144 HOW CROPS GKOW. 



The Caebonates which occur in the ashes of plants 

 are those of Potassium, Sodium, and Calcium. The 

 Carbonates of Magnesium, Iron, and Manganese are de- 

 composed by the heat at which ashes are prepared. 



Potassium Carbonate, or Carbonate of Potash, 

 KaOOs, 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 carbon- 

 ate. If wood-ashes are repeatedly 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 dafk or 

 brown mass. This, when somewhat purified, yields 

 pearl-ash. Potassium carbonate, when pure, is white, and 

 has a bitter, biting taste — ^the so-called alkaline taste. It 

 has such attraction for water, that, when exposed to the 

 air, it absorbs moisture and becomes a liquid. 



If hydrochloric acid be poured upon this carbonate a 

 brisk effervescence immediately takes place, owing to the 

 escape of carbon dioxide gas, and potassium chloride and 

 water are formed, which remain behind. 



KjCOj + 2 HCl = 2 KCl + HjO + CO^. 



Potassium Bicarbonate, KHCOs. — A solution of 

 potassium carbonate, when exposed to carbon dioxide, ab- 

 sorbs the latter, and the potassium bicarbonate is pro- 

 duced, so called because to a given amount of potassium 

 it contains twice as much carbonic acid as the carbonate. 

 Potash-salmratus consists essentially of this salt. It 

 probably exists in the juices of various plants. 



Sodium Carbonate, or Carbonate of Soda, 

 NaaCOa, 106. — This substance, so important in the arts, 

 was formerly made from the ashes of certain marine 

 plants {Salsola and Salicornia), in a manner similar to 

 that now employed in wooded countries for the prepara- 

 tion of potash. It is at present almost wholly obtained 



