498 Sir David Brewster on the Existence of Crystals 



last spring, when I discovered cavities in topaz filled with the 

 most beautiful crystals of various form, that 1 was induced to 

 undertake a new investigation of their nature and properties. 

 In this investigation I have examined, with various magnifying 

 powers, and both in common and polarized light, more than 

 900 specimens of topaz from Scotland, New Holland, and the 

 Brazils ; and I have had the good fortune to observe many 

 new phaenomena connected with mineralogy, chemistry, and 

 physics, which, in addition to the interest which they may 

 possess as scientific facts, promise to throw a strong light upon 

 the existing theories of crystallization, and to bring before us 

 some of those recondite operations which had been going on 

 in the primitive rocks of our globe, before the commencement 

 of vegetable or animal life. 



1. On the Form and Position of the Strata ifi which the Cavities 



lie. 



The cavities which contain the two new fluids, and their 

 accompanying crystals, sometimes occur single, and in groups 

 more or Jess numerous ; but, in general, they exist in millions, 

 occupying extensive strata, which affect the transparency of 

 the mineral, and render it unfit for the use of the jeweller, or 

 even for the cabinet of the collector, who has not learned that 

 it is in the deviations from her ordinary laws that Nature often 

 discloses her deepest mysteries. 



Although the strata of cavities sometimes occur, as in arti- 

 ficial salts, in planes parallel to the primary or secondary 

 forms of the crystal, yet they occupy every possible position m 

 reference to these planes ; and we therefore cannot account 

 for them by supposing that certain spaces have been left in 

 the crystal, without the primitive molecules which ought to 

 have been there deposited. The strata of cavities, too, have 

 every possible curvature. From a plane surface they pass 

 into a curved one, sometimes of variable curvature, and some- 

 times of contrary flexure, cutting and intersecting each other 

 in the most capricious manner. 



In the shape of the strata the same irregularity presents 

 itself; their outline is sometimes rectilineal, sometimes curved, 

 and sometimes singularly irregular. In some specimens the 

 whole crystal is intersected with the strata; and it is extremely 

 probable, though it is impossible to determine the fact, that in 

 every specimen some edge or angle of the stratum touches the 

 surface. 



The succession of the cavities in composing the stratum, 

 and their form in relation to the character of the stratum, 

 present interesting phasnomena. I have found specimens in 



