600 Research Staff of the Cr. E. C, London, on the 



for this reason that the gases of greater molecular weight have 

 the smaller glow potentials, for the smaller potential doubt- 

 less corresponds to the smaller number of ions. But as yet 

 no simple numerical relation has been found between the 

 molecular weight and the glow potential at a given pres- 

 sure: doubtless the mean free path and the ionization 

 potential of the gas are also effective in determining that 

 relation. 



However, though it is not permissible to conclude that, if 

 the current increases 10- or 100-fold at the moment the glow 

 starts, the ionization increases in the same proportion, the 

 ionization when there is a glow must be greater than when 

 there is not. For the neutralization ot the space-charge 

 implies the emergence of more electrons from the cathode ; 

 and, unless the number of ions produced by each electron 

 decreases very greatly, the greater number of electrons 

 must produce a greater number o£ ions. Such considerations 

 are of importance if it is asked whether the appearance of 

 the glow is merely an indication of a great increase of 

 ionization or whether it represents the entry of some new 

 form of ionization. If the first alternative is adopted, it 

 must be concluded that there is a glow even at potentials 

 lower than the glow potential although it is too faint to be 

 seen ; for at such potentials there is certainly some ionization. 

 The matter can only be decided definitely by determining how 

 great is the increase of ionization, and whether it is great 

 enough to account for a change from a state in which th e 

 glow cannot be seen with the most elaborate precautions to 

 one in which it can be seen in bright daylight. The fact 

 that the increase of current is not a measure of the increase 

 of ionization shows that much more inquiry must be made 

 before the matter can be decided ; but all the evidence that 

 has been accumulated so far tends to show that the appear- 

 ance of the glow is not merely due to an increase in the 

 number of ions produced. 



On the other hand it is clear that the glow potential is 

 not, like the ionization potential, a direct property of the 

 individual atoms of the gas ; it must also be a function of 

 their mode of reaction with each other or with the walls of 

 the vessel ; for the glow potential is not, like the ionization 

 potential, independent of the pressure. There is no evidence, 

 therefore, that the glow represents a new form of ionization 

 of the individual atoms. 



