Alum was first employed for that purpose by Constantini, a physician of Melle near Osnaburgh, about 1650. The process does not succeed but at a low temperature. Sulphate of lime decomposes common salt when formed with it into balls, and exposed to a strong heat. Much discussion has taken place among the German chemists about the possibility of decomposing common salt by sulphate of iron. That sulphate of soda may be obtained by exposing a mixture of these two salts to a strong heat was first announced by Vander Ballen. This was contradicted by Hahneman, but confirmed by the experiments of Tuhten, Lieblein, and Wiegleb. It succeeded completely with the French commissioners, De Lievre, Pelletier, Darcet, and Girond, who were appointed, in 1794, to examine the different processes for obtaining soda from common salt. They ascertained also that pyrites or sulphureted oxide of iron may be employed for the same purpose. After obtaining the sulphate of soda it is necessary to expel the acid, to obtain the soda separately. This is done by calcining the salt mixed with a certain proportion of charcoal or pit-coal. By this process it is converted into sulphuret of soda, and the sulphur may be abstracted by the intervention of iron or chalk. When the sulphuret of soda is nearly in fusion, small bits of iron (the parings of tin-plate answer best) are thrown in gradually in sufficient quantity to decompose the sulphate. The fire is raised till the mixture melts. The iron, having a stronger affinity for the sulphur, combines with it, and leaves the soda, which may be separated by solution in water, filtration, and evaporation.

SALT, COMMON, METHOD OF PREPARING. Without entering into any particular detail of the processes used for the preparation of bay salt, in different parts of the world, we shall only give a brief account of the best methods of preparing common salt. At some convenient place near the sea-shore is erected the saltern. This is a long low building, consisting of two parts; one of which is called the fore-house, and the other the pan-house or boiling-house. The fore-house serves to receive the fuel, and cover the workmen; and in the boiling-house are placed the furnace and pan, in which the salt is made. Sometimes they have two pans, one at each end of the saltern; and the part appropriated for the fuel and workmen is in the middle. The furnace opens into the fore-house by two mouths, beneath each of which is a mouth to the ash-pits. To the mouths of the furnace doors are fitted; and over them a wall is carried up to the roof, which divides the fore-house from the boilinghouse, and prevents the dust of the coal and the ashes, and smoke of the furnace from falling into the salt pan. The fore-house communicates with the boiling-house by a door placed in the wall which divides them. The body of the furnace consists of two chambers, divided from each other by a brick partition called the midfeather; which from a broad base terminates in a narrow edge nigh the top of the furnace, and, by means of short pillars of cast iron erected upon it, supports the bottom of the salt pan: it also fills up a considerable part of the furnace,

The

which otherwise would be too large, and would consume more coals than by the help of this contrivance are required. To each chamber of the furnace is fitted a grate, through which the ashes fall into the ash-pits. The grates are made of long bars of iron, supported underneath by strong cross bars of the same metal. They are not continued to the farthest part of the furnace, it being unnecessary to throw in the fuel so far; for the flame is driven from the fire on the grate to the farthest part of the furnace, and thence passes, together with the smoke, through two flues into the chimney; and thus the bottom of the salt pan is every where equally heated. The salt pans are made of an oblong form, fiat at the bottom, with the sides erected at right angles: the length of some of these pans is fifteen feet, the breadth twelve feet, and the depth sixteen inches; but at different works they are of different dimensions. They are commonly made of plates of iron, joined together with nails, and the joints are filled with a strong cement. Within the pan five or six strong beams of iron are fixed to its opposite sides at equal distances, parallel to each other and to the bottom of the pan, from which they are distant about eight inches. From these beams hang down strong iron hooks, which are linked to other hooks or clasps of iron, firmly nailed to the bottom of the pan; and thus the bottom of the pan is supported, and prevented from bending down or changing its figure. plates most commonly used are of malleable iron, about four feet and a half long, a foot broad, and the third of an inch in thickness. The Scots prefer smaller plates, fourteen or fifteen inches square. Several make the sides of the pan, where they are not exposed to the fire, of lead; those parts, when made of iron, being found to consume fast in rust from the steam of the pan. Some have used plates of cast iron, five or six feet square, and an inch in thickness; but they are very subject to break when unequally heated, and shaken (as they frequently are) by the violent boiling of the liquor. The cement most commonly used to fill the joints is plaster made of lime. The pan thus formed is placed over the furnace, being supported at the four courners by brick work, but along the middle, and at the sides and ends, by round pillars of cast iron, called taplins, which are placed at three feet distance from each other, being about eight inches high, and at the top, where smallest, four inches in diameter. By means of these pillars the heat of the fire penetrates equally to all parts of the bottom of the pan, its four corners only excepted. Care is also taken to prevent the smoke of the furnace from passing into the boiling-house, by bricks and strong cement, which are closely applied to every side of the salt pan. In some places, as at Blyth in Northumberland, besides the common salt-pans here described, they have a preparing pan placed between two salt-pans, in the middle part of the building, which in other works is the fore-house. The sea-water, being received into this preparing pan, is there heated and in part evaporated by the flame and heat conveyed under it through flues from the two furnaces of the salt-pans; and the hot water, as occasion requires, is con

