424 CHEMICAL SOURCES OF HEAT. 



When we know the weight of any of the above-named substances 

 consumed by a man in twenty-four hours, a simple calculation enables 

 us to determine how many heat-units are formed in the body by 

 oxidation i.e., provided the substance is completely oxidised. 



Sources of Heat. The individual sources of heat are to be found in 

 the following : 



(1.) In the transformation of the chemical constituents of the food, 

 endowed with a large amount of potential energy, into such substances 

 as have little or no energy. 



The organic substances used as food consist of C, H, 0, N, so that there 

 takes place (a) Combustion of C into C0 2 , of H into H 2 0, whereby heat 

 is produced; 1 grm. C burned to produce C0 2 yields 8,080 heat-units, 

 while 1 grm. H oxidised to H 2 yields 34,460 heat-units. The neces- 

 sary for these processes is absorbed during respiration, so that, to a cer- 

 tain extent at least, the amount of heat produced may be estimated from 

 the amount of consumed. The same consumption of gives rise 

 to the same amount of heat whether it is used to oxidise H or C 

 (Pfliiger). There is a relation amounting to cause and effect, between 

 the amount of heat produced in the body and the consumed. The 

 cold-blooded animals, which consume little have a low temperature ; 

 amongst warm-blooded animals, 1 kilo, of a living rabbit takes up 

 within an hour 0'914 grm. 0, and its body is heated to a mean of 

 38C. 1 kilo, of a living fowl uses 1*186 grms. 0, and gives a mean 

 temperature of 43'9C. (Eegnault and Keiset). The amount of heat 

 produced is the same whether the combustion occurs slowly or quickly; 

 the rapidity of the metabolism, therefore, affects the rapidity, but not 

 the absolute amount of heat production. The combustion of inorganic 

 substances in the body, such as the sulphur into sulphuric acid, the 

 phosphorus into phosphoric acid, is another, although very small, source 

 of heat. 



(&.) In addition to the processes of combustion or oxidation, all 

 those chemical processes in our body, by which the amount of the avail- 

 able potential energy which is present is diminished, in consequence of 

 a greater satisfaction of atomic affinities, lead to the production of 

 heat. In all cases where the atoms assume more stable positions with 

 their affinities satisfied, chemical energy passes into kinetic thermal 

 energy, as in the alcoholic fermentation of grape-sugar, and other 

 similar processes. 



Heat is also developed during the following chemical processes : 

 (a) During the union of bases with acids (Andrews). The nature of the base 

 determines the amount of heat produced, while the nature of the acid is without 

 effect. Only in those cases where the acid, e.g., CO 2 , is unable to set aside the 

 alkaline reaction, the amount of heat produced is less. The formation of com- 

 pounds of chlorine (e.g., in the stomach) produces heat. 



