*cobaltous*

a. (Chem.) Pertaining to, derived from, or containing, cobalt; -- said esp. of cobalt compounds in which the metal has its lower valence. [ 1913 Webster ]

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. [ 1913 Webster ]