2l6 NUTRITION OF FARM ANIMALS 



no account of the excretion through the skin and the alimentary 

 canal, it is only by indirect methods that it is possible to com- 

 pute the total balance of carbon, hydrogen and oxygen by its 

 use. 



301. Balance of water and of ash ingredients. The res- 

 piration apparatus of either type serves to determine the ex- 

 cretion of water vapor by the subject as well as that of carbon 

 dioxid and other gases and thus, in connection with the neces- 

 sary analyses of the feed and visible excreta, to establish the 

 gain or loss of hydrogen. Unfortunately, more or less difficulty 

 is experienced in determining accurately the hydrogen balance 

 owing in part to the liability to condensation of water in the 

 apparatus and in part to the fact that the amount of organic 

 hydrogen actually entering into the metabolism of the animal 

 is small as compared with the amounts of water consumed and 

 simply evaporated again. 



The ash ingredients, of course, with the possible exception 

 of minute amounts of sulphur, all leave the animal in the visible 

 excreta and the balance of these elements may therefore be 

 determined according to the same principles as the balance of 

 nitrogen. 



4. THE BALANCE OF ENERGY 



302. Balance of nutrition includes energy. Since the 

 animal body is essentially a transformer of energy (207), the 

 balance of nutrition is not only concerned with the income and 

 outgo of matter but also, corresponding to the dual function of 

 feed (263), with the gain or loss of energy by the body. The 

 study of the balance of energy is a method of investigating 

 some of the important problems of nutrition which has been 

 especially developed in recent years and which has proved 

 fruitful of results. Before entering upon its consideration, 

 however, a brief review of some of the elementary concepts of 

 energetics as related to physiological processes may not prove 

 superfluous. 



Elementary principles 



303. Energy. Up to this point the word energy has been 

 used without any precise definition. In a specific study of the 



