in the Cavities of Minerals. 27 



matter which composes the thin plate that exhibits the rings, 

 discharges itself rapidly in gaseous bubbles. 



When the drops quit the point where the vacuity vanishes, 

 and pass over one of the summits of the cavity, they often leave 

 an irregular streak, which also gives the colours of thin plates ; 

 and sometimes the circular expansion of the drops extends with- 

 in the circular vacuity, and thus displays two intersecting sys- 

 tems of coloured rings, which proves, in the most incontrover- 

 tible manner, that the vapour within the vacuity will not mix 

 with the fluid which composes the drops. The drops now de- 

 scribed often quit the vacuity before it is filled up by the ex- 

 pansion of the fluid, and one of them will sometimes remain on 

 the margin of the vacuity, which can be easily seen through it. 



SECT. VI. On the Phenomena of the two New Fluids when taken 

 out of the Cavities. 



From the extreme minuteness of the cavities in topaz, my 

 first attempts to extract the fluid were not attended with much 

 success ; but I at last fell upon a method by which I have open- 

 ed more than a hundred cavities. 



When the most expansible of the new fluids first runs from 

 the cavity upon the surface of the topaz, it neither remains still, 

 like the fixed oils, nor disappears, like evaporable fluids. Under 

 the influence, no doubt, of heat and moisture, it is in a state of 

 constant motion, now spreading itself in a thin plate over a large 

 surface, and now contracting itself into a deeper and much less 

 extended drop *. These contractions and extensions are mark- 



A round hemispherical drop often stretches itself into a plane of more than 

 twelve times its original area. 



D 2 



