Theory of CnjtalTijation. 39* 



Let us make this more ftriking by an example taken from 

 a plane figure. What I am going to obferve refpe6ting this 

 figure may be eafiiy applied to a folid, fince we may always 

 conceive a plane figure as a feelion of a folid. Let E R FN 

 then (Jig. 74.) be an anbrtment of fmall fquares, in which 

 the fquare ABCD, compofed of forty-nine partial fquares, 

 rcprefents a feclion of the nucleus, and the extreme fquares 

 R, S, G, A, I, L, &c. that of the kind of flair formed by 

 the laminae of fuperpofition. It may be readily conceived 

 that the anbrtment began by the fquare ABCD; and that 

 different files of fmall fquares were afterwards applied on 

 each of the fides of the central fquare, for example, on the 

 fide A B, firft the five fquares comprehended between I and M, 

 next the three fquares comprehended between L and O, and 

 then the fquare E. This increment corresponds with tha^ 

 which would take place if the dodecaedron began by being 

 a cube, proportioned to its bulk, and which increafed af- 

 terwards by the addition of continually decreafing laminae. 



But, on the other hand, we may conceive that the affort- 

 ment was at firft: like that reprefentcd by fig. 76, in which 

 the fquare abedis compofed of only nine moleculae, and 

 bears upon each of its fides only one fquare e, n K f\ or r ; 

 and that afterwards, by means of the application of new 

 fquares, arranged around the former, the anbrtment has 

 become that of figure 75, where the central fquare a' V c'-df 

 is formed of twentv-five fmall fquares, and bears on each 

 of its fides a file of three fquares, plus a terminating fquare 

 /,n',f or/, and that, in fiiort, by a farther application, 

 the aiibrtment of fig. 75 is converted into that of fig. 74, 

 Thefe different tranfitions will give an idea of the manner in 

 which fecondary cryftals may augment in bulk, yet retain 

 their form; from which it is feen that the ftru&ure is com- 

 bined with that augmentation of bulk, fo that the law, ac- 

 cording to which all the laminae applied in the nucleus of 

 the cryftal.when arrived at its greateft dimenGons,fucceflivcly 

 C c 4 decreafe, 



