70 HEAT OR CALORIC. 



radiate, are heated by it, but they derive little heat from the solar rays, 

 which permeate them easily. 



For the most important facts respecting the radiation of heat, see 

 the section on the nature of heat and light. 



A few facts may be added here. 



(iv.) Polished surfaces, of all bodies that are not transparent, re- 

 flect radiant solar heat, and do not transmit it.* 



(a?.) Caloric not only radiates freely in a vacuum,^ but it is not 

 impeded by currents or agitation of the air. Winds do not disturb 

 sunshine, and the solar focus is equally distinct and powerful, in a 

 windy as in a still day. Bellows blowing across a current of radiant 

 culinary heat, do not divert the rays. 



(Y.) Surface has a great effect on the radiation and reception of 

 heat independently of the nature of the material. 



Blackf and rough surfaces, radiate and receive heat the best ; 

 bright and polished surfaces, the worst. Glass, however, although 

 naturally polished, radiates and receives heat very well, and so do 

 paper, skin, and animal membrane ; the latter radiates and receives 

 twenty five times as powerfully as polished metal. 



(2.) The radiating and absorbing powers are alike and equal; 

 but the radiating and reflecting powers are directly opposed, and 

 are inversely as each other. In a cubical vessel of tin, one of whose 

 sides was blackened, another papered, and another glazed, the radi- 

 ation was in the following proportion 



from the black side, 100 



" " papered, - 98 



" " glazed, - 90 



" " bright metallic, - 12 Leslie. 



(aa.) The thermometer indicates more or less of heat, according as 

 'its surface is blackened, covered with tinfoil or other good reflector, 

 or is in its natural state. For a comparative result, it should be at 

 the same temperature, in the beginning of different experiments. 



Jbb.) All mirrors lose their power of reflecting heat if blackened 

 become heated. Glass mirrors, not reflecting culinary heat, do 

 reflect it, if covered with tin foil. 



* In order that this should be strictly true, the solids must be supposed to be per- 

 fectly smooth, of which we have perhaps n'o examples. Scratched metallic surfaces 

 receive and emit more heat, if the scratches cross one another, than if they are 

 parallel ; the difference is attributed to the formation of points, by the intersection, 

 through which points, the heat more readily passes. 



t As ascertained by Pictet and Rumford. In the experiments of the latter it per- 

 vaded the Torricellian vacuum. Sir Humphrey Davy found that a thermometer 

 was heated by radiation, from charcoal, ignited by galvanism in a vacuum, three 

 times as much as it would have been in the air ; there being no cooling effect from 

 currents. 



t Dr. Turner doubts whether color has any effect on the absorption of heat unless 

 the latter is accompanied by light, in which case he calls it luminous caloric : but 

 then he allows that the effect is great. 



