FORMATION OF SOILS. 



All silicates soluble in water are decomposed by acids. If the 

 solution of the silicate contains silica corresponding to more 

 than - 3 X - the weight of the water, the addition of an acid causes 

 the formation of a precipitate of a very gelatinous appearance. 

 This precipitate, being a compound of silica with water, is 

 termed the hydrate of silica. But, if the solution contains less 

 silica than the above proportion, no precipitate is formed on the 

 addition of an acid, the whole remaining perfectly clear. This 

 circumstance proves that silica, in the state in which it is preci- 

 pitated by an acid, possesses a certain degree of solubility in 

 pure water. Indeed, by washing with water the gelatinous pre- 

 cipitate of silica formerly alluded to, its volume diminishes, and 

 silica may be detected in solution by evaporating the wate> 

 which has passed through. 



From these facts we perceive, that silica possesses two distirf 

 chemical characters. In the form in which it is separated fror»' 

 a silicate, it possesses quite different properties from those vvhic 1 - 

 it has when in the state of sand, quartz, or rock crystal. Whei* 

 sufficient water is present during its separation from a base, U 

 effect its solution, the whole remains dissolved ; in certaip 

 conditions, silica is more soluble in water than gypsum. 



On drying, silica loses completely its solubility in wato 

 The solution of silica in acids acquires, at a certain degree o; 

 concentration after cooling, such a gelatinous consistence tha* 

 the vessel containing it may be turned upside down withoiM 

 spilling a drop of the transparent jelly. By drying it stilV 

 further, the water which retained it in the gelatinous condition 

 escapes along with that which had served to hold it in solution ■■« 

 When the water has been one? removed in this way, the silica 

 is no longer soluble in water. But, although it has thus lost its 

 solubility, it does not acquire all the properties of crystallized 

 silica, such as sand and quartz, for it still possesses the power o' 

 dissolving in alkalies and alkaline carbonates at the ordinary 

 temperature of the air, and this power it retains even when i* 

 has been heated to redness. 



There is scarcely any other mineral substance which can b* 

 compared to silica for the possession of such remarkable proper 

 ties as those now described- 



