most part transformed and incorporated into the sterilised woody tissue. The vulcanisation is most effective if fresh, unseasoned wood is treated.

The timber to be operated on is brought in from the yard on a narrow tramway and run into one of the cylinders, in which vulcanisation takes place. There are four such cylinders, each 112 ft. in length (consisting of four sections of 28 ft. each), so as to admit of treating the largest size of telegraph posts. The timber being operated on when I visited the yard (1898) were sleepers, of which some 1410 cub. ft., or 28 tons, could be railed on little waggons into each cylinder. When deals are dealt with, the feeding consists of from 1000 to 1415 cub. ft., according to the way they pack on the small trollies.

The cylinders are built of boiler plate, and are anchored at the top end; while the other portions are supported by rollers on runners, to allow play for the expansion of between 14 inch and 2 inches, due to the hot air applied within.

When the load of timber has been trollied into one of the cylinders, the open end is hermetically closed by means of a heavy steel lid hinged at the top and heavily counterpoised (weighing in all about 9 tons), which is opened and shut by means of a cog-wheel worked by the leverage of a hand-wheel. On the lower end being brought into position, the complete closure against air-leakage is effected by means of numerous curved levers radiating like the spokes of a wheel from the centre of the lid, whence the levers are actuated, and pressing powerfully on the lid so as to seal it hermetically against the airpressure of 200 lb. per square inch from within. To facilitate air-tight closure, the lid of the cylinder is coated with asbestos packing. There are forty of such spoke-like levers, and each fits into a strong steel socket or slot. The opening or closure of the lid can take place in less than two minutes, only two men being required to work the wheel and apply all the requisite leverage. The lid having been closed on the filled cylinder, steam is turned into the pipes and surplus moisture from the outside of the wood is expelled and run off through cocks at bottom of the cylinder. The cocks being again shut, superheated compressed air, between 200° and 300° Fahr., is then driven into the cylinder. This is provided by an air-compressing machine in an adjoining chamber. The air-compressing engine (weighing about 35 tons) has steam and air cylinders, each 18 inches in diameter, and with a 30 inches piston-stroke, in which the air is compressed to a pressure of 200 lb. per square inch, while a spray of water is at the same time injected into it to absorb the heat evolved by compression. This moist compressed air is next filtered through a water separator which dries it thoroughly (?), after which it is pumped by the circulating engine (also of about 35 tons weight) first through cylinders containing tubes heated by steam and then through a kind of pipe-stove heated by a coke furnace, whence it is conducted to the cylinders where vulcanisation takes place. It is essential that there should be constant circulation of the superheated air in the vulcanising cylinders, and each complete stroke of the piston of the air-circulating machine displaces 35 cub. ft. of air. Fresh superheated air is forced into one end of the vulcanisation cylinder, and a corresponding quantity is drawn off from the other end. From the cylinder the air passes to a tubular cooler, and is conducted thence to the circulating pump for redistribution. Any loss of air-pressure due to circulation is replaced by the occasional use of the air-compressing machine. Both the air-compressing and the air-circulating

machines are worked at a slow rate.

The time taken in vulcanisation depends of course on the dimensions of the timber to be operated on, but for sizes like sleepers it occupies about eight hours; and no advantage has been found to accrue from continuing the treatment beyond this time. When the plant is not fully employed, the cylinders are fed in the morning, operated on during the day, and allowed to cool at night, the load being taken out early next morning and replaced by fresh wood for treatment. If worked under pressure of a large business, cooling can be facilitated by gradually lowering the temperature of the air pumped into the cylinders.

The effects claimed to be produced on wood by vulcanisation are-(1) that it will not decay; (2) that the cheaper kinds can be utilised for sleepers, mine-props, piles, &c. ; (3) that the technical properties of wood are improved, especially as regards diminished shrinkage; (4) that the resinous, oleaginous, and other properties of the sap within the cells and fibre become solidified; (5) that all "fungi," "germs," or "insects" are killed ;

(6) that the germinative principle inherent in the fluids is destroyed; and (7) that the wood is indurated, strengthened, perfected, and rendered less inflammable.

