Spectra of Carbon Compounds. -191 



the Swan spectrum and the carbon line 

 spectrum being supposed to pertain to carbon in the ele- 

 mentary state, there remains only one spectrum, the oxycarbon 

 spectrum, peculiar to the two oxides. It is concluded that 

 this spectrum pertains to carbon monoxide for the following 

 reasons : — (1) carbon dioxide is known to be dissociable by 

 heat into carbon monoxide and oxygen ; (2) it is the simpler 

 oxide ; (3) the flame of carbon monoxide which contains 

 glowing carbon dioxide gives a continuous spectrum. 



A complete explanation of the appearance of the different 

 spectra under different conditions has not, so far as I am 

 aware, been given by anyone. Liveing and Dewar infer that 

 a low pressure is unfavourable to the stability of the mole- 

 cular combination which gives the Swan spectrum. Wesen- 

 donck believes that the Swan spectrum is always produced by 

 a strong spark-discharge, though it may appear under other 

 conditions. 



I think the facts admit of an altogether different explana- 

 tion, which is at the same time simpler and more rational. I 

 consider that the Swan spectrum is the spectrum of carbon 

 monoxide, and the oxycarbon spectrum the spectrum of carbon 

 dioxide — that there are in fact two spectra dependent on the 

 association of carbon with oxygen. In support of this view I 

 adduce the following summarized evidence : — 



1. There is a striking resemblance between the Swan 

 spectrum and the oxycarbon spectrum. The actual difference 

 is such as might reasonably be expected from the molecular 

 difference of the two oxides. 



2. Carbon monoxide gives the Swan spectrum predomi- 

 nantly, except under extreme conditions. If, as is now 

 generally believed, the conduction of a gas is electrolytic in 

 character, the passage of the discharge should be associated 

 with the alternate dissociation and recombination of carbon 

 and oxygen. The process occurring in the tube has therefore 

 a chemical feature in common with that occurring in all 

 those flames which show the Swan spectrum. 



3. Under extreme conditions of gaseous pressure or elec- 

 trical tension, carbon monoxide gives either the oxycarbon 

 spectrum or the carbon line spectrum, or both. Now these 

 extreme conditions correspond also to maximum heating 

 effects of the discharge, and it is under these circumstances 

 that the gas may be expected to give spectra of decomposition 

 products. 



Carbon monoxide is a gas easily altered by heat. Deville 

 showed long since (Compt. Bend. lix. p. 873, 186-1) that 

 when carbon monoxide is heated in a tube to a hio-U 



2K2 



