366 Dr Brewster’s Reply to Mr Brooke's Observations 
graphy concur so perfectly as they do in this instance, in determin- 
ing the species to which a mineral belongs, it will be difficult to ad- 
mit a variation of optical character as a sufficient ground to alter that 
determination.” 
This syllogism is full of errors, and all its parts are incorrect. 
It is assumed, 
1. That Tesselite and Apophyllite are chemically the same. 
& That they are crystallographically the same. 
3. That Tesselite is only marked by a variation of optical 
character. 
The first of these propositions has been already shewn to be an 
assumption ; the second is positively erroneous ; and the third 
is in every point of view incorrect. 
When Mr Brooke says that Tesselite and Apophyllite are 
crystallographically the same, he must mean not only that they 
have exactly the same angles, but that they have the same clea- 
vages, and the same primary form. Nothing, however, is said 
about cleavage, but the identity of crystallographic structure is 
inferred from equality of angles. Mr Brooke will, no doubt, 
be surprised to learn (what a perusal of my papers on Apophyl- 
lite could have informed him of long ago) that Tesselite pos- 
sesses cleavage planes, and faces of composition , totally different 
from those of ordinary Apophyllite , and totally irreconcileable 
with the crystallographic structure which he assigns to it. These 
faces of composition are not inferred from optical phenomena ; 
they are visible by common light , and with the aid of a simple 
microscope , — an instrument which ought to be more frequently in 
the hands of crystallographers. Mr Brooke has, therefore, not 
determined, because he has not studied, the crystallographic struc- 
ture of Tesselite. The note which we have reprinted at the foot of 
this and the preceding page, affords a demonstration that he has 
no more idea of the structure of Tesselite than he has of one of 
the fixed stars. The very phrase, indeed, by which Mr Brooke 
marks the difference of the two minerals, viz. a variation of optical 
crystals ; so that a line passing through these in the direction of their greatest 
lengths, would in fact be perpendicular to the axis of the primary form. Sections 
perpendicular to the axes of these apparently similar prisms, would certainly pre- 
sent very different optical phenomena. But it is not probable that the practised 
eye of Dr Brewster should have been misled by their apparent similarity, and the 
differences he has observed will still remain to be explained.” 
2 
