346 Mr. W. B. Haines on 



At higher pressures the contamination is largely that from 

 the walls of the apparatus. These facts are in the direction 

 that would be expected, since at lower pressures there are 

 fewer encounters per second between the molecules of the 

 gas and therefore less opportunity for the ionic aggregates to 

 increase their size. In other words, there is greater stability 

 of light ions at lower pressures. For similar reasons the 

 foreign molecules present in the hydrogen have greater 

 influence at higher pressures. Since the electrons do not 

 readily attach themselves to hydrogen molecules, the first 

 step in the formation of most of the hydrogen ions is, no 

 doubt, the capturing of an electron by a foreign molecule 

 having the necessary affinity. And the greater the frequency 

 of collision the greater the chance of the electrons being 

 quickly taken captive. 



The formula for the mobility of an ion as derived from the 

 theory of gases is given by Townsend * as : 



Q1 . Xet 

 mx 



t/V being the mean time between collisions. For a hydrogen 

 molecule with a single electronic charge e/m = 4:'78 x 10 3 , 

 X = 10 8 . The values f for t/V vary between 1-052 x 10" 10 

 (N. T. P.) and 1-186 x lO" 10 (15° C, Kundt and Warburg). 

 The former value gives w = 4l"0 cm./sec, which corresponds 

 very nearly to the measurements for ion C (u = 40 '6 at 

 76 cm.). The second value gives u = 4z6'2, and this bears a 

 close integral relationship to the measurements for the other 

 ions. This is made plain in the accompanying Table, in which 

 the theoretical mobilities for various molecular aggregates 

 are shown, based on the assumption that the time between 

 collisions is the same for aggregates as for single molecules, 

 and the measured values placed beside them. 



Ion. Neg. C. Neg. B. Neg. A. Pos. 



Experimental values u ... 



40-6 



15-9 



7-9 



5-4, 





Theoretical values u 



4G-2 



15-4 



7*7 



5-1 



76 cm. press. 



No. of mols. per electronic 











and 15° 0. 



charge 



1 



3 



6 



9 J 





We conclude that the ions neg. C, neg. B, neg. A, and 



* Townsend, 'Electricity in Gases,' 1915, p. 84. 

 t See Meyer, ' Theory of Gases,' p. 192. 



