CHAPTER FOUR 



HEAT 



SECTION 15. Heat makes things expand. 



How does a thermometer work? What makes the mer- 

 cury rise in it ? 



Why does heat make things get larger ? 



When we look at objects through a microscope, they 

 appear much larger and in many cases we are able to 

 see the smaller parts of which they are made. If we 

 had a microscope so powerful that it made a tiny speck 

 of dust look as big as a mountain (of course no such 

 microscope exists), and if we looked through this im- 

 aginary microscope at a piece of iron, we should find to 

 our surprise that the particles were not standing still. 

 The iron would probably look as if it were fairly alive 

 with millions of tiny specks moving back and forth, back 

 and forth, faster than the flutter of an insect's wings. 



These tiny moving things are molecules. Everything 

 in the world is made of them. It seems strange that 

 we should know this, since there really are no microscopes 

 nearly powerful enough to show the molecules to us. 

 Yet scientists know a great deal about them. They 

 have devised all sorts of elaborate experiments very 

 accurate ones and have tested the theories about 

 molecules in many ways. They have said, for instance, 

 " Now, if this thing is made of molecules, then it will 

 grow larger when we make the molecules move faster 

 by heating it." Then they heated it in your next 

 experiment you will see what happened. This is only 

 one of thousands of experiments they have performed, 

 measuring over and over again, with the greatest care, 



