in the Cavities of Minerals, 11 



While I was applying a heat not above 170 to some of these 

 specimens, they frequently leapt from the plate of glass on which 

 they lay, and at other times threw off, with an explosion, consi- 

 derable fragments. In one of these experiments, I was fortunate 

 enough to observe a phenomenon which will be considered a 

 very remarkable one. When the compound microscope was ad- 

 justed to a distinct view of a stratum of globules containing the 

 new fluid, but particularly to a vast number of minute specks, 

 which the microscope had not power to resolve, a heat of about 

 150, which happened to be applied to the specimen, produced 

 a sort of crackling noise, which arose from the bursting of the 

 cavities near the surface. Upon looking into the microscope, I 

 was astonished to observe a great number of darkish brown glo- 

 bules rising through the solid quartz, like globules of air in wa- 

 ter. In examining them more minutely, I observed, as shewn 

 in Fig. 5., that they took their origin from the minute specks or 

 cavities, which gradually enlarged and went off in the form of a 

 globule. This phenomenon lasted fully five minutes, when the 

 specimen burst into two or three pieces. While examining the 

 cavities of one of the fragments, I found that several of the large 

 ones, with flat faces, had been emptied of their contents, through 

 a fissure parallel to their flat faces, and that the faces of the fis- 

 sure had closed up, so as to transmit a brownish light, while a 

 bright light was freely transmitted through the polished faces of 

 the cavities as shewn in Fig. 6 *. 



Had only one of the cavities shewn in Fig. 6. existed, the 

 fluids which it contained might have escaped through a narrow 

 fissure, not wider than its own breadth ; and this narrow fissure 



' In a specimen of Topaz which split in the fire, I found that a quantity 

 of the new fluid had got into a fissure, where it has been permanently detained 

 without reaching the surface. It exhibits the same brown tint as the globules, at 



particular inclinations. 



* 



