and Electronic Potential Energy. 2G3 



their compounds, except the metals, es for the atom is nearly 

 proportional to the volume of the atom, so for the molecules 

 of most ordinary liquids es is proportional to the volatile mjp. 

 So the change of energy on mixing is proportional to 



±p(j>ilPi +W.02) 2 - fyjpi - 4 WP2- 

 If this is 0, then l\o=lh\pi+P'2\p2i 



which states that mixture occurs without change of volume. 

 Thus the absence of change of volume and of tnermal effect 

 connected with potential energy are related. As the limiting 

 volume of the molecule is proportional to its electric moment, 

 and as the molecules of ordinary liquids at ordinary tempe- 

 ratures occupy nearly their limiting volumes, we may conclude 

 that constancy of volume on mixing and constancy of electric 

 moment are connected. 



10. The relation of Mills. 



This has been discussed recently in " The Electric Origin 

 of Molecular Attraction " (Phil. Mag. [6] xvii. 1909, p. 657), 

 but requires to be considered further in the present connexion. 

 In the simplest case the change of attractional potential 

 energy for an element gas when unit mass is changed from 

 liquid to saturated vapour is 4ze 2 s^(p 1 — p 2 )lm 2 . For a typical 

 compound the change is 



or nearly 4^f {Pi/2 — PslO- + P*\Pc)}\m 2 . 



The simplest hypothesis that we could make concerning 

 the internal latent heat of vaporization would be to equate it 

 to these changes of potential energy. For an element gas at 

 different temperatures we should have the internal latent 

 heats proportional to the difference of the densities of liquid 

 and saturated vapour, and for a typical compound proportional 

 to pij^— /o 2 /(l + /cp 2 ), or nearly to pi/2— jo 2 /(l -f p 2 [pc). -^ u ^ the 

 relation of Mills makes latent heat proportional to p^ — p^. 

 The interpretation of- these results leads to an important 

 principle concerning the motion of the electrons which form 

 a molecule. In an investigation of the nature of dielectric 

 capacity (Phil. Mag. [6] xix. 1910, p. 1) the molecule was 

 treated as made of pairs of opposite electrons, each pair 

 having an electric moment ea. The sum of the components 

 of ea parallel to the axis of electrization of the whole atom 

 or molecule forms es. For the maintenance of this state of 

 affairs we must imagine each pair of electrons in motion 



