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In our last communication on this subject we observed (“Proc. Roy. Soc.,” vol. 29, p. 405) that iron introduced as metal, or as chloride, into the electric arc, in a lime crucible, in the way which had proved successful in the case of many other metals, gave us no reversals. We succeeded, however, in reversing some ten of the brightest lines of iron, mostly in the blue and violet, by passing an iron wire through one of the carbons, so as to keep up a constant supply of iron in the arc. Considering the great number of iron lines, and that so many of them are strongly represented amongst the Fraunhofer lines, it seemed somewhat surprising that it should be difficult to obtain a reversing layer of iron vapour in the arc inclosed as we use it in an intensely heated crucible. A like remark might be made respecting titanium, which is almost as well represented as iron in the Fraunhofer lines, but has heretofore given us no reversals. Almost the same might be said of chromium, except that the number of chromium lines is so much less than that of either of the other two metals. We have since found that most, if not all, of the strong lines of these three metals may be reversed by proper management of the atmosphere and supply of metal in the crucible. Indeed, with regard to iron we have found that the method employed with other metals was successful so far as the ultra-violet rays were concerned, though it failed for less refrangible rays. Then 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 O, all the strong lines from S2 to U inclusive, and two strong groups still more refrangible.

XII. On the reversal of the lines of metallic vapours. No. VIII. (iron, titanium, chromium, and aluminium.)