Page:The American Cyclopædia (1879) Volume XIV.djvu/595

Rh SALT, sodic chloride, sea salt, or common salt, the substance which is always denoted when the word “salt” is used in ordinary language. The word is derived from the Greek, in which in the feminine is used for the sea, and in the masculine for the solid product left when sea water evaporates. It is sometimes also called muriate of soda. It may be formed by burning sodium in chlorine gas, or by neutralizing hydrochloric acid with sodium carbonate, and evaporating. It occurs very abundantly in nature, both in the solid state, as rock salt, and in solution in sea water, salt lakes, and salt springs; also in smaller quantity in river water. Mines of rock salt have been recently explored in the Caucasus, in which the stone implements were found as they were left at a date so remote that no tradition exists of the time when they were worked. The mines of Wieliczka in Austrian Galicia have been worked for at least six, but probably for upward of eight centuries. The springs of Droitwich in England were worked by the Romans, and in Cheshire “the Wiches” were very productive in the reign of Edward the Confessor. Little is understood of the origin of rock salt. Some beds, as those of Cheshire, appear to have been produced by the drying up of bodies of sea water cut off from the ocean, while in other cases, as at Bex, where the salt forms a perpendicular vein or dike, its origin is altogether obscure. Salt lakes are derived either from the partial drying up of isolated bodies of sea water, as the Dead sea, or by the evaporation of lakes without outlets, and fed by streams which have passed over beds of salt, or plains impregnated with it, as Great Salt lake, Lake Urumiah in Persia, and many of the lakes of South America. Saline incrustations often overspread the surface of plains in Russia, India, the South American pampas, and the regions E. and W. of the Rocky mountains. The salt of the ocean has doubtless resulted from the chemical changes which have taken place between the elements that constitute the earth's crust during former geological epochs. The waters of the open ocean contain on an average 33.8 parts of salt in 1,000, of which 26.8 in 1,000 are common salt, equal to about 4 oz. in a gallon, or a bushel from 300 or 350 gallons. The entire quantity of salt in the ocean is estimated by Schafhäutl at 3,000,000 cubic miles. The water of landlocked seas like the gulf of Mexico or the Mediterranean sea contains more salt than that of the open ocean, and it is also found that the water of the bottom of such seas is salter than that upon the surface.—Salt crystallizes in colorless, transparent, anhydrous crystals, belonging to the isometric system, and has a very perfect cubic cleavage, which generally displays itself even in the great masses of rock salt, parts of which however are frequently massive and granular, and rarely fibrous or columnar. But the most characteristic peculiarity of the crystallization of salt is the formation of the hopper-shaped crystals on the surface of a saline solution during evaporation. A single cube appears at first, which partially sinks in the liquid, and new cubes then form and attach themselves to its upper edge, till by a repetition of this process a hollow rectangular pyramid, sometimes of considerable dimensions, and with the apex downward, is finally produced. Crystals of this form occur in some salt mines, and casts of them in clay are found in the New York salt region and some other places. Salt has a specific gravity of 2.1 to 2.257, and a hardness between gypsum and calc spar. It is transparent to translucent, and its color varies from white to yellowish, reddish, bluish, and purplish. It is of all substances the most perfectly diathermanous or transparent to heat of every degree of refrangibility. (See .) At 32° F. 100 parts of water dissolve 36.52 parts of pure salt; and at 229.5°, the boiling point of a saturated solution, only 40.35 parts are dissolved. This almost uniform solubility at all temperatures furnishes the means of separating it from many of the foreign salts with which it is associated in sea water and brine springs. Rock salt dissolves much more slowly, even in fine powder, than sea salt and that from springs, and the coarsely crystallized salt than the finer varieties. These differences are of economical importance, especially in curing provisions. For the principle of its curative properties see. The freezing and boiling points of solutions rise with the degree of concentration. Salt is fusible at a red heat, and volatile at a still higher temperature. Its volatility is made use of in the process of “salt-glazing” common earthenware. (See, vol. xiii., p. 788.) Artificial crystals generally decrepitate when heated, from the presence of water mechanically enclosed between their layers. Some specimens of rock salt from Wieliczka decrepitate when dissolved in water, and disengage a gas, which is sometimes pure carburetted hydrogen, and sometimes a mixture of this with hydrogen and oxide of carbon.—Salt is a compound of one atom of chlorine combined with one atom of sodium; chemical symbol, NaCl; molecular weight, 58.5. When it is heated to redness with silica, silicate of sodium and hydrochloric acid are formed. A process for manufacturing soluble glass is based on this reaction. With oil of vitriol it gives sulphate of sodium and hydrochloric acid. This is the first step in Le Blanc's process for soda ash. Salt is rarely if ever ob- tained pure. The chief impurities in rock salt are sulphate of lime, oxide of iron, and clay; but besides these the chlorides of potassium, calcium, and magnesium, the sulphates of soda and magnesia, and bituminous matters are occasionally met with, and some varieties are even colored by the presence of infusoria. In salt made from sea water, the salts of magnesia with a little sulphate of lime are the principal