in the Cavities of Minerals. 19 



vity which it occupied appears of a brownish-blue colour, while 

 the part occupied by the second fluid is perfectly transparent. 

 This phenomenon explains, in many cases, the apparent opacity 

 of the cavities, which become perfectly transparent by inclining 

 the specimen. When the stratum of cavities is very much in- 

 clined, all of them appear like black specks, and hence they have 

 been generally considered by lapidaries as opaque particles. 



Two immiscible fluids, possessing the properties now descri- 

 bed, exist also in Quartz, Amethyst, and Cymophane, and I have 

 reason to conclude that the one never occurs without the other, 

 as I have in almost every case discovered the second fluid in ca- 

 vities, where the difficulties of observation had at first prevented 

 me from detecting it. 



SECT. III. On the Phenomena of Two Immiscible Fluids without 

 a Vacuity in the Cavities of Minerals. 



The preceding results conduct us gradually to the develop- 

 ment and explanation of phenomena, which, had they been ob- 

 served alone, would have occasioned no inconsiderable per- 

 plexity. 



In the same specimen of topaz, I have noticed the two classes 

 of cavities which form the subject of the two preceding sections ; 

 and, along with them, I have likewise found a third class, such 

 as AB, Fig. 13., which differs in no respect from those of the 

 first class, shewn in Fig. 1., when examined by the microscope 

 alone. Their difference, however, becomes very manifest by the 

 agency of heat and light. 



When heat is applied to these cavities, the circular space N, 

 Fig. 14., in place of diminishing, as it does in Fig. 1., actually 

 increases, as in Fig. 15., as if the fluid WW had contracted with 



c 2 



