s also ionize the air and cause the formation of ozone. This can be
detected near a quartz mercury-arc, for example, by the characteristic
odor.

The foregoing are only a few of the multitude of photochemical reactions
and other effects of radiant energy. The development of this field
awaits to some extent the production of so-called actinic rays more
efficiently and in greater quantities, but there are now many practical
applications of artificial light for these purposes. In the extensive
fields of photography various artificial light-sources have served for
many years and they are constantly finding more applications. Artificial
light is now used to a considerable extent in the industries in
connection with chemical processes, but little information is available,
owing to the secrecy attending these new developments in industrial
processes. However, this brief chapter has been introduced in order to
indicate another field of activity in which artificial light is serving.
It is agreed by scientists that photochemistry has a promising future.
Mankind harnesses nature's forces and produces light and this light is
put to work to exert its influence for the further benefit of mankind.
Science has been at work systematically for only a century, but the
accomplishments have been so wonderful that the imagination dares not
attempt to prophesy the achievements of the next century.

XX

LIGHT AND HEALTH

The human being evolved without clothing and the body was bathed with
light throughout the day, but civilization has gone to the other extreme
of covering the body with clothing which keeps most of it in darkness.
Inasmuch as light and the invisible radiant energy which is associated
with it are known to be very influential agencies in a multitude of
ways, the question arises: Has this shielding of the body had any marked
influence upon the human organism? Although there is a vast literature
upon the subject of light-therapy, the question remains unanswered,
owing to the conflicting results and the absence of standardization of
experimental details. In fact, most investigations are subject to the
criticism that the data are inadequate. Throughout many centuries light
has been credited with various influences upon physiological processes
and upon the mind. But most of the early applications had no foundation
of scientific facts. Unfortunately, many of the claims pertaining to the
physiological and psychological effe