THE ASH OF PLANTS. 131 



carbonate of potash when exposed to carbonic acid gas 

 absorbs the latter, and the bicarbonate of potash is pro- 

 duced, so called because to a given amount of potassium 

 it contains twice as much carbonic acid as the carbonate. 

 P otash-salceratus consists essentially of this salt. It 

 probably exists in the juices of various plants. 



Carbonate Of Soda, Na 2 O CO 2 , 106. This substance, so 

 important in the arts, was formerly made from the ashes 

 of certain marine plants (Salsola and Salicornia), in a man- 

 ner similar to that now employed in wooded countries for 

 the preparation of potash. It is at present almost wholly 

 obtained from common salt by a somewhat complicated 

 process. It occurs in commerce in an impure state under 

 the name of Soda-ash. When nearly pure it forms sal- 

 soda, which usually exists in transparent crystals or crys- 

 tallized masses. These contain 63 per cent of water, which 

 slowly escapes when the salt is exposed to the air, leaving 

 the anhydrous (water-free) carbonate as a white, opaque 

 powder. 



Carbonate of soda has a nauseous alkaline taste, not 

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

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



Bicarbonate of Soda, NaHO CO 2 . The supercarbon- 

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

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

 this with some simple carbonate. It is prepared in the 

 same way as the bicarbonate of potash. The bicarbonates, 

 both of potash and soda, give off half their carbonic acid 

 at a moderate heat, and lose all of this ingredient by con- 

 tact 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 inflating it with 

 carbonic acid gas. 



Carbonate of Lime, CaO CO 2 , 112. This compound is 



