M. CLAPEYRON ON THE MOTIVE POWER OF HEAT. 361 
We shall add that the equation 
Q=R(B —- Clogp) 
gives the law of the specific calorics at a constant pressure and volume, 
The expression of the first is 
aqdB dC 
—_ ~~ los 5 
R(F dt a3 
of the second, 
1 dp 
R(G- gp-Co SF), 
equal to 
(GE F 8? ~ aarzi) 
BP x67 4 
The first may be obtained i differentiating Q with relation to ¢, 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- 
R 
cific caloric of aconstant yolume, is thesame for all, and equal to eT Lt 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 ¢, 
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 d¢ than the temperature of the body A. We 
have seen that if cd (fig. 4.) represents the pressure of the vapour 
| 
Fig. 4. 
ia 
Q 
i~] 
te 
a 
a 
