388 GENERAL SCIENCE 



and other animals. Plants make the foods which they 

 need and also all the food for animals. They are the only 

 real food factories. 



Kinds of Foods. The number of substances used as 

 food is large, but these substances may be conveniently 

 grouped according to their chemical composition and 

 sources, as protein, carbohydrates, fats, vitamines, ash 

 constituents, and water. 



Proteins. The proteins are frequently spoken of as 

 the nitrogenous foods. They always contain carbon, 

 hydrogen, oxygen, and nitrogen. Proteins are always 

 rich in one or more such organic substances as albumen, 



FIG. 346. Some Building Foods that Make Bones and Muscles. 



casein, fibrin, gelatin, gluten, legumin, and myosin. 

 The principal protein food materials are milk, eggs, flesh 

 foods, legumes, and cereals (Figures 346, 347). They 

 are the foods which serve to build up the body and keep 

 it in repair. 



The presence of protein in a food may be detected by 

 the following simple experiments. 



Nitric Acid Test. Boil the food to be tested in nitric 

 acid. When cool add enough ammonia to neutralize the 

 acid. If the yellow nitric acid solution turns to a deep 

 orange when the ammonia is added, protein is present. 



Biuret Test. - - To a ten per cent solution of caustic 

 soda add a dilute solution of copper sulphate, drop by 



