MILK. 371 



exact determination wf total solids and of the various con- 

 stituents, is required. 



Analysis of milk. The total solids are determined by placing 

 a weighed quantity (from 5 to 10 c.c.) of the well-mixed milk 

 in a weighed platinum dish and heating for several hours in a 

 water-bath until no more weight is lost. The loss in weight 

 represents the water, the weight of the residue the total solids. 

 The fat is determined by extracting the solid residue repeatedly 

 with warm ether, filtering this solution through a small filter, 

 which is to be well washed with ether, and evaporating the 

 ethereal solution in a weighed platinum dish. 



Milk-sugar is next determined by treating the residue (from 

 which fat has been extracted) with hot diluted alcohol; lactose 

 and a few soluble salts enter into solution; the liquid evaporated 

 to dryness in a weighed dish, gives the quantity of sugar plus 

 some salts. Upon igniting the milk-sugar a residue of salts is 

 left, which is also weighed and this weight deducted from the 

 first one. 



Casein. The residue now left (after treatment with ether and 

 alcohol) contains chiefly casein with some albumin and salts. 

 If any casein should have been washed upon the filters acci- 

 dentally, it has to be transferred back to the dish, the contents 

 of which are dried and weighed. By burning off the casein 

 and reweighing the dish plus the salts, the quantity of the casein 

 is determined. 



The remaining salts added to those previously obtained from 

 the alcoholic solution form the total ash or inorganic solids, an 

 analysis of which may be made according to the methods given 

 heretofore. 



Casein may also be determined directly by precipitating it 

 from milk, by the addition of acetic acid and boiling. The 

 precipitated casein is filtered off, and has to be well washed, 

 first with water, and then with ether, as it contains most of the 

 fat. 



QUESTIONS. 



511. Mention the five principal constituents of milk. 



512. Give the average composition of human and of cow's milk. 



513. What compounds constitute milk-ashes? 



