given by Carbon and some of its Compounds. 127 



at a pressure of 100 mm. the " Swan " spectrum and the 

 carbonic-oxide spectrum simultaneously. As the pressure 

 was decreased the " Swan" disappeared and the carbonic- 

 oxide spectrum remained ; but in the intermediate condition 

 the two ends of the capillary were of different colours, — the 

 negative end decidedly blue showed the " Swan" brightest, 

 whereas at the positive end the carbonic-oxide spectrum was 

 the brightest. 



Liveing and Dewar*, who observed the " Swan " spectrum 

 in an incandescent carbon lamp at the moment at which the 

 filament gave way, also observed a sort of flame giving the 

 carbonic-oxide spectrum at the point where the filament 

 joined the positive wire. 



Baldwin! (in " a photographic study of arc-spectra ") 

 says, " The central arc is violet, outside this a blue sheath 

 strongest at the negative carbon and surrounded by an outer 

 yellow sheath shading into orange at the outside. The flame 

 which starts at the negative carbon extends some distance 

 up the positive carbon." 



HagenbachJ, describing the arc between copper poles in 

 an atmosphere of carbon dioxide, says: — " The positive elec- 

 trode was always hotter than the negative electrode ; with 

 decreasing pressure this difference became more marked, and 

 at 10 to 20 mm. the negative electrode never became red-hot. 

 The bands of carbon oxide 66220, 6078"0, 5607*5, 5197*0, 

 4836-5, 45090, 4394'0, 4209*0, and 4130-0 were seen espe- 

 cially at the positive pole. The * Swan ' spectrum 6188, 

 5634, 5164, 4736, 4375 to 4325, 4315, and 3890 to 3873 

 became more intense and sharper with decreasing pressure, 

 but was absent from the positive pole." 



A study of the results obtained by Sir J. J. Thomson § 

 leads to the conclusion that the -'Swan" spectrum is due to 

 the negatively-charged carbon atom, and not to either carbon 

 monoxide or carbon dioxide. 



The Swan spectrum is given by carbonic oxide at atmo- 

 spheric pressure, but not by carbon dioxide. In carbon 

 monoxide as the pressure is reduced || the Swan dies away, 

 and at low pressures is completely replaced by the second 

 spectrum. This indicates that carbon monoxide contains 

 something not present in carbon dioxide, but that this 

 something gets smaller in amount as the pressure is reduced. 



* Liveing and Dewar, Proc. Rot. Soc. xxxiii. p. 403 (1882). 



t Baldwin, Phvs. Rev. iii. p. 370 (1895). 



t Hageubach, Pkys. Zs. x. p. 649 (1910). 



§ Thomson, 'Nature,' lxxxvi. p. 460 (1911). 



ij Deslandres, C. 11. cvi. p. 842 (1888) ; Ann. Chim. Phys. xv. p. 72. 



