BOILING WATER. 115 



Q. Why does a KETTLE sometimes BOIL OVER? 



A. Liquids expand very much by 

 heat ; if, therefore, a kettle be filled with 

 cold water, some of it must run over as 

 soon as it is expanded by heat. 



Q. But I have seen a KETTLE BOIL OVER, 

 although it has not been filled FULL of WATER ; how 

 do you account for THAT ? 



A. If a fire be very fierce, the air is 



expelled so rapidly, that the bubbles are 



very numerous ; and (towering one above 



the other) reach the top of the kettle, and 



fall over. 



Q. Why is a pot, which is full to OVERFLOWING 

 (while the water is boiling HOT), NOTHING' LIKE 

 FULL, when it has been taken off thejire for a short 

 time ? 



A. When the water was swelled by 

 boiling heat y it filled the pot even to 

 overflowing ; but as soon as the water is 

 condensed by cold, it contracts again, 

 and occupies a much less space. 



Q. Wliy does the water of a KETTLE run out 

 of the SPOUT when it BOILS ? 



A. Because the steam cannot escape 

 50 fast as it is formed, and (being con- 

 fined in the kettle) presses on the water 



