210 REPORT OF THE CHEMIST OF THE 
(2) with acids of different concentrations, (3) for different periods 
of time, (4) at different temperatures, and (5) with dilute solutions 
of neutral salts. 
The acids used were hydrochloric, lactic, sulphuric and acetic. 
The concentrations of the acids used were, approximately, N-125, 
N-500, N-1o00 and N-2000. 
The periods of time used for keeping casein and acid in contact 
varied from one minute to forty-eight hours; the periods usually 
employed were 5, 15 and 45 minutes, 114, 3 and 6 hours, in the case 
of experiments at room temperature or higher; while additional 
periods of 12, 24, and sometimes 48 hours were used in the case of 
experiments at 0°. C. The temperatures used were 0°, about 25° 
(room temperature) and 45° C. The study of hydrochloric and 
sulphuric acids was made to cover more details than in the case of 
the other acids. 
DESCRIPTION OF METHODS, APPARATUS AND RE- 
AGENTS USED. 
METHOD OF PREPARING CASEIN. 
To 1 liter of fresh, separator skim-milk, we add about 6 liters 
of distilled water and enough dilute acetic acid (10 to 15 cc. of 
strong acid diluted with water to a liter) to cause complete pre- 
cipitation of casein, avoiding any marked excess of acid. The 
mixture is kept vigorously agitated while the acid is being grad- 
ually added. The precipitate is allowed to settle, the superna- 
tant liquid is decanted or siphoned off, after which the precipi- 
tate is washed with copious amounts of distilled water, until the 
wash-water no longer shows an acid reaction to litmus. The pre- 
cipitate is then treated with just enough dilute ammonia water 
(about 5 cc. of strong ammonia diluted to a liter) to dissolve the 
casein, forming a solution neutral to litmus. The solution is diluted 
to about 6 liters with distilled water and reprecipitated by dilute 
acetic acid. Much less acid is required for the second and subsequent 
precipitations than for the first. The precipitate is washed free 
from acid as before and then redissolved in dilute ammonia. Re- 
precipitation and redissolving should be performed five or six times. 
The casein should not be allowed to stand longer than necessary 
