6 PHYSIOGRAPHY. . 256. 



rules be strictly followed which have been mentioned in 

 . 244., in respect to the Characteristic. 



. 250. ARRANGEMENT OF THE GENERAL DESCRIP- 

 TION. 



The general or collective descriptions require to 

 be so arranged, as to facilitate their use as much as 

 possible, and to produce, in fact, a complete gene- 

 ral view of the species. 



An example taken from one of the general descriptions 

 contained in this Treatise, will be the best means to shew 

 their arrangement in a greater detail ; for which purpose, 

 we select that of rhombohedral Lime-haloide, which is par- 

 ticularly well calculated for illustration, on account of the 

 numerous varieties which this species comprehends. 



In order to determine the series of crystallisation of a 

 species in general, it is necessary to indicate its funda- 

 mental form with its dimensions expressed by numbers. 

 The authority has been given in the forms of variable di- 

 mensions. Those which have been re-examined, or newly 

 or more accurately determined, are likewise indicated by the 

 addition of R. G. (Reflective Goniometer) or AP. (approxi- 

 mation) ; the first expressing a higher degree of exactitude. 



The angles or dimensions of every one of these which 

 can possibly occur in the species, may be calculated from 

 the fundamental form, agreeably to the rules laid down 

 for the derivations of other simple forms. For the sake of 

 greater convenience in calculating the angles of rhombo- 

 hedral Lime-haloide, the value of a, the axis of the funda- 

 mental form, has been given ; its horizontal projection being 

 supposed =5= 1. The same is observed in forms of the Py- 

 ramidal system. In the fundamental forms belonging to 

 the Prismatic system, the ratio of the axis and the two dia- 

 gonals, a : b : c has been given ; in Hemi-prismatic forms 

 the ratio of the four lines a : b : c : d, as explained in 

 .98. 