veyed through troughs from the preparing pan into the salt-pans. Various other contrivances have been invented to lessen the expense of fuel, and several patents have been obtained for that purpose; but the salt-boilers have found their old methods the most convenient. Between the sides of the pan and walls of the boiling-house there runs a walk five or six feet broad, where the workmen stand when they draw the salt, or have any other business in the boiling-house. The same walk is continued at the end of the pan, next to the chimney; but the pan is placed close to the wall at the end adjoining to the fore-house. The roof of the boiling-house is covered with boards fastened on with nails of wood, iron nails quickly mouldering into rust. In the roof are several openings to convey off the watery vapors; and on each side of it a window or two, which the workmen open when they look into the pan whilst it is boiling. Not far distant from the saltern on the sea-shore, between full sea and low-water marks, they also make a little pond in the rocks, or with stones on the sand, which they call their sump. From this pond they lay a pipe, through which, when the tide is in, the seawater runs into a well adjoining to the saltern; and from this well they pump it into the troughs, by which it is conveyed into their ship or cistern, where it is stored up until they have occasion to use it. The cistern is built close to the saltern, and may be placed most conveniently between the two boiling-houses, on the back side of the fore-house; it is made either of wood or brick and clay it sometimes wants a cover, but ought to be covered with a shed, that the saltwater contained therein may not be weakened by rains, nor mixed with soot and other impurities. It should be placed so high that the water may conveniently run out of it through a trough into the salt pans. Besides the buildings already mentioned, several others are required; as storehouses for the salt cisterns for the bittern, an office for his majesty's salt-officers, and a dwellinghouse for the salt-boilers. All things being thus prepared, and the sea-water having stood in the cistern till the mud and sand are settled to the bottom, it is drawn off into the salt-pan. And at the four corners of the salt-pan, where the flame does not touch its bottom, are placed four small lead pans, called scratch-pans, which, for a saltpan of the size above-mentioned, are usually about a foot and a half long, a foot broad, and three inches deep; and have a bow or circular handle of iron, by which they may be drawn out with a hook when the liquor in the pan is boiling. The salt-pan being filled with sea-water, a strong fire of pit-coal is lighted in the furnace; and then, for a pan which contains about 400 gallons, the salt-boiler takes the whites of three eggs, and incorporates them well with two or three gallons of sea-water, which he pours into the salt-pan while the water contained therein is only lukewarm; and immediately stirs it about with a rake, that the whites of the eggs may every where be equally mixed with the salt water. Instead of whites of eggs, at many salterns, as at most of these near Newcastle, they use blood from the butchers, either of sheep or black cattle, to clarify the seawater; and at many others they do not give

