16 Dr Brewster on the Optical Analysis of Minerals. 
limited to transparent or translucent minerals. It is no doubt 
true, that in certain substances, ol* almost absolute opacity, we 
cannot discover their axes of double refraction, or examine their 
action upon transmitted light ; but there are very few minerals 
of this description, from which I have not been able to obtain 
an optical character , and I have no doubt, that when we are 
better acquainted with the superficial action of bodies upon light, 
all these will be embraced by this general method. But while 
the optical method is limited to this extent, the chemical and 
the crystallographic methods have also their limitations. In 
very scarce minerals, which exist only in small specimens, such 
as the Stilbite from Aachen, the chemist and the crystallogra- 
pher are often unable to find a sufficient portion for their an^ 
lyses, while the optical observer can work with the smallest frag- 
ments, and still preserve them for the cabinet of the mineralo- 
gist. In granular minerals, and those which are disseminated 
through rocks, the chemist is equally baffled, while the resources 
of the optical method remain unexhausted. 
From the similarity in the composition of the Faroe and Uto 
Apophyllite, M. Berzelius has concluded,. that the first of these 
substances was not entitled to a separate name. If this distin- 
guished chemist meant to say, that in a chemical system of mi- 
neralogy the two substances should be arranged as one, we en- 
tirely agree with him ; but if he intended to state, that, in a ge- 
neral system of mineralogy, the Tesselite should not appear as 
a separate mineral, and under a separate name, we must object to 
the adoption of such an opinion. If the chemist exercises the 
right of distinguishing, in his nomenclature, minerals externally 
the same, but differing by only a few per cents, of some solid or 
gaseous ingredient, it would be a strange abridgment of the privi- 
leges of the optical observer, if he were not permitted to designate 
by a new name a body which possesses one of the most remarkable 
structures that has ever come under the notice of the mineralo- 
gist. The leading object of mineralogy is to explain the phy- 
sical properties of mineral bodies ; and their artificial classifica^ 
tion is to be considered in no other light than as a means of at- 
taining this object. But how can the mineralogist either pro- 
secute his inquiries, or teach his science, if he does not attach 
different names to bodies of different properties. Were he to 
