Fluids in the Cavities of Minerals. 223 
“ts constant disappearance when it has accumulated to a cer- 
tain degree, and its constant reproduction while the tempera- 
ture remains the same. 
nese views respecting the vaporisation of the expansible 
fluid, have been fully confirmed by the discovery of cavities, 
in which the expansible fluid occupies only one-third or one- 
fourth of the cavity. These cavities are represented in fig. 
8, where AB is the cavity, V the vacuity in the expansible 
fluid m nop, and Amn, B po the second fluid. When heat 
is applied to this cavity, the vacuity V does not contract, as 
in ordinary cases, but expands, till its circumference coincides 
with the boundary mn o p. his unexpected effect might 
have arisen from the expansible fluid occupying the lower 
part of the cavity below V, as in the section, fig. 9. In this 
case c e f d might have been the vacuity, and the surface of 
the fluid e f might have risen by heat, and gradually filled 
the vacuity V, while its boundary at ¢ and d retired to m and 
n as e f ascended. _ In order to determine if this supposition 
both the surfaces mp, n 0, which was now uniform, was not 
that of total reflexion, nor yet that of the expanded fluid, but 
~ of an intermediate intensity, corresponding to that of a dense 
vapour, with a refractive power much lower than 1.211. 
There is another set of phenomena of exquisite beauty to 
an optical observer, which seem to arise either from the de- 
composition of the fluid, or the condensation of gaseous mat- 
ter in the vacuity. ; 
When heat ad applied to the cavity, the new fluid has its 
surface in a state of constant agitation, resembling, in the 
closest manner, a surface into which a fluid is discharging it- 
self by drops. When the vacuity is just filled up, one or 
