he
sails when the spray that has wet them evaporates.

But crystals may form also in the air. There is always some moisture
in the air, and when it becomes very cold, some of this moisture forms
crystals of ice. If they form up in the clouds, they fall as snow.
If they form around blades of grass or on the sidewalk, as the alum
crystals formed on the string, we have frost.

Still another place that crystals occur is in the earth. When the
rocks in the earth were hot enough to be melted and then began to
cool, certain substances in the rocks crystallized. Some of these
crystals that are especially hard and clear constitute precious and
semi-precious stones.

_APPLICATION 64._ Explain why you beat fudge as it cools; why
the paper around butter becomes encrusted with salt if it is
exposed to the air for some time.

INFERENCE EXERCISE

Explain the following:

391. Dynamos have _copper_ brushes to lead the current from
the coils of wire to the line wires.

392. A megaphone makes the voice carry farther than usual.

393. Copper wire is used to conduct electricity, although iron
wire costs much less.

394. A flute gives notes that differ in pitch according to the
stops that are opened.

395. There are usually solid pieces of sugar around the mouth
of a syrup jar.

396. You can beat eggs quickly with a Dover egg beater.

397. When ocean water stands in shallow open tanks for some
time, salt begins to form before the water has all evaporated.

398. In a coffee percolator the boiling water goes up through
a tube. As this water drips back through the ground coffee
beans, it becomes brown and flavored, and the coffee is made.

399. Kerosene will clean off the rim of soap and grease that
forms in bathtubs.

400. Beating cake frosting or candy causes it to sugar.

SECTION 43. _Diffusion._

How does food get into the blood?

Why can you so quickly smell gas that is escaping at the
opposite side of a room?

On our imaginary switchboard the DIFFUSION switch would not be safe
to tamper with. It would be near the SOLUTION switch, and almost as
dangerous. For if you were to make diffusion cease in the world, the
dissolved food and oxygen in your blood would do no good; it could not
get out of the blood vessels or into the cells of your body. You might
breathe all you liked, but breathing would not help you; the