ASPHALT. (Fa., Asphalte ; GER., Asphalt.)—Asphalt, or native bitumen, is believed to be pro duced by the decomposition of organic, and probably vegetable, matter, under the earth's surface, in places where water is present, but air is almost entirely excluded. These organic remains being deposited in the beds of seas, rivers, and lakes, have been gradually enclosed in sand or clay, and have then undergone a process of decomposition, by which asphalt, petroleum, naphtha, pit-coal, and other substances of a similar nature are formed. The true varieties of bitumen have probably been formed from coal or lignite by subterraneous heat.
When pure, asphalt is a black, or brownish-black solid, possessing a bright, eenchoidal fracture. It fuses at 100°, emitting a strong smell of pitch, and burns with a bright flame. It dissolves in five times its weight of naphtha, but is insoluble in water and alcohol. It has a specific gravity of from 1.0 to 1.2.
Asphalt was formerly found in considerable quantities and in a very pure state on the shores of the Dead Sea, or Locus Asphaltites, whence it derives its name. Only a small quantity is now derived from this source. There are also beds of pure asphalt at Altona, in Albania ; nt Coxitambo, in South America ; in the Island of Barbadoes; and in the great Tar Lake of Trinidad. This wonderful lake, or rather plain, is circular in form, about a mile and a halt in diameter, and consists of bituminous scoriae, vitrified sand, and earth, cemented together. From its colour and smooth ness it resembles a lake of water, and in hot weather its surface to a depth of some inches is covered with a thick, cohesive liquid, which emits powerful sulphurous or bituminous odour. When dry, the surface is covered with little cracks or fissures, which when filled with water are almost invisible. The edges of the lake are almost always hard and cold, but the bitumen gradually becomes warm and softer as the centre is approached, where it is at last found to be boiling. According to Sir Charles Lyell, this lake is probably formed from the woody and vegetable matter carried by the River Orinoco into the surrounding seas. where, by the influence of currents,
it is accumulated in particular spots. The decomposition of these beds of vegetable matter is then effected by subterranean volcanic action, and the bitumen so formed is by the same agency forced to the surface, where it becomes indurated by exposure to the air, thus giving rise to the different varieties of pure and earthy pitch so abundant on the island.
The substance now commonly called asphalt is a perfectly pure carbonate of lime (containing sometimes slight traces of silica), impregnated naturally with bitumen ; this impregnation has been effected by geological agencies, of the precise date and particulars of which we know but little. It is in the proportion of about 7 or 8 parts of bitumen to 93 or 94 parts of the limestone. This mineral, which is found in layers interposed between beds of ordinary limestone, especially in the upper Jurassic formation, presents the following physical characteristics:— Its colour is a deep chocolate, almost black. Its fracture also resembles chocolate in appearance and colour ; it is granular and irregular, without any plane of cleavage ; its colour is deeper according as it is worked in the direction of the stratification or perpendicularly to that direction ; it is deeper and more floury in the first case, and drier and clearer in the second. Each individual mine has its own particular shade, which is always that of chocolate.
In consistence, it varies with the temperature ; it is very bard and sonorous when cold, but softens when heated ; until at 50° or 60° it falls to powder. In winter, it may be broken easily with a hammer, like ordinary limestone ; in summer, it is softened by blows into a kind of paste. Its average specific gravity is Its structure varies in different samples, the produce of each mine being somewhat different from that of the others.