62 ON THE PRIMITIVE FOE MS OF CRYSTALS. 



20. Muriate of gold. 



of silver. 



of iron. 



of lead. 



Nitrate of magnesia and ammonia. 

 25. Acetate of soda. 



— of potash. 



Phosphate of iron. 



of copper. 



of lead. 



30. of magnesia. 



Arseniate of lead. 



Calomel. 



Oxymuriate of mercury. 

 Emerald copper. 

 35. Topazulite. 

 Hatiyne. 

 Meionite. 

 Wernerite. 



Hyposulphite of strontian. 

 40. Specular iron. 

 Petalite. 



Subsulphate of alumine. 



The connection between the external forms of 

 crystals and their optical properties, naturally leads 

 us to enquire, if any reason can be assigned, why 

 particular forms should be distinguished by a parti- 

 cular number of axes. 



It appears from the general principle of the reso- 

 lution and composition of polarising forces which I 



