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Petroleum

Petrolem，like coal，is found in sedimentary rocks，and was probably formed form long-dead living organisms.The rocks in which it is found are almost always of ocean origin and the petroleum-forming organisms must have been ocean creatures rather than trees.

Instead of originating in accumulating woody matter，petroleum may be the product of the acumulating fatty matter of ocean organisms such as plankton，the myriads of single-celled creatures that float in the surface layers of the ocean.

The fat of living organisms consists of atom combinations that are chiefly made up of carbon and hydrogen atoms.It does not take much in the way of chemical change to turn that into petroleum.It is only necessary that the organisms settle down into the ooze underlying shallow arms of the ocean under conditions of oxygen shortage.Instead of decomposing and decaying，the fat accumulates，is trapped under further layers of ooze，undergoes minor rearrangements of atoms，and finally is petroleum.

Petroleum is lighter than water and，beng liquid，bends to ooze upward through the porous rock that covers it.There are regions on Earth where some reaches the surface and the ancients spoke of pitch，bitumen，or asphalt.In ancient and medieval times，such petroleum seepages were more often looked on as medicines rather than fuels.

Of course，the surface seepages are in very minor quantities.Petroleum stores，however，are sometimes overlain with nonporous rock.The petroleum seeping upward reaches that rock and them remains below it in a slowly acumulating pool.If a hole can be drilled through the rock overhead，the petroleum can move up through the hole.Sometimes the pressure on the pool is so great that the petroleum gushes high into the air.The first successful drilling was carried through in 1859 in Titusville，Pennsylvania，by Edwin Drake.

If one found the right spot then it was easy to bring up the liquid material.It was much easier to do that than to send men underground to chip out chunks of solid coal.Once the petroleum was obtained，it could be moved overland through pipes，rather than in fright trains that had to be laboriously loaded and unloaded，as was the case with coal.

The convenience of obtaining and transporting petroleum encouraged its use.The petroleum could be distilled into separate fractions，each made up of molecules of a particular size.The smaller the molecules，the easier it was to evaporate the fraction.

Through the latter half of the nineteenth century，the most important fraction of petroleum was“kerosene，”made up of middlesized molecules that did not easily evaporate.Kerosene was used in lamps to give light.

Toward the end of the nineteenth century，however，engines were developed which were powered by the explosions of mixtures of air and inflammable vapors within their cylinders.The most convenient inflammable vapor was that derived from“gasoline，”a petroleum fraction made up of small molecules and one that therefore vaporized easily.

Such“internal combustion engines”are more compact that earlier steam engines and can be made to start at a moments’ notice，whereas steam engines require a waiting period while the water reserve warms to be boiling point.

As automobiles，trucks，buses，and aircraft of all sorts came into use，each with internal combustion engines，the demand for petroleum zoomed upward.Houses began to be heated by burning fuel oil rather than coal.Ships began to use oil；electricity began to be formed from the energy of burning oil.

In 1900，the energy derived from burning petroleum was only 4 percent that of coal.After World WarⅡ，the energy derived from burning the various fractions of petroleum exceeded that of coal，and petroleum is not the chief fuel powering the world’s technology.

The greater convenience of petroleum as compared with coal is，however，balanced by the fact that petroleum exist on Earth in far smaller quantities than coal does.（This is not surprising，since the fatty substances from which petroleum was formed are far less common on Earth than the woody substances from which coal was formed.）

The total quantity of petroleum now thought to exist on Earth is about 14 trillion gallons.In weight that is only one-ninth as much as the total existing quantity of coal and，at the present moment，petroleum is being used up much more quickly.At the present rate of the use，the world’s supply of petroleum may last for only thirty years or so.

There is another complication in the fact that petroleum is not nearly so evenly distributed as coal is.The major consumers of energy have enough local coal to keep going but are，however，seriously short of petroleum.The United Stated has 10 percent of the total petroleum reserves of the world in its own territory，and has been a major producer for decades.It still is，but its enormous consumption of petroleum products is now making it an oil importer，so that it is increasingly dependent on foreign nations for this vital resource.The Soviet Union has about as much petroleum as the United States，but it uses less，so it can be an exporter.

Nearly three-fifths of all known petroleum reserves on Earth is to be found in the territory of the various Arabic-speaking countries.Kuwait，for instance，which is a small nation at the head of the Persian Gulf，with an area only three-fourths that of Massachusetts and a population of about half a million，possesses about one-fifth of all the known petroleum reserves in the world.

The political problems this creates are already becoming crucial.

28?Petroleum is unlike coal in the way.

A.petroleum is found in sedimentary rocks and was probably formed from long-dead living organisms.

B.once the petroleum was obtained，it could be moved overland in freight trains.

C.petroleum is not nearly so evenly distributed as coal is.

D.petroleum exists on Earth in far greater quantities than coal does.

29?The use of petroleum is greatly encouraged by.

A.the fact that petroleum is lighter than water

B.the fact that petroleum is the produce of the accumulating fatty matter of ocean organisms.

C.the fact that obtaining and transporting petroleum is very convenient.

D.the fact that the energy derived from burning petroleum is only 4 percent that of coal.

30?Which of the following is a petroleum fraction made up of small molecules and one that therefore vaporized easily？

A.kerosene. B.gasoline C.asphalt D.vapor