a compound of chlorine and sodium, known in chemical language as chloride of sodium. Its symbol is Na Cl, and its equivalent 58.5. It is the only mineral food of man, and forms an essential constituent of the blood, the loss of saline particles therefrom by the secretions, the tears, the bile, &c., being repaired by the use of common salt as a condiment. The gastric juice of the stomach contains free hydrochloric acid, which is doubtless derived from the salt taken with food; while the blood and some of the secretions contain soda, also referable to the same source. The unwholesomeness of salted provisions is probably due rather to their hardness and indigestible nature than to the salt with which they are impregnated.
The use of salt must have been nearly coeval with man's existence on the earth, frequent references to it, or to customs connected with it, occurring in the sacred writings.
All animals appear to be more or less fond of salt; even bees will sip a solution of it with avidity. Mungo Park says that in the interior of Africa, "the greatest of all luxuries is salt." It would appear strange to a European to see a child suck a piece of rock-salt as if it were sugar. This, however, I have frequently seen; although in the inland parts the poorer class of inhabitants are so very rarely indulged with this precious article, that to say a man eats salt with his victuals is the same as saying he is a rich man. I have myself suffered great inconvenience from the scarcity of this article. The long use of vegetable food creates so painful a longing for salt that no words can sufficiently describe it." (Travels, i. 280.) Burchell, in his Travels in South Africa, states that he had to send 90 miles for a gallon of salt, which he and his party regarded as a "valuable and important acquisition."
Salt exists in inexhaustible quantities in the waters of the ocean, in the proportion of about 2.7 per cent., or nearly 4 oz. per gallon, or a bushel in from 300 to 350 gallons. Salt is also found in immense masses of what is called rock-salt, or sal gem, in rocks of all ages, but chiefly in the new red sandstone, or saliferous formation, or the trias, where it is associated with a set of red sandstones and pebbly conglomerates, yellow magnesian limestones, and variegated shales and marls, which inclose irregular masses of rock-salt and gypsum. The following section (fig. 1) represents the deposit at Wimpfen in Württemberg, where the gypsum is inclosed by a deep layer of shell limestone containing the rock-salt as a separate mass. There are considerable difficulties in accounting for these deposits, but the discussion of them belongs to Geology.
The subterranean streams of water percolating through saliferous strata become impregnated with salt, and give rise to brine springs, so abundant in the great plain of the red marls and sandstones of Cheshire. The salt is not uniform in extent, but occupies limited areas. The brine springs occur at various depths. At Nantwich the brine is met with about 10 or 12 yards from the surface; and in sinking for fresh water, caution is required to avoid the brine. At Winsford it is generally necessary to sink from 55 to 60 yards before it is met with, and it then rises to within 12 yards of the surface. It occurs at other places at various depths. Droitwich in Worcestershire furnished salt from its brine springs in the time of the Romans; and it is probable that the supply was procured from such springs as found their way to the surface. In the time of Edward the Confessor, as appears from Doomsday-Book, brine pits were wrought at all the wicles in Cheshire. We read of several attempts made to improve the manufacture; and soon after the formation of the Royal Society reports of the methods of manufacture were published by that body. The salt made in England was long considered to be inferior to foreign salt; but at the commencement of the eighteenth century Parliament granted a reward to Mr Lowndes, a Cheshire gentleman, for improvements in the manufacture. In 1748 Dr Brownrigg published his Art of Making Common Salt, and by this time the manufacture had made some progress. The River Weaver was made navigable for vessels of large size from Northwich and Winsford to Liverpool, whereby means for distributing the salt of Cheshire were increased, and the manufacture gradually became important, the salt being distributed throughout the country, and also exported. About the year 1670 the beds of rock-salt, whence the springs originated, were discovered while searching for coal in Marbury, about a mile to the north of Northwich. They were found about 34 yards from the surface, in a bed 30 yards thick, resting on a stratum of indurated clay. It was afterwards found that, on sinking a shaft at any point within half a mile of Marbury, the salt was met with at the same depth. This was the only deposit discovered until 1779, when, in searching for brine near Lawton, it was met with about 42 yards from the surface in a stratum 4 feet in thickness; but on penetrating through the clay beneath it, a second stratum of rock-salt, 12 feet in thickness, was found. On continuing the sinking through 15 yards of clay, a third stratum of rock-salt was discovered, which was sunk into to the depth of 24 yards, the lowest 14 of which were found to be the purest. The existence of this pure salt at so great a depth induced the Northwich owners to sink deeper, which they had not hitherto done, for fear of meeting with freshwater springs. Accordingly, in 1781, they passed through the indurated clay below the rock-salt which had so long been worked. Immediately below this clay, which was 10 or 11 yards in thickness, they came upon a second stratum of rock-salt, the upper portion of which was about equal in purity to the higher stratum; but on penetrating to the depth of about 25 yards it was found to be much more free from earthly admixture. This increased purity, however, only extended to 4 or 5 yards.
