Salt

water brine rock chloride magnesium sodium sulphate salts yards evaporation

SALT. Common salt, or simply salt, is the name given to the native and industrial forms of sodium chloride (NaC1). 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: - North Sea 32.80 grammes per litre.

Cattegat and Sound 15'12 fl Mediterranean 37.50 Atlantic 34'30 Black Sea 15'89 Caribbean Sea 36.10 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, &c., 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 "Salzgarten" 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 rock-salt 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 1-02. 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 P1315 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 rt-h-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 11461 a little sodium bromide has begun also to deposit. At specific gravity 1.311 the volume of the water is only Tito-ths of what it was at first, and it is thus composed: - Magnesium sulphate 11'45 per cent.

Magnesium chloride 19.53 „ Sodium chloride 15 '98 „ Sodium bromide 2.04 „ Potassium chloride. 3.30 „ 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 - Calcium sulphate. 0'0283 per cent.

Magnesium sulphate 0 '0624 „ Magnesium chloride. 0.0153 „ Sodium chloride .. 2'7107 „ Sodium bromide 0.0222 ..

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 11965, 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 laud, 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 i 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 laud is partitioned off into large basins (culerncs or muants) and others (called in France aires, (allets, 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 (espe- cially 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 grin, 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 : - Sodium chloride.. 87'97 per cent.

Magnesium chloride 1'58 „ Magnesium sulphate 0'50 ,, Calcium sulphate 1.65 „ Insoluble 0.80 „ Water.. 7.50 ..

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: - Sodium chloride 95.11 per cent.

Magnesium chloride 0'23 „ Magnesium sulphate 1.30 Calcium sulphate 0.91 Insoluble 0'10 „ Water 2'35 „ 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 11407 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'Istria. It is a parallelogram of 2 to 3 acres in extent SALT 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, 1,1. 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, a, which serve to hold it till the return of fine weather.

308 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 inagnesian 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 Tries. 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 Thas ; 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 Bastenes 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 he 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 MoldoWalachian, 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 the 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 Bothnia. 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 1270 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 fairypalaces 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, lschl, Hallstadt, and Hallein, and the Bavarian includes Berchtesgaden, Reiehenhall, Traunstein, and Rosenheim. In the last-named salt is made from brine conveyed in pipes from Berchtesgaden, passing by Ilkichenhall, 15 miles in all, with a total fall of 1552 feet. There are also large salt works at Hall near Innsbruck. Here, as in the Carpathian region, most of the rock-salt 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 cast and west from Kreuznach to Halle. Brine springs and small workings I lie scattered all over the country. But two formations of special importance are Stassfurt in Saxony and the Liineburg 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, 308 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 ride and with it other deliquescent chlorides. Next to this conies e, the "kieserite" region, about 301i 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. It has been computed that a sea depth of 121- 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 18701871, 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 Beg, the last being the most important.

The Pyrenees are rich on both sides in brine springs and rock-salt formations. In the south-west of France we have the rock-salt 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), Villafii.fila (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 Castellano ; in Italy, Volterra ; in Sicily, Nicosia and Mussomeli ; in Croatia, Szambor ; in Bosnia, Tnsia ; in Russia, Bachmutz on the Donets, Balachna on the Volga, Staraya-Russa near Lake llmen, Eupatoria and other places in the Crimea ; in Prussia, Waltersdorff, Sperenberg, &c.

