262 



ON THE NATURE OF HEAT. 



Other differ. 

 ences. 



Why solar 

 heat passes 

 through trans'- 

 parent bodies 

 •wholly J 



other h«at only 

 in part. 



Other differ- 

 ences. 



Opaque bo- 

 dies. 



" counteract or diminish in a certain measure the peculiar 

 " effect of the heat emitteJ from the same source." 



Another ditierence still between ihe two kinds of heat was 

 discovered by Mr. Leslie. A very considerable aberration 

 takes place in the reflection of culinary heat, which is not I 

 believe the case with the solar rays, Kor is the etlect of 

 colour, in absorbing- the two kinds of heat, the same. — 

 ♦* Stained paper has very nearly the same action as white 

 *' paper, and it is only when cjjvered by a pigment super- 

 " induced, that th'e diversity ol' tffect becomes conspicu- 

 "ousV* 



I shall now attempt to explain this remarkable difference 

 between solar and culinary heat. Solar heat may consist of 

 vibrations in that medium or fluid, which we^ suppose to fill 

 space. This fluid is one of extreme tenuity, and pervades 

 all bodies without exception ; vibration therefore, which 

 subsists in this fluid, does and ought to pass through such 

 bodies as are transparent, "witii little or no interruption. 

 Radiation from other bodies, that is radiant culinary heat, 

 is very different : the radiator is in a state of vibr;ition ; this 

 vibration is communicattd to all surrounding bodies, the 

 most iiQfiortant of which is the atmosphere ; the subtile fluid 

 too must be taken into consideration ; these, with other bor 

 dies, which are within the vicinity of the source of heat, take 

 on vibration, and convey it to distant surfaces. In as much 

 as the vibration subsists in the more subtile medium, it will, 

 as it did in the case of solar heat, pervade transparent bodies ; 

 but the chief conductor of heat in this operation is the 

 *' ambient air;'* this fluid does not pervade transparent or 

 other bodies, its vibrations will therefore be intercepted by I 

 thecj. 



It is easy to conceive, that, as the two kinds of radiant 

 heat are so extremely different, the laws which are observed 

 in their other motions shall be very different; a difference in 

 their reflection and absorption is what might have been ria- 

 u rally expected. 



From some cause, the pervading fluid of certain bodie> 

 does not propagate its vibrations; these bodies are therefoie 



Inquiry, p.. ^4. 



opacwe 



