New York AGRICULTURAL EXPERIMENT STATION. 205 
6. Equilibrium ratio of reaction— The maximum amount of 
acid absorbed by a gram of casein varies almost directly with the 
concentration of the acid within the limits used (N-125 and 
N-1000) in the case of hydrochloric, lactic and acetic acids, so that 
at equilibrium the ratio of acid in 1 gram of casein to the acid in 
I cc. of surrounding solution is nearly constant. In the case of 
sulphuric acid the ratio increases with dilution of acid. The 
equilibrium ratio reached when 1 gram of casein is treated with 
100 cc. of N-500 solution at o°, is 675 for sulphuric acid, 147 for 
hydrochloric, 80 for lactic and 36 for acetic. Equilibrium is reached 
in 2 to 24 hours according to (1) the acid used, (2) the tempera- 
ture, and (3) the degree of agitation. The greater part of the acid 
reacting is generally taken up in the first half hour or hour. In- 
crease in temperature increases the rate at which equilibrium 1s 
approached, but decreases the final amount of acid taken up, when 
solution of casein does not result at the higher temperature. 
7. Extraction of acid from casein.— The acid taken up by casein 
may be extracted by shaking with water. The extraction con- 
tinues until the equilibrium ratio (concentration of acid in I gram 
of casein divided by concentration of acid in 1 cc. of surrounding 
solution) is established, and then ceases. 
8. Neutral salts— Neutral salts (potassium chloride and mag- 
nesium sulphate) are not adsorbed from dilute solution by casein. 
g. The behavior of casein with dilute acids, when no solution 
occurs, suggests three different explanations: (1) an adsorption 
compound, (2) solution of acid in casein, or (3) a hydrolyzible 
salt of casein. A careful application of the experimental results 
to each of these hypotheses indicates that the action appears to cor- 
respond in detail only with the characteristics which apply to an 
adsorption. 
10. On the basis of this explanation, the proteid precipitated 
when milk sours is free casein in which lactic acid is held by ad- 
sorption. 
INTRODUCTION. 
In Bulletin No. 261 it was shown that when base-free casein 
is suspended for a time in dilute hydrochloric acid and then fil- 
tered, the acid is not all recovered in the filtrate. According to 
the results then obtained, it appeared that one gram of casein, when 
