MOLECULES. 37 



The figures of the solid molecules require to be 

 explained in reference to each of the five following 

 classes of primary forms. 



1. The cube and all the other classes of parallelo- 

 pipeds, or solids contained within six planes. 



2. The regular octahedron and all the other classes 

 of octahedrons. 



3. The regular tetrahedron. 



4. The rhombic dodecahedron. 



5. The hexagonal prism. 



If we attempt to fracture a piece of galena, it will 

 split into rectangular fragments. But we find by 

 observing the secondary forms of galena, that its 

 primary crystal may be a cube, and we know also 

 that by supposing this cube to be composed of cubic 

 molecules, the angles at which the secondary planes 

 incline upon the primary, may be computed and de- 

 termined with mathematical precision. We are there- 

 fore led to infer, that if the rectangular fragments ob- 

 tained bij cleavage could be reduced to single molecules , 

 those molecules would be cubes.^ 



are found crystallized, are similar in form to the molecules of other crys- 

 tallized substances which present similar primary forms. 



Now according to two suppositions, the first being that entertained 

 by the Abbe Haiiy, the other arising out of a theory which will be pre- 

 sently stated, the molecule of sulphur may be an irregular tetrahedron, or a 

 right rhombic prism , and the molecule of silver a regular tetrahedron^ or a cute. 

 But the compound of sulphur and silver crystallizes in the form of a cuke. 

 Hence the molecule of sulphuret of silver, arising out of the chemical 

 union of irregular tetrahedrons with the regular tetrahedrons or cubes t accord- 

 ing to one supposition, or of right rhombic prisms and cubes y according to 

 the ether supposition, performs the function of a cube. If this subject were 

 pursued it might be shewn that the cubic function is performed by 

 molecules very variously composed. 



f Whether these little cubes would consist of one or more atoms of 

 lead and of sulphur, or how these elementary particles would be com- 

 bined in the production of a cubic molecule, are circumstances not im- 

 mediately relating to Crystallography, however interesting they may 

 be as separate branches of enquiry. 



