﻿516 Prof. J. A. Pollock on the 



The large ion thus affords an interesting example of the 

 adsorption of water vapour at a rigid surface. In connexion 

 with such an idea it is interesting to recall a statement of 

 Lord Rayleigh. Referring to the rise of a liquid in a 

 capillary tube Lord Rayieigh says *: " Above that point (the 

 meniscus) the walls of the tube are coated with a layer of 

 fluid, of gradually diminishing thickness, less than the range 

 of forces, and extending to an immense height. At every 

 point the layer of fluid must be in equilibrium with the 

 vapour to be found at the same level. The data scarcely 

 exist for anything like a precise estimate of the effect to be 

 expected, but the argument suffices to show that a solid body 

 brought into contact with vapour at a density which may be 

 much below the so-called point of saturation will cover itself 

 with a layer of fluid, and that this layer may be retained in 

 some degree even in what passes for a good vacuum. The 

 fluid composing the layer, though denser than the surrounding 

 atmosphere of vapour, Ctvnnot properly be described as either 

 liquid or gaseous/' 



In the large ion, according to the foreo-oino- suo-o-estion. 

 we have similar conditions, modified perhaps by the electri- 

 fication, the equilibrium vapour-pressure depending on the 

 thickness of the adsorbed fluid surrounding the rigid core. 



To obtain some idea of the nature of the relation between 

 mobility and vapour-pressure which is to be expected in 

 connexion with such an ion, consider unit mass of a mixture 

 of ions and water vapour as the working substance in a 

 Carnot's engine. A cycle may be performed involving only 

 reversible processes, so we have, for the mere change of state, 

 the well known relation, 



/#\ .__(dv\ 



\dpjg W,' 



where <£ = the entropy, 



^>=the vapour-pressure, 



# = the absolute temperature, 



y=fche volume of the working substance. 



Let a and o-'=the density of water vapour at the saturated 

 pressure P, and at the pressure p, respec- 

 tively. 



p, p and A, A/ = the densities and latent heats of vaporiza- 

 tion of water, and of the adsorbed fluid, 

 respectively. 



* Rayleigb, Phil. Mag. xxxiii. p. 220 (1892) ; Scientific Papers, iii. 

 p. 523. ' 



