126 Dr. W. Marshall Watts on the Spectra 



It may be concluded, I think, that the three lines farm a 

 part o£ the " Swan " spectrum; but the shaded band /upon 

 which they are superposed probably does not belong to the 

 same spectrum, though always seen in the combustion of 

 hydrocarbons, in the discharge in liquids containing car on 

 and hvdrogen, or carbon, hydrogen, and oxygen, and occa- 

 sionally in vacuum-tubes. There is, I believe, no evidence 

 that it is due to carbon alone ; but there is evidence that 

 hydrogen is necessary for its production, and therefore it 

 may, provisionally at least, be termed the hydrocarbon band. 



The group/ is seen in Vogel's photograph of the spectrum 

 of the cyanogen flame {Berliner Berichte, xxi. 1888). 



In my early observations of the direct discharge in carbon 

 monoxide at atmospheric pressure which shows the " Swan " 

 spectrum, I sometimes observed /and sometimes the cyanogen 

 bands £ and 6 instead of/, a slight alteration of the electrical 

 conditions producing the change. It is now clear that the 

 gas used in these experiments contained both water-vapour 

 and nitrogen as impurities. The " Swan " spectrum in 

 carbonic oxide usually shows /, unless the gas is very 

 thoroughly dried ; but that it can be so completely dried that 

 / disappears is shown by the photograph of the spark in 

 the gas at atmospheric pressure (PL II. fig. 6), for which 

 I am indebted to my friend Mr. E. E. Brooks. The two 

 outside strips are the spectrum of the Bunsen flame, and the 

 central strip that of dry carbonic oxide together with the iron 

 arc. It can be seen that /has disappeared, but that " the 

 three" lines 4381, 4371, and 4364 remain. 



Fig. 2 shows Mr. Brooks's spectrum of magnesium in a 

 coal-gas vacuum*, and in fig. 3 portions of the Bunsen-flame 

 spectrum are fitted on to this to show the presence o£ the 

 groups / and G 6 in the electric spectrum. 



Figs. 4 and 5 are photographs for which I am indebted to 

 my friend Mr. 0. W. Raftety, the first of the /group from 

 a Meeker burner, and the second of a hydrogen vacuum-tube 

 in which the group /is seen amongst the lines of the secondary 

 hydrogen spectrum. 



We have thus evidence of the production of the /group in 

 carbonic oxide with a trace of hydrogen, and in hydrogen with 

 a trace of carbonic oxide. 



The u Swan" spectrum predominates at the negative pole 

 both in vacuum-tubes and in the arc, whereas the spectrum of 

 carbonic oxide is seen most brightly at the positive pole, 



In 1902 I observed in a vacuum-tube containing coal-gas 



* See P. R S. Ixxx. p. 218 (1968), and Kayser, Hdb. Sped. v. p. 232. 



