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XLIX. Reply to some remarks by G. Kirchhoff in his Paper " On 

 the History of Spectrum Analysis." By Balfour Stewart, 

 M.A., F.R.S. 



My dear Sir David Brewster, 



IN the last Number of the Philosophical Magazine I have 

 perused an article by Professor Kirchhoff, of Heidelberg, "On 

 the History of Spectrum Analysis and of the Analysis of the 

 Solar Atmosphere." In the course of his remarks the learned 

 author has reviewed in a somewhat disparaging manner some 

 researches of mine on radiant heat, in consequence of which I 

 am forced to reply, although very unwillingly, and desiring 

 much to avoid a scientific controversy, especially with Professor 

 Kirchhoff as an opponent. 



The experiments in confirmation of my views were made in 

 the physical laboratory of the University of Edinburgh in the 

 beginning of the year 1858, and my account of them was, in 

 March of the same year, communicated to the Royal Society of 

 Edinburgh by Principal Forbes, whose name is well known in 

 connexion with radiant heat, and who gave me much valuable 

 assistance and advice during the progress of these experiments. 



I entertain a hope that you, as a distinguished office-bearer 

 in both of these scientific bodies, will afford me the opportu- 

 nity of briefly vindicating in your Journal the claims of these 

 researches to a greater measure of completeness than the learned 

 Heidelberg Professor seems disposed to allow. 



It was the theoretical extension of Prevost's law of exchanges 

 which first presented itself to my mind ; nor did I omit to obtain 

 the best possible experimental verification of my views, or to 

 present this to men of science as the chief feature, grounding 

 the theory upon the experiments, rather than deducing the ex- 

 periments from the theory. 



It was thus that dark heat came to form the subject of these 

 investigations ; for as the point to be proved was an extension of 

 Prevost's theory, it was necessary to deal with rays which were 

 universally acknowledged to produce heat by their absorption ; 

 and hence luminous rays were excluded, since it was still an open 

 question whether these were in all cases capable of producing 

 heat. I shall only add that it was attempted, as far as possible, 

 to disengage the proof, theoretical and experimental, from the 

 embarrassment of considering surface reflexion. 



I first endeavoured to show, not only as a simple deduction 

 from the theory of exchanges, but also as the result of experi- 

 ments, that in a field of uniform temperature the absorption of a 

 plate or particle is equal to its radiation. Now, since a thick 

 plate absorbs more heat than a thin one, it will also radiate more. 



