in the Cavities of Minerals. 18 



careful experiment, I found that the vacuity disappeared at a 

 temperature of 83^ ; and when it was made to reappear by ra- 

 pid cooling, an ebullition took place, as in the deep cavities of 

 topaz. 



As the cavities in this specimen were terminated with cry- 

 stallised summits at a, b, c, d, I was enabled to observe a curious 

 optical phenomenon, which accompanied the expansion of the 

 fluid. Whenever the vacuity was so much reduced by the ex- 

 pansion of the fluid, that it could be made to occupy one of the 

 crystallised summits, and afterwards to vanish, it left behind it, 

 on that summit, a system of beautiful concentric coloured rings, 

 which were constantly varying in tint, in diameter, and in num- 

 ber. These rings had the highest order of colours in their cen- 

 tre, and continued while the fluid preserved its expanded state ; 

 but they invariably disappeared when the fluid was allowed to 

 contract by cold, as if the substance which formed them had as- 

 sumed a gaseous form, and entered into the vacuity. 



SECT. II. On the coexistence of two Immiscible Fluids, of different 

 Physical Properties, in the Cavities of Minerals, and accom- 

 panied with a vacuity. 



Although many of the cavities which have been described in 

 the preceding section, contain only the new fluid, yet in a very 

 great number, particularly in Topaz, another phenomenon pre- 

 sents itself, which requires a very minute examination. This 

 phenomenon, as exhibited in Topaz, is represented in Figures 8, 

 9, and 10, where V is the vacuity, NNN the new fluid, and 

 WWW another fluid, which we shall distinguish by the name 

 of the Second Fluid. This second fluid WW commonly occupies 

 the angles of triangular cavities, as in Fig. 8., or the terminations 

 of longitudinal ones. It is always separated from the new fluid 

 by a curved surface m n, m n, &c. It never expands perceptibly 



