Cady and Arnold — Electric Arc. 



389 



change in the iron arc, or else it is possible to obtain both first 

 and second stages between terminals of magnetic oxide of iron 

 even when no free oxygen is present in the surrounding gas.* 



The question was settled conclusively by starting an arc 

 between carefully cleaned iron terminals in pure nitrogen. 



The apparatus shown in fig. 2 was used for this purpose. The 

 terminals A and A" were located inside a glass bulb i?15 cm in 

 diameter. They were made from iron rod 4'5 mm thick, and the 



Fig. 2. Apparatus for the arc in nitrogen. 



ends from which the discharge took place were turned down 

 to a diameter of 3 mm . They extended into the bulb through 

 vertical glass tubes about 80 cm long, which were provided with 

 ground joints at D and E. The glass tubes dipped into vessels 

 of mercury, which rose to barometric height when the air was 

 exhausted. It was possible to raise and lower each electrode 

 independently, as well as to rotate it about a vertical axis in 

 order to secure exact alignment. The rod K was jointed 

 at F. The short portion beyond F was bent outward by a 

 suitable device when K was to be taken from the bulb or intro- 



* When the iron arc was formed under a bell-jar containing nitrogen in 

 which only a small trace of free oxygen can have been present, a surpris- 

 ingly large quantity of nitric acid was produced. Traces of moisture or 

 other impurities from organic substances in the jar may have contributed 

 toward this. 



