. 256. PHYSIOGRAPHY. 11 



A great number of the different varieties of certain spe- 

 cies is produced by the composition of their individuals. 

 The species of rhombohedral Lime-haloide is one of the 

 most remarkable in this respect ; and this has been the 

 reason of its having been divided and subdivided into a 

 great number of sub-species and kinds. If the collective 

 description of the simple varieties has been drawn up with 

 sufficient accuracy, it will not be difficult to join the com- 

 pound varieties within a narrow compass which can be easily 

 surveyed, and annexed to the former. This has been done 

 in rhombohedral Lime-haloide. Among the compound va- 

 rieties, the most remarkable are those in which the compo- 

 sition follows a certain rule, that is the twin-crystals. The 

 general consideration of the twin-crystals (. 179-) contains 

 the principles of a method, according to which those be- 

 longing to any particular species may be indicated with 

 precision and convenience, and provided with an appro- 

 priate designation. This method consists in expressing 

 the form of the regularly joined individuals by means of 

 their crystallographic signs, and in determining in the same 

 way the axis of revolution and the face of composition. 

 It will be still more general if we conceive, that not any 

 particular observed crystalline variety, but indeed every 

 simple or compound form belonging to the species, may be- 

 long to the individuals, which are joined in the regular 

 composition. 



It will be sufficient only to mention the imitative forms, 

 in order to recal their properties to the memory, these be- 

 ing commonly so much alike in every instance, that they 

 allow of a general explanation, which has been given in its 

 proper place (. 181, &c.). The condition of their surface, 

 or of the faces of composition in their interior, the shape of 

 the particles of composition, and the mode of that compo- 

 sition itself, may likewise be indicated. It is necessary 

 to remark here that the occurrences of composition are 

 not mentioned for the purpose of recognising or distin- 

 guishing the compound varieties of rhombohedral Lime- 

 haloide from those of any other species, which in fact 



