VteliDS of the theory of Crystallography. 217 



pie solid of the system, or that which has the least number possible 

 of faces, and on the other, the solid having the greatest number, 

 namely a sphere or an ellipsoid. Although it is more convenient in 

 the calculation of forms to start from the most simple polyhedral 

 forms in order to arrive at the more complex, nothing proves that 

 such has been the route which nature has followed. As long as we 

 considered the integral molecules as polyhedral, it appeared natural 

 to vie\v them as grouping in polyhedrons ; but when once we cease 

 to admit polyhedral molecules, it then becomes most natural to sup- 

 pose, that ellipsoidal molecules should have a tendency, more or less 

 decided, to group in solids of the same form as themselves, when no 

 extraneous circumstances interpose an obstacle to this tendency. 



In order to give an idea of the kind of effect which Avould be pro- 

 duced on the form of the solid by these obstacles, such as the nature 

 of the medium in which crystallization takes place, a hurried or tu- 

 multuous crystallization, &c., the author conceives that each mole- 

 cule, as well as each solid formed by their union, has different axes 

 of attraction, endued with different degrees of energy, and symme- 

 trically disposed in groups, the vveaker and the most numerous round 

 the stronger, which are, at the same time, the smallest in number ; 

 all, in short, symmetrically arranged around the principal axes of 

 crystallization, which are the most energetic of all. Thus we shall 

 conceive that sort of polarity by which crystallization is distin- 

 guished from molecular attraction. The effect of obstacles, such 

 as the attraction exerted by mediums, by interposed bodies, by the 

 molecular atti"action of the molecules themselves, when they arrive 

 both in too great numbers and too rapidly towards the same jDoint, 

 will be the annihilation of the weaker axes ; whence will follow the 

 formation of a tangent plane to the spherical or elliptical surface. 

 If the action of the obstacle goes on increasing, axes of attraction, 

 which, by their intensity, had resisted the first obstacles, are destroyed 

 by the new ones ; and new tangential planes are produced, in which 

 those that had been first formed finish by being confounded : thus it 

 will happen that, by the increase of obstacles, the surface of the solid 

 from being curved has become polyhedral, and finishes by presenting 

 only an assemblage of a small number of plane faces, separated by 

 edges, and j^laced tangentially at the extremity of the axes whose 

 forces have longest resisted the action of the obstacles. But since 

 the most energetic axes are necessarily the least numerous, the greater 

 the energy they possess, the number of faces which bound the solid 

 will continually decrease according as the obstacles increase ; until, 

 at lengtli, the solid, reduced to its most simple form, no longer ]ux:- 

 sents any but that constituted by the principal axes of crystallization, 

 terminating at the summits of the solid angles of the simple poly- 

 licdron, which axes alone have been capable of withstanding the ac- 

 tion of all the obstacles opposed to the tendency of the molecules to 

 unite in the form of an ellipsoid. 



On this hypothesis, the author explains how common salt, alum, 

 sulphate of iron, &c., crystallize in pure water in the most simj)le 

 forms, the reciprocal attraction of their molecules being controlled 



