210 THE FEEDING OF ANIMALS 



safe conclusions. In every instance, one or more assump- 

 tions are involved where definite proof is not furnished. 

 Nothing short of a complete record of the income and 

 outgo of the animal organism during the experimental 

 period is conclusive evidence as to whether there has been 

 a gain or loss of body substance and what is the kind and 

 extent of the growth or waste. The securing of such a 

 record is an expensive and laborious task. It requires not 

 only complete information in regard to the quantity and 

 composition of the food, but also an acccurate measure- 

 ment of the excreta, including the feces, the urine, the 

 respiratory products, and the matter given off through 

 the skin. Such measurements are taken by means of a 

 respiration apparatus, a costly and complicated mechan- 

 ism, a detailed description of which would be of little use 

 to most readers. It is sufficient to state that this appa- 

 ratus makes possible the collection and analysis of all the 

 excretory products, whether solid or gaseous. The 

 experimental man or animal lives in a closed chamber 

 into which is introduced food and fresh air and from 

 which is pumped the vitiated air, the water and carbon 

 dioxid of which are absorbed and weighed. 



All conclusions drawn from experiments with the 

 respiration apparatus are based largely upon the in- 

 come and outgo of nitrogen and carbon. As carbon 

 is a constituent of all possible compounds of the ani- 

 mal body except the mineral, it is certain that when 

 the body gains in carbon it gains in organic substance 

 of some kind, and if it loses in carbon there is a waste 

 of organic body substance. The general character of 

 the gain or loss can be determined by the nitrogen bal- 

 ance. If more nitrogen is taken in by the experimental 

 animal than is given off, it is clear that the nitrogen 



