the wood before the destructive distillation begins. And when resinous Coniferwood is being used, this is here treated with superheated steam, in order to carry off the oil of turpentine, before the wood is let down into the carbonising-shaft when thus warmed and partially dried. Distillation in the shaft is carried out by pumping in from below just sufficient air to maintain the fire. The carbonic acid (CO2) thus formed becomes reduced to carbon oxide (CO) in passing over the incandescent mass, and distillation is effected by the upward current of de-oxygenised air. The temperature varies at different levels in the shaft, being greatest down below where the air is pumped in, and decreasing gradually at the higher levels. Low down, where the heat is greatest, the last remnants of gaseous constituents are volatilised, the carbonisation of the wood is completed, and the charcoal is drawn and cooled by being plunged in water. Above that, in the zone throughout which the temperature ranges from about 550° to 850° Fahr., tarvapour is emitted; and in the upper portion of the shaft, the pyroligneous or crude acetic acid (from which pure acetic acid and wood-naphtha are subsequently obtained by fractional distillation) is mainly given off where the temperature ranges between about 300° to 550° Fahr. The tar and the pyroligneous acid can be collected in separate condensers, while the inflammable gases ascending to the top of the shaft are led off from there for heating-purposes-either throughout the factory, or to the foot of the carbonising-shaft of the furnace.

(2) Closed Iron Retorts give a larger yield of pyroligneous acid and tar than can be got from furnaces. The retorts may either be horizontal or vertical, and these latter may either be cylindrical and fixed in masonry, or somewhat conical and movable. The retorts now mostly used are horizontal cylinders of strong malleable iron, about 10 ft. long by 3 to 3 ft. in diameter (or about 70 to 80 cub. ft. capacity), closing with a cast-iron door in front, and having a tube at the back for carrying off the products of distillation. They are built into, or rest upon, masonry supports, which contain the fire-place. In place of the flames from this being allowed to come in direct contact with the retort, the hot air and the inflammable gases, generated during distillation and utilised for heating, are made to circulate around the retort before escaping from the chimney. While still hot from the last charge, the cylinder is packed as full of wood as possible; and to facilitate the drawing of the hot charcoal, an iron frame is fitted into the back with a long iron handle extending to the front. The process of distillation takes from about 12 to 16 hours, and the charcoal is then drawn quickly into sheet-iron cooling-cases mounted on wheels, which can contain the yield of one retort and be hermetically closed and allowed to cool down for about 36 hours.

The vertical cylinders are built into masonry, and are filled from above and emptied from below. The heating takes place from the side, to prevent the actual contact of the furnace-flames, and the heating-gases circulate spirally round the retort before escaping from the chimney, while the tube for leading off the pyroligneous acid and tar is at the top, just below the lid.

The somewhat conical and movable retorts are attached to their masonry supports by a strong cast-iron flange round their broad upper end. They are fired from below. The flames from the furnace do not come directly in contact with the retort, but holes in the arched brickwork allow the heating-gases to ascend and heat the walls of the retort before escaping above. When distillation is conpleted, the retort is raised and removed by a travelling crane, the lid at the top is opened, the whole tilted up, and the hot charcoal emptied into an iron coolingThe retort is then filled with wood, ready for replacing the next retort to be moved.

case.

The different vapours and gases generated in any of the above kinds of retorts escape through a cooling-worn, which acts as a condenser and leads the fluid, raw acid to a collecting tank, while the inflammable gases are conveyed by pipes to the

furnaces, to be used in heating the retorts. On movable retorts the condensing pipe can of course be unscrewed from each retort when removed, and attached to that taking its place.

In the collecting tanks, most of the tar sinks to the bottom, while the crude pyroligneous acid may be drawn off from above.

(a) Wood-tar, known commercially as Stockholm or Archangel tar, is of a dark-brown colour and a treacly consistence, and has a characteristic tarry smell if made from the wood of Conifers; while it is grey-brown to dark-brown in colour, tallowy or fatty to the touch, and has a strong burnt smell when made from the wood of broad-leaved trees. It is mostly used in its crude condition for coating fishing-boats and bridge-piles, fencing, &c.1

