338 RADIANT HEAT. 



radiations emitted from six different substances, each intercepted 

 successively by 24 minerals and 10 colored glasses, from which it 

 appears that the transmission is very different, according to the 

 nature of the first medium. 



He afterwards describes an experirAnt with the solar rays trans- 

 mitted by a green glass, and then intercepted by other media. They 

 pass copiously through rock salt, but feebly through alum. Hence he 

 concludes that there are among the solar rays some which resemble those 

 of terrestrial heat, and, in general, that " the differences observed between 

 solar and terrestrial heat, as to their properties of transmission, are there- 

 fore to be attributed merely to the mixture in different proportions of these 

 severed species of rays." 



In a note to this memoir, M. Melloni refers to my original experi- 

 ment, (Phil. Trans., 1825,) in which the action of the rays on surfaces 

 is observed in connexion with their transmissibilty . 



He confirms the accuracy of my result by a careful repetition of the 

 experiment with the thermo-multiplier, but makes no reference to the 

 conclusion I Lad drawn, viz: the coexistence of two distinct sorts of 

 heat in the radiation from luminous sources, one of which is the same 

 as that from dark sources. He explains the result by supposing the 

 transmitted rays to acquire, in and by the act of transmission through 

 the glass screen, new properties in their relation to the surfaces on 

 which they fall, i. e., to the degree of absorption they undergo re- 

 spectively on a black and a white surface. 



He extends the investigation b} r a table of results of the same kind 

 with a series of screens, both transparent, and of various degrees of 

 opacity. The ratio of the effects on the black and white surfaces is 

 nearer to equality as the screen is more opaque. 



On this subject there appears a short paper by M. Melloni in the 

 London and Edinburgh Journal of Science, vol. vii, p. 175, to which 

 I replied in the same journal, January, 183G. 



While referring to my own experiments, I may be allowed to add 

 that in Dr. Thomson's Treatise oir Heat, &c, first edition, the bearing 

 of my investigation was incorrectly represented, and accordingly I 

 pointed this out in the London and Edinburgh Journal of Science, Nov., 

 1830. 



In the second edition of Dr. Thomson's work, which has lately 

 appeared, the author omits all mention of the subject whatever. 



Transmission and Refraction of Heat: Forbes. 



The subjects of transmission and refraction of heat were taken up 

 by Professor Forbes, and Melloni's experiments repeated and extended 

 by him, the details being given in the first and part of the second sec- 

 tions of his first Memoir " on the Refraction and Polarization of Heat," 

 read to the Royal Society of Edinburgh, January 5 and 19. 1835, and 

 published in their Transactions, vol. xiii; also in the London and 

 Edinburgh Journal of Science, vol. vi. 



The first section contains an account of various experiments with 

 the thermo-multiplier. The principal object was to verify the several 



