moon.

406. Although the carbon dioxid you breathe out is heavier
than the rest of the air, part of it goes up and mixes with
the air above.

407. On a cold day wood does not feel as cold as iron.

408. To make mayonnaise dressing, the oil, egg, and vinegar
are thoroughly beaten together.

409. A solution of iodin becomes stronger if it is allowed to
stand open to the air.

410. A drop of milk in a glass of water clouds all the water
slightly.

SECTION 44. _Clouds, rain, and dew: Humidity._

Why is it that you can see your breath on a cold day?

Where do rain and snow come from?

What makes the clouds?

There is water vapor in the air all around us--invisible water
vapor, its molecules mingling with those of the air--water that has
evaporated from the oceans and lakes and all wet places.

This water vapor changes into droplets of water when it gets cool
enough. And those droplets of water make up our clouds and fogs; they
join together to form our rain and snow high in the air, or gather as
dew or frost on the grass at night.

If the water vapor should suddenly lose its power of changing into
droplets of water when it cooled,--well, let us pretend it has lost
this power but that any amount of water can evaporate, and see what
happens:

What fine weather it is! There is not a cloud in the sky. As evening
closes in, the stars come out with intense brightness. The whole sky
is gleaming with stars--more than we have ever seen at night before.

The next morning we find no dew or frost on the grass. All the green
things look dry. As the day goes on, they begin to wilt and wither.
We all wish the day were not quite so fine--a little rain would help
things wonderfully. Not a cloud appears, however, and we water as
much of our gardens as we can. They drink the water greedily, and that
night, again no dew or fog, and not the faintest cloud or mist to dim
the stars. And the new day once more brings the blazing sun further
to parch the land and plants. Day after day and night after night the
drought gets worse. The rivers sink low; brooks run dry; the edges of
the lakes become marshes. The marshes dry out to hardened mud. The
dry leaves of the trees rustle and crumble. All the animals and
wood creatures gather around the muddy pools that once were lakes or
rivers. People begin saving water and buying it and selling it as the
most precious of articles.

As t