in the Cavities (/Minerals. 5 



terminations, and at other times their form is not far from that 

 of a circle. 



The cavities now described, are filled with a colourless and 

 transparent fluid, as shewn at ABCD, Fig, 1. Plate I., and have 

 almost always a vacuity V, of a circular form, which moves by an 

 inclination of the plate to different parts of the cavity. The 

 depth of the cavity may be easily estimated, by the breadth of 

 its bounding line ABCD, which, in the flat cavities, is generally 

 the same as that of the circle V. In very shallow cavities, this 

 boundary is a narrow line, scarcely visible, and in deep ones it is 

 broad, with a penumbral termination inwards, arising from the 

 deviation of the light at the separating surfaces of the fluid and 

 the topaz, and at that of the fluid and the vacuity. 



When the hand is applied to the crystal, the heat of it gra- 

 dually expands the fluid. The vacuity V consequently dimi- 

 nishes, and being in a short time reduced to a physical point, it 

 entirely disappears. When the fluid again cools, by withdraw- 

 ing the hand, it of course contracts, and quits the sides of the 

 cavity. The vacuity V reappears, increasing till it resumes its 

 former magnitude ; and it deserves particular notice, that the 

 evanescence and reappearance of the vacuity takes place simul- 

 taneously in many hundred cavities, of the same general form, 

 which may be seen in the field of view. 



In order to obtain an accurate measure of the temperature 

 at which the vacuity reappears, which is almost the same as that 

 at which it vanishes, I plunged the topaz in heated water, and 

 by means of an accurate thermometer, I obtained the following 

 results : 



\r A fAi n * Temperature at which the 



Nature of the Cavities. 



Vacuity reappeared. 



1. Topaz from New Holland, with shallow cavities, 74 | Fahr. 



{74 

 77 

 78 \ 

 82 



