in the Cavities of Minerals. 509 



5. On Solid Crystals and Crystalline Masses imbedded in 

 Topaz. 



Among the new phaenomena which this section embraces, 

 there is at least one intimately connected with the subject of 

 the fluid cavities. How far the other phaenomena may have 

 any such connexion, it remains to be seen. 



The imbedded crystals to which I refer, presented them- 

 selves to me while the specimens which contain them were 

 exposed to polarized light. Mineralogists have been long 

 familiar with the beautiful crystals of titanium, imbedded in 

 quartz, and I have found the same mineral imbedded under 

 still more interesting circumstances in the Brazilian amethysts. 



In topaz, however, the imbedded crystals have never been 

 noticed, and I have fortunately obtained specimens, in which 

 they are displayed with singular beauty. Their axes of double 

 refraction are not coincident with those of the topaz; and 

 hence they are seen in the obscure field of the microscope 

 splendent with all the colours of polarized light. These cry- 

 stals are equally transparent with the topaz, with a ^evt slight 

 exceptions. They sometimes polarize five or six orders of 

 colours ; and, in general, they have very beautiful crystalline 

 forms, which can be seen by the microscope in common light. 

 In some cases they are mere crystalline masses, often of a 

 reniform shape, but still with regular axes of double refraction. 



In some specimens of Brazil topaz, the crystals occur in 

 branches or groups of singular beauty, consisting of prisms and 

 hexagonal plates, connected apparently by filaments of some 

 opake matter. 



I have occasionally met with another interesting variety 

 of them, which have no visible outline by common light, and 

 which could never have been detected but by the polarizing 

 microscope. In one of these cases, the crystalline mass, which 

 is nearly spherical, lies in a crowded group of small fluid ca- 

 vities, none of which enters its mass ; a complete proof that 

 the cavities were formed in the soft mass of topaz, when it en- 

 circled the indurated crystal. 



Along with these interesting phaenomena, another occasion- 

 ally occurs, which may still require a further examination. I 

 have observed apparent doubly refracting crystals, which differ 

 in some essential points from those which have been described. 

 They depolarize a uniform, or nearly a uniform tint, notwith- 

 standing the different thicknesses through which the polarized 

 light passes ; and that tint is less brilliant than in the real im- 

 bedded crystals. I conceive, therefore, that they are crystal- 

 lized cavities, having their inner surfaces coated with a doubly 



