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guished from molecular attraction. The effect of obstacles, such 

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

 molecular attraction of the molecules themselves, when they arrive 

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

 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 placed 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 length, the solid, reduced to its most simple form, no longer pre- 

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

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

 hedron, 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 simple 

 forms, the reciprocal attraction of their molecules being controlled 

 and diminished by the affinity exerted on them by the molecules of 

 the water; whilst if some of these molecules of water are neutralized 

 by mixture with another soluble principle, they cease to act as an 

 obstacle to the crystallization of the body, which then takes forms 

 more complicated and approaching nearer to that of the normal solid 

 with a curved surface. 



M. Necker considers that the new views he has sketched require, 

 for their complete developement, many ulterior details, as well as 

 many new experiments and new facts ; but that the tendency which 

 the crystals of all systems present, to progress towards the curved 

 surface form appropriate to each system, by the complication of their 

 forces, is a fundamental fact of the first importance ; and that an ad- 

 vance has been made by showing the bearing of the important ex- 

 periments of MM. Leblanc and Beudant, and by having brought 

 the theory of crystallography nearer to those views which the pro- 

 gress of chemistry and of physics have led us to adopt, relative to 

 the form of the elementary molecules of bodies. 



January 24, 1839. 

 FRANCIS BAILY, Esq., V.P., in the Chair. 

 Charles Darwin, Esq., was elected a Fellow of the Society, 



