one of the great divisions of natural bodies. The characteristic marks of salt have usually been reckoned its power of affecting the organs of taste, and of being soluble in water. But this will not distinguish salt from quicklime, which also affects the sense of taste, and dissolves in water; yet quicklime has been universally reckoned an earth, and not a salt. The only distinguishing property of salts, therefore, is their crystallization in water; but this does not belong to all salts; for the nitrous and marine acids, though allowed on all hands to be salts, are yet incapable of crystallization, at least. least by any method hitherto known. Several of the imperfect neutral salts also, such as combinations of the nitrous, muriatic, and vegetable acids, with some kinds of earths, crystallize with very great difficulty. However, by the addition of spirit of wine, or some other substances which absorb part of the water, keeping the liquor in a warm place, &c., all of them may be reduced to crystals of one kind or other. Salt, therefore, may be defined a substance affecting the organs of taste, soluble in water, and capable of crystallization, either by itself or in conjunction with some other body; and, universally, every salt capable of being reduced into a solid form, is also capable of crystallization per se. Thus the clas of saline bodies will be sufficiently distinguished from all others; for quickly, though soluble in water, cannot be crystallized without addition either of fixed air or some other acid; yet it is most commonly found in a solid state. The precious stones, basaltites, &c., though supposed to be formed by crystallization, are nevertheless distinguished from salts by their insipidity and insolubility in water.

But acids and alkalies, and combinations of both, when in a concrete form, are salts, and of the purest form. Hence we conclude, that the bodies, to which the name of salts more properly belongs, are the concretions of those substances; which are accordingly called acid salts, alkaline salts, and neutral salts. These last are combinations of acid and alkaline salts, in such proportion as to render the compounds neither sour nor alkaline to the taste. This proportionate combination is called saturation: thus common kitchen salt is a neutral salt, composed of muriatic acid and soda combined together to the point of saturation. The appellation of neutral salts is also extended to denote all those combinations of acids, and any other substance with which they can unite, so as to lose, wholly or in great measure, their acid properties.

But although this general definition of salts is commonly received, yet there are many writers, especially mineralogists, who confine the denomination of salts in the manner we first mentioned, viz., to those substances only which, besides the general properties of salts, have the power of crystallizing, that is, of arranging their particles so as to form regular shaped bodies, called crystals, when the water superfluous to their concrete existence has been evaporated.

Common Salt, or Sea Salt, the name of that salt extracted from the waters of the ocean, which is used in greater quantities for preserving provisions, &c.

It is a perfect neutral salt, composed of marine or muriatic acid, saturated with mineral alkali. It has a saline but agreeable flavour. It requires about four times its weight of cold water to be dissolved, and nearly the same quantity of boiling water, according to Macquer. But according to Kirwan, it only requires 2.5 its weight of water to be dissolved in the temperature of sixty degrees of Fahrenheit. This salt always contains some part formed with a calcareous base; and, in order to have it pure, it must be dissolved in distilled water; then a solution of mineral alkali is to be poured in it until no white precipitation appears; then by filtrating and evaporating the solution, a pure common salt is produced. Its figure is perfectly cubic, and those hollow pyramids, or tremies as the French call them, as well as the parallelopipeds formed sometimes in its crystallization, consist all of a quantity of small cubes, disposed in those forms. Its decrepitation on the fire, which has been reckoned by some as a characteristic of this salt, although the vitriolated tartar, nitrous lead, and other salts, have the same property, is owing chiefly to the water, and perhaps also to the air of its crystallization.

Its specific gravity is 2.120 according to Kirwan. The acid of tartar precipitates nothing from it. One hundred parts of common salt contain thirty-three of real acid, fifty of mineral alkali, and seventeen of water. It is commonly found in salt water, and salt springs, in the proportion of even thirty-five per cent. It is found also in coals, and in beds of gypsum. This salt is unalterable by fire, though it fuses, and becomes more opaque: nevertheless a violent fire, with the free access of air, causes it to evaporate in white flowers, which adhere to the neighboring bodies. It is only decomposed, as Macquer affirms, by the sulphuric and nitric acids; and also by the boracic or sedative salt. But although nitre is decomposed very easily by arsenic, this neutral marine salt is nowise decomposed by the same. According to Monge, the fixed vegetable alkali, when caustic, decomposes all this marine salt. It preserves from corruption almost all sorts of animal food much better for use than any other salt, as it preserves them without destroying their taste and qualities; but when applied in too small a quantity, it then promotes putrefaction.

