THE ASH OF PLANTS. 145 



from common salt by somewhat complicated processes. 

 It occurs in commerce in an impure state under the name 

 of Soda-ash. United to water, it forms sal-soda, which 

 usually exists in transparent crystals or crystallized 

 masses. These contain 63 per cent of water, which 

 partly escapes when the salt is exposed to the air, leav- 

 ing a white, opaque powder. 



Sodium carbonate has a nauseous alkaline taste, not 

 nearly so decided, however, as that of the carbonate of 

 potassium. It is often present in the ashes of plants. 



Sodium Bicarbonate, NaHC0 3 . The supercarbon- 

 ate of soda of the apothecary is this salt in a nearly pure 

 state. The cooking-soda of commerce is a mixture of 

 this with some simple carbonate. It is prepared in the 

 same way as potassium bicarbonate. The bicarbonates, 

 both of potassium and sodium, give off half their carbonic 

 acid at a moderate heat, and lose all of this ingredient 

 by contact with excess of any acid. Their use in baking 

 depends upon these facts. They neutralize any acid 

 (lactic or acetic) that is formed during the " rising " of 

 the dough, and assist to make the bread " light " by in- 

 flating it with carbon dioxide. 



Calcium Carbonate, or Carbonate of Lime, 

 CaC0 3 , 112. This compound is the white powder formed 

 by the contact of carbon dioxide with lime-water. When 

 slacked lime is exposed to the air, the water it contains 

 is gradually displaced by carbon dioxide, and carbonate of 

 lime is the result. Air-slacked lime always contains 

 much carbonate. This salt is distinguished from lime 

 by its being destitute of any alkaline taste. 



In nature carbonate of lime exists to an immense ex- 

 tent as coral, chalk, marble, and limestone. These 

 rocks, when strongly heated, especially in a current of 

 air, part with carbon dioxide, and quick-lime remains 

 behind. 



Calcium carbonate occurs largely in the ashes of most 

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