New York AGRICULTURAL EXPERIMENT STATION. 209 
acid in the filtrate, measured as free acid, is too high, since the 
titration represents not only the acid that has not been taken up by 
casein but also that which has combined to form soluble compounds 
and, in addition, the acidity of the dissolved proteid itself. When 
the work previously published was done, the ease with which casein 
dissolves in very dilute acids was not fully appreciated, and the 
conditions then employed permitted some solution, as we have since 
learned by repeating some of the former work. 
When no solution occurs and conditions permit the use of acids 
sufficiently concentrated for accurate titration, results by titration 
agree closely with those obtained by measuring electrical con- 
ductivity. In order to avoid solution, it was necessary to use dilu- 
tions as high as N-1ooo or to have the reaction occur at low 
temperature. In the case of high dilution, accurate titration was 
impossible, even when solution was prevented, and therefore con- 
ductivity measurements were used in all cases as the basis of our 
work. 
When we began the work embodied in this article, we expected 
simply to measure more accurately the amount of acid that would 
combine with one gram of casein to form a definite compound in- 
soluble in water; but we soon found the problem less simple than 
we had supposed. In one set of experiments, we treated varying 
amounts of casein with a fixed amount of acid, the larger amounts 
of casein being, theoretically, more than sufficient to take up all the 
acid, judging from previous results. However, we found that some 
free acid remained in the filtrate in every case and, moreover, the 
acid was taken up in different proportions by the casein. Also, in 
treating casein with acids of different concentrations, it was found 
that the amount taken up varied with the different concentrations of 
solution. The phenomena were such as to suggest something else 
than simple, definite chemical combination ; and our study practically 
resolved itself into an effort to decide what hypothesis was best sup- 
ported by facts,— whether that of (1) definite chemical combina- 
tion accompanied by hydrolysis, or (2) solution of acid in proteid, or 
(3) adsorption. 
In order to obtain data which might be utilized in coming to a 
decision in regard to the kind of action involved, the behavior of 
casein was studied (1) with acids of different dissociating power, 
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