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 2 , 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 t^.e free state and 

 in combination w r ith 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. In 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 lire, 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, i. 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 arc soluble in water, like 



