1:4 Dr Brewster, oil tfte Optical Analysis of Aimer ah. 
bodies, and must, on this account alonCj be held to be inferior 
to the other two. 
The imperfections of chemical analysis are so well known, 
and so candidly admitted by the most distinguished chemists, 
that it would be an unprofitable task to enumerate the discor- 
dant results obtained even by masters of the analytic’ art. It 
may be sufficient to state, that the chemist examines a mineral 
in its ruins, and after its parts have been separated by the dis- 
integrating power either of heat or acids. He may deter- 
mine the quantity, but he is frequently ignorant of the na- 
ture of those volatile parts which escape during the destroying 
process. The fluoric acid, for example, in certain micas eluded 
the penetration of Vauquelin, and the same gas was not disco- 
vered by Berzelius in his first analyses of apophyllite. 
With regard to the method of optical analysis^ the case is 
quite different. We examine the mineral in its perfect and un- 
disturbed condition. We determine its various properties, as 
modified by the elements of which it is composed, by the pro- 
portions in which they combine, and by the mechanical or crys- 
tallographic structure into which they are arranged. We there- 
fore determine properties, and measure actions, and observe 
structures, which vary with the elementary parts of the mineral, 
as well as with their mode of combination ; and though we do 
not know either the names or proportion of the elements whose 
agency is under our observation, yet we are in reality observing 
the direct effects of chemical composition and crystallographic 
structure. In proof of these positions, I might refer in general 
to the universality of the law which I have established between 
the primitive forms of crystals and the number of their axes of 
double refraction, and to the various new mineral species which 
have been detected by optical analysis alone ; but as the papers 
in which these results are contained, are printed in various 
works, I shall direct the attention of the reader to the influence 
of optical analysis in determining the mineral species of the 
Zeolites, one of the most perplexing families of mineral sub- 
stances, and upon which the chemist and the crystallographer 
have exliausted all the resources of their respective arts. 
When Haiiy published his Traite de Mineralogie in 1804, 
the two species of Sxilbite and Mesotype embraced the follow- 
