Mr. B. Stewart on the Light radiated from Heated Bodies. 539 



In conclusion, I may be permitted to remark regarding these laws 

 of light, that from their simple nature some of them may have been 

 observed before, but I think they are now for the first time con- 

 nected with a theory of radiation. 



Supplement (added March 7, 1860). — Since writing the above, 

 the law which asserts that the absorption of a thin plate or particle 

 is proportional to its radiation for every description of light, has 

 received a very beautiful confirmation. 



In the Philosophical Magazine for this month, pages 194-196, 

 Professor Stokes has noticed some very interesting experiments of 

 M. Foucault, and also of Professor Kirchhoff. M. Foucault finds as 

 the result of his experiments, " that the voltaic arc formed between 

 charcoal poles presents us with a medium which emits the ray D of 

 the solar spectrum on its own account, and which at the same time 

 absorbs it when it comes from another quarter." Professor Kirchhoff, 

 again, finds as the result of his experiments on the spectra of coloured 

 flames, " that coloured flames, in the spectra of which bright sharp 

 lines present themselves, so weaken rays of the colour of these lines 

 when such rays pass through the flames, that, in place of the bright 

 lines, dark ones appear as soon as there is brought behind the flame 

 a source of light of sufficient intensity, in the spectrum of which these 

 lines are otherwise wanting." 



We thus see that the same media which in a heated state emit 

 rays of a certain refrangibility in great abundance, have also the 

 property of stopping these very rays when they fall upon them from 

 another quarter, or, in other words, their absorption of such rays is 

 proportional to their radiation of them. 



Supplement (added March 8, 1860). — The following fact noticed 

 by Professor Kirchhoff is also in accordance with the theory brought 

 forward in this paper. 



" The spectrum of the Drummond light," he remarks, " contains, 

 as a general rule, the two bright lines D, if the luminous spot of the 

 cylinder of lime has not long been exposed to the white heat ; if the 

 cylinder remains unmoved these lines become weaker, and finally vanish 

 altogether. If they have vanished, or only faintly appear, an alcohol 

 flame into which salt has been put, and which is placed between 

 the cylinder of lime and the slit, causes two dark lines of remarkable 

 sharpness and fineness to show themselves in their stead ; but the 

 Drummond light requires, in order that the lines D should come out 

 in it dark, a salt-flame of lower temperature. The flame of alcohol 

 containing water is fitted for this, but the flame of Bunsen's gas -lamp 

 is not. With the latter the smallest mixture of common salt, as 

 soon as it makes itself generally perceptible, causes the bright lines 

 of sodium to show themselves." 



Now, when we heat a piece of ruby glass in the fire, we have an 

 analogous phenomenon. As long as the ruby glass is of a lower 

 temperature than the coals behind it, the light given out is of a red 

 description, because the ruby glass stops the green : the green here 



2N2 



