104' Sir David Brewster ott the Structure of Topaz. 



differing little in their refractive power from topaz, and ex- 

 hibiting in polarized light the most beautiful colours, varying 

 with the thickness of the crystal, and diminishing in intensity 

 as their axes approach to the plane of primitive polarization. 



It is impossible to review the preceding facts without arri- 

 ving at the conclusion, that the topaz must have been in a 

 soft and plastic state when it yielded to the compressing force 

 which emanated from the cavities ; and that a mineral body 

 thus acted upon could not have been formed, according to 

 the received theory, by the aggregation of molecules having 

 the primitive form of the crystal. 



In a letter to Sir Joseph Banks, printed in the Philosophical 

 Transactions for 1805, I deduced, from my experiments on 

 depolarization, the existence of a new " species of crystalliza- 

 tion, which is the effect of time alone, and which is produced 

 by the slow action of corpuscular forces;" and I have re- 

 marked that "this kind of crystallization will probably be 

 found to have had an extensive influence in those vast arrange- 

 ments which must have attended the formation of our globe." 

 These views have been confirmed by various new facts, wholly 

 independent of each other ; — by the existence of crystals im- 

 bedded in topaz, and having their axes in all possible direc- 

 tions, but especially by the nature and form of the strata of 

 fluid cavities in that mineral. These strata cut at all inclina- 

 tions the primary and secondary planes of the crystal. They 

 are bent in the most capricious manner, forming planes of 

 double curvature; and, what is also true of individual cavities 

 stretching in every possible direction, they could never have 

 been formed but when the topaz was in a soft and plastic 

 state. 



An objection to these views may be drawn from the fissures 

 which proceed from the pressure cavities. The topaz must, 

 doubtless, have been indurated when these fissures took place ; 

 but it is equally obvious that the depolarization produced by 

 compression must have previously existed, and it is probable 

 that the fissures were produced after the crystal had been 

 removed from its matrix, and when, from cleavage or other- 

 wise, its cohesive forces had been diminished. 



St. Leonard's College, St. Andrew's, 

 January 16, 1845. 



