in the Cavities of Minerals. 33 



fluid. In a crystal of quartz belonging to Mr ALLAN, and con- 

 taining a large cavity, with water and an air-bubble, he observed 

 a little black globule which adhered to the air-bubble. Upon 

 looking at it afterwards, he remarked that the black globule had 

 separated into a great number of minute black particles. This 

 opaque matter is likely to have had the same origin as that 

 which is described in page 23. 



One of the most remarkable specimens of quartz which I 

 have ever met with, was shewn to me by Mr SIVRIGHT. The 

 cavities are of the most singular shape, and are almost all nearly 

 filled with a fluid, accompanied with a small air-bubble, which 

 does not perceptibly expand with heat. Some of the cavities 

 contain a yellow fluid, with various air-bubbles, which seem to 

 be naphtha apparently in a very viscid state. This specimen is 

 shewn, though very imperfectly, in Fig. 30. 



3. TOPAZ. There are many topazes from Brazil, New Hol- 

 land, and Scotland, which contain a single fluid, with an air-bub- 

 ble. In these the fluid does not perceptibly expand with heat ; 

 and I have ascertained that it is aqueous, and that the vacuity is 

 filled with a gas. 



In several topazes, both from Aberdeenshire and Brazil, the 

 form of the cavities is extremely curious, resembling the writ- 

 ing in Eastern MSS. These grotesque forms generally contain 

 the new fluid ; but many of them have no vacuity at all, while 

 some of them contain a fluid of a decided yellow colour, which I 

 have never found accompanied with a vacuity. These cavities 

 are shewn in Fig. 31. The cavities in topaz containing the two 

 new fluids are shewn in Fig. 40. 



In a particular specimen of topaz, I observed a regular rhom- 

 boidal space apparently filled with particles of dust suspended 

 in it. This rhomboidal space appeared green by reflected, and 

 red by transmitted, light. 



VOL. x. P. i. E 



