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history of salt
Salt, NaCl, is an ionic compound made of sodium and chloride ions which has been exceptionally important to humans for thousands of years, because it is one of the substances upon which all of life evolved to depend. Humans, like all life, need a supply of salt in order to simply survive. Salt's ability to preserve food was a foundation of civilization. It helped to eliminate the dependence on the seasonal availability of food and it allowed travel over long distances. However, salt was difficult to obtain, and so it was a highly valued trade item to the point of being considered a form of currency by certain peoples. Many salt roads, such as the via Salaria in Italy, had been established by the Bronze age.
Today, salt is almost universally accessible, relatively cheap and often iodized.
== Sources ==
There have been two main sources for salt: sea water and the sodium chloride mineral halite (also known as rock salt). Rock salt occurs in vast beds of sedimentary evaporite minerals that result from the drying up of enclosed lakes, playas, and seas. Salt beds may be up to 350 m thick and underlie broad areas. In the United States and Canada extensive underground beds extend from the Appalachian basin of western New York through parts of Ontario and under much of the Michigan basin. Other deposits are in Texas, Ohio, Kansas, New Mexico, Nova Scotia, and Saskatchewan. In the United Kingdom underground beds are found in Cheshire and around Droitwich. Salzburg, Austria, was named "the city of salt" for its mines.〔Kurlansky, Mark. ''Salt: A World History''. Penguin Books, 2003 ISBN 0-14-200161-9〕 High-quality rock salt was cut in medieval Transylvania, Maramureş and Southern Poland (Wieliczka). Tuzla in Bosnia and Herzegovina was named in Hungarian Só (salt) from the twelfth century on and later "place of salt" by Turks.
Salt is extracted from underground beds either by mining or by solution mining using water to dissolve the salt. In solution mining the salt reaches the surface as brine, from which the water is evaporated leaving salt crystals.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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