On Crystallography. 333 



T)'tfference letti'een the Structure and the Increment. 



In the preceding development of the theory, we havfe 

 suppojed that the component laminae of crystals originally 

 of one and the same species, issue from one common nu- 

 cleus, undergoing decrements subjected to certain laws, 

 upon which the forms of these crystals depended. But here 

 it is only a conception, adopted to make us more easily per- 

 ceive the mutual connections of the form in question. 

 Properly speaking, a crystal in its entire state is only a 

 regular group of similar molecules. It does not commence 

 by a nucleus of a size proportioned to the volume which 

 it ought to acquire, or, what comes to the same thing, by 

 a nucleus equal to that which we extract by the aid of me- 

 chanical division ; and the latnin^ which cover this nu- 

 cleus are not applied successively over each other in the 

 same order in which the theory regards them. The proof 

 of this is, that among crystals of different dimensions 

 which are frequently attached to the same support, those 

 W'hich can only be distinguished by the microscope are as 

 complete as the most bulky; from which it follows that 

 They have the same structure, i. e. they already contain a 

 small nucleus proportioned to their diameter, and envelop- 

 ed by the nutriber of decreasing laminae necessary, in ordef 

 that the polyhedron should be provided with all its faces. 

 We do not perceive these various transitions of the primi- 

 tive to the secondary forms, which ought to take place if 

 crystallization constructed, as if by courses, the species of 

 pyramids superadded to the nucleus, m going from the base 

 to the summit *. 



We must therefore conceive, for example, that from the 

 first instant a crystal, similar to the dodecahedron with 

 rhomboidal planes derived from the cube (figs. 11 and 12), 

 is already a very small dodecahedron, which contains a cu- 

 bical nucleus proportionally small, and that in the follow- 

 ing instants this kind of embryo increases without changing 

 its form, by new strata which envelop it on all sides ; so 

 that the nucleus increases on its part, always preserving the 

 same relation with the entire crystal. 



We shall make this idea apparent by a construction re- 

 lative to the dodecahedron now mentioned, and represented 

 by means of a plane figure. What we shall say of this 

 figure may easily be applied to a solid, since we may al- 



• This, however, is only generally true; for it sometimes happens, in 

 arti^-ial crybtaliizalion, (and it is very priibable that we may say .is much 

 of tii.it of iialiiral hcdics,) that a form which had at[ai:)ed a certain dcirree 

 of increment cuddcnly undergoes variaiiuus by the effect of tome particular 

 eircu instance. 



Vol. 34. No. 139. Nov. I80f). Z ways 



