Henri Becquerel was interested in uranium minerals which were used at the time to make fluorescent glasses :
Having put in a drawer a sample of this mineral by chance on a photographic plate wrapped in black paper and not yet exposed, he realized that a radiation from uranium had impregnated the plaque. As the fluorescence could not pass through the paper he concluded at a new type of radiation!
Marie Curie, the pupil student of Henri Becquerel, together with her husband Pierre, realized that this radiation could not come from uranium, since for equal contents of uranium, the intensity could be very different. She concluded at a new chemical element. She found by chemical analysis that this element possesses properties similar to barium (so it is an alkaline earth). She called it radium (Ra). The difference in solubility between radium chloride and barium chloride allowed her to separate it from the latter.
