MOLECULES. 45 



The regular dodecahedron may be cleaved into 

 obtuse rhomboids, obtuse octahedrons, and irregular 

 tetrahedrons, as will be shewn in the section on clea- 

 vage. Of these the Abbe Hauy has chosen the irregu- 

 lar tetrahedron for the molecule of the dodecahedron, 

 and he has supposed that the decrements on this form 

 are produced by the abstraction, not of single molecules, 

 but of masses of single molecules packed into the figure 

 of those obtuse rhomboids which are produced from its 

 cleavage.* 



The very complicated system of molecules which 

 the Abbe Haiiy has, by this view of the structure of 

 the octahedron and dodecahedron, introduced into 

 his otherwise beautiful theory of crystals, arid the 

 apparent improbability that the molecules of the 

 cube, the regular octahedron, tetrahedron and dode- 

 cahedron, among whose primary and secondary forms 

 so perfect an identity subsists, should really differ from 

 each other, have induced me to propose a new theory 

 of molecules in reference to all the classes of octa- 

 hedrons, to the tetrahedrons, and the rhombic dode- 

 cahedron, which 1 shall now state. 



Fluate of lime, as we have seen, has for its primary 

 form a regular octahedron, under which it sometimes 

 occurs in nature ; but it is generally found in the 

 form of a cube, and sometimes as a rhombic dodeca- 

 hedron, and it has a cleavage in the direction of Us 

 primary planes. 



Galena, whose primary form is a cube, is also found 

 under the forms of an octahedron, and rhombic dode- 

 cahedron, with a cleavage parallel to its cubic planes. 



* Under the head of cleavage I shall endeavour to explain the nature 

 of the relation which the different solids obtained by cleavage from the 

 tetrahedron, octahedron, and rhombic dodecahedron, respectively bear 

 to those primary forms, and to each other; and to shew that they da not 

 in eithtr case represent th: molecules of those farms. 



