hammer. Becquerel has recently brought forward evidence to show that the spark is due to a cleavage produced in the zinc sulphide crystal by the impact of the alpha particles. This explains why the effect is not observable with all kinds of screens.

The Continuous Emission of Light and Heat by Radio-active Substances.

After learning that the radio-active substances uranium, thorium and radium are, for some reason or other, continuously projecting with enormous velocities two kinds of particles, the alpha and the beta particles, one is not surprised to find that these substances maintain a temperature above the temperature of the surrounding atmosphere. This has been proved experimentally only for radium, which was found last year by M. Curie and M. Laborde to remain permanently at a temperature between one and two degrees centigrade above that of its surroundings, and to give out for each gram of weight enough heat per hour to raise a hundred grams of water through one degree. Since radium radiates more than a million times more actively than either of the other substances, it is not likely that any one will ever be able to show experimentally that uranium and thorium also maintain a temperature above that of their surroundings. Nevertheless, the same causes which operate to hold up the temperature of radium, operate also to hold up the temperature of both the other radio-active substances, the only difference being one of degree. Hence it is probable that all radio-active substances are continuously emitting, in a greater or less degree, heat energy. This is not surprising in view of the conclusion that such substances are continually projecting particles with enormous velocities, for if these particles are projected from all the molecules of the active substance, it would be expected that the temperature of the mass of the substance would rise under this unceasing internal bombardment. But whence comes this energy which is represented in the projected particles, and of which this heat and light are the ultimate manifestation?

Radio-activity a Manifestation of Subatomic Energy.

The answer to this last question has not yet been fully given. This much, however, can be said, that, thanks to the splendid work of Rutherford and Soddy, of McGill University, of Sir William Crookes, of the Curies and Becquerel in Paris, and of one or two German physicists, a fairly satisfactory answer is at least in sight. Whatever be the cause of this ceaseless emission of particles by radio-active substances, it is certain that it is not due to any ordinary chemical reactions, such as those with which we have heretofore been familiar; for Madame Curie showed, when she originally discovered the activity of thorium, that the activity of all the active substances is proportional simply to the amount of the active element present and has nothing whatever to do with the nature of the chemical compound in which that element is found. Thus, thorium may be changed from a nitrate to a chloride, or from a chloride to a sulphide, or it may undergo any sort of a chemical change, without any change whatever being noticeable in its activity. Furthermore, radio-activity has been found to be absolutely independent of all changes in physical as well as chemical condition. A radio-active substance may be subjected to the lowest temperatures known, or to the highest temperature obtainable, without showing in either case any alteration whatever in the amount of its activity. Radio-activity seems therefore to be as unalterable a property of the atom of the radio-active substances as is weight itself. It is certainly something which is entirely beyond the range of ordinary molecular forces. This is strong evidence in favor of the view that radio-active change, i. e., the change, whatever it be, which is responsible for the expulsion of the alpha and beta particles, involves a change in the nature of the atom itself. This is the first time in the history of science that any subatomic store of energy has been tapped by man, although, as stated above, the possibility of breaking up the atom was first proved by the study of cathode rays.

The Production of Uranium X.

The view that radio-activity consists in some change going on in the nature of the atom has received powerful support from a series of discoveries which were started in 1900 by an experiment performed" by Sir William Crookes. He found that if uranium nitrate were precipitated by ammonium carbonate and then enough of the ammonium carbonate added to redissolve the uranium nitrate, there remained behind an undissolved precipitate which contained a large part of the original activity which had been possessed by the uranium nitrate. He called this undissolved precipitate (or better, the portion of it which was responsible for the activity, for when chemically tested, it showed nothing but iron, aluminum and other impurities) uranium X. But he soon afterward discovered that the uranium nitrate, which had partially lost its activity through the separation from it of this unknown substance, uranium X, in the course of a few months had regained completely its original activity, while the uranium X had lost its power to radiate.

A little more than a year ago Rutherford tried the same experiment with thorium and found quite similar results. But more important still, he found that in both cases the rate of loss of activity of the separated substance, that is, of the uranium X or the thorium X, was equal to the rate of recovery of the uranium or the thorium from which the new substance had been extracted. To state this result in a slightly different way, he found that if all the uranium X were removed from a sample of uranium by this process, so that further precipitation by ammonium carbonate would bring down no more uranium X, and if the uranium were then allowed to stand till it had recovered one half of the lost activity, and if then the uranium X was again removed, the amount of this uranium X which could be obtained was now Just one half as much as the amount obtained at first. If the uranium had regained three fourths of its original activity, just three fourths as much uranium X could be obtained from it as at first. This result seems capable of but one possible interpretation, namely, this: the uranium is continually producing, by some change which goes on within itself, some radio-active substance uranium X, which, however, is formed in such minute quantities that it can be detected and measured only by means of its radio-activity. Further, this uranium X itself is unstable, for it undergoes a change by which it loses its activity. Rutherford further found that in this separation of uranium X from uranium the part of the activity which was left behind in the uranium consisted entirely of the alpha type of radiation, while the part which was separated out in the uranium X consisted wholly of the beta type. This seems to show that the first step in the process of radio-active change consists in the expulsion from the uranium atom of the big alpha particles, while the beta particles are expelled only from some product which is formed by the disintegration of the uranium atom.

