502 On the Spectra of Carbon Compounds. 



Evidence from the General Character of the Swan 

 Spectrum. 



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It is a remarkable fact that Angstrom and Thai en, who 

 ascribed the Swan spectrum to a hydrocarbon, should have 

 been led to do so, in the first instance, by its general resem- 

 blance to the spectrum of the oxides of alkaline earth metals. 

 After describing the Swan spectrum they say " cette descrip- 

 tion de Faspect des raies est tout-a-fait identique a celle 

 donnee auparavant par rapport aux raies des oxydes metal- 

 liques, et nous pensons que celte analogie remarquable ne 

 peut etre entierement fortuite." They insist further that 

 carbon in the elementary state is characterized by a line 

 spectrum, and that the Swan spectrum is conditioned by the 

 atmosphere in which the discharge takes place, it being found 

 only in the aureole and not in the line of spark. Although 

 they ascribe the spectrum to a hydrocarbon the general 

 argument is equally cogent for the doctrine advanced in this 

 paper. 



It is also important to remember that it was the inherent 

 probability of the correctness of the above views that led 

 Professors Liveing and Dewar to their exhaustive and elabo- 

 rate researches on the spectrum of carbon. At the outset of 

 their experiments and for long afterwards these investigators 

 inclined most strongly to the view that the Swan spectrum 

 was due to a hydrocarbon, and they found an additional 

 argument in the resemblance of the Swan spectrum to that 

 of a compound of hydrogen and magnesium. 



Although so many investigators have found themselves 

 compelled to ascribe the Swan spectrum to elementary carbon, 

 I am not aware that any rational explanation has been given 

 of the relation of this spectrum to the line spectrum of carbon. 

 Gases are not uncommon of an element giving more than one 

 spectrum, but the conditions for transition are in most cases 

 known and, in a sense, intelligible. In the case of carbon no 

 such conditions are known ; the line spectrum of carbon is 

 sometimes but not always obtained simultaneously with the 

 Swan spectrum, and the dependence of the Swan spectrum 

 on the nature of the atmosphere in which the discharge takes 

 place is certain. If it could be shown that the development 

 of the Swan spectrum from the discharge between carbon 

 poles in different atmospheres followed the conductivity or 

 some other physical property it might be maintained that 

 both it and the line spectrum were due to the element carbon. 

 But this has not been shown, and, I believe, cannot be shown ; 

 on the other hand, there is the strongest evidence that the 



