186 PEACTICAL PHYSIOLOGY 



required to produce neutralisation with the aid of different indicators 

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

 Titration Methods, p. 245.) 



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. 240) the specific 

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

 is about 1*0345, in the latter 1'028. 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 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 it* 

 surface. A similar skin is produced when any emulsion containing 

 proteid 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 proteid at the surface of the milk. 



THE CHEMICAL CONSTITUENTS OF MILK. 



I. Proteids. The chief proteid of milk is a pseudo-nuclein called 

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

 a weak acid, or by saturating it with a neutral salt. 



EXPERIMENT III. Place about five cubic centimetres of skimmed 

 milk in a test tube, and dilute with an equal bulk of water. To this 

 diluted milk add, drop by drop, a weak solution of acetic acid; 

 a precipitate of caseinogen, entangling fat, falls down. Filter off 

 this precipitate and wash it with water. Now add to it a weak 

 solution of Na 2 C0 3 ; the precipitate dissolves, and an opalescent 

 solution of caseinogen, still however containing some fat, passes through 

 the filter. By repeated reprecipitation and filtration comparatively 

 pure caseinogen can be obtained, from which the last traces of fat can 

 be removed by treating with ether. 



The chief property of caseinogen is its power to clot when treated 

 with rennin (a ferment contained in gastric juice) in the presence of 

 soluble calcium salts. 



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 proteids will be produced. 



