18 SIR DAVID BREWSTER ON THE EXISTENCE OF CRYSTALS 
that the plane, passing through the optical axis of the topaz, could be readily 
placed either parallel or 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 crystals, 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 examination 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 doubly refracting crystals melt readily, 
others with very 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 already 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 jour 
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 from the total re- 
flection of the light which fell upon it, I observed three white openings, a, b, c, of 
a crystalline form, see Fig. 14. These appeared to be fixed crystals, or rather parts 
of the topaz, surrounded by a cavity. .1 found, however, that the hexagonal one 
