Recoil of Radium C from Radium B. 



105 



air, any radium B or radium C found on the disk after 

 exposure could only have reached it by secondary mechanical 

 projection during the more energetic recoil of radium D 

 from radium C. Moreover, the number of particles of 

 radium B and radium C expelled from the plate should have 

 been in the ratio of the number of particles of radium B to 

 that of radium C on the plate at the time of an experiment. 



The point was tested in the following way : — A disk was 

 suspended 7 millimetres above a strongly active plate with 

 no radium A left on it and exposed to the radiation from it 

 for 10 minutes, the pressure of the air between the plate and 

 disk being maintained at 3*3 mm. of mercury. After 

 exposure, the disk was removed and the time period of the 

 activity received by it tested. As may be seen from fig. 3, 



Fig. 3. 



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the activity was found to fall to half value in about 40 minutes, 

 and this may easily be shown to mean that the numbers of 

 radium B and radium C particles on the disk, when tested, 

 were nearly equal. This was to be expected since, under 

 the experimental conditions, the amounts of radium B and 

 radium C present on the plate must also have been almost 

 equal. It will be noticed that experiments were made with 

 the plate charged with positive and the disk with negative 

 electricity and vice versa, and also with the plate and disk at 

 the same potential. This was done because it was thought 

 that possibly the radium C produced from radium B by 

 recoil might become charged after being stopped by the air. 





