# 1911 Encyclopædia Britannica/Salt

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​SALT (a common Teutonic Word, cf. Dutch zout, Ger. Salz, Scand. salt; cognate with Gr. ἅλς, Lat. sal). In chemistry the term salt is given to a compound formed by substituting the hydrogen of an acid by a metal or a radical acting as a metal, or, what comes to the same thing, by eliminating the elements of water between an acid and a base (see Acid; Chemistry).
Common Salt.
Common salt, or simply salt, is the name given to the native and industrial forms of sodium chloride, NaCl. Pure sodium chloride, which may be obtained by passing hydrochloric acid gas into a saturated solution of the commercial salt, whereupon it is precipitated, forms colourless, crystalline cubes (see also below under Rock salt) which melt at 815·4°, and begins to volatilize at slightly higher temperatures. It is readily soluble in water, 100 parts of which dissolve 35·52 parts at 0° and 39·16 parts at 100°. The saturated solution at 109·7° contains 40·35 parts of salt to 100 of water. On cooling a saturated solution to −10°, or by cooling a solution in hot hydrochloric acid, the hydrate NaCl.2H$_2$O separates; on further cooling an aqueous solution to −20° cryohydrate containing 23·7% of the salt is deposited. The consideration of this important substance falls under two heads, relating respectively to sea salt or “bay” salt and “rock” salt or mineral salt. 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.
Sea Salt.—Assuming that each gallon of sea water contains 0·2547 ℔ of salt, and allowing an average density 2·24 for rock-salt, it has been computed that the entire ocean if dried up would yield no less than four and a half million cubic miles of rock-salt, or about fourteen and a half times the bulk of the entire continent of Europe above high-water mark. 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 3·33%, ranging from 2·9% for the polar seas to 3·55% 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 (see Ocean).
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 there are salt works, at Setubal, Alcacer do Sal, Figueira and Aveiro. Spain has salt works at the Bay of Cadiz, the Balearic Islands, &c.; Italy at Sicily, Naples, Tuscany and Sardinia. France has its “marais salants du midi” and also works on the Atlantic seaboard; whilst Austria has “Salzgärten” at various places on the Adriatic (Sabbioncello, Trieste, Pirano, Capo d'Istria, &c.). In England and Scotland the industry has greatly fallen off under the competition of the rock-salt works of Cheshire.
The process of the spontaneous evaporation of sea water was 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 calcium carbonate and ferric oxide. This goes on till a density of 1·1315 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 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 contained—
| Magnesium sulphate | 11·45% |
| 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 % |
| Magnesium sulphate | 0·0624 % |
| Magnesium chloride | 0·0153 % |
| Sodium chloride | 2·7107 % |
| Sodium bromide | 0·0222 % |
| | 2·8389 % |
Of this about 95% is sodium chloride. Up to this point the separation of the salts has taken place in a fairly regular manner, but now the temperature be ins 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 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 carnal lite 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, barely above high-water mark, is levelled, and if necessary puddled with clay. In tidal seas a “jas” (or storage reservoir) is constructed alongside, similarly rendered impervious, in which the water is 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 aires, œuillets 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 urilication, 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. The salt is collected from the surface by means of a sort of wooden scoop or scraper, 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.
Generally speaking this salt, which may contain up to 15% of impurities, 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, containing about 5% of impurities. In northern Russia and in Siberia sea water is concentrated by freezing, the ice which separates containing little salt; the brine is then boiled down when an impure sea salt is deposited.
