Considered industrially, by far the most important bodies are the carbonates, the sulphates, and caustic soda (sodium hydrate), the manufacture of which forms the basis of the soda industry. Immense quantities of these bodies are used in the manufacture of soap and glass, and under the name of “washing soda” or “soda crystals” the consumption of the hydrated carbonate for domestic washing is also very great. There are indeed few chemical industries in which soda in one form or another does not play an important part, and the combinations of economic value into which it enters are numberless. It will be convenient to treat of the manufacture as a whole by itself, after speaking of sodium and its salts in their chemical relations.

Sodium, in German Natrium (symbol, Na; atomic weight, 23·053; O = 16), is a univalent metal. It occurs nowhere in nature in an uncombined condition, and was first isolated in 1807 by Sir Humphrey Davy through the medium of voltaic electricity. It is prepared by distillation of an intimate mixture of carbonate of soda with charcoal. The process is quite analogous to that followed in making metallic potassium (Potassium, vol. xix. p. 590), but much easier of execution, and free from certain dangers which attend the preparation of the other. The distillation is conducted in cylindrical iron retorts protected against the fire by means of fireclay tubes fitting closely round them. In the charge is included a certain proportion of chalk, which, giving off carbonic acid, aids in driving over the metallic vapour and protects the distillate against oxidation. The metal cast into the form of cakes or ingots is protected from the air by a coating of paraffin and secured in closely fitting soldered-up tinned-iron boxes. Metallic sodium is very similar in properties and appearance to potassium. The principal points of difference are its pure white colour, its specific gravity (0·9735 at 13°·5 C.), and its fusing point (95°·6 C.). In thin layers its vapour is colourless, but dense fumes have a purple tint. It decomposes water violently, but the hydrogen evolved does not take fire, although the reaction is more dangerous than the corresponding phenomena developed by potassium, because it leads frequently to most violent explosions. Yet the process serves in practice for the preparation of pure soda hydrate. In this operation a piece of sodium is placed in a silver basin standing in a shallow cold-water bath. Drop by drop water is added—the metal between the additions being allowed to expend its energy—till the desired quantity of hydrate is formed. The process, in short, is so conducted that, except at first, the metal never touches water in any other form than that of a strong soda lye. Sodium is largely made for use as an agent in the manufacture of aluminium and magnesium, and as a reagent in laboratory operations. The metal does not affect carbonic oxide at any temperature; it acts on hydrogen as potassium does; but the compound is less stable. On ammonia gas it acts, as in the parallel case of potassium, with the formation of NH 2 Na, only the reaction is less energetic. Sodium has less powerful affinity to oxygen than potassium; in dry air or oxygen it burns into the dioxide Na 2 O 2 ,—a product obtainable also by heating the nitrate or nitrite. A white solid soluble in cold water, forming a hydrate, Na 2 O 2 + 8H 2 O, is obtainable in crystals, the solution of which is strongly alkaline. With acids it yields sodium salts and peroxide of hydrogen. Sodium tetroxide (Na 2 O 4 ) is not known to exist.

Caustic Soda (NaHO) is prepared from carbonate by means of caustic lime, just as caustic potash is made from its carbonate (see Potassium). The analogy between the two caustic alkalis is so perfect that we need only summarize the points of difference between them. Compared with caustic potash, caustic soda is less easily soluble in water, less caustic, less energetically basilous, less prone when fused in air to pass into peroxide, hence less destructive to platinum, iron, nickel, and silver vessels.

Sodium Chloride (NaCl) occurs in nature in a nearly pure state. Absolutely pure salt is made from commercial salt by precipitating from a solution the lime and magnesia by pure carbonate of soda, filtering, neutralizing with pure hydrochloric acid, concentrating by evaporation, and then precipitating the pure salt by a stream of hydrochloric acid gas. The crystalline precipitate is collected over glass wool, washed with pure fuming hydrochloric acid, and dried by heating in a platinum basin. It forms non-hygroscopic crystals, free from combined water, having a specific gravity at 16° C. of 2·162 (Stolba), and according to Carnelley fusing at 776° C. The solubility of pure salt in water is almost independent of temperature; 100 parts of water dissolve—

at 0° 14° 60° 100 β 109°∙7

(boiling)

35∙52 35∙87 37·25 39∙61 40·53 parts of NaCl.

