82 GENERAL SCIENCE 



position by expansion and contraction as the weather 

 changed. For this reason it is often dangerous to walk 

 along such places. Some rock may lose its equilibrium 

 at any time and plunge to the bottom. The once barren, 

 rocky plateaus of the western part of the United States 

 have been so changed by the action of heat and water 

 that it is now possible to cultivate them. 



Water, unlike metals and rock, expands when it freezes 

 and so assists in breaking up stones. It runs into the 

 cracks of stones and freezes, breaking the stones apart 

 and shoving the outer pieces off the cliff. It runs in be- 

 tween the grains of a rock, and when it freezes it causes 

 the rock to expand so much that the rock crumbles and 

 becomes fine earth after it thaws out again. In this way, 

 in the temperate climates, the stones on the surface are 

 being continually broken up. Even the bricks of a 

 house are made to crumble by the freezing of the water 

 in them. Paint will keep the water out of brick, and on 

 this account it is used as a preservative. 



In general, all substances solids, liquids, and gases 

 - expand when heat is applied to them, and they con- 

 tract when heat is allowed to pass from them. Water, 

 however, is an exception to this rule. Water is most 

 dense at 4 C. That is, the molecules of water are closer 

 together at 4 C. than they are at any other temperature, 

 and hence a gram or pound of water will occupy less 

 space at 4 C. than it will at any other temperature. 

 If a gallon of water at 4 C. is heated, it will expand and 

 require more space. If it is cooled below 4 C., it will 

 expand. When water freezes it expands about one- 

 ninth. Of what value is this property of water to man 

 and to the animals that live in water? 



