OF COPPER-PYRITES, IS 
Now OP : PQ = NA : AQ, or, what follows fmm 
it, NA = AQ. 
But since the angle OAQ is = 135*^, 
ON = NA, 
and NQ: NO =: 2 : 1, 
which gives the angle OQN = 26^ S3' 5^, and 
ORO' = 126^ IT, equal to the angle of the 
transverse section of (F-\-ny . * 
16. P— 2. ^~P. P+oo. [P+Qo]. fig. 24, 
Crystals of this form are very easily taken for tessu- 
lar, particularly if they are fixed to some support, so 
as to allow only a few of their faces to be observed. 
17. P~0D. P-2. P. g-^P. g^^P+l. P-h OD. fig. 25. 
P Q CP r»^wi 
18. 5^-2^. P~l. P. P+1. P+oo. (P+oc)^. 
fig. 26. There occur in the present combination 
only the alternating faces of the pyramid P — 2. By 
their enlargement we arrive at a solid contained un- 
der four isosceles triangles, fig. 27., analogous to the 
tetrahedron, and to that in var. 1., which is pro- 
duced by the increasing of the alternating faces of P, 
The form is in the designation marked with — , to 
denote its faces to be alternating with the enlarged 
faces of P. 
Encyclopaedia Britannica, article Mineralogy. 
