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THEORY AND PRACTICE OF ENGINEERING.
Rook 11
CHAP. VII.
Bitumen or asphaltum is used in the composition of hydraulic cements, and for varnishesand japans : it is a black substance found in the earth ; in external character it bears someresemblance to coal ; it is a compound of carbon, hydrogen, and oxygen, and generallyobtained from the secondary and alluvial formations; its average density is 116, and it meltsat the temperature of boiling water.
The island of Trinidad contains a tar lake, 3 miles in circumference, and asphaltum isproduced in great abundance from springs in many parts of Asia ; it is also obtained in con-siderable quantities near Antibes in France , where it is made into a varnish by dissolving2 portions of asphaltum with 12 parts of fused amber, 2 parts of resin, 6 parts of linseedoil varnish, and 12 parts of oil of turpentine.
Bitumen has been applied as a covering for roofs and floors, and lining cisterns, and con-siderable quantities have been imported from the borders of the Lower Rhine : to everypound of bitumen when melted is added 4 or 5 pounds of powdered limestone, chalk orburnt clay ; when thoroughly mixed and combined, it is poured out and spread intomoulds, previously smeared with a thin coating of loam to prevent adhesion ; after thewhole has cooled, the mould is taken to pieces, and the bricks or contents are 18 inchesin length, a foot broad, and 4 inches thick, weighing about 70 lbs.
Asphaltum may lie decomposed by alcohol and caustic potash ; its origin is but little known ;by some chemists it is supposed to be the product of coals decomposed by volcanic heat orby spontaneous combustion.
Resins — The Juice of Pine and Fir 7 Wes, like that of the Pistacea terebinthus, has an austerestringent taste ; it is viscid and transparent, readily inflammable, and easily becomes concrete.In distillation with water, it yields a highly penetrating essential oil, and the liquor isfound to be impregnated with an acid, a brittle resinous matter remaining behind : diges-tion with rectified spirit of wine completely dissolves all the resinous parts, with which someportion of the insipid gum, or mucilage is also taken up. If this solution be filtered, anddiluted largely with water, it becomes turbid, and throws oft' the greatest part of the oil,the gummy substance being retained : if the solution be subjected to distillation, the spiritcarries with it some of the lighter oil, so as to be sensibly impregnated with its terebinthi-nate odour, and it leaves behind an extract, differing from the resin separated by water, inhaving an admixture of mucilage. The native juice becomes miscible in water by the me-diation of the yolk or white of an egg, or by that of vegetable mucilage, and forms a milkyliquor: exposed to the immediate action of fire, the roots and other hard parts of the treeproduce a thick, black, empyreumatic fluid, which, containing a proportion of saline andother matter, mixed with the resinous and oily, proves soluble in aqueous liquors, and ac-cording to its several modifications constitutes the varieties of tar and pitch. The resinousresidue of the several processes to which the matter extracted from pines may be subjectedconstitutes the varieties of resin, colophony, &c. &c. There are also other products, bothnatural and artificial, much employed in medicine and the arts.
Tar and Pitch are extensively used for the purpose of retarding the decomposition ofwood, cordage, and other articles.
Tar mixed with grease or clay is used for greasing wheels, &c.
Yellow Resin in the proportion of 3 cwt. to 10 cwt. of tallow, for common yellow soap.
Shoemakers' Wax is a composition of pitch, oil, and suet, but it is also made of resin, bees-wax and tallow.
Turpentine in all its different forms, is extensively employed in painting.
Tar and Pitch, with a mixture of tow or beaten cables, used for paxjing over the seamsof the sides and decks of ships after they are caulked, to preserve the oakum from any wet.Oakum is formed of untwisted old ropes steeped in tar, and in ship-building is indis-pensible.
Lampblack is used by painters, modellers, and other artists.
Turpentine made from the Scotch fir is so inferior to that obtained from the silver firthat the latter is generally preferred.
Tar is procured from the Scotch pine in great quantities in the north of Europe , and isconsidered superior to that produced in the United States from P. resinosa, StrobusAustralis, &c. The process by which tar is obtained is very simple; it is chiefly from theroots of the Scotch pine. A conical cavity is made in the ground, generally on the side ofa bank ; and the roots, together with logs and billets of wood, neatly trussed in a stack of