Osmious \Os"mi*ous\, 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]