702

PRODUCTS OF DESTRUCTIVE DISTILLATION.

logwood bath, it will come out dyed of a brown colour in every part except the places to which the citric acid was applied.

Occasionally with substantive colours, such as indigo, what are called resist-pastes are used: if, for instance, patterns be printed upon the cloth, with cupric nitrate properly thickened, and the goods thus prepared be passed through the indigo vat, the nitrate will act by oxidizing the indigo in the parts to which the salt is applied, and the dye-stuff will not soak into the cloth in its soluble form at those points: hence, the colouring matter may be readily washed away from the spots to which the resist-paste was applied, whilst it remains fixed firmly everywhere else.

A peculiar modification of the mode of fixing the colours has received the name of steam colour-printing. For steam colours stannic oxide is generally employed as the mordant. The stannic chloride is decomposed by an excess of potash, which dissolves the precipitated oxide, and into this solution, when properly diluted, the cloth is dipped, and allowed to dry. After this it is immersed in very dilute sulphuric acid, to neutralize the potash, whilst the stannic oxide remains attached to the fibre: the colours, properly thickened, are then printed on in patterns, and the whole exposed to the action of steam. At this temperature the colour combines completely with the mordant, producing a brilliant effect.

CHAPTER IX.

PRODUCTS OF DESTRUCTIVE DISTILLATION.

(1536) (a) Products of the Distillation of Wood and of Resins.— The substances which are formed during the destructive distillation of wood in closed vessels are very numerous. They vary with the nature of the wood operated on, and the temperature at which the distillation is effected. The decomposition commences slowly at a temperature of about 284° (140° C.). The volatile portion of the products includes substances which are gaseous, as well as those which are liquid or solid, whilst a considerable residue is left in the form of charcoal in the retort. Amongst the gaseous products of the distillation, the most abundant are carbonic anhydride, carbonic oxide, and hydrogen. Of the liquid products one portion is soluble in water; the other portion is insoluble, and is of an oily consistence. The soluble portion consists of acetic acid, wood spirit, methyl acetate, and acetone, accompanied by water, and by other bodies but imperfectly known. The insoluble portion, which constitutes wood tar, is

DISTILLATION OF WOOD-STOCKHOLM TAR.

703

composed of a mixture of various liquids holding solid matters in solution, or in suspension: its most important constituents are several forms of hydrocarbon, such as toluol, xylol, cymol, and other bodies, such as eupion, which have been less perfectly examined, besides a number of oxidized compounds, including kreasote, picamar, and kapnomor. Amongst the solid portions are resinous matters more or less resembling colophony, and a waxy body, which has been termed paraffin; there are also many other bodies, such as naphthalin, cedriret, pittacal, pyrene, chrysene, and pyroxanthin.

The products of the distillation of wood obtained at the lowest temperature are those into the composition of which oxygen enters abundantly, such, for example, as water, acetic and carbonic acids. As the temperature rises, compounds containing less oxygen are gradually formed, such as wood spirit, acetone, and kreasote; at a still more elevated temperature various hydrocarbons, such as toluol, xylol, eupion, and the different forms of paraffin, are produced, whilst as the temperature approaches to redness, pure hydrogen predominates.

In the distillation in iron cylinders of air-dried hard woods, such as oak or beech, the charcoal reaches about 25 per cent. of the weight of the wood employed, or about three-fifths of the quantity of carbon present in the wood. The liquid portion, amounting to about 53 per cent., contains about one-fifth of the entire quantity of carbon in the wood; and the remaining portion of the distillate, consisting of uncondensed gases, carries off the remainder of the carbon. Among the liquid products acetic acid amounts to between 3 and 4 per cent. of the weight of the harder woods, naphtha to about 1 per cent., and the tar to 75 per cent.

