their mutual attraction, unite into the solid state.
As it was long thought that the air had great influence upon the
quantity of fluid evaporated, it will be proper to point out the errors
which this opinion has produced. There certainly is a constant slow
evaporation from fluids exposed to the free air; and, though this
species of evaporation may be considered in some degree as a solution in
air, yet caloric has considerable influence in producing it, as is
evident from the refrigeration which always accompanies this process;
hence we may consider this gradual evaporation as a compound solution
made partly in air, and partly in caloric. But the evaporation which
takes place from a fluid kept continually boiling, is quite different in
its nature, and in it the evaporation produced by the action of the air
is exceedingly inconsiderable in comparison with that which is
occasioned by caloric. This latter species may be termed _vaporization_
rather than _evaporation_. This process is not accelerated in proportion
to the extent of evaporating surface, but in proportion to the
quantities of caloric which combine with the fluid. Too free a current
of cold air is often hurtful to this process, as it tends to carry off
caloric from the water, and consequently retards its conversion into
vapour. Hence there is no inconvenience produced by covering, in a
certain degree, the vessels in which liquids are evaporated by continual
boiling, provided the covering body be of such a nature as does not
strongly draw off the caloric, or, to use an expression of Dr
Franklin's, provided it be a bad conductor of heat. In this case, the
vapours escape through such opening as is left, and at least as much is
evaporated, frequently more than when free access is allowed to the
external air.
As during evaporation the fluid carried off by caloric is entirely lost,
being sacrificed for the sake of the fixed substances with which it was
combined, this process is only employed where the fluid is of small
value, as water, for instance. But, when the fluid is of more
consequence, we have recourse to distillation, in which process we
preserve both the fixed substance and the volatile fluid. The vessels
employed for evaporation are basons or pans of copper, silver, or lead,
Pl. II. Fig. 13. and 15. or capsules of glass, porcellain, or stone
ware, Pl. II. A, Fig. 1. and 2. Pl. III. Fig. 3 and 4. The best utensils
for this purpose are made of the bottoms of glass retorts and matrasses,
as their equal thinness renders them more fit than any other kind of
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