^88 Ml" Haidinger on the Forms (^'Crystallisation 
others, equally interesting, with which I had been favoured by 
Dr Brewster, Mr Irving, and Mr T. Dowler, have enabled me 
to ascertain the forms of this species with a considerable degree 
of accuracy. The results of that examination are very remark- 
able. They exclude the rhombohedron and the regular six- 
sided prism from the range of forms, which the individuals of 
the species may assume, and thus perfectly confirm the inference 
drawn by Dr Brewster from his optical observations, while they 
are at variance with the crystallographic statements both of 
Count Bournon and of Mr Brooke. 
By means of the reflective goniometer, I found that the six- 
sided prisms are not regular, but that they are combinations of 
thi’ee different simple forms, a, 6, and c, Plate X. Fig. 1. ; the 
inclination of a upon h being = 90° 29'; that of c upon c = 
120° SO'; and that of h upon c = 119° 50'. Though slight,, 
these differences are easily ascertained, and their consequences, 
in the disposition of the crystalline faces, are so obvious, that they 
would certainly not have escaped the practised eye of the crys- 
tallographers who described them, had not a particular mode 
of regular composition seemed to establish a kind of symmetry 
round a rhombohedral axis supposed to be perpendicular to the 
faces of cleavage. 
The system of crystallisation to which the forms of Axoto- 
mous Lead-baryte belong, is not, therefore, the rhombohedral 
system, nor do these forms enter into that class of prismatic 
forms which exhibit the full number of faces of every simple 
form in the combinations, but they must be considered as hemi- 
prismatic, the axis of crystallisation, which is parallel to the 
edges of the prism c, being inclined to the base at an angle of 
90° 29'. 
There are two observations which can be very easily institu- 
ted on almost every group of well-pronounced crystals of the 
species, and which evidently prove that the forms of these are 
really hemiprismatic. The first of them refers to oblique trunca- 
tions of the lateral edges, between h and c, as d, d, in Fig. 2., 
which are inclined to h at an angle of 156° 27', and to c at an 
angle of 143° 23'. The other refers to the slightly, but very 
distinctly, marked planes of conjunction, between two indivi- 
duals in the regular compositions, in a direction joining alterna- 
