."254 M. G. Kirchhoff on the History of Spectrum Analysis 



der Willigen, and Pliicker upon the spectra of the electric spark 

 or electric light (to which I have already referred in my e Re- 

 searches on the Solar Spectrum and Spectra of the Chemical 

 Elements/ MacMillan, London, 1862, p. 8), as well as those of 

 Despretz*, from which this physicist concluded that the posi- 

 tions of the bright lines in the spectrum of the light from a 

 galvanic battery were unaltered by variation of the intensity of 

 the current, might serve to support the view that the bright 

 lines in the spectrum of an incandescent gas are solely depend- 

 ent upon the several chemical constituents of the gas ; but they 

 could not be considered as proof of such an opinion, as the con- 

 ditions under which they were made were, for this purpose, too 

 complicated, and the phenomena occurring in an electric spark 

 too ill understood. The demonstrative power of the above ex- 

 periments as regards the question at issue is rendered less cogent 

 t)y the difference visible in the colour of the electric light in dif- 

 ferent parts of a Geissler's tube ; by the circumstance noticed by 

 Van der Willigen, who obtained different spectra by passing an 

 electric spark from the same electrodes through gas of constant 

 chemical composition if the density of the gas was varied within 



o 



sufficient limits ; and lastly by an observation which Angstrom 

 cursorily mentions. This physicist saysf, "Wheatstone has 

 already noticed that when the poles consist of two different 

 metals the spectrum contains the lines of both metals. Hence 

 it became of interest to see whether a compound of these metals, 

 especially a chemical compound, also gives the lines of both 

 metals, or whether the compound is distinguished by the occur- 

 rence of new lines. Experiment shows that the first supposition 

 is correct. The sole difference noticed is, that certain lines were 

 wanting or appeared with less distinctness ; but when they were 

 observed they always appeared in the position in which they oc- 

 curred in the separate metals." In the following sentence, how- 

 ever, he states " That in the case of zinc and tin the lines in the 

 blue were somewhat displaced in the direction of the violet end, 

 but the displacement was very inconsiderable." Had such a 

 displacement, however small, really occurred, we must conclude 

 either that the bright lines of the electric spark obey other laws 

 than those of a glowing gas, or that these latter are not solely 

 dependent on the separate chemical constituents of the gas. 



The question at issue respecting the lines of incandescent 

 gases could only be satisfactorily solved by experiments carried 



British Association, 1835; Chemical News, March 23, 1861; Chemical 

 News, March 30, 1861.) 



* Compies Rendus, vol. xxxi. p. 419 (1850). 



t Fogg. Ann. vol. xciv. p. 150. [Translated in Phil. Mag. for May, 1855.] 



