Expansion

an increase in volume due to the action of heat. Theoretically, at least, heat causes the molecules of a substance to move to and fro with great energy, and jostle each other farther apart. If, for instance, a bar of iron be measured at a freezing temperature, and again in boiling water, it will be found to have increased sensibly in length. In case of an iron rail or bridge truss 1,000 feet in length this gain amounts to over one foot. Wrought iron and steel expand a little faster than cast iron. If we let the number 11 represent the expansion of cast iron under certain conditions, that of glass is represented by 7; platinum, 8; steel and wrought iron, 12; gold, 15; copper, 17; brass and silver, 19; tin, 22; lead and zinc, 29. It is but fair to say that these values differ according to the author consulted. A gain in length is, of course, accompanied by an increase in surface and in volume. The increase in surface may be represented by the square of the increase in length; that of volume by the cube. The expansion of mercury is about ten times that of silver, copper, or brass; that of olive oil and turpentine is .00080; platinum, .000008. The expansion of turpentine and olive oil is therefore about 100 times as great as that of platinum.

It has been ascertained that the rate of expansion for the important gases is practically the same. Starting with the volume at the freezing point, 0 deg. C. or 32 deg. F., it is found that air, oxygen, hydrogen, nitrogen, and other standard gases gain 1/273 of their volume for every increase of 1 deg. C. in temperature, and 1/490 for every increase of 1 deg. F. According to this statement, an increase of 273 deg. C. doubles the volume of a gas. Starting at the freezing point again, it is found that gases lose correspondingly in volume with a loss of temperature. Theoretically a gas would lose its entire volume, that is, be extinguished, if its temperature were reduced 273 deg. C. below the freezing point. This theoretical temperature at which a gas would go out of existence is called an absolute zero.

Mechanics and engineers make allowance for expansion. Railroad irons are laid with open joints to prevent the bulging of rails. The same allowance ' is made in the construction of iron bridge trusses. The walls of a bulging building may be brought together by large rods passing through them from side to side. If the centers of these rods be heated and the expansion taken up by nuts on the outside ends, a physical cooling and contracting of the rods will bring the building together powerfully. Wagon makers heat tires over circular fires of burning chips and put them on the wheels while hot. The tire contracts in cooling and becomes tight.

The following table gives the increase in volume of various building material if heated from 32 deg. to 212 deg. F. or from 0 deg. to 100 deg. C. The decimal indicates such a part of the volume at a freezing temperature.

Expansion and contraction, due to heating and cooling, does much to convert rock into soil. Water entering the surface and crevices of rock in the summer time is frozen in winter and in expanding not only acts like a wedge, opening vast crevices in the rock, but it also splits off little particles. Experiments with a block of granite one hundred feet long have shown that, if the temperature be increased 150 deg., the length of the block will be increased three-fourths of an inch.

Where enormous areas of granite lie exposed to the sun, this expansion can only take place in an upward direction. Explorers in northern Labrador, where such exposures are frequent, report that hundreds of acres of granite rock are covered by flakes of stone. In many places the granite surface is covered by sheets of coarse granite about as thick as a pane of glass. The effects of expansion and contraction are, of course, greatest in a country like Labrador, where an extremely cold winter is succeeded by the blistering heat of a short but intense summer.