Source : Webster's Revised Unabridged Dictionary (1913)

Cobaltous \Co*balt"ous\, a. (Chem.)
   Pertaining to, derived from, or containing, cobalt; -- said
   esp. of cobalt compounds in which the metal has its lower
   valence.

   {Cobaltous chloride}, a crystalline compound, {CoCl2}, of a
      pale rose color when hydrous, blue when dehydrated. Its
      solution is used for a sympathetic ink, the writing being
      nearly colorless when dried in the air, owing to absorbed
      moisture, and becoming bright blue when warmed.