The strata passed through in sinking for brine or rock-salt are usually clay and gypsum, mingled in various proportions, the latter predominating in nearing the brine or rock-salt. The miners (named wallers, from the bank or wall which they raise round the pit with the rubbish of the works) call the clay, according to its colour, red, brown, or blue metal, and the gypsum they name plaster. The strata are usually sufficiently compact to exclude fresh water; but in some places are so broken and porous (shaggy metal, as the wallers call it) as to lead to the discontinuance of the sinking. The use of the steam-engine in pumping, and improved methods of sinking, have, however, in modern times, obviated the difficulty. The accompanying section (fig. 2) of the Wharton salt-mine, on the River Weaver, will further illustrate the nature of the workings.
The different degrees of purity of the rock-salt are represented by a horizontal section of one of the beds, in which various irregular circular, oval, or pentagonal figures, varying from 2 or 3 to 10 or 12 feet in diameter, may be seen. The boundary-lines of these figures are white, and from 2 to 6 inches wide: they consist of pure rock-salt, while the other portions are of salt mixed with earth in varying proportions. In passing through the indurated clay or stone, small veins of rock-salt are found running in various directions; and wherever a crevice occurs it is filled up with rock-salt, to which the clay and oxide of iron have given a deep red tinge.
The rock-salt of Cheshire is obtained in masses of considerable size, differing in form and purity. They are separated by blasting, and with the aid of the usual tools. In extending the workings, a good roofing is secured for the intended cavity, and in doing this the men work horizontally with common picks, so as to leave a roofing of the rock as plane as possible. A few feet above the purer portion the rock is of inferior quality, and is used in the refineries. The purer rock is called Prussian rock, from its being largely exported to the shores of the Baltic. The cavity thus formed, when illuminated by candles, presents a striking and brilliant appearance. In some cases pillars 8 or 10 yards square are left to support the roof; in others the salt is worked out in aisles. The salt is raised to the surface by steam-power, but horses are employed under-ground for conveying it to the bottom of the shaft. The shafts are usually square, and constructed of timber.
When, from any circumstance, water has reached a deposit of rock-salt, it forms a solution, and should the supply of water be constant, we thus have a brine spring. In order to reach it, a shaft is sunk down to a strong flagstone over the brine; and in order to exclude fresh-water, a second inner shaft is formed, and space between the two filled with clay. When the clay puddle becomes solid, the flag is broken, and the brine usually flows up the shaft. In Camden's time the brine was raised by human labour. He speaks of a "deep and plentiful brine-pit at Northwich, with stairs about it, by which, when the people have drawn the water in their leathern buckets, they ascend half-naked to their troughs, and fill them, from whence it is conveyed to the wick-houses." In places where a stream of water could be commanded, a water-wheel has been used for working the pumps; wind-mills and horse-power have been employed; but steam-power has now superseded all other methods of pumping.
The writer of this article has lately visited the salt-making town of Droitwich in Worcestershire, and will now describe the modes of manufacture adopted in that place, as a type of this branch of industry. The brine containing one-fourth of its weight of salt, rises from springs situated about 150 feet below the surface. A steam-engine is employed to pump it into reservoirs attached to each of the salt-works of the town, of which there are about five, producing about 60,000 tons of salt per annum. The reservoirs are large wooden cisterns, pitched within, or they may be ponds formed in clay and lined with brick. In Cheshire, where rock-salt is easily procured, a quantity is kept in the reservoir in order to ensure the saturation of the brine. The object of the manufacture is to evaporate the liquid portion of the brine in such a manner as to produce the variety of salt required. The different varieties were named by our Droitwich informant as follows:—Fine or square salt, the term square being due to the shape of the moulds; Butter salt; brisk salt; basket salt; broad or coarse salt; bough salt, made once a year in cold winter weather, and sometimes crystallized on boughs of trees, or on ornamental baskets, like alum baskets, for toys or presents; lastly, agricultural salt, consisting of the sweepings of the salt-works, and the deposit formed at the bottom of the evaporating pan.
