18 



ANIMAL MECHANISM. 



with the combustion of the coal which he has burned ; he will 

 prove that the heat which he has obtained is less than this, 

 which is made evident by the disappearance of a certain 

 number of units ; this disappearance he will attribute to the 

 transformation of heat into work. Now as he knows the 

 number of kilogrammetres produced by the machine, he will 

 only have to divide this number by that of the units of heat 

 which have disappeared, in order to find the number of 

 kilogrammetres equivalent to each of them. 



Hirn believed that the combustion effected, the heat given 

 out, and the mechanical work produced by a man could be 

 estimated at the same time. He enclosed the subject in a 

 hermetically closed chamber, and made him turn a wheel 

 which could, at choice, revolve with or without doing work. 



The air of the chamber being analysed, showed what 

 quantity of carbonic acid had been given out ; from thence 

 were deduced the combustion produced and the number of 

 units of heat to which that combustion ought to have corre- 

 sponded. 



The heat given out in the chamber was ascertained by the 

 ordinary calorimetric processes ; it was, in proportion to the 

 work produced, sensibly inferior to that which ought to 

 have been found according to the quantity of carbonic acid 

 exhaled. 



This disappearance of a certain number of units of heat 

 was explained by their transformation into mechanical work. 



From these experiments Hirn deduced a valuation of the 

 mechanical equivalent of heat -for animated motors ; but the 

 number which he obtained differed considerably from that 

 which has been established by physicists. This difference is in 

 no wise surprising when we think of all the causes of error 

 which are united in such an experiment. There may have been 

 an error concerning the quantity of carbonic acid exhaled ; an 

 error concerning the nature of the chemical actions which 

 disengaged this carbonic acid, and therefore respecting the 

 quantity of heat which ought to have accompanied the disen- 

 gagement ; an error in the estimation of the heat diffused 

 through the calorimetric chamber ; finally, an error as to the 

 quantity of mechanical work produced by the subject. In 



