\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.