18 Dr Brewster oil the Optical Analysis qf Minerals. 
ters; and we should think little of any system that founded its 
decisions on the variations of colours or of transparency, or in 
numerous other accidents of light which are familiar to mine- 
ralogists. The system of optical analysis disclaims^ all such tri- 
vial distinctions, and founds its results, upon characters as es- 
sentially necessary to the existence of the mineral as the most 
prominent of its chemical elements. 
There are some circumstances in the comparison of the Faroe 
and the Uto Apophy Hites, which require to be carefully kept in 
view, before we pronounce the Tesselated Apophyllite to be 
chemically the same with that which is not tesselated. I have 
reason to believe, that the Tesselite exists among the Uto apo- 
phyllite, and, therefore, it is not improbable that M. Berzelius 
may have analysed the Tesselite of Uto. The two kinds of 
Apophyllite which are set in peculiar contrast with one another 
by the optical method, are the Apophyllite surcomposee from 
Fassa, and the Tesselite of Faroe; and therefore, the ana- 
lysis of these two crystals, when the former is perfectly pure, 
should be well compared. But even if they should yield the 
same ingredients, we still would not admit their chemical 
identity ; for in analysing the Tesselite of Faroe, we actually 
analyse a combination of crystals with one and two axes, 
and therefore, it is essentially necessary to an accurate compa- 
rison of the two substances, that the hiaxal part of the Tesse- 
lite should be analysed separately from the uniaxal part, the 
last of which has the same general optical properties as the 
Apophyllite stircomposce. Let us suppose that such a nice ope- 
ration is capable of being executed by a dexterous analyst, and 
that the hiaxal part is found to have the same composition as 
the uniaxal part, then we shall be forced to the conclusion, 
that minerals may have differeni optical structures^ and yet 
be chemically the same suhsiance. But such a conclusion, in- 
stead of being imfavourablo to the optical metlmdj tends only tc 
prove the superior delicacy of its indications, and to increase oui 
confidence in its results. 
Edinburgh, ) 
May 15. 1822. | 
