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XLII. Interfacial Tension and Complex Molecules. 

 By Prof. G. N. Antonoff*. 



§ 1. A 1 he or y of Surface Tension. 



N order to explain the phenomena of surface tension, it 

 is usual to postulate the existence of forces, sensible 

 only at very small distances, between the molecules of a 

 liquid. The distance at which these forces are still effective 

 is known as the radius of molecular action. They are as- 

 sumed to be inversely proportional to a sufficiently high 

 power of the distance apart of the molecules. Several 

 writers, after Lord Kelvin, have regarded them as propor- 

 tional to the inverse fifth power of distance, but Sutherland f 

 has given strong evidence, based mainly on experimental 

 results, that the inverse fourth power is more suitable. 



Theories of surface tension based on the existence of such 

 forces involve of necessity the conception of " Molecular 

 pressure. " i3ut while surface tension is a real and tangible 

 phenomenon, the same cannot be said of the molecular pres- 

 sure. The absence of direct methods for its determination 

 has even led some writers J to regard it as a purely meta- 

 physical magnitude, and no clear account has apparently 

 been given which expresses precisely the mutual dependence 

 of molecular pressure and surface tension. 



One of the most widely known of such theories, which 

 undoubtedly plays an important role, is that of Laplace, 

 which is, however, of a very general type. But at the 

 present time, our knowledge of the nature of the molecule 

 and the forces which can be associated with it, is much more 

 definite, and it is desirable to work out the consequences of 

 a more definite hypothesis of molecular action which is in 

 general agreement with the present conception of the mole- 

 cule. For it is possible to explain the existence of attractive 

 forces between molecules, diminishing rapidly with distance, 

 without making any special trypothesis for the purpose §. 

 We may suppose that the forces exerted by atoms and mole- 

 cules are essentially of electromagnetic origin, for into the 

 composition of atoms and molecules apparently enter only 



* Communicated by Prof. J. W. Nicholson, F.R.S. 



f Phil. Mag-. [5] xxvii. p. 305 ; ibid. [5] xxxv. p. 112 (1693). 



% Kapillarchemie, Leipzig-, 1909, p. 9. 



§ See also Crehore, Phil. Mag. xxvi. p. 25 (1913). 



