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, 7. e. they contain water as a large and 
essential ingredient. 
Hydrated Silica.—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, isknown 
only in a state of solution. It is formed when the solution 
of an alkali-silicate is déeomposed by means of a large ex- 
cess of some strong acid, like the chlorhydric or sulphuric. 
Exe. 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, uniting 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 evavorat- 
6 
