Hantzel has communicated to the Saxon Royal Society of Science some
interesting observations on the production of electricity by light
in colored fluor-spar. The centers of the fluor-spar cubes become
negatively electric by the action of light. The electric tension
diminishes toward the edges and angles, and frequently positive polarity
is produced there. With very sensitive crystals a short exposure to
daylight is sufficient; by a long exposure to light the electric current
increases. The direct rays of the sun act much more powerfully than
diffused daylight, and the electric carbon light is more powerful even
than sunlight. The photo-electric action of light belongs principally
to the "chemically active" rays; this is shown by the fact that the
production of electricity is extremely small behind a glass colored with
cuprous oxide, and behind a film of a solution of quinine sulphate;
while it is not appreciably diminished by a film of a solution of alum.
The photo-electric excitability of fluor-spar crystals is increased by a
moderate heat (80° to 100° C.).
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