SILICA. 315 



times the prism is very short or disappears entirely, and the 

 pyramid only is seen, as in ordinary quartz. 



Soluble Silicic Acid. The preceding description applies to 

 silica after it has been dried or heated, but silica can also be 

 obtained in a state in which it is soluble in dilute acids and 

 even in water. The oxidation of the sulphuret of silicon, in 

 water, gives silica in this condition; the solution when con- 

 centrated, becomes a gelatinous mass, like size. When the 

 gaseous fluoride of silicon is absorbed by water, silica separates 

 in large quantity in that gelatinous condition, and this jelly is 

 soluble in water although it requires a large quantity to dis- 

 solve it. The solution of silica was found by Berzelius to be 

 insipid, and not to redden litmus ; by evaporation of the liquor 

 the silica is deposited in the form of an earthy mass without a 

 trace of crystallization, and capable of dissolving again in water. 

 It is observed however, that when sulphuric or hydrochloric 

 acid is added to the solution during evaporation, the silica ob- 

 tained is no longer the soluble, but the former insoluble va- 

 riety. The fixed alkalies and their carbonates, it is curious, 

 effect a transmutation of the opposite kind, for when insoluble 

 silica is boiled with them, it is gradually converted into the 

 soluble species and dissolves. Berzelius finds that this change 

 supervenes, without decomposition of the alkaline carbonate or 

 any escape of carbonic acid. The alkali in this solution may be 

 saturated completely with an acid, without any silica precipi- 

 tating, which proves that that body is dissolved in the water 

 and not in the alkaline carbonate. 



The water of springs and wells always contains a little solu- 

 ble silica, which can only be obtained by evaporating the water 

 to dryness. In some mineral waters the proportion of silica is 

 very considerable, and it is often associated with an alkaline 

 carbonate, as in the hot alkaline spring of Reikum in Iceland, 

 and in the boiling jets of the Geyser, which deposit about their 

 crater an incrustation of silica. There can be no doubt like- 

 wise that much of the crystalline quartz in nature besides all 

 the agates, calcedonies and siliceous petrifactions have been 

 formed from an aqueous solution. 



The soluble silica seems to exist in the class of minerals 

 called zeolites, which also contain w r ater, and many of which 

 dissolve entirely in dilute hydrochloric acid. But it may be 

 obtained from any silicate by fusing it with an alkaline carbo- 