Rock-salt.—To mineralogists rock-salt is often known as halite-a name suggested in 1847 by E. F. Glocker from the Greek ἅλς (salt). The word halite, however, is sometimes used not only for the species rock-salt but as a group-name to include a series of haloid minerals, of which that species is the type. Halite or rock-salt crystallizes in the cubic system, usually in cubes, rarely in octahedral; the cubes being solid, unlike the skeleton-cubes obtained by rapid evaporation of brine. The mineral has perfect cubic cleavage. Percussionfigures, readily made on the cleavage-faces, have rays parallel to faces of the rhombic dodecahedron; whilst figures etched with water represent the four-faced cube. Rock-salt commonly occurs in cleanable masses, or sometimes in laminar, granular or fibrous forms, the finely fibrous variety being known as “hair-salt.” The hardness is 2 to 2·5 and the spec. grav. 2·1 to 2·6. Rock-salt when pure is colourless and transparent, but is usually red or brown by mechanical admixture with ferric oxide or hydroxide. The salt is often grey, through bituminous matter or other impurity, and rarely green, blue or violet. The blue colour, which disappears on heating or dissolving the salt, has been variously ascribed to the presence of sodium sub chloride, sodium, sulphur or of a certain compound of iron, or again to the existence of minute cavities with parallel walls. Halite occasionally exhibits double refraction, perhaps due to natural pressure. It is remarkably diathermanous, or capable of transmitting heat-rays, and has therefore been used in certain physical investigations. Pure halite consists only of sodium chloride, but salt usually contains certain magnesium compounds rendering it deliquescent. Minute vesicular cavities are not infrequently present, sometimes as negative cubes, and these may contain saline solutions or carbon dioxide or gaseous hydrocarbons. Some salt decrepitates on solution (Knistersalz), the phenomenon being due to the escape of condensed gases.
Halite may occur as a sublimate on lava, as at Vesuvius and some other volcanoes, Where it is generally associated with potassium chloride; but its usual mode of occurrence is in bedded deposits, often lenticular, and sometimes of great thickness. The salt is commonly associated with gypsum, often also with anhydrite, and occasionally with sylvite, carnal lite and other minerals containing potassium and magnesium. Deposits of rock-salt have evidently been formed by the evaporation of salt water, probably in areas of inland drainage or enclosed basins, like the Dead Sea and the Great Salt Lake of Utah, or perhaps in some cases in an arm of the sea partially cut off, like the Kara Bughaz, which forms a natural salt-pan on the east side of the Caspian. Such beds of salt are found in strata of very varied geological age; the Salt Range of the Punjab, for instance, is probably of Cambrian age, while, the famous salt deposits of Wieliczka, near Cracow, have been referred to the Pliocene period. In many parts of the world, including the British area, the Triassic age offered conditions especially favourable for the formation of large salt-deposits.
In England extensive deposits of rock-salt are found near the base of the Keuper marl, especially in Cheshire. The mineral occurs generally in lenticular deposits, which may reach a thickness of more than 100 ft.; but it is mined only to a limited extent, most of the salt being obtained from brine springs and wells which derive their saline character from deposits of salts. Much salt is obtained from north Lancashire, as also from the brine pits of Staffordshire, Worcestershire, Yorkshire, Durham and the Isle of Man (Point of Ayre). The salt of N.E. Yorkshire and S. Durham is regarded by some authorities as Permian, but that near Carrickfergus in Co. Antrim, Ireland, is undoubtedly of Triassic age. The Antrim salt was discovered in 1850 during a search for coal: one of the beds at Duncrue mine has a thickness of 80 ft. Important deposits of rocksalt occur in the Keuper at Berchtesgaden, in the Bavarian Alps; at Hall in Tirol and at Hallein, Hallstatt, Ischl and Aussee in the Salzkammergut in Austria. Salt occurs in the Muschelkalk at F riedrichshall and some other localities in Württemberg and Thuringia; and in the Bunter at Schöningen near Brunswick. The Permian system (Zechstein) yields the great salt-deposits worked at Stassfurt and at Halle in Prussian Saxony. The Stassfurt deposits are of special importance for the sake of the associated salts of potassium and magnesium, such as carnal lite and kainite. These deposits, in addition to having a high commercial importance, present certain problems which have received much attention, more particularly at the hands of van't Hoff and his collaborators, whose results are embodied in his Zur Bildung der ozeanischen Salzablagerungen, vol. i. (1905), vol. ii. (1209). (A summary is given in A. W. Stewart, Recent Advances in Physical and Inorganic Chemistry, 1909; see also van’t Hoff, Lectures on Theoretical and Physical Chemistry, vol. i.) A typical section is as follows: Beneath the surface soil of sandstone there is a layer up to 100 ft. in thickness of carnallite, MgCl;·KCl·6H$_2$O, mixed with a little salt; this is followed by a thicker deposit of kieserite, MgSO$_4$·H$_2$O, containing rather more salt than the upper bed. Dee er down there are successively strata of polyhalite, MgSO$_4$·K$_2$SO$_4$·2CaSO$_4$·2H$_2$O, and anhydrite, CaSO$_4$, interspersed with regular layers of rock-salt; whilst below the anhydrite we have the main rock-salt deposits. A bed of rock-salt in the Zechstein at Sperenberg near Berlin has been proved by boring to have a thickness of upwards of 4000 ft. The salt of Bex in Switzerland is jurassic, whilst Cretaceous salt occurs in Westphalia and Algiers. Important deposits of salt are developed in many parts of the Tertiary strata. At Cardona, near Barcelona, Tertiary salt forms hill-masses, while the Carpathian ​ sandstone in Galicia and Transylvania is rich in salt. The extensive mines at Wieliczka are in this rock-salt, as also is the salt of Kalusz in Galicia, which is associated with sylvite, KCl.
