Soda, a name given to sodic monoxide, or common oxide of sodium, Na2O, the base of the important series of sodium salts; also to the hydrated oxide, or caustic soda, NaHO, and in commerce to the normal carbonate, Na2 CO3 + 10H2O. Anhydrous sodic monoxide, or the soda of the chemist, Na20, is formed when the metal is burned in dry air or oxygen gas, by exposing the dioxide to a high heat, or by heating sodic hydrate with an equivalent quantity of metallic sodium, whereby NaHO + Na is converted into Na20 + H. Whcn sodium is burned in oxygen gas till its weight is constant a dioxide, Na202, is formed. When exposed to the air it deliquesces, and, uniting with carbon dioxide, resolidifies as carbonate. When a heap of it is moistened it becomes heated and evolves oxygen gas. The monoxide attracts moisture as powerfully as the corresponding potassic oxide, forming sodic hydrate or caustic soda, from which the water cannot be expelled by heat alone. The properties of caustic soda resemble those of caustic potash, and it may be prepared from the carbonate by a similar method (see Potash, vol. xiii., p. 756); but its action upon acids is rather less energetic.

Its specific gravity is 2.13. It is manufactured on a large scale in the alkali works according to a process proposed by Mr. Gossage, by which advantage is taken of the presence of caustic soda in the black ash solution. The crude solution of black ash vats is evaporated to a specific gravity of 1.5 or 1.6, during which operation most of the carbonate, sulphate, and chloride crystallize out. The " red liquor," as it is technically called, which owes its color to a compound of sulphide of sodium and sulphide of iron, and which is also contaminated with ferrocyanide and sometimes with sulphocyanide of potassium, has air forced through it while hot, which causes the precipitation of the iron as sesquioxide and the conversion of the sulphur compounds into sulphates. The addition of sodic nitrate completes the oxidation, and this salt may be used for the whole process. After its addition the evaporation is carried further until the whole mass is heated nearly to redness. When the temperature rises to 311° large quantities of ammonia are evolved, and as it increases nitrogen escapes abundantly.

The fused soda is poured into sheet-iron vessels, in which it solidifies. - The normal carbonate, existing in certain lakes in Egypt and Hungary, and in the volcanic springs of Iceland and North America, often containing sesquicarbonate, was long known in commerce as natron. Large quantities of it and of other soda salts occur in the form of an efflorescence on the " alkali plains " of the western territories. It was formerly prepared artificially from kelp, or the ashes of seaweeds and fuci, and also from barilla, the semi-fused ash of the Salsola soda, a plant which has been cultivated with great care by the Spaniards, especially in the vicinity of Alicante, the seed being sown in light low soils which are irrigated by sea water. Barilla yields much more soda than kelp, the latter being now principally used for obtaining iodine. But the quantity of soda obtained from barilla is small in comparison with that manufactured by the process of Leblanc, which consists in first converting chloride of sodium or common salt into sulphate of sodium or Glauber's salt, and then converting the sulphate into carbonate by heating it with carbonate of lime and coal. The conversion of common salt into sulphate or "salt cake" is called the "salt-cake process," and is effected in a salt-cake furnace.

One of the best forms of furnace contains two iron vessels or retorts placed in separate heating apartments or furnaces, but connected with each other by a neck. Into the first vessel, called the decomposer, which is oval, are introduced 5 or 6 cwt. of common salt and a rather less weight of sulphuric acid of sp. gr. 1.78, and a gentle heat is applied. Hydrochloric acid is evolved and passes off by a flue to condensing towers containing fragments of coke or stone, through which water is allowed to trickle. There are two towers, the first one receiving the vapors at the bottom, passing what are not absorbed to the top of the other, from the bottom of which the residue, mostly air and some impurities, issues and passes into a large chimney. In the first vessel about half the salt is decomposed, when the pasty mass, consisting of acid sulphate of sodium and undecomposed salt, is thrust into the second vessel or roaster, which is heated to a higher degree, and the decomposition completed. The reaction in the first vessel is as follows : 2NaCl + H2SO4 = NaCl + NaH,S04 + HC1. In the second vessel the acid sodic sulphate reacts upon the unchanged salt, the hydrogen taking the chlorine to form hydrochloric acid, leaving two molecules of sodium to unite with the sulphion, S04; thus, NaCl + NaHSO4. =HC1 + Na2S04. The hydrochloric acid gas from both vessels passes through the same flue and condensing towers.

The neutral sulphate or "salt cake" is then removed from the second chamber, reduced to powder, and mixed with powdered chalk and coal, in the proportion of two parts each of sulphate and chalk and one part of coal. This mixture is then thrown in quantities of from 2 to 3 cwt. into a reverberatory furnace, and melted while being stirred. The mass is then raked out into a mould from which it is turned when cold, forming ball soda, or black ash, which contains from 20 to 27 per cent, of pure soda or neutral carbonate, minus its water of crystallization, and mixed with calcium sulphate, quicklime, and unburned coal. The reaction is represented as follows: Na2SO4 + CaCO3 + 40 =Na2C03 + CaS + 4CO, the chemical changes consisting firstly in the deoxidation of the salt cake, and its conversion into disodic sulphide with evolution of carbonic oxide, and secondly in the formation of sodic carbonate and calcic sulphide by interchange of the constituents of the disodic sulphide and calcic carbonate. The sodium salts are extracted in a series of vats, by warm water which passes from one to the other.

Calcium sulphide, which is formed in large quantities, was formerly a waste product, but is now partly utilized in the preparation of hyposulphite of soda, which has been employed to a considerable extent as an "antichlor" for removing the last traces of chlorine from bleached paper pulp. The black solution obtained by the lix-iviation of the black ash is allowed to settle, when it is pumped into iron pans and evaporated by the waste heat from the furnaces. Much of the salt crystallizes during ebullition and is removed by perforated ladles. The mother liquor retains a portion of caustic soda, which may be converted into carbonate by mixing it with sawdust and roasting in a reverberatory furnace. At present, however, this conversion into carbonate is not much practised, but the caustic soda is extracted according to the plan of Mr. Gossage, already described. The crude carbonate is crystallized by redissolving it in hot water, allowing this to become clear by standing, and then running it into deep pans, having a capacity to yield about one ton of crystallized carbonate. The solution cools in five or six days, and large crystals are formed.

The mother liquor yields an inferior ash. - Sodic carbonate, or commercial neutral carbonate of soda, has a nauseous alkaline taste, and crystallizes in large transparent rhomboidal prisms, containing 10 molecules of water, which melt in their water of crystallization, are soluble in any proportion of hot water, and are also very soluble in cold water. The salt easily parts with its water, and melts at a red heat. If it is crystallized at a temperature of - 4° F., 15 molecules of water of crystallization are taken up. Mit-scherlich obtained sodic carbonate with six molecules of water of crystallization. Above 93.2° the salt crystallizes in forms derived from the square-based octahedron, containing five molecules of water; but between 158° and 176° it crystallizes in four-sided prisms containing only one moleculo of water. The maximum solubility of soda in water is at 100.4°. - The principal uses of commercial carbonate of soda are in the preparation of the bicarbonate and of caustic soda; in the manufacture of ham soap, for which purpose it is better adapted than potash on account of not being deliquescent like the latter alkali (see Soap); and also very largely in the preparation of paper pulp from various materials.

The paper maker uses it in connection with quicklime, which reduces it to caustic soda. (See Paper.) It is also used in the laundry, and for domestic and cleansing purposes generally.