342 REPORT OF THE CHEMIST OF THE 
for some time with mercuric chlorid, usually from 5 to 10 ce. 
or even more may be required. One can usually add 2 or 3 
cc. of alkali at the start and then add it in smaller portions, 
until the milk begins to show signs of neutrality. After that 
the alkali is added a drop at a time, the flask being shaken 
and the color being observed after each addition. The opera- 
tion of neutralizing should be performed only in a good light, 
sufficiently strong to enable one to observe slight changes of 
coloration. Persons accustomed to the determination of the 
acidity of milk, cream, etc., should have no difficulty in obsery- 
ing the proper color point that indicates neutrality. A little 
experience, especially under proper instruction, will enable 
one to perform this step with rapidity and accuracy. 
(3) Precipitation of casein. (a) Addition of acid.—Into the 
neutralized sample of diluted milk, which should be at a tem- 
perature of 65° to 75° F., one now runs from a burette some 
of the standardized acetic acid, adding the acid approximately 
in 5 cc. portions and agitating vigorously for a few seconds 
after each addition. It is usually safe to add about 25 ce. 
of acid before examining the milk to see if the casein sepa- 
rates in the form of white flakes. After adding 20 to 25 ce. 
and shaking, the mixture is allowed to come to rest. If 
enough acid has been added, the casein separates promptly in 
large, white flakes, and, on standing a short time, the liquid 
above the settled casein appears clear and not at all milky. 
If the addition of 25 cc. of acid is insufficient to separate the 
casein properly, add 1 cc. more of acid and shake; continue 
the addition of acid, 1 cc. at a time, until the casein is observed 
to separate promptly and completely on standing at rest a 
short time. The number of cubic centimeters of acid used to 
effect precipitation is noted and this result is recorded as A. 
(b) Infiuence of temperature.—For convenience of work and 
uniformity of results, the temperature of the mixture at the 
time of the addition of acid may be between 65° and 75° F. 
Under these conditions, we have found that in most of the 
milks with which we have worked 30 cc. of acid gives satis- 
factory resuits. In some cases, especially with the milk of 
