SALT. Common salt, or simply salt, is the name given to the native and industrial forms of sodium chloride (NaCl). The consideration of this important substance naturally falls under two heads, relating respectively to sea salt or “bay” salt and “rock” salt or mineral salt. As actually found, however, the one is probably derived from the other, most rock salt deposits bearing evidence of having been formed by the evaporation of lakes or seas at former (often remote) geological periods. This is seen from their stratified nature, with their interposed beds of clay, which could only have been deposited from solution. The crystals of selenite (hydrated calcium sulphate), moreover, which they contain can only have been formed in water and can never since have been subjected to any considerable amount of heat, otherwise their water of crystallization would have been driven off. The beds also of potassium and magnesium salts found at Stassfurt and other places, interposed between or overlying the rock salt deposits, are in just the position in which one would naturally expect to find them if deposited from salt water. Finally, the marine shells often occurring abundantly in the surrounding rocks of contemporary periods also testify to the former existence of large neighbouring masses of salt water....
Sea Salt.— Assuming a degree of concentration such that each gallon of sea water contains 0·2547 lb. of salt, and allowing an average density of 2·24 for rock salt, it has been computed that the entire ocean if dried up would yield no less than 4,419,360 cubic miles of rock salt, or about fourteen and a half times the bulk of the entire continent of Europe above high-water mark, mountain masses and all. The proportion of sodium chloride in the water of the ocean, where it is mixed with small quantities of other salts, is on the average about 33·3 per 1000 parts, ranging from 29 per 1000 for the polar seas to 35·5 per 1000 or more at the equator. Enclosed seas, such as the Mediterranean, the Red Sea, the Black Sea, the Dead Sea, the Caspian, and others, are dependent of course for the proportion and quality of their saline matter on local circumstances. Forchhammer found the following quantities of solid matter in the water of various seas:—
[table]
Of this sodium chloride constitutes about four-fifths.
See Sea Water .
At one time almost the whole of the salt in commerce was produced from the evaporation of sea water, and indeed salt so made still forms a staple commodity in many countries possessing a seaboard, especially those where the climate is dry and the summer of long duration. In Portugal a total of over 250,000 tons is annually made in the salt works of St Ubes (Setubal), Alcacer do Sal, Oporto, Aneyro, and Figueras. Spain, with the salt works of the Bay of Cadiz, the Balearic Islands, Ac., makes 300,000 tons. Italy has salt works in Sicily, Naples, Tuscany, and Sardinia, producing 165,000 tons. In France, between the “marais salants du midi” and those on the Atlantic, 250,000 to 300,000 tons are annually produced, besides those of Corsica. The “Salzgiirten” of Austria produce collectively from 70,000 to 100,000 tons annually at various places on the Adriatic (Sabioncello, Trieste, Pirano, Capo d’Istria, &c.). In England and Scotland the industry has of late years greatly fallen off under the competition of the rocksalt works of Cheshire, but some small manufactories still exist, at North Shields and elsewhere, where salt is made by dissolving rock-salt in sea water, and evaporating the solution to crystallization by artificial heat.
The process of the spontaneous evaporation of sea water has been very carefully studied by Usiglio on Mediterranean water at Cette. The density at first was l·02. Primarily but a slight deposit is formed (none until the concentration arrives at specific gravity 1·0509), this deposit consisting for the most part of calcic carbonate and ferric oxide. This goes on till a density of ·1315 is attained, when hydrated calcium sulphate begins to deposit, and continues till specific gravity 1·2646 is reached. At a density of 1·218 the volume of the sea water has become reduced to 95 / 1000 ths of what it was at first, and from this moment the deposit becomes augmented by sodium chloride, which goes down mixed with a little magnesium chloride and sulphate. At specific gravity 1 ·2461 a little sodium bromide has begun also to deposit. At specific gravity 1·311 the volume of the water is only 16 / 1000 ths of what it was at first, and it is thus composed:—
[table]
Up to the time then that the water became concentrated to specific gravity 1'218 only 0·150 of deposit had formed, and that chiefly composed of lime and iron, but between specific gravity 1·218 and 1·313 there is deposited a mixture of—
[table]
And of this we see that about 95 per cent. is sodium chloride. Up to this point the separation of the salts has taken place in a fairly regular manner, but now the temperature begins to exert an influence, and some of the salts deposited in the cold of the night dissolve again partially in the heat of the day. By night the liquor gives nearly pure magnesium sulphate; in the day the same sulphate mixed with sodium and potassium chlorides is deposited. The mother-liquor now falls a little in density to a specific gravity of 1·3082 to 1·2965, and yields a very mixed deposit of magnesium bromide and chloride, potassium chloride, and magnesium sulphate, with the double magnesium and potassium sulphate, corresponding to the kainite of Stassfurt. There is also deposited a double magnesium and potassium chloride, similar to the carnallite of Stassfurt, and finally the mother-liquor, which has now again risen to specific gravity 1·3374, contains only pure magnesium chloride.