The chief centres of manufacture in England are at Northwich, Middlewich, Winsford, and Sandbach in Cheshire, Weston-on-Trent in Staffordshire, Stoke Prior and Droitwich in Worcestershire, and Middlesbrough in Yorkshire.1 Duncrue near Carrickfergus in Ireland also possesses a large deposit of salt. The Cheshire and Worcestershire salt deposits are up to the present time by far the most important, the Duncrue deposit being only partially worked. Although brine springs have been known to exist in both these counties ever since the Roman occupation, and salt had been made there from time immemorial, it was not till 1670 that rock-salt about 30 yards thick was discovered at Marbury near Northwich by some men exploring for coal, at a depth of 34 yards. In 1779 three beds of rock-salt were discovered at Lawton, separated from one another by layers of indurated clay. The Marston mine, the property of Messrs Rigby and Fletcher of Northwich, is the largest and perhaps the oldest (there are twenty-five in England altogether where rock-salt is raised). It was worked for about a hundred years in only its upper bed, but in 1781 its owners decided on sinking farther, and, after traversing a layer of indurated clay intersected with small veins of salt Di yards thick, they came on another of rock-salt. This - the bed which has continued to be worked ever since - is 33 to 37 yards thick. Beneath it are others, but they are thin and impure. The total depth of the mine to the bottom of the lower level is 120 yards. At IN insford, where the same formation seems to recur, it is 159 yards from the surface. The Marston mine covers an arca of about 40 acres. The salt is first reached at 35-40 yards in the Northwich district, and the upper layer is 25-50 yards in thickness (Marston 23-26 yards) ; it has above it, apparently lying in the recesses of its surface, a layer of saturated brine. This is the brine which is raised at the various pumping stations in Northwich and elsewhere around, and which serves when evaporated to produce white salt. The beds are reached by sinking through the clays and variegated marls typical of this formation. The salt is blasted out with gunpowder. The Middlesbrough deposit bids fair soon to become of very great importance. It was discovered by Messrs Bolckow and Vaughan in boring for water in 1862 at a depth of 400 yards, but was not utilized, and was again found by Messrs Bell Bros. at Port Clarence at a depth of 376 yards, and is being now worked by them, the heat used for evaporation being the waste gases of their blast furnaces. Encouraged by their success the Newcastle Chemical Company have also bored on the opposite side of the river. They failed at first to find the salt, but ultimately succeeded by a fresh boring. The extent of the bed is not yet ascertained, but evidently by the failure of the Newcastle Chemical Company at first it cannot extend far to the north. Its thickness has been proved in so far as the spot where Messrs Bell Bros. made their boring is concerned. These gentlemen have introduced the method employed at Nancy of raising the salt in the form of brine without the trouble or expense of sinking a shaft. In Cheshire the surface-water trickling through the overlying strata dissolves the salt, which is subsequently pumped as brine, but here the great depth and impermeability of the strata precludes this, so another method has been resorted to. A bore is made into the salt, and lined with tubing in the usual manner, and this tube where it traverses the salt is pierced with holes. Within this is hung loosely a second tube of much smaller dimensions so as to leave an annular space between the two. Through this space the fresh surface water finds its way,. and dissolving the salt below rises in the inner tube as brine, but only to such a level that the two columns bear to one another the relation of ten to twelve, this being the inverse ratio of the respective weights of saturated brine and fresh water. For the remaining distance the brine is raised by a pump. At first, while the cavity remains small, there is some difficulty in getting a continuous supply of brine of full strength, but this ceases to be the case as the solution chamber (as it is called) becomes enlarged. The fresh water, however, as it descends rises to the surface of the salt, tending rather to dissolve its upper layers and extend superficially, so that after a time the superincumbent soil, being without support, falls in. These interior landslips, besides choking the pipes and breaking the communication, often produce sinkings at the surface, such as occurred some time ago at Dieuze (Lorraine). The same inconvenience is beginning to make itself felt in the environs of Nancy, and a similar one produces on a larger scale the sinking and subsidences at Winsford and Northwich so much complained of. The deposits of salt in the United States are unimportant. The country possesses no really considerable salt industry, but is supplied so far as interior consumption is concerned to a small extent by brine springs. The principal supplies, however, are derived from England and the shores of Spain and Portugal. The same remark applies to Canada. South America possesses several salt deposits and brine springs, but also takes all its supplies from Europe. Asiatic Russia is very abundantly supplied with salt, as likewise is China ; and Persia is perhaps one of the countries most abundantly endowed with this natural and useful product. British India cannot be said to be similarly favoured. In the north, it is true, is the great salt range of the Punjab, as well as the Sambhur Lake, and salt is obtained from sea-water at many places along its extensive seaboard ; but India is not well supplied in many parts, and is dependent largely for this article on the Cheshire salt works. In tact this export is one of the most important branches of their trade.

Table 11. (see next page) is from Spon's Encyclopmdia of the Industrial Arts, &c. The clay and insoluble matters given for the Stassfurt salt seem to be somewhat abnormally large.