Of this most useful commodity there are ample stores on land as well as in the ocean. There are few countries which do not afford vast quantities of rock or fossil salt. Mines (A) of it have long been discovered and wrought in England, Spain, Italy, Germany, Hungary, Poland, and other countries of Europe. In several parts of the world, there are huge mountains which wholly consist of fossil salt. Of this kind are two mountains in Russia, near Astrakan; several in the kingdoms of Tunis and Algiers, in Africa, and several also in Asia; and the whole island of Ormus in the Persian gulf almost entirely consists of fossil salt. The new world is likewise stored with treasures of this useful mineral, as well as with all other kinds of subterranean productions. Moreover, the sea affords such vast plenty of common salt, that all mankind might thence be supplied with quantities sufficient for their occasions. There are also innumerable springs, ponds, lakes, and rivers, impregnated with common salt, from which the inhabitants of many countries are plentifully supplied therewith. In some countries which are remote from the sea, and have little commerce, and which are not blessed with mines of salt or salt-waters, the necessities of the inhabitants have forced them to invent a method of extracting

(A) Amongst the salt mines of chief note are those of Northwich in Cheshire, Altemonte in Calabria, Halle in Tyrol, Cardona in Catalonia; also those stupendous mines at Wilczek in Poland, to be noticed in the sequel of this article, and Soowar in Upper Hungary; of which see accounts in Phil. Trans. No. 61, and 413. ting their common salt from the ashes of vegetables. The muriatic salt of vegetables was described by Dr Grew under the title of "lixiviated marine salt." Leeuwenhoek obtained cubic crystals of this salt from a lixivium of soda or kelp, and also from a solution of the lixivial salt of carduus benedictus; of which he hath given figures in a letter to the Royal Society, published in No. 175. of their Transactions. Dr Dagner, in Act. Acad. N. C. vol. v. obf. 152., takes notice of great quantities of it which he found mixed in potashes. And the ingenious Dr Fothergill extracted plenty of it from the ashes of fern: See Medical Essays, vol. v. article 13.

The muriatic salt which the excellent Mr Boyle extracted from sandiver, and supposed to be produced from the materials used in making glass, was doubtless separated from the kelp made use of in that process. Kunckel also informs us, that he took an alkaline salt; and after calcining it with a moderate fire, dissolved it in pure water, and placing the solution in a cool cellar, obtained from it many crystals of a neutral salt. He supposes, that the alkaline salt was by the process converted into this neutral salt. But it is more reasonable to believe, that the alkaline salt which he applied was not pure, but mixed with the muriatic salt of vegetables, which by this process was only separated from it.

It is doubtless chiefly this muriatic salt which, in some of the inland parts of Asia, they extract from the ashes of duck-weed and of Adam's fig-tree, and use for their common salt.

That they are able in those countries to make common salt to profit from vegetables, ought not to be wondered at, since in Delhi and Agra, capitals of Indostan, salt is so scarce as usually to be sold for half-a-crown a pound. We may therefore give some credit to Marco Polo, when he informs us, that in the inner parts of the same quarter of the world, in the province of Caindu, lying west of Tibet, the natives used salt instead of money, it being first made up in cakes, and sealed with the stamp of their prince; and that they made great profit of this money by exchanging it with the neighbouring nations for gold and milk. We are also told by Ludolfus, in his Historia Ethiopica, that in the country of the Abyssines there are mountains of salt, the which when dug out is soft, but soon grows hard; and that this salt serves them instead of money to buy all things. The same is confirmed by Ramusio.

Mr Boyle discovered common salt in human blood and urine. "I have observed it (says Mr Brownrigg), not only in human urine, but also in that of dogs, horses, and black cattle. It may easily be discovered in these, and many other liquids impregnated with it, by certain very regular and beautiful starry figures which appear in their surfaces after congelation. These figures I first observed in the great frost in the year 1739. The dung of such animals as feed upon grass or grain, doth also contain plenty of common salt."

Naturalists, observing the great variety of forms under which this salt appears, have thought fit to rank the several kinds of it under certain general classes; distinguishing it, most usually, into rock or fossil salt, sea-salt, and brine or fountain salt. To which classes, others might be added, of those muriatic salts which are found in vegetable and animal substances. These several kinds of common salt often differ from each other in their outward form and appearance, or in such accidental properties as they derive from the heterogeneous substances with which they are mixed. But when perfectly pure, they have all the same qualities; so that chemists, by the exactest inquiries, have not been able to discover any essential difference between them; for which reason we shall distinguish common salt after a different manner, into the three following kinds, viz. into rock or native salt, bay salt, and white salt.

By rock salt, or native salt, is understood all salt dug out of the earth, which hath not undergone any artificial preparation. Under the title of bay salt may be ranked all kinds of common salt extracted from the water wherein it is dissolved, by means of the sun's heat, and the operation of the air; whether the water from which it is extracted be sea-water, or natural brine drawn from wells and springs, or salt water stagnating in ponds and lakes. Under the title of white salt, or boiled salt, may be included all kinds of common salt extracted by coking from the water wherein it is dissolved; whether this water be sea water, or the salt water of wells, fountains, lakes or rivers; or water of any sort impregnated with rock-salt, or other kinds of common salt.

