Hydrogen and Nitrogen by Electron Impacts. 791 



It appears then that the simplest explanation of the dis- 

 appearance of the hydrogen in this investigation is that it 

 is due to the dissociation of the hydrogen into atomic 

 hydrogen which condenses on the walls, especially at low 

 temperatures. The experimental value u b" for the yield of 

 atomic hydrogen per collision is probably too low for several 

 reasons. 



(a) Some atoms may collide with other atoms forming 

 ordinary molecular hydrogen which does not condense. 



(b) Some atoms may collide with hydrogen molecules 

 forming H 3 which may not condense as readily. 



(c) An atom may hit a spot on the cold surface already 

 occupied by another atom. A hydrogen molecule will be 

 formed and will leave the surface. Hence this dissociation 

 does not contribute to the pressure decrease. 



(d) If, after an appreciable clean up of hydrogen, the gas 

 is completely pumped out, and electrons are accelerated so 

 as to bombard the gauze and glass, it is found that there 

 is a considerable evolution of hydrogen, much more than 

 after a thorough outgassing. This effect no doubt goes on 

 all the time during a clean up, and tends to reduce the rate 

 of clean up, especially towards the end of a run. (This 

 evolution due to bombardment increases, for a given electron 

 current, with the accelerating potential, but not so quickly. 

 Calculation shows that the total heating effect at the gauze 

 of such a bombardment is absolutely negligible.) These 

 effects are more clearly marked at the higher pressures, and 

 no doubt influence somewhat even the initial values of the 

 clean up. 



(e) It may be that not every atom which strikes a cold 

 surface condenses. Langmuir states that the measurements 

 of the clean up in a cold bulb in which hydrogen is being 

 dissociated thermally by an incandescent filament, gives 

 under the most favourable conditions a value only 1/7 of the 

 amount of dissociation actually occurring as deduced from 

 the measurements oi heat transfer through the gas. This 

 factor is no doubt determined largely by the dimensions and 

 shape of the bulb and the conditions of the experiment, 

 and it would not be justifiable to multiply our experimental 

 determination of " b " by 7 to get the real amount dissociated. 

 Nevertheless it is significant that the highest value of vw 6,' J 

 viz. '18 (Tables III. and IV.), is roughly 1/6 of the maximum 

 possible yield, i.e., one dissociation per collision. 



The hydrogen curve in fig. 3 shows the yield for different 

 accelerating potentials. It attains a value not far from its 

 maximum at about 70-100 volts. As (he electrons under 



