an elementary substance, not metallic in its nature, which is present in all organic compounds. atomic weight 11.97. symbol c. it is combustible, and forms the base of lampblack and charcoal, and enters largely into mineral coals. in its pure crystallized state it constitutes the diamond, the hardest of known substances, occuring in monometric crystals like the octahedron, etc. another modification is graphite, or blacklead, and in this it is soft, and occurs in hexagonal prisms or tables. when united with oxygen it forms carbon dioxide, commonly called carbonic acid, or carbonic oxide, according to the proportions of the oxygen; when united with hydrogen, it forms various compounds called hydrocarbons. compare diamond, and graphite.