112 EFFECTS OF HEAT. 



the chimney by the wind) would fall 

 down the flue, and drive the smoke with 

 it back into the room. 



Q. Why does a SMOKE-JACK turn round in 

 a chimney ? 



A. The current of hot air up the 

 chimney, striking against the oblique 

 vanes of the smoke-jack, drives them 

 round and round; in the same way as 

 the sails of a wind-mill are driven round 

 by the wind. 



Q. Why are some things SOLID, others LIQUID, 

 and others GASEOUS ? 



A. As heat enters any substance, it 

 drives its particles further asunder ; and 

 a solid (like ice) becomes a liquid ; and 

 a liquid (like water} becomes a gas. 



Q. Why does WATER SIMMER before it boils ? 



A. The particles of water near the 

 bottom of the kettle (being formed into 

 steam sooner than the rest) shoot 

 upwards; but are condensed again (as 

 they rise) by the colder water, and 

 produce what is called "simmering." 



Q. What is meant by SIMMERING ? 



A. A gentle tremor or undulation on 



