[MCLENNAN-EVANS] MOBILITIES OF IONS 



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III. Purification of the Helium 



The gas which was used in the experiments was purified by passing 

 it through three stout copper tubes filled with cocoanut charcoal and 

 maintained at the temperature of liquid air. From these it was passed 

 into the steel cylinder which had been highly exhausted previously, 

 and when the desired pressure was reached, a carefully constructed 

 gas-tight valve was closed. Attached to the steel chamber was a 

 fourth copper tube filled with charcoal. This was then surrounded 

 with liquid air for some hours in order to remove any oxygen or nitro- 

 gen which might have come away from the walls of the cylinder, after 

 the gas had been introduced to the latter. Finally, this latter was also 

 cut off by closing a well-fitting valve, and measurements were then 

 made on the conductivity of the gas. 



IV. Conductivity Measurements 



In making the conductivity measurements on the gas, various 

 positive and negative potentials up to 80 volts were applied to the 

 plate AB, the pressure of the gas being 81 atmospheres. In no case, 



