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 Kubner l 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, under 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. 



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 O47 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 KELATION 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 Kubner. 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, Miinchen, 1894, Bd. xxx. S. 135. 



2 Favre and Silbermann, Ann. de chim. et phys., Paris, 1842, Se*r. 3, tome xxxiv. 

 p. 357 ; Woods, London, Edinburgh, and Diiblin Phil. Mag., London, 1851, vol. ii. p. 268, 

 1852, vol. iv. p. 370 ; Joule, ibid., 1852, vol. iii. p. 481. 



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