A soft, bright, silvery element of the lanthanide series occurring in three allotropic forms and used as an x-ray source for portable irradiation devices, as a dopant for laser materials, and in solar cells and some special alloys. Atomic number 70; atomic weight 173.05; melting point 824°C; boiling point 1,196°C; specific gravity 6.903 (alpha form), 6.966 (beta form); valence 2, 3. See Periodic Table.