16 SIR DAVID BREWSTER ON THE EXISTENCE OE CRYSTALS 



moveable. Some of the fixed crystals are often beautifully crystallized. They 

 have their axes of double refraction coincident with those of the crystal, and, as I 

 have ascertained by the examination of exploded cavities, they actually form part 

 of the solid topaz, though they exist in the fluid cavity. One or two of these are 

 shewn in Fig. 4, Plate XIX., of my paper of 1826,* and they may be distinguished 

 by their attachment to the sides of the cavity. In the same figure, as well as in 

 Figs. 10, 13, 20, and 21 of my Paper of 1823,f I have drawn others which I then 

 believed to be fixed, but which I have no doubt are moveable, and produced from 

 one or other of the new fluids. 



In re-examining my specimens of topaz, I have been surprised at the great 

 number of cavities which contain crystals. In some there are only one ; in very 

 many there are two, three, and four ; and in a great number of specimens the ca- 

 vity is so crammed with them, like a purse full of money, that the circular vacuity 

 has not room to take its natural shape, and often can scarcely be recognised, in its 

 broken-do-mi condition, among the jostling crystals. 



The crystals of which I am treating are sometimes found in the volatile, and 

 sometimes in the dense fluid, but chiefly in the latter. They are often found in 

 an amorphous state in the narrow necks and narrow extremities of cavities, posi- 

 tions in which they remain fixed while they continue solid ; and sometimes re- 

 gularly formed crystals remain fixed between the prismatic edges of cavities, in con- 

 sequence of having either fallen into that position, or of having been formed there. 



The crystals in topaz cavities are, in general, beautifully crystaUized, and have 

 a great variety of forms. I have observed the following : — 



1. The Tetrahedron. 



2. The Cube. 



3. The Cube, truncated on its edges and angles. 



4. The Rhombohedron. 



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 would be desirable to submit these crystals, as well as the fluids 

 which contain them, to chemical analysis, yet the task is too difficult to be ac- 



* Edinburgh Transactions, vol. x. | Ibid., Plates I. and 11. 



