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214 MECHANICAL DENTISTRY AND METALLURGY.

fusible alloy is composed of lead parts, tin 2 parts, and
3
bismuth 5 parts, which fuses at 197° F.
Soft solder is an alloy composed of lead and tin in the
])r()i)()rti<)n of 1\vo parts of the former to one of the latter.
Use in the Laboratory.—Lead, either in its pure state or
when alloyed with certain other metals, serves important
purposes in the dental laboratory. In its simple or uncom-
bined state it is useful only in forming- counter-dies, in
swaging plates, and for striking up cusps in crown- and
bridge-work. Alloyed with antimony, w-ith the addition,
sometimes, of very small portions of copper, tin, and bis-
muth, it forms different grades of type-metal, which is
harder than lead and very brittle, and is sometimes used
for dies.
TIN.

Symbol, Sn (Stannum). Atomic zveiglit, 117.5.
Occurrence.—Tin is found in nature chiefly as dioxid,
SnOo. This is practically the only ore of tin, and it occurs
in large quantities in comparatively few localities. This
ore is known as tin-stoiic or cassitcritc, and is usually mixed
with other minerals.
The finely crushed ore is roasted to expel any sulphur or
arsenic which may be present. The tin oxid is then mixed
with po\\dered coal and smelted; carbon from the anthracite
combines with the oxygen, forming carbon monoxid, liber-
ating the tin.
General Properties.—Tin is a brilliant, silver-white
metal, the luster of which is not sensibly affected by ex-
posure to the air, but is easily oxidized by heat. It has
a slightly disagreeal)le taste, and emits, when rubbed, a
peculiar odnr. It is soft, inelastic, and when bent emits
a peculiar cracking sound called " the creaking of tin." It
is inferior in tenacitv and ductilitv, but is verv malleable.