the Spectrum of Cyanogen. 387 



severely*, because in the general opinion of spectroscopists 

 carbon dioxide has no spectrum at all. 



A great number of experiments, however, had been 

 previously carried out with a view of obtaining a mixture 

 of hydrogen and carbon monoxide, or rather a mixture which 

 showed only the Swan spectrum together with that of 

 hydrogen. This was found at the time to be impossible, as 

 the Swan spectrum was at once changed into the carbon- 

 oxide spectrum. 



In order to obtain the Swan spectrum in a pure state, it 

 was found necessary to fill a vacuum-tube with pure carbon 

 monoxide, using the greatest precautions to eliminate all 

 impurities, as the smallest trace of oxygen at once changed 

 the spectrum to the carbon-oxide spectrum, which itself was 

 always obtained when carbon dioxide was used. 



These results were not published at the time as they did 

 not actually bear on the work in hand ; but in view of Pro- 

 fessor Smithells's recent paper, it seems worth while to 

 describe them and others more recently carried out, since 

 they support very strongly the view he puts forward. There 

 seems, indeed, no room for doubt that the true explanation 

 of these spectra is that they are due to carbon monoxide 

 and dioxide respectively, but at the same time it is easy to 

 see how confusion could arise. In the first place, there is 

 the extreme difficulty of obtaining a vacuum-tube containing 

 pure carbon monoxide ; and in the second place, a very 

 small quantity of carbon dioxide in a mixture gives a very 

 decided spectrum, and can easily mask that of carbon mon- 

 oxide. These two facts can account for all the difficulty 

 connected with the vacuum-tube spectra of the gases, because 

 under ordinary circumstances carbon monoxide is changed 

 so far into carbon dioxide that it shows none, or only very 

 little, of the Swan spectrum, but practically entirely the 

 carbon-oxide spectrum. Naturally, therefore, the carbon- 

 oxide spectrum was attributed to carbon monoxide. 



As Professor Smithells has shown, if proper precautions 

 are taken, then carbon monoxide gives the Swan spectrum, 

 and when pure only the Swan spectrum. The chief difficulty 

 lies in removing all the condensed air from the walls of the 

 vacuum-tube, and all the occluded gases from the electrodes. 

 This can be done quite easily by exhausting as far as possible, 

 keeping the discharge passing and heating the vacuum-tube 

 with a Bunsen-burner. The carbon monoxide must be made 

 from formic acid and sulphuric acid, and should preferably 



* Kayser, Handbuch der Sj:eotroscopie, p. 198. 



