On the cryJIaUine Forms of CovunduDi. c^j 



to different clrcumftances, which have exifted at the time of their cryilallization. \fe may 

 conceive, for inftance, that if before the progreflive decreafe of the cryltalline laminae^ 

 in the manner above mentioned, the increafe of the rhomboidal parallelopiped had taken 

 place, by a fuperpofition of laminx, in which the rovi's of cryflalline molecules experienced 

 a progreflive decreafe along the edges of the acute angle of the bafe only (fig. 6.), and that 

 (the fides of the prifm having already acquired a certain length) the fucceeding cryltalline 

 laminse had experienced a decreafe at the acute angle of the fummit, the fame regular 

 hexaedral prifm would have refulted from this procefs ; but the proportion between the 

 height, and the line drawn from two of the oppofite fides of the planes, on the extremities, 

 would have been much greater than that of 6,45 : 5, and confequently this prifm would 

 have been longer than that of the rhomboidal parallelopiped, which ferved as its nucleus. 

 On the other, hand, if the increafe of the rhomboidal parallelopiped had taken place, by a 

 fuperpofition of cryflalline laminse, decreafing at the acute angle of the fummit, and fomc 

 time after decreafing alfo along the fides of the acute angle of the bafe (fig. 7.), the regular 

 hexaedral prifm refulting from this procefs, would have been (horter, in proportion to the 

 duration of the mode of decreafe in the cryflalline laminse, which were firfl: depofited. There 

 are fome of the hexaedral prifms, in corundum'cryftals, which are fo Ihort, that they appear 

 no more than fegments. Calcareous fpar ofiers the fame phenomenon ; as do likewife all 

 the fubftances in which the hexsedral prifin has any analogy of formation, with that which we 

 have here defcribed. 



It happens frequently, when the fuperpofition of the cryftalUne laminae does not go oti 

 equally on all the faces of the rhomboidal parallelopiped, that one or two only of the folid 

 angles of the hexxdral prifm, taken alternately, ftill fhew, by fmall ifofceles triangular 

 planes> fome remains of the faces of the parallelopiped, while the others do not fhew any 

 at all. 



Mr. Greville, in his colle£lion of this fubftance, has a cryftal of corundum, upon one 

 -fide of which only two of the planes of the rhomb have experienced an equal and perfe£l 

 fuperpofition, while there has been but a very fmall number of cryflalline laminje depofited on 

 die third plane. Confequently this cryftal prefents a regular hexaedral prifm, one of whofe 

 folid angles is fo much truncated, that the half of the plane of the end of the hexaedral 

 prifms difappears (fig- 8.); and this cut, or feflion, forms an angle of 122° 34' with the 

 plane on the extremity. 



It is unneceflary to obfcrve, that the regularity of the hexaedral prifm depends on that of 

 the rhomboidal parallelopiped, on which it is formed. 



When, by detaching the laminse from the alternate folid angles of the regular hexaedral 

 prifm, the planes refulting from this operation begin to run into one another -, and the cryftal 

 begins to afTume the form of the rhomboidal parallelopiped, to which it owes its origin ; we 

 frequently fee the furface of thefe new planes divided into an immenfe number of fmall 

 rhombs, formed thereon by the interfe£tion of lines, that are parallel to the fides, which be- 

 long to the rhomboidal form of the new faces (fig. 9.). 



Thefe lines are owing to the extremities of the laminse, which have been depofited on the 

 •"ferior faces, correfponding with thofe on which we obferve them ; and they ferve to cor- 



4 A 2 , roborate 



