M. CLAPEYRON ON THE MOTIVE POWER OF HEAT. 363 



With regard to the generality of vapours, the ratio — of the density 



of the vapour to that of the liquid from which it is formed may be neg- 

 lected before it arrives at unity, so long as the temperature is not very 

 high ; we shall have therefore, sensibly, 



dt 



This equation expresses that the latent caloric contained in equal 

 volumes of the vapour of different liquids at the same temperature, and 



under the corresponding pressure, is proportional to the coefficient -^ 



dt 

 of the pressure with regard to the temperature. 



Whence it results, that the latent caloric contained in the vapours of 

 liquids which commence boiling only at high temperatures, as mercury 



for example, is very feeble, since for these vapours the quantity ^ is 



dt 

 verj' small. 



We shall not insist upon the consequences which result from the equa- 

 tion 



\ p ) dt 



We shall simply remark that if, as eveiything leads us to believe, C 

 ^"^ rff ^" "*^* become infinite for any value of the temperature, k will 

 become null when we have g = p,that is, that when the pressure is stron"- 

 enough, and the temperature sufficiently elevated to render the density 

 of the vapour equal to that of the liquid, the latent caloric is reduced to 

 zero. 



§ V. 



Variation is produced in the volume of all the substances of nature 

 by changes in the temperature and pressure to which they are subjected; 

 liquids and solids are amenable to this law, and serve equally to dev e- 

 lop the motive power of heat; it has been proposed to substitute them 

 for the vapour of water, in order to render this motive force available ; 

 they have even occasionally been advantageously employed, particularly 

 when it was necessary to develop a very considerable momentaneous 

 effort, exerted within narrow limits. 



In bodies of these kinds, as in the gases, it may be remarked, that of 

 the four quantities, the volume v, the pressure j9, the temperature T, and 

 the absolute quantity of heat Q, two being determined, the others are 



Vol. I.— Part III. 2 c 



