Dr Brewster on the Optical Analysis of Minerals. 13 
their axes of double refraction ; and to distinguish mineral spe- 
cies by differences in the position of the axes of .double refrac- 
tion ; — in the nature of these axes, whether positive or nega- 
tive ; — in the absolute intensity of their action upon light, when 
the crystals have one or two axes; — in the dispersive power of 
the axes ; — in the relative intensity of the axes, when they have 
two ; — or in the imperfect equilibrium of the axes when they 
have three ; — and by detecting in polarised light compound and 
remarkable structures, which neither the crystallographer nor 
the chemist have the means of discovering. 
Of these three methods (f analysis^ we conceive the crystallo- 
graphic method to hQ tho least perfect; the chemical method 
next to it ; and the optical method decidedly superior to both. 
It appears from the beautiful experiments of M. Mitscherlich, 
that there are certain bases which he calls isomorphous^ such as 
the neutral phosphates and arseniates, which are substituted, as 
it were, in place of each other in mineral bodies, while the ex- 
ternal form of the mineral remains the same. If this result is 
rigorously true, the crystallographic method must be considered 
as incapable of affording essential characters of all such mineral 
relation between the optical structure and Chemical Composition of different kinds 
of mica were begun by M. Biot. M. Berzelius has, no doubt, been misled by the 
circumstance of M. Biot’s paper appearing in the Memoirs of the Institute for 1816, 
although it was neither written till 1818, nor read till the 22d June 1818, and 
of its not containing the slightest allusion to my previous labours. This singu- 
lar anachronism in the Transactions of a public body, has a tendency to do 
much mischief, as no person is likely to suspect that Memoirs for 1816 will con- 
tain papers written in 1818. In the Transactions of the Royal Society of London, 
the effect is quite the reverse, as the volumes never can contain papers of a subse- 
quent date. In the volume for 1818, for example, the paper of mine, which 
stands in contrast with that of M. Biot bears the date of June 1. 1817, and was in 
Sir Joseph Banks’s possession iii December 1817. The experiments themselves 
w'ere signed by Sir George Mackenzie on the 24th November 1816 ; — the 2d De- 
cember 1816, and the 6th January 1817. The general results of these experiments 
were also communicated to M. Biot personally, when he w'as in Scotland, in the 
summer of 1817. If M. Berzelius will therefore have the goodness to compare 
my papers in the Phil. Trans- 1818, p. 223, 224, 225, 231., and the remarks in 
this Journal., Vol. V,, p. 1., w'ith M. Biot’s Traite de Physique^ vol. iv. p. 553, 
554., published in 1816, and his Memoir on Mica, published in 1818, he will 
form a correct opinion on the point under consideration. In an early number of 
this w'ork, we shall have an opportunity of rectifying these and other errors rela- 
tive to this branch of optics, by the publication of various documents. 
