446 Prof. Rutherford and Mr. Soddy on the 



knowing the time constants of the processes involved, it is 

 possible to predict from general principles the whole course 

 of any series of changes of the radioactivity of radium under 

 any given conditions. The main point of distinction is in the 

 rate at which the emanations in the two cases lose their 

 activity. The intensity of the radiation from the thorium 

 emanation falls to half value in one minute, while that of the 

 radium emanation falls to half value in about four days. In 

 one case, therefore, the change occurs about 6000 times faster 

 than in the other. 



On the other hand, the excited activity from radium decays 

 much faster than that produced from thorium. The former 

 is almost inappreciable a few hours after the removal of the 

 exciting cause, whereas the latter continues for several days. 

 In the case of radium the emanation and the excited activity 

 produced by it contribute the major proportion of the total 

 activity, while in the case of thorium these effects are, for 

 various reasons, not so marked. 



The experimental analysis of the processes that give rise to 

 the radioactivity of radium have so far not given any evidence 

 of the existence of a stage corresponding to that of thorium X 

 in the case of thorium. It will be recalled that the first stage 

 in the disintegration of thorium is not directly into the 

 emanation, but into an intermediate system named Thorium X. 

 which then gives rise to the emanation by further change. In 

 the case of radium it has not yet been found possible to 

 separate any system intermediate between the radium and the 

 radium emanation. From analogy to thorium one would 

 expect that a Radium X or RaX, analogous to ThX, existed ; 

 but the quantity of radium at our disposal has been too small 

 to enable us to obtain a definite answer to this question. 



After the removal of the emanation and excited activity 

 radium retains about 25 per cent, of its normal activity, which 

 is not affected by chemical processes, and constitutes a " non- 

 separable " activity, analogous to that possessed by thorium 

 and uranium. But whether this is in reality the non-separated 

 activity of " Radium X " superimposed on the true non- 

 separable activity has not yet been determined. 



In the present paper the radioactivity has been examined 

 in detail, and the results are in accordance with the view that 

 the radium is changing spontaneous!}' at a constant rate into 

 the radium emanation, whose further changes give rise to the 

 phenomenon of excited activity. In addition, the compa- 

 rative study of the two emanations has settled some points 

 left over in the discussion (Phil. Mag. 1902, iv. p. 582) of the 

 nature of the emanating-power of thorium (see § 3). 



