124 THE CHEMISTRY OP THE FARM 



and the intestinal gases. Any internal work, mechani- 

 cal or chemical, following the consumption of the 

 food, does not disturb its heat value to the animal, 

 provided the energy thus developed does not exceed 

 in its heat value the immediate requirements of the 

 animal. All internal work appears finally as heat ; 

 such work is thus only a mode of providing heat, and 

 if the work were not performed the same quantity 

 of digested food would have to be burnt in the body 

 to provide the heat necessary for the animal. The 

 consumption of energy during the digestion of fibrous 

 foods is thus not to be regarded as waste when the 

 animal is merely kept on a maintenance diet. The 

 waste begins as soon as the labour of digestion results 

 in more heat than the animal requires. The excess 

 of heat then produced is waste, for the energy has not 

 been developed in the muscles of the limbs and cannot 

 therefore appear as useful work, nor does it in any way 

 aid in the production of animal increase. 



Production Value of Poods.— Kellner has not only 

 determined the true heat values to the ox of the foods 

 already mentioned, he has also ascertained their pro- 

 duction values. For this purpose he chose rather lean 

 oxen, and gave them a fixed moderate ration, which 

 resulted in a small continuous increase in weight. He 

 then added to this ration the food to be experimented 

 with, and determined the amount of increase produced. 

 The increase in the nitrogenous tissues was calculated 

 from the amount of nitrogen stored up in the animal 

 body (excess of nitrogen in food over nitrogen in ex- 

 crements). The increase in fat was calculated from 

 the amount of carbon stored up in the body (excess of 

 carbon in food over carbon in excrements and breath), 



