in the Cavities of Minerals. 503 



3. On the Form and Position of Crystals in the Cavities of 



Topaz. 



In a former paper I have described a moveable group of 

 crystals of carbonate of lime, which I discovered in a cavity 

 in quartz from Quebec, containing a fluid with the properties 

 of water. The crystals to which I am about to call attention 

 are of a very different kind, and possess a very different kind 

 of interest. 



The crystals which occupy the fluid cavities of topaz are 

 either fixed or moveable. Some of the fixed crystals are often 

 beautifully crystallized. They have their axes of double re- 

 fraction coincident with those of the crystal, and, as I have 

 ascertained by the examination of exploded cavities, they ac- 

 tually form part of the solid topaz, though they exist in the 

 fluid cavity. One or two of these are shown in fig. 4, plate 

 19, 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 

 1823tj 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 sur- 

 prised 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 

 cavity is so ci*ammed 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-down con- 

 dition, 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, positions in 

 which they remain fixed while they continue solid ; and some- 

 times regularly formed crystals remain fixed between the pris- 

 matic edges of cavities, in consequence of having either fallen 

 into that position, or of having been formed there. 



The crystals in topaz cavities are, in genei'al, beautifully 

 crystallized, and have a great variety of forms. I have ob- 

 served the following : — 



1. The tetrahedron. 



2. The cube. 



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



4. The rhombohedron. 



• Edinburgh Transactions, vol. x. f Ibid, plates 1 and 2. 



