through the openings of the dome and fire-place. The external air enters
through the openings of the ash-hole for supporting the combustion, and
escapes by the superior opening or chimney at EE; and air is admitted
through the door of the muffle GG for oxydating the contained metal.
Very little reflection is sufficient to discover the erroneous
principles upon which this furnace is constructed. When the opening GG
is shut, the oxydation is produced slowly, and with difficulty, for want
of air to carry it on; and, when this hole is open, the stream of cold
air which is then admitted fixes the metal, and obstructs the process.
These inconveniencies may be easily remedied, by constructing the muffle
and furnace in such a manner that a stream of fresh external air should
always play upon the surface of the metal, and this air should be made
to pass through a pipe of clay kept continually red hot by the fire of
the furnace. By this means the inside of the muffle will never be
coolled, and processes will be finished in a few minutes, which at
present require a considerable space of time.
Mr Sage remedies these inconveniencies in a different manner; he places
the cuppel containing lead, alloyed with gold or silver, amongst the
charcoal of an ordinary furnace, and covered by a small porcelain
muffle; when the whole is sufficiently heated, he directs the blast of a
common pair of hand-bellows upon the surface of the metal, and completes
the cuppellation in this way with great ease and exactness.
SECT. III.
_Of increasing the Action of Fire, by using Oxygen Gas instead of
Atmospheric Air._
By means of large burning glasses, such as those of Tchirnausen and Mr
de Trudaine, a degree of heat is obtained somewhat greater than has
hitherto been produced in chemical furnaces, or even in the ovens of
furnaces used for baking hard porcelain. But these instruments are
extremely expensive, and do not even produce heat sufficient to melt
crude platina; so that their advantages are by no means sufficient to
compensate for the difficulty of procuring, and even of using them.
Concave mirrors produce somewhat more effect than burning glasses of the
same diameter, as is proved by the experiments of Messrs Macquer and
300