362 Dr Brewster's Reply to Mr Brooke's Observations 
stated all the exceptions to it which were then known, 
exceptions were as follows : 
Sulphate of Magnesia. 
Chromate of Lead. 
Mesotype. 
Carbonate of Barytes. 
5 Strontites. 
Cryolite. 
Harmotome. 
Chabasie. 
10 Sulphate of Iron. 
Essonite. 
Iolite. 
These 
These Eleven exceptions to a law otherwise general and com- 
prehensive, were ascribed by myself and others to the circum- 
stance of the primitive forms of the minerals having been erro- 
neously determined ; and as I was then aware, from actual ex- 
periment, that the optical structure of a crystal was dependent 
on the mechanical condition of its parts, I did not scruple to 
predict, and to publish the prediction, that all these eleven ex- 
ceptions had their foundation in crystallographic errors , and 
that these minerals would be found to possess primitive forms, 
in perfect conformity with the general optical law which I had 
discovered. 
Crystallography and Optics were thus fairly placed at issue, 
and there was no method of evading the decision of the mi- 
neralogists of Europe. If these Eleven bodies were found to 
possess the primitive forms assigned to them by Haiiy, either 
the optical law which I promulgated must have instantly fallen 
to the ground as an imposing though imperfect generalisation, 
or the crystallographic method must have enjoyed the proudest 
triumph. 
The result, however, of this trial of strength was very dif- 
ferent indeed. All the minerals out of the eleven which have 
been car fully re-examined by mineralogists , have been found to 
have different primitive forms from those assigned to them by 
Hauy , and in all of them has my prediction been accomplished. 
These minerals are, 
Sulphate of Magnesia. 
Chromate of Lead. 
Mesotype. 
Carbonate of Barytes. 
Carbonate of Strontites. 
Harmotome. 
Sulphate of Iron. 
Essonite. 
The remaining three, viz. Iolite , Cryolite , and Chabasie , have 
not been re-examined. Mr Brooke, however, considers that 
Cryolite may have a different primitive form. I have deter- 
