KADIATIOX OF HEAT. 277 



in tbe same way that light is reflected by a looking-glass. 

 A plate of bright tin held near the fire will not for a long 

 time become hot, the heat being reflected from it without 

 entering and heating it. But if it be blackened with 

 smoke, it Avill no longer reflect, but absorb the heat, and 

 consequently will speedily become hot. This experiment 

 may be easily tried by placing a new tin cup containing 

 water over a charcoal fire, which yields no smoke. The 

 heat will be reflected into the fire by the tin, and the w^a- 

 ter will scarcely become warm. But if a few pine shav- 

 ings be throwm on this fire, to smoke the surface of the tin, 

 it will then absorb the heat rapidly, and soon begin to 

 boil. This explains the reason that bread bakes more 

 slowly in a new tin dish, and that a polished andiron be- 

 fore a fire is long in becoming hot. 



A concave burning-mirror, which throws the rays of 

 heat to a focus or point, may be made of sheet-tin, by 



Fiff. 294. 



beating it out concave so as to fit a regularly curved 

 gauge. If a foot in diameter, and carefully made, it will 

 condense the rays of heat so powerfully at the focus, when 

 held several feet from the fire, as to set fire to a pine stick 

 or to flash gunpowder (fig. 294). 



The reflection of radiated heat may be beautifully ex- 

 hibited by using two such concave tin mirrors. Place 

 them on a long table several feet apart, and ascertain the 

 focus of each by means of the light of a candle. Then 

 place in the focus of one a red-hot iron ball, or a small 

 chafinof-dish of burninsj charcoal. In the focus of the 



