IN THE CAVITIES OF MINERALS. Lal 
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, v, other two 
bubbles, one of which v7 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. 138, 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 state. 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 difficulty ; 
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 polarising microscope, so constructed 
VOL. XVI. PART I. E 
