mentioning Prof. B. Silliman's report on Pennsylvania petroleum to Messrs. Eveleth, Bissell & Reed, 1855.
He examined the rock-oil or petroleum of Venango County, and, long before the present processes of refining had been introduced, suggested several very important processes, which have been since followed in its treatment; such as distillation by steam, "cracking," or breaking up of the heavier oils into lighter compounds, its use for making gas, for illuminating purposes, for lubricating, etc.
Composition.—Petroleum is a mixture of several hydrocarbons, and contains also bituminous materials, sulphur, carbonaceous matter, sand, and clay. Its odor is generally offensive. The color and specific gravity vary greatly. The crude petroleum of Pennsylvania is generally dark-green with a brownish tinge by reflected light; the color of thin layers by transmitted light varies from dark-yellowish to reddish-brown. The oil of Enniskillen is blackish-brown; of Mecca, Ohio, yellow; in the neighborhood of Shamburg, Venango County, Pennsylvania, "black" and "green" oils occur side by side in the same districts; the lubricating oil of White Oak, West Virginia, is yellow; that from Amiano, Italy, is red to straw-color; at Baku the light oil is clear and faint yellow. Pennsylvania petroleum is somewhat thick, like thin sirup, but, although stiffened somewhat by cold, is always fluid. The oil of Pagan, Burmah, is very light, resembling naphtha, as is some of that from Baku.
The specific gravities of different petroleums are as follows: White Oak, West Virginia, 28° to 40° Baumé; Mecca, Ohio, 26° to 27°; Franklin, Pennsylvania, 30° to 32°; Cuba, New York, 32°; Tidioute, 43°; Pit-Hole, 51°; Pomeroy, Ohio, 51°; Russia, 28° to 40°. The heavy oils command, as a rule, a higher price. Although there is no certainty about their occurrence, the heavy oils have been frequently found at a higher level than the light oils in Pennsylvania, so that this was at one time supposed to be the rule.
The constituents of the mixture known as petroleum are separated from each other by fractional distillation; with care they can be isolated in quite a pure state, but in practice they undergo various decompositions, and are frequently to be regarded rather as products than as educts of the operations. Some are gaseous at ordinary temperatures, others are liquid, and others solid. They are divided into two classes: one having the formula CnH2n+2, and belonging to the marsh-gas. or paraffine series; the other, with the formula CnH2n, belonging to the ethylene series (olefines). They have been carefully investigated by Pelouze and Cahours, Warren, Schorlemmer, and Ronalds, and the results obtained by them are given in the following table, partly compiled from the review of the subject by Prof. S. P. Sadtler, in Prof. Genth's "Report on the Mineralogy of Pennsylvania" ("Second Geological Survey, 1874"). The letters F, R, W, P and C, and S, indicate the observers, Fouqué, Ronalds, Warren, Pelouze and
