CRYSTALS CONTAINING FLUID. 189 



ciously distributed when the substance of the crystal was 

 in a soft or plastic state. This conclusion derives con- 

 siderable 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 igne- 

 ous origin of these minerals, and of the rocks which con- 

 tain 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 impos- 

 sible 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 be^n 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 ex- 

 posed to changes of temperature, compresses the circum- 

 jacent 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/' 



Mr. Sorby discovered numerous fluid-cavities in the 

 quartz of granite, in the quartz of volcanic rocks, and also 

 in the feldspar ejected from the crater of Vesuvius ; from 

 this it is inferred that these researches tend to confirm the 

 theory of the igneous origin of granite and eruption-rocks 

 in general. 



Mr. Sorby, in a paper read at the Leeds Meeting of the 



