Salt

SALT. In chemistry the term salt is applied generically to any compound formed by substituting the hydrogen of an acid by a metal or a group of elements acting as a metal. (See ACID.) Com mon salt, or, simply, salt, is the name given to the varied natural and industrial forms of sodium chloride, NaCl. Pure sodium chlo ride is made by passing dry hydrogen chloride gas into a saturated solution of commercial salt, when the purified substance is de posited in the form of a colourless crystalline powder. It crystal lizes in cubes (see below, under Rock Salt) which melt at about 800° C, that is, at bright red heat, and begins to volatilize at a slightly higher temperature. It dissolves readily in cold water and a little more readily in hot water; ioo parts of water dissolve parts of salt at o° C, 39.16 parts at oo° C, and 40•35 parts at 109.7° C, the boiling point of the saturated solution. If a saturated solution in water be cooled to — io° C, a crystalline hy drate, separates. (See HYDRATE.) Solution of salt in water is accompanied by reduction of temperature: 36 parts of salt in dissolving in ioo parts of water at 12.6° C lower the tem perature to me C. If the same proportion of salt and snow be intimately mixed the temperature falls to —21.3° C.

Salt occurs in the sea, in natural brines and in the crystalline form, as rock salt. Its most abundant source is the ocean. Assum ing that each gallon of sea water contains 0.2547 lb. of salt, and allowing an average density of 2.24 for rock salt, it has been corn puted that if dried up the entire ocean would yield no less than 41 million cubic miles of rock salt or about 141- times the bulk of the entire continent of Europe above high-water mark. Natural brines having commercial importance are those of Austria, France, Germany, of Kharaghoda and Kuda in India, of Michigan, New York State, Ohio, Pennsylvania, West Virginia, and the salt lake of Utah in the United States, and the Dead sea. In Great Britain salt brines are met with in Cheshire, Worcestershire, Lancashire and Yorkshire, and have been found by deep boring in Derby shire, Staffordshire and Midlothian. Salt in solution is accom

panied by the chlorides and sulphates of potassium, calcium and magnesium ; in many cases the valuable element bromine occurs, probably in the form of magnesium bromide.

Sea water contains on the average about 3.33% of solids, but the concentration of salts varies from about 2.9% in the polar seas to 3.55% and upwards at the equator. Enclosed seas such as the Mediterranean and Red seas contain a higher proportion of salt than the open ocean at the same latitude. (See OCEAN.) From Dittmar's analyses of sea water taken during the "Challenger" ex pedition, the average composition of the solids in sea water may be considered to be: sodium chloride 2.6o%, magnesium chloride 0.31%, magnesium sulphate 0.22%, calcium sulphate 0.12%, po tassium chloride 0.07% and magnesium bromide 0.007%. The mixed salt obtained by evaporation of sea water has, however, the following composition irrespective of the source of the sea water : sodium chloride 77.82%, magnesium chloride 9.44%, magnesium sulphate 6.57%, calcium sulphate 3.44%, potassium chloride 2.11%, magnesium bromide 0.22%, and some calcium carbonate.

Natural Brines.

The Dead sea, which covers an area of about square miles, contains approximately 11,600 million tons of salt, and the River Jordan, which contains only 35 parts of salt per ioo,000 of water, adds each year 850,000 tons of salt to this total. The composition of Dead sea water is given in the following table: Certain natural brines occurring in England and the United States are of interest, not only from the economic point of view, but also because they contain salts not usually found in brines, such as the chlorides of barium and strontium; when salt is pro duced from such brines special methods of manufacture are adopted. In Great Britain these brines were found at great depth in boreholes in Derbyshire, Staffordshire and Midlothian during the search for petroleum wells. In America they occur in the Ohio valley district of West Virginia and in Ohio, at depths ranging from ijoo to 1,600 feet.