188 Mr Brooke's Remarks on Dr Brewster's Reply 
Ing to the same species of mineral, which are termed secon- 
dary. 
But a primitive form , according to Dr Brewster’s use of the 
term, seems to be something much less definite. 
It is true, that in p. 12. Vol. VII. of this Journal, Dr 
Brewster alludes to the 66 method of crystallographic analysis, 
by which we ascertain the mechanical structure of crystals by 
cleavage, and thus obtain for each mineral species a primitive 
form , to which all its secondary forms may be referred/’ I have, 
however, endeavoured to shew, in the volume on crystallography 
referred to by Dr Brewster, that cleavage does not perform this 
crystallographic analysis, except in particular instances. For 
we can frequently obtain by cleavage from the same crystals se- 
veral forms, to any of which its secondary forms may be referred; 
and hence we may be said to obtain several primitive forms from 
one crystal. But the supposed primitive forms of Tesselite do 
not appear to have any relation even to cleavage, nor, as we shall 
presently observe, is it very evident what Dr Brewster’s notion 
of primitive form is, in reference to that substance. 
What Dr Brewster understands by a mineral species , is also 
very indistinctly seen in his papers. Berzelius considers all mi- 
nerals which are composed of similar elementary particles , united 
in equal proportions , to belong to one species. Haiiy adds to 
this chemical character the crystallographic condition, that these 
elements shall be combined into molecules which are similar in 
form in all the individuals belonging to the species. 
Perhaps Dr Brewster will either add to this definition, or sub- 
stitute for it, a condition, that the minerals comprised in one 
species shall all resemble each other in their optical properties. 
This, at least, is what we might expect in an optical system of 
mineralogy. But a crystal of Tesselite would, according to ei- 
ther of these definitions, belong to several species. 
It would appear, however, from Dr Brewster’s experiments 
on glass, which will be afterwards more particularly alluded to, 
that the optical character of every known crystal may be imi- 
tated in that substance. For, Dr Brewster asserts in p. 260. of 
the Phil. Trans, for 1818, that there is not one phenomenon 
belonging to regular crystals which he has not been able to imi- 
tate in glass. If optical phenomena, therefore, are to constitute 
specific characters among minerals, does it not follow, either that 
