200 Mr Brooke's Remarks on Dr Brewster s Reply 
which evidently did not relate merely to variations of' colour or 
of transparency , or of' other accidents of light which are fami- 
liar to mineralogists , but to that class of' optical phenomena 
which had induced Dr Brewster to separate tesselite from the 
apophyllites , and, therefore , to Dr Brewster's Optical System 
generally . 
The salt of Nickel, already alluded to, is one of the Sulphates 
described in the Annals of Philosophy for December 1823. 
The form of this salt is a square prism , and hence ought to 
possess only one axis of double refraction, according to one of 
Dr Brewster’s optical systems ; but, according to his theory, 
which assigns two axes to all the sulphates , it should possess two 
axes. When Dr Brewster has examined the crystals of this 
salt which have been sent him by Mr Cooper, he will be en- 
abled to state whether its optical character be at variance with 
any of his theories, and, if so, whether one of them must not 
give way. 
Another fact which impugns Dr Brewster’s optical system, re- 
lates to the Ferro-prussiate of potash. This salt was examined 
both by Mr Levy and myself, and found, by goniometrical 
measurement, to have for its primary form an octahedron with 
a square base, which, on reference to Mohs’s classification of 
crystals, will be found to belong to his pyramidal system. Dr 
Brewster states, in No. XVIII. of this Journal, p. 371., that the 
optical method places this salt in the prismatic system , and he 
adds, “ Mr Levy will, we have no doubt , see reason to correct his 
determination.” 
Dr Brewster has, however, rendered this correction unneces- 
sary, by the discovery that some of the crystals possess one , and 
others two axes of double refraction. 
The fact is thus announced, in a letter to Mr Cooper. “ Per- 
haps you could explain to me a fact regarding prussiate of pot- 
ash, which is rather perplexing. The large crystals, which are 
generally very compound in their structure, have, for their pri- 
mitive form the octahedron with a square base ; but when they 
are recrystallized, the most perfect of the small crystals belong 
to the prismatic series , and have two axes of double refraction ” 
On this discovery being announced, I dissolved a portion of 
a very large crystal of this substance which had been previously 
