98 MOTIVE POWER OF HEAT. 



with no more experimental data than we now pos- 

 sess, the law according to which the motive power 

 of heat varies at different points on the ther mo- 

 metric scale. This law is intimately connected 

 with that of the variations of the specific heat of 

 gases at different temperatures a law which ex- 

 periment has not yet made known to us with suffi- 

 cient exactness.* 



We will endeavor now to estimate exactly the 

 motive power of heat, and in order to verify our 

 fundamental proposition, in order to determine 

 whether the agent used to realize the motive power 

 is really unimportant relatively to the quantity of 

 this power, we will select several of them succes- 

 sively: atmospheric air, vapor of water, vapor of 

 alcohol. 



Let us suppose that we take first atmospheric 

 air. The operation will proceed according to the 

 method indicated on page 70. We will make the 

 following hypotheses : The air is taken under 

 atmospheric pressure. The temperature of the 

 body A is y^r ^ a degree above zero, that of the 

 body B is zero. The difference is, as we see, very 

 slight a necessary condition here. 



The increase of volume given to the air in our 



* Note E, Appendix B. 



