填空题

Iron production was revolutionized in the early eighteen century when coke was first used (36) of charcoal for refining iron ore. Previously the poor (37) of the iron had restricted its use in architecture to items such as chains and tie bars for supporting arches, vaults, and walls. With the (38) in refining ore, it was now possible to make cast-iron beams, columns, and girders. During the nineteenth century further (39) were made, notably Bessemer’s process for converting iron into steel, which made the material more commercially viable. Iron was rapidly (40) for the construction of bridges, because its (41) was far greater than that of stone or timber, but its use in the architecture of buildings developed more slowly. By 1800 a complete internal iron skeleton for buildings had been developed in (42) architecture replacing traditional timber beams, but it (43) remained concealed. Apart from its low cost, the appeal of iron as a building material lay in its strength, its resistance to fire, and its potentialtospan vast areas. As a result, (44) , but it was invariably concealed. Significantly, the use of exposed iron occurred mainly in the new building types spawned by the Industrial Revolution: in factories, warehouses, commercial offices, exhibition hall, and railroad stations, (45) .Iron, despite its structural advantages, had little aesthetic status. (46) .