to his Observations on the Optical System of Mineralogy. 199 
I do not stand alone in opinion upon this point. In VoL VII. 
p. 4. of this Journal, Berzelius says, “ It appears to me, that 
mineralogical characters, drawn from optical phenomena, pre- 
sented by transparent crystallised minerals, may be denied from 
several sources, 1 st J From the nature of their elements; 2d, 
From the number of atoms of each element, on which depends 
the crystalline system to which the mineral belongs ; 3d, From 
accidental mixtures, often inconsiderable, which alter, in an es- 
sential manner, the transparency, the forms, and the crystalli- 
sation. These are what are called, in artificial crystals, impuri- 
ties. To these we may add the variable transparency of diffe- 
rent parts of the same crystal. Differences of this kind, how- 
ever great be the influence which they exercise upon light, can 
never constitute differences of species in mineralogy, — differ- 
ences which can only be founded on a real diversity of compo- 
sition. To distinguish among the optical phenomena produced 
by accidental circumstances, and those which are derived from 
a difference in the elements, or in the number of their atoms, is 
to carry to its maximum the employment of optical phenomena 
as distinctive characters in mineralogy.’’" 
Upon this Dr Brewster has the following observations in 
page 17. of the same volume : Mr Berzelius has remarked, 
that while the optical characters of minerals may be derived 
from the nature of their elements, and the number of atoms of 
each element, they may arise also from carnal mixtures and 
impurities , and that to employ optical phenomena , produced by 
accidental circumstances , is to carry the use of this method to its 
maximum. In this opinion we heartily concur ; but we are 
not aware that any such optical phenomena have ever been pro- 
posed, even as subsidiary characters ; and we should think little 
of any system that founded its decisions on the variations of co- 
lours or of transparency , or on numerous other accidents of 
light which are familiar to mineralogists. The system of op- 
tical analysis disclaims all such trivial distinctions, and founds 
all its results upon characters as essentially necessary to the ex- 
istence of the mineral , as the most prominent of its chemical 
elements y 
This, however, takes the question at issue for granted, and is 
any thing but an answer to the objections raised by Berzelius, 
