54) Count de Bournon's Description of a 
of it. The above-mentioned vertical stride are sometimes so 
strongly marked, as to give reason to suppose we might, by a 
very trifling effort, succeed in separating the laminae from each 
other, according to their natural direction ; but every attempt I 
have made with this intention, has constantly been fruitless. 
Second Modification. This modification is produced by a de- 
crease at the solid angles of the primitive crystal, in consequence 
of which, each of these angles is replaced by a plane, which 
makes, at its meeting with the terminal faces, an angle of 130°; 
and, with the vertical edges of the prism, an angle of 140°, as 
in Fig. 5. I have never yet observed the planes produced by 
this modification, simply combined (as is shown in the figure) 
with those of the primitive crystal ; but the crystals I shall now 
describe, will demonstrate the probability of the existence of such 
a combination. I have, however, seen these planes combined 
with those of the secondary prism ; and, when that happens, 
the planes caused by this second modification, instead of being 
situated at the solid angles of the prism, (as is the case when 
they are combined with the planes of the primitive crystal,) are 
situated on the edges of the terminal faces. 
Third Modification. This modification is also produced by a 
decrease at the solid angles of the primitive crystal ; but this 
decrease is greater than the former, that is to say, the edges of 
the laminae, w'hich are deposited one upon the other, recede in 
a greater degree from the edges of those which have preceded 
them. This gives rise to a plane, which, in a similar manner, 
replaces the solid angles of the primitive crystal, and makes, at 
its meeting with the terminal faces, an angle of 150°; and, with 
the edges of the prism, one of 120®. See Fig. 6 . I have not in- 
deed hitherto seen the planes produced by this third modification 
