_Iridium_, when pure, is of small value, being difficult to manipulate on account of its brittleness. It is principally used in alloy with platinum for hardening purposes. Jewellers’ platinum usually contains 10 per cent. iridium, and in the electrical industry an alloy composed of from 15 to 50 per cent. iridium is usually employed. Alloys with up to 10 per cent. of iridium are ductile and malleable, but with over that amount are hard and difficult to work. The metal is used in the manufacture of fountain-pen points, for which purpose the grains require careful selecting[8] p. 106; also for standard weights and for contact points. Iridium black, an oxide, is of value as a pigment for chinaware[1] p. 1001. Iridium is suitable for sharp surgical instruments, and gold needles with soldered iridium ends are employed for stitching wounds. It is also used in photography. Iridium is of greater scarcity than platinum, hence its greater value. _Osmium._—There is now little or no market for osmium. It was formerly in considerable use for the manufacture of incandescent lamps. Osmic acid is used for staining anatomical preparations in microscopic work. On account of the poisonous nature of its vapour the extraction of osmium is costly and dangerous. _Ruthenium_ is also of little or no commercial value. Both these metals possess the disadvantages of being brittle and easily oxidized. _Rhodium_ is of small commercial use. It is used principally in alloy with platinum. An alloy containing 10 per cent. rhodium is used for some thermo-couples of pyrometers, and in the making of laboratory utensils. _Platinum Alloys._—Platinum forms alloys with a number of metals, but only a few are of industrial importance[3] p. 400. Platinum and iridium form a hard and elastic alloy, which is unaffected by air, and takes a high polish. Alloyed with 10 per cent. iridium platinum is used for one of the wires in thermo-couples of pyrometers; and with 10 to 20 per cent. iridium for making standard measures of length and weight. Platinum and copper form various alloys. An alloy with 18·75 per cent. copper, called “coopers’ gold,” takes a high polish and closely resembles 18–carat gold. An alloy of platinum and silver containing 66 per cent. silver is used as a standard of electrical resistance. An alloy containing 20 to 30 per cent. silver is used in dentistry. Platinum alloys with lead, zinc and other metals at low temperatures; it is usually recovered from these alloys by cupellation. Platinum alloys with steel in all proportions. With 10 per cent. platinum, rusting is prevented. A very elastic metal is produced by alloying platinum with from 5 to 10 per cent. gold. The melting-point of silver is raised by alloying it with platinum, but its thermal conductivity is lowered. The following table gives the composition of the principal platinum and palladium alloys[4] p. 561: