2

As is so often the case, it was in unexpected directions that this wave of experimentation upon the X-rays bore

fruit.

The Discovery of Radio-activity

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-commonly known as a "fluores

PROF. R. A. MILLIKAN.

than to any immediate influence of the electrical discharge; and, if so, that the rays ought to be emitted not simply by a vacuum tube, but also by uranium when exposed to sunlight.

cent" light. It had long been known that there are some natural substancesnotably the mineral uranium and its compounds-which possess possess a

It was in 1896, within a year of the discovery of X-rays, that Henri Becquerel, the fourth illustrious possessor of that illustrious name, was following up this idea. His method was to expose uranium to strong sunlight for a long time, and to note whether a photographic plate which was wrapped up carefully in a perfectly opaque paper received any impression from the uranium through this opaque covering. He found that it did; but, almost by accident, he found also something that was of far greater significance-namely, that the exposure of uranium to sunlight was altogether unnecessary; that the uranium itself, in a perfectly dark room, would, in the course of about twenty days, affect a photographic plate from which it was separated both by opaque black paper and by a thin sheet of metal. In fact, he succeeded in obtaining in this way a radiograph of a metallic object, similar in all respects to the pictures which Röntgen had obtained with the X-rays. This demonstrated, first, 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. Becquerel named these rays uranium rays (rayons uraniques).

similar property of emitting a 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 that sunlight produces upon photographic plates. It accordingly, very naturally, occurred to some scientists that the X-rays might perhaps be due to the fluorescent light which came from a vacuum bulb, rather

This discovery, which has since been found to be one of the most fruitful in the history of science, was 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 top of it, to await more favorable weather. When 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, and see if anything had happened to it. The discovery of radioactivity 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.

The Discovery of Radium

It was but a few months after this that Madame Curie, one of the few women who have attained eminence in the pursuit of science, and who, together with her husband and collaborator, deserves a large share of the credit for our present knowledge of radium, set about investigating all the known elements to see if any of the rest of them possessed the remarkable property which Becquerel had discovered in the case of uranium. She found that one, and but one, of the rest of the known elements—namely, thorium, which is one of the chief constituents of the well-known Welsbach mantles-was capable, as were also all thorium compounds, of producing the effects that Becquerel had found to result from uranium.

In connection with this investigation, however, she noticed something further which appeared to her very remarkable. This was that pitchblende, the crude ore from which uranium is extracted, which consists chiefly of uranium oxide, would make an impression upon a photographic plate or would discharge an electrified body in about one-fourth the time that an equal weight of pure uranium would require to effect the same result. She inferred, therefore, that the activity of pitchblende in emitting rays could not be due solely to the uranium it contained: on the contrary, pitchblende 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 entering into the composition of pitchblendesuch, for example, as uranium, barium, lead, copper, arsenic, antimony, etc.and, after each analysis, testing the two portions separated to find which part carried with it the activity, that is, the ability to affect a sensitized plate or to discharge an electrically charged body. The search was a long and arduous one; but it ended triumphantly in the separation, from several tons of pitchblende, of

two or three grains of the new element that has become the wonder of the world. Its radio-active property is apparently not different in kind from that of uranium or thorium, but, as compared with these, it radiates as much 1,500,000 times more actively.

as

Having followed in this way the steps. whereby 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 about which it is necessary to understand something before we can intelligently discuss the nature of the radiations from radium and other radio-active substances.

The Nature of Cathode Rays

We have said that X-rays are emitted from an exhausted bulb through which an electrical discharge is passing; but the very existence of the X-rays themselves is found to depend upon rays of another kind, which also are connected with the electrical discharge from an exhausted tube. exhausted tube. These are called "cathode" rays because they originate in the cathode, or the negative electrode

D

FIG. 1.

(indicated at C in Figure 1), of a discharge-tube when the tube is put into connection with an induction coil or static machine. These cathode rays were discovered long before X-rays, and have been the subject of much investigation. In fact, Röntgen was experimenting upon them when he made the discovery of X-rays.

Figure I will give some idea of how these rays manifest themselves. If A and B are two diaphragms in the middle of which are two horizontal slits, when an induction coil is connected to the points marked † and —, and is set in operation, a spot of yellowish-green light will appear on the glass, just as though rays of some sort originated at C and passed through the two openings O in much the same way as light would do. There are a great many substances which, if placed anywhere in the line. OP so that the cathode rays from C can strike upon them, will light up with a

characteristic glow. The photograph reproduced in Figure 2 shows the distinctive outline traced upon a zinc sulphide screen along which the cathode rays graze in passing from O to P.

The nature of the cathode rays was the subject of much dispute between the year 1880, when they first began to be studied, and the year 1898. Some thought them to be streams of minute, negatively charged particles shot off from the cathode C with enormous velocities; while others maintained that these rays

FIG. 2. EXHAUST TUBE SHOWING BEAM OF CATHODE RAYS.

did not consist of particles at all, but were waves in the ether somewhat similar to light waves.

In 1898 this dispute was ended by two experiments, the first performed 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 in all cases a body placed along the path O P 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 particies. Thomson's experiments consisted in showing that if a charge of positive electricity was placed upon the plate E (see Figure 1), and

one of negative electricity upon the plate D, the rays were deflected from the line O P into the path O P'. This, too, was exactly what would be expected if the rays consisted of negatively charged particles, for these particles would then be repelled by the negative electricity upon D and attracted by the positive electricity upon E. These two experiments-taken in connection with the fact, long known, that when a magnet is brought near the path of the cathode rays in the manner shown in the photograph, Figure 3, the cathode beam is deflected by it also, just as it would be if this beam consisted of negatively charged particles in motion-these two experiments settled the question in favor of the projectedparticle theory. Physicists, accordingly, are now all agreed in regarding the cathode rays as streams of minute, charged particles shot forth in straight lines from the surface of the negative electrode in a direction at right angles to that surface, and traveling through the tube with exceedingly great velocity.

