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 name of “Seneca oil,” and the “American Medicinal Oil” of Kentucky was largely sold after its discovery in 1829. The Pennsylvania Rock Oil Company was formed in 1854, but its operations were unsuccessful, and in 1858 certain of the members founded the Seneca Oil Company, under whose direction E. L. Drake started a well on Oil Creek, Pennsylvania. After drilling had been carried to a depth of 69 feet, on the 28th of August 1859, the tools suddenly dropped into a crevice, and on the following day the well was found to have “struck oil.” This well yielded 25 barrels a day for some time, but at the end of the year the output was at the rate of 15 barrels. The production of crude petroleum in the United States was officially reported to have been 2000 barrels in 1859, 4,215,000 barrels in 1869, 19,914,146 barrels in 1879, 35,163,513 barrels in 1889, 57,084,428 barrels in 1899, and 126,493,936 barrels in 1906. From Oil Creek, development spread first over the eastern United States and then became general, subsequently embracing Canada (1862), recently discovered fields being those of Illinois, Alberta and California (44,854,737 barrels in 1908).

For about 10 years Pennsylvania was the one great oil producer of the world, but since 1870 the industry has spread all over the globe. From the time of the completion on the Baku field of the first flowing well (which was unmanageable and resulted in the loss of the greater part of the oil), Russia has ranked second in the list of producing countries, whilst Galicia and Rumania became prominent in 1878 and 1880 respectively Sumatra, Java and Borneo, where active development began in 1883, 1886 and 1896, bid fair to rank before long among the chief sources of the oil supplies of the world. Similarly, Burma, where the Burmah Oil Company have, since 1890, rapidly extended their operations, is rising to a position of importance. Oil fields are being continually opened up in other parts of the world, and whilst America still maintains her position as the largest petroleum producer, the world's supplies are now being derived from a steadily increasing number of centres.

Physical and Chemical Properties.—Although our information respecting the chemical composition of petroleum has been almost entirely gained since the middle of the 18th century, a considerable amount of empirical knowledge of the substance was possessed by chemists at an earlier date, and there was much speculation as to its origin. In his Sylva sylvarum (1627), Francis Bacon states that “the original concretion of bitumen is a mixture of a fiery and watery substance,” and observes that flame “attracts” the naphtha of Babylon “afar off.” P. T. Macquer (1764), T. O. Bergman (1784) Charles Hatchett (1798) and others also expressed views with regard to the constitution and origin of bitumens. Of these early writers, Hatchett is the most explicit, the various bituminous substances being by him classified and defined. Jacob Joseph Winterl, in 1788, appears to have been the first to examine petroleum chemically, but the earliest systematic investigation was that carried out by Professor B. Silliman, Jun., in 1855, who then reported upon the results which he had obtained with the “rock oil or petroleum” of Venango county, Pennsylvania. This report has become a classic in the literature of petroleum.

The physical properties of petroleum vary greatly. The colour ranges from pale yellow through red and brown to black or greenish, while by reflected light it is, in the majority of cases, of a green hue. The specific gravity of crude petroleum appears to range from ·771 to 1·06, and the flash point from below 0° to 370°F. Viscosity increases with density, but oils of the same density often vary greatly; the coefficient of expansion, on the other hand, varies inversely with the density, but bears no simple relation to the change of fluidity of the oil under the influence of heat, this being most marked in oils of paraffin base The calorific power of Baku oil appears to be highest, while this oil is poorest in solid hydrocarbons, of which the American petroleums contain moderate quantities, and the Upper Burma oils the largest amount. The boiling point, being determined by the character of the constituents of the oil, necessarily varies greatly in different oils, as do the amounts of distillate obtained from them at specified temperatures.

