490 Mr. FoKSTER on the Molecular Forniation of Crystals. 



and basis of the theory of Haut. We shall presently prove how his theory 

 also signally failed. 



The theory of Hctgens only applied to the third system ; but the con- 

 struction of crystals in the first follows as an easy consequence from it. (In 

 order to avoid error, it is necessary to state that the systems and names made 

 use of are those of Eose.) 



HooKE, in his "Micrographia," advanced a similar hypothesis, except that he 

 considered the atoms to be spherical — a supposition which would have ac- 

 counted for forms in the first or regular system, but which would have utterly 

 failed in case of the third, or rhombohedral, to which he applied it : nor does 

 he perceive that the molecules, if left to themselves, would not assume a defi- 

 nite arrangement. He does not seem to consider these spheres as the ultimate 

 atoms : he says that, having already shown how a fluid will naturally assume 

 the spherical form, he will proceed to show how these spheres will unite to 

 form a crystal. His experience, in common with many old writers, seems to 

 have been confined to crystals of quartz; — in fact, some of them went so far as ' 

 to think that everything crystallized in virtue of the quartz it contained. 



The next who commanded attention was M. Pkeciitl de Brun, whose ideas 

 were, to a certain extent, the same as those of Hooke, inasmuch as he consi- 

 dered a fluid to be made up of soft molecules ; but he also considered, that 

 while the body was undergoing its change of state, they suffered a change 

 of form ; and that under different degrees of pressure different crystals were 

 produced. 



Dr. WoLLASTON has fully demonstrated that this theory is totally erroneous 

 in a mathematical point of view. Dr. Wollaston also, in the same article, 

 which is published in the " Philosophical Transactions," propounded a theory 

 to account for the formation of the ordinary octahedron and tetrahedron. He 

 considered the molecules to be simply spheres mutually attracting each other, 

 and he stated that such molecules will combine, as shown in Fig. 3, and thus 

 form a tetrahedron. This, liowever, is certainly not the case ; for if we consi- 

 der the first spherical atoms which unite to form the crystal, it is evident that 

 the first four will assume the form shown in Fig. 1; and a fifth atom will attach 

 itself, as shown in Fig. 2, that being evidently the position of equilibrium ; but 

 if we examine Fig. 3, we find that any five adjacent spheres occupy such posi- 



