142 Mr Haidinger on the Series of Crystallisation of Apatite. 
not take place in reality* The combination of the same forms, 
as produced by Nature, is that in Fig. 2., where those faces of 
which are inclined towards the upper extremity of the crys- 
tal, appear only at the left, those which are inclined towards the 
lower one, if the crystal be inverted, only appear at the right 
hand edges of combination between 2(R) and P-{-oo . Among 
all those mineral species, whose forms belong to the rhombohe- 
dral system, there is only one, which, in some respect, may be com- 
pared to the species of apatite, as exhibiting a peculiarly remark- 
' able character in the combinations containing the faces of scalene 
six-sided pyramids. The varieties here alluded to are those of 
rhombohedral quartz, called plagiMre^ coordonne^ &c. by Haiiy, 
which, a long time since, have fixed the attention of naturalists, 
and lately received an additional degree of interest, by the re- 
cent discovery of the exact agreement between this external form, 
and the phenomena of the circular polarisation, as demonstrated 
by Herschel. Yet there is one striking difference between the 
two species, in the mode in which che opposite extremities of their 
crystals are terminated ; in quartz, notwithstanding the crystal 
may be inverted, yet, on the opposite end, the inclined faces of 
the scalene pyramids will continue to appear at the same side of 
the faces of R : on both ends the right, or on both ends the left. 
In apatite, the right hand faces, contiguous to one of the extremi- 
ties, co-exist with the left hand ones of the other ; and hence there 
is no such difference of right or Ifb individuals^ as in the other spe- 
cies. This difference appears perfectly evident, in comparing the 
variety of apatite in Fig. 2., with those of quartz in Fig. 3. and 
Fig. 4., the former of these being a right, the latter a left in- 
dividual. If all these inclined faces be duly enlarged, till they 
intersect each other, and limit the space by themselves, they 
produce in apatite a form, which, but for its position in respect 
to other forms of the series, might be called an isosceles six-sided 
pyramid, whilst the result of this enlargement of the faces in 
quartz would be solids contained under twelve, or if th^y ap- 
pear only at the alternating angles of the common combination, 
under six trapezoidal faces, which solids, even by themselves, 
continue to shew that relation between right and left, in which 
they appear in the combinations. The varieties of apatite, in 
which I first remarjced this singular configuration of the crys- 
