A brittle, silvery-white, rare metallic element usually found in combination with gold and other metals, produced commercially as a byproduct of the electrolytic refining of copper and used in compact discs, semiconductors, ceramics, and blasting caps and (in the form of bismuth telluride) in thermoelectric devices. In alloys it improves the machinability of stainless steel or copper, and increases the durability and hardness of lead. Atomic number 52; atomic weight 127.60; melting point 449.5°C; boiling point 988°C; specific gravity 6.23 (20°C); valence 2, 4, 6. See Periodic Table.