Theory of CttfaUifatioti. W5 



duce the fix planes of the prifm. For example, If we con- 

 fer the effea of the decrement on the angle dfx, it is 

 neceflary alfo that the laminae applied on the faces afdb, 

 a f x g {ji g . 4 . ), fhould experience towards their lateral angles 

 afd, afx, adjacent to the angle dfx, variations which fe- 

 cond the effea of the generating decrement. But here 

 thefe variations are intermediary decrements by ranges o£ 



double moleculse. 



To conceive a better idea of thefe variations, let us re- 

 fume the face a b df (fg. 50A The variations in queftion 

 will take place parallel to the lines c e, r x, gz, v v, && 

 that is to fay, by one range of double molecule, and in fuch 

 a manner that there will always be two lamina? on a level at 

 their edges in the direftion of the height. By this it is evi- 

 dent why the lamins taken from the prifm by the firft fee- 

 tions are trapeziums, fuch zsplus {fg. i.), « which the 

 aifortment of the fmall compofing rhombufes will be the 

 fame as on the trapezium u sop {fg- 50.). We may in the 

 like manner affign the reafon of the different figures through 

 which the laminae, fucceffwely detached, before arriving at 

 the nucleus, are obliged to pafs. But this detail would lead 

 us too far. In a word, I mud here repeat, that every thing 

 is included in the effect of the principal decrements : that 

 is to fay, in the prefent cafe, of thofe which take place on the 

 fuperior and inferior angles, or parallel to the horizontal 

 diagonals ; and after the firft lamina of fuperpofition, the 

 figure of the cryftal is given according to this Ongle condi- 

 tion, that the initial faces be prolonged fo as to interfea 

 each other. 



The prifm is fufcept'ible of varying in the length of its 

 axis compared with its thicknefs, which depends on the dif- 

 ferent epochs at which the decrements commence, or are 

 fuppofed to commence. For example : if we fuppofe that 

 the decrement, which takes place towards the inferior angle, 

 acts alone at firft on a certain number of lamina?, the axis 

 of the cryftal will be fo much the longer as the commence- 



U 4 ment 



