JfaiiLhc'sk'r Memoirs, Vol. liv. (1910), No. 8. 5 



(2) That produced b}' the Radium B which was on 

 the disc at the moment of removal. 



Let R' be the quantity of Radium C due to this cause 

 at any time T. 



Then, b)- analogy with the case considered by Ruther- 

 ford* — 



Hence the total quantity of Radium C present at any 

 time T after removal of the disc is R^ + R'^ due to the 

 Radium R projected from the Radium A at the bottom 

 of the tube. 



There will, however, be some Radium C due to the 

 Radium A projected from the Emanation itself, and this 

 quantity ma\' be calculated ; the change of activity with 

 time due to the Radium A and Radium C together may, 

 however, be read off from the well-known decay curve for 

 the particular time of exposure. 



This has been done for the exposures of 10 minutes 

 and 3 hours under consideration here. 



Let w = number of Radium A particles at the bottom 

 of the tube. 



Then the number obtained on the disc after a time / 

 will be proportional to n{i —c~^i'). The factor of propor- 

 tionality depends, as already stated, on the solid angle 

 subtended by the disc, and also, perhaps, on the nature of 

 the surface upon which the Emanation is condensed, for 

 it has been shownf that only about one-eleventh part of 



• " Radio-activity," p. 331. 



t loc. cit. 



