the Occurrence of Enhanced Lines in the Arc. 845 



radiations both of the arc in nitrogen and of that in air are 

 of like origin, and the explanation which we have offered for 

 the air arc will equally w r ell hold in the case of the nitrogen 

 arc. 



No stable arcs are formed in oxygen, hydrogen, or coal-gas 

 under electrical conditions similar to those used in air and 

 nitrogen — namely, with continuous current at SO volts. The 

 arcs consist merely of luminous flashes which show the spark 

 lines strongly during the first phase. The prevention of a 

 stable arc in oxygen is possibly caused by the very vigorous 

 chemical actions which set in as soon as the arc is started. As 

 a consequence of this the electrodes become rapidly covered 

 with a non-conducting layer of oxide which invariably leads 

 to the extinction of the arc. With the oxygen arc between 

 magnesium poles the flame line X 4571 and the bands at 

 A. 5007 are particularly strong at the beginning of the 

 second phase. The lines of the Rydberg series are winged 

 on the red side, especially at the commencement of the arc 

 flash. The spark line A, 4481 is intense and symmetrically 

 widened in the first phase, but it remains visible as a fairly 

 strong though narrow line throughout the flash. Whether 

 or in how far the emission of the spark lines in the oxygen 

 arc is caused or facilitated by the process of oxidation of 

 the magnesium metal, our present results do not allow us to 

 suggest. But as for the cause of emission of these lines 

 during the first phase — namely, whilst there is paucity of 

 oxygen in the space between the electrodes, — it is probablv 

 the same as that in the case of the air and nitrogen arcs 

 mentioned above. 



A prominent feature of the hydrogen and coal gas arcs 

 between magnesium poles is the relatively great intensity of 

 the spark line X 4481. In the first phase, which also in 

 these two arcs is probably caused by a film of rarefied gas, 

 the line is considerably broadened out on both sides. It 

 then suddenly falls off sharply in intensity, although still 

 remaining strong throughout the second phase. The lines 

 of the Rydberg series are winged on the red side and verv 

 nebulous in appearance during the whole of the arc flash. 

 The flame line A, 4571 and the hydride bands are well 

 developed. The series triplets present the usual normal 

 appearance and, as was to be expected, the band at X 5007 

 is absent. The hydrogen lines H a and H^ are very in- 

 tense during the first phase but much fainter during the 

 second. 



The following data have been obtained for the principal 



