106 REl'ORT — 1861. 



meter a small portion of phosphate of soda, and heated it in the dull flame of 

 Bunsen's lamp. The salt melted, formed a fluid lens, and remained perfectly 

 transparent ; it, however, emitted no light, while the platinum ring with which 

 it was in contact glowed brilliantly. Kirchhoff" also showed that a plate of 

 tourmaline cut parallel to the axis which absorbs the ordinary rays in excess, 

 radiates the same in excess. These results are similar to those which I com- 

 municated shortly afterwards to the Royal Society, and which have been 

 already mentioned. 



It was likewise stated by KirchhoS" in this paper, that Bunsen and he had 

 reversed the brighter lines of the spectra of potassium, calcium, strontium, 

 and barium, by exploding before the slit of the spectral instrument mixtures 

 of sugar of milk and chlorates of the respective metals during the passage of 

 the sun's ra3's. 



Allusion has already been made to Kirchhoff"s application of this law of 

 reversal, in order to determine the constituents of the solar atmosphere. By 

 means of this principle he has been enabled, he believes, to trace the presence 

 of iron and other metals in the photosphere of our luminary, having found 

 that the bright lines whicii occur in the electric spectra of those metals cor- 

 respond in position with dark lines in tiie solar spectrum. " Iron," he says, 

 *' is remarkable on account of the number of the lines which it causes in 

 the solar spectrum. Less striking, but still quite distinctly visible, are tiie 

 dark solar lines coincident with the bright lines of chromium and nickel. 

 The occurrence of these substances in the sun may therefore be regarded as 

 certain. Many metals, however, appear to be absent ; for although silver, 

 copper, zinc, aluminium, cobalt, and antimony possess very characteristic 

 spectra, still these do not coincide with any (or at least with any distinct) 

 dark lines of the solar spectrum." 



It has been shown, in the course of this Report, how the law which connects 

 together the radiating and absorbing power of bodies for individual descrip- 

 tions of heat or light follows immediately from the theory of exchanges. 

 But physicists have been anxious to establish this law as the result of some 

 simple fundamental property of matter. Euler, we have seen, and Angstrom 

 after him, predicted its existence, assuming as a fundamental principle, that 

 a body absorbs all the series of oscillations which it can itself assume. 



Professor Stokes also, in commenting on the discovery of Foucault and 

 Kirchhoff" (Philosophical JVIagazine, March 1860), uses these words : — " The 

 remarkable phenomenon discovered by Foucault, and rediscovered and ex- 

 tended by Kirchhoff, that a body may be at the same time a source of light 

 giving out rays of a definite refrangibility, and an absorbing medium ex- 

 tinguishing rays of the same refrangibility which traverse it, seems readily 

 to admit of a dynamical illustration borrowed from sound. We know that 

 a stretched string which on being struck gives out a certain note (suppose 

 its fundamental note) is capable of being thrown into the same state of vibra- 

 tion by aerial vibrations corresponding to the same note. Suppose now a 

 portion of space to contain a great number of such stretched strings, forming 

 thus the analogue of a 'medium.' It is evident that such a medium on 

 being agitated would give out the note above mentioned, while on the other 

 hand, if that note were sounded in air at a distance, the incident vibrations 

 would throw the strings into vibration, and consequently would themselves 

 be gradually extinguished, since otherwise there would be a creation ot vis 

 viva. The optical application of this illustration is too obvious to need com- 

 ment." 



Professor Tyndall also, in the Bakerian Lecture for this year, " On the 

 Absorption and Radiation of Heat by Gases and Vapours, and on the Physical 



