e 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
glass vessel for bearing a brisk fire and sudden alterations of heat and
cold without breaking.

As the method of cutting these glass vessels is no where described in
books, I shall here give a description of it, that they may be made by
chemists for themselves out of spoiled retorts, matrasses, and
recipients, at a much cheaper rate than any which can be procured from
glass manufacturers. The instrument, Pl. III. Fig. 5. consisting of an
iron ring AC, fixed to the rod AB, having a wooden handle D, is employed
as follows: Make the ring red hot in the fire, and put it upon the
matrass G, Fig. 6. which is to be cut; when the glass is sufficiently
heated, throw on a little cold water, and it will generally break
exactly at the circular line heated by the ring.

Small flasks or phials of thin glass are exceeding good vessels for
evaporating small quantities of fluid; they are very cheap, and stand
the fire remarkably. One or more of these may be placed upon a second
grate above the furnace, Pl. III. Fig. 2. where they will only
exper