In drawing towards the close of these lectures I must say a few words
about some dim and mysterious objects to which we have not yet alluded.
They are what are called nebulæ, or little clouds; and in one sense
they are justly called little, for each of them occupies but a very
small spot in the sky as compared with that which would be filled by
an ordinary cloud in our air. The nebulæ are, however, objects of the
most stupendous proportions. Were our earth and thousands of millions
of bodies quite as big all put together, they would not be nearly so
great as one of these nebulæ. Astronomers reckon up the various nebulæ
by thousands, but I must add that most of them are apparently faint and
uninteresting. A nebula is sometimes liable to be mistaken for a comet.
The comet is, as I have already explained, at once distinguished by the
fact that it is moving and changing its appearance from hour to hour,
while scores of years elapse without changes in the aspect or position
of a nebula. The most powerful telescopes are employed in observing
these faint objects. I take this opportunity of showing a picture of an
instrument suitable for such observations. It is the great reflector of
the Paris Observatory (Fig. 87).
[Illustration: FIG. 86.--The Ring Nebula in Lyra, under Different
Telescopic Powers.]
There are such multitudes of nebulæ that I can only show a few of the
more remarkable kinds. In Fig. 86 will be seen pictures of a curious
object in the constellation of Lyra seen under different telescopic
powers. This is a gigantic ring of luminous gas. To judge of the size
of this ring let us suppose that a railway were laid across it, and
the train you entered at one side was not to stop until it reached
the other side, how long do you think this journey would require?
I recollect some time ago a picture in _Punch_ which showed a train
about to start from London to Brighton, and the guard walking up and
down announcing to the passengers the alarming fact that “this train
stops nowhere.” An old gentleman was seen vainly gesticulating out of
the window and imploring to be let out ere the frightful journey was
commenced. In the nebular railway the passengers would almost require
such a warning.
[Illustration: FIG. 87.--A Great Reflecting Telescope.]
Let the train start at a speed of a mile a minute, you would think,
surely, that it must soon cross the ring. But the minutes pass, an
hour has elapsed; so the distance must be sixty miles, at all events.
The hours creep on into days, the days advance into years, and still
the train goes on. The years would lengthen out into centuries, and
even when the train had been rushing on for a thousand years with an
unabated speed of a mile a minute, the journey would certainly not have
been completed. Nor do I venture to say what ages must elapse ere the
terminus at the other side of the ring nebula would be reached.
A cluster of stars viewed in a small telescope will often seem like a
nebula, for the rays of the stars become blended. A powerful telescope
will, however, dispel the illusion and reveal the separate stars. It
was, therefore, thought that all the nebulæ might be merely clusters
so exceedingly remote that our mightiest instruments failed to resolve
them into stars. But this is now known not to be the case. Many of
these objects are really masses of glowing gas; such are, for instance,
the ring nebulæ, of which I have just spoken, and the form of which I
can simulate by a pretty experiment.
[Illustration: FIG. 88.--How to make the Smoke-rings.]
We take a large box with a round hole cut in one face, and a canvas
back at the opposite side. I first fill this box with smoke, and there
are different ways of doing so. Burning brown paper does not answer
well, because the supply of smoke is too irregular and the paper
itself is apt to blaze. A little bit of phosphorus set on fire yields
copious smoke, but it would be apt to make people cough, and, besides,
phosphorus is a dangerous thing to handle incautiously, and I do not
want to suggest anything which might be productive of disaster if the
experiment was repeated at home. A little wisp of hay, slightly damped
and lighted, will safely yield a sufficient supply, and you need not
have an elaborate box like this; any kind of old packing-case, or even
a band-box with a duster stretched across its open top and a round hole
cut in the bottom, will answer capitally. While I have been speaking,
my assistant has kindly filled this box with smoke, and in order to
have a sufficient supply, and one which shall be as little disagreeable
as possible, he has mixed together the fumes of hydrochloric acid and
ammonia from two retorts shown in Fig. 88. A still simpler way of doing
the same thing is to put a little common salt in a saucer and pour
over it a little oil of vitriol; this is put into the box, and over
the floor of the box common smelling-salts is to be scattered. You see
there are dense volumes of white smoke escaping from every corner of
the box. I uncover the opening and give a push to the canvas, and you
see a beautiful ring flying across the room; another ring and another
follow. If you were near enough to feel the ring, you would experience
a little puff of wind; I can show this by blowing out a candle which
is at the other end of the table. These rings are made by the air
which goes into a sort of eddy as it passes through the hole. All the
smoke does is to render the air visible. The smoke-ring is indeed
quite elastic. If we send a second ring hurriedly after the first, we
can produce a collision, and you see each of the two rings remains
unbroken, though both are quivering from the effects of the blow. They
are beautifully shown along the beam of the electric lamp, or, better
still, along a sunbeam.
We can make many experiments with smoke-rings. Here, for instance, I
take an empty box, so far as smoke is concerned, but air-rings can be
driven forth from it, though you cannot see them, but you can feel them
even at the other side of the room, and they will, as you see, blow out
a candle. I can also shoot invisible air-rings at a column of smoke,
and when the missile strikes the smoke it produces a little commotion
and emerges on the other side, carrying with it enough of the smoke to
render itself visible, while the solid black looking ring of air is
seen in the interior. Still more striking is another way of producing
these rings, for I charge this box with ammonia, and the rings from
it you cannot see. There is a column of the vapor of hydrochloric
acid that also you cannot see; but when the invisible ring enters the
invisible column, then a sudden union takes place between the vapor
of the ammonia and the vapor of the hydrochloric acid; the result is
a solid white substance in extremely fine dust which renders the ring
instantly visible.