Every chemical element known, then, has a distinctive spectrum of its
own when it is raised to incandescence, and this distinctive spectrum is
as reliable a means of identification for the element as a human face is
for its owner. Whether it is a substance glowing in the laboratory or in
a remote star makes no difference to the spectroscope; if the light of
any substance reaches it, that substance will be recognised and
identified by the characteristic set of waves.
The spectrum of a glowing mass of gas will consist in a number of bright
lines of various colours, and at various intervals; corresponding to
each kind of gas, there will be a peculiar and distinctive arrangement
of bright lines. But if the light from such a mass of glowing gas be
made to pass through a cool mass of the _same_ gas it will be found that
dark lines replace the bright lines in the spectrum, the reason for this
being that the cool gas absorbs the rays of light emitted by the hot
gas. Experiments of this kind enable us to reach the important general
statement that every gas, when cold, absorbs the same rays of light
which it emits when hot.
Crossing the solar spectrum are hundreds and hundreds of dark lines.
These could not at first be explained, because this fact of
discriminative absorption was not known. We understand now. The sun's
white light comes from the photosphere, but between us and the
photosphere there is, as we have seen, another solar envelope of
relatively cooler vapours--the reversing layer. Each constituent
element in this outer envelope stops its own kind of light, that is, the
kind of light made by incandescent atoms of the same element in the
photosphere. The "stoppages" register themselves in the solar spectrum
as dark lines placed exactly where the corresponding bright lines would
have been. The explanation once attained, dark lines became as
significant as bright lines. The secret of the sun's composition was
out. We have found practically every element in the sun that we know to
be in the earth. We have identified an element in the sun before we were
able to isolate it on the earth. We have been able even to point to the
coolest places on the sun, the centres of sun-spots, where alone the
temperature seems to have fallen sufficiently low to allow chemical
compounds to form.
It is thus we have been able to determine what the stars, comets, or
nebulæ are made of.