102 Dr. W. Makower and Dr. S. Russ on the 



The method of experimenting was as follows : — A platinum 

 plate, mounted on an iron disk to give it rigidity, was 

 exposed in an electric field to radium emanation over mercury 

 for at least three hours to allow the maximum amount of 

 active deposit to be collected on the platinum surface. After 

 removal from the emanation the iron-platinum plate was 

 allowed to stand for a time varying in different experiments 

 from half an hour to three-quarters of an hour before 

 mounting on the support A (tig. 1). This interval was used 

 to get rid of the emanation dragged out, from the vessel 

 containing the emanation, by the disk and adhering to it. 

 It was no easy matter to do this satisfactorily, but after a 

 number of trials the best plan was found to be to put the 

 plate in an evacuated tube kept at about 360° C. by an 

 electrically heated furnace. Air was occasionally admitted 

 and pumped out again to remove the emanation as it was 

 given off by the plate. In this way, the adhering emanation 

 was almost completely removed from the plate without 

 appreciable loss of radium B by volatilization. The quantity 

 of active deposit on the plate after this treatment was then 

 tested by a y ray electroscope in the usual manner by com- 

 parison with a standard quantity of radium. 



Conditions for obtaining Pure Radium C by Recoil. 

 Attention has already been drawn to the fact that it is only 

 under special conditions that the recoil of radium C from 

 radium B can be studied by itself without the interference 

 of disturbing causes. It was soon found that if the disk B 

 (fig. 1) was exposed to the active plate P in a vacuum, there 

 was in general radiated to the disk, not only radium C, as 

 was to be expected, but also a certain amount of radium B. 

 Now, since in all the experiments to be described in this 

 paper, a long interval of time was allowed to elapse between 

 withdrawing the active plate from the emanation and mounting- 

 it for an experiment, the proportion of radium A left on the 

 plate must have been very small. It was therefore some- 

 what surprising that any radium B should reach the disk, 

 and the facts seem capable of only two explanations. Either 

 the proportion of radium C particles shot off from an active 

 plate by recoil is exceedingly small compared with the total 

 number of radium B particles breaking up on the plate, or 

 the radium B is in some way detached from the plate and 

 carried away during the recoil of radium D from radium C. 

 If the first explanation is correct, then, although the ratio of 

 the number of radium A particles on the plate to that of 

 radium B is small, the actual number of radium B particles 



