THE ASH OF PLANTS. 121 



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 alkali and lime, alumina, or iron, with a large 

 proportion of silica, is nearly or altogether insoluble, not 

 only in water, but in most acids — constitutes, in fact, ordi- 

 nary 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 are of two classes, viz., the 

 acid silicates, (containing a preponderance of silica,) and 

 basic silicates, (with large proportion of base): the former 

 are but slowly dissolved or decomposed by acids, while 

 the latter are readily attacked even by carbonic acid. 

 Many native silicates are anhydrous, or destitute of water ; 

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

 essential ingredient. 



Hydrated Silicai — ^Various compounds of silica with 

 water are known to the chemist. Of these but three need 

 be mentioned here. 



Soluble Silica. — ^This body, doubtless a hydrate, is known 

 only in a state of solution. It is formed when the solution 

 of an alkali-silicate is decomposed by means of a large ex- 

 cess of some strong acid, like the chlorhydrje or sulphuric. 



Exp. 57. — Dilute half the solution of silicate of potash obtained in 

 Exp. 56 with ten times its volume of water, and add diluted chlorhydric 

 acid gradually until the liquid tastes sour. In this Exp. the chlorhydric 

 acid decomposes and destroys the silicate of potash, 110111115 itself with 

 the base with production of chloride of potassium, which dissolves in 

 the water present. The silica thus liberated unites chemically with wa- 

 ter, and remains also in solution. 



By appropriate methods Doveri and Graham have re- 

 moved from solutions like that of the last Exp. everything 

 but the silica, and obtained solutions of silica in pure wa- 

 ter. Graham prepared a liquid that gave, when evaBorat^ 

 6 Digitized by Microsoft® 



