CHEMICAL CHANGE AND HEAT PRODUCTION. 833 
Helmholtz, Ludwig, Pfliiger, and others, by their investigations upon the 
production of heat in muscle, glands, and other tissues, and their determina- 
tions of the respiratory exchange of animals, have indicated where and how 
heat is produced. Finally, the exact determinations made by Rubner 1 upon 
heat production and metabolism have proved that chemical change is the 
cause of animal heat. Simultaneous determinations of the exchange of 
material and the production of heat in dogs, undei different conditions as 
regards diet, were made, and the results show that the heat of combustion of 
the food, as determined in a calorimeter, is equal to the heat given off by the 
animal; in fact, the animal must be looked upon as a living calorimeter, in 
which the food is burnt. The results are so exact that they prove the con- 
servation of energy in a vital process. 
Condition of the Animal. 
Heat as Calculated. 
Heat as found 
by Calorimeter. 
Percentage 
Difference. 
Fasting 
Diet of fat . 
Diet of flesh nnu fat 
Diet of flesh 
1296-3 cal. 
1510-1 ,, 
2492-4 ,, 
4780-8 ,, 
1305-2 
1495-3 
24S8-0 
4769-3 
+ 0-69 
-0-97 
-0-17 
-0-24 
The above figures only give some of the results, but the mean of all the 
experiments shows that the amount of heat, as determined directly by the 
animal calorimeter, is only 047 per cent, less than the amount as calculated 
from the heats of combustion of the different substances which have been 
decomposed in the animal's body. 
The Relation of Chemical Change to Heat Production. 
A consideration of the law of the conservation of energy leads to the 
conclusion that the sole cause of animal heat is a chemical process, a 
combustion of food substances by the oxygen taken in by the animal ; 
as just mentioned, the experimental proof of this conclusion has been 
recently given by Bubner. The chemical energy of the ingesta 
manifests itself chiefly in two forms, heat and motion. 
In this connection it is important to consider the heats of combus- 
tion of the various substances which form part of an animal's body or 
food, for it will thereby be possible to determine indirectly the amount 
of heat produced by an animal. A given amount of chemical action is 
accompanied by the production or the absorption of a definite quantity 
of heat. The accurate determination of this quantitative relation is 
beset with considerable difficulties, for the chemical changes in the 
complex substances of animal tissues or food are rarely simple, and are 
accompanied by physical changes, which have to be measured and taken 
into account before the amount of heat due to the chemical change can 
be estimated. Chemical decomposition is attended with the absorption 
of a quantity of heat equal to that which would be evolved by the 
combination of the same chemical substances. 2 Therefore, in the 
1 Ztschr.f. Biol, Munchen, 1894, Bd. xxx. S. 135. 
- Favre and Silbermann, Ann. de chim. et 2 } hys., Paris, 1842, Ser. 3, tome xxxiv. 
p. 357 ; Woods, London, Edinburgh, and Dublin Phil. May., London, 1851, vol. ii. p. 268, 
1852, vol. iv. p. 370 ; Joule, ibid., 1852, vol. iii. p. 481. 
VOL. I.— 53 
