8 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
is due. deals mainly with bodies of known molecular weight, which can 
be measured by the density of a vapor or by cryoscopic methods. To 
the mineral chemist such knowledge is not available, for the compounds 
which interest him are neither volatile nor soluble, and their molecular 
weights can only be inferred. The simplest empirical formula of a sili- 
cate is not necessarily its true formula; the latter may be a multiple 
or polymer of the former; and here we find a difficulty which is at pres- 
ent almost insuperable. Strong evidence can be brought to bear upon 
this side of the question, but it is only partial evidence and not finally 
conclusive. The case, however, is by no means hopeless, for even the 
partial solution of a problem is better than no solution at all. An 
approximation is some gain, and it is possible so to investigate the 
constitution of the silicates as to bring many relations to light, devel- 
oping formulae which express those relations and indicate profitable 
lines for future research. 
The problem is open to attack from several points of view, and 
various methods of investigation can be brought to bear upon it. 
First, of course, the empirical formula of each silicate must be definitely 
ascertained, which involves the discussion of sufficiently numerous 
analyses and the elimination of possible errors due to impurity, alter- 
ation, and isomorphous admixtures. In this work the microscope ren- 
ders important service to the analyst, and makes his results much more 
certain. By the aid of the microscope many supposed mineral species 
have been proved to be mixtures, and the problem of the silicates has 
been thereby simplified. Indeed, the final outcome of such investiga- 
tion generally indicates, for any given natural silicate, simplicity of 
composition; and this is what should be expected. These compounds 
are, as a rule, exceedingly stable salts, while complex substances are 
commonly characterized by instability. The mineral silicates are 
formed in nature under conditions of high temperature or are deposited 
from solutions in which many reactions are simultaneously possible, 
and these circumstances are strongly opposed to any great complica- 
tions of structure. Furthermore, they are quite limited in number, only 
a few hundred at most being known : whereas, if complexity were the 
rule among them, slight variations in origin should produce correspon ding 
variations in character, and millions of different minerals would be 
generated. That few variations exist is presumptive evidence that 
only few are possible, and hence simplicity of constitution is reason- 
ably to be inferred. In fact, we find the same small range of mineral 
species occurring under the same associations in thousands of widely 
separated localities; a few typical forms containing a few of the com- 
monest metals being almost universally distributed. The longer the 
evidence is considered, the stronger the argument in favor of simple 
silicate structures becomes. 
Having established the empirical formula of a silicate, its physical 
properties may next be considered; and of these the crystalline form 
