﻿294 Drs. K. Fajans and W. Makower on the 



collecting-plate was exposed as the negative electrode in an 

 electric field to the recoil from the first plate for a few 

 seconds so as to receive a deposit of RaB by recoil ; the 

 collecting-plate was then quickly transferred to a /3-ray 

 electroscope of the usual type and its activity measured at 

 successive times through a thick aluminium screen, the 

 thickness of which was varied in different experiments. The 

 case of a screen 0"93 mm. thick is taken as typical and 

 shown in figure 1, curve C, in which the time is reckoned 

 from the end of the recoil. 



It will be noticed that the experimental curve does not 

 coincide with either of the calculated curves ; it lies above 

 both curves and does not start from zero but from 10 per 

 cent, of the maximum value. The cause of this discrepancy 

 must in part be due to the fact that the collecting of the 

 RaB by recoil did not occur instantaneously, but occupied 

 25 seconds. In this time a small quantity of RaC is formed 

 and due allowance must be made in the curve for the RaC 

 present at the beginning of the measurements ; but the 

 corrections to be introduced from this cause are too small to 

 account for the observed differences. Thus a simple calcu- 

 lation shows that after an exposure of 25 seconds the quantity 

 of RaC on the plate at time is only 1*7 per cent, of the 

 maximum after 32'8 minutes. As shown by H. W. Schmidt *, 

 and as confirmed below, the remainder of the initial radiation 

 is to be ascribed to RaB itself, which is thus seen to emit 

 rays capable of penetrating 0*93 mm. of aluminium. Hence 

 to obtain a curve showing the growth of activity of RaO 

 from RaB it is necessary to subtract at each time, from the 

 experimental values of the ionization, both the ionization due 

 to the RaC originally present, which falls off with a period 

 of 19*5 minutes, and the ionization due to RaB, which falls 

 off with a period of 26*7 minutes. If this is done for the 

 curve C and if the maximum value after 32*8 minutes is 

 again taken as 100, a curve is obtained which coincides 

 closely with the theoretical curve A, which was calculated 

 on the assumption that RaC\ was responsible for the major 

 part of the radiation emitted by RaC. The points marked 

 with crosses have been calculated in this way. It is therefore 

 clear that RaC 2 does not contribute materially to the hard 

 /3 radiation from RaC. 



In order to extend the investigation to the case of the soft 

 (3 rays emitted by RaC a similar experiment to that described 

 above was made with an absorbing layer of aluminium 



* H. W. Schmidt, Annalen der Physik. xxi. (19C6). 



