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 clastic 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 ivhite 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 shown 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 six 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 pcculiai' 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 



