882 SPECIAL PHYSIOLOGY. 



and Silbermann, prove that the heat evolved by carbon in its combus- 

 tion, is greater than that estimated by Dulong and Despretz. It is 

 now known, that more oxygen is absorbed in respiration, than is re- 

 turned in the form of carbonic acid, and this oxygen doubtless is also 

 combined in-the system, partly with hydrogen, and partly with sulphur 

 and phosphorus, in either case evolving a certain amount of the heat 

 of chemical combination. The hydrogen of the carbhydrates being 

 already associated with oxygen in the proportions to form water, is 

 not supposed to be able to give out further heat ; but some of that 

 contained in the fat and albuminoid bodies, must be oxidized in the 

 system, as was first suggested by Barral, though much of the hydro- 

 gen of the nitrogenous substance, appears in the urea. The larger part 

 of the heat is, however, due to the oxidation of carbon in the system. 

 The nitrogen is never oxidized, but passes out almost entirely in the 

 form of urea, and supplies no animal heat. Experiments have shown 

 that carbon, in different states of aggregation, yields slightly different 

 quantities of heat in being burnt, wood charcoal giving out more heat 

 than the more compact coke. The combination of carbon and hydro- 

 gen in the animal economy, is not like that which occurs in artificial 

 combustion, simple and direct, but complex, and marked by intermedi- 

 ate decompositions, and most varied products. By many, it has been 

 supposed that these conditions might modify, in some way, the quan- 

 tity of heat evolved ; but it seems more probable, that no number of 

 intermediate stages of decomposition can alter the total quantity of 

 heat given out ; and that, according to the degree of oxidation which 

 . occurs, the same amount of heat is evolved, whether this be direct and 

 rapid, or complex arid slow. Such decompositions in the body may 

 affect the amount of heat evolved in particular organs or parts of the 

 system, as in the liver, kidneys, or the muscles when in action ; but 

 they cannot modify the ultimate or total heat product. It has even 

 been conjectured that the oxygen of the atmosphere used in respira- 

 tion, being partly ozonized, might evolve a larger amount of heat 

 than in ordinary combustion'; but the same air is employed in both 

 cases. 



The idea entertained by Dulong and Despretz, that a balance of 

 heat might be evolved in the conversion of fluid into solid substances 

 during the act of nutrition, has been mentioned as fallacious. If, 

 indeed, new nutrient matter be solidified in the act of deposition, the 

 process of disintegration of tissue which precedes or accompanies this, 

 implies a precisely similar amount of liquefaction which would involve 

 a disappearance of heat. In the digestive and secretory processes, 

 too, the numerous acts of liquefaction imply absorption of heat. Ber- 

 tholet has recently advocated the view, that molecular as well as chem- 

 ical changes in the body, may give rise to heat. The mode in which 

 -a particular, complex, organic compound splits up, may influence the 

 amount of heat which it gives off, and, locally, this would affect the 

 temperature ; but there is a balance in these actions, and the total re- 

 sult is that of simple change. Certain processes of hydration, or the 

 assumption and fixation of constitutional wafer, which are supposed by 

 Bertholet to be constantly occurring in the system, and to be the 



