UNIVERSITY 



MOTIVE POWER OF HEAT. 49 



of permanent gases, that is, without ever return- 

 ing to a liquid state. Most of these substances 

 have been proposed, many even have been tried, 

 although up to this time perhaps without remark- 

 able success. 



We have shown that in steam-engines the motive- 

 power is due to a re- establishment of equilibrium 

 in the caloric ; this takes place not only for steam- 

 engines, but also for every heat-engine that is, 

 for every machine of which caloric is the motor. 

 Heat can evidently be a cause of motion only by 

 virtue of the changes of volume or of form which 

 it produces in bodies. 



These changes are not caused by uniform tem- 

 perature, but rather by alternations of heat and 

 cold. Now to heat any substance whatever requires 

 a body warmer than the one to be heated; to cool 

 it requires a cooler body. We supply caloric to 

 the first of these bodies that we may transmit 

 it to the second by means of the intermediary 

 substance. This is to re-establish, or at least to 

 endeavor to re-establish, the equilibrium of the 

 caloric. 



It is natural to ask here this curious and impor- 

 tant question : Is the motive power of heat invari- 

 able in quantity, or does it vary with the agent 

 employed to realize it as the intermediary sub- 



