What property did Becquerel originally believe caused uranium to emit rays?

Becquerel originally believed that phosphorescence caused uranium to emit its mysterious rays.

In early 1896, Henri Becquerel was investigating phosphorescent materials after Wilhelm Röntgen announced the discovery of X-rays. Becquerel suspected that uranium salts might absorb sunlight and then release penetrating, X-ray-like radiation. His initial experiments therefore focused on whether the salts’ phosphorescence was responsible for fogging photographic plates.

A chance observation overturned that idea. On cloudy days, Becquerel stored uranium samples and wrapped photographic plates in darkness. When he developed the plates, they were strongly fogged even though the samples had not been exposed to sunlight. This showed that the uranium was emitting radiation spontaneously, without needing phosphorescent excitation.

The phenomenon was later called radioactivity, a term associated with Marie Curie’s work. Phosphorescence, fluorescence, and bioluminescence are all forms of light emission, but none is the correct explanation for uranium’s spontaneous radioactive emissions.