The material Henri Becquerel worked with was potassium uranyl sulfate. He exposed it to sunlight and placed on photographic plates wrapped in black paper. When it was developed, the plates revealed an image of the uranium crystals. Finally, Becquerel concluded "that the phosphorescent substance in question emits radiation which penetrates paper opaque to light". Initially, he believed that the sun's energy was being absorbed by the uranium which then emitted x-rays.
In the same year on the 26th and 27th of Feburuary, an investigation was delayed because the skies over Paris were overcast and the uranium-covered plates, which were to be exposed to the sun, were returned to a drawer. In the beginning of March, he took the plates out, expecting them to have developed only faint images to appear. To his surprise, the images were clear and strong. This meant that the uranium emitted radiation without an external source of energy such as the sun. He had just discovered radioactivity, the spontaneous emission of radiation by a material!
