BOILING WATER. 233 



Q. How do these two currents PASS each 

 other? 



A. The hot ascending current passes 

 close by the metal sides of the kettle ; 

 while the cold descending current passes 

 down the centre. 



Q. Why does BOILING WATER BUBBLE ? 



A. The bubbles are portions of 

 steam (formed at the bottom of the 

 vessel) which rise to the surface, and 

 escape into the air. 



Q. Why does a KETTLE RUN OVER, when the 

 water BOILS ? 



A. As the heat insinuates itself 

 between the particles of water, it drives 

 them asunder ; and (as the particles of 

 water are driven apart from each other J 

 the same vessel will no longer hold the 

 expanded water, and some runs over. 



Q. Why does a KETTLE SING, when it is 



ABOUT tO BOIL ? 



A. Water contains a great deal of 

 air, which (being expanded by the heat 

 of the fire) escapes by fits through the 

 spout of the kettle ; which sings in the 

 same way as a trumpet does, when a 

 person blows in it. x 3 