The pans (fig. 3) used for evaporating the brine are made of wrought iron, and may contain from 600 to 1000 or more superficial feet; the usual form is that of an oblong square, and the depth from 12 to 16 or 18 inches. There are from two to four fires to each pan, according to its size; and the furnace extends far beneath the pan, and is so capacious as to require that iron supports should be fixed within it, on which the bottom of the pan rests. Thus, on looking into the furnace, the flames are seen to play around a number of short columns, spreading out at their bases and at their tops, so as to present a broad surface to the floor of the oven, and to the under part of the pan respectively. A supply of coal is kept near the fur-
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1 Engravings illustrative of the above descriptions are given in Mr Henry Holland's General View of the Agriculture of Cheshire; but the most complete and indeed the only separate work on salt is by Mr Charles Tomlinson, entitled The Natural History of Common Salt, published by the Christian Knowledge Society, 1850. nace doors, and a man works with long-handled iron tools at feeding the fire, stirring and spreading the fuel, and raking out the slag or clinkers. The fires are at one of the narrower sides of the oblong pan, and the flues conveying the heat into the drying-room (supposing the manufacture to be fine salt) are at the other; thus the two longer sides of the pan are left free for the operations of the work-people. For their convenience, the floor is raised, and presents a long walk on each side the pan, with a bench or a floor, with apertures, along side of it, where the moulds are set to drain. Standing on one of these raised walks, we watched the boiling of a panful of brine, which had been brought to the boil soon after daybreak, and was then (ten o'clock) ready for the moulds. The steam rose in dense white clouds to the roof, where it passed through apertures left for its escape, and the manipulators (in this case women) were bleached, by constantly living in an atmosphere of steam, to salt-like whiteness, but declared themselves to be perfectly healthy. The first step was slightly to lower the temperature of the boiling brine. This was effected by placing a wooden shoot beneath a tap in connection with the reservoir, and thus conveying into the pan a small quantity of cold brine, enough to check the boiling. Each woman now took a long-handled tool, which she called a rake, but which rather resembled the scrapers with which mud is collected in roads; this she flung forward towards the middle of the pan, and then dragged towards her, thus collecting the salt in large white heaps at the sides of the pan. The weight of the rake is so great as to make this a laborious part of the woman's work. At the back of the workers, ranged along beneath the side windows of the pan-room, stood a number of empty moulds or tubs, as the work-people call them. These are made of wood, and are of the well-known size and form of our squares of salt, or rather parallelopipeds. One of these moulds was now lifted into the pan, and set up on one of its narrow ends, which was perforated, the other being open. The woman now took a second long-handled tool called the skimmer, being a large iron disk, also perforated, and with which she lifted up masses of salt, and poured them into the mould. A few of these ladlesful seemed to fill the mould to the brim; but she took a short, thick stick, called a rammer, and worked the salt within the mould until it had subsided to half its former height, the brine meanwhile gushing out at every crevice. More salt was now poured into the mould, until a conical heap rose above the top; this was beaten down by a flat wooden tool called the beater, and was then lifted out of the pan and set aside to drain and consolidate. When all the salt had been thus removed from the pan and collected in moulds, fresh brine was added to that which remained in the pan, the fires were made up, and the boiling commenced anew. Such was the simple process of making fine or square salt, repeated at each pan about three times daily. The consolidation of the salt was completed in a hot room, where the squares, released from their moulds, and trimmed by means of a smooth piece of wood called the tupper, were arranged on shelves, with small spaces between each, that the air heated by the flues might have free access to them. The quantity of salt made at one pan, and that not the largest, amounted to about twenty tons per week.
The form of the salt-crystal obtained by slow spontaneous evaporation is a solid cube; but when procured at a boiling heat from the surface of the solution, the crystals are very small, and collect together in groups in hollow four-sided pyramids or hoppers, as the work-people call them, with the sides graduated in steps (fig. 4), in consequence of the small lines of cubical crystals gradually retreating inwards. One of these groups, according to Regnault, is thus formed:—Suppose a small cubical crystal to be produced at the surface of the solution, this crystal, from its superior density, tends to sink, but is prevented from doing so by capillary attraction. Around this first crystal other crystals are quickly formed, and become attached to its four upper edges, so as to form a hollow four-sided frame above the first cube; the group descending in the liquid, other crystals form along the upper and outer edges of the first frame, so as to present a second hollow frame; another frame forms on the second; and in this way the group enlarges, chiefly at the surface, since the salt, being equally soluble in hot and cold water, does not tend to deposit crystals on cooling, but only by evaporation, which takes place at the surface alone. The bases and altitudes of these little pyramids are generally equal, thus showing the disposition of the salt to form a cube. The cubes themselves, as obtained by slow evaporation (fig. 5), are well described by Bergmann. He says,—“These cubes exhibit diagonal markings or striæ, but frequently on each side produce squares parallel to the external surface, gradually decreasing inwards; circumstances which show the vestiges of their internal structure, for every cube is composed of six quadrangular hollow pyramids, joined by their apices and external surfaces; each of these pyramids filled up by others similar, but gradually decreasing, completes the form. By a due degree of evaporation, it is no difficult matter to obtain these pyramids separate and distinct, or six of such, either hollow or more or less solid, joined together round a centre. If we further examine the hollow pyramid of salt, we shall find it to be composed of four triangles, each of which is formed of threads parallel to the base, which threads, upon accurate examination, are found to be nothing more than series of small cubes.”
