liquor in the one containing fresh black-ash, while the other two have liquors of intermediate but unequal strengths. The level of the liquor differs in each, being highest where weakest and lowest where strongest. When communication is opened between these tanks circulation is caused by hydrostatic pressure. The soda liquor from the black

where it solidifies into blocks of crude soda, termed ball soda or black-ash.

Black-ash is now extensively made in a revolving furnace, which does away with the arduous manual labour required in a stationary furnace, and which was first patented in 1853, though many difficulties had to be overcome before it became a success. Mr J. C. Stevenson, of the Jarrow Chemical Works, after much labour succeeded about 1870 in establishing its superiority over the older kinds of balling furnaces. A longitudinal section of this furnace is shown in fig. 4. It is either cylindrical or barrel-shaped, about 18 feet long and 10 feet in diameter, lined with firebrick. The furnace is driven by steam and the necessary gearing, a spurwheel being placed round and fixed to the cylinder, which turns on friction rollers. At one end it is

Fig. 4. Revolving Black-ash Furnace, partly in section: F, fire; R, revolving cylinder; W, friction wheels; H, tanks, &c., for evaporating soda-lye by waste heat of furnace.

furnished with a large fireplace, the fire gases from which pass through the barrel,' and onwards to heat the boiling-down pans, which are placed at the opposite end and arranged much in the same way as in the stationary furnace. The largersized revolving furnaces produce 30 tons of blackash in twenty-four hours.

Lixiviation of the Black-ash.-The crude soda so named requires to be porous, so that water will easily penetrate the broken lumps of it when placed in iron tanks. Formerly a series, say of four of these, was placed in a step-like arrangement in which the lowest contained the fresh black-ash and the highest that which was nearly exhausted of its soda. Fresh water flowed in at the top, and, as it dissolved out the soda, became gradually stronger in descending from tank to tank, till it reached its full strength in the lowest one. more recent plan is to have the tanks all on a level and communicating with each other by tubes; but the exhaustion of the black-ash takes place in a similar way. Weak soda liquor is present in the tank with the nearly exhausted ash, and strong

Α

ash is treated differently, according to whether sodaash or caustic-soda is to be made from it.

the

Caustic-soda. - As tank liquor consists of a strong solution of carbonate of soda, it requires to be diluted before it can be causticised with lime. Long iron cylinders contain this diluted liquor, into which lime is placed, and at the same time it is heated and agitated. After being allowed to settle, the clear liquor is drawn off and pumped into liquor settlers, the lime mud in these being saved and used in the black-ash furnace. Several iron concentrating cisterns are successively used with the aid of heat to bring up by degrees the causticised soda liquor to the required strength. From the last of these cisterns (boat-pans) the liquor, having a specific gravity of 1550, is run into cast-iron pots, each of a capacity of 10 tons, which are heated by strong fires, and here the concentration of the liquor is completed, and the caustic-soda ladled into sheetiron 'drums' containing 6 cwt. each. On cooling it solidifies into a white mass of sodium hydrate or caustic-soda, NaOH, which is now manufactured in large quantities, containing as much as 77 per cent. of sodium monoxide, Na2O. During the concentration nitrate of soda is used to decompose any sodium sulphide present in the liquor. Causticsoda is most largely used in soap-making and paper-making, but also in the manufacture of some coal-tar dyes and oxalic acid.

The

Soda-ash.-When this substance (carbonate of soda) and not caustic-soda is to be made, the black-ash liquor is differently treated. In referring to the black-ash furnaces (see figs. 3, 4) it has been stated that their waste heat is used to boil down the black-ash liquor. As the concentration of the liquor proceeds granular crystals of soda are deposited and scraped out into drainers. When heated these crystals yield soda-ash; but impure soda remains in the boiling-down vessel. This soda, which is mostly carbonate but also contains caustic-soda and sodium sulphide, is mixed with some sawdust and evaporated to dryness. black-salt, as this residual substance is called, is then heated in a carbonating oven in which the burning off of the sawdust generates carbonic acid, and this converts the caustic-soda and sodium sulphide present into carbonate, and sodaash is the result. The composition of commercial soda-ash is very variable, but it frequently contains about 80 per cent. of the carbonate, the remainder consisting of other compounds of sodium and small quantities of other substances. None of these, however, interfere with its use for the purposes for which it is usually employed. When it is sold the available percentage of soda (sodium oxide or Na2O) is quoted. For certain purposes soda-ash is refined by dissolving, settling, evaporating, and calcining. It is then called refined or white alkali, which should be free of caustic-soda and contain no trace of sodium sulphide, sodium sulphite, or of iron.

