On the Silica Strata of the Lower Chalk, 227 



element within our observation exist in the combined or com- 

 pound form — that is to say, as silicates of alumina, iron, potash, 

 soda, lime, <Scc., constituting the different kinds of granitic, 

 basaltic, and other rocks, or as clay, which is a simple silicate of 

 alumina derived from the disintegration of these rocks. 



But silica, as it occurs in sand or flint, is not in what chemists 

 call the state of " soluble silica." By that term they mean that 

 kind of silica which is in an active chemical state disposing it to 

 unite with bases (silica, it should be remembered, being an 

 acid body) to form salts as other acids do. If common sand 

 or powdered flint be strongly heated with an alkali at a certain 

 temperature, the mixture melts and becomes glass more or less 

 transparent — when an excess of silica is employed, the glass is 

 insoluble in water like ordinary glass, but, when the proportion 

 of alkali is excessive, the compound produced is soluble in water 

 — and the solution is that of silicate of soda or potash, accord- 

 ing as one or the other alkali has been employed. 



If to this solution a sufficiently strong acid is added, a pre- 

 cipitate of a gelatinous character is produced, and this precipitate, 

 which is slightly soluble in water — more so in acids, and above 

 all in caustic alkalies — is called "soluble" or "gelatinous" 

 silica : when dry, it is a white powder of a gritty character, which 

 dissolves with the greatest facility in alkaline solutions at the 

 boiling temperature, and it is rather in relation to them than to 

 any other solvent that this form of silica is called " soluble." 



Now if silica in this state be strongly heated it will be found 

 to lose its property of dissolving in alkalies except by fusion — in 

 other words, it will be reduced to the same chemically inactive 

 condition as sand or flint, neither of which bodies can be dis- 

 solved to any appreciable extent in solutions of caustic alkalies 

 at the ordinary boiling temperature. There are, therefore, two 

 distinct conditions under which the element silica can exist in the 

 uncombined state — the inactive or insoluble, and the active or 

 soluble condition : sand and flint are, under ordinary circum- 

 stances, in the first, and silica, when prepared as before described, 

 or by other artificial processes, in the second. 



Flint can indeed be dissolved in alkaline solutions in high- 

 pressure boilers, and consequently at an elevated temperature ; 

 whilst quartz rock or white sand, which are other forms of more 

 or less pure silica, cannot be dissolved by these means, or at all 

 events not at the temperature which will dissolve flint. 



We have here, then, another condition of silica, making three 

 varieties in all, possessing different degrees of chemical intensity, 

 as evidenced by their disposition to combine with alkalies — 



1st. Soluble gelatinous or chemical silica, which readily dis- 

 solves in boiling solutions of potash or soda. 



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