Page:Handbook of Meteorology.djvu/23

Rh earth. Many of the chemical elements that compose the earth have been discovered in the sun and other heavenly bodies, and no chemical element has been discovered in any heavenly body that does not occur in the earth. Air and water vapor occur on the planet Mars, and it is not unreasonable to assume that the meteorology of this planet has much in common with the meteorology of the earth. The occasional occurrence of dust storms on Mars adds weight to the reasonableness of such an assumption.

Forms of Matter.—For practical purposes it may be assumed that matter exists in three forms—solid, liquid and gaseous. Most of the metals and some of the non-metals may be changed easily from one form to another. Thus, iron is a solid at ordinary temperatures; it “melts” or liquefies at a temperature somewhat above 2100° F (1150° C); at a still higher temperature it gives off a reddish-brown vapor. Mercury is ordinarily a liquid; it “boils” or becomes a vapor at 675 F (375° C) and “freezes” or solidifies at &#8722;38° F (&#8722;39° C). Water is the most common illustration of all; it solidifies at 32° F (0° C) and gradually becomes a vapor at ordinary temperatures; but at 212° F (100° C) the vapor pressure is that of the air at sea level. Practically all the ordinary gases have been liquefied and solidified. Liquid air and carbon dioxide are articles of commerce.

The conditions which surround the liquefaction of ice and snow, the evaporation of water, and the condensation of the water vapor of the air are fundamental factors in the science of weather. The distribution of precipitation—that is, rain, snow, hail, and the floating forms of fog and cloud—affect the habitability of the earth and human activities to a very great degree.

Matter may be changed in physical form, but it cannot be annihilated. Thus, the coal in the fire-box is changed to carbon dioxide, a gas, instead of a solid; but the chemist may separate the carbon from the oxygen. Nothing, not even the energy, is lost; to nature nothing can be added, and from nature nothing can be taken away.

Properties of Matter.—All forms of matter have certain