h2so3

Sulphurous \Sul"phur*ous\, a. [L. sulphurosus, sulfurosus: cf.
   F. sulfureux.]
   1. Of or pertaining to sulphur.
      [1913 Webster]

   2. (Chem.)
      (a) Derived from, or containing, sulphur; specifically,
          designating those compounds in which the element has a
          lower valence as contrasted with the sulphuric
          compounds.
      (b) Having the characteristic odor of sulphur dioxide, or
          of hydrogen sulphide, or of other sulphur compounds.
          [1913 Webster]

   {Sulphurous acid}.
      (a) Sulphur dioxide. See under {Sulphur}. [Obs.]
      (b) An acid, {H2SO3}, not known in the free state except
          as a solution of sulphur dioxide in water, but forming
          a well-known series of salts (the sulphites).

   {Sulphurous anhydride} (Chem.), sulphur dioxide. See under
      {Sulphur}.
      [1913 Webster]