ng a bit of sodium less than a pea in magnitude is plunged
into the flame. The sodium soon volatilises and burns with brilliant
incandescence. The beam crosses the flame, and at the same time the
yellow band of the spectrum is clearly and sharply cut out, a band of
intense darkness occupying its place. On withdrawing the sodium, the
brilliant yellow of the spectrum takes its proper place, while the
reintroduction of the flame causes the band to reappear.

Let me be more precise: The yellow colour of the spectrum extends
over a sensible space, blending on one side with the orange and on the
other with the green. The term 'yellow band' is therefore somewhat
indefinite. This vagueness may be entirely removed. By dipping the
carbon-point used for the positive electrode into a solution of common
salt, and replacing it in the lamp, the bright yellow band produced by
the sodium vapour stands out from the spectrum. When the sodium flame
is caused to act upon the beam it is that particular yellow band that
is obliterated, an intensely black streak occupying its place.

An additional step of reasoning leads to the conclusion that if,
instead of the flame of sodium alone, we were to introduce into the
path of the beam a flame in which lithium, strontium, magnesium,
calcium, &c, are in a state of volatilisation, each metallic vapour
would cut out a system of bands, corresponding exactly in position
with the bright bands of the same metallic vapour. The light of our
electric lamp shining through such a composite flame would give us a
spectrum cut up by dark lines, exactly as the solar spectrum is cut up
by the lines of Fraunhofer.

Thus by the combination of the strictest reasoning with the most
conclusive experiment, we reach the solution of one of the grandest of
scientific problems--the constitution of the sun. The sun consists of
a nucleus surrounded by a flaming atmosphere. The light of the
nucleus would give us a continuous spectrum, like that of our common
carbon-points; but having to pass through the photosphere, as our beam
had to pass through the flame, those rays of the nucleus which the
photosphere can itself emit are absorbed, and shaded spaces,
corresponding to the particular rays absorbed, occur in the spectrum.
Abolish the solar nucleus, and we should have a spectrum showing a
bright line in the place of every dark line of Fraunhofer. These
lines are therefore not absolutely dark, but dark by an amount