rdinary table salt is sodium
chloride; that is to say, a compound of the metal sodium and the gas
chlorine. Now if other compounds of sodium be experimented with in the
same manner, it will soon be found that these two yellow lines are
characteristic of sodium when turned into vapour by great heat. In the
same manner it can be ascertained that every element, when heated to a
condition of vapour, gives as its spectrum a set of lines peculiar to
itself. Thus the spectroscope enables us to find out the composition of
substances when they are reduced to vapour in the laboratory.

[Illustration: FIG. 9.--The Solar Spectrum.]

In order to increase the power of a spectroscope, it is necessary to
add to the number of prisms. Each extra prism has the effect of
lengthening the coloured strip still more, so that lines, which at first
appeared to be single merely through being crowded together, are
eventually drawn apart and become separately distinguishable.

On this principle it has gradually been determined that the sun is
composed of elements similar to those which go to make up our earth.
Further, the composition of the stars can be ascertained in the same
manner; and we find them formed on a like pattern, though with certain
elements in greater or less proportion as the case may be. It is in
consequence of our thus definitely ascertaining that the stars are
self-luminous, and of a sun-like character, that we are enabled to speak
of them as _suns_, or to call the sun a _star_.

In endeavouring to discover the elements of which the planets and
satellites of our system are composed, we, however, find ourselves
baffled, for the simple reason that these bodies emit no real light of
their own. The light which reaches us from them, being merely reflected
sunlight, gives only the ordinary solar spectrum when examined with the
spectroscope. But in certain cases we find that the solar spectrum thus
viewed shows traces of being weakened, or rather of suffering
absorption; and it is concluded that this may be due to the sunlight
having had to pass through an atmosphere on its way to and from the
surface of the planet from which it is reflected to us.

Since the sun is found to be composed of elements similar to those which
go to make up our earth, we need not be disheartened at this failure of
the spectroscope to inform us of the composition of the planets and
satellites. We are justified, indeed, in assuming that more or less the
s