Origin of Molecular Attraction. 



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molecules being brought nearer to one another by their attrac- 

 tion tend to increase its amount, while a pair repelling one 

 another cause the repulsion to diminish by increase of the 

 distance between them. There is thus a tendency for the 

 attractive effect to preponderate. To show that the prepon- 

 derance must be considerable it was argued that a pair of 

 neighbours attracting one another would approach one 

 another so comparatively closely as to introduce a compara- 

 tively large attraction. This principle taken with the ten- 

 dency of the directive couples to promote similarity of 

 direction in electric axes, was held in general to explain the 

 preponderance of attraction, though no attempt was made to 

 calculate definitely the amount of that preponderance. In a 

 general way it was argued that, in reasoning about molecular 

 attraction, we can replace the complex medley of Nature by 

 a representative pair of neighbour molecules having similarly 

 directed electric axes along the line joining their centres, 

 and therefore attracting one another. 



The attractive and repulsive effects of the more remote 

 molecules in Nature are assumed to neutralize one another. 

 In this way I sought to give a new interpretation and use to 

 the idea of a finite range of molecular force. The actual 

 range of the electric forces is infinite, but their effective 

 range is the distance between our two schematic represen- 

 tative neighbours having their electric axes similarly directed 

 in the same straight line, or nearly so. The object of the 

 present communication is to establish this conception more 

 rigorously, and to formulate it more systematically in detail. 

 In "The Pressure of Gases and the Equation of Virial" 

 (Phil. Mag. [0'] ix. 1905, p. 104) Rayleigh has investigated 

 some of the general conditions under which the preponderance 

 of attraction over repulsion can come to pass, and obtains 

 strong confirmation of its reality. He considered briefly the 

 important point as to the time during which any particular 

 value of the virial prevails. Van der Waals, Jr., has 

 recently attacked the problem of the law of attraction for 

 electrical double points in the molecules of a gas (Kon. Akad. 

 van Wet. te Amsterdam, 8 Sept. 1908, p. 132). He takes 

 any two molecules at random and writes down the expression 

 for their potential energy in terms of their distance apart 

 and the directions of their axes. He then uses the law of 

 Boltzmann to express the number of pairs of molecules which 

 are nearly in this same relative position within a range ex- 

 pressed in the usual way by differentials, and then evaluates 

 the average attraction between two molecules at distance r 

 apart, finding it to be a sum of powers of r"\ the lowest of 

 which is the seventh. He asserts, therefore, that the theorv 



2 Y 2 



