4 
ON 1?HE CRYSTALLISATIONS 
The immediate measurement was nevertheless not re- 
quired for proving these forms to be pyramidal. This is 
evident by their mere inspection, observing only how their 
edges of combination are situated ; and it might have been 
found that the angles of the pyramid differed from 109° 
28' 16'' even less than could be determined by any measure- 
ment; without making the form by this only to cease 
to be what it was, an isosceles four-sided pyramid, and to 
change it into the regular octahedron. Naturalists thought 
themselves, however, still more confirmed in the last opinion 
by a sort of twin-crystals, very common in copper-pyrites, 
which bear a strong resemblance to those of octahedral and 
dodecahedral corundum, and of octahedral iron-ore, andy 
therefore, had been supposed to take their rise from the 
same forms, as in these species. 
It will not be without interest to compare the chief mi- 
neralogical authors on the present subject, before we turn 
to the observation of Nature, 
The first author that treats more at large of copper- 
pyrites, is Henckel, who wrote at a period when the fu- 
ture importance of crystallographical knowledge for mine- 
ralogy scarce was thought of. He gives in the Pyritologia 
(Leipzig, 17S5, p. 157.) the description of those tetrahedral 
varieties whose forms have since been taken for the regular 
tetrahedron. The twelfth plate, however, " Pyrites deca- 
edrosj''' excepting only Fig. 2., represents a variety of cop- 
per-pyrites not uncommon in nature, and which we shall 
return to afterwards. From these drawings, founded upon 
correct observations, it might have been argued, that some 
one pyrites or other had a pyramidal form, though it could 
not be determined to which of them these observations re- 
ferred. 
Rome' de l'Isle, and, after him, Hauy, consider 
the crystalline forms of this mineral as deriveable from the 
