Bitumen, a generic name for a variety of substances found in the earth, or exuding from it upon the surface, in the form of springs. The liquid varieties become inspissated by exposure, and eventually harden into the solid form, which is asphaltum. The bitumens burn with a flame and thick black smoke, giving out the peculiar odor called bituminous. Some of the impure fluid bitumens, and the solid variety when melted, closely resemble coal tar. They are distinguished from bituminous coal in giving no ammonia, or mere traces of it, by distillation, and in developing negative electricity by friction without being insulated; also, when ignited upon a grate, the bitumens melt and run through at the temperature of about 220° F., but the coals burn to ashes. In melting, volatile fluids escape from them with no swelling up other than that due to ebullition. This property of dividing by heat into fluids and solid residues having a porous form, assimilates the bitumens to ordinary turpentine and tar, and renders them unsuitable for producing gas economically. In boiling water the bitumens soften, adhere to the sides of the vessel, and give off naphtha; coal undergoes no change.

The bitumens, again, dissolve perfectly in spirits of turpentine, benzole, rosin oil, linseed oil, and sulphuric ether; while coal, after long digestion in the oils, only colors the liquid brown, and to the sulphuric ether imparts a naphtha-like fluid and a resinous body. The bitumens decompose nitric acid, coal does not; they combine with sulphuric acid, coal is not affected by it. Dropped upon melted tin with a temperature of 442° F., the bitumens decompose and give off copious fumes; coal is unaltered. Most of these points of difference were given in evidence by Dr. A. A. Hayes and Dr. C. T. Jackson of Boston, in an important suit tried in New Brunswick, to test the title to the Albert coal-mining property, this turning on the point whether the product was coal or asphaltum. Dr. Ure notices that the fluid bitumens differ from coal tar in not producing the six substances extracted from the latter by Mr. Mansfield, and named by him alliole, benzole, toluole, camphole, mortuole, and nitro-benzole. - The varieties of bitumen commonly described are: the liquid oil, naphtha, or, in its more impure form, petroleum; the viscid pitchy bitumen, which passes into the black resinous asphaltum; and the elastic bitumen, or elaterite of the mineralogists.

The last is also called mineral caoutchouc, from its property of rubbing out pencil marks. It was first found in the deserted lead mine of Odin, in Derbyshire, England, by Dr. Lister, in 1673, and was called by him a subterranean fungus. It occurs in soft flexible masses of blackish brown colors and resinous lustre, and consists of about 85 per cent. of carbon, and the remainder hydrogen with probably some oxygen. Compact bitumen, or asphaltum, has been noticed under Asphaltum; but further consideration will be given to it in this article in treating of the uses of the bitumens. Gra-hamite, found in West Virginia, and albertite, in Nova Scotia, are supposed to be inspissated and oxygenated petroleums. Chapapote is an asphaltum found in abundance near Havana, and elsewhere in the island of Cuba. It appears to be a consolidated petroleum, a liquid variety of which is often seen near it oozing through the fissures of the limestone rocks. The solid product is of jet-black color, and gives a brown powder and a strong but not unpleasant odor. Its specific gravity is given by Dr. Hayes at from 1.165 to 1.170. It melts in boiling water into a thick liquor, and forms a scum upon the surface.

Alone, it melts at 214° F. into a uniform fluid, which may be poured from one vessel to another; calcined in close vessels, it swells and leaves a very light coke; dissolved in spirits of turpentine, it makes a coarse varnish. Brown-colored and viscid oils are extracted from it. Petroleum and naphtha are fluid substances, called also rock oil, which flow up through fissures in the rocks, and collect in low places, and are found floating upon the surface of the waters of lakes. When indurated and oxidized by exposure, they are asphaltum. The purer form, called naphtha, is very common in many parts of the world, and in numerous places is turned to good account as a fuel, and also for illumination. (See Naphtha, and Petroleum.) These different varieties of bitumen are found only in the secondary and tertiary formations. If they occur at all in the primary rocks, it is merely in veins and fissures, which probably have been filled long after their formation. They are very generally met with in connection with salt springs, or mines of rock salt. Near volcanoes, petroleum is often seen issuing with the waters of springs, or floating upon the sea, furnished from springs at its bottom.

The ancient Babylonians obtained the imperishable cement for their structures from the fountains of Is, which is the modern Hit, on the right bank of the Euphrates. These still continue to pour out inexhaustible supplies, mingled with the strongly saline and sulphurous wafers. Common salt is also prepared here from the brine springs. The water of the springs has a temperature of about 160° F. As it flows slowly along a conduit, the oily bitumen gathers on the surface, and is skimmed off and laid in pits exposed to the air, in which it speedily gardens into flakes of about an inch thick, which are sold at Hit for about five cents the cwt. It is much used for covering the houses and boats of the region. The rock formation is an argillaceous limestone, over which is found in some places a coarsely granular gypsum. These fountains are celebrated as having attracted the attention of Alexander the Great Trajan, and Julian. The bituminous products of the Dead sea in Palestine are collected on the E. and W. sides of the lake, and are supposed to be derived from a bed of bitumen at the bottom.

