part i

. 287.

[2305] Chabræus in his _Stirpium Sciagraphia_ (1666) remarks that he had seen the _Pesse_ (_P. Abies_ L.) in great plenty “_in Burgundicis montibus_,” yet makes no particular allusion to its yielding resin.

[2306] _Pharm. Journ._ ix. (1876) 164; also in Hanbury’s _Science Papers_, pp. 46 to 53.

[2307] _Oesterreichischer Ausstellungs-Bericht_, x. (Wien, 1868) 471.

[2308] I spent several days in the localities in 1873.—F. A. F.

[2309] _Traité des Arbres_, etc. i. (1775) 12.

=Description=—Pure Burgundy pitch, of which we have numerous authentic specimens, is a rather opaque, yellowish-brown substance, hard and brittle when cold, yet gradually taking the form of the vessel in which it is kept. It is strongly adhesive, breaks with a clear conchoidal fracture, and has a very agreeable, aromatic odour, especially when heated. It does not exhibit a crystalline structure, although, as we have frequently observed, the resin on the stem of the tree is distinctly crystalline.

Burgundy pitch is readily soluble in glacial acetic acid, acetone, absolute alcohol, and even in alcohol of 75 per cent. (sp. gr. 0·860), yet its solubility in these liquids is considerably altered by the presence of water or essential oil; and still more by the formation of abietic acid in the resin itself. The same influences also affect the melting point.

The crude resin of _Pinus Abies_,[2310] deprived of essential oil and dissolved in one part of absolute alcohol, was found to deviate a ray of polarized light 3° to the left, in a column of 50 mm.; the essential oil deviated 8·5° to the same direction. The oil contains a small amount of an oxygenated oil. After treatment with sodium the oil which remains does not form a solid compound if saturated with hydrochloric acid.

[2310] Collected by myself.—F. A. F.

=Chemical Composition=—The investigations of Maly mentioned at p. 607 afford a satisfactory elucidation of the chemical properties of the pinic resinous exudations. They all, according to that chemist, are mixtures of the same amorphous resin, C₄₄H₆₂O₄, with essential oils of the composition C₁₀H₁₆. These terebinthinous juices are collected and sold either in their natural state as _turpentine_, or deprived more or less completely of their volatile oil, in which condition they are represented by _Burgundy Pitch_, and finally by _rosin_ or _colophony_.

The turpentines flowing down the stems of the trees gradually lose their transparency if allowed to dry slowly in the air, becoming at the same time harder and somewhat granular. This alteration is due to the incorporation of water, which at last is not only mixed with the components of the resinous juice, but to some extent combines chemically with the resin so as to transform it into a crystalline body having the characters of an acid. The fact is easily observed if clear drops of the turpentine of _Pinus silvestris_, _P. Abies_ or _P. Picea_ are collected in vials and kept perfectly dry. Thus treated these turpentines remain transparent, but the addition of water causes after a short time the formation of microscopic crystals of abietic acid, rendering them more or less opaque.

If turpentines are collected before they lose their essential oil by evaporation and oxidation, and before they have become crystalline, they can be retained perfectly transparent by distilling off the volatile oil without water. The distillation being most commonly carried on _with water_, the remaining resin is opaque.

Maly is of opinion that the same amorphous resin occurs in all the _Coniferæ_, and that it yields by hydration the same acid, namely _Abietic_, which has been described by former chemists as _Pinic_, _Silvic_, and _Pimaric_ acids, all of which indeed are admitted to have the same composition. We must however remember that several sorts of turpentine, as Canada Balsam, appear incapable, according to our experiments, of yielding any crystalline resinoid compound whatever; and that their amorphous resin being but partially soluble is certainly not a homogeneous substance.

The crystals as formed naturally in the common turpentines do not exhibit precisely the same forms as those obtained artificially when the resins are agitated with warm diluted alcohol, as in the preparation of abietic acid. As to _Pimaric Acid_, we have prepared it in quantity from _galipot_, the resin of _Pinus Pinaster_, but have always found its crystalline character entirely different from that of abietic acid.[2311]

[2311] _Jahresbericht_ of Wiggers and Husemann for 1867. 37.

