Page:Proceedings of the Royal Society of London Vol 1.djvu/104

Rh perfect kind, will emit a light of a very deep ﬁre-colour, similar to that of a red hot iron.

5. The speciﬁc gravity of this stone varies in all its different kinds. The means deduced from a great number of observations afford the following numbers.

Imperfect corundnm 3931. Perfect corundum, in the instance of Oriental ruby, 3977; and of sapphire, 4158. The difference seems to be proportionate to the degree of perfection of the crystallization, and consequently of the transparency of the stone.

6. We come’now to the most extensive and most elaborate sec- tion_ of the paper which treats of the crystalline forms of the different kinds of corundum. The primitive form of all the kinds, whatever be their degree of perfection, we are here told is a rhomboid slightly acute, the obtuse angles of the planes measuring 94°, and the acute ones 86° ; and it is asserted, that whatever the form of an individual crystal may be, it may always, by dividing it according to the lately established rules of crystallography, be ultimately reduced to this rhomboidal form. The manner in which crystals deviate from their primitive form. by the substitution of planes for the angles, effected by the retrmt of rows of molecules, which constitute the crystalline laminae, is amply discussed in a note; and nine modiﬁcations are described, forming a great variety of prismatic, pyramidal. and other- crystals, of which some idea can only be formed by an inspection of the ﬁgures that accompany the paper.

7. The next section treats of the {mature and tertwe of this stone. In general we are told that all the inds have a lamellated texture, the layers being in a direction parallel to the faces of the rhomboid, and that they break in a direction parallel to those faces. The ease. however, with which these laminae may be divided, diﬂ’ers greatly in the different varieties; and this is ascribed to the degree of force ex- isting in the attraction of the molecules which compose these cry- stals, as well as to the perfect adhesion of the crystalline laminae composed of those molecules at all points of their surface. This at— traction and adhesion, it is thought, varies with the colour of the stone, the blue or sapphire possessing those qualiﬁes in the highest degree, which accounts for the fracture of this stone being often in a direction oblique, and even at right angles to the planes of the la- mmae.

8. The 8th section contains some observations on the phenomena of light exhibited by this stone. The prismatic, as well as the pyramidal crystals of corundum, when their extremities are terminated by planes which are perpendicular to their axes, very frequently exhibit on these planes a changeable variety of colours, known by the name of chatoyant. This property is ascribed to the reﬂection of light in the small intervals which remain between the crystalline laminae in those parts Where these laminae are not in perfect contact. It follows hence that the most compact sorts of corundum will not exhibit this appearance. To the same property is also ascribed that beautiful reﬂection of light in the form of a star of six rays, frequently pro-‘