hich we have been looking,
is characteristic of sodium. Wherever you see that particular kind of
light, you know that sodium must have been present in the body from
which it came.

We have accordingly learned to recognize two substances, namely,
strontium and sodium, by the different lights which they give out when
burning. To these two metals we may add a third. Here is a strip of
white metallic ribbon. It is called magnesium. It seems like a bit of
tin at the first glance, but indeed it is a very different substance
from tin; for, look, when I hold it in the spirit-lamp, the strip of
metal immediately takes fire, and burns with a white light so dazzling
that it pales the gas-flames to insignificance. There is no other
substance which will, when kindled, give that particular kind of light
which we see from magnesium. I can recommend this little experiment as
quite suitable for trying at home; you can buy a bit of magnesium
ribbon for a trifle at the opticians; it cannot explode or do any
harm, nor will you get into any trouble with the authorities provided
you hold it when burning over a tray or a newspaper, so as to prevent
the white ashes from falling on the carpet.

There are, in nature, a number of simple bodies called elements.
Every one of these, when ignited under suitable conditions, emits a
light which belongs to it alone, and by which it can be distinguished
from every other substance. I do not say that we can try the
experiments in the simple way I have here indicated. Many of the
materials will yield light which will require to be studied by much
more elaborate artifices than those which have sufficed for us. But
you will see that the method affords a means of finding out the actual
substances present in the sun or in the stars. There is a practical
difficulty in the fact that each of the heavenly bodies contains a
number of different elements; so that in the light it sends us the
hues arising from distinct substances are blended into one beam. The
first thing to be done is to get some way of splitting up a beam of
light, so as to discover the components of which it is made. You might
have a skein of silks of different hues tangled together, and this
would be like the sunbeam as we receive it in its unsorted condition.
How shall we untangle the light from the sun or a star? I will show
you by a simple experiment. Here is a beam from the electric light;
beautifully white and bright, is it not? It looks so