and connected by off-flow pipes. Water is made to flow into the top cistern, and then, after percolating through the cellulose with which it is filled, drains through the perforated bottom and flows through the off-flow pipe on to the cellulose with which the next cistern is filled; and so on, from cistern to cistern, the solution strengthening as it percolates through more and more cellulose, until at last it issues from the last cistern in about a 6 to 8 per cent solution, which can be used again when strengthened by the addition of whatever fresh caustic soda may be necessary. When the cellulose in the top cistern has been thoroughly washed its contents are removed for bleaching; cistern No. 2 thus becomes No. 1, and the empty cistern taking its place as the last of the set is refilled with raw cellulose slid down from the boiler in a gutter-shaped shoot.

(2) The Acid or Calcium Sulphite process was invented by Mitscherlich about 1870, but has been much improved on since, and is now the method in general use, with special variations protected by patents. Its general principle is based on the maceration of the wood-chips by the agency of calcium bisulphite, Ca(HSO3)2, obtained by dissolving sulphate of lime (CaSO) in hydrated sulphurous acid (H2SO3). The active agent is here the sulphurous acid, while the use of lime is mainly subservient in enabling it to be employed in a convenient form; and it has the advantages over caustic soda of being cheaper and of dissolving the incrusting substance without wasting much of the cellulose itself. But the cellulose is not so soft and pliable as that obtained by the soda process.

In order to prepare the lye, sulphur dioxide (SO2) has first to be obtained-either by burning sulphur (when S + O2 = SO2), or by roasting iron pyrites (FeS2) in specially constructed kilns (when 2FeS2 + 110 = Fe„O3 + 4SO2)—and then hydrated (when SO2 + H2O = H2SO3), after which it can be made to act on limestone or carbonate of lime (CaCO3) so as to produce calcium bisulphite in setting free carbonic acid and water (when 2H2SO3 + CACO3 = Ca(HSO3)2 + CO2 + H2O). This preparation generally takes place in high towers, 60 to 100 ft. high, and 5 to 6 ft. square, made of planking closed with tow, tarred, and bounded with iron bands to strengthen it. The limestone, broken into pieces from 3 or 4 up to 9 or 10 in. in diameter, rests on a strong staging of oak-beams, below which the pipe enters, bringing the gas from the burning sulphur or roasted pyrites after this has been cooled in tubing, while from a reservoir at the top of the tower a shower of water is allowed to play over the limestone. Fresh limestone can be filled in below this reservoir as required, and of course the purer the limestone, the better is the lye, and the less often is it necessary to clean out the tower thoroughly by deluging it with water. The lye obtained by the action of the sulphurous acid on the limestone collects in a masonry tank under the staging upon which the limestone rests, and is led off from there by lead piping to a wooden tank outside. It usually consists of a 5 to 7 per cent solution, the strength being rather less during summer than in winter. The average quantity of sulphurous acid (SO2) contained is usually about 34 per cent, of which about two-thirds are free and one-third combined with the lime.

The shower of water requires to be so regulated that only a faint smell of sulphurous acid should be noticeable at the top of the tower, but this condition can only be satisfied with very high towers. One of Kellner's improvements consisted in treating the limestone simultaneously in two towers, by leading off in an earthenware pipe the superfluous acid from the top of the first tower to the bottom of the second, and then pumping up the weak lye from the bottom of the latter to the top of the first tower, to trickle down over the limestone there and be acted on by the fresh acid coming direct from the furnace-tube. The inconvenience of these high towers has also led to various methods of preparing the lye in batteries of four or five wooden cases or cisterns arranged at different levels, and which can be closely shut. The limestone rests on a wooden framework in each, below which the water or lye collects. The sulphurous acid is pumped into the bottom of the lowest case, from the top of this it is led by a pipe to the bottom of the second case, and so on to the fourth or fifth, on reaching the top of which the whole of the active part of the acid will have been absorbed. From each of these cisterns the weak lye, after reaching a certain level, is carried away by an off-flow pipe to the next cistern below, till it reaches its maximum strength in the lowest cistern, from which it is drawn off for use.

