596 Research Staff of the G. E. C, London, on the 



the glow potentials against the reciprocal of the pressure 

 (which is proportional to the mean free path) ; this method 

 is adopted in fig. 4, which shows Vg for various gases. It 

 is to be observed that the glow potentials of hydrogen are 

 much greater than those of nitrogen, carbon monoxide, or 

 argon, and that the order of the glow potentials is roughly 

 the inverse of the order of the molecular weights ; if 

 the glow potential were determined only by the molecular 

 weight, nitrogen would have the same glow potential as 

 carbon monoxide ; actually the glow potentials are not very 

 different. At the lower pressures all the curves seem to be- 

 come straight lines, and at the higher all appear to tend 

 towards nearly the same limit. The observations shown on one 

 curve indicate the accuracy obtainable in the best conditions; 

 the irregularities appear to be due to errors in the measure- 

 ment of the pressure rather than to errors in V</; if the 

 pressure was constant, the determination of ~Vg could be 

 repeated to a few tenths of a volt. 



Lastly, the glow potential depends on the form of the 

 electrodes and of the vessel. A full consideration of this 

 influence is reserved for later discussion ; but it may be 

 recorded here that the phenomena are changed completely 

 if there is substituted for the wire anode a cylinder sur- 

 rounding the filament, and that — contrary to what might 

 have been expected — the glow potential at the lower 

 pressures is apparently independent within wide limits of 

 the size of the vessel, whereas at high pressures it depends 

 upon that size. 



Effect of Impurities. 



7. If the gases are not pure very different results may be 

 obtained. The " pure " hydrogen was prepared by allowing 

 the vapour of water (that had been boiled to one-tenth of 

 its volume in a vacuum) to act on metallic sodium in a 

 vacuum ; the measurements were made after all possible 

 precautions had been taken to free the apparatus from 

 residual gas. "Hydrogen 99 per cent." refers to gas taken 

 from a cylinder ; it had been prepared from water-gas, and 

 contained about 1 per cent, of impurity, chiefly methane, 

 carbon monoxide, and nitrogen ; the remaining curve to the 

 pure gas in the presence of the vapour of special " vacuum " 

 wax. 



It is clear that the glow potential of hydrogen is very 

 greatly affected by the presence of small traces of other 

 gases. On the other hand, no change in the glow potentials 