We are inclined, therefore, to think that the composition of the resins of _Coniferæ_ is not so uniform as Maly suggests. The remarkable variety of their essential oils is a fact which seems in favour of our view.

=Uses=—Burgundy pitch is prescribed as an ingredient of plasters, and thus employed is useful as a mild stimulant. In Germany it has some economic applications, one of which is the lining of beer casks, for which purpose a composition is used called _Brauerpech_ (brewers’ pitch), made by mixing it with colophony or _galipot_.

=Adulteration=—No drug is the subject of more adulteration than Burgundy pitch, so much so that the very name is understood by some pharmacologists to be that of a manufactured compound. The substance commonly sold in England is made by melting together colophony with palm oil or some other fat, water being stirred in to render the mixture opaque. In appearance it is very variable, different samples presenting different shades of bright or dull yellow or yellowish-brown. Many when broken exhibit numerous cavities containing air or water; all are more or less opaque, becoming in time transparent on the surface by the loss of water. Artificial Burgundy pitch is offered for sale in bladders; it has a weak terebinthinous odour, and is devoid of the peculiar fragrance of the genuine. The presence of a fatty oil is easily discovered by treatment with double its weight of glacial acetic acid, which forms a turbid mixture, separating by repose into two layers, the upper being oily.

PIX LIQUIDA.

_Wood-Tar_; F. _Goudron végétal_, _Poix liquide_; G. _Holztheer_, _Fichtentheer_.

=Botanical Origin=—Tar is obtained by submitting the wood of the stems and roots of coniferous trees to dry or destructive distillation. That found in commerce is produced in Northern Europe, chiefly from two species, namely _Pinus silvestris_ L. and _P. Ledebourii_ Endl. (_Larix sibirica_ Ledeb.). These trees constitute the vast forests of Arctic Europe and Asia.

=History=—Theophrastus gives a circumstantial description of the preparation of tar, which applies with considerable accuracy to the processes still practised in those districts where no improved methods of manufacture have yet been introduced.

=Production=—The great bulk of the vegetable tar used in Europe, and known in commerce as _Archangel_ or _Stockholm Tar_, is prepared in Finland, Central and Northern Russia, and Sweden.

The process is conducted in the following manner:—vast stacks of pine wood consisting chiefly of the roots and lower portions of the trunks (the more valuable parts of the trees being used as timber), and containing as much as 30,000 to 70,000 cubic feet, are carefully packed together, and then covered with a thick layer of turf, moss, and earth, beaten down with heavy stampers. The whole stack of billets is constructed over a conical or funnel-like cavity made in the ground, if possible on the side of a hill, this arrangement being adopted for the purpose of carrying on a downward distillation. Fire being applied the combustion of the mass of wood has to be carried on very slowly and without flame in order to obtain the due amount of tar and a charcoal of good quality. During its progress the products, chiefly tar, collect in the funnel-like cavity, from which they are discharged by a tube into a cast-iron pan placed beneath the stack, or simply into hollow tree trunks. The time required for combustion varies from one to four weeks, according to the size of the stack.

During the last few years this rude process has been improved and accelerated by the introduction of rationally constructed wrought-iron stills, furnished with refrigerating condensers, as proposed in Russia by Hessel in 1861. By this mode of manufacture the yield in tar of pine wood is about 14 per cent. from stems, dried by exposure to the open air; and 16 to 20 per cent. from roots. Large quantities of pyroligneous acid and oil of turpentine are at the same time secured. The wood of the beech and of other non-coniferous trees appears not to afford more than 10 per cent. of tar, while turf yields only from 3 to 9 per cent.

=Description=—The numerous empyreumatic products which result from the destructive distillation of pine wood, and which we call tar, constitute a dark brown or blackish semi-liquid substance, of peculiar odour and sharp taste. When deprived of water and seen in thin layers, tar is perfectly transparent. The magnifying glass shows some of the varieties to contain colourless crystals of _Pyrocatechin_, scattered throughout the dark viscid substance, and to these tar owes its occasionally granular, honey-like consistence.[2312] A gentle heat causes them to melt and mix with the other constituents.

