204. _Why should the bottoms and back parts of kettles and saucepans be allowed to remain black?_ Because a _thin_ coating of soot acts as a _good absorber of heat_, and overcomes the _non-absorbing_ quality of the _bright surface_. [Verse: "And the foolish said unto the wise, Give us of your oil, for our lamps are gone out."] 205. _But why should soot be prevented from accumulating in flakes at the bottom and sides of kettles and saucepans?_ 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 kettles and saucepans 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. _Does 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 surrounding it_. It is, therefore, _incorrect_ to speak of the _radiation of cold_. CHAPTER XI. 208. _Why, if you hold a piece of looking-glass at an angle towards the sum, 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 a fire, would heat fall upon an object opposite to the plate?_ Because rays of heat may be _reflected_ in the same manner as the rays of light. [Verse: "But the wise answered saying, Not so; lest there be not enough for us and you: but go ye rather to them that sell, and buy for yourselves."--MATT. XXV.] 211. _Why would not the same effect arise if the plate were of a black or dark substance?_ Because black and dark substances are not _good reflectors of heat_. 212. _What are the best reflectors of heat?_ Smooth, light-coloured, and highly polished surfaces, especially those of _metal_. 213. _Why does meat become cooked more thoroughly and quickly when a tin screen is placed before the fire?_ Because the bright tin reflects the rays of heat back again to the meat. 214. _Why is reflected heat less intense than the primary heat?_ Because it is impossible to collect all the rays, and also because a portion of the caloric, imparting heat to the rays, is absorbed by the air, and by the various other bodies with which the rays come in contact. 215. _Can heat be reflected in any great degree of intensity?_