No. 6.] STUDY OF ARC SPECTRA. 457 



Summary. 



(i) The spectrum obtained from the electric arc is not the 

 same for all parts of the arc and the surrounding flame. On the 

 contrary, there is a decided difference in the spectrum of the 

 several sheaths. The difference is largely due to a fading out of 

 the carbon bands and of all lines from the positive carbon, and an 

 increase of intensity in the outer sheath of those lines which are 

 most brilliant near the negative carbon. Certain lines and bands 

 which are invisible in the center are seen in the outer part of the 

 flame. 



(2) The total number of metallic lines diminishes rapidly, as 

 we explore the arc from the center outwards. 



(3) When the spectrum is changed by the introduction of 

 metals into the carbons, it is observed that the more positive 

 metals, such as K, Na, Li, and Ba, Ca, Sr, greatly weaken, but 

 do not destroy, the characteristic band spectrum of the arc. The 

 lines which are most affected by these metals are those which in 

 the ordinary spectrum are strongest near \\\q. positive carbon. 



Certain metals, as silver and copper, do not materially alter the 

 original arc spectrum, while others, as zinc and cadmium, affect 

 the lines near the negative carbon, and thus give the band spec- 

 trum, which is no longer obscured by the metallic lines usually 

 present, an unwonted brilliancy. 



The causes of the phenomena are not easily discovered, but it is 

 hoped that the points brought out in this paper may throw some 

 light upon the intricacies of this complex problem. 



