CASEIN 95 



from them by prolonged trituration with water (23) (16). To 

 argue from this fact that these combinations are not chemical in 

 character would be analogous to concluding that, because salicylic 

 acid can be completely removed from its solution in water by 

 repeated extraction with ether, therefore the salicylic acid was not 

 truly dissolved in the water. 



To return to the more specific problem of the occurrence of 

 combinations between inorganic acids and casein. Van Slyke and 

 Hart (59) and Van Slyke and Van Slyke (60) have described a 

 series of compounds of casein with inorganic acids which are 

 insoluble in distilled water. When solid casein is suspended in a 

 dilute solution of an inorganic acid, the protein abstracts some of 

 the acid from the solution, forming these insoluble compounds. 

 Van Slyke and Van Slyke measured the velocity with which the 

 acid is abstracted from the solution and the final equilibrium at- 

 tained by abstracting portions of the well-stirred suspension, from 

 time to time filtering, and measuring the electrical conductivity 

 of the filtrate. The decrease in conductivity, from the moment of 

 introduction of the casein, afforded a measure of the amount of 

 acid bound. They found that the acid is bound at first rapidly 

 and later more slowly (Cf . Chap. XII) and at equilibrium the ratio 



acid bound by one gram casein 

 acid in one cc. of solution 



is nearly constant between the limits of concentration of the acid 

 employed (A^/125 and A^/1000) for hydrochloric acid at constant 

 temperatures. When sulphuric acid is employed the ratio increases 

 with dilution of the acid, the quantity of acid bound by one gram 

 varying as the square root* instead of directly as the concentration 

 of the acid. The equilibrium is the same from whichever direction 

 it is approached, whether by suspending the protein compound in 

 more dilute acid than that in which it was formed, in which case 

 it yields acid to the solution, or by suspending uncombined casein 

 directly in the solution of acid. When one gram of casein is sus- 

 pended in 100 cc. of iV/500 acid at 0° C, it takes up 17.4 X 10-^ 

 equivalents of sulphuric, 11.9 X 10~^ equivalents of hydrochloric, 

 8.9 X 10~^ equivalents of lactic or 5.3 X 10~^ equivalents of acetic 



* The exact exponent obtained by Van Slyke and Van Slyke is 1/1.95; this 

 experimental value is so nearly I that it affords no grounds for preferring the 

 experimental value to that which is indicated by stoichiometry. 



