THE ASH OF PLANTS. 131 
¢arbonate 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. 
Potash-saleratus consists essentially of this salt. It 
probably exists in the juices of various plants. 
Carbonate of Soda, Na,O CO,, 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 sa/- 
soda, which usually exists in transparent crystals or erys- 
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,.—The supercarbon- 
ate of soda of the apothecary is this salt in a nearly pure 
state. The soda-saleratus 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 
pon 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 
‘arbonic acid gas. 
Carbonate of Lime, CaO CO,, 112.—This compound is 
