210 Professor Deivar [June 6, 



defined, and has a wave-length about 6,140, and it fades away towards 

 the blue. 



These absorptions are all seen both when potassium and sodium are 

 used along with mgnesium, and may be fairly ascribed to magnesium, 

 or to magnesium together with hydrogen. 



But besides these, other absorptions are seen which appear to be 

 due to mixed vapours. 



(4) Thus 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 some- 

 times 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. 



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 

 between electrodes of magnesium. 



The coincidences of the series of the solar spectrum hitherto 

 observed have, for the most part, been with lines given by dense 

 electric sparks; while it is not improbable that the conditions of 

 temperature, and the admixtures of vapours in the upper part of the 

 solar atmosphere, may resemble much more nearly those in our tubes. 



It became a question of interest to find the conditions under which 

 the same mixtures would give luminous spectra, consisting of the lines 

 which had been seen reversed. On observing sparks from an induc- 

 tion-coil taken between magnesium points in an atmosphere of hydro- 

 gen, a bright line regularly appeared, with a wave-length about 5,210, 

 in the same position as one of the most conspicuous of the dark 

 lines observed to be produced by vapour of magnesium with hydrogen 

 in our iron tubes. This line is best seen, i. e. is most steady, when 

 no Leyden jar is used, and the rheotome is screwed back, so that it 

 will but just work. It may, however, be seen when the coil is in 

 its ordinary state, and when a' small Leyden jar is interposed ; but 

 it disappears (except in flashes) when a larger Leyden jar is used, 

 if the hydrogen be at the atmospheric pressure. This line does not 

 usually extend across the whole interval between the electrodes, and is 

 sometimes only seen near the negative electrode. Its presence seems 

 to depend on the temperature, as it is not seen continuously when a 

 large Leyden jar is employed, until the pressure of the hydrogen and 

 its resistance is very much reduced. When well-dried nitrogen or 

 carbonic oxide is substituted for hydrogen, this line disappears en- 



