270 Sir David Brewster on the Optical Phanomena of 



the particles of dust were seen by the microscope to consist each 

 of several spicular crystals of titanium, crossing one another at 

 angles of 60° and 30 , and forming distinct groups. In one 

 crystal there were two interior pyramids composed of these 

 groups ; and it will be seen, from the explanation which I shall 

 presently give of this phenomenon, that there may be any num- 

 ber of such pyramids. 



As the crystals of amethyst are supposed to have been pro- 

 duced by the gradual enlargement of a small crystal placed in 

 an amethystine solution, we have only to assume that a solution 

 containing titanium has been introduced into the amethystine 

 solution at different times during the growth of the crystal. 

 The small crystals of titanium will deposit themselves on each of 

 the surfaces of the pyramid ; and when the whole of the intro- 

 duced titanium has been thus deposited, the enlargement of the 

 amethyst will go on, leaving a pyramid of titanium crystals in its 

 interior. If a second solution of titanium is introduced, a second 

 pyramid of its particles will be formed in the same manner ; and 

 this process may be repeated any number of times. 



If we now suppose that the amethystine solution is exhausted 

 just when the 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 pyramid of tita- 

 nium groups will be on the veiy outside of the pyramid of ame- 

 thyst. 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 shown 

 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 cry- 

 stals 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. 