Crystals of Soda, Washing-soda, Na2CO3,10H2O. -The soda-ash used for making soda-crystals is previously calcined and dissolved in hot water in fron vessels, the solution being then allowed to cool.

From this solution large crystals of almost pure carbonate of soda separate. Ordinary washing soda consists of these crystals, which are of uniform composition and easily dissolved. They contain ten molecules of water-that is to say, they are composed of 37 per cent. of carbonate of soda and 63 per cent. of water. Crystal soda being quite free from caustic-soda and other compounds acting on the skin, the hands of washerwomen suffer less from it than from other kinds of alkali.

Bicarbonate of Sodu, 2NaHCO3.-As will be presently explained, this salt is now prepared on a large scale as a stage in the ammonia-soda process.

See SODIUM.

Sulphur Recovery.-The recovery of sulphur from the exhausted black-ash, which forms the waste heaps of the alkali-maker, is now, after many unsuccessful attempts to do so economically, practised on a large scale by Chance's process patented so recently as 1888. This residue, as has been stated, is essentially calcium sulphide, which when brought into contact with carbonic acid in the presence of water is converted into carbonate of lime, and sulphuretted hydrogen is liberated. The practical difficulty had long been the getting of hydrogen sulphide in a sufficiently concentrated state. Mr Chance's process is as follows: The vat mud (black-ash waste) has the coarser extraneous pieces removed by a sifting process, and is then made into a creamy consistency with water. In this state it is distributed into a series of cylindrical iron vessels for the purpose of having carbonic acid passed through it. These cylinders have three main pipes passing over them, with branches to each. By one pipe the carbonic acid is introduced at the bottom of the liquid, and the other two lead the gases away from the top of the cylinder.

The carbonic acid is produced in a limekiln, and passes, unavoidably mixed with nitrogen, into the cylinders, which successively become richer in sulphur compounds. The result is that for a time little else than nitrogen comes away from the last cylinder. But when the reactions in the cylinders are completed the final vessel of the series gives off gas rich in sulphuretted hydrogen. By means of stopcocks one of the pipes at the top of each cylinder conveys the nearly unmixed nitrogen to an open chimney, and the other takes the rich mixture of nitrogen and sulphuretted hydrogen to a gasholder. When the carbonic acid from the limekiln passes into the first cylinder containing the black-ash waste, carbonate of lime is produced with evolution of sulphuretted hydrogen, H2S. The latter passes on with the excess of nitrogen into the second cylinder, where there is formed sulphydrate of calcium, CaH2S2, which is a compound of H2S and CaS. In this way we have the sulphur concentrating from first to last in the series of cylindrical vessels until it is finally expelled as sulphuretted hydrogen gas. In the process the

carbonic acid combines with the calcium of the sulphydrate, giving off the two atoms of sulphur as H2S, so that for a given amount of carbonic acid used we get a double quantity of sulphur. All the time the vessels at the beginning of the series remain unsaturated, the nitrogen, amounting to about 70 per cent. of the gases pumped in, passes away in pipes, and is allowed to escape. It contains little or no sulphur; but by-and-by the gas in the vessels consists of from 30 to 35 per cent. of sulphuretted hydrogen and from 1 to 2 per cent. of carbonic acid, the remainder being nitrogen. This mixed gas is collected in a gasholder to enable it to be treated as required. The carbonated mud left in the vessels is drained, and used in place of limestone in the black-ash furnaces, so that any soda this dried mud contains is recovered.

553

The sulphur is obtained from the gas in the gasholder in a very pure state in cakes and flowers of sulphur by this operation. A definite mixture of the sulphuretted hydrogen (present in this gas) and air is passed through a layer of anhydrous oxide of iron in a Claus kiln, the oxygen present being only enough to unite with the hydrogen (of the H2S) to form water, the sulphur being set free. Iron oxide has the power of producing the combination without itself suffering change, the bed of this material becoming (without the use of fuel) sufficiently hot, by the chemical change which goes on, to volatilise the sulphur vapour along with the steam produced. The change is represented by the formula H2S + O = H2O+ S. But the sulphuretted hydrogen may also be itself burned to make vitriol, which is obtained of the same purity as when made from sulphur.

