126 Profs. Gr. D. Liveing and J. Dewar. [June 15, 



of the De Meritens intermittent current, taken in air. We have 

 observed in previous experiments that the De Meritens arc in 

 hydrogen produces a sufficient temperature to render the C line of 

 hydrogen permanently visible. The continuous Siemens current, 

 under the same circumstances, only shows this line when the arc is 

 produced by breaking the contact of the poles, not with the steady 

 arc. 



Approximate Wave-lengths of Carbon Arc-lines. 



2434* 8 absent from spark. 

 2478-3 strongest line. 

 2506-6. 



2514- 1. 



2515- 8. 

 2518-8. 

 2523-9. 

 2528-1. 



2881*1 not in spark. 



We have here another instance of the lines of high refrangibility 

 appearing, under certain circumstances, when no trace of strong lines 

 belonging to the less refrangible portion of the spectrum can be 

 detected. Thus the strong carbon line in the blue at 4266 does not 

 appear in the photographs of the arc spectra. Of course it is possible 

 that a very long exposure of the photographic plate might reveal some 

 of the missing lines, as we have shown in other cases. The presence 

 of these carbon lines is a proof that carbon vapour of a definite, but 

 probably low, tension exists in the arc discharge, and this is doubtless 

 the reason why under such cond itions carbon combines with hydrogen 

 and nitrogen with such facility. By a careful series of experiments 

 carried out at different pressures with varying electric power we 

 hope to ascertain with greater precision the variations in the carbon 

 line spectrum. 



Bye Observation of Spectra. 



When the spectrum of different parts of a magnified image of the 

 electric arc is examined, all the more refrangible cyanogen groups may 

 be seen near the positive pole, together with a series of channellings 

 in the red. When the arc is steady the cyanogen spectrum is per- 

 manently visible at the negative pole, when no trace of the hydro- 

 carbon series can be seen. In the same way the arc in the middle of 

 a magnesia crucible often shows no trace of the hydrocarbon set, 

 although the cyanogen are strong. If, however, puffs of air or 

 carbonic acid are passed into the arc, the hydrocarbon lines are pro- 

 duced. There is always, under these circumstances, far greater 



