208 RADIATION. SECT. XXV. 



vacuo ; it is altogether imperceptible when the hot 

 body is inclosed in a solid or liquid. Heated substances, 

 when exposed to the open air, continue to radiate 

 caloric till they become nearly of the temperature of 

 the surrounding medium. The radiation is very rapid 

 at first, but diminishes according to a known law with 

 the temperature of the heated body. It appears, also, 

 that the radiating power of a surface is inversely as its 

 reflecting power ; and bodies that are most impermea- 

 ble to heat radiate least. 



Rays of heat, whether they proceed from the sun, 

 from flame, or other terrestrial sources, luminous or 

 non-luminous, are instantaneously transmitted through 

 solid and liquid substances, there being no appreciable 

 difference in the time they take to pass through layers 

 of any nature or thickness whatever. They pass also 

 with the same facility whether the media be agitated 

 or at rest; and in these respects the analogy between 

 light and heat is perfect. Radiant heat passes through 

 the gases with the same facility as light ; but a remark- 

 able difference obtains in the transmission of light and 

 heat through most solid and liquid substances, the same 

 body being often perfectly permeable to the luminous 

 and altogether impermeable to the calorific rays. For 

 example, thin and perfectly transparent plates of alum 

 and citric acid sensibly transmit all the rays of light 

 from an argand lamp, but stop eight or nine tenths of 

 the concomitant heat ; while a large piece of brown 

 rock crystal gives a free passage to the radiant heat, 

 but intercepts almost all the light. M. Melloni has 

 established the general law in uncrystalized substances 

 such as glass and liquids, that the property of instanta- 

 neously transmitting heat is in proportion to their re- 

 fractive powers. The law, however, is entirely at fault 

 in bodies of a crystaline texture. Carbonate of lead, 

 for instance, which is colorless, and possesses a very 

 high refractive power with regard to light, transmits 

 less radiant heat than Iceland spar or rock-crystal, 

 which are very inferior to it in the order of refran- 

 gibility ; while rock-salt, which has the same transpa- 

 rency and refractive power with alum and citric acid, 

 transmits six or eight times as much caloric. This 



