General Science | Page 3

pole, sag in hot weather and are much too long. In
summer they are exposed to the fierce rays of the sun, become strongly
heated, and expand sufficiently to sag. If the wires were stretched taut
in the summer, there would not be sufficient leeway for the contraction
which accompanies cold weather, and in winter they would snap.
[Illustration: FIG. 2--When the ball is heated, it become too large to
slip through the ring.]
Air expands greatly when heated (Fig. 3), but since air is practically
invisible, we are not ordinarily conscious of any change in it. The
expansion of air can be readily shown by putting a drop of ink in a thin
glass tube, inserting the tube in the cork of a flask, and applying heat to
the flask (Fig. 4). The ink is forced up the tube by the expanding air.
Even the warmth of the hand is generally sufficient to cause the drop to
rise steadily in the tube. The rise of the drop of ink shows that the air in
the flask occupies more space than formerly, and since the quantity of
air has not changed, each cubic inch of space must hold less warm air
than| it held of cold air; that is, one cubic inch of warm air weighs less
than one cubic inch of cold air, or warm air is less dense than cold air.
All gases, if not confined, expand when heated and contract as they
cool. Heat, in general, causes substances to expand or become less
dense.

[Illustration: FIG. 3--As the air in A is heated, it expands and escapes in
the form of bubbles.]
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
heat.]
In the construction of long wire fences provision must be made for
tightening the wire in summer, otherwise great sagging would occur.
Heat plays an important part in the splitting of rocks and in the

formation of débris. Rocks in exposed places are greatly affected by
changes in temperature, and in regions where the changes in
temperature are sudden, severe, and frequent, the rocks are not able to
withstand the strain of expansion and contraction, and as a result crack
and split. In the Sahara Desert much crumbling of the rock into sand
has been caused by the intense heat of the day followed by the sharp
frost of night. The heat of the day causes the rocks to expand, and the
cold of night causes them to contract, and these two forces constantly at
work loosen the grains of the rock and force them out of place, thus
producing crumbling.
[Illustration: FIG. 6.--Splitting and crumbling of rock caused by
alternating heat and cold.]
The surface of the rock is the most exposed part, and during the day the
surface, heated by the sun's rays, expands and becomes too large for the
interior, and crumbling and splitting result from the strain. With the
sudden fall of temperature in the late afternoon and night, the surface of
the rock becomes greatly chilled and colder than the rock beneath; the
surface rock therefore contracts and shrinks more than the underlying
rock, and again crumbling results (Fig. 6).
[Illustration: FIG. 7.--Debris formed from crumbled rock.]
On bare mountains, the heating and cooling effects of the sun are very
striking(Fig. 7); the surface of many a