Consider for a moment the explanation of the changes in radium. A minute fraction of the radium atoms is supposed each second to become unstable, breaking up with explosive violence. A fragment of the atom—and α-particle—is ejected at a high speed, and the residue of the atom, which has a lighter weight than before, becomes an atom of a new substance, the radium emanation. The atoms of this substance are far more unstable than those of radium and explode again with the expulsion of an α-particle. As a result the atom of radium A makes its appearance and the process of disintegration thus started continues through a long series of stages.
I can only refer in passing here to the large amount of work done by various experimenters in analysing the long series of transformations of radium and thorium and actinium; the linking up of radium with uranium and the discovery by Boltwood of the long looked-for and elusive parent of radium, viz. ionium. This phase of the subject is of unusual interest and importance but has only an indirect bearing on the subject of my lecture. It has been shown that the great majority of the transition elements produced by the transformation of uranium and thorium break up with the expulsion of α-particles. A few, however, throw off only β-particles, while some are "rayless", i.e. undergo transformation without the expulsion of high-speed α– and β-particles. It is necessary to suppose that in these latter cases the atoms break up with the expulsion of α-particles at a speed too low to be detected, or, as is more probable, undergo a process of atomic rearrangement without the expulsion of material particles of atomic dimensions.
Another striking property of radium was soon seen to be connected with the expulsion of α-particles. In 1903 P. Curie and Laborde showed that radium was a self-heating substance and was always above the temperature of the surrounding air. It seemed probable from the beginning that the effect must be the result of the heating effect due to the impact of the α-particles on the radium. Consider for a moment a pellet of radium enclosed in a tube. The α-particles are shot out in great numbers equally from all parts of the radium and in consequence of their slight penetrating power are all stopped in the radium itself or by the walls of the tube. The energy of motion of the α-particles is converted into heat. On this view the radium is subject to a fierce and unceasing bombardment by its own particles and is heated by its own radiation. This was confirmed by the work of Rutherford and Barnes in 1903, who showed that three quarters of the heating effect of radium was not directly due to the radium but to its product, the emanation, and that each of the different substances produced in radium gave out heat in proportion to the energy of the α-particles expelled from it. These experiments brought clearly to light the enormous energy, compared with the weight of matter involved, which was emitted during the transformation of the emanation. It can readily be calculated that one kilogram of the radium-emanation and its products would initially emit energy at the rate of 14,000 horse-power, and during its life would give off energy corresponding to about 80,000 horse-power for one day. It was thus clear that the heating effect of radium was mainly a secondary phenomenon resulting from the bombardment by its own α-particles. It was evident also that all the radioactive substances must emit heat in proportion to the number and energy of the α-particles expelled per second.
We must now consider another discovery of the first importance. In discussing the consequences of the disintegration theory, Rutherford and Soddy drew attention to the fact that any stable substances produced during the transformation of the radio-elements should be present in quantity in the radioactive minerals, where the processes of transformation have been taking place for ages. This suggestion was first put forward in 1902.[3] "In the light of these results and the view that has already been put forward of the nature of radioactivity, the speculation naturally arises whether the presence of helium in minerals and its invariable association with uranium and thorium, may not be connected with their radioactivity, and again[4]." "It is therefore to be expected that if any of the unknown ultimate products of the changes of a radioactive element are gaseous, they would be found occluded, possibly in considerable quantities, in the natural minerals containing that element. This lends support to the suggestion already put forwards, that possibly helium is an ultimate product of the disintegration of one of the radioactive elements, since it is only found in radioactive minerals."
It was at the same time recognized that it was quite possible that the α-particle itself might prove to be a helium atom. As only weak preparations were then available, it did not seem feasible at that time to test whether helium was produced from radium. About a year later, thanks to Dr. Giesel of Braunschweig, preparations of pure radium bromide were made available to experimenters. Using 30 milligrams of Giesel's preparation, Sir William Ramsay and Soddy in 1903 were able to show conclusively that helium was present in radium some months old and that the emanation produced helium. This discovery was of the greatest interest and importance, for it brought to light that in addition to a series of transition elements, radium also gave rise in its transformation to a stable form of matter.
A fundamental question immediately arose as to the position of helium in the scheme of transformations of radium. Was the helium the end or final product of transformation of radium or did it arise at some other stage or stages? In a letter to Nature[5] I pointed out that probably helium was derived from the α-particles fired out by the α-ray products of radium and made an approximate estimate of the rate of production of helium by radium. It was calculated that the amount of helium produced per gram of radium should lie between 20 and 200 cubic millimetres per year and probably nearer the latter estimate. The data available for calculation at that time were imperfect, but it is of interest to note that the rate of production of helium recently found by Sir James Dewar, in 1908, viz. 134 cubic millimetres per year, is not far from the value calculated as most probable at that time.
These estimates of the rate of production of helium were later modified as new and more accurate data became available. In 1905, I measured the charge carried by the α-particles from a thin film of radium. Assuming that each α-particle carried the ionic charge measured by J.J. Thomson, I showed that 6.2 x 1010 α-particles were expelled per second per gram of radium itself and four times this number when radium was in equilibrium with its three α-ray products. The rate of production of helium calculated on these data was 240 cubic millimetres per gram per year.
In the meantime, by the admirable researches of Bragg and Kleeman in 1904, our knowledge of the character of the absorption of the α-particles by matter had been much extended. It had long been known that the absorption of α-particles by matter was different in many respects from that of the β-rays. Bragg showed that these differences arose from the fact that the α-particle, on account of its great energy of motion, was not deflected from its path like the β-particle, but travelled in nearly a straight line, ionizing the molecules in its path. From a thin film of matter of one kind, the α-particles were all projected at the same speed and lost their power of producing ionization suddenly, after traversing a certain definite distance of air. The velocity of the α-particles in this view were reduced by their passage through matter by equal amounts. These conclusions of Bragg were confirmed by experiments I made by the photographic method. As a source of rays, a thin film of radium C, deposited from the radium-emanation on a thin wire, was used. By examining the deflection of the rays in a magnetic field, it was found that the rays were homogeneous and were expelled from the surface of the wire at an identical speed. By passing the rays through a screen of mica or aluminium, it was found that the velocity of all the α-particles were reduced by the same amount and the issuing beam was still homogeneous.
A remarkable result was noted. All α-particles apparently lost their characteristic properties of ionization, phosphorescence and photographic action, at exactly the same point while they were still moving at a speed of about 9,000 kilometres per second. At this critical speed, the α-particle suddenly vanishes from our ken and can no longer be followed by the methods of observation at our command.
The use of a homogeneous source of α-rays like radium C at once suggested itself as affording a basis for a more accurate determination of the value of e/m for the α-particle and for seeing whether the value was consistent with the view that the α-particle was a charged atom of helium. In the course of a long series of experiments, I proved that the α-particles, whether expelled from radium, thorium or actinium, were identical in mass and must consist of the same kind of matter.