The Collaborative International Dictionary of English v.0.48:

Stannous \Stan"nous\ (-n[u^]s), a. (Chem.)
   Pertaining to, or containing, tin; specifically, designating
   those compounds in which the element has a lower valence as
   contrasted with stannic compounds.
   [1913 Webster]

   Stannous chloride (Chem.), a white crystalline substance,
      SnCl2.(H2O)2, obtained by dissolving tin in hydrochloric
      acid. It is used as a mordant in dyeing.
      [1913 Webster]