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
Hauy has thus denominated the gem which is known takes place, for example, on the angle g, the three rows
under the different names of ruby, sapphire, oriental which remain unoccupied between that angle and the
topaz, according as the colour is red, blue, or yellow. corresponding edge of the first lamina of superposition,
The different varieties of this gem have not been ac- are, 1. the small rhombus go i p, which alone forms the
curately described, aud the nature of the particular first row ; 2. the two rhombuses osti,px di; 3. the

P
angles of each has not been precisely indicated, on ac- three rhombuses situated on the same line behind the
count of the rare occurrence of regularly formed cry- two preceding
stals, or when such have been found, on account of Crystals of this gem are chiefly found in the kingdom
their being defaced by being water-worn, or otherwise of Pegu. Some have been found in France, which have
injured. But from some crystals which were suffi- received the name of sapphires of Puy. They have
ciently characterised, Hauy obtained the following re- been also found at a little distance from Velay, on the
sults.

banks of a rivulet near the village of Expailly, where
1. Primitive Oriental,

they are mixed with garnets and hyacinths. These have

all the characters of the stone which is denominated ori-
This mineral crystallizes in the form of a regular ental sapphire.
hexahedral prism, which is divisible parallel to its bases.
According to theory, which points out other joinings V. DIFFERENCE BETWEEN STRUCTURE and INCRE-
parallel to the planes, the molecule is an equilateral

MENT.
triangular prism. The height of this prism, calculated
by theory, is a little less than three times the height of In what has been said respecting the decrements to
the triangle of the base.

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-
Geometric Character.—Respective inclinations of the lieving that in the formation of a dodecahedral crystal,

triangles IAS, IBS, 139° 54'. Angles of the tri- or one of any other form, having a cube for a nucleus,
angle IAS, A=22° 54'. I or S=78° 47'.

the crystallization has originally produced that nu-
This form is the result of a decrement by a simple by the successive application of all the laminæ of super-

cleus such as it is extracted from the dodecahedron,
row of small quadrangular prisms on all the edges of the position with which it is covered. It seems proved, on
bases of the nucleus. Let q d (fig. 75.) be the supe-
rior base, subdivided into small triangles, which repre- already a very small dodecahedron, containing a cubi-

the contrary, that from the first moment the crystal is
sent the analogous bases of so many molecules. The
edges of the laminæ of superposition will correspond suc-

cal nucleus proportioned to its small size, and that the
cessively to the hexagons h ilmnr, e ku x yv, &c.; its form, by new layers which envelope it on all sides,

crystal afterwards increases by degrees without changing
from which it follows that the subtractions take place
by rows of small parallelopi peds of quadrangular prisms same relation with the whole dodecahedron.

so that the nucleus increases also, preserving always the
composed each of two triangular prisms.

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
4. Enneagonal Oriental, (fig. 81.)

M, next the three squares comprehended between L
Geometric Character.-Inclination of each small tri-

and O, and then the square E. This increment cor-
angle, such as c q i, to the adjacent base ac iplbged, cahedron began by being a cube proportioned to its

responds with that which would take place if the dode-
122° 18'.

bulk, and which increased afterwards with the addition
This is the elongated oriental, whose summits are re. of continually decreasing laminæ.
placed by two faces parallel to the bases of the nucleus,

But, on the other hand, the arrangement may be
with the addition of six small isosceles triangles cqi, conceived to be such as is represented in fig. 84. in
16 f, v % m, &c. the three superior of which are alter which the square a b c d is composed of only nine mo-

8 :
pate in position with the three inferior. These triangles lecules, and bears upon each of its sides only one