56 PROPERTIES OF ELEMENTS OF SOIL. 



(68), dissolves in water, and gives silica, in solution. If this 

 is evaporated, a jelly-like, sizy mass is obtained, which' may 

 be again dissolved in water. Acid, added to the solution, 

 when evaporating, renders silica insoluble. Alkalies, boiled 

 with insoluble silica, render it soluble, no change occurring 

 in the alkali. These singular changes are due, probably, to 

 a new arrangement of the particles of silica, produced by 

 that power called catalysis^ or the action of presence, that 

 is, by the presence of a third body, taking no part itself 

 in the action, but simply influencing the changes which 

 occur. 



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 quantity 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 exhibits 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 example of the last, and so 

 are the rocks. Green bottle 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 

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

 origin of these several elements of soil can now be con- 

 veniently 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 