Samples of Pine sleepers, said to have been in the ground for about ten years, were shown to me in a thorough state of preservation; and if they really were what they were said to be, they furnished a striking proof of the utility of vulcanisation. During my visits to the works I had opportunities of examining Baltic Pine baulks and sleepers under treatment for the Natal Government, and of comparing them with the unvulcanised wood; and I could not but remark the highly resinous condition of the vulcanised wood in comparison with that awaiting treatment. This resinification, this transformation of sap and of the less durable portions of cell-walls into substances which become incorporated into the woody tissue, must increase considerably the resistance which the saturated fibres are able to offer to the natural processes of decay.

While it is easily intelligible how the transformation of albuminous substances, sap, and portions of cell-walls capable of undergoing transformation under heat and pressure can and probably must increase most of the technical properties of timber, yet it is almost impossible to accept in its entirety the statement put forward that vulcanisation renders wood less inflammable. Certainly for non-resinous woods this will be so, but in the case of Conifers resinification must produce greater inflammability; and in all woods operated on the heating-power must become enhanced.

The cost of vulcanising is 2d. per cubic foot for softwoods and about 4d. for hardwoods, a reduction being made for larger quantities. It is therefore somewhat cheaper than creosoting. The difference in the scale of charges between softwoods and hardwoods does not arise from any difference in treatment, but is due to hardwoods being usually sent in smaller quantities and in forms (e.g., veneers, cabinet-woods, sounding-boards for pianos, &c.) which involve more handling in the yard and during the process of vulcanisation than sleepers and baulks of Pine and Fir. So far as the actual treatment is concerned, the vulcanisation process is identical for both classes of timber, and mixed timber can be run into the cylinders. The actual cost of vulcanising must be very small if there are regular supplies of timber to operate on; but the first cost for machinery is very heavy.

3. Dissolving the Sap not only reduces the amount of moisture in the wood, but also weakens its hygroscopicity, so that it seasons quicker and shrinks less, while the shrinkage is more even and uniform in the various directions. It is not, however, a method capable of application on a large scale.

water.

The simplest way of dissolving the sap is to completely immerse converted timber, or barked sections of logs, for several months in running But large stems may require to be left for several years in water, as the sap can only be washed out by diffusion, which is a very slow and gradual process. This method can only be applied immediately after the logs have been felled, as certain constituents in the sap become insoluble if the logs have been long exposed to the air. Apart from the length of time this takes, and the amount of capital it therefore locks up, this method of treatment by immersion in river-beds does not fulfil the object in view.

Oak for flooring, which would require two years' seasoning in the open air, may be dried in four months after having been immersed in a stream for three or four months; and if immersed in water of a temperature of about 85° Fahr., the time of immersion may be reduced to fifteen or twenty days. After rafting the wood dries more rapidly, because of the sap having been partially driven out and replaced by water, which evaporates more readily than the sap.

For small timber, such as is used for turnery and carriage-making, the sap

can be better dissolved by boiling the wood in water, though even then the sap is never thoroughly got rid of. It prevents warping and splitting, however; but is far less effective than steaming.

4. Steaming. The converted timber is usually put into thick wooden boxes or cases 10 to 12 ft. long and 5 to 7 ft. broad and high, strongly bound with iron and capable of being hermetically closed at each end by means of iron screws and bolts. The box rests somewhat slantingly on strong supports, so that the condensed water can be run off at the lowest part by a turn-cock, while the mouth of the steam-pipe enters at the opposite end. To economise steam, the wood is packed as closely as possible into the box, care being taken, however, that the surfaces of the different pieces of timber should come into contact as little as possible, boards being set on edge.

