CONSTITUENTS OF MILK. 45 



with cow's milk ; it dissolves entirely, with the exception of an 

 inconsiderable quantity of coagulated casein, if woman's milk 

 is used. The solution is then evaporated at a gentle tempera- 

 ture to the consistence of a thin syrup, and is treated with ten 

 or twelve times its volume of alcohol of O85, which precipitates 

 the casein. As the casein may retain a little sugar, it is expe- 

 dient to digest it once or oftener with a little water, and to treat 

 the dilute pulp with spirit ; the casein that remains must be 

 thoroughly dried and weighed. The spirituous solution con- 

 tains the sugar, and the greater part of the extractive matter, 

 from which the sugar cannot be easily separated. A partial 

 separation may be effected in this way : we may dissolve the 

 impure sugar in a little water ; on the addition of strong alcohol, 

 the sugar with a very little extractive matter, is precipitated, 

 while the alcoholic solution contains extractive matters and a 

 little sugar. On evaporating this solution to the consistence 

 of a syrup, and adding strong alcohol to it while still hot, some 

 more sugar separates on cooling. 



I usually estimate the salts by incinerating a weighed por- 

 tion of the dried residue of the milk ; and, in some cases, I 

 have separated the soluble from the insoluble salts. 



This analysis of milk does not yield, as Berzelius 1 justly 

 observes, any very accurate results, since casein is slightly soluble 

 in alcohol; although strong cold alcohol takes up only a very 

 small portion, dilute hot alcohol dissolves a considerable quan- 

 tity. The determination of the sugar and of the extractive 

 matters by the course that I have indicated is still more inac- 

 curate. Berzelius proposes to precipitate the casein (and the 

 butter) by rennet ; but it must be observed that, by this means, 

 we do not obtain results of greater accuracy, since a portion of 

 the casein always remains in solution in the whey. This 

 amounts to a considerable quantity in woman's milk, but is 

 comparatively slight in the milk of the cow, 2 and has always to 

 be obtained by means of alcohol from the evaporated solution. 

 In order to precipitate the casein thoroughly by rennet, it 

 would be requisite to supersaturate the free alkali of the milk 

 by acetic or lactic acid ; we should then obtain the casein in a 

 state of combination with these acids ; in fact, casein precipi- 



1 Thierchemie, p. 698. 2 Die Frauenmilch, &c. p. 33. 



