8 SIR DAVID BREWSTER ON A MODIFICATION OF THE DOUBLY 
intersections of the arms of the black cross, from which the compressing force 
had emanated. One of these cavities is shewn at E., Fig. 2. It is of a quad- 
rangular form, like the section of a rhomboidal prism, sometimes elongated, and 
sometimes of a slightly irregular shape. When perfectly regular, these cavities 
are between the 3000th and the 4000th of an inch in diameter. They are always 
dark, as if the elastic substance which they contained had collapsed into an opaque 
powder; and I have met with only one case in which there seemed to be a speck 
of light in the centre. The degree of compression to which the topaz has been 
subjected is measured by the polarised tint developed in the luminous quadrants. 
It varies from the faintest pale blue to the white of the first order. In one case 
I found the luminous quadrant of one cavity coinciding with a luminous quad- 
rant of another cavity, and thus producing the sum of their separate tints. This 
effect is shewn in Fig. 3. 
In the phenomenon now described, the elastic force has spent itself in the com- 
pression of the topaz. The cavity itself has remained entire, without any fissure 
by which a gas or a fluid could escape. I have discovered, however, other cavi- 
ties, and these generally of a larger size, in which the sides have been rent by the 
elastic force; and fissures, from one to siz in number, propagated to a small dis- 
tance around them. These fissures have modified the doubly refracting structure 
produced by compression ; but, what is very interesting, no solid matter has been 
left on the faces of fracture, such as that which is invariably deposited, when an 
ordinary cavity, containing one or both of the two new fluids, is exploded by heat. 
The form of some of the cavities which have suffered this disruption is shewn in 
Figs. 4, 5, and 6. 
The influence of the compressing forces in altering the density, and conse- 
quently the refractive power of the topaz, is so distinctly seen in common light 
as to indicate the phenomena that are seen under polarised light. When the cavity 
is most distinctly perceived, it is surrounded with luminous and shaded circles, 
as shewn in Fig. 7; and traces of these are distinctly seen, as shewn in Fig. 8, 
when the specimen is examined in polarised light. 
The cavities now described have obviously no resemblance whatever to those 
which I have described in previous papers as containing two new fluids. When 
any of the latter are either burst by heat, or exposed under high temperatures to 
the compressing forces of the fluids which they contain, they exhibit none of the 
phenomena peculiar to the former. The doubly refracting structure suffers no 
change ; and when the cohesive forces of the crystal are overpowered, the faces of 
most eminent cleavage separate, and are covered with translucent crystalline par- 
ticles, which the evaporated or discharged fluids leave behind. 
The peculiar character of the pressure cavities, as we may call them, is still 
farther evinced by the nature of the specimens in which they occur. I have never 
found them accompanying the ordinary cavities with two fluids. The specimens 
