TABLES OF MODIFICATIONS. 



To ascertain the class of primary forms to which 

 such secondary crystal belongs, we should first ob- 

 serve whether there be on the crystal any series of 

 planes whose edges are parallel to each other. If we 

 observe such a series of planes, we should then hold 

 the crystal in such a manner, that the series of 

 parallel edges may be vertical.) or upright. And while 

 it is in this position, we should observe whether there 

 be any plane at right angles to the series of vertical 

 planes we have noticed.* 



But on examining the secondary forms of crystals, 

 we may sometimes find that there are two sets of 

 parallel edges, either of which being held upright, 

 the crystal would present a series of vertical planes. 

 We should in this case endeavour to ascertain whether 

 the planes belonging to one set, are not so symmetric 

 cally arranged with respect to those of the other, as 

 to possess the character of modifications of the ter-> 

 minal edges of a primary form ; if we find them so, 

 we should not make that the vertical series. 

 . If there be a series of vertical planes, and a horizontal 

 plane, we should observe whether any of the vertical 

 planes are at right angles to each other, and whether 

 there be any oblique planes lying between some of 

 the vertical planes, and the horizontal plane. 



We should remark the equality, or inequality, of 

 the angle at which any of the vertical or oblique 

 planes incline on the several adjacent planes. 



We should notice whether there be any such sym- 

 metrical arrangement of the vertical planes, or of 

 the oblique planes, if there be any, as would induce 



* When the series of planes with parallel edges are held verii^Uyn 

 the plane at right angles to them will of course be horizontal. These 

 may therefore be called the vertical and horizontal planes ; all other planes 

 ivill be termed oblique ; and the edges of the horizontal and vertical planes, 

 will be termed horizontal and vertical edges. 



SF 