Rock-salt is probably the origin of more than half the salt manufactured in the world. It occurs in all degrees of purity, from that of mere salty clay to that of the most transparent crystals. In the former case it is often difficult to obtain the brine at a density even approaching saturation, and, as at Moutiers in Savoy and in several of the German salt works, chambers and galleries are excavated within the saliferous bed to increase the dissolving surface, and water let down fresh is pumped up as brine. Many brine springs also occur in a more or less saturated condition. In such cases the water is sometimes caused to trickle over faggots arranged under large open sheds called "graduation houses" (Gradirhituser), whereby a more extensive surface of evaporation is obtained, and 308 the brine becomes rapidly concentrated. Fig. 3 shows one of these "Gradirliduser." It consists of a long shed, the floor of which is a shallow cistern kept filled with the brine to be concentrated, the body of the house being occupied by a single or double row of 308 faggots of blackthorn t, and above these a trough or troughs b, into which the brine is pumped; escaping from these into the channel c, it is allowed to flow or drip slowly over the faggots, and finds its way back to the basin beneath. The shed has its sides open and exposed to the prevailing winds, and, the brine being thus spread over a largo surface, there is much scope for evaporation, and it becomes rapidly concentrated. Several such sheds are often built in series, and the brine, being conveyed from one to the other as it becomes denser, attains at last a specific gravity of about 1.18, when it is stored in large cisterns till required for evaporation. This is done in large iron pans by the method to be hereafter described when speaking of rock-salt brine. The use, however, of the "graduation houses" is dying out, except in particular localities where competition from sea salt or purer rock-salt is difficult, as both their construction and their maintenance are expensive. The purer roek-salt is often simply ground for use, as we have seen to be the case at Wieliezka and elsewhere, but it is more frequently pumped as brine, produced either by artificial solution as at Middlesborough and other places, or by natural means as in Cheshire and Worcestershire. One great drawback to the use of even the _purest rock-salt simply ground is its tendency to revert to a hard unwieldy mass, when kept any length of time in sacks. This is partly but not wholly obviated by packing in casks, which, however, are dear and not always obtainable. As usually made, white salt from rock-salt may be classified into two groups : - (1) boiled: known as fine, table, lump, staved lump, superfine, basket, butter, and cheese salt (Fr. sel fn fin,sel a la minute, dr.c.); (2) unboiled : common, chemical, fishery, Scotch fishery, extra fishery, double extra fishery, and bay salt (Fr. sel de 12, 2.y, .4Y, 60, and 7.9.. heures). All these names are derived from the size and appearance of the crystals, their uses, and the modes of their production. The boiled salts, the crystals of which are small, are formed in a medium constantly agitated by boiling. The fine or stored table salts are those white masses with which we are all familiar. Basket salt takes its name from the conical baskets from which it is allowed to drain when first it is " drawn " from the pan. Butter and cheese salts are not stove-dried, but left in their more or less moist condition, as being thus more easily applied to their respective uses. Of the unboiled salts the first two, corresponding to the Fr. sel de 1 heures and sel de f4 heures, show by their English names the uses to which they are applied, and the others, the applications of which are equally shown by their names, merely depend for their quality on the length of time which elapses between successive "drawings," and the temperature of the evaporation. The time varies for the unboiled salts from twelve hours to three or four weeks, the larger crystals being allowed a longer time to form, and the smaller ones being formed more quickly. The temperature varies from 55° to 180° Fahr.

Oue striking difference between the manufacture of salt from rock-salt brine as carried on in Britain and on the Continent lies in the almost exclusive use in the latter case of closed or covered pans, except in the making of fine salt, whereas in Britain open ones are employed. With open pans the vapour is free to diffuse itself into the surrounding atmosphere, and the evaporation is perhaps more rapid. When covered pans are used, the loss of heat by radiation is less, and the salt made is also cleaner. In works published in France and Germany the statement is frequently made that it would be impossible to sell there a grain of salt manufactured by English methods, but one is fairly justified in doubting this assertion, seeing the •ease with which the public are induced to purchase the sel gris of the marais salants. In fact, it is customary in some places to make a special article, which is sold in competition with sea salt, by mixing with the purer one 10 or 12 per cent. of mud or earth. The most advantageous mode of evaporation would evidently be to cause the heated. gases from the furnace to pass over the surface of the liquid itself. No wearing-out of the pans need thus be feared, no lowering of the conductive power by incrustation, but the vapour as fast as formed would diffuse itself into heated air in rapid motion, this air being far from its point of saturation and greedy of moisture. The plan, however, which was tried in Britain by Otto Pohl and in Germany by Born has hitherto been a failure, the salt being for one thing very much soiled with the soot and other products of combustion. Again, this mode of evaporation hardly consorts with the slow progress and perfect stillness required for the production of the larger-grained salts, and gives only fine salt.

