CONVECTION CURRENTS. 



fore lighter. When a given quantity of water 

 occupies, in its expanded state, more space, 

 there is a smaller quantity of it in a given space 

 that is, it is lighter. The now lighter water 

 is forced sideways and upwards by the weight 

 of the heavier water from the top which presses 



LFiG. 29.-ICE UNMELTED IN VESSEL OF BOILING WATER. 



downwards under it, forcing it upwards. While 

 the heat is applied these convection-currents 

 are maintained, and the water keeps getting 

 hotter until it boils. 



The effect is made more striking if ice be 

 placed in the bottom of the test-tube. Ice, 

 however, being lighter than water, tends to 

 float, just as the lighter water itself floats on 



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