370 PHILOSOPHICAL TRANSACTIONS. [ANNO 17Q8. 



other angles, they may be broken, but it is impossible to detach them. When, 

 instead of the solid angles of a hexaedral prism, small triangular planes are met 

 with, (which frequently happens, whether caused by violence or otherwise,) they 

 are always placed in the direction above- mentioned. 



If, by following this indication of nature, we continue to detach the crystalline 

 laminae, we shall at last cause the form of the hexaedral prism to disappear totally, 

 and, instead of it, a rhomboidal parallelopiped will be obtained, (fig. 2,) of which 

 the plane angles at the rhombs will be 86° and Q4°; the solid angles at the summit* 

 will measure 84° 31'; and that taken at the re-union of the bases will be 95° 20'. 

 We can split this parallelopiped only in a direction parallel to its faces; it will still 

 consequently preserve the same form, which is that of the nucleus of this sub- 

 stance, and its primitive form. 



It is, therefore, by a modification of the rhomboidical parallelopiped, (fig. 2,) 

 that nature has formed the regular hexaedral prism (fig. 1,) which this substance 

 presents. For if we conceive, that in any period whatever of the increase of the 

 rhomboidal parallelopiped, a series of laminae or crystalline plates has been deposited 

 on all sides of the parallelopiped; and that these laminae have all undergone a pro- 

 gressive decrease of 1 row of crystalline molecules, at the acute angle which tends 

 to form the summit, and also along the sides of the opposite acute angle, (fig. 3 

 and 4,) there will necessarily result from the continuation of this superposition, 

 to a certain period, an hexaedral prism, terminated by 2 triedral pyramids, placed 

 in a contrary direction; and their planes or faces, which form a solid angle of 

 147° 26', with the sides of the prism, will be either pentagonal, (fig. 3,) or tri- 

 angular, (fig. 4.) They will also have, instead of a summit, an equilateral trian- 

 gular plane, sometimes greater and sometimes smaller. 



If the superposition continues, the equilateral triangular plane on the summit 

 will become nonagonal, and there will remain no other traces of the primitive 

 planes of the rhomboidal parallelopiped, than small isoceles triangular planes, 

 (fig. 5 :) if the superposition still continues, till the last crystalline lamina is reduced 

 to a single molecule or point, no appearance of the rhomboidal parallelopiped will 

 then remain ; and the crystal resulting from this operation of nature will be a regular 

 hexaedral prism, (fig. 1.) In the same manner, viz. by a decrease on the lower 

 edges of the laminae, the primitive rhomboidal parallelopiped of calcareous spar 

 passes to a regular hexaedral prism of that substance; though more frequently it 

 does so by a decrease on the lower angles of the laminae. 



When the laminae of the corundum crystal have, during their superposition on 

 the planes of the primitive rhomboidal parallelopiped, experienced a progressive de- 

 crease at 1 of their acute angles, and along the sides of the other, at the same 



* For greater clearness, this rhomboidal parallelopiped may be considered as being formed by the 

 junction of 2 triedral pyramids, base to base ; and the 2 solid angles (each of which is formed by the 

 re-union of 3 of the acute angles on the planes of the rhomb) will then be considered as the summits 

 of these pyramids. — Orig. 



