260 TRANSMISSION OF HEAT. SECT. XXVII. 



by the experiments of M. Melloni on heat 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 coloured rays do which constitute white 

 light. This explains the reason of the loss of heat as it pene- 

 trates 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 violet, blue, green, orange, and yellow 

 rays are extinguished, and the red are transmitted. 



M. Melloni employed four sources of heat, 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 178 of Fahrenheit. 

 Rock-salt transmitted heat in the proportion 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 heat 

 from sources of high temperature than from such as are low. 

 For instance, limpid 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 

 transparent 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 the 

 other three sources. Out of 39 different substances, 34 were 

 pervious to the calorific rays from hot water, 14 excluded those 

 from the hot copper, and 4 did not transmit those from the 

 platinum. 



Thus it appears that heat proceeding from these four sources 

 is of different kinds : this difference in the nature of the calorific 



