120 HOW CROPS GROW. 
in three modifications: one, a brown, powdery substance ; 
another, resembling black-lead, (p. 31,) and a third, that 
occurs in crystals, having the form and nearly the hard- 
ness of the diamond. 
Anhydrous Silicic Acid, Sym. Si O,, mo. wt. 60.—This 
compound, known also as Silica, and anciently termed 
Silex, is widely diffused in nature, and occurs to an enor- 
mous extent in rocks and soils, both in the free state and 
in combination with other bodies. 
Free silica exists in nearly all soils, and in many rocks, 
especially in sandstones and granites, in the form known 
to mineralogists as quartz. The glassy, white or trans- 
parent, often yellowish or red fragments of common sand, 
which are hard enough to scratch glass, are almost inva- 
riably this mineral. Jn the purest state, it is rock-crystal. 
Jasper, flint, and agate, are somewhat less pure silica. 
Silicates.—Anhydrous silicic acid is extremely insoluble 
in pure water and in most acids, It has, therefore, none 
of the sensible qualities of acids, but is nevertheless ca- 
pable of union with bases. It is slowly dissolved by strong, 
and especially by hot solutions of potash and soda, form- 
ing soluble silicates of these alkalies. 
Exp. 56.—Formation of silicate of potash, Heat a piece of quartz or 
flint, as large as a chestnut, as hot as possible in the fire, and quench 
suddenly in cold water. Reduce it to fine powder in a porcelain mortar, 
and boil it in a porcelain dish with twice its weight of caustic potash, 
and eight or ten times as much water, for two hours, taking care to sup- 
ply the water as it evaporates. Pour off the whole into a tall narrow 
bottle, and leave at rest until the undissolved silica-has settled. The 
clear liquid is a basic silicate of potash, 7. e. a silicate which contains a 
number of molecules of base for each molecule of silica. It has, in fact, 
the taste and feel of potash solution. The so-called water-glass, now em- 
ployed in the arts, is a similar silicate of potash or soda, 
When silica is strongly heated with potash or soda, or 
with lime, magnesia, or oxide of iron, it readily melts to- 
gether and unites with these bodies, though nearly infus- 
ible by itself, and silicates are the result. The silicates 
thus formed with potash and soda are soluble in water, like 
