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.

[Illustration: FIG. 7. HOW TO MAKE THE SMOKE RINGS.]

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 visible 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.

What the Nebulae are made of.

There is a fundamental difference between the illumination of these
little rings that I have shown you and the great rings in the heavens.
I had to illuminate our smoke with the help of the electric light,
for, unless I had done so, you would not have been able to see them.
This white substance formed by the union of ammonia and hydrochloric
acid has, of course, no more light of its own than a piece of chalk;
it requires other light falling upon it to make it visible. Were the
ring nebula in Lyra composed of this material, we could not see it.
The sunlight which illuminates the planets might, of course, light up
such an object as the ring, if it wrere comparatively near us; but
Lyr