$2 Mr. E. M. Wellisch on the Motion of 



in experimenting with regard to the variation of: ionic mobility 

 with changes in temperature, concluded that his results were 

 not explicable by the notion of clusters. In the present 

 experiments it has been shown that an electron passes un- 

 encumbered through ordinary gases at considerable pressures 

 notwithstanding the strong electric field which is associated 

 with it ; it is hard to reconcile this fact with the basic idea 

 of the cluster theory, viz., that the cluster of molecules is 

 held together by the electric field associated with the ion. 



Leaving the question as to the nature of the gas ion, we 

 may now with advantage consider another outstanding- 

 problem of ionic theory, viz., the explanation of the difference 

 in the experimental values obtained for the mobilities of the 

 positive and negative ions in a gas. The greater mobility 

 of the negative ion in most gases has usually been regarded 

 as indicating that this ion is constituted by a smaller cluster 

 of molecules. On another view* we could explain the greater 

 mobility of the negative ion by supposing that the electron 

 is able occasionally to leave the ion, so that the increased 

 velocity would arise during the free motion of the electron. 

 The present experiments show, however, that this view is 

 untenable as an explanation : it was shown that the electrons 

 pass through the gas independently of the negative ions, and 

 still the latter have a mobility greater than that of the 

 positive ions. 



If we regard the ion as consisting of a single charged 

 molecule, it seems evident that the difference in the mobilities 

 of the two kinds of ions must be ascribed to a difference 

 in the attractive forces between each kind of ion and the 

 uncharged molecules. In the Bakerian Lecture of 1890 f 

 Schuster remarked that " if the law of impact is different 

 between the molecules of the gas and the positive and 

 negative ions respectively, it follows that the rate of diffusion 

 of the two sets of ions will in general be different." 



Franck and Hertz J were the first to bring out clearly the 

 possibility of great differences existing in the nature of 

 the collisions between an electron and the molecules of 

 different gases. On their view the electrons are regarded as 

 possessing different degrees of elasticity when in collision 

 with the molecules of different gases, the collisions being 

 extremely elastic in the case of the inert gases, but only 

 partially elastic or even almost inelastic for most other gases. 



* Cf. J. J. Thomson, l Conduction of Electricity through Gases/ 2nd 

 edit. pp. 28, 29. 



t Schuster, Proc. Roy. Soc. vol. xlvii. p. 553 (1890). 

 t Franck and Hertz, he. cit. 



