are intended to perform. Several of the most common kinds are
represented Pl. VII. Fig. 7. 8. 9. and 10. the one represented at Fig.
9. is almost shut at its mouth.
Though fusion may often take place without changing the nature of the
fused body, this operation is frequently employed as a chemical means of
decomposing and recompounding bodies. In this way all the metals are
extracted from their ores; and, by this process, they are revivified,
moulded, and alloyed with each other. By this process sand and alkali
are combined to form glass, and by it likewise pastes, or coloured
stones, enamels, &c. are formed.
The action of violent fire was much more frequently employed by the
ancient chemists than it is in modern experiments. Since greater
precision has been employed in philosophical researches, the _humid_ has
been preferred to the _dry_ method of process, and fusion is seldom had
recourse to until all the other means of analysis have failed.
SECT. II.
_Of Furnaces._
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
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