Petroleum (Lat. petra, a rock, and oleum, oil), rock oil, a natural product of certain geological formations, sometimes rising to the surface through natural channels, forming springs, but chiefly obtained by boring. It was known to the ancient Greeks and Romans, and by Pliny, Tacitus, Vitruvius, and other Roman writers was designated bitumen, a word derived from the Greek irirra, nicca, pitch, and probably first written pitumen. Among the localities cited where the liquid bitumen was found is Zacynthus (now Zante), one of the Ionian islands. As it is referred to by Herodotus, this spring must have been flowing more than 2,000 years. At Agrigentum in Sicily the petroleum was collected and burned in lamps. (Diosco-rides, i. 99.) It was but little noticed during the middle ages, excepting in certain localities where it occurs in large quantities; but in modern times it has become one of the most important of natural productions. It occurs in rocks or deposits of nearly all geological ages, from the lower Silurian to the tertiary epoch. It is associated most abundantly with argillaceous shales and sandstones, but is found also permeating limestones, giving them a bituminous odor; and from these it often exudes, floating upon the streams and lakes of the region, or rising in springs.

It often exists in subterranean cavities, situated along gentle anticlinals in the barren rocks of the region, the oil having collected in them from the subjacent strata, and having been retained by the impervious overlying sandstones. If the oil existed already formed, a slight elevation of temperature or hydrostatic pressure would force it into these cavities or bring it to the surface; but if it was formed by destructive distillation from materials within the rocks, considerable heat would be necessary. - Petroleum is very widely distributed, yet there are a few localities especially noted for its occurrence, among which are the following: Amiano and other places in the north of Italy, which have furnished the supplies used for lighting the cities of Parma and Genoa; Baku on the borders of the Caspian; the slopes of the Caucasus; Rangoon in Burmah; the island of Trinidad; and portions of the province of Ontario, Pennsylvania, Ohio, New York, West Virginia, and California. In northern Italy, in the former duchies of Parma and Modena, petroleum has been extracted from the earth since its first discovery in 1640, the method pursued being merely to sink pits, and collect the fluid that exuded from the soil in little basins or reservoirs at the bottom.

Various sorts were gathered from different localities, and their peculiar properties appear to have been correctly observed; but no methods of purifying them were employed; on the contrary, the lighter and better oils were made the medium of utilizing the poorer sorts by mixture. The W. shore of the Caspian (see Baku) has been celebrated from a remote period for the extraordinary quantities of inflammable gases and liquids that rise from the surface of the ground. They are met with over a tract of country about 25 m. long and about 1/2 m. wide, in sjtrata of a porous argillaceous sandstone belonging to the tertiary period. In the vicinity are hills of volcanic rocks through which flow out springs of the heavier sorts of petroleum. The oil is collected by means of large open wells 16 to 20 ft. in depth. It is introduced very largely into Persia, and over large districts no other material is used for artificial light. During the past few years the attention of the Russian government has been directed to this possible source of mineral wealth in this region and the neighboring slopes of the Caucasus. An accomplished engineer was sent to the United States to investigate our methods of operation, and on his return numerous wells were bored in the vicinity of the city of Tiflis, which have proved remarkably productive, even rivalling those of Pennsylvania. The Rangoon district on the Ir-rawaddy is quite as wonderful for its immense production of rock oil as Baku. For an unknown period the whole Burman empire and a considerable portion of India have been supplied with oil from this source.

The trade is carried on by means of large boats that come up the Ir-rawaddy to the town of Rainanghoung, a place inhabited by potters, who are constantly making the earthen jars in which the oil is kept. These are piled up in great pyramids about the town, ready for use. The wells are in beds of sandy clays which rest on sandstones and argillaceous slates, and are sometimes sunk to the depth of 60 ft. Under the slates is said to be coal; but this and the other strata may be of the tertiary epoch. Symes ("Embassy to Ava," vol. ii.) says the number of wells in this district exceeded 520, and the annual yield of petroleum was more than 400,000 hogsheads. The natives use the oil in lamps, for preserving timber against insects, and as a medicine. Petroleum is found at several places in upper Burmah. In 1873 about 150 wells were worked at Yeynaugyoung, and about 50 at Pagau. The oil of the latter closely resembles naphtha. The annual production in this region is about 65,000 bbls. - The occurrence of petroleum about the head waters of the Alleghany river in New York and Pennsylvania was known to the early settlers. The Indians collected it on the shores of Seneca lake, and it was sold as a medicine by the name of Seneca or Genesee oil.

