On a frosty night pools of water are covered with a hard, transparent crust, of what is called ice. You may break this crust into pieces, but if the cold continues, a new crust will soon be formed with bits of the old one firmly cemented in it. And the greater the cold the thicker will the crust be, until perhaps the whole of the water in the pools may become solid. If you take a piece of this solid substance, you find it to be cold, brittle, and transparent. Brought into a warm room it soon melts into water, and you may drive off the water as before into vapor. Ice is the general name given to water when it is in the solid state, such forms as snow and hail being only different appearances which ice puts on. Whenever water becomes colder than a certain temperature it passes into ice, or freezes, and this temperature is consequently known as the freezing-point. The upper layers of the atmosphere are much colder than the freezing-point of water. In the condensation which takes place there, the clouds do not resolve themselves into rain. The vapor of the up-streaming currents of warm air from the earth’s surface is condensed and frozen in these high regions, and passes into little crystals, which unite into flakes of snow. Even in summer the fine white cloudlets which you see floating at great heights are probably formed of snow. But in those countries, such as ours, where in winter the air even at the surface is sometimes very cold, the snow falls to the ground, and lies there as a white covering, until returning warmth melts it away. Besides rain and snow, the moisture of the air takes sometimes the form of hail, which consists of little lumps of ice like frozen rain; and of sleet, which is partially melted snow. But rain and snow are the most important, and it is these two forms which we must follow a little further.