FAT — SUGAR — ^PEOTEINS. 7 



point ; tie solidification is, however, a process which takes a 

 considerable amount of time (some hours), and it appears pro- 

 bable that an apparent reversal of the laws of nature takes 

 place. When a substance cools heat is given out or energy is 

 evolved. When a very small globule cools it contracts, and 

 the surface energy is increased — that is to say, energy is absorbed. 

 As this energy can only be supplied as heat abstracted from the 

 aqueous portion of the milk, it follows that, as the fat globules 

 and aqueous portion are at approximately the same temperature, 

 the passage of energy from one to the other will be slow, and, 

 therefore, that solidification of the globules will be but a slow 

 process, 



Burri and Nussbaumer find that the surface tension of milk is 

 diminished by cooling, and Bauer attributes this to the solidifica- 

 tion of the fat, as the original value can be very nearly restored 

 by heating above the melting point of fat. 



Sugar. — The sugar in milk is of a peculiar nature ; that of 

 cow's milk is called " lactose," or, more commonly, sugar of milk. 

 It is generally assumed that all milks contain the same sugar, 

 but of this there is some doubt. The author, in conjunction 

 with Pappel, has identified in the milk of the " gamoose," or 

 Egyptian buffalo, a sugar distinct from lactose, to which the 

 name of " tewfikose " has been given. The sugar of the milk of 

 the mare has the property of easily undci\Li;oing alcoholic fer- 

 mentation, a property not possessed by Iacto.se. According to 

 the experiments of Carter and the author, the sugar of human 

 milk is not identical with that of the milk of the cow. 



The sweetness of milk is enthely due to the suuai' contained 

 in it ; the sugar of milk is many times less sweet than cane 

 sugar. It is very easy of digestion, even by young chUdi-en. 



Proteins. — It is in the proteins that the milk of different animals 

 show the most marked variations. They may be divided broadly 

 into two classes — those which give a curd on the addition of an 

 acid, and those which do not. In the first class are included 

 the milk yielded by the cow, the goat, the gamoose, etc. ; and in 

 the second human milk, that of the mare, and that of the ass 

 may be cited as examples. In the first class the curd is composed 

 of casein, which is combined with phosphates of the alkaline 

 earths ; while in the second this is replaced by a similar protein, 

 which is not, however, combined with phosphates. It is possible 

 that the difference between the proteins of the two classes is 

 simply dependent on the presence or absence of the phosphates, 

 but the chemistry of these bodies is not yet sufliciently advanced 

 to decide this. Besides casein, or a similar body, there exists 

 in all milks a second protein called albumin ; this differs from 

 casein by not being precipitated by acids, and by being coagulated 



