Wellisch — Motion of Ions and Electroyis through Gases. 27 



It does not seem advantageous to discuss in great detail the 

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

 is to hidicate here the leading features of this outstanding 

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

 as consisting at moderately high pressures of a cluster of mole- 

 cules grouped round a charged nucleus was first introduced 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^ that the observed values were 

 consistent with the view tliat 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 neighboring 

 molecules.f A definite decision in favor of the cluster theory 

 appeared to be given by the results of the series of experiments 

 which indicated a departure from the law pic = const, even 

 when the gas was at a pressure of several cm.; the abnormally 

 high mobility values were naturally interpreted as cori-espond- 

 ing 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 

 an ionic 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/^ = 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 mean free path. In the 

 present experiments the positive ions are shown to have a nor- 

 mal mobility for values of X/^ as great as 11, even if we take 

 for X only the small values of the critical field from which the 

 mobility was estimated. The normal character of the complete 

 curves which were obtained in the process of determining the 

 critical potentials indicates that neither the positive nor the 

 negative ion is appreciably altered in nature for much greater 



♦Phil. Trans., loc. cit., p. 272. 



fit was the author's idea that this extra resistance was due entirely to 

 increased frequency of collision between the ion and the molecules. Suther- 

 land 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 com- 

 munications, (v. Phil. Mag., xix, pp. 201, 817, 1910) 



