58 MOTIVE POWER OF HEAT. 



sequently to a change of volume. Finally, conden- 

 sation took place also without contact of bodies of 

 different temperatures. It occurred whiJe exert- 

 ing a constant pressure on the steam brought in 

 contact with the body B of the same temperature 

 as itself. The conditions for a maximum are thus 

 found to be fulfilled. In reality the operation 

 cannot proceed exactly as we have assumed. To 

 determine the passage of caloric from one body to 

 another, it is necessary that there should be an 

 excess of temperature in the first, but this excess 

 may be supposed as slight as we please. We can 

 regard it as insensible in theory, without thereby 

 destroying the exactness of the arguments. 



A more substantial objection may be made to 

 our demonstration, thus : When we borrow caloric 

 from the body A to produce steam, and when this 

 steam is afterwards condensed by its contact with 

 the body B, the water used to form it, and which 

 we considered at first as being of the temperature 

 of the body A, is found at the close of the opera- 

 tion at the temperature of the body B. It has 

 become cool. If we wish to begin again an opera- 

 tion similar to the first, if we wish to develop a 

 new quantity of motive power with the same in- 

 strument, with the same steam, it is necessary first 

 to re-establish the original condition to restore 



