SECT, xxv.] MELLON1 : S EXPERIMENTS. 243 



radiated from sources of different temperatures, whence it 

 appears that the calorific rays pass less abundantly not only 

 through glass, but through rock-crystal, Iceland-spar, and 

 other diaphanous bodies, both solid and liquid, according as 

 the temperature of their origin is diminished, and that they 

 are altogether intercepted when the temperature is about 

 that of boiling water. 



In fact, he has proved that the heat emanating from the 

 sun or from a bright flame consists of rays which differ from 

 each other as much as the red, yellow, and blue rays do 

 which constitute white light. This explains the reason of 

 the loss of heat as it penetrates deeper and deeper into a 

 solid mass, or in passing through a series of plates ; for, of 

 the different kinds of rays which dart from a vivid flame, all 

 are successively extinguished by the absorbing nature of the 

 substance through which they pass, till those homogeneous 

 rays alone remain which have the greatest facility in passing 

 through that particular substance ; exactly as in a red liquid 

 the blue and yellow rays are extinguished, and the red are 

 transmitted. 



M. Melloni employed four sources of caloric, two of 

 which were luminous and two obscure ; namely, an oil-lamp 

 without a glass, incandescent platina, copper heated to 696, 

 and a copper vessel filled with water at the temperature of 

 178i of Fahrenheit. Rock-salt transmitted heat in the pro- 

 portion of 92 rays out of 100 from each of these sources; 

 but all other substances pervious to radiant heat, whether 

 solid or liquid, transmitted more caloric from sources of high 

 temperature than from such as are low. For instance, lim- 

 pid and colourless fluate of lime transmitted in the proportion 

 of 78 rays out of 100 from the lamp, 69 from the platina, 42 

 from the copper, and 33 from the hot water ; while transpa- 

 rent rock-crystal transmitted 38 rays in 100 from the lamp, 

 28 from the platina, 6 from the copper, and 9 from the hot 

 water. Pure ice transmitted only in the proportion of 6 rays 

 in the 100 from the lamp, and entirely excluded those from 



K 2 



