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 fiuids 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 with 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 incommon 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. PART I. F 
