Recoil of Radium C from Radium B. 107 



radium emanation for some hours, when it was removed and 

 transferred to an evacuated vessel placed in a furnace at 

 360° C. for nearly half-an-hour to remove emanation. The 

 plate was then tested by its 7 radiation and found to be 

 equivalent in activity to *0b'6 milligram of radium bromide. 

 A disk was then suspended above the plate in vacuo, and 

 after an exposure of 20 minutes, removed and tested by a 

 sensitive «-ray electroscope. The decay of the activity on 

 the disk is shown in fig. 4. It will be seen that the activity 



Fig. 4. 



k 25 



I 

 1, 



10 



30 40 c5i? 



M/NUT£S 



CO 



70 



80 



at first began to decay with the period of Ra< ', viz. 19 minutes, 

 which gradually became longer as time proceeded, indicating 

 that even in these circumstances there was present on the 

 disk a small quantity of radium B. 



The Proportion of Radium ( ' projected from a Plate 

 by Recoil. 



In a previous paper * it was shown that when a disk was 

 exposed above some emanation condensed by liquid air at 

 the bottom of a tube, the amount of radium A and radium B 

 projected on to the disk was about one-eleventh of the 

 quantity which would have reached it if every a particle had 

 been effective in causing a recoil as it left a disintegrating 

 atom. Having regard to the easy absorption of this radiant 

 active matter, this was perhaps as large a fraction as was to 

 be expected, for any slight film of condensed water-vapour 

 or other gas over the surface of the condensed emanation 

 would tend to prevent the escape cf the active matter from 

 the surface. A similar result has since been obtained for 



* Loc. cit. 





