to his Observations on the Optical System of Mineralogy. 20 1 
measured in distilled water, and have found the smali ones pro- 
duced from it correspond in measurement with the large crystal. _ 
The large and small crystals are therefore similar in external 
O %j 
form. 
In the Phil. Trans, for 1816, Dr Brewster announces the 
discovery, that muriate erf soda , fluorspar , and diamond , 
which belong to the Tessular system of Mohs, and hence ought 
to possess no double refraction , may receive that property by 
compression or dilatation. And in the volumes for 1816 and 
1818, it is stated, that all the properties of regular crystals 
might be communicated permanently to glass , and other crystal- 
lized bodies , by heat. 
The experiments made by Sir Humphry Davy, on the state 
of water and aeriform matter, in cavities found in certain crys- 
tals, and detailed in a communication printed in the Phil. Trans, 
for 1822, afford some ground to believe that crystals may some- 
times be formed under different states of both pressure and tem- 
perature : And hence, judging from the experiments just refer- 
red to, on fluor-spar, glass, &c., we may conceive, that minerals 
which are chemically similar , but crystallized under different 
circumstances, may exhibit considerable differences in their opti- 
cal phenomena, without possessing any other specific distinction. 
But Dr Brewster’s experiments upon glass, and upon muriate 
of soda , fluorspar, diamond , & c. appear to affect the very foun- 
dation of his system, which professes to distinguish the primi- 
tive forms of crystals by their optical phenomena. Glass, in 
its ordinary state, is denominated by Dr Brewster an uncrys- 
tallized body , and consequently can have no primitive form , 
in any sense of that term ; but, by heat, he says it becomes 
crystallized , and may be impressed permanently with the 
doubly refractive property of crystals, so as to possess one or 
two axes of double refraction. Does not the system require us 
to admit, that it acquires at the same time the primitive forms 
of the crystals whose optical phenomena it is made to represent. 
For otherwise the certainty of the indication of primitive 
forms by optical phenomena must be abandoned. According to 
Dr Brewster’s system, we must also suppose that muriate of 
soda , fluorspar , the diamond , &c. acquire bv pressure or dila- 
tation new primitive forms , with their new optical character , 
