gear cases, engine crank cases, covers, fittings, and wherever lightness
and moderate strength are desirable.

Aluminum is about one-third the weight of iron and about the same weight as
glass and porcelain; it is a good electrical conductor (about one-half as
good as copper); is fairly strong itself and gives great strength to other
metals when alloyed with them. One of the greatest advantages of aluminum
is that it will not rust or corrode under ordinary conditions. The granular
formation of aluminum makes its strength very unreliable and it is too soft
to resist wear.

_Copper_ is one of the most important metals used in the trades, and
the best commercial conductor of electricity, being exceeded in this
respect only by silver, which is but slightly better. Copper is very
malleable and ductile when cold, and in this state may be easily worked
under the hammer. Working in this way makes the copper stronger and harder,
but less ductile. Copper is not affected by air, but acids cause the
formation of a green deposit called verdigris.

Copper is one of the best conductors of heat, as well as electricity, being
used for kettles, boilers, stills and wherever this quality is desirable.
Copper is also used in alloys with other metals, forming an important part
of brass, bronze, german silver, bell metal and gun metal. It is about
one-eighth heavier than steel and has a tensile strength of about 25,000 to
50,000 pounds per square inch.

_Lead._--The peculiar properties of lead, and especially its quality
of showing but little action or chemical change in the presence of other
elements, makes it valuable under certain conditions of use. Its principal
use is in pipes for water and gas, coverings for roofs and linings for vats