OF COPPER-PYRITES. 
3 
regular octahedron^ or any other form of the Tessular 
System. 
By considerations of this kind, it became very probable 
that the angles of the four-sided pyramid of copper-pyrites 
would, by an accurate investigation, prove different from 
those of the regular octahedron. Such a difference has in- 
deed been found by an immediate measurement with Dr 
Wollaston's reflecting goniometer, which yielded the 
angle at the basis of the four-sided pyramid = 108° 40' ; 
due attention being given to all the necessary corrections 
required in the use of that instrument. The difference be» 
tween this angle of 108° 40', and that of the regular octa- 
hedron = 109° 28' 16", amounts only to 0° 48', a quantity 
very easily missed in employing the common goniometer *, 
for the determination of the angles. Moreover the crystals 
are commonly very small, which adds to the difficulty of 
having correct results from the application of this instru- 
ment. The little difference existing between the angles of 
the pyramid of copper-pyrites and those of the regular 
octahedron, the smallness^ and after all the scarcity of dis- 
tinct crystals, so as to have all their homologous faces of 
equal extent, sufficiently account for the incorrect ideas 
hitherto current among mineralogists respecting the system 
of crystallisation of the present species. 
* The Abbe Hauy allows errors to be possible in measurements of this 
kind to the amount of a whole degree. He says, (Traite, t. ii. p. 325 
" On volt que les differences entre les angles donnes par les deux hypotheses 
ne vont pas audela d'un degre (being 0° 0', 1° 1', 0° 9', 1° 0'), et que cello 
qui appartient a la troisieme, n'est que de 9'. Or, quoique ces differences 
resultent de mesures prises avec beaucoup de soin sur des cristaux d'une 
forme tres-nette, il est possible, apres tout, qu'une erreur due a I'observation 
en produise de semblables." 
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