78 PRINCIPLES OF ANIMAL NUTRITION. 
sarily, then, on this assumption, each gram of carbon gained in 
excess of that stored in the form of protein will represent 1.3 grams 
of fat stored. 
Formation of Glycogen.—Granting the substantial accuracy of 
the computation of the gain or loss of protein, the only serious 
criticism to which the above method of computing the gain or loss 
of fat is subject is that it does not take account of the possible stor- 
age of carbon in other forms, and particularly as glycogen. In 
other words, it may be contended that the schematic body should 
be regarded as consisting of water, ash, fat, and carbohydrates. 
There is undoubtedly a certain degree of justification for this con- 
tention, and the significance of small gains of carbon, or of gains 
observed during short periods, is by no means unambiguous. But 
when such a gain is observed to continue day after day for weeks 
on an unchanged ration, as in some of the experiments cited on 
subsequent pages, the objection loses all force. 
Computation oF Toran Merrasorism.—The same principle 
may be applied to the computation of the total amount of protein 
and fat metabolized. From the urinary nitrogen (plus that of the 
feces if the latter be regarded as a metabolic product) by multipli- 
cation by the conventional factor we obtain, as already explained, 
the total proteid metabolism. Subtracting the amount of carbon 
corresponding to this quantity of protein from the total carbon 
excretion leaves a remainder which must have been derived from 
non-nitrogenous material. If carbohydrates are absent from the 
food, this material, in an experiment of any length, must be 
substantially fat, and the amount of the latter can be computed 
from the carbon by the use of the factor 1.3. In the presence of 
any considerable amount of carbohydrates, however, the results 
are ambiguous unless we know also the quantity of oxygen con- 
sumed. 
Other Determinations.—The great majority of investigations 
upon the metabolism of matter have been confined to determi- 
nations of the nitrogen and carbon balance. Occasionally, how- 
ever, other determinations have been made. 
HypRocEN BaLancre.—Determinations of water and of hydrogen 
in organic combination in food and excreta enable us, after making 
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