The Ammonia-soda process has within the last few years come into competition with and threatens to supersede that of Leblanc. It is based on the mutual reaction which takes place at ordinary temperatures between common salt and bicarbonate of ammonia in strong aqueous solutions. The sodium of the salt combines with the carbonic acid and the chlorine with the ammonia, giving bicarbonate of soda, which is insoluble, and chloride of ammonium, which remains dissolved in the liquid, thus:

NaCl + (NH4) HCO3 =NH4Cl + NaHCO3. The ammonia is recovered from the chloride and one-half of the carbonic acid from the bicarbonate for future use. Where possible natural brine is used, and this is brought to a specific gravity of nearly 1200, either by the addition of salt if too weak or by adding water if too strong. Ammonia in the free gaseous state is now passed into the brine until the required quantity is present, which is known by the amount of increase in the volume of the liquid. The ammonia enters a mixing tank under a perforated diaphragm, and the liquid is kept in agitation. A great rise of temperature is caused by the condensation of the gaseous ammonia, and this necessitates the running of a stream of water through a coil of piping inside the mixer to keep the heat as low as possible. The brine in running off passes through a filter to retain solid impurities, and then through another worm of piping surrounded by cold water. To form the bicarbonate of soda the ammoniacal brine requires to be saturated with carbonic acid. Air-pumps draw the carbonic acid from the limekiln and force it (after being properly cooled) at a pressure of nearly two atmospheres in at the bottom of a tower 50 feet high, which is kept nearly full of the liquid. This tower has perforated plates at every three feet of height to make sure that the gaseous bubbles are spread equally through the liquid. Every half-hour some of the pasty mixture in the tower is run off at the bottom. This is full of the small crystals of bicarbonate of soda, and these are separated by running the mass over a wiregauze filter covered by a cloth, a vacuum being maintained below. The bicarbonate of soda on the filter is nearly pure, and the liquid which passes through is ammonium chloride. The bicarbonate thus obtained is washed with water and carefully dried in apparatus of which there are various forms. As there is a comparatively limited demand for this kind of soda, it is afterwards heated in close vessels in which half of its carbonic acid is given off, thereby reducing it to the normal or common carbonate of soda (soda-ash). The gas given off is pumped back to the tower and used along with the kiln gas for carbonating fresh material. To expel any ammoniacal salts adhering to the carbonate of soda and render it denser for packing, the heat is

See Lunge's Treatise on the Manufacture of Sulphuric Acid and Alkali (1880); Diagram, with Key of the Leblanc Soda Process, by J. J. Miller, 1891 (for students); Journal of Chemical Industry (vol. for 1888), containing a paper by Mr Chance on Sulphur Recovery.

Soda Water. See AERATED WATERS. Söderhamn, a seaport of Sweden, on a bay of the Gulf of Bothnia, 13 miles N. of Gefle, exports some 250,000 tons of iron and timber (in 600 vessels) annually. It has been frequently burned down, the last time in 1865. Pop. 9044.

Sodium (sym. Na; equiv. 22:29; sp. grav. 0.973) is one of the metals of the alkalies, its oxide being soda. Its properties closely resemble those of the allied metal potassium. It is of a bluish-white colour, is somewhat more volatile than potassium, and further differs from that metal in having a higher fusing-point-about 208° (97° C.), a greater specific gravity, and in not catching fire when dropped in water (unless the water is heated), although, like potassium under similar conditions, it partially decomposes it and liberates hydrogen, and at the same time communicates a strong alkaline reaction to the solution. If, however, a piece of unsized paper is placed on the surface of cold water, and the sodium is placed on the paper, the metal takes fire and burns with a deep yellow flame. Strictly speaking, it is the liberated hydrogen rather than the metal which burns; but a little sodium, volatilised by the heat, burns with the hydrogen. When heated in the air it burns with its characteristic yellow flame, and is converted into soda. posed in vacuo to a red heat it assumes the form of vapour, and admits of distillation. Like potassium, it must be kept immersed in naphtha,

SO as

When ex

to exclude the oxidising action of the air. As a reducing agent it is little inferior to potassium; and as its combining power is lower, and it is obtained much more cheaply, it may usually be advantageously substituted for potassium in reducing operations. Sodium does not occur in the metallic form in nature, but its compounds are very widely distributed. It is found by far the most abundantly in the form of chloride of sodium (or common salt), but it likewise occurs as albite or soda felspar, cryolite (the double fluoride of sodium and aluminium), borax (the biborate of soda), trona (the sesquicarbonate of soda), and Chili saltpetre (nitrate of soda). Duhamel in 1736 discovered that potash and soda (now known to be the oxides of potassium and sodium) were distinct bodies. Sir H. Davy first obtained the metal Sodiun in 1807. The symbol of this metal, Na, is the abbreviation of Natrium, which is derived from Natro, one of the old names of native carbonate of soda.

