118 THE PRINCIPLES OF AGRICULTURE. 



body becomes too warm, it is cooled by the evaporation 

 of perspiration from the skin. 



As the air is generally cooler than the bodies of ani- 

 mals, it is constantly cooling them. To take the place 

 of the heat thus withdrawn, it is necessary that a new 

 supply be constantly furnished in the system. This is 

 produced by burning, or oxidizing, a portion of the food, 

 or particles of the body. The oxygen for this process is 

 obtained from the air through the lungs. 



The Composition of Foods. The food of animals con- 

 tains six different classes of substances, as follows : 



1. Water. All articles of food contain more or less 

 moisture. Hay and meal, which appear to be perfectly 

 dry, contain from ten to twenty per cent, water. Succu- 

 lent food, like green fodder and roots, may contain from 

 seventy to ninety-five per cent, water. In addition to that 

 contained in the food, the animal must drink enough to 

 supply the wants of the system. 



2. Albuminoids. The albuminoids of food, sometimes 

 called protein, are the parts containing nitrogen. They 

 are often called "flesh formers." They serve to form 

 the flesh, or muscle, and the other nitrogenous parts of 

 the body. 



The albuminoids contain not only nitrogen, which is 

 used in forming the muscles, but also other elements of 

 an entirely different nature, which serve to supply heat 

 or force. The albuminoids may thus be made to supply 

 all the wants of the body ; but to confine animals to a 

 diet composed entirely of nitrogenous food would be a 

 wasteful practice. The albuminoids contain a larger 

 proportion of nitrogen than animals require. After a 

 sufficient quantity of such food has been consumed to 

 supply enough nitrogen, it is necessary to consume an 



