Ions and Electrons through Gases, 53 



For each gas the value of K : is less and that of K 2 

 is greater than the value usually assigned ; for the sake of 

 comparison the values of the mobilities obtained by Zeleny* 

 are given below: — 





K r 



K, 



K 2 /K x . 



Air 



1-36 



•76 



6-70 



1-87 



•81 

 7-95 



1-375 



1-07 



1-19 



C0 o 



H., 





Zeleny's values for air and C0 2 are in fair agreement with 

 those obtained by the writer in former experiments and by 

 several other observers. The cause of the discrepancy is 

 not apparent : it is scarcely probable that any defect either 

 in the method employed in the present experiments or in the 

 determination of any of the constants involved would cause 

 the ascertained values of K] and K 2 to vary in different 

 directions. It should be mentioned that the present values 

 for C0 3 and H 2 are in fair agreement with those obtained by 

 Blanc f, who used Franck and PohFs method with X-rays 

 as the ionizing agent. Blanc's values are : 





K v 



K, 



K 2 /K r 



co 2 



H 9 



•83 

 533 



1-027 

 10-00 



1-24 



1-88 





Haines % has recently obtained . the value 5*4 for the 

 mobility of the positive ion in pure hydrogen ; the negative 

 ions did not appear till a trace of impurity was present, and 

 under these conditions their mobility was about 8. 



Effect of Water Vapour. 



A few experiments were performed to ascertain the effect 

 on the ionic mobilities of saturating with water vapour the 

 gas under consideration. In these experiments the water 

 vapour was introduced by ebullition when the gas in the 

 vessel was at a low pressure ; after this operation the gas 

 was admitted till the desired pressure was attained. As a 

 result of condensation of the vapour, the insulation was 

 extremely defective and the currents could not be determined 

 by observation of the rate of deflexion of the electrometer- 

 needle. However, it was observed that the spot of light 



* Zeleny, Phil. Trans. A. cxcv. p. 193 (1900). 



t Blanc, Journ. de Phys. vii. p. 825 (1908). 



t Haines, loc. cit., also Phil. Mag. xxxi. p. 339 (1916). 



