the Occurrence of Enhanced Lines in the Arc. 847 



of magnesium A, 4571, which is only feebly excited under 

 these conditions. In stable arcs burning between the same 

 metal in gaseous air or nitrogen and in which the tempe- 

 rature reaches a high value, this line is more strongly 

 emitted. The series triplets at X 3838 and A, 5184 show 

 a normal development in the liquid air arc, thus indicating 

 that under arc conditions they are little, i£ at all, affected by 

 temperature changes. The lines of the Rydberg series are 

 well brought out and are markedly winged on the red side 

 throughout the length of the arc flash. The spark line 

 A 4481 is strongly emitted, not only at the moment of striking 

 the arc, but evenly along the whole path of the flash. Although 

 at no stage of the liquid air arc does this line attain the re- 

 markable breadth which characterises it in the first phase of 

 the water or hydrogen arc, it nevertheless shows unmistakable 

 signs of being symmetrically winged. It may also be useful 

 to place on record the fact that the band at A. 5007 is emitted, 

 though only faintly, in the liquid air arc. 



With zinc electrodes in liquid air the arc flash obtained 

 brings out the spark lines A, 4912 and X4924 strongly during 

 the first phase ; they then fall off in intensity very rapidly 

 and only traces remain visible till the end of the second phase. 

 The red line and the triplet in the blue, which are very intense 

 along the whole length of the arc flash, appear strengthened 

 and symmetrically widened in the first phase. 



Our experiments with a cadmium arc under liquid air were 

 not very successful inasmuch as we could neither observe 

 visually nor obtain photographically any evidence of the 

 emission of the two spark lines X5338 and X5379. It 

 should, however, be mentioned that the whole spectrum 

 of cadmium as obtained by us under these conditions was 

 only feebly developed, possibly owing to the fact that poles 

 of this metal fuse together rather readily when bringing 

 them into contact on starting the arc. Although our obser- 

 vations have not brought out anj r interesting facts bearing 

 on the emission of the principal spark lines of cadmium, they 

 do, however, show up in a striking manner the difference in 

 the behaviour of the arc line A. 4413 and the third type 

 enhanced line A 4416. The arc line, which is but very 

 feebly emitted in the first phase, becomes strong as the 

 arc flash develops during the second phase. The enhanced 

 line, on the other hand, is strong in the first phase and 

 changes almost abruptly into a feeble line as the second 

 phase sets in. Thus there is here shown up a marked dif- 

 ference in the mode of excitation prevailing at the moment 



