60 TE EEASON WHY. 



'And the foolish said unto the wise, Give us of your oil, for our mmps are 

 gone out. 



205. But why should soot be prevented from accumulating 

 inflates at the bottom and sides of kettles and saucepans T 



Because, although soot is a good absorber of heat, it is a very 

 bad conductor ; an accumulation of it, therefore, would cause a waste 

 of fuel, by retarding the effects of heat. 



206. Why should the lids and fronts of Icettles and sauce- 

 pans be kept bright ? 



Because bright metal will not radiate heat ; therefore, the heat 

 which is taken up readily through the absorbing and conducting 

 power of the bottom of the vessel, is kept in and economised by the 

 non-radiating property of the bright top and front. 



207. I)oes cold radiate as well as heat ? 



It was once thought that cold radiated as well as heat. But a 

 mass of ice can only be said to radiate cold, by its radiating heat 

 in less abundance than that which is emitted from other bodies 

 turrounding it. It is, therefore, incorrect to speak of the 

 radiation of cold. 



CHAPTER XL 



208. Why, if you hold a piece of looking-glass at an angle 

 towards the sun, will light fall upon an object opposite to the 

 looking-glass ? 



Because the rays of the sun are reflected by the looking-glass. 



209. Why, when we stand before a mirror, do we see our 

 features therein ? 



Because the rays of light that fall upon us are reflected upon the 

 bright surface of the mirror. 



210. Why, if a plate of bright metal were held sideways 

 before afire, would heat fall upon an object opposite to the 

 plate? 



