New York AGRICULTURAL EXPERIMENT STATION. 201 
28 hours. In no case was change in conductivity observed. In the 
case of N-125 MgSO, solution similarly treated, there was a de- 
crease of only a few tenths of a per ct. in conductivity. From these 
experiments, it is apparent that the H* ion is the factor in solution 
on which depends the fixation of the large amounts of electrolyte 
previously determined in the experiments with the mineral acids. 
DISCUSSION OF RESULTS. 
In the foregoing pages, it has been shown (1) that when casein 
and a dilute acid, as, for example, hydrochloric acid, are shaken 
together, the final amount of acid taken up by a gram of casein 
without solution of the proteid is not constant but varies contin- 
uously with the concentration of the surrounding acid solution ; 
(2) that, however small the amount of acid used, it is never com- 
pletely taken up; (3) that the amount of acid taken up varies also 
with the temperature; and (4). that the acid passes as readily from 
the casein into the surrounding solution as from the solution into 
the casein, moving either way according to the change required to 
establish the equilibrium ratio, amount of acid in I gram of casein 
divided by the amount of acid in 1 cc. of surrounding solution, as 
shown in the cases of hydrochloric and sulphuric acids. It is evi- 
dent that no acid-casein compound of constant composition 1s 
formed. Three different explanations may be offered to account 
for the facts observed: 
(1) The reaction may be regarded as one of adsorption, the 
insoluble acid-casein being what van Bemmelen calls an adsorption 
compound. | 
(2) Applying Witte’s explanation of the fixing by fibers of sub- 
stantive dyes, the reaction may be one of solution, the acid dividing 
itself between casein and water as resorcinol divides itself between 
ether and water, according to its solubility in each. 
(3) The insoluble acid-casein may be regarded as a hydrolyzible 
salt, its inconstant composition being due to hydrolysis, which varies 
with the temperature and concentration of the acid. 
We will consider each of these hypotheses in their relation to the 
facts that have been developed. 
