200 SCIENCE PROGRESS 



which, after having been employed for many years to cover 

 a variety of phenomena apparently similar in character, has 

 attached to itself within recent years a much more restricted, 

 but at the same time much more definite meaning. The 

 result is that although the term is less comprehensive, its 

 application is one of much greater precision, and therefore of 

 more scientific value. The kind of phenomenon we are con- 

 sidering depends essentially upon the existence of a surface 

 or interface separating two homogeneous phases. It is known 

 that all such surfaces are characterised by a peculiar distribu- 

 tion of intermolecular forces, which give rise to surface-tension. 

 These surfaces are also characterised by the existence of an 

 electrical potential difference acting across the surface. Adsorp- 

 tion is primarily a capillary surface-tension phenomenon. 

 When the electrical forces enter into the result, the extra 

 effect produced is termed electrocapillary adsorption. It is 

 theoretically possible to eliminate the latter effect or reduce it 

 to negligible dimensions by working under conditions in which 

 the pure capillary effects predominate. In the present survey 

 of the problem the two effects will be considered separately. 



Capillary Adsorption — Gibbs' Generalisation 



Willard Gibbs, in his now celebrated memoir on equilibrium 

 in heterogeneous systems, was the first to consider the problem 

 of phase-equilibrium when modified by capillary forces. The 

 method of investigation involves a very generalised thermo- 

 dynamic treatment. The important conclusion reached by 

 Gibbs may be stated as follows : If a substance (a gas or a 

 dissolved substance) possesses the property of lowering the 

 interfacial tension which exists between the gaseous or solu- 

 tion phase and some second phase which may be either liquid 

 or solid, then the substance will be attracted to the interface, 

 so that its concentration in the interfacial layer becomes 

 greater than its average concentration throughout the bulk of 

 the gaseous or solution phase. If, on the other hand, the 

 substance considered possesses the property of raising the 

 interfacial tension, the substance will be repelled from the 

 surface and its concentration in the surface-layer will be less 

 than that in the bulk of the homogeneous phase. In the 

 particular case in which the substance exerts no effect upon 



