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.