ON THE CRYSTALLISATIONS 
Now GO : 01 r= GP : PK, and from the similarity 
of the triangles KPO and AOR, 
OA : PK = OR : OP. 
From the triangles OKP and RKP being equal 
and similar, it follows that OP = PR, and OP = 
I OR, therefore PK = J OA. 
Thus, the first proportion becomes 
GO : 01 = I GO : J OA, 
and, OA : 01 = 3 : 1. 
The horizontal projections being equal, the axis 
of the more acute pyramid is three times as long as 
the axis of the more obtuse ; from which follow the 
angles as they have been stated in variety 5. 
The pyramid which appears with parallel edges of 
3 
combination on the terminal edges of ^g^+^j 
3 
evidently g^P? being the subsequent more obtuse 
member of the same subordinate series to which the 
other pyramid belongs. 
Its axis is = V8.7354 ; 
terminal edge 108° 18' ; 
lateral edge =111° 50'. 
p_oo. P— 1. R P-fl. fig. a The same variety, 
which in the preceding we have made use of for the 
development of the three pyramids of the principal 
series. It is one of the most common varieties of 
copper-pyrites, and occurs more or less distinct in a 
great many localities^ 