In all these particulars Rutherford found that thorium and uranium acted essentially alike, the chief difference being that while the uranium X loses one half of its activity in about twenty-two days, it requires but four days for the activity of the thorium X to decay to half its initial value.

Nor did physicists have long to look in order to discover this substance into which the emanation from radium is transformed. The Curies found as early as 1899 that when this gas comes into contact with a solid object, the object becomes coated with a film of radioactive matter which can be dissolved with hydrochloric or sulphuric acid, and which is left in the dish when the acid is evaporated. Or which may be rubbed off with leather and found, by means of the property of activity which it possesses, in the ash of the leather after the leather has been burned. This radio-active matter is so infinitesimal in amount that in no case is it detected in any other way than by its radio-activity. It might, at first, look as though it were nothing but the active gas itself condensed on the surface of the solid object, but since the rate at which it loses its activity is altogether different from the rate at which the activity of the emanation decays; and, more important still, since it is found to emit both alpha and beta rays while the emanation emits only alpha rays, it seems necessary to conclude that this film of active matter is a product of the emanation rather than the emanation itself. In fact it appears to bear in all respects the same relation to the emanation which the emanation does to radium. That is, it is the result of the disintegration of the atom of the emanation, just as the emanation is the result of the disintegration of the atom of radium.

In the case of thorium this continuous change from one radioactive substance into another has been followed with certainty through as many as four different stages, thus; first, thorium produces thorium X; second, thorium X produces an active gas or emanation which is very like the radium emanation; third, the thorium emanation gives rise to a radio-active substance which is responsible for the induced radio-activity which is observable whenever the emanation comes in contact with a solid object; fourth, this induced radio-active matter due to the thorium emanation gradually loses its radiating power, and hence must undergo at least one further change into some other substance.

The Disintegration of the Atom of Radio-active Substances.

The Birth of Helium.

It appears, therefore, that all the three heaviest atoms known are slowly disintegrating into simpler atoms. The process is, however, extremely slow. Despite the incessant projection of particles from radium, so strikingly shown by the Crookes spinthariscope, no one has as yet been able to detect with certainty any loss whatever in its weight, nor any diminution in its activity. Yet we may be certain that in fact it is both losing weight and diminishing in activity; for otherwise the principle of the conservation of energy, the corner-stone of modern science, would be violated. From a knowledge of the amount of heat energy given off by radium per hour, viz., 100 calories, and a knowledge of energy represented by each projected particle (this knowledge we possess, since we know the mass and velocity of the alpha particles, the energy contained in the beta particles being wholly negligible in comparison), we can easily estimate certain limits within which we may expect all the radium now in existence to pass out of existence as radium. In the first place we obtain the number of alpha particles projected per second from one gram weight of radium atoms by dividing the 100 gram-calories by the kinetic energy of each alpha particle. The result of this calculation is 200,000,000,000 ${\displaystyle \left(=2\times {10^{11}}\right)}$. Now there are ${\displaystyle 3\times {10^{21}}}$ atoms of radium in a gram of radium chloride. Hence if each atom of radium which becomes unstable threw off but one alpha particle, then the fractional part of any given number of radium atoms which become unstable per second would be simply ${\displaystyle 2\times {10^{11}}}$ divided by ${\displaystyle 3\times {10^{21}}}$ This amounts to but one in fifteen thousand million. On the other hand, if each atom of radium which becomes unstable produces the maximum possible number of alpha particles, viz., 225/2, 235 being the atomic weight of radium and two the atomic weight of the alpha particles, then only one atom in sixteen hundred thousand million would become unstable per second. These two numbers represent then respectively the maximum and minimum possible rates at which the atoms of radium are becoming unstable. At the first rate radium would lose about one one-hundredth of its activity in five years, ninety-nine one-hundredths in 2,200 years and in 9,000 years it would possess no more than one hundred-millionth part of its present activity, i. e., it would no longer be measurably active. Since we have brought forward good evidence in the foregoing paragraphs that each atom of radium which becomes unstable throws off at least as many as four alpha particles before it again reaches a condition of stability, it is probable that the above lowest possible limit to the life of radium, viz., 9,000 years, should be replaced by 36,000. At the second or minimum rate radium would lose one-hundredth of its activity in about 500 years and in 900,000 years would be no longer measurably active. It appears then that within a period of a million years at most all the radium now in existence will have ceased to be radio-active, i. e., will have ceased to be radium. The life of uranium and thorium would be from one to two million times as much, since they are radiating only about a millionth as actively.