But although water in freezing expands only to this small amount, it resembles steam in the tremendous force with which it expands. If you fill a hollow iron shell quite full of water, screw down the opening tight, and then put it in a cold place where the water may freeze, the water as it freezes will burst the iron walls of the shell. You know that when the winter is severe, the pipes by which water is brought to a house often burst. This is because the water in them freezes, and, being unable to get out of the pipe, bursts it, just as you may burst a jacket that is too tight for you by stretching yourself. Among the bare hill-tops, or on the face of cliffs exposed to the weather, the strongest and hardest rocks are every winter split and broken, just as if quarrymen had been at work at them. In the summer the rain-water gets into the little cracks and rifts in the stone and lodges there. Then the winter comes with its cold and freezes the water. And the water bursts the rocks asunder just as it bursts our waterpipes. 42. =Hoar Frost is the Gaseous Water which exists in the Atmosphere, condensed and converted into Ice Crystals.= In the winter-time you often notice, on a clear sharp night, that the tops of the houses and the trees are covered with a white powder called =hoar frost=; and, on the windows of the room when you wake up, you see most beautiful figures, like delicate plants. Take a little of the hoar-frost, or scrape off some of the stuff that makes the window look like ground glass, and you find that it melts in your hand and turns to water. It is in fact ice. And if you look at the figures on the window pane with a magnifying glass you will see that they are made up bits of ice which have a definite shape, and are arranged in a regular pattern. Each of these definitely shaped bits of ice has been formed in the following way. The air in the room is much warmer than that outside, and there is mixed with it nearly as much water, derived from the breath and the evaporation of moist surfaces, as can maintain itself in the gaseous state at the temperature. The windowpanes, being thin, are cooled by the outside air, and of course the gaseous water inside the room, when it comes in contact with the cold windowpanes, becomes condensed on them into fine drops of cold water. The panes becoming colder and colder, these minute drops at last freeze, and the water not only becomes solid, but it =crystallises=; that is to say, the little solid masses take on more or less regular geometrical forms with flat faces, inclined to one another at constant angles, so that they resemble bits of glass cut according to particular fixed patterns. All ice is in fact crystalline, but in ice which has been formed from thick sheets of water, the crystals are so packed together that they cannot be distinguished separately. 43. =When Ice is warmed it begins to change back into Water as soon as the Temperature reaches 32°.=