[L, from Gk. petra, a rock; and oleum, oil.] Rock-oil, an inflammable liquid which exudes from the earth in various parts of the world. Petroleum has been known since the most ancient times, but it is only recently that its importance as a commercial production has been discovered. It is found in great quantities in the United States and at Baku, Russia, and in smaller quantities in several other countries. The oil is generally got by sinking deep holes, called wells, into the earth. In some of these wells the oil rises up and flows over, being forced out by a kind of gas; but in others the oil has to be pumped out. In the oil-region in Pennsylvania there are now several thousand wells, some of which are more than a thousand feet deep. There is always a good deal of what is known as natural gas associated with petroleum. This gas is made up of carbon and hydrogen, and burns very brightly. It is carried in pipes to neighboring towns and used for domestic and manufacturing purposes. At one time Pittsburg used 500,000,000 cubic feet daily in its factories and houses.

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The oil from the wells flows into large tanks, from which it is carried in iron pipes to the shipping places and places where it is to be refined or purified. There are more than two thousand miles of these pipes laid in the Pennsylvania oil-region, and they reach from thereto Philadelphia. At the refineries the oil is distilled and separated into oil for illuminating purposes, commonly called kerosene oil; naphtha, used in making oil-cloths, and sometimes as a burning fluid; benzine, used in making paints and varnishes; gasolene, used for making gas and for mixing with coal gas. (See Naphtha and Paraffin.)