themselves the trouble of clarifying it. As the water grows hot, the whites of eggs separate from it a black frothy scum, which arises to the surface of the water, and covers it all over. As soon as the pan begins to boil this scum is all risen, and it is then time to skim it off. The most convenient instruments for this purpose are skimmers of thin ash boards, six or eight inches broad, and so long that they may reach above half way over the salt-pan. These skimmers have handles fitted to them; and the salt-boiler and his assistant, each holding one of them on the opposite sides of the pan, apply them so to each other that they overlap in the middle, and, beginning at one end of the pan, carry them gently forward together, along the surface of the boiling liquor to the other end; and thus, without breaking the scum, collect it all to one end of the pan, from whence they easily take it out. After the water is skimmed, it appears perfectly clear and transparent; and they continue boiling it briskly, till so much of the fresh or aqueous part is evaporated that what remains in the pan is a strong brine almost fully saturated with salt, so that small saline crystals begin to form on its surface; which operation, in a pan filled fifteen inches deep with water, is usually performed in five hours. The pan is then filled up a second time with clear sea-water drawn from the cistern; and, about the time when it is half filled, the scratch-pans are taken out, and, being emptied of the scratch found in them, are again placed in the corners of the salt-pan. The scratch taken out of these pans is a fine white calcareous earth found in the form of powder, which separates from the sea-water during its coction, before the salt begins to form in grains. This subtile powder is violently agitated by the boiling liquor, until it is driven to the corners of the pan, where, the motion of the liquor being more gentle, it subsides into the scratch-pans placed there to receive it, and in them it remains undisturbed; and thus the greatest part of it is separated from the brine. After the pan has again been filled up with sea-water, three whites of eggs are mixed with the liquor, by which it is clarified a second time, in the manner above described; and it is afterwards boiled down to a strong brine as at first; which second boiling may take up about four hours. The pan is then filled up a third time with clear sea-water; and after that a fourth time; the liquor being each time clarified and boiled down to a strong brine, as before related; and the scrach-pans being taken out and emptied every time that the pan is filled up. Then, at the fourth boiling, as soon as the crystals begin to form on the surface of the brine, they slacken the fire, and only suffer the brine to simmer, or boil very gently. In this heat they constantly endeavour to keep it all the time that the salt corns or granulates, which may be nine or ten hours. The salt is said to granulate, when its minute crystals cohere together into little masses or grains, which sink down in the brine and lie at the bottom of the salt-pan. When most of the liquor is evaporated, and the salt thus lies in the pan almost dry on its surface, it is then time to draw it out. This part of the process is performed by raking the salt to one side of the pan into a long hea, wher it drains a

while from the brine, and is then emptied out into barrows or other proper vessels, and carried into the store-house, and delivered into the custody of his majesty's officers. And in this manner the whole process is performed in twenty-four hours, the salt being usually drawn every morning. In the store-house the salt is put hot into drabs, which are partitions like stalls for horses, lined on three sides and at the bottom with boards, and having a sliding board on the foreside to put in or draw out as occasion requires. The bottoms are made shelving, being highest at the back side, and gradually inclining forwards; by which means the saline liquor, which remains mixed with the salt, easily drains from it; and the salt in three or four days becomes sufficiently dry, and is then taken out of the drabs and laid up in large heaps, where it is ready for sale. The saline liquor which drains from the salt is not a pure brine of common salt, but has a sharp and bitter taste, and is therefore called bittern; this liquor at some works they Save for particular uses, at others throw away. A considerable quantity of this bittern is left at the bottom of the pan after the process is finished; which, as it contains much salt, they suffer to remain in the pan, when it is filled up with sea-water. But at each process this liquor becomes more sharp and bitter, and also increases in quantity; so that, after the third or fourth process is finished, they are obliged to take it out of the pan, otherwise it mixes in such quantities with the salt as to give it a bitter taste, and disposes it to grow soft and run in the open air, and renders it unfit for domestic uses. After each process there also adheres to the bottom and sides of the pan a white stony crust, of the same calcareous substance with that be fore collected from the boiling liquor. This the operators call stone-scratch, distinguishing the other found in the lead pans by the name of powder-scratch. Once in eight or ten days they saparate the stone-scratch from their pans with iron picks, and in several places find it a quarter of an inch in thickness. If this stony crust is suffered to adhere to the pan much longer, it grows so thick that the pan is burnt by the fire, and quickly wears away.

