10 THE CONSTITUTION OF THE SILICATES. [bull 125. 
thermal decomposition of silicates, a line of investigation which has 
been successfully followed by several investigators. Thus garnet, when 
fused, yields anorthite and an olivine ; natrolite gives nepheline; talc, 
upon ignition, liberates silica; and the prolonged heating of ripidolite 
produces an insoluble residue having the empirical composition of spi- 
nel. All of these facts have relevancy to the problem of chemical con- 
stitution, and their number could easily be enlarged by experiment. 
As yet the field has been barely scratched upon the surface; upon 
deeper cultivation a goodly crop may be secured. 
The artificial synthesis of mineral species, with the allied study of 
crystalline slags and furnace products, furnishes still more evidence of 
pertinent utility. But, here again, caution is needed in the interpreta- 
tion of results. A compound may be produced in various ways, and it 
does not follow that the first method which is successful in the labora- 
tory is the method pursued by nature in the depths of the earth. The 
data yielded by synthesis are undoubtedly helpful in the determination 
of chemical constitution, but they furnish only a small part of the proof 
needed for complete demonstration, and their applicability to geological 
questions is limited in the extreme. For the latter purpose they are 
only suggestive, not final. 
Suppose now that the empirical formula of a silicate has been accu- 
rately fixed, and that a mass of data such as I have indicated are avail- 
able for combination with it. Suppose the physical properties to be 
determined, the natural relations known, the alteration products 
observed, its chemical reactions and the results of fusion ascertained ; 
what then? It still remains to combine these varied data into one 
expression which shall symbolize them all, and that expression will 
be a constitutional formula. To develop this, the established principles 
of chemistry must be intelligently applied, with due regard to recog- 
nized analogies. The grouping of the atoms must be in accord with 
other chemical knowledge; they must represent known or probable 
silicic acids; and any scheme which fails to take the latter considera- 
tion into account is inadmissible. Not merely composition, but func- 
tion also is to be represented, and the atomic linking which leaves that 
disregarded may be beautiful to see, but is scientifically worthless. A 
good formula indicates the convergence of knowledge; if it fulfills that 
purpose it is useful, even though it may be supplanted at some later 
day by an expression of still greater generality. Every formula should 
be a means toward this end, and the question whether it is assuredly 
final is of minor import. Indeed, there is no formula in chemistry 
to-day of which we can be sure that the last word has been spoken. 
In the development of constitutional formuhe for the silicates it 
sometimes happens that alternatives offer between which it is difficult 
to decide. Two or more distinct expressions may be possible, with the 
evidence for each so strong that neither can be accepted or abandoned. 
In such cases nothing cau be done but to state the facts and await the 
