Here is a piece of stone. If I wanted to know what it was composed
of, I should ask a chemist to tell me. He would take it into his
laboratory, and first crush it into powder, and then, with his test
tubes, and with the liquids which his bottles contain, and his
weighing scales, and other apparatus, he will tell all about it;
there is so much of this, and so much of that, and plenty of this, and
none at all of that. But now, suppose you ask this chemist to tell
you what the sun is made of, or one of the stars. Of course, you have
not a sample of it to give him; how, then, can he possibly find out
anything about it? Well, he can tell you something, and this is the
wonderful discovery that I want to explain to you. We now put down the
gas, and I kindle a brilliant red light. Perhaps some of those whom
I see before me have occasionally ventured on the somewhat dangerous
practice of making fireworks. If there is any boy here who has ever
constructed sky-rockets, and put the little balls into the top which
are to burn with such vivid colors when the explosion takes place, he
will know that the substance which tinged that red fire must have been
_strontium_. He will recognize it by the color; because _strontium_
gives a red light which nothing else will give. Here are some of these
lightning papers, as they are called; they are very pretty and very
harmless; and these, too, give brilliant red flashes as I throw them.
The red tint has, no doubt, been produced by _strontium_ also. You
see we recognized the substance simply by the color of the light it
produced when burning.
Perhaps some of you have tried to make a ghost at Christmas by dressing
up in a sheet, and bearing in your hand a ladle blazing with a mixture
of common salt and spirits of wine, the effect produced being a most
ghastly one. Some mammas will hardly thank me for this suggestion,
unless I add that the ghost must walk about cautiously, for otherwise
the blazing spirit would be very apt to produce conflagrations of a
kind more extensive than those intended. However, by the kindness of
Professor Dewar, I am enabled to show the phenomenon on a splendid
scale, and also free from all danger. I kindle a vivid flame of an
intensely yellow color, which I think the ladies will unanimously
agree is not at all becoming to their complexions, while the pretty
dresses have lost their variety of colors. Here is a nice bouquet,
and yet you can hardly distinguish the green of the leaves from the
brilliant colors of the flowers, except by trifling differences of
shade. Expose to this light a number of pieces of variously colored
ribbon, pink and red and green and blue, and their beauty is gone; and
yet we are told that this yellow is a perfectly pure color; in fact,
the purest color that can be produced. I think we have to be thankful
that the light which our good sun sends us does not possess purity
of that description. There is one substance which will produce that
yellow light; it is a curious metal called sodium--a metal so soft
that you can cut it with a knife, and so light that it will float on
water; while, still more strange, it actually takes fire the moment it
is dropped on the water. It is only in a chemical laboratory that you
will be likely to meet with the actual metallic sodium, yet in other
forms the substance is one of the most abundant in nature. Indeed,
common salt is nothing but sodium closely united with a most poisonous
gas, a few respirations of which would kill you. But this strange metal
and this noxious gas, when united, become simply the salt for our eggs
at breakfast. This pure yellow light, wherever it is seen, either in
the flame of spirits of wine mixed with salt or in that great blaze at
which 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 optician’s; 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 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 pure and simple, but yet that beam is composed of all sorts of
colors mingled together, in such proportions as to form white light. I
take a wedge-shaped piece of glass called a prism, and when I introduce
it into the course of the beam, you see the transformation that has
taken place (Fig. 85). Instead of the white light you have now all
the colors of the rainbow--red, orange, yellow, green, blue, indigo,
violet, marked by their initial letters in the figure. These colors are
very beautiful, but they are transient, for the moment we take away
the prism they all unite again to form white light. You see what the
prism has done; it has bent all the light in passing through it; but it
is more effective in bending the blue than the red, and consequently
the blue is carried away much further than the red. Such is the way in
which we study the composition of a heavenly body. We take a beam of
its light, we pass it through a prism, and immediately it is separated
into its components; then we compare what we find with the lights
given by the different elements, and thus we are enabled to discover
the substances which exist in the distant object whose light we have
examined. I do not mean to say that the method is a simple one; all
I am endeavoring to show is a general outline of the way in which we
have discovered the materials present in the stars. The instrument that
is employed for this purpose is called the spectroscope. And perhaps
you may remember that name by these lines, which I have heard from an
astronomical friend:--
[Illustration: FIG. 85.--How to split up a Ray of Light.]
“Twinkle, twinkle, little star,
Now we find out what you are,
When unto the midnight sky,
We the spectroscope apply.”
I am sure it will interest everybody to know that the elements which
the stars contain are not altogether different from those of which the
earth is made. It is true there may be substances in the stars of which
we know nothing here; but it is certain that many of the most common
elements on the earth are present in the most distant bodies. I shall
only mention one, the metal iron. That useful substance has been found
in some of the stars which lie at almost incalculable distances from
the earth.