﻿520 Prof. J. A. Pollock on the 



of the mobility, reduced to standard atmospheric pressure, 

 and the water-vapour pressure for a temperature of 20° C. 

 The single observations have been included, but they are 

 not to be considered as of equal weight with the grouped 

 measures. 



A second approximation to the value of n is now possible 

 for the part of the curve in fig. 1 corresponding to a pressure 

 of about 15 millimetres. It appears here that the fit of the 

 points to the line is perceptibly better when n is taken as 1 

 than when it is taken as 1 + *01, and noticeably better than 

 when it is put equal to l + *03. 



For the large ions, then, to a considerable degree of 

 accuracy the mobility remains the same as the temperature 

 varies, if the equilibrium vapour-pressure is a constant 

 fraction of the saturated vapour-pressure for a plane surface; 

 or, in other words, the mobility at standard atmospheric 

 pressure is a function of the relative humidity only. 



This relation may be written 



l(dp\ _1^P 

 p\dOJ m ~P d6> 



and in this form is similar to the relation found by Trouton* 

 between the equilibrium vapour-pressure and the mass of 

 contained moisture in the case of flannel and cotton- wool, 

 and independently discovered by Masson and JRichards t 

 during a very accurate investigation with this latter material. 



The determination that the ratio of X to V is approxi- 

 mately unity in the case of the large ions enables a deduction 

 to be made as to the condition of the adsorbed moisture, for 

 with its latent heat nearly equal to that of ordinary water it 

 seems impossible to consider the adsorbed fluid other than in 

 the liquid state. 



Trouton % has shown that there are two possible modes of 

 condensation of water vapour on rigid surfaces. If special 

 precautions are taken in drying the surfaces, on exposure to 

 water vapour, a gaseous form of condensation occurs, which 

 changes somewhat abruptly to the liquid form at a vapour- 

 pressure depending on the nature of the surface. In the 

 case of the gaseous form of condensation, one would imagine 

 that the ratio of the latent heat of water to the latent heat 

 of vaporization of the dense vapour, or n in the formula of 

 reduction, would be much greater than unity. Insofar as 



* Trouton, Proc. Roy. Soc. A. lxxvii. p. 292 (1906). 

 t Masson and Richards, Proc. Roy. Soc. A. lxxvii. p. 412 (1907). 

 \ Trouton, Proc. Roy. Soc. A. lxxix. p. 383 (1907) ; Chem. News, 

 96. p. 92 (1907). 



