SURFACES OF DISCONTINUITY 541 



accent refers we increase the amount of the r*'^ component in 

 the conceptual system, in which the two homogeneous phases 

 are assumed to extend right up to the dividing surface, by 

 ^(7r' — 7r") estimated per unit area of the surface, and so we 

 diminish the value of Tr by this amount, so that the new Tr is 

 equal to the old Tr — X(Tr' — y/'); if we choose X to be 

 equal to Ti/iyi — 7/'), this obviously makes the new Ti zero, 

 and the new Tr, i.e. Tra), equal to 



71 ~ Ti 



which is the result [515] once more. 



VII. Other Adsorption Equations 



Having commented on the derivation and form of Gibbs' 

 adsorption equation we will refer briefly to other equations, 

 which have been suggested empirically or derived in other ways, 

 concerning the concentration of components at a surface of dis- 

 continuity. Some of these refer to adsorption at solid surfaces 

 just as much as at liquid surfaces; indeed in their derivation the 

 conditions at solid surfaces have been more in the minds of 

 their originators when developing their views. In such cases 

 the concept of surface tension hardly has any bearing on the 

 matter; but of course surface energy is a wholly justifiable term to 

 use, although in the nature of things it is only at liquid-vapor 

 or liquid-liquid interfaces that measurements of change of 

 interfacial energy are practicable. This, however, is a minor 

 matter, as it happens that the surface tension does not enter 

 into many of these laws, apart from the one derived by J. J. 

 Thomson, and a few others. Nevertheless, in the discussions 

 concerning the validity of the Gibbs relation it is hardly possible 

 to avoid making some reference to a few of these other proposed 

 forms of adsorption laws, and that must serve as an excuse for 

 making a brief reference to two or three of the most important 

 of them. For a very adequate account of the complete group 

 of laws the reader is referred to a rev^iew of the literature by 

 Swan and Urquhart in the Journal of Physical Chemistry, 

 31, 251-276 (1927). 



