208 REPORT OF THE CHEMIST OF THE 
acid equivalents of the proteids, but, as von Rohrer points out, 
physico-chemical methods must be employed in order to ascertain 
the degree of hydrolysis of the proteid and the amount of acid it 
holds as it exists in solution. 
Several physico-chemical methods have been employed to meas- 
ure the acid combined when no precipitation occurs. Sjéqvist used 
the method of electrical conductivity. Bugarszky and Liebermann 
measured, by means of the electromotive force of concentration 
cells containing acid, the changes in concentration of the latter 
caused by contact with proteid; they also measured the change in 
freezing point due to the addition of various amounts of proteid 
to dilute acid. Cohnheim measured the free acid by the velocity 
of the catalysis of cane sugar. 
However, in the work done by most of the investigators men- 
tioned above, the compounds of proteids and acids which they 
studied are soluble, unlike the insoluble substance formed by treat- 
ing casein with dilute acids. Osborne! has studied the action of 
hydrochloric acid upon neutral edestin suspended in water, con- 
cluding that it forms a monochloride insoluble in water. Leo? 
states that fibrin forms a series of insoluble compounds with hydro- 
chloric acid. Laxa? and Richet* are, so far as we know, the only 
workers who have worked with casein, proceeding upon lines similar 
to our previous work. Laxa regards the insoluble substance formed 
by treating casein with lactic acid as a case of chemical combination. 
He concludes that lactates of casein which contain one per ct. of 
lactic acid or less are insoluble in water, while lactates of casein with 
higher content of acid are soluble. 
When the proteid-acid substance is insoluble, as in the case of 
casein with dilute acids, the amount of acid taken up by the casein 
may be measured by determining through titration how much acid 
remains in the filtrate after the removal of the insoluble casein-acid 
substance. The titration method, as used in previous work, gives 
misleading results, unless the conditions of treatment are such that 
no casein goes into solution. In case of solution, the amount of 
*Jour. Amer. Chem. Soc., 24239. 1902. 
*Ztschr. Physiol. Chem., 46:286. 1905. 
* Milchw. Zentbl., 12538. 10905. 
“Compt. Rend. Soc. Biol. [Paris], 60:650. 1906. 