The application of these results to the production of salt from sea water is obvious. A large piece of land, varying from one or two to several acres, barely above high-water mark, is levelled, and if necessary puddled with clay so as to prevent the water from percolating and sinking away. In tidal seas a “jas” (as the storage reservoir is called) is constructed alongside, similarly rendered impervious, in which the water is stored and allowed to settle and concentrate to a certain extent. In non-tidal seas this storage basin is not required. The prepared land is partitioned off into large basins (adernes or muants) and others (called in France aircs, oeuillets, or tables salantes) which get smaller and more shallow in proportion as they are intended to receive the water as it becomes . more and more concentrated, just sufficient fall being allowed from one set of basins to the other to cause the water to flow slowly through them. The flow is often assisted by pumping. The sea salt thus made is collected into small heaps on the paths around the basins or the floors of the basins themselves, and here it undergoes a first partial purification, the more deliquescent salts (especially the magnesium chloride) being allowed to drain away. From these heaps it is collected into larger ones, where it drains further, and becomes more purified. Here it is protected by thatch till required for sale.
The salt is collected from the surface by means of a sort of wooden scoop or scraper which the workman pushes before him, but in spite of every precaution some of the soil on which it is produced is inevitably taken up with it, communicating a red or grey tint. Sea salt is thence known in many of the French markets as sel gris, and frequently contains as much as 15 per cent. of impurity. Yet such is the ignorance and prejudice of many people that they will buy it in preference to the purer article from the evaporation of rock-salt brine, asserting its action to be milder and more even. Even if this were true they forget that mud ought to be cheaper than salt. The salt made on the coast of Brittany possesses the following composition :—
[table]
Generally speaking this salt goes into commerce just as it is, but in some cases it is taken first to the refinery, where it either is simply washed and then stove-dried before being sent out or is dissolved in fresh water and then boiled down and crystallized like white salt from rock-salt brine. The salt of the “salines du midi” of the south-east of France is far purer than the above, however, its composition being as follows :—
[table]
This is perhaps partly owing to the fact that of late years, by way of obviating the above-mentioned cause of impurity, a species of moss has been introduced there with some success from Portugal and forms a bed on which the salt is deposited. The mother-liquors from the crystallization of the common salt contain still a little sodium chloride and most of the bromine and iodine of the sea water, all the potassium salts, much magnesium sulphate, and a large quantity of magnesium chloride. They are often thrown away as useless, but lately, in the south of France, in the “salines du midi,” they have been used for the production of certain chemicals by a system of ulterior treatment introduced by M. Merle and still continued by his successor M. Pechinet.
As soon as the water arrives at specific gravity 1·2407 and has deposited most of its salt, it is drawn off and stored in large tanks of 50,000 or 60,000 cubic metres capacity. From these it is withdrawn in successive portions, and artificially cooled to 0·4° Fahr. Under these circumstances, indeed at any temperature below 26° Fahr., a double decomposition takes place between the sodium chloride and the magnesium sulphate—crystallized sodium sulphate being thus separated. After being withdrawn and freed from the mother-liquor by a hydro-extractor, this sulphate, which contains two atoms of water, is then rendered anhydrous by heating in a reverberatory furnace. From the refrigerating vessel the water now passes to an ordinary evaporating pan, where the remaining salt is precipitated by boiling, collected, and purified by the hydro-extractor. Here the water attains a specific gravity 1·2680, and, being spread out in a thin layer on a smooth level bed of cement or concrete, deposits on cooling all its potassium as the double chloride of potassium and magnesium, the same as the carnallite of Stassfurt.
