M. CLAPEYRON ON THE MOTIVE POWER OF HEAT. 



361 



We shall add that the equation 



Q = R (B - C \ogp) 

 gives the law of the specific calorics at a constant pressure and volume. 

 The expression of the first is 



of the second, 



equal to 



\dt dt ^^ 261 +t) 



267 + t 

 The first may be obtained by difierentiating Q with relation to t, sup 

 posing p constant ; the second, by supposing v constant. If we take 

 equal volumes of different gases at the same temperature and under the 

 same pressure, the quantity R will be the same for all; and accordingly we 

 see that the excess of specific caloric at a constant pressure, over the spe- 

 cific caloric of a constant volume, is the same for all, and equal to 



267 + t 



C. 



§ IV. 



The same method of reasoning applied to vapours enables us to esta- 

 blish a new relation between their latent caloric, their volume, and their 

 pressure. 



we have shown in the second paragraph how a liquid passing into 

 the state of vapour may serve to transmit the caloric from a body main- 

 tained at a temperature T, to a body maintained at a lower temperature t, 

 and how this transmission develops the motive force. 



Let us suppose that the temperature of the body B is lower by the 

 infinitely small quantity dt than the temperature of the body A. We 

 have seen that if c i (fig. 4.) represents the pressure of the vapour 

 ! 



Fig. 4. 



Di, 



* 3 A i 



