SOURCES OF KNOWLEDGE 211 



compounds of the body have received an accession. 

 Knowing as we do the proportions of nitrogen and car- 

 bon in the various tissues of the animal, we can calculate 

 how much of the gain or loss of carbon belongs in the 

 nitrogenous substance deposited or wasted. If more 

 carbon is gained or lost than can possibly be associated 

 with the nitrogen gained or lost, then there has been a 

 gain or loss of fat, because protein and fat being the main 

 constituents of the animal carcass, any considerable 

 retention of carbon must be in one of these forms. If 

 there has been nitrogen equilibrium, all excess or deficit 

 of carbon belongs to a deposit or waste of fat. By such 

 searching methods as these, it is possible to ascertain 

 with a good degree of accuracy how food is used and what 

 quantity and kind of nutrients are needed in maintain- 

 ing an animal under given conditions. 



296. Determination of energy values. — ^We have 

 reached a point in our study of animal nutrition where 

 we realize that food Values are to some extent commen- 

 surable with energy values and that it is desirable to 

 know the energy product of different compounds and 

 feeding-stuffs. Moreover, we cannot possess sufficiently 

 full knowledge concerning the energy needs of the several 

 classes of animals imtil we have measiu-ed energy use in 

 terms of heat given ofiF under the various conditions of 

 work and of production. The mere determination of the 

 income and outgo of the animal body does not neces- 

 sarily measure energy needs or use. We may go so far 

 as to ascertain that a certain amount of carbon from a 

 certain source was consumed in a given time, but from 

 this alone we do not learn the extent to which this com- 

 bustion has supported the internal and external work 

 of the body. 



