in the Cavities of Minerals. 505 



persons ; and it is very remarkable that they generally reap- 

 pear in this specimen of the same form, though with consi- 

 derable modifications. 



Upon applying heat to other cavities containing several 

 crystals, I obtained very different results. Some of them 

 melted easily, others with greater difficulty ; and some were 

 not in the slightest degree affected by the most powerful heat 

 I could apply. When the crystals melted easily, they were 

 as quickly reproduced; sometimes reappearing more perfectly 

 formed than before, but frequently running into amorphous 

 and granular crystallizations. 



In some specimens of topaz all the crystals in the cavities 

 refuse to melt with heat, and seem not to suffer the slightest 

 change in their form. Hence we are entitled to conclude, 

 that the crystals possessing such different properties must be 

 different substances; and this conclusion is amply confirmed 

 by an examination of their optical properties. 



In making this examination, I used a polarizing microscope, 

 so constructed that the plane, passing through the optical axis 

 of the topaz, could be readily placed either parallel or per- 

 pendicular to the plane of primitive polarization. In this case 

 the field of the microscope is wholly obscure, in so far as the 

 depolarizing action of the plate of topaz is concerned ; but if 

 there is any crystal in the topaz, either imbedded in its mass, 

 or included in its cavities, that crystal will exhibit its doubly 

 refracting structure, if it has any, by its depolarizing action. 

 It may, indeed, happen, — and it does happen, — that the plane 

 passing through their optical axes coincides, either accurately, 

 or so nearly, with that of the topaz, that its depolarizing action 

 is a minimum ; but an experienced observer will have no dif- 

 ficulty in distinguishing this want of depolarization by position, 

 from the v/ant of it by structure. 



When the specimen of topaz is rich in cavities full of cry- 

 stals, the display of luminous and coloured crystalline forms in 

 the dark field of the microscope, indicating, too, the impri- 

 sonment of fluids, and the condensation of gases before vege- 

 table or animal life had visited our primaeval globe, was as 

 interesting to the imagination and the judgement as it was 

 beautiful to the eye. Having had the privilege of being the 

 first to see it, I felt the full influence of the sight ; and 1 have 

 again and again contemplated it with renewed wonder and 

 delight. When the cavities are so numerous as to mock cal- 

 culation, and so infinitely small as to yield no visible outline 

 to the highest powers, the bright twinkle of a crystalline atom 

 within them reveals to us their nature as well as their contents. 



In the examination of the individual crystals, many interest- 



