18 SIR DAVID BREWSTEU ON THE EXISTENCE OF CRYSTALS 



that the plane, passinj;- through the optical axis of the topaz, could be readily 

 placed cither parallel oj- perpendicular to the plane of primitive polarisation. In 

 this case, the field of the microscope is wholly obscure, in so far as the depolaris- 

 ing 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 depolarising 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 depolarising action is a minimum ; but an experienced observer will have no 

 difficulty in distinguishing this want of depolarisation by position, from the want 

 of it by structure. 



When the specimen of topaz is rich in cavities full of crj^stals, the display of 

 luminous and coloured crystalline forms in the dark field of the microscope, in- 

 dicating, too, the imprisonment of fluids, and the condensation of gases before 

 vegetable or animal life had visited our primeval globe, was as interesting to the 

 imagination and the judgment 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 I 

 have again and again contemplated it with renewed wonder and delight. When 

 the cavities are so numerous as to mock calculation, 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 examinatii)n of the individual crystals, many interesting facts present 

 themselves to our notice. The crystals of the tessular class, which are modifica- 

 tions of the cube, are very numerous, and have no action upon polarised light. 

 Many of them melt easily, while others refuse to yield to the action of heat ; and 

 hence, there must be two different substances in the cavities which assume the 

 same shape. In like manner, some of the doublj^ refracting crystals melt readily, 

 others with A^ery great difficulty, and others not at all ; so that there must be three 

 different substances, which belong to the classes of forms that give double refrac- 

 tion ; a conclusion which is confirmed by the different secondary forms which I 

 have ah-eady enumerated. 



I have seldom found any crystals in these cavities which depolarise white 

 light, or the highest order of colours. I have found some that depolarise four 

 orders of colours ; and when the crystal which does this is a flat hexagonal plate, 

 it is highly interesting to see it pass through all the tints which these orders in- 

 clude, while slowly melting, and again reproducing them during its re-crystal- 

 lization. 



In a cavity which was so placed as to be entirely black fi'om the total re- 

 flection of the light which fell upon it, I observed three white openings, a, b, c, of 

 a crystalline form, st e Fig. 14. These appeared to be fixed cr3'stals, or rather parts 

 of the topaz, suiTounded by a cavity. . I found, however, that the hexagonal one 



