this operation, which usually takes about three or four hours. The product is an excellent article for the plate-glass manufacturer. It is not usual at the present time to mix the carbonate with coal, sawdust, or charcoal, in this part of the process, as was formerly practised.

To obtain the crystallized carbonate of soda, the purified ash is merely dissolved in hot water to saturation, and the solution is run into shallow crystallizing pans of cast iron or of wood lined with sheetlead, and allowed to become cold. If the ash has been calcined with saw-dust, coal, or charcoal, it is lixiviated with water, the solution is allowed to settle, and the clear liquor is drawn off and evaporated until a pellicle forms upon its surface, when it is run into the crystallizing pans. The crystallization may be considered complete in a week or less; the mother-liquor, which contains nearly the whole of the foreign salts, is then withdrawn and evaporated to dryness; the residue generally contains about thirty per cent. of alkali, and serves for the crownglass and soap manufactures.

A patent was obtained in 1841 by Mr. James Shanks, of St. Helen's, Lancashire, for processes for impregnating black-ash and white-ash with carbonic acid gas, for the purpose of converting the caustic soda and silicate of soda contained therein, into carbonate. In one of these processes the black-ash is broken into fragments and disposed in several layers each of three or four inches in depth, in a stone or iron vessel called the "carbonator." The layers having been moistened with water, carbonic acid gas is admitted into the

carbonator by a pipe, the air being allowed to escape by another pipe. When the odour of sulphuretted hydrogen is strongly perceptible at the exit pipe, the whole of the soda is considered to be carbonated; the introduction of carbonic acid is then discontinued, and the black-ash is removed to the vats, and lixiviated in the ordinary manner. The ley prepared from blackash may be exposed to the action of carbonic acid, instead of the solid ash itself. The carbonator used for this purpose should have a capacity of sixteen cubic feet for every ton of white-ash, or six cubic feet for every ton of soda-crystals, to be made daily. Across the bottom of the carbonator is thrown an arch of open brick-work, and the space above is filled with small pebbles, over which flows the solution to be carbonated, which is admitted at the top. The carbonic acid gas is introduced below the arch, and allowed to rise in the carbonator between the pebbles, the air of the vessel escaping by a pipe at the top. The liquor loses its greenish-yellow colour (which seems to be due to traces of sulphuret of iron held in solution by sulphuret of sodium) when sufficiently carbonated. In another process, the last or weakest soda liquor from the lixiviating vats is supersaturated with carbonic acid gas in the carbonator, and then used for lixiviating fresh charges of black-ash. Mr. John Wilson obtained in 1840 a patent for some improvements in the manufacture of carbonate of soda, which included the application of as much bicarbonate of soda to blackash, or white-ash, as might be necessary to convert the whole of the caustic soda in the ash into carbonate.*

* Mr. Wilson's patent also included a mode of preparing carbonate of soda from sulphuret of sodium, by mixing with a solution of forty-eight

Besides the modifications of the common soda process which have been already described, several more considerable changes have been proposed, neither of which, however, has yet come into successful competition with the process of Leblanc.

A method of procuring caustic soda directly from common salt was long ago contrived by Mr. Turner, which consisted in intimately mixing and rubbing together one part of salt, from four to six parts of litharge (protoxide of lead), and a little water. Double decomposition takes place between the salt and a portion of the oxide of lead, with formation of caustic soda and chloride of lead: the former is dissolved out by water, and the latter remains in an insoluble state in combination with another portion of litharge as an oxi-chloride of lead. After being fused and levigated, the oxi-chloride forms the pigment known as patent yellow." This mode of procuring soda has been abandoned as a manufacturing process, in consequence of the limited sale of the pigment.

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To procure caustic soda directly from sulphate of soda, it has been proposed to decompose the sulphate by a solution of caustic barytes. When a solution of sulphate of soda is mixed with a solution of caustic barytes, the sulphuric acid of the sulphate unites with the barytes to form the insoluble sulphate of that earth; and if the mixture is made in the proper

parts of the sulphuret, eighty-five parts of bicarbonate of soda. If the sulphuretted hydrogen gas thus disengaged is to be collected, the materials may be mixed in a covered iron pan furnished with a pipe to conduct the gas to a gas-holder; but if the gas is not required, the operation may be carried on in a common white-ash furnace. The mixture is evaporated to dryness, the residuary carbonate is dissolved in water, and the clear solution is decanted and concentrated by evaporation.

proportion, nothing should remain in the solution but caustic soda. If an excess of barytes has been applied, it is precipitated as carbonate on evaporating the solution with access of air; or it may be more conveniently removed by adding an additional quantity of sulphate of soda. The difficulty of procuring with facility the requisite quantity of caustic barytes, will probably always prevent any process of this kind from becoming general.

An interesting process for obtaining carbonate of soda from common salt has been proposed by Messrs. Hemming and Dyer, which is founded on the comportment of a solution of salt with a solution of bicarbonate of ammonia. When the carbonate of ammonia of the shops, which contains bicarbonate of ammonia, is added in a state of fine powder to a solution of about an equal weight of salt in three parts of water, the mixture being well stirred, a white crystalline precipitate appears in a few hours, which causes the liquid to become thick. The precipitate is bicarbonate of soda, and the liquid contains in solution muriate of ammonia, these two substances being the products of the double decomposition of bicarbonate of ammonia and chloride of sodium.*

In the manufacturing process founded on this reaction it is proposed to commence by mixing with

* This decomposition is most simply stated on the ammonium theory, which supposes the muriate of ammonia and bicarbonate of ammonia to contain a radical composed of NH4, which is named ammonium, and which occupies in the ammoniacal salts the place of a metal, as potassium and sodium in the salts of potash and soda. Bicarbonate of ammonia may then be considered as a double carbonate of water and oxide of ammonium, or HO, CO, + NH,O, CO2, as bicarbonate of soda is the double carbonate of water and oxide of sodium; and muriate of am

salt an impure carbonate of ammonia, such as the liquor furnished by the distillation of bones; and having separated the solid bicarbonate of soda by pressure in a screw-press, to reconvert the muriate of ammonia in the mother-liquor to carbonate, by evaporating the liquid to dryness, and heating the residue in a distillatory apparatus with carbonate of lime, as in the ordinary mode of preparing carbonate of ammonia. Unless particular precautions are adopted, the loss of ammonia in this process must be very great and inevitable, from two reasons: first, when a mixture of muriate of ammonia and carbonate of lime is heated, a portion of the ammonia is always disengaged in the free state, the product not being a neutral carbonate, but a sesquicarbonate, containing, to three equivalents of carbonic acid, two equivalents of ammonia. In this process, one eq. of ammonia out of three is disengaged in the free state as a gas, and lost, unless particular means are adopted to retain it. Again, when the sesquicarbonate of ammonia is dissolved in water, it is resolved into the neutral carbonate and the bicarbonate; but the common salt is decomposed by the bicarbonate only; the neutral carbonate remains in the mother-liquor, and is decomposed during the evaporation into free ammonia and the bicarbonate, both of which are volatilized. By making use of such an apparatus as would allow the ammoniacal gas and the bicarbonate of ammonia thus disengaged to be conducted into a condenser of some

was the chloride of ammonium, NH, Cl; or the analogue of chloof sodium. On this view, the mutual decomposition of bicarbonate wia and chloride of sodium is nothing more than the transpothe ammonium in the former, and the sodium in the latter.