ELEMENTARY PHYSIOLOGICAL CHEMISTEY. 185 



CHAPTER VI. 

 MILK. 



MILK consists of a watery solution of various proteids, a carbohydrate 

 and salts, containing in suspension emulsified fat. 



The percentage composition varies somewhat in different animals. 

 The more quickly the young animal grows the higher is the percentage 

 of proteid and salts in the mother's milk. Thus a puppy doubles its 

 weight in eight days, and bitches' milk contains 7'1 per cent, proteid 

 and 1*3 per cent. ash. On the other hand, a child takes one hundred 

 and eighty days to double its weight, and human milk contains only 

 1-6 per cent, proteid and O2 per cent. ash. It forms a perfect food 

 for the young growing animal, but as time goes on it requires to be 

 supplemented by other food stuffs in order that the tissues may be 

 properly developed. 



From a medical point of view the t\\ 7 o kinds of greatest importance 

 are cow's milk and human milk. These differ from one another only in 

 their quantitative composition: 



Proteid. Fat. Carbohydrate. Salts. 



Cow's milk, - - 3-5 3-7 4-9 -1 



Human milk, - -1-7 3'4 6'2 -23 



(Bunge). 



The amount of fat and carbohydrate is nearly the same in both, there 

 being, however, twice as much proteid and nearly three times as much 

 salts in cow's as in human milk. To bring cow's milk to the same com- 

 position as human milk it is therefore necessary to dilute the former with 

 an equal bulk of water, at the same time adding some fat and carbohydrate. 



In order to study the chemistry of milk, we usually employ cow's 

 milk because it is easily obtainable. 



Cow's Milk. This is an opalescent solution, possessing a character- 

 istic taste, and of amphoteric reaction. 



EXPERIMENT I. Place a drop of fresh milk on a piece of red litmus 

 paper, and wash it off with distilled water ; a blue stain is left : if the 

 drop be placed on blue litmus, a red stain is left. This peculiar reaction 

 is due to the fact that milk contains a mixture of acid and alkaline 

 salts. By ascertaining how much decinormal acid or alkali 1 are 



1 A normal acid is one in which the amount of acid in 1000 c.c. corresponds 

 to its molecular weight. A normal solution of HC1 would therefore contain 

 1+35-5 = 36-5 gr. HC1 in 1000 c.c. water. A decinormal (^.\ solution would con- 

 tain 3*65 gr. HC1. A normal alkaline solution is prepared in a similar way. 



