Production of Helium by Radium. 591 



When the experiments on the .determination of the pro- 

 duction of helium had been completed a further measurement 

 of the amount of radium present in the salt used was made 

 as follows :— The salt was dissolved in distilled water con- 

 taining a small quantity of hydrochloric acid and the solution 

 was diluted to a definite volume (50 c.c). A small, definite 

 fraction (1*22 per cent.) of this solution was removed, 

 evaporated to dryness in a small glass tube, and hermetically 

 sealed by fusing the glass. After a month's time the small 

 fraction w r as directly compared by means of the <y radiation 

 with the 3*69 milligram radium bromide standard of this 

 Laboratory, and found to be equivalent to 4*02 milligrams 

 RaBr 2 . The results of this comparison gave the amount ot 

 radium present in the salt used as 193 milligrams. 



Collection and Measurement of the Helium. 



The helium produced by the radium salt was collected, 

 purified, and measured in the apparatus shown in tig. 2. 



Fiff. 2. 



This consisted of a Giessler pump, P, which was used for the 

 evacuation of the other portions of the apparatus. The gases 

 removed by this pump could be collected when desired at D. 

 The vessel M contained phosphorus pentoxide for the removal 

 of water vapour. The vessel A with the attached capillary 

 tubes E and e was essentially a MacLeod gauge in its design 

 and operation, but in these experiments was used for the 



