Many varieties of petroleum, and perhaps all, become thicker by exposure to the air, and finally solid, resembling asphaltum. Bitumen, and doubtless petroleum also, was known from the earliest ages, being the "pitch" which Noah used in building the Ark, and the "slime" used for mortar in the Tower of Babel, being dug from pits in the Valley of Sodom, precisely as is done in the same region at the present day, where the Arabs annually extract considerable quantities
The fluid petroleum has been collected in Burmah for at least 16 centuries. It is used by the inhabitants for light and fuel. The product obtained at the present time, from 520 wells, is said to be 420,000 hogsheads annually. In the United States, petroleum is not, as many suppose, a new discovery. Years ago springs of it were known at many localities, but its use was very limited. No method of purifying it was known, so that it was looked upon as valueless, and several wells bored for salt water were abandoned on account of the oil rendering the salt impure. In 1861 it was purified, and introduced extensively as an illuminating oil, to take the place of burning fluid (camphene and alcohol), the price of which was greatly enhanced, and which, by the explosive qualities of its vapor, was causing many severe accidents. The trade increased, new wells were bored, and some of them yielding several hundred barrels a day, and making their possessors at once wealthy, started what has been known as the oil fever. Lands sold for fabulous prices, sometimes for 500 times as much as 2 or 3 years before.
Petroleum has probably been formed by a slow decomposition of organic matter under the earth's surface. It is found in cavities and crevices, and through the substance of the rock. In mining