of a small number of chemical constituents, namely, protein, fat, car- 

 bohydrates, and ash, together with a larger or smaller amount of 

 water. The latter can be often seen, as in the juice of fresh plants. 

 In dry hay no water or juice is visible. A small amount is, however, 

 contained in minute particles in the plant tissues. 



Protein is a name given to the total group of nitrogenous materials 

 present. The group is made up mainly of the true proteids and albu- 

 mens such as the gluten of wheat, and of nitrogenous materials such 

 as amids, which are believed to have a lower feeding value than the 

 albumens. 



The group "fat" includes the true vegetable fats and oils, like the 

 oil in cotton seed or corn, as well as vegetable wax, some chlorophyl 

 (the green coloring matter in leaves, etc.), and other coloring matters; 

 in l)rief, all the materials which are extracted by ether in the usual 

 laboratory method of estimating fat. The name "ether extract" is 

 often and quite properly applied to this group. 



The group "carbohydrates" includes starches, sugars, crude fiber, 

 cellulose, pentosans, and other bodies of a similar chemical structure. 

 This group is usually subdivided, according to the analytical methods 

 followed in estimating it, into "nitrogen-free extract" and "crude 

 fiber;" the former subdivision including principally sugar, starches, 

 and most of the pentosans, and the latter cellulose, lignin, and other 

 woody substances which very largel}^ make up the rigid structure of 

 plants. 



The group "mineral matter" includes the inorganic bodies present 

 in the form of salts in the juices and tissue of the different feeding 

 stuffs, the principal chemical elements found being sodium, potassium, 

 calcium, chlorin, fluorin, phosphorus, and sulphur. The term "ash" 

 is often and very appropriately used for this group, since the mineral 

 matter represents the incombustible portion which remains when any 

 given feeding stuff is burned. 



The functions of food are (1) to supply material to build and repair 

 the body, and (2) to yield energy. The chemical composition of a 

 feeding stuff serves as a basis for judging of its value for building and 

 repairing body tissue. Its value as a source of energy must, however, 

 be learned in another way. The most usual way of measuring energy 

 is in terms of heat, the calorie being taken as a unit. This is the 

 amount of heat which would raise the temperature of 1 kilogram of 

 water 1" C. , or 1 pound of water 4^ F. Instead of this the unit of 

 mechanical energj^, the foot-ton (the force which would lift 1 ton 1 

 foot), may be used, but it is not as convenient. One calorie corresponds 

 very nearly to 1.54 foot- tons. 



The fuel value of any food is equal to its heat of combustion less 

 the energy of the excretory products derived from it, and ma}^ be 

 learned by taking into account the chemical composition of the food 



170 



