42 PROPERTIES OF ELEMENTS OF SOIL. 



70. Soluble silica exists in some minerals, and is 

 produced, when a silicate is melted with an alkali, 

 and dissolved in dilute acid. It is in consequence 

 of this ready solubility of silica, that a small quanti- 

 ty is contained in all natural waters ; associated with 

 alkaline carbonates in mineral springs, it is often an 

 abundant product. 



71. The general properties, which silicic acid ex- 

 hibits in its combinations, are these : 



1st. All its compounds, with excess of alkali, are 

 caustic, and soluble in water. Those with an excess 

 of silica are mild, and insoluble. Glass is an ex- 

 ample of the last, and so are the rocks. Green bot- 

 tle glass, is but a fused rock, a mixture of silicates of 

 potash, soda, alumina, lime, magnesia, and iron. 

 These are the silicates which have been already enu- 

 merated, (60) as composing rocks ; and the amount, 

 and origin of these several elements of soil, can now 

 be conveniently understood. This is practical 

 ground, and shows the value of chemical analysis of 

 rocks. Whatever opinion respecting their origin, 

 is adopted, and whether or not, granite is supposed 

 to have produced the soil above it, or that it is only 

 overlaid by granite drift, it is evident, from the table 

 (59) that all granite rocks contain lime and alkali. 

 These will be in proportion to the mica and felspar, 

 for granite (35) is composed of these and quartz. 



