388 Mr. Baly and Dr. Syers on 



be let into the vacuum-tube directly from the generating 

 apparatus. 



Even with the above precautions, the first admission of 

 the gas often gives no Swan spectrum; but after re-exhaustion 

 and re-admission once or twice more, the Swan spectrum is 

 seen unmixed with the carbon-oxide spectrum. This experi- 

 ment has been repeated very many times in various ways, 

 and with always identically the same result. 



It is interesting also to note that, under certain conditions, 

 the carbon monoxide deposits carbon under the influence of 

 the discharge ; this is always at once accompanied by a change 

 in the spectrum to the carbon-oxide spectrum. Again, the 

 admission of a small trace of oxygen into a vacuum-tube 

 showing the Swan spectrum, instantaneously changes it to 

 the carbon-oxide spectrum. These facts are strongly con- 

 firmatory of the theory. 



As regards the carbon-oxide spectrum, this is invariably 

 obtained when the electric discbarge is passed through either 

 carbon dioxide itself, or a mixture of carbonic oxide and 

 oxygen. 



As Professor Smithells has pointed out, considerable sup- 

 port would be gained to the above view of the carbon spectra, 

 if it could be shown that a carbon compound such as cyanogen 

 gives no Swan spectrum when under the influence of the 

 electric discharge. Smithells was unable to satisfy himself 

 as regards this, owing to the inherent difficulties in working 

 with cyanogen ; but, by taking similar precautions as were 

 described above, we have succeeded in filling vacuum-tubes 

 with cyanogen which showed no trace of either of the carbon 

 spectra, but only a very beautiful and characteristic cyanogen 

 spectrum. In these experiments, the first difficulty to be 

 overcome was in connexion with the purity of the cyanogen 

 itself. This was overcome as follows : — About half a litre of 

 the gas was prepared as pure as possible by heating mercuric 

 cyanide, and this was then frozen in a small bulb immersed 

 in liquid air. The bulb containing the cyanogen was then 

 exhausted as far as possible with a mercury-pump, and the 

 liquid air being then removed, the cyanogen was allowed to 

 boil off into a gas-holder. A second difficulty was the very 

 i*apid polymerization to paracyanogen, which takes place when 

 the discharge is passed through the gas ; this was, however, 

 surmounted by allowing a slow stream of cyanogen to flow 

 into the vacuum-tube. 



The apparatus used is shown in the figure, where A is the 

 tube containing the mercuric cyanide, and B a mercury gas- 

 holder to which was fitted a reservoir and indiarubber tubino- 



