384 Prof. G. N. Antonoff on Interfacial 



liquids in so far as it vanishes at the critical point. More- 

 over, di — d 2 becomes indefinitely small near the critical 

 point and appears in the expression to the second power, 

 while f[t) also approaches zero at the critical point. The 

 formula, in fact, indicates the same phenomenon which is 

 found in practice, for the surface tension is effectively zero* 

 somewhat before the critical point. Laplace's theory, while 

 embracing a whole series of phenomena, is not satisfactory 

 in this respect, and the theory of van der Waals, which is 

 based on the conception of a continual passage from the 

 liquid to the gaseous state, appears to be more suitable ; we 

 must admit also that the density of the liquid is variable, 

 and that near the surface it passes by degrees into the 

 density of the vapour of the same liquid. 



Let us consider how such phenomena can be represented 

 from the point of view of the kinetic theory. 



The particles of a liquid are in motion like those of a gas, 

 but are characterized by a much smaller mean free path. 

 Some particles, with a velocity greater than the mean, 

 detach themselves from the liquid surface and enter the 

 surrounding medium to form a saturated vapour. When 

 equilibrium is reached, equal numbers of particles enter the 

 surface and are detached from it. In this manner, the sur- 

 face is in continual bombardment on two sides, and it is 

 therefore quite natural to attribute special properties to it. 



But in this case the properties of the surface must change 

 radically, if it is in contact with another liquid instead of its 

 own vapour. In fact, in the latter case, particles approach 

 the surface which have, in the two media, very different free 

 paths, whereas at the boundary of two liquids, the molecules 

 in the two media have mean paths of the same order of 

 magnitude and characteristic for the two liquids. 



The available evidence appears to support the opinion, that 

 the surface of a liquid, when in contact with another liquid, 

 retains the same properties which it had while in contact 

 with its own vapour (opinion of Planck) *. The opposite 

 view does not, in fact, lead us to results in agreement with 

 experiment (Kantor) f . Tamman J considers that if the 

 liquid passed to the gaseous state by jumps the law 



— = const. 

 a 



should be true. But according to the theory developed in 



* Thermodynamik, Leipzig-, 1905, p. 175. 

 f Wied. Ann. lxvii. p. 687 (1899). 

 X TJeber die Beziehungeu, p. 175. 



i 