In North America salt is widely distributed at various geological horizons. In New York it occurs in the Salina beds of the Onondaga series, of Silurian age; and Silurian salt is found also in parts of Michigan and in Ontario, Canada. Some of the salt of Michigan is regarded as Carboniferous. Rock-salt is mined in several states, as New York, Kansas and Louisiana; but American salt is mostly obtained from brine. Deposits of salt, regarded as either Cretaceous or Tertiary, occur in the island of Petite Anse, west of Vermilion Bay, in Louisiana. Salt often occurs in association with petroleum and natural gas, and extensive beds were discovered in the Wyoming valley in boring for petroleum. In the dry regions of the West salt occurs as an incrustation on the surface of the soil—a mode of occurrence found in desert areas in various parts of the world.
Cubic pseudomorphs representing rock-salt are sometimes seen in strata which have been deposited in shallow water, especially on the margin of a salt-lake. The salt has been dissolved out of its original matrix, and the cavity so formed has then been filled with fine clayey or other mineral matter, forming a cubic cast. Such casts are not infrequent in the Keuper marls and sandstones, and in the Purbeck beds of England.
Manufacture.—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. The Cheshire and Worcestershire salt deposits are by far the most important. 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 yds. thick was discovered at Marbur near Northwich by some men exploring for coal, at a depth of, 34 yds. In 1779 three beds of rock-salt were discovered at Lawton, separated from one another by layers of indurated clay. The old Marston or Marston Rock mine is the largest and perhaps the oldest in England. It was worked for about a hundred years in only its upper bed, but in 1781, after traversing a layer of indurated clay intersected with small veins of salt 10½ yds. thick, a layer of rock-salt 33 to 37 yds. thick was found. 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 yds. At Winsford, where the same formation seems to recur, it is 159 yds. from the surface. The Marston mine covers an area of about 40 acres. The salt is first reached at 35–40 yds. in the Northwich district, and the upper layer is 25–50 yds. in thickness (Marston 23–26 yds.); 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 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 was discovered by Bolckow and Vaughan in boring for water in 1862 at a depth of 400 yds., but was not utilized, and was again found by Messrs Bell Brothers at Port Clarence at a depth of 376 yds. In Cheshire the surface-water trickling through the overlying strata dissolves the salt, which is subsequently pumped as brine, but at Middlesbrough 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, 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. 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. The same inconvenience is felt in the environs of Nancy, and a similar one produces on a larger scale the sinking and subsidences at Winsford and Northwich.
In the United States extensive deposits and brine springs are worked, and also incrustations (see above). Canada also is a producer. South America possesses several salt deposits and brine springs. 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. In India there 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.
