needles. You will thus discover that the pipes become longer when 

 heated. This is one reason why the pipes are hung in brackets at- 

 tached to the board instead of being fastened more solidly, and the 

 noise you hear when the pipes are being heated up is partly due to 

 their slipping in the brackets. 



III. 



HEAT EXPANDS GASES. 



We have now seen that heat expands solids and liquids. "Will it 

 also expand a gas like air? You will find the following a beautiful 

 experiment: Hold a glass bottle by the neck and dip it quickly a few 

 times in very hot water merely to warm it up gradually so as not to 

 break it. Finally hold it in the hot water until it becomes thoroughly 

 heated up. Then invert it quickly in a pan containing water to the 

 depth of about an inch and watch what happens. You will find that 

 as the bottle cools, water is quickly sucked up into the bottle until 

 finally the bottle becomes about one-fourth full of water. This shows 

 that the air in the bottle contracted as it cooled. Will it also expand 

 when it is heated? To test this, all you have to do is to pour hot water 

 over the bottle as it stands inverted in the pan, and you will find that 

 the air expands again and drives out some of the water. (Why does 

 it not drive all the water out again?) To vary the experiment, invert 

 the cold bottle in the pan and pour hot water over it, and notice how 

 bubbles are driven out by the expansion of the air. When you have 

 driven out as many bubbles as possible, let the bottle cool again, and 

 you will find that water is sucked up as before. (Is there as much 

 sucked lip this time? Why?) 



WHY HOT AIU RISES. 



You will now be able to understand better why hot air rises. It is 

 because on being heated it expands", and therefore becomes less dense, 

 and so it rises above the cold air just as a piece of wood rises in water 

 because it is less dense than the water. An interesting proof of the 

 risino- of hot air can be obtained by making a little paper mill, fasten- 

 ing it to the end of a stick by a pin and holding it over a hot stove. 

 You will find that the current of hot air, rising, will turn it, just as 

 wind turns a windmill. Other examples of the rising of hot air will 

 occur to you, such as the rising of hot air and smoke in a chimney, and 

 the fact that the air in a room is much warmer near the ceiling than 

 near the floor. See if you can think of some more examples. 



2—19 



