still larger amount of it than this to get 4000 grains of carbon, 

 and there would be a still greater unnecessary consumption of 

 nitrogen. So that, although a man could derive all the neces- 

 sary elements from proteid food (with water and salts), such a 

 diet would throw much unnecessary labour on the digestive and 

 excretory organs, and some disorder of them would soon occur. 

 \\Y thus see the advantages of a mixed diet, containing some 

 food-stuffs rich in carbon and others rich in nitrogen ; and 

 the same applies to the other elements. Some articles of food 

 contain in themselves the various food-stuffs mixed in a suitable 

 proportion for the bodily wants. Milk is one of these. 



Milk. Milk consists of water holding proteids, sugar, and 

 certain salts in solution, and having suspended in it a large 

 amount of fat in a state of division into fine globules. To the 

 fat its whiteness is due. On standing some of the fat rises as 

 cream, and < ream shaken till the fat globules run together forms 

 butter. There are two proteids in milk, casein and albumin, 

 which are in solution in much 'the same way as globulin and 

 albumin are in solution in blood. If a drop of acetic acid is 

 added to diluted milk the casein is precipitated, and as it falls 

 it carries down with it the fat, leaving a clear colourless fluid 

 in which the albumin, sugar, and salts remain in solution. 

 The chief salts present are the chlorides of sodium and 

 potassium and the phosphates of sodium and calcium. 



Relative Percentage of Food-Stuffs in certain Foods 



The usefulness of foods does not depend merely on the 

 amount of useful food-stuffs they contain, for their digestibility 



