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V. Researches in Spectrum- Analysis in connexion with the Spectrum of the Sun. 
By J. Norman Lockyer, F.R.S. 
Received November 6, — Read December 12, 1872, 
The researches of which an account is given in the present communication have been 
undertaken in continuation of those carried on by Dr. Frankland and myself at the 
Royal College of Chemistry, from which we arrived at the conclusion that the 
thickening of spectral lines was due to pressure, and not to temperature per .sc*. In our 
joint communications we pointed out that this held good for hydrogen in Geissler’s 
tubes and for magnesium vapour f when the spark was taken in air, by means of a 
method which enabled us to spectroscopically examine its various portions. 
The magnesium experiment was important not only so far as the decrease of thickness 
of lines with decrease of density was concerned, but because it showed that one of the 
well-known triple lines in the spectrum of magnesium absolutely vanished altogether 
from the spectrum at some distance from the source of the supply of the vapour — that 
is, the pole of metallic magnesium. This result we also obtained, as stated in our note, 
when we observed the spectrum of the spark between two magnesium poles enclosed in 
a Geissler’s tube in an atmosphere of hydrogen in which the pressure of the gas was 
gradually reduced. 
In some experiments with sodium vapour, which were not referred to in the papers in 
question £, Dr. Frankland and myself observed the same phenomena. The experiments 
were conducted as follows : — 
(1) Into a piece of hard glass combustion-tube, thoroughly cleaned and closed at one 
end, a few pieces of metallic sodium, clean and as free as possible from naphtha, were 
introduced. The end of the tube was then drawn out and connected with a Sprengel 
pump and exhausted as rapidly as possible. Flydrogen was then admitted, and the tube 
reexhausted and, when the pressure was again reduced to a few millimetres, carefully 
sealed up. The tube thus prepared was placed between the slit plate of a spectroscope 
and a source of light giving a continuous spectrum. 
Generally, unless the atmosphere of the laboratory was very still and free from dust, 
the two bright D lines could be distinctly seen on the background of the bright con- 
tinuous spectrum. 
The tube containing the sodium was then heated with a Bunsen flame and the 
spectrum carefully watched. Soon after the application of the heat, a dark line thin, and 
delicate as a spider’s thread, was observed to be slowly creeping down each of the bright 
* Proceedings of the Royal Society, vol. xvii. p. 289. t Ibid. vol. xviii. p. 79. 
t This experiment was first exhibited at a Lecture given by me at the Royal Institution in May, 1 869. 
MDCCCLXXI1I. 2 M 
