130 HOW CROPS GROW. 
iron, and oxide of manganese, they all exist in the ash in 
the form of salts, (compounds of acids and bases). In the 
living agricultural plant it is probable, that of them all, 
only silica occurs in the uncombined state. 
We shall notice in the first place the salts which may 
occur in the ash of plants, and shall consider them under 
the following heads, viz.: Carbonates, Sulphates, Phos- 
phates, and Chlorides. As to the Silicates, it is unneces- 
sary to add anything here to what has been already men- 
tioned, 
Tux Carsonates which occur in the ashes of plants 
are those of Potash, Soda, and Lime. (Carbonate of 
Rubidia, similar to carbonate of soda, and Carbonate of 
Lithia, rather insoluble in water, may also be present, but 
in exceedingly minute quantity.) The Carbonates of Mag- 
nesia, Iron, and Manganese, are decomposed by the heat 
at which ashes are prepared. 
Carbonate of Potash, K,O CO,, 114.—The peartl-ash 
of commerce is a tolerably pure form of this salt. When 
wood is burned, the potash which it contains is found in 
the ash, chiefly as carbonate. If wood-ashes are repeat- 
edly washed or leached with water, all the salts soluble in 
this liquid are removed; by boiling this solution down to 
dryness, which is done in large iron pots, crude potash is 
obtained, as a dark or brown mass. This, when somewhat 
purified, yields pearl-ash. Carbonate of potash, when pure, 
is white, has a bitter, biting taste—the so-called alkaline 
taste. It has such attraction for water, that, when expos- 
ed to the air, it absorbs moisture and becomes. liquids — 
If chlorhydric acid be poured upon carbonate of potash 
a brisk effervescence immediately takes place, owing to the 
escape of carbonic acid gas, and chloride of potassium and 
water are formed which remain behind. 
K,O CO, + 2H Cl = 2K Cl + H,O + CO, 
Bicarbonate of Potash, KHO CO,—A solution of 
