into a chimney through the arched flue bb, the bottom of which is as wide as the decomposing bed. The flue is separated from the decomposing bed by cast-iron plates and slate stones supported by iron bearers.

As the consumption of muriatic acid bears a small proportion to that of soda, many alkali manufacturers consider it to be to their advantage to allow the acid vapours to escape entirely, the inconveniences which attend the condensation of the acid being of greater consequence than the value of the liquid acid. Some means of condensation, however, should always be applied where the works are situated in or near a cultivated district; as the acid vapours, if allowed to escape into the air, never fail to sterilize the surrounding vegetation. In many places, the vapours might easily be conveyed to a vault through which a current of water might be conducted to a neighbouring river. To carry off the smoke and uncondensed gases, and to maintain a proper draught in the furnaces, a chimney might be erected over the farther end of the vault, provided no ingress is allowed for air except through the furnaces.

As soon as the fumes disengaged from the mixture of sulphuric acid and salt cease to be very copious, and the mixture attains a pasty consistence, which generally happens in about two hours from the time of charging, the mass is pushed out of the decomposing bed, through the opening at the back of the furnace, into a place supplied with a dome or chimney for the purpose of carrying off the acid vapours, which would suffocate the workmen if diffused into the air. The fire in the furnace having been checked a

little, another charge is introduced into the decomposing bed in the same manner as before; after which the product of the first charge, by this time somewhat cooled, is shovelled into the "roasting bed," b, fig. 43. There the sulphate is exposed to a much higher degree of heat, which serves, in an hour or so, to dissipate all traces of muriatic acid. When it has ceased to emit acid fumes, and has changed from a yellowish colour to white, it may be raked out of the furnace to make room for another charge. The dry sulphate is technically known by the name of "salt cake."

Several other methods of procuring sulphate of soda for the alkali manufacture have been proposed, but neither of them has yet come into successful competition with the above. Sulphate of iron (copperas), which may be obtained at a very moderate expense by the oxidation of iron pyrites, may be made available in the conversion of common salt into sulphate of soda, in two ways:

1o. By calcining at a red heat an intimate mixture of the two salts (in the proportion of single equivalents) in a reverberatory furnace, when the volatile perchloride of iron is disengaged, and a mixture of sulphate of soda and peroxide of iron left as a residue, from which the sulphate may be separated by This mode of procuring sulphate of soda has long been proposed, but a few years since it was made one of the subjects of a patent for this country.

water.

2o. By dissolving copperas and salt together in water, when a double decomposition takes place, with formation of sulphate of soda and chloride of iron. The former may be obtained in hydrated crystals by exposing the solution to a low temperature; or in the

anhydrous state, as an opaque precipitate, by concentrating the solution at the boiling point.* In either case the chloride of iron remains in the motherliquor.

Common salt has also been converted into sulphate of soda by the agency of sulphate of magnesia. When these two salts are mixed in solution, a double decomposition takes place, and sulphate of soda may be separated from the liquor, either by exposure to cold, or by concentration at the boiling point. Chloride of magnesium remains in the mother-liquor.

Another method is by the calcination of a mixture of common salt and finely-powdered pyrites. One hundred parts of pyrites are mixed with forty parts of common salt, and roasted in a proper furnace for sixty hours; the brownish-red mass which results is lixiviated in water, and the solution evaporated to afford crystals. The product consists of about fortyfive parts of crystallized sulphate, which correspond to about twenty parts of dry sulphate. The addition of carbonaceous matter is said to accelerate the process, and increase the product. (Brande's Manual.)

The sulphate of soda being obtained, the next step in the process consists in calcining it with chalk or broken limestone, and small coal.

The furnace in which this operation is conducted is called the "fluxing" or "black ash furnace." It is quite similar to a common reverberatory; but its

* Sulphate of soda is considerably less soluble in water at the boiling point than at some degrees below: at the temperature 91° Fahr., 100 parts of water dissolve 50·65 parts of the anhydrous sulphate, but at the boiling temperature only 42.65 parts.

floor is divided into two parts, one of which, the farthest from the fire, is four or five inches higher than the other. The annexed figure is a horizontal Fig. 45.

section, or ground-plan, of a furnace of this kind: a is called the "preparatory bed," because the cold materials are first placed there, and thoroughly heated, to avoid cooling the furnace, when the mass is afterwards placed on b, which is called the "fluxing bed." The preparatory bed and fluxing bed have each a door-way in the side of the furnace, and a small peephole through the door to view the progress of the operation. The usual length of black-ash furnaces is from eighteen to twenty feet, and the breadth from eight to ten feet. The "fluxing bed" sinks towards the centre to the depth of one inch and a half.

The proportions in which the mixture of sulphate of soda, limestone or chalk, and coal is made, are varied at different works; but the success of the operation is influenced to a considerable extent by the exactness observed in the ratio. In an extensive alkali work at St. Helen's in Lancashire, the proportions taken are thirteen parts of salt cake, nine and a half parts of coal, and fourteen parts of limestone. Dr. Ure recommends ten parts of the sulphate, from eleven to twelve parts of chalk or limestone, according

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to its purity, and five parts of coal. In some alkali works the materials are ground and sifted separately, and then carefully mixed; but the trouble of grinding and sifting may be avoided without much inconvenience, for the operation goes on successfully when the materials are merely broken into small fragments, and mixed in that state.

The furnace having been heated, from two hundredweight to a ton of the mixture (according to the size of the furnace) is shovelled in upon the preparatory bed, spread evenly over the surface, and transferred, when it has become hot, to the fluxing bed, which by this time should be at a full red heat. The mixture is transferred with the assistance of an iron tool, shaped somewhat like an oar. Whenever the first charge is shifted down upon the fluxing bed, a second charge should immediately be placed upon the preparatory bed.

As soon as the mass on the fluxing bed becomes ignited, and begins to clot upon the surface, it is completely turned over by the oar or "spreader;" and in the course of a few minutes afterwards, when jets of inflamed sulphuretted hydrogen and carbonic oxide gases, called "candles," begin to issue from various parts of the mass, the whole must be expertly worked about with the "spreader" and an iron rake, in order to equalize the mass, and constantly expose fresh surfaces to the action of the flame. When the temperature of the mixture seems to fall below the proper decomposing heat, the stirring and spreading may be discontinued for a few minutes, and the door closed to allow the mass again to attain a state of bright ignition. The agitation is then recommenced with