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About 6,000,000 bushels of salt are produced annually in New York. When the brine is not a saturated solution, it is usually concentrated by partially evaporating it in large shallow pans by the air and the sun. It is then further concentrated and crystallized by artificial heat.

103. Sodic Carbonate. — Sodic carbonate, Na CO2, is known in commerce as soda-ash, and is manufactured on an enormous scale. It is largely used in glass-making, soap-making, bleaching, and many

other

processes in the arts.

Before the French Revolution, the Continental nations of Europe derived their soda-ash mainly from Spain, where it was made from the ashes of certain marine plants, which contain a large amount of soda. The sodaash thus obtained was called barilla.

In England, the soda-ash used was mainly obtained from the ashes of a sea-weed called kelp, which grows abundantly on the north and west coast of Ireland, and on the west coast and the islands of Scotland.

One of the first effects of the war of the French Revolution was to cut off the supply of alkali from Spain. About this time, a French chemist, Le Blanc, discovered a process by which sodic carbonate, or soda-ash, could be obtained from common salt, or sodic chloride, NaCl. This process became publicly known through a commission appointed, during the first year of the Republic, to investigate the subject of alkali manufacture.

Le Blanc's method is the same as that now used in the making of sodic carbonate, which has come to be one of the most important branches of chemical manufacture. The process may be divided into two stages :

(1) Manufacture of sodic sulphate, or salt-cake, from sodic chloride ; called the salt-cake process.

(2) Manufacture of sodic carbonate, or soda-ash, from salt-cake; called the soda-ash process.

(1) Salt-cake Process. — This consists in the decomposition of salt, by means of sulphuric acid. This is effected in a furnace, called the salt-cake furnace, a section of which is represented in Figure 17. It consists

Fig. 17.

of a large covered iron pan, placed in the centre of the furnace, and heated by a fire placed underneath ; and two roasters, or reverberatory furnaces, placed one at each side, on the hearths of which the salt is completely decomposed. The charge of half a ton of salt is first placed in the iron pan, and the sulphuric acid allowed to run in upon it. Hydric chloride (muriatic acid) is evolved, and escapes through a flue with the products of combustion into towers, or scrubbers, filled with coke or bricks moistened with a stream of water; the acid vapors are thus condensed, while the smoke and heated air pass up the chimney. The reaction is as follows:

2Naci+H,80,=Na_SO + 2HCI. After the mixture of salt and acid has been heated for some time in the iron pan, and has become solid, it is raked out upon the hearths of the roasters, where the flame and heated air of the fire complete the decomposition into sodic sulphate and hydric chloride.

(2) Soda-ash Process. This consists (1) in the preparation of the sodic carbonate, and (2) in the separation and purification of the same. The first chemical change which the salt-cake undergoes is its reduction to sodic sulphide, by heating it with powdered coal:

Na S04+C=Na,S + 4CO.

The second decomposition is the conversion of the sodic sulphide into sodic carbonate, by heating it with chalk or limestone (calcic carbonate) :

Na,S+ CaCO3=Na_CO2+ CaS. These two reactions are, in practice, carried on at once; a mixture of 10 parts of salt-cake, 10 parts of chalk or limestone, and 7.5 parts of coal, being heated in a reverberatory furnace, called the balling furnace, until it fuses, and the decomposition is complete, when it is raked out into iron wheelbarrows to cool. This process is generally called the black-ash process, from the color of the fused mass.

The next operation consists in the separation of the sodic carbonate from the insoluble calcic sulphide and other impurities. This is easily accomplished by lixiviation, or dissolving the former salt out in water. On evaporating down the solution, for which the waste heat of the balling furnace is used, and calcining the residue, the soda-ash of commerce is obtained.

No less than 200,000 tons of common salt are annually consumed in the alkali works of Great Britain, for the preparation of nearly the same weight of soda-ash, the value of which is about $10,000,000.

104. Bicarbonate of Soda. · The salt known as bicarbonate of soda, HNaCO3, is obtained by exposing the crystallized sodic carbonate in an atmosphere of carbonic acid. It is a white crystalline powder, used in medicine and for making effervescing drinks; also in bread-making, as a substitute for yeast.

105. Other Sodic Salts. Sodic nitrate (soda saltpetre), NaNO3, is found in northern Chili in large beds. It is used in making nitric acid, and as a manure.

Sodic sulphate (sulphate of soda), Na, SO4, is familiarly known as Glauber's salts.

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Sodic hyposulphite (hyposulphite of soda) is used in photography (92).

Sodic borate, or borax, is considerably used in the arts, especially as a flux in soldering. It dissolves the oxide formed upon the surfaces to be united, and thus keeps them clean and bright.

Sodic silicate (silicate of soda) is called soluble glass, and is used to some extent as t paint or varnish for fixing fresco colors, and for preserving stone-work, and quite largely in calico printing and dyeing.

106. SALTS OF MAGNESIUM. — Magnesium has but one oxide, MgO, or magnesia. The most important salt of this oxide is the magnesic sulphate or Epsom salts, used in medicine.

107. SALTS OF ALUMINIUM. — The only oxide of aluminium is alumina, Al,0g. It occurs native in a nearly pure and crystalline state as corundum, ruby, sapphire, and emery. Alumina is largely used as a mordant in dyeing and calico-printing, as it has the power of forming insoluble compounds, called lakes, with vegetable coloring-matter, and thus fixes the color in the pores of the cloth so that it cannot be washed out (85).

The soluble aluminic sulphate, Al,3SO,, is prepared on a large scale for the use of the dyer, by decomposing clay by the action of sulphuric acid. The solid mixture of silica and aluminic sulphate thus obtained goes by the name of alum-cake. The most useful compounds of alumina are, however, the alums, a series of double salts, which aluminic sulphate forms with the alkaline sulphates. Common potash alum has the composition KA12SO. It may be prepared by dissolving the two sulphates together, and allowing the compound salt to crystallize, but it is usually obtained from the decomposi

tion of a shale or clay containing iron pyrites, FeS This substance gradually undergoes oxidation when the shale is roasted (taking oxygen from the air), producing sulphuric acid, which unites with the alumina of the clay, and, on the addition of a potassic compound, alum crystallizes out. A salt called ammonium alum, and containing H.N, instead of K, is at present prepared on a large scale ; the ammonia liquor of the gas-works, together with sulphuric acid, being added to the burnt shale, instead of a potash salt.

There are a great many other alums known, in which Fe,Og, Cr,Og, or MnO2, oxides of iron, chromium, and manganese, are substituted for the alumina in common alum. Clay is an aluminic silicate resulting from the disintegration and decomposition of felspar by the action of air and water. Kaolin or porcelain clay is the purest form of clay, containing no iron or other impurities. There are many very beautifully crystalline minerals, as garnet and mica, consisting of aluminic silicates combined with silicates of other metals.

108. Glass, Porcelain, and Earthen-ware. - The silicates of the alkalies are soluble in water and noncrystalline; those of the alkaline earths (Ba, Sr, and Ca), are soluble in acid and crystalline ; while compounds of the two are insoluble in water and acids, and do not assume a crystalline form. Such a compound, when fused, is termed a glass. There are four different kinds of glass used in the arts, differing in their chemical composition, and exhibiting corresponding differences in their properties: (1) Crown or window and plate glass, composed of silicates of soda and lime; (2) Bohemian glass, consisting of silicates of potash and lime; (3) flint glass or crystal, containing silicates of potash and lead; and (4) common green bottle-glass, composed of silicates of soda, lime, iron, and alumina. The first and third of these

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