33 2 AGRICULTURAL CHEMISTRY 



total digestible energy, the calories contained in the 

 products, as urea, which are not completely oxidized. In 

 the determination of the available energy of foods, the 

 principle is the same as that explained in the section re- 

 lating to the digestibility of nutrients. The total number 

 of calories in a food is determined by the calorimeter ; 

 the number of calories in the feces is likewise deter- 

 mined and deducted from the total, as well as the caloric 

 value of the liquid excrements containing urea. This 

 gives the energy of the food available to the animal. 



453. Net Energy of Foods. In the process of diges- 

 tion, particularly of coarse fodders containing much 

 crude fiber, energy varying with the density of the tissue 

 is required to render the food available to the body. Of 

 the total available energy, a portion, and in some cases, 

 a large amount is used up in rendering the food available, 

 that is, in carrying on the process of digestion. Means 

 have been devised whereby the approximate amount of 

 work required on the part of the animal to render the 

 food available can be determined. When the energy 

 used in this way is deducted, it leaves the net energy. 

 In the case of coarse fodders, a considerable portion of 

 the energy is used up in the digestion of the food leav- 

 ing, in some cases, only comparatively little net energy 

 while less force is required to digest the grains, and a 

 larger amount of energy is available to the body. The 

 total, available, and net energy which foods produce are 

 important factors because one of the objects of food is to 

 supply nutrients for the production of energy. 



454. Digestion of Proteids. The insoluble proteids 



