CHLORIDE OF SODIUM.
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the old red sandstone group {Trans. Geol. Society, vol. i. p. 38, and vol. ii.p. 94). The salt of Ischel, in the Austrian Alps , belongs to the ooliticgroup (Sedgwick and Murchison, Phil. Mag. N. S. vol. viii. p. 102), asdoes also that found in the lias in Switzerland (Bakewell, op. cit. p. 253).The immense mass or bed of salt near Cardona in Spain , and which hasbeen described by Dr. Traill {Trans. Geolog . Society, vol. iii. p. 404)occurs in the cretaceous group (De la Beche, op . cit. p. 293). The saltdeposit of Wieliczka, near Cracow, belongs to the supracetaceous group (De la Beche, p. 270). Lastly, in the Crimea , salt is said to be daily accu-mulating in the inland lakes.
{b.) In the organized kingdom. —It is found in plants which grow bythe sea side, in the blood and urine of man, &c.
Preparation. —The salt consumed in this country is procured by theevaporation of the water of brine springs. The salt districts are North-wich, Middlewich, and Nantwicli,in Cheshire; Shirleywich, in Stafford-shire ; and Droitwich, in Worcestershire. In Cheshire the rock salt (calledalso fossil salt,sal fossilis or sal gemnue) constitutes two beds which vary inthickness from! to 130 feet, and are separated by a bed of clay, 10 or 12feet thick ; the uppermost bed of salt being 30 or more feet from the sur-face of the earth. It is for the most part of a reddish colour, but is alsomet with in transparent colourless masses. It is called in commercePrussia rock, and is largely exported for purification. Brine springs aremet with both above and below the level of the beds of rock salt.
The brine is pumped up into cisterns or reservoirs, from which it isdrawn when wanted into large, oblong, wrouglit-iron evaporating pans,which are usually worked with four or more fires. If the brine be notcompletely saturated with chloride of sodium, a little rock salt is addedto it. By the evaporation of the water the salt deposits in crystals. Theimpurities separate in the form of a scum (which is removed by a skim-ming dish), and of a sediment called pan-scale, or pan-scratch. As all partsof the pan are not equally heated, the crystallization of the salt takesplace in different parts at different times, and consequently in the samepan the grains or crystals vary considerably in size. The small-grainedsalt is formed by the strongest heat, and constitutes the butter, stored,lump, or basket salt of commerce: while the larger crystals, forming thebay wad fishery salts of commerce, are deposited in the coolest part of thepan. (For further information on the manufacture of common salt, consultAikin’s Dictionary of Chemistry, vol. ii. p. 118; Holland’s AgriculturalSurvey of Cheshire ; Mr. Furnival’s Wharton and Marston Patent SaltRefineries, 1836 ; Dr. Brownrigg’s Art of making Common Salt, 1748.)
Tn some parts of the world chloride of sodium is obtained from sea-water : but the mode of extracting it varies according to circumstances.In warm countries it is procured by solar evaporation, and is then calledbay salt. In cold countries congelation is resorted to as a means ofconcentrating sea water; for when a weak saline solution is exposed togreat cold, it separates into two parts ; one almost pure water, whichfreezes, and the other which remains liquid, and contains the larger pro-portion of salt. Another method of concentration is by graduatinghouses: these are skeletons of houses, in which the water is pumped up,and allowed to fall on heaps of brushwood, thorns, &c., by which it isdivided and agitated with the air, and evaporation promoted. The fur-ther concentration is effected by heat.