1S8 Mr Brooked Remarks on Dr Brewster'' s Reply 
a structure that defies all the laws of crystallography , must in- 
fallibly harmonise with the sound deductions of ‘ that science. 
In page 14. of Vol. VII. Dr Brewster, in alluding to the powers 
of optical analysis , says, 44 We examine the mineral in its perfect 
and undisturbed condition ; we determine its various jjroper- 
ties as modified by the elements of which it is composed , by the 
proportions in which they combine , and by the mechanical or cry- 
stallographic structure into which they are arranged : we there- 
fore determine properties , and measure actions , and observe 
structures , which vary with the elementary parts of the mineral , 
as well as with their mode of combination .” This conclusion 
will certainly not be denied to Dr Brewster, by those who admit 
his nearly identical premises. A few lines farther on, Dr 
Brewster adds, 46 In proof of these positions , I might refer in 
general to the universality of the law which I have established 
between the primitive forms of crystals, and the number of their 
axes of double refraction 
I might dismiss these passages at once by observing, that 
they are hypothetical from beginning to end. I shall, how- 
ever, make them the subject of one or two remarks. It is not 
very clear how the observed relation between the primitive 
forms of crystals , and the number of their axes of double 
refraction, should enable the observer to determine all the va- 
rious properties alluded to in the preceding extracts. In the 
first of these, it will be also seen, that Dr Brewster says, 
64 we examine the mineral in its perfect and undisturbed condi- 
tion. - ” It may, I think, be inferred from the context, that the 
term perfect here means pure , unmixed with foreign matter. 
But where is a specimen of this nature to be found for exami- 
nation ? And how many experiments has Dr Brewster made 
on perfectly pure and uncontaminated minerals ? If the term 
perfect is not to be understood in this sense, but is taken to im- 
ply no more than the actually entire state of the ci^stal which 
is examined, be its impurities and composition what they may, 
the generality of any law, derived from the application of opti- 
cal analysis, to any number of such specimens, might surely be 
received with some degree of distrust ; as we may perhaps rea- 
sonably entertain a suspicion, that the results of optical analysis 
are liable to be influenced by the accidental presence of foreign 
iHiatter in a crystal. It affords me some satisfaction to find that 
