228 ON THE APPLICATION OF THE 



hedron, as appears by a paper in the Annals of Phi- 

 losophy, new series, vol. 3. p. 297. 



When crystals of this irregular character occur, it 

 is generally, only by cleavage, and by using the 

 goniometer, that we can be led to an accurate deter- 

 mination of their true forms. 



Having ascertained the class of primary forms to 

 which our supposed crystal belongs, our next step 

 would be to measure the angles of its primary or 

 secondary planes, in order to determine the species 

 to which the mineral itself belongs. 



If the crystal belongs to one of the four regular 

 solids, whose angles, when the forms are similar, are 

 always equal, its hardness, or specific gravity, or some 

 other character, will the most readily lead to a de- 

 termination of its mineral species. 



But it may happen that the secondary crystal we 

 are examining, may be referred with equal propriety 

 to either of the two, or more, classes of primary forms. 

 If we turn to the modifications of the octahedron 

 with a square base, and to those of the square prism, 

 and imagine the modifying planes of the square prism 

 much enlarged, we shall observe such a resemblance 

 between them, that either form may be taken as the 

 primary, in reference to the secondary forms of both. 

 The same remark will apply to the octahedron with 

 a rectangular or a rhombic base, and the right rect- 

 angular, or rhombic prism. 



In these cases it has been usual to adopt that as 

 the primary form which is developed by cleavage. 

 But if there be no practicable cleavage, or if there 

 be two sets ,of cleavages, parallel to the planes of 

 two primary forms, we are then at liberty, as it has 

 been already stated, to adopt either of these, and 



