in Topaz, Beryl, and Diamond. 179 



the cavities containing the two new fluids were formed by highly 

 elastic substances, when the mineral itself has been either in a 

 state of fusion or rendered soft by heat." At this time I was 

 acquainted only with the two new fluids, and some of their 

 chemical and physical properties ; but when I had studied their 

 arrangement in strata, this opinion acquired additional weight. 

 Had these cavities been arranged in planes parallel to the primitive 

 or secondary faces of the crystal, some argument might be urged 

 in favour of their aqueous formation ; but when it was found 

 that the strata of cavities traversed the crystal in all possible 

 directions, that they were bent also into curves of contrary 

 flexure, and that even individual cavities had a curvilinear shape, 

 it was impossible to resist the conclusion that the cavities were 

 formed, and thus capriciously distributed, when the substance 

 of the crystal was in a soft or plastic state. This conclusion 

 derives additional strength from the fact that the water-cavities 

 in crystals deposited from an aqueous solution are never thus 

 arranged. 



The discovery of pressure cavities in topaz and diamond may 

 be considered as completing the evidence for the igneous origin 

 of these minerals, and of the rocks which contain them, We know 

 that gas in a state of compression exists in minerals. In the 

 pressure cavities we have not only the seat of an elastic force, 

 but its direct action upon the substance of the crystal. Though 

 of equal density throughout, as is proved by the equality of its 

 polarized tints, the crystal has its density increased round the 

 pressure cavity, the density being a maximum close to the cavity. 

 Such a structure is impossible in crystals formed by aqueous 

 deposition ; and hence there is not a single example of a pressure 

 cavity in any of them. They exist, however, in amber and in 

 glass, substances that have once been in a plastic state ; and I 

 have produced them artificially by compressing a solution of 

 gum-arabic between two plates of glass so as to include some 

 bubbles of air. The air in these cavities, being exposed to 

 changes of temperature, compresses the circumjacent gum, and 

 gives it that variation of density which produces four luminous 

 sectors in polarized light, exactly of the same character as those 

 which are found in topaz and diamond. 



The existence of crystals of different physical properties in the 

 cavities of minerals, and of imbedded crystals either shooting 

 through their mass, or occurring in groups, or lying singly with 

 their optical axes in every direction, admit of no other explana- 

 tion than that which is afforded by supposing the surrounding 

 mineral to have been in a state of fusion, and to have either 

 contained the elements of the imbedded crystals, or to have 

 surrounded them when previously formed. 



N2 



