MOTIVE POWER OF HEAT. 71 



Let us suppose that operations 1 and 2 be per- 

 formed on two gases of different chemical natures 

 but under the same pressure under atmospheric 

 pressure, for example. These two gases will be- 

 have exactly alike under the same circumstances, 

 that is, their expansive forces, originally equal, 

 will remain always equal, whatever may be the 

 variations of volume and of temperature, provided 

 these variations are the same in both. This results 

 obviously from the laws of Mariotte and MM. Gay- 

 Lussac and Dalton laws common to all elastic 

 fluids, and in virtue of which the same relations 

 exist for all these fluids between the volume, the 

 expansive force, and the temperature. 



Since two different gases at the same tempera- 

 ture and under the same pressure should behave 

 alike under the same circumstances, if we subjected 

 them both to the operations above described, they 

 should give rise to equal quantities of motive power. 



Now this implies, according to the fundamental 

 proposition that we have established, the employ- 

 ment of two equal quantities of caloric; that is, it 

 implies that the quantity of caloric transferred from 

 the body A to the body B is the same, whichever 

 gas is used. 



The quantity of caloric transferred from the 

 body A to the body B is evidently that which is 



