M. CLAPEYRON ON THE MOTIVE POWER OF HEAT. 363 
(=: dp 
p/dt 1 a _ 6\dp 
ams; sae whence A= (1 )ee 
With regard to the generality of vapours, the ratio = of the density 
p 
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, 
d 
a1 i 
dt 
This equation expresses that the latent calorie contained in equal 
volumes of the vapour of different liquids at the same temperature, and 
under the corresponding pressure, is proportional to the coefficient a 
of the pressure with regard to the temperature. 
Whence it results, that the latent calorie 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 
very small. 
We shall not insist upon the consequences which result from the equa- 
tion 
0 \dp C 
i ( 2) , 
We shall simply remark that if, as everything leads us to believe, C 
and = do not become infinite for any value of the temperature, & will 
become null when we have 6=,, that is, that when the pressure is strong 
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 deve- 
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 
: 
: 
: 
4 
= 
t 
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 p, the temperature T, and 
the absolute quantity of heat Q, two being determined, the others are 
Vor. I.—Panrt JII. 2c 
