101 
in the Cavities of Minerals, 
second fluid in the terminations and angles of cavities. When 
the fluids again cool, the surface n n' approaches to c d, and when 
n is near c, the two surfaces n n>, and those of the same fluid in 
c F and d F, suddenly start into union, in virtue of their mu- 
tual attraction, and the portions B and C are again separated. 
y 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 ap- 
proached nearer to topaz than the new fluid. By the same 
means, he determined, that the angle at which total reflexion 
took place at the separating surface from the topaz, was very 
nearly the same as if it were w r ater. 
Two immiscible fluids, possessing the properties now described, 
exist also in Quartz , Amethyst , and Cymophane , and there is rea- 
son to conclude that the one never occurs without the other, as 
the second fluid has, in almost every case, been discovered in ca- 
vities where the difficulties of observation had at first prevented 
it from being detected. 
Passing over the third section, in which our author explains 
the phenomena of two immiscible fluids coexisting without a 
vacuity; and also th e 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 low Temperatures , when enclosed in the Cavities of 
Minerals. 
Let ABCD, Fig. 7., 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 expansible fluid, bounded 
by a circular line abed, and V the vacuity in the new fluid, 
bounded by the circle e fg h. Let the face ABCD be placed 
under a compound microscope, so that the rays of a luminous 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 vacuity VV with 
