Sodium, the most abundant of the alkali metals, its chloride composing the principal part of the saline matter of the ocean, and also existing in extensive beds in geological strata. Large quantities of nitrate and carbonate of sodium are found in beds, and in some rocks it is combined with silica. The metal was obtained by Sir Humphry Davy soon after his discovery of potassium, and by a similar method. Gay-Lussac and Thenard afterward prepared it by decomposing sodic hydrate with metallic iron at a white heat. It may be prepared readily by the process of Brunner, which consists in distilling a mixture of the carbonate with powdered charcoal. The process has been improved by Deville and others, and employed on a large scale in manufacturing. The carbonate of soda used in the process is prepared by calcining the crystallized neutral carbonate. It is thoroughly dried, pounded, and mixed with a slight excess of charcoal. Ground chalk is also added, to preserve a pasty condition and prevent the carbonate of soda from separating from the charcoal.

The following proportions are recommended by Deville for manufacturing operations: dry carbonate of soda, 30 kilogrammes; charcoal, 13; chalk, 3. The materials should be thoroughly mixed, and it is well to calcine the mixture before putting it into the distilling apparatus, by which it is made more compact, so that a greater quantity can be introduced. It is put into cylindrical iron retorts covered with clay, which are heated in a reverberatory furnace. The retorts have movable ends, so that at the close of the operation the charge may be withdrawn and a fresh one introduced without removing the cylinders or putting out the fire. The receivers are of the form used in the preparation of potassium. (See Potassium, vol. xiii., p. 758.) The same precautions are necessary as in the preparation of that metal. The chalk is employed to prevent the charcoal from separating the carbonate of soda when it fuses.

The charcoal combines with oxygen when the heat is sufficient to weaken the affinities between the constituents of the salt, and the metallic sodium is left free, when it distils over and is condensed in the receiver, nearly pure if the operation is well conducted. It is perfectly purified by melting it under naphtha, when it may be run into moulds like those used for lead. - Sodium is a brilliant silver-white metal, resembling potassium in its physical and in most of its chemical properties. It is a good conductor of heat and electricity. Its specific gravity is 0'972, its atomic weight 23, and its symbol Na (natrium). It is soft at common temperatures, fuses at 207.7° F., and oxidizes rapidly in the air. At the freezing point of water it is very ductile, and at the zero of Fahrenheit it is quite hard. If a small quantity of the metal is melted in a sealed tube filled with coal gas, and cooled till crystallization begins, when the liquid portion is turned off shining octahedral crystals will remain. When dropped into cold water it decomposes it with violence, evolving hydrogen gas, but does not produce, enough heat to inflame it unless the metal is held in one spot so that the heat shall not be dissipated.

If the water is previously warmed, the gas will take fire, burning with a bright characteristic yellow flame. Sodium is widely diffused in the mineral, animal, and vegetable kingdoms, united with silicic and carbonic acid in many minerals, forms a large share of the saline portions of animal fluids, and enters largely into the composition of marine plants. It unites with oxygen to form two well known oxides: the monoxide, Na20, the soda of the chemists, and the dioxide, Na202. These two oxides are formed when sodium is burned in common air. When burned in oxygen gas till it no longer increases in weight, it is wholly converted into the dioxide. With water it forms a hydrate, NaHO, which corresponds in composition to the monoxide, a molecule of hydrogen replacing one of sodium. This hydrate is the caustic soda of commerce. (See Soda.) - Salts. The salts of sodium are among the most important of all compounds, not excepting those of potassium. The principal one is the chloride, or common salt. (See Salt.) The iodide, NaT, and the bromide, NaBr, are analogous to the corresponding potassium compounds. At temperatures above 86° the bromide crystallizes in anhydrous cubes, but-at lower temperatures it unites with two molecules of water and forms hexagonal tables.

The iodide, at temperatures above 101°, crystallizes in anhydrous cubes; but at ordinary temperatures large, transparent, striated, oblique rhombic prisms are formed, containing two molecules of water. The small proportion of sodic iodide which is contained in sea water furnishes the commercial supply of iodine, the kelp from which iodine is obtained being the ashes of marine plants which assimilate the iodide from the sea water. (See Iodine.) The sulphides of sodium correspond to those of potassium, and may be prepared by similar processes. The fluoride, NaF, exists in combination with alu-minic fluoride in the mineral cryolite 6(NaF), Al2F6, found in Greenland and the Ural, which is the chief source of metallic aluminum. (See Aluminum, and Cryolite.) - Sodic sulphate, the well known Glauber's salt, is described under that title. Sodium unites with sulphurous acid to form a neutral and an acid sulphite. The neutral salt, Na2S03 + 10H2O, is procured by passing sulphurous anhydride (see Sll-piiue), the product of sulphur burned in air, over moistened crystals of sodic carbonate as long as the gas is absorbed, dissolving the mass in water and crystallizing.

It is extensively employed for the preparation of the hyposulphite of soda, which is largely used under the name of "antichlor" to remove the last traces of chlorine from bleached paper pulp. (See Paper, vol. xiii.,' p. 46.) The acid sulphite, NaHS03, is of little importance. The hyposulphite, Na2S2O3 + 5H2O, was formerly made to some extent from impure sodic sulphide, or sulphuret of sodium, by passing sulphurous anhydride through it until it ceased to be absorbed; but it is now largely prepared from neutral sulphite of soda by digesting this salt with sulphuric acid for several days, at a moderate heat. It may also be prepared by digesting a solution of the sulphite with flowers of sulphur. The sulphur is gradually dissolved, forming a clear solution which yields crystals on evaporation; these are oblique prisms belonging to the right prismatic system, freely soluble in water, but insoluble in alcohol. Hyposulphite of soda possesses the property of forming double salts with silver compounds, and in photography it is employed in dissolving away ordinary insoluble compounds of silver, such as chloride and iodide. A mixed solution of sulphite and hyposulphite of soda dissolves malachite and blue copper ore, and Stromeyer has employed it in the hydro-metallurgical extraction of copper.

