prepared. A
little gas was allowed into each tube, and then almost all the gas was
taken out again, so that only a mere trace was left. I pass a current
of electricity through these tubes, and now you see they are glowing
with beautiful colors. The different gases give out lights of
different hues, and the optician has exerted his skill so as to make
the effect as beautiful as possible. The electricity, in passing
through these tubes, heats the gas which they contain, and makes it
glow; and just as this gas can, when heated sufficiently, give out
light, so does the great nebula, which is a mass of gas poised in
space, become visible in virtue of the heat which it contains.

We are not left quite in doubt as to the constitution of these gaseous
nebulae, for we can submit their light to the prism in the way I
explained when we were speaking of the stars. Distant though that ring
in Lyra may be, it is interesting to learn that the ingredients from
which it is made are not entirely different from substances we know on
our earth. The water in this glass, and every drop of water, is formed
by the union of two gases, of which one is hydrogen. This is an
extremely light material, as you see by a little balloon which ascends
so prettily when filled with it. Hydrogen also burns very readily,
though the flame is almost invisible. When I blow a jet of oxygen
through the hydrogen, I produce a little flame with a very intense
heat. For instance, I hold a steel pen in the flame, and it glows and
sputters, and falls down in white-hot drops. It is needless to say
that, as a constituent of water, hydrogen is one of the most important
elements on this earth. It is, therefore, of interest to learn that
hydrogen in some form or other is a constituent of the most distant
objects in space that the telescope has revealed.

Photographing the Nebulae.

[Illustration: FIG. 8. THE PLEIADES.]

Of late years we have learned a great deal about nebulae, by the help
which photography has given to us. Look at this group of stars which
constitutes that beautiful little configuration known as the Pleiades
(Fig. 8). It looks like a miniature representation of the Great Bear;
in fact, it would be far more appropriate to call the Pleiades the
Little Bear than to apply that title to another quite different
constellation, as has unfortunately been done. The Pleiades form a
group containing six or seven stars visible to the ordinary eye,
though persons