108 CHEMISTRY OF NATURAL WATERS. [IX. 



By varying somewhat the conditions of temperature, the sul- 

 phate of magnesia and the chloride of sodium of the mother- 

 liquor undergo mutual decomposition, with the production of 

 sulphate of soda and chloride of magnesium. Hydrated sul- 

 phate of soda crystallizes out from such a mixed solution at 

 C., and by reducing the temperature to 18 C. the greater 

 part of the sulphates may be separated in this form from the 

 mother-liquor of 1.24, previously diluted with one tenth of 

 water ; without which addition a mixture of hydrated chloride 

 of sodium would separate at the same time. If, on the other 

 hand, the temperature of the mixed solution be raised above 

 50 C., the sulphate of soda crystallizes out in the anhydrous 

 form, as thenardite. By the spontaneous evaporation during 

 the heats of summer of the mother-liquors of density 1.35, a 

 double sulphate of potassium and magnesium separates. These 

 reactions are taken advantage of on a great scale in Balard's 

 process, as modified by Merle,* for extracting salts from sea- 

 water. 



23. The results of the evaporation of sea-water would 

 however be widely different if an excess of lime-salt were 

 present. In this case the whole of the sulphates present would 

 be deposited in the form of gypsum at an early stage of the 

 evaporation, and the mother-liquor, after the separation of the 

 greater part of the common salt, would contain little else than 

 the chlorides of sodium, potassium, calcium, and magnesium. 



24. A consideration of the conditions of the ocean in 

 earlier geological periods will show that it must have con- 

 tained a much larger quantity of lime-salts than at present. 

 The alkaline carbonates, whose origin has been described in 

 13, and which from the earliest times have been flowing 



strata of the saliferous formation of Stassfurth in Germany ; where it is as- 

 sociated with a hydrous double chloride of calcium and inacrncsiuni, tachydrite, 

 and also with a sparingly soluble sulphate of magnesia, kioscritc. which con- 

 tains a small and variable amount of water, and is supposed to be, in its nor- 

 mal condition, an anhydrous salt. When heated to irdm-^ in a current of 

 steam this sulphate loses all its acid, which passes off umlerom] 



* See my paper in Amer. Jour. Science (2), XXV. 361 ; also Report of the 

 Juries of the Exhibition of 1862, Class II. page 48. 



