iRIDIUM. 7 3. Iridium. Iridium, Wollaston. Philosophical Transactions for 1805. External Characters. Its colour is very pale steel-grey. It occurs in very small irregular flat grains; The lustre is shining and metallic. The fracture is foliated. It is brittle. It is harder than platina. Specific gravity, 19.5. Chemical Characters. By fusion with nitre, it acquires a dull black colour, but recovers its original colour and lustre by heating with Charcoal. Constituent Parts. It is composed of Osmium and Iridium, alloyed toge ther. Geognostic and Geographic Situations. It occurs in alluvial soil in South America, along with platina. Observations. It was first examined, and introduced to the notice of mnturalists, by Dr'Wollaston. * 4 II. ORDER