THE REASON WHY. 59 



; T will teach vou by the hand of God ; that which is with the Almighty will I 

 not conceal." JOB xxvn. 



And some of its rays, falling upon a mirror, or any other bright 

 surface, might be diffused by reflection. 



198. Do some substances absorb heat? 



Yes ; those substances which are the best radiators are also the 

 best absorbers of heat. 



199. Why does scratching a bright metal surface increase 

 its power of radiation ? 



Because every irregularity of the surface acts as a point of 

 radiation, or an outlet by which the heat escapes. 



200. Why does a bright metal tea-pot produce better tea 

 than a brown or black earthenware one ? 



Because bright metal radiates but little heat, therefore the 

 water is kept hot much longer, and the strength of the tea is 

 extracted by the heat. 



201. But if the earthenware tea-pot were set by the fire, 

 why would it then make the best tea ? 



Because the dark earthenware tea-pot is a good absorber of heat, 

 and the heat it would absorb from the fire would more than 

 counterbalance the loss by radiation. 



202. How would the bright metal tea-pot answer if set upon 

 the hob by the fire-? 



The bright metal tea-pot would probably absorb less heat than it 

 would radiate. Therefore it would not answer so well, being set 

 upon the hob, as the earthenware tea-pot. 



203. Why should dish covers be plain in form, and have 

 bright surfaces ? 



Because, being bright and smooth, they will not allow heat to 

 escape by radiation. 



204. Why should the bottoms and bade parts of kettles and 

 saucepans be allowed to remain black ? 



Because a thin coating of soot acts as & good aosorber of heat, 

 Wid overcomes the non-absorbing quality of the bright surface. 



