THE CONSERVATION OF ENERGY IN THE ANIMAL BODY. 265 
agree very closely and, granting the entire validity of the numerous 
assumptions involved in this method, would seem to approach a 
demonstration of the applicability of the law of the conservation of 
energy to the metabolism of the animal. Aside from errors in the 
estimation of the carbon of the excreta from their nitrogen, which 
are probably small, the chief elements of uncertainty are the 
assumptions as to the nature of the material metabolized in the 
body and as to the heat of combustion of the excreta. As regards 
the former point, Rubner himself points out (loc. cit., pp. 118-121) 
that a portion of the carbon of the respiration may be derived from 
glycogen, and even bases upon the calorimetric results in one case 
a computation of the extent to which this may have occurred. The 
latter, however, is obviously begging the question, and in his main 
computations Rubner assumes that only protein and fat were meta- 
bolized. 
LAULANI£’S EXPERIMENTS.—By means of his differential water- 
calorimeter, Laulanié * has determined the respiratory exchange 
and the heat production of animals, both fasting and fed. The 
nitrogen excretion does not appear to have been determined. 
From the respiratory exchange the heat production is computed, 
using the data given on p. 250, and compared with that obtained 
calorimetrically. In the fasting experiments an evolution of 4.6 
Cals. of heat is computed per liter of oxygen consumed. In the 
experiments in which food was given the author computes from the 
respiratory quotient the distribution of the oxygen between fat and 
carbohydrates, neglecting the protein because it yields the same 
amount of heat per unit of oxygen as does fat, and thence calculates 
the heat production. Preliminary tests of the calorimeter, by 
allowing water to cool in it, gave respectively 101.3 per cent., 100.9 
per cent., and 99.7 per cent. of the theoretical results. The experi- 
ments show a close agreement between the observed and computed 
amounts of heat, as appears from the table at the top of page 266. 
Arwater & Brnepicr’s InvesTications.— By far the most 
extensive and complete data regarding the conservation of energy 
in the animal body are those afforded by the investigations of 
Atwater & Benedict + upon man. The experiments were made 
* Archives de Physiol , 1898, p. 748. 
+U. S. Dept. Agr, Office of Experiment Stations, Bull. 109; Memoirs 
Nat Acad. Sci., 8, 235. 
