THE BALANCE OF NUTRITION 227 



which reach it from its environment, any more than the gasoline 

 engine can use the energy of falling water or of an electric cur- 

 rent. Chemical energy is not merely a source but the only source 

 from which the animal body can derive its supply. 



315. Gross energy. The income of energy may be ascer- 

 tained, therefore, by determining the chemical energy con- 

 tained in the various compounds present in the feed in the 

 manner already indicated (309), viz., by converting it into heat 

 and measuring the amount of the latter by means of a suitable 

 calorimeter. In other words, the income of chemical energy is 

 measured by the heat of combustion of the feed. In order to 

 avoid the implication that this is the total amount of energy 

 associated with the feed (311), it will be convenient to use the 

 term gross energy as equivalent to the amount of energy mani- 

 fested as heat when the feed is completely oxidized. 



Since the chemical energy of a feeding stuff is converted into 

 heat for purposes of measurement, its amount is usually ex- 

 pressed in heat units. It should be clearly understood, however, 

 that this is simply a matter of convenience and that it is the 

 chemical energy of feeding stuffs and not the heat produced by 

 their combustion which is of use to the animal. 



It is scarcely necessary to point out that the gross energy of 

 the feed does not measure its nutritive value. Otherwise, 

 anthracite coal, with a heat of combustion of some 7.9 Cals. 

 per gram, would outrank most feeding stuffs, while hydrogen 

 gas, with a heat of combustion of more than 34 Cals. per gram 

 would stand still higher in the list. Obviously, the feed value 

 of a substance depends not only upon its content of gross energy 

 but upon the proportion of the latter which the body can 

 utilize. 



316. Heats of combustion. The heats of combustion of a 

 great variety of organic substances have been determined. 

 Atwater l in 1895 published a compilation of results upon a 

 large number of compounds of importance in nutrition, Fries 2 

 has prepared a rather more extensive list, and Benedict and 

 Osborne 3 have determined the heats of combustion of nineteen 

 vegetable proteins. 



1 U. S. Dept. Agr., Office Expt. Stas., Bull. 21 (1895). 



2 U. S. Dept. Agr., Bur. Anim. Indus., Bui. 94 (1907). 



3 Jour. Biol. Chem., 3 (1907), 119. 



