244 PRINCIPLES OF ANIMAL NUTRITION. 
These results tend strongly to substantiate the belief that the 
combustible gases practically consist of methane only. 
Perspiration.—In view of the relatively minute amounts of 
organic matter contained in the perspiration it has generally been 
regarded as a negligible quantity. The data given on p. 48 for 
the nitrogen of the sensible perspiration would afford some approxi- 
mate data for computing the amount of energy contained in it. 
The Energy of Tissue Gained.—The amount of potential energy 
stored up in a gain of tissue, or the amount liberated in the kinetic 
form in case the gain is negative, cannot, of course, be made the 
subject of a direct determination. The amounts of protein and of 
fat gained or lost can, however, be determined by the methods 
described in Chapter III, and their energy content computed from 
average figures. The errors involved are those incident to the 
method of computation from the carbon and nitrogen balance, 
which have already been considered in the chapter cited, and those 
arising from uncertainty as to the exact energy content of the 
material gained by the body. 
ProTern.—Just as computations of the gain or loss of protein 
by the body have been based upon the average composition of the 
proteids, so computations of its energy content have been based 
upon the average heat of combustion of these substances. The 
compilation by Atwater on pp. 237-9 contains the available data 
up to 1894. 
For approximate computations the value 5.7 Cals. per gram has 
been commonly used, while in more exact computations it has 
been assumed that the gain of protein by the animal has substan- 
tially the heat value as well as the chemical composition of fat-free 
muscular tissue (see p. 63), and the average of Stohmann’s two 
determinations, viz., 5.652 Cals. per gram, has been employed. 
Kéhler’s investigation * of the composition of fat and ash-free 
muscular tissue (p. 64) included determinations of the heats of 
combustion which are reproduced on the opposite page. 
Far.—Rubner, in his computations, employs the round number 
9.4 Cals. per gram for fat, while Kellner uses the value 9.5 Cals. 
Benedict & Osterberg,} whose determinations of the composition of 
* Zeit. physiol. Chem., 31, 479. 
+ Amer. Jour. Physiol., 4, 69. 
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