176 PHYSICS OF THE ELECTRON 



is 3, 21, and 28 minutes respectively. Radium A gives only a rays, B 

 gives out no rays at all, while C gives out a, /?, and j rays. These 

 results have been deduced by the comparison of the change of activity 

 with time, with the mathematical theory of successive changes. The 

 variation of the activity with time depends upon whether the activity 

 is measured by the a, /?, or 7- rays. The complicated curves are very 

 completely explained on the hypothesis of three successive changes 

 of 'the character already mentioned. 



The activity of a vessel in which the radium emanation has been 

 stored for some time rapidly falls to a very small fraction after the 

 emanation is withdrawn. There, however, always remains a slight 

 residual activity. The writer has recently examined the activity in 

 detail. The residual activity at first mainly consists of /? rays, and the 

 activity measured by them does not change appreciably during the 

 period of one year. The a ray activity is at first small, but increases 

 uniformly with the time for the first few months that the activity has 

 been examined. These results receive an explanation on the hypothe- 

 sis that radium C changes into a product D which emits only /? rays. 

 D changes into a product E, which emits only a rays. This view has 

 been confirmed by separating the a ray product by dipping a bismuth 

 plate into the solution containing radium D and E. The probable 

 period of these changes can be deduced from observations of the mag- 

 nitude of the a and {3 ray activity at any time. It has been deduced 

 that radium D is probably half transformed in forty years, and 

 radium E is half transformed in about one year. The evidence at 

 present obtained points to the conclusion that radium E is the active 

 constituent present in Marckwald's radio-tellurium, and probably 

 also in the polonium of Mme. Curie. 



The changes in the active deposit of thorium have been analyzed 

 by the writer, and the corresponding changes in actinium by Miss 

 Brooks. 



The occurrence of a " rayless change " in the active deposits from the 

 emanation of radium, thorium, and actinium is of great interest and 

 importance. As these products do not emit either a or /? or f rays, 

 their presence can only be detected by their effect on the amount of 

 the succeeding products. The action of the rayless change is most 

 clearly brought out in the examination of the variation of activity 

 with time of a body exposed for a very short interval in the presence 

 of the emanations of thorium and actinium. Let us consider, for 

 simplicity, the variation of activity with time for thorium. The activ- 

 ity (measured by the a rays) observed at first is very small, but gradu- 

 ally increases with the time, passes through a maximum, and finally 

 decays according to an exponential law with the time falling to half 

 value in 11 hours. The shape of this curve can be completely ex- 

 plained on the assumption of the two successive changes, the second 



