638 Miss J. M. W. Slater on the 



of the disk to the discharge (diagram 2, Curve I. second 

 branch) to that given by a surface exposed for a short time 

 to the thorium emanation* will be at once observed; and the 

 explanation of the rise is undoubtedly the same in the two 

 cases. Two stages in the decomposition of the active deposit 

 are present simultaneously ; they have different rates of 

 decay, and are not at first in the equilibrium proportions 

 which are reached after a few hours. 



Prof. Rutherford has shown f from the curve for short 

 exposures that in this case there are two stages present, the 

 first of which changes into the second without giving off 

 rays, while the second in decomposing gives rise to the radia- 

 tion which is responsible for the excited activity. Each 

 change may be expressed by an equation of the form 



i,=v- w , 



where I , I*, are proportional to the number of particles of a 

 given kind changing per second at the beginning and end of 

 the time t, e is the base of the natural logarithms, and X is 

 a constant. The values of A, for the two substances are 75 

 and '063 (if the time is measured in hours), giving decay to 

 half value in 55 minutes and 11 hours respectively. The 

 same constants agree very well with the curves obtained in 

 the present work, using the general equation for the activity 

 (I) at any time t, 





^ = ,-v+^V(,-v-- e -v), 



so that the same two substances are evidently present. 



One alteration must, however, be made in Rutherford's 

 theory. He assumes that the first change is the rapid one, 

 so that the regular exponential curve is obtained when the 

 first stage has almost entirely disappeared. But the separa- 

 tion of activity obtained in the discharge-tube always occurred 

 after the stage of regular decay had set in ; in one case the 

 active disk was exposed to the discharge 30 hours after its 

 removal from the thorium, and a separation of activities was 

 still obtained. This shows that the two stages are always 

 present simultaneously, and it is evident that the first change 

 (occurring without emission of rays) is the sloiv one, and the 

 second the rapid one. This is confirmed by the possibility 

 of obtaining the rapidly-decaying activity almost pure on the 

 box ; in some cases this transferred activity fell to half value 

 in about an hour, and after two or three hours the box had 

 become almost inactive. This assumption agrees just as well 

 as the other with Rutherford's data, for it only necessitates 



* "Rutherford, ' Radioactivity/ p. 260. t Loc. cit. p. 269. 



