90 



THE SUN'S GIFT OF HEAT 



(a poor conductor of heat) remains cool for a time. When 

 the lamp was made, air was taken from the bulb, and so 

 the white-hot filament is surrounded by almost empty space 

 (vacuum). The heat, therefore, cannot travel to the hand 

 by convection currents, because there is no air nor other 

 substance in contact with the filament. The hand is not 

 warmed by convection currents from the glass, because the 

 bulb is still cool. The sensation of heat can- 

 not be due to conduction, because the air which 

 surrounds the bulb is not in contact with the 

 hot filament. Besides, air is an even poorer 

 conductor of heat than glass, and the glass 

 itself does not become hot for some little time. 

 There must, therefore, be another mode of 

 transferring heat besides conduction and con- 

 vection. It also appears that in this method 

 of transferring no material substance is neces- 

 sary. This is shown by the fact that the hot 

 filament is surrounded by an almost perfect 

 vacuum. Astronomers tell us that there is 

 no material medium between our atmosphere 

 and the sun. 



The heat of the sun travels to us with the tremendous 

 speed of light, 186,000 miles a second, but does not warm 

 the intervening space because there is no matter in it to 

 be warmed. Radiation is the name given to this method of 

 heat transference. If heat did not travel in this way, the 

 earth would be uninhabitable. The conduction process is 

 very slow when compared with radiation. 



FIGURE 45 



Conserving Heat. Heat is so essential to life and 

 happiness that it is often necessary to provide means for 



