1 76 Sir David Brewster on the Pressure Cavities 



crystals which melted easily were quickly reproduced, some- 

 times reappearing in a more perfect form, but frequently run- 

 ning into amorphous shapes or granular crystallizations. While 

 some of the crystals were resuming a tabular form, their tints, 

 under the polarizing microscope, gradually rose in the scale of 

 colours as their thickness increased ; and when there happened 

 to be numerous crystals in the specimen, the whole field of 

 the microscope was filled with brilliant portions of light which 

 they polarized. 



While making these observations, crystals of a different kind 

 presented themselves to me when the specimens which contained 

 them were exposed to polarized light. These crystals were 

 imbedded in the topaz ; and as their axes of double refraction 

 were not coincident with those of the mineral, they were seen 

 in the obscure field of the microscope, brilliant with all the 

 colours of polarized light. They often polarize five or six orders 

 of colours ; and in general they have beautiful crystalline forms, 

 which are visible in the microscope even in common light. In 

 some specimens of Brazil topaz, the imbedded crystals occur in 

 groups of singular beauty, consisting of prisms and hexagonal 

 plates, connected apparently by filaments of opake matter. In all 

 these specimens the crystals had a distinct outline, whether they 

 were examined in common or in polarized light; but I have 

 met with topazes in which the imbedded crystals had no visible 

 outline in common light, and which never could have been 

 detected but by the polarizing microscope. In one of these an 

 amorphous crystal, nearly spherical, lay in a crowded group of 

 small fluid-cavities, none of which had entered it — a proof that 

 the cavities had been formed in the topaz when soft, and when 

 it imprisoned the previously indurated crystal. 



The other class of phenomena to which I have referred is of a 

 still more remarkable nature, and has a more direct bearing on 

 geological theories. About thirty years ago I communicated to 

 the Geological Society the singular fact that I had found in a 

 diamond a small cavity, round which four luminous sectors were 

 seen in polarized light — a phenomenon which clearly proved 

 that the diamond, when in a soft state, had been compressed by 

 an elastic force proceeding from the cavity. This inference 

 countenances the opinion that the diamond was of vegetable 

 origin ; and as this gem was a sort of outlaw in the mineral 

 world, the idea that it had once been in a plastic state, like 

 amber and other gums, and susceptible of compression, did not 

 startle the mineralogists who believed in the ordinary doctrine 

 of crystallization. The insulated fact, therefore, and the pro- 

 bable inference from it, excited no notice ; and it was not till the 

 same phenomenon had been observed more frequently in the 