When the box is packed and closed hermetically, the steaming is begun. At first the condensed water runs off fairly clear and colourless, but later on it gets much darker and discoloured, and has a peculiar woody smell from the extracts dissolved. The steaming is continued until this stage has passed and the condensed water again runs clear and colourless, showing that the sap has been dissolved and withdrawn to the full extent possible.

In steaming timber in wooden boxes great superpressure can of course not be applied; nor is this desirable, because then the timber loses in strength. After being steamed, the wood is either dried in the open air, or in special drying-chambers.

Steaming takes from about forty to eighty hours, according to the size of the timber operated on. Steamed wood differs in colour from wood naturally seasoned, and is generally darker. Oak turns dark-brown, Maple reddish, and Cherry yellow to red, while Beech turns brown to pinkish, and is then preferred for such work as parquet-flooring. Steamed wood dries quicker, is lighter, and is less liable to warp and split than unsteamed wood of the same degree of dryness. While still warm and moist as it comes from the steaming-box, it is very flexible; and in cooling and drying it retains the form given to it while still warm and moist. This quality is made use of in bending wood for ship's planking, carriage- and waggon-making, cooperage, chair-making, &c.

Some processes are, however, much more rapid than the above, as, for example, that known as the "S.S." (superheated steam), practised in London by the Wood Syndicate, Ltd., as follows:

:

The wood is placed in a specially constructed chamber and treated with superheated steam at atmospheric pressure for a time varying from 10 to 40 hours, according to the kind and thickness of the timber-softwoods with an open tissue taking from 10 to 18 hours, while closer hardwoods take from 18 to 40 hours. Superheated exhaust-steam may be used, and the total cost is said not to exceed 2d. per cubic foot. The timber may be used immediately after treatment without fear of shrinkage, warping, or twisting, and it can be planed just as when naturally seasoned.

Superheated steam is also employed under great pressure in almost precisely the same way as superheated air in the Haskin process of vulcanisation, above

described. The wood is enclosed in two strong iron cylinders, hermetically sealed and packed round with asbestos; then steam is introduced after passing through a system of pipes to free it from water carried along with it and to superheat it. Under the pressure of the steam the sap is expelled and runs off with the condensed water, and under longer exposure to the superheated steam the water contained in the wood is reduced to 10 per cent. This kills all fungi in the wood, and makes it much less liable to warp and split, but it diminishes its strength very considerably, as the steam affects the tenacity of the fibres.

In Leclerc's process the wood is arranged in a close chamber of masonry (being piled upon the open-work floor, and inclined slightly to induce the sap to move), and the steam is brought in for forty-eight hours by perforated pipes. Under this action part of the sap is drawn from the wood, and part is coagulated, but the result is not complete. The steamed wood is then dried by causing a current of air heated to from 85° to 95° Fahr. to circulate throughout the chamber for a fortnight in the case of planks of ordinary thickness. Each piece of wood is separate, to allow the steam and air to circulate freely over the whole surface. For drying, the warm air is alternately introduced from above and from below, and at one or the other extremity of the chamber, and is drawn in through the wood and exhausted by a ventilator woking at the opposite extremity.

5. Superficial Application of Preservatives.-This can take place in three different ways-by (a) polishing, (b) painting, or (c) charring and then coating with antiseptics.

(a) Polishing with shellac or wax is for the direct purpose of improving the ornamental appearance of the wood, although indirectly it also preserves it by hermetically closing the superficial pores. The polish used for furniture consists of shellac dissolved in alcohol, which is applied to the smooth prepared surface with a soft ball of rag, and well rubbed in with a little oil. For parquet-flooring the polish consists of bee's-wax and turpentine, and is rubbed on with a hard brush.

(b) Painting or Coating superficially with oil-paint, earth-oil, coaltar, wood-tar, carbolineum and other tar-products, linseed oil, or varnish also closes the superficial pores of the wood. It therefore prevents warping and splitting, due to hygroscopic changes, and hinders the entrance of fungous spores. Even when such a coating is only given for an ornamental purpose (e.g., painting of interior wood-work of Pine or Fir, or rubbing over Oak wainscotting with linseed-oil), it acts as a preservative if applied to wellseasoned wood. But if the wood painted be still green, the effect is then only to make it rot quicker, by preventing the evaporation of the excess moisture still contained above what there should be in an air-dried condition.