Figs. 4 and 5 represent a French pan, while fig. 6 is a British pan, only differing from the Continental ones in not being covered in, and in usually having three or four fires in place of two or three, and a separate chamber beyond the pan in which the salt is staved, heated by the flues conveying the furnace gases to the chimney after leaving the pan. The first two represent a pan of 64 feet long by 217i feet wide filled with brine, &c., and with circulating flues beneath for economy of heat. This pan, a, is supported all round its lower edges on a wall and on the pillars b, b, and heated by two fires c, e. The flame and the heated gases of each fire circulate in the flues p, p, p, in which are holes at various convenient points for cleaning ; thus then these gases are made to traverse the length of the pan three times before arriving at the chimneys n, n or the drying floors o, o. The channels e, e beneath the flues (fig. 5) serve to warm the air which feeds the fires, and, entering at the further end of the pan, traverses them and issues warm into the ash pit g, which is of course otherwise closed by the door h. The steam, collecting beneath the cover m, of which the upper portion i is attached to the timbers of the roof, issues by the chimney k, while below a series of shutters allow access for the various manipulations.

The two drying floors o, o are each heated by three flues q, q, q, 308 continuations of those below the pan, within which circulate the heated gases on their way to the main chimney, and on this floor is spread the salt to be dried. The floor of a pan is generally at first slightly arched towards the centre, so that when new a pan 308 308 is rather deeper at the sides than in the middle, but they soon flatten out and warp in all directions on being fired. This warping is a great inconvenience, opening communications between the 308 flues and interfering sadly with the arrangements of these latter just described, so much so that some makers prefer simple iron or brick supports placed here and there, without any definite arrangement. On the Continent the pan is often suspended by iron rods from the beams of the roof. The 308 warping or buckling, the scaling, and the formation of "cats," as the workmen call the sort of stalactites of salt which form in the flues, arising from leaks in the pan, are perhaps among the worst annoyances of the saltmakers. The pans are of ordinary boiler plates riveted together. The plates vary in size, but usually are 2 feet by 4 feet, and rather smaller over the fire. The grate, which should be such as to produce a moderate and diffused heat, is of the ordinary kind, and the firing is usually done from a pit below the end of the pan. In England they use "slack" sometimes called " burgey"; abroad they use all kinds of fuel - wood, coal, lignite, and turf ; and they also in many places are in the habit of protecting the pan from the more intense heat immediately over the fire c by a guard t at that particular part. As a means of producing a diffused and gentle heat without smoke, water gas will probably come to be used by and by. On the Continent the flues are often 2 or 2i feet high, and in Britain they are usually half that height. As, however, a slow and regular draught is to be aimed at, on the principle enunciated by Mr Fredk. Siemens, the Continental plan seems the more rational. Space does not here admit of a description of the so-called machine pans - the clay pans of the Cheshire Amalgamated Salt Company or Otto Pohl's system.' In Britain the brine is so pure that, keeping a small stream of it running into the pan to replace the losses by evaporation and the removal of the salt, it is only necessary occasionally (not often) to reject the mother-liquor when at last it becomes too impure with magnesium chloride ; but in some of the works on the Continent, especially those of North Germany, the mother-liquor not only contains more of this impurity but becomes quite brown from organic matter on concentration, and totally unfit for further service after yielding but two or three crops of salt crystals. Sometimes, to get rid of these impurities, the brine is treated in a large tub (bessoir) with lime ; on settling it becomes clear and colourless, but the dissolved lime forms a skin on its surface in the pan, retards the evaporation, and impedes the crystallization. At times sodium sulphate is added to the brine, producing sodium chloride and magnesium sulphate by double decomposition with the magnesium chloride. A slight degree of acidity seems more favourable to the crystallization of salt than alkalinity ; thus it is a practice to add a certain amount of alum, 2 to 12 lb. per pan of brine, especially when, as in fishery salt, fine crystals are required. The salt is " drawn " from the pan and placed (in the case of boiled salts) in small conical baskets hung round the pan to drain, and thence moulded in square boxes, and afterwards stove-dried, or (in case of unboiled salts) " drawn " in a heap on to the "hurdles," on which it drains, and thence is carried to the store.

