1878.] on the Reversal of the Lines of Metallic Vapours. 353 



(4.) When sodium and magnesium are used together a dark line, 

 with ill-defined edges, is seen in the green, with a wave length about 

 5,300. This is the characteristic absorption of the mixed vapours of 

 sodium and magnesium, it is not seen with either vapour separately, 

 nor is it seen when potassium is used instead of sodium. 



(5.) When potassium and magnesium are used together, a pair of 

 dark lines are seen in the red. The less refrangible of these sometimes 

 broadens into a band with ill-defined edges, and has a mean wave 

 length of about 6,580. The other is always a fine sharp line, with a 

 wave length about 6,475. These lines are as regularly seen with the 

 mixture of potassium and magnesium as the above-mentioned line 

 (5,300) is seen with the mixture of sodium and magnesium, but are 

 not seen except with that mixture. 



(6.) On one occasion, with a mixture of potassium and magnesium, 

 another dark line was seen in the blue, with a wave length nearly 

 4,820. This line is very near one of the bright lines, seen when sparks 

 from an induction-coil, without a Ley den jar, are taken between 

 electrodes of magnesium, and may very likely be due to magnesium 

 alone, and not to the mixture of vapours, as we only observed it on 

 one occasion. 



There is a certain resemblance between the absorptions above 

 ascribed to magnesium, and the emission spectrum seen when the 

 sparks of a small induction-coil, without Leyden jar, are taken be- 

 tween electrodes of magnesium. This emission spectrum is the same, 

 with the addition of some blue lines, as that seen when the sparks are 

 taken from a solution of magnesium chloride, as accurately described 

 by Lecoq de Boisbaudran {Spectres lumineux, p. 86), and as that seen 

 in burning magnesium (Dr. W. M. Watts, Phil. Mag., 1875), and 

 consists, as we have observed, of lines with the following wave 

 lengths :— 4,481, 4,590 (Thalen 4,586), 4,570 (Thalen 4,565), 4,797, 

 4,930, 4,945, 4,957,4,969, 4,980, 4,990, 5,000, the well-known b group, 

 and 5,528 (Thalen). 



The pair of lines (1) correspond nearly with the b group, but 

 slightly displaced towards the red ; the shaded band (3) corresponds 

 less closely to the series of seven lines 5,000 to 4,930, which pro- 

 gressively decrease in brightness towards the blue, and is also a little 

 less refrangible than that series ; the broad line in the blue (2) cor- 

 responds to the pair of lines 4,570 and 4,590 ; and the remaining line 

 (6) with the line 4,797, also both displaced a little towards the red. 

 No absorption corresponding to the extreme lines 4,481 and 5,528 was 

 observed. There is plainly no exact reversal except of the line b x , and 

 even in that case it may be an accident if we suppose the two dark 

 lines (1) to represent the extreme line of the group b. It may be 

 noted in connexion with this that the absorption lines described by us 

 in our former communication as seen with sodium and potassium 