The methods of obtaining sodium are similar to those already described for obtaining potassium. Intimately mix 30 parts of common soda-ash with 13 parts of small coal and 3 parts of chalk, knead them into a stiff paste with oil, heat them in a covered iron pot till the oil is decomposed, and finally distil them in an iron retort with the precautions which are noticed in describing the preparation of potassium. The object of adding the chalk is to prevent the separation of the charcoal from the carbonate of soda when the latter fuses. This mixture ought to yield nearly oneseventh of its weight of sodium.

Sodium combines with all the elementary gaseous

SODIUM

bodies, and two of these combinations, those with oxygen and chlorine, are of extreme importance and value. With oxygen sodium forms two compounds-an oxide, NaO, and a peroxide, NaO The latter is of no practical value. The oxide (soda) was formerly known as fossil or mineral alkali, to distinguish it from potash, which, from the source from which it was procured, was termed vegetable alkali. Anhydrous soda, NaO, is procured by burning the metal in dry air; it is of a yellowish-white colour, powerfully attracts moisture, and retains the water so firmly that it cannot be expelled by heat. Hydrated or caustic soda, NaHO, closely resembles, both in its properties and in the mode of procuring it, the corresponding potash compound. It is, however, not so fusible as the latter, and is gradually converted, by exposure to the air, into carbonate of soda, which is also an infusible salt in its anhydrous state. tion of hydrate of soda (or soda lye) is largely employed in the arts. It is prepared by boiling a tolerably strong solution of carbonate of soda in milk of lime until a portion of the filtrate ceases to effervesce on the addition of an acid. The solid hydrate has a specific gravity of 2:13, and the quantity of anhydrous soda in any solution may be closely approximated to by determining the specific gravity of the fluid and referring to a table indicating the strength corresponding to the specific gravity.

Solu

Many of the combinations of the oxide of sodium (soda) with acids-constituting soda-salts-are of great importance. Carbonic acid forms three salts with soda-a normal carbonate, a sesquicarbonate, and a bicarbonate of soda.

Carbonate of Soda, Na, CO + 10H,O, the Soda of commerce, is a colourless, inodorous salt, with a nauseous alkaline taste. It crystallises in large transparent rhomboidal prisms, which contain nearly 63 per cent. of water, but it readily parts with all this water on the application of heat. The crystals also lose the greater part of their water on mere exposure to the air, when they effloresce, and fall to powder. Water at 60° (15° C.) dissolves half its weight of the crystals, and boiling water considerably more, the solution acting like an alkali on vegetable colours. This salt, the natron of commerce, occurs native. in the natron-lakes of Hungary, Armenia, &c., in associa tion with sulphate of soda and chloride of sodium. In other regions it appears in an efflorescent form on the surface of the earth. It is now, however, almost entirely manufactured from sea-salt. For its manufacture, see Soda.

Sesquicarbonate of Soda, NaCO3 + 2NaHCO3 + 3H,O, occurs native in the form of large, hard, nonefflorescent prisms, in Hungary, Egypt, Mexico, &c., under the name of Trona. When strongly heated it loses one-third of its carbonic acid, and becomes converted into the preceding salt.

Bicarbonate of Soda, NaHCO,, may be formed by passing a current of carbonic acid through a strong solution of carbonate of soda, till saturation takes place, and allowing the mixture to crystallise; or it may be produced on a large scale by exposing crystals of carbonate of soda to a prolonged current of carbonic acid. Most of the bicarbonate in commerce is now, however, prepared by the ammoniasoda process (see SODA). In this a current of carbonic acid gas is passed through a solution of salt in aqueous ammonia, when chloride of ammonium and bicarbonate of soda are produced. The bicarbonate crystallises in four-sided prisms, which require 10 parts of water at an ordinary temperature for their solution. This salt is used largely in medicine. See AERATED WATERS.

Sulphuric acid forms with soda a normal and an acid sulphate. The normal Sulphate of Soda,

MOUNTAINS OF SODOM AND SOUTHERN EXTREMITY OF THE DEAD SEA.