THE FOOD AS A SOURCE OF ENERGY. 273 
assumption is at least in harmony with current opinion. Rubner’s 
experiments were therefore directed to the determination of the 
metabolizable energy of the proteids. 
The earlier computations of the metabolizable energy of the 
proteids by Frankland, Traube, Danilewski, and others * were af- 
fected by two sources of error. First, the heats of combustion as 
determined by the imperfect calorimetric methods then available 
were seriously in error. Second, the manner of computing the 
metabolizable energy from these data has been shown by Rubner 
to be incorrect. Previous to his investigations the metabolizable 
energy of the proteids had been very generally computed by deduct- 
ing from their gross energy the energy of the corresponding amount 
of urea. In other words, it was assumed that all the nitrogen of the 
proteids was split off in the form of urea and excreted in the urine, 
which was accordingly regarded as being practically an aqueous 
solution of urea, and that the non-nitrogenous residue of the proteids 
was completely oxidized to carbon dioxide and water. Rubner’s 
results show that this assumption is seriously erroneous and gives 
too high results for the metabolizable energy. 
In the first place, it neglects entirely one of the waste products, 
‘viz., the feces. The latter are to be regarded in the carnivora, 
especially on a proteid diet, as a true excretory product. comparable 
to the organic matter of the urine and containing at most but traces 
of undigested food. This was early pointed out by Bischoff & 
Voit + and is now generally admitted by physiologists. (Compare 
p. 47.) In Rubner’s experiments somewhat over 3 per cent. of 
the energy of the proteid food was found in the feces. 
In the second place, Rubner shows that the urine is far from 
being a simple solution of urea.{ His previous investigations § had 
shown that the extractives of lean meat, the form of proteid most 
commonly used in such experiments, pass through the system un- 
changed and are excreted in the urine, thus increasing its content of 
energy. By feeding meat previously treated with water to remove 
* Cf, Rubner, loc. cit., p. 341. 
+ Ernahrung des Fleischfressers, p. 291; compare also Miller, Zeit. f. 
Biol., 20, 327; Rieder, ibid., 20, 378; Tsuboi, zbid., 85, 68. 
{ Compare Chapter VIII, p. 241. 
§ Zeit. f. Biol., 20, 265. 
