THEOEY OF ELECTRIC DISCHARGES 81 



that the large forces developed at the surfaces of con- 

 ductors separated by a fraction of the wave length of 

 sodium light, and differing in potential by 300 volts, are 

 not sufficient to make electricity pass from the conductor 

 to the surrounding gas. 



35. Remarks on processes of ionization which account for 

 various phenomena. 



In the preceding account of the development of 

 currents in gases it has been shown that the conduc- 

 tivities may be explained on the theory that all the 

 ionization is to be attributed to ions produced in the gas 

 by positive and negative ions. Some of the phenomena 

 that have been discussed might also be fairly well 

 accounted for by supposing that the negative ions ionize 

 the molecules of the gas, and that the positive ions have 

 only the power of setting free negative ions when they 

 impinge on the negative electrode. There are good 

 reasons, however, for preferring the former method of 

 explaining the phenomena that occur at pressures above 

 the critical pressure, and of showing the connection 

 between them. In the first place, if the positive ions 

 acted by setting free negative ions from the cathode, 

 their effect would presumably depend on the metal of the 

 electrode, and the sparking potentials would show varia- 

 tions depending on the metal of which the electrode 

 was made. This point has been examined recently by 

 Carr (loc. cit.), and the results of several careful experi- 

 ments have shown that the sparking potential is 

 independent of the metal. This conclusion is in agree- 

 ment with the observations of most other physicists, but 

 some have found that with aluminium, and possibly also 

 with magnesium, the sparking potentials are somewhat 

 lower than with other metals. 



I.O. G 



