in the Cavities of Minerals . 95 
Sect. VI. — On the Phenomena of the two New Fluids when 
taken out of the Cavities. 
VII.— On the Existence of Moveable Crystals in a Fluid 
Cavity of Quartz. 
VIII. — On die Phenomena of a single Fluid in the Ca- 
vities of Minerals and Artificial Crystals. 
Sect. I. On the Existence of a New Fluid in the Cavities 
of Minerals . 
While examining the cavities of crystallised bodies, our au- 
thor observed such remarkable differences in the phenomena of 
the fluids which they enclosed, that he found it impossible to 
explain them upon the supposition of their being fluids possess- 
ing the ordinary properties of that class of bodies. Hence he 
was led, by the following train of reasoning, to ascribe these phe- 
nomena to new fluids, possessing new physical properties. 
In examining the Topazes from New Holland, Scotland and 
Brazil, he observed the cavities arranged in strata. These ca- 
vities are sometimes beautifully crystallised, and sometimes 
amorphous, sometimes extremely shallow, and at other times 
deep. 
They are filled with a colourless and transparent fluid* as 
shewn at ABCD, Fig. 1 . Plate II., and have almost always a 
vacuity V, of a circular form, which moves by an inclination of 
the plate to different parts of the cavity. The depth of the ca~ 
vity may be easily estimated, by the breadth of its bounding 
line ABCD, which, in the flat cavities, is generally the same as 
that of the circle V. In very shallow cavities, this boundary is 
a narrow line, scarcely visible, and in deep ones it is broad, 
with a penumbral termination inwards, arising from the devia- 
tion of the light at the separating surfaces of the fluid, and the 
topaz, and at that of the fluid and the vacuity. 
When the hand is applied to the crystal, the heat of it gra- 
dually expands the fluid. The vacuity V consequently dimi- 
nishes, and being in a short time reduced to a physical point, it 
entirely disappears. When the fluid again cools, by withdrawing 
the hand, it of course contracts, and quits the sides of the cavity. 
The vacuity V reappears, increasing till it resumes its former 
