Petroleum, called also mineral tar, is less fluid and less transparent than naphtha. It has an oily consistence, more or less viscid. It occurs of a black or brown colour. It burns rapidly, but not so readily as naphtha, and exhales a black smoke. By distillation, it forms a liquid like naphtha, and leaves a thick tar in the retort.
It exudes from rocks, is found in wells, &c. In Pegu, the wells furnish annually 400,000 hogsheads. It is used there in the place of oil for lamps. When boiled with rosin, it is used for painting houses, and the bottoms of vessels. In the embalming of dead bodies, it was employed by the ancient Egyptians; and, in some countries, clay, soaked in it, is used as fuel.
It is found in the United States, in Kentucky, Ohio, the western parts of Pennsylvania, in New York at the Seneca lake, &c. The Seneca or Genessee oil is the same bitumen.
When petroleum is exposed to the atmosphere, it acquires a greater degree of consistence, and passes into another bituminous substance, called maltha. This has the properties, and frequently the appearance of pitch. When burnt, it yields more smoke and soot than petroleum. According to its original meaning, it signifies a kind of cement; and the maltha mentioned by Pliny, Heineccius, Festus, and others, which was employed in the same manner as our modern sealing wax, was a mixture of pitch and wax, and was also used to make reservoirs, pipes, &c. water-tight. Maltha also sometimes resembles wax. Mr. Kirwan, however, gave it the name of mineral tallow.
Mineral or Barbadoes tar is somewhat thicker than petroleum, and nearly of the consistence of common tar. It is used for the same purposes as the ordinary petroleum. Elastic bitumen, a variety between the softer and harder bitumens, resembles caoutchouc. It burns with a bright flame, and bituminous odour.
Asphaltum, or solid bitumen, is much harder than pitch, brittle, and of a brownish-black colour. It burns freely, and leaves but little residue. In Judea, it is found on the waters of the Dead sea, or the lake of Asphaltes. It is also called Jews' pitch. It was employed by the Egyptians for embalming under the name of mumia mineralis.
Both maltha and asphaltum were used by the ancients as a cement. The walls of Babylon were cemented with these substances, as obtained from the river Is, which falls into the Euphrates. It may be observed, that those countries, which yield bitumen, contain salt springs, and it frequently accompanies pyrites. Limestone, particularly the black, contains it, and the colour is often owing to its presence. The stink stone, or bituminous carbonate of lime, is of this kind. The retinasphaltum, a combination of bitumen and earth, having a yellow colour, burns with a bright flame, and fragrant odour, which at last becomes bituminous. Many stones, and particularly some of the black marbles, owe their colour to bitumen; hence they burn white. The bituminous schistus, or bituminous shale, sometimes contains so much of this substance as to burn in the fire. Jet is a mineral of a black colour, and resembles the cannel coal. It is inflammable, producing a green flame, with a strong bituminous odour.
With respect to bitumens, we may observe, that they all possess one character, that of being inflammable; and that they are more or less so in proportion as they partake of the principle of naphtha; or, at least, the rapidity of their combustion depends upon the presence of this oil. The following additional facts, therefore, with respect to naphtha, may be interesting: Certain liquids have the property of uniting with naphtha, which has also the property of dissolving and combining with solid substances, of which the following examples may be stated:
At the degree of ebullition, it dissolves sulphur, which, on cooling, is in part deposited in needle-form crystals. At the same temperature, it also dissolves phosphorus, part of which is again separated.
It unites also with iodine. With camphor, it also combines, and in large quantity. It takes up a much larger proportion of pitch. In the cold, its action on wax is feeble, but assisted by heat, it unites with it in all proportions. On lac and copal, its action is feeble. In the cold, it does not dissolve caoutchouc; but when assisted by heat, it dissolves this substance, though not completely. These facts may determine its action in certain mixtures.