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; 
496 REPORT OF THE CHEMIST OF THE 
cipitation will generally be found incomplete and such a deter- 
mination, if carried out, will give low results. 
(8) Detailed description of method used in determining the 
casein of cows’ milk.—I. In fresh milk: Weigh out about ten 
grams of milk, dilute in a beaker with about ninety cc. of water 
at 40°-42° C., and add at once 1.5 cc. of a solution containing 
ten per cent. of acetic acid by weight. Stir with a glass rod and 
let stand three to five minutes or longer. Then decant on filter, 
wash two or three times with cold water by decantation, and 
then transfer precipitate completely to filter. Wash once or 
twice on filter. The filtrate should be clear or very nearly so. 
If the filtrate is not clear when it first runs through, it can gen- 
erally be made so by two or three repeated filtrations, after which 
the washing of the precipitate can be completed. The washed 
precipitate and filter paper are then digested as in the regular 
Kjeldahl method for the determination of nitrogen and the deter- 
mination of nitrogen is completed as usual. To calculate the — 
nitrogen into an equivalent amount of casein, multiply the 
amount of nitrogen found by the factor 6.25. Ordinarily, for 
this purpose, milk can be regarded as “fresh” when it does not 
show marked development of lactic acid. 
If. In old milk: When milk has undergone such change as to 
show marked development of lactic acid, the method above 
given can not generally be relied upon to give accurate results in 
determining casein. So far as our results go, they indicate that 
we can not with positive accuracy determine casein in such milk 
by any method now known, and results obtained with changed 
milk must be regarded as only approximate. If, however, one 
part of finely powdered mercuric chloride is added to 2,000 
parts of milk, when fresh, the changes, which would otherwise 
take place, are prevented or greatly retarded, so that milk 
treated in this manner may be used after standing some days for 
the determination of its casein. In such cases, the method given 
above for fresh milk may be followed, except that the acetic 
acid should be added in small portions, a few drops at a time, 
stirring after each addition, and continuing the addition of acetic 
acid until the liquid above the precipitate becomes clear or very 
nearly so. 

