268 Sir David Brewster on the Optical Phenomena of 



most interesting specimen of this kind I owe to Professor Fle- 

 ming, who pointed out to me one of the peculiarities which it 

 contains. This specimen is accurately represented, of the natural 

 size, in fig. 2. The largest of the five crystals is 0'28 of an inch 

 broad, and the smallest 0*08 of an inch. Their thickness can- 

 not greatly exceed the thousandth of an inch, and yet it is with 

 difficulty that the strongest sun-light can be seen through them. 

 The form of the smallest is a perfect hexagon, and in the rest 

 the same form is more or less distinct. In the oval crystal 

 there are numerous holes, and in all of them there are numbers 

 of rectilineal cracks parallel to the sides of the hexagon, and 

 some of them so narrow that light can scarcely pass through 

 them. When we look at the sun through one of these crystals, 

 a curious optical phenomenon is seen ; a luminous hexagonal 

 surface, composed of lines of light, parallel to the 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 crystallography. It proves that the crystals were in a soft 

 state after they had attained their present form j and that, in 

 the process of induration, the fissures were produced by the 

 shrinking of the tourmaline, in the same manner as similar fis- 

 sures 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 

 laminae. 



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 Penicillium glaucum of Link, 

 with its sporules scattered about between the laminse, and some- 

 times beautifully moniliform, as in the Penicillium glaucum ob- 

 tained from milk by M. Turpin*. 



2. On the Distribution of Titanium in Mica, 



In examining a remarkable specimen of mica from Irkutsk in 

 Siberia, I found titanium between the laminse 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 

 * See Comptes Rendus, vol. v. p. 822, December 11, 1837. 



