MOLECULES. 





The tetrahedron thus obtained, may be reduced 

 again to an octahedron, as shewn in fig. 69, by 

 removing a smaller tetrahedron, as a b c d, from each 

 of its solid angles. 



And all the fragments separated from the octa- 

 hedron by the cleavages just described, may also be 

 reduced, by cleaving in the proper directions, to 

 regular octahedrons and tetrahedrons. 



In this case two distinct solids are obtained from the 

 cleavage of an octahedral crystal ; and the Abbe Hauy 

 has chosen to assume the tetrahedron as the molecule of 

 the octahedral crystal, upon the supposition that if the 

 cleavage were continued until only single molecules 

 remained to be separated, these molecules would be 

 tetrahedrons ; and the octahedron is, according to his 

 theory, conceited to be composed of tetrahedral solids 

 united by their points, and octahedral spaces. * 



From considerations analogous to these, the Abbe 

 Haiiy has concluded that the tetrahedron, when it 

 occurs as a primary form, is constituted also of tetra- 

 hedral molecules and octahedral spaces. 



* The same imaginary structure has also been supposed by the Abbe 

 Haiiy to exist in every class of octahedrons, the molecules peculiar to 

 each being distinct irregular tetrahedrons, varying in their angles and 

 relative dimensions in each particular case. 



But it will be attempted to be shewn presently that this imaginary 

 structure does not belong to the octahedron, and that the tetrahedral solid dees not 

 rtprestnt the molecule of that form* 



