given by Carbon and some of its Compounds. 119 



nitrogen or chlorine, were present in the arc, whatever the 

 atmosphere. Nevertheless, putting aside the natural con- 

 clusion to be drawn from these experiments, they attribute the 

 spectrum to hydrocarbon, the hydrogen being supposed to 

 come from impurities. They also examined the spark in 

 various gases with every precaution to exclude impurities — 

 in carbon tetrachloride and trichloride, in naphthaline and 

 between carbon poles in nitrogen. " In all the^e expe- 

 riments the bands which Angstrom and Thalen ascribe to 

 hydrocarbons were always more or less plainly seen"*. 

 Again, Liveing and Dewar put aside the obvious conclusion, 

 and attribute their results to the presence of hydrogen as an 

 impurity. 



Later t, as the result o£ further experiments, Liveing and 

 Dewar abandon the view that the " Swan " spectrum depends 

 upon the presence of hydrogen. In these further expe- 

 riments they found that " the spark between electrodes near 

 together in wide tubes filled with saturated vapour of carbon 

 disulphide or carbon tetrachloride dried with phosphoric 

 anhydride and deprived as completely as possible of air by 

 pumping or boiling out, shows the spectrum of the flame of 

 hydrocarbons brightly. The flame of cyanogen from liquid 

 cyanogen which had remained in contact with phosphoric 

 anhydride or sulphuric acid gave the green line of the Swan 

 spectrum together with the cyanogen bands; but when oxygen 

 was used to raise the temperature of the flame all the hydro- 

 carbon-flame sets appeared with marked brilliancy." 



In another experiment the spark was observed in carbonic 

 oxide at various pressures. The carbonic oxide was prepared 

 by heating potassium oxalate and lime, and was dried over 

 phosphoric anhydride. At atmospheric pressure the spark 

 (without condenser) showed the spectrum of the hydrocarbon 

 flame. On exhausting, the spectrum of carbonic oxide makes 

 its appearance, superposed on the former ; and as the ex- 

 haustion proceeds it increases in brilliance until it overpowers, 

 and at last entirely supersedes the flame-spectrum. If the 

 pressure of the gas is increased above the atmospheric, the 

 hydrocarbon-flame spectrum grows brighter. [The line- 

 spectrum of carbon also appears at high pressures : the 

 effect of increasing pressure being thus similar to that of 

 introducing a condenser.] On letting down the pressure 

 the same phenomena occur in the reverse order. 



* Dewar, Proc. Roy. Inst. June 10th, 1881. 



+ Lireinp and Dewar, Proc. Boy. Roc. No. '22S (1882), xxxiv. pp. 123, 

 41°. 



