PASSAGE INTO SOLUTION 287 



suspended particles of casein takes place in accordance with the 

 equation x = Kt"", although in this case either no soluble com- 

 pound of casein is formed or, which is more probable, the soluble 

 compound which is formed is unable to issue forth from the 

 colloid particles within which it arises (88) (89). 

 Differentiating the equation: 



X = KV^ 

 we find 



dx 



dt ^"^^ ' 



in other words, the product Km, which has been termed by- 

 Robertson and Miyake the coefficient of penetration, expresses 

 the constant proportionality between the velocity of solution and 

 an exponent (peculiar to each solvent) of the time during which 

 the protein has been exposed to the action of the solvent. The 

 coefficient of penetration is, in the case of the solution of casein 

 by dilute sodium hydroxide, very characteristically affected by 

 the presence in the solvent of inorganic salts. The magnitude 

 of the coefficient of penetration decreases with increasing con- 

 centrations of NaCl, KCl, CaCl2, SrCl2 or BaCl2, the acceleration 

 of the decrease being positive in the cases of NaCl and KCl and 

 negative in the cases of CaCL, SrCl2 or BaCl2. Lithium chloride, 

 on the other hand, increases the value of the coefficient of pene- 

 tration with a negative acceleration, so that at concentrations 

 of this salt lying above 0.33 normal the algebraic sum of these 

 two opposite effects results in a decrease of the value of the 

 coefficient of penetration (89). 



If the rate of solution of casein by alkalies were primarily 

 determined by the rate of penetration of the capillary pores in 

 the suspended colloid particles by the solvent, then the addition 

 to the solvent of any substance which markedly reduces the 

 tension of a solid-water interface should retard the rate of 

 penetration and, consequently, the rate of solution of the casein. 

 It has been found by Robertson and Miyake (90) that alcohol 

 and glycerol, both of which reduce the tension of a solid-water 

 interface, also retard the solution of casein by dilute sodium 

 hydroxide. The penetration-formula x = Kt^ expresses the re- 

 lationship between the quantity of casein dissolved and the 

 time of stirring of the mixture in all mixtures of glycerol and water 



