the sunlight_. The dampness
quickens the action.

WHY SOME PEOPLE FRECKLE IN THE SUN. When the sunlight falls for a long
time on the skin, it often causes the cells in the under part of
the skin to produce some dark coloring matter, or pigment. This dark
pigment shows through the outer layer of skin, and we call the little
spots of it _freckles_. Some people are born with these pigment spots;
but when the freckles come out from long exposure to the sunlight,
they are an example right in our own skins of chemical change caused
by the action of light. Tan also is due to pigment in the skin and is
caused by light.

The next experiments with their explanations will show you how
cameras can take pictures. If you are not interested in knowing how
photographs are made, do the experiments and skip the explanations
down to the middle of page 332.

EXPERIMENT 100. Dissolve a small crystal of silver nitrate
(AgNO_3) in about half an inch of pure water in the bottom
of a test tube. Distilled water is best for this purpose.
Now add one drop of hydrochloric acid (HCl). The white powder
formed is a silver salt, called _silver chlorid_ (AgCl); the
rest of the liquid is now a diluted nitric acid (HNO_3).

Pour the suspension of silver chlorid (AgCl) on a piece of
blotting paper or on a paper towel, so that the water will be
absorbed. Spread the remaining white paste of silver chlorid
(AgCl) out over the blotter as well as you can. Cover part of
it with a key (or anything that will shut off the light), and
leave the other part exposed. If the sun is shining, put the
blotter in the sunlight for 5 minutes. Otherwise, let as much
daylight fall on it as possible for about 10 minutes. Now take
the key off the part of the silver chlorid (AgCl) that it was
covering and compare this with the part that was exposed
to the light. What has the light done to the silver chlorid
(AgCl) that it shone on?

What has happened is that the light has made the silver (Ag)
_separate_ from the chlorine (Cl) of the silver chlorid (AgCl).
Chemists would write this:

AgCl -> Ag + Cl.

That is, silver chlorid (AgCl) has changed into silver (Ag) and
chlorine (Cl). Chlorine, as you know, is a poisonous gas, and it
floats off in the air, leaving the fine particles of silver behind.
When silver is divided into very tiny particles, it absorbs light
instead of reflecting it; so it looks d