8 Mr Wellisch, The Laws of Mobility and Diffusion 



have not time to be completely polarised by the adjacent ionic 

 charge during a collision. It is worthy of mention that the 

 large values of K which have been experimentally obtained for 

 ammonia aud vapours have been ascribed by some authorities 

 as being partially due to traces of conductivity which they possess. 

 Without dwelling further on the cause of these departures 

 from the mobility law, it appears that, on the whole, the experi- 

 mental values of the mobilities of the ions can be explained 

 approximately on the supposition that the ion consists of a single 

 molecule of the gas, with which is associated a charge equal to 

 that carried by the monovalent ion in electrol3'sis. It has been 

 shown from considerations based on the kinetic theory of gases 

 that, irrespective of any theory as to the structure of the ion, the 

 ascertained mobility values lead of necessity to the conclusion that 

 the volume of the ion is in all cases greater than that of the 

 corresponding gaseous molecule. The question arises : what is 

 the nature of this volume ? On the one hand, if we neglect the 

 influence of the charge on the mean free path of the ion, we are 

 led to the conception of the ion as a cluster of molecules held 

 together by forces arising from the polarisation due to the electric 

 charge. On the other hand, the effect of the charge on the 

 collision frequency has been shown to be equivalent to an increase 

 in the molecular sphere of force such that the resultant effective 

 volume is sufficient to explain approximately the observed 

 mobilities. On this view the effect of the charge is to cause 

 the ion itself and the neighbouring molecules to deviate from 

 their rectilineal free paths; Sutherland*, by assuming such 

 deviations to occur in the case of gaseous molecules by reason 

 of attractive forces between them, was able to explain accurately 

 the observed variation of the viscosity of gases with temperature. 

 Langevinf has obtained an expression for the ionic mobility by 

 using the dynamical method employed by Maxwell in the kinetic 

 theory of gases ; he concluded that the experimental values of the 

 mobilities lead to the necessity of regarding the ion as a cluster 

 of molecules. The question as to the nature of the volume of the 

 ion as determined from the experimental mobility values could be 



e 

 decided if the ratios — for the different gaseous ions were known. 



In this connection it is worthy of mention that Professor Sir J. J. 

 Thomson J has recently measured this ratio for the positive ions 

 formed by cathode particles in oxygen at low pressures and came 

 to the conclusion that the ion consisted of a single charged oxygen 

 molecule. 



* Phil. Blag. v. 36, p. 507, 1893. 



t Ann. de Chim. et de Phys. viii. 5, p. 270, 1905. 



t Phil. Mag. v. 16, p. 680, 1908. 



