550 SIR DAVID BREWSTER ON THE OPTICAL PHENOMENA OF 
sides of the hexagon, and six beautiful radiations, like those of the Asterial 
Sapphire, perpendicular to the sides of the hexagon. 
The existence of these rectilineal fissures is an important fact in crystallogra- 
phy. It proves that the crystals were in a soft state after they had attained their 
present form; and that, in the process of induration, the fissures were produced 
by the shrinking of the tourmaline, in the same manner as similar fissures are 
produced during the induration of clay. In the mica which surrounds some of 
the crystals, there is the appearance of considerable disturbance; but I can find 
no trace of any cavity from which the tourmaline may have been ejected in a 
fluid state. The faces of these crystals are not everywhere in optical contact 
with the mica, and it is very probable that they could be removed without any 
adhering mica, as I have occasionally found crystals of tourmaline that were 
moveable between the lamine. 
In the same specimen which contains these tourmalines, and in others, I have 
found, what I believe has never before been observed, the woolly filaments of the 
Penicillum glaucum of Linx, with its sporules scattered about between the lamine, 
and sometimes beautifully moniliform, as in the Penicillum glaucum obtained 
from milk by M. Turrin.* 
2. On the Distribution of Titanium in Mica. 
In examining a remarkable specimen of mica from Irkutsk, in Siberia, I 
found ¢itanium between the lamineze in various forms, sometimes in amorphous 
plates, sometimes in a powdery state adhering to the mica, and most frequently 
in beautiful dendritic forms, of various degrees of thickness. At a thickness of 
about the hundredth of an inch, the titanium, in all these forms is ‘opaque; but 
at less thicknesses, it has a brownish transparency, becoming almost perfectly 
transparent at thicknesses which do not seem to exceed the 2000th part of an 
inch. In Fig. 3 I have given a drawing of an opaque group executed for me 
with minute accuracy by my celebrated friend Mr Haminerr of Vienna, during 
his residence in Edinburgh. The transparent groups are much more beautiful 
than the opaque ones, the crystalline ramifications having the most diversified 
forms, resembling often regular organisations. 
When the mica is removed from above the titanium, so that only an exceed- 
ingly thin film of it is left, the reflected light is extremely brilliant, and consists 
of the most splendid colours. These colours, which have always the form of the 
titanium, are those which are produced by the thin film of mica which covers 
the titanium, and are not produced, as has been supposed, by a vacuity in the 
mica. 
In some specimens of mica from Bengal, the imbedded titanium is spread out 
* See Comptes Rendus, tom. vy., p. 822, 1837. Dee. 11. 