Rock-salt is the origin of the greater part of 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 chambers and galleries are sometimes excavated within the saliferous beds 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 cases where the atmospheric conditions are suitable the brine is run into large tanks and. concentrated merely by solar heat, or it may be caused to trickle over faggots arranged under large open sheds called “ graduation houses " (Gradirhäuser), whereby a more extensive surface of evaporation is obtained and the brine becomes rapidly concentrated. After settling it is evaporated in iron pans. The use, however, of the “ graduation houses ” is dying out, as both their construction and their maintenance are expensive. The purer rock-salt is often simply ground for use, as at Wieliczka and elsewhere, but it is more frequently pumped as brine, produced either by artificial solution as at Middlesbrough 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. As usually made, white salt from rock-salt may be classified into two groups: (1) boiled: known as fine, table, lump, stoved lump, superfine, basket, butter and cheese salt (Fr. sel fin-fin, sel à la minute, &c.); (2) unboiled: common, chemical, fishery, Scotch fishery, extra fishery, double extra fishery and bay salt (Fr. sel de 12, 24, 48, 60 and 72 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 stoved 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 12 heures and sel de 24 heures, show by their English names the use to which they are applied, and the others 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° F.
One difference between the manufacture of salt from rock-salt brine as carried on in Britain and on the Continent lies in the 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 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. It has also been proposed to concentrate the brines under diminished pressure. In S. Pick's system a triple effect is obtained by evaporating in these connected vessels, so that the steam from one heats the second into which it is led (see Soc. of Eng., 1891, p. 115).
In Britain the brine is so pure that, keeping a small stream of it running into the an to replace the losses by evaporation and the removal of the sali, 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 works 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 ℔ 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 European countries a 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 in the salt manufacture in England is very nearly saturated, containing 25 or 26% of sodium chloride, the utmost water can take up being 27%; and it ranges from 38 to 42 oz. of salt per gallon. In some other countries the brine has to be concentrated before use.
Saltmaking 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.
A parliamentary commission was appointed in 1881 to investigate the causes of the disastrous subsidence's 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 “shaps,” “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 annual abstraction of rather more than a mass of rock equal to a foot in thickness spread over a square mile. The table gives the outputs in metric tons of the most important producers in 1900 and 1905 (from Rothwell, Mineral Industry, 1908).
See F. A. Fürer, Salzbergbau- und Salinenkunde (Braunschweig, 1900); J. O. Freiherr von Buschmann, Das Salz: dessen Vorkommen und Verwertung (Leipzig, vol. 1, 1909, vol. 2, 1906).(X.)
Ancient History and Religious Symbolism.—Salt must have been quite unattainable to primitive man in many parts of the world. Thus the Odyssey (xi. 122 seq.) speaks of islanders (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 the 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 Hadramut 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, Met. ii. 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 (Tac. Ann. xiii. 57), in Gaul and in Spain (Plin. H.N. xxxi. 7. 82 seq.); but these were imperfect surrogates. Among inland peoples a salt spring was regarded as a special gift of the gods. The Chaonians in Epirus had one which flowed into a stream where there were no fish; and the legend was that Heracles had allowed their forefathers to have salt instead of fish (Arist. ut 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” (Timaeus, p. 60; cf. 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 element of the covenant meal as that which was regarded as sealing 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 ἅλας καὶ πράπεζαν παραβαίνεν, the Arab phrase “there is salt between us,” the expression “to eat the salt of the palace” (Ezra iv. 14, R.V), the modern Persian phrase namak harām, “untrue to salt,” i.e. disloyal or ungrateful, and many others. Both early in the history of the Roman army and in later times an allowance of salt was made to officers and men. In imperial times, however, this salarium was an allowance of money for salt (see Salary).
It has been conjectured that some of 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 we 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 Ostia was carried up into the Sabine country. Herodotus's account of the caravan route uniting the salt oases of the Libyan desert (iv. 181 seq.) 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), and long after the glory of the great merchant city was past “the salt of Tadmor” retained its reputation (Mas'ūdi viii. 398). In like manner the ancient trade between the Aegean 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 medieval 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 seldom lacking and are 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” (Matt. 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 Colonel 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 seq. In the same work interesting details are given as to the importance of salt in the financial system of the Mongol emperors (ii. 200 seq.). (W. R. S.)