SECTION IX. 



HEMITROPE AND INTERSECTED 

 CRYSTALS. 



BESIDES the secondary forms referred to in the 

 preceding- section, there is another class of crystals 

 which were denominated macks by Rome de 1'Isle, 

 but which the Abbe Haiiy has called hemitrope crys- 

 tals^ having assigned the term made to a species of 

 mineral more generally known by the name of chi- 

 astolite. 



The term hemitrope has been derived from the 

 resemblance of this class of forms to crystals which 

 might be conceived to have been slit in a particular 

 direction, and then to have had one half partly 

 turned round on an imaginary axis, passing through 

 the centre of, and perpendicular to, the planes of 

 section. 



This kind of structure may be readily understood 

 from one or two examples. 



Fig. 115. 



If we conceive an octahedron a b c d e /, fig. 1 15, 

 to be cut through in the direction g //, 



