Mr Haidinger on the Series qf‘ Crystallisation of Apatite. 145 
finitely small axis, is represented by a single plane perpendicular 
to the axis of K ; whilst R+oo , which has the same transverse 
section as R, but an infinite axis, will have its faces disposed 
parallel to the direction of the same line, and, therefore, in no 
respect differ from a regular six-sided prism. On account of the 
identity of the transverse sections, R+oo is that regular six- 
sided prism, which produces horizontal edges of combination 
with any rhombohedron of the series. 
From the preceding development we may conclude, that, con- 
trary to Mr Brooke’s assertion, the idea of infinity (which cer- 
tainly is not a consideration of infinite lines) thus introduced, 
would be particularly recommendable in the method of M. Mohs, 
both for the elegance in the expression, and for the simplicity 
which it imparts to the method altogether. I am so much con- 
vinced of the extensive crystallographic knowledge displayed by 
the author of the preceding quotation, that I do not doubt he 
will perfectly concur in the latter opinion, when the system of 
M. Mohs shall have been sufficiently developed to the public, 
to enable him to judge fairly of its merits.” 
, But let us return to the further development of the combina- 
tion. The faces a?, &c. if they limit a space by themselves, 
form an isosceles six-sided pyramid. On account of the paral- 
lelism of its terminal edges with those in which it intersects the 
faces of R, x is the isosceles pyramid belonging to that rhombo- 
hedron, and P therefore is its crystallographic sign. The faces 
of M, lastly, are the limits of the series of isosceles six-sided 
pyramids, or the isosceles six-sided pyramid belonging to R-f-oo , 
and as such designated by P+go . The designation of the whole 
form therefore is, 
R— X . P. 2(R). P-f X . 
P a s, s' M 
The measures of the angles relative to the simple forms con- 
tained in this combination are the following. The angle at the 
terminal edges of R is = 88° 41'. R (fig. 6.) is obtained, as 
mentioned above, by enlarging the faces s^ s, s^ Sec. The side 
of its horizontal projection being supposed = 1 , the length of 
VOL. X. NO. 19. JAN. 18S4. 
K 
