378 PROCEEDINGS OF THE AMERICAN ACADEMY. 



are slightly changed. Of these twenty-six lines seventeen are from two 

 to ten times as strong in nitrogen as in air. But these seventeen lines 

 were all found to be due to impurities, fifteen belonging to manganese, 

 one to chromium, and one to cobalt. The remaining nine lines are 

 reduced by nitrogen to from one-half to one-tenth their intensity in air. 

 Part, if not all, of these lines are iron spark lines. 



Effect of Excluding Nitrogen. 



The effect of pure nitrogen being so slight, it seemed possible that the 

 presence of so large a percentage of nitrogen in the air about the ordi- 

 nary arc might account for the smallness of the change. I therefore 

 attempted to determine the effect of nitrogen by a process of exclusion. 

 This was done by substituting for air an atmosphere of commercial oxy- 

 gen taken from an ordinary stereopticon gas-cylinder. A stream of this 

 oxygen was kept flowing through the hood of the arc. 



If the chemical affinity of the electrodes for the atmosphere has any 

 effect on the spectrum, one might certainly expect this effect to be exhib- 

 ited when such easily oxidizable metals as iron, magnesium, and tin are 

 employed in the atmosphere of oxygen. 



Mr. A. S. King* found that the metallic lines were intensified by 

 increasing the supply of oxygen about the carbon arc. But just the con- 

 trary seems to be true with reference to the metallic arc. A current as 

 large as ten amperes was tried with chemically pure magnesium elec- 

 trodes, but the oxygen had little effect on the working of the arc or the 

 appearance of the so-called "flame." To the eyes the *' flame," espe- 

 cially with iron electrodes, appears to be less blue and more yellow. 



Of eighteen exposures made with the iron, tin, and magnesium arcs in 

 oxygen, all but one show a greater average intensity for the same length 

 of exposure in air than in oxygen. This is not the only respect in 

 which the action of oxygen on the metallic arc resembles that of hydrogen 

 and ammonia. The metallic lines that have been noted as being rela- 

 tively enhanced or reduced by hydrogen are precisely the ones which are 

 so affected by oxygen. The changes produced by oxygen are not so 

 great as those produced by hydrogen, but they are in the same direction. 



Summary. 



The results of these experiments may therefore be summarized as 

 follows : 



* Astroph. Jour., 14, 329 (1901). 



