Spectra of Carbon Compounds. 501 



The observations which have been made on the spectra of 

 rarefied cyanogen and hydrocarbon gases are no more satis- 

 factory. Wesendonck makes the highly significant observa- 

 tion in special reference to hydrocarbons that the pure sub- 

 stances do not give good spectra. The admission of a little 

 air, he remarks, greatly improves the spectrum, and hydrogen 

 also improves the spectrum without accentuating its own. 

 From many carban compounds he could observe little else 

 than a continuous spectrum. 



The difficulty of freeing cyanogen from moisture may be 

 overcome, but the difficulties of freeing it from other oxygen 

 compounds have appeared to me insuperable, and have de- 

 terred me from undertaking any lengthened experiments with 

 this gas. 



Whilst fully conscious of the importance of observations 

 on the electric spectra of the compounds of carbon with 

 chlorine, hydrogen, and nitrogen, I can only repeat that the 

 difficulties of obtaining and maintaining these compounds 

 free from oxygen are of the most formidable kind, and I 

 believe that the recorded observations go so far at least as to 

 show that when oxygen is known to be present the Swan 

 spectrum is correspondingly enforced. 



The Experiments of Prof . J. J. Thomson. 



In a paper read before the Royal Society (Proc. R. S. xlv. 

 p. 244) in 1895 Prof. J. J. Thomson describes some highlv 

 interesting observations on the spectra of carbon compounds, 

 which he summarizes as follows : — " The view which seems 

 most in accordance with the results of observations on the 

 discharge through these vapours is that the candle spectrum 

 (*. e. Swan spectrum) is the spectrum of carbon when the 

 atom is charged with negative electricity or of some com- 

 pound of carbon in which its atom is negatively charged, 

 while the carbonic oxide spectrum is the spectrum of carbon 

 when the atom is charged with positive electricity or some 

 compound in which the carbon atom is positively charged. ' J 

 I do not propose at present to enter upon a discussion of 

 these results, lor they are not only in the nature of a pre r 

 liminary communication, but they scarcely involve the main 

 question dealt with in this paper, and in any case I cannot 

 find that they present any serious obstacle to the view which 

 I have advanced concerning the origin of the Swan spectrum. 

 The experiments, however, are so important that 1 am bound 

 to mention them, and it may well be that their further pro- 

 secution will throw much light on the immediate subject of 

 this paper. 



