CONVECTION. 37 



CONVECTION. 



72. It has been already shown that 



Explain how 



liquids become liquids and gases are non-conductors. 

 This implies that they cannot be heated, 

 like a mass of metal or other solid, by communi- 

 cation of heat from particle to particle. Each parti- 

 cle, on the contrary, receives its heat directly from the 

 source of heat, and conveys it away, making room for 

 others. Hence the term convection. In the process of 

 boiling water, for example, the first effect of the fire is 

 to heat the lower layer of liquid, and thereby to expand 

 and make it lighter. It then rises as a cork would in 

 water, and gives place to another portion, which be- 

 comes heated and rises in. its turn. Thus a circula- 

 tion is commenced, the warmer portions ascending and 

 the cooler descending, which continues until the water 

 boils. Before this happens, each particle will have 

 made many circuits, accumulating heat with each re- 

 turn, but not communicating it to others. Air and 

 gases become heated in the same way. 



73. CONVECTION MADE VISIBLE. The 



How can the ., , , 



circulation in circulation above described may be ren- 



dered visible b addin s a little of the " flow ~ 



berenderedvis- erS of Slllphlir" to Water , 

 iblc? . . 



and then heating it in 

 a test-tube over a spirit lamp. The 

 suspended particles will be found 

 to move in the direction indicated 

 by the arrows, showing that the 

 water has the same motion. The 