Stockholm tar, so largely used in ship-building, is obtained by a rude kind of distillation of the resinous wood of the pine. A conical cavity is formed in the side of a hillock, the apex of the cone being below, and terminating in an aperture that opens into a trough leading to a reservoir for the tar. The kiln is filled with wood, and covered over incompletely with turf; the pile is lighted at the top, and the combustion regulated by covering it in more or less completely; the wood is thus charred from above downwards, and the tar flows off at the bottom, charged with a considerable quantity of resin, and mixed with acetic acid and oil of turpentine. When heated, an impure essence of turpentine is distilled, leaving a black resinous substance, which constitutes ordinary pitch. The specific gravity of the ordinary wood tar of commerce is about 1040.

In preparing pyroligneous acid, wood which contains but little

resin is used. The distillation is conducted in iron cylinders, and the condensation is more complete. The tar is of a different kind, and contains a variety of products, which have been already enumerated.

(1537) Paraffin (xЄH2) ; Sp. gr. 0·870.-This name was given by Reichenbach to a crystalline substance first found amongst the solid constituents of wood tar, and particularly in beech tar. It comes over during the last stages of the distillation, when the crude tar is rectified. Reichenbach also found it in the tar of both animal and vegetable substances, as well as in that of pit-coal. Paraffin is an abundant constituent of Rangoon petroleum, and is likewise obtained in considerable quantity from the products of the distillation of peat, as well as of certain forms of lignite, and of some varieties of bituminous shale. Α large proportion of the solid paraffin of commerce is obtained from the distillate of the bituminiferous substance known as Boghead Cannel (note, p. 148).

At ordinary temperatures beechwood paraffin is a hard crystalline white substance, without either taste or odour, somewhat brittle, and resembling spermaceti both to the touch and in appearance. It fuses at 110°7 (43°8 C.) to a colourless oily liquid, which remains transparent after solidification, and at a higher heat may be distilled unchanged. It does not dissolve camphor, naphthalin, or pitch when fused with them, but may readily be mixed with stearin, spermaceti, bees' wax, and common resin. Cold olive oil dissolves it sparingly till heated with it, but the hot oil dissolves it freely. Oil of turpentine and benzol dissolve it abundantly; so also does ether. Boiling absolute alcohol does not take up more than 3'45 per cent. of its weight, and deposits the greater part on cooling in crystalline plates. Paraffin is insoluble in water. It burns with a bright smokeless flame; candles made of it burn like those made of the finest The strongest acids and alkalies, and even chlorine, unless at a high temperature, are without effect upon it; hence it has received the name of paraffin (from parum, little, affinis, akin). In order to isolate the paraffin from wood tar, the heaviest portion of the tar oils is gradually mixed with from to its weight of oil of vitriol. The temperature rises rapidly, and a black liquid is formed, which emits sulphurous acid. The temperature must be raised to 212°, if it does not reach that point spontaneously; the mixture must then be maintained at rest in a place the temperature of which is not allowed to fall below 120°. A colourless liquid rises to the surface; it must be allowed to cool, when it will become solid. If now it be washed

wax.

PARAFFIN-PARAFFIN OIL.

705

and submitted to pressure, a liquid oil will be removed, and the paraffin may be purified by boiling with a solution of caustic soda, from which it separates on standing, and solidifies on cooling. The bodies termed cerene and melene, which Brodie obtained during the distillation of wax (1226), are very similar to this substance, and contain carbon and hydrogen in equal atomic proportions. It is probable that many substances termed paraffin consist of a mixture of several polymeric hydrocarbons, which form the higher terms of the series to which olefiant gas belongs; the melting point of different bodies presenting the appearance and properties of paraffin varying between 110° and 149° (43° and 65° C.). It is, however, very difficult to determine by analysis of the higher terms of the series, whether their formula should be Є„H or (E„H2n + 2), which latter hypothesis Anderson is inclined to favour, and the same view is supported by Pelouze and Cahours. Paraffin is an admirable insulator of electricity.

