^91 
^ Sulphato-tri- Carbonate of Lead. 
Fig. 4. They are parallel to one face of ( p -f ^ )'^? = 119'" lO', 
a prism which is likewise found in the crystals of this species. 
Upon this supposition, the angles are — 90® ^0', and 89® 40'. 
Of the individuals AA'BCDE, and AA'BC'D'E', which meet 
in the plane of composition passing through A B, nothing will 
remain but the rhomb-like trapezium AA'BA", the angles of 
which are A 60® SO' ; A' and A'' each~119® 50', and B=60®. 
If a third individual A'A"CGFC' joins the regular composition of 
the preceding two, being applied to BMA', the remainder of 
BAA', Fig. 4. in the line A'M, there will also arise a face of 
composition between A'MA" and BMA', and the angles of the 
remaining triangular figure A'A''B 3 Fig. 5, will be exactly = 60"*. 
In the compound crystals of axotomous lead-baryte, each of the 
edges A'xM, A"M, and BM is 179° 10'. 
The regular composition of three individuals, if the faces of 
:F 
Pr -P 1 or a are considerably increased, while those of ^ or P 
disappear, assumes very nearly the form of an acute rombohedron, 
whose apices and lateral solid angles are truncated, as in Fig. 6. 
The incidence of e upon g' is =; 72® S9\ almost the same as the 
angle given by Mr Brooke for the terminal edge of his acute 
rhombohedron. Even in crystals most perfectly formed, it is 
very easy to overlook the small salient angle of 179® 10' upon 
the faces RST, supposed perpendicular to the axis of the rhom- 
bohedron ; but this composition is often so intricate, particularly 
in larger crystals, that it sometimes becomes difficult to point out 
the direction and extent of each separate individual, though the 
existence of the composition is indicated by small, salient and re- 
entering angles, and proves, with the highest degree of evidence, 
that the forms of axotomous lead-barvte are not Bhombohedrai 
tj 
but Hemiprismatic. 
Although the observation of the optical properties of minerffis 
can never supersede the study of their regular forms, yet the 
preceding examination of the forms of axotomous lead-baryte, 
affords an ample proof that they may be highly useful in guiding 
us through the latter, particularly if these regular forms nearly 
coincide with certain limits. The crystallographic researches re- 
lative to this species are attended with considerable difficulties^. 
