318 Fluids in the Cavities of Minerals. 
could be examined by a microscope, whose object lens is Li» 
Upon making this arrangement, the stratum of cavities was 
seen in the most beautiful manner. The vacuity V, fig. 3, 
of a cavity seen in this way, shone with all the brilliancy of 
total reflexion, the separating surface of the new fluid ABCD, 
and the cavity, exhibited a faint gray tint, while the surround- 
ing portions of the solid topaz were comparatively black. 
The variations which the vacuity V undergoes by heat, are 
now finely seen, and, at a temperature of 80°, it vanishes in 
a brilliant speck, leaving the whole of the cavity ABCD of 
e same uniform tint as in fig. 4. 
h a specimen of amethyst, our author was enabled to 
Speesmine tha, the refractive power of the expansible fluid was 
In the remainder of this section, the author describes anal- 
ogous phenomena in cymophane, quartz-crystals from Que- 
_ bec, and amethyst from Siberia, the last of which is a speci 
men of very great interest, from the cabinet of Mr. Allan. 
In these crystals the vacuities re-appear as follows : 
~ Cymophane §3}° Fahr. 
ar: ? : 4 
: Quartz from Quebec, different cavities in the “ 
SEES tpecimen, 76° 80° 125° 
Amethyst from Siberia, 834° 
