54 



Mr. E. M. Wellisch on the Motion of 



assumed for each value of the potential a definite position on 

 the scale: in this position the ionization current is balanced 

 by the current due to the leak through the condensed vapour, 

 and is thus proportional to the steady reading of the electro- 

 meter. The conductivity o£ the condensed vapour remained 

 constant over a sufficient interval to enable the critical 

 potential to be determined in this manner. Typical curves 

 obtained for saturated H 2 are given in fig. 7*. The results 

 of the mobility determinations for saturated H 2 and C0 2 

 together with the calculated values for the dry gases at the 

 same pressure are given below : — 





mm. l 



K, 



H 2 dry 



saturated 



809 4-80 

 809 5-03 



769 -72 

 769 ! -77 



9-09 

 6-105 



1-06 



•88 



C0 2 dry 



saturated 





In addition, other experiments were performed in which 

 small quantities of water vapour and ether vapour were 

 mixed with hydrogen: in these experiments the mobilities 

 were determined in the ordinary manner. The following 

 results were obtained : — 





p. 



mm. 



K r 



K 2 . 



H dry 



517 



517 

 766 



7-51 

 7-43 

 4-13 



14-21 



10-27 

 7-25 



H 2 with water vapour . . . 

 H 2 with ether vapour ... 



The presence of water vapour appears thus to be without 

 effect on the mobility of the positive ion, but occasions a 

 marked diminution in that of the negative ion. This is in 

 accordance with the results obtained by previous investigators. 

 The diminution in the mobility of the negative ion is too 

 great to be accounted for by the extra resistance to the motion 

 of the ion which arises when the vapour is mixed with the gas. 

 The diminution may be explained in part by assuming that 

 the effect of the water molecules is to cause inelastic impacts 

 with the negative ions, and thus prevent them from acquiring 

 the larger terminal velocities which they attain in the dry gas. 



* The current scale for these curves is different from that for the other 

 curves in fig. 7. 



