68 On Crystallography. 



5. SOLID WITH 24 EQUAL AND SIMILAR TRAPEZOIDS. 

 Names of the Substances. Primitive Forms. 



Muriated ammonia Regular octahedron 



Garnet . Rhomboidal dodecahedron 



Amphigene Cube 



Analcime Ditto 



Sulphurated iron Ditto, 



Explanation of the plan which has been adopted in the de- 

 scriptions of the different species of minerals. 



After having given the sytfonymy of (he best known 

 authors, we have successively presented the essential cha- 

 racter of the substance, and the physical, geometrical, and 

 chemical characters, the assemblage of which forms the 

 specific character. 



We have excluded from this character every thing con- 

 nected with fugitive accidents, such as colours, when they 

 are owing to a principle which is only interposed in the 

 substance. 



In the detail of the geometrical characters, care has been 

 taken to indicate not only the direction of the natural joints, 

 Jbut also the greater or less facility of obtaining them, their 

 difference of neatness in one and the same crystal, and 

 those, in short, whose positions are only presumed. In 

 addition to this, we haVe made known, in a note, the re- 

 spective dimensions of the molecule, and all that may serve 

 as data for applying the theoretical calculus to the laws of 

 decrements upon which the secondary forms depend. 



After the indication of the chemical characters, we have 

 "given the result of the analyses of the substance which 

 seem to have merited most confidence. 



The table of varieties, which follows the characters, is 

 generally divided into two sections, one of which contains 

 the descriptions of the forms, and the other refers to the 

 accidents of lights. The forms are either determinable, 

 i.e. susceptible of being described geometrically, from the 

 number, the disposition and the mutual incidences of their 

 faces, or indeterminable, i. e. produced by a confused or 

 precipitated crystallization, so that geometry cannot de- 

 scribe them, and we can at most indicate the vague rela- 

 tions which exist between them and known objects ; as 

 when we say of a mineral that it is cylindrical, globular, 

 granular, &c. and the last term of this kind of degradation 

 of forms is expressed by the word amorphous, which de- 

 signates a substance in masses of a certain volume com- 

 pletely irregular. The 



