Tension and Complex Molecules. 383 



particular therefore at absolute zero which is a corresponding 

 temperature for all liquids. 



When the temperature rises the effect on the liquid can 

 be represented by an equivalent diminution in the effective 

 value of /, which approaches zero in the neighbourhood of 

 the critical point. In order to express the diminution of the 

 attractive forces between the doublets when the temperature 

 rises it is sufficient, for example, to suppose that the doublets, 

 instead of being arranged vertically, commence to move in 

 such a manner that their charges describe circles while their 

 centres remain stationary, so that their areas in fact describe 

 oones. 



As the temperature rises, these cones tend to become 

 flatter, until finally the two charges are describing the 

 same circle, and the entire doublet moves in a horizontal 

 plane. If we recollect that this type of movement must 

 increase with the temperature, it is evident that the effec- 

 tive value of I must decrease, and reach the value zero 

 when the doublet no longer exercises attractive forces on 

 the average. 



As for the magnitude p, it is merely the specific gravity 

 of the liquid divided by its molecular weight. For certain 

 reasons, however, it is necessary to replace this specific 

 gravity by a smaller value, the difference between the 

 density of the liquid d x and that of its saturated vapour d 2 = 

 In this case the formula for the normal pressure becomes 



J/«(^) 2 > (2) 



where f(t) replaces Z 2 , and J is constant. The molecular 

 weight is M. In a paper by Kleeman * a formula very 

 similar to this is derived from considerations of a quite 

 different character. 



According to Kleeman, the surface tension is 



=K '"(^) 2(SGa)2 ' 



where (£(;a) 2 is a constant, K'" is a quantity which is the 

 same for all liquids at corresponding temperatures, pi and p 2 

 are the densities of the liquid and of its saturated vapour, 

 and m is the molecular weight of the liquid. This expres- 

 sion for the surface tension accords with the properties of 



* Phil. Mag. xix. p. 784 (1910). 

 2 D 2 



