Crystallisation of Eyidote and qf‘ Glaiiber-Salt. 811 
face QONP, at the lateral solid angle B, Fig. 7., formed by the 
meeting of four different faces. Since the diagonals of any rhomb 
bisect each other, QR will be equal to NR. Draw QT parallel to 
BM, and TN parallel to MC. The triangles QBR and NSR 
are equal, and similar to each other ; QB, therefore, = SN, 
and TN — % SN. From this rhombic figure it follows, there- 
fore, that the ratio of the lines QT and TN, or h' and d in a 
section of the prism o, parallel to the base of P, must be equal to 
that of BM : 2 MC, Fig. 10., or = 6 : 2c, if, by b and c, we denote 
the long and the short diagonal of the base of the fundamental 
form. The sign of the prism o is therefore (Pr + go )^, and its 
angles of intersection, if the faces meet above the face M, is 
rr: 68° 8'. Since the faces of a horizontal prism have exactly the 
same situation at the other angle of combination, between che 
faces w, r, and T, the ratio of a' :d for y will be 1 6? : c, 
being expressed by the analogous lines in P ; and the form, 
therefore, to which these faces belong must be Pr — 1. The 
faces noted cc have first been described by Weiss. In or- 
der to develope the form to which they belong. 
Fig. 9. to represent a combination of 4- ^ 
— 2 
{7 
and (Pr 4 00 ( 0 ), projected oil a plane parallel to that which 
passes through the axis and the long diagonal of the fundamen- 
tal form. As the edges of combination between x and n are 
parallel to those between n and z, if this face passes through 
the point A, the edge of combination will coincide with the ter- 
minal edge AM of P. Likewise, on account of the parallelism 
between a?, o, and z, the same face will also pass through the 
edge MO, and AO will be the projection of the terminal edge 
of the pyramid to which the faces x belong. In this pyramid 
the ratio of a' \d is — a : c, on account of the coincidence of the 
two terminal edges in AM. But for the other diagonal we have 
ft' : = OQ : QA =:ift:|<x, r=:ft:8a, and consequently for the 
whole pyramid d \ V d — %a \ h \ which makes the sign of 
Xz=z 
(Pf 
on 
^ , and since d possesses a situation analogous to x.^ 
the opposite side of the fundamental form, the sign of this part 
2 ' 
of the same pyramid will be 
For the pyramid w, the 
