102 REPORT — 1861 



It was also mentioned that polished metal gives out less light than tar- 

 nished metal, and that, when a piece of black and white porcelain is heated 

 in the fire, the black parts give out much more light than the white, thereby 

 producing a curious reversal of the pattern. 



All these facts are comprehended in the statement that in a constant tem- 

 perature the absorption of a particle is equal to its radiation, and that for 

 every description of light. 



It was also noticed that all coloured glasses ultimately lose their colour in 

 the fire as they approach in temperature the coals around them, the expla- 

 nation being, that while red glass, for instance, gives out a greenish light, it 

 passes red light from the coals behind it, while it absorbs the green, in such 

 a manner that the light which it radiates precisely makes up for that which it 

 absorbs, so that we have virtually a coal radiation coming partly from and 

 partly through the glass. 



In another paper communicated to the Royal Society in May of the same 

 year, it was shown that tourmaline, which absorbs in excess the ordinary ray 

 of light, also radiates, when heated, this description of light in excess, but that 

 when the heated tourmaline is viewed against an illuminated background of 

 the same temperature as itself this peculiarity disappears. 



It is now time to advert to the spectrum observations which have recently 

 excited so much attention, and which are intimately connected with the sub- 

 ject of this Report. Our countryman Wollaston*, and after him Fraunhofer, 

 were the first to show that in the solar spectrum numerous dark bands occur 

 which indicate the absence of light of certain definite refrangibility. Other 

 new bands were artificially produced by Sir David Brewsterf in his remark- 

 able experiment, in which the spectrum was made to pass through nitrous- 

 acid gas; and it was thus rendered probable that those which occur in tlie 

 solar spectrum are also in some way due to absorption. Professor W. H. 

 Miller of Cambridge, and the late Professor DaniellJ, extended this property 

 to chlorine, iodine, bromine, euchlorine, and indigo. 



When the spectra produced by the ignition of various substances were 

 examined by Sir D. Brewster§, Sir J. Herschel||, Messrs. Talbot^, Wheat- 

 stone**, W. A. Millerff, and others, their contrast to the solar spectrum 

 was exceedingly remarkable. 



While the latter may be described as a continuous spectrum intersected 

 with dark bands, the spectra of artificial substances are for the most part 

 made up of bright, discontinuous, highly characteristic bands of light in a 

 dark background, and their general appearance is that of the solar spectrum 

 reversed :}:$. I think Fraunhofer was the first to notice that a bright band 

 corresponding in refrangibility to the double dark band Dof the solar spectrum 

 was produced by the yellow light of a flame containing sodium ; and this ray 

 was shown by Professor W. A. Miller §§ to occur in the flames of lime, 

 strontia, baryta, zinc, iron, and platinum, while, according to Angstrom, it 

 was found in the electric flames of every metal examined by him. Professor 

 Swan II II afterwards showed that an exceedingly small proportion of common 

 salt called forth this line. All these philosophers, but particularly Angstrom, 



* Philosophical Transactions 1802, p. 378. 



t London and Edinb. Philosophical Magazine, vol. ii. p. 381. 



X Philosophical Magazine, 1833. § Edinburgh Phil. Trans. 1822. 



II Edinburgh Phil. Trans. 1822. "jf Brewster's Journal of Science, vol. v. 



** British Association Report for 1835. 



tt British Association Report for 1845, or Philosophical Magazine, vol. xxvii. p. 81. 



Jt Professor W. A. Miller exhibited at this Meeting of the British Association (Manches- 

 ter 1861) photographs of the spectra of several metals, and I have since been informed that 

 he is pm-suing the subject with success. 



§§ Philosophical Magazine, August 1845. |||| Edinburgh Transactions, 1856. 



