22 Wellisch — Motion of Ions and Eleotrons through Gases. 



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 vapor 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. It 

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

 vapor, tliat condensation occurs round the negative ion and 

 that the diminution in mobility is to a large extent due to this 

 process. We would 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 cluster by the electrostatic forces due to the charge on 

 the ion, the function of the charge being merely to determine 

 the act of condensation. 



Vapors. 



A number of measurements were made of the mobilities of 

 the positive and negative ions in a few vapors ; 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 corre- 

 sponding values taken from the previous research. The figures 

 in the second column give the minimum and maximum pres- 

 sures employed ; the mobilities given correspond as usual to a 

 pressure of 1 atmosphere. 



Vapor 



Pressure 

 range 

 mm. 



1915 



1909 



K, 



K^ 



; 1 



Ethyl ether 

 Ethyl alcohol 



do (saturatedf) 

 Petroleum ether 

 Sulphur dioxide 

 Methyl iodide 



67-126 

 23- 39 

 38- 42 

 73-115 

 74- 94 

 63- 65 



•27 



•39 



•365 



•370 



•415 



•24 



•346 

 •412 

 •392 

 •440 

 •414 

 •233 



•29 

 •34 



•36t 



•44 



•21 



•31 



•27 



•35| 

 •41 



•22 



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



\ Measured by the method employed with saturated water vapor. 



X Pentane. 



