BAUXITE AND ALUMINUM 215

 The choice of any metal for a particular use is determined largely by its
chemical and physical properties, and the outstanding property of aluminum
is its lightness. This was a factor in the early use of aluminum for kitchen
utensils and in its subsequent use in automobiles, airplanes,jairships, and
other means of transportation. Aluminum competes with copper as an electrical
conductor; it is a noteworthy adjunct to the metallurgy of steel. Lately
it has become important as a light structural material made strong by alloying
and heat treatment. It is marketed in die castings; sand castings; permanent
structural shapes; foil; paints; powder; round, square, flat, and hexagonal
wire; rivets; bolts; nuts; screen; perforated sheets; bottle- caps; drums;
barrels; collapsible tubes; tanks and tank cars; pans; shingles; corrugated
sheets; and many other forms.
 When the wbight of transportation equipment is reduced by the use of aluminum
a corresponding increase in the weight of the pay load is permitted; or,
if the pay load is not increased, lighter equipment decreases the inertia
to be overcome in starting into motion and the momentum to be overcome in
stopping. Reduction in operating costs and maintenance expense therefore
results. Because of these advantages there has been an increasing trend toward
the use of aluminum in motor trucks and the rolling stock of railroads and
street railways. It is reported that in 1932 transportation continued to
account for the largest percentage of aluminum consumption.
 Aluminum alloys are now employed in the chassis frames of motor trucks as
well as in the body and the cab, in trucks of the heavyduty dump type having
25 tons capacity, and in rapid delivery vans. For the latter class of vehicles
there was an innovation in 1932 of trucks having pressed aluminum-alloy frames
that were especially light.
 In recent years there has been a decline in the use of aluminum for pleasure
cars, except those of the higher price range, and for pistons. In the past
year a new design of aluminum piston called the T-slot piston, so named because
of the shape of a slot cut in its edge, was placed on the market. It is said
to permit close clearance due to a tendency of this piston to conform to
the cylinder. Likewise the more extensive use of high-compression motors
led in the past year to the adoption of aluminum cylinder heads by some motorcar
manufacturers. In this use the thermal conductivity of aluminum is said to
be an advantage.
 For several years past there has been gradual extension in the use of aluminum
in the construction of railroad passenger cars, by which 5 or 6 tons of weight
are saved in an ordinary car. Tank cars for chemicals, saving 4 tons of weight,
have also been built. In 1931,
- experiments were made in buildrng Pullman cars in which aluminum was used
as extensively as possible, and the success of this experiment led in 1932
to the development of a light, stream-line railway car for main-line service.
It is constructed almost entirely of light, strong aluminum alloys and is
motored by a 16-cylinder gasoline engine.
 It is doubtful whether the present development of aircraft would have been
possible but for aluminum and its alloys. Until recently the demand for aluminum
in this field showed rapid increase. With a marked curtailment in the production
of aircraft, the consumption