refore a good reducing agent. Hydrochloric acid acts upon
it, forming aluminium chloride: nitric acid and dilute sulphuric acid
have almost no action on it, but hot, concentrated sulphuric acid acts
upon it in the same way as upon copper:

2Al + 6H_{2}SO_{4} = Al_{2}(SO_{4})_{3} + 6H_{2}O + 3SO_{2}.

Alkalis readily attack the metal, liberating hydrogen, as in the case of
zinc:

Al + 3KOH = Al(OK)_{3} + 3H.

Salt solutions, such as sea water, corrode the metal rapidly. It alloys
readily with other metals.

~Uses of aluminium.~ These properties suggest many uses for the metal. Its
lightness, strength, and permanence make it well adapted for many
construction purposes. These same properties have led to its extensive
use in the manufacture of cooking utensils. The fact that it is easily
corroded by salt solutions is, however, a disadvantage. Owing to its
small resistance to electrical currents, it is replacing copper to some
extent in electrical construction, especially for trolley and power
wires. Some of its alloys have very valuable properties, and a
considerable part of the aluminium manufactured is used for this
purpose. Aluminium bronze, consisting of about 90% copper and 10%
aluminium, has a pure golden color, is strong and malleable, is easily
cast, and is permanent in the air. Considerable amounts of aluminium
steel are also made.

~Goldschmidt reduction process.~ Aluminium is frequently employed as a
powerful reducing agent, many metallic oxides which resist reduction by
carbon being readily reduced by it. The aluminium in the form of a fine
powder is mixed with the metallic oxide, together with some substance
such as fluorspar to act as a flux. The mixture is ignited, and the
aluminium unites with the oxygen of the metallic oxide, liberating the
metal. This collects in a fused condition under the flux.

An enormous quantity of heat is liberated in this reaction, and a
temperature as high as 3500 deg. can be reached. The heat of the reaction is
turned to practical account in welding car rails, steel castings, and in
similar operations where an intense local heat is required. A mixture of
aluminium with various metallic oxides, ready prepared for such
purposes, is sold under the name of _thermite_.

[Illustration: Fig. 83]

~Preparation of chromium by the Goldschmidt method.~ A mixture of
chromium oxide and aluminium powder is placed in a Hessian
crucible (A, Fig. 83), and on top