40 SIR DAVID BREWSTER ON THE 



taining the nature and properties of certain crystalline deposits which I had 

 noticed, and to which I had refeiTed in my earliest observations.* In these new 

 researches, the results of which were published in two papers in the Transactions 

 of this Society for 1845, I discovered two new classes of phenomena which had 

 escaped the notice of preceding observers, and which threw much light on the 

 formation of the minerals in which they were exhibited. 



In many specimens of topaz from Brazil and New Holland, I discovered numer- 

 ous cavities, filled with crystals of various primitive forms, and with diflferent 

 physical properties. These crystals are either fixed or moveable. Some of the 

 fixed crystals are beautifully crystallised, and have their axes of double refrac- 

 tion coincident with those of the specimen which contains them. In some cavi- 

 ties there is only one crystal, in many two, three, and/o?fr, and in a great number 

 the crystals actually fill the cavities to such a degree, that the circular vacuity 

 in the fluid cannot take its natural shape, and can often be scarcely recognised 

 among the jostling crystals. 



Upon the application of heat to these crystals, some of them gradually lost 

 their angles, and melted slowly, till not a trace of them was visible. Others 

 melted with greater difficulty, and some resisted the most powerful heat I could 

 apply. The crystals which melted easily were quickl}^ reproduced, — sometimes 

 reappearing in a more perfect form, but frequently running into amorphous 

 shapes or granular crystallisations. While some of the crystals were resuming a 

 tabular form, their tints, under the polarising 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 polarised. 



While making these observations, crystals of a different kind presented them- 

 selves to me when the specimens which contained them were exposed to polarised 

 light. These crystals were embedded 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 polarised light. 

 They often polarise five or six orders of colours, and in general they have beauti- 

 ful crystalline forms, which are visible in the microscope even in common light. 

 In some specimens of Brazil topaz, the embedded crystals occur in groups of sin- 

 gular beauty, consisting of prisms and hexagonal plates, connected apparently 

 by filaments of opaque matter. In all these specimens the crystals had a distinct 

 outline, whether they were examined in common or in polarised light ; but I 

 have met with topazes in which the embedded crystals had no visible outline in 

 common light, and which never could have been detected but by the polarising 

 microscope. In one of these, an amorphous crystal, nearly spherical, lay in a 



* See Edinburgh Transactions, vol. x. p. 21, note, and plate i. fig. 10, plate ii. figs. 20, 21 ; 

 p. 419, note, and plate xix. fig. 4. 



