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 diffused through air like a gas of heavy molecular weight. At a later date Rutherford and Soddy showed that the radium and thorium emanations behaved like chemically inert gases, since they were unaffected by the most drastic physical and chemical treatment.

On the other hand, P. Curie, who, in conjunction with Debierne, had made a series of researches on the radium emanation, expressed dissent from this view. P. Curie did not consider that there was sufficient evidence that the emanation was material in nature, and pointed out that no spectroscopic evidence of its presence had yet been obtained, and also that the emanation disappeared when contained in a sealed vessel. It was pointed out by the writer that the failure to detect spectroscopic lines was probably a consequence of the minute quantity of the emanation present, under ordinary conditions, although the electrical and phosphorescent actions produced by this small quantity are very marked. This contention is borne out by later work. P. Curie at first took the view that the emanation was not material, but consisted of centres of condensation of energy attached to the gas molecules and moving with them.

M. and Mme Curie have throughout taken a very general view of the phenomena of radio-activity, and have not put forward any definite theory. In Jan. 1902, they gave an account of the general working theory which had guided them in their researches. Radio-activity is an atomic property, and the recognition of this fact had created their methods of research. Each atom acts as a constant source of emission of energy. This energy may either be derived from the potential energy of the atom itself, or each atom may act as a mechanism which instantly regains the energy which is lost. They suggested that this energy may be borrowed from the surrounding air in some way not accounted for by the principle of Carnot.

In the course of a detailed study of the radio-activity of thorium, Rutherford and Soddy found that it was necessary to suppose