A stream in Allegany co., N. Y., was named Oil creek in consequence of the appearance of oil in its banks; and the same name was given to another branch of the Alleghany river in Venango co., Pa. At points along the latter springs issued from the banks of the stream, bringing up oil, which collected on the surface of the water as it stood in the pools below the springs. The inhabitants were accustomed to collect the oil by spreading woollen cloths upon the water, and wringing them when saturated. Down the valley of this creek are numerous ancient pits which appear to have been excavated for collecting oil, but by whom no one can now tell. From the fact that logs have been found in them notched as if with an axe, some have supposed that the work was done by the French, who occupied this region in the early part of the last century; others believe that the Indians, who are known to have valued the oil, dug the pits. Day, in his "History of Pennsylvania" (1844), gives ah account of the estimation in which they held this product, using it mixed with, paint to anoint themselves for war, and also employing it in their religious rites.

He quotes an interesting letter from the commander of Fort Duquesne to Gen. Montcalm, describing an assembly of the Indians by night on the banks of the creek, and in the midst of the ceremonies their firing the scum of oil that had collected upon the surface of the water. As the flames burst forth, illuminating the dark valley, triumphant snouts rose from the Indians. The quantities of oil collected by the early settlers were unimportant, the largest amount, which was from the lower spring on Oil creek, reaching sometimes 20 barrels in a year. When, in boring for salt near Tarentum, 35 m. above Pittsburgh on the Alleghany river, springs of petroleum were struck in 1845, the material was valued only as a medicine, and for this use it was long retailed in small quantities at high prices. In Ohio, on the Little Muskingum, the inhabitants narrowly missed learning the importance of this product as far back as the year 1819. Dr. S. P. Hildreth of Marietta, in the "American Journal of Science" (1826), speaking of the borings for salt water, says: "They have sunk two wells, which are now more than 400 ft. in depth; one of them affords a very strong and pure water, but not in great quantity.

The other discharges such vast quantities of petroleum, or as it is vulgarly called ' Seneka oil,' and besides is subject to such tremendous explosions of gas as to force out all the water and .afford nothing but gas for several days, that they make but little or no salt. Nevertheless the petroleum affords considerable profit, and is beginning to be in demand for lamps in workshops and manufactories. It affords a clear,.brisk light, when burnt in this way, and will be a valuable article for lighting the street lamps in the future cities of Ohio." It is not a little singular that, with the sources of supply thus pointed out and the useful application of the petroleum understood, its value should have remained unappreciated, and at the expiration of more than 35 years be at last perceived through the progress of experiments made upon the distillation of bituminous shales and coal. The success attending these, and the similarity of the crude oil to the natural petroleum, caused attention to be directed to the sources of this with the view of testing the capacity of the supplies, and applying to the natural oil the methods of purification invented for the artificial.

The first movement made in this direction was in 1854, by Messrs. Eveleth and Bis-sell of New York, who secured the right to the upper spring on Oil creek, and organized a company in New York. The quality of the oil was tested and a report made upon it by Prof. B. Silliman, jr. No progress was made in establishing the business until December, 1857, when Messrs. Bowditch and Drake of New Haven undertook to search for the oil. Ool. E. L. Drake removed to Titusville on Oil creek, and in the winter of 1858-9 completed his arrangements for boring into the rock below the bed of the creek. The work advanced very slowly, and it was not until Aug. 26,1859, that oil was struck at the depth of 71 ft. The drill suddenly sank into a cavity in the rock, and the oil rose within 5 in. of the surface. A small pump being introduced, a supply of oil was obtained, amounting to 400 gallons a day; and with a larger pump the flow was increased to 1,000 gallons a day. Though a steam engine was applied to the work and kept in constant operation, the supply continued uninterrupted for weeks. This success gave a new value to every spot where oil had ever been found or which was thought likely to produce it.