In most Continental countries a heavy tax is laid on salt ; and the coarser as well as the finer crystals are therefore often dried so as not to pay duty on more water than can be helped.

The brine used iu the salt manufacture in England is very nearly saturated, containing 25 or 26 per cent. of sodium chloride, the utmost water can take up being 27 per cent.; and it ranges from 38 to 42 ounces of salt per gallon. Iii seine other countries, as has been explained, the brine has to be concentrated before use, and every ounce per gallon by which the brine is below saturation indicates a difference of cost in the production from it of salt of about 4d. to 40. per ton. Subjoined are four analyses of brine taken from Messrs Richardson and Watts's Chemistry applied to the Arts and Manufactures: - The price of salt at the works may be said to range /rem 49. oa. to 6s. per ton, the former being less than the cost price as given before the British parliamentary commission in 1881. It is there stated to be - brine, 6d. ; labour, 10d. ; fuel 3s. ; rent, interest, &c., ls. ; total, 5s. 4d. Thus the margin for profit is but small, almost the only gain being said to accrue from the lightering, most of the salt manufacturers doing the carriage in their own " flats."

Saltmaldng is by no means an unhealthy trade, some slight soreness of the eyes being the only affection sometimes complained of ; indeed, the atmosphere of steam saturated with salt in which the workmen live seems specially preservative against colds, rheumatism, neuralgia, &c. It is said that wages are rather better and employment more regular in Worcestershire than in Cheshire.

The parliamentary commission above referred to was appointed with a view to the investigation of the causes of the disastrous subsidences which are constantly taking place in all the salt districts, and the provision of a remedy. It led to no legislative action ; but the evil is recognized as a grave one. At Northwich and Winsford scarcely a house or a chimney stack remains straight. Houses are keyed up with "chaps," " face plates," and "bolts," and only kept from falling by leaning on one another. The doors and windows have become lozenge-shaped, the walls bulged, and the floors crooked. Buildings have sunk, - some of them disappearing altogether. Lakes have been formed where there was solid ground before, and incalculable damage done to property in all quarters. At the same time it is difficult to see how this grievance can be remedied without inflicting serious injury, almost ruin, upon the salt trade. The workings in Great Britain represent the abstraction of rather more than a cubic mile of rock every five years, and of this by far the larger part is in Cheshire.

Manley gives the following statistics of the production of salt in England for 1881 :- Northwich 500,000 tons.

Winsford 1,000,000 „ Cheshire........ Middlewieh 30,000 „ Wheelock and Lawton 100,000 „ Staffordshire... Shirleywick and Weston -on-Tren t 4,000 „ Droitwich 115,000 „ 1Vorcesterthiro Stoke Prior 105,000 „ Total 1,854,000 „ He also gives the following details of the salt exported for years ending Dec. 31, 1881 to 1883 inclusive, quoted from the archives of the Salt Chamber of Commerce, whence the importance of the salt trade in England may be judged : - 308 (F. M. L.) Ancient History and Religious Symbolism. - Indispensable as tho use of salt appears to us, it must have been quite unattainable to primitive man in many parts of the world. Thus the Odyssey (xi. 122 sq.) speaks of inlanders (in Epirus ?) who do not know the sea and use no salt with their food. In some parts of America, and even of India (among the Todas), salt was first introduced by Europeans ; and there are still parts of central Africa where the use of it is a luxury confined to the rich. Indeed, where men live mainly on milk and flesh, consuming tho latter raw or roasted, so that its salts are not lost, it is not necessary to add sodium chloride, and thus we understand how the Numidian nomads in the time of Sallust and the Bedouins of Hadramaut at the present day never eat salt with their food. On the other hand, cereal or vegetable diet calls for a supplement of salt, and so does boiled meat. The important part played by the mineral in the history of commerce and religion depends on this fact ; at a very early stage of progress salt became a necessary of life to most nations, and in many cases they could procure it only from abroad, from the sea-coast, or from districts like that of Palmyra where salty incrustations are found on the surface of the soil. Sometimes indeed a kind of salt was got from the ashes of saline plants (e.g., by the Umbrians, Aristotle, Het., ii. p. 459), or by pouring the water of a brackish stream over a fire of (saline) wood and collecting the ashes, as was done in ancient Germany (Tae., Ann., xiii. 57), in Gaul, and in Spain (Plin., H. N., xxxi. 7, 82 sq.); but these were imperfect surrogates. Among inland peoples a salt spring was regarded as a special gift of the gods. The Cliaonians in Epirus had one which flowed into a stream where there were no fish ; and the legend was that Heraeles had allowed their forefathers to have salt instead of fish (Arist., set supra). The Germans waged war for saline streams, and believed that the presence of salt in the soil invested a district with peculiar sanctity and made it a place where prayers were most readily heard (Tac., ut sup.). That a religious significance was attached to a substance so highly prized and which was often obtained with difficulty is no more than natural. And it must also be remembered that the habitual use of salt is intimately connected with the advance from nomadic to agricultural life, i.e., with precisely that step in civilization which had most influence on the cults of almost all ancient nations. The gods were worshipped as the givers of the kindly fruits of the earth, and, as all over the world "bread and salt" go together in common use and common phrase, salt was habitually associated with offerings, at least with all offerings which consisted in whole or in part of cereal elements. This practice is found alike among the Greeks and Romans and among the Semitic peoples (Lev. ii. 13); Homer calls salt "divine," and Plato names it "a substance dear to the gods" (Timmus, p. 60 ; comp. Plutarch, Sympos., v. 10). As covenants were ordinarily made over a sacrificial meal, in which salt was a necessary element, the expression "a covenant of salt" (Numb. xviii. 19) is easily understood ; it is probable, however, that the preservative qualities of salt were held to make it a peculiarly fitting symbol of an enduring compact, and influenced the choice. of this particular clement of the covenant meal as that which was regarded as scaling an obligation to fidelity. Among the ancients, as among Orientals down to the present day, every meal that included salt had a certain sacred character and created a bond of piety and guest friendship between the participants. Hence the Greek phrase Baas Kai Tpd7rEcav wapPaively, the Arab phrase "there is salt between us," the expression " to eat the salt of the palace" (Ezra iv. 14, Rev. Ver.), the modern Persian phrase namak hardm, "untrue to salt," i.e., disloyal or ungrateful, and many others.

