' 504 Sir David Brewster on the Existence of Crystals 



5. The prism, with plain and pyramidal summits. 



6. The flat octohedron, truncated on its edges and angles, 



7. Rhomboidal plates. 



8. Hexagonal plates. 



9. Long rectangular plates. 



Besides these, there are amorphous crystals and crystallized 

 masses of various characters. 



4. On the Physical Properties of the Crystals in Topaz Cavities. 



Although it vk^ould be desirable to submit these crystals, as 

 well as the fluids which contain them, to chemical analysis, 

 yet the task is loo difficult to be accomplished in the present 

 state of chemical science. I must therefore limit my obser- 

 vations 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 considera- 

 tion, I employed a very fine specimen, with large and nume- 

 rous crystallized cavities, ofaprismatical form, containing both 

 the new fluids. In this specimen there wei'e seven cavities 

 unlike all the rest, and each of them containing a single cry- 

 stal, 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 appear- 

 ance shown in fig. 3, where V was the vacuity, and v^ x/ other 

 two bubbles, one of which v soon joined the principal one V. 

 In all the other six cavities the crystals were speedily repro- 

 duced, always at the point where they disappeai'ed, provided 

 a small speck remained unmelted ; but otherwise in diflferent 

 parts of the cavity. In the cavity AB, however, fig. 3, 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. 4, and to them the principal crystal 

 attached itself, as in fig. 5, 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 expan- 

 sible fluid, which existed in a very minute quantity in all the 

 seven cavities. The expansible fluid is shown 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 

 phaenomenon now described, of the melting of the crystals, 

 and their subsequent recrystallization, I have shown to various 



