etort lid door provided at
the side of the furnace. Openings provided at each end of the furnace
permit the passage of the flame through it, and the revolution of the
furnace turns over the powdered ore and brings it into more or less
sustained contact with the oxidising flame. The exposure of the ore to
this action is continued sufficiently long to ensure the more or less
complete oxidation of the ore particles.

_Third or C Process._--In this process the powdered ore is allowed to
fall in a shower from a considerable height, through the centre of a
vertical shaft up which a flame ascends; the powdered ore in falling
through the flame is heated to an oxidising temperature, and the
sulphides are thus depleted of their sulphur and become oxides.

Another modification of this direct fall or shaft furnace is that in
which the fall of the ore is checked by cross-bars or inclined plates
placed across the shaft; this causes a longer oxidising exposure of the
ore particles.

When the sulphur contents of pyritous ores are sufficiently high, and
after the ore has been initially fired with auxiliary carbonaceous fuel,
it is unnecessary, in a properly designed roasting furnace, to add
fuel to the ore to enable the heat for oxidation to be obtained. The
oxidation or burning of the sulphur will provide all the heat necessary
to maintain the continuity of the process. The temperature necessary for
effecting the elimination of both sulphur and arsenic is not higher
than that equivalent to dull red heat; and provided that there is a
sufficient mass of ore maintained in the furnace, the potential heat
resulting from the oxidation of the sulphur will alone be adequate to
provide all that is necessary to effect the calcination.

TYPES OF FURNACES OF THE DIFFERENT CLASSES THAT ARE IN ACTUAL USE.

"A" OR REVERBERATORY CLASS.

The construction of this furnace has already been sufficiently
described. If the roasting is performed in a muffle chamber, the
arrangement employed by Messrs. Leach and Neal, Limited, of Derby, and
designed by Mr. B. H. Thwaite, C.E., can be advantageously employed in
this furnace, which is fired with gaseous fuel. The sensible heat of the
waste gases is utilised to heat the air employed for combustion; and by
a controllable arrangement of combustion, a flame of over 100 feet in
length is obtained, with the result that the furnace from end to end is
maintained at a uniform temperature. By this system, and with