SALT, NATIVE, or Rock SALT, or fossil salt, is common salt dug out of the earth. This kind of salt is in several countries found so pure that it serves for most domestic uses, without any previous preparation (triture excepted); for, of all natural salts, rock-salt is the most abundantly furnished by nature in various parts of the world, being found in large masses, occupying great tracts of land. It is generally formed in strata under the surface of the earth, as in Hungary, Muscovy, Siberia, Poland, Calabria, Egypt, Ethiopia, and the East Indies. In England, says Magellan, the salt mines at Northwich are in a high ground, and contain it in layers or strata of various colors, of which the yellow and brown are the most plentiful, as I have observed on the spot, which I visited in June 1782, in company with my worthy and learned friend Mr. Volta, professor of natural philosophy in the university of Pavia, and well known by his great abilities and many discoveries in that branch of

knowledge. The mine into which we descended was excavated in the form of a vast dome or vault under ground, supported by various columns of the salt, that were purposely left to support the incumbent weight. And, the workmen having lighted a number of candles all round its circumference, it furnished us with the most agreeable and surprising sight, whilst we were descending in the large tub which serves to bring up the lumps that are broken from the mine, &c. See the description of the famous salt mines of Wieliczka in Poland, by Mr. Bernard, in the Journal de Physique, vol. xvi., for 1780, p. 459, in which the miraculous tales concerning these subterraneous habitations, villages, and towns, are reduced to their proper magnitude and estimate.' But the English fossil salt is unfit for the uses of the kitchen, until by solution and coction it is freed from several impurities, and reduced into white salt. The British white salt is not so proper as several kinds of bay salt for curing fish and such flesh meats as are intended for sea provisions, or for exportation into hot countries. So that for these purposes we are obliged, either wholly or in part, to use bay salt, which we purchase in France, Spain, and other foreign countries.

It is remarked by the writer of the account of the Agriculture of Cheshire, that, from some experiments made on different specimens of the rock-salt of that county, it would appear that the transparent kind of it is an almost pure muriate of soda, which contains no admixture of either earth or earthy salts; and that the color of the less transparent and brown specimens is derived from the earth that enters, in greater or less proportions, into their compositions. That on 480 grains of transparent rock-salt being dissolved in four ounces of distilled water, there was, first, no precipitate let fall, on the addition of carbonate of potash. Secondly, no alteration was produced by this solution on blue vegetable juices. Thirdly, on the addition of a few drops of tincture of galls, a slight purple tinge was given to the solution; and after standing some hours, there was a brown sediment at the bottom of the vessel. Fourthly, on the addition of muriate of barytes, there was no precipitate thrown down. From the first of these trials, it is supposed that rock-salt has no muriate of lime, or muriate of magnesia, combined with it; from the second, that it has no uncombined acid or alkali; from the third, that it contains some portion of iron; and from the fourth, or last, that there is no sulphate of lime contained in it. And that, on examining different specimens of the less transparent, and the brown rock-salt, with the same re-agents as in the above trials, it was found that these consisted of muriate of soda, or sea-salt, in combination with a certain proportion of earth, varying in quantity from one to thirty per cent.; also, that the earth was wholly the argillaceous or common clay; but that some of the specimens contained a few grains of sulphate of lime, in 480 of those of the rock-salt.

The beds of this salt are now well known to be the principal cause of the salt-brine springs in this county; and, in connexion with some

of rock-salt, an irregular stratification, such as noticed by Jars, may, he says, by frequent accurate examination, be observed, the general appearance of the sides of the openings, whence the rock-salt is taken, is that of a confused and irregular red mass; in which some portions of salt have a greater, others a less, proportionate admixture of earth; while here and there they may be seen perfectly pure and transparent. He, therefore, asks, is it likely that this irregularity and confusion would have existed, had the beds of rock-salt in this district been formed by the evaporation of sea-water inundating the land at certain intervals of time, as the above theory supposes? On the contrary, says he, would it not be natural to expect from reasonings à priori, that the salt, thus deposited from sea-water, would be disposed in layers perfectly regular, and differing from one another merely in thickness, or a few other circumstances of inferior moment? Another fact which, it is supposed, invalidates, in some measure, the notion that the rock-salt has been deposited from the waters of the sea, is the great disproportion of quantity shown by analysis to exist between the earthy salts contained in the brine of this district, and those held in solution by sea-water; the ratio here being as one to ten, or the proportion which the earthy salts bear to the pure muriate of soda in sea-water is ten times greater than that which prevails in the Cheshire brine. The ascertaining of this fact proves, it is supposed, that the rocksalt (from the solution of which the brine is formed) is combined with a much smaller proportion of earthy salts than exists in sea-water; a circumstance difficult to be accounted for, on the supposition that the beds of this substance were formed by the evaporation of the sea-water, occupying the valleys and lowest parts of the land. It must be noticed, however, as worthy of attention, that the earthy salts intermixed with the rock-salt in the above district are the same which are held in solution by sea-water, being principally muriated magnesia and sulphate of lime.

