On Crystallogi-aphy, 453 



it were a point which frequently escapes in the process 6f 

 crystallization, amid that multitude of circumstances which 

 infliience in so many ways the progress of this operation* 



The diversity of the primitive forms ought to be re- 

 garded as a certain indication of a difierence in nature be- ' 

 tween two substances, and the identity of primitive form 

 indicates that of nature, at all times when this form is not 

 one of those which have a marked character of regularity, 

 such as the cube, the regular octahedr«)n, &c. 



Secondary forms. In order to describe more easily the 

 secondary iornis, we shall suppose them always situated in 

 such >a mariner that the line which may be considered as 

 their axis has a vertical position, and then the faces parallel 

 to this axis will themselves bear the name oi vertical faces ; 

 we shall call hmizotital faces those which will be perpcn- 

 dicular^ and oblique faces [host which will be inclined to- 

 wards it. 



We are sometimes in the situation of indicating the in- 

 cidence of a face which is presented in front in the pro* 

 jection of a crystal, on that which is adjacent to it behind 

 the same crystal. We shall then give to the latter the 

 name oi returned fnceL Suppose, for example, that in the 

 distich topaz represented flJg. 61, PI. VII) it is requisite lo 

 indicate the angle formed by one or other of the panes o, o, 

 with that which is contiguous to it in the posterior part, 

 we shall say that the incidence of o on the returned pane 

 is 93° 6'. 



The forms of crystals are subject to various kinds of al- 

 terations purely accidental. One consists in certain laces 

 being nearer to, or more distant from, the centre in one 

 crystal than in another which belongs to the same variel^'i 

 in such a way, however, as constantly to preserve a certain 

 character of symmetry. In several cases these variations only 

 fall on the dimensions of the faces, and not on the number 

 of their sides. This happens with certain dodecahedral 

 garnets, which in the case of perfect symn^etry would have 

 ttieir surface composed of twelve equal and j^imilar rhombs, 

 and which are lengthened in the direction of an axis which 

 would pass by two of their opposite solid angles taken among 

 those which are formed of three plane angles. The dode- 

 cahedron is then presented under the appearance of a solid 

 with six panes which are elongated rhombs, with summits 

 of three faifes each which are true rnombs. In other cases, 

 the faces tnemselve*^, or some of them at least, change theit 

 figure, by the increase or diminution of the number of 

 F f 3 their 



