circumstance the retort would only be heated in its bottom, the vapours
raised from the contained substance would condense in the upper part,
and a continual cohabitation would take place without any thing passing
over into the receiver, but, by means of the dome, the retort is equally
heated in every part, and the vapours being forced out, can only
condense in the neck of the retort, or in the recipient.
To prevent the bottom of the retort from being either heated or coolled
too suddenly, it is sometimes placed in a small sand-bath of baked clay,
standing upon the cross bars of the furnace. Likewise, in many
operations, the retorts are coated over with lutes, some of which are
intended to preserve them from the too sudden influence of heat or of
cold, while others are for sustaining the glass, or forming a kind of
second retort, which supports the glass one during operations wherein
the strength of the fire might soften it. The former is made of
brick-clay with a little cow's hair beat up alongst with it, into a
paste or mortar, and spread over the glass or stone retorts. The latter
is made of pure clay and pounded stone-ware mixed together, and used in
the same manner. This dries and hardens by the fire, so as to form a
true supplementary retort capable of retaining the materials, if the
glass retort below should crack or soften. But, in experiments which are
intended for collecting gasses, this lute, being porous, is of no manner
of use.
In a great many experiments wherein very violent fire is not required,
the reverberatory furnace may be used as a melting one, by leaving out
the piece called the laboratory, and placing the dome immediately upon
the fire-place, as represented Pl. XIII. Fig. 3. The furnace represented
in Fig. 4. is very convenient for fusions; it is composed of the
fire-place and ash-hole ABD, without a door, and having a hole E, which
receives the muzzle of a pair of bellows strongly luted on, and the dome
ABGH, which ought to be rather lower than is represented in the figure.
This furnace is not capable of producing a very strong heat, but is
sufficient for ordinary operations, and may be readily moved to any part
of the laboratory where it is wanted. Though these particular furnaces
are very convenient, every laboratory must be provided with a forge
furnace, having a good pair of bellows, or, what is more necessary, a
powerful melting furnace. I shall describe the one I use, with the
principles upon which it is constructed.
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