3. Amount of Expansion and Contraction. While most substances expand
when heated and contract when cooled, they are not all affected
equally by the same changes in temperature. Alcohol expands more than
water, and water more than mercury. Steel wire which measures 1/4 mile
on a snowy day will gain 25 inches in length on a warm summer day, and
an aluminum wire under the same conditions would gain 50 inches in
length.

[Illustration: FIG. 4.--As the air in _A_ is heated, it expands and
forces the drop of ink up the tube.]

4. Advantages and Disadvantages of Expansion and Contraction. We owe
the snug fit of metal tires and bands to the expansion and contraction
resulting from heating and cooling. The tire of a wagon wheel is made
slightly smaller than the wheel which it is to protect; it is then
put into a very hot fire and heated until it has expanded sufficiently
to slip on the wheel. As the tire cools it contracts and fits the
wheel closely.

In a railroad, spaces are usually left between consecutive rails in
order to allow for expansion during the summer.

The unsightly cracks and humps in cement floors are sometimes due to
the expansion resulting from heat (Fig. 5). Cracking from this cause
can frequently be avoided by cutting the soft cement into squares, the
spaces between them giving opportunity for expansion just as do the
spaces between the rails of railroads.

[Illustration: FIG. 5: A cement walk broken by expansion due to sun