254 Mr, Norman Shaw on Charged Surface Layers in 



steady in the presence of dry air than in moist air. If the 

 presence of water-vapour was an essential factor in the 

 phenomena of contact potential it would have been expected 

 that the continued presence of dry ionized air would lower 

 the value of the contact potential by a much larger amount, 

 and that it would at least render it uncertain. 



(4) When pure dry hydrogen was used as the gaseous 

 medium it was possible to produce changes such as those 

 shown by curves (d) and {e) in fig. 5 for lead-brass, and 

 curves (1) and (2) in fig. 6 for zinc-brass. Curve (d) in 



Fig. 6. 





15 30 4-5 60 75 90 105 120 135 150 165 180 195 2\0 Sscomds 



fig. 5 is for lead-brass taken in dry air, while curve (e) shows 

 the effect after passing pure dry hydrogen through the 

 apparatus for several days. In this case the hydrogen was 

 prepared electrolytically, and passed through an alkaline 

 pyrogallic solution and the usual drying-flasks. Curves (1) 

 and (2) in iifr. 6 represent the change from dry air to 

 hydrogen in the case of zinc-brass. It will be seen that 

 under the influence of pure dry hydrogen it was not possible 



