68 MOTIVE POWER OF HEAT. 



We have chosen atmospheric air as the instru- 

 ment which should develop the motive power of 

 heat, but it is evident that the reasoning would 

 have heen the same for all other gaseous substances, 

 and even for all other bodies susceptible of change 

 of temperature through successive contractions and 

 dilatations, which comprehends all natural sub- 

 stances, or at least all those which are adapted to 

 realize the motive power of heat. Thus we are led 

 to establish this general proposition : 



The motive power of heat is independent of the 

 agents employed to realize it ; its quantity is fixed 

 solely by the temperatures of the bodies between 

 which is effected, finally, the transfer of the caloric. 



We must understand here that each of the 

 methods of developing motive power attains the 

 perfection of which it is susceptible. This condi- 

 tion is found to be fulfilled if, as we remarked 

 above, there is produced in the body no other 

 change of temperature than that due to change of 

 volume, or, what is the same thing in other words, 

 if there is no contact between bodies of sensibly 

 different temperatures. 



Different methods of realizing motive power may 



principles on which the theory of heat rests require the 

 most careful examination. Many experimental facts ap- 

 pear almost inexplicable in the present state of this theory. 