It is also used for preparing antimonial cinnebar and aniline green. Hyposulphite of soda fuses at comparatively low temperatures in its water of crystallization, and advantage is taken of this property in the sealing of glass tubes containing explosive compounds to be used under water in torpedoes. Mr. M. Carey Lea employs it as a new test for ruthenium. If a salt of this metal is made alkaline with ammonia and boiled with the hyposulphite, it first acquires a rose color, and then a-magnificent carmine. Employed in medicine, it appears to have deoxidizing powers, in consequence,'it has been suggested, of conversion of hyposulphurous into sulphuric acid. It diminishes urea and increases uric acid in the urine, and also increases the sulphates and causes the appearance of sugar and oxalic acid. It has been used, in accordance with the suggestions of Dr. Polli, in zymotic diseases, or those which are supposed to be caused by ferments in the blood, the development of which it has the power of arresting. It has also been used in cases of yeasty vomiting, on account of its destructive effect on the sarcenia ventriculi which infests the stomach in that disease, and as a local application in parasitic affections of the skin and mucous membranes.

It may be given in doses of from 10 to 20 grains three times a day, dissolved in water. For external use a dram may be dissolved in an ounce of water. - The nitrate, called also cubic nitre, is described in the article Nitrates. - The neutral carbonate, commonly called soda in commerce, is treated under Soda. Bicarbonate of soda, acid sodic carbonate, or mono-sodic carbonate, may be formed by saturating a strong solution of the neutral carbonate or sal soda with carbonic acid. It is also manufactured on a large scale by passing a current of carbonic acid gas over crushed and moistened crystals of commercial carbonate, exposed two or three inches in depth in a chamber upon cloths stretched horizontally above one another. The carbonate passes into the ses-quicarbonate, and then into the bicarbonate, which may be redissolved and crystallized on evaporation in rectangular four-sided prisms, soluble in 10 parts of water at 50°. If the solution is heated, four molecules of bicarbonate lose one of carbonic acid and are converted into the sesquicarbonate (4NaHC03 = 2Na2C03, H2CO3 + H2CO3), which by heating to redness, or by continued boiling, is converted into normal carbonate.

Bicarbonate of soda is much used in medicine as an antacid and promoter of mucous secretions and perspiration, and as an ingredient in effervescing powders. (See Effervescence.) It is also used in bread making, as was formerly the sesquicarbonate. There are several compounds of sodium with boracic acid, but only one is of any practical importance, the acid borate (biborate of soda, or common borax), which is described in the article Borax. - Sodium forms with the three varieties of phosphoric acid orthophosphates, metaphosphates, and pyrophosphates. Among the orthophosphates are trisodic phosphate, or subphosphate of sodium, Na3P04 + 12H20, prepared from rhombic phosphate by adding caustic soda to its solution; and the hydric disodic phosphate, or rhombic phosphate of sodium, Na2HP04+ 12H20, commonly called phosphate of soda, and the salt from which most of the phosphates are obtained. The latter is prepared by adding sodic carbonate to acid calcic phosphate, one of the salts formed in obtaining phosphorus. (See Phosphorus, vol. xiii., pp. 4G4 and 405.) Tricalcic phosphate is precipitated while the disodic phosphate is held in solution.

When decanted and evaporated it forms large, transparent, efflorescent, rhombic prisms, soluble in four parts of cold water, but fusing at 90° F. in their water of crystallization. It has an alkaline reaction, and corrodes flint glass, causing white silicious' scales to separate from the surface. When evaporated at temperatures above 90° it combines with seven molecules of water of crystallization, and does not effloresce. On adding free phosphoric acid to a solution of rhombic phosphate, triphosphate of soda, NaH2+PO4 + H2O, is formed, which crystallizes in right rhombic prisms having a strongly acid reaction. There are several metaphos-phates of sodium, and also double salts of the same constitution in which another metal is one of the basyles. There are several pyrophosphates, embracing also both single and double salts, for a description of which the reader is referred to the larger works on chemistry. - The silicates of sodium are glasses of various degrees of fusibility, and also of solubility in water. (See Concrete, Glass, and Glass, Soluble.) There are several organic salts of sodium, the principal of which are acetates, citrates, oxalates, tartrates, and valerianates; but they do not possess sufficient general interest to require notice here. - General Characteristics of Sodium Salts. There are no good direct tests of sodium salts, because they are nearly all soluble, so that the presence of sodium is often inferred when the absence of every other metal is proved, and yet a saline substance remains which yields yellow, striated, prismatic crystals on addition of chloride of platinum and evaporating the solution, a double salt of sodium and platinum being formed.

The detection of this double salt is more certain by microscopic examination with polarized light, which tinges the crystals with va-rious characteristic colors. Before the blowpipe the salts of sodium impart an intense yellow to the outer flame. Spectroscopic examination reveals pure yellow light having the same position in the solar spectrum as the double line D. The chief distinguishing characteristics between sodium and potassium salts are, that the latter impart a violet color to flames, and are generally more insoluble, as shown in the slight solubility of sulphate of potassium and the great solubility of Glauber's salt. Many sodium salts moreover effloresce on exposure to the air, while potassium salts generally deliquesce, a fact markedly shown in the carbonates.