« PreviousContinue »
As is so often the case, it in than to any immediate influence of the unexpected directions that this wave of electrical discharge; and, if so, that the experimentation upon the X-rays bore rays ought to be emitted not simply by fruit.
a vacuum tube, but also by uranium The Discovery of Radio-activity
when exposed to sunlight.
It was in 1896, within a year of the It was noticed that an exhausted bulb discovery of X-rays, that Henri Becquewhich is emitting X-rays under the influ
rel, the fourth illustrious possessor of ence of electrical discharges is always
that illustrious name, was following up aglow with a peculiar greenish-yellow this idea. His method was to expose light-commonly known as a "fluores 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 PROF. R. A. MILLIKAN.
in just the way the X-rays do. Becquerel
named these rays uranium rays (rayons cent" light. It had long been known uraniques). that there are some natural substances This discovery, which has since been notably the mineral uranium and its found to be one of the most fruitful in compounds—which possess a similar the history of science, was immediately property of emitting a yellowish-green due to the accident of a few cloudy days light, not only when they are in a vacuum in Paris, during which Becquerel, since tube through which electrical discharges lie could not expose his uranium to are passing, but also when they are sunlight, set away his plate, with the exposed to the invisible radiation from
uranium on top of it, to await more the sun, that is, to the so-called actinic favorable weather. When fair weather or ultra-violet rays which are chiefly returned, and he was ready to continue responsible for the effects that sunlight his experiments, it fortunately occurred produces upon photographic plates. It to him that it might be worth while to accordingly, very naturally, occurred to develop the plate upon which the urasome scientists that the X-rays might nium had rested, and see if anything had perhaps be due to the fluorescent light happened to it. The discovery of radiowhich came from a vacuum bulb, rather activity was the result. Those who recall
the story of the discovery of photography two or three grains of the new element will remember that it was made quite as that has become the wonder of the world. accidentally and under quite similar Its radio-active property is apparently circumstances.
not different in kind from that of
uranium or thorium, but, as compared The Discovery of Radium
with these, it radiates much It was but few months after this that 1,500,000 times more actively. Madame Curie, one of the few women Having followed in this way the steps who have attained eminence in the pur whereby radium was discovered as early suit of science, and who, together with as 1898, let us turn to some of the other her husband and collaborator, deserves a results which followed close upon the large share of the credit for our present discovery of the X-rays, and about which knowledge of radium, set about investi it is necessary to understand something gating all the known elements to see if before we can intelligently discuss the any of the rest of them possessed the nature of the radiations from radium and remarkable property which Becquerel other radio-active substances. had discovered in the case of uranium.
The Nature of Cathode Rays She found that one, and but one, of the rest of the known elements—namely, We have said that X-rays are emitted thorium, which is one of the chief con from an exhausted bulb through which stituents of the well-known Welsbach an electrical discharge is passing; but mantles—was capable, as were also all the very existence of the X-rays themthorium compounds, of producing the selves is found to depend upon rays of effects that Becquerel had found to result another kind, which also are connected from uranium.
