6 l)r Brewster on the Connexion between the Optical Structure 
when I found a specimen of mica that had the inclination of its 
resultant axes only 14°, or in which the axis in the plane of the 
laminae was much feebler than in the Siberian mica, it became 
still more difficult to understand how irregular and confused 
crystallisation could have the power of weakening and removing 
one polarising axis, without affecting the other. The general 
law of polarisation and double refraction which I afterwards 
discovered, proved that such an explanation was inadmissible. 
From the external characters of the different Micas, I concluded 
that they were different substances, and I began to make a very 
large collection of them, for the purposes of analysis. M. Biot, 
however, seems afterwards to have abandoned this explanation, 
from having found several varieties of mica, in which the incli- 
nation of the resultant axes had different values * *, and which, 
when analysed by M. Vauquelin, exhibited a difference in their 
chemical composition -f*. 
Among some specimens of Sulphate of Nickel which I recei- 
ved from Mr Brande, and others for which I am indebted to 
Mr Badams of Birmingham, I found one set of crystals which 
had two axes of polarisation and double refraction, and in which 
the inclination of the resultant axes was 4T, and another set 
which had only one negative axis of double refraction. Nay, 
in one of these crystals, of a very uncommon size, the external 
portion had one axis of double refraction, while the central part 
of the crystal had two , the inclination of the resultant axes being 
about 3° J. The first set of these specimens, which I conceived 
to be pure sulphate of Nickel, from its having two axes, efflo- 
resced by exposure to the air ; while the second set were in no 
way affected. I transmitted both to M. Berzelius, but I have 
not yet received his analysis of them. Since this paper was sent 
to press, Dr Fyfe |) has had the goodness to analyse the speci- 
men with one axis, and he has found it to be a new triple salt, 
viz. a Sulphate of Nickel and Copper, composed as follows : 
* Memoir read before the Institute, June 22 . 1818. 
*f* It appears strange that M. Vauquelin found no fluoric acid in any of these 
specimens of mica. See this Journal , vol. iv. p. 22. 
$ The results in this paragraph are given without* any commentary in the 
tables printed in pages 211. and 230. of the Phil. Trans, for 1818. 
|j Dr Fyfe’s analysis is published in a subsequent article of this Number, p. 210. 
