on Evaporation. 403 
a liquid in which steam is continually formed within itself, 
notwithstanding the external pressure ; and as such an expan- 
sive power in steam depends on a certain degree of density, 
its production requires, in the liquid, a certain degree of heat, 
which thus is determined by the degree of pressure. As for 
the fixity of the degree of heat in water boiling under a con- 
stant pressure, it proceeds from an equilibrium constantly 
produced between the quantity of fire which continues to 
penetrate the water, and that which goes off in the steam ; 
the differences which happen in the first of those quantities, 
having no other sensible effect, than to produce a more or 
less rapid formation of steam. 
7. Now from the same law derives also the difference be- 
tween the phaenomena of common evaporation and ebullition. 
If the latter requires a determined degree of heat, it is only 
because the steam cannot be formed within the water, with- 
out having at once a sufficient degree of density for overcom- 
ing alone the actual pressure exercised over that water by 
the atmosphere. But in common evaporation the steam is 
formed at the surface of the water by every temperature, 
because there it meets with no resistance but what it can al- 
ways overcome : for it only mixes with the air, and expands 
it in proportion to its quantity, as would a new quantity of 
air. 
8. Steam formed by common evaporation is absolutely of 
the same nature with that of boiling water ; and in respect 
of the pressure that it undergoes, it is in the same state as 
when produced by evaporation under an exhausted receiver. 
In this case, where the pressure of the atmosphere is suppress- 
ed, the resistance which steam meets with in the space is its 
