Spectroscoped and Spectrometed in 1879. 109 



but it manifests itself excessively in tubes of 



Cyanogen (carbo-nitrogen). 



Carbonic acid and carbonic oxide (carbo-oxygen). 



Also with extreme brightness in tubes of 



Alcohol , 



Olefiant gas, and 



Marsh-gas (all carbo-hydrogens). 



Whence we may safely conclude that the spectrum appears 

 in the six first tubes solely as an impurity of some kind ; and 

 the abundant carbonate of soda used by the glass-makers for 

 such tube-glass may be its origin. Hence it is the excess of 

 intensity between ^hose six first, and the six last, tubes which 

 we are to look to alone as the possible carbon-spectrum of the 

 carbonic compound gas proper to the tube. Pure hydrogen- 

 lines (probably derived from infinitesimal traces of moisture) 

 also appear in every one of the 12 tubes, except the cyanogen ; 

 and there they are strikingly absent. 



Confining our attention now, for the sake of greater accu- 

 racy with economy of time and labour, when using a high dis- 

 persion (33° from A to H, and mag. power = 10), to the green- 

 band region alone of the tube-carbon-spectrum, that band 

 does seem to be, throughout all the tubes, identical — even mi- 

 croscopically identical in every one of its earlier and brighter 

 minute constructional lines ; and it varies only, at least in that 

 earlier part, in the mere matter of general development or 

 strength and intensity from tube to tube. 



Those constructional lines of the vacuum-tubes' green band 

 are closest and brightest on that side of it which is toward the 

 direction of least refrangibility (i. e. towards the red), begin- 

 ning at Wave-Number place 48,861 ; and they include, after 

 a few of their lines towards the direction of greater refrangi- 

 bility (i. e. towards the violet), or at Wave-Number place 

 48,969, a peculiar crossing over each other of two sets of lines. 

 That spot, marked extra brightly by one line just overlapping 

 another, is very easily identifiable by the eye, at least in my 

 spectroscope, even when the spectrum is but faintly deve- 

 loped ; and it forms a convenient step for an observer, when 

 first roughly journeying optically from the strong and sharp 

 beginning or least refrangible and the red-ward edge of the 

 said vacuum-tube carbon's green band, onwards to its more 

 refrangible and fainter, and violet-ward regions, in search of 

 any thing possibly abnormal in any one tube or another. 



Instituting this quest, if we examine all our first mentioned 



