206 REPORT OF THE CHEMIST OF THE 
thus treated, takes from solution about 5 cc. of N-10oo0 HCl and 
forms with it a definite compound, which, like the casein itself, 
is insoluble. ‘The method of determining by titration with standard 
alkali the amount of acid remaining in the filtrate did not, under 
the conditions employed, yield results that were more than ap- 
proximate. It was thought that more satisfactory results could 
be obtained by measuring the changes in electrical conductivity of 
acids caused by treating the acids with casein. 
The special object of the work described in this bulletin was to 
secure additional data bearing on the question as to whether casein 
combines with acids to form insoluble casein salts of acids, a ques- 
tion which has been in dispute for over fifty years and which has 
an intimate connection with the coagulation of sour milk and also 
with certain stages of cheese-making. While the basic property 
of proteids has long been recognized, it is within the past fifteen 
years that evidence has accumulated, furnishing more specific proof 
that acids combine with proteids to form chemical compounds; and 
the more intimate knowledge gained in regard to the constitution 
of proteids has constituted an essential part of this evidence. This 
view is supported by the work of the following investigators: 
Sjoqvist,! Cohnheim,? Cohnheim and Krieger,? Erb, Bugarszky 
and Liebermann,®> Matthews,* Osborne,’ von Rohrer,’ Panormoff,? 
Hardy,” Mellanby'! and Robertson.” 
It was found that in general the amount of acid combining with 
unit mass of a given proteid in solution was not constant, but de- 
pendent upon the concentrations of acid and proteid. This was 
explained by the hypothesis that the proteids act as weak bases, 
and consequently their combinations with acids hydrolyze. Spiro 

*Skand. Arch. Physiol., 5:277. 1804. 
*Ztschr. Biol., 33:489. 1896. 
*Ztschr. Bioi., 40:05. 1900. 
*Ztschr, Biol., 412300. 1001; 
° Arch. Physiol. (Pfliiger), 72:51. 1808. 
* Amer. Jour. Phystol., 7:445. (1808.) 
"Jour. Amer. Chem. Soc., 24:39. 1902. 
* Arch. Physiol. (Pfiliiger), 90:368. 1902. 
°Jour. Russ. Physiol. Chem. Gesell., 312556. 
Jour. Physiol, 33:333. 1005. 
* Jour. Physiol., 33:373. 1905. 
Jour. Biol. Chem., 2:317. 
