142 THE POPULAR SCIENCE MONTHLY. 



mentioning Prof. B. Silliman's report on Pennsylvania petroleum to 

 Messrs. Eveleth, Bissell & Reed, 1855. 



He examined the rock-oil or petroleum of Yenango County, and, 

 long before the present jjrocesses 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 mat- 

 ter, 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, Yenango 

 County, Pennsylvania, " black " and " green " oils occur side by side 

 in the same districts ; the lubricating oil of White Oak, West Vir- 

 ginia, 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 Beaume ; Mecca, Ohio, 26 to 

 27 ; Franklin, Pennsylvania, 30 to 32 ; Cuba, New York, 32 ; Tidi- 

 oute,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 fre- 

 quently 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 iso- 

 lated in quite a pure state, but in practice they undergo various de- 

 compositions, 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 C n H n + a , and belonging to 

 the marsh-gas. or paraffine series; the other, with the formula C n H on , 

 belonging to the ethylene series (olefines). They have been carejfully 

 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, Fouque, Ronalds, Warren, Pelouze and 



