[ 117 ] 



XIV. On the Spectra given by Carbon and some of its 

 Compounds ; and, in particular, the " Swan " Spectrum. 

 By W. Marshall Watts, D.Sc* 

 [Plate II.] 



ALTHOUGH more than a century has passed since 

 Wollaston f first observed the spectrum given by 

 the base o£ a candle-flame, now known as the " Swan " 

 spectrum, no complete agreement as to its origin has yet 

 been reached. Swan J, who observed it in 1857, attributed 

 the spectrum to a hydrocarbon. He obtained the spectrum 

 only by combustion of hydrocarbons. Van der Willigen§ 

 found that the spark-discharge between carbon poles gave 

 the same spectrum as burning olefiant gas, and ascribes the 

 spectrum to carbon. Attfield|| obtained the spectrum not 

 only by the combustion of hydrocarbons, but also, most 

 brilliantly, from the flame of dry cyanogen in dry oxygen, 

 and also by the electric discharge in dry cyanogen, carbonic 

 oxide, and carbon disulphide vapour at atmospheric pressure. 

 Since these substances have only carbon in common, " unless 

 the experiments are vitiated by impurities, they prove un- 

 doubtedly that this spectrum is due to the element carbon "1". 

 Dibbits** arrived at the same conclusion as Attfield. In 

 answer to the objection raised that carbon could not exist 

 as vapour in the flame of a candle or of a Bunsen burner, 

 Dibbits argues that carbon is combined with hydrogen 

 before the combustion, and after the combustion that carbon 

 is combined with oxygen, and during the combustion it may 

 have been in an uncombined condition : a flame of carbonic 

 oxide does not show the same spectrum because the carbon 

 is already combined with oxygen. In the ease of cyanogen 

 the carbon is at first combined with nitrogen, and after the 

 combustion it is combined with oxygen, so that the same 

 explanation appliesft- 



* Communicated by the Author. 



+ Wollaston, Phil. Trans. 1802, p. 365. 



X Swan, Phil. Trans. Edinb. xxi. p. 411 (1857). 



§ Van der Willigen, Pogg. Ann. cvii. p. 473 (1859). 



|| Attfield, Phil. Trans, clii. p. 221 (1862) : Phil. Mag. xlix. p. 106 

 (1875). 



1J Schuster, B. A. Keport, 1880. 



** Dibbits, Pogg. Ann. cxxii. p. 497 (1864). 



tt Note. In the case of cyanogen burning in oxygen the temperature 

 probably reaches the volatilization-point of carbon, or that of the electric 

 arc, viz. 3500° to 3700° C. " The temperature of individual molecules in 

 the respective flames of cyanogen and acetylene may reach a temperature 

 of from six to seven thousand degrees." (Liveingand Dewar, Proc. Kuy. 

 Soc. No. 223, 1882.) 



