The sulphateis next reduced topow and mixed with an equal weight of chalk or limestone, and half as much small coal, both 'ground or crushed. The mixture is thrown into a reverberatory furnace, and heated to fusion, with con stant stirring ; 2 cwt. is about the quan tity operated on at once. When the de composition is judged complete, the melted matter is raked from the furnace into an iron trough, where it is al lowed to cool. When cold, it is brok en up into little pieces and lixiviated with cold or tepid water. The solution is evaporated to dryness, and the salt calcined with a little saw-dust in a suitable furnace. The product is the soda-ash of commerce, which, when of good quality, contains from 48 to 52 per cent. of pure soda, partly in the state of carbonate, and partly as hydrate, the remainder being chiefly sulphate of soda and common salt, with occasional traces of sulphite or by posulphite, and also cyanide of sodium. By dissolving soda-ash in hot water, fil tering the solution, and then allowing it to cool slowly, the carbonate is deposited in large transparent crystals.
The re-action which takes place in the calcination of the sulphate with chalk and coal-dust, seems to consist, first, in the conversion of the sulphate of soda into sulphuret of sodium by the aid of the combustible matter, and secondly, in the double interchange of elements between that substance and the carbonate of lime. The sulphuret of calcium, thus pro duced, combines with another proportion of lime to form a peculiar compound, which is insoluble in cold or slightly warm water.
Other processes have been proposed, and even carried into execution, but the above is found most advantageous. The ordinary crystals of carbonate of soda contain 10 equivalents of water, but by particular management the same salt may be had with seven equivalents, or sometimes with only one—these differ in figure from the precedine.. The common form of the crystal is derived from an oblique rhombic prism: they effloresce in dry air, and crumble to white powder.
Heated, they fuse in their water of crys tallization : when the latter has been ex pelled, and the dry salt exposed to a full red heat, it melts without undergoing change. The common crystals dissolve in two parts of cold, and in less than their own weight of boiling water ; the solution has a strong, disagreeable, alka line taste, and a powerful alkaline re action.
Bicarbonate of Soda.—NaO, salt is prepared by passing carbonic acid gas into a cold solution of the neutral carbonate, or by placing the crystals in an atmosphere of the gas, which is rapidly absorbed, while the crys tals lose the greater part of their water, and pass into the new compound.
Bicarbonate of soda, prepared by either process, is a crystalline white powder, which cannot be re-dissolved in warm water without partial decomposition. It requires 10 parts of water at 60° for solu tion ; the liquid is feebly alkaline to test paper, and has a much milder taste than that of the simple carbonate,. It does not precipitate a solution of magnesia. By exposure to heat, the salt is converted into neutral carbonate.
A sesquicarbonate of soda containing 27a0, 3COI +41I0 has been described by Mr. Phillips ; like the sesquicarbonate of potash, it cannot be formed at pleasure. This salt occurs native on the banks of the soda-lakes of Sakena in Africa, whence it is exported under the name of Trona.
A newprocess for manufacturing bicar bonate of soda has been patented in this country, which consists in exposing the crude carbonate of soda, moistened in trays laid on shelves, in air-tight apartment, into which steam and car bonic acid, from the flue of an anthra cite stove, are driven in for 7 or 10 days, until the soda will take up no more acid. It is then found that the soda has ab sorbed one half an equivalent more of carbonic acid, forming a sesquicarbon ate. The mass is taken and ground to powder in a mill. This substance is now used in making domestic bread, and hence is called bread soda.