HOW TO EXPRESS FOOD VALUE 337 



take into consideration the food constituents winch are 

 consumed bv the involuntary activities of the body. An 

 animal absolutely at rest or asleep is constantly using up 

 food constituents. The beating of the heart, expansion and 

 contraction of the lungs, movements of the digestive ap- 

 paratus are going on constantly. When food is masticated 

 work is done and the oxidation of bodv fuel is necessarv to 

 obtain this energy. 



Most of the food taken in by the animal is used up in the 

 production of energy, and since energy can be expressed in 

 terms of heat, it is customary to value food on the basis of 

 heat equivalents. Since the combustion of food material 

 in the body results in the production of exactly as much 

 heat as is derived from the combustion of the same con- 

 stituents in the air, it is possible to determine the fuel value 

 of foods bv well known methods of analvsis. And, finally, 

 since an apparatus has been devised for determining the 

 energy value of foods in the animal, and at the same time 

 permitting a complete analysis of all food income and outgo, 

 it is possible to overcome in large measure the defects just 

 mentioned for determining the value of foods. 



For the determination of the heat value of substances, 

 an instrument called a calorimeter is used, which consists 

 essentially of a closed chamber in which organic material 

 can be burned in an atmosphere of oxygen and the resulting 

 heat accurately measured by the rise in temperature of a 

 surrounding body of water after making certain necessary 

 corrections. Modern scientific ingenuity has gone one step 

 further and devised calorimeters which will contain a 

 living animal and in this way the amount of heat developed 

 bv the combustion of food in the bodv can be accurately 

 measured. In addition these animal calorimeters are 

 equipped with elaborate apparatus for measuring the intake 

 of oxygen and the output of carbon dioxide and other gases. 

 These factors, together with the weighing and analyzing 

 of all foods consumed and all solid and liquid material 

 excreted, make it possible by a series of calculations to 

 arrive very accurately at the proper value of any food for 

 any particular animal. The respiration calorimeter devised 



99 



