aped, cast-iron cylinder about fourteen
feet long, the outlet end being the larger to allow for the expansion of
the gases. Internal studs are so arranged as to keep the ore agitated;
and spiral flanges convey it to the outlet end continually, shooting
it across the cylinder. The cylinder is encased in a brick furnace. The
firing is provided from _outside_, the inventor maintaining that the
products of combustion are inimical to rapid oxidisation, to specially
promote which he introduces an excess of oxygen produced in a small
retort set in the roof of the furnace and fed from time to time with
small quantities of nitrate of soda and sulphuric acid. Ores containing
much sulphur virtually calcine themselves. I have seen this appliance
doing good work. The difficulties appeared to be principally mechanical.

There are other furnaces which work with outside heat, but I have not
seen them in action.

"C."--SHAFT TYPE OF FURNACE

In one form of this furnace, instead of allowing the ore to descend in
a direct clear fall the descent is impeded by inclined planes placed at
different levels in the height of the shaft, the ore descending from one
plane to the other.

_The Stetefeldt Shaft Furnace._--Although very expensive in first cost,
has many advantages. No motive power is required and the structure of
the furnace is of a durable character. Its disadvantages are:--Want
of control, and the occasionally imperfect character of the roasting
originating therefrom.

Three sizes of Stetefeldt's furnaces are constructed:

The largest will roast from 40 to 80 tons per diem.

The intermediate will roast from 20 to 40 tons per diem.

The smallest will roast from 10 to 20 tons per diem.

A good furnace should bring down the sulphur contents even of
concentrates so as to be innocuous to mercuric amalgamation. The sulphur
left in the ore should never be allowed to exceed two per cent.

A forty per cent pyritous or other sulphide ore should be roasted in a
revolving furnace in thirty to forty minutes, and without any auxiliary
fuel.

For ordinary purposes a 40-foot chimney is adequate for furnace work;
such a chimney four feet square inside at the base, tapering to 2' 6" at
the summit, will require 12,000 red bricks, and 1500 fire-bricks for
an internal lining to a height of 12 feet from the base of the chimney
shaft.

When second-hand Lancashire or Cornish boiler flues are available, they
make admirable and inexpensive chimn