256 Messrs. A. Fowler and H. Payn. [June 10, 



intensity, though they are less striking than the principal enhanced 

 lines which appear in the case of magnesium (Plate 14). 



Strong flutings also appear, the principal one commencing with a 

 sharp head near X 4300 and grading off towards the violet. These 

 are identical with flutings which appear in the spectrum of the arc 

 when zinc poles are surrounded by an atmosphere of hydrogen, 

 and may therefore be provisionally attributed to "zinc hydride." 

 These flutings have also been observed in the zinc arc in hydrogen 

 by Basquin,* who also " in lieu of a better hypothesis " attributes them 

 to a compound of hydrogen with the metal. 



In one experiment the stronger flutings of nitrogen appeared in the 

 spectrum, but they were apparently restricted to the region surrounding 

 one of the poles. The F line of hydrogen also appears in some of the 

 photographs. 



As in the case of magnesium, there is evidence that the flutings and 

 enhanced lines do not originate in the same part of the arc. 



Cadmium. 



The phenomena are generally similar to those observed in the case 

 of zinc, but some of the features are less marked. The strong triplet 

 XX 5086-06, 4800-09, 4678-37 and the adjacent line 4662-69 are of 

 reduced intensity, but more sharply defined than in the ordinary 

 arc, while most of the other arc lines in the region D to K disap- 

 pear. The spark lines 5379'3, 5338'6 are certainly present but not 

 conspicuous. 



Strong flutings, fading towards the violet, are seen, the principal 

 heads having approximate wave-lengths, 4491 and 4298. Here, 

 again, the flutings are identical with those which appear when the 

 cadmium arc is surrounded by hydrogen, and may therefore be pro- 

 visionally regarded as originating in " cadmium hydride." 



The F line of hydrogen, if present, is too feeble to be shown in the 

 photographs. 



Iron. 



It is only after careful examination that the spectrum of an iron 

 arc in an exhausted globe is seen to be materially different from that 

 of the arc in air at atmospheric pressure. Nevertheless, there is no 

 doubt as to the appearance of the principal enhanced lines, among 

 them being 5018*13 and 4584-02. 



The F line of hydrogen is not certainly present, but it could not be 

 distinguished from the neighbouring iron line with the dispersion 

 employed unless comparatively strong. 



* ' Astrophys. Journ.,' vol. 14, p. 10, 1901. 



