SOURCES OF KNOWLEDGE 213 



The differences between the food heat units and those 

 found for the excreta might seem to represent the energy 

 value of that portion of the ration digested by the animal. 

 This would be an accurate measurement of the available 

 or metabolizable energy of the ration if it were not for the 

 loss of unoxidized gases, chiefly methane, which contrib- 

 ute nothing to the maintenance of the animal. Accurate 

 work requires that these gases be measured. But even 

 then it does not appear to what extent the digested 

 nutrients have been oxidized with a corresponding radia- 

 tion of heat or whether there has been a gain or loss of 

 body substance. If there has been a gain of body sub- 

 stance, then the ration is productive, but if there has been 

 a loss of body substance, then the ration is below the 

 required standard for the maintenance of the particular 

 animal under investigation. In a study of energy rela- 

 tions, it is therefore even more necessary to resort to a 

 respiration apparatus of some sort than in determining 

 food balances. We must learn the actual extent of the 

 food combustion which occurs if we would have all the 

 data necessary for measuring energy used, and here we 

 come to the third and most accurate method of determin- 

 ing energy expenditure, viz., experiments with a respira- 

 tion apparatus. 



299. Measurement of food combustion. There are 

 two general ways of ascertaining the extent to which 

 food is burned by any living organism. One is to measure 

 the products of combustion and the other is to measure 

 the amount of oxygen used. It is self-evident that no 

 combustion can occur without the use of oxygen, and so 

 if the experimenter is able to learn just how much of this 

 element is taken up in uniting with the carbon and hydro- 

 gen of the food, he has a direct and accurate means of 



