Spectra of Carbon Compounds. 499 



the Swan spectrum with carbon monoxide, it appears to me 

 that the phenomena just described are best explained by the 

 gradual accumulation of this gas in the tube. For, if the 

 Swan spectrum be due to a compound of carbon and hydrogen, 

 I can see no explanation of the gradual disappearance of the 

 spectrum as the hydrogen is purified, nor any reason why 

 the spectrum should take twenty minutes to appear on a par- 

 ticular occasion. This, however, is quite consistent with the 

 gradual accumulation of carbon monoxide. How the carbon 

 monoxide arises I am not prepared to say with certainty. 

 The possibilities of gradual contamination with oxygen are, 

 however, very considerable in glass apparatus in which the 

 electric discharge is being passed. h\ some cases I have 

 found that where the Swan spectrum has appeared without 

 obvious contamination vxith oxygen a modification of the dis- 

 charge would produce the oxycarbon spectrum, which on any 

 hypothesis points to the presence of oxygen. 



1 have tried to detect carbon monoxide in the tubes by 

 ordinary analytical tests, but the quantity of gas is very small, 

 and in presence of acetylene it is hardly possible to work with 

 certainty. 



In order to get rid of possible oxygen contamination from 

 the glass I had sparking tubes made of copper by electro- 

 deposition. The difficulty of getting these provided with 

 electrodes and attached to the pump without leaving some 

 small leak was, I think, eventually overcome, but the results 

 obtained were not satisfactory. The difficulty in getting rid 

 of the Swan spectrum was greater than in glass tubes, indeed, 

 I never succeeded in getting rid of the last traces. Futile 

 attempts extending over many weeks led me to think that 

 perhaps there had been some error in the experiments with 

 the glass apparatus, but on repeating these I succeeded, as 

 before, in getting entirely rid of the Swan spectrum. 



Electric Spectra of other Carbon Compounds. 



It would seem at first sijdit as if the examination of the 

 spectrum of the vapour of carbon tetrachloride in a Pliicker 

 tube should afford a simple crucial test as to the connexion 

 of the Swan spectrum with carbon monoxide. This experi- 

 ment has been performed by Watts, Lockyer, Liveing and 

 Dewar, and by myself. 



According to Lockyer the electric discharge in the vapour 

 of carbon tetrachloride gives the Swan spectrum brightly, 

 together with the more refrangible groups associated also 

 according to Lockyer with elementary carbon. No sign of a 

 hydrogen or nitrogen spectrum could be seen in the tube 



