I. On the Distribution of the Colouring Matter, and 
on certain peculiarities in the Structure and Optical 
Properties of the Brazilan Topaz. 
By DAVID BREWSTER, L.L.D. F.R.S. L. & E. 
AND HONORARY MEMBER OF 
THE CAMBRIDGE PHILOSOPHICAL SOCIETY. 
[Read May 6, 1822.]} 
In the early experiments which I made upon Topaz,* I em- 
ployed some very fine plates of the Blue Topaz of Aberdeenshire 
with the natural polish of their cleavage planes. ‘The colourless 
Topazes of New Holland, which I subsequently examined, dif- 
fered very slightly from those in their optical properties; and in 
comparing with them the Topazes of Brazil, I proceeded no 
farther than to investigate the absorbent power of the latter, and 
the changes which were superinduced by heat on the matter with 
which they were coloured.+ The Brazilian Topazes which I used 
in these experiments were all crystallized, and did not exhibit 
any of the phenomena which I have since discovered during the 
examination of a large quantity of these crystals, which accident 
put into my possession. 
RAE Oh ie eee ES 1 Eee Bae ii) ee) 2 
* Phil. Trans. Lond. 1814. p. 202. 
+ Id. Id. 1819. pp. 19, 125, &e. 
Vol. If. Part L. A 
