That light can effect radical changes in many substances was known to
the ancients. Its destructive action on artists' pigments, e.g., the
blackening of vermilion, was recorded 2,000 years ago by Vitruvius.
Since that time it has been well established, by numerous observations
and experiments, that light possesses, in a high degree, the power of
exerting chemical action, i.e., causing the combination or
decomposition of a large number of substances. The union of chlorine
with hydrogen gas, the blackening of silver salts, the reduction of
bichromate of potash and of certain ferric salts in contact with
organic substances, are all familiar instances of the action of light.
In illustration of this, I show here some calico prints produced by
first preparing the calico with a solution of potassium bichromate,
then exposing the dried calico under a photographic negative, and,
after washing, dyeing with alizarin or some similar coloring matter.
During the exposure under the negative, the light has reduced and
fixed the chromium salt upon certain parts of the fiber as insoluble
chromate of chromium (Cr_{2}O_{3}CrO_{3}) in the more protected
portions, the bichromate remains unchanged, and is subsequently
removed by washing.
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