14 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
will be followed throughout this memoir. As opposed to Groth's view 
we have also to consider the theory of Becker* that nietasilicates may 
be really mixtures or compounds of ortho- and trisilicates, with H 4 Si0 4 -f 
H 4 Si 3 G 8 =4H 2 Si0 3 . This theory may easily be pushed to an extreme, 
for the weight of evidence shows that true metasilicates exist; but in 
many cases it has real significance; that is, mixed salts are known 
having pseudometasilicate ratios, and these are most simply explained 
by Becker's theory, which, in the case of the micas, had been independ- 
ently developed by myself. As to the constitution of trisilicic acid, we 
have no positive evidence, and at least two independent structures are 
conceivable, thus: 
= Si-OH 
I 
O 
I 
Si=(OH) 2 
I 
O 
I 
G=Si-CH - 
and 
(OH) 2 =Si< >Si( >Si=(OH) 2 
MX XK 
The first of these formulae has the closer analogy with the formulae 
of the other silicic acids, and is, therefore, in default of evidence, to be 
preferred. Furthermore, it indicates possibilities of isomerism, such 
as we find in the case of orthoclase and microcline, a consideration 
which is well worth bearing in mind. _ 
So far, then, there are five silicic acids whose salts occur in the crust 
of the earth. They are : 
Orthosilicic acid H 4 Si0 4 
Metasilicic acid H 2 Si0 3 
Diorthosilicic acid H c Si 2 7 
Dimetasilicic acid H 2 Si 2 5 
Trisilicic acid H 4 Si 3 8 
and unless we admit the possible polymerization of metasilicic acid, 
as suggested by Troost and Hautefeuille's ether, no others need be 
invoked in order to account for the facts which are now available. 
The possibility of other silicic acids is an entirely distinct question. 
If the natural silicates were simple normal salts of the five silicic 
acids the problem of their constitution would not be difficult. But 
relatively few of the known species are of this description; the greater 
number are double salts, and even triple replacements are not uncom- 
mon. Furthermore, there are acid and basic salts to be interpreted, 
* Am. Jour. Sci., (3) XXXVIII, p. 154, 1889. 
