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LXVTI. On the "Swan" Spectrum. By W. Marshall Watts, 

 D.Sc, and H. M. Wilkinson, M. A* 



PROBABLY no spectra have been so often observed and 

 so much discussed as those obtainable from carbon and 

 carbon compounds ; and, in particular, the chemical origin 

 of the Swan spectrum, or that observed at the blue base of 

 the flame of a Bunsen burner, has been a subject of controversy 

 for the last titty years. This spectrum, first noticed bv 

 Wollaston in 1802, was examined by Swan in 1857, and by 

 him attributed to a hydrocarbon. Attrfield in 18b*2 came to 

 the conclusion that the spectrum was that of the element 

 carbon, since it was common to compounds of carbon with 

 hydrogen, nitrogen, oxygen, and sulphur. 



This conclusion was combated by several writers. Morren, 

 in 1865, undertook a series of experiments to prove that 

 Attfield was wrong, but in the end became convinced that he 

 was right. Morren particularly describes the brilliant spec- 

 trum produced by burning cyanogen with oxygen. Dibbits 

 arrived at the same conclusion as to the origin of the spectrum, 

 He reasons that carbon exists in combination with hydrogen 

 in the gas which is burnt, and after combustion that carbon 

 is in combination with oxygen ; it is possible, therefore, that 

 in some intermediate stage it existed as uncombined carbon. 



Pliicker and Hittorf, Van der Willigen and Wullner all 

 arrived at the same conclusion. 



Thus, from 1862 till 1875 this view of the origin of the 

 Swan spectrum was all but universally held, but in the latter 

 year Thalen published a paper in which it was contended 

 that the spectrum was due to acetylene, and this view was 

 supported by Profs. Liveing and Dewar. They dwelt much 

 upon the difficulty of getting rid of all hydrogen from gases 

 like cyanogen, carbonic oxide, and carbon tetrachloride, from 

 all of which the Swan spectrum can be obtained ; or from 

 the carbon electrodes used in the arc, in the spectrum of 

 which the Swan bands were always seen, whatever the 

 atmosphere in which the arc was taken. 



This view of the origin of the Swan spectrum which 

 Profs. Liveing and Dewar held so strongly, they abandoned 

 later on as the result of their own further experiments. 



It seems, then, that the theory which ascribes the Swan 

 spectrum to elementary carbon was finally adopted by nearly 

 every experimenter — whether chemist or physicist — who 



* Communicated by* the Authors. 

 Phil. Mag. S. 6. Vol. 12, No. 72. Dec. 1906. 2 Q 



