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COPPER. 235

The fusing point of tin is about 440° F. ; it boils at a
white heat, aiul Inirns with a blue flame to binoxid.
Dental Uses.—In its pure state, it is sometimes used for
counter-dies, and occasionally for dies. When employed
for the latter purpose in connection with a lead counter, the
latter should not be obtained directly from the die, as the
high temperature of melted lead would produce, when
poured upon the tin, partial fusion of the latter, and con-
sequent adhesion of the two pieces. Tin is also used by
many operators as a trial base plate for artificial dentures
instead of wax, gutta-percha, or other more pliable ma-
terials.
COPPER.
Symbol, Cu (Cuprum). Atomic weight, 63.2.
Occurrence.—Copper ores are found in many parts of
the world. The metal is found native in some localities
in large quantities, notably in the Lake Superior region,
where the richest copper mine in the world (Calumet and
Hecla) is located.
The ore most commonly employed for the production
of this metal is copper pyrites, CuFeSg, a combination of
sulphid of copper and iron. This ore contains about 34.5
per cent, copper. It is heated in a reverberatory furnace
to obtain a compound of sulphur and copper. The iron
sulphid is converted into oxid. The roasted mass is then
smelted, when a portion of the copper is oxidized to copper
oxid, which as the temperature increases, reacts upon the
remaining copper sulphid, resulting in sulphur dioxid,
which escapes, and metallic copper. The copper secured
is further purified by a refining process.
General Properties.-—Copper is of a brownish-red color,
with a tinge of yellow; has a faint but nauseous and dis-
agreeable taste, and imparts when exposed to friction a