Salt, or CHLORIDE OF SODIUM (sym. ; sp. gr. 2.15). This substance is called by chemists common salt to distinguish it from a great number of other bodies termed Salts (q.v.) in scientific language. Rock-salt or halite crystallises in cubes, colourless and usually transparent when pure, and sometimes measuring an inch on the side. It is, however, generally coloured by the presence of some foreign body, and occurs of yellow, red, blue, and purple tints. When decidedly impure it is often of a dull-gray colour. Salt is one of the few substances which are nearly as soluble in cold as in hot water. A saturated solution has a specific gravity of 1.205. Sea-water, which contains salt to the extent of from 2.5 to 2.7 per cent., has a mean specific gravity of 1.027. Salt has a saline but not a bitter taste, and is inodorous. Some of the physical properties of rock-salt are remarkable (see HEAT, Vol. V. p. 610). Salt is of great importance as a condiment and antiseptic, and equally so in some chemical manufactures. As rock-salt it is found in most countries, and in some extensively; but salt from any source is scarce and costly in some places—certain parts of the interior of Africa, for example. It can, of course, be obtained by evaporating sea-water. Some beds or deposits of rock-salt have no doubt been formed by the slow evaporation of large bodies of salt water, which by one means or another—by a sandbank, for example—have been cut off from communication with the sea. This process would sometimes be repeated by sea-water again getting in through a breach in the bank, and this again being filled up. There are instances where it has certainly been formed in this way on a limited scale. But geologists seem to be of opinion that beds of rock-salt have more generally arisen from the long-continued evaporation of large inland lakes without outlets; these had long been fed by rivers or streams which dissolved salt out of the soil or strata over which they flowed. The Dead Sea and the Great Salt Lake of Utah are examples of lakes without outlets at the bottoms of which rock-salt is forming. All salt, wherever found, has probably come originally in some way or other from the sea. Rock-salt beds are usually associated with deposits of sulphate of lime (calcium sulphate), and are more common in the Trias than in other geological formations.
Valuable beds of rock-salt, usually accompanied with brine, occur in England. The Cheshire deposits, which are in the basin of the river Weaver, have been long worked and are still productive. At Northwich in that county rock-salt is found at from 200 to 250 feet below the surface. There are four beds of an aggregate thickness of 240 feet, but the two lower ones are mixed with marls. The two upper beds each average about 90 feet in thickness, with some brine just above the top bed. At Winsford and Wheelock in the same county thick beds of rock-salt also occur at moderate depths. In Worcestershire, at Stoke Prior and at Droitwich, rock-salt occurs, 154 feet thick including marls in the former locality, and brine is likewise found at both places. At Stafford Common in Staffordshire the salt is 78 feet thick; and in these two counties the deposits are not far below the surface. In Cheshire some rock-salt is still mined at Northwich, but here, as in other places in the west of England, most of the salt is obtained from brine naturally overlying the rock-salt or from inundated salt-mines. In 1889 an important discovery of rock-salt was made at Walney Island and at Fleetwood (Preesall), in Lancashire, where the deposits are roundly from 100 to 500 feet in thickness, and at the former place from 270 to 360 feet below the surface. This Lancashire salt was first worked (as Preesall brine) in 1890. On the east side of England, between Middlesborough and Hartlepool, there is an extensive deposit of rock-salt, which was accidentally discovered in boring for water in 1862. Its proved area is between 30 and 40 sq. m., the salt is from 50 to 115 feet thick, and its depth below the surface varies from 800 to 1600 feet. This is greatly deeper than the beds in the west of England. Hitherto in this district the rock-salt has not been mined. A borehole is made, into which a double iron pipe—one within the other—is inserted, and in the jacket or annulus thus formed fresh water is let down to form brine, which is pumped up through the inner tube by a steam-engine. In 1890 the rock-salt mined in England (Cheshire) amounted to 159,000 tons, while the salt obtained from brine was as much as 1,958,000 tons, Cheshire producing more than two-thirds of the latter. Some rock-salt is mined at Carrickfergus in Ireland.
Rock-salt is found in Galicia, the mines at Wieliczka, which have been worked since the 13th century, being the most famous in the world. The system of mines extends over an area of about 6 miles from east to west, and 2 miles from north to south, with underground streets, squares, &c., and over 30 miles of tramway; the greatest depth reached being about 12,000 feet. In Wallachia, Transylvania, Hungary, Upper Austria, Styria, Salzburg, and the Tyrol important salt-mines are worked. Rock-salt is also found in Russia, Italy, and largely at Stassfurth in Germany and Cordova in Spain. Of eastern countries Persia is perhaps the best supplied with salt; India has productive mines in the Salt Range (q.v.), but salt is also largely imported; and both rock-salt and salt lakes occur in Asiatic Russia. The Wieliczka rock-salt is of Tertiary age; that of Cheshire, Bavaria, and the Austrian Alps Triassic: in Permian deposits near Berlin borings have gone down in rock-salt for nearly 4000 feet. Deposits of the same age occur also in Holstein, and in eastern Russia. Brine-springs rise from many other geological systems: thus, in Northumberland and Leicestershire they issue from Carboniferous strata, while in some parts of the Alps they come from rocks of Jurassic age. Similarly in North America saliferous formations occur on several geological horizons. Thus, the salt-works at Syracuse, New York, and in the neighbourhood of Goderich on the Canadian side of Lake Huron, are in the well-named 'Salina group' of the Upper Silurian. Brine-springs also rise from the Carboniferous strata of Michigan, Ohio, and Virginia, while a thick bed of rock-salt of apparently Cretaceous age occurs in southern Louisiana.
