Note on Chemical Analysis by Spectrum-observations, 173 



13. We are thus furnished with a test by means of which we 

 may in all probability ascertain whether such groups of bodies 

 exist. In applying this test, it will be necessary to assume the 

 truth of the hypothesis of art. 6. Our method will be to con- 

 struct screens of the bodies under analysis such that they all 

 stop the same proportion of some one kind of incident heat ; and 

 if these bodies possess the property we are in search of, that is, 

 if their ultimate particles always radiate the same quality of 

 heat at the same temperature, the screens will all stop the same 

 proportion of any other description of heat. It is almost unne- 

 cessary to remark that the common proportion stopped by the 

 screens for one kind of heat will generally be different from that 

 stopped by them for heat of another description. 



14. This test has possibly a still wider application. For let 

 it be supposed that we are comparing together two bodies 

 which possess in common the same characterizing or influential 

 element, but one of which possesses in addition another element 

 also influential, but relating to an entirely different set of rays. 

 It seems likely that for those kinds of heat which are character- 

 istic of the common element the bodies will conform to the test ; 

 but, on the other hand, for those kinds of heat which refer to 

 the element which the one body possesses and not the other, the 

 test will fail. 



Some tables furnished by Melloni appear to confirm this idea; 

 but the subject will doubtless require a special experimental in- 

 vestigation. 



Kew Observatory, 

 August 7, I860. 



XXI. Note on Professors Kirchhoff and Bunsen's Paper " On 

 Chemical Analysis by Spectrum-observations" By Professor 



Swan. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



ON perusing Professors Kirchhoff and Bunsen's paper " On 

 Chemical Analysis by Spectrum-observations," I find it 

 stated that I have " already remarked upon the small quantity 

 of sodium necessary to produce the yellow line in a flame-spec- 

 trum." As I have not only remarked that a small quantity of 

 sodium suffices to produce the yellow line in a flame-spectrum, 

 but have distinctly announced the opinion that in all flames the 

 yellow line is due to the presence of sodium, you will perhaps 

 allow me to state the observations I adduced in support of that 

 view, as published in the Edinburgh Transactions for 1856. 



