﻿298 Drs. K. Fajans and TV\ Makower on the 



present is greater than after 32*8 minutes in a known ratio 

 and is, for the ease of six minutes considered, about double. 

 The quantity of RaC formed will, on the other hand, be less 

 than after 32*8 minutes, but also in a ratio calculable from 

 equation (1). Let B* and C* be the ionization due to the 

 RaB and RaC present after a time t reckoned from the time 

 at which the recoil was concluded. Then, since B^ = «B and 

 C, = /30, we have that 



a B + /3C + £C = 6, (4) 



where b is a quantity to be determined from the curve and 

 the term £C is introduced for the same reason as the term 

 eO in equation (3). From equations (3) and (4) B and C 

 can be found. ISlow to get the absorption curve for the rays 

 from RaB it is necessary to determine the ratio of B for each 

 thickness of aluminium to the value of B obtained without 

 any screen. Let the latter quantity be represented by B 

 and let C be the ionization produced by RaC in equilibrium 

 with RaB under the same conditions. Then the required 

 ratio B/B is given by the equation 



_B = B Co C_ 



B C ' B C " 



(5) 



Now all the quantities on the right-hand side of the equation 

 c;in be obtained from the curves given in figure 3 except the 

 ratio 0/C , which has to be determined by a separate experi- 

 ment on the absorption of the radiation from RaC by 

 aluminium. 



To determine the ratio of C/C for different thicknesses, 

 a nickel plate was rendered active with pure RaC by the 

 method of von Lerch and a thin aluminium plate then rubbed 

 on the nickel surface so as to detach some of the activity. 

 This was done in order to have an aluminium plate of the same 

 thickness as the absorbing plate behind the RaC, and thus to 

 eliminate errors due to secondary radiation *. The aluminium 

 plate was then placed in the same position with regard to 

 the electroscope as the plates previously used in the experi- 

 ments with RaB. Foils of aluminium of different thicknesses 

 were then interposed between the active plate and the 

 electroscope and the ionization produced measured in the 

 usual manner. A plate of aluminium of the same thickness 

 as the absorbing foil was in every case placed behind the 

 active plate. After making due allowance for the decay of 

 the RaC between successive measurements, a curve could be 



* Kovafik, Phil. Mag. Nov. 1910. 



