92 MOTIVE POWEH OF HEAT. 



same time t = 0, log v = 0, whence A . = ; ^ will 

 then express not only the increase of temperature, 

 but the temperature itself above the thermometric 

 zero. 



We need not consider the formula that we have 

 just given as applicable to very great changes in 

 the volume of gases. We have regarded the ele- 

 vation of temperature as being in inverse ratio to 

 the specific heat; which tacitly supposes the specific 

 heat to be constant at all temperatures. Great 

 changes of volume lead to great changes of tem- 

 perature in the gas, and nothing proves the con- 

 stancy of specific heat at different temperatures, 

 especially at temperatures widely separated. This 

 constancy is only an hypothesis admitted for gases 

 by analogy, to a certain extent verified for solid 

 bodies and liquids throughout a part of the ther- 

 mometric scale, but of which the experiments of 

 MM. Dulong and Petit have shown the inaccuracy 

 when it is desirable to extend it to temperatures 

 far above 100.* 



According to a law of MM. Clement and De- 

 sormes, a law established by direct experiment, the 

 vapor of water, under whatever pressure it may 

 be formed, contains always, at equal weights, the 



* Note C, Appendix B. 



