162 ON THE OPTICAL PROPERTIES OF MURIATE OF SODA, 



would thus acquire the three different structures which we 

 have actually found it to possess. 



If this view of the subject be correct, it will follow, that the 

 cubical and octahedral forms are intermediate between those 

 which belong to the attractive and repulsive classes of doubly 

 refracting crystals ; — that a deviation from these forms on one 

 side will produce the structure of the attractive class, and a de- 

 viation on the other side the structure of the repulsive class ; 

 — that the force of double refraction increases with that devi- 

 ation, and that there is a certain primitive structure belonging 

 to each mineral, by which its class may be ascertained. 



The imperfect state of crystallography does not enable us to 

 determine what this structure is ; but when this science shall 

 have made farther progress, we may probably be able to 

 ascertain from the crystalline forms of minerals, both the cha- 

 racter and the intensity of their doubly refracting force. 



APPENDIX. 



Edinburgh, July 4. 1816. 

 M. Biot, who had an opportunity of examining the greater 

 part of the preceding paper in MS. in October 1815, has sta- 

 ted in his TraitS de Physique •, torn. iv. p. 573, newly pub- 

 lished, that the polarising structure which I have discovered in 

 Muriate of Soda, Fluate of Lime, and the Diamond, appears 

 to be owing either to the effects of heat, or rapid evaporation. 

 " C'est je crois a eela," says this eminent mathematician, 

 " qu'il faut attribuer les indices de polarisation observees acci- 

 " dentellement par M. Brewster dans certains echantillons de 



" cristaux 



