﻿JS 7 ew Type of Ion in the Air. 645 



water-vapour round the nucleus become sufficiently tightly 

 packed. The advent of a liquid surface involves a diminished 

 rate of molecular escape ; rapid condensation will therefore 

 occur, with a decreasing unit-surface energy, until further 

 increase in the size of the ion means an increase in the total 

 energy of the mixture of ions and vapour. The final result 

 is no other than the large ion of Langevin, where, as I have 

 shown, the surrounding moisture is in the liquid state. 



There is independent evidence from cloud condensation 

 experiments that the large ion has a rigid core, but, as yet, 

 no such evidence exists in the case of the intermediate ion. 

 If, however, the intermediate ion becomes the large one by a 

 change of fluid state only, it must have the same nucleus as 

 the larger aggregation. The mobility which the core alone 

 would have may be estimated by extrapolation in connexion 

 with the curve in fig. 4. Judging from the comparatively 

 large value which is indicated the nucleus may be, at most, a 

 collection of not many molecules. In this connexion it is 

 interesting- to remember that the mobilities of the fully 

 developed large ions, under given atmospheric conditions, 

 appear to lie within narrow limits; the explanation depends, 

 no doubt, on some characteristic of the nuclei. 



Sutherland, in his paper on the Ions of Gases in the 

 Philosophical Magazine for September 1909, definitely makes 

 the suggestion of an ion of intermediate mobility " consisting 

 of an envelope of vapour, such as that of H 2 0, surrounding 

 a small ion which is the central nucleolus," and he applies 

 the conception to the experiments of Moreau on the cooled 

 gases of flames sprayed with electrolytic solutions. The 

 discussion in this present paper is on different lines to that so 

 ably developed by Sutherland. 



Summary. 



A description is given of certain characteristics of an ion 

 in the air with a mobility intermediate between that of the 

 small gas ion and that of the large ion of Lano-evin. 



The mobility is found to depend on the water-vapour 

 pressure rather than on the relative humidity. 



Both the intermediate and large ions exist in the air at 

 the same time provided the vapour-pressure is below seven- 

 teen millimetres. Above this pressure only the large ion is 

 found. 



On the whole evidence it seems probable that the inter- 

 mediate ion consists of a rigid nucleus enveloped by a dense 

 atmosphere of water-vapour. The mass of the ion becomes 

 greater as the vapour-pressure increases, until at a critical 



