136 TECHNOLOGY. 
bricks upon which the finely stamped arsenic mineral is spread. The fire 
passes from the grate beneath the hearth out at the opening, g, thence back 
through the channels, 2, to the double chimney, g, and thus the flat muffle- 
formed space above the hearth is heated without bringing the flame into 
contact with the arsenic, and the arsenic acid passes by the passage, A, to 
the condensing-chamber. Whilst the furnace is in operation a second 
charge is being warmed upon the top of the furnace, and is fed to the © 
muffle at the hole, e. 
Crucible-furnaces, which serve simply for melting, are of the simplest con- 
struction. 1.28, figs. 8a and 8d, is a Sefstrom furnace, and consists of two 
cylinders of sheet-metal so united by the ring, e, as to leave a vacant space 
between them. The interior cylinder is lined with fire-clay, and there is 
also a support of the same material for the crucible; at @ is an opening for 
the entrance of the blast, which passes through a circle of holes, 6, to the 
fire on every side. 
For handling the crucibles in this and other similar furnaces, the tongs 
shown in pl. 27, jigs. 17 and 18, and pl. 28, jigs. 9 and 10, are used. Pl. 
28, figs. 6a and 6b, represent an English muffle of sheet-iron lined with 
fire-clay ; @ is the fire space, ¢ the fire grate, e the fire door. The box, d, in 
the space, 5, is the muffle. 
Fig. 5a is a longitudinal section, and 56 a transverse section of a 
muffle-furnace used for burning enamels and colors upon porcelain and 
elass ; a is the ash-pit through which the draught passes to the fire; 0 is the 
fuel; g the grate. The muffle, A, is shoved into the furnace from front. 
When the operation is completed the chimney is closed at d, and the whole 
permitted to cool gradually. 
Sand-baths are another species of furnace of this general character. They 
are used where materials are to be warmed or heated to a moderate degree 
without coming in contact with the fire, for which purpose the vessels 
which contain them or the substances themselves are burned in the sand 
or simply laid upon its surface. $l. 28, jigs. 4a and 46, represent a sand- 
bath-furnace, in which @ is the ash-pit, f the grate, b the fire space, A the 
fire door; behind the fire space is a bridge over which the flame and smoke 
pass to the chimney, ¢c, which may be closed by the plate, d, to regulate the 
cooling of the sand-bath. Over the fire space is a cast-iron plate, and upon 
this a frame, 2, which supports the sand-bath. 
4. Cyemicat Merartiturcic APPARATUS. 
The apparatus used in the chemical processes of metallurgy are the same 
as those required for this branch in the laboratory, modified only by the 
amount of materials operated upon in each case. As an example, however, 
of the difference which occurs in the construction of some of the apparatus, 
we will present the gold amalgam mill (pl. 27, jig. 16) used for extracting 
gold from auriferous sand. As but a portion of the metal can be extracted 
from the sand by a single mill, it is usual to unite several of them together, 
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