374 PROCEEDINGS OP THE AMERICAN ACADEMY. 



a study of the effects of hydrogen and nitrogen on certain spark lines of 

 magnesium, Liveing and Dewar * were led to remark that " it is possible 

 that the atmosphere may, besides the resistance it offers to the discharge, 

 in' some degree affect the vibrations of the metallic particles." This 

 conclusion in regard to the spark seems to have been borne out with 

 reference to the arc by the results obtained with a hydrogen f atmosphere. 

 Nitrogen resembling hydrogen in its inability to combine directly with 

 metals, it seemed reasonable to expect that it would have a very similar 

 effect on the iron, zinc, and tii^ arcs. As magnesium :|: combines directly 

 with nitrogen a different effect might be expected in the magnesiun arc. 



Method and Apparatus.§ 



In order to eliminate the effect of gases other than the one whose 

 effect was being studied, the enclosed rotating metallic arc, || with "chemi- 

 cally pure " zinc, magnesium, and tin electrodes, was used. The iron 

 used, however, was "commercial." The speed of the rotating electrode 

 was approximately eleven hundred revolutions per minute. Excepting 

 in two instances noted later, the arc was operated by a 104 volt 

 alternating circuit. 



The spectrum was photographed with a Rowland ten-foot concave 

 grating, four exposures being made on each plate, as shown in the 

 diagram. 



Near the top of the plate was photographed the spectrum of the arc in 

 air ; just below this, was photographed the same arc operated in an 

 atmosphere of the gas being studied ; below this, again, was made a short 

 exposure with the arc in the same gas ; and at the bottom, a short 



* Proc. Roy. Soc, 32, 203 (1881). 



t Astroph. Jour., 12, 167-175 (1900). 



J Liveing and Dewar, Proc. Roy. Soc, 32, IGl. 



§ The funds to meet the expense of this experiment were kindly appropriated 

 by the Committee in charge of the Rumford Fund of the American Academy of 

 Arts and Sciences. 



II Crew and Tatnall, Phil. Mag., 38, 379-386 (1894). 



