CARBONATE OF SODA. 465 



heat, and condensing upon the surface of the pottery causes its 

 vitrification, which is attended with the formation of hydro- 

 chloric acid, and perchloride of iron. 



The hromide and iodide of sodium crystallize in cubes, and 

 resemble in properties the corresponding compounds of potas- 

 sium. The other compounds of sodium are not of particular 

 interest. 







SALTS OF SODA. 



Carbonate of soda; NaO, CO 2 -f 10HO ; 667.3 + 1125, or 

 53.47 + 90. This useful salt is found nearly pure in commerce, 

 in large crystals, which effloresce when exposed to air. These 

 crystals contain 1 equivalents of water, and consist in 100 parts, 

 of 21.81 soda, 15.43 carbonic acid, and 62.76' water. According 

 to Dr. Thomson, they generally contain about \ per cent of 

 sulphate of soda, as an accidental impurity. Their form ap- 

 pears to belong to the oblique prismatic system. Their density 

 is 1.623 ; 100 parts of water dissolve 20.64 of the crystals at 

 58.25, and more than an equal weight at the boiling tempera- 

 ture (Dr. Thomson). In warm w r eather the carbonate of soda 

 sometimes crystallizes in another form of crystal, which is not 

 efflorescent, and of which the proportion of water is variously 

 stated by Mitscherlich and Thomson at 7 and 8 equivalents. 

 A third hydrate was obtained by Mohs, on allowing a solution 

 of carbonate of soda, saturated between 68 and 86, to cool; 

 which was found to contain 17-74 percent, of water, a result 

 somewhat exceeding 1 equivalent. On evaporating a solution 

 of carbonate of soda at the boiling point, the salt precipitates in 

 a powder, which contains nearly the same proportion of water. 



This salt has a disagreeable alkaline taste. When heated it 

 undergoes the watery fusion ; its water is soon dissipated, and 

 a white anhydrous salt remains, which again becomes liquid at 

 a red heat, undergoing then the igneous fusion. A mixture of 

 carbonates of potash and soda is more fusible than either salt 

 separately. 



Carbonate of soda is prepared by a process which will be 

 described immediately, under the head of sulphate of soda. 

 Much of the carbonate of commerce is not crystallized, but 

 simply evaporated to dryness, and is then known as salts of 

 soda. In this form it always contains chloride of sodium, sul- 



H H 2 



