makes a very much louder snap. The snap is caused
in this way: As the electric spark leaps through the air, it leaves
an empty space or vacuum immediately behind it. The air from all sides
rushes into the vacuum and collides there; then it bounces back. This
again leaves a partial vacuum; so the air rushes in once more, coming
from all sides at once, and again bounces back. This starts the air
vibrations which we call _sound_. Then the sound is echoed from cloud
to cloud and from the clouds to the earth and back again, and we call
it _thunder_.

The electricity you have been reading about and experimenting with in
this section is called _static electricity_. "Static" means standing
still. The electricity you rubbed up to the surface of the comb or
glass stayed still until it jumped to the bit of paper or hair; then
it stayed still on that. This was the only kind of electricity most
people knew anything about until the nineteenth century; and it is
not of any great use. Electricity must be flowing through things to do
work. That is why people could not invent electric cars and electric
lights and telephones before they knew how to make electricity flow
steadily rather than just to stand still on one thing until it jumped
across to another and stood there. In the next chapter we shall take
up the ways in which electrons are made to flow and to do work.

_APPLICATION 48._ Explain why the stroking of a cat's back
will sometimes cause sparks and make the cat's hairs stand
apart; why combing sometimes makes your hairs fly apart. Both
of these effects are best secured on a dry day, because on a
damp day the water particles in the air will let the electrons
pass to them as fast as they are rubbed up to the surface of
the hair.

INFERENCE EXERCISE

Explain the following:

291. If you shuffle your feet on a carpet in clear, cold
weather and then touch a person's nose or ear, a slight spark
passes from your finger and stings him.

292. If you stay out in the cold long, you get chilled
through.

293. The air and earth in a greenhouse are warmed by the sun
through the glass even when it is cold outside and when the
glass itself remains cold.

294. When you hold a blade of grass taut between your thumbs
and blow on it, you get a noise.

295. Shadows are usually black.

296. Some women keep magnets with which to find lost needles.

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