ed most
successful consists in the electrolysis of the oxide dissolved in melted
cryolite.

~Metallurgy.~ An iron box A (Fig. 82) about eight feet long and
six feet wide is connected with a powerful generator in such a
way as to serve as the cathode upon which the aluminium is
deposited. Three or four rows of carbon rods B dip into the
box and serve as the anodes. The box is partially filled with
cryolite and the current is turned on, generating enough heat
to melt the cryolite. Aluminium oxide is then added, and under
the influence of the electric current it decomposes into
aluminium and oxygen. The temperature is maintained above the
melting point of aluminium, and the liquid metal, being heavier
than cryolite, sinks to the bottom of the vessel, from which it
is tapped off from time to time through the tap hole C. The
oxygen in part escapes as gas, and in part combines with the
carbon of the anode, the combustion being very brilliant. The
process is carried on at Niagara Falls.

The largest expense in the process, apart from the cost of
electrical energy, is the preparation of aluminium oxide free
from other oxides, for most of the oxide found in nature is too
impure to serve without refining. Bauxite is the principal ore
used as a source of the aluminium because it is converted into
pure oxide without great difficulty. Since common clay is a
silicate of aluminium and is everywhere abundant, it might be
expected that this would be utilized in the preparation of
aluminium. It is, however, very difficult to extract the
aluminium from a silicate, and no practical method has been
found which will accomplish this.

~Physical properties.~ Aluminium is a tin-white metal which melts at 640 deg.
and is very light, having a density of 2.68. It is stiff and strong, and
with frequent annealing can be rolled into thin foil. It is a good
conductor of heat and electricity, though not so good as copper for a
given cross section of wire.

~Chemical properties.~ Aluminium is not perceptibly acted on by boiling
water, and moist air merely dims its luster. Further action is prevented
in each case by the formation of an extremely thin film of oxide upon
the surface of the metal. It combines directly with chlorine, and when
heated in oxygen burns with great energy and the liberation of much
heat. It is the