MILK. 89 



mosphere upon the caseine. Although a small quantity of 

 albumen exists in the milk, this does not coagulate on the 

 surface by the action of heat, for the scum does not form 

 when the fluid is heated in an atmosphere of carbonic acid, 

 or of hydrogen, or in a yacuum. 1 



AVhen the milk is coagulated by any substance acting 

 upon the caseine, or when it coagulates spontaneously, it 

 separates into a curd, composed of caseine with most of 

 the fatty particles, and a nearly clear, greenish-yellow serum, 

 called whey. This separation occurs spontaneously, at a 

 yariable time after the discharge of the milk, taking place 

 much more rapidly in warm than in cold weather. It is a 

 curious fact that fresh milk is frequently coagulated during 

 a thunder-storm, a phenomenon which has neyer been sat- 

 isfactorily explained. 



On being allowed to stand for a short time, the milk 

 separates, without coagulating, into two tolerably distinct 

 portions, A large proportion of the globules rise to the top, 

 forming a yellowish- white, and very opaque fluid, called 

 cream, leaying the lower portion poorer in globules and 

 of a decidedly bluish tint. In healthy milk the stratum 

 of cream forms from one-fifth to one-third of the entire 

 mass of the milk. In the human subject the skim-milk is 

 not white and opaque, but is nearly as transparent as the 

 whey. This is a yery good method of testing the richness 

 of milk; and little graduated glasses, called lactometers, 

 haye been, constructed for measuring the thickness of the 

 layer of cream. The specific grayity of the cream from 

 milk of the ayerage specific gravity of 1032 is about 1024. 

 The specific gravity of the skim-milk is about 1034. 



Microscopical Characters of the Milk. If a drop of milk 

 be examined with a magnifying power of from three hun- 

 dred to six hundred diameters, the cause of its opacity will 

 be apparent. It contains an immense number of minute 



1 ROBIN, Lemons surles humeurs, Paris, 186?, p. 388. 



