In another example, osmium tetroxide is used to detect fingerprints and to stain fatty tissue for optical and electron microscopy. Osmium occurs in the platinum-bearing river sands in North America, South America, and in the Ural Mountains in Russia. The latter, in fact, is known to be the site of the second largest alluvial deposit, which today is still mined. There are three known kinds of gold standard that have been adopted since the early 1700s - the gold specie, gold exchange, and gold bullion standards. Following is the definition and a brief historical account of each. Gold Specie Standard: In this gold standard, the unit of currency is linked to the gold coins that are in circulation. Other uses of iridium include the following: 1. Electrical contacts for spark plugs (due to its resistance to arc erosion); 2. Computer memory devices; 3. Direct-ignition engine (as a catalyst); 4. Radiotherapy (as a source of radiation); 5. X-ray telescopes. In 2007, worldwide demand for iridium reached 3,701 kilograms (119,000 troy ounces). They have higher melting points compared with other metals. The high-ductility property of gold and silver means they can be easily damaged as coins for circulation. Coins intended for circulation must be highly resistant to corrosion and wear. For this reason, gold or silver must be alloyed with other metals (example, manganese) so that the resulting coins will come out harder, more wear-resistant, and not easily damaged or deformed. Owning a silver certificate: In lieu of storing actual silver bullion, an investor may opt for ownership of a silver certificate. A silver certificate allows an investor to buy and sell the security sans the inconveniences associated with the physical silver's transfer. The Perth Mint Certificate Programme, which is fully guaranteed by the Government of Western Australia, is the only silver certificate program in the world that is guaranteed by a national government. Rhodium - VIIIb/5; Rh -> Rh2+ + 2 e-; 0.600 V 3. Palladium - VIIIb/5; Pd -> Pd2+ + 2 e-; 0.987 V 4. Silver - Ib/5; Ag -> Ag+ + e-; 0.7996 V 5. Osmium - VIIIb/6; Os + 4 H2O -> OsO4 + 8 H+ + 8 e-; 0.838 V 6. Iridium - VIIIb/6; Ir -> Ir3+ + 3 e-; 1.156 V 7. Platinum - VIIIb/6; Pt -> Pt2+ + 2 e-; 1.18 V 8.