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. 7 
I have already had occasion to state, that the cavities often burst when too 
much heat is applied to the specimen. This generally takes place by a separa- 
tion of the laminze, 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 
are 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 laminze. The fluid is merely 
driven between the laminee to 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 lamine is great, it resists the 
explosive force over a large cavity, and the contents of the cavity are thrown to 
a considerable distance around it, and remains between the laminee, 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 lamin separate, we find this 
crystalline matter either fluid or indurated; exhibiting, when fluid, the extraordi- 
nary properties described in my former papers. If we breathe upon the indurated 
matter it becomes fluid, re-crystallizes 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 avery 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, which I have described in the Cambridge Trans- 
actions,* and the singular optical figure formed by which, I have represented in 
a late volume of the Transactions of this Society.t+ 
* Cambridge Transactions, vol. ii. Plate i. fig. 15. 
t Edinburgh Transactions, vol. xiv. Plate x., fig. 1, 2. 
