J. A. Prescott 121 



Similarly a silica gel was treated with calcium hydroxide and after- 

 wards with potassium chloride solution. The potassium removed much 

 of the calcium but the chloride radical remained in solution. 



He concludes^ that the phenomena of absorption in soils are of the 

 same nature as in artificial calcium aluminium silicates brought into 

 contact with the solutions of salts of the alkalies. Thus the soil 

 absorptions fall into line with other absorption phenomena. 



Adsorption. 



Much of the recent work on the absorptive power of soils has been 

 of a physico-chemical nature as it has been recognised that more 

 definite knowledge as to the nature of the phenomena will be obtainable 

 by this means. Before proceeding to the discussion of these develop- 

 ments it will be necessary to deal with the general phenomenon known 

 as "adsorption." This subject has received considerable attention 

 during the last ten years from the physical chemists and more recently 

 from the physicists^. "Adsorption" is the concentration of any sub- 

 stance on the surface of another. The power of charcoal to absorb 

 gases and to remove colouring matter from solution is perhaps the 

 most familiar example. 



Certain phenomena occur at the surface bounding any two phases of a 



system which are generally associated with the effect of Surface Tension. 



In the case of a liquid in contact with a gas this tension is a well defined 



and measurable constant. The surface energy of any system tends to 



become a minimum ; in the case of a liquid in contact with a gas, the 



liquid tends to become a perfect sphere, which has the minimum surface 



and hence the minimum surface energy. Where the bulk of the liquid 



is too great and the force of gravity prevents the liquid becoming 



spherical the surface remains practics^rily constant but the surface 



energy nevertheless falls if the opportunity occurs. The addition of 



soluble substances to a liquid usually changes the surface tension. 



If an increase of concentration decreases the surface tension, then a 



concentration of the solute occurs in the surface layer ; in other words 



adsorption takes place. If however the addition of a solute increases 



the surface tension, the concentration of the solute in the surface layer 



diminishes, giving rise to "negative adsorption." These deductions 



from thermodynamical laws have been confirmed by experiments. 



Miss C. C. Benson^ has shown that the froth obtained from a solution 



' Die Absorption, p. 433. ^ See Trouton, Brit, Atsn. Report, 1914. 



3 Journ. Phys. Chem. 1903, 7, 532. 



