178 



MILK 



The determination of the specific gravity of milk is of great 

 importance in milk analysis, and is generally carried out by means 

 of the lactometer or Westphal's balance, although the most 

 scientifically exact method, and that by which the lactometer 

 and Westphal's balance are graduated, is the employment of 

 the density bottle or pyknometer (Figs. 21 and 22). 



The specific gravity or density of a sample of fresh milk should 

 be determined by each of the three following methods and the 

 results compared. 



176. Determination of Specific Gravity by 



Density Bottle. 



A density bottle similar to that shown in Fig. 21 

 which has a capacity of 100 cc, is washed successively 

 with water, alcohol, and ether, and is 

 then dried internally by drawing a 

 current of air through it. The bottle 

 and stopper is then weighed (wt. = a 

 grams). 



It is then filled with cold distilled 

 water, the stopper replaced, and im- 

 mersed up to the neck for some 

 minutes in a beaker containing water 

 at 15° C. Any liquid passing through 

 the hole in the stopper is carefully 

 removed, and the bottle is kept in 

 the bath until no further expansion 

 of the water within takes place. 



It is then carefully dried exter- 

 nally, care being taken not to warm 

 it with the hand while so doing, and is again weighed 

 (wt. = b grams). 



The water is poured out, and the bottle thoroughly 

 dried as before, and then filled with milk, immersed in 

 a bath at 15° C, and treated exactly as described above. 

 It is then weighed (wt. = c grams). 



Fig. 21. — Specific 

 gravity bottle. 



