TABLES OF MODIFICATIONS. 



227 



parisoii of the crystals themselves with each other, 

 assisted by the tables of modifications already given. 



A circumstance, which has not yet been alluded to, 

 will also frequently render it very difficult to read a 

 crystal. This is the unequal extension of some of its 

 parallel planes. A very remarkable instance of this 

 character prevails in copper pyrites, and has been 

 the occasion of the erroneous opinions entertained 

 until very lately, respecting the primary form of that 

 substance. 



In all the works on mineralogy, except that by 

 Professor Mohs, its primary form is stated to be a 

 regular tetrahedron. Mohs, however, discovered that 

 its form was an octahedron with a square base. The 

 two following figures, for the drawings of which, 

 made from crystals in his own possession, I am obliged 

 to Mr. W. Phillips, exhibit crystals containing equal 

 numbers of similar planes ; fig. 300 having these planes 

 regularly placed on the primary form, and fig. 301 

 representing the same crystal as it frequently occurs 

 in nature, with some of its planes considerably enlarged. 



The same letters are placed on the corresponding 

 planes of each. 



Fig. 300. 



Fig. 301. 



Mr. Phillips had discovered from the cleavage of 

 this substance, before the publication of Professor 

 Mohs's book, that its primary form was not a tetra- 

 2 F2 



