﻿10 Mr. W. Sutherland on the Molecular 



negative ion, when they come so near one another as to cause 

 a specially intense field lying between them. By the principle 

 of energy, (H 2 0) 3 having a larger dielectric capacity than 

 (H 2 0) 2 will tend to move into such a field from the weaker 

 field around more quickly than (H 2 0) 2 . For the same reason 

 such a field will be a place favouring the change of (H 2 0) 2 

 into (H 2 0) 3 . The number of such fields will be proportional 

 to the number of each sort of ion, therefore to p 2 . Also the 

 force drawing a mobile dielectric into a field of electric force 

 of intensity R is proportional to R 2 . Hence, since a number 

 of fields of force proportional to p 2 and of average effect 

 proportional to R 2 act on an amount of dihydrol p^pi, we 

 have an amount of trihydrol formed proportional to p z p^R 2 p x ' . 

 The rate of variation of this will be proportional to dpi/dt 

 chiefly. Hence we see how the term — bp i comes into (18) 

 and how be is proportional to R 2 . But as the ions of different 

 compounds are similarly distributed through their solutions^ 

 R for different solutes must be proportional to the difference 

 of electric potential between positive and negative ions, say V. 

 But the heat of dissociation of the solute molecule or equi- 

 valent into ions is eV, where e is the electron charge. If this 

 heat of dissociation is the principal part of the heat of for- 

 mation H of the solid compound from its elements, then V is 

 approximately proportional to H. So it comes abontthat R 2 

 is roughly proportional to H 2 . The fuller investigation of this 

 relation can hardly be undertaken in the present connexion. 



5. Surface Tension. 



In the case of surface tension we have a still more compli- 

 cated set of conditions. It has been shown in " The Molec. 

 Const, of "Water" (he. cit. p. 473) that the surface film of 

 water between 0° and 40° consists of pure trihydrol 3 but that 

 at higher temperatures the surface tension is sufficiently 

 reduced to allow some dihydrol to form in the surface film. 

 In a theory of the surface tension of solutions we are con- 

 fronted with the additional difficulty of having to find the 

 solubility of the solute in the surface film. Moreover, tbc 

 dissociation of (H 2 0) 3 in the surface takes place under dif- 

 ferent physical conditions, which affect the value of r to an 

 unknown extent. Accordingly the present investigation will 

 be devoted chiefly to establishing the broad principle that the 

 anomalies in the surface tensions of aqueous solutions are 

 caused by the action of the solute in changing (H 2 0) 3 into 

 (H 2 0) 2 to about the same extent as has been demonstrated 

 already in the previous sections. Jn u Molecular Force and the 

 Surface Tension of Solutions" (Phil. Mag. [5] vol. 1. p. 477) 



