Refracting and Physical Structure of' Topaz. 103 



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 polarized 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 quadrant of another cavity, and thus producing 

 the sura of their separate tints. This effect is shown in fig. 3. 



In the phaenomenon now described, the elastic force has 

 spent itself in the compression 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 cavities, 

 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 distance around them. These 

 fissures have modified the doubly refracting structure pro- 

 duced 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 disrup- 

 tion is shown in fig. 4. 



The influence of the compressing forces in altering the 

 density, and consequently the refractive power of the topaz, is 

 so distinctly seen in common light as to indicate the phseno- 

 mena that are seen under polarized light. When the cavity 

 is most distinctly perceived, it is surrounded with luminous 

 and shaded circles, as shown in fig. 5; and traces of these are 

 distinctly seen, as shown in fig. 6, when the specimen is ex- 

 amined in polarized 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 phaenomena 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 trans- 

 lucent crystalline particles, which the evaporated or discharged 

 fluids leave behind. 



The peculiar character of the pressure cavities, as we may 

 call them, is still further evinced by the nature of the speci- 

 mens in which they occur. I have never found them accom- 

 panying the ordinary cavities with two fluids. The specimens 

 which contain them have imbedded in them numerous crystals, 



