will thereby be so much less coolled; hence baked earth is much to
be preferred to plate iron. It would be even of consequence to make the
tube double, and to fill the interval with rammed charcoal, which is one
of the worst conductors of heat known; by this the refrigeration of the
air will be retarded, and the rapidity of the stream of air consequently
increased; and, by this means, the tube may be made so much the longer.

As the fire-place is the hottest part of a furnace, and the part where
the air is most dilated in its passage, this part ought to be made with
a considerable widening or belly. This is the more necessary, as it is
intended to contain the charcoal and crucible, as well as for the
passage of the air which supports, or rather produces the combustion;
hence we only allow the interstices between the coals for the passage of
the air.

From these principles my melting furnace is constructed, which I believe
is at least equal in power to any hitherto made, though I by no means
pretend that it possesses the greatest possible intensity that can be
produced in chemical furnaces. The augmentation of the volume of air
produced during its passage through a melting furnace not being hitherto
ascertained from experiment, we are still unacquainted with the
proportions which should exist between the inferior and superior
apertures, and the absolute size of which these openings should be made
is still less understood; hence data are wanting by which to proceed
upon principle, and we can only accomplish the end in view by repeated
trials.

This furnace, which, according to the above stated rules, is in form of
an eliptical spheroid, is represented Pl. XIII. Fig. 6. ABCD; it is cut
off at the two ends by two plains, which pass, perpendicular to the
axis, through the foci of the elipse. From this shape it is capable of
containing a considerable quantity of charcoal, while it leaves
sufficient space in the intervals for the passage of the air. That no
obstacle may oppose the free access of external air, it is perfectly
open below, after the model of Mr Macquer's melting furnace, and stands
upon an iron tripod. The grate is made of flat bars set on edge, and
with considerable interstices. To the upper part is added a chimney, or
tube, of baked earth, ABFG, about eighteen feet long, and almost half
the diameter of the furnace. Though this furnace produces a greater heat
than any hitherto employed by chemists, it is still