420 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



suffer a loss of energy in the urine, but at present little is known 

 as to the nature of this loss and what factors modify it. 



Available Energy. A portion of the energy in the food fed to 

 the animal remains in the undigested matter of the solid excre- 

 ment, a portion appears in the urine, and a portion is lost in the 

 form of marsh gas. None of the energy so lost is of any value 

 to the animal, and may be termed unavailable energy. The 

 available energy is the total energy in the nutrients less the losses 

 in the solid, liquid, and gaseous excrements. A portion of the 

 available energy is expended in the processes of mastication, 

 moving the food, preparation of digestive juices, digestion, and 

 other operations necessary to bring the digested nutrients into 

 the body, and convert them into forms suitable for its use. This 

 energy does not, of course, come from the food actually in pro- 

 cess of digestion, but the digested food must replace the energy 

 so used. The energy used in digestion, etc., appears as heat, and 

 may aid to keep the animal warm, but any excess over the amount 

 so required, serving no useful purpose, is evolved as heat. We 

 will term the energy consumed in digestion of the food thermal 

 energy. 



The available energy remaining after the thermal energy has 

 been subtracted, may be used for processes requisite to the life 

 of the animal, such as to keep the animal warm, to furnish energy 

 for beating of the heart, breathing, and necessary movements of 

 the body. Any excess over that needed by the animal body may 

 be used for productive purposes for fat, flesh, milk, etc. We 

 will term this portion of the energy of the food its kinetic energy. 



Estimation of the Energy Expended in Digestion. Two meth- 

 ods may be used to estimate the energy expended in digestion of 

 food. The first method can be applied only to animals, such as 

 the dog, having small digestive organs, which can be completely 

 emptied by starving for a few days. The heat evolved from a 

 starving dog at a temperature of 33 is measured by placing the 

 animal in a calorimeter. Sufficient food is then given to supply 

 an amount of available energy equal to that lost daily while 

 starving, and the heat liberated is again measured. Any increase 



