on the Optical System of Mineralogy. * 867 
character , must convince those who have studied the subject, that 
he entertains very inadequate conceptions of those singular optical , 
and physical , and crystallographic differences, which separate the 
Tesselite from all other substances in nature. What would 
we say of a traveller, who should maintain that a house built 
with a variety of kinds of granite, and consisting of apartments 
beautifully and symmetrically arranged, was the same thing with 
one of the granite blocks which rest upon the flanks of Jura? 
In this comparison, we give the traveller the advantage of ha- 
ving the angles as well as the composition of both the masses 
assumed as identical ; but our surprise would be still greater, if 
he should persist in his opinion, when a chemist should inform 
him, that not one of the supposed granites which compose the 
house had ever been separately analysed. 
We come now to Mr Brooke’s objection to the optical sys- 
tem, as derived from the Sulphato-tri-carhonate of lead. 
“ A paragraph (says he) published by Dr Brewster in the 6th vo- 
lume of the same Journal, relative to the crystalline form of the suU 
phato-tri-carbonate of lead, furnishes an additional motive to believe 
that the connection between the optical character of minerals, and 
their crystalline forms, is not yet sufficiently understood. 
“ Dr Brewster admits, what I believe is not liable to question, 
that the crystals of this substance are acute rhomboids . But, he adds, 
“ Upon examining their optical structure, I find that they have two 
axes of double refraction, the principal one of which is coincident 
with the axis of the rhomb. The sulphato-tri-carbonate, therefore, 
cannot have the acute rhomboid for its primitive form, but must belong 
to the Prismatic System of Mohs” 
“ But it appears,” adds Mr Brooke, “ from the outline of Pro- 
fessor Mohs’ new System of Crystallography, published in vol. iii» 
of the same Journal, that a rhomboid cannot belong to his Prismatic 
System. For, it is stated in p. 173., “that the rhomboid and the 
four-sided oblique based pyramid (the fundamental form of the Pris- 
matic System), are forms which cannot by any means be derived 
from each other ; the ( two ) groups of simple for?ns, as well as their 
combination , must each be always distinct from the (other) *. 
“ If, therefore, in the hands of Dr Brewster, the use of optical 
characters cannot at present be relied upon for the determination of 
a mineral species, it may be doubted whether they can be success- 
fully employed by less accurate and less intelligent observers.” 
* Had Mr Brooke read my papers on the subject on which he writes, he would 
have found (See Wernerian Transactions, vol, iii. p. 341.) that I admit all this. 
None 
