Silver was known to ancient civilisations. Silver is a soft, malleable metal with a characteristic sheen. It has the highest thermal and electrical conductivities of all metals. It is generally found uncombined, or in the sulphide or arsenide ores from which it can be recovered as a cyanide complex which is subsequently reduced to the metal, in aqueous solution, by the use of zinc. The pure metal is stable to water and oxygen but is attacked in air by sulphur bearing compounds to form the characteristic black layer of silver sulphide. It is soluble in sulphuric and nitric acids. Some silver salts are sensitive to light (e.g. AgI, AgCl and AgBr) and are of fundamental importance to photography. Other applications and industries in which silver is used include the manufacture of jewellery (both as the pure metal and as a constituent of various alloys), the electrical industry (e.g. in the manufacture of contacts) and for the silvering of glass.Tube - A hollow length of material normally circular in section. Most tubes are straight except those made of flexible polymer.