32 



TRANSMISSION OF HEAT. 



TRANSMISSION OF HEAT THROUGH MEDIA, AND THE EFFECT OF 



SCREENS. 



IT has been shewn by Dulong and Petit that hot bodies ra- 

 diate equally in all gases, or exactly as they radiate in a 

 vacuum. Hot bodies certainly cool more rapidly in some gases 

 than in others, but this is owing to the mobility and conducting 

 powers of the gases being different. 



Light of every colour, and from every source, is equally 

 transmitted by all transparent bodies in the liquid or solid form, 

 but this is not the case with heat. The heat of the sun passes 

 through any transparent body without loss, but of heat from 

 terrestrial sources, a certain variable proportion only is allowed 

 to pass, which increases as the temperature of the radiant body 

 is elevated. Thus, it was observed by Delaroche that, from a 

 body heated to 182, only l-40th of all the heat emitted passed 

 through a glas's screen: from a body at 346, 1-1 6th of the 

 whole; and from a body at 960, so large a proportion as l-4th 

 appeared to pass through the screen. M. Melloni has, within 

 the last few years, greatly extended our knowledge respecting 

 the transmission of heat through media, in a series of the most 

 profound researches.* In his experiments, he made use of the 

 thermo-electric pile to detect changes of temperature, an instru- 

 ment which in his hands exhibited a sensibility to the impres- 

 sions of heat vastly greater than that of the most delicate 

 mercurial, or air thermometer. 



His instrument, or the thermo-multiplier, (see the annexed 

 figure) consists of an arrangement of thirty pairs of bismuth and 



* The complete series of Melloni's Memoirs given in Mr. R. Taylor's Scientific 

 Memoirs, Nos. 1 and 3. 