The chemical composition of wood-tar is very complicated, over twenty constituents being already known. If subjected to fractional distillation—by the application of slow heat gradually rising to a very high temperature, and thus allowing the different constituents to escape into condensers in a series of fractions on their respective distilling-points being reached-it can be reduced to form three other marketable products—namely, (1) 10-15 per cent of light oil (sp. gr. 0·90-0·97), "pine-oil," or naphthas; (2) heavy oil (sp. gr. 1·01-1·02) or crude creosote; and (3) 40-50 per cent of pitch as residuum-the other 15-35 per cent evaporated being water and a little acetic acid.—The light oil, which consists chiefly of a series of carbo-hydrogen compounds (such as Benzol (C6H6), Toluol (CH), Xylol and Cymol), is separated as the temperature gradually rises to 320° Fahr. ; and when refined, by elimination of the acetised water mixed with it, this can be used either for illuminating purposes, or in making solutions of fats, resins, &c.—The heavy oil distilled off when the heat increases to about 350°-500° Fahr. consists of creosote mixed with various other substances. This crude oil is sometimes used for impregnating wood, or for making waggon-grease, &c.; but it is generally refined to obtain the pure creosote by re-distillation, the oil which sinks in water being separated and mixed and shaken with concentrated sodium-lye, thus dissolving the creosote while leaving the other constituents undissolved. When this alkaline solution is cleared and treated with sulphuric acid, the heavy, oily creosote separates from the watery fluid, and can then be collected and refined by re-distillation.-The pure creosote thus obtained (which is, however, not a pure chemical substance, but a very complicated mixture of phenol, creosol, paracresol, phloral, &c.) is an oily and highly refractive fluid, colourless at first, then gradually turning yellow with exposure to light; it has a smoky smell, and burns the skin if applied to it. Its specific gravity varies from 1'03 to 1.08, its reaction is neutral, and its boiling-point lies between 400° and 430° Fahr., while it retains its fluidity to close upon 0° Fahr. It dissolves easily in alkaline lyes, alcohol, and æther, but not readily in water.-The pitch left as residuum hardens, as it cools down, into a lustrous black substance, which breaks with a shell-like fracture. It consists chiefly of paraffin and similar carbo-hydrogen compounds, and is largely used for caulking the seams between deck-planks and other wood-work of ships and boats, wood-paving-blocks, &c.

(b) Crude Pyroligneous Acid or Wood-vinegar, the crude commercial form of acetic acid, contains tar, creosote, wood-naphtha, and other impurities, which can only be removed by fractional distillation, and of which only the acetic acid and wood-naphtha have then a commercial value. This acid is largely used in making the acetates employed by dyers and calico-printers; and when purified and diluted with five times its volume of water, it is used as a substitute for vinegar in pickling and for table use; while it gives a "smoked" flavour to fish cured with it. As drawn off from the collecting tank the raw acid is a dull, reddish-brown fluid, having a tarry, pungent odour, a strong acid reaction, and a specific gravity of

In Russia a thin, light, volatile, oily, greyish- to blackish-blue, opalescent, fluid tar, smelling very much like petroleum, is made from Birch-bark. It consists mainly of Toluol (C-Hs, to about 50 per cent), Benzol (CH), and similar substances, and is used for obtaining these and for preparing "Russia leather."

102-105. Beech, Oak, other hardwoods, and Birch give the best form of acid. The crude acid drawn off from the collecting tank is usually at once prepared for market. The more primitive way of doing this is to neutralise it with quicklime, then enclose it in a retort and distil off the spirituous substances it contains; and the dry residuum left after evaporating the water consists of grey acetate of lime— though not in a pure form, owing to its being mixed with tarry substances.

In large factories an improved method is used, by which acetate of lime and crude wood-naphtha are obtained without vinegar being formed. These purer products are got by re-distilling the raw acid, after first neutralising it, in three stills or retorts ranged one above the other, sideways, to form a battery. The lowest and largest, which is heated by steam, contains the raw acid for distillation. The acetic acid vapour here produced is conveyed to the second retort, filled with dilute milk of lime, with which the acetic acid combines. The vapour escaping from this is conducted to the third retort, also filled with dilute milk of lime, which absorbs the rest of the acetic acid. The spirituous and watery vapour now left is led off into a cooling-vessel, where it is completely condensed and run off as crude wood-naphtha, wood-spirit, or methyl alcohol, ready for refining and rectification. The acetate of lime solutions in the second and third retorts are clarified by being pumped through a filter-press, and are then led off to be dried by first evaporating the water and then heating the residual grey powder in shallow iron vessels. This purified grey acetate of lime (Ca(C2H3O2)2) is the commercial product used as the raw material for the preparation of acetic acid, acetic salts (acetates), and acetone.

