ANIMAL FOODS AND FEEDING 293 



Thus, with any food of which an analysis has been made, we 

 can determine the amount digested, provided we have coefficients 

 of digestibiHty which apply. In some cases where the identical 

 food or animal has not been subject to experiment it is necessary 

 to use the data most nearly corresponding; e.g. wheat for 

 horses, in the table of coefficients, is given as "figures obtained 

 with oats.'' 



Energy Value of Food Constituents 



Having then determined the amount of food constituents 

 which an animal digests, the next step is to find out how much 

 energy this digested food will yield. We have previously said 

 that whether animal food yields energy directly or is used to 

 build body substance, eventually, in most cases, the final result 

 is energy, as the body substance is later torn down and energy 

 is set free. Also that whether this energy is manifested as heat 

 or as muscular work, the units for expressing the energy are 

 always in terms of heat. The liberation of energy in the 

 animal body is by the reaction of oxidation and this reaction is 

 easily carried out in accurate physical apparatus which allows 

 the exact determination of the energy liberated. In such re- 

 actions the substance burns and the heat produced by this 

 burning is the measurement of the energy of the substance. 



Fuel Value. — This we term the combustion value or fuel 

 value of the substance, and this fuel value, which we express in 

 heat units, represents the energy value which, in a food substance, 

 means th.t, food value. Therefore the fuel value of a food con- 

 stituent or any substance used as food is equivalent to its pos- 

 sible food value. 



Bomb Calorimeter. — This determination of the fuel value or 

 food value of food substances is made in a piece of apparatus 

 known as a homh calorimeter. Speaking very generally, this 

 consists of a heavy thick-walled metallic cylinder or bomb. 

 This is so constructed that a substance placed within it may 

 be ignited and burned, usually in pure oxygen gas under pres- 

 sure, and the heat generated by this combustion is determined 



