Royal Institution. 539 



Among the most recent researches on the subject are those of 

 Mr. Knoblauch (of which a translation is given in Taylor's Foreign 

 Scientific Memoirs/Part xviii. and xix.), and they are not to be sur- 

 passed for extent and accuracy of detail. 



One series is devoted to the examination of the alleged differences 

 in radiation of heat proportioned to the temperature of the source. 

 This, as before observed, is an untenable hypothesis, but Mr. Knob- 

 lauch distinctly refutes it by a series of experiments on alcohol flame, 

 red-hot metal, hydrogen flame, and an Argand lamp, whose tempera- 

 tures are in the order of enumeration beginning with the highest ; 

 but the power of their heat to penetrate screens is found to follow 

 exactly the reverse order. And even with lower stages of heat, the 

 effects bear no proportion to the temperatures as such. Hence the 

 effect is evidently not due to a mere extrication of the heat of tem- 

 perature, but is of a peculiar kind. In a word, agreeably to the pre- 

 ceding remarks, the different species of rays, more or less compounded 

 together in the several cases, exhibit their diversities of character in 

 developing heat by their absorption. One very peculiar result is, that 

 platinum, at a stage intermediate between red and white heat, trans- 

 mits through all the screens employed rather less heat than when 

 at a red heat. That is, these intermediate rays are of such a wave- 

 length as to be subject to a peculiar absorption by these screens ; 

 while at the same time possibly less of the former may be emitted. 



In another section Mr. Knoblauch adverts to the effects of surfaces 

 on the absorption of rays, and particularly remarks (p. 205), •' The 

 experiments of B. Powell and Melloni have shown that one and the 

 same body is not uniformly heated bj r rays from different sources, 

 which exert the same direct action on a blackened thermoscope ; " 

 a statement which does not very intelligibly express any conclusion 

 of the author's. Mr. Knoblauch however supports it by elaborate 

 experiments, showing, as might be anticipated, that an Argand lamp 

 affects a surface of carmine less, and one of black paper more, while 

 a cylinder heated to 212° affects the carmine more and the black 

 paper less. 



Another extensive series, on the effect of surfaces on radiation, is 

 directed to show that the effect is independent of the source whence 

 the heat so radiated was originally obtained. 



Among the very multifarious results referring to screens and sur- 

 faces obtained by Mr. Knoblauch, it can here only be remarked that 

 none of those varied facts appear to present anything at variance 

 with the principles here advocated, while in the general conclusions 

 which he indicates at the close of his memoir, the author, though 

 professedly avoiding all hypothesis, yet distinctly intimates his con- 

 viction of the heterogeneity of the heating rays increasing as the 

 condition of the source rises in the scale from a low heat up to lumi- 

 nosity or combustion : and that the diversities of heating effect on 

 different media are due to a selective absorption of particular species 

 of rays, from peculiarities in the nature of those substances, and 

 analogous to the absorption of particular rays of light by coloured 

 media. 



