358 On Crystallography/. 



that they are subjected to certain laws always analogous ta 

 those of the structure; and that these crystals, instead of 

 being tuniultuously precipitated on each other, have in 

 some measure concerted their arrangement. 



Let us also on this occasion choose a very simple exam- 

 ple. Let A C ^fig. 46) be a cube, and M N r an equi- 

 lateral triangular facet produced in the place of the angle A, 

 in virtue of a decrement by one range round this same 

 angle. Let us suppose a second cube modified in the same 

 maniKT, and fastened to the former by the facet which re- 

 sults from the decrement indicated. We shall thus have 

 the assortment represented by fig. 47. 



We may now conceive that one of the two cubes, that 

 for example which is placed below, is increased in all its 

 dimensions, except at the places where the other forms an 

 obstacle to it. In proportion as this increment becomes 

 more considerable, the upper cube will be more and more 

 engaged in the inferior one, and it may even finish by being 

 entirely masked or concealed by it. We observe crystals 

 effectually sunk into each other at various depths ; but 

 which have always a plane of junction situated like a face 

 produced by a decrement, in such a manner that the two 

 structures follow their ordinary progress, each on its own 

 part, the length of this same plane, which serves as their 

 respective limit. I have divided cubes of fluatcd lime in- 

 serted into each other; and 1 have remarked that the la- 

 minae of each extended without interruption, until suddenly 

 stopped by the common plane of junction. 



The example now quoted relates to a very simple and 

 wry regular Jaw of decrement. But frequently the laws 

 which determine the plane of junction are more or less re- 

 liiote from this simplicity, and there are a few which are 

 somewhat extraordinary. 



When two prisms cross towards the middle of their axes, 

 there are two planes of junction, which unite, crossing 

 each other on one common line, as we shall find under the 

 an\c]e slaurotide. and both these planes also have positions 

 analogous to tlu)sc which would be determined immediately 

 by laws of decrements. 



To conclude : I have here presented the results of but a 

 small number of particular observations. I propose after- 

 wards to resume the subject now glanced at, and to' give a 

 fuller development to the theory of v'hich I think it sus- 

 ceptible, 



[To be continued.] 



L. An 



