New York AGRICULTURAL EXPERIMENT STATION. 235 
of the acid is removed from solution before appreciable solution 
of casein occurs. The fact that the acid is thus taken up to form 
an insoluble substance with casein being shown, the question 
remains: Is the taking up of acid by casein a chemical reaction and 
is the resulting insoluble casein-acid substance a definite chemical 
compound, or is the process one of adsorption? The results which 
follow appear to indicate that the behavior of the reaction places it 
in the class of adsorption reactions, of which the adsorption of dyes 
and dilute acids by silk forms a parallel example.t. Our results 
point to the following general conclusions: 
(1st) Contrary to the former belief of Van Slyke and Hart, 
there is no definite insoluble chemical compound formed; but the 
amount of acid taken up by casein varies, temperature remaining 
constant, in proportion to the concentration of the acid in the 
surrounding solution. The ratio found by dividing the amount 
of acid taken up by 1 gram of casein by the amount of acid in 
I cc. of the surrounding solution is a constant under uniform condi- 
tions and can be established either by acid-casein giving up acid 
to surrounding water, or vice versa, the acid going either way 
according to the initial concentration. 
(2d) The variation in the amount of acid taken up by casein 
does not appear to be due to the hydrolysis of a casein-acid salt 
but rather appears to occur in accordance with the behavior of 
adsorption reactions, as will be pointed out in later discussion. 
In carrying on our study, it was important to employ conditions 
under which the reaction reaches equilibrium without solvent action 
on the casein. We found it necessary to use acids not stronger than 
N-125 and to carry on the experiments at 0° C. in order to avoid 
solution before the casein had taken up all the acid it could. Even 
under these conditions, long standing in contact with acids usually 
resulted in perceptible solution of casein, but to a degree so slight 
that it did not appear to affect the results, except possibly in the 
case of the 24-hour periods with N-125 acid. 
*Schmidt. Zischr. Phys. Chem., 15:56. 1804. 
Walker and Appleyard. Jour. Chem. Soc. [London], Trans., 69:1334. 
1806. 
