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.