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.