clabke.] THEORY OF THE SILICATES. 13 
tioiis have shown them to be ortho compounds, iwssibly more or less 
impure. Troost and Hautefeuille, however, have described an ether 
having the formula, (C 2 H 5 ) 8 Si 4 Oi 2 , which is a polymer of a metasilicate ; 
but its true nature has not been determined. The simplest formulae 
for typical metasilicates are as follows : 
Types. 
Examples. 
R l 2 Si0 3 
Na^iO, 
R"Si0 3 
MgSi() 3 
B"S(SiOs)a 
Al 2 (Si0 3 ) 3 ' 
R lv (Si0 3 ) 2 
Zr(SK) 3 ) 2 * 
By eliminating a molecule of water from two molecules of ortho- 
silicic or metasilicic acid, a disilicic acid may be produced. From 
orthosihcic acid we have 
Si=(OH) 3 
Si(OH) 4 | 
— H 2 = O 
Si(OH) 4 | 
Si=(OH) 3 
or, H 6 Si 2 7 ; a sexbasic acid of which several ethers have been obtained. 
It is, therefore, a well-established acid, and a number of silicates appear 
to be salts of it. It may be called diorthosilicic acid. From metasilicic 
acid, in the same way, we get 
0=Si— OH 
0=Si=(OH) a | 
— H 2 = O 
0=Si=(OH) 2 | 
0=Si— OH 
• 
or dimetasilicic acid, H 2 Si 2 5 . No ethers of this acid are known, but 
among its salts are the minerals petalite, milarite, ptilolite, and 
mordenite. 
By a similar process, that is, the elimination of water from three or 
four molecules of orthosilicic acid, a series of tri- and tetrasilicic acids 
may be theoretically developed. These higher acids would present 
many possibilities for isomerism, and with their long chains of atoms 
would presumably be unstable. At all events, with a single excep- 
tion, it is unnecessary to consider them in order to account for the 
natural silicates. The one exception is the tetrabasic acid H 4 Si 3 0(i, 
which appears most notably in eudidymite, tbe feldspars, the micas, 
and the scapolite group. The feldspars of this type, however, albite 
and orthoclase, are represented by Groth as mixed salts of two other 
silicic acids, when Si 2 5 +Si0 3 =Si 3 8 ; so that the trisilicic acid in 
question can be left out of consideration. But a general view of the 
whole field seems to point to its existence; at least greater simplicity 
is secured by retaining it in the scheme of silicic acids; and that policy 
* Salts not actually known, but theoretically possible. 
