62 MOTIVE POWER OF HEAT. 



When a gaseous fluid is rapidly compressed its 

 temperature rises. It falls, on the contrary, when 

 it is rapidly dilated. This is one of the facts best 

 demonstrated by experiment. We will take it for 

 the basis of our demonstration.* 



If, when the temperature of a gas has been 

 raised by compression, we wish to reduce it to its 

 former temperature without subjecting its volume 

 to new changes, some of its caloric must be re- 

 moved. This caloric might have been removed in 

 proportion as pressure was applied, so that the 

 temperature of the gas would remain constant. 

 Similarly, if the gas is rarefied we can avoid lower- 

 ing the temperature by supplying it with a cer- 

 tain quantity of caloric. Let us call the caloric 

 employed at such times, when no change of tem- 

 perature occurs, caloric due to change of volume. 

 This denomination does not indicate that the 

 caloric appertains to the volume : it does not ap- 

 pertain to it any more than to pressure, and 

 might as well be called caloric due to the change 

 of pressure. We do not know what laws it 

 follows relative to the variations of volume : it is 

 possible that its quantity changes either with the 

 nature of the gas, its density, or its temperature. Ex- 



* Note 13, Appendix B. 



