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- 
bout acid, 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. 
Tue SutpHaTes 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,O SO,, 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,O So,, 142.—G@lauber’s salt is 
