84, 



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 corundum 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 retreat 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 fracture and texture of this stone. 

 In general we are told that all the kinds 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, differs 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 qualities 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- 

 minae. 



8. The 8th section contains some observations on the phaenomena 

 of light exhibited by this stone. The prismatic, as well as the pyra- 

 midal 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 fol- 

 lows hence that the most compact sorts of corundum will not exhibit 

 this appearance. To the same property is also ascribed that beauti- 

 ful reflection of light in the form of a star of six rays, frequently pro- 



