1878.] on the Reversal of the Lines of Metallic Vapours. 135 



such phenomena as the increased fusibility of mixtures of salts of 

 potassinm and sodium, and the well-known fluidity of the alloy of 

 those metals. 



As we have had occasion to use sodium and potassium in our tubes, 

 we have had opportunities of observing the absorption spectra of these 

 metals, and we find that there is a great deal yet to be observed in 

 regard to these spectra. Up to the present time, we have not observed 

 any of the appearances noted by Lockyer, " On a new class of absorp- 

 tion phenomena," in the Proceedings of the Royal Society, vol. xxii, 

 but we have repeatedly noted the channelled-space spectrum of sodium 

 described by Roscoe and Schuster in the same volume of the Proceed- 

 ings. We observed in our tubes no channelled-space absorption by 

 potassium, but continuous absorption in the red, and one narrow 

 absorption band, with a wave-length of about 5,730, not correspond- 

 ing with any bright line of that metal. 



With reference to the absorption spectrum of sodium vapour, we 

 may remark that it is by no means so simple as -has been generally 

 represented. The fact that the vapour of sodium, in a flame shows 

 only the reversal of the D lines, while the vapour, volatilized in tubes, 

 shows a channelled-space absorption corresponding to no known 

 emission spectrum, appears to be part of a gradational variation of the 

 absorption spectrum, which may be induced with perfect regularity. 

 Experiments with sodium, carried out in the way we have described, 

 exhibit the following succession of appearances, as the amount of 

 vapour is gradually diminished, commencing from the appearance 

 when the tube is full of the vapour of sodium, part of it condensing 

 in the cooler portion of the tube, and some being carried out by the 

 slow current of hydrogen. During this stage, although the lower 

 part of the tube is at a white heat, we have always noticed, as long as 

 the cool current of hydrogen displaced metallic vapour, that, on looking 

 down the tube, it appeared perfectly dark. The first appearance of 

 luminosity is of a purple tint, and, with the spectroscope, appears as 

 a faint blue band, commencing with a wave-length of about 4,500, 

 and fading away into the violet. Next appears a narrow band in the 

 green, with a maximum of light, with a wave-length of about 5,420, 

 diminishing in brightness so rapidly on either side as to appear like a 

 bright line. This green band gradually widens, and is then seen to be 

 divided by a dark band, with a wave-length of about 5,510. Red 

 light next appears, and between the red and green light is an enormous 

 extension of the D absorption line, while a still broader dark space 

 intervenes between the green and the blue light. The dark line in 

 the green (wave-length about 5,510) now becomes more sharply 

 defined. This line appears to have been observed by Roscoe and 

 Schuster, and regarded by them as coinciding with the double sodium 

 line next in strength to the D lines, but it is considerably more refran- 



