TRANSFER OF HEAT 



23 



water are poor conductors, heat is readily communi- 

 cated from one part of a liquid to another, or from 

 one part of a room to another. The air in a cold room 

 becomes warm very quickly if a hot fire is built. 



The explanation for this is 

 simple. When a liquid or a 

 gas is heated, it expands. The 

 same volume of it will then 

 weigh less, and in conse- 

 quence, it rises. The cooler 

 substance around it, being 

 heavier, sinks beneath the 

 lighter part and forces it up. 

 This process is called con- 

 vection. 



A kettle of water on a 

 stove is heated in this manner. 

 The water at the bottom of 



duced in tea kettles or coffee 

 pots when placed over a fire. 

 The heated water expands 

 and becomes lighter. The 

 colder water above is heavier 

 and is drawn down beneath 

 the warm water which in 

 consequence is forced upward. 



la F c?d Vn aVsf tu^ulfof 

 water, a* continuous 6 current 

 can be easily seen when the 



water is heated. 



the kettle expands and be- 

 comes lighter. The colder 

 water gravitates downward, 

 causing it to rise. The colder 

 water soon becomes heated, 

 and in turn rises. In this 

 way all the water is soon 

 heated. 



^ ^ ^^ ^"^ *** ^ 

 ^ Y i n a TOOm becomes heated. 

 mi . ... 



Ihe air immediatelv around 



