IN THE CAVITIES OF MINERALS. 19 



C depolarised white light, while the rest had no action upon polarised light. 

 Upon applying heat, the crystal c melted, and took up a position at c Fig. 15, in 

 a nari-ower part of the cavity, where it remains of an irregular form, having heen 

 repeatedlymelted and re-crj'stallized. Upon turning the cavityinto 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- 

 minae 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 surprishig 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 A B, Fig. 16. This cavity is fiUed 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 smaU 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 a few 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 tho 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 octoheriral 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 



