RADIATION OF HEAT. 31 



radiating power of lead with its surface tarnished is 45, and with 

 its surface bright, only 19; but glass and porcelain radiate most 

 powerfully although their surface is smooth. When the actual 

 radiating surface is metallic, it is not affected in a sensible 

 manner by the substance under it. Thus, glass covered with 

 gold leaf possesses the radiating power of a bright metal. 



It appears, from the recent experiments of Dr. Bache, that 

 the radiating power of any surface is not affected by its colour, 

 at least in an appreciable degree. Hence, no particular colour 

 of clothes can be recommended for superior warmth in winter. 

 But the absorbent power of bodies for the heat of the sun de- 

 pends entirely upon their colour.* 



The faculty which different surfaces possess of absorbing or of 

 reflecting heat radiated against them, is connected with their 

 own radiating power. Those surfaces which radiate heat freely, 

 such as lamp-black, glass, &c., also absorb a large proportion 

 of the heat falling upon them, and reflect little of it ; while sur- 

 faces which have a feeble radiating and absorbing faculty, such 

 as the bright metals, reflect a large proportion, as they absorb 

 little, and form the most powerful reflectors. So that the good 

 absorbents are found at the top, and the good reflectors at the 

 bottom of the preceding table. The efficiency of a reflector 

 depending upon its low absorbing power, reflectors of glass are 

 totally useless in conducting experiments upon radiant heat. 

 Metallic reflectors remain cold, although they collect much heat 

 in their foci. 



These laws of the radiation of heat admit of some practical 

 applications. If we wish to retard, as much as possible, the 

 cooling of a hot fluid or other substance, in what sort of vessel 

 should we inclose it ? In a metallic vessel, of which the surface 

 is not dull and sooty, but clean and highly polished ; for it has 

 been observed that hot water cools twice as fast in a tin globe 

 of which the surface is covered with a thin coating of lamp- 

 black, as in the same globe when the surface is bright and 

 clean. Hence, the advantage of bright metallic covers at table, 

 and the superiority of metallic tea-pots over those of porcelain 

 and stoneware. 



* Journal of the Franklin Institute, May and November 1835. 



