MOTIVE POWER OF HEAT. 51 



We have already remarked upon this self-evident 

 factj or fact which at least appears evident as soon 

 as we reflect on the changes of volume occasioned 

 by heat : wherever there exists a difference of tem- 

 perature, motive-power can be produced. Recipro- 

 cally, wherever we can consume this power, it is 

 possible to produce a difference of temperature, 

 it is possible to occasion destruction of equilibrium 

 in the caloric. Are not percussion and the fric- 

 tion of bodies actually means of raising their tem- 

 perature, of making it reach spontaneously a 

 higher degree than that of the surrounding bodies, 

 and consequently of producing a destruction of 

 equilibrium in the caloric, where equilibrium pre- 

 viously existed ? It is a fact proved by experience, 

 that the temperature of gaseous fluids is raised by 

 compression and lowered by rarefaction. This is 

 a sure method of changing the temperature of 

 bodies, and destroying the equilibrium of the 

 caloric as many times as may be desired with the 

 same substance. The vapor of water employed in 

 an inverse manner to that in which it is used in 

 steam-engines can also be regarded as a means of 

 destroying the equilibrium of the caloric. To be 

 convinced of this we need but to observe closely 

 the manner in which motive power is developed by 

 the action of heat on vapor of water. Imagine 



