RADIANT HEAT. 365 



tion might be expected to occur, but only as applying to that portion 

 of the rays which consists of Species II. 



Thus, in a multitude of such cases, these effects might be very 

 small, or altogether disguised and hidden. And, moreover, if they 

 were real and sensible for Species (II), it would not follow that they 

 existed for Species (I), yet the wave analogy would render it 

 highly probable; and experiment has now proved it to be the fact. 

 This constitutes the peculiar value of Professor Forbes' s researches. 

 In the instance of luminous bodies, then, all the combined heteroge- 

 neous species of rays, undergo these modifications, though possibly in 

 different degrees, and liable to modifications from the different nature 

 of the media employed. 



Yet many recent researches seem altogether to ignore these dis- 

 tinctions; and the " radiant heat" from different sources is commonly 

 spoken of as if all of one kind, and of which a certain per centage is 

 stopped or transmitted, in particular cases, by the interposition of 

 certain substances; whereas the body of rays is heterogeneous, and 

 certain integrant rays only are totally stopped or totally transmitted 

 in the respective cases, showing not only quantitative but qualitative 

 differences. 



In many researches of this kind the "diathermancy" of bodies is 

 spoken of, and experiments made to measure it, as if it applied indif- 

 ferently to all species of heating rays, and without any reference to 

 the consideration that for certain species of rays bodies are diather- 

 manous simply in proportion as they are diaphanous to the particular 

 luminous ray in question; while for other species of rays their diather- 

 maneity follows some totally different, and as yet unknown law. The 

 confusion of ideas introduced by the adoption of that term, unqual- 

 ified by any reference to this distinction, and applied to general con- 

 clusions respecting "radiant heat," ought to be sufficiently manifest, 

 yet has been but little considered: And in cases where the rays 

 transmitted by a partially diathermanous body are in fact different in 

 kind (agreeably to the above distinction) from those stopped, there is 

 an ambiguity in the mode of expression; in fact it is not merely a 

 per centage of rays, but an analysis of the whole heterogeneous body 

 into more homogeneous or simple elements, i. e., rays of different 

 wave-lengths, or combinations of several such rays. 



In my former report (1832, p. 332) it is mentioned that I had then 

 repeated my original experiment (Phil. Trans., 1825) with a pair of 

 thermometers of a peculiar construction made on purpose, in which 

 very small differences were appreciable; but I did not there state any 

 of the results. As attention has now been recalled to my original 

 experiment, both by the repetition of it by Melloni with the thermo- 

 multiplier, and the more recent confirmatory remarks of Knoblauch, 

 and as results obtained in this way (notwithstanding the superior claims 

 of the thermal pile) may still possess some interest, it may not be 

 irrelevant here briefly to state the results of a set of observations 

 which have been lying by me since that period. The instrument was 

 unfortunately broken soon afterwards. These observations were made 

 on the rays of an Argand lamp; the thermometers, fixed on one frame 