The brine is not a pure solution of chloride of sodium, but is contaminated with carbonate of lime and sulphate of lime, which, not being soluble, subside to the bottom of the pan, and form an incrustation, called by the workmen pan scratch or scale. This gradually accumulates, together with a portion of salt, and becoming thicker from day to day, it is necessary every week, or, where the evaporation is slower, once in three or four weeks, to clear it away by means of sharp iron picks. Heaps of this substance may be seen in the vicinity of the pan-house ready to be mixed with refuse salt, and ground up as agricultural salt. The larger fragments are sometimes given to cattle to lick instead of rock-salt. This pan-scratch in appearance resembles a thick layer of opaque ice. According to the analysis of Dr Henry, 480 parts of this substance contained 40 of chloride of sodium, 60 of carbonate of lime, and 380 of sulphate of lime; but the proportions vary in different brines. The greatest accumulation of pan-scratch is towards the close of the evaporation, for when much salt is deposited in the pan, it forms such a heavy mass at the bottom that the water cannot penetrate it, and the scale or deposit undergoes a kind of calcination and fusion which makes it very hard, and causes it to adhere strongly to the pan.
It was long supposed that the salt of Great Britain was inferior as a preserver of food to the salt produced in France, Spain, and Portugal, by the evaporation of sea-water, and hence we were accustomed to pay large sums of money for an article which we possessed in boundless profusion. Many years ago Dr Henry made a careful inquiry into the subject, in order to ascertain whether this preference of foreign salt was the result of accurate experience or merely prejudice, and in the former case whether any chemical difference could be detected to explain the superiority. The result of this investigation is given in an admirable paper contained in the Philosophical Transactions, vol. c.
Dr Henry found that the various descriptions of salt might be divided into stoved or lump salt, common salt, the large-grained flaky, and large-grained or fishery salt. In making the stoved or lump salt, the brine is raised to the temperature of boiling, or $225^\circ$ F. When the water has nearly all evaporated, the fires are slackened, the salt is drawn to the sides of the pan, and placed in wicker baskets or barrows (fig. 6), which are set aside to drain, and afterwards dried in stoves. It loses in drying about one-seventh of its weight.
In making this salt, according to Dr Henry's observation, the pan is filled twice in 24 hours, and for common salt only once. In the latter case, the brine is brought to a boiling heat in order to get a state of saturation as quickly as possible, and also to throw down the earthy matters. The fires are then slackened, and the crystallization is carried on at the temperature of from $160^\circ$ to $170^\circ$. The salt is in quadrangular pyramids of close and compact texture. After it is drained in baskets, it is carried to the storehouse, and is not exposed to heat. The large-grained flaky salt is formed at $130^\circ$ or $140^\circ$. It is somewhat harder than common salt, and approaches nearer the natural form of the crystals of chloride of sodium. The pan is filled once in 48 hours. Salt of this grain is sometimes made by slackening the fires between Saturday and Monday, and allowing the crystallization to proceed slowly on Sunday, whence it has obtained the name of Sunday salt. For the large-grained or fishery salt, the brine is heated to $100^\circ$ or $110^\circ$, at which temperature the evaporation is comparatively slow, and as there is no agitation produced in the brine, the salt forms in large cubical crystals. Five or six days are required for the process.
Now it is on the difference in size and hardness of the crystals that their adaptability to different uses depends, and not on any differences in chemical composition, for these are too minute to have any influence. The large-grained salt is peculiarly fitted for the packing of fish and other provisions, a purpose to which the small-grained salts are much less suitable. Their different powers of preserving food depend on the size of the crystals and their degrees of hardness. Quickness of solution, other circumstances being equal, is nearly proportional to the quantity of surface exposed; and since the surfaces of cubes are as the squares of their sides, it follows that a salt, the crystals of which are of a given magnitude, will dissolve four times more slowly than one whose cubes are only half the size. The kind of salt, then, which is distinguished for hardness, compactness, and perfection of crystals, will be best suited for packing fish and other provisions, since it will remain permanently between the different layers, or will be very gradually dissolved by the fluids that exude from the provisions, thereby furnishing a slow but constant source of saturated brine; whereas for preparing the pickle or striking meat, which is done by immersion in a saturated solution of salt, the small-grained varieties answer equally well; or, on account of their greater solubility, even better. The specific gravity of various samples of salt depending to a great extent on hardness and compactness of crystal, was found to be almost the same in the large-grained British salt as in that of foreign manufacture. Dr Henry remarks that "if no superiority be claimed for British salt, as applicable to economical purposes, on account of the greater degree of chemical purity which unquestionably belongs to it, it may safely, I believe, be asserted that the larger-grained varieties are, as to their mechanical properties, fully equal to the foreign hay salt."
