Explain the following:

451. When you want bread dough to rise, you put it in a warm
place.

452. Ink left long in an open inkwell becomes thick.

453. A ball bounces up when you throw it down.

454. When the warm ocean air blows over the cool land in the
early morning, there is a heavy fog.

455. Striking a match makes it burn.

456. When you have something hard to cut, you put it in the
part of the scissors nearest the handles.

457. A magnet held over iron filings makes them leap up.

458. Dishes in which flour thickening or dough has been mixed
should be washed out with cold water.

459. A woolen sweater is liable to stretch out of shape after
being washed.

460. When a telegraph operator presses a key in his set, a
piece of iron is pulled down in the set of another operator.

SECTION 49. _Chemical change caused by light._

How can a camera take a picture?

Why does cloth fade in the sun?

What makes freckles?

If light could not help chemical change, nothing would ever fade when
hung in the sun; wall paper and curtains would be as bright colored
after 20 years as on the day they were put up, if they were kept
clean; you would never become freckled, tanned, or sunburned; all
photographers and moving-picture operators would have to go out of
business; but worst of all, every green plant would immediately stop
growing and would soon die. Therefore, all cows and horses and other
plant-eating animals would die; and then the flesh-eating animals
would have nothing to eat and they would die; and then all people
would die.

You will be able better to understand why all this would happen after
you do the following experiments, the first of which will show that
light helps the chemical change called bleaching or fading.

EXPERIMENT 99. Rinse two small pieces of light-colored cloth.
(Lavender is a good color for this experiment.) Lay one piece
in the bright sun to dry; dry the other in a dark cabinet or
closet. The next day compare the two cloths. Which has kept
its color the better? If the difference is not marked, repeat
the experiment for 2 or 3 days in succession, putting the same
cloth, wet, in the sun each time.

Bleaching is usually a very slow kind of burning. It is the dye that
is oxidized (burned), not the cloth. Most dyes will combine with the
oxygen in the air _if they are exposed to