340 ON THE PRIMITIVE FORM OF CRYSTALS. 



Molybdate of Lead. 

 Oxide of Tin. 

 Octohedrite. 

 Tungstate of Lime. 



5. Right Prism with a Square Base, 



Idocrase. 



Titanite. 



Meionite. 



Uranite*. 



Wernerite. 



6. Bipyramidal Dodecahedron. 



Quartz. 



Phosphate of Lead. 



As Crystals with one axis of Double Refraction 

 are divisible into two great classes, viz. Positive and 

 Negative, according to the nature of the action 

 which they exercise upon the extraordinary ray, I 

 was anxious to ascertain if any relation existed be- 

 tween this character and their primitive form. Al- 

 though no general relation has presented itself, yet 

 it is worthy of remark, that not one positive crys- 

 tal is found in the three first primitive forms; 

 and that the three last contain both positive and 

 negative crystals. 



• This mineral has a very weak double refraction. In con- 

 sequence of its imperfect transparency, I was formerly obliged 

 to use very thin plates of it ; and as its axis is perpendicular 

 to the laminae, the double refraction was not then perceptible. 



