« AnteriorContinuar »
THE GROWTH OF RADIUM
and the process proceeds through a long succession of more or less unstable intermediate elements, until the final stable product is reached. In this process energy is evolved of the order of a million times greater than the energy ever liberated in ordinary chemical changes, in which the groups of atoms, or the molecules, change, but not the constituent atoms themselves. The energy evolved by an ounce of radium, in the course of its life, equals that evolved from the burning of ten tons of coal. The period of average life in this case is about 2500 years, which means that both part of any quantity of radium changes per annum.
The rate at which the various radioactive products change varies very widely. It may be slow or rapid, a matter of seconds or even billionths of a second on the one hand, or of years or centuries or æons on the other. It was reasonable to interpret what Mme. Curie had done for pitchblende in exactly the same way as had been done for thorium, merely extending the time scale. The radium, polonium, actinium and the other new intensely active radio-elements she discovered in such infinitesimal amount in pitchblende were in all probability the products of the change of the parent element uranium. The view carries with it the corollary that, if you separated uranium from radium and everything else completely and left it to itself, in the course of years or centuries a new crop of radium would be gradually formed. The case of radium is specially interesting as it has been established that it is an ordinary element resembling barium, with definite spectrum, atomic weight, chemical properties and position in the Periodic Table. It was one of very many startling predictions of a similar character made as soon as the new point of view was attained. But it has been the last to receive confirmation and the difficulties have been great. Were radium the first direct product, the growth of radium in uranium, initially purified completely from it, could be observed in the course of an hour, so excessively delicate are the radioactive tests for this new element. Experiments were started in 1903 in London, continued on a very much larger and more thorough scale in Glasgow, with the aid of Mr T. D. Mackenzie. Yet in 1914 the expected confirmation was still not clearly forthcoming. Long before that time it was known that radium was not the direct product of uranium, and that another new radio-element, ionium, intervened in the series. The uranium changes into radium, via ionium, and this ionium is an exceedingly slowly changing element in comparison even with radium, not more than about 100,000th part changing every year. This retards enormously the initial rate of growth of radium and makes it proceed at first not linearly with the lapse of time, but according to the square of the lapse of time. That is, the growth after ten years would be 100 times, and after 100 years 10,000 times, that in the initial year from purification. The oft-tested preparations of uranium were transplanted to Aberdeen in safety, and tests since carried out, in conjunction with Miss Ada Hitchins, last year satisfactorily established a growth of radium beyond all doubt in the largest preparation, and showed that the rate was proceeding as nearly as can yet be seen according to the square of the time. The growth of radium was not large. In three years it amounted to 150,000,000,000,000 th of the quantity of uranium experimented upon, and in six years to just four times this quantity. The experiments gave, moreover, indirectly a maximum estimate of the rate of change of ionium as at most 100,000th part per year. This estimate has now been confirmed and made more definite by some very fine direct work on ionium
THE a-, B- AND y-RAYS
97 itself at the Radium Institute of Vienna a few months ago, which gives the rate of change as 16,007th part per year. This is more than fifty times slower than the rate of change of radium itself, which has long been established to be about 2500th part per year. On the other hand the original uranium is estimated with fair probability to be changing 50,000 times more slowly than ionium, or not much more than 10,000,000,000 th part changing per annum.
In the course of 1,000,000,000 yearsma period beyond what even the geologists claim as the total age of the earthhardly more than 10 per cent. of a given quantity of uranium would change-through ionium, radium and so on-into other elements. Yet, as has been mentioned, so delicate are our methods, that had radium been the first direct product of the change, an hour's observation on a kilogram of purified uranium would have sufficed to have established the growth beyond all doubt. As it is, the problem took thirteen years. Uranium and thorium are the only two primary radio-elements in the process of change. All the other radio-elements known, and they number thirty-three, are produced from one or other of them in the course of their long sequence of changes.
But what of the rays themselves, the expulsion of which first drew attention to the phenomenon, and which have furnished the necessary experimental means for the study of the whole problem? Like the X-rays, they do not recognise the optical properties, transparency and opacity, nor, to a great extent, the chemical nature of the matter in their path. They plough through everything, affected primarily only by the density of the absorbing medium, or by the actual mass of the material in their way. Physicists recognise three distinct types of rays—the as, the B- and the y-rays, the first stopped completely by a sheet of notepaper, but by
far the most energetic and important of all, the second capable of penetrating perhaps fth of an inch of glass or aluminium without being totally stopped, and the third reduced to half their original intensity by about 1 inch of lead, though not absolutely completely stopped even by 20 inches. The y-rays are far the most penetrating rays known and are really X-rays, but far more penetrating than any that can be artificially produced. They are light waves of wave-length thousands of times shorter than those of visible light, and are probably a secondary phenomenon accompanying the expulsion of the B-rays. The B-rays, or B-particles, are electrons—the atoms of negative electricity divorced from matter, recognised as such by Sir J. J. Thomson in 1897, but previously well-known in the phenomena of the Crookes' tube. They travel at a speed varying from a third up to nearly the velocity of light itself, which is very much greater than any that can be produced artificially. The a-rays, or a-particles, are atoms of matter, carrying two atomic charges of positive electricity-just twice the charge of positive electricity that the B-particles carry of negative electricity-and travelling with a velocity varying from oth to ith that of light, about a hundred times faster than matter had ever been known to travel previously. Their mass is several thousand times as great as that of the B-particle, and in spite of their feeble penetrative power, and, at first sight, less showy qualities, over 90 per cent. of the energy evolved in the change of an atom is emitted in the form of these a-particles. Much of Rutherford's finest work has been in connection with these a-particles.
The early measurements of the mass of the atom constituting the a-particle left a choice as to its nature, whether it was an atom of helium or of hydrogen, but strong indirect evidence of a very
HELIUM PRODUCED BY RADIUM
remarkable character favoured helium. Thus helium, though it forms no compounds, is found in minerals containing uranium and thorium, only in the minerals containing uranium and thorium, and always in them. Might not this helium be the a-particles fired off from the uranium and thorium in the mineral, and, unable to escape from the glassy minerals, accumulating in the material over long periods of geological time, until its presence was obvious and striking even to the relatively rough tests of chemistry and the spectroscope ? Naturally, if one could only get enough radium the point might be tested directly, for the spectroscopic test for helium is very sensitive, a bubble of the gas, roboth of a cubic millimetre in volume, that is, Ioooth part of a large pin's head, being sufficient to give the characteristic spectrum. This was in 1903, at the time when pure radium compounds were being put on the market for the first time by the enterprise of the German technical chemist, Dr Giesel. The first thing done with it in the late Sir William Ramsay's laboratory in London was to see whether helium was being generated by it continuously, as should be the case if the a-particles were really positively charged atoms of helium. A few milligrams of radium only was available, but it proved sufficient, and the growth of helium from radium was established by the spectroscope by the aid of the beautiful methods of manipulation of gases, devised by Sir William in the course of his investigations on the rare gases of the atmosphere. Later, the writer established the continuous production of helium from uranium and thorium, though here, from a ton of either element in a year, the quantity of helium produced is only odoth of a milligram by weight—a quantity unweighable on the most sensitive chemical balance-or ii cubic millimetres by volume. Helium has also been detected as a