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
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 poun