5. Page 320
AN INTRODUCTION TO METALLURGY.
the zinc oxide is collected. The retorts are placed in rows in a
vertical arched chamber, with a fire-place at the base. In recent
furnaces of this kind the fire-place is arranged for gaseous firing.
The furnaces are usually built in pairs, back to back. At
Angleur* they contain 160 retorts on either side. The charge
for each retort is 25 lbs. of a mixture of equal weights of pow-
dered roasted blende and non-caking coal.
An external view of the Belgian zinc furnace is shown in Fig. 70.
Retort-furnaces are also used for smelting mercury ores, the
condensers sometimes consisting of earthen pipes, or aludels
(Fig. 71), thrust one into another. The ore is roasted in a cir-
cular kiln with a system of openings connecting with a series of
ranges of aludels resting on the doubly inclined surface of a
* See article Zinc, by de Lalande, in Wurtz, Dict. de Chem., p. 754.