666 Mr. W. Sutherland on the Electric 



treatment of a gas as a completely ionized fluid, and is one 

 by which Mills might have given a dynamical interpretation 

 of his discovery, instead o£ connecting it with gravitation. 

 A notable fact in this relation is the disappearance of K, 

 "because in assuming K constant during the change of state 

 from liquid to vapour, we have virtually made it disappear. 

 Now in the laws of the parameter a in 'da 1 a 2 /r i for molecular 

 attraction in my various Phil. Mag. papers on this subject, 

 it has never been necessary or advantageous to consider K 

 as exercising any influence ; it disappears from the scene. 

 These two instances of the disappearance of K are most 

 easily accounted for by the hypothesis that each molecule 

 behaves as an electrized sphere. The external mutual effect 

 of two such neighbour spheres depends only upon the electric 

 moment assigned to each, just as the mutual effect of two 

 neighbour magnetized spheres depends only upon the mag- 

 netic moment of each, and not upon the permeability of the 

 matter of the spheres, if mutual induction is not operative. 

 Thus in (1) K may be allowed to disappear by being merged 

 in e 2 (m/p ) 2 / s , the square of the electric moment of a mole- 

 cule. Let us denote the electric moment bv es and replace 



(i) bj 



This discarding of K or this putting it equal to 1 expresses 

 definitely the principle that cohesion is the attraction of 

 immediate neighbours through the aether. In the theory of 

 ions in solution K plays a prominent part, because neighbour 

 ions are separated by the solvent of dielectric capacity K. 



This equation (4) gives the electrostatic energy per unit 

 volume: that per molecule is 27r<?V/3(?n/p). If now s were 

 equal to (m/d) l/3 in the liquid and to (m/D) 1 ^ in the vapour, 

 we should obtain at once the formula of Mills. The most 

 important point to understand then is this : How does the 

 molecule of vapour behave as if its uniform electrization 

 extends not only through its volume m/p or m/d, but through 

 its domain m/T) ? The answer is to be found by considering 

 the electrostatic energy of a molecule in two parts, the inter- 

 molecular and the intramolecular, or the mutual and the 

 self energy. For many purposes it is convenient with 

 Maxwell to localize so much electric or magnetic energy in 

 a given element of volume. Thus in our collection of 

 molecules at absolute zero we assign electrostatic energy 

 — 27re 2 s 2 j3(mlp) to each molecule, though we may drop the 

 negative sign, as above, when it is not essential. But this 



