Properties of Two New Fluids in Minerals. 417 



one immediately below it, and connect the whole together as in 

 Fig. 2. We do not mean to say, that the cavities bC in Fig. 7. 

 were actually formed in this manner, because this is rendered im- 

 probable by their connection with the rectilineal ones AB, but 

 merely to explain how cavities having the forms shewn in Figs. 

 1 7. may have their origin from the union of a great number 

 of cavities arranged as in Fig. 9. 



When the cavities are regularly crystallised, which is frequently 

 the case in quartz and topaz, the homologous sides of the hollow 

 crystals are parallel to one another, and also to those of the pri- 

 mitive or secondary form which they resemble. In some very 

 curious but amorphous specimens of quartz from Brazil, belong- 

 ing to Mr SPADEN, the hollow crystals terminate in six-sided py- 

 ramids, with flat summits, and the axes of these pyramids is pa- 

 rallel to the axis of the system of polarised rings, and conse- 

 quently to the axis of the crystal. 



3. On the Condition of the Fluids within the Cavities. 



The phenomena of the expansible fluid have been so fully 

 described in my former paper, that I have only a few observa- 

 tions to add upon this part of the subject. In some specimens 

 of quartz, the expansible fluid seems to exert a very consider- 

 able elastic force, even at the ordinary temperature of the atmo- 

 sphere, and when a very small heat is applied, it sometimes has 

 sufficient force to burst the specimen. A very remarkable case 

 of this kind happened to a son of Mr SANDERSON, who put one 

 of the Quebec crystals of quartz into his mouth. Even with this 

 small accession of heat the specimen burst with great force, and 

 cut his mouth. The fluid which was discharged had a very dis- 

 agreeable taste. 



The extreme volubility of the expansible fluid, and its power 

 of penetrating even the hard topaz in which it is inclosed, were 



3 G 2 



