In the Cavities of Minerals. 23 



? The most unequivocal proofs, however, of a change in the 

 fluid, are obtained from various topazes, where the induration of 

 the fluid is perfectly obvious to the eye. It resembles a resinous 

 substance, and has a sort of cellular structure, like that shewn 

 at d, Fig. 24., where the vacuity retains its circular form. No 

 change whatever is produced upon these appearances by heat. 

 In the figure at e, the fluid N, with its vacuity V, still exists, 

 and the latter vanishing with heat ; but the induration is dis- 

 tinctly seen at the lower end of the cavity. 



In other specimens the same cellular structure appears, but 

 the vacuity has lost, in different degrees, its circular form, as 

 shewn atf. ,? 



Similar phenomena occur in cymophane and felspar, in the 

 last of which the induration of the fluid is most distinct 



SECT. V. On the Vaporisation and Decomposition of the New 

 Fluid at low Temperatures, when enclosed in the Cavities of 

 Minerals. 



Let ABCD, Plate II. Fig. 30. be the summit of a crystallised 

 cavity in topaz, and let the length of the cavity be in a vertical 

 direction, so that SS is the second fluid, NN the new fluid, 

 bounded by a circular line abed, and V the vacuity in the new 

 fluid, bounded by the circle efgh. Let the face ABCD be 

 placed under a compound microscope, so that the rays of a lumi- 

 nous body incident upon it, may be reflected at an angle less than 

 that of total reflexion. When the observer now looks through 

 the microscope, the temperature of the room being 50, he will 

 see the second fluid SS shining with a very feeble reflected light, 

 the new fluid NN with a light perceptibly brighter, and the va- 

 cuity V V with a light of considerable brilliancy. The boundaries 

 abed, efgh, are marked by a well defined outline, and also by 



