Theory of Cryfiallifation. 3 Of 



fmall parallelograms, and which are always extremely fmooth 

 and brilliant. Moreover, if the diagonal O R be drawn, it 

 .will be found perpendicular to OA and RN; or, what: 

 amounts to the fame, will be fituated horizontally, by fup- 

 .pofing that the ridges O A, B H, &c. have a vertical 

 pofition. We fhall foon have occafion to make ufe of this 

 obfervation. 



The polynomous petunze prefent.s the mpft complicated 

 variety which I have obferved among cryftals of this kind. 

 To form an idea of its ftru&ure, let us fuppofe that hpyr 

 {fig. 57) reprefents a fedtion of the nucleus A R (jig. 58) 

 made by a plane perpendicular to the parallelograms 

 GOAD, BOAH, and fubdivided into a multitude of fmall 

 parallelograms, which are the analogous fections of fo many 

 moleculse. Here the fide yr (fig. 57) which is the fam# 

 fedtion of the cutting plane as GOAD, is greater than it 

 ought to be in regard to the Gde c r (jig. 57), which is the 

 fame fe£tion as BOAH (jig. 58): but thefe dimenfions 

 are fuited to thofe of the fe.condary cryftal, and here occafion 

 no difficulty, becaufe we may fuppofe that the primitive 

 form has been extended more in one direction than in 

 another ; for this form, as 1 have already remarked, is only 

 a convenient datum for the explanation of the ftruclure, and 

 the cryftal confifts merely in an afTemblage of fimilar mole- 

 cule ; fo that it is the dimenfions of thefe moleculae which 

 remain invariable. 



This being premifed, we fhall find, by comparing the 

 figures 56 and 57 ; ift, that the vhnefabtik/ih (fig. 56) 

 and its oppofite, which correfpond to mn, dg y (fig. 57), are 

 parallel to two of the planes of the nucleus, viz. GOAD, 

 BRN H (fig. 58), and confequently do not refult from any 

 Jaw of decrement ; 2d, that the plane P m N, and its op- 

 pofite (fig. 56), which correfpond to ao, eg (fig. 57), are 

 alfo parallel to two of the planes of the nucleus, viz. 

 BOAH, RGDN (fig. 58); 3d, that the plane on km 

 and its oppofite ( %. 56), which correfpond to o», e &% 



