HgO

Precipitate \Pre*cip"i*tate\, n. [NL. praecipitatum: cf. F.
   pr['e]cipit['e].] (Chem.)
   An insoluble substance separated from a solution in a
   concrete state by the action of some reagent added to the
   solution, or of some force, such as heat or cold. The
   precipitate may fall to the bottom (whence the name), may be
   diffused through the solution, or may float at or near the
   surface.
   [1913 Webster]

   2. atmospheric moisture condensed as rain or snow, etc.; same
      as {precipitation[5]}.
      [PJC]

   {Red precipitate} (Old. Chem), mercuric oxide ({HgO}) a heavy
      red crystalline powder obtained by heating mercuric
      nitrate, or by heating mercury in the air. Prepared in the
      latter manner, it was the {precipitate per se} of the
      alchemists.

   {White precipitate} (Old Chem.)
      (a) A heavy white amorphous powder ({NH2.HgCl}) obtained
          by adding ammonia to a solution of mercuric chloride
          or corrosive sublimate; -- formerly called also
          {infusible white precipitate}, and now {amido-mercuric
          chloride}.
      (b) A white crystalline substance obtained by adding a
          solution of corrosive sublimate to a solution of sal
          ammoniac (ammonium chloride); -- formerly called also
          {fusible white precipitate}.
          [1913 Webster]