TAPPING, or Paracenttsis (in Surgery), is the operation usually employed for the removal of fluid from any of the serous cavities of Ore body in which it has collected in a dangerous quantity. It is ammaplished by means of an instrument called a trocar, and a tube, or eswsla, in which it exactly fits. The trocar is of steel, cylindrical through the chief part of its length, and terminated by a three-aide I pyramid which ends in a very sh..up point. The cantata being placed upon its shaft, the trocar is thrust into the cavity containing the fluid, and being then withdrawn through the cumin, the latter is retained in the aperture till all the fluid is discharged. The diseases for which tapping is chiefly performed are excites, hydrothorax, hydrocele, and, occasionally, hydrocephalus, and effusions of fluid on the pericardium.
TAR To give a concise definition of this familiar substance is difficult inasmuch as it varies in colour, composition, and consistence, and is derived equally from the animal, vegetable, and mineral kingdoms. Prom the colourless oil-like 'Nei-alms, on the one hand, to the hard, black, resin-like BITUMEN, or pitch, on the other, we have mixtures of the two, containing more or less of either, and to which the term tar is applied.
Tar, then, is a coloured oleo-resin. Deposits of it are frequently met with in nature, rarely however, in large quantities'. The most impor tant basins of it are found in Bunnell, especially at Rangoon. Wells, about sixty feet deep, are sunk in the soil, and from their walls oozes out the tar and collects at the bottom ; it has a brownish green colour, a goose-grease consistence, and is a mixture of several well-defined matters that will be referred to presently. Names, other than tar, have been given to this naturally occurring oleo-resin. Thus, we have rock-oil or petroleum, black naphtha, liquid pitch, liquid bitumen, fluid asphalt, and mineral tar.
But the vegetable kingdom is also a source of tar ; indeed. it is the most important one, and wood and coal are the members which yield it In commerce we meet with wood-tar in barrels holding about thirty gallons. Under the name of Stockholm tar it ie imported from Russia, Sweden, Norway, Denmark and other northern parts of Europe ; while that from the States of the New World is distinguished as American tar. The wood, especially that of the root, of pines, is the kind most profitably used in the production of tar; and it is subjected to a crude yet effective process of destructive distillation. A large neatly-trimmed hole of conical shape is made in a bank, hill-side or other sloping ground, and into thin is lowered a similarly shaped bundle of pine-billets. The wood being kindled the whole is covered with turf. Slow partial combustion now goes on, the resin naturally existing in the wood melts, is slightly decomposed and darkened in colour by the heat, and flowing down to the bottom of the hole is there received in an iron dish having a long tubular spout which conveys it a few feet through the ground to the mouth of the barrel ; the latter being placed for its reception in a cavity some two or three yards lower down the slope.
Coal-tar has already been treated of in a separate article [Coma TAR.] In appearance and complexity of composition it much resembles mineral and wood tar.
Remembering the vegetable nature of coal, it is easy to conceive that the above-described varieties of tar have a common origin. The pro duction of, first, wood, then coal, and finally mineral tar are possibly sequential operations in nature which are simply hastened by the restless energy of man when, in the rude Macedonian fashion he half burns wood in the forest, or submits coal to destructive distillation aided by all the appliances of refined modern ingenuity.
Animal tar has already been treated of It is chiefly used for lubricating machinery.
Pitch.—When tar is heated in retorts it partially volatilises. The first portions of the distillate constitute crude naphtha turpentine and impure pyrolifineoutt (acetic) acid, and next oils containing paraffin come over; the residue in the retort is pitch, a hard, black, vitreous resin.
Constituents of Tar.—By tedious processes of fractional distillation, each portion of the distillate being redistilled and its products col lected in several separate quantities, and these again rectified and acted upon by powerful chemical reagents—coal-tar has been shown to consist of ;—first, a number of very inflammable liquids, containing either the elements carbon and hydrogen only, as Cumoi,e, Enews, (from *3 beautiful, and whey fat) TOLL'OLE, and XYLOLE ; or oxygen also, as in KREABOTE, CAPNOSI011 (from narrbr, smoke, and aolpo part), and primmer. The latter is an oily body, of bitter taste, and specific gravity 1.10: it forms a crystalline compound with potash. The second series contains solid bodies, namely, PARAFFIN, NAPHTHALIN CFDRIIIET, (from cedrium, the old name for "acid tar-water," and tale a net, in allusion to the reticulated appearance of its crystals), Pv HEN, CHRTSEN, Pr ROXANTRIN and pittacal ; the latter has a deep blue colour, and is insoluble in water, alcohol, or ether. The liquid called naphtha chiefly consists of hydro-carbon ; the oils contain the oxidised bodice ; and pitch is a mixture of the various solids, together with other fixed matters that are probably decomposed when distillation is effected at very high temperatures, and which yield the charcoal that under these circumstances is always left in the retorts.
Mineral tar and coal-tar have been proved to consist of the same substances as wood-tar. As might be suspected, they contain less non volatile matter than wood-tar.
Animal tar, besides the compounds already referred to, contains METHYLAMINE, ETHYLAMINE and other Isaacs of the same class ; Ammo: and its homologues; and some nitrilee of fatty acids.