MOTIVE POWER OF HEAT. 57 



in the bodies employed to realize the motive power 

 of heat there should not occur any change of tem- 

 perature which may not be due to a change of 

 volume. Reciprocally, every time that this condi- 

 tion is fulfilled the maximum will be attained. 

 This principle should never be lost sight of in the 

 construction of heat-engines ; it is its fundamental 

 basis. If it cannot be strictly observed, it should 

 at least be departed from as little as possible. 



Every change of temperature which is not due 

 to a change of volume or to chemical action (an 

 action that we provisionally suppose not to occur 

 here) is necessarily due to the direct passage of the 

 caloric from a more or less heated body to a colder 

 body. This passage occurs mainly by the contact 

 of bodies of different temperatures; hence such 

 contact should be avoided as much as possible. It 

 cannot probably be avoided entirely, but it should 

 at least be so managed that the bodies brought in 

 contact with each other differ as little as possible 

 in temperature. When we just now supposed, in 

 our demonstration, the caloric of the body A em- 

 ployed to form steam, this steam was considered as 

 generated at the temperature of the body A ; thus 

 the contact took place only between bodies of equal 

 temperatures ; the change of temperature occurring 

 afterwards in the steam was due to dilatation, con- 



