48 Proceedings of the Royal Irish Academy. 



it was investigated under suitable conditions. It is very likely that 

 this is the gas-molecule with two positive charges. The ion of* 

 mobility 16, that appears with the second apparatus, is possibly the ion 

 of mobility 8 with a double charge, while the uncertainty as to the correct 

 value for the latter ion may be due to the fact that doubly charged ions of 

 original mobility 4*2 are present. Whatever uncertainty may be felt as to 

 these latter points — and the clearing up of these depends upon accurate 

 observations of the mobilities and of the condibions under which they occur — 

 the positive ion of mobility 24 remains well established ; and we can iind no 

 reasonable explanation for it other than that of a double positive charge. 

 This would mean that the n-particle can in the process of ionisation detach 

 two electrons from the molecule. This agrees with the results claimed by 

 Townsend^ and by Franck and WestphaP from measurements on diffusion 

 and mobility. Millikan, hovrever, claims that his oil-drop experiments show 

 conclusively that doubly charged ions, if they occur at all, occur very rarely. 

 The question of ionisation by a-rays is dealt with specially in a recent paper,' 

 and this conclusion is reafifirmed. Millikan and his collaborators state that 

 singly charged positive ions are produced in certainly more than 99 per cent. 

 of the cases of a-particle ionisation. The ions which we consider to be doubly 

 charged positives appeared only with extreme drying, Millikan's oil-drop 

 experiment would be decisive on the point if repeated under similar conditions 

 as to drying. 



In view of the results given in this paper, it is difficult to see how the 

 mono-molecular theory of the ordinary small ion, so ably urged by such 

 workers as Wellisch^ and I.oeb,' can be maintained. Much yet remains 

 doubtful, but the composite nature of ionisation in air and the general 

 nature of the changes produced by drying are quite clear. Eeference has 

 been made in a previous paper'' to the work of Haines,' who found separate 

 groups of negative ions of high mobility in freshly prepared and very pure 

 hydrogen. With the lapse of time these ions disappeared. It was suggested 

 by one of us that the impurity which found its way into the hydrogen and 

 caused the disappearance of the mobile icuis was water. This explanation is 

 rendered very probable by the work now reported. In fact, there is a very 

 close resemblance between Haines' results and ours. Haines' work has been 



1 Townsend, Proc. Royal Soc, See. A, Ixxx, 1908. 



^Franck and Westphal, Verb. d. deutsch. phys. Ges., xi, 1909. 



3 Millikan, Gottschalk, and Kelly, " Physical Review," xv, 1920. 



« VVellisoh, "Phil. Mag.," July, 1917, and other papers. 



5 Loeb, "Phys. Rev.," viii, 1916, and xvii, 1921. 



SP.R.S., loc. cit. 



7 Haines, "Phil. Mag.," Oct., 1915, and July, 1916. 



