Uranium radiations

While studying the fluorescent substance potassium uranyl sulfate, Becquerel thought sunlight would make the crystal fluoresce and emit x-rays. He accidentally found that no sunlight was needed, and that the crystal was radiating x-rays on its own.
 Antoine-Henri Becquerel France 1896 Uranium radiations

While studying the fluorescent substance potassium uranyl sulfate, Becquerel thought sunlight would make the crystal fluoresce and emit x-rays. He accidentally found that no sunlight was needed, and that the crystal was radiating x-rays on its own.