314 



SALTS. 



What is said 

 of carbonate 

 of potassa ? 



Describe car- 

 bonate of 

 soda. 



793. CARBONATE OF POTASSA. POTASH. 

 The method of preparing potash and 

 pearlash, from wood ashes, has already 

 been considered in the paragraph on Potassa. Saleratus 

 is a carbonate containing a large proportion of carbonic 

 acid. Its use, for u raising" bread and cake, is familiar. 

 The acid employed with it, sets the carbonic acid gas 

 at liberty and thus puffs up the " sponge." 



794. CARBONATE or SODA. SODA. 

 Carbonate of soda is commonly known 

 under the name of soda. It is a white 

 soluble salt, familiar from its use in Seidlitz and soda 

 powders. Its carbonic acid is the source of the effer- 

 vescence in these preparations. 



795. Carbonate of soda is prepared from 



flow i.s carbo- . . 



note of soda the sulphate of soda. 1 his salt being 

 prepared? heated with charcoal is converted into 

 sulphide of sodium. On heating the latter with car- 

 bonate of lime, a double de- 

 composition occurs, and car- 

 bonate of soda is produced, 

 with sulphide of calcium as 

 an incidental product. Both 

 parts of the process are com- 

 bined in practice. Sulphate 

 of soda, chalk, and coal, are heated together in a rever- 

 beratory furnace, the carbonate of soda is then dissolved 

 out from the fused mass, dried, purified, and subse- 

 quently crystallized. The sulphide of calcium would 

 dissolve at the same time, and thus defeat the process, 

 were it not rendered insoluble by combination with a 

 certain quantity of lime. 