Not infrequently rock-salt occurs in considerable abundance at the surface; some of these accumulations are of quite recent formation, while others are of great geological age. Thus, at Cardona (Montserrat) in Spain rock-salt forms hills some 400 and 500 feet in height—the deposits being probably of Cretaceous age; and similar masses occur near Orenburg in Russia, and in the Punjab. Superficial saline deposits are met with covering extensive areas in many more or less desert regions, where they evidently indicate the sites of dried-up lakes and inland seas of comparatively recent age. As examples may be cited the rock-salt deposits of the Kirghiz Steppes, those near Lake Urumiah, and in various parts of South America. Salt lakes from the bottoms and shores of which rock-salt is obtained are met with in many regions, both in the Old and New Worlds. Thus, it is collected in Cyprus, Melos, the Crimea, and the Aral-Caspian area; and many saline lakes, pools, and marshes occur in the great western region of North America. Even in dry warm regions where no salinas may happen to exist, an efflorescence of salt not infrequently covers the ground after rains—the salt having doubtless been derived by capillary attraction from the underlying subsoils and rocks. Rock-salt also occurs as a product of sublimation in volcanic regions, from which it has been inferred that much of the steam emitted by volcanoes has come from water introduced by underground fissures from the sea.
When brine is pumped up to the surface it is run into cisterns or reservoirs situated at a higher level than the evaporating pans, into which it descends through pipes. These pans are large shallow iron vessels, heated either by coal-fires placed beneath them or by other means, such as waste steam. With a high heat and a short time in the pan a fine salt is produced; with a comparatively low heat and a longer time in the pan a coarse salt forms. Agitation of the brine tends also to produce a fine salt. Brine boils at 226° F., and it is at this temperature that ordinary table-salt, called also 'lump' or 'lumped' salt, is produced. At about 165° F. what is called 'common' salt is obtained; large-grained salt forms at between 130° and 140° F.; and fishery-salt, also large grained, at from 100° to 110° F. The salt crystallises on the surface of the brine in the pan, floats about a little, and then falls to the bottom, leaving the surface free for a fresh crop of crystals. Twice or thrice in twenty-four hours fine salt is drawn from the pans, which are kept nearly full of brine, by raking it to the side and lifting it out with perforated scoops. It is then put into moulds called 'tubs,' and left for about half an hour to let the water drain off, after which the shaped lumps are put into a stove, where they remain till they are quite dry. Sometimes even fine salt is not stoved, and it is then called butter salt or cheese salt. To make salt of the largest grain the brine is left for nearly a fortnight before removing the crystals. In this process the 'bittern' or magnesium chloride remains in solution after the common salt separates.
At different places around the British coasts salt was formerly obtained by evaporating sea-water, and to such salt the name bay-salt (often used for coarse salt) properly belongs. In Spain, Italy, and southern France the manufacture of salt in this way is practised on a large scale. Besides common salt sea-water contains much smaller quantities of potassium chloride, magnesium chloride and sulphate, calcium sulphate, and more minute quantities of other bodies. The water of the Mediterranean, which is slightly more salt than the open ocean, contains 2.72 per cent. of sodium chloride. As the evaporation goes on calcium carbonate separates first and then calcium sulphate. When the water reaches the density of between 1.22 and 1.31 sodium chloride separates along with small quantities of other salts; and when the specific gravity becomes 1.33 very little of anything but magnesium chloride remains. A series of shallow rectangular basins are usually so arranged that the water can flow from basin to basin, and when it has reached the lowest the partially concentrated sea-water is collected in a well. From this it is raised to the first of another series of basins, and finally into the series where the salt crystallises. The salt is then stacked in heaps, which are protected from rain and left for months to drain. This to a large extent gets rid of deliquescent and bitter impurities. It then contains about 95 per cent. of chloride of sodium, in which state much of it is sent into the market.
In some northern countries sea-water, suitably placed for the purpose, is frozen over in winter in order to increase the proportion of salt in what remains liquid, since the ice so formed is fresh. The water is then evaporated by artificial heat till the salt is obtained. The chief markets abroad for British salt are India and the United States. To the former country about 300,000 tons are now annually exported, and to the latter 120,000 tons were sent in 1889, or only about one-half the average quantity exported to that country a few years before. It is estimated that London consumes daily 11 tons of salt.
Besides its universal use as a condiment, salt is an all-important substance in the manufacture of soda, hydrochloric acid being obtained from it in large quantities in the process as a by-product. It is also employed in the manufacture of soap and for producing what is called a salt-glaze on stone-ware. Agriculturists destroy slugs and grubs with salt, and it appears to improve the fertility of soils when mixed with certain manures. Brine is used in the pipes of freezing-machines. Rock-salt is carved into cups and vases in the Punjab, and it is said that houses are built of it in certain very dry regions, as in the Desert of Caramania, where this material is abundant.
The salt-tax was or is in some countries an important source of revenue, though it is recognised by economists as one that presses unfairly on the poor. The salt-tax of the ancient regime in France is referred to at GABELLE. In modern Italy tobacco and salt are government monopolies. In British India the revenue from the salt duty comes next in value to that from land and opium. In Burma it is now 1 rupee per maund of 82½ lb.; elsewhere (since 1888) it is 2½ rupees. In Britain salt duties were first exacted in 1702; the rate was 5s. per bushel in 1798, during the great French war, but was ultimately increased to 15s. per bushel, thirty or forty times the cost of the article! The duty was finally remitted in 1825. Salt makers or salters used in some parts of Britain to be, like colliers, serfs (see SLAVERY). For the value of salt in preserving food, see ANTISEPTICS, PRESERVED PROVISIONS. For the varying saltness of the sea at various depths, see ATLANTIC, PACIFIC, SEA, &c.
See J. J. L. Raton's Common Salt (1879); E. M. Boddy's History of Salt (1881); C. G. Gümpel, Common Salt (1898).