Slyucture of are the result of a decrement, by three roirs of small sirncture of Crystals. Crystals whose Molecules are Triangular Prisms. quadrangular prisms on the three angles of the superior Crystals. base of the nucleus, such as b, d, : (fig. 75.), and on Oriental. the intermediate angles of the inferior base. It may be readily conceived, that in the decrement which P banks of a rivulet near the village of Expailly, where they are mixed with garnets and hyacinths. These have all the characters of the stone which is denominated ori- MENT. which the laminæ of superposition are subjected, the author observes, that it was his view only to unfold the 2. Elongated Oriental, (fig. 8o.). laws of structure, and he adds, that he is far from be- triangles IAS, IBS, 139° 54'. Angles of the tri- or one of any other form, having a cube for a nucleus, the crystallization has originally produced that nu- cleus such as it is extracted from the dodecahedron, the contrary, that from the first moment the crystal is cal nucleus proportioned to its small size, and that the crystal afterwards increases by degrees without changing so that the nucleus increases also, preserving always the An example taken from a plane figure will make 3. Minor Oriental. this more striking; and what is said respecting this fie gure may be easily applied to a solid, since a plane fiGeometric Character.-Dodecahedron formed of two gure may be always conceived as a section of a solid. right pyramids less elongated than those of the pre Let ERFN (fig. 82.) be an arrangement of small ceding variety. The triangles corresponding to squares, in which the square ABCD, composed of 49 IAS, IBS, are inclined to each other 122° 36'. In partial squares, represents a section of the nucleus, and each of these triangles the angle of the summit is 31°, the extreme squares R, S, G, A, I, L, &c. that of the and each of the angles at the base is 74° 30'. kind of stair formed by the laming of superposition. The law of which this variety is the result, differs It may be readily conceived, that the arrangement befrom that which produces the preceding, as it deter- gan with the square ABCD ; and that different files of mines a mixed decrement by three rows in breadth and small squares were afterwards applied on each of the two rows in height. sides of the central square : for example, on the side AB, first the five squares comprehended between I and M, next the three squares comprehended between L and O, and then the square E. This increment cor- responds with that which would take place if the dode- bulk, and which increased afterwards with the addition But, on the other hand, the arrangement may be 8 : square Structure of square e, n, f, or r; and that afterwards, by means of Crystals. the application of new squares arranged round the former, the assortment has become that of fig. 83. where the central square a'b'd d' is formed of 25 small squares, and bears on each side of its sides a file of three squares, plus a terminating square e', n', f', or '; and that, in short, by a farther application, the assortment of fig. 83. is converted into that of fig. 82. These different transitions will give some notion of the manner in which secondary crystals may increase in bulk, and yet retain their form; and from this it will appear, that the structure is combined with that augmentation of bulk, so that the law, according to which all the lamina applied in the nucleus of the crystal, when arrived at its greatest dimensions, successively decrease, in departing from this nucleus, existed already in the rising crystal. Such is the ingenious theory of the structure of crystals, which the author observes, is in this similar to other theories, that it sets out from a principal fact, on which it makes all facts of the same kind to de- Structure of pend, and which are only as it were corollaries. This Crystals. fact is the decrement of the laminæ superadded to the primitive form; and it is by bringing back this decrement to simple and regular laws, susceptible of accurate calculation, that theory arrives at results, the truth of which is proved by the mechanical division of crystals, and by observation of their angles. But new researches are still wanting, in order to ascend a few steps farther towards the primitive laws by which crystallization is regulated. The object of one of these researches would be to explain how these small polyhedrons, which are as it were the rudiments of crystals of a sensible bulk, sometimes represent the primitive form, without modification; sometimes a secondary form produced in virtue of a law of decre ment; and to determine the circumstances which produce decrements on the edges, as well as those which give rise to decrements on the angles. |