200 The Feeding of Animals 



suits in the two cases.* Even if this method gives 

 us a correct estimation of the energy equivalent of 

 the food used, it furnishes no definite information as 

 to the manner of use. It does not appear to what ex- 

 tent the digested nutrients have been oxidized with a 

 corresponding radiation of heat or whether there has 

 been a gain or loss of body substance. If there has 

 been a gain of body substance, then the needs of the 

 work horse or milch cow, if these are under considera- 

 tion, are less than the heat units of the ration, but if 

 there has been a loss of body substance, then the 

 ration is below the required standard for the par- 

 ticular animal under investigation. In a study of 

 energy relations, 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 accu- 

 rate method of determining energy expenditure; viz., 

 experiments with a respiration apparatus. 



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 hydrogen of the food, he 

 has a direct and accurate means of measuring actual 



*This is now regarded as established. 



