The boiling point of metals is much higher than the considered parameter. Mercury can then enter into the human food chain in the form of methylmercury. From the 6th or 5th century BC, the Chu (state) circulated the Ying Yuan, one kind of square gold coin. The melting point of a substance depends on pressure and is usually specified at standard pressure. Many cyanide spills[124] from gold mines have occurred in both developed and developing countries which killed aquatic life in long stretches of affected rivers. [8] This is equal to a cube with each side measuring roughly 21.7 metres. Gold does not react with oxygen at any temperature[33] and, up to 100 °C, is resistant to attack from ozone.[34]. Gold is unaffected by most acids. Gold is metallic, with a yellow colour when in a mass, but when finely divided it may be black, ruby, or purple. Christie, A and Brathwaite, R. 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The purity of a gold bar or coin can also be expressed as a decimal figure ranging from 0 to 1, known as the millesimal fineness, such as 0.995 being nearly pure. Gold ions in solution are readily reduced and precipitated as metal by adding any other metal as the reducing agent. For a long period, the United States government set the value of the US dollar so that one troy ounce was equal to $20.67 ($0.665 per gram), but in 1934 the dollar was devalued to $35.00 per troy ounce ($0.889/g). From April 2001 to August 2011, spot gold prices more than quintupled in value against the US dollar, hitting a new all-time high of $1,913.50 on 23 August 2011,[151] prompting speculation that the long secular bear market had ended and a bull market had returned. Because of the softness of pure (24k) gold, it is usually alloyed with base metals for use in jewelry, altering its hardness and ductility, melting point, color and other properties. The second-largest producer, Australia, mined 300 tonnes in the same year, followed by Russia with 255 tonnes. A number of people have claimed to be able to economically recover gold from sea water, but they were either mistaken or acted in an intentional deception. The Second Boer War of 1899–1901 between the British Empire and the Afrikaner Boers was at least partly over the rights of miners and possession of the gold wealth in South Africa. These gold ore dumps are long term, highly hazardous wastes second only to nuclear waste dumps.[127]. In 2007 China (with 276 tonnes) overtook South Africa as the world's largest gold producer, the first time since 1905 that South Africa has not been the largest.[106]. 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