686 Research Staff of the Gr. E. C, London, on the 



1300° K. and the greatest potential difference in the vessel 

 as small as 10 volts ; in these circumstances there is no 

 glow discharge. No other gas is known to disappear in the 

 absence of the discharge, unless, like oxygen and chlorine, 

 it can react chemically with the tungsten to form well- 

 known compounds, or unless the temperature of the tungsten 

 is so high that there is rapid volatilization of the metal. 

 Consequently, the phenomenon differs from all those which 

 form the main object of this investigation. But since 

 hydrogen is one of the gases the disappearance of which is 

 of most importance, and since the form of discharge dis- 

 cussed here involves the presence of an incandescent cathode 

 which would cause the gas to disappear even if there were 

 no discharge, the matter requires attention. 



The experiments made on the disappearance of hydrogen 

 in the absence of a glow discharge confirmed completely 

 those of Langmuir. If the gas has access to glass cooled in 

 liquid air, some of the gas is liberated again when the cooled 

 glass is warmed to ro> m temperature (the filament being 

 now cold), and is not re-absorbed when the glass is cooled 

 again. In a few experiments all the gas that had dis- 

 appeared was liberated on warming the cooled glass ; in 

 others only a part of it reappeared. When only part 

 reappeared, a further portion could be liberated by heating 

 to about 600° K. that part of the glass which had been at 

 room temperature during the disappearance. It was always 

 impossible to recover all the hydrogen by this method ; 

 possibly the residue was adhering to portions of the glass 

 which, for mechanical reasons, could not be heated, such as 

 the connecting-tubes and the taps or cut-offs which were 

 necessary to isolate the part of the apparatus in which the 

 disappearance occurred ; but the general results of the work 

 here described indicated that this is not the only explanation. 

 Increasing the temperature of the oven from 600° K. to 

 730° K. (when the glass began to soften) did not increase the 

 quantity restored. By baking the vessel before warming- 

 up the cooled glass, then warming up the cotaled glass, and, 

 lastly^ baking again, it could be shown that some of the gas 

 liberated from the cooled glass recondensed on the glass at 

 room temperature and could be liberated thence by baking. 



If the gas that disappeared was pure hydrogen, the gas 

 restored was also pure hydrogen. This important result 

 could be established with great certainty by measurements 

 of the glow potential. It was pointed out in § 6 that the 

 glow potential of hydrogen differs so greatly from that of. 



