224. ° Fluids in the Cavities of Minerals. 
more drops quit the point where the vacuity disappeared, and 
pass along the surface of the cavity, like a drop of oil adher- 
ing to it in close contact, and never mixing with the fluid. 
Each of these drops begins in a short time to spread circu- 
larly, and to exhibit within its disc an immense number of 
close coloured rings. By slow cooling, the drops become 
_rature. When the cooling is effected quickly, the matter 
which composes the thin plate that exhibits the rings, dis- 
charges itself rapidly in gaseous bubbles. 
Sect. VI—On the Phenomena of the two new fluids when 
taken out of the cavities. 
From the extreme minutenes of the cavities in topaz, our 
author’s first attempts to extract the fluid were not attended 
with much success ; but he at last fell upon a method by 
which he has opened more than a hundred cavities. 
When the most expansible of the new fluids first runs from 
the cavity upon the surface of the topaz, it neither remains 
still, like the fixed oils, nor disappears, like evaporable fluids. 
nder the influence, no doubt, of heat and moisture, it is in 
a State of constant motion, now spreading itself in a thin plate 
over a large surface, and now contracting itself into a deeper 
and much less extended drop.* These contractions and ex- 
tensions are marked by a very beautiful optical phenomenon. 
When the fluid has extended itself into a thin plate, it ceases 
to reflect light, like the most attenuated part of the soap-bubble, 
and when it is again accumulated into a thicker drop, it is 
covered with all the coloured rings of thin plates. When one 
of the drops of fluid is very minute and perfectly circular, it 
es, in the most accurate manner, the small drops which 
pass from the vacuity, and which have been described in the 
precedin ion. 
ten or twelve minutes, the fluid suddenly disa d 
- . . . ud prmears i 
leaves behind it a residue of minute and - rate articles, 
uen are opaque by reflected, but transparent by transmitted 
tght. Upon examining this residue with a single micros- 
cope held in the hand, it again started into a fluid state, and 
ee et acon ial ed — stretches itself into @ plane of more 
