346 REPORT OF THE CHEMIST OF THE 
chlorid may be approximately measured by taking the quantity 
that will easily lie on the surface of a silver dime for one 
quart of milk or, more conveniently, the amount held by a 
(.22-inch, pistol cartridge-shell 14 inch long, when loosely 
filled. A stiff wire soldered to such a shell makes it convenient 
to handle. Commercial mercuric chlorid tablets containing 
coloring-matter can not be used. 
(8) Summary of precautions. Below we give in outline the 
special points to be observed with care in performing the 
operations of the test, assuming that the graduated glassware 
is accurate and the solutions of correct strength. 
(a) Preliminary neutralization. In the neutralization of 
the sample of milk, excess of alkali must be avoided, which can 
be controlled by the use of a properly prepared color-standard. 
(b) Conditions of precipitation. Before precipitating with 
acid, have the dilute, neutralized milk at a temperature be- 
tween 65° and 75° F. Add enough acid to cause the casein 
to separate promptly in large flakes, leaving the supernatant 
liquid clear. Shake the mixture vigorously at intervals during 
the addition of acid; also after complete precipitation and 
again after dilution to the 200 cc. mark. 
(c) Filtration. Allow most of the liquid to run through the 
filter before making the final titration with alkali. 
(d) Titration with alkali. In titration the filtrate with 
alkali, avoid an excess of alkali. Add the alkali solution 
cautiously until, after thorough agitation, a faint but distinct 
pink color remains through the solution half a minute or 
longer. The same uniform shade and duration of pink color 
should be obtained as nearly as possible in all cases. 
(e) Acid milk. Milk that is sour or that coagulates on heat- 
ing should not be used. 
(f) Use of preservatives. Milk treated, when fresh, with 
a small amount of powdered mercuric chlorid and then kept 
in a cool place gives good results for two or three weeks. 