The narrow valleys of the watercourses, excavated 300 or 400 ft. through the piles of horizontal,strata, had been its natural outlets, and along these great numbers of wells were soon commenced. Oil creek below Titusville, the valley of the Alleghany from below Franklin up into Warren co., and the banks of French creek, were soon explored by wells, and around the most successful of these villages rapidly sprung up, and extraordinary business activity was introduced into regions that had been among the most retired and quiet portions of the state. Next to Oil creek the valley of the Alleghany, from Tidioute in Warren co. S. to the Venango line, contained the most productive wells, and others of great yield were opened in the town of Franklin. So numerous were these undertakings, that the village presented a curious aspect with the numbers of tall derricks, employed in boring the artesian wells, scattered among the gardens and house lots. Before the close of the year 1860, according to one published statement, the number of wells had amounted to full 2,000, and 74 of these were producing daily 1,165 barrels of 40 gallons each.

In Allegany co., N. Y., about a mile N; W. of the town of Cuba, operations were begun about the first of January, 1861, near a famous great pool, which had always been known as the oil spring. Before the iron pipe driven into the ground had reached the rock, oil mixed with water gushed violently up through it. On the margin of the" coal field in Trumbull co., Ohio, at a place called Mecca, 44 m. from Cleveland and 60 from Erie, Pa., wells were first sunk in the spring of 1860, the encouragement for making the trial consisting in the fact of the water in the wells being strongly impregnated with oil. In West Virginia wells have been successful in Eitchie and Wirt cos. In 1840 a spouting well of oil at Burkesville, Ky., was described, and in 1844 Mx. Murray mentioned the petroleum of Enniskillen, Canada. - Various opinions have been expressed concerning the origin of petroleum. Until quite recently, all of these theories were based upon the assumption that it has been derived from vegetable or animal organisms. Some have supposed that it is the product of the decomposition of woody fibre, by which more of the carbon and less of the hydrogen has been evolved than by the decomposition which has produced coal.

Again, it has been supposed to be the product of the natural distillation of pyrobituminous shales and coals. Lesque-reux attributes its origin to the partial decomposition of low forms of marine vegetation. Berthelot has advanced the theory that by complex chemical changes at present taking place in the interior of the earth, petroleum is being continually set free. It may be assumed that petroleum is the normal or primary product of the decomposition of marine animal or vegetable organisms, chiefly the former, and that nearly all other varieties of bitumen are products of a subsequent decomposition of petroleum, differing both in kind and degree. The occurrence of petroleum in the lower palaeozoic rocks of Pennsylvania and Canada, which contain no traces of land plants, shows that it has not in all cases been derived from terrestrial vegetation, but may have been formed from marine plants or animals; an opinion further strengthened when we find in rocks of tertiary age, in which fossil remains of the higher marine animals occur in abundance, a petroleum comparatively rich in nitrogen.

Such is the character of the petroleum issuing from the miocene of the coast ranges of southern California. In Trinidad a thick oil (maltha), with asphalt, occurs with lignite, and specimens of the vegetable material are found partly changed to oil and penetrated by it, and having its cells looking as if it had been corroded by it. Though we obtain oils resembling petroleum by the destructive distillation of coals, shales, and even animal substances, its occurrence is not confined to localities contiguous to large deposits of these substances. On the contrary, springs of it issue in different parts of the world from all the stratified rocks, and from the volcanic and metamorphic formations. In Ohio and West Virginia it is found in the coal measures, and the wells are in some instances sunk through these into the sandstones and slates before they become productive. In N. W. Pennsylvania or in New York the wells are entirely outside of the coal field, and so remote from it that we cannot well imagine any connection between the oil and the coal beds. The strata in which the oil is found dip S. and pass below the coal measures at least 500 or 600 ft., the nearest coal bed to the more northern springs occurring in the tops of the highest hills, perhaps 30 m. distant.

