The Production of Helium from Radium.

§ 94. It had been observed already that the radioactive minerals on heating give off Helium—a gaseous element, characterised by a particular yellow line in its spectrum—and it seemed not unlikely that helium might be the ultimate decomposition product of the emanation. A research to settle this point was undertaken by Sir William Ramsay and Mr. Soddy,[110] and a preliminary experiment having confirmed the above speculation, they carried out further very careful experiments. “The maximum amount of the emanation obtained from 50 milligrams of radium bromide was conveyed by means of oxygen into a U-tube cooled in liquid air, and the latter was then extracted by the pump.” The spectrum was observed; it “was apparently a new one, probably that of the emanation itself. . . . After standing from July 17 to 21 the helium spectrum appeared, and the characteristic lines were observed.” Sir William Ramsay performed a further experiment with a similar result, in which the radium salt had been first of all heated in a vacuum for some time, proving that the helium obtained could not have been occluded in it; though the fact that the helium spectrum did not immediately appear, in itself proves this point. Sir William Ramsay’s results were confirmed by further careful experiments by Sir James Dewar and other chemists. It was suggested, therefore, that the α-particle consists of an electrically charged helium-atom, and not only is this view in agreement with the value of the mass of this particle as determined experimentally, but it has been completely demonstrated by Professor Rutherford and Mr. Royds. These chemists performed an experiment in which the emanation from about one-seventh of a gramme of radium was enclosed in a thin-walled tube, through the walls of which the α-particles could pass, but which were impervious to gases. This tube was surrounded by an outer jacket, which was evacuated. After a time the presence of helium in the space between the inner tube and the outer jacket was observed spectroscopically.[111] Now, the emanation-atom results from the radium-atom by the expulsion of one α-particle; and since this latter consists of an electrically charged helium-atom, it follows that the emanation must have an atomic weight of 226 - 4, i.e., 222. This value is in agreement with Sir William Ramsay’s determination of the density of the emanation. We may represent the degradation of the radium-atom, therefore, by the following scheme:—

Radium-atom 226		α-particle (Helium-atom) 4
	↗

	↘	Emanation (Niton-atom) 222	↗	α-particle (Helium-atom) 4

			↘	Radium-A, &c.

[110] Sir William Ramsay and Frederick Soddy: “Experiments in Radioactivity and the Production of Helium from Radium,” Proceedings of the Royal Society of London, vol. lxxii. (1903), pp. 204 et seq.

[111] E. Rutherford, F.R.S., and T. Royds, M.Sc.: “The Nature of the α-Particle from Radio-active Substances,” Philosophical Magazine [6], vol. xvii. (1909), pp. 281 et seq.