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Rh 0.00000648 of its length for each degree F of increase in temperature, this being its “coefficient of linear expansion.” The coefficient of expansion of air is 0.00367; of ethyl alcohol, 0.0005; of mercury, 0.0002. In meteorology the principles of this property are fundamental. The expansion of mercury and of alcohol are used to determine the intensity of heat; and to the unequal heating of the air in different localities are due the movements of the air—that is, the winds.

Magnetism is a property pertaining chiefly to iron and steel, but possessed to a lesser degree by other metals. When in the condition known as magnetized, a piece of iron or of steel attracts and holds other pieces of iron and steel. Steel retains its magnetism permanently; iron is sensibly magnetic only when within magnetic influence—that is, a “magnetic field.” Nickel, cobalt, certain manganese alloys and tungsten alloys exhibit magnetism very sensibly. A bar of magnetized steel, suspended by a thread attached at its center of gravity, comes to rest pointing nearly or quite north and south, the negative or marked end pointing in a general way to the earth’s north magnetic pole. A few substances, chiefly bismuth, similarly suspended come to rest across the magnetic field when between the poles of a horseshoe magnet. The investigations concerning the earth’s magnetic properties are carried on in the United States by the Coast and Geodetic Survey.

Properties of Gases.—Gases are perfectly elastic. A gas fills any space within which it is confined. A cubic inch of a gas whose density has been measured, if set free in a space whose dimensions are a cubic foot, or a cubic yard, will fill the space. Manifestly its density and tension will be lessened in proportion. It is the custom to say, therefore, that a gas has no specific volume of its own; its volume is that of the container.

Equal volumes of a gas, temperature and density remaining the same, contain an equal number of molecules. If hydrogen, the lightest known gas, be taken as the unit of measurement,