To obtain acetic acid (C2H4O2) the pure acetate of lime is dissolved with just sufficient dilute hydrochloric acid as may be needed, and acetic acid is distilled off at a temperature of 212°-250° Fahr.-when Ca(CH3O2)2+2HCl=CaCl2 + 2(C2H4O2). Though colourless, the acid thus distilled has a faint tarry smell, which is removed by re-distillation with an addition of 2-3 per cent. of bichromate of potassium. The distilled product contains about 35-40 per cent of pure acetic acid.

Since soda became cheaper, however, acetate of soda is often used in place of acetate of lime, and it has the advantage of producing almost pure acetic acid. The pyroligneous acid or wood-vinegar is neutralised with soda; and after the tarry substances are removed, this solution is distilled and the wood-naphthas are separated. The residual fluid is run off into a flat pan and reduced by heat to a 27 per cent solution (weighed warm), when it is still coloured red from tarry constituents. While still hot it is run into iron crystallising cases, where the acetate of soda crystallises; and on the crystals being separated centrifugally from the parent lye, they are melted in a boiler. Here the water of crystallisation is evaporated, and the heat is gradually increased till the whole mass becomes fluid and stands the test of forming a colourless solution in water. When this point is reached, the contents of the boiler are emptied into boiling-hot water, and the solution is filtered and spread out for crystallisation. These crystals are also separated centrifugally from the parent lye and distilled along with sulphuric acid, when almost chemically pure acetic acid is given off (here 2(NaC2H3O2) + H2SO ̧⇒ Na2SO1+C2H4O2).

Pure acetic acid has an extremely sharp, pungent smell, and blisters the skin if applied to it. It mixes readily in water, alcohol, and æther, and dissolves essential oils, camphor, resins, gums, &c. It is a monobasic acid, the salts of which are called acetates. Many of these, formed either from acetate of lime to begin with, or from free acetic acid, are used in dyeing, calico-printing, colour-making, and for chemical and pharmaceutical purposes, the chief being the acetates of lead, copper, iron, and aluminium.

By the dry distillation of acetate of lime at from 570° to 750° Fahr., acetone (CHO) is obtained. This is a fluid having a peculiar odour, a specific gravity of 0·79, and a boilingpoint of 133° Fahr.; its vapour is combustible, and forms an explosive mixture with air. It is chiefly used for the manufacture of smokeless powder, and as a solvent for oleo-resins. The crude acetone obtained by the first distillation is mixed with water to eliminate the tar-oils, and then re-distilled with the addition of soda-lye-the first, the medium, and the final products being collected separately. The medium product gives pure acetone on being

rectified with permanganate of potash; while the impure first and final products are again re-distilled, and then treated similarly. Acetate of lime gives 24-25 per cent of crude and 20-22 per cent of pure acetone.

The crude wood-naphtha or raw wood-spirit has to be subjected to several complicated processes before soluble naphtha and methylated alcohol can be produced for commercial purposes. To render it suitable for preparing aniline colours, the acetone has first to be got rid of by boiling it in a closed vessel, into which chlorine gas is introduced. This unites with the acetone, and forms a chloride having a high boiling-point, so that the woodspirit distils off. This is again rectified with lime, and then forms the strong-smelling commercial product known as methylated spirits, and used for many industrial purposes. In order to obtain the colourless, chemically pure methylated alcohol (CHO), however, this common chemical product has to be raised to the boiling-point again, and mixed with an equal quantity of oxalic acid to form crystallised oxalated-methyl-æther; and this is washed in water (in which it is insoluble) to clear away impurities, then distilled along with hydrate of potash, when the pure methylated alcohol is given off (Schwackhöfer, op. cit., pp. 340-344).

The percentage of acetate of lime and of crude wood-naphtha obtained from dry wood by its destructive distillation in closed furnaces and retorts varies considerably. The drier the wood and the slower the distillation, the larger is the yield for each of the products; the wood of broad-leaved trees furnishes more than that of Conifers, and the wood of the trunk more than branchwood; barked wood gives more acid, than unbarked, and sound wood more than unsound. It is important that the wood should be used as dry as possible, in order to carry out the distillation economically; hence the necessity for having it thoroughly airdried and well heated artificially (under proper ventilation) before distilling it.