[2312] _Jahresbericht_ of Wiggers and Husemann for 1867. 37.

True vegetable tar has always a decidedly acid reaction. It is readily miscible with alcohol, glacial acetic acid, ether, fixed and volatile oils, chloroform, benzol, amylic alcohol or acetone. It is soluble in caustic alkaline solutions, but not in pure water or watery liquids. The sp. gr. of tar from the roots of conifers is about 1·06 (Hessel) yet at a somewhat elevated temperature, it becomes lighter than warm water.

Water agitated with tar acquires a light yellowish tint, and the taste and odour of tar, as well as an acid reaction. On evaporation the solution becomes brown, and at last microscopic crystals are obtained with a brown residue like tar itself, which is no longer soluble in water. A microscopical examination of tar which has been exhausted with water, shows that all crystals have disappeared.

=Chemical Composition=—Dry wood may be heated to about 150° C. without decomposition; but at a more elevated temperature, it commences to undergo a change, yielding a large number of products, the nature and comparative quantity of which depend upon circumstances. If the process is carried on in a closed vessel, a residue will be got which has more or less resemblance to coal. By heating fir-wood enclosed with some water to 400° C., Daubrée (1857) obtained a coal-like substance, which yielded by a subsequent increase of temperature scarcely any volatile products.

The results are widely different if a process is followed which permits the formation of volatile bodies; and these substances are formed in largest proportion, if the heat acts quickly and intensely. At lower degrees of heat, more charcoal results and more water is evolved.

Among the volatile products of destructive distillation, those alone which are condensed at the ordinary temperature of the air are of pharmaceutical interest; and of these, chiefly the portion not soluble in water, or that which is called _Tar_ or _Liquid Pitch_. The aqueous portion of the products consist principally of empyreumatic acetic acid, to which tar owes its acid reaction.

The tissue of wood is chiefly formed of cellulose, intimately combined with a saccharine substance, which may be separated if the wood is boiled with dilute acids. The remaining cellulose is however not yet pure, but is still united to a substance which, as shown by Erdmann,[2313] is capable of yielding pyrocatechin.

[2313] Liebig, _Annalen der Chemie u. Pharmacie_, Suppl. v. (1867) 229.

It is well known that sugar subjected to an elevated temperature, yields a series of pyrogenous products; and the same fact is observed if purified cellulose is heated in similar manner. But for tar-making, wood is preferred which is impregnated with resins and essential oils, and these latter furnish another series of empyreumatic products. From these circumstances, the components of wood-tar are of an extremely complicated character, which is still more the case when other woods than those of conifers form part of the material submitted to distillation. In the case of beech-wood, _Creasote_ is formed, which is obtained only in very small quantity from the _Coniferæ_. Volatile alkaloids and carbolic acid, which are largely produced in the destructive distillation of coal, appear not to be present in wood-tar.

The components of the latter may be considered under two heads:—first, the _lighter aqueous portion_, which separates from the other products of distillation, forming what is called _Impure Pyroligneous Acid_. This contains chiefly acetic acid and _Methyl Alcohol_ or _Wood Naphtha_, CH₄O; _Acetone_, C₃H₆O; besides other liquid products abundantly soluble in water and acetic acid. In this portion, some pyrocatechin also occurs.

The second class of pyrogenous products of wood consists of a homologous series of liquid hydrocarbons, sparingly soluble in water, and which therefore are chiefly retained in the heavy layer below the pyroligneous acid, forming the proper wood-tar. The liquid in question furnishes _Toluol_ or _Toluene_, C₇H₈ (boiling point 114° C.), _Xylole_ C₈H₁₀, and several other analogous substances.

If tar is redistilled, an elevated temperature being used towards the end of the process, some crystallizable solid bodies are obtained, the most important of which is that called _Paraffin_, having the formula CₙH₍₂ₙ₊₂₎, _n_ varying from 20 to 24.

The crystals already mentioned as occurring in tar are _Pyrocatechin_. They are easily sublimed at some degrees above their fusing point (104° C.), or removed by acetic acid, in which as well as in water they are readily soluble. Hence in some sorts of tar this substance does not occur, it having probably been removed by water.

