50 MOTIVE POWER OF HEAT. 



stance, selected as the subject of action of the 

 heat? 



It is clear that this question can be asked only 

 in regard to a given quantity of caloric,* the differ- 

 ence of the temperatures also being given. We 

 take, for example, one body A kept at a tempera- 

 ture of 100 and another body B kept at a tempera- 

 ture of 0, and ask what quantity of motive power 

 can be produced by the passage of a given portion 

 of caloric (for example, as much as is necessary to 

 melt a kilogram of ice) from the first of these 

 bodies to the second. We inquire whether this 

 quantity of motive power is necessarily limited, 

 whether it varies with the substance employed to 

 realize it, whether the vapor of water offers in this 

 respect more or less advantage than the vapor of 

 alcohol, of mercury, a permanent gas, or any other 

 substance. We will try to answer these questions, 

 availing ourselves of ideas already established. 



* It is considered unnecessary to explain here what is 

 quantity of caloric or quantity of heat (for we employ 

 these two expressions indifferently), or to describe how we 

 measure these quantities by the calorimeter. Nor will we 

 explain what is meant by latent heat, degree of temperature, 

 specific heat, etc. The reader should be familiarized with 

 these terms through the study of the elementary treatises 

 of physics or of chemistry. 



