of the Ions formed in Gaseous Media. 7 



considerable. Column 8 gives the values of the mobilities under 

 a potential gradient of 1 volt per cm. which would be possessed 

 by a molecule carrying a charge E if there were no retarding 

 effect due to this polarisation. The remaining columns give the 

 values of the mobilities as deduced from expression (a) together 

 with the observed experimental values of the positive and negative 

 ionic mobilities ; these values correspond to a real or hypothetical 

 pressure of 760 mm. of mercury. 



Unfortunately the values of K have been determined experi- 

 mentally for only a very limited number of gases and vapours ; 

 in consequence there do not appear in the table several vapours 

 whose mobilities have been ascertained. 



Considering that we are comparing the observed values of the 

 mobility with absolute values calculated from various physical 

 constants of the substances the agreement is in several cases 

 quite satisfactory ; however, in the case of ammonia and the 

 vapours, there is a marked divergence between the calculated 

 and observed values, the former values being invariably smaller. 

 The values of the dielectric constants for the vapours appear to be 

 inordinately large, and, in this connection, it is interesting to note 

 that it is also in the case of vapours that there is a marked 

 departure from Maxwell's law, K = n^, where n is the refractive 

 index. The following table will serve as an illustration : 



Table II. 



In the case of Maxwell's law the reason for the departures 

 is probably the existence in the molecule of free periods with 

 durations long compared with that of the incident wave period*. 

 It is possible that, in the case under consideration, owing to free 

 periods of long duration in the molecules of the vapours, these 



* Vide Drude, Theory of Optics, Eng. trans, p. 389. 



