IN THE CAVITIES OF MINERALS, 



17 



complished in the present state of chemical science. I must, therefore, limit my 

 observations to such of the physical properties of these crystals as can be rendered 



visible to the eye. 



When I first applied heat to the crystals under consideration, I employed a 

 very fine specimen, with large and numerous crystallized cavities, of a prismatical 

 form, containing both the new fluids. In this specimen, there were seven cavities 

 unlike all the rest, and each of them containing a single crystal, and apparently 

 but one fluid, namely, the dense one. The cavities were exceedingly flat, and 

 irregular in their shape, and very unlike one another. Upon applying the heat of 

 only a lighted paper match beneath the plate of glass on which the specimen lay, 

 I was surprised to see the crystals gradually lose their angles, and then slowly 

 melt, till not a trace of them was visible. In this state, one of the cavities had 

 the appearance shewn in Fig. 11, where V was the vacuity, and v, z', other two 

 bubbles, one of which v soon joined the principal one V. In all the other six 

 cavities^ the crystals were speedily reproduced, always at the point where they 

 disappeared, provided a small speck remained unmelted ; but otherwise in different 

 parts of the cavity. In the cavity A B, however, Fig. 11, the crystal was very long 

 in appearing. In the course of an hour, however, a fasciculus of minute crystals 

 appeared in the centre of the vacuity, as in Fig. 12, and to them the principal 

 crystal attached itself, as in Fig. 13, which exhibits a perfect rhomboidal plate, 

 truncated on its obtuse angles. The elliptical vacuity was pressed into the shape 

 of a heart ; and, by the application of ice, I succeeded in precipitating the vapour 

 of the expansible fluid, which existed in a very minute quantity in all the seven 

 cavities. The expansible fluid is shewn between the two heart-shaped outlines 

 in the figure, and I repeatedly threw it into vapour, and reduced that vapour to 

 a fluid st^ate. The phenomenon now described, of the melting of the crystals, and 

 their subsequent re-crystallization, I have shewn to various persons ; and it is very 

 remarkable that they generally reappear in this specimen of the same form, though 

 with considerable modifications. 



Upon applying heat to other cavities, containing several crystals, I obtained 

 very different results. Some of them melted easily, others with greater diificulty ; 

 and some were not in the slightest degree affected by the most powerful heat I 

 could apply. When the crystals melted easily, they were as quickly reproduced ; 

 sometimes reappearing more perfectly formed than before, but frequently running 

 into amorphous and granular crystallizations. 



In some specimens of topaz, all the crystals in the cavities refuse to melt 

 with heat, and seem not to suffer the slightest change in their form. Hence we 

 are entitled to conclude, that the crystals possessing such different properties 

 must be different substances ; and this conclusion is amply confirmed by an exa- 

 mination of their optical properties. 



In making this examination, I used a polarismg microscope, so constructed 



VOL. XVI. PART I. ^ 



