1879.] on the Reversal of the Lines of Metallic Vapours. 473 



appear steadily as sliarp black lines, when no trace of the other lines 

 of these metals, either dark or bright, can be detected. Dark bands 

 also frequently appear, with ill- defined edges, in the positions of the 

 well-known bright green and orange bands of lime. 



In the case of sodium, using the chloride, we have repeatedly 

 reversed the pair of lines (5687, 5681) next more refrangible than 

 the D group. In every case the less refrangible of the two was the 

 first to be seen reversed, and was the more strongly reversed, as has 

 also been observed by Mr. Lockyer. But our observations on this 

 pair of lines differ from his in so far as he says that " the double 

 green line of sodium shows scarcely any trace of absorption when the 

 lines are visible," while we have repeatedly seen the reversal as dark 

 lines appearing on the expanded bright lines ; a second pair of faint 

 bright lines, like ghosts of the first, usually coming out at the same 

 time on the more refrangible side. 



Using potassium carbonate, besides the violet and red lines which 

 had been reversed before, we saw the group, wave-lengths 5831, 

 5802, and 5782, all reversed, the middle line of the three being the 

 first to show reversal. Also the lines wave-lengths 6913, 6946, well 

 reversed, the less refrangible remaining reversed the longer. Also 

 the group, wave-lengths 5353, 5338, 5319 reversed, the most 

 refrangible not being reversed until after the others. Also the line 

 wave-length 5112 reversed, while two other lines of this group, wave- 

 lengths 5095 and 5081, were not seen reversed. 



Using lithium chloride, not only were the red and blue lines, as 

 usual, easily reversed, and the orange line well reversed for a long 

 time, but also the green line was distinctly reversed ; the violet line 

 still unreversed, though broad and expanded. Had this green line 

 belonged to ceesium, the two blue lines of that metal which are so easily 

 reversed could not have failed to appear ; but there was no trace of 

 them. 



In the case of rubidium, we have seen the less refrangible of the 

 red lines well reversed as a black line on a continuous background, 

 but it is not easy to get, even from the arc in one of our crucibles, 

 sufficient light in the low red to show the reversal of the extreme ray 

 of this metal. 



With charred barium tartrate, and also with baryta and aluminium 

 together, we have obtained the reversal of the line with wave-length 

 6496, besides the reversals previously described. The less refrangible 

 line, wave-length 6677, was not reversed. 



With charred strontium tartrate, the lines with wave-lengths 4812, 

 4831, and 4873, were reversed, and by the addition of aluminium, 

 the line wave-length 4962 was reversed for a long time, and lines 

 wave-lengths 4895, 4868, about, were also reversed. 



On putting calcium chloride into the crucible, the line wave-length 



