Strontium, chemical element, symbol Sr; for physical constants see Periodic Table. Strontium was discovered by Adair Crawford in 1790. In nature it is found as celestite and strontianite. Strontium is prepared by electrolysis of its fused chloride mixed with potassium chloride. Strontium is a silvery-white, soft, reactive metal, a member of the alkaline earth metals. It reacts with water more vigorously than calcium, and quickly oxides in air. Strontium-90, present in nuclear fallout, is a strong, long-lived beta emitter and is of potential use in nuclear-electric power devices. Strontium and its compounds are used in fireworks, flares, glass for color television picture tubes, and optical materials.