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. 



TIIE CARBONATES 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 2 O CO 2 , 114. The pearl-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 a liquid. 



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 re-main behind. 



K 2 O CO 3 + 2H Cl = 2K Cl + H a O + CO 2 . 



Bicarbonate of Potash, KHO CO,. A solution of 



