THERE is something very fascinating about crystals. It is not merely the intrinsic beauty of their forms, their picturesque grouping, and the play of light upon their faces, but there is a feeling of wonder at the power of Nature, which causes substances, in passing from the fluid to the solid state, to assume regular shapes bounded by plane faces, each substance with its own set of forms, and its faces arranged with characteristic symmetry: some, like alum, in perfect octahedra; others, like blue vitriol, in shapes which are regularly oblique. It is this power of Nature which is the subject of this discourse. I hope to show that crystalline forms, with all their regularity and symmetry, are the outcome of the accepted principles of mechanics. I shall invoke no peculiar force, but only such as we are already familiar with in other facts of Nature. I shall call in only the same force that produces the rise of a liquid in a capillary tube and the surface-tension at the boundary of two substances which do not mix. Whether this force be different from gravity I need not stop to inquire, for any attractive force which for small masses, such as we suppose the molecules of matter to be, is only sensible at insensible distances is sufficient for my purpose.