Properties of the Two New Fluids in Minerals. 133 



when it first flows out of its cavity upon the surface of the 

 plate of topaz which contains it, that I have nothing to add 

 upon this subject.* It did not then occur to me that these 

 movements might be owing to electricity, till I read an ac- 

 count of the following experiment made both by Professor 

 Erman and Mr Herschel. When a globule of water, drop- 

 ped on the surface of a flat dish of mercury, is brought into 

 connection with the positive pole of a galvanic battery, while 

 the mercury is connected with the negative pole, it instantly 

 flattens and spreads to twice its diameter, regaining its former 

 sphericity when the circuit is broken. This extension and 

 subsequent re-aggregation of the globule of water, is precise- 

 ly the same effect as that exhibited by the drop of expansible 

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

 to an electrical cause. In separating the particles of bodies, 

 electricity is always produced ; and in the cleavage of topaz 

 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 

 undergoing during its rapid evaporation, and its partial con- 

 version into a powdery residue. 



5. On some Miscellaneous Phenomena connected with the For- 

 mation of Fluid Cavities. 



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

 single fluid in the cavities of various minerals and artificial 

 crystals. Since that paper was written, I have seen many 

 specimens of this kind ; but as the fluid has always, when ex- 

 amined, been found to be water, such specimens possess no 

 peculiar interest, unless their cavities are opened, in the man- 

 ner first adopted by Sir Humphry Davy. One of these spe- 

 cimens, however, which was kindly sent to me for exami- 

 nation by W. C. Trevelyan, Esq. is so peculiar as to deserve 

 notice. In the drawing of it, in Fig. 13, which is of the 



• Some of the fluids in quartz seem to be entirely gaseous, while in 

 sulphate of bary tes, it appears to the mineral itself in a fluid state. — See 

 p. 134, note, and note on p. 135. 



