552 SIR DAVID BREWSTER ON THE OPTICAL PHENOMENA OF 
titanium solution has deposited all its crystals, the completed crystal of amethyst 
will have its outer surfaces covered with spicular crystals of titanium, or the pyra- 
mid of titanium groups will be on the very outside of the pyramid of amethyst. 
I had the good fortune to find such a crystal, in which the coat containing the 
titanium is laid like varnish on all the faces of the pyramid, but only on the 
upper end of three of them; the lower end of these three faces having lain on the 
solution protected from the deposition of the titanium. This crystal is, I believe, 
unique, and possesses the great interest of exhibiting the very process by which 
it was formed. 
The two phenomena which I have just described are shewn in Figs. 4 and 5. 
5. Distribution of Titanium in Brazil Topaz. 
In examining a great number of very imperfect crystals of Brazil topaz, I 
found many which contained crystals of titanium of a brilliant scarlet colour, 
with a tinge of yellow. These crystals were perfectly transparent, and occurred 
in seven different forms. 
1. In flat amorphous plates, which were highly transparent. 
2. In hexagonal plates, lying in different planes. 
3. In transparent lines running in different directions, and, though continuous, 
lying in different planes. 
4. In lines running inwards from the margin of the specimen, and terminating 
in small flat plates. See Fig. 6. 
5. In the most remarkable symmetrical forms like sceptres or maces, resem- 
bling some of those symmetrical cavities which I had previously found in the 
white topazes of New Holland.* See Fig. 7. 
6. In some specimens the plates of titanium are actually bent, as in Fig. 8. 
7. In little groups of transparent circular plates of a scarlet colour, and hay- 
ing concentric rings. 
When light is reflected from the separating faces of the titanium and topaz, 
it is almost completely polarised; and at greater angles than that of maximum 
polarisation, colours of singular brilliancy cross the reflected images. These 
colours are doubtless connected with the fact, that at some of these faces there 
are three images of a luminous object seen by reflexion, one of the two outer ones 
being polarised oppositely to one of the double middle images, as in the case of 
the multiplication of images in composite crystals of calcareous spar. 
6. On the Crystals and Cavities in Garnet. 
In the greater number of the crystals of garnet which I have had occasion to exa- 
mine, I have found many crystals and cavities, and much amorphous matter. In 
* See Edinburgh Transactions, 1826, vol. x., Plate XX. 
t See Phil. Trans., 1815, Plate XV., Fig. 2. 
ae ee 
