﻿/ d> 



Thus 



P 9 / S l-V 1/E 



Breaking Stress of Crystals of Rock- Salt. 63 



For the surface tension, the expression was given as 



d 



or assuming ~r=P where p is the number of molecules per 

 unit volume, the expression for the surface tension a becomes 



u = kl 2 p^; 



instead of p we may put l 2 , where 6\ is the density of 



the liquid, B 2 that of the saturated vapour, and M the 

 molecular weight. 



-=«■(¥•)" (i > 



It was also shown that the internal pressure P can be 

 calculated by the formula 



P = 2«p 1 / 3 (2) 



) (3) 



In other words, the intrinsic pressure can be calculated 

 from the surface tension if the molecular weight of the 

 liquid is known. Thus for normal or non-associated liquids 

 it should be possible to calculate the normal pressure from 

 the value of the surface tension. 



It should be pointed out that the assumption made in the 

 above theory, that the length of the doublet is small compared 

 with the intramolecular distance, is not necessarily the right 

 one. 



In the expression (2) this law is, however, eliminated, and 

 the same expression is obtained for any law of molecular 

 attraction. The figures for P obtained from the above 

 expression agree with those from indirect evidence. How- 

 ever, experimentally it is not possible to determine P 

 directly, owing to the mobility of the particles of liquids 

 which always adjust themselves so that the molecular 

 pressure is inappreciable. 



It is not so in the solid or crystalline state, in which the 

 particles have a definite orientation, and where the internal 

 pressure can be determined by a direct experiment. It is 

 sufficient to apply to a crystalline body such a weight as 

 would overcome the attraction of the molecular forces and 

 cause the disruption of the body. The force applied is not 



