Before1840architectsandbuildersmadebuildingsfromstone,b...

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Before 1840 architects and builders made buildings from stone, brick, or wood. The outside walls held up the weight of the building. A building that had more than a few stories became very heavy. A tall building, such as a castle, had very thick walls.

As time went by, architecture began to change. By 1850 builders had begun to use a metal called iron for the fronts of some large buildings. Iron is lighter than stone and stronger than wood. people dig raw iron from the ground, but raw iron is not enough to use in buildings. Builders must use another type of iron called an alloy.

An alloy is a metal that contains different metals and other compounds, or mixtures of chemicals. The metals and compounds are melted and mixed together to form the alloy. To make an iron alloy, workers use a blast furnace. This machine creates great heat. It has a tall chamber that is lined with brick and open at the top. Workers put limestone, raw iron, and coke (a substance made from coal) into the top of the furnace. They force hot air into the bottom of the furnace. The hot air and a chemical reaction turn the materials into a liquid mass. Workers pour off waste liquid, called slag, and save the iron. Then they form the hot iron into large blocks called pigs, which can be formed into objects. This formed iron is called cast iron.

Cast iron is strong, but it still contains unwanted materials. People wanted to remove these impurities. To do this, they needed to heat the iron to very high temperature. Blast furnaces could not make that much heat.

In England in 1855, Sir Henry Bessemer found the solution. He forced air through liquid pig iron. The oxygen in the air mixed with the impurities in the iron in a process called oxidation. Oxidation creates heat. The heat turns impurities into slag, which is poured off so that only pure steel remains. Pure steel is much stronger than iron. The Bessemer process took place in an egg-shaped machine called a converter. With this process, workers began to make large amounts of steel quickly and cheaply.

By the late 1800s, many people lived in big cities. The price of land increased, one way builders could lower costs was to build taller buildings, which used less land. In Chicago in 1889, George A. Fuller completed the Tacoma Buildings, which had steel beams to hold up its weight. Soon people built more tall buildings with 10 to 20 stories. These were the first skyscrapers.

50. Alloys are important because they ________.

A. have no impurities

B. are the raw materials that people use to make iron

C. may be more useful than the separate metals in the alloy

D. contain different metals

51. The Bessemer process was a success because it produced __________.

A. iron from steel

B. steel quickly and cheaply

C. less slag than a blast furnace

D. impurities in the iron

52. Sir Henry Bessemer probably __________.

A. avoided difficult problems

B. created slag into pure steel

C. was not familiar with metals such as steel

D. knew how to make cast iron

53. From the article, you can conclude that architects throughout history __________.

A. thought carefully about materials when they designed buildings

B. could not design creative buildings until steel was invented

C. disliked stone, brick, and wood

D. liked to design tall buildings

54. Which of the statements below expresses the main idea of the article?

A. Discoveries about iron and steel have allowed people to build taller buildings.

B. Sir Henry Bessemer developed a process to make steel.

C. Architects use many kinds of materials in buildings.

D. Changes about designing building.

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BDAA

知识点：阅读理解

题型：阅读理解