Preparation

Tar is procured by the slow combustion of pine wood, so covered with earth as to admit but a scanty supply of atmospheric air. The heat caused by the burning of a portion of the wood serves to char the remainder; and the resinous ingredient, melted and partially changed, mixes with the condensed products arising from the decomposition of the wood, and flows out in a semiliquid form; charcoal being left behind. Almost all the tar of the United States is prepared in North Carolina, and other pine regions of the South.

Properties

This is a semiliquid, very tenacious substance, nearly black, of a peculiar, empyreumatic, not disagreeable odour, and a bitterish, resinous, acidulous taste. it is highly inflammable. Water dissolves a small proportion of its constituents. it is soluble in alcohol, ether, and the fixed and volatile oils.

Composition

Tar is a very complex substance, containing resin, acetic acid, oil of turpentine, water, and various volatile products of the decomposition of ligneous and resinous matter, condensed in the liquid form. When distilled, it yields two products; one consisting mainly of acetic acid with water and various impurities, called pyroligneous acid; the other an oily liquid called the empyreumatic oil of tar. This oil is extremely complex, yielding to analysis no less than seven distinct substances, of which two only, picamar and creasote, have any particular interest for the physician; the former as the bitter principle of tar, the latter as an important remedial agent. I have always, however, doubted whether creasote exists in a free state in tar; for this has none whatever of its distinctive odour, unless treated with an alkali, when the smell of creasote is strongly developed. Any one may convince himself of this by applying tar to his hands, and then washing them with soap and water. I infer that creasote is either neutralized in tar by combination, or that in fact it is a product of reaction among the constituents of tar; and we are not, therefore, to expect the same physiological and remedial effects from the two substances.

Medical Properties and Uses

Tar is very analogous in its effects to the turpentines. it is locally irritant, and, admitted into the system, proves stimulant to the circulation, and to the secretory functions, especially that of the kidneys. Along with its diuretic operation, it sometimes irritates the urinary passages, producing pain and other symptoms of strangury. it is also probably excitant to the bronchial mucous membrane, and, like turpentine, exercises a stimulant influence over the ultimate tissues everywhere. That a portion of its active matter is absorbed, is evinced by its odour in the urine and other excretions, when largely swallowed. The presence of carbolic acid has been chemically detected in the urine, even after its external application in the form of ointment. (B. arid F. Medico-chirurg. Rev., July, 1856, p. 181.) in large quantities, tar is capable of exciting high vascular irritation or inflammation in the stomach and bowels.

Though possessed of most valuable properties as a local stimulant, tar is seldom used internally. it may, however, be employed, either in substance or infusion, in the various affections of the urinary organs to which turpentine is adapted; and has been used, to a considerable extent, in chronic bronchial inflammation, with a view to its stimulant and alterative influence on the mucous membrane. it has also been used in obstinate piles, and is thought by some to exercise a very favourable influence over certain ulcerative or chronic inflammatory conditions of the mucous membrane of the bowels. When we come to the consideration of the rubefacients, it will be seen that tar acts almost like a specific in some cutaneous eruptions. A close analogy has been supposed to exist between certain obstinate diseases of the bowels, and these affections of the skin; and it has not unreasonably been supposed that tar might produce, in the former, effects analogous to those which it so obviously produces in the latter. Hence, it has been used by Drs. Simpson and Cumming, of Edinburgh, and with supposed benefit, in an obstinate variety of pseudomembranous inflammation of the bowels, with which the inhabitants of that town are afflicted, and of which I have met with examples in this country. (See my Treatise on (he Practice of Medicine, 5th ed., i. 621.) it may, indeed, be given, with reasonable hope of benefit, in obstinate chronic inflammation of the intestines generally, whether enteritic or dysenteric, and with or without ulceration or false membrane; provided all symptoms of acuteness are absent.

The vapour of tar is often highly useful in chronic bronchial inflammation, or other pectoral disease attended with copious mucous, or muco-purulent expectoration. I have seen it do much apparent good in these cases; but it should be long and steadily persevered with. it probably acts as a mild stimulant and alterative to the diseased membrane, and the surface of cavities. I have no idea, however, that it can materially modify tuberculous disease of the lungs. For the method of using the vapour, the reader is referred to the general observations on inhalations (i. 74).

Administration

The dose of tar in substance is from half a drachm to a drachm; and from two drachms to half an ounce may be taken daily. it may be given either in the form of pill, made with some absorbent substance, as wheat flour, or powdered liquorice root, or in electuary made by mixing it with sugar.

Tar Water (infusum Picis Liquids, U. S.; Aqua Picis Liquids) has been a good deal used in pectoral and urinary disorders, and as a wash in cutaneous eruptions; more, however, formerly than at present; though I have no doubt that it is occasionally efficacious, especially in diseases of the urinary passages. it is made by stirring together a pint of tar and half a gallon of water, and filtering after the subsidence of the tar. it has a reddish-brown colour, somewhat of the odour of tar, and an acidulous, sharp, empyreumatic taste, which it owes to the acetic acid, and the oil of tar held in solution by means of the acid. One or two pints of it may be used daily.

For a preparation of tar called beer or wine of tar, see U. S. Dispensatory.