METHODS OF INVESTIGATION. 67 
Second, in the nitrogenous products of the proteid metabolism, 
contained chiefly in the urine but including also the small quanti- 
ties of nitrogenous metabolic products contained in the feces and 
perspiration. 
The nitrogen of urine and perspiration, then, together with the 
metabolic nitrogen of the feces, will indicate the extent of proteid 
katabolism, while the difference between total income and total 
outgo of nitrogen will show whether the body is gaining or losing 
protein. Finally, since the losses of metabolic nitrogen in feces and 
perspiration are relatively small, and often not readily determinable, 
in cases where the greatest accuracy is not required, and particularly 
in comparative experiments, we may regard the total urinary nitro- 
gen as representing with a fair degree of accuracy the amount of 
protein broken down by the organism. 
In the foregoing statements, however, it has been tacitly assumed 
that the protein of the food consists of true proteids. If, how- 
ever, the latter are accompanied by amides and other non-proteid 
nitrogenous bodies, which do not appear to contribute to the forma- 
tion of proteid tissue (compare p. 53), the corresponding amount 
of nitrogen will appear in the urine and be added to that derived 
from the actual katabolism of body or food proteids. This, how- 
ever, does not, of course, affect any conclusions as to the gain or loss 
of protein by the body. 
Factor For Protrin.—It is thus comparatively easy to deter- 
mine in terms of nitrogen both the proteid katabolism and the 
gain or loss of protein, the principal precaution necessary, aside 
from technical details, being that the experiment shall extend over 
a sufficient length of time to eliminate the influences of irregulari- 
ties in ingestion and excretion. 
Knowing approximately the ultimate composition of the pro- 
tein of the body, we may take a step further and infer from the 
amounts of nitrogen determined the corresponding amounts of 
protein, the accuracy of the result depending, of course, upon the 
accuracy of the figures on which it is based. The composition 
commonly assumed for the body protein has been that given on 
page 65, and the same conventional factor, 6.25, has been used 
to convert nitrogen into protein which has been employed in case 
of feeding-stuffs. Kéhler’s investigations (p. 64) show that the 
