in the Cavities of Minerals. 
99 
Sect. II. — On the coexistence of two Immiscible Fluids , of dif- 
ferent Physical Properties , in the Cavities of Minerals , and 
accompanied with a Vacuity. 
The phenomenon of two immiscible fluids, as exhibited in 
Topaz, is represented in Fig. 5. where V is the vacuity, NNN 
the new fluid, and WWW another fluid, which we shall dis- 
tinguish by the name of the Second Fluid. This second 
fluid WW commonly occupies the angles of triangular cavities, 
as in Fig. 5, or the terminations of longitudinal ones. It is 
always separated from the new fluid by a curved surface mn 9 
m n , & c. It never expands perceptibly with heat, and never 
mixes with the new fluid NN. By a little management, the 
vacuity V may be made to come in contact with the bounding 
lines m n, mn, &c. ; but it never affects its curvature, and seldom 
enters the fluid W. When the vacuity V has been made to 
vanish by heat, these bounding lines remain exactly the same. 
Having at first observed this second fluid only in the angles 
of cavities, as in Fig. 5., considerable difficulty was experienced 
in proving that it was a fluid. The difficulty of conceiving 
two fluids existing in a transparent state, in absolute contact* 
without mixing in the slightest degree, induced several persons 
to refer it to an optical illusion, and to consider the line which 
separated it from the new fluid as a septum or partition in the 
cavity. The beautiful curvature of the bounding line, how- 
ever, and its perfect similarity to that of two contiguous fluids, 
rendered this supposition untenable. 
Having found specimens in which the second fluid occupied 
a large part of the cavity, most of the difficulties which had for- 
merly presented themselves were removed ; but something was 
still wanting to prove its fluidity. This desideratum was for- 
tunately obtained in a specimen of topaz belonging to Mr Siv- 
right. In examining this specimen, I observed a very remark- 
able cavity, of the form shewn in Fig. 6., where A, B and C 
are three separate portions of the new fluid (shaded lightly), 
insulated by the interposition of the second fluid DEF (sha- 
ded darkly). The first portion A of the new fluid had four 
vacuities V, X, Y, Z, while the other two portions B, C, had 
no vacuity. Having often succeeded in making the vacuities 
g & 
