the Corundum Stone , and its Varieties , &c. 267 
As it is by no means uncommon, in corundum, (in the same 
manner as is observed in the beryl,) to meet with elongated 
prisms, formed merely by the connection or contact of several 
prisms at their terminal faces, it frequently happens that these 
prisms, after being separated from each other, exhibit, upon the 
terminal faces which were in contact, a polish or lustre that 
might easily cause those faces to be taken for fractures, in a 
direction perpendicular to the axis. But this appearance is an 
illusion we must guard against : for, if we endeavour to make 
any fractures at the extremities of these crystals, they will take 
place, as usual, upon three of the alternate solid angles ; and we 
shall find it impossible to succeed in making any fractures 
perpendicular to the axis, except such as are extremely irre- 
gular, and exhibit an appearance very different from that exhi- 
bited by natural ones. It sometimes happens also, that, by means 
of the above connection, as well as by some causes of com- 
pression, which must necessarily have been frequent with respect 
to crystals inclosed in their matrix, in the manner those of 
felspar are inclosed in granite or porphyry, that the terminal 
faces have varied from their natural position, and have as- 
sumed another, which inclines more or less upon the sides of 
the prism. We must, however, distinguish these accidental 
varieties, from those crystals in which such an inclination really 
belongs to the mode of crystallization, and which I have already 
described, in speaking of the sixth modification. In this latter 
case, the inclination of the terminal faces is constantly the same; 
whereas, in the accidental case here treated of, it varies consi- 
derably. 
There exists also, in this substance, and even among the 
same crystals, (when hexaedral prisms,) not only of imperfect 
