on the Optical System of Mineralogy. $6$ 
mined, that the biaxal Chabasie is a new mineral different from 
the ordinary Chabasie ; and therefore, Iolite is now the only 
exception to the law, or rather it is no exception at all, as its 
crystallographic structure was never determined with care *. In 
a short time it must follow the fate of the rest, and will be handed 
over with the other ten exceptions to the dominion of the optical 
system. 
I would now venture to ask the candid inquirer after truth, 
if he is acquainted with any law in Chemistry or Physics which 
is supported by a sounder or a more extensive induction, than 
that optical law which connects the primitive forms of minerals 
with the number of their axes of double refraction ? It stands 
pre-eminent among crystallographic generalisations, without a 
single ascertained exception ; — it has corrected the deductions of 
the most distinguished crystallographers of Europe; — it has en- 
abled those who confide in its accuracy, to predict the results 
of crystallographic researches ; — and, what has not been suffi- 
ciently attended to, it is a law founded on experimental evi- 
dence, that the optical phenomena are the necessary results of a 
mechanical structure, and that their indications must infallibly 
harmonize with the sound deductions of crystallography. 
Under such circumstances, I experienced no inconsiderable 
surprise at finding in Mr Brooke’s Familiar Introduction to 
Crystallography, what every person who understands the sub- 
ject must regard as a groundless animadversion upon the Opti- 
cal System of Mineralogy. In discussing those well known and 
now numerous examples where chemistry and crystallography 
are at direct variance, as mineral ogical methods, Mr Brooke 
has treated the great host of chemical analysts with the defe- 
rence which they so well merit ; and has very properly al- 
lowed u that all such anomalies will probably be reconciled 
by the future investigations of science.” He has declined, how- 
ever, to extend to the optical system the same courtesy, and has 
opened all the trenches of chemistry and crystallography 
against a new and incomplete fortress, reared and defended by 
a single man at arms. The reader is not even told what the 
system is which is assailed ; he is not made aware that it has 
* I understand that Professor Mohs has made Iolite Prismatic, solely on the 
authority of Haiiy. 
