Heat 99 



became very cold it would begin to expand a little im- 

 mediately before it froze. 



And it is very lucky for us that water does expand 

 when it freezes, because if it did not, ice 'would be 

 heavier than water is. But since the water expands 

 as it freezes, ice weighs less than water and floats. 

 And that is why lakes and oceans and rivers freeze 

 over the top and do not freeze at the bottom. If they 

 froze from the bottom up, as they would if the ice 

 sank as it formed, every river and lake would be solid 

 ice in the winter. All the harbors outside the tropics 

 would probably be ice-bound all winter long. And the 

 ice in the bottom of the lakes and rivers and in the ocean 

 would probably never melt. 



So in the case of freezing water, and in the case of a 

 couple of metals, there is a point where coldness, not 

 heat, makes things expand. 



Experiment 32. Take a ketchup bottle with a screw cap 

 and a cork that fits tightly. Fill it to the top with water; 

 put a long pin beside the cork while you insert it, so that the 

 water can be crowded out as the cork goes down; then 

 when you have pushed the cork in tightly, pull out the pin. 

 Screw the cap on the bottle so as to hold the cork fast. Put 

 the bottle in a pail or box, and pack ice and salt around it. 

 Within an hour you should be able to see what the freezing 

 water does to the bottle. 



Application 27. Explain why ice is lighter than water; 

 why we have no snow in summer. 



Inference Exercise 



Explain the following : 



141. Sealing wax is held over a candle flame before it is applied 

 to a letter. 