Tarring the surface of wood is only to be recommended when the objects coated are to be used in the ground, or in or under water, and not when they are to be exposed to the direct action of the sunlight, because then the black surfaces absorb a great deal of heat, and the wood is apt to split and crack. Coal-tar is preferable to wood-tar as a preservative, and some of the other tar-products (Carbolineum, Stop-rot, &c.) enter farther into the wood than tar.

A coat of tar is durable, and is a good protection against damp; but its action is almost purely mechanical, and thus applied it has little really antiseptic effect. It forms an impervious layer, and does not penetrate into the wood. It is warmed to make it sufficiently fluid for application, but its consistence is increased in

drying by the addition of a little slaked lime burned and powdered, or of a little

cement.

The two preparations of tar most suitable for this class of work are carbolin and antinonnin. Carbolin is a tar-oil containing about 10 to 15 per cent of creosote, of a light-brown to deep dark-brown colour, which smells strongly of tar, and has a sp. gr. of 1·13 to 1·19, and a boiling-point of 480° to over 640° Fahr. It can be used for coating bridge-piles, gate- and fence-posts, and other outside woodwork, but it should not contain above 15 per cent of creosote, as it then eats into the woody substance. Owing to its strong tarry smell it is not suitable for coating interior woodwork. Antinonnin, so called because it was originally used for "greasebanding" stems to prevent the caterpillars of the nun-moth from re-ascending them, is a combination of nitrous oxide, creosote and potash, with glycerine soap or other substance added to obviate the risk of explosiveness common to nitrous products. It is sold as a yellow paste, soluble in water to the extent of 5 per cent. It is better, however, to use only a 2 per cent solution, and to give two, or if necessary three, coats. These soak into the wood and have a direct antiseptic action. It is used for coating interior woodwork, and protects it against Merulius lacrymans and other fungi (see p. 187). It is almost odourless, and is therefore preferable to carbolin for inside woodwork; but it is less suitable for outside work, as it gets gradually washed out by rain. It is also mixed with lime for the whitewashing of walls (2 per cent solution).

Linseed oil is one of the oils which dry; and it possesses the property of absorbing atmospheric oxygen, and thus of thickening and forming a thin hard layer over the surface of the wood. This thickening takes place more rapidly if the oil be oxidised artificially by heating it to about 480° Fahr. and then rubbing in soluble metallic combinations (chiefly lead and manganese), when the oil becomes a varnish. In consequence of the metals and colours added varnish is thicker than oil, and has to be mixed with a little turpentine to enable it to be easily applied with a brush, whereas linseed oil can, like polish, best be applied with a soft ball of rag.

(e) Charring the wood when it is to be inserted in the ground, perhaps the oldest preservative process of all, ensures good results, but has its drawbacks and its limitations. It is not so much that the charred surface protects the interior of the wood from the action of either dryness or moisture, or heat or cold, as that the heat developed during the charring process transforms the fluid substances into preservative solids, and at the same time destroys saprophytic fungi, although the general character and appearance of the wood, however, remain unimpaired. But in order to get the heat to penetrate the wood beyond the charred surface, much of the timber has often to be burned away, thus reducing the size of the timber. Unless, however, the charred surface be tarred or treated with some antiseptic, it remains strongly hygroscopic; and this is of course a disadvantage for poles and posts used in the ground.

Superficial carbonisation is suitable to hardwoods which cannot be impregnated with antiseptic substances. The charring is produced by a flaming jet in a current of compressed air, which forms a kind of blow-pipe, producing great heat. The flame draws out the water from the superficial layers, dries the fermentable portions, carbonises the external part completely, and produces a thin torrefied surface impregnated with the distilled products, which are creosoted substances.