with the electrical discharge from an In connection with this investigation, exhausted tube. These are called “cathhowever, she noticed something further ode” rays because they originate in the which appeared to her very remarkable. cathode,
electrode 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
FIG. 1. plate or would discharge an electrified body in about one-fourth the time that (indicated at C in Figure 1), of a an equal weight of pure uranium would discharge-tube when the tube is put into require to effect the same result. She connection with an induction coil or inferred, therefore, that the activity of static machine. These cathode rays were pitchblende in emitting rays could not be discovered long before X-rays, and have due solely to the uranium it contained: been the subject of much investigation. on the contrary, pitchblende must contain In fact, Röntgen was experimenting some hitherto unknown element which upon them when he made the discovery had the property of emitting Becquerel of X-rays. rays more powerfully than uranium itself. Figure I will give some idea of how She therefore immediately set about the these rays manifest themselves. If A task of separating as carefully as possible and B are two diaphragms in the middle the dozen or so of substances entering of which are two horizontal slits, when into the composition of pitchblende an induction coil is connected to the such, for example, as uranium, barium, points marked + and —, and is set in lead, copper, arsenic, antimony, etc.--- operation, a spot of yellowish-green light and, after each analysis, testing the two
the glass, just as portions separated to find which part though rays of some sort originated at carried with it the activity, that is, the C and passed through the two openings ability to affect a sensitized plate or to O in much the same way as light would discharge an electrically charged body. do. There are a great many substances The search was a long and arduous one; which, if placed anywhere in the line but it ended triumphantly in the separa O P so that the cathode ravs from C can tion, from several tons of pitchblende, of strike upon them, will light up with a
characteristic glow. The
The photograph one of negative electricity upon the plate reproduced in Figure 2 shows the dis D, the rays were deflected from the line tinctive outline traced upon zinc O P into the path O P'. This, too, was sulphide screen along which the cathode exactly what would be expected if the rays graze in passing from 0 to P. rays consisted of negatively charged
The nature of the cathode rays was the particles, for these particles would then subject of much dispute between the year be repelled by the negative electricity 1880, when they first began to be studied, upon D and attracted by the positive and the year 1898. Some thought them electricity upon E. These two experito be streams of minute, negatively ments—taken in connection with the fact, charged particles shot off from the long known, that when a magnet is cathode C with enormous velocities; brought near the path of the cathode rays while others maintained that these 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
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
of the FIG. 2. EXHAUST TUBE SHOWING BEAM
smallest atom known, namely, the atom OF CATHODE RAYS.
The calculation by which this concludid not consist of particles at all, but sion is obtained is based upon a comwere waves in the ether somewhat similar parison of the amount of deflection of to light waves.
the cathode rays produced by a magnet In 1898 this dispute was ended by two and the amount produced by the electric experiments, the first performed by charges D and E. It is based upon other Perrin, a Frenchman, and the other by experiments also, which will not here be J. J. Thomson, professor of physics in
described. Suffice it to say that more Cambridge University, England. Per than a dozen well-known physicists have rin's experiment consisted in proving made the observations and the calculathat in all cases a body placed along the tions upon which this theory is based; path O P so that the cathode rays could and that, although they have worked by fall upon it became charged with nega as many as three different methods, the tive electricity, just as would be expected results are all in substantial agreement. if the cathode rays consisted of nega
Radio-active Substances Emit Cathode Rays tively charged particies. Thomson's experiments consisted in showing that if We are now in a position to understand a charge of positive electricity was placed the nature of the experiments that were upon the plate E (see Figure 1), and performed with radio-active substances-
namely, with uranium, thorium, and all emit other rays besides cathode rays, radium—in order to discover the nature which are distinguishable from the latter, of their radiations. It was at first sus first, by possessing a very much smaller pected that these rays were similar to penetrating power, and second, in the X-rays, because, like the latter, they fact that they are not ordinarily deflected possessed the power of penetrating either by a magnet or by an electrically opaque objects and of affecting a charged body. Rutherford named these photographic plate. X-rays, however, rays the Alpha rays, while he designated differ radically from cathode rays in this the cathode rays emitted by radio-active respect, that they are not deflected either substances as the Beta rays. In order to by a magnet or by electrically charged separate the Alpha from the Beta rays, it bodies, nor do they impart negative was necessary only to lay over the radiocharges to bodies upon which they fall. active substance a very thin sheet of 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
FIG, 3. DEFLECTION OF CATHODE the cathode rays.
RAYS BY MAGNET. The velocity with which these minute particles, of about one-thousandth the aluminum-for example, a sheet .005 size of the hydrogen atom, are shot off centimeter thick. This opposed almost from radio-active substances, is found to no obstruction to the passage of the Beta be even more stupendous than the rays, but it cut off entirely the Alpha ve'ocity of the similar particles in the rays.
