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 Ibs., this can 

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

 is heavier than water; the specific gravity of this milk 

 will then be l -* =1.032. 



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 Ibs., 1 gal. of milk will weigh 

 S.34Xl- ft 32 or 8.6 Ibs.; 1 gal. of skim milk, 8.7 Ibs., and 1 gal. of 

 cream frpm 8.1 to 8.5 Ibs., according to its richness. (See Table XVI, 

 Appendix.) 



