[Illustration: _Henri Becquerel_
Courtesy Journal of Chemical Education, Discovery of the Elements,
Mary Elvira Weeks.]
In 1896 the French physicist Henri Becquerel was investigating the
relation between fluorescence and X rays, a puzzling kind of penetrating
radiation discovered a few months earlier by the German, Wilhelm
Roentgen. Various chemical compounds fluoresce, or glow, when exposed to
ultraviolet rays and other types of radiation. While experimenting with
a large number of chemicals, Becquerel discovered, quite by accident,
that a compound containing the element uranium can, without being
exposed to any kind of radiation, darken a photographic plate completely
wrapped in heavy black paper.
Although no one realized it at the time, Becquerel had discovered that
atoms of some elements will at random times transform themselves into
atoms of a different element by emitting certain extremely high-speed
charged particles. Atoms that can do this are said to be radioactive,
and it was the radiation from transforming uranium atoms that darkened
Becquerel’s photographic plate.