IN THE CAVITIES OF MINERALS. 13 
2. Additional Observations on the Nature and Properties of the two New Fluids. 
In re-examining the phenomena exhibited by the two new fluids, I have 
found no occasion to modify or to correct any of the results contained in my 
former papers. In the cavities which appear to contain only one fluid, namely, 
the dense fluid, I have sometimes found a very small quantity of the volatile 
fluid, which, with a slight rise of temperature, passes into vapour, and prevents 
the apparent vacuity from disappearing by the application of a strong heat. — 
When there is no volatile fluid present in such cavities, the vacuity is a real one, 
and disappears entirely by the application of such a heat. If the heat is not 
instantly withdrawn on the disappearance of the vacuity, the crystal never fails 
to burst with great violence. 
In some specimens of Brazil topaz I have found cavities with two fluids, and 
without any vacuity in the volatile fluid at the ordinary temperature of an apart- 
ment. In such cases [ have generally produced a vacuity by the application of 
ice. Had heat been applied, the crystals would have burst, as there were no 
empty spaces into which the fluids could expand. 
When the cavities are flat, and have their faces perpendicular to the axis of 
the crystal, or parallel to the planes of easy cleavage, the application of heat does 
not burst the crystal, but produces a very remarkable phenomenon. The cavity 
opens at its weakest point, and the fluid passes by starts, through a succession of 
resting places, to another part of the crystal where it finds the readiest exit. The 
fluid penetrates, as it were, the solid gem, and the laminze which it has forced 
asunder in its passage, again close into optical if not into mechanical contact. If the 
heat is withdrawn when the first minute drop has passed, the laminze unite, and 
we can discharge the rest of the fluid whenever we please till the cavity is ex- 
_hausted. This phenomenon is represented in Plate, Fig. 9, where ABCD isa 
shallow cavity in a plate of topaz MN, and EF another cavity, which has been 
emptied of its fluid contents by reaching the surface at N, where it had been broken 
through. Upon looking at the cavity A B C D when slightly heated, I observed dark 
portions of fluid rushing from its sharp termination at D through the cavity at 
a, and then reappearing at ) and ¢, and then passing into the empty cavity E F. 
The small lakes, as we may call them, at a, 6, and c, disappeared entirely when the 
discharged portions of fluid had passed, and reappeared with a change of form 
and size when the operation was repeated. 
In a specimen of topaz possessed by Major Playfair, and seen by many indi- 
viduals, a white ball passed from one cavity to the edge of the specimen, as if 
projected from a mortar; but by the application of too strong a heat it was 
shattered in pieces. 
In my first paper of 1823,* I have described and figured a phenomenon of an 
* Edinburgh Transactions, vol. x. p. 11, Plate I. Fig. 5, 6. 
VOL. XVI. PART I. D 
