FUEL VALUE. 293 



of evolving heat when oxidised. If the food of an animal con- 

 tain sufficient nitrogenous matter to replenish the waste of 

 tissue necessitated by the vital processes, the heat-producing 

 power of the food then becomes a measure of the amount of 

 mechanical work which the animal can perform or of the 

 quantity of heat which the food can supply. These quantities 

 are related to the capacity for producing fat, since fat may be 

 regarded as stored energy. The calorific power of a food, 

 therefore, if calculated from the digestible constituents, affords 

 a fair measure of its feeding power ; due attention, however, 

 must be paid to the capability of the food for replenishing 

 waste of tissue (i.e., to its containing a due proportion of 

 albuminoids) . 



In such a calculation the heat of combustion of albuminoids, 

 carbohydrates, and fat, already given, may be employed. For 

 example, the heat-yielding power of maize is thus calculated : 



1 gramme of maize contains '079 digestible protein 



667 ,, carbohydrates 

 043 fat. 



Multiplying these by their respective heats of combustion 



079 x 4100 = 324 calories 

 667 x 4100 = 2735 

 043 x 9300 = 400 



3459 ,, 



or 1 gramme of maize when used as food is theoretically 

 able to produce 3459 units of heat ; or, to express it more 

 generally, one part by weight of maize when used as food is 

 able to produce sufficient heat to raise the temperature of 3459 

 parts by weight of water through one degree Centigrade. In 

 fact, the results obtained by such calculations are really the 

 calorific powers of the food stuffs when oxidised in the animal. 

 As would be expected, foods richest in fat are generally of 

 highest calorific power. 



The following table gives the approximate fuel values of 

 many of the commoner food stuffs. The numbers are calcu- 

 lated from a table of the digestible ingredients of various foods> 



