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LEAD. 233

the air is not excluded, witli a hrillianl. L;reenisb-\vliite
llanie, the oxid hein^- formed.
Use for Dental Purposes.—Zinc has l)een lont;- employed
in the formation of thes used in swagiug- metallic plates,
this being- its principal use in the dental laboratory.
Several of the compounds of zinc are employed in opera-
tive dentistry in the preparation of plastic iillings. Zinc
oxid, ZnO, is the only known compound of zinc and oxygen,
and is the basis of most of the plastic iillings, known as
oxy-phosphate, oxy-chlorid, and oxy-sulphate of zinc.

LEAD.
Symbol, Pb (Plumbum). Atomic zveighf, 206.5.

Occurrence.—The chief source of lead is from the ore
known as galena, PbS, or lead sulphid. This ore is broken
up and roasted at a dull red heat when it becomes oxidized
and converted into sulphate. At this stage of the operation
the contents of the furnace are thoroughly mixed and the
temperature raised, which causes the sulphid and the sul-
phate to act upon each other, producing sulphurous oxid,
and metallic lead.*
General Properties.—Lead has a grayish-blue color, with
a bright, metallic luster when melted or newly cut, but it
soon becomes tarnished wdien exposed to the air. It is
both malleable and ductile, but soft and perfectly inelastic.
The fusing point of lead is 617° F. Exposed to a high
heat, it absorbs oxygen rapidly, forming on its surface a
gray film of protoxid and metallic lead.
Fusible Alloys.—The alloy known as Rose's Fusible
Metal is composed of two parts of bismuth to one of lead
and one of tin, and melts at about 200° F. A still more
* Essig's Atetallurgy.
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