Before proceeding to notice foreign methods of manufacture, we may give the following statistics of salt, derived from the Mineral Statistics of the Museum of Practical Geology:
| Cheshire | | --- | | The quantities of white salt carried on the River Weaver from the 5th April 1857 to 5th April 1858, was | 647,437 | | Do. do. of rock-salt | 65,773 | | Salt carried by railway from the districts of Winsford and Northwich, estimated | 525,000 |
| Worcestershire | | --- | | Stoke and Droitwich | 195,500 |
| Ireland | | --- | | Duncon, near Carrickfergus, belonging to the Belfast Mining Company, shipped | 16,650 | | Do. do. used for manufacturing purposes | 5,798 | | Do. do. white salt manufactured | 4,877 |
Total produce of the United Kingdom ... 1,462,045
The quantities of salt exported from the United Kingdom in the years 1855, 1856, and 1857, with the declared values, were as follows:
| 1855 | 1856 | 1857 | | --- | --- | --- | | 639,154 tons. | 745,513 tons. | 651,765 tons. | | L.205,857 | L.276,242 | L.239,959 |
There was formerly a duty on salt, which originated as a war-tax in the ninth year of the reign of William III., and was not removed until the year 1823. The price of salt, in consequence of the duty, was raised from 6d. a bushel to more than 20s.
In countries situated near the sea-coast common salt is frequently obtained by the evaporation of sea-water. There is not more than from 3 to 4 per cent. of saline matter in sea-water, and of this quantity common salt forms nearly two-thirds. Dr Schweitzer found in 1000 grains of the water of the English Channel, near Brighton:
| Water | Grains. | | --- | --- | | Chloride of sodium | 964.74372 | | Chloride of magnesium | 27.05948 | | Chloride of potassium | 3.66558 | | Bromide of magnesium | 0.75552 | | Sulphate of magnesia | 0.02929 | | Sulphate of lime | 2.29578 | | Carbonate of lime | 1.40662 | | Chloride of calcium | 0.03301 |
The specific gravity of the water at the surface, and from a depth of 10 fathoms, was 1.0274. A minute quantity of iodine was also found; and Professor George Wilson has discovered fluorine to be an element of sea-water. Dr Porchammer has also detected minute quantities of manganese, ammonia, barvta, strontia, iron, and silica.
In the preparation of salt from sea-water the water is ex- posed in a series of shallow ponds, called salt-gardens or salterns, to the action of the sun and air. The salterns are laid out on a clay soil on the sea-coast, and are protected from the influence of the tides; they are worked during the warmer months, from March to September. They are arranged in such a way that, as the salt is deposited in the hindermost pools, the foremost ones receive constant supplies of sea-water. In the first place a collecting pond A (fig. 7) is filled at the flow of the tide by means of a flood-gate to the depth of from 2 to 6 feet. In this pond the mud and mechanically-suspended substances are deposited, while the clear water is conveyed by means of a pipe to the front pool B, which is divided by a central embankment and arms proceeding alternately from it and the sides of the pool. This pool is horizontal and very shallow, and the salt water moves slowly through it in the direction of the arrows as far as the pipe C, which conveys it into a channel running along the four sides of the saltern. Arriving at the point D, it enters the ponds E into a third series of ponds F, and thence by channels h into the crystallizing ponds G. During all this time the brine has become stronger by evaporation of the water, so that when it reaches the crystallizing ponds it is ready to deposit its salt. This is indicated by a reddish tint on the brine. The ponds G are filled from channels at the corners, which admit of being closed with wooden plugs. The salt forms on the surface of the water, and is collected by means of rakes into small heaps i at the sides. Here the mother liquor flows off, and is collected in proper channels; and when no more salt separates by crystallization the spent liquor is allowed to flow off through K into the sea. The salt thus collected is contaminated with chloride of magnesium, so that the small heaps i are made up into large ones j, and covered with straw for keeping off the rain; the moisture of the air liquefies the chloride of magnesium, which gradually becomes separated from the saline mass. The process depends so much on the state of the weather that at the beginning of the season eight days may be required for the deposit of salt; but in fine dry weather salt may be collected two or three times a week, and in very favourable cases every day.
