234 CONVECTION. 



Q. Why does WATER EOIL ? 



A. Boiling is the effect of a more 

 violent escape of air from the heated 

 water; when, therefore, the air is not 

 permitted to escape, water will never 

 boil 



Q. Why is HEAT applied to the BOTTOM, and 

 not to the top of a KETTLE ? 



A. Because the heated water always 

 ascends to the surface, heating the 

 water through which it passes : if, there- 

 fore, heat were applied to the top of a 

 vessel, the water below the surface would 

 never be heated. 



Q. As the lower part of a GEATE is made 

 KED-HOT by the fire ABOVE, why would not the 

 WATER boil, if fire were applied to the TOP ? 



A. The iron of a grate is an excellent 

 conductor ; and, therefore, if one part be 

 heated, the heat is conducted to every 

 other part : but water is a very bad con- 

 ductor, and will not diffuse heat in a 

 similar way. 



Q. How do you know that WATER is a BAD 

 CONDUCTOR of heat ? 



A. When a blacksmith immerses 



