482 



Prof. A. Smithells on the 



Consideration of other Hypotheses as to the Origin of the 

 Swan Spectrum in Flames. 



The evidence just recorded appears to point very directly 

 to carbon monoxide as the source of the Swan spectrum, and 

 1 know of no other evidence derived from flames that is 

 incompatible with this view. But it is now necessary to con- 

 sider how far the facts are in harmony with other views that 

 have been held as to the identity of the substance w r hich yields 

 the Swan spectrum. 



Swan considered that the spectrum was due to hydro- 

 carbons ; but an investigation by Attfield, in 1862, gave 

 currency to the view that the spectrum really belonged to 

 carbon itself, and this view was finally adopted by Dibbits, 

 Morren, Watts, and Salet, all of whom investigated the 

 subject. Jn 1875 Angstrom and Thalen (Nov. Act. Reg. 

 Soc. Ups. [3], ix. 1875) concluded that the luminous substance 

 was a hydrocarbon — probably acetylene. It is between these 

 two views that the opinion of investigators has from time to 

 time been divided, and, as already stated, the amount of 

 experimental evidence that has been recorded is most volumi- 

 nous. The most recent investigations bearing directly on 

 the origin of the Swan spectrum are those of Professors 

 Liveing and Dewar (Proc. Roy. Soc. xxx. pp. 152 & 494, 1880 ; 

 ibid, xxxiii. pp. 3 & 403, 1882) and Herr Wesendonck (Unter- 

 suchungen iiber die Spectra der Koldenverbindungen. Inaug. 

 Dissert. Berlin, Schade, 1881). Liveing and Dewar at 

 first were strongly of opinion that the spectrum was due to a 

 hydrocarbon, but later were led to recede from that view in 

 consequence of their own further experiments. 



I will first consider the view which attributes the Swan 

 spectrum to elementary carbon. 



Most observers appear to have felt the prima facie ob- 

 jections to this hypothesis. Writing iu 1880 Professors 

 Liveing and Dewar said " the evidence that carbon uncom- 

 bined can take the state of vapour at the temperature of the 

 electric arc is at present very imperfect. Carbon shows at 

 such temperatures only incipient fusion, if so much as that*, 

 and that carbon should be volatilized at the far lower tem- 

 perature of the flame of cyanogen is so incredible an 

 hypothesis that it ought not to be accepted if the phenomena 

 admit of any other probable explanation." 



Although in a later paper these authors are led to admit 

 the existence of carbon vapour in a cyanogen flame the words 



* Recent researches indicate that the temperature of the positive pole- 

 is about 3500° C. M. Moissan {Le Four Electrique, p. 159) shows that 

 at this temperature carbon is distinctly volatile, but without fusion). 