Regarding its commercial relations, &c., see Salt. Chlorate of Soda (NaClO 3 ) is a salt of some industrial importance, from its use in the manufacture of aniline black. It may be made from the potash salt (a ) by decomposing this with hydrofluosilicic acid (which precipitates the potassium as fluosilicate and yields a solution of chloric acid), and neutralizing the chloric acid solution with soda, or (b ) by double decomposition with bitartrate of soda, NaHC 4 H 4 O 6 ,—the cream of tartar (KHC 4 H 4 O 6 ) separating out almost completely, whilst the chlorate of soda remains in solution. According to Weldon, it is best manufactured from caustic soda by the direct action of chlorine, the two salts NaCl and NaClO 3 being separated by crystallization. Chlorate of soda forms cubes which often exhibit tetrahedral faces; 100 parts of water dis solve at 0° 81·9, at 20° 99, and at 100° C. 233 parts of the salt. Hence it is much more soluble than the potash salt, on which account it is preferred for aniline black printing.

Sulphate (Na 2 SO 4 ) is the most largely produced of all soda salts in manufacturing operations, although it is regarded principally as an intermediate product. The anhydrous salt readily combines with water into a crystalline soluble compound, Na 2 SO 4 + 10H 2 O, known as Glauber’s salt. This forms large transparent monoclinic crystals, and is characteristically prone to form supersaturated solutions. 100 parts of water dissolve—

at 0° 11∙67 25°·05 32°∙73 33°∙88

12·17 26∙38 99·18 32·21 3121 parts of Na 2 SO 4 +10 H 2 O,

5∙02 10·12 28·11 50·65 50·1 parts of NaSO 4 .

As the temperature rises beyond about 33°, the solubility decreases (Gay-Lussac). Glauber’s salt, when exposed to dry air, especially in summer heat, gradually falls into a powder of anhydrous sulphate. It is much less volatile than the chloride. The thiosulphate, Na 2 S 2 O 3 (commercial hyposulphite of soda), and soda salts of other lower sulphur acids, are reserved for Sulphur.

Of all sodium compounds, except common salt, the carbonates are by far the most important, both industrially and chemically. These comprise the following.

Normal Carbonate, Na 2 CO 3 .—The anhydrous salt usually presents itself in the form of a white opaque porous solid, specific gravity 2·65 (Karsten). According to Carnelley, it fuses at 818° C. (dull red heat) into a colourless liquid. On fusing it loses some of its carbonic acid, and at a bright red heat it volatilizes appreciably. The porous salt absorbs water from the air; when moistened with water it gives off heat and unites into crystalline hydrates, of which the important compound is Decahydrate, Na 2 CO 3 + 10H 2 O.—This salt, known as soda crystals or washing soda, forms large transparent monoclinic rhomboidal prisms or double pyramids. The salt dissolves readily in water, forming strongly alkaline solutions, which emulsionize fats, though less readily than is done by caustic lyes. 100 parts of water dissolve—

at 0° 15° 20° 30° 38° 104°

21∙33 63∙20 92·82 273·6 1142·2 539·6 of crystals,

6·97 16∙20 21∙71 37·24 51·67 46·17 of Na 2 CO 3

(Löwel). According to this observer, the dissolved salt from 38° C. upwards assumes the form of lower hydrates, hence the diminution in solubility at higher temperatures. A saturated solution, when evaporated down by heat, deposits a granular salt of the composition Na 2 CO 3 (H 2 O or 2H 2 O). The decahydrate, when exposed to dry air even at ordinary temperatures, loses water, with the formation of monohydrate, Na 2 CO 3 H 2 O. It also readily takes up carbonic acid with the formation of Bicarbonate, NaHCO 3 .—Its formation may be thus formulated—Na 2 CO 3 + 10H 2 O + CO 2 = 2NaHCO 3 + 9H 2 O. The bicarbonate remains as a crystalline mass, while the liberated water runs off with more or less of dissolved carbonate and the saline impurities which may be present. Bicarbonate forms small four-sided monoclinic plates with a feebly alkaline taste and reaction. 100 parts of water dissolve—

