Properties of the Two New Fluids in Minerals. 419 



where there are large vacuities V, V, and there is a globular por- 

 tion of it at n, without a vacuity. When heat is applied, the fluid 

 at N N and N' N' quickly goes off into vapour ; the portion at n 

 expands into an elliptical globule, but its force is not sufficient 

 to displace the mass of the second fluid between n and N, and n 

 and N' ; and being kept in equilibrio by the opposite and nearly 

 equal expansive forces of the vapour in N N, and N' N', it conse- 

 quently remains fluid at n *. In a plate of Topaz shewn to me 

 by Mr SIVRIGHT, where the expansible fluid consists of two por- 

 tions floating in a large quantity of the dense fluid, one of the 

 portions is a spherical drop which expands with heat, and con- 

 tracts with cold, exhibiting by transmitted light an effect similar 

 to the opening and shutting of the pupil. 



In re-examining the phenomena of the second or denser 

 fluid, several very curious facts have come under my notice. 



I had previously shewn, that, when several cavities communi- 

 cated with each other, the narrow necks, or lines which joined 

 them, were filled with the dense fluid, which shifted its position 

 when the equilibrium of the adjacent portions was destroyed by 

 heat ; but I have since had occasion to examine the phenomena 

 of the second fluid with more attention. The particles of this 

 fluid have a very powerful attraction for themselves, like those of 

 water, and they are also powerfully attracted by the mineral 

 which contains it. The particles of the expansible fluid have, 

 on the contrary, a very slight attraction for one another, and also 

 for the mineral which incloses them. Hence it follows, that, as 

 the two fluids never in the slightest degree mix with one ano- 

 ther, the dense fluid is either attracted to the angles of angular 



* In Fig. 4>. of Plate XIX, I have represented another vapour cavity, which is 

 remarkable for having a very small portion of the expansible fluid, and also, for ha- 

 ving several crystalline forms within the dense fluid. 3 