Pyrocatechin, C₆H₄(OH)₂, can be obtained by the destructive distillation of many other substances, as catechu, kino, the extracts of rhatany and bearberry leaves, and other extracts rich in that form of tannin which produces _greenish_ (not _blue-black_) precipitates in salts of iron. It is extracted from the granular sorts of wood-tar, by exposing them at a proper temperature to a current of heated dry air, or by exhausting them with water. Ether when shaken with the concentrated aqueous solution and left to evaporate, leaves colourless crystals of pyrocatechin which after purification are devoid of acid reaction. They have a peculiar burning persistent taste, and are very pungent and irritating when allowed to evaporate. A solution of pyrocatechin yields with perchloride of iron a dark green coloration changing to black after a few moments, and becoming red on the addition of potash. This mixture finally acquires a magnificent violet hue, like a solution of alkaline permanganate. No alteration is produced in a solution of pyrocatechin by protosalts of iron.

Among the few medicinal preparations of tar, is _Tar Water_, called _Aqua vel Liquor Picis_, made by agitating wood-tar with water. The presence in it of pyrocatechin is easily proved by the above-mentioned reactions, or by a few drops of red chromate of potassium, which produces a brownish black colouration. It may hence be inferred that pyrocatechin is perhaps the active ingredient in tar-water, and that for making this liquid the granular, crystalline sorts of tar should be preferred.[2314]

=Commerce=—Tar as well as pitch is manufactured in Finland, and shipped from various ports in the Gulf of Bothnia, as Uleaborg, Gamla Carleby, Jacobstad, Ny Carleby and Christinestad; also from Archangel and Onega on the White Sea. Some tar is also produced in Volhynia, and finds its way by the Dnieper to the Black Sea.

The North of Sweden likewise produces tar, chiefly about Umea and Lulea, the distillation being now performed in well-constructed apparatus of iron.

The pine forests of North America afford tar and pitch. Wilmington in North Carolina exported in 1871, 25,260 barrels of tar, and 3788 barrels of pitch.[2315]

[2314] We may suppose that the authors of the French _Codex_ were not of this opinion, inasmuch as in making _Eau de Goudron_, they order that the liquid obtained by the first maceration of the tar, shall be thrown away.

[2315] Consul Walker, _Report on the Trade of North and South Carolina—Consular Reports_ presented to Parliament, May, 1872.

The imports of tar into the United Kingdom in 1872, were 189,291 barrels, valued at £218,339. Of this quantity 145,483 barrels were shipped from the northern ports of Russia.

The barrels in which tar arrives hold about 30 gallons. Smaller sized vessels termed _half-barrels_ are also used, though less frequently.

=Uses=—In medicine of no great importance: an ointment of tar is a common remedy in cutaneous diseases, and tar water is sometimes taken internally. The consumption of tar in shipbuilding and for the preservation of fences, sufficiently explains the large importations.

Other Varieties of Tar.

_Juniper Tar_, _Pyroleum Oxycedri_, _Oleum Juniperi empyreumaticum_, _Oleum Cadinum_, _Huile de Cade_.—This is a tar originally obtained by the destructive distillation of the wood of the _Cade_, _Juniperus Oxycedrus_ L., a shrub or small tree, native of the countries bordering the Mediterranean. It was for centuries used in the South of France as an external remedy, chiefly for domestic animals, but had fallen into complete oblivion until ten years ago, when it began to be prescribed in skin complaints.

The _Huile de Cade_ now in use, is transparent and devoid of crystals. It is somewhat thinner than Swedish tar, but closely agrees with it in other respects. It is imported from the Continent, but where made and from what wood we know not. _Huile de Cade_ is mentioned by Olivier de Serres,[2316] a celebrated French writer on agriculture of the 16th century; it is named by Parkinson[2317] in 1640; also by Pomet,[2318] in whose time (1694) it was rarely genuine, common tar being sold in its place.

[2316] _Théâtre d’Agriculture_, Paris, 1600. 941.

[2317] _Theatrum Botanicum_, 1033.

[2318] _Hist. des Drogues_, Paris, 1694.