REFRACTING AND PHYSICAL STRUCTURE OF TOPAZ. 9 



which contain them have imbedded in them numerous crystals, differing little in 

 their refractive power from topaz, and exhibiting in polarised light the most beau- 

 tiful colom-s, varying with the thickness of the crystal, and diminishing in inten- 

 sity as their axes approach to the plane of primitive polarisation. 



It is impossible to review the preceding facts without an-iving at the conclu- 

 sion, 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' depolarisation, the existence of a 

 new " species of crystallization, which is the effect of time alone, and which is 

 produced by the slow action of corpuscular forces ;" and I have remarked that 

 " this kind of crystallization wiU j)robably be found to have had an extensive 

 influence in those vast arrangements which must have attended the formation of 

 our globe." These views have been confirmed by various new facts, whoUy inde- 

 pendent of each other ; — by the existence of crystals imbedded in topaz, and having 

 their axes in all possible directions, but especially by the nature and form of the 

 strata of fluid cavities in that mineral. These strata cut at aU inclinations the 

 primary and secondary planes of the crystal. They are bent in the most capri- 

 cious manner, forming planes of double curvature ; and, what is also true of indi- 

 vidual 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 depolarisation pro- 

 duced 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 otherwise, its cohesive forces had been diminished. 



St Leonard's College, St Andbews, 

 Januwty 16. 1845, 



VOL. XVI. PART 1. 



