﻿Neio Type of Ion in the Air. 637 



an ion with a mobility intermediate between that o£ the 

 small gas ion and that o£ the large ion of Langevin. Under 

 average atmospheric conditions this new ion has a mobility 

 of about 1/50, and like the Langevin ion its mobility depends 

 on the hygrometric condition of the air. The large ion, 

 however, judging from cloud condensation experiments, 

 retains its stability even if the vapour-pressure becomes 

 slightly greater than that of saturation for a plane water 

 surface, irrespective of the temperature, whereas the ion 

 of intermediate mobility disappears if the vapour-pressure 

 exceeds a certain value, less than that of saturation for 

 summer temperatures. 



In a paper on the Nature of the Large Ion*, recently 

 published in the Philosophical Magazine, I put forward the 

 view that in the Langevin ion we have an instance of the 

 adsorption in the liquid state of water-vapour by a rigid 

 nucleus, as from the relation between mobility and vapour- 

 pressure it was deduced that the adsorbed fluid had a latent 

 heat very little different from that of water. In this paper 

 I propose to show that the ion of intermediate mobility 

 consists of a rigid core surrounded by adsorbed moisture 

 which, on the whole evidence, is certainly not in the liquid 

 condition. 



Troutont, in 1907, made the interesting discovery that 

 there are two modes of condensation of water-vapour on 

 rigid surfaces. If special precautions are taken in drying 

 the surfaces, on exposure to water- vapour, adsorption occurs 

 as a dense atmosphere of water molecules, in a state, no 

 doubt, intermediate between that of a gas and that of a 

 liquid. At any rate, a change to the liquid condition some- 

 what abruptly takes place, in these circumstances, when, 

 according to Trouton, the humidity is about 50 per cent, in 

 the case of glass, and about 90 per cent, in that of shellac. 



In the intermediate ion, the state of the fluid, doubtless, 

 corresponds to that of the moisture condensed at low pressures 

 on carefully dried surfaces in Trouton's experiments. The 

 two classes of ions thus appear to illustrate in a somewhat 

 striking way Trouton's discovery of the two modes of con- 

 densation. Further, the intermediate ion is not to be found 

 when the vapour-pressure exceeds 17 millimetres, and it 

 seems not unlikely that at a critical pressure, by a change 

 in the state of the fluid surrounding the nucleus, it develops 

 into the large ion of Langevin. 



* Pollock, Phil. Mag. April 1915. 



t Trouton, Proc. lloy. Soc. A. lxxix. p. 383 (1907) ; Chem. News, 

 xcvi. p. 92:1907). 



