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230 MECHANICAL DENTISTRY AND METALLURGY.

properties of aliiniinum is its extreme lightness, it being the
Hghtest of commercial metals, having a specific gravity of
2.6, whilst that of platinum is 21.5 and gold 19.5. It is
very malleable and ductile, and can be reduced to thin sheets
or drawn into very fine threads. Its tenacity, though
superior to that of silver, is less than that of copper, but
no very accurate experiments have been made in this respect.
When pure it is about as hard as silver, is readily manip-
ulated, and is capable of taking and retaining a very high
polish.
Fusing Point.—Aluminum melts at a temperature be-
tween silver and zinc, or about 1160° F. (according to the
latest experiments). When casting aluminum it should
not be heated much above the fusing point, or be allowed
to remain melted for any great length of time.
Corrodibility.—One of the most marked qualities of
aluminum is its resistance to oxidation, from the influence
of the air. As now manufactured, this metal is also found
to withstand the action of organic secretions fully as well
as silver, and is receiving a much larger use as a base for
dental plates.
Solubility.—The natural solvent for aluminum is hydro-
chloric acid. Concentrated sulphuric acid also dissolves
aluminum, while nitric acid, either concentrated or diluted,
has very little action upon it. Aluminum is also soluble
in solutions of caustic potash or soda.
Electrical Conductivity.—Pure aluminum has an elec-
trical conductivity of about one-half that possessed by silver
or copper, and about two-thirds that of gold.
Manner of Annealing.—For the purpose of annealing
aluminum the surface of the plate may be coated with oil,
and then passed over the flame of a spirit lamp or Bunsen
burner until the oil is entirely burned off and the plate be-
comes white, when it is instantly withdrawn. Or it may