These results are most instructive, and their application in the art of salt production from sea water will presently be seen. Usiglio further describea the reactions which follow on coptiauing the evaporation of the mother-liquor ; how they become more complicated, and the composition of the material which salts out commences to vary with alternations of temperature ; how, if the temperature of this rnother-liquor of sp. gr. 1.3 be lowered, as by exposure during the night, magnesium sulphate alone will crystallize, whereas if the liquor be concentrated by further evaporation during the day, a mixture of sodium and potassium chlorides with magnesium sulphate goes down. By this deposition, the solution slightly loses in density, and its sp. gr. may possibly fall to about 1-28. Magnesium bromide also separates with the potiussium and magnesium chlorides, and a double potassium and magnesium sulphate forms, corresponding with the kainite of Stassfurt (KaSO,MgSO460112). There likewise separates another double salt, corresponding to the Stassfurt carnallite (potassium and mag,nesium chloride, KC1MgC1,601-10. Filially, the mothers, whieli now have attained a sp. gr. of 1.333, retain scarcely any sodium chloride or magnesium sulphate, vory little potassium chloride, and are in point of fact a saturated solution of nearly pure magnesium chloride. This last salt crystallizes if the temperature be lowered to about 4r (40° F.).
The works in which the sea-salt industry is carried on consist of several series of basins communicating with one another, and possessing extensive evaporating surfaces. Through these, the sca-water is led until arriving in the last, which are very shallow; the already concentrated salt water is allowed to stand till most of the salt has crystallized out.
The mother-liquor or "bittern " is drawn off, and the salt is collected and drained to dryness. The first of the series of basin.s is usually a large shallow pond, into which the sea-water is admitted, and where it is allowed to settle, and is stored for use. Sometimes two sad large basins are employed, one for settling, the other for storage. Hence the water is carried through a series of other basins, each set in its turn being smaller and shallower. In the last, the salt principally deposits ; it is then collected, drained, and stacked for sale.
These works are called by various names, according to the countries in which they are situate. In England, they were known as " salt-marshes," " salterns," " salt-gardens," and by other local names. In France, they are called marais salants or satins ; in Portugal, marinhas ; in Germany, Meersalinen or Salzgarten.
Fig. 1203 shows a marais salant as now in use on the Atlantic coast of France. The spot chosen is generally some little bay or creek protected from the direct action of the waves ; from this, is led a small canal, through which at spring-tides the sea water can be conducted into the large reservoir A, the jas or vasiere ("settler "), where the water is allowed to clarify. This reservoir is usually placed higher than the rest of the marais salant, so that the water can be run off at pleasure into the first set of basins or couches c, without pumping. The jas may be of any moderate dimen sions, and often covers 2i acres, the depth varying from a yard to a fathom. The water, having Ill1/1 U11611 the jas, is allowed to run by the underground channel B, fitted with a suitable sluice, to the couches, which are frequently about 23-24 ft. long, 12 ft. wide, and 1—li ft. deep, ar ranged in sets of 8 or 10 in a double row, as shown, separated by low walls or dams, but com municating with each other in such a manner that the water entering from A by the sluice B can circulate slowly through them, as shown by the lines aud arrows, and be drawn oft' by the sluice G. In fine weather, the water has already undergone some degree of concentration by the time it has settled in the jas A, and as it passes in an almost insensible current through the couches, it continues to evaporate. It is led by the sluice G into a, canal D, which nearly encircles the marais salant, and serves to conduct the water on to tho t«bles E, arranged similarly to the couches ; over these, it flows as before in an almost insensible current into other basins R, called adernes or 2nuants, whence it is fed as required by small channels cut in the soil into the ceuillets f g, small basins -svhere the salt crystallizes, or, as the French peasants say,, l'eau commence saliner."
On the shores of the Mediterranean, about Cette, Marseilles, and the Etang de Berre, immense quantities of salt are produced by a somewhat similar arrangement. As, however, there are no tides iu that sca, the arrangement with the separate reservoir A is not essential. A series of basins, whose bottoms are levelled and pugged with clay, are made by sets in gradients (usually 3) so arranged with channels and sluices that the water can flow from basin to basin and from one set to another. The general principles involved aro much the same as on the Atlantic coast. They differ, however, in the degree of circulation of the water. In the western works, the water is allowed to almost stagnate, as it were, no differences of level being maintained so as to promote its flow, except in respect of the jas, which is usually placed on a rather higher level. In the salins du midi, on the contrary, when the flowing water has reached its lowest gradient, it is collected in large wells, whence it is drawn up and thrown back by a pump or water-wheel to its former level, and again traverses a like set of gradients, to return once more to another set of wells. The first set are called " wells of green water," the second are called " salt water " wells. On arriving at these latter, the water should have attained a sp. gr. of 1.18-1.20, and be nearly at crystallizing-point. By this means, a greater eiroulath al is maintained, and the evaporation is more rapid. Each set of basins in each griulieut is made a little smaller than the previous set, to correspond with the diminishing bulk of the water as it undergoes evaporation. Finally, in a set of basins placed at the lowest level of all, the salt crystallizes ; these are called tables salantes or crystallisoirs, and receive the water so soon as it has attained a sp. gr. of 1.95-2.00. lu the salt marshes of the west, the mother-liquor (" bittern ") is always left in tbe =Wets, and has a tendency to render the salt rather bitter and deliquescent, by reason of contamination with magnesia') salts. In the salins du midi, as the French Mediterranean works are called, the bittern is drawn off, and stored in special reservoirs, with a view to its subsequent further evaporation, alternated with refrigera tion, for the extraction of the potassium and magnesiuin salts. When the salt has formed in tho ceuillets or on the tables salantes, as the case may be, more ready-coneentrated water is rue on, the bittern being drawn off in tho ease of the sulins du midi, or left in the marws salants of the west. Here the water is main tained at a depth of 3-4 in., and a fresh supply is run in every 2 days or so during fine weather, and when enough salt (say 3-1 in.) has accumulated, it is collected by means of a sort of scoop or hand plough B, Fig. 1201, which the sautnicr, as these workmen aro culled, pushes along before him. Notwith standing that he does this work with surprising dexterity, ho never fails to piek up a con siderable portion of clay with the salt, render ing the latter impure. Of late years, species of moss has been introduced from the Portu guese marshes into the satins du midi. This is grown under fresh water, with which the basins aro flooded ler the purpose, and fia.ms a clean bed, whereon the salt crystallizes, and is thus obtained far whiter and purer. The salt of the French Atlantic coast often contains not over 88 per cont. of sodium chloride ; the Marseilles and Cate salt, over 95 par cent. The salt, after collection, is stacked in heaps II around the mantis salant, these often being thatched over for protection from rain, and it there drains and loses much of its deliquescence and bitterness by long exposure to the atmosphere. In France, this work lasts in lino summers from April till September.