12 ELEMENTARY SCIENCE 



Nearly all substances expand when they are heated, and 

 contract when they are cooled. But in the case of water 

 this applies only up to a certain point. Water contracts 

 as it cools nearly up to the point when ice is formed, and 

 then it expands. If it did not, all the fish in our streams 

 and lakes would be killed. Can you see why this is ? Sup- 

 pose ice was contracted water instead of expanded water. 

 Then a cubic foot of ice would weigh more than a cubic 

 foot of water, and all the ice would form at the bottoms 

 of streams and lakes instead of at the surface. As it is, the 

 ice on top really protects the water beneath from becoming 

 cold enough to freeze. If the ice began to form at the bot- 

 tom, our lakes and streams would all be solid ice before the 

 winter was over. The only inconvenient thing about the 

 expansion of water when it turns to ice is that sometimes 

 this causes the bursting of unprotected water-pipes. 



These things that you have been reading about are very 

 easy to see, but they are not so easy to explain. For hun- 

 dreds and hundreds of years men observed these things 

 and were unable to explain them. But now, in a very short 

 time, you can learn explanations of things that your great- 

 grandfathers could not explain at all. Take this matter 

 of evaporation that is so important to our lives. You 

 know what it is, but how are you going to explain it ? Here 

 is a dish of water. You know that after a while, even 

 when nobody touches it, the water will be gone. It will 

 pass into the air. How did this happen? To understand 

 this you must understand that water and all other sub- 

 stances are composed of very, very small parts, so small 

 that they are invisible. These invisible and very small 

 parts of substances are called molecules. Though they are 



