42 
Steam Engine, 
This eminent elasticity of caloric tends continually to pro- 
duce expansion ; on the other hand, this fluid, by a par- 
ticular destination of nature, is more or less disseminated 
between the moleculae of all bodies, in such sort that we 
may say with M. Lavoisier that even in the solid state 
these moleculae do not touch, but, as it were, swim in the 
caloric at a certain distance from each other. There must, 
therefore, be a perpetual contest between the expansive 
force of caloric which tends to disseminate the moleculae, 
and the cohesive attraction of the moleculae which 
tends to join them together. From the reciprocal in-= 
intensity of these two powers results the solid and liquid 
states of bodies : thus, water only differs from ice by the 
greater or less condensation of caloric, which permits 
more or less of the moleculae of the liquid to yield to the 
effect of their attraction or reciprocal cohesion. 
When substances pass from the liquid to the aeriform, 
state, there is a third power to combine with the expan. 
sive effort of caloric, and the aggregative or attractive ef- 
fort of the moleculae ; namely, the pressure of the atmos» 
phere, or of any elastic fluid whatever which compresses 
the fluid, and opposes itself to the separation of its parts. 
This third powder has a certain influence also upon the pas.= 
sage from the solid to the fluid state, but it is most fre- 
quently (in this case) very small, and even evanescent in 
comparison of the resistance arising from the mutual co- 
hesion of the molecule. The contrary effect has place in 
the course of the passage from the liquid to the gaseous 
or aeriform state ; the cohesion of the fluid moleculae be- 
ing extremely small, the elasticity of the caloric has scarce- 
ly any thing to surmount to produce volatilisation besides 
the pressure of the atmosphere, or gas which actually 
compresses it. 
Hence it results that the same liquid under different pres- 
sures ought to volatilise at different temperatures. M, 
Lavoisier proved the truth of this result, by placing ether 
