These are instruments of most universal use in chemistry; and, as the
success of a great number of experiments depends upon their being well
or ill constructed, it is of great importance that a laboratory be well
provided in this respect. A furnace is a kind of hollow cylindrical
tower, sometimes widened above, Pl. XIII. Fig. 1. ABCD, which must have
at least two lateral openings; one in its upper part F, which is the
door of the fire-place, and one below, G, leading to the ash-hole.
Between these the furnace is divided by a horizontal grate, intended
for supporting the fewel, the situation of which is marked in the figure
by the line HI. Though this be the least complicated of all the chemical
furnaces, yet it is applicable to a great number of purposes. By it
lead, tin, bismuth, and, in general, every substance which does not
require a very strong fire, may be melted in crucibles; it will serve
for metallic oxydations, for evaporatory vessels, and for sand-baths, as
in Pl. III. Fig. 1. and 2. To render it proper for these purposes,
several notches, m m m m, Pl. XIII. Fig. 1. are made in its upper
edge, as otherwise any pan which might be placed over the fire would
stop the passage of the air, and prevent the fewel from burning. This
furnace can only produce a moderate degree of heat, because the quantity
of charcoal it is capable of consuming is limited by the quantity of air
which is allowed to pass through the opening G of the ash-hole. Its
power might be considerably augmented by enlarging this opening, but
then the great stream of air which is convenient for some operations
might be hurtful in others; wherefore we must have furnaces of different
forms, constructed for different purposes, in our laboratories: There
ought especially to be several of the kind now described of different
sizes.

The reverberatory furnace, Pl. XIII. Fig. 2. is perhaps more necessary.
This, like the common furnace, is composed of the ash-hole HIKL, the
fire-place KLMN, the laboratory MNOP, and the dome RRSS, with its funnel
or chimney TTVV; and to this last several additional tubes may be
adapted, according to the nature of the different experiments. The
retort A is placed in the division called the laboratory, and supported
by two bars of iron which run across the furnace, and its beak comes out
at a round hole in the side of the furnace, one half of which is cut in
the piece called the laboratory, and the other in the dome. In most of