12S

BITUMEN.

has not been observed among the primitive or older strata, but only in the secondaryand alluvial formations. It constitutes sometimes considerable beds, as in the isle ofTrinidad , where it occurs over an extensive district, in scattered masses. The greaterpart of the asphaltum to be met with in commerce comes from the Dead Sea , on whoseshores it is cast up and gathered ; whence it has got the name of Jewish bitumen. Inits black color and fiacture it resembles ordinary pitch. By friction it affords negativeelectricity. Its average density is 1T6. It melts at the temperature of boiling water,kindles very readily at the flame, burns brightly with a thick smoke, and leaves littleashes. Distilled by itself, it yields a peculiar bituminous oil, very little water, some com-bustible gases, and traces cf ammonia. It leaves about one third of its weight of char-coal after combustion, and ashes, containing silica, alumina, oxyde of iron, sometimes .alittle lime, and oxyde of manganese. According to John, asphaltum may be decomposed,by different solvents, into three distinct substances. Abater dissolves nothing; alcohol(anhydrous) dissolves out a yellow resin equal to 5 per cent, of the weight of the asphal-tum ; that resin is soluble in dilute alcohol and in ether. The portion not soluble in thealcohol gives up a brown resin to ether, amounting to 70 per cent, of the weight of theasphaltum. On evaporating off the ether, the resin remains of a brownish-black color,which dissolves readily in the volatile oils, and in the oil of petroleum. The portion ofasphaltum which does not dissolve in ether is very soluble in oil of turpentine, and in oilof petroleum; but less so in oil of lavender. These three resinous principles dissolve alltogether by digestion in Hie oils of anise, rosemary, turpentine,'olive, hemp-seed, nut, andlinseed. Caustic potash dissolves a notable quantity of asphaltum; hut carbonate of pot-ash has no effect upon it.

Asphaltum enters into the composition of hydraulic cements, and into that of black var-nishes, called japans, for coating iron trays, &c. A similar varnish may be prepared by-dissolving 12 parts of fused amber, 2 parts of rosin, and 2 parts of asphaltum, in 6 partsof linseed oil varnish, to which 12 parts of oil of turpentine have been added.

There is a kind of bitumen found at Aniches, in France , in the department of thenorth, which is black, very fusible, and soft. It burns with flame. Alcohol, ether, andoil of turpentine extract from it a fatty substance, which may be saponified with alkalis.

The bitumen of Murindd, near Choco, in Columbia, is of a brownish-black color, soft,and has an earthy fracture. It has an acrid taste, burns with a smell of vanilla, and issaid to contain a large quantity of benzoic acid. It appears to be the result of the decom-position of trees containing benzoin.

Asphaltum occurs abundantly at the surface of the salt lake Aspliallites, in Judea ,produced from springs in the neighborhood; it is floated down, gathers consistence andaccumulates upon the surface of the lake; the winds drive it on the shores, and the in-habitants collect it for sale. Its inspissation diffuses a disagreeable smell in the air ofthat region, which is supposed by the natives to be powerful enough to kill birds whenthey attempt to fly across the lake.

But probably the most remarkable locality of asphaltum in the world is the entirebasin, or rather plain of it, in the island of Trinidad , called the Tar Lake. It lies on thehighest land in the island, and emits a strong smell, sensible at ten miles’ distance. Itsfirst appearance is that of a lake of water, but when viewed more nearly, it seems to bea surface of glass. In hot weather its surface liquefies to the depth of an inch, and itcannot then be walked upon. It is of a circular form, about three miles in circumference,and of a depth not ascertained. Large fissures frequently open and close up in it, whencethe pitch has been supposed to float upon a body' of water. The soil, for a considerabledistance round it, consists of cinders and burnt earth, and presents in many points indi-cations of convulsions by subterranean fire. In several parts of the neighboring woods,there are round holes and fissures in the ground, containing liquid bitumen to the depthof two inches.

Mr. Hatchett examined some specimens from Trinidad , and concluded that what hadbeen heretofore supposed to be a pure mineral pitch was in reality only a porous stone ofthe argillaceous kind, much impregnated with bitumen.

These various bitumens belong exclusively to the secondary and tertiary geologicalformations, and are not found among primitive rocks, except very rarely in veins. Theyoccur most generally in calcareous, argillaceous, and sandy strata, and also in volcanicdistricts. Petroleum frequently floats on the waters which issue from the volcanic moun-tains, or which lie at their base ; even the sea is at times covered with it near the vol-canic islands of Cape Verd. Mr. Breislack observed a petroleum spring rising from thebottom of the sea near the southern base of Vesuvius . ,

The substance with which bitumen seems to have the most constant and most remark-able relations, is sea-salt; so that almost all the countries most abundant in petroleum,as Italy, Transylvania, Persia, the environs of Babylon, the region of the Dead Sea , & c ->contain salt mines, or lakes, or exhibit saline efflorescences. Iron pyrites is often im-pregnated with petroleum, or contains a bituminous nucleus.