326 PRACTICAL PHYSIOLOGY 



an immunised cadaver, yet the young mouse can be immunised through 

 its food. This points either to a special condition of the antitoxins in 

 the milk of the mother or to a special condition of digestion in the 

 young. 



In order to study the chemistry of milk, we usually employ cow's 

 milk, because it is easily obtainable. 



Cow's Milk. This is an opalescent solution, possessing a character- 

 istic taste, and of amphoteric reaction. 



EXPERIMENT I. Place a drop of fresh milk on a piece of red litmus 

 paper, and wash it off with distilled water ; a blue stain is left : if the 

 drop be placed on blue litmus, a red stain is left. This peculiar 

 reaction is due to the fact that milk contains a mixture of acid and 

 alkaline salts. By ascertaining how much decinormal acid or alkali are 

 required to produce neutralisation with the aid of different indicators 

 the amount of each of these kinds of salt can be determined. (See 

 Titration Methods.) 



The specific gravity of fresh milk varies between 1*028 and 1*0345. 

 The more fat (i.e. cream) the milk contains the lower is the specific 

 gravity. 



EXPERIMENT II. Estimate by a hydrometer (p. 409) the specific 

 gravity (a) in skimmed milk and (b) in fresh milk. In the former it 

 is about T0345, in the latter T028. By adding water to (a) the 

 specific gravity obviously falls, and by removing the cream from (b) it 

 rises. In dairy hygiene, a rough estimate of the richness of milk in 

 cream is obtained by ascertaining its specific gravity, but it is clear 

 from the above experiment that some of the cream can be removed 

 and the consequent rise in specific gravity masked by the addition of 

 water. This fraudulent trick of some dairymen must, therefore, be 

 borne in mind before giving an opinion of the quality of the milk. 



Fresh milk does not coagulate on boiling, but a skin forms on its 

 surface. A similar skin is produced when any emulsion containing 

 protein is boiled, and in the case of milk it is composed chiefly of 

 caseinogen entangling some fat globules. 1 Its formation is due to 

 drying of the protein at the surface of the milk. 



THE CHEMICAL CONSTITUENTS OF MILK. 



I. Proteins. The chief protein of milk is a phospho-protein called 

 Caseinogen. This can be precipitated by adding to the diluted milk a 



1 An emulsion of cod-liver oil in diluted blood-serum is given round ; warm it to 

 about 50 C., and a skin will form on the surface. Be careful not to heat above 

 50 C. , as then coagulation of the proteins will be produced. 



