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substitutable one for the other. Copper is useful because
It is ductile and is an excellent conductor of electricity.
Aluminum is a good electrical conductor, too, although
less well suited to the task than is copper. Xt is
lighter and stronger than copper, however, and therefore
is useful where strength and lightness are important.
Steel is still stronger and at times can be used in
place of aluminum; and plastics or ceramics may replace
copper, aluminum, or steel in some uses. Packaging
materials may be made of steel, wood, plastic, paper,
or glass. Either aluminum or iron may be used for
engine blocks. Either copper or aluminum may he used
for electrical transmission. Thus, there exists a
veriety of products and uses for which any one of
several materials might be employed.
Whether one material is used in preference to
others depends often upon which material produces the
best results for the least cost, either because of the
low relative cost of the material itself or because
of engineering advantages obtained when a particular
material is used. Scarcity of one material may raise
its cost relative to others, resulting in substitution
of a lower cost (and possibly more abundant) material
for the higher cost one. Substitutability therefore
makes the exhaustion of one or a few materials a less
critical problem than it otherwise would be.