The repeal of the duty on salt enabled the Cheshire manufacturers to sell the article at so low a rate that the proprietors of the salterns could not compete with them. This will readily be understood when it is considered that the price of fuel, strength of brine, and facilities of carriage by river and canal, were all in favour of Cheshire. There was a saltern at Lymington in Hampshire, in which it was the practice to concentrate the sea-water to about one-sixth of its bulk, and complete the operation in boilers. The chief variety of salt manufactured resembled in grain the stoved salt of Cheshire. In preparing it, the salt was not raked out of the boiler and grained, but the water was entirely evaporated, and the salt taken out every eight hours, and removed into perforated troughs, through which the bittern, or mother-liquor, drained off. Below the troughs, and in a line with the holes, were upright stakes, on which a portion of the salt crystallized, and formed in the course of ten or twelve days on each stake a mass or lump, called a salt cat, weighing 60 or 80 lb. During the winter, when the manufacture of salt could not be carried on, sulphate of magnesia (Epsom salts) was manufactured from the bittern.
Salterns can only flourish in countries which have no natural deposits of salt or brine, and where foreign salt is excluded by a high protective duty. The method of evaporating the sea-water differs somewhat in different localities, and attempts have even been made to increase the mineral contents of sea-water before the process of evaporation is commenced. A contrivance of this kind has been in use in Lower Normandy from the ninth century, and it consists in allowing the sea-water to percolate through a filter of sand collected on the sea shore after the tide has gone out, by means of a long broad scoop drawn by a horse. The strength of the sea-water is thus considerably increased, and the evaporation is conducted in leaden boilers with wood fuel, the scum being removed during the boiling. The boiler is repeatedly filled up with sea-water during the evaporation, and the salt which forms is kept in motion by means of long rakes, to prevent the lead from fusing. When the whole of the water has been driven off it is removed by means of a perforated tool (fig. 8), and placed in baskets suspended over the boilers, the steam from which in the next charge penetrates the baskets, and removes much of the bitter deliquescent salts. The salt is next taken to warehouses, and by means of the tool (fig. 9) is piled up on the floor, which is made of hard cement. The salt remains here for about two months, during which time it loses from 20 to 25 per cent. of bittern. The salt is now fine and pure, and quite white. According to Dumas, from 700 to 800 litres of salt water are required to produce from 150 to 225 kilogrammes of salt. According to Sir Stamford Raffles a method of obtaining salt similar in principle to the above is adopted on the south coast of the island of Java.
There are vast deposits of rock salt at Bochnia and Vie-liczka in Galicia, and others equally important along each side of the great Carpathian range, extending at various intervals from Moldavia to Swabia. In this extensive tract are the celebrated salt-mines of Wallachia, Transylvania, Galicia, Upper Hungary, Upper Austria, Styria, Salzburg, and the Tyrol. In some cases culinary salt is prepared from brine, formed by letting down fresh water through a bore to the middle of a salt-bed, and pumping it up as a saturated solution, which may be treated in the manner already described for the Droitwich works. In some inland districts which are not so fortunate as to possess deposits of salt, and where land carriage and fiscal regulations render the imported article costly, advantage is taken of natural salt-wells where such occur. These are for the most part but slightly impregnated with salt, arising probably from their contact with fresh water after leaving the salt- Some of these springs have only half the strength of sea-water, and yet by judicious arrangements it has been found profitable to work them. The method of evaporation by artificial heat is of course out of the question, even supposing fuel to be abundant, which it is not. We will first describe the method which has been in use, it is said, from the year 1550 at Moutiers, the capital of the province of Tarentaise in Sardinia, taking as our chief authority Mr Bakewell's *Travels in the Tarentaise*, published in 1823.
