Which uranium compound was used in Becquerel's accidental dark-drawer experiment?

The uranium compound used in Henri Becquerel’s accidental dark-drawer experiment was potassium uranyl sulfate. In February 1896, Becquerel was investigating whether fluorescent uranium salts might emit X-ray-like radiation after exposure to sunlight. He placed potassium uranyl sulfate crystals and photographic plates together, wrapping the plates in protective black paper.

Cloudy weather prevented the planned sunlight exposure, so Becquerel stored the prepared materials in a drawer. When he later developed the plates, they were strongly darkened despite the crystals having remained in darkness. The result showed that the uranium compound emitted penetrating radiation spontaneously rather than merely releasing stored sunlight.

The experiment is often described as the accidental discovery of radioactivity, although Becquerel initially called the emission “uranic rays.” The radiation was not the same as X-rays, and later work showed that it originated from uranium atoms. Marie Curie subsequently studied the phenomenon systematically and coined the term “radioactivity.”