subject to similar changes. A metal ball which when cool will just
slip through a ring (Fig. 2) will, when heated, be too large to slip
through the ring. Telegraph and telephone wires which in winter are
stretched taut from pole to 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.]