Fig. 1 represents the usual form of an Austrian “Salzgarten” at Capo d’lstria. It is a parallelogram of 2 to 3 acres in extent surrounded by a dyke or sea-wall a. The sea water enters by the sluice b, and passes into the wide fosse c , where, clarifying by settlement, it passes by the openings f into a sextuple series of large basins divided by the separations d, first of all entering the largest ones g, h, i, and then passing by the canals n into the other basins k, k, l, l. The flow of the water from one set of basins to the other is regulated by the sluices e, e, e. As it passes from one set of basins to another it becomes more and more concentrated, till at last in the basins m, m the salt deposits. The mother-liquor or
“bittern” is then run off into p, and thence into the sea. In France it is often stored as already stated for future treatment. In case of heavy rain, the already concentrated water is run into the covered cisterns s , s, which serve to hold it till the return of fine weather.
[table]
Rock-Salt.— This appears to occur in almost every formation, except in the Primary rocks, strictly so called. The oldest deposit of which the age may be considered to have been anything like precisely determined may be said to be the great salt range of the Punjab, which is regarded as belonging to the Permian; and that lately discovered at Middlesbrough in Yorkshire, immediately overlying the magnesian limestone, may be probably referred to the same period. In the northern counties of England there are frequent instances of brine springs rising from the Carboniferous and contiguous formations. The Cheshire and Worcestershire salt-beds are by some attributed to the Permian; more generally, however, they are referred to the Trias. Those of West New York and Gooderich (Canada) are said to belong to the Salina period of the Upper Silurian. The deposits of the Vosges, Salzburg, and others of central Germany and Austria are considered to belong to the Trias; that of Bex in Switzerland to the Lias. Those of Wieliczka in Poland, Cardona in Spain, and some Algerian formations are admitted to be Cretaceous. Those of Bayonne, Dax, and Camarade, in the Pyrenees, are probably Tertiary, while the Dead Sea, Lake Elton in Astrakhan, the Bitter Lakes of the Isthmus of Suez, the Kara Boghaz on the shores of the Caspian, the Limans of Bessarabia south of Odessa, the Runn of Cutch, and certain formations of the Sea of Azoff, &c., are instances of salt formations now in actual progress. The frequent association of bitumen and petroleum with rock-salt and brine is one of the most noticeable features in the geology of those substances, and seems to point to some unknown condition of the formation of the two first named. The Dax salt is close to the bitumen deposits of Bastenès and Gaujac. Borings made at Dax, as well as at Salies about 20 miles distant (where also salt exists), gave vent to an efflux of inflammable gas which continued for several weeks, and the water of several springs in that neighbourhood is tainted with petroleum. Bitumen and petroleum occur near Volterra in Tuscany, where a large deposit of salt is being worked. In Walachia the two occur in the same formation. In the United States of America and in the south of Russia petroleum and brine are found in many places either actually associated or in near proximity; petroleum has recently been discovered not far from the salt deposits of Hanover, and one of the beds of rock-salt at Nancy is strongly coloured by bitumen, while almost all rock-salt has a more or less perceptible bituminous odour when struck or rubbed. In the province of Szechuen, China, are some remarkable salt springs, where the brine is accompanied by such an efflux of inflammable gas that the latter serves as fuel for its evaporation; and other springs accompanied by the same phenomenon exist in the same region. In fact, instances without end might be cited of the two occurring together, and it would appear that petroleum for some mysterious reason can only be formed in presence of salt.
The chief rock-salt districts of Europe may be classified as follows :—(1) the Carpathians; (2) Austrian and Bavarian Alps; (3) West Germany; (4) Vosges; (5) Jura; (6) Swiss Alps; (7) Pyrenees and the Spanish or Celtiberian Mountains; (8) the British salt deposits; (9) isolated deposits and springs in Russia, Turkey, Italy, &c.
The Carpathian district may be subdivided into the Moldo-Walachian, Transylvanian, Galician, and Hungarian sections. They form probably the richest and most extensive of the European salt fields and by them alone the entire continent might be supplied for ages. The Transylvanian and Walachian mines are specially numerous and rich. Thousands of tons of salt, in the form of brine from the springs which are common throughout tho country, are allowed to run to waste, no important factory existing in the country for its evaporation. The rock is in fact in itself so pure that simply ground it meets all requirements of public consumption. In Galicia the principal mines and those of most historical interest are at Wieliczka and Bochnia. The former, which is justly the most celebrated in the world, is situated 9 miles from Cracow and has been worked continuously for six hundred years. The mass of salt is calculated to be 500 miles long, 20 miles broad, and 1200 feet thick. It is on the north-west side of a ridge of hills, an offset of the Carpathians. The salt is stoped out in longitudinal and transverse galleries, and large vaulted chambers, supported by massive pillars. Explosives are not used in this or any of the other mines of the district. The salt is sold just as it comes from the mine, or else finely ground and packed in casks or sacks. The mine is divided into four levels, and is 284 yards deep and 1 mile 1279 yards long by 830 yards wide. All the grinding and packing is done within it.
