t the beautiful hues of velvet, satin, and
plush tapestry may not be marred by loss in brilliancy and sheen.
Bright carpets and rugs are sometimes bought in preference to more
delicately tinted ones, because the purchaser knows that the latter
will fade quickly if used in a sunny room, and will soon acquire a
dull mellow tone. The bright and gay colors and the dull and somber
colors are all affected by the sun, but why one should be affected
more than another we do not know. Thousands of brilliant and dainty
hues catch our eye in the shop and on the street, but not one of them
is absolutely permanent; some may last for years, but there is always
more or less fading in time.

Sunlight causes many strange, unexplained effects. If the two
substances, chlorine and hydrogen, are mixed in a dark room, nothing
remarkable occurs any more than though water and milk were mixed, but
if a mixture of these substances is exposed to sunlight, a violent
explosion occurs and an entirely new substance is formed, a compound
entirely different in character from either of its components.

By some power not understood by man, the sun is able to form new
substances. In the dark, chlorine and hydrogen are simply chlorine and
hydrogen; in the sunlight they combine as if by magic into a totally
different substance. By the same unexplained power, the sun frequently
does just the opposite work; instead of combining two substances to
make one new product, the sun may separate or break down some
particular substance into its various elements. For example, if the
sun's rays fall upon silver chloride, a chemical action immediately
begins, and as a result we have two separate substances, chlorine and
silver. The sunlight separates silver chloride into its constituents,
silver and chlorine.

121. The Magic Wand in Photography. Suppose we coat one side of a
glass plate with silver chloride, just as we might put a coat of
varnish on a chair. We must be very careful to coat the plate in the
dark room,[B] otherwise the sunlight will separate the silver chloride
and spoil our plan. Then lay a horseshoe on the plate for good luck,
and carry the plate out into the light for a second. The light will
separate the silver chloride into chlorine and silver, the latter of
which will remain on the plate as a thin film. All of the plate was
affected by the sun except the portion protected by the horseshoe
which, because it is opaque, would not allow light to pass t