CHEMICAL SOURCES OF HEAT. 317 



heat-units are formed in the body by oxidation, i.e., provided the substance is com- 

 pletely oxidised. 



[Several sources of heat-production or thermogenesis are to be found in all 

 tissues wherever oxidation is going on. The metabolism of protoplasm is always 

 associated with the evolution of heat.] 



(I) 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, II, O, 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 8080 heat-units, while 1 grm. H oxidised 

 to H 2 yields 34,460 heat-units. The O necessary for these purposes is absorbed 

 during respiration, so that, to a certain extent at least, the amount of heat produced 

 may be estimated from the amount of O consumed. The same consumption of O 

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

 (Pfluger). There is a relation, amounting to cause and effect, between the amount 

 of heat produced in the body and the O consumed. The cold-blooded animals, 

 which consume little O, 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 38 C. 1 kilo, of a living fowl uses 1*186 grms. O, and gives a 

 mean temperature of 43*9 C. 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 com- 

 bustion of inorganic substances in the body, e.g., of the sulphur into sulphuric acid, 

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

 heat. 



[The muscles form about the half of the whole mass of the body and the bones 

 nearly the other half. In the latter, oxidation does not go on actively, so that the 

 muscles must be the great seats of heat-production or thermogenesis in the body. 

 This view is supported by the fact that the blood leaving a muscle at rest contains 

 more C0 2 than the blood in the right ventricle. Muscular exercise greatly 

 increases the metabolism and the C0 2 excreted ( 127), but at the same time, there 

 is a great increase in heat-production. The muscles, therefore, are the great ther- 

 mogenic tissues, and they yield iths of the heat in health. The several secreting 

 glands, especially the liver and the alimentary canal, during digestion, are also foci 

 of heat-formation.] 



(6) In addition to the processes of combustion or oxidation, all those chemical 

 processes in our body, by which the amount of the available 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. 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., C0 2 , is unable to set aside the alkaline reaction, the amount of heat produced is less. 

 The formation of compounds of chlorine {e.g., in the stomach) produces heat. 



(j8) When a neutral salt is changed into a basic one. In the blood the sulphuric and 

 phosphoric acids derived from the combustion of S and P are united with the alkalies of the 

 blood to form basic salts. The decomposition of the carbonates of the blood by lactic and 

 phosphoric acids forms a double source of heat, on the one hand, by the formation of a new 

 salt, and, on the other, by the liberation of C0 2 , which is partly absorbed by the blood. 



(7) The combination of haemoglobin with ( 36). 



During those chemical processes, whereby the heat of the body is produced, 

 heat-absorbing intermediate compounds are not unfrequently formed. Thus, in 



