IN THE CAVITIES OF MINERALS. 19 
C depolarised white light, while the rest had no action upon polarised light. 
Upon applying heat, the crystal ¢ melted, and took up a position at ¢ Fig. 15, in 
a narrower part of the cavity, where it remains of an irregular form, having been 
repeatedly melted and re-crystallized. Upon turning the cavity into a position where 
it became transparent, I found that there was no fluid whatever in the cavity ; so 
that we have here an example of a crystal melting and re-crystallizing without 
having been dissolved in one of the fluids. From the irregular state of the la- 
minze close to this cavity, there is every appearance of the fluids having escaped 
from one of its extremities. 
In the course of these observations, I observed a phenomenon, produced by 
heat, of the most novel and surprising kind, and one which I feel myself utterly 
unable to explain. It presented itself when I was studying the very interesting 
collection of crystals in the cavity AB, Fig. 16. This cavity is filled with the 
dense fluid, in which there is a vacuity V: the fluid swells to such a degree with 
heat as to diminish very perceptibly the size of this vacuity; and as I can find 
no trace of any portion of the volatile fluid, I have no doubt that this vacuity 
would disappear by an increased degree of heat. The fear, however, of bursting 
so rare and interesting a cavity, has prevented me from making this experiment. 
The cavity contains a great number of crystals of different forms, not one of which 
melts with heat, and almost all of which possess double refraction. When I first 
submitted this cavity to the microscope, there were jive small crystals lying be- 
tween D and the vacuity V; one a flat prism, another a hexagonal plate, a third 
amorphous, and a fourth and fifth two irregular halves of a hexagon. Upon the 
first application of heat, one or two of these crystals leapt from their resting place, 
and darted to the opposite side of the cavity. In afew seconds, the others quitted 
their places one after another, performing the most rapid and extraordinary rota- 
tions. One crystal joined another, and, at last, four of them thus united revolved 
with such rapidity as completely to efface their respective shapes. They then 
separated on the withdrawal of the heat, and took the position which their gravity 
assigned them. On another occasion, a long flat prism performed the same rota- 
tion round its middle point ; and I have repeated the experiment so often, in shew- 
ing it to others, that the small crystals have been driven between the inclined 
edges of the cavity, from which I cannot extricate them. I have succeeded, how- 
ever, in conducting a fine octohedral crystal, truncated on its edges and angles, 
into the arena at D, where I have just seen it perform its rotation, as indicated 
by the concentric circles on the right hand of D. 
In seeking for the cause of so extraordinary a phenomenon, we are reminded 
of the rotations of camphor and other volatile substances; but, in this case, no 
gas or matter of any kind could be thrown off without becoming visible in the 
fluid. The pyro-electricity of topaz next suggests itself as a moving power; but 
though it might produce attractions and repulsions, we cannot see how it could 
