IN THE CAVITIES OF MINERALS. 21 



The only chemical experiment on the contents of these cavities, which I have 

 had occasion recently to make, is perhaps worth reporting. One angle of a cavity 

 was blown off by its explosion, and though the fluids escaped, a pretty large pris- 

 matic crystal remained within the cavity. I introduced water and alcohol succes- 

 sively into the cavity, and raised them to a considerable heat; but they had no 

 effect in dissolving the crystal. 



5. On Solid Crystals and Crystalline Masses imbedded in Topaz. 



Among the new phenomena which this section embraces, there is at least 

 one intimately connected with the subject of the fluid cavities. How far the 

 other phenomena may have any such connexion, it remains to be seen. 



The imbedded crystals to which I refer, presented themselves to me while the 

 specimens which contain them were exposed to polarised 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 circum- 

 stances 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 witli all 

 the colours of polarised light. These crystals are equally transparent with the 

 topaz, with a few slight exceptions They sometimes polarise 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 opaque 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 polarising microscope. In one of these cases, the crystalline mass, 

 which is nearly spherical, lies in a crowded group of small fluid cavities, none of 

 which enters its mass ; a complete proof that the cavities were formed in the soft 

 mass of topaz, when' it encircled the indurated crystal. 



Along with these interesting phenomena, another occasionally occurs, which 

 may still require a farther examination. I have observed apparent doubly re- 

 fracting crystals, which differ in some essential points from those which have 

 been described. They depolarise a uniform, or nearly a uniform tint, notwith- 

 standing the different thicknesses through which the polarised light passes; and 



VOL. XVI. PAET I. ^ 



