IONS PRODUCED BY RONTGEN RAYS IN GASES AND VAPOURS. 277 



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 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 and 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 experimental 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 electrolysis. 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. LANGEVIN 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 



st 



from the experimental mobility values could be decided if the ratios for the 



different gaseous ions were known. In this connection it is worthy of mention that 

 Prof. Sir J. J. THOMSON || 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. 



* Fide DRUDE, ' Theory of Optics,' Eng. trans., p. 389. 



t Fide BADEKER, 'Zeits. Phys. Chem.,' Bd. 36, p. 321, 1901. 



t 'Phil. Mag.,' vol. 36, p. 507, 1893. 



' Ann. de Chim. et de Phys.,' vol. 5, p. 284, 1905. 



|| 'Phil. Mag.,' vol. 16, p. 680, 1908. 