There is, however, a still stronger proof, it is supposed, against the notion that the beds of rock-salt in this county are depositions from the sea-water, in the circumstance that no marine exuvia have ever been discovered in the strata. This, it is imagined, would almost indubitably have been the case, had the land been covered with sea-water during a period of sufficient length for the deposition of beds of salt of such prodigious thickness; and the fact, that no such exuvi do actually exist, is supposed in itself sufficient to induce a suspicion that the theory in question cannot be well founded. Other objections too, it is observed, offer themselves to its validity; such as the enormous depth of seawater necessary to the production of a body of rock-salt forty yards in thickness; the difficulty, if not impossibility, on such principles, of accounting for the formation of the singular insulated mountain of rock-salt at Cordova in Spain; with others of a more trivial nature, which will readily present themselves in this enquiry. It is, however, at the same time candidly acknowledged, that there are many facts and circumstances of actual observation, that confer a strong degree of

plausibility on the opinion, which it has been here contended against. The certainty that the surface of the county was at some former period much lower than it is at present, and the diminution of the thickness of the strata of rock-salt in proportion as they recede from the sea, are circumstances which undoubtedly range themselves on this side of the question: and, upon the whole, it is thought, that it may be doubted whether the theory, which regards the beds of rock-salt as deposits from sea-water, does not accord more exactly with existing appearances than any other which has been adduced on the subject.

According to the satement of Mr. Holland, in his Agricultural Survey, the first bed and pit of rock-salt was found and wrought in Marbury, at a small distance from the town of Northwich, at the depth of about thirty yards from the surface, in the year 1670, when searching for coal. The bed was thirty yards in thickness, and rested upon a stratum or layer of hard clay. In consequence of this discovery, other similar attempts were made; and, on sinking shafts or pits any where in the vicinity of it within the space of half a mile, it was found to exist at about the same depth from the surface of the earth, when not prevented from being dug down to by brinesprings or those of common water. This continued the only place in which it was found until the year 1779, when this sort of rock was again met with in searching for brine in the neighbourhood of Lawton, at the depth of about forty-two yards, but only of the thickness of about four feet; there being beneath it a bed of indurated clay ten yards in thickness, which, being penetrated through, a second stratum of rock-salt was discovered, twelve feet in thickness; and, on continuing the sinking of the pit, another layer of indurated clay, fifteen yards in thickness, was passed through; below which appeared a third stratum of rock-salt, which was unk into not less than twenty-four yards; the lowest fourteen yards, being the purest, or the least mixed with other substances, were the only parts that were wrought.

Until this period, in the neighbourhood of Northwich, no attempts had, however, been made to sink pits in order to find a lower stratum of rock-salt; as the one which had been first met with was so thick, and furnished such an abundant supply for every demand, there could be no other inducement to this than the expectation of meeting with a stratum, at a greater depth, which might contain a less admixture of earthy matters. It would seem, too, that the fear of meeting with springs below, which might impede the working out of the materials from the pits, and even render this wholly impracticable, prevented the proprietors of them from sinking deeper. As, however, no inconvenience or interruption of this nature had occurred, on sinking through different alternate strata of rock-salt and clay at Lawton; and it had been found that there was a wer stratum of rock-salt there, which was more pure than those nearer the surface, the owners of one of the works or pits in this vicinity were induced, a little time after the trials at Lawton, as in 1781, to sink deeper than had yet been done, and to