CHAPTER VI. 



THE LACTOMETER AND ITS APPLICATION. 



109. The lactometer is used for determining the spe- 

 cific gravity of milk. The term specific gravity means 

 the weight of a certain volume of a solid or a liquid 

 substance compared with the weight of the same vol- 

 ume of water at 4° C. (39.2° Fahr.) ; for gases the 

 standard of comparison is air or hydrogen. If the milk 

 which a can will hold weighs exactly 103.2 Ifts., this can 

 will hold a smaller weight of water, say 100 lbs., as milk 

 is heavier than water; the specific gravity of this milk 



will then be 1 "^-^ =1.032. 

 100 



The specific gravity of normal cow's milk will vary 

 in different samples between 1.029 and 1.035 at 60° F., 

 the average being about 1.032. The specific gravity of 

 skim milk is about 1.036-1.038, and of sweet cream 1.01 

 to .95, according to the per cent, of fat contained there- 

 in; average specific gravity 1.0 (see p. 77).^ 



The lactometer enables us to determine rapidly the 

 relative weight of milk and water. Its application rests 

 on well-known laws of physics : When a body floats in 

 a liquid, the weight of the amount of liquid which it 

 replaces is equal to the weight of the body. It will sink 

 further into a light liquid than into a heavy one, be- 



1 Since a gallon of water weighs 8.34 Iba., 1 gal. of milk will weigb 

 S..S4 X 1 .OHi' or S.fi Ihs. : 1 gal. of skim milk. 8.7 lbs., and 1 gal. of 

 cream from 8.1 to 8.5 lbs., according to Its richness. (See Table XVI, 

 Appendix.) 



