60 SCIENCE PRIMERS. [MATERIAL 



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 water- 

 pipes. 



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 



