Spectra of Carbon Compounds. 493 



monoxide at that temperature is largely dissociated. It is 

 true that carbon dioxide is dissociated at lower temperatures 

 when passed through a heated tube, especially if the tube be 

 packed with broken glass or porcelain ; but these are not the 

 conditions in a Plucker tube. 



5. Liveing* and Dewar found that the carbon arc-discharge 

 in carbon monoxide gave the Swan spectrum with no trace of 

 the oxycarbon spectrum, and they point to this as a remarkable 

 fact. 



The non-appearance of the oxycarbon spectrum under 

 these circumstances would indeed be remarkable if that 

 spectrum were really due to carbon monoxide. But if, as I 

 main tarn, the true spectrum of carbon monoxide is the Swan 

 spectrum, the observation is what would be expected. It is 

 to be remarked also that whilst the high temperature of the 

 discharge might favour the dissociation of the gas into carbon 

 and carbon dioxide, and so lead to the oxycarbon spectrum, 

 the fact that the electrodes are glowing carbon rods would 

 act in the opposite direction, tending to conserve the carbon 

 monoxide. This last influence appears to operate in the case 

 of the carbon arc-discharge in carbon dioxide, for here also 

 the Swan spectrum is brilliantly developed. 



0*. The spectrum of the carbon arc in air contains the 

 Swan spectrum together with groups of lines whose origin 

 has been traced by Liveing and Dewar, by Kayser and 

 Kunge, and by Tietz to cyanogen, or to carbon in association 

 with nitrogen. The appearance of the Swan spectrum in 

 this case 1 attribute to the formation of carbon monoxide in 

 the arc- The generation of this gas in the arc is well- 

 known to occur, and is, indeed, a recognized source of danger 

 to health. The appearance of the Swan spectrum in the arc, 

 far from presenting an anomaly, removes one if we ascribe 

 this spectrum to carbon monoxide. For otherwise it would 

 be highly remarkable that the arc-spectrum should contain 

 components attributable to the action between the carbon of 

 the poles and the nitrogen of the air, but none attributable 

 to the action beween carbon and the oxygen of the air with 

 which it unites so readily. Carbon monoxide is known to be 

 formed abundantly in the arc, and according to any hypothesis 

 readily yields a characteristic spectrum under the electric 

 discharge. 



The validity of the view which I have just given is borne 

 out in a remarkable manner by observations of Liveing and 

 Dewar. They say: " the arc in the middle of a magnesia 

 crucible often shows no sign of the (Swan) spectrum although 

 the cyanogen is strong. U\ however, puffs of air or carbon 



