. 78. OF THE CONNEXION OF FORMS. 65 



culiar to certain mineral species, whilst others are never 

 found in the same substances. Thus hexahedral Gold is 

 found in hexahedrons, but never in rhombohedrons ; rhom- 

 bohedral Lime-haloide in rhombohedrons, never in hexa- 

 hedrons. 



Experience proves quite as generally that varieties of one 

 and the same accurately determined mineral species, may as- 

 sume several different forms of crystallization ; hexahedral 

 Gold, beside the form of the hexahedron, assumes also that 

 of the octahedron, of the dodecahedron, of the digrammic 

 tetragonal -icositetrahedron, &c. ; rhombohedral Lime-ha- 

 loide, besides rhombohedrons, exhibits also several isosceles 

 and scalene six-sided pyramids, and regular six-sided prisms; 

 and we may frequently observe, that even in one and the 

 same individual of such species, several of those simple 

 forms appear at the same time, or in connexion with each 

 other : thus, in hexahedral Gold, the hexahedron occurs in 

 one individual with the octahedron; in rhombohedral Lime- 

 haloide, rhombohedrons are found with pyramids, with 

 prisms, &c. 



It is likewise demonstrated by experience, that two or 

 more simple forms, if they appear at the same time, in a 

 species or an individual, do really possess certain dimen- 

 sions or relations towards each other, and that other forms, 

 though of the same kind with the preceding, are exclud- 

 ed from such species, merely on account of their dimen- 

 sions. Thus the species of rhombohedral Lime-haloide does 

 not present indiscriminately the forms of any rhombohe- 

 dron, or of any six-sided pyramid whatever ; but we find 

 only such as possess certain dimensions, upon which the 

 symmetry of their combinations depends. 



Natural History does not lead us to inquire into the fi- 

 nal cause of that remarkable fact, why the crystals of hexa- 

 hedral Gold should be hexahedrons, octahedrons, &c. ; and 

 why those of rhombohedral Lime-haloide should be rhom- 

 bohedrons and six-sided pyramids of certain dimensions. 

 Such questions, supposing even that they were capable of 

 being answered, are not within the province of Natural 



VOL. I. E 



