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,