Another mark of difference cathode rays. The latter were found to between the two kinds of ray, was that move with a speed of about 20,000 miles while the Beta rays were very much per second, which is about one-tenth of more effective than the Alpha rays in the speed at which light travels; but the penetrating opaque objects and in affectvelocity of the particles shot off from ing a photographic plate, their influence radio-active substances is estimated to be in rendering a gas electrically conductive as high as 175,000 miles per second, or was very small in comparison with that only a little lower than that of light itself. of the Alpha rays, so that if the thin
sheet of aluminum were taken away, the Other Radiations of Radio-active Substances
gas above the radio-active substance It was also discovered by Professor became a hundred times as good a conRutherford of McGill University, Can ductor as when the Alpha rays were
There is also a third kind of ray emitted as high as 20,000 miles per second, or by radio-active substances, to which has about one-third that of the Beta-ray been given the name Gamma rays. particles. Their great size in compariThese are very much more penetrative son with the Beta particles accounts for than even the Beta rays, but so far little their smaller penetrating power, as well is known about their nature. Since the as for their much greater effectiveness in energy carried by them, however, is very knocking the gas to pieces, or disintegratinsignificant as compared with that in the ing it, thus rendering it electrically Alpha and the Beta rays, we may leave conductive. them entirely out of account in most of
The Crookes Spinthariscope our computations upon the energy of the radiations from radio-active substances. We have attempted to follow thus far
the evidence upon which is based the The Nature of the Alpha Rays
conclusion that the radiations from radio It was at first conjectured that possibly active substances consist, to a large the Alpha rays might be X-rays, since,
extent at least, of projected particles of like the latter, they are not deflected by
matter shot forth with exceedingly high a magnet, and also are very effective in
velocity from the active substances, and rendering a gas electrically conductive.
that some of these particles have a mass Only last year, however, Rutherford one-thousandth that of the hydrogen contrived a very ingenious experiment by
atom, while others have a mass twice as which he succeeded in showing conclu
great as that of the hydrogen atom. But sively that the Alpha rays are deflected
no amount of reasoning of the sort thus very slightly by a magnet if the magnet
far given will be found half so convincing is sufficiently powerful. He also suc
to the ordinary mind as the sight of a ceeded in showing that they are deflected
piece of radium actually at work. in the same way by a very strong elec
Radium itself in the dark glows with a trical charge. But in both of these cases
light resembling that of a glow-worm; the direction of the deflection is opposite and when placed near certain substances, to that obtained under the same condi
as willemite or zinc sulphide, it causes tions with Beta ravs. These observations
them to light up with a glow more or made by Rutherford are of the utmost
less brilliant according to the amount of importance, and have recently been
the radium at hand. In the spring of confirmed both by Becquerel in Paris and
1903 Sir William Crookes first exhibited by a German physicist of the name of
this most beautiful and wonderful experiDes Coudres. The only possible inter
ment at the Soirée of the Royal Society pretation that can be put upon them is
in London. A small quantity of radium that the Alpha rays also consist of
is placed about one millimeter above a particles of matter shot off from radio
zinc sulphide screen, and the latter is active substances; but that, while the
then viewed through a microscope of Beta-ray particles are charged with
from ten to twenty diameters' magnificanegative electricity, the Alpha-ray parti
tion. The continuous soft glow of the cles carry charges of positive electricity.
screen, which is all that one sees with the Further, when from the amounts of
naked eye, is resolved by the microscope the deflection produced by the magnet into innumerable tiny flashes of light. It and by the electric charge the side and is as though one were viewing a swamp welocity of the Alpha-ray particles are
full of fireflies, or, better still, a sky full calculated, the results are again most
of shooting stars. It is probable that interesting, for these particles are found
these sparks or scintillations of light are •to have a mass, not one-thousandth that
caused by the impact of the Alpha-ray of the hydrogen atom, like the cathode
particles upon the screen, just as sparks rays, but approximately twice as great as
fly from an iron when it is sharply that of the hydrogen atom. That is, the
struck with a hammer. Alpha-ray particles are about 2,000 times as heavy as those of the cathode rays. The Continuous Emission of Light and Heat Their mass is about the same as that of
by Radio-active Substances the atom of helium; but despite this After learning that the radio-active great mass, their velocity is found to be substances uranium, thorium, and radium