JACQUES LOEB AND ROBERT F. LOEB 199 



ing at a short distance, while the originally rough surface of the grain of 

 casein becomes smooth. These phenomena of solution accompanied 

 by spreading occur soon all over the surface. The small particles of 

 casein carried away in the spreading motion are gradually completely 

 dissolved and the result is a solution of Na caseinate of a high osmotic 

 pressure, indicating that the solution consists to a large extent of 

 isolated protein ions. 



The mechanism of the solution of casein in NaOH is essentially 

 that of the solution of a crystal except for the purely accidental and 

 secondary phenomenon that the surface tension of the Na caseinate 

 solution is considerably lower than that of a watery NaOH solution, 

 and that thus the orderly process of solution noticeable in the case 

 of crystals is disturbed by phenomena of spreading. The mechanism 

 of solution of grains of solid casein in NaOH resembles the solution of 

 sodium oleate in water, which is also accompanied by a lowering 

 of the surface tension at the interface. This interference of the 

 phenomena of spreading in the process of solution of Na caseinate 

 may account for the fact that Robertson^ was unable to confirm in 

 this. case the solution theory of Noyes and Whitney,^^ according to 

 which there exists at the boundary of crystals and water a film of 

 solution which is always saturated, the velocity of solution being 

 determined by the rate of dift'usion of the dissolved crystalloid out 

 of this film into the body of the fluid. It is natural that this theory 

 should be masked whenever the process of solution is accompanied 

 by phenomena of spreading due to a lowering of surface tension at 

 the interface. The forces dragging the particles of Na caseinate 

 into solution are to all appearances those forces of residual valency 

 between certain groups of the molecule of Na caseinate and water 

 which also cause the solution of crystalloids in water, while the 

 forces dragging the particles of casein chloride into water are forces 

 which have their origin in the Donnan equilibrium. A quotation 

 from Langmuir will illustrate the role which active groups of a com- 

 plex molecule may play in the process of true solution. 



"Acetic acid is readily soluble in water because the COOH group has a strong 

 secondary valency by which it combines with water. Oleic acid is not soluble 

 because the affinity of the hydrocarbon chains for water is less than their affinity 



1=" Noyes, A. A., and Whitney, W. R., Z. physik. Chem., 1897, x.xiii, 689. 



