HEAT. 145 



Experiment. Take two tin or Brittania tumblers, and 

 cover the outside of one of them with lamp-black, fill 

 them both with boiling hot water, and place a thermome- 

 ter in each ; it will be seen that the water in the coated 

 tumbler will cool to the temperature of the room in about 

 one half of the time required to cool that in the bright 

 one. The same effect will be observed whatever be the 

 nature of the metallic bodies, whether the tumblers be 

 iron or silver, provided their surfaces have the same polish 

 as the tin, and coated in the same manner. 



The metals radiate heat less than any other substances. 

 Glass radiates 7^ times more heat than polished tin. 



Mr Leslie found that the surface of a body produced 

 the greatest effect upon radiation when the air was still ; 

 and that in a strong wind the polished and coated 

 vessels filled with hot water cooled in nearly the same 

 time. This proves that the lamp-black does not conduct 

 the heat off, but radiates it. The shape of a vessel has 

 no effect upon the radiating power of a body, except that 

 the greater the surface with the same solid contents, the 

 greater the radiation. 



The color has but a slight effect upon radiation. If 

 instead of coating the tumbler with lamp-black, we cover 

 it with white paper, nearly the same effect is produced 

 it therefore depends mainly upon the nature of the surface. 



The following substances radiate heat in the order in 

 which they are placed. Lamp-black writing paper 

 glass tarnished lead bright lead polished iron tin 

 plate silver gold copper polished brass. 



Bright metallic bodies cool the slowest ; culinary uten- 

 sils, therefore, which are required to keep whatever is in 

 them hot, should be of metal, with bright clean surfaces. 

 For the same reason, pipes, which are used to conduct 

 heat from a furnace to an apartment, should be made of 

 bright metal. Stove pipes and others, which are used 

 to heat apartments by distributing the heat, should be 

 covered with lamp-black, or some other similar substance, 

 which will cause them to radiate more heat. And for 

 the same reasons, such vessels as are intended to receive 

 heat, should be black, as they heat much sooner than 

 they would if bright; of course, such culinary vessels 

 should not be scoured on the outside, where the beat 

 comes to them. Heat is reflected in the same manner as 



