Electric Origin of Molecular Attraction. 631 



centres, and an effect due to the large departure from average 

 conditions when a pair o£ neighbours collide. For example, 

 two molecules A and B, separated by a large number of 

 others, may have their axes so directed that they have the 

 maximum mutual inductive effect and maximum attraction 

 at that distance, but it will be possible to find near A a 

 molecule C whose effect on B is nearly equal and opposite to 

 that of A, and a molecule D near B which neutralizes the 

 effect of B on A. But if A and B are on the point of col- 

 lision it is not in general possible to find another pair C and D 

 capable of neutralizing the mutual effects of A and B. For 

 molecules whose distance apart is several times the mean 

 molecular interval, the preponderance of the attractive over 

 the repulsive forces diminishes rapidly with increasing dis- 

 tance. To take account of the average effect of this pheno- 

 menon we can replace the perpetually varying actual forces 

 by a fictitious molecular attraction f(r)/f*, in which f(r) can 

 be assigned a form which best represents the average facts, 

 and introduces a fairly definite range beyond which mole- 

 cular attraction is negligible. A speculation of van der 

 TVaals (Ann. d. Ph. Beibl. xviii. p. 734) suggests one form 

 that/(V) might conveniently have assigned to it provisionally. 

 He assumes that molecules attract one another according to 

 Xewton's law of gravitation, but that the lines of force are 

 absorbed by the medium in such a way that the potential 

 energy of two molecules may be written —fe~ rX /r, where A. 

 is a parameter characteristic of the substance and is equal to 

 H/K, the ratio of Laplace's two capillary parameters. But 

 we shall see immediately that molecular attraction has no 

 direct connexion with gravitation. Moreover, the absorption 

 of lines of force would be difficult to reconcile with the 

 absence of any known gravitational property corresponding 

 to the electric one of dielectric capacity. But van der Waals* 

 factor e~ rK becomes intelligible if taken as representing our 

 f(r). In the absence of knowledge as to the form of f(r) 

 perhaps the simplest way of taking account of it is to remove 

 it and assume that the force 1/r 4 acts from a distance r=v, 

 where v is of the order of the distance between contiguous 

 molecules, up to a distance ?* = L and not beyond, L being so 

 chosen that the effects due to distances greater than L are 

 allowed for by exaggerating the effects up to distance L 

 through treating the function /(?•) as 1. This simple method 

 of treating the unknown f(r) has the temporary advantage 

 of agreeing with that which I have already adopted in 

 investigating the law l/>- 4 while providing a more definite 

 meaning for L than that formerly suggested. In one place 



