A metal recently discovered, and contained in the ore of platinum. A native alloy of this metal with iridium is found in grains along the rivers in South America. osmium has a dark gray color; it is not volatile when heated in close vessels, but heated in the open air, it absorbs oxygen and forms a volatile oxyd. It is insoluble in the acids, readily soluble in potassa and very volatile. It takes its name from the singular smell of its oxyd.

A rare metallic element of the platinum group, found native as an alloy in platinum ore, and in iridosmine. It is a hard, infusible, bluish or grayish white metal, and the heaviest substance known. Its tetroxide is used in histological experiments to stain tissues. Symbol Os. Atomic weight 191.1. Specific gravity 22.477.