Petroleum, an flammable liquid which occurs naturally in the earth in various localities in Central Europe, the Caucasus, and in the United States. Such liquids have been known from early times, and were employed by the Chinese at a very remote period, while mention of them is also made by several of the classical writers. The liquid is usually obtained from its sources by boring deep wells or bore-holes. In some cases the petroleum is forced up by natural pressure, and issues in a jet at the top of the bore-hole. Usually, however, the pressure soon sinks and the petroleum has to be pumped out artificially. The chief sources now are in the United States, notably in Pennsylvania, where the industry developed very rapidly from 1861 to 1865. Chemically petroleum consists chiefly of a mixture of hydrocarbons of the series known as the paraffins (q.v.). Other compounds of carbon and hydrogen are, however, frequently present. The crude liquor is purified by careful distillation. As obtained it has a specific gravity of about '75. When it is distilled, portions are separately collected. First, those portions that boil over below 65° Fahr. with specific gravity -62, then those that boil between 65° and 100° having a specific gravity of about '66, etc. In this way a number of separate fractions are obtained known under the names of rhigolene, gasolene, naphtha, etc. The lower boiling portions are not much used as illuminants, but are very largely employed as solvents for gums, caoutchouc, etc. The intermediate portions are largely used for illumination and fuels, the highest portions as fuels and lubricants, while a quantity solidifies on cooling, yielding solid paraffin.