232 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



off the cream or adding skimmed milk increases the spe- 

 cific gravity, while the addition of water decreases it. It 

 is not possible to detect these forms of adulteration from 

 the change in specific gravity alone, on account of the 

 wide range in the specific gravity of normal milk, but 

 when considered in connection with the per cent, of fat 

 and solids not fat the specific gravity is of great assist- 

 ance. The specific gravity of skimmed milk may be 

 lowered to normal by the addition of a suitable quantity 

 of water. When this is suspected, 

 the determination of the specific 

 gravity of the dry matter or solids 

 and the per cent, of fat in the total 

 solids will throw further light on 

 the subject. The test for nitrates 

 and nitrites will assist in discover- 

 ing the addition of water (see page 

 247). The refraction number of 

 the milk serum is also of value in 

 this connection (see page 250). 



It is necessary to refer to the 

 New York Board of Health lac- 

 tometer because it is still used to some extent in 

 the East, although it is constructed on the incorrect 

 theory that 1.029 is the lowest specific gravity of pure 

 milk. Its scale is divided into 120 divisions. One hun- 

 dred corresponds to 29 Quevenne degrees, or a specific 

 gravity of 1.029. Milk testing 90 on this scale is sup- 

 posed to be 90 per cent, pure, i.e., 10 per cent, of water 

 is supposed to have been added, which, of course, is 

 not always true. Readings on this scale may be con- 

 verted into Quevenne degrees by multiplying by 0.29 

 or the following table may be used: 



Fio. 32. Westphal balance. 

 (From Chemical Testing of Milk 

 and Cream, by R. H. Shaw, 

 U. 8. Dept. of Agriculture.) 



