8 Dr. Brewster on the Optical Properties 
In many specimens of the Brazilian Topazes, I have 
observed another substance of a very singular kind. It is of a 
brilliant red colour, and in general perfectly transparent. Some- 
times it appears in thin plates between the lamine, and some- 
times in long stripes parallel with the axis of the prism. By 
holding the neutral axis of the crystal in the plane of primitive 
polarization, and examining these red portions with polarizing 
Microscopes, I have found parts of them crystallized so as to 
produce four sectors of light round a black cross. When the 
crystal is broken, the surfaces of these red films have a high 
metallic lustre like Realgar or Cinnabar; but I have not been 
able to collect enough of the substance to determine what it is. 
V. On the probable Difference in the Chemical composition 
of the Brazilian and other Topazes. 
When I had ascertained the very marked difference between 
the optical properties of the Brazilian Topazes, and those of 
Saxony, New Holland and Scotland, I could not for a moment 
doubt that a difference would be found in their chemical com- 
position. I accordingly sent specimens, that I had examined, to 
M. Berzelius, with the request that he would favour me with an 
analysis of them. This distinguished chemist, however, had 
previously analysed other specimens of the same minerals, and 
he informed me, “that his analyses gave exactly the same re- 
“sults for the New Holland and Brazilian Topazes, with the 
“exception of a small quantity of hydrate of iron, with which 
“the latter was coloured, and which, when decomposed by heat, 
“gave the fine pink colour to burned Topaz in consequence of 
“‘ the oxide of iron being set at liberty.” 
High as M. Berzelius’s authority undoubtedly is, I cannot 
avoid placing the most unbounded confidence in the general 
principle, that every difference in optical structure is accompanied 
