Heat 107 



158. When you shoot an air gun, the air is compressed at first; 



then when it is released it springs out to its original volume 

 and throws the bullet ahead of it. 



159. Leather soles get wet through in rainy weather, while rubbers 



remain perfectly dry on the inside. 



1 60. When you want to clean a wooden floor, you scrub it with a 



brush. 



SECTION 19. Boiling and condensing. 



What makes a geyser spout ? 

 How does a steam engine go ? 



Once more let us imagine we are looking at molecules 

 of water through our magical microscope. But this 

 time suppose that the water has been made very hot. 

 If we could watch the molecules smash into each other 

 and bound about more and more madly, suddenly we 

 should see large numbers of them go shooting off from 

 the rest like rifle bullets, and they would fly out through 

 the seemingly great spaces between the slower mole- 

 cules of air. This would mean that the water was 

 boiling and turning to steam. 



Here are a couple of experiments that will show you 

 how much more room water takes when it turns to 

 steam than while it remains just water : 



Experiment 36. Pour a half inch of water into the bottom 

 of a test tube. Put a cork in the test tube so tightly that 

 it will not let any steam pass it, but not too tightly. Hold 

 the test tube with a test-tube clamp at arm's length over a 

 flame, pointing the cork away from you. Wait for results. 



The reason the cork flew out of the test tube is this : 

 Steam takes a great deal more room than water does, 

 many times as much room ; so when the water in the 



