MOTIVE POWER OF HEAT. 131 



of force. The amount of mechanical effect thus 

 developed will depend not only on the calorific 

 agency concerned, but also on the alteration in the 

 physical condition of the body. Hence, after al- 

 lowing the volume and temperature of the body to 

 change, we must restore it to its original tempera- 

 ture and volume; and then we may estimate the 

 aggregate amount of mechanical effect developed 

 as due solely to the thermal origin. 



6. Now the ordinarily-received, and almost uni- 

 versally-acknowledged, principles with reference 

 to "quantities of caloric" and "latent heat" lead 

 us to conceive that, at the end of a cycle of opera- 

 tions, when a body is left in precisely its primitive 

 physical condition, if it has absorbed any heat dur- 

 ing one part of the operations, it must have given 

 out again exactly the same amount during the re- 

 mainder of the cycle. The truth of this principle 

 is considered as axiomatic by Carnot, who admits 

 it as the foundation of his theory ; and expresses 

 himself in the following terms regarding it, in a 

 note on one of the passages of his treatise :* 



" In our demonstrations we tacitly assume that 

 after a body has experienced a certain- number of 

 transformations, if it be brought identically to its 



* Carnot, p. 67. 



