146 HOW CROPS GROW. 



plants, particularly of trees. In the manufacture ol 

 potash it remains undissolved, and constitutes a chief 

 part of the residual leached ashes. 



The calcium carbonate found in the ashes of plants is 

 supposed to come mainly from the decomposition by heat 

 of organic calcium salts (oxalate, tartrate, malate, etc. ), 

 which exist in the juices of the vegetable, or are abun- 

 dantly deposited in its tissues in the solid form. Car- 

 bonate of lime itself is, however, not an unusual compo- 

 nent of vegetation, being found in the form of minute, 

 rhombic crystals, in the cells of a multitude of plants. 



THE SULPHATES which we shall notice at length are 

 those of Potassium, Sodium, and Calcium. Sulphate of 

 Magnesium is well known as Epsom salts, and Sulphate 

 of Iron is copperas or green vitriol. 



Potassium Sulphate, or Sulphate of Potash, 

 K 2 S0 4 , 174. This salt may be procured by dissolving 

 potash or carbonate of potash in diluted sulphuric acid. 

 On evaporating its solution, it is obtained in the form of 

 hard, brilliant crystals, or as a white powder. It has a 

 bitter taste. Ordinary potash, or pearl-ash, contains 

 several per cent of this salt. 



Sodium Sulphate, or Sulphate of Soda, N"a 2 S0 4 , 

 142. Glauber's salt is the common name of this famil- 

 iar substance. It has a bitter taste, and is much em- 

 ployed as a purgative for cattle and horses. It exists, 

 either crystallized and transparent, containing 10 mole- 

 cules, or nearly 56 per cent of water, or anhydrous. 

 The crystals rapidly lose their water when exposed to the 

 air, and yield the anhydrous salt as a white powder. 



Calcium Sulphate, or Sulphate of Lime, CaS0 4 , 

 136. The burned Plaster of Paris of commerce is this 

 salt in a more or less pure state. It is readily formed by 

 pouring diluted sulphuric acid on lime or marble. It is 

 found in the ash of most plants, especially in that of 

 clover, the bean, and other legumes. 



