Wellisch — Motion of Ions and Electrons throuyh Gases. 29' 



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

 experinaental values obtained for the mobilities of the positive 

 and negative ions in a gas. The greater mobility of the nega- 

 tive ion in most gases has nsiialiy 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 independ- 

 ently of the negative ions and still the latter have a mobility 

 greater than that of the positive ions. 



If we re2:ard the ion as consisting of a single charged mole- 

 cule 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 ISOOf Schuster 

 remarked that ' if the law of impact is different between the 

 molecules of the gas and the positive and negative ions respec- 

 tivel}^ it follows that the rate of diffusion of the two sets of ion& 

 will in general be different.' 



Franck and HertzJ 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 mole- 

 cules 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. It seems to the writer to 

 be perfectly natural and logical to extend this conception so as 

 to apply to the collisions between the ions and the neutral 

 molecules. The difference in the mobilities of the two kinds 

 of ions is thus regarded as being due to the different degrees 

 of elasticity between the neutral molecules and the positive 

 and negative ions respectively. 



If we regard the collisions between neutral gas molecules as 

 being moderately elastic we would expect that collisions 

 between an ion and a gas molecule would have a smaller degree 

 of elasticity on account of the attractive forces resulting from 

 the charge on the ion. These forces vrould result in a small 

 fraction of the translational energy at collision being trans- 

 formed into energy inside the ion or molecule. A very high 

 degree of elasticity would imply ('y. sec. 4:B) either an acceler- 



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

 p. 28,29. y ^ . _ r 



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

 X Franck and Hertz, loc. cit. 



