ORDER I. RHOMBOHEDRAL FLUOR- II ALOIDE. 77 



As has been shewn in Vol. I. . 115., this equality of the 

 angle is a general relation in the rhombohedral system, 

 where, from the formula m = 3m/+1 it appears that m = 3 



3(tn'--i) 



gives m' = , that m = 2 gives m' = , &c. It is deserv- 

 ing of notice, that if T i , the first variety of tetraconta-oc- 

 tahedrons (Vol. I. . 77-)> be placed in an upright position, 

 so as to have one of the rhombohedral axes vertical, and 

 the hexahedron be supposed = R, as the fundamental form 

 of the series ; the vertical faces will be expressed by the 

 sign (P + co) 5 . The inclined faces would be, upon the 

 same supposition, P 1. (P I) 2 . (P I) 5 . 



2. The varieties of the present species, by no means less 

 distinct than those of the preceding one, decidedly sepa- 

 rate from others and connected among themselves, have 

 been considered by some mineralogists as forming two or 

 even three different species, and farther subdivided into 

 several sub-species or kinds. The distinctive marks be- 

 tween them, however, are so slight, that they cannot be 

 indicated with precision, nor usefully employed in an 

 actual distinction. It would be superfluous, therefore, to 

 attempt their explanation. The varieties of Apatite, 

 Asparagus-stone, and Phosphorite, are so intimately con- 

 nected, that they pass insensibly into each other ; and thus 

 demonstrate that in fact they form but one and the same 

 species, which is an immediate consequence of the transi- 

 tions, and in the present species was first ascertained by 

 the Abbe HAUY. 



3. Rhombohedral Fluor-haloide consists of 



Lime 55-0. 



Phosphoric Acid 45-0. KLAPROTH. 



It is Ca 3 P 2 , according to which formula the proportion of 

 lime to that of phosphoric acid will be as 54-48 : 45-52. 

 It is slowly soluble in nitric acid, and without effervescence. 

 Some varieties are phosphorescent upon ignited charcoal 

 and before the blowpipe, others even when rubbed with 

 hard bodies. In a very strong heat of the blowpipe, the 



