clarke.] THEORY OF THE SILICATES. 17 
gous to that offered by the benzene ring, prism, or nucleus, a concep- 
tion of which the utility is fully recognized, in spite of outstanding 
uncertainties. For practical purposes, that is, for the coordination of 
known facts, expressions like the following are sufficient: 
Si0 4 Al Si0 4 =KH 2 Si0 4 ~CaH 
Al -SK) 4 =Ai Al -SiO^EAl Al-Si0 4 E!CaH 
\siQ 4 =Al ^Si0 4 =Al \siQ 4 =Al 
These expressions indicate the observed replaceability of aluminum 
atoms by other atoms or groups, and have no ulterior significance. So 
long as their limitations are kept in mind they are useful, but beyond 
this it would be unreasonable to go. With prolonged discussion and 
more evidence we may get a deeper insight into the nature of the 
fundamental molecule 5 at present, speculation concerning it would be 
premature. The relations expressed are clear, no matter what others 
may be revealed in the future. As a working hypothesis, the concep- 
tion of substitution from a normal salt may be applied to many non- 
alummous silicates, as in the magnesian series, the silicates of quadriv- 
alent metals, and so on. These points will be developed in subsequent 
chapters. In the systematic discussion I shall begin with the orthosili- 
cates, starting with those of aluminum, for the reason that they show 
most clearly the evidence relative to structure. In a classification of 
the silicates the starting point would be different, but this is not a classi- 
fication. The trisilicates will be considered with the orthosilicates, for 
reasons which are obvious. In the feldspar, zeolite, and mica groups 
the ortho- and trisilicates can not well be separated. 
Bull. 125 2 
