After Marie Curie discovered polonium in the bismuth salt component of pitchblende residue in July 1898, she discovered that barium salts were more radioactive in the more insoluble sulfates. In December of the same year, she predicted the existence of the new element radium. It took her 4 years to extract 0.1 grams of radium chloride crystals from several tons of slag. Use sodium carbonate to convert insoluble sulfate into carbonate, remove SO4--, and then use hydrochloric acid to convert into soluble chloride. Taking advantage of the difference in solubility (BaCl2 1041g/L RaCl2 706 g/L), fractional crystallization was used for purification, and the long-cherished wish was achieved after untold hardships. Later, RaCl2 was used to electrolyze elemental radium.