812 RADIATING POWER. 



Polished metals are found accordingly to reflect the 

 greatest part of the heat rays which impinge upon 

 them, while the darkest of all known substances, lamp- 

 black, absorbs heat rays as completely as luminous 

 rays. 



A piece of bright tinfoil upon which the snn's rays are brought to 

 a focns by means of a lens, will be fused with difficulty or not at all ; 

 but if the surface is blackened with lamp-black it will melt in the 

 focus at once. The tinfoil purchased of the dealers is usually brighter 

 on one side than on the other ; the bright surface should be used for 

 the reflection of the rays, and the other surface should be blackened 

 by holding it over a lighted splinter of wood which has been 

 dipped in oil of turpentine or paraffine. To prevent the tinfoil 

 being fused by the heat of the flame it should be rolled round a 

 bottle filled with water or a rather thick cylinder of metal. 



A small tin pan is blackened on one side by a tur- 

 pentine or paraffine flame; some water is then made to 

 boil in it and the vessel removed from the flame. If 

 now the back of each hand be held about l cm from the 

 sides of the pan, one hand opposite to the blackened 

 side, and the other to the bright side, the hand opposite 

 to the blackened side will feel the heat much more than 

 the other hand. The blackened surface radiates much 

 more heat than the bright surface. The same fact holds 

 in the case of all other bodies. Substances which have 

 the greatest absorbing power for heat have also the 

 greatest radiating or emissive power. On the other 

 hand, those substances which have the greatest reflect- 

 ing power possess the least emissive power. 



Whenever one part of a body is hotter than the 

 remainder, heat flows from the hottest part to the 

 neighbouring colder part, until this part has the same 

 temperature as the first; from the second part heat 

 flows to the neighbouring third part, and so on. There 