The first of these kinds of salt is in several countries found so pure, that it serves for most domestic uses, without any previous preparation (trituration 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 found 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 colours, 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 light, whilst we were descending in the large tub, which serves to bring up the lumps that are broken from the mine," &c.

Wraxall gives the following description of the famous salt mines near Cracow in Poland.

"After being let down (says he) by a rope to the depth of 230 feet, our conductors led us through galleries, which, for loftiness and breadth, seemed rather to resemble the avenues to some subterraneous palace, than passageways cut in a mine. They were perfectly dry in every part, and terminated in two chapels composed entirely of salt, hewn out of the solid mass. The images which adorn the altars, as well as the pillars and ornaments, were all of the same transparent material; the points and spars of which, reflecting the rays of light from the lamps which the guides held in their hands, produced an effect equally novel and beautiful. Descending low- Salt.

Into the earth by means of ladders, I found myself in an immense hall or cavern of salt, many hundred feet in height, length, and dimensions, the floor and sides of which were cut with exact regularity. A thousand persons might dine in it without inconvenience, and the eye in vain attempted to trace or define its limits. Nothing could be more sublime than this vast subterranean apartment, illuminated by flambeaux, which faintly discover its prodigious magnitude, and leave the imagination at liberty to enlarge it indefinitely. After remaining about two hours and a half under ground, I was drawn up again in three minutes with the greatest facility."

See also an account of the same mines by Mr Bernard, Journal de Physique, vol. xvi. for 1780, in which the miraculous tales concerning those subterranean habitations, villages, and towns, are reduced to their proper magnitude and climate.

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 also 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.

However, it does not appear that there is any other thing requisite in the formation of bay salt than to evaporate the sea-water with an exceedingly gentle heat; and it is even very probable, that our common sea-salt by a second solution and crystallization might attain the requisite degree of purity. Without entering into any particular detail of the processes used for the preparation of bay-salt in different parts of the world, we shall content ourselves with giving a brief account of the best methods of preparing common salt.

At some convenient place near the sea-shore is erected the faltern. 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 faltern; 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 boiling-house, 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, 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 from thence passes, together with the smoke, through two flues into the chimney; and thus the bottom of the salt pan is everywhere equally heated.

The salt pans are made of an oblong form, flat at the bottom, with the sides erected at right angles; the length of some of these pans is 15 feet, in breadth 12 feet, and the depth 16 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. The 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, 14 or 15 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 corners 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 part 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 conveyed 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 boil- ing-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 vapours; 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 faltern, 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 fump. From this pond they lay a pipe, through which, when the tide is in, the seawater runs into a well adjoining to the faltern; and from this well they pump it into 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 faltern, 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 salt water contained therein may not be weakened by rains, nor mixed with soil 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 store houses for the salt, cisterns for the bittern, an office for his majesty's salt-officers, and a dwelling-house for the salt-boilers.

All things being thus prepared, and the sea-water having flooded 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 salt pan 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 eggs may everywhere be equally mixed with the salt water.

Instead of whites of eggs, at many falterns, as at most of those nigh Newcastle, they use blood from the butchers, either of sheep or black cattle, to clarify the sea-water: And at many of the Scots falterns 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 15 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 scum found in them, are again placed in the corners of the salt pan. The scum 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 into grains. This subtle 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 hath 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 before 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 scratch-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, then 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 heap, where it drains a while from the brine, and is then filled 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 24 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 fore side 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 hath a sharp and bitter taste, and is therefore called bitters; this liquor, at some works, they save for particular uses, at others throw away. A considerable quantity of this bitter 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 before 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 separate 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.

In M. de Pagés's Travels round the World, we find the following important fact. "I had been anxious (says that author) to ascertain by comparison, whether sea-water contains salt in greater quantity under the torrid than under the other zones; and my experiments on this subject served to show, contrary to what I expected, that sea-water is impregnated with salt in less quantity within than without the tropics." These experiments were made on a hundred pounds of sea-water, taken at the depth of ten fathoms, and weighed in water-cales. M. de Pagés has given a table of these experiments, from which it appears that 100 lb. of seawater in 40° 12' S. lat. gave 4½ lb. of salt, and in 1° 16" only 3½ lb.; and that in 74° N. Lat. it gave 4½ lb., and in 4° 22' only 3½ lb. these being the highest and lowest latitudes in which the experiments were made, and also the greatest and least quantities of salt.

Duty on Salt, is a distinct branch of his majesty's extraordinary revenue, and consists in an excise of 3s. 4d. per bushel imposed upon all salt, by several statutes of King William and other subsequent reigns. This is not generally called an excise, because under the management of different commissioners: but the commissioners of the salt-duties have, by statute 1 Ann. c. 21. the same powers, and must observe the same regulations, as those of other excises. This tax had usually been only temporary: but by statute 26 Geo. II. c. 3. was made perpetual.