The Collaborative International Dictionary of English v.0.48:

Chromous \Chro"mous\, a.
   Of, pertaining to, or derived from, chromium, when this
   element has a valence lower than that in chromic compounds.
   [1913 Webster]

   Chromous acid, a bluish gray powder, CrO.OH, of weak acid
      properties and regard as an acid.
      [1913 Webster]