500 Sir David Brewster on the Existence of Crystals 



no volatile fluid present in such cavities, the vacuity is a real 

 one, and disappears entirely by the application of such a heat. 

 It* the heat is not instantly withdrawn on the disappearance of 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 apartment. 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 have their faces perpendi- 

 cular to the axis of the crystal, or parallel to the planes of 

 easy cleavage, the application of heat does not burst the cry- 

 stal, but produces a very remarkable phaenomenon. 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 laminae which it has forced 

 asunder in its passage, again close into optical if not into me- 

 chanical contact. If the heat is withdrawn when the first 

 minute drop has passed, the laminae unite, and we can 

 discharge the rest of the fluid whenever we please till the 

 cavity is exhausted. This phaenomenon is represented in 

 Plate III. fig. 1, where ABCD is a 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 ABCD 

 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 EF. The small lakes, as we may call them, at «, 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 individuals, 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 182:3*, I have described and figured 

 a phaenomenon of an analogous kind ; but as it appeared un- 

 expectedly, and was instantly followed by the explosion of the 

 crystal, I could neither observe it accurately, nor confirm what 

 I did observe, by a repetition of the experiment. I have, 

 therefore, some satisfaction in describing a similar phaeno- 

 * Edinburgh Transactions, vol. x. p. 11, plate 1. fig. 5, 6. 