2n

Paraffin Oil.-Amongst the products of the distillation of Boghead Cannel, and of certain other bituminous shales, is a large quantity of an oily matter, the proportion of which is greater, the lower the temperature at which the distillation is effected. The oil is almost entirely free from oxygen. When rectified, a volatile portion may be separated from it, which, according to G. Williams, contains several of the radicles of the alcohols, including tetryl and amyl. The compounds which come over at a higher temperature appear to belong to the hydrocarbons of the form of (EH), or possibly (H2n+2). For practical purposes they may be separated into three portions, one of which remains liquid at very low temperatures; the sp. gr. of this is about o'820, and it boils at about 419° (215° C.) or upwards: it is largely used in lamps as a source of light as paraffin oil, and also under a variety of other names. The second portion, which is distilled at a higher temperature, deposits crystals of a solid hydrocarbon (one of the forms of paraffin), when exposed to a great reduction of temperature, and the last portions of the distillate are semisolid at ordinary temperatures. It is obvious, therefore, that the products of the distillation of this mineral contain a mixture of several substances, the less volatile of which are found by analysis to consist of hydrocarbons of the same class as paraffin. The less volatile liquid portion of the oil still retains paraffin in solution in a mixture of liquid hydrocarbons of nearly the same ultimate composition as paraffin. This oil is frequently used for the purpose of lubricating machinery, to which use it is well adapted, since it

.

706

PYRENE AND CHRYSENE-PYROXANTHIN-EUPION.

does not become oxidized or thickened by exposure to the air, and it evaporates but slowly.

15

(1538) Pyrene and Chrysene.-In the latter stages of the distillation of fatty and of resinous bodies, and in that of coal tar, a yellow powder is sublimed, which contains two substances termed by Laurent, pyrene and chrysene. Pyrene (H12), so called from up, fire, to indicate its origin from the action of heat, is soluble in hot ether, and may be separated from chrysene by means of this solvent, which at a low temperature deposits it in microscopic rhomboidal plates. It is insoluble in water, and is destitute of odour; it fuses at about 350° (177° C.), and may be distilled unaltered. Chrysene (EH),? is a crystalline substance of a beautiful yellow colour, whence its name, from Xpuosos, golden. It is inodorous, insoluble in water and alcohol, and only slightly soluble in ether: boiling oil of turpentine dissolves it more readily than ether. It fuses at about 450° (232° C.), and is partially decomposed when distilled.

4

(1539) Pyroxanthin (21H18? Gregory); Fusing pt. 291° (144° C.).--This substance was discovered by Scanlan amongst the products of the action of caustic potash upon crude woodspirit. When crude wood-spirit is rectified with lime, a dark brown residue remains in the retort, which when treated with hydrochloric acid leaves a brown insoluble matter, consisting of pyroxanthin and a resinous substance. If this residue be boiled in alcohol it is in great measure dissolved, and as the liquid cools, crystals of pyroxanthin are deposited in long yellow needles; hence its name-from up, fire, Eavfòs, yellow. It is insoluble in water and in alkaline solutions, but soluble in hot alcohol, ether, and acetic acid. It may be sublimed in a current of air at 273°. If it be sublimed in a closed tube it undergoes partial decomposition. It is soluble in sulphuric acid with a deep bluish red colour, and in hydrochloric acid with a fine purple, which soon passes into dark brown. Pyroxanthin appears to be the result of the action of potash upon one of the tar oils, which Voelckel has hence termed pyroxanthogen.

(1540) Eupion.-When the tar of hard woods is submitted to distillation, an acid liquid comes over, upon the surface of which a light yellowish oil floats; and on continuing the distillation a volatile oil, heavier than water, passes over. The most volatile portion of the lighter oil consists principally of a hydrocarbon, to which Reichenbach gave the name of eupion (from Euç, good, Tiwv, fat; Sp. gr. 0740 at 68°). He did not submit it to ultimate analysis, and it is probable that more than