﻿840 Messrs. E. W. B. Gill and F. B. Pidduck on' 



gases had been previously driven by heating to about 250 in 

 a vacuum and pumping off the gases set free. 



The mercury pump, shown in conjunction with the three- 

 way tap D, could transfer the gas from the apparatus or 



Kk. 1. 



Pump etc. 



£/lffTH 



\JJ 



the storage-chamber to the charcoal chamber. A tap A was 

 also inserted for convenience. In all cases the charcoal 

 chamber was first of all surrounded by liquid air and left for 

 some time in order that the charcoal might reabsorb any 

 gases which might have been evolved from it since the 

 previous purification. The tap D was then opened connecting 

 to the apparatus, so that any impurities present there might 

 be absorbed. Finally the helium was admitted to the cold 

 chamber a little at a time, so that the pressure was low, and 

 after being there for an interval was allowed to enter the 

 apparatus by again opening D. It was then pumped back 

 again to B and the process repeated. The effect of each 

 washing out of the apparatus with pure helium was to reduce 

 very much the amount of any impurity which had not 

 been already taken out. 



