CHAPTER VI 



A NOTE ON ADSORPTION 



An important corollary to, or amplification of, the theory of 

 surface-tension is to be found in the chemico-physical doctrine of 

 Adsorption; which means, in a word, the concentration of a 

 substance at a surface, by reason of that surface-energy of which 

 we have had so much to say*. Charcoal, with its vast internal 

 surface-area of carbonised cell- walls, is the commonest and most 

 famihar of adsorbents, and of it Du Bois Reymond first used the 

 name. In its full statement this subject becomes very complicated, 

 and involves physical conceptions and mathematical treatment 

 which go far beyond our range. But it is necessary for us to take 

 account of the phenomenon, even though it be in the most elemen- 

 tary way. 



In the brief account of the theory of surface-tension with which 

 our last chapter began, it was shewn that, in a drop of liquid, the 

 potential energy of the system could be diminished, and work mani- 

 fested accordingly, in two ways. In the first place we saw that, 

 at our liquid surface, surface-tension tends to set up an equilibrium 

 of form, in which the surface is reduced or contracted either to the 

 absolute minimum of a sphere, or at any rate to the least possible 

 area which is permitted by the various circumstances and conditions ; 

 and if the two bodies which comprise our system, namely the drop 

 of Uquid and its surrounding medium, be simple substances, and 

 the system be uncomphcated by other distributions of force, then 

 the energy of the system will have done its work when this 

 equihbrium of form, this minimal area of surface, is once attained. 

 This phenomenon of the production of a minimal surface-area we 

 have now seen to be of fundamental importance in the external 



* Some define adsorption as surface-condensation, without reference to the 

 forces which produce it; in other words they recognize chemical, electrical and 

 other forces, including cohesion, as producing analogous or indistinguishable 

 results: of. A. P. Mathews, in Physiological Reviews, i, pp. 553-597, 1921. 



