500 Dr. J. R. Mayer on the Mechanical Equivalent of Heat, 



Whether the heat directly evolved is alone to be laid to the 

 account of the process of combustion, or whether it is the sum of 

 the heat evolved both directly and indirectly that is to be taken into 

 calculation. 



This is a question that touches the very foundations of science ; 

 and unless it receives a trustworthy answer, the healthy develop- 

 ment of the doctrine concerned is not possible. For it has been 

 already shown, by various examples, what are the consequences 

 of neglecting primary quantitative determinations. No wit of 

 man is able to furnish a substitute for what nature offers. 



The physiological theory of combustion starts from the funda- 

 mental proposition, that the quantity of heat which results from 

 the combustion of a given substance is invariable — that is, that 

 its amount is uninfluenced by the circumstances which accompany 

 the combustion ; whence we infer, " in specie" that the chemi- 

 cal effect of combustible matter can undergo no alteration in 

 amount even by the vital process, or that the living organism, 

 with all its riddles and marvels, cannot create heat out of 

 nothing. 



But if we hold firm to this physiological axiom, the answer to 

 the question started above is already given. For, unless we wish 

 to attribute again to the organism the power of creating heat 

 which has just been denied to it, it cannot be assumed that 

 the heat which it produces can ever amount to more than the 

 chemical action which takes place. On the combustion-theory 

 there is, then, no alternative, short of sacrificing the theory itself, 

 but to admit that the total amount of heat evolved by the 

 organism, partly directly, and partly indirectly by mechanical 

 action, corresponds quantitatively or is equal to the amount of 

 combustion. 



Hence it follows, no less inevitably, that the heat produced 

 mechanically by the orgamsm must bear an invariable quantitative 

 relation to the work expended in producing it. 



For if, according to the varying construction of the mecha- 

 nical arrangements which serve for the development of the heat, 

 the same amount of work, and hence the same amount of organic 

 combustion, could produce varying quantities of heat, the quan- 

 tity of heat produced from one and the same expenditure of 

 material would come out smaller at one time and larger at 

 another, which is contrary to our assumption. Further, inas- 

 much as there is no difference in kind between the mechanical 

 performances of the animal body and those of other inorganic 

 sources of work, it follows that 



AN INVARIABLE QUANTITATIVE RELATION BETWEEN HEAT AND 

 WORK IS A POSTULATE OF THE PHYSIOLOGICAL THEORY OF COM- 

 BUSTION. 



