104 Dr. W. Makower and Dr. S. Buss on the 



radium A on the plate must have fallen to a hundred 

 millionth of its initial value by the time this exposure was 

 begun. A third series of observations was therefore made 

 with another disk beginning 184 minutes after the removal 





Fig. 2. 







IS 













~-*-^L 



T= 33 mats.- 





^^^^o^~ 



J^ 



r * Zemins 



''$ 







T^xotnins. 



IS 









/O ZO 30 UO 



/1/A/UTES, 



of the plate from the emanation. By this time the quantity 

 of active deposit on the plate had become so small, that on 

 testing the disk its activity was found to fall to half value in 

 20 minutes, indicating that the matter radiated to the disk 

 now consisted of practically pure radium C (see curve III.). 

 The experiments just described indicate that even when 

 there was practically no radium A on the plate, a certain 

 amount of radium B reached the disk suspended above it 

 in vacuo ; and, as we have seen, this phenomenon can be 

 explained if we suppose that radium B is mechanically 

 removed from the plate during the recoil of radium D from 

 radium C. Since, in addition to the interest of the pheno- 

 menon itself, this action is liable to produce disturbances 

 when studying the recoil of radium C from radium B, 

 the matter was investigated in greater detail. Now, since 

 the recoil of radium D from radium C takes place as the 

 result of the emission of an a particle of high velocity, 

 whereas in the transformation of radium B into radium 

 no such particles have been detected, it is evident that 

 the energy with which the radium D residue leaves the 

 disintegrating atom must far surpass that of radium C when 

 expelled from radium B. If, therefore, using the same 

 apparatus as previously, the pressure of the air between the 

 radiating plate and the disk were increased to such a value 

 that all the radium C leaving the plate by recoil from radium 

 B was prevented from reaching the disk by the intervening 