Even prior to the discovery of petroleum in commercial quantities, a number of chemists had made determinations of the chemical composition of several different varieties, and these investigations, supplemented by those of a later date, show that petroleum consists of about 84% by weight of carbon with 12% of hydrogen, and varying proportions of sulphur, nitrogen and oxygen. The principal elements are found in various combinations, the hydrocarbons of the Pennsylvania oils being mainly s (q.v.), while those of Caucasian petroleum belong for the most part to the naphthenes, isomeric with the s (q.v).

Paraffins are found in all crude oils, and olefines in varying proportions in the majority, while acetylene has been found in Baku oil, members of the benzene group and its derivatives, notably benzene and toluene, occur in all petroleums. Naphthenes are the chief components of some oils, as already indicated, and occur in varying quantities in many others. Certain crude oils have also been found to contain camphenes, naphthalene and other aromatic hydrocarbons. It is found that transparent oils under the influence of light absorb oxygen, becoming deeper in colour and opalescent, while strong acidity and a penetrating odour are developed, these changes being due to the formation of various acid and phenylated compounds, which are also occasionally found in fresh oils. The residues from petroleum distillation have been shown to contain very dense solids and liquids of high specific gravity, having a large proportion of carbon and possessed of remarkable fluorescent properties. Natural gas is found to consist mainly of the lower paraffin's, with varying quantities of carbon dioxide, carbon monoxide, hydrogen, nitrogen and oxygen, in some cases also sulphuretted hydrogen and possibly ammonia. This mixture dissolves in petroleum, escaping when the oil is stored, and conversely it invariably carries a certain amount of water and oil, which is deposited on compression.

Occurrence.—Bitumen is, in its various forms, one of the most widely-distributed of substances, occurring in strata of every geological age, from the lowest Archean rocks to those now In process of deposition, and in greater or less quantity throughout both hemispheres, from Spitzbergen to New Zealand, and from California to Japan. The occurrence of commercially valuable petroleum is, however, comparatively limited, hitherto exploited deposits being confined to rocks younger than the Cambrian and older than the Quaternary, while the majority of developed oilfields have been discovered north of the equator.

The main requisites for a productive oil or gas field are a porous reservoir and an impervious cover. Thus, while the mineral ma be formed in a stratum other than that in which it is found, though in many cases it is indigenous to it, for the formation of a natural reservoir of the fluid (whether liquid or gas) it is necessary that there should be a suitable porous rock to contain it. Such a rock is typically exemplified by a coarse-grained sandstone or conglomerate, while a limestone may be naturally porous, or, like the Trenton limestone of Ohio and Indiana, rendered so by its conversion into dolomite and the consequent production of cavities due to shrinkage—a change occurring only in the purer limestones. Similarly it is necessary, in view of the hydrostatical relations of water and mineral oils, and the volatile character of the latter, that the porous stratum should be protected from water and air by an overlying shale or other impervious deposit. Water, often saline or sulphurous, is also found in these porous rocks and replaces the oil as the latter is withdrawn.

In addition to these two necessary factors, structural conditions play an important part in determining the accumulation of oil and gas. The main supplies have been obtained from strata unbroken and comparatively undisturbed, but the occurrence of anticlinal or terrace structure, however slightly marked or limited in extent, exerts a powerful influence on the creation of reservoirs of petroleum. These tectonic arches often extend for long distances with great regularity, but are frequently crossed by subsidiary anticlines, which themselves play a not unimportant part in the aggregation of the oil. Owing to difference of density the oil and water in the anticlines separate into two layers, the upper consisting of oil which fills the anticlines, while the water remains in the sync lines. Any as which may be present rises to the summits of the anticlines. When the slow folding of the strata is accompanied by a gradual local descent, a modified or “arrested” anticlinal structure, known as a “terrace” is produced, the up heaving action at that part being sufficient only to arrest the descent which would otherwise occur. The terraces may thus be regarded as flat and extended anticlines. They need not be horizontal. and sometimes have a dip of a few feet per mile, as in the case of the Ohio and Indiana oil fields, where the amount varies from