SURFACES 19 



ganic salts increase the surface tension of water and they are, ac- 

 cordingly, less concentrated at the surface than in the interior of 

 the solution. This latter occurrence, negative adsorption, is of 

 significance for the distribution of salts in cells, as will be shown on 

 page 25. 



"Adsorption," which is of the greatest significance in colloid 

 research, will, in the subsequent paragraphs, be considered from the 

 standpoint of the distribution of a dissolved substance between two 

 phases. In this connection also reference should be made to page 

 33 (surface pellicles). 



CHEMICAL COMBINATION, SOLUTION, ADSORPTION. 



We have seen that colloids are diphasic systems, and the ques- 

 tion must arise as to how a third substance will be distributed 'between 

 the two phases. 1 



Chemical Combination. 



If we take as the dispersed phase, a suspension of calcium carbon- 

 ate (precipitated chalk) in water, this will represent the solution of a 

 hydrophobe colloid. If we add sulphuric acid to the suspension, the 

 acid will be immediately and completely bound. It is impossible to 

 detect free sulphuric acid in the suspension by any reagent if any 

 calcium carbonate still remains in the suspension. 2 If we continue 

 with the addition of sulphuric acid, a point will suddenly occur when, 

 no matter how much is added, sulphuric acid is no longer bound by 

 the chalk; all the excess remains in the water. We are accustomed 

 to say that a chemical reaction has occurred between the calcium 

 carbonate and the sulphuric acid and that there is a chemical union 

 of Ca and 864. Ca unites firmly with a definite quantity of 864. 

 We may add as much water as we want; it cannot abstract any 

 SC^from the CaSC>4. The process is irreversible (cannot be reversed). 



Solution. 



If we emulsify carbon disulphid in water, and add a little bromin, 

 the entire fluid is colored brown. If we allow the carbon disulphid 

 to settle, the water is light brown and the carbon disulphid is colored 



1 In this instance "distribution" is used in the most general sense, though in 

 physical chemistry it is employed only to indicate the distribution of a sub- 

 stance between two solvents. 



2 Although the formation of salts on mixing dissolved acids and bases results 

 with infinite rapidity, it takes an appreciable time in the case of colloid solutions 

 (and of course with coarse suspensions). VORLANDER and HABERLE, 



