The Collaborative International Dictionary of English v.0.48:

Mercurous \Mer*cu"rous\, a. (Chem.)
   Of, pertaining to, or derived from, mercury; containing
   mercury; -- said of those compounds of mercury in which it is
   present in its highest proportion.
   [1913 Webster]

   Mercurous chloride. (Chem.) See Calomel.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mercurous chloride
    n 1: a tasteless colorless powder used medicinally as a
         cathartic [syn: calomel, mercurous chloride]