The method here adopted is based upon the physical fact, that the rate of evaporation depends, other things being equal, on the amount of surface exposed; and the method practically consists in raising the weak brine to a height, and allowing it to fall in the form of rain, the single drops being retarded in their fall by a peculiar contrivance. In this way 3,000,000 lbs. of salt are produced every year, from a source which in most other countries would scarcely be noticed except for medicinal purposes. The salt-springs rise at the base of a limestone rock, and are passed along an open canal, through the distance of about a mile, to the salt-works. In this canal the brine deposits a red ochreous incrustation, and also gives off a mixture of carbonic acid and sulphuretted hydrogen. The water has an acidulous and slightly saline taste; its temperature at its rising is 99° F., and it contains only 1·83 per cent. of saline matter, other springs only 1·50. In addition to common salt, the water contains small proportions of sulphate of lime, sulphate of soda, sulphate and muriate of magnesia, and oxide of iron. Thus there is about 1½ lb. of common salt in 13 gallons of water. The first method of evaporating the water was by allowing it to trickle repeatedly through pyramids of rye-straw arranged in open galleries. A portion of the sulphate of lime was deposited on the straw, and the water attained a certain degree of concentration. The process was then completed at a salt-pan with fuel. This plan continued in operation for nearly two centuries. In 1730 the modern plan came into use. There are four evaporating houses, called *maison d'épines* or "thorn-houses," the first two of which are 350 yards long, about 25 feet high, and 7 feet wide. They consist of frameworks of wood supported on stone buttresses, and containing double rows of fagots of black thorn, placed loosely so as to admit the air, but supported firmly by transverse pieces of wood. In the centre of each house is a stone building containing the pumping apparatus, moved by a water-wheel. By this means the water is raised to the top of the thorn-house, and is received in channels on each side, extending the whole length: these long channels distribute it to smaller ones, from which it trickles through a multitude of small holes in a very gentle shower upon the fagots, where it is further divided into innumerable drops, falling from one point to another, and is received in troughs below, from which it is again pumped up, until by repeated exposure to the air it is deemed sufficiently concentrated to be passed to the evaporating house No. 2, where it undergoes similar treatment. These thorn-houses are placed at different angles, so as to catch the different currents of air that flow down the valley. The sulphate of lime is deposited in incrustations on the twigs. The thorn-house No. 3 is covered to preserve the salt water from the rain: it is 370 yards in length, and has 12 pumps on each side to distribute the water more equally. After passing through No. 3, the water, reduced to one-seventh of its original bulk, is conveyed along channels to the thorn-house No. 4, which is 70 yards in length, where it is concentrated to saturation. The time occupied in obtaining this result, from the first commencement, is in summer about one month; in wet seasons of course much longer. A good idea will be formed of the quantity of water evaporated by the following statements:—8000 hogsheads, when received at the thorn-houses Nos. 1 and 2, contain about 1½ per cent. of salt, and are reduced by evaporation to 4000 hogsheads; 4000 hogsheads, when received at No. 3, contain about 3 per cent. of salt, and are reduced to 1000 hogsheads; 1000 hogsheads, when received at No. 4, contain about 12 per cent. of salt, and are reduced to 550 hogsheads; 550 hogsheads received at the pans contain nearly 22 per cent of salt. The process is completed in the usual way, and it is calculated that only ¼-th part of the fuel is consumed that would be required for evaporating the whole of the water by fire. The fagots last from four to seven years, those in Nos. 1 and 2 decaying sooner than those in Nos. 3 and 4. The chief deposit of sulphate of lime is in No. 3. Sulphate of soda is also manufactured at the works, and the other alkaline refuse goes to the glass-maker.
The method of graduation invented at Moutiers was introduced into Saxony in 1539, and has received considerable attention on the part of modern chemists. In Knapp's *Technology* (translated by Ronalds) is an engraving of a portion of one of the evaporating houses which we here copy (fig. 10.) The weak brine is pumped up into a large reservoir, from which it flows into the troughs b, b of the thorn-house. From these troughs the brine is conveyed in a thin stream to a perforated channel c, from which it falls in drops on the wall of black-thorn fagots t. There is a sloping board placed so as to prevent the wind from giving a wrong direction to the drops, and the whole arrangement is protected from the weather by a roof r, a portion of which has been removed in our engraving to show the troughs. The building is erected in an airy place, in a direction at right angles to that of the prevailing wind; but should the wind blow slantingly upon one of the faces of the wall, so as to dissipate the brine, certain channels are closed by means of a lever, and others are opened, whereby the supply of brine is carried to the opposite side of the wall. At Schönebeck the thorn wall exposes a surface of 390,000 square feet, and this evaporates on an average 3½ths cubic feet of water from each square foot of wall per day, or nearly a million and a quarter hogsheads of water in a year of 258 working days, which include the most favourable portion of the year, frost being found to be injurious, for below 27° Fahr. sulphate of magnesia, and a portion of chloride of sodium, become converted into chloride of magnesium and sulphate of soda, and this decomposition, once begun, is not reversed when the weather becomes warmer; so that not only is salt lost, but the quantity of chloride of magnesium, which is injurious in the boiling process, is increased. Some loss is also entailed by the evaporation of salt with the water, an observation made in 1770 by Pallas, in the neighbourhood of the salt lakes of Asiatic Russia, where he found the dew to have a decidedly salt taste. The deposit upon the thorns, known as *thorn-stone*, consists of the carbonates of lime, magnesia, manganese, and iron, with traces of chlo- These deposits gradually fill up the interstices of the thorn wall, and stop the draught; so that it is necessary to renew the wall every five, six, or eight years. In the brine cisterns the deposit forms a fine mud, with a greyish kind of scum filled with bubbles; this consists chiefly of living infusoria, evolving pure oxygen. The brine is generally considered fit for boiling when it contains 23 per cent. of salt. If the natural spring contain as much as this, it is boiled down at once without being graduated.
