318 RADIANT HEAT. 



cepting the one kind of heat which follows no proportion to the other, 

 this last being wholly transmitted. Also by comparison of the 

 latter experiments with the two first of the series, it is probable that, 

 throughout, a certain degree of heat was in this case also absorbed 

 and radiated again by the screen. 



The existence of this distinction, and the proportion between the 

 two species of heat in the radiation from different sources, as various 

 kinds of flame, metal at successive stages of incandescence, &c, 

 afford many topics of inquiry, on some of which I attempted some 

 rough determinations, coiifessedly very imperfect. — (Annals of Phil., 

 N. S. liii, 359; liv, 401.) The distinction applies to some results of 

 Mr. Brande on the flames of different gases, (Phil. Trans., 1820, Part 

 I, p. 22,) and of Count Rumford on increased intensity of combustion 

 and on the coalescing of several flames. — (Essays, i, 304.) 



7.) Melloni states (Ann. de Chim., December, 1831, p. 385,) that 

 by using his thermo-multiplier he has found the permeability of trans- 

 parent bodies to heat to be also dependent on their refractive poiver. 

 He has compared twenty such media, and finds the order of permea- 

 bility constantly the same, whatever be the temperature of the source. 

 Chloruret of sulphur has the greatest power, oil next, and water least; 

 he exposed them to the rays of a candle, an Argand lamp, or the sun. 

 He finds the differences of permeability less, the higher the tempera- 

 ture. The full account is promised in another memoir. 



All this obviously applies only to luminous hot bodies. 



MM. Melloni and Nobili, in their former paper, (Annales de Chimie, 

 October, 1831, p. 211,) also speakof the heat from phosphorus hav- 

 ing been by these means found sensible, though it is often supposed 

 to give light without heat. 



8.) For information on various points connected with the subject, 

 and on the theories of the evolution of light and heat, the following 

 references may be useful: 



Wedgewood (Phil. Trans., 1792, p. 28) thinks that light from 

 attrition is produced by a heat of from 400° to 600° Fahrenheit. 



Dize on Heat as the Cause of Shining. — (Journ. de Phys., xlix, 177. 

 Gilbert, Ann. iv, 410.) 



Fordyce on Light from Inflammation. — (Phil. Trans., 177G, p. 504. 

 Morgan. Phil. Trans., 1785, p. 190. M. Hermstaedt, Nicholson's 

 Quarto Journal, v, 187.) 



Mr. Davies on Flame. — (Annals of Phil., December, 1825.) 



Mr. Deuchar on Flame. — (Edinb. Phil. Journal, iv, 374.) 



M. Seguin on Heat and Motion, &c. — (Edinb. Journ. of Science, xx, 

 280.) 



Division III. 



HEAT OF THE SUN'S RAYS. 



Speaking according to our ordinary sensations, we are accustomed 

 to say that the sun communicates both light and heat. Light is trans, 

 mitted in a way which we term radiation. The heat from non-lumi^ 



