Spectra of Carbon Compounds. 487 



substance being present in infinitesimal quantity in a 

 cyanogen flame. 



It is also wortb remarking that a flame-spectrum is by no 

 means so likely to be due to an undiscoverable or irremovable 

 impurity as is an electric spectrum. A flame may be 

 developed at the margin of contact of two gaseous streams, 

 away from solid surfaces and without the intense localized 

 temperature-effects of the electric discharge which render 

 visible the minutest traces of matter. It appears very unlikely 

 that so brilliant a spectrum as that of the Swan bands in 

 the flame of cyanogen burning in oxygen can depend on an 

 irremovable quantity of an impurity. 



The evidence afforded by the flame of cyanogen is so 

 strongly opposed to the hydrocarbon view of the Swan 

 spectrum that it is at present hardly necessary to discuss the 

 same view as applied to hydrocarbon flames. It can scarcely 

 be maintained that either acetylene or marsh -gas is likely to 

 be made incandescent by the general temperature of its 

 surroundings in a flame. If they were generated in the flame 

 by a chemical action involving a considerable liberation of 

 energy there might be reason to ascribe luminosity to the 

 newly-formed molecules, but we have no right to suppose that 

 this is the case. Acetylene may be formed from hydrocarbons 

 by passing them through a heated tube, and the quantity so 

 formed is greater than that which occurs in flame gases. 

 I have viewed such a tube " end-on/' but there is no sign of 

 a spectrum. • 



I have now discussed all the material evidence bearing on 

 the Swan spectrum that is to be derived from a study of the 

 flames of carbon compounds. This evidence appears to be 

 equally inconsistent with the hypothesis that the glowing- 

 substance is elementary carbon, or that it is a hydrocarbon, 

 whilst it points directly to carbon monoxide as the source of 

 the spectrum. 



So simple a view would no doubt have been long since put 

 forward had there not been some obvious reasons opposing it. 

 These reasons are, first that the direct formation of carbon 

 monoxide in the combustion of carbon and its compounds 

 was not established until recently, and secondly, the behaviour 

 of carbon compounds under the electric discharge appeared 

 inconsistent with such a view. 



I will therefore proceed to consider the electric spectra of 

 carbon compounds. 



Electric Spectra of the Oxides of Carbon. 



The spectra of the oxides of carbon under the action of the 

 electric discharge have been described frequently. Broadly 



