of the Brazilian Topaz. 3 
3. In some specimens the colouring matter is deposited on 
the faces of a rhomb parallel to the natural faces of the prism, 
as shewn in Fig. 3; and this space sometimes extends outwards 
to the natural faces as in Fig. 4, and sometimes inwards to the 
centre, as in Fig. 5. 
4. The pink colouring matter frequently occupies only a 
part of the external rhomboidal space, as in Fig.6; and at 
other times it is arranged as in Fig.7, both the obtuse and the 
acute angles being free from the pink colour. 
5. In some crystals which are very perfect, the pink 
colouring matter is uniformly distributed as in Fig. 8. 
6. Among the mass of Topazes, amounting to some thousands, 
which I inspected, there were only éwo which possessed a 
decidedly green colouring matter. These crystals are represented 
in Figs. 10. and 12. In Fig.10. the outer portion is of an 
orange yellow colour, and the inner prism is a mixture of 
green and pale pink. In Fig. 12. the outer portion is a mixture 
of green and pale pink, and the inner portion a light orange 
yellow. When the longer diagonals of both these prisms are in 
the plane of primitive polarization, the green tint is a maximum 
as shewn in the figures. For the sake of comparison, I have 
represented in Fig. 11. the distribution of the colouring matter 
im some of the finest and most valuable of the Scotch Topazes. 
II. On the Tesselated Structure of the Brazilian Topaz, and 
the singular Superposition of its external Lamine. 
In the year 1819 Mr. Herschel informed me that he had 
observed in some Brazilian Topazes a hemitrope or tesselated 
structure, “It is a structure he remarked which never occurs 
“in fine crystals; but when properly examined, inferior spe- 
*“‘cimens often present the phenomenon represented in the figure 
“‘ (Fig. 13.), the principal sections of the different parts making 
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