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说明晶体结构为何不存在5次或高于6次的对称轴
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第三篇 Oil Refining An important new industry, oil refining, grew after the civil war. Crude oil, or petroleum --- a dark, thick ooze from the earth --- had been known for hundreds of years. But little use had ever been made of it. In the 1850’s Samuel M. Kier a manufacturer in western Pennsylvania, began collecting the oil from local seepages (油苗) and refining it into kerosene. Refining, like smelting is a process of removing impurities from a raw material. Kerosene was used to light lamps. It was a cheap substitute for whale oil, which was becoming harder to get. Soon there was a large demand for kerosene. People began to search for new supplies of petroleum. The first oil well was drilled by E.L.Drake, a retaired railroad conductor. In 1859 he began drilling in Titusvill, Pennsylvania. The whole venture seemed so impractical and foolish that onlookers called it “ Drake’s Folly”. But when he had drilled down about 70 feet (21 meters ), Drake struck oil and his well began to yield 20 barrels of crude oil a day. News of Drake’s success brought oil prospectors to the scene. By the early 1860’s these wildcatters were drilling for “black gold “ all over western Pennsylvania. The boom rivaled the California gold rush of 1848 in its excitement and Wild West atmosphere. And it brought far more wealth to the prospectors than any gold rush. Crude oil could be refined into many products. For some years kerosene continued to be the principal one. It was sold in grocery stores and door-to-door. In the 1880’s and 1890’s refiners learned how to make other petroleum products such as waxes and lubricating oils. Petroleum was not then used to make gasoline or heating oil.
The council are going to merge our school in the big new county school.
This proved an important contribution toward the public health.
The truck gained momentum as it rolled down the steep road.
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