464 SODIUM. 



times sufficiently pure for its ordinary uses, as it is taken from 

 these deposits, but more generally it is coloured brown from an 

 admixture of clay, and requires to be purified by solution and 

 filtration. Instead of sinking a shaft to the bed of rock salt, 

 and mining it, the superior strata are often pierced by a bore 

 of merely a few inches in diameter, by which water is admitted 

 to the bed, and the brine formed drawn off by a pump and pipe 

 of copper suspended in the same tubular opening. 



Chloride of sodium crystallizes from solution in water in 

 cubes, and sometimes from urine and liquids containing phos- 

 phates in the allied form of the regular octohedron. Its crys- 

 tals are anhydrous, but decrepitate when heated, from the ex- 

 pansion of water confined between their plates. According to 

 the experiments of Fuchs, pure chloride of sodium has exactly 

 the same degree of solubility in hot and cold water, requiring 

 2.7 parts of water to dissolve it ; or 100 parts of water dissolve 37 

 of salt at all temperatures. The composition of such a solution 

 corresponds exactly with 1 eq. of salt to 1 8 eq. of water. Gay- 

 Lussac makes the boiling point of a saturated solution 229.5, 

 but that temperature is too high (I believe,) for a solution of 

 pure chloride of sodium. When a saturated solution is exposed 

 to a low temperature, between 14 and 5, the salt crystallizes in 

 hexagonal tables, which have two sides larger than the others. 

 Fuchs found these crystals to contain 6, and Mitscherlich 4 

 equivalents of water. If their temperature is allowed to rise 

 above 14, they undergo decomposition, and are converted into 

 a congeries of minute cubes, from which water separates. 



Pure chloride of sodium has an agreeable saline taste, deli- 

 quesces slightly in damp weather, and dissolves largely in recti- 

 fied spirits, but is very slightly soluble in absolute alcohol. Its 

 density is 2.557 (Mobs). It fuses at a bright red heat, and at 

 a higher temperature rises in vapour. It is immediately decom- 

 posed by oil of vitriol, with the evolution of hydrochloric acid. 

 Besides being used as a seasoning for food, chloride of sodium 

 is employed in the preparation of the sulphate and carbonate of 

 soda. When ignited in contact with clay containing oxide of 

 iron, the sodium of this salt becomes soda, and unites with the 

 silica of the clay, while the chlorine combines with iron, and is 

 volatilised. On this decomposition is founded the mode of 

 communicating the salt-glaze to pottery : a quantity of salt is 

 thrown into the kiln, where it is converted into vapour by the 



