SALT, culinary, or MIIRIATE of SODA. This salt is one of the most abundant pro ductions of nature, and exists native in much greater quantity than any other neutral salt. The waters of the ocean owe their saltness to it : it is found in a number of mineral springs, and it forms immense strata in the bowels of the earth, or rising on the surface, even to the height of mountains. According as it is produced from these sources, it is named sea-salt, or rock-salt. Rock-salt is solid, hard, and more or less transparent, of a white, grey, or reddish colour, sometimes of a bright or deep red, or yellow, and more rarely with spots of blue. Its frac ture is foliated or fibrous ; generally it is massive, but sometimes crystallized in cubes, and its fragments are always of a cubical form. The colours have been supposed to depend on the oxide or mu riate of iron. In general it is pure, and hence its taste is purely saline ; but some-' times it is bitter from the presence of fo reign salts. There are immense mines of it in different countries. Those of Cracow, in Gallicia, have been long cele brated. It abounds in the east and south of Germany, is found in large quantities in rin, and likewise in Cheshire, in En and. In Africa, Asia, and America, it is not less extensively distributed, forming hills above the surface, or very extensive beds. It is always connected with rocks of secondary formation, and generally with gypsum or sulphate of lime.
Dr. Watson, in the second volume of his " Essays," speaking of the salt mines, says, There are several mines of rock salt near Northwich, in Cheshire, the first of which was discovered as they were boring for coal in the year 1670. The springs which are met with, both above and below the level of the Northwich bed of rock-salt, are strongly impregnat ed with salt. This is easily accounted for: the rain-water, in sinking through the ground which lies over the rock-salt, at last arrives at the salt ; its further de scent is in a great measure obstructed by the solid body of salt ; it rests upon it, and in resting upon it dissolves it, and thus constitutes a brine spring above the level of the bed of rock-salt. The brine springs, which are found below that level, probably arise from the water which has dissolved a portion of rock-salt, in sinking to that depth in the earth. I have," continues the Doctor, " had the curiosity to go to the bottom of some of the most famous mines in England, but I never thought my labour in these sub. terraneous expeditions so well rewarded, as in the sight of the rock-salt mines at Northwich. These are superior to the mines at Cracow, in Poland, which have, for many centuries, been the subject of general admiration." A single pit, at Northwich, yields, at a medium, 4,000 tons of salt in a year.
In different countries, the process of obtaining salt is different. In very cold climates, the water, being received into shallow ditches during the winter, is fro zen, by which a great part of the super fluous water is removed, and the remain ing liquor affords salt, by artificial evapo ration. In warm climates it is obtained by spontaneous evaporation. The water is received into broad, shallow trenches at the sea side, without the reach of the tide. The bottom of these is made of clay, well beaten, and they are divided into several departments. The-fluid be ing thus spread out on an extensive sur face, quickly evaporates, and by sluices it is removed from one department to another, so that when it arrives at the last, it is a strong brine, and the salt is soon deposited. It is necessarily mixed with the clay of the ground, and with se veral of the neutral salts, and other im purities, which sea-water contains. Salt,
prepared in this manner, is known by the name of bay-salt. In colder climates, re course must be had to artificial evapora tion. The water is heated in shallow iron pans. Muriate of soda possesses the sin gular property, that it is as soluble in cold as in hot water; after due evaporation, therefore, it begins to crystallize on the surface of the hot liquor; the crystals, as they increase, fall to the bottom of the vessel, are raked out, and set to drain. This is the process by which it is obtain ed in this country. Sometimes this me thod is conjoined with natural evapora tion. The sea-water, before it is receiv ed into the boiler, is pumped into a large reservoir, under which faggots of thorns &c. are suspended It is allowed to drop over these, and a large surface being thus presented to the atmosphere, while the air is also rapidly renewed, a considerable part of the water is evaporated. It is then conveyed to the boiler, and evaporated in the usual manner. Or. in some of the northern departments of France, the sea water is made to flow over a bottom of clay covered with sand, which favours both the evaporation of the water and the concretion of the salt ; the saline deposit, which is at length formed, is lixiviated with sea-water, which, becoming thus more impregnated with salt, is "concen trated by boiling, so as to afford it by has ty crystallizationi Sea.salt, obtained by any of these processes, is never perfect ly pure. Sea-water, by its analysis, is found to contain, besides muriate of soda, several other neutral salts, particularly muriate of magnesia, muriate of lime, and sulphate of soda. These, being much more soluble in hot than in cold water, remain dissolved in the hot liquor from which the salt crystallizes. A small quantity of them, however, still adheres to the muriate of soda; they render it de liquescent, give it a bitter taste, and con siderably impair its antiseptic power. Dif ferent processes have therefore been con trived to obtain the salt free from these mixtures. The most simple is, merely to procure the salt by a slow artificial eva poration. It then crystallizes with scarce ly any mixture of the others. This is the cause of the superior purity of the bay salt. Honce, also, the larger the crystals sf sea-salt are, they may be justly suppos ed to be the purer, as the largeness of the crystals is owing to the slowness of the evaporation by which they are formed. For chemical purposes, miiriate of so da is most easily purified by dissolving it in water, and adding to its solution a so lution of carbonate of soda, drop by drop, till no cloudiness is produced by the ad dition. Every foreign salt is thus decom posed and precipitated, and the strained solution will contain the pure muriate of soda, which may be crystallized. Mori ate of soda has a salt, rather agreeable, taste, being, when pure, free from all bit terness; it is soluble in rather less than three parts of water, at the temperature of 60°. The crystals neither deliquesce nor effloresce, on exposure to the air ; the common sea-salt, indeed, is deliques cent ; but this is owing to the muriates of magnesia and lime which adhere to it. Exposed to heat, the crystals of muriate of soda decrepitate, from the sudden con version of their water of crystallization into vapour. If the temperature is rais ed to a red heat, the salt melts ; in an in tense heat, it is volatilized in white va pours, without having undergone any de composition.