It has been plausibly conjectured that the oldest trade routes were created for traffic in salt ; at any rate salt and incense, the chief economic and religious necessaries of the ancient world, play a great part in all that wo know of the ancient highways of commerce. Thus one of the oldest roads in Italy is the Via Salaria, by which the produce of the salt pans of Oaths was carried up into the Sabine country. Herodotns's account of the caravan route uniting the salt-oases of the Libyan desert (iv. 181 sq.) makes it plain that this was mainly a salt-road, and to the present day the caravan trade of the Sahara is largely a trade in salt. The salt of Palmyra was an important element in the vast trade between the Syrian ports and the Persian Gulf (see PALMYRA, vol. xviii. p. 200), and long after the glory of the great merchant city was past "the salt of Tadmor" retained its reputation (Mas.6.di, viii. 398). In like manner the ancient trade between the iEgean and the coasts of southern Russia was largely dependent on the salt pans at the mouth of the Dnieper and on the salt fish brought from this district (Herod., iv. 53 ; Dio Chrys., p. 437). In Phoenician commerce salt and salt fish - the latter a valued delicacy in the ancient world - always formed an important item. The vast salt mines of northern India were worked before the time of Alexander (Strabo, v. 2, 6, xv. 1, 30) and must have been the centre of a widespread trade. The economic importance of salt is further indicated by the almost universal prevalence in ancient and mediaeval times, and indeed in most countries down to the present day, of salt taxes or of Government monopolies, which have not often been directed, as they were in ancient Rome, to enable every one to procure so necessary a condiment at a moderate price. In Oriental systems of taxation high imposts on salt are never lacking and aro often carried out in a very oppressive way, one result of this being that the article is apt to reach the consumer in a very impure state largely mixed with earth. "The salt which has lost its savour" (Mat. v. 13) is simply the earthy residuum of such an impure salt after the sodium chloride has been washed out.

Cakes of salt have been used as money in more than one part of the world, - for example, in Abyssinia and elsewhere in Africa, and in Tibet and adjoining parts. See the testimony of Marco Polo (bk. ii. ch. 48) and Col. Yule's note upon analogous customs elsewhere and on the use of salt as a medium of exchange in the Shan markets down to our own time, in his translation of Polo, ii. 48 sq. In the same work interesting details are given as to the importance of salt in the financial system of the Mongol emperors (ii. 200 sq.). (W. R, S.)

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