Masterpieces of Science 



perature below, or by lowering it above, currents 

 are at once established; and, if light matter, 

 such as sawdust, be suspended in the liquid, the 

 direction of these currents becomes very evident. 

 Thus in Fig. 46, where heat is applied at the 

 bottom of a vessel, the liquid becomes specifically 

 lighter and therefore rises, whilst the surrounding 



Fig. 46. Currents in water Pig. 47. Currents in water 

 by heat by cold 



colder water being denser, runs down in streams 

 to supply the place of that which has ascended 

 to the surface. This is, in fact, the ordinary way 

 in which heat is propagated through a body of 

 liquid, and the process is called convection, to 

 distinguish it from conduction, or the method by 

 which heat is propagated through solid bodies. 

 In conduction, the heat is passed on from particle 

 to particle, and thus travels on through the mass, 

 while in convection the heated particles them- 

 selves move. Again, if a piece of ice be dropped 

 into a tumbler of slightly warm water, a system 

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