In Continental factories 100 parts of dry barked Beech-wood usually gives about 7-8 per cent acetate of lime ("grey lime "), 1-14 per cent crude naphtha, 7-8 per cent tar, and 24-25 per cent charcoal, or in all about 40 per cent of marketable products; while the wood of Conifers gives about 2-3 per cent less acetate of lime, and per cent less naphtha, but more tar (up to 12-16 per cent in very resinous wood). In some places there is so little market for the tar, that it is used for heating the furnaces.

IV. Small Waste Wood. In large saw-mills there is necessarily a considerable quantity of small waste wood and of sawdust which cannot always be used as fuel, and which it is desirable to use profitably, if possible. This is often used in preparing acetate of lime, &c., in retorts (as above described, but on a smaller scale), or in making Wood-wool and Manila Shavings, while sawdust, besides being largely used for petty purposes (e.g., stuffing pin-cushions and dolls, cleaning dusty floors, &c.), is utilised extensively in making oxalic acid, and also for pressing into briquettes, and in the "carbonating stage" of preparing soda ash.

The production of the fine shavings known as Wood-wool and Manila Shavings, used for packing, for cattle litter, for rough filtering, &c., is another industry only profitable with wood of little value-e.g., for utilising waste pieces in saw-mills. After being cut into lengths of about 18-20 in. long and 6 in. broad, the wood is put between two rollers and cut longways by a number of parallel knives worked with a crank, while a plane at the side shaves off these cut portions, which then fall down below. The shavings are finer or coarser according as the parallel knives are set closer or wider apart.

Another form of wood-wool machine is shown in Fig. 283, and may be used with profit in saw-mills where many short cross-cut ends are left, and about 25 to 30 cwts, of "wool" of various degrees of fineness can be turned out per day. The pieces of wood are

placed one above the other, held down by weighted levers, and advanced to the knives by a continuous feed motion, actuated by a screw, bevel wheels, and cone pulleys. The rate of feed varies according to the wood and the fineness of wool required. The knives are fitted into a reciprocating slide and arranged to cut both ways. The feed stops automatically when as much wool is cut from the wood as is possible. The pieces of wood may be 24 × 12 x 12 in.; but they need not all be of the full length, and they may vary in thickness.

Oxalic Acid (H2C2O4+ 2H2O) is made by first fusing the sawdust with its own weight of a mixture of caustic alkalies (hydrates of potassium and sodium in proportion 2:3) in shallow iron pans, keeping the whole continually stirred at a temperature of 470° Fahr. The grey-brown powder thus obtained is dissolved in water (which leaves the sodium oxalate undissolved) and boiled with milk of lime to form oxalate of lime, which is dissolved with sulphuric acid, while the filtrate is evaporated sufficiently to allow the oxalic acid to crystallise. From 100 parts sawdus: about 80 per cent of raw oxalic acid is obtainable, which can be used for dyeing, calico-printing, &c., either in this form or as oxalates (salts).

In Briquette-making sawdust is used, or waste wood is reduced to pieces as small as practicable. One of the best processes is Heidenstam's patent, now used Fig. 283.

in Sweden. In this the sawdust and chips are passed under pressure between rollers to get rid of as much moisture as possible; then further dried in an apparatus heated by the uncondensed gases coming from the dry-distillationcylinder; and then moulded in briquette-presses and passed into the carbonising and dry-distilling apparatus, consisting of an upright iron cylinder built into a masonry furnace and provided with a pipe at the lower end for running off the distilled products. The briquettes are kept under pressure in a press during the whole time of their dry distillation, to prevent them falling to pieces. By regulating the temperature and also the pressure that is being applied to the briquettes, the dry distillation and carbonisation proceed in such a way that all the more volatile products are distilled off, and little else but the pitchy residuum of the tar is left behind, which binds and gives good consistence to each briquette. A cylinder containing a charge of about 2 tons of such material takes 14 or 15 hours for treatment. This method is said to yield (from every 100 parts Pineand Spruce-wood containing 18-20 per cent of water) about 5 per cent acetate of lime, 0.75 per cent wood-naphtha, 8-9 per cent tar, and 333 per cent of charcoal (or one-third of the original quantity of wood) in the form of briquettes; and these are said to have a heating-power of 7800 heat-units (compare with table on p. 593).

V. Potashes and all other kinds of salts of potassium were, until about forty years ago, made entirely from the ashes of wood and from various