Origin of Molecular Attraction. 669 



to its electrization. As each atom is equivalent to an in- 

 finitely short bipole of equal moment at its centre, the law 

 becomes rigorous that one atom acts on any other with a 

 force inversely proportional to the fourth power of the dis- 

 tance between their centres. If the relative directions of 

 the two electric axes of two atoms are random, repulsion 

 will occur as frequently as attraction. This is the great 

 difficulty in accounting for cohesion by means of electric 

 polarity of atoms and molecules, especially in the case of 

 gases in which the kinetic theory has made random distri- 

 bution the ideal of generality. It was previously pointed 

 out that as the attractive forces tend to increase themselves, 

 while the repulsive tend by their own action to diminish 

 themselves, there must be a tendency even in a random dis- 

 tribution of polarities for the attractive forces to prepon- 

 derate. But it is now suggested that the electrization of 

 neighbour molecules is not randomly directed, but is so 

 directed that an atom is attracted by each of its six nearest 

 neighbours. This satisfies the main condition for minimum 

 electric potential energy in a set of moving molecules whose 

 axes of electrization are free from constraint except that of 

 being translated with the molecule. Beyond the range of 

 the six nearest neighbours the attractions and repulsions of a 

 molecule tend to become more nearly equal and opposite the 

 greater the distance. Thus we have a range of force which 

 is actually infinite but is effectively an attraction reaching 

 only the six nearest neighbours of a molecule. The effective 

 range of molecular attraction is the distance between a mole- 

 cule and its immediate neighbours. This principle is verified 

 by the fact that dielectric capacity does not appear in values 

 of cohesion. Although in a small group of molecules, say a 

 molecule audits six nearest neighbours, there is at any instant 

 a prevailing direction of electrization, this varies with time, 

 and at a given instant of time varies gradually from one 

 group to the next, so that throughout a large number of 

 molecules the directions of electrization are as many as if 

 distributed at random. The electrostatic energy N per unit 

 volume of a collection of such molecules of mass m and 

 density /o in contact at absolute zero is —27re 2 /3(m/p o y :i r 

 and the energy per molecule is — 'Ittc^/ZQu/pq) 1 ' 3 , the dielectric 

 capacity K not appearing. But Mills has discovered that 

 the internal latent heat of evaporation of a liquid being L — E 

 at any temperature at which the densities of liquid and 

 saturated vapour are p = d and /> = D, (L — E)/(V/' 3 — D 1 3 ) is 

 independent of temperature. This can be accounted for most 

 readily by generalizing this last formula and making the 



