in a
glass tube a mixture of gases related to each other in a certain way
chemically, they will combine into small globules or particles similar
to moisture in the air. If now a beam of light is thrown upon this tube
and a dark screen put behind it, we shall, in the beginning of the
experiment, simply see the dark screen. As soon, however, as the
molecules of the gases have combined in sufficient numbers to produce
particles of sensible size we begin to have a reflection of light from
them, the color of which is constantly changing as the combining
particles grow in size. At a certain stage in its progress the color
which the mixture of gases assumes is a beautiful azure blue, rivaling
in purity the finest skies of Greece or southern Italy.

The sun is the great lamp that illuminates the world, while the
atmosphere, which is filled with particles of various substances,
becomes the shade of the lamp which diffuses and softens the light and
gives it its color tones, whether of warmth or coldness. We could not
well do without the reflected light of the sky. The poetry of life would
be sadly marred. The beautiful effects of color and purity of tone would
be wanting. We need to bathe in light as much as in water, and the
character of the light is almost as important as the character of the
water. Imagine a world with an atmosphere devoid of all substances that
would in any way reflect light or give to it softness or color tone.
Imagine a sun or a moon without visible rays--for without a reflecting
atmosphere there would be none. Imagine a sky that was no sky at all,
but only a dark void, with no protecting vault. Think of the shadows, so
dark that you could see nothing in them. These would be some of the
effects that would come from an atmosphere that had no sky substance in
it. Imagine the world lighted by one great arc light. The reflex action
upon the race living in such a light would be anything but desirable.
The world would develop into an arc-light civilization--if one can
imagine what that would be like; certainly one of intensely violent
contrasts. Look on this picture and let us be thankful for the blue sky
and golden sunsets.

"But," you ask, "why is the sky blue?"

In one of the chapters on the subject of light in Vol. II. the
properties of soap bubbles are discussed. It is shown that when a film
is stretched across the mouth of a tumbler held in a position so that
the film is perpendicular, by the action of g