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
the ready made reverberatory furnaces which are sold by the potters at
Paris, the openings both above and below are too small: These do not
allow a sufficient volume of air to pass through; hence, as the quantity
of charcoal consumed, or, what is much the same thing, the quantity of
caloric disengaged, is nearly in proportion to the quantity of air which
passes through the furnace, these furnaces do not produce a sufficient
effect in a great number of experiments. To remedy this defect, there
ought to be two openings GG to the ash-hole; one of these is shut up
when only a moderate fire is required; and both are kept open when the
strongest power of the furnace is to be exerted. The opening of the dome
SS ought likewise to be considerably larger than is usually made.
It is of great importance not to employ retorts of too large size in
proportion to the furnace, as a sufficient space ought always to be
allowed for the passage of the air between the sides of the furnace and
the vessel. The retort A in the figure is too small for the size of the
furnace, yet I find it more easy to point out the error than to correct
it. The intention of the dome is to oblige the flame and heat to
surround and strike back or reverberate upon every part of the retort,
whence the furnace gets the name of reverberatory. Without this
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