CHAPTEE II. 
THE THEORY OF THE SILICATES. 
If all of the silicates were salts of a single silicic acid, the problem of 
their constitution would be relatively simple; but this is not the case. 
Many silicic acids are theoretically possible, and several of them have 
representatives in the mineral kingdom, although the acids themselves, 
as such, are not known. Their nature must be inferred from their salts, 
and especially from their ethers, and this side of the problem is the 
first to be considered. 
Since silicon is quadrivalent, its ortho acid is necessarily represented 
by one atom of the element united with four hydroxyl groups, thus — 
Si(OH) 4 , or, structurally: 
H 
1 
O 
I 
H— O— Si— O— H 
I 
O 
I 
H 
To this acid ? orthosilicic acid, the normal silicic ethers and many com- 
mon minerals correspond. Its normal salts, reduced to their simplest 
expressions, may be typically represented as follows : 
Types. 
Examples. 
R i 4 Si0 4 
(0 2 H 5 ) 4 SiO 4 
R^SiCX 
Mg 2 Si0 4 
R i,l 4 (Si0 4 ) 3 
Al 4 (Si0 4 ) 3 
R iv Si0 4 
ZrSiQ 4 
Any silicate in which the oxygen atoms outnumber the silicon atoms 
by more than four to one, as for example the compound Al 2 Si0 5 , must 
be regarded as a basic salt. 
By elimination of water orthosilicic acid may be conceived as yielding, 
first, metasilicic acid, H 2 Si0 3 , and, secondly, the anhydride, Si0 2 , thus : 
Si(OH) 4 = Si=(OH) 2 0=Si=0 
Corresponding to metasilicic acid many salts are known, but as yet 
no ethers have been certainly obtained. The ethers first described by 
Ebelmen were supposed to be metasilicates, but all recent investiga- 
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