. 256. PHYSIOGRAPHY. 7 



Yet a calculation would always be required, if we intended 

 to find the angles of the simple forms from the given value 

 of the axis, or from that of the mentioned ratios between 

 the axis and the diagonals ; and as it is useful and interesting 

 to know what simple forms have already been observed, or 

 wliich among these most commonly occur in the species ; 

 the derived forms have also been indicated, along with the 

 measure of their angles, and expressed by means of their 

 crystallographic signs. In respect to the indication of their 

 angles, a general exception has been made in those limit- 

 ing forms, in which a similar determination is not neces- 

 sary, since their angles are the same wherever they occur, 

 and have been given in their proper place, under the head 

 of Terminology. An asterisk over the crystallographic 

 sign of a simple form denotes, that it has been observed 

 in nature, without any additional faces. 



The peculiar mode in which the simple forms of a spe- 

 cies join in combinations, is another very important subject 

 in the general outline of that species; it is the Character of 

 Combinations (. 145., &<%). The simple forms of octahe- 

 dral Fluor- haloide, those of hexahedral Iron-pyrites and 

 of tetrahedral Copper-glance, belong to one and the same 

 series of crystallisation, and yet every one of these species 

 contains some simple forms not to be met with in the others, 

 and which nevertheless impart a peculiar aspect to the com- 

 binations in which they enter, so that the crystallisations 

 of the three species differ most essentially from each other. 

 This difference is expressed in the Character of Combina- 

 tions, which is tessular (. 15C.) in the Fluor-haloide, semi, 

 tessular of parallel faces (. 157-) in the Iron-pyrites, and semi- 

 Icssiilar of inclined faces (. 15?.) in the Copper-glance. In the 

 rhombohedral system, the combinations are either rhombo- 

 hedral or di-rhombohedral, or hemi-rhombohedral, &c. 

 agreeably to . 145., &c. The first of these applies to the 

 species of rhombohedral Liuie-haloide, and its Character of 

 Combinations therefore is rhombohedral. 



Next to this are indicated some of the most common or 

 remarkable combinations occurring in the species, expressed 



