Formation of Compound or Twin Crystals. 279 



To produce those regular solids which are the primary forms of 

 crystals, the spherical and spheroidal molecules of matter must be 

 endowed with some peculiar power of attraction competent to cause 

 that arrangement of them necessary to the production of these sol- 

 ids. We cannot suppose with Wollaston, that spheres aggrega- 

 ting themselves as they are "naturally disposed" that is under the 

 influence of the attraction of cohesion, w r ould ever give rise to a 

 cube or an octahedron.* This is contradicted in every globule of 

 mercury or drop of water, instances which may be cited as examples 

 of the forms resulting from the mutual action of spheres, gifted with 

 an equal attraction in every direction. If then the regular crystal- 

 line solids cannot result from so general and equable a diffusion of 

 attraction in the molecules, must we not suppose the cohesive pow- 

 er to be exerted in certain directions only ? This hypothesis has 

 been lately advanced in Germany by M. VoLTz.f As stated cor- 

 rectly by this author, attraction in one direction, will cause an addi- 

 tion of particles in a straight line, in two directions will give rise to 

 a plane ; a third is required and is sufficient to produce a regular 

 solid. Although this supposition is made by M. Voltz, the de- 

 velopment of the proposed theory will manifest several points of dis- 

 crepancy with his views.J If there were no other argument on this 

 point, it would seem sufficient to prove the inadequateness of the at- 

 traction of cohesion to produce regular solids, that an axial attrac- 

 tion will accomplish this result. 



The molecules probably assume their axes, and take on a corres- 

 ponding degree of eccentricity, at the moment the mineral leaves 

 the fluid state, for we see no evidence of their existence previous to 

 that time ; and we may suppose this to be that great change which 



* In Thomson's Annals of Philosophy, Vol. I, new Series, p. 84, there is an at- 

 tempt by Mr. Emmeti to prove on Mathematical principles, attraction of cohesion 

 to be sufficient to account for the formation of crystals. His reasoning however is 



quite unsatisfactory. 



t Transactions of the Strasburgh Natural History Society for 1833, and L'ln- 

 stitut, for 29th of March, and 8th of August, 1834. 



X Some suggestions have lately been thrown out on the existence of axes of at- 

 traction in the ultimate particles of matter, by Prout in a valuable work entitled 

 "Chemistry, Meteorology and the function of digestion" forming Vol. viii of the 

 Bridgewater Treatises. But although I had supposed on first perusing if, that his 

 views coincided with those here adopted, the similarity is in fact very slight. 

 According to the theory of Mr. Prout, atoms have axes of attraction. Only two 

 of them however are cohesive; the other is chemical and in its direction juxta- 

 position, in the formation of Chemical Compounds, takes place. 





