Internal Pressure of a Liquid to its Dielectric Capacity. 109 



If we consider only those liquids whose molecular volumes 

 and refractive indices (for long waves) are of the same order 

 of magnitude, it is evident that in such cases it would be 

 directly proportional to K. This is suggested as the theo- 

 retical basis for the Obach-Walden generalization, the more 

 correct relation between it and K being given by the above 

 equation. 



The Obach-Walden relation is therefore of considerable 

 significance as regards the origin of cohesion between mole- 

 cules. If cohesion were electrostatic in nature, one would 

 expect 7r to vary inversely as K ; the fact that there is ap- 

 proximate direct proportionality appears to be explicable only 

 upon an electromagnetic basis of molecular attraction. 



The significance of the K ternras employed above calls 

 for remark. It refers evidently to the dielectric capacity of 

 the medium separating the molecules. If the molecules were 

 very far apart (i. e. a dilute gas) the medium would be 

 practically unaffected by the presence of the molecules, and 

 K would be a constant having the value unity (E.S.U.). In 

 such a case the above expression becomes identical with 

 van der Waals* term (the numerical value being at the same 

 time very small compared to its value in the liquid state). 

 In the case of liquids with closely packed molecules, the 

 intervening space will be modified as regards dielectric 

 capacity, and the assumption involved above is that either 

 this quantity is actually evaluated when a dielectric constant 

 measurement is carried out, or at any rate that the dielectric 

 capacity of the medium between the molecules is proportional 

 to the observed dielectric capacity. 



That cohesion between molecules is magnetic in origin has 

 been recently suggested by A. P. Mathews (Journ. Physical 

 Chem. xvii. p. 481, 1913), who writes: " In this view the 

 atoms [in a molecule] would be united by their electrostatic 

 affinities, and these same valencies and the other atomic 

 electrons by their magnetic effects produce the molecular 

 cohesion." Whether this is actually the relative mechanism 

 of the two kinds of valency or not is rather a matter of 

 speculation. 



We may then consider the question of cohesion from the 

 standpoint of the electron theory as applied to liquid dielec- 

 trics. The force — due to surrounding molecules — tending 

 to draw an electron out of a given molecule, is i7rne 2 .u, where 

 x is the distance through which the electron is displaced, 

 n the number of electrons per unit volume of the substance, 

 and e the charge upon an electron. If the force exerted upon 

 the electron by the molecule to which it belongs is fx the 