at 0° 20° 40° 60º

6∙90 9∙60 12∙70 16·10 of NaHCO 3

(Dibbits). From about 70° C. upwards the solution gives off carbonic acid with formation of Sesquicarbonate,— 2Na 2 O + 3CO 2 + 4H 2 O = Na 2 CO 3 2NaHCO 3 + 3H 2 O. This salt, found in nature as trona or urao, forms in its pure state crystals which do not lose water in the air. At 20° C. 100 parts of water dissolve 18·3 parts of the salt calculated as anhydride, 2Na 2 O3CO 2 . Both bicarbonate and sesquicarbonate when heated dry break up readily (below redness) into normal carbonate, water, and carbonic acid. Other salts of soda which are of importance on account of their acid constituents—nitrate, silicate, phosphate— are dealt with under Nitrogen, Silica, Phosphorus.

The estimation and analysis of alkalis are sufficiently dealt with at the close of the article Potassium. Soda Manufacture.

The ancients probably did not know soda in other than its native forms, and till about the end of the 18th century potash was, of the two alkalis, the more abundant and generally used substance. Indeed it was not till well into that century that the chemical distinctions of the alkalis were established; they were previously spoken of indifferently as nitrum, natron, kali, alkali, and soda, names simply meaning a fixed alkali. Soda has properly a separate history only from 1736, when Duhamel established the fact that common salt and mineral alkali have the same base,—a body different from the salt of tartar or vegetable alkali. Yet soda, from both mineral and vegetable sources, had long previously been used in Europe, the Arabs having probably brought into Spain a knowledge of the alkali and its sources. Apart from the trona and soda lakes, &c., already alluded to, the only source till the close of the 18th century was the ashes of certain plants which grow on the sea-coast and in salt-impregnated soils. These plants, principally species of Salsola and a few other genera of the natural order Atripliceae, on incineration yield an ash containing in some cases about 50 per cent. of carbonate of soda. The ash of these plants, fluxed by red heat into a pasty mass and broken into convenient lumps, forms the barilla of commerce, which in former times was a product of the first importance on the shores of Spain, Sicily, Sardinia, and other coasts of the Mediterranean. In 1834 as much as 12,000 tons of barilla were introduced into England from Spain alone, and, in spite of the cheapness of artificial soda, the manufacture of barilla is still—or at least was till recently—carried on at various localities on the Mediterranean. On the west coasts of Scotland and Ireland large amounts of impure soda carbonate were obtained from the kelp burned from certain sea-weeds; but the introduction of artificial soda early extinguished that industry, although in connexion with the production of iodine and other products a small quantity of sea-weed soda continues to be made in Scotland till the present day.

The increasing price of potash salts and the discovery of Duhamel led to strenuous attempts to produce the carbonate from common salt. In 1775 the French Academy of Sciences offered a prize of 2400 livres for a practical method of converting salt into soda. But it was never awarded, although the problem was soon triumphantly solved. Indeed in that same year Scheele succeeded in making soda from common salt by means of litharge. Several claims were made for the prize, the first being based on a process invented by a Benedictine monk, Malherbe, in 1778, which was worked with some success for several years. Some time about or before 1787 De la Métherie proposed a plan for calcining with charcoal the sulphate prepared from chloride,—an impracticable proposal, because nearly all the resulting product is sulphide. But this proposal has historical importance, because from the published account of it Le Blanc received in 1787, according to his own statement, the first suggestion of his process,—probably the most valuable and fertile chemical discovery of all times. Nicolas Le Blanc, born at Issoudun (Indre) in 1753, was private surgeon to the duke of Orleans. He was a chemist as well as a surgeon, and the prospect of the Academy prize attracted his attention to the soda problem. He added to the sulphate and charcoal mixture proposed by De la Methérie a certain proportion of chalk, and by fluxing the mixture in crucibles succeeded in effecting the desired transformation. The chemist D’Arcet and his assistant Dizé, having recognized the soundness of the process, the duke of Orleans, early in 1790, agreed to provide a capital of 200,000 francs for working the process. In September 1791 the National Assembly granted Le Blanc a patent for fifteen years, and under the superintendence of Le Blanc himself, with Dizé and Henri Shée, the steward of the duke of Orleans, a work was established at Saint Denis. But on the fall and execution of his patron Le Blanc, and all others owning and working soda processes, were ordered to resign them to the state for the public benefit, he receiving the miserable compensation of 4000 francs. In 1800 his works were reconveyed to him, but in 1806, broken in hope, health, spirit, and resource, he perished by his own hand in a workhouse.

