THE 
EDINBURGH 
PHILOSOPHICAL JOURNAL. 
Art. I .—On the Connexion between the Optical Structure and 
Chemical Composition of Minerals. By David Brewster, 
LL. D. F. R. S. Lond. & Sec. R. S. E. 
In the course of an extensive examination of mineral bodies, 
in which I was engaged in the years 1816 and 1817, for the 
purpose of investigating the laws of Polarisation and Double 
Refraction, I was led to the discovery of two general principles* 
which connected the optical condition of crystals with their mi- 
neralogical structure and their chemical composition. From 
the number of Axes of Double Refraction which any mineral 
possessed, I was enabled to determine the Class of Primitive 
Forms to which it belonged ; and while every variation in the 
position, the intensity, and the character of these axes in similar 
minerals, was found to be accompanied with a difference of che- 
mical composition, a difference of composition was also found to 
be accompanied with a difference of optical structure. All the 
combinations, too, of the sulphuric and tartaric acids, with a single 
earthy, alkaline and metallic base, were found to have Two axes 
of double refraction. The new paths which these determinations 
seemed to throw open to the philosophical mineralogist, promised 
to introduce an unexpected degree of precision into the science, 
and to decide many of those contested points which had been 
left unsettled, either from the imperfection of chemical analysis, 
or the indefinite indications of external characters. In the years 
1816, 1817, and the beginning of 1818, I had opportunities of 
VOL. v. NO. 9. JULY 1821, 
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