. 246. CHARACTERISTIC. 375 



character of the species should contain marks, which, if not 

 .always, at least very often may be superfluous in respect to 

 the mere process of distinction. The genus Emerald con- 

 tains two species, prismatic and rhombohedral Emerald. 

 For the sake of mere distinction, the character of each of 

 these species need not to contain any thing besides the 

 system ef crystallisation, or the limits of specific gravity, 

 for these likewise would suffice for distinguishing the two 

 species. A third species, however, might exist, besides the 

 two species above mentioned ; a species which, on account 

 of its natural-historical properties, did belong to the genus 

 Emerald, and that species might agree with one or the 

 other of these in the above mentioned characteristic marks. 

 In order to assure ourselves that an individual belonging to 

 the genus Emerald enters either within the species of the 

 prismatic or of the rhombohedral Emerald, their characters 

 are made to contain a greater number of marks, whose pro- 

 perties leave no doubt, upon the supposition of the indi- 

 vidual being an Emerald, whether and to which of the two 

 species the individual belongs. This arrangement, more- 

 over, produces an uniformity in the specific characters, 

 which, according to what has been stated above, is one of 

 their principal qualities. 



The specific characters, therefore, consist chiefly of three 

 marks of this kind, which, wherever the quality of the 

 species would allow, have been given in all instances. 

 These are the crystalline forms (including cleavage), the 

 degrees of hardness, and the specific gravity. The first 

 characteristic mark in the specific character is the system 

 of crystallisation. Then follows, together with its angles 

 (if these be known), the fundamental form, from which all 

 the other simple and compound forms of the species may 

 be derived. Of rhombohedrons the terminal edge is given ; 

 for instance, in rhombohedral Lime-haloide, R = 105 5' ; 

 of an isosceles four-sided pyramid, first the terminal edge, 

 then the lateral edge ; as in pyramidal Zircon, P = 123 

 19', 84 20' ; of a scalene four-sided pyramid the obtuse 

 terminal edge, the acute terminal edge, and the lateral 



