634 PROPERTIES AND COMPOSITION OF MILK. 



turns or acquires a sour taste, and at 70° or 80° it sours with still greater 

 rapidity. If sour milk be gently warmed it undergoes fermentation, and 

 may be made to yield an intoxicating liquor. By longer exposure to the 

 air it gradually begins to putrify, becomes disagreeable to the taste, 

 emits an unpleasant odour, and ceases to be a wholesome article of 

 food. 



The milk of each species of animal is distinguished by some charac- 

 ters peculiar to itself. 



Ewe's milk does not differ in appearance from that of the cow, but it 

 is generally more dense and thicker, and gives a pale yellow butter, 

 which is soft, and soon becomes rancid. The curd is separated from 

 this milk with greater difficulty than from that of the cow. 



Goafs milk generally possesses a characteristic unpleasant odour and 

 taste, which is said to be less marked in animals of a white colour or 

 that are destitute of horns. The butter is always white and hard, and 

 keeps long fresh. The milk is considered to be very wholesome, and is 

 often recommended to invalids. 



Ass's milk has much resemblance to that of the woman. It yields 

 little cream, and the butter is white and light, and soon becomes rancid. 

 It contains much sugar, and hence soon passes to the state of fermenta- 

 tion. 



2°. Composition of milk. — Milk, like the numerous vegetable products 

 we have had occasion to consider, consists, besides water, of organic sub- 

 stances destitute of nitrogen — sugar and butter ; of an organic substance 

 containing nitrogen in considerable quantity — the curd or casein ; and 

 of inorganic or saline matter, partly soluble and partly insoluble in pure 

 water. 



The proportions of these several cc stituents vary in different animals. 

 This appears in the following table, which exhibits the composition of 

 the milk of several animals in its ordinary state, as found by Henry and 

 Chevallier : — 



Woman. 



Casein (cheese) . . 1-52 



Butter 3-55 



Milk sugar . . . 6-50 



Saline matter . . . 0-45 



Water 87-98 



100 100 100 100 100 



From the numbers in the above table, it appears that the milk of the 

 cow, the goat, and the ewe, contains much more cheesy matter than that 

 of the woman or the ass. It is probably this similarity of asses' milk to 

 that of the human species, together with its deficiency in butter, which, 

 from the most remote times, has recommended it to invalids, as a light 

 and easily digested drink. 



§ 2. Of the circumstances by which the composition or quality of the milk 

 is modified. 

 ^ But the composition or quality of milk varies with a great variety of 

 circumstances. Let me direct your attention to a few of these. 



1°. Distance from the time of calving. — The most remarkable depar- 



