26 THE ROYAL SOCIETY OF CANADA 



thin layer close to the surface of the plate CD. As indicated above, 

 this condition did not strictly obtain in our experiments. It will be 

 seen, however, that the effect of this departure from the requirements 

 of the formula, would be to make the effective value of d, the distance 

 between GH and CD, to be slightly less than 1-0 cm., which in turn 

 would lead to values of ki and k2 even less than those found. This 

 result would make the difference between our values for the mobilities 

 and those calculated from Franck and Pohl's numbers even more 

 marked, and would go to emphasise still further the inapplicability 

 of the inverse pressure law for the mobilities of ions in helium, especi- 

 ally at high pressures. 



The measurements were not extended to lower pressures, because 

 by so doing the thickness of the ionisation layer would be increased 

 still further. This would render the mobility formula more inapplic- 

 able than ever. Pressures higher than 81 atmospheres were not avail- 

 able at the time the experiments were carried out. 



The Physical Laboratory, 

 University of Toronto. 

 May 15. 1920. 



