Heat 97 



ice molecules would be different from the iron molecules 

 in size, but they would be vibrating back and forth in 

 exactly the same way, only with less motion. It is be- 

 cause they have less motion that we say the ice is colder 

 than the iron. Then let us suppose that the sun was 

 shining on the ice while we watched the ice molecules. 



First we should see movements of the ice molecules be- 

 come gradually more rapid, just as the iron molecules 

 did when the iron was warmed. Then, as they moved 

 faster and faster, they would begin to bump into each 

 other and go around every which way, each molecule 

 bumping first into one neighbor, then into another, and 

 bouncing back in a new direction after each collision. 

 This is what causes the ice to melt. When its molecules 

 no longer go back and forth in the same path all the time, 

 the ice no longer keeps its shape, and we call it water 

 a liquid. 



Almost all solid substances will melt when they are 

 heated. Or, to put it the other way around, every 

 liquid will freeze solid if it gets cold enough. Even 

 liquid air (which is ordinary air cooled and compressed 

 until it runs like water) can be frozen into a solid chunk. 

 Some things will melt while they are still very cold ; solid 

 air, for instance, melts at a temperature that would freeze 

 you into an icicle before you could count ten. Other 

 things, such as stones, are melted only by terrific heat. 



When the little particles of water that make up the 

 clouds become very cold, they freeze as they gather and 

 so make snowflakes. When the little particles of water 

 in the air, that usually make dew, freeze while they are 

 gathering on a blade of grass, we call it frost. When 



