THE ASH OF PLANTS. 135 



ash, and eight or ten times as much water, for two hours, taking care 

 to supply 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 potassium silicate, i. e., a silicate which con. 

 tains 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 employed in the arts, is a similar sodium silicate. 



When silica is strongly heated with potast or soda, or 

 with lime, magnesia, or oxide of iron, it readily melts to- 

 gether and unites with these hodies, though nearly infus- 

 ible by itself, and silicates are the result. The silicates 

 thus formed with potash and soda are soluble in water, 

 like the product of' Exp. 56, when the alkali exceeds a 

 certain proportion — ^when highly basic ; but, with silica 

 in excess (acid silicates), they dissolve with difficulty. 

 A mixed silicate of sodium, calcium, and aluminum, with 

 a large proportion of silica, is nearly or altogether insol- 

 uble, not only in water, but in most acids — constitutes, 

 in fact, ordinary glass. 



A multitude of silicates exist in nature as rocks and 

 minerals. Ordinary clay, common slate, soapstone, mica, 

 or mineral isinglass, feldspar, -hornblende, garnet, and 

 other compounds of.frequent and abundant occurrence, 

 are silicates. The natural silicates may be roughly dis- 

 tinguished as belonging to two classes, yiz., the acid sil- 

 icates (containing a preponderance of silica) and lasic 

 silicates (with large proportion of base). The former are 

 but slowly dissolved or decomposed by acids, while the 

 latter are readily attacked, even by carbon dioxide acid. 

 Many native silicates are anhydrous, or destitute of 

 water ; others are hydrous, i. e., they contain water as a 

 large and essential ingredient. 



The Silicic Acids. — ^Various silicic acids — compounds 

 of silica with water — are known to the chemist, or are 

 represented by the silicates found in nature. The silicic 

 acids themselves have little stability and are readily re- 

 solved into water and silica. 



Soluble Silica, Si(0H)4? — This body is known only in 



