60 ON THE PRIMITIVE FORMS OF CRYSTALS. 



HID CLASS OF PRIMITIVE FORMS. 



Nitrate of lead. 



■ strontian, octohedral crystals. 



barytes 



Nitrite of lead. 

 5 Muriate of potash. 

 Uranite. 

 Sodalite. 

 Essonite*. 



Sulphate of alumina and ammonia. 

 10 Cinnamon-stone. 



The examination of the optical structure of mi- 

 nerals, enables us to go still a step farther in ap- 

 proximating, by exclusion, to their primitive forms. 



The crystals in the following table, I have ascer- 

 tained only to possess double refraction, without ha- 



* Haiiy makes the primitive form of Essonite a. right rhom- 

 boidal prism, which is quite incompatible with its optical 

 structure. I would propose to restrict the name of Essonite to 

 those pure and perfectly transparent Hyacinths, which are 

 distinguished from the Zirconian Hyacinths, by their being 

 destitute of double refraction ; and to apply the name of Cin- 

 namon-stone to a class of Hyacinths, in which I have discover- 

 ed an imperfect transparency, like that which arises from the 

 mixture of alcohol and water, or any other fluids of different 

 refractive powers. The portions of the cinnamon-stone thus 

 imperfectly combined have a slight depolarising structure; 

 and I consider the Essonite to have the same relation to Ciiu 

 namon-stvfie, as Quartz has to Chalcedony* 



