Ions and Electrons through Gases. 57 



to this question was that the carrier altered in nature during 

 its motion between the electrodes, but in such a manner that 

 for a given pressure it possessed an "average" mass. If, 

 for instance, we regard the ion as being constituted at high 

 pressures by a cluster of molecules, then we should have to 

 assume that as the pressure was reduced the average number 

 of molecules in the cluster decreased ; as the pressure was 

 still further reduced, any individual negative carrier would 

 be for part of the time in the ionic state (say now as a single 

 molecule), and for the remainder would exist as a free electron; 

 at this pressure we should have at any given instant a number 

 of free electrons and a certain number of ions, but if we were 

 to follow one electron throughout its motion we should find it 

 associated on the avenige with a mass intermediate between 

 that of an electron and that of a molecule Ultimately at 

 very low pressures the carriers would be all free electrons. 

 Prof. Townsend's* point of view differed only slightly from 

 this in that he regarded the average nature of the carrier to 

 be determined by electric force as well as gas pressure. 



The answer afforded by the present experiments is funda- 

 mentally different. We now regard the electrons and ions as 

 passing independently through the gas, each kind of carrier 

 remaining constant in nature throughout. The transition 

 from the ionic conduction at high pressures to the electronic 

 conduction at low pressures is effected by means of an 

 increase in the number of free electrons relative to the 

 number of negative ions without any alteration in the 

 nature of either kind of carrier. The appearance of the 

 phenomenon of ionization by collision would further affect 

 the relative numbers of carriers, but would not influence the 

 nature of the conduction. 



Looked at from this point of view it seems clear that, as 

 far as the so-called permanent gases are concerned, we must 

 regard the free electrons as occurring theoretically at all 

 pressures. These gases differ, of course, considerably in the 

 relative number of free electrons and ions for any given pres- 

 sure, and, practically speaking, there is for each gas a pressure 

 at which the number of free electrons is negligibly small, but 

 the general rule is in no way invalidated on this account. 



In the above illustration we considered the electric current 

 passing through air. It was shown, however, by Franck f 

 that for certain gases, viz. the inert gases and nitrogen 

 (which behaves often as an inert gas), the negative carriers 



* Townsend, ' Electricity in Gases,' Oxford (1915), Chap. IV., VIII. 

 Cf. also Pidduck, < Electricity,' Cambridge (1£>16), Arts. 214-215. 

 t Franck, loc. cit. 



