Disappearance of Gas in the Electric Discharge. 691 



apparent diminution while the discharge lasts. In 48 hours 

 enough gas to fill the lamp to a pressure of 1 cm. has been 

 evolved without any signs of fatigue, so long as the gas 

 formed is continually pumped away. Meanwhile the filament 

 wastes rapidly, and the experiment is brought to a close by 

 its breakage. Other experiments conducted in this laboratory 

 have shown that it is probable that the gas is evolved from 

 the walls in the first instance as water, which is reduced to 

 hydrogen by the tungsten filament. If the supply of 

 mercury vapour is not unlimited, the action seems to come 

 to an end after many hours ; the mercury has disappeared. 

 It is not known what becomes of the mercury, and indeed it 

 is not absolutely certain that it disappears. The process is 

 interesting as showing definitely another action which must 

 be taken into account in considering the disappearance of 

 gases under the discharge. The discharge here evolves gas 

 from the walls. It has been already noted that this action 

 occurs aho in argon, and probably in hydrogen ; it may also 

 occur in nitrogen and CO. And it should he noted that the 

 gas thus liberated cannot be liberated by mere heating of the 

 walls to their softening point; oas can be attached to the 

 walls in some such way that ir, can be liberated by the 

 discharge, but not by heating. Of course, the attachment 

 may consist of chemical combination; it is possible that 

 glass contains hydrogen chemically combined, probably as 

 water. But it should be observed that the hydrogen liberated,, 

 if piled up on the glass, w r ould form a layer at least 25 

 molecules thick ; some of it must therefore have come from 

 a layer at least 25 molecules deep. Since the potential 

 driving the discharge in these experiments was often as low 

 as 50 volts, it is hardly to be expected that the electrons or 

 ions could penetrate so far into the glass simply in virtue of 

 the energy which they receive from the discharge. It seems 

 easier to believe that a layer on the surface, subject to the- 

 action of these particles, is constantly renewed by diffusion 

 from within. 



18. Conclusions. — The following conclusions are based on 

 these experiments, though some of the evidence for them is 

 derived from the next section : — 



(1) All gases can be made to adhere to glass by the dis- 

 charge in such a way that part, at least, can be restored by 

 heating" the jdass. 



(2) The amount of gas that can be made so to adhere 

 depends on the nature of the gas and on the state of the 

 glass. 



