Mr. FoRSTiiR on the Molecular Fonmitlon of Crystals. 491 



tions as shown in Fig. 4, or such as shown in Fig. 5, neither of which are posi- 

 tions of equiUbrium. 



WoLLASTON himself seems to have some lurking doubts of the validity of 

 this assumption, as he begins by showing how the particles will become aggre- 

 gated if on a plane., and then from this basis builds up the tetrahedron: this, 

 however, is the very way in which crystals do not form. 



The octahedron, of course, was easily deduced by removing the corners of 

 the tetrahedron. He also formed the rhombohedron, by placing a pyramid of 

 spheres on two opposite faces of the octahedron, and accounted for the different 

 rhombohedrous by considering the spheres to become spheroids. He did not 

 perceive that the planes of cleavage would in such a case be four in number ; 

 nor is it to be supposed that he would have advanced such an hypothesis if he 

 had been acquainted with the theory of Huygens, which not only accounts for 

 this form, but is in strict accordance with the known phenomena of cleavage. 

 He also makes some observations with regard to the cube, but they are vague 

 and indefinite, and indeed cannot be said to be a theory at all. 



Next in order of time, and first in merit, was the Abbe HAtrv. His theory, 

 as we have already stated, had its origin and groundwork in the phenomena of 

 cleavage. His great merit lies in being the first to advance the theory of 

 decrements, which is, perhaps, one of the most successful in the whole range of 

 physical science. He perceived that many crystals were liable to cleavage, 

 and that in many cases new solids were thus obtained : he was led from this 

 to consider, that if this cleavage were continued long enough, we should ulti- 

 mately arrive at the element itself ; and he assumed, without sufiicient founda- 

 tion, tliat this element should have the same form with the solid obtained by 

 cleavage. This element itself he considered to be further divisible into what 

 he called the absolute atom. 



The forms which he considered these elements or nuclei to have were the 

 tetrahedron, parallelopiped, and the three-sided prism. With these he clearly 

 accounted for nearly all primary forms ; and by means of his theory of decre- 

 ments ably included the secondary : but in case of the tetrahedral cleavage, 

 his theory signally failed, for no one soUd can be obtained by this cleavage, in- 

 asmuch as tetrahedra will not, however united, fill space. The only way in 

 which this difficulty could be got over was by supposing that the tetrahedral 