The pieces resemble pitch, and, though one seventh heavier than pure water, float upon the saline water of the Dead sea, the specific gravity of which is 1.23. They melt in boiling water, and when distilled yield a volatile oil, some water, and traces of ammonia. The residue consists of charcoal, amounting to one eighth of the weight of the asphaltum, its ashes composed of silica, alumina, oxide of iron, and traces of lime and manganese. It is from this locality that the name Jews' pitch has been given to asphaltum. In the island of Trinidad, in the West Indies, there is a famous lake of asphaltum and petroleum called Tar lake, or by the French Le Brai, from its material answering the purposes of pitch, and possessing this additional advantage, that it keeps off the teredo or borer, which in warm climates is so destructive to the timber of ships. The lake is near the sea, about 3 m. in circumference. It appears at a distance like water, but near by like a lake of glass. In approaching, a strong sulphurous smell is perceived at the distance of 8 or 10 miles. When the weather is hot and dry, the surface of the lake is so soft and sticky one cannot walk upon it. A foot below the surface it becomes softer, and contains an oily substance in little cells.

Specimens of this bitumen, which were regarded as pure, and taken to Europe, were examined by Mr. Hatch-ett, who found them to consist of a porous and argillaceous stone thoroughly impregnated with bitumen. It does not burn readily, but becomes plastic by a slight increase of temperature. Bitumen is also found disseminated through calcareous and sandstone rocks, and saturating slates and shales. Nearly all the varieties of it are liable to have many impurities mixed with them, and all contain volatile oils and water. - The bitumens are purified by first boiling them with water. The sand and other mineral substances fall to the bottom, and the bitumen floating or sticking to the sides of the boiler is skimmed off and put into another boiler, by which more water is separated. It is then boiled by itself for some time, and is entirely freed from water and oils and the solid impurities, which subside to the bottom. It is thus obtained in the form of a thick fatty pitch, ready to be barrelled for the market or applied to its uses. - The results of the ultimate analysis of the pure natural bitumens, whether liquid or solid, vary but little from 88 per cent. of carbon and 12 of hydrogen.

A solid bitumen of Coxitambo, near Cuenca in Ecuador, gave 88.7 per cent, of carbon and 9.7 of hydrogen, with 1.6 of oxygen and nitrogen. Nitrogen is usually present to the extent of a trace, and in the solid asphaltum it has been found to the extent of 12 per cent., and oxygen also in the same variety about 8 per cent. By treating asphaltum with different solvents, three distinct bodies may be separated. Water dissolves nothing. Anhydrous alcohol dissolves a yellow resin equal to 1/20 of the weight of the asphaltum; this is soluble also in ether. The residue, insoluble in alcohol, treated with ether, yields a dark brown resin, which is separated by evaporating the ether. It amounts to 7/10 the weight of the asphaltum. It dissolves easily in volatile oils, and in oil of petroleum. The latter also, as well as turpentine oil, takes up the residue which the ether leaves. - The following formulas, exhibiting the composition of petroleum and asphalt, are given by Dr. Mus-pratt, as setting forth in a striking manner the derivation of the latter by oxidation of the former:

Naphtha, or petroleum............C20H16, or C40H32

Asphalt, or bitumen.. .. ..................C40H32O6

Great expectations have been entertained of the important uses to which the natural bitumens might be applied; they have proved to be admirably adapted for the construction of walks, terraces, roofs, and every kind of hydraulic work. The material most successfully employed in France for producing the bituminous mastic is liquid bitumen mixed with a bituminous limestone, which is ground to powder, sifted and stirred into the boiling asphaltum, four parts of the stone to one of the bitumen. Dry, common limestone, or broken bricks, will answer as well. The mixture, when of homogeneous consistency, is poured out upon a table covered with sheets of paper, and upon which a square frame is placed for receiving the sheets of mastic. It is spread smoothly by a heated iron roller, sprinkled with sand, and left to cool. When laid, they are united by soldering with a hot iron. Coal tar is often substituted for the natural bitumen, but it is considered far inferior to it in durability and strength. The bituminous limestone is found at Val de Travers, in the canton of Neufchatel, in the Jura limestone formation, corresponding to the English oolite. It consists of 80 per cent, carbonate of lime and 20 per cent, of bitumen. It is tough, difficult to break with a hammer, and is excavated by blasting.

Slightly heated, it exhales a fragrant odor, quite different from that of the factitious compounds. The carbonate of lime is so protected by the bitumen that it does not effervesce with muriatic acid. In any artificial mixture it would be impossible to produce so intimate a combination of these substances as is found in this natural asphalt rock. Silicious matters, as sand and smooth pebbles, are not so well adapted for the preparation of durable mastic as calcareous substances, because they have little attraction for the bitumen, and the mixture is liable to crack and crumble. Bitumen is applied also in the form of an external coating of mastic to give strength and protection to thin sheet-iron pipes and glass tubes used for conveying water, also for roofing. To some extent asphaltum may be used as a fuel, especially for heating meters in gas works, when blown into the grate in the form of powder. It appears to have been a principal ingredient in the destructive Greek fire. (See Greek Fire.) Bricks of poor quality saturated with it are rendered strong ,and impervious to water. It answers most of the purposes for which coal tar is used. It makes the strongest cement for laying brick and stone work. The ancient Egyptians used some form of it for embalming bodies.

The hardness of the mummies is probably owing to the combination of bitumen with the animal substances. In France a process has been patented for spreading fluid bitumen upon canvas sheets or netting and passing it between metallic rolls, thus coating the cloth on one or both sides, and to any desired thickness. The use of the material is for lining buildings. - The origin of the bitumens has been regarded as very doubtful. The composition would seem to refer them to vegetable matters, though they possess very marked differences from the coals.