through different solid and liquid bodies. 231 



An inspection of these tables, shews at once, that the aptitude of 

 a body to transmit radiant heat, has scarcely any connection with its 

 aptitude to transmit light. The difference between these two prop- 

 erties is enormous, since in the case in which the bodies possess the 

 same degree of transparency, the quantities of heat transmitted, vary 

 from one to eight, and frequently the quantity transmitted by certain 

 substances of a deep tint is four or five times as great as that by other 

 substances perfectly diaphanous. 



This faculty of giving passage to calorific rays, diminishes by an 

 increase of the quantity of matter to be traversed, but not to so much 

 as might be supposed. Thus pieces of Iceland spar and rock crys- 

 tal, smoked, from 86 to 1000 millemetres thick, transmitted from 52 

 to 54 out of 100. The crystal was of so dark a tint that, laid on 

 printed paper, it was impossible to perceive the least trace of the 

 letters through it, even when exposed to the strongest light. Now 

 a plate of alum 2.8""" thick, allows only 12 rays in a hundred to pass 

 through, by reducing it to one mellimetre Melloni could obtain only 

 an increase of 4 to 5 hundredths, the material notwithstanding being 

 as pure as the finest glass. Here then is a very transparent plate 

 which transmits 3 or 4 times less radiant heat than another plate al- 

 most opake and nearly a hundred times thicker. 



The author finally ascertained that rays of heat are sensibly trans- 

 mitted through the perfectly opake glass employed in the fabrication 

 of the mirrors which are used in polarizing light. 



Not the least doubt therefore can remain of the almost entire in- 

 dependence of the two transparences — calorific and luminous, and it 

 becomes now indispensable to distinguish by a special denomination, 

 the bodies which transmit much radiant heat. Melloni proposes to 

 call them transcaloric or diathermanes, in imitation of the terms trans- 

 parent or diaphanous, indicating the analogous property relative to 

 light. 



It may now be asked whether the faculty of transmiting rays of 

 heat has any relation to other properties of matter, in these trials on 

 liquids and the vitrification of crystalline bodies properly so called. 

 An inspection of the first table assures us that a liquid is diather- 

 mous in proportion to its refrangibility. Hence the curburet and the 

 chloride of sulphur, transmit more caloric than oils, — oils more than 

 acids, — acids more than aqueous solutions, and the latter more than 

 pure water, which is the least refringent of the whole series, and also 

 the least diathermauous. Melloni also proved by exi)eriment that 



