QO Common Science 



To go back, then : if we looked at a piece of iron under 

 a microscope that would show us the molecules, and 

 remember, no such powerful microscope could exist, 

 we should see these quivering particles, and nothing 

 more. Then if some one heated the iron while we 

 watched the molecules, or if the sun shone on it, we 

 should see the molecules move faster and faster and 

 separate farther and farther. That is why heat ex- 

 pands things. When the molecules in an object move 

 farther apart, naturally the object expands. 



Heat is the motion of the molecules. When the mole- 

 cules move faster (that is, when the iron grows hotter), 

 they separate farther and the iron swells. 



How we can tell the temperature by reading a ther- 

 mometer. The mercury (quicksilver) in the bulb of 



FIG. 42. A thermometer made of a flask of water. It does not show the exact 

 degree of heat of the water, but it does show whether the water is hot or cold. 