The conglomerate which underlies the coal formation caps some of the hills in the oil region, and attains a thickness of from 100 to 300 ft. The shales and sandstones that succeed below this rock belong to the Chemung and Portage groups of the New York geologists, and extend over a large portion of southern New York, west of Binghamton, and of northwestern Pennsylvania. The oil wells are bored in this group through alternating layers of shales and sandstones, and an occasional stratum of a bluish sandy limestone. The next group below is that known as the Hamilton shales in New York, and in Ohio as the black slate. Prof. Newberry considers this the source which affords the petroleum. It contains much carbonaceous matter, and is supposed by him to be amply sufficient for generating the supplies that are forced upward from it by the water that finds its way beneath the oil, and by the pressure of the carburetted hydrogen gas also furnished from the same source. It is from these slates that the oil springs of Ontario issue, and these are far distant from the coal formation. In southern California the oil issues from shales interstratified with coarse sandstones of enormous thickness, and nowhere containing coal.

In the northern part of that state it issues from serpentine and other metamorphic rocks of cretaceous age. Around volcanoes petroleum is often seen floating upon the surface of the water, as around the volcanic isles of Cape Verd; and to the south of Vesuvius a spring of it rises up through the sea. Springs also issue beneath the sea upon the coast of Venezuela, and in the Santa Barbara channel off the coast of southern California. Wells sunk near together vary in their yield, in the depth at which they become productive, and even sometimes, as in the Kanawha region of Virginia, in the quality of the oil they afford. - Petroleum from different localities varies considerably in character. The substance is ordinarily of a greenish hue by reflected light, and brown by transmitted light, more or less deep and opaque; but some varieties of light clear oils are reddish. The oils of northern Italy are of the latter class. At Baku it is observed that the oil from the central portion of the tract is clear and pure as if distilled, and by its faint yellow tint resembles Sauterne wine. That obtained nearer the sides of the tract is darker, gradually changing to a yellowish green, then reddish brown, and finally to asphalt.

Those of southern California are leek-green when taken fresh from the rock; but they soon become brown and black, changing to maltha and asphalt. The Canadian oils are black, but those of Pennsylvania, Ohio, West Virginia, Rangoon, and many other localities, are for the most part of the ordinary color. All have a disagreable, pungent odor. In the province of Ontario oil has been obtained remarkable for its garlicky odor. The quality of petroleums is indicated by their specific gravity, and this is taken by Baume's hydrometer, the higher degrees of which mark the lighter oils, such as are most esteemed. The best are some of those of Oil creek, of 46° B. Others of the same district increase in density to 38°. At Tidioute oils are obtained of 43°. At Franklin they rate from 33° to 36°, and on French creek also they are heavy. At Mecca, Pa., they are dark, thick, and heavy, so that when cold they refuse to flow. Their density is 26° or 27°, corresponding to a specific gravity of about 0.90. The oil from Cuba, N. Y., resembles that of Franklin, marking 32°. Rangoon petroleum is described as of semifluid consistence, like goose grease.

As it occurs in nature it is of no definite composition, but consists of various oily hydrocarbons, which hold in solution paraffine and other substances, some of which contain nitrogen. When of the greatest fluidity they resemble naphtha, and have been called by this name, and also oil of naphtha. As the proportion of carbon increases, the mixture becomes thicker and darker, resembling tar in appearance; and at length, by further diminution of the fluid ingredients, it passes into asphalt. There are two classes of petroleums which are quite distinct. The first may be termed paraffine oils, as they invariably yield paraffine by distillation; the second never yield paraffine. The first are exceedingly stable compounds, changing but little if at all on exposure to the atmosphere; these form the greater portion of the petroleums of commerce. The second are very unstable, and change rapidly on exposure to the atmosphere to maltha and asphalt. While they are found in some localities in large quantities, they have as yet proved of limite value for commercial purposes when compared with the other variety. The oils of California and Trinidad are of this class. - Comparatively little is known of the chemical composition of petroleum.

Warren and Storer in this country and Pelouze and Cahours in France have investigated the more volatile portion of the Pennsylvania and Rangoon oils, but whether the substances they obtained are educts or products is uncertain. These two petroleums were found to be similar. Two groups of hydrocarbons were isolated, having the constitution of the hydrides of the alcohol radicals, the consecutive members of which were isomeric and differed in their boiling points by 30° 0. There were also found the more volatile members of the ethylene series, and" in the Rangoon petroleum some of the members of the benzole series. Very little is known concerning the denser constituents, whose boiling points are above 200° C. Some petroleums contain more than one per cent, of nitrogen, others contain sulphur. The amount of carbon increases with the density, and that of hydrogen decreases. - The process of sinking the wells is described in the article Aetesian "Wells. The productive wells vary greatly in depth. In some large supplies have been afforded at 60 or 70 ft., and in others at greater depths to over 1,000 ft. Most of the oil is from wells over 180 ft. deep. Shallow wells, that are exhausted by pumping, are successively made to yield again by sinking them deeper.

