ON CRYSTALLOGRAPHY. 147 



can be demonstrated that no subsequent divi-ion can alter its fi- 

 gure. Consequently it can be continued till the figure which it 

 assumes is similar to that of its integrant particles. 



Hauy Ins found that the figure of the integrant particles of 

 bodies, as far as experiment has gone, may be reduced to three ; 

 namely, 



1. The parallelepiped, the simplest of the solids, whose faces 

 are six in number, and parallel two and two. 



2. The triangular prism, the simplest of prisms. 



3. The tetrahedron, the simplest of pyramids. Even this small 

 number of primitive forms, if we consider the almost endless di- 

 versity of size, proportion, and density, to which particles of 

 diflVn-nt bodies, though they have the same figure, may still be 

 liable, will be found fully sufficient to account for all the differ, 

 ences in cohesion and heterogeneous affinity, without having re. 

 course to dinVrent absolute forces. 



These integrant particles, when they unite to form the primitive 

 crystals, do not always join together in the same way. Sometimes 

 they unite by their faces, and at other times by their edges, leav- 

 ing considerable vacuities between each. This explains why inte. 

 grant particles, though they have the same form, may compose pri- 

 mitive crystals of different figures. 



Mr. Hauy has ascertained that the primitive forms of crystals are 

 six in number ; namely, 



1. The parallelepiped, which includes the cube, the rhomboid, 

 and all solids terminated by six faces, parallel two and two. 



2. The regular tetrahedron. 



3. The octahedron with triangular faces. 



4. The six. sided prism. 



5. The dodecahedron, terminated by rhombs. 



6. The dodecahedron, with isosceles triangular faces. 



Each of these may be supposed to occur as the primitive form, or 

 the nucleus in a variety of bodies ; but those only which are regu- 

 lar, as the cube and the octahedron, have hitherto been found in 

 any considerable number. 



But bodies, when crystallized, do not always appear in tht 

 primitive form ; some of them indeed very seldom affect that form : 

 and all of them have a certain latitude and a certain number of 

 forms, which they assume occasionally as wellas the primitive form. 



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