Fluids in the Cavities of Minerals. 221 
In order to examine the refractive power of the second 
fluid, our author made the arrangement represented in fig. 2, 
and found that the second fluid W always reflected less light 
than the new fluid, and consequently that its refractive power 
approached nearer to topaz than the new fluid. the same 
means, he determined that the ote at which total reflexion 
took place at the separating surface from the topaz, was very 
nearly the same as if it were water 
wo immiscible fluids, possessing the properties now de- 
scribed, exist also in Quartz, Amethyst, and Cymophane, and 
there is reason to conclude that the one never occurs without 
the other, as the second fluid has, in almost every case, been 
discovered in cavities where the difficulties of observation had 
at first prevented it from hs detected. 
assing over the third section, in which our author ex- 
plains the phenomena of two immiscible fluids coexisting 
without a vacuity; and also the fourth section, in which he 
shews that the fluids are sometimes indurated like a resinous 
substance within the cavities, we come to 
Sect. V.—On the Vaporisation and Decomposition of the 
New Fluid at ie Ba lick, when enclosed in the 
Cavities of Minera 
Let ABCD, fig. 7, be the summit of a crystallised cavity 
in topaz, and let the Jength of the cavity be in a vertical di- 
rection, so that SS is the seeond fluid, NN the expansible flu- 
id, bounded by a circular line a 6 ¢ d, and V the vacuity in 
fluid, bounded by the circle e fg h. Let the face 
rays 0 fiected a 
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 oat with 
a very feeble reflected light, the new fluid NN with a light 
perceptibly brighter, and the vacuity VV with a considerable 
brilliancy. The boundaries a b ¢ d, e f g h, are marked by 
a well-defined outline, and also by concentric coloured rings 
of thin plates, produced by the extreme thinness of each of the 
fluids at the edges. 
If we now raise the temperature of the room gradually to 
58°, we shall observe a brown spot appear in the centre of 
the yacuity Ve fgh. This spot marks the visible com- 
