MRS. B:...There is... a beautiful green salt too curious to be omitted; it is produced by the combination of cobalt with muriatic acid, which has the singular property of forming what is called sympathetic ink. Characters written with this solution are invisible when cold, but when a gentle heat is applied, they assume a fine bluish green colour.

Conversations on Chemistry In Which the Elements of that Science Are, Familiarly Explained and Illustrated by Experiments (Volume 1), Conversation X (p. 159), Oliver D. Cooke, 1822