Bitumens, are inflammable mineral bodies, not sulphureous, or only casually impregnated with sulphur. They are of various degrees of consistence, ' and appear, in the mineral kingdom, to correspond with the oils and resins in the vegetable. By their peculiar smell, they are easily distinguished from either purely animal or vegetable productions. When the native rock-oils are mixed with concentrated mineral acids, they become thick, and at length consistent: in which state they are called bitumens.
There is a thin fluid bitumen called naphtha, which is found on the surface of waters, or oozing from clefts of rocks in the eastern countries, particularly Persia. It has a strong smell, very different from that of vegetable or animal oils, is highly inflammable, not soluble in spirit of wine, and almost as limpid as water, with which it is more averse to unite than any other oil. Next to naphtha, in consistence, is petroleum, or rock
T oil; oil : the former is collected for making varnishes, and the latter is used for lamps and torches. Genuine naphtha is sometimes recommended in diseases of the nerves, but it is seldom obtained in a pure state.
The solid bitumens are, amber, jet, asphaltum, or bitumen of Ju-dea, and fossil or pit-coal. By distillation, they all yield an odorous water, more or less coloured and saline ; an acid frequently in a concrete state, an oil similar to the native rock-oils, but which soon increases in weight, and becomes thicker ; and, lastly, a quantity of volatile alkali. The residuum is a charry matter, differing in appearance, according to the nature of the analyzed bitumen.
It is conjectured by naturalists, , that all bitumens are of animal or vegetable origin ; and that the circumstances by which they differ from the resinous and other oily matters of vegetables and animals, are the natural effects of time ; or of an alteration produced on them by mineral acids ; or of both causes combined. This opinion is the more probable, as bitumens, on a chemical analysis, afford oil and volatile alkali, neither of which is found in any other minerals.