%osmious%

a. (Chem.) 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. ] [ 1913 Webster ]

Osmious acid (Chem.), an acid derived from osmium, analogous to sulphurous acid, and forming unstable salts. It is a brown amorphous substance. [ 1913 Webster ]