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 Lyra is at such a stupendous distance that any light which the sun could send out there would be just as feeble as the light we receive from a fixed star.  Should we be able to show our smoke-rings, for instance, if, instead of having the electric light, I merely cut a hole in the ceiling and allowed the feeble twinkle of a star in the Great Bear to shine through?  In a similar way the sunbeams would be utterly powerless to effect any illumination of objects in these stellar distances.  If the sun were to be extinguished altogether, the calamity would no doubt be a very dire one so far as we are concerned, but the effect