The oil is found at several zones or oil-producing belts at different depths. Several wells may continue in successful operation near together without seeming to draw upon each other; and again wells may be sunk near others that are producing largely, or near the natural springs of oil, and prove unsuccessful. The pumps are sunk deeper into the wells as the supply goes down; and it is observed that if the pumping is interrupted for a day, the product obtained when it is renewed will be water, which is more or less salt. At some wells the flow of water has continued during several days' pumping before the oil was recovered. This never seems to fail entirely, unless it be from some obstruction arresting the flow, and then recourse is had to sinking deeper or enlarging the bore of the hole. Salt water commonly comes up with the oil, and is separated from it by standing in the vats into which the products are received. The proportion of this to the oil is very variable, and the quantity of oil daily pumped from a single well is far from being regular. Sometimes the oil, when first struck, rushes up with great .violence by reason of the pressure of the carburetted hydrogen gas that accompanies it.

This produces a spouting or flowing well, from some of which the yield has been more than 1,000 barrels a day for a long time; but the quantity gradually diminishes until they cease to be flowing wells, and they are then pumped. In a few instances the oil has leaped forth with such violence as to be beyond control, and immense quantities have been lost. These fountains of oil have sometimes taken fire, producing terrific conflagrations and presenting scenes of appalling grandeur. - Petroleum acquired a reputation as a medicine before it was used for other purposes; and there is no doubt it possesses some virtue, especially as an outward application in diseases of the skin, chilblains, rheumatism, etc. Taken internally in doses of 30 to 60 drops, it acts as a sudorific and stimulating anti-spasmodic. It has been recommended for disorders of the chest, and in Germany as a remedy for tapeworm. Its use for illuminating purposes has become of vast importance; and it is becoming each year of more extended application in technology, the waste products of its manufacture yielding a great variety of useful products. (See Pe-troleum Products.) Very large quantities are used raw or unrefined; but the purposes to which it is applied are few, and the consumption is almost exclusively confined to the denser varieties, such as are found in Ohio and West Virginia. It is used chiefly as a lubricator, for fuel, and in the manufacture of gas.

As a lubricator for heavy journals, either pure or mixed with tallow or animal oils, it is of great value, especially for the axles of railroad cars, in rolling mills, etc. For fuel it has been made the subject of many elaborate experiments and voluminous reports from the government engineers of this and other countries, many of which have been highly favorable. Several furnaces have been contrived for its combustion for steam and other purposes, yet it has been but little used as fuel. The reason for this apparent neglect of such an abundant, cheap, and efficient source of artificial heat is no doubt thus far to be attributed to its comparatively dangerous properties and to other difficulties attending its transportation and storage. These difficulties and the prejudices attending them are gradually disappearing, and we seem to be rapidly approaching a time when the consumption of crude petroleum, especially for steam purposes, will become enormous. It has been used to a limited extent for the manufacture of illuminating gas.

For this purpose it is allowed to drip on to coke heated red hot in a retort, and the gaseous products conducted to a gasometer. - The table below will give the reader some idea of the vast amount of this material which has been yielded by the Pennsylvania oil region alone, from 1860 to 1873 inclusive:

YEARS.

Bbls. of 40 gallons.

1860............

500,000

1861............

2,118,000

1862............

3,056,000

1863............

2,631,000

1864............

2,116,000

1865............

2,497,000

1866............

3,597,000

YEARS.

Bbls. of 40 gallons.

1867...........

3,347,000

1868...........

3,583,660

1869...........

4,210,720

1870...........

5.673,195

1871...........

5.715,900

1872...........

6.531,675

1873...........

9,884,719

The average daily product of this region from the discovery of petroleum to Jan. 1, 1874, is 10,852 bbls. The total product for the same time is 55,461,319 bbls. During the year 1873, 379,634 bbls. of crude oil was exported from New York city, the product of the Pennsylvania wells.