It is stated that the collective length of the galleries and chambers is no less than 30 English miles and the total yield 55,067 tons per annum. These mines employ from eight hundred to one thousand persons, many of whom live permanently under ground; the lower levels contain streets and houses and constitute a complete village. Travellers have given glowing descriptions of the crystal vaults, sparkling aisles, and fairy palaces of this mine. The salt is greyish, and somewhat resembles granite in appearance.
In the well-known district of the Austrian and Bavarian Alps the mine of Salzburg (Salzkammergut) is perhaps the most familiar. The Austrian portion of the district includes the towns of Aussee, Ischl, Hallstadt, and Hallein, and the Bavarian includes Berchtesgaden, Reichenhall, Traunstein, and Rosenheim. In the last-named salt is made from brine conveyed in pipes from Berchtesgaden, passing by Reichenhall, 15 miles in all, with a total fall of 1552 feet. There arc also large salt works at Hall near Innsbruck. Here, as in the Carpathian region, most of the rocksalt is sold merely ground, or in lumps, and the trade is, as in other parts of Austria-Hungary, a strict Government monopoly, , producing an annual revenue of two and a quarter to two and a half millions sterling.
The German mines are numerous; they extend north and south from Segeberg in Holstein to Sulz on the Neckar, and east and west from Kreuznach to Halle. Brine springs and small workings lie scattered all over the country. But two formations of special importance are Stassfurt in Saxony and the Lüneburg Heath in Hanover. Fig. 2 represents a section of the Stassfurt beds, and will give an idea of their formation. It appears less than most others to have been subjected to denudation since being formed, and consequently better than many others illustrates the formation of such deposits. Overlying the salt properly so called (e ) is a thin band of anhydrite, and above this b, c, d, beds of variously - coloured clay, red and grey, containing highly deliquescent salts, forming three distinct layers. The lowest, d , called “polyhalite,” contains some sodium chloride and with it other deliquescent chlorides. Next to this comes c, the “kieserite” region, about 30½ yards thick—here are chiefly potassium and magnesium sulphates; and lastly we have b, the upper layer or “carnallite” region, 23 yards thick, containing almost exclusively the double potassium and magnesium chlorides, together with other deliquescent salts, nodules of boracite, &c. lt has been computed that a sea depth of 12½ miles would be required for the production of such a series as this.
The Vosges, which is a very important district, supplied a large part of the east of France with salt, till lost by the war of 1870- 1871, since which time Nancy has gained considerably in importance. Geologically speaking, Nancy is included in this basin.
In Switzerland the chief salt district lies on the right bank of the Rhone, near the Lake of Geneva. The principal centres are Aigle, Roche, and Bex, the last being the most important.
The Pyrenees are rich on both sides in brine springs and rocksalt formations. In the south-west of France we have the rocksalt of Dax and Villefranche, and the brine springs of Salies and Briscous, as well as that of Camarade. In Spain both rock-salt and brine are plentiful, as is indicated by the frequent recurrence of the syllable “Sal” in the names of towns (Salinas, Salinillas, Poza de la Sal, &c.).
The Celtiberian or exclusively Spanish district includes various towns scattered over Spain—Salinas de Saelices (Guadalajara), Villafáfila (Zamora), Torreximeno, Cazorla, and Hinojares (Jaen), &c.; but perhaps the most remarkable deposit of salt in Spain is that of Cardona in the province of Barcelona, 45 miles north-west of that city. Here is a veritable mountain composed of a bed of remarkably pure salt 142 to 164 yards thick, and forming two masses, each about a mile in circumference. The salt is as usual stratified, and bears very strong evidence of denudation. It is chiefly pure white, but in parts varies from light-blue to brick-red. It is extracted by an open-air working like stone from a quarry.
There are some brine and rock-salt deposits which can hardly be classified as belonging to any particular district. Such are—in France, at the foot of the Alps, the brine springs of Moutiers and Castellane; in Italy, Volterra;...
