254 REPORT OF THE CHEMIST OF THE 
The tenacity with which sulphuric acid is held by casein is inter- 
esting as explaning the difficulty which Hammarsten! found in try- 
ing to remove sulphuric acid from casein by rubbing in a mortar 
with successive portions of water. Van Slyke and Hart? found 
that it required 80 or 90 triturations with successive portions of 
fresh water to remove completely the sulphuric acid from casein. 
It also appears, from the readiness with which a given amount of 
water extracts the maximum amount of acid by simply shaking 
with the acid-casein, that a few minutes of agitation with each of a 
number of successive portions of pure water should remove the 
acid as well as trituration. 
From the varying levels approached by the curves of Fig. 13, 
it is apparent that the equilibrium ratio of acid in I gram casein 
to acid in I cc. water is, as already mentioned above, not a constant, 
even within the limits of concentration tested, but increases with 
the dilution. In order to follow the variation of the ratio, the fol- 
lowing experiment was performed: 
.4795 g. of casein was shaken for two and one-half hours with 
100 cc. of N-250 H, SO,. As indicated by Table XV and the curves, 
this time is sufficient for equilibrium to become practically complete, 
and not long enough to result in appreciable solution of proteid. The 
suspension was allowed to settle until the supernatant solution was 
perfectly clear, then 50 cc. of the latter drawn off with a pipette and 
replaced by an equal volume of conductivity water. The mixture 
was then shaken two hours and 50 cc. of the solution again replaced 
by conductivity water; and this process was repeated twice more. 
The 50 cc. portions drawn off were first tested for conductivity, 
then titrated with N-100 KOH. In no case was a precipitate noticed 
on titrating, and the titration results, considering the dilution of 
the solutions, agree throughout satisfactorily with those obtained 
by conductivity. Evidently no appreciable solution of proteid 
occurred. 
* Jahresber. Tier-Chem., 7:160. 1877. 
*N. Y. Agrl. Expt. Sta. Bull. No. 261, p. 14. 
