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



gas would raise the glow potential above that which would be 

 obtained if the gas were not added. But it is important to 

 notice that an impurity consisting- of a gas or vapour, which 

 is characterised by a low glow potential throughout the 

 curve, may reduce the glow potential of a gas with which it 

 is mixed far below that which the impurity would have in 

 the absence of the gas ; the glow potential of a mixture may 

 be lower than that of either of the constituents. Thus, if all 

 the hydrogen were removed from the impure hydrogen from 

 the cylinder, the residual gas, which would have a pressure 

 only 1 per cent, of that of the original hydrogen, would 

 have a glow potential a great deal higher than that of the 

 mixture. Again, if mercury vapour in contact with frozen 

 mercury (234° K.), and not with mercury at room temperature, 

 is allowed access to the lamp, the glow potential, when all 

 permanent gases are evacuated, is greater than 300 volts. 

 Nevertheless, this same quantity of mercury vapour, when 

 mixed with nitrogen at a pressure of 0*01 mm. having a 

 glow potential of about 45 volts, causes a notable diminution 

 of its glow potential ; it makes the glow potential of the 

 nitrogen almost identical with that of carbon monoxide (see 

 fig. 4). Even when the temperature of the mercury was 

 reduced to 200°K., its effect could still be detected in a slight 

 but consistent diminution of the glow potential of nitrogen 

 at low pressures. It may be added that the wax vapour 

 which has so marked an effect on the glow potential of 

 hydrogen had a glow potential greater than 600 volts in the 

 absence of other gas. 



Since, as we shall see, the glow potential is a very im- 

 portant factor in determining the disappearance of the gas, 

 these observations, proving the large effect of very minute 

 traces of impurity, are of great importance. The impurities 

 that are important are those which, like mercury, are 

 characterized by low glow potentials at all pressures; and 

 the effect of the impurities is greatest in gases which, like 

 hydrogen, have a high glow potential. The glow potential 

 of hydrogen is probably one of the most delicate methods 

 for detecting any common impurities, which are usually 

 characterized by low glow potentials. It is not certain, of 

 course, that the hydrogen of fig. 4 is absolutely pure ; all 

 that can be said is that two samples agreed in giving the 

 same glow potentials, and that these are higher than those of 

 any other sample examined. 



The reduction of the glow potential by impurities is 

 associated with the development of the spectrum of the 

 impurities in the glow ; such an association is natural, for the 



