TAR , a product of the destructive
See also:distillation of organic sub-stances . It is a highly complex material, varying in its composition according to the nature of the
See also:body from which it is distilled, different products, moreover, being obtained ac-cording to the temperature at which the
See also:process of distillation is carried on . As commercial products there are two
See also:principal classes of tar in use—(1)
See also:wood tar, the product of the
See also:special distillation of several varieties of wood, and (2)
See also:coal tar (q.v.), which is primarily a by-product of the distillation of coal during the manufacture of
See also:gas for
See also:illuminating purposes . These tars are intimately related to
See also:mineral pitch and petroleum . Wood Tar.—Wood tar, known also as
See also:Stockholm and as Archangel tar, is principally prepared in the
See also:pine forests of central and
See also:northern Russia, Finland and Sweden . The material chiefly employed is the resinous stools and roots of the Scotch
See also:fir (Pines sylvestris) and the Siberian larch (Larix sibirica), with other less
See also:common fir-
See also:tree roots . A large amount of tar is also prepared from the roots of the swamp pine (P. australis) in
See also:North and South Carolina,
See also:Georgia and
See also:Alabama, in the
See also:United States . In the distillation of wood a series of products, including gas, tar, pyroligneous acid,
See also:acetone, wood spirit (see METHYL
See also:ALCOHOL) and
See also:charcoal may be obtained, and any of these may be the
See also:object of the operation . The carbonization of wood can be effected in two ways: (1) by stacking and firing as in the manufacture of charcoal: this method is very wasteful as it is impossible to recover the valuable by-products; and (2) by distilling from retorts, ovens or kilns (after the manner of
See also:coke production from coal): this method is more economical as it leads to the
See also:isolation of all the by-products . The retorts may be
See also:horizontal or vertical and the
See also:heating effected by any available fuel, or by the inflammable gases and less valuable grades of tar obtained in previous operations . The condensing plant is also of variable design; a common
See also:pattern consists of a connected series of slightly inclined copper pipes contained in a rectangular tank of
See also:water (see COAL TAR) . After settling the distillate separates into three layers: the lowest consists chiefly of tar and
See also:creosote oils with a little acetic acid; the
See also:middle layer consists of water, containing pyroligneous acid, wood spirit, acetone with a little tarry
See also:matter; whilst the upper consists of
See also:light hydro-carbons .
The tarry layer is run off by means of a
See also:cock near the
See also:base of the tank, and is then distilled from retorts resembling coal tar stills . At first, between to° and 12o° C., water and acetic acid comes over; then, between 120°—230° C., the heavy or creosote oils; the
See also:residue in the still is wood pitch, which finds application in making briquettes, artificial asphalts, certain varnishes, &c . The crude tar and pitch are also largely used as protective coatings for woodwork exposed to atmospheric conditions . The heavy oils on further fractional distillation yield more acetic acid, and then mixtures of carbolic acid, creosols, &c . Wood tar is a semi-fluid substance, of a dark
See also:brown or black
See also:colour, with a strong pungent odour and a
See also:sharp taste . Owing to the presence of acetic acid, it has an acid reaction; it is soluble in that acid, as well as in alcohol and the fixed and essential oils, &c . Some varieties of tar have a granular appearance, from the presence of minute crystals of
See also:pyrocatechin, which dissolve and disappear on heating the substance . See P Dumesny and J . Noyer,- Wood Products, Distillates and Extracts (Engl. trans . 1908) .
See also:Medicine.—Wood tar is used in medicine under the name of Pix liquida . Its preparation unguentum picis liquidae is composed of wood tar and yellow beeswax .
Externally tar is a valuable stimulating dressing in scaly skin diseases, such as
See also:psoriasis and chronic eczema . Internally wood tar is a popular remedy as an expectorant in subacute and chronic
See also:bronchitis . It is usually given as tar water, 1
See also:part of wood tar being stirred into 4 parts of water and filtered . Given internally tar is likely to upset the digestion; taken in large quantities it causes
See also:pain and vomiting and dark urine, symptoms similar to carbolic acid poisoning . Coal tar is used in medicine as Pix liquida preparara . From it is made Liquor picis carbonis, prepared with tincture of quillaia . Coal tar is rarely prescribed for
See also:internal use . Its
See also:external use is similar to that of wood tar: the Liquor carbonis detergens, a proprietary preparation, owes its properties chiefly to the contained phenol . It is used in water as a lotion for skin diseases, and also in an inhaler in the treatment of whooping-cough,
See also:croup and bronchitis .
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