The wood-chips are prepared and chopped in the same way as for the soda process, but the boilers in which they are macerated are of an entirely different description. While able to stand a pressure of at least 6 atmospheres, or 84 lb. per square inch, they must also be able to resist the corroding action of the sulphurous acid; and these conditions are best satisfied by cylindrical, oval, or spherical vessels of cast-iron or steel, lined with lead (Kellner's patent), or with well-burned acid-proof tiles made of porcelain or glass. The cylindrical boilers may either be horizontal, and these mostly rotating, or else upright and immovable; while the spherical boilers are always made for rotating, but are mostly used only in preparing straw-cellulose.

The boilers are of enormous size, usually having a capacity of from 2000 to 3500 cub. ft.; and some of those recently brought into use have over 7000 cub. ft. capacity, in order to profit by the advantage of reducing the proportion of their wall-surface to their contents, because the plates soon get corroded by the acid. Each boiler has its own doors for filling in the chips and taking out the raw cellulose, together with all the necessary piping for filling in and leading off the lye solution and steam, and carrying off gases evolved, taps for drawing off samples, safety-valve, pressure-gauge and thermometer, &c. The heating takes place either by bringing in hot steam, or else by means of a hard-lead pipe serpenting round the inside walls, or suspended from the apex and hanging free inside the boiler (Offenheimer's patent, with rectangular piping).

When the boiler has been packed with wood-chips up to within 15 or 16 in. of the top, these are steamed at 212° Fahr., and sometimes more, to drive out the air, so that the lye may penetrate easily into the woody substance, and to make the chips settle down firmly so that more wood can also be added for treatment. When the condensed water has been run off, the boiler is filled with lye from the top, and the temperature is gradually raised to about 260° Fahr., although this, as well as the length of time the chips are allowed to boil, depends of course on the kind of wood under treatment. The boiling generally lasts either for about 60 hours at a pressure of 3 atmospheres (or 50 lb. per square inch), or for about 26 to 30 hours at a pressure of 4 to 5 atmospheres (or 60 to 70 lb. per square inch). Here, however, the pressure is not regulated solely by the temperature, as the expansive power of the gas evolved from the lye also comes into play. When the boiling is ended this free sulphurous acid gas is carried off to the towers in which the lye is prepared, and is used again for acting on the limestone there. The boiler is then washed out several times with water to carry off as much as possible of the lye and of the calcium monosulphite it forms, and the raw cellulose is taken out. No attempt is here made, as in the soda process, to recover part of the lye for strengthening and using a second time.

The waste here is, however, so strongly charged with noxious chemicals as to pollute streams, and this question must be fully considered before any pulp-mill is erected. From a large boiler of the size generally in use over 2000 cub. ft., or 12,500 gallons, of used-up lye are run off, containing close on 5 tons of organic substances and 3 tons of salts of lime. By running the waste lye into cisterns and mixing it with milk of lime, monosulphite of lime is formed and deposited, and the water can then be led off to irrigate fields or meadows before discharging itself into any public watercourse.

(3) The Electro-chemical process was invented by Kellner, and is one of his several patents. It is based on the fact that when a strong electric current is passed through a solution of sodium chloride or common salt (NaCl), the chlorine and the sodium become separated at the two poles; and as water is present, the former is partly transformed into hydrogen chloride or hydrochloric acid (HCl), and the latter completely into hydrated sodium or caustic soda (NaHO). Kellner's method consists of an earthenware cistern encased in masonry and

divided into two parts, each of which contains one electrode; and when the woodchips are alternately subjected to the action of soda-lye and of chlorine and hydrochloric acid, they become macerated and bleached at the same time.

As pulping by the calcium sulphite process is cheaper, however, this electrochemical method is practically only used for bleaching purposes.

The raw cellulose taken from the washing-cistern in the soda process, or direct from the boiler in the calcium sulphite process, is a soft, crumbling, reddishyellow mass, consisting mostly of bundles of vessels which require to be separated. This is now usually done by placing it in drums of a slightly conical shape, revolving round an axle set with wheels of spikes or finger-like processes. Through the friction thus produced when the drum revolves the bundles of vessels are separated, while any hard unmacerated pieces remain entire. The thin pulpy broken-up mass is then removed to a second drum formed by pieces of wood or vulcanite, through the narrow slits between which the water drains off and the pulp empties itself at the lower end. It is then passed down a gutter of from 30 to 60 ft. in length, in which the heavier bits of unmacerated wood and impurities of one sort and another are separated, while the fine pulp flows into a vat, from which it is raised by a bucket-wheel to have the water further drained off in fine-meshed sieves. The drying and pressing of the pulp then takes place under cast-iron cylinders heated by steam and made to rotate slowly.

Bleaching. When the pulp has to be bleached at once, as when it is being prepared for filtering purposes, a clear watery extract of chloride of lime is almost always used, the active constituent in which is the hypochloride of lime (CaCl2O2). The pulp prepared by the acid process is easiest to bleach, and (for 100 parts of dry cellulose) about 8 per cent of chlorine (Cl) is sufficient; while for that made by the alkali process, from 10 to 12 per cent of chlorine is required for the sulphate-cellulose, and about 20 per cent for the soda-cellulose. Bleaching is confined merely to the necessary degree, because it diminishes the strength and elasticity of the felty fibres.

The ultimate out-turn in cellulose from the different kinds of wood is, weight for weight of raw material used, about the same. It is usually, however, slightly higher for Conifers than for broad-leaved trees, though the difference in the various kinds of trees is mainly due to variations as regards waste in cleaning and preparing the wood for treatment. One ton weight of seasoned wood yields on the average about 6 cwt., or 30 per cent, of cellulose by the soda process, and about 10 cwt., or 50 per cent, by the calcium sulphite process; and by the latter it has been found that every cubic foot of wood treated gives from 12 to 13 lb. of cellulose, or between 5 and 6 cwt. per ton of 50 cub. ft.

III. Charcoal-burning is probably, next to the conversion of timber, the most ancient of woodland industries; and it is one which was formerly of very great importance throughout Britain, before coal began to be used for iron-smelting. As early as 1558, timber-trees of Oak, Beech, and Ash were prohibited from being made into charcoal, and in 1581 and 1585 further Acts were passed to protect other trees in woods and underwoods (see vol. i., Introduction, pp. 18, 19). But even after the last charcoal iron-smelting furnace was shut down at Ashburnham, in Sussex, in 1809, the use of charcoal long continued to be of special importance in many industries,such as gunpowder-factories, glassmakers, blacksmiths, locksmiths, &c. Nowadays, however, charcoal-burning plays but an unimportant part in British woodlands, being still practised on a larger scale in the Forest of Dean and in the Midlands than in any other parts of the United Kingdom. And of

course with such a bulky, though light, material it is a special necessity that the place of production should be as near as possible to the place of consumption. Besides being still used by glassmakers, locksmiths, and blacksmiths, it is much preferable to coal for the preparation of rolled steel and of armour-plates for warships; while for making the finer sorts of gunpowder Alder-buckthorn (Rhamnus) and Dogwood (Cornus) are still largely used when obtainable, with Alder for commoner sorts. In a glowing condition charcoal has a strong affinity for and readily absorbs gases and fluids, and also colouring matter and perfumes. To this property it owes its use for such purposes as extracting fusel-oil from spirits, and for clearing solutions, &c. When used for filtering water, such chemical action soon ceases, and only the purely mechanical action continues. Like other commodities, however, charcoal has felt the irresistible effects of the competition of coke from gas-works and of cheaper foreign imports both from east and west, so that now, in most parts of Great Britain and Ireland, charcoal-burning, when still carried on at all, is chiefly done to use up the lop and top and other waste wood in making small quantities for ordinary estate and household purposes.

Even in the great forests on the Continent, however, charcoal-burning has undergone quite a revolution during the last thirty years. When the author served his apprenticeship in forestry in Hanover (1873-4) a fortnight's course of charcoal-burning in the Harz mountains was considered a necessary part of one's work; but now that even within the depth of large forests wood has gradually risen in value for pulp, cellulose, &c., charcoal-burning in stacks or kilns built up in the open air with billets of wood and covered with a coating of brushwood and charcoal-dust has given place to more elaborate methods of dry or destructive distillation in retorts, which enable other products, such as acetic acid and wood-tar, to be obtained for commercial purposes, while still leaving the charcoal as a marketable residuum. Like the more primitive charcoal-burning itself, however, even these improved methods can now only prove profitable where the wood has a small value, owing to difficulty or cost of transport to timber-consuming centres.

