Mr Haidinger on the Series (^Crystallisation of Apatite. 14S 
tals, were those from Schlaggenwald in Bohemia^ I have since 
had the opportunity of examining a great number of crystals 
from different localities, which uniformly exhibited the same ap- 
pearance. 
The forms of apatite belong to the rhombohedral system of 
Professor Mohs. The character of their combinations is di- 
rhombohedral ; that is to say, the combinations contain the faces 
of the rhombohedrons in both, the parallel and the turned posi- 
tion, whenever one of them is met with in a compound form. 
Thus, in Fig. 5., the faces s, s, &c. belong to R, the fundamen- 
tal rhombohedron of the species. Fig. 6. ; whilst s\ s', &c. if 
duly enlarged, till they limit the space by themselves, will pro- 
duce, Fig. 7*, a rhombohedron of exactly the same dimensions, 
only in a position 60° different from that of R. Of the other 
forms contained in the combination P, being the plane perpen- 
dicular to the principal or rhombohedral axis, is evidently R — go , 
or the limit on one side of the series of rhombohedrons. 
Many persons, not sufficiently acquainted with every part of 
the system of crystallography of M. Mohs, have found fault with 
the signs dependent, like that of R — go, upon the idea of infinity. 
Thus, Mr Brooke says, in his Introduction to Crystallography, 
that the consideration of infinite lines, ,which M. Mohs has in- 
troduced into his system, and his notation founded on this cha- 
racter, are parts of his theory which will probably render its 
public reception less general than it might have been, from its 
merits in other respects.” In order to enable the reader to form 
an opinion in this matter, I shall subjoin a few explanatory re- 
marks, in respect to that idea of infinity, as introduced by M. 
Mohs ; and this cannot be done more satisfactorily, than by de- 
veloping the principle upon which it is founded. 
If we suppose, Fig. 8., tangent planes, equally inclined to both 
the adjacent faces of a rhombohedron, to be applied to the termi- 
nal edges of that form, and these planes to be enlarged, till they 
intersect each other, they will produce a new rhombohedron^ 
which is more obtuse than the given one. The axis AX is com- 
mon to both the forms, the side of the horizontal projection 
OR, of the more obtuse, is equal to 2 O' R', or double the side 
of the horizontal projection of the more acute rhombohedron f 
2 
