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 back parts of kettles and saucepans be allowed to remain black?