€0 Mr. E. M. Wellisch on the Motion oj 



results for the inert gases and hydrogen, although the value 

 assigned by Franck and Hertz * for the ionization potential 

 in nitrogen (viz. 7*5 volts) would not indicate on this view a 

 very large percentage of free electrons. 



It does not seem advantageous to discuss in great detail the 

 questioa as to the nature of the gas ion ; all that is proposed 

 is to indicate here the leading features of this outstanding 

 problem. It should be remembered that the notion of the ion 

 as consisting at moderately high pressures of a cluster of 

 molecules grouped round a charged nucleus was first intro- 

 duced in order to account for the observed mobility and 

 diffusion values, which were found to be considerably smaller 

 than the values which were to be expected from theoretical 

 considerations if we regard the ion as consisting of a single 

 molecule. It was shown, however, by the author t that 

 the observed values were consistent with the view that the 

 ion was a single molecule, provided we took into account 

 the extra resistance to the motion of the ion resulting from 

 the attraction between the charge on the ion and the charges 

 induced on neighbouring molecules J. A definite decision in 

 favour of the cluster theory appeared to be given by the 

 results of the series of experiments, which indicated a 

 departure from the law p& = const. even when the gas was 

 at a pressure of several cm.; the abnormally high mobility 

 values were naturally interpreted as corresponding to the 

 disintegration of the ionic cluster. The fallacy of this series 

 of results has already been discussed in the present paper; 

 it is sufficient here to repeat that no indication has been 

 obtained of any change in the nature of either the positive 

 or the negative ion as the pressure of the gas changes over a 

 wide range. 



We would certainly expect at least a partial disintegration 

 of anionic cluster when the electric field and the gas pressure 

 were such that the ion acquired energy comparable with 

 that required to ionize a neutral molecule. According to 

 Townsend ionization by collision in air commences to be 

 appreciable when X/p = 60 (X being measured in volts/cm. 

 and p in mm. Hg). X//? is proportional to the energy 

 acquired by an ion after traversing a distance equal to its 



* Franck and Hertz, Verh. Deutsch. Phys. Ges. xv. p. 34 (1913). 



t Phil. Trans, he. cit. p. 272. 



% It was the author's idea that this extra resistance was due entirely 

 to increased frequency of collision between the ion and the molecules. 

 Sutherland later maintained that the increased frequency was responsible 

 only for part of the extra resistance and that it was necessary to introduce 

 another type of electric viscosity. These points were discussed further 

 in two communications (v. Phil. Mag. xix. pp. 201, 817, 1910). 



