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 foimd a very small quantity of the volatile 

 fluid, which, with a slight rise of temperature, passes into vapour, and prevents 

 the apparent vacuity fi'om 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 instantly withdrawn on the disappearance of the vacuity, the crystal never fails 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 I 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 ha-ue 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 flnds the readiest exit. The 

 fluid penetrates, as it were, the solid gem, and the laminae 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 heat is withdrawn when the first minute drop has passed, the laminte unite, and 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 A B C D is a 

 shallow cavity in a plate of topaz M N, and E F 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 b and c, and then passing into the empty cavity E F. 

 The small lakes, as we may call them, at a, b, 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 w^as 

 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. PAET I. D 



