THE MECHANICAL EQUIVALENT OF HEAT. 



that the living organism, with all its riddles and marvels, can- 

 not create heat out of nothing. 



But if we hold firm to this physiological axiom, the an- 

 swer 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 sa- 

 crificing 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 organism must bear an invariable quantita- 

 tive relation to the work expended in producing it. 



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

 chanical 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 quanti- 

 ties of heat, the quantity 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 assump- 

 tion. Further, inasmuch as there is no difference in kind be- 

 tween 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 OP THE PHYSIOLOGICAL THEORY 

 OF COMBUSTION. 



While following in general the direction indicated, it was 

 accordingly needful for me in the end to fix my attention 

 chiefly on the physical connection subsisting between motion 

 and heat ; and it was thus impossible for the existence of the 

 mechanical equivalent of heat to remain hidden from me. 

 But, although I have to thank an accident for this discovery, 



