48

COMMON SALT.

when it is called rock-salt, to distinguish it from thesalt obtained hy the evaporation of sea-water, called seaor bay salt.

141. Common salt has already been stated to be achloride of sodium (122), a compound of chlorine witha white brilliant metal called sodium. It used formerlyto be called a muriate of soda, because it was knownthat sulphuric acid poured on salt expels muriatic acid,whilst on the other hand there are substances whichcan take away the acid and leave caustic soda. It isnow known that dry salt contains nothing but chlorineand sodium ; and when it is decomposed by sulphuricacid or in any other way, the muriatic acid and sodaevolved were not contained in the salt, but are formedby the decomposition of a portion of water, the oxygenof which unites with the sodium to form soda, whilstthe hydrogen combines with the chlorine to formmuriatic acid. Hence, though strictly speaking com-mon salt is a chloride of sodium, yet for practicalpurposes it may be considered as a muriate of soda ;because whatever change it undergoes, or in whatevermanner it is decomposed, soda and muriatic acid aregiven out just as if they were really the components ofsalt.

142. The only other compound of soda, which occursnative in any quantity, is the nitrate or cubic nitre, asit is commonly called. It is found native in largequantities in certain parts of South America , and isused to a considerable extent as a manure. When it