424 Dr BREWSTER on the Refractive Powers and other 



is precisely the same effect as that exhibited by the drop of ex- 

 pansible fluid ; and it is therefore very likely that the latter is 

 owing to an electrical cause. In separating the particles of bo- 

 dies, electricity is always produced ; and in the cleavage of To- 

 paz and Mica, even electric light is developed. But experiments 

 are still wanting to determine, whether, in the present case, the 

 electricity is derived from the separation of the cleavage planes, 

 or from the change of condition which the new fluid is under- 

 going during its rapid evaporation, and its partial conversion in- 

 to a powdery residue. 



5. On some Miscellaneous Phenomena connected with the For- 

 mation of Fluid Cavities. 



In my former paper, 1 have described the phenomena of a 

 single fluid in the cavities of various minerals and artificial crys- 

 tals *. Since that paper was written, I have seen many specimens 

 of this kind ; but as the fluid has always, when examined, been 

 found to be water, such specimens possess no peculiar interest, un- 

 less their cavities are opened, in the manner first adopted by 

 Sir HUMPHRY DAVY. One of these specimens, however, which 

 was kindly sent to me for examination by W. C. TREVELYAN, 

 Esq. is so peculiar as to deserve notice. In the drawing of it, 

 in Plate XX. Fig. 8., which is of the real size, A B is a cavity 

 in quartz, which is filled with a fluid, excepting the vacuity a It, 

 which may be made to move to different parts of the cavity. The 



* Mr WILLIAM NICOL, Lecturer on Natural Philosophy and Chemistry, has 

 shewn me some fine specimens of Amber containing cavities. The inner surface of 

 these cavities is rough, like finely ground glass, and many of them contain a fluid 

 with a moveable globule of air. In a specimen of calcareous spar, in the possession 

 of Mr SANDEBSON, there is a fluid cavity about two inches long, an inch wide, and 

 one-eighth of an inch deep. 



