SALTS. 73 



comprise the chlorides, iodides, bromides, &c., and are named haloid 

 salts, from a Greek word signifying sea salt. 



The notion of a salt has been still farther extended. It is known 

 that two metallic sulphurets occasionally unite together to forma 

 compound called a double sulphuret. To such compounds Berzelius 

 gave the name of sulphur -salts. They are precisely analogous in 

 their composition to a common oxy-salt, as may be seen by simply 

 substituting for the sulphur an equivalent quantity of oxygen. In 

 these salts, the sulphur -acid is a sulphuret of some one of the 

 electro-negative metals, as arsenic, antimony, tungsten, &c. ; the 

 sulphur-base is a sulphuret of an electro-positive metal, as potassium, 

 sodium, mercury, &c. 



It has also been found that the haloid salts will unite together, to 

 form what is termed a double haloid salt ; which also may be con- 

 sidered precisely analogous to an oxy-salt, the halogen element 

 (chlorine, iodine, &c.), simply taking the place of oxygen. 



By a double salt is meant one in which the same acid is united 

 with two separate bases, as tartar emetic, (tartrate of oxide of anti- 

 mony and potassa.) 



Nearly all salts are solid at common temperatures ; most are 

 capable of crystallizing ; their colour is variable ; the soluble ones 

 are more or less sapid ; very few are odorous ; they differ much in 

 their affinity for water : some attract moisture from the air and be- 

 come liquid ; such salts are called deliquescent ; they differ very 

 much in their solubility in water, which is in the direct ratio of their 

 affinity for water, and in the inverse ratio of their cohesion. In 

 many salts water acts as a base, and is hence termed basic water. 



As a general rule, every salt has its own distinct crystalline form, 

 by which it may be recognised. Crystallization may be effected in 

 various ways, as by solution and evaporation : the slower the evapo- 

 ration, the larger and more regular the crystals ; if the evaporation 

 be made very rapid by heat, a confused crystalline mass is obtained. 

 Fusion and slow cooling may sometimes be employed ; thus crystals 

 of sulphur and bismuth may be procured. A third condition under 

 which crystals form, is in passing from the gaseous to the solid state, 

 as in the case of iodine. 



Many salts, in crystallizing, unite chemically with a definite por- 

 tion of water, which belongs to the crystal, but not to the salt ; this 

 is termed ivater of crystallization. By a strong heat, all this water 

 is expelled, and the salt is said to undergo the watery fusion. Such 

 salts, when exposed to the air, are liable to part with a portion of 

 this water, and crumble down into a powder ; they are said then to 

 be effiorescent. 



Some salts, again, in crystallizing, enclose a portion of water 

 mechanically within their texture ; which, by its expansion, when 



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