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200 MECHANICAL DENTISTRY AND METALLURGY.

series of Ixxlies known as luctallifcrons minerals. The art
of extracting metals from their ores is an important part
of mctallnrg}'.
Methods of Extracting.—The process of extracting
metals from their ores may be divided into two classes,
known as the dry and the wet methods.
The first, or dry method is where ores are treated in
furnaces at high temperatures. This operation is usually
referred to as smelting. The second or wet method is
where, after the ore has been crushed and converted into
a readily soluble compound, the metal is extracted by a
suitable solvent.
Furnaces.—There are various forms of furnaces em-
ployed in smelting. (i) The blast fnrnacc, usually a
vertical chamber or stack, the ore and fuels being intro-
duced at the top, the molten metal being drawn off at
intervals at the base. (2) A horizontal furnace known as
a reverberatory furnace. This consists of three distinct
parts,—a fireplace at one end, a stack or chimney at the
other, and a bed between them on which the ore is heated.
It is owing to the reverberation of the flame on the ore
spread over the bed that the furnace takes its name. (3)
Mufile and crucible furnaces ; these, however, are for lighter
metallurgical work in the laboratory,—the melting of
metals, and alloys, and for assaying.
In the following chapters the metals will be classified and
considered according to their usefulness in the dental labo-
ratory, gold being the first, as it is in many respects the
chief among metals, and to the dental practitioner of the
greatest importance.