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 asa 
white powder. 
Sulphate of Lime, CaO SO,, 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 SO, 2H,0, 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 requires 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. 
Tuer Pxospnates 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 3H,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- 
