132 HOW CROPS GROW. 



the white powder formed by the contact of carbonic acid 

 with lime-water. When hydrate of lime is exposed to the 

 air, the water it contains is gradually displaced by car 

 bonic acid &amp;gt; and carbonate of lime is the result. Air- 

 slaked lime always contains much carbonate. This salt 

 is distinguished from hydrate of lime by its being destitute 

 of any alkaline taste. 



In nature carbonate of lime exists to an immense extent 

 as coral, chalk, marble, and limestone. These rocks, when 

 strongly heated, especially in a current of air, part with 

 their carbonic acid, and quick-lime remains behind. 



Carbonate of lime occurs largely in the ashes of most 

 plants, particularly of trees. In the manufacture of pot 

 ash, it remains undissolved, and constitutes a chief part 

 of the residual leached ashes. 



The carbonate of lime found in the ashes of plants is 

 supposed to come mainly from the decomposition by heat 

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

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

 dantly deposited in its tissues in the solid form. Carbonate 

 of lime itself is, however, not an unusual component 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 Potash, Soda, and Lime. Sulphate of Magnesia 

 is well known as epsom salts, and Sulphate of Iron is 

 copperas or green- vitriol. (Sulphate of Lithia is very 

 similar to sulphate of potash.) 



Sulphate of Potash, K a O SO 3 , 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. 



Sulphate of Soda, Na a O So,, U2. Glauber s salt i 



