THE ASH OF PLANTS. 133 



the common name of this familiar substance. It has a 

 bitter taste, and is much employed as a purgative for cat- 

 tle and horses. It exists, either crystallized and transpar- 

 ent, containing 10 molecules, 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. 



Sulphate of Lime, CaO SO3, 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. 



In nature, sulphate of lime is usually combined with 

 two molecules of water, and thus constitutes Gypsum, 

 CaO SO3 SH^O, which is a rock of frequent and extensive 

 occurrence. In the cells of many plants, as for instance 

 the bean, gypsum may be discovered by the microscope 

 in the shape of minute crystals. It I'equires 400 times its 

 weight of water to dissolve it, and being almost univer- 

 sally distributed in the soil, is rarely absent from the water 

 of wells and springs. 



The Phosphates which require special description are 

 those of Potash, Soda, and Lime. 



There exist, or may be prepared artificially, numerous 

 phosphates of each of these bases. The chemist is ac- 

 quainted with no less than thirteen different phosphates of 

 soda. But three classes of phosphates have any immedi- 

 ate interest to the agriculturist. As has been stated (p. 

 117), hydrated phosphoric acid prepared by boiling anhy- 

 drous phosphoric acid with water, is represented by the 

 symbol SH^O, P^O^. The phosphates may be regarded as 

 hydrated phosphoric acid in which one, two, or all the 

 molecules of water are substituted by the same number 

 of molecules of one or of several bases. We may illus- 

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