g 6 Count de Bournon’s Description of a 
with the terminal faces, and with the vertical faces of the prism, 
an angle of 135°, as in Fig. 13. 
If the planes produced by this fourth modification were com- 
bined with those of the secondary prism, they would then be 
situated at the solid angles of this prism, as is represented in 
Fig. 14. I have not yet seen these two last varieties in so simple 
a state as that in which the figures represent them ; but the two 
most common forms in which this substance has hitherto been 
found, are those shewn in Figs. 15 and 1 6. Now, Fig. 15 is 
nothing more than the combination of the planes of the primitive 
crystal with those of this modification, and with those of the 
first and second modifications. In the same way, Fig. 16 is 
likewise the result of the same combination ; but with the planes 
of the third modification added to it. 
Lastly, Fig. 1 7 exhibits a detached crystal, of a perfectly de- 
terminate form, and of about an inch in length, seven lines in 
breadth, and five lines in height, which makes part of the col- 
lection of Mr. Phillips, who was so kind as to allow me to 
examine it, as well as every other specimen of this substance in 
his possession. It is, in fact, the variety represented in Fig. 1 6, 
but in which the planes belonging to the fourth modification 
have acquired a more considerable extent; while those which 
belong to the third modification exist only on two of the op- 
posite edges of the terminal faces, as was observed in speaking 
of the crystal represented in Fig. 10. 
Before I proceed to those observations which this triple sul- 
phuret (a most interesting substance in mineralogy) affords me 
an opportunity of making, with regard to the various ores that 
are produced by the combination of sulphur and copper, of the 
nature of which, neither mineralogy nor chemistry have yet 
