Investigation with Respiration Apparatus 197 



duced 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 sub- 

 stance 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 balance. If more nitrogen is taken in by the 

 experimental animal than is given off, it is clear that 

 the nitrogen compounds of the body have received an 

 accession. Knowing as we do the proportions of nitro- 

 gen and carbon in the various tissues of the animal, 

 we can calculate how much of the gain or loss of 

 carbon belongs in the nitrogenous substance deposited 

 or wasted. If more carbon is gained or lost than can 

 possibly be associated with the nitrogen gained or lost, 

 then there has been a gain or loss of fat, because protein 

 and fat being the main constituents of the animal car- 

 cass, any considerable retention of carbon must be in one 

 of these forms. If there has been nitrogen equilibrium, 

 all excess or deficit of carbon belongs to a deposit or 

 waste of fat. By such searching methods as these, it 

 is possible to ascertain with a good degree of accuracy 

 how food is used and what quantity and kind of nu- 

 trients are needed in maintaining an animal under 

 given conditions. 



