2 Dr Brewster on ike Connexion between the Optical Structure 
explaining my views on these subjects to several foreign mine- 
ralogists, particularly to Major Petersen, Professor Gmelin of 
Tubingen, Professor Mohs of Prey berg, and Count Breunner, 
who took a deep interest in a subject so intimately connected 
with their own. The difficulty, however, of procuring analyses 
of the minerals and crystals which I had examined, prevented 
me from publishing in detail the results of my inquiry ; but al- 
though the same difficulty still exists to a great degree, I have 
thought it necessary to lay before the public a notice of the prin- 
cipal results which I have obtained. 
In August 1814, when I was in Paris, M. Biot presented 
to me a plate of Arragonite , from which he had determined 
that this mineral had one axis of double refraction, like Calca- 
reous-spar * *. Upon examining this plate, however, with care, 
I discovered that it had actually two axes of double refraction ; 
and as all the other specimens which I subjected to examination 
had also two axes, a result which the primitive form of Arrago- 
nite rendered necessary, it appeared to me probable that the 
Carbonate of Strontian was not an essential ingredient in this mi- 
neral-)*; since those specimens in which it did not occur, had the 
same crystalline structure as those which contained it. I re- 
commended it, however, to those who had the necessary speci- 
mens, to try if chere was any difference in the optical struc- 
ture of those Arragonites which did not contain the Carbonate 
of Strontian* 
In the beginning of the year 1817, I received from Mr 
Henry Thomson of Cheltenham a quantity of crystals of the 
Murio-sulphate of Magnesia and Iron. They had the same 
crystallographic structure as Sulphate of Magnesia, but the in- 
clination of the resultant axes was 51° 16' in the Murio-sulphate, 
whereas it was only 87° 24' in the Sulphate J. Some time after 
these experiments were made,, this salt was analysed by Mr 
Richard Phillips |j, who found it to consist of 
• See Traite de Physique , tom. Iv. p, 473, 478. 
*f* The same conclusion was afterwards deduced, on other grounds, by Hauy.=—= 
See the Journal of the Royal Institution , vol. iv. p. 112., and the Journal de Phy* 
sique, vol. Ixxxv. p, 333, & 341, October and November 1817. 
$ See Phil. Trans . 1818, p. 30. 
!J Annals of Philosophy, Jan. 1818, vol. xi. p. 30, 