After the brine has been graduated it is passed into vast reservoirs of masonry covered over and protected from the frost. Here the brine makes a further deposit of suspended matters. The boiling is carried on in winter only, in flat four-sided pans of sheet-iron, somewhat deepened towards the middle. The bottom of each pan is supported by brick-work (fig. 11), which contains the flues for distributing the fires and heating the drying-chambers. The pan is covered with a roof-shaped hooding of boards, with a steam or vapour trunk s, furnished at the bottom with wooden shutters d, which can be turned back so as to allow free circulation of air over the surface of the brine. The vapour is said to contain about 1 per cent. of salt, so that means are taken for condensing it, or rather that portion of it which trickles down the sides of the chimney, at the bottom of which is fixed a sort of channel connected with a tube t, leading into a tank. The process of boiling consists of first, the schlotage, or the further purification and evaporation of the brine, up to the point of saturation; and secondly, the soccage, or crystallization of the salt. During the boiling the scum which rises to the surface is removed, while the other impurities form pan-scale. A pan containing 1600 feet of brine, or 176 cwt. of salt, is re-filled as often as one-fourth of the quantity is evaporated; so that when, after twenty or twenty-four hours, a pellicle of crystals begins to form on the surface the fire is slackened until the temperature of the brine falls to 194° Fahr., and from that to 167°, when with slow evaporation the soccage begins, and is continued for several days. In this way salt of coarse grain is produced. During the soccage the salt is raised to the edge of the pan and placed in baskets of peeled willow, or heaped upon boards to drain, after which it is dried and packed for sale. The salt thus formed is not quite pure; it usually contains a minute portion of one or other of the following salts:—Chloride of magnesium, chloride of calcium, sulphate of soda, sulphate of magnesia, and sulphate of lime. The chloride of magnesium has the greatest influence on the quantity of the salt, on account of its highly saline taste and deliquescent nature in the air. Pure chloride of sodium does not attract moisture, but if it contain only a minute portion of chloride of magnesium, it becomes wet in damp weather. The greater pungency, however, of this salt, causes it to be preferred in places where salt is costly, as a smaller quantity of it will suffice. The chloride of magnesium can be removed during the soccage by the addition of slaked lime to the brine.
It is remarkable that in the neighbourhood of these inland salt-works plants which usually grow on the sea-shore are met with,—such as the Salicornia, the Salsola kali, the Aster tripolium, the Glaux maritima, &c.—because in such situations they find food adapted to their habits. Liebig supposes that the seeds of these plants must have been carried by the wind or by birds, and scattered over the several hundred miles which separate the salt-works from the sea, but that they only germinated in those places where they found the conditions essential to their existence. Small fishes (Gasterosteus aculeatus) are found in the salt reservoirs at Nidda in Hesse-Darmstadt.
There is yet another method of procuring salt, based upon the remarkable property of ice to exclude foreign substances from its composition, so that when brine is exposed to a low temperature it resolves itself into two portions,—one consisting of pure water, which crystallizes or freezes, and can be removed in the form of ice; the other portion or brine remaining liquid, and becoming intensely salt by the removal of the fresh water. The salt can be separated by the usual method of boiling. The product, however, is very impure, since the effect of low temperature, as already noticed, is to convert the sulphate of magnesia of the brine into sulphate of soda and chloride of magnesium at the expense of the salt. A portion of the brine formed in this way from the water of the sea of Okhotsk was found by Hess to contain—
| Substance | Percentage | |--------------------|------------| | Common salt | 77·60 | | Sulphate of soda | 13·60 | | Chloride of aluminium | 6·20 | | Chloride of calcium | 0·94 | | Chloride of magnesium | 1·65 |
In places where the salt prepared from such brine is used, the people are subject to scorbutic diseases, which Hess attributes to the presence of these chlorides. Such brines should therefore in all cases be purified by means of lime. It is remarked by Dumas that this is the first analysis of bay salt in which chloride of aluminium occurs. (c.r.)
SÁLTA. See PLATA, La.