Iron was known and used by ancient civilizations. One of the most abundant metals (41000 ppm in the earth's crust), iron is probably one of the most important, being used on the largest scale of any metal. Its production in the blast furnace is well documented. When pure, iron is a lustrous white metal which is soft and very workable. However, it is reactive and easily forms a coating of hydrated oxide on its surface in the presence of moist air. This is non-coherent and flakes easily to reveal fresh surfaces for attack. Iron is soluble in dilute acids, where Fe(II) is produced in solution; more oxidising acids produce Fe(III) solutions and strongly oxidising agents (e.g. dichromate or concentrated nitric acid) produce a passive form of the metal, probably as a result of the formation of a coherent surface film of oxide. Depending upon the temperature, pure iron can exist in three forms, namely alpha-, gamma- and delta-iron; alpha iron is a polymorphic form of iron which is stable below 906C; it has a body centred cubic lattice (bcc) and is magnetic up to 768C. Gamma iron is a polymorphic form of iron which is stable between 906C and 1403C; it has a face centred cubic lattice (fcc) and is nonmagnetic (n.b. its range of stability is reduced by the presence of carbon, manganese and nickel and it is the basis of the austenite solid solutions). Delta iron is the polymorphic form of iron which is stable between 1403C and the melting point; it has the same lattice structure as alpha iron. Iron is the basis for many types of steel, the properties being achieved by the alloying of iron with carbon, nickel, chromium and other elements in varying proportions which results in materials with vastly differing mechanical and physical properties. Iron is also an essential element for all life forms, the average human body containing 4 gm of the element. The majority of iron in the body is present within haemoglobin, the respiratory pigment which provides the mechanism for the transportation of oxygen by red blood cells.Tube - A hollow length of material normally circular in section. Most tubes are straight except those made of flexible polymer.