274 PRINCIPLES OF ANIMAL NUTRITION. 
these extractives, he demonstrates that in this case also the urine 
is far from being a simple solution of urea. With a daily excretion 
of 13.22 grams of total urinary nitrogen, there was found in the urine 
0.105 gram of kreatinin, 0.656 gram of cyanuric acid, and an un- 
determined amount of phenol. The proportion of carbon to nitro- 
gen in the urine was also notably higher than in urea, viz., 0.523: 1 
in place of 0.428:1, or an excess of about 20 per cent. Rubner 
concludes that the only sure method of ascertaining the amount of 
potential energy carried off in the urine is the direct determination 
of its heat of combustion. Accordingly, in the experiments under 
consideration, the urine was dried on pumice-stone and burned in 
the calorimeter, a correction being made for the urea decomposed 
during the drying. Danilewski,* about the same time, also re- 
ported determinations of the heat of combustion of the dry matter 
of human urine which, like Rubner’s, show an excess over that 
computed from the urea present. 
The materials experimented on by Rubner were prepared lean 
meat, such as has been commonly used in feeding experiments, 
and meat with the extractives removed by treatment with water, 
the gross energy of each being determined by burning the dried 
material in the calorimeter after having removed the fat by extrac- 
tion with alcohol and ether.t The prepared material (in the moist 
state) was fed to dogs for from five to eight days, during all or a 
portion of which time the feces and urine were collected and their 
content of nitrogen and energy determined. The amounts fed are 
not stated, but the percentage of the total nitrogen fed which 
reappeared in the feces is given. A third experiment on a fasting 
dog was added in which the urine of the second, third, and fourth 
days was collected and examined. : 
So far as the proteids are metabolized in the body all their nitro- 
gen which does not reappear in the feces will be found in the urine. 
On this basis the nitrogen per gram of dry proteids metabolized in 
these «xperiments was divided as shown in the following table. In 
the case of the fasting animal, Rubner believes himself justified, on the 
basis of other experiments, in assuming that the nitrogenous tissue 
* Arch. ges. Physiol., 36, 230. 
} Subsequent investigations have shown that the material thus prepared 
still contains traces of fat. 