Le Blanc’s process continued to hold the field against all competing schemes till within the last few years, and that essentially in on an endless chain of plates. The salt so prepared is distributed in a range of not less than eight cast-iron cylinders, which are pervaded successively by a current of mixed superheated steam and sulphurous acid given off by pyrites. The reaction begins at about 400° C. and increases in energy with the rise of temperature, but it is impracticable to force the heat beyond 500° to 550°, as the charge then begins to fuse and ceases to be permeable by gases.

The condensation of the hydrochloric acid given off in these processes is effected by a variety of means according to the purpose in view in securing the gas. In Continental works a favourite method is to pass the gas through a range of Woulfe’s bottles arranged in an ascending series,—fresh water entering the topmost and passing through the whole, till in the last and lowest, highly charged with acid, it meets the gas coming direct from the roaster. After leaving the last and highest bottle the gas is generally washed thoroughly out by passing it into a small coke tower, in which it meets a downward trickle of water. In English works Woulfe’s bottles are not employed, and the gases are commonly conducted by a long range of piping, in which they are cooled, to one or a series of coke towers, in which they are exposed to an enormously extended condensing surface of water trickling over the coke, stones, or brick with which the interiors of the towers are filled.

Black Ash Making.— The conversion of the crude sulphate or salt cake into carbonate of soda, peculiarly the process of Le Blanc, is conducted in what is termed a “balling furnace.” In its simplest form this consists of a long reverberatory furnace in which the fuel occupies a lower grating at one extremity of the flat sole, on which the whole reaction is carried out. This sole has usually two beds, that more distant from the fire-bridge being a little higher than the front division; and on front and back beds two separate charges at different stages of advancement are treated simultaneously. The salt-cake is taken as it comes from the roaster, mixed with limestone or chalk (crushed to pieces not bigger than a walnut) and with coal or anthracite in the form of slack or culm. The proportions of these ingredients used in different works vary widely; but, generally stated, to each 100 parts of sulphate there are added from 100 to 140 parts of chalk or limestone and from 40 to 70 of coal. These ingredients roughly mixed are passed by a hopper into the back bed of the furnace, where they become dried and heated, while a previous charge is being finished on the hot front or working bed. When the charge on the working bed has been withdrawn, the mixture on the back bed is pushed forward and spread over the highly heated...

It is said that Bazzi jeered at the History of the Painters written by Vasari, and that Vasari consequently traduced him; certainly he gives a bad account of Bazzi’s morals and demeanour, and is niggardly towards the merits of his art. According to Vasari, the ordinary name by which Sodona was known was “Il Mattaccio” (the Madcap, the Maniac),—this epithet being first bestowed upon him by the monks of Monte Oliveto. He dressed gaudily, like a mountebank; his house was a perfect Noah’s ark, owing to the strange miscellany of animals which he kept there. He was a cracker of jokes and fond of music, and sang some poems composed by himself on indecorous subjects. In his art Vasari alleges that Bazzi was always negligent,—his early success in Siena, where he painted many portraits, being partly due to want of competition. As he advanced in age he became too lazy to make any cartoons for his frescos, but daubed them straight off upon the wall. Vasari admits, nevertheless, that Bazzi produced at intervals some works of very fine quality, and during his lifetime his reputation stood high.

The general verdict is that Sodona was an able master in expression, motion, and colour. His taste was something like that of Da Vinci, especially in the figures of women, which have grace, sweetness, and uncommon earnestness. He is not eminent for drawing, grouping, or general elegance of form. His easel pictures are rare. His most celebrated works are in Siena. In S. Domenico, in the chapel of St Catherine of Siena, are two frescos painted in 1526, showing Catherine in ecstasy and fainting as she is about to receive the Eucharist from an angel,—a beautiful and pathetic treatment. In the oratory of S. Bernardino, scenes from the history of the Madonna, painted by Bazzi in conjunction with Pacchia and Beccafumi (1536 to 1538)—the Visitation and the Assumption—are noticeable. In S. Francesco are the Deposition from the Cross (1513) and Christ Scourged; by many critics one or other of these paintings is regarded as Bazzi’s masterpiece. In the choir of the cathedral at Pisa is the Sacrifice of Abraham, and in the gallery of Florence a St Sebastian.