1881.] On the Reversal of the Lines of Metallic Vapours. 403 



failed for less refrangible rays. When iron has been put into the 

 crucible through which the arc of a Siemens' dynamo-electric machine 

 is passing, and then fragments of magnesium dropped in from time to 

 time, most of the strong ultra-violet lines of iron are reversed. The 

 magnesium seems to supply a highly reducing atmosphere, and to some 

 extent carry with it the iron vapour. It also produces a good deal of 

 continuous spectrum, at least in certain regions, and against this the 

 iron lines are often depicted on the photographic plates sharply 

 reversed. In this way we have observed the reversal of the strong 

 iron lines about the solar lines L and M, four strong lines below N, the 

 line 0, all the strong lines from S 2 to U inclusive, and two strong 

 groups still more refrangible. 



Potassium ferrocyanide introduced into the arc instead of magnesium 

 gives a reversal of the same lines as are mentioned in the foregoing 

 paragraph. 



Iron wire fed in through a perforated pole gives reversals of the 

 highest group (wave-length 2492 to 2480), but with, the lines so 

 much expanded as to form broad absorption-bands instead of lines. 



With a vertical arrangement of the carbons, similar to that used by 

 us for resolving the fine double lines (" Proc. Roy. Soc," vol. 32, p. 231), 

 and a stout iron wire in the axis of the lower (positive) carbon, many 

 more lines in the visible part of the spectrum are seen expanded and 

 reversed. This effect is sometimes enhanced by leading into the 

 crucible through the upper carbon, which is perforated for the purpose, 

 a very gentle stream of hydrogen gas ; the stream must be no more 

 than is just sufficient to give a tiny flame at the mouth of the crucible ; 

 a stronger stream diminishes the amount of metallic vapour, probably 

 by its cooling action, and lessens the effect. By this treatment, some 

 of the strongest lines of iron remain reversed for some time, the 

 weaker lines are seen to expand and be reversed for a few seconds at a 

 time, when, from a change in the intensity of the current, or some 

 other reason, a larger amount of metal is volatilized and shows itself 

 by burning in brilliant scintillations at the mouth of the crucible. 



A list of the iron lines reversed, designated by their approximate 

 wave-lengths, is subjoined. 



5614-5 



5585-5 



5445-6 



5428-5 



5405 



5396 



5370-5 



5327 the more refrangible of this pair. 

 5268-5 E 



