COMMUNICATION OF HEAT. 223 



part, good radiators of heat ; but bright and polished sub- 

 stances are generally bad radiators.* Color, however, 

 alone, has no elFect on the radiation of heat. 



If a metal surface be scratched, its radiating power is increased. A liquid 

 contained iu a bright, highly-polislied metal pot, will retain its beat much 

 longer than in a dull and blackened one. This is not due to the polish or 

 briglitness of the surface, but to the foct that, by polishing, the surface is ren- 

 dered dense and smooth, and such surfaces do not allow the heat to escape 

 readily. If we cover the polished metal surface with a thin cotton or linen 

 cloth, so as to render the surface less dense, the radiation of heat, and conse- 

 quent cooling, will proceed rapidh'. 



Black lead is one of the best known radiators of heat, and on this account 

 is generally employed for the blackening of stoves and hot-air flues. As a 

 high polish is unfivorable to radiation, stoves should not be too highly polished 

 with this substance. 



Heat radiated from the sun is all radiant heat. 



506. Heat is propagated throug;!! space by 



How is heat ... . \ , '^ ^. , ? .' . ■^ 



propagated by radiatiou in straitrht lines, and its intensity 



radiation? . T * *1 1 1 ' T 



vanes according to the same law which governs 

 the- attraction of gravitation, that is, inversely as to the 

 square of the distance.f 



Thus the heating effects of any hot body is nine times less at three feet than 

 at one ; sixteen times less at four feet ; and twenty-live times less at five. 



The velocity with which radiant heat moves through space 

 l^Tdty does la- is, in all probabihty, the same as the velocity of light. Somo 

 dirint heat authorities, however, consider it to be only four fifths of that 

 of light, or about 164,000 miles in a second of time. 



Does radiation 507. Thc radiatiou of heat goes on at all 

 mTiuiyfromrii tiuies, aud from all surfaces, whether their 

 ""^'^^^ temperature be the same or different from 



that of surrounding objects : therefore the temperature 

 of a body falls when it radiates more heat than it absorbs; 

 its temperature is stationary when the quantities emitted 

 and received are eo^^ual ; and it grows warm when the ab- 

 sorption exceeds the radiation. 



• The action of a blackened surface of tin being assumed as TOO, it has been found that 

 that of a steel plate was 15 ; of clean tin, 12 ; of tin scraped bright, 16 ; wlien scraped with 

 the edge of a fine file in one direction, -6; wlien scraped again across, al)oiit IH; a sur- 

 face of clean lead, 19 ; covered with a gray crust, 45 ; a tliin crust of isinglass, 80 ; rosin 

 96 ; wrjting-paper, 9$ ; ice. 85. 



t It is an exceedingly curious fact that this liw applies to all physical influencos that 

 spread from a center, such as gravitation, heat, light, electrical forces, magnetism and 

 sound ; and to all central forces, when not weakened by any resistance or opposing force. 