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 the rays consist of rapidly moving particles, these particles are not ordinary atoms or molecules, but are bodies whose mass is only about 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 of the cathode rays produced by a magnet and the amount produced by the electric charges D and E. It is based upon other experiments also, 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 this theory is based; and that, although they have worked by as many as three different methods, the results are all in substantial agreement.

Radio-active Substances Emit Cathode Rays

We are now in a position to understand the nature of the experiments that were performed with radio-active substances-

namely, with uranium, thorium, and radium-in order to discover the nature of their radiations. It was at first suspected that these rays were similar to X-rays, because, like the latter, they possessed the power of penetrating opaque objects and of affecting a photographic plate. X-rays, however, differ radically from cathode rays in this respect, that they are not deflected either by a magnet or by electrically charged bodies, nor do they impart negative charges to bodies upon which they fall.

As soon as the tests which distinguished X-rays from cathode rays were applied to Becquerel rays-that is, as soon as the magnet was placed so that it would be able to distort photographs. produced by means of Becquerel rays in case they, like cathode rays, were deflected by a magnet-it was found that the photographs did indeed indicate such deflection. It was further found that the Becquerel rays could be bent out of their course by electrical charges, just as the cathode rays; and lastly, that also, like them, they imparted negative charges of electricity to objects upon which they fell. Moreover, when the mass of these particles was calculated from the amounts of deflection by a magnet and by electric charge, it was found, strangely enough, to be the same as that of the cathoderay particles. It seems certain, therefore, that radio-active substances spontaneously emit particles that are identical in all respects with those that constitute the cathode rays.

The velocity with which these minute. particles, of about one-thousandth the size of the hydrogen atom, are shot off from radio-active substances, is found to be even more stupendous than the velocity of the similar particles in the cathode rays. The latter were found to move with a speed of about 20,000 miles per second, which is about one-tenth of the speed at which light travels; but the velocity of the particles shot off from radio-active substances is estimated to be as high as 175,000 miles per second, or only a little lower than that of light itself. Other Radiations of Radio-active Substances

It was also discovered by Professor Rutherford of McGill University, Can

all emit other rays besides cathode rays, which are distinguishable from the latter, first, by possessing a very much smaller penetrating power, and second, in the fact that they are not ordinarily deflected either by a magnet or by an electrically charged body. Rutherford 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 necessary only to lay over the radioactive substance a very thin sheet of

FIG, 3. DEFLECTION OF CATHODE
RAYS BY MAGNET.

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 ray, 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 conductive 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

There is also a third kind of ray emitted by radio-active substances, to which has been given the name Gamma rays. These are very much more penetrative than even the Beta rays, but so far little is known about their nature. Since the energy carried by them, however, is very insignificant as compared with that in the Alpha and the Beta rays, we may leave them entirely out of account in most of our computations upon the energy of the radiations from radio-active substances.

The Nature of the Alpha Rays

It was at first conjectured that possibly the Alpha rays might be X-rays, since, like the latter, they are not deflected by a magnet, and also are very effective in rendering a gas electrically conductive. Only last year, however, Rutherford contrived a very ingenious experiment by which he succeeded in showing 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 in the same way by a very strong electrical charge. But in both of these cases the direction of the deflection is opposite to that obtained under the same conditions with Beta rays. These observations made by Rutherford are of the utmost importance, and have recently been confirmed both by Becquerel in Paris and by a German physicist of the name of Des Coudres. The only possible interpretation that can be put upon them is that the Alpha rays also consist of particles of matter shot off from radioactive substances; but that, while the Beta-ray particles are charged with negative electricity, the Alpha-ray particles carry charges of positive electricity.

Further, when from the amounts of the deflection produced by the magnet and by the electric charge the size and velocity of the Alpha-ray particles are calculated, the results are again most interesting, for these particles are found to have a mass, not one-thousandth that of the hydrogen atom, like the cathode rays, but approximately twice as great as that of the hydrogen atom. That is, the Alpha-ray particles are about 2,000 times as heavy as those of the cathode rays. Their mass is about the same as that of the atom of helium; but despite this great mass, their velocity is found to be

as high as 20,000 miles per second, or about one-third that of the Beta-ray particles. Their great size in comparison with the Beta particles accounts for their smaller penetrating power, as well as for their much greater effectiveness in knocking the gas to pieces, or disintegrating it, thus rendering it electrically conductive.

The Crookes Spinthariscope

We have attempted to follow thus far the evidence upon which is based the conclusion that the radiations from radio

active substances consist, to a large extent at least, of projected particles of matter shot forth with exceedingly high velocity from the active substances, and that some of these particles have a mass one-thousandth that of the hydrogen atom, while others have a mass twice as great as that of the hydrogen atom. But no amount of reasoning of the sort thus far given will be found half so convincing to the ordinary mind as the sight of a piece of radium actually at work.

Radium itself in the dark glows with a light resembling that of a glow-worm; and when placed near certain substances, as willemite or zinc sulphide, it causes them to light up with a glow more or less brilliant according to the amount of the radium at hand. In the spring of 1903 Sir William Crookes first exhibited this most beautiful and wonderful experiment at the Soirée of the Royal Society in London. A small quantity of radium is placed about one 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 innumerable tiny flashes of light. It is as though one were viewing a swamp full of fireflies, or, better still, a sky full of shooting stars. It is probable that these sparks or scintillations of light are caused by the impact of the Alpha-ray particles upon the screen, just as sparks fly from an iron when it is sharply struck with a hammer.

The Continuous Emission of Light and Heat by Radio-active Substances After learning that the radio-active substances uranium, thorium, and radium