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XVIII. The Tlveory of the Small Ton in Gases. 



To the Editors of the Philosophical Magazine, 

 Gentlemen, — 



IX a paper on the Ions of Gases published in the September 

 number of your Magazine, Mr. Sutherland refers to 

 some theoretical considerations which I had previously 

 advanced (Phil. Trans. A. vol. ccix. p. 272. 1 1H >0). and as a 

 result of which I was led to the conclusion that the values of 

 the mobilities of the ions produced by Rontgcn rays in oases 

 could be approximately explained on the supposition that the 

 ion consisted of a single charged molecule provided we took 

 account of the increased collision frequency resulting from 

 the inductive attraction between the ion and the neutral 

 molecules. Mr. Sutherland's view i- that this " small " ion 

 •consists of a single charged atom, and to this extent i- in 

 accordance with my conclusion that there is no necessity for 

 introducing molecular clusters to explain the mobility values. 

 At the same time he asserts that the increased collision 

 frequency which I considered i- wholly inadequate as an expla- 

 nation : but that in addition to the viscous resistance (F) to 

 the ion resulting from collisions with the molecules, there 

 must be introduced a resistance (6) resulting from a new 

 type of viscosity which he had previously applied to the case 

 of electrolytic ion-. 



I would be £*lad if von would allow me to make a few 

 observations in reference to these points. 



The following considerations will perhaps serve t<> explain 

 how it is that, although our expressions lor the factor re- 

 sulting from the increased collision frequency arc identical, 

 Mr. Sutherland is led to the necessity of introducing this new 

 and additional resistance (^=7*6 F for air). 



(1) Mr. Sutherland's expression for F, to judge from its 

 form, seem- to be based on a method in which the dynamical 

 conditions attending collision are taken into consideration. 



las in veil 



4 /2w 



It involves a coefficient ., \ / ".,-( =1"93). Stefan " 



.>V ,) 



•expressions for F of exactly the same form but with co- 

 efficients ., ( =4*1 0) in the case when the molecular velocities 



8 /' } 7T 



•of agitation are taken as equal, and ., \/ -' (=3*86) when 

 Maxwell's law of distribution of velocities is assumed. 

 * WUn. Ber. II. Bd. Ixv. p. 360 (U 



