20 SIR DAVID BREWSTER ON THE EXISTENCE OF CRYSTALS 



turn a crystal upon its axis. The experiments of Libri and Fresnel, on the re- 

 pulsions which heated bodies exert upon each other at sensible distances, afford 

 us as little aid. They may enable us to account for the mere displacement of the 

 crystals by the application of heat, or for their sudden start from their places of 

 rest, but they do not supply us with a force fitted to give and to sustain a rapid 

 rotatory movement. 



I have already had occasion to state, that the cavities often burst when too 

 much heat is applied to the specimen. This generally talces place by a separa- 

 tion of the lamina), which fly off in splinters ; but when the burst cavity is large 

 and insulated, a piece of the solid crystal is scooped out on its weakest side. 

 Sometimes a great number of cavities explode at the same time, and when they 

 ai-e small, or exist in a part of the crystal where there are no large ones, the ex- 

 plosive force is not strong enough to separate the laminae. The fluid is merely 

 driven between the laminse tv a small distance around the cavity, and shews 

 itself as a dark brown powdery matter, encircling the cavity as the burr of a 

 comet does its nucleus. When the cohesion of the laminaj is great, it resists the 

 explosive force over a large cavity, and tlie contents of the cavity are thrown to 

 a considerable distance around it, and remains between the lamina;, either as a 

 sort of powder, or as a congeries of minute crystals, which are sometimes large 

 enough to shew their depolarising action. When the laminae separate, we find this 

 crystalline matter either fluid or indurated ; exhibiting, when fluid, the extraordi- 

 nary properties described in my former papers. If Ave breathe upon the indurated 

 matter it becomes fluid, re-crystalUzes in new spicule and crystals ; and, on 

 several occasions, I have found fine examples of circular crystallization. 



After the explosion of cavities containing only the dense fluid, I have been 

 surprised to find, and that in large cavities, that no trace of matter was left upon 

 the sides of the cavity or around it. Whether this arose, as the fact seems to indi- 

 cate, from the dense fluid being a condensed gas, or from some other cause, it 

 will require new experiments to determine. 



In a very remarkable specimen, in which the cleavage plane passed through a 

 great number of large flat cavities, the brown matter has been lodged near to the 

 edges of each cavity, and marks them them out even to the unassisted eye. 

 These cavities were filled almost solely with the volatile fluid ; and since the faces 

 of the cavities are corroded as if by the action of a solvent, developing crystalline 

 forms, there is reason to think that the fluid has exercised this action, and that 

 the phenomenon is analogous to that external action, on the faces of hundreds of 

 Brazil topazes in my possession, Avhich I have described in the Cambridge Trans- 

 actions,* and the singular optical figure formed by which, I have i-epresented in 

 a late volume of the Transactions of this Society.f 



• Cambridge Transactions, vol. ii. Plate i. fig. 15. 



t Edinburgh Transactions, vol. xiv. Plate x., fig. 1, 2. 



