196 On the Simultaneous Emission and Absorption of Rays. 
“J conclude from these observations, that coloured flames in the 
spectra of which bright sharp lines present themselves, so weaken 
rays of the colour of these lines, when such rays pass through 
the flames, that in place of the bright lines dark ones appear 
as soon as there is brought behind the flame a source of light of 
sufficient intensity, in the spectrum of which these lines are 
otherwise wanting. I conclude further, that the dark lines of 
the solar spectrum which are not evoked by the atmosphere of the 
earth, exist in consequence of the presence, in the incandescent 
atmosphere of the sun, of those substances which in the spectrum 
of a flame produce bright lines at the same place. We may 
assume that the bright lines agreeing with D in the spectrum of 
a flame always arise from sodium contained in it ; the dark line 
D in the solar spectrum allows us, therefore, to conclude that 
there exists sodium in the sun’s atmosphere. Brewster has 
found bright lines in the spectrum of the flame of saltpeter at 
the place of Fraunhofer’s lines A, a, B; these lines point to the 
existence of potassium in the sun’s atmosphere. From my ob- 
servation, according to which no dark line in the solar spectrum 
answers to the red line of lithium, it would follow with proba- 
bility that in the atmosphere of the sun lithium is either absent, 
or is present in comparatively small quantity. 
“The examination of the spectra of coloured flames has 
accordingly acquired a new and high interest; I will carry it 
out in conjunction with Bunsen as far as our means allow. In 
connexion therewith we will investigate the weakening of rays of 
light in flames that has been established by my observations. 
In the course of the experiments which have at present been insti- 
tuted by us in this direction, a fact has already shown itself 
which seems to us to be of great importance. The Drummond 
light requires, in order that the lines D should come out in it 
dark, a salt-flame of lower temperature. The flame of alcohol 
containing water is fitted for this, but the flame of Bunsen’s gas- 
lamp is not. With the latter the smallest mixture of common 
salt, as soon as it makes itself generally perceptible, causes the 
bright lines of sodium to show themselves. We reserve to our- 
selves to develope the consequences which may be connected with 
this fact.” 
Note.—The remarkable phenomenon discovered by Foucault, 
and rediscovered and extended by Kirchhoff, that a body may be 
at the same time a source of light giving out rays of a definite 
refrangibility, and an absorbing medium extinguishing rays of 
that same refrangibility which traverse it, seems readily to admit 
of a dynamical illustration borrowed from sound. 
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