5. Pix liquida and Pix solida.—Tar and Pitch.
1. Pix Liquida.—Vegetable Tar.
(Bitumen liquidum e ligno igne praeparatum, L.—Pix liquida, L. E. D.)
History.—This is the πίττα of Theophrastus, [Hist. Plant. lib. ix. cap. ii. and iii.] the πίσσα υγρά (liquid pitch), or κώνος of Dioscorides, [Lib. i. cap. xciv.] and the pix liquida of Pliny. [Hist. Nat. lib. xxiv. cap. 24, ed. Valp.]
Preparation.—Two kinds of tar are known in commerce; namely, coal tar and wood tar. They are obtained in the destructive distillation—the first of coal, the second of wood.
Of wood tar there are two sorts: one procured in the northern parts of Europe and in America, from the waste of fir timber, and known in commerce as Stockholm tar, Archangel tar, American tar, &c.; the other obtained as a secondary product in the manufacture of pyroligneous acid and gunpowder charcoal. The former is the kind used in medicine. That which is procured from Pinus silvestris, in the northern parts of Europe, is considered to be much superior to American tar.
The process now followed seems to be identical with that practised by the Macedonians, as described by Theophrastus. It is a kind of distillalio per descensum of the roots and other woody parts of old pines. As now carried on in Bothnia, it is thus described by Dr. Clarke: [Travels in Scandinavia, part iii. p. 251; see also Duhamel, Traité des Arbres.] "The situation most favourable to the process is in a forest near to a marsh or bog, because the roots of the fir, from which tar is principally extracted, are always most productive in such places. A conical cavity is then made in the ground (generally in the side of a bank or sloping hill); and the roots of the fir, together with logs and billets of the same, being neatly trussed in a stack of the same conical shape, are let into this cavity. The whole is then covered with turf, to prevent the volatile parts from being dissipated, which, by means of a heavy wooden mallet and wooden stamper, worked separately by two men, is beaten down, and rendered as firm as possible about the wood. The stick of billets is then kindled, and a slow combustion of the fir takes place, without flame, as in working charcoal. During this combustion the tar exudes, and a cast-iron pan being at the bottom of the funnel, with a spout which projects through the side of the bank, barrels are placed beneath this spout to collect the fluid as it comes away. As fast as the barrels are filled, they are bunged, and ready for immediate exportation.
Wood-tar is also obtained as a secondary product, in the manufacture of acetic acid, by the dry distillation of wood.
Commerce.—Wood-tar is imported into this country chiefly from the northern parts of Europe (Russia, Sweden, Norway, Denmark, and North Germany)—partly from the United States of America. It usually comes in barrels, each holding 31 ½ gallons; twelve barrels constituting a last. Tar is also produced in this country.
Properties.—It is a dark brown, viscid, semi-liquid substance, which preserves during a long period its softness. It is soluble in alcohol, ether, and the oils both fixed and volatile. Submitted to distillation, it yields an aqueous acid liquor (pyro-ligneous acid), and a volatile oily matter (oil of tar); the residue in the still is pitch.
Composition.—Wood-tar is a very complex substance. It consists principally of pyretine (pyrogenous or empyreumatic resin), pyroliene (pyrogenous oil), acetic acid, and water.—Reichenbach has obtained from it creosote, paraffin, eupion, picamar, kapnomor, pittacal, and cedriret.—Pyren and chrysen have likewise been found in it.
The tar obtained from coniferous woods contains also colophony and oil of turpentine.
Physiological Effects.—The effects of tar are analogous to those of turpentine, but modified by the presence of acetic acid and the pyrogenous products. Locally it acts as a stimulant; and when applied to chronic skin diseases and indolent ulcers, it frequently induces a salutary change in the action of the capillary and secerning vessels, evinced by the improved quality of the secretions, and the rapid healing of the sores. In such cases, it is termed detergent, digestive, or cicatrisant. Swallowed, it acts as a local irritant and stimulant, becomes absorbed, and stimulates the secreting organs, especially the kidneys, on which it operates as a diuretic. Slight [Wibmer, Wirk. d. Arzneim. Bd. iv. S. 215.] states that a sailor swallowed a considerable quantity of liquid tar, which caused vomiting, great lassitude, and violent pain in bowels and kidneys. The urine was red, and, as well as the other evacuations, had the odour of tar. The head and the pulse were unaffected. The vapour of tar, inhaled, acts as a stimulant and irritant to the bronchial membrane, the secretion of which it promotes.
Uses.—Tar is rarely employed internally. It has, however, been administered in chronic bronchial affections, and in obstinate skin diseases.
The inhalation of tar vapour was recommended by Sir Alex. Crichton [Practical Observations on the Treatment and Cure of several varieties of Pulmonary Consumption and on the Effects of the Vapour of boiling Tar in that Disease, 1823.] in phthisis; but at best it proves only a palliative, and it frequently, perhaps generally, fails to act even thus, and in some cases occasions a temporary increase of cough and irritation. [Dr. Forbes. Translation of Laennec's Treatise on Diseases of the Chest, p. 365.] In chronic laryngeal and bronchial affections it has more chance of doing good. [Trousseau und Pidoux, Traité de Thérap. t. i. p. 459.] Sir A. Crichton's directions for using it in phthisis are as follows: The tar employed should be that used in the cordage of ships; to every pound of which half an ounce of carbonate of potash must be added, in order to neutralize the pyroligneous acid generally found mixed with the tar, the presence of which will necessarily excite coughing. The tar thus prepared is to be placed in a suitable vessel over a lamp, and to be kept slowly boiling in the chamber during the night as well as the day. The vessel, however, ought to be cleansed and replenished every twenty-four hours, otherwise the residuum may be burned and decomposed—a circumstance which will occasion increased cough and oppression on the chest.
Applied externally, tar is used in various forms of obstinate skin diseases, especially those which affect the scalp, lepra, &c.
Administration.—Internally, tar is administered either in substance, in the form of pills, made up with wheat flour, or of electuary, with sugar; or in the form of tar water. In substance, it may be taken to the extent of several drachms daily.
1. AQUA PICIS LIQUIDAE; Tar Water.—(Tar Oij; Water Cong, j [wine measure]. Mix, stirring with a stick for a quarter of an hour; then, as soon as the tar subsides, strain the liquor, and keep it in well-stoppered jars.)—Tar water has the colour of Madeira wine, and a sharp empyreumatic taste. It reddens litmus, but does not effervesce on the addition of a solution of carbonate of potash, though its colour becomes deepened. With a solution of bicarbonate of potash a very slight effervescence takes place. By persulphate of iron, tar water is rendered very dark, or even blackish. The volatile oil contained in tar water is partly held in solution by acetic acid, which, as is well known, dissolves creasote. It consists of water holding in solution acetic acid, and pyrogenous oil and resin. Notwithstanding the high eulogies passed on it by Bishop Berkeley, [Siris, a Chain of Phil. Reflex. and Inq. concerning Tar Water, new edit. Lond. 1744.] tar water is now rarely employed. It is occasionally administered in chronic catarrhal and nephritic complaints, to the extent of one or two pints daily. As a wash in chronic skin diseases, especially those affecting the scalps of children, I have frequently seen it used, and sometimes with apparent benefit.
2. UNGUENTUM PICIS LIQUIDAE, L. E. D. [U. S.]; Tar Ointment.—(Tar, Mutton Suet, of each lb j. Melt them together, and press through a linen cloth. The Edinburgh College takes of Tar ℥v, and Beeswax ℥ij; melt the wax with a gentle heat, add the tar, and stir the mixture briskly, while it concretes on cooling. The Dublin College orders of Tar Oss; Yellow Wax ℥iv.)—Its principal use is as an application to ringworm of the scalp and scalled head, in which it sometimes succeeds, but more frequently fails to cure. It is now and then applied to foul ulcers.
3. OLEUM PICIS LIQUIDAE; Oleum Pini rubrum; Oil of Tar.—This is obtained by distillation from tar. It is a reddish, limpid fluid, having the odour of tar. It is a mixture of various volatile constituents of tar. By redistillation it may be rendered colourless, and then becomes very similar to oil of turpentine. It is occasionally used as an application to ringworm of the scalp and scalled head. Swallowed in a large dose it has proved fatal. [Lancet for 1832-3, vol. ii. p. 598; also March 8, 1834.]
2. Pix nigra.—Black Pitch.
(Pix. L.; Bitumen aridum e Pice liquida praeparata; Pix arida.)
History.—This is the πίσσα ξηρά (dry pitch) of Dioscorides, [Lib. i. cap. 97.] which, he says, some call παλίμπισσα (pitch re-boiled).
Preparation.—The residuum in the still after the distillation of wood-tar is pitch (pix nigra, L.)
Properties.—At ordinary temperatures it is a black solid, having a brilliant fracture. It softens at 99° F. and melts in boiling water. It dissolves in alcohol, and in solutions of the alkalies and of the alkaline carbonates.
Composition.—Pitch is composed of pyrogenous resin and colophony.
Physiological Effects.—Made into pills with flour or any farinaceous substance, pitch may be taken to a great extent, not only without injury, but with advantage to the general health. It affords one of the most effectual means of controlling the languid circulation, and the inert and arid condition of the skin. [Bateman, Cynopsis of Cutaneous Diseases, p. 53, 6th ed.] As a local remedy it possesses great adhesiveness, and when applied to wounds and ulcers acts as a stimulant and digestive.
Uses.—Bateman [Op. cit.] speaks favourably of the internal use of pitch in ichthyosis. It has been employed also in other obstinate skin diseases. But the principal use of pitch is in the form of ointment, as an application to cutaneous affections of the scalp.
Administration.—Dose from grs. x to ʒj, made into pills with flour. The unpleasant pitchy flavour of the pills is materially diminished by keeping them for some time.
UNGUENTUM PICIS, L.; Unguentum Picis nigri; Unguentum Basilicum nigrum vel Tetrapharmacum.—(Black Pitch, Wax, Resin, of each ℥xj; Olive Oil Oj. Melt them together, and press through a linen cloth.)—Stimulant and digestive; used in the obstinate cutaneous eruptions of the scalp. [Vide Unguentum Picis liquidae.]