OSMIOUS
Adjective
osmious (not comparable)
(chemistry) Denoting those compounds of osmium in which the element has a valence relatively lower than in the osmic compounds.
Source: Wiktionary
Os"mi*ous, a. (Chem.)
Definition: Denoting those compounds of osmium in which the element has a
valence relatively lower than in the osmic compounds; as, osmious
chloride. [Written also osmous.] Osmious acid (Chem.), an acid
derived from osmium, analogous to sulphurous acid, and forming
unstable salts. It is a brown amorphous substance.
Source: Webster’s Unabridged Dictionary 1913 Edition