272 



THEORY OF COLLOIDAL BEHAVIOR 



isoelectric casein were prepared in 0, M/2,048, M/ 1,024, to 

 M/4 NaCl. The pH of a solution of 1 gm. of casein in 100 c.c. 

 containing 12.5 c.c. of 0.1 N HC1 was 2.12 and this pH was the 

 same in all solutions made up in NaCl. The solution was kept 

 at 20C. for 16 hours and then allowed to settle for 24 hours at 

 20 in 100 c.c. graduated cylinders. The dry weight of the sedi- 

 ment was determined and this weight when deducted from the dry 

 weight of 1 gm. isoelectric casein, namely, 0.870 gm., was the 

 amount that had gone into solution after a correction was made 

 for the free NaCl held in 2 c.c. solution which was arbitrarily 

 assumed not to have been removed. Though this latter correc- 

 tion was somewhat arbitrary, it could have caused a noticeable 

 error only when the concentration of the salt solution exceeded 

 M/64. For the solutions of M/64 and below this error was 

 negligible. Table LII gives the number of milligrams of casein 

 which had gone into solution. 



TABLE LII 



The main fact is that a slight increase in the concentration of 

 NaCl causes a noticeable drop in the rate of solution. Thus, 

 M/1,024 NaCl causes a noticeable diminution in the solubility 

 of a 1 per cent solution of casein chloride of pH 2.12 at 24. 



These observations then indicate that the solution of solid 

 particles of casein chloride is brought about by the ultimate 

 elements being forced apart mechanically through the process of 

 swelling. The force acting in this swelling is the hydrostatic 

 pressure of the water which is forced into the interstices of the 

 solid particles by the osmotic pressure of the solution in the 

 interstices between the casein ions. Procter and Wilson have 

 shown that the application of Donnan's theory of membrane 

 equilibrium accounts quantitatively for this swelling on the 

 assumption that swelling is caused by the excess of the osmotic 

 pressure inside the gel over that of the surrounding solution 



