Ions and Electrons through Gases. 



55 



It seems probable, however, when the gas is saturated with 

 water vapour, that condensation occurs round the negative 

 ion and that the diminution in mobility is to a large extent 

 due to this process. We should thus have the negative ion 

 constituted by a cluster of water molecules round a charged 

 nucleus : it should be carefully noticed that the existence of 

 such a cluster in a moist gas affords no evidence as to the 

 nature of the ion in a dry gas. In a later paper experimental 

 evidence will be given which indicates that the water molecules 

 are not held together in the duster by the electrostatic forces 

 due to the charge on the ion, the function of the charge being 

 merely to determine the act of condensation. 



Vapours. 



A number of measurements were made of the mobilities 

 of the positive and negative ions in a few vapours ; this was 

 of interest as affording a comparison with the results obtained 

 by the different method employed in a previous investigation*. 

 The mobilities were determined in the usual manner ; the 

 average values estimated from a number of determinations 

 in good agreement are recorded below, together with the 

 corresponding values taken from the previous research. The 

 figures in the second column give the minimum and maximum 

 pressures employed ; the mobilities given correspond as usual 

 to a pressure of 1 atmosphere. 



Vapour. 



Pressure 



range. 



ram. 



1915. 



1909. 



K v 



K 2 . 



K r 



K 2 . 



Ethyl ether 



67—126 

 23— 39 

 38— 42 

 74—115 

 73— 94 

 63- 65 



•27 



•39 



•365 



•370 



•415 



•24 



•346 

 •412 

 •392 

 •440 

 •414 

 •233 



•29 

 •34 



•36+ 



•44 

 •21 



•31 



•27 



•35i 



•41 



•22 



Ethyl alcohol 



do. (saturated t) ... 



Petroleum ether 



Sulphur dioxide 





The agreement in the case of the positive ion is as good 

 as could reasonably be expected in view of the difficulties 

 attendant upon experimenting with vapours ; we can say 

 with a high degree of certainty that to each vapour there 

 corresponds a definite value of the mobility of the positive 

 ion. The mobilities of the negative ions in alcohol and 

 petroleum ether are, however, in greater disagreement than 



* Phil. Trans, ser. A. vol. ccix. p. 249 (1909). 



t Measured by the method employed with saturated water vapour. 



% Pentane. 



