16 



Monday, 3cl February 1845. 

 Sir T. M. BRISBANE, Bart., President, in the Chair. 



The following Communications were read : — 



1. On a Peculiar Modification of the Doubly Refracting 

 Structure of Topaz. By Sir D, Brewster, K.H. 



While examining, in polarised light, some of the crystals which 

 he had discovered in Topaz, the author observed certain optical phe- 

 nomena, depending on a peculiarity of structure. This peculiarity is 

 manifestedeither inthedepolarisationof light, when it gives rise to four 

 quadrants of light, separated by the radii of a black rectangular cross si- 

 milar to the central portion, or the tints of the first order in the uni- 

 axal system of polarised rings, or in the unequal refraction of com- 

 mon light, which gives rise to the mirage of a luminous point, in the 

 form of concentric circles surrounding the centre of force. In every 

 case there was found a quadrangular cavity in the centre of the in- 

 tersection of the cross, generally dark and opaque, but in one case 

 having a luminous spot in the centre. These cavities are from the 

 5ffo to t^6 j^Vo of an inch in diameter. 



These cavities are quite distinct from all those formerly described 

 by the author ; and from the phenomena above described, he con- 

 cludes that the contents of each cavity have exerted an elastic force 

 on the surrounding mineral while in a plastic state. In some cases 

 fissures are seen proceeding from the central cavities, but these are 

 supposed to have been produced after the mineral had become indu- 

 rated, and had already been subjected, in the plastic state, to the 

 pressure or force above indicated. 



These cavities never accompany the cavities with two fluids, but 

 occur in specimens containing numerous embedded crystals, differ- 

 ing little fi-om Topaz in refracting power. 



Since the mineral must have been plastic when it yielded to the 

 pressure here noticed, it cannot have been formed by the aggrega- 

 tion of molecules having the primary form of the crystal. 



These considerations, along with others connected with the crys- 

 tals, which occur in the cavities of Topaz, have led the author to 

 adopt the idea of a new and peculiar kind of crystallization, to which 

 he will soon direct attention. 



