148 



ELEMENTARY GENERAL SCIENCE 



CHAP, 



If we wish to lift a hot plate we hold it with a folded cloth for 

 the same reason. Cylinders of engines are encased in a packing 

 of some badly conducting material. 



Convection. How Liquids and Gases become heated. 

 The process by which water and other liquids are heated must 

 now be considered. The water nearest the source of heat gets 

 heated, expands, and in consequence gets lighter ; it therefore 

 rises through the general mass of the liquid. Something must 

 take its place, and the cold water at the top being heavier sinks 

 and occupies the space of the water which has risen. This water 



in its turn gets heated and 

 rises, and more cold water 

 from the surface sinks. This 

 gives rise to upward currents 

 of heated water and down- 

 ward currents of cool water, 

 until by and by the whole of 

 the water is heated. These 

 currents are known as con- 

 vection currents, and the 

 process of heating in this 

 manner is called convection, 



EXPT. 147. Heat over 

 a small flame a round- 

 bottomed flask full of 

 water, as in Fig. 65. 

 Throw into the water 

 some solid colouring 

 matter, like cochineal, 



aniline dye, litmus, &c. Notice how the hot and coloured 



water ascends. 



Gases are similarly heated by this process of convection, 

 which may be thus defined ; Convection is the process by 

 which fluids (liquids and gases) become heated by the actual 

 movement of the particles of the fluid. 



Applications of Heating by Convection. Heating Build- 

 ings by Hot Water. One of the commonest ways of heating 

 large buildings is by means of hot-water pipes, and the efficacy 

 of this plan is due to the facts we have just learnt. In Fig. 66 

 we have the condition of things in such a building very simply 

 represented. We will suppose, to begin with, that the boiler B, 



FIG. 65. Convection Currents. 



