678 Professor Bewar [June 10, 



longed series of experiments to arrive at definite conclusions on this 

 matlier ; but, in tlie meantime, it is highly probable that one of the 

 main factors in producing these remarkable variations in the arc will 

 be found to be the relative facility with which the carbon of the poles 

 combines with the gaseous medium. 



On a review of the above series of observations, certain points 

 stand out plainly. In the first place, the indigo, violet, and ultra- 

 violet bands, characteristic of the flame of cyanogen, are conspicuous 

 in the arc taken in an atmosphere of nitrogen, air, nitric oxide, or 

 ammonia, and they disappear almost, if not quite, when the arc is 

 taken in a non-nitrogenous atmosphere of hydrogen, carbonic oxide, 

 carbonic acid, or chlorine. These same bands are seen brightly in the 

 flames of cyanogen and hydrocyanic acid, but are not seen in those of 

 hydrocarbons, carbonic oxide, or carbon disulphide. The conclusion 

 seems irresistible that they belong to cyanogen ; and this conclusion 

 does not seem to us at all invalidated by the fact that they are seen 

 weakly, or by flashes, in the arc or spark taken in gases supposed free 

 from nitrogen on account of the extreme difficulty of removing the last 

 traces of air. They are never, in such a case, the principal or pro- 

 minent part of the spectrum, and in a continuous experiment they are 

 seen to fade out in proportion as the nitrogen is removed. This con- 

 clusion is strengthened by the recent discovery that cyanogen is always 

 generated in the electric arc in atmospheric air. 



The green and blue bands, characteristic of hydrocarbon flames, 

 are well seen when the arc is taken in hydrogen ; but, though less 

 strong when the arc is taken in nitrogen or in chlorine, they seem 

 to be always present in the arc, whatever the atmosphere. This 

 is what we should expect, if they be due, as Angstrom and Thalen 

 suppose, to acetylene ; for we have found that the carbon electrodes 

 always contain, even when they have been long treated with chlorine 

 at a white heat, a notable quantity of hydrogen. 



The hydrocarbon bands are well developed in the blowpipe flame, 

 that is, under conditions which appear, at first sight, unfavourable to 

 the existence of acetylene. We have, however, satisfied ourselves, by 

 the use of Deville's aspirator, that acetylene may be withdrawn from 

 the interior of such a flame, and from that part of it which shows the 

 hydrocarbon bands brightly. 



The question as to whether these bands are due to carbon itself or 

 to a compound of carbon with hydrogen, has been somewhat simplified 

 by the observations of Watts and others on the spectrum of carbonic 

 oxide. There is, we suppose, no doubt now that that compound has 

 its own spectrum quite distinct from the hydrocarbon flame spectrum. 

 The mere presence of the latter spectrum feebly developed in the 

 electric discharge in compounds of carbon supposed to contain no 

 hydrogen, appears to us to weigh very little against the series of 

 observations which connect this spectrum directly with hydrocarbons. 



In the next place, it appears, from experiments, that the de- 

 velopment of the violet bands of cyanogen, or the less refrangible 



