﻿Growth of Radium C from Radium B. 297 



started from different initial values, the curves with "73 mm. 

 and greater thicknesses all rose from 10 per cent, of the 

 maximum obtained after 32 minutes. It w;is therefore 

 evident that at the end of the recoil there was present on 

 the plate a substance emitting j3 rays whose coefficient of 

 absorption was the same as that of the hard rajs from RaC. 

 Hence, either RaB gives hard rays whose coefficient is the 

 same as that of the hard rays from RaC, or else the initial 

 activity is due to RaC mechanically carried over with the 

 RaB during recoil *. The latter explanation seemed, how- 

 ever, unlikely, as the curve (fig. 3) taken with no screen, 

 so that the ionization was mostly due to a rays, starts more 

 nearly from zero than any of the /3-ray curves ; moreover, 

 similar results were obtained in experiments made with 

 widely different quantities and concentrations of RaA. The 

 possibility of explaining the observed initial activity by 

 contamination with RaC mechanically carried over during 

 the recoil is, however, rendered still more unlikely by 

 experiments made with 7 rays f. 



The curves given in figure 3 can be used to find the ratio 

 of the ionization produced by the {3 rays of RaB and RaC, 

 in equilibrium, when measured through different thicknesses 

 of aluminium. From the data so obtained the absorption of 

 the /3 rays of RaB by aluminium can be deduced if the 

 absorption curve for the ft rays from RaC x is known. The 

 calculation is carried out as follows. 



Let the total ionization in the testing-vessel after 

 32*8 minutes be taken as a and the ionization after any time, 

 say 2, 4, or 6 minutes, be represented by b. Now the ioniza- 

 tion at any time is made up partly of the radiation from 

 RaB and partly by that from RaC. Consider, first, the state 

 of affairs after 32*8 minutes and let B be the ionization due 

 to the RaC formed from it at the time considered. Then we 

 have the equation 



B + + 6C=a (3) 



where e is a small fraction and eC represents the quantity of 

 RaC clue to the RaC produced during the time of recoil, 

 which was never infinitely short and varied in different 

 experiments from 5 seconds to 25 seconds. The quantity 

 could easily be calculated for any given experiment. 



Consider now the state of affairs at some fixed time after 

 the end of the recoil, say 6 minutes. The quantity of RaB 



* See Makower and Russ, Phil. Mag:. January 1010. 

 t Moseley and Makower, infra, p. 302. 



Plat. Mag. S. 6. Vol. 23. No. 134. Feb, 1912. X 



