232 Count de Bournon*s Description of 
dicular upon the sides, as in Fig. 40. (Plate IX.) This form, 
which was before unknown in the thallite, and which might at 
first view be taken for a primitive one, was very likely to lead to 
an erroneous idea ; it may however be explained by another form, 
which is also met with in perfectly determined crystals. In these 
last, the prism is hexaedral, with two edges of 1 14,° 30', two others 
of 128° 30', and the two last of 117 0 ; its terminal faces are also 
perpendicular upon the sides of the prism, as in Fig. 41. Now 
this form is exactly the same as one of those already observed 
in the prism of the common thallite, and is produced in the fol- 
lowing manner, viz. the primitive rhomboid, the edges of which 
are 114 0 30' and 65° 30', has each of its acute edges replaced 
by a plane, inclined, in a contrary direction, upon one of the sides 
of the prism, so as to make with it an angle of 128° 30k I 
have often found this hexaedral prism terminated, in the same 
way, by planes perpendicular to its sides, among the crystals of 
thallite from the Alps of Dauphiny. The preceding rhomboidal 
tetraedral prism, consequently, is produced by an increase of the 
faces which have replaced the edges of 65° 30' ; which increase 
has been such as to cause the sides of the primitive rhomboidal 
prism, on which each of them incline, to disappear: this is 
represented by the dotted lines in Fig. 42. The direction of 
the laminae, in these crystals, strongly supports the foregoing 
explanation. Sometimes the rhomboidal prisms become of an 
indeterminate form, by being flattened so as to render the edges 
of 128° 30' much more obtuse: when that happens, they have 
no longer any regular measure. 
In this first state of the thallite which accompanies the imper- 
fect corundum from the Carnatic, the pieces, whether they are 
crystallized or of an indeterminate form, have their surface co- 
vered with little asperities, thereby exhibiting an appearance 
