258 MELLONFS EXPERIMENTS. SECT. XXVII. 



stautaneously 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 remarkable difference obtains in the 

 transmission of light and heat through most solid and liquid sub- 

 stances, 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 ; 

 whilst a large piece of brown rock-crystal gives a free passage to 

 the radiant heat, but intercepts almost all the light. Alum 

 united to green glass is also capable of transmitting the brightest 

 light, but it gives not the slightest indication of heat ; while rock- 

 salt covered thickly over with soot, so as to be perfectly opaque 

 to light, transmits a considerable quantity of heat. M. Melloni 

 has established the general law in uncrystallized substances such 

 as glass and liquids, that the property of instantaneously trans- 

 mitting heat is in proportion to their refractive powers. The 

 law, however, is entirely at fault in bodies of a crystalline tex- 

 ture. Carbonate of lead, for instance, which is colourless, 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 refrangibility ; 

 whilst rock-salt, which has the same transparency and refractive 

 power with alum and citric acid, transmits six or eight times as 

 much heat. This remarkable difference in the transmissive power 

 of substances having the same appearance is attributed by M. 

 Melloni to their crystalline form, and not to the chemical com- 

 position of their molecules, as the following experiments prove. 

 A block of common salt cut into plates entirely excludes calorific 

 radiation ; yet, when dissolved in water, it increases the transmis- 

 sive power of that liquid : moreover, the transmissive power of 

 water is increased in nearly the same degree, whether salt or 

 alum be dissolved in it ; yet these two substances transmit very 

 different quantities of heat in their solid state. Notwithstanding 

 the influence of crystallization on the transmissive power of 



