418 MR WILLIAM SWAN ON THE PRISMATIC SPECTRA OF THE 



all due to the combustion of the carbohydrogen compounds, and not to the pre- 

 sence of foreign matter. The alcohol, ether, and other liquids, were burned in 

 lamps made of small phials, — a glass tube furnished with a cotton wick serving 

 as a burner. 



Taking the spectrum of the Bunsen lamp as a standard, the spectra of the 

 other flames were compared with it, by viewing both simultaneously, — the light 

 from the two flames passing through the same narrow slit. 



The result of this comparison has been, that, in all the spectra produced by 

 substances, either of the form C r H 3 or of the form C r H 3 O t , the bright lines have 

 been identical. In some cases, indeed, certain of the very faint lines, which occur 

 in the spectrum of the Bunsen lamp, were not seen. The brightness of the lines 

 varies with the proportion of carbon to hydrogen in the substance which is burned, 

 being greatest where there is most carbon. Thus, in the spectra of light carburet- 

 ted hydrogen, pyroxylic spirit, and glycerine — substances which contain compa- 

 ratively little carbon — certain of the fainter lines of the Bunsen lamp spectrum 

 were not seen; but all those that were seen were identical with the lines of the coal- 

 gas flame. I have no doubt that the fainter lines were really present, but were 

 invisible, merely owing to their feeble luminosity ; and this is rendered more pro- 

 bable by the fact that the number of lines visible in any spectrum varies with the 

 brightness of the light. Thus in the solar spectrum, or in that of the Bunsen 

 lamp, the fainter lines disappear when the intensity of the light is diminished. 



The absolute identity which is thus shown to exist between the spectra of 

 dissimilar carbohydrogen compounds is not a little remarkable. It proves, 1st, 

 that the position of the lines in the spectrum does not vary with the proportion 

 of carbon and hydrogen in the burning body ; as when we compare the spectra of 

 light carburetted hydrogen, C H 2 , olefiant gas, C 2 H 2 , and oil of turpentine, C 10 H g ; 

 and, 2dly, that the presence of oxygen does not alter the character of the spec- 

 trum ; thus, ether, C + EL 0, and wood spirit, C 2 H 4 2 , give spectra which are 

 identical with those of paraffin, C 20 H 20 , and oil of turpentine, C l0 H 8 . 



In certain cases, at least, the mechanical admixture of other substances with 

 the carbohydrogen compound does not affect the lines of the spectrum. Thus I 

 have found that a mixture of alcohol and chloroform burns with a flame having 

 a very luminous green envelope — an appearance characteristic of the presence 

 of chlorine — and no lines are visible in the spectrum. When, however, the flame 

 is urged by the blowpipe, the light of the envelope is diminished, and the ordinary 

 lines of the hydrocarbon spectrum become visible. 