Large quantities of wood-tar are produced in Scandinavia and Russia (see p. 603), and both these and the charcoal are exported. Even in Central Germany, at Laubach, in Hesse, it pays to use Beech for the dry distillation of acetic acid, leaving charcoal as a byproduct, and the Friedrichshütte factory there consumes 280,000 cub. ft. of Beech yearly for this purpose. But of course it is only where wood is abundant and cheap that such an industry can possibly be profitable.

By the dry distillation of wood charcoal is produced either as (1) the chief product, or (2) merely as a by-product. In the former case, the carbonisation of the wood takes place under partial exclusion of air, but with direct application of fire, either in pits dug in the ground, or in kilns or stacks built of pieces of wood and covered with brushwood, earth, and charcoal-dust; while in the latter case it takes place in retorts, under complete exclusion of atmospheric air and without the direct application of fire. In the former processes there is, and always must be, a partial combustion of the carbon contained in the wood, simultaneously with the dry distillation; while in the latter, the more thoroughly the atmospheric oxygen is excluded, the more completely will the objects in view be effected, and the larger will be the yield both of condensible products of distillation and of the charcoal or carbonised wood forming the residuum.

When wood is heated under partial or total exclusion of air, first of all only the water it contains is driven off till the boiling-point is reached (212° Fahr.), but it is not until a temperature of about 300° Fahr. is exceeded that the decomposition of the woody substance begins. At higher temperatures than 300° Fahr., three stages of decomposition may be distinguished, viz.-(1) at from 300° to 500° Fahr., (2) at from 500° to 625°, (3) at from 625° to 800°.

During the first of these stages of great superheating (300° to 500° Fahr.) watery substances are evolved to the extent of about 60 per cent of the weight of the dry wood. These contain a small proportion of acetic acid, wood-naphtha, and similar products (combined in the form of pyroligneous acid or wood-vinegar), while tar and non-condensible gases are only emitted to a very limited extent.

During the second stage (500° to 625° Fahr.), the evolution of the watery substances decreases, and carbo-hydrates, such as marsh-gas (CH4), acetyline (C2H2), aetyline (C2H ̧), &c., are given out along with carbon oxide (CO) and carbonic acid (CO2), while the small quantity of nitrogen contained in the wood combines with the hydrogen to form ammonia (NH3), and partly also methylamine (CHN). The total loss in weight now amounts to about 70 per cent.

It is during the third stage (625° to 800° Fahr.) that tar is chiefly produced, in the form of a dark-brown viscous mass, which mostly sinks down to the bottom of the watery products of distillation. Its chief constituents are paraffin, cresole, carbolic acid, benzole, toluole, &c. Almost the only gases now evolved are methan and hydrogen. By this time the original weight of the wood has been decreased by about 80 per cent, and the residuum, weighing about one-fifth of its original weight, consists of charcoal. If the temperature is superheated still more, the process of decomposition can be still further carried on, as an absolute elimination of all gases is impossible owing to small quantities of hydrogen and oxygen always remaining in the residuum; but for all practical purposes the carbonisation is fully completed at 750° to 840° Fahr. Ordinary charcoal, however, made at a temperature up to about 650° Fahr., weighs on the average about 25 per cent or only one-fourth of the weight of the wood used in forming the kiln.

In Violette's investigations into the carbonisation of the wood of the Alder-buckthorn (Rhamnus frangula), he obtained the following results (Schwackhöfer, op. cit., pp. 325, 326, 340).

By whatever process it may be produced, good charcoal should possess the following qualities:

(1) Its colour should be deep black, with a steel-blue metallic sheen, and lustrous across the transverse surface; and it should not change in appearance. The brownish or reddish colour of "foxy" charcoal is a sign of carbonisation having been incomplete ;