1866.] 



Prof. Tyndall on Badiation and Absorption. 



5 



I. " Sixth Memoir on Eadiation and Absorption By Prof. J. 

 Tyndall, F.R.S. Received December 21, 1865. 



(Abstract.) 



In this paper the author considers the deportment of certain additional 

 elementary bodies towards Radiant Heat. He exposes powders and liquids 

 of the same physical character, but differing from each other chemically, 

 at a focus of dark rays, and describes the different elBFects produced. He 

 examines and explains the experiments of Franklin on the absorption of 

 solar heat. Pie then determines the radiative power of a great number 

 of substances in the state of fine powder, and finds, contrary to the current 

 belief, that in this state also chemical constitution exercises a paramount 

 influence. The results obtained by previous experimenters in connexion 

 with this subject are illustrated and explained. The reciprocity of radia- 

 tion and absorption on the part of fine powders is also illustrated. It is 

 moreover shown that the heat emitted from different sources, at a tem- 

 perature of 100° C, varies in quality, this being proved by its unequal 

 transmission through plates of rock-salt of perfect purity. The absorption 

 by such plates varies from 4 to 30 per cent, of the incident radiation. 



II. "On the Spectrum of Comet 1, 1866.^^ By William Huggins, 

 F.R.S. Received January 11, 1866. 



The successful application of prismatic analysis to the light of the ne- 

 bulae showed the great importance of subjecting the light of comets to a 

 similar examination, especially as we possess no certain knowledge of the 

 intimate nature of those singular and enigmatical bodies, or of the cosmi- 

 cal relations which they sustain to our system. The importance of a pris- 

 matic analysis of cometary light is enhanced by the consideration of the 

 general resemblance which some of the nebulae present to the nearly round 

 vaporous masses of which some comets, in some positions at least in their 

 orbits, appear to consist, — a resemblance which suggests the possible ex- 

 istence of a close relation between nebulous and cometary matter. 



I made several unsuccessful attempts to obtain a prismatic observation 

 of Comet 1, 1864. The position of the comet and the weather were un- 

 favourable. M. Donati succeeded in making an examination of the spec- 

 trum of this comet. " It resembles," says M. Donati, " the spectra of the 

 metals ; in fact the dark portions are broader than those which are more 

 luminous, and we may say these spectra are composed of three bright 

 lines"*. 



Yesterday evening, January 9, 1SG6, I observed the spectrum of Comet 

 * Monthly Notices, Eoyal Astronoiincal Society